Monomers and Oligomers of the M2 Muscarinic Cholinergic Receptor Purified from Sf9 Cells

G protein-coupled receptors are known to form oligomers. To probe the nature of such aggregates, as well as the role and prevalence of monomers, epitope-tagged forms of the M(2) muscarinic receptor have been isolated as oligomers and monomers from Sf9 cells. Membranes from cells coexpressing the c-Myc- and FLAG-tagged receptor were solubilized in digitonin-cholate, and the receptor was purified by successive passage through DEAE-Sepharose, the affinity resin 3-(2'-aminobenzhydryloxy)tropane (ABT)-Sepharose, and hydroxyapatite. Coimmunoprecipitation of the two epitopes indicated the presence of oligomers at each stage of the purification up to but not including the fraction eluted specifically from ABT-Sepharose. The affinity-purified receptor therefore appeared to be monomeric. The failure to detect coimmunoprecipitation was not due to an ineffective antibody, nor did the conditions of purification appear to promote disaggregation. Receptor at all stages of purification bound N-[(3)H]methylscopolamine and [(3)H]quinuclidinylbenzilate with high affinity, but the capacity of receptors that were not retained on ABT-Sepharose was only 4% of that expected from densitometry of western blots probed with an anti-M(2) antibody. Similarly low activity was found with oligomers isolated by successive passage of coexpressed receptor on anti-c-Myc and anti-FLAG immunoaffinity columns. M(2) muscarinic receptors therefore appear to coexist as active monomers and largely or wholly inactive oligomers in solubilized extracts of Sf9 cells. A different pattern emerged when coinfected cells were treated with quinuclidinylbenzilate prior to solubilization, in that ABT-purified receptors from those cells exhibited coimmunoprecipitation. Treatment with the antagonist therefore led to oligomers in which at least some of the constituent sites were active and were retained by ABT-Sepharose.

Evidence for a tandem two-site model of ligand binding to muscarinic acetylcholine receptors

After short preincubations with N-[(3)H]methylscopolamine ([(3)H]NMS) or R(-)-[(3)H]quinuclidinyl benzilate ([(3)H]QNB), radioligand dissociation from muscarinic M(1) receptors in Chinese hamster ovary cell membranes was fast, monoexponential, and independent of the concentration of unlabeled NMS or QNB added to reveal dissociation. After long preincubations, the dissociation was slow, not monoexponential, and inversely related to the concentration of the unlabeled ligand. Apparently, the unlabeled ligand becomes able to associate with the receptor simultaneously with the already bound radioligand if the preincubation lasts for a long period, and to hinder radioligand dissociation. When the membranes were preincubated with [(3)H]NMS and then exposed to benzilylcholine mustard (covalently binding specific ligand), [(3)H]NMS dissociation was blocked in wild-type receptors, but not in mutated (D99N) M(1) receptors. Covalently binding [(3)H]propylbenzilylcholine mustard detected substantially more binding sites than [(3)H]NMS. The observations support a model in which the receptor binding domain has two tandemly arranged subsites for classical ligands, a peripheral one and a central one. Ligands bind to the peripheral subsite first (binding with lower affinity) and translocate to the central subsite (binding with higher affinity). The peripheral subsite of M(1) receptors may include Asp-99. Experimental data on [(3)H]NMS and [(3)H]QNB association and dissociation perfectly agree with the predictions of the tandem two-site model.

Endocytosis and recycling of muscarinic receptors

Agonist stimulation causes the endocytosis of many G protein-coupled receptors, including muscarinic acetylcholine receptors. In this study we have investigated the agonist-triggered trafficking of the M3 muscarinic receptor expressed in SH-SY5Y human neuroblastoma cells. We have compared the ability of a series of agonists to generate the second messenger Ins(1,4,5)P3 with their ability to stimulate receptor endocytosis. We show that there is a good correlation between the intrinsic activity of the agonists and their ability to increase the rate constant for receptor endocytosis. Furthermore, on the basis of our results, we predict that even very weak partial agonists should under some circumstances be able to cause substantial receptor internalization. Receptor endocytosis occurs too slowly to account for the rapid desensitization of the Ca2+ response to carbachol. Instead, receptor endocytosis and recycling appear to play an important role in resensitization. After an initial agonist challenge, the response to carbachol is fully recovered when only about half of the receptors have been recycled to the cell surface, suggesting that there is a receptor reserve of about 50%. Removal of this reserve by receptor alkylation significantly reduces the extent of resensitization. Resensitization is also reduced by inhibitors of either endocytosis alone (concanavalin A) or of endocytosis and recycling (nigericin). Finally, the protein phosphatase inhibitor calyculin A also reduces resensitization, possibly by blocking the dephosphorylation of the receptors in an endosomal compartment.

Allosteric effects of four stereoisomers of a fused indole ring system with 3H-N-methylscopolamine and acetylcholine at M1-M4 muscarinic receptors

We previously demonstrated that brucine and some analogues allosterically enhance the affinity of ACh at muscarinic receptor subtypes M1, M3 or M4. Here we describe allosteric effects at human M1-M4 receptors of four stereoisomers of a pentacyclic structure containing features of the ring structure of brucine. All compounds inhibited 3H-NMS dissociation almost completely at all subtypes with slopes of 1, with similar affinity values at the 3H-NMS-occupied receptor to those estimated from equilibrium assays, consistent with the ternary complex allosteric model. Compound 1a showed positive cooperativity with H-NMS and small negative or neutral cooperativity with ACh at all subtypes. Its stereoisomer, 1b, showed strong negative cooperativity with both 3H-NMS and ACh across the subtypes. Compound 2a was positive with 3H-NMS at M2 and M4 receptors, neutral at M3 and negative at M1 receptors; it was negatively cooperative with ACh at all subtypes. Its stereoisomer, 2b, was neutral with 3H-NMS at M1 receptors and positive at the other subtypes; 2b was negatively cooperative with ACh at M1, M3 and M4 receptors but showed 3-fold positive cooperativity with ACh at M2 receptors. This latter result was confirmed with further 3H-NMS and 3H-ACh radioligand binding assays and with functional assays of ACh-stimulated 35S-GTPgammaS binding. These results provide the first well characterised instance of a positive enhancer of ACh at M2 receptors, and illustrate the difficulty of predicting such an effect.

Identification and subcellular distribution of muscarinic acetylcholine receptor-related proteins in rabbit corneal and Chinese hamster ovary cells

PURPOSE

The authors examined the muscarinic acetylcholine receptor (mAChR) subtypes in rabbit corneal epithelial and endothelial cells and in subcellular fractions of these cell types. A Chinese hamster ovary (CHO) cell line (nontransfected CHO K1), expected to be a negative control, also was investigated.

METHODS

Whole cell homogenate and subcellular fractions were labeled with the covalent-binding, mAChR-specific ligand [3H]propylbenzilylcholine mustard ([3H]PrBChM) and were analyzed by a combination of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, or SDS-PAGE, and autoradiography.

RESULTS

A pattern of multiple PrBChM-binding proteins was detected in homogenates of corneal epithelial and endothelial cells and, surprisingly, in the CHO cells. Ligand binding to all of these proteins is inhibited by the mAChR antagonists atropine sulfate and quinculidinyl benzilate. The sizes of four of the labeled protein bands are the same as the molecular masses deduced from mAChR sequence data for subtypes m3, m4, m5, and either m1 or m2. One band of 47 kd, smaller than any reported sequence, was also observed. Two of the [3H]PrBChM-binding proteins, one at 59 to 62 kd (corresponding to m5 in size) and another at 47 kd, clearly were present when highly purified nuclei were analyzed.

CONCLUSIONS

The presence of multiple mAChR-like proteins at low concentrations in these disparate cell types suggests the possibility of a more general regulatory role for this type of receptor than was considered previously. Combined with other reports, the identification of proteins with the characteristics of mAChRs in purified nuclei adds support to data indicating the likelihood of G-protein-coupled signaling across the nuclear envelope.

Development of muscarinic acetylcholine receptors in the ferret retina

The development of muscarinic acetylcholine receptor protein in the ferret retina was studied using biochemical, autoradiographic, and light and electron microscopic immunohistochemical techniques. The development of retinal muscarinic cholinergic receptor proteins involves transient shifts in their number and distribution, as well as changes in the relative abundance of two molecular weight variants. Receptor binding assays demonstrate changes in the number and affinity of retinal binding sites for the muscarinic cholinergic ligand [3H]quinuclidinylbenzilate ([3H]QNB). Light microscopic immunohistochemical studies reveal the presence of muscarinic acetylcholine receptor-like (mAChR-like) immunoreactivity in the adult inner plexiform layer. During development, the mAChR-like immunoreactivity appears in a number of other retinal layers. Electron microscopic immunohistochemical studies indicate that muscarinic acetylcholine receptor-like immunoreactivity is found at amacrine-amacrine cell contacts. Both autoradiographic and gel slice electrophoretic studies were carried out after labeling of developing and adult retinal muscarinic receptors with [3H]propylbenzilylcholine mustard ([3H]propylbenzilylcholine mustard ([3H]PrBCM), which irreversibly labels the muscarinic acetylcholine receptor. Polyacrylamide gel electrophoresis under reducing, denaturing conditions resolved two peaks of radioactivity corresponding to [3H]PrBCM-labeled protein; both were eliminated by pre- and co-incubation of labeled adult retinas with excess atropine. Combined with the results of earlier studies, these observations suggest that the subtypes, number and distribution of muscarinic receptor proteins changes during retinal synaptogenesis.

Nuclear muscarinic acetylcholine receptors in corneal cells from rabbit

PURPOSE

Previous studies have indicated that muscarinic acetylcholine receptors (mAChR) may be present in an unexpected, unique location and play a singular role in cellular growth regulation of rabbit corneal epithelium that may be of general physiologic significance if found in other cells. The purpose of this study was to examine rabbit corneas and corneal cells in culture to determine mAChR location and tissue distribution.

METHODS

Using [3H]-propylbenzilylcholine mustard ([3H]PrBChM), which binds covalently to the active site of mAChR, rabbit corneal cross-sections, cultured corneal keratocytes, epithelial and endothelial cells, as well as nuclei isolated from these cultured corneal cells were labeled, stained, and autoradiographed. Nuclei labeled with [3H]PrBChM were further analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis.

RESULTS

Direct visual confirmation of the localization of mAChRs was obtained. MAChR were found in epithelial and endothelial layers of fresh-frozen corneal cross-sections, in cultured rabbit epithelial and endothelial cells, and on isolated rabbit epithelial and endothelial cell nuclei. mAChR were not detectable in keratocytes with these techniques. When [3H]PrBChM-labeled nuclei from cultured corneal cells were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis, epithelial and endothelial samples showed specific mAChR binding, whereas binding to keratocyte nuclei was not detectable.

CONCLUSIONS

As a result of these findings, a revised hypothesis is suggested for the locations and possible functions of mAChR in regulation of growth in corneal and other cells.

Characterization of subtype of propylbenzilylcholine mustard (PrBCM)-sensitive and -resistant muscarinic cholinoceptors in guinea pig ileal muscle

The subtype of propylbenzilylcholine mustard (PrBCM)-sensitive and -resistant muscarinic cholinoceptors in guinea pig ileal muscle was examined using four selective muscarinic antagonists, pirenzepine, AF-DX 116, himbacine and 4-DAMP. The pA2 values of the four antagonists against pilocarpine were not different from their values against carbachol after the treatment with PrBCM and was identified with the values for the m3 subtype. These results suggest that the subtype of PrBCM-sensitive and -resistant muscarinic cholinoceptors in guinea pig ileal muscle is the m3 subtype only and not other subtypes.

[Transition of drug receptor mechanisms]

It is generally accepted that the agonists, full agonist and partial agonist, interact with the same receptors according to the classical receptor mechanisms. We tried to modify the drug receptor mechanisms in muscarinic cholinoceptors, alpha 1-adrenoceptors and beta-adrenoceptors. In the muscarinic cholinoceptor, there are two subtypes of M3-cholinoceptors, propylbenzilylcholine mustard (PrBCM)-sensitive receptors and PrBCM-resistant ones. The full agonists contract the longitudinal muscle through the interaction of two cholinoceptors, PrBCM-sensitive and-resistant ones, while the partial agonists produce the contraction through only the activation of PrBCM-sensitive ones. Upon activation PrBCM-sensitive receptors may use cytosolic Ca2+ more effectively than PrBCM-resistant receptors. In the alpha 1-adrenoceptor, the full agonist induces contraction through both alpha 1A and alpha 1B subtypes and the partial agonist through only alpha 1A subtype. The adrenoceptors activated by full agonist may be partly different from that by partial agonist in the arteries. In both the common iliac artery and thoracic aorta treated with the irreversible antagonist, phenoxybenzamine the slopes of schild plots of the results obtained from an antagonism between full agonist (phenylephrine) and alpha 1A-selective competitive antagonist (WB4101) equal to 1, suggesting that phenoxybenzamine preferably interacts with alpha 1B subtype. In the beta-adrenoceptor, the pD2-values of the partial agonists obtained from the concentration-response curves are significantly different from their pA2-values against full agonist (isoprenaline). The Scatchard plot of the specific [3H]befunolol (the partial agonist) binding showed two affinity sites of the receptors in the absence of Gpp(NH)p but the low affinity site was reduced while the high affinity site was not affected in the presence of Gpp(NH) p. The beta-adrenergic partial agonists are able to discriminate these two different binding sites of the beta-adrenoceptors. Our results suggest that the receptors activated by full agonists are partly different from those by partial agonists in muscarinic cholinoceptors, alpha 1- and beta-adrenoceptors, and that the irreversible antagonist can discriminate between the sites interact with full agonists and those with partial agonists in muscarinic cholinoceptors and alpha 1-adrenoceptors.

Identification of muscarinic receptor subtypes present in cerebellar granule cells: prevention of [3H]propylbenzilyl choline mustard binding with specific antagonists

Subtypes of muscarinic receptors (possible m1 to m5) can be identified by their molecular size, specific effector systems and antagonist specificity. In membranes prepared from primary cultures of cerebellar granule cells, [3H]propylbenzilylcholine mustard [( 3H]PBCM) irreversibly binds to muscarinic receptive proteins, having two major molecular sizes, 92 and 66 kDa. With relatively short periods of incubation (approx. 30 min, 30 degrees C) of [3H]PBCM with atropine, a nonspecific competitive receptor antagonist, the irreversible labeling of these muscarinic proteins by [3H]PBCM could be prevented. Methoctramine, a specific competitive antagonist at muscarinic receptors coupled to inhibition of adenylate cyclase, protected most of the muscarinic receptors having a molecular size of 66 kDa from binding of [3H]PBCM. These 66 kDa receptive proteins are suggested to be muscarinic m2 and m4 subtypes. (-)Quinuclidinyl xanthene-9-carboxylate [(-)QNX], a somewhat specific competitive antagonist at muscarinic receptors coupled to hydrolysis of phosphatidylinositol, prevented the binding of [3H]PBCM to 92 kDa muscarinic receptive proteins and some 66 kDa muscarinic receptive proteins. The 92 kDa receptive proteins are suggested to be the muscarinic m3 subtype and the 66 kDa proteins could be either the m2 or m4 receptor subtype. Lastly, pirenzepine, a nonspecific antagonist at muscarinic receptors mediating inhibition of adenylate cyclase and hydrolysis of PI in these cultures, resembled (-)QNX in preventing binding of [3H]PBCM to the 92 kDa receptive proteins and some 66 kDa receptive proteins. The suggested subtypes of muscarinic receptors, specifically bound by pirenzepine should be the m3 (92 kDa) and the m4 (66 kDa) subtypes, since pirenzepine reportedly exhibits a low affinity for the muscarinic m2 subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic acetylcholine receptors. Peptide sequencing identifies residues involved in antagonist binding and disulfide bond formation

Muscarinic acetylcholine receptors (mAChR) were purified from rat brain and labeled either with the site-directed affinity label [3H]propylbenzilylcholine mustard (PrBCM) or with the sulfhydryl-specific label [3H]N-ethylmaleimide (NEM), using a protocol designed to give selective incorporation of the label into disulfide-bonded cysteines. m1 mAChRs were purified from CHO-K1 cells stably expressing the cloned receptor sequence and labeled with [3H]PrBCM. The labeled receptors were cleaved with the lysine-specific protease Lys-C and, after fractionation of the products, subcleaved with cyanogen bromide. Two major CNBr cleavage products were found with a molecular mass of approximately 3.9 and approximately 2.4 kDa, labeled either by [3H]PrBCM or [3H]NEM. The results obtained from CNBr cleavage of purified forebrain receptors were consistent with those obtained from the purified cloned m1 mAChR. Edman degradation was applied to the CNBr peptides. The results were compatible with the attachment of the [3H]PrBCM label to a conserved aspartic acid residue in transmembrane helix 3 of the mAChR (corresponding to Asp-105, m1 sequence) and of [3H]NEM to a conserved cysteine residue (corresponding to Cys-98, m1 sequence). These results support the hypothesis that the cysteine residue participates in a disulfide bond on the extracellular surface of the mAChRs and related G-protein-coupled receptors, while the aspartic acid residue is involved in binding the positively charged headgroup of muscarinic antagonists.

Location in muscarinic acetylcholine receptors of sites for [3H]propylbenzilylcholine mustard binding and for phosphorylation with protein kinase C

Muscarinic acetylcholine receptors purified from porcine cerebra or atria were covalently labeled with [3H]propylbenzilylcholine mustard ([3H]PrBCM), and then the labeled receptors were subjected to limited hydrolysis with trypsin, V8 protease, and lysyl endopeptidase, followed by analysis involving sodium dodecyl sulfate-polyacrylamide gel electrophoresis, fluorography, autoradiography, or immunostaining. The labeled peptides were located on the basis of their reactivity with antibodies raised against three synthetic peptides with partial sequences of the m1 or m2 receptor, and of their sensitivity to endoglycosidase F, which was taken as evidence that they contain glycosylation sites near the N terminus. The [3H]PrBCM-binding site in both cerebral and atrial receptors was found to be located between the N terminus and the second intracellular loop, because the size of the smallest deglycosylated peptide that contained both the [3H]PrBCM-binding and glycosylation sites was approximately 16 kDa. Cerebral receptors were 32P-phosphorylated with protein kinase C, and the major phosphorylation sites in cerebral muscarinic receptors were found to be located in a C-terminal segment including a part of the third intracellular loop, because a 32P-labeled peptide of 12-14 kDa reacted with anti-(m1 C-terminal peptide) antiserum. The presence of an intramolecular disulfide bond, probably between Cys 98 and Cys 178 in the first and second extracellular loops, respectively, was suggested by the finding that a peptide of approximately 17 kDa containing the [3H]PrBCM-binding site, but not the glycosylation sites, was partly converted to a peptide of approximately 12 kDa on treatment with beta-mercaptoethanol.

Autoradiographic visualization of muscarinic receptors on rat paratracheal neurons in dissociated cell culture

An autoradiographic method was used to determine the distribution of muscarinic receptors on cells cultured from the trachealis muscle of 12-13-day-old rats. Cells identified in these culture preparations included neurones, fibroblasts, smooth muscle, and glial and epithelial cells. The cultured cells were incubated with the specific, irreversible ligand [3H]propylbenzylylcholine mustard, and the autoradiographs generated showed that most, if not all, of the paratracheal neurones observed in these cultures were specifically labelled. Both the neuronal cell body and associated neurites were evenly labelled over their entire surface. Neither the pattern nor the density of neuronal labelling appeared to be influenced by close association with other cultured cell types. Autoradiographic grains for muscarinic receptors also appeared to be uniformly distributed over smooth muscle cells and epithelial cell groups in culture. In contrast, no specific labelling was associated with cultured fibroblasts, glial cells and other non-neuronal supporting cells. The precise localization of muscarinic receptors on different cell types in culture may prove to be useful knowledge in the design of an effective and specific antimuscarinic bronchodilator.

Distribution of muscarinic acetylcholine receptors on processes of isolated retinal cells

Binding of propylbenzilylcholine mustard, a muscarinic acetylcholine receptor antagonist, to isolated retinal cells was examined with light microscopic autoradiography. Dissociation of the adult tiger salamander retina yielded identifiable rod, cone, horizontal, bipolar, amacrine/ganglion, and Müller cells. Preservation of fine structure was assessed with conventional electron microscopy. For all cell types, the plasmalemma was intact and free of adhering debris; in addition, presynaptic ribbon complexes were present in photoreceptor and bipolar axon terminals indicating that synaptic structures were retained. Specific binding to cell bodies and processes was analyzed separately by using morphometric and statistical techniques. The highest grain densities occurred on processes of amacrine/ganglion cells and axons and 2 degrees and 3 degrees dendrites of bipolar neurons. Bipolar cells, however, seemed to be a heterogeneous population because there was great variation in the density of binding sites on both their axons and distal dendrites. Intermediate levels of binding were found on bipolar 1 degree dendrites and horizontal cells. No specific binding was detected on Müller cells and most parts of photoreceptors. Comparisons between cells showed that grain densities were similar for bipolar axons and amacrine/ganglion cell processes but bipolar dendrites were richer in binding sites than horizontal cell dendrites. Thus, muscarinic receptors in the salamander retina are located on amacrine/ganglion, bipolar, and horizontal cells and primarily confined to the processes which compose the two synaptic layers. In the inner plexiform layer, muscarinic receptors reside on processes from all three inner retinal neurons: in the outer synaptic layer, receptors are only on second-order cells and are more numerous along bipolar than horizontal cell dendrites.

Site-directed mutagenesis of m1 muscarinic acetylcholine receptors: conserved aspartic acids play important roles in receptor function

Muscarinic acetylcholine receptors contain a region encompassing the second and third transmembrane domains that is rich in conserved aspartic acid residues. To investigate the role of four conserved aspartic acids at positions 71, 99, 105, and 122 in muscarinic receptor function, point mutations in the rat m1 muscarinic receptor gene were made that converted each Asp to Asn, and wild type or mutant genes were stably expressed in Chinese hamster ovary cells that normally lack muscarinic receptors. Substitution of Asp71 or Asp122 with Asn produced mutant receptors that displayed high affinity for carbachol but decreased efficacy and potency, respectively, in agonist-induced activation of phosphoinositide hydrolysis, suggesting that these residues may mediate receptor-GTP binding protein interactions. Substitution of Asp99 or Asp105 with Asn produced marked decreases in ligand binding affinities and/or covalent incorporation of [3H] propylbenzilylcholine mustard, suggesting that these residues may be involved in receptor-ligand interactions.

Muscarinic cholinergic receptors on the endothelium of human cerebral arteries

To characterize the muscarinic cholinergic receptors on the endothelium of human cerebral arteries, isometric tension measurement and receptor autoradiographic studies were performed. Acetylcholine (ACh) induced dose-dependent relaxation of human cerebral arteries precontracted by 10(-5) M serotonin, with an EC50 of 1.9 +/- 0.6 X 10(-6) M (n = 7). The relaxation was abolished by 10(-5) M hemoglobin. Autoradiography, using the muscarinic antagonist [3H]propylbenzilycholine mustard, demonstrated the high density of muscarinic cholinergic receptors on the endothelial cells of human cerebral arteries, especially on the luminal surface of the endothelium. These findings suggest that ACh-induced relaxation mediated by muscarinic cholinergic receptors on the endothelium has a physiological function in human cerebral arteries.

Propylbenzilylcholine mustard labels an acidic residue in transmembrane helix 3 of the muscarinic receptor

Muscarinic acetylcholine receptors were purified from rat forebrain and labeled with [3H]N-(2-chloroethyl)N-(2',3'-[3H2]propyl)-2-aminoethylbenzilate. Cleavage of the labeled muscarinic acetylcholine receptors with a lysine-specific protease yielded labeled, glycosylated peptides about 130 and 200 residues in length, which came from different receptor sequences. The probable cleavage sites are in the second intracellular loop and in the second extracellular or third intracellular loop. The N-terminal 130 residues are disulfide-bonded to another part of the receptor structure, supporting the presence of a link between the second and third extracellular loops. The [3H]propylbenzilylcholine mustard-receptor link is cleaved by nucleophiles, acids, and bases under denaturing conditions, suggesting modification of an acidic residue. Cyanogen bromide cleavage points to transmembrane helix 3 as the site of label attachment.

Inhibition and labeling of the rat renal Na+/H+-exchanger by an antagonist of muscarinic acetylcholine receptors

A covalently binding label for muscarinic acetylcholine receptors, propylbenzilylcholine mustard (PrBCM), irreversibly inhibits the Na+/H+ exchanger in rat renal brush-border membrane vesicles. Substrates of the antiporter, Na+ and Li+, as well as inhibitors, amiloride, 5-(N-ethyl-N-isopropyl)amiloride (EIPA) and propranolol, protect the antiporter from inactivation by PrBCM. With [3H]PrBCM a band with an app. Mr of 65 kDa is predominantly labeled. Amiloride protects this band from labeling with [3H]PrBCM and [14C]-N,N'-dicyclohexylcarbodiimide (DCCD) proving its identity with the renal Na+/H+ exchanger. Our data reveal a specific interaction of PrBCM with the Na+/H+ exchanger and suggest structural relations between antiporter and receptors.

Localization of acetylcholine receptors on isolated CNS neurons: cellular and subcellular differentiation

Significant progress has been made in determining regional expression of neurotransmitter receptors within the CNS, but little information is available at the neuronal level. In the current study, to begin characterizing cellular and subcellular aspects of receptor differentiation, we have localized ACh receptors on neurons isolated from the chicken CNS. Localization was determined autoradiographically using 2 cholinergic receptor probes: 3H-propylbenzilylcholine mustard (PrBCM) a muscarinic antagonist, and 125I-alpha-bungarotoxin (BTX), a putative nicotinic ligand in the avian CNS. To isolate neurons, we incubated embryonic chicken retinas (E13-E19) in Ca2+/Mg2+-free buffer containing 8 units/ml papain for 20 min and then gently agitated the tissue by trituration. Large numbers of dendrite-rich neurons, belonging to recognizable morphological subpopulations (e.g., multipolar neurons of various sizes, small bipolar and unipolar neurons), were collected on slides for autoradiography. Cell isolation had no effect on ligand binding levels, and a high ratio of specific to nonspecific binding allowed us to associate silver grains with receptor position. Muscarinic-receptor-positive cells comprised a recognizable subpopulation that had small rounded cell bodies (6-7 micron) and a single emergent arbor. The cells had an axial or elongated appearance. Muscarinic receptors were abundant over dendrites but absent from cell bodies. Segregation to dendrites was complete by E13, the age when synapses reportedly first begin to appear. Cells labeled with 125I-BTX were more heterogeneous in morphology. The most striking BTX-positive cells comprised neurons with large cell bodies (approximately 15 micron) and multiple processes. Dendrites were profusely labeled, but only sporadic labeling was seen on cell bodies, and often this was at sites crossed by labeled dendrites. Maximum labeling occurred in the distal, smallest-caliber ends of the dendritic arbors. All limbs of BTX-positive multipolar neurons expressed abundant receptors. Occasionally, a thin uniform-caliber process was seen branching from a primary dendrite, and such processes, which may have been axons, were never labeled. Very small neurons with bipolar morphology also showed minimal or no labeling on one process, despite dense labeling on the other. Photoreceptors and Mueller cells were never labeled with BTX or PrBCM. The data show that, within a discrete CNS region, specific subpopulations of neurons independently regulate expression of ACh receptors and that, even early in development, control mechanisms segregate receptors to physiologically appropriate regions of the cell surface.

Muscarinic acetylcholine receptors from avian retina and heart undergo different patterns of molecular maturation

Muscarinic acetylcholine receptors (mAChRs) from the avian CNS exist in two molecular weight forms whose concentrations change during development. Here, we have compared the development of mAChRs from embryonic hearts with those of the CNS. Analysis of [3H]-propylbenzilylcholine mustard (PrBCM)-labeled retina and heart mAChRs by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed two atropine-sensitive peaks for each tissue. Apparent molecular masses of retina mAChRs, 86 +/- 0.7 kilodaltons (kDa) and 72 +/- 0.7 kDa, were different from those of heart mAChRs, 77 +/- 1.0 kDa and 52 +/- 0.9 kDa. During retina development, the major receptor type changed from 86 kDa to 72 kDa. No such change occurred during heart development. Furthermore, the 52-kDa species appeared to be generated by endogenous proteolysis, as prolonged incubation of heart membranes at 37 degrees C increased the amount of 52-kDa peptide with a decrease of 77-kDa peptide. Protease inhibitors blocked this conversion. Incubation of retina membranes at 37 degrees C did not result in a conversion of the 86-kDa peptide into the 72-kDa peptide, but it did cause the appearance of a minor amount of 52-kDa peptide. The proteolysis of retina mAChRs was not enhanced by cohomogenizing them with heart tissue, arguing against the presence of releasable proteases in heart. Membrane-bound retina and heart mAChRs displayed similar sensitivity to exogenous (Staphylococcus aureus V8) protease, indicating that heart receptors were not unusually susceptible to proteolytic attack; analysis of the labeled polypeptides with the V8 protease showed different patterns of digestion for the retina and heart receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Distance between the propylbenzilylcholine mustard attachment site and carbohydrates and thiol groups in muscarinic acetylcholine receptor protein from rat cerebral cortex

When rat cerebral-cortex membranes were labelled with [3H]propylbenzilylcholine mustard ([3H]PrBCM), a single protein of Mr 68,000 was found to carry the atropine-sensitive covalent label. After trypsinolysis of the receptors solubilized in 0.075% SDS, the resulting fragments were submitted to size analysis in combination with wheat-germ agglutinin (WGA)-Sepharose and organomercurial-agarose chromatography. Peptides of Mr 75,000, 50,000, 30,000, 18,000 and 8000 were specifically released from the receptor. All fragments above Mr 8000 were able to bind WGA-Sepharose and therefore the peptide of Mr 18,000 was taken as the upper limit of the distance between the antagonist and the glycan moieties. The limit fragment of Mr 8000 carried chemical groups which were modified by N-ethylmaleimide and reacted with an immobilized organomercurial. About 65-80% of the labelled receptors were adsorbed on concanavalin A-Sepharose with low affinity, generating two further components after sequential application to WGA-Sepharose. About 50% of the receptors were susceptible to neuraminidase treatment, with a concomitant slight modification of the SDS/polyacrylamide-gel-electrophoretic pattern.

Experimental localization of muscarinic receptor subtypes to cingulate cortical afferents and neurons

A technique was developed to evaluate the potency of ligand binding at M2 ACh receptors and to experimentally localize the M1 and M2 subtypes to specific neuronal processes. Normal and experimental material was prepared with tritiated ligand binding to cryostat-sectioned area 29c of posterior cingulate cortex in rat, coverslip autoradiography, and single-grain-counting techniques. Three fundamental issues were addressed. 1. A morphological criterion termed an index of heterogeneity was developed by which the specificity of M2 binding by different ligands could be assessed. The index was calculated by first determining the laminar distribution of pirenzepine (PZ) binding sites and then summing absolute laminar variations from this distribution for each ligand. According to this measure the most efficient protocol for assaying M2 sites was tritiated oxotremorine-M (OXO) coincubated in unlabeled PZ (5 x 10(-8) M). The classical muscarinic antagonist propylbenzilylcholine mustard (Pr-BCM), however, when coincubated in PZ, was almost as efficient as PZ-blocked OXO binding. 2. Terminal axons of neurons in the anterior thalamic nuclei (ATN) have M2 receptors based on the following observations. First, specific binding of the M1 ligand PZ was unaffected by ATN lesions. Second, tritiated OXO and PrBCM binding blocked with unlabeled PZ, conditions favoring M2 receptor binding, showed significant reductions in binding in layers la, lb, and IV following ATN ablations. Third, IC50 values as determined by competition of PZ for PrBCM binding sites were shifted to lower concentrations in superficial layers by ATN lesions but not in deep layers where the thalamus does not terminate. Finally, in contrast to PZ-blocked OXO and PrBCM binding, binding of PZ-blocked 3H-quinuclidinyl benzilate (QNB) was reduced to homogeneity following ATN lesions. 3. Cortical pyramidal neurons have dendritic receptors that are primarily of the M1 subtype but may also include M2 sites. Thus, full depth ibotenic acid lesions reduced PZ binding by almost 70%. Neurotoxin lesions of neurons in layers II-IV or Vb-VI were followed by degeneration of the apical dendrites of pyramids in layer I and 78 and 15% reductions, respectively, in PZ binding. Also, full-depth neurotoxin lesions combined with ATN ablations completely abolished heterogeneities in PrBCM and PZ-blocked OXO binding. These data demonstrate that experimental techniques can be used in conjunction with normal material to make morphological assessments of the efficiency of binding of putative M2 ligands.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunoprecipitation of an affinity-alkylated fragment of the muscarinic acetylcholine receptor with an anti-ligand monoclonal antibody

A monoclonal antibody raised against the muscarinic acetylcholine affinity-alkylating antagonist propylbenzilylcholine mustard was tested for its ability to recognize affinity-alkylated muscarinic receptors. We demonstrate here that although the antibody will not recognize the mustard when it is covalently linked to the native muscarinic receptor, trypsinization of affinity-labeled membranes releases a proteolytic labeled fragment that can be specifically immunoprecipitated by the antibody. Electrophoretic analysis of the immunoprecipitate indicates that the ligand was associated with a polypeptide of molecular weight 5,000. The recognition of this fragment by the antibody provides a means to immunopurify a portion of the muscarinic receptor that is at or near the ligand binding site.

Autoradiographic localisation of muscarinic receptors on guinea pig intracardiac neurones and atrial myocytes in culture

Muscarinic receptors were localised on cells cultured from the atria and interatrial septum of newborn guinea pig heart by autoradiography using [3H]propylbenzilylcholine mustard. All of the intracardiac neurones observed in these cultures were specifically labelled: both the neuronal cell body and processes were evenly labelled over their entire surface. Autoradiographic grains were also uniformly distributed over atrial myocytes in culture. This even pattern of labelling of both neurones and atrial myocytes was not changed by the substitution of serum-supplemented growth medium with serum-free, hormone-supplemented, defined medium. The demonstration of muscarinic receptors on intracardiac neurones has important implications for studies on the roles of these neurones in the mammalian heart.

Phosphorylation of brain muscarinic receptor: evidence of receptor regulation

Muscarinic receptor, from porcine synaptic membrane, was purified by affinity chromatography. Molecular weight analysis by SDS-gel electrophoresis revealed one major peptide with an apparent Mr of 68 +/- 2 Kda. The purified receptor was phosphorylated by the catalytic subunit of cAMP-dependent protein kinase resulting in a concomitant loss in specific binding, and this loss was reversed by calcineurin.

Structural microheterogeneity of the muscarinic cholinergic receptor is not related to functional diversity identified by differences in affinity for pirenzepine

The muscarinic receptor for acetylcholine shows a diversity in ligand binding properties and effector mechanisms which have suggested the existence of two subtypes (M1 and M2), to which the selective antagonist pirenzepine binds with markedly different affinities. The receptor from rat brain, covalently labelled with the alkylating antagonist tritiated propylbenzilylcholine mustard, displays a structural microheterogeneity on electrophoresis, covering the region of apparent molecular weight 66,000-76,000, with dominant components at 68,000 and 73,000. Selective inhibition by pirenzepine of labelling of the M1 receptor with tritiated mustard has been analysed on fluorographs of sodium dodecyl sulphate-polyacrylamide gels and shown to cause a uniform reduction in radioactive labelling of the broad receptor peak, rather than selectively inhibiting either the high- or low-molecular-weight regions of the band. It is further shown that although this receptor microheterogeneity is found for each of four brain regions studied, it is not found for the heart receptor, which gives a discrete labelled band of apparent molecular weight 72,000. It is therefore suggested that the structural microheterogeneity is the result of tissue-specific, posttranslational modification of the molecule, such as glycosylation, and is not directly related to the functional diversity of the receptor.

A novel class of conformationally restricted heterocyclic muscarinic agonists

A series of conformationally restricted compounds containing the 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol (THPO) skeleton, including O-methyl-THPO (10a) and O,5-dimethyl-THPO (11a), were synthesized. The compounds were designed by bioisosteric replacement of the methyl ester groups of the muscarinic cholinergic agonists norarecoline and arecoline by the 3-methoxyisoxazole group, and their interactions with central and peripheral muscarinic receptors were tested in vitro. The compounds 10a, 11a, O-ethyl-THPO (10b), O-propargyl-THPO (10j), and O-ethyl-5-methyl-THPO (11b) were inhibitors of the binding of the muscarinic mustard [3H]PrBCM to rat brain membranes with an increasing order of potency. There was, however, a very low degree of correlation between these binding data and the effects of the compounds on peripheral (ileal) muscarinic receptors, where 11a, 10j, 11b, and 10a were agonists with a decreasing order of potency, whereas O-isopropyl-THPO (10e) showed antagonistic effects. The relatively low pKa values of the compounds (7.5-7.7 for compounds with secondary and 6.1-7.0 for compounds with tertiary amino groups) are likely to allow the compounds to penetrate the blood-brain barrier.

Parallel postnatal development of choline acetyltransferase activity and muscarinic acetylcholine receptors in the rat olfactory bulb

The development of cholinergic synapses in the rat olfactory bulb was investigated by measuring changes in the activity of choline acetyltransferase (ChAT; EC 2.3.1.6.), a presynaptic cholinergic marker, and in the concentration of muscarinic receptors, components of cholinoceptive membranes. Three biochemical properties of the muscarinic system also were examined for possible differentiation: ligand binding, molecular weight, and isoelectric point. Receptors from embryonic (day 18), neonatal (postnatal day 3), and adult rat olfactory bulbs exhibited identical complex binding (nH = 0.45) of the agonist carbachol. For each age, the relative proportions of high-affinity (Ki approximately equal to 1.0 microM) and low-affinity (Ki approximately equal to 100 microM) binding states were 60% and 40%, respectively. The antagonist pirenzepine also bound to high-affinity (Ki approximately equal to 0.15 microM, RH approximately equal to 70%) and low-affinity (Ki approximately equal to 2.0 microM, RL approximately equal to 30%) sites in neonatal and adult rats. Sodium dodecyl sulfate/urea-polyacrylamide gel electrophoresis of [3H]propylbenzilylcholine mustard-labeled receptors from neonatal and adult rats showed a single electrophoretic form with an apparent molecular weight of 65,000. In contrast, analytical isoelectric focusing indicated high pI (4.50) and low pI (4.00) receptor forms were present. Neonatal rats contained approximately equal proportions of the two receptor forms, whereas adult rats contained mainly the low pI form, indicating that molecular alteration of the receptor population had occurred during development. Comparison of postnatal changes in acetylcholine receptors and ChAT activity showed a striking correlation between the development of cholinergic terminals and muscarinic receptors. Throughout the first postnatal week, ChAT activity remained at 5% of adult levels; activity began to rise on postnatal day 6 and gradually reached adult levels (56 +/- 4 mumol of [3H]acetylcholine/h/g) during the fourth week. Similarly, muscarinic receptor concentration was low (30-50 fmol/mg) throughout the first week, began to rise at postnatal day 7; and reached 90% of adult levels (317 +/- 17 fmol/mg) by the fourth week. In contrast, there was little increase in the concentration of nicotinic acetylcholine receptors (30 fmol/mg) during this period. The parallel postnatal development of ChAT activity and muscarinic receptors suggests the existence of factors that couple the differentiation of presynaptic cholinergic terminals and postsynaptic cholinoceptive elements.

Localization of muscarinic receptors on cultured myenteric neurons: a combined autoradiographic and immunocytochemical approach

In order to localize the distribution of muscarinic receptors over the surface of cultured myenteric neurons, an autoradiographic procedure for detecting muscarinic receptors (using [3H]propylbenzilylcholine mustard; [3H]PrBCM) has been combined with an immunofluorescence procedure visualizing neuronal morphology (using an antibody raised against rat brain synaptosomes; anti-CTX). Using this technique, muscarinic receptors were localized over discrete areas of the neuronal cell surface. Receptors were seen to be widely distributed over the somata and neurites of 10-20% of cultured myenteric neurons. The greatest concentration of receptors occurred over the cell somata and proximal neurites. Receptors appeared evenly distributed over labeled cell somata, where their density was estimated to be between 30 and 100 receptors/micron2. Distal regions of neurites were labeled intermittently with some varicosities and intervaricosities being labeled while others were unlabeled. Growth cones and nerve endings of labeled neurites were consistently labeled. The ability to localize muscarinic receptors on a subpopulation of immunocytochemically identified neurons enhances our understanding of muscarinic neurotransmission in myenteric neurons and provides an experimental system for the investigation of regulatory influences on neuronal neurotransmitter receptor expression and distribution.

Patterns of muscarinic cholinergic binding in the striatum and their relation to dopamine islands and striosomes

The distribution of muscarinic cholinergic binding sites in the striatum was studied in relation to the locations of other neurochemical markers in the developing rat, cat, ferret, and human. In addition, patterns of striatal muscarinic binding were studied in the adult cat. Receptor binding autoradiography was carried out with tritiated propylbenzilylcholine mustard [( 3H]-PrBCM), an irreversible muscarinic antagonist, and subsequent serial section analyses involved comparisons among patterns of muscarinic binding, catecholamine histofluorescence, acetylcholinesterase (AChE) staining, Nissl staining, and cell labeling with [3H]-thymidine. Muscarinic binding in the immature striatum was characterized by local patchiness as well as regional density gradients in all species, with the most complex patterns appearing in the human. Patches of dense muscarinic binding were shown to lie in register with fluorescent dopamine islands (rat, cat, ferret), with AChE-positive patches (all species), and with clusters of neurons pulse-labeled by exposure to [3H]-thymidine on embryonic day 27 (ferret). At the developmental stages examined, the [3H]-PrBCM-positive patches were roughly aligned with regions of weak Nissl staining (cat, human). Striatal [3H]-PrBCM binding in the adult cat was dense, and though it usually appeared nearly homogeneous, in some sections patches of elevated binding were present. These had as counterparts, in neighboring sections, AChE-poor striosomes. We conclude that during development muscarinic cholinergic function is compartmentalized in the striatum in association with dopamine-containing afferents, and that this compartmentalization may persist to some degree in the adult.

Purification of the muscarinic acetylcholine receptor from porcine brain

The muscarinic acetylcholine receptor of porcine cerebrum has been purified to apparent homogeneity by affinity chromatography, with conjugated 3-(2'-aminobenzhydryloxy)tropane (ABT) as described previously (Haga, K., and Haga, T. (1983) J. Biol. Chem. 258, 13575-13579). In a single step purification using 900 ml of digitonin/cholate-solubilized preparations and 300 ml of the ABT-agarose gel, we obtained, in a yield of 10-15%, more than 250 pmol of muscarinic receptors which bind [3H]N-methylscopolamine with a specific activity of 1,000-5,000 pmol/mg of protein (1,000-5,000-fold purification). The muscarinic receptors eluted from the ABT-agarose gel with 0.1 mM atropine were adsorbed to hydroxylapatite and then recovered as a concentrated solution. Muscarinic receptors were further purified by rechromatography with the same gel or by gel permeation high pressure liquid chromatography. The amino acid composition of the purified receptor was determined, and the specific activity of the purified preparation was estimated to be 13,100 pmol/mg of protein on the basis of amino acid composition. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified receptors with or without radioiodination revealed a single, major band with an apparent Mr of 70,000 either by silver staining or radioautogram. The major band corresponded to the band which specifically bound [3H]propylbenzylcholine mustard (irreversible muscarinic ligand). The purified receptor showed essentially the same specificity for muscarinic ligands as unpurified receptors.

Muscarinic acetylcholine receptor structure in acinar cells of mammalian exocrine glands

Characterization of muscarinic acetylcholine receptors in acinar cells from rat pancreas and lacrimal and parotid glands was achieved by binding of the reversible muscarinic antagonist [3H]quinuclidinyl benzilate (QNB) and the specific alkylating reagent [3H]propylbenzilylcholine mustard (PrBCM) to intact acini or dispersed acinar cells. Binding studies with [3H]QNB showed that acinar cells from pancreas contain 26,400, from parotid 21,400, and from lacrimal gland 25,700 binding sites/cell. To assess molecular size of the receptor in each gland, acini were prepared by digestion with purified collagenase and singly dispersed acinar cells were prepared by a combination of digestion with crude collagenase, hyaluronidase, and alpha-chymotrypsin and divalent cation chelation using EDTA. Muscarinic receptors on acini or dispersed cells were covalently labeled with 5 nM [3H]PrBCM, solubilized directly in hot sodium dodecyl sulfate buffer, and resolved by polyacrylamide gel electrophoresis. When solubilized acini were electrophoresed, a major labeled peak was observed on gels along with a smaller peak of lower apparent molecular weight. For pancreatic acini, the apparent molecular weights of these peaks were 117,600 and 85,700; for parotid acini, 104,800 and 74,500; and for lacrimal acini, 87,200 and 63,100. Addition of muscarinic antagonists to the labeling medium abolished both peaks. When dispersed acinar cells were labeled, the larger peak was eliminated, and all radioactivity was concentrated in a single peak: 87,600 for pancreas, 78,000 for parotid gland, and 62,800 for lacrimal gland. Digestion of prelabeled acini with the mixture of enzymes used to produce dispersed acinar cells similarly shifted all radioactivity into this second peak. Limited digestion of acini or dispersed cells with 1 mg/ml of papain resulted in the disappearance of these higher molecular weight peaks and the appearance of a broad peak at Mr = 40,000. Cells of nonepithelial origin, IM-9 lymphocytes and NG108 neuroblastoma X glioma hybrids, also were labeled with [3H]PrBCM and electrophoresed.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic cholinergic receptor of rat brain. Factors influencing migration in electrophoresis and gel filtration in sodium dodecyl sulphate

The muscarinic cholinergic receptor present in synaptosomal membranes of rat brain was covalently labelled with the alkylating muscarinic antagonist, tritiated propylbenzilylcholine mustard. The labelled receptor was then solubilized in sodium deoxycholate and sodium dodecyl sulphate, and its migration in polyacrylamide gel electrophoresis and gel filtration in the presence of sodium dodecyl sulphate analysed. Provided both proteolysis and inter-chain disulphide bond formation were vigorously prevented, the receptor from rat forebrain (cerebral cortex plus caudate putamen) migrated, in sodium dodecyl sulphate/polyacrylamide gel electrophoresis, as a broad band of apparent Mr 66000-76000. Two dominantly labelled polypeptides, of apparent Mr 68000 and 73000, could be distinguished as the major components of this band. These multiple species seen in electrophoresis may reflect a structural diversity related to the different binding properties, and modes of action, of this receptor. In electrophoresis using discontinuous buffer systems the labelled receptor readily formed intermolecular disulphide bonds and so aggregated. In particular, if solubilized membranes were reduced with 2-mercaptoethanol, and reformation of disulphide bonds during electrophoresis not prevented, then formation of a dimeric species (apparent Mr 119000-128000) occurred. This probably explains previous reports in the literature of larger-Mr species seen in electrophoresis. During gel filtration, the receptor formed intra-chain disulphide bonds which produced conformational heterogeneity, leading to polydisperse migration. In addition, extensive proteolytic degradation of the receptor occurred due to a protease migrating slightly ahead of the receptor. Both effects were eliminated by alkylation of the solubilized membranes with iodoacetamide before gel filtration. Alkylated receptor migrated on Sephacryl S-300 in 0.5% sodium dodecyl sulphate with an equivalent Stokes' radius of 6.1 nm. This is identical to that of reduced ovalbumin, a molecule with an apparent Mr in gel electrophoresis of 43000. On a different gel matrix, TSK HW 55(S), the receptor migrated with a somewhat larger Stokes' radius, eluting just behind reduced bovine serum albumin (Stokes' radius 8.5 nm; apparent Mr in electrophoresis 67000). Thus the receptor appears to adsorb to the Sephacryl matrix, although even on the TSK gel the receptor eluted as a somewhat smaller protein than expected from its behaviour in gel electrophoresis. Solubilized, alkylated receptor, partly purified by gel filtration so that it was not degraded by endogenous proteases, was not cleaved by mild hydroxylamine treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

The citalopram/5-HTP-induced head shake syndrome is correlated to 5-HT2 receptor affinity and also influenced by other transmitters

Combination of the 5-HT-uptake inhibitor, citalopram, with 1-5-HTP induced a characteristic head shake syndrome in rats. This was blocked by a range of 5-HT antagonists, including the selective 5-HT2 antagonists, ketanserin and pirenperone and was also blocked by the alpha 1-adrenoceptor antagonists, prazosin and WB 4101, and the alpha 2-adrenoceptor agonist, clonidine. 1-5-HTP-antagonistic effect was also recorded for 26 neuroleptic drugs. Their inhibitory potencies showed close correlation to 5-HT2-receptor affinity in vitro and, slightly weaker, to alpha 1-adrenoceptor affinity. In contrast, no correlation to dopamine D-2 receptor affinity was found, indicating that the cataleptogenic and motility-inhibitory properties of neuroleptics did not unspecifically influence 1-5-HTP-induced head shakes. These were not influenced by a histaminic antagonist, muscarinic antagonist or alpha 2-adrenoceptor antagonist, but were inhibited by beta-adrenoceptor blockers, GABA agonists, a benzodiazepine and morphine. The results indicate that 1-5-HTP-induced head shakes are most sensitive to 5-HT2 antagonists, but that the syndrome is influenced by other neuronal systems. Since 5-HT2 affinity and alpha 1-adrenoceptor affinity of many compounds is found concomitantly, caution is needed to evaluate the relative importance of these properties for 1-5-HTP antagonism.

Distribution of muscarinic receptors on cultured myenteric neurons

The distribution of neurotransmitter receptors over the neuronal cell surface remains an outstanding question in neurobiology. In this study we have used autoradiographic procedures to localize muscarinic receptors on living cultures of enteric neurons, using the specific irreversible muscarinic ligand, [3H]propylbenzilylcholine mustard ([3H]PrBCM). Most of the label was associated with nerve processes surrounding the cell bodies and along the lengths of many of the neurites radiating from the cell bodies; a small proportion of the nerve cell bodies was also seen to be labeled. The distribution of autoradiograph silver grains along both varicose and non-varicose neurites demonstrated that muscarinic receptors were present on both pre-terminal and terminal regions of the nerve fibres.

Autoradiographic identification of muscarinic receptors in human iris smooth muscle

We have used the specific, irreversible muscarinic ligand [3H]-propylbenzilylcholine mustard to localize putative muscarinic cholinergic receptors in the smooth muscle tissue of the human iris. Analysis of autoradiograms from labeled irides reveals high grain densities over the iris sphincter muscle, consistent with the well-known pharmacology of this muscle. In addition, a smaller but significant population of muscarinic binding sites was seen in the iris dilator muscle as well. Grain densities in both muscles are substantially reduced in control tissue treated with relatively high concentrations of the muscarinic antagonists quinuclidinyl benzilate (QNB) and atropine. This is, to our knowledge, the first report of autoradiographic localization of putative muscarinic receptors in the human iris.

Determination of dissociation constants and relative efficacies of oxotremorine analogs at muscarinic receptors in the guinea-pig ileum by pharmacological procedures

The muscarinic activities in the isolated guinea-pig ileum of oxotremorine, its acetamide analog, I, and of carbachol were resolved into affinity and efficacy components. The method used involved irreversible blockade of spare receptors with propylbenzilylcholine mustard (PrBCM). This method also was employed to determine dissociation constants (KA) and relative efficacies of the enantiomers of two oxotremorine analogs (II and III) with partial agonist properties. The KA values thus obtained for the enantiomers of Compounds II and III were almost identical to those estimated pharmacologically by two independent methods, one of which did not make use of an irreversible antagonist. The dissociation constants of the enantiomers of the competitive antagonist IV, determined against carbachol, were the same before and after inactivation of about 90% of the receptors with PrBCM. These results appear to justify the use of PrBCM for the determination of dissociation constants and relative efficacies of muscarinic agonists, despite claims in the recent literature to the contrary. The KA values of oxotremorine (6.79 X 10(-7) M) and carbachol (1.64 X 10(-5) M) were in good agreement with those determined pharmacologically and biochemically in other laboratories. The efficacy of carbachol was 7.2-fold higher than that of oxotremorine which was only slightly less efficacious than Compound I. A survey of structure-activity relationships among the eight oxotremorine analogs studied suggested that the structural requirements for achieving high affinity are independent of those leading to high efficacy.

Muscarinic receptors: evidence for a nonuniform distribution in tracheal smooth muscle and exocrine glands

Muscarinic receptor distribution in smooth muscle, exocrine glands, and epithelium of the ferret trachea was determined using [3H]propylbenzilylcholine mustard ([3H]PrBCM) binding and autoradiography. Specific, atropine-sensitive [3H]PrBCM binding was quantified autoradiographically in the trachealis muscle (approximately 21 binding sites/microns2), surface epithelium (approximately 6 binding sites/microns2), and submucosal glands (approximately 5 binding sites/microns2). Serous and mucous cells in the glands did not differ in receptor density. Binding sites on gland and epithelial cells were associated with basolateral membranes. In the trachealis muscle, a gradient in receptor density was observed, with outer layers of muscle containing 3 to 10 times more receptors per unit area than inner layers. Receptor distribution in both glands and muscle paralleled the distribution of cholinergic axons. However, at the light microscope level, there was no evidence for the presence of receptor "hot spots" related to the position of individual axons. The parallelism in the distribution of axons and receptors suggests the possibility of neural control of the genesis and/or maintenance of receptor distribution in these tissues.

The effects of p-chloromercuribenzoate on muscarinic receptors in the cerebral cortex

The action of p-chloromercuribenzoate (PCMB) on the ligand binding properties of the muscarinic receptors in the rat cerebral cortex has been examined. At low concentrations, PCMB produces a selective change in the binding of agonists without any effect on the binding of antagonists. At higher concentrations, the structure-binding profile for binding antagonists is changed. The affinity of agonists is greatly reduced and the heterogeneity of binding eliminated. The effects of both high and low concentrations of PCMB can be reversed by dithiothreitol. Inactivation of receptors proceeds in parallel and is kinetically complex. It can only be partially reversed by dithiothreitol. Evidence is presented connecting the low affinity agonist binding site with the high affinity pirenzepine binding site. The changes produced by PCMB have been interpreted in terms of the modification of receptor conformation.

Presynaptic muscarinic receptors on dopaminergic terminals in the nucleus accumbens

Levels of muscarinic receptors were measured in the nucleus accumbens of rat following 0.8 microgram 6-hydroxydopamine or vehicle injections (0.2 microliter into the ventral tegmental area to investigate whether the dopaminergic terminals destroyed by this procedure bear muscarinic receptors. Dopamine levels in the nucleus accumbens ipsilateral to the injection of 6-hydroxydopamine were substantially reduced by 83% as compared to the unlesioned side after 7 days. Significant decreases in the specific binding of [3H]N-methylscopolamine of 9 and 15% were also seen in the nucleus accumbens ipsilateral to the lesion after 7 and 14 days respectively. The class of muscarinic receptor depleted by the lesion was further investigated using [3H]oxotremorine-M to label the 'super high' affinity binding sites. The percentage occupancy of total muscarinic receptors by [3H]oxotremorine-M was significantly decreased by lesion e.g. 23% after 7 days indicting a selective loss of 'super high' affinity binding sites. The lesion caused no change in the affinity constant for the muscarinic antagonist, propylbenzilylcholine. Studies of the binding of the agonist carbachol and oxotremorine-M by competition with [3H] propylbenzilylcholine showed little change in the concentrations or affinity constants of the 'high' and 'low' affinity binding sites with the 6-hydroxydopamine lesion.

Muscarinic cholinergic receptor localization in brain by electron microscopic autoradiography

Muscarinic cholinergic receptors were labeled by the potent, specific, irreversible muscarinic cholinergic antagonist propylbenzilylcholine mustard. Slices of rat hippocampus and cerebral cortex were labeled in vitro under conditions such that workable specific to non-specific ratios of receptor binding were obtained. Light microscopic autoradiography of 1 micrometer sections revealed a limited penetration of drug into the tissue and, by grain counts, confirmed the specific to non-specific ratios observed in preliminary biochemical studies. Examination of thin sections with the electron microscope revealed a significant fraction of (but not all) autoradiographic grains associated with synapses. This fraction was reduced in tissues coincubated with quinuclidinylbenzilate to produce blanks. These results indicate an enrichment of cholinergic muscarinic receptors at synapses.

Persistent effects of high concentrations of chlorpromazine on 3H-propylbenzilylcholine mustard binding to muscarinic receptors in guinea-pig intestinal muscle strips

Low concentrations of chlorpromazine inhibited the binding of 3H-propylbenzilylcholine mustard (3H-PrBCM) to muscarinic receptors in strips of longitudinal muscle from guinea-pig small intestine in an apparently competitive manner. Higher concentrations of chlorpromazine, corresponding to the prelytic-lytic range on hypotonic erythrocyte haemolysis, produced an inhibition of the binding of 3H-PrBCM which persisted after washing for 1 hr. Neither 10(-6) M atropine nor 10(-3) M carbachol afforded any protection against this effect. There was no significant inhibition of 3H-PrBCM binding after washing for 1 hr when atropine and carbachol were added alone. The curve for the inhibition of 3H-PrBCM binding by methylatropinium in strips pretreated with chlorpromazine showed a small shift to higher concentrations compared with untreated strips, without any significant effect on the Hill coefficient. In contrast, the Hill coefficient for carbachol binding was significantly increased in the chlorpromazine-treated strips.

The character of the muscarinic receptors in different regions of the rat brain

The binding characteristics of muscarinic receptors have been critically examined in six regions of the rat brain. The binding curves of antagonists are similar for all six areas but the binding curves of agonists show large differences. It is shown that in all regions there are three classes of receptors with similar binding characteristics but that these are present in different proportions. The binding constants to the three receptor types of a range of agonists were examined and evidence was produced in support of the theory that the subclasses of brain receptors are due to a single receptor subunit subject to different conformational constraints.

Muscarinic receptors in the central nervous system of the rat. IV. A comparison of the effects of axotomy and deafferentation on the binding of [3H]propylbenzilylcholine mustard and associated synaptic changes in the hypoglossal and pontine nuclei

The reaction of axotomy has been studied in the rat hypoglossal nucleus by quantitative electron microscopical counts of numbers of synapses and by changes in muscarinic receptors assessed by counting silver grains in light microscope autoradiographs of the specific (atropine-sensitive) binding of [3H]propylbenzilylcholine mustard in cryostat sections. For the first 5 days after unilateral peripheral hypoglossal nerve axotomy the muscarinic ligand binding falls to 50% of control levels and then shows no further fall for up to 30 days. Synapse numbers decrease progressively over the first 10 days after operation, by which time they reach 50% of normal. Thus receptor changes reach completion at a time when synapse loss is still continuing. Later, both muscarinic ligand binding and synapse numbers recover to an extent which depends at least in part on the effectiveness of the peripheral nerve regeneration, suggesting that both the receptor and synapse changes may be dependent upon neuromuscular contacts. The reactions of muscarinic receptors to axotomy and deafferentation have been studied in the rat basilar pontine nuclei. Cerebellectomy, which causes axotomy of the pontine neurones and also removes their postsynaptic targets (the granule cells), causes no change in pontine muscarinic receptor over the first week after operation. This differs from the rapid fall in hypoglossal muscarinic receptors induced by axotomy. At longer survivals after cerebellectomy there is a partial loss of pontine muscarinic receptors associated with atrophy of the pontine neurones. Destruction of the neocortical afferents causes a loss of at least half of the synapses in the pontine neuropil. However, the light microscopic autoradiographic study revealed no obvious changes in the dentisy or distribution of the pontine muscarinic receptors from 4 days to more than 6 months after operation.

Muscarinic receptors in the central nervous system of the rat. II. Distribution of binding of [3H]propylbenzilylcholine mustard in the midbrain and hindbrain

The distribution of muscarinic receptors has been studied in the rat midbrain and hindbrain by counting silver grains in light microscope autoradiographs of the specific (atropine-sensitive) binding of [3H]propylbenzilylcholine mustard in cryostat sections. Of the 78 areas studied 6 had grain counts between 6 and 9 times the nonspecific level ("high"), and a further 15 had counts 4-6 times non-specific ("intermediate"). The basilar pontine nuclei and the ventral nuclei of the lateral lemniscus had high counts. Among the cranial nerve motor nuclei the facial and hypoglossal nuclei had high counts and the motor trigeminal nucleus and nucleus ambiguus had medium counts. The interpeduncular nucleus as a whole had low counts but there were two bands of intense staining on each side around the entry zone of the bundles of afferent cholinergic fibres from the habenula. Intermediate levels of binding occurred over the inferior colliculus and the superficial and intermediate grey layers of the superior colliculus. The molecular layer of the vestibulocerebellar vermis was distinctly labelled.

Muscarinic receptors in the central nervous system of the rat. I. Technique for autoradiographic localization of the binding of [3H]propylbenzilylcholine mustard and its distribution in the forebrain

[3H]Propylbenzilylcholine mustard ([3H]PrBCM) is a synthetic, potent muscarinic antagonist, which binds specifically and irreversibly by means of a covalent linkage to muscarinic receptors. Ten micrometer coronal cryostat sections taken through unfixed rat brain at the level of the maximum extent of the caudate nucleus were mounted on glass slides and incubated with 2.4 nM [3H]PrBCM at 30 degrees C for 25 min. They showed a total binding of 3250 pmol/g protein, of which 2130 pmol/g protein was sensitive to pretreatment with 10-6 M atropine. The specific (atropine-sensitive) binding was saturable. Saturation was reached at 15 min, with a rate constant of 1.3 x 106 M-1 sec-1. Binding was unaffected by drugs acting at nicotinic receptors (D-tubocurarine, hexamethonium), or by physostigmine, but was inhibited by muscarinic drugs (pilocarpine, oxotremorine, 3-quinuclidinylbenzilate). Postfixation for 15 min in Carnoy's fixative reduced the specific binding by 10% and the non-specific by 50%. Prefixation (i.e. before incubation with [3H]PrBCM) with any fixatives containing formaldehyde largely prevented specific binding, but a range of concentrations of glutaraldehyde (2% to 0.05%) caused only small reductions in specific binding (e.g. 0.1% glutaraldehyde caused only a 6% reduction). Clear, regionally specific patterns of localization of specific label in light microscope autoradiographs could be obtained from cryostat sections prefixed with 0.1% glutaraldehyde, incubated with 2.4 nM [3H]PrBCM for 15 min at 30 degrees C, and postfixed for 15 min in Carnoy's solution. Of the 105 forebrain areas studied 12 had grain counts between 6 and 9 times the non-specific level and a further 30 had counts 4 to 6 times non-specific. The higher grain counts were in the external plexiform layer of the olfactory bulb, anterior olfactory nucleus, olfactory tubercle, pyriform cortex, stratum radiatum of the hippocampus, stratum moleculare of the dentate gyrus, lateral amygdaloid nucleus, cortico-amygdaloid transition zone, anteroventral thalamic nucleus, hypothalamic supraoptic nucleus, caudate-putamen, nucleus accumbens, and in laminae 3 and 6 of the neocortex (parietal region). There were high grain densities over the choroid plexus the lateral but not the third or fourth ventricles.

Muscarinic receptofs in the central nervous system of the rat. III. Postnatal development of binding of [3H]propylbenzilylcholine mustard

The postnatal development of muscarinic receptors has been studied in 7 selected areas from the brains of 1-17-day-old rats by counting silver grains in light microscope autoradiographs of the specific (atropine-sensitive) binding of [3H]propylbenzilylcholine mustard in cryostat sections. A major part of the adult receptor density is present at 1 day of age, a time when only a small fraction of the adult number of synapses has yet been formed. Of the areas studied the hypoglossal nucleus is the most precocious in muscarinic receptor development, and the dentate gyrus the latest (associated with the late development of the dentate granule cells). The pattern of receptor distribution changes with development. The caudate-putamen first develops receptor in patches, beginning at the lateral (ventricular) surface. The pontine nuclei develop receptor in a medial to lateral sequence. The maturation of the adult laminar pattern of the olfactory bulb depends on the alignment of cells (especially the mitral cells). The neocortex initially has uniform labelling throughout its depth, and later the labelling in layer 4 becomes relatively less dense (probably associated with the ingrowth of afferent fibres). The hippocampal formation first develops receptor evenly over the pyramidal cell dendrites; later receptor appears over the newly formed dentate stratum moleculare and becomes much reduced over the hippocampal stratum lucidum and stratum lacunosum-moleculare (probably associated with the ingrowth of afferent fibres from the dentate gyrus and entorhinal area). In the cerebellum muscarinic receptor is found only in the lobules which receive the primary vestibular afferents. In the neonate it is present in the granular layer, but this later disappears and is replaced by the adult pattern of labelling in the molecular layer.

Changes in muscarinic ligand binding to intestinal muscle strips produced by pre-exposure to hypotonic conditions

The extent of the binding of [3H]propylbenzilylcholine mustard (3H-PrBCM) to muscarinic receptors in longitudinal muscle strips from guinea-pig small intestine is increased by nearly 50% when the strips are preexposed to distilled water before measurement of 3H-PrBCM binding in Krebs-Henseleit solution. The apparent rate constant for 3H-PrBCM-receptor complex formation is more than double that of intact strips. The curves for the inhibition of 3H-PrBCM binding by methylatropinium bromide in normal and treated strips are superimposable, but, in contrast, distilled water pretreatment shifts the inhibition curve for carbachol to lower concentrations by a factor of 5-6. The inhibition curve for methylfurmethide is also shifted, by a factor of approximately 4, but the effect on the curve for hexyltrimethylammonium (C6TMA) is slight. The relative inhibition produced by benzhexol in the two preparations was variable. Comparison of the rate of equilibration of benzhexol with muscarinic receptors in intact and in distilled water pretreated muscle indicates that this inconsistency is unlikely to be due to incomplete equilibration.

Peripheral and central muscarinic receptor affinity of psychotropic drugs

The muscarinic receptor affinity of 27 psychotropic and 5 anticholinergic substances was examined in 2 in-vivo and 2 in-vitro models. A highly significant correlation was obtained between the effect of all compounds examined on the atropine sensitive binding of 3H-PrBCM and the effect in the conventional guinea-pig ileum preparation. Antagonism of oxotremorine induced tremors in mice by anticholinergics and neuroleptics was also significantly correlated to the corresponding data obtained in the in-vitro tests. Due to very low potency in the physostigmine induced mortality test in mice too few ED50 values were obtained to perform statistical comparisons. It is concluded, that the conventional guinea-pig ileum model and the 3h-prBCM binding model are equally predictive as tests for antimuscarinic properties. When in-vivo anticholinergic data for neuroleptics are used it must be considered that a possible dopamine receptor blockade may diminish the antimuscarinic effect of the substance.