Octopamine receptors on Aplysia neurones mediate hyperpolarisation by increasing membrane conductance.

Abstract

OCTOPAMINE is a phenylethylamine synthesised by β-hydroxylation of tyramine and differs from noradrenaline in lacking one hydroxyl group on the phenyl ring. Although Aplysia nervous tissue, like that of most molluscs, contains little or no noradrenaline1,2, it has between 0.20 and 1.85 pmol octopamine per mg of tissue in the various ganglia. Individual identified neurones also contain detectable octopamine and in neurone R14 it reaches 1.5×10−4 M (ref. 2). Although octopamine-activated adenyl cyclases have been found in Aplysia nervous tissue3 and insects4,5, there has been no electrophysiological demonstration of specific octopamine receptors. We now report receptors in the nervous system of Aplysia that are most sensitive to octopamine and mediate a hyperpolarising conductance-increase response. This provides further evidence that octopamine can function as a neurotransmitter.