Apoptotic neuronal loss in the postnatal mouse retina (P5 and P8) occurs in spatially coherent clusters (Cusato and Reese, 2000, SFN Abs. 119.12), suggesting bystander cell killing. To examine whether gap junctions contribute to the spread of cell death, we induced apoptosis in P13 rat retina by scrapeloading with Cytochrome c (Cc) in the presence of rhodamine dextran (RD, to identify Cc loaded cells). Control retinas were scrapeloaded with RD only. Retinas were maintained in vitro for 1 hour, fixed, sectioned, DAPI stained, and pyknotic cells were counted. While only 1.5% of control scrapeloaded cells showed apoptotic morphology, 97% of Cc scrapeloaded cells were pyknotic. Moreover, in Cc retinas, many cells were pyknotic that were not RD-labeled. For Cc, the bystander index (the proportion of pyknotic cells not RD labeled) was 0.67, which was significantly reduced by the gap junction inhibitors octanol, carbenoxolone, and SNAP. Because neither Cc and RD are gap junction-permeant, experiments with biotiny...