Fig. 3. The effect of nitrous oxide (N2O) concentration (90-min exposure) on the total number of c-Fos–positive cells in the spinal cord at the lumbar level (L5). Each bar represents the mean number of c-Fos–positive cells from four animals, and the error bar indicates the SD. *P < 0.05 compared with basal c-Fos expression (0%, control).