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Catecholamines in the Central Nervous System
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CATECHOLAMINES IN T H E CENTRAL NERVOUS SYSTEM
A. A. Grace,* C. R. Gerfen,t and G. Aston-Jones$ *Departments of Neuroscience and Psychiatry University of Pittsburgh Pittsburgh, Pennsylvania I5260 tLaboratory of Systems Neuroscience National institute of Mental Health Bethesda, Maryland 20892
*Department of Psychiatry Allegheny University Philadelphia, Pennsylvania I 9 I02
Overview Studies concentrating on catecholaminergic systems in the central nervous system (CNS) have initially lagged far behind the more aggressive approaches used to examine their function in the periphery. However, the preponderance of studies over the past several decades demonstrating a role for these neurotransmitter systems in neurological and psychiatric disorders has been a driving force for studies focusing o n central aspects of catecholaminergic function. Indeed, it is becoming increasingly clear that dopamine, norepinephrine (NE), and epinephrine exhibit essential roles in the regulation and coordination of activity among widespread central systems. A multitude of methodological approaches was applied to study a comparatively diverse collection of topic materials; nonetheless, there was a remarkable degree of convergence among the Advances 111 Pharmacology. Volume 42
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experimental findings and conceptual models advanced. The papers presented in this part generally could be divided along two foci: the anatomy, pharmacology, and molecular regulation of catecholamines in the normal brain; and how these relationships may be altered in disease states.
1. Molecular Regulation of Dopaminergic Responses An effective approach for studying the functional role of dopamine in the basal ganglia has been to measure changes in the expression levels of various genes and their products in the basal ganglia following pharmacological manipulations. Gerfen and his colleagues (p. 670) describe studies of dopamine regulation of striatal output neurons, which follow from their demonstration of the segregation of D1 and D2 dopamine receptor subtypes, respectively, to the so-called directstriatonigral and indirect-striatopallidal striatal projection neurons. One issue surrounding the segregation of these receptor subtypes stems from models that synergy between the subtypes requires colocalization. To address this issue, single-cell measurements were made of changes in the immediate early gene zif 268, which has a constitutive level of expression and thus allows both increases and decreases in expression levels to be examined in identified direct ( D l ) or indirect (D2) striatal projection neurons in animals treated with a D1-receptor agonist alone or in combination with D2 agonists. Results demonstrated that combined treatment with D1 and D2 agonists results in a selective increase in gene regulation in D1-expressing “direct” projection neurons and decreased gene regulation in D2-expressing “indirect” projection neurons. Thus, the synergistic interaction between these receptor subtypes appears to occur through intercellular interactions between different neurons, rather than through intracellular interactions between receptors expressed by the same neurons. While a variety of generegulation effects are seen by manipulation of dopaminergic neurotransmission, the functional significance of these effects remains for the most part obscure. Work by Steiner and Gerfen was described, which has shown that elevated expression of the opiate neuropeptide dynorphin following repeated overstimulation of D 1 receptors (e.g., by cocaine)is an adaptive response of these neurons to blunt the effects of such overstimulation. While there are many effects of dopamine receptor-mediated action within the striatum, other parts of the basal ganglia circuitry are necessarily involved in the behavioral consequences of dopaminergic system manipulation. Chesselet and her coworkers (p. 674) describe studies in which such extrastriatal basal ganglia effects are examined by measurement of changes in GAD 67 mRNA in basal ganglia nuclei downstream from the striatum. Although it might be anticipated that dopamine-mediated changes in the output of the striatum would be reflected in changes in the target nuclei of that output, the direction of such changes does not always appear to follow from the expected changes in activity levels of the neurons. For example, elevated markers in striatopallidal neurons resulting from striatal dopamine depletion or neuroleptic treatment resulted in increased GAD 67 mRNA levels in globus pallidus neurons, which may be considered surprising because increased striatopallidal output would
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be expected to decrease activity in such pallidal neurons. A number of possible mechanisms are examined that might account for these results. Rather than simply altering the level of activity in pallidal neurons, changes in striatal outputs might alter the pattern of activity, which might result from altered membrane properties. Along these lines, results are described in which altered dopaminergic neurotransmission results in altered expression of the Kv3.1 “shaw-like” potassium channel. Moreover, nuclei that are connected with, but not necessarily considered part of, the basal ganglia are shown to display alterations in gene expression and are examined in dopaminergic manipulation paradigms. One result of interest is that of a change in the level of GAD 67 in the thalamic reticular nucleus. Such results are discussed in terms of the circuitry and mechanisms through which dopamine exerts effects on behavior. Marshall and his colleagues (p. 678) describe studies from their laboratory that have examined D1 and D2 dopamine receptor regulation of striatal output neurons in paradigms utilizing psychostimulant drugs such as amphetamine. They provide a number of examples of an obligatory dependence on D2 receptor mechanisms of D1-receptor-mediated induction of Fos in striatonigral neurons. Such studies point out the importance of D1- and D2-receptor interactions in the effect of dopamine in the striatum, particularly in the normal striatum in animals treated with psychostimulant drugs. An additional set of studies is described in which the effects of alteration of the striatopallidal pathways on the target neurons in the globus pallidus are examined. The most interesting result from these studies is that distinct populations of pallidal neurons, characterized on the basis of their expression of the calcium-binding protein parvalbumin and on their axonal projections, display different immediate early gene responses to dopamine agonists or antagonists. Liu and Graybiel (p. 682) describe work from their laboratory that examined dopamine- and calcium-signaling regulation of cyclic response elementbinding (CREB) phosphorylation in striatal neurons. A developmental model was described in which organotypic cultures of the striatum were used as a substrate for dopaminergic- and calcium-signaling pharmacological treatment paradigms and in which the measured effect is that of phosphorylation of CREB. One of the intriguing results described was the difference in the kinetics of phosphorylation of CREB (pCREB) in different striatal compartments. Whereas D1 dopamine receptor agonist treatment for a short period (7 min) resulted in striosomal-patch and matrix pCREB, sustained agonist incubation resulted in only striosomal-patch labeling. A different pattern of pCREB kinetics was observed when an L-type calcium channel activator (BAY K 8644) was used. Similar to the D1 agonist, short-term treatment resulted in pCREB in neurons of both compartments; however, sustained treatment resulted in labeling primarily in the matrix. These results are discussed in terms of the convergence of different signaling pathways through CREB for gene regulation in striatal neurons.
II. Physiology of Dopaminergic Neurons Studies were presented of the neurophysiological activity of dopaminergic neurons of the midbrain spanning the environmental and behavioral contexts
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that are represented in the activity of these neurons and the afferent glutamatergic, cholinergic, and GABAergic receptor-mediated membrane mechanisms responsible for such activity. Midbrain dopaminergic neurons in the substantia nigra and ventral tegmental area are generally accepted as playing a role in the motivational control of voluntary behavior. Schultz describes his group’s work, which has helped define that role. Using the powerful approach of single-unit recording in awake behaving primates, Schultz (p. 686) has examined the relationship between the reward value of specific behavior-evoking stimuli and dopamine neuron activity. While most dopamine neurons respond to primary appetitive stimuli, it is the relationship between such reward and stimulus parameters that provides the most interesting insight into the function of dopamine neuronal activity. Among the observations that Schultz provides is the relationship between the novelty of a stimulus and dopamine neuron activity. Once a reward becomes fully predictable, responses of dopamine neurons diminish. However, this aspect of the novelty, or unpredictability, in the relationship between a stimulus and associated reward has an important corollary function in that it allows for the representation of a reward to be transferred to secondary, intrinsically neutral conditional stimuli. Such determinants of dopamine neuron activity are suggested by Schultz to indicate that dopamine neuron activity codes errors in reward prediction as an appropriate signal for appetitive learning. In a series of three chapters, Johnson (p. 691), Tepper et al. (p. 694), and Kitai (p. 700) describe membrane physiological properties underlying glutamatergic, GABAergic, and cholinergic afferent synaptic regulation of dopamine neuron activity. Johnson has used a slice preparation to study the effects of synaptic inputs, membrane potentials, or currents on dopamine neurons using intracellular recording from a slice preparation. Results demonstrate that in dopamine neurons, both N-methy-D-aspartate (NMDA) and non-NMDA glutamatergic inputs evoke fast excitatory synaptic currents, whereas metabotropic glutamatergic receptors mediate slow-onset long-duration responses. GABAergic synaptic inputs evoke inhibitory responses, producing fast and slow IPSPs through GABAAand GABABreceptors, respectively. Tepper and his colleagues expand on the differential actions of GABAergic synaptic input through the Aand B-receptor subtypes on dopamine neuron activity. Their studies demonstrate that GABAergic inputs are provided to dopamine neurons through multiple sources, from the striatum, from the globus pallidus, and from the GABAergic neurons in the substantia nigra pars reticulata. GABAA- and GABAB-receptor subtypes appear to be involved not only in synaptic input from each of these sources, but also in regulating release through presynaptic mechanisms. Importantly, the two receptor subtypes appear to have distinct roles in determining transitions between burst and pacemaker patterns of activity: GABAA antagonists increase burstiness, whereas GABABantagonists decrease burstiness and increase pacemaker activity. Examining the circuits involved in regulating afferent GABAergic regulation of dopamine neuron activity, Tepper and his group determined that altered patterns of pallidal output affect, at least in part, dopamine neuron activity through multisynaptic mechanisms involving the GABAergic neurons of the substantia nigra pars reticulata, which themselves provide inhibition to the dopamine neurons. In addition to the GABAergic
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input are other afferents thought to be responsible for excitatory responses of midbrain dopamine neurons. Kitai describes work in his laboratory that has examined two such inputs: glutamatergic inputs from the subthalamic nucleus and cholinergic input from the pedunculopontine nucleus. Kitai and his colleagues have pioneered work examining the importance of glutamatergic input to both GABAergic and dopaminergic neurons of the substantia nigra. Such input is thought to be responsible, in large measure, for the tonic activity of these neurons. In this chapter, Kitai focuses on the contribution of such glutamatergic input to the dopamine neurons and on studies that examine the interaction of cholinergic receptor-mediated mechanisms on such excitatory input. Conclusions drawn from these studies suggest that such glutamatergic-cholinergic interactions may be involved in the modification of dopamine neuron activity patterns from an intrinsic, regular, rhythmic pattern to an irregular, bursty pattern.
111. Catecholamines in the Prefrontal Cortex One focus of recent research efforts delves into the anatomy and pharmacology of neuronal interactions within the prefrontal cortex (PFC). This interest has been driven by recent studies implicating the PFC as an important limbic component related to the pathophysiology of schizophrenia. Details of the anatomy of dopamine input to the frontal cortex are provided in the chapter by Lewis et al. (p. 703).This group showed that the distribution of dopamine in the frontal cortex is dependent on the region of the cortex examined, that dopamine is selectively distributed within different laminae of different frontal cortical regions, and moreover that this innervation exhibits distinct maturational modifications throughout development. Thus, dopamine innervation is densest in the dorsomedial prefrontal cortex, particularly within the superficial and deep layers; in contrast, in area 9, the innervation is trilaminar. Moreover, the type of elements that these axons contact differs depending on the layer of innervation, reinforcing the proposal that dopamine exerts potent direct and indirect actions over the pyramidal cell population. Indeed, Goldman-Rakic (p. 707) suggests that the distinct types of dopamine innervation may play unique roles in the modulation of working memory function within the frontal cortex. She suggests that the direct dopaminergic input onto pyramidal neuron spines and distal dendrites may underlie its ability to intensify and focus inputs related to memory fields, as shown by her work on D1 agonist modulation of this behavior. In contrast, the more pervasive nonsynaptic actions of dopamine on pyramidal neurons may also modulate working memory via a unique interaction with D1 receptors. Finally, the ability of dopamine to indirectly modulate pyramidal cells via a D4-mediated action on GABAergic interneurons may act in concert with pyramidal neurons to hold information in working memory. These relationships may have important functional consequences during development, because Lewis et al. showed that these patterns of innervation reveal unique alterations during development in a region-specific manner. In addition to dopamine’s modulatory action over intrinsic neuronal elements within PFC, Thierry et al. (p. 717) provide evidence that dopamine also
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provides a modulatory influence over long-loop afferents that control activity states within this region. Stimulation of the mediodorsal nucleus was shown to exert both orthodromic and antidromic excitation of PFC neurons. However, it appears that dopamine selectively modulates only the local axon collaterals activated by the antidromic stimulus and, therefore, may enable a focusing of activity within this region by selectively blocking local interactions. Dopamine was also found to exert direct inhibitory actions within the PFC via D1 and D2 receptor stimulation. However, it appears that the D2-mediated response is dependent on intact noradrenergic a1transmission. Indeed, this interdependence of the dopaminergic and noradrenergic systems within the PFC is supported by studies by Tassin (p. 712). In a complex series of studies, he extends this correlation to suggest that even subcortical dopaminergic actions are dependent on intact a-adrenergic activation within the prefrontal cortex. This may occur via a tonic influence of norepinephrine over dopaminergic transmission in the PFC. Furthermore, the presence of this noradrenergic innervation may be essential for the expression of dopamine denervation-induced supersensitivity to D 1stimulation. This noradrenergic-dopaminergic interdependence may provide a cellular basis for the proposal by Breier et d. that clozapine exerts its unique antipsychotic actions via an effect on noradrenergic systems.
IV. Interaction among Glutamatergic Afferents and Dopamine in the Striatum In addition to the effects of dopamine within the frontal cortex, an increasing amount of evidence has implicated the PFC in the regulation of subcortical dopaminergic transmission. In the chapter by Thierry et al., findings are presented that dopamine agonists injected into the PFC exert an opposite action to those injected systemically, and that this appears to be mediated by a D1 action. Furthermore, the effects of prefrontal glutamatergic input onto subcortical structures are also modulated by dopaminergic agonists. Within the nucleus accumbens, Grace et aE. (p. 721) have shown that dopamine acting at very low concentrations exerts a tonic inhibitory influence on PFC afferents. In contrast, in the dorsal striatum, Levine and Cepeda have shown that dopamine exerts a pharmacologically selective modulation of glutamate via distinct postsynaptic mechanisms. Thus, dopamine acting on D1 receptors potentiates postsynaptic glutamate responses mediated via the NMDA receptor, whereas D2 agonists attenuate nonNMDA-mediated glutamatergic excitation of striatal neurons. This was found to be a directly mediated action, because it was also observed in whole-cell clamp studies in the presence of tetrodotoxin. Indeed, Thierry et al. provide evidence that suggests that dopamine can modulate glutamatergic responses within the PFC as well. Konradi (p. 729) provides a molecular mechanism through which such a glutamatergic-dopaminergic interaction may take place. In an elegant series of studies, she shows that c-fos expression mediated by dopamine-receptor stimulation is dependent on functional NMDA receptors and is inhibited by the glutamate antagonist MK 801. This interaction is proposed to occur via a D1-stimulated, cyclic adenosine monophosphate-dependent phosphorylation of CREB that is additive with glutamate-mediated activation of CREB.
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The relevance of dopaminergic-glutamatergicinteraction within subcortical structures is further underscored in two models that examine the functional impact of these systems. Arbuthott et al. attempt to bridge studies examining the electrophysiological actions of dopamine in a model system that also examines ultrastructure. As they point out, it is clear from many studies that dopamine does not exert a simple inhibitory or excitatory action in the striatum. Indeed, studies by Schultz have shown that dopamine acts in reward-related events as a way to increase the probability that an unexpected reward will be generated by increasing the type of behavior that is associated with the rewarding event. Arbuthnott et al. (p. 733)show physiological evidence for such a role, in that repetitive corticostriatal stimulation tends to induce an LTD-like inhibition of responses to subsequent stimuli; however, in the presence of dopamine, repetitive responses are potentiated. Examined at the ultrastructural level, data are presented to suggest that dopamine may be involved in the selective survival of potentiated synaptic connections within this highly integrative structure. Another aspect of dopamine function as it relates to motor systems is examined by Rebec (p. 737), who used the elegant technique of combined extracellular recordings from striatal cells in freely moving animals with microiontophoresis. This approach allows his laboratory to examine dopamine-glutamate interactions at a single-cell level and to correlate these results with behavior. In these studies, locally applied amphetamine was found to selectively potentiate the activation of motor-related striatal units; furthermore, it was discovered that this activation is dependent on corticostriatal inputs. This work has important parallels with that of Levine and Cepeda (p. 724), in that it demonstrates a behavioral function for the potentiation of glutamatergic responses by dopamine that occurs at the cellular level. Finally, as Svensson et al. point out, an important component of glutamatergic actions on dopamine systems also occurs with respect to afferents to the dopamine cell body region, particularly when the drugs are administered systemically. Dopaminergic systems may also play a role in modulating the interaction of other types of inputs within the striatum. In particular, Grace et al. have shown a unique type of modulatory interaction among afferents to the accumbens that may be related to the regulation of information flow within these structures. Thus, accumbens neurons receive overlapping inputs from the PFC, hippocampus, and amygdala. Moreover, the hippocampus and amygdala appear to exert a facilitatory action on prefrontal inputs, which they propose underlies selective gating of information flow from the PFC through the accumbens to eventually influence thalamocortical activity. In particular, the hippocampal input is proposed to provide a context-dependent bias over selective prefrontal cortical inputs, whereas the amygdala would provide input selection based on the affective state of the organism. The ability of dopamine agonists and phenycyclidine (PCP)to interfere with this information-gating process may, therefore, provide a mechanism by which these systems contribute to at least a subset of the cognitive deficits observed in schizophrenia.
V. Systems Level Analyses of Brain Noradrenergic Systems
There was notable coherence in the systems-level analyses of NE in the CNS. Regulation of behavioral state, sensory processing, attention, and memory
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were particularly noteworthy themes. Interestingly, these themes are closely related, interdependent functions of the forebrain, and, therefore, it should perhaps be no surprise that different investigators would conclude such related functions for NE in the CNS. Nonetheless, this convergence of results is encouraging and suggests that we may be closing in on fundamental aspects of the functions of this enigmatic brain system.
A. Behavioral State Two reports deal directly with the role of forebrain NE in behavioral state regulation. Rajkowski et al. (p. 740) recorded from locus ceruleus (LC)neurons in monkeys during spontaneously occurring changes in alertness. As previously reported in New World monkeys as well as other species, this study in Old World (Cynomolgus)monkeys found that LC neurons dramatically decreased tonic impulse activity during drowsiness and slow-wave sleep. This effect appears to be especially pronounced in Old World monkeys because these cells became virtually quiescent, even during relatively early stages of drowsiness, As also previously found in other species, LC neurons in monkeys increased activity abruptly either just preceding or coincident with the transition from drownsiness or slow-wave sleep to waking. Recently, this group has also found that LC neurons in Old World monkeys are completely silent during paradoxical sleep, the first results for LC neurons in monkeys during this important stage of sleep, again similar to properties of these cells found in other species. Thus, these findings confirm for the Old World monkey properties that were also prominent for LCNE neurons in rats and cats. These results indicate that, across species, LC neurons may play a permissive role in sleep but an executive role in waking from slow-wave sleep. Additional studies in monkeys indicate that such state control extends to the attentional state within continuous waking, so that regulation of sleep and waking is only a portion of the state regulation produced by this system. Berridge (p. 744) reports studies in rats that were very compatible with the view that increased arousal and waking behaviors are associated with increased tonic LC impulse activity. In lightly anesthetized rats, he found that microinfusion of the 6-adrenoceptor agonist isoproterenol (Iso) into the area of the medial septum reliably activated cortical and hippocampal electroencephalographic (EEG) signs of arousal. In addition, microinfusions of the Padrenoceptor antagonist timolol into this same region were effective in shifting these EEG measures toward activities associated with less alert states. Additional studies in unanesthetized rats confirmed that Is0 injections into the medial septal area increased waking, indicating that the former results are not limited to the anesthetized preparation. These findings are entirely compatible with those from LC recordings in behaving animals (as the LC densely innervates the medial septal area) and indicate that the LC system may regulate alertness, at least in part via P-adrenoceptor mechanisms in the medial septal area.
6. Sensory Processing Waterhouse and colleagues (p. 749) report a series of anatomical and physiological experiments aimed at further delineating the role of the LCNE
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system in somatosensory processing. LC neurons are perhaps best known for their highly divergent axonal projections. Recent anatomical studies from this group call for a re-examination of the resulting widely held assumption that these cells are also nonspecific in their wiring. Instead, Devilbiss et al. report the intriguing result that individual LC neurons preferentially innervate functionally connected circuit elements at different levels of the neuraxis. Thus, for example, neurons that project to the sonlatosensory cortex are more likely to also project to the somatosensory thalamus (ventroposterior nucleus) than to, for example, the visual thalamus (lateral geniculate nucleus). Thus, although LC neurons may project widely, they exhibit much greater specificity in their projections than typically assumed. These investigators also found evidence for substantial specificity in the physiological actions of NE on somatosensory cortical neurons. In experiments using the in vitro cortical slice preparation, they found that low concentrations of NE or a,-adrenoceptor agonists facilitate spike production in layer 5 neurons in response to near threshold synaptic inputs, while in other neurons, P-adrenoceptor activation augments GABAergic synaptic responses recorded in similar neurons. Together, these results indicate that the LC efferent system exhibits substantial anatomical and physiological specificity in its impact on target cortical circuits. Thus, instead of a simple “gain” control over a process such as arousal, the LC may specifically and coordinately regulate processing of sensory information by virtue of actions on select neurons via different receptors along circuits that mediate responses to certain sensory modalities of stimulation.
C. Attention Selective processing of sensory information is a hallmark of attention, and, therefore, the aforementioned findings for modulation of senory circuits would be consistent with a role for the LC in attentional activity. In fact, results reported by Aston-Jones and colleagues (p. 755) from their recording of LC neurons in monkeys during performance of a visual discrimination-attention task strongly implicate the LC in attention. This task required animals to stably foveate a central fixation spot on a video monitor and selectively release a lever after presentation of a target cue (10-20% of trials) to receive a drop of juice. Responses to nontarget cues (80-90% of trials) resulted in a 3-sec time-out. LC neurons exhibited tonic and phasic activities that varied in relation to task performance. Tonically, LC activity varied among low, intermediate, and high levels. Low activity (near zero) was strongly associated with drowsiness (as noted in the report by Rajkowski et u!.) and poor task performance characterized by frequently missed stimuli. High tonic LC activity (-3-5 spikeskec) was also associated with poor task performance, in this case characterized by frequent false alarms (incorrect responses to nontarget stimuli), poor foveation of fix spots, and increased scanning eye movements. Optimal performance was associated with an intermediate level of tonic LC discharge (-1-2 spikeshec). Phasic LC activation was evoked at short latencies (-100 ms) selectively by target stimuli but not by other task events. Moreover, the phasic responses to target stimuli occurred only during epochs of excellent task performance associated with intermediate tonic LC activity. Aston-Jones et al. conclude that
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the LC exhibits two modes of activity. The aphasic mode is characterized by moderate tonic activity and robust phasic responses to attended sensory stimuli; this mode corresponds to focused (selective) attention and good task performance. These cells also exhibit a tonic mode characterized by high tonic activity and no phasic responsiveness; this mode corresponds to scanning, labile attention, and poor performance on this task. Additional studies by this group, using connectionist modeling, indicated that these different modes of LC operation may play a causal role in determining the associated level of behavioral performance. These modeling studies also indicate that these two LC modes may result from different levels of electrotonic coupling among LC neurons, consistent with recent results from Williams and colleagues for such coupling in the adult LC.
D. Memory A system involved in sensory processing and attention may also be expected to play a role in memory. Indeed, two separate reports indicate that actions of NE in the dorsolateral PFC (area 46) of behaving monkeys may play a significant role in memory. Sawaguchi and Kikuchi (p. 759) used iontophoretic methods to study the effects of NE agents on activity of area 46 neurons in monkeys during a delay task. Previous studies by Fuster and colleagues and by Goldman-Rakic’s group revealed that neurons in this area exhibit prolonged activity during the delay period in a delayed-response task. Goldman-Rakic trained monkeys on an oculomotor delayed-response task, in which they were required to remember the spatial location of a visual stimulus during a delay period and to subsequently move their center of gaze to that location in the absence of the stimulus. Local iontophoretic application of the a2-adrenoceptor antagonist yohimbine attenuated delay period activity of area 46 neurons, whereas iontophoresis of the padrenoceptor antagonist propranolol had no consistent effects on such activity. Further analysis revealed that this az-adrenoceptor antagonist attenuated the sharpness of tuning of the delay-period activity (relative selectivity of delayperiod activity for the specific location in space to be recalled) more than the baseline firing of delay-period activity. These results indicate that activation of axadrenoceptors plays a modulatory role in the coding of impulse activity thought to underlie working memory processes in the primate PFC. This proposed role of a2-adrenoceptor activity in the PFC is consistent with behavioral studies in monkey carried out by Arnsten and her colleagues. They summarize experiments in which az-adrenoceptor agonists given systemically improved performance in a delayed-response task in aged monkeys with naturally occurring NE depletion. The beneficial effects of a2-adrenoceptor agonists was particularly evident in conditions of high interference (e.g., distracting stimuli), and the pharmacologic profile indicated that these effects primarily involved actions at the azA-receptorsubtype. Additional studies indicate that the a2-adrenoceptor agonist clonidine is more effective in monkeys that have suffered presynaptic NE depletion (aged animals, reserpine treatment, 6-hydroxydopamine [6-OHDA], or MPTP lesions) than in normal animals, implicating postsynaptic rather than presynaptic a*-adrenoceptor sites in these behavioral effects. Local microinfusion experiments with az-adrenoceptor ago-
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nists and antagonists support this general view. Recent studies by this group indicate that, in contrast to results with a2 adrenoceptors, al-adrenoceptor activation in monkey PFC impairs working memory performance. This leads Arnsten et al. (p. 764) to propose that a balance of a2 and al-adrenoceptor activation in the PFC may be involved in regulating the functional status of this area and its ability to maintain normal working memory activity.
E. Functional Organization of Inputs to the Locus Ceruleus Ennis and colleagues (p. 767) describe neuroanatomical tract-tracing studies in which the organization of afferents to the core and pericellular shell of the LC has been examined. A number of afferents specifically target dendrites of noradrenergic neurons that extend into the pericellular shell region surrounding the LC core region, in which the cell bodies are located. Among these afferents, two specific systems, those from the midbrain periaqueductal gray and from the medial preoptic area, have been studied with both neuroanatomical tracing methods and functional activation studies. Specific pharmacological activation of each of these sources of afferent input to the pericellular shell of the LC resulted in induction of the immediate early gene Fos in the noradrenergic neurons. These functional studies demonstrate that the afferents to the pericellular shell into which noradrenergic neurons extend their dendrites may functionally activate these neurons. Nakamura and his colleagues (p. 772) describe their neurophysiological studies of the noradrenergic neurons of the LC. First, they describe the development of sensory-evoked responses. Specifically, they relate the patterns of sensory-evoked activity to the postnatal development of a negative-feedback mechanism, which replaces the more predominant positive-feedback system of early development. Second, they examine the role of GABAergic inhibition of LC neurons, using caloric vestibular stimulation as a paradigm for examining this phenomenon. Their results suggest that vestibular-mediated inhibition of the LC is mediated by the ventral lateral medullary afferents, which because they are primarily excitatory, presumably activate GABAergic afferents connected with the noradrenergic neurons.
F. Ultrastructural Localization of N E and N E Receptors in Primate Cortex Finally, detailed electron microscopic studies by Aoki et al. (p. 777) have begun to shed light on the possible microcircuitry involved in such NE functions in the PFC. These investigators used immunohistochemistry to localize NE fibers and receptors at the ultrastructural level in tissue from monkey area 46. Laminar analysis indicated that NE fibers form synapses most frequently in layers 2 through 5, and that such synapses occur at about every micrometer along the length of an NE fiber in this area. This reveals a high connectivity from the LC/NE system to the PFC. Staining, using antibodies against the subtypes of a2 adrenoceptors in the PFC, revealed that the (Y2A subtype was
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most prevalent in perikarya and proximal dendrites of neurons throughout layers, and that (Y2B and azc-receptor staining was more scarce in area 46. These findings are consistent with the results of behavioral studies with preferential a2-adrenoceptor subtype agents by Arnsten et al. These studies also revealed that azA-adrenoceptorsare more frequently localized to presynaptic terminals than to postsynaptic sites, but that receptors associated with identified synapses were more often postsynaptic than presynaptic. These results may indicate that synaptically released NE may operate preferentially at postsynaptic sites but that exogenous as well as endogenously released NE may also have substantial effects mediated via presynaptic a2-adrenoceptors. Immunohistochemical localization of p adrenoceptors revealed a surprisingly similar pattern for and aZAadrenoceptor activation, which have opposite effects on the adenylate cyclase cascade. However, dual labeling was much more frequently observed for fl adrenoceptors and GABA than for azA-adrenoceptors in GABA+ profiles, indicating that the cortical NE system (which is derived exclusively from the LC) interacts with GABA interneurons primarily via p adrenoceptors. Thus, converging evidence from several levels of analysis indicates roles for forebrain NE in related functional processes. First, state control is an important factor in the control of sensory processing and memory. Similarly, regulation of sensory processing is at the heart of attentional function. Finally, attention is an essential prerequisite for proper memory functioning. Indeed, late attentional stage processing can be readily and closely related to working memory. This group of papers, then, provides a glimpse at how functional attributes of the NE system at different levels of analysis may be involved in integrative behavioral attributes of this system. It is notable, for example, that the aforementioned results may provide an anatomically and physiologically specific mechanism underlying the well-known linkage between attention and memory functions from psychological work. Such brain substrates for psychological phenomena may also hold important insights for clinical application. Indeed, there are several clinical implications of this work. Perhaps most obvious are the parallels between LC function and attentional dysfunctions such as attention deficit hyperactivity disorder (ADHD). The results of these studies indicate that ADHD may be associated with the tonic mode of LC function (see Aston-Jones et al.), which may produce overstimulation of a1adrenoceptors in cortical sites (see Arnsten et al.). These results also indicate that NE in the PFC may play an important role in memory processes, much as it may play a role in sensory processing when acting in more sensory-related cortical areas. Roles of the LC system in the PFC in attention and in memory may become disrupted in clinical conditions associated with decreased NE availability in cortical regions, such as that found in aging and Alzheimer’s disease.
VI. Catecholamines: Involvement in Clinical Disorders A. Stress Because of the diverse nature of catecholaminergic projections, these systems are believed to play a role in general activational processes related to
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adaptation and survival of the organism. Such responses are particularly evident in the CNS activation produced by stress-related stimuli, often referred to as the fight or flight response. However adaptive such a response may be in dealing with immediately threatening environments, the continuous presence of stress can also exert pathological changes within the CNS. Valentino et al. (p. 781) point to evidence that the noradrenergic system is a principal component of the acute stress response, which is likely to be mediated via an excitatory amino acid transmitter input to the LC. In contrast, Valentino et al. describe some convincing experiments to suggest that corticotropin-releasing factor (CRF) may exert a major role in the long-term activation of the noradrenergic system in both normal and pathological states. Thus, CRF has been shown to mediate stress-induced activation of the LC, particularly when certain classes of stimuli serve as triggers. Moreover, there is clear evidence that CRF afferents contact NE neuron dendrites near the LC. Finally, CRF injection into the LC exerts functional actions. Therefore, CRF exerts a clear activation of LC noradrenergic cell firing in a manner that is sufficient to stimulate postsynaptic targets. In doing so, this system may be capable of maintaining arousal states in threating environments and may serve to link certain peripheral effects on the autonomic nervous system with CNS activity. Another treatment that appears to elevate noradrenergic activity is administration of the atypical antipsychotic drug clozapine, as outlined by Breier et al. (p. 785). Indeed, it appears that clozapine is unique in activating LC neuron firing. The ability of clozapine to alter this system could provide a functional link to account for the known involvement of stressful stimuli in the exacerbation of schizophrenia, and it provides a unique perspective on the potentially important tonic influence of NE on dopaminergic systems, as outlined by Thierry et al. and Tassin. Of course, it is also evident from studies by Svensson et al. that noradrenergic input to the ventral tegmental dopamine system plays an important role in modulating the activity of these neurons, and it also must be taken into consideration when analyzing the impact of noradrenergic-dopaminergic interactions in mediating CNS responses to stress.
B. Parkinson’s Disease Zigmond and Hastings (p. 788) review studies of the changes in various measures of dopamine neurotransmission following 6-OHDA lesions of the nigrostriatal dopaminergic pathway. These studies have attempted to address two main issues related to the pathophysiology of Parkinson’s disease, for which 6-OHDA lesions are a useful animal model. One issue involves why the behavioral effects of dopamine depletion develop over an extended period, rather than abruptly with the onset of the lesion. Studies are described that demonstrate increases in dopamine release from remaining terminals as an example of mechanisms that are temporarily able to compensate for the early phases of dopamine afferent destruction. A second issue addressed is why dopamine neurons are susceptible to toxicity. Studies are described that raise the possibility that dopamine itself, like the analogue 6-OHDA, may be toxic to dopaminergic neurons. Although L-dopa therapy has provided an effective treatment for Parkinson’s disease, the fact that this pharmacological treatment does not remain
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effective indefinitely and that, despite this treatment, the degeneration of the dopaminergic system progresses, make clear the necessity for an alternative treatment approach. Bankiewicz et al. (p. 801) describe work from their group that is exploring the use of gene therapy for the treatment of Parkinson’s disease. Studies are described in which a cell-based delivery system is being tested. The strategy is to deliver the protein machinery necessary to synthesize dopamine into the deafferented striatum. Two alternative strategies are discussed. One is based on using immortalized cell lines that have been genetically engineered to produce dopamine. Another is to used adeno-associated virus as a means of chronic delivery of therapeutic agents. Preliminary data using these approaches suggest that gene therapy may one day provide an effective strategy for the treatment of Parkinson’s disease.
C. Schizophrenia One issue raised by Lipska and Weinberger (p. 806) points out a major difficulty in this line of research-developing an animal model that can accurately reflect at least some aspects of a disorder that is uniquely human in nature. This is particularly true for a disorder such as schizophrenia, in which the neuropathological underpinnings of the disease are not obvious at the ultrastructural or neurochemical level, and the major components of the disorder involve disturbances of higher cognitive processes. Experimental approaches can nonetheless be employed toward this end. For example, one can focus on examining the basic physiological properties and types of network interactions that occur among neuronal systems that are likely to play a major role in a specificdisorder. Such an approach has been described in the chapters by Thierry et al., Goldman-Rakic, Lewis et al., and Grace et al., in which a systems-interaction approach is utilized to derive models related to the pathophysiology of schizophrenia. One important aspect of models as they relate to schizophrenia is the concept that this disorder is developmental in origin. Indeed, the developmentally related changes that take place within the dopaminergic system and its innervation of the PFC, as outlined by Lewis et al., demonstrate the complex nature and potential for pathological insult that is present throughout the formation and elaboration of the frontal cortical dopaminergic system. As such, there is the potential for a number of developmental insults to impact on the functional development of this system. Such an approach was used by Lipska and Weinberger to investigate potential developmental pathologies related to this disorder. Given the evidence that insults to fetal development that occur within the second trimester of pregnancy tend to increase the incidence of schizophrenic offspring, they embarked on a series of studies to examine potential pathologies produced at an equivalent rat developmental stage that may impact on limbic system function. By lesioning the hippocampus, Lipska and Weinberger found that, although the prepubertal rat appeared normal along several dimensions, when tested as an adult, the animals displayed distinct alterations in behavioral testing and pharmacological responsiveness that are consistent with some of the observations made in schizophrenic humans. Indeed, such alterations impact on the same systems that Grace et al. have shown to be involved in limbic interactions within the nucleus accumbens. As such, the
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model by Lipska and Weinberger is the first developmentally relevant animal model of schizophrenia advanced to date, and it is likely to have an important impact on our ability to examine systems-based developmental interactions as they relate to human diseases. Another method that is often employed to utilize animal models as approaches to understanding the pathophysiology of schizophrenia is the use of pharmacological tools. This approach can be divided into two categories: (1) examining the mode of action of agents that are known to mimic schizophrenia-like symptoms and (2) examining the actions of antipsychotic drugs as a method for analyzing what actions may correspond to the clinical response profile of these compounds. One particularly important pharmacological model of schizophrenia relates to PCP. As reviewed by Jentsch et al. (p. 810), unlike other pharmacological models, PCP is a psychotomimetic that can reproduce both the positive and negative symptoms of schizophrenia at doses related to its NMDA-antagonistic properties. In an elegant series of studies, Jentsch et al. have investigated the short-term and long-term consequences of PCP administration on nonhuman primates. Their work has shown that PCP given acutely can exert significant influences on both prefrontai cortical and subcortical metabolism of dopamine. Furthermore, they have shown that agents that block stress-induced increases in dopamine also block these actions of PCP, again showing the importance of stress, norepinephrine-dopamine interactions, and frontal cortical function. Perhaps more importantly, repeated administration of PCP appears to induce long-term changes in both behavioral tests and the PFC that may be related to schizophrenic pathophysiology. Svensson et al. have utilized a different approach to the study of schizophrenia: one that focuses on the mode of action of antipsychotic drugs. As Svensson et al. point out, dopamine D2-receptor occupancy cannot account for the efficacy of antipsychotic drugs, particularly as it relates to the atypical neuroleptics. Their work focuses on the importance of ventral tegmental area dopaminergic neuron activity in assessing pharmacological responses to drugs. In particular, they show that, in addition to actions in forebrain structures, as demonstrated by Jentsch et al. and Grace et al., indirect glutamatergic blockers also have important and selective actions on dopamine neuron activity states. Perhaps more importantly, the research by Svensson etal. (p. 814) suggests that atypical antipsychotic drugs may achieve their unique therapeutic profile due to their actions on serotonin-lAand noradrenergic cu,-receptorsystems, which in turn produce potent modulatory actions on dopaminergic neuron activity.
VII. Summary A goal of this part is to examine the functional impact of catecholaminergic systems on CNS function as it relates to normal and pathological states. The participants achieve their objectives individually by relating their high-quality work on this expansive topic, while maintaining a focus on the functional implications of their findings. Nonetheless, despite the necessarily broad nature of the topics presented, there is a remarkable degree of convergence of informa-
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tion. Several subthemes have emerged as a consequence of considering this work in its entirety. First is the importance of examining neurotransmitter effects, not in isolation, but in terms of interactions with other neurotransmitter systems. History has shown that a limited focus often produces confusing or inconsistent results, which become increasingly clear on consideration of the state of the organism. Second is the importance of examining pharmacological and pathophysiological interactions in light of the anatomy of the system and how developmental influences can alter this relationship. Such very general considerations have been found to provide an essential ingredient in understanding the nature of catecholamine function within this complex system.
Charles R. Gerfen, Kristen A. Keefe, and Heinz Steiner Laboratory of Systems Neuroscience National Institute of Mental Health Bethesda, Maryland 20892
Dopamine-Mediated Gene Regulation in the Striatum Dopamine’s critical role in the basal ganglia is evident in clinical disorders such as Parkinson’s disease and the implication of striatal dopamine effects of drugs of abuse, including cocaine. The response of genes encoding transcription factors and neuropeptides to manipulation of dopamine receptors may be used to study the dynamic modulatory role that dopamine plays in affecting basal ganglia function. Basal ganglia output is antagonistically determined by two separate striatal output systems, the direct and indirect pathways, which are oppositely modulated by their respective expression of the D1 and D2 dopamine receptor subtypes (1).Two sets of studies are reviewed that demonstrate how dopamine receptor-mediated gene-regulation effects provide insight into the functional role of dopamine in the striatum.
1. D I -D2 Dopamine Receptor Segregation in Direct and Indirect Striatal Output Pathways While there is now considerable support for the original observations of the segregation of D1 and D2 dopamine receptor subtypes t o the neurons contributing to the “direct” and “indirect” striatal output pathways (2), some physiological studies suggest a functional colocalization of these receptors. For Advances in Pharmacology, Volume 42 Copyright 0 1998 by Academic Press. All rights of reproduction in any form reserved. 1054-3589/98 $25.00
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CATECHOLAMINES IN T H E CENTRAL NERVOUS SYSTEM
A. A. Grace,* C. R. Gerfen,t and G. Aston-Jones$ *Departments of Neuroscience and Psychiatry University of Pittsburgh Pittsburgh, Pennsylvania I5260 tLaboratory of Systems Neuroscience National institute of Mental Health Bethesda, Maryland 20892
*Department of Psychiatry Allegheny University Philadelphia, Pennsylvania I 9 I02
Overview Studies concentrating on catecholaminergic systems in the central nervous system (CNS) have initially lagged far behind the more aggressive approaches used to examine their function in the periphery. However, the preponderance of studies over the past several decades demonstrating a role for these neurotransmitter systems in neurological and psychiatric disorders has been a driving force for studies focusing o n central aspects of catecholaminergic function. Indeed, it is becoming increasingly clear that dopamine, norepinephrine (NE), and epinephrine exhibit essential roles in the regulation and coordination of activity among widespread central systems. A multitude of methodological approaches was applied to study a comparatively diverse collection of topic materials; nonetheless, there was a remarkable degree of convergence among the Advances 111 Pharmacology. Volume 42
Copyright 0 1998 by Academic Press. All rights ot reprriducrion
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experimental findings and conceptual models advanced. The papers presented in this part generally could be divided along two foci: the anatomy, pharmacology, and molecular regulation of catecholamines in the normal brain; and how these relationships may be altered in disease states.
1. Molecular Regulation of Dopaminergic Responses An effective approach for studying the functional role of dopamine in the basal ganglia has been to measure changes in the expression levels of various genes and their products in the basal ganglia following pharmacological manipulations. Gerfen and his colleagues (p. 670) describe studies of dopamine regulation of striatal output neurons, which follow from their demonstration of the segregation of D1 and D2 dopamine receptor subtypes, respectively, to the so-called directstriatonigral and indirect-striatopallidal striatal projection neurons. One issue surrounding the segregation of these receptor subtypes stems from models that synergy between the subtypes requires colocalization. To address this issue, single-cell measurements were made of changes in the immediate early gene zif 268, which has a constitutive level of expression and thus allows both increases and decreases in expression levels to be examined in identified direct ( D l ) or indirect (D2) striatal projection neurons in animals treated with a D1-receptor agonist alone or in combination with D2 agonists. Results demonstrated that combined treatment with D1 and D2 agonists results in a selective increase in gene regulation in D1-expressing “direct” projection neurons and decreased gene regulation in D2-expressing “indirect” projection neurons. Thus, the synergistic interaction between these receptor subtypes appears to occur through intercellular interactions between different neurons, rather than through intracellular interactions between receptors expressed by the same neurons. While a variety of generegulation effects are seen by manipulation of dopaminergic neurotransmission, the functional significance of these effects remains for the most part obscure. Work by Steiner and Gerfen was described, which has shown that elevated expression of the opiate neuropeptide dynorphin following repeated overstimulation of D 1 receptors (e.g., by cocaine)is an adaptive response of these neurons to blunt the effects of such overstimulation. While there are many effects of dopamine receptor-mediated action within the striatum, other parts of the basal ganglia circuitry are necessarily involved in the behavioral consequences of dopaminergic system manipulation. Chesselet and her coworkers (p. 674) describe studies in which such extrastriatal basal ganglia effects are examined by measurement of changes in GAD 67 mRNA in basal ganglia nuclei downstream from the striatum. Although it might be anticipated that dopamine-mediated changes in the output of the striatum would be reflected in changes in the target nuclei of that output, the direction of such changes does not always appear to follow from the expected changes in activity levels of the neurons. For example, elevated markers in striatopallidal neurons resulting from striatal dopamine depletion or neuroleptic treatment resulted in increased GAD 67 mRNA levels in globus pallidus neurons, which may be considered surprising because increased striatopallidal output would
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be expected to decrease activity in such pallidal neurons. A number of possible mechanisms are examined that might account for these results. Rather than simply altering the level of activity in pallidal neurons, changes in striatal outputs might alter the pattern of activity, which might result from altered membrane properties. Along these lines, results are described in which altered dopaminergic neurotransmission results in altered expression of the Kv3.1 “shaw-like” potassium channel. Moreover, nuclei that are connected with, but not necessarily considered part of, the basal ganglia are shown to display alterations in gene expression and are examined in dopaminergic manipulation paradigms. One result of interest is that of a change in the level of GAD 67 in the thalamic reticular nucleus. Such results are discussed in terms of the circuitry and mechanisms through which dopamine exerts effects on behavior. Marshall and his colleagues (p. 678) describe studies from their laboratory that have examined D1 and D2 dopamine receptor regulation of striatal output neurons in paradigms utilizing psychostimulant drugs such as amphetamine. They provide a number of examples of an obligatory dependence on D2 receptor mechanisms of D1-receptor-mediated induction of Fos in striatonigral neurons. Such studies point out the importance of D1- and D2-receptor interactions in the effect of dopamine in the striatum, particularly in the normal striatum in animals treated with psychostimulant drugs. An additional set of studies is described in which the effects of alteration of the striatopallidal pathways on the target neurons in the globus pallidus are examined. The most interesting result from these studies is that distinct populations of pallidal neurons, characterized on the basis of their expression of the calcium-binding protein parvalbumin and on their axonal projections, display different immediate early gene responses to dopamine agonists or antagonists. Liu and Graybiel (p. 682) describe work from their laboratory that examined dopamine- and calcium-signaling regulation of cyclic response elementbinding (CREB) phosphorylation in striatal neurons. A developmental model was described in which organotypic cultures of the striatum were used as a substrate for dopaminergic- and calcium-signaling pharmacological treatment paradigms and in which the measured effect is that of phosphorylation of CREB. One of the intriguing results described was the difference in the kinetics of phosphorylation of CREB (pCREB) in different striatal compartments. Whereas D1 dopamine receptor agonist treatment for a short period (7 min) resulted in striosomal-patch and matrix pCREB, sustained agonist incubation resulted in only striosomal-patch labeling. A different pattern of pCREB kinetics was observed when an L-type calcium channel activator (BAY K 8644) was used. Similar to the D1 agonist, short-term treatment resulted in pCREB in neurons of both compartments; however, sustained treatment resulted in labeling primarily in the matrix. These results are discussed in terms of the convergence of different signaling pathways through CREB for gene regulation in striatal neurons.
II. Physiology of Dopaminergic Neurons Studies were presented of the neurophysiological activity of dopaminergic neurons of the midbrain spanning the environmental and behavioral contexts
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that are represented in the activity of these neurons and the afferent glutamatergic, cholinergic, and GABAergic receptor-mediated membrane mechanisms responsible for such activity. Midbrain dopaminergic neurons in the substantia nigra and ventral tegmental area are generally accepted as playing a role in the motivational control of voluntary behavior. Schultz describes his group’s work, which has helped define that role. Using the powerful approach of single-unit recording in awake behaving primates, Schultz (p. 686) has examined the relationship between the reward value of specific behavior-evoking stimuli and dopamine neuron activity. While most dopamine neurons respond to primary appetitive stimuli, it is the relationship between such reward and stimulus parameters that provides the most interesting insight into the function of dopamine neuronal activity. Among the observations that Schultz provides is the relationship between the novelty of a stimulus and dopamine neuron activity. Once a reward becomes fully predictable, responses of dopamine neurons diminish. However, this aspect of the novelty, or unpredictability, in the relationship between a stimulus and associated reward has an important corollary function in that it allows for the representation of a reward to be transferred to secondary, intrinsically neutral conditional stimuli. Such determinants of dopamine neuron activity are suggested by Schultz to indicate that dopamine neuron activity codes errors in reward prediction as an appropriate signal for appetitive learning. In a series of three chapters, Johnson (p. 691), Tepper et al. (p. 694), and Kitai (p. 700) describe membrane physiological properties underlying glutamatergic, GABAergic, and cholinergic afferent synaptic regulation of dopamine neuron activity. Johnson has used a slice preparation to study the effects of synaptic inputs, membrane potentials, or currents on dopamine neurons using intracellular recording from a slice preparation. Results demonstrate that in dopamine neurons, both N-methy-D-aspartate (NMDA) and non-NMDA glutamatergic inputs evoke fast excitatory synaptic currents, whereas metabotropic glutamatergic receptors mediate slow-onset long-duration responses. GABAergic synaptic inputs evoke inhibitory responses, producing fast and slow IPSPs through GABAAand GABABreceptors, respectively. Tepper and his colleagues expand on the differential actions of GABAergic synaptic input through the Aand B-receptor subtypes on dopamine neuron activity. Their studies demonstrate that GABAergic inputs are provided to dopamine neurons through multiple sources, from the striatum, from the globus pallidus, and from the GABAergic neurons in the substantia nigra pars reticulata. GABAA- and GABAB-receptor subtypes appear to be involved not only in synaptic input from each of these sources, but also in regulating release through presynaptic mechanisms. Importantly, the two receptor subtypes appear to have distinct roles in determining transitions between burst and pacemaker patterns of activity: GABAA antagonists increase burstiness, whereas GABABantagonists decrease burstiness and increase pacemaker activity. Examining the circuits involved in regulating afferent GABAergic regulation of dopamine neuron activity, Tepper and his group determined that altered patterns of pallidal output affect, at least in part, dopamine neuron activity through multisynaptic mechanisms involving the GABAergic neurons of the substantia nigra pars reticulata, which themselves provide inhibition to the dopamine neurons. In addition to the GABAergic
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input are other afferents thought to be responsible for excitatory responses of midbrain dopamine neurons. Kitai describes work in his laboratory that has examined two such inputs: glutamatergic inputs from the subthalamic nucleus and cholinergic input from the pedunculopontine nucleus. Kitai and his colleagues have pioneered work examining the importance of glutamatergic input to both GABAergic and dopaminergic neurons of the substantia nigra. Such input is thought to be responsible, in large measure, for the tonic activity of these neurons. In this chapter, Kitai focuses on the contribution of such glutamatergic input to the dopamine neurons and on studies that examine the interaction of cholinergic receptor-mediated mechanisms on such excitatory input. Conclusions drawn from these studies suggest that such glutamatergic-cholinergic interactions may be involved in the modification of dopamine neuron activity patterns from an intrinsic, regular, rhythmic pattern to an irregular, bursty pattern.
111. Catecholamines in the Prefrontal Cortex One focus of recent research efforts delves into the anatomy and pharmacology of neuronal interactions within the prefrontal cortex (PFC). This interest has been driven by recent studies implicating the PFC as an important limbic component related to the pathophysiology of schizophrenia. Details of the anatomy of dopamine input to the frontal cortex are provided in the chapter by Lewis et al. (p. 703).This group showed that the distribution of dopamine in the frontal cortex is dependent on the region of the cortex examined, that dopamine is selectively distributed within different laminae of different frontal cortical regions, and moreover that this innervation exhibits distinct maturational modifications throughout development. Thus, dopamine innervation is densest in the dorsomedial prefrontal cortex, particularly within the superficial and deep layers; in contrast, in area 9, the innervation is trilaminar. Moreover, the type of elements that these axons contact differs depending on the layer of innervation, reinforcing the proposal that dopamine exerts potent direct and indirect actions over the pyramidal cell population. Indeed, Goldman-Rakic (p. 707) suggests that the distinct types of dopamine innervation may play unique roles in the modulation of working memory function within the frontal cortex. She suggests that the direct dopaminergic input onto pyramidal neuron spines and distal dendrites may underlie its ability to intensify and focus inputs related to memory fields, as shown by her work on D1 agonist modulation of this behavior. In contrast, the more pervasive nonsynaptic actions of dopamine on pyramidal neurons may also modulate working memory via a unique interaction with D1 receptors. Finally, the ability of dopamine to indirectly modulate pyramidal cells via a D4-mediated action on GABAergic interneurons may act in concert with pyramidal neurons to hold information in working memory. These relationships may have important functional consequences during development, because Lewis et al. showed that these patterns of innervation reveal unique alterations during development in a region-specific manner. In addition to dopamine’s modulatory action over intrinsic neuronal elements within PFC, Thierry et al. (p. 717) provide evidence that dopamine also
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provides a modulatory influence over long-loop afferents that control activity states within this region. Stimulation of the mediodorsal nucleus was shown to exert both orthodromic and antidromic excitation of PFC neurons. However, it appears that dopamine selectively modulates only the local axon collaterals activated by the antidromic stimulus and, therefore, may enable a focusing of activity within this region by selectively blocking local interactions. Dopamine was also found to exert direct inhibitory actions within the PFC via D1 and D2 receptor stimulation. However, it appears that the D2-mediated response is dependent on intact noradrenergic a1transmission. Indeed, this interdependence of the dopaminergic and noradrenergic systems within the PFC is supported by studies by Tassin (p. 712). In a complex series of studies, he extends this correlation to suggest that even subcortical dopaminergic actions are dependent on intact a-adrenergic activation within the prefrontal cortex. This may occur via a tonic influence of norepinephrine over dopaminergic transmission in the PFC. Furthermore, the presence of this noradrenergic innervation may be essential for the expression of dopamine denervation-induced supersensitivity to D 1stimulation. This noradrenergic-dopaminergic interdependence may provide a cellular basis for the proposal by Breier et d. that clozapine exerts its unique antipsychotic actions via an effect on noradrenergic systems.
IV. Interaction among Glutamatergic Afferents and Dopamine in the Striatum In addition to the effects of dopamine within the frontal cortex, an increasing amount of evidence has implicated the PFC in the regulation of subcortical dopaminergic transmission. In the chapter by Thierry et al., findings are presented that dopamine agonists injected into the PFC exert an opposite action to those injected systemically, and that this appears to be mediated by a D1 action. Furthermore, the effects of prefrontal glutamatergic input onto subcortical structures are also modulated by dopaminergic agonists. Within the nucleus accumbens, Grace et aE. (p. 721) have shown that dopamine acting at very low concentrations exerts a tonic inhibitory influence on PFC afferents. In contrast, in the dorsal striatum, Levine and Cepeda have shown that dopamine exerts a pharmacologically selective modulation of glutamate via distinct postsynaptic mechanisms. Thus, dopamine acting on D1 receptors potentiates postsynaptic glutamate responses mediated via the NMDA receptor, whereas D2 agonists attenuate nonNMDA-mediated glutamatergic excitation of striatal neurons. This was found to be a directly mediated action, because it was also observed in whole-cell clamp studies in the presence of tetrodotoxin. Indeed, Thierry et al. provide evidence that suggests that dopamine can modulate glutamatergic responses within the PFC as well. Konradi (p. 729) provides a molecular mechanism through which such a glutamatergic-dopaminergic interaction may take place. In an elegant series of studies, she shows that c-fos expression mediated by dopamine-receptor stimulation is dependent on functional NMDA receptors and is inhibited by the glutamate antagonist MK 801. This interaction is proposed to occur via a D1-stimulated, cyclic adenosine monophosphate-dependent phosphorylation of CREB that is additive with glutamate-mediated activation of CREB.
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The relevance of dopaminergic-glutamatergicinteraction within subcortical structures is further underscored in two models that examine the functional impact of these systems. Arbuthott et al. attempt to bridge studies examining the electrophysiological actions of dopamine in a model system that also examines ultrastructure. As they point out, it is clear from many studies that dopamine does not exert a simple inhibitory or excitatory action in the striatum. Indeed, studies by Schultz have shown that dopamine acts in reward-related events as a way to increase the probability that an unexpected reward will be generated by increasing the type of behavior that is associated with the rewarding event. Arbuthnott et al. (p. 733)show physiological evidence for such a role, in that repetitive corticostriatal stimulation tends to induce an LTD-like inhibition of responses to subsequent stimuli; however, in the presence of dopamine, repetitive responses are potentiated. Examined at the ultrastructural level, data are presented to suggest that dopamine may be involved in the selective survival of potentiated synaptic connections within this highly integrative structure. Another aspect of dopamine function as it relates to motor systems is examined by Rebec (p. 737), who used the elegant technique of combined extracellular recordings from striatal cells in freely moving animals with microiontophoresis. This approach allows his laboratory to examine dopamine-glutamate interactions at a single-cell level and to correlate these results with behavior. In these studies, locally applied amphetamine was found to selectively potentiate the activation of motor-related striatal units; furthermore, it was discovered that this activation is dependent on corticostriatal inputs. This work has important parallels with that of Levine and Cepeda (p. 724), in that it demonstrates a behavioral function for the potentiation of glutamatergic responses by dopamine that occurs at the cellular level. Finally, as Svensson et al. point out, an important component of glutamatergic actions on dopamine systems also occurs with respect to afferents to the dopamine cell body region, particularly when the drugs are administered systemically. Dopaminergic systems may also play a role in modulating the interaction of other types of inputs within the striatum. In particular, Grace et al. have shown a unique type of modulatory interaction among afferents to the accumbens that may be related to the regulation of information flow within these structures. Thus, accumbens neurons receive overlapping inputs from the PFC, hippocampus, and amygdala. Moreover, the hippocampus and amygdala appear to exert a facilitatory action on prefrontal inputs, which they propose underlies selective gating of information flow from the PFC through the accumbens to eventually influence thalamocortical activity. In particular, the hippocampal input is proposed to provide a context-dependent bias over selective prefrontal cortical inputs, whereas the amygdala would provide input selection based on the affective state of the organism. The ability of dopamine agonists and phenycyclidine (PCP)to interfere with this information-gating process may, therefore, provide a mechanism by which these systems contribute to at least a subset of the cognitive deficits observed in schizophrenia.
V. Systems Level Analyses of Brain Noradrenergic Systems
There was notable coherence in the systems-level analyses of NE in the CNS. Regulation of behavioral state, sensory processing, attention, and memory
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were particularly noteworthy themes. Interestingly, these themes are closely related, interdependent functions of the forebrain, and, therefore, it should perhaps be no surprise that different investigators would conclude such related functions for NE in the CNS. Nonetheless, this convergence of results is encouraging and suggests that we may be closing in on fundamental aspects of the functions of this enigmatic brain system.
A. Behavioral State Two reports deal directly with the role of forebrain NE in behavioral state regulation. Rajkowski et al. (p. 740) recorded from locus ceruleus (LC)neurons in monkeys during spontaneously occurring changes in alertness. As previously reported in New World monkeys as well as other species, this study in Old World (Cynomolgus)monkeys found that LC neurons dramatically decreased tonic impulse activity during drowsiness and slow-wave sleep. This effect appears to be especially pronounced in Old World monkeys because these cells became virtually quiescent, even during relatively early stages of drowsiness, As also previously found in other species, LC neurons in monkeys increased activity abruptly either just preceding or coincident with the transition from drownsiness or slow-wave sleep to waking. Recently, this group has also found that LC neurons in Old World monkeys are completely silent during paradoxical sleep, the first results for LC neurons in monkeys during this important stage of sleep, again similar to properties of these cells found in other species. Thus, these findings confirm for the Old World monkey properties that were also prominent for LCNE neurons in rats and cats. These results indicate that, across species, LC neurons may play a permissive role in sleep but an executive role in waking from slow-wave sleep. Additional studies in monkeys indicate that such state control extends to the attentional state within continuous waking, so that regulation of sleep and waking is only a portion of the state regulation produced by this system. Berridge (p. 744) reports studies in rats that were very compatible with the view that increased arousal and waking behaviors are associated with increased tonic LC impulse activity. In lightly anesthetized rats, he found that microinfusion of the 6-adrenoceptor agonist isoproterenol (Iso) into the area of the medial septum reliably activated cortical and hippocampal electroencephalographic (EEG) signs of arousal. In addition, microinfusions of the Padrenoceptor antagonist timolol into this same region were effective in shifting these EEG measures toward activities associated with less alert states. Additional studies in unanesthetized rats confirmed that Is0 injections into the medial septal area increased waking, indicating that the former results are not limited to the anesthetized preparation. These findings are entirely compatible with those from LC recordings in behaving animals (as the LC densely innervates the medial septal area) and indicate that the LC system may regulate alertness, at least in part via P-adrenoceptor mechanisms in the medial septal area.
6. Sensory Processing Waterhouse and colleagues (p. 749) report a series of anatomical and physiological experiments aimed at further delineating the role of the LCNE
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system in somatosensory processing. LC neurons are perhaps best known for their highly divergent axonal projections. Recent anatomical studies from this group call for a re-examination of the resulting widely held assumption that these cells are also nonspecific in their wiring. Instead, Devilbiss et al. report the intriguing result that individual LC neurons preferentially innervate functionally connected circuit elements at different levels of the neuraxis. Thus, for example, neurons that project to the sonlatosensory cortex are more likely to also project to the somatosensory thalamus (ventroposterior nucleus) than to, for example, the visual thalamus (lateral geniculate nucleus). Thus, although LC neurons may project widely, they exhibit much greater specificity in their projections than typically assumed. These investigators also found evidence for substantial specificity in the physiological actions of NE on somatosensory cortical neurons. In experiments using the in vitro cortical slice preparation, they found that low concentrations of NE or a,-adrenoceptor agonists facilitate spike production in layer 5 neurons in response to near threshold synaptic inputs, while in other neurons, P-adrenoceptor activation augments GABAergic synaptic responses recorded in similar neurons. Together, these results indicate that the LC efferent system exhibits substantial anatomical and physiological specificity in its impact on target cortical circuits. Thus, instead of a simple “gain” control over a process such as arousal, the LC may specifically and coordinately regulate processing of sensory information by virtue of actions on select neurons via different receptors along circuits that mediate responses to certain sensory modalities of stimulation.
C. Attention Selective processing of sensory information is a hallmark of attention, and, therefore, the aforementioned findings for modulation of senory circuits would be consistent with a role for the LC in attentional activity. In fact, results reported by Aston-Jones and colleagues (p. 755) from their recording of LC neurons in monkeys during performance of a visual discrimination-attention task strongly implicate the LC in attention. This task required animals to stably foveate a central fixation spot on a video monitor and selectively release a lever after presentation of a target cue (10-20% of trials) to receive a drop of juice. Responses to nontarget cues (80-90% of trials) resulted in a 3-sec time-out. LC neurons exhibited tonic and phasic activities that varied in relation to task performance. Tonically, LC activity varied among low, intermediate, and high levels. Low activity (near zero) was strongly associated with drowsiness (as noted in the report by Rajkowski et u!.) and poor task performance characterized by frequently missed stimuli. High tonic LC activity (-3-5 spikeskec) was also associated with poor task performance, in this case characterized by frequent false alarms (incorrect responses to nontarget stimuli), poor foveation of fix spots, and increased scanning eye movements. Optimal performance was associated with an intermediate level of tonic LC discharge (-1-2 spikeshec). Phasic LC activation was evoked at short latencies (-100 ms) selectively by target stimuli but not by other task events. Moreover, the phasic responses to target stimuli occurred only during epochs of excellent task performance associated with intermediate tonic LC activity. Aston-Jones et al. conclude that
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the LC exhibits two modes of activity. The aphasic mode is characterized by moderate tonic activity and robust phasic responses to attended sensory stimuli; this mode corresponds to focused (selective) attention and good task performance. These cells also exhibit a tonic mode characterized by high tonic activity and no phasic responsiveness; this mode corresponds to scanning, labile attention, and poor performance on this task. Additional studies by this group, using connectionist modeling, indicated that these different modes of LC operation may play a causal role in determining the associated level of behavioral performance. These modeling studies also indicate that these two LC modes may result from different levels of electrotonic coupling among LC neurons, consistent with recent results from Williams and colleagues for such coupling in the adult LC.
D. Memory A system involved in sensory processing and attention may also be expected to play a role in memory. Indeed, two separate reports indicate that actions of NE in the dorsolateral PFC (area 46) of behaving monkeys may play a significant role in memory. Sawaguchi and Kikuchi (p. 759) used iontophoretic methods to study the effects of NE agents on activity of area 46 neurons in monkeys during a delay task. Previous studies by Fuster and colleagues and by Goldman-Rakic’s group revealed that neurons in this area exhibit prolonged activity during the delay period in a delayed-response task. Goldman-Rakic trained monkeys on an oculomotor delayed-response task, in which they were required to remember the spatial location of a visual stimulus during a delay period and to subsequently move their center of gaze to that location in the absence of the stimulus. Local iontophoretic application of the a2-adrenoceptor antagonist yohimbine attenuated delay period activity of area 46 neurons, whereas iontophoresis of the padrenoceptor antagonist propranolol had no consistent effects on such activity. Further analysis revealed that this az-adrenoceptor antagonist attenuated the sharpness of tuning of the delay-period activity (relative selectivity of delayperiod activity for the specific location in space to be recalled) more than the baseline firing of delay-period activity. These results indicate that activation of axadrenoceptors plays a modulatory role in the coding of impulse activity thought to underlie working memory processes in the primate PFC. This proposed role of a2-adrenoceptor activity in the PFC is consistent with behavioral studies in monkey carried out by Arnsten and her colleagues. They summarize experiments in which az-adrenoceptor agonists given systemically improved performance in a delayed-response task in aged monkeys with naturally occurring NE depletion. The beneficial effects of a2-adrenoceptor agonists was particularly evident in conditions of high interference (e.g., distracting stimuli), and the pharmacologic profile indicated that these effects primarily involved actions at the azA-receptorsubtype. Additional studies indicate that the a2-adrenoceptor agonist clonidine is more effective in monkeys that have suffered presynaptic NE depletion (aged animals, reserpine treatment, 6-hydroxydopamine [6-OHDA], or MPTP lesions) than in normal animals, implicating postsynaptic rather than presynaptic a*-adrenoceptor sites in these behavioral effects. Local microinfusion experiments with az-adrenoceptor ago-
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nists and antagonists support this general view. Recent studies by this group indicate that, in contrast to results with a2 adrenoceptors, al-adrenoceptor activation in monkey PFC impairs working memory performance. This leads Arnsten et al. (p. 764) to propose that a balance of a2 and al-adrenoceptor activation in the PFC may be involved in regulating the functional status of this area and its ability to maintain normal working memory activity.
E. Functional Organization of Inputs to the Locus Ceruleus Ennis and colleagues (p. 767) describe neuroanatomical tract-tracing studies in which the organization of afferents to the core and pericellular shell of the LC has been examined. A number of afferents specifically target dendrites of noradrenergic neurons that extend into the pericellular shell region surrounding the LC core region, in which the cell bodies are located. Among these afferents, two specific systems, those from the midbrain periaqueductal gray and from the medial preoptic area, have been studied with both neuroanatomical tracing methods and functional activation studies. Specific pharmacological activation of each of these sources of afferent input to the pericellular shell of the LC resulted in induction of the immediate early gene Fos in the noradrenergic neurons. These functional studies demonstrate that the afferents to the pericellular shell into which noradrenergic neurons extend their dendrites may functionally activate these neurons. Nakamura and his colleagues (p. 772) describe their neurophysiological studies of the noradrenergic neurons of the LC. First, they describe the development of sensory-evoked responses. Specifically, they relate the patterns of sensory-evoked activity to the postnatal development of a negative-feedback mechanism, which replaces the more predominant positive-feedback system of early development. Second, they examine the role of GABAergic inhibition of LC neurons, using caloric vestibular stimulation as a paradigm for examining this phenomenon. Their results suggest that vestibular-mediated inhibition of the LC is mediated by the ventral lateral medullary afferents, which because they are primarily excitatory, presumably activate GABAergic afferents connected with the noradrenergic neurons.
F. Ultrastructural Localization of N E and N E Receptors in Primate Cortex Finally, detailed electron microscopic studies by Aoki et al. (p. 777) have begun to shed light on the possible microcircuitry involved in such NE functions in the PFC. These investigators used immunohistochemistry to localize NE fibers and receptors at the ultrastructural level in tissue from monkey area 46. Laminar analysis indicated that NE fibers form synapses most frequently in layers 2 through 5, and that such synapses occur at about every micrometer along the length of an NE fiber in this area. This reveals a high connectivity from the LC/NE system to the PFC. Staining, using antibodies against the subtypes of a2 adrenoceptors in the PFC, revealed that the (Y2A subtype was
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most prevalent in perikarya and proximal dendrites of neurons throughout layers, and that (Y2B and azc-receptor staining was more scarce in area 46. These findings are consistent with the results of behavioral studies with preferential a2-adrenoceptor subtype agents by Arnsten et al. These studies also revealed that azA-adrenoceptorsare more frequently localized to presynaptic terminals than to postsynaptic sites, but that receptors associated with identified synapses were more often postsynaptic than presynaptic. These results may indicate that synaptically released NE may operate preferentially at postsynaptic sites but that exogenous as well as endogenously released NE may also have substantial effects mediated via presynaptic a2-adrenoceptors. Immunohistochemical localization of p adrenoceptors revealed a surprisingly similar pattern for and aZAadrenoceptor activation, which have opposite effects on the adenylate cyclase cascade. However, dual labeling was much more frequently observed for fl adrenoceptors and GABA than for azA-adrenoceptors in GABA+ profiles, indicating that the cortical NE system (which is derived exclusively from the LC) interacts with GABA interneurons primarily via p adrenoceptors. Thus, converging evidence from several levels of analysis indicates roles for forebrain NE in related functional processes. First, state control is an important factor in the control of sensory processing and memory. Similarly, regulation of sensory processing is at the heart of attentional function. Finally, attention is an essential prerequisite for proper memory functioning. Indeed, late attentional stage processing can be readily and closely related to working memory. This group of papers, then, provides a glimpse at how functional attributes of the NE system at different levels of analysis may be involved in integrative behavioral attributes of this system. It is notable, for example, that the aforementioned results may provide an anatomically and physiologically specific mechanism underlying the well-known linkage between attention and memory functions from psychological work. Such brain substrates for psychological phenomena may also hold important insights for clinical application. Indeed, there are several clinical implications of this work. Perhaps most obvious are the parallels between LC function and attentional dysfunctions such as attention deficit hyperactivity disorder (ADHD). The results of these studies indicate that ADHD may be associated with the tonic mode of LC function (see Aston-Jones et al.), which may produce overstimulation of a1adrenoceptors in cortical sites (see Arnsten et al.). These results also indicate that NE in the PFC may play an important role in memory processes, much as it may play a role in sensory processing when acting in more sensory-related cortical areas. Roles of the LC system in the PFC in attention and in memory may become disrupted in clinical conditions associated with decreased NE availability in cortical regions, such as that found in aging and Alzheimer’s disease.
VI. Catecholamines: Involvement in Clinical Disorders A. Stress Because of the diverse nature of catecholaminergic projections, these systems are believed to play a role in general activational processes related to
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adaptation and survival of the organism. Such responses are particularly evident in the CNS activation produced by stress-related stimuli, often referred to as the fight or flight response. However adaptive such a response may be in dealing with immediately threatening environments, the continuous presence of stress can also exert pathological changes within the CNS. Valentino et al. (p. 781) point to evidence that the noradrenergic system is a principal component of the acute stress response, which is likely to be mediated via an excitatory amino acid transmitter input to the LC. In contrast, Valentino et al. describe some convincing experiments to suggest that corticotropin-releasing factor (CRF) may exert a major role in the long-term activation of the noradrenergic system in both normal and pathological states. Thus, CRF has been shown to mediate stress-induced activation of the LC, particularly when certain classes of stimuli serve as triggers. Moreover, there is clear evidence that CRF afferents contact NE neuron dendrites near the LC. Finally, CRF injection into the LC exerts functional actions. Therefore, CRF exerts a clear activation of LC noradrenergic cell firing in a manner that is sufficient to stimulate postsynaptic targets. In doing so, this system may be capable of maintaining arousal states in threating environments and may serve to link certain peripheral effects on the autonomic nervous system with CNS activity. Another treatment that appears to elevate noradrenergic activity is administration of the atypical antipsychotic drug clozapine, as outlined by Breier et al. (p. 785). Indeed, it appears that clozapine is unique in activating LC neuron firing. The ability of clozapine to alter this system could provide a functional link to account for the known involvement of stressful stimuli in the exacerbation of schizophrenia, and it provides a unique perspective on the potentially important tonic influence of NE on dopaminergic systems, as outlined by Thierry et al. and Tassin. Of course, it is also evident from studies by Svensson et al. that noradrenergic input to the ventral tegmental dopamine system plays an important role in modulating the activity of these neurons, and it also must be taken into consideration when analyzing the impact of noradrenergic-dopaminergic interactions in mediating CNS responses to stress.
B. Parkinson’s Disease Zigmond and Hastings (p. 788) review studies of the changes in various measures of dopamine neurotransmission following 6-OHDA lesions of the nigrostriatal dopaminergic pathway. These studies have attempted to address two main issues related to the pathophysiology of Parkinson’s disease, for which 6-OHDA lesions are a useful animal model. One issue involves why the behavioral effects of dopamine depletion develop over an extended period, rather than abruptly with the onset of the lesion. Studies are described that demonstrate increases in dopamine release from remaining terminals as an example of mechanisms that are temporarily able to compensate for the early phases of dopamine afferent destruction. A second issue addressed is why dopamine neurons are susceptible to toxicity. Studies are described that raise the possibility that dopamine itself, like the analogue 6-OHDA, may be toxic to dopaminergic neurons. Although L-dopa therapy has provided an effective treatment for Parkinson’s disease, the fact that this pharmacological treatment does not remain
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effective indefinitely and that, despite this treatment, the degeneration of the dopaminergic system progresses, make clear the necessity for an alternative treatment approach. Bankiewicz et al. (p. 801) describe work from their group that is exploring the use of gene therapy for the treatment of Parkinson’s disease. Studies are described in which a cell-based delivery system is being tested. The strategy is to deliver the protein machinery necessary to synthesize dopamine into the deafferented striatum. Two alternative strategies are discussed. One is based on using immortalized cell lines that have been genetically engineered to produce dopamine. Another is to used adeno-associated virus as a means of chronic delivery of therapeutic agents. Preliminary data using these approaches suggest that gene therapy may one day provide an effective strategy for the treatment of Parkinson’s disease.
C. Schizophrenia One issue raised by Lipska and Weinberger (p. 806) points out a major difficulty in this line of research-developing an animal model that can accurately reflect at least some aspects of a disorder that is uniquely human in nature. This is particularly true for a disorder such as schizophrenia, in which the neuropathological underpinnings of the disease are not obvious at the ultrastructural or neurochemical level, and the major components of the disorder involve disturbances of higher cognitive processes. Experimental approaches can nonetheless be employed toward this end. For example, one can focus on examining the basic physiological properties and types of network interactions that occur among neuronal systems that are likely to play a major role in a specificdisorder. Such an approach has been described in the chapters by Thierry et al., Goldman-Rakic, Lewis et al., and Grace et al., in which a systems-interaction approach is utilized to derive models related to the pathophysiology of schizophrenia. One important aspect of models as they relate to schizophrenia is the concept that this disorder is developmental in origin. Indeed, the developmentally related changes that take place within the dopaminergic system and its innervation of the PFC, as outlined by Lewis et al., demonstrate the complex nature and potential for pathological insult that is present throughout the formation and elaboration of the frontal cortical dopaminergic system. As such, there is the potential for a number of developmental insults to impact on the functional development of this system. Such an approach was used by Lipska and Weinberger to investigate potential developmental pathologies related to this disorder. Given the evidence that insults to fetal development that occur within the second trimester of pregnancy tend to increase the incidence of schizophrenic offspring, they embarked on a series of studies to examine potential pathologies produced at an equivalent rat developmental stage that may impact on limbic system function. By lesioning the hippocampus, Lipska and Weinberger found that, although the prepubertal rat appeared normal along several dimensions, when tested as an adult, the animals displayed distinct alterations in behavioral testing and pharmacological responsiveness that are consistent with some of the observations made in schizophrenic humans. Indeed, such alterations impact on the same systems that Grace et al. have shown to be involved in limbic interactions within the nucleus accumbens. As such, the
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model by Lipska and Weinberger is the first developmentally relevant animal model of schizophrenia advanced to date, and it is likely to have an important impact on our ability to examine systems-based developmental interactions as they relate to human diseases. Another method that is often employed to utilize animal models as approaches to understanding the pathophysiology of schizophrenia is the use of pharmacological tools. This approach can be divided into two categories: (1) examining the mode of action of agents that are known to mimic schizophrenia-like symptoms and (2) examining the actions of antipsychotic drugs as a method for analyzing what actions may correspond to the clinical response profile of these compounds. One particularly important pharmacological model of schizophrenia relates to PCP. As reviewed by Jentsch et al. (p. 810), unlike other pharmacological models, PCP is a psychotomimetic that can reproduce both the positive and negative symptoms of schizophrenia at doses related to its NMDA-antagonistic properties. In an elegant series of studies, Jentsch et al. have investigated the short-term and long-term consequences of PCP administration on nonhuman primates. Their work has shown that PCP given acutely can exert significant influences on both prefrontai cortical and subcortical metabolism of dopamine. Furthermore, they have shown that agents that block stress-induced increases in dopamine also block these actions of PCP, again showing the importance of stress, norepinephrine-dopamine interactions, and frontal cortical function. Perhaps more importantly, repeated administration of PCP appears to induce long-term changes in both behavioral tests and the PFC that may be related to schizophrenic pathophysiology. Svensson et al. have utilized a different approach to the study of schizophrenia: one that focuses on the mode of action of antipsychotic drugs. As Svensson et al. point out, dopamine D2-receptor occupancy cannot account for the efficacy of antipsychotic drugs, particularly as it relates to the atypical neuroleptics. Their work focuses on the importance of ventral tegmental area dopaminergic neuron activity in assessing pharmacological responses to drugs. In particular, they show that, in addition to actions in forebrain structures, as demonstrated by Jentsch et al. and Grace et al., indirect glutamatergic blockers also have important and selective actions on dopamine neuron activity states. Perhaps more importantly, the research by Svensson etal. (p. 814) suggests that atypical antipsychotic drugs may achieve their unique therapeutic profile due to their actions on serotonin-lAand noradrenergic cu,-receptorsystems, which in turn produce potent modulatory actions on dopaminergic neuron activity.
VII. Summary A goal of this part is to examine the functional impact of catecholaminergic systems on CNS function as it relates to normal and pathological states. The participants achieve their objectives individually by relating their high-quality work on this expansive topic, while maintaining a focus on the functional implications of their findings. Nonetheless, despite the necessarily broad nature of the topics presented, there is a remarkable degree of convergence of informa-
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tion. Several subthemes have emerged as a consequence of considering this work in its entirety. First is the importance of examining neurotransmitter effects, not in isolation, but in terms of interactions with other neurotransmitter systems. History has shown that a limited focus often produces confusing or inconsistent results, which become increasingly clear on consideration of the state of the organism. Second is the importance of examining pharmacological and pathophysiological interactions in light of the anatomy of the system and how developmental influences can alter this relationship. Such very general considerations have been found to provide an essential ingredient in understanding the nature of catecholamine function within this complex system.
Charles R. Gerfen, Kristen A. Keefe, and Heinz Steiner Laboratory of Systems Neuroscience National Institute of Mental Health Bethesda, Maryland 20892
Dopamine-Mediated Gene Regulation in the Striatum Dopamine’s critical role in the basal ganglia is evident in clinical disorders such as Parkinson’s disease and the implication of striatal dopamine effects of drugs of abuse, including cocaine. The response of genes encoding transcription factors and neuropeptides to manipulation of dopamine receptors may be used to study the dynamic modulatory role that dopamine plays in affecting basal ganglia function. Basal ganglia output is antagonistically determined by two separate striatal output systems, the direct and indirect pathways, which are oppositely modulated by their respective expression of the D1 and D2 dopamine receptor subtypes (1).Two sets of studies are reviewed that demonstrate how dopamine receptor-mediated gene-regulation effects provide insight into the functional role of dopamine in the striatum.
1. D I -D2 Dopamine Receptor Segregation in Direct and Indirect Striatal Output Pathways While there is now considerable support for the original observations of the segregation of D1 and D2 dopamine receptor subtypes t o the neurons contributing to the “direct” and “indirect” striatal output pathways (2), some physiological studies suggest a functional colocalization of these receptors. For Advances in Pharmacology, Volume 42 Copyright 0 1998 by Academic Press. All rights of reproduction in any form reserved. 1054-3589/98 $25.00