Abstracts

Rationale: It has been reported that NMDA receptor (NR) activation has neuroprotective role against kainic acid (KA)-induced excitotoxicity in adult murine hippocampus (Ogita et al., 2003). However, subfield-specific roles of hippocampal NRs in the neuroprotection against epileptic insult have not been investigated. Here we investigated the functional role of CA3 NRs in the KA-induced excitotoxicity using CA3 pyramidal cell-restricted NR subunit 1 knockout (CA3 NR1-KO or mutant) mice and its control littermates [i.e., C57BL/6N (B6), floxed-NR1 (fNR1) and CA3 pyramidal cell-cre (CA3-cre) mice; Nakazawa et al., 2002].Methods: KA was administered i.p. to adult mice (12-20 wk of age) at 20 mg/kg body weight. Animals were monitored continuously for 2 h by video camera, and the onset and extent of behavioral seizure activity were recorded. Animals were killed at 3 h, 1 d, 7 d and 4 wk after KA treatment for the following histological studies. Neuronal damage was evaluated by Nissl staining, Fluoro Jade B and silver staining, and Mac-2 immunostaining. To detect the seizure-induced changes in GABAergic cells, brain sections were stained with the antibodies against GAD67 and palvalbumin (PV), respectively. c-fos immunoreactivity (IR) was monitored to evaluate neuronal excitability after the seizure.Results: After KA injection, CA3 NR1-KO mice showed a higher severity of seizure and a higher occurrence of status epilepticus (continuous clonic seizures) than any other control genotypes. The c-fos induction 3 h after injection was higher in the mutant CA1 pyramidal cells and dentate granule cells than that of control mice. Surprisingly, mutant mice showed an extensive cell loss in the CA1 pyramidal cell layer, but not in CA3 cell layer at 7 d and 4 wk after injection. In contrast, seizure-induced neurodegeneration was rarely found in other control genotypes, which is consistent with empirical evidence that B6 strain is resistant to excitotoxic cell death. Further, we found differential changes in the expressions of GAD67-IR between mutant and control mice. One day after injection, the number of GAD67-positive cells was decreased at strata radiatum and oriens of field CA1 among all genotypes examined. However, mutants showed an increased GAD67-IR in the CA1 pyramidal layer suggesting enhanced feed-forward inhibition. Nevertheless, the mutant GAD67- and PV-IR were mostly absent from CA1 field 4 wk after injection, suggesting neurodegeneration of majority of CA1 GABAergic cells. In contrast, the expression of GAD67 and PV of CA1 field, once decreased 1 day after injection in the controls, was gradually returned to baseline from 7 d to 4 wk after injection.Conclusions: The ablation of CA3 NR1 induced a higher susceptibility and an extensive cell loss in CA1 subfield after KA injection compared with the controls. The results suggest that CA3 NRs have a protective role of CA1 subfield, but not CA3 subfield against KA-induced excitotoxicity, which may be mediated by activation of feed-forward inhibition. This research was supported by the Intramural Research Program of the NIH.