Opioids and sexual behavior in the male rabbit: the role of central and peripheral opioid receptors.

The purpose of the present series of experiments was to analyze the effects of morphine and naloxone on sexual behavior in the male rabbit, and to evaluate the role of central and peripheral opioid receptors. Morphine was found to inhibit sex behavior in a dose dependent way. The effects were slight at 5 min postinjection. At 1 hr all aspects of sexual behavior were reduced. This effect lasted at least until 3 hrs postinjection. Subcutaneous (s.c.) injection produced effects at lower doses than intraperitoneal (i.p.) injection. Minimal effective doses were 1.25 and 5 mg/kg, respectively. Naloxone also inhibited sexual behavior. Again, s.c. administration had effects at lower doses than i.p. administration (0.25 vs 16 mg/kg). The effects of morphine were reduced but not completely antagonized by several doses of naloxone, independently of whether s.c. or i.p. administration were used. An opioid kappa agonist, bremazocine, inhibited sexual behavior at a low dose (30 micrograms/kg). It is suggested that the inhibitory effects of morphine may be mediated by the kappa receptor. A peripheral opioid antagonist, methylnaloxone, had no effects by itself and was unable to modify the effects of morphine. It is concluded that the effects of morphine are localized within the central nervous system. This is further supported by the observation that loperamide, a peripheral opiate agonist, had only marginal effects on sex behavior.