The Contribution of Endogenous Modulatory Systems to TMS- and tDCS-Induced Analgesia: Evidence from PET Studies
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
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
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
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
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
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.
