Pain relief from transcranial magnetic stimulation is possibly opioid-mediated

Repetitive transcranial magnetic stimulation (rTMS) is approved by the FDA for the treatment of depression and anxiety. We have been using it to treat migraine headaches and other neurological conditions that are not responsive to standard therapies. Improvement in headaches and pain may be at least in part due to improvement in depression. However, additional mechanisms play a role since we see patients who are not depressed but whose pain improves with rTMS.

A new study by Chinese and Australian researchers published in Pain suggests that opioid mechanisms (endorphins, encephalin, and other peptides) may underlie the mechanism of pain relief produced by rTMS.

This was a double-blind, placebo-controlled study. 45 healthy participants were randomized into 3 groups: one receiving rTMS over the primary motor cortex (M), dorsolateral prefrontal cortex (DLPFC), or sham stimulation. Experimental pain was induced by applying capsaicin (hot pepper extract) over the skin of the right hand followed by application of heat.

Participants received intravenous naloxone (an opioid receptor antagonist) or saline before the first rTMS session to block or allow opioid effects, respectively. After 90 minutes to allow naloxone metabolism, participants received a second rTMS session.

For the M1 group, naloxone abolished the analgesic effects of the first rTMS session compared to saline. Pain relief returned in the second session after naloxone was washed out of the body. For the DLPFC group, only the second prolonged rTMS session induced significant analgesia in the saline condition compared to naloxone. rTMS over M1 selectively increased plasma ?-endorphin levels, while rTMS over DLPFC increased encephalin levels.

The results suggest that opioid mechanisms mediate rTMS-induced analgesia. The specific opioid peptides and rTMS dosage requirements differ between M1 and DLPFC stimulation.

However, these results are far from definitive. The study was small and the study protocol was complicated (e.g. using a double dose of rTMS to DLPFC), which increases the likelihood of an error. Also, these results apply to conditions of acute pain. In patients with chronic pain and headaches, rTMS likely provides relief by improving network connectivity between different parts of the brain.