Isoflurane, but not halothane, depresses c-fos expression in rat spinal cord at concentrations that suppress reflex movement after supramaximal noxious stimulation

We investigated the effects of isoflurane and halothane on the induction of fos-like immunoreactivity (FLI) in the rat lumbosacral spinal cord after supramaximal noxious mechanical stimulation of the hindpaw. Compared with unstimulated controls (0.9% isoflurane), noxious stimulation at 0.9%-1.5% elicited significant (0.9%-1.5% isoflurane) increases in FLI bilaterally. FLI was distributed mainly in the superficial dorsal horn (laminae I-III) and, to a lesser extent, in the deep dorsal horn (laminae IV-VI) and intermediate zone (lamina VII), with three- to fivefold greater labeling ipsilaterally. At 1.8% isoflurane, mean FLI counts in all laminar regions were significantly smaller (1.7 ± 1.3 per section) compared with the other concentrations (11.4 ± 9.5, 7.5 ± 6.8, and 9.7 ± 6.6 at 0.9%, 1.2%, and 1.5%, respectively) but were not different from unstimulated controls. At sacral levels, we observed a bilateral distribution of FLI primarily in superficial laminae in unstimulated controls that was not significantly different at any isoflurane concentration. FLI counts were not significantly different across groups receiving halothane (0.9%-1.5%). FLI was reduced only at isoflurane concentrations that depressed both gross, purposeful movement and reflex withdrawal, whereas halothane did not cause depression even at concentrations that depressed withdrawal reflexes. Isoflurane and halothane may have differing effects on neuronal function and responses to noxious stimulation.