Concomitant increase in nociceptive flexion reflex threshold and plasma opioids following transcutaneous nerve stimulation

In order to evaluate the role of endogenous opioids in sustaining analgesia induced by transcutaneous nerve stimulation (TNS), we measured plasma β-lipotropin (BLPH), β-endorphin (BEP), ACTH and cortisol changes concomitantly with nociceptive flexion reflex (RIII) threshold after TNS (80 μsec rectangular waves at 85 Hz) in a group of healthy volunteers (A). The same protocol was carried out in another group of volunteers using placebo stimulation (0.5 Hz) (B). RIII threshold significantly increased 0.5 h after TNS in group A and no changes were recorded in group B. Similarly, both BLPH and BEP plasma levels increased at the end of TNS only in group A. ACTH and cortisol concentrations show only random variations after both high and low frequency TNS. A positive linear correlation was found between the maximum percentage increase of RIII threshold after high frequency TNS and the maximum percentage increase of BLPH plasma levels occurring 20 min beforehand (r = 0.856, P <0.001). A less positive correlation was found between RIII and BEP levels (r = 0.574, P <0.05). These data indicate that the so-called post-stimulation analgesia could be supported by the enhancement of the endogenous opioid system.