Melatonin and serotonin N-acetyltransferase activity in developing eggs of the cricket Gryllus bimaculatus

Melatonin (N-acetyl-5-methoxytryptamine) and serotonin N-acetyltransferase (NAT), a key regulatory enzyme in melatonin synthesis, are present in the adults and larvae of several insect species, as well as in vertebrates. To determine when melatonin and NAT first appear in insects ontogenetically, melatonin levels and NAT-like activity were measured in developing eggs of the cricket Gryllus bimaculatus. When the eggs were incubated under a 12-h light/12-h dark (LD) cycle at 24-26 degrees C, melatonin was detected in the egg extracts at all of the developmental stages examined. NAT-like activity was first found in the eggs 3 days after oviposition. From 5 to 11 days after oviposition, both NAT-like activity and melatonin levels showed significant day/night changes with the high levels occurring during the dark period of the LD cycle. By contrast, significant day/night changes were not detected in eggs just before hatching. To determine more detailed temporal changes, NAT-like activity was assayed in eggs 6 to 7 days after oviposition at 2- or 4-h intervals over a 48-h period. The activity in the eggs clearly exhibited a diurnal rhythm, peaking in the dark period of the LD cycle, and the rhythm persisted in constant darkness. These results suggest that the cricket egg (probably the embryo) synthesizes melatonin, and that its melatonin synthesis may fluctuate with a circadian rhythm. In addition, the results of the present study strongly suggest that a circadian clock controlling NAT activity functions in the cricket at the embryonic stage.