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

Ligand bias at the muscarinic acetylcholine receptor family: Opportunities and challenges

Muscarinic acetylcholine receptors (mAChRs) are G protein-coupled receptors (GPCRs) that are activated by the endogenous neurotransmitter, acetylcholine (ACh). Disruption of mAChR signalling has been associated with a variety of neurological disorders and non-neurological diseases. Consequently, the development of agonists and antagonists of the mAChRs has been a major avenue in drug discovery. Unfortunately, mAChR ligands are often associated with on-target side effects for two reasons. The first reason is due to the high sequence conservation at the orthosteric ACh binding site among all five receptor subtypes (M1-M5), making on-target subtype selectivity a major challenge. The second reason is due to on-target side effects of mAChR drugs that are associated with the pleiotropic nature of mAChR signalling at the level of a single mAChR subtype. Indeed, there is growing evidence that within the myriad of signalling events produced by mAChR ligands, some will have therapeutic benefits, whilst others may promote cholinergic side effects. This paradigm of drug action, known as ligand bias or biased agonism, is an attractive feature for next-generation mAChR drugs, as it holds the promise of developing drugs devoid of on-target adverse effects. Although relatively simple to detect and even quantify in vitro, ligand bias, as observed in recombinant systems, does not always translate to in vivo systems, which remains a major hurdle in GPCR drug discovery, including the mAChR family. Here we report recent studies that have attempted to detect and quantify ligand bias at the mAChR family, and briefly discuss the challenges associated with biased agonist drug development. This article is part of the Special Issue on "Ligand Bias".

Activation of M4 muscarinic receptors in the striatum reduces tic-like behaviours in two distinct murine models of Tourette syndrome

BACKGROUND AND PURPOSE

Current pharmacotherapies for Tourette syndrome (TS) are often unsatisfactory and poorly tolerated, underscoring the need for novel treatments. Insufficient striatal acetylcholine has been suggested to contribute to tic ontogeny. Thus, we tested whether activating M1 and/or M4 receptors-the two most abundant muscarinic receptors in the striatum-reduced tic-related behaviours in mouse models of TS.

EXPERIMENTAL APPROACH

Studies were conducted using CIN-d and D1CT-7 mice, two TS models characterized by early-life depletion of striatal cholinergic interneurons and cortical neuropotentiation, respectively. First, we tested the effects of systemic and intrastriatal xanomeline, a selective M1/M4 receptor agonist, on tic-like and other TS-related responses. Then, we examined whether xanomeline effects were reduced by either M1 or M4 antagonists or mimicked by the M1/M3 agonist cevimeline or the M4 positive allosteric modulator (PAM) VU0467154. Finally, we measured striatal levels of M1 and M4 receptors and assessed the impact of VU0461754 on the striatal expression of the neural marker activity c-Fos.

KEY RESULTS

Systemic and intrastriatal xanomeline reduced TS-related behaviours in CIN-d and D1CT-7 mice. Most effects were blocked by M4, but not M1, receptor antagonists. VU0467154, but not cevimeline, elicited xanomeline-like ameliorative effects in both models. M4, but not M1, receptors were down-regulated in the striatum of CIN-d mice. Additionally, VU0467154 reduced striatal c-Fos levels in these animals.

CONCLUSION AND IMPLICATIONS

Activation of striatal M4, but not M1, receptors reduced tic-like manifestations in mouse models, pointing to xanomeline and M4 PAMs as novel putative therapeutic strategies for TS.

Neuroprotective effects of chlorogenic acid against oxidative stress in rats subjected to lithium-pilocarpine-induced status epilepticus

Epilepsy is a condition marked by sudden, self-sustained, and recurring brain events, showcasing unique electro-clinical and neuropathological phenomena that can alter the structure and functioning of the brain, resulting in diverse manifestations. Antiepileptic drugs (AEDs) can be very effective in 30% of patients in controlling seizures. Several factors contribute to this: drug resistance, individual variability, side effects, complexity of epilepsy, incomplete understanding, comorbidities, drug interactions, and no adherence to treatment. Therefore, research into new AEDs is important for several reasons such as improved efficacy, reduced side effects, expanded treatment options, treatment for drug-resistant epilepsy, improved safety profiles, targeted therapies, and innovation and progress. Animal models serve as crucial biological tools for comprehending neuronal damage and aiding in the discovery of more effective new AEDs. The utilization of antioxidant agents that act on the central nervous system may serve as a supplementary approach in the secondary prevention of epilepsy, both in laboratory animals and potentially in humans. Chlorogenic acid (CGA) is a significant compound, widely prevalent in numerous medicinal and food plants, exhibiting an extensive spectrum of biological activities such as neuroprotection, antioxidant, anti-inflammatory, and analgesic effects, among others. In this research, we assessed the neuroprotective effects of commercially available CGA in Wistar rats submitted to lithium-pilocarpine-induced status epilepticus (SE) model. After 72-h induction of SE, rats received thiopental and were treated for three consecutive days (1st, 2nd, and 3rd doses). Next, brains were collected and studied histologically for viable cells in the hippocampus with staining for cresyl-violet (Nissl staining) and for degenerating cells with Fluoro-Jade C (FJC) staining. Moreover, to evaluate oxidative stress, the presence of malondialdehyde (MDA) and superoxide dismutase (SOD) was quantified. Rats administered with CGA (30 mg/kg) demonstrated a significant decrease of 59% in the number of hippocampal cell loss in the CA3, and of 48% in the hilus layers after SE. A significant reduction of 75% in the cell loss in the CA3, shown by FJC+ staining, was also observed with the administration of CGA (30 mg/kg). Furthermore, significant decreases of 49% in MDA production and 72% in the activity of SOD were seen, when compared to animals subjected to SE that received vehicle. This study introduces a novel finding: the administration of CGA at a dosage of 30 mg/kg effectively reduced oxidative stress induced by lithium-pilocarpine, with its effects lasting until the peak of neural damage 72 h following the onset of SE. Overall, the research and development of new AEDs are essential for advancing epilepsy treatment, improving patient outcomes, and ultimately enhancing the quality of life for individuals living with epilepsy.

Anticholinergic action is rarely a good thing

The evidence for the risks associated with anticholinergic agents has grown considerably in the last two decades. Not only are they associated with causing peripheral side effects such as dry mouth, blurred vision and constipation, but they can also cause central effects such as cognitive impairment; and more recently, they have consistently been linked with an increased risk of dementia and death in older people. This paper reviews the evidence for the associations of anticholinergic agents and the risk of dementia and increased mortality in dementia.

Preclinical Evaluation of Novel PET Probes for Dementia

The development of novel PET imaging agents that selectively bind specific dementia-related targets can contribute significantly to accurate, differential and early diagnosis of dementia causing diseases and support the development of therapeutic agents. Consequently, in recent years there has been a growing body of literature describing the development and evaluation of potential new promising PET tracers for dementia. This review article provides a comprehensive overview of novel dementia PET probes under development, classified by their target, and pinpoints their preclinical evaluation pathway, typically involving in silico, in vitro and ex/in vivo evaluation. Specific target-associated challenges and pitfalls, requiring extensive and well-designed preclinical experimental evaluation assays to enable successful clinical translation and avoid shortcomings observed for previously developed 'well-established' dementia PET tracers are highlighted in this review.

Long-Term-But Not Short-Term-Plasticity at the Mossy Fiber-CA3 Pyramidal Cell Synapse in Hippocampus Is Altered in M1/M3 Muscarinic Acetylcholine Receptor Double Knockout Mice

Muscarinic acetylcholine receptors are well-known for their crucial involvement in hippocampus-dependent learning and memory, but the exact roles of the various receptor subtypes (M1-M5) are still not fully understood. Here, we studied how M1 and M3 receptors affect plasticity at the mossy fiber (MF)-CA3 pyramidal cell synapse. In hippocampal slices from M1/M3 receptor double knockout (M1/M3-dKO) mice, the signature short-term plasticity of the MF-CA3 synapse was not significantly affected. However, the rather unique NMDA receptor-independent and presynaptic form of long-term potentiation (LTP) of this synapse was much larger in M1/M3-deficient slices compared to wild-type slices in both field potential and whole-cell recordings. Consistent with its presynaptic origin, induction of MF-LTP strongly enhanced the excitatory drive onto single CA3 pyramidal cells, with the effect being more pronounced in M1/M3-dKO cells. In an earlier study, we found that the deletion of M2 receptors in mice disinhibits MF-LTP in a similar fashion, suggesting that endogenous acetylcholine employs both M1/M3 and M2 receptors to constrain MF-LTP. Importantly, such synergism was not observed for MF long-term depression (LTD). Low-frequency stimulation, which reliably induced LTD of MF synapses in control slices, failed to do so in M1/M3-dKO slices and gave rise to LTP instead. In striking contrast, loss of M2 receptors augmented LTD when compared to control slices. Taken together, our data demonstrate convergence of M1/M3 and M2 receptors on MF-LTP, but functional divergence on MF-LTD, with the net effect resulting in a well-balanced bidirectional plasticity of the MF-CA3 pyramidal cell synapse.

Cross-diagnostic determinants of cognitive functioning: the muscarinic cholinergic receptor as a model system

Cognitive impairment is a predictor of disability across different neuropsychiatric conditions, and cognitive abilities are also strongly related to educational attainment and indices of life success in the general population. Previous attempts at drug development for cognitive enhancement have commonly attempted to remedy defects in transmitters systems putatively associated with the conditions of interest such as the glutamate system in schizophrenia. Recent studies of the genomics of cognitive performance have suggested influences that are common in the general population and in different neuropsychiatric conditions. Thus, it seems possible that transmitter systems that are implicated for cognition across neuropsychiatric conditions and the general population would be a viable treatment target. We review the scientific data on cognition and the muscarinic cholinergic receptor system (M1 and M4) across different diagnoses, in aging, and in the general population. We suggest that there is evidence suggesting potential beneficial impacts of stimulation of critical muscarinic receptors for the enhancement of cognition in a broad manner, as well as the treatment of psychotic symptoms. Recent developments make stimulation of the M1 receptor more tolerable, and we identify the potential benefits of M1 and M4 receptor stimulation as a trans-diagnostic treatment model.

Treatment of Clozapine-Associated Sialorrhea: The Role of Benzamide Derivatives

PURPOSE

Hypersalivation is one of the most prevalent and distressing adverse effects associated with clozapine treatment. Currently, there is no standard therapeutic approach toward how to overcome it. Clinicians use various medications for managing this adverse effect. However, some of the agents are not effective enough, whereas others can induce other adverse effects. Recently, several reviews have been published on the treatment of clozapine-associated hypersalivation, in which the focus was on drugs from various pharmacological groups, and little attention was paid to drugs from the group of substituted benzamides. The intention of this brief narrative review is to draw the attention of clinicians to the use of the benzamide group for the treatment of this unpleasant adverse effect.

METHODS

A MEDLINE search was conducted to identify published treatment studies and case reports in the literature from 2000 to September 2021, concerning a treatment of clozapine-associated hypersalivation, mainly substituted benzamides.

RESULTS

Accumulating evidence during the last 2 decades indicates that agents derived from the benzamide group may be effective and safe agents for treatment of clozapine-associated hypersalivation. Whether with a psychotropic effect or without, medications from this group may produce a beneficial response.

CONCLUSIONS

Substitute benzamide derivatives have emerged as effective and well-tolerated agents for treatment clozapine-associated hypersalivation.

Novel shaking exercises for hippocampal and medial prefrontal cortex functioning maintain spatial working memory

INTRODUCTION

The decline in spatial working memory is one of the earliest signs of normal brain aging.

OBJECTIVE

We developed a novel physical exercise method, termed the "shaking exercise," to slow down this process.

METHODS

The experimental protocol included administering the shaking exercise for 8-32 weeks in male senescence-accelerated mouse prone 10 (SAMP-10). They were subjected to the T-maze test, followed by immunohistochemical analysis, to assess the influence of the shaking exercise on the M1 muscarinic acetylcholine receptor (CHRM1) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) of the dorsal hippocampus and medial prefrontal cortex (dHC-mPFC).

RESULTS

The T-maze test demonstrated that the shaking group had less hesitation in the face of selecting direction at week 24. In the immunohistochemical analysis, more CHRM1s were in the CA3 subregion and more AMPARs were in the subiculum. CHRM1s and AMPARs were maintained in the CA1 region and the mPFC. The CHRM1s seem to have a positive effect on the AMPAR in the dentate gyrus (DG) region and the CA3 region. In the CA1 region, CHRM1s were negatively correlated with AMPARs. In addition, high-density neurons were expressed in the shaking group in the upstream DG, the middle part and the distal part of CA3, the distal part of CA1, and the mPFC.

CONCLUSIONS

Our results raise the possibility that maintenance of the spatial working memory effect observed with the shaking exercise is driven in part by the uneven affection of CHRM1s and AMPARs in the dHC-mPFC circuit system and significantly maintains the neuronal expression in the dHC-mPFC.

Muscarinic Acetylcholine Receptor Agonists as Novel Treatments for Schizophrenia

Schizophrenia remains a challenging disease to treat effectively with current antipsychotic medications due to their limited efficacy across the entire spectrum of core symptoms as well as their often burdensome side-effect profiles and poor tolerability. An unmet need remains for novel, mechanistically unique, and better tolerated therapeutic agents for treating schizophrenia, especially those that treat not only positive symptoms but also the negative and cognitive symptoms of the disease. Almost 25 years ago, the muscarinic acetylcholine receptor (mAChR) agonist xanomeline was reported to reduce psychotic symptoms and improve cognition in patients with Alzheimer's disease. The antipsychotic and procognitive properties of xanomeline were subsequently confirmed in a small study of acutely psychotic patients with chronic schizophrenia. These unexpected clinical findings have prompted considerable efforts across academia and industry to target mAChRs as a new approach to potentially treat schizophrenia and other psychotic disorders. The authors discuss recent advances in mAChR biology and pharmacology and the current understanding of the relative roles of the various mAChR subtypes, their downstream cellular effectors, and key neural circuits mediating the reduction in the core symptoms of schizophrenia in patients treated with xanomeline. They also provide an update on the status of novel mAChR agonists currently in development for potential treatment of schizophrenia and other neuropsychiatric disorders.

Biased Profile of Xanomeline at the Recombinant Human M4 Muscarinic Acetylcholine Receptor

Many Food and Drug Administration (FDA)-approved drugs are structural analogues of the endogenous (natural) ligands of G protein-coupled receptors (GPCRs). However, it is becoming appreciated that chemically distinct ligands can bind to GPCRs in conformations that lead to different cellular signaling events, a phenomenon termed biased agonism. Despite this, the rigorous experimentation and analysis required to identify biased agonism are often not undertaken in most clinical candidates and go unrealized. Recently, xanomeline, a muscarinic acetylcholine receptor (mAChR) agonist, has entered phase III clinical trials for the treatment of schizophrenia. If successful, xanomeline will be the first novel FDA-approved antipsychotic drug in almost 50 years. Intriguingly, xanomeline's potential for biased agonism at the mAChRs and, in particular, the M₄ mAChR, the most promising receptor target for schizophrenia, has not been assessed. Here, we quantify the biased agonism profile of xanomeline and three other mAChR agonists in Chinese hamster ovary cells recombinantly expressing the M₄ mAChR. Agonist activity was examined across nine distinct signaling readouts, including the activation of five different G protein subtypes, ERK1/2 phosphorylation, β-arrestin recruitment, calcium mobilization, and cAMP regulation. Relative to acetylcholine (ACh), xanomeline was biased away from ERK1/2 phosphorylation and calcium mobilization compared to Gα_i2 protein activation. These findings likely have important implications for our understanding of the therapeutic action of xanomeline and call for further investigation into the in vivo consequences of biased agonism in drugs targeting the M₄ mAChR for the treatment of schizophrenia.

Cholinergic modulation of persistent inward currents is mediated by activating muscarinic receptors of serotonergic neurons in the brainstem of ePet-EYFP mice

Persistent inward currents (PICs) play important roles in regulating neural excitability. Results from our previous studies showed that serotonergic (5-HT) neurons of the brainstem expressed PICs. However, little is known about cholinergic (ACh) modulation of PICs in the 5-HT neurons. The whole-cell patch-clamp recordings were performed in the brainstem slices of ePet-EYFP mice to investigate the electrophysiological properties of PICs with cholinergic modulation. PICs in 5-HT neurons were activated at - 51.4 ± 3.7 mV with the amplitude of - 171.6 ± 48.9 pA (n = 71). Bath application of 20-25 μM ACh increased the amplitude by 79.1 ± 42.5 pA (n = 23, p < 0.001) and hyperpolarized the onset voltage by 2.2 ± 2.7 mV (n = 23, p < 0.01) and half-maximal activation by 3.6 ± 2.7 mV (n = 6, p < 0.01). Muscarine mimicked the effects of ACh on PICs, while bath application of nicotine (15-20 μM) did not induce substantial change in the PICs (n = 9). Muscarine enhanced the amplitude of PICs by 100.0 ± 27.4 pA (n = 28, p < 0.001) and lowered the onset voltage by 2.8 ± 1.2 mV (n = 28, p < 0.001) and the half-maximal activation by 2.9 ± 1.4 mV. ACh-induced increase of amplitude and hyperpolarization of onset voltage were blocked by 3-5 μM atropine. Furthermore, the muscarine-induced enhancement of the PICs was antagonized by 5 μM 4-DAMP, the antagonist of M3 receptor, while the antagonists of M1 (Telenzepine, 5 μM) and M5 (VU6008667, 5 μM) receptors did not significantly affect the PIC enhancement. This study suggested that ACh potentiated PICs in 5-HT neurons of the brainstem by activating muscarinic M3 receptor.

Changes in cortical gene expression in the muscarinic M1 receptor knockout mouse: potential relevance to schizophrenia, Alzheimer's disease and cognition

Postmortem and neuroimaging studies show low levels of cortical muscarinic M1 receptors (CHRM1) in patients with schizophrenia which is significant because CHRM signalling has been shown to change levels of gene expression and cortical gene expression is altered in schizophrenia. We decided to identify CHRM1-mediated changes in cortical gene expression by measuring levels of RNA in the cortex of the Chrm1^-/- mouse (n = 10), where there would be no signalling by that receptor, and in wild type mouse (n = 10) using the Affymetrix Mouse Exon 1.0 ST Array. We detected RNA for 15,501 annotated genes and noncoding RNA of which 1,467 RNAs were higher and 229 RNAs lower in the cortex of the Chrm1^-/- mouse. Pathways and proteins affected by the changes in cortical gene expression in the Chrm1^-/- are linked to the molecular pathology of schizophrenia. Our human cortical gene expression data showed 47 genes had altered expression in Chrm1^-/- mouse and the frontal pole from patients with schizophrenia with the change in expression of 44 genes being in opposite directions. In addition, genes with altered levels of expression in the Chrm1^-/- mouse have been shown to affect amyloid precursor protein processing which is associated with the pathophysiology of Alzheimer's disease, and 69 genes with altered expression in the Chrm1^-/- mouse are risk genes associated with human cognitive ability. Our findings argue CHRM1-mediated changes in gene expression are relevant to the pathophysiologies of schizophrenia and Alzheimer's disease and the maintenance of cognitive ability in humans.

Effects of adolescent substance use disorders on central cholinergic function

Adolescence is a transitional period between childhood and adulthood, in which the individual undergoes significant cognitive, behavioral, physical, emotional, and social developmental changes. During this period, adolescents engage in experimentation and risky behaviors such as licit and illicit drug use. Adolescents' high vulnerability to abuse drugs and natural reinforcers leads to greater risk for developing substance use disorders (SUDs) during adulthood. Accumulating evidence indicates that the use and abuse of licit and illicit drugs during adolescence and emerging adulthood can disrupt the cholinergic system and its processes. This review will focus on the effects of peri-adolescent nicotine and/or alcohol use, or exposure, on the cholinergic system during adulthood from preclinical and clinical studies. This review further explores potential cholinergic agents and pharmacological manipulations to counteract peri-adolescent nicotine and/or alcohol abuse.

Reduced acetylcholine and elevated muscarinic receptor 2 in duodenal mucosa contribute to the impairment of mucus secretion in 6-hydroxydopamine-induced Parkinson's disease rats

Patients with Parkinson's disease (PD) have a higher incidence rate of duodenal ulcers. The mucus barrier provides the first line of defense for duodenal mucosal protection. However, it is unknown whether duodenal mucus secretion is affected in PD. In the present study, we used the rats microinjected 6-hydroxydopamine (6-OHDA) into the bilateral substantia nigra to investigate duodenal mucus secretion and potential therapeutic targets in duodenal ulcer in PD. Alcian blue-periodic acid-Schiff, transmission electron microscopy, immunofluorescence, duodenal mucosal incubation, and enzyme-linked immunosorbent assays were used. The 6-OHDA rats exhibited mucin accumulation and retention in duodenal goblet cells. Mucin granules were unable to fuse with the apical membranes of goblet cells, and the exocytosis ratio of goblet cells was significantly reduced. Moreover, decreased acetylcholine and increased muscarinic receptor 2 (M₂R) levels were detected in the duodenal mucosa of 6-OHDA rats. Bilateral vagotomy rats were also characterized by defective duodenal mucus secretion and decreased acetylcholine with increased M₂R levels in the duodenal mucosa. Application of the cholinomimetic drug carbachol or blocking M₂R with methoctramine significantly promoted mucus secretion by goblet cells and increased MUC2 content in duodenal mucosa-incubated solutions from 6-OHDA and vagotomy rats. We conclude that the reduced acetylcholine and increased M₂R contribute to the impaired duodenal mucus secretion of 6-OHDA rats. The study provides new insights into the mechanism of duodenal mucus secretion and potential therapeutic targets for the treatment of duodenal ulcers in PD patients.

An investigation into nicotinic receptor involvement in mood disorders uncovers novel depression candidate genes

BACKGROUND

We have previously reported reduced expression of the cholinergic autoreceptor CHRM2 in Brodmann's Area (BA) 24 of the anterior cingulate cortex from subjects with major depressive disorder (MDD) and bipolar disorder (BD), consistent with a hypercholinergic state. This led us to investigate whether levels of the high affinity nicotinic acetylcholine receptors are also altered in BA 24.

METHODS

We measured the binding levels of a high-affinity nicotinic receptor-selective radioligand, [³H]epibatidine, in BA 24 from subjects with MDD (n = 20), BD (n = 18) and age- and sex-matched controls (n = 20). We used qPCR to measure mRNA expression of the high affinity nicotinic acetylcholine receptor subunit CHRNB2 in these subjects.

RESULTS

[³H]Epibatidine binding density and CHRNB2 mRNA expression were not significantly altered in either MDD or BD compared to control levels. While validating reference genes for our qPCR experiments, we found that the mRNA levels of 3 putative reference genes, TFB1M, PPIA and SNCA, were increased in MDD but not BD compared to controls. Further investigations in other cortical regions showed that these changes were specific to BA24.

LIMITATIONS

Cohort size and available patient data were limited due to standard constraints associated with post-mortem studies.

CONCLUSION

Our data suggest that decreased CHRM2 in BA24 in mood disorders is not associated with a corresponding change in high affinity nicotinic acetylcholine receptor expression. Our findings of increased TFB1M, PPIA and SNCA expression in MDD point to a broader derangement of several homeostatic pathways in MDD that are distinct from BD.

Autonomic and cholinergic mechanisms mediating cardiovascular and temperature effects of donepezil in conscious mice

Donepezil is a centrally acting acetylcholinesterase (AChE) inhibitor with therapeutic potential in inflammatory diseases; however, the underlying autonomic and cholinergic mechanisms remain unclear. Here, we assessed effects of donepezil on mean arterial pressure (MAP), heart rate (HR), HR variability, and body temperature in conscious adult male C57BL/6 mice to investigate the autonomic pathways involved. Central versus peripheral cholinergic effects of donepezil were assessed using pharmacological approaches including comparison with the peripherally acting AChE inhibitor, neostigmine. Drug treatments included donepezil (2.5 or 5 mg/kg sc), neostigmine methyl sulfate (80 or 240 μg/kg ip), atropine sulfate (5 mg/kg ip), atropine methyl bromide (5 mg/kg ip), or saline. Donepezil, at 2.5 and 5 mg/kg, decreased HR by 36 ± 4% and 44 ± 3% compared with saline (n = 10, P < 0.001). Donepezil, at 2.5 and 5 mg/kg, decreased temperature by 13 ± 2% and 22 ± 2% compared with saline (n = 6, P < 0.001). Modest (P < 0.001) increases in MAP were observed with donepezil after peak bradycardia occurred. Atropine sulfate and atropine methyl bromide blocked bradycardic responses to donepezil, but only atropine sulfate attenuated hypothermia. The pressor response to donepezil was similar in mice coadministered atropine sulfate; however, coadministration of atropine methyl bromide potentiated the increase in MAP. Neostigmine did not alter HR or temperature, but did result in early increases in MAP. Despite the marked bradycardia, donepezil did not increase normalized high-frequency HR variability. We conclude that donepezil causes marked bradycardia and hypothermia in conscious mice via the activation of muscarinic receptors while concurrently increasing MAP via autonomic and cholinergic pathways that remain to be elucidated.

Fine Tuning Muscarinic Acetylcholine Receptor Signaling Through Allostery and Bias

The M1 and M4 muscarinic acetylcholine receptors (mAChRs) are highly pursued drug targets for neurological diseases, in particular for Alzheimer's disease and schizophrenia. Due to high sequence homology, selective targeting of any of the M1-M5 mAChRs through the endogenous ligand binding site has been notoriously difficult to achieve. With the discovery of highly subtype selective mAChR positive allosteric modulators in the new millennium, selectivity through targeting an allosteric binding site has opened new avenues for drug discovery programs. However, some hurdles remain to be overcome for these promising new drug candidates to progress into the clinic. One challenge is the potential for on-target side effects, such as for the M1 mAChR where over-activation of the receptor by orthosteric or allosteric ligands can be detrimental. Therefore, in addition to receptor subtype selectivity, a drug candidate may need to exhibit a biased signaling profile to avoid such on-target adverse effects. Indeed, recent studies in mice suggest that allosteric modulators for the M1 mAChR that bias signaling toward specific pathways may be therapeutically important. This review brings together details on the signaling pathways activated by the M1 and M4 mAChRs, evidence of biased agonism at these receptors, and highlights pathways that may be important for developing new subtype selective allosteric ligands to achieve therapeutic benefit.

Regulation of Glutamatergic Activity via Bidirectional Activation of Two Select Receptors as a Novel Approach in Antipsychotic Drug Discovery

Schizophrenia is a mental disorder that affects approximately 1-2% of the population and develops in early adulthood. The disease is characterized by positive, negative, and cognitive symptoms. A large percentage of patients with schizophrenia have a treatment-resistant disease, and the risk of developing adverse effects is high. Many researchers have attempted to introduce new antipsychotic drugs to the clinic, but most of these treatments failed, and the diversity of schizophrenic symptoms is one of the causes of disappointing results. The present review summarizes the results of our latest papers, showing that the simultaneous activation of two receptors with sub-effective doses of their ligands induces similar effects as the highest dose of each compound alone. The treatments were focused on inhibiting the increased glutamate release responsible for schizophrenia arousal, without interacting with dopamine (D₂) receptors. Ligands activating metabotropic receptors for glutamate, GABA_B or muscarinic receptors were used, and the compounds were administered in several different combinations. Some combinations reversed all schizophrenia-related deficits in animal models, but others were active only in select models of schizophrenia symptoms (i.e., cognitive or negative symptoms).

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.

Discovery of [11C]MK-6884: A Positron Emission Tomography (PET) Imaging Agent for the Study of M4Muscarinic Receptor Positive Allosteric Modulators (PAMs) in Neurodegenerative Diseases

The measurement of receptor occupancy (RO) using positron emission tomography (PET) has been instrumental in guiding discovery and development of CNS directed therapeutics. We and others have investigated muscarinic acetylcholine receptor 4 (M4) positive allosteric modulators (PAMs) for the treatment of symptoms associated with neuropsychiatric disorders. In this article, we describe the synthesis, in vitro, and in vivo characterization of a series of central pyridine-related M4 PAMs that can be conveniently radiolabeled with carbon-11 as PET tracers for the in vivo imaging of an allosteric binding site of the M4 receptor. We first demonstrated its feasibility by mapping the receptor distribution in mouse brain and confirming that a lead molecule 1 binds selectively to the receptor only in the presence of the orthosteric agonist carbachol. Through a competitive binding affinity assay and a number of physiochemical properties filters, several related compounds were identified as candidates for in vivo evaluation. These candidates were then radiolabeled with ¹¹C and studied in vivo in rhesus monkeys. This research eventually led to the discovery of the clinical radiotracer candidate [¹¹C]MK-6884.

Simultaneous activation of mGlu2 and muscarinic receptors reverses MK-801-induced cognitive decline in rodents

The activity of an allosteric agonist of muscarinic M₁ receptor, VU0357017, and a positive allosteric modulator (PAM) of M₅ receptor, VU0238429, were investigated alone or in combination with the mGlu₂ receptor PAM, LY487379 using the following behavioural tests: prepulse inhibition (PPI), novel object recognition (NOR), and spatial delayed alternation (SDA). VU0357017 (10 and 20 mg/kg) and VU0238429 (5 and 10 mg/kg) reversed deficits in PPI while VU0238429 (2.5 and 5 mg/kg) was effective in SDA. The simultaneous administration of subeffective doses of M₁ or M₅ activators (5, 1, or 0.25 mg/kg) with LY487379 (0.5 mg/kg) induced the same effect as that observed for the active dose of each compound. Selective M₁ or M₅ receptor blockers antagonized the effect exerted by these combinations, and pharmacokinetic studies confirmed independent transport through the blood-brain barrier. The expression of both receptors (M₁ and M₅) was established in brain structures involved in cognition (neocortex, hippocampus, and entorhinal cortex) in both the rat and the mouse brains by immunofluorescence staining. Specifically, double neuronal staining of mGlu₂-M₁ and mGlu₂-M₅ receptors was observed in many areas of the rat brain, while the number of double-stained mGlu₂-M₁ receptors was moderate in the mouse brain with no mGlu₂-M₅ colocalization. Finally, the combined administration of subeffective doses of the compounds did not alter prolactin levels or motor coordination, in contrast to the compounds given alone at the highest dose or in combination with standard neuroleptics.

PF-06827443 Displays Robust Allosteric Agonist and Positive Allosteric Modulator Activity in High Receptor Reserve and Native Systems

Positive allosteric modulators (PAMs) of the M₁ subtype of muscarinic acetylcholine receptor have attracted intense interest as an exciting new approach for improving the cognitive deficits in schizophrenia and Alzheimer's disease. Recent evidence suggests that the presence of intrinsic agonist activity of some M₁ PAMs may reduce efficacy and contribute to adverse effect liability. However, the M₁ PAM PF-06827443 was reported to have only weak agonist activity at human M₁ receptors but produced M₁-dependent adverse effects. We now report that PF-06827443 is an allosteric agonist in cell lines expressing rat, dog, and human M₁ and use of inducible cell lines shows that agonist activity of PF-06827443 is dependent on receptor reserve. Furthermore, PF-06827443 is an agonist in native tissue preparations and induces behavioral convulsions in mice similar to other ago-PAMs. These findings suggest that PF-06827443 is a robust ago-PAM, independent of species, in cell lines and native systems.

Probing the binding site of novel selective positive allosteric modulators at the M1 muscarinic acetylcholine receptor

Subtype-selective allosteric modulation of the M1 muscarinic acetylcholine (ACh) receptor (M1 mAChR) is an attractive approach for the treatment of numerous disorders, including cognitive deficits. The discovery of benzyl quinolone carboxylic acid, BQCA, a selective M1 mAChR positive allosteric modulator (PAM), spurred the subsequent development of newer generation M1 PAMs representing diverse chemical scaffolds, different pharmacodynamic properties and, in some instances, improved pharmacokinetics. Key exemplar molecules from such efforts include PF-06767832 (N-((3R,4S)-3-hydroxytetrahydro-2H-pyran-4-yl)-5-methyl-4-(4-(thiazol-4-yl)benzyl)pyridine-2-carboxamide), VU6004256 (4,6-difluoro-N-(1S,2S)-2-hydroxycyclohexyl-1-((6-(1-methyl-1H-pyrazol-4-yl)pyridine-3-yl)methyl)-1H-indole-3-carboxamide) and MIPS1780 (3-(2-hydroxycyclohexyl)-6-(2-((4-(1-methyl-1H-pyrazol-4-yl)-benzyl)oxy)phenyl)pyrimidin-4(3H)-one). Given these diverse scaffolds and pharmacodynamics, the current study combined pharmacological analysis and site-directed mutagenesis to explore the potential binding site and function of newer M1 mAChR PAMs relative to BQCA. Interestingly, the mechanism of action of the novel PAMs was consistent with a common model of allostery, as previously described for BQCA. Key residues involved in the activity of BQCA, including Y179 in the second extracellular loop (ECL) and W4007.35 in transmembrane domain (TM) 7, were critical for the activity of all PAMs tested. Overall, our data indicate that structurally distinct PAMs share a similar binding site with BQCA, specifically, an extracellular allosteric site defined by residues in TM2, TM7 and ECL2. These findings provide valuable insights into the structural basis underlying modulator binding, cooperativity and signaling at the M1 mAChR, which is essential for the rational design of PAMs with tailored pharmacological properties.

The deletion of M4 muscarinic receptors increases motor activity in females in the dark phase

OBJECTIVES

M4 muscarinic receptors (MR) presumably play a role in motor coordination. Previous studies have shown different results depending on genetic background and number of backcrosses. However, no attention has been given to biorhythms.

MATERIAL AND METHODS

We therefore analyzed biorhythms under a light/dark cycle obtained telemetrically in intact animals (activity, body temperature) in M4 KO mice growth on the C57Bl6 background using ChronosFit software. Studying pure effects of gene knockout in daily rhythms is especially important knowledge for pharmacological/behavioral studies in which drugs are usually tested in the morning.

RESULTS

We show that M4 KO mice motor activity does not differ substantially from wild-type mice during light period while in the dark phase (mice active part of the day), the M4 KO mice reveal biorhythm changes in many parameters. Moreover, these differences are sex-dependent and are evident in females only. Mesor, night-day difference, and night value were doubled or tripled when comparing female KO versus male KO. Our in vitro autoradiography demonstrates that M4 MR proportion represents 24% in the motor cortex (MOCx), 30% in the somatosensory cortex, 50% in the striatum, 69% in the thalamus, and 48% in the intergeniculate leaflet (IGL). The M4 MR densities were negligible in the subparaventricular zone, the posterior hypothalamic area, and in the suprachiasmatic nuclei.

CONCLUSIONS

We conclude that cholinergic signaling at M4 MR in brain structures such as striatum, MOCx, and probably with the important participation of IGL significantly control motor activity biorhythm. Animal activity differs in the light and dark phases, which should be taken into consideration when interpreting the results.

Structure-Activity Relationships of Pan-Gαq/11 Coupled Muscarinic Acetylcholine Receptor Positive Allosteric Modulators

Recent years have seen a large increase in the discovery of allosteric ligands targeting muscarinic acetylcholine receptors (mAChRs). One of the challenges in screening such compounds is to understand their mechanisms of action and define appropriate parameter estimates for affinity, cooperativity and efficacy. Herein we describe the mechanisms of action and structure-activity relationships for a series of "pan-G_q-coupled" muscarinic acetylcholine (ACh) receptor (mAChR) positive allosteric modulators (PAMs). Using a combination of radioligand binding, functional inositol phosphate accumulation assays, receptor alkylation and operational data analysis, we show that most compounds in the series derive their variable potency and selectivity from differential cooperativity at the M₁, M₃ and M₅ mAChRs. None of the PAMs showed greater than 10-fold subtype selectivity for the agonist-free receptor, but VU6007705, VU6007678, and VU6008555 displayed markedly increased cooperativity compared to the parent molecule and M₅ mAChR-preferring PAM, ML380 (αβ > 100), in the presence of ACh. Most of the activity of these PAMs derives from their ability to potentiate ACh binding affinity at mAChRs, though VU6007678 was notable for also potentiating ACh signaling efficacy and robust allosteric agonist activity. These data provide key insights for the future design of more potent and subtype-selective mAChR PAMs.

Assessment of the Molecular Mechanisms of Action of Novel 4-Phenylpyridine-2-One and 6-Phenylpyrimidin-4-One Allosteric Modulators at the M1 Muscarinic Acetylcholine Receptors

Positive allosteric modulators (PAMs) that target the M1 muscarinic acetylcholine (ACh) receptor (M1 mAChR) are potential treatments for cognitive deficits in conditions such as Alzheimer disease and schizophrenia. We recently reported novel 4-phenylpyridine-2-one and 6-phenylpyrimidin-4-one M1 mAChR PAMs with the potential to display different modes of positive allosteric modulation and/or agonism but whose molecular mechanisms of action remain undetermined. The current study compared the pharmacology of three such novel PAMs with the prototypical first-generation PAM, benzyl quinolone carboxylic acid (BQCA), in a recombinant Chinese hamster ovary (CHO) cell line stably expressing the human M1 mAChR. Interactions between the orthosteric agonists and the novel PAMs or BQCA suggested their allosteric effects were solely governed by modulation of agonist affinity. The greatest degree of positive co-operativity was observed with higher efficacy agonists, whereas minimal potentiation was observed when the modulators were tested against the lower efficacy agonist, xanomeline. Each PAM was investigated for its effects on the endogenous agonist ACh on three different signaling pathways [extracellular signal-regulated kinases 1/2 phosphorylation, inositol monophosphate (IP1) accumulation, and β-arrestin-2 recruitment], revealing that the allosteric potentiation generally tracked with the efficiency of stimulus-response coupling, and that there was little pathway bias in the allosteric effects. Thus, despite the identification of novel allosteric scaffolds targeting the M1 mAChR, the molecular mechanism of action of these compounds is largely consistent with a model of allostery previously described for BQCA, suggesting that this may be a more generalized mechanism for M1 mAChR PAM effects than previously appreciated.

Muscarinic M2 receptor promotes vasopressin synthesis in mice supraoptic nuclei

Muscarinic acetylcholine receptors have been suggested to be implicated in arginine-vasopressin secretion because intracerebroventricular muscarinic agonist administration induces arginine-vasopressin release into the circulation. Although which subtype is involved in the regulation of arginine-vasopressin secretion is unclear, M₂ receptors have been reported to be highly expressed in the hypothalamus. In the present study, M₂ receptor-knockout mice were used to elucidate whether M₂ receptor regulates arginine-vasopressin synthesis in the paraventricular nuclei and supraoptic nuclei of the hypothalamus. The number of arginine-vasopressin-immunoreactive neurons in M₂ receptor-knockout mice was significantly decreased in the supraoptic nuclei, but not in the paraventricular nuclei compared with wild-type mice. Plasma arginine-vasopressin level in M₂ receptor-knockout mice was also significantly lower than in the wild-type mice. Urinary volume and frequency as well as water intake in M₂ receptor-knockout mice were significantly higher than those in wild-type mice. The V₂ vasopressin receptor expression in kidneys of M₂ receptor-knockout mice was comparable with that of wild-type mice, and increased urination in M₂ receptor-knockout mice was significantly decreased by administration of desmopressin, a specific V₂ receptor agonist, suggesting that V₂ receptors in the kidneys of M₂ receptor-knockout mice are intact. These results suggest that M₂ receptors promote arginine-vasopressin synthesis in the supraoptic nuclei and play a role in the regulation and maintenance of body fluid.

In Vitro Pharmacological Characterization and In Vivo Validation of LSN3172176 a Novel M1 Selective Muscarinic Receptor Agonist Tracer Molecule for Positron Emission Tomography

In the search for improved symptomatic treatment options for neurodegenerative and neuropsychiatric diseases, muscarinic acetylcholine M1 receptors (M1 mAChRs) have received significant attention. Drug development efforts have identified a number of novel ligands, some of which have advanced to the clinic. However, a significant issue for progressing these therapeutics is the lack of robust, translatable, and validated biomarkers. One valuable approach to assessing target engagement is to use positron emission tomography (PET) tracers. In this study we describe the pharmacological characterization of a selective M1 agonist amenable for in vivo tracer studies. We used a novel direct binding assay to identify nonradiolabeled ligands, including LSN3172176, with the favorable characteristics required for a PET tracer. In vitro functional and radioligand binding experiments revealed that LSN3172176 was a potent partial agonist (EC₅₀ 2.4-7.0 nM, E_max 43%-73%), displaying binding selectivity for M1 mAChRs (K_d = 1.5 nM) that was conserved across species (native tissue K_d = 1.02, 2.66, 8, and 1.03 at mouse, rat, monkey, and human, respectively). Overall selectivity of LSN3172176 appeared to be a product of potency and stabilization of the high-affinity state of the M1 receptor, relative to other mAChR subtypes (M1 > M2, M4, M5 > M3). In vivo, use of wild-type and mAChR knockout mice further supported the M1-preferring selectivity profile of LSN3172176 for the M1 receptor (78% reduction in cortical occupancy in M1 KO mice). These findings support the development of LSN3172176 as a potential PET tracer for assessment of M1 mAChR target engagement in the clinic and to further elucidate the function of M1 mAChRs in health and disease.

Autoradiography of 3H-pirenzepine and 3H-AFDX-384 in Mouse Brain Regions: Possible Insights into M1, M2, and M4 Muscarinic Receptors Distribution

Autoradiography helps to determine the distribution and density of muscarinic receptor (MR) binding sites in the brain. However, it relies on the selectivity of radioligands toward their target. 3H-Pirenzepine is commonly believed to label predominantly M1MR, 3H-AFDX-384 is considered as M2MR selective ligand. Here we performed series of autoradiographies with 3H-AFDX-384 (2 nM), and 3H-pirenzepine (5 nM) in WT, M1KO, M2KO, and M4KO mice to address the ligand selectivity. Labeling with 3H-pirenzepine using M1KO, M2KO, and M4KO brain sections showed the high selectivity toward M1MR. Selectivity of 3H-AFDX-384 toward M2MR varies among brain regions and depends on individual MR subtype proportion. All binding sites in the medulla oblongata and pons, correspond to M2MR. In caudate putamen, nucleus accumbens and olfactory tubercle, 77.7, 74.2, and 74.6% of 3H-AFDX-384 binding sites, respectively, are represented by M4MR and M2MR constitute only a minor portion. In cortex and hippocampus, 3H-AFDX-384 labels almost similar amounts of M2MR and M4MR alongside significant amounts of non-M2/non-M4MR. In cortex, the proportion of 3H-AFDX-384 binding sites attributable to M2MR can be increased by blocking M4MR with MT3 toxin without affecting non-M4MR. PD102807, which is considered as a highly selective M4MR antagonist failed to improve the discrimination of M2MR. Autoradiography with 3H-QNB showed genotype specific loss of binding sites.

IN CONCLUSION

while 3H-pirenzepine showed the high selectivity toward M1MR, 3H-AFDX-384 binding sites represent different populations of MR subtypes in a brain-region-specific manner. This finding has to be taken into account when interpreting the binding data.

Mutual activation of glutamatergic mGlu4 and muscarinic M4 receptors reverses schizophrenia-related changes in rodents

RATIONALE

Metabotropic glutamate receptors and muscarinic M₄ receptors have been proposed as novel targets for various brain disorders, including schizophrenia. Both receptors are coupled to G_o/i proteins and are expressed in brain circuits that are important in schizophrenia. Therefore, their mutual activation may be an effective treatment and allow minimizing the doses of ligands required for optimal activity.

OBJECTIVES

In the present studies, subactive doses of mGlu₄ and M₄ activators (LSP4-2022 and VU152100, respectively) were administered to investigate the mutual interaction between mGlu₄ and M₄ receptors in animal models of schizophrenia.

METHODS

The behavioral tests used were MK-801-induced hyperactivity, (±)-2.5-dimethoxy-4-iodoamphetamine hydrochloride (DOI)-induced head twitches, the modified forced swim test, and MK-801-induced disruptions of social interactions and novel object recognition. DOI-induced spontaneous excitatory postsynaptic currents (sEPSCs) in brain slices and positron emission tomography (PET) in were used to establish the ability of these compounds to modulate the glutamatergic and dopaminergic systems. Rotarod was used to assess putative adverse effects.

RESULTS

The mutual administration of subactive doses of LSP4-2022 and VU152100 exerted similar antipsychotic-like efficacy in animals as observed for active doses of both compounds, indicating their additive actions. VU152100 inhibited the DOI-induced frequency (but not amplitude) of sEPSCs in the frontal cortex, confirming presynaptic regulation of glutamate release. Both compounds reversed amphetamine-induced decrease in D₂ receptor levels in the striatum, as measured with [¹⁸F]fallypride. The compounds did not induce any motor impartments when measured in rotarod test.

CONCLUSIONS

Based on our results, the simultaneous activation of M₄ and mGlu₄ receptors is beneficial in reversing MK-801- and amphetamine-induced schizophrenia-related changes in animals.

Identification, expression and functional characterization of M4L, a muscarinic acetylcholine M4 receptor splice variant

Rodent genomic alignment sequences support a 2-exon model for muscarinic M4 receptor. Using this model a novel N-terminal extension was discovered in the human muscarinic acetylcholine M₄ receptor. An open reading frame was discovered in the human, mouse and rat with a common ATG (methionine start codon) that extended the N-terminus of the muscarinic acetylcholine M₄ receptor subtype by 155 amino acids resulting in a longer variant. Transcriptional evidence for this splice variant was confirmed by RNA-Seq and RT-PCR experiments performed from human donor brain prefrontal cortices. We detected a human upstream exon indicating the translation of the mature longer M₄ receptor transcript. The predicted size for the longer two-exon M₄ receptor splice variant with the additional 155 amino acid N-terminal extension, designated M₄L is 69.7 kDa compared to the 53 kDa canonical single exon M₄ receptor (M₄S). Western blot analysis from a mammalian overexpression system, and saturation radioligand binding with [³H]-NMS (N-methyl-scopolamine) demonstrated the expression of this new splice variant. Comparative pharmacological characterization between the M₄L and M₄S receptors revealed that both the orthosteric and allosteric binding sites for both receptors were very similar despite the addition of an N-terminal extension.

Discovery of the first low-shift positive allosteric modulators for the muscarinic M1 receptor

Positive modulation of the muscarinic M1-receptor has for a long time attracted scientists and drug developers for the potential treatment of Alzheimer's disease or Schizophrenia. The precognitive potential of M1 activation has however not been clinically demonstrated as a result of side effects associated both with agonists and positive allosteric modulators (PAM's) of the M1-receptor. To avoid excessive activation of the M1-receptor we have designed a new screening format and developed the first low-shift positive allosteric modulators for the M1 receptor. Low-shift PAM's offer the potential of "use-dependent" attenuation of transmitter-signaling while avoiding pseudo-agonistic behavior in vivo as a common limitation of the so far described high-shift PAM's. With these novel M1-PAM's, the M1 receptor is potentially the first GPCR for which both, high- and low shift PAM's have become available.

The effect of the muscarinic M1 receptor antagonist biperiden on cognition in medication free subjects with psychosis

The acetylcholine muscarinic M₁ receptor has been implicated in both psychosis and cognition. Post-mortem research has shown reduced muscarinic M₁ receptor density in 25% of chronic patients with schizophrenia. It is unknown whether reduced M₁ receptor density is related to cognitive symptoms of psychosis. We investigated the role of the M₁ receptor in separate cognitive domains in subjects with a psychotic disorder using a muscarinic M₁ antagonist as an acute pharmacological challenge. 33 young subjects with a psychotic disorder and 30 gender, age and IQ matched healthy controls were enrolled. All participants completed a comprehensive cognitive test battery twice: once after placebo and once after oral administration of 4mg. biperiden (M₁ antagonist). The order of drug administration was counterbalanced. Biperiden significantly negatively influenced both verbal (p< 0.001 and p=0.032) and visual learning and memory (p=0.028) in both groups. A medication x group interaction effect was found for reasoning and problem solving (p=0.005). No main or interaction effects were found for other cognitive domains. These results provide further in-vivo evidence that the M₁ receptor is involved in cognitive functioning, particularly verbal and visual memory processes. Lack of differential effects of biperiden between psychotic subjects and healthy controls may suggest that decreased M₁ receptor density is only present in chronic, older schizophrenia patients. However, it remains possible that differential effects of biperiden would be present in more severe cognitive impaired subjects with psychosis after several doses of biperiden instead of a single administration.

DL0410 Ameliorates Memory and Cognitive Impairments Induced by Scopolamine via Increasing Cholinergic Neurotransmission in Mice

Deficiency of the cholinergic system is thought to play a vital role in cognitive impairment of dementia. DL0410 was discovered as a dual inhibitor of acetylcholinesterase (AChE) and butyrylcholinestease (BuChE), with potent efficiency in in-vitro experiments, but its in vivo effect on the cholinergic model has not been evaluated, and its action mechanism has also not been illustrated. In the present study, the capability of DL0410 in ameliorating the amnesia induced by scopolamine was investigated, and its effect on the cholinergic system in the hippocampus and its binding mode in the active site of AChE was also explored. Mice were administrated DL0410 (3 mg/kg, 10 mg/kg, and 30 mg/kg), and mice treated with donepezil were used as a positive control. The Morris water maze, escape learning task, and passive avoidance task were used as behavioral tests. The test results indicated that DL0410 could significantly improve the learning and memory impairments induced by scopolamine, with 10 mg/kg performing best. Further, DL0410 inhibited the AChE activity and increased acetylcholine (ACh) levels in a dose-dependent manner, and interacted with the active site of AChE in a similar manner as donepezil. However, no difference in the activity of BuChE was found in this study. All of the evidence indicated that its AChE inhibition is an important mechanism in the anti-amnesia effect. In conclusion, DL0410 could be an effective therapeutic drug for the treatment of dementia, especially Alzheimer’s disease.

Inositol Phosphate Accumulation in Vivo Provides a Measure of Muscarinic M1 Receptor Activation

The rationale for using M₁ selective muscarinic acetylcholine receptor activators for the treatment of cognitive impairment associated with psychiatric and neurodegenerative disease is well-established in the literature. Here, we investigate measurement of inositol phosphate accumulation, an end point immediately downstream of the M₁ muscarinic acetylcholine receptor signaling cascade, as an in vivo biochemical readout for M₁ muscarinic acetylcholine receptor activation. Five brain penetrant M₁-subtype selective activators from three structurally distinct chemical series were pharmacologically profiled for functional activity in vitro using recombinant cell calcium mobilization and inositol phosphate assays, and a native tissue hippocampal slice electrophysiology assay, to show that all five compounds presented a positive allosteric modulator agonist profile, within a narrow range of potencies. In vivo characterization using an amphetamine-stimulated locomotor activity behavioral assay and the inositol phosphate accumulation biochemical assay demonstrated that the latter has utility for assessing functional potency of M₁ activators. Efficacy measured by inositol phosphate accumulation in mouse striatum compared favorably to efficacy in reversing amphetamine-induced locomotor activity, suggesting that the inositol phosphate accumulation assay has utility for the evaluation of M₁ muscarinic acetylcholine receptor activators in vivo. The benefits of this in vivo biochemical approach include a wide response window, interrogation of specific brain circuit activation, an ability to model responses in the context of brain exposure, an ability to rank order compounds based on in vivo efficacy, and minimization of animal use.

Anticholinergic activity in the nervous system: Consequences for visuomotor function

Acetylcholine is present in the peripheral and central nervous system, where it is involved in a number of fundamental physiological and biochemical processes. In particular, interaction with muscarinic receptors can cause adverse effects such as dry mouth, drowsiness, mydriasis and cognitive dysfunction. Despite the knowledge that exists regarding these common side-effects, little is known about how anticholinergic medications influence central motor processes and fine motor control in healthy individuals. This paper reviews critical visuomotor processes that operate in healthy individuals, and how controlling these motor processes are influenced by medications that interfere with central cholinergic neurotransmission. An overview of receptor function and neurotransmitter interaction following the ingestion or administration of anticholinergics is provided, before exploring how visuomotor performance is affected by anticholinergic medications. In particular, this review will focus on the effects that anticholinergic medications have on fixation stability, saccadic eye movements, smooth pursuit eye movements, and general pupil dynamics.

The M1 Muscarinic Receptor Antagonist VU0255035 Delays the Development of Status Epilepticus after Organophosphate Exposure and Prevents Hyperexcitability in the Basolateral Amygdala

Exposure to organophosphorus toxins induces seizures that progress to status epilepticus (SE), which can cause brain damage or death. Seizures are generated by hyperstimulation of muscarinic receptors, subsequent to inhibition of acetylcholinesterase; this is followed by glutamatergic hyperactivity, which sustains and reinforces seizure activity. It has been unclear which muscarinic receptor subtypes are involved in seizure initiation and the development of SE in the early phases after exposure. Here, we show that pretreatment of rats with the selective M₁ receptor antagonist, VU0255035 [N-(3-oxo-3-(4-(pyridine-4-yl)piperazin-1-yl)propyl)-benzo[c][1,2,5]thiadiazole-4 sulfonamide], significantly suppressed seizure severity and prevented the development of SE for about 40 minutes after exposure to paraoxon or soman, suggesting an important role of the M₁ receptor in the early phases of seizure generation. In addition, in in vitro brain slices of the basolateral amygdala (a brain region that plays a key role in seizure initiation after nerve agent exposure), VU0255035 blocked the effects produced by bath application of paraoxon-namely, a brief barrage of spontaneous inhibitory postsynaptic currents, followed by a significant increase in the ratio of the total charge transferred by spontaneous excitatory postsynaptic currents over that of the inhibitory postsynaptic currents. Furthermore, paraoxon enhanced the hyperpolarization-activated cation current I_h in basolateral amygdala principal cells, which could be one of the mechanisms underlying the increased glutamatergic activity, an effect that was also blocked in the presence of VU0255035. Thus, selective M₁ antagonists may be an efficacious pretreatment in contexts in which there is risk for exposure to organophosphates, as these antagonists will delay the development of SE long enough for medical assistance to arrive.

Positive Allosteric Modulation of the Muscarinic M1 Receptor Improves Efficacy of Antipsychotics in Mouse Glutamatergic Deficit Models of Behavior

Current antipsychotics are effective in treating the positive symptoms associated with schizophrenia, but they remain suboptimal in targeting cognitive dysfunction. Recent studies have suggested that positive allosteric modulation of the M₁ muscarinic acetylcholine receptor (mAChR) may provide a novel means of improving cognition. However, very little is known about the potential of combination therapies in extending coverage across schizophrenic symptom domains. This study investigated the effect of the M₁ mAChR positive allosteric modulator BQCA [1-(4-methoxybenzyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid], alone or in combination with haloperidol (a first-generation antipsychotic), clozapine (a second-generation atypical antipsychotic), or aripiprazole (a third-generation atypical antipsychotic), in reversing deficits in sensorimotor gating and spatial memory induced by the N-methyl-d-aspartate receptor antagonist, MK-801 [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine]. Sensorimotor gating and spatial memory induction are two models that represent aspects of schizophrenia modeled in rodents. In prepulse inhibition (an operational measure of sensorimotor gating), BQCA alone had minimal effects but exhibited different levels of efficacy in reversing MK-801-induced prepulse inhibition disruptions when combined with a subeffective dose of each of the three (currently prescribed) antipsychotics. Furthermore, the combined effect of BQCA and clozapine was absent in M₁^-/- mice. Interestingly, although BQCA alone had no effect in reversing MK-801-induced memory impairments in a Y-maze spatial test, we observed a reversal upon the combination of BQCA with atypical antipsychotics, but not with haloperidol. These findings provide proof of concept that a judicious combination of existing antipsychotics with a selective M₁ mAChR positive allosteric modulator can extend antipsychotic efficacy in glutamatergic deficit models of behavior.

Double-Blind, Randomized, Placebo-Controlled Trial of Metoclopramide for Hypersalivation Associated With Clozapine

Hypersalivation is a frequent, disturbing, and uncomfortable adverse effect of clozapine therapy that frequently leads to noncompliance. The aim of this study was to examine the efficacy of metoclopramide (dopamine D2 antagonist, antiemetic medication) as an option for management of hypersalivation associated with clozapine (HAC). A 3-week, double-blind, placebo-controlled trial was conducted in university-based research clinics from January 2012 to May 2014, on 58 inpatients treated with clozapine who were experiencing hypersalivation. The subjects were randomly divided into placebo and metoclopramide groups. The starting dose was 10 mg/d. Participants who did not respond were up-titrated 10 mg/d weekly to a total of 30 mg/d during the third week. The number of placebo capsules was increased accordingly up to 3 capsules per day. Primary outcome was the change from baseline to the end of study in the severity of hypersalivation as measured with the Nocturnal Hypersalivation Rating Scale and the Drooling Severity Scale. Secondary outcomes included Clinical Global Impression of Improvement scale and adverse effect scales. Significant improvement on the Nocturnal Hypersalivation Rating Scale was demonstrated in the metoclopramide group from the end of the second week (P < 0.004), and on the Drooling Severity Scale (P < 0.02) in the third week. Clinical Global Impression-Improvement scale scores revealed major improvement. Twenty subjects (66.7%) treated with metoclopramide reported significant decline or total disappearance of HAC in comparison to 8 patients (28.6%) who received placebo (P = 0.031). No adverse effects to metoclopramide were reported. Metoclopramide was found to be safe and effective for the treatment of HAC.

Mouse Models of Primary Sjogren's Syndrome

Sjogren's syndrome (SjS) is a chronic autoimmune disorder characterized by immune cell infiltration and progressive injury to the salivary and lacrimal glands. As a consequence, patients with SjS develop xerostomia (dry mouth) and keratoconjunctivitis sicca (dry eyes). SjS is the third most common rheumatic autoimmune disorder, affecting 4 million Americans with over 90% of patients being female. Current diagnostic criteria for SjS frequently utilize histological examinations of minor salivary glands for immune cell foci, serology for autoantibodies, and dry eye evaluation by corneal or conjunctival staining. SjS can be classified as primary or secondary SjS, depending on whether it occurs alone or in association with other systemic rheumatic conditions, respectively. Clinical manifestations typically become apparent when the disease is relatively advanced in SjS patients, which poses a challenge for early diagnosis and treatment of SjS. Therefore, SjS mouse models, because of their close resemblance to the human SjS, have been extremely valuable to identify early disease markers and to investigate underlying biological and immunological dysregulations. However, it is important to bear in mind that no single mouse model has duplicated all aspects of SjS pathogenesis and clinical features, mainly due to the multifactorial etiology of SjS that includes numerous susceptibility genes and environmental factors. As such, various mouse models have been developed in the field to try to recapitulate SjS. In this review, we focus on recent mouse models of primary SjS xerostomia and describe them under three categories of spontaneous, genetically engineered, and experimentally induced models. In addition, we discuss future perspectives highlighting pros and cons of utilizing mouse models and current demands for improved models.

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.

Changes in Muscarinic M2 Receptor Levels in the Cortex of Subjects with Bipolar Disorder and Major Depressive Disorder and in Rats after Treatment with Mood Stabilisers and Antidepressants

BACKGROUND

Increasingly, data are implicating muscarinic receptors in the aetiology and treatment of mood disorders. This led us to measure levels of different muscarinic receptor-related parameters in the cortex from people with mood disorders and the CNS of rats treated with mood stabilisers and antidepressant drugs.

METHODS

We measured [(3)H]AF-DX 384 binding in BA 46 and BA 24 from subjects with bipolar disorders (n = 14), major depressive disorders (n = 19), as well as age- and sex-matched controls (n = 19) and the CNS of rats treated with fluoxetine or imipramine. In addition, we used Western blots to measure levels of CHRM2 protein and oxotremorine-M stimulated [(35)S]GTPγS binding as a measure of CHRM 2 / 4 signaling.

RESULTS

Compared with controls, [(3)H]AF-DX 384 binding was lower in BA 24 and BA 46 in bipolar disorders and major depressive disorders, while CHRM2 protein and oxotremorine-M stimulated [(35)S]GTPγS binding was only lower in BA 24. Compared with vehicle, treatment with mood stabilisers, antidepressant drugs for 10 days, or imipramine for 28 days resulted in higher levels of in [(3)H]AF-DX 384 binding select regions of rat CNS.

CONCLUSIONS

Our data suggest that levels of CHRM2 are lower in BA 24 from subjects with mood disorders, and it is possible that signalling by that receptor is also less in this cortical region. Our data also suggest increasing levels of CHRM2 may be involved in the mechanisms of action of mood stabilisers and tricyclic antidepressants.

A Genetic Animal Model of Alcoholism for Screening Medications to Treat Addiction

The purpose of this review is to present up-to-date pharmacological, genetic, and behavioral findings from the alcohol-preferring P rat and summarize similar past work. Behaviorally, the focus will be on how the P rat meets criteria put forth for a valid animal model of alcoholism with a highlight on its use as an animal model of polysubstance abuse, including alcohol, nicotine, and psychostimulants. Pharmacologically and genetically, the focus will be on the neurotransmitter and neuropeptide systems that have received the most attention: cholinergic, dopaminergic, GABAergic, glutamatergic, serotonergic, noradrenergic, corticotrophin releasing hormone, opioid, and neuropeptide Y. Herein, we sought to place the P rat's behavioral and neurochemical phenotypes, and to some extent its genotype, in the context of the clinical literature. After reviewing the findings thus far, this chapter discusses future directions for expanding the use of this genetic animal model of alcoholism to identify molecular targets for treating drug addiction in general.

Clozapine in Parkinsonian Rest Tremor: A Review of Outcomes, Adverse Reactions, and Possible Mechanisms of Action

BACKGROUND

The pathogenesis of rest tremor in Parkinson's disease (PD) is incompletely understood. This symptom can be resistant to typical anti-PD medications. Therefore, new treatments are needed given the concern that this symptom causes to patients and family. Limited experience suggests that clozapine can have an important antitremor effect in PD. The mechanism(s) underlying this effect is not well understood, but could provide insight and impetus to the development of more-effective and safer antitremor therapies.

METHODS AND RESULTS

Exemplifying the antitremor effects of clozapine, we describe a patient with tremor-predominant PD who obtained prominent reduction of rest tremor with clozapine treatment. We review the responses to this treatment in another 7 of our PD patients with treatment-resistant rest tremor. We also review the published literature on clozapine for tremor in PD and discuss its potential mechanisms of action and possible adverse effects. In our case series, there was a 64% reduction of tremor score after clozapine was initiated. The mechanism of tremor reduction remains unclear with possible involvement of anticholinergic, serotonergic, antihistaminergic, antiadrenergic, and antidopaminergic effects. Clozapine does have potential serious adverse effects.

CONCLUSIONS

Clozapine may be effective in controlling rest tremor in PD. Given the potential fatal side effects, if clozapine is to be initiated in PD patients, it has to be used cautiously with proper monitoring, preferably in specialized centers. We acknowledge that the number of patients in this case series is small. Further studies are needed to understand clozapine's mechanism of action in reducing tremor.

The role of glutamatergic, GABA-ergic, and cholinergic receptors in depression and antidepressant-like effect

Depression is one of the most common mental disorders and social issue worldwide. Although there are many antidepressants available, the effectiveness of the therapy is still a serious issue. Moreover, there are many limitations of currently used antidepressants, including slow onset of action, numerous side effects, or the fact that many patients do not respond adequately to the treatment. Therefore, scientists are searching for new compounds with different mechanisms of action. Numerous data indicate the important role of glutamatergic, GABA-ergic, and cholinergic receptors in the pathomechanism of major depressive disorder. This review presents the role of glutamatergic, GABA-ergic, and cholinergic receptors in depression and antidepressant-like effect.