Abstract Caspases play crucial roles in the inflammatory response and in the cell pathway leading to apoptosis. Caspase 1 (ICE), 2 (Nedd2), 3 (CPP32), 6 (Mch2) and 8 (Mch5, FLICE) expression was examined using immuno- histochemistry in the brains of rats and gerbils following systemic administration of kainic acid (KA). The distribu- tion of caspase expression was compared with the distrib- ution of c-Fos expression, a transcription factor that is produced in response to the excitotoxic insult. Strong cas- pase 2 immunoreactivity was found in microglia up to 6 h following KA administration. Focal strong expression of caspases 1, 2, 3, 6 and 8 was observed in astrocytes and neurons, from 12 to 48 h after KA injection, in areas in which a number of neurons were committed to die. This distribution was in contrast with the generalised distribu- tion of c-Fos expression following KA administration. Only a minority of neurons in the entorhinal cortex, amygdala and hilus, but a majority of neurons in selected thalamic nuclei, exhibited strong caspase expression in KA-treated rats. Similar findings, although minimised, were observed in KA-treated gerbils. Double-labelling caspase immuno- histochemistry and in situ end-labelling of nuclear DNA fragmentation disclosed co-localisation of strong caspase expression and nuclear DNA breaks in a small percentage of neurons but no co-localisation in astrocytes. Western blots of entorhinal cortex and neocortex homogenates showed cleavage of certain caspase substrates in KA- treated rats. The intensity of the bands corresponding to lamin B and protein kinase C-δ was decreased in the en- torhinal cortex following KA administration. Several bands appeared in the entorhinal cortex and neocortex in Western blots processed for the demonstration of poly(ADP-ribose) polymerase (PARP), thus indicating that other proteases, in addition to caspases, cleaved PARP following KA administration. Taken together, these findings indicate that KA excitotoxicity triggers caspase expression which, although predominant in regions sub- jected to irreversible cell damage, has only a weak associ- ation with the presence of nuclear DNA breaks and neu- ron cell death. Although these results suggest caspase ac- tivation, further studies have to be performed to elucidate whether caspase activation plays a crucial role in KA ex- citotoxicity. Key words Caspase · Poly(ADP-ribose) polymerase · Caspase substrates · Excitoxicity · Kainic acid Introduction Systemic kainic acid (KA) administration at convulsant doses causes epileptic seizures and excitotoxic cell death in the entorhinal cortex, amygdala, CA1 and CA3 regions of the hippocampus, and selected thalamic nuclei in the adult rat [3, 25, 40, 42]. Excitotoxic cell death, in this model, has morphological characteristics of necrosis. However, it is associated with discrete apoptotic features, including the early appearance of internucleosomal DNA fragmentation, as revealed by a ladder pattern on agarose gel electrophoresis of isolated DNA, and the positive staining of certain cell subpopulations using in situ end- labelling of nuclear DNA fragmentation [11, 33]. This is accompanied by a more extensive response that involves, in addition to the mentioned vulnerable regions, the cere- bral neocortex and the dentate gyrus. Such a response is manifested as a generalised expression of the inducible isoform of the heat shock protein-70 [34] and of various members of the Fos and Jun families of transcription fac- tors, particularly c-Fos and c-Jun [20, 21, 36]. Several genes and their products are involved in the process of cell death and cell survival, including tran- scription factors c-Fos and c-Jun, and members of the bcl-2 I. Ferrer · E. López · R. Blanco · R. Rivera · J. Krupinski · E. Martí Differential c-Fos and caspase expression following kainic acid excitotoxicity Acta Neuropathol (2000) 99 : 245–256 © Springer-Verlag 2000 Received: 2 April 1999 / Revised: 17 June 1999 / Accepted: 8 July 1999 REGULAR PAPER I. Ferrer () Unitat de Neuropatologia, Servei d’Anatomia Patològica, Hospital Princeps d’Espanya (Bellvitge), Carrer Feixa Llarga sn, E-08907 Hospitalet de Llobregat, Spain e-mail: iferrer@sakma.es, Fax: +34-93-2045065 I. Ferrer · E. López · R. Blanco · R. Rivera · J. Krupinski · E. Martí Departament de Biologia Cellular i Anatomia Patològica, Campus de Bellvitge, Universitat de Barcelona, E-08907 Hospitalet de Llobregat, Spain