1-Methyl-4-phenylpyridinium accumulates in cerebellar granule neurons via organic cation transporter 3

1-Methyl-4-phenylpyridinium (MPP⁺), the toxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, induces apoptosis in cerebellar granule neurons (CGNs). We have tested the hypothesis that organic cation transporter (OCT) 3 mediates the accumulation and, hence, the toxicity of MPP⁺ in CGNs. CGNs in primary culture express OCT3 but do not express mRNA for OCT1, OCT2 or the dopamine transporter. Cerebellar astrocytes are negative for OCT3 protein by immunocytochemistry. [³H]MPP⁺ accumulation by CGNs exhibits first-order kinetics, and a K_t value of 5.3 ± 1.2 μM and a T_max of 0.32 ± 0.02 pmol per min per 10⁶ cells. [³H]MPP⁺ accumulation is inhibited by corticosterone, β-estradiol and decynium 22 with K_i values of 0.25 μM, 0.17 μM and 4.0 nM respectively. [³H]MPP⁺ accumulation is also inhibited by desipramine, dopamine, serotonin and norepinephrine, but is not affected by carnitine (10 mM), mazindol (9 μM) or GBR 12909 (1 μM). MPP⁺-induced caspase-3-like activation and cell death are prevented by pretreatment with 5 μM β-estradiol. In contrast, the neurotoxic effects of rotenone are unaffected by β-estradiol. Interestingly, GBR 12909 protects CGNs from both MPP⁺ and rotenone toxicity. In summary, CGNs accumulate MPP⁺ in manner that is consistent with uptake via OCT3 and the presence of this protein in CGNs explains their sensitivity to MPP⁺ toxicity.