Noradrenergic modulation of human visual cortex excitability assessed by paired-pulse visual-evoked potentials

Paired-pulse paradigms are common tools to explore excitability in the human cortex. Although the underlying mechanisms of intracortical inhibition and facilitation in the motor system assessed by paired transcranial magnetic stimulation are well understood, little is known about the physiology of excitability in the human cortex measured by paired-pulse visual-evoked potentials (VEPs). We therefore aimed to explore the noradrenergic influence on paired-pulse VEPs. We recorded and analysed VEPs following a single and paired-pulse stimulation in healthy individuals before and after they received single doses of 60 mg atomoxetine and in a control group. Paired-pulse suppression was expressed as a ratio of the amplitudes of the second and the first peaks. We found that the selective norepinephrine reuptake inhibitor atomoxetine reduced paired-pulse suppression significantly, indicating a facilitatory effect on visual cortex excitability, whereas in the control group, no significant effects were found. Single-pulse VEPs were unaffected. We conclude that single-dose atomoxetine is able to increase excitability in the visual cortex, indicating an involvement of the noradrenergic system.