Effects of Typical and Atypical Antipsychotic Drugs on Rat Brain Muscarinic Receptors

Low levels of muscarinic M1 receptor-positive neurons in cortical layers III and V in Brodmann areas 9 and 17 from individuals with schizophrenia

BACKGROUND

Results of neuroimaging and postmortem studies suggest that people with schizophrenia may have lower levels of muscarinic M1 receptors (CHRM1) in the cortex, but not in the hippocampus or thalamus. Here, we use a novel immunohistochemical approach to better understand the likely cause of these low receptor levels.

METHODS

We determined the distribution and number of CHRM1-positive (CHRM1+) neurons in the cortex, medial dorsal nucleus of the thalamus and regions of the hippocampus from controls (n = 12, 12 and 5, respectively) and people with schizophrenia (n = 24, 24 and 13, respectively).

RESULTS

Compared with controls, levels of CHRM1+ neurons in people with schizophrenia were lower on pyramidal cells in layer III of Brodmann areas 9 (-44%) and 17 (-45%), and in layer V in Brodmann areas 9 (-45%) and 17 (-62%). We found no significant differences in the number of CHRM1+ neurons in the medial dorsal nucleus of the thalamus or in the hippocampus.

LIMITATIONS

Although diagnostic cohort sizes were typical for this type of study, they were relatively small. As well, people with schizophrenia were treated with antipsychotic drugs before death.

CONCLUSION

The loss of CHRM1+ pyramidal cells in the cortex of people with schizophrenia may underpin derangements in the cholinergic regulation of GABAergic activity in cortical layer III and in cortical/subcortical communication via pyramidal cells in layer V.

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.

Effects of olanzapine on muscarinic M3 receptor binding density in the brain relates to weight gain, plasma insulin and metabolic hormone levels

The second generation antipsychotic drug (SGA) olanzapine has an efficacy to treat schizophrenia, but can cause obesity and type II diabetes mellitus. Cholinergic muscarinic M3 receptors (M3R) are expressed on pancreatic β-cells and in the brain where they influence insulin secretion and may regulate other metabolic hormones via vagal innervation of the gastrointestinal tract. Olanzapine's M3R antagonism is an important risk factor for its diabetogenic liability. However, the effects of olanzapine on central M3Rs are unknown. Rats were treated with 0.25, 0.5, 1.0 or 2.0 mg olanzapine/kg or vehicle (3×/day, 14-days). M3R binding densities in the hypothalamic arcuate (Arc) and ventromedial nuclei (VMH), and dorsal vagal complex (DVC) of the brainstem were investigated using [3H]4-DAMP plus pirenzepine and AF-DX116. M3R binding correlations to body weight, food intake, insulin, ghrelin and cholecystokinin (CCK) were analyzed. Olanzapine increased M3R binding density in the Arc, VMH and DVC, body weight, food intake, circulating plasma ghrelin and CCK levels, and decreased plasma insulin and glucose. M3R negatively correlated to insulin, and positively correlated to ghrelin, CCK, food intake and body weight. Increased M3R density is a compensatory up-regulation in response to olanzapine's M3R antagonism. Olanzapine acts on M3R in regions of the brain that control food intake and insulin secretion. Olanzapine's M3R blockade in the brain may inhibit the acetylcholine pathway for insulin secretion. These findings support a role for M3Rs in the modulation of insulin, ghrelin and CCK via the vagus nerve and provide a mechanism for olanzapine's diabetogenic and weight gain liability.

Effects of benzodiazepine treatment on cortical GABA(A) and muscarinic receptors: studies in schizophrenia and rats

Changes in cortical γ-aminobutyric acid A (GABA(A)) receptors and muscarinic receptors have been reported in schizophrenia, a disorder treated with antipsychotic drugs and benzodiazepines. As there is a reported functional relationship between the GABAergic and cholinergic systems in the human central nervous system we have investigated whether there are changes in the GABA(A) and muscarinic receptors in the cortex of subjects from APD-treated subjects with schizophrenia and whether changes were different in subjects who had also received benzodiazepine treatment. We failed to show any strong correlations between changes in GABA(A) and muscarinic receptors in the CNS of subjects with schizophrenia. We showed that subjects with schizophrenia treated with benzodiazepines had lower levels of muscarinic receptors; which was not the case in rats treated with APDs, benzodiazepines or a combination of both drugs. Further, the benzodiazepine binding site, but not the muscimol binding site, was decreased in the parietal cortex of subjects with schizophrenia independent of benzodiazepine status at death. These data would therefore support our previously stated hypotheses that changes in the cortical cholinergic and GABAergic systems are involved in the pathophysiology of schizophrenia.

Decreased cortical muscarinic receptors define a subgroup of subjects with schizophrenia

Schizophrenia is widely acknowledged as being a syndrome, consisting of an undefined number of diseases probably with differing pathologies. Although studying a syndrome makes the identification of an underlying pathology more difficult; neuroimaging, neuropsychopharmacological and post-mortem brain studies all implicate muscarinic acetylcholine receptors (CHRM) in the pathology of the disorder. We have established that the CHRM1 is selectively decreased in the dorsolateral prefrontal cortex of subjects with schizophrenia. To expand this finding, we wanted to ascertain whether decreased cortical CHRMs might (1) define a subgroup of schizophrenia and/or (2) be related to CHRM1 genotype. We assessed cortical [(3)H]pirenzepine binding and sequenced the CHRM1 in 80 subjects with schizophrenia and 74 age sex-matched control subjects. Kernel density estimation showed that [(3)H]pirenzepine binding in BA9 divided the schizophrenia, but not control, cohort into two distinct populations. One of the schizophrenia cohorts, comprising 26% of all subjects with the disorder, had a 74% reduction in mean cortical [(3)H]pirenzepine binding compared to controls. We suggest that these individuals make up 'muscarinic receptor-deficit schizophrenia' (MRDS). The MRDS could not be separated from other subjects with schizophrenia by CHRM1 sequence, gender, age, suicide, duration of illness or any particular drug treatment. Being able to define a subgroup within schizophrenia using a central biological parameter is a pivotal step towards understanding the biochemistry underlying at least one form of the disorder and may represent a biomarker that can be used in neuroimaging.

The receptor mechanisms underlying the disruptive effects of haloperidol and clozapine on rat maternal behavior: a double dissociation between dopamine D(2) and 5-HT(2A/2C) receptors

Many antipsychotic drugs disrupt active components of maternal behavior such as pup approach, pup retrieval and nest building at clinically relevant doses in postpartum female rats. However, the neurochemical mechanisms underlying such a disruptive effect remain to be determined. This study examined the neurochemical mechanisms that mediate the disruptive effects of haloperidol (a typical antipsychotic) and clozapine (an atypical antipsychotic) on rat maternal behavior. Postpartum rats were administered with haloperidol (0.2 mg/kg, sc) or clozapine (10.0 mg/kg, sc) together with either vehicle (saline or water), quinpirole (a selective dopamine D(2)/D(3) agonist, 0.5 or 1.0 mg/kg, sc), or 2,5-dimethoxy-4-iodo-amphetamine (DOI, a selective 5-HT(2A/2C) agonist, 1.0 or 2.5 mg/kg, sc), and their maternal behaviors were tested at different time points before and after drug administration. Haloperidol and clozapine treatment disrupted pup approach, pup retrieval, pup licking and nest building. Pretreatment of quinpirole, but not DOI, dose-dependently reversed the haloperidol-induced disruptions. In contrast, pretreatment of DOI, but not quinpirole, dose-dependently reversed the clozapine-induced disruptions. Quinpirole pretreatment even exacerbated the clozapine-induced disruption of pup retrieval and nest building. These findings suggest a double dissociation mechanism underlying the disruption of haloperidol and clozapine on rat maternal behavior. Specifically, haloperidol disrupts maternal behavior primarily by blocking dopamine D(2) receptors, whereas clozapine exerts its disruptive effect primarily by blocking the 5-HT(2A/2C) receptors. Our findings also suggest that 5-HT receptors are involved in the mediation of rat maternal behavior.

Effects of antipsychotic medication on muscarinic M1 receptor mRNA expression in the rat brain

Alterations in muscarinic M1 receptor protein and mRNA expression have been revealed in post-mortem brains of schizophrenia patients. Most patients had been treated with antipsychotics, so medication effects cannot be excluded as a possible explanation for these results. With in situ hybridization, this study investigated M1 receptor mRNA expression in rats treated with the typical antipsychotic haloperidol (0.3 mg/kg/day) and the atypical antipsychotics olanzapine (1.5 mg/kg/day) and aripiprazole (2.25 mg/kg/day) for 1 or 12 weeks. Compared with the control group, haloperidol significantly increased (approximately 13-21%, P < 0.05) M1 mRNA expression in the CA1, CA2, and CA3 regions of the hippocampus after both 1 and 12 weeks of treatment, and it also increased (approximately 17%, P < 0.01) M1 mRNA expression in the substantia nigra compacta after 1 week of treatment. Olanzapine significantly increased (14-22%, P < 0.05) M1 mRNA expression in the hippocampus (CA1, CA2, and CA3) and substantia nigra compacta after 12 weeks of treatment, but not after 1 week. Aripiprazole significantly increased (17%, P < 0.01) M1 mRNA expression in the hippocampus (CA1) after both 1 and 12 week treatments and increased (12%, P < 0.05) M1 mRNA expression in the nucleus accumbens after 1 week of treatment. Despite their different affinities for muscarinic M1 receptors, all three antipsychotic medications induced a similar trend of change in M1 mRNA expression in selected brain regions. These data suggest that the decreased M1 receptor protein and mRNA expression observed in schizophrenia patients is unlikely to be a consequence of drug treatments and implicates muscarinic M1 receptors in the pharmacotherapy of the disease.

Effect of chronic treatment with clozapine and haloperidol on 5-HT2A and 2C receptor mRNA expression in the rat brain

This study examined regional changes of 5-HT(2A and 2C) receptor mRNA expression in the rat brain after chronic administration of clozapine (1.5 mg/kg/day) and haloperidol (2.0 mg/kg/day) for 36 days. 5-HT(2A and 2C) receptor mRNA expression and distributions were detected by in situ hybridization after rats were sacrificed either 2 or 48 h after the last drug administration to examine both immediate and delayed effects following drug withdrawal. Following 2 h of drug withdrawal, it showed that clozapine administration significantly decreased 5-HT(2A) receptor mRNA, predominantly in the nucleus accumbens (65%), hippocampus (80%), lasteral septal nucleus (61%) and striatum (68%) compared to controls, whilst rebound increases were observed in most of these regions 48 h later. In contrast, no change in 5-HT(2A) receptor mRNA expression was found in the haloperidol treated groups either 2 h or 48 h after drug withdrawal. Clozapine also decreased 5-HT(2C) receptor mRNA expression in the posteromedial cortical amygdala (32%) and substantia nigra (35%) 2 h after the last drug administration, while rebound effects were also observed 48 h later. 5-HT(2C) receptor mRNA was only decreased in the substantia nigra at both 2 h (42%) and 48 h (54%) after the last haloperidol administration. Alterations in serotonin receptor expression in limbic system region such as the nucleus accumbens, hippocampus and lateral septal nucleus as well as the striatum may represent the specific regional targets that mediate the clinical effects of antipsychotics via the serotonin system.