Abstracts

RATIONALE: The dentate gyrus of the hippocampus is unique in having a region of neuronal proliferation, termed the subgranular zone (SGZ), that continues to generate dentate granule cells (DGC) throughout life. The nature of normal DGC migration and maturation, and the migration of newborn DGCs to ectopic locations following pilocarpine-induced status epilepticus (SE), is poorly understood. The aim of this study was to examine the morphology of DGCs and their patterns of migration in control versus SE animals.
METHODS: Adult, male, Sprague-Dawley rats (180-200g) were given i.p. atropine methylbromide followed 20 min later by i.p. pilocarpine hydrochloride to induce SE. Seizure activity was monitored behaviorally and terminated with diazepam after 2h of convulsive SE. Control rats received saline instead of pilocarpine. 14-28 days later, animals were killed and immunocytochemistry was performed on coronal sections of the hippocampus using antibodies to the neural precursor marker doublecortin (DCX) and the mature neuronal marker NeuN.
RESULTS: In control rats, a subset of putative, recently born DGCs (labeled with DCX) were located at the SGZ and appeared as isolated, single cells or small clusters of cells with cell bodies and dendritic trees oriented horizontal to the main axis of the DGC layer. Some cells had basal dendrites. Another subset of cells at the SGZ had dendrites perpendicular to the DGC layer that extended toward the molecular layer; examples of cells intermediate between the horizontal and perpendicular orientation were seen as well. Labeled cells in the middle of the DGC layer typically had decreased DCX staining in the soma relative to that seen in the dendrites. In pilocarpine-treated rats, an increased number of DCX-positive cells were observed in the SGZ, and many of these cells had basal dendrites that extended deep into the hilus. Other cell soma were located within the hilus or the molecular layer, with dendrites oriented opposite to the direction seen in cells located within the DGC layer.
CONCLUSIONS: The reliance on static images of immature and mature DGCs does not provide direct evidence regarding the dynamic nature of DGC migration. Nonetheless, with this caveat in mind, the morphology and orientation of the DCX-positive cells observed in both the normal and pathological states suggest that DGC dendrites may [dsquote]guide[dsquote] developing DGCs during the later stages of their migration into a final location. Anomalies in the interaction between dendrites and adjacent cells or the extracellular matrix may underlie the appearance of ectopic cells in certain pathological conditions.
[Supported by: NIH, RO1NS NS39950]