Previous studies showed that neurogenesis occurs in the dentate gyrus of the adult rodent. Recent evidence suggests that the resulting newly born neurons integrate into pre-existing hippocampal circuitry. Newly born neurons in the developing and adult dentate gyrus exhibit a transient basal dendrite. In adult pilocarpine-induced epileptic rats, basal dendrites persist and are ectopically located in the hilus where they receive synaptic input from mossy fiber axons. We hypothesize that these hilar basal dendrites are derived from newly born neurons that are born after the pilocarpine-induced seizures. To test this hypothesis, the length of basal dendrites from epileptic rats was compared with that from control rats using doublecortin immunocytochemistry, which labels newly born neurons and their processes for up to 3 weeks after their genesis. The data on hilar basal dendrites in pilocarpine animals indicate that those from newly born neurons are significantly longer than those found in the control rats. We also demonstrate that 20% of newly born neurons in the epileptic rat have a basal dendrite that enters the hilus at an angle greater than 30 degrees from its cell body as compared with <2% in the control rats. Lastly, we provide evidence that the hilar basal dendrites in the epileptic rats are adjacent to glial fibrillary acidic protein-labeled astrocytic processes in the hilus and suggest that an ectopic glial scaffold in the hilus is involved with the formation of hilar basal dendrites. In conclusion, the data show that newly born neurons from epileptic rats have longer hilar basal dendrites and their formation might relate to gliosis which occurs as a result of hilar neuronal cell loss after status epilepticus.
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