Immunohistochemical detection of MDR1 expression in human drug-refractory epileptic brain. Panels A-B show MDR1 expression at the BBB. Widespread GFAP immunoreactivity (green) co-localizes with MDR1 (red). Nuclei are stained blue with DAPI. Note that both parenchymal and perivascular astrocytes express MDR1 (arrowheads), as do endothelial cells of the brain capillaries (thin arrows). The box graph in B illustrates the percentage of GFAP-positive astrocytes (GFAP) in samples from 11 patients (Table 1, 1–11) that also expressed MDR1. Data points (circles), mean value (triangle), range (horizontal bars) and standard errors (° SE) are shown together with the 50th percentile value. Panels C-D show the neuronal expression of MDR1. Double immunostaining with MDR1 and two neuronal markers (neurofilament, NF and NeuN) reveals that MDR1 is expressed in a subpopulation of epileptic neurons. Arrows point to MDR1 negative neurons, while arrowheads indicate the more frequently-occurring MDR1 positive neurons. Approximately 64% of cortical neurons (n = 264) were positive for MDR1. Quantitative analysis in D was obtained from 11 patients. Patient values (circles), mean (triangle), range (horizontal bars) and standard errors (± SE) are shown. The 50th percentile is also indicated in the box graph.