The effect of ACEA-1328, a competitive and systemically bioavailable NMDA receptor/glycine site antagonist, was studied on morphine-induced antinociception and tolerance in CD-1 mice using the tail flick test. To study the effect of acute administration of ACEA-1328 on morphine-induced antinociception, mice were injected with either ACEA-1328 (1, 5, and 10 mg kg(-1)) or Bis-Tris (0.2 m) immediately followed by an injection of morphine and tested for antinociception 30 min later. ACEA-1328 significantly increased the antinociceptive potency of morphine. To study the effect of chronic administration of ACEA-1328 on morphine-induced antinociception and tolerance, mice were treated, either once per day for 9 days or twice daily for 4 days, with ACEA-1328 or with the vehicle. Mice were then, within 1 min, injected daily with either morphine or saline. On the day of the test, mice were injected with only morphine and tested for antinociception 30 min later. In comparison to the acute effect of ACEA-1328, chronic treatment with the NMDA receptor/glycine site antagonist did not affect the antinociceptive potency of morphine. Chronic treatment with morphine, by both methods, produced a significant degree of tolerance. Concurrent administration of ACEA-1328 with the opioid analgesic completely blocked morphine tolerance. Our results demonstrate that acute, but not chronic, treatment with ACEA-1328 increased the antinociceptive potency of morphine. Furthermore, co-administration of the NMDA receptor antagonist with morphine abolished the development of tolerance. Overall, the data support a growing body of evidence showing that activation of the NMDA receptor plays a functional role in opioid-induced antinociception and tolerance.
Copyright 1999 Academic Press.