Clonidine suppresses copulatory behavior and erectile reflexes in male rats: lack of effect of naloxone pretreatment.

Adrenergic transmitters and opioid peptides are implicated in the regulation of male sexual behavior. In the present studies we examine a possible interaction between these two neurochemical systems in the regulation of components of male sexual behavior. In mating tests, clonidine (0.25 mg/kg, 6 min pretest) induced a profound deficit in intromissive and ejaculatory behavior whereas naloxone (5 mg/kg, 20 min pretest) evinced a facilitation of ejaculatory behavior, evidenced by decreases in the ejaculation latency in the initial copulatory series and by decreases in ejaculation latency and intercopulatory interval in the second copulatory series. Importantly, prior treatment with naloxone did not prevent or attenuate the copulatory suppression induced by clonidine. In ex copula penile reflex tests, clonidine (0.25 mg/kg, 6 min pretest) decreased the incidence of seminal emission and the number of penile responses (erections, cups and flips). Naloxone (5 mg/kg, 20 min pretest) was without effect on any of the parameters of penile reflex activity and, further, failed to prevent or attenuate the erectile suppression induced by clonidine. A final study evaluated the dose-response relationship of clonidine-induced erectile dysfunction. A similar degree of erectile dysfunction was observed after 0.005, 0.025 or 0.25 mg/kg clonidine, whereas 0.0005 mg/kg was without effect. Previous studies demonstrated a dose-dependent inhibition of mounting, intromissive and ejaculatory behavior, with 0.25 mg/kg selectively eliminating ejaculatory behavior in mating tests. These data demonstrate a dose-dependent inhibition of penile reflexes, with inhibition occurring at doses lower than those required to induce copulatory dysfunction. Further, we suggest that opioid receptors sensitive to naloxone blockade are not involved in the clonidine-induced suppression of copulatory or erectile function.