mRNA for the m4 muscarinic receptor subtype is expressed in adult rat brain cholinergic neurons

Optimized Administration of the M4 PAM VU0467154 Demonstrates Broad Efficacy, but Limited Effective Concentrations in Mecp2+/- Mice

Hypofunction of cholinergic circuits and diminished cholinergic tone have been associated with the neurodevelopmental disorder Rett syndrome (RTT). Specifically, deletion of Mecp2 in cholinergic neurons evokes the same social and cognitive phenotypes in mice seen with global Mecp2 knockout, and decreased choline acetyltransferase activity and vesamicol binding have been reported in RTT autopsy samples. Further, we recently identified significant decreases in muscarinic acetylcholine receptor subtype 4 (M₄) expression in both the motor cortex and cerebellum of RTT patient autopsies and established proof of concept that an acute dose of the positive allosteric modulator (PAM) VU0467154 (VU154) rescued phenotypes in Mecp2^+/- mice. Here, we expand the assessment of M₄ PAMs in RTT to address clinically relevant questions of tolerance, scope of benefit, dose response, chronic treatment, and mechanism. We show that VU154 has efficacy on anxiety, social preference, cognitive, and respiratory phenotypes in Mecp2^+/- mice; however, the therapeutic range is narrow, with benefits seen at 3 mg/kg concentrations, but not 1 or 10 mg/kg. Further, sociability was diminished in VU154-treated Mecp2^+/- mice, suggestive of a potential adverse effect. Compound efficacy on social, cognitive, and respiratory phenotypes was conserved with a 44-day treatment paradigm, with the caveat that breath rate was moderately decreased with chronic treatment in Mecp2^+/+ and Mecp2^+/- mice. VU154 effects on respiratory function correlated with an increase in Gsk3β inhibition in the brainstem. These results identify the core symptom domains where efficacy and adverse effects may present with M₄ administration in RTT model mice and advocate for the continued evaluation as potential RTT therapeutics.

Identification of the Risk Genes Associated With Vulnerability to Addiction: Major Findings From Transgenic Animals

Understanding risk factors for substance use disorders (SUD) can facilitate medication development for SUD treatment. While a rich literature exists discussing environmental factors that influence SUD, fewer articles have focused on genetic factors that convey vulnerability to drug use. Methods to identify SUD risk genes include Genome-Wide Association Studies (GWAS) and transgenic approaches. GWAS have identified hundreds of gene variants or single nucleotide polymorphisms (SNPs). However, few genes identified by GWAS have been verified by clinical or preclinical studies. In contrast, significant progress has been made in transgenic approaches to identify risk genes for SUD. In this article, we review recent progress in identifying candidate genes contributing to drug use and addiction using transgenic approaches. A central hypothesis is if a particular gene variant (e.g., resulting in reduction or deletion of a protein) is associated with increases in drug self-administration or relapse to drug seeking, this gene variant may be considered a risk factor for drug use and addiction. Accordingly, we identified several candidate genes such as those that encode dopamine D₂ and D₃ receptors, mGluR₂, M₄ muscarinic acetylcholine receptors, and α₅ nicotinic acetylcholine receptors, which appear to meet the risk-gene criteria when their expression is decreased. Here, we describe the role of these receptors in drug reward and addiction, and then summarize major findings from the gene-knockout mice or rats in animal models of addiction. Lastly, we briefly discuss future research directions in identifying addiction-related risk genes and in risk gene-based medication development for the treatment of addiction.

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'.

Effects of dopamine D1-like and D2-like antagonists on cocaine discrimination in muscarinic receptor knockout mice

Muscarinic and dopamine brain systems interact intimately, and muscarinic receptor ligands, like dopamine ligands, can modulate the reinforcing and discriminative stimulus (S(D)) effects of cocaine. To enlighten the dopamine/muscarinic interactions as they pertain to the S(D) effects of cocaine, we evaluated whether muscarinic M1, M2 or M4 receptors are necessary for dopamine D1 and/or D2 antagonist mediated modulation of the S(D) effects of cocaine. Knockout mice lacking M1, M2, or M4 receptors, as well as control wild-type mice and outbred Swiss-Webster mice, were trained to discriminate 10mg/kg cocaine from saline in a food-reinforced drug discrimination procedure. Effects of pretreatments with the dopamine D1 antagonist SCH 23390 and the dopamine D2 antagonist eticlopride were evaluated. In intact mice, both SCH 23390 and eticlopride attenuated the cocaine discriminative stimulus effect, as expected. SCH 23390 similarly attenuated the cocaine discriminative stimulus effect in M1 knockout mice, but not in mice lacking M2 or M4 receptors. The effects of eticlopride were comparable in each knockout strain. These findings demonstrate differences in the way that D1 and D2 antagonists modulate the S(D) effects of cocaine, D1 modulation being at least partially dependent upon activity at the inhibitory M2/M4 muscarinic subtypes, while D2 modulation appeared independent of these systems.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Cortical neuronal activity does not regulate sleep homeostasis

The neural substrate of sleep homeostasis is unclear, but both cortical and subcortical structures are thought to be involved in sleep regulation. To test whether prior neuronal activity in the cortex or in subcortical regions drives sleep rebound, we systemically administered atropine (100mg/kg) to rats, producing a dissociated state with slow-wave cortical electroencephalogram (EEG) but waking behavior (e.g. locomotion). Atropine injections during the light period produced 6h of slow-wave cortical EEG but also subcortical arousal. Afterward, rats showed a significant increase in non-rapid eye movement (NREM) sleep, compared to the same period on a baseline day. Consistent with the behavioral and cortical EEG state produced by systemic atropine, c-Fos expression was low in the cortex but high in multiple subcortical arousal systems. These data suggest that subcortical arousal and behavior are sufficient to drive sleep homeostasis, while a sleep-like pattern of cortical activity is not sufficient to satisfy sleep homeostasis.

Enhanced self-administration of alcohol in muscarinic acetylcholine M4 receptor knockout mice

Modulation of cholinergic neurotransmission via nicotinic acetylcholine receptors is known to alter alcohol-drinking behavior. It is not known if muscarinic acetylcholine receptor subtypes have similar effects. The muscarinic M4 receptor is highly expressed in the brain reinforcement system and involved in regulation of cholinergic and dopaminergic transmission. Here we investigate, for the first time, the role of the M4 receptor in alcohol consumption using M4 knockout (M4(-/-)) and wild-type (M4(+/+)) mice. Experimentally naïve M4(-/-) and M4(+/+) mice were trained to orally self-administer 5%, 8% and 10% alcohol in 60min sessions, 6 days/week, after having undergone a standard sucrose fading training procedure on a fixed ratio schedule. The mice were further subjected to an extinction period followed by a 1 day reinstatement trial. M4(-/-) mice consumed more alcohol at 5% and 8% compared to their M4(+/+) littermates. The highest alcohol concentration used (10%) did not immediately result in divergent drinking patterns, but after 4 weeks of 10% alcohol self-administration, baseline levels as well as a pattern of M4(-/-) mice consuming more alcohol than their M4(+/+) controls were re-established. Moreover, the M4(-/-) mice displayed a reduced capacity to extinguish their alcohol-seeking behavior. Taken together, alcohol consumption is elevated in M4(-/-) mice, indicating that the M4 receptor is involved in mediating the reinforcing effects of alcohol. The M4 receptor should be further explored as a potential target for pharmacological (positive allosteric modulators or future agonists) treatment of alcohol use disorders.

Interactions between the neuromodulatory systems and the amygdala: exploratory survey using the Allen Mouse Brain Atlas

Neuromodulatory systems originate in nuclei localized in the subcortical region of the brain and control fundamental behaviors by interacting with many areas of the central nervous system. An exploratory survey of the cholinergic, dopaminergic, noradrenergic, and serotonergic receptor expression energy in the amygdala, and in the neuromodulatory areas themselves was undertaken using the Allen Mouse Brain Atlas. The amygdala was chosen because of its importance in cognitive behavior and its bidirectional interaction with the neuromodulatory systems. The gene expression data of 38 neuromodulatory receptor subtypes were examined across 13 brain regions. The substantia innominata of the basal forebrain and regions of the amygdala had the highest amount of receptor expression energy for all four neuromodulatory systems examined. The ventral tegmental area also displayed high receptor expression of all four neuromodulators. In contrast, the locus coeruleus displayed low receptor expression energy overall. In general, cholinergic receptor expression was an order of magnitude greater than other neuromodulatory receptors. Since the nuclei of these neuromodulatory systems are thought to be the source of specific neurotransmitters, the projections from these nuclei to target regions may be inferred by receptor expression energy. The comprehensive analysis revealed many connectivity relations and receptor localization that had not been previously reported. The methodology presented here may be applied to other neural systems with similar characteristics, and to other animal models as these brain atlases become available.

Knockouts reveal overlapping functions of M(2) and M(4) muscarinic receptors and evidence for a local glutamatergic circuit within the laterodorsal tegmental nucleus

Cholinergic neurons in the laterodorsal tegmental (LDT) and peduncolopontine tegmental (PPT) nuclei regulate reward, arousal, and sensory gating via major projections to midbrain dopamine regions, the thalamus, and pontine targets. Muscarinic acetylcholine receptors (mAChRs) on LDT neurons produce a membrane hyperpolarization and inhibit spike-evoked Ca(2+) transients. Pharmacological studies suggest M(2) mAChRs are involved, but the role of these and other localized mAChRs (M(1-)-M(4)) has not been definitively tested. To identify the underlying receptors and to circumvent the limited receptor selectivity of available mAChR ligands, we used light- and electron-immunomicroscopy and whole cell recording with Ca(2+) imaging in brain slices from knockout mice constitutively lacking either M(2), M(4), or both mAChRs. Immunomicroscopy findings support a role for M(2) mAChRs, since cholinergic and noncholinergic LDT and pedunculopontine tegmental neurons contain M(2)-specific immunoreactivity. However, whole cell recording revealed that the presence of either M(2) or M(4) mAChRs was sufficient, and that the presence of at least one of these receptors was required for these carbachol actions. Moreover, in the absence of M(2) and M(4) mAChRs, carbachol elicited both direct excitation and barrages of spontaneous excitatory postsynaptic potentials (sEPSPs) in cholinergic LDT neurons mediated by M(1) and/or M(3) mAChRs. Focal carbachol application to surgically reduced slices suggest that local glutamatergic neurons are a source of these sEPSPs. Finally, neither direct nor indirect excitation were knockout artifacts, since each was detected in wild-type slices, although sEPSP barrages were delayed, suggesting M(2) and M(4) receptors normally delay excitation of glutamatergic inputs. Collectively, our findings indicate that multiple mAChRs coordinate cholinergic outflow from the LDT in an unexpectedly complex manner. An intriguing possibility is that a local circuit transforms LDT muscarinic inputs from a negative feedback signal for transient inputs into positive feedback for persistent inputs to facilitate different firing patterns across behavioral states.

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.

Increased cocaine self-administration in M4 muscarinic acetylcholine receptor knockout mice

RATIONALE

The reinforcing effects of cocaine are mediated by the mesolimbic dopamine system. Behavioral and neurochemical studies have shown that the cholinergic muscarinic M(4) receptor subtype plays an important role in regulation of dopaminergic neurotransmission.

OBJECTIVES

Here we investigated for the first time the involvement of M(4) receptors in the reinforcing effects of cocaine using chronic intravenous cocaine self-administration in extensively backcrossed M(4) receptor knockout (M(4) (-/-)) mice.

METHODS

We evaluated acquisition of cocaine self-administration in experimentally naïve mice. Both cocaine self-administration and food-maintained operant behavior were evaluated under fixed ratio 1 (FR 1) and progressive ratio (PR) schedules of reinforcement. In addition, cocaine-induced dopamine release and cocaine-induced hyperactivity were evaluated.

RESULTS

M(4) (-/-) mice earned significantly more cocaine reinforcers and reached higher breaking points than their wild-type littermates (M(4) (+/+)) at intermediate doses of cocaine under both FR 1 and PR schedules of reinforcement. Under the PR schedule, M(4) (-/-) mice exhibited significantly higher response rates at the lowest liquid food concentration. In accordance with these results, cocaine-induced dopamine efflux in the nucleus accumbens and hyperlocomotion were increased in M(4) (-/-) mice compared to M(4) (+/+) mice.

CONCLUSIONS

Our data suggest that M(4) receptors play an important role in regulation of the reward circuitry and may serve as a new target in the medical treatment of drug addiction.

Antipsychotic-induced catalepsy is attenuated in mice lacking the M4 muscarinic acetylcholine receptor

A delicate balance exists between the central dopaminergic and cholinergic neurotransmitter systems with respect to motor function. An imbalance can result in motor dysfunction as observed in Parkinson's disease patients and in patients treated with antipsychotic compounds. Cholinergic receptor antagonists can alleviate extrapyramidal symptoms in Parkinson's disease and motor side effects induced by antipsychotics. The effects of anticholinergics are mediated by muscarinic receptors of which five subtypes (M(1)-M(5)) exist. Muscarinic M(4) receptors are found at high concentrations in motor parts of the striatum, suggesting a role for muscarinic M(4) receptors in the motor side effects of antipsychotics, and in the alleviation of these side effects by anticholinergics. Here we investigated the potential role of the muscarinic M(4) receptor in catalepsy induced by antipsychotics (haloperidol and risperidone) as well as the anti-cataleptic effects of the non-selective anticholinergic drug scopolamine in fully backcrossed muscarinic M(4) receptor knockout mice. The drug-induced catalepsy was strongly attenuated, but not abolished, in M(4) knockout mice as compared to wild-type controls. Scopolamine further attenuated the cataleptic response in M(4) knockout mice, suggesting that non-M(4) muscarinic receptors also participate in the anti-cataleptic effects. In conclusion, these data indicate an important role for M(4) receptors in antipsychotic-induced motor side effects and suggest that M(4) receptors could be a target for future pharmacological treatment of antipsychotic-induced as well as idiopathic parkinsonism.

Coexpression of glutamate vesicular transporter (VGLUT1) and choline acetyltransferase (ChAT) proteins in fetal rat hippocampal neurons in culture

A very small population of choline acetyltransferase (ChAT) immunoreactive cells is observed in all layers of the adult hippocampus. This is the intrinsic source of the hippocampal cholinergic innervation, in addition to the well-established septo-hippocampal cholinergic projection. This study aimed at quantifying and identifying the origin of this small population of ChAT-immunoreactive cells in the hippocampus at early developmental stages, by culturing the fetal hippocampal neurons in serum-free culture and on a patternable, synthetic silane substrate N-1 [3-(trimethoxysilyl) propyl] diethylenetriamine. Using this method, a large proportion of glutamatergic (glutamate vesicular transporter, VGLUT1-immunoreactive) neurons, a small fraction of GABAergic (GABA-immunoreactive) neurons, and a large proportion of cholinergic (ChAT-immunoreactive) neurons were observed in the culture. Interestingly, most of the glutamatergic neurons that expressed glutamate vesicular transporter (VGLUT1) also co-expressed ChAT proteins. On the contrary, when the cultures were double-stained with GABA and ChAT, colocalization was not observed. Neonatal and adult rat hippocampal neurons were also cultured to verify whether these more mature neurons also co-express VGLUT1 and ChAT proteins in culture. Colocalization of VGLUT1 and ChAT in these relatively more mature neurons was not observed. One possible explanation for this observation is that the neurons have the ability to synthesize multiple neurotransmitters at a very early stage of development and then with time follows a complex, combinatorial strategy of electrochemical coding to determine their final fate.

M4 muscarinic receptors regulate the dynamics of cholinergic and dopaminergic neurotransmission: relevance to the pathophysiology and treatment of related CNS pathologies

Dopaminergic dysfunction is an important pathogenetic factor for brain pathologies such as Parkinson's disease, ADHD, schizophrenia, and addiction as well as for metabolic disorders and anorexia. Dopaminergic neurons projecting from the midbrain to forebrain regions, such as the nucleus accumbens and the prefrontal cortex, regulate motor and cognitive functions and coordinate the patterned response of the organism to sensory, affective, and rewarding stimuli. In this study, we showed that dopaminergic neurotransmission is highly dependent on M4 cholinergic muscarinic receptor function. Using in vivo microdialysis, we found elevated dopamine (DA) basal values and enhanced DA response to psychostimulants in the nucleus accumbens of M4 knockout mice. We also demonstrated impaired homeostatic control of cholinergic activity that leads to increased basal acetylcholine efflux in the midbrain of these animals. Thus, loss of M4 muscarinic receptor control of cholinergic function effectuates a state of dopaminergic hyperexcitability. This may be responsible for pathological conditions, in which appetitive motivation as well as affective and cognitive processing is impaired. We propose that M4 receptor agonists could represent an innovative strategy for the treatment of pathologies associated with hyperdopaminergia.

Muscarinic cholinergic receptor subtypes in the hippocampus of aged rats

In spite of the suggestion of impaired muscarinic function in adult-onset cognitive disorders, data on the expression of muscarinic receptors in the hippocampus as a function of age are inconsistent. One reason may be that the majority of investigations were unable to differentiate the five brain muscarinic receptors subtypes. In this study, using a protocol based on a combination of both kinetic and equilibrium binding approaches, we have assessed the expression and the density of M1-M5 muscarinic cholinergic receptors in the hippocampus of Fisher 344 rats aged 6, 15 and 22 months. An age-related decrease of the density of M1 receptor was found in pyramidal neurons of the CA1 subfield. In this area, other subtypes of muscarinic receptors were unchanged with the exception of a loss of M2 receptor in the radial layer. In the CA3 subfield, receptor changes involved M2, M3 and M5 subtypes, whereas in the dentate gyrus, the main changes affected M1 and M2 receptors of the granular layer and M2 and M3 receptors of the molecular layer. The above findings indicate that analysis of age-related changes of different muscarinic cholinergic receptors might represent a useful contribution to identifying the basis of cholinergic neurotransmission impairment in adult-onset cognitive dysfunction.

Synaptic contacts between serotonergic and cholinergic neurons in the rat dorsal raphe nucleus and laterodorsal tegmental nucleus

We examined synaptic connectivity between cholinergic and serotonergic neurons in the dorsal raphe nucleus and the laterodorsal tegmental nucleus of the rat. To this purpose we employed two variations (the combination of pre-embedding immunogold-silver intensification with avidin-biotin-peroxidase complex technique and the combination of avidin-biotin-peroxidase/3, 3'-diaminobenzidine/silver-gold intensification with avidin-biotin-peroxidase/3,3'-diaminobenzidine reaction) of a double pre-embedding immunoelectron procedure, using primary antibodies against vesicular acetylcholine transporter and serotonin. At the light-microscopic level, serotonin-like immunoreactive neurons in the dorsal raphe nucleus appeared as reddish black and vesicular acetylcholine transporter-like immunoreactive axon terminals were brown colored using a combination of pre-embedding immunogold-silver technique and avidin-biotin-peroxidase complex technique. Serotonin-like immunoreactive fibers projected to the laterodorsal tegmental nucleus. At the electron microscopy level, with both methods we observed in the dorsal raphe nucleus vesicular acetylcholine transporter-immunopositive axon terminals in synaptic contact with serotonin-like immunoreactive dendrites and, to a lesser degree, with serotonin-like immunoreactive cell bodies. These synapses usually were of the symmetrical type. Occasionally we noted, next to vesicular acetylcholine transporter-immunopositive axon terminals, also immunonegative terminals synapsing with the serotonin-like immunoreactive dendrites. In the laterodorsal tegmental nucleus we found serotonin-like immunoreactive axon terminals and immunonegative terminals forming synapses with vesicular acetylcholine transporter-immunoreactive dendrites. Most synapses formed by the serotonin-like immunopositive terminals were of the asymmetrical type. Our results suggest that serotonergic neurons in the dorsal raphe nucleus and cholinergic neurons in the laterodorsal tegmental nucleus may reciprocally influence each other by means of synaptic connectivity. Such connectivity may serve to regulate pain sensation, or be involved in the regulation of the sleeping-waking cycle.

Hippocampal neurotrophin and trk receptor mRNA levels are altered by local administration of nicotine, carbachol and pilocarpine

Cholinergic receptor agonists nicotine (nicotinic), carbachol (nicotinic/muscarinic) and pilocarpine (muscarinic) were administered into the hippocampus and mRNA levels of neurotrophins and their receptors determined using in situ hybridisation. Drug doses were carefully chosen to avoid the potentially confounding effects of seizure and cell death. Nicotine caused a long-lasting increase in nerve growth factor (NGF) mRNA in all subfields of the hippocampus. The increase was evident from 24 h up to 72 h after drug administration. This increase was dependent on excitatory amino acid neurotransmission as it was blocked by administration of an AMPA or NMDA receptor antagonist. In contrast, carbachol and pilocarpine produced a transient increase in NGF mRNA levels present 4-8 h after drug administration. Pilocarpine caused a transient increase in hippocampal brain-derived neurotrophic factor (BDNF) levels, with carbachol and nicotine showing the same trend. Nicotine and carbachol caused transient decreases in NT-3 mRNA levels in dentate gyrus and CA2 with pilocarpine showing a similar trend. Increases in mRNA encoding full-length trkB were seen 8 h after nicotine, with nicotine also causing elevations in a mRNA encoding a truncated isoform (trkB.T2). TrkC mRNA was not altered by any of the conditions used. The study suggests that muscarinic and nicotinic receptor activation in the hippocampus causes transient changes in all of the neurotrophins, but that NGF levels are selectively up-regulated by nicotinic receptor stimulation. The reciprocal interaction between NGF and ascending cholinergic systems may be a component of the cognitive enhancing effects of nicotine.

Control elements of muscarinic receptor gene expression

Studies describing the structures of the M1, M2 and M4 muscarinic acetylcholine receptors (mAChR) genes and the genetic elements that control their expression are reviewed. In particular, we focus on the role of the neuron-restrictive silencer element/restriction element-1 (NRSE/RE-1) in the regulation of the M4 mAChR gene. The NRSE/RE-1 was first identified as a genetic control element that prevents the expression of the SCG-10 and type II sodium channel (NaII) genes in non-neuronal cells in culture. The NRSE/RE-1 inhibits gene expression by binding the repressor/silencer protein NRSF/REST, which is present in many non-neuronal cell lines and tissues. Our studies show that although the expression of the M4 mAChR gene is inhibited by NRSF/REST, this inhibition is not always complete. Rather, the efficiency of silencing by NRSF/REST is different in different cells. A plausible explanation for this differential silencing is that the NRSF/RE-1 interacts with distinct sets of promoter binding proteins in different types of cells. We hypothesize that modulation of NRSF/REST silencing activity by these proteins contributes to the cell-specific pattern of expression of the M4 mAChR in neuronal and non-neuronal cells. Recent studies that suggest a more complex role for the NRSE/RE-1 in regulating gene expression are also discussed.

The role of N-, Q- and R-type Ca2+ channels in feedback inhibition of ACh release from rat basal forebrain neurones

1. The Ca2+ channel subtypes controlling ACh release from basal forebrain neurones and the ionic basis underlying muscarinic receptor-mediated autoinhibition were studied using skeletal myoballs to detect ACh release from individual rat basal forebrain neurones in culture. 2. Somatic Ca2+ currents evoked using a simulated action potential waveform revealed that Ca2+ entry was primarily through N-, Q- and to a lesser extent R-, T- and L-type Ca2+ channels. 3. Muscarine (10 microM) inhibited N- and Q- but not R-, T- or L-type somatic Ca2+ channels. Agonist inhibition was totally blocked by pre-treatment with pertussis toxin (500 ng ml-1). 4. ACh release from discrete sites along basal forebrain neurites (1. 2 mM extracellular Ca2+) could be largely abolished by blocking Ca2+ entry through either N-type or Q-type Ca2+ channels. Inhibition of Ca2+ entry through L- or T-type channels had no effect upon release. Following inhibition of either N- or Q-type Ca2+ channels, release could be restored to near control levels by raising [Ca2+]o. After selectively blocking N-, Q-, L- and T-type channels, low levels of release could still be evoked as a result of Ca2+ entry through R-type Ca2+ channels. 5. Muscarinic receptor activation reversibly inhibited ACh release due to Ca2+ entry through N-, Q- and R-type Ca2+ channels. In contrast, inhibition of inwardly rectifying K+ channels using Ba2+ (3-10 microM) or substance P (0.03-0.1 microM), or block of SK or BK Ca2+-activated K+ channels with apamin (100 nM) or charbydotoxin (100 nM) respectively, had no effect upon either ACh release or its modulation by muscarinic agonists. 6. These results show that ACh release from individual release sites on basal forebrain neurones is controlled by multiple Ca2+ channel subtypes with overlapping Ca2+ microdomains and that autoinhibition of release results from M2 muscarinic receptor-mediated inhibition of these presynaptic Ca2+ channels rather than as a consequence of K+ channel activation.