VENTROMEDIAL HYPOTHALAMUS AS A TARGET FOR ESTRADIOL ACTION ON PROCEPTIVITY, RECEPTIVITY AND LUTEINIZING-HORMONE SURGE OF THE EWE

Most of the literature suggests that in sheep as in rodents nervous structures involved in female sexual behaviour are not necessarily identical to those involved in the LH surge. In rodents, oestradiol triggers female sexual behaviour by acting on a restricted area of the mediobasal hypothalamus whereas the concomitant induction of the preovulatory LH surge is at least partially under the control of more anterior structures. The central sites of oestradiol action, however, remained poorly defined in sheep. To provide this definition, 37 ovariectomized ewes were stereotaxically implanted unilaterally or bilaterally with a guide cannula in preoptic area (POA), anterior, mediobasal, lateral, or posterior hypothalamus (AH, MBH, LHT, PH). Experiments were made during the breeding season (Br) and the anoestrous period (An: unilat only) and females were primed with a peripheral treatment of progesterone and a dose of 17-beta-oestradiol subthreshold for both the LH surge and sexual behaviour. Intracranial implants (i.d. = 0.45 mm) of crystalline E2 were lowered 16 h after progesterone removal and left in the brain for 48 h. Whereas POA implants never had any significant effects on either the behaviour or the LH surge, all MBH implants caused receptivity (11 bilat, 5 unilat Br and 5 unilat An). Bilateral MBH implants also induced proceptivity in 9 of 11 ewes and increased the LH levels in 7 of them. These proportions do not differ significantly from those observed after a 25-mu-g peripheral injection of E2. Unilateral MBH implants had no significant effect on proceptivity and LH increase but oestrous behaviour was induced by some implants placed laterally to the MBH (2/5 recept and 3/5 procept). The quality of oestrous behaviour after brain implants was not significantly different from that observed after peripheral steroid treatment, showing that all brain areas needed for full expression of oestrus have been stimulated. The amplitude of the LH increase, however, was lower after implantation than after i.m. injection of the steroid; we propose that this is due to an insufficient stimulation of the hypophysis. These results show that in the ewe, a restricted are in the mediobasal hypothalamus is an important site of oestradiol action to trigger both sexual behaviour and the LH surge.