Abstract
N-methyl-D-aspartate(NMDA) receptor has been well known as an important mediator of several forms of neural and behavioral plasticity. But different results were reported about the effect of MK-801 or dextromethorphan on opioid dependence. The present studies examined whether NMDA receptor antagonists can alter the opioid dependence and tolerance in rodents. Naloxone precipitated withdrawal symptoms and changes of locomotor activities were observed in MK-801 or dextromethorphan pretreated morphine-dependent rats. Tail-flick assay was used for morphine analgesia and tolerance was found after 4 day's consecutive injections (10 mg/kg, s.c., twice/day) of morphine in mice. Locomotor activity was increased and the withdrawal symptoms were decreased by the pretreatment of MK-801 in morphine-dependent rats. But 0.3 mg/kg i.p. of MK-801 intensified the body weight loss and produced severe ataxia and rotation although some withdrawal signs were attenuated. Morphine induced analgesic tolerance was inhibited by the pretreatment of MK-801 and dextromethorphan. Dextromethorphan was more potent than MK-801 in inhibiting the development of the analgesic tolerance in mice. These results suggest that NMDA system may be involved in opioid withdrawal and analgesic tolerance but appropriate caution should be requested when MK-801 is used in combination with opioid because of untoward neurologic signs.