Table 1.

A summary of the main findings of the studies investigating the effects of tDCS and TMS on the opioidergic system.

Opioidergic system
Reference	Design	Population (n)	Intervention	Result
Gabis et al., 2003	Randomized double-blind placebo-controlled study	Chronic back pain patients N=20	Active or placebo. Transcranial electrical stimulation (TCES).	Increased levels of beta-endorphin in seven out of the ten patients from the treatment group.
De Andrade et al., 2011	Crossover randomized double-blind placebo-controlled study	Healthy volunteers N=36	Two groups of active TMS (right M1 or DLPFC/PMC) and one group of sham TMS (M1 or DLPFC/PMC), after a pretreatment with intravenous saline or naloxone.	Naloxone injection significantly reduced the analgesic effects of M1-TMS. However, it did not affect the effects of DLPFC-rTMS or sham rTMS.
Taylor et al., 2012	Crossover randomized double-blind placebo-controlled study	Healthy volunteers N=14	Active or sham left DLPFC-TMS, after a pretreatment with intravenous saline or naloxone.	Naloxone pretreatment significantly decreased the analgesic effects of active TMS.
DosSantos et al., 2014	Observational study	Healthy volunteers N=9	Study investigating the effects of M1-tDCS on the mu-opioid system through PET.	Placebo tDCS induced a reduction in the availability of MOR in the thalamus, precuneus, and PAG. Active tDCS induced MOR activation in the PAG and precuneus and left prefrontal cortex.
Lamusuo et al., 2017	Crossover randomized double-blind placebo-controlled study	Healthy volunteers N=10	Active or sham rTMS applied to the right M1/S1 cortex, combined with opioidergic/dopaminergic evaluation though PET.	Lower opioid receptor availability associated with active rTMS, when compared to sham, in the right ventral striatum, PFC, medial orbitofrontal cortex, ACC, DLPFC, insula, and precentral and superior temporal gyrus. No changes in striatal dopamine D2 receptor.