Repetitive transcranial magnetic stimulation as a universal modulator of synaptic plasticity: Bridging the gap between functional and structural plasticity.

Repetitive transcranial magnetic stimulation (rTMS) is a painless and non-invasive technique for neuromodulation that has shown great potential in therapy of several neurodegenerative and neuropsychiatric disorders both in patients and animal models. In addition to its non-invasiveness, the main rationale for using it for these disorders is that the positive effects extend beyond the stimulation period and can last up to several minutes, hours or even days after the last application. While the mechanisms underlying these long-lasting positive effects have not yet been fully deciphered, current literature supports hypothesis of modulation of both functional and structural plasticity. Dendritic spines are structures on dendritic branches that regulate synaptic transmission at the level of postsynapse and represent one of the structural and functional carriers of synaptic plasticity. Since rTMS has been proposed to induce long-term potentiation/long-term depression-like effects, based on the existing literature in animal studies, we suggest several molecular mechanisms which could underpin rTMS-induced structural plasticity manifested at the level of dendritic spines that include processes starting from spinogenesis to gradual spine maturation and eventual spine shrinkage and loss. The results gathered in this review postulate rTMS as a universal modulator of synaptic plasticity, which could guide future research and help in optimizing appropriate protocols of transcranial magnetic stimulation for adequate disorders and pathologies.