Immunohistochemistry of cholinergic receptors

Cellular, Synaptic and Network Effects of Acetylcholine in the Neocortex

The neocortex is densely innervated by basal forebrain (BF) cholinergic neurons. Long-range axons of cholinergic neurons regulate higher-order cognitive function and dysfunction in the neocortex by releasing acetylcholine (ACh). ACh release dynamically reconfigures neocortical microcircuitry through differential spatiotemporal actions on cell-types and their synaptic connections. At the cellular level, ACh release controls neuronal excitability and firing rate, by hyperpolarizing or depolarizing target neurons. At the synaptic level, ACh impacts transmission dynamics not only by altering the presynaptic probability of release, but also the magnitude of the postsynaptic response. Despite the crucial role of ACh release in physiology and pathophysiology, a comprehensive understanding of the way it regulates the activity of diverse neocortical cell-types and synaptic connections has remained elusive. This review aims to summarize the state-of-the-art anatomical and physiological data to develop a functional map of the cellular, synaptic and microcircuit effects of ACh in the neocortex of rodents and non-human primates, and to serve as a quantitative reference for those intending to build data-driven computational models on the role of ACh in governing brain states.

Regulation of glutamate release by heteromeric nicotinic receptors in layer V of the secondary motor region (Fr2) in the dorsomedial shoulder of prefrontal cortex in mouse

We studied how nicotinic acetylcholine receptors (nAChRs) regulate glutamate release in the secondary motor area (Fr2) of the dorsomedial murine prefrontal cortex, in the presence of steady agonist levels. Fr2 mediates response to behavioral situations that require immediate attention and is a candidate for generating seizures in the frontal epilepsies caused by mutant nAChRs. Morphological analysis showed a peculiar chemoarchitecture and laminar distribution of pyramidal cells and interneurons. Tonic application of 5 µM nicotine on Layer V pyramidal neurons strongly increased the frequency of spontaneous glutamatergic excitatory postsynaptic currents. The effect was inhibited by 1 µM dihydro-β-erythroidine (which blocks α4-containing nAChRs) but not by 10 nM methyllicaconitine (which blocks α7-containing receptors). Excitatory postsynaptic currents s were also stimulated by 5-iodo-3-[2(S)-azetidinylmethoxy]pyridine, selective for β2-containing receptors, in a dihydro-β-erythroidine -sensitive way. We next studied the association of α4 with different populations of glutamatergic terminals, by using as markers the vesicular glutamate transporter type (VGLUT) 1 for corticocortical synapses and VGLUT2 for thalamocortical projecting fibers. Immunoblots showed higher expression of α4 in Fr2, as compared with the somatosensory cortex. Immunofluorescence showed intense VGLUT1 staining throughout the cortical layers, whereas VGLUT2 immunoreactivity displayed a more distinct laminar distribution. In Layer V, colocalization of α4 nAChR subunit with both VGLUT1 and VGLUT2 was considerably stronger in Fr2 than in somatosensory cortex. Thus, in Fr2, α4β2 nAChRs are expressed in both intrinsic and extrinsic glutamatergic terminals and give a major contribution to control glutamate release in Layer V, in the presence of tonic agonist levels.

Cellular and subcellular localization of the 2B-subunit of the NMDA receptor in the adult rat telencephalon

NMDA receptors (NR) are encoded by a family of genes including those of the NR1 and NR2A-D subunits. In situ hybridization has revealed that NR1, comprising eight splice variants, is ubiquitously expressed in the central nervous system (CNS) while the expression of NR2 isoforms is restricted to particular CNS regions. We report on the cellular and ultrastructural distribution of the NR2B polypeptide in rat telencephalon. In the telencephalon, the hippocampus represented the most intensively immunolabeled region. Here, predominantly the CA pyramidal neurons were heavily stained. Intense immunoreactivity (IR) was also detected in cortical neurons, in particular in pyramidal-like ones of layers II/III and V. On the ultrastructural level, the NR2B subunit was present not only in synaptic complexes where it usually was present in postsynaptic sites but in addition could be located at extrasynaptic sites. Furthermore, preliminary evidence indicates a presynaptic location of NR2B in some rare cases. NR2B antigen distribution is consistent with that of corresponding transcripts. Indeed, NR2B immunoreactivity coincides largely with that for NR1, indicating that both subunits are coexpressed in numerous cortical and hippocampal neurons.

Peptidergic innervation and the nicotinic acetylcholine receptor in the primate basal nucleus

Peptidergic innervation and localization of the neuronal nicotinic acetylcholine receptor (nAChR) was studied in the basal forebrain of Macaca fascicularis in order to provide microstructural proofs for the theory (Changeux et al., 1992) that calcitonin gene-related peptide (CGRP) is responsible for the maintenance of the acetylcholine receptor. Distribution and localization of five neuropeptides, namely substance P (SP), CGRP, neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP) neurotensin (NT), and the neuropeptides parvalbumin (PV) and the alpha-bungarotoxin- (alpha-BTX-) binding protein was studied by means of light- and electron microscopic pre-embedding immunocytochemistry. Immunohistochemical double staining revealed that large cholinergic principal nerve cells in the basal forebrain, corresponding to cell group Ch4 constituting Meynert's basal nucleus (BNM), and exerting intense choline acetyltransferase (ChAT) immunoreactivity, are synaptically innervated by axons displaying CGRP immunoreactivity. While SP, NPY, PV and CGRP establish dense networks in BNM, innervation by NT and VIP is sparse. Biotinylated alpha-BTX visualizes beaded axons that surround dendrites and perikarya of cholinergic principal cells. Electron microscopic organization of the neuropil in BNM is characterized by a glomerular (or rather cartridge-like) arrangement of axons surrounding dendrites of non-cholinergic principal nerve cells. At least one of the axons establishing the glomerulus (cartridge) exerts CGRP immunopositivity while alpha-BTX-immunopositive axons, presynaptic to dendrites of principal cells, are attached to the glomeruli (cartridges) from outside. As alpha-BTX-binding indicates localization of the alpha7 subunit of the neuronal nAChR, the microtopographical arrangement supports the idea that, in a manner similar to that in the neuromuscular junction, CGRP might contribute to the maintenance of nAChR also in BNM. Our results suggest that presynaptic nAChR-s are involved in the regulation of acetylcholine release from a feed-forward amplification mechanism of cholinergic principal cells of BNM.

Cholinergic immunolesions by 192IgG-saporin--useful tool to simulate pathogenic aspects of Alzheimer's disease

Alzheimer's disease, the most common cause of senile dementia, is characterized by intracellular formation of neurofibrillary tangles, extracellular deposits of beta amyloid as well as cerebrovascular amyloid accumulation and a profound loss of cholinergic neurons within the nucleus basalis Meynert with alterations in cortical neurotransmitter receptor densities. The use of the cholinergic immunotoxin 192IgG-saporin allows for the first time study of the impact of cortical cholinergic deafferentation on cortical neurotransmission, learning, and memory without direct effects on other neuronal systems. This model also allows the elucidation of contributions of cholinergic mechanisms to the establishment of other pathological features of Alzheimer's disease. The findings discussed here demonstrate that cholinergic immunolesions by 192IgG-saporin induce highly specific, permanent cortical cholinergic hypoactivity and alterations in cortical neurotransmitter densities comparable to those described for Alzheimer's disease. The induced cortical cholinergic deficit also leads to cortical/hippocampal neurotrophin accumulation and reduced amyloid precursor protein (APP) secretion, possibly reflecting the lack of stimulation of postsynaptic M1/M3 muscarinic receptors coupled to protein kinase C. This immunolesion model should prove useful to test therapeutic strategies based on stimulation of cortical cholinergic neurotransmission or amelioration of pathogenic aspects of cholinergic degeneration in the basal forebrain. Application of the model to animal species that can develop beta-amyloid plaques could provide information about the contribution of cholinergic function to amyloidogenic APP processing.

The somatosensory cortex of human: cytoarchitecture and regional distributions of receptor-binding sites

The aim of this study is to characterize the regional and laminar distribution patterns of various neurotransmitter binding sites in areas 3a, 3b, 1, and 2 of the human primary somatosensory cortex, and to compare these receptor-based "maps" with the cytoarchitectonic parcelation. Cryostat sections from a dorsomedial region of the postcentral gyrus close to the interhemispheric fissure and from a ventrolateral region close to the Sylvian fissure were examined. Neurotransmitter-binding sites were analyzed with quantitative in vitro receptor autoradiography. Different muscarinic-binding sites were labeled with [3H]pirenzepine and [3H]oxotremorine-M, noradrenergic-binding sites with [3H]prazosin, different serotoninergic-binding sites with [3H]5-hydroxytryptamine and [3H]ketanserine, glutamate-binding sites with l-[3H]glutamate, and GABA-binding sites with [3H]muscimol. Adjacent sections were stained with a modified Nissl method for cytoarchitectonic analysis. The binding sites either were preferentially localized in the superficial layers ([3H]5-hydroxytryptamine, [3H]prazosin, l-[3H]glutamate, [3H]muscimol, and [3H]pirenzepine) or were more homogeneously distributed with highest densities in layers III-V ([3H]oxotremorine-M and [3H]ketanserine). Changes in the distribution patterns of [3H]oxotremorine-M- and [3H]ketanserine-binding sites precisely matched the borders between areas 4/3a, 3b/1, and 1/2, as defined cytoarchitectonically. In addition, the autoradiographs showed that area 1 possibly consists of two subregions which cannot be distinguished cytoarchitectonically. The results demonstrate that the regional and laminar distribution patterns of some, but not all, transmitter-binding sites are precisely correlated with the cytoarchitectonic parcelation of the human primary somatosensory cortex. In addition, binding sites may reveal new borders not detectable in Nissl-stained sections. Finally, the human primary somatosensory cortex differs clearly from the primary motor cortex due to higher densities of l-[3H]glutamate-, [3H]muscimol-, [3H]pirenzepine-, [3H]oxotremorine-M-, and [3H]ketanserine-binding sites.

Non-selectivity of the monoclonal antibody M35 for subtypes of muscarinic acetylcholine receptors

The monoclonal antibody M35, one of the first monoclonal antibodies successfully raised against muscarinic acetylcholine receptors, has been widely used to study the distribution of this protein in a variety of tissues and cell types of different species. It is not fully known, however, to which muscarinic acetylcholine receptor subtypes M35 binds. Knowledge of subtype-selectivity of M35 is a necessary step towards a functional interpretation of the obtained immunocytochemical data. The aim of the present study was to determine the subtype-selectivity of M35 employing transfected CHO-K1 cells stably expressing human m1-m5 muscarinic acetylcholine receptors separately, and to study M35 immunoreactivity in areas of rat central and peripheral tissues known to be specifically enriched in a single muscarinic acetylcholine receptor subtype. The results show that (a) all five transfected cell lines were immunopositive for M35, (b) nontransfected control cells were immunonegative, (c) the number of mAChRs expressed per cell correlated positively with the intensity of M35 immunoreactivity, and (d) cell types in aldehyde-fixed rat tissue enriched in a single m1-m4 subtypes revealed clear M35 immunoreactivity. Taken together, the present results show that M35 does not discriminate between muscarinic acetylcholine receptor subtypes. Evidently, the epitope of M35 on the receptor-protein is preserved on all muscarinic acetylcholine receptor subtypes. The epitope for M35 must, therefore, be localized on a homologous part of each subtype.

Expression of alpha subunit genes of nicotinic acetylcholine receptors in human lymphocytes

Using immunohistochemistry and in situ hybridisation, we have studied whether alpha-subunits of nicotinic acetylcholine receptors (nAChRs) are expressed in human lymphocytes. Cells were isolated by differential low speed gradient centrifugation from heparinised venous blood of 10 healthy volunteers. Receptor sites were visualised using the monoclonal antibody WF6 which specifically recognises alpha-isoforms from several species including man. For visualisation of transcripts, digoxigenin-labelled cRNA probes for alpha 4- and alpha 3-subunits were used. Immunostaining revealed specific binding of WF6 to isolated human lymphoid cells. The antibody was bound to most cells and concentrated preferentially in the perinuclear/surface region. The immunoreactivity resembled that observed after application of an antibody recognising CD4 surface proteins which was conducted for comparison. In situ-hybridisation revealed that the alpha 4-subunit genes of nAChRs was expressed in lymphocytes of all probands. The alpha 3-subunit was found, with lower intensity than alpha 4-transcripts, in eight of the 10 individuals. Control incubations with corresponding sense cRNAs were negative. It is concluded that human lymphocytes are able to express alpha-subunit genes of nAChRs.

Postsynaptic and extrasynaptic localization of kappa-opioid receptor in selected brain areas of young rat and chick using an anti-receptor monoclonal antibody

kappa-opioid receptors were visualized by light and electron microscopical immunohistochemistry in young rat and chick brains, using a monoclonal antibody KA8 (IgG1, kappa) raised against a kappa-opioid receptor preparation from frog brain, which recognizes selectively the kappa-type receptor with preference for the kappa-2 subtype. The most pronounced kappa-opioid receptor-like immunoreactivity was observed in the hypothalamic nuclei of the rat brain and in the chick optic tectum, in regions where the functional significance of kappa-opioid receptors is well documented. Both neurons and glia were stained, the former on both somata and dendrites. At the ultrastructural level, the receptor-like immunoreactivity was similar in both species. Immunoprecipitate decorated the inner surface of the plasma membrane of glial cells, neuronal somata and dendrites, in a discontinuous arrangement. In the cytoplasm, labelling was associated with ribosomes, polyribosomes and rough endoplasmic reticulum membranes but not with Golgi cisternae. In the neuropil, the immunoprecipitate was observed along the dendritic microtubules and was also associated with postsynaptic sites. Nuclei and axons were devoid of label and immunoreactivity was never visible presynaptically. Our findings indicate that the antibody used in the present study marks various forms of the kappa-opioid receptor protein including those synthesised in ribosomes, transported along dendritic microtubules and incorporated into postsynaptic and non-synaptic membranes. The antibody also recognizes glial opioid receptors. The observed subcellular distribution appears to be conserved in phylogenetically distant species.

Cellular distribution in the rat telencephalon of mRNAs encoding for the alpha 3 and alpha 4 subunits of the nicotinic acetylcholine receptor

Pharmacological and electrophysiological studies provide evidence for the involvement of different nicotinic acetylcholine receptor isoforms in rat neocortical and hippocampal signal transduction. Yet, rather little is known on the cellular localization of these isoforms. With the availability of isoform specific nucleic acid probes and sensitive non-isotopic detection systems, nicotinic receptors can be studied on the mRNA level in individual neurons. In this way, we have paradigmatically studied the distribution of the alpha 3 and alpha 4 isoform mRNAs of the nicotinic receptor in the rat telencephalon. In the cerebral cortex, alpha 3 transcripts were mainly located in pyramidal neurons of layers V and VI and in some non-pyramidal cells in layer IV, while alpha 4 mRNA was detected in different types of neurons located in almost all layers. In the hippocampus, local distribution of both transcripts was comparable. Only very few labeled neurons were observed in the dentate gyrus. In the CA region, the specific mRNAs were detected in pyramidal perikarya and individual neurons in the strata oriens and lacunosum-moleculare. Our data show that the applied method is sufficiently sensitive and isoform-selective in order to study the differential expression of nicotinic receptors on the cellular level in the mammalian brain.

Subcellular innervation patterns of the calcitonin gene-related peptidergic efferent terminals in the chinchilla vestibular periphery

We examined the ultrastructural distribution of calcitonin gene-related peptide immunoreactivity in the peripheral vestibular system of the chinchilla to study the innervation patterns of this efferent neuropeptide. Immunoelectron microscopic localization of calcitonin gene-related peptide immunoreactive terminals in the maculae and cristae revealed an extensive innervation pattern on the afferent vestibular pathway. Calcitonin gene-related peptide immuno-reactive terminals made synaptic contacts with the unmyelinated portions of the primary afferent vestibular dendrites innervating both type I and type II hair cells. Abundant synaptic contact between calcitonin gene-related peptide immunoreactive terminals and the chalices surrounding type I hair cells was observed. Direct contact between calcitonin gene-related peptide immunoreactive terminals and type II hair cells was observed. In addition, vesiculated efferent terminals without calcitonin gene-related peptide immunoreactivity were seen synapsing on the chalices of type II hair cells and on the surrounding type I hair cells. The primary afferent somata in the vestibular ganglion of Scarpa did not contain calcitonin gene-related peptide immunoreactivity. Unmyelinated calcitonin gene-related peptide immunoreactive axons passed among the primary afferent fibers in Scarpa's ganglion, and these fibers continued through the subepithelial regions of the vestibular end-organs. The calcitonin gene-related peptide immunoreactive axons ramified to produce numerous calcitonin gene-related peptide immunoreactive terminals throughout the neurosensory epithelium of the maculae and cristae. These data suggest that calcitonin gene-related peptide-mediated modulation of the afferent vestibular system is functionally important.

Immunohistochemical localization of kappa opioid receptors in the human frontal cortex

The cellular and subcellular kappa opioid receptor distribution in human frontal cortex was studied using the monoclonal antibody (KA8). kappa opioid receptor-like immunoreactivity was mainly localized in pyramidal neurons of layers II/III and V. In addition, some round and ovoid neurons were found immunolabeled mainly in layer VI. At the ultrastructural level the immunoprecipitate was attached to the cell membrane but was not associated with synaptic specializations. Furthermore, labeling was present in the neuronal perikarya associated with free ribosomes and in the dendrites partly decorating microtubular structures. Previous autoradiographic findings remained restricted to the level of laminar distribution. By contrast, the monoclonal antibody KA8 provides a unique tool to study the cerebral kappa opioid receptor distribution on the cellular and subcellular level.

Cellular distribution of nicotinic acetylcholine receptor subunit mRNAs in the human cerebral cortex as revealed by non-isotopic in situ hybridization

The pharmacology of telencephalic nicotinic acetylcholine receptors (nAChRs) has become an important issue in recent years. While in the human brain a direct pharmacological assessment is difficult to achieve the visualization of nAChRs has been enabled by histochemical techniques providing an ever increasing and improving resolution. Receptor autoradiography was used to visualize binding sites on the level of cortical layers whereas immunohistochemistry has allowed for the cell type-specific and ultrastructural localization of receptor protein. Further investigations have to elucidate the cellular sites of NAChR biosynthesis by visualizing subunit-specific transcripts. Using autopsy samples of the human precentral cortex (Area 4) as a paradigm we have applied digoxigenin-labeled cRNA probes to localize transcripts for the alpha 3- and alpha 4-1-subunits of the nAChR. In accordance with findings in the monkey cortex, the alpha 3-subunit seems to be expressed mainly in pyramidal neurons of layers III-VI of the human cerebral cortex. Transcripts for the alpha 4-1-subunit, by contrast, appear to be present in a large number of neurons throughout all layers of the cerebral cortex, consonant with its ubiquitous distribution in the rodent brain. The present findings show that also in human autopsy brains the cell type-specific detection of nAChR transcripts is possible. For the future, this technique will enable to investigate the expression of receptor transcripts in diseased human brains as compared to controls.