1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine Metabolism and l-Methyl-4-Phenylpyridinium Uptake in Dissociated Cell Cultures from the Embryonic Mesencephalon

Abstract

1‐Methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP), a contaminant found in a synthetic illicit drug, can elicit in humans and monkeys a severe extrapyramidal syndrome similar to Parkinson's disease. It also induces alterations of the dopamine (DA) pathways in rodents. MPTP neurotoxicity requires its enzymatic transformation into 1‐methyl‐4‐phenylpyridinium (MPP⁺) by monoamine oxidase followed by its concentration into target cells, the DA neurons. Here, we show that mesencephalic glial cells from the mouse embryo can take up MPTP in vitro, transform it into MPP⁺, and release it into the culture medium. MPTP is not taken up by neurons from either the mesencephalon or the striatum in vitro (8 days in serum‐free conditions). However, mesencephalic neurons in culture revealed a high‐affinity uptake mechanism for the metabolite MPP⁺, similar to that for DA. The affinity (K_m) for DA uptake is fivefold higher than that for MPP⁺ (0.2 and 1.1 μM, respectively), whereas the number of uptake sites for MPP⁺is double (V_max= 25 and 55 pmol/mg of protein/min for DA and MPP⁺, respectively). Mazindol, a DA uptake inhibitor, blocks the uptake of DA and MPP⁺ equally well under these conditions. Moreover, by competition experiments, the two molecules appear to use the same carrier(s) to enter DA neurons. Small concentrations of MPP⁺ are also taken up by striatal neurons in vitro. The amount taken up represented + uptake in mesencephalic neurons. Depolarization induced by veratridine released comparable proportions of labeled DA and MPP⁺ from mesencephalic cultures. These data support the view that MPTP is transformed into MPP⁺ by astrocytes and concentrated in DA neurons by a relatively specific uptake system, similar to that for DA.