NEUROENDOCRINE CONTROL OF OVULATION IN SH EEP

The preovulatory surge of LH, or positive feedback response to oestrad iol, is related to the steroid's direct stimulating action on the pitu itary gland, but we have demonstrated that, in sheep, the central nerv ous system plays an essential role in its action : a preovulatory GnRH surge. Since GnRH neurons appear to contain few, if any receptors for oestradiol the question is raised: Where in the central nervous syste m does oestradiol act to stimulate GnRH secretion ? A series of experi ments were conducted with a combination of techniques for sampling pit uitary portal blood and stereotaxic brain micro-implantations of of oe stradiol. Castrated Ile-deFrance breed ewes were treated during the ps eudo follicular phase of successive artifical cycles, receiving either peripheral oestradiol implants or brain implants containing oestradio l or cholesterol. Administration of oestradiol implants into the ventr omedial region of the hypothalamus induced a preovulatory GnRH surge i n 5 out of 10 animals. The interval between oestradiol and the GnRH su rge was comparable with that observed in animals treated with a periph eral implant although the amplitude of the surge was smaller. No GnRH surges were observed in animals treated with cholesterol. Finally, the re was an inverse correlation between the response intensity (amplitud e of the GnRH/LH surges) and the distance separating implants from cel ls in the region carrying oestradiol receptors. These results show tha t the mediobasal hypothalamus is a site of action for the oestradiol-i nduced preovulatory GnRH surge in the ewe. Immunohistochemical tests s hould identify the nature of the cells forming the relay for the stero id action.