Muscarinic Receptor Sites Research Articles

Cardiac muscarinic cholinergic receptor sites: Opposing regulation by divalent cations and guanine nucleotides of receptor-agonist interaction

Cardiac muscarinic cholinergic receptor sites: Opposing regulation by divalent cations and guanine nucleotides of receptor-agonist interaction

Agonist-antagonist interaction with rat atrial muscarinic cholinergic receptor sites: Differential regulation by guanine nucleotides

Agonist-antagonist interaction with rat atrial muscarinic cholinergic receptor sites: Differential regulation by guanine nucleotides

Dendro axonic transmission. I. Evidence from receptor binding of dopaminergic and cholinergic agents

Spiroperidol binding in the substantia nigra and QNB and a-bungarotoxin ( a-Btx) binding in the neostriatum were measured in rats with lesions designed to produce selective alterations in pre- and postsynaptic neuronal processes. The objective was to determine whether presynaptic receptor sites may exist for dendritically released transmitters. Spiroperidol was used to measure dopaminergic receptor sites while QNB and a-Btx were used to measure muscarinic and nicotinic cholinergic receptor sites, respectively. The binding results were correlated with measurements on the individual striata of tyrosine hydroxylase (as an index of the integrity of nigrostriatal dopaminergic neurons) and of glutamic acid decar☐ylase (as an index of the integrity of striatal interneurons and descending striatonigral pathways). The results indicate that muscarinic receptor sites in the striatum are on neuronal perikarya or dendrites but nicotinic receptor sites are on afferent dopaminergic and corticostriatal neurons; in the substantia nigra spiroperidol binding is on afferent striatonigral neurons. The results, together with available biochemical and morphological data, are used to support the hypothesis that communication at many CNS synapses may involve a neurotransmitter dialog rather than a monolog. If true, this will affect interpretations of many pharmacological, behavioral and physiological experiments.

Muscarinic acetylcholine receptors in cat iris

The binding of tritium labeled N- methyl-4- piperidylbenzilate([ 3 H]-4- NMPB ) to fractions obtained from cat irides was investigated. The binding of this highly potent muscarinic antagonist to the cat iris consisted of two components: (a) high affinity binding sites with low capacity and (b) low affinity binding sites with high capacity. By a simple fractionation procedure those two components were reasonably separated, and a relatively pure fraction with high affinity binding sites was obtained. The high affinity binding sites are attributed to the muscarinic receptor sites, since they showed characteristic behaviour: saturability, high affinity, reduction of the binding by muscarinic ligands, and no effect on binding by non-muscarinic ligands (at a pharmacological range of concentration). Hill coefficients for the antagonists were approximately 1, and for the agonists less than 1. The dissociation constant of [ 3H]-4-NMPB (0.4–0.6 nM) is similar to that found in albino rabbit irides and in mouse brain. Furthermore, the capacity of the high affinity binding sites (468 fmole/iris) was similar to that found in albino rabbit irides (447 fmole/iris) and in pigmented rabbits irides (512 fmole/iris). According to these findings, the low affinity binding sites cannot be related to the muscarinic receptor and are probably related to the presence of abundant melanin pigment. The interactions of [ 3H]-4-NMPB with the components of the cat irides and their possible physiological significance are discussed.

Cholinergic False Transmitters

Publisher Summary A false neurotransmitter is a substance that is not normally present in a nerve ending, but which can accumulate in the sites that are usually occupied by the physiological neurotransmitter, and can be released by stimuli that normally release the physiological transmitter. This chapter discusses cholinergic false transmitters. All putative cholinergic false transmitters studied so far are formed from choline analogues. The criteria of presence, subcellular localization, and release have been demonstrated for acetyldiethylcholine, acetyltriethylcholine, acetylpyrrolidinecholine, and acetylhomocholine. The criteria of presence and release have been demonstrated for acetylmonethylcholine, and the criterion of presence for acetylsulfocholine. All of these presumed false transmitters are less active than is acetylcholine (ACh) at nicotinic and muscarinic receptor sites and all can be hydrolysed by acetylcholinesterase. Studies on the uptake, acetylation, storage, release and turnover of choline analogues and the false transmitters formed therefrom are likely to be of some use in clarifying presynaptic cholinergic mechanisms.

Properties of the muscarinic cholinergic receptors in rat atrium.

1. Properties of the muscarinic cholinergic receptor sites in the rat atrial homogenate and microsomal fraction were studied by the use of tritium labelled 3′-quinuclidinyl benzilate ([3H]-QNB), a potent muscarinic antagonist. 2. The specific [3H]-QNB binding to the receptor sites displayed saturability, stereospecificity as well as reversibility. 3. The competition studies showed that muscarinic antagonists were more potent than muscarinic agonists. 4. Certain neuromuscular blocking agents, antipsychotics, antiarrhythmics and antihistamines also were capable of interacting with the [3H]-QNB binding sites. However, α- and β-adrenergic agents, calcium antagonist (D-600) and ionophore (A-23187) failed to show any effect. 5. Analyses by the double reciprocal plot, Hill plot and Scatchard plot of the dependence of the specific [3H]-QNB binding on the concentration of QNB suggested that binding was occurring to a single population of receptor sites in the atrial homogenate or microsomal fraction. Further, there was no evidence of any detectable site to site interactions (positive or negative cooperative type). From the Scatchard plot, the equilibrium dissociation constant (KD) of 1.1 nM was calculated and the Hill coefficient was close to 1.0 6. Interaction of the muscarinic antagonists with the [3H]-QNB sites showed the Hill coefficients close to 1.0 whereas for the agonists, the coefficients were much less than 1.0 indicating that agonist-receptor site interactions have some different characteristics from those following antagonist-receptor site interaction. 7. The rate and the maximal level of QNB binding to the receptor sites was markedly influenced by the temperature; various cations, on the other hand, displayed no effect either on the association or dissociation of QNB binding. The specific QNB binding exhibited a broad pH optimum from pH 6.0–8.5. 8. Treatment of the membrane fraction (or homogenate) with either phospholipase A or C and with p-chloromercuribenzoate caused significant inhibition of [3H]-QNB binding. 9. The QNB binding site was resistant to tryptic digestion. Even when about 40% of the membrane proteins were removed by the tryptic proteolysis, the [3H]-QNB binding ability of the membrane remained unaffected; in fact, the removal of tryptic proteolytic products by centrifugation markedly increased the specific QNB binding to the membrane.

Cardiovascular response to intracerebroventricular administration of acetylcholine in rats treated with physostigmine

After central administration of physostigmine, the pressor response to intracerebroventricular (i.c.v.) injection of acetylcholine (ACh) was reversed into a short-lasting depressor response, with or without a secondary rise in arterial blood pressure, followed sometimes by a late prolonged depressor response. The initial depressor effect was accompanied by a bradycardia. The general characteristics of the latter effects of ACh and mechanisms mediating them were studied. The decrease in arterial blood pressure and heart rate were abolished after intracerebroventricular administration of atropine or nicotine and intravenous injection of methylatropine. Bilateral cutting of the vagosympathetic trunks and intracerebroventricular administration of hexamethonium did not affect the hypotension, but strongly reduced the bradycardia. Accordingly, the depressor effect of ACh results from the activation of central muscarinic receptor sites, which appears to evoke the excitation of a peripheral cholinergic mechanism followed by active vasodilation. The bradycardia is due to the activation by ACh of the central intermediate type of cholinergic receptor sites. It is tentatively suggested that ACh takes part in the physiological control of the activity of the rat cardiovascular system, as both a central cholinergic excitatory and inhibitory regulator.

Cardiovascular response to intracerebro-ventricular administration of acetylcholine in rats

General characteristics of the cardiovascular response to intracerebroventricular (i.c.v.) injection of acetylcholine (ACh) and the mechanisms mediating this response were studied. Acetylcholine produced a pressor response which was sometimes followed by a prolonged depressor response. The vascular response to ACh. in the majority of experiments, was not accompanied by marked changes in heart rate. The pressor effect of ACh was abolished after transsection of the spinal cord, intracerebroventricular administration of atropine or nicotine and intravenous administration of phenoxybenzamine, tolazoline or methylatropine. Furthermore, the pressor response to ACh was reduced after bilateral adrenalectomy or intravenous injection of bretylium. However, bilateral cutting of the vagosympathetic trunks and intracerebroventricular or intravenous administration of hexamethonium did not affect the pressor response to ACh. It is concluded that the pressor effect of ACh results from the activation of the central muscarinic receptor sites, which evokes the activation of the muscarinic receptors in the sympathetic ganglia and the adrenal medulla followed by the release of catecholamines from adrenergic nerve terminals and the adrenal medulla. In the rat sympathetic ganglia and the adrenal medulla, in addition to the nicotinic receptors, the muscarinic receptors can play the role of the main pathway in ganglionic transmission.

Modulation of the aversive qualities of shock through a central inhibitory cholinergic system in the rat

Evidence has been supplied which suggests that a central inhibitory cholinergic (i.e., muscarinic) system may be involved in modulating the aversive qualities of electric shock in the rat. Central cholinergic stimulation via the administration of pilocarpine or arecoline elevated the threshold for grid shock, while central acting anticholinergics (i.e., scopolamine and atropine) produced decrements in the threshold. Peripheral acting anticholinergics (e.g., methyl scopolamine, methyl atropine) were less potent than central acting drugs given in equivalent doses, while peripheral cholinergic stimulants (i.e., neostigmine, carbachol) were inactive. In addition, only the central acting stimulant pilocarpine, and not carbachol, was able to block the decrements noted in response to scopolamine hydrobromide administration. Finally, only arecoline, and not nicotine, was able to elevate the aversive threshold indicating that muscarinic receptor sites are probably involved in mediating the effects of central cholinergic stimulants.

IRREVERSIBLE INHIBITORY EFFECT OF ATROPINE ON CONTRACTILE RESPONSES TO DRUGS IN ISOLATED RABBIT ILEUM

Effect of atropine sulfate (atropine) on the contractile responses to acetylcholine chloride (ACh), potassium chloride (K) and barium chloride (Ba) was investigated in isolated rabbit ileum. Tension of the strips suspended in the bath medium (Locke solution, 30 °C) was isotonically recorded. The K(ED50)- and Ba(ED50)-induced contractions were not affected by atropine at 3.0 × 10−4 mM, which reversibly abolished the ACh(ED100)-contraction. After washout of 6.0 × 10−4 mM atropine, the phasic, but not the tonic, component of ACh(ED50)-, K(ED50)- and Ba(ED50)-contractions was to some extent inhibited irreversibly. On the contrary, such irreversible inhibition of the phasic component was not produced by atropine methyl-bromide even in the high concentration of 3.0 × 10−3 mM. The irreversible inhibitions by atropine of 6.0 xlO−4 mM on the phasic component of ACh- and K-contractions were protected by pretreatment with a high concentration of ACh or K. Further, these irreversible inhibitions by atropine were potentiated by the absence of Ca and were abolished by the increase of Ca content in bath media. These results suggest that the irreversible inhibition of the contractile responses to drugs by atropine in high concentration may be due to the interference with the mobilization of Ca in the deep layer of the membrane, rather than by a blockade of muscarinic receptor sites.

In Vitro Studies on the Human Renal Calices

Smooth muscle preparations taken from the minor calices of human kidneys were investigated by an isolated tissue technique. The existence of alpha-adrenoceptor and muscarinic receptor sites was demonstrated and it was shown that stimulation with the appropriate agonist resulted in an excitatory response. No evidence could be found to support the presence of beta-adrenoceptors in such tissues. The results of transmural electrical stimulation of similar preparations were highly suggestive of an effective innervation of the alpha-adrenoceptors.

Antischizophrenic drugs: Affinity for muscarinic cholinergic receptor sites in the brain predicts extrapyramidal effects

Publisher Summary This chapter discusses the dosage of antischizophrenic drugs and the affinity for muscarinic cholinergic receptor sites in the brain to predict extrapyramidal effects. The dopamine hypothesis of schizophrenia is predicated in large part upon the interactions of the antischizophrenic phenothiazines and butyrophenones with brain catecholamines, especially dopamine. Besides accounting for their antischizophrenic effects, blockade by phenothiazines and butyrophenones of dopamine receptors can explain their extrapyramidal side effects including akathisia, abnormal movements, and parkinsonian-like symptoms. If dopamine receptors throughout the brain respond similarly to various phenothiazines, at therapeutic antischizophrenic doses similar receptor blockade should occur in all the dopamine systems. The therapeutic antischizophrenic dopamine receptor blockade requires almost ten times higher brain levels of thioridazine than of trifluoperazine.

Spider cardiac physiology, II. responses of a tarantula heart to cholinergic compounds

1. 1. Acetylcholine produces a marked increase in the rate and amplitude of the tarantula heart-beat by its actions on the cardiac ganglion. These effects are greatly potentiated by eserine and are considerably reduced by atropine, hexamethonium, and D-tubocurarine. 2. 2. Methacholine produces a moderate increase in heart-rate and a pronounced increase in the amplitude of the contractions. Nicotine produces a marked increase in heart-rate, but either no change or else a reduction in heart-beat amplitude. 3. 3. The effects of methacoline are blocked completely by atropine, but are unaffected by hexamethonium. The effects of nicotine are reduced considerably by hexamethonium, but not by atropine. 4. 4. It is suggested that both nicotinic and muscarinic receptor sites for acetylcholine may be present in the cardiac ganglion.

BRL 1288-a new anti-Parkinson drug.

THE symptomatic treatment of Parkinson's disease usually involves anti-acetylcholine drugs which may block muscarinic receptor sites on neurones in the reticular formation. Unfortunately, all of the drugs in common use have pronounced peripheral anti-acetylcholine actions which result in unpleasant side effects, such as dryness of the mouth and mydriasis.

Conformationally rigid analogues of acetylcholine.

IN spite of the simplicity of its chemical structure, details of the mode of action of acetylcholine (ACh) remain unknown. We have considered stereochemical attachment of ACh at its nicotinic and its muscarinic receptor sites, and compared these two stereochemical attachments. A logical working hypothesis, proposed by many in the past (and so far not proved), is that the flexible ACh molecule assumes different conformations and hence different molecular shapes and different interatomic distances at the nicotinic and muscarinic receptors. These conformations are imposed on the ACh molecule by the chemical and the steric nature of the receptor area.

A partition theory for the action of quaternary ammonium compounds at the muscarinic receptor site

The transition from agonist through partial agonist to antagonist observed in ascending a series of quaternary ammonium compounds, ▪, is interpreted in terms of the partitioning of the drug-receptor complex into active and inactive forms. A biological response requires ion-pair formation (active form) whereas in the inactive form the predominance of non-ionic binding forces, inherent in a longer alkyl chain, displaces the quaternary head from the anionic site.