Abstracts

Rationale: Mechanisms that transform single seizures into prolonged continuous seizure activity of status epilepticus (SE) are not known. We determined whether the plasticity GluA1 subunit-containing AMPA receptors played a critical role in initiation and sustenance of SE. Methods: SE was induced in adult male rats by pilocarpine or by continuous hippocampal stimulation (CHS). AMPAR-mediated synaptic neurotransmission of CA1 pyramidal neurons (PN) was studied by whole-cell patch clamp technique in animals in SE (60 min from the first stage 5 behavioral seizure). Cell surface expression of GluA1 subunit in CA1 region was determined using biotinylation assay and Western blotting. The susceptibility to SE was determined in adult male and female mice lacking the GluA1 subunit (KO) and wild-type (WT) by continuous video-EEG monitoring. Results: The AMPAR-mediated mEPSCs and sEPSCs recorded from CA1 PNs of animals in SE were larger than those recorded from control animals. Furthermore, the AMPAR currents evoked in CA1 neurons by electrical stimulation of Schaffer collaterals were inwardly rectifying in SE animals; their rectification index was significantly smaller than that recorded from control CA1 neurons (0.35 0.079, n= 8 vs 0.96 0.065,n= 8, p < 0.0001) . In addition, bath application of AMPA (2.5 microM) evoked a larger current in CA1 PNs of SE animals (20.6 3.8 pA/pF, n=7 vs 12.5 1.5 pA/pF, n=10, p < 0.05). The biotinylation assay showed increased surface expression of GluA1 subunits in the CA1 region of SE animals (51 10% of that in control, n=7). Furthermore, a BS3 assay revealed lower intracellular expression of GluA1 subunit in the proteins isolated from CA1 region of SE animals. In order to determine the functional significance of GluA1 subunit plasticity observed in SE animals, we tested whether SE was altered in KO mice. The KO and WT mice were equally susceptible to electrically or chemically-evoked seizures. However, the KO mice resisted developing SE. Thirty min of CHS did not induce SE in the KO mice (n=7), whereas it induced SE in 64% of the WT mice (n=14). CHS for 60 min triggered SE in only half of the KO mice (n=11) compared to 77% of the WT mice (n=13). The SE also terminated early in KO mice compared to WT mice (83 12 min, n=5 vs 342 64 min, n=8, p < 0.05). Administration of pilocarpine triggered seizures in KO and WT mice; the latency to first seizure from the time of injection was similar in both the strains. However the seizures became continuous in only half of KO mice (n=11). Furthermore, the time from injection to continuous seizure activity was much longer in KO mice than that in WT mice (27 4 min, n=6 vs 20 2 min, n=10, p < 0.05). Conclusions: These studies revealed that seizure activity increased the expression of functional of GluA1 subunit-containing AMPARs on CA1 PNs. The GluA1 subunit plasticity played a critical role in induction and sustenance of SE and is also likely to contribute to excitotoxicity of CA1 PNs. Funding: NIH, Epilepsy foundation