Comparison of the inhibitory and excitatory effects of ADHD medications methylphenidate and atomoxetine on motor cortex

Stimulant and norepinephrine (NE) reuptake inhibitor medications have different effects at the neuronal level, but both reduce symptoms of attention deficit hyperactivity disorder (ADHD). To understand their common physiologic effects and thereby gain insight into the neurobiology of ADHD treatment, we compared the effects of the stimulant methylphenidate (MPH) and NE uptake inhibitor atomoxetine (ATX) on inhibitory and excitatory processes in human cortex. Nine healthy, right-handed adults were given a single, oral dose of 30 mg MPH and 60 mg ATX at visits separated by 1 week in a randomized, double-blind crossover trial. We used paired and single transcranial magnetic stimulation (TMS) of motor cortex to measure conditioned and unconditioned motor-evoked potential amplitudes at inhibitory ( 3 ms) and facilitatory ( 10 ms) interstimulus intervals (ISI) before and after drug administration. Data were analyzed with repeated measures, mixed model regression. We also analyzed our findings and the published literature with meta-analysis software to estimate treatment effects of stimulants and NE reuptake inhibitors on these TMS measures. There were no significant pretreatment differences or effects of treatment order. Both agents produced a significant increase in facilitation and a decrease in inhibition. Effects of ATX and MPH did not differ significantly. Pooled estimates from published studies show similar results for stimulants and NE reuptake inhibitors. In conclusion, in healthy adults, both stimulant and nonstimulant medications for ADHD decrease cortical inhibition and increase cortical facilitation. Cortical inhibition, shown previously to be abnormal in ADHD, may play a key role producing behavioral pathology.