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

GnRH neurons active in the preovulatory LH surge have been identified in se veral 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 subs et of GnRH neurons is active when pulsatile LH secretion is acutely stimula ted 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. The se 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 increas ed in GnRH neurons found in any other area. In experiment 2, episodic LH se cretion was induced in rams by introduction of estrous ewes. This treatment increased Fos/FRA expression in MBH GnRH neurons approximately 10-fold com pared with control rams. Again, this increase in Fos/FRA expression in GnRH neurons was limited to the MBH. This selective activation of MBH GnRH neur ons could reflect the preferential inhibition of these perikarya by endogen ous opioid peptides. It also raises the possibility that a subset of GnRH n eurons in the MBH may be responsible for episodic GnRH secretion in sheep.