Hippocampal CA1 neurons have been shown to be highly susceptible to excitotoxicity produced by various forms of insult. CA1 neurotoxicity is partly dependent on over activity of N-methyl-D-aspartate (NMDA) receptors. It is unclear, however, if sensitivity of this region to excitotoxicity is related to inherent properties of CA1 neurons and/or network activation of polysynaptic pathways. The present studies examined the role of mossy fiber and Schaffer collateral function in promoting NMDA-induced neurodegeneration. Organotypic hippocampal cultures were subjected to transection of mossy fibers, Schaffer collaterals, or CA1 efferent fibers and then exposed to NMDA (20 microM) for 1 h. Hippocampal damage was assessed 24 h later via fluorescent microscopy. NMDA exposure produced significant excitotoxicity in all regions (160-500% of control), particularly in the CA1. In each region, toxicity was reduced by co-exposure to NMDA with MK-801 (20 microM), to near-control levels. Surgical transection of mossy fibers and Schaffer collaterals significantly reduced NMDA-induced neurotoxicity in the CA1 ( approximately 20%), and to a lesser extent, the CA3 and dentate regions. Conversely, transection of CA1 efferent fibers did not reduce the neurotoxicity in these regions. These data indicate that CA1 neurotoxicity caused by excitotoxic insult depends, in part, on 'network activation' of intrinsic polysynaptic pathways.