The effect of naloxone on ketamine-induced effects on hyperalgesia and ketamine-induced side effects in humans

Background The (NMDA) receptor plays a significant role in wind-up and spinal hypersensitivity and is involved in the occurrence of secondary hyperalgesia. Ketamine is an NMDA-receptor antagonist and has proven effective in alleviating secondary hyperalgesia in humans. Although it is disputed, the actions of ketamine have been ascribed not only to NMDA receptor antagonism, but also to opioid receptor agonism. A study therefore was designed in which the abolishment of a previously demonstrated effect of ketamine on secondary hyperalgesia was sought by pretreatment with naloxone. Methods: Twenty-five volunteers were subjected to three treatment regimens. A standardized first-degree burn injury was induced. On appearance of primary and secondary hyperalgesia, one of the following infusion schemes was applied in a randomized, double-blind, cross-over fashion: (1) infusion of naloxone 0.8 mg/15 min followed by 0.4 mg/h), succeeded by infusion of ketamine (0.3 mg kg(-1) 15 min(-1) followed by 0.3 mg kg(-1) h(-1)); (2) infusion of placebo, succeeded by infusion of ketamine (0.5 mg kg(-1) 15 min(-1) followed by 0.3 mg kg(-1) h(-1); and (3) infusion of placebo, succeeded by infusion of placebo. Heat-pain detection thresholds, magnitude of secondary hyperalgesia around the burn injury, and side effects mere determined. Results: Ketamine reduced secondary hyperalgesia. Naloxone did not affect the action of ketamine. The magnitudes of side effects were equal if the subjects received ketamine, regardless of preceding infusion of naloxone, Conclusions: In this experimental setting, opioid receptor blockade does not inhibit ketamine-induced reductions of secondary hyperalgesia.