Comparison of the effects of yohimbine, naloxone and 8-OH-DPAT on the diminished ejaculatory capacity induced by repeated ejaculation

In various species, repeated ejaculation over a short period of time results in a marked decrease in the capacity of sexual responses and the activity of sexual arousal/motivation. The purpose of this study was to compare the effects of the alpha(2)-adrenergic receptor antagonist yohimbine, the opioid receptor antagonist naloxone, and the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (all of which have been shown to stimulate male sexual arousal/motivation in sexually exhasuted rats), on the diminished ejaculatory capacity induced by repeated ejaculation in dogs. The data obtained show that both the decrease in the amount of ejaculate and the delay onset in ejaculation latency elicited by antecedent ejaculation were completely prevented by a low dose (0.1 mg/kg, i.p.) of yohimbine, but not naloxone (1.0-3.0 mg/kg, i.p.) and 8-OH-DPAT (0.01-0.1 mg/kg, i.p.). Unlike the effects on male sexual arousal/motivation, the present results suggest that yohimbine, naloxone, and 8-OH-DPAT, differentially affect on the diminished ejaculatory capacity and that yohimbine only may be effective for the treatment of ejaculatory dysfunction, which accompanies by the diminished ejaculatory capacity.