NMDA receptor-related mechanisms of dopaminergic modulation of tDCS-induced neuroplasticity.

Dopamine is a key neuromodulator of neuroplasticity and an important neuronal substrate of learning, and memory formation, which critically involves glutamatergic N-methyl-D-aspartate (NMDA) receptors. Dopamine modulates NMDA receptor activity via dopamine D1 and D2 receptor subtypes. It is hypothesized that dopamine focuses on long-term potentiation (LTP)-like plasticity, i.e. reduces diffuse widespread but enhances locally restricted plasticity via a D2 receptor-dependent NMDA receptor activity reduction. Here, we explored NMDA receptor-dependent mechanisms underlying dopaminergic modulation of LTP-like plasticity induced by transcranial direct current stimulation (tDCS). Eleven healthy, right-handed volunteers received anodal tDCS (1 mA, 13 min) over the left motor cortex combined with dopaminergic agents (the D2 receptor agonist bromocriptine, levodopa for general dopamine enhancement, or placebo) and the partial NMDA receptor agonist D-cycloserine (dosages of 50, 100, and 200 mg, or placebo). Cortical excitability was monitored by transcranial magnetic stimulation-induced motor-evoked potentials. We found that LTP-like plasticity was abolished or converted into LTD-like plasticity via dopaminergic activation, but reestablished under medium-dose D-cycloserine. These results suggest that diffuse LTP-like plasticity is counteracted upon via D2 receptor-dependent reduction of NMDA receptor activity.