Inhibition of sexual behaviour and the luteinizing hormone surge by intracerebral progesterone implants in the female sheep

In female sheep, progesterone blocks the induction by oestradiol of both sexual behaviour and the pre-ovulatory surges of gonadotrophin releasing hormone (GnRH) and luteinising hormone (LH). However, the central sites of action of progesterone remain poorly defined, so we attempted to locate them by implanting progesterone intracerebrally in ovariectomised ewes treated with exogenous steroids to induce oestrous behaviour and the LH surge. Single bilateral implants or a double bilateral implants filled with progesterone or cholesterol were placed in the ventromedial hypothalamus (VMH) or the preoptic area (POA). Control ewes were not implanted. To determine the inhibitory capacity of the central progesterone implants, ewes received an injection (i.m.) of 8 mu g or 16 mu g of oestradiol. The single bilateral implants of progesterone failed to block oestrous behaviour and the LH surge induced by 8 mu g of oestradiol. Double bilateral progesterone implants in the VMH blocked the sexual behaviour (P < 0.05) and the LH surge (P < 0.05), but implants in the POA blocked only sexual receptivity (P < 0.05). No changes were observed after central implantation of cholesterol. Our results support the hypothesis that progesterone acts centrally in the VMH and the POA to inhibit the induction of LH surge and sexual behaviour by oestradiol.