Overexposure to manganese (Mn) has been known to induce neuronal damage. However, the mechanisms of Mn-induced neuronal cell death are incompletely understood. The objective of this study is to explore mechanisms that contribute to Mn-induced neuronal apoptosis focusing on the alteration of intracellular Ca(2+) homeostasis and expression of NMDA receptor subunits in primary cultured neurons. Treatment of neuronal cells with Mn (0-400 microM) for 6-48 h resulted in the damages of primary cultured neurons concentration- and time-dependently, which were determined by methyl thiazolyl tetrazolium (MTT) and lactate dehydrogenase (LDH) release assay and supported by morphological examination. After neurons treated with Mn (25, 100, 400 microM) for 12 h, there was a significant increase in apoptosis rate [Ca(2+)](i) and decrease in Na(+)-K(+)-ATPase and Ca(2+)-ATPase activities in a concentration-dependent manner. Moreover, Mn could inhibit expression of NMDA receptor subunits in neuron and expression of NR2A mRNA and protein were much more sensitive to Mn than those of NR1 and NR2B. In conclusion, the present results showed that Mn-induced neuronal damage by increasing [Ca(2+)](i) and altering expression of NMDA receptor subunits mRNAs and proteins.