Electrophysiological approach to the study of pain control system during antalgic treatments

Nociceptive flexion reflex represents an interesting objective method for investigating pain and the mechanism of antalgic treatments in man. This method is based, in fact, on the existence of a strong correlation between the threshold of RIII reflex and the subjective pain threshold. RIII reflex can be recorded from biceps femoris muscle and elicited by electrical stimulation of the ipsilateral sural nerve at the retromalleolar level. Several analgesic drugs (e. g. acetylsalicylic acid) or local anesthetics were able to increase the threshold of RIII reflex, but the most interesting findings concern the effect of morphine. This substance was found to depress in a dose-dependent fashion the nociceptive flexion reflex in normal subjects. Interestingly, this effect was also seen in spinal subjects, thus suggesting that morphine exerts its depressive effect by acting directly at spinal level. Similar results were obtained using demorphin, a new potent opiate-like peptide and FK33-824, a synthetic enkephal in analogue.