Stimulation of opioid receptors suppresses penile erectile reflexes and seminal emission in rats.

The effects of several doses of morphine and naloxone, given alone or in combination, on ex copula tests for rat penile erectile responses and seminal emission were studied. Morphine (0.1, 0.5, 1 and 5 mg/kg IP, 30 min before the test) reduced the proportion of animals showing erections in a dose-related fashion. Seminal emission was apparently more sensitive to opioid inhibition than erectile responses, since it was virtually suppressed by all the doses of morphine tested. Naloxone given alone (0.1, 1 and 10 mg/kg IP, 15 min before testing) was largely ineffective on these genital responses although a significant decrease in the display of erection was observed with the lowest dose. Naloxone (1 mg/kg) efficiently antagonized the effects of morphine (1, 5 and 25 mg/kg) on erectile responses and all but the largest dose of the opiate agonist on seminal emission. These results indicate that, in addition to the well-documented effects of opioids on sexual drive, their effects on the genital reflex potential could play a major role in the sexual deficits associated to opiate intake.