Agonist-mediated conformational changes of muscarinic receptors in rat brain

Effects of N-ethylmaleimide on conformational equilibria in purified cardiac muscarinic receptors

Muscarinic receptors purified from porcine atria and devoid of G protein underwent a 9-27-fold decrease in their apparent affinity for the antagonists quinuclidinyl benzilate, N-methylscopolamine, and scopolamine when treated with the thiol-selective reagent N-ethylmaleimide. Their apparent affinity for the agonists carbachol and oxotremorine-M was unchanged. Conversely, the rate of alkylation by N-ethylmaleimide, as monitored by the binding of [(3)H]quinuclidinyl benzilate, was decreased by antagonists while agonists were without effect. The receptor also underwent a time-dependent inactivation that was hastened by N-ethylmaleimide but slowed by quinuclidinyl benzilate and N-methylscopolamine. The destabilizing effect of N-ethylmaleimide was counteracted fully or nearly so at saturating concentrations of each antagonist and the agonist carbachol. Similar effects occurred with human M(2) receptors differentially tagged with the c-Myc and FLAG epitopes, coexpressed in Sf9 cells, and extracted in digitonin/cholate. The degree of coimmunoprecipitation was unchanged by N-ethylmaleimide, which therefore was without discernible effect on oligomeric size. The data are quantitatively consistent with a model in which the purified receptor from porcine atria interconverts spontaneously between two states (i.e. R R*). Antagonists favor the R state; agonists and N-ethylmaleimide favor the comparatively unstable R* state, which predominates after purification. Occupancy by a ligand stabilizes both states, and antagonists impede alkylation by favoring R over R*. Similarities with constitutively active receptors suggest that R and R* are akin to the inactive and active states, respectively. Purified M(2) receptors therefore appear to exist predominantly in their active state.

Opposing effects of N-ethylmaleimide on the affinity of carbachol for muscarinic cholinoceptors of guinea-pig atrium

1. Inhibition of the binding of [3H]quinuclidinyl benzilate to homogenates of guinea pig right atrium (M2 receptors) by varying concentrations of carbachol was studied. 2. Pretreatment of membranes with 5 x 10(-5) M N-ethylmaleimide at 2 degrees C shifted the carbachol inhibition curve to the right, indicating decreased affinity of the receptor for carbachol. However pretreatment at 37 degrees C moved the curve to the left. 3. The ability of guanyl-5'-yl imidodiphosphate to reduce agonist affinity was largely eliminated by treatment with N-ethylmaleimide at both temperatures. 4. Conflicting reports in the literature and the present results can be explained by invoking a model in which N-ethylmaleimide has a high affinity for a heat-labile site and a lower affinity for a heat-insensitive site. Reaction with the first site decreases agonist affinity, but at 37 degrees C this site is largely inactivated and reaction with the second site, which leads to increased agonist affinity, predominates.

Decreased receptor activation with age. Can it be explained by desensitization?

In the three endocrine/neuroendocrine systems discussed, there are demonstrable declines in post-maturational responsiveness. Parathyroid hormone stimulation of 1,25-dihydroxyvitamin D production declines with age in the kidney as does calcium absorption in the intestine. Chronotropic and inotropic responsiveness to beta-adrenergic agonists decreases with age in the myocardium, and performance on passive avoidance tasks related to memory dysfunction declines with age in rodents. In each case there is a corresponding decrease in receptor activation with age. Parathyroid hormone receptors are less able to activate adenylate cyclase in older rat kidneys; beta-adrenergic receptors have reduced density in some tissues, demonstrate reduced agonist affinity (are uncoupled), and are less able to activate adenylate cyclase in most tissues with age; and muscarinic receptors demonstrate mixed agonist affinity (are uncoupled) with age in rat hippocampal cells. This reduction in receptor activation can be attributed to desensitization to increased agonist concentrations. Parathyroid hormone receptor activation is restored by parathyroidectomy, beta-adrenergic agonists no longer desensitize in older animals, and muscarinic receptors from senescent rats pharmacologically mimic desensitized receptors. However, desensitization of receptor activation cannot fully account for the reduced hormonal responsiveness in any of these systems. Parathyroidectomy does not restore 1,25-dihydroxyvitamin D production or intestinal calcium absorption. There are age-related post receptor deficits in beta-adrenergic pathway that are not mediated by changes in serum catecholamines. In conclusion, there are significant changes in receptor and post-receptor biochemistry with age. The overall decreases in hormonal responsiveness are not due to a single biochemical defect in the system and are probably multiple in nature.

Muscarinic M1 receptors in the lateral septal area mediate cardiovascular responses to cholinergic agonists and bradykinin: supersensitivity induced by chronic treatment with atropine

The infusion of pilocarpine, acetylcholine, bradykinin and the selective M1 muscarinic agonist McNeil-A-343 into the lateral septal area produced a dose-dependent increase of arterial blood pressure and heart rate. The M1 muscarinic agonist carbamylcholine that causes a rise in arterial blood pressure when injected into the anterior lateral ventricles did not produce any cardiovascular effects when infused into the lateral septal area. Chronic treatment with atropine induced supersensitivity to the muscarinic agonists and a significant increase in the number of muscarinic receptors. In this study bradykinin failed to produce any significant change in cardiovascular activity. Pirenzepine, a M1 muscarinic blocking agent, inhibited completely the effect of both muscarinic agonists and bradykinin on cardiovascular activity. In fact, in vitro studies shows that the displacement of the binding of [3H]QNB by pirenzepine is compatible with the presence of the M1 subtype of muscarinic receptor in the lateral septal area, where it may play a major role on cardiovascular regulation.

Interaction between acetylcholine and bradykinin in the lateral septal area of the rat brain: involvement of muscarinic receptors in cardiovascular responses

The lateral septal area was used as a model to study the interaction between acetylcholine (Ach) and bradykinin on arterial blood pressure, since both mediators are present in this region. In the lateral septal area, the administration of the peptide or Ach produced a long-lasting, sympathetic-mediated increase of arterial blood pressure which was blocked by atropine. Pretreatment of the lateral septal area with hemicholinium-3, which depletes stores of acetylcholine, partially blocked the pressor effect of bradykinin but not that of Ach. Captopril--an inhibitor of kininase II--enhanced the pressor effects of bradykinin and Ach. Synaptosomal studies showed that bradykinin increased sodium-dependent, high-affinity uptake of choline and the conversion of [3H]choline to [3H]acetylcholine. Competition experiments using the highly specific muscarinic antagonist [3H]quinuclidinyl benzilate, demonstrated that bradykinin displaced the muscarinic antagonist from its receptor-ligand complexes. These results suggest that in the lateral septal area acetylcholine and bradykinin interact in a positive feed-back which amplifies pressor responses.

Molecular distinction between calf heart and brain muscarine receptors: different N-ethylmaleimide modulation of agonist binding

Muscarinic acetylcholine receptors in calf heart and forebrain membranes were identified by binding of 1-[benzilic-4,4'-3H]quinuclidinyl benzilate ([3H]QNB). We were able to solubilise these receptors with a yield of 50% of the proteins by treatment of the membranes with digitonin. The existence of two or more receptor subclasses with different agonist affinity in the heart membranes was evidenced by the shallow carbachol/[3H]QNB competition binding curves. The receptors displayed only low agonist affinity, in the presence of GTP as well as after solubilisation. The alkylating reagent N-ethylmaleimide (NEM) caused a 70-fold increase in agonist affinity for the solubilised receptors whereas GTP was ineffective. A similar difference in affinity was observed for the membranes when agonist competition curves in the presence of NEM were compared to those in the presence of GTP. NEM caused only a 2- to 3-fold increase of the agonist affinity for solubilised brain cortex membranes. These data suggest that heart and brain muscarine receptors are structurally different.

Desensitization of muscarinic acetylcholine receptors: possible relation to receptor heterogeneity and phosphoinositides

The increases in firing rates of hippocampal cells were examined following microiontophoretic application of several muscarinic cholinergic receptor agonists. The agonists studied had been pharmacologically characterized previously and divided into two classes: class A agonists (e.g. acetylcholine, carbamylcholine, and oxotremorine-M) which maximally stimulate PI turnover and reveal mAChR heterogeneity, and class B agonists (e.g. bethanecol and oxotremorine-1) which poorly stimulate PI turnover and do not alter mAChR conformation/orientation in the hippocampus. While comparable stimulatory effects on hippocampal pyramidal cell firing rates were seen with both classes of agonists during short (20 s) ejection periods, longer applications (greater than 25 s) produced class-dependent differential firing patterns. Prolonged ejection of class A agonists selectively desensitized cells to further, continued application in the same ejection period, and the firing rates declined. Class B agonists produced stimulatory responses in hippocampal cells during the entire ejection period, and DE was not observed. This desensitization effect (DE) was observed only for bursts and not for simple spikes.

Conformational changes in muscarinic receptors may produce diminished cholinergic neurotransmission and memory deficits in aged rats

Both clinical and laboratory studies suggest that age-related memory deficits may be due, at least in part, to disturbances in muscarinic acetylcholine (mAChR) receptors. In order to further evaluate this premise, the present studies examined the electrophysiological responses rates of hippocampal pyramidal cells to iontophoretically applied ACh in young, middle-age and aged animals. The relationship between age and muscarinic agonist and antagonist binding in the hippocampus was also examined. In addition, possible age-related changes in receptor-effector coupling were assessed by determining calmodulin levels and the activities of phospholipid methyl-transferase I and II. Analysis of electrophysiological data showed selective age-related decrements in the ability of ACh to alter burst rate but not simple spike rate. These age-related decreases in the efficacy of ACh to increase burst rate were not paralleled by decreases in mAChR density as assessed by 3H-QNB binding, but they were temporally paralleled by age-related changes in the ability of oxotremorine to inhibit 3H-QNB binding. In the young animals, the resultant Hill coefficients derived from these analyses approached 1, while in the middle and old aged animals, the Hill coefficients deviated significantly from 1, indicating the possible existence of 2 or more receptor states with differential affinity for oxotremorine in the 2 older age groups. When carbamylcholine was used to inhibit 3H-QNB, these complex binding patterns were seen even in the young, since carbamylcholine induces conformational/orientational changes in the mAChR while oxotremorine does not.(ABSTRACT TRUNCATED AT 250 WORDS)

Mediation of passive avoidance learning by nicotinic hippocampo-entorhinal components in young rats

Young rats, 11, 16, and 20 days of age, received bilateral injections of three antinicotinic agents into the posteroventral hippocampo-subiculo-entorhinal area, and were trained to learn a cool-draft-stimulus, passive-avoidance task shortly after (17 min). Gallamine triethiodide had no action at low doses and provoked convulsions at higher concentrations. Pempidine tartrate produced age- and dose-dependent impairments of the passive avoidance, and was much more effective in younger groups (11 and 16 days) than at 20 days. alpha-bungarotoxin also induced dose-dependent deficits. These results, together with the mecamylamine-induced deficits already reported, suggest that nicotinic cholinergic synapses located in the posteroventral part of the hippocampal complex play a role in passive-avoidance learning in the young rat as soon as this type of conditioning is possible, but become relatively less important at older ages, when muscarinic mechanisms also become involved.

Arginine and cysteine residues in the ligand binding site of alpha 2-adrenergic receptors

alpha 2-Adrenergic receptors were identified in calf brain, human platelet and human uterus membranes by [3H]-rauwolscine binding. The reagents phenylglyoxal (selective for guanidino groups), p- hydroxy mercuribenzoate and N-ethylmaleimide (selective for sulfhydryl groups) caused a time- and dose- dependent decrease in the number of receptor sites. alpha 2-Adrenergic agonists and antagonists mediated efficient protection of the receptors against these reagents. These data suggest that essential arginine and cysteine residues are present at or near the alpha 2-adrenergic binding site.

Direct and indirect effects of sulfhydryl blocking agents on agonist and antagonist binding to central alpha 1- and alpha 2-adrenoceptors

The effects of p-chloromercuribenzoate and N-ethylmaleimide were evaluated on the binding of (3H)-p-aminoclonidine, (3H)-rauwolscine and (3H)-prazosin on rat brain alpha-adrenergic receptors. Pretreatment of the particulate fraction with increasing concentrations of p-chloromercuribenzoate indicated that the binding of all three radioligands was similarly inhibited with an IC50 of about 30 microM. This effect was then reduced when agonist [(-)-norepinephrine] or antagonist (phentolamine) were present during the pretreatment. Pretreatment of the particulate fraction at N-ethylmaleimide concentrations less than 100 microM specifically decreased the (3H)-p-aminoclonidine binding while binding of antagonist was unchanged. N-ethylmaleimide produced binding changes similar to those induced by GTP in control membranes, i.e. interconversion of the alpha 2-adrenoceptors states from a high affinity to a low affinity for agonists. Norepinephrine but not phentolamine reduced the effects of N-ethylmaleimide when present during the pretreatment. Taken together, these results suggest that the alpha 1- and alpha 2-adrenoceptors possess, within or close by the recognition site, an --SH group which can be blocked at low concentrations by p-chloromercuribenzoate but not by N-ethylmaleimide. In contrast, the group alkylated by the latter does not seem to be located in the recognition site domain but rather at a site important for the coupling between the alpha 2-receptor and the GTP-binding protein.

The proliferative response of low-density human cell cultures to tumor promoters and its relevance to carcinogenic mechanisms in vitro

The effects of the tumor promoter -12-O-tetradecanoyl phorbol-13-acetate, its analogues, and other tumor promoters, on the proliferation of human skin fibroblasts (SF) have been investigated. We have previously shown (Kopelovich and Bias, 1979), that TPA caused a biphasic (upward concave) dose effect in the cloning efficiency assay of normal and mutant human fibroblastic cell strains. Here we report that the biphasic dose response pattern, consisting of an inhibitory phase and a stimulatory phase, was shared by the active analogues of TPA, e.g., phorbol-12,13-dibutyrate and phorbol-12,13-dibenzoate. This biphasic dose effect relationship, however, was not seen with phorbol-12,13-diacetate or the inactive analogues of TPA such as phorbol and 4-O-methyl-12-O-tetradecanoyl phorbol-13-acetate, nor was it seen with mezerein, teleocidin, or bile-acid derivatives of humans. An analysis of the cloning efficiency data by the median-effect equation (Chou and Talalay, 1981) showed that in low-density cultures both the inhibitory phase and the stimulatory phase of the dose-effect relationships of TPA, its analogues, mezerein, and teleocidin exhibited a linear median-effect plot and thus closely followed the basic mass-action principle. The median-effect plot of these data allowed quantitative determination of growth curve characteristic such as regression coefficient, slope (a measure of sigmoidicity), median-effect concentrations such as I50 for the inhibitory effect, and A50 for the stimulatory effect (i.e., the relative potency of the analogues) and the transition point of the biphasic phenomenon in the case of the phorbol esters. In addition, we have demonstrated a relationship between the dose response effect of TPA on the proliferation of various human cells and tumor progression in vitro.

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

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

Agonist mediated conformational changes of solubilized calf forebrain muscarinic acetylcholine receptors

Muscarinic receptors in calf forebrain membranes can be identified by the specific binding of the radiolabelled antagonist [3H]dexetimide. These receptors (2.8 pM/mg protein) comprise two non-interconvertible subpopulations with respectively high and low agonist affinity but with the same antagonist affinity. For all the agonists tested the low affinity sites represent 85 +/- 5% of the total receptor population. 0.5% Digitonin solubilized extracts contain 0.8 pM muscarinic receptor/mg protein. In contrast with the membranes, these extracts contain only sites with low agonist affinity. The alkylating reagent N-ethylmaleimide causes an increase of the acetylcholine affinity for the low affinity sites in membranes as well as for the solubilized sites. This effect is time dependent until a maximal 3-fold increase in affinity is attained. The rate of N-ethylmaleimide action is enhanced by the concomitant presence of agonists. In contrast, N-ethylmaleimide does not affect antagonist binding. This suggests that agonists mediate a conformational change of both the membrane bound low affinity muscarinic sites and of the solubilized sites, resulting in their increased susceptibility towards NEM alkylation.

Monoclonal antibodies to purified muscarinic receptor display agonist-like activity

Monoclonal antibody M-35, which immunoprecipitates native calf brain acetylcholine muscarinic receptor, mimics agonist stimulation of the intact guinea pig myometrium: the antibody, just like carbamoylcholine hydrochloride, causes a rise in intracellular cyclic GMP content, an inhibition of cyclic AMP accumulation due to prostacyclin, and induces uterine contractions. Another antibody, M-23, which reacts with the denatured muscarinic receptor, is devoid of agonist-like activity at the cyclic nucleotide level but is still able to induce contractions of both rat and guinea pig myometrium. The cyclic nucleotide changes caused by both carbamoylcholine and antibody M-35 are inhibited by atropine; this antagonist, which blocks carbamoylcholine-mediated contractions, fails however, to prevent contractions induced by antibodies M-35 and M-23. These results suggest that the information necessary to transmit muscarinic signals is entirely contained in the receptor and that ligands only act to trigger the biological response. The data also imply that the muscarinic receptors of the myometrium are coupled to multiple effector systems.

Differentiation between ligand trapping into intact cells and binding on muscarinic receptors

Binding properties of [3H] dexetimide , L-quinuclidinyl[phenyl-4-3H] benzilate and [3H]methylscopolamine were compared with intact 108 CC 15 cells and membrane preparations of those. The ability of the three ligands to label specifically muscarinic receptors on membrane fractions was quite similar. By contrast, when performed with intact cells, [3H] dexetimide and L-quinuclidinyl [phenyl-4-3H]benzilate revealed higher nonspecific binding which was prevented by methylamine, suggesting a trapping of the ligands within the cells presumably in the lysosomes. To the contrary, such nonspecific 'binding' or trapping was not detectable when [3H]methylscopolamine was used as ligand, a fact which makes this ligand particularly appropriate for labelling cell surface muscarinic receptors. It is concluded that more caution is needed in binding studies when performed with intact cells; indeed, besides specific binding on receptor sites, [3H]ligand can be entrapped within the cell and can even sometimes give the illusion of specific binding. The use of lysosomal agents which do not interfere with specific receptors on membrane preparations should allow one, in most cases, to discard the possibility of a trapping phenomenon in intact cells.

Copper ions and diamide induce a high affinity guanine-nucleotide-insensitive state for muscarinic agonists

The binding capacity of [3H]-acetylcholine for muscarinic receptors of rat cerebral cortex membranes is increased in the presence of Cu2+ ions from 690 to 1320 fmol/mg protein with no significant change in affinity. Membranes treated with 50 microM Cu2+ and washed retain the increased binding capacity. Agonist binding in copper-treated membranes is insensitive to guanylylimidodiphosphate even at high concentrations (greater than 200 microM). Similar results were obtained when the sulfhydryl oxidizing agent, diamide (2 mM) was substituted for Cu2+ in the treatment of membranes. These data suggest the involvement of inter- or intra-molecular SH/S-S transitions in the interaction between the muscarinic receptor and a guanine nucleotide binding regulatory protein.

Quantitative analysis of dose-effect relationships: the combined effects of multiple drugs or enzyme inhibitors

A generalized method for analyzing the effects of multiple drugs and for determining summation, synergism and antagonism has been proposed. The derived, generalized equations are based on kinetic principles. The method is relatively simple and is not limited by whether the dose-effect relationships are hyperbolic or sigmoidal, whether the effects of the drugs are mutually exclusive or nonexclusive, whether the ligand interactions are competitive, noncompetitive or uncompetitive, whether the drugs are agonists or antagonists, or the number of drugs involved. The equations for the two most widely used methods for analyzing synergism, antagonism and summation of effects of multiple drugs, the isobologram and fractional product concepts, have been derived and been shown to have limitations in their applications. These two methods cannot be used indiscriminately. The equations underlying these two methods can be derived from a more generalized equation previously developed by us (59). It can be shown that the isobologram is valid only for drugs whose effects are mutually exclusive, whereas the fractional product method is valid only for mutually nonexclusive drugs which have hyperbolic dose-effect curves. Furthermore, in the isobol method, it is laborious to find proper combinations of drugs that would produce an iso-effective curve, and the fractional product method tends to give indication of synergism, since it underestimates the summation of the effect of mutually nonexclusive drugs that have sigmoidal dose-effect curves. The method described herein is devoid of these deficiencies and limitations. The simplified experimental design proposed for multiple drug-effect analysis has the following advantages: It provides a simple diagnostic plot (i.e., the median-effect plot) for evaluating the applicability of the data, and provides parameters that can be directly used to obtain a general equation for the dose-effect relation; the analysis which involves logarithmic conversion and linear regression can be readily carried out with a simple programmable electronic calculator and does not require special graph paper or tables; and the simplicity of the equation allows flexibility of application and the use of a minimum number of data points. This method has been used to analyze experimental data obtained from enzymatic, cellular and animal systems.

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

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