Distribution of opioid peptides in the preoptic region: immunohistochemical evidence for a steroid-sensitive enkephalin sexual dimorphism.

The distribution of cells and fibers that contain opioid peptides within the preoptic region of the rat was examined immunohistochemically. Cells and/or fibers that contain peptides derived from each of the three major opioid peptide families were differentially stained by using antisera that recognize unique derivatives of each precursor molecule and do not cross-react with members of the other opioid peptide families. A beta-endorphin (beta E) antiserum was used to stain fibers that contain peptides derived from the proopiomelanocortin molecule, and dynorphin-containing cells were identified by using an antiserum directed toward dynorphin B (Dyn B) that does not show detectable cross-reactivity with enkephalin-related peptides. An antiserum raised against peptide E (PE), which does not appear to cross-react significantly with dynorphin peptides, was used to localize enkephalin cells and fibers. Each family of opioid peptides showed a unique distribution in the preoptic region. beta E-immunoreactive fibers were primarily localized to the preoptic part of the periventricular nucleus, with moderate densities of fibers contained in the anteroventral periventricular nucleus (AVPv) and medial preoptic nucleus (MPN). Dyn B-immunoreactive fibers showed a somewhat more uniform distribution throughout the region, and only a few Dyn B-stained cells bodies were found within the medial preoptic area. In contrast, the preoptic region contained hundreds of PE-immunoreactive cells, which were particularly numerous within the AVPv, MPN, and anterodorsal preoptic nucleus. The AVPv and MPN also contained discretely localized plexuses of PE-stained fibers. Although the overall distributions of opioid peptide-containing fibers within the preoptic region were quite similar in male and female rats, differential distributions of fibers were found in certain nuclei such as the AVPv and MPN, and they were correlated with previously identified cytoarchitectonic sexual dimorphisms. Such differential distributions were particularly distinct for enkephalin-containing fibers. Although the AVPv is larger in female rats, it contained more PE-immunoreactive cell bodies in male rats, and we have shown here that this sexual dimorphism appears to be at least partially dependent on perinatal levels of gonadal steroids. In contrast, no difference in the number of PE-stained cells was found within the anterodorsal preoptic nucleus of male and female animals, indicating that sexual differences are not a general characteristic of enkephalinergic cells in the preoptic region of the rat.(ABSTRACT TRUNCATED AT 400 WORDS)