Increased muscarinic cholinergic receptors in prefrontal cortices of medicated schizophrenics

Functional coupling of M1 muscarinic acetylcholine receptor to Gαq/11 in dorsolateral prefrontal cortex from patients with psychiatric disorders: a postmortem study

Accumulating studies have implicated intracellular signaling through muscarinic acetylcholine receptors (mAChRs) in psychiatric illness. In the present study, carbamylcholine chloride (carbachol)-induced Gα_i/o and Gα_q/11 activation was identified in postmortem human prefrontal cortical membranes. The following two sample cohorts were used: subjects [1], consisting of 40 controls without neuropsychiatric disorders, and subjects [2], consisting of 20 with bipolar disorder (BP), 20 major depressive disorder (MDD), 20 schizophrenia, and 20 controls, strictly sex- and age-matched. Carbachol-stimulated [³⁵S]GTPγS binding to human brain membranes was assessed by the two methods, i.e., conventional method using filtration techniques (Gα_i/o activation coupled to M₂/M₄ mAChRs) applied to subjects [1], and [³⁵S]GTPγS binding/immuno precipitation assay (Gα_q/11 activation coupled to M₁ mAChR) applied to subjects [1] and [2]. The concentration eliciting the half-maximal effect (EC₅₀), maximum percent increase (%E_max), and slope factor were obtained from concentration-response curve of carbachol-induced Gα_i/o and Gα_q/11 activation. The pEC₅₀ values of both carbachol-induced Gα_i/o and Gα_q/11 activations in subjects [1] were significantly correlated, though its implications or underlying molecular processes are unclear. The results of M₁ mAChR-mediated Gα_q/11 activation in subjects [2] indicated no significant disorder-specific alterations. However, the distribution patterns of the pEC₅₀ values showed unequal variances among the groups. There was a significant inverse correlation between the %E_max values and the pEC₅₀ values in subjects with schizophrenia, but not in those with BP or MDD, or controls. These data support the notion that schizophrenia patients consist of biologically heterogeneous subgroups with respect to M₁ mAChR-mediated signaling pathways.

High-Frequency Neuronal Oscillatory Abnormalities in the Phospholipase C-β1 Knockout Mouse Model of Schizophrenia

BACKGROUND

Schizophrenia is a complex neuropsychiatric disorder characterized by psychoses, socioaffective disturbances, and cognitive deficits. The phosphodiesterase enzyme phospholipase C-β1 has been reported to be reduced in postmortem tissue of schizophrenia patients. Dysregulation of neuronal oscillations, particularly those in the higher frequency range such as beta (12-30 Hz) and gamma (30-80 Hz), are also associated with this disorder. We investigated the influence of phospholipase C-β1 gene deletion on cortical oscillatory activity and sensorimotor gating behavior.

METHODS

Adult phospholipase C-β1 knockout and wild-type C57Bl/6J control mice (total n = 26) underwent surgical implantation of extradural electrodes to allow electrocorticography recordings. Electrocorticography was recorded during prepulse inhibition behavior sessions to measure ongoing and auditory-evoked electrophysiological responses. Mice were also pretreated with antipsychotic drugs haloperidol (0.25 mg/kg), clozapine (2.5 mg/kg), and olanzapine (5 mg/kg).

RESULTS

Phospholipase C-β1 knockout mice exhibited reduced prepulse inhibition and diminished power and phase synchrony of beta and gamma oscillatory responses to auditory stimuli as well as elevated ongoing beta oscillations. Reductions in prepulse inhibition were highly correlated with the power and phase synchrony of evoked oscillations. Clozapine and olanzapine ameliorated the prepulse inhibition deficit in phospholipase C-β1 knockout mice, but not the electrophysiology abnormalities.

CONCLUSIONS

Phospholipase C-β1 reduction leads to disturbances to beta and gamma oscillatory dynamics and prepulse inhibition behavior. The strong relationships between these measures demonstrate the importance of event-related oscillatory activity to sensorimotor gating behavior. However, dissociation of these measures observed in the drug studies suggests that abnormalities in neuronal networks may not necessarily need to be corrected for behavioral improvement.

Changes in BQCA Allosteric Modulation of [(3)H]NMS Binding to Human Cortex within Schizophrenia and by Divalent Cations

Stimulation of the cortical muscarinic M1 receptor (CHRM1) is proposed as a treatment for schizophrenia, a hypothesis testable using CHRM1 allosteric modulators. Allosteric modulators have been shown to change the activity of CHRMs using cloned human CHRMs and CHRM knockout mice but not human CNS, a prerequisite for them working in humans. Here we show in vitro that BQCA, a positive allosteric CHRM1 modulator, brings about the expected change in affinity of the CHRM1 orthosteric site for acetylcholine in human cortex. Moreover, this effect of BQCA is reduced in the cortex of a subset of subjects with schizophrenia, separated into a discrete population because of a profound loss of cortical [(3)H]pirenzepine binding. Surprisingly, there was no change in [(3)H]NMS binding to the cortex from this subset or those with schizophrenia but without a marked loss of cortical CHRM1. Hence, we explored the nature of [(3)H]pirenzepine and [(3)H]NMS binding to human cortex and showed total [(3)H]pirenzepine and [(3)H]NMS binding was reduced by Zn(2+), acetylcholine displacement of [(3)H]NMS binding was enhanced by Mg(2+) and Zn(2+), acetylcholine displacement of [(3)H]pirenzepine was reduced by Mg(2+) and enhanced by Zn(2+), whereas BQCA effects on [(3)H]NMS, but not [(3)H]pirenzepine, binding was enhanced by Mg(2+) and Zn(2+). These data suggest the orthosteric and allosteric sites on CHRMs respond differently to divalent cations and the effects of allosteric modulation of the cortical CHRM1 is reduced in a subset of people with schizophrenia, a finding that may have ramifications for the use of CHRM1 allosteric modulators in the treatment of schizophrenia.

Phospholipase C-β1 Hypofunction in the Pathogenesis of Schizophrenia

Schizophrenia is a mental disorder that is characterized by various abnormal symptoms. Previous studies indicate decreased expression of phospholipase C-β1 (PLC-β1) in the brains of patients with schizophrenia. PLC-β1-null (PLC-β1(-/-)) mice exhibit multiple endophenotypes of schizophrenia. Furthermore, a study of PLC-β1 knockdown in the medial prefrontal cortex of mice has shown a specific behavioral deficit, impaired working memory. These results support the notion that disruption of PLC-β1-linked signaling in the brain is strongly involved in the pathogenesis of schizophrenia. In this review, we broadly investigate recent studies regarding schizophrenia-related behaviors as well as their various clinical and biological correlates in PLC-β1(-/-) and knockdown mouse models. This will provide a better understanding of the pathological relevance of the altered expression of PLC-β1 in the brains of patients with schizophrenia. Evidence accumulated will shed light on future in-depth studies, possibly in human subjects.

Neuropathological changes in the nucleus basalis in schizophrenia

The nucleus basalis has not been examined in detail in severe mental illness. Several studies have demonstrated decreases in glia and glial markers in the cerebral cortex in schizophrenia, familial bipolar disorder and recurrent depression. Changes in neocortical neuron size and shape have also been reported. The nucleus basalis is a collection of large cholinergic neurons in the basal forebrain receiving information from the midbrain and limbic system, projecting to the cortex and involved with attention, learning and memory, and receives regulation from serotonergic inputs. Forty-one cases aged 41-60 years with schizophrenia or major depressive disorder with age-matched controls were collected. Formalin-fixed paraffin-embedded coronal nucleus basalis sections were histologically stained for oligodendrocyte identification with cresyl-haematoxylin counterstain, for neuroarchitecture with differentiated cresyl violet stain and astrocytes were detected by glial fibrillary acid protein immunohistochemistry. Cell density and neuroarchitecture were measured using Image Pro Plus. There were larger NB oval neuron soma in the combined schizophrenia and major depression disorder groups (p = 0.038), with no significant change between controls and schizophrenia and major depression disorder separately. There is a significant reduction in oligodendrocyte density (p = 0.038) in the nucleus basalis in schizophrenia. The ratio of gemistocytic to fibrillary astrocytes showed a greater proportion of the former in schizophrenia (18.1 %) and major depressive disorder (39.9 %) than in controls (7.9 %). These results suggest glial cell abnormalities in the nucleus basalis in schizophrenia possibly leading to cortical-limbic disturbance and subcortical dysfunction.

Human electrophysiological correlates of learned irrelevance: effects of the muscarinic M1 antagonist biperiden

Learned irrelevance (LIrr) refers to a reduction in associative learning after pre-exposure of the conditioned and unconditioned stimulus in a non-contingent fashion. This paradigm might serve as a translational model for (pre)attentive information processing deficits in schizophrenia. This is the first study to investigate the event-related potentials (ERPs) of a within-subject LIrr paradigm in humans. Furthermore, the effects of the muscarinic M1 antagonist biperiden on LIrr were assessed. As expected, LIrr was found to be intact in young healthy volunteers after placebo. Furthermore, in the placebo condition P3b latency was decreased for target stimuli, which were pre-cued. This suggests that the predictability of the occurrence of these stimuli is mainly reflected by this ERP component. Biperiden had no effect on the behavioural LIrr measures, although prolonged reaction times were evident. Biperiden increased the N1 amplitude of the pre-exposed predictor letters, suggesting an effect of this drug on early perceptual processing. In conclusion, the within-subject paradigm used in the current study in combination with electroencephalography can reveal brain mechanisms involved in LIrr. M1 antagonism did not affect LIrr performance but seemed to influence early information processing.

Deletion of PLCB1 gene in schizophrenia-affected patients

A prevalence of 1% in the general population and approximately 50% concordance rate in monozygotic twins was reported for schizophrenia, suggesting that genetic predisposition affecting neurodevelopmental processes might combine with environmental risk factors. A multitude of pathways seems to be involved in the aetiology and/or pathogenesis of schizophrenia, including dopaminergic, serotoninergic, muscarinic and glutamatergic signalling. The phosphoinositide signal transduction system and related phosphoinositide-specific phospholipase C (PI-PLC) enzymes seem to represent a point of convergence in these networking pathways during the development of selected brain regions. The existence of a susceptibility locus on the short arm of chromosome 20 moved us to analyse PLCB1, the gene codifying for PI-PLC β1 enzyme, which maps on 20p12. By using interphase fluorescent in situ hybridization methodology, we found deletions of PLCB1 in orbito-frontal cortex samples of schizophrenia-affected patients.

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.

Muscarinic receptors: their roles in disorders of the central nervous system and potential as therapeutic targets

Phylogenetically, acetylcholine is an ancient neurochemical. Therefore, it is not surprising that cholinergic neurons project extensively throughout the central nervous system, innervating a wide range of structures within the brain. In fact, acetylcholine is involved in processes that underpin some of our most basic central functions. Both muscarinic and nicotinic receptor families, which mediate cholinergic transmission, have been implicated in the pathophysiology of psychiatric and neurological disorders. The question that remains to be definitively answered is whether or not these receptors are viable targets for the development of future therapeutic agents.

Role of the central cholinergic system in the therapeutics of schizophrenia

The therapeutic agents currently used to treat schizophrenia effectively improve psychotic symptoms; however, they are limited by adverse effects and poor efficacy when negative symptoms of the illness and cognitive dysfunction are considered. While optimal pharmacotherapy would directly target the neuropathology of schizophrenia neither the underlying neurobiological substrates of the behavioral symptoms nor the cognitive deficits have been clearly established. Abnormalities in the neurotransmitters dopamine, serotonin, glutamate, and GABA are commonly implicated in schizophrenia; however, it is not uncommon for alterations in the brain cholinergic system (e.g., choline acetyltransferase, nicotinic and muscarinic acetylcholine receptors) to also be reported. Further, there is now considerable evidence in the animal literature to suggest that both first and second generation antipsychotics (when administered chronically) can alter the levels of several cholinergic markers in the brain as well as impair memory-related task performance. Given the well-established importance of central cholinergic neurons to information processing and cognition, it is important that cholinergic function in schizophrenia be further elucidated and that the mechanisms of the chronic effects of antipsychotic drugs on this important neurotransmitter system be identified. A better understanding of these mechanisms would be expected to facilitate optimal treatment strategies for schizophrenia as well as the identification of novel therapeutic targets. In this review, the following topics are discussed: 1) the central cholinergic system in schizophrenia 2) effects of antipsychotic drugs on central cholinergic neurons 3) important neurotrophins in schizophrenia, especially those that support central cholinergic neurons; 4) novel strategies to optimize the therapeutics of schizophrenia via the use of cholinergic compounds as primary (i.e., antipsychotic) treatments as well as adjunctive, pro-cognitive agents.

PLC-beta1 knockout mice as a model of disrupted cortical development and plasticity: behavioral endophenotypes and dysregulation of RGS4 gene expression

The complexity of the genetics underlying schizophrenia is highlighted by the multitude of molecular pathways that have been reported to be disrupted in the disorder including muscarinic, serotonergic, and glutamatergic signaling systems. It is of interest, therefore, that phospholipase C-beta1 (PLC-beta1) acts as a point of convergence for these pathways during cortical development and plasticity. These signaling pathways, furthermore, are susceptible to modulation by RGS4, one of the more promising candidate genes for schizophrenia. PLC-beta1 knockout mice were behaviorally assessed on tests including fear conditioning, elevated plus maze, and the Y maze. In situ hybridization was used to assess RGS4 expression. We found that PLC-beta1 knockout mice display abnormal anxiety profiles on some, but not all measures assessed, including decreased anxiety on the elevated plus maze. We also show memory impairment and a complete absence of acquisition of hippocampal-dependent fear conditioning. Furthermore, at a molecular level, we demonstrate dramatic changes in expression of RGS4 mRNA in selective regions of the PLC-beta1 knockout mouse brain, particularly the CA1 region of the hippocampus. These results validate the utility of the PLC-beta1 knockout mouse as a model of schizophrenia, including molecular and cellular evidence for disrupted cortical maturation and associated behavioral endophenotypes.

Muscarinic receptors: do they have a role in the pathology and treatment of schizophrenia?

The high affinity of antipsychotic drugs for the dopamine D2 receptor focused attention onto the role of these receptors in the genesis of psychoses and the pathology of schizophrenia. However, psychotic symptoms are only one aspect of the complex symptom profile associated with schizophrenia. Therefore, research continues into other neurochemical systems and their potential roles in key features associated with schizophrenia. Modulating the cholinergic system in attempts to treat schizophrenia predates specific neurochemical hypotheses of the disorder. Cholinergic modulation has progressed from the use of coma therapy, through the use of anti-cholinergic drugs to control side-effects of older (typical) antipsychotic medications, to the development of drugs designed to specifically activate selected muscarinic receptors. This review presents data implicating a decrease in muscarinic receptors, particularly the M1 receptor, in the pathology of schizophrenia and explores the potential physiological consequences of such a change, drawing on data available from muscarinic receptor knockout mice as well as clinical and pre-clinical pharmacology. The body of evidence presented suggests that deficits in muscarinic receptors are associated with some forms of schizophrenia and that targeting these receptors could prove to be of therapeutic benefit to patients with the disorder.

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.

Phospholipase C-beta1 knockout mice exhibit endophenotypes modeling schizophrenia which are rescued by environmental enrichment and clozapine administration

Phospholipase C-beta1 (PLC-beta1) is a rate-limiting enzyme implicated in postnatal-cortical development and neuronal plasticity. PLC-beta1 transduces intracellular signals from specific muscarinic, glutamate and serotonin receptors, all of which have been implicated in the pathogenesis of schizophrenia. Here, we present data to show that PLC-beta1 knockout mice display locomotor hyperactivity, sensorimotor gating deficits as well as cognitive impairment. These changes in behavior are regarded as endophenotypes homologous to schizophrenia-like symptoms in rodents. Importantly, the locomotor hyperactivity and sensorimotor gating deficits in PLC-beta1 knockout mice are subject to beneficial modulation by environmental enrichment. Furthermore, clozapine but not haloperidol (atypical and typical antipsychotics, respectively) rescues the sensorimotor gating deficit in these animals, suggesting selective predictive validity. We also demonstrate a relationship between the beneficial effects of environmental enrichment and levels of M1/M4 muscarinic acetylcholine receptor binding in the neocortex and hippocampus. Thus we have demonstrated a novel mouse model, displaying disruption of multiple postsynaptic signals implicated in the pathogenesis of schizophrenia, a relevant behavioral phenotype and associated gene-environment interactions.

Neuroreceptor imaging studies in schizophrenia

The ability of SPECT and PET to image specific biomolecules in the living brain provides a unique tool for clinical researchers. It is therefore not surprising that the use of neuroreceptor-imaging techniques has become more widespread over the past decade. This article reviews the application of these techniques to the study of schizophrenia. The design of neuroreceptor-imaging studies performed in the field of schizophrenia research can be broadly divided into two categories: (1) studies of pathophysiology and (2) studies of pharmacology. The former examines neuroreceptor and neurotransmitter parameters in individuals with schizophrenia compared to control subjects in order to provide a better understanding of the disease process. Studies of pharmacology seek to elucidate the mechanism of action for the treatments utilized in schizophrenia. This review will consider both studies of pathophysiology and pharmacology, with a discussion of the application of these techniques to drug development.

Molecular targets for treating cognitive dysfunction in schizophrenia

Cognitive impairment is a core feature of schizophrenia as deficits are present in the majority of patients, frequently precede the onset of other positive symptoms, persist even with successful treatment of positive symptoms, and account for a significant portion of functional impairment in schizophrenia. While the atypical antipsychotics have produced incremental improvements in the cognitive function of patients with schizophrenia, overall treatment remains inadequate. In recent years, there has been an increased interest in developing novel strategies for treating the cognitive deficits in schizophrenia, focusing on ameliorating impairments in working memory, attention, and social cognition. Here we review various molecular targets that are actively being explored for potential drug discovery efforts in schizophrenia and cognition. These molecular targets include dopamine receptors in the prefrontal cortex, nicotinic and muscarinic acetylcholine receptors, the glutamatergic excitatory synapse, various serotonin receptors, and the gamma-aminobutyric acid (GABA) system.

Altered hippocampal muscarinic M4, but not M1, receptor expression from subjects with schizophrenia

BACKGROUND

Having shown a decrease in [3H]pirenzepine binding in the hippocampus from subjects with schizophrenia, we wished to determine whether such a change in radioligand binding was associated with changes in hippocampal mRNA for the muscarinic1 (M1) and muscarinic4 (M4) receptors in tissue from different cohorts of subjects.

METHOD

The [3H]pirenzepine binding using autoradiography and in situ hybridization with oligonucleotides specific for muscarinic M1 and M4 receptors were completed using hippocampal tissue obtained postmortem from 20 control subjects and 20 subjects with schizophrenia.

RESULTS

The [3H]pirenzepine binding was decreased in the dentate gyrus (p < .05), CA3 (p < .01), CA2 (p < .05), and CA1 (p < .01) regions of the hippocampus from subjects with schizophrenia. Levels of M4 mRNA varied with the diagnosis of schizophrenia (p = .01), but significant region-specific changes were not apparent. Changes in levels of mRNA for the muscarinic M1 receptor were not detected with diagnosis.

CONCLUSIONS

This study suggests that decreases in hippocampal [3H]pirenzepine binding in subjects with schizophrenia are most likely associated with widespread changes in expression levels of the M4 receptor. These data further implicate the hippocampal formation in the pathology of schizophrenia.

Towards a muscarinic hypothesis of schizophrenia

Although the neurotransmitter dopamine plays a prominent role in the pathogenesis and treatment of schizophrenia, the dopamine hypothesis of schizophrenia fails to explain all aspects of this disorder. It is increasingly evident that the pathology of schizophrenia also involves other neurotransmitter systems. Data from many streams of research including pre-clinical and clinical pharmacology, treatment studies, post-mortem studies and neuroimaging suggest an important role for the muscarinic cholinergic system in the pathophysiology of schizophrenia. This review will focus on evidence that supports the hypothesis that the muscarinic system is involved in the pathogenesis of schizophrenia and that muscarinic receptors may represent promising novel targets for the treatment of this disorder.

Chronic treatment with first or second generation antipsychotics in rodents: effects on high affinity nicotinic and muscarinic acetylcholine receptors in the brain

Several postmortem and neuroimaging studies suggest that central nicotinic and muscarinic acetylcholine receptors are important in both the pathophysiology and pharmacotherapy of schizophrenia. However, while antipsychotic drugs are routinely used in the therapeutics of schizophrenia, little is known about their effects on the densities of these receptors when they are administered for extended periods of time (a common practice in the clinical setting). In the present study in rats, the residual effects of prior chronic exposure to representative first generation antipsychotics and second generation antipsychotics on the densities of high affinity nicotinic acetylcholine receptors and muscarinic acetylcholine receptor in the brain were investigated. Test subjects were treated with the first generation antipsychotics, haloperidol (2.0 mg/kg/day) or chlorpromazine (10.0 mg/kg/day) or the second generation antipsychotics, risperidone (2.5 mg/kg/day) or olanzapine (10.0 mg/kg/day) in drinking water for periods of 90 or 180 days, given a drug-free washout period (i.e. returned to normal drinking water) for two weeks and then killed. Quantitative receptor autoradiography was subsequently performed using 16 mum sagittal slices of whole brain incubated with [3H]-epibatidine, [3H]-pirenzepine or [3H]-AFDX-384 to measure high affinity nicotinic acetylcholine receptors, M1 and M2 muscarinic acetylcholine receptors, respectively. The most notable experimental result was a moderate, but significant (P<0.01) increase in [3H]-AFDX-384 binding sites in a number of brain regions (including cortex, hippocampus, subiculum, substantia innominata, and thalamus) associated with prior exposure to olanzapine for 90, but not 180 days. Olanzapine was also associated with a significantly higher density of [3H]-pirenzepine binding sites in cortex lamina I after 90 days of prior drug exposure. These data indicate that chronic treatment with a commonly used second generation antipsychotic, olanzapine is associated with modest increases in M2 muscarinic acetylcholine receptors in memory-related brain regions that may eventually abate with longer periods of chronic drug exposure.

Effects of rivastigmine on sustained attention in schizophrenia: an FMRI study

This study assessed the neural correlates of the effects of rivastigmine, a CNS-selective cholinesterase inhibitor, given as an add-on therapy to antipsychotics-treated patients with schizophrenia who displayed moderate cognitive impairments, using functional magnetic resonance imaging (fMRI) during a sustained attention task. The study used a placebo-controlled, randomized, double-blind longitudinal design. Twenty patients stable on antipsychotics, 11 assigned to receive rivastigmine and 9 to receive placebo, underwent fMRI and clinical assessments at baseline and after 12 weeks. The fMRI task used a periodic block design and involved 3 conditions: rest, detecting a nonzero number ("nonzero" condition), and detecting a specific number ("specific number" condition) among a series of 6-digit numbers. Online data (via button presses) were acquired on both occasions. Behavioral results showed a trend (P = 0.075) for the rivastigmine-treated patients to have more correct responses and the placebo group to have fewer correct responses at 12 weeks compared with baseline in the "nonzero" condition. There was also an increase in regional brain activity in the cerebellum in the rivastigmine group at 12 weeks in both conditions, which was only partially explained by change in behavioral measures; no change was observed in the placebo group. Our results showed that rivastigmine treatment increased cerebellar activity and influenced attentional processes.

Membrane phospholipid composition, alterations in neurotransmitter systems and schizophrenia

This review addresses the relationship between modifications in membrane phospholipid composition (MPC) and alterations in dopaminergic, serotonergic and cholinergic neurotransmitter systems in schizophrenia. The main evidence in support of the MPC hypothesis of schizophrenia comes from post-mortem and platelet studies, which show that in schizophrenia, certain omega-3 and omega-6 polyunsaturated fatty acid (PUFA) levels are reduced. Furthermore, examination of several biochemical markers suggests abnormal fatty acid metabolism may be present in schizophrenia. Dietary manipulation of MPC with polyunsaturated fatty acid diets has been shown to affect densities of dopamine, serotonin and muscarinic receptors in rats. Also, supplementation with omega-3 fatty acids has been shown to improve mental health rating scores, and there is evidence that the mechanism behind this involves the serotonin receptor complex. This suggests that a tight relationship exists between essential fatty acid status and normal neurotransmission, and that altered PUFA levels may contribute to the abnormalities in neurotransmission seen in schizophrenia.

Cholinergic targets for cognitive enhancement in schizophrenia: focus on cholinesterase inhibitors and muscarinic agonists

RATIONALE

Alterations in the central cholinergic system of patients with schizophrenia such as reduced numbers of muscarinic and nicotinic receptors in the cortex and hippocampus may contribute to the cognitive impairment of schizophrenia. Therefore, pharmacological treatments that enhance central cholinergic function may be useful as cognitive enhancers in schizophrenia.

METHODS

Searches were conducted for articles which investigated alterations of central cholinergic systems in patients with schizophrenia. Additional searches were conducted for animal and human trials of potential cognitive enhancing compounds that target the cholinergic system and any preliminary trials conducted with schizophrenic patients.

RESULTS

Currently available treatments which are potentially suitable for this purpose include acetylcholinesterase inhibitors, muscarinic agonists, nicotinic agonists, and allosteric potentiators of nicotinic receptor function. Although some open label studies demonstrate modest cognitive improvements of schizophrenic patients treated with donepezil, data from a blinded, placebo controlled study demonstrate no effect. Data from a controlled trial of galantamine, a combined acetylcholinesterase inhibitor and allosteric potentiator of the nicotinic receptor, indicates that this may be an effective alternative. In addition, some preclinical data indicates that selective M(1) muscarinic agonists under development may have potential as cognitive enhancers and antipsychotic treatments for schizophrenic patients.

CONCLUSIONS

A cholinergic approach to ameliorating the cognitive dysfunction of schizophrenia appears viable. There is some preliminary data to support the efficacy of combined acetylcholinesterase inhibitors and allosteric potentiators of the nicotinic receptor, whereas future trials are awaited for more specific muscarinic agonists currently under development.

Investigation of m1/m4 muscarinic receptors in the anterior cingulate cortex in schizophrenia, bipolar disorder, and major depression disorder

Abnormal cholinergic neurotransmission has been suggested to occur in psychiatric illness. Therefore, this study investigated cholinergic muscarinic receptors in the anterior cingulate cortex (ACC) of schizophrenia, bipolar disorder and major depression disorder (n=15 per group). We used quantitative autoradiography to measure [(3)H]pirenzepine binding to M1 and M4 receptors. Brain tissue was obtained from the Stanley Foundation Neuropathology Consortium. [(3)H]pirenzepine binding was higher in superficial laminae (I-II) than in deep laminae (III-VI) of the ACC. There was a significant 24% reduction in the density of [(3)H]pirenzepine in the deep laminae and a significant 19% reduction in the upper laminae of the ACC in the schizophrenia group compared to the control group. There were no differences in [(3)H]pirenzepine binding in any laminae of the ACC in the bipolar or major depression groups compared with the control group, except for a trend towards decreased [(3)H]pirenzepine binding in subjects with major depression relative to control subjects. We also detected a significant effect of suicide on [(3)H]pirenzepine binding in the ACC in subjects who died as a result of suicide relative to those who did not, which was more evident in patients with schizophrenia. A significant effect of the onset of the disease was also observed that was more evident in patients with bipolar disorder. The study provides evidence of decreased muscarinic receptor density in the ACC in schizophrenia but no evidence for significant changes in these receptors in the bipolar and major depression groups. The changes observed in schizophrenia may contribute to dysfunctional ACC neural circuits.

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.

Decreased muscarinic1 receptors in the dorsolateral prefrontal cortex of subjects with schizophrenia

To test the hypothesis that muscarinic receptors are involved in the pathology of schizophrenia, we measured muscarinic(1) (M1R) and muscarinic(4)(M4R) protein and mRNA as well as [(3)H]pirenzepine binding in Brodmann's areas (BA) 9 and 40 obtained postmortem from 20 schizophrenic and 20 age/sex-matched control subjects. There was a significant decrease in [(3)H]pirenzepine binding to BA 9 (mean +/- SEM: 151 +/- 15 vs 195 +/- 10 fmol mg(-1) ETE; P< 0.02), but not BA 40 (143 +/- 13 vs 166 +/- 11 fmol mg(-1) ETE), from subjects with schizophrenia. The level of M1R protein (0.11 +/- 0.007 vs 0.15 +/- 0.008 OD; P < 0.01), but not M4R protein, was decreased in BA9 from schizophrenic subjects with neither receptor protein being altered in BA 40. The level of M1R mRNA was decreased in BA 9 (30 +/- 7.0 vs 79 +/- 14 dpm x 10(3) mg(-1) ETE, P < 0.01) and BA 40 (28 +/- 5.9 vs 99 +/- 14, P < 0.01) with schizophrenia but M4R mRNA was only decreased in BA 40 (48 +/- 6.6 vs 89 +/- 9.9, P < 0.005). These data suggest that the M1R, at least in the dorsolateral prefrontal cortex, may have a role in the pathology of schizophrenia.

Reduced expression of the muscarinic 1 receptor cortical subtype in schizophrenia

The involvement of specific pathways mediated through muscarinic receptor activity has been widely implicated in schizophrenia. Extensive pharmacological evidence supports the systems role in mediating antipsychotic drug efficacy, while mounting physiological evidence demonstrates the presence of significant alterations to normal muscarinic receptor integrity in the disorder. The mechanisms that facilitate the systems involvement and their magnitude, however, remain poorly understood. We have proposed that alterations to normal muscarinic receptor expression exist in schizophrenia, and that these significantly influence the physiological changes often reported for the system amongst patients. In this study, we investigate this potential, and have selected to examine the muscarinic 1 receptor, which constitutes a major target for antipsychotic action and plays a conspicuous role in those regions central to the disorders pathophysiology. Using relative gene quantification, we measured post-mortem levels of steady-state cortical muscarinic 1 receptor cDNA in patients (N = 20) and unaffected controls (N = 20), and examined group differences in expression levels. Commensurate with our hypothesis, we observed significant reductions in muscarinic 1 receptor cDNA in our patient sample (F(1,37) = 4.73, P = 0.036) and have estimated this to constitute a 28% decrease compared to the control subjects (95% CI from 2 to 47%). These results provide evidence in support of altered muscarinic 1 receptor expression in schizophrenia, though further work is needed to corroborate these findings.

Cholinergic systems and schizophrenia: primary pathology or epiphenomena?

Post mortem schizophrenia research has been driven first by the dopamine and then the glutamate hypotheses. These hypotheses posit primary pathology in pathways dependent upon dopamine or glutamate neurotransmission. Although the dopamine and glutamate hypotheses retain considerable theoretical strength, neurobiological findings of altered dopamine or glutamate activity in schizophrenia do not explain all features of this disorder. A more synthetic approach would suggest that focal pathological change in either the prefrontal cortex or mesial temporal lobe leads to neurochemical changes in multiple neurotransmitter systems. Despite the limited experimental evidence for abnormal cholinergic neurotransmission in psychiatric disorders, increased understanding of the role of acetylcholine in the human brain and its relationship to other neurotransmitter systems has led to a rapidly growing interest in the cholinergic system in schizophrenia. This review focuses on the basic anatomy of the mammalian cholinergic system, and its possible involvement in the neurobiology of schizophrenia. Summaries of cholinergic cell groups, projection pathways, and receptor systems, in the primate and human brain, are followed by a brief discussion of the functional correlations between aberrant cholinergic neurotransmission and the signs and symptoms of schizophrenia.

Signal transmission, rather than reception, is the underlying neurochemical abnormality in schizophrenia

OBJECTIVE

This review aims to summarise the outcome of studies on changes in the molecular architecture of the brain of subjects with schizophrenia and formulate a hypothesis on mechanisms involved in the pathology of the illness.

METHOD

The outcomes from key studies using neuroimaging techniques and tissue obtained post-mortem that have been directed toward identifying abnormalities in the molecular architecture of the brain in subjects with schizophrenia were summarised. Using the results from these studies hypotheses were formulated on the underlying pathological process that precipitate schizophrenia.

RESULTS

Studies using neuroimaging techniques or tissue obtained post-mortem have revealed changes in the dopaminergic, serotoninergic, glutamatergic, GABAergic and cholinergic systems of the brain in schizophrenia. Some of these studies have identified abnormalities in presynaptic proteins or functioning that may be central to the pathology of schizophrenia.

CONCLUSIONS

There appears to be diverse changes in the molecular cytoarchitecture of the brains from subjects with schizophrenia. It could be that it is by affecting these multiple systems that the atypical antipsychotic drugs produce their improved clinical outcomes. Abnormal functioning of presynaptic processes could be central to the pathology of schizophrenia. If the 'presynaptic' hypothesis is proven, future antipsychotic drug design should be directed away from post-synaptic receptor antagonism toward the modulating the functions of presynaptic neurones.

Muscarinic1 and 2 receptor mRNA in the human caudate-putamen: no change in m1 mRNA in schizophrenia

Studies using tissue obtained at autopsy suggest that changes in cholinergic neurons could be important in the pathology of schizophrenia.1-4 We have previously reported a decrease in [3H]pirenzepine binding5 and [3H]AF-DX 384 binding6 to caudate-putamen (CP) from subjects who had schizophrenia. Under the conditions chosen, [3H]pirenzepine would predominately bind to muscarinic1 (M1) and muscarinic4 (M4) receptors,7whereas [3H]AF-DX 384 would mainly bind to muscarinic2 (M2) and M4 receptors.8 Given the relative concentrations of M1, M2 and M4 receptors in the human CP and the magnitude of the decreases in radioligand binding in schizophrenia, our results most likely reflected a change in the density of M1 and M2 receptors in the CP from the schizophrenic subjects. In situ hybridisation has now been used to determine levels of m1 and m2 mRNA in CP from 14 schizophrenic and 16 control subjects previously used for radioligand binding. m2 mRNA in the CP from the schizophrenic and control subjects was below the sensitivity of in situhybridisation. There was no difference in the levels of m1 mRNA in CP from schizophrenic and control subjects (mean +/- SEM: 103 +/- 16 vs106 +/- 17 fmol [35S]oligonucleotide probe g-1estimated tissue equivalents, P = 0.91). In conclusion, data from our radioligand binding studies show decreases in [3H]pirenzepine binding that are likely to reflect a decrease in the density of M1 receptors in CP from schizophrenic subjects. Our data in this study show the absence of a concomitant change in mRNA coding for that receptor.

The binding of [3H]AF-DX 384 is reduced in the caudate-putamen of subjects with schizophrenia

Clinical studies of cholinergic pharmacotherapy, together with the putative role of the muscarinic receptor system in the neurophysiology of human behavior, support a possible muscarinic cholinergic involvement in schizophrenia. The present study has measured the density of [3H]AF-DX 384 labelled receptors (muscarinic M2 and M4) in the caudate-putamen, obtained at autopsy, from 19 subjects who had schizophrenia, and 20 subjects who did not have schizophrenia. [3H]AF-DX 384 binding was reduced in caudate-putamen from schizophrenic subjects (104 +/- 10.3 vs 145 +/- 901 fmol mg(-1) TE; mean +/- s.e.; p = 0.007). Preliminary analysis of patient drug data as well as rat studies suggest that the reduced [3H]AF-DX 384 binding in caudate-putamen of schizophrenic subjects is not wholly due to antipsychotic drug treatment, or anticholinergic medication for the treatment of extrapyramidal effects. These data suggest that the muscarinic cholinergic system may be involved in the pathology of schizophrenia.

Effect of sequence of administration on the pharmacokinetic interaction between the anticholinergic drug biperiden and [3H]quinuclidinyl benzylate or [3H]N-methylscopolamine in rats

In rats the pharmacokinetic interactions between the anticholinergic drug biperiden and [3H]quinuclidinyl benzylate ([3H]QNB) or [3H]N-methylscopolamine ([3H]NMS) is affected by the sequence in which the drugs are administered. Drug concentrations in various tissues were determined after intravenous administration of [3H]QNB or [3H]NMS (325 ng kg(-1)). Biperiden (6.4 mg kg(-1)) was administered either 5 min before, concomitantly with or 20 min after injection of [3H]QNB or [3H]NMS. When biperiden was administered concomitantly with or before [3H]QNB, distribution of [3H]QNB among the regions of the brain and other tissues was reduced; at 4 h the ratio of the distribution of [3H]QNB for experimental animals to that for control animals ranged from 0.15 to 0.9. When biperiden was administered after [3H]QNB, the distribution of [3H]QNB in the brain and other tissues was significantly higher than for the other two treatments (P < 0.01). However, for [3H]NMS the sequence of administration had no effect on the distribution of the drug in the brain and other tissues except for the kidney. In-vitro, in crude synaptosomal membranes, the amount of [3H]QNB at 2 h relative to the control concentration at equilibrium was 87% when biperiden was added before [3H]QNB and 56% when biperiden was added after [3H]QNB. In both instances the concentration of [3H]NMS reached equilibrium within 30 min. These findings suggest that the difference between the rate constant of association and dissociation at the possible site of action gives rise to the effect of the sequence of administration on the pharmacokinetic interaction.

Neurochemical studies of schizophrenia in Japan

Neurobiological research in schizophrenia has been hampered by several confounding factors such as the heterogeneity of the illness and the paucity of biological markers. Recent progress in research methods, however, has enabled the improvement in our understanding its pathophysiology. This paper reviews recent neurochemical investigations of schizophrenia and its animal models which were conducted in Japan in the last decade. The research areas reviewed are (i) monoamine and their metabolites in body fluids, (ii) phospholipids and prostaglandins, (iii) neurochemistry in autopsy brains, (iv) immunological measures, (v) magnetic resonance spectroscopy, (vi) regional cerebral blood flows (rCBF), (vii) molecular genetics, and (viii) animal models. It is worth noting that there exist abnormalities of amino acidergic (glutamatergic and GABAergic) neurotransmission as well as monoaminergic (dopaminergic and serotonergic) one in postmortem schizophrenic brains. These abnormalities and also the findings of altered rCBF indicate the existence of disturbed neuronal circuits that contribute to the diverse symptoms of schizophrenia. Also, dysfunction of membrane phospholipids derived from studies on magnetic resonance spectroscopy may underlie negative symptoms in schizophrenia. Given that schizophrenia is considered to comprise a group of disorders with a diverse heterogeneity of etiologies, research in the next decade is expected to identify putative genes that are involved in vulnerability to schizophrenic phenotype.

Evidence in postmortem brain tissue for decreased numbers of hippocampal nicotinic receptors in schizophrenia

This study tests the hypothesis that nicotinic cholinergic receptors, including those sensitive to the antagonist alpha-bungarotoxin, are decreased in the hippocampus of schizophrenics. The hypothesis is derived from the finding that alpha-bungarotoxin causes a defect in the inhibitory gating of auditory-evoked potentials in laboratory animals that resembles a defect in auditory sensory gating observed in schizophrenics. Nicotine transiently normalizes this psychophysiological deficit in schizophrenic patients. Postmortem brain tissue was obtained from eight schizophrenic and eight age-matched nonschizophrenic subjects. Sections of the hippocampus were labeled with [125I alpha-bungarotoxin and imagined by autoradiography. Binding of the nicotinic agonist [3H]-cytisine was determined in tissue homogenates. alpha-Bungarotoxin labeled a population of putative interneurons in the hippocampus, primarily in the dentate gyrus and the CA3 region of Ammon's horn. This labeling was significantly decreased in the tissue from the schizophrenic patients, with seven or eight patients below the range of the nonschizophrenic subjects. There was also a significant decrease in the binding of cytisine. The results were not related to generalized hippocampal cell loss, drug exposure at time of death, or smoking history. This initial study suggests that schizophrenic patients have fewer nicotinic receptors in the hippocampus, a condition which may lead to failure of cholinergic activation of inhibitory interneurons, manifest clinically as decreased gating of response to sensory stimulation.

Growth hormone responses to pyridostigmine in schizophrenia: evidence for cholinergic dysfunction

The hypothesis that increased central cholinergic neurotransmitter function may be present in schizophrenic illness and may underlie negative symptoms was tested using a neuroendocrine challenge approach. The cholinergic challenge used was the anticholinesterase pyridostigmine, thought to cause the release of growth hormone (GH) from the anterior pituitary by diminishing inhibitory somatostatin tone. Eleven patients, six neuroleptic-naive and five neuroleptic-free, satisfying DSM-III-R criteria for schizophrenia and 11 matched controls took part. Subjects received pyridostigmine (120 mg orally) and blood was sampled at 0, 60, 90, 120, and 180 min for GH estimation. Peak GH responses were significantly increased in the schizophrenic group compared to controls. There was no relationship between individual peak GH values and negative symptom ratings (Scale for the Assessment of Negative Symptoms). Neither could a relationship be established between other aspects of psychopathology or dyskinesias and GH responses. An increased pyridostigmine/GH response is also found in affective disorders and could be related to nonspecific symptoms common to all these diagnostic groups. This study suggests that schizophrenia may be associated with increased cholinergic neurotransmitter function but the relationship between this cholinergic dysfunction and schizophrenia may involve psychopathology not specific to schizophrenia.

Muscarinic cholinergic hyperactivity in schizophrenia. Relationship to positive and negative symptoms

Based on the implication of increased muscarinic ACh activity in the production of negative symptoms, the association of decreasing cholinergic activity with positive symptoms, and the covariance of positive and negative symptoms in the psychotic phase of schizophrenia, a model of (DA) dopaminergic/(ACh) cholinergic interactions in schizophrenia was recently formulated. It suggests that DA/ACh balance is of central importance in schizophrenic pathophysiology and that muscarinic ACh activity increases in an attempt to maintain this balance in the face of increasing DA activity that occurs in the psychotic phase of the illness. The model further suggests that the muscarinic system exerts a damping influence on the emergence of positive symptoms associated with DA hyperactivity, but that this compensatory increase in muscarinic activity is accompanied by an intensification of negative symptoms. In the present study, we tested two important postulates of this model. We tested the prediction that muscarinic activity is increased in schizophrenia by comparing the effect of biperiden, an antimuscarinic M-1 agent, on REM latency in 12 drug-free schizophrenic inpatients and matched normal controls. We found that biperiden caused a smaller increase in REM latency in schizophrenic patients, suggesting that muscarinic activity is increased in schizophrenia. We tested the prediction that an anticholinergic agent would increase positive symptoms and decrease negative symptoms by studying the effect of 8 mg of biperiden/day for 2 days on positive and negative symptoms (assessed by the BPRS) in 30 medication-free schizophrenic inpatients.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitters, receptors and neuropeptides in post-mortem brains of chronic schizophrenic patients

In the analysis of post-mortem brains of 14 chronic schizophrenic patients and 10 controls, biochemical evidence of a hyperdopaminergic state was found in the basal ganglia of schizophrenics; tyrosine hydroxylase activity was increased with a concomitant increase of homovanillic acid. Unusually high tyrosine hydroxylase activity was noted in 2 schizophrenic cases. The Bmax value of 3H-spiperone binding for schizophrenics was higher than the controls. We also found increased specific binding of 3H-kainic acid to the prefrontal cortex in schizophrenics. A negative correlation existed between 3H-kainic acid binding in the medial frontal cortex, and glutamic acid content in various brain areas. Increased immunoreactivity of substance P was found in more than ten brain areas. Methionine-enkephalin was also increased in three areas of the prefrontal cortex of schizophrenics. These results suggest that the hyperdopaminergic state co-existed with glutamatergic hypofunction and increased neuropeptides in various brain areas of chronic schizophrenic patients.

Decreased muscarinic receptor binding in cerebral cortex and hippocampus in Alzheimer's disease

Muscarinic (cholinergic) receptor binding sites (MRB) were studied by determining the 3H-QNB binding in four cortical areas and hippocampus of 20 histologically confirmed Alzheimer patients and comparing these with corresponding controls. Alzheimer patients dying at younger age (less than or equal to 80) with profound decrease in choline-acetyltransferase activity (by 61-85%) and without any, possibly MRB modifying, drug treatment showed 30% decrease in MRB in the frontal cortex (p less than 0.05), 28% in the temporal cortex (p less than 0.05) and 37% in the hippocampus (p less than 0.01). These findings further suggest that muscarinic receptors are affected in Alzheimer's disease, at least in advanced state of the disease.

Dopamine receptors and the dopamine hypothesis of schizophrenia

The discovery of neuroleptic drugs in 1952 provided a new strategy for seeking a biological basis of schizophrenia. This entailed a search for a primary site of neuroleptic action. The Parkinsonian effects caused by neuroleptics suggested that dopamine transmission may be disrupted by these drugs. In 1963 it was proposed that neuroleptics blocked "monoamine receptors" or impeded the release of monoamine metabolites. The neuroleptic concentration in plasma water or cerebrospinal fluid was of the order of 2 nM for haloperidol in clinical therapy. A systematic research was made between 1963 and 1974 for a primary site of neuroleptic action which would be sensitive to 2 nM haloperidol and stereoselective for (+)-butaclamol. Direct evidence that neuroleptics selectively blocked dopamine receptors occurred in 1974 with the finding that nanomolar concentrations of these drugs stereoselectively inhibited the binding of [3H]-dopamine or [3H]-haloperidol. These binding sites, now termed D2 dopamine receptors (which inhibit adenylate cyclase), are blocked by neuroleptics in direct relation to the antipsychotic potencies of the neuroleptics. No such correlation exists for D1 receptors (which stimulate adenylate cyclase). Based on the fact that dopamine-mimetic drugs elicited hallucinations, and that neuroleptics caused rigidity, Van Rossum in 1966 had suggested a hypothesis that dopamine pathways may be overactive in schizophrenia. The D2-selective blockade by all neuroleptics (except the monoamine-depleting reserpine) provided strong support for the dopamine hypothesis. Further support now comes from postmortem data and in vivo positron tomographic data, both of which indicate that the density of D2 receptors are elevated in the schizophrenic brain. The postmortem data indicate a bimodal pattern with half the schizophrenics having striatal D2 densities of 14 pmol/g (control is 13 pmol/g) and the other half having 26 pmol/g. Current positron tomographic data indicate D2 densities of 14 pmol/g in control subjects, but values of 34 pmol/g in drug-naive schizophrenics. Future tests of the dopamine hypothesis of schizophrenia may entail an examination of the amino acid composition and genes for D2 receptors in schizophrenic tissue, an examination of the ability of the D2 receptor to become phosphorylated and to desensitize into the low-affinity state, and an examination of the interaction of D2 receptors with D1 receptors or other neurotransmitters.