Colocalization of progesterone receptors and thyroid hormone receptors alpha in the ovine diencephalon: No effect of estradiol

Progesterone and thyroid hormones are key hormones in the control of reproduction in the ewe since both have been shown to inhibit GnRH secretion. GnRH neurons do not express progesterone receptors (PR) but half of them contain thyroid hormone receptors alpha (THRalpha), two nuclear receptors potentially able to act on gene transcription. PR and THRalpha distributions overlap in most regions of the ovine preoptic area (POA) and hypothalamus. To determine whether progesterone and thyroid hormones may also have common neuronal targets for the control of GnRH cell activity, we searched for coexpression of PR with THRa within neurons of the POA and hypothalamus of ovariectomized ewes treated with estradiol plus progesterone or with progesterone alone. Double-labeled cells were found throughout the medial POA, the periventricular part of the paraventricular nucleus (PVNpe), the arcuate nucleus (ARC) and the ventrolateral ventromedial nucleus (VMNvl). Colocalization ratios were not statistically different between steroid treatment groups. 84% of PR-immunoreactive cells in VMNvl, 90% in POA and ARC and 95% in PVNpe contain THRalpha. More than two-thirds of THRalpha-immunoreactive cells contain PR in each area examined. This study provides evidence that progesterone and thyroid hormones may act within the same cells to modulate physiological functions such as reproduction. Copyright (C) 2003 S. Karger AG, Basel.