Nicotinic receptors: Therapeutic potential in neurological disease

X28 Cholinergic nicotinic receptors as drag targets in psychiatv

-1

Nicotinic receptors: Therapeutic potential in neurological disease

A. Nordberg. Department of Clinical Neuroscience and Family Medicine, Division of Molecular Neuropharmacology, Karolinska Institute, Geriatric Clinic, Huddinge University Hospital, Huddinge, Sweden There is a growing interest for the involvement of neuronal nicotinic acetylchohne receptors (nAChRs) in neurodegenerative disorders. Experimental and clinical data suggest that the human nAChRs seem to be important for cognitive processes as well as neuroprotective mechanisms in brain. Pathogenic roles of nAChRs have been revealed in neurodegenerative diseases. Deficits in nAChRs have been shown in living Alzheimer patients using imaging techniques as positron emission tomography (PET). Neurochemical and molecular biology studies in autopsy brain tissue show deficits in subtypes of nAChRs in neurodegenerative disorders including Alzheimer’s disease and Parkinson’s disease. The nAChRs show rich abundance in human brain. Three nAChRs with super-high, high and low affinities have been identified in autopsy brain tissue using nicotinic agonists with different affinity to reveal receptor subtype specificity. Nicotine, cytisine, epibatidine has been useful ligands for this type of receptor binding studies (including receptors autoradiography) in human brain. The neuronal nicotinic receptors (nAChRs) represent an archetype of transmitter-gated ion channel from the gene super family of homologous receptors for GABA, glycine and 5HTa receptors. Molecular biology studies have identified seven nAChRs subunits (a3-a5, a7, fi2-(34)in human brain. The nAChR is formed by a combination of a and S subunits and a huge number of permutations and combinations of subtypes are possible but probably a few predominate including a3,a4,a7 in combination with 82 or p4. Data suggest that the nAChRs play a modulatory role for several neurotransmitters and thus participate in communicative processes engaging signal substances as dopamine, noradrenaline and glutamate. The presynaptic nAChRs regulating dopamine release have been suggested to be both the a3 and a4 subunits while data suggest that the a7 nAChR subunit might facilitate the glutamate release. The nAChR receptor response occurs in the milliseconds and it is quite possible that the nAChRs can be tuned regarding channel opening time, agonists sensitivity and desensitization properties to fulfil requirements for a certain neurotransmitter that appear to be regulated by presynaptic nAChRs. The modulatory role of nAChRs in signal system communication make them attractive to study in several neurological diseases. Chromosomal aberrations The nAChRs are associated with a growing list of brain disorders including Alzheimer’s disease (AD), Parkinson’s disease, schizophrenia and Tourette syndrome. Some mutations have been found in nAChRs. Autosomal dominant nocturnal frontal lobe epilepsy is caused by an mutation in the a4 subunit and was the first mutation shown to cause a form of epilepsy. In schizophrenia a defect in auditory response that maps to the a7 nAChR gene locus may be predisposing factor reflecting attentional disturbances. The marked loss of nAChRs which recently was found in cortical regions of AD patients with the Swedish 670/671 APP brains (Marutle et al. submitted) make the influence of various chromosomal aberrations of the APP gene on chromosome 21 and preseniline genes in AD important to further study. Effect of nieotinic agonists on nicotinic receptor subtypes Chronic exposure of nicotine is known to upregulate the nicotinic receptors in brain of rodent and man. 1s it possible to restore receptor function and transmitter activtiy by long-term nicotinic agonist treatment? Studies in transfected fibroblast a@2 Ml0 cells suggest that the nicotine-induced upregulation occur via postranslational mechanisms probably reflecting altered turnover of the receptors, Studies of the effect of chronic treatment with nicotinic agonists in cell cultures indicate that the upregulation of a4@2 nAChRs in Ml0 cell cells is influcenced by the affinity of the nicotinic agonists and they also are more readily upregulated than the a3 nAChRs subunits expressed in SH-SYSY neuroblastoma cells (Warpman et al. 1998). The observation suggests a difference in effect of nAChRs on a4 and a3 nAChRs. A

S129

and neurology

further understanding of the underlying mechansism for this difference in rate of sensitization/resensitization mechanisms, receptor conformational states are needed for further progress in the design of selective nicotinic agonists as novel tools in the treatment of neurodegenerative disorders. Alzheimer’s disease AD is the most common for of dementia and one of the most devastating disease of the middle-aged and elderly. AD is characterized by gradual onset and progression of cognitive dysfuncion including forgetfulness, spatial disorientation, language disturbances but also depression and aggressiveness. The etiology and patbogenesis still remain elusive although there has been a great progress during recent years in further understanding of polyetiology and the underlying basic brain mechanisms of the disease. An imporant goal will be to initiate drug therapy early in the course of the disease but this necessitates early assessments, diagnosis and use of drugs that can be administered on long-term basis. The nAChRs are important for cognitive processes in human brain. Deficits in nAChRs in brain early in the course of the AD disease which correlate to impaitment in cognitive function have been revealed by PET (Nordberg et al. 1997). Similary studies in autopsy brain tisseu show a mark reduced in nAChRs in cortical regions of AD brains. A selective loss has been measured in a+2 nAChRs when using nicotinic agonists.as epibatidine and ABT-418 (Warpman et al. 1985). Quantification of mRNA levels for various nAChRs subunits have so far shown relatively unchanged levels in AD brain except for increase in a7 mRNA in hippocampus (Hellstrom-Lindahl et al. to be submitted) which suggest that the loss of nAChRs seen in AD patients reflect translational and/or posttranslational processes. Presently, cholinesterase inhibitors such as tacrine and donepezil are clinical used for symptomatic treatment of AD. An improvement of nAChRs (measured as “C-nicotine) has been observed by PET early (weeks) after initiation of tacrine treatment while improvement in glucose metabolism was increased after several months of tacrine treatment (Nordberg et al. 1998). Neuropsychological investigations reveal an improvement in attention during tacrine treatment. Interestingly, other studies suggest that nicotinic receptor stimulation as a major component involve improvement of attention. The number of nAChRs have been observed to increase in cell cultures expressing a4 nAChRs when incubated with cholinesterase inhibitors such as tacrine in clinical relevant concentrations (Svensson and Nordberg 1996). This increase in nAChRs seem to be induced via an interaction with an allosteric activator site on the nAChR separate from the ACh binding site on the nAChR (Svensson and Nordberg 1996). Interaction via the allosteric nAChR site might be of importance for the clinical drug efficacy. Direct activation of the allosteric site on the nAChR may be beneficial and cause less desensitization phenomena than a direct interaction via the ACh binding site. The experience of nicotinic agonist treatment in AD patients are still rare except for some few acute and chronic treatments with nicotine where nicotine has been shown to cause improvement in attentional driven tasks. Other treatment strategies where an improvement of nicotinic receptors as well as in cerebra1 blood flow and glucose metabolism have been observed includes chronic intraventricular infusion with nerve growth factors (NGF) in AD patients. The P-amyloid (A@ peptide may have a modulatory function on neurotransmittor activity in brain independently of apparent neurotoxicity. A@may thereby contribute to the vulnerability of cholinergic neuronal cells in AD. We have observed a different susceptibility for Ap between a7 and a3 nAChR subtypes in PC12 cells (Zhang et al. to be submitted). Nicotinic agonists can attenuate the AS toxicity The effect appears to be exerted via the nAChRs of a7 subtype which is known to play a role in modulating calcium homeostasis and cell to sell signals. Modifying the excitability of neurons in brain might be of one of the primary roles of nAChRs which means that they may regulate excitatory and inhibitory transmitter systems. It is important to obtain further insight into the coupling between the amyloid cascade and synaptic plasticity and function in brain in order to understand neurotoxic events in brain and possible neuroprotective mechanisms exerted via nAChRs. For AD it is of great interest to focus on the a4 and a7 nAChRs to develop drugs with symptomatic and neuroprotective effects. We have recently observed that cholinesterase

inhibitors

as tacrine

and donepezil

can prevent fi-

s130

S.28 Cholinergic nicotinic receptors as drag targets in psychiatry

amyloid neurotoxicity in PC12 cells and that the mechanisms seem to be mediated via nAChRs (Svensson and Nordberg 1998). PrukInson’s disease Cholinergic deficits are seen in brains of patients with Parkinson’s disease. The cholinergic deficits appear to be more pronounced in Parkinsonian patients with dementia than without dementia. The cognitive impairments seen in Parkinson’s disease are considered to reflect impaired function of the prefrontal cortex. Attentional deficits have been suggested to be secondary to disturbances in neuronal pathways involving prefrontal, parietal-basal ganglia and thalamus. Similar to AD Parkinson’s disase is associated with a large loss of high affitity nAChRs in brain. Stimulation of nAChRs with nicotinic agonists may improve attentional functioning in Parkinson’s disease. The exact involvement of nACbR subtypes has however not yet been revealed in the disease although there is suggstions that the a6 subunit may play a role and might provide a useful pharmacological target especially the subunit has been shown to be present in the substantia nigra. Also for Parkinson’s disease compounds that induce neuroprotective effects via interaction with nAChRs are of high therapeutic interest. Schizophrenia Schizophrenia is characterized by psychosis including hallucinations, delusions, apathy but also attentional deficits and cogntive disturbances. The dopamine hypothesis suggest that schizophrenia is due to excess dopamine. Involvement of other transmitter system including the glutamate system is presently discussed. It has been shown that patients with schizophrenia don’t inhibit the PSO auditory-evoked repsonse to a second stimuli. Interestingly, it was recently reported that a defect in auditory response maps to the a7 gene locus on chromosome 15 (see Freedman this symposium). Further characterization of the impairment of nAChRs in schizophrenia has to be performed. Toorette’s syndrome Patients with hyperkinetic motor disorder Tour&e’s syndrome which is characterized by frequent occurrence of motor and verbal tics but also hyperactivity and anxiety have in some few cases been reported to be improved by nicotine treatment. The tinding suggsts that nicotinic agonists treatment might be beneficial in this disease. References

[I] Warpman U, Nordbcrg A. Epibatidine and ABT 418 reveal selective losses [2]

[3]

[4] [5]

of a@2 nicotinic receptors in Alzheimer brains. NeuroReport 1995: 6: 24192423. Warpman U, Friberg L, Gillespie A, Hellstrom-Lindahl E, Zhang X, Nordherg A. Regulation of nicotinic receptor subtypes following chronic nicotinic agonist exposure in Ml0 and SH-SYSY neuroblastoma cells. J Neurochem 1998: 70: 2028-2037. Nordberg A, Lnndqvist H, Hartvig P, Anderson J, Johansson M, HellstriimLindahl E, L.&ngstim B. Imaging of nicotinic and muscarinic receptors in Alzheimer’s disease: effect of tacrine treatment. Dementia and Geriatric Cognitive Disorders 1997: 8: 78884. Svensson AL, Nordbetg A. Tacrineinteracts with an allosteric activator site on a4@2 nAChRs in Ml0 cells. NeuroReport 1996: 7: 2201-2205. Svensson AL, Nordberg A. Tacrine and donepezil attenuate the neurotoxic effect of A$ (25-35) in rat PC12 cells. NemoReport 1998, in press.

(s.28.051 Schizophrenia and nkotinic receptors: Gene expression and genetic linkage studies S. Leonard, C.R. Breese, J. Gault, L. Adler, A. Olincy, M. Lee, R. Freedman. Department of Psychiatry University of Colorado Health Sciences Center and The Department of Vetemns Affairs Medical Cente,: Denver, Colorado, USA

Gene expression and biological function of the neuronal nicotinic acetylcholine receptor gene family have been examined in schizophrenics and subjects with no history of mental illness. Schizophrenia is a relatively common mental illness, affecting more than 1% of the general population worldwide. It presents in early adulthood and is life-long, representing a large burden to both the families and to society. This disease is partially characterized by deficits in the processing of sensory information. We have studied two of these deficits, a failure to filter or gate extraneous auditory stimuli and abnormal eye tracking. Although both of these deficits arc found in about 10% of the population, most all subjects

and neurology

who suffer from schizophrenia exhibit these defects, as do approximately one-half of their first-degree relatives who do not have schizophrenia. These deficits are, thus, inherited in an autosomal dominant manner and represent endophenotypes predisposing the subject to schizophrenia (1) (Leonard et aZ., 1996). The incidence of smoking in the mentally ill is known to be higher (70-90%) than in the normal subjects (20-30%), and is particularly high in schizophrenics (85-90%). Schizophrenics often smoke high tar cigarettes and use multiple forms of tobacco. Nicotine, in the form of gum or cigarettes has been found to normalize both the auditory sensory gating deficit, seen in most affected subjects, as well as their abnormal eye-tracking, suggesting that a nicotinic receptor mechanism is involved in the neuronal pathways regulating the filtering of sensory information (1). Further studies in an animal model of auditory gating, using antagonists of specific nicotinic that only a-bungarotoxin and methyllycaconitine, specific antagonists of the a7 nicotinic receptor, produced a loss of auditory gating similar to that seen in schizophrenics. Fimbria-fomix lesions in the rat that lesion the cholinergic input into the hippocampus, also resulted in a loss of auditory gating. This deficit was normalized by nicotine (1). These pharmacological experiments suggest that a decrease in nicotinic receptor number, and specifically a reduction in a7 nicotinic receptor expression may result in a loss of auditory gating. A mouse strain (DBA) normally exhibits lower levels of [1251]a-bungarotoxin, which binds to the a7 receptor in mammalian tissue. DBA mice have a deficit in gating of auditory stimuli that is normalized by both nicotine and GTS-2 1. The latter is a partial agonist, specific for the a7 nicotinic receptor. Ligand binding studies in human postmortem brain, for both high and low affinity nicotinic receptors, show that expression of more than one type of a subunit may be decreased in schizophrenic subjects. In normal subjects, [3H]-nicotine binding in both hippocampus and thalamus is correlated with the number of cigarettes smoked per day (2). In smokers who had quit for varying periods before death, we found receptor levels in the non-smoking range. In schizophrenics, however, we did not see the same up-regulation in smokers. At every smoking level schizophrenics had fewer high afhnity receptors than control subjects. We had previously shown that binding to [‘25r]-a-bungarotoxin, which identifies the low-a&&y nicotinic receptor, a7, in mammalian brain, was also decreased in schizophrenics (1). These decreases in both high and low affinity nicotinic receptor numbers do not appear to be related to typical neuroleptic medication. We treated rats with nicotine and haloperidol, separately and together, finding that haloperidol did not affect up-regulation of nicotinic receptors by nicotine. A detailed regional study of a7 expression in human postmortem brain indicates that it is expressed in many sensory nuclei, including lateral and medial geniculates, hippocampus and the reticular thalamic nucleus (1). The reticular thalamic nucleus (RTN), a capsular structure surrounding the thalamus, provides the principal inhibitory input into the dorsal area of the thalamus. Localization of a7 receptors appears to be mainly on non-principal cells, but labeling was otten seen on dendrites as well as on cell somata. Genetic evidence also suggests a role for the a7 nicotinic receptor in schizophrenia. The a7 nicotinic receptor subunit maps to human chromosome 15q14. A full genome scan using more than 500 polymorphic markers at a 10 CM resolution, including a polymorphic dinucleotide repeat D15Sl360 that lies within 120 kb of the a7 gene, was run on nine pedigrees with familial schizophrenia in which status of the auditory gating deficit had been determined. Linkage was found with the auditory gating deficit to the D15Sl360 marker near the a7 gene with a lod of 5.3, 0 = 0.0 (3). We have examined linkage of the a7 locus to the schizophrenia phenotype in a second cohort, collected by the NIMH Genetics Initiative, and also find evidence for genetic linkage in this independent sample. These genetic studies complement the biological data supporting the involvement of the a7 receptor in schizophrenia. We have isolated both cDNA (GenBank Accession # U40983) and genomic clones for the human a7 gene (4). The full-length gene has 10 exons and is more than 65 kb in size. The exon/intron borders appear to be conserved across species, as they are identical to those found in the chick. The human a7 promoter is G/C rich and has no TATA box. Consensus sequences for several transcription factor binding