EVIDENCE THAT CANNABINOID-INDUCED INHIBITION OF ELECTRICALLY-EVOKED CONTRACTIONS OF THE MYENTERIC PLEXUS - LONGITUDINAL MUSCLE PREPARATION OF GUINEA-PIG SMALL-INTESTINE CAN BE MODULATED BY CA2+ AND CAMP

Cannabinoid receptor agonists inhibit electrically evoked isometric co ntractions of the myenteric plexus - longitudinal muscle preparation o f the guinea-pig small intestine (MPLM), probably by reducing release of acetylcholine (ACh) through the activation of prejunctional CB1 rec eptors. As CB1 receptors are thought to be negatively coupled through G(i/o) proteins to both N-type Ca2+ channels and adenylate cyclase, we have now further investigated the involvement of CB1 receptors by mon itoring the effects of forskolin, 8-bromo-cAMP, 3-isobutyl-1-methylxan thine (IBMX), and extracellular Ca2+ on the ability of the cannabinoid agonist, (+)-WIN 55212 To inhibit electrically evoked contractions of the MPLM (0.1 Hz, 0.5.ms, and 110% maximal voltage). Some experiments were performed with normorphine instead of (+)-WIN 55212. At 10(-7) M , forskolin, 8-bromo-cAMP, and IBMX were found to reduce significantly the maximum inhibitory response to (+)-WIN 55212 by 49.4, 48.4, and 4 0.2%, respectively, without affecting control contractions or response s to exogenous ACh. Low external Ca2+ (0.64 mM) significantly increase d the maximum response to (+)-WIN 55212 and shifted the curve slightly leftwards, whereas high external Ca2+ (5.08 mM) reduced the maximum r esponse by 27.2%. The concentration-response curve to normorphine, whi ch also reduces evoked contractions of this preparation as a result of a presynaptic inhibition of ACh release via opioid mu receptors, was affected similarly. These results support the hypothesis that cannabin oid-induced inhibition in the MPLM is mediated by CB1 receptors.