Late Cortical Disinhibition in Human Motor Cortex: A triple-pulse transcranial magnetic stimulation

Por: Estimulação Magnética Transcraniana repetitiva (EMTr)  25/11/2009
Palavras-chave: Depressão, Estimulação Magnética, EMTr

 Late Cortical Disinhibition in Human Motor Cortex: A triple-pulse transcranial magnetic stimulation (TMS) study. Cash R, Ziemann U, Murray K, Thickbroom GW. J Neurophysiol. 2009 Nov 18.

In human motor cortex transcranial magnetic stimulation (TMS) has been used to identify short-interval intracortical inhibition (SICI) corresponding to GABAA effects, and long-interval intracortical inhibition (LICI) and the cortical silent period (SP) corresponding to post-synaptic GABAB effects. Pre-synaptic GABAB effects, corresponding to disinhibition, can also be identified with TMS, and have been shown to be acting during LICI by measuring SICI after a supra-threshold priming stimulus (PS). The duration of disinhibition is not certain, and guided by studies in experimental preparations we hypothesised that it may be longer-lasting than post-synaptic inhibition, leading to a period of late cortical disinhibition (LCD) and consequently a net increase in corticospinal excitability. We tested this first by measuring the motor evoked potential (MEP) to a test stimulus (TS), delivered after a PS at inter-pulse intervals (IPIs) up to 300ms that encompassed the period of PS-induced LICI and its aftermath. MEP amplitude was initially decreased, but then increased at IPIs of 190-210 ms, reaching 160+/-17% of baseline 200ms after PS (p<0.05). SP duration was 181+/-ms. A second experiment established that the onset of the later period of increased excitability correlated with PS intensity (r(2)=0.99) and with the duration of the SP (r(2)=0.99). The third and main experiment demonstrated that SICI was significantly reduced in strength at all IPIs up to 220ms after PS. We conclude that TMS-induced LICI is associated with a period of disinhibition that is at first masked by LICI, but that outlasts LICI and gives rise to a period where disinhibition predominates and net excitability is raised. The identification of this period of late period of disinhibition in human motor cortex may provide an opportunity to explore or modulate the behaviour of excitatory networks at a time when inhibitory effects are restrained.

Palavras-chave: Depressão, EMTr, Estimulação Magnética

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