Multiplicity of muscarinic autoreceptor subtypes? Comparison of the distribution of cholinergic cells and cells containing mRNA for five subtypes of muscarinic receptors in the rat brain

Peripheral biomarkers of treatment-resistant schizophrenia: Genetic, inflammation and stress perspectives

Treatment-resistant schizophrenia (TRS) often results in severe disability and functional impairment. Currently, the diagnosis of TRS is largely exclusionary and emphasizes the improvement of symptoms that may not be detected early and treated according to TRS guideline. As the gold standard, clozapine is the most prescribed selection for TRS. Therefore, how to predict TRS in advance is critical for forming subsequent treatment strategy especially clozapine is used during the early stage of TRS. Although mounting studies have identified certain clinical factors and neuroimaging characteristics associated with treatment response in schizophrenia, the predictors for TRS remain to be explored. Biomarkers, particularly for peripheral biomarkers, show great potential in predicting TRS in view of their predictive validity, noninvasiveness, ease of testing and low cost that would enable their widespread use. Recent evidence supports that the pathogenesis of TRS may be involved in abnormal neurotransmitter systems, inflammation and stress. Due to the heterogeneity of TRS and the lack of consensus in diagnostic criteria, it is difficult to compare extensive results among different studies. Based on the reported neurobiological mechanisms that may be associated with TRS, this paper narratively reviews the updates of peripheral biomarkers of TRS, from genetic and other related perspectives. Although current evidence regarding biomarkers in TRS remains fragmentary, when taken together, it can help to better understand the neurobiological interface of clinical phenotypes and psychiatric symptoms, which will enable individualized prediction and therapy for TRS in the long run.

Prefrontal cortical distribution of muscarinic M2 and cannabinoid-1 (CB1) receptors in adult male mice with or without chronic adolescent exposure to Δ9-tetrahydrocannabinol

Chronic adolescent administration of marijuana's major psychoactive compound, ∆9-tetrahydrocannabinol (Δ9-THC), produces adaptive changes in adult social and cognitive functions sustained by prelimbic prefrontal cortex (PL-PFC). Memory and learning processes in PL-PFC neurons can be regulated through cholinergic muscarinic-2 receptors (M2R) and modulated by activation of cannabinoid-1 receptors (CB1Rs) targeted by Δ9-THC. Thus, chronic exposure to Δ9-THC during adolescence may alter the expression and/or distribution of M2Rs in PL-PFC neurons receiving CB1R terminals. We tested this hypothesis by using electron microscopic dual CB1R and M2R immunolabeling in adult C57BL/6 J male mice that had received vehicle or escalating dose of Δ9-THC through adolescence. In vehicle controls, CB1R immunolabeling was mainly localized to axonal profiles virtually devoid of M2R but often apposing M2R-immunoreactive dendrites and dendritic spines. The dendrites received inputs from CB1R-labeled or unlabeled terminals, whereas spines received asymmetric synapses exclusively from axon terminals lacking CB1Rs. Adolescent Δ9-THC significantly increased plasmalemmal M2R-immunogold density exclusively in large dendrites receiving input from CB1R-labeled terminals. In contrast, cytoplasmic M2R-immunogold density decreased in small spines of the Δ9-THC-treated adult mice. We conclude that Δ9-THC engagement of CB1Rs during adolescence increases M2R plasmalemmal accumulation in large proximal dendrites and decreases M2R cytoplasmic expression in small spines of PL-PFC.

Effects of Muscarinic Acetylcholine m1 and m4 Receptor Blockade on Dyskinesia in the Hemi-Parkinsonian Rat

Standard treatment for Parkinson's disease (PD) is L-DOPA, but with chronic administration the majority of patients develop L-DOPA-induced dyskinesia (LID). Emerging evidence implicates the cholinergic system in PD and LID. Muscarinic acetylcholine receptors (mAChR) are known to modulate movement and of late have been implicated as possible targets for LID. Therefore the current study investigated the role of M₁ and M₄ mAChRs in LID, on motor performance following L-DOPA treatment, and sought to identify brain sites through which these receptors were acting. We first administered M₁R-preferring antagonist trihexyphenidyl (0, 0.1, and 1.0 mg/kg, i.p.) or the M₄R-preferring antagonist tropicamide (0, 10, and 30 mg/kg, i.p.) before L-DOPA, after which LID and motor performance were evaluated. Both compounds worsened and extended the time course of LID, while M₁R blockade improved motor performance. We then evaluated the effects of tropicamide and trihexyphenidyl on dyskinesia induced by D₁R agonist SKF81297 or D₂R agonist quinpirole. Surprisingly, both M₁R and M₄R antagonists reduced D₁R agonist-induced dyskinesia but not D₂R agonist-induced dyskinesia, suggesting that mAChR blockade differentially affects MSN firing in the absence of postsynaptic DA. Finally, we evaluated effects of striatum- or PPN-targeted tropicamide microinfusion on LID and motor performance. Despite prior evidence, M₄R blockade in either site alone did not affect the severity of LID via local striatal or PPN infusions. Taken together, these data suggest M₄R as a promising therapeutic target for reducing LID using more selective compounds.

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'.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Electron microscopic localization of M2-muscarinic receptors in cholinergic and noncholinergic neurons of the laterodorsal tegmental and pedunculopontine nuclei of the rat mesopontine tegmentum

Muscarinic m2 receptors (M2Rs) are implicated in autoregulatory control of cholinergic output neurons located within the pedunculopontine (PPT) and laterodorsal tegmental (LTD) nuclei of the mesopontine tegmentum (MPT). However, these nuclei contain many noncholinergic neurons in which activation of M2R heteroceptors may contribute significantly to the decisive role of the LTD and PPT in sleep-wakefulness. We examined the electron microscopic dual immunolabeling of M2Rs and the vesicular acetylcholine transporter (VAchT) in the MPT of rat brain to identify the potential sites for M2R activation. M2R immunogold labeling was predominately seen in somatodendritic profiles throughout the PPT/LTD complex. In somata, M2R immunogold particles were often associated with Golgi lamellae and cytoplasmic endomembrannes, but were rarely in contact with the plasma membrane, as was commonly seen in dendrites. Approximately 36% of the M2R-labeled somata and 16% of the more numerous M2R-labeled dendrites coexpressed VAchT. M2R and M2R/VAchT-labeled dendritic profiles received synapses from inhibitory- and excitatory-type axon terminals, over 88% of which were unlabeled and others contained exclusively M2R or VAchT immunoreactivity. In axonal profiles M2R immunogold was localized to plasmalemmal and cytoplasmic regions and showed a similar distribution in many VAchT-negative glial profiles. These results provide ultrastructural evidence suggestive of somatic endomembrane trafficking of M2Rs, whose activation serves to regulate the postsynaptic excitatory and inhibitory responses in dendrites of cholinergic and noncholinergic neurons in the MPT. They also suggest the possibility that M2Rs in this brain region mediate the effects of acetylcholine on the release of other neurotransmitters and on glial signaling. J. Comp. Neurol. 524:3084-3103, 2016. © 2016 Wiley Periodicals, Inc.

Evidence of D2 receptor expression in the nucleus incertus of the rat

The nucleus incertus (NI), located in the caudal brainstem, mainly consists of GABAergic neurons with widespread projections across the brain. It is the chief source of relaxin-3 in the mammalian brain and densely expresses corticotropin-releasing factor type 1 (CRF1) receptors. Several other neurotransmitters, peptides and receptors are reportedly expressed in the NI. In the present investigation, we show the expression of dopamine type-2 (D2) receptors in the NI by reverse transcriptase-polymerase chain reaction (RT-PCR), western blotting (WB) and immunofluorescence (IF). RT-PCR did not show expression of D3 receptors. D2 receptor short isoform (D2S)-like, relaxin-3, CRF1/2 receptor and NeuN immunoreactivity were co-expressed in the cells of the NI. Behavioural effects of D2 receptor activation by intra-NI infusion of quinpirole (a D2/D3 agonist) were evaluated. Hypolocomotion was observed in home cage monitoring system (LABORAS) and novel environment-induced suppression of feeding behavioural paradigms. Thus the D2 receptors expressed in the NI are likely to play a role in locomotion. Based on its strong bidirectional connections to the median raphe and interpeduncular nuclei, the NI was predicted to play a role in modulating behavioural activity and the present results lend support to this hypothesis. This is the first evidence of expression of a catecholamine receptor, D2-like immunoreactivity, in the NI.

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.

The M-current contributes to high threshold membrane potential oscillations in a cell type-specific way in the pedunculopontine nucleus of mice

The pedunculopontine nucleus is known as a cholinergic nucleus of the reticular activating system, participating in regulation of sleep and wakefulness. Besides cholinergic neurons, it consists of GABAergic and glutamatergic neurons as well. According to classical and recent studies, more subgroups of neurons were defined. Groups based on the neurotransmitter released by a neuron are not homogenous, but can be further subdivided. The PPN neurons do not only provide cholinergic and non-cholinergic inputs to several subcortical brain areas but they are also targets of cholinergic and other different neuromodulatory actions. Although cholinergic neuromodulation has been already investigated in the nucleus, one of its characteristic targets, the M-type potassium current has not been described yet. Using slice electrophysiology, we provide evidence in the present work that cholinergic neurons possess M-current, whereas GABAergic neurons lack it. The M-current contributes to certain functional differences of cholinergic and GABAergic neurons, as spike frequency adaptation, action potential firing frequency or the amplitude difference of medium afterhyperpolarizations (AHPs). Furthermore, we showed that high threshold membrane potential oscillation with high power, around 20 Hz frequency is a functional property of almost all cholinergic cells, whereas GABAergic neurons have only low amplitude oscillations. Blockade of the M-current abolished the oscillatory activity at 20 Hz, and largely diminished it at other frequencies. Taken together, the M-current seems to be characteristic for PPN cholinergic neurons. It provides a possibility for modulating gamma band activity of these cells, thus contributing to neuromodulatory regulation of the reticular activating system.

Role of acetylcholine in control of sexual behavior of male and female mammals

The results of studies using systemic or central applications of cholinergic drugs suggest that acetylcholine makes important contributions to the neurochemical control of male- and female-typical reproductive behaviors. In males, cholinergic control seems largely specific to some elements or aspects of copulatory behavior that can vary significantly across species. Synapses in or near the medial preoptic area represent part of this mechanism, but the entire system appears to extend more widely, perhaps especially to one or more structures flanking some part of the lateral ventricle. In females, the lordosis response that essentially defines sexual receptivity is clearly responsive to cholinergic drugs. The same seems likely to be true of other elements of female sexual behavior, but additional studies will be needed to confirm this. Changes in cholinergic activity may help to mediate estrogenic effects on female sexual behavior. However, estrogen exposure can increase or decrease cholinergic effects, suggesting a relationship that is complex and requires further analysis. Also presently unclear is the localization of the cholinergic effects on female sexual responses. Though periventricular sites again have been implicated, their identity is presently unknown. This review discusses these and other aspects of the central cholinergic systems affecting male and female sexual behaviors.

Muscarinic modulation of high frequency oscillations in pedunculopontine neurons

We previously reported that persistent application of the non-specific cholinergic agonist carbachol (CAR) increased the frequency of calcium channel-mediated oscillatory activity in pedunculopontine nucleus (PPN) neurons, which we identified as dependent on voltage-gated, high-threshold P/Q-type channels. Here, we tested the hypothesis that M2 muscarinic receptors and G-proteins associated with M2 receptors mediate the increase in oscillatory frequency in PPN neurons. We found, using depolarizing ramps, that patch clamped 9-12 day old rat PPN neurons (n = 189) reached their peak oscillatory activity around -20 mV membrane potential. Acute (short duration) application of CAR blocked the oscillatory activity through M2 muscarinic receptors, an effect blocked by atropine. However, persistent (long duration) application of CAR significantly increased the frequency of oscillatory activity in PPN neurons through M2 receptors [40 ± 1 Hz (with CAR) vs. 23 ± 1 Hz (without CAR); p < 0.001]. We then tested the effects of the G-protein antagonist guanosine 5'-[β-thio] diphosphate trilithium salt (GDP-β-S), and the G-protein agonist 5'-[γ-thio] triphosphate trilithium salt (GTP-γ-S). We found, using a three-step protocol in voltage-clamp mode, that the increase in the frequency of oscillations induced by M2 cholinergic receptors was linked to a voltage-dependent G-protein mechanism. In summary, these results suggest that persistent cholinergic input creates a permissive activation state in the PPN that allows high frequency P/Q-type calcium channel-mediated gamma oscillations to occur.

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.

Pharmacological validation of candidate causal sleep genes identified in an N2 cross

Despite the substantial impact of sleep disturbances on human health and the many years of study dedicated to understanding sleep pathologies, the underlying genetic mechanisms that govern sleep and wake largely remain unknown. Recently, the authors completed large-scale genetic and gene expression analyses in a segregating inbred mouse cross and identified candidate causal genes that regulate the mammalian sleep-wake cycle, across multiple traits including total sleep time, amounts of rapid eye movement (REM), non-REM, sleep bout duration, and sleep fragmentation. Here the authors describe a novel approach toward validating candidate causal genes, while also identifying potential targets for sleep-related indications. Select small-molecule antagonists and agonists were used to interrogate candidate causal gene function in rodent sleep polysomnography assays to determine impact on overall sleep architecture and to evaluate alignment with associated sleep-wake traits. Significant effects on sleep architecture were observed in validation studies using compounds targeting the muscarinic acetylcholine receptor M3 subunit (Chrm3) (wake promotion), nicotinic acetylcholine receptor alpha4 subunit (Chrna4) (wake promotion), dopamine receptor D5 subunit (Drd5) (sleep induction), serotonin 1D receptor (Htr1d) (altered REM fragmentation), glucagon-like peptide-1 receptor (Glp1r) (light sleep promotion and reduction of deep sleep), and calcium channel, voltage-dependent, T type, alpha 1I subunit (Cacna1i) (increased bout duration of slow wave sleep). Taken together, these results show the complexity of genetic components that regulate sleep-wake traits and highlight the importance of evaluating this complex behavior at a systems level. Pharmacological validation of genetically identified putative targets provides a rapid alternative to generating knock out or transgenic animal models, and may ultimately lead towards new therapeutic opportunities.

Muscarinic receptors in brain stem and mesopontine cholinergic arousal functions

All five muscarinic receptor subtypes and mRNAs are found widely in the brain stem, with M₂ muscarinic receptors most concentrated in the hindbrain. Three cholinergic cell groups, Ch5: pedunculopontine (PPT); Ch6: laterodorsal tegmental (LDT); Ch8: parabigeminal (PBG), are found in the tegmentum. Ch5,6 neurons are activated by arousing and reward-activating stimuli, and inhibited via M₂-like autoreceptors. Ch5,6 ascending projections activate many forebrain regions, including thalamus, basal forebrain, and orexin/hypocretin neurons (via M₃ receptors) for waking arousal and attention. Ch5,6 activation of dopamine neurons of the ventral tegmental area and substantia nigra (via M₅ receptors) increases reward-seeking and energizes motor functions. M₅ receptors on dopamine neurons facilitate brain-stimulation reward, opiate rewards and locomotion, and male ultrasonic vocalizations during mating in rodents. Ch5 cholinergic activation of superior colliculus intermediate layers facilitates fast saccades and approach turns, accompanied by nicotinic and muscarinic inhibition of the startle reflex in pons. Ch8 PBG neurons project to the outer layers of the superior colliculus only, where M₂ receptors are associated with retinotectal terminals. Ch5,6 descending projections to dorsal pontine reticular formation contribute to M₂-dependent REM sleep.

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

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

Cholinergic inputs to laryngeal motoneurons functionally identified in vivo in rat: a combined electrophysiological and microscopic study

The intrinsic laryngeal muscles are differentially modulated during respiration as well as other states and behaviors such as hypocapnia and sleep. Previous anatomical and pharmacological studies indicate a role for acetylcholine at the level of the nucleus ambiguus in the modulation of laryngeal motoneuron (LMN) activity. The present study investigated the anatomical nature of cholinergic input to inspiratory- (ILM) and expiratory-modulated (ELM) laryngeal motoneurons in the loose formation of the nucleus ambiguus. Using combined in vivo intracellular recording, dye filling, and immunohistochemistry, we demonstrate that LMNs identified in Sprague-Dawley rat receive several close appositions from vesicular acetylcholine transporter-immunoreactive (VAChT-ir) boutons. ELMs receive a significantly greater number of close appositions (mean ± standard deviation [SD]: 47 ± 11; n = 5) than ILMs (32 ± 9; n = 8; t-test P < 0.05). For both LMN types, more close appositions were observed on the cell soma and proximal dendrites compared to distal dendrites (two-way analysis of variance [ANOVA], P < 0.0001). Using fluorescence confocal microscopy, almost 90% of VAChT-ir close appositions (n = 45 boutons on n = 4 ELMs) were colocalized with the synaptic marker synaptophysin. These results support a strong influence of cholinergic input on LMNs and may have implications in the differential modulation of laryngeal muscle activity.

Cholinergic control of male mating behavior in hamsters: effects of central oxotremorine treatment

The responses of rats to intracranial injections of cholinergic drugs implicate acetylcholine in the control of male mating behavior and suggest specific brain areas as mediators of these effects. In particular, past work has linked the medial preoptic area (MPOA) to the control of intromission frequency but implicated areas near the lateral ventricles in effects on the initiation and spacing of intromissions. Studies of responses to systemic cholinergic treatments suggest that acetylcholine is even more important for the control of mating behavior in male hamsters but provide no information on the relevant brain areas. To fill this gap, we observed the effects of central injections of the cholinergic agonist oxotremorine that approached the MPOA along contrasting paths. Both studies suggest that increased cholinergic activity in or near the MPOA can facilitate behavior by reducing the postejaculatory interval and possibly affecting other parts of the mechanisms controlling the initiation of copulation and the efficiency of performance early in an encounter. In addition, oxotremorine caused other changes in behavior that could not be tied to the MPOA and may reflect actions at more dorsal sites, possibly including the bed nucleus of the stria terminalis and medial septum. These effects were notably heterogeneous, including facilitatory and disruptive effects on male behavior along with a facilitation of lordosis responses to manual stimulation. These results emphasize the number and diversity of elements of sexual behavior in hamsters that are under the partial control of forebrain cholinergic mechanisms.

Cholinergic control of male mating behavior in hamsters: effects of systemic agonist or antagonist treatment

Sexual behavior in male rats is thought to depend in part on central cholinergic activity. In particular, previous studies of responses to systemically administered cholinergic drugs suggest that male rat behavior can be facilitated by the muscarinic agonist oxotremorine but is disrupted by the muscarinic antagonist scopolamine. However, it is not clear how broadly these effects generalize across species. To address this issue, we observed the impact on sexual behavior in male hamsters of systemic treatment with oxotremorine or scopolamine. In each case, the peripheral muscarinic antagonist methylscopolamine was used as an auxiliary or control treatment to better isolate central cholinergic effects. Both oxotremorine and scopolamine disrupted male behavior in hamsters. For example, both increased the likelihood of failure to achieve intromission or ejaculation. Further, even on completed tests oxotremorine treatment led to changes including increases in mount latency and postejaculatory interval while scopolamine treatment caused changes including increases in ejaculation latency and intromission frequency. The many changes caused by these treatments suggest that acetylcholine helps to control many elements of male behavior, probably by acting at multiple brain sites. The generally similar responses to a cholinergic agonist and antagonist suggest the dependence of efficient mating behavior on optimal levels of central cholinergic activity.

Nucleus incertus--an emerging modulatory role in arousal, stress and memory

A major challenge in systems neuroscience is to determine the underlying neural circuitry and associated neurotransmitters and receptors involved in psychiatric disorders, such as anxiety and depression. A focus of many of these studies has been specific brainstem nuclei that modulate levels of arousal via their ascending monoaminergic projections (e.g. the serotonergic dorsal raphé, noradrenergic locus ceruleus and cholinergic laterodorsal tegmental nucleus). After years of relative neglect, the subject of recent studies in this context has been the GABAergic nucleus incertus, which is located in the midline periventricular central gray in the 'prepontine' hindbrain, with broad projections throughout the forebrain. Nucleus incertus neurons express receptors for the stress hormone, corticotropin-releasing factor (CRF), are activated by psychological stressors, and project to key nuclei involved in stress responses and behavioral activation. The nucleus incertus is also a node in neural circuits capable of modulating hippocampal theta rhythm, which is related to control of spatial navigation and memory. A significant population of nucleus incertus neurons express the recently discovered, highly conserved neuropeptide, relaxin-3; and the recent availability of structurally-related, chimeric peptides that selectively activate or inhibit the relaxin-3 receptor, RXFP3, is facilitating studies of relaxin-3/RXFP3 networks and associated GABA and CRF systems. It is predicted that such targeted research will help elucidate the functions of ascending nucleus incertus pathways, including their possible involvement in arousal (sleep/wakefulness), stress reponses, and learning and memory; and in the pathology of related psychiatric diseases such as insomnia, anxiety and depression, and cognitive deficits.

Muscarinic type 2 receptors in the lateral dorsal tegmental area modulate cocaine and food seeking behavior in rats

The cholinergic input from the lateral dorsal tegmental area (LDTg) modulates the dopamine cells of the ventral tegmental area (VTA) and plays an important role in cocaine taking. Specific pharmacological agents that block or stimulate muscarinic receptors in the LDTg change acetylcholine (ACh) levels in the VTA. Furthermore, manipulations of cholinergic input in the VTA can change cocaine taking. In the current study, the ACh output from the LDTg was attenuated by treatment with the selective muscarinic type 2 (M2) autoreceptor agonist oxotremorine.sesquifumarate (OxoSQ). We hypothesized that OxoSQ would reduce the motivation of rats to self-administer both natural and drug rewards. Animals were tested on progressive ratio (PR) schedules of reinforcement for food pellets and cocaine. On test days, animals on food and on cocaine schedules were bilaterally microinjected prior to the test. Rats received either LDTg OxoSQ infusions or LDTg artificial cerebrospinal fluid (aCSF) infusions in a within-subjects design. In addition, infusions were delivered into a dorsal brain area above the LDTg as an anatomical control region. OxoSQ microinjection in the LDTg, compared to aCSF, significantly reduced both the number of self-administered pellets and cocaine infusions during the initial half of the session; this reduction was dose-dependent. OxoSQ microinjections into the area just dorsal to the LDTg had no significant effect on self-administration of food pellets or cocaine. Animals were also tested in locomotor activity chambers for motor effects following the above microinjections. Locomotor activity was mildly increased by OxoSQ microinjection into the LDTg during the initial half of the session. Overall, these data suggest that LDTg cholinergic neurons play an important role in modifying the reinforcing value of natural and drug rewards. These effects cannot be attributed to significant alterations of locomotor behavior and are likely accomplished through LDTg muscarinic autoreceptors.

The M5 muscarinic receptor as possible target for treatment of drug abuse

Two reports published in the latter 1980s are generally given credit for being the first to announce the discovery of a new subtype of muscarinic acetylcholine receptor (mAChR), designated m5 or M5, and now officially M(5) (1). Both identifications were assigned using molecular biology techniques. Then - as now - no selective high-affinity ligands or toxins were available. In situ hybridization and reverse-transcriptase PCR have found M(5) AChR expression in brain to be distinct from that of the four other G protein-coupled mAChR subtypes and primarily localized to the substantia nigra, ventral tegmental area, hippocampus (CA1 and CA2 subfields), cerebral cortex (outermost layer) and striatum (caudate putamen). M(5) AChR brain region localization and involvement in the regulation of striatal dopamine release and in rewarding brain stimulation suggests a possible role for M(5) AChR as a target for novel therapy to treat excess hedonic drive, including drug abuse.

Cholinergic modulation of fast inhibitory and excitatory transmission to pedunculopontine thalamic projecting neurons

The pedunculopontine nucleus (PPN) is part of the cholinergic arm of the reticular activating system, which is mostly active during waking and rapid-eye movement sleep. The PPN projects to the thalamus and receives cholinergic inputs from the laterodorsal tegmental nucleus and contralateral PPN. We employed retrograde labeling and whole cell recordings to determine the modulation of GABAergic, glycinergic, and glutamatergic transmission to PPN thalamic projecting neurons, and their postsynaptic responses to the nonspecific cholinergic agonist carbachol. M2 and M4 muscarinic receptor-modulated inhibitory postsynaptic responses were observed in 73% of PPN output neurons; in 12.9%, M1 and nicotinic receptor-mediated excitation was detected; and muscarinic and nicotinic-modulated fast inhibitory followed by slow excitatory biphasic responses were evident in 6.7% of cells. A significant increase in the frequency of spontaneous excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents during carbachol application was observed in 66.2% and 65.2% of efferent neurons, respectively. This effect was blocked by a M1 antagonist or nonselective muscarinic blocker, indicating that glutamatergic, GABAergic, and/or glycinergic neurons projecting to PPN output neurons are excited through muscarinic receptors. Decreases in the frequency of miniature EPSCs, and amplitude of electrical stimulation-evoked EPSCs, were blocked by a M2 antagonist, suggesting the presence of M2Rs at terminals of presynaptic glutamatergic neurons. Carbachol-induced multiple types of postsynaptic responses, enhancing both inhibitory and excitatory fast transmission to PPN thalamic projecting neurons through muscarinic receptors. These results provide possible implications for the generation of different frequency oscillations in PPN thalamic projecting neurons during distinct sleep-wake states.

Acetylcholine-dopamine interactions in the pathophysiology and treatment of CNS disorders

Dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area of the midbrain form the nigrostriatal and mesocorticolimbic dopaminergic pathways that, respectively, project to dorsal and ventral striatum (including prefrontal cortex). These midbrain dopaminergic nuclei and their respective forebrain and cortical target areas are well established as serving a critical role in mediating voluntary motor control, as evidenced in Parkinson's disease, and incentive-motivated behaviors and cognitive functions, as exhibited in drug addiction and schizophrenia, respectively. Although it cannot be disputed that excitatory and inhibitory amino acid-based neurotransmitters, such as glutamate and GABA, play a vital role in modulating activity of midbrain dopaminergic neurons, recent evidence suggests that acetylcholine may be as important in regulating dopaminergic transmission. Midbrain dopaminergic cell tonic and phasic activity is closely dependent upon projections from hindbrain pedunculopontine and the laterodorsal tegmental nuclei, which comprises the only known cholinergic inputs to these neurons. In close coordination with glutamatergic and GABAergic activity, these excitatory cholinergic projections activate nicotinic and muscarinic acetylcholine receptors within the substantia nigra and ventral tegmental area to modulate dopamine transmission in the dorsal/ventral striatum and prefrontal cortex. Additionally, acetylcholine-containing interneurons in the striatum also constitute an important neural substrate to provide further cholinergic modulation of forebrain striatal dopaminergic transmission. In this review, we examine neurological and psychopathological conditions associated with dysfunctions in the interaction of acetylcholine and dopamine and conventional and new pharmacological approaches to treat these disorders.

Hypoglossal premotor neurons of the intermediate medullary reticular region express cholinergic markers

The inspiratory drive to hypoglossal (XII) motoneurons originates in the caudal medullary intermediate reticular (IRt) region. This drive is mainly glutamatergic, but little is known about the neurochemical features of IRt XII premotor neurons. Prompted by the evidence that XII motoneuronal activity is controlled by both muscarinic (M) and nicotinic cholinergic inputs and that the IRt region contains cells that express choline acetyltransferase (ChAT), a marker of cholinergic neurons, we investigated whether some IRt XII premotor neurons are cholinergic. In seven rats, we applied single-cell reverse transcription-polymerase chain reaction to acutely dissociated IRt neurons retrogradely labeled from the XII nucleus. We found that over half (21/37) of such neurons expressed mRNA for ChAT and one-third (13/37) also had M2 receptor mRNA. In contrast, among the IRt neurons not retrogradely labeled, only 4 of 29 expressed ChAT mRNA (P < 0.0008) and only 3 of 29 expressed M2 receptor mRNA (P < 0.04). The distributions of other cholinergic receptor mRNAs (M1, M3, M4, M5, and nicotinic alpha4-subunit) did not differ between IRt XII premotor neurons and unlabeled IRt neurons. In an additional three rats with retrograde tracers injected into the XII nucleus and ChAT immunohistochemistry, 5-11% of IRt XII premotor neurons located at, and caudal to, the area postrema were ChAT positive, and 27-48% of ChAT-positive caudal IRt neurons were retrogradely labeled from the XII nucleus. Thus the pre- and postsynaptic cholinergic effects previously described in XII motoneurons may originate, at least in part, in medullary IRt neurons.

The developmental decrease in REM sleep: the role of transmitters and electrical coupling

STUDY OBJECTIVES

This mini-review considers certain factors related to the developmental decrease in rapid eye movement (REM) sleep, which occurs in favor of additional waking time, and its relationship to developmental factors that may influence its potential role in brain development.

DESIGN

Specifically, we discuss some of the theories proposed for the occurrence of REM sleep and agree with the classic notion that REM sleep is, at the least, a mechanism that may play a role in the maturation of thalamocortical pathways. The developmental decrease in REM sleep occurs gradually from birth until close to puberty in the human, and in other mammals it is brief and coincides with eye and ear opening and the beginning of massive exogenous activation. Therefore, the purported role for REM sleep may change to involve a number of other functions with age.

MEASUREMENTS AND RESULTS

We describe recent findings showing that morphologic and physiologic properties as well as cholinergic, gamma amino-butyric acid, kainic acid, n-methyl-d-aspartic acid, noradrenergic, and serotonergic synaptic inputs to mesopontine cholinergic neurons, as well as the degree of electrical coupling between mostly noncholinergic mesopontine neurons and levels of the neuronal gap-junction protein connexin 36, change dramatically during this critical period in development. A novel mechanism for sleep-wake control based on well-known transmitter interactions, as well as electrical coupling, is described.

CONCLUSION

We hypothesize that a dysregulation of this process could result in life-long disturbances in arousal and REM sleep drive, leading to hypervigilance or hypovigilance such as that observed in a number of disorders that have a mostly postpubertal age of onset.

Effects of pilocarpine on the cortical and hippocampal theta rhythm in different vigilance states in rats

It has been suggested that theta rhythm gates the flow of information between the hippocampus and cortex during memory processes. The cholinergic system plays an important role in regulating vigilance states and in generating theta rhythm. This study aims to analyse the effects of the muscarinic agonist pilocarpine (120 and 360 microg, i.c.v.) on hippocampal and frontal cortical theta rhythm during several vigilance states in rats. Pilocarpine injection increased the duration and number of episodes with theta activity, particularly when theta rhythm appeared during waking states in the cortex and hippocampus simultaneously. It seems that the effects of pilocarpine are related to the appearance of cortical theta activity in waking states, and suggest that pilocarpine could modify the transference rate of information from the hippocampus to cortex in rats during wakefulness states, in relation to the postulated effect of cholinergic system modulating memory consolidation.

Morphological characterization of electrophysiologically and immunohistochemically identified basal forebrain cholinergic and neuropeptide Y-containing neurons

The basal forebrain (BF) contains cholinergic as well as different types of non-cholinergic corticopetal neurons and interneurons, including neuropeptide Y (NPY) containing cells. BF corticopetal neurons constitute an extrathalamic route to the cortex and their activity is associated with an increase in cortical release of the neurotransmitter acetylcholine, concomitant with low voltage fast cortical EEG activity. It has been shown in previous studies (Duque et al. in J Neurophysiol 84:1627-1635, 2000) that in anesthetized rats BF cholinergic neurons fire mostly during low voltage fast cortical EEG epochs, while increased NPY neuronal firing is accompanied by cortical slow waves. In this paper, electrophysiologically and neurochemically characterized cholinergic and NPY-containing neurons were 3D reconstructed from serial sections and morphometrically analyzed. Cholinergic and NPY-containing neurons, although having roughly the same dendritic surface areas and lengths, were found to differ in dendritic thickness and branching structure. They also have distinct patterns of dendritic endings. The subtle differences in dendritic arborization pattern may have an impact on how synaptic integration takes place in these functionally distinct neuronal populations. Cholinergic neurons exhibited cortically projecting axons and extensive local axon collaterals. Elaborate local axonal arbors confined to the BF also originated from NPY-containing neurons. The presence of local axon collaterals in both cholinergic and NPY neurons indicates that the BF is not a mere conduit for various brainstem inputs to the cortex, but a site where substantial local processing must take place.

Muscarinic and nicotinic responses in the developing pedunculopontine nucleus (PPN)

The pedunculopontine nucleus (PPN), the cholinergic arm of the reticular activating system (RAS), is known to modulate waking and rapid eye movement (REM) sleep. REM sleep decreases between 10 and 30 days postnatally in the rat, with the majority occurring between 12 and 21 days. We investigated the possibility that changes in the cholinergic, muscarinic and/or nicotinic, input to PPN neurons could explain at least part of the developmental decrease in REM sleep. We recorded intracellularly from PPN neurons in 12-21 day rat brainstem slices maintained in artificial cerebrospinal fluid (aCSF) and found that application of the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) depolarized PPN neurons early in development, then hyperpolarized PPN neurons by day 21. Most of the effects of DMPP persisted following application of the sodium channel blocker tetrodotoxin (TTX), and in the presence of glutamatergic, serotonergic, noradrenergic and GABAergic antagonists, but were blocked by the nicotinic antagonist mecamylamine (MEC). The mixed muscarinic agonist carbachol (CAR) hyperpolarized all type II (A current) PPN cells and depolarized all type I (low threshold spike-LTS current) and type III (A+LTS current) PPN cells, but did not change effects during the period known for the developmental decrease in REM sleep. The effects of CAR persisted in the presence of TTX but were mostly blocked by the muscarinic antagonist atropine (ATR), and the remainder by MEC. We conclude that, while the nicotinic inputs to the PPN may help modulate the developmental decrease in REM sleep, the muscarinic inputs appear to modulate different types of cells differentially.

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.

Roles of pedunculopontine tegmental cholinergic receptors in brain stimulation reward in the rat

RATIONALE

The brainstem pedunculopontine tegmental nucleus (PPTg) is proposed to mediate hypothalamic self-stimulation reward via cholinergic activation of the ventral tegmental area (VTA). However, to date there is little direct evidence to support this hypothesis.

OBJECTIVES

To further study the role of PPTg in hypothalamic self-stimulation reward.

METHODS

By using in vivo microdialysis, the levels of extracellular acetylcholine (ACh) in the PPTg and VTA were detected during lateral hypothalamic (LH) self-stimulation in rats. Rate-frequency curve shift procedure was used to evaluate the effects of nonselective muscarinic antagonist scopolamine (1 approximately 100 microg/microl) and nicotinic antagonist mecamylamine (5 approximately 100 microg/microl) microinjected into the PPTg on the rewarding efficacy of LH self-stimulation. Subsequently, the drugs were injected into the PPTg, and the extracellular ACh in the VTA was measured.

RESULTS

LH self-stimulation produced a concurrent ACh release in the PPTg and VTA. Intra-PPTg injection of scopolamine (100 microg/microl) significantly reduced the frequency threshold for LH self-stimulation reward, but nicotinic antagonist mecamylamine did not shift the threshold. However, mecamylamine (10, 25 microg/microl) injected into the PPTg robustly diminished the nicotine-potentiated LH self-stimulation reward. The extracellular ACh in the VTA was dramatically increased by intra-PPTg scopolamine (10, 100 microg/microl), but not by mecamylamine.

CONCLUSIONS

Results confirm that PPTg plays an important role in brain stimulation reward by modulating the cholinergic activity of the VTA. The PPTg muscarinic receptors contribute to an inhibitory modulation of reward effects by self-stimulation, whereas nicotinic receptors seem to be more involved in nicotine potentiation of brain stimulation reward.

Differential effects on [35S]GTPgammaS binding using muscarinic agonists and antagonists in the gerbil brain

In this work, we studied the in vitro G-protein activation induced by muscarinic agonists using [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) autoradiographic methods to characterize the M(2) and M(4) muscarinic subtypes response. Thus, we describe a detailed characterization of the increases in [(35)S]GTPgammaS binding elicited by carbachol (Cch) and oxotremorine (OXO) (binding in the presence minus binding in the absence of agonist) throughout the gerbil brain (Meriones unguiculatus). For both agonists, the strongest stimulations were found in the superficial gray layer of the superior colliculus, the anteroventral and anteromedial thalamic nuclei, the anterior paraventricular thalamic nucleus, and the caudate-putamen. The comparative study using OXO and Cch suggested that OXO is able to detect differences in the response of structures enriched in M(4) muscarinic receptors, showing a lower potency to stimulate these brain areas. Furthermore, using increasing concentrations of selective M(2) (AF-DX 116) and M(1)/M(4) (pirenzepine) antagonists to inhibit specific Cch- or OXO-induced [(35)S]GTPgammaS binding, significant differences were observed in M(2)-enriched structures but not in M(4)-enriched ones such as the caudate-putamen. These data indicate that appropriate muscarinic agonist stimulation, together with selective inhibition of this effect using functional autoradiography, can be used as a tool to unravel the M(2)- and M(4)-muscarinic subtype-mediated response.

Aging and subcellular localization of m2 muscarinic autoreceptor in basalocortical neurons in vivo

By using immunohistochemical approaches at the light and electron microscopic levels, we have shown that aging modifies the subcellular distribution of the m2 muscarinic autoreceptor (m2R) differentially at somato-dendritic postsynaptic sites and at axonal presynaptic sites in cholinergic basalocortical neurons, in vivo. In cholinergic perikarya and dendrites of the nucleus basalis magnocellularis (NBM), aging is associated with a decrease of the density of m2R at the plasma membrane and in the cytoplasm, suggesting a decrease of the total number of m2R in the somato-dendritic field. In contrast, the number of substance P receptors per somato-dendritic surface was not affected. In the frontal cortex (FC), we have shown a decrease of cytoplasmic m2R density also leading to a decrease of the number of m2R per surface of varicosities but with no change of the density of m2R at the membrane. Our results suggest that the decrease of m2R in the somato-dendritic field of the NBM, but not a modification of the number of presynaptic m2 autoreceptors at the plasma membrane in the FC, could contribute to the decrease of the efficacy of cholinergic transmission observed with aging in the rat.

Degeneration of the basalocortical pathway from the cortex induces a functional increase in galaninergic markers in the nucleus basalis magnocellularis of the rat

The present work aimed 1) to evaluate whether an increase in galanin or galanin receptors could be induced in the nucleus basalis magnocellularis (nbm) by degeneration of the basalocortical neurons from the cortex and 2) to analyze the consequences of such an increase on cortical activity. First, a mild ischemic insult to the frontoparietal cortex was performed to induce the degeneration of the basalocortical system; galanin immunoreactivity, galanin binding sites, and cholinergic muscarinic receptors were quantified through immunocytochemistry and autoradiography. Second, galanin infusions in the nbm were undertaken to mimic a local increase of the galaninergic innervation; cortical acetylcholine release, cerebral glucose use, and cerebral blood flow were then measured as indices of cortical activity. As a result of the cortical ischemic lesion, the postsynaptic M1 and presynaptic M2 muscarinic receptors were found to be reduced in the altered cortex. In contrast, galaninergic binding capacity and fiber density were found to be increased in the ipsilateral nbm in parallel with a local decrease in the cholinergic markers such as the muscarinic M1 receptor density. Galanin infusion into the nbm inhibited the cortical acetylcholine release and cerebral blood flow increases elicited by the activation of the cholinergic basalocortical system but failed to affect acetylcholine release, cerebral blood flow, and cerebral glucose use when injected alone in the nbm. These results demonstrate that degeneration of the basalocortical system from the cortex induces an increase in galaninergic markers in the nbm, a result that might suggest that the galaninergic overexpression described in the basal forebrain of patients with Alzheimer's disease can result from a degeneration of the cholinergic basalocortical system from the cortex. Because galanin was found to reduce the activity of the basalocortical cholinergic system only when this one is activated, galanin might exert its role rather during activation deficits than under resting conditions such as the resting cortical hypometabolism, which is characteristic of Alzheimer's disease.

Pontine cholinergic mechanisms and their impact on respiratory regulation

Activation of pontomedullary cholinergic neurons may directly and indirectly cause depression of respiratory motoneuronal activity, activation of respiratory premotor neurons and acceleration of the respiratory rate during REM sleep, as well as activation of breathing during active wakefulness. These effects may be mediated by distinct subpopulations of cholinergic neurons. The relative inactivity of cholinergic neurons during slow-wave sleep also may contribute to the depressant effects of this state on breathing. Cholinergic muscarinic and nicotinic receptors are expressed in central respiratory neurons and motoneurons, thus allowing cholinergic neurons to act on the respiratory system directly. Additional effects of cholinergic activation are mediated indirectly by noradrenergic, serotonergic and other neurons of the reticular formation. Excitatory and suppressant respiratory effects with features of natural states of REM sleep or active wakefulness can be elicited in urethane-anesthetized rats by pontine microinjections of the cholinergic agonist, carbachol. Carbachol models help elucidate the neural basis of respiratory disorders associated with central cholinergic activation.

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.

Nigrostriatal dopamine release modulated by mesopontine muscarinic receptors

The present study investigated the regulation of substantia nigra pars compacta (SNc) dopamine neuronal activity by pedunculopontine (PPT) cholinergic neurons. Changes in dopamine efflux following chemical activation or blockade of muscarinic acetylcholine receptors in the PPT were measured at stearate-carbon paste electrodes in the striatum of urethane (1.5 g/kg) anaesthetized male rats using in vivo chronoamperometry (30 s sampling rate). Intra-PPT infusions of a mixed muscarinic/nicotinic (carbachol 8 microg/microl) or M2/4-selective muscarinic (oxotremorine 0.5 microg/microl) receptor agonist attenuated striatal dopamine efflux, whereas a non-selective (scopolamine 100 microg/microl) or M2/4-selective (methoctramine 50 microg/microl) muscarinic receptor antagonist enhanced striatal dopamine efflux. These results suggest that M2/4 muscarinic receptors in the mesopontine tonically influence SNc basal dopamine cell activity and striatal dopamine release.

Pedunculopontine tegmental stimulation evokes striatal dopamine efflux by activation of acetylcholine and glutamate receptors in the midbrain and pons of the rat

The pedunculopontine tegmental nucleus appears to influence striatal dopamine activity via cholinergic and glutamatergic afferents to dopaminergic cells of the substantia nigra pars compacta. We measured changes in striatal dopamine oxidation current (dopamine efflux) in response to electrical stimulation of the pedunculopontine tegmental nucleus using in vivo electrochemistry in urethane-anaesthetized rats. Pedunculopontine tegmental nucleus stimulation evoked a three-component change in striatal dopamine efflux, consisting of: (i) an initial rapid increase of 2 min duration; followed by (ii) a decrease below prestimulation levels of 9 min duration; then by (iii) a prolonged increase lasting 35 min. Intra-nigral infusions of the ionotropic glutamate receptor antagonist kynurenate (10 microg/ microL) or the nicotinic cholinergic receptor antagonist mecamylamine (5 microg/0.5 microL) selectively attenuated the rapid first component, while systemic injections of the muscarinic cholinergic antagonist scopolamine (5 mg/kg, i.p.) diminished the second and third components. In addition, intra-pedunculopontine tegmental nucleus infusions of the M2 muscarinic antagonist methoctramine (50 microg/ microL) selectively abolished the inhibitory second component, while intranigral infusions of scopolamine (200 microg/ microL) selectively abolished the prolonged third component. Intra-nigral infusions of the metabotropic glutamate receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine (2 microg/ microL) had no effect on pedunculopontine tegmental nucleus-elicited striatal dopamine efflux. These results suggest that the pedunculopontine tegmental nucleus utilizes nicotinic and ionotropic glutamate receptors in the substantia nigra to mediate rapid activation, M2-like muscarinic autoreceptors in the pedunculopontine tegmental nucleus to mediate decreased activation, and muscarinic receptors in the substantia nigra (probably of the M5 subtype) to mediate prolonged activation, of the nigrostriatal dopaminergic system.

Increased drinking in mutant mice with truncated M5 muscarinic receptor genes

The rarest and least understood of the muscarinic receptors is the M5 subtype. Recombinant methods were used to create mutant mice with a deletion in the third intracellular loop of the M5 receptor gene. Salivation induced by the nonselective muscarinic agonist pilocarpine (1 mg/kg s.c.) was reduced in homozygous mutants from 15 to 60 min after injection as compared with wild-type mice. After 18-h food and water deprivation, drinking was increased in homozygous mutants, but feeding was not increased. The mutant and wild-type mice had similar responses in tests of open-field exploration, seizures induced by pilocarpine (300 mg/kg) or hypothermia induced by pilocarpine (1-3 mg/kg). These results indicate that M5 muscarinic receptors are important for fluid intake and suggest that M5 receptors are involved in slow secretory processes.

Characterization of central inhibitory muscarinic autoreceptors by the use of muscarinic acetylcholine receptor knock-out mice

Forebrain muscarinic acetylcholine (ACh) receptors (mAChRs; M1-M5) are predicted to play important roles in many fundamental central functions, including higher cognitive processes and modulation of extrapyramidal motor activity. Synaptic ACh levels are known to be regulated by the activity of presynaptic muscarinic autoreceptors mediating inhibition of ACh release. Primarily because of the use of ligands with limited receptor subtype selectivity, classical pharmacological studies have led to conflicting results regarding the identity of the mAChR subtypes mediating this activity in different areas of the brain. To investigate the molecular identity of hippocampal, cortical, and striatal inhibitory muscarinic autoreceptors in a more direct manner, we used genetically altered mice lacking functional M2 and/or M4 mAChRs [knock-out (KO) mice]. After labeling of cellular ACh pools with [3H]choline, potassium-stimulated [3H]ACh release was measured in superfused brain slices, either in the absence or the presence of muscarinic drugs. The nonsubtype-selective muscarinic agonist, oxotremorine (0.1-10 microm), inhibited potassium-stimulated [3H]ACh release in hippocampal, cortical, and striatal slices prepared from wild-type mice by up to 80%. This activity was totally abolished in tissues prepared from M2-M4 receptor double KO mice. Strikingly, release studies with brain slices from M2 and M4 receptor single KO mice indicated that autoinhibition of ACh release is mediated primarily by the M2 receptor in hippocampus and cerebral cortex, but predominantly by the M4 receptor in the striatum. These results, together with additional receptor localization studies, support the novel concept that autoinhibition of ACh release involves different mAChRs in different regions of the brain.

Regional distribution and cellular localization of 5-HT2C receptor mRNA in monkey brain: comparison with [3H]mesulergine binding sites and choline acetyltransferase mRNA

The distribution of serotonin 5-HT(2C) receptor mRNA in monkey brain was studied by in situ hybridization and compared with the distribution of [3H]mesulergine binding sites as visualized by receptor autoradiography. 5-HT(2C) receptor transcripts showed a widespread and heterogeneous distribution. The strongest hybridization signal was detected in choroid plexus. In neocortex, 5-HT(2C) mRNA was detected in layer V of all cortical regions examined except in the calcarine sulcus, which was devoid of signal. Several structures within the striatum and basal forebrain were strongly labeled: nucleus accumbens, ventral aspects of anterior caudate and putamen, septal nuclei, diagonal band, ventral striatum, and extended amygdala. Several thalamic, midbrain, and brainstem nuclei also contained 5-HT(2C) mRNA. Comparison of the distributions of 5-HT(2C) mRNA and specific [3H]mesulergine binding sites showed a good agreement in the majority of brain regions, suggesting a predominant somatodendritic localization of 5-HT(2C) receptors. A possible localization to axon terminals of 5-HT(2C) receptors is suggested by the disagreement observed in some regions such as septal nuclei and horizontal limb of the diagonal band (presence of mRNA with apparent absence of binding sites) and interpeduncular nucleus (presence of binding sites with apparent absence of mRNA). Comparison of 5-HT(2C) receptor and choline acetyltransferase mRNA distributions indicate that some regions where cholinergic cells are located are also enriched in cells containing 5-HT(2C) mRNA. Although the present methodology does not allow strict colocalization of both mRNA species to the same cells, the codistribution observed in several regions provides a possible anatomical substrate for the described modulation of acetylcholine release by 5-HT(2C) receptors.

Muscarinic tone sustains impulse flow in the septohippocampal GABA but not cholinergic pathway: implications for learning and memory

Systemic infusions of the muscarinic cholinergic receptor antagonists atropine and scopolamine (atr/scop) produce an amnesic syndrome in humans, subhuman primates, and rodents. In humans, this syndrome may resemble early symptoms of Alzheimer's disease. Behavioral studies in rats have demonstrated that the medial septum/diagonal band of Broca (MSDB), which sends cholinergic and GABAergic projections to the hippocampus, is a critical locus in mediating the amnesic effects of atr/scop. The amnesic effects of atr/scop in the MSDB have been presumed but not proven to be caused by a decrease in hippocampal acetylcholine (ACh) release after blockade of a muscarinic tone in the MSDB. Using electrophysiological recordings and fluorescent-labeling techniques to identify living septohippocampal neurons in rat brain slices, we now report that, contrary to current belief, a blockade of the muscarinic tone in the MSDB does not decrease impulse flow in the septohippocampal cholinergic pathway; instead, it decreases impulse flow in the septohippocampal GABAergic pathway via M(3) muscarinic receptors. We also report that the muscarinic tone in the MSDB is maintained by ACh that is released locally, presumably via axon collaterals of septohippocampal cholinergic neurons. As such, cognitive deficits that occur in various neurodegenerative disorders that are associated with a loss or atrophy of septohippocampal cholinergic neurons cannot be attributed solely to a decrease in hippocampal acetylcholine release. An additional, possibly more important mechanism may be the concomitant decrease in septohippocampal GABA release and a subsequent disruption in disinhibitory mechanisms in the hippocampus. Restoration of impulse flow in the septohippocampal GABA pathway, possibly via M(3) receptor agonists, may, therefore, be critical for successful treatment of cognitive deficits associated with neurodegenerative disorders such as Alzheimer's and Parkinson's disease.