Ontogeny of Leu-enkephalin and β-endorphin innervation ofthe 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 leucine-enkephalin (Leu-enk) and β-endorphin (β-endo) innervation was studied using antisera directed against these compounds. The distribution of Leu-enk and β-endo differed at each age examined from birth to postnatal day 12 (P12). Furthermore, the patterns of fiber innervation differed across development. Leu-enk-like immunoreactivity was initially densest in the lateral preoptic region of both sexes, ultimately expanding into the medial preoptic region to become densest in the lateral portion of the medial preoptic nucleus by P12. This latter pattern was observed only in males, however, as females continued to exhibit diffuse medial preoptic Leu-enk-like immunoreactivity at P12. In contrast, the distribution and developmental pattern of β-endo-like immunoreactivity was similar in both sexes; diffuse staining was observed in the medial preoptic region at birth, later becoming dense only in the periventricular and parastrial nuclei. The time course of development of Leu-enk and β-endo innervation of the medial preoptic area suggests that the sexually dimorphic expression of opioid immunoreactivity occurs after preoptic neurons appear in their sexually dimorphic configuration. Therefore, although the development of opioid-containing pathways could be influenced by the perinatal gonadal steroid hormone environment, medial preoptic Leu-enk and β-endo innervation might not contribute directly to the sexually dimorphic neuronal organization of the preoptic area.