A SUBGROUP OF LHRH NEURONS IN GUINEA-PIGS WITH PROGESTIN RECEPTORS ISCENTRALLY POSITIONED WITHIN THE TOTAL POPULATION OF LHRH NEURONS

Although the role of gonadal steroids in inducing the LH surge is undi sputed, the mechanism(s) whereby steroids induce the release of the hy pothalamic luteinizing hormone-releasing hormone (LHRH) remain(s) enig matic. In this study we examined the issue of the presence of steroid receptors in LHRH neurons using a mammalian species that has a true lu teal phase, namely, guinea pigs. Progestin receptors (PR) were localiz ed in LHRH neurons of ovariectomized guinea pigs administered estradio l(10-20 mu g estradiol benzoate) for 3-4 days, using several different immunocytochemical protocols. The subgroup of LHRH neurons containing PR, although small, was strategically positioned within the core of t he total population of LHRH neurons. This central position was visuali zed in simultaneous views of three-dimensional computer reconstruction s of the populations of LHRH/PR neurons and LHRH neurons. The subgroup of LHRH/PR neurons formed a thread permeating the population of LHRH neurons. We propose that in guinea pigs, LHRH neurons containing proge stin receptors, are foci of activity, capable of activating a larger c omponent of the LHRH population of cells in certain endocrine conditio ns, such as prior to the LH surge.