Inhibitory effects of anti-prolactin receptor antibodies on prolactin binding in brain and prolactin-induced feeding behavior in ring doves.

Abstract

Although binding sites for prolactin (PRL) have been mapped and partially characterized in the brains of several species, there is as yet no direct evidence that the effects of intracranial PRL on brain function are receptor-mediated events. We addressed this question by testing whether antibodies generated against rat liver PRL receptors can effectively antagonize the ability of PRL to enhance feeding behavior in male ring doves (Streptopelia risoria). Both agents were administered directly to the ventromedial hypothalamic nucleus (VMN), which is an effective site of PRL action in promoting hyperphagia in this species. In the initial study, affinity-purified gamma-globulin (IgG) from the receptor antiserum preparation was tested for its ability to compete with 125I-ovine PRL for binding to receptors in rat liver, dove choroid plexus, and 6 PRL-sensitive dove brain regions using in vitro quantitative film autoradiography. Although the binding affinity of the anti-PRL receptor antibodies was at least 50 times lower in dove brain than in rat liver, a 40-50% inhibition of specifically bound 125I-ovine PRL was observed in choroid plexus and in 5 of 6 brain regions with anti-receptor IgG concentrations of 5.8 x 10(-7) M and 1.2 x 10(-6) M, using sections incubated with normal rabbit serum (NRS) IgG as a control. In a second study, anti-PRL receptor IgG or NRS IgG (2.4 micrograms) was injected unilaterally into the VMN at 45-60 min prior to VMN injection of ovine PRL (50 ng) or saline vehicle. This procedure was repeated at twice-daily intervals for 5 days. When compared to the feeding behavior of PRL-injected birds given NRS IgG, antireceptor antibody-treated animals showed a marked reduction in PRL-induced hyperphagia. The magnitude of this reduction was calculated to be approximately 50% after corrections were made for a mild hypophagia induced by the anti-receptor IgG treatment alone, as reflected in the feeding behavior of the anti-PRL receptor IgG + vehicle-treated group. These results suggest that PRL receptors in dove brain and rat liver exhibit structural similarities as well as differences and that the hyperphagia induced by intracranial injections of PRL is mediated, at least in part, by interactions with PRL receptors in the brain.