Endomorphin-1, effects on male sexual behavior

We examined the effect of endomorphin-1 (EM-1), an endogenous opioid peptide that binds selectively to the μ receptor, on male copulatory behavior in sexually vigorous Wistar rats. In the first experiment, four doses of EM-1 (1, 10, 50, and 100 μM) injected intracerebroventricularly produced a marked increase in ejaculation latency and interintromission interval and reduced the number of ejaculations during the test. In experiment 2 the effects of EM-1 were completely blocked by pretreatment with naloxone (5 mg/kg) 30 min prior to intracerebroventricular injection of EM-1 (100 μM). Collectively these results indicate that the activation of μ receptors by EM-1 modifies parameters associated with ejaculation (increases ejaculation latency and reduces the number of ejaculations) confirming that opioids are released during sexual behavior.