Short Communication Release of Octopamine from the Photomotor Neurones of the Larval Firefly Lanterns

The biogenic amine, octopamine, has been proposed as the transmitter in the firefly light organ (Evans, 1980; Robertson, 1981). Whereas initial studies implicated adrenaline (Smalley, 1965; Carlson, 1968a), octopamine was found to be more potent in inducing luminescence than any amine other than synephrine (Carlson, 19686). Octopamine, but not synephrine, was found in adult lantern segments (Robertson & Carlson, 1976; Copeland & Robertson, 1982) and in larval lantern tissue (Christensen, Sherman, McCaman & Carlson, 1983). Evidence that cyclic nucleotides mediate the effect of octopamine was obtained by Oertel & Case (1976). An octopamine-sensitive adenylate cyclase has been found in the larval lantern and application of exogenous octopamine strongly stimulates cyclic AMP production in adult (Nathanson, 1979) and larval (Nathanson & Hunnicutt, 1979) light organs. The firefly photomotor neurones are dorsal unpaired median (DUM) neurones residing in the last (eighth) abdominal ganglion of the larva (Christensen & Carlson, 1982) and the last two (sixth and seventh) abdominal ganglia of the adult male (Christensen & Carlson, 1981). They are the only nerve cells in the lantern and have many features in common with DUM neurones described in locusts (Hoyle, Dagan, Moberly & Colquhoun, 1974) which have been shown to be octopaminergic (Evans & O'Shea, 1978). Synaptic profiles in adult (Smith, 1963) and larval (Peterson, 1970) photomotor nerve terminals are very similar to those in locust DUM neurone terminals (Hoyle et al. 1974). To be considered an unequivocal transmitter in the photomotor neurones octopamine must be released and detected under conditions associated with light production. In this report, experiments are described which demonstrate that octopamine can be detected after synaptic activation and that this release can be accomplished only in lanterns with an intact innervation. Photuris versicolor larvae were collected in early autumn 1984 and maintained at 10°C on moistened paper. The paired lanterns were removed from each larva and transferred to firefly saline (Carlson, 1968a).