Potential role of muscarinic receptors in schizophrenia

A growing understanding of the role of muscarinic receptors in the molecular pathology and treatment of schizophrenia

Pre-clinical models, postmortem and neuroimaging studies all support a role for muscarinic receptors in the molecular pathology of schizophrenia. From these data it was proposed that activation of the muscarinic M1 and/or M4 receptor would reduce the severity of the symptoms of schizophrenia. This hypothesis is now supported by results from two clinical trials which indicate that activating central muscarinic M1 and M4 receptors can reduce the severity of positive, negative and cognitive symptoms of the disorder. This review will provide an update on a growing body of evidence that argues the muscarinic M1 and M4 receptors have critical roles in CNS functions that are dysregulated by the pathophysiology of schizophrenia. This realization has been made possible, in part, by the growing ability to visualize and quantify muscarinic M1 and M4 receptors in the human CNS using molecular neuroimaging. We will discuss how these advances have provided evidence to support the notion that there is a sub-group of patients within the syndrome of schizophrenia that have a unique molecular pathology driven by a marked loss of muscarinic M1 receptors. This review is timely, as drugs targeting muscarinic receptors approach clinical use for the treatment of schizophrenia and here we outline the background biology that supported development of such drugs to treat the disorder.

Antipsychotic-like effect of the muscarinic acetylcholine receptor agonist BuTAC in non-human primates

Cholinergic, muscarinic receptor agonists exhibit functional dopamine antagonism and muscarinic receptors have been suggested as possible future targets for the treatment of schizophrenia and drug abuse. The muscarinic ligand (5R,6R)-6-(3-butylthio-1,2,5-thiadiazol-4-yl)-1-azabicyclo[3.2.1]octane (BuTAC) exhibits high affinity for muscarinic receptors with no or substantially less affinity for a large number of other receptors and binding sites, including the dopamine receptors and the dopamine transporter. In the present study, we wanted to examine the possible antipsychotic-like effects of BuTAC in primates. To this end, we investigated the effects of BuTAC on d-amphetamine-induced behaviour in antipsychotic-naive Cebus paella monkeys. Possible adverse events of BuTAC, were evaluated in the same monkeys as well as in monkeys sensitized to antipsychotic-induced extrapyramidal side effects. The present data suggests that, the muscarinic receptor ligand BuTAC exhibits antipsychotic-like behaviour in primates. The behavioural data of BuTAC as well as the new biochemical data further substantiate the rationale for the use of muscarinic M₁/M₂/M₄-preferring receptor agonists as novel pharmacological tools in the treatment of schizophrenia.

Pharmacological approaches to the management of schizophrenia: 10 years on

OBJECTIVE

To review the contemporary landscape regarding pharmacological treatments for schizophrenia.

METHOD

Selective literature review.

RESULTS

Newer antipsychotic agents include aripiprazole, asenapine, paliperidone, sertindole and ziprasidone. Each has some particular benefits and some shortcomings. Overall treatment efficacy (for positive symptoms at least) has not advanced substantially but some newer agents might have a better profile than older typical agents for negative and cognitive symptoms. Metabolic side effects and hyperprolactinaemia remain a problem with some of the newer agents and appropriate monitoring is required.

CONCLUSIONS

Whilst newer antipsychotics have been welcome additions to our pharmacological armamentarium, mostly in terms of tolerability, we have still not seen a 'quantum leap' agent brought to market. Mechanisms of action apart from post-synaptic dopamine blockade appear worthy of further investigation in this regard.

Potential action of betel alkaloids on positive and negative symptoms of schizophrenia: a review

BACKGROUND

The Areca catechu Linn. is the fourth most used drug in the world after nicotine, ethanol and caffeine. This plant contains nine alkaloids with muscarinic and nicotinic action, which could have an antipsychotic effect.

AIM

The aim of this work is reviewing literature data about the potential action of betel alkaloids on positive, negative and cognitive symptoms of schizophrenia.

METHOD

We reviewed the clinical literature data since 1980 via a PubMed search for the terms: arecoline, arecaidine, guvacine, guvacoline, betel, Areca catechu, positive and negative symptoms, cognitive symptoms, psychosis and schizophrenia in combination.

CONCLUSION

Male high consumption of betel had significantly lower positive symptoms than low consumers or non-betel users.

Muscarinic mechanisms in psychotic disorders

Schizophrenia is a devastating disease with several broad symptom clusters and the current monoamine-based treatments do not adequately treat the disease, especially negative and cognitive symptoms. A proposed alternative approach for treating schizophrenia is through the use of compounds that activate certain muscarinic receptor subtypes, the so-called muscarinic cholinergic hypothesis theory. This theory has been revitalized with a number of recent and provocative findings including postmortem reports in schizophrenia patients showing decreased numbers of muscarinic M(1) and M(4) receptors in brain regions associated with schizophrenia as well as decreased muscarinic receptors in an in vivo imaging study. Studies with M(4) knockout mice have shown that there is a reciprocal relationship between M(4) and dopamine receptor function, and a number of muscarinic agonists have shown antidopaminergic activity in a variety of preclinical assays predictive of antipsychotic efficacy in the clinic. Furthermore, the M(1)/M(4) preferring partial agonist xanomeline has been shown to have antipsychotic-like and pro-cognitive activity in preclinical models and in clinical trials to decrease psychotic-like behaviors in Alzheimer's patients and positive, negative, and cognitive symptoms in patients with schizophrenia. Therefore, we propose that an agonist with M(1) and M(4) interactions would effectively treat core symptom clusters associated with schizophrenia. Currently, research is focused on developing subtype-selective muscarinic agonists and positive allosteric modulators that have reduced propensity for parasympathetic side-effects, but retain the therapeutic benefit observed with their less selective predecessors.

The muscarinic M(4) receptor is the functionally predominant subtype in rat and mouse striatum as demonstrated using [(35)S] GTPγS binding

We have used selective muscarinic receptor antagonists and M(2) and M(4) receptor knockout (KO) mouse tissue to define the functional muscarinic acetylcholine receptor populations in rodent striatum. [(3)H] NMS binding studies in rat and mouse striatum demonstrated that approximately 30% of muscarinic acetylcholine receptors expressed are M(1) receptors. Radioligand binding studies suggest that the remaining muscarinic acetylcholine receptor population is largely M(4) with small levels of M(2). In agreement, carbachol-induced GTPγS binding studies in M(2) and M(4) receptor KO mouse striatum implicated the M(4) receptor as the predominant functional receptor subtype. Based on these data we have developed a novel, native tissue M(4) receptor [(35)S] GTPγS binding assay. Pharmacological assessment of M(4) receptor agonist and positive 3modulators revealed clear differences in the potencies observed in a human recombinant CHO-M(4) receptor [(35)S] GTPγS binding assay as compared to the native tissue [(35)S] GTPγS binding assay. These differences are believed to reflect differences in receptor reserve between the assay systems as well as differences in compound pharmacology (relative contribution of compound affinity and efficacy to observed potency). These studies have demonstrated the importance of understanding the pharmacology of test compounds in a native environment when predicting in vivo response.

Acetylcholine-dopamine interactions in the pathophysiology and treatment of CNS disorders

Dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area of the midbrain form the nigrostriatal and mesocorticolimbic dopaminergic pathways that, respectively, project to dorsal and ventral striatum (including prefrontal cortex). These midbrain dopaminergic nuclei and their respective forebrain and cortical target areas are well established as serving a critical role in mediating voluntary motor control, as evidenced in Parkinson's disease, and incentive-motivated behaviors and cognitive functions, as exhibited in drug addiction and schizophrenia, respectively. Although it cannot be disputed that excitatory and inhibitory amino acid-based neurotransmitters, such as glutamate and GABA, play a vital role in modulating activity of midbrain dopaminergic neurons, recent evidence suggests that acetylcholine may be as important in regulating dopaminergic transmission. Midbrain dopaminergic cell tonic and phasic activity is closely dependent upon projections from hindbrain pedunculopontine and the laterodorsal tegmental nuclei, which comprises the only known cholinergic inputs to these neurons. In close coordination with glutamatergic and GABAergic activity, these excitatory cholinergic projections activate nicotinic and muscarinic acetylcholine receptors within the substantia nigra and ventral tegmental area to modulate dopamine transmission in the dorsal/ventral striatum and prefrontal cortex. Additionally, acetylcholine-containing interneurons in the striatum also constitute an important neural substrate to provide further cholinergic modulation of forebrain striatal dopaminergic transmission. In this review, we examine neurological and psychopathological conditions associated with dysfunctions in the interaction of acetylcholine and dopamine and conventional and new pharmacological approaches to treat these disorders.

Associations between betel nut (Areca catechu) and symptoms of schizophrenia among patients in Nepal: A longitudinal study

Betel nut is one of the mostly widely used substances in the world, particularly across Asia. Arecoline, a partial muscarinic agonist, has been hypothesized to have beneficial effects on both positive and negative symptoms of schizophrenia. This study aims to further explore associations between betel use and symptoms of schizophrenia in a 4-month longitudinal study in Nepal. Sixty Nepali patients with schizophrenia were recruited from regional outpatient clinics. The Positive and Negative Syndrome Scale (PANSS) and the Social Adaptation Self-Evaluation Scale were used to assess symptoms and social functioning in regular betel users and non-users. No significant group differences or dose-response relationships were noted on either initial or follow-up assessments. Stratifying by sex also failed to reveal an association between symptoms and betel use, which stands in contrast with previously reported data from Micronesia. There were no differences seen in social functioning other than a significantly higher proportion of betel users holding jobs. It was also noted that significantly fewer betel chewers were taking anti-cholinergic medication, which may tentatively indicate a potentially therapeutic role in the future for partial muscarinic agonists in the treatment of medication-induced movement disorders.

Neuroprotective role of curcumin in the cerebellum of streptozotocin-induced diabetic rats

AIMS

Chronic hyperglycaemia in diabetes involves a direct neuronal damage caused by intracellular glucose which leads to altered neurotransmitter functions and reduced motor activity. The present study investigated the effect of curcumin in the functional regulation of muscarinic and alpha7 nicotinic acetylcholine receptors, insulin receptors, acetylcholine esterase and Glut3 in the cerebellum of streptozotocin (STZ)-induced diabetic rats.

MAIN METHODS

All studies were done in the cerebellum of male Wistar rats. Radioreceptor binding assays were done for total muscarinic, M(1) and M(3) receptors using specific ligands, and the gene expression was also studied using specific probes.

KEY FINDINGS

Our results showed an increased gene expression of acetylcholine esterase, Glut3, muscarinic M1, M3, alpha7 nicotinic acetylcholine and insulin receptors in the cerebellum of diabetic rats in comparison to control. Scatchard analysis of total muscarinic, M1 and M3 receptors showed an increased binding parameter, B(max) in diabetic rats compared to control. Curcumin and insulin inhibited diabetes-induced elevation in the gene expression of acetylcholine esterase, Glut3, insulin and cholinergic receptors in the cerebellum of diabetic rats.

SIGNIFICANCE

Our studies suggest that curcumin plays a vital role in regulating the activity of cholinergic and insulin receptors and mechanism of glucose transportation through Glut3, which results in normalizing the diabetes-mediated cerebellar disorders. Thus, curcumin has a significant role in a therapeutic application for the prevention or progression of diabetic complications in the cerebellum.

Decreased M1 muscarinic receptor density in rat amphetamine model of schizophrenia is normalized by clozapine, but not haloperidol

There is increasing evidence supporting the involvement of the muscarinic-cholinergic system in schizophrenia. We examined the M1 muscarinic receptor density and mRNA expression in brains of a rat amphetamine model of schizophrenia. We also assessed the effect of the model and chronic treatment with haloperidol and clozapine on brain M1 receptor density and gene expression. A significant decrease of about 20% in the density of M1 receptor was detected in the cortex and in the striatum of amphetamine model rats. A significant increase of 33% in the density of the M1 receptor was found in the cortex and striatum of rats treated chronically with clozapine (0.5 mg/kg), but not with haloperidol (25 mg/kg). Chronic clozapine, but not haloperidol, normalized the decrease in M1 receptors observed in amphetamine model rats, in both cortex and striatum. Regulation of the M1 receptor may occur in a post-transcriptional phase. Our findings suggest involvement of both dopaminergic and cholinergic-muscarinic systems in the pathophysiology and pharmacotherapy of schizophrenia.

Evaluation of N-desmethylclozapine as a potential antipsychotic--preclinical studies

There is growing interest in N-desmethylclozapine (NDMC), the major metabolite of clozapine, as a unique antipsychotic because it acts in vitro as a 5-HT(2) antagonist and as a partial agonist to dopamine D(2) and muscarinic receptors. To explore this, we compared NDMC to a typical (haloperidol), atypical (clozapine), and partial-agonist atypical (aripiprazole) antipsychotic in preclinical models. The comparison was carried out using: brain D(2) and 5-HT(2) receptor occupancy; animal models predictive of antipsychotic efficacy (amphetamine-induced hyperlocomotion (AIL) and conditioned avoidance response (CAR) models); measures predictive of side effects (catalepsy and prolactin elevation); and molecular markers predictive of antipsychotic action (striatal Fos induction). NDMC (10-60 mg/kg/s.c.) showed high 5-HT(2) (64-79%), but minimal D(2) occupancy (<15% at 60 mg/kg) 1 h after administration. In contrast to other antipsychotics, NDMC was not very effective in reducing AIL or CAR and showed minimal induction of Fos in the nucleus accumbens. However, like atypical antipsychotics, it showed no catalepsy, prolactin elevation, and minimal Fos in the dorsolateral striatum. It seems unlikely that NDMC would show efficacy as a stand-alone antipsychotic, however, its freedom from catalepsy and prolactin elevation, and its unique pharmacological profile (muscarinic agonism) may make it feasible to use this drug as an adjunctive treatment to existing antipsychotic regimens.

F15063, a compound with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. III. Activity in models of cognition and negative symptoms

BACKGROUND AND PURPOSE

The D(2)/D(3) receptor antagonist, D(4) receptor partial agonist, and high efficacy 5-HT(1A) receptor agonist F15063 was shown to be highly efficacious and potent in rodent models of activity against positive symptoms of schizophrenia. However F15063 induced neither catalepsy nor the 'serotonin syndrome'. Here, we evaluated its profile in rat models predictive of efficacy against negative symptoms/cognitive deficits of schizophrenia.

EXPERIMENTAL APPROACH

F15063, given i.p., was assessed in models of behavioural deficits induced by interference with the NMDA/glutamatergic (phencyclidine: PCP) or cholinergic (scopolamine) systems.

KEY RESULTS

Through 5-HT(1A) activation, F15063 partially alleviated (MED: 0.04 mg kg(-1)) PCP-induced social interaction deficit between two adult rats, without effect by itself, underlining its potential to combat negative symptoms. At doses above 0.16 mg kg(-1), F15063 reduced interaction by itself. F15063 (0.16 mg kg(-1)) selectively re-established PCP-impaired 'cognitive flexibility' in a reversal learning task, suggesting potential against adaptability deficits. F15063 (0.04-0.63 mg kg(-1)) also reversed scopolamine-induced amnesia in a juvenile-adult rat social recognition test, indicative of a pro-cholinergic influence. Activity in this latter test is consistent with its D(4) partial agonism, as it was blocked by the D(4) antagonist L745,870. Finally, F15063 up to 40 mg kg(-1) did not disrupt basal prepulse inhibition of startle reflex in rats, a marker of sensorimotor gating.

CONCLUSIONS AND IMPLICATIONS

The balance of D(2)/D(3), D(4) and 5-HT(1A) receptor interactions of F15063 yields a promising profile of activity in models of cognitive deficits and negative symptoms of schizophrenia.

SSR180711, a novel selective alpha7 nicotinic receptor partial agonist: (II) efficacy in experimental models predictive of activity against cognitive symptoms of schizophrenia

SSR180711 (4-bromophenyl 1,4diazabicyclo(3.2.2) nonane-4-carboxylate, monohydrochloride) is a selective alpha7 nicotinic receptor (n-AChR) partial agonist. Based on the purported implication of this receptor in cognitive deficits associated with schizophrenia, the present study assessed efficacy of SSR180711 (i.p. and p.o.) in different types of learning and memory involved in this pathology. SSR180711 enhanced episodic memory in the object recognition task in rats and mice (MED: 0.3 mg/kg), an effect mediated by the alpha7 n-AChR, as it was no longer seen in mice lacking this receptor. Efficacy was retained after repeated treatment (eight administrations over 5 days, 1 mg/kg), indicating lack of tachyphylaxia. SSR180711 also reversed (MED: 0.3 mg/kg) MK-801-induced deficits in retention of episodic memory in rats (object recognition). The drug reversed (MED: 0.3 mg/kg) selective attention impaired by neonatal phencyclidine (PCP) treatment and restored MK-801- or PCP-induced memory deficits in the Morris or linear maze (MED: 1-3 mg/kg). In neurochemical and electrophysiological correlates of antipsychotic drug action, SSR180711 increased extracellular levels of dopamine in the prefrontal cortex (MED: 1 mg/kg) and enhanced (3 mg/kg) spontaneous firing of retrosplenial cortex neurons in rats. Selectivity of SSR180711 was confirmed as these effects were abolished by methyllycaconitine (3 mg/kg, i.p. and 1 mg/kg, i.v., respectively), a selective alpha7 n-AChR antagonist. Additional antidepressant-like properties of SSR180711 were demonstrated in the forced-swimming test in rats (MED: 1 mg/kg), the maternal separation-induced ultrasonic vocalization paradigm in rat pups (MED: 3 mg/kg) and the chronic mild stress procedure in mice (10 mg/kg o.d. for 3 weeks). Taken together, these findings characterize SSR180711 as a promising new agent for the treatment of cognitive symptoms of schizophrenia. The antidepressant-like properties of SSR180711 are of added interest, considering the high prevalence of depressive symptoms in schizophrenic patients.

Cholinergic circuits and signaling in the pathophysiology of schizophrenia

Central cholinergic signaling has long been associated with aspects of memory, motivation, and mood, each affected functions in neuropsychiatric disorders such as schizophrenia. In this chapter, we review evidence related to the core hypothesis that dysregulation of central cholinergic signaling contributes to the pathophysiology of schizophrenia. Although central cholinergic circuits are resistant to simplification-particularly when one tries to parse the contributions of various classes of cholinergic receptors to disease related phenomena--the potential role of ACh signaling in Schizophrenia pathophysiology deserves careful consideration for prospective therapeutics. The established role of cholinergic circuits in attentional tuning is considered along with recent work on how the patterning of cholinergic activity may modulate corticostriatal circuits affected in schizophrenia.

Performance of F2 B6x129 hybrid mice in the Morris water maze, latent inhibition and prepulse inhibition paradigms: Comparison with C57Bl/6J and 129sv inbred mice

Assessment of cognition and information processing in mice is an important tool in preclinical research that focuses on the development of cognitive enhancing drugs. Analysis of transgenic (TG) and knockout (KO) mice is usually performed on a F2 B6x 129 background. In the present study, we have compared performance of F2 B6x 129 hybrid mice (F2 mice) with that of the two parental inbred strains (C57Bl/6J and 129sv mice), and a wild-type (WT) strain (with a combined B6x 129 background) in three cognitive/information processing paradigms. It was found that the F2 mice outperformed either of the parental strains and provide a control sample with good baseline performance in the Morris water maze (MWM). Reliable deficits could be obtained in learning and memory in this paradigm following injections with scopolamine (0.16 mg/kg) in the F2 mice, which can potentially be used to test effects of reference and novel compounds in order to develop cognitive enhancing drugs. Furthermore, it was shown that the four genotypes showed normal latent inhibition (LI) using the conditioned taste aversion (CTA) paradigm and exhibited no differences in prepulse inhibition (PPI) levels. Following the setup of these procedures in mice, we are now able to compare the effects of gene knockout/mutations used for target validation with results in the present study as a frame of reference.

Elongation factor 1A family regulates the recycling of the M4 muscarinic acetylcholine receptor

In this study, we tested the hypothesis that the elongation 1A (eEF1A) family regulates the cell surface density of the M4 subtype of the muscarinic acetylcholine receptors (mAChR) following agonist-induced internalization. Here, we show that mouse brains lacking eEF1A2 have no detectable changes in M4 expression or localization. We, however, did discover that eEF1A1, the other eEF1A isoform, is expressed in adult neurons contrary to previous reports. This novel finding suggested that the lack of change in M4 expression and distribution in brains lacking eEF1A2 might be due to compensatory effects of eEF1A1. Supporting this theory, we demonstrate that the overexpression of either eEF1A1 or eEF1A2 inhibits M4 recovery to the cell surface after agonist-induced internalization in PC12 cells. Furthermore, eEF1A1 or eEF1A2 had no effect on the recovery of the M1 subtype in PC12 cells. These results demonstrate the novel ability of the eEF1A family to specifically regulate the M4 mAChR.

Regulation of m1 muscarinic receptors and nNOS mRNA levels by autoantibodies from schizophrenic patients

In this paper we demonstrate that, circulating antibodies from schizophrenic patients interacting with cerebral M1 muscarinic acetylcholine receptors (M1 mAChRs), can act as an inducer of m1 mAChR-mRNA, and neuronal nitric oxide synthase (nNOS) mRNA gene expression of rat frontal cortex. The different signaling pathways involved in the autoantibody's actions, were characterized. As previously reported serum autoantibodies from schizophrenic patients reacted against neural cells surface inhibiting the binding of the specific mAChR radioligand to rat cerebral frontal cortex membrane. Moreover, by ELISA using M1 synthetic peptide (with identical aminoacid sequence to human M1 mAChR) as coating antigen we demonstrated the reactivity against the second extracellular loop of human cerebral M1 mAChR. The corresponding affinity-purified anti M1 peptide IgG (anti M1 peptide IgG) from schizophrenic patients by stimulation of M1 mAChR exerted an increase in m1 mAChR-mRNA and nNOS-mRNA levels, that significantly correlated with the accumulation of phosphoinositides (IPs) and activation of NOS (alpha = 0.05). All these effects were blunted by pirenzepine and mimicked the action of the authentic agonist. Concurrent analysis of the effects of nNOS, phospholipase C (PLC) and calcium/calmodulin (CaM) inhibition on both, m1 mAChR-mRNA and nNOS-mRNA levels, showing that antibody up-regulation mRNA level is under the control of endogenous nitric oxide (NO) signaling system. On the basis of our results, the activation of M1 mAChR by schizophrenic autoantibody appears to induce nNOS-mRNA expression and reciprocally, the activation of NOS up-regulates m1 mAChR gene expression. These results gave support to the participation of an autoimmune process in a particular group of chronic schizophrenic patients.

Regulation of synaptic plasticity in a schizophrenia model

The pathology of schizophrenia is characterized by increased hippocampal activity at baseline and during auditory hallucinations. Animal-model studies in which the flow of activity to the hippocampus is increased through decreased amygdalar GABAergic inhibition have shown alterations of hippocampal circuitry similar to schizophrenia, but the functional importance of this phenomenon remains unclear. We provide evidence of decreased hippocampal feed-forward and tonic GABA-mediated inhibition in this animal model, complementing increased hippocampal activity seen in neuroimaging and postmortem studies. We demonstrate that GABA dysfunction increases long-term potentiation through activation of the cholinergic system, offering a new mechanism for pharmacological strategies of this disorder.

Midbrain muscarinic receptor mechanisms underlying regulation of mesoaccumbens and nigrostriatal dopaminergic transmission in the rat

Laterodorsal (LDT) and pedunculopontine (PPT) tegmental nuclei in the mesopontine project cholinergic inputs to the midbrain ventral tegmental area (VTA) and substantia nigra pars compacta (SNc), respectively, to directly and indirectly influence the activity of dopamine neuronal cells via actions on muscarinic and nicotinic receptors. The present study investigated the role of midbrain muscarinic receptors in the functional modulation of VTA and SNc dopamine cell activity as reflected by alterations in, respectively, nucleus accumbens (NAc) and striataldopamine efflux. In vivo chronoamperometry was used to measure changes in basal dopamine efflux via stearate-graphite paste electrodes implanted unilaterally in the NAc or striatum of urethane-anaesthetized rats, following blockade or activation of, respectively, VTA or SNc muscarinic receptors. Intra-VTA or -SNc infusion of the muscarinic antagonist scopolamine (200 microg/microL) reduced, respectively, NAc and striatal dopamine efflux while infusion of the muscarinic and nicotinic agonist carbachol (0.5 microg/microL) or the prototypical muscarinic agonist muscarine (0.5 microg/microL) increased NAc and striatal dopamine efflux. Transient decreases in dopamine efflux preceded these increases selectively in the striatum, suggesting a reduction in excitatory or increase in inhibitory drive to the SNc by preferential activation of M3 muscarinic receptors on GABA interneurons and glutamatergic inputs. This was confirmed by showing that selective blockade of M3 receptors with p-F-HHSiD (0.5 microg/microL) increased striatal, but not NAc, dopamine efflux. Together, these findings suggest that midbrain muscarinic receptors, probably M5 subtypes on VTA and SNc dopamine neurons, contribute to the tonic excitatory regulation of forebrain basal dopamine transmission whereas presynaptic M3 receptors serve to counter excessive excitation of nigral dopamine cell activity.

Decreased amphetamine-induced locomotion and improved latent inhibition in mice mutant for the M5 muscarinic receptor gene found in the human 15q schizophrenia region

M5 muscarinic receptors are coexpressed with D2 dopamine receptors in the ventral tegmentum and striatum, and are important for reward in rodents. Previously, we reported that disruption of the M5 receptor gene in mice reduced dopamine release in the nucleus accumbens. In this study, we established a polymerase chain reaction (PCR) genotyping method for M5 mutant mice, and, using RT-PCR, found that M5 mRNA expression was highest in the ventral tegmentum, striatum, and thalamus in wild-type mice. In the M5 mutant mice, D2 mRNA expression was increased in several brain structures, including the striatum. Genome mapping studies showed the M5 gene is localized to chromosome 2E4 in mice, and to 15q13 in humans in the region that has been linked to schizophrenia. Amphetamine-induced locomotion, but not baseline locomotion or motor functions, decreased in M5 mutant mice, consistent with lower accumbal dopamine release. Previous reports found latent inhibition improvement in rats following nucleus accumbens lesions, or blockade of dopamine D2 receptors with neuroleptic drugs. Here, latent inhibition was significantly increased in M5 mutant mice as compared with controls, consistent with reduced dopamine function in the nucleus accumbens. In summary, our results showed that M5 gene disruption in mice decreased amphetamine-induced locomotion and increased latent inhibition, suggesting that increased M5 mesolimbic function may be relevant to schizophrenia.

Autoantibodies against cerebral muscarinic cholinoceptors in Sjögren syndrome: functional and pathological implications

Previous studies have demonstrated that antibodies against muscarinic acetylcholine receptors (mAChRs) from exocrine glands, correlates with Sjögren syndrome (SS) in the majority of patients. The aim of the present investigation was to establish if serum IgG antibodies present in SS interacts with cerebral mAChRs. Results show that anti-cerebral IgG are present in the sera of 40% SS patients studied. Autoantibodies were able to interact with mAChRs of cerebral frontal cortex membranes inhibiting the [(3)H]QNB binding to its specific receptor. Moreover, tested by ELISA and dot blot they recognized the synthetic peptides corresponding to the second extracellular loop of human M(1) and M(3) mAChR. In addition, the corresponding affinity-purified anti-M(1) and anti-M(3) peptide IgGs displayed an agonistic activity, stimulating phosphoinositide hydrolysis. The results support the notion that serum IgG autoantibodies in SS patients target cerebral mAChRs may have some role in the pathogenesis of higher cognitive dysfunction present in SS patients.

Loss of cortical acetylcholine enhances amphetamine-induced locomotor activity

Cholinergic disturbances have been implicated in schizophrenia. In a recent study we found that intracerebroventricular (i.c.v.) delivery of the immunotoxin 192 IgG-saporin, that effectively destroys cholinergic projections from the basal forebrain to hippocampus and cortex cerebri, leads to a marked facilitation of amphetamine-induced locomotor activity in adult rats. The aim of the present experiments was to evaluate the contribution of the septohippocampal versus the basalocortical cholinergic projections for the amphetamine hyper-response seen previously in i.c.v. 192 IgG-saporin injected rats. Since i.c.v. delivery of 192 IgG-saporin also destroys a population of Purkinje neurons in cerebellum, this cell loss needs to be taken into consideration as well. Cortex cerebri and hippocampus were selectively cholinergically denervated by intraparenchymal injections of 192 IgG-saporin into nucleus basalis magnocellularis and the medial septum/diagonal band of Broca, respectively. Selective loss of Purkinje cells in cerebellum was achieved by i.c.v. delivery of OX7 saporin. Possible effects of these three lesions on spontaneous and amphetamine-induced locomotor activity were assessed in locomotor activity cages. We find that selective cholinergic denervation of cortex cerebri, but not denervation of hippocampus or damage to cerebellum can elicit dopaminergic hyper-reactivity similar to that seen in previous i.c.v. 192 IgG-saporin experiments. Our data are compatible with the hypothesis that disturbances of cholinergic neurotransmission in cortex cerebri may be causally involved in forms of schizophrenia.

Association of muscarinic m1 receptor genetic polymorphisms with psychiatric symptoms and cognitive function in schizophrenic patients

OBJECTIVE

The cholinergic system is important in the search for the pathophysiology of schizophrenia due to its role in cognitive function, interaction with the dopamine system in brain regions relevant to schizophrenia, side effects of antipsychotic medication and potential antipsychotic effect of muscarinic receptor antagonists. This study investigated the association of type I muscarinic receptor (CHRM1) genetic polymorphisms with the clinical characteristics of chronic schizophrenic inpatients.

METHODS

We determined the genotype of CHRM1 genetic polymorphisms in 243 schizophrenic patients hospitalized in chronic care wards. Psychotic symptoms were assessed using the Brief Psychiatric Rating Scale (BPRS), and cognitive function was assessed using the Folstein Mini-Mental Status Examination (MMSE) test. Sixty of the 243 subjects also completed the Wisconsin Card Sorting Test (WCST).

RESULTS

There was a significant difference in the number of correct responses and the percentage of perseverative errors in the WCST in the CHRM1 C267A genotype group of schizophrenia patients. There was no significant association between age at onset, chlorpromazine equivalents, BPRS scores, MMSE or schizophrenia per se in patients with the CHRM1 C267A genotype. The full exon of the CHRM1 gene was screened out with single-strand conformation polymorphism, and 2 single nucleotide polymorphisms (C267A and C1353T) were identified in our patients and control subjects. These 2 single nucleotide polymorphisms were linked together without exception.

CONCLUSION

This study demonstrated that in schizophrenic patients, the heterozygote group of CHRM1 C267A polymorphism (267C/A) had more correct responses and less perseverative errors on the WCST performance than the 267C/C homozygote group, implicating that this polymorphism may be related to prefrontal cortical function. Our results also suggested that the C267A polymorphism plays no major role in the susceptibility to and clinical manifestations of schizophrenia.

Antibodies against astrocyte M1and M2muscarinic cholinoceptor from schizophrenic patients' sera

We demonstrated the presence of circulating antibodies from schizophrenic patients able to interact with cultured astrocytes activating muscarinic acetylcholine receptors (mAChRs). Sera and purified IgG from 15 paranoid schizophrenic and 15 age-matched normal subjects were studied by indirect immunofluorescence (IFI), flow cytometry, dot blot, enzyme immunoassay (ELISA), and radioligand competition assays. Astrocyte membranes and/or a synthetic peptide, with identical amino acid sequence of human M(1) and M(2) mAChR, were used as antigens. By IFI and flow cytometry procedures, we proved that serum purified IgG fraction from schizophrenic patients, reacted to astrocyte cell surface. The same antibodies were able to inhibit the binding of the specific mAChR radioligand (3)H-QNB. Using synthetic peptide for dot blot and ELISA, we demonstrated that these antibodies reacted against the second extracellular loop of human cerebral M(1) and M(2) mAChR. Also, the corresponding affinity-purified antipeptide antibody displayed an agonistic-like activity associated to specific M(1) and M(2) mAChR activation, increasing inositol phosphates accumulation and decreasing cyclic AMP production, respectively. This article gives support to the participation of an autoimmune process in schizophrenia disease.

Muscarinic mechanisms of antipsychotic atypicality

The interactions of the atypical antipsychotic drugs (APD) clozapine, olanzapine, risperidone, quetiapine and ziprasidone with muscarinic receptors were reviewed. Only clozapine and olanzapine have marked affinity for muscarinic receptors in radioligand binding studies; however, the affinity of these compounds is considerably lower than classical muscarinic antagonists. Although functional assays in cell lines transfected with muscarinic receptors suggest that olanzapine and clozapine have weak partial agonist activity at muscarinic receptors, particularly M4 receptors, studies in vitro and in vivo indicate that the compounds function as antagonists. In animal studies and in humans, clozapine has pronounced antimuscarinic effects whereas olanzapine has weak antimuscarinic effects. However, olanzapine significantly occupies central muscarinic receptors in humans. Overall, the role of muscarinic receptors in the antipsychotic effects of clozapine and olanzapine is controversial and complex.

Xanomeline and the antipsychotic potential of muscarinic receptor subtype selective agonists

Binding studies initially suggested that the muscarinic agonist, xanomeline, was a subtype selective muscarinic M(1) receptor agonist, and a potential new treatment for Alzheimer's disease. However, later in vitro and in vivo functional studies suggest that this compound is probably better described as a subtype selective M(1)/M(4) muscarinic receptor agonist. This subtype selectivity profile has been claimed to explain the limited classical cholinomimetic side effects, particularly gastrointestinal, seen with xanomeline in animals. However, in both healthy volunteers and Alzheimer's patients many of these side effects have been reported for xanomeline and in the patient population this led to a >50% discontinuation rate. Clearly, the preclinical studies have not been able to predict this adverse profile of xanomeline, and this suggests that either xanomeline is not as subtype selective as predicted from preclinical research or that there are differences between humans and animals with regard to muscarinic receptors. Nevertheless, in Alzheimer's patients xanomeline dose-dependently improves aspects of behavioral disturbance and social behavior including a reduction in hallucinations, agitation, delusions, vocal outbursts and suspiciousness. The effects on cognition are not as robust and mainly seen at the highest doses tested. These effects in Alzheimer's patients have given impetus to the suggestion that muscarinic agonists have potential antipsychotic effects. The current review assesses the antipsychotic profile of xanomeline within the framework of the limited clinical studies with cholinergic agents in man, and the preclinical research on xanomeline using various models commonly used for the assessment of new antipsychotic drugs. In general, xanomeline has an antipsychotic-like profile in various dopamine models of psychosis and this agrees with the known interactions between the cholinergic and dopaminergic systems in the brain. Moreover, current data suggests that the actions of xanomeline at the M(4) muscarinic receptor subtype might mediate its antidopaminergic effects. Particularly intriguing are studies showing that xanomeline, even after acute administration, selectively inhibits the firing of mesolimbic dopamine cells relative to dopamine cell bodies projecting to the striatum. This data suggest that xanomeline would have a faster onset of action compared to current antipsychotics and would not induce extrapyramidal side effects. The preclinical data on the whole are promising for an antipsychotic-like profile. If in a new formulation (i.e., transdermal) xanomeline has less adverse effects, this drug may be valuable in the treatment of patients with psychosis.

Role of specific muscarinic receptor subtypes in cholinergic parasympathomimetic responses, in vivo phosphoinositide hydrolysis, and pilocarpine-induced seizure activity

Muscarinic agonist-induced parasympathomimetic effects, in vivo phosphoinositide hydrolysis and seizures were evaluated in wild-type and muscarinic M1-M5 receptor knockout mice. The muscarinic agonist oxotremorine induced marked hypothermia in all the knockout mice, but the hypothermia was reduced in M2 and to a lesser extent in M3 knockout mice. Oxotremorine-induced tremor was abolished only in the M2 knockout mice. Muscarinic agonist-induced salivation was reduced to the greatest extent in M3 knockout mice, to a lesser degree in M1 and M4 knockout mice, and was not altered in M2 and M5 knockout mice. Pupil diameter under basal conditions was increased only in the M3 knockout mice. Pilocarpine-induced increases in in vivo phosphoinositide hydrolysis were completely absent in hippocampus and cortex of M1 knockout mice, but in vivo phosphoinositide hydrolysis was unaltered in the M2-M5 knockout mice. A high dose of pilocarpine (300 mg/kg) caused seizures and lethality in wild-type and M2-M5 knockout mice, but produced neither effect in the M1 knockout mice. These data demonstrate a major role for M2 and M3 muscarinic receptor subtypes in mediating parasympathomimetic effects. Muscarinic M1 receptors activate phosphoinositide hydrolysis in cortex and hippocampus of mice, consistent with the role of M1 receptors in cognition. Muscarinic M1 receptors appear to be the only muscarinic receptor subtype mediating seizures.

Use of M1-M5 muscarinic receptor knockout mice as novel tools to delineate the physiological roles of the muscarinic cholinergic system

In this review we report recent findings on the physiological role of the five known muscarinic acetylcholine receptors (mAChRs) as shown by gene targeting technology. Using knockout mice for each mAChRs subtype, the role of mAChRs subtypes in a number of physiological functions was confirmed and new activities were discovered. The M1 mAChRs modulate neurotransmitter signaling in cortex and hippocampus. The M3 mAChRs are involved in exocrine gland secretion, smooth muscle contractility, pupil dilation, food intake, and weight gain. The role of the M5 mAChRs involves modulation of central dopamine function and the tone of cerebral blood vessels. mAChRs of the M2 subtype mediate muscarinic agonist-induced bradycardia, tremor, hypothermia, and autoinhibition of release in several brain regions. M4 mAChRs modulate dopamine activity in motor tracts and act as inhibitory autoreceptors in striatum. Thus, as elucidated by gene targeting technology, mAChRs have widespread and manifold functions in the periphery and brain.

Role of the cholinergic muscarinic system in bipolar disorder and related mechanism of action of antipsychotic agents

The evidence for the involvement of cholinergic muscarinic receptors in mania and depression is reviewed. Small pilot trials with cholinesterase inhibitors and muscarinic agonists suggest that stimulation of muscarinic receptors may produce an antimanic effect, possibly by activation of muscarinic M(4) receptors. It is concluded that it is not likely that currently used mood stabilizers, such as lithium, valproic acid and carbamazepine, work directly through muscarinic receptor mechanisms. Furthermore, the evidence indicates that antipsychotic agents used for mania are working through the common mechanism of antagonism of dopamine D(2) receptors, and interactions with muscarinic receptors do not play a key role. Finally, it is hypothesized that olanzapine has robust antimanic activity, due to blockade of dopamine D(2) receptors and antagonism of other monoaminergic receptors. Olanzapine may normalize mood due to antidepressant-like activities, such as 5-HT(2A) receptor antagonism and increasing cortical norepinephrine and dopamine.

Novel interaction between the M4 muscarinic acetylcholine receptor and elongation factor 1A2

The activation of the muscarinic acetylcholine receptor (mAChR) family, consisting of five subtypes (M1-M5), produces a variety of physiological effects throughout the central nervous system. However, the role of each individual subtype remains poorly understood. To further elucidate signal transduction pathways for specific subtypes, we used the most divergent portion of the subtypes, the intracellular third (i3) loop, as bait to identify interacting proteins. Using a brain pull-down assay, we identify elongation factor 1A2 (eEF1A2) as a specific binding partner to the i3 loop of M4, and not to M1 or M2. In addition, we demonstrate a direct interaction between these proteins. In the rat striatum, the M4 mAChR colocalizes with eEF1A2 in the soma and neuropil. In PC12 cells, endogenous eEF1A2 co-immunoprecipitates with the endogenous M4 mAChR, but not with the endogenous M1 mAChR. In our in vitro model, M4 dramatically accelerates nucleotide exchange of eEF1A2, a GTP-binding protein. This indicates the M4 mAChR is a guanine exchange factor for eEF1A2. eEF1A2 is an essential GTP-binding protein for protein synthesis. Thus, our data suggest a novel role for M4 in the regulation of protein synthesis through its interaction with eEF1A2.

5-HT(1A) and 5-HT(2A) receptors minimally contribute to clozapine-induced acetylcholine release in rat medial prefrontal cortex

The atypical antipsychotic drugs (APDs) clozapine, olanzapine, risperidone, and ziprasidone preferentially increase dopamine (DA) release in rat medial prefrontal cortex (mPFC). These effects have been shown to depend upon potent 5-HT(2A) relative to weak D(2) antagonism, and 5-HT(1A) agonism as well. Atypical APDs also increase acetylcholine (ACh) release in the mPFC, but not the nucleus accumbens (NAC) or striatum (STR), whereas typical APDs such as haloperidol, S(-)-sulpiride and thioridazine do not produce either effect in the mPFC. This study examined the role of 5-HT(1A) agonism, 5-HT(2A) and D(2) antagonism, and the combination thereof, in the ability of clozapine to increase ACh release in rat mPFC. R(+)-8-OH-DPAT (0.2 mg/kg), a 5-HT(1A) agonist, WAY100635 (0.2-0.5 mg/kg), a 5-HT(1A) antagonist, and DOI (0.6-2.5 mg/kg), a 5-HT(2A/2C) agonist, increased ACh release in the mPFC, whereas M100907 (0.03-1 mg/kg), a 5-HT(2A) antagonist, did not. DOI (2.5 mg/kg) and M100907 (0.1 mg/kg) had no effect on ACh release in the NAC or STR. WAY100635 and M100907 inhibited the ability of R(+)-8-OH-DPAT and DOI, respectively, to increase ACh release in the mPFC. WAY100635, which inhibits clozapine-induced DA release in the mPFC, failed to inhibit clozapine (20 mg/kg)-induced ACh release in that region. Similarly, the combination of M100907 and haloperidol (0.1 mg/kg), which enhances DA release in the mPFC, failed to increase ACh release in that region. These results suggest that 5-HT(1A) agonism and 5-HT(2A) antagonism, as well as DA release, contribute minimally to the ability of clozapine, and perhaps other atypical APDs, to increase ACh release in the mPFC.

Antibodies against cerebral M1 cholinergic muscarinic receptor from schizophrenic patients: molecular interaction

We demonstrated the presence of circulating Abs from schizophrenic patients able to interact with cerebral frontal cortex-activating muscarinic acetylcholine receptors (mAChR). Sera and purified IgG from 21 paranoid schizophrenic and 25 age-matched normal subjects were studied by indirect immunofluorescence, flow cytometry, immunoblotting, dot blot, ELISA, and radioligand competition assays. Rat cerebral frontal cortex membranes and/or a synthetic peptide, with an amino acid sequence identical with that of human M(1) mAChR, were used as Ags. By indirect immunofluorescence and flow cytometry procedures, we proved that serum-purified IgG fraction from schizophrenic patients reacted to neural cell surfaces from rat cerebral frontal cortex. The same Abs were able to inhibit the binding of the specific M(1) mAChR radioligand [(3)H]pirenzepine. Immunoblotting experiments showed that IgG from schizophrenic patients revealed a band with a molecular mass coincident to that labeled by an anti-M(1) mAChR Ab. Using synthetic peptide for dot blot and ELISA, we demonstrated that these Abs reacted against the second extracellular loop of human cerebral M(1) mAChR. Also, the corresponding affinity-purified antipeptide Ab displayed an agonistic-like activity associated to specific receptor activation, increasing cyclic GMP production and inositol phosphate accumulation, and protein kinase C translocation. This paper gave support to the participation of an autoimmune process in schizophrenia.

Hyperactivity, elevated dopaminergic transmission, and response to amphetamine in M1 muscarinic acetylcholine receptor-deficient mice

Acetylcholine serves an important modulatory role in the central nervous system. Pharmacological evidence has suggested that cholinergic activity can modulate central dopaminergic transmission; however, the nature of this interaction and the receptors involved remain undefined. In this study we have generated mice lacking the M1 muscarinic acetylcholine receptor and examined the effects of M1 deletion on dopaminergic transmission and locomotor behavior. We report that M1 deficiency leads to elevated dopaminergic transmission in the striatum and significantly increased locomotor activity. M1-deficient mice also have an increased response to the stimulatory effects of amphetamine. Our results provide direct evidence for regulation of dopaminergic transmission by the M1 receptor and are consistent with the idea that M1 dysfunction could be a contributing factor in psychiatric disorders in which altered dopaminergic transmission has been implicated.

Hyperactivity and intact hippocampus-dependent learning in mice lacking the M1 muscarinic acetylcholine receptor

Members of the muscarinic acetylcholine receptor family are thought to play key roles in the regulation of a large number of important functions of the CNS. However, the precise roles of the individual muscarinic receptor subtypes in modulating these processes are not well understood at present, primarily because of the lack of ligands with sufficient receptor subtype selectivity. To investigate the behavioral significance of the M(1) muscarinic receptor (M(1)R), which is abundantly expressed in the forebrain, we subjected M(1) receptor-deficient mice (M(1)R(-/-) mice) to a battery of behavioral tests. M(1)R(-/-) mice showed no significant impairments in neurological reflexes, motor coordination, pain sensitivity, and prepulse inhibition. Strikingly, however, M(1)R(-/-) mice consistently exhibited a pronounced increase in locomotor activity in various tests, including open field, elevated plus maze, and light/dark transition tests. Moreover, M(1)R(-/-) mice showed reduced immobilization in the Porsolt forced swim test and reduced levels of freezing after inescapable footshocks, suggesting that M(1)R(-/-) mice are hyperactive under stressful conditions as well. An increased number of social contacts was observed in a social interaction test. Surprisingly, M(1)R(-/-) mice displayed no significant cognitive impairments in the Morris water maze and in contextual fear conditioning. M(1)R(-/-) mice showed slight performance deficits in auditory-cued fear conditioning and in an eight-arm radial maze, most likely because of the hyperactivity phenotype displayed by the M(1)R(-/-) mice. Our results indicate that M(1) muscarinic receptors play an important role in the regulation of locomotor activity but appear to be less critical for cognitive processes, as generally assumed.

The muscarinic receptor agonist BuTAC, a novel potential antipsychotic, does not impair learning and memory in mouse passive avoidance

(5R,6R)-6-(3-butylthio-1,2,5-thiadiazol-4-yl)-1-azabicyclo[3.2.1]octane) (BuTAC) is a novel, selective muscarinic receptor ligand with partial agonist mode of action at muscarinic M2 and M4 and antagonist mode of action at M1, M3 and M5 receptor subtypes in cloned cell lines. BuTAC exhibits functional dopamine receptor antagonism despite its lack of affinity for dopamine receptors, and parasympathomimetic effects in mice are produced only at doses well beyond the doses exhibiting the antipsychotic-like effects. In the present study we investigated the effects of BuTAC and the antipsychotic compounds clozapine, sertindole and olanzapine using one trial passive avoidance with mice as a model of learning and memory. Pharmacologically relevant doses of BuTAC and reference antipsychotics were identified, based on inhibition of apomorphine-induced climbing in mice as an assay measuring antidopaminergic potency. When ratios between the minimum effective dose (MED) for impairment of retention in passive avoidance and the MED for inhibition of apomorphine-induced climbing were calculated, BuTAC displayed a high ratio of >10, compared with clozapine (0.3), sertindole (3) and olanzapine (3). These data suggest that BuTAC is a potential novel antipsychotic which may have favourable effects on aspects of learning and memory.

The muscarinic agonist xanomeline increases monoamine release and immediate early gene expression in the rat prefrontal cortex

BACKGROUND

The muscarinic agonist xanomeline has been shown to reduce antipsychotic-like behaviors in patients with Alzheimer's disease. Because atypical antipsychotic agents increase dopamine release in prefrontal cortex and induce immediate early gene expression in prefrontal cortex and nucleus accumbens, the effect of xanomeline was determined on these indices.

METHODS

The effect of xanomeline on extracellular levels of monoamines in brain regions was determined using a microdialysis technique, and changes in expression of the immediate early genes c-fos and zif/268 in brain regions were evaluated using in situ hybridization histochemistry.

RESULTS

Xanomeline increased extracellular levels of dopamine in prefrontal cortex and nucleus accumbens but not in striatum. Xanomeline increased expression of c-fos and zif/268 in prefrontal cortex and nucleus accumbens. There was no change in immediate early gene expression in striatum.

CONCLUSIONS

Xanomeline increased extracellular levels of dopamine, which is similar to the effects of the atypical antipsychotics clozapine and olanzapine. The regional pattern of immediate early gene expression induced by xanomeline resembled that of atypical antipsychotic agents. Based on the antipsychotic-like activity of xanomeline in Alzheimer's patients and the similarity to atypical antipsychotic agents, we suggest that xanomeline may be a novel antipsychotic agent.

Muscarinic receptor agonists decrease cocaine self-administration rates in drug-naive mice

(5R,6R)-6-(3-Propylthio-1,2,5-thiadiazol-4-yl)-1-azabicyclo[ 3.2.1]octane (PTAC) is a selective muscarinic receptor ligand. The compound exhibits high affinity for central muscarinic receptors with partial agonist mode of action at muscarinic M(2) and M(4) and antagonist mode of action at muscarinic M(1), M(3) and M(5) receptor subtypes. The compound was earlier reported to exhibit functional dopamine receptor antagonism in rodents despite its lack of affinity for dopamine receptors. In the present study, we report that PTAC, as well as the muscarinic receptor agonists pilocarpine and oxotremorine, dose-dependently decreased rates of intravenous self-administration (fixed ratio 1) of the indirect dopamine receptor agonist cocaine in drug naive mice. Similar decreases in cocaine self-administration rates were obtained with the dopamine receptor antagonists olanzapine, clozapine, risperidone, fluphenazine and haloperidol. These findings suggest that compounds with partial muscarinic receptor agonist mode of action may be used in the medical treatment of cocaine abuse.