Production and disposition of 1‐methyl‐4‐phenylpyridinium in primary cultures of mouse astrocytes

Abstract

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.

Proliferation of microglial cells induced by 1‐methyl‐4‐phenylpyridinium in mesencephalic cultures results from an astrocyte‐dependent mechanism: role of granulocyte macrophage colony‐stimulating factor

“…Consistent with this view, an extra-neuronal monoamine transporter has been reported to be responsible for the uptake of MPP + in astrocytes (Inazu et al 2003). MPP + exerts toxic effects on astroglia comparable to those seen in dopaminergic neurones, in particular oxidative stress and ATP depletion (Di Monte et al 1992) as MPP + inhibits complex I of the mitochondrial respiratory chain in any cell type it enters. Therefore, a stimulatory effect of MPP + on GM-CSF synthesis, through the cellular stress events described above is likely.…”

Mitochondrial dysfunction in Parkinson's disease

“…MPDP is capable of passing across cell membranes and so the conversion of MPDP to MPP þ probably occurs in the extracellular space by autooxidation. 73 MPP þ is a substrate for the dopamine reuptake system. 74,75 Blockade of MPP þ uptake by, for instance, nomifensine can prevent MPTP toxicity.…”

The Neurotoxin 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine Induces Apoptosis in Mouse Nigrostriatal Glia

“…The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces in man a clinical syndrome closely resembling PD and is widely employed as a Parkinson's disease model in experimental animals (2). Astrocytes are the primary site of bioactivation of the innocuous MPTP to the toxic metabolite 1-methyl-4-phenyl-pyridinium ion (MPP ϩ ) via the monoamine oxidase-dependent formation of a dihydropyridinium intermediate (3). The dihydropyridinium intermediate is a reactive radical that undergoes autoxidation with formation of superoxide anion (O 2 . )…”