PREVENTION BY MORPHINE OF N-METHYL-D-ASPARTIC ACID-INDUCED PENILE ERECTION AND YAWNING - INVOLVEMENT OF NITRIC-OXIDE

The effect of morphine on the increase of NO2- and NO3- concentration in the dialysate obtained with a microdialysis probe implanted in the paraventricular nucleus of the hypothalamus, and penile erection and y awning induced by N-methyl-D-aspartic acid (NMDA) was studied in male rats. NMDA (50 ng) injected in the paraventricular nucleus of the hypo thalamus, induced penile erection and yawning and increased NO; from 1 .10 +/- 0.28 mu M to 7.30 +/- 1.10 mu M and NO3- from 5.05 +/- 0.71 mu M to 11.03 +/- 1.61 mu M, Morphine (1-10 mu g), but not U-69,593 (10 mu g), a selective agonist of the kappa opiate receptor subtype, preve nted in a dose-dependent manner NMDA-induced increase in NO2- and NO3- concentration when injected in the paraventricular nucleus 15 min bef ore NMDA, Morphine prevention of NMDA-induced NO2- and NO3- increase w as related to a concomitant decrease in the number of penile erection and yawning episodes induced by the excitatory amino acid, Morphine ef fect was not observed in male rats treated with the opiate receptor an tagonist naloxone (10 mu g) microinjected in the paraventricular nucle us 15 min before morphine, The present results suggest that morphine p revents an NMDA-induced increase in paraventricular NO production, pen ile erection, and yawning by inhibiting NO synthase activity in the pa raventricular nucleus of the hypothalamus through the stimulation of o pioid receptors of the mu subtype. (C) 1997 Elsevier Science Inc.