ONTOGENY OF LEU-ENKEPHALIN AND BETA-ENDORPHIN INNERVATION OF THE PREOPTIC AREA IN MALE AND FEMALE RATS

The distribution of endogenous opioid peptide-containing fibers in the medial preoptic area of developing male and female rats was examined using immunohistochemical methods. In Particular, the ontogeny of leuc ine-enkephalin (Leu-enk) and beta-endorphin (beta-endo) innervation wa s studied using antisera directed against these comPounds. The distrib ution of Leu-enk and beta-endo differed at each age examined from birt h to postnatal day 12 (P12). Furthermore, the patterns of fiber innerv ation differed across development. Leu-enk-like immunoreactivity was i nitially densest in the lateral preoptic region of both sexes, ultimat ely expanding into the medial preoptic region to become densest in the lateral portion of the medial preoptic nucleus by P12. This latter pa ttern was observed only in males, however, as females continued to exh ibit diffuse medial preoptic Leu-enk-like immunoreactivity at P12. In contrast, the distribution and developmental pattern of beta-endo-like immunoreactivity was similar in both sexes; diffuse staining was obse rved in the medial preoptic region at birth, later becoming dense only in the periventricular and parastrial nuclei. The time course of deve lopment of Leu-enk and beta-endo innervation of the medial preoptic ar ea suggests that the sexually dimorphic expression of opioid immunorea ctivity occurs after preoptic neurons appear in their sexually dimorph ic configuration. Therefore, although the development of opioid-contai ning pathways could be influenced by the perinatal gonadal steroid hor mone environment, medial preoptic Leu-enk and beta-endo innervation mi ght not contribute directly to the sexually dimorphic neuronal organiz ation of the preoptic area.