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 intake sustains thyroidal rhythmicity and that time of feeding may influence the amplitude, but not the phase, of diurnal thyroid hormone cycles. Several experiments were conducted to characterize the nature of thyroidal rhythmicity in a more derived perciform teleost, the red drum. These studies were designed 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 hormone rhythm. Circulating T(4) levels in this species exhibit high-amplitude diurnal 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 effect on the phase of the T(4) cycle, but altered the amplitude of the cycle. Dawn-fed fish had significantly greater mean peak levels of T(4) than dusk-fed fish, although there was no difference in daily mean levels in both groups of fish. When red drum were deprived of food, significant declines in plasma glucose, HSI, and liver glycogen content occurred within 3 days. When red drum were sampled once per day after 3, 7, or 11 days of food deprivation there were no consistent changes in circulating T(4) and T(3) levels compared to those of fed controls. However, significant declines in circulating T(4) and T(3) levels in response to food deprivation were detected with a diurnal sampling protocol. Within 3 days of food deprivation, T(4) levels were significantly reduced compared to those in fed controls and not significantly different from T(4) levels after 10 days of food deprivation. T(3) levels exhibited 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 cycles. These data also underscore the importance of incorporating a consideration of endocrine rhythmicity into sampling protocols.