Male sexual activities were tested in androgentreated castrated male rats with lesions of the raphe obscurus nucleus (ROBL) or lesions of the raphe magnus nucleus (RMGL). The ROBL male rats showed low levels of mounting, intromission and ejaculation frequencies, and prolonged mount latencies compared to castrated and sham-operated control males. The sexual activity in the RMGL group was comparable to that of the controls. The results suggest that the raphe obscurus nucleus is involved in the neural mechanisms mediating copulatory behavior in male rats, and that the raphe magnus nucleus is not. In several castrated control and ROBL males, serotonin-synthesis inhibitor, p-chlorophenylalanine (PCPA) was injected before the behavioral test, because the raphe obscurus nucleus contains a large number of serotonergic neuronal cells. PCPA-treated control males showed higher frequencies of copulatory patterns than did control males without PCPA. In contrast, the frequencies of ejaculation and intromission were not increased by PCPA in the ROBL males, compared to PCPA-untreated ROBL males, although the mount latency was shortened and mount frequency was increased. This indicates that PCPA facilitates male sexual behavior. However, the suppressive effect of ROBL still remained even after deprivation of serotonin. Moreover, PCPA acts on serotonergic neurons other than those in the raphe obscurus nucleus, thereby facilitating mount activities.
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