A method for objectively quantifying propidium iodide exclusion in organotypic hippocampal slice cultures. 
Organotypic hippocampal slice cultures (OHSCs) are an attractive in vitro model to examine mechanisms of neuronal injury, because the normal hippocampal architecture, function and cellular diversity are mostly preserved.  The effects of exposure to excitotoxins such as N-methyl-d-aspartate (NMDA) on cell viability can be determined by propidium iodide (PI) staining.  We describe a simple method to objectively quantify cell death in NMDA exposed slice cultures using PI that provides a standardized means of quantifying cell death in hippocampal subfields without the need to induce maximal cell death in each slice.  The method employs separation of subfields using simple landmarks and densitometric quantification of PI intensity in 10 template-oriented counting fields.  We show that exposure to increasing concentrations of NMDA results in a dose-dependent increase in PI uptake.  Additionally, our method facilitates the comparison of cell death in different hippocampal subfields, such as dentate gyrus, CA1 and CA3.  Our results show marked differences of PI uptake in the hippocampal regions with the CA1 area being most sensitive to NMDA-induced injury.  The method provides a standardized format for quantifying PI exclusion in OHSCs that can be applied to cultures of differing shapes and sizes, permits comparisons between hippocampal subfields and does not require induction of maximal cell death.  The method of quantifying PI uptake described herein allows for an objective, quantitative and reproducible analysis and comparison of cell death in distinct regions of OHSCs.