Imipramine does not affect argon-laser-induced pin-prick pain thresholds and laser-evoked cerebral potentials

The tricyclic antidepressant imipramine has shown analgesic effect in human clinical and experimental pain studies. The aim of the present study was to test the effect of imipramine on a pure short-term nociceptive stimulus with pin-prick pain quality. In a randomized, placebo-controlled, double-blind, crossover study, the hypoalgesic effect of a single oral dose of 100 mg imipramine was investigated in 10 healthy volunteers. Test procedures performed before and 2, 4, 6, 8, 10, 12 and 14 h after medication included determination of warmth and pin-prick pain thresholds to high-energy argon laser light stimulation on the hand, as well as laser-evoked cerebral potentials to suprathreshold stimulation. Both the warmth and the pin-prick pain thresholds (p=0.49 and 0.85) and the root mean square of the laser-evoked potentials (p=0.89) were unaltered by imipramine. It is concluded that a single oral dose of 100 mg imipramine has no effect on pin-prick pain. This study demonstrates the important fact that a drug may show clear analgesic effect in some experimental pain models while it is without effect in other models; e.g. imipramine is known to affect pain tolerance and summation thresholds. Pre-clinical tests of potentially analgesic drugs should therefore be based on different pain-stimulation modalities. Copyright 1998 European Federation of Chapters of the International Association for the Study of Pain.