A neuroendocrine releasing effect of melatonin in the brain of an insect, Periplaneta americana (L.).

Melatonin exists in nearly all organisms, but little is known of its function in non-vertebrates. Long-term perifusions as well as short-term batch incubations of brains and molting glands of the cockroach Periplaneta americana were used to test the influence of melatonin on the prothoracicotropic hormone, a glandotropic neuropeptide in the brain, which stimulates the production of the molting hormone ecdysone in the molting gland. Changes of ecdysteroid production in molting glands were determined by radioimmunoassay as ecdysone equivalents. Melatonin (10 nmol/L) was without effect on the prothoracic gland but stimulated the prothoracicotropic effect of brains in both in vitro investigations, long-term perifusions and short-term batch incubations. The effect was dose-dependent. The melatonin effect on the release of prothoracicotropic hormone in the brain was suppressed by luzindole (10 nmol/L), a pre-synaptic receptor antagonist of melatonin. The retro-cerebral complex (corpora cardiaca-corpora allata) did not seem to be involved in the effect of melatonin on the brain. Serotonin (10 nmol L) suppressed the release of prothoracicotropic hormone. This is the first experimental evidence of a neurohormonal releasing effect of melatonin in the insect nervous system.