Muscarinic receptor subtypes involved in hippocampal circuits

Muscarinic Modulation of Network Excitability and Short-Term Dynamics in the Dorsal and Ventral Hippocampus

Cholinergic transmission fundamentally modulates information processing in the brain via muscarinic receptors. Using in vitro electrophysiological recordings of population spikes from the CA1 region, we found that the muscarinic receptor agonist carbachol (CCh, 1 μM) enhances the basal excitation level in the dorsal but not ventral hippocampus. Using a frequency stimulation protocol, we found that CCh transforms depression of neuronal output into facilitation (at 3-30 Hz) in the ventral hippocampus while only lessening depression in the dorsal hippocampus, suggesting that muscarinic transmission boosts basal neuronal activation in the dorsal hippocampus and strongly facilitates the output of the ventral hippocampus in a frequency-dependent manner.

Cav3 T-Type Voltage-Gated Ca2+ Channels and the Amyloidogenic Environment: Pathophysiology and Implications on Pharmacotherapy and Pharmacovigilance

Voltage-gated Ca2+ channels (VGCCs) were reported to play a crucial role in neurotransmitter release, dendritic resonance phenomena and integration, and the regulation of gene expression. In the septohippocampal system, high- and low-voltage-activated (HVA, LVA) Ca2+ channels were shown to be involved in theta genesis, learning, and memory processes. In particular, HVA Cav2.3 R-type and LVA Cav3 T-type Ca2+ channels are expressed in the medial septum-diagonal band of Broca (MS-DBB), hippocampal interneurons, and pyramidal cells, and ablation of both channels was proven to severely modulate theta activity. Importantly, Cav3 Ca2+ channels contribute to rebound burst firing in septal interneurons. Consequently, functional impairment of T-type Ca2+ channels, e.g., in null mutant mouse models, caused tonic disinhibition of the septohippocampal pathway and subsequent enhancement of hippocampal theta activity. In addition, impairment of GABA A/B receptor transcription, trafficking, and membrane translocation was observed within the septohippocampal system. Given the recent findings that amyloid precursor protein (APP) forms complexes with GABA B receptors (GBRs), it is hypothesized that T-type Ca2+ current reduction, decrease in GABA receptors, and APP destabilization generate complex functional interdependence that can constitute a sophisticated proamyloidogenic environment, which could be of potential relevance in the etiopathogenesis of Alzheimer's disease (AD). The age-related downregulation of T-type Ca2+ channels in humans goes together with increased Aβ levels that could further inhibit T-type channels and aggravate the proamyloidogenic environment. The mechanistic model presented here sheds new light on recent reports about the potential risks of T-type Ca2+ channel blockers (CCBs) in dementia, as observed upon antiepileptic drug application in the elderly.

Modulation of arousal and sleep/wake architecture by M1 PAM VU0453595 across young and aged rodents and nonhuman primates

Degeneration of basal forebrain cholinergic circuitry represents an early event in the development of Alzheimer's disease (AD). These alterations in central cholinergic function are associated with disruptions in arousal, sleep/wake architecture, and cognition. Changes in sleep/wake architecture are also present in normal aging and may represent a significant risk factor for AD. M₁ muscarinic acetylcholine receptor (mAChR) positive allosteric modulators (PAMs) have been reported to enhance cognition across preclinical species and may also provide beneficial effects for age- and/or neurodegenerative disease-related changes in arousal and sleep. In the present study, electroencephalography was conducted in young animals (mice, rats and nonhuman primates [NHPs]) and in aged mice to examine the effects of the selective M₁ PAM VU0453595 in comparison with the acetylcholinesterase inhibitor donepezil, M₁/M₄ agonist xanomeline (in NHPs), and M₁ PAM BQCA (in rats) on sleep/wake architecture and arousal. In young wildtype mice, rats, and NHPs, but not in M₁ mAChR KO mice, VU0453595 produced dose-related increases in high frequency gamma power, a correlate of arousal and cognition enhancement, without altering duration of time across all sleep/wake stages. Effects of VU0453595 in NHPs were observed within a dose range that did not induce cholinergic-mediated adverse effects. In contrast, donepezil and xanomeline increased time awake in rodents and engendered dose-limiting adverse effects in NHPs. Finally, VU0453595 attenuated age-related decreases in REM sleep duration in aged wildtype mice. Development of M₁ PAMs represents a viable strategy for attenuating age-related and dementia-related pathological disturbances of sleep and arousal.

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).

Unexpected Effects of Acetylcholine Precursors on Pilocarpine Seizure- Induced Neuronal Death

BACKGROUND

Choline alfoscerate (α-GPC) and Cytidine 5'-diphosphocholine (CDPCholine) are both acetylcholine precursors and are considered to act as pro-cholinergic nootropic agents. Acetylcholine precursors have also recently found frequent use in the neurology clinic. Stroke and many types of dementia have been shown to respond favorably after treatment with these agents, not only in terms of cognitive dysfunction but also behavioral and psychological symptoms. The primary mechanisms of Acetylcholine precursors are the following: 1) Acetylcholine precursors themselves are used in the biosynthesis of acetylcholine and 2) byproducts like glycerophosphate have protective functions for neuronal phospholipids. However, whether acetylcholine precursors have a similar effect in treating cognitive impairment in patients with epilepsy remains controversial.

METHODS

Our previous studies investigating acetylcholine precursors in seizure-experienced animals have produced variable results that were dependent on the timing of administration.

RESULTS

Early administration of CDP-choline immediately after seizure increased neuronal death, blood-brain barrier (BBB) disruption and microglial activation in the hippocampus. However, administration of α-GPC starting 3 weeks after seizure (late administration) improved cognitive function through reduced neuronal death and BBB disruption, and increased neurogenesis in the hippocampus.

CONCLUSION

These seemingly contradictory results may be attributed to both epileptogenic features and neuroprotective functions of several acetylcholine precursors.

Muscarinic receptor subtype distribution in the central nervous system and relevance to aging and Alzheimer's disease

Muscarinic acetylcholine receptors (mAChRs) are G proteincoupled receptors (GPCRs) that mediate the metabotropic actions of acetylcholine (ACh). There are five subtypes of mAChR, M₁ - M₅, which are expressed throughout the central nervous system (CNS) on numerous cell types and represent promising treatment targets for a number of different diseases, disorders, and conditions of the CNS. Although the present review will focus on Alzheimer's disease (AD) and amnestic mild cognitive impairment (aMCI), a number of conditions such as Parkinson's disease (PD), schizophrenia, and others represent significant unmet medical needs for which selective muscarinic agents could offer therapeutic benefits. Numerous advances have been made regarding mAChR localization through the use of subtype-selective antibodies and radioligand binding studies and these efforts have helped propel a number of mAChR therapeutics into clinical trials. However, much of what we know about mAChR localization in the healthy and diseased brain has come from studies employing radioligand binding with relatively modest selectivity. The development of subtype-selective small molecule radioligands suitable for in vitro and in vivo use, as well as robust, commercially-available antibodies remains a critical need for the field. Additionally, novel genetic tools should be developed and leveraged to help move the field increasingly towards a systems-level understanding of mAChR subtype action. Finally, functional, proteomic, and genetic data from ongoing human studies hold great promise for optimizing the design and interpretation of studies examining receptor levels by enabling patient stratification. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Cognitive enhancement and antipsychotic-like activity following repeated dosing with the selective M4 PAM VU0467154

Although selective activation of the M₁ muscarinic acetylcholine receptor (mAChR) subtype has been shown to improve cognitive function in animal models of neuropsychiatric disorders, recent evidence suggests that enhancing M₄ mAChR function can also improve memory performance. Positive allosteric modulators (PAMs) targeting the M₄ mAChR subtype have shown therapeutic potential for the treatment of multiple symptoms observed in schizophrenia, including positive and cognitive symptoms when assessed in acute preclinical dosing paradigms. Since the cholinergic system has been implicated in multiple stages of learning and memory, we evaluated the effects of repeated dosing with the highly selective M₄ PAM VU0467154 on either acquisition and/or consolidation of learning and memory when dosed alone or after pharmacologic challenge with the N-methyl-d-aspartate subtype of glutamate receptors (NMDAR) antagonist MK-801. MK-801 challenge represents a well-documented preclinical model of NMDAR hypofunction that is thought to underlie some of the positive and cognitive symptoms observed in schizophrenia. In wildtype mice, 10-day, once-daily dosing of VU0467154 either prior to, or immediately after daily testing enhanced the rate of learning in a touchscreen visual pairwise discrimination task; these effects were absent in M₄ mAChR knockout mice. Following a similar 10-day, once-daily dosing regimen of VU0467154, we also observed 1) improved acquisition of memory in a cue-mediated conditioned freezing paradigm, 2) attenuation of MK-801-induced disruptions in the acquisition of memory in a context-mediated conditioned freezing paradigm and 3) reversal of MK-801-induced hyperlocomotion. Comparable efficacy and plasma and brain concentrations of VU0467154 were observed after repeated dosing as those previously reported with an acute, single dose administration of this M₄ PAM. Together, these studies are the first to demonstrate that cognitive enhancing and antipsychotic-like activity are not subject to the development of tolerance following repeated dosing with a selective M₄ PAM in mice and further suggest that activation of M₄ mAChRs may modulate both acquisition and consolidation of memory functions.

Prefrontal Cortex-Mediated Impairments in a Genetic Model of NMDA Receptor Hypofunction Are Reversed by the Novel M1 PAM VU6004256

Abnormalities in the signaling of the N-methyl-d-aspartate subtype of the glutamate receptor (NMDAR) within cortical and limbic brain regions are thought to underlie many of the complex cognitive and affective symptoms observed in individuals with schizophrenia. The M₁ muscarinic acetylcholine receptor (mAChR) subtype is a closely coupled signaling partner of the NMDAR. Accumulating evidence suggests that development of selective positive allosteric modulators (PAMs) of the M₁ receptor represent an important treatment strategy for the potential normalization of disruptions in NMDAR signaling in patients with schizophrenia. In the present studies, we evaluated the effects of the novel and highly potent M₁ PAM, VU6004256, in ameliorating selective prefrontal cortical (PFC)-mediated physiologic and cognitive abnormalities in a genetic mouse model of global reduction in the NR1 subunit of the NMDAR (NR1 knockdown [KD]). Using slice-based extracellular field potential recordings, deficits in muscarinic agonist-induced long-term depression (LTD) in layer V of the PFC in the NR1 KD mice were normalized with bath application of VU6004256. Systemic administration of VU6004256 also reduced excessive pyramidal neuron firing in layer V PFC neurons in awake, freely moving NR1 KD mice. Moreover, selective potentiation of M₁ by VU6004256 reversed the performance impairments of NR1 KD mice observed in two preclinical models of PFC-mediated learning, specifically the novel object recognition and cue-mediated fear conditioning tasks. VU6004256 also produced a robust, dose-dependent reduction in the hyperlocomotor activity of NR1 KD mice. Taken together, the current findings provide further support for M₁ PAMs as a novel therapeutic approach for the PFC-mediated impairments in schizophrenia.

Cholinergic regulation of fear learning and extinction

Cholinergic activation regulates cognitive function, particularly long-term memory consolidation. This Review presents an overview of the anatomical, neurochemical, and pharmacological evidence supporting the cholinergic regulation of Pavlovian contextual and cue-conditioned fear learning and extinction. Basal forebrain cholinergic neurons provide inputs to neocortical regions and subcortical limbic structures such as the hippocampus and amygdala. Pharmacological manipulations of muscarinic and nicotinic receptors support the role of cholinergic processes in the amygdala, hippocampus, and prefrontal cortex in modulating the learning and extinction of contexts or cues associated with threat. Additional evidence from lesion studies and analysis of in vivo acetylcholine release with microdialysis similarly support a critical role of cholinergic neurotransmission in corticoamygdalar or corticohippocampal circuits during acquisition of fear extinction. Although a few studies have suggested a complex role of cholinergic neurotransmission in the cellular plasticity essential for extinction learning, more work is required to elucidate the exact cholinergic mechanisms and physiological role of muscarinic and nicotinic receptors in these fear circuits. Such studies are important for elucidating the role of cholinergic neurotransmission in disorders such as posttraumatic stress disorder that involve deficits in extinction learning as well as for developing novel therapeutic approaches for such disorders. © 2016 Wiley Periodicals, Inc.

Induction of Anti-Hebbian LTP in CA1 Stratum Oriens Interneurons: Interactions between Group I Metabotropic Glutamate Receptors and M1 Muscarinic Receptors

An anti-Hebbian form of LTP is observed at excitatory synapses made with some hippocampal interneurons. LTP induction is facilitated when postsynaptic interneurons are hyperpolarized, presumably because Ca(2+) entry through Ca(2+)-permeable glutamate receptors is enhanced. The contribution of modulatory transmitters to anti-Hebbian LTP induction remains to be established. Activation of group I metabotropic receptors (mGluRs) is required for anti-Hebbian LTP induction in interneurons with cell bodies in the CA1 stratum oriens. This region receives a strong cholinergic innervation from the septum, and muscarinic acetylcholine receptors (mAChRs) share some signaling pathways and cooperate with mGluRs in the control of neuronal excitability.We therefore examined possible interactions between group I mGluRs and mAChRs in anti-Hebbian LTP at synapses which excite oriens interneurons in rat brain slices. We found that blockade of either group I mGluRs or M1 mAChRs prevented the induction of anti-Hebbian LTP by pairing presynaptic activity with postsynaptic hyperpolarization. Blocking either receptor also suppressed long-term effects of activation of the other G-protein coupled receptor on interneuron membrane potential. However, no crossed blockade was detected for mGluR or mAchR effects on interneuron after-burst potentials or on the frequency of miniature EPSPs. Paired recordings between pyramidal neurons and oriens interneurons were obtained to determine whether LTP could be induced without concurrent stimulation of cholinergic axons. Exogenous activation of mAChRs led to LTP, with changes in EPSP amplitude distributions consistent with a presynaptic locus of expression. LTP, however, required noninvasive presynaptic and postsynaptic recordings.

SIGNIFICANCE STATEMENT

In the hippocampus, a form of NMDA receptor-independent long-term potentiation (LTP) occurs at excitatory synapses made on some inhibitory neurons. This is preferentially induced when postsynaptic interneurons are hyperpolarized, depends on Ca(2+) entry through Ca(2+)-permeable AMPA receptors, and has been labeled anti-Hebbian LTP. Here we show that this form of LTP also depends on activation of both group I mGluR and M1 mAChRs. We demonstrate that these G-protein coupled receptors (GPCRs) interact, because the blockade of one receptor suppresses long-term effects of activation of the other GPCR on both LTP and interneuron membrane potential. This LTP was also detected in paired recordings, although only when both presynaptic and postsynaptic recordings did not perturb the intracellular medium. Changes in EPSP amplitude distributions in dual recordings were consistent with a presynaptic locus of expression.

Distribution and effects of the muscarinic receptor subtypes in the primary visual cortex

Muscarinic cholinergic receptors modulate the activity and plasticity of the visual cortex. Muscarinic receptors are divided into five subtypes that are not homogeneously distributed throughout the cortical layers and cells types. This distribution results in complex action of the muscarinic receptors in the integration of visual stimuli. Selective activation of the different subtypes can either strengthen or weaken cortical connectivity (e.g., thalamocortical vs. corticocortical), i.e., it can influence the processing of certain stimuli over others. Moreover, muscarinic receptors differentially modulate some functional properties of neurons during experience-dependent activity and cognitive processes and they contribute to the fine-tuning of visual processing. These functions are involved in the mechanisms of attention, maturation and learning in the visual cortex. This minireview describes the anatomo-functional aspects of muscarinic modulation of the primary visual cortex's (V1) microcircuitry.

Impaired functional organization in the visual cortex of muscarinic receptor knock-out mice

Acetylcholine modulates maturation and neuronal activity through muscarinic and nicotinic receptors in the primary visual cortex. However, the specific contribution of different muscarinic receptor subtypes in these neuromodulatory mechanisms is not fully understood. The present study evaluates in vivo the functional organization and the properties of the visual cortex of different groups of muscarinic receptor knock-out (KO) mice. Optical imaging of intrinsic signals coupled to continuous and episodic visual stimulation paradigms was used. Retinotopic maps along elevation and azimuth were preserved among the different groups of mice. However, compared to their wild-type counterparts, the apparent visual field along elevation was larger in M2/M4-KO mice but smaller in M1-KO. There was a reduction in the estimated relative receptive field size of V1 neurons in M1/M3-KO and M1-KO mice. Spatial frequency and contrast selectivity of V1 neuronal populations were affected only in M1/M3-KO and M1-KO mice. Finally, the neuronal connectivity was altered by the absence of M2/M4 muscarinic receptors. All these effects suggest the distinct roles of different subtypes of muscarinic receptors in the intrinsic organization of V1 and a strong involvement of the muscarinic transmission in the detectability of visual stimuli.

Modulation of NMDA receptor function as a treatment for schizophrenia

Schizophrenia is a devastating mental illness that afflicts nearly 1% of the world's population. Currently available antipsychotics treat positive symptoms, but are largely ineffective at addressing negative symptoms and cognitive dysfunction. Thus, improved pharmacotherapies that treat all aspects of the disease remain a critical unmet need. There is mounting evidence that links NMDA receptor hypofunction and the expression of schizophrenia, and numerous drug discovery programs have developed agents that directly or indirectly potentiate NMDA receptor-mediated neurotransmission. Several compounds have emerged that show promise for treating all symptom sub-domains in both preclinical models and clinical studies, and we will review recent developments in many of these areas.

Novel M(1) allosteric ligands: a patent review

INTRODUCTION

There is substantial evidence from preclinical and early proof-of-concept studies suggesting that selective modulation of the M(1) muscarinic receptor is efficacious in cognitive models of Alzheimer's disease (AD) and antipsychotic models of schizophrenia. For example, a number of nonselective M(1) muscarinic agonists have previously shown positive effects on cognitive function in AD patients, but were limited due to cholinergic adverse events thought to be mediated by pan activation of the M(2) to M(5) subtypes. Thus, there is a need to identify selective activators of the M(1) receptor to evaluate their potential in cognitive disorders. One strategy to confer selectivity for M(1) is the identification of allosteric agonists or positive allosteric modulators, which would target an allosteric site on the M(1) receptor rather than the highly conserved orthosteric acetylcholine binding site.

AREAS COVERED

This review discusses the M(1) muscarinic receptor and its potential therapeutic value in the treatment of CNS disorders such as AD and schizophrenia. Specifically, novel allosteric ligands that activate or positively modulate the M(1) receptor are examined and peer-reviewed articles associated with these patents publications are also described.

EXPERT OPINION

There is substantial evidence supporting activation of the M(1) receptor might be effective in treating symptoms of AD and schizophrenia, but therapeutic success has been elusive and is hypothesized to be due to the lack of selectivity among orthosteric agonists. During the past decade, allosteric modulation of GPCRs has evolved as a viable strategy toward generating subtype selective molecules. A number of novel, selective ligands in the form of allosteric agonists and positive allosteric modulators of the M(1) receptor have been identified offering the potential for clinical evaluation of M(1)-specific receptor activation.

Differential effects of m1 and m2 receptor antagonists in perirhinal cortex on visual recognition memory in monkeys

Microinfusions of the nonselective muscarinic antagonist scopolamine into perirhinal cortex impairs performance on visual recognition tasks, indicating that muscarinic receptors in this region play a pivotal role in recognition memory. To assess the mnemonic effects of selective blockade in perirhinal cortex of muscarinic receptor subtypes, we locally infused either the m1-selective antagonist pirenzepine or the m2-selective antagonist methoctramine in animals performing one-trial visual recognition, and compared these scores with those following infusions of equivalent volumes of saline. Compared to these control infusions, injections of pirenzepine, but not of methoctramine, significantly impaired recognition accuracy. Further, similar doses of scopolamine and pirenzepine yielded similar deficits, suggesting that the deficits obtained earlier with scopolamine were due mainly, if not exclusively, to blockade of m1 receptors. The present findings indicate that m1 and m2 receptors have functionally dissociable roles, and that the formation of new visual memories is critically dependent on the cholinergic activation of m1 receptors located on perirhinal cells.

Xanomeline modulation of the blood oxygenation level-dependent signal in awake rats: development of pharmacological magnetic resonance imaging as a translatable pharmacodynamic biomarker for central activity and dose selection

In vivo translational imaging techniques, such as positron emission tomography and single-photon emission-computed tomography, are the only ways to adequately determine that a drug engages its target. Unfortunately, there are far more experimental mechanisms being tested in the clinic than there are radioligands, impeding the use of this risk-mitigating approach in modern drug discovery and development. Pharmacological magnetic resonance imaging (phMRI) offers an approach for developing new biomarkers with the potential to determine central activity and dose selection in animals and humans. Using phMRI, we characterized the effects of xanomeline on ketamine-induced activation on blood oxygen level-dependent (BOLD) signal. In the present studies, xanomeline alone dose-dependently increased the BOLD signal across several regions of interest, including association and motor and sensory cortical regions. It is noteworthy that xanomeline dose-dependently attenuated ketamine-induced brain activation patterns, effects that were antagonized by atropine. In conclusion, the muscarinic 1/4-preferring receptor agonist xanomeline suppressed the effects of the N-methyl-D-aspartate channel blocker ketamine in a number of brain regions, including the association cortex, motor cortex, and primary sensory cortices. The region-specific brain activation observed in this ketamine challenge phMRI study may provide a method of confirming central activity and dose selection for novel antipsychotic drugs in early clinical trials for schizophrenia, if the data obtained in animals can be recapitulated in humans.

Muscarinic and nicotinic acetylcholine receptor agonists and allosteric modulators for the treatment of schizophrenia

Muscarinic and nicotinic acetylcholine (ACh) receptors (mAChRs and nAChRs) are emerging as important targets for the development of novel treatments for the symptoms associated with schizophrenia. Preclinical and early proof-of-concept clinical studies have provided strong evidence that activators of specific mAChR (M(1) and M(4)) and nAChR (α(7) and α(2)β(4)) subtypes are effective in animal models of antipsychotic-like activity and/or cognitive enhancement, and in the treatment of positive and cognitive symptoms in patients with schizophrenia. While early attempts to develop selective mAChR and nAChR agonists provided important preliminary findings, these compounds have ultimately failed in clinical development due to a lack of true subtype selectivity and subsequent dose-limiting adverse effects. In recent years, there have been major advances in the discovery of highly selective activators for the different mAChR and nAChR subtypes with suitable properties for optimization as potential candidates for clinical trials. One novel strategy has been to identify ligands that activate a specific receptor subtype through actions at sites that are distinct from the highly conserved ACh-binding site, termed allosteric sites. These allosteric activators, both allosteric agonists and positive allosteric modulators, of mAChR and nAChR subtypes demonstrate unique mechanisms of action and high selectivity in vivo, and may provide innovative treatment strategies for schizophrenia.

Muscarinic agonists and antagonists in schizophrenia: recent therapeutic advances and future directions

Existing therapies for schizophrenia have limited efficacy, and significant residual positive, negative, and cognitive symptoms remain in many individuals with the disorder even after treatment with the current arsenal of antipsychotic drugs. Preclinical and clinical data suggest that selective activation of the muscarinic cholinergic system may represent novel therapeutic mechanisms for the treatment of schizophrenia. The therapeutic relevance of earlier muscarinic agonists was limited by their lack of receptor selectivity and adverse event profile arising from activation of nontarget muscarinic receptors. Recent advances in developing compounds that are selective to muscarinic receptor subtypes or activate allosteric receptor sites offer tremendous promise for therapeutic targeting of specific muscarinic receptor subtypes in schizophrenia.

Muscarinic receptor pharmacology and circuitry for the modulation of cognition

The muscarinic cholinergic system constitutes an important part of the neuronal circuitry that modulates normal cognition. Muscarinic receptor antagonists are well known to produce or exacerbate impairments in attention, learning, and memory. Conversely, both direct-acting muscarinic receptor agonists and indirect-acting muscarinic cholinergic agonists, such as acetylcholinesterase inhibitors, have shown cognition-enhancing properties, including improvements in normal cognitive function, reversal of cognitive deficits induced by muscarinic receptor antagonists, and attenuation of cognitive deficits in psychiatric and neurological disorders, such as Alzheimer's disease and schizophrenia. However, until recently, the lack of small molecule ligands that antagonize or activate specific muscarinic acetylcholine receptor (mAChR) subtypes with high selectivity has been a major obstacle in defining the relative contributions of individual mAChRs to different aspects of cognitive function and for the development of novel therapeutic agents. These limitations may be potentially overcome by the recent discovery of novel mAChR subtype-selective compounds, notably allosteric agonists and positive allosteric modulators, which exhibit greater selectivity for individual mAChR subtypes than previous mAChR orthosteric agonists. In preclinical studies, these novel ligands have shown promising efficacy in several models for the enhancement of cognition. In this chapter, we will review the muscarinic cholinergic circuitry and pharmacology of mAChR agonists and antagonists relevant to the modulation of different aspects of cognition in animals and clinical populations.

Neuronal localization of M2 muscarinic receptor immunoreactivity in the rat amygdala

Muscarinic cholinergic neurotransmission in the amygdala is critical for memory consolidation in emotional/motivational learning tasks, but little is known about the neuronal distribution of different receptor subtypes. Immunohistochemistry was used in the present investigation to localize the m2 receptor (M2R). Differential patterns of M2R-immunoreactivity (M2R-ir) were observed in the somata and neuropil of the various amygdalar nuclei. Neuropilar M2R-ir was strongest in rostral portions of the basolateral nuclear complex (BLC). M2R-positive (M2R+) somata were seen in low numbers in all nuclei of the amygdala. Most M2R+ neurons associated with the BLC were in the lateral nucleus and external capsule. These cells were nonpyramidal neurons that contained glutamatic acid decarboxylase (GAD), somatostatin (SOM), and neuropeptide Y (NPY), but not parvalbumin (PV), calretinin (CR), or cholecystokinin (CCK). Little or no M2R-ir was observed in GAD+, PV+, CR+, or CCK+ axons in the BLC, but it was seen in some SOM+ axons and many NPY+ axons. M2R-ir was found in a small number of spiny and aspiny neurons of the central nucleus that were mainly located along the lateral and ventral borders of its lateral subdivision. Many of these cells contained SOM and NPY. M2R+ neurons were also seen in the medial nucleus, including a distinct subpopulation of neurons that surrounded its anteroventral subdivision. The latter neurons were negative for all neuronal markers analyzed. The intercalated nuclei (INs) were associated with two types of large M2R+ neurons, spiny and aspiny. The small principal neurons of the INs were M2R-negative. The somata and dendrites of the large spiny neurons, which were actually found in a zone located just outside of the rostral INs, expressed SOM and NPY, but not GAD. These findings indicate that acetylcholine can modulate a variety of discrete neuronal subpopulations in various amygdalar nuclei via M2Rs, especially neurons that express SOM and NPY.

M1 muscarinic receptors boost synaptic potentials and calcium influx in dendritic spines by inhibiting postsynaptic SK channels

Acetylcholine release and activation of muscarinic cholinergic receptors (mAChRs) enhance synaptic plasticity in vitro and cognition and memory in vivo. Within the hippocampus, mAChRs promote NMDA-type glutamate receptor-dependent forms of long-term potentiation. Here, we use calcium (Ca) imaging combined with two-photon laser glutamate uncaging at apical spines of CA1 pyramidal neurons to examine postsynaptic mechanisms of muscarinic modulation of glutamatergic transmission. Uncaging-evoked excitatory postsynaptic potentials and Ca transients are increased by muscarinic stimulation; however, this is not due to direct modulation of glutamate receptors. Instead, mAChRs modulate a negative feedback loop in spines that normally suppresses synaptic signals. mAChR activation reduces the Ca sensitivity of small conductance Ca-activated potassium (SK) channels that are found in the spine, resulting in increased synaptic potentials and Ca transients. These effects are mediated by M1-type muscarinic receptors and occur in a casein kinase-2-dependent manner. Thus, muscarinic modulation regulates synaptic transmission by tuning the activity of nonglutamatergic postsynaptic ion channels.

Presynaptic muscarinic receptor subtypes involved in the enhancement of spontaneous GABAergic postsynaptic currents in hippocampal neurons

We investigated the effects of muscarinic acetylcholine receptor (mAChR) activation on GABAergic synaptic transmission in rat hippocampal neurons. Current-clamp recordings revealed that methacholine produced membrane depolarization and action potential firing. Methacholine augmented the bicuculline-sensitive and GABA(A) -mediated frequency of spontaneous inhibitory postsynaptic currents (sIPSCs); the action of methacholine had a slow onset and longer duration. The increase in methacholine-evoked sIPSCs was completely inhibited by atropine and was insensitive to glutamatergic receptor blockers. Interestingly, methacholine action was not inhibited by intracellular perfusion with GDP-β-S, suggesting that muscarinic effects on membrane excitability and sIPSC frequency are mainly presynaptic. McN-A-343 and pirenzepine, selective agonist and antagonist of the m1 mAChR subtype, respectively, neither enhanced sIPSCs nor inhibited the methacholine effect. However, the m3-m5 mAChR antagonist 4-DAMP, and the m2-m4 mAChR antagonist himbacine inhibited the methacholine effect. U73122, an IP(3) production inhibitor, and 2APB, an IP(3) receptor blocker, drastically decreased the methacholine effect. Recording of miniature events revealed that besides the effect exerted by methacholine on membrane firing properties and sIPSC frequency, muscarinic receptors also enhanced the frequency of mIPSCs with no effect on their amplitude, possibly modulating the molecular machinery subserving vesicle docking and fusion and suggesting a tight colocalization at the active zone of the presynaptic terminals. These data strongly suggest that by activating presynaptic m2, m3, m4 and m5 mAChRs, methacholine can increase membrane excitability and enhance efficiency in the GABA release machinery, perhaps through a mechanism involving the release of calcium from the endoplasmic reticulum.

A component of Premarin(®) enhances multiple cognitive functions and influences nicotinic receptor expression

In women, ovarian hormone loss at menopause has been related to cognitive decline, and some studies suggest that estrogen-containing hormone therapy (HT) can mitigate these effects. Recently, the Women's Health Initiative study found that conjugated equine estrogens, the most commonly prescribed HT, do not benefit cognition. Isolated components of conjugated equine estrogens (tradename Premarin(®)) have been evaluated in vitro, with delta(8,9)-dehydroestrone (∆(8)E1) and equilin showing the strongest neuroprotective profiles. It has not been evaluated whether ∆(8)E1 or equilin impact cognition or the cholinergic system, which is affected by other estrogens and known to modulate cognition. Here, in middle-aged, ovariectomized rats, we evaluated the effects of ∆(8)E1 and equilin treatments on a cognitive battery and cholinergic nicotinic receptors (nAChR). Specifically, we used (125)I-labeled epibatidine binding to assay brain nicotinic receptor containing 4α and 2β subunits (α4β2-nAChR), since this nicotinic receptor subtype has been shown previously to be sensitive to other estrogens. ∆(8)E1 enhanced spatial working, recent and reference memory. ∆(8)E1 also decreased hippocampal and entorhinal cortex α4β2-nAChR expression, which was related to spatial reference memory performance. Equilin treatment did not affect spatial memory or rat α4β2-nAChR expression, and neither estrogen impacted (86)Rb(+) efflux, indicating lack of direct action on human α4β2 nAChR function. Both estrogens influenced vaginal smear profiles, uterine weights, and serum luteinizing hormone levels, analogous to classic estrogens. The findings indicate that specific isolated Premarin(®) components differ in their ability to affect cognition and nAChR expression. Taken with the works of others showing ∆(8)E1-induced benefits on several dimensions of health-related concerns associated with menopause, this body of research identifies ∆(8)E1 as a new avenue to be investigated as a potential component of HT that may benefit brain health and function during aging.

Blockade of different muscarinic receptor subtypes changes the equilibrium between excitation and inhibition in rat visual cortex

We have shown that cortical acetylcholine modulates the balance between excitation and inhibition evoked in layer 5 pyramidal neurons of rat visual cortex [Lucas-Meunier E, Monier C, Amar M, Baux G, Frégnac Y, Fossier P (2009) Cereb Cortex 19:2411-2427]. Our aim is now to establish a functional basis for the role of the different types of muscarinic receptors (MRs) on glutamate fibers and on GABAergic interneurons and to analyse their contribution to the modulation of excitation-inhibition balance in the rat visual cortex. To ascertain that there was a basis for our functional study, we first checked for the presence of the various MR subtypes by single cell RT-PCR and immunolabeling experiments. Then, recording the composite responses in layer 5 pyramidal neurons to layer 1-2 stimulation (which also recruits cholinergic fibers) in the presence of specific antagonists of the different types of MR allowed us to determine their modulatory role. We show that the specific blockade of the widely distributed M1R (with the mamba toxin, MT7) induced a significant increase in the excitatory conductance without modifying the inhibitory conductance, pointing to a localization of M1R on glutamatergic neurons where their activation would decrease the release of glutamate. From our functional results, M2/M4Rs appear to be located on glutamatergic neurons afferent to the recorded layer 5 pyramidal neuron and they decrease glutamate release. The extended distribution of M4Rs in the cortex compared to the restricted distribution of M2R (layers 3-5) is in favour of a major role as a modulator of M4R. The selective antagonist of M3Rs, 4-DAMP, decreased the inhibitory conductance, showing that activated M3Rs increase the release of GABA and thus are located on GABAergic interneurons. The activation of the different types of MRs located either on glutamatergic neurons or on GABAergic interneurons converges to reinforce the dominance of inhibitory inputs thus decreasing the excitability of layer 5 pyramidal neurons.

Carbachol-induced long-term synaptic depression is enhanced during senescence at hippocampal CA3-CA1 synapses

Dysregulation of the cholinergic transmitter system is a hallmark of Alzheimer's disease and contributes to an age-associated decline in memory performance. The current study examined the influence of carbachol, a cholinergic receptor agonist, on synaptic transmission over the course of aging. Extracellular excitatory postsynaptic field potentials were recorded from CA3-CA1 synapses in acute hippocampal slices obtained from young adult (5-8 mo) and aged (22-24 mo) male Fischer 344 rats. Bath application of carbachol elicited a transient depression of synaptic transmission, which was followed by a long-lasting depression (CCh-LTD) observed 90 min after carbachol cessation in both age groups. However, the magnitude of CCh-LTD was significantly larger in senescent animals and was attenuated by N-methyl-D-aspartate receptor blockade in aged animals. Blockade of L-type Ca(2+) channels inhibited CCh-LTD to a greater extent in aged animals compared to young adults. Finally, the expression of CCh-LTD was dependent on protein synthesis. The results indicate that altered Ca(2+) homeostasis or muscarinic activation of Ca(2+) signaling contribute to the enhanced CCh-LTD during senescence.

A novel derivative of xanomeline improves fear cognition in aged mice

The M1/M4 muscarinic acetylcholine receptor agonist xanomeline can significantly improve the cognitive function, but the intolerable side effects limit its clinical usefulness. Our recent study has reported a novel derivative of xanomeline, 3-[3-(3-(3-florophenyl)-2-propyn-1-ylthio)-1,2,5-thiadiazol-4-yl]-1,2,5,6-tetrahydro-1-methylpyridine oxalate (EUK1001), exhibited higher affinity of mAChRs and less side effects relative to xanomeline. In the present study, we further utilized behavioral and electrophysiological techniques to investigate the effects of EUK1001 on fear cognition and hippocampal long-term potentiation (LTP) in aged mice. Behavioral testing showed that 0.1, 0.5 or 1.0mg/kg EUK1001 group, like 1.0mg/kg xanomeline group, exhibited better performance in contextual fear conditioning and passive avoidance test than vehicle-controls. In the cued fear conditioning test, just 0.5 or 1.0, but not 0.1mg/kg EUK1001, significantly enhanced the levels of freezing response. In addition, theta-burst stimulation (TBS) induced the significant larger hippocampal LTP in brain slices perfused with artificial cerebrospinal fluid (ACSF) containing 0.01microM EUK1001 or in brain slices from aged mice injected intraperitoneally (i.p.) with EUK1001. This enhancing effect was blocked by 0.25microM pirenzepine, a selective M1 antagonist. Together, these results show that EUK1001 can enhance fear cognition and synaptic plasticity via the activation of M1 muscarinic acetylcholine receptors. Thus, EUK1001 may possibly represent a promising lead compound for the treatment of Alzheimer's disease and age-related cognitive deficits.

A novel selective muscarinic acetylcholine receptor subtype 1 antagonist reduces seizures without impairing hippocampus-dependent learning

Previous studies suggest that selective antagonists of specific subtypes of muscarinic acetylcholine receptors (mAChRs) may provide a novel approach for the treatment of certain central nervous system (CNS) disorders, including epileptic disorders, Parkinson's disease, and dystonia. Unfortunately, previously reported antagonists are not highly selective for specific mAChR subtypes, making it difficult to definitively establish the functional roles and therapeutic potential for individual subtypes of this receptor subfamily. The M(1) mAChR is of particular interest as a potential target for treatment of CNS disorders. We now report the discovery of a novel selective antagonist of M(1) mAChRs, termed VU0255035 [N-(3-oxo-3-(4-(pyridine-4-yl)piperazin-1-yl)propyl)-benzo[c][1,2,5]thiadiazole-4 sulfonamide]. Equilibrium radioligand binding and functional studies demonstrate a greater than 75-fold selectivity of VU0255035 for M(1) mAChRs relative to M(2)-M(5). Molecular pharmacology and mutagenesis studies indicate that VU0255035 is a competitive orthosteric antagonist of M(1) mAChRs, a surprising finding given the high level of M(1) mAChR selectivity relative to other orthosteric antagonists. Whole-cell patch-clamp recordings demonstrate that VU0255035 inhibits potentiation of N-methyl-D-aspartate receptor currents by the muscarinic agonist carbachol in hippocampal pyramidal cells. VU0255035 has excellent brain penetration in vivo and is efficacious in reducing pilocarpine-induced seizures in mice. We were surprised to find that doses of VU0255035 that reduce pilocarpine-induced seizures do not induce deficits in contextual freezing, a measure of hippocampus-dependent learning that is disrupted by nonselective mAChR antagonists. Taken together, these data suggest that selective antagonists of M(1) mAChRs do not induce the severe cognitive deficits seen with nonselective mAChR antagonists and could provide a novel approach for the treatment certain of CNS disorders.

Novel selective allosteric activator of the M1 muscarinic acetylcholine receptor regulates amyloid processing and produces antipsychotic-like activity in rats

Recent studies suggest that subtype-selective activators of M(1)/M(4) muscarinic acetylcholine receptors (mAChRs) may offer a novel approach for the treatment of psychotic symptoms associated with schizophrenia and Alzheimer's disease. Previously developed muscarinic agonists have provided clinical data in support of this hypothesis, but failed in clinical development because of a lack of true subtype specificity and adverse effects associated with activation of other mAChR subtypes. We now report characterization of a novel highly selective agonist for the M(1) receptor with no agonist activity at any of the other mAChR subtypes, termed TBPB [1-(1'-2-methylbenzyl)-1,4'-bipiperidin-4-yl)-1H-benzo[d]imidazol-2(3H)-one]. Mutagenesis and molecular pharmacology studies revealed that TBPB activates M(1) through an allosteric site rather than the orthosteric acetylcholine binding site, which is likely critical for its unprecedented selectivity. Whole-cell patch-clamp recordings demonstrated that activation of M(1) by TBPB potentiates NMDA receptor currents in hippocampal pyramidal cells but does not alter excitatory or inhibitory synaptic transmission, responses thought to be mediated by M(2) and M(4). TBPB was efficacious in models predictive of antipsychotic-like activity in rats at doses that did not produce catalepsy or peripheral adverse effects of other mAChR agonists. Finally, TBPB had effects on the processing of the amyloid precursor protein toward the non-amyloidogenic pathway and decreased Abeta production in vitro. Together, these data suggest that selective activation of M(1) may provide a novel approach for the treatment of symptoms associated with schizophrenia and Alzheimer's disease.

Interaction between glutamatergic-NMDA and cholinergic-muscarinic systems in classical fear conditioning

A number of studies have suggested that the glutamatergic and cholinergic systems are both involved in learning and memory processes and that they interact in order to facilitate these processes. However, the role of M1-muscarinic receptors in mediating this interaction has not been elucidated. The aim of this study was to determine whether the concomitant administration of MK-801 (non-competitive NMDA antagonist) and dicyclomine (M1-muscarinic antagonist--DIC) in sub-effective doses impairs contextual fear conditioning (hippocampal-dependent task) and tone fear conditioning tasks (hippocampal-independent task). The results showed that concomitant pre-training administration of DIC (8.0 mg/kg) and MK-801 (0.07 mg/kg)--two sub-effectives doses for the contextual fear conditioning task--does impair the performance of animals on this task (as measured by freezing behavior time). Tone fear conditioning tasks were not affected by the drugs either administered separately or concurrently. The pre-training administration of sub-effective doses of MK-801 and DIC in combination impairs performance on contextual fear conditioning task (hippocampal-dependent), but not on tone fear conditioning task (hippocampal-independent). These data support the hypothesis that the interaction between glutamatergic and cholinergic systems in hippocampus-dependent learning and memory processes probably occurs through M1 receptor.

Hypoxia modulates cholinergic but not opioid activation of G proteins in rat hippocampus

Intermittent hypoxia, such as that associated with obstructive sleep apnea, can cause neuronal death and neurobehavioral dysfunction. The cellular and molecular mechanisms through which hypoxia alter hippocampal function are incompletely understood. This study used in vitro [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTP gamma S) autoradiography to test the hypothesis that carbachol and DAMGO activate hippocampal G proteins. In addition, this study tested the hypothesis that in vivo exposure to different oxygen (O(2)) concentrations causes a differential activation of G proteins in the CA1, CA3, and dentate gyrus (DG) regions of the hippocampus. G protein activation was quantified as nCi/g tissue in CA1, CA3, and DG from rats housed for 14 days under one of three different oxygen conditions: normoxic (21% O(2)) room air, or hypoxia (10% O(2)) that was intermittent or sustained. Across all regions of the hippocampus, activation of G proteins by the cholinergic agonist carbachol and the mu opioid agonist [D-Ala(2), N-Met-Phe(4), Gly(5)] enkephalin (DAMGO) was ordered by the degree of hypoxia such that sustained hypoxia > intermittent hypoxia > room air. Carbachol increased G protein activation during sustained hypoxia (38%), intermittent hypoxia (29%), and room air (27%). DAMGO also activated G proteins during sustained hypoxia (52%), intermittent hypoxia (48%), and room air (43%). Region-specific comparisons of G protein activation revealed that the DG showed significantly less activation by carbachol following intermittent hypoxia and sustained hypoxia than the CA1. Considered together, the results suggest the potential for hypoxia to alter hippocampal function by blunting the cholinergic activation of G proteins within the DG.

An allosteric potentiator of M4 mAChR modulates hippocampal synaptic transmission

Muscarinic acetylcholine receptors (mAChRs) provide viable targets for the treatment of multiple central nervous system disorders. We have used cheminformatics and medicinal chemistry to develop new, highly selective M4 allosteric potentiators. VU10010, the lead compound, potentiates the M4 response to acetylcholine 47-fold while having no activity at other mAChR subtypes. This compound binds to an allosteric site on the receptor and increases affinity for acetylcholine and coupling to G proteins. Whole-cell patch clamp recordings revealed that selective potentiation of M4 with VU10010 increases carbachol-induced depression of transmission at excitatory but not inhibitory synapses in the hippocampus. The effect was not mimicked by an inactive analog of VU10010 and was absent in M4 knockout mice. Selective regulation of excitatory transmission by M4 suggests that targeting of individual mAChR subtypes could be used to differentially regulate specific aspects of mAChR modulation of function in this important forebrain structure.

Cholinergic dysfunction in temporal lobe epilepsy

The entorhinal cortex-hippocampus complex is believed to be the site of origin of seizure activity in the majority of patients with temporal lobe epilepsy (TLE). Both these regions are enriched with cholinergic innervation, which plays a key role in the normal control of neuronal excitability and in higher cognitive processes. In TLE, anatomical and functional changes occur in all cellular components of the local neural circuit. Thus, while it is not surprising that cholinergic functions are altered in the epileptic temporal lobe, the exact nature and role of these changes in the pathogenesis of the disease are not known. In this report, we summarize the scientific background and experimental data supporting a "cholinergic hypothesis of TLE." We conclude that while the exact role of cholinergic dysfunction in TLE is not known, there is a firm basis for suggesting that changes in the expression of key cholinergic proteins-and the associated cholinergic dysfunction-are key factors in the basic mechanisms underlying TLE.

The regulation of M1 muscarinic acetylcholine receptor desensitization by synaptic activity in cultured hippocampal neurons

To better understand metabotropic/ionotropic integration in neurons we have examined the regulation of M1 muscarinic acetylcholine (mACh) receptor signalling in mature (> 14 days in vitro), synaptically-active hippocampal neurons in culture. Using a protocol where neurons are exposed to an EC(50) concentration of the muscarinic agonist methacholine (MCh) prior to (R1), and following (R2) a desensitizing pulse of a high concentration of this agonist, we have found that the reduction in M(1) mACh receptor responsiveness is decreased in quiescent (+tetrodotoxin) neurons and increased when synaptic activity is enhanced by blocking GABA(A) receptors with picrotoxin. The picrotoxin-mediated effect on M1 mACh receptor responsiveness was completely prevented by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor blockade. Inhibition of endogenous G protein-coupled receptor kinase 2 by transfection with the non-G(q/11)alpha-binding, catalytically-inactive (D110A,K220R)G protein-coupled receptor kinase 2 mutant, decreased the extent of M1 mACh receptor desensitization under all conditions. Pharmacological inhibition of protein kinase C (PKC) activity, or chronic phorbol ester-induced PKC down-regulation had no effect on agonist-mediated receptor desensitization in quiescent or spontaneously synaptically active neurons, but significantly decreased the extent of receptor desensitization in picrotoxin-treated neurons. MCh stimulated the translocation of diacylglycerol- sensitive eGFP-PKCepsilon, but not Ca2+/diacylglycerol-sensitive eGFP-PKCbetaII in both the absence, and presence of tetrodotoxin. Under these conditions, MCh-stimulated eGFP-myristoylated, alanine-rich C kinase substrate translocation was dependent on PKC activity, but not Ca2+/calmodulin. In contrast, picrotoxin-driven translocation of myristoylated, alanine-rich C kinase substrate was accompanied by translocation of PKCbetaII, but not PKCepsilon, and was dependent on PKC and Ca2+/calmodulin. Taken together these data suggest that the level of synaptic activity may determine the different kinases recruited to regulate M1 mACh receptor desensitization in neurons.

Allosteric modulation of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors (mAChRs) are prototypical Family A G protein coupled-receptors. The five mAChR subtypes are widespread throughout the periphery and the central nervous system and, accordingly, are widely involved in a variety of both physiological and pathophysiological processes. There currently remains an unmet need for better therapeutic agents that can selectively target a given mAChR subtype to the relative exclusion of others. The main reason for the lack of such selective mAChR ligands is the high sequence homology within the acetylcholine-binding site (orthosteric site) across all mAChRs. However, the mAChRs possess at least one, and likely two, extracellular allosteric binding sites that can recognize small molecule allosteric modulators to regulate the binding and function of orthosteric ligands. Extensive studies of prototypical mAChR modulators, such as gallamine and alcuronium, have provided strong pharmacological evidence, and associated structure-activity relationships (SAR), for a "common" allosteric site on all five mAChRs. These studies are also supported by mutagenesis experiments implicating the second extracellular loop and the interface between the third extracellular loop and the top of transmembrane domain 7 as contributing to the common allosteric site. Other studies are also delineating the pharmacology of a second allosteric site, recognized by compounds such as staurosporine. In addition, allosteric agonists, such as McN-A-343, AC-42 and N-desmethylclozapine, have also been identified. Current challenges to the field include the ability to effectively detect and validate allosteric mechanisms, and to quantify allosteric effects on binding affinity and signaling efficacy to inform allosteric modulator SAR.

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

BACKGROUND

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

METHOD

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

RESULTS

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

CONCLUSIONS

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

Opposing effects on muscarinic acetylcholine receptors in the piriform cortex of odor-trained rats

We combined pharmacological studies and electrophysiological recordings to investigate modifications in muscarinic acetylcholine (ACh) receptors (mAChR) in the rat olfactory (piriform) cortex, following odor-discrimination rule learning. Rats were trained to discriminate between positive and negative cues in pairs of odors, until they reached a phase of high capability to learn unfamiliar odors, using the same paradigm ("rule learning"). It has been reported that at 1-3 d after the acquisition of odor-discrimination rule learning, pyramidal neurons in the rat piriform cortex show enhanced excitability, due to a reduction in the spike-activated potassium current I(AHP), which is modulated by ACh. Further, ACh and its analog, carbachol (CCh), lost the ability to reduce the I(AHP) in neurons from trained rats. Here we show that the reduced sensitivity to CCh in the piriform cortex results from a decrease in the number of mAChRs, as well as a reduction in the affinity of the receptors to CCh. Also, it has been reported that 3-8 d after the acquisition of odor-discrimination rule learning, synaptic transmission in the piriform cortex is enhanced, and paired-pulse facilitation (PPF) in response to twin stimulations is reduced. Here, intracellular recordings from pyramidal neurons show that CCh increases PPF in the piriform cortex from odor-trained rats more than in control rats, suggesting enhanced effect of ACh in inhibiting presynaptic glutamate release after odor training.

Role of the cholinergic system in regulating survival of newborn neurons in the adult mouse dentate gyrus and olfactory bulb

Neurogenesis in the subgranular zone of the hippocampal dentate gyrus and olfactory bulbs continues into adulthood and has been implicated in the cognitive function of the adult brain. The basal forebrain cholinergic system has been suggested to play a role in regulating neurogenesis as well as learning and memory in these regions. Herein, we report that highly polysialylated neural cell adhesion molecule (PSA-NCAM)-positive immature cells as well as neuronal nuclei (NeuN)-positive mature neurons in the dentate gyrus and olfactory bulb express multiple acetylcholine receptor subunits and make contact with cholinergic fibers. To examine the function of acetylcholine in neurogenesis, we used donepezil (Aricept), a potent and selective acetylcholinesterase inhibitor that improves cognitive impairment in Alzheimer's disease. Intraperitoneal administrations of donepezil significantly enhanced the survival of newborn neurons, but not proliferation of neural progenitor cells in the subgranular zone or the subventricular zone of normal mice. Moreover, donepezil treatment reversed the chronic stress-induced decrease in neurogenesis. Taken together, these results suggest that activation of the cholinergic system promotes survival of newborn neurons in the adult dentate gyrus and olfactory bulb under both normal and stressed conditions.

Muscarinic enhancement of R-type calcium currents in hippocampal CA1 pyramidal neurons

The "toxin-resistant" R-type Ca2+ channels are expressed widely in the CNS and distributed mainly in apical dendrites and spines. They play important roles in regulating signal transduction and intrinsic properties of neurons, but the modulation of these channels in the mammalian CNS has not been studied. In this study we used whole-cell patch-clamp recordings and found that muscarinic activation enhances R-type, but does not affect T-type, Ca2+ currents in hippocampal CA1 pyramidal neurons after N, P/Q, and L-type Ca2+ currents selectively were blocked. M1/M3 cholinergic receptors mediated the muscarinic stimulation of R-type Ca2+ channels. The signaling pathway underlying the R-type enhancement was independent of intracellular [Ca2+] changes and required the activation of a Ca(2+)-independent PKC pathway. Furthermore, we found that the enhancement of R-type Ca2+ currents resulted in the de novo appearance of Ca2+ spikes and in remarkable changes in the firing pattern of R-type Ca2+ spikes, which could fire repetitively in the theta frequency. Therefore, muscarinic enhancement of R-type Ca2+ channels could play an important role in modifying the dendritic response to synaptic inputs and in the intrinsic resonance properties of neurons.

Temporal and region-dependent changes in muscarinic M4 receptors in the hippocampus and entorhinal cortex of adrenalectomized rats

Long-term adrenalectomy induces a dramatic loss of cells in the dentate gyrus and CA1-CA4 fields of the hippocampus resulting in an impairment of cognitive functions such as spatial learning, memory and exploratory behaviour. Muscarinic M1 and M4 receptor levels in the hippocampus and entorhinal cortex of adult male Wistar rats were examined 3, 14, 30, 90, and 150 days after adrenalectomy. Receptor levels in the entorhinal cortex and the hippocampus were determined by quantitative autoradiography using 125I-M1-toxin-1 and 125I-M4-toxin-1, M1 and M4 subtype selective antagonists, respectively. Moreover, the level of hippocampal M1 and M4 muscarinic receptors were evaluated 1 month after adrenalectomy by immunoblot analysis. Adrenalectomy induced apoptotic processes were examined by analysing apoptotic markers using Western blot analysis. No significant changes were observed in the level of muscarinic M1 receptors in the entorhinal cortex, the dentate gyrus and in the different CA fields of the hippocampus of adrenalectomized (ADX) rats. However, M4 receptors showed a significant decrease in the entorhinal cortex (at 3 days), dentate gyrus and CA4 (at 14 days), CA3 (at 30 days), and CA2 and CA1 (at 90 days) after adrenalectomy. Moreover, a decrease in the level of M4 receptors was detected in ADX rats 1 month after adrenalectomy as compared with sham groups using M4 specific antibody. Apoptotic markers such as PARP and p53 were significantly increased whereas Bcl-2 marker was decreased in ADX rat brain homogenates compared to controls. Our results show that M1 and M4 receptors are differentially affected by adrenalectomy and indicate that these subtypes have different functions in the hippocampus. Our data on time and region-dependent decreases in hippocampal M4 receptors indicate that the M4 receptor subtype is influenced by adrenal hormones and suggest that the M4 receptor might be linked to memory function in the hippocampus.

Enhancement of the synchronized firing of CA1 pyramidal cells by medial septum preconditioning: Time-dependent involvement of muscarinic cholinoceptors and GABAB receptors

Activation of the medial septum (MS) enhances the synchronized firing of pyramidal cells in the hippocampus. At least two distinct mechanisms might be recruited in this process: GABAergic inhibition of hippocampal inhibitory interneurons and cholinergic enhancement of pyramidal cell excitability. In the present study, a field potential recording in urethane anaesthetised rats was used to show that MS preconditioning with brief high-frequency burst stimulation markedly enhanced Schaffer collateral/commissural (SC/C) synaptically evoked population spikes in the CA1 area without significant alteration in the initial slope of SC/C field excitatory postsynaptic potentials (fEPSPs). An early phase (< approximately 125 ms) of the facilitation of neuronal discharge was inhibited by intracerebroventricular (i.c.v.) injection of the GABA(B) receptor antagonist CGP 52432 (200 nmol/5 microl). In contrast, the muscarinic cholinoceptor antagonist scopolamine (20 nmol/5 microl) inhibited a later phase ( approximately 150-300 ms) of the population spike facilitation. These findings support an important role for both cholinergic and GABAergic mediated septal drive in the tuning of signal conversion within the hippocampus as well as in securing the cortical feedback from the hippocampus.

Decreased expression and impaired function of muscarinic acetylcholine receptors in the rat hippocampus following transient forebrain ischemia

In this study, we investigated whether transient cerebral ischemia affects the function and molecular expression of specific muscarinic cholinergic receptors. Our results show that in contrast to the GABA-B and A1 adenosine receptor systems, the ability of muscarinic receptors to attenuate evoked excitatory responses at vulnerable CA1 synapses is significantly attenuated by 18 h following reperfusion. This attenuation in efficacy was restricted to the vulnerable CA1 subfield, as no significant change in muscarinic receptor-mediated attenuation of evoked responsiveness was observed within post-ischemic dentate granule cell synapses. Expression analysis revealed that the mRNA and immunoreactive protein levels for individual types of muscarinic receptors respond differently and uniquely to transient cerebral ischemia insult. Of particular interest is the m4 subtype of receptor, whose mRNA and protein expression levels were significantly diminished within the hippocampus by 12 and 24 h following reperfusion, respectively. As the m4 muscarinic receptor localizes to presynaptic terminals within the hippocampus, a decrease in its expression could account for the impaired functional responsiveness of the muscarinic receptor system following ischemic insult. Taken together, these results demonstrate that transient forebrain ischemia leads to dynamic alterations in the gene expression, protein prevalence, and functionality of muscarinic receptors in the post-ischemic hippocampus at times preceding the degeneration of the vulnerable neurons.

Muscarinic acetylcholine receptor activation enhances hippocampal neuron excitability and potentiates synaptically evoked Ca(2+) signals via phosphatidylinositol 4,5-bisphosphate depletion

Using single cell Ca(2+) imaging and whole cell current clamp recordings, this study aimed to identify the signal transduction mechanisms involved in mACh receptor-mediated, enhanced synaptic signaling in primary cultures of hippocampal neurons. Activation of M(1) mACh receptors produced a 2.48 +/- 0.26-fold enhancement of Ca(2+) transients arising from spontaneous synaptic activity in hippocampal neurons. Combined imaging of spontaneous Ca(2+) signals with inositol 1,4,5-trisphosphate (IP(3)) production in single neurons demonstrated that the methacholine (MCh)-mediated enhancement required activated G(q/11)alpha subunits and phospholipase C activity but did not require measurable increases in IP(3). Electrophysiological studies demonstrated that MCh treatment depolarized neurons from -64 +/- 3 to -45 +/- 3 mV and increased action potential generation. Depletion of plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) enhanced neuronal excitability and prolonged the action of MCh. These studies suggest that, in addition to producing the second messengers IP(3) and diacylglycerol, mACh receptor activation may directly utilize PIP(2) hydrolysis to regulate neuronal excitability.

Interaction between M1-muscarinic and glutamatergic NMDA receptors on an inhibitory avoidance task

It has been demonstrated that MK-801 potentiates the effects of the non-selective muscarinic antagonist scopolamine on memory in rats. In this study, we investigated the role of the M1-muscarinic receptor in this interaction, by administering different doses of dicyclomine (DIC) and MK-801 in combination to male Wistar rats before training on the inhibitory avoidance task. MK-801 and DIC in sub-effective doses were administered in combination. It was observed that MK-801 at a dose of 0.1125 mg/kg with a sub-effective dose of 8 mg/kg of DIC significantly impaired the retention test when compared with saline-treated animals, i.e. MK-801 potentiated the effects of dicyclomine on memory impairment. Our results suggest an important role for the M1-muscarinic receptor in the synergistic interaction between cholinergic muscarinic and glutamatergic NMDA receptors, which is in line with the findings that the interactive modulation between these two neurotransmitters systems constitutes an important mechanism in cognitive functions.

The effects on place cells of local scopolamine dialysis are mimicked by a mixture of two specific muscarinic antagonists

Using a dialysis probe near CA1 hippocampal recording electrodes, we infused nonspecific (scopolamine) and specific (methoctramine, pirenzepine) antagonists of muscarinic cholinergic transmission to determine their effects on the positional firing properties of place cells. Both low (0.5 mM) and high (2.0 or 3.0 mM) scopolamine significantly decreased in-field firing rate, increased the ratio of out-of-field to in-field rate, and reduced the smoothness of rate maps, while tending to increase out-of-field rate. Thus, local nonspecific muscarinic blockade mimicked the effects seen with intracerebroventricular application, suggesting that blockade of receptors local to the recorded cells plays an essential role. Unexpectedly, dialysis of scopolamine reduced locomotor activity, again duplicating the effects of intracerebroventricular administration. Most effects of methoctramine (1.0 mM), which blocks presynaptic m2 and m4 receptors, were initially strong but then diminished over hours. Methoctramine produced a significant increase only in out/in ratio and out-of-field rate, whereas it tended to increase in-field rate and monotonically decrease smoothness. Pirenzepine (3.0 mM), which blocks postsynaptic m1 receptors, produced a significant increase only in out/in ratio, whereas it tended to increase out-of-field rate and decrease in-field rate; all these effects were monotonic with respect to time. A mixture of methoctramine plus pirenzepine recapitulated the place-cell effects of scopolamine, although neither the mixture nor its separate components affected behavior. We conclude that the effects of scopolamine on place cells likely result from a combination of blockade of postsynaptic m1 receptors, leading to reduced excitability, with blockade of presynaptic m2 and m4 receptors, leading to increased out-of-field firing.