title: Dosage-dependent non-linear effect of L-dopa on human motor cortex plasticity.
abstract: The neuromodulator dopamine affects learning and memory formation and their likely physiological correlates, long-term depression and potentiation, in animals and humans. It is known from animal experiments that dopamine exerts a dosage-dependent, inverted U-shaped effect on these functions. However, this has not been explored in humans so far. In order to reveal a non-linear dose-dependent effect of dopamine on cortical plasticity in humans, we explored the impact of 25, 100 and 200 mg of L-dopa on transcranial direct current (tDCS)-induced plasticity in twelve healthy human subjects. The primary motor cortex served as a model system, and plasticity was monitored by motor evoked potential amplitudes elicited by transcranial magnetic stimulation. As compared to placebo medication, low and high dosages of L-dopa abolished facilitatory as well as inhibitory plasticity, whereas the medium dosage prolonged inhibitory plasticity, and turned facilitatory plasticity into inhibition. Thus the results show clear non-linear, dosage-dependent effects of dopamine on both facilitatory and inhibitory plasticity, and support the assumption of the importance of a specific dosage of dopamine optimally suited to improve plasticity. This might be important for the therapeutic application of dopaminergic agents, especially for rehabilitative purposes, and explain some opposing results in former studies.
authors.affiliation: Department of Clinical Neurophysiology, Georg- August-University Göttingen, Robert-Koch-Strasse 40, 37099 Göttingen, Germany.
authors.forename: Katia
authors.initials: K
authors.lastname: Monte-Silva
authors.forename: David
authors.initials: D
authors.lastname: Liebetanz
authors.forename: Jessica
authors.initials: J
authors.lastname: Grundey
authors.forename: Walter
authors.initials: W
authors.lastname: Paulus
authors.forename: Michael A
authors.initials: MA
authors.lastname: Nitsche
mesh_terms: D000328:Adult; D018491:Dopamine Agonists / Q000008:administration & dosage* / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D007980:Levodopa / Q000008:administration & dosage* / Q000494:pharmacology*; D008297:Male; D009044:Motor Cortex; D009473:Neuronal Plasticity / Q000187:drug effects* / Q000502:physiology; D050781:Transcranial Magnetic Stimulation
chemical_list: D018491:Dopamine Agonists; D007980:Levodopa