Interactions between short latency afferent inhibition and long interval intracortical inhibition

Peripheral nerve stimulation inhibits the motor cortex and the process has been termed afferent inhibition. Short latency afferent inhibition (SAI) at interstimulus intervals (ISI) of similar to 20 ms likely involves central cholinergic transmission and was found to be altered in Alzheimer's disease and Parkinson's disease. Cholinergic and GABA(A) receptors are involved in mediating SAI. The effects of SAI on other intracortical inhibitory and facilitatory circuits have not been examined. The objective of the present study is to test how SAI interacts with long interval cortical inhibition (LICI), a cortical inhibitory circuit likely mediated by GABA(B) receptors. We studied 10 healthy volunteers. Surface electromyogram was recorded from the first dorsal interosseous muscle. SAI was elicited by median nerve stimulation at the wrist followed by transcranial magnetic stimulation (TMS) at ISI of N20 somatosensory evoked potential latency + 3 ms. The effects of different test motor-evoked potential (MEP) amplitudes (0.2, 1, and 2 mV) were examined for LICI and SAI. Using paired and triple-pulse paradigms, the interactions between SAI and LICI were investigated. Both LICI and SAI decreased with increasing test MEP amplitude. Afferent stimulation that produced SAI decreased LICI. Thus, the present findings suggest that LICI and SAI have inhibitory interactions.