Sexual behavior induces naloxone-reversible hypoalgesia in male rats.

The sensitivity to painful and sexual stimuli in male rats was markedly suppressed immediately after ejaculation and enhanced after the postejaculatory refractory period. The suppression of pain sensitivity induced by sexual activity was reversed, but sexual behavior was only slightly affected by intraperitoneal (i.p.) injection of the opioid antagonist naloxone (5 mg). Plasma concentrations of beta-endorphin-like immunoreactivity were unaffected by sexual activity. Injection of beta-endorphin (10 micrograms s.c.) markedly raised plasma concentrations of beta-endorphin-like immunoreactivity within 1 min of injection but did not affect the sensitivity to painful stimulation or the display of sexual behavior. It is suggested that while ejaculation may activate opioid receptor mechanisms, which affect the sensitivity to painful, but not sexual, stimuli, elevation of beta-endorphin in the blood does not affect the sensitivity to either sexual or painful stimuli in male rats.