J. Anim Sci.
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J. Anim Sci. 1986. 62:1012-1020.
© 1986 American Society of Animal Science
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Serum Concentrations of Prolactin, Thyroxine and Triiodothyronine Relative to Season and the Estrous Cycle in the Mare1,2,

A. L. Johnson

Rutgers, The State University3, New Brunswick, NJ 08903

Abstract

Studies were conducted to characterize circulating concentrations of prolactin, thyroxine (T4) and triiodothyronine (T3) relative to season and the estrous cycle, and to evaluate the ability of thyrotropin-releasing hormone (TRH) to induce prolactin secretion in the horse mare. The increase in serum prolactin following the iv injection of 0, 5, 50 and 500 µg TRH was dose-related, while all three doses of TRH induced a significant and comparable increase in serum T4. Seasonal variations in serum prolactin were directly correlated (P<.001) with changes in photoperiod (r=.80) and temperature (r=.61). By comparison, concentrations of T3 were inversely related to temperature (r=-.49; P<.001), while changes in T4 were less closely associated with photoperiod (r=-.29; P=.04). There were no significant fluctuations in prolactin, T3 or T4 at any stage of the estrous cycle. These results demonstrate that concentrations of prolactin in the mare are highest during the summer and lowest during the winter, but it is unknown whether this annual rhythm is controlled by photoperiod, temperature and(or) other environmental factors.

Footnotes

1 Publication D-06106-01-85 of the New Jersey Agr. Exp. Sta. This research was supported in part by Biomed. Res. Support Grant (PHS RR 07058-17).

2 The author thanks C. Brown, S. Becker and J. Weithenauer for technical assistance and C. J. Uckele for manuscript preparation.

3 Dept. of Anim. Sci.






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Copyright © 1986 by the American Society of Animal Science.