Ribonucleic Acid Levels Induced by Mating or Ovariectomy in a Reflex Ovulator, Changes in Mediobasal Hypothalamic Gonadotropin-Releasing Hormone Messenger

The ferret is a reflex-ovulating species in which receipt of an intromission induces a prolonged (612 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 S-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. (Endocrinology 140: 595– 602, 1999) I THE FERRET, a reflex ovulator, receipt of an intromission induces a preovulatory LH surge in the estrous female (1, 2). This elevation in circulating LH begins around 1.5 h after the onset of intromission, peaks approximately 6 h later, and is sustained for at least 12 h (2). The preovulatory LH surge in the female ferret is probably stimulated by a large, sustained release of GnRH from the mediobasal hypothalamus (MBH) into the pituitary portal vessels. It was previously found that the in vitro release from perifused MBH slices and MBH tissue content of GnRH were significantly reduced in estrous females killed 0.25 h after receipt of an intromission (3). Also, fewer GnRH-immunoreactive perikarya were detected in the MBH of ovariectomized, estradiol-primed female ferrets killed 20 min after receiving mechanical vagino-cervical stimulation (4). In the vole, another reflex ovulating species, a similar depletion in hypothalamic GnRH content was found in females 5 min after mating (5). These findings suggest that in these species mating induces a large release of GnRH from the MBH that initially depletes GnRH neuronal terminals of peptide. Interestingly, no decrease in the MBH release of GnRH was observed in estrous female ferrets killed 1 or 2.6 h after the receipt of an intromission (3), suggesting that releasable GnRH stores in the MBH are replenished as early as 1 h after mating. This replenishment could reflect a mating-induced increase in the biosynthesis of GnRH peptide as a result of increased GnRH gene expression. In Exp 1, we addressed this question by comparing GnRH messenger RNA (mRNA) levels in MBH neurons of estrous female ferrets killed at different times during the course of the mating-induced pre-