Morphine injected into the paraventricular nucleus of the hypothalamus prevents noncontact penile erections and impairs copulation: involvement of nitric oxide

Male rats show four to six penile erection episodes when put in the presenc e of an inaccessible receptive female for 80 min. These noncontact erection s occur concomitantly with an increase in nitric oxide production in the pa raventricular nucleus of the hypothalamus. This is shown by the increases i n the NO2- and No-3(-) concentrations in the paraventricular dialysate obta ined from these males by in vivo microdialysis, The NO2- concentration incr eased from 0.75 +/- 0.10 mu M to 2.89 +/- 0.39 mu M and that of NO3- from 4 .13 +/- 0.58 mu M to 9.5 +/- 1.2 mu M. Morphine (0.5, 1 and 5 mu g), given unilaterally into the paraventricular nucleus 15 min before the introductio n of the receptive female, prevented the NO2- and NO3- increases, and nonco ntact erections, dose-dependently. In contrast, the kappa opioid receptor a gonist U-69 593 (5 mu g) was ineffective. The effects of morphine on NO2- a nd No-3(-), and on noncontact erections, were prevented by the opiate recep tor antagonist naloxone (10 mu g) injected into the paraventricular nucleus 15 min before morphine. The NO2- and NO3- concentrations were also increas ed in the paraventricular dialysate of male rats during copulation, i.e. wh en in copula penile erections occurred. As found with noncontact erections, morphine, but not U-69 593, injected into the paraventricular nucleus prev ented the NO2- and NO3- increases and impaired copulatory behaviour, and na loxone prevented these responses when given before morphine. Although some diffusion of the opiate to surrounding brain areas cannot be completely rul ed out, the present results suggest that morphine acts through mu receptors in the paraventricular nucleus to impair noncontact erections and copulati on. These effects of morphine are apparently mediated by a prevention of th e increased nitric oxide production that occurs in the paraventricular nucl eus of the hypothalamus of male rats during sexual activity.