INHIBITION OF SEXUAL-BEHAVIOR AND THE LUTEINIZING-HORMONE SURGE BY INTRACEREBRAL PROGESTERONE IMPLANTS IN THE FEMALE SHEEP

In female sheep, progesterone blocks the induction by oestradiol of bo th sexual behaviour and the pre-ovulatory surges of gonadotrophin rele asing hormone (GnRH) and luteinising hormone (LH). However, the centra l sites of action of progesterone remain poorly defined, so we attempt ed to locate them by implanting progesterone intracerebrally in ovarie ctomised ewes treated with exogenous steroids to induce oestrous behav iour and the LH surge. Single bilateral implants or a double bilateral implants filled with progesterone or cholesterol were placed in the v entromedial hypothalamus (VMH) or the preoptic area (POA). Control ewe s were not implanted. To determine the inhibitory capacity of the cent ral progesterone implants, ewes received an injection (i.m.) of 8 mu g or 16 mu g of oestradiol. The single bilateral implants of progestero ne failed to block oestrous behaviour and the LH surge induced by 8 mu g of oestradiol. Double bilateral progesterone implants in the VMH bl ocked 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 ch anges were observed after central implantation of cholesterol. Our res ults support the hypothesis that progesterone acts centrally in the VM H and the POA to inhibit the induction of LH surge and sexual behaviou r by oestradiol.