Inhibition of field stimulation-induced contractions of rabbit vas deferens by muscarinic receptor agonists: selectivity of McN-A-343 for M1 receptors

Abstract

Inhibition of the field stimulation-induced twitch responses of the rabbit vas deferens by the muscarinic receptor agonist, McN-A-343, has been attributed to presynaptic muscarinic receptors of the M1 subtype located on noradrenergic nerve terminals. Stimulation of these receptors causes inhibition of transmitter release and inhibition of the contractile response. However, the selectivity of McN-A-343 for M1 receptors has been questioned and this throws doubt on whether the prejunctional receptors of the rabbit vas deferens are of the M1 subtype. In this study we have undertaken a comprehensive re-evaluation of the inhibition of prostatic and epididymal portions of the rabbit isolated field-stimulated vas deferens by several agonists, including McN-A-343, and quantified the antagonism by M1-selective antagonists, pirenzepine and telenzepine. Prostatic and epididymal portions of vasa deferentia from New Zealand White rabbits were immersed in a low Ca2+ Krebs solution at 32+/-0.5 degrees C gassed with 5% CO2 in oxygen. Yohimbine (1.0mM) was present throughout to block prejunctional alpha2-adrenoceptors. Field stimulation was applied by repeated application of single pulses (30 V, 0.05 Hz, 0.5 ms) and isometric contractions recorded. Carbachol and oxotremorine initially potentiated the epididymal contractions but at higher concentrations there was inhibition. In the prostatic portion, oxotremorine only inhibited. McN-A-343 produced inhibitory responses only in both epididymal and prostatic portions. Pirenzepine shifted the concentration-response curves forthe inhibitory responses to oxotremorine to the right. However, the potentiation of the twitches also became more apparent with the lower concentrations of oxotremorine. Schild plots for the antagonism by pirenzepine yielded pA2 values of 7.96+/-0.004 and 7.7+/-0.02 for the epididymal and prostatic portions, respectively. The concentration-response curves for the inhibition of twitches by McN-A-343 were displaced to the right in a parallel manner by pirenzepine in both prostatic and epididymal portions with no potentiation of the twitches. The Schild plot for this antagonism generated pA2 values of 7.68+/-0.01 and 8.07+/-0.01, respectively. Telenzepine caused parallel shifts of the McN-A-343 concentration-response curves to the right in prostatic portions, the pA2 value being 8.70+/-0.13. Telenzepine (10(-7) M) abolished the inhibitory effect of carbachol to reveal only concentration-dependent potentiation of the contractions. The Schild plot for antagonism of this contractile effect yielded a pA2 value (7.07+/-0.09) that was significantly less by almost two orders of magnitude (1.70) than the value for the antagonism by telenzepine of the McN-A-343-induced inhibitory response. The pA2 values of pirenzepine and telenzepine against the inhibitory responses of the rabbit vas deferens are consistent with the involvement of M1 receptors. This leads to the conclusion that McN-A-343 causes inhibition through this receptor type. The doubts concerning the selectivity of Mc...

The spinal antinociceptive effects of cholinergic drugs in rats: receptor subtype specificity in different nociceptive tests

“…This conclusion is based upon the selectivity of McN-A-343 for that receptor subtype and there is some controversy about this. Although there is the suggestion that it also binds with M 2 and M 3 receptor subtypes this has been refuted by recent studies [10]. However the suggestion for involvement of the M1 subtype in spinal cord by the current study does disagree with binding studies that concluded that there are no M 1 receptors in the rat spinal cord [22].…”

“…McN-A-343 has been proposed as a highly selective M 1 agonist [24] and this was confirmed in a recent study designed to settle doubts about the selectivity of McN-A-343 [10]. The results reported herein suggest that a spinal cord action of McN-A-343 causes significant dose dependent antinociception when determined by ECT (t).…”

“…That is also true for the novel muscarinic agonist McN-A-343 (M 1 -agonist) that was used in the study reported here. However, a recent study confirmed that McN-A-343 is a selective M 1 agonist [10]. One study used that selectivity of McN-A-343, which was injected intrathecally, to determine the receptor subtype involved with cholinergic antinociception at the level of the spinal cord.…”

Muscarinic suppression in stratum radiatum of CA1 shows dependence on presynaptic M1 receptors and is not dependent on effects at GABAB receptors

“…MCN-A-343 is highly debated, as reviewed by CaulWeld and Birdsall (1998) and by Wess (2003) and the more recent Davies and associates research (Davies et al, 2001). Experiment two focused on use of antagonists to superWcially test the contribution of the M2 receptor to ACh suppression by using the M2 antagonist gallamine as well as the potential indirect inhibition of transmission by ACh through GABA B receptor activity by utilizing a GABA B antagonist, SGS-742 (Green et al, 2000;Lacey & Curtis, 1994;Pozza, Manuel, Steinmann, Froestl, & Davies, 1999).…”

“…The Wrst experiment was designed to clarify the role of the m1 receptor and its potential contribution to presynaptic inhibition in stratum radiatum of CA1 compared to previous work from this laboratory using the ACh agonist muscarine. Further, the M2-selective agonist oxotremorine (Puolivali, Jakala, Koivisto, & Riekkinen, 1998;Ringdahl & Jenden, 1983a, 1983b and purported M1-selective agonist MCN-A-343 (CaulWeld & Birdsall, 1998;Davies, Scholes, Virdi, & Broadley, 2001;Wess, 2003) were also administered to allow comparisons and incorporation of behavioral eVects in the neural models. However, the selectivity of…”