Abstracts

Rationale: Granule cells of the dentate gyrus are relatively resistant to seizure-induced neuronal damage compared to adjacent hippocampal neurons, but the reasons for this resistance are unclear. Because granule cells express a higher concentration of the neurotrophin brain-derived neurotrophic factor (BDNF) than other hippocampal neurons, and BDNF has been shown to be neuroprotective, we explored BDNF-mediated mechanisms that might explain granule cell resistance. BDNF binds and signals through the TrkB receptor, and one signaling substrate of TrkB that may play a role in granule cell resistance is ARMS (ankyrin repeat-rich membrane spanning protein, also called KIDINS-220). Therefore, we compared the damage following experimental status epilepticus in mice with a heterozygous mutation in ARMS (+/-; Het) to wild type (WT) mice.Methods: Status epilepticus was induced at 12-13 weeks of age using the chemoconvulsant pilocarpine hydrochloride (300 mg/kg, s.c.) 30 minutes after an injection of 5 mg/kg (s.c.) atropine methylbromide. One hour after the onset of status, determined using the Racine scale, the anticonvulsant diazepam was administered (5 mg/kg, i.p.). Animals were perfusion-fixed 3, 7, 14 days or 2 months after status and 50 μm sections were cut with a vibratome. Immunocytochemistry was conducted to examine neuronal loss using an antibody to NeuN, a neuronal marker (mouse monoclonal; Chemicon). Fluorojade B was used to detect neurodegeneration. Neuropeptide Y (NPY; rabbit polyclonal; Chemicon) was used to identify mossy fibers. Degenerating hilar neurons were quantified using an optical fractionator (StereoInvestigator; Microbrightfield, Inc.).Results: Granule cell and hippocampal pyramidal cell morphology and density appeared identical in WT and Het mice that had received pilocarpine but did not have status (n=5/group). Two months after status, there was greater loss of granule cells in the Het mice compared to WT mice (n=5/group). However, latency to status, incidence of status, and the latency to the first seizure were not statistically different, and differences in the behavioral signs of status could not be discriminated, suggesting that the results could not be explained by more severe status in Het mice. Moreover, stereological estimates of hilar neuronal loss were not statistically different in WT and Het mice (n=5/group), and fluorojade labeling 3 (n=5/group), 7 (n=2/group) and 14 days (n=2/group) after status was comparable in pyramidal cell layers of both WT and Het mice. NPY labeling showed more mossy fiber sprouting in Het mice when they were examined 2 months after status despite the loss of granule cells (n=6/group). Conclusions: The results suggest a role for ARMS in the resistance of dentate gyrus granule cells to seizure-induced neuronal damage. The high expression of BDNF, its receptor TrkB, and downstream molecular components of neurotrophin signaling, such as ARMS, could be a powerful combination that ensures survival of the granule cell population even after insults that damage the hippocampal pyramidal cells and hilar neurons.