Muscarinic M1 receptor agonist actions of muscarinic receptor agonists in rabbit vas deferens

Expression and function of G-protein-coupled receptorsin the male reproductive tract

This review focuses on the expression and function of muscarinic acetylcholine receptors (mAChRs), α1-adrenoceptors and relaxin receptors in the male reproductive tract. The localization and differential expression of mAChR and α1-adrenoceptor subtypes in specific compartments of the efferent ductules, epididymis, vas deferens, seminal vesicle and prostate of various species indicate a role for these receptors in the modulation of luminal fluid composition and smooth muscle contraction, including effects on male fertility. Furthermore, the activation of mAChRs induces transactivation of the epidermal growth factor receptor (EGFR) and the Sertoli cell proliferation. The relaxin receptors are present in the testis, RXFP1 in elongated spermatids and Sertoli cells from rat, and RXFP2 in Leydig and germ cells from rat and human, suggesting a role for these receptors in the spermatogenic process. The localization of both receptors in the apical portion of epithelial cells and smooth muscle layers of the vas deferens suggests an involvement of these receptors in the contraction and regulation of secretion.

Maturational and maintenance effects of testosterone on terminal axon density and neuropeptide expression in the rat vas deferens

Testosterone causes growth of many pelvic ganglion cells at puberty and their maintenance during adulthood. Here we have focused on two populations of pelvic ganglion cells that project to the rat vas deferens: noradrenergic neurons that innervate the smooth muscle and synthesize neuropeptide Y, and cholinergic neurons that primarily innervate the mucosa and contain vasoactive intestinal peptide. We have assessed the muscle innervation after pre- or postpubertal castration, using immunohistochemistry to determine axon density and radioimmunoassay to quantify levels of neuropeptides in tissue extracts. Our results show that androgen deprivation in each period causes substantial effects. Noradrenergic axons in the muscle increase in density after castration, partly due to organ size being smaller than age-matched controls. However, when corrected for target size, there is an overall decrease in total number of axons. This implies that androgen exposure at puberty has a direct effect on neurons to ensure that the adult pattern of innervation is attained, and that this is not simply by matching terminal field to target size. Similar effects of pre- and postpubertal castration imply that continued exposure to testosterone is necessary to maintain normal target innervation. Castration in both time periods increased the density of axons containing vasoactive intestinal peptide, however the effects of castration on the total number of these axons in the muscle were more variable. The concentration of vasoactive intestinal peptide increased substantially following either pre- or postpubertal castration although absolute amounts per vas deferens were decreased. Effects on neuropeptide Y concentration were less pronounced but the total amount per vas deferens was decreased after pre- or postpubertal castration. Our study shows that the action of testosterone (or a metabolite) on a pelvic ganglion cell soma is likely to reflect a change in its terminal field, but that these effects are not mediated simply by testosterone influencing the size of its target organ.

Receptor reserve of phosphoinositide-coupled muscarinic receptors in mouse hippocampus in vivo

The ability of the partial muscarinic agonist pilocarpine to increase in vivo phosphoinositide (PI) hydrolysis in mouse brain was compared to two full agonists. Pilocarpine increased in vivo phosphoinositide (PI) hydrolysis in cortex, striatum, and to the greatest extent in the hippocampus. Pilocarpine injected either subcutaneously or intracerebroventricularly robustly increased in vivo PI hydrolysis in hippocampus up to 500% of control levels and the increases were blocked by the muscarinic antagonist scopolamine. The increases in vivo PI hydrolysis induced by pilocarpine were 60-75% of the magnitude of the full muscarinic agonists oxotremorine-M and cis-dioxolane. The muscarinic M(1) preferring antagonist pirenzepine potently blocked pilocarpine-induced increases in in vivo PI hydrolysis, consistent with the increase being mediated by M(1) receptors. Since pilocarpine is a relatively weak partial agonist, these data suggest a substantial level of receptor reserve for the PI response in mouse hippocampus.

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

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 McN-A-343 for M1 receptors are therefore unfounded. The fact that McN-A-343 does not display a selective binding profile suggests that its selectivity does not arise from affinity differences but probably resides in its intrinsic efficacy.

Pharmacologic reversal of pertussis toxin-induced thermal allodynia in mice

We have previously demonstrated that the intrathecal administration of pertussis toxin produces a long-lasting thermal allodynia in mice. The purpose of the present studies was to compare the antinociceptive and the antiallodynic effects of drugs that are commonly used in treating neuropathic allodynia in untreated mice and in mice which had been administered vehicle or pertussis toxin intrathecally 7 days previously. In untreated mice, morphine, fentanyl, clonidine, oxymetazoline, desipramine and lidocaine, but not MK801, produced dose-related antinociception when tested using a 55 degrees C water tail-flick test. However, 7 days after the intrathecal injection of pertussis toxin, which induced a condition of thermal allodynia when tested using a 45 degrees C water bath, the full opioid and the full alpha(2)-adrenergic receptor agonists fentanyl and clonidine, but not the partial opioid nor the partial alpha(2)-adrenergic receptor agonists morphine and oxymetazoline, reversed the pertussis toxin-induced thermal allodynia. Moreover, lidocaine, desipramine, carbamazepine and MK801 failed to reverse the pertussis toxin-induced thermal allodynia. The present results suggest that decrements in G(i)/G(o)-protein function may be involved in initiating and/or maintaining some neuropathic pain states. Moreover, the results of the present study suggest that the use of full, but not partial, opioid or alpha(2)-agonists may be useful in the treatment of thermal allodynic pain states which may be due at least in part to inhibitory second messenger system dysfunction. Further, the underlying biochemistry of the apparent allodynic pain state induced by intrathecal administration of pertussis toxin warrants further investigation.

Direct pharmacological comparison of the muscarinic receptors mediating relaxation and contraction in the rabbit thoracic aorta

The purpose of the present studies was to directly compare the pharmacology of the muscarinic cholinergic receptors coupled to carbachol-induced relaxation and contraction of the intact and the endothelium-denuded rabbit thoracic aorta, respectively. The order of potencies of agonists for producing relaxation in the intact aorta was similar to that for producing contraction in the denuded aorta. In both preparations, the partial agonists pilocarpine, McN-A-343, and RS86 functioned as antagonists, indicating a lack of receptor reserve in both preparations. Further, the pA2 values for antagonists in both tissues were virtually identical and were consistent with the pharmacology of M3 receptors.

Xanomeline compared to other muscarinic agents on stimulation of phosphoinositide hydrolysis in vivo and other cholinomimetic effects

Activation of muscarinic m1 receptors which are coupled to the phosphoinositide (PI) second messenger transduction system is the initial objective of cholinergic replacement therapy in Alzheimer's disease. Thus, we evaluated the ability of the selective muscarinic receptor agonist (SMRA) xanomeline to stimulate in vivo phosphoinositide (PI) hydrolysis and compared it to a number of direct acting muscarinic agonists, two cholinesterase inhibitors and a putative m1 agonist/muscarinic m2 antagonist. Using a radiometric technique, it was determined that administration of xanomeline robustly stimulated in vivo PI hydrolysis and the effect was blocked by muscarinic antagonists, demonstrating mediation by muscarinic receptors. The non-selective muscarinic agonists pilocarpine, oxotremorine, RS-86, S-aceclidine, but not the less active isomer R-aceclidine, also effectively stimulated PI hydrolysis in mice. Amongst the putative m1 agonists, thiopilocarpine, hexylthio-TZTP as well as xanomeline effectively stimulated PI hydrolysis, but milameline, WAL 2014, SKB 202026 and PD 142505 did not significantly alter PI hydrolysis. Furthermore, WAL 2014 and SKB 202026 inhibited agonist-induced PI stimulation, suggesting that they act as antagonists at PI-coupled receptors in vivo. The cholinesterase inhibitors, tacrine and physostigmine, and the mixed muscarinic m1 agonist/m2 antagonist LU25-109 did not activate in vivo PI hydrolysis. Xanomeline, hexylthio-TZTP and thiopilocarpine were relatively free of cholinergic side effects, whereas milameline, WAL 2014 and SKB 202026 produced non-selective effects. Therefore, these data demonstrate that xanomeline selectively activates in vivo PI hydrolysis, consistent with activation of biochemical processes involved in memory and cognition and xanomeline's beneficial clinical effects on cognition in Alzheimers patients.

Comparison of cholinergic vasodilator responses to acetylcholine and methacholine in the human forearm

In order to study the contribution of the nitric oxide (NO)-pathway to cholinergic vasodilatation in the resistance vessels of the human forearm, we infused acetylcholine (ACh; 0.1 1000 ng/kg/min) or methacholine (MCh; 0.1 A 100 ng/kg/min) in the presence of saline, the NO-scavenger and guanylate cyclase inhibitor methylene blue (MB; 1000 ng/kg/min), or the NO-synthase inhibitor NG-monomethyl-L-arginine (L-NMMA; 30 micrograms/kg/min) into the brachial artery of normotensive volunteers (n = 32), using venous occlusion plethysmography. We calculated the plasma concentrations of the infused compounds to obtain EC50-values (-log mol/l). ACh and MCh both caused concentration-dependent vasodilatation (EC50-values of 6.43 +/- 0.05 and 7.24 +/- 0.08, respectively). MB (13 mumol/l) did not change basal forearm blood flow (FBF) when administered alone, but it markedly potentiated the vasodilator response to ACh, shifting the concentration-response curve (CRC) leftwards by 1.5 log-step (p < 0.001). MB did not affect MCh-induced vasodilatation. L-NMMA (1 mmol/l) alone caused dose-dependent vasoconstriction that was subject to tachyphylaxis. In addition, L-NMMA caused a steepening of the slopes of the CRCs of ACh, and MCh L-NMMA attenuated the ACh-/MCh-induced vasodilator responses in the lowest concentration ranges (p < 0.05) only, but did not alter the response at higher concentrations. The 10-fold higher potency of MCh compared to ACh can be explained by the more rapid degradation of ACh by cholinesterases. The observation that high concentrations of L-NMMA only affect vasodilation mediated by low concentrations of ACh or MCh, suggests a second mechanism in cholinergic vasodilatation, such as a direct effect on smooth muscle cells or the release of a relaxing factor other than NO.

Pathways involved in muscarinic M1 and M2 receptor stimulation in rabbit vas deferens

Pretreatment of the field-stimulated rabbit isolated vas deferens for 30 min with LiCl (2 x 10(-2) and 4 x 10(-2) M) attenuated the inhibition of neurogenic twitch contractions due to muscarinic M1 receptor stimulation by 4-(4-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium iodide (4-Cl-McN-A-343), and enhanced the muscarinic M2 receptor-mediated potentiation of contractions evoked by carbachol. When the tissues were preincubated for 5 min with the adenylate cyclase activator, forskolin (3 x 10(-8)-3 x 10(-7) M), the response to carbachol was attenuated whereas that to 4-Cl-McN-A-343 remained unchanged. 1,9-Dideoxy-forskolin (3 x 10(-7) and 10(-6) M), which fails to activate cyclase, did not abolish the carbachol effect. In addition, desensitization of the response to 4-Cl-McN-A-343 but not to carbachol occurred in preparations incubated for 90 min with the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA, 3 x 10(-8)-3 x 10(-7) M), whereas its inactive 4 alpha-stereoisomer (4 alpha-PMA, 3 x 10(-7) M) was without effect. In unstimulated preparations, LiCl, forskolin and PMA did not impair contractions due to exogenous ATP (10(-3) M). These findings are consistent with the hypothesis that, in rabbit vas deferens, inhibitory muscarinic M1 receptors stimulate LiCl-sensitive phosphatidylinositol turnover (IP3 pathway) involving protein kinase C, whilst excitatory muscarinic M2 receptors are coupled to inhibition of adenylate cyclase, resulting in reduced levels of cyclic AMP.