Interaction between octopamine and proctolin on the oviducts of Locusta migratoria

The biogenic amine octopamine and the pentapeptide proctolin are two important neuroactive chemicals that control contraction of the oviducts of the African locust Locusta migratoria. The physiological responses and signal transduction pathways used by octopamine and proctolin have been well characterized in the locust oviducts and this therefore provides the opportunity to examine the interaction between these two pathways. Octopamine, via the intracellular messenger adenosine 3′,5′-cyclic monophosphate (cyclic AMP), inhibits contraction of the oviducts, while proctolin, via the phosphoinositol pathway, stimulates contraction. We have examined the physiological response of the oviducts to combinations of octopamine and proctolin and also looked at how combinations of these affect one of the main intracellular mediators of the octopamine response, namely cyclic AMP. It was found that application of octopamine to the oviducts led to a dose-dependent reduction in tonus of the muscle and also a decrease in the amplitude and frequency of spontaneous phasic contractions. Octopamine-induced relaxation was enhanced in the presence of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). Octopamine was also able to inhibit proctolin-induced contractions of the oviducts in a dose-dependent manner. A 10−9 M proctolin-induced contraction was inhibited by 83% in the presence of 10−5 M octopamine, and was completely inhibited in the presence of 10−5 M octopamine plus 5×10−4 M IBMX. Octopamine led to a dose-dependent increase in cyclic AMP content as measured by radioimmunoassay. In the presence of 10−9 M proctolin, this octopamine-induced increase in cyclic AMP was reduced by as much as 60%. Proctolin also caused a dose-dependent decrease in the cyclic AMP elevation produced by 5×10−6 M octopamine. These results indicate that octopamine and proctolin can antagonize each otherʹs physiological response when added in combination, and that proctolin is able to modulate the response of the oviducts to octopamine by influencing cyclic AMP levels.