Effects of ketamine on formalin-induced activity in the spinal dorsal hornof spinal cord-transected cats: differences in response to intravenous ketamine administered before and after formalin

Background: Although formalin has been widely used as an algesic substance in rodent studies, the unique biphasic effect seen in rats is not present i n humans. Humans, like cats, have a monophasic behavioral response to forma lin injection. Electrophysiologically, spinal dorsal horn neurons in cats a lso have what could be considered a monophasic response after the initial b urst of activity following formalin injection. Although several studies of the effects of ketamine on formalin responses have been carried out in rode nts, we are unaware of similar studies in cats. We hypothesize that such sp ecies differences may explain observed differences in preemptive analgesic effects. Therefore, we examined the effects of ketamine on activity of spin al wide dynamic range (WDR) neurons evoked by formalin injection in cats. Methods: We investigated in cats the effect of ketamine on the activity of WDR neurons in the spinal dorsal horn that was evoked by formalin. In addit ion, we studied the effects of pre- and post-administration of ketamine on the maintained phase of the formalin response. Each dose was a subanestheti c, anesthetic or high anesthetic dose (3.0 mg kg(-1), 10 mg kg(-1), and 30 mg - kg(-1)). Results: Intravenously administered ketamine produced a dose-dependent depr ession of evoked activity that was significantly greater when the drug was administered before formalin. Conclusion: In spite of the species differences in responses to formalin, t here still appears to be a clear preemptive effect of ketamine in the cat. Species differences may not explain apparent differences between human and animal preemptive analgesia.