The biochemical and toxic effects of the two monoamine oxidase-generated metabolites of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine were investigated using primary cultures of mouse astrocytes. After the addition of equimolar concentrations (25 microM) of these metabolites, namely 1-methyl-4-phenyl-2,3-dihydropyridinium (MPDP+) ion or 1-methyl-4-phenylpyridinium (MPP+) ion, similar levels of MPP+ accumulated within the astrocytes. Both MPDP+ and MPP+ caused cytotoxicity which was preceded by increased glucose utilization and lactate accumulation. The metabolites were equipotent in producing a rapid decrease in cellular ATP which correlated well with the intracellular accumulation of MPP+ and the loss of cell viability. When astrocytes were incubated in glucose-free medium, both ATP depletion and loss of viability occurred more rapidly. Formation of MPP+ from MPDP+ was not affected by the presence of astrocytes, because MPP+ concentrations increased over time at the same rate regardless of the presence or absence of cells. In contrast to pretreatment of cells with monoamine oxidase inhibitors before addition of MPTP, pretreatment of astrocytes with deprenyl and clorgyline had no effect on intracellular levels of MPP+ after exposure to MPDP+ or MPP+ and did not protect against ATP depletion or cytotoxicity. These results indicate that 1) MPP+ and MPDP+ cause similar metabolic changes leading to cell death probably via ATP depletion; (2) intracellular levels of MPP+ are directly correlated to cytotoxicity; and (3) MPDP+ toxicity is also correlated to intracellular MPP+ accumulation, further confirming that MPP+ rather than MPDP+ is responsible for cell damage.