Abu Syed Md Anisuzzaman

Novel contribution of cell surface and intracellular M1-muscarinic acetylcholine receptors to synaptic plasticity in hippocampus.

Muscarinic acetylcholine receptors (mAChRs) are well known to transmit extracellular cholinergic signals into the cytoplasm from their position on the cell surface. However, we show here that M1‐mAChRs are also highly expressed on intracellular membranes in neurons of the telencephalon and activate signaling cascades distinct from those of cell surface receptors, contributing uniquely to synaptic plasticity. Radioligand‐binding experiments with cell‐permeable and ‐impermeable ligands and immunohistochemical observations revealed intracellular and surface distributions of M1‐mAChRs in the hippocampus and cortex of rats, mice, and humans, in contrast to the selective occurrence on the cell surface in other tissues. All intracellular muscarinic‐binding sites were abolished in M1‐mAChR‐gene‐knockout mice. Activation of cell surface M1‐mAChRs in rat hippocampal neurons evoked phosphatidylinositol hydrolysis and network oscillations at theta rhythm, and transiently enhanced long‐term potentiation. On the other hand, activation of intracellular M1‐mAChRs phosphorylated extracellular‐regulated kinase 1/2 and gradually enhanced long‐term potentiation. Our data thus demonstrate that M1‐mAChRs function at both surface and intracellular sites in telencephalon neurons including the hippocampus, suggesting a new mode of cholinergic transmission in the central nervous system.

Intracellular distribution of functional M1‐muscarinic acetylcholine receptors in N1E‐115 neuroblastoma cells

J. Neurochem. (2011) 118, 958–967.

Different affinities of native α1B-adrenoceptors for ketanserin between intact tissue segments and membrane preparations

The pharmacological profiles of alpha1-adrenoceptors for ketanserin, prazosin, silodosin, and BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]decane-7,9-dione dihydrochloride) were examined under different assay conditions. Among the tested antagonists and alpha1-adrenoceptors subtypes, ketanserin showed significantly lower affinity for the alpha1B-adrenoceptor subtype in intact tissue sampled from the rat tail artery, thoracic aorta, and cerebral cortex (functional pKB and binding pKi were approximately 6), than in cerebral cortex membrane preparations or whole cell and membrane preparations of alpha1B-adrenoceptor transfected human embryonic kidney 293T (HEK 293T) cells (pKi was approximately 8). In these tissues and cells, however, ketanserin showed a similar affinity (pKi = approximately 8) for alpha1A- and alpha1D-adrenoceptors even though the assays were conducted under different conditions. In contrast, the affinities of alpha1A-, alpha1B-, and alpha1D-adrenoceptors for prazosin, silodosin, and BMY 7378 did not significantly change under different assay conditions and in different tissues. The present study reveals that the pharmacological profiles of native alpha 1B-adrenoceptors for ketanserin is strongly influenced by the assay conditions and suggest that antagonist affinity is not necessarily constant.

Influence of tissue integrity on pharmacological phenotypes of muscarinic acetylcholine receptors in the rat cerebral cortex

Distinct pharmacological phenotypes of muscarinic acetylcholine receptors (mAChRs) have been proposed. We compared the pharmacological profiles of mAChRs in intact segments and homogenates of rat cerebral cortex and other tissues by using radioligand binding assays with [3H]N-methylscopolamine ([3H]NMS). Recombinant M1 and M3 mAChRs were also examined. The density of mAChRs detected by [3H]NMS binding to rat cerebral cortex segments and homogenates was the same (approximately 1400 fmol/mg tissue protein), but the dissociation constant of [3H]NMS was significantly different (1400–1700 pM in segments and 260 pM in homogenates). A wide variation in [3H]NMS binding affinity was also observed among the segments of other tissues (ranging from 139 pM in urinary bladder muscle to 1130 pM in the hippocampus). The mAChRs of cerebral cortex were composed of M1, M2, M3, and M4 subtypes, which showed typical subtype pharmacology in the homogenates. However, in the cortex segments the M3 subtype showed a low selectivity for M3 antagonists (darifenacin, solifenacin) and was not distinguished by the M3 antagonists from the other subtypes. Recombinant M1 and M3 mAChRs showed high affinity for [3H]NMS and subtype-specific pharmacology for each tested ligand. The present binding study under conditions where tissue integrity was kept demonstrates a wide variation in [3H]NMS binding affinity among mAChRs of many rat tissues and the presence of an atypical M3 phenotype in the cerebral cortex, suggesting that the pharmacological properties of mAChRs are not necessarily constant, rather they may be significantly modified by tissue integrity and tissue type.

Activation of focal adhesion kinase via M1 muscarinic acetylcholine receptor is required in restitution of intestinal barrier function after epithelial injury

Impairment of epithelial barrier is observed in various intestinal disorders including inflammatory bowel diseases (IBD). Numerous factors may cause temporary damage of the intestinal epithelium. A complex network of highly divergent factors regulates healing of the epithelium to prevent inflammatory response. However, the exact repair mechanisms involved in maintaining homeostatic intestinal barrier integrity remain to be clarified. In this study, we demonstrate that activation of M1 muscarinic acetylcholine receptor (mAChR) augments the restitution of epithelial barrier function in T84 cell monolayers after ethanol-induced epithelial injury, via ERK-dependent phosphorylation of focal adhesion kinase (FAK). We have shown that ethanol injury decreased the transepithelial electrical resistance (TER) along with the reduction of ERK and FAK phosphorylation. Carbachol (CCh) increased ERK and FAK phosphorylation with enhanced TER recovery, which was completely blocked by either MT-7 (M1 antagonist) or atropine. The CCh-induced enhancement of TER recovery was also blocked by either U0126 (ERK pathway inhibitor) or PF-228 (FAK inhibitor). Treatment of T84 cell monolayers with interferon-γ (IFN-γ) impaired the barrier function with the reduction of FAK phosphorylation. The CCh-induced ERK and FAK phosphorylation were also attenuated by the IFN-γ treatment. Immunological and binding experiments exhibited a significant reduction of M1 mAChR after IFN-γ treatment. The reduction of M1 mAChR in inflammatory area was also observed in surgical specimens from IBD patients, using immunohistochemical analysis. These findings provide important clues regarding mechanisms by which M1 mAChR participates in the maintenance of intestinal barrier function under not only physiological but also pathological conditions.

Identification of M1 muscarinic receptor subtype in rat stomach using a tissue segment binding method, and the effects of immobilization stress on the muscarinic receptors

Distinct muscarinic acetylcholine receptor subtypes widely distribute in stomach tissues and are involved in many physiological functions. Although mRNA of M(1) subtype was found in gastric mucosa, the M(1) subtype has not been detected by conventional membrane binding assays. In the present study, muscarinic receptor subtypes in the rat stomach were reevaluated by using the tissue segment binding technique recently developed to recognize the inherent/native profiles of receptors without receptor environment perturbation. [(3)H]-N-methylscopolamine (NMS) bound to muscarinic receptors in the intact segments of rat gastric mucosa and muscle layers. The muscarinic receptors in the mucosal segments were composed of M(1), M(2) and M(3) subtypes, among which the M(1) subtype selectively showed high affinity for pirenzepine. However, in the membrane preparations, binding sites with high affinity for pirenzepine could not be detected. In the muscle layer, M(2) and M(3) subtypes, but not M(1), were identified in tissue segment and conventional membrane binding assays. Western blotting analysis recognized the M(1) subtype in the membrane preparations of mucosal but not muscle layers. Chronic immobilization stress increased the M(3) subtype in mucosal and muscle layers and decreased the M(2) subtype in the muscle layer, whereas M(1) and M(2) subtypes in mucosal layer did not change after the stress. The current study shows that M(1) subtype occurs as a pirenzepine-high affinity entity in intact segments of rat gastric mucosa, but that it loses the affinity for pirenzepine upon homogenization. Careful identification of native in vivo muscarinic receptors may further elucidate their functions in stomach.

Activation of muscarinic cholinoceptor ameliorates tumor necrosis factor-α-induced barrier dysfunction in intestinal epithelial cells.

Impaired intestinal barrier function is one of the critical issues in inflammatory bowel diseases. The aim of this study is to investigate muscarinic cholinoceptor (mAChR)‐mediated signaling for the amelioration of cytokine‐induced barrier dysfunction in intestinal epithelium. Rat colon challenged with TNF‐α and interferon γ reduced transepithelial electrical resistance (TER). This barrier injury was attenuated by muscarinic stimulation. In HT‐29/B6 intestinal epithelial cells, muscarinic stimulation suppressed TNF‐α‐induced activation of NF‐κB signaling and barrier disruption. Finally, muscarinic stimulation promoted the shedding of TNFR1, which would be a mechanism for the attenuation of TNF‐α/NF‐κB signaling and barrier disruption via mAChR.

Pharmacologically distinct phenotypes of α1B-adrenoceptors: variation in binding and functional affinities for antagonists

The pharmacological properties of particular receptors have recently been suggested to vary under different conditions. We compared the pharmacological properties of the α1B‐adrenoceptor subtype in various tissue preparations and under various conditions.

Comparison of subcellular distribution and functions between exogenous and endogenous M1 muscarinic acetylcholine receptors

AIMS Recombinant systems have been used for evaluating the properties of G-protein-coupled receptors (GPCRs) on the assumption of cell surface expression. However, many GPCRs, including muscarinic acetylcholine receptors (mAChRs), have also been reported to be distributed in intracellular organelles in native tissues and cell lines. In this study, we compared the pharmacological profiles of exogenously and endogenously expressed M1-mAChRs, and evaluated the functional properties of these receptors. MAIN METHODS Recombinant M1-mAChRs were expressed exogenously in Chinese hamster ovary cells (CHO-M1 cells) and compared with endogenously expressed M1-mAChRs in N1E-115 neuroblastoma cells. The pharmacological and functional profiles were evaluated using cell-permeable antagonists (1-quinuclidinyl-benzilate (QNB), pirenzepine and atropine) and cell-impermeable antagonists (N-methylscopolamine (NMS) or MT-7). KEY FINDINGS M1-mAChRs were seen at the cell surface and intracellular sites in both cell lines. Under whole cell conditions, intracellular M1-mAChRs were mainly recognized by cell-permeable ligands, but scarcely by cell-impermeable ligands (at less than 100nM). In CHO-M1 cells, M1-mAChR activation by carbachol resulted in Ca(2+) mobilization, ERK1/2 phosphorylation and a reduction in thymidine incorporation, all of which were completely inhibited by MT-7, indicating the involvement of surface M1-mAChRs. In N1E-115 cells, Ca(2+) mobilization occurred through surface M1-mAChRs, whereas ERK1/2 phosphorylation and acceleration of thymidine incorporation were mediated through intracellular M1-mAChRs. SIGNIFICANCE Exogenous and endogenous M1-mAChRs are present at both the cell surface and the intracellular organelles, and the pharmacological properties of geographically distinct M1-mAChRs are different, and may depend on cell background and/or exogenous or endogenous origin.

Re-Evaluation of Nicotinic Acetylcholine Receptors in Rat Brain by a Tissue-Segment Binding Assay

Nicotinic acetylcholine receptors (nAChRs) of the cerebral cortex and cerebellum of rats were evaluated by a radioligand binding assay, employing tissue segments, or homogenates as materials. [3H]-epibatidine specifically bound to nAChRs in rat cortex or cerebellum, but the dissociation constants for [3H]-epibatidine differed between segments and homogenates (187 pM for segments and 42 pM for homogenates in the cortex and 160 pM for segments and 84 pM for homogenates in the cerebellum). The abundance of total nAChRs was approximately 310 fmol/mg protein in the segments of cortex and 170 fmol/mg protein in the segments of cerebellum, which were significantly higher than those estimated in the homogenates (115 fmol/mg protein in the homogenates of the cortex and 76 fmol/mg protein in the homogenates of the cerebellum). Most of the [3H]-epibatidine binding sites in the cortex segments (approximately 70% of the population) showed high affinity for nicotine (pKi = 7.9), dihydro-β-erythroidine, and cytisine, but the binding sites in the cerebellum segments had slightly lower affinity for nicotine (pKi = 7.1). An upregulation of nAChRs by chronic administration of nicotine was observed in the cortex segments but not in the cerebellum segments with [3H]-epibatidine as a ligand. The upregulation in the cortex was caused by a specific increase in the high-affinity sites for nicotine (probably α4β2). The present study shows that the native environment of nAChRs is important for a precise quantitative as well as qualitative estimation of nAChRs in rat brain.