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Film autoradiography and densitometry were employed to obtain a detailed map of prolactin (PRL) receptor distribution in ring dove brain and to characterize the affinity, capacity, specificity, and in vivo saturability of ¹²⁵I-ovine PRL binding in individual brain regions. Significant specific binding was detected in choroid plexus and in a variety of forebrain and midbrain sites, with highest levels observed in the preoptic area, tuberoinfundibular region, lateral hypothalamic area, and the suprachiasmatic, paraventricular, and ventromedial hypothalamic nuclei. A comparison of PRL receptors in the choroid plexus, preoptic area, ventromedial nucleus, suprachiasmatic nucleus, and tuberal hypothalamus revealed significant regional differences in binding capacity, but no differences in binding affinity (K_d = 0.4-0.9 nM; B_max = 6-14 fmol/mg protein). Unlabelled ovine PRL proved to be an effective competitor of labelled ovine PRL at concentrations of 43 pM, 0.43 nM, and 4.3 nM in the choroid plexus and in 5 diencephalic areas. In contrast, no significant competition was observed in any of these brain areas in tests with unlabelled turkey GH, ovine GH, or ovine LH at similar concentrations. Unlabelled ovine PRL (37 nmol) significantly reduced the uptake of ¹²⁵I-ovine PRL (37 pmol) in the preoptic area, tuberal hypothalamus, and bed nucleus of the pallial commissure at 2 h after intravenous injection. In addition, specific binding of ¹²⁵I-ovine PRL in vitro was reduced by over 67% in these brain areas and in several other brain regions following a 7 day period of systemic ovine PRL administration. These results indicate that PRL receptors in different regions of the dove brain exhibit similar binding characteristics. In addition, they suggest that at least some of these PRL-sensitive brain loci are accessible to blood-borne hormone.

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