The epithelial cells of the choroid plexus possess specific, high affinity prolactin receptors whose functional significance is unknown. The role of these binding sites in a receptor-mediated transport system for prolactin from blood to cerebrospinal fluid (CSF) will be investigated by determining the competitive effects of structurally related and unrelated hormones on vascular I125-prolactin entrance into the cerebral ventricular system. In addition, the response of the choroid plexus to various prolactin endocrinologic states will be assessed by evaluating the prolactin binding capacity of the choroid plexus, the chemical fate of bound hormone, and the morphologic pattern of hormone internalization and translocation. These parameters will be compared between acute and chronic hyperprolactinemic states, within stages of the estrous cycle, and between males and females. The techniques utilized include in vitro competitive binding assay, electron microscopic autoradiography, gel filtration column chromatography, and high resolution two-dimensional gel electrophoresis. The data acquired on choroid plexus will be compared to the results of comparable studies on prolactin target cells in liver and adrenal gland. This approach will determine similarities and dissimilarities in prolactin-target cell interactions among structurally and functionally different cell types. These studies may serve to elucidate the functional significance of prolactin receptors at choroid plexus and will define the response of the choroid plexus to different prolactin environments. The proposal will provide insight into a major route by which prolactin interacts with the central nervous system. Comparison of results to liver and adrenal target cells will contribute to an understanding of prolactin-target cell interactions in general.