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

Abstract

The ferret is a reflex-ovulating species in which receipt of an intromission 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 GnRH 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 intromission from a male. Coronal brain sections ranging from the rostral preoptic area caudally to the posterior hypothalamus were processed for in situ hybridization using a 35S-labeled oligoprobe complementary to the human GnRH-coding region. We found no evidence of increased MBH GnRH mRNA levels during 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. Furthermore, cellular GnRH mRNA levels decreased in the arcuate region toward the end of the preovulatory LH surge. In Exp 2 we asked whether ovarian hormones regulate MBH GnRH mRNA levels in the female ferret. Ovariectomy of estrous females significantly reduced the number of GnRH mRNA-expressing cells in the arcuate region. This decrease was probably not due to the absence of circulating estradiol. Gonadally intact anestrous females had levels of MBH GnRH mRNA similar to those in estrous females even though plasma estradiol levels were equally low in anestrous females and ovariectomized females. Ovarian hormones other than estradiol may stimulate MBH GnRH mRNA levels in anestrous and estrous females.