Abstract
Dopamine improves learning and memory formation. The neurophysiological basis for these effects might be a focusing effect of dopamine on neuroplasticity: Accordingly, in humans l-dopa prolongs focal facilitatory plasticity, but turns nonfocal facilitatory plasticity into inhibition. Here we explore the impact of D1 receptors on plasticity. Nonfocal plasticity was induced by transcranial direct current stimulation (tDCS), and focal plasticity by paired associative stimulation (PAS). Subjects received sulpiride, a D2 antagonist, to increase the relative contribution of D1 receptors to dopaminergic activity, combined sulpiride and l-dopa, to increase the relation of D1/D2 activity further, or placebo medication. Under placebo, anodal tDCS and excitatory PAS (ePAS) increased motor cortex excitability. Cathodal tDCS and inhibitory PAS (iPAS) reduced it. Sulpiride abolished iPAS-induced inhibition, but not ePAS-generated facilitation, underlining the importance of D1-receptor activity for focal facilitatory neuroplasticity. Combining sulpiride with l-dopa reestablished iPAS-induced inhibition, but did not affect ePAS-induced plasticity. tDCS-induced plasticity, which was abolished by sulpiride in a former study, also recovered. Thus enhancing D1 activity further relative to D2 activity is relevant for facilitatory and inhibitory plasticity. However, comparison with former results show that an appropriate balance of D1 and D2 activity seems necessary to (1) consolidate the respective excitability modifications and (2) to elicit a focusing effect.
