PLACENTAL-LACTOGEN BINDING-SITES IN THE PREGNANT RABBIT CHOROID-PLEXUS

Prolactin has direct effects on the CNS. The highest concentration of prolactin receptors resides within the choroid plexus where they proba bly function to transport prolactin from blood into CSF. Another membe r of the lactogen family of hormones, placental lactogen (PL), also af fects CNS activity and may similarly employ the cerebroventricular sys tem as an intermediary. In order to determine whether the choroid plex us was a PL target tissue, in vitro autoradiography was used to identi fy specific PL binding sites in the choroid plexus of pregnant New Zea land White rabbits. Frozen brain sections were incubated in a medium c ontaining I-125 human PL (hPL) alone (total binding) or with a 500-fol d excess of unlabelled hPL (nonspecific binding). The specificity of t he binding was assessed with unlabelled human growth hormone (hGH) and ovine luteinising hormone (oLH). An intense autoradiographic reaction occurred over the choroid plexus of tissue sections incubated with I- 125 hpL alone. Excess unlabelled hPL and hGH, which is lactogenic in t he rabbit, caused a significant reduction (P < 0.001) in the binding o f radiolabelled hPL to the choroid plexus. In contrast, unlabelled oLH had no effect on radiolabelled hPL binding to this tissue. The result s support a role for the choroid plexus in the interactions between PL and the CNS.