Dual modulating effects of amphetamine on neuronal excitability and stimulation-induced plasticity in human motor cortex

Objectives: The objective of the present study is to test the modulating effects of dextro-amphetamine (d-AMP) on excitability and stimulation-induced plasticity in human motor cortex. Methods: Transcranial magnetic stimulation (TMS) was used to measure motor threshold, motor evoked potential (MEP) size and paired-pulse intracortical facilitation (ICF) in the biceps muscle of 7 healthy subjects before and after two different experimental manipulations: temporary forearm ischemic nerve block (INB) alone, or INB plus 0.1Hz repetitive TMS (INB+rTMS) of the motor cortex contralateral to INB. Both manipulations were run after treatment with 10mg of d-AMP or placebo (PBO). Results: In the PBO experiments, INB alone had no significant effect on MEP size or ICF, while INB+rTMS produced long-lasting (>60min) increases. Compared with PBO, d-AMP led to a short-lasting (∼10min) increase in MEP size in the INB alone experiment, but suppressed the long-lasting increases of MEP size and ICF in the INB+rTMS experiment. Conclusions: The present findings suggest that d-AMP increases neuronal excitability but suppresses long-lasting stimulation-induced plasticity in human motor cortex. These dual effects may be relevant when using d-AMP to modulate human cortex function.