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 contractions of the myenteric plexus - longitudinal muscle preparation of the guinea-pig small intestine (MPLM), probably by reducing release of acetylcholine (ACh) through the activation of prejunctional CB 1 receptors. As CB 1 receptors are thought to be negatively coupled through G i/o proteins to both N-type Ca 2+ channels and adenylate cyclase, we have now further investigated the involvement of CB 1 receptors by monitoring the effects of forskolin, 8-bromo-cAMP, 3-isobutyl-1-methylxanthine (IBMX), and extracellular Ca 2+ 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 40.2%, respectively, without affecting control contractions or responses to exogenous ACh. Low external Ca 2+ (0.64 mM) significantly increased the maximum response to (+)-WIN 55212 and shifted the curve slightly leftwards, whereas high external Ca 2+ (5.08 mM) reduced the maximum response by 27.2%. The concentration-response curve to normorphine, which also reduces evoked contractions of this preparation as a result of a presynaptic inhibition of ACh release via opioid µ receptors, was affected similarly. These results support the hypothesis that cannabinoid-induced inhibition in the MPLM is mediated by CB 1 receptors.