Preemptive effect of intravenous ketamine in the rat: Concordance between pain behavior and spinal fos-like immunoreactivity

Background: The purpose of this study was to compare behavioral antinociceptive responses with spinal fos-like immunoreactivity (FLI) for a intravenous ketamine injection between pre vs. postformalin administration in rats. Methods: Sprague-Dawley rats (250-300 g) were prepared to receive either saline or ketamine. All rats were randomly divided into three groups: control, pretreatment and post-treatment group. Formalin (5%) 100 μl was injected into the hindpaw. Pain related behavior and FLI in the lumbar spinal cord was examined. Results: Flinches of phase 2 were 239.3 (22,8), 118.6 (7,5) (P < 0.05 vs. control and post-treatment group), and 186.7 (16,6) in the control, pre and, post-treatment groups, respectively. Fos-like immunoreactivity expression was significantly correlated with phase 2 flinching behavior (P < 0.001). Conclusion: Pretreatment with intravenous ketamine inhibits inflammatory pain behavior and FLI expression following a formalin injection in rats, suggesting that pretreatment of ketamine plays an important role in preemptive analgesia.