Met-enkephalin and beta-endorphin are not involved in the analgesic action of transcutaneous vibratory stimulation.

Although the analgesic effects observed during the application of vibration may be attributable to neuronal inhibition of the pain pathways, this does not account for the fact that pain relief sometimes persists for a long time after the end of vibration treatment. Two experiments were carried out in order to determine whether pain relief might involve the release of endogenous opioids. In the first experiment, we studied the effects of injecting either a morphine antagonist, naloxone (0.4 mg), or a placebo, on the analgesia resulting from vibratory stimulation in 12 patients suffering from acute or chronic pain. In the second experiment, the Met-enkephalin and beta-endorphin levels were determined before and after 30 min vibratory stimulation in the cerebrospinal fluid of 8 patients suffering from chronic pain and 1 control subject, all of whom had been fitted with a ventriculo-peritoneal drain which made it possible to collect samples of cerebrospinal fluid painlessly. The results of these experiments show, on the one hand, that the effects of naloxone on the vibration-induced analgesia did not differ from those of the placebo and, on the other hand, that no increase in the Met-enkephalin or beta-endorphin levels occurred concomitantly with pain relief. It will therefore be necessary to investigate other mechanisms as possible means of explaining the post-vibratory analgesic effects.