Gonadotropin-releasing hormone receptor mRNA expression in the ovaries of neonatal and adult rats

Recent studies have shown that gonadotropin-releasing hormone (GnRH) can ex ert various effects on the rat ovary by acting through its specific recepto rs. To determine the cell types responsive to the action of GnRH under phys iological conditions in the ovary, distribution of the GnRH receptor mRNA w as studied histologically by in situ hybridization in neonatal and adult ra ts. Expression of the luteinizing hormone receptor mRNA was also examined t o judge the growing state of follicles and the corpora lutea. In neonatal r at ovaries, no significant GnRH receptor mRNA signal was detected until 5 d ays after birth. The expression was first observed at 10 days in the inters titial cells. At 15 days of age, the receptor mRNA was expressed in the gra nulosa cells of most preantral and early antral follicles, while no hybridi zation signal was detected in oocyte and theca cells. In adult cycling rats , GnRH receptor mRNA was detected mainly in the granulosa cells of most fol licles and luteal cells. The granulosa cells of atretic follicles showed a very high level of the mRNA expression throughout their degenerating proces s. A strong hybridization signal was also detected in the mural granulosa c ells of mature follicles. Newly formed (developing) corpora lutea exhibited signals with moderate intensity in the luteal cells, and the older ones sh owed weaker signals. The finding that the initial expression of GnRH recept or mRNA was seen in the interstitial cells of neonatal ovaries implies an u nknown function of the ovarian GnRH receptor in ovarian development The hig h level expression of GnRH receptor mRNA in atretic and mature follicles su pports the putative roles of GnRH in the induction of follicular atresia an d ovulation in rat ovaries.