Abstract
The effect of morphine on the increase of NO 2 − and NO 3 − concentration in the dialysate obtained with a microdialysis probe implanted in the paraventricular nucleus of the hypothalamus, and penile erection and yawning induced by N -methyl- d-aspartic acid (NMDA) was studied in male rats. NMDA (50 ng) injected in the paraventricular nucleus of the hypothalamus, induced penile erection and yawning and increased NO 2 − from 1.10 ± 0.28 μM to 7.30 ± 1.10 μM and NO 3 − from 5.05 ± 0.71 μM to 11.03 ± 1.61 μM. Morphine (1–10 μg), but not U-69,593 (10 μg), a selective agonist of the κ opiate receptor subtype, prevented in a dose-dependent manner NMDA-induced increase in NO 2 − and NO 3 − concentration when injected in the paraventricular nucleus 15 min before NMDA. Morphine prevention of NMDA-induced NO 2 − and NO 3 − increase was related to a concomitant decrease in the number of penile erection and yawning episodes induced by the excitatory amino acid. Morphine effect was not observed in male rats treated with the opiate receptor antagonist naloxone (10 μg) microinjected in the paraventricular nucleus 15 min before morphine. The present results suggest that morphine prevents an NMDA-induced increase in paraventricular NO production, penile erection, and yawning by inhibiting NO synthase activity in the paraventricular nucleus of the hypothalamus through the stimulation of opioid receptors of the μ subtype.
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