ELISA Measurement of Plasma Melatonin Levels in the Fish Fundulus Heteroclitus Housed Under Two LD Cycles

The purpose of these studies was to (1) gain information on the feasibility of using a commercially available melatonin ELISA kit to measure melatonin in Fundulus heteroclitus and (2) determine the plasma melatonin levels throughout a 24-hr period. Field-collected male F. heteroclitus were housed in diluted artificial seawater in the laboratory for approximately four weeks in each study. Fluorescent bulbs suspended above the aquaria provided a daily cycle of 12 hrs light:12 hrs darkness (LD 12:12) or LD 14:10. In the LD 12:12 study, fish were sacrificed at mid-day (in the middle of the light period) and mid-dark (in the middle of the dark period). In the LD 14:10 study, fish were sacrificed at three-hour intervals throughout the LD cycle. Blood was collected from the caudal vein, and blood plasma was frozen until assay. An ELISA kit designed for humans was used to measure melatonin. In the preliminary study using LD 12:12, F. heteroclitus plasma melatonin was undetectable at mid-day and elevated at mid-dark. The more detailed melatonin profile obtained under LD 14:10 revealed low plasma melatonin levels during the light period and higher melatonin levels in the darkness (as expected). Mean melatonin levels were markedly elevated by 1 hr after lights off, and the greatest concentration of melatonin was measured at 4 hrs after lights off. Melatonin levels had begun to decline by 7 hrs after lights off