Higher levels of different muscarinic receptors in the cortex and hippocampus from subjects with Alzheimer’s disease

Muscarinic M1 and M4 receptor agonists for schizophrenia: promising candidates for the therapeutic arsenal

INTRODUCTION

Successful phase 3 trials of KarXT in people with schizophrenia herald a new era of treating the disorder with drugs that do not target the dopamine D2 receptor. The active component of KarXT is xanomeline, a muscarinic (CHRM) M1 and M4 agonist, making muscarinic receptors a viable target for treating schizophrenia.

AREAS COVERED

This review covers the process of taking drugs that activate the muscarinic M1 and M4 receptors from conceptualization to the clinic and details the mechanisms by which activating the CHRM1 and 4 can affect the broad spectrum of symptoms experienced by people with schizophrenia.

EXPERT OPINION

Schizophrenia is a syndrome which means drugs that activate muscarinic M1 and M4 receptors, as was the case for antipsychotic drugs acting on the dopamine D2 receptor, will not give optimal outcomes in everyone within the syndrome. Thus, it would be ideal to identify people who are responsive to drugs activating the CHRM1 and 4. Given knowledge of the actions of these receptors, it is possible treatment non-response could be restricted to sub-groups within the syndrome who have deficits in cortical CHRM1 or those with one of the cognitive endophenotypes that may be identifiable by changes in the blood transcriptome.

Lower levels of cortical [3H]pirenzepine binding to postmortem tissue defines a sub-group of older people with schizophrenia with less severe cognitive deficits

Multiple lines of evidence argue for lower levels of cortical muscarinic M1 receptors (CHRM1) in people with schizophrenia which is possibly due to a sub-group within the disorder who have a marked loss of CHRM1 (muscarinic receptor deficit sub-group (MRDS)). In this study we sought to determine if the lower levels of CHRM1 was apparent in older people with schizophrenia and whether the loss of CHRM1 was associated with symptom severity by measuring levels of cortical [³H]pirenzepine binding to CHRM1 from 56 people with schizophrenia and 43 controls. Compared to controls (173 ± 6.3 fmol / mg protein), there were lower levels of cortical [³H]pirenzepine binding in the people with schizophrenia (mean ± SEM: 153 ± 6.0 fmol / mg protein; p = 0.02; Cohen's d = - 0.46). [³H]pirenzepine binding in the people with schizophrenia, but not controls, was not normally distributed and best fitted a two-population solution. The nadir of binding separating the two groups of people with schizophrenia was 121 fmol / mg protein and levels of [³H]pirenzepine binding below this value had a 90.7 % specificity for the disorder. Compared to controls, the score from the Clinical Dementia Rating Scale (CDR) did not differ significantly in MRDS but were significantly higher in the sub-group with normal radioligand binding. Positive and Negative Syndrome Scale scores did not differ between the two sub-groups with schizophrenia. Our current study replicates and earlier finding showing a MRDS within schizophrenia and, for the first time, suggest this sub-group have less severe cognitive deficits others with 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.

Sex, Age, and Regional Differences in CHRM1 and CHRM3 Genes Expression Levels in the Human Brain Biopsies: Potential Targets for Alzheimer's Disease-related Sleep Disturbances

BACKGROUND

Cholinergic hypofunction and sleep disturbance are hallmarks of Alzheimer's disease (AD), a progressive disorder leading to neuronal deterioration. Muscarinic acetylcholine receptors (M1-5 or mAChRs), expressed in hippocampus and cerebral cortex, play a pivotal role in the aberrant alterations of cognitive processing, memory, and learning, observed in AD. Recent evidence shows that two mAChRs, M1 and M3, encoded by CHRM1 and CHRM3 genes, respectively, are involved in sleep functions and, peculiarly, in rapid eye movement (REM) sleep.

METHODS

We used twenty microarray datasets extrapolated from post-mortem brain tissue of nondemented healthy controls (NDHC) and AD patients to examine the expression profile of CHRM1 and CHRM3 genes. Samples were from eight brain regions and stratified according to age and sex.

RESULTS

CHRM1 and CHRM3 expression levels were significantly reduced in AD compared with ageand sex-matched NDHC brains. A negative correlation with age emerged for both CHRM1 and CHRM3 in NDHC but not in AD brains. Notably, a marked positive correlation was also revealed between the neurogranin (NRGN) and both CHRM1 and CHRM3 genes. These associations were modulated by sex. Accordingly, in the temporal and occipital regions of NDHC subjects, males expressed higher levels of CHRM1 and CHRM3, respectively, than females. In AD patients, males expressed higher levels of CHRM1 and CHRM3 in the temporal and frontal regions, respectively, than females.

CONCLUSION

Thus, substantial differences, all strictly linked to the brain region analyzed, age, and sex, exist in CHRM1 and CHRM3 brain levels both in NDHC subjects and in AD patients.

The PET tracer [11C]MK-6884 quantifies M4 muscarinic receptor in rhesus monkeys and patients with Alzheimer's disease

[Figure: see text].

Muscarinic M1 and M4 receptors: Hypothesis driven drug development for schizophrenia

The finding that the drug KarXT, a formulation of xanomeline and tropsium which targets muscarinic receptors, has given a positive result in reducing the positive and negative symptoms of schizophrenia in a phase II trial suggests targeting muscarinic receptors is a new approach to treating the disorder. This review will detail the synergistic interplay between studies to understand the role of muscarinic receptors in the aetiology of schizophrenia and drug development and how this has supported the hypothesis that activating the muscarinic M1 and M4 receptors is critical to the efficacy of KarXT, in schizophrenia. The discovery of an intermediate phenotype within schizophrenia which is characterised by the presence of a marked loss of cortical muscarinic M1 receptors will be reviewed. Highlighted will be progress in understanding the biochemistry of that intermediate phenotype and evidence to suggest that those with the intermediate phenotype may resist treatment with agonist to the orthosteric site on the muscarinic M1 and M4 receptor. Finally, the possibility of using drugs targeting the allosteric binding sites on muscarinic receptors to treat schizophrenia will be discussed. This timely review will therefore highlight how research can influence hypothesis driven drug discovery that should produce new treatments for schizophrenia.