Further evidence that nitric oxide modifies acute and chronic morphine actions in mice

The effects of the nitric oxide (NO) synthase inhibitor, N(omega)-nitro-L-arginine (L-NNA, 2.5-10 microg i.c.v.), and the NO synthesis precursor, L-arginine (L-Arg, 2.5-10 microg i.c.v.), on morphine-induced analgesia, and on morphine-induced tolerance and dependence were examined in mice. Administration of L-NNA diminished the morphine-induced analgesia. L-Arg pretreatment increased the analgesic effect of morphine. Repeated pretreatment (three times, at 24-h intervals) with L-NNA diminished both acute and chronic tolerance to morphine, whereas both the acute and the chronic morphine-induced tolerance increased after the repeated (three times, at 24-h intervals) administration of L-Arg. Neither L-NNA nor L-Arg affected the signs of morphine dependence, as assessed by naloxone (1 mg/kg, s.c.)-precipitated withdrawal. Our data suggest that increased NO synthesis potentiates morphine analgesia and enhances the development of morphine tolerance in mice.