Pharmacological modulations on the nociceptive flexion reflex in man

Abstract

Pharmacological actions on 14 normal human hindlimb nociceptive flexion reflexes were studied. Reflexes used as an index of pain were recorded in the biceps femoris muscle, elicited by electrical stimulation of the ipsilateral sural nerve (RIII,su) and of the skin in the distal receptive field of this nerve (RIII,Cu). The threshold ratio of RIII,Cu/RIII,Su was calculated as an indication of the drug mechanism and efficacy. During control periods, the mean values of RIII,Su threshold (10 mA) and of RII,Cu threshold (5 mA) remained stable, as RIII,Cu/RIII,Su threshold (50%). Cutaneous application of naphthalene provoked a decrease in the RIII,Cu threshold and relative pain threshold; lignocaine increased them. No changes in the RIII,Su threshold and relative pain were observed. The ratio RIII,Cu/RIII,Su was lowered to 10% with naphthalene and increased to 110% with lignocaine. I.V. injection of acetylsalicylic acid increased only the RIII,Cu threshold and relative pain threshold. The ratio RIII,Cu/RIII,Su was 90% at the maximal effect of the drug. Pethidine provoked a decrease in the RIII,Su threshold and an increase in RIII,Cu threshold, parallel with an increase in pain threshold sensation. The ratio was 190% at the maximal effect. Practical implications of the method for testing analgesic efficacy and mechanisms was discussed.