Immunocytochemical localization in rat brain of a prolactin release-inhibiting sequence of gonadotropin-releasing hormone prohormone

The structure of a precursor protein for gonadotropin-releasing hormone (GnRH) of relative molecular mass 10,000 has recently been deduced from cloned complementary DNA sequences derived from human placental messenger RNA. The 56-amino-acid peptide representing residues 14-69 of this prohormone exhibits potent inhibition of prolactin secretion. To investigate whether the same prohormone is synthesized in mammalian brain and describe the anatomical distribution of the prolactin-inhibiting region of this molecule, we have generated antiserum to a synthetic peptide containing residues 40-53 of the human placental precursor. We report here that a substance recognized by this antibody is present in GnRH-containing neurones of the rat brain and appears to coexist with GnRH in secretory granules of nerve terminals in the median eminence. These results indicate homology between hypothalamic and placental prohormones for GnRH and are consistent with the suggestion elsewhere in this issue that a prolactin-inhibiting factor (PIF) is generated from this prohormone and cosecreted with GnRH by nerve terminals in the median eminence.