Prolactin receptors in the rat hypothalamus: autoradiographic localization and characterization

A precise mapping of prolactin (PRL) receptors in the rat brain has been achieved. Localization of binding sites for both 125I-human growth hormone (125I-hGH) and 125I-monoclonal anti-PRL receptor (125I-U5) was studied by in vitro autoradiography on brain sections in female rats (n = 7). The analysis of autoradiograms generated from 12 adjacent sections at 11 different brain levels (bregma 0.2 to -4.8 mm) revealed 9 distinctive localizations for 125I-hGH binding sites: preoptic suprachiasmatic nucleus, medial preoptic area, periventricular, supraoptic, paraventricular, arcuate and vetromedial nuclei and also the median eminence and the infundibulum. Specificity for PRL binding was assessed by competition experiment of 125I-hGH with unlabeled hGH and ovine PRL. Binding sites were similarly localized by 125I-U5 indicating the presence of PRL receptors moiety. The quantitative analysis with 0.6 nM 125I-hGH demonstrated maximal densities in the preoptic suprachiasmatic and arcuate nuclei and minimal densities in the median eminence and the infundibulum. Due to ample antero-posterior variations no significant changes were observed during the estrous cycle. Saturation analysis of binding in the arcuate nucleus indicated a single class of high affinity (Kd from 0.9 to 2.2 nM) receptors (Bmax from 34 to 44 fmol/mg of proteins). The present data provide the hypothalamic cartography of PRL receptors in the female rat brain and support all the physiological evidence for the existence of a direct action of PRL in the hypothalamus.