Regulation of muscarinic agonist binding by cations and guanine nucleotides

Action of the muscarinic toxin MT7 on agonist-bound muscarinic M1 receptors

The muscarinic toxin MT7 is the most selective ligand for the muscarinic M(1) receptors. Previous studies have shown that the toxin interacts with the antagonist-receptor complex and slows the antagonist dissociation rate, possibly by binding to an allosteric site and impeding the access to and egress from the orthosteric binding pocket. In the present study, we investigated the action of MT7 on agonist-occupied receptors in functional and radioligand binding assays of the cloned human muscarinic M(1) receptor expressed in Chinese hamster ovary cells. In time-course experiments, the addition of MT7 rapidly blocked the acetylcholine-stimulated guanosine-5'-O-(3-[(35)S]thio)triphosphate binding to membrane G proteins. Similarly, in acetylcholine-treated cells MT7 completely stopped the agonist-stimulated [(3)H]inositol phosphate accumulation. In dissociation experiments using membranes pre-equilibrated with [(3)H]acetylcholine, the addition of MT7 increased the rate of radioligand dissociation. The data indicate that MT7, while partially stabilizing the antagonist-receptor complex, effectively destabilizes the agonist-occupied muscarinic M(1) receptors.

Conformation of ligands bound to the muscarinic acetylcholine receptor

Many biogenic amines evoke a variety of physiological responses by acting on G protein-coupled receptors. We have determined the conformation of two acetylcholine analogs, (S)-methacholine and (2S,4R,5S)-muscarine, bound to the M(2) muscarinic acetylcholine receptor (M(2) mAChR) by NMR spectroscopy. The analysis of the transferred nuclear Overhauser effect indicated that the receptor selectively recognized the conformers of (S)-methacholine and (2S,4R,5S)-muscarine with the gauche O-C2-C1-N dihedral angle at +60 degrees. This is distinct from the predominant conformations of these ligands in solution with O-C2-C1-N dihedral angle (+80 to approximately 85 degrees ) in the absence of the M(2) mAChR, as assessed by analyses of the coupling constants and nuclear Overhauser effect spectroscopy. We have also built a molecular model of the M(2) mAChR-(S)-methacholine complex, based on the X-ray crystallographic structure of rhodopsin. This model indicated that the conformation with the gauche O-C2-C1-N dihedral angle at +55.5 degrees, which is similar to the one determined by NMR measurement, is energetically favored in the binding of (S)-methacholine to the receptor. We suggest that this conformation represents the binding of the agonist to the M(2) mAChR in the absence of G protein.

Transducer abstraction: a novel approach to the detection of partial agonist efficacy in radioligand binding studies

The properties of the ternary complex model (TCM) of drug action at G protein-coupled receptors (GPCRs) were examined, using theoretical computer simulations, with regard to the predicted effects of the presence of a fixed concentration of one agonist on the competition binding profile of another. Subsequently, the binding properties of the full muscarinic acetylcholine receptor (mAChR) agonists acetylcholine (ACh) and carbachol (CCh), and the partial agonists pilocarpine and McN-A-343, were investigated in competition experiments against [(3)H]N-methylscopolamine using homogenate preparations from Chinese hamster ovary cells, stably expressing the human M(1) or M(2) mAChR. At the M(2) mAChR, all agonists displayed biphasic binding curves and were readily modulated by the non-hydrolyzable GTP analogue, Gpp(NH)p, in accordance with previously established experimental observations. In contrast, agonist binding at the M(1) mAChR showed no significant change in the presence of Gpp(NH)p, even in the case of a full agonist. This phenomenon precludes using the "GTP-shift" to assess agonist efficacy at the M(1) mAChR. When the ACh competition curves were reconstructed in the presence of graded concentrations of either a full or a partial agonist, a significant redistribution of the fraction of the high-affinity state recognized by ACh was observed. However, when the procedure was repeated using the antagonist, atropine, no significant effect on the fraction of either the high or low affinity ACh binding components at the mAChR was observed. Taken together, these results indicate that changes in the profile of full agonist binding isotherms, when constructed in the presence of a partial agonist, may be more sensitive indicators of partial agonist efficacy than regular assays that directly measure partial agonist binding.

Structural determinants for binding to CGRP receptors expressed by human SK-N-MC and Col 29 cells: studies with chimeric and other peptides

Structure-activity relationships for the binding of human alpha-calcitonin gene-related peptide 8-37 (halphaCGRP8-37) have been investigated at the CGRP receptors expressed by human SK-N-MC (neuroblastoma) and Col 29 (colonic epithelia) cells by radioligand binding assays and functional assays (halphaCGRP stimulation of adenylate cyclase). On SK-N-MC cells the potency order was halphaCGRP8-37 > halphaCGRP19-37 = AC187 > rat amylin8-37 > halpha[Tyr0]-CGRP28-37 (apparent pKBs of 7.49+/-0.25, 5.89+/-0.20, 6.18+/-0.19, 5.85+/-0.19 and 5.25+/-0.07). The SK-N-MC receptor appeared CGRP1-like. On Col 29 cells, only halphaCGRP8-37 of the above compounds was able to antagonize the actions of halphaCGRP (apparent pKB=6.48+/-0.28). Its receptor appeared CGRP2-like. halpha[Ala11,18]-CGRP8-37, where the amphipathic nature of the N-terminal alpha-helix has been reduced, bound to SK-N-MC cells a 100 fold less strongly than halphaCGRP8-37. On SK-N-MC cells, halphaCGRP8-18,28-37 (M433) and mastoparan-halphaCGRP28-37 (M432) had apparent pKBs of 6.64+/-0.16 and 6.42+/-0.26, suggesting that residues 19-27 play a minor role in binding. The physico-chemical properties of residues 8-18 may be more important than any specific side-chain interactions. M433 was almost as potent as halphaCGRP8-37 on Col 29 cells (apparent pKB=6.17+/-0.20). Other antagonists were inactive.

CCK-B receptor-mediated stimulation of polyphosphoinositide turnover in GH3 pituitary cells in response to cholecystokinin and pentagastrin

CCK-B receptor-mediated polyphosphoinositide (PPI) turnover in GH3 pituitary cells has been examined and comparisons are made with Ca2+ mobilisation and receptor binding data, previously described. Sulphated cholecystokinin octapeptide (CCK-8s) and the CCK-B-selective agonist pentagastrin dose-dependently stimulated PPI turnover in GH3 cells with similar maximal increases of 3.0 fold and 3.3 fold, respectively, in production of [3H]inositol phosphates over control. Responses, measured over 30min periods in the presence of 10mM LiCl, were generally maximal for both agonists at 100nM. Consistent with their [125]Bolton Hunter CCK-8s (BHCCK) binding affinities and with effects on Ca2+ mobilisation, CCK-8s was slightly more potent than pentagastrin in stimulating PPI turnover (EC50s 1.3nM and 3.9nM respectively). Both peptides showed higher potency in the PPI assay than in Ca2+ studies. 100nM pentagastrin-induced PPI turnover was dose-dependently inhibited by the CCK-B receptor-selective antagonist L-365,260 (IC50 470nM) whilst the CCK-A receptor antagonist, devazepide, only produced weak partial inhibition (18% at 10,000nM). Antagonists alone were observed to depress control activity in PPI turnover but not in Ca2+ mobilisation assays. The selectivity of L-365,260 compared to devazepide was similar in binding studies to that for both 100nM pentagastrin-induced functional responses. Schild analysis of antagonism of PPI turnover by L-365,260 yielded a line with slope close to unity (1.07) and a pKB of 8.27+/-0.05.

Zinc supplementation is associated with improved neurologic recovery rate and visceral protein levels of patients with severe closed head injury

Sixty-eight patients were entered into a randomized, prospective, double-blinded controlled trial of supplemental zinc versus standard zinc therapy to study the effects of zinc supplementation on neurologic recovery and nutritional/metabolic status after severe closed head injury. One month after injury, the mortality rates in the standard zinc group and the zinc-supplemented group were 26 and 12%, respectively. Glasgow Coma Scale (GCS) scores of the zinc-supplemented group exceeded the adjusted mean GCS score of the standard group at day 28 (p = 0.03). Mean motor GCS score levels of the zinc-supplemented group were significantly higher on days 15 and 21 than those of the control group (p = 0.005, p = 0.02). This trend continued on day 28 of the study (p = 0.09). The groups did not differ in serum zinc concentration, weight, energy expenditure, or total urinary nitrogen excretion after hospital admission. Mean 24-h urine zinc levels were significantly higher in the zinc-supplemented group at days 2 (p = 0.0001) and 10 (p = 0.01) after injury. Mean serum prealbumin concentrations were significantly higher in the zinc-supplemented group (p = 0.003) at 3 weeks after injury. A similar pattern was found for mean serum retinol binding protein level (p = 0.01). A significantly larger number of patients in the standard zinc group had craniotomies for evacuation of hematoma; thus a bias may have been present. The results of this study indicate that zinc supplementation during the immediate postinjury period is associated with improved rate of neurologic recovery and visceral protein concentrations for patients with severe closed head injury.

Purification of recombinant porcine m2 muscarinic acetylcholine receptor from Chinese hamster ovary cells. Circular dichroism spectra and ligand binding properties

The recombinant porcine m2 muscarinic acetylcholine receptor (rPm2R) from Chinese hamster ovary cells has been purified to homogeneity. Two mg of purified rPm2R, with a specific activity of 12 nmol of R-(-)-quinuclidinyl benzilate/mg of protein, were obtained from 30 ml of packed Chinese hamster ovary cells. The apparent molecular mass (78.5 kDa) and specific activity for the rPm2R preparation were the same as that for the Pm2R purified from atrial tissue, but the yield was 100 times greater. Purified rPm2R bound agonist and antagonist with the same affinities and coupled to the inhibitory guanine nucleotide-binding protein with the same efficiency as the purified native atrial Pm2R. Ligand binding studies were consistent with a single class of antagonist binding sites but two subclasses of agonist binding sites. The fraction of rPm2R having high affinity for agonists was increased by mM Mg2+, low detergent concentration, and low temperature. Circular dichroism spectra obtained for the purified rPm2R with and without agonists were indistinguishable, but spectra for the antagonist-occupied receptor showed reproducibly deeper characteristic negative deflections at 208 and 220 nm. Secondary structure analysis of the CD spectra predicted 53% alpha-helix for the free receptor and 49% alpha-helix for the R-(-)-quinuclidinyl benzilate-receptor complex.

The function of a highly-conserved arginine residue in activation of the muscarinic M1 receptor

Arg123 in the rat muscarinic M1 receptor is part of the highly conserved triad Asp-Arg-Tyr found at the junction of transmembrane helix 3 with the second intracellular loop. Mutation of Arg123 to Lys, Ala, Leu, Glu and Gln had no effect on levels of receptor expression in COS-7 cells, or on affinities for the antagonist N-methylscopolamine. Acetylcholine stimulation of the Lys123 receptor evoked the same maximum phosphoinositide response as the wild type, although the potency was reduced six-fold, but mutation to other residues strongly disrupted receptor function. Mutation of Arg123 always decreased the ratio of the high affinity to the low affinity agonist binding constant, but the absolute effect on the latter varied from a 4-fold increase for the Lys123 to a small decrease for the Leu123 mutation. These results suggest that a positive charge at position 123 is central to the activation of G-proteins by the muscarinic M1 receptor.

Regulation of the CHAPS-solubilized muscarinic receptors by an inhibitory GTP binding protein (Gi) in the brain of neonatal and adult rats

CHAPS-solubilized muscarinic receptors (mAChRs) retain selective agonist or antagonist binding and sensitivity to GTP and ADP-ribosylation by pertussis toxin that ribosylates a group of G protein called Gi. This suggests that CHAPS solubilizes the mAChR-Gi complex both from neonatal and adult neurons. The gradient centrifugation of solubilized samples indicated that the neonatal samples contain mostly Gi-coupled mAChR (mAChRn) exhibiting a high affinity for [3H]OxM, but the adult samples contain a comparable quantity of Gi coupled mAChR (mAChRa1) and uncoupled mAChRs (mAChRa2) exhibiting low affinity for the agonist. The binding of [3H]OxM to or the dissociation of [3H]OxM from mAChRn and mAChRa1 was sensitive to GTP. However, the effects of GTP were sensitive to Na+ for mAChRn but not for mAChRa1. ADP-ribosylation by pertussis toxin but not by cholera toxin abolished the effects of Na+ on agonist binding to solubilized mAChRn, suggesting that NaCl affects mAChRn not by interacting with the receptor but by interacting with the mAChR-Gi complex. The subtype composition of mAChRn, mAChRa1 and mAChRa2 was studied by determining the IC50 and the Ki values for the inhibition by subtype-selective antagonists in the binding of [3H]OxM to the receptors. The neonatal and adult samples exhibited differences in the distribution of the mAChR subtypes, the neonatal samples containing mostly the M1-mAChR subspecies but the adult samples containing M1-, M2- and M3-mAChR subspecies. The pirenzepine-induced inhibition in the binding of [3H]OxM both to M1-mAChRn and M1-mAChRa1 was sensitive to GTP. The GTP-induced effects on mAChRn were modulated by Na+, but the effect on M1-mAChRa1 was not. This suggests functional differences between Gi coupled M1-mAChRn and M1-mAChRa1.

Differences between rat dorsal and ventral hippocampus in muscarinic receptor agonist binding and interaction with phospholipase C

Carbachol, a full muscarinic receptor agonist, stimulated [3H]inositol phosphate accumulation in both the ventral and dorsal hippocampus, but its efficacy and affinity were higher in the former area. The partial agonist oxotremorine had a weak stimulatory effect in both regions. The affinity profiles of pirenzepine and AF-DX 116 in antagonizing carbachol-stimulated [3H]inositol phosphate accumulation indicated that M1 and M3 receptors contributed equally to the response in either region. On the other hand, there were no differences in the receptor density, or in the distribution of muscarinic receptor subtypes between the two regions of the hippocampus which could account for the effect as determined in binding experiments with selective antagonists. Analysis of carbachol binding curves did, instead, indicate a difference in the way the agonist interacted with the receptors within the hippocampus, i.e., carbachol recognized three agonist affinity states (superhigh, high and low) in the ventral hippocampus, and only two (high and low) in the dorsal part. The findings thus suggested that the regional diversity in the efficacy of carbachol in stimulating phosphoinositide turnover was related to the complexity with which it bound to muscarinic receptors. Transduction processes that intervene between changes in the muscarinic receptors' conformation and activation of phospholipase C might be relevant to these differences.

Agonist-independent inhibition of G protein activation by muscarinic acetylcholine receptor antagonists in cardiac membranes

Agonist activation of muscarinic acetylcholine (mACh) receptors in porcine atrial membranes stimulates binding of the GTP analog, guanosine 5'-O-[gamma-thio]triphosphate (GTP[S]), to membrane G proteins. In contrast, atropine as well as several other mACh receptor antagonists reduced the binding of GTP[S] below basal values, both in the absence and presence of the agonist carbachol, by a similar maximal extent (about 25%). Evidence is presented that this inhibitory action of atropine was not due to an antagonism of endogenous acetylcholine. Similar to agonist-induced stimulation, antagonist-induced inhibition of GTP[S] binding required the presence of GDP and Mg2+. On the other hand, addition of salts, e.g. NaCl, amplified agonist but reduced antagonist effects on GTP[S] binding. The data presented suggest that agonist-unliganded mACh receptors interact with and activate G proteins in native cardiac membranes and that antagonist binding induces a conformational change of the receptor, which then either does not interact with G proteins or which prevents GDP release from and subsequent GTP[S] binding to G proteins.

Magnesium-dependent enhancement of endogenous agonist binding to A1 adenosine receptors: a complicating factor in quantitative autoradiography

Quantitative autoradiography was used to investigate the effects of Mg2+ on agonist and antagonist binding to A1 receptors in rat striatum. A1 receptors were labelled with the selective agonist N6-[3H]cyclohexyladenosine ([3H]CHA) or the selective antagonist 1,3-[3H]dipropyl-8-cyclopentylxanthine ([3H]DPCPX). Mg2+ had no significant effect on equilibrium binding constants for [3H]CHA [control: KD (95% confidence interval) of 0.34 (0.15-0.80) nM and Bmax of 267 +/- 8 fmol/mg of gray matter; with 10 mM Mg2+: KD of 0.8 (0.13-4.9) nM and Bmax of 313 +/- 8.9 fmol/mg of gray matter] or [3H]DPCPX [control: KD of 0.54 (0.30-0.99) nM and Bmax of 256 +/- 2.3 fmol/mg of gray matter; with 10 mM Mg2+: KD of 1.54 (0.2-11.0) nM and Bmax of 269 +/- 35.7 fmol/mg of gray matter]. In contrast, Mg2+ slowed the apparent association rate for both ligands; this was observed as a shift from a one-component to a two-component model for [3H]DPCPX. Mg2+ also affected the dissociation rates of both ligands; for [3H]CHA, dissociation in the presence of Mg2+ was not detected. Mg2+ produced a concentration-dependent inhibition of [3H]CHA binding only prior to equilibrium. HPLC was performed on untreated sections, sections preincubated with adenosine deaminase (ADA), and sections preincubated with ADA and incubated with ADA in the absence or presence of Mg2+. Adenosine was found in measurable quantities under all conditions, and the concentration was not influenced by Mg2+ or by the inclusion of GTP in the preincubation medium. From these data, we conclude the following: (a) adenosine is present and may be produced continuously in brain sections; (b) ADA is not capable of completely eliminating the produced adenosine; (c) Mg2+ apparently does not influence adenosine production or elimination; (d) A1 receptor-guanine nucleotide binding protein coupling is maximal in this preparation; and (e) Mg2+ decreases the dissociation rate of bound endogenous adenosine from A1 receptors, thus limiting the access of [3H]CHA and [3H]DPCPX to the receptors. Thus, enhancement of endogenous adenosine binding to A1 receptors by Mg2+ is a complicating factor in receptor autoradiography and may be so in other preparations as well.

Pharmacological characterization of a receptor for calcitonin gene-related peptide on rat, L6 myocytes

1 The L6 myocyte cell line expresses high affinity receptors for calcitonin gene-related peptide (CGRP) which are coupled to activation of adenylyl cyclase. The biochemical pharmacology of these receptors has been examined by radioligand binding or adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation. 2 In intact cells at 37 degrees C, human and rat alpha- and beta-CGRP all activated adenylyl cyclase with EC50s of about 1.5 nM. A number of CGRP analogues containing up to five amino acid substitutions showed similar potencies. In membrane binding studies at 22 degrees C in 1 mM Mg2+, the above all bound to a single site with IC50s of 0.1-0.4 nM. 3 The fragment CGRP(8-37) acted as a competitive antagonist of CGRP stimulation of adenylyl cyclase with a calculated Kd of 5 nM. The Kd determined in membrane binding assays was lower (0.5 nM). 4 The N-terminal extended human alpha-CGRP analogue Tyro-CGRP activated adenylyl cyclase and inhibited [125I]-iodohistidyl-CGRP binding less potently than human alpha-CGRP (EC50 for cyclase = 12 nM, IC50 for binding = 4 nM). 5 The pharmacological profile of the L6 CGRP receptor suggests that it most closely resembles sites on skeletal muscle, cardiac myocytes and hepatocytes. The L6 cell line should be a stable homogeneous model system in which to study CGRP mechanisms and pharmacology.

Calcitonin gene-related peptide: multiple actions, multiple receptors

Calcitonin gene-related peptide (CGRP) shows diversity both in its effects and its receptors. It is likely to have roles as a neurotransmitter, neuromodulator, local hormone and trophic factor. Its effects include rapid changes in neuronal activity, relaxation of many types of smooth muscle, actions on metabolism and changes in gene expression. Receptor heterogeneity has been revealed from experiments comparing agonist potency ratios and antagonist affinities. The evidence from these approaches is reviewed in this article and a speculative receptor classification scheme is proposed. Some of the likely future directions for CGRP research are discussed.

Age-dependent decrease in the affinity of muscarinic M1 receptors in neocortex of rhesus monkeys

In vitro autoradiography on tissue sections and receptor assay in cortical membrane homogenates revealed that pirenzepine high-affinity muscarinic sites (M1) decrease in affinity in the prefrontal cortex and in other cortical areas of aged rhesus monkey (Macaca mulatta). Carbachol competition experiments detected only a single, low-affinity class of sites in old monkeys, while two classes of sites (low and high affinity) were observed in young adults. The change in affinity in the aged monkeys is not accompanied by a decrease in the density of these sites and, further, the age-related decline in the affinity of the M1 site is reversible. In the presence of Mg2+, the M1 muscarinic receptors in the aged monkeys were capable of forming carbachol high-affinity sites. These results provide evidence for age-dependent functional changes in receptor activity in cerebral cortex and indicate that these receptors maintain a degree of plasticity that could be a strategic target for research aimed at treatment of memory disorders in aged humans.

Guanine nucleotide sensitivity of [3H]iloprost binding to prostacyclin receptors

A component of the displaceable binding of the stable prostacyclin analogue, [3H]iloprost, to membranes from human platelets and the somatic hybrid cell lines NG108-15 and NCB20, was inhibited by guanine nucleotides. The order of potency of a range of nucleotides for this effect was GTP gamma S greater than GppNHp greater than GTP greater than GDP = GMP; ATP, UTP and CTP were ineffective at concentrations up to 1 mM. In the presence of 100 microM GppNHp, iloprost binding curves were displaced to the right of curves obtained in the absence of guanine nucleotide, and their Hill slopes were greater. This was consistent with a conversion of a minor population of high affinity agonist binding sites to lower affinity sites in the presence of guanine nucleotides. These effects of guanine nucleotides on the binding of the agonist ligand [3H]iloprost were consistent with an interaction with a G protein coupled receptor.

Modulation of the structure-binding relationships of antagonists for muscarinic acetylcholine receptor subtypes

1. Membranes from rat cerebral cortex, myocardium and extraorbital lacrimal gland were used as sources of M1, M2 and M3 muscarinic acetylcholine receptors respectively and the affinities of seven antagonists for the three subtypes were examined under different experimental conditions. 2. The affinities for the membrane-bound receptors were measured at different ionic strengths and temperatures and compared with those determined on the receptor solubilised in the neutral detergent digitonin or the zwitterionic detergent, CHAPSO. 3. The range of measured affinity constants of a given antagonist for a specific subtype varied from 2 (atropine at M1 receptors) to 1000 (AF-DX 116 at M2 receptors). 4. As a consequence of these changes in affinity, which were dependent on the drug, the subtype and the experimental conditions, both the structure-binding relationships of a given subtype can be markedly changed as well as the selectivity of a drug for the different subtypes. For example it is possible to change the relative affinities of AF-DX 116 and gallamine at membrane-bound M1 receptors from 50:1 to 1:60. 5. Experimental conditions for the observation of high selectivity of pirenzepine, AF-DX 116, gallamine and hexahydrosiladiphenidol for the three subtypes are given. 6. When the receptors are removed from their membrane environment by solubilisation in detergent, antagonist affinities are changed but the subtypes still retain different structure-binding relationships. 7. In general, AF-DX 116 and the allosteric antagonist, gallamine, behave differently from the other antagonists, suggesting that they bind in different ways to muscarinic receptors. Careful attention should therefore be paid to the experimental conditions in binding assays used to assess the affinities and selectivities of new muscarinic antagonists in order to avoid misleading results. 9. The ability to produce enhanced or attenuated affinities and selectivities of antagonists, resulting from the induction of different conformations of the receptor by a variety of physical, chemical or molecular biological perturbations may lead to a better understanding of the structural basis of drug receptor interactions.

Divalent cation-sensitive antagonist binding to heart muscarinic receptors

1. The binding of (-) [3H]quinuclidinylbenzilate (QNB), a potent muscarinic antagonist, to cardiac muscarinic receptors was examined in washed particles and microsomes isolated from rat heart atria. 2. Addition of divalent cations (Mg2+, Mn2+ and Ca2+) to the binding assays increased (6 to 8-fold) the antagonist binding to microsomes but exerted little or only weak (up to 35%) stimulation of the antagonist binding to washed particles. 3. Mg2+ ions present only in the dilution and washing buffer prior to rapid filtration of the binding reaction were capable of markedly increasing the antagonist binding to microsomes but not to washed particles. 4. Such differential action of Mg2+ on antagonist binding to subcellular fractions was seen when fractions were either isolated and or incubated in media containing either 10 or 50 mM imidazole-Cl, Tris-phosphate or Tris-Cl buffer, each at pH 7.5. While the modulatory effect of Mg2+ was not seen in Na-K-phosphate buffer, pH 7.5, the antagonist binding per se was much higher in Na-K-phosphate buffer compared to other buffers.

Lithium-selective alteration of the function of brain versus cardiac Gs protein

Lithium was recently demonstrated to inhibit the coupling of both muscarinic cholinergic receptors and beta-adrenergic receptors to pertussis toxin-sensitive and cholera toxin-sensitive G proteins, respectively, thus suggesting alteration of the function of G protein by lithium, as the single site for both the antimanic and antidepressant effects of this drug. One of the most puzzling aspects of the ability of lithium to ameliorate the manic-depressive condition, is its relatively selective action upon the central nervous system (CNS). In the present study, it was shown that lithium selectively attenuated the function of Gs proteins in the CNS, assessed through isoproterenol-induced increases in the binding of guanosine triphosphate (GTP) to these proteins. Therapeutic concentrations of lithium (1-1.5 mM in vitro) inhibited the function of Gs protein in the cerebral cortex of the rat, while 4- to 6-fold larger concentrations of lithium were required to alter the function of Gs protein equivalently in the cardiac ventricles of the rat. Chronic administration of lithium via rat chow, containing lithium carbonate, to rats totally abolished the effect of isoproterenol on the binding of GTP in the CNS but did not affect the function of peripheral cardiac Gs protein. The lithium-selective action on the function of Gs protein in the CNS may stem from the heterogeneity of the alpha s subunit proteins: in the heart, the major species is a 45 kDa molecule, while in the brain, a 52 kDa molecular weight species predominates. The heterogeneity in alpha s subunits may thus be the biochemical basis for the selective action of lithium on the CNS and for the scarcity of peripheral side effects.

MgCl2-sensitive and Gpp(NH)p-sensitive antagonist binding states of rat heart muscarinic receptors: preferential detection at ambient temperature assay and location in two subcellular fractions

Some novel observations dealing with antagonist binding to cardiac particulate muscarinic receptors are described. Gpp(NH)p increased (2-3 fold) the specific binding of [3H]-QNB or [3H]-NMS, both potent muscarinic antagonists, to washed particles (WP), but not microsomes (MIC), when the binding was conducted at 30 degrees C. Magnesium, on the other hand, increased (2-3 fold) the binding of these antagonists to MIC, but not to WP, under the same condition. The treatment of subcellular fractions with 0.2 mM N-ethylmaleimide (NEM), a sulfhydryl reagent, failed to significantly modify the respective stimulatory actions of either Gpp(NH)p on WP binding or of magnesium on MIC binding of these antagonists; treatment with dithiothreitol (1 mM) was also ineffective in this regard. Gpp(NH)p decreased Kd (WP) while magnesium increased Kd (MIC) for [3H]-QNB. Repeated freezing/thawing of isolated subcellular fractions abolished the stimulatory effect of magnesium on antagonist binding to MIC but not of Gpp(NH)p on WP antagonist binding; the freeze/thaw procedure per se increased MIC binding but not WP binding of these antagonists. When the binding was conducted at 4 degrees C (24 hr), the stimulatory effect of Gpp(NH)p on [3H]-QNB binding was enhanced (6-fold) in the case of WP and was detectable (80%) in the case of MIC. Under this condition, the stimulatory effect of magnesium on [3H]-QNB binding was also enhanced (5-fold) in the case of MIC and became evident (200%) in the case of WP. The results of this work support the following views: (a) antagonist-occupied cardiac muscarinic receptors are capable of interaction with guanine nucleotide binding proteins (G protein like Gi, Go) and such interaction influences antagonist binding properties (e.g. increased affinity) of the cardiac membrane-associated muscarinic receptors (b) magnesium influences (decreased affinity) antagonist binding properties by interacting with multiple sites of which some are likely associated with components other than G proteins of the particulate fractions (c) a pool of NEM-sensitive sulfhydryls involved in the regulation of Gpp(NH)p-sensitive agonist binding to cardiac muscarinic receptors is not involved in the regulation by either Gpp(NH)p or magnesium of antagonist binding in these subcellular fractions and (d) membrane fluidity and microenvironment surrounding the receptor and G proteins contribute to the actions of Gpp(NH)p and magnesium on antagonist binding.

Calcium antagonizes the magnesium-induced high affinity state of the hepatic vasopressin receptor for the agonist interaction

1. The present study describes the role of Ca2+ in the regulation of the hepatic vasopressin V1 receptor. With low concentrations of Ca2+, there was a small increase in [3H]-arginine vasopressin [( 3H]-AVP) binding, but above 10 mM, Ca2+ decreased the binding of this agonist. In contrast, low concentrations of Mg2+ were associated with a dramatic concentration-dependent increase in [3H]-AVP binding, reaching a maximal effect of 650% above control at concentrations ranging between 1-5 mM. At higher concentrations of Mg2+, the stimulatory effect of this cation was less pronounced, falling to 210% of control at 100 mM Mg2+. Strikingly, Ca2(+)-inhibited the stimulatory effect of Mg2+ in a concentration-dependent fashion. 2. Saturation binding data revealed that Ca2+ (2 to 10 mM) per se promotes the high affinity conformation of the V1 receptor for the agonist binding with the KD decreased from a control value of 2.3 nM to 0.5 nM in the presence of 10 mM Ca2+. This effect was attenuated with an increase in Ca2+ above 10 mM. With an increase in Ca2+ to 20 mM, however, the Bmax for [3H]-AVP binding was decreased. Ca2+ also decreased the high affinity/high capacity state (KD 100 pM) of the receptor induced by 1 mM Mg2+ for agonist interaction. 3. [3H]-V1 antagonist binding was inhibited by both Ca2+ and Mg2+. The IC50 values (mean +/- s.e. mean) for Ca2+ and Mg2+ were 32 +/- 8 and 53 +/- 9 mM respectively. Maximal inhibition achieved at 100 mM was 29% for Ca2+ and 42% for Mg2+. Both cations decreased the affinity and increased the capacity of the V1 receptor for the antagonist. 4. The results suggest that the divalent metal ion binding site(s) modulated by Mg2 + is also accessible to Ca2 +. Although Ca2 + opposes the powerful stimulatory effects of Mg2 + on agonist binding, the effects of Ca2+ and Mg2 + on the B,,x of [3H]-AVP binding were different, suggesting that the divalent cations may bind to two different sites, thereby regulating the affinity and the capacity characteristics of the V1 receptor.

Two distinct, divalent cation-sensitive, antagonist binding states of heart muscarinic receptors: differential modulation by guanine nucleotide

1. The binding of [3H]quinuclidinylbenzilate (QNB), a muscarinic antagonist, to cardiac muscarinic receptors was investigated in two subcellular fractions (washed particles and microsomes) isolated from rat heart atria and ventricles. 2. 5'-guanylylimidodiphosphate (Gpp(NH)p, 0.1 mM), increased (2-3-fold) the binding to washed particles, but not to microsomes, whereas Mg2+ (1-20 mM) increased (up to 5-fold) the binding to microsomes, but not to washed particles. Gpp(NH)p modestly increased the affinity while Mg2+ decreased the affinity towards the radiolabelled antagonist. 3. Treatment with N-ethylmaleimide (NEM, 2 mM) increased the antagonist binding to either fraction. The stimulatory effect of Gpp(NH)p was not evident while that of Mg2+ survived in the NEM-treated fractions. 4. The treatment of fractions with divalent cations chelators (EDTA, EGTA; 10 mM) augmented the stimulatory effect of Mg2+ on [3H]QNB binding to microsomes while that of Gpp(NH)p on the washed particle [3H]QNB binding was decreased. Such treatment further revealed an inhibitory action (about 40%) of Mg2+ on the washed particle binding.

Regional distribution of somatostatin receptor affinity states in rat brain: effects of divalent cations and GTP

In the present work, we have characterized by film radioautography the effects of divalent cations and guanine nucleotide on specific receptor for somatostatin (SRIF) using 125I-TyrO-DTrp8-SRIF14 (125I-ToD8-SRIF) as a ligand. The experiments were performed on coronal 20-microM-thick sections cut at the level of the amygdala, thus allowing to study binding sites in several regions enriched in binding sites (frontal cortex, hippocampus CA1 and dentate gyrus, habenula, basolateral nucleus of the amygdala). In a preliminary set of experiments using brain cortical membranes it was found that 3 mM Mg2+ ions doubled the specific binding of 125I-ToD8-SRIF. However, Mg2+ enhanced equally by a factor of 3 affinities of high- and low-affinity binding sites as evidenced by SMS 201.995 displacement curves without modifying the ratio between high and low affinity sites. In radioautographic studies while SRIF14 and SRIF28 elicited monophasic displacement curves, SMS 201.995 displaced 125I-ToD8-SRIF binding in a biphasic manner in all regions tested but the baso-lateral nucleus of the amygdala. Radioautographic distribution of 125I-ToD8-SRIF binding sites was identical whether the sections were incubated with MgCl2 or with MnCl2 and almost undetectable in the absence of ions. In all structures investigated increasing concentrations of GTP totally inhibited 125I-ToD8-SRIF binding with an IC50 value of 3 microM. In conclusion, our results demonstrate that 125I-ToD8-SRIF-binding sites in brain occur on two different affinity states as assessed by a displacement curve using endogenous ligands and SMS 201.995. According to the comparable effects of divalent cations and GTP, the two subtypes of 125I-ToD8-SRIF-binding sites discriminated by SMS 201.995 are likely to correspond to interconvertible forms of the same receptor coupled to a G protein-transducing system.

Agonist binding to M1 muscarinic receptors is sensitive to guanine nucleotides

Putative M1 (high-affinity pirenzepine) muscarinic receptors in rabbit hippocampal membranes, treated with 0.1 mM N-ethylmaleimide (NEM), were selectively labeled with [3H]pirenzepine. A single class of binding sites was labeled with a Kd of 3.4 nM, consistent with the pharmacologically-defined M1 subtype of muscarinic receptors. While full muscarinic agonists bound to high- and low-affinity states of [3H]pirenzepine-labeled M1 sites with a KL/KH ratio of approximately 100, the ratio for partial muscarinic agonists was approximately 10. The high-affinity binding of all agonists tested required divalent cations, and was interconverted to low-affinity binding in the presence of the non-hydrolyzable GTP analogue, guanylyl imidodiphosphate (GppNHp). Direct labeling of the high-affinity agonist state of M1 receptors was achieved with 5 nM [3H]oxotremorine-M by selectively uncoupling the high-affinity agonist state of M2 (low-affinity pirenzepine) receptors with NEM. The rate of dissociation of [3H]Pxotremorine-M from M1 receptors was accelerated 6-fold by GppNHp. These results provide further evidence which suggests that putative M1 muscarinic receptors activate second messenger systems by coupling to NEM-insensitive guanine nucleotide-binding proteins.

Membrane phospholipid polar heads influence the coupling of M2 muscarinic receptors to G proteins

Treating membranes from rat heart with phospholipase C (phosphatidylcholine choline phosphohydrolase) from Clostridium perfringens increased the affinity of muscarinic acetylcholine receptors (M2) for the agonists carbachol and oxotremorine. The affinity for antagonists was not affected. Phospholipase C activity, i.e., the cleavage of polar heads of membrane phospholipids, led to the disappearance of the guanine nucleotide-dependent rightward shift of the isotherm for agonist binding. The treatment of tracheal smooth muscle with phospholipase C led to a decrease in the maximum contractile effect of muscarinic (M2) stimulation with no modification of the agonist EC50, i.e., to the uncoupling of the stimulation-contraction process. These results demonstrate that when phospholipid polar heads are hydrolysed by phospholipase C, M2 receptors are uncoupled from G proteins, which enhances their affinity for agonists but prevents information transfer.

An [3H]oxotremorine binding method reveals regulatory changes by guanine nucleotides in cholinergic muscarinic receptors of cerebral cortex

A rapid, reliable filtration method for [3H]oxotremorine binding to membranes of the cerebral cortex that allows the direct study of regulation by guanine nucleotides of muscarinic receptors was developed. [3H]Oxotremorine binds to cerebral cortex membranes with high affinity (KD, 1.9 nM) and low capacity (Bmax, 187 pmol/g protein). These sites, which represent only about 18% of those labeled with [3H]quinuclidinyl benzilate, constitute a population of GTP-sensitive binding sites. Association and dissociation binding experiments revealed a similar value of KD (2.3 nM). Displacement studies with 1-4000 nM oxotremorine showed the existence of a second binding site of low affinity (KD, 1.2 microM) and large capacity (Bmax, 1904 pmol/g protein). Gpp(NH)p, added in vitro, produced a striking inhibition of [3H]oxotremorine binding with an IC 50 of 0.3 microM. Saturation assays, in the presence of 0.5 microM Gpp(NH)p, revealed a non-competitive inhibition of the binding with little change in affinity. These results are discussed from the viewpoint of conflicting reports in the literature about guanine nucleotide regulation of muscarinic receptors in reconstituted systems and membranes from different tissues.

Muscarinic cholinergic receptor subtypes in hippocampus in human cognitive disorders

Total muscarinic receptor levels, the levels of the subtypes exhibiting high and low affinity for pirenzepine, and the high- and low-affinity agonist states of the receptor were investigated in hippocampal tissue obtained at autopsy from mentally normal individuals and the following pathological groups: Alzheimer's disease, Parkinson's disease, Down's syndrome, alcoholic dementia, Huntington's chorea, and motor-neurone disease. A moderate decrease in the density of both high-affinity pirenzepine and high-affinity agonist subtypes was found in Alzheimer's disease, whereas a trend towards an increase in the overall muscarinic receptor density was apparent in the parkinsonian patients without dementia, mainly due to an increase in the low-affinity agonist state; the differences between the Alzheimer's disease and nondemented parkinsonian cases were highly significant. As previously reported, the levels of both choline acetyltransferase and acetylcholinesterase were markedly reduced in both Alzheimer's disease and Parkinson's disease--with a greater loss of both enzymes in the demented subgroup of parkinsonian patients. Activities of the cholinergic enzymes were also extensively reduced in Down's syndrome, accompanied by a loss of high-affinity pirenzepine binding. There were no significant receptor or enzyme alterations in the other groups studied. These observations suggest that in the human brain, extensive degeneration of cholinergic axons to the hippocampus, as indicated by a loss of cholinergic enzymes, is not necessarily accompanied by extensive muscarinic receptor abnormalities (as might be expected if a major subpopulation were presynaptic). Moreover, the opposite changes in muscarinic binding in Parkinson's and Alzheimer's diseases may be related to the greater severity of dementia in the latter disease.

Relative affinities of drugs acting at cholinoceptors in displacing agonist and antagonist radioligands: the NMS/Oxo-M ratio as an index of efficacy at cortical muscarinic receptors

1. Radioligand binding assays using [3H]-N-methylscopolamine (NMS) and [3H]-oxotremorine M (Oxo-M) have been devised to predict the efficacy of test compounds at muscarinic receptors in rat cerebral cortex. 2. Muscarinic antagonists, including non-selective and both M1- and M2-selective compounds, displayed similar affinity for both binding assays. 3. Full agonists such as carbachol and muscarine possessed a ratio of potencies against the antagonist versus the agonist ligand (NMS/Oxo-M ratio) of greater than 4000. 4. Compounds which have been shown previously to display partial agonist activity in functional assays e.g. pilocarpine and RS86 had intermediate NMS/Oxo-M ratios of 100-150. A second group of compounds which included oxotremorine had somewhat higher ratios (500-1400). 5. The ratio of affinity constants for the two assays predicted the ability of agonists to stimulate cortical phosphatidyl-inositol turnover. 6. These results suggest that the NMS/Oxo-M ratio may be a useful prediction of efficacy for novel compounds acting at cortical muscarinic receptors.

GTP effects in rat brain slices support the non-interconvertability of M1 and M2 muscarinic acetylcholine receptors

GTP (guanosine-5'-triphosphate) markedly reduced high-affinity 3H-oxotremorine-M binding to M2 receptors on brain slices in autoradiographic experiments while 3H-pirenzepine binding to M1 receptors was largely unaffected. The distribution of M1 receptors so labelled was also not altered by GTP to include former M2-rich regions, thus indicating that GTP could not, by itself, interconvert high agonist-affinity M2 receptors to M1 receptors.

Guanine nucleotide modulation of muscarinic cholinergic receptor binding in postmortem human brain--a preliminary study in Alzheimer's disease

The coupling of cortical muscarinic receptors to guanosine triphosphate (GTP) binding proteins, as defined by changes in agonist affinity states of the receptor in the presence of magnesium ions (Mg2+) and a GTP analogue has been investigated using carbachol in competition experiments with either N-methylscopolamine (NMS) or pirenzepine (PZ). The stability of the system with regard to autopsy delay and freezing was first established in membrane preparations from mouse brain. Applying the same methods to human autopsy tissue from the parietal cortex of Alzheimer's diseased cases and controls, matched for age and postmortem delay, there was no significant difference in the detectable coupling of the total (NMS-labelled) muscarinic receptor population. However, coupling of the 'M1' muscarinic receptor subtype, selectively labelled by PZ, appeared to be more labile than that of the receptor population as a whole and the modulation of this subtype by the GTP analogue was significantly attenuated in Alzheimer's disease.

Effects of halothane on high affinity agonist binding and guanine nucleotide sensitivity of muscarinic acetylcholine receptors from brainstem of rat

The influence of halothane on muscarinic receptors with a high affinity for agonists was studied using [3H]oxotremorine-M. [3H]Oxotremorine-M bound with high affinity (KD = 2.8 nM) to a subpopulation of muscarinic receptors in the brainstem of rat, representing 32% of the total receptor pool. Agonist affinity for binding sites for [3H]oxotremorine-M was not affected by a guanine nucleotide (5'-guanylylimidodidiphosphate; Gpp(NH)p), although the level of binding was decreased, presumably due to the conversion of receptors to lower affinity conformations. However, only 58% of 3 nM binding of [3H]oxotremorine-M was sensitive to Gpp(NH)p. Halothane had two effects on the binding of [3H]oxotremorine-M: halothane (1) decreased the level of binding of [3H]oxotremorine-M without affecting agonist affinity for the surviving sites, and (2) lowered the sensitivity of the binding of [3H]oxotremorine-M to Gpp(NH)p by a factor of 120. The decrease in binding of [3H]oxotremorine-M binding was nonselective with regard to the sensitivity of the receptors to the guanine nucleotide, insofar as Gpp(NH)p inhibited the binding of [3H]oxotremorine-M to the same extent in the presence and absence of halothane. These results suggest that halothane (1) converts both G protein-coupled and -uncoupled muscarinic receptors to states of lower agonist affinity and (2) lowers the affinity of receptor-G protein complexes for guanine nucleotides.

Binding studies with [3H]cis-methyldioxolane in different tissues. Under certain conditions [3H]cis-methyldioxolane labels preferentially but not exclusively agonist high affinity states of muscarinic M2 receptors

Special conditions--tricine buffer containing Ca2+ and Mg2+, 22 degrees C (TCM)--allow to label a much higher proportion of muscarinic receptors by [3H]cis-methyldioxolane (CD) than hitherto described (Vickroy et al. 1984a). Taking the maximum number of binding sites, Bmax, of [3H]QNB as 100%, Bmax of [3H]CD amounts to 83% in the rat heart instead of the reported 17%, 33% in the cerebral cortex instead of 6%, 20% in hippocampus and 55% in pons/medulla. In the salivary glands specific binding was negligible. The affinities of a number of muscarinic agonists and antagonists to [3H]CD and [3H]QNB binding sites in different tissues of the rat are compared. Apparent affinities of agonists are much higher in the [3H]CD system, affinities of antagonists are slightly higher in the [3H]QNB system. In both assay systems receptors of heart and pons/medulla membranes seem to have similar drug specificity. They differ somewhat from those in the cortex. Receptors in the salivary glands, however, seem to be completely different from those in the other three tissues. In the heart [3H]CD binding can be abolished almost completely by GppNHp. In the cortex about half of the [3H]CD binding is susceptible to GppNHp. The reduction of binding in the cortex is due to a change in Bmax and not in the dissociation constant KD. Competition of unlabelled pirenzepine with [3H]CD: In heart and pons/medulla only low affinity sites for pirenzepine (M2-receptors) are labelled by [3H]CD.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of veratridine on [3H]-L-quinuclidinyl benzilate ([3H]QNB) binding in mouse hindbrain

The neurotoxin veratridine is well known for its ability to open sodium channels in neuronal and muscle tissues in micromolar concentrations. It has also been shown that veratridine is an inhibitor of the potent muscarinic receptor antagonist L-quinuclidinyl benzilate (QNB) at these concentrations. These findings prompted us to examine the relationships between action potential sodium channels and muscarinic receptors in a glass-fiber filtration assay for [3H]QNB binding to mouse hindbrain membranes using agents known to affect interconversion of the affinity states in some muscarinic receptor populations, i.e. guanosine triphosphate (GTP) and magnesium (Mg2+). The actions of the sodium channel antagonist tetrodotoxin (TTX) were also examined. Veratridine inhibited [3H]QNB binding with a Ki value of approximately 2.5 microM. This inhibition exhibited a competitive mechanism at higher concentrations (5-10 microM), while showing an apparent non-competitive action at low concentrations (1 microM). Magnesium caused a parallel shift to the right in the inhibition curve with a 32% increase in the veratridine Ki. GTP caused a non-parallel shift to the left with the greatest displacement occurring at lower veratridine concentrations (2-5 microM). The addition of magnesium to GTP did not alter the action of GTP significantly. TTX (5 microM) caused a parallel shift of the veratridine inhibition curve to the right. In addition, TTX alone inhibited the binding of [3H]QNB. Therefore, it appears that there may be more than one binding site for veratridine which may be linked to the muscarinic system and that these may be action potential sodium channels.

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

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

Assessment of a ternary model for the binding of agonists to neurohumoral receptors

A frequently cited variant of the "mobile receptor" hypothesis has been examined for its ability to describe the binding of agonists at neurohumoral receptors that operate via a guanylyl nucleotide binding protein. The model involves a reversible association between the receptor (R) and the G protein (G). Agonists (A) bind with different affinity to R and to the RG complex; similarly, G differentiates between R and the AR complex. Theoretical binding curves calculated according to the model have been analyzed in terms of the Hill equation and as a mixture of independent and noninteracting sites. The model is shown to be compatible in some respects with reported data on the binding of agonists to the beta-adrenergic receptor but not to the muscarinic cholinergic or D2 dopaminergic receptors. It is difficult to reconcile with the reported effects of guanylyl nucleotides, magnesium, and N-ethylmaleimide on the binding of agonists at any neurohumoral receptor.

Effects of divalent metal ions on the uptake of glutamate and GABA from synaptosomal fractions

The effects of divalent metal ions on high affinity uptake glutamate and GABA were examined, using crude and purified synaptosomal fractions prepared from brains of DBA/2CBI. The uptake velocities of both amino acids are severely reduced in the presence of Cu2+, Fe2+ and Zn2+ but remain unaffected by Co2+.

Effects of N-ethylmaleimide on muscarinic acetylcholine receptor subtype autoradiography and inositide response in rat brain

Chemical modification of brain muscarinic acetylcholine receptors (mAChr) with N-ethylmaleimide (NEM) has been employed to investigate mAChr-subtype distribution and mediation of the inositide response. 3H-Pirenzepine and 3H-oxotremorine-M were used to autoradiographically localize the M1- and M2-AChr subtypes, respectively, in brain slices. M1- and M2-AChr distribution were observed to be distinct from each other. The presence of 1 mM NEM selectively reduced the labeling of M2-, but not of M1-AChr. These data support the notion that NEM converts the high-affinity M2-AChr to a lower affinity state, without affecting the affinity of the M1-AChr. Also, regional analysis indicated that the M1- and M2-AChr subtypes were not interconvertible by NEM. NEM at 30 microM enhanced the carbamylcholine stimulated labeling of phosphatidic acid from 32Pi in nerve endings from rat forebrain, suggesting that the low affinity M2-AChr may mediate at least a part of the inositide response to cholinergic stimulation.

Observations on the muscarinic activation of catecholamine secretion in the chicken adrenal

Cells were isolated by collagenase digestion of chicken adrenal glands. Catecholamine secretion could be stimulated by acetylcholine, carbamylcholine, potassium or veratridine. Methacholine, muscarine and oxotremorine were also effective secretagogues whereas nicotine was not. Secretion evoked by acetylcholine was blocked by low concentrations of atropine but was relatively insensitive to hexamethonium. Atropine-sensitive secretion required both external sodium and calcium, was unaffected by tetrodotoxin, blocked by methoxy verapamil and nifedipine, and potentiated by BAY-K-8644. These data suggest that muscarinic activation of these cells facilitates tetrodotoxin insensitive depolarization, thereby opening conventional voltage-sensitive calcium channels. The mechanism by which calcium activates catecholamine secretion was investigated in cells that had been made permeable by exposure to brief intense electric fields. Catecholamine release required Mg-adenosine 5' triphosphate, was half-maximally activated by 1 microM Ca2+ and could be inhibited by high concentrations of Mg2+. At low Ca2+ concentrations, release was potentiated by 12-O-tetradecanoylphorbol 13-acetate, dioctanoylglycerol, guanosine 5'-O-(3-thiotriphosphate) and 5'-guanylylimidodiphosphate, all of which increased the apparent affinity of exocytosis for Ca2+.

Direct autoradiographic determination of M1 and M2 muscarinic acetylcholine receptor distribution in the rat brain: relation to cholinergic nuclei and projections

The autoradiographic distributions of receptors with high affinity for [3H]oxotremorine-M (the M2 receptor) and [3H]pirenzepine (the M1 receptor) were studied in the rat brain. M1 receptors were seen in highest density only in telencephalic structures: cerebral cortex (layers I-II), hippocampus, dentate gyrus, medial and basolateral amygdala, nucleus accumbens and caudate/putamen. M2 receptors were detected throughout the brain, with highest levels observed in cerebral cortical layers III and V, forebrain cholinergic nuclei, caudate/putamen, various thalamic areas, inferior and superior colliculus, interpeduncular and pontine nuclei, brainstem cholinergic nuclei and cervical spinal cord regions. M2 receptors were found to be good markers for cholinergic cell groups and the majority of cholinergic projection areas, whereas M1 receptors were only found in a large sub-group of telencephalic cholinergic projection areas, and the pattern of distribution of receptors in these areas differed from that of M2 receptors. Scatchard analysis of [3H]oxotremorine-M binding to inferior collicular slices revealed one site with a dissociation constant (Kd) of 1.9 nM and a receptor density (Bmax) of 1.4 pmol/mg protein. Our data support the hypothesis that M1 and M2 receptors are physically distinct sub-types of the muscarinic acetylcholine receptor.

Characterization of choline efflux from the perfused heart at rest and after muscarine receptor activation

The resting efflux of choline from perfused chicken hearts varied from 0.4 to 2.6 nmol/g min, but was constant for at least 80 min in the individual experiments. The rate of choline efflux was found to be equal to the rate of choline formation in the heart, which, from the following reasons, was essentially due to hydrolysis of choline phospholipids. Cardiac content of choline phospholipids (7,200 nmol/g) was much higher than that of acetylcholine (5.5 nmol/g). Resting release of acetylcholine was 0.016 nmol/g min and, after inhibition of cholinesterase, only about 0.1 nmol/g min. Resting efflux of choline was reduced by mepacrine, a phospholipase A2 inhibitor, by perfusion with a Ca2+-free Tyrode's solution containing EGTA and by the combination mepacrine plus Ca2+-free/EGTA solution. In all experiments the reduced choline efflux levelled off within 10 min at about 50%. Omission or elevation of Mg2+ from 1.05 to 10.5 mmol/l had no effect. Resting efflux was increased to 150% by oleic acid (as sodium salt; 2 X 10(-5) mol/l) which is known to activate phospholipase D. Likewise muscarinic agonists (carbachol and acetylcholine) caused facilitation of the efflux of endogenous choline that was blocked by 3 X 10(-7) mol/l atropine. This effect was not reduced, but even slightly enhanced, by mepacrine and by infusion of EGTA in a modified Tyrode's solution (Ca2+-free, 10.5 mmol/l Mg2+). It is concluded that the resting efflux of choline from the heart is essentially due to hydrolysis of choline phospholipids, that half of the efflux is insensitive to mepacrine and is Ca2+-independent (excluding an involvement of phospholipase A2).(ABSTRACT TRUNCATED AT 250 WORDS)

The binding of pirenzepine to digitonin-solubilized muscarinic acetylcholine receptors from the rat myocardium

The binding of pirenzepine to digitonin-solubilized rat myocardial muscarinic acetylcholine receptors has been examined at 4 degrees C. Solubilization produced only small changes in the binding of N-methylscopolamine and atropine. In contrast to the low affinity binding of pirenzepine found to be present in in the membranes, high affinity binding was detected in the soluble preparation. In both preparations, pirenzepine binding was complex. High affinity pirenzepine binding (KD approximately 3 X 10(-8)M) to the soluble myocardial receptors could be monitored directly using [3H]-pirenzepine. [3H]-pirenzepine-labelled soluble myocardial receptors have a sedimentation coefficient of 11.1 s. This indicates that [3H]-pirenzepine binds predominantly to the uncoupled form of the receptor. However, [3H]-pirenzepine-agonist competition experiments indicated that the high affinity pirenzepine binding sites are capable of coupling with a guanosine 5'-triphosphate (GTP)-binding protein. Pirenzepine affinities for the soluble myocardial receptors were unaffected by their state of association with the GTP-binding proteins found in the heart. The equilibrium binding properties of the soluble cortical and myocardial receptors were very similar. However, the binding kinetics of the myocardial receptor were much slower. It appears that the membrane environment can affect the affinity of pirenzepine for the rat myocardial muscarinic receptor. Removal of the constraint by solubilization allows the expression of high affinity pirenzepine binding.

Rat brain and heart muscarinic receptors: modification with tetranitromethane

Tetranitromethane at a concentration of 50 microM modifies the muscarinic receptors in membrane preparations from rat striatum, hippocampus and heart atrium, but not from the rat brain stem. While the binding of antagonists is only slightly altered, the modified receptor possesses an increased affinity of up to 8-fold for [3H]-acetylcholine binding to the high affinity state. This effect is absent if the nitration is carried out in the presence of an antagonist, but not in the presence of an agonist. The affinity for carbamylcholine is increased for both the high and the low affinity state of the receptor, as is evident from its ability to compete with a labeled antagonist. In addition, the proportion of binding sites (alpha) exhibiting the high affinity state for [3H]-acetylcholine or for carbamylcholine is increased upon nitration. This increase cannot be protected against by an antagonist, and is enhanced when nitration takes place in the presence of an agonist. With the agonists oxotremorine and [3H]-oxotremorine-M only the latter effect (i.e., increase in alpha) is observed following nitration, while their dissociation constants for the receptor are unchanged. Data are discussed with respect to the proposed existence of subtypes of muscarinic receptors, as well as the importance of the agonist chosen for studies of ligand-receptor interactions.

Solubilization and characterization of high and low affinity pirenzepine binding sites from rat cerebral cortex

An apparently monomeric form of the digitonin-solubilized muscarinic acetylcholine receptor from the rat cerebral cortex retains a high affinity of 7 X 10(7) M-1 for pirenzepine. Muscarinic receptor binding sites in the rat cerebral cortex with a low affinity for pirenzepine are solubilized with relatively little change in affinity. The ability of pirenzepine to distinguish between subtypes of muscarinic binding site in the cerebral cortex is manifest in both the membrane-bound and soluble state.