Translator Disclaimer
1 March 2002 Thyroid Hormones Mediate Steroid-Independent Seasonal Changes in Luteinizing Hormone Pulsatility in the Ewe
Author Affiliations +

Thyroid hormones permit the increase in response to estradiol negative feedback in ewes at the transition to anestrus. In this study, we tested whether the thyroid hormones are also required for steroid-independent seasonal changes in pulsatile LH secretion. In experiment 1, Suffolk ewes were ovariectomized and thyroidectomized (THX) or ovariectomized only (controls) in late November. LH pulse frequency and amplitude were measured for 4 h in December, April, May, June, and August. Pulse frequency was also measured in the presence of estradiol-containing implants during the breeding (December) and early anestrus (March) seasons. As expected, in the presence of estradiol, pulse frequency declined between December and March in control but not THX ewes. In the absence of estradiol, a seasonal decline in frequency and an increase in amplitude occurred in control ewes, concurrent with lengthening photoperiod. A similar trend was seen in THX ewes, but the seasonal changes were lower in magnitude and not significant. In experiment 2, the same protocol was used (pulse measurements in December, May, and June) with a larger THX group size (n = 7). Results were similar to those of experiment 1 for controls. In THX ewes, pulse frequency did not change over time and was significantly elevated relative to that of controls during the summer. Pulse amplitude in THX ewes tended to increase during summer and did not differ from pulse amplitudes in control ewes. These results demonstrate that thyroid hormones are required for steroid-independent cycles in LH pulse frequency; however, some seasonal changes in amplitude still occur in the absence of thyroid hormones. This finding contrasts with the changes in estradiol negative feedback at the transition to anestrus, which are entirely thyroid hormone dependent.

Greg M. Anderson, John M. Connors, Steven L. Hardy, Miroslav Valent, and Robert L. Goodman "Thyroid Hormones Mediate Steroid-Independent Seasonal Changes in Luteinizing Hormone Pulsatility in the Ewe," Biology of Reproduction 66(3), 701-706, (1 March 2002).
Received: 29 June 2001; Accepted: 1 October 2001; Published: 1 March 2002

This article is only available to subscribers.
It is not available for individual sale.

Get copyright permission
Back to Top