The interaction of McN-A-343 with pirenzepine and other selective muscarine receptor antagonists at a prejunctional muscarine receptor

Allosteric modulation of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors (mAChRs) are prototypical Family A G protein coupled-receptors. The five mAChR subtypes are widespread throughout the periphery and the central nervous system and, accordingly, are widely involved in a variety of both physiological and pathophysiological processes. There currently remains an unmet need for better therapeutic agents that can selectively target a given mAChR subtype to the relative exclusion of others. The main reason for the lack of such selective mAChR ligands is the high sequence homology within the acetylcholine-binding site (orthosteric site) across all mAChRs. However, the mAChRs possess at least one, and likely two, extracellular allosteric binding sites that can recognize small molecule allosteric modulators to regulate the binding and function of orthosteric ligands. Extensive studies of prototypical mAChR modulators, such as gallamine and alcuronium, have provided strong pharmacological evidence, and associated structure-activity relationships (SAR), for a "common" allosteric site on all five mAChRs. These studies are also supported by mutagenesis experiments implicating the second extracellular loop and the interface between the third extracellular loop and the top of transmembrane domain 7 as contributing to the common allosteric site. Other studies are also delineating the pharmacology of a second allosteric site, recognized by compounds such as staurosporine. In addition, allosteric agonists, such as McN-A-343, AC-42 and N-desmethylclozapine, have also been identified. Current challenges to the field include the ability to effectively detect and validate allosteric mechanisms, and to quantify allosteric effects on binding affinity and signaling efficacy to inform allosteric modulator SAR.

Application of an allosteric ternary complex model to the technique of pharmacological resultant analysis

A simple ternary complex model of drug-receptor interaction has been used to extend the procedure of pharmacological resultant analysis, enabling the quantitation of interactions between allosteric modulators and orthosteric antagonists. Equations derived in the theoretical treatment were used to analyse functional data for the interaction between the allosteric modulator gallamine and the orthosteric antagonist scopolamine, with oxotremorine as the agonist, at rat tracheal muscarinic acetylcholine receptors. Quantitative estimates of the affinity of gallamine for the allosteric site (pKz = 4.7) and the extent of negative, heterotropic co-operativity between gallamine and scopolamine (alpha' = 13.1) were obtained. Furthermore, an alternative direct, model-fitting approach, that does not rely on the determination of concentration ratios, was also developed, and yielded similar results. It is suggested that the approach presented in this paper is useful for quantifying interactions between orthosteric antagonists and allosteric modulators, particularly when the extent of co-operativity is low or the modulators possess multiple pharmacological properties, or both.

Evidence for facilitatory and inhibitory muscarinic receptors on postganglionic sympathetic nerves in mouse isolated atria

1. McNeil A 343 (10 microM-30 microM) enhanced the fractional stimulation-induced (S-I) outflow of radioactivity from mouse isolated atria which had been incubated with [3H]-noradrenaline. The enhancing effect of McNeil A 343 was not altered by hexamethonium (300 microM) suggesting that it was not due to an action at nicotinic receptors. It is also unlikely that McNeil A 343 enhanced the S-I outflow of radioactivity in mouse atria by blocking neuronal reuptake of noradrenaline since the effect persisted in the presence of cocaine (30 microM). 2. The facilitatory effect of McNeil A 343 on the S-I outflow of radioactivity was attenuated by atropine (0.3 microM), pirenzepine (0.2 microM or 1.0 microM), dicyclomine (1.0 microM) and methoctramine (1.0 microM) and was thus due to activation of muscarinic receptors. 3. In contrast to the effect of McNeil A 343, another muscarinic receptor agonist, carbachol (3.0 microM) significantly decreased the S-I outflow of radioactivity. The receptors through which McNeil A 343 acts to enhance the S-I outflow of radioactivity appear to be distinct from inhibitory prejunctional muscarinic receptors. The relatively M 1-selective antagonist, pirenzepine (0.2 microM), attenuated the facilitatory effect of McNeil A 343 whereas a higher concentration (1.0 microM) was required to block the inhibitory effect of carbachol. Conversely, the relatively M2-selective antagonist, methoctramine (0.1 microM), blocked the inhibitory effect of carbachol but a higher concentration of methoctramine (1.0 microM) was required to block the facilitatory effects of McNeil A 343. These results tentatively ascribe facilitatory muscarinic receptors as belonging to the Ml subtype and inhibitory muscarinic receptors as belonging to the M2 subtype. 4. The non-selective muscarinic receptor antagonist, atropine, enhanced the S-I outflow of radioactivity, suggesting that there was tonic activation of inhibitory prejunctional muscarinic receptors by endogenous acetylcholine released from parasympathetic nerves. However, pirenzepine (0.03 pM-LO microM) did not decrease the S-I outflow of radioactivity, suggesting that under the conditions of the present study, facilitatory muscarinic receptors are not tonically activated by endogenous acetylcholine.

Selective inhibition of responses to carbachol and McN-A-343 in the rabbit vas deferens

1. The effect of several selective muscarine receptor antagonists were evaluated on the responses of carbachol (CCh) and McN-A-343 (McN) during sympathetic nerve stimulation in the rabbit vas deferens. 2. The muscarine M1 receptor antagonist pirenzepine exhibited similar apparent pKB values for antagonism of the prejunctional inhibitory response of either CCh (pKB, 8.2) or McN (pKB, 8.5) on sympathetic nerve stimulation. 3. The muscarine M2 receptor antagonists, pancuronium and the bisalkyl ammonium compound 'C7/3-phth' were selective inhibitors of the postjunctional facilitatory response produced by CCh on sympathetic nerve stimulation. They were also 17- and three-fold, respectively, less potent against the inhibitory responses of McN on sympathetic nerve stimulation. The apparent pKB value of pancuronium on the inhibitory response produced by CCh did not differ significantly (P greater than 0.05) from that using McN. A similar finding was made for C7/3-phth. 4. Selective blockade of the inhibitory response to CCh with pirenzepine (0.03 or 0.5 mumol/L) did not significantly (P greater than 0.05) modify the apparent pKB value for pancuronium on the facilitatory response of CCh. 5. Selective blockade of the facilitatory response to CCh with a low concentration of pancuronium (0.5 mumol/L) did not significantly (P greater than 0.05) modify the apparent pKB value for pancuronium (30 mumol/L) on the inhibitory response of CCh. 6. It is suggested that CCh and McN activate the same prejunctional M1 muscarine receptor and that pancuronium is the most selective of the muscarine M2 receptor antagonists presently tested in this preparation for distinguishing between muscarine M1 and M2 receptors.

Heterogeneity of vascular muscarinic receptors

Muscarinic receptors mediate diverse effects on the vasculature. Recently, a consensus has been arrived at with regard to muscarinic receptor classification (Levine & Birdsall, 1989). As a result, it may now be possible to clarify the role of each subtype in the responses of vascular tissues to muscarinic agonists. It is apparent that vascular muscarinic receptors form a heterogeneous population. M1 receptors contract canine venous tissue, whilst M3 receptors contract porcine and bovine coronary arteries. M3 receptors also mediate EDRF-dependent relaxant responses in the majority of tissues studied to date. M2 receptors elicit relaxations by a decrease in sympathetic outflow in canine femoral vein, rabbit ear artery and rat portal vein. These conclusions are primarily derived from functional estimations of equilibrium dissociation constants, since comparable radioligand binding data are both scarce and contradictory. It is concluded that all three major subtypes of receptors are present in the vasculature. However, the limited selectivity of the available antagonists, the lack of extensive use of such compounds and the unavailability of selective agonists clearly indicate the need for more definitive studies to be undertaken.

Muscarinic M1- and M2-receptors mediating opposite effects on neuromuscular transmission in rabbit vas deferens

Twitch contractions of the rabbit vas deferens elicited by electrical field stimulation were inhibited by tetrodotoxin, guanethidine, bretylium and alpha,beta-methylene-ATP but were unaffected by hexamethonium, physostigmine, 1,1-dimethyl-4-phenylpiperazinium and prazosin, suggesting that they resulted from ATP released following postganglionic sympathetic nerve stimulation. McN-A-343 inhibited but carbachol and several other muscarinic agonists potentiated the twitch contractions; these effects were not modified by hexamethonium or physostigmine. Muscarinic agonists had no effect on the tension in unstimulated organs whereas contractions elicited by ATP, noradrenaline and KCl were potentiated by carbachol but remained unaffected by McN-A-343. The responses of the twitch contractions to McN-A-343 and carbachol were inhibited to different degrees by antimuscarinic drugs: the affinity (pA2) of atropine, secoverine and himbacine against McN-A-343 and carbachol was similar. However, pirenzepine, telenzepine, trihexyphenidyl, dicyclomine and hexahydro-sila-difenidol displayed preferential antagonism of the responses to McN-A-343 whereas the converse was true for AF-DX 116 and gallamine. The highly significant correlation between the pA2 values obtained for 10 antagonists against carbachol responses in rabbit vas deferens and rat left atrium suggests that the receptors may be similar. The data support the presence of a presynaptic M1-receptor mediating inhibition and a postsynaptic, cardiac-like M2-receptor responsible for enhancing neurogenic contractions in rabbit vas deferens.

A quantitative study of the effects of some muscarinic antagonists on the guinea-pig olfactory cortex slice

1. Muscarinic depression of the electrically-evoked surface-negative field potential (N-wave) was measured in guinea-pig olfactory cortex slices maintained in vitro. 2. The effects of three muscarinic receptor antagonists, pirenzepine, atropine and gallamine on this muscarinic response were analysed in detail. 3. Pirenzepine was a potent competitive antagonist of carbachol (CCh)-evoked responses. Schild plot analysis yielded a pA2 value of 7.9 (Schild slope constrained to unity). A similar analysis for atropine versus CCh responses gave a pA2 of 8.9. 4. Combination experiments using pirenzepine and atropine produced dose-ratio shifts close to those expected for two antagonists competing for a similar receptor site. 5. Gallamine was only a weak antagonist of responses to CCh. 6. Oxotremorine behaved as a competitive antagonist at this muscarinic receptor (pA2 = 6.1). 7. It is concluded that the presynaptic muscarinic receptor mediating depression of the N-wave in the olfactory cortex slice is of the M1-subtype.

The interaction of nicotinic receptor antagonists at muscarinic receptors

1. The interaction of some neuromuscular blocking drugs such as gallamine, pancuronium and stercuronium with muscarinic receptors has several features which distinguish these compounds from competitive antagonists in functional and binding studies. 2. They also differentiate between muscarinic receptors and may produce effects on ion channels.

Interaction of some muscarinic agonists and antagonists at the prejunctional muscarinic receptor in the rabbit ear artery preparation

The inhibitory effect of several muscarinic agonists on responses to sympathetic nerve stimulation of the isolated perfused ear artery of the rabbit was compared to that of acetylcholine in preparations pretreated with dyflos, cocaine and yohimbine. In general the potency of the agonists was similar to that observed at peripheral muscarinic sites except for arecaidine propargyl ester and 4-(m-chlorophenylcarbamoyloxy)-2-butynyl trimethylammonium chloride (McN-A-343). The inhibitory effect observed with N-benzyl-3-pyrrolidyl acetate methobromide (AHR-602) was not exerted via muscarinic receptors. With carbachol (CCh) as an agonist, the antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) was found to have a pKB value of 7.74 and thus was 19 fold less active as an antagonist of responses to the agonist, than previously reported for guinea-pig ileum. When McN-A-343 was used as the agonist, the slope of the Schild plot with the antagonist was significantly less than unity. It is suggested that an allosteric interaction of 4-DAMP may be involved with this agonist. By use of an allosteric model, a pKB of 8.56 for 4-DAMP was obtained. Secoverine produced similar pKB values with either CCh (8.19) or McN-A-343 (8.13) as the agonist.