The oncogene, c-fos, is a useful marker of nociceptive neurons activated by various types of pain. However, the relationship between c-fos and acute pain induced by surgical incision has not been studied. The purpose of this study was to describe the expression of spinal c-fos in a rat model of postoperative pain.

Sprague-Dawley rats were used to enumerate spinal fos-like immunoreactive (fos-LI)-positive neurons after making sham, skin, or skin-muscle incisions on the plantar surface of the rat hindpaw. The spinal cords were divided into the following 4 segments for immunohistochemical detection of fos-LI at 3 time points after the incision (4 hours, POD1, and POD3): thoracic, upper lumbar, lower lumbar, and sacral. For analysis of the laminar distribution of fos-LI, the dorsal horn was divided into three regions (I-II, III-IV, and V-VI).

The number of fos-LI was increased in the ipsilateral spinal dorsal horn in the lower lumbar segment 4 hours post-incision and was significantly greater in rats with skin-muscle incisions than in rats with skin-alone incisions. In the skin-muscle group, laminae I-II and V-VI had a greater number of fos-LI neurons than laminae III-IV. The increase in number of spinal fos-LI neurons normalized by POD1 and POD3.

c-fos expression is confined to neurons of the spinal dorsal horn known to be activated by nociceptive stimulation. The larger the injury elicited, the greater the expression of c-fos, suggesting that the expression of c-fos can serve as a useful marker of activated nociceptive neurons caused by acute pain following surgical incision. Further study is required to elucidate why the degree of tissue injury affects the laminar distribution of c-fos expression.