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

Medial septal microinfusion of scopolamine disrupts hippocampal activity and trace jaw movement conditioning

This study investigated the effects of microinfusion of scopolamine into the medial septum (MS Scp) on hippocampal neurophysiology and learning of the rabbit's classically conditioned jaw movement response. The percentage of hippocampal theta slow waves (2-8 Hz) decreased after drug infusion in the MS Scp group but did not change in control groups that received infusion of saline into the MS or scopolamine into the cortex. Unit recordings from the MS Scp group showed significantly smaller conditioning-related hippocampal neural responses than seen in controls, and during conditioning, rabbits in the MS Scp group took significantly longer to reach learning criterion than either control group. Thus, the neural and behavioral impairments previously reported for systemic muscarinic blockade were reproduced by microinfusions restricted to the medial septal nucleus.

Cholinergic modulation of the sleep state-dependent P13 midlatency auditory evoked potential in the rat

Injections into the pedunculopontine nucleus (PPN) of the cholinergic receptor agonist, carbachol (CAR), were found to reduce the amplitude of the vertex-recorded, sleep state-dependent P13 midlatency evoked potential in a dose- and time-dependent manner. This effect was blocked or reduced by pretreatment with the muscarinic receptor antagonist, scopolamine, injected into the PPN.

Muscarinic receptor subtypes expression in rat and chick dorsal root ganglia

In the present work we have analyzed by Northern blot, RT-PCR and in situ hybridization the expression of muscarinic receptor subtype mRNAs in rat and chick dorsal root ganglia. Northern blot analysis performed on rat total RNA revealed a strong signal for M(2) while a faint band was observed for M(3) and M(4) subtypes; no signal was evident for M(1) and M(5), while in chick total RNA no signal was detected for any of the analyzed subtypes (M(2), M(3), M(4)). On the other hand, RT-PCR revealed that all muscarinic subtype mRNAs were present both in rat and chick DRG, although the level of their expression may be different. In chick DRG, the presence of various muscarinic subtypes was confirmed by competition binding experiments. In situ hybridization in rat DRG showed that M(3) and M(4) transcripts, similarly to what has been previously described for M(2) mRNA, were preferentially localized in medium-small neurons. Large neurons were usually negative or faintly labelled. No hybridization signal was detected in rat DRG with probes for M(1) and M(5) muscarinic subtypes. The presence of various muscarinic receptors in DRG and their preferential expression in the medium-small sensory neurons suggest their possible involvement in the modulation of nociceptive stimuli transduction.

Laterodorsal tegmental stimulation elicits dopamine efflux in the rat nucleus accumbens by activation of acetylcholine and glutamate receptors in the ventral tegmental area

Cholinergic and glutamatergic neurons in the laterodorsal tegmentum (LDT) and neighbouring mesopontine nuclei are thought to influence mesolimbic dopaminergic neuronal activity involved in goal-directed behaviours. We measured the changes in dopamine oxidation current (corresponding with dopamine efflux) in the nucleus accumbens (NAc) in response to electrical stimulation of the LDT using in vivo chronoamperometry in urethane-anaesthetized rats. LDT stimulation (35 Hz pulse trains for 60 s, 1 s intertrain interval) evoked a three-component change in dopamine efflux in the NAc: (i) an initial stimulation time-locked increase in the dopamine signal above baseline, followed by (ii) an immediate decrease below baseline, and thereafter by (iii) a prolonged increase in the dopamine signal above baseline. Intra-VTA infusion of the nicotinic receptor antagonist mecamylamine (5 microg/0.5 microL) or the ionotropic glutamate receptor antagonist kynurenate (10 microg/microL) attenuated the first LDT-elicited component. The second suppressive component was abolished by intra-LDT infusions of either the nonselective or the M2-selective muscarinic receptor antagonists scopolamine (100 microg/microL) and methoctramine (50 microg/microL), respectively. In contrast, intra-VTA infusions of scopolamine (200 microg/microL) resulted in a selective attenuation of the third facilitatory component, whereas both second and third components were abolished by systemic injections of scopolamine (5 mg/kg). These results suggest that the initial increase, subsequent decrease, and final prolonged increase in extracellular dopamine levels in the NAc are selectively mediated by LDT-elicited activation of (i) nicotinic and glutamatergic receptors in the VTA, (ii) muscarinic M2 autoreceptors on LDT cell bodies, and (iii) muscarinic receptors in the VTA, respectively.

Dendritic and axonal targeting of the vesicular acetylcholine transporter to membranous cytoplasmic organelles in laterodorsal and pedunculopontine tegmental nuclei

Autoregulation of cholinergic neurons in the laterodorsal tegmental (LDT) and pedunculopontine (PPT) nuclei has been implicated in many functions, most importantly in drug reinforcement and in the pathophysiology of schizophrenia. This autoregulation is attributed to the release of acetylcholine, but neither the storage or release sites are known. To determine these sites, we used electron microscopy for the immunocytochemical localization of antipeptide antiserum raised against the vesicular acetylcholine transporter (VAchT) that is responsible for the uptake of acetylcholine into storage vesicles. The cellular and subcellular distribution of VAchT was remarkably similar in the two regions by by using each of two methods, immunogold and immunoperoxidase. In both PPT and LDT nuclei, VAchT labeling was seen mainly on membranous organelles including the trans-Golgi network in many somata. VAchT-immunoreactive tubulovesicles resembling saccules of smooth endoplasmic reticulum were often seen near the plasma membrane in dendrites. The VAchT-containing dendrites comprised almost 50% of the labeled profiles (1027/2129) in PPT and LDT nuclei. The remaining VAchT-immunoreactive profiles were primarily small unmyelinated axons and axon terminals. In axon terminals, VAchT was densely localized to membranes of small synaptic vesicles. The VAchT-immunoreactive axon terminals formed either symmetric or asymmetric synapses. The postsynaptic targets of these axon terminals included dendrites that were with (36/110) or without (74/110) VAchT immunoreactivity. Our results suggest that dendrites, as well as axon terminals, have the potential for storage and release of acetylcholine in the LDT and PPT nuclei. The released acetylcholine is likely to play a major role in autoregulation of mesopontine cholinergic neurons.

Role of the laterodorsal tegmental nucleus in scopolamine- and amphetamine-induced locomotion and stereotypy

Scopolamine (1.5 mg/kg; i.p.) or amphetamine (3 mg/kg; i.p.) increases locomotion and stereotyped behavior patterns in rats. Previous studies suggest that scopolamine acts via muscarinic receptors near the midbrain-pons border. In this study, unilateral microinjections in N-methyl-scopolamine (2.5-10 microg) into the laterodorsal tegmental nucleus (LDT) increased locomotion. Bilateral ibotenate lesions of the LDT attenuated scopolamine-induced locomotion by 68% 7 days postlesion, and by 35% 28 days postlesion. LDT lesions reduced scopolamine-induced stereotypy less than locomotion. The sensitization to amphetamine observed on repeated tests was attenuated by LDT lesions for stereotypy, but not for locomotion. These findings suggest that scopolamine induces locomotion largely, but not exclusively, by blocking muscarinic receptors in LDT.

Antisense oligodeoxynucleotides against the muscarinic m2, but not m4, receptor supports its role as autoreceptors in the rat hippocampus

Antisense oligodeoxynucleotides against muscarinic m2 and m4 receptors were used to investigate the role of these receptor subtypes as negative autoreceptors in the regulation of acetylcholine (ACh) release in the rat hippocampus. Following the continuous infusion of antisenses into the third ventricle (1 microgram microliter-1 h-1, 3 days), 3H-AF-DX 384/muscarinic M2-like binding was significantly decreased in the medial septum by the antisense against the m2 receptor whereas M2-like binding in the dorsal striatum was decreased by the antisense against the m4 receptor. In contrast, 3H-pirenzepine/muscarinic M1-like binding was unaffected by either antisense treatment in any of the brain areas investigated. When perfused into the hippocampus via a dialysis probe, the purported muscarinic M2 receptor antagonist AF-DX 384 (100 nM) increased hippocampal ACh release in freely moving rats. This effect of AF-DX 384 was significantly attenuated by the m2, but not the m4, receptor antisense treatment. Hippocampal choline acetyltransferase activity was not affected by either antisense treatments. Taken together, these results suggest that the molecularly defined muscarinic m2 receptor regulates hippocampal ACh release by acting as a negative autoreceptor. In contrast, the molecularly defined m4 receptor is unlikely to be directly involved in the negative regulation of ACh release in the rat hippocampus. Therefore, inhibiting muscarinic m2 receptor function may be an alternative approach to regulate the release of ACh in neurodegenerative diseases associated with impaired cholinergic functions.

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.

m2 muscarinic receptor immunolocalization in cholinergic cells of the monkey basal forebrain and striatum

Pharmacological studies have suggested that the m2 muscarinic receptor functions as an autoreceptor in the cholinergic axons which innervate the cerebral cortex and striatum. To test this hypothesis in the macaque monkey, we used a subtype-specific antibody to the m2 muscarinic receptor. Immunoreactive cells were well visualized in the nucleus basalis, where some of these cells displayed dense m2 immunoreactivity, while others were lightly labeled. This heterogeneity of labeling intensity was not based on peculiarities of the methodology, because cholinergic cells of the striatum expressed uniformly dense m2 immunoreactivity. Concurrent labeling with choline acetyltransferase immunoreactivity proved that most of the heavily m2-labeled cells in the nucleus basalis were also choline acetyl-transferase positive. The findings demonstrate that at least 10-25% of the cholinergic neurons in the nucleus basalis of the monkey are densely m2 immunoreactive. In the striatum, concurrent labeling demonstrated that the majority, if not all, choline acetyltransferase-positive cells also contained m2 immunoreactivity. In addition, these experiments identified a population of smaller striatal cells which were m2 immunoreactive and choline acetyltransferase negative. Consecutive labeling with m2 immunoreactivity and NADPH-diaphorase histochemistry demonstrated that many of these m2-immunoreactive non-cholinergic neurons belonged to the population of nitric oxide-synthesizing medium aspiny neurons. The findings indicate that the m2 muscarinic receptor may be expressed at high levels in only a subset of cholinergic basal forebrain neurons. In contrast, m2 receptors appear to be expressed by all cholinergic cells of the striatum.

Cholinergic neurons of the nucleus basalis of Meynert receive cholinergic, catecholaminergic and GABAergic synapses: an electron microscopic investigation in the monkey

An electron microscopic analysis of the nucleus basalis in the macaque monkey was carried out following the immunohistochemical labeling of choline acetyltransferase, either by itself or in conjunction with glutamate decarboxylase or tyrosine hydroxylase. Cholinergic axon varicosities were frequently encountered, and formed large, usually asymmetric, synapses on both choline acetyltransferase-immunopositive and -immunonegative dendrites of nucleus basalis neurons. Catecholaminergic (tyrosine hydroxylase-immunoreactive) axon varicosities formed synapses which in most cases were classified as asymmetric, and glutamate decarboxylase-immunoreactive (GABAergic) axons formed clearly symmetric synapses, each on to choline acetyltransferase-immunopositive or -immunonegative dendrites. These findings indicate that cholinergic cells in the nucleus basalis of the monkey, also known as Ch4 neurons, receive numerous synaptic inputs from cholinergic, catecholaminergic and GABAergic axons.

Amygdala-kindled and electroconvulsive seizures alter hippocampal expression of the m1 and m3 muscarinic cholinergic receptor genes

Expression of m1 and m3 muscarinic cholinergic receptors mRNAs was examined in rat hippocampus following either: (1) kindling to five Stage 5 amygdala-kindled seizures; or (2) eight electroconvulsive shock (ECS) seizures. Twenty-four hours after the last seizure of either type, there was a significant decrease in both m1 and m3 mRNAs in CA1, CA3 and the dentate gyrus subfields of the hippocampus. Twenty-eight days after the last seizure of either type, there was a significant increase in m1 mRNAs in CA1, CA3, and the dentate gyrus; for m3 mRNAs, there was a significant increase in CA3 28 days after the last ECS seizure, and in CA1 and CA3 28 days after the last kindled seizure. These results suggest that seizures alter the cholinergic system in the hippocampus, and that some of the alterations are very long-lasting.

Excitatory effects of muscarine on septohippocampal neurons: involvement of M3 receptors

Cholinergic mechanisms in the septohippocampal pathway contribute to several cognitive functions and impaired cholinergic transmission in this pathway may be related to the memory loss and dementia that accompanies normal aging and Alzheimer's disease and behavioral studies suggest that muscarinic mechanisms in the medial septum/diagonal band of Broca (MSDB) may contribute to these functions. The goal of the present study was to begin a characterization of the physiological and pharmacological effects of muscarine on antidromically identified septohippocampal neurons (SHNs). Muscarinic agonists produced a concentration-dependent excitation in >90% of SHNs tested using extracellular recordings in an in vitro rat brain slice preparation. The SHNs excited by muscarine had a broad range of conduction velocities (0.2 to 3.7 m/s; mean: 1.6+/-0.06 m/s; n=110), suggesting involvement of neurons with both slow (possibly cholinergic) and fast (possibly GABAergic) conducting fibers. The muscarine-induced excitations in SHNs were found not to be mediated via M1, M2 or M4 receptors, as they were not blocked by the M1-selective antagonists, pirenzepine or telenzepine or by the M2/M4-selective antagonist, methoctramine. In contrast, the M3-selective antagonist, 4-DAMP-mustard, blocked muscarinic excitations in a majority of SHNs, indicating the presence of M3 as well as non-M3-type responses. McN-A-343, an M1 and M5-selective agonist, excited 33% of neurons tested, confirming involvement of non-M3 receptors (possibly M5) and M3 receptors. Since the cholinergic and GABAergic MSDB neurons together innervate almost every type of hippocampal neuron, the effects of muscarine on SHNs would also have a profound effect on hippocampal circuitry.

Expression of GTPase-deficient Gialpha2 results in translocation of cytoplasmic RGS4 to the plasma membrane

The members of a recently identified protein family termed regulators of G-protein signaling (RGS) act as GTPase-activating proteins for certain Galpha subunits in vitro, but their physiological effects in cells are uncertain in the face of similar biochemical activity and overlapping patterns of tissue expression. Consistent with its activity in in vitro GTPase-activating protein assays, RGS4 interacts efficiently with endogenous proteins of the Gi and Gq subclasses of Galpha subunits but not with G12alpha or Gsalpha. Unlike other RGS proteins such as RGS9, RGS-GAIP, and Sst2p, which have been reported to be largely membrane-associated, a majority of cellular RGS4 is found as a soluble protein in the cytoplasm. However, the expression of a GTPase-deficient Gialpha subunit (Gialpha2-Q204L) resulted in the translocation of both wild type RGS4 and a non-Gialpha-binding mutant (L159F) to the plasma membrane. These data suggest that RGS4 may be recruited to the plasma membrane indirectly by G-protein activation and that multiple RGS proteins within a given cell might be differentially localized to determine a physiologic response to a G-protein-linked stimulus.

Neuropharmacological characterization of basal forebrain cholinergic stimulated cataplexy in narcoleptic canines

Basal forebrain (BF) cholinergic regulation of cataplexy was investigated in narcoleptic canines. Specific cholinergic agonists and antagonists, and excitatory or inhibitory amino acid neurotransmitter receptor agonists, were perfused through microdialysis probes implanted bilaterally in the BF of narcoleptic canines. Cataplexy was monitored using the food-elicited cataplexy test (FECT) and recordings of electroencephalogram, electrooculogram, and electromyogram. In narcoleptic canines, carbachol and oxotremorine (10(-5)-10(-3) M), but not McN-A-343 or nicotine (10(-4)-10(-3) M), produced a dose-dependent increase in cataplexy. In addition, N-methyl-d-aspartate (10(-4)-10(-3) M) and kainic acid (10(-5)-10(-4) M) did not have any effects, while muscimol (10(-3) M) produced a weak (P < 0.10) increase in cataplexy. In control canines, carbachol (10(-5)-10(-3) M), but not oxotremorine (10(-4)-10(-3) M), produced muscle atonia after the highest concentration in one of three animals. Carbachol (10(-3) M)-induced cataplexy in narcoleptic canines was blocked by equimolar perfusion with the muscarinic antagonists atropine, gallamine, and 4-DAMP but not pirenzepine. These findings indicate that carbachol-stimulated cataplexy in the BF of narcoleptic canines is mediated by M2, and perhaps M3, muscarinic receptors. The release of acetylcholine in the BF was also examined during FECT and non-FECT behavioral stimulation in narcoleptic and control canines. A significant increase in acetylcholine release was found in both narcoleptic and control BF during FECT stimulation. In contrast, simple motor activity and feeding, approximating that which occurs during an FECT, did not affect acetylcholine release in the BF of narcoleptic canines. These findings indicate that BF acetylcholine release is enhanced during learned emotion/reward associated behaviors in canines.

Mesolimbic component of the ascending cholinergic pathways: electrophysiological-pharmacological study

The cholinergic input from the pontomesencephalic cholinergic neurons to the diencephalic and basal forebrain structures has been implicated in a number of limbically controlled overt behaviors. The cellular mechanism by which the cholinergic terminals initiate behavioral manifestations is not clear. The objective of this study was to investigate the effects of the ascending cholinergic projection from the laterodorsal tegmental nucleus (LDT) on neuronal firing in the anterior hypothalamic-medial preoptic region (AHMP), known to be involved in agonistic behavior. Experiments were performed on urethan-anesthetized rats. Iontophoretic application of carbachol (CCh) into the vicinity of single cells in the AHMP caused a dose-dependent decrease in the mean firing rate of 83% of units and an increase in 10% of units. The inhibitory effect of CCh, but not the excitatory effect, was reversed by iontophoretic pretreatment with scopolamine. The inhibition of the firing rate was repeatable for the same dose of CCh and dose dependent. Electrical stimulation of neurons in the LDT caused a comparable, current-dependent decrease in the mean firing rate of AHMP neurons that also was reversed by pretreatment of neurons in the AHMP with scopolamine. The antagonizing effects of scopolamine were reversible with time. The same units in the AHMP that inhibited their firing to stimulation of the LDT also responded with a similar inhibition to local iontophoretic CCh. Finally, the fluorescent carbocyanine dye, 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide, (DiA), has been used as a retrograde axonal tracer and was injected into the recording sites immediately after the electrophysiological recordings. After 1 wk, DiA dye was found in numerous neurons in the LDT as shown by the fluorescence confocal microscopy. Results of the study suggest that LDT cholinergic neurons project and terminate in the AHMP and that their activation causes a decrease in the mean firing rate of the AHMP neurons. It is postulated that this inhibitory effect is implicated in the initiation of some of the behavioral patterns like defensive or alarm vocalization and behavioral inhibition.

Expression and localization of muscarinic receptors in P19-derived neurons

The muscarinic acetylcholine receptors are important in a variety of physiological processes such as induction of secretion from various glands and regulation of pacemaker activity, muscle tone, and neurotransmission. To date, the muscarinic receptor family includes five members (designated m1-m5), of which m1-m4 are abundant in brain and in peripheral tissues, and m5 is found exclusively in brain, and even there at very low levels. The expression of m1-m5 receptor subtypes was studied in neurons derived from the murine embryonal carcinoma cell line P19. These cells serve as a model system for differentiation and maturation of neurons resembling CNS neurons. Our results show that P19 neurons express mainly the m2, m3, and m5 subtypes. Low levels of m1 receptors are also detected and m4 subtype is practically absent. Furthermore, muscarinic receptors in P19 neurons are functional in activating second messenger signaling pathways. The localization of m2 receptors is predominantly presynaptic, whereas the m5 subtype is mainly postsynaptic. Consequently, P19 cells provide a model system for the study of pre- and postsynaptic muscarinic acetylcholine-receptor subtypes in a proper neuronal context. This is particularly valid for the rare m5 receptors.

Kainic acid-induced generalized seizures alter the regional hippocampal expression of the rat m1 and m3 muscarinic acetylcholine receptor genes

We investigated the gene expression responses using in situ hybridization with radiolabelled riboprobes for the m1 and m3 subtypes of muscarinic cholinergic receptors in the rat hippocampus following a brief (5-min) kainic acid-induced behavioral seizure. The kainic acid was intraperitoneally administered, and the ensuing generalized convulsive seizure terminated with diazepam. Our results demonstrate that the expression of the m1 subtype was significantly reduced in the CA1, CA3 and the dentate granule cells by 3 h after the administration of kainic acid while no significant change was observed in any hippocampal subfield for the m3 subtype. By 6 h post challenge, the m1 subtype was still decreased in all hippocampal subfields examined, while the m3 subtype remained unchanged from vehicle injected control. At 24 h post challenge, both the m1 and m3 subtypes were significantly reduced in the CA1 and CA3 subfields; the expression of the m1 subtype in the dentate granule cells, however, had recovered to levels indistinguishable from vehicle-injected control. These results demonstrate that epileptiform activity induced by kainic acid administration promotes alterations in the expression levels for both the m1 and m3 muscarinic receptor genes, and suggest that the activity of this neuromodulatory system in the hippocampus may be altered through activity-dependent mechanisms at early times following seizures.

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

A number of pharmacological, anatomical, and immunological studies have previously addressed the subtype identity of the hippocampal muscarinic pre-synaptic autoreceptor. A preponderance of findings indicate that it is of the M2 pharmacological type. Both the m2 and m4 molecular subtypes exhibit M2 pharmacology and there are few drugs that differentiate between these receptors. Pharmacological attempts at defining the hippocampal autoreceptor have yielded conflicting results. The basal forebrain is relatively enriched in m2 muscarinic receptor mRNA and protein, and lesions that denervate the hippocampus of its basal forebrain cholinergic input have shown a decrement in m2, but not m4, receptor protein in the hippocampus. Thus, the anatomical data obtained to date tend to support the view that the m2 subtype is expressed as the hippocampal autoreceptor. We have combined in situ hybridization histochemistry (ISHH) with immunocytochemistry to choline acetyltransferase to examine whether mRNA for the m4 subtype of muscarinic receptor is expressed in central cholinergic neurons. The m4 muscarinic mRNA was found at moderate levels in all subdivisions of the cholinergic basal forebrain, including the medial septum/diagonal band complex (MS/DB). The m4 mRNA was also found in striatal cholinergic interneurons, in the cholinergic reticular core of the upper brainstem, and in brainstem cholinergic motor neurons. Muscarinic m4 receptor mRNA was also found in many non-cholinergic cells in the brain. For example, the hippocampal pyramidal neurons, dentate gyrus granule cells, and entorhinal cortical pyramidal neurons express relatively high levels of m4 mRNA, while in the brainstem the dorsal raphe and pontine reticular nuclei express relatively high levels of this mRNA. The finding of m4 mRNA in the MS/DB cholinergic neurons suggests that this receptor protein might be expressed as an autoreceptor in hippocampal cholinergic terminals.

Regulation of cellular tyrosine phosphorylation by stimulatory and inhibitory muscarinic acetylcholine receptors

Tyrosine phosphorylation is a key signaling event in transmembrane and cytoplasmic signal transduction. The m5 muscarinic receptor (m5AChR) responds to ligand stimulation with calcium influx and protein phosphorylation. In contrast, neither of these responses has been associated with m4AChR signaling. We hypothesized that activation of the m5AChR would alter tyrosine phosphorylation patterns spatially within the cell and in a calcium influx-sensitive manner. CHO cells stably transfected with m4- or m5AChRs were assessed for spatial localization and quantity of phosphotyrosylated proteins in response to receptor activation. Results were confirmed by immunoblot of whole cell lysates and cytosol and membrane fractions. m5AChR activation increased tyrosine phosphorylation in all subcellular compartments; coincubation with CAI, a calcium influx inhibitor, reduced phosphorylation below basal levels. Western blot confirmed the change of phosphotyrosylated proteins of M(r) 70, 85, 120, and 180 kDa in whole and fractionated cells. PLC-gamma, used as a marker of m5AChR activity, was increased in quantity and degree of phosphorylation in CHOm5 cell membranes and microvilli in response to receptor activation. Both the quantitative increase and tyrosine phosphorylation of PLC-gamma in membrane fractions was inhibited by CAI. In contrast, CC treatment of CHOm4 cells reduced tyrosine phosphorylation throughout the cell. CC-stimulation of m5AChR cells caused a calcium influx-sensitive increase in phosphotyrosylated proteins throughout the cell, though predominantly in the membrane and microvilli. Activation of the m5AChR induces tyrosine phosphorylation, whereas activation of the m4AChR inhibited tyrosine phosphorylation below baseline, further demonstrating the dichotomy between signaling of these two AChRs.

Ultrastructural study of cholinergic and noncholinergic neurons in the pars compacta of the rat pedunculopontine tegmental nucleus

A group of medium-to-large cholinergic neurons situated in the dorsolateral mesopontine tegmentum comprises the pedunculopontine tegmental nucleus (PPT). The PPT pars compacta (PPT-pc), which occupies the lateral part of the caudal two-thirds of the nucleus, contains a dense aggregation of cholinergic neurons. In the present study, we have employed immunohistochemistry for choline acetyltransferase (ChAT) and electron microscopy to investigate the ultrastructure and synaptic organization of neuronal elements in the PPT-pc. Our results demonstrate that: (1) ChAT-immunoreactive (i.e., cholinergic) PPT-pc neurons are characterized by abundant cytoplasm and organelles, and have few axosomatic synapses (both asymmetric and symmetric); (2) ChAT-immunoreactive dendrites comprise 6-15% of total dendritic elements in the neuropil; the mean percentage of dendritic membrane covered by synaptic terminals is approximately 15%, and nearly all synapses with ChAT-immunoreactive dendrites are asymmetric; (3) within the boundaries described by cholinergic PPT-pc, there are noncholinergic neurons which, in contrast, exhibit a lucent cytoplasm and a higher frequency of axosomatic synapses (10.5% versus 3.7% for cholinergic neurons); and (4) noncholinergic neurons are morphologically heterogeneous with one subpopulation exhibiting a mean diameter that approximates that of cholinergic cells (i.e., > 15 microns and < 20 microns) and a very high frequency of axosomatic synapses (> 20%). Only 0.2-0.7% of terminal elements in the neuropil were ChAT-immunoreactive and these were not observed to synapse with cholinergic dendrites or somata. This relative paucity of terminal labeling and lack of cholinergic-cholinergic interactions seems inconsistent with the recognized and prominent physiological actions of acetylcholine on cholinergic PPT-pc neurons, and suggests a methodological limitation and/or a potential paracrine-like action of nonsynaptically released acetylcholine in the PPT region.

Differential regulation of expression of rat hippocampal muscarinic receptor subtypes following fimbria-fornix lesion

Quantitative RNase protection assays were performed to determine the levels of muscarinic receptor subtype (m1-m5) mRNAs in rat hippocampi. Results showed that the m1, m3, and m4 subtype mRNAs were expressed at relatively high levels, but the levels of the m2 and m5 subtype were very low. Three weeks following aspiration lesions of the fimbria-fornix to produce cholinergic denervation of the hippocampus, non-M1 receptors (non-pirenzepine displaceable [3H]quinuclindinyl benzilate binding sites) in the hippocampus were increased significantly, which correlated with increases in the levels of hippocampal m3 and m4 receptor mRNAs (m3: +24% and m4: +41%). These findings indicate that multiple muscarinic receptor subtypes are expressed in the hippocampus with the m3 and m4 subtypes predominantly postsynaptic to the septohippocampal cholinergic terminals.

Critical role for M3 muscarinic receptors in paradoxical sleep generation in the cat

In the cat, microdialysis application of 200 microM carbachol to the peri-locus coeruleus alpha (peri-LC alpha) of the mediodorsal pontine tegmentum produced a marked (< or = 5-fold) increase in paradoxical sleep. This effect was blocked by 5-50 microM 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), an M1/M3-selective muscarinic receptor antagonist. In contrast, the effect was not reversed by methoctramine, an M2-selective antagonist, or pirenzepine, an M1-selective antagonist, even at concentrations as high as 500 microM. In addition, unilateral application of 5 microM 4-DAMP alone to the peri-LC alpha induced both a > 60% decrease in paradoxical sleep and a state of paradoxical sleep without atonia, whereas 50 microM pirenzepine and 500 microM methoctramine had no effect. Our findings are further evidence for the important role played by the peri-LC alpha and demonstrate a critical role for M3 muscarinic cholinergic receptors in the generation of paradoxical sleep.

Muscarinic control of amyloid precursor protein secretion in rat cerebral cortex and cerebellum

It was previously shown by us and by others that activation of muscarinic acetylcholine receptors evoke amyloid precursor protein (APP) secretion in various cell lines. Here we examined if such muscarinic control of APP secretion occurs also in normal brain tissues. We found that the secretion of APP from rat cerebrocortical slices (rich in M1 receptors) was significantly increased by K+ depolarization, the non-selective agonist, carbachol (CCh), and the M1-selective agonist, AF102B. CCh also increased APP secretion from cerebellar slices (rich in M2 receptors) while AF102B had no significant effect in this brain region. Despite of its stimulatory effect on APP release in the cerebellum, CCh had no effect on phosphoinositide (PI) metabolism in this brain region. In the cerebral cortex PI metabolism was significantly increased by CCh but only partially increased by AF102B. These results suggest that APP secretion in the brain is mediated via muscarinic receptors. In the cerebral cortex APP secretion seems to be regulated via M1 receptors. Our results also suggest that PI metabolism is not a pronounced step in mediating APP processing.

Expression of m1-m4 muscarinic acetylcholine receptor immunoreactivity in septohippocampal neurons and other identified hippocampal afferents

Muscarinic cholinergic transmission plays an important role in modulating hippocampal activity and many higher brain functions. Many of the modulatory effects of acetylcholine on hippocampal function result from direct effects in the hippocampus or from actions on the hippocampal afferent neurons. At each site, the differential expression of a family of five distinct but related receptor subtypes governs the nature of the response. The aim of the present study was to identify the subtypes expressed in the hippocampal afferent neurons by combining retrograde tracing with immunocytochemistry. The retrograde tracer, wheat germ agglutinin conjugated to horseradish peroxidase, was injected into the hippocampus unilaterally to label afferent neurons, and was combined with muscarinic (m) acetylcholine (ACh) receptors (mAChRs) with immunocytochemistry to identify the m1-m4 subtypes expressed. The retrogradely labeled cells in the basal forebrain that contribute to the septohippocampal pathway were found to express m2, m3, and, to a lesser extent, m1. Commissural/associational pathway neurons, which were identified by retrogradely labeled cells in the ipsi- and contralateral dentate gyrus, expressed m1, m3, and m4. The retrogradely labeled cells in the entorhinal cortex of the perforant pathway expressed predominantly m1 and m3, with fewer neurons expressing m2 and m4. Raphe-hippocampal cells were found to express m1. Thus, this study provides evidence for the diversity of mAChR subtypes expressed in neurons that project to the hippocampus. The complex modulation by acetylcholine of hippocampal function, therefore, is governed not only by the variety of mAChRs expressed in the hippocampus but also by their differential expression in extrinsic hippocampal afferents.

Axotomy-induced loss of m2 muscarinic receptor mRNA in the rat facial motor nucleus precedes a decrease in concentration of muscarinic receptors

The abundance of muscarinic receptors and m2 muscarinic receptor mRNA in the facial nuclei of rats was evaluated by autoradiographic procedures at various times up to 14 days after transection of the right facial nerve. Receptors were labelled by in vitro incubation of brain sections with L-[3H]quinuclidinyl benzilate, while in situ hybridization with a 35S-labelled oligonucleotide was used to identify m2 muscarinic receptor mRNA in neighbouring sections. The right and left facial nuclei of non-operated control rats appeared equivalent in abundance of muscarinic receptors (359 +/- 8 versus 376 +/- 9 fmol per mg tissue, n = 5) and the presence of m2 mRNA. Axotomy had no effect on the concentration of receptors in the contralateral facial nucleus but caused a gradual loss of receptors from the ipsilateral side. No change was detected at 1 day after nerve transection, but a 23% decrease relative to the contralateral facial nucleus had occurred by 3 days. A maximum decrease of 51% was achieved by 1 week after nerve transection. By comparison, m2 mRNA was nearly eliminated from the ipsilateral facial nucleus at 1 day post-taxonomy and remained depleted for the duration of study. Previous work has established that no significant loss of motoneurons occurs within this period. Accordingly, it is postulated that axonal injury inhibits transcription of the m2 muscarinic receptor gene, resulting in a later decrease in muscarinic receptor protein expression.

Autoradiographical and immunohistochemical analysis of receptor localization in the central nervous system

Quantitative receptor autoradiographic methods have been widely used over the past two decades. Some of the advantages and limitations of these techniques are reviewed here. Comparison with immunohistochemical and in situ hybridization methods is also highlighted, as well as the use of these approaches to study receptor gene over-expression in cell lines. Together, data obtained using these various methodologies can provide unique information on the potential physiological roles of a given receptor protein and/or binding sites in various tissues.

Putative cholinergic neurons of the pedunculopontine tegmental nucleus projecting to the superior colliculus consist of sensory responsive and unresponsive populations which are functionally distinct from other mesopontine neurons

We examined the sensory properties of putative cholinergic neurons of the pedunculopontine tegmental nucleus projecting to the superior colliculus. Projection neurons were identified by antidromic activation from the contralateral posterior superior colliculus; stimulation of the anterior half was essentially ineffective. Identified neurons fell into two groups, one with a somatosensory input (39%) and one without a sensory input. Somatosensory responsive projection neurons were low threshold and rapidly adapting. Receptive fields were contralateral (94%) and predominantly orofacial (57%). Sensory responsive and unresponsive projection neurons were intermingled within the pedunculopontine tegmental nucleus as identified histologically by reduced nicotinamide adenine dinucleotide phosphate diaphorase or acetylcholinesterase. The properties of neurons outside the nucleus differed significantly. They could not be activated antidromically from the superior colliculus; many had ipsi- or bilateral receptive fields (75%) and wide dynamic range or nociceptive response patterns (52%). The presence of two functionally distinct groups of projection neurons implies a dual or more complex modulation of tectal neurons by the pedunculopontine tegmental nucleus. The pedunculopontine tegmental nucleus has been implicated in a multiplicity of behaviors and, in particular, in rapid eye movement sleep and alerting or arousal functions. By virtue of its many connections with the basal ganglia, limbic system and reticular structures, the projection to the superior colliculus of two distinct groups may provide an important differentiating element of the tectal organization of orienting and spatial cognitive behavior.

The pedunculopontine nucleus--auditory input, arousal and pathophysiology

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

Neurotransmitter receptor histochemistry: the contribution of in situ hybridization

Molecular cloning of neurotransmitter receptors has enabled the application of in situ hybridization histochemistry to the study of the regional distribution and cellular localization of receptor mRNAs with an unprecedented degree of selectivity. This has resulted in a large body of information including detailed maps of the distribution of receptor subtype transcripts, the establishment of neurotransmitter phenotype and connectivity of receptor-expressing cells, and the relationship between receptor transcripts and binding sites. In this minireview, we discuss and illustrate with a number of examples the contributions of in situ hybridization histochemistry to the study of receptor distribution in brain tissue.

Toxins from mamba venoms: small proteins with selectivities for different subtypes of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors exist as five subtypes that are widely distributed throughout the body. Conventional pharmacological agents are not highly selective for particular subtypes, making investigations on the functional significance of the subtypes difficult. Recent findings indicate that mamba snake venoms contain several small proteins ('muscarinic toxins') that are highly specific for muscarinic receptors, and are discussed in this review by Diana Jerusalinsky and Alan Harvey. Some of these toxins act selectively and irreversibly on individual subtypes of receptor, and some are antagonists, while others activate muscarinic receptors. The toxins should be useful tools in studies of the functions of individual receptor subtypes, and comparisons of their three-dimensional structures should give clues about how selective binding to muscarinic receptor subtypes can be obtained.