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Ultrastructural evidence of a sexual dimorphism in the neuropil of the medial preoptic nucleus of the rat: a quantitative study.

In the rat, a new structural sex difference of the nervous system was found: the density of axodendritic synapses (ADS) is larger in the male central component of the medial preoptic nucleus (MPNc), as proven by quantitative electron microscopy. The study included 24 animals divided into four groups (n = 6): control males (CM), control females (CF), males castrated as newborn (MGx) and males castrated as newborn and treated 10 days with daily injections of testosterone. No significant differences were found in the number of axospinous synapses among all groups; however, the number of ADS was significantly larger in CM when compared to that of the CF. This difference was abrogated by castration of the newborn male and restored by perinatal treatment with testosterone. Further, no difference in ADS was encountered between CF and MGx. The present data, together with previous studies which indicate that sex steroids do modify the number of synapses in other nuclei of the brain, led us to conclude that the sex difference in the number of ADS within the MPNc is determined by the organizational effect(s) of testosterone.