Muscarinic receptor binding in mouse brain: regulation by guanine nucleotides

Modulatory role of dopamine on excitatory amino acid receptors

1. In extensively washed synaptic membrane preparations from rat prefrontal cortex, the "in vitro" addition of either the D1 (SKF 38393) or the D2 (LY 171555) specific agonists markedly decreased the apparent affinity of the NMDA receptor antagonist [3H]-MK801 specific binding. In the same membrane preparation, the concentration of L-glutamate required to produce half maximal enhancement of [3H]-MK801 binding was approximately the same both in the presence or in the absence of dopaminergic drugs. 2. I.c.v. administration of the neurotoxin 6-OHDA resulted in a dramatic reduction of dopamine (DA) prefrontal cortex levels, whilst repeated administrations (21 consecutive days) with either the D1 (SCH 23390) or the D2 (YM 09151-2) selective antagonist failed to change DA and DOPAC contents. 3. Repeated administrations with the D1 receptor blocker SCH 23390 selectively increased the Bmax values of [3H]-SCH 23390 binding while [3H]-spiroperidol binding was increased both by repeated administrations of YM 09151-2 and by i.c.v. injection of 6-OHDA. 4. Although both chronic D2 blockade and 6-OHDA lesions consistently increased D2 receptor number, in extensively washed synaptic plasma membranes (SPM) of rats repeatedly administered with YM 09151-2 but not with 6-OHDA, the [3H]-MK801 binding was increased. 5. It is concluded that the effects of NMDA receptor activation could not be directly mediated by stimulation of DA release, but are highly dependent upon the presence of DA axon terminals.

GNAZ in human fetal cochlea: expression, localization, and potential role in inner ear function

Dissociation of an activated alpha-subunit from the beta-gamma complex directly regulates secondary messenger proteins. To address the potential role of G proteins expressed in human fetal cochlea, degenerate oligonucleotide primers corresponding to the 3'-end of the conserved region of alpha-subunits were used for polymerase chain reaction amplification of reverse-transcribed total human fetal cochlear mRNAs; GNAZ and GNAQ were isolated. These two G proteins are unique among the G-protein family because they lack a typical pertussis modification site. GNAZ is expressed in high levels in neural tissue while GNAQ is ubiquitously expressed. We characterized GNAZ expression using Northern blots, tissue in-situ hybridization and immunohistochemistry techniques to elucidate the potential role of this protein in inner ear function. Our data suggest that GNAZ may play a role in maintaining the ionic balance of perilymphatic and endolymphatic cochlear fluids.

Agonist-induced muscarinic cholinergic receptor internalization, recycling and degradation in cultured neuronal cells. Cellular mechanisms and role in desensitization

Short-term incubation of intact neuronal cells with muscarinic cholinergic agonists resulted in a rapid decrease of the specific binding of [3H]methylscopolamine to cell surface receptors indicative of receptor internalization. The agonists induced the internalization of both the muscarinic receptor subtypes coupled to adenylyl cyclase and those coupled to phosphoinositide turnover. Receptor internalization, which was inhibited at 0-4 degrees and by depletion of intracellular K+, is thought to occur through coated pits formation and was rapidly reversible. Receptor recycling did not imply protein synthesis. Down-regulation of muscarinic receptors occurred slowly in the presence of agonists, needed intact cytoskeleton (demonstrated by the inhibitory effect of colchicine) and involved lysosomal activity. Both receptor internalization and down-regulation were prevented by muscarinic receptor antagonists. Receptor internalization and down-regulation are agonist-induced cellular mechanisms that with receptor phosphorylation and uncoupling, may induce desensitization. These processes may contribute to complex intracellular regulatory processes and may be involved in some of the long-term effects of neurotransmitters (mainly neuropeptides and growth hormones) or drugs.

Selective amplification and partial sequencing of cDNAs encoding G protein alpha subunits from cochlear tissues

An approach utilizing the polymerase chain reaction (PCR) was devised to clone members of a family of cDNAs encoding the alpha subunit of G proteins in the cochlea. RNA was extracted from the whole cochlea of the mouse and from the organ of Corti or the lateral wall of the cochlea microdissected from the guinea pig cochlea. The RNA was reverse-transcribed to cDNA which was selectively amplified by PCR using degenerate primers corresponding to two conserved regions of the G protein coding sequence. PCR products were cloned into a plasmid for sequencing. The following seven cDNA clones of particular interest were obtained: three clones putatively coding for part of the alpha-subunit of a stimulatory G protein (Gs), one clone putatively coding for part of the alpha-subunit of an inhibitory G protein (Gi) and three clones putatively coding for part of the alpha-subunit of a transducin (Gi)-like protein. Possible functions in the cochlea of putative G proteins with alpha-subunits partly encoded by these cDNA clones are briefly discussed and future studies are suggested.

Effects of GTP?S and other nucleotides on phosphoinositide metabolism in crude rat brain synaptosomal preparations

Crude nerve-ending preparations from rat brain were labeled with either [(32)P]phosphate or myo[2-(3)H]inositol in order to observe effects of guanosine 5?-[?-thio]triphosphate (GTP?S) and other nucleotides on phosphoinositides, phosphatidate and inositol phosphates. This system exhibited typical responses to muscarinic agonists, including acetylcholine-a decrease in net labeling of [(32)P]polyphosphoinositides, an increase in labeling of [(32)P]phosphatidate, and a stimulation of [(3)H]inositol phosphate formation. GTP?S and other nucleotides may not readily penetrate intact synaptosomal membranes to cause activation of phospholipase C via an interaction with G proteins, and, as might be expected, there was no indication that G-protein interaction occurred in these preparations. However, other effects were observed. GTP?S decreased net [(32)P] incorporation in phosphatidylinositol (PI) and polyphosphoinositides in a dose-dependent manner. GTP?S also caused an initial marked stimulation of [(32)P] labeling of phosphatidate, suggesting a possible inhibition in the conversion of phosphatidic acid to PI. Other nucleotides [GTP, ATP, Gpp(NH)p, GMP] produced qualitatively similar effects on phosphoinositides. Thus GTP?S and other nucleotides, at physiologically relevant concentrations, may influence phosphoinositide turnover via extracellular or other mechanisms, in addition to the proposed interaction of GTP with G-proteins within membranes.

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

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

A model for the interaction of muscarinic receptors, agonists, and two distinct effector substances

The binding of the agonist carbamylcholine to muscarinic receptors in rat heart myocytes from young and aged cultures and in rat atrial membranes has been measured in the absence and presence of GppNHp, pertussis toxin, and/or batrachotoxin. The effect of each of the added substances upon agonist binding was accounted for by a model according to which the receptor may form an equilibrium complex with agonist and either of two distinct effector substances, one of which is postulated to increase the affinity of receptor for agonist and the other of which is postulated to decrease the affinity of receptor for agonist.

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.

Aging of rat heart myocytes disrupts muscarinic receptor coupling that leads to inhibition of cAMP accumulation and alters the pathway of muscarinic-stimulated phosphoinositide hydrolysis

The biochemical responses to muscarinic stimulation (inhibition of isoproterenol-stimulated cAMP accumulation and stimulation of phosphoinositide turnover) were investigated in intact myocyte cultures prepared from the hearts of newborn rats. The studies employed young (5 days after plating) and aged (14 days old) myocyte cultures. Aging of the myocyte cultures was accompanied by marked alterations in both the inhibition of cAMP accumulation and the stimulation of the phosphoinositide metabolism via the muscarinic receptors. However, the effects on the two muscarinic responses were different. The first response was disrupted at the level of the coupling of the muscarinic receptors with adenylate cyclase through Gi. On the other hand, muscarinic stimulation of phosphoinositide hydrolysis still occurred in the aged myocyte cultures; however, the inositol trisphosphate generated was not converted to inositol 1-phosphate as in young cultures or as in aged cultures stimulated by norepinephrine. This raises the possibility that muscarinic activation of aged myocyte cultures shifts the metabolic state of the cells and alters the pathway of phosphoinositide hydrolysis. Treatment of aging cultures with phosphatidylcholine liposomes under conditions that yielded aged myocyte cultures with a lipid composition resembling that of young ones restored the muscarinic effect on cAMP accumulation, where the impairment in aged cultures was at the coupling stage (which takes place in the plasma membrane). This treatment had no effect on the response of the phosphoinositide metabolism to muscarinic stimulation.

Increases in muscarinic stimulated hydrolysis of inositol phospholipids in rat hippocampus following cholinergic deafferentation are not parallelled by alterations in cholinergic receptor density

The effect of electrolytic fimbria/fornix lesions on muscarinic receptor subtypes and putative nicotinic binding sites in the hippocampus has been studied using [3H]N-methylscopolamine, [3H]pirenzepine and [3H]L-nicotine, respectively. In parallel experiments the carbachol-stimulated hydrolysis of inositol phospholipids was measured after incorporation of [3H]inositol into hippocampal slice preparations. Ten days after lesioning there were no apparent changes in either receptor density or affinities despite extensive reductions in choline acetyltransferase. In contrast a significant increase in carbachol stimulated turnover of inositol phospholipids was measured. These observations suggest that whilst loss of cholinergic afferents may not affect receptor density per se, the efficacy of the post synaptic muscarinic receptors can be up-regulated at least in the short term.

Two affinity states of M1 muscarine receptors

1. The binding of oxotremorine-M to M1 muscarine receptors was examined by measuring competition between the agonist and 3H-pirenzepine, using rabbit hippocampal membranes suspended in 20 mM Tris buffer containing 1 mM Mn2+. 2. Both ligands interacted with a single class of receptors. The receptors could assume two affinity states for oxotremorine-M, with equal numbers of high-affinity (KH) and low-affinity (KL) sites. 3. KH interconverted reversibly to KL in the absence of divalent cations and interconverted reversibly to a state similar to KL in the presence of guanyl 5'-yl imidodiphosphate. 4. The results are compatible with a model in which a pair of receptor molecules can be stabilized by a guanine nucleotide-binding "G protein" and have one site each of KH and KL affinity.

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.

Temperature as a dependent variable in the study of cholinergic mechanisms

1. Change in core temperature over time can be used as a dependent variable when studying the effects of manipulations on neurotransmitter systems. This article focuses on the measurement of core temperature as a strategy for detecting changes in the status of cholinergic systems. 2. Cholinergic neurons participate in the process of thermoregulation and interventions affecting them alter the thermal response to cholinomimetics. For example, chronic treatment with amitriptyline, chronic swim stress and inescapable footshock supersensitize rats to the hypothermic effects of oxotremorine. 3. This is consistent with the hypothesis that the pathophysiologies of tricyclic antidepressant withdrawal phenomena and stress involve supersensitivity of muscarinic mechanisms. 4. Uses of thermoregulation paradigms for investigating the actions of lithium ion, electroconvulsive shock and substances of abuse on muscarinic mechanisms are discussed. Applications to problems in the arena of clinical research are highlighted.

Different agonist binding properties of M1 and M2 muscarinic receptors in calf brain cortex membranes

The muscarinic antagonist 1-[benzilic 4,4'-3H]-quinuclidinyl benzilate [3H]-QNB) bound to a single class of non-cooperative sites in calf cerebral cortex membranes (KD = 0.29 nM and Bmax = 1.06 pM/mg protein). Computer-assisted analysis of the shallow pirenzepine/[3H]-QNB competition binding curves indicated that 68% of these sites were of the M1-subtype and the remaining 32% of the M2 subtype. Respective Ki-values for pirenzepine were 27 nM and 1.14 microM. Binding characteristics of the antagonist atropine and of the agonist carbachol for M2 were evaluated by performing competition binding with 0.5 nM [3H]-QNB in the presence of 2 microM pirenzepine. The binding characteristics for the M1 receptors were obtained indirectly by subtracting the curve for M2 from the total curve, or directly by competition binding with 0.3 nM [3H]-pirenzepine. Atropine competition curves were steep for M1 and M2 and were not affected by 1 mM GTP nor by 1 mM N-ethylmaleimide. The carbachol competition curve was shallow for M2. The steep curves for M1 indicate that this receptor subclass was only composed of low agonist affinity sites. GTP, which caused a rightward shift and a steepening of the carbachol competition curve for M2, did not affect the curves for M1. N-ethylmaleimide provoked a leftward shift and a steepening of the carbachol competition curve for M2 and abolished GTP modulation. A leftward shift was also observed for M1, but of a smaller magnitude (i.e. 3-4-fold for M1 compared to 17-fold for M2). These data suggest that, in calf brain cortex, M1 and M2 receptors show different susceptibility towards GTP and N-ethylmaleimide modulation.

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.

Guanine nucleotide regulation of receptor binding of thyrotropin-releasing hormone (TRH) in rat brain regions, retina and pituitary

Guanine nucleotides inhibited the specific binding of [3H](3-Me-His2)thyrotropin-releasing hormone ([3H]MeTRH) to receptors for TRH in washed homogenates of rat anterior pituitary gland in a dose-related manner. The order of potency (at 100 and 500 microM final) was Gpp(NH)p (a stable analog of GTP) greater than GTP much greater than GDP much greater than cGMP (with the adenine nucleotides being inactive) in the pituitary and various brain regions. Gpp(NH)p at 1 mM caused 17-35% inhibition of [3H]MeTRH binding to different tissues including the pituitary, hypothalamus, retina and nucleus accumbens. A statistically significant nucleotide effect was not observed, however, in the olfactory bulb and medulla/pons membranes. Gpp(NH)p (1 mM) increased the dissociation constants for [3H]MeTRH binding by 1.9- to 2.4-fold in the pituitary, n. accumbens and retinal preparations without altering the apparent binding capacity. These data suggest that TRH receptor binding can be allosterically regulated by guanine nucleotides and provide further evidence for the existence of guanine nucleotide binding protein(s) coupled to the TRH receptor.

Ontogeny of muscarinic receptors in the rat brain with emphasis on the differentiation of M1- and M2-subtypes--semi-quantitative in vitro autoradiography

The ontogeny of muscarinic acetylcholine receptors (mAChR) in the rat brain was studied with emphasis on the differentiation of M1- and M2-receptor subtypes through semi-quantitative in vitro autoradiography. [3H]Quinuclidinyl benzilate (QNB) and [3H]pirenzepine (PZ) were used for labeling total mAChR and M1-receptors, respectively. In the cerebral cortex of adult rats, [3H]QNB binding sites were more richly present in the superficial and deeper layers than in the middle layer, while M1-receptors were diffusely observed in all the layers. This means that M2-receptors are highly concentrated in the superficial and deeper layers. The ontogenetical differentiation of the laminar distribution between M1- and M2-receptors first appeared at 14 days of postnatal age. In the hippocampus and striatum whose mAChR were predominantly of the M1-type in the adult rat brain, ontogenic patterns of M1-receptors were almost identical to those of total mAChR. On the other hand, mAChR in the cerebellar cortex and lower brainstem of the adult rat were mainly of the M2-subtype. In these areas, the ontogeny of total mAChR was apparently observed. However, M1-receptors were not observed at any stage of the ontogeny. The above-mentioned results indicate that M1- and M2-receptors show distinct developmental behaviors in the rat brain.

Temperature-dependence and heterogeneity of muscarinic agonist and antagonist binding

The binding parameters of muscarinic agonists and antagonists in rat central (brain, cerebellum and striatum) and peripheral (heart and lung) tissues were determined at 2 degrees and 37 degrees from competitive binding experiments with (3H)-quinuclidinylbenzylate (QNB). Muscarinic ligands binding affinities for cerebellum, heart and lung were tightly correlated. These tissues were also characterized by their low concentration of muscarinic receptors with high affinity for agonists. In contrast, brain and striatum contained a higher concentration of muscarinic receptors with a lower affinity for agonists. The affinity of QNB was lower at 2 degrees whereas that of other ligands was higher. The temperature-dependent shifts of competition curves differed from tissue to tissue and from compound to compound. The shifts were highest with gallamine and carbachol. The binding isotherms of muscarinic ligands were tentatively studied with a two-site binding model. The percentage of high- and low-affinity binding components of agonists differed with the compound. Guanine nucleotides and the temperature increase lowered agonist affinity without changing the proportions of the high- and low-affinity binding components. These results corroborate that the binding heterogeneity of muscarinic ligands does not depend only on the presence of two distinct receptors. Neither guanine nucleotides nor temperature changes allow conversion between the different putative conformational states of the muscarinic receptors.

Agonist and antagonist binding to rat brain muscarinic receptors: influence of aging

The objective of the present study was to determine the binding properties of muscarinic receptors in six brain regions in mature and old rats of both sexes by employing direct binding of [3H]-antagonist as well as of the labeled natural neurotransmitter, [3H]-acetylcholine [( 3H]-AcCh). In addition, age-related factors were evaluated in the modulation processes involved in agonist binding. The results indicate that as the rat ages the density of the muscarinic receptors is altered differently in the various brain regions: it is decreased in the cerebral cortex, hippocampus, striatum and olfactory bulb of both male and female rats, but is increased (58%) in the brain stem of senescent males while no significant change is observed for females. The use of the highly sensitive technique measuring direct binding of [3H]-AcCh facilitated the separate detection of age-related changes in the two classes (high- and low-affinity) of muscarinic agonist binding sites. In old female rats the density of high-affinity [3H]-AcCh binding sites was preserved in all tissues studied, indicating that the decreases in muscarinic receptor density observed with [3H]-antagonist represent a loss of low-affinity agonist binding sites. In contrast, [3H]-AcCh binding is decreased in the hypothalamus and increased in the brain stem of old male rats. These data imply sexual dimorphism of the aging process in central cholinergic mechanisms.

Is the effect of somatostatin on muscarinic receptors selective to M1 type?

The effect of somatostatin on muscarinic receptors (mAchR) was investigated through saturation experiments of [3H]oxotremorine-M-acetate and oxotremorine/[3H]N-methyl-scopolamine competition experiments. Somatostatin converted oxotremorine high affinity binding sites to low affinity sites in the hippocampus and cerebral cortex whose mAchR were dominantly of M1 type. Somatostatin did not alter agonist binding in the medulla-pons where M2 receptors were abundant. Therefore, the effect of somatostatin on mAchR seems to be selective to high affinity binding sites of M1 receptors.

Pathophysiology of "cholinoceptor supersensitivity" in affective disorders

Phenomenological and physiological variables demonstrate supersensitive changes to cholinergic challenge in affective disorder subjects. Theorists generally assume the primary defect is the postsynaptic muscarinic receptor. However, in addition to defectiveness or up-regulation of this receptor, the appearance of postsynaptic "cholinoceptor supersensitivity" can result from abnormal presynaptic mechanisms, membrane "pathology," derangement of intrasystolic mechanisms that amplify effects of receptor-agonist coupling, or aberrant cholinergic-monoaminergic interaction. This article discusses abnormalities of the postsynaptic receptor, regulation of postsynaptic receptor density, the presynaptic muscarinic receptor, and other mechanisms regulating the release of acetylcholine, membrane dynamics, and "cascade" mechanisms-specifically the phosphatidylinositol (PI) cycle, Ca2+ mobilization, and cyclic guanosine monophosphate (GMP) generation-as causes of cholinergic system "supersensitivity." It is suggested that an approach to the topic emphasizing site of abnormality will encourage greater clarity of thought in the study of the cholinergic component of the pathophysiology of affective illness.

Muscarinic cholinergic receptor subtypes in the rat brain. I. Quantitative autoradiographic studies

The binding characteristics of N[3H]methylscopolamine ([3H]NMS) to slide-mounted tissue sections were studied using quantitative autoradiography. Binding of [3H]NMS was saturable, reversible and of high affinity (Kd = 0.26 nM). The inhibition of [3H]NMS binding produced by several muscarinic agonists and antagonists was analyzed in 29 discrete brain regions by constructing complete displacement curves. Comparison of IC50 values obtained both biochemically and by autoradiography demonstrated a very close agreement, supporting the validity of the autoradiographic approach. The competition curves for the agonists carbachol, oxotremorine and 2-ethyl-8-methyl-2,8-diazaspiro-[4,5]-decan-1,3-dion-h ydrobromide (RS 86) fitted to a two-site model, with comparable affinity values from region to region, although different proportions of high- and low-affinity sites were seen in the different areas studied. The distribution of high- and low-affinity sites was similar for the three agonists. Atropine showed monophasic curves presenting similar affinities in all regions studied. In contrast, pirenzepine differentiated between high- and low-affinity sites which showed a distribution opposite to that observed for the agonists. Gallamine, a ligand for a putative regulatory site in the muscarinic receptor, inhibited [3H]NMS binding in a biphasic manner. The calculated IC50 values for the gallamine high- and low-affinity sites did not vary from region to region and the distribution of these sites correlated well with that observed for the agonists. High-affinity pirenzepine sites (also called M1 sites) were localized mainly in forebrain areas, such as striatum, hippocampus and cortex, and their regional distribution correlated with that of the low-affinity sites for the agonists and gallamine. On the other hand, low-affinity sites for pirenzepine (named M2 sites) were mainly found in the brainstem and parts of the thalamus. A good correlation was found between pirenzepine low-affinity sites and agonist and gallamine high-affinity sites. The significance of these findings is discussed in relation to the known and possible effects of selective M1 and M2 centrally acting agents.

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.

Heterogeneity of solubilized muscarinic cholinergic receptors: binding and hydrodynamic properties

Previous studies have described the conversion, after detergent solubilization, of the multiple populations of membrane-bound muscarinic agonist binding sites to a population of uniform affinity. This paper describes the solubilization of at least two receptor species, distinct in their agonist binding characteristics, which are capable of interconversion by transition metal ions. This finding enabled a more detailed examination of the molecular properties and regional differences of brain muscarinic receptors than was previously possible. Muscarinic receptors (mAChR) obtained from the rat cerebral cortex or medulla pons were solubilized using digitonin or the zwitterion detergent, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (Chaps). The equilibrium binding of the antagonist [3H]-4-N-methylpiperidyl benzilate ([3H]4NMPB) to detergent-solubilized receptors resembled binding to neural membranes and exhibited subnanomolar affinity, saturability, and simple mass action kinetics. Agonist binding to soluble preparations was measured by competition of [3H]4NMPB binding sites. Saturation isotherms for agonist binding to digitonin- and Chaps-solubilized mAChR obtained from various brain regions appear flattened and have Hill coefficients in the range 0.52-0.78. Computerized modelling techniques indicate that the best fit to the experimental data is provided by a model specifying two soluble muscarinic agonist binding sites with differing dissociation constants, KH and KL, respectively. Solubilization of cerebral cortex membranes with Chaps or digitonin resulted in a population with a composition of high- and low-affinity sites similar to that found in the membrane-bound state. In contrast, solubilization of the medulla pons resulted in an approximately 40% loss of high-affinity sites. Solubilized receptors retained the sensitivity to transition metals ions, but were insensitive to guanine nucleotides. Density gradient centrifugation indicated that Chaps-solubilized mAChR are composed of two molecular forms with S20,W equal to 9.9 S and 14.9 S. The 14.9 S species comprises approximately 30% of the total binding activity in the cortex and approximately 40% in the medulla. We identify the 14.9 S species as being associated with a guanylnucleotide binding protein because treatment of medulla membranes with guanylylimidodiphosphate prior to solubilization results in disappearance of 14.9 S with 9.9 S unchanged. Sedimentation of cortical mAChR in the presence of Cu+2 leads to an increase in 14.9 S to almost 50% of the total binding activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of submaxillary gland muscarinic receptors during heat acclimation

Binding properties of submaxillary gland muscarinic receptors and agonist-induced saliva secretion were studied in rats subjected to heat acclimation. The maximal binding capacity for the muscarinic antagonist N-[3H]methyl-4-piperidyl benzilate was increased from control value of 0.21 to 0.40 pmol/mg protein within 1-2 days of heat acclimation. The increase in the number of muscarinic receptors per gland (100%) was by far higher than the increase in tissue weight (20%), indicating higher density of receptors in the acinar cells of the treated rats. High levels of receptors coincided with the appearance of high-affinity binding sites for muscarinic agonists (oxotremorine, pilocarpine and carbamylcholine), and with reduced tissue sensitivity to pilocarpine. After 4-8 weeks of heat acclimation, the number of receptors as well as tissue response to pilocarpine returned to control levels. These results suggest a functional correlation between the transient upregulation muscarinic receptors in the submaxillary gland and the physiological activity in salivary secretion, and indicate that the high-affinity muscarinic receptors may attenuate saliva secretion during the initial phase of heat acclimation.

Allosteric interactions between muscarinic agonist binding sites and effector sites demonstrated by the use of bisquaternary pyridinium oximes

Agonist binding to muscarinic receptors from rat brain stem and cerebral cortex was studied using bisquaternary pyridinium oximes for detecting possible interactions between agonist binding sites and sites of the effector guanosine 5' (beta, gamma-imino) triphosphate (Gpp(NH)p) and Co2+. Pretreatment of either brain stem or cortical homogenates with 200 microM 1-(2-hydroxyiminoethylpyridinium) 1-(3-phenylcarboxypyridinium) dimethylether (HGG-12) reduced the affinity of muscarinic agonists. No change was observed in the relative proportions of high (RH) and low (RL) affinity agonist binding sites. However, the oxime affected the processes of interconversion between these sites. Thus, unlike in control membranes, HGG-12 treated brain stem membranes, Gpp(NH)p could not induce conversion of RH to RL, and in cortical membranes Co2+ could not induce conversion of RL to RH. These results suggest that HGG-12 inactivates a component which is involved in both processes of induced-interconversion. Induced-interconversion between RH and RL was not affected in membranes treated with HGG-12 in the presence of carbamylcholine in concentrations at which mainly RH is occupied by the agonist. The occupation of RH by carbamylcholine protected both RH and RL from the effects of the oxime. The possible role of the molecular events involved is discussed.

Ultrahistochemical localization of adenylate cyclase activity in the electric organ of Torpedo marmorata

The lead pyrophosphate precipitation technique was used to visualize adenylate cyclase activity with the electron microscope in unfixed electric organ and synaptosomes of Torpedo marmorata, with special attention to presynaptic membranes. Specificity of the deposition of reaction product was ensured by using 5'-adenylyl imidodiphosphate as substrate and 5'-guanylyl imidodiphosphate and sodium fluoride as activators. Under suitable conditions a reaction product was deposited on the Schwann cell, on presynaptic vesicles, on the inner side of membranes of cisternae and on glycogen granules of the presynaptic region of the endplate. In some cases, a precipitate was also found on postsynaptic membranes of the synaptic cleft and on mitochondria. In isolated synaptosomes localization of the reaction product was identical with that of minced tissue. However, most strikingly, on presynaptic membranes no precipitate was ever found, neither in pieces of electric organ nor in isolated synaptosomes. Furthermore, the extended membrane system of the postsynaptic region of the electroplax remained always free of lead pyrophosphate precipitate.

In vivo and in vitro studies on the potentiation of muscarinic receptor stimulation by alaproclate, a selective 5-HT uptake blocker

Alaproclate (10-60 mg/kg) injected i.p. into male mice potentiated and prolonged the oxotremorine and physostigmine-induced tremor in a dose-dependent manner. Atropine completely blocked the tremor caused by oxotremorine or physostigmine both in the presence and absence of alaproclate. Pretreatment with the 5-HT receptor antagonist metitepine completely blocked the enhancement of oxotremorine-induced tremor caused by alaproclate. Biochemical studies indicated that the above effects cannot be explained by assuming that alaproclate a) acts as a cholinergic agonist, b) inhibits the acetylcholine esterase, c) interferes with choline uptake or acetylcholine synthesis, or d) directly potentiates the release of acetylcholine. In ligand binding studies alaproclate was found to be a weak competitive inhibitor of muscarinic antagonist binding to membranes from the rat cerebral cortex, rat striatum, human cerebral cortex and human striatum. (Ki approximately 28-40 microM in all four tissues). The present results suggest that alaproclate may potentiate muscarinic responses by a mechanism involving serotonergic receptor mechanisms rather than by a direct interaction with the muscarinic cholinergic receptors.

Muscarinic cholinergic receptor in the human heart evidenced under physiological conditions by positron emission tomography

The muscarinic receptor was studied in vivo in the human heart by a noninvasive method, positron emission tomography (PET). The study showed that the binding sites of 11C-labeled methiodide quinuclidinyl benzilate [( 11C]-MQNB), a muscarinic antagonist, were mainly distributed in the ventricular septum (98 pmol/cm3 of heart) and in the left ventricular wall (89 pmol/cm3), while the atria were not visualized. A few minutes after a bolus intravenous injection, the concentration of [11C]MQNB in blood fell to a negligible level (less than 100th of the concentration measured in the ventricular septum). When injected at high specific radioactivity, the concentration of [11C]MQNB in the septum rapidly increased and then remained constant with time. This result was explained by rebinding of the ligand to receptors. It was the major difference observed between the kinetics of binding of [11C]MQNB to receptor sites after intravenous injection in vivo and that of [3H]MQNB to heart homogenates in vitro. The MQNB concentrations in the ventricular septum of different individuals were found to be highest when the heart rate at the time of injection was slow. This result suggests that the antagonist binding site is related to a low-affinity conformational state of the receptor under predominant vagal stimulation. Thus, positron emission tomography might be the ideal method to study the physiologically active form of the muscarinic acetylcholine receptor in man.

Fatty acid incorporation increases the affinity of muscarinic cholinergic receptors for agonists

Incorporation of unsaturated fatty acids into membrane fragments from rat brain cortex and medulla pons selectively increased the affinity of the muscarinic agonist, carbamylcholine. The affinity and number of binding sites for the labeled antagonist, N-[3H]methyl-4-piperidyl benzilate was unchanged. The effect on agonist binding was most prominent in the cortex, in which carbamylcholine IC50 values were decreased up to 5-fold. Selectivity of the effect was observed with fatty acids of chain length 18-20 carbons, unsaturation in position 11-12, and a cis conformation of the double bond being most effective. The effects of fatty acids on agonist binding were due primarily to alterations in the affinity constants for the binding reaction, with minor increases in the proportion of high-affinity sites. Transition metals selectively increased the percentage of high-affinity sites in the cortex, but in cis-vaccenic-acid-treated membranes more than additive effects of the metal were observed; both were reversed by GTP. GTP also reversed binding parameters in cis-vaccenic-acid-treated medulla membranes to control level. We conclude that the primary effect of the active fatty acids is to alter the thermodynamic properties of muscarinic agonist binding without markedly inducing interconversion.

Effects of copper on the binding of agonists and antagonists to muscarinic receptors in rat brain

Studies were performed to assess the effects of copper treatment in vitro on muscarinic binding parameters in rat brain homogenates. Brainstem, an area low in copper, was found to be insensitive to copper treatment as compared to forebrain, a region of relatively high copper content. Inclusion of 3 microM copper in forebrain homogenates decreased the number of sites seen by [3H]-l-quinuclidinyl benzilate (QNB) by 40-50%. Copper-enhanced displacement of bound QNB was noted for agonists and antagonists. Both ligands showed maximal effects at 6 microM copper, although quantitative differences could be determined at any copper level. At levels of maximal effect, the increase in QNB displacement was greater than or less than 50% for agonists and antagonists respectively. Two-site analyses of carbamylcholine (CCH) binding showed that the addition of 1 microM copper to forebrain homogenates increased the percentage of high affinity sites (alpha) from 42 to 70%. The IC50 decreased from 3.1 to 1.7 microM, but the dissociation constants for the high and low affinity sites were not changed. The effect of added copper on CCH binding to muscarinic receptors was reversible with the addition of the copper-chelating agent triethylene tetramine.

Guanine nucleotide sensitivity of muscarinic acetylcholine receptors from rat brainstem is eliminated by endogenous proteolytic activity

The sensitivity to guanine nucleotides of agonist binding to muscarinic acetylcholine receptors was eliminated by incubating rat brainstem membranes at 37 degrees C for 30 min. Pretreatment with any of a variety of proteinase inhibitors prevented this loss of sensitivity. In contrast to other treatments which inactivate guanine nucleotide regulatory mechanisms of muscarinic receptors, incubation at 37 degrees C did not alter agonist binding measured in the absence of guanine nucleotides. Endogenous proteolytic activity appears to inactivate the nucleotide regulatory subunit without engendering its dissociation from the receptor binding subunit.

Recognition of the muscarinic receptor by its endogenous neurotransmitter: binding of [3H]acetylcholine and its modulation by transition metal ions and guanine nucleotides

Agonist binding to the muscarinic receptor in rat cerebral cortex membranes was studied by using the neurotransmitter itself, [3H]acetylcholine [( 3H]AcCho). By using 10 microM atropine or oxotremorine to define specific binding, it was possible to demonstrate specific binding of [3H]AcCho that was sensitive to muscarinic but not to nicotinic ligands. Equilibrium binding experiments with 5-240 nM [3H]AcCho indicated specific binding of the ligand to a saturable population of muscarinic receptors (361 +/- 29 fmol/mg of protein; Kd = 76 +/- 17 nM). This value represented 25% of the available binding sites for a labeled antagonist in the same preparation and corresponds to the proportion of high-affinity agonist binding sites observed previously in competition experiments with labeled antagonists. Inclusion of transition metal ions (e.g., 2 mM Ni2+) in the assay increased the equilibrium binding of [3H]AcCho (628 +/- 38 fmol/mg of protein, Kd = 86 +/- 21 nM) but did not affect equilibrium binding of 3H-labeled antagonists, indicating conversion of low- into high-affinity muscarinic agonist binding sites. The increase developed slowly over 30 min of incubation at 25 degrees C but could be reversed rapidly (approximately equal to 2 min) by the chelating agent EDTA or by guanine nucleotides. These data directly reveal a slow though quickly reversible interconversion of low- into high-affinity muscarinic agonist binding sites.

Specific and non-specific desensitization of guinea-pig ileal smooth muscle

The effects of cis-2-methyl-4-dimethylaminomethyl-1-3-dioxolane methiodide (CD), a muscarinic agonist, histamine, substance P and K+-stimulation on the mechanical responses, Ca2+-dependence and desensitization in guinea-pig ileal longitudinal smooth muscle have been studied. The mechanical responses to all four stimulants are highly dependent upon extracellular Ca2+(Ca2+EXT) and are blocked by the Ca2+ channel antagonist nicardipine. The tonic (slow) components of response are more dependent on Ca2+EXT and are more sensitive to nicardipine (IC50 values 5.0 X 10(-8) - 2.5 X 10(-9)M) than are the phasic (fast) components of response. Tissue exposure to CD (5 X 10(-7)M, 10 min) or histamine (3 X 10(-6)M and 3 X 10(-4)M, 10 min) produces short term nonspecific desensitization but substance P (5 X 10(-8)M, 10 min) produces only specific desensitization. K+-induced responses neither desensitize nor are desensitized. Desensitization is concentration- and time-dependent for both specific and nonspecific processes. Nonspecific desensitization is protected by elevation of K+ concentration (5.36mM) in the incubating medium, by dithiothreitol and by inhibitors (mepacrine,p-bromophenacyl bromide and phenylgloxal) of phospholipase A2 and is potentiated by mellitin, an activator of phospholipase A2. Desensitization produced by the muscarinic agonist CS is protected by Gpp(NH)p (10(-4)M), but histamine-induced desensitization is unaffected. There is no loss of muscarinic receptors, measured by [3H]QNB binding following tissue exposure to low concentrations of CD (5.0 X 10(-7)M) for up to 72 h. However, an apparent loss of receptors (20-30%) is measured following 10-90 min exposure of tissue to 10(-3)M CD. It is suggested that contractions of guinea-pig ileal longitudinal smooth muscle elicited by CD, histamine, substance P or K+ mobilize a common pool of Ca2+ through a Ca2+ channel antagonist (nicardipine) sensitive pathway. However, the existence of short term nonspecific desensitization (CD and histamine), specific desensitization (substance P) or no desensitization (K+ stimulation) indicates that significant differences exist in the pathways linking initial stimulus to mechanical response. The ability of elevated K+ to protect against nonspecific desensitization suggest that post stimulus membrane hyperpolarization may represent one contributing component to nonspecific desensitization. Products of phospholipid degradation may also contribute to desensitization since inhibitors or activators of phospholipase A2 prevented or potentiated respectively, nonspecific desensitization.

Effects of guanine nucleotide and sulfhydryl reagent on subpopulations of muscarinic acetylcholine receptors in mammalian hearts: possible evidence for interconversion of super-high and low affinity agonist binding sites

Multiple site models of muscarinic acetylcholine receptors (mAChR) for agonist binding were applied to curves for the inhibition of QNB binding by carbachol by using nonlinear least square regression analysis. The effects of a guanine nucleotide guanyl-5'-yl imidodiphosphate (Gpp(NH)p) and a sulfhydryl reagent 5,5'-dithiobis(2-nitrobenzoic acid (DTNB) on the curves were also analyzed. The results suggested that mAChR of dog and guinea pig heart had three types of sites with different affinities for carbachol (super-high (SH), high (H) and low (L]. In the presence of Gpp(NH)p, SH sites were eliminated and L sites increased, indicating conversion of SH sites to L sites. On the contrary, in the presence of DTNB, L sites were converted to SH sites. These results were obtained at both 37 degrees C and 0 degrees C incubation although the affinity of each site was high at 0 degrees C than at 37 degrees C. These data suggest the interconversion of SH and L sites. The possible existence of two subtypes (GTP-regulated mAChR(SH-L type) and GTP-independent mAChR (H type] is discussed.

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.