The ontogeny of specific prolactin binding sites in the rat choroid plexus

The development of prolactin receptors in the choroid plexus of the rat was examined using the in vivo autoradiographic approach employing the principle of competitive binding. Experimental animals were injected with [125I]prolactin alone (total binding) while control animals received [125I]prolactin and a 500-fold excess of unlabelled prolactin (non-specific binding). Newborns as well as animals 10, 14 and 18 days postnatal were studied. Three minutes following hormone injection animals received an intracardiac perfusion with fixative and tissues were prepared for quantitative light microscopic autoradiography. The choroid plexus first demonstrated specific binding of prolactin, i.e. a statistically significant difference in the autoradiographic reactions between experimental and control animals, at 14 days postnatal. The lactogen specificity of these binding sites was further defined by the ability of [125I]prolactin to be displaced by unlabelled human growth hormone, which is lactogenic in rats, and not by unlabelled insulin, which is structurally dissimilar to prolactin. Morphometric analyses were performed on electron micrographs of choroid plexus from 10- and 14-day postnatal rats. The volume densities of constituents known to be enriched in polypeptide hormone receptors were measured and compared. Small cytoplasmic vesicles and tubules were statistically significantly more abundant in 10-day-old rats than in 14-day-old animals. It is conjectured that these vesicles and tubules contain an intracellular pool of prolactin receptors whose decrease at 14 days parallels the expression of specific binding sites at the cell surface.