EFFECTS OF ANTIEPILEPTIC DRUGS ON MOTOR CORTEX EXCITABILITY IN HUMANS- A TRANSCRANIAL MAGNETIC STIMULATION STUDY

The effect of a single oral dose of various antiepileptic drugs on the excitability of the motor system was studied in healthy volunteers by means of transcranial magnetic stimulation. Motor threshold, duration of the cortical silent period, and intracortical excitability after d ouble-shock transcranial stimulation were tested before and at defined intervals after drug intake. Antiepileptic drugs that support the act ion of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in the neocortex (vigabatrin, baclofen) reduced intracortical excitabi lity but had no effect on motor threshold, Gabapentin, whose mechanism of action has not yet been unequivocally identified, showed a similar profile. By contrast, sodium and calcium channel blockers without con siderable neurotransmitter properties (carbanazepine, lamotrigine, los igamone) elevated motor threshold but did not change intracortical exc itability. The cortical silent period was lengthened by gabapentin and carbamazepine. Changes in peripheral motor excitability (maximum M wa ve, peripheral silent period) were not observed. We conclude that the changes in intracortical excitability are caused by GABA-controlled in terneuronal circuits in the motor cortex while changes in motor thresh old are dependent on ion channel conductivity and may reflect membrane excitability. Transcranial magnetic stimulation may be a promising no ninvasive approach to study the selective effects of antiepileptic dru gs on brain function.