Cyclic variations in the increased responsiveness of the pituitary to luteinizing hormone-releasing hormone (LHRH) induced by LHRH.

Following a single iv injection of 50 ng of LH-releasing hormone (LHRH), a small increase in plasma LH titers was observed and this response was greater on proestrus and estrus than in other stages of the cycle. When a second injection of a similar dose of the neurohormone was given one hour later, the response to the second injection was markedly enhanced on proestrus but only slightly increased at other stages of the cycle. The response to LHRH in androgenized females was similar to that observed on proestrus, and, as on proestrus, an enhanced response to a second injection occurred. Only small increases in FSH followed these injections and there was no increase in responsiveness to the second injection in any of the groups. It is postulated that rising estrogen titers can bring about a priming action of LHRH which enhances the response of the gland to a second injection on proestrus. The priming action of LHRH may magnify the response of the pituitary to endogenous decapeptide release to further increase the release of LH on the afternoon of proestrus.