Differential presynaptic and postsynaptic expression of m1-m4 muscarinic acetylcholine receptors at the perforant pathway/granule cell synapse

Presynaptic Inhibitory Effects of Acetylcholine in the Hippocampus: A 40-Year Evolution of a Serendipitous Finding

Cholinergic regulation of hippocampal circuit activity has been an active area of neurophysiological research for decades. The prominent cholinergic innervation of intrinsic hippocampal circuitry, potent effects of cholinomimetic drugs, and behavioral responses to cholinergic modulation of hippocampal circuitry have driven investigators to discover diverse cellular actions of acetylcholine in distinct sites within hippocampal circuitry. Further research has illuminated how these actions organize circuit activity to optimize encoding of new information, promote consolidation, and coordinate this with recall of prior memories. The development of the hippocampal slice preparation was a major advance that accelerated knowledge of how hippocampal circuits functioned and how acetylcholine modulated these circuits. Using this preparation in the early 1980s, we made a serendipitous finding of a novel presynaptic inhibitory effect of acetylcholine on Schaffer collaterals, the projections from CA3 pyramidal neurons to dendrites of CA1 pyramidal cells. We characterized this effect at cellular and pharmacological levels, published the findings in the first volume of the Journal of Neuroscience, and proceeded to pursue other scientific directions. We were surprised and thrilled to see that, nearly 40 years later, the paper is still being cited and downloaded because the data became an integral piece of the foundation of the science of cholinergic regulation of hippocampal function in learning and memory. This Progressions article is a story of how single laboratory findings evolve through time to be confirmed, challenged, and reinterpreted by other laboratories to eventually become part of the basis of fundamental concepts related to important brain functions.

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.

Knockin' on heaven's door: Molecular mechanisms of neuronal tau uptake

Since aggregates of the microtubule‐binding protein tau were found to be the main component of neurofibrillary tangles more than 30 years ago, their contribution to neurodegeneration in Alzheimer's disease (AD) and tauopathies has become well established. Recent work shows that both tau load and its distribution in the brain of AD patients correlate with cognitive decline more closely compared to amyloid plaque deposition. In addition, the amyloid cascade hypothesis has been recently challenged because of disappointing results of clinical trials designed to treat AD by reducing beta‐amyloid levels, thus fuelling a renewed interest in tau. There is now robust evidence to indicate that tau pathology can spread within the central nervous system via a prion‐like mechanism following a stereotypical pattern, which can be explained by the trans‐synaptic inter‐neuronal transfer of pathological tau. In the receiving neuron, tau has been shown to take multiple routes of internalisation, which are partially dependent on its conformation and aggregation status. Here, we review the emerging mechanisms proposed for the uptake of extracellular tau in neurons and the requirements for the propagation of its pathological conformers, addressing how they gain access to physiological tau monomers in the cytosol. Furthermore, we highlight some of the key mechanistic gaps of the field, which urgently need to be addressed to expand our understanding of tau propagation and lead to the identification of new therapeutic strategies for tauopathies.

Diverse glutamatergic inputs target spines expressing M1 muscarinic receptors in the basolateral amygdala: An ultrastructural analysis

Although it is known that acetylcholine acting through M1 muscarinic receptors (M1Rs) is essential for memory consolidation in the anterior basolateral nucleus of the amygdala (BLa), virtually nothing is known about the circuits involved. In the hippocampus M1R activation facilitates long-term potentiation (LTP) by potentiating NMDA glutamate receptor (NMDAR) currents. The majority of NMDAR+ profiles in the BLa are spines. Since about half of dendritic spines of BLa pyramidal neurons (PNs) receiving glutamatergic inputs are M1R-immunoreactive (M1R+) it is possible that the role of M1Rs in BLa mnemonic functions also involves potentiation of NMDAR currents in spines. However, the finding that only about half of BLa spines are M1R+ suggests that this proposed mechanism may only apply to a subset of glutamatergic inputs. As a first step in the identification of differential glutamatergic inputs to M1R+ spines in the BLa, the present electron microscopic study used antibodies to two different vesicular glutamate transporter proteins (VGluTs) to label two different subsets of glutamatergic inputs to M1R+ spines. These inputs are largely complimentary with VGluT1+ inputs arising mainly from cortical structures and the basolateral nucleus, and VGluT2+ inputs arising mainly from the thalamus. It was found that about one-half of the spines that were postsynaptic to VGluT1+ or VGluT2+ terminals were M1R+. In addition, a subset of the VGluT1+ or VGluT2+ axon terminals were M1R+, including those that synapsed with M1R+ spines. These results suggest that acetylcholine can modulate glutamatergic inputs to BLa spines by presynaptic as well as postsynaptic M1R-mediated mechanisms.

Selective allosteric modulation of muscarinic acetylcholine receptors for the treatment of schizophrenia and substance use disorders

Muscarinic acetylcholine receptor (mAChRs) subtypes represent exciting new targets for the treatment of schizophrenia and substance use disorder (SUD). Recent advances in the development of subtype-selective allosteric modulators have revealed promising effects in preclinical models targeting the different symptoms observed in schizophrenia and SUD. M₁ PAMs display potential for addressing the negative and cognitive symptoms of schizophrenia, while M₄ PAMs exhibit promise in treating preclinical models predictive of antipsychotic-like activity. In SUD, there is increasing support for modulation of mesocorticolimbic dopaminergic circuitry involved in SUD with selective M₄ mAChR PAMs or M₅ mAChR NAMs. Allosteric modulators of these mAChR subtypes have demonstrated efficacy in rodent models of cocaine and ethanol seeking, with indications that these ligand may also be useful for other substances of abuse, as well as in various stages in the cycle of addiction. Importantly, allosteric modulators of the different mAChR subtypes may provide viable treatment options, while conferring greater subtype specificity and corresponding enhanced therapeutic index than orthosteric muscarinic ligands and maintaining endogenous temporo-spatial ACh signaling. Overall, subtype specific mAChR allosteric modulators represent important novel therapeutic mechanisms for schizophrenia and SUD.

Contribution of M1 and M2 muscarinic receptor subtypes to convulsions in fasted mice treated with scopolamine and given food

Treatment of fasted mice and rats with the nonselective muscarinic antagonist, scopolamine or atropine, causes convulsions after food intake. This study evaluated the effect of fasting on the expression of M₁ and M₂ muscarinic receptors in the brain regions, the relationship between receptor expression and seizure stages, and the muscarinic receptor subtype which plays a role in the occurrence of convulsions. Mice were grouped as allowed to eat ad lib (fed) and deprived of food for 24h (fasted). Fasted animals developed convulsions after being treated with scopolamine (60%) or the selective M₁ receptor antagonist pirenzepine (10mg/kg; 20% and 60mg/kg; 70%) and given food. Fasting increased expression of M₁ receptors in the frontal cortex and M₂ receptors in the hippocampus, but produced no change in the expression of both receptors in the amygdaloid complex. Food intake after fasting decreased M₁ receptor expression in the frontal cortex and M₁ and M₂ receptor expression in the hippocampus. Seizure severity was uncorrelated with muscarinic receptor expression in the brain regions. Taken together, these findings provide evidence for the role of M₁ muscarinic receptor antagonism and fasting-induced increases in M₁ and M₂ expression possible underlying mechanism in the occurrence of convulsions in fasted animals.

In Vivo Modulation of Hippocampal Excitability by M4 Muscarinic Acetylcholine Receptor Activator: Implications for Treatment of Alzheimer's Disease and Schizophrenic Patients

Abnormal hippocampal activity has been linked to impaired cognitive performance in Alzheimer's disease and schizophrenia, leading to a hypothesis that normalization of this activity may be therapeutically beneficial. Our work suggests that one approach for hippocampal normalization may be through activation of the M4 muscarinic acetylcholine receptor. We used a brain penetrant M4 muscarinic acetylcholine receptor selective activator, PT-3763, to show dose-dependent attenuation of field potentials in Schaffer collateral (CA3-CA1) and recurrent associational connections (CA3-CA3) ex vivo in hippocampal slices. In vivo, systemic administration of PT-3763 led to attenuation of glutamate release in CA3 as measured by amperometry and to a dose-dependent decrease in population CA1 pyramidal activity as measured by fiber photometry. This decrease in population activity was also evident with a localized administration of the compound to the recorded site. Finally, PT-3763 reversed scopolamine-induced deficit in Morris water maze. Our results suggest that M4 muscarinic acetylcholine receptor activation may be a suitable therapeutic treatment in diseases associated with hyperactive hippocampal activity.

Metabotropic Acetylcholine and Glutamate Receptors Mediate PI(4,5)P2 Depletion and Oscillations in Hippocampal CA1 Pyramidal Neurons in situ

The sensitivity of many ion channels to phosphatidylinositol-4,5-bisphosphate (PIP2) levels in the cell membrane suggests that PIP2 fluctuations are important and general signals modulating neuronal excitability. Yet the PIP2 dynamics of central neurons in their native environment remained largely unexplored. Here, we examined the behavior of PIP2 concentrations in response to activation of Gq-coupled neurotransmitter receptors in rat CA1 hippocampal neurons in situ in acute brain slices. Confocal microscopy of the PIP2-selective molecular sensors tubbyCT-GFP and PLCδ1-PH-GFP showed that pharmacological activation of muscarinic acetylcholine (mAChR) or group I metabotropic glutamate (mGluRI) receptors induces transient depletion of PIP2 in the soma as well as in the dendritic tree. The observed PIP2 dynamics were receptor-specific, with mAChR activation inducing stronger PIP2 depletion than mGluRI, whereas agonists of other Gαq-coupled receptors expressed in CA1 neurons did not induce measureable PIP2 depletion. Furthermore, the data show for the first time neuronal receptor-induced oscillations of membrane PIP2 concentrations. Oscillatory behavior indicated that neurons can rapidly restore PIP2 levels during persistent activation of Gq and PLC. Electrophysiological responses to receptor activation resembled PIP2 dynamics in terms of time course and receptor specificity. Our findings support a physiological function of PIP2 in regulating electrical activity.

Modulation of Hippocampal Circuits by Muscarinic and Nicotinic Receptors

This article provides a review of the effects of activation of muscarinic and nicotinic receptors on the physiological properties of circuits in the hippocampal formation. Previous articles have described detailed computational hypotheses about the role of cholinergic neuromodulation in enhancing the dynamics for encoding in cortical structures and the role of reduced cholinergic modulation in allowing consolidation of previously encoded information. This article will focus on addressing the broad scope of different modulatory effects observed within hippocampal circuits, highlighting the heterogeneity of cholinergic modulation in terms of the physiological effects of activation of muscarinic and nicotinic receptors and the heterogeneity of effects on different subclasses of neurons.

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'.

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.

Localization of the M2 muscarinic cholinergic receptor in dendrites, cholinergic terminals, and noncholinergic terminals in the rat basolateral amygdala: An ultrastructural analysis

Activation of M2 muscarinic receptors (M2Rs) in the rat anterior basolateral nucleus (BLa) is critical for the consolidation of memories of emotionally arousing events. The present investigation used immunocytochemistry at the electron microscopic level to determine which structures in the BLa express M2Rs. In addition, dual localization of M2R and the vesicular acetylcholine transporter protein (VAChT), a marker for cholinergic axons, was performed to determine whether M2R is an autoreceptor in cholinergic axons innervating the BLa. M2R immunoreactivity (M2R-ir) was absent from the perikarya of pyramidal neurons, with the exception of the Golgi complex, but was dense in the proximal dendrites and axon initial segments emanating from these neurons. Most perikarya of nonpyramidal neurons were also M2R-negative. About 95% of dendritic shafts and 60% of dendritic spines were M2 immunoreactive (M2R(+) ). Some M2R(+) dendrites had spines, suggesting that they belonged to pyramidal cells, whereas others had morphological features typical of nonpyramidal neurons. M2R-ir was also seen in axon terminals, most of which formed asymmetrical synapses. The main targets of M2R(+) terminals forming asymmetrical (putative excitatory) synapses were dendritic spines, most of which were M2R(+) . The main targets of M2R(+) terminals forming symmetrical (putative inhibitory or neuromodulatory) synapses were unlabeled perikarya and M2R(+) dendritic shafts. M2R-ir was also seen in VAChT(+) cholinergic terminals, indicating a possible autoreceptor role. These findings suggest that M2R-mediated mechanisms in the BLa are very complex, involving postsynaptic effects in dendrites as well as regulating release of glutamate, γ-aminobutyric acid, and acetylcholine from presynaptic axon terminals. J. Comp. Neurol. 524:2400-2417, 2016. © 2016 Wiley Periodicals, Inc.

Regulation of neuronal communication by G protein-coupled receptors

Neuronal communication plays an essential role in the propagation of information in the brain and requires a precisely orchestrated connectivity between neurons. Synaptic transmission is the mechanism through which neurons communicate with each other. It is a strictly regulated process which involves membrane depolarization, the cellular exocytosis machinery, neurotransmitter release from synaptic vesicles into the synaptic cleft, and the interaction between ion channels, G protein-coupled receptors (GPCRs), and downstream effector molecules. The focus of this review is to explore the role of GPCRs and G protein-signaling in neurotransmission, to highlight the function of GPCRs, which are localized in both presynaptic and postsynaptic membrane terminals, in regulation of intrasynaptic and intersynaptic communication, and to discuss the involvement of astrocytic GPCRs in the regulation of neuronal communication.

Chemogenetic synaptic silencing of neural circuits localizes a hypothalamus→midbrain pathway for feeding behavior

Brain function is mediated by neural circuit connectivity, and elucidating the role of connections is aided by techniques to block their output. We developed cell-type-selective, reversible synaptic inhibition tools for mammalian neural circuits by leveraging G protein signaling pathways to suppress synaptic vesicle release. Here, we find that the pharmacologically selective designer Gi-protein-coupled receptor hM4D is a presynaptic silencer in the presence of its cognate ligand clozapine-N-oxide (CNO). Activation of hM4D signaling sharply reduced synaptic release probability and synaptic current amplitude. To demonstrate the utility of this tool for neural circuit perturbations, we developed an axon-selective hM4D-neurexin variant and used spatially targeted intracranial CNO injections to localize circuit connections from the hypothalamus to the midbrain responsible for feeding behavior. This synaptic silencing approach is broadly applicable for cell-type-specific and axon projection-selective functional analysis of diverse neural circuits.

Extrasynaptic muscarinic acetylcholine receptors on neuronal cell bodies regulate presynaptic function in Caenorhabditis elegans

Acetylcholine (ACh) is a potent neuromodulator in the brain, and its effects on cognition and memory formation are largely performed through muscarinic acetylcholine receptors (mAChRs). mAChRs are often preferentially distributed on specialized membrane regions in neurons, but the significance of mAChR localization in modulating neuronal function is not known. Here we show that the Caenorhabditis elegans homolog of the M1/M3/M5 family of mAChRs, gar-3, is expressed in cholinergic motor neurons, and GAR-3-GFP fusion proteins localize to cell bodies where they are enriched at extrasynaptic regions that are in contact with the basal lamina. The GAR-3 N-terminal extracellular domain is necessary and sufficient for this asymmetric distribution, and mutation of a predicted N-linked glycosylation site within the N-terminus disrupts GAR-3-GFP localization. In transgenic animals expressing GAR-3 variants that are no longer asymmetrically localized, synaptic transmission at neuromuscular junctions is impaired and there is a reduction in the abundance of the presynaptic protein sphingosine kinase at release sites. Finally, GAR-3 can be activated by endogenously produced ACh released from neurons that do not directly contact cholinergic motor neurons. Together, our results suggest that humoral activation of asymmetrically localized mAChRs by ACh is an evolutionarily conserved mechanism by which ACh modulates neuronal function.

Muscarinic cholinergic receptor M1 in the rat basolateral amygdala: ultrastructural localization and synaptic relationships to cholinergic axons

Muscarinic neurotransmission in the anterior basolateral amygdalar nucleus (BLa) mediated by the M1 receptor (M1R) is critical for memory consolidation. Although knowledge of the subcellular localization of M1R in the BLa would contribute to an understanding of cholinergic mechanisms involved in mnemonic function, there have been no ultrastructural studies of this receptor in the BLa. In the present investigation, immunocytochemistry at the electron microscopic level was used to determine which structures in the BLa express M1R. The innervation of these structures by cholinergic axons expressing the vesicular acetylcholine transporter (VAChT) was also studied. All perikarya of pyramidal neurons were labeled, and about 90% of dendritic shafts and 60% of dendritic spines were M1R+. Some dendrites had spines suggesting that they belonged to pyramidal cells, whereas others had morphological features typical of interneurons. M1R immunoreactivity (M1R-ir) was also seen in axon terminals, most of which formed asymmetrical synapses. The main targets of M1R+ terminals forming asymmetrical synapses were dendritic spines, most of which were M1R+. The main targets of M1R+ terminals forming symmetrical synapses were M1R+ perikarya and dendritic shafts. About three-quarters of VAChT+ cholinergic terminals formed synapses; the main postsynaptic targets were M1R+ dendritic shafts and spines. In some cases M1R-ir was seen near the postsynaptic membrane of these processes, but in other cases it was found outside of the active zone of VAChT+ synapses. These findings suggest that M1R mechanisms in the BLa are complex, involving postsynaptic effects as well as regulating release of neurotransmitters from presynaptic terminals.

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.

Presynaptic muscarinic M(2) receptors modulate glutamatergic transmission in the bed nucleus of the stria terminalis

The anterolateral cell group of the bed nucleus of the stria terminalis (BNST(ALG)) serves as an important relay station in stress circuitry. Limbic inputs to the BNST(ALG) are primarily glutamatergic and activity-dependent changes in this input have been implicated in abnormal behaviors associated with chronic stress and addiction. Significantly, local infusion of acetylcholine (ACh) receptor agonists into the BNST trigger stress-like cardiovascular responses, however, little is known about the effects of these agents on glutamatergic transmission in the BNST(ALG). Here, we show that glutamate- and ACh-containing fibers are found in close association in the BNST(ALG). Moreover, in the presence of the acetylcholinesterase inhibitor, eserine, endogenous ACh release evoked a long-lasting reduction of the amplitude of stimulus-evoked EPSCs. This effect was mimicked by exogenous application of the ACh analog, carbachol, which caused a reversible, dose-dependent, reduction of the evoked EPSC amplitude, and an increase in both the paired-pulse ratio and coefficient of variation, suggesting a presynaptic site of action. Uncoupling of postsynaptic G-proteins with intracellular GDP-β-S, or application of the nicotinic receptor antagonist, tubocurarine, failed to block the carbachol effect. In contrast, the carbachol effect was blocked by prior application of atropine or M(2) receptor-preferring antagonists, and was absent in M(2)/M(4) receptor knockout mice, suggesting that presynaptic M(2) receptors mediate the effect of ACh. Immunoelectron microscopy studies further revealed the presence of M(2) receptors on axon terminals that formed asymmetric synapses with BNST neurons. Our findings suggest that presynaptic M(2) receptors might be an important modulator of the stress circuit and hence a novel target for drug development.

The cholinergic antagonist gymnodimine improves Aβ and tau neuropathology in an in vitro model of Alzheimer disease

Gymnodimine (GYM) is a marine phycotoxin with a macrocyclic imine structure, isolated from extracts of the dinoflagellate Karenia selliformis known to act as a cholinergic antagonist with subtype selectivity. However, no data on the chronic effects of this compound has been reported so far. In this work, we evaluated the effect of long term exposure of cortical neurons to gymnodimine in the progress of Alzheimer disease (AD) pathology in vitro. Treatment of cortical neurons with 50 nM gymnodimine decreased the intracellular amyloid beta (Aβ) accumulation and the levels of the hyperphosphorylated isoforms of tau protein recognized by AT8 and AT100 antibodies. These results are suggested to be mediated by the increase in the inactive isoform of the glycogen synthase kinase-3 (phospho GSK-3 Ser9), the decrease in the levels of the active isoform of the ERK1/2 kinase and the increase in acetylcholine (Ach) synthesis elicited by long term exposure of cortical neurons to the toxin. Moreover, gymnodimine decreased glutamate-induced neurotoxicity in vitro. Altogether these results indicate that the marine phycotoxin gymnodimine may constitute a valuable tool for the development of drugs to treat neurodegenerative diseases.

Preferential localization of muscarinic M1 receptor on dendritic shaft and spine of cortical pyramidal cells and its anatomical evidence for volume transmission

Acetylcholine (ACh) plays important roles for higher brain functions, including arousal, attention, and cognition. These effects are mediated largely by muscarinic acetylcholine receptors (mAChRs). However, it remains inconclusive whether the mode of ACh-mAChR signaling is synaptic, so-called "wired," transmission mediated by ACh released into the synaptic cleft, or nonsynaptic, so-called "volume," transmission by ambient ACh. To address this issue, we examined cellular and subcellular distribution of M(1), the most predominant mAChR subtype in the cerebral cortex and hippocampus, and pursued its anatomical relationship with cholinergic varicosities in these regions of adult mice. M(1) was highly expressed in glutamatergic pyramidal neurons, whereas it was low or undetectable in various GABAergic interneuron subtypes. M(1) was preferentially distributed on the extrasynaptic membrane of pyramidal cell dendrites and spines. Cholinergic varicosities often made direct contact to pyramidal cell dendrites and synapses. At such contact sites, however, synapse-like specialization was infrequent, and no particular accumulation was found at around contact sites for both M(1) and presynpatic active zone protein Bassoon. These features contrasted with those of the glutamatergic system, in which AMPA receptor GluA2 and metabotropic receptor mGluR5 were recruited to the synaptic or perisynaptic membrane, respectively, and Bassoon was highly accumulated in the presynaptic terminals. These results suggest that M(1) is so positioned to sense ambient ACh released from cholinergic varicosities at variable distances, and to enhance the synaptic efficacy and excitability of pyramidal cells. These molecular-anatomical arrangements will provide the evidence for volume transmission, at least in M(1)-mediated cortical cholinergic signaling.

Endocannabinoid-mediated control of synaptic transmission

The discovery of cannabinoid receptors and subsequent identification of their endogenous ligands (endocannabinoids) in early 1990s have greatly accelerated research on cannabinoid actions in the brain. Then, the discovery in 2001 that endocannabinoids mediate retrograde synaptic signaling has opened up a new era for cannabinoid research and also established a new concept how diffusible messengers modulate synaptic efficacy and neural activity. The last 7 years have witnessed remarkable advances in our understanding of the endocannabinoid system. It is now well accepted that endocannabinoids are released from postsynaptic neurons, activate presynaptic cannabinoid CB(1) receptors, and cause transient and long-lasting reduction of neurotransmitter release. In this review, we aim to integrate our current understanding of functions of the endocannabinoid system, especially focusing on the control of synaptic transmission in the brain. We summarize recent electrophysiological studies carried out on synapses of various brain regions and discuss how synaptic transmission is regulated by endocannabinoid signaling. Then we refer to recent anatomical studies on subcellular distribution of the molecules involved in endocannabinoid signaling and discuss how these signaling molecules are arranged around synapses. In addition, we make a brief overview of studies on cannabinoid receptors and their intracellular signaling, biochemical studies on endocannabinoid metabolism, and behavioral studies on the roles of the endocannabinoid system in various aspects of neural functions.

Synergistic effects of genetic variation in nicotinic and muscarinic receptors on visual attention but not working memory

It is widely appreciated that neurotransmission systems interact in their effects on human cognition, but those interactions have been little studied. We used genetics to investigate pharmacological evidence of synergisms in nicotinic/muscarinic interactions on cognition. We hypothesized that joint influences of nicotinic and muscarinic systems would be reflected in cognitive effects of normal variation in known SNPs in nicotinic (CHRNA4 rs1044396) and muscarinic (CHRM2 rs8191992) receptor genes. Exp. 1 used a task of cued visual search. The slope of the cue size/reaction time function showed a trend level effect of the muscarinic CHRM2 SNP, no effect of the nicotinic CHRNA4 SNP, but a significant interaction between the 2 SNPs. Slopes were steepest in individuals who were both CHRNA4 C/C and CHRM2 T/T homozygotes. To determine the specificity of this synergism, Exp. 2 assessed working memory for 1-3 locations over 3 s and found no significant effects on either SNP. Interpreting these results in light of Sarter's [Briand LA, et al. (2007) Modulators in concert for cognition: Modulator interactions in the prefrontal cortex. Prog Neurobiol 83:69-91] claims of tonic and phasic modes of cholinergic activity, we argue that reorienting attention to the target after invalid cues requires a phasic response, dependent on the nicotinic system, whereas orienting attention to valid cues requires a tonic response, dependent on the muscarinic system. Consistent with that, shifting and scaling after valid cues (tonic) were strongest in CHRNA4 C/C homozygotes who were also CHRM2 T/T homozygotes. This shows synergistic effects within the human cholinergic system.

Synthesis, trafficking, and localization of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors are members of the G-protein coupled receptor superfamily that are expressed in and regulate the function of neurons, cardiac and smooth muscle, glands, and many other cell types and tissues. The correct trafficking of membrane proteins to the cell surface and their subsequent localization at appropriate sites in polarized cells are required for normal cellular signaling and physiological responses. This review will summarize work on the synthesis and trafficking of muscarinic receptors to the plasma membrane and their localization at the cell surface.

Subcellular distribution of M2 muscarinic receptors in relation to dopaminergic neurons of the rat ventral tegmental area

Acetylcholine can affect cognitive functions and reward, in part, through activation of muscarinic receptors in the ventral tegmental area (VTA) to evoke changes in mesocorticolimbic dopaminergic transmission. Among the known muscarinic receptor subtypes present in the VTA, the M2 receptor (M2R) is most implicated in autoregulation and also may play a heteroreceptor role in regulation of the output of the dopaminergic neurons. We sought to determine the functionally relevant sites for M2R activation in relation to VTA dopaminergic neurons by examining the electron microscopic immunolabeling of M2R and the dopamine transporter (DAT) in the VTA of rat brain. The M2R was localized to endomembranes in DAT-containing somatodendritic profiles but showed a more prominent, size-dependent plasmalemmal location in nondopaminergic dendrites. M2R also was located on the plasma membrane of morphologically heterogenous axon terminals contacting unlabeled as well as M2R- or DAT-labeled dendrites. Some of these terminals formed asymmetric synapses resembling those of cholinergic terminals in the VTA. The majority, however, formed symmetric, inhibitory-type synapses or were apposed without recognized junctions. Our results provide the first ultrastructural evidence that the M2R is expressed, but largely not available for local activation, on the plasma membrane of VTA dopaminergic neurons. Instead, the M2R in this region has a distribution suggesting more indirect regulation of mesocorticolimbic transmission through autoregulation of acetylcholine release and changes in the physiological activity or release of other, largely inhibitory transmitters. These findings could have implications for understanding the muscarinic control of cognitive and goal-directed behaviors within the VTA.

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.

Nitric oxide, cAMP and the biphasic muscarinic modulation of ACh release at the lizard neuromuscular junction

In this study, we characterized the pharmacology and physiology of the automodulation of ACh release at the lizard neuromuscular junction (NMJ). The activation of muscarinic ACh receptors generated a biphasic modulation of synaptic transmission. Muscarine-induced activation of M3 receptors (0-12 min) decreased release, whereas M1 activation (> 12 min) enhanced release. Both phases of the biphasic effect are dependent on nitric oxide. However, cAMP acting via protein kinase A is also necessary for the M1 effect. In summary, we present a novel biphasic role for muscarine and implicate M3 receptors in the inhibition and M1 receptors in the enhancement of transmitter releaseat the cholinergic lizard NMJ.

Septal modulation of excitatory transmission in hippocampus

Application of the acetylcholinesterase inhibitor physostigmine to conventional hippocampal slices caused a significant reduction of field excitatory postsynaptic potentials (EPSPs) elicited by single pulse stimulation to the medial perforant path. Similar but smaller effects were obtained in the lateral perforant path and other excitatory pathways within hippocampus. The reductions were blocked by atropine, were not accompanied by evident changes in the EPSP waveform, and were eliminated by lesions to the cholinergic septo-hippocampal projections. Antidromic responses to mossy fiber stimulation, recorded in stratum granulosum, were not affected by the drug. However, paired-pulse facilitation was reliably increased, indicating that the depressed synaptic responses were secondary to reductions in transmitter release. The absence of cholinergic axo-axonic connections in the molecular layer suggests that physostigmine reduces presynaptic release by increasing retrograde signaling from the granule cells. In accord with this, an antagonist of the CB1 cannabinoid receptor eliminated the effects of physostigmine on synaptic responses, while an antagonist of the presynaptically located m2 muscarinic acetylcholine receptor did not. This is in contrast to previously reported effects involving application of cholinergic agonists, in which presynaptic inhibition likely results from direct activation of presynaptically located muscarinic receptors. In summary, it is proposed that the cholinergic inputs from the septum to the middle molecular layer modulate, via endocannabinoid release, the potency of the primary excitatory afferent of hippocampus.

Altered hippocampal muscarinic receptors in acetylcholinesterase-deficient mice

A primary therapeutic strategy for Alzheimer's disease includes acetylcholinesterase (AChE) inhibitors with the goal of enhancing cholinergic transmission. Stimulation of muscarinic acetylcholine receptors (mAChRs) by elevated levels of ACh plays a role in the effects of AChE inhibitors on cognition and behavior. However, AChE inhibitors only demonstrate modest symptomatic improvements. Chronic treatment with these drugs may cause mAChR downregulation and consequently limit the treatment efficacy. AChE knockout (-/-) mice were utilized in this study as a model for investigating the effects of selective, complete, and chronic diminished AChE activity on mAChR expression and function. In AChE -/- mice, the M(1), M(2), and M(4) mAChRs showed strikingly 50 to 80% decreased expression in brain regions associated with memory. In addition, mAChRs showed decreased presynaptic, cell surface, and dendritic distributions and increased localization to intracellular puncta. Furthermore, mAChR agonist-induced activation of extracellular signal-regulated kinase, a signaling pathway associated with synaptic plasticity and amyloidogenesis, is diminished in the hippocampus and cortex of AChE -/- mice. Therefore, chronic diminished ACh metabolism produces profound effects on mAChR expression and function. The alterations of mAChRs in AChE -/- mice suggest that mAChR downregulation may contribute to the limited efficacy of AChE inhibitors in Alzheimer's disease treatment.

Cell-specific extracellular signal-regulated kinase activation by multiple G protein-coupled receptor families in hippocampus

Several families of G protein-coupled receptors (GPCR) have been shown to activate extracellular signal-regulated kinase (ERK) in transfected cells and non-neuronal systems. However, little is known about GPCR activation of ERK in brain. Because ERK is an important component in the regulation of synaptic plasticity, in this study we examined ERK activation by three families of GPCR that respond to major neuromodulatory neurotransmitters in the hippocampus. We used an immunocytochemical approach to examine ERK activation by muscarinic acetylcholine (mAChR), metabotropic glutamate (mGluR), and beta-adrenergic (beta-AR) receptors in CA1 neurons of mouse hippocampal slices. Because these GPCR families comprise receptors coupling to each of the major heterotrimeric G proteins, we examined whether ERK activation differs according to G-protein coupling. By using immunocytochemistry, we were able to examine not only whether each family of receptors activates ERK, but also the cellular populations and subcellular distributions of activated ERK. We demonstrated that M1 mAChRs and group I mGluRs, both of which are Gq-coupled receptors, activate ERK in CA1 pyramidal neurons, although activation in response to mAChR is more robust. The G(i/o)-coupled group II mGluRs activate ERK in glia scattered throughout CA1, and Gs-coupled beta-AR receptors activate ERK in scattered interneurons. Thus, we demonstrated that GPCR coupling to Gq, G(i/o), and Gs all activate ERK in the hippocampus, although each does so with unique properties and distributions.

PET imaging of ischemic neuronal death in the hippocampus of living monkeys

The aim of the present study was to visualize postischemic hippocampal neuronal death in the living monkey brain, using a high-resolution positron emission tomography (PET) and novel radioligands. In preceding papers, we reported on postischemic hippocampal neuronal death in a model of Japanese monkeys (Macaca fuscata) undergoing a 20-min complete whole-brain ischemia. Using the same model here, we investigated the in vivo bindings of two radiotracers, [11C]Ro15-4513 (a type II benzodiazepine receptor ligand) and [11C](+)3-MPB (a muscarinic cholinergic receptor ligand), in the hippocampus on day 7 after ischemia, as compared to the normal hippocampus. A significant decrease in the in vivo binding of [11C]Ro154513 and [11C(+)3-MPB was observed in the postischemic monkey hippocampus on day 7 after ischemia compared to controls. Light and electron microscopic analyses of postischemic CA1 neurons showed typical features of coagulation necrosis, as associated with a marked reduction of postsynaptic densities and presynaptic vesicles. These results suggest that semiquantification of hippocampal neuronal death is possible in the living primate brain using PET, and that the same procedures can be applied for evaluating neuronal cell loss in patients with ischemic injuries and/or dementia.

Pilocarpine-induced seizure-like activity with increased BNDF and neuropeptide Y expression in organotypic hippocampal slice cultures

Organotypic hippocampal slice cultures were treated with the muscarinic agonist pilocarpine to study induced seizure-like activity and changes in neurotrophin and neuropeptide expression. For establishment of a seizure-inducing protocol, 2-week-old cultures derived from 6-8-day-old rats were exposed to 0.1 mM to 5 mM of pilocarpine for 4 h to 7 days. Other cultures were treated with pilocarpine for 7 days and left for 7-14 days in normal medium. Age-matched, non-treated cultures served as controls. Intracellular recordings from CA1 pyramidal cells revealed increased spontaneous activity in 31 of 35 cultures superfused with 0.1 or 5 mM pilocarpine. Epileptiform discharges were recorded in 17 of the 31 cultures, and 19 displayed frequencies specifically in the 6-12-Hz (Theta rhythm) range when superfused with pilocarpine. The pilocarpine effect was blocked by simultaneous superfusion with the muscarinic receptor antagonist atropine (100 microM). Regardless of dose and exposure time, the pilocarpine treatment induced very limited neuronal cell death, recorded as cellular propidium iodide uptake. Cultures exposed to 5 mM pilocarpine for up to 7 days displayed increased BDNF expression when analyzed by Western blot and ELISA. This BDNF increase correlated with increased neuropeptide Y immunoreactivity, known to accompany seizure activity. Addition of BDNF (200 ng/ml) to otherwise untreated cultures also upregulated NPY expression. The pilocarpine-induced seizure-like activity in hippocampal slice cultures, with concomitant increase in BDNF and NPY expression, is compared with in vivo observations and discussed in terms of the potential use of the easily accessible slice cultures in experimental seizure research.

A CalDAG-GEFI/Rap1/B-Raf cassette couples M(1) muscarinic acetylcholine receptors to the activation of ERK1/2

In this study we examine signaling pathways linking the M(1) subtype of muscarinic acetylcholine receptor (M(1) mAChR) to activation of extracellular signal-regulated kinases (ERK) 1 and 2 in neuronal PC12D cells. We first show that activation of ERK1/2 by the M(1) mAChR agonist carbachol takes place primarily via a Ras-independent pathway that depends largely upon Rap1, another small GTP-binding protein in the Ras family. Rap1 in turn activates B-Raf, an upstream activator of ERK1/2. Consistent with these results, carbachol was found to activate Rap1 more potently than Ras. Similar to other small GTP-binding proteins, activation of Rap1 requires a guanine nucleotide exchange factor (GEF) to promote its conversion from the GDP- to GTP-bound form. Using specific antibodies, we show that a recently identified Rap1 GEF, calcium- and diacylglycerol-regulated guanine nucleotide exchange factor I (CalDAG-GEFI), is expressed in PC12D cells and that carbachol stimulates the formation of a complex containing CalDAG-GEFI, Rap1, and activated B-Raf. Finally, we show that expression of CalDAG-GEFI antisense RNA largely blocks carbachol-stimulated activation of hemagglutinin (HA)1-tagged B-Raf and formation of the CalDAG-GEFI/Rap1/HA1-tagged B-Raf complex. Together, these data define a novel signaling pathway for M(1) mAChR, where increases in Ca(2+) and diacylglycerol stimulate the sequential activation of CalDAG-GEFI, Rap1, and B-Raf, resulting in the activation of MEK and ERK1/2.

Identification of a basolateral sorting signal for the M3 muscarinic acetylcholine receptor in Madin-Darby canine kidney cells

Muscarinic acetylcholine receptors (mAChRs) can be differentially localized in polarized cells. To identify potential sorting signals that mediate mAChR targeting, we examined the sorting of mAChRs in Madin-Darby canine kidney cells, a widely used model system. Expression of FLAG-tagged mAChRs in polarized Madin-Darby canine kidney cells demonstrated that the M(2) subtype is sorted apically, whereas M(3) is targeted basolaterally. Expression of M(2)/M(3) receptor chimeras revealed that a 21-residue sequence, Ser(271)-Ser(291), from the M(3) third intracellular loop contains a basolateral sorting signal. Substitution of sequences containing the M(3) sorting signal into the homologous regions of M(2) was sufficient to confer basolateral localization to this apical receptor. Sequences containing the M(3) sorting signal also conferred basolateral targeting to M(2) when added to either the third intracellular loop or the C-terminal cytoplasmic tail. Furthermore, addition of a sequence containing the M(3) basolateral sorting signal to the cytoplasmic tail of the interleukin-2 receptor alpha-chain caused significant basolateral targeting of this heterologous apical protein. The results indicate that the M(3) basolateral sorting signal is dominant over apical signals in M(2) and acts in a position-independent manner. The M(3) sorting signal represents a novel basolateral targeting motif for G protein-coupled receptors.

A predicted cortical serotonergic/cholinergic/GABAergic interface as a site of pathology in schizophrenia

1. The pathological process that precipitates schizophrenia has yet to be identified. However, many lines of evidence suggest that a change in the functioning of the frontal cortex is an important abnormality that underlies schizophrenia. 2. Studies in Brodmann's area 9, obtained post-mortem, have shown changes in 5-hydroxytryptamine 5-HT2A, muscarinic M1 and GABA(A) receptors in tissue from subjects with schizophrenia. 3. Animal studies suggest a site in the cortex where there would be an interaction between serotonergic and cholinergic innervation and that this interaction would involve the 5-HT2A and the M1 receptor. This site, in turn, would be a potent modulator of GABA activity and, hence, levels of GABA(A) receptors. 4. From combining these data, a theoretical site is proposed that, if proven to exist in human cortex, is likely to be central to the pathology of that illness.

Subcellular localization of m2 muscarinic receptors in GABAergic interneurons of the olfactory bulb

We analysed the ultrastructural distribution of the m2 muscarinic receptor (m2R) in the rat olfactory bulb (OB) using immunohistochemical techniques and light and electron microscopy. m2R was differentially distributed within the cellular compartments of gamma-aminobutyric acid (GABA)ergic bulbar interneurons. It is located in the gemmules of granule cells and in the synaptic loci of the interneurons of the external plexiform layer, suggesting that m2R activation could modulate the release of GABA from these interneurons onto principal cells by a presynaptic mechanism. By contrast, the receptor appears in the somata and dendritic trunks of second-order short-axon interneurons located in the inframitral layers, suggesting that postsynaptic muscarinic activation in these cells could elicit the inhibition of granule cells, leading to a disinhibition of principal cells. We also detail the anatomical substrate for a new putative muscarinic modulation that has not been previously described, and that could influence the reception of sensory information within the olfactory glomeruli. m2R appears in a subset of GABAergic/dopaminergic juxtaglomerular cells innervated by olfactory axons but is absent in juxtaglomerular cells that do not receive sensory inputs. This finding suggests that m2R activation could modify, through dopaminergic local circuits, the strength of olfactory nerve inputs onto principal cells. Activation of the muscarinic receptor may modulate the olfactory information encoding within olfactory glomeruli and may facilitate the bulbar transmission to superior centres influencing the GABA release by presynaptic and postsynaptic mechanisms. Taken together, our data provide the neuroanatomical basis for a complex action of m2R at different levels in the mammalian OB.

Muscarinic toxins: novel pharmacological tools for the muscarinic cholinergic system

Muscarinic receptors are widely spread throughout the body, and are involved in the regulation of fundamental physiological processes, like the modulation of the heart rate, control of motor systems and modulation of learning and memory. In the central nervous system the cholinergic transmission is mainly mediated by muscarinic receptors; there are five subtypes that are all expressed in the brain of mammals (m1-m5). There are regional differences in their concentrations in the brain and more than one subtype is expressed in the same cell. It has been difficult to study their localization and function in vivo due to the lack of ligands that exclusively act on one subtype of the receptor. We studied the action of the muscarinic toxins MT1, MT2 and MT3, from the venom of the snake Dendroaspis angusticeps, on muscarinic receptors, by using the classical muscarinic radioligand 3H-NMS as reporter of the inhibition of its own binding, to either native or cloned receptors. We have also studied the in vivo effects on memory retention of the injection of the toxins into discrete brain regions. The muscarinic toxins appear to be invaluable tools to study receptor pharmacology, physiology and structure/function relationships. They would enable the design of new, more selective, pharmacological agents.

Activation of the genetically defined m1 muscarinic receptor potentiates N-methyl-D-aspartate (NMDA) receptor currents in hippocampal pyramidal cells

Evidence suggests that cholinergic input to the hippocampus plays an important role in learning and memory and that degeneration of cholinergic terminals in the hippocampus may contribute to the memory loss associated with Alzheimer's disease. One of the more prominent effects of cholinergic agonists on hippocampal physiology is the potentiation of N-methyl-D-aspartate (NMDA)-receptor currents by muscarinic agonists. Here, we employ traditional pharmacological reagents as well as m1-toxin, an m1 antagonist with unprecedented selectivity, to demonstrate that this potentiation of NMDA-receptor currents in hippocampal CA1 pyramidal cells is mediated by the genetically defined m1 muscarinic receptor. Furthermore, we demonstrate the colocalization of the m1 muscarinic receptor and the NR1a NMDA receptor subunit at the electron microscopic level, indicating a spatial relationship that would allow for physiological interactions between these two receptors. This work demonstrates that the m1-muscarinic receptor gene product modulates excitatory synaptic transmission, and it has important implications in the study of learning and memory as well as the design of drugs to treat neurodegenerative diseases such as Alzheimer's.