CATECHOLAMINE TRANSPORTERS AND 1-METHYL-4-PHENYL-1,2,3,6-TETRAHYDROPYRIDINE NEUROTOXICITY - STUDIES COMPARING THE CLONED HUMAN NORADRENALINE AND HUMAN DOPAMINE TRANSPORTER

The uptake and cytotoxicity of 1-methyl-4-phenylpyridinium (MPP(+)), t he toxic metabolite of the parkinsonism inducing agent 1-methyl-4-phen yl-1,2,3,6-tetrahydropyridine (MPTP), were studied in COS-7 cells tran siently transfected with the cloned human noradrenaline and dopamine t ransporters and in permanently transfected SK-N-MC neuroblastoma cells . MPP(+) had a 10- to 20-fold lower K-m value for the noradrenaline th an for the dopamine transporter. In dopamine transporter expressing ce lls, the maximal transport rate (V-max) of MPP(+), dopamine and noradr enaline was the same, but in noradrenaline trans porter expressing cel ls the V-max of MPP(+) and dopamine was only one-half of the V-max of noradrenaline. The turnover numbers (V-max of uptake/maximal binding s ites of binding) were 5 times higher for the dopamine transporter (as measured with [H-3]dopamine and [H-3]-2 beta-carbomethoxy-3 beta-(4-fl uorophenyl) tropane than for the noradrenaline transporter (as measure d with [H-3]noradrenaline and [H-3]nisoxetine). In SK-N-MC cells with similar V-max values for both catecholamines, noradrenaline transporte r expressing cells were killed by lower concentrations of MPP(+) in th e medium than dopamine transporter expressing cells. Desipramine block ed the toxicity of MPP(+) toward the noradrenaline transporter, but no t the dopamine transporter expressing cells. We conclude that the toxi c effect of MPTP at the striatal dopamine system in the MPTP primate m odel of Parkinson's disease is not correlated with the affinity profil e of MPP(+) for catecholamine transporters, but rather with the higher turnover number of MPP(+) at the dopamine transporter. In contradisti nction, the toxicity of MPTP at the noradrenaline neurons in the prima te cerebral cortex (Pifl et al., 1991) may involve the higher affinity of MPP(+) for the noradrenaline transporter.