Submission Number: HEA-4-20-11

Abstract Number: 54

THYROID HORMONES AND SEASONAL REPRODUCTION: TEMPORAL ASPECTS OF THYROID HORMONE ACTION AND MODE OF INTERACTION WITH A CIRCANNUAL RHYTHM.

Heather J Billings* ¹, John M Connors ², Robert L Goodman* ², Fred J Karsch* ¹, Catherine Viguie* ¹ and Greg M Anderson* ²

Reproductive Sciences Program, University of Michigan, Ann Arbor, MI ¹
Department of Physiology, West Virginia University, Morgantown, WV ²

Abstract:
Thyroid hormones are required for expression of one stage of the circannual reproductive rhythm in the ewe, the transition to anestrus. Thyroid hormones appear to act in this regard during a limited "window" of time beginning mid-late breeding season. This experiment was conducted: 1) to determine when this window of responsiveness ends, and 2) to distinguish whether thyroid hormone plays a role in generating the circannual rhythm or in relaying its output to the GnRH/LH system. Suffolk ewes were ovariectomized and thyroidectomized (THX) late in the breeding season and treated with estradiol implants. Four groups of ewes (n=5-7) were injected s.c. with thyroxine (T4) for 100 days beginning progressively later in the year (Jan, Apr, Jun, Aug; dose 3g/kg daily for 1 week, then 5g/kg 3 times/week). Controls were either thyroid intact or THX without T4 treatment. Serum collected twice weekly was used to monitor changes in the LH response to estradiol negative feedback, indicative of seasonal reproductive condition. In thyroid-intact controls, LH fell to undetectable levels in mid-Jan, marking the transition to anestrus. As expected, this fall was prevented in 5 of 6 THX controls. LH fell to undetectable levels in 4 of 5 ewes in the Jan group and all ewes in the Apr group, although latency to the LH fall was longer in the Apr group (37.0 4.7 vs. 62.3 7.3 days; p<0.05). In contrast, LH fell to undetectable levels in only 3 of 5 ewes in the Jun group and in none of the ewes in the Aug group, indicating loss of responsiveness to T4. In those ewes in which LH fell, LH rose again during the next breeding season, but this rise was delayed in the Apr (Oct 8 6.7 days, p<0.05) and June (Nov 8 8.1 days, p<0.05) groups, compared to the Jan group (Sep 6 7.8 days) and thyroid-intact controls (Sep 4 4.7 days). Our results indicate that the seasonal loss in responsiveness of the reproductive neuroendocrine axis to T4 occurs gradually throughout the spring, and by Aug the window has ended. Further, the delay in onset of the next breeding season in the Apr and Jun groups indicates that, in the absence of T4 in winter and spring, the circannual rhythm itself is delayed. Thus, T4 appears to play a fundamental role in the endogenous reproductive rhythm of the ewe. (Supported by USDA 97-35203-4908, NIH P30 HD18258 and NIH HD17864).

Keywords: seasonality, thyroid hormone, endogenous rhythm, sheep

This abstract is being presented at: 8:00 AM in session:
TRAINEE RESEARCH COMPETITION