Fig. 3. Number (mean ± SEM, n = 4) of c-Fos–positive cells at the lumbar level in response to formalin injection from the four age groups of animals receiving either air (black bars ) or 70% Xe–20% O2–10% N2(Xe) (dotted bars ) or in response to saline injection from the four age groups receiving air (white bars ). *P < 0.01, **P < 0.001 relative to air + formalin group at the corresponding region. +P < 0.01, ++P < 0.001 relative to xenon + formalin. The figures in the left column represent c-Fos expression ipsilaterally associated with injection, and those in the right column represent c-Fos expression contralaterally associated with injection. From 19 days old to adult, laminae I–II (superficial area), laminae II–IV (nucleus proprius area), laminae V–VI (neck area), and laminae VII–X (ventral area) in the spinal cord section are presented by A / B , C , D , and E , respectively, as equivalent to the five regions in 7-day-old pups.