Age-related differences in how tDCS priming modulates cortical inhibition during fatigue recovery

Metaplasticity is homeostatic regulation of brain excitability through which neurons change their threshold for response based on synaptic history. Transcranial direct current stimulation (tDCS) induced metaplastic shifts in cortical excitability have been shown to improve motor learning in older adults. However, this has not been explored in the context of a fatiguing contraction. We aimed to explore modulation of fatiguing exercise performance, corticospinal excitability and gamma-aminobutyric acid (GABA_B)-mediated inhibition in older and younger adults, guided by a metaplasticity-based framework. Fifteen young and fifteen older adults completed three counterbalanced, double-blind tDCS sessions: sham-primed sham tDCS (stDCS - stDCS), sham-primed anodal tDCS (stDCS - atDCS) and cathodal-primed anodal tDCS (ctDCS - atDCS). Each session involved 15 min of priming stimulation (stDCS/ctDCS) followed by 15 min of stDCS/atDCS delivered during a 15% MVC of the elbow flexors held to task failure. Motor evoked potential (MEP; representing corticospinal excitability) and long interval cortical inhibition (LICI; representing GABA_B-mediated intracortical inhibition) were assessed using transcranial magnetic stimulation, with EMG responses recorded from biceps brachii before and after fatiguing exercise. Time to task failure was not different between sessions and groups. At recovery, young adults had greater peak-to-peak MEPs (normalised to baseline) than older adults during both stDCS-atDCS and ctDCS-atDCS (P < 0.05), whereas older adults had greater normalised to baseline LICI ratio (i.e. less inhibition) compared to young adults during ctDCS-atDCS (P < 0.05). The findings demonstrate age-related differences in cortical response to tDCS during fatigue. Although exercise performance was preserved with age, the underlying cortical responses differed, potentially reflecting age-related modifications in brain plasticity.