Sexual experience reduces neuronal activity in the central part of the medial preoptic nucleus in male rats during sexual behavior

The medial preoptic area (MPN) plays an important role in the control of male sexual behavior. In rats, the central part of the MPN (MPNc) is sexually dimorphic and contains a sexually dimorphic nucleus composed of neurons expressing calbindin-D28 K (CALB-SDN). Although the functions of the MPNc are not well understood, surgical destruction of the MPNc adversely affects the performance of sexual behavior in sexually naive males, but not in sexually experienced males, supporting the notion that the MPNc changes functionally with sexual experience. In this study, we aimed to determine the effects of sexual experience on the neuronal activity of the MPNc and CALB-SDN. Sexual behavior in sexually inexperienced males that had no experience of ejaculation, and experienced males that had ejaculated once was observed. After they displayed sexual behavior, the brains were sampled, and immunohistochemical analysis of c-Fos, a neuronal activity marker, in the MPNc and CALB-SDN was performed. The numbers of c-Fos-immunopositive cells with or without calbindin-D28K-immunoreactivity increased significantly in the MPNc and CALB-SDN following ejaculation in both sexually inexperienced and experienced males, although the numbers did not change significantly with exposure to estrous female odors, the first mount, and the first intromission before and after the first ejaculation. We further found that the number of c-Fos-immunopositive and calbindin-D28K-immunonegative cells in the MPNc, but not in the CALB-SDN, was significantly smaller in sexually experienced males than in sexually inexperienced males. These results suggest that a population of MPNc neurons, which is located outside the CALB-SDN and do not express calbindin-D28 K, are activated during the first copulation and then silent after acquisition of sexual experience.