Primary structure of porcine cardiac muscarinic acetylcholine receptor deduced from the cDNA sequence

Muscarinic m1 receptor-stimulated adenylate cyclase activity in Chinese hamster ovary cells is mediated by Gs alpha and is not a consequence of phosphoinositidase C activation

The mechanism underlying muscarinic m1 receptor-mediated increases in adenosine 3',5'-cyclic monophosphate (cAMP) was investigated in Chinese hamster ovary (CHO) cells expressing human recombinant m1 muscarinic receptors (CHO-ml cells). Stimulation of CHO-ml cells with carbachol resulted in marked accumulation of Ins(1,4,5)P3 and cAMP, in an atropine-sensitive manner, with EC50 values (log M) of -5.16 +/- 0.06 and -3.93 +/- 0.07 respectively. Basal and agonist-stimulated cAMP accumulation were unaffected by a 5 min pretreatment with l microM phorbol 12,13-dibutyrate and were not attenuated by pertussis toxin (100 ng/ml, 20h). Agonist-stimulated cAMP accumulation was also observed in CHO-ml cell membranes incubated in a buffer containing 100 nM free Ca2+. Guanosine 5'- [gamma-thio]triphosphate (10 microM) potentiated agonist-stimulated cAMP accumulation in CHO-ml cell membranes, implicating a G-protein involvement in this response. Co-incubation of carbachol with forskolin (10 microM) produced a greater than additive accumulation of cAMP in CHO-ml cells. Furthermore, a C-terminal-directed anti-Gs alpha serum attenuated both carbachol-stimulated (in CHO-ml cell membranes) and isoprenaline-stimulated (in CHO-beta 2 cell membranes) cAMP accumulation with a similar dose-dependency. These results suggest that muscarinic agonist-stimulated cAMP accumulation in CHO-ml cells occurs via activation of Gs alpha and not as a consequence of phosphoinositidase C activation.

Muscarinic cholinergic receptor subtypes in the pigeon bursa of Fabricius: a radioligand binding and autoradiographic study

The pharmacological profile and the anatomical localisation of muscarinic cholinergic receptor subtypes were studied in the pigeon bursa of Fabricius, using radioligand binding and autoradiographic techniques with [3H]quinuclidinyl benzilate (QNB) as a ligand. [3H]QNB was specifically bound to sections of bursa of Fabricius. The binding was time-, temperature- and concentration-dependent. The dissociation constant was 0.31 +/- 0.02 nM, and the maximum density of binding sites averaged 38 +/- 2.5 fmol/mg protein. The pharmacological profile of [3H]QNB binding to sections of pigeon bursa of Fabricius was consistent with the labelling of M2, M3 and M4 muscarinic receptor subtypes. Light microscope autoradiography showed the localisation of [3H]QNB binding sites in the medulla, in follicular septa, in the cortico-medullary border and in lesser amounts in the cortical layer. The functional significance of these receptors should be clarified in future studies.

Muscarinic cholinergic receptor subtypes in human peripheral blood lymphocytes

The subtypes of muscarinic cholinergic receptors were studied in human peripheral blood lymphocytes with radioligand binding techniques and the non-selective muscarinic cholinergic receptor antagonist [3H]quinuclidinyl benzylate (QNB) as a ligand. [3H]QNB was bound to human peripheral lymphocytes in a manner consistent with the labelling of muscarinic cholinergic receptors. The dissociation constant (Kd) value was 0.60 +/- 0.08 nM and the maximum density of binding sites (Bmax) was 2.33 +/- 0.03 fmol/2.2 x 10(6) cells. The binding was time-, temperature- and concentration-dependent, belonging to a single class of high affinity sites. Analysis of the pharmacological profile of [3H]QNB binding in the presence of compounds specific for the different muscarinic receptor subtypes suggests that human peripheral blood lymphocytes express mainly muscarinic cholinergic M2 and M3 receptor subtypes and to a lesser extent muscarinic M4 receptors. The characterization of the subtypes of muscarinic cholinergic recognition sites expressed by human peripheral blood lymphocytes may represent a tool for investigating the possible relationships between immune and cholinergic systems in normal and pathologic conditions.

Autoradiographic localization of functional muscarinic receptors in the rat superior cervical sympathetic ganglion reveals an extensive distribution over non-synaptic surfaces of neuronal somata, dendrites and nerve endings

Fast synaptic transmission in sympathetic ganglia is mediated by acetylcholine, acting on nicotinic receptors, yet muscarinic receptors are also present and are involved in the production of slow postsynaptic potentials. In order further to elucidate the role of muscarinic receptors in ganglionic transmission their distribution in the rat superior cervical sympathetic ganglion was investigated autoradiographically by use of the tritiated irreversible muscarinic ligand propylbenzilylcholine mustard. It was observed that this agent blocked the carbachol-evoked hydrolysis of inositol phospholipids in the ganglion and that this response to carbachol is itself inhibitable by selective muscarinic antagonists with a potency sequence which indicates involvement primarily of M1 receptors. Light microscope autoradiography showed that labelling inhibitable by atropine and by the M1-selective muscarinic antagonist pirenzepine was essentially confined to the margins of neuronal somata and regions of dendritic arborization, which include synaptic contacts. Quantitative electron microscope autoradiography showed that binding of the radioligand, of which approximately 70% was inhibitable by atropine and 68% by pirenzepine, was associated predominantly with surface membranes of neuronal somata, dendrites, other neurites (including axons and uncharacterized dendrites) and nerve terminal profiles, in the approximate ratios 95:85:52:45. Of the inhibitable binding over neuronal membranes in the ganglion little more than 3% was found to be synaptically located, and this involved para- or peri-synaptic regions of nerve terminal contacts rather than the specialized synaptic zone. About 5% of the inhibitable binding over neuronal membranes involved non-synaptic surfaces of nerve terminals and preterminal axon segments; almost 70% was distributed over non-synaptic surfaces of neuronal somata and dendrites, and about 21% upon other neurites. Binding sites were found not to be more highly concentrated at or adjacent to synapses than over other regions of neuronal surface membranes. About 50%, possibly more, of the binding on non-synaptic surfaces of nerve endings, and about 7% of binding upon dendritic membranes, was of non-M1, possibly M2 type, inhibitable by atropine but not by pirenzepine. Non-synaptic neuro-neuronal appositions, which involve dendrites and somata and often lie adjacent to synapses, showed rather more than twice the binding expected for each membrane individually; and neuronal membrane exposed to basal lamina lining ganglionic tissue spaces showed high levels of binding. Little inhibitable binding was seen over membranes of satellite and Schwann cells, or over cytoplasmic territories or ganglionic interstitial tissue. A model was constructed of the distribution of label, which showed that the observed results for total binding could be approximately matched by assuming the following relative densities of ligand binding sites: interstitial tissue space and supporting cells 1, soma cytoplasm 3, cytoplasm of dendrites, neurites and nerve terminals 4.5, surfaces of mesodermal elements 15, surfaces of neurites and nerve endings including sites of synapse 45, surfaces of dendrites 90, surfaces of neuronal somata 120, non-synaptic neuro-neuronal appositions 180. It is concluded that functional muscarinic receptors in this sympathetic ganglion, predominantly of the M1 type linked with slow depolarizations, but including some non-M1 receptors, are widely distributed over non-synaptic surfaces of the neuronal somata and dendrites and are not concentrated at synapses. Presynaptic autoreceptors are also present, of which half or more are of non-M1, possibly M2, type which might be inhibitory. The presence of M4 receptors is not excluded...

Evaluation of 1-azabicyclo[2.2.2]oct-3-yl alpha-fluoroalkyl-alpha-hydroxy-alpha-phenylacetates as potential ligands for the study of muscarinic receptor density by positron emission tomography

Both 1-azabicyclo[2.2.2]oct-3-yl alpha-(1-fluoroeth-2-yl)-alpha-hydroxy-alpha-phenylacetate (FQNE, 5) and 1-azabicyclo[2.2.2]oct-3-yl alpha-(1-fluoropent-5-yl)-alpha-hydroxy-alpha-phenylacetate (FQNPe, 6) were prepared and evaluated as potential candidates for the determination of muscarinic cholinergic receptor (mAChR) density by positron emission tomography (PET). The results of in vitro binding assays demonstrated that although both 5 and 6 had high binding affinities for m1 and m2 mAChR subtypes, 6 displayed a higher affinity (nM, m1; KD, 0.45, m2; KD, 3.53) as compared to 5 (nM, m1; KD, 12.5, m2; KD, 62.8). It was observed that pretreatment of female Fisher rats with either 5 or 6 prior to the i.v. administration of Z-(-)(-)-[131I]-IQNP, a high-affinity muscarinic ligand, significantly blocked the uptake of radioactivity in the brain and heart measured 3 h postinjection of the radiolabeled ligand. These new fluoro QNB analogues represent important target ligands for evaluation as potential receptor imaging agents in conjunction with PET.

S-(-)-ET 126: a potent and selective M1 antagonist in vitro and in vivo

The pharmacological profile of the competitive muscarinic antagonist S-(-)-alpha-(hydroxymethyl)benzeneacetic acid 1-methyl-4-piperidinyl ester (S-(-)-ET 126) was evaluated on M1 (rabbit vas deferens; pA2=8.99), M2 (rat left atrium; pA2=8.21) and M3 (rat ileum; pA2=6.84) muscarinic receptors in comparison with pirenzepine. The drug shows a subtype selectivity (M1/M2=8; M1/M3=178; M2/M3=22) that proposes it as a useful pharmacological tool for receptor studies. S-(-)-ET 126, like pirenzepine, prevents the antinociception induced by M1 agonists (McN-A-343 and AF-102B). Unlike pirenzepine and spirotramine, the compound is able to cross the blood brain barrier which makes it useful for in vivo investigations.

Desensitization and internalization of the m2 muscarinic acetylcholine receptor are directed by independent mechanisms

The phenomenon of acute desensitization of G-protein-coupled receptors has been associated with several events, including receptor phosphorylation, loss of high affinity agonist binding, receptor:G-protein uncoupling, and receptor internalization. However, the biochemical events underlying these processes are not fully understood, and their contributions to the loss of signaling remain correlative. In addition, the nature of the kinases and the receptor domains which are involved in modulation of activity have only begun to be investigated. In order to directly measure the role of G-protein-coupled receptor kinases (GRKs) in the desensitization of the m2 muscarinic acetylcholine receptor (m2 mAChR), a dominant-negative allele of GRK2 was used to inhibit receptor phosphorylation by endogenous GRK activity in a human embryonic kidney cell line. The dominant-negative GRK2K220R reduced agonist-dependent phosphorylation of the m2 mAChR by approximately 50% and prevented acute desensitization of the receptor as measured by the ability of the m2 mAChR to attenuate adenylyl cyclase activity. In contrast, the agonist-induced internalization of the m2 mAChR was unaffected by the GRK2K220R construct. Further evidence linking receptor phosphorylation to acute receptor desensitization was obtained when two deletions of the third intracellular loop were made which created m2 mAChRs that did not become phosphorylated in an agonist-dependent manner and did not desensitize. However, the mutant mAChRs retained the ability to internalize. These data provide the first direct evidence that GRK-mediated receptor phosphorylation is necessary for m2 mAChR desensitization; the likely sites of in vivo phosphorylation are in the central portion of the third intracellular loop (amino acids 282-323). These results also indicate that internalization of the m2 receptor is not a key event in desensitization and is mediated by mechanisms distinct from GRK phosphorylation of the receptor.

Inhibition of muscarinic stimulated phosphoinositide hydrolysis in the rat parotid gland by cAMP

The ability of agents that increase or mimic cAMP to affect muscarinic receptor mediated phosphoinositide hydrolysis was investigated in the rat parotid gland. Forskolin (10 microM) and isoproterenol (10 microM) elevated cAMP in the parotid gland by 2-fold and 7-fold, respectively, and these agents also inhibited oxotremorine-M (3 microM) mediated phosphoinositide hydrolysis by 14% and 26%, respectively. Forskolin (1, 4.3, 18, and 75 microM) increased cAMP accumulation and inhibited PIP2 hydrolysis in a concentration-dependent manner. Forskolin (75 micrometers) shifted the concentration-response curve for the full agonist oxotremorine-M rightward by 4.2-fold. Pre-treatment with the phosphodiesterase inhibitor isobutylmethylxanthine (1 mM) reduced the maximum effect of oxotremorine-M by 31%. The inhibitory effect of isoproterenol and forskolin on muscarinic receptor-mediated phosphoinositide hydrolysis was unaffected by the removal of extracellular Ca2+. Moreover, isoproterenol and forskolin dampened sodium fluoride and oxotremorine-M mediated phosphoinositide hydrolysis to the same extent suggesting that the inhibitory effect of cAMP is downstream from the muscarinic receptor.

Subtype-specific regulation of muscarinic receptor expression and function by heterologous receptor activation

Incubation of cultured embryonic chicken heart cells with the beta-adrenergic agonist isoproterenol resulted in a dose-dependent increase in the number of mAChR on the surface of intact cells. The isoproterenol-mediated increase in mAChR number was time dependent and reached a maximum by 48 h. Chick heart cells treated with isoproterenol exhibited a greater than 6-fold increase in the sensitivity for carbachol-mediated inhibition of adenylyl cyclase activity as compared to control. Stimulation of cultured heart cells for 24 h with isoproterenol resulted in a 25-35% increase in cm2 mRNA levels as compared to control cm2 mRNA levels. In contrast, the level of cm4 mRNA was not significantly affected by isoproterenol treatment. cm2 mRNA levels were maximally elevated by 15 h following isoproterenol stimulation and remained elevated for up to 72 h. Incubation of cells with isoproterenol in the presence of Rp-cAMP, an inhibitor of cAMP-dependent protein kinase, blocked the increase in the level of cm2 mRNA. Thus, prolonged activation of beta-adrenergic receptors results in an increase in mAChR number and muscarinic responsiveness in chick heart cells due to a cAMP-dependent protein kinase mediated increase in cm2 mRNA levels.

Cooperativity manifest in the binding properties of purified cardiac muscarinic receptors

Muscarinic receptors were solubilized from porcine atria in digitonin-cholate and were purified by chromatography on DEAE-Sepharose and 3-(2'-aminobenzhydryloxy)tropane-Sepharose. The product identified on Western blots migrated with an apparent molecular mass of 60-75 kDa, with additional bands indicative of homotrimers (190 kDa) and homotetramers (240 kDa). Receptor eluted from the affinity column was accompanied by a mixture of guanyl nucleotide-binding proteins (G-proteins) identified on Western blots as Gi1/2, G(o), Gq/11, and Gs (preparation M2G); the G-proteins were largely removed by further processing on hydroxyapatite (preparation M2). Solubilized purified receptors bound muscarinic ligands in an apparently cooperative manner. In studies at equilibrium, the antagonists [3H]AF-DX 384, N-[3H]methylscopolamine (NMS), and [3H]quinuclidinylbenzilate (QNB) revealed Hill coefficients between about 0.8 and 1.2. Also, the apparent capacity for [3H]QNB exceeded that for [3H]AF-DX 384 and [3H]NMS by about 1.5-fold in M2 and by 2-fold in M2G. Binding to M2G at high concentrations of [3H]QNB was fully inhibited by unlabeled NMS, which therefore affected sites not labeled at similar concentrations of [3H]NMS. Oxotremorine-M displayed a biphasic inhibitory effect on the binding of [3H]AF-DX 384 in M2 and M2G, suggesting that multiple states of affinity are intrinsic to the receptor; 5'-guanylylimidodiphosphate was without appreciable effect in M2 but resulted in a bell-shaped binding profile for the agonist in M2G. All of the data can be described in terms of cooperative interactions within a receptor that is at least tetravalent and presumably an oligomer. In the context of the model, copurifying G-proteins and guanyl nucleotides serve to regulate the degree of cooperativity between successive equivalents of muscarinic ligands.

Differential coupling of m2 and m4 muscarinic receptors to inhibition of adenylyl cyclase by Gi alpha and G(o)alpha subunits

We compared the G-protein requirements for coupling of human and chicken m2 and m4 muscarinic acetylcholine receptors (mAChRs) to inhibition of adenylyl cyclase, using a luciferase reporter gene under the transcriptional control of a cAMP response element as a sensitive monitor of intracellular cAMP levels. Previously, we used this system to demonstrate that the chick m4 receptor preferentially coupled to Gi alpha-2 and G(o)alpha over Gi alpha-1 and Gi alpha-3. We found that both the chick and human m2 mAChRs can couple to Gi alpha-1, Gi alpha-2, Gi alpha-3, and G(o)alpha, while the human m4 mAChR preferentially couples to Gi alpha-2 and G(o)alpha. Both the G(o)1 and G(o)2 forms of the G(o)alpha subunit were effective in reconstituting coupling of the m2 and m4 mAChRs to inhibit adenylyl cyclase activity. The m2 and m4 mAChRs thus couple to inhibition of adenylyl cyclase by overlapping but different sets of G-protein alpha subunits.

Coupling of muscarinic m1, m2 and m3 acetylcholine receptors, expressed in Chinese hamster ovary cells, to pertussis toxin-sensitive/insensitive guanine nucleotide-binding proteins

Chinese hamster ovary (CHO) cells expressing recombinant human m1 (CHO-m1 cells), m2 (CHO-m2 cells), or m3 (CHO-m3 cells) muscarinic receptors were characterised pharmacologically with [3H]N-methylscopolamine. Agonist-stimulated coupling of these receptors with guanine nucleotide-binding proteins (G proteins) was measured by guanine nucleotide- and pertussis toxin-modification of carbachol competition-binding curves, and pertussis toxin-sensitivity of agonist-stimulated [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTP gamma S) binding, in membrane preparations of the CHO cell clones. High affinity agonist binding and agonist-stimulated [35S]GTP gamma S binding was abolished in CHO-m2 cell membranes (expressing 99 +/- 25 fmol of [3H]N-methylscopolamine binding sites/mg protein) after pertussis toxin pretreatment of cells, suggesting that muscarinic m2 receptors expressed in these cell membranes couple predominantly with pertussis toxin-sensitive G proteins. CHO-m1 (713 +/- 102 fmol/mg protein) and CHO-m3 (1212 +/- 279 fmol/mg protein) cell membranes produced smaller elevations in agonist-stimulated [35S]GTP gamma S binding considering the higher receptor levels, compared with CHO-m2 cell membranes. Pertussis toxin pretreatment of these clones also resulted in a significant attenuation of agonist-stimulated [35S]GTP gamma S binding suggesting that, under these experimental conditions, muscarinic m1 and m3 receptors can couple with both pertussis toxin-sensitive and pertussis toxin-insensitive G proteins. Guanine nucleotide-modification of agonist binding in CHO-m1 and CHO-m3 cell membranes was comparatively smaller than in CHO-m2 cell membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Light and electron microscopic study of m2 muscarinic acetylcholine receptor in the basal forebrain of the rat

The m2 muscarinic acetylcholine receptor gene is expressed at high levels in basal forebrain, but the paucity of information about localization of the encoded receptor protein has limited the understanding of cellular and subcellular mechanisms involved in cholinergic actions in this region. The present study sought to determine the cellular localization of m2 protein, its relationship to cholinergic neurons, and its pre- and postsynaptic distribution in the rat medial septum-diagonal band complex using immunocytochemistry with polyclonal rabbit antibodies and a newly developed rat monoclonal antibody specific to the m2 receptor. Light microscopic colocalization studies demonstrated that m2 was present in a subset of choline acetyltransferase immunoreactive neurons, in choline acetyltransferase-negative neurons, and in more neuropil elements than was choline acetyltransferase. Intraventricular injections of 192 IgG-saporin, an immunotoxin directed to the low-affinity nerve growth factor receptor, resulted in depletion of choline acetyltransferase-immunoreactive neurons in the medial septum-diagonal band complex, whereas m2 immunoreactivity in neurons and in the neuropil was unchanged. By electron microscopy, m2 receptor in medial septum-diagonal band complex was localized to the plasmalemma of a small population of small to medium-sized neurons, and it was also found in dendrites, axons, and axon terminals in the neuropil. Neurons expressing m2 immunoreactivity received synaptic contacts from unlabelled axon terminals. A small distinct subpopulation of large neurons, unlabelled by m2 immunoreactivity, received synaptic contacts from m2-immunoreactive terminals. Thus, m2 receptor is situated to mediate the local effects of acetylcholine on basal forebrain cholinergic and noncholinergic neurons and, also, at both pre- and postsynaptic sites.

Characterization of the rat m3 muscarinic acetylcholine receptor produced in insect cells infected with recombinant baculovirus

The m3 muscarinic acetylcholine receptor from rat heterologously produced in insect cells after infection with a recombinant baculovirus has an apparent molecular mass of approximately 75 kDa. Polyclonal antibodies raised against a carboxy-terminal nonapeptide that is unique to the m3 subtype can detect the receptors produced in the insect cells by Western blot and can also immunoprecipitate solubilized receptor. Immunofluorescence microscopy as well as electron microscopy revealed that the receptor was located intracellularly, visualized as a ring around the nucleus of the infected insect cells. Solubilization of the receptor was accomplished with digitonin which was added in increments (over 10 min) to a final concentration of 0.8% (mass/vol). The solubilized receptor is unstable when the ligand-binding site is not protected by a ligand. Here the low-affinity ligand propylbenzilylcholine (approximately 10 nM) has demonstrable protective ability during solubilization, but the usefulness of this ligand is limited by a very slow off rate. From the behaviour of the solubilized receptor during DEAE-Sephacel chromatography and lectin-affinity chromatography it can be deduced that the receptor produced in insect cells is heterogeneously glycosylated in the producing insect cells.

Cultured neurons from mouse brain reproduce the muscarinic receptor profile of their tissue of origin

These studies investigate the regional variations in the muscarinic acetylcholine receptor (mAChR) profiles in neuron populations of the CNS using primary neuron cultures derived from three areas of the mouse brain--the cerebral hemispheres, the mesencephalon and the medulla-pons--that have distinct mAChR systems. We first assessed the extent to which neurons reproduced their in vivo properties in culture by monitoring the binding capacity, the pharmacological profiles and the levels of mAChR transcripts in neuron cultures and their tissues of origin. We showed that the primary neuron cultures accumulated mAChRs with initial rates similar to those in vivo, had pharmacological profiles very close to those of their area of origin, and accumulated m1, m2, m3, m4 and m5 receptor transcripts according to patterns resembling those in the tissues. We conclude that most of the characteristics of the mAChRs in a given area are proper to the neuron population of that area, that the pattern is established early in ontogenesis, and that it is reproduced in vitro. We also show that the stimulation of phosphoinositide turnover is mediated by mAChRs with distinct pharmacological profiles in neuron cultures from the three brain areas.

Location of agonist-dependent-phosphorylation sites in the third intracellular loop of muscarinic acetylcholine receptors (m2 subtype)

Muscarinic acetylcholine receptors (mAChR, human m2 subtype) expressed in Sf9 (Spodoptera frugiperda) cells using the baculovirus system were purified and subjected to phosphorylation by a mAChR kinase, which was partially purified from porcine cerebrum. Two bands with apparent molecular masses of 59 kDa and 39 kDa as determined by SDS/PAGE were found to be phosphorylated in an agonist-dependent manner. Both bands were labeled by the irreversible muscarinic ligand [3H]propylbenzilylcholine mustard. Molecular masses of the [32P]phosphorylated or [3H]propylbenzilylcholine-mustard-labeled bands decreased following treatment with N-glycanase. The 59-kDa and 39-kDa bands were converted to 52-kDa and 32-kDa bands, respectively, indicating that both the 59-kDa and 39-kDa bands contain the amino-terminal region where glycosylation sites are present. The ratio of incorporated [32P]phosphate and bound [3H]propylbenzilylcholine mustard was essentially the same for the 59-kDa and 39-kDa bands, indicating that all the phosphorylation sites reside in the sequence of 39 kDa from the amino-terminal region. The amounts of incorporated [32P]phosphate were estimated to be 10-11/receptor, with 7-8 serine and 3-4 threonine, but no phosphorylated tyrosine residues. Further treatment of [32P]phosphorylated or [3H]propylbenzilylcholine-mustard-labeled receptors with V8 protease indicated that the phosphorylation sites were not present in 30-kDa amino-terminal segment. These results indicate that the phosphorylation sites are localized in the range 30-39 kDa from the amino terminus, which consists of primarily the central part of the third intracellular loop. Consistent with this conclusion, a fusion protein containing glutathione S-transferase linked to a peptide corresponding to residues 227-324 of the central part of the third intracellular loop was found to be phosphorylated by the mAChR kinase in a heparin-sensitive manner.

Muscarinic receptors and novel strategies for the treatment of age-related brain disorders

The muscarinic class of acetylcholine receptors is widely distributed throughout the body and mediates numerous vital functions in both the brain and autonomic nervous system. Within the brain, muscarinic receptors play an important role in learning, memory and the control of posture. There is a decrease in the synthesizing enzyme for acetylcholine in Alzheimer's disease, and damage to the ascending cholinergic system is thought to be an important determinant of the loss of memory and other functional deficits of this disease. Five subtypes of the muscarinic receptor (m1-m5) have been identified, and these receptors have a differential distribution throughout the body. The differential distribution of subtypes of the muscarinic receptor in the body suggests that centrally acting m1 and m4 muscarinic agonists might be efficacious in the treatment of age-related memory disorders, without causing peripheral side effects. In addition to the primary ligand binding site, muscarinic receptors also possess a secondary allosteric site that appears to be the target for some novel cardioselective muscarinic antagonists including the neuromuscular blocking agent gallamine. The existence of a secondary allosteric site on the muscarinic receptor suggests that it might be possible to develop novel allosteric muscarinic agonists that potentiate the effects of endogenous acetylcholine much in the same way that benzodiazepines potentiate GABA. Although no such allosteric muscarinic agonists have been identified to date, they could be very efficacious in the treatment of Alzheimer's disease.

M1 muscarinic receptors increase calcium current and phosphoinositide turnover in guinea-pig ventricular cardiocytes

1. Physiological and molecular evidence for the presence and functional role of M1 muscarinic cholinergic receptors (mAChRs) in adult guinea-pig ventricular cells is presented. 2. Whole-cell clamp measurements of the L-type calcium current (ICa) in isolated myocytes were performed. Caesium was used to suppress potassium currents. ICa was increased by the muscarinic agonist carbachol in cells pretreated with pertussis toxin which blocked the M2 mAChR-triggered cascade of intracellular signalling, while it was not changed in untreated cells. 3. If the M2-mediated regulation of ICa was blocked by directly saturating the cell with cyclic adenosine monophosphate (cAMP) through the patch pipette, application of carbachol induced a further small increase of the current above the level reached after cAMP perfusion. This increase was more pronounced in cells pretreated with pertussis toxin. 4. The carbachol-induced increase of ICa was blocked by the selective M1 mAChR antagonist pirenzepine. 5. The application of high concentrations of carbachol increased the accumulation of [3H]inositol monophosphate up to 240% above control levels. This increase was reduced by application of pirenzepine. 6. The expression of M1 receptor mRNA in ventricular cardiocytes was shown by reverse transcriptase-polymerase chain reaction. 7. These results suggest that M1 mAChR regulation of ICa can be a component of the paradoxical positive inotropism induced by high concentrations of muscarinic agonists.

Co-expression of four muscarinic receptor genes by the intrinsic neurons of the rat and guinea-pig heart

Expression of the messenger RNAs encoding the five different muscarinic acetylcholine receptor subtypes was examined in intracardiac neurons from the rat and guinea-pig heart by in situ hybridization techniques. Newborn guinea-pig intracardiac neurons were studied in dissociated cell culture preparations employing both 35S- and digoxigenin-labelled oligonucleotide probes specific for the m1, m2, m3, m4 or m5 muscarinic receptor messenger RNAs. When 35S-tailed oligonucleotides were used, all intracardiac neurons in culture were found to express m1, m2, m3 and m4, but not m5 messenger RNAs. However after hybridization with digoxigenin-tailed probes, only m1 and m2 transcripts were detected. This may reflect differences in the sensitivity of the two techniques. Further to these experiments, intracardiac ganglia in sections of adult rat heart were studied employing m1-, m2-, m3- or m4-specific, 35S-labelled oligonucleotides, and again, all intracardiac neurons expressed messenger RNA for each of these four muscarinic receptor subtypes. Atrial myocytes in culture were only labelled by [35S]- and digoxigenin-tailed m2 oligonucleotides. No other heart cell type seen expressed messenger RNA for any of the muscarinic receptors. The expression of four different muscarinic receptor transcripts by intrinsic neurons of the heart provides the molecular basis for the diverse muscarinic actions observed in these and other autonomic ganglia.

M3 muscarinic acetylcholine receptor coupling to PLC in rat exorbital lacrimal acinar cells

This study was designed to characterize the muscarinic acetylcholine receptor (mAChR) subtype present in rat exorbital lacrimal gland as well as its biochemical coupling. The nonselective muscarinic antagonist [N-methyl-3H]scopolamine ([3H]NMS) binds with high affinity to a homogeneous population of binding sites in both membranes [dissociation constant (Kd) = 82.3 +/- 3.2 pM] and acinar cell (Kd = 170.3 +/- 20 pM) preparations. Muscarinic antagonist inhibition of [3H]NMS binding is homogeneous with the following order of potency: atropine > or = 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) > pirenzepine > 11-([2-(diethylamino)-ethyl]-1-piperidinyl)-acetyl- 5,11-dihydro-6H-pirido[2,3-b]1,4,benzo diazepine-6-one (AFDX 116). Both the affinity of the selective antagonists 4-DAMP, pirenzepine, and AFDX 116 and Northern blot analysis of lacrimal gland mRNAs show a single mAChR population of the M3 subtype. Muscarinic agonist inhibition of [3H]NMS binding displays both high (approximately 20%)- and low-affinity sites (approximately 80%). Both the receptor occupancy and the stimulation by agonists or the inhibition by antagonists of the accumulation of [3H]inositol phosphate were examined under identical conditions with respect to tissue preparations (acinar cells) and buffer (Krebs-Ringer). Results demonstrate 1) the efficient coupling of the M3 mAChR subtype with the phosphatidylinositol (4,5))bisphosphate-specific phospholipase C activity and 2) that the efficacy of a muscarinic agonist is dependent on its structure. Lastly, comparison of the agonists affinity and potency to trigger the [3H]inositol phosphate accumulation suggests that the occupation of the high-affinity agonist binding state of the M3 mAChR was involved in the cellular response.

Improving effects of FG-7080, a serotonin reuptake inhibitor, on scopolamine-induced performance deficits of memory tasks in rats

A novel serotonin reuptake inhibitor, (-)trans 4-(4-fluorophenyl)-3-(4-methoxyphenoxy-methyl)piperidine hydrochloride (FG-7080), was investigated for its effects on scopolamine-induced impairments in memory tasks of rats. In the radial-arm maze task, FG-7080 (3 mg/kg, p.o.) improved the impaired performance. FG-7080 (1 mg/kg, p.o.) administered before the acquisition trial reduced the acquisition deficits in the passive avoidance task. Thus, the compound showed improving effects in the appetitive and aversive memory tasks. These findings suggest that FG-7080 may ameliorate the cognitive impairments caused by the cholinergic dysfunction.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

Regional binding of 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) to muscarinic receptors in rat brain and comparative analysis of minimum energy conformations

The binding of the muscarinic antagonist 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP), which has been suggested as an M3-selective antagonist in peripheral tissues, was examined through quantitative autoradiographic techniques in brain. The ability of 4-DAMP to displace [3H](R)-quinuclindinyl benzilate (QNB) binding to rat brain sections was compared with the known distribution of M1 and M2 muscarinic receptor subtypes as measured previously with pirenzepine and AF-DX 116 (Messer et al., 1989a). 4-DAMP displayed a high affinity for [3H](R)-QNB binding sites in rat brain sections. Analysis of 4-DAMP binding to various brain regions revealed heterogeneous binding profiles, suggesting an interaction with multiple receptor sites. Quantification of the autoradiograms indicated that 4-DAMP bound with the highest affinity to muscarinic receptors in the midline thalamus (IC50 values < 30 nM), and had a slightly lower affinity for hippocampal receptors (IC50 values between 30 and 46 nM). 4-DAMP also displayed a lower affinity for cortical receptors with IC50 values between 30 and 50 nM. The binding profile of the putative M3 muscarinic antagonist did not exhibit a marked selectivity for any single region of brain. The data suggest that whereas 4-DAMP may be selective for M3 receptors in peripheral tissues, it has limited selectivity in the CNS. Minimum energy conformations for 4-DAMP were calculated using molecular mechanics calculations. 4-DAMP displayed two global minimum energy conformations, differing in the relative position of the piperidine ring with respect to the aromatic rings. The minimum energy conformations of 4-DAMP were compared with conformations generated for pirenzepine (Messer et al., 1989a). The lowest energy conformation of 4-DAMP was superimposable on the lowest energy conformation of pirenzepine (RMS = 0.297 A). It is suggested that the conformations available to 4-DAMP permit binding to several muscarinic receptors in the CNS.

Immunological localization of m1-m5 muscarinic acetylcholine receptors in peripheral tissues and brain

Knowledge of the distributions and functions of native m1-m5 muscarinic acetylcholine receptors in tissues is limited. To characterize the family of m1-m5 proteins directly, a panel of subtype-selective antibodies was generated against divergent i3 loop-fusion proteins. Each antibody was shown to bind a single cloned receptor specifically. In peripheral tissues and brain, four receptor proteins (m1-m4) were found to account for the vast majority of the muscarinic binding sites using immunoprecipitation studies with the subtype-specific antibodies. The subtypes were differentially distributed, although most tissues were comprised of a complex mixture of receptors. Moreover, within tissues there were major differences in the precise localization of the subtypes, as determined by immunocytochemistry. The immunological methods described offer a novel approach with exquisite sensitivity and specificity for delineating the distribution of m1-m5 receptors in animal and human tissues.

Muscarinic acetylcholine receptors in invertebrates: comparisons with homologous receptors from vertebrates

The pharmacology, physiology and molecular biology of invertebrate muscarinic acetylcholine receptors are compared with current knowledge concerning vertebrate muscarinic acetylcholine receptors. Evidence for the existence of multiple receptor subtypes in invertebrates is examined, emphasizing what is presently known about the sensitivity of invertebrate preparations to subtype selective ligands previously defined in vertebrate studies. Other evidence for muscarinic receptor subtypes which is examined includes: heterogeneous responses to classical muscarinic ligands and evidence for coupling of invertebrate muscarinic receptors to several different classes of second messenger systems. Clues regarding possible functions for invertebrate muscarinic receptors are discussed, including evidence from both physiological studies and in situ localization studies which reveal patterns of receptor protein and mRNA expression. A detailed analysis of the structural similarities between a cloned Drosophila muscarinic receptor and vertebrate muscarinic receptors is also presented. Regions of the receptors that may be involved in ligand binding, effector coupling and receptor regulation are identified in this comparison. Future directions for invertebrate muscarinic receptor research are considered including: methods for cloning other receptor subtypes, methods for cloning homologous receptors from other species and genetic approaches for determining the physiological roles of muscarinic receptors.

Muscarinic acetylcholine receptor subtypes: localization and structure/function

Based on the sequence of the five cloned muscarinic receptor subtypes (m1-m5), subtype selective antibody and cDNA probes have been prepared. Use of these probes has demonstrated that each of the five subtypes has a markedly distinct distribution within the brain and among peripheral tissues. The distributions of these subtypes and their potential physiological roles are discussed. By use of molecular genetic manipulation of cloned muscarinic receptor cDNAs, the regions of muscarinic receptors that specify G-protein coupling and ligand binding have been defined in several recent studies. Overall, these studies have shown that amino acids within the third cytoplasmic loop of the receptors define their selectivities for different G-proteins and that multiple discontinuous epitopes contribute to their selectivities for different ligands. The residues that contribute to ligand binding and G-protein coupling are described, as well as the implied structures of these functional domains.

Mutational analysis of muscarinic acetylcholine receptors: structural basis of ligand/receptor/G protein interactions

Molecular cloning studies have revealed the existence of five molecularly distinct muscarinic acetylcholine receptors (m1-m5), which differ in their tissue distribution, ligand binding properties, and functional profiles. Structurally (and functionally), the muscarinic receptors are members of the superfamily of G protein-coupled receptors. A variety of different mutagenesis techniques have been used to study the molecular basis of muscarinic receptor function. This approach has led to the identification of distinct receptor domains (or individual amino acids) predicted to play key roles in ligand binding, agonist-dependent receptor activation, and G protein coupling. Since all G protein-linked receptors share a similar molecular architecture, the information gained from the mutational analysis of muscarinic receptors should help delineate functionally important regions of other members of this receptor family.

Muscarinic receptor subtypes: modulation of ion channels

The discovery of five muscarinic receptor subtypes by molecular genetic techniques has resulted in new approaches to understanding their function. This involves the expression of the individual genes encoding each receptor subtype in isolation, such that their effects and mechanisms of action can be studied. The coupling of the receptors with G-proteins and ion channels is the subject of this review and emphasis is placed upon the assignment of genetically defined receptor subtypes with a given physiological function. Activation of inwardly rectifying potassium conductances by m2 and m4 and inhibition by m1, as well as stimulation of calcium-dependent conductances by m1, m3 and m5 are discussed.

Structure/function relationships of muscarinic acetylcholine receptors

The regions of muscarinic receptors that specify G-protein-coupling and ligand-binding have been defined in several recent studies. Overall, these studies have shown that amino acids within the third cytoplasmic loop of the receptors define their selectivity for different G-proteins, and that multiple, discontinuous epitopes contribute to their selectivities for different ligands. In fact, several competitive and allosteric antagonists can be classified into groups based on which of these epitopes contribute to their subtype selectivity. Site-directed mutagenesis, combined with covalent-labeling studies have suggested that ligands bind to a hydrophobic core of the receptors that is formed by multiple transmembrane (TM) domains. An aspartic acid located in TM3 is likely to bind to the ammonium headgroup of muscarinic ligands, and multiple hydroxyl-containing amino acids contribute to agonist but not antagonist binding. These data are discussed in the context of a computational model of a muscarinic receptor. Our model is based on a sequence alignment with bacteriorhodopsin, a seven TM protein for which a higher resolution structure is available. Most of the mutagenic data can be rationalized in the context of this model, and predict testable hypotheses concerning the mechanism by which ligands control the activity of muscarinic receptors.

ATP-independent deactivation of squid rhodopsin

Deactivation of light-activated squid rhodopsin was studied in vitro using GTP gamma S binding by G-protein as a direct measure of rhodopsin activity. Deactivation was inhibited by dilution of the retinal suspension or by removal of soluble components. Deactivation could be restored by addition of soluble material to washed membranes. These results indicate that the deactivation is not due entirely to a conformational transition within rhodopsin itself, but depends on the interaction with other molecules. The possibility that phosphorylation is involved in the deactivation was studied. Deactivation occurred in the presence and absence of added ATP. Deactivation also occurred in the presence of kinase inhibitors and after addition of apyrase, which reduced residual ATP levels to below 1 microM. These results indicate that light-induced phosphorylation is not required for deactivation of squid rhodopsin. In this regard deactivation of squid rhodopsin is different from that of vertebrate rhodopsin, which requires phosphorylation.

Characterization of muscarinic receptor subtypes inhibiting Ca2+ current and M current in rat sympathetic neurons

Muscarinic receptors mediating suppression of Ca2+ current and of M-type K+ current in rat superior cervical ganglion neurons were subclassified pharmacologically by using the muscarinic receptor antagonists pirenzepine and himbacine. Our voltage clamp experiments previously distinguished fast and slow intracellular signaling pathways coupling muscarinic receptors to calcium channels. We now establish that the fast, pertussis toxin-sensitive suppression of Ca2+ current is mediated primarily by muscarinic receptors of the M4 subtype, whereas the slow, bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetate (BAPTA)-sensitive suppression of Ca2+ current is mediated primarily by muscarinic receptors of the M1 subtype. Both actions on Ca2+ current are blocked by guanosine 5'-[beta-thio]diphosphate. Muscarinic suppression of M current is slow, BAPTA-sensitive, and mediated by receptors of the M1 subtype. Hence the two muscarinic pathways use different receptors and different guanine nucleotide binding proteins to produce different actions on channels.

Effects of muscarinic agonists and depolarizing agents on inositol monophosphate accumulation in the rabbit vagus nerve

The effects of muscarinic agonists and depolarizing agents on inositol phospholipid hydrolysis in the rabbit vagus nerve were assessed by the measurement of [3H]inositol monophosphate production in nerves that had been preincubated with [3H]inositol. After 1 h of drug action, carbachol, oxotremorine, and arecoline increased the inositol monophosphate accumulation, though the maximal increase induced by these agonists differed. Addition of the muscarinic antagonists atropine or pirenzepine shifted the carbachol dose-response curves to the right, without decreasing the carbachol maximal stimulatory effects. The KB for pirenzepine was 35 nM, which is characteristic of muscarinic high-affinity binding sites coupled to phosphoinositide turnover and often associated with the M1 receptor subtype. On the other hand, agents known to depolarize or to increase the intracellular Ca2+ concentration, e.g., elevated extracellular K+, ouabain, Ca2+, and the Ca2+ ionophore A23187, also increased inositol monophosphate accumulation. These effects were not mediated by the release of acetylcholine, as suggested by the fact that they could not be potentiated by the addition of physostigmine nor inhibited by the addition of atropine. The Ca(2+)-channel antagonist Cd2+, also known to inhibit the Na+/Ca2+ exchanger, was able to block the effects of K+ and ouabain, but did not alter those of carbachol. These results suggest that depolarizing agents increase inositol monophosphate accumulation in part through elevation of the intracellular Ca2+ concentration and that muscarinic receptors coupled to phosphoinositide turnover are present along the trunk of the rabbit vagus nerve.

Regulation of muscarinic acetylcholine receptor mRNA expression by activation of homologous and heterologous receptors

Muscarinic acetylcholine receptors (mAChR) in the embryonic chicken heart undergo agonist-induced internalization and a subsequent decrease in receptor number (downregulation). Cloning studies have identified two subtypes of mAChR expressed in the embryonic chicken heart, the cm2 and cm4 receptors. We report here that persistent activation of the mAChR in cultured chicken heart cells with the cholinergic agonist carbachol causes significant decreases in the levels of both cm2 and cm4 mRNA, as measured by solution hybridization analyses. The half-lives of the cm2 and cm4 mRNAs are not altered by agonist treatment, indicating that agonist most likely regulates mRNA levels by regulating the rate of gene transcription. Activation of mAChR in chicken heart causes both inhibition of adenylate cyclase activity and stimulation of phospholipase C activity. To test whether changes in the levels of intracellular second messengers were involved in the changes in mAChR mRNAs observed following agonist exposure, we determined the effects of incubation with agonists for the A1 adenosine receptors (which inhibit adenylate cyclase in chicken heart) and angiotensin II receptors (which stimulate phospholipase C) on mAChR receptor number and mRNA levels. Activation of these pathways together through heterologous receptors resulted in decreased mAChR number and mRNA levels, although these changes were not as large as those seen with direct activation of the mAChR. These results suggest that regulation of adenylate cyclase and phospholipase C activities may be involved in the regulation of mAChR gene expression.

In vitro mutagenesis and the search for structure-function relationships among G protein-coupled receptors

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The ligand-binding domain of rhodopsin and other G protein-linked receptors

Rhodopsin is a member of the very large family of G protein-linked receptors. The members of this family show clear signs of evolutionary relatedness, primarily in amino acid sequence homology, topographical structure of the proteins in the membrane, and the fact that all of the receptors function through the intermediary action of a GTP-binding regulatory protein or G protein. Recently, it has become clear that the structural similarity of these receptors extends well beyond the rather crude comparison of membrane topography. Reviewed here are several studies in which site-directed mutagenesis and active-site-directed reagents were used to show that the ligand-binding pockets of these receptors are highly similar. They are similar despite the fact that the structures of their various ligands are very different.