Relationship between NMDA receptor expression and MPP⁺ toxicity in cultured dopaminergic cells

It has been suggested that excitotoxicity could be contributing to dopamine cell loss after methylphenylpyridinium ion (MPP⁺) exposure, although the literature regarding this is contradictory. Given that in cell culture excitotoxicity has been reported to be dependent on culture age, we postulated that these discrepant results might be explained by a difference in developmental expression of N-methyl-D-aspartate (NMDA) receptors. To test this, mesencephalic cells were cultured and the number of dopaminergic neurons (tyrosine hydroxylase-immunoreactive cells [TH-IR] cells) expressing the NMDA R1 subunit (NR1) was determined using double-label immunofluorescence microscopy. An increase in the percentage of TH-IR cells expressing NR1 occurred over time in culture and this correlated with the toxicity of NMDA. At 7 days in vitro (DIV 7), only 17% (n = 167 cells/4 experiments) of TH-IR cells expressed NR1 and these cells were insensitive to NMDA toxicity. This increased to 80% (n = 254 cells/6 experiments) by DIV 11 and cultures were now susceptible to NMDA-induced injury. Cultures grown for either 7 or 11 days were treated for 48 hr with increasing concentrations of MPP⁺ (0.5-20 μM) and the loss of dopaminergic neurons was determined by cell counting. Cultures at DIV 7 were more sensitive to MPP⁺ than 11-day-old cultures (LD₅₀ = ∼0.75 μM vs. 15 μM, respectively). Co-exposure to MK-801 (5 μM) did not protect against MPP⁺ toxicity in young cultures, but attenuated MPP⁺ toxicity in the older cultures, becoming statistically significant at 20 μM MPP⁺. These data indicate that the activation of NMDA receptors is not required for, but can contribute to, MPP⁺-induced neurodegeneration of dopaminergic cells in culture.