The selective vulnerability of striatopallidal neurons

Progress in Neurobiology Vol. 59, pp. 691 to 719, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0301-0082/99/$ - see front matter

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THE SELECTIVE VULNERABILITY OF STRIATOPALLIDAL NEURONS I. J. MITCHELL*}, A. J. COOPER$ and M. R. GRIFFITHS* *School of Psychology, University of Birmingham, Birmingham, B15 2TT, UK $Department of Pharmacology, University of Birmingham, Birmingham, B15 2TT, UK (Received 12 March 1999) AbstractÐThe di€erent types of striatal neuron show a range of vulnerabilities to a variety of insults. This can be clearly seen in Huntington's disease where a well mapped pattern of pathological events occurs. Medium spiny projection (MSP) neurons are the ®rst striatal cells to be a€ected as the disease progresses whilst interneurons, in particular the NADPH diaphorase positive ones, are spared even in the late stages of the disease. The MSP neurons themselves are also di€erentially a€ected. The death of MSP neurons in the patch compartment of the striatum precedes that in the matrix compartment and the MSP neurons of the dorsomedial caudate nucleus degenerate before those in the ventral lateral putamen. The enkephalin positive striatopallidal MSP neurons are also more vulnerable than the substance P/dynorphin MSP neurons. We review the potential causes of this selective vulnerability of striatopallidal neurons and discuss the roles of endogenous glutamate, nitric oxide and calcium binding proteins. It is concluded that MSP neurons in general are especially susceptible to disruptions of cellular respiration due to the enormous amount of energy they expend on maintaining unusually high transmembrane potentials. We go on to consider a subpopulation of enkephalinergic striatopallidal neurons in the rat which are particularly vulnerable. This subpopulation of neurons readily undergo apoptosis in response to experimental manipulations which a€ect dopamine and/or corticosteroid levels. We speculate that the cellular mechanisms underlying this cell death may also operate in degenerative disorders such as Huntington's disease thereby imposing an additional level of selectivity on the pattern of degeneration. The possible contribution of the selective death of striatopallidal neurons to a number of clinically important psychiatric conditions including obsessive compulsive disorders and Tourette's syndrome is also discussed. # 1999 Elsevier Science Ltd. All rights reserved

CONTENTS 1. Introduction 2. Striatal anatomy 2.1. Striatal neurons 2.2. Striatal a€erents 2.3. Striatal e€erents 2.4. Direct and indirect striatal projection pathways 2.5. Patch/matrix organisation of the striatum 2.6. Parallel pathways within the striatum and basal ganglia 2.7. Overview of the anatomical/Functional organisation of the striatum 3. Huntington's disease 3.1. Heritability, symptomatology and basic pathology 3.2. Selective loss of striatal neurons in Huntington's disease 3.2.1. Di€erential loss of MSP neurons in the patch and matrix 3.2.2. Di€erential loss of GABA/enkephalin and GABA substance P/dynorphin MSP neurons 3.2.3. Preservation of striatal interneurons in Huntington's Disease 3.3. Huntingtin protein 3.4. Overview of striatal degeneration in Huntington's disease 4. Animal models of Huntington's disease 4.1. Introduction 4.2. Excitotoxins and models of Huntington's disease 4.2.1. Role of glutamate receptor sub-types in striatal excitotoxicity 4.2.2. Selective sparing of interneurons by excitotoxic lesions 4.2.3. Role of dopamine in glutamate mediated excitoxicity of the striatum 4.2.4. Extrastriatal changes induced by excitotoxic lesions of the striatum 4.2.5. Type of cell death induced by excitotoxins 4.2.6. Selective vulnerability of striatal neurons and calcium binding proteins

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4.2.7. Role of microglia in mediating excitotoxic death of striatal neurons 4.2.8. Protection from excitotoxin induced striatal degeneration by neurotrophic factors 4.3. Striatal toxicity and inhibitors of mitochondrial respiration 4.3.1. Introduction 4.3.2. Mechanism of toxic action of mitochondrial respiration inhibitors 4.3.3. Speci®city of neuronal damage induced by mitochondrial inhibitors 4.3.4. Selective attenuation of mitochondrial inhibitor induced striatal damage by trophic factors 4.3.5. Amphetamines potentiate the toxic e€ects of mitochondrial inhibitors 4.3.6. Behavioural de®cits following administration of mitochondrial inhibitors 4.4. Transgenic animals as a model for Huntington's disease 4.4.1. Molecular studies of the Huntingtin gene 4.4.2. Genetic composition of transgenic animals and distribution of mutant Htt 4.4.3. Pathology, neurochemistry and neurology of transgenic animals 4.4.4. Overview of animal model of Huntington's disease 5. Dopaminergic manipulations and striatal toxicity 5.1. Introduction 5.2. Amphetamines and striatal toxicity 5.3. Phencyclidine induced striatal apoptosis 5.4. Reserpine and haloperidol induced striatal apoptosis 5.5. Overview of dopaminergic manipulations and striatal toxicity 6. Stress and striatal toxicity 6.1. Introduction 6.2. Stress and the striatum 6.3. Corticosteroids and apoptosis 6.4. Mechanisms mediating dexamethasone induced striatal apoptosis 6.5. Overview of stress and striatal toxicity 7. Conclusions Acknowledgements References

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ABBREVIATIONS AOAA BDNF bFGF DHEAS EAA MPTP MSP NADPH

Amino-oxyacetic acid Brain derived neurotrophic factor basic ®broblast growth factor Dehydroepiandrosterone sulphate Excitatory amino acid 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine Medium spiny neuron Nicotinamide adenine dinucleotide phosphate

1. INTRODUCTION Dysfunction of the striatum is often associated with disorders of movement. Thus, loss of striatal dopamine is known to result in Parkinsonism whilst the degeneration of striatal neurons seen in Huntington's disease is associated with choreiform movements. Similarly, striatal lesions are associated with dystonia and the motor tics which occur in Tourette's patients are assumed to be of striatal origin (Salloway and Cummings, 1996). There is, however, an increasing realisation that the striatum is involved in cerebral functions other than the control of movement (Calabresi et al., 1997b). These nonmotor functions of the striatum include roles in the control of attention, executive function and motivated behaviours (Alexander et al., 1986). Consequently, striatal dysfunction is now being implicated in neuropsychiatric conditions such as obsessive compulsive disorders, psychoses and addictive behaviours (Calabresi et al., 1997b). The striatum is not a homogenous structure with respect to either its internal organisation or its a€erent and e€erent connections. This anatomical heterogeneity is re¯ected in functional heterogeneity and it thus follows that de®cits seen as a conse-

NGF 3NP NO NOS NMDA PCP SOD

Nerve growth factor 3-Nitropropionic acid Nitric acid Nitric oxide synthase N-Methyl-D-aspartate Phencyclidine Superoxide dismutase.

quence of striatal dysfunction must depend in large part on the precise location of the damage incurred. Speci®c parts of the striatum show marked di€erential vulnerability to di€erent types of insult. Thus, amphetamines can result in damage to dopaminergic striatal terminals (Sonsalla et al., 1989; Eisch et al., 1992; Hirata et al., 1995; Hirata et al., 1996) whilst mitochondrial inhibitors cause the degeneration of projection neurons but not interneurons (Beal, 1992a, 1992b; Bossi et al., 1993; Bolanos et al., 1997). Similarly, the neuronal degeneration seen in Huntington's disease follows a set pattern with the di€erent populations of striatal projection neurons dying in a progressive manner. Indeed there is now considerable evidence to show that it is the enkephalin positive striatopallidal neurons which are most vulnerable in Huntington's disease (Albin et al., 1992; Rich®eld et al., 1995). Rat striatopallidal neurons also appear to be particularly vulnerable to certain pharmacological manipulations. Thus, we have recently demonstrated that acute administration of both phencyclidine (PCP) and dexamethasone induces apoptosis of striatopallidal neurons in the rat (Mitchell et al., 1998).

The Selective Vulnerability of Striatopallidal Neurons

The purpose of this paper is to review some of the literature on the mechanisms underlying striatal damage in both Huntington's disease and following the administration of certain drugs in order to advance hypotheses as to why striatopallidal neurons should show this selective vulnerability. The roles of endogenous glutamate, nitric oxide (NO), immediate early genes, corticosteroids and dopamine in the process of striatal degeneration will be discussed and the possible contribution of the selective death of striatopallidal neurons to a number of clinically important neurological and psychiatric conditions is considered.

2. STRIATAL ANATOMY 2.1. Striatal Neurons There are minor species di€erences in the anatomical organisation of the striatum and associated structures in rodents and primates (Parent, 1986). The primate striatum contains a structurally distinct caudate nucleus and putamen whereas these structures are not di€erentiated in the rodent brain and are collectively referred to as the caudateputamen or neostriatum. The striatum of both primates and rodents contains a rostroventral extension which is referred to as the nucleus accumbens (Nauta, 1979). The histological organisation of striatal neurons does not vary across these major anatomical subdivisions. It is now accepted that 95% of striatal neurons are projection neurons and only 5% are interneurons. Striatal projection neurons are GABAergic medium spiny (MSP) neurons. These neurons have unusual electrophysiological properties in that they show very low levels of spontaneous discharge and maintain a hyperpolarised state. MSP neurons, however, will discharge phasically in response to excitatory inputs from the neocortex. (Kawaguchi, 1997; Kawaguchi et al., 1995). These phasic discharges suppress the activity of neurons in the output structures of the basal ganglia, which in contrast to the MSP neurons, tend to discharge continuously. Striatal interneurons have been identi®ed on the basis of their morphology and cytochemical staining characteristics (Kawaguchi et al., 1995). The most studied group of these interneurons are cholinergic, have a large soma, widespread dendritic trees, receive direct dopaminergic inputs and form most of their synaptic contacts with MSP neurons (Kawaguchi et al., 1995). A second group of striatal interneurons are distinguished in terms of being nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase positive, this enzyme being identical to nitric oxide synthase (NOS). These interneurons also contain somatostatin and neuropeptide Y. The NADPH diaphorase positive striatal interneurons are of particular interest in the context of neurodegeneration as these neurons appear to be resistant to excitotoxin induced damage.

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2.2. Striatal A€erents Fig. 1 shows the principle connections of the striatum. The major a€erent inputs to the striatum arise from the cerebral cortex. All areas of the cerebral cortex, both neocortex and allocortex, project to the striatum and all striatal areas receive a cortical input (Nauta, 1979; Nauta and Domesick, 1984; Goldman-Rakic and Selemon, 1990). The corticostriatal ®bres are topographically organised so that each striatal area preferentially receives an input from a certain area of the cerebral cortex (McGeorge and Faull, 1989). There is, however, considerable overlap in the terminal ®elds of the cortical a€erents which ensures that each striatal area receives information from more than one cortical area (Kemp and Powell, 1970; Goldman-Rakic and Selemon, 1990; Selemon and Goldman-Rakic, 1985). The corticostriatal neurons are glutamatergic and make direct synaptic contact with the MSP neurons (Bouyer et al., 1984). The dopaminergic neurons of the midbrain innervate all areas of the striatum. In general it can be argued that the caudateputamen is innervated by dopaminergic neurons which lie within the substantia nigra pars compacta whilst the nucleus accumbens receives an input from the dopamine cells which lie in the nearby ventral tegmental area (Nauta and Domesick, 1984; Anden et al., 1964; Oades and Halliday, 1987). A third major source of striatal a€erents arises from the thalamus. The caudateputamen receives glutamatergic ®bres from the centromedian-parafascicular nuclei of the intralaminar complex and the rostral intralaminar thalamic nuclei whilst the nucleus accumbens also receives an input from the midline and intralaminar nuclei including the nucleus reunions (Groenewegen et al., 1980; Druga et al., 1991). 2.3. Striatal E€erents The principle projection targets of the MSP neurons in the primate are the lateral and medial segments of the globus pallidus and the substantia nigra (Parent et al., 1984). An equivalent pattern of projections is seen in the rodent brain though a di€erent nomenclature is used to describe the globus pallidus. In the rat the term globus pallidus is used to refer to the lateral segment of the globus pallidus and the entopeduncular nucleus is considered to be equivalent to the medial pallidal segment of the primate. The substantia nigra pars reticulata shares many anatomical and histological features with the medial pallidal segment which has caused some to consider them to be functionally related (Nauta, 1979). All MSP neurons are GABAergic. In addition to this classical transmitter, however, MSP neurons also contain neuropeptides. Those MSP neurons which project to the globus pallidus are enkephalinergic whereas those which project to the entopeduncular nucleus and the substantia nigra pars reticulata have substance P and/or dynorphin colocalised with GABA (Gerfen and Young, 1988). The enkephalinergic and substance P/dynorphin

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Fig. 1. The major a€erent and e€erent connections of the MSP neurons of the striatum. MSP neurons can be di€erentiated into two major populations on the basis of their neurochemistry. The enkephalin positive MSP neurons project to the lateral segment of the globus pallidus whereas the substance P/ dynorphin positive ones project to the medial segment of the globus pallidus. Both populations of striatal neurons receive a glutamatergic input from the cerebral cortex and a dopaminergic input from the substantia nigra. The two populations di€er, however, in the type of dopamine receptor which they express. The enkephalin positive MSP neurons predominantly express D-2 receptors whilst the substance P/dynorphin positive MSP neurons tend to express D-1 receptors.

MSP neurons are distributed throughout the striatum though cells with common projection sites tend to be clustered in small aggregates of neurons (Kawaguchi, 1997). There is also a well organised projection from the striatum to the dopaminergic neurons of the substantia nigra pars compacta (Nauta and Domesick, 1984; Gerfen, 1992) as will be described in Section 2.5. The principle projection target of the nucleus accumbens is an ill-de®ned area which lies beneath the rostral aspect of globus pallidus. This area, termed the ventral pallidum, receives inputs from both types of MSP neurons (Haber et al., 1990). 2.4. Direct And Indirect Striatal Projection Pathways The striatoentopeduncular (striatonigral) and striatopallidal neurons are said to give rise to two complementary pathways, the so called direct and indirect pathways respectively (Penney and Young, 1986). It is generally assumed that many forms of basal ganglia dysfunction stem from imbalances in the activity of these two pathways. For example, degeneration or hypoactivity of MSP neurons which

contribute to the indirect pathway are associated with choreiform movements (Crossman, 1987; Crossman et al., 1988; Mitchell, 1990; Mitchell et al., 1985a; Mitchell et al., 1985b). The direct/indirect pathway terminology re¯ects the fact that the entopeduncular nucleus (or medial pallidal segment in primates) represents the major output nucleus of the basal ganglia, its projections being principally directed to the ventromedial nucleus of the thalamus (ventral anterior-ventrolateral thalamic nuclei in the primate), the lateral habenula and the pedunculopontine nucleus of the midbrain (Nauta, 1979; Nauta and Mehler, 1966). The striatoentopeduncular neurons can thus enable the striatum to in¯uence the output of the basal ganglia directly. The striatopallidal neurons can also in¯uence the activity of the entopeduncular nucleus but only via an indirect route. These neurons synapse on pallidal neurons which in turn project to the subthalamic nucleus. The subthalamic nucleus neurons then project to the entopeduncular nucleus (Smith et al., 1998). The MSP neurons which contribute to the direct and indirect pathways express markedly di€erent dopamine receptor subtypes. Striatoentopeduncular

The Selective Vulnerability of Striatopallidal Neurons

neurons principally express D-1 receptors whereas striatopallidal neurons preferentially express D-2 receptors (Gerfen et al., 1990). Decreasing striatal dopamine mediated transmission tends to increase the activity of striatopallidal neurons (Mitchell et al., 1989b). As a product of molecular biological studies it is now recognised that there are families of D-1 and D-2 like receptors (Missale et al., 1998) which show speci®c regional distributions within the striatum. For example, D-3 receptors are not expressed by MSP neurons of either type in the neostriatum but they are found within the nucleus accumbens (Kawaguchi, 1997). MSP neurons of the direct and indirect pathways also respond di€erently to cholinergic stimulation. Thus cholinergic agonists appear to stimulate striatopallidal neurons but inhibit striatonigral neurons (Harrison et al., 1996). These di€ering responses to cholinergic drugs may re¯ect the di€erential location of muscarinic receptor subtypes as the M4 receptor subtype is found preferentially on striatonigral neurons (Harrison et al., 1996). Additionally, indirect neurons express the A2a adenosine receptor subtype whereas neurons of the direct pathway do not (Ferre et al., 1993). 2.5. Patch/matrix Organisation Of The Striatum In addition to being classi®ed as being part of the direct or indirect pathway MSP neurons can be classed as being part of the patch (also referred to as the striosome) or matrix compartment (Graybiel, 1990; Gerfen, 1992). This classi®cation stems from observations on the di€ering neurochemical content of subregions of the striatum. Several neurochemical markers can be used to distinguish the matrix from the patch compartments (Graybiel, 1990; Gerfen, 1992). For example, the calcium binding protein calbindin D28K is found in neurons which lie in the matrix but not the patch whereas mu opiate receptors are found only in neurons which lie in the patch (Herkenham and Pert, 1981; Kawaguchi, 1997). The striosome and matrix striatal compartments have slightly di€erent a€erents and e€erents to each other. For example, the cortical projections to the patch compartment originate mainly from the medial and orbital frontal cortices whilst the matrix receives inputs from the remainder of the neocortex (Gerfen, 1989). Similarly the major output from the striosome is to the dopamine cells of the substantia nigra whereas the projections from the matrix tends to be directed to the globus pallidus, entopeduncular nucleus and the substantia nigra pars reticulata (Gerfen, 1985). This anatomical arrangement places the MSP neurons of the striosomal compartment in a position to regulate the activity of the mesencephalic dopamine neurons. The dendrites of individual MSP neurons tend not to extend beyond the compartment in which their soma lies (Penny et al., 1988; Kawaguchi, 1997). In contrast the soma of cholinergic interneurons lie predominately in the matrix but their dendrites extend into nearby patches. NADPH diaphorase positive and parvalbumin positive striatal interneurons similarly have dendritic trees which extend into both compartments (Kubota and

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Kawaguchi, 1993). This arrangement enables striatal interneurons, unlike MSP neurons, to directly interact with neurons of both striatal compartments. The dissociation of MSP neurons into patch and matrix neurons may be of signi®cance to the understanding of the selective vulnerability of speci®c subpopulations of striatal neurons as neurons in the patch appear to be some of the ®rst to degenerate in Huntington's disease (Hedreen and Folstein, 1995). 2.6. Parallel Pathways Within The Striatum And Basal Ganglia The caudateputamen and the nucleus accumbens clearly share many common anatomical features including their internal organisation, histology and a€erent and e€erent connections. There is, however, a wealth of evidence to show that these two striatal regions also di€er from each other in terms of the precise nature of their input/output connections and, consequently, their function. The major cortical inputs to the caudateputamen arise from the neocortex whereas the nucleus accumbens receives inputs from the limbic cortex, in particular the hippocampus, and the cingulate cortex (Groenewegen et al., 1980; McGeorge and Faull, 1989). It is generally assumed that the nucleus accumbens, in keeping with these strong limbic connections, plays a critical role in the control of motivated behaviours (Mogenson et al., 1980). Thus manipulations of the nucleus accumbens in experimental animals tend to result in changes in motivated behaviours such as locomotion and exploratory behaviours. There is also a wealth of evidence to show that the nucleus accumbens forms part of the neural mechanisms which underlie reward or reinforcement. Dysfunction of the nucleus accumbens in humans has been hypothesised to account for schizophrenia and addictive behaviours (Iversen, 1995; Wise et al., 1995; Bardo, 1998). It is generally assumed that the caudateputamen is not concerned with the control of motivated behaviours (though this simple functional dissociation is not supported by the intracranial self stimulation studies of White and Hiroi (1998) nor the electrophysiological studies of Schultz and co-workers (Apicella et al., 1991; Schultz et al., 1992; Schultz et al., 1995). Neostriatal pathology in humans is often associated with disorders of movements and manipulations of the neostriatum in experimental animals can similarly induce movement disorders. This has led in part to the assumption that the caudateputamen is solely concerned with motor control. Whilst the strict functional dissociation of the caudateputamen and nucleus accumbens in terms of motor control verses control of motivated behaviours has been tremendously in¯uential it is no longer tenable. It is now clear that parts of the neostriatum, in particular the caudate nucleus, are primarily concerned with tasks other than the control of movement. This conclusion is based on observations of cognitive de®cits following damage to the neostriatum of both experimental animals and humans (Brown et al., 1997). Delong and colleagues have devised a model of basal ganglia organisation which views the striatum as being part of a series of

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parallel structurally and functionally distinct circuits that link the cortex, basal ganglia and thalamus (Alexander et al., 1986). Each circuit focuses on a di€erent portion of the frontal lobe. These authors de®ne ®ve such circuits, namely a motor circuit, an oculomotor circuit, a dorsolateral prefrontal circuit (concerned with executive functions), lateral orbitofrontal circuit and an anterior cingulate circuit.

2.7. Overview Of The Anatomical/Functional Organisation Of The Striatum The anatomical and functional organisation of the striatum can be described from several perspectives. At the simplest level the functional organisation of the striatum can be said to be based in large part on the topographical nature of the cortical glutamatergic and mesencephalic dopaminergic a€erents which synapse directly onto the MSP neurons. Several functionally distinct and anatomically segregated circuits link speci®c areas of the cerebral cortex, basal ganglia and thalamus. It is assumed that the cortex conveys speci®c, temporally and spatially patterned messages to the MSP neurons (Bjorklund et al., 1987). In contrast the dopaminergic inputs convey less speci®c information serving as either a reinforcement signal (Schultz et al., 1995), an error signal (Barto, 1995) or a gain control signal (Servan-Schreiber et al., 1990). Two major types of MSP neurons can be distinguished in terms of their neurochemical properties. These are the substance P/dynorphin positive neurons which project to the entopeduncular nucleus and substantia nigra pars reticulata and the enkephalin positive neurons which provide a€erents to the globus pallidus (Gerfen, 1992). This anatomical dissociation of the MSP neurons forms the basis of the direct and indirect pathways. The balance of activity between the two types of MSP neuron and thus the respective activities of the direct and indirect pathways ultimately determine the pattern and level of output of the basal ganglia (Penney and Young, 1986). An additional level of organisation is imposed upon the striatum by the existence of a patch or striosomal compartment which appears to regulate the activity of MSP neurons within the complementary matrix compartment via the action of cholinergic interneurons (Kawaguchi, 1997). One striking feature of the striatum is the way in which these di€erent anatomical units respond to a range of insults. Each of the sub-components of the striatum, whether it be striosomes or matrix, MSP neurons in the direct or indirect pathways, MSP neurons in a given functional circuit, projection neurons or interneurons, intrinsic cells or terminals, show a vulnerability to a particular type of insult. This speci®city will dictate in large part the nature of the de®cit that will follow from damage to any of these speci®c anatomical units. The following sections will detail how the di€erent components of the striatum respond to a variety of insults, including the administration of drugs and toxins, beginning with a consideration of the pattern of striatal degeneration seen in Huntington's disease.

3. HUNTINGTON'S DISEASE 3.1. Heritability, Symptomatology And Basic Pathology Huntington's disease is the best characterised clinical condition which results from striatal degeneration. It is a progressive autosomal dominant disorder. The locus of the defective gene, IT15, has been mapped to the short arm of chromosome 4 (Huntington's Disease Collaborative Research Group, 1993; Albin and Tagle, 1995). Huntington's disease belongs to an ever increasing group of diseases characterised by the presence of trinucleotide repeats (Paulson and Fischbeck, 1996). Chromosomes from both normal and Huntington's disease patients have an expansion of a trinucleotide (CAG) repeat sequence which encodes a polyglutamine tract. Individuals with in excess of approximately 40 repeats will almost certainly develop Huntington's disease (Andrew et al., 1993). Onset of symptoms is usually around the age of 40, the predominant clinical symptom being chorea. Most patients, however, go onto develop dystonia and some show rigidity and akinesia. Psychiatric symptoms often present. Initially these tend to be obsessive behaviours and psychoses but many patients become demented in the later stages of the disease (Lauterbach et al., 1998). More subtle cognitive problems have been detected in Huntington's patients by means of neuropsychological investigations. These problems include de®cits on tests of executive function and implicit learning (Knowlton et al., 1996a, 1996b; Lawrence et al., 1998). In general terms the striatum can be considered as bearing the brunt of the pathology seen in Huntington's disease. As the disease progresses, however, the degeneration can spread beyond the striatum and the ®nal stage of Huntington's is associated with cortical atrophy (Ellison et al., 1987; Heinsen et al., 1994). It should be noted, however, that degeneration of the cerebral cortex of the temporal lobe in the early stages of Huntington's disease has been reported (Macdonald et al., 1997). The striatum is not uniformly a€ected in Huntington's disease. Indeed, the pattern of striatal cell loss unfolds in a complex manner as the disease progresses. As will be detailed in the next sections the striatal damage begins with MSP neurons in the striosomes before spreading to the matrix in dorsomedial parts of the caudate nucleus. MSP neurons giving rise to the indirect pathway tend to die before those in the direct. The wave of degeneration spreads ventrally such that the nucleus accumbens is only a€ected in the late stages of the disease and some populations of interneuron remain intact throughout. 3.2. Selective Loss Of Striatal Neurons In Huntington's Disease 3.2.1. Di€erential Loss Of MSP Neurons In The Patch And Matrix Hedreen and Folstein (1995) described the progressive pattern of striatal degeneration in Huntington's disease across the striosomes and

The Selective Vulnerability of Striatopallidal Neurons

matrix, the striosomal compartment being de®ned in terms of low levels of calbindin immunoreactivity. They noted that the pathology in early stage Huntington's disease, as revealed by glial ®brillary acidic protein (GFAP) immunoreactivity (a marker of astrocytes and indicator of the presence of neuronal damage), correlated well with the calbindin poor striosomes. They further noted GFAP immunoreactivity and neuronal loss along the medial edge of the caudate nucleus, adjacent to the lateral ventricle. This extended progressively, ventrally and laterally across the matrix compartment of the whole of the striatum, as the disease state progressed. Preferential neuronal loss in the striosomes in early stage Huntington's disease has been reported by other workers. Augood et al. (1996) noted reductions in markers for neuropeptides in the striosomes from such patients. They describe reductions in mRNA for both enkephalin and substance P in the striosomes. These authors similarly note a loss of D-1 and D-2 dopamine mRNA in early stage Huntington's disease (Augood et al., 1997). MSP neurons in the striosomes project mainly to the substantia nigra pars compacta where they synapse onto dopaminergic neurons (Gerfen, 1985). Loss of striosomal MSP neurons would thus be predicted to lead to loss of GABAergic inhibition of dopaminergic cells and thus lead to increases in striatal dopamine levels. Other workers, however, have failed to ®nd changes in dopamine turnover. For example, Pearson et al. (1990) demonstrated that there are substantial losses of homovanillic acid in Huntington's patients which is indicative of reduced dopamine turnover in the nigrostriatal system. The reasons for this decrease in dopamine turnover are unclear as it is generally assumed that there is no neuronal loss in the substantia nigra pars compacta in Huntington's disease (Albin et al., 1990; Ferrante et al., 1997). Why the striosome compartment should degenerate before the matrix is currently unclear. It is, however, possible that this di€erential vulnerability re¯ects the distribution of calbindin, the concentration of which is higher in the matrix than the patch (Gerfen, 1985). Calbindin plays an important role in regulating intracellular calcium levels and problems with calcium homeostasis are strongly implicated in some forms of neurodegeneration (Gerlach et al., 1996). 3.2.2. Di€erential Loss Of GABA/enkephalin And GABA Substance P/dynorphin MSP Neurons Several studies have described a wave of degeneration spreading through the striatum as Huntington's disease progresses. The caudate nucleus is ®rst a€ected, then the putamen with the nucleus accumbens only degenerating in the late stages of the disease (Vonsattel et al., 1985; Ellison et al., 1987). This wave of degeneration does not a€ect all types of striatal neuron to the same extent. A series of studies have cited evidence which shows that the ®rst neurons to die as this wave of degeneration passes are the enkephalin positive MSP neurons, that is, those neurons which project to the lateral segment of the globus pallidus.

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Early studies have noted that markers for GABA are di€erentially lost in the two segments of the globus pallidus with the losses being most marked in the lateral segment in early stage Huntington's disease (Spokes, 1980; Ellison et al., 1987; Pearson et al., 1990). In keeping with these observations Albin et al. (1992) noted decreases in enkephalin immunoreactivity in the lateral pallidal segment but no changes in substance P immunoreactivity in the medial pallidal segment in a presymptomatic case of Huntington's disease. Others have noted a similar reduction in mRNA for preproenkephalin in the striatum of early stage Huntington's disease with losses in preprotachykinin mRNA only being seen in late stages (Rich®eld et al., 1995). These latter studies also give support to the conclusion that the previously reported losses of GABA and enkephalin are due to degeneration of striatal neurons rather than the more restricted loss of striatal terminals within the globus pallidus. Studies of the progressive loss of dopamine receptors also suggests that the bulk of the degeneration of enkephalin positive MSP neurons tends to precede that of the substance P/dynorphin positive neurons (Augood et al., 1997). This preferential loss of enkephalinergic striatal MSP neurons is thought to correlate with the appearance of choreic movements (Reiner et al., 1988; Albin et al., 1992). Loss of substance P/dynorphin MSP neurons in the later stages of the disease correlate temporally with the appearance of dystonia. This interpretation of postmortem studies is in keeping with models of the pathophysiology of movement disorders which have been derived from studies of primates with experimentally induced dyskinesias (Crossman, 1987; Crossman et al., 1988). These studies have demonstrated that choreiform movements can be elicited by the injection of bicuculline directly into the lateral pallidal segment and parts of the putamen. Metabolic mapping studies subsequently revealed that the induction of chorea by these direct injections correlated with decreased 2-deoxyglucose uptake in the subthalamic nucleus (Mitchell et al., 1989a; Mitchell, 1990; Mitchell et al., 1992). This implies that the dyskinesia was the product of abnormal discharges in the indirect pathway, that is, the striatal outputs which originate from the enkephalin positive MSP neurons. The preferential loss of enkephalin positive MSP neurons may also contribute to some of the cognitive and psychiatric symptoms seen in the early stages of Huntington's disease. Loss of this subtype of striatal neuron would only be expected to result in disordered movements if the neurons formed part of a motor circuit. As noted above, the wave of degeneration spreads from the caudate nucleus to the putamen. Whilst parts of the caudate nucleus may be concerned with the control of movement, in particular the control of eye movements, large portions of the nucleus are given over to more cognitive functions. For example, parts of the medial aspect of the caudate nucleus receive inputs, not from the motor cortices, but from the cingulate and orbitofrontal cortex (Alexander et al., 1986). Dysfunction of this striatal area has been implicated in neuropsychiatric problems including obsessional behaviour (Salloway and Cummings, 1996).

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Table 1. Di€erential vulnerability of striatal neurons in Huntington's disease Factor Location Compartment Neuron type MSP type

Most vulnerable

Least vulnerable

Dorsomedial Patch MSP Enkephalin positive (projects to lateral segment of the globus pallidus)

Ventrolateral Matrix Interneuron Substance P/dynorphin positive (projects to medial segment of the globus pallidus/substantia nigra pars reticulata)

3.2.3. Preservation Of Striatal Interneurons In Huntington's Disease There is considerable evidence to show that some types of striatal interneurons, in contrast to MSP neurons, do not degenerate in Huntington's disease. Ellison and colleagues, for example, showed that the concentrations of the neuropeptides somatostatin and neuropeptide Y, which are co-localised in medium aspiny NADPH diaphorase positive interneurons, are increased in the striatum of Huntington's patients (Ellison et al., 1987). This increase in concentration is interpreted as suggesting that this subpopulation of interneurons have survived whilst the surrounding MSP neurons had died. The large cholinergic interneurons also survive (Ferrante et al., 1987). Parent and colleagues have further described a novel class of interneuron that survives in Huntington's disease. This subpopulation of interneurons are large and medium sized and are calretinin and substance P receptor immunopositive (Cicchetti and Parent, 1996; Cicchetti et al., 1996). In addition, parvalbumin containing interneurons are spared (Harrington and Kowall, 1991). 3.3. Huntingtin Protein The sequencing of the Huntington's gene, IT15, and the identi®cation of the associated protein, Huntingtin, has o€ered a means of gaining fresh insights into the mechanisms which underlie the di€erential loss of striatal neurons in Huntington's disease. Surprisingly, huntingin mRNA is expressed throughout the brain by all neurons (Li et al., 1993; Strong et al., 1993; Landwehrmeyer et al., 1995a). Similarly, both mutant and normal Huntingtin proteins are widely distributed throughout the brain (Trottier et al., 1995). This strongly implies that Huntingtin has a normal function in brain. The relationship between Huntingtin and striatal degeneration is, therefore, far from straightforward. The Huntingtin protein has been reported to be expressed at similar levels in patients and controls (Li et al., 1995). A correlation has, however, been established between the number of CAG repeats in the a€ected gene and the degree of striatal atrophy (Penney et al., 1997). Detailed immunohistochemical studies to localise Huntingtin to speci®c striatal components have been undertaken. These studies has shown Huntingtin to be mainly con®ned to neurons and the neuropil within the calbindin rich matrix compartment of the striatum, with low levels of expression in the patch and virtually none in the NADPH diaphorase posi-

tive interneurons (Ferrante et al., 1997). Conversely, Kosinski et al. (1997) have reported that Huntingtin immunoreactivity is highest in striosomes with only a few matrix neurons containing high levels. This lack of clarity concerning the relationship between the presence of Huntingtin in striosomal neurons and their tendency to degenerate in early stage Huntington's disease has led to the search for additional mechanisms which must regulate the toxicity of the protein. Attention has focused on other cellular proteins, such as Huntingtin associated protein-1 (HAP-1). HAP-1 binds to Huntingtin, the binding being enhanced by the presence of the polyglutamine repeats (Li et al., 1995). This could lead to the aggregation of Huntingtin protein within the cell (Ferrante et al., 1997). Huntingtin, possibly in conjunction with associated proteins, may also act so as to increase the susceptibility of neurons to excitotoxicity by compromising metabolic activity. This view is supported by the observation that Huntingtin and the dentatorubral-pallidoluysian atrophy gene product, Atrophin, can both bind to the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase which is a critical enzyme for glycolysis in Kreb's cycle (Ferrante et al., 1997). 3.4. Overview Of Striatal Degeneration In Huntington's Disease The pattern of neuronal loss within the striatum is far from homogeneous in Huntington's disease. Several di€erent dimensions can be used to describe those striatal neurons which are most vulnerable. These are summarised in Table 1. Neurons which tend to be most vulnerable in Huntington's disease lie dorsomedially rather than ventrally. Enkephalin positive MSP neurons are more vulnerable that substance P/dynorphin positive MSP neurons and NADPH diaphorase neurons are the most robust of striatal neurons. Neurons in the striosomal compartment tend to degenerate before those in the matrix. This di€erential vulnerability may not correlate with the presence of Huntingtin protein. 4. ANIMAL MODELS OF HUNTINGTON'S DISEASE 4.1. Introduction Postmortem studies of the pathological mechanisms underlying Huntington's disease have been supplemented by work on animal models of the disease. These experimental studies have typically sought to replicate the pattern of degeneration seen in the

The Selective Vulnerability of Striatopallidal Neurons

human condition in experimental animals, thereby giving insights into the possible pathological mechanisms that operate in the disease. The validity of these animal models can be judged in terms of the degree to which the pathological changes seen in Huntington's disease are reproduced and in terms of the neurological/behavioural de®cits that are elicited. Three di€erent approaches have been used, namely, excitotoxic striatal lesions, systemic administration of inhibitors of mitochondrial respiration and transgenic animals. These di€erent approaches to modelling Huntington's disease result in di€erent forms of striatal damage and are consequently of pertinence to our understanding of the selective vulnerability of subpopulations of striatal neurons. 4.2. Excitotoxins And Models Of Huntington's Disease Excitotoxins have been extensively used to model Huntington's disease. This approach was pioneered by Coyle and Schwarcz (1976) who induced striatal damage in rat by the intrastriatal injection of the glutamate analogue, kainic acid. This procedure resulted in a clear loss of striatal GABA markers whilst sparing axons of passage. This early model of Huntington's disease, however, is limited in that there is not a selective loss of MSP neurons nor do the animals exhibit chorea. Furthermore, there are reports which question the axon sparing capabilities of kainic acid and other excitotoxic lesioning agents (Co€ey et al., 1988). Nonetheless, the last two decades have seen the development and characterisation of a variety of excitotoxins which have subtly di€erent lesioning properties. These di€erent properties depend in large part upon the anities the compounds show for the di€erent sub-types of glutamate receptor. 4.2.1. Role Of Glutamate Receptor Sub-types In Striatal Excitotoxicity Lesion work with glutamate analogues has demonstrated that the type of excitotoxic induced striatal damage depends in large part upon the type of excitotoxin used. For example, Beal et al. (1986, 1991) compared the e€ects of quinolinic acid, kainic acid and AMPA lesions of rat striatum following a prolonged survival period of several months. All three excitotoxins induced striatal damage but intrastriatal quinolinic acid injection was uniquely associated with increased concentrations of somatostatin and neuropeptide Y as a result of preferential sparing of striatal interneurons. The e€ects of intrastriatal quinolinic acid injections have now been extensively studied in rats (Beal et al., 1986, 1991; Beal, 1992a, 1992b; Dunnett et al., 1991; Figueredo-Cardenas et al., 1994; Malcon et al., 1997) and to a lessor extent in primates (Ferrante et al., 1993). This work has demonstrated that quinolinic acid can reliably induce lesions of striatal neurons with relative sparing of ®bres of passage and some interneuron populations. Such lesions are also associated with behavioural de®cits including hyperactivity but not chorea (Sanberg et al., 1989). There are both similarities and di€erences

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in the pathological and neurochemical changes induced by the intrastriatal injection of excitatory amino acids (EAAs) and those seen in Huntington's disease (Beal, 1992a, 1992b). One study has addressed the issue of the di€erential vulnerability of rat striatal projection neuron subtypes to quinolinic acid by using retrograde anatomical tracers to distinguish between the di€erent types of striatal neuron (Figueredo-Cardenas et al., 1998). The results are dicult to interpret as the use of retrograde tracers requires injection sites which cover the whole extent of the structure. Taking this into consideration the authors report that striatopallidal neurons are more vulnerable than striatoentopeduncular neurons which is consistent with previous ®ndings for early stage Huntington's disease (Albin et al., 1992; Rich®eld et al., 1995). These authors further report that striatonigral neurons are the most vulnerable to quinolinic acid. Previous studies in humans have suggested that substance P containing striatonigral neurons are also more vulnerable than MSP neurons in the direct striatopallidal pathway (Reiner et al., 1988). 4.2.2. Selective Sparing Of Interneurons By Excitotoxic Lesions The relative sparing of some interneuron populations in Huntington's disease (see Section 3.2.3.) has prompted much investigation into the relative vulnerability of striatal interneurons to quinolinic acid. Beal and colleagues (Beal, 1992a, 1992b) have demonstrated the relative survival of NADPH/somatostatin/neuropeptide Y interneurons over projection neurons and the invulnerability of cholinergic interneurons. Other authors (Koh and Choi, 1988) have reported similar results in cultured striatal neurons though some researchers dispute this ®nding. Figueredo-Cardenas et al. (1994) suggest that somatostatin/neuropeptide Y neurons are more vulnerable than projection neurons and Davies and Roberts (1987) report that all neuronal subtypes are vulnerable to quinolinic acid. This apparent contradiction may re¯ect the dose of quinolinic acid used (Malcon et al., 1997), the method of injection or the age of animals (FigueredoCardenas et al., 1994). Striatal interneurons have also been shown to be less vulnerable than MSP neurons in models of ischemia where neuronal death is thought to result from excess N-methyl-D-aspartate (NMDA) receptor mediated transmission. For example, Uemura et al. (1990) demonstrated that bilateral transient ligation of the common carotid arteries in the gerbil results in severe loss of striatal neurons but selective sparing of NADPH diaphorase positive neurons and unchanged concentrations of neuropeptide Y and somatostatin immunoreactivity. The sparing of striatal interneurons following quinolinic acid injection is similar to the survival of striatal interneurons seen in Huntington's disease. This has led to the suggestion that the process of neurodegeneration in Huntington's disease may result from an NMDA receptor mediated excitotoxic process (Beal et al., 1991). This suggestion has since been supported by data from imaging studies

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which have shown Huntington's patients to have disordered glutamate metabolism (Taylor-Robinson et al., 1996). The mechanisms underlying the apparent robustness of some types of striatal interneurons have been considered by Chen et al. (1996). They have argued that the selective vulnerability of some striatal neuron types in Huntington's disease may re¯ect di€erences in the expression of NMDA receptor sub-types. They demonstrate that virtually all MSP neurons and all cholinergic interneurons express NMDAR2A/2B subunits whereas somatostatin interneurons do not. Similarly, Landwehrmeyer et al. (1995b) have shown that cholinergic interneurons express low levels of NMDAR1 and 2B compared with MSP neurons. The lack of NMDAR2A/2B subunits of NMDA receptors on striatal somatostatin interneurons may thus confer resistance to NMDA receptor mediated excitotoxicity on these neurons. In support of this hypothesis it has been demonstrated that MSP neurons and cholinergic interneurons show di€erent responses to glutamate receptor activation (Calabresi et al., 1998). In summary, the selective sparing of NADPH positive striatal interneurons seen in Huntington's disease can be modelled by excitotoxic lesions of the striatum. To date, however, excitotoxic striatal lesions have not reproduced the preferential degeneration of the striosomal compartment of the striatum nor the selective degeneration of the enkephalin positive MSP neurons seen in the early stages of Huntington's disease. 4.2.3. Role Of Dopamine In Glutamate Mediated Excitoxicity Of The Striatum The mechanisms by which glutamate and glutamate analogues can induce excitotoxic lesions of the striatum are complicated by the action of dopamine. The regulation of striatal glutamate transmission and striatal dopamine transmission are intertwined. For example dopamine acting at proximal synapses on the spines of dendrites of MSP neurons can gate the action of glutamate acting at more distal synapses (Smith and Bolam, 1990). In addition, glutamate acting at presynaptic glutamate receptors on dopaminergic nigrostriatal neurons can regulate the release of striatal dopamine (Glowinski et al., 1989). Furthermore, the existence of dopamine receptors on glutamatergic corticostriatal terminals has been demonstrated (Calabresi et al., 1996). These complex neuroanatomical arrangements ensure that dopamine transmission can a€ect excitotoxic striatal damage by regulating intrastriatal glutamate levels. In support of this, several reports have shown that excitotoxic striatal damage induced by transient ischemia can be attenuated by prior manipulations to reduce striatal dopamine levels including lesions of the nigrostriatal pathway (Globus et al., 1987; Hashimoto et al., 1994). The role of dopamine in striatal toxicity has been further complicated by the realisation that dopamine metabolites can be neurotoxic in their own right. Dopamine or its precursor can, under some circumstances, be converted to toxic superoxides and quinones (Walkinshaw and Waters, 1995;

Cheng et al., 1996; O€en et al., 1996). Indeed. Chapman et al. (1989) postulate that conversion of dopamine to a superoxide anion contributes to the excitotoxic induced damage within the striatum. These observations raise the possibility that the selective vulnerability of di€erent subpopulations of striatal neurons may re¯ect the local anatomical organisation of dopamine terminals and receptors. 4.2.4. Extrastriatal Changes Induced By Excitotoxic Lesions Of The Striatum It is now generally assumed that the judicial use of dose and type of excitotoxin enables relatively speci®c striatal lesions to be made which spare both ®bres of passage and some populations of interneurons. It should be noted, however, that glutamate analogues, even under `optimal conditions', can induce changes in extrastriatal receptor densities and even extrastriatal degeneration. Some of these changes re¯ect some of the pathological changes seen in Huntington's disease. For example, Huntington's disease and striatal excitotoxic lesions are associated with decreases in GABA levels in the substantia nigra and globus pallidus but increases in GABA receptor densities (Pasinetti et al., 1991; Nicholson et al., 1995). This similarity in GABA activity in the human disease and experimental model has been interpreted as supporting a role for excitotoxin induced degeneration in Huntington's disease. Excitotoxic lesions of the striatum have also been shown to result in delayed death of neurons in the substantia nigra pars reticulata (Pasinetti et al., 1991; Saji and Volpe, 1993). This secondary damage is presumed to result from excessive glutamate mediated transmission in the subthalamonigral pathway and can be prevented by the intraventricular administration of GABA agonists. This transneuronal damage, however, may be secondary to damage to the globus pallidus as a consequence of spread of the excitotoxin from the striatum. 4.2.5. Type Of Cell Death Induced By Excitotoxins The last decade has seen a dramatic growth in interest in the ways in which cells die. This interest has extended to cover both the manner of neuronal death in Huntington's disease and that induced by excitotoxins. Exposure of neurons to high concentrations of glutamate and other EAAs for prolonged periods of time causes swelling of dendrites and soma due to osmotic overload caused by in¯ux of Na+ and Clÿ through EAA gated channels followed by a redistribution of water and Clÿ (Olney et al., 1986; Colwell and Levine, 1996). This process can ultimately lead to rupture of the cell membrane and death. Briefer exposure to NMDA agonists results in a slower form of neurotoxicity which is Ca2+ dependent (Choi et al., 1987). Non-NMDA agonists have also been shown to damage neurons in a Ca2+ dependent manner during prolonged agonist exposure. This damage can be attenuated by voltagegated Ca2+ channel blockers, suggesting the depolarisation caused by these non-NMDA agonists is causing activation of Ca2+ channels (Weiss et al., 1990).

The Selective Vulnerability of Striatopallidal Neurons

701

Some experimental studies have speci®cally investigated whether careful adjustment of lesioning parameters can enable excitotoxins to induce neuronal death by apoptosis. These studies have produced mixed ®ndings. Some authors have claimed that excitotoxic induced neurodegeneration does not result in apoptosis (Deshpande et al., 1992; Dessi et al., 1993; Van Lookeren Campagne and Gill, 1996) whilst others have presented evidence to show that it does (Kure et al., 1991; Macmanus et al., 1993; Islam et al., 1995; Linnik et al., 1995; Macmanus et al., 1994). This disparity may re¯ect the dose of excitotoxin administered and the duration of the exposure. Postmortem analysis of brains from late stage Huntington's patients have typically shown a mass of necrotic damage accompanied by reactive gliosis (Vonsattel et al., 1985). Recent studies, however, have shown that some of the neuronal death seen in Huntington's disease occurs by a process of naturally occurring cell death or apoptosis (Dragunow et al., 1995; Portera-Cailliau et al., 1995; Thomas et al., 1995) and that the degree of DNA fragmentation observed is proportional to the CAG repeat length (Butterworth et al., 1998). In our own work we have induced apoptosis of striatal neurons by manipulating glutamatergic striatal transmission indirectly. We have thus demonstrated that administration of the monoamine depleting agent reserpine induced apoptosis of striatopallidal neurons in the dorsomedial caudateputamen (Mitchell et al., 1994). The reserpine induced apoptosis was blocked by cortical aspiration or administration of low doses of the NMDA antagonist ketamine. Collectively, these results suggest that the striatal apoptosis resulted from excess corticostriatal glutamate mediated transmission.

The calcium binding protein, calbindin, is present in high concentrations within many striatal cells. This protein is, however, heterogeneously distributed and is one of the criteria for distinguishing the matrix from the patch. There is some evidence which suggests that calbindin contributes to the resistance of the striatum to neurotoxins. For example hypoxic ischemic injury can result in preferential damage to the calbindin poor areas of the striatum (Burke and Baimbridge, 1993). These di€erential toxic e€ects may re¯ect the potential of calbindin to bu€er intracellular calcium levels. Along similar lines, the medium sized striatal interneurons which are relatively spared in Huntington's disease are calretinin positive (Cicchetti et al., 1996). Calretinin has a high capacity for binding free intracellular calcium ions and may thus confer a degree of protection against excitotoxicity. However, there are numerous lines of evidence to show that the presence or absence of speci®c calcium binding proteins cannot solely account for the pattern of degeneration seen in Huntington's disease. For example discrete populations of both calbindin positive and parvalbumin positive striatal interneurons do degenerate in Huntington's disease (Ferrer et al., 1994). These calcium binding protein containing interneurons are also sensitive to excitotoxins (Waldvogel et al., 1991). It should be noted, however, that stimulation of NMDA receptors can lead to neurotoxic e€ects which are not directly dependent upon NMDA receptor mediated Ca2+ in¯ux. For example, stimulation of NMDA receptors can lead to activation of NO synthase (Sato et al., 1995; Szabo, 1996). The resultant release of NO within the striatum could lead to damage of surrounding neurons irrespective of their calcium binding protein content.

4.2.6. Selective Vulnerability Of Striatal Neurons And Calcium Binding Proteins

4.2.7. Role Of Microglia In Mediating Excitotoxic Death Of Striatal Neurons

As noted above, elevated intracellular Ca2+ levels are strongly implicated in the mechanisms which mediate excitotoxic neuronal death. As a consequence of this potential toxic mechanism, intracellular calcium levels are normally tightly regulated by a series of calcium binding proteins including calbindin, parvalbumin and calretinin. It is possible that the di€erential distribution of these various calcium binding proteins could contribute to the selective vulnerability of striatal neurons (Ferrer et al., 1994). The role of Ca2+ in excitotoxin induced neuronal death has been extensively studied. There is strong evidence to show that excitotoxicity is preceded by an in¯ux of Ca2+ (Beal, 1992a, 1992b). Elevated intracellular Ca2+ levels may result in activation of Ca2+ sensitive proteases. This view is supported by the observation that intraventricular infusion of the calpain inhibitor, leu-peptin can prevent excitotoxic induced neuronal damage (Lee et al., 1991). It should be noted, however, that the critical Ca2+ dependent signal which induces neuronal damage may not simply be the absolute level of intracellular Ca2+ but rather the distribution of Ca2+ amongst a series of intracellular Ca2+ pools (Beal, 1992a, 1992b).

It is well established that a variety of neuronal insults leads to activation of microglia (Banati and Graeber, 1994; Gebicke-Haerter et al., 1996). Evidence is now accumulating, however, to show that the activation of microglia can also contribute to the death of neurons (Lees, 1993). The localised activation of microglia could thus potentially confer some speci®city on the neuronal damage induced by excitotoxins within the striatum. The initial observations of microglia inducing death of neurons came from work on ischemia. Delayed neuronal death of hippocampal neurons was seen 72 hr after reperfusion in transient models of ischemia (Lees, 1993). This second wave of death was temporally correlated with the activation of microglia and blood borne macrophages. The mechanisms by which activation of microglia can lead to neuronal death are unclear but microglia are known to release a variety of toxins including glutamate, NO, hydrogen peroxide and superoxide. NO levels reach toxic concentrations during ischemia and NOS inhibitors can markedly attenuate ischemic induced neuronal damage (Strijbos, 1998). Microglia and astrocytes, once activated release several types of cytokines some of which can induce the endogen-

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ous production of quinolinic acid which can in turn result in excitotoxic mediated neuronal death (Lees, 1993). 4.2.8. Protection From Excitotoxin Induced Striatal Degeneration By Neurotrophic Factors Di€erential vulnerability of striatal neurons to excitotoxins may re¯ect the di€erential distribution of neuroprotective agents including neurotrophic factors. Elevations of endogenous neurotrophic factor levels have been shown to ameliorate the extent of glutamate induced neuronal death (Lindholm, 1994). Work of this type has emphasised that the neuroprotection conferred by speci®c trophic factors is selective for speci®c neuronal phenotypes. For example, Martinez-Serrano and Bjorklund (1996) have shown that implantation of genetically modi®ed neural stem cell lines producing trophic factors can reduce quinolinic acid induced striatal damage. The degree of protection, however, was critically dependent upon the trophic factor produced. Implantation of nerve growth factor (NGF) producing cells led to protection of MSP neurons and cholinergic interneurons whereas brain derived neurotrophic factor (BDNF) producing implants had no e€ect. Similar results have been observed following the implantation of immortalised rat ®broblasts which secrete NGF (Frim et al., 1993) and to a lessor extent basic ®broblast growth factor (bFGF). Others have demonstrated that quinolinic acid induced striatal damage can be attenuated by neurotrophin-4/5 which exerts a protective e€ect on parvalbumin positive striatal neurons, and transforming growth factor-a, which has some protective e€ect on NADPH-diaphorase neurons (Alexi et al., 1997). The molecular mechanisms which enable NGF to protect striatal neurons from excitotoxic induced death are currently unclear. There is some evidence, however, to show that this trophic factor helps control the levels of free radicals by inhibiting the synthesis of NO and decreasing the levels of superoxide radicals (Galpern et al., 1996). The extent to which the normal distribution of trophic factors and their receptors within the striatum contributes to the pattern of cell loss seen in Huntington's disease is currently unclear. 4.3. Striatal Toxicity And Inhibitors Of Mitochondrial Respiration 4.3.1. Introduction There has been considerable interest in the possibility of mimicking the pathological features of Huntington's disease by the use inhibitors of mitochondrial respiration to induce the selective destruction of MSP neurons. This approach has been dominated by the work of Beal and colleagues (Beal, 1992a, 1992b). Many mitochondrial inhibitors have been used including 3NP (3-nitropropionic acid), AOAA (amino-oxyacetic acid), malonate and succinate all of which act to inhibit the actions of succinate dehydrogenase, a key enzyme in the mitochondrial respiratory chain. There is considerable evidence from experimental models to show that striatal neurons are particularly vulnerable to the

toxic e€ects of mitochondrial inhibition. Furthermore, there is evidence to show that mitochondrial respiration in striatal neurons is abnormal in Huntington's disease (Brown et al., 1997). 4.3.2. Mechanism Of Toxic Action Of Mitochondrial Respiration Inhibitors A critical requirement for neurons is the maintenance of the transmembrane ionic gradients. In normal circumstances this is a highly energy dependent process mediated by sodium-potassium-ATPase. Compromising the activity of this enzyme will result in the collapse of transmembrane gradients and associated in¯ux of extracellular Ca2+ (Lees, 1991). Impaired cellular energy metabolism would also exacerbate the damaging e€ects of excitotoxins and oxidative stress (Galpern et al., 1996). There is some evidence to show that the striatal degeneration which is induced by inhibitors of mitochondrial respiration is mediated, at least in part, by NO. 3NP induced lesions of rat striatum are accompanied by activation of NOS (Nishino et al., 1996). NO exposure is further known to irreversibly inhibit mitochondrial complex II±III and IV and succinate dehydrogenase which could lead to the formation of the highly toxic peroxynitrites (Connop et al., 1996; Bolanos et al., 1997). This pathological mechanism could also interact with excitotoxic induced damage of the striatum due to the release of endogenous EAAs as NO formation can be induced by activation of NMDA receptors on NOS containing neurons (Connop et al., 1996). Similarly, administration of 1-methyl-4-phenyl1,2,3,6-tetrahydropyridine (MPTP), which results in a transient decrease in ATP levels, potentiates the toxic e€ects of malonate in mice (Albers et al., 1996). 4.3.3. Speci®city Of Neuronal Damage Induced By Mitochondrial Inhibitors Bossi et al. (1993) account for the speci®city of the toxic e€ects of mitochondrial inhibitors in terms of the individual energy demands of speci®c neural structures. They argue that the brain regions with higher metabolic rates will be more susceptible to mitochondrial inhibitor induced toxicity as those regions will have a lower tolerance for mitochondrial dysfunction. The work of Ray and colleagues lends indirect support to this hypothesis. They demonstrated that administration of the toxin 1,3-dinitrobenzene induced death of neurons in structures which are metabolically very active as measured by the 2-deoxyglucose uptake procedure (Ray et al., 1992). No damage was seen in the striatum whilst considerable neuronal death was observed in the auditory structures, including the cochlear nuclei and inferior colliculus, which normally show high levels of cerebral glucose utilisation. Remarkably, continuous activation of the auditory system resulted in an exacerbation of the toxic e€ects. The argument that individual energy requirements determines susceptibility to mitochondrial inhibitors raises the issue of how metabolically active MSP neurons are. These neurons are renowned for their

The Selective Vulnerability of Striatopallidal Neurons

low spontaneous discharge rates which may lead to the assumption that their metabolic needs are small. The metabolic energy requirements of MSP neurons has been addressed by Calabresi et al. (1995, 1997a). They claim that >40% of the energy released by respiration in the CNS is used by Na+/K+ ATPase to maintain transmembrane ionic gradients. They demonstrated that application of ouabain and strophanthidin (inhibitors of the Na+/K+ ATPase) on identi®ed MSPs resulted in an irreversible inward current coupled to an increase in conductance which led to cell deterioration. They further demonstrated that lower doses of the Na+/K+ ATPase inhibitors both dramatically increased the membrane depolarisation and inward current produced by subcritical concentrations of EAAs and increased the membrane responses induced by repetitive cortical activation. This implies that impairment of the activity of Na+/K+ ATPase may render striatal neurons more sensitive to the action of glutamate and may thus lower the threshold for the excitotoxic events. Similarly, increased metabolic demands by glucose deprivation, results in the depolarisation of MSP neurons but the hyperpolarisation of cholinergic interneurons, thus providing a mechanism for the selective vulnerability of projection neurons over cholinergic interneurons (Calabresi et al., 1997a). Bossi and colleagues have advanced similar arguments to account for the late onset of Huntington's disease (Bossi et al., 1993). These authors argue that the disease might begin to present following reduced oxidative brain metabolism and mitochondrial metabolism which decline with age. In keeping with this argument, experimentally induced mitochondrial inhibitor toxicity is known to be age dependent (Bossi et al., 1993). Local impairments in cerebral mitochondrial respiration have been noted in Huntington's disease. For example, activity of complex II±III has been shown to be reduced within the striatum but not cortex or cerebellum in Huntington's disease (Browne et al., 1997; Bossi et al., 1993). Studies of the cellular actions of Huntingtin, the protein produced by the abnormal gene in Huntington's disease, have also suggested that the striatal degeneration associated with the disease is due to impaired mitochondrial respiration. Thus, Huntingtin has been shown to bind to, and possibly inhibit glyceraldehyde-3-phosphate dehydrogenase, a key enzyme in energy metabolism (Guyot et al., 1997). Mitochondrial inhibitors can, without doubt, lead to striatal damage though systemic administration of them is also known to result in damage to the hippocampus and the thalamus (Bossi et al., 1993). The precise pattern of neural degeneration re¯ects the particular type of mitochondrial inhibitor used and the dosing regime. Administration of both 3NP and AOAA have been reported to induce neuronal death which is primarily con®ned to the striatum (Martinou et al., 1994; Chang and Jang, 1995). Others have reported that 3NP lesions of the striatum are speci®c to MSP neurons whilst dopaminergic axons and NADPH diaphorase positive interneurons are spared (Guyot et al., 1997). A recent report has demonstrated that there are

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marked strain/species di€erences in the sensitivity to 3NP (Alexi et al., 1998). In depth studies of the subtle di€erences between these strains/species of rodent may give insights into the factors which confer neuronal vulnerability to mitochondrial inhibitors. 4.3.4. Selective Attenuation Of Mitochondrial Inhibitor Induced Striatal Damage By Trophic Factors Inhibition of mitochondrial respiration could induce selective degeneration of speci®c subpopulations of striatal neurons if the vulnerable subpopulations have a di€erential requirement for neuroprotective factors. There is limited experimental evidence to suggest that this may occur. NGF secreting ®broblasts implanted into the corpus callosum can o€er protection against 3NP induced striatal damage (Galpern et al., 1996). The protective action of NGF may result from the inhibition of NO synthesis or by increasing the activity of antioxidant enzymes. The latter would result in decreased levels of superoxide radicals which in turn would decrease the generation of oxidative agents such as peroxynitrite (Galpern et al., 1996). 4.3.5. Amphetamines Potentiate The Toxic E€ects Of Mitochondrial Inhibitors Some studies have suggested that amphetamine and methamphetamine can potentiate the toxic e€ects of mitochondrial inhibitors in the striatum (Albers et al., 1996; Bowyer et al., 1996; Reynolds et al., 1998). The mechanisms underlying this interaction are, however, unclear. One possibility is that administration of methamphetamine results in a transient decline in striatal ATP levels which will exaggerate the energy de®ciency induced by mitochondrial inhibitors. This view is supported by the observation that the resultant striatal damage is potentiated by co-administration of 2-deoxyglucose which acts to inhibit glucose uptake and utilisation (Albers et al., 1996; Bowyer et al., 1996). 4.3.6. Behavioural De®cits Following Administration Of Mitochondrial Inhibitors There have been numerous attempts to model the motor and cognitive de®cits seen in Huntington's disease in experimental animals by inducing striatal lesions with mitochondrial inhibitors. This work has been conducted in both rodents and primates. Administration of 3NP to rats can induce either a hyperactive syndrome or a hypoactive one (Borlongan et al., 1995). Which syndrome is elicited seems to depend upon the dosing regime used with the hyperactive condition resulting from a restricted dosing regimen. The two types of behavioural outcome bear at least super®cial resemblance to the early and late stages of Huntington's disease where patients pass from a hyperactive choreic state into a hypoactive dystonic state. 3NP administration to rats has also been shown to result in de®cits in sensorimotor gating (Kodsi and Swerdlow, 1997) as manifested by reduced prepulse inhibition of the acoustic startle re¯ex. Analogous de®cits have been observed in humans in a number of conditions

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including Huntington's disease (Swerdlow et al., 1995). Guyot and colleagues administered subacute doses of 3NP to primates for several days (Guyot et al., 1997). This procedure resulted in relatively selective lesions of the lateral striatum which correlated with the development of dystonia. Smaller lesions were associated with choreiform movements. Why the lateral striatum should be more vulnerable than the medial striatum to the systemic administration of this mitochondrial inhibitor is unclear but it may re¯ect the heterogeneous accumulations of 3NP in the lateral striatum from the lateral striatal artery. Other dosing regimens with 3NP in primates have induced preferential damage to the dorsal aspects of the striatum with relative sparing of NADPH-diaphorase positive interneurons (Pal® et al., 1996). Following this type of treatment animals have shown cognitive de®cits which are typical of the fronto-striatal problems shown by Huntington's patients (Pal® et al., 1996; Lawrence et al., 1998). In summary, these studies collectively imply that administration of mitochondrial inhibitors to experimental animals can elicit behavioural syndromes which are similar to some of the motor and cognitive abnormalities seen in Huntington's disease. There is, however, a tendency for this procedure to result in dystonia and hypoactivity (Borlongan et al., 1995). These symptoms are normally associated with the late stages of the disease in humans and are assumed to result from degeneration of MSP neurons in both the direct and indirect pathways (Penney and Young, 1986). In keeping with this hypothesis 3NP does not appear to preferentially damage enkephalin positive MSP neurons and spare substance P/dynorphin positive MSP neurons. 4.4. Transgenic Animals As A Model For Huntington's Disease 4.4.1. Molecular Studies Of The Huntingtin Gene Identi®cation of the locus of the IT15 gene has allowed detailed analysis at the molecular level of the protein, huntingtin (Htt). The protein appears to be highly conserved, with the zebra®sh homolog having 70% amino acid identity to the human form (Karlovich et al., 1998). The function of the normal htt has been investigated in knock-out animals. Deletion results in early embryonic lethality with increased levels of apoptosis (Zeitlin et al., 1995), implying that htt has a critical role in development, possibly as an anti-apoptotic factor. Fragments of the htt gene have been expressed in Drosophila with the purpose of characterising the molecular pathways which are activated by the expanded CAG repeats (Jackson et al., 1998). 4.4.2. Genetic Composition Of Transgenic Animals And Distribution Of Mutant Htt To date, two transgenic mouse lines have been produced expressing either a portion of or the full length of cDNA for the mutant gene. The ®rst mouse line transgenic for the mutant form of huntingtin was reported by Mangiarini et al. (1996). These transgenic animals were produced by insertion

of the portion of the human IT15 gene (the ®rst exon), which contains the CAG repeat region, into the mouse genome. Detailed molecular analysis of the transgenic mice indicated that they contained more than 115 CAG repeats, which is much greater than the number typically observed in the human condition. The authors report that both mRNA and protein for the mutant huntingtin can be detected in both neural and non-neural tissue. A second transgenic model has recently been reported (Reddy et al., 1998), whereby the animals have the full-length cDNA for the htt gene with either 16,48 or 89 copies of the trinucleotide. This is more in-keeping with the repeat number seen in the human condition. 4.4.3. Pathology, Neurochemistry And Neurology Of Transgenic Animals Neuropathological investigation of animals with the CAG repeat-containing fragment indicated that, although transgenic brains were smaller than normal, gross brain morphology appeared normal and there was no evidence of cell loss or increased expression of glial markers in any part of the brain, including the striatum (Mangiarini et al., 1996). Cellular analysis of these fragment-expressing animals has provided insights into the possible pathogenic mechanism of action of the CAG repeats. Scherzinger et al. (1997) have demonstrated the presence of protein aggregates in the nuclear fraction from transgenic mice, which resemble the amyloid ®brils of Alzheimer's disease when viewed with an electron microscope. Additionally, Davies et al. (1997) have reported that transgenic mice have neuronal intranuclear inclusions which can be demonstrated immunohistochemically to be composed of the N-terminal of huntingtin (the polyglutamine expansion containing portion) and ubiquitin. These inclusions are present in neurons prior to the onset of neurological dysfunction. Subsequent to the identi®cation of these intranuclear inclusions in transgenic animals, similar abnormalities were found in brain tissue of HD patients, where the frequency of inclusions correlated with the CAG repeat length (Becher et al., 1998). Interestingly, inclusions were observed in striatal projection neurons but not striatal cholinergic interneurons. Similarly, animals which contained the full mutant gene with 16 copies of the trinucleotide showed no neuropathological abnormalities (Reddy et al., 1998). In contrast, neuronal loss and gliosis were evident in the striatum, cortex, thalamus and hippocampus of animals with 48 or 89 CAG repeats, especially if the animals were killed in the hypo/akinetic end-stages (Reddy et al., 1998). Neuronal loss was particularly high in the striatum, where small/medium sized neurons had decreased in number by approximately 20%, whilst large neurons were una€ected. Evidence for apoptosis was also obtained for the striatum and hippocampus using the TUNEL technique. The results of Mangiarini et al. (1996) and Reddy et al. (1998) taken in conjunction suggest that the CAG expansion is sucient to produce neurological de®cits but that other parts of the gene are required to account for the selective vulnerability of neuronal populations. This view is supported by the work of

The Selective Vulnerability of Striatopallidal Neurons

Ordway et al. (1997) who have produced transgenic animals where CAG repeats (146 units) are targeted to genes other than those which normally have CAG repeats. These animals progressively develop a neurological phenotype characterised by ataxia, tremor, seizures and decreased locomotor activity. However, no neurodegeneration was observed. Analysis of neurotransmitter receptors using in situ hybridisation or binding techniques in mice expressing a fragment of the huntingtin gene (Cha et al., 1998) indicate that glutamatergic, dopaminergic and cholinergic systems are a€ected. Hence, mRNA/binding levels for some metabotropic glutamate receptor subtypes, AMPA receptors, kainate receptors, D-1 and D-2 dopamine receptors and muscarinic cholinergic receptors are reduced when compared to controls. Neither GABA receptors nor NMDA receptor levels were di€erent from controls. Interestingly, the receptor decreases often preceded the onset of clinical symptoms. Despite a lack of pathology, animals transgenic for the CAG repeat fragment did, however, exhibit a neurological phenotype reminiscent of the human condition. The animals showed a progressive onset of symptoms including involuntary stereotypic and choreiform movements and resting tremor (Mangiarini et al., 1996). In addition, when these animals were challenged with methamphetamine (Carter et al., 1998), the resultant increase in locomotor activity was considerably less than that observed in normal animals. The authors attribute this to a functional impairment of the nigrostriatal dopaminergic system. Motor dysfunction was also observed in animals containing the full length cDNA with 89 repeats, but not 16 or 48 repeats (Reddy et al., 1998). This motor dysfunction was also progressive and initially included feet-clasping when suspended by the tail and hyperactivity. With time, the animals exhibited decreased exploratory behaviour and alertness, ®nally becoming akinetic just before death. 4.4.4. Overview Of Animal Model Of Huntington's Disease Attempts to model the pattern of degeneration seen in Huntington's disease in experimental animals have given extensive insights into the di€erential vulnerability of subpopulations of striatal neurons. Models of Huntington's disease based on the intrastriatal injection of EAAs have shown that the degree of the striatal damage induced is dependent upon the type of excitotoxin used. There seems to be a general consensus that intrastriatal injections of quinolinic acid can spare somatostatin and neuropeptide Y positive striatal interneurons provided that an appropriate dose of the toxin is used. There is also a little evidence to show that the subtypes of MSP neurons are di€erentially vulnerable to intrastriatal excitotoxins with striatopallidal neurons being more vulnerable than striatonigral neurons which in turn are more vulnerable than striatoentopeduncular neurons. The di€erential vulnerability of striatal neurons to excitotoxins could arise via a variety of mechanisms. For example, there is di€erential expression of

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NMDA receptor subtypes by striatal neuronal types such that all MSP neurons and all cholinergic interneurons express NMDAR2A/2B subunits whereas somatostatin interneurons do not. The presence or absence of protective calcium binding proteins could confer further speci®city upon the response of the neuron to EAAs as could the proximity of interneurons and glia which can release NO. Animal models of Huntington's disease based on the systemic administration of inhibitors of mitochondrial respiration have consistently shown MSP neurons to be extremely sensitive to this type of toxin. The susceptibility of speci®c neuronal types to mitochondrial inhibitors is generally assumed to re¯ect the energy requirements of that cell type. It is thought that the metabolic requirements of neurons is extremely high, despite their very low spontaneous discharge rates, due to the maintenance of their transmembrane ionic gradients at unusually high levels of hyperpolarisation. This line of research has, however, not revealed any striking di€erences in the vulnerability of the di€erent types of MSP neuron. Analysis of transgenic models of Huntington's disease have reproduced neuropathology reminiscent of that seen in the human condition. However, this pathology is only seen in animals which contain the full gene with high repeat numbers of the trinucleotide tract, that is, where numbers of repeats are comparable to that seen in Huntington's disease. No di€erence in the time of onset was observed between animals with 48 or 89 repeats. This contrasts with the human condition where there is a high degree of correlation between the CAG repeat length and age of onset. Transgenic animals with only an expanded CAG repeat region showed neurological abnormalities but no neurodegeneration. Hence, the animals do not provide a speci®c model of Huntington's disease but rather could be considered as a model for all diseases where expanded CAG repeats have been implicated. These include dentatorubral pallidoluysian atrophy and some of the spinocerebellar ataxias. This suggests that a common molecular composition, that is expanded CAG repeats, may underlie several neurodegenerative diseases, with the structure of the brain a€ected being determined by either other parts of the gene or other genes.

5. DOPAMINERGIC MANIPULATIONS AND STRIATAL TOXICITY 5.1. Introduction Dopamine is implicated in the processes underlying striatal degeneration in a number of ways. For example, dopamine can regulate corticostriatal glutamate release and thus a€ect excitotoxin induced striatal damage (Chapman et al., 1989). Furthermore, amphetamines are reported to have neurotoxic e€ects and their principle pharmacological action is to increase extracellular dopamine levels (Eisch et al., 1992). Dopamine too can be converted into toxic metabolites as discussed in Section 4.2.3. There is also some evidence to show that D-1 dopa-

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mine agonists and D-2 dopamine antagonists can have neurotoxic e€ects (Kelley et al., 1990; Cooper et al., 1997). The mechanisms underlying the toxic e€ects of such manipulations which a€ect dopaminergic transmission and the speci®c pattern of neuronal damage induced are discussed below. 5.2. Amphetamines And Striatal Toxicity There is considerable evidence to show that amphetamines, in particular substituted amphetamines such as methamphetamine, are toxic to the striatum. For the most part, however, their toxic e€ects seem to be limited to terminals, especially monoaminergic terminals, rather than striatal neurons (Eisch et al., 1992; Albers et al., 1996; Bowyer et al., 1996; Cadet and Brannock, 1998). Methamphetamine, for example, severely a€ects many markers of dopaminergic neurons including striatal dopamine levels, tyrosine hydroxylase activity, numbers of tyrosine hydroxylase immunoreactive striatal ®bres and dopamine uptake sites (Eisch et al., 1992; Albers et al., 1996; Cadet and Brannock, 1998). The neurotoxic e€ects of methamphetamine extend to 5HT neurons (Eisch et al., 1992; Hirata et al., 1995; Cadet and Brannock, 1998) with administration of the drug being associated with inhibition of 5HT synthesis, decreased concentrations of 5HT and its metabolite, 5-HIAA, and destruction of 5HT uptake sites and terminals. Methamphetamine induced damage to striatal dopaminergic ®bres is, however, heterogeneous with more damage being sustained by the caudateputamen than the nucleus accumbens (Eisch et al., 1992). Neuronal damage induced by amphetamines, to both dopamine and 5HT terminals, is thought to be due to the e‚ux of dopamine from presynaptic terminals and blockade of the high anity dopamine transporter site (Eisch et al., 1992; Sprague and Nichols, 1995; Bowyer et al., 1996). Evidence in favour of this view comes from studies where protection against methamphetamine induced neurotoxicity has been blocked by the administration of the dopamine synthesis inhibitor, a-methyl paratyrosine and monoamine oxidase B inhibitors (Eisch et al., 1992). Furthermore, the extent of methamphetamine induced neuronal damage correlates with the drug induced striatal over¯ow of dopamine (Eisch et al., 1992). The elevated levels of striatal dopamine induced by amphetamines could potentially induce their toxic e€ects by increasing the production of toxic free radicals (Albers et al., 1996; Bowyer et al., 1996; Cadet and Brannock, 1998). Catecholamines can be broken down by monoamine oxidases to form hydrogen peroxide, undergo autoxidation to toxic quinones and be converted into oxygen based free radicals. Evidence in favour of this free radical hypothesis comes from several sources. For example, the toxic e€ects of methamphetamine are attenuated by antioxidants which prevent the formation of free radicals (Cadet and Brannock, 1998). Manipulation of superoxide dismutase (SOD), which normally acts to remove free radicals, can also a€ect toxicity induced by amphetamines. Inhibition of SOD exacerbates this drug induced

neuronal damage (Cadet and Brannock, 1998) whereas the damage to both dopaminergic and 5HT neurons is diminished in transgenic mice which over-express copper±zinc SOD (Hirata et al., 1995, 1996). An alternative mechanism whereby elevated striatal dopamine levels could induce neuronal damage would be via an excitotoxic process. There is some evidence to show that the toxic e€ects of methamphetamine can be blocked by NMDA receptor antagonists (Sonsalla et al., 1989). One interpretation of this observation is that the amphetamine induced damage results from increased release of glutamate from corticostriatal terminals. Amphetamine induced striatal toxicity could also involve the production of NO. Administration of 7nitroindazole, a NO inhibitor, attenuates neuronal methamphetamine induced damage to dopaminergic terminals without preventing the initial release of dopamine (Di Monte et al., 1996). Amphetamines could induce NO synthesis via stimulation of NMDA receptors following an indirect potentiation of glutamate release. Alternatively, NO could be released by astrocytes which are known to be activated following methamphetamine administration (Miller and O'Callaghan, 1995). The postulated mechanisms which mediate amphetamine induced neurotoxicity (production of free radicals and NO) and excitotoxicity are remarkably similar to those mechanisms which are thought to underlie mitochondrial inhibitor induced striatal damage. It should be noted, however, that mitochondrial inhibitors induce loss of striatal neurons whereas amphetamines induce loss of dopaminergic terminals. The reasons for these two remarkably di€erent pathologies cannot be readily explained but may re¯ect the site of metabolic overload. 5.3. Phencyclidine Induced Striatal Apoptosis Phencyclidine (PCP) can exert both neuroprotective and neurotoxic e€ects. This compound acts principally as a non-competitive NMDA receptor antagonist. It also has an amphetamine like action and its administration results in elevations of extracellular striatal dopamine levels. This action of PCP on dopamine levels is thought to result from increasing the discharge rates of nigrostriatal and mesoaccumbal dopamine neurons (Zhang et al., 1992; Miller and O'Callaghan, 1995; Venero et al., 1996; Mathe et al., 1998) and from stimulating vesicular release of dopamine from synaptic terminals and blocking reuptake (Raja and Guyenet, 1980; Vickroy and Johnson, 1982; Vignon et al., 1988; French and Ceci, 1990). There is a vast literature detailing how NMDA receptor antagonists can have marked neuroprotective e€ects. Thus, NMDA receptor antagonists can inhibit acute and delayed NMDA receptor mediated neuropathology (Olney et al., 1986; Meldrum and Garthwaite, 1991; Deupree et al., 1996; Haghighi et al., 1996; Hermenegildo et al., 1996; Ikeda et al., 1996), pilocarpine induced seizures and necrosis secondary to high-potassium induced increases in glutamate levels (Fugikawa, 1997; Lee et al., 1997). NMDA antagonists also a€ord protection in in vitro

The Selective Vulnerability of Striatopallidal Neurons

models of ischemia (Pringle et al., 1997) and following electrical stimulation of the perforant pathway (Thompson and Wasterlain, 1997). These drugs can also attenuate methamphetamine induced damage of striatal dopamine terminals as detailed above (Sonsalla et al., 1989). In addition to a€ording neuroprotection, NMDA receptor antagonists can also induce neuronal damage. Unlike amphetamines, which are primarily toxic to dopaminergic terminals, PCP induces degenerative changes in neuronal cell bodies. Thus, PCP, MK-801 and ketamine dose-dependently induce cytoplasmic vacuolisation reactions in neurons. When administered acutely this damage has been reported to be limited to the retrosplenial and posterior cingulate cortices (Olney et al., 1989). A recent report has, however, demonstrated that acute administration of PCP results in induction of haem oxygenase-1, a marker of neuronal injury, in the medial striatum (Rajdev et al., 1998). The neuropathomorphological changes are reversible at low doses, but at higher doses the neurotoxicity is irreversible (Olney et al., 1991; Wozniak et al., 1996; Zhang et al., 1996a; Zhang et al., 1996b; Hetman et al., 1997). This neurotoxicity occurs by both necrosis and apoptosis (Wozniak et al., 1996; Zhang et al., 1996a; Zhang et al., 1996b). The necrotic neuronal degeneration can be e€ectively antagonised by a series of pharmacologically active agents. These include: anticholinergics, various antipsychotic agents such as clozapine and olanzapine, and agents which act at the GABA-A receptor channel complex (Olney et al., 1991; Farber et al., 1993 Olney and Farber, 1994; Jevtovic-Todorovic et al., 1997). These observations have led to the hypothesis that NMDA receptor antagonist induced neuronal toxicity results from these glutamate antagonists blocking excitation of GABAergic interneurons. This then leads to increased activity of pyramidal neurons in the retrosplenial cortex and consequent excitotoxicity via actions at non-NMDA glutamate receptors and cholinergic receptors (Olney and Farber, 1995a, 1995b). Our own studies have investigated whether or not non-competitive NMDA receptor antagonists can induce damage to striatal projection neurons. Acute administration of large single doses of PCP (80 mg/ kg) in rat can induce death of a small number of striatal neurons (Mitchell et al., 1998). The dying neurons tend to lie in the large calbindin-poor region of the dorsomedial caudateputamen and show the morphological characteristics of cells undergoing apoptosis. The majority of the dying neurons could also be labelled retrogradely with a colloidal gold based anatomical tracer which was injected into the globus pallidus some weeks before the administration of the PCP. This procedure demonstrates, therefore, that PCP is inducing apoptosis of striatopallidal neurons. This pattern of neuronal damage is remarkably similar to that seen in the early stages of Huntington's disease when the pathology is largely limited to the loss of enkephalin positive striatopallidal neurons in the patch compartment of the dorsomedial caudate nucleus (Albin et al., 1992; Hedreen and Folstein, 1995).

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We have conducted extensive investigations to determine the pharmacological mechanisms by which PCP can induce the selective loss of striatopallidal neurons within the dorsomedial striatum. These experiments have included trying to induce a similar neuronal loss with compounds which are pharmacologically related to PCP and attempting to block the PCP induced striatal death by the co-administration of other agents. Acute administration of very large single doses of amphetamine (up to 12 mg/kg) does not induce loss of these neurons (unpublished observations). This implies that the PCP induced striatal apoptosis is not solely due to PCP acting as an indirect dopamine agonist. Similarly, systemic administration of the D-1 agonist, SKF38393, does not reliably induce striatal apoptosis though this drug has been reported to be neurotoxic when injected directly into the striatum (Kelley et al., 1990). Systemic injection of MK-801 at doses up to 5.0 mg/kg are ine€ective in inducing signi®cant levels of death of striatal cells (Griths et al., in press). This suggests that the PCP induced striatal apoptosis is not due solely to the drug acting as a non-competitive NMDA receptor antagonist. Coadministration of scopolamine, a cholinergic antagonist, has been shown to block MK-801 and PCP induced cytoplasmic vacuolisation within the retrosplenial cortex (Olney et al., 1991). This procedure was, however, ine€ective in blocking PCP induced striatal apoptosis implying that the apoptosis is induced by a cholinergic independent mechanism and that di€erent pathological mechanisms are operating within the striatum and the retrosplenial cortex. An alternative approach to investigating the mechanisms underlying PCP induced striatal apoptosis has involved studying the action of PCP on the expression of immediate early genes. The rationale behind this approach is two-fold. Firstly, the apoptotic death pro®le has been reported to be associated with the prolonged expression of certain immediate early genes, speci®cally c-fos and c-jun (Smeyne et al., 1993; Gass and Herdegen, 1995; Kasof et al., 1995; Schreiber and Baudry, 1995; Ferrer et al., 1996). Secondly, pharmacologically induced expression of c-fos within the striatum has been extensively mapped. Thus the characterisation of PCP induced c-fos expression could give insights into the pharmacological and cellular mechanisms which would ultimately lead to cell death. Following PCP administration, immediate early gene induction, including c-fos and its protein product, has been observed in the retrosplenial cortex which suggests that c-fos could mediate neurotoxic events in the brain (Tamminga et al., 1980; Nakki et al., 1996aNakki et al., 1996b; Sato et al., 1997; Gao et al., 1998). Previous work has noted only a modest expression of Fos-like immunoreactivity in the striatum following a low dose of PCP (Sato et al., 1997). In contrast, our own work has shown that doses of PCP sucient to induce apoptosis of striatal neurons result in dense Fos-like immunoreactivity within restricted striatal areas (Griths et al., 1999). Striatal Fos-like immunoreactivity induced by large doses of PCP could be blocked by the coadministration of the D-1 antagonist SCH23390

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(Griths et al., 1999). As amphetamine and D-1 agonists can induce striatal c-fos (Robertson et al., 1991; Wirtshafter and Asin, 1994; Le Moine et al., 1997) it is conceivable that PCP elicits Fos immunoreactivity by stimulation of D-1 receptors as a consequence of raised extracellullar striatal dopamine levels. However, amphetamines are thought to elicit c-fos expression in striatal projection neurons which express D-1 receptors not D-2 receptors (Robertson et al., 1991), that is, in striatonigral and striatoentopeduncular neurons not striatopallidal neurons. In contrast Fos immunoreactivity can be induced in striatopallidal neurons by decreasing dopamine levels, by either administering the monoamine depleting agent reserpine or lesioning the nigrostriatal pathway with 6-hydroxydopamine (Cooper et al., 1995a, 1995b) or by the systemic administration of D-2 receptor antagonists such as haloperidol (Robertson et al., 1991). This interpretation of the attenuation of PCP induced striatal Fos like immunoreactivity critically assumes that the Fos immunoreactivity is limited to the D-1 receptor expressing neurons. However, subsequent experiments using retrograde anatomical tracers to characterise the Fos positive striatal cells has shown this not to be the case (Griths et al., 1999). We have stereotaxically injected a retrograde colloidal gold anatomical tracer into the major projection areas of the striatum prior to the administration of PCP. Striatal sections were then processed to reveal the presence/absence of the tracer combined with Fos immunohistochemistry. This work has conclusively demonstrated that PCP elicits Fos immunoreactivity in both striatoentopeduncular

neurons and striatopallidal neurons, with the majority of the Fos immunoreactive neurons being of the latter type (Griths et al., 1999). A potential explanation of the unexpected observation of PCP induced Fos immunoreactivity in both striatopallidal and striatoentopeduncular neurons concerns the ability of PCP to depress extracellular dopamine levels as well as elevating them. Large doses of PCP have been shown to result in rebound decreases in striatal dopamine levels (Lillrank et al., 1994). Furthermore, PCP can act as a sigma ligand (Gundlach et al., 1985). Sigma binding sites are found in high densities within the densocellular part of the substantia nigra pars compacta (Graybiel et al., 1989), that is, that part of the substantia nigra which preferentially projects to the striosomal compartment of the caudateputamen (Graybiel et al., 1987; Graybiel et al., 1989). Thus PCP could elevate striatal Fos immunoreactivity in striatopallidal neurons by depression of striatal dopamine levels. In keeping with this hypothesis, we have also demonstrated that acute administration of both reserpine and haloperidol induce apoptosis of striatopallidal neurons as described below (Mitchell et al., 1994; Cooper et al., 1997). Interestingly, PCP induced striatal apoptosis can be markedly attenuated by the coadministration of RU38486, a corticosteroid receptor antagonist (unpublished observations). This implies that the PCP induced striatal apoptosis may be secondary to activation of striatal corticosteroid receptors. As will be detailed in Section 6, the dorsomedial caudateputamen is particularly sensitive to stress. Injection of saline is sucient to induce Fos immunoreactivity

Fig. 2. Haloperidol induced microglial activation and apoptosis of MSP neurons. Photomicrograph of a section of striatum following the administration of haloperidol (12 mg/kg). Note the mass of OX42 immunopositive labelling indicative of an activated microglia. A striatal cell undergoing apoptosis can be seen in the centre of the glia. Equivalent microglia responses can be seen following the administration of lower doses of haloperidol (1±4 mg/kg) or dexamethasone. Scale bar=10 mm.

The Selective Vulnerability of Striatopallidal Neurons

within this brain region (Griths et al., 1999) and striatopallidal neurons will undergo apoptosis following injection of the synthetic corticosteroid dexamethasone (Mitchell et al., 1998). 5.4. Reserpine And Haloperidol Induced Striatal Apoptosis Administration of the monoamine depleting agent reserpine, as noted above, induces Fos immunoreactivity in the striatum (Cooper et al., 1995a). The distribution of Fos like immunoreactivity is not homogeneous but tends to lie in a crescent which runs through the dorsomedial and dorsolateral aspects of the striatum close to the overlying corpus callosum. Double labelling studies using retrograde anatomical tracers have shown that the reserpine induced striatal Fos like immunoreactivity is found exclusively in striatopallidal neurons (Cooper et al., 1995a). It has been hypothesised that the reserpine induced striatal Fos immunoreactivity results from loss of striatal dopamine which removes the tonic inhibition normally mediated by activation of D-2 receptors. This loss of inhibition will enable glutamatergic corticostriatal inputs to more easily drive striatopallidal neurons which preferentially express the D-2 receptor (Gerfen et al., 1990). Support for this hypothesis has been provided by studies which have shown that prolonged low doses of ketamine (NMDA receptor antagonist) can block the dopamine depletion induced Fos like immunoreactivity (Cooper et al., 1995b). The observation that reserpine elevates Fos immunoreactivity in striatopallidal neurons by an NMDA receptor mediated process led us to speculate that reserpine may induce toxic e€ects within the striatum. More speci®cally, we hypothesised that the loss of D-2 mediated inhibition of striatopallidal neurons may be sucient to raise corticostriatal glutamatergic transmission to excitotoxic levels. Subsequent experiments showed that this was indeed the case. Systemic administration of reserpine resulted in apoptosis of small numbers of striatal neurons (Mitchell et al., 1994). These dying cells were clustered in the dorsomedial striatum with a distribution similar to that of reserpine induced striatal Fos like immunoreactivity and PCP induced striatal apoptosis. Double labelling studies with retrograde tracers con®rmed that the reserpine induced dying cells were striatopallidal neurons and their death could be attenuated by ketamine or by cortical aspiration. Taken collectively, these data demonstrate that reserpine induced striatal apoptosis is mediated by excess corticostriatal glutamatergic transmission. We further investigated whether other means of blocking dopamine mediated transmission can induce striatal apoptosis. We have accordingly administered single large doses of the D-2 receptor antagonist haloperidol to adult rats. These experiments have shown that haloperidol (4±12 mg/kg) can induce signi®cant levels of striatal apoptosis (see Fig. 2). As was the case with PCP and reserpine the majority of the dying cells lay in the dorsomedial aspect of the caudateputamen. Double labelling studies with retrograde tracers showed the majority of

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the dying cells to be striatopallidal neurons but appreciable numbers of the apoptotic cells were striatoentopeduncular neurons (Cooper et al., 1997). The injection of haloperidol was also associated with activation of microglia as revealed by elevated OX42 immunoreactivity (see Fig. 2). 5.5. Overview Of Dopaminergic Manipulations And Striatal Toxicity Pharmacological manipulations which a€ect striatal dopamine levels/transmission can induce toxic e€ects. Amphetamines can damage dopaminergic terminals within the striatum and can result in genomic activity, for example the induction of c-fos in striatoentopeduncular neurons via the stimulation of D-1 receptors. Prolonged expression of c-fos has been implicated in the molecular cascades which control the initiation of apoptosis. However, there is no evidence to date to show that amphetamines can induce loss of striatal neurons. In contrast, PCP, which has amphetamine like properties with respect to striatal dopamine, can induce apoptosis in a subpopulation of striatopallidal neurons in the dorsomedial aspect of the caudateputamen. The pharmacological mechanisms underlying this PCP induced striatal apoptosis are unclear but may re¯ect an action at striatal corticosteroid receptors as the death can be attenuated by the coadministration of a corticosteroid receptor antagonist. Large doses of PCP can paradoxically result in decreases in striatal dopamine levels. This could provide a mechanism for the striatal apoptosis as both systemic administration of haloperidol and reserpine induce similar patterns of cell death.

6. STRESS AND STRIATAL TOXICITY 6.1. Introduction Several lines of anecdotal evidence suggest that stress could play a role in toxin induced damage to the striatum. Stress is known to elevate circulating corticosteroid levels in both the periphery and the brain and corticosteroids are known to have marked cytotoxic properties within the immune system (Coplan et al., 1996). Corticosteroids have also been shown to be toxic to hippocampal neurons (Sapolsky, 1985; Sapolsky et al., 1985; Uno et al., 1994). In our own in vivo studies on striatal apoptosis we have noted that systemic injections of saline are associated with the death of a small number of striatal cells (Mitchell et al., 1998). Furthermore, as noted above, PCP induced apoptosis of enkephalin positive striatopallidal neurons can be attenuated by a corticosteroid receptor antagonist. Natural stressors can also elevate extracellular striatal dopamine levels and induce Fos immunoreactivity within speci®c regions of the striatum (Griths et al., 1999). 6.2. Stress And The Striatum Exposure of rats to transient mild stressors such as tail pinch or brief restraint can cause marked

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transient increases in extracellular dopamine as detected by in vivo dialysis (Finlay and Zigmond, 1997). These stress induced rises in extracellular dopamine are, however, reported to a€ect the prefrontal cortex and the nucleus accumbens more pronouncedly that the caudateputamen (Abercrombie et al., 1989; Iversen, 1995; Castro et al., 1996; Tidey and Miczek, 1996). The regional nature of the stress induced elevation of extracellular dopamine appears to be dependent in part upon the type of stressor used. Mild and short term stressors preferentially increase cortical dopamine levels whilst striatal dopamine levels are a€ected by periods of more severe or long lasting stress (Tidey and Miczek, 1996). Application of natural stressors can result in c-fos expression within restricted regions of the striatum and other neuronal structures (Handa et al., 1993; Chen and Herbert, 1995; Liste et al., 1997; Griths et al., 1999). For example, a 20 min exposure to a novel environment induced c-fos expression in the dorsal part of the caudateputamen, prefrontal and limbic cortices and speci®c nuclei of the thalamus and hypothalamus (Handa et al., 1993). The mechanisms which mediate stress induced striatal c-fos are unclear but may be secondary to stress induced elevations of extracellular dopamine levels. Support for this hypothesis comes from the observation that striatal c-fos expression induced by treadmill running can be blocked by the administration of a D-1 antagonist (Liste et al., 1997). We have similarly demonstrated that intraperitoneal injections of saline can elicit Fos like immunoreactivity in the dorsomedial caudateputamen and the nucleus accumbens via a D-1 dependent mechanism (Griths et al., 1999). It should be noted, however, that double labelling studies using retrograde neuroanatomical tracers injected into either the globus pallidus or the entopeduncular nucleus have demonstrated that this stress induced Fos like immunoreactivity is seen in both types of striatal projection neuron. As striatopallidal neurons do not express signi®cant numbers of D-1 dopamine receptors this ®nding suggests that the stress induced elevation of striatal dopamine levels must be exerting an indirect e€ect upon these neurons. Stress induced striatal c-fos expression could alternatively be elicited via the actions of corticosteroids. Mild stressors result in elevated systemic levels of corticosteroids. These steroids pass into the brain where they can exert an action by binding to membrane bound receptors or by binding within the nucleus to DNA regulatory sites (Vreugdenhil et al., 1996). Two types of corticosteroid receptor are found within the brain which are referred to as type I and type II corticosteroid receptors (Ahima and Harlan, 1990; Ahima et al., 1991). These are equivalent to the mineralo and glucocorticoid receptors found in the periphery. Type I corticosteroid receptors are con®ned mainly to the hippocampus whereas type II are distributed throughout many brain structures including the striatum (Ahima and Harlan, 1990; Ahima et al., 1991; Seckl, 1997). Corticosteroids, however, do not appear to play a direct role in inducing striatal c-fos expression as:

1. the levels of striatal Fos like immunoreactivity seen following the systemic injection of the synthetic corticosteroid dexamethasone are not signi®cantly di€erent to those seen following the injection of saline; and 2. striatal Fos like immunoreactivity induced by the injection of saline is not attenuated by coadministration of the corticosteroid receptor antagonist RU38486 (unpublished observations). Although corticosteroids do not appear to play a direct role upon stress induced striatal c-fos expression there is considerable evidence to show that there are complex interactions between corticosteroids and dopamine within the striatum. This evidence stems from initial observations of amphetamine induced behavioural sensitisation being potentiated by mild stressors (Piazza et al., 1996; Piazza and Le Moal, 1997). Thus, animals given repeated intermittent exposure to stressors such as foot shock, tail pinch and restraint show behavioural responses to a subsequent injection of amphetamine that is increased or sensitised in a manner similar to that seen after repeated intermittent exposure to amphetamine itself. This behavioural sensitisation phenomenon may re¯ect an action of corticosteroids upon type II receptors expressed on striatal neurons or on dopaminergic neurons in the mesencephalon (Overton et al., 1996; Piazza et al., 1996). Alternatively, the behavioural e€ects may not re¯ect the action of corticosterone per se but that of a related hormone such as corticotropin releasing factor (Badiani et al., 1996). 6.3. Corticosteroids And Apoptosis Raised corticosteroid levels are associated with damage to the hippocampus. This e€ect has been noted following chronic exposure to natural stressors, for example, in primates at the bottom of dominance hierarchies (Uno et al., 1994). Similarly, scanning studies of humans who have been exposed to chronic stressors, for example war veterans, have shown reduced temporal lobe volumes (Bremner et al., 1995; Wang et al., 1997). Experimental studies have demonstrated that administration of corticosterone to rats induces damage of hippocampal neurons, particularly in the CA3 sub®eld (Woolley et al., 1990). Similarly, administration of dexamethasone is reported to result in loss of granule cells within the dentate gyrus (Hassan et al., 1996). It should be noted, however, that removal of the adrenal cortex with subsequent loss of endogenous corticosteroids can also result in the loss of cells in the granule layer of the dentate gyrus (Sloviter et al., 1989). In our own studies we have demonstrated that administration of a single dose of dexamethasone (20 mg/kg, i.p.) to rats induces apoptosis of striatal cells which lie in the dorsomedial quadrant of the caudateputamen (Mitchell et al., 1998). Using retrograde tracers we have gone onto show that these cells are striatopallidal neurons. The striatal neurons which die following dexamethasone administration may represent an equivalent subpopulation to those which ®rst die in Huntington's disease. In both con-

The Selective Vulnerability of Striatopallidal Neurons

ditions cell loss is primarily limited to enkephalin positive striatopallidal neurons which lie in the dorsomedial caudateputamen (Albin et al., 1990; Hedreen and Folstein, 1995). Interestingly, Huntington's patients can show abnormalities in neuroendocrine function, including corticosteroid function. Thus, Leblhuber et al. (1995) argue that Huntington's disease is accompanied by hypothalamic disturbances which lead to imbalances in prolactin, growth hormone, somatotropic, thyrotropic and gonadotropin releasing hormone levels. These authors further note that Huntington's patients have elevated levels of cortisol and ACTH and abnormal levels of dehydroepiandrosterone sulphate (DHEAS) which antagonises the e€ects of glucocorticoids. It is conceivable, therefore, that altered corticosteroid function might contribute to the pathology seen in Huntington's disease. 6.4. Mechanisms Mediating Dexamethasone Induced Striatal Apoptosis Dexamethasone induced striatal apoptosis can be blocked by coadministration of the corticosteroid receptor antagonist RU 38486 which thus demonstrates that the cell death is mediated via a corticosteroid receptor dependent mechanism (unpublished observations). Why activation of striatal corticosteroid receptors should result in selective apoptosis is unclear. It is possible, however, that corticosteroids compromise local cellular energy levels which can leave the neuron more susceptible to the e€ects of endogenous excitotoxins. Such a hypothetical mechanism has been advanced to account for corticosteroid induced damage in the hippocampus (Sapolsky, 1985). Dexamethasone could also exert a direct e€ect upon intracellular Ca2+ levels as it has been shown to increase Ca2+ currents in neuronal cell lines (Fomina et al., 1996). An additional mechanism by which corticosteroids can induce toxic e€ects on striatal neurons is via the regulation of calcium binding proteins as corticosterone regulates the expression of calbindin D28K in the rat hippocampus (Strauss et al., 1995). Similarly, prolonged administration of corticosterone can result in reductions of calretinin mRNA in speci®c hypothalamic and thalamic nuclei (Strauss et al., 1995). Corticosteroids could thus both increase the intracellular Ca2+ levels and limit the ability of the neuron to bu€er these ions which would thus leave the cell prone to damage from excessive Ca2+ in¯ux. An alternative mechanism by which corticosteroids could induce damage in speci®c neuronal populations is via an interaction with trophic factors. Immobilisation stress can cause dramatic reductions in BDNF expression within the hippocampus which would leave BDNF dependent neurons more susceptible to atrophy (Vaidya et al., 1997). A stress induced reduction in the production of BDNF has been postulated to occur in depression where scanning studies have shown that chronic depression can be associated with reduced hippocampal volume (Duman et al., 1997). Corticosteroids could also mediate neurotoxic e€ects by exerting an action upon microglia. Both glucocorticoid and mineralocorticoid receptors are

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found in microglia (Tanaka et al., 1997) and corticosteroids can both cause cultured microglia to shrink and block their cytokine induced rami®cation (Tanaka et al., 1997). Antagonism of corticosteroid receptors with RU 38486 in microglia can attenuate the action of inducible NO and acid phosphatase and reduce the formation of lysosomes (Tanaka et al., 1997). 6.5. Overview Of Stress And Striatal Toxicity Acute stress can result in elevated levels of corticosteroids which can induce death of striatal cells. Similarly, a single administration of dexamethasone can induce apoptosis of striatopallidal neurons in the dorsomedial caudateputamen. This cell death can be blocked by the coadministration of corticosteroid receptor antagonists which demonstrates that the toxic e€ect is a receptor mediated process. Acute stress can also elicit elevations of striatal Fos like immunoreactivity. This e€ect, however, is seen in both striatopallidal and striatoentopeduncular neurons and appears to be dopamine dependent. The stress induced Fos like immunoreactivity can thus be blocked by administration of D-1 antagonists but not by corticosteroid receptor antagonists. The cellular mechanism underlying dexamethasone induced apoptosis of striatopallidal neurons is unclear but several lines of indirect evidence suggest increased vulnerability to the excitotoxic e€ects of glutamate as being the causative factor. The established vulnerability of MSP neurons to compromised metabolic activity lend support to this view.

7. CONCLUSIONS Striatal neurons show a range of vulnerabilities to a variety of insults. This can be clearly seen in Huntington's disease where a well mapped pattern of pathological events occurs. MSP neurons are the ®rst striatal cells to be a€ected as the disease progresses whilst interneurons, in particular the NADPH diaphorase positive ones, are spared even in the late stages of the disease. The MSP neurons, however, are not uniformly a€ected in Huntington's disease. MSP neurons in the patch compartment degenerate before those in the matrix and those in the dorsomedial caudate nucleus before those in the ventral lateral putamen. The enkephalin positive striatopallidal MSP neurons are also more vulnerable than the substance P/dynorphin MSP neurons. Several factors could potentially contribute to the selective vulnerability of striatopallidal neurons (these are summarised in Fig. 3). MSP neurons appear to be sensitive to the toxic e€ects of EAAs. There is little data, however, to show that excitotoxins are more toxic to a particular subgroup of MSP neurons de®ned either in terms of their projection target or location within the striatum. Striatal neurons show similar pro®les of vulnerability to systemically administered inhibitors of mitochondrial respiration. Consideration of the pattern of striatal damage induced by EAAs and mitochondrial inhibitors does, however, highlight the unusually high vulnerability of MSP neurons to disruptions of cellular

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respiration. This may re¯ect the enormous metabolic expenditure of MSP neurons in maintaining unusually high transmembrane potentials. It is possible that the additional vulnerability of striatopallidal MSP neurons in the dorsomedial striosome which is observed in Huntington's disease arises from an interaction of this susceptibility to reductions in energy supply with additional factors. These additional factors could include the distribution of calcium binding proteins, the local release of NO, the neurotoxic action of glia and the local densities of trophic factors. Consideration of such interactions has, however, failed to generate a simple model to account for the data. Our own work has revealed the presence of a subpopulation of striatopallidal MSP neurons in the calbindin poor region of the dorsomedial caudateputamen which are particularly prone to undergoing apoptosis. These neurons will undergo apoptosis following acute exposure to both dexamethasone and PCP. The cell death can, in both cases, be blocked by the coadministration of the corticosteroid receptor antagonist RU 38486 which implies that the drug induced apoptosis is a corticosteroid receptor mediated event. Corticosteroids are known to compromise cellular energy reserves and it has been previously postulated that the toxic e€ects of corticosteroids on hippocampal neurons result from increasing the e€ectiveness of excitotoxins. A similar mechanism could act within the striatum and local di€erences in the activity or regulation of corticosteroids and their receptors within the striatum could confer the observed pro®les of vulnerability. In support of this hypothesis it is interesting to note that abnormalities of corticosteroid function have been observed in Huntington's disease (Leblhuber et al., 1995). The selective vulnerability of striatopallidal neurons, particularly those in the dorsomedial caudate-

putamen, is both surprising and extreme. The apoptosis of small numbers of these neurons induced by single injections of saline, as noted in our work, implies that relatively small and transient elevations of corticosteroid levels are sucient to induce signi®cant levels of death. Such an extreme level of vulnerability leads one to question the potential adaptive signi®cance of this cell loss. Neuronal apoptosis is considered to be advantageous during the early development of the nervous system. It is generally assumed that restricted supplies of speci®c trophic factors released by neuronal targets ensures the survival of only those neurons which have grown to appropriate targets (Ra€ et al., 1993). This further ensures the establishment of optimal input/output ratios of neurons (Ra€ et al., 1993). Our own theoretical studies using connectionist architectures have also highlighted a potential further advantage of this early neuronal attrition by demonstrating how the selective deletion of arti®cial neurons from a neuronal network does not necessarily lead to a fall in computational performance and can actually lead to an increase in the ability to respond appropriately to novel inputs (Brown et al., 1994). It is dicult, however, to envisage how similar advantages could arise from the death of striatopallidal neurons in the adult. There is now strong evidence to show that the striatum plays a role in implicit learning. Patients with striatal dysfunction due to Parkinsonism or Huntington's disease show de®cits on tasks of implicit learning which are thought to involve the learning of habits (Knowlton et al., 1996a, 1996b). It is interesting to speculate how the death of striatopallidal neurons would a€ect this type of learning. The striatopallidal neurons of the dorsomedial caudateputamen, with their strong inputs from the anterior cingulate cortex (Berendse et al., 1992), are concerned with cognitive processes including the

Fig. 3. The major factors conferring selective vulnerability on NSP neurons. MSP neurons expend enormous amounts of energy in maintaining hyperpolarised ionic gradients which leaves them particularly susceptible to the pathological consequences of energy depletion. Loss of metabolic resources can result in elevated intracellular Ca2+ levels which can in turn result in death by either necrosis or apoptosis. Elevated levels of endogenous glutamate and corticosteroids as well as mitochondrial dysfunction can contribute to this compromised state. Dopamine, which is intimately involved in the regulation of striatal glutamate and corticosteroid levels, can also be toxic to MSP neurons following oxidation to free radicals. MSP neurons can, in addition, be damaged by NO which can be released by nearby striatal interneurons and glia. Protection of MSP neurons from the consequences of energy depletion and raised intracellular Ca2+ levels is a€orded by a number of factors including the presence of calcium binding proteins and trophic factors.

The Selective Vulnerability of Striatopallidal Neurons

control of covert attention (Alexander et al., 1986). Apoptosis of these neurons would thus be expected to induce immediate and long lasting changes in the control of attention. It is conceivable that the judicial use of such a system might be bene®cial in cases where fast, irreversible learning is required. Inappropriate operation of this system would similarly be predicted to result in the tendency to maintain attention inappropriately which could present as obsessive behaviours. In support of this speculation it is interesting to note that striatal dysfunction is associated with obsessive compulsive behaviours (Ebert et al., 1997; Escalona et al., 1997; Rosenberg et al., 1997; Purcell et al., 1998). Such compulsive behaviours are frequently reported in Huntington's and Parkinson's disease, and in Gilles de la Tourette's syndrome which is also assumed to result from striatal dysfunction (Charlett et al., 1998; Lauterbach et al., 1998). Similarly, children with Sydenham's chorea, which results from autoimmune induced damage of the striatum secondary to streptococcal infections, also show obsessive behaviours (Emery and Vieco, 1997; Asbahr et al., 1998; Garvey et al., 1998). An additional tantalising possibility to account for the selective loss of striatopallidal neurons in our experiments concerns neurogenesis. It is striking that the manipulations which we have used to induce apoptosis of striatopallidal neurons usually do not a€ect other neuronal structures with the exception of the hippocampus. The hippocampus of the adult is now thought to have a relatively privileged ability to compensate for lost neurons by neurogenesis (Kuhn et al., 1996; Liu et al., 1998). Given the similarities in the vulnerability of hippocampal neurons and striatopallidal neurons it will be interesting to know if neurogenesis also occurs in the caudateputamen of the adult rat. AcknowledgementsÐSome of the original work presented in this article was supported by the Medical Research Council, The Wellcome Trust and the University of Birmingham.

REFERENCES Abercrombie, D. A., Keefe, K. A., DiFrischia, D. S. and Zigmond, M. J. (1989) Di€erential e€ect of stress on in vivo dopamine release in striatum, nucleus accumbens and medial frontal cortex. J. Neurochem. 52, 1655±1658. Ahima, R. S. and Harlan, R. E. (1990) Charting of type II glucocorticoid receptor like immunoreactivity in the rat CNS. Neuroscience 39, 579±604. Ahima, R. S., Krozowski, Z. S. and Harlan, R. E. (1991) Type I corticosteroid receptor like immunoreactivity in the rat CNS: distribution and regulation by corticosteroids. J. Comp. Neurol. 313, 522±538. Albers, D. S., Zeevalk, G. D. and Sonsalla, P. K. (1996) Damage to dopaminergic nerve terminals in mice by combined treatment of intrastriatal malonate with systemic methamphetamine or MPTP. Brain Res. 718, 217±220. Albin, R. and Tagle, D. A. (1995) Genetics and molecular biology of Huntington's disease. TINS 18, 11±14. Albin, R. L., Reiner, A., Anderson, K. D., Penney, J. B. and Young, A. B. (1990) Striatal and nigral neuron subpopulations in rigid Huntington's disease: implications for the functional anatomy of chorea and rigidity-akinesia. Ann. Neurol. 27, 357± 365.

713

Albin, R. L., Reiner, A., Anderson, K. D., Leon, S., Dure, I. V., Handelin, B., Balfour, R., Whetsell, W. O., Penney, J. B. and Young, A. B. (1992) Preferential loss of striato-external pallidal projection neurons in presymptomatic Huntington's disease. Ann. Neurol. 31, 425±430. Alexander, G. E., Delong, M. R. and Strick, P. L. (1986) Parallel organisation of functionally segregated circuits linking basal ganglia and cortex. Ann. Rev. Neurosci. 9, 357±381. Alexi, T., Venero, J. L. and Hefti, F. (1997) Protective e€ects of neurotrophin-4/5 and transforming growth factor-alpha on striatal neuronal phenotypic degeneration after excitotoxic lesioning with quinolinic acid. Neuroscience 78, 73±86. Alexi, T., Hughes, P. E., Knusel, B. and Tobin, A. J. (1998) Metabolic compromise with systemic 3-nitropropionic acid produces striatal apoptosis in Sprague±Dawley rats but not in BALB/c ByJ mice. Exp. Neurol. 153, 74±93. Anden, N. E., Carlsson, A., Dahlstrom, A., Fuxe, K., Hillarp, N. A. and Larsson, K. (1964) Demonstration and mapping out of nigro-neostriatal dopamine neurons. Life Sci. 3, 523±530. Andrew, S. E., Goldberg, Y. P., Kremer, B., Telenius, H., Theilmann, J., Adam, S., Starr, E., Squitieri, F., Lin, B. Y., Kalchman, M. A., Graham, R. K. and Hayden, M. R. (1993) The relationship between trinucleotide (CAG) repeat length and clinical features of Huntington's disease. Nature Genet. 4, 398± 403. Apicella, P., Ljungberg, T., Scarnati, E. and Schultz, W. (1991) Responses to reward in monkey dorsal and ventral striatum. Exp. Brain Res. 85, 491±500. Asbahr, F. R., Negrao, A. B., Gentil, V., Zanetta, D. M. T., daPaz, J. A., MarquesDias, M. J. and Kiss, M. H. (1998) Obsessive-compulsive and related symptoms in children and adolescents with rheumatic fever with and without chorea: a prospective 6-month study. Am. J. Psychiat. 155, 1122±1124. Augood, S. J., Faull, R. L. M., Love, D. R. and Emson, P. C. (1996) Reduction in enkephalin and substance P mRNA in the striatum of early grade Huntington's disease: a detailed cellular in situ hybridization study. Neuroscience 72, 1023±1036. Augood, S. J., Faull, R. L. M. and Emson, P. C. (1997) Dopamine D1 and D2 receptor gene expression in the striatum in Huntington's disease. Ann. Neurol. 42, 215±221. Badiani, A., Jakob, A., Rodaros, D. and Stewart, J. (1996) Sensitization of stress induced feeding in rats repeatedly exposed to brief restraint: the role of corticosterone. Brain Res. 710, 35± 44. Banati, R. B. and Graeber, M. B. (1994) Surveillance, intervention and cytotoxicity: is there a protective role of microglia? Dev. Neurosci. 16, 114±127. Bardo, M. T. (1998) Neuropharmacological mechanisms of drug reward: beyond dopamine in the nucleus accumbens. Crit. Rev. Neurobiol. 12, 37±67. Barto, A. G. (1995) Adaptive critics and the basal ganglia. In: Models of Information Processing in the Basal Ganglia, pp. 215± 232. Eds J. C. Houk, J. L. Davis and D. G. Beiser. MIT Press, Massachusetts. Beal, M. F. (1992a) Does impairment of energy-metabolism result in excitotoxic neuronal death in neurodegenerative illnesses. Ann. Neurol. 31, 119±130. Beal, M. F. (1992b) Role of excitotoxicity in human neurological disease. Curr. Opin. Neurobiol. 2, 657±662. Beal, M. F., Kowall, N. W., Ellison, D. W., Mazurek, M. F., Swartz, K. J. and Martin, J. B. (1986) Replication of the neurochemical characteristics of Huntington's disease by quinolinic acid. Nature 321, 168±171. Beal, M. F., Ferrante, R. J., Swartz, K. J. and Kowall, N. W. (1991) Chronic quinolinic acid lesions in rats closely resemble Huntington's disease. J. Neurosci. 11, 1649±1659. Becher, M. W., Kotzuk, J. A., Sharp, A. H., Davies, S. W., Bates, G. P., Price, D. L. and Ross, C. A. (1998) Intranuclear neuronal inclusions in Huntington's disease and dentatorubral and pallidoluysian atrophy: correlation between the density of inclusions and IT15 CAG triplet repeat length. Neurobiol. Dis. 4(6), 387± 397. Berendse, H. W., Galis-de Graaf, G. and Groenewegen, H. J. (1992) The topographical organization and relationship with ventral striatal compartments of prefrontal corticostriatal projections in the rat. J. Comp. Neurol. 316, 314±347. Bjorklund, A., Lindvall, O., Isacson, O., Brudin, P., Wictorin, K. and Strecker, R. E. (1987) Mechanisms of action of intracerebral neural implants: studies on nigral and striatal grafts to the lesioned striatum. TINS 10, 509±516. Bolanos, J. P., Almeida, A., Stewart, V., Peuchen, S., Land, J. M., Clark, J. B. and Heales, S. J. R. (1997) Nitric oxide-mediated

714

I. J. Mitchell et al.

mitochondrial damage in the brain: mechanisms and implications for neurodegenerative disease. J. Neurochem. 68, 2227± 2240. Borlongan, C. V., Koutouzis, T. K., Freeman, T. B., Cahill, D. W. and Sanberg, P. R. (1995) Behavioural pathology induced by repeated systemic injections of 3-nitropropionic acid mimics the motoric symptoms of Huntington's disease. Brain Res. 697, 254±257. Bossi, S. R., Simpson, J. R. and Isacson, O. (1993) Age dependence of striatal neuronal death caused by mitochondrial dysfunction. NeuroReport 4, 73±76. Bouyer, J. J., Park, D. H., Joh, T. H. and Pickel, V. M. (1984) Chemical and structural-analysis of the relation between cortical inputs and tyrosine hydroxylase-containing terminals in rat neostriatum. Brain Res. 302, 267±275. Bowyer, J. F., Clausing, P., Schmued, L., Davies, D. L., Binienda, Z., Newport, G. D., Scallet, A. C. and Slikker, W. (1996) Parenterally administered 3-nitropropionic acid and amphetamine can combine to produce damage to terminals and cell bodies in the striatum. Brain Res. 712, 221±229. Bremner, J. D., Randall, P., Scott, T. M., Bronen, R. A., Seibyl, J. P., Southwick, S. M., Delaney, R. C., McCarthy, G., Charney, D. S. and Innis, R. B. (1995) MRI based measurement of hippocampal volume in patients with combat related post traumatic stress disorder. Am. J. Psychiat. 152, 973±981. Brown, G. D. A., Hulme, C., Hyland, P. D. and Mitchell, I. J. (1994) Programmed cell death in the developing nervous system: a functional neural network model. Cognitive Brain Res. 2, 71± 75. Brown, L. L., Schneider, J. S. and Lidsky, T. I. (1997) Sensory and cognitive functions of the basal ganglia. Curr. Opin. Neurobiol. 7, 157±163. Browne, S. E., Bowling, A. C., MacGarvey, U., Baik, M. J., Berger, S. C., Muqit, M. M. K., Bird, E. D. and Beal, M. F. (1997) Oxidative damage and metabolic dysfunction in Huntington's disease: selective vulnerability of the basal ganglia. Ann. Neurol. 41, 646±653. Burke, R. E. and Baimbridge, K. G. (1993) Relative loss of the striatal striosome compartment de®ned by calbindin D-28K immunostaining following developmental hypoxic-ischemic injury. Neuroscience 56, 305±315. Butterworth, N. J., Williams, L., Bullock, J. Y., Love, D. R., Faull, R. L. M. and Dragunow, M. (1998) Trinucleotide (CAG) repeat length is positively correlated with the degree of DNA fragmentation in Huntington's disease striatum. Neuroscience 87, 49±53. Cadet, J. L. and Brannock, C. (1998) Free radicals and the pathobiology of brain dopamine systems. Neurochem. Int. 32, 117± 131. Calabresi, P., De Murtas, M., Pisani, A., Stefani, A., Sancessario, G., Mercuri, N. B. and Bernardi, G. (1995) Vulnerability of medium spiny striatal neurons to glutamate: role of Na+/K+ ATPase. Eur. J. Neurosci. 7, 1674±1683. Calabresi, P., Pisani, A., Mercuri, N. B. and Bernardi, G. (1996) The corticostriatal projection: from synaptic plasticity to dysfunctions of the basal ganglia. TINS 19, 19±24. Calabresi, P., Ascone, C. M., Centonze, D., Pisani, A., Sancesario, G., D'Anelo, V. and Bernardi, G. (1997a) Opposite membrane potential changes induced by glucose deprivation in striatal spiny neurons and in large aspiny interneurons. J. Neurosci. 17, 1940±1949. Calabresi, P., De Murtas, M. and Bernardi, G. (1997b) The neostriatum beyond the motor function: experimental and clinical evidence. Neuroscience 78, 39±60. Calabresi, P., Centonze, D., Pisani, A., Sancessario, G., Gubellini, P., Mar®a, G. A. and Bernardi, G. (1998) Striatal spiny neurons and cholinergic interneurons express di€erential ionotropic glutamatergic responses and vulnerability: implications for ischemia and Huntington's disease. Ann. Neurol. 43, 586±597. Carter, R. J., Hickey, M. A., Mangiarini, L., Mahal, A., Bates, G. P. and Morton, A. J. (1998) Abnormal motor response to (+) methamphetamine in mice transgenic for the Huntington's disease mutation. Eur. J. Neurosci. 10(S10), 15603. Castro, S. L., Sved, A. F. and Zigmond, M. J. (1996) Increased neostriatal tyrosine hydroxylation during stress: role of extracellular dopamine and excitatory amino acids. J. Neurochem. 66, 824±833. Cha, J. H. J., Kosinski, C. M., Kerner, J. A., Alsdorf, A. S., Manigiarini, L., Davies, S. W., Penney, J. B., Bates, G. P. and Young, A. B. (1998) Altered brain neurotransmitter receptors in transgenic mice expressing a portion of an abnormal human

Huntington disease gene. Proc. natl Acad. Sci. USA 95, 6480± 6485. Chang, C. and Jang, T. (1995) Age-dependent neurotoxicity of striatal lesions produced by aminooxyacetic acid: quantitative in vitro NMR spectroscopic studies. J. Neurochem. 65, 1192±1198. Chapman, A. G., Durmuller, N., Lees, G. J. and Meldrum, B. S. (1989) Excitotoxicity of NMDA and kainic acid is modulated by nigrostriatal dopaminergic ®bres. Neurosci. Lett. 107, 256± 260. Charlett, A., Dobbs, R. J., Purkiss, A. G., Wright, D. J., Peterson, D. W., Weller, C. and Dobbs, S. M. (1998) Cortisol is higher in parkinsonism and associated with gait de®cit. Acta Neurol. Scand. 97, 77±85. Chen, Q., Veenman, C. L. and Reiner, A. (1996) Cellular expression of ionotropic glutamate receptor subunits on speci®c striatal neuron types and its implication for striatal vulnerability in glutamate receptor-mediated excitotoxicity. Neuroscience 73, 715±731. Chen, X. and Herbert, J. (1995) Regional changes in c-fos expression in the basal forebrain and brain-stem during adaptation to repeated stressÐcorrelations with cardiovascular, hypothermic and endocrine responses. Neuroscience 64, 675± 685. Cheng, N., Maeda, T., Kume, T., Kaneko, S., Kochiyama, H., Akaike, A., Goshima, Y. and Misu, Y. (1996) Di€erential neurotoxicity induced by L-DOPA and dopamine in cultured striatal neurons. Brain Res. 743, 278±283. Choi, D. W., Maulucci-Gedde, M. and Kriegstein, A. R. (1987) Glutamate neurotoxicity in cortical cell culture. J. Neurosci. 7, 357±368. Cicchetti, F. and Parent, A. (1996) Striatal interneurons in Huntington's disease: selective increase in the density of calretinin-immunoreactive medium sized neurons. Movement Disorders 6, 619±626. Cicchetti, F., Gould, P. V. and Parent, A. (1996) Sparing of striatal neurons coexpressing calretinin and substance P (NK1) receptors in Huntington's disease. Brain Res. 730, 232±237. Co€ey, P. J., Perry, V. H., Allen, Y., Sinden, J. and Rawlins, J. N. P. (1988) Ibotenic acid induced demyelination in the CNS: a consequence of a local in¯ammatory response. Neurosci. Lett. 84, 178±184. Colwell, C. S. and Levine, M. S. (1996) Glutamate receptorinduced toxicity in neostriatal cells. Brain Res. 724, 205±212. Connop, B. P., Boegman, R. J., Beninger, R. J. and Jhamandas, K. (1996) Attenuation of malonate-induced degeneration of the nigrostriatal pathway by inhibitors of nitric oxide synthase. Neuropharmacology 35, 459±465. Cooper, A. J., Moser, B. and Mitchell, I. J. (1995a) A subset of striatopallidal neurons are Fos-immunopositive following acute monoamine depletion in the rat. Neurosci. Lett. 187, 189±192. Cooper, A. J., Wooller, S. and Mitchell, I. J. (1995b) Elevated striatal Fos immunoreactivity following 6-hydroxydopamine lesioning of the rat is mediated by excitatory amino acid transmission. Neurosci. Lett. 194, 73±76. Cooper, A. J., Fraser, M., Foreshaw, M., Barber, D. J. and Mitchell, I. J. (1997) Acute administration of D-1 dopamine agonists and a D-2 antagonist induce apoptosis of a sub-population of striatal neurons. Brain Res. Assoc. Abstr. 14, 58. Coplan, J. D., Andrews, M. W., Rosenblum, L. A., Owens, M. J., Friedman, S., Gorman, J. M. and Nemero€, C. B. (1996) Persistent elevations of cerebrospinal ¯uid concentrations of corticotropin-releasing factor in adult nonhuman primates exposed to early-life stressors: implications for the pathophysiology of mood and anxiety disorders. Proc. natl Acad. Sci. USA 93, 1619±1623. Coyle, J. T. and Schwarcz, R. (1976) Lesion of striatal neurons with kainic acid provides a model for Huntington's chorea. Nature 263, 244±246. Crossman, A. R. (1987) Primate models of dyskinesiaÐthe experimental approach to the study of basal ganglia-related involuntary movement disorders. Neuroscience 21, 1±40. Crossman, A. R., Mitchell, I. J., Jackson, A. and Sambrook, M. A. (1988) Chorea and myoclonus in the monkey induced by gamma-aminobutyric acid antagonism in the lentiform complex: the site of drug action and a hypothesis of the neural mechanisms of chorea. Brain 111, 1211±1233. Davies, S. W. and Roberts, P. J. (1987) No evidence for preservation of somatostatin-containing neurons after intrastriatal injections of quinolinic acid. Nature 327, 326±329. Davies, S. W., Turmaine, M., Cozens, B. A., DiFiglia, M., Sharp, A. H., Ross, C. A., Scherzinger, E., Wanker, E. E., Mangiarini, L. and Bates, G. P. (1997) Formation of neuronal intranuclear

The Selective Vulnerability of Striatopallidal Neurons inclusions underlies the neurological dysfunction in mice transgenic for the HD mutation. Cell 90, 537±548. Deshpande, J., Bergstedt, K., Linden, T., Kalimo, H. and Wieloch, T. (1992) Ultrastructural changes in the hippocampal CA1 region following transient cerebral ischemia: evidence against programmed cell death. Exp. Brain Res. 88, 91±105. Dessi, F., Charriaut-Marlangue, C., Khrestchatisky, M. and Benari, Y. (1993) Glutamate-induced neuronal death is not a programmed cell death in cerebellar culture. J. Neurochem. 60, 1953±1955. Deupree, D. L., Tang, X. W., Yarom, M., Dickman, E., Kirch, R. D., Schloss, J. V. and Wu, J. Y. (1996) Studies of NMDA- and non-NMDA-mediated neurotoxicity in cultured neurons. Neurochem. Int. 29, 255±261. Di Monte, D. A., Royland, J. E., Jakowec, M. W. and Langston, J. W. (1996) Role of nitric oxide in methamphetamine neurotoxicity: protection by 7-nitroindazole, an inhibitor of neuronal nitric oxide synthase. J. Neurochem. 67, 2443±2450. Dragunow, M., Faull, R. L. M., Lawlor, P., Beilharz, E. J., Singleton, K., Walker, E. B. and Mee, E. (1995) In-situ evidence for DNA fragmentation in Huntington's disease striatum and Alzheimer's disease temporal lobes. Neuroreport 6, 1053±1057. Druga, R., Rokyta, R. and Benes, V. (1991) Thalamocaudate projections in the macaque monkey. J. Hirnforsch 32, 765±774. Duman, R. S., Heninger, G. R. and Nestler, E. J. (1997) A molecular and cellular theory of depression. Arch. Gen. Psychiat. 54, 597±606. Dunnett, S. B., Everitt, B. J. and Robbins, T. W. (1991) The basal forebrain-cortical cholinergic system: interpreting the functional consequences of excitotoxic lesions. TINS 14, 494±501. Ebert, D., Speck, O., Konig, A., Berger, M., Hennig, J. and Hohagen, F. (1997) H-1-magnetic resonance spectroscopy in obsessive-compulsive disorder: evidence for neuronal loss in the cingulate gyrus and the right striatum. Psychiat. Res.Ð Neuroimaging 74, 173±176. Eisch, A. J., Ga€ney, M., Weihmuller, F. B., O'Dell, S. J. and Marshall, J. F. (1992) Striatal subregions are di€erentially vulnerable to the neurotoxic e€ects of methamphetamine. Brain Res. 598, 321±326. Ellison, D. W., Beal, M. F., Mazurek, M. F., Malloy, J. R., Bird, E. D. and Martin, J. B. (1987) Amino acid neurotransmitter abnormalities in Huntington's disease and the quinolinic acid animal model of Huntington's disease. Brain 110, 1657±1673. Emery, E. S. and Vieco, P. T. (1997) Sydenham chorea: magnetic resonance imaging reveals permanent basal ganglia injury. Neurology 48, 531±533. Escalona, P. R., Adair, J. C., Roberts, B. B. and Graeber, D. A. (1997) Obsessive±compulsive disorder following bilateral globus pallidus infarction. Biol. Psychiat. 42, 410±412. Farber, N. B., Price, M. T., Labruyere, J., Nemnich, J., St Peter, H., Wozniak, D. F. and Olney, J. W. (1993) Antipsychoticdrugs block phencyclidine receptor-mediated neurotoxicity. Biol. Psychiat. 34, 119±121. Ferrante, R. J., Beal, M. F., Kowall, N. W., Richardson, E. P. and Martin, J. B. (1987) Sparing of acetylcholinesterase-containing striatal neurons in Huntington's disease. Brain Res. 411, 162±166. Ferrante, R. J., Kowall, N. W., Cipolloni, P. B., Storey, E. and Beal, M. F. (1993) Excitotoxin lesions in primates as a model for Huntington's diseaseÐhistopathologic and neurochemical characterization. Exp. Neurol. 119, 46±71. Ferrante, R. J., Gutekunst, C. A., Persichetti, F., McNeil, S. M., Kowall, N. W., Gusella, J. F., MacDonald, M. E., Beal, M. F. and Hersch, S. M. (1997) Heterogeneous topographic and cellular distribution of Huntingtin expression in the normal human neostriatum. J. Neurosci. 17, 3052±3063. Ferre, S., O'Connor, W. T., Fuxe, K. and Ungerstedt, U. (1993) The striopallidal neuron: a main locus for adenosine-dopamine interactions in the brain. J. Neurosci. 13, 5402±5406. Ferrer, I., Kulisevsky, J., Gonzalez, G., Escartin, A., Chvite, A. and Casas, R. (1994) Parvalbumin-immunoreactive neurons in the cerebral cortex and striatum in Huntington's disease. Neurodegeneration 3, 169±173. Ferrer, I., Olive, M., Ribera, J. and Planas, A. M. (1996) Naturally occurring (programmed) and radiation-induced apoptosis are associated with selective c-Jun expression in the developing rat brain. Eur. J. Neurosci. 8, 1286±1298. Figueredo-Cardenas, G., Anderson, K. D., Chen, Q., Veenman, C. L. and Reiner, A. (1994) Relative survival of striatal projection neurons and interneurons after intrastriatal injection of quinolinic acid in rats. Exp. Neurol. 129, 37±56.

715

Figueredo-Cardenas, G., Harris, C. L., Anderson, K. D. and Reiner, A. (1998) Relative resistance of striatal neurons containing calbindin or parvalbumin to quinolinic acid-mediated excitotoxicity compared to other striatal neuron types. Exp. Neurol. 149, 356±372. Finlay, J. M. and Zigmond, M. J. (1997) The e€ects of stress on central dopaminergic neurons: possible clinical implications. Neurochem. Res. 22, 1387±1394. Fomina, A. F., Levitan, E. S. and Takimoto, K. (1996) Dexamethasone rapidly increases calcium channel subunit messenger RNA expression and high voltage-activated calcium current in clonal pituitary cells. Neuroscience 72, 857±862. French, E. D. and Ceci, A. (1990) Non-competitive N-methyl-Daspartate antagonists are potent activators of ventral tegmental A-10 dopamine neurons. Neurosci. Lett. 119, 159±162. Frim, D. M., Uhler, T. A., Short, M. P., Ezzedine, Z. D., Klagsbrun, M., Breake®eld, X. O. and Isacson, O. (1993) E€ects of biologically delivered NGF, BDNF and bFGF on striatal excitotoic lesions. Neuroreport 4, 367±370. Fugikawa, D. G. (1997) E€ects of N-methyl-D-aspartate-receptor blockade on high-potassium-induced neuronal death and glutamate release. Neurosci. Lett. 226, 25±28. Galpern, W. R., Matthews, R. T., Beal, M. F. and Isacson, O. (1996) NGF attenuates 3-nitrotyrosine formation in a 3-NP model of Huntington's disease. NeuroReport 7, 2639±2642. Gao, X. M., Hashimoto, T. and Tamminga, C. (1998) Phencyclidine (PCP) and dizocilpine (MK801) exert time-dependent e€ects on the expression of immediate early genes in rat brain. Synapse 29, 14±28. Garvey, M. A., Giedd, J. and Swedo, S. E. (1998) PANDAS: the search for environmental triggers of pediatric neuropsychiatric disorders. Lessons from rheumatic fever. J. Child Neurol. 13, 413±423. Gass, P. and Herdegen, T. (1995) Neuronal expression of AP-1 proteins in excitotoxic-neurodegenerative disorders and following nerve ®ber lesions. Prog. Neurobiol. 47, 257±290. Gebicke-Haerter, P. J., Van Calker, D., Norenberg, W. and Illes, P. (1996) Molecular mechanisms of microglial activation. Implications for regeneration and neurodegenerative diseases. Neurochem. Int. 29, 1±12. Gerfen, C. R. (1985) The neostriatal mosaic. I. Compartmental organization of projections from the striatum to the substantia nigra in the rat. J. Comp. Neurol. 236, 454±476. Gerfen, C. R. (1989) The neostriatal mosaic±striatal patch-matrix organization is related to cortical lamination. Science 246, 385± 388. Gerfen, C. R. (1992) The neostriatal mosaic: multiple levels of compartmental organisation. TINS 15, 133±138. Gerfen, C. R. and Young, W. S. (1988) Distribution of striatonigral and striatopallidal peptidergic neurons in both patch and matrix compartments: an in situ hybridization histochemistry and ¯uorescent retrograde tracing study. Brain Res. 460, 161± 167. Gerfen, C. R., Engber, T. M., Mahan, L. C., Susel, Z., Chase, T. N., Monsma, F. J. and Sibley, D. R. (1990) D-1 and D-2 dopamine receptor-regulated gene expression of striatonigral and striatopallidal neurons. Science 250, 1429±1432. Gerlach, M., Riederer, P. and Youdim, M. B. H. (1996) Molecular mechanisms for neurodegenerationÐsynergism between reactive oxygen species, calcium and excitatory amino acids. Adv. Neurol. 69, 177±194. Globus, M. Y. T., Ginsberg, M. D., Harik, S. I., Busto, R. and Dietrich, W. D. (1987) Role of dopamine in ischemic striatal injury. Neurology 37, 1712±1719. Glowinski, J., Barbeito, L. and Cheramy, A. (1989) In¯uence of corticostriatal glutamatergic neurons on dopaminergic transmission in the striatum. In: The Basal Ganglia, Vol. III. Eds G. Bernardi, M. B. Carpenter, G. DiChiara and P. Stanzione. Plenum Press. Goldman-Rakic, P. S. and Selemon, L. D. (1990) New frontiers in basal ganglia research. TINS 13, 241±244. Graybiel, A. M. (1990) Neurotransmitters and neuromodulators in the basal ganglia. TINS 13, 244±254. Graybiel, A. M., Hirsch, E. and Agid, Y. A. (1987) Di€erences in tyrosine hydroxylase like immunoreactivity characterise the mesostriatal innervation of striosomes and extrastriosomal matrix at maturity. Proc. natl Acad. Sci. USA 84, 303±307. Graybiel, A. M., Besson, M. J. and Weber, E. (1989) Neuroleptic sensitive binding sites in the nigrostriatal system: evidence for di€erential distribution of sigma sites in the substantia nigra, pars compacta of the cat. J. Neurosci. 9, 326±338.

716

I. J. Mitchell et al.

Griths, M. R., Cooper, A. J. and Mitchell, I. J. (1999) Phencyclidine induces D-1 dopamine receptor mediated Fos-like immunoreactivity in discretely localised populations of striatopallidal and striatoentopeduncular neurons of the rat. Brain Res. 21, 177±189. Griths, M.R., Cooper, A.J., Barber, D.J., and Mitchell, I.J. (1999) Pharmacological mechanisms mediating phencyclidineinduced apoptosis of striatopallidal neurons: the roles of glutamate, dopamine, acetylcholine and corticosteroids. Brain Res. (in press). Groenewegen, H. J., Becker, N. E. H. M. and Lohman, A. H. M. (1980) Subcortical a€erents of the nucleus accumbens septi in the cat, studied with retrograde axonal transport of horseradish peroxidase and bisbenzimid. Neuroscience 5, 1903±1916. Gundlach, A. L., Largent, B. L. and Snyder, S. H. (1985) Phencyclidine and sigma opiate receptors in brain: biochemical and autoradiographical di€erentiation. Eur. J. Pharmac. 113, 465±466. Guyot, M. C., Hantraye, P., Dolan, R., Pal®, S., Maziere, M. and Brouillet, E. (1997) Quanti®able bradykinesia, gait abnormalities and Huntington's disease like striatal lesions in rats chronically treated with 3-nitropropionic acid. Neuroscience 79, 45±56. Haber, S. N., Lynd, E., Klein, C. and Groenewegen, H. J. (1990) Topographic organisation of the ventral striatal e€erent projections in the rhesus monkey: an anterograde tracing study. J. Comp. Neurol. 293, 282±298. Haghighi, S. S., Johnson, G. C., de Vergel, C. F. and Vergel Rivas, B. J. (1996) Pretreatment with NMDA receptor antagonist MK801 improves neurophysiological outcome after an acute spinal cord injury. Neurol. Res. 18, 509±515. Handa, R. J., Nunley, K. M. and Bollnow, M. R. (1993) Induction of c-fos mRNA in the brain and anterior pituitary gland by a novel environment. Neuroreport 4, 1079±1082. Harrington, K. M. and Kowall, N. W. (1991) Parvalbumin immunoreactive neurons resist degeneration in Huntingtons-disease striatum. J. Neuropathol. Exp. Neurol. 50, p309. Harrison, M. B., Tissot, M. and Wiley, R. G. (1996) Expression of M1 and M4 muscarinic receptor mRNA in the striatum following a selective lesion of striatonigral neurons. Brain Res. 734, 323±326. Hashimoto, N., Matsumoto, T., Mabe, H., Hashitani, T. and Nishino, H. (1994) Dopamine has inhibitory and accelerating e€ects on ischemia-induced neuronal cell damage in the rat striatum. Brain Res. Bull. 33, 281±288. Hassan, A. H. S., VonRosenstiel, P., Patchev, V. K., Holsboer, F. and Almeida, O. F. X (1996) Exacerbation of apoptosis in the dentate gyrus of the aged rat by dexamethasone and the protective role of corticosterone. Exp. Neurol. 140, 43±52. Hedreen, J. C. and Folstein, S. E. (1995) Early loss of neostriatal striosome neurons in Huntington's disease. J. Neuropath. Exp. Neurol. 54, 105±120. Heinsen, H., Strik, M., Bauer, M., Luther, K., Ulmar, G., Gangus, D., Jungkunz, G., Eisenmenger, W. and Gotz, M. (1994) Cortical and striatal neurone number in Huntington's disease. Acta Neuropathol. 88, 320±333. Herkenham, M. and Pert, C. B. (1981) Mosaic distribution of opiate receptors, parafascicular projections and acetylcholinesterase in rat striatum. Nature 291, 415±418. Hermenegildo, C., Marcaida, G., Montoliu, C., Grisolia, S., Minana, M. D. and Felipo, V. (1996) NMDA receptor antagonists prevent acute ammonia toxicity in mice. Neurochem. Res. 21, 1237±1244. Hetman, M., Danysz, W. and Kaczmarek, L. (1997) Increased expression of cathepsin D in retrosplenial cortex of MK-801-treated rats. Exp. Neurol. 147, 229±237. Hirata, H., Ladenheim, B., Rothman, R. B., Epstein, C. and Cadet, J. L. (1995) Methamphetamine-induced serotonin neurotoxicity is mediated by superoxide radicals. Brain Res. 677, 345± 347. Hirata, H., Ladenheim, B., Carlson, E., Epstein, C. and Cadet, J. L. (1996) Autoradiographic evidence for methamphetamineinduced striatal dopaminergic loss in mouse brain: attenuation in CuZn-superoxide dismutase transgenic mice. Brain Res. 714, 95±103. Huntington's Disease Collaborative Research Group (1993) A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes. Cell 72, 971±983. Ikeda, J., Terakawa, S., Murota, S., Morita, I. and Hirakawa, K. (1996) Nuclear disintegration as a leading step of glutamate excitotoxicity in brain neurons. J. Neurosci. Res. 43, 613±622.

Islam, N., Aftabuddin, M., Moriwaki, A. and Hori, Y. (1995) Detection of DNA damage induced by apoptosis in the rat brain following incomplete ischemia. Neurosci. Lett. 188, 159± 162. Iversen, S. D. (1995) Interactions between excitatory amino acids and dopamine systems in the forebrain: implications for schizophrenia and Parkinson's disease. Behav. Pharmac. 6, 478±491. Jackson, G. R., Salecker, I., Dong, X. Z., Yao, X., Arnheim, N., Faber, P. W., MacDonald, M. E. and Zipursky, S. L. (1998) Polyglutamine-expanded human huntingtin transgenes induce degeneration of Drosophila photoreceptor neurons. Neuron 21, 633±642. Jevtovic-Todorovic, V., Kirby, C. O. and Olney, J. W. (1997) Iso¯urane and propofol block neurotoxicity caused by MK-801 in the rat posterior cingulate/retrosplenial cortex. J. Cerebral Blood Flow Metabolism 17, 168±174. Karlovich, C. A., John, R. M., Ramirez, L., Stainier, D. Y. R. and Myers, R. M. (1998) Characterization of the Huntington's disease (HD) gene homolog in the zebra®sh Danio rerio. Gene 217, 117±125. Kasof, G. M., Mandelzys, A., Maika, S. D., Hammer, R. E., Curran, T. and Morgan, J. I. (1995) Kainic acid-induced neuronal death is associated with DNA damage and a unique immediate early gene response in c-fos-lacZ transgenic rats. J. Neurosci. 15, 4238±4249. Kawaguchi, Y. (1997) Neostriatal cell subtypes and their functional roles. Neurosci. Res. 27, 1±8. Kawaguchi, Y., Wilson, C. J., Augood, S. J. and Emson, P. C. (1995) Striatal interneurons: chemical, physiological and morphological characterization. TINS 18, 527±535. Kelley, A. E., Delfs, J. M. and Chu, B. (1990) Neurotoxicity induced by the D-1 agonist SKF38393 following microinjection into rat brain. Brain Res. 532, 342±346. Kemp, J. M. and Powell, T. P. S. (1970) The corticostriate projection in the monkey. Brain 93, 525±546. Knowlton, B. J., Squire, L. R., Paulsen, J. S., Swerdlow, N. R., Swenson, M. and Butters, N. (1996a) Dissociation within nondeclarative memory in Huntington's disease. Neuropsychology 10, 538±548. Knowlton, B. J., Mangels, J. A. and Squire, L. R. (1996b) A neostriatal habit learning system in humans. Science 273, 1399± 1402. Kodsi, M. H. and Swerdlow, N. R. (1997) Mitochondrial toxin 3nitropropionic acid produces startle re¯ex abnormalities and striatal damage in rats that model some features of Huntington's disease. Neurosci. Lett. 231, 103±107. Koh, J. Y. and Choi, D. W. (1988) Cultured striatal neurons containing NADPH-diaphorase or acetylcholinesterase are selectively resistant to injury by NMDA receptor agonists. Brain Res. 46, 374±378. Kosinski, C., Cha, J. H., Young, A. B., Persichetti, F., MacDonald, M., Gusella, J. F., Penney, J. B. and Standaert, D. G. (1997) Huntingtin immunoreactivity in the rat neostriatum: di€erential accumulation in projection and interneurons. Exp. Neurol. 144, 239±247. Kubota, Y. and Kawaguchi, Y. (1993) Spatial distributions of chemically identi®ed intrinsic neurons in relation to patch and matrix compartments of rat neostriatum. J. Comp. Neurol. 332, 499±513. Kuhn, H. G., Dickinson-Anson, H. and Gage, F. H. (1996) Neurogenesis in the dendate gyrus of the adult rat: age related decreases of neuronal progenitor proliferation. J. Neurosci. 16, 2027±2033. Kure, S., Tominaga, T., Yoshimoto, T., Tada, K. and Narisawa, K. (1991) Glutamate triggers internucleosomal DNA cleavage in neuronal cells. Biochem. Biophys. Res. Commun. 179, 39±45. Landwehrmeyer, G. B., Mcneil, S. M., Dure, L. S., Ge, P., Aizawa, H., Huang, Q., Ambrose, C. M., Duyao, M. P., Bird, E. D., Bonilla, E., Deyoung, M., Avilagonzales, A. J., Wexler, N. S., Di®glia, M., Gusella, J. F., Macdonald, M. E., Penney, J. B., Young, A. B. and Vonsattel, J. P. (1995a) Huntington's disease gene regional and cellular expression in brain of normal and a€ected individuals. Ann. Neurol. 37, 218±230. Landwehrmeyer, G. B., Standaert, D. G., Testa, C. M., Penney, J. B. and Young, A. B. (1995b) NMDA receptor subunit messenger-RNA expression by projection neurons and interneurons in rat striatum. J. Neurosci. 15, 5297±5307. Lauterbach, E. C., Cummings, J. L., Du€y, J., Co€ey, C. E., Kaufer, D., Lovell, M., Malloy, P., Reeve, A., Royall, D. R., Rummans, T. A. and Salloway, S. P. (1998) Neuropsychiatric correlates and treatment of lenticulostriatal diseases: a review of the literature and overview of research opportunities in

The Selective Vulnerability of Striatopallidal Neurons Huntington's, Wilson's and Fahr's diseasesÐa report of the ANPA committee on research. J. Neuropsychiat. Clin. Neurosci. 10, 249±266. Lawrence, A. D., Hodges, J. R., Rosser, A. E., Kershaw, A., €rench-Constant, C., Rubinsztein, D. C., Robbins, T. W. and Sahakian, B. J. (1998) Evidence for speci®c cognitive de®cits in preclinical Huntington's disease. Brain 121, 1329±1341. Leblhuber, F., Peichl, M., Neubauer, C., Reisecker, F., Steinparz, F. X., Windhager, E. and Maschek, W. (1995) Serum dehydroepiandrosterone and cortisol measurements in Huntington's chorea. J. Neurolog. Sci. 132, 76±79. Lee, K. S., Frank, S., Van der Klish, P., Arai, A. and Lynch, G. (1991) Inhibition of proteolysis protects hippocampal neurons from ischemia. Proc. natl Acad. Sci. USA 88, 7233±7237. Lee, M. G., Chou, J. Y., Lee, K. H., Choi, B. J., Kim, S. K. and Kim, C. Y. (1997) MK-801 augments pilocarpine-induced electrographic seizure but protects against brain damage in rats. Prog. Neuro-Psychopharmac. Psychiat. 21, 331±344. Lees, G. J. (1991) Inhibition of sodium-potassium-ATPase: a potentially ubiquitous mechanism contributing to central nervous system neuropathology. Brain Res. Rev. 16, 283±300. Lees, G. J. (1993) The possible contribution of microglia and macrophages to delayed neuronal death after ischemia. J. Neurol. Sci. 114, 119±122. Le Moine, C., Svenningsson, P., Fredholm, B. and Bloch, B. (1997) Dopamine-adenosine interactions in the striatum and globus pallidus: inhibition of striatopallidal neurons through either D2 or A2a receptors enhances D1 receptor-mediated e€ects on c-fos expression. J. Neurosci. 17, 8038±8048. Li, S. H., Schilling, G., Young, W. S., Li, X. J., Margolis, R. L., Stine, O. C., Wagster, M. V., Abbott, M. H., Franz, M. L., Ranen, N. G., Folstein, S. E., Hedreen, J. C. and Ross, C. A. (1993) Huntingtons-disease gene (IT-15) is widely expressed in human and rat-tissues. Neuron 11, 985±993. Li, X. J., Li, S. H., Sharp, A. H., Nucifora, F. C., Schilling, G., Lanahan, A., Worley, P., Snyder, S. H. and Ross, C. A. (1995) A huntington-associated protein enriched in brain with implications for pathology. Nature 378, 398±402. Lillrank, S. M., O'Connor, W. T., Saransaari, P. and Ungerstedt, U. (1994) In vivo e€ects of local and systemic phencyclidine on the extracellular levels of catecholamines and transmitter amino acids in the dorsolateral striatum of anaesthetized rats. Acta. Physiol. Scand. 150, 109±115. Lindholm, D. (1994) Role of neurotrophins in preventing glutamate induced neuronal cell death. J. Neurol. 241, S16±S18. Linnik, M. D., Miller, J. A., Sprinkle-Caballo, J., Mason, P. J., Thompson, F. Y., Montgomery, L. R. and Schroeder, K. K. (1995) Apoptotic DNA fragmentation in the rat cerebral cortex induced by permanent middle cerebral artery occlusion. Mol. Brain Res. 32, 116±124. Liste, I., Guerra, M. J., Caruncho, H. J. and Labandeira-Garcia, J. L. (1997) Treadmill running induces striatal Fos expression via NMDA glutamate and dopamine receptors. Exp. Brain Res. 115, 458±469. Liu, J. L., Solway, K., Messing, R. O. and Sharp, F. R. (1998) Increased neurogenesis in the dentate gyrus after transient global ischemia in gerbils. J. Neurosci. 18, 7768±7778. Macdonald, V., Halliday, G. M., Trent, R. J. and McCusker, E. A. (1997) Signi®cant loss of pyramidal neurons in the angular gyrus of patients with Huntington's disease. Neuropathol. Appl. Neurobiol. 23, 492±495. Macmanus, J. P., Buchan, A. M., Hill, I. E., Rasquinha, I. and Preston, E. (1993) Global ischemia can cause DNA fragmentation indicative of apoptosis in rat brain. Neurosci. Lett. 164, 89± 92. Macmanus, J. P., Hill, I. E., Huang, Z. G., Rasquinha, I., Xue, D. and Buchan, A. M. (1994) DNA damage consistent with apoptosis in transient focal ischemic neocortex. Neuroreport 5, 493± 496. Malcon, C., Achaval, M., Komlos, F., Partata, W., Sauressig, M., Ramirez, G. and Souza, D. O. (1997) GMP protects against quinolinic acid induced loss of NADPH-diaphorase positive cells in the rat striatum. Neurosci. Lett. 225, 145±148. Mangiarini, L., Sathasivam, K., Seller, M., Cozens, B., Harper, A., Hetherington, C., Lawton, M., Trottier, Y., Lehrach, H., Davies, S. W. and Bates, G. P. (1996) Exon 1 of the HD gene with an expanded CAG repeat is sucient to cause a progressive neurological phenotype in transgenic mice. Cell 87, 493±506. Martinez-Serrano, A. and Bjorklund, A. (1996) Protection of the neostriatum against excitotoxic damage by neurotrophin-producing, genetically modi®ed neural stem cells. J. Neurosci. 16, 4604±4616.

717

Martinou, J. C., Dubois-Dauphin, M., Staple, J. K., Rodriguez, I., Frankowski, H., Missotten, M., Albertini, P., Talabot, D., Catsicas, S., Pietra, C. and Huarte, J. (1994) Over expression of bcl-2 in transgenic mice protects neurons from naturally occurring cell death and experimental ischemia. Neuron 13, 1017± 1030. Mathe, J. M., Nomikos, G. G., Schilstrom, B. and Svensson, T. H. (1998) Non-NMDA excitatory amino acid receptors in the ventral tegmental area mediate systemic dizocilpine (MK-801) induced hyperlocomotion and dopamine release in the nucleus accumbens. J. Neurosci. Res. 51, 583±592. McGeorge, A. J. and Faull, R. L. M. (1989) The organisation of the projection from the cortex to the striatum in the rat. Neuroscience 29, 503±537. Meldrum, B. and Garthwaite J. (1991) Excitatory amino acid neurotoxicity and neurodegenerative disease. TIPS Special Report 11, 54±61. Miller, D. B. and O'Callaghan, J. P. (1995) The role of temperature, stress, and other factors in the neurotoxicity of the substituted amphetamines 3,4-methylenedioxymethamphetamine and fen¯uramine. Mol. Neurobiol. 11, 177±192. Missale, C., Nash, S. R., Robinson, S. W., Jaber, M. and Caron, M. G. (1998) Dopamine receptors: from structure to function. Physiol. Rev. 78, 189±225. Mitchell, I. J. (1990) Striatal outputs and dyskinesia. J. Psychopharmacol. 4, 188±197. Mitchell, I. J., Sambrook, M. A. and Crossman, A. R. (1985a) Subcortical changes in the regional uptake of [3H]-2-deoxyglucose in the brain of the monkey during experimental choreiform dyskinesia elicited by injection of a gamma-aminobutyric acid antagonist into the subthalamic nucleus. Brain 108, 405±422. Mitchell, I. J., Jackson, A., Sambrook, M. A. and Crossman, A. R. (1985b) Common neural mechanisms in experimental chorea and hemiballismus in the monkey. Evidence from 2-deoxyglucose autoradiography. Brain Res. 339, 346±350. Mitchell, I. J., Jackson, A., Sambrook, M. A. and Crossman, A. R. (1989a) The role of the subthalamic nucleus in experimental chorea: evidence from 2-deoxyglucose metabolic studies and horseradish peroxidase tracing studies. Brain 112, 1533±1548. Mitchell, I. J., Clarke, C. E., Boyce, S., Robertson, R. G., Peggs, D., Sambrook, M. A. and Crossman, A. R. (1989b) Neural mechanisms underlying parkinsonian symptoms based upon regional uptake of 2-deoxyglucose in monkeys exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Neuroscience 32, 213±226. Mitchell, I. J., Boyce, S., Sambrook, M. A. and Crossman, A. R. (1992) A 2-deoxyglucose study of the e€ects of dopamine agonists on the parkinsonian primate brain: implications for the neural mechanisms that mediate dopamine agonist-induced dyskinesia. Brain 115, 809±824. Mitchell, I. J., Lawson, S., Moser, B., Laidlaw, S. M., Cooper, A. J., Walkinshaw, G. and Waters, C. M. (1994) Glutamateinduced apoptosis results in a loss of striatal neurons in the Parkinsonian rat. Neuroscience 63, 1±5. Mitchell, I. J., Cooper, A. J., Griths, M. R. and Barber, D. J. (1998) Phencyclidine and corticosteroids induce apoptosis of a subpopulation of striatal neurons: a neural substrate for psychosis? Neuroscience 84, 489±501. Mogenson, G. J., Jones, D. L. and Yim, C. (1980) From motivation to action: functional interface between the limbic system and motor system. Prog. Neurobiol. 14, 69±97. Nakki, R., Sharp, F. R. and Sagar, S. M. (1996a) Fos expression in the brainstem and cerebellum following phencyclidine and MK801. J. Neurosci. Res. 43, 203±212. Nakki, R., Sharp, F. R., Sagar, S. M. and Honkaniemi, J. (1996b) E€ects of phencyclidine on immediate early gene expression in the brain. J. Neurosci. Res. 45, 13±27. Nauta, W. J. H. (1979) A proposed conceptual reorganization of the basal ganglia and telencephalon. Neuroscience 20, 1875± 1881. Nauta, W. J. H. and Domesick, V. B. (1984) A€erent and e€erent relationships of the basal ganglia. In: Functions of the Basal Ganglia, Ciba Foundation Symposium, Vol. 107, pp. 3±23. Eds D. Evered and M. O'Conner. Pitman, London. Nauta, W. J. H. and Mehler, W. R. (1966) Projections of the lentiform nucleus in the monkey. Brain Res. 1, 3±42. Nicholson, L. F. B., Faull, R. L. M., Waldvogel, H. J. and Dragunow, M. (1995) GABA and GABAA receptor changes in the substantia nigra of the rat following quinolinic acid lesions in the striatum closely resemble Huntington's disease. Neuroscience 66, 507±521.

718

I. J. Mitchell et al.

Nishino, H., Fujimoto, I., Shimano, Y., Hida, H., Kumazaki, M. and Fukuda, A. (1996) 3-Nitropropionic acid produces striatum selective lesions accompanied by iNOS expression. J. Chem. Neuroanat. 10, 209±212. Oades, R. D. and Halliday, G. M. (1987) Vental tegmental (A10) system: neurobiology. 1. Anatomy and connectivity. Brain Res. Rev. 12, 117±165. O€en, D., Ziv, I., Sternin, H., Melamed, E. and Hochman, A. (1996) Prevention of dopamine induced cell death by thiol antioxidants: possible implications for treatment of Parkinson's disease. Exp. Neurol. 141, 32±39. Olney, J. W. and Farber, N. B. (1994) Ecacy of clozapine compared with other antipsychotics in preventing NMDA-antagonist neurotoxicity. J. Clin. Psychiat. 55, 43±46. Olney, J. W. and Farber, N. B. (1995a) Response to commentaries and to the challenge of building a perfect theory to explain schizophrenia. Arch. Gen. Psychiat. 52, 1019±1024. Olney, J. W. and Farber, N. B. (1995b) NMDA antagonists as neurotherapeutic drugs., psychotogens, neurotoxins, and research tools for studying schizophrenia. Neuropsychopharmacology 113, 335±345. Olney, J. W., Price, M. T., Fuller, T. A., Labruyere, J., Sampson, L., Carpenter, M. and Mahan, K. (1986) The anti-excitotoxic e€ects of certain anesthetics., analgesics and sedative-hypnotics. Neurosci. Lett. 68, 29±34. Olney, J. W., Labruyere, J. and Price, M. T. (1989) Pathological changes induced in cerebrocortical neurons by phencyclidine and related drugs. Science 244, 1360±1362. Olney, J. W., Labruyere, J., Wang, G., Wozinak, D. F., Price, M. T. and Sesma, M. A. (1991) NMDA antagonist neurotoxicity: mechanism and prevention. Science 254, 1515±1518. Ordway, J. M., Tallaksen-Greene, S., Gutekunst, C. A., Bernstein, E. M., Cearley, J. A., Wiener, H. W., Dure, L. S., Lindsey, R., Hersch, S. M., Jope, R. S., Albin, R. L. and Detlo€, P. J. (1997) Ectopically expressed CAG repeats cause intranuclear inclusions and a progressive late onset neurological phenotype in the mouse. Cell 91, 753±763. Overton, P. G., Tong, Z. Y., Brain, P. F. and Clark, D. (1996) Preferential occupation of mineralocorticoid receptors by corticosterone enhances glutamate-induced burst ®ring in rat midbrain dopaminergic neurons. Brain Res. 737, 146±154. Pal®, S., Ferrante, R. J., Brouillet, E., Beal, M. F., Dolan, R., Guyot, M. C., Perchanski, M. and Hantraye, P. (1996) Chronic 3-nitropropionic acid treatment in baboons replicates the cognitive and motor de®cits of Huntington's disease. J. Neurosci. 16, 3019±3025. Parent, A. (1986) Comparative Neurobiology of the Basal Ganglia. Wiley. Parent, A., Bouchard, C. and Smith, Y. (1984) The striatopallidal and striatonigral projections: two distinct ®bres systems in primate. Brain Res. 303, 385±390. Pasinetti, G. M., Morgan, D. G. and Finch, C. E. (1991) Disappearance of GAD mRNA and tyrosine hydroxylase in substantia nigra following striatal ibotenic acid lesions. Exp. Neurol. 112, 131±139. Paulson, H. L. and Fischbeck, K. H. (1996) Trinucleotide repeats in neurogenetic disorders. Ann. Rev. Neurosci. 19, 79±107. Pearson, S. J., Heath®eld, K. W. G. and Reynolds, G. P. (1990) Pallidal GABA and chorea in Huntington's disease. J. Neural Transm. [GenSect.] 81, 241±246. Penney, J. B. and Young, A. B. (1986) Striatal inhomogeneities and basal ganglia function. Movement Disorders 1, 3±15. Penney, J. B., Vonsattel, J. P., MacDonald, M. E., Gusella, J. F. and Myres, R. H. (1997) CAG repeat number governs the development rate of pathology in Huntington's disease. Ann. Neurol. 41, 689±692. Penny, G. R., Wilson, C. J. and Kitai, S. T. (1988) Relationship of the axonal and dendritic geometry of spiny projection neurons to the compartmental organization of the neostriatum. J. Comp. Neurol. 269, 273±289. Piazza, P. V. and Le Moal, N. (1997) Glucocorticoids as a biological substrate of reward: physiological and pathophysiological implications. Brain Res. Rev. 25, 359±372. Piazza, P. V., RougePont, F., DeRoche, V., Maccari, S., Simon, S. and Le Moal, M. (1996) Glucocorticoids have state dependent stimulant e€ects on the mesencephalic dopaminergic transmission. Proc. natl Acad. Sci. USA 93, 8716±8720. Portera-Cailliau, C., Hedreen, J. C., Price, D. L. and Koliatsos, V. E. (1995) Evidence for apoptotic cell death in Huntington's diseae and excitotoxic animal models. J. Neurosci. 15, 3775±3787. Pringle, A. K., Iannotti, F., Wilde, G. J. C., Chad, J. E., Seeley, P. J. and Sundstrom, L. E. (1997) Neuroprotection by both

NMDA and non-NMDA receptor antagonists in in vitro ischemia. Brain Res. 755, 36B46. Purcell, R., Maru€, P., Kyrios, M. and Pantelis, C. (1998) Cognitive de®cits in obsessive-compulsive disorder on tests of frontal-striatal function. Biol. Psychiat. 43, 348±357. Ra€, M. C., Barres, B. A., Burne, J. F., Coles, H. S., Ishizaki, Y. and Jacobson, M. D. (1993) Programmed cell death and the control of cell survival: lesson from the nervous system. Science 262, 695±700. Raja, S. N. and Guyenet, P. G. (1980) E€ects of phencyclidine on the spontaneous activity of monoaminergic neurons. Eur. J. Pharmac. 63, 229±233. Rajdev, S., Fix, A. S. and Sharp, F. R. (1998) Acute phencyclidine neurotoxicity in rat forebrain: induction of haem oxygenase-1 and attenuation by the antioxidant dimethylthiourea. Eur. J. Neurosci. 10, 3840±3852. Ray, D. E., Brown, A. W., Cavanagh, J. B., Nolan, C. C., Richards, H. K. and Wylie, S. P. (1992) Functional/metabolic modulation of the brainstem lesions caused by 1,3-dinitrobenzene in the rat. Neurotoxicology 13, 379±388. Reddy, P. H., Williams, M., Charles, V., Garrett, L., PikeBuchanan, L., Whetsell, W. O., Miller, G. and Tagle, D. A. (1998) Behavioural abnormalities and selective neuronal loss in HD transgenic mice expressing mutated full-length HD cDNA. Nature Genet. 20, 198±202. Reiner, A., Albin, R. L., Anderson, K. D., Damato, C. J., Penney, J. B. and Young, A. B. (1988) Di€erential loss of striatal projection neurons in Huntington's disease. Proc. natl Acad. Sci. USA 85, 5733±5737. Reynolds, D. S., Carter, R. J. and Morton, A. J. (1998) Dopamine modulates the susceptibility of striatal neurons to 3-nitropropionic acid in the rat model of Huntington's disease. J. Neurosci. 18, 10116±10127. Rich®eld, E. K., Maguire-Ziess, K. A., Vonkeman, H. E. and Voorn, P. (1995) Preferential loss of preproenkephalin verses preprotachykinin neurons form the striatum of Huntington's disease patients. Ann. Neurol. 38, 852±861. Robertson, H. A., Paul, M. L., Moratalla, R. and Graybiel, A. M. (1991) Expression of the immediate early gene c-fos in basal ganglia: induction by dopaminergic drugs. Can. J. Neurol. Sci. 18, 380±383. Rosenberg, D. R., Keshavan, M. S., O'Hearn, K. M., Dick, E. L., Bagwell, W. W., Seymour, A. B., Montrose, D. M., Pierri, J. N. and Birmaher, B. (1997) Frontostriatal measurement in treatment-naive children with obsessive-compulsive disorder. Arch. Gen. Psychiat. 54, 824±830. Saji, M. and Volpe, B. T. (1993) Delayed histologic damage and neuron death in the substantia nigra reticulata following transient forebrain ischemia depends on the extent of initial striatal injury. Neurosci. Lett. 155, 47±50. Salloway, S. and Cummings, J. (1996) Subcortical structures and neuropsychiatric illness. Neuroscientist 2, 66±75. Sanberg, P. R., Calderon, S. F., Giordano, M., Tew, J. M. and Norman, A. B. (1989) The quinolinic acid model of Huntington's diseaseÐlocomotor abnormalities. Exp. Neurol. 105, 45±53. Sapolsky, R. M. (1985) A mechanism for glucocorticoid toxicity in the hippocampus: increased neuronal vulnerability to metabolic insults. J. Neurosci. 5, 1228±1232. Sapolsky, R. M., Krey, L. C. and McEwen, B. S. (1985) Prolonged glucocorticoid exposure reduces hippocampal neuron number: implications for aging. J. Neurosci. 5, 1222±1227. Sato, I., Kim, Y., Himi, T. and Murota, S. (1995) Induction of calcium independent nitric oxide synthase activity in cultured cerebellar granule neurons. Neurosci. Lett. 184, 145±148. Sato, D., Umino, A., Kaneda, K., Takigawa, M. and Nishikawa, T. (1997) Developmental changes in distribution patterns of phencyclidine-induced c-fos in rat forebrain. Neurosci. Lett. 239, 21±24. Scherzinger, E., Lurz, R., Turmaine, M., Mangiarini, L., Hollenbach, B., Hasenbank, R., Bates, G. P., Davies, S. W., Lehrach, H. and Wanker, E. E. (1997) Huntingtin-encoded polyglutamine expansions form amyloid-like protein aggregates in vitro and in vivo. Cell 90, 549±558. Schreiber, S. S. and Baudry, M. (1995) Selective neuronal vulnerability in the hippocampusÐa role for gene expression? TINS 18, 446±450. Schultz, W., Apicella, P., Scarnatti, E. and Ljungberg, T. (1992) Neuronal activity in monkey ventral striatum related to the expectation of reward. J. Neurosci. 12, 4595±4610. Schultz, W., Romo, R., Ljungberg, T., Mirenowicz, J., Hollerman, J. R. and Dickinson, A. (1995) Reward-related signals carried

The Selective Vulnerability of Striatopallidal Neurons by dopamine neurons. In: Models of Information Processing in the Basal Ganglia, pp. 233±248. Eds J. C. Houk, J. L. Davis and D. G. Beiser. MIT Press, Massachusetts. Seckl, J. R. (1997) 11-b Hydroxysteroid dehydrogenase in the brain: a novel regulator of glucocorticoid action? Frontiers Neuroendocrinol. 18, 49±99. Selemon, L. D. and Goldman-Rakic, P. S. (1985) Longitudinal topography and interdigitation of corticostriatal projections in the rhesus monkey. J. Neurosci. 5, 776±794. Servan-Schreiber, D., Printz, H. W. and Cohen, J. D. (1990) A network model of catecholamine e€ects: gain, signal to noise ratio and behaviour. Science 249, 892±895. Sloviter, R. S., Valiquette, G., Abrams, G. M., Ronk, E. C., Sollas, A. L., Paul, L. A. and Neubort, S. (1989) Selective loss of hippocampal cells in the mature rat brain after adrenalectomy. Science 243, 535±538. Smeyne, R. J., Vendrell, M., Hayward, M., Baker, S. J., Miao, G. C., Schilling, K., Robertson, L. M., Curran, T. and Morgan, J. I. (1993) Continuous c-fos expression preceeds programmed cell death in vivo. Nature 363, 166±169. Smith, A. D. and Bolam, J. P. (1990) The neural network of the basal ganglia as revealed by the study of synaptic connections of identi®ed neurons. TINS 13, 259±265. Smith, Y., Bevan, M. D., Shink, E. and Bolam, J. P. (1998) Microcircuitry of the direct and indirect pathways of the basal ganglia. Neuroscience 86, 353±387. Sonsalla, P. K., Nicklas, W. J. and Hiekkila, R. E. (1989) Role for the excitatory amino acids in methamphetamine-induced nigrostriatal dopaminergic toxicity. Science 243, 398±400. Spokes, E. G. S. (1980) Neurochemical alterations in Huntington's disease. Brain 103, 179±210. Sprague, J. E. and Nichols, D. E. (1995) Inhibition of MAO-B protects against MDMA-induced neurotoxicity in the striatum. Psychopharmacology 118, 357±359. Strauss, K. I., Schulkin, J. and Jacobowitz, D. M. (1995) Corticosterone e€ects on rat calretinin mRNA in discrete brain nuclei and the testes. Mol. Brain Res. 28, 81±86. Strijbos, P. J. L. M. (1998) Nitric oxide in cerebral ischemia neurodegeneration and excitotoxicity. Crit. Rev. Neurobiol. 12, 223± 243. Strong, T. V., Tagle, D. A., Valdes, J. M., Elmer, L. W., Boehm, K., Swaroop, M., Kaatz, K. W., Collins, F. S. and Albin, R. L. (1993) Widespread expression of the human and rat Huntingtons-disease gene in brain and nonneural tissues. Nature Genet. 5, 259±265. Swerdlow, N. R., Paulsen, J., Bra€, D. L., Butters, N., Geyer, M. A. and Swenson, M. R. (1995) Impaired prepulse inhibition of acoustic and tactile startle response in patients with Huntingtons-disease. J. Neurol. Neurosurg. Psychiat. 58, 192± 200. Szabo, C. (1996) Physiological and pathophysiological roles of nitric oxide in the central nervous system. Brain Res. Bull. 3, 131±141. Tamminga, C. A., Holcomb, H. H., Gao, X. M. and Lahti, A. C. (1980) Glutamate pharmacology and the treatment of schizophreniaÐcurrent status and future-directions. Int. Clin. Psychopharmac. 10, 29±37. Tanaka, J., Fujita, H., Matsuda, S., Toku, K., Sakanaka, M. and Maeda, N. (1997) Glucocorticoid- and mineralocorticoid receptors in microglial cells: the two receptors mediate di€erential e€ects of corticosteroids. Glia 20, 23±37. Taylor-Robinson, S. D., Weeks, R. A., Bryant, D. J., Sargentoni, J., Marcus, C. D., Harding, A. E. and Brooks, D. J. (1996) Proton magnetic resonance spectroscopy in Huntington's disease: evidence in favour of the glutamate excitotoxic theory? Movement Dis. 11, 167±173. Thomas, L. B., Gates, D. J., Rich®eld, E. K., Obrien, T. F., Schweitzer, J. B. and Steindler, D. A. (1995) DNA end labeling (tunel) in Huntington's disease and other neuropathological conditions. Exp. Neurol. 133, 265±272. Thompson, K. and Wasterlain, C. (1997) Partial protection of hippocampal neurons by MK-801 during perforant path stimulation in the immature brain. Brain Res. 755, 96±101. Tidey, J. W. and Miczek, K. A. (1996) Social defeat stress selectively alters mesocorticolimbic dopamine release: an in vivo microdialysis study. Brain Res. 721, 140±149. Trottier, Y., Devys, D., Imbert, G., Saudou, F., An, I., Lutz, Y., Weber, C., Agid, Y., Hirsch, E. C. and Mandel, J. L. (1995) Cellular-localization of the Huntington's disease protein and discrimination of the normal and mutated form. Nature Genet. 10, 104±110.

719

Uemura, Y., Kowall, N. W. and Beal, M. F. (1990) Selective sparing of NADPH-diaphorase-somatostatin-neuropeptide Y neurons in ischemic gerbil striatum. Ann. Neurol. 27, 620±625. Uno, H., Eisele, S., Sakai, A., Shelton, S., Baker, E., Dejesus, O. and Holden, J. (1994) Neurotoxicity of glucocorticoids in the primate brain. Hormones Behav. 28, 336±348. Vaidya, V. A., Marek, G. J., Aghajanian, G. K. and Duman, R. S. (1997) 5-HT2A receptor-mediated regulation of brain-derived neurotrophic factor mRNA in the hippocampus and the neocortex. J. Neurosci. 17, 2785±2795. Van LookerenCampagne, M. and Gill, R. (1996) Ultrastructural morphological changes are not characteristic of apoptotic cell death following cerebral ischemia in the rat. Neurosci. Lett. 213, 111±114. Venero, J. L., Santiago, M., Machado, A. and Cano, J. (1996) MK-801 partially protects against the acute MPP+ depleting e€ect on dopamine levels in rat striatal slices. Neurochem. Int. 29, 411±416. Vickroy, T. W. and Johnson, K. M. (1982) Similar dopaminereleasing e€ects of phencyclidine and nonamphetamine stimulants in striatal slices. J. Pharmac. exp. Ther. 223, 669±674. Vignon, J., Pinek, V., Cerruti, C., Kamenda, J. and Chicheportiche, R. (1988) [3H]-N-(1-(2-benzo-(b)-thiophenyl)cyclohexyl)piperidine ([3H]BTCP): a new phencyclidine analogue selective for the dopamine uptake complex. Eur. J. Pharmac. 148, 427±436. Vonsattel, J. P., Myers, R. H., Stevens, T. J., Ferrante, R. J., Bird, E. D. and Richardson, E. (1985) Neuropathological classi®cation of Huntington's disease. J. Neuropath. exp. Neurol. 44, 559±577. Vreugdenhil, E., DeJong, J., Schaaf, M. J. M., Meijer, O. C., Busscher, J., Vuijst, C. and DeKloet, E. R. (1996) Molecular dissection of corticosteroid action in the rat hippocampus. J. Molec. Neurosci. 7, 135±146. Waldvogel, H. J., Faull, R. L. M., Williams, M. N. and Dragunow, M. (1991) Di€erential sensitivity of calbindin and parvalbumin immunoreactive cells in the striatum to excitotoxins. Brain Res. 546, 329±335. Walkinshaw, G. and Waters, C. M. (1995) Induction of apoptosis in catecholaminergic PC-12 cells by L-DOPA. Implications for the treatment of Parkinson's disease. J. Clin. Invest. 95, 2458± 2464. Wang, S., Mason, J., Charney, D., Yehuda, R., Riney, S. and Southwick, S. (1997) Relationship between hormonal pro®le and novelty seeking in combat-related posttraumatic stress disorder. Biol. Psychiat. 41, 145±151. Weiss, J. H., Hartley, D. M., Koh, J. and Choi, D. W. (1990) The calcium channel blocker nifedipine attenuates slow excitatory amino acid neurotoxicity. Science 247, 1474±1477. White, N. and Hiroi, N. (1998) Preferential localization of selfstimulation sites in striosomes/patches in the rat striatum. Proc. natl Acad. Sci. USA 95, 6486±6491. Wirtshafter, D. and Asin, K. E. (1994) Interactive e€ects of stimulation of D-1 and D-2 dopamine-receptors on fos-like immunoreactivity in the normosensitive rat striatum. Brain Res. Bull. 35, 85±91. Wise, R. A., Leone, P., Rivest, R. and Leeb, K. (1995) Elevations of nucleus accumbens dopamine and DOPAC levels during intravenous heroin self-administration. Synapse 21, 140±148. Woolley, C. S., Gould, E. and McEwen, B. S. (1990) Exposure to excess glucocorticoids alters dendritic morphology of adult hippocampal pyramidal neurons. Brain Res. 531, 225±231. Wozniak, D. F., Brosnan-Watters, G., Nardi, A., McEwen, M., Corso, T. D., Olney, J. W. and Fix, A. S. (1996) MK-801 neurotoxicity in male mice: histological e€ects and chronic impairment in spatial learning. Brain Res. 707, 165±179. Zeitlin, S., Liu, J. P., Chapman, D. L., Papaioannou, V. E. and Efstratiadis, A. (1995) Increased apoptosis and early embryonic lethality in mice. Nature Genet. 11, 155±163. Zhang, X., Chiodo, L. A. and Freeman, A. S. (1992) Electrophysiological e€ects of MK-801 on rat nigrostriatal and mesoaccumbal dopaminergic neurons. Brain Res. 590, 153±163. Zhang, X., Boulton, A. A. and Yu, P. H. (1996a) Expression of heat shock protein-70 and limbic seizure-induced neuronal death in the rat brain. Eur. J. Neurosci. 8, 1432±1440. Zhang, X., Boulton, A. A., Zuo, D. M. and Yu, P. H. (1996b) MK-801 induces apoptotic neuronal death in the rat retrosplenial cortex: prevention by cycloheximide and R(ÿ)-2-hexyl-Nmethylpropargylamine. J. Neurosci. Res. 46, 82±89.