Age-dependent susceptibility of developing hippocampal neurons to HIV/MDM neurotoxicity is predicted by NMDA receptor expression. Hippocampal cultures were maintained for DIV 7, DIV 14, or DIV 21 and preincubated with the noncompetitive NMDA receptor antagonist MK801 (10 μm; A) or the competitive NMDA receptor antagonist AP-5 (100 μm; B) for 1 h before exposure to MDM supernatants. Cultures were incubated for 24 h and assayed for the number of surviving neurons (*p < 0.01 same day HIV-Jago). C, Western blots for NMDA receptor subunits (NR1, NR2A, and NR2B) and for β-tubulin III as a protein loading control.