Simultaneous effect of melatonin on supraoesophageal ganglion spontaneous electrical activity and photoreceptor electroretinogram amplitude in crayfish

Abstract

In crayfish, the supraoesophageal ganglion and the retina are neuronal structures whose electrical activities follow a circadian rhythm. The hormone melatonin (MEL) is an indoleamine, conserved from unicellular organisms to mammals, that is released rhythmically with a peak occurring in the darkness period. In vertebrates, MEL acts on numerous central and peripheral structures and participates in the modulation of circadian rhythms. The main purpose of this study was to present evidence of the role of MEL in crayfish circadian physiology, simultaneously recording the electrical activity of both the supraoesophageal ganglion and the retina under the free-running condition. We found that at circadian time (CT) 2, when the concentration of endogenous MEL is low, injection of this indoleamine into intact animals elicited a decrease in the level of spontaneous electrical activity recorded from the supraoesophageal ganglion and enhancement of electroretinogram amplitude. The MEL-induced effect mimics the level of these activities that was observed when the endogenous MEL concentration is high. These results suggest that MEL acts, simultaneously on both central and peripheral circadian oscillators, as a mediator to maintain the crayfish circadian system organization under the free-running condition.