Immature hippocampal neurons (E-18) were maintained in defined medium for up to 3 weeks and their susceptibility to N-methyl-D-aspartic acid (NMDA)-induced cell death was studied at various days in vitro. Upon acute exposure to NMDA (5 min), hippocampal neurons in vitro (8-12 days after plating) showed cell body swelling and dendritic degeneration that preceded cell death 24 h later. NMDA-induced neurodegeneration could be prevented by MK-801 treatment but not by tetrodotoxin. In contrast, immature (5-7 days old) neurons were unaltered by exposure to 500 microM NMDA for either 5 min or 24 h. One explanation for the resistance of immature neurons to glutamate neurotoxicity may be related to maturation of the NMDA receptor complex. Glutamate binding to the NMDA receptor in vivo increased from 14.6 +/- 1.6% (0 day) to 55.2 +/- 4.5% (day 7), 79 +/- 4.9% (day 14), 93.8 +/- 2.8% (day 21) until it reached the adult Sprague-Dawley value of 100 +/- 0.8% (day 90).