Lack of effect by substance P at sites in the substantia gelatinosa where Met-enkephalin reduces the transmission of nociceptive impulses.

In alpha-chloralose anaesthetized spinal cats, Met-enkephalin amide (M-ENKA) and substance P were administered electrophoretically in the substantia gelatinosa while studying the excitation of deeper spinal neurones 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 fibres.