The effects of photoperiod and feeding on the diurnal rhythm of circulating thyroid hormones in the red drum, Sciaenops ocellatus

Available data in cyprinid and salmonid species indicate that nutrient inta ke sustains thyroidal rhythmicity and that time of feeding may influence th e amplitude, but not the phase, of diurnal thyroid hormone cycles. Several experiments were conducted to characterize the nature of thyroidal rhythmic ity in a more derived perciform teleost, the red drum. These studies were d esigned to test the following hypotheses: (1) that feeding time will alter the amplitude of the thyroid hormone rhythm without altering its phase and (2) that food deprivation will diminish the amplitude of the thyroid hormon e rhythm. Circulating T-4 levels in this species exhibit high-amplitude diu rnal rhythms, whereas circulating T-3 levels fluctuate within a more narrow range. Fish were reared under a 12L:12D photoperiod and fed 5% body weight once daily either at dawn or at dusk. Feeding time had no discernible effe ct on the phase of the T-4 cycle, but altered the amplitude of the cycle. D awn-fed fish had significantly greater mean peak levels of T-4 than dusk-fe d fish, although there was no difference in daily mean levels in both group s of fish. When red drum were deprived of food, significant declines in pla sma glucose, HSI, and liver glycogen content occurred within 3 days. When r ed drum were sampled once per day after 3, 7, or 11 days of food deprivatio n there were no consistent changes in circulating T-4 and T-3 levels compar ed to those of fed controls. However, significant declines in circulating T -4 and T-4 levels in response to food deprivation were detected with a diur nal sampling protocol. Within 3 days of food deprivation, T-4 levels were s ignificantly reduced compared to those in fed controls and not significantl y different from T-4 levels after 10 days of food deprivation. T-3 levels e xhibited a stepwise decline in circulating levels during food deprivation. These data indicate that both feeding time and nutrient status exert their effects on thyroid hormone rhythms by modifying the amplitude of these cycl es. These data also underscore the importance of incorporating a considerat ion of endocrine rhythmicity into sampling protocols. (C) 2000 Academic Pre ss.