Abstracts

Rationale: Previous studies on the pathophysiology of status epilepticus (SE) have focused on dynamic changes in GABA A receptor mediated inhibition. However, plasticity of glutamatergic synaptic neurotransmission in SE has not been investigated in detail. Since expression of calcium permeable, GluR2 lacking AMPA receptors (CP-AMPARs) occurs in a variety of physiological and pathophysiological conditions such as LTP and hypoxia, animals were studied at two different stages of SE: 10 or 60 minutes after first stage 5 seizure. Methods: Changes in AMPAR mediated synaptic transmission was assessed by whole-cell voltage-clamp recordings of excitatory post synaptic currents (EPSCs) from DGCs in hippocampal slices from animals sacrificed 10 minutes after first grade 5 seizure (10 min SE-DGCs) or untreated control animals (control DGCs). Evoked EPSCs (eEPSCs) were obtained by stimulating the perforant pathway fibers (100-300 μA; 0.69Hz) with a concentric bipolar electrode. Rectification of EPSCs, used to determine the presence of CP-AMPARs, were obtained by measuring peak averaged eEPSCs from -70 to +40 mV. The rectification index (RI) was calculated by dividing the peak amplitude of the average EPSC measured at a potential of +40 mV to that measured at -40 mV. Results: The frequency of sEPSCS recorded from 10 min SE-DGCs (0.38 ± 0.09 Hz; n = 13 cells / 7 animals) was less compared to control-DGCs (0.79 ± 0.08 Hz, n = 17 cells / 9 animals; p=0.004, unpaired t test). The amplitude of sEPSCs was similar in both control- (13.08 ± 0.4 pA) and 10 min SE-DGCs (14.03 ± 0.6 pA). The IV relationship obtained in 10 out of 12 10-min SE-DGCs demonstrated inward rectification of eEPSCs at depolarized potentials. In contrast, IV relationship of eEPSCs in control DGCs was linear. Rectification index (RI) was calculated by dividing the peak amplitude of the averaged eEPSC measured at a potential of +40 mV to that measured at -40 mV. The mean RI was 0.45 ± 0.05 in 10 min SE-DGCs (n = 12 cells / 5 animals) and in control-DGCs it was 0.84 ± 0.03 (n = 9 cells / 4 animals p = 0.002, unpaired t test). Philanthotoxin-433 (10 μM), a selective blocker of GluR-2 lacking AMPARs inhibited eEPSCs in 5 out of 7 10-min SE-DGCs; but not in any control-DGCs (6 cells, p = 0.02, Fisher’s exact test) further confirming the presence of CP-AMPARs. The frequency of sEPSCs obtained from DGCs in hippocampal slices from animals sacrificed 60 min after first grade 5 seizure (60-min SE-DGCs; 0.59 ± 0.12 Hz; n = 12 cells / 6 animals) was similar to that recorded from control-DGCs (0.79 ± 0.08 Hz, n = 17 cells / 8 animals; p = 0.01 unpaired t test). However, the mean amplitude of sEPSCs recorded from 60-min SE-DGCs (16.9 ± 0.7 pA) was larger than in control-DGCs (13.08 ± 0.4 pA, p = 0.0001, unpaired t test). Interestingly, in 60-min SE-DGCs, there we did not observe rectification of AMPAR mediated currents. The IV relationship was linear (mean RI = 0.6 ± 0.05; n = 7 cells / 6 animals; p = 0.40, unpaired t test) and similar to that in control DGCs. Conclusions: Together, these data demonstrate a transient expression CP-AMPARs during SE.