Central regulation of thyroidal status in a teleost fish: Nutrient stimulation of T 4 secretion and negative feedback of T 3

• Published in: Journal of Experimental Zoology Part A: Comparative Experimental Biology
• Main Authors: Leiner, Kevin A., Mackenzie, Duncan S.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2003
• Subjects: Red drum; Sciaenops ocellatus
• Online Access: http://dx.doi.org/10.1002/jez.a.10255; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fjez.a.10255; https://onlinelibrary.wiley.com/doi/pdf/10.1002/jez.a.10255

Abstract 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. J. Exp. Zool. 298A:32–43, 2003. © 2003 Wiley‐Liss, Inc.