ENDOGENOUS RELEASE OF OPIATES BY REPETITIVE ELECTRICAL FIELD STIMULATION IN THE GUINEA-PIG AND RAT ILEAL LONGITUDINAL MUSCLE

The effect of repetitive electrical field stimulation and the response of the guinea-pig and rat ileal longitudinal muscle to single pulse stimulations was examined. Single pulse field stimulation produced twitch contraction which was inhibited by repetitive field stimulation (10 Hz, 40V, 0.5 msec for 5 m). This inhibition was largely, though never completely, reversed by naloxone. Contractions due to exogenous acetylcholine and histamine were also inhibited after repetitive field stimulation. The inhibition of acetylcholine response was party reversed by naloxone whereas that of histamine was not. Contractions due to single pulse field stimulation or to either acetylcholine or histamine were inhibited by prior exposure to high concentrations of acetylcholine as a substitute for high frequency stimulation. The inhibitory responses were resistant to naloxone. The inhibitory responses to acetylcholine and histamine after exposure to the lowest concentration of acetylcholine was seen in preparations treated with tetrodotoxin or hemicholinium. The inhibition of the histamine response by acetylcholine pretreatment was prevented by mepyramine. Response to histamine, but not those to single pulse field stimulation or acetylcholine, were inhibited by prior exposure to histamine. It is concluded that repetitive field stimulation possibly initiates two distinct inhibitory processes. One involves the release of endogenous opiates and is probably mediated by inhibition of acetylcholine release. The second type of inhibition is not mediated by endogenous opiates and can be explained by post-junctional desensitization. The non-specific desensitization to histamine is probably a consequence of histamine release from mast cells by acetylcholine.