Increase in substance P in the dorsal horn during a chemogenic nociceptive stimulus.

Substance P has been implicated as a neuromediator of nociception in the dorsal horn of the spinal cord. This is the first report of quantitative increases in substance P following an acute nociceptive stimulus. Female Wistar rats were injected subcutaneously in the plantar aspect of the right hindpaw with 5% formalin. Substance P levels were determined by quantitative immunohistochemistry 1 h after the injection. The present results show that substance P levels in the upper two laminae of the ipsi-and contralateral dorsal horn were significantly increased 1 h after the formalin injection. These results suggest a direct functional relationship between substance P levels in the upper dorsal horn and an acute chemogenic nociceptive stimulus.