The distribution of lactogen receptors in the mammalian hypothalamus: an in vitro autoradiographic analysis of the rabbit and rat.

The hypothalamus contains a high concentration of lactogen receptors as detected with in vitro radioreceptor assay techniques. In an effort to define the location of the lactogen receptors relative to specific hypothalamic nuclei, an in vitro autoradiography technique was applied to frozen sections of rat and rabbit brains. Three lactogenic hormones, i.e. human growth hormone (hGH), ovine prolactin (oPRL), and rat prolactin (rPRL), were radiolabeled with iodine-125. Competition for observed binding sites was assessed with unlabeled hGH, oPRL, and bovine growth hormone (bGH). Analysis of the autoradiographs with a microcomputer-based densitometry system revealed that the rabbit hypothalamus contains specific lactogen binding sites within the supraoptic, paraventricular, suprachiasmatic, ventromedial, arcuate, and dorsomedial nuclei and the medial preoptic area. Unlabeled bGH was effective in competing for binding sites in all areas when hGH but not oPRL was used as the radiolabeled ligand, suggesting the presence of growth hormone receptors in the rabbit hypothalamus with a distribution similar to that of the lactogen binding sites. In contrast to the rabbit, no lactogen binding sites were detected in the rat hypothalamus regardless of the ligand used in the assay. All of the ligands were successful, however, in detecting lactogen receptors within the rat choroid plexus and liver. The results from the rabbits indicate that the influences of prolactin on hypothalamic activity are mediated via lactogen receptors that are widely distributed throughout the various pertinent hypothalamic nuclei. The broad distribution of lactogen receptors in the rabbit hypothalamus attests to the extensive influence of prolactin on hypothalamic regulatory systems. The results from the rat raise questions as to the nature of rat brain prolactin receptors in comparison to prolactin receptors in rat peripheral tissues.