The noradrenaline transporter as site of action for the anti-Parkinson drug amantadine.

Amantadine is an established antiparkinsonian drug with a still unclear molecular site of action. In vivo studies on rodents, in vitro studies on tissue of rodents as well as binding studies on post mortem human tissue implicate monoamine transporters and NMDA receptors. In order to re-examine its action at human variants of these proteins on intact cells we established cells stably expressing the human NR1/2A NMDA-receptor, noradrenaline transporter (NAT) or dopamine transporter (DAT) and tested the activity of amantadine in patch-clamp, uptake, release, and cytotoxicity experiments. Amantadine was less potent in blockade of NMDA-induced inward currents than in blockade of noradrenaline uptake and in induction of inward currents in NAT expressing cells. It was 30 times more potent in blocking uptake in NAT- than in DAT cells. Amantadine induced NAT-mediated release at concentrations of 10-100 μM in superfusion experiments and blocked NAT-mediated cytotoxicity of the parkinsonism inducing neurotoxin 1-methyl-4-phenyl-pyridinium (MPP(+)) at concentrations of 30-300 μM, whereas 300-1000 μM amantadine was necessary to block NMDA-receptor mediated cytotoxicity. Similar to amphetamine, amantadine was inactive at α(2A)-adrenergic receptors and induced reverse noradrenaline transport by NAT albeit with smaller effect size. Thus, amantadine acted as "amphetamine-like releaser" with selectivity for the noradrenergic system. These findings and differences with memantine, which had been reported as less efficient antiparkinsonian drug than amantadine but in our hands was significantly more potent at the NMDA-receptor, suggest contributions from a noradrenergic mechanism in the antiparkinsonian action of amantadine.