To the Editor:—

I read with much interest the recent report by Hashimoto et al.  1regarding the analgesic mechanism of nitrous oxide in the spinal cord. They found that inhalation of nitrous oxide itself induced the c-Fos protein expression in the rat spinal dorsal horn (especially in laminae III and IV).

However, I did not find such induction of c-Fos expression in my experiment. 2Sun et al.  3also showed no increase of c-Fos expression by inhalation of nitrous oxide. Although Sun et al.  and I investigated the effect of nitrous oxide on c-Fos expression evoked by noxious stimuli, we found no changes in the control side, namely the nonstimulated side. I found that nitrous oxide suppressed c-Fos expression evoked by noxious stimulation only in the deeper layer, whereas in laminae III and IV, there were few Fos-like immunoreactive cells, even with noxious stimulation, and no changes were observed in this region. Is the induction of c-Fos expression by nitrous oxide due to the difference in species? I used Wistar rats and Sun et al.  used Sprague-Dawley rats, whereas Hashimoto et al.  used Fischer rats.

Second, many anatomic and electrophysiologic studies confirmed that generally neurons, which exist in laminae III and IV, which receive only proprioceptive inputs and do not involve noxious input processing. Hunt et al.  4showed that noxious stimuli evoked c-Fos expression on neurons in laminae I, II, and V, whereas nonnoxious stimuli evoked c-Fos expression on neurons in laminae III and IV, which also supports the concept of functional organization in the spinal dorsal horn. Furthermore, immunohistochemical studies 5,6revealed that there are many noradrenergic terminals in laminae I and II but few in laminae III and IV. Consequently, even c-Fos expression would be induced by inhalation of nitrous oxide; it would not involve pain processing or descending noradrenergic control.

Third, many reports (Hunt et al. , 4Bullitt, 7Menétrey et al. , 8and others) showed that there were no or few Fos-like immunoreactive neurons in the control animals in the spinal cord. Bullitt wrote that almost no immunoreactivity was present in the lumbar or cervical spinal cord in normal controls, whereas Hashimoto et al.  reported that a fair number of Fos-like immunoreactive neurons existed in the control animals. What is the difference between those previous reports and that of Hashimoto et al. ? Is it because of the difference in species or characteristics of antibodies used in the experiments?

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Hagihira S, Taenaka N, Yoshiya I: Inhalation anesthetics suppress the expression of c-fos protein evoked by noxious somatic stimulation in the deeper layer of the spinal cord in the rat. Brain Res 1997; 751: 124–30
Sun WZ, Shyu BC, Shieh JY: Nitrous oxide or halothane, or both, fail to suppress c-fos  expression in rat spinal cord dorsal horn neurones after subcutaneous formalin. Br J Anaesth 1996; 76: 99–105
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