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Although the role of gonadal steroids in inducing the LH surge is undisputed, the mechanism(s) whereby steroids induce the release of the hypothalamic luteinizing hormone-releasing hormone (LHRH) remain(s) enigmatic. In this study we examined the issue of the presence of steroid receptors in LHRH neurons using a mammalian species that has a true luteal phase, namely, guinea pigs. Progestin receptors (PR) were localized in LHRH neurons of ovariectomized guinea pigs administered estradiol (10–20 µg 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 the total population of LHRH neurons. This central position was visualized in simultaneous views of three-dimensional computer reconstructions 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 progestin receptors, are foci of activity, capable of activating a larger component of the LHRH population of cells in certain endocrine conditions, such as prior to the LH surge.

1. There are numerous sites in the nervous system where steroid hormones dramatically influence development. Increasing interest in mechanisms in neural development is providing avenues for understanding how gonadal steroids alter the ontogeny of these regions during sexual differentiation. 2. An increasing number of researchers are examining effects of gonadal steroids on neurite outgrowth, cell differentiation, cell death, cell migration, and synaptogenesis. The interrelated timing of these events may be a key aspect influenced by gonadal steroids throughout development. 3. The preoptic area and hypothalamus are characteristically heterogeneous in terms of cell type (e.g., different neuropeptides) and cell derivation. Perhaps a major reason for the ontogeny of sexual differences in the preoptic area and hypothalamus lies in the convergence of many different cell types from diverse sources (i.e., proliferative zones surrounding the lateral and third ventricles, and the olfactory placodes) that can be influenced in an interactive manner by gonadal steroid mechanisms. 4. The characterization of multiple mechanisms (e.g., trophic, migratory, apoptotic, fate, etc.,) that contribute to permanent changes in brain structure and ultimately function is essential for unraveling the process of sexual differentiation.