A subset of gonadotropin-releasing hormone neurons in the ovine medial basal hypothalamus is activated during increased pulsatile luteinizing hormone secretion.

GnRH neurons active in the preovulatory LH surge have been identified in several species using the early intermediate gene product, Fos, but the GnRH neurons active during episodic LH secretion remain unknown. In this study, we have used Fos and Fos-related antigens (FRA) to determine whether a subset of GnRH neurons is active when pulsatile LH secretion is acutely stimulated in sheep. In experiment 1, episodic LH secretion was stimulated in five of six ewes by injection of an opioid antagonist to luteal phase ewes. These five ewes had a 6-fold increase in the percentage of GnRH neurons in the medial basal hypothalamus (MBH) expressing Fos/FRA, compared with control ewes that had no LH pulses before death. Fos/FRA expression was not increased in GnRH neurons found in any other area. In experiment 2, episodic LH secretion was induced in rams by introduction of estrous ewes. This treatment increased Fos/FRA expression in MBH GnRH neurons approximately 10-fold compared with control rams. Again, this increase in Fos/FRA expression in GnRH neurons was limited to the MBH. This selective activation of MBH GnRH neurons could reflect the preferential inhibition of these perikarya by endogenous opioid peptides. It also raises the possibility that a subset of GnRH neurons in the MBH may be responsible for episodic GnRH secretion in sheep.