Transcranial Magnetic Stimulation: From Basic Mechanisms to Clinical Application for Addiction Medicine

Transcranial magnetic stimulation is a noninvasive method of brain stimulation, which enables to electrically stimulate neurons of the human cerebral cortex by electromagnetic induction. When applied through repeated stimulation protocols, rTMS is able to produce long-lasting modulations of cortical excitability. These long-term effects have been ascribed to neuroplastic changes affecting the synaptic properties, raising great interest in the use of rTMS as therapeutic intervention in a wide range of neurological and psychiatric disorders. There is currently a growing interest in the application of rTMS in the addiction medicine. The rationale is grounded on the possibility of restoring the normal functioning of the meso-cortico-limbic dopamine pathway, which may be directly accessed targeting rTMS to the superficial frontal nodes of the network. Therapeutic effects should be driven by excitability modulations of both the local stimulated sites and of the connected subcortical areas, where rTMS effects spread along cortico-subcortical projections. Compelling evidence from preclinical studies and from single sessions of rTMS in healthy subjects support this theoretic reasoning. Despite promising, results of the clinical studies are still not conclusive to endorse rTMS as an effective intervention to reduce craving or substance consumption. Remarkably, the role played by patients' expectations about treatment's efficacy has never been taken into account. This issue assumes a strong importance when testing treatments for addiction, because the neural mechanisms generating expectations rely on the same fronto-striatal pathways impaired in addicted patients. To conclude, rTMS treatments may represent a concrete chance to develop innovative treatment options in addiction medicine beyond available conventional therapies. Nevertheless, future research should aim at elucidating all the factors that contribute to rTMS response and their interaction, in order to design rTMS interventions with stronger and more persistent effects.