Opioid κ-receptor agonists suppress sexual behaviors in male rough-skinned newts (Taricha granulosa)

Four experiments were performed to evaluate a possible opioid involvement in the regulation of sexual behavior (amplectic clasping of a female) in intact adult male rough-skinned newts (Taricha granulosa) during the breeding season. It was found that an ip injection of bremazocine, a κ-receptor opiate agonist, can markedly reduce sexual activity and that an ip injection of naloxone can reverse this inhibition in a dose-dependent fashion. In contrast, in male newts that were sexually inactive before treatment, injections of naloxone failed to induce sexual behavior, suggesting that opioid mechanisms do not normally exert a tonic inhibition of amphibian sexual behavior. In addition, an injection of ethylketocyclazocine (another κ-receptor agonist), but not morphine (a μ-receptor agonist) suppressed sexual behaviors of male newts. These results indicate that opioid mechanisms that include κ-type opioid receptors may contribute to the regulation of sexual behavior in nonmammalian vertebrates.