INHIBITION OF HUMAN MOTOR CORTEX BY ETHANOL - A TRANSCRANIAL MAGNETICSTIMULATION STUDY

The non-invasive technique of focal transcranial magnetic stimulation (TMS) was used in sis healthy volunteers to evaluate the effect of eth anol on four aspects of human central motor system excitability: (i) t hreshold intensity for the motor evoked potential (MEP) in the vesting and active abductor digiti minimi muscle (ADM); (ii) variation of MEP amplitude with increasing stimulus intensity; (iii) duration of the T MS induced cortical silent period (CSP); (iv) intracortical inhibition and facilitation after paired magnetic stimulation In addition, the m aximum M wave (M(max)) after supra-maximal electrical ulnar nerve stim ulation and the peripheral silent period (PSP) have been tested as par ameters of peripheral motor excitability. Baseline measurements were c ompared with data obtained on a mean ethanol blood concentration of 0. 8 ml/l. Resting and active motor thresholds remained unchanged. Also, the absolute M(max) values and the MEP/M(max) ratio at various TMS int ensities were not affected by ethanol. The mean CSP duration was prolo nged by up to 35 ms while the PSP duration showed no change. The princ ipal effect of ethanol was an. enhancement of intracortical inhibition and a suppression of intracortical facilitation. These results ave di scussed in relation to data from animal experiments which have shown a potentiation of gamma-aminobutyric acid (GABA) receptor-mediated curr ents as the main mode of action of ethanol in the mammalian cortex. It is suggested that CSP and intracortical inhibition reflect the activa tion of cortical GABAergic mechanisms by TMS, while motor threshold do es not.