Participation of Endogenous Opioids in the Inhibition of the Spinal Generator for Ejaculation in Rats

Introduction. A spinal pattern generator controls the expression of ejaculation. When this ejaculation generator is activated it can be phasically controlled, at a spinal level, by intrinsic mechanisms that eventually lead to the establishment of both short- and long-lasting inhibitory processes. Aim. To evaluate the hypothesis that endogenous opioids participate in the control of ejaculation by exerting an inhibitory influence upon the spinal generator for ejaculation. Main Outcome Measures. Electromyographic recordings of the ejaculatory motor pattern recorded in the bulbospongiosus muscles were obtained as physiological markers of ejaculation. Methods. By using a model for the study of ejaculation in spinal male rats, we analyze the effects of the intravenous injection of the opioid agonist morphine and the opioid antagonist naloxone on the expression of the ejaculatory motor pattern. In addition, the effect of pre-treatment with systemic naloxone on the establishment of the inhibition of the ejaculatory motor pattern resulting from its repeated sensory-induced elicitation was evaluated. Results. Data obtained show that: (i) the i.v. injection of morphine (1-10 mu g/rat) inhibits whereas that of naloxone (1-10 mu g/rat) induces the expression of the genital ejaculatory motor pattern; (ii) naloxone pretreatment dose-dependently blocks the inhibitory effects of the high dose of morphine upon the rhythmic motor pattern of ejaculation; (iii) the inhibition of the ejaculatory response induced by repeated urethral stimulation can be delayed, and the ejaculatory capacity augmented, by naloxone injection (10 mu g/rat). Conclusion. Together, these evidences support the notion that endogenous opioids modulate the activity of the spinal generator for ejaculation by exerting an inhibitory influence. Carro-Juarez M, and Rodriguez-Manzo G. Participation of endogenous opioids in the inhibition of the spinal generator for ejaculation in rats. J Sex Med 2009;6:3045-3055.