Substance P effects on spinal nociceptive neurones.

In decerebrated spinal cats, the effects of iontophoretically applied acetylcholine (ACh) and substance P were examined on the responses of dorsal horn neurones to noxious stimulation and touch of the skin. Both agents, in amounts that did not have a significant direct effect on the neuronal firing rate, prolonged the response of the cells to noxious stimulation but did not alter that to touch stimulation. The peptide and ACh potentiated the late, but not the early, responses of dorsal horn neurones to sural A_δ and C afferent stimulation. Substance P-induced potentiation of the above responses was observed even when the agent did not produce a significant depolarization of nociceptive cells. In greater amounts, the peptide depolarized the neurones, an effect that was not associated with a detectable change in the membrane resistance. These results indicate that substance P facilitates nociceptive pathways by potentiating the subliminal fringe and, in greater amounts, by depolarizing the cells. The failure by the peptide to potentiate touch-induced excitation of the nociceptive neurones appears not to be due to the selectivity of the drug effect but due to the absence of subliminal fringe.