Glioneuronal MDR1 expression and resistance to Phenytoin induced cytotoxicity. Histograms in A show data (mean ± SE, n = 3) obtained using cortical slices from human epileptics (Table 1, ID# 9,10,11), or naïve rat cortical slices, treated for 5 h with 375 μM Phenytoin (PHE) ± 2 h preincubation with 3 μM XR9576. Note that PHE toxicity (as illustrated in the micrographs) was apparent in normal rat brain slices, and was greatly exacerbated in human epileptic tissue treated with the MDR1 blocker (p < 0.01). Micrographs depict immunohistochemical evidence of PHE-induced cytotoxicity in GFAP-positive astrocytes and NeuN-positive neurons as assessed by their co-localization with EthD-1, a marker of cell damage. All the cells (astrocytes and neurons) were assessed by DAPI staining. Note that the combination of PHE + XR9576 increased the percentage of injured cells (red) as compared to PHE alone. Panel B shows enlarged nuclei (identified by DAPI) in cells expressing (MDR1 positive) or not expressing (MDR1 negative) MDR1 protein. Note that small condensed nuclei (seen in MDR1 negative cells) reflect apoptosis or irreversible cell damage. In contrast, large nuclei with diffuse DNA staining (seen in MDR1 positive cells) are typical of healthy cells. The graph shows the positive correlation between cells with healthy nuclei and MDR1 expression (n = 34 independent values from slices obtained from 11 patients; p < 0.006).