SUBSTANCE-P RELEASED ENDOGENOUSLY BY HIGH-INTENSITY SENSORY STIMULATION POTENTIATES PURINERGIC INHIBITION OF NOCICEPTIVE DORSAL HORN NEURONS INDUCED BY PERIPHERAL VIBRATION

To investigate the interaction at the spinal level of endogenously released substance P with the effects of endogenously released adenosine, extracellular single-unit activity was recorded from dorsal horn neurons in the anesthetized cat. Vibration to the skin inhibited on-going activity of nociceptive neurons; 20 mg/kg caffeine reversibly blocked this inhibition, indicating mediation via adenosine receptors. In half of the cases, this inhibition was potentiated by iontophoretic application of substance P. High-intensity electrical stimulation to a sensory nerve produced excitation which was blocked by an NK-1 (substance P) receptor antagonist, implicating an endogenous neurokinin. When electrical stimulation preceded the vibrational stimulus, the inhibitory effect of vibration was potentiated. Thus, we suggest that endogenous substance P may potentiate the inhibitory response to endogenous adenosine. The results have important implications for integration of inputs from different sensory modalities, especially as they relate to nociception and pain.