P 85. Resistance to perturbance by anodal tDCS indicates diminished plasticity of GABAergic intracortical inhibition in old age

Abstract

Question GABAergic inhibitory circuits are changed as a function of age marked by a prevailing level of disinhibition during resting-state and deficient event-related modulatory capacity, potential mechanisms underlying age-related speed and dexterity deficits. The driving question for the present work was whether anodal transcranial direct current stimulation (atDCS) can influence age-related deficiency of inhibition and “open a window” for event-related modulatory capacity within the intracortical inhibitory network. Furthermore, if intracortical inhibition is to be influenced by atDCS, do direction and amount of the effect correlate with stimulation-induced changes in dexterous motor behaviour? Methods N = 20 healthy right-handed participants (N = 10 young 25.2 ± 1.6 years, range 23–28, 5 female, N = 10 old 73.6 ± 7.5 years, range 65–83, 6 female) volunteered in a double-blind cross-over design. Assessment of short-interval intracortical inhibition (SICI), dexterous hand function, attention and fatigue was performed before, immediately, 45, and 90 min (P, P45, P90) after atDCS (1 mA, 20 min.) or sham stimulation. Dexterous hand function was evaluated with solitary index finger tapping (1FT), alternating index and little finger tapping (2FT), simple reaction time task (SRT), and choice-reaction time task (CRT). Results Our main finding was that resting-state intacortical inhibition was significantly diminished after a single session of 20 min atDCS exclusively in the young but not in old participants (TIME X STIMULATION X GROUP (F (3, 2408) = 8.345, p .9). No atDCS stimulation-effect was observed for event-related SICI modulation, neither in young nor in old. The effect of atDCS on motor behaviour differed among age groups and was dependent on the character of the task performed: While the young showed a similar temporal pattern of performance irrespective of stimulation condition, old subjects tended to improve in SRT and 2FT but showed a significant decline in performance speed in CRT accompanied by a marked improvement in CRT error rate under atDCS. Stimulation-induced release of inhibition was associated with faster performance in SRT after atDCS in the old subgroup (R = −.71, p Conclusions Performance improvements with atDCS in old age cannot be explained solely by changes to intracortical inhibition mediated by GABAergic inhibitory interneurons. When prevailing disinhibition dominates resting-state, no perturbation as seen in young is achieved with atDCS. Event-related modulation of intracortical inhibition, i.e. rapid release of inhibition, seems to follow a hard-coded process, which cannot be influenced by atDCS.