Pharmacological Analysis of Cannabinoid Receptor Activity in Isolated Nerve-smooth Muscle and Epithelial Preparations

This study was directed at characterising the cannabinoid receptor activity modulating the electrical field stimulation (EFS) evoked contractions of the rat isolated ileum myenteric plexus longitudinal muscle (MPLM) preparation, and the capsaicin, nicotine and veratridine evoked secretory responses of the rat isolated colonic submucosal plexus-mucosal (SPM) preparation. EFS of the MPLM preparation with single pulses at a repetition frequency of 0.05 Hz elicited a transient twitch contraction immediately in response to each electrical pulse. In contrast, stimulation of the MPLM preparation with 2 second trains of pulses every minute at a frequency of 30 Hz elicited a rapid transient rebound contraction on termination of each train of EFS. The non-selective cannabinoid receptor agonists AEA, CP 55,940, Δ-THC and WIN 55,212-2 inhibited both EFS-evoked twitch and rebound contractions of the rat ileum MPLM elicited by 0.05 Hz and 30 Hz EFS respectively. The inhibition of the twitch contractions was competitively antagonised by the cannabinoid CB1 receptor antagonist / inverse agonist SR 141716 with pKB values of 8.60. In contrast, SR 141716 only antagonised the ability of AEA, Δ-THC and WIN 55,212-2 but not CP 55,940 to inhibit the rebound contractions with pA2 values of 6.60. These observations extended to the inhibitory effect of WIN 55,212-2 on the twitch and rebound contractions of the guinea-pig ileum MPLM. The CB2 antagonist / inverse agonist SR 144528 did not alter the effects of the agonists. Additionally, the inhibitory effect of AEA was refractory to the vanilloid TRPV1 receptor antagonist capsazepine. WIN 55,212-3 a stereoisomer of WIN 55,212-2 was without effect on the rat MPLM. SR 141716 alone concentration-dependently increased the twitch contractions but inhibited the rebound contractions. Both types of the EFS-evoked contractions were abolished by the Na channel blocker tetrodotoxin or the muscarinic acetylcholine (ACh) receptor antagonist atropine but not the nicotinic ACh receptor antagonist hexamethonium. None of the cannabinoids altered the contractions to exogenously applied ACh. These data suggested that the cannabinoid agonists inhibited the twitch contractions through a stereospecific presynaptic CB1 receptor-mediated reduction in the release of