Delayed apoptotic pyramidal cell death in CA4 and CA1 hippocampal subfields after a single intraseptal injection of kainate

We have performed a detailed time-course analysis of cell death in the hippocampal formation, basal forebrain and amygdala following a single intraseptal injection of kainate in adult rats. Acetylcholinesterase histochemistry revealed a profound loss of staining in the medial septum but not in the diagonal band, and cholinergic fiber density was highly reduced in the hippocampus and amygdala at 10 days postinjection. Terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphatebiotin nick end labeling (TUNEL) histochemistry was performed for precise location of apoptotic cells. Both the medial septum and amygdala exhibited numerous TUNEL-positive nuclei after the intraseptal injection of kainate, while the lateral septum exhibited a lower but significant incidence in terms of apoptotic cells. In the medial septum, the presence of apoptotic cells was at a location displaying acetylcholinesterase staining. TUNEL histochemistry revealed a time-dependent sequential apoptotic cell death in hippocampal pyramidal cells. During the first two days postinjection, apoptosis in the hippocampus was only evident in the CA3 region. At five days postinjection, the entire CA4 region became apoptotic. At 10 days postinjection, the whole extent of the CA1 pyramidal cell layer exhibited numerous TUNEL-positive nuclei. The time-course of kainate-induced apoptosis in Ammons's horn correlated with the disappearance of hippocampal pyramidal neurons as detected by Nissl staining, which is suggestive of a prominent apoptotic death for these cells. The temporal delayed distant damage to CA4 and CA1 hippocampal subfields after a single intraseptal kainate injection is not seen in other models employing kainate and may be a valuable tool for exploring the cellular mechanisms leading to cell death in conditions of status epilepticus.