The long-term objective of this work is to train MBRS students to conduct research investigations which will elucidate the hypothalamic opioid systems involved in the regulation of reproduction in the rat. In particular, the opioid circuitry of the medial preoptic area will be defined and the effect of gonadal steroid hormone exposure on preoptic mu-opiate receptors and the opioid neurons projecting to this region will be characterized. The specific aims are to (1) define the precise morphology of medial preoptic mu-opiate receptors in relation to the underlying sexually dimorphic neuronal structure, (2) investigate the influence of estrogen and progesterone treatment on the expression of opioid peptides in neurons which supply medial preoptic mu-receptor systems, and (3) examine the mechanism and timing of medial preoptic mu- receptor regulation. Potential imbalances in these systems could affect reproductive hormonal and behavioral processes in humans. Combined in vitro receptor autoradiographic/histological methods will be used to precisely localize hormone-dependent medial preoptic mu-opiate receptors in adult female rats. Once the location of these receptors is known, retrograde tracer/in situ hybridization histochemistry methods will be employed to identify the opioid peptide-expressing neurons which supply these receptors, and the effect of hormone treatment on these identified neurons will be determined using quantitative autoradiographic techniques with cellular resolution. Furthermore, the mechanism of hormonal regulation of receptor level will be examined using protein synthesis blockade at various critical periods in receptor expression, and the normal physiological diurnal variation of these receptors will be determined across the estrous cycle. Characterization of the reciprocal interaction between gonadal steroid hormones and opioid peptides in the preoptic area will ultimately lead to a better understanding of how opioid peptides regulate reproductive behavior and hormonal cyclicity.