Abstract
Despite the abundance of research reporting the neurophysiological and behavioral effects of transcranial direct current stimulation (tDCS) in healthy young adults and clinical populations, the extent of potential neuroplastic changes induced by tDCS in healthy older adults is not well understood. The present study compared the extent and time course of anodal tDCS-induced plastic changes in primary motor cortex (M1) in young and older adults. Furthermore, as it has been suggested that neuroplasticity and associated learning depends on the brain-derived neurotrophic factor (BDNF) gene polymorphisms, we also assessed the impact of BDNF polymorphism on these effects. Corticospinal excitability was examined using transcranial magnetic stimulation before and following (0, 10, 20, 30 min) anodal tDCS (30 min, 1 mA) or sham in young and older adults. While the overall extent of increases in corticospinal excitability induced by anodal tDCS did not vary reliably between young and older adults, older adults exhibited a delayed response; the largest increase in corticospinal excitability occurred 30 min following stimulation for older adults, but immediately post-stimulation for the young group. BDNF genotype did not result in significant differences in the observed excitability increases for either age group. The present study suggests that tDCS-induced plastic changes are delayed as a result of healthy aging, but that the overall efficacy of the plasticity mechanism remains unaffected.
Highlights
Transcranial direct current stimulation is a non-invasive brain stimulation (NIBS) technique that induces transmembrane neuronal potential and influences the level of cortical excitability
It is thought that the neuronal changes associated with the persisting effects of Transcranial direct current stimulation (tDCS) are analogous to activity-dependent synaptic plasticity, i.e., long-term potentiation (LTP) and long-term depression (LTD; Di Lazzaro et al, 2012)
Due to a technical error resulting in motor evoked potentials (MEPs) not being recorded, data from one young participant was excluded from all analyses
Summary
Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation (NIBS) technique that induces transmembrane neuronal potential and influences the level of cortical excitability (see review Nitsche et al, 2008; Zaghi et al, 2010). Fritsch et al (2010) showed that the synaptic effects of direct current stimulation which induced LTP in mouse motor cortex (M1) slices is dependent on N -methyl-D-aspartate (NMDA) receptor activation. Pharmacological studies have shown that tDCS after-effects are affected by NMDA receptor antagonist dextromethorphane (Nitsche et al, 2003a; Ranieri et al, 2012). These results strongly indicate that the effects induced by tDCS share similarities with activity-dependent synaptic plasticity, such as LTP and LTD (Di Lazzaro et al, 2012). TDCS is considered to have capacity to induce LTP/LTD-like plasticity
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