The selective effects of metorphamide on dorsal horn neurones of the cat spinal cord

Abstract

In anaesthetized cats, the proenkephalin A derivative metorphamide was administered microelectrophoretically while recording the excitation of lumbar dorsal horn neurones by noxious and innocuous cutaneous stimuli. Administered in the substantia gelatinosa or more ventrally near the cell bodies, metorphamide reduced the nociceptive responses and spontaneous firing of laminae IV and V neurones. These effects were reversed by electrophoretic administration of naloxone at the same site. Responses to innocuous stimulation were partially but inconsistently reduced by metorphamide administration near cell bodies, and not reduced by administration in the substantia gelatinosa. These results suggest that metorphamide may be an endogenous ligand at opioid μ-receptors, and may also act at κ-receptors.