Effects of substance P on functionally identified units in cat spinal cord

A correlation was made between the effects of synthetic substance P (ssP) on spinal dorsal horn neurones and the responses of these neurones to natural peripheral stimulation. It was found that ssP caused excitation only of those units which were excited by noxious radiant heat applied to the skin. ssP also caused a small facilitation of the excitatory response to this noxious stimulus and, in two cases, led to a response to noxious heat of units which had previously been unaffected by this stimulus. The ratio of ssP sensitive to insensitive units was highest in the deeper parts of the dorsal horn. Excitation by ssP showed a positive correlation to excitation by bradykinin. These results suggest that substance P may be involved in excitatory spinal processes and that its actions may be associated specifically with nociception.