Changes in mediobasal hypothalamic gonadotropin-releasing hormone messenger ribonucleic acid levels induced by mating or ovariectomy in a reflex ovulator, the ferret

The ferret is a reflex-ovulating species in which receipt of an intromissio n induces a prolonged (+/-12 h) preovulatory LH surge in the estrous female . This LH surge is probably stimulated by a large release of GnRH from the mediobasal hypothalamus (MBH). In Exp 1 we asked whether GnRH messenger RNA (mRNA) levels increase in response to mating so as to replenish the MBH Gn RH stores needed to sustain the preovulatory LH surge. Estrous females were killed 0, 0.25, 0.5, 1, 3, 6, 14, or 24 h after the onset of a 10-min intr omission from a male. Coronal brain sections ranging from the rostral preop tic area caudally to the posterior hypothalamus were processed for in situ hybridization using a S-35-labeled oligoprobe complementary to the human Gn RH-coding region. We found no evidence of increased MBH GnRH mRNA levels du ring the ferret's mating-induced preovulatory LH surge. Instead, the number of GnRH mRNA-expressing cells dropped significantly in the arcuate region beginning 6 h after onset of intromission and remained low thereafter. Furt hermore, cellular GnRH mRNA levels decreased in the arcuate region toward t he end of the preovulatory LH surge. In Exp 2 we asked whether ovarian horm ones regulate MBH GnRH mRNA levels in the female ferret. Ovariectomy of est rous females significantly reduced the number of GnRH mRNA-expressing cells in the arcuate region. This decrease was probably not due to the absence o f circulating estradiol. Gonadally intact anestrous females had levels of M BH GnRH mRNA similar to those in estrous females even though plasma estradi ol levels were equally low in anestrous females and ovariectomized females. Ovarian hormones other than estradiol may stimulate MBH GnRH mRNA levels i n anestrous and estrous females.