Recent studies have shown that gonadotropin-releasing hormone (GnRH) can exert various effects on the rat ovary by acting through its specific receptors. To determine the cell types responsive to the action of GnRH under physiological conditions in the ovary, distribution of the GnRH receptor mRNA was studied histologically by in situ hybridization in neonatal and adult rats. Expression of the luteinizing hormone receptor mRNA was also examined to judge the growing state of follicles and the corpora lutea. In neonatal rat ovaries, no significant GnRH receptor mRNA signal was detected until 5 days after birth. The expression was first observed at 10 days in the interstitial cells. At 15 days of age, the receptor mRNA was expressed in the granulosa cells of most preantral and early antral follicles, while no hybridization signal was detected in oocyte and theca cells. In adult cycling rats, GnRH receptor mRNA was detected mainly in the granulosa cells of most follicles and luteal cells. The granulosa cells of atretic follicles showed a very high level of the mRNA expression throughout their degenerating process. A strong hybridization signal was also detected in the mural granulosa cells of mature follicles. Newly formed (developing) corpora lutea exhibited signals with moderate intensity in the luteal cells, and the older ones showed weaker signals. The finding that the initial expression of GnRH receptor mRNA was seen in the interstitial cells of neonatal ovaries implies an unknown function of the ovarian GnRH receptor in ovarian development. The high level expression of GnRH receptor mRNA in atretic and mature follicles supports the putative roles of GnRH in the induction of follicular atresia and ovulation in rat ovaries.

Keywords:
Ovary, Development, Follicular atresia, Gonadotropin-releasing hormone receptor, Hybridization, in situ, Rat
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