Distribution Of Muscarinic Cholinergic Receptors Research Articles

Differential cholinoceptor subtype-dependent activation of signal transduction pathways in neonatal versus adult rat atria

In this study, we investigated the expression and distribution of muscarinic cholinergic receptors (mAChRs) and the different signaling pathways associated with mAChR activation in atria isolated from adult and neonatal rats. Carbachol stimulation of mAChRs in both neonatal and adult rat atria led to a negative inotropic response with activation of phosphoinositide hydrolysis, an increase in cyclic GMP levels, and a decrease in cyclic AMP production. However, compared with adult atria, neonatal atria showed hypersensitivity in the contractile effect induced by carbachol. Pharmacological analysis with mAChR antagonists indicated that M 1 and M 2 mAChR subtypes are important mediators of the response to carbachol in neonatal atria. In contrast, in adult atria the effect of the agonist was coupled only to the M 2 mAChR subtype. Moreover, an increased number of total mAChRs was labeled in neonatal atrial membranes compared with those of adults. Although a predominant M 2 mAChR population is expressed in atria at both stages of development studied, competition binding parameters calculated for carbachol indicated the presence of high-affinity binding sites, with higher affinity in neonates than in adults. These results suggest that the differences observed between neonatal and adult atria in their response to a cholinergic agonist may be related to differential expression of mAChR subtypes and/or changes in functional coupling of mAChR subtypes during development.

Motor activity and quantitative autoradiographic analysis of muscarinic receptors in the brain of rats subjected to the forced swimming test.

A cholinergic dysfunction has been involved in the neurobiological mechanisms of stress and depression. In the present study, we determined the autoradiographic distribution of muscarinic cholinergic receptors in the brain of rats subjected to the forced swimming test for 15 days. Motor activity was automatically analyzed daily before swimming. In the forced swimming test group, both total horizontal activity and ambulatory movements exhibited a significant decrease, when the data from 1st and 15th day were compared. Neither the affinity of [3H]-quinuclidinyl benzilate nor the maximal number of receptors were affected by the forced swimming test in the caudate-putamen, cortex, and hippocampus. The distribution of [3H]-quinuclidinyl benzilate binding sites did not show significant differences in the 30 analyzed areas. Further analysis of muscarinic receptor subtypes after forced swimming test would be necessary to discard any cholinergic involvement.

Three‐dimensional distribution of [3H] quinuclidinyl benzilate binding to muscarinic cholinergic receptors in the developing human brainstem

Acetylcholine has been implicated in brainstem mechanisms of cardiac and ventilatory control, arousal, rapid eye movement (REM) sleep, and cranial nerve motor activity. Virtually nothing is known about the developmental profiles of cholinergic perikarya, fibers, terminals, and/or receptors in the brainstems of human fetuses and infants. This study provides baseline information about the quantitative distribution of muscarinic cholinergic receptors in fetal and infant brainstems. Brainstem sections were analyzed from 6 fetuses (median age: 21.5 postconceptional weeks), 4 premature infants (median age: 26 postconceptional weeks), and 11 infants (median age: 53 postconceptional weeks). One child and three adult brainstems were examined as indices of maturity for comparison. The postmortem interval in all cases was less than or equal to 24 hours (median: 10 hours). Muscarinic receptors were localized by autoradiographic methods with the radiolabeled antagonist [3H]quinuclidinyl benzilate ([3H]QNB). Computer-based methods permitted quantitation of [3H]QNB binding in specific nuclei and three-dimensional reconstructions of binding patterns. By midgestation, muscarinic cholinergic receptor binding is already present and regionally distributed, with the highest binding levels in the interpeduncular nucleus, inferior colliculus, griseum pontis, nucleus of the solitary tract, motor cranial nerve nuclei, and reticular formation. During the last half of gestation, [3H]QNB binding decreases in most, but not all of the nuclei sampled. The most substantial decline occurs in the reticular formation of the medulla and pons, a change that is not fully explained by progressive myelination and lipid quenching. Binding levels remain essentially constant in the inferior olive and griseum pontis. Around the time of birth or shortly thereafter, the relative distribution of binding becomes similar to that in the adult, with the highest levels in the interpeduncular nucleus and griseum pontis, although binding levels are higher overall in the infant. In the rostral pontine reticular formation, paramedian bands of high muscarinic binding are present which do not correspond to a cytoarchitectonically defined nucleus. By analogy to animal studies, these bands may comprise a major cholinoreceptive region of the human rostral pontine reticular formation involved in REM sleep. In the human interpeduncular nucleus in all age periods examined, muscarinic binding localizes to the lateral portions bilaterally, indicative of a heterogeneous chemoarchitecture. Muscarinic binding is high in the arcuate nucleus, a component of the putative respiratory chemosensitive fields along the ventral surface of the infant medulla. This observation is consistent with the known effects of muscarinic agents on chemosensitivity and ventilatory responses applied to the ventral medullary surface in animal models. The nonuniform distribution of muscarinic binding in the caudorostral plane in individual brainstem nuclei, as illustrated by three-dimensional reconstructions, underscores the need for rigorous sampling at precisely matched levels in quantitative studies. This study provides basic information toward understanding the neurochemical basis of brainstem disorders involving dysfunction of autonomic and ventilatory control, arousal, and REM sleep in preterm and full-term newborns and infants and for developing cholinergic drugs for such disorders in the pediatric population.

3-D computer simulations of resolution effects of multidetector point focusing SPECT imaging

The authors present an efficient algorithm and the results of its application in simulating the three-dimensional (3-D) projection data resulting from a 3-D distribution of radioactivity. The algorithm was applied to a series of geometrical mathematical phantoms and to a realistic mathematical brain phantom. The authors simulated the projection data from a multidetector single-photon emission computed tomography (SPECT) system with point focusing collimators. The simulated projection data were then reconstructed using the manufacturer's software. The objects simulated included simple geometrical solids such as spheres and sheets, as well as the distribution of muscarinic cholinergic receptors in a realistic brain slice. Spheres were chosen as a model for brain structures such as caudate nucleus, thalamus, and cerebellum; sheets were selected as representing lateral cortical gray matter regions. The results of these simulations indicate the existence of significant qualitative and quantitative artifacts in reconstructed human brain images.

Muscarinic cholinergic receptors in the songbird and quail brain: A quantitative autoradiographic study

In order to clarify the neuroanatomical basis for postulated muscarinic cholinergic control of a wide array of physiological processes in birds, the distribution of muscarinic cholinergic receptors in the brain of three avian species was investigated by quantitative autoradiography. The species consisted of two passerines (songbirds), the European starling (Sturnus vulgaris) and the song sparrow (Melospiza melodia), and one galliform, the Japanese quail (Coturnix coturnix japonica). [3H]N-methyl scopolamine (NMS), a muscarinic cholinergic antagonist was used as the ligand to label the receptors. Initial experiments demonstrated that the binding of this ligand in the three species is saturable in the nanomolar range and has a high affinity (Kd = +/- 0.6 nM). Displacement experiments revealed that three muscarinic ligands competed in an order of potency characteristic of the mammalian muscarinic receptor (i.e., atropine greater than oxotremorine greater than carbachol) for NMS binding in the avian brain. In all three species, portions of the basal ganglia, such as the parolfactory lobe and the paleostriatum augmentatum, exhibited the highest density of binding. On the other hand, the paleostriatum primitivum, the avian homologue of the mammalian globus pallidus, contained very few binding sites. Other telencephalic sites, such as the ventral and dorsal hyperstriatum, also revealed relatively high receptor density. However, the neostriatum and especially the ectostriatum showed much lower levels. In the hypothalamus, in all three species, specific binding could be observed in the ventromedial nucleus and adjacent areas. The paraventricular nucleus also showed moderate levels of binding density, especially in the two songbird taxa. At a more rostral level, the preoptic area showed low levels of binding. In the quail, the sexually dimorphic nucleus of the preoptic area was clearly outlined in the autoradiograms by the low level of binding sites compared to the surrounding areas. In the two passerine species, nuclei of the song system were identified by either high or low levels of NMS binding. High binding defined area X and the mesencephalic nucleus, intercollicularis (ICo). In contrast, the robust nucleus of the archistriatum and the magnocellular nucleus of the anterior neostriatum showed low levels of binding in comparison with the surrounding tissue. None of these nuclei were visible in the quail autoradiograms except for ICo, which appeared as in the passerines as a heavily labelled area surrounding the lightly labelled nucleus mesencephalicus lateralis pars dorsalis. In all three species, the hippocampal complex was devoid of NMS binding except for two lateral dark bands that were present along the entire rostral to caudal extent of the hippocampus.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional differences in the distribution of muscarinic cholinergic receptors in the macaque cerebral cortex.

The in vitro autoradiographic technique was used to characterize the density and laminar distribution of muscarinic cholinergic receptors in 12 cytoarchitectonic areas in the frontal, parietal, and occipital lobes of the rhesus monkey. The entire population of muscarinic receptors was labeled with [3H]quinuclidinyl-benzilate; the M1 receptor subtype was labeled with [3H]pirenzepine; and the density of the M2 receptor subtype was estimated by subtracting the density of M1 receptors from the total population. The overall density of M1 and M2 receptor subtypes was similar throughout the cerebral cortex. However, their laminar distribution varied regionally. In cortical regions of the parietal and occipital lobes and in the primary motor cortex of the frontal lobe, both M1 and M2 receptor subtypes were concentrated in the supragranular layers. By contrast, in prefrontal cortical areas, the combined population of M1 and M2 receptors was evenly distributed across the cortical layers, though M1 receptors were most dense and M2 receptors least dense in layer IV. The difference in the distribution of cholinergic receptors in the prefrontal cortex compared to other neocortical areas reveals a degree of chemoarchitectural specificity of this region with respect to cholinergic markers that has escaped immunohistochemical and other anatomical and functional techniques.

Autoradiographic Analysis of Alpha-Adrenoceptors and Muscarinic Cholinergic Receptors in the Hyperplastic Human Prostate

Radioligand receptor binding and autoradiography were used to characterize, localize and compare alpha-1 and alpha-2 adrenoceptors and muscarinic cholinergic receptor populations in human benign prostatic hyperplastic tissue. The binding of selective alpha-1 and alpha-2 ligands, [3H]- prazosin and [3H]-UK 14,304, to homogenates of human central and peripheral prostate was saturable and of high affinity. Scatchard analysis produced an equilibrium dissociation constant (KD) of 0.51 ± 0.10 nM for alpha-1 adrenoceptors, and 2.34 ± 0.40 nM for alpha-2 adrenoceptors. The mean densities, Bmax, of alpha-1 and alpha-2 adrenoceptors identified in the human adenomatous prostate were 65.9 ± 12.9 and 36.1 ± 7.0 fmoles/mg. protein respectively.Receptor autoradiography was used to examine the distribution of muscarinic cholinergic receptors ([3H]-QNB), alpha-1 adrenoceptors ([3H]-prazosin]), and alpha-2 adrenoceptors ([3H]-rauwolscine) on consecutive sections of benign hyperplastic prostatic tissue. Although both subtypes of adrenoceptor were seen in the stromal component of the hyperplastic prostate, there was a substantial predominance of alpha-1 adrenoceptors. A densitometric computer-assisted analysis was performed on the autoradiographic slides to determine the mean ratio of specific alpha-1: alpha-2 adrenoceptors in the stromal compartment of the hyperplastic tissue. The ratio, expressed as % grain occupancy/unit area, was 3.9 ± 0.75, which is in agreement with a functional alpha-1 adrenoceptor predominance shown in previous studies. Although sparsely distributed in the stroma, a dense alpha-2 adrenoceptor population was seen in association with blood vessels, and in close proximity to the base of some of the [3H]-QNB-labelled prostatic glandular epithelial cells.These results clearly illustrate a differential localization of specific receptors to precise prostatic regions, and future correlation between function and location might enable more selective therapeutic treatment of benign prostatic hyperplasia.

Differential modification of muscarinic cholinergic receptors in the hippocampus of patients with Alzheimer's disease: an autoradiographic study

We have used quantitative light microscopic autoradiographic techniques to analyze changes in muscarinic cholinergic receptors in the hippocampus in Alzheimer type dementia (ATD). The density and distribution of muscarinic cholinergic receptors has been correlated with the density of neurons, neuritic plaques and neurofibrillary tangles in the CA1 subfield of the hippocampus of control and ATD patients. The number of pyramidal cells per mm 2 in the CA1 sector was significantly decreased in ATD cases as compared to controls, although there were large variations among cases. The most marked reductions in cell counts were observed in patients with a history of profound dementia. The densities of muscarinic receptors, as well as the proportions of M 1 and M 2 subtypes, in the CA1 sector and dentate gyrus were not significantly different between ATD and old non-demented patients. Neuritic plaques, even in high numbers, did not affect the density of muscarinic receptors; moreover, the densities of receptors over the neuritic plaques did not differ from the surrounding neuropil. However, in some ATD cases there was a marked decrease in the concentration of these receptors in the CA1 sector and subiculum, with no change in the proportions of muscarinic receptor subtypes. These patients exhibited frequent extracellular remnants of neurofibrillary tangles (ghost tangles), but scarce neuritic plaques, and were those showing severe losses of pyramidal cells. There was a significant positive correlation between the total concentration of muscarinic receptors in the CA1 and the density of pyramidal cells, suggesting that decreases in receptor concentration result from a severe neuronal loss. We observed that the ratio of muscarinic receptors per pyramidal cell was significantly increased in ATD patients. This might indicate a possible upregulatory mechanism for muscarinic receptors in the population of remaining neurons in ATD. However, decreases of receptor numbers following severe neuronal fall out suggest that compensatory mechanisms are no longer possible in such cases. The question is raised whether these differences between cases reflect different diseases or different stages of the same disease.

Distribution of muscarinic cholinergic receptors in the dorsal vagal complex and other selected nuclei in the human medulla

Muscarinic cholinergic receptors were localized in human brainstem by quantitative autoradiography, using the radioligand [ 3H]quinuclidinyl benzilate. Receptor densities were highest in the hypoglossal nucleus. The second highest density was found in the medial region of the nucleus of the solitary tract (NTS). Moderately high numbers of receptors were present in the dorsal motor nucleus of the vagus, the dorsal NTS, subpostremal NTS, lateral NTS and ventral NTS. Intermediate densities were present in the dorsal and medial accesssory nuclei of the inferior olive and the spinal trigeminal nucleus pars interpolaris. Low densities were found in the area postrema, principle nucleus of the inferior olive, gracile nucleus, cuneate nucleus and the tractus of the NTS. Muscarinic cholinergic receptors in the dorsal vagal complex are an important component of the neural substrate governing visceral function. These receptors may be the central site of action of anticholinergic medications in suppressing emesis.

On the cellular localization of cerebellar muscarinic receptors: An autoradiographic analysis of weaver, reeler, purkinje cell degeneration and staggerer mice

Light microscopic autoradiography of [ 3H]quinuclidinyl benzilate binding sites was used to study the distribution of muscarinic cholinergic receptors in mouse mutants which have abnormalities affecting specific cerebellar cell types. In the normal C57BL/6J mouse, binding sites were distributed throughout the cerebellar cortex, with the highest levels in the granule cell layer and deep cerebellar nuclei. Normal binding site density was observed in the cerebellum of the weaver mutant in which the majority of granule cells had degenerated. The density of [ 3H]quinuclidinyl benzilate binding sites was elevated in the cortex of the reeler, despite a reduction in the number of granule cells. The concentration of binding sites was also high over the Purkinje cell masses where granule cells were largely absent. No significant reduction in cortical [ 3H]quinuclidinyl benzilate binding site density was detected in the Purkinje cell degeneration mutant, in which essentially all Purkinje cells had degenerated. In contrast, receptor binding in the deep cerebellar nuclei of this mutant was significantly increased. A substantial increase in labeling was observed in the cortex and deep nuclei of the staggerer cerebellum in which a large fraction of Golgi II cells, Purkinje cells, granule cells and mossy fibers have degenerated. We discuss the possibility that the persistence of [ 3H]quinuclidinyl benzilate binding sites in all four mutants may imply a non-neuronal localization for a large proportion of muscarinic receptors in the mouse cerebellar cortex.

Neurotransmitter receptors in the avian brain. II. Muscarinic cholinergic receptors

The characteristics and distribution of muscarinic cholinergic receptors were examined in the pigeon brain using in vitro receptor autoradiography. The antagonist N-[ 3 H] methylscopolamine was used as ligand and the presence of the putative subtypes M 1 and M 2 of the muscarinic cholinergic receptors examined using carbachol and pirenzepine as displacers. The highest densities of muscarinic cholinergic receptors in the pigeon brain were localized in the paleostriatum augmentatum and the lobus parolfactorius, areas homologous to the mammalian corpus striatum. In contrast, the paleostriatum primitivum, corresponding to the mammalian globus pallidus, was poor in muscarinic cholinergic receptors. The rest of the telencephalon was also rich in muscarinic cholinergic receptors, while thalamic, hypothalamic and brainstem areas, as well as the tectum, presented intermediate densities, similar to the mammalian brain. Exceptions to that were the hippocampus, which was poorly labeled in the bird brain and the cerebellum, which presented intermediate to high densities of muscarinic cholinergic receptors in the bird brain. Preliminary pharmacological studies suggest differences between avian and mammalian receptor subtypes, since carbachol and pirenzepine apparently did not recognize two different receptor subpopulations.

Quantitative light microscopic autoradiographic localization of cholinergic muscarinic receptors in the human brain: Forebrain

The distribution of muscarinic cholinergic receptors in the human forebrain and cerebellum was studied in detail by quantitative autoradiography using N-[ 3H]methylscopolamine as a ligand. Only postmortem tissue from patients free of neurological diseases was used in this study. The highest densities of muscarinic cholinergic receptors were found in the striatum, olfactory tubercle and tuberal nuclei of the hypothalamus. Intermediate to high densities were observed in the amygdala, hippocampal formation and cerebral cortex. In the thalamus muscarinic cholinergic receptors were heterogeneously distributed, with densities ranging from very low to intermediate or high. N-[ 3H]Methylscopolamine binding was low in the hypothalamus, globus pallidus and basal forebrain nuclei and very low in the cerebellum and white matter tracts. The localization of the putative muscarinic cholinergic receptors subtypes M 1 and M 2 was analysed in parallel using carbachol and pirenzepine at a single concentration to partially inhibit N-[ 3H]methylscopolamine binding. Mixed populations of both subtypes were found in all regions. M 1 sites were largely predominant in the basal ganglia, amygdala and hippocampus and constituted the majority of muscarinic cholinergic receptors in the cerebral cortex. M 2 sites were preferentially localized in the diencephalon, basal forebrain and cerebellum. In some areas such as the striatum and substantia innominata there was a tendency to lower densities of muscarinic cholinergic receptors with increasing age. In general, we observed a slight decrease in M 2 sites in elderly cases. Muscarinic cholinergic receptor concentrations seemed to be reduced following longer postmortem periods. The distribution of acetylcholinesterase was also studied using histochemical methods and compared with the localization of muscarinic cholinergic receptors and other cholinergic markers. The correlation between the presence of muscarinic cholinergic receptors and the involvement of cholinergic mechanisms in the function of specific brain areas is discussed. Their implication in neurological diseases is also reviewed.

Muscarinic cholinergic receptors in the rat deep cerebellar nuclei: A quantitative autoradiographic study

The distribution of muscarinic cholinergic receptors within the rat deep cerebellar nuclei was analyzed using in vitro receptor binding of [ 3H]quinuclidinylbenzilate (QNB) in conjunction with autoradiography. The highest density of QNB binding sites occurred in the lateral cerebellar (dentate) nucleus. Interpositus nuclei displayed an intermediate density of muscarinic cholinergic binding sites with the posterior interpositus nucleus demonstrating higher binding than the anterior nucleus. The fastigial (medial) cerebellar nucleus exhibited the lowest levels of QNB binding among the four cerebellar nuclei. These results indicate that muscarinic cholinergic receptors are present in the deep cerebellar nuclei and that differences in receptor density occur among the four nuclear groups.

Characterization of an altered membrane form of the beta-adrenergic receptor produced during agonist-induced desensitization.

Incubation of 1321N1 human astrocytoma cells with 1 microM isoproterenol rapidly results in the conversion of a portion of the beta-adrenergic receptors to a membrane form that can be separated from markers for the plasma membrane by sucrose density gradient or differential centrifugation. This "light peak" form of the receptor reaches a maximal level within 10 min of incubation of cells with catecholamine. Two types of experiments suggest that the early phase of catecholamine-induced desensitization of the beta-adrenergic receptor-linked adenylate cyclase can be separated into at least two reactions. First, the agonist-induced loss of catecholamine-stimulated adenylate cyclase activity precedes the appearance of beta-adrenergic receptors in the light peak fraction by 1-2 min. Second, pretreatment of cells with concanavalin A prior to induction of desensitization blocks the formation of the light peak form of beta-adrenergic receptors without blocking the "uncoupling" reaction as measured by catecholamine-stimulated adenylate cyclase activity. Specificity for the reaction that converts beta-adrenergic receptors to the light peak form is indicated by the lack of a catecholamine-induced alteration in the sucrose density gradient distribution of muscarinic cholinergic receptors, adenylate cyclase or the guanine nucleotide-binding proteins, Ns and Ni. The light peak of beta-adrenergic receptors migrates at a density similar to that of at least a portion of the activity of galactosyltransferase, a marker for Golgi. Enzyme marker activities for lysosomes and endoplasmic reticulum are not associated with this population of beta-adrenergic receptors. Taken together, these and other data suggest that incubation of 1321N1 cells with isoproterenol results in a rapid uncoupling of beta-adrenergic receptors from adenylate cyclase which is followed by a change in the membrane form of the receptor. This latter step most likely represents internalization of receptors into a vesicular form which may then serve as the precursor state from which receptors are eventually lost from the cell.

Muscarinic acetylcholine receptors of the small intestine and pancreas of the rat: distribution and the effect of vagotomy.

1. The distribution of muscarinic cholinergic receptors (mAChR), detected by atropine-inhibitable binding of [3H] quinuclidinyl benzilate, was examined in membrane fractions of pancreas, small intestinal muscle, mucosa, villi and crypts of sham-operated and vagotomized rats. 2. Specific (atropine inhibitable) [3H] quinuclidinyl benzilate binding was greater to the ileal mucosa than to jejunal mucosa or to duodenal mucosa, but binding crypt and villus fractions was not significantly different. This distribution of specific [3H] quinuclidinyl benzilate binding suggests that cholinergic mucosal innervation is more important in the ileum than the jejunum. 3. Vagotomy produced a decrease in the amount of specific [3H] quinuclidinyl benzilate binding to duodenal mucosa only, suggesting that parasympathetic denervation of the small intestine does not cause mucosal hypersensitivity to acetylcholine by an increase in mAChR.

The effect of entorhinal cortical ablation on the distribution of muscarinic cholinergic receptors in the rat hippocampus

Removal of the entorhinal cortical projection to the hippocampus in adult rats decreased the density of muscarinic cholinergic receptors in the denervated dentate gyrus outer molecular layer at two days postlesion. Thirty days following the lesion (in adults and neonates) there is a small receptor density increase in the outer molecular layer (may be due to tissue shrinkage), and a larger increase in the lacunosum-moleculare. The receptor density decrease seen two days postlesion suggests the presence of presynaptic muscarinic receptors on the lost entorhinal cortical fibers. The distribution and extent of the receptor changes seen at 30 days postlesion are inconsistent with the cholinergic fiber reorganization which follows an entorhinal cortical lesion, but are consistent with a proposed model of non-cholinergic afferent mediated control of muscarinic receptor density in the rat hippocampus.

Properties and distribution of muscarinic cholinergic receptors in rat striatal micropunched tissue homogenates

The properties of muscarinic receptors in rat striatal micropunched tissue homogenates were studied with a binding technique using [ 3H]quinuclidinyl benzilate (QNB) as ligand. Among the agonists, oxotremorine was the most potent inhinitor (ID 50 value, 2.20 × 10 −5 M), followed by pilocarpine, acetylcholine, carbachol and bethanechol. No marked differences were found between the binding properties of QNB in the micropunched tissue homogenates and membrane fractions. Therefore, the detailed distribution of muscarinic receptors in the striatum was examined using the micropunched tissue homogenates. High levels of QNB binding in the striatum were found at dorso-lateral sites and low levels in the center regions of the striatum. The technique presented here for studying small regions of the striatum should be useful in further studies of the function of cholinergic neurons.

Regional and subcellular distribution of myocardial muscarinic cholinergic receptors

Regional and subcellular distribution of muscarinic cholinergic receptors were investigated in the myocardium of commonly used laboratory animals. The density of receptor sites (expressed in terms of either pmol/g protein or pmol/g tissue), amongst the regions examined, was found much higher in right and left atrium in the case of rat and rabbit whereas for the guinea pig and dog, the distribution was diffuse. However irrespective of the species and/or region studies, the microsomal fraction, amongst the subcellular fractions, showed the highest enrichment of receptors.

Regional distribution of muscarinic cholinergic receptors in rat brain

Using the tritiated ligand [3H]quinuclidinyl benzilate ([3H]QNB), a specific muscarinic cholinergic antagonist, the distribution of muscarinic receptors was determined in 75 specific brain regions and nuclei removed by microdissection as well as in large brain areas. [3H]QNB binding was highest in the caudate-putamen; other regions with high binding included the nucleus accumbens, cerebral cortex and hippocampus, certain limbic and thalamic nuclei, superior colliculus, pontine nuclei and the hypoglossal nucleus. Muscarinic cholinergic receptors may be concentrated in these regions. [3H]QNB binding generally correlated well with acetylcholinesterase staining in terminals and is consistent with certain sites as particularly enriched in cholinergic synapses.