Analgesic mechanisms of ketamine in the presence and absence of peripheral inflammation.

BACKGROUND The studies on the mechanisms of ketamine antinociception have led to conflicting results. In this study, the authors investigated the contribution of supraspinal monoaminergic descending inhibitory system to ketamine analgesia for acute nociception and inflammation-induced hyperalgesia. METHODS Male Sprague-Dawley rats were used. The paw withdrawal latencies to radiant heat stimuli were measured to assess the thermal nociceptive threshold. The analgesic effects of intrathecal or intraperitoneal ketamine were examined in the rats that received unilateral intraplantar carrageenan and in those that were untreated. In addition, it was examined whether pretreatment with intrathecal yohimbine or methysergide inhibited the analgesic effects of ketamine. Using an intrathecal microdialysis method, noradrenaline and 5-hydroxytryptamine concentrations in lumbar cerebrospinal fluid were measured after intraperitoneal ketamine in both saline- and carrageenan-treated rats. RESULTS In the untreated rats, intraperitoneal but not intrathecal ketamine produced antinociceptive effects in a dose-dependent manner. Pretreatment with intrathecal yohimbine or methysergide inhibited these antinociceptive effects. Intraplantar carrageenan significantly reduced paw withdrawal latencies on the injected paw but not on the contralateral paw. Both intraperitoneal and intrathecal ketamine reversed the shortened paw withdrawal latencies on the injected side in a dose-dependent manner without any effects on the contralateral side. Neither yohimbine nor methysergide inhibited these antihyperalgesic effects. In analyses of monoamines, the magnitude of increase in monoamines after intraperitoneal ketamine was significantly smaller in the carrageenan-treated rats than in the saline-treated rats. CONCLUSION These results demonstrated that ketamine produced antinociceptive effects through an activation of the monoaminergic descending inhibitory system, whereas, in a unilateral peripheral inflammation-induced hyperalgesic state, the monoaminergic system did not contribute to the antihyperalgesic effects of ketamine. The mechanisms of the antinociceptive and antihyperalgesic properties of ketamine are different.