Cortical inhibition assessed using paired-pulse TMS-EEG is increased in older adults

Alterations in inhibitory processes mediated by gamma-aminobutyric acid type B (GABA_B) receptors may contribute to age-related functional impairments. However, investigation of these circuits using conventional paired-pulse transcranial magnetic stimulation (TMS) at long interstimulus intervals (∼100-200ms) have produced conflicting results in older adults, possibly due to the indirect nature of the TMS motor evoked potential (MEP).

To utilise electroencephalography and TMS coregistration (TMS-EEG) to more directly assess age-related changes in GABA_B-mediated long-interval intracortical inhibition (LICI).

In 17 young (24.2 ± 1.1 years) and 17 older (71.4 ± 1.4 years) subjects, the TMS-evoked potential (TEP) was used to assess the global scalp response to single-pulse TMS and LICI applied at two interstimulus intervals of 100 ms (LICI₁₀₀) and 150 ms (LICI₁₅₀).

For single-pulse stimulation, P30 amplitude was unaffected by age. Despite this, N45 amplitude was increased in older adults and both N100 and P180 showed altered spatial distributions. Furthermore, the latency of P30 was shorter, while the latency of P180 was longer, in the elderly. In addition, inhibition of the N100 and P180 was increased in older adults following both LICI₁₀₀ and LICI₁₅₀.

These findings with TMS-EEG suggest that the ageing process is associated with a potentiation of GABAergic inhibition, particularly for the GABA_B-receptor subtype.