Opiate antagonists and copulatory behavior of male hamsters

A possible role of endogenous opioids in male copulatory behavior was examined in six experiments which studied the effects of opiate antagonists on the copulatory behavior of male hamsters (Mesocricetus auratus). In Experiment 1, the acute administration of naloxone hydrochloride (4 mg/kg, SC) ten minutes before testing significantly decreased mount frequency, intromission frequency, and ejaculation latency. In Experiment 2 males were tested weekly for three weeks. Half of the males were injected with naloxone ten minutes before each test and half with physiological saline. Naloxone administration reduced mount frequency, and intromission frequency while increasing the postejaculatory interval and the proportion of males displaying behavioral signs of satiety. In Experiment 3 similar effects were obtained following daily administration of naltrexone hydrochloride (10 mg/kg/day SC). In Experiment 4 males were allowed to mate to satiety. Naloxone treated males were more likely to display behavioral signs of satiety during the first ten minutes of these tests. In Experiment 5 it was demonstrated that naloxone administration did not alter the duration of insertion during either intromissions or ejaculations. In Experiment 6 the administration of naloxone did not facilitate the display of copulatory behavior by sexually inactive males. Overall the results are consistent with the hypothesis that opiate antagonists alter male copulatory behavior by enhancing the impact of stimuli occurring during the sexual interaction.