Pre- versus postinjury effects of intravenous GABAergic anesthetics on formalin-induced Fos immunoreactivity in the rat spinal cord

We evaluated the suppression of spinal Fos-like immunoreactivity (FLI) by I V anesthetics in the rat formalin model. Preformalin injection (1.5% subcut aneously) treatment groups included IV saline controls and three IV GABAerg ic anesthetic groups (pentobarbital 20 mg/kg, propofol 10 mg/kg, or alphaxa lone 1.5 mg/kg; n = 12 per group). After perfusion 2 h postformalin, spinal cords were dissected, sliced at 30 mu m, and processed by immunoperoxidase staining with an antibody against the Fos protein. Quantification and dete rmination of the laminar distribution of Fos-labeled nuclei were performed at the L4-5 spinal level ipsilateral to formalin injection. Drug groups dem onstrating FLI suppression were comparatively studied in a 5-min postformal in treatment group. Pentobarbital pretreatment failed to suppress FLI. Howe ver, significant reductions (percent decrease) of FLI were observed with pr opofol (63%) and alphaxalone (30%) compared with saline controls. Pre- vers us postformalin comparison studies showed that propofol, but not alphaxalon e, suppressed FLI more effectively when given preformalin. Given the observ ed inconsistencies between this study of Fos expression and our previous be havioral study, it is questionable whether anesthetic modulation of noxious stimulus-induced FLI parallels that of behavioral responses. Implications: In this study, we examined whether IV general anesthetics (propofol, alpha xalone, and pentobarbital) prevent injury-induced spinal cord changes. We m easured spinal Fos protein after rats received anesthetics before versus af ter a formalin injection. Fos inhibition patterns were inconsistent with be havioral studies of these anesthetics, suggesting that Fos inhibition does not always correlate with behavioral analgesia.