Radioautographic identification of lactogen binding sites in rat median eminence using 125I-human growth hormone: evidence for a prolactin "short-loop" feedback site.

The binding characteristics of human growth hormone were exploited to identify radioautographically lactogen binding sites in the rat median eminence. Following systemic injection 125I-human growth hormone bound preferentially to the lateral palisade zone, a region of median eminence rich in dopamine and LHRH. Coinjection of 125I-human growth hormone with an excess of unlabeled human growth hormone or ovine prolactin, but not bovine grown hormone, competitively blocked 125I-human growth hormone binding to the external median eminence. These observation provide direct evidence of recognition sites for lactogenic hormones in a discrete region of the median eminence associated with hypothalamic regulation of hypophyseal prolactin and luteinizing hormone secretion. Median eminence lactogen binding sites may mediate presumed direct effect of lactogenic hormones on the reproductive functions of the hypophysiotropic hypothalamus.