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

Characterization and functional expression in mammalian cells of genomic and cDNA clones encoding a Drosophila muscarinic acetylcholine receptor

Genomic and cDNA clones encoding a muscarinic acetylcholine receptor from Drosophila melanogaster have been isolated. Sequence analysis demonstrates that this gene encodes a receptor with a high degree of amino acid identity to the mammalian muscarinic acetylcholine receptors and has three introns in the portion of the gene encoding the third putative cytoplasmic loop. A full-length cDNA clone has been placed under the control of the mouse metallothionein promotor and transfected into mouse Y1 adrenal cells. The receptor expressed in these cells exhibits the high-affinity binding for the antagonists quinuclidinyl benzilate and atropine expected of a muscarinic receptor. The Drosophila muscarinic receptor, when expressed in Y1 cells, is physiologically active, as measured by agonist-dependent stimulation of phosphatidylinositol metabolism.

Deletion analysis of the mouse m1 muscarinic acetylcholine receptor: effects on phosphoinositide metabolism and down-regulation

Deletions have been constructed in the putative third cytoplasmic loop of the mouse m1 muscarinic acetylcholine receptor (mAChR) gene, and the effects of these mutations on mAChR coupling to phosphoinositide metabolism and agonist-induced down-regulation have been examined following expression in Y1 adrenal carcinoma cells. Deletion of up to 123 of the 156 amino acids in this loop has no effect on antagonist or agonist binding, or on coupling to stimulation of phosphoinositide metabolism. These results suggest that the membrane proximal portions of this loop are involved in determining the specificity of functional coupling of the receptor. Deletion of 75% of the loop has no effect on short-term agonist-induced internalization but does cause a significant decrease in the magnitude of agonist-induced down-regulation of receptor number. Thus, this portion of the receptor may be involved in mediating the response to long-term agonist exposure.

Human HT-29 colon carcinoma cells contain muscarinic M3 receptors coupled to phosphoinositide metabolism

Five different muscarinic receptor subtypes can be distinguished by the differences in their amino acid sequence, the coupled signal transduction system, pharmacological binding properties and activation of ionic fluxes. The present study served to characterize the binding profile of muscarinic receptors in human colon carcinoma cells (HT-29) using selective muscarinic antagonists. The affinities of the compounds were compared with their potency to inhibit cholinergically-activated phosphoinositide metabolism. Pirenzepine displaced [3H]N-methyl-scopolamine binding and inhibited inositolphosphate (IP) release with potencies typical of those of non-M1 receptors. The M3 subtype-selective antagonists sila-hexocyclium and hexahydro-sila-difenidol had high affinity to the muscarinic receptors in HT-29 cells (KD = 3.1 nM and 27 nM, respectively) and inhibited IP release at nanomolar concentrations. The M2 receptor antagonists, AF-DX 116 and methoctramine, had low antimuscarinic potencies. Our results demonstrate that HT-29 human colon carcinoma cells contain an apparently pure population of M3 receptors. These cells could serve as a model system for further investigations concerning regulatory and signal transduction mechanisms associated with glandular muscarinic M3 receptors.

Antipeptide antibodies to the beta 2-adrenergic receptor confirm the extracellular orientation of the amino-terminus and the putative first extracellular loop

We developed site-directed rabbit antisera against synthetic peptides selected from the deduced amino acid sequence of the hamster lung beta 2-adrenergic receptor (amino acids 16-31 and 174-189, respectively). All antisera directed against peptide 1 (four of four rabbits) as well as two antisera directed against peptide 2 (two of four rabbits) recognized the purified beta 2-adrenergic receptor in immunoblot conditions when used at a dilution of 1:500. Antisera directed against peptide 1 as well as peptide 2 were able to immunoprecipitate iodinated as well as 125I-cyanopindolol labeled beta 2-adrenergic receptor. This last result implies that the recognized epitopes do not contain the 125I-cyanopindolol binding domain of the beta 2-adrenergic receptor. Immunoblot experiments performed on membrane fractions from hamster lung tissue showed that immunoreactive bands at 64,000, 57,000, 47,000, 44,000 and 38,000 daltons were specifically detected. When purified beta 2-adrenergic receptor was iodinated and submitted to glycolytic and/or tryptic treatments, species with similar molecular weights could be recovered. Then, the immunoreactive bands probably correspond to native beta 2-adrenergic receptor and to degradative or nonglycosylated species of this molecule. The antisera were also able to detect immunoreactive molecules in murine and human cell lines, suggesting conservation of the probed sequences between these species. Enzymatic linked immunosorbent assay tests on intact cells and immunofluorescence studies confirmed that the amino-terminus and putative first extracellular loop are extracellularly located. Immunofluorescence studies on mouse brain primary cultures showed that cells expressing beta 2-adrenergic receptor-like molecules exhibited a neuronal phenotype.

Muscarinic suppression of the M-current in the rat sympathetic ganglion is mediated by receptors of the M1-subtype

1. Under voltage-clamp dissociated adult and foetal rat superior cervical ganglion (s.c.g.) cells exhibited a non-inactivating voltage- and time-dependent component of K+ current termed the M-current (IM). IM was detected and measured from the current decay during hyperpolarizing voltage steps applied from potentials where IM was pre-activated. 2. Neither the resting membrane current nor the amplitude of these current decay relaxations were reduced by omitting Ca from the bathing fluid, showing that the M-current was not a 'Ca-activated' K-current dependent on a primary Ca-influx. Concentrations of (+)-tubocurarine sufficient to block the slow Ca-activated K-current IAHP did not inhibit IM or antagonize the effect of muscarinic agonists on IM, showing that IM was not contaminated by IAHP. Tetraethylammonium (1 mM), which blocks the fast Ca-activated K-current IC, produced a small inhibition of IM. This was not due to contamination of IM by IC since muscarinic agonists did not consistently block IC. 3. The muscarinic agonists muscarine, oxotremorine, McN-A-343 and methacholine reversibly suppressed IM, resulting in an inward (depolarizing) current. The rank order of potency was: oxotremorine greater than or equal to muscarine greater than McN-A-343 greater than methacholine. 4. The suppression of IM by muscarine was similar in cultured cells derived from adult and foetal tissue to that seen in the intact ganglia. 5. IM-suppression by muscarine was inhibited by pirenzepine (Pz) and AF-DX 116 with mean pKB values of 7.53 +/- 0.13 (n = 3) and 6.02 +/- 0.13 (n = 4) respectively. 6. The suppression of IM by muscarinic agonists was not affected by gallamine (10-30 microM). 4-Diphenylacetoxy-N-methylpiperidine methiodide inhibited the response at 300 nM. 7. Pirenzepine inhibited the contractions of the guinea-pig isolated ileum produced by muscarine with a mean pKB of 6.37 +/- 0.03 (n = 8). 8. These results suggest that the receptors mediating suppression of the M-current accord with those designated pharmacologically as M1 and that these receptors reach maturity at a very early stage in the development of the rat s.c.g.

Cloning, sequence analysis and chromosome localization of a Drosophila muscarinic acetylcholine receptor

Two cDNA clones (3.7 kb and 4.8 kb) encoding a Drosophila muscarinic acetylcholine receptor were isolated from a Drosophila head cDNA library and characterized by automated DNA sequence analysis. The Drosophila muscarinic receptor contains 788 amino acids with a calculated Mr of 84,807 and displays greater than 60% homology with mammalian muscarinic receptors. The muscarinic receptor maps to the tip of the right arm of the second chromosome of the Drosophila genome.

Polyclonal anti-propylbenzilylcholine mustard antibodies selectively recognize labeled muscarinic receptors from rabbit brain

Polyclonal antibodies were produced in rabbits to propylbenzilylcholine mustard-bovine serum albumin (PBCM-BSA) conjugates. The antibodies selectively recognized and bound to an apparent single class of sites with high affinity for PBCM. The antibodies also demonstrated high to intermediate affinity for muscarinic ligands which have benzhydryl or dicyclohexane ring structures similar to PBCM. The antibodies had little or no affinity for ligands which lacked the double cyclohexane structure. Antibodies recognized muscarinic receptors specifically labeled with [3H]PBCM, but not unlabeled receptors, on immunoblots. These results suggest that the polyclonal antibodies produced in this study react specifically with the dicyclohexane structure on PBCM and that this antigenic determinant remains available for interaction with the antibodies, even when the ligand is bound to the receptor.

The H-L subgroup of guinea-pig cardiac M2 receptors (M2 beta) regulates inositol phosphate formation

In previous studies, we showed that cardiac muscarinic receptors (M2) are composed of two subgroups, M2 alpha and M2 beta, with different affinities for agonists and that the M2 alpha subgroup is coupled with inhibition of adenylate cyclase. We now studied which subgroup was responsible for the formation of inositol mono- (IP), bis- (IP2), tris- (IP3) and tetrakis- (IP4) phosphates in guinea pig heart. Carbachol (1 mM) significantly stimulated the formation of all four IPs in [3H]myoinositol-preloaded slices of guinea-pig ventricles. Acetylcholine (1 mM) also stimulated the formation of IP2, IP3 and IP4. However, oxotremorine (1 mM) only slightly stimulated the formation of IP2, and pilocarpine did not stimulate the formation of any IP. The pED50 values of carbachol for IP2 and IP3 formation were 3.76 and 4.23, respectively, which coincided with the pKd values of the low-affinity agonist binding site (L site) measured by competition of carbachol with [3H]quinuclidinyl benzilate [( 3H]QNB) binding while the pKd value for inhibition of adenylate cyclase coincided with the pKd value of the high-affinity agonist binding site (H site). Treatment of animals with pertussis toxin decreased the formation of IP2 and IP3 by carbachol to 66 and 54%, respectively, but resulted in complete inhibition of adenylate cyclase. These results suggested that muscarinic stimulation of the formation of IPs was manifested through a different receptor subgroup (M2 beta) and GTP binding protein different from those for inhibition of adenylate cyclase.

New photoaffinity labels for rat brain muscarinic acetylcholine receptors

Localization of the ligand binding site on muscarinic acetylcholine receptors is one of the new fields of interest opened by the recent determination of their primary structures. Owing to their interesting photochemical properties, aryldiazonium salts may be considered as appropriate tools for "tagging" the agonist/antagonist binding domain and to get precise identification and positioning of covalently labelled residues along the primary sequence of these receptors. A series of aryldiazonium derivatives and some of their azido-analogs were synthesized and their reversible muscarinic binding component was assessed through competition experiments involving either the whole population of receptor sites [( 3H]QNB assay) or the super high affinity of their agonist binding sites [( 3H]OXO-M assay). Three compounds fulfilled the criteria for efficient photolabels, allowing substantial and irreversible occupation of the receptor sites to be obtained. Interestingly, the two diazonium derivatives which were selected have been previously described as potent photoprobes of the peripheral nicotinic receptor and of acetylcholinesterase, though displaying lower binding affinities for these acetylcholine binding proteins than for the muscarinic receptors. These findings, together with the all-to-none photolabelling efficiency observed for a quinuclidine derivative, substituted either by an azido or a diazonium group, are discussed. Finally, the apparent lack of binding selectivity of these new photo-affinity probes towards muscarinic receptor affinity states or subtypes should allow comparative studies of the acetylcholine binding site on different muscarinic receptor proteins, obtained either through purification procedures or expression of separate gene products.

A molecular modelling study of the interaction of noradrenaline with the beta 2-adrenergic receptor

A model of the beta 2-adrenergic receptor binding site is built from the primary structure of the receptor, experimental evidence for key binding residues and analogy with a homologous protein of partially determined structure. It is suggested that residues Trp-109, Thr-110 and Asp-113 are involved in ligand binding. Noradrenaline is successfully docked into this model, and the results of an INDO molecular orbital calculation on the complex indicate that a charge transfer interaction between Trp-109 and noradrenaline is possible.

Mapping of beta-adrenoceptor coupling domains to Gs-protein by site-specific synthetic peptides

Peptides corresponding to the known sequence of turkey erythrocyte beta 1-adrenergic receptor were synthesized and the effects on receptor-mediated cyclase activation were measured. Peptides corresponding to the first and second intracellular loops (T61-71 and T138-159) inhibited at micromolar concentrations the hormone-dependent cyclase activation in turkey erythrocyte membranes. In contrast, the peptide corresponding to the C-terminal part of the third intracellular loop (T284-295) increased the cyclase activity in a hormone-independent manner. Peptides T338-353 and T2-10 and a number of synthetic peptides unrelated to the beta-adrenoceptor had no effect.

Molecular genetics of human blue cone monochromacy

Blue cone monochromacy is a rare X-linked disorder of color vision characterized by the absence of both red and green cone sensitivities. In 12 of 12 families carrying this trait, alterations are observed in the red and green visual pigment gene cluster. The alterations fall into two classes. One class arose from the wild type by a two-step pathway consisting of unequal homologous recombination and point mutation. The second class arose by nonhomologous deletion of genomic DNA adjacent to the red and green pigment gene cluster. These deletions define a 579-base pair region that is located 4 kilobases upstream of the red pigment gene and 43 kilobases upstream of the nearest green pigment gene; this 579-base pair region is essential for the activity of both pigment genes.

Hydrophobic organization of membrane proteins

Membrane-exposed residues are more hydrophobic than buried interior residues in the transmembrane regions of the photosynthetic reaction center from Rhodobacter sphaeroides. This hydrophobic organization is opposite to that of water-soluble proteins. The relative polarities of interior and surface residues of membrane and water soluble proteins are not simply reversed, however. The hydrophobicities of interior residues of both membrane and water-soluble proteins are comparable, whereas the bilayer-exposed residues of membrane proteins are more hydrophobic than the interior residues, and the aqueous-exposed residues of water-soluble proteins are more hydrophilic than the interior residues. A method of sequence analysis is described, based on the periodicity of residue replacement in homologous sequences, that extends conclusions derived from the known atomic structure of the reaction center to the more extensive database of putative transmembrane helical sequences.

Direct and energy-transfer photolabelling of brain muscarinic acetylcholine receptors

Efficient photolabelling of muscarinic acetylcholine receptors was obtained using either two aryldiazonium salts or an azido derivative. These probes did not discriminate between muscarinic binding subtypes or affinity states and became irreversibly bound to the receptor sites, in an entirely atropine-protectable manner, upon ultraviolet irradiation. The extent of labelling was dependent both on probe concentration and on time of irradiation and reached up to 80% of the receptor population, under optimal alkylating conditions. In contrast to the azido derivative, both diazonium salts behave as potent irreversible labels of muscarinic receptors, provided energy-transfer photolabelling conditions were followed. Such an indirect activation of diazonium ligands, through an energy transfer from photoexcited tryptophan residues, has been previously found to increase the site-specificity and the rate of labelling of other acetylcholine binding proteins. Analogies in the photolabelling process of acetylcholinesterase or of nicotinic and muscarinic receptors by the two diazonium salts are discussed. Altogether, these findings suggest that these new probes may be promising tools to investigate the location and the topography of the agonist-antagonist binding domain on purified muscarinic receptors, through amino acid and/or sequence analyses of radioactive, photolabelled residues.

Cytoplasmic loop of beta-adrenergic receptors: synaptic and intracellular localization and relation to catecholaminergic neurons in the nuclei of the solitary tracts

Pharmacological studies suggest that beta-adrenergic receptors (beta AR) in the medial nuclei of the solitary tracts (m-NTS) facilitate presynaptic release of catecholamines and also function at postsynaptic sites. We have localized the antigenic sites for a monoclonal antibody against a peptide corresponding to amino acids 226-239 of beta AR in the m-NTS of rat brain. By light microscopy, immunoperoxidase labeling for this antibody was detected in somata and proximal processes of many small cells that were distributed throughout the rostrocaudal extent of the m-NTS. Electron microscopy confirmed the cytoplasmic localization of beta AR in perikarya and proximal dendrites of neurons. Immunoreactivity occurred as discrete patches associated with cytoplasmic surfaces of plasma membrane and with irregularly-shaped saccules with clear lumen in the immediate vicinity. Select regions of nuclear envelopes, mitochondrial membranes, and rough endoplasmic reticulum were also immunoreactive along their cytoplasmic surfaces. In contrast, the Golgi apparatus was labeled, but infrequently. Immunoreactivity was also detected at numerous post- and occasional presynaptic membrane specializations of select axodendritic junctions. Dual labeling for the beta AR-antibody by the immunoperoxidase method and for a rabbit antiserum against the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), by the immunoautoradiographic method within the same sections, further established the precise cellular relations between beta AR and catecholaminergic neurons. Immunoreactivity for beta AR was detected in numerous perikarya and proximal dendrites that did not show detectable levels of TH. However, a few cells were dually labeled for both antigens, as seen by both light and electron microscopy. The TH-labeled terminals formed synapses at junctions both with and without beta AR-like immunoreactivity. These results from the single and dual labeling studies: (1) confirm biochemical predictions that amino acids 226-239 of beta AR protein reside intracellularly; (2) provide the first ultrastructural evidence for beta AR localization within both pre- and postsynaptic membrane specializations of a subset of catecholaminergic synapses; and (3) suggest select intracellular sites that may be involved with synthesis and/or internalization and degradation of the receptor protein.

Inhibition of cyclic AMP formation in N1E-115 neuroblastoma cells is mediated by a non-cardiac M2 muscarinic receptor subtype

The cardioselective muscarinic antagonist, AF-DX 116 [11[2-[(diethyl-amino)-methyl]-O-1-piperidinyl]-5,11-dihydro-6H-pyrido- [2,3-b][1,4]-benzodiazepine-6-one), was weak at blocking the M2 muscarinic receptor-mediated inhibition of cyclic adenosine monophosphate (cAMP) formation in mouse neuroblastoma cells (clone N1E-115). In contrast, the glandular-selective antagonists, hexahydro-sila-difenidol (HHSiD) and 4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP), were quite potent at inhibiting this response, being 14- and 318-fold more potent than AF-DX 116 in this regard, respectively. According to the rank order of potency of these two classes of antagonists, these data provide the first pharmacological evidence that inhibition of cAMP formation in a neuronal tissue is mediated by a non-cardiac M2 muscarinic receptor subtype.

An analysis of the periodicity of conserved residues in sequence alignments of G-protein coupled receptors. Implications for the three-dimensional structure

Twenty-three sequences from the family of G-protein coupled receptors have been aligned according to the 'historical alignment' procedure of Feng and Doolittle. Fourier transform analysis of this reveals that parts of five of the seven putative membrane-spanning regions exhibit a periodicity of conserved/nonconserved residues which is compatible with the periodicity of the alpha-helix. This would place the conserved residues on one side of the helix, which may face the inside of the proposed seven membered helical bundle.

Acetylcholine analogue stimulates DNA synthesis in brain-derived cells via specific muscarinic receptor subtypes

Little is known about the factors which regulate the growth and development of the mammalian brain. Although proliferation of neuronal cells ceases relatively early in development, certain types of glial cells proliferate and differentiate mainly perinatally. In the perinatal period, the ability of acetylcholine to stimulate phosphoinositide (PI) hydrolysis in brain reaches peak levels, and indeed the stable acetylcholine analogue carbachol can stimulate PI hydrolysis of primary neonatal astroglial cells. As PI hydrolysis is thought to be important in the regulation of cell proliferation, we investigated whether cellular DNA synthesis can be induced by carbachol. Our results show that carbachol stimulates DNA synthesis via muscarinic acetylcholine receptors (mAChRs), in primary astrocytes derived from perinatal rat brain, in an age-dependent fashion. Carbachol is also mitogenic in certain brain-derived astrocytoma and neuroblastoma cell lines, as well as in chinese hamster ovary (CHO) cells expressing recombinant muscarinic receptors. DNA synthesis is strongly activated by carbachol in those brain-derived cell lines and transfected CHO cells that express mAChR subtypes which activate PI hydrolysis efficiently, and poorly activated in cells expressing mAChR subtypes which only weakly activate PI hydrolysis. These results strongly support a role for acetylcholine in regulating astroglial cell growth in the developing brain, and indicate that the specificity of acetylcholine-induced cell proliferation may be determined by the expression of those mAChR subtypes which activate PI hydrolysis.

Replica filter screening technique to detect transfected cells expressing beta 2-adrenergic receptor

We have utilized a replica transfer technique to develop a novel screening assay for the identification of transfectants expressing beta 2-adrenergic receptors (beta 2-AR). The hamster beta 2-AR gene flanked by either its natural promoter or the zinc-inducible mouse metallothionein (MMT) promoter was cotransfected with plasmids conferring neomycin resistance (pRSVneo) into beta 2-AR-deficient mouse L cells. Transfectant colonies were grown on polyester nylon filters and screened by filter binding with [125I]iodohydroxybenzylpindolol to identify colonies expressing beta 2-AR. Individual colonies were isolated and examined to determine beta 2-AR gene dosage, mRNA expression, and receptor densities and affinities. Analysis of cell lines expressing beta 2-AR indicates that this method can identify transfectants containing only a single beta 2-AR gene copy and expressing as few as 4,000 beta 2-receptors per cell. This method may be useful as a tool for the molecular cloning of neurotransmitter receptor genes and for the measurement of transfection efficiencies and expression of receptor genes in cells.

Muscarinic receptor modulation of glucose-induced electrical activity in mouse pancreatic B-cells

Acetylcholine (1-10 microM) depolarized the membrane and stimulated glucose-induced bursts of electrical activity in mouse pancreatic B-cells. The acetylcholine effects were mimicked by muscarine while nicotine had no effect on membrane potential. Pirenzepine, an antagonist of the classical M1-type muscarinic receptors, but not gallamine (1-100 microM), an antagonist of the classical M2-type receptors, antagonized the acetylcholine action on glucose-induced electrical activity (IC50 = 0.25 microM). Bethanechol, an agonist of the classical M2-type muscarinic receptors, was approximately 100 times less effective than acetylcholine in stimulating the electrical activity. In addition, acetylcholine (1 microM) induced a marked increase (25%) in input resistance to the B-cell membrane. The results indicate that acetylcholine exerted its effects on the B-cell membrane by inhibiting K+ conductance via activation of a muscarinic receptor subtype distinct from the classical M2-type receptor.

Muscarinic receptor subclassification and G-proteins: significance for lithium action in affective disorders and for the treatment of the extrapyramidal side effects of neuroleptics

The classification of muscarinic receptors into M1 and M2 subtypes and the involvement of guanine nucleotide binding proteins (G-proteins) as major mediators of receptor information transduction in the cholinergic and other neurotransmitter systems have prompted us to undertake studies both at receptor and postreceptor levels that may shed light on the importance of these new findings to the pharmacotherapy of manic-depressive illness and of extrapyramidal syndromes. We searched for patterns of muscarinic selectivity among the commonly used anticholinergics (biperiden, procyclidine, trihexyphenidyl, benztropine, and methixen) through radioligand receptor studies in various rat tissues. The drugs showed a range of selectivity, from the totally nonselective methixen to the highly M1-selective biperiden. Sinus arrhythmia measurements were undertaken in psychiatric patients treated with different antiparkinsonian anticholinergics. The extent of sinus arrhythmia suppression was inversely correlated with the degree of M1 selectivity of the drugs used, advocating the use of M1-selective antiparkinsonian anticholinergics like biperiden in the treatment of extrapyramidal side effects. The implications of muscarinic receptor subclassification were further extended to include postreceptor phenomena. We have directly studied G-protein function by measuring cholinergic agonist-induced increases in guanosine triphosphate (GTP) binding to these proteins. This cholinergic agonistic effect was shown to be exerted by G-proteins other than Gs (the adenylate cyclase stimulatory G-protein), i.e., Gi (the adenylate cyclase inhibitory G-protein) or Gp [the G-protein activating phosphatidylinositol (PI) turnover], as ribosylation by pertussis toxin abolished this cholinergic effect, whereas it was unaffected by cholera toxin. Pertussis toxin-blockable, carbamylcholine-induced increases in GTP binding capacity were found to be mediated through M1 muscarinic receptors, as M1-selective antagonists were 100-fold more effective than M2 selective antagonists in blocking carbamylcholine effects. Moreover, carbamylcholine effect was exclusively detected in tissues predominantly populated by M1 receptors. Our results thus suggest that carbamylcholine-induced increases in GTP binding are exerted through M1 receptors interacting with Gp. At therapeutically efficacious concentrations, lithium completely blocked carbamylcholine-induced increases in GTP binding capacity in both in vitro and in vivo experiments.(ABSTRACT TRUNCATED AT 400 WORDS)

The molecular basis of muscarinic receptor diversity

The cloning of cDNAs and genes for five different muscarinic acetylcholine receptors provides a new basis for characterizing muscarinic receptor function. Studies of the cloned receptors when introduced into cells not expressing endogenous receptors have allowed the initial identification of two classes of functional response. The m1, m3 and m5 receptors belong to a class characterized by agonist-induced stimulation of phosphatidylinositol metabolism and are structurally more related to each other than they are to the m2 and m4 receptors, which belong to a class associated with agonist-induced inhibition of adenylate cyclase. While functional differences within these classes may yet be found, it appears likely that much of the difference between functionally similar receptors will be found to lie in their regulation.

Consensus residues at the acetylcholine binding site of cholinergic proteins

The nicotinic (nAcChR) and muscarinic (mAcCh) acetylcholine receptors and acetylcholinesterase (AcChEase) are structurally unrelated but share a common functional property: interaction with acetylcholine (AcCh). Alignment of the probable AcCh binding site regions of the nAcChR and mAcChR protein sequences revealed the presence of ten nearly identically spaced consensus residues, six of which contain potentially ligand-interactive side chains. Important elements of the consensus residues also were found in one unique sequence region of the AcChEases. Alignments among the two receptors and AcChEase outside the apparent binding region were rare, and the consensus AcCh binding residues were largely substituted in the homologous proteins, which do not bind AcCh. The consensus residues include two possible anionic subsite Asp residues and a Ser that may hydrogen bond to the AcCh carbonyl in the receptors. These residues correspond to positions Asp-166, Ser-173, and Asp-200 in the neuromuscular nAcChR; Asp-71, Ser-78, and Asp-105 in the M1 mAcChR; and Asp-93 and Asp-128 in Torpedo AcChEase. No corresponding consensus Ser is found in the AcChEase sequence; this is expected because of a downstream esterase active-site Ser-200 (Torpedo). A receptor-conserved and disulfide-linked Cys corresponding to neuromuscular nAcChR residue 193 and M1 mAcChR residue 97 may be important in energy transduction associated with agonist-mediated events. The presence of additional binding-site aromatic residues that may form a hydrophobic environment near the anionic subsite are aligned within, but not between, the three cholinergic protein groups. These observations target specific regions and residues within these proteins for structure-function studies of the cholinergic binding domain.

Autoradiographic analyses of agonist binding to muscarinic receptor subtypes

The binding of four muscarinic receptor agonists to regions of rat brain was examined through quantitative autoradiographic techniques. Oxotremorine, arecoline, pilocarpine and bethanechol were chosen based on their different potencies and efficacies in muscarinic second messenger systems. Overall, the order of potency for inhibition of [3H]-l-quinuclidinyl benzilate ([3H]-l-QNB) binding to rat brain slices was oxotremorine greater than pilocarpine = arecoline much greater than bethanechol. Regional assays of agonist potency indicated that all agonists were more selective for brainstem and thalamic regions than for hippocampal and cortical regions. The high selectivity of agonists for areas such as the paraventricular thalamus and the superior colliculus, which also display low affinity for pirenzepine, suggests that muscarinic agonists bind with higher affinity to M2 receptors. Of the four agonists examined, pilocarpine displayed the lowest selectivity for M2 receptors in that IC50 values for pilocarpine were only 3-fold higher in the hippocampal and striatal regions (e.g. CA3: 40.6 +/- 9.4 microM) than in thalamic and brainstem regions (e.g. paraventricular thalamus: 14.9 +/- 6.2 microM). Oxotremorine was 8-fold more potent in the brainstem and thalamus, while arecoline and bethanechol were, respectively, 19- and 100-fold more selective for brainstem and thalamic receptors. Scatchard analyses revealed heterogeneous binding profiles for some agonists within single brain regions, suggesting that multiple agonist sites exist even within regions of predominantly M1 or M2 receptors. For example, arecoline displayed curved Scatchard plots within the external layers of the cerebral cortex, layer CA1 of the hippocampus (predominantly M1 subtype), and the paraventricular thalamus (predominantly M2 subtype). The ability of agonists to recognize multiple sites within a single region may reflect the ability to recognize receptors coupled or uncoupled to second messenger systems through G-proteins.

Functionally distinct G proteins selectively couple different receptors to PI hydrolysis in the same cell

The number of G proteins identified by molecular cloning exceeds the number of known G protein functions. Here we show that a cell can possess multiple G proteins that carry out a similar function, the activation of phospholipase C, but couple selectively to different receptors, which are endogenous to the cell or introduced by DNA transfection. These G proteins (termed Gp) can be distinguished by their sensitivity to pertussis toxin. The assignment of a given Gp pathway to specific receptors is confirmed by the additivity relationships of the PI hydrolysis response mediated by the different receptors. Significantly different amounts of PI hydrolysis are activated through each Gp pathway, suggesting that Gp proteins also differ in their coupling to phospholipase C. These results indicate that distinct Gp pathways in a given cell exist to couple different receptors to PI hydrolysis selectively, and may specify the nature of the cellular response to different receptors by determining the magnitude of PI hydrolysis.

Molecular biology of 5-HT receptors

Within the past six months, isolation of cDNA or genomic clones has been reported for three 5-HT receptors, the 5-HT1C, 5-HT1A and 5-HT2 subtypes. As members of the G protein receptor superfamily, all three 5-HT receptor clones encode single-subunit proteins containing approximately 450 amino acids arrayed as seven interconnected transmembrane segments. Comparisons of 5-HT receptor sequence data with data from other G protein receptors provide suggestions for which amino acids may be involved in the binding of 5-HT and 5-HT antagonists to these receptors, and for certain key amino acids which may help confer 5-HT properties on these receptors. These rapid advances in our molecular understanding of 5-HT receptors also have significant implications for the ongoing debate over 5-HT receptor classification. In this article, Paul Hartig assembles the available molecular data and proposes a speculative model for the structure of G protein-coupled 5-HT receptors.

Bretylium tosylate binds preferentially to muscarinic receptors labelled with [3H]oxotremorine M (SH or 'high affinity' receptors) in rat heart and brain cortex

Bretylium tosylate is an antiarrhythmic agent. In guinea pig atria it showed the properties of a competitive muscarinic (cholinergic) antagonist and could distinguish between two muscarinic receptor classes or states in cardiac membranes. We decided to further investigate its binding properties at muscarinic cholinergic receptors of the rat heart and brain (cortex), keeping in mind the recently discovered heterogeneity of muscarinic receptor protein. Bretylium tosylate recognized two receptor classes or states in the heart with Ki values of 0.9 and 11 microM. All cardiac membrane receptors showed a homogeneous (11 microM) Ki value for the drug in the presence of GTP in the incubation medium, or after in vivo pretreatment with islet activating protein (IAP). Bretylium tosylate was able (but only at a high concentration, 1 mM) to slow the dissociation kinetics of the tracer, which suggests that it also bound to an allosteric site on the muscarinic receptor, or that it affected the receptor environment. In the brain cortex, as in the heart, bretylium tosylate displayed a high affinity for receptors labelled with the agonist [3H]oxotremorine M (Ki value: 0.8 microM for the SH-or cardiac-type high-affinity receptors), and a 8- to 10-fold lower affinity for cortex M and L receptors. These data suggest that the antagonist bretylium tosylate had binding properties in rat cardiac membranes analogous to those of the partial agonist pilocarpine and that it interacted with a single type of receptor.

Molecular pharmacology of muscarinic receptor heterogeneity

Muscarinic receptors can be pharmacologically classified into 3 types at the present time, however, five genes for the receptor have been identified. The muscarinic receptor types have unique antagonist selectivity, distribution and are linked to specific second messenger systems. The interaction between the muscarinic receptor types and G proteins may depend on the systems in which the receptors are integrated. Expression of the cloned gene in mammalian cells will be useful in delineating the relationships between the pharmacological types of muscarinic receptors and their genes and studying the interactions between the receptor, G proteins, and second messenger coupling.

Location of a region of the muscarinic acetylcholine receptor involved in selective effector coupling

Chimaeric muscarinic acetylcholine receptors (mAChR) in which corresponding portions of mAChR I and mAChR II are replaced with each other have been produced in Xenopus oocytes by expression of cDNA constructs encoding them. Functional analysis of the chimaeric mAChRs indicates that a region mostly comprising the putative cytoplasmic portion between the proposed transmembrane segments V and VI is involved in selective coupling of mAChR I and mAChR II with different effector systems. In contrast, the exchange of this region between mAChR I and mAChR II does not significantly affect the antagonist binding properties of the two mAChR subtypes.

Different sensitivities to agonist of muscarinic acetylcholine receptor subtypes

Muscarinic acetylcholine receptor (mAChR) III expressed in Xenopus oocytes, like mAChR I, mediates activation of a Ca2+-dependent Cl- current, whereas mAChR IV, like mAChR II, principally induces activation of Na+ and K+ currents in a Ca2+-independent manner. mAChR III has a sensitivity to agonist of about one order of magnitude higher than that of mAChR I in mediating the Ca2+-dependent current response in Xenopus oocytes and in stimulating phosphoinositide hydrolysis in NG108-15 neuroblastoma-glioma hybrid cells. The agonist-binding affinity of mAChR III is also about one order of magnitude higher than that of mAChR I.

Carboxyl residue(s) at the ligand-binding site of rat muscarinic receptors

Chemical modification of muscarinic receptors of rat cerebral cortex, brain stem and atria by a carboxyl-group-specific reagent, namely trimethyloxonium ion (TMO+) reduces the number of tritium-labeled antagonist- and agonist-binding sites in a dose-dependent way. No such effect is observed when modification is carried out in the presence of atropine, oxotremorine or carbamylcholine. These findings suggest that TMO+ specifically methylates the carboxyl residue(s) positioned at the binding site in members of the M1 and M2 receptor family.

Tissue distribution of mRNAs encoding muscarinic acetylcholine receptor subtypes

The tissue distribution of the mRNAs encoding muscarinic acetylcholine receptors (mAChRs) I, II, III and IV has been investigated by blot hybridization analysis with specific probes. This study indicates that exocrine glands contain both mAChR I and III mRNAs, whereas smooth muscles contain both mAChR II and III mRNAs. All four mAChR mRNAs are present in cerebrum, whereas only mAChR II mRNA is found in heart.

Stimulation of arachidonic acid release and inhibition of mitogenesis by cloned genes for muscarinic receptor subtypes stably expressed in A9 L cells

A family of genes encoding four distinct muscarinic receptors (designated m1-m4) has been cloned and stably expressed in A9 L cells. When the m1 and m3 receptors were stimulated with carbachol, there was a rapid rise of liberated arachidonic acid, inositol phosphates, and cAMP, while m2 and m4 receptor stimulation had no detectable stimulation of these second messengers. Pretreatment with phorbol 12-myristate 13-acetate (PMA) caused a marked acceleration and amplification of m1 and m3 receptor-mediated arachidonic acid release. In contrast, m1- and m3-mediated inositol phosphate formation was inhibited by the same PMA pretreatment. Arachidonic acid release was unaffected by manipulations of cAMP levels. Arachidonic acid production was inhibited by calcium-free medium and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8; an inhibitor of cytosolic calcium mobilization) yet was unaffected by verapamil, a calcium-channel blocker. These experiments show that arachidonic acid release induced by the m1 and m3 receptors is regulated independently of phospholipase C and cAMP accumulation. Carbachol stimulation of the m1 and m3 cAMP accumulation. Carbachol stimulation of the m1 and m3 receptors also markedly decreased mitogenesis as measured by thymidine incorporation. The m1 receptor-mediated inhibition of mitogenesis could be partially blocked by indomethacin, a cyclooxygenase inhibitor. The inhibition of mitogenesis could be mimicked by cAMP elevation.

Heterogeneity of muscarinic autoreceptors and heteroreceptors in the rat brain: effects of a novel M1 agonist, AF102B

The effects of oxotremorine and AF102B (cis-2-methylspiro-(1,3-oxathiolane-5,3')-quinuclidine), a novel M1-selective muscarinic agonist, on acetylcholine (ACh) and dopamine (DA) release from superfused rat hippocampal and striatal synaptosomes were investigated. Synaptosomes that had been prelabeled with [3H]choline or [3H]DA were depolarized by high K+. Oxotremorine and AF102B decreased the K+-evoked [3H]ACh release from hippocampal synaptosomes and increased the K+-evoked [3H]DA release from striatal synaptosomes. The dose-response curves showed that AF102B was far less potent than oxotremorine at the hippocampal presynaptic muscarinic receptors (autoreceptors). On the other hand, AF102B was more potent than oxotremorine at the muscarinic receptors on the striatal dopaminergic terminals (heteroreceptors). Pirenzepine, a selective M1 antagonist, counteracted the effects of oxotremorine on [3H]DA release more potently than it did the effects of oxotremorine on [3H]ACh release. Our results suggest that AF102B and pirenzepine discriminate pharmacologically between muscarinic autoreceptors and heteroreceptors.

Molecular cloning and expression of the cDNA for the hamster alpha 1-adrenergic receptor

The cDNA for the Syrian hamster alpha 1-adrenergic receptor has been cloned with oligonucleotides corresponding to the partial amino acid sequence of the receptor protein purified from DDT1MF-2 smooth muscle cells. The deduced amino acid sequence encodes a 515-residue polypeptide that shows the most sequence identity with the other adrenergic receptors and the putative protein product of the related clone G-21. Similarities with the muscarinic cholinergic receptors are also evident. Expression studies in COS-7 cells confirm that we have cloned the alpha 1-adrenergic receptor that couples to inositol phospholipid metabolism.

Selective coupling with K+ currents of muscarinic acetylcholine receptor subtypes in NG108-15 cells

The primary structures of two muscarinic acetylcholine receptor (mAChR) species, designated as mAChR I and mAChR II, have been elucidated by cloning and sequence analysis of DNAs complementary to the porcine cerebral and cardiac messenger RNAs, respectively. mAChR I and mAChR II expressed in Xenopus oocytes differ from each other both in acetylcholine-induced response and in antagonist binding properties. These results, together with the differential tissue location of the two mAChR mRNAs, have indicated that pharmacologically distinguishable subtypes of the mAChR represent distinct gene products. The primary structures of two additional mammalian mAChR species, designated as mAChR III and mAChR IV, have subsequently been deduced from the nucleotide sequences of the cloned cDNAs or genomic DNAs. We report here that mAChR I and mAChR III expressed in NG108-15 neuroblastoma-glioma hybrid cells, but not mAChR II and mAChR IV, efficiently mediate phosphoinositide hydrolysis, activation of a Ca2+-dependent K+ current and inhibition of the M-current, a voltage-dependent K+ current sensitive to muscarinic agonists.

The genomic clone G-21 which resembles a beta-adrenergic receptor sequence encodes the 5-HT1A receptor

The recent cloning of the complementary DNAs and/or genes for several receptors linked to guanine nucleotide regulatory proteins including the adrenergic receptors (alpha 1, alpha 2A, alpha 2B, beta 1, beta 2), several subtypes of the muscarinic cholinergic receptors, and the visual 'receptor' rhodopsin has revealed considerable similarity in the primary structure of these proteins. In addition, all of these proteins contain seven putative transmembrane alpha-helices. We have previously described a genomic clone, G-21, isolated by cross-hybridization at reduced stringency with a full length beta 2-adrenergic receptor probe. This clone contains an intronless gene which, because of its striking sequence resemblance to the adrenergic receptors, is presumed to encode a G-protein-coupled receptor. Previous attempts to identify this putative receptor by expression studies have failed. We now report that the protein product of the genomic clone, G21, transiently expressed in monkey kidney cells has all the typical ligand-binding characteristics of the 5-hydroxytryptamine (5-HT1A) receptor.

Heterologous expression of excitability proteins: route to more specific drugs?

Many clinically important drugs act on the intrinsic membrane proteins (ion channels, receptors, and ion pumps) that control cell excitability. A major goal of pharmacology has been to develop drugs that are more specific for a particular subtype of excitability molecule. DNA cloning has revealed that many excitability proteins are encoded by multigene families and that the diversity of previously recognized pharmacological subtypes is matched, and probably surpassed, by the diversity of messenger RNAs that encode excitability molecules. In general, the diverse subtypes retain their properties when the excitability proteins are expressed in foreign cells such as oocytes and mammalian cell lines. Such heterologous expression may therefore become a tool for testing drugs against specific subtypes. In a systematic research program to exploit this possibility, major considerations include alternative processing of messenger RNA for excitability proteins, coupling to second-messenger systems, and expression of enough protein to provide material for structural studies.

The muscarinic receptor subtype in mouse pancreatic B-cells

Isolated mouse islets were used to identify the muscarinic receptor subtype present in pancreatic B-cells. We thus compared the inhibitory potencies of atropine (non-specific), of pirenzepine (specific for M1 receptors) and of compound AF-DX 116 (specific for cardiac M2 receptors) on acetylcholine-induced insulin release, 86Rb+ efflux and 45Ca2+ efflux. The three antagonists inhibited all effects of acetylcholine, but EC50 values were markedly different: atropine = 1.5-5 nM, pirenzepine = 0.6-1.7 microM and AF-DX 116 = 1.7-11 microM. The results did not suggest that the various effects of ACh could result from the activation of different subtypes of receptors. It is concluded that muscarinic receptors of pancreatic B-cells belong to an M2 subtype distinct from the cardiac M2 receptors.