Lithium‐pilocarpine‐induced status epilepticus produces necrotic neurons with internucleosomal DNA fragmentation in adult rats

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Abstract
Prolonged and continuous epileptic seizures [status epilepticus (SE)] produce a widespread pattern of neuronal death, primarily in limbic brain regions. Because it has been suggested that seizure‐induced neuronal death may be apoptotic in nature, we tested the hypothesis that lithium‐pilocarpine‐induced status epilepticus (LPCSE) produces apoptotic neurons. LPCSE lasting 3 h was induced in male Wistar rats which were allowed to recover for 24 or 72 h before perfusion‐fixation. Neuronal death was assessed by light microscopy with the haematoxylin‐and‐eosin stain (H&E), with in situ DNA nick‐end labelling (TUNEL stain), by electron microscopy, and by agarose gel electrophoresis of DNA extracted from vulnerable brain regions. Ultrastructurally, acidophilic neurons identified with H&E were dark, shrunken and necrotic in appearance, exhibiting pyknotic nuclei, irregular, dispersed chromatin clumps and cytoplasmic vacuolization. No cells with apoptotic features were seen. Acidophilic neurons were found in 21 out of 23 brain regions examined, and comprised 26–45% of the total number of neurons examined. A subset of these neurons (< 10% of the total number of neurons) were TUNEL‐positive at 72 h, but not 24 h, after SE. Internucleosomal DNA cleavage (DNA ‘laddering’) was found in the six brain regions examined ultrastructurally 24 and 72 h after SE. These results indicate that, in adult rats, LPCSE produces neuronal injury with the appearance of necrosis rather than apoptosis. The necrotic neurons show nuclear pyknosis, chromatin condensation and internucleosomal DNA fragmentation, confirming the nonspecificity of these nuclear changes. Internucleosomal DNA cleavage and other programmed cell death mechanisms can be activated by SE in neurons which become necrotic.