Central regulation of thyroidal status in a teleost fish: nutrient stimulation of T4 secretion and negative feedback of T3.
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Several experiments were conducted to investigate the dynamics of central regulation of thyroid function in the red drum, Sciaenops ocellatus, by manipulating a well-characterized circadian rhythm of T(4) secretion. In the first experiment, red drum were reared under either a long (16L:8D) or short (8L:16D) photoperiod and fed at the same time relative to dawn. The same feeding time under different photoperiods maintained the same phase relationship between T(4) cycles under each photoperiod. This suggests that the circadian clock that determines when the hypothalamus-pituitary-thyroid (HPT) axis is activated is comprised of a feeding-entrained oscillator and a light-entrained oscillator that interact to determine the phase of the T(4) rhythm. Additionally, the amplitude of the main T(4) peak of the cycle was inversely related to the frequency of feeding, while the duration of the main T(4) peak was directly related to feeding frequency under a long photoperiod. Feeding time appears to modify the diurnal profile of circulating T(4) by stimulating post-prandial T(4) secretion that subsequently results in negative feedback on the HPT axis to regulate thyroidal status. In following experiments, red drum immersed in T(3), in lieu of a meal at a specific time that would diminish the main T(4) peak, exhibited a dose-dependent decline in amplitude of the T(4) cycle. This demonstrates that T(3) can exert negative feedback on the HPT axis of red drum to maintain appropriate thyroid hormone concentrations. These data are consistent with a dynamic and physiologically important central component of the regulation of thyroid function in fish.