Fig. 4. (A and B ) Representative cross-sections of the spinal cord at the lumbar level L5 stained for c-Fos using a 3,3′-diaminobenzidine (DAB) reaction and (C and D ) illustrations of each section including the borders of each lamina. (A and C ) Control (air for 90 min). (B and D ) Seventy-five percent nitrous oxide (90 min). The c-Fos–positive cells are seen as block nuclear staining in A and B and are emphasized with large black dots in C and D .