Complete prevention but stimulus-dependent reversion of morphine tolerance by the glycine/NMDA receptor antagonist (+)-HA966 in neuropathic rats.

BACKGROUND Tolerance to the analgesic effect of morphine complicates the management of chronic pain states. The authors studied the ability of the glycine/N-methyl-D-aspartate receptor antagonist (+)-HA966 to modify morphine tolerance in a rat model of neuropathic pain. METHODS Mononeuropathy was induced by placing four ligatures around the common sciatic nerve. The 4-day pretreatment regimens with two daily subcutaneous injections of saline and saline, saline and morphine (10 mg/kg), (+)-HA966 (2.5 mg/kg) and morphine, or (+)-HA966 and saline were begun on post-operative day 12 to test the ability of (+)-HA966 to prevent the development of tolerance. Behavioral experiments were performed on postoperative day 16, when the pain-related behavior reached a stable maximum. The effect of an acute dose of morphine (1 mg/kg intravenously) was tested against both mechanical (vocalization threshold to paw pressure) and thermal (struggle latency to hind paw immersion into a 46 degrees C hot-water bath) stimuli. In addition, to test the ability of a single injection of (+)-HA966 to reverse established morphine tolerance, groups of morphine-pretreated rats received injections of either (+)-HA966 (2.5 mg/kg subcutaneously) and morphine (1 mg/kg intravenously), saline and morphine, or (+)-HA966 and saline. RESULTS Baseline vocalization thresholds and struggle latencies did not differ in the various pretreatment groups, indicating that the pretreatments had no effect on pain-related behavior. Coadministration of (+)-HA966 prevented the reduction of the effect observed with morphine alone in both the mechanical test and the thermal test. (+)-HA966 reversed morphine tolerance in the thermal but not in the mechanical test. CONCLUSION (+)-HA966 prevented morphine tolerance in both mechanical and thermal tests but reversed established morphine tolerance in the thermal test only.