Abstracts

RATIONALE: It has been shown that seizures increase neurogenesis of granule cells in the dentate gyrus. If the survival of new granule cells exceeds their death, one would expect that the granule cell layer would increase in size. To test this hypothesis, the dentate gyrus was evaluated morphologically in animal models of epilepsy. At the end of this activity the participants should understand changes that accompany seizures in the rat dentate gyrus.
METHODS: Adult male Sprague-Dawley rats were injected with pilocarpine or kainic acid i.p., followed by diazepam after 1 hr of status epilepticus. Controls received identical treatment, except they received saline instead of a convulsant. Other rats were amygdala-kindled (up to 50 kindled seizures) and perfused after the last kindled seizure. Immunocytochemistry was performed 1-10 months after status using antisera to calbindin D28K, a marker of granule cells, NeuN, a marker of adult neurons, and neuropeptide Y, a peptide expressed in GABAergic neurons, and in granule cells after seizures. Timm stain was used to examine sprouting. In some pilocarpine-treated rats, bromodeoxyuridine (BrdU) was injected (50mg/kg i.p., once/ day, on days 4-9 after status). Neurons born after seizures were visualized using antisera to BrdU and NeuN.
RESULTS: In the posterior half of the hippocampus, all pilocarpine- or kainic acid-treated animals (n=35) showed infoldings. The folds were similar to gyri of the primate neocortex, but less extensive. These [dsquote]microgyri[dsquote] were mostly in the ventral blade. There was little evidence of changes in the width of the cell layer, extent of mossy fiber sprouting, or NPY staining along a microgyrus. There often were clusters of calbindin-immunoreactive neurons near infoldings, presumably ectopic granule cells born after seizures. BrdU-labeled granule cells were common in the microgyrus. Similar invaginations were obtained after amygdala-kindling. In saline controls (n=12), there was little sign of invaginations except in 4 animals, where small folds were present in the ventral blade at extreme posterior levels of the hippocampus.
CONCLUSIONS: The results suggest that the granule cell layer of epileptic rats undergoes dramatic changes after repetitive seizures. In addition to the previously described reports of granule cell dispersion and increased neurogenesis, the granule cell layer appears to develop folds in the ventral blade. Like adult neurogenesis, the infoldings after seizures appear to reflect an increase in a phenomenon that is present at a low level normally. The observation of robust BrdU labeling along infoldings, previous reports that neurogenesis is greater in the ventral blade, and spatial association of calbindin-immunoreative granule-like hilar cells with the infoldings, raise the possibility that the invaginations develop as a result of increased numbers of granule cells that are born after seizures. This is analogous to the argument that neocortical gyri developed in primates because the number of neurons increased disproportionally to the increase in skull size. The corollary to this hypothesis is that seizure-induced neurogenesis is greater than seizure-related granule cell death, as least in some animal models of epilepsy.
[Supported by: NS 38285 and the Human Frontiers Science Program to H.E.S.]; (Disclosure: Grant - Contract with Neuropace Inc.)