Previous work has shown that 6h-hypoxia followed by reoxygenation for up to 96h induces delayed apoptotic death through changes in synthesis of specific proteins in cultured neurons from the fetal rat forebrain (Pediatr Res, 1997, 41:289A). The inducible AP-1 transcription factor includes Fos, Fos-related antigen (FRA) and Jun protein families that form dimers which bind to specific DNA sequences in the promotor region of target genes to modulate their expression. In order to study the expression of specific AP-1- related gene products Fos and Jun, cultured neurons, after 6 days in vitro, were maintained for 1h to 6h in hypoxia (95% N2-5% CO2) and reoxygenated for 96h under normoxia. Control neurons were kept under normoxia. As a function of time during hypoxia and reoxygenation, expression of these proteins was assessed by immunohistochemistry and western blotting. Fos immunoreactivity was analyzed by a polyclonal antibody against p62 C-Fos protein and also by a polyclonal antibody against four proteins of the Fos family, namely C-Fos, FRA-1, FRA-2 and FosB. Jun immunoreactivity was studied using a monoclonal antibody against p39 C-Jun protein. Experiments were performed in triplicate. Immunoreactivity for C-Fos, FRA-1, FRA-2 and FosB increased during hypoxia (1h and 6h). At 48h post-reoxygenation, it was inhibited compared to normoxic controls, and finally increased again at 96 h. Immunohistochemical staining with p62 C-Fos antibody also showed an increase during hypoxia itself, but p62 C-Fos expression was still increased at 48h and finally similar to controls at 96h after reoxygenation. Western blotting, however, revealed two distinct immunoreactivity bands corresponding to MW of 62 and 45 kDa, respectively, and their specific alterations with time were different. Expression of 62 kDa protein was enhanced at 6h and 48h and then similar to controls at 96h, whereas the expression of 45 kDa protein was similar to controls at 6h, inhibited at 48h, and increased at 96h. Finally, C-Jun protein was highly expressed during hypoxia and reoxygenation. This study outlines the role of fos/jun gene products in hypoxia-induced apoptosis, and shows specific changes in the composition of the transcription factor AP-1 during hypoxia and reoxygenation. Such changes could be directly related to neuronal vulnerability following transient brain hypoxia and require further studies to determine the respective contribution of the various nuclear proteins.