PRODUCTION AND DISPOSITION OF 1-METHYL-4-PHENYLPYRIDINIUM IN PRIMARY CULTURES OF MOUSE ASTROCYTES

Dopaminergic neurons are a primary target for 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity. However, the conversion of MPTP to its neurotoxic 1-methyl-4-phenylpyridinium metabolite (MPP+) is likely to occur in astrocytes via the monoamine oxidase (MAO)-dependent formation of the 1-methyl-4-phenyl-2,3-dihydropyridinium intermediate (MPDP+). The main purpose of this study was to characterize the molecular mechanism(s) by which MPP+, once generated by astrocytes, may reach the extracellular space to become available for the active accumulation into dopaminergic neurons. Primary cultures of mouse astrocytes were used as an in vitro model system. After the addition of MPTP, levels of MPP+ were found to increase at constant rates both intracellularly and extracellularly at time points when no sign of cytotoxicity was evident. In contrast, MPDP+ levels remained quite stable during 4 days of incubation in the presence of MPTP. Finally, when astrocytes were allowed to accumulate MPP+ by pretreatment with either MPTP or MPP+ and then were incubated in fresh medium not containing MPTP or MPP+, intracellular levels of MPP+ rapidly declined and corresponding amounts of this compound were found in the incubation medium. Results of this study are compatible with the following conclusions: 1) the MPP+ accumulated in the extracellular compartment during incubations with MPTP is not released from astrocytes as a consequence of its own cytotoxic effects; 2) MPP+ can be formed extracellularly presumably via autoxidation of MPDP+ after this latter compound has been generated within astrocytes and has crossed astrocyte membranes; and 3) despite its charged chemical structure, MPP+ can cross the plasma membrane toward the extracellular space after being formed within astrocytes.