Interactions between two different inhibitory systems in the human motor cortex

1. Intracortical inhibition in the human motor cortex has been previously d emonstrated using paired-pulse transcranial magnetic stimulation (TMS) prot ocols at short intervals (1-6 ms; short interval intracortical inhibition, SICI) with a subthreshold conditioning pulse preceding a suprathreshold tes t pulse, and at long intervals (50-200 ms; long interval intracortical inhi bition, LICI) with suprathreshold conditioning and test pulses. 2. We investigated whether different circuits mediate these inhibitory phen omena and how they interact. In nine healthy volunteers, we applied TMS to the motor cortex and recorded motor evoked potentials from the first dorsal interosseous muscle. 3. With increasing test pulse strength, LICI decreases but SICI tends to in crease. There was no correlation between the degree of SICI and LICI. 4. We tested the interactions between SICI and LICI. SICI was reduced or el iminated in the presence of LICI. Loss of SICI was seen even with a conditi oning stimulus too weak to induce significant LICI. 5. Our findings demonstrate that different cell populations mediate SICI an d LICI. The results are consistent with the hypothesis that LICI inhibits S ICI through presynaptic GABA(B) receptors. Testing of SICI in the presence of LICI may be a non-invasive way of evaluating inhibitory interactions in the human motor cortex.