Substance P and somatostatin modulate spinal cord excitability via physiologically different sensory pathways.

The effect of intrathecally injected substance P and somatostatin on spinal flexion reflex excitability was examined in decerebrate, spinalized, unanaesthetized rats. Substance P increased the excitability of the spinal cord to mechanical and thermal stimuli suprathreshold for C-afferents. Somatostatin had a similar effect with thermal, but not with mechanical stimuli. It is suggested that both peptides are released in association with C-afferent activation. Substance P may be released by polymodal nociceptors whereas somatostatin may be released by thermosensitive C-afferents.