TILDEN, A.R.*; BRAUCH, R.; BALL, R.; SWEENEY, K.; YUREK, J.; COOPER, R.L.; Colby College; University of Kentucky; University of Kentucky; Colby College; Colby College; University of Kentucky: Modulatory Effects of Melatonin on Neurotransmitter Release and Behavior in Crayfish

The vertebrate pineal hormone melatonin, produced during darkness, affects a variety of circadian processes such as neurotransmitter release and behavioral activity. Crayfish melatonin production occurs in the eyestalks, and the cycle of production may change seasonally. Furthermore, previous studies have shown that melatonin affects the sensitivity of the visual system. To date, however, melatonin’s roles and mechanisms of action of melatonin in crustacean physiology are unclear. We examined the influence of melatonin on neurotransmitter release at the neuromuscular junction, and its effects on locomotor activity. Synaptic transmission is slightly enhanced with the exposure of 10 uM of melatonin on excised neuromuscular junctions of crayfish. Some of the enhancement is due to a presynaptic action on the motor terminals since more evoked release occurs. Next, we injected melatonin into crayfish at various times of day (1200, 1500, 1800, and 2100 h) and monitored locomotor activity with a videomonitoring system. Control crayfish were most active at mid-photophase and early scotophase. Crayfish injected with melatonin at mid-photophase had significantly greater activity than controls; those injected at late photophase were not influenced by melatonin; and those injected during early scotophase had significantly reduced activity. These results suggest a phase-response curve for melatonin sensitivity. Our study indicates that melatonin may have a phylogenetically conserved role in the transduction of circadian information in invertebrates as well as vertebrates.