Differential effects of yohimbine and naloxone on copulatory behaviors of male rats

Prior research has demonstrated that both yohimbine, an alpha2-adrenergic antagonist, and naloxone, an opiate antagonist, facilitate components of copulatory behaviors in nonstressed male rats. In the present experiments, we demonstrate that these drugs differentially affect copulatory behaviors when the behavioral testing situation contained an aversive element. Male rats received an injection of lithium chloride (0.3 M, 20 ml/kg, ip) immediately after each encounter with an estrous female. Consequently, male copulatory behaviors gradually declined during successive test sessions. These male rats also received ip injections of either yohimbine (2 mg/kg/ml), naloxone (4 mg/kg/ml), or isotonic saline 20 min prior to each copulation test. Yohimbine-treated rats were more likely to copulate than control rats during both acquisition and extinction of lithium chloride-induced associative inhibition of copulatory behavior. Conversely, naloxone-treated rats were less likely to copulate than control rats during both acquisition and extinction. These data are consistent with the hypothesis that yohimbine increases sexual motivation in the male rat and limit the generality of the excitatory effects of naloxone on copulatory behaviors.