A Critical Period for Thyroid Hormone Action on Seasonal Changes in Reproductive Neuroendocrine Function in the Ewe

Abstract

Thyroid hormones are obligatory for the annually recurring termination of reproductive activity in a spectrum of seasonal breeders, including sheep. Previous studies involving thyroidectomy and T4 replacement have led to the hypothesis that, in the ewe, thyroid hormones are necessary only during a limited interval late in the breeding season for the neuroendocrine processes that cause the transition to anestrus. The present series of experiments tested this hypothesis by assessing the influence of thyroidectomy, with or without T4 replacement for specific durations and at different times of the year, on the transition to anestrus. Seasonal alterations in reproductive neuroendocrine activity were monitored by changes in serum LH concentration in ovariectomized ewes bearing s.c. SILASTIC brand silicon tubing implants containing estradiol. Thyroidectomy in mid-December, just before the putative period of thyroid hormone action, prevented the development of the neuroendocrine anestrous season (fall in LH in this animal model). T4 replacement for 90 days beginning in late December (i.e., during the postulated period of thyroid hormone action) overcame the blockade of anestrus, causing LH to fall in ewes thyroidectomized several months previously. The minimal effective duration of exposure to thyroid hormones required for the transition to anestrus was estimated to be 60-90 days. Further, exposure to T4 for 60-90 days beginning in late December was found to be the only time of the year that thyroid hormones were required to maintain seasonal changes in reproductive neuroendocrine activity. Finally, replacement of T4 for 90 days at a different time of year (beginning in August) failed to provoke development of neuroendocrine anestrus in thyroidectomized ewes. These results support the hypothesis that thyroid hormones are necessary only during a limited interval late in the breeding season to promote seasonal reproductive suppression in the ewe. Further, the reproductive neuroendocrine axis is not equally responsive to thyroid hormone at all times of the year. This suggests there is a critical period of responsiveness during which thyroid hormones must be present for anestrus to develop.

Thyroid hormones in small ruminants: effects of endogenous, environmental and nutritional factors

“…Similar but less-pronounced effects have been obtained in sheep rendered hypothyroid, in which the end of the reproductive season occurred later than in controls (Follett and Potts, 1990; Hernandez et al, 2003). TH are necessary during a limited period late in the breeding season to permit transition to seasonal anoestrus (Thrun et al, 1996 and1997a), acting primarily within the brain to promote inhibition of neuroendocrine reproductive function (Viguié et al, 1999). TH permit the increase of the responsiveness to the oestradiol negative feeback, but are also required for steroid-independent seasonal cycles in luteinising hormone pulse frequency (Anderson et al, 2002).…”

Thyroid hormones in the cerebrospinal fluid of the third ventricle of adult female sheep during different periods of reproductive activity

“…In seasonal sheep, the responsiveness of the neuroendocrine system, which regulates reproductive activity to thyroid hormones is strongly limited to the particular time of the year. The minimal effective duration of exposure to THs required for the transition to anestrus was estimated to be 60-90 days (Thrun et al 1997). In thyroidectomized ewes, replacement of T4 beginning in late December was found to be the only time of year that THs were effective in seasonal changes of reproductive activity (Thrun et al 1997).…”

“…The minimal effective duration of exposure to THs required for the transition to anestrus was estimated to be 60-90 days (Thrun et al 1997). In thyroidectomized ewes, replacement of T4 beginning in late December was found to be the only time of year that THs were effective in seasonal changes of reproductive activity (Thrun et al 1997). In our study we began the CSF collection in December and finished in March, when all ewes entered anestrus according to the decrease in progesterone levels.…”

“…In sheep, THs are obligatory for at least one aspect of this endogenous rhythm: neuroendocrine changes that lead to the transition from breeding season to anestrus , Webster et al 1991, Dahl et al 1994. In this process, THs act during a very limited time of the year (Thrun et al 1997) and primarily within the brain (Viguie et al 1999).…”

The effects of exogenous melatonin on wool quality and thyroid function in Rasa Aragonesa ewes

“…In tropical sheep breeds, the highest T 4 profiles were recorded in the summer (Ashutosh et al, 2001), andRhind et al (2000) observed that T 4 concentrations were higher in the spring and summer than in the autumn. Thrun et al (1997) hypothesized that, to promote seasonal reproductive suppression in the ewe, thyroid hormones are necessary only during a short interval late in the breeding season and the reproductive neuroendocrine axis is not equally responsive to thyroid hormones at all times of the year. If that is true, it suggests there is a critical period of responsiveness during which thyroid hormones must be present for anoestrus to develop, and it might explain the different seasonal patterns in the secretion of T 4 reported in the literature.…”