Regulation of diurnal rhythms of circulating thyroid hormones in the red drum, Sciaenops ocellatus /
Diurnal rhythms of circulating thyroid hormones have been observed in several fish species, although the regulation of these cycles is still largely unknown. To determine the relative contribution of the light-cycle or nutrient intake to the maintenance of diurnal thyroid hormone rhythms in the red...
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| Format: | Thesis Book |
| Language: | English |
| Published: |
[Place of publication not identified] :
[publisher not identified] ;
2000.
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| Subjects: | |
| Online Access: | http://proxy.library.tamu.edu/login?url=http://proquest.umi.com/pqdweb?did=731990211&sid=1&Fmt=2&clientId=2945&RQT=309&VName=PQD |
| Summary: | Diurnal rhythms of circulating thyroid hormones have been observed in several fish species, although the regulation of these cycles is still largely unknown. To determine the relative contribution of the light-cycle or nutrient intake to the maintenance of diurnal thyroid hormone rhythms in the red drum, a series of experiments involving the manipulation of photoperiod or feeding time was conducted. Red drum exhibited a high amplitude bimodal rhythm of circulating T₄ levels, that in some cases increased ten-fold above basal values within twelve hours. In contrast, plasma T₃ levels were maintained within a relatively narrow range, and rarely fluctuated more than two-fold above basal values. Plasma T₄ and T₃ levels declined significantly during a time course of food deprivation, and the significant decline in plasma T₄ levels observed within 3 days of food deprivation suggests an important central component of prohormone supply to conserve nutrient stores. Light was the main environmental cue that determined the phase of the T₄ cycle, while the amplitude and duration of the component peaks of the T cycle were modified by feeding time. The amplitude of the T₄ rhythm was inversely related to feeding frequency, while the duration of the main T₄ peak was directly related to feeding frequency. These results indicated an interplay between nutrient stimulation of T₄ secretion, resulting in post-prandial T₄ peaks, and subsequent negative feedback. This was confirmed by immersing red drum in T₃, which resulted in the complete ablation of the main T₄ peak, demonstrating that the HPT axis in this fish is capable of dynamic regulation to maintain appropriate plasma thyroid hormone levels. Additionally, these studies provide the first evidence in a fish that this diurnal T₄ rhythm is an endogenous circadian rhythm that is entrained by dawn. Red drum were acclimated to a 12L:12D photoperiod and fed once daily at variable times before the lighting was switched to constant dim illumination. The T₄ rhythm persisted and free-ran for three complete cycles under constant conditions, further implicating a central component in the regulation of thyroidal status. |
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| Item Description: | Vita. "Major Subject: Zoology". |
| Physical Description: | xv, 151 : illustrations ; 28 cm. Issued also on microfiche from University Microfilm Inc. |
| Bibliography: | Includes bibliographical references (leaves 135-150 |