Intravenous injection of a partially purified extract of ovine hypothalami into rats at various times during the estrous cycle caused significant LH release in vivo as indicated by elevations of plasma LH titers. Addition of the same extracts during in vitro incubation of pituitaries from similar rats resulted in increased release of both LH and FSH. Pronounced cyclic patterns of pituitary responsiveness to LH-RF/FSH-RF were noted for LH in vitro and in vivo and for FSH in vitro. Pituitary responsiveness to the releasing factor preparation attained a peak at. proestrus, declined somewhat at estrus and reached the lowest value at diestrus. Examination of adenohypophysial LH and FSH contents at the end of each incubation revealed a cyclic pattern similar to that observed for pituitary responsiveness. The changes in adenohypophysial sensitivity could not be attributed entirely to the variation in gonadotropin content since they were still apparent when the evoked LH and FSH releases were expressed as percentages of the original hormone content. It is concluded that an augmentation of pituitary responsiveness to neurohumoral stimulation occurs between diestrus day 2 and the afternoon of proestrus in the 4-day cyclic rat. (Endocrinology95: 1293, 1974)

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