Pharmacological modulations on the nociceptive flexion reflex in man

Pharmacological actions on the nociceptive flexion flexes of the hindlimb were investigated in 14 normal subjects. These reflexes were used as an index of pain and 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 ratio of the threshold of RIII,Cu/RIII,Su was calculated since it gives an indication on the mechanism and the efficacy of the drug. During control periods, the mean values of RIII,Su threshold (10 mA) and of RIII,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, whereas lignocaine increased them. No changes in the RIII,Su, threshold and relative pain were observed. The ratio RIII,Cu/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. Intravenous 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. In contrast, 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 found to be 190% at the maximal effect. Practical implications of these results, concerning a method for testing the efficacy and mechamisms of an analgesic, are then discussed.