LACK OF EFFECT BY SUBSTANCE-P AT SITES IN THE SUBSTANTIA GELATINOSA WHERE MET-ENKEPHALIN REDUCES THE TRANSMISSION OF NOCICEPTIVE IMPULSES

In .alpha.-chloralose anesthetized spinal cats, met-enkephalin amide (M-ENKA) and substance P were administered electrophoretically in the substantia gelatinosa while studying the excitation of deeper spinal neurons by noxious and non-noxious cutaneous stimuli. At sites where met-enkephalin selectively reduced excitation by noxious skin stimuli, substance P was without effect. This result does not support the hypothesis that enkephalins are released at axo-axonic synapses on the terminals of substance P releasing primary afferent fibers.