Demonstration of a sexual dimorphism in the distribution of serotonin-immunoreactive fibers in the medial preoptic nucleus of the rat.

The distribution of serotonergic fibers in the medial preoptic nucleus (MPN) and adjacent areas was evaluated with an indirect immunohistochemical method in the normal adult male and female albino rat. Sections through the MPN were processed for immunofluorescence with an anti-serum directed toward serotonin and were counterstained with the fluorescent Nissl stain ethidium bromide. The distribution of serotonin-immunoreactive fibers in the MPN was correlated with cytoarchitectonic features of the nucleus. On the basis of the results, we have subdivided the MPN into three parts: a medial cell-dense part ( MPNm ), a lateral cell-sparse part ( MPNl ), and a central very cell-dense part ( MPNc ) that is embedded in the medial part. The MPNc corresponds to the sexually dimorphic nucleus of the preoptic area identified by Gorski et al. ('80). A marked sexual dimorphism was found in the relative size of each part of the MPN. In the male, the volumes of the cell-dense MPNm and MPNc appear to be notably larger, while in the female more than half of the nucleus is occupied by the cell-sparse lateral part. The MPN as a whole appears to be slightly larger in the male. Each subdivision contains a characteristic pattern of serotonin-immunoreactive fibers. In each sex, the MPN is surrounded by a low to medium density of serotonin-stained fibers, while the MPNl is filled with a dense plexus of varicose immunoreactive fibers. In contrast, the MPNm contains a low density of stained fibers, and the MPNc is virtually devoid of serotonin-stained fibers. Since both the MPNm and the MPNc are larger in the male, a correspondingly larger region of very low serotonin-stained fiber density is found in the male. It appears then that the MPN is a sexually dimorphic complex composed of at least three cytoarchitectonically distinct subdivisions, each of which contains a characteristic density of serotonin-immunoreactive fibers.