Identified octopaminergic neurons modulate contractions of locust visceral muscle via adenosine 3',5'-monophosphate (cyclic AMP)

Abstract

D,L-Octopamine elevates the cyclic AMP content of the lateral oviduct of the locust, Locusta migratoria, in a dose-dependent manner with a threshold of about 10(-8) M. The effect of octopamine is potentiated by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). The response is specific for octopamine and synephrine with an order of potency being octopamine = synephrine greater than metanephrine greater than tyramine greater than norepinephrine = dopamine = 5-hydroxytryptamine and the effect of octopamine is inhibited by the alpha-adrenergic receptor antagonist phentolamine. The diterpene adenylate cyclase activator forskolin also elevates cyclic AMP levels and IBMX potentiates the action of forskolin. Stimulation of the two identified octopaminergic neurons which project to the lateral oviducts results in an elevation in cyclic AMP and again this effect is blocked by phentolamine. Elevation of cyclic AMP levels in the lateral oviducts by means of IBMX, forskolin or dibutyryl cyclic AMP mimics the physiological effects of octopamine on this preparation. The results indicate that the octopaminergic control of this insect's visceral muscle is mediated via cyclic AMP.