Effect of 6-hydroxydopamine and 5,6-dihydroxytryptamine on the response of the coaxially stimulated guinea-pig ileum to morphine.

Studies were conducted to determine the role of norepinephrine and 5-hydroxytryptamine in the action of morphine in the coaxially stimulated guinea-pig ileum. 6-hydroxydopamine produced supersensitivity to norepinephrine and decreased the levels of norepinephrine in the ileum. 6-Hydroxydopamine did not interfere with the acute effects of morphine but did interfere with the degree of tolerance developed to morphine, which is in contrast to reported results of the effect of 6-hydroxydopamine on the analgesic response to morphine. No evidence was found that 5,6-dihydroxytryptamine altered the acute or chronic response of the ileum to morphine. Again this is in contrast to results for the analgesic receptor where 5,6-dihydroxytryptamine has been reported to inhibit the development of tolerance to morphine. Thus, the role of the biogenic amines in the action of morphine in the ileum appears to differ from their reported role in the action of morphine in the central nervous system.