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In male rats copulation has antinociceptive effects as measured either by shock-induced
vocalizations or hindlimb withdrawal to pinch. Prolonged mating reduces the content
of endogenous opioids in midbrain but not in hypothalamus or caudate nucleus. Blockage
of opiate receptors with the narcotic antagonist naloxone (4 mg/kg) significantly
extends the postejaculatory interval. The results indicate that mating is a biological
stimulus for the release of endogenous opoids, possibly to (a) prevent intense sexual
stimulation from becoming aversive, and (b) increase its reward value.