Muscarinic Ligand Binding Research Articles

Agonist-Specific Conformations of the M2 Muscarinic Acetylcholine Receptor Assessed by Molecular Dynamics.

Binding of muscarinic ligands, both antagonists and agonists, and their effects on the conformation of the M2 acetylcholine receptor were modeled in silico and compared to experimental data. After docking of antagonists to the M2 receptor in an inactive conformation (3UON, 5ZK3, 5ZKB, or 5ZKB) and agonists in an active conformation (4MQS), 100 ns of conventional molecular dynamics (MD) followed by 500 ns of accelerated MD was run. Conventional MD revealed ligand-specific interactions with the receptor. Antagonists stabilized the receptor in an inactive conformation during accelerated MD. The receptor in complex with various agonists attained different conformations specific to individual agonists. The magnitude of the TM6 movement correlated with agonist efficacy at the non-preferential Gs pathway. The shape of the intracellular opening where the receptor interacts with a G-protein was different for the classical agonist carbachol, super-agonist iperoxo, and Gi/o-biased partial agonists JR-6 and JR-7, being compatible with experimentally observed agonist bias at the G-protein level. Moreover, a wash-resistant binding of the unique agonist xanomeline associated with interactions with membrane lipids was formed during accelerated MD. Thus, accelerated MD is suitable for modeling of ligand-specific receptor binding and receptor conformations that is essential for the design of experiments aimed at identification of the secondary binding sites and understanding molecular mechanisms underlying receptor activation.

Systematic development of high affinity bis(ammonio)alkane-type allosteric enhancers of muscarinic ligand binding.

Bis(ammonio)alkane compounds carrying lateral phthalimidopropyl substituents on the nitrogen atoms belong to the archetypal muscarinic allosteric agents. Herein, a series of symmetrical and nonsymmetrical compounds was synthesized in which the phthalimide residues were replaced by differently substituted imide moieties. The allosteric action was measured in porcine heart muscarinic M(2) receptors using [(3)H]N-methylscopolamine (NMS) as a ligand for the orthosteric receptor site in equilibrium binding and dissociation experiments. 1,8-Naphthalimido residues conferred an up to 100-fold gain in affinity leading into the low nanomolar range, while the inhibition of NMS binding was maintained. Additional propyl chain methylation was accompanied by an allosteric elevation of orthosteric ligand binding. In general, the gain in allosteric activity achieved by ring variation plus propyl chain methylation on one side of the molecule could not be augmented by symmetrical variations. The elevation of the ligand binding can be explained by different quantitative structure-activity relationships for the affinities to the free and the orthoster-liganded receptor.

Site-directed mutagenesis reveals two epitopes involved in the subtype selectivity of the allosteric interactions of gallamine at muscarinic acetylcholine receptors.

Gallamine allosterically modulates the binding of classical muscarinic ligands with a potency order of M(2) > M(1),M(4) > M(3), M(5). We have suggested previously that the M(2)/M(5) and M(2)/M(3) selectivities are attributable to an epitope in the sixth transmembrane region or third outer loop (o3) region of the receptor. In this study, analysis of numerous point mutations in this region of the M(5) receptor found that a mutation of V --> N resulted in an increased affinity toward gallamine, suggesting that the asparagine residue at M(2)(419) is responsible for gallamine's M(2)/M(5) selectivity. Mutations in the other subtypes indicated that the acidic residues found at this position in M(1) and M(4) are associated with slightly higher affinity toward gallamine, whereas the valine and lysine residues of M(5) and M(3), respectively, are associated with significantly lower affinity. In the o2 region, replacement of an acidic sequence of M(2) (EDGE) by the corresponding neutral sequence of M(1) (LAGQ) reduced the affinity toward gallamine, as reported previously by others; the converse substitution of the acidic sequence into M(1) significantly increased affinity for gallamine. Substitution of the M(1) sequence into this region of M(5) markedly reduced affinity toward gallamine, whereas substitution into M(4) had no effect. All of the above mutations are consistent with gallamine binding with a similar orientation at each subtype, such that it interacts with acidic residues in the o2 region of M(3) and M(5) and with acidic residues in the o3 region of M(1) and M(4); gallamine appears to interact with both regions of the M(2) subtype.

Identification and localization of muscarinic acetylcholine receptors in the ocular tissues of the chick

The purpose of this study was to characterize the distribution of muscarinic acetylcholine receptors (mAChRs) in the ocular tissues of hatched chicks. In the chick, different isoforms of these receptors have been detected in the brain, heart, and retina, and mAChRs in ocular tissues have been implicated in the pathogenesis of form-deprivation myopia. However, the precise anatomical distribution of mAChRs within the retina, retinal pigment epithelium, choroid, ciliary body, and ciliary ganglion remains unknown. We used affinity-purified, type-specific antibodies directed to three different chick mAChR subtypes (cm2, cm3, and cm4) to detect receptor immunoreactivity in sections and extracts of these ocular tissues. We found cm2, cm3, and cm4 in the retina, retinal pigment epithelium, choroid, and ciliary body. Within the retina, cm2 was expressed in numerous amacrine and ganglion cells; cm3 was expressed in many bipolar cells and small subsets of amacrine cells; and cm4 was found in most, if not all, amacrine and ganglion cells. Each mAChR was localized to distinct strata within the inner plexiform layer that cumulatively form three broad bands that closely match previously described localizations of subtype-nonspecific muscarinic ligand binding. Only cm3 was detected in the outer plexiform layer, and only cm4 was detected in the ciliary ganglion. We propose that each mAChR subtype has unique functions in each ocular tissue.

Covalent labeling of muscarinic acetylcholine receptors by tritiated aryldiazonium photoprobes

p-dimethylamino (A) and p-dibutylamino (B) benzenediazonium salts, previously characterized as efficient labels of membrane-bound and solubilized muscarinic receptor sites, are endowed with overall interesting photochemical and alkylating properties that allow their use as structural probes of the muscarinic ligand binding domain to be considered. Under reversible binding conditions, these antagonists display no binding selectivity towards the 5 muscarinic acetylcholine receptor (mAChR) subtypes. They were used here, in a tritiated form, as photoaffinity labels of purified muscarinic receptors from porcine striatum, and their irreversible binding was assessed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. When irradiated under energy transfer conditions, [ 3H]A and [ 3H]B were both found to covalently label purified muscarinic receptor sites in a light-dependent and atropine-protectable manner. The electrophoretic migration properties of the alkylated sites were similar to those of [ 3H]propylbenzilylcholine mustard (PrBCM)-labeled mAChRs. Specific radioactive incorporation showed a clear dependency on probe concentration. Labeling efficiency was rather high, with up to 30% and even 60% of the receptor population being photolabeled by [ 3H]A and [ 3H]B, respectively. These two photoactivatable ligands have proven to be powerful tools for the structural analysis of other cholinergic targets (acetylcholinesterase and the nicotinic acetylcholine receptor) by allowing the characterization of a number of different residues belonging to their acetylcholine-binding domain. Altogether, these results reinforce the interest of our site-directed labeling approach because [ 3H]A- and [ 3H]B-alkylated mAChRs may now be considered as suitable materials to investigate the muscarinic receptorbinding pocket through peptide mapping, sequence analyses, and identification of radiolabeled amino acid residues.

Characterization of muscarinic binding sites in the central nervous system of larval Manduca sexta

Putative muscarinic receptors (mAChRs) were characterized in the CNS of larval Manduca sexta through the binding of 3H-quinuclidyl benzilate ([ 3H]-QNB). Specific binding isotherms revealed a high affinity binding site in both crude homogenates (K a=3.22±0.62 nM 1 Kd 0.311 nM), B max=65.4±9.8 fmoles/mg protein) and in purified membrane preparations (K a=7.61±1.78 nM 1(K d 0.130 nM), B max=22.8±2.15 fmoles/mg protein). In purified membrane preparations the binding was complex, consisting of at least two sites (Hill coefficient =0.514±0.041). Because of the high proportion of non-specific binding at QNB concentrations greater than 10 nM, the binding parameters for the additional low affinity site could not be determined accurately but the Kd was estimated to be greater than 8 nM. Complex binding was also exhibited in the kinetics of [ 3H]-QNB association and dissociation in purified membranes. Analysis resolved two high affinity sites (dissociation rate constants =80.4 and 1.2 min, Kds=0.272 and 0.909 nM, consisting of 65.9±3.7 and 26±4.9% of the sites respectively). Presumably, the high affinity site identified in saturation studies consists of these two components. The primary high affinity site was isolated using a dissociative method and its pharmacology determined in competition studies. [ 3H]-QNB binding at this site was displaced by pirenzepine and methoctramine (Kis=248 and 707 nM) closely matching the pharmacology of the cloned Drosophila mAChR. Competition data for 4-DAMP were better fitted by a two site model (K/s=37 and 547 nM). These results demonstrate unexpected complexity in muscarinic ligand binding and are consistent with mAChR heterogeneity in the CNS of Manduca.

Regulation of PNMT gene promoter constructs transfected into the TE 671 human medulloblastoma cell line

The human medulloblastoma TE 671 cell line has been evaluated as a model for studying expression of transiently transfected phenylethanolamine N-methyltransferase (PNMT) promoter-fusion gene constructs. Because TE 671 cells are one of few continuous lines exhibiting a neuronal phenotype, possess both nicotinic and muscarinic ligand binding properties, and express PNMT mRNA, they represent a likely candidate system for study of cholinergic-regulated PNMT gene expression. When transfected with constructs containing from 0.1 to 3 kb of the region 5′ to the initiation site for PNMT gene transcription, TE 671 cells express at detectable levels only those constructs containing the −391 to −863 by portion of the rat PNMT promoter fused to the chloramphenicol acetyltransferase (CAT) gene. Both general and selective cholinergic agents regulate expression of the −863 to −391 by PNMT region in a TK promoter-CAT reporter construct. With this construct, carbachol stimulates CAT expression 1.5-fold in transfected TE 671 cells. The selective agonists nicotine and muscarine each stimulate expression of the upstream TK-CAT constructs 2.8- and 1.9-fold respectively, while antagonists for these receptors block induction. Because TE 671 cells respond to both nicotinic and muscarinic stimuli, this continuous line represents a useful model system for studying PNMT gene regulation by cholinergic stimuli.

Localization of muscarinic receptors in human heart biopsies using rabbit anti-peptide antibodies

Immunocytochemistry of muscarinic receptors on human heart biopsies from patients with heart disease was studied using rabbit antibodies against a synthetic peptide corresponding to amino acids 168-192 of the second extracellular loop of the human M2 muscarinic receptor. By using both light and electron microscopic immunocytochemistry techniques, muscarinic receptors were visualized on sarcolemma of human myocytes from patients with different heart diseases such as coronary heart disease and dilated cardiomyopathy in adults and congenital heart disease in children. The patchy distribution of immunoreactivity suggests a muscarinergic activity in vivo. These reactivities were abolished by preincubation of antibodies with antigenic peptide and were not shown in the absence of antibodies. Moreover, these antibodies were able to interfere with muscarinic ligand binding in myocardium from human dilated cardiomyopathy as shown by decreases in binding sites and antagonist affinity. These results demonstrate that the antibodies against the second extracellular loop of the human M2 muscarinic receptor can specifically recognize muscarinic receptors in human tissue and display pharmacological activity in human diseased myocardium, confirming their usefulness for the study of localization and function of muscarinergic activity in the human heart.

Acute ethanol dependence or long-term ethanol treatment and abstinence do not reduce hippocampal responses to carbachol

In the hippocampus of human alcoholics, prolonged ethanol treatment reduces the number of muscarinic ligand binding sites present at autopsy suggesting a decrease in functional muscarinic receptors. Whether these changes are due to alcohol-induced brain damage or ethanol dependence and represent a reduced level of cholinergic function is unknown. The present studies tested the impact of ethanol dependence or long-term ethanol treatment and subsequent withdrawal on the function of pre- and postsynaptic muscarinic receptors in the CA1 region of the rat hippocampus. Field excitatory postsynaptic potentials (EPSPs) were inhibited in a concentration-dependent manner by 0.1–100 μM carbachol. This presynaptic inhibitory action of carbachol involving muscarinic receptors was not significantly reduced either by ethanol treatment (12 days), causing physical dependence, or by long-term ethanol treatment (97–120 days) and abstinence (3–6 months). Postspike after hyperpolarizations (AHPs) were inhibited in a concentration-dependent manner by carbachol (6–2000 nM). This postsynaptic excitatory action of muscarinic receptors also was not significantly reduced either by 12-day ethanol treatment or by long-term ethanol treatment. Taken together, these results suggest that neither pre- nor postsynaptic muscarinic receptor function measured electrophysiologically is reduced by either ethanol dependence or long-term ethanol consumption and abstinence in the rat as suggested by reduced muscarinic ligand binding in the hippocampus of human alcoholics.

Functional epitope analysis of the second extracellular loop of the human heart muscarinic acetylcholine receptor

Two synthetic peptides corresponding to amino acids 172-181 and 169-193 of the second extracellular loop of the human M2 muscarinic receptor respectively were used to raise antibodies in rabbits. Affinity-purified antibodies were able not only to recognize a major band with a molecular weight of about 80 kDa on the electrotransferred membrane proteins of rat ventricular membranes but also to localize the muscarinic receptors on the sarcolemma and t-tubules of rat cardiomyocytes. Antibodies were also able to mimic muscarinic agonist stimulation as demonstrated by a negative chronotropic effect on cultured neonatal cardiomyocytes. In contrast with the antibodies raised against the peptide 169-193, the antibodies against the peptide 172-181 were unable to inhibit muscarinic ligand binding. These results suggest that the decapeptide 172-181 contains the B-cell epitope responsible for the functional effect of antibodies directed against the second extracellular loop of the receptor. Coupling this peptide by cystein 177 blocks the induction of antibodies with pharmacological effects but induces antibodies which are able to recognize the denatured receptor protein and to exert a negative chronotropic effect.

Perinatal low-level lead exposure and the septo-hippocampal cholinergic system: selective reduction of muscarinic receptors and cholineacetyltransferase in the rat septum

We investigated the effects of low-level lead exposure on the postnatal development of cholinergic muscarinic receptors (mAChR) and a cholinergic marker enzyme cholineacetyltransferase (ChAT) activity in the rat septum and hippocampus. Rat pups were maternally lead-exposed by giving 0.2% lead acetate in drinking water to dams from one week before parturition (gestational day 16) through weaning at postnatal day 28. The lead-exposed litters had blood Pb in the range 20 μg/dl and tissue Pb< 0.2 μg/g in both the septum and hippocampus. Associated with this level of lead exposure there was a significant 30–40% reduction in the ChAT activity in the septa and hippocampi of PN7 through PN28 animals. In contrast, the levels of glutamic acid decar☐ylase (GAD) activity, a GABAergic neuron marker enzyme, were not altered in either brain region. Associated with the selective reduction of ChAT activity there was a parallel 30–40% reduction of the [ 3H]quinuclidinyl benzilate, [ 3H]AF-DX 384, and [ 3H]pirenzepine binding in the septum, however muscarinic ligand binding in the hippocampus of lead exposed animals was not affected. These results indicate preferential vulnerability of septal cholinergic neurons to adverse effects of low-level Pb exposure and suggest that impaired expression of muscarinic receptors and disruption of muscarinic transmission in the septum may be an important factor in cognitive and learning deficits associated with developmental low-level lead exposure.

Reorganization of cingulate cortex in Alzheimer's disease: neuron loss, neuritic plaques, and muscarinic receptor binding.

Pathology related to dementia of the Alzheimer type (DAT) develops later in cingulate cortex than in medial temporal areas. Therefore, end-stage cases have earlier forms of pathology in cingulate cortex, and postmortem studies of this region may provide a window on processes that temporal cortices pass through decades before death. Five classes of DAT have been described on the basis of neuron degeneration and receptor binding in posterior cingulate cortex. The present study assessed binding of 3H-oxotremorine-M with pirenzepine (OXO-M/PZ), a protocol for presynaptic muscarinic receptors, and thioflavin S-stained neuritic plaques (NPs) in cingulate area 23a in 12 DAT cases distributed over four classes of pathology and in nine age-matched control cases. OXO-M/PZ binding was significantly elevated in layers I, II, IV, and VI of all DAT cases and was very high in layer V compared to control cases. Almost 75% of the layer Va increase was due to binding in classes 2 and 3, while classes 1 and 4 were least affected. In class 3 cases, neuron density in layer Va was inversely correlated with OXO-M/PZ binding (r = -0.98) and primitive NP densities (r = -0.93). The close association between neuron densities and presynaptic muscarinic ligand binding in some classes confirms that there are independent classes of DAT. The high and inverse correlations between cortical pathology and ligand binding in class 3 cases suggest that there is a progression in class 3 pathology. Finally, elevated OXO-M/PZ binding and a report of increased choline uptake suggest that cholinergic axons sprout in DAT, and this sprouting may be associated with a progressive loss of postsynaptic elements.

Site-directed mutagenesis of the m3 muscarinic receptor: identification of a series of threonine and tyrosine residues involved in agonist but not antagonist binding.

The hydrophobic core of all muscarinic receptors contains several conserved serine, threonine and tyrosine residues, most of which do not occur in any other G-protein coupled receptor. Since these amino acids can serve as potential hydrogen bond donors or acceptors, we have tested the hypothesis that they may be involved in the selective binding of muscarinic ligands. To eliminate the OH groups present in these residues, we have created nine single point mutations in the rat m3 muscarinic receptor by converting serine and threonine residues to alanine, and tyrosine residues to phenylalanine. The ligand binding and functional properties of these receptors were studied after transient expression in COS-7 cells. Six out of the nine mutant receptors (threonine and tyrosine mutations) showed strong reductions (approximately 10- to 40-fold lower than the wild-type receptor) in agonist binding affinities and reduced potencies in agonist-induced activation of phosphoinositide hydrolysis. Their antagonist binding properties, however, were similar to those of the wild-type m3 receptor. Despite their location on different transmembrane domains (III, V, VI and VII), all six mutations are positioned at a similar level (one to two helical turns away from the membrane surface) within the outer leaflet of the plasma membrane and may thus define the plain in which muscarinic agonists (but not antagonists) bind to their target receptor.

The cysteine residue in the carboxyl-terminal domain of the m2 muscarinic acetylcholine receptor is not required for receptor-mediated inhibition of adenylate cyclase.

Muscarinic acetylcholine receptors (mAChRs) share with many other receptors of the guanine nucleotide-binding protein-coupled receptor family a highly conserved cysteine residue in the putative cytoplasmic carboxyl-terminal region of the protein. Because elimination of this cysteine in the beta 2-adrenergic receptor has been reported to decrease functional responsiveness, we determined if this cysteine residue is essential for mAChR-effector coupling by replacing Cys457 of the m2 mAChR with glycine and expressing wild-type and mutant receptor in Chinese hamster ovary (CHO) cells. The mutant and wild-type receptors exhibited similar affinities for binding of muscarinic ligands. In addition, the mutation did not affect cell surface localization or receptor-mediated inhibition of adenylate cyclase. These results indicate that the cysteine residue in the carboxyl-terminal domain of the m2 mAChR is not required for ligand binding or mAChR-mediated inhibition of adenylate cyclase in CHO cells.

Developmental expression of muscarinic cholinergic receptors and coupling to phospholipase C in rat sweat glands are independent of innervation

During development, the innervation of rat sweat glands undergoes a striking change from noradrenergic to cholinergic function. The acquisition of secretory responsiveness by the glands is temporally correlated with the appearance of cholinergic properties. In addition, responsiveness fails to appear in the absence of innervation. To investigate the basis of the onset of functional transmission and secretory responsiveness and its possible relationship to innervation, we analyzed the development of muscarinic cholinergic receptors in sweat glands, examined their expression in the glands of adult rats sympathectomized at birth, and assayed the ability of muscarinic agonists to increase phosphoinositide (PI) turnover. Autoradiographic and in situ hybridization analysis revealed that muscarinic ligand binding sites were first detectable as glands begin to form on postnatal day 4 (P4). Between P4 and P14, receptor concentration increased in parallel with mRNA for the m3 receptor subtype. On P14, the concentration of ligand binding sites approached adult levels, although only a small proportion of glands at this age secrete in response to nerve stimulation or cholinergic agonists. When the pharmacological properties of muscarinic receptors in sweat glands of adult rats sympathectomized at birth were compared to those of normal glands, the concentration and affinity determined with [N-methyl-3H]-scopolamine and the Ki values determined with the subtype-selective muscarinic antagonists 4-DAMP, pirenzepine, and AF DX-116 were similar. In addition, the molecular subtype was unchanged as was the level of m3 message. Studies of PI turnover in response to muscarinic stimulation indicated that the receptors expressed in sweat glands isolated from sympathectomized and acutely denervated, as well as control, rats were functionally coupled to phospholipase C. The absence of sympathetic innervation therefore does not appear to influence either the development of muscarinic receptors or their coupling to PI turnover. Our results suggest that functional sympathetic cholinergic innervation plays a central role in the development and maintenance of secretory function at a step distal to signal transduction across the cell membrane.

Two types of intrinsic muscarinic responses in Xenopus oocytes

Oocytes of 40% of Xenopus laevis frogs respond to acetylcholine (ACh). Oocytes of the majority of responders exhibit the common two-component depolarizing muscarinic response (mean amplitude of the rapid component, 54 nA). Oocytes of approximately 10% of the responders ("variant" donors) exhibit a muscarinic response characterized by a very large transient, rapid current (mean amplitude 1242 nA, reversal potential -33 mV). Responses in oocytes of variant donors exhibit further qualitative differences: pronounced desensitization (absent in oocytes of common donors), characteristic prolonged latency (5.4 vs 0.9 s in oocytes of common donors) and marked inhibition of the response by activators of protein kinase C. Rapid responses in oocytes of variant donors are usually increased by treatment with collagenase, which, in common oocytes, often results in a complete loss of the response that correlates with the loss of muscarinic ligand binding. The number of muscarinic receptors was similar in oocytes of both types of donors (2.2 vs 3.0 fmol/oocyte). Also, the responses of oocytes of variant donors to microinjections of CaCl2 or inositol 1,4,5-trisphosphate were similar to those found in cells of common donors. These findings imply that altered receptor number, calcium stores and/or chloride channel density are not responsible for the variant responses. However, ACh caused an sixteen-fold greater efflux of 45Ca in oocytes of variant donors (35 vs 2.2% of total label in oocytes of common donors). Hence, the characteristics of the variant response may be related to a more efficient coupling between receptor stimulation and the mobilization of cellular calcium.

Possible Involvement of Pertussis Toxin‐Sensitive G Proteins and D2 Dopamine Receptors in the A1 Adenosine Receptor‐Adenylate Cyclase System in Rat Cerebral Cortex

To identify the involvement of dopamine receptors in the transmembrane signaling of the adenosine receptor-G protein-adenylate cyclase system in the CNS, we examined the effects of pertussis toxin (islet-activating protein, IAP) and apomorphine on A1 adenosine agonist (-)N6-R-[3H]phenylisopropyladenosine ([3H]PIA) and antagonist [3H]xanthine amine congener ([3H]XAC) binding activity and adenylate cyclase activity in cerebral cortex membranes of the rat brain. Specific binding to a single class of sites for [3H]XAC with a dissociation constant (KD) of 6.0 +/- 1.3 nM was observed. The number of maximal binding sites (Bmax) was 1.21 +/- 0.13 pmol/mg protein. Studies of the inhibition of [3H]XAC binding by PIA revealed the presence of two classes of PIA binding states, a high-affinity state (KD = 2.30 +/- 1.16 nM) and a low-affinity state (KD = 1.220 +/- 230 nM). Guanosine 5'-(3-O-thio)triphosphate or IAP treatment reduced the number of the high-affinity state binding sites without altering the KD for PIA. Apomorphine (100 microM) increased the KD value 10-fold and decreased Bmax by approximately 20% for [3H]PIA. The effect of apomorphine on the KD value increase was irreversible and due to a conversion from high-affinity to low-affinity states for PIA. The effect was dose dependent and was mediated via D2 dopamine receptors, since the D2 antagonist sulpiride blocked the phenomenon. The inhibitory effect of PIA on adenylate cyclase activity was abolished by apomorphine treatment. There was no effect of apomorphine on displacement of [3H]quinuclidinyl benzilate (muscarinic ligand) binding by carbachol. These data suggest that A1 adenosine receptor binding and function are selectively modified by D2 dopaminergic agents.

Modulatory effects of phosphatidylserine on the binding of muscarinic cholinergic receptor ligands

The modulation of the binding of muscarinic cholinergic receptor ligands by phosphatidylserine purified from bovine cerebral cortex (BC-PS) was examined in vitro and in vivo. The enrichment of bovine cerebral cortical synaptosomal membranes with BC-PS, using a fusion technique, produced a concentration-dependent decrease in the affinity (increase in Kd) of [3H]quinuclidinyl benzylate (3H-QNB) specific binding to muscarinic acetylcholine receptors (mAChR), without changes in their maximal number (Bmax). Similar results were observed when [3H]oxotremorine (3H-OXO) was used to label a high affinity subpopulation of mAChR. On the other hand, preincubation of BC-PS liposomes with synaptosomal membranes in a nonoptimum fusion condition (at pH 7.4) did not alter the binding properties of both radioligands. Fusion experiments using a pure phosphatidylserine preparation from spinal cord revealed a similar decrement in the affinity of 3H-QNB specific binding. Five day's intraperitoneal (i.p.) administration of 15 mg/kg of BC-PS liposomes in rats increased the maximal number of cerebral cortical binding sites for 3H-OXO. Scatchard analysis revealed no changes in the apparent dissociation constant. This modification is selective in relation to the neural structure studied. Thus, BC-PS treatment did not modify 3H-OXO binding in the hippocampal formation and cerebellum. In contrast, parallel experiments using the muscarinic antagonist 3H-QNB showed no alteration in the binding properties of mAChR. Five day's i.p. administration of 15 mg/kg/d of phosphatidylcholine from bovine cerebral cortex (BC-PC) liposomes produced quite similar results to those obtained with BC-PS. These results indicate that mAChR are under the modulatory action of phosphatidylserine (PS) and phosphatidylcholine (PC), and suggest that this endogenous phospholipids may play a regulatory role on the mAChR. The possible implications of these findings on the effects of PC or PS treatment in neurological disorders involving a decrease in central cholinergic functions are discussed.

Non-selectivity of amitriptyline for subtypes of brain muscarinic receptors demonstrated in binding and functional assays

The characteristics of interaction of amitriptyline, a tricyclic antidepressant, with rat brain muscarinic receptors were assessed using both radioligand binding and functional assays. In competition studies, amitriptyline displaced muscarinic ligand binding from a single high-affinity site in homogenates of various brain regions which have a different distribution of M 1 and M 2 receptor subtypes. The affinity of amitriptyline for muscarinic receptors was also comparable in all brain regions. Furthermore, amitriptyline indentified a single species of muscarinic receptors in intact cells dissociated from the cerebral cortex and in cerebrocortical slices. The non-selectivity of amitriptyline for muscarinic receptor sybtypes in these preparations was in contrast to the selectivity exhibited by pirenzepine. This non-selective nature of amitriptyline was also evident in functional assays, since this antidepressant was equipotent at antagonizing M 1-mediated increase in phosphoinositide hydrolysis and M 2-mediated inhibition of cyclic AMP formation in dissociated cortical cells. Atropine was also equipotent at blocking both responses but was 20- to 30-fold more potent than amitriptyline. These results demonstrate that amitriptyline behaves as a non-selective muscarinic antagonist using both radioligand binding and functional measurements.

Rat thymocytes release a factor which inhibits muscarinic ligand binding

Rat thymocytes incubated in RPMI medium for 120–300 min release a soluble factor of molecular weight below 10 000 Da which inhibits the binding of 3H-labeled muscarinic antagonists in an uncompetitive manner, i.e. it reduces maximal specific binding ( B max) without changing the affinity of the ligand ( K D). This factor inhibited muscarinic antagonist binding on thymocytes and rat cerebral cortex cellular membranes. Thymocytes from hydrocortisone-treated rats produced more factor per mg cell protein than thymocytes from untreated rats. The activity of the factor was unaffected by incubation with Cd 2+ (1 mM) or Zn 2+ (1 mM) or EDTA (1 mM), and its protein nature is supported by the following findings: it was trypsin sensitive, heat denaturated at 56°C and precipitated by (NH 4) 2SO 4 (40%, w/v). Inhibition by the factor was apparently irreversible after 1 h of incubation.