Immunocytochemistry of luteinizing hormone releasing hormone (LHRH) in the sheep hypothalamus during variuos reproductive stages: correlation with the gonadotropic hormones of the pituitary.

Using the labeled and unlabeled immunoperoxidase metods, the distribution and concentration (1) of immunoreactive LFRH-material in the hypothalamus, and (2) of gonadotropic hormones in the adenohypophysis of the ewe were determined during various reproductive stages, including two phases of the estrous cycle, anestrus, lactation, and the state after ovariectomy. The concentration of LHRH-immunoreactive material varied in particular regions of the median eminence (ME) and was closely dependent on the physiological state. The immunoreactive material was most abundant in hypothalami of lactating animals, exceeding gradually the corresponding deposits in ovariectomized animals, in ewes during the 16th day of the estrous cycle (before ovulation), and in anestrous ewes. A severe depletion of LHRH from the rostral and central parts of the ME was observed 24 h after ovulation. This depletion was accompanied by a degranulation of LH- and prolactin-producing cells in the adenophypophysis. It is postulated that LHRH is stored in the ME of the sheep in all examined endocrine stage. Only circumscribed regions of the ME take part in the cyclic release of LHRH during the ovulatory phase; this suggests a functional differentiation of the nerve terminals in this neurohemal area.