Abstracts

Rationale: Hypoxic encephalopathy is the most common cause of neonatal seizures, and is associated with long term sequela such as epilepsy. In our rat model of hypoxia-induced neonatal seizures we have observed long-term increase in hippocampal excitability and susceptibility to seizure induced neuronal injury(Jensen et al., J Neurophysiol. 1998;79:73-81; Koh and Jensen, Ann Neurol. 2001;50:366-72). Moreover, we have demonstrated rapid increases in hippocampal network excitability and enhanced spontaneous AMPAR EPSCs in hippocampal CA1 neurons, and that these are in part associated with post-translational modification of AMPAR GluR1 subunits (Sanchez et al., J Neurosci. 2005;25:3442-51; Rakhade et al., AES abstract 2006). We have shown that post-insult treatment with the AMPAR antagonists topiramate and NBQX attenuate the long term increases in seizure susceptibility (Koh et al., Epilepsia. 2004;45:569-75). Here we hypothesized that AMPAR activation may mediate these early functional alterations of the AMPAR currents. Methods: Rat pups were subjected to hypoxia (15 minutes at 4-7% O2) on postnatal day (P)10. Immediately following hypoxia, the rats received topiramate (30 mg/kg i.p) or vehicle (PBS) and compared to normoxic rats. After 1 hour, hippocampal slices were prepared from the rats. The whole-cell patch clamp technique was used to examine spontaneous(s) and miniature(m) EPSCs in CA1 pyramidal neurons. The pipette solution contained 110 mM Cs, 10 mM TEA and 5 mM QX-314 to block voltage-sensitive K+ and Na+ currents. ACSF contained picrotoxin (30 µM) and DL-AP5 (100 µM) to block GABA and NMDA receptor currents, respectively. TTX (1 µM) was used to block sodium channels when miniature EPSCs were studied. Results: Similar to our previous reports, following hypoxic seizures (vehicle treatment), AMPAR-mediated sEPSCs were increased in their amplitude (158 ± 15.9%, n = 15) and frequency (261.6 ± 83.1%, n =15), compared with those from normoxic rats (100%, n = 10). Consistently, AMPAR-mediated mEPSCs also showed similar increase in amplitude (188 ± 22.3%, n = 5) and frequency (226 ± 47%, n = 5). In contrast, with in vivo injection of NBQX or topiramate following hypoxia exposure, sEPSCs were decreased in both amplitude (topiramate 116.3 ± 10.9%, n = 5) and frequency (topiramate 48.0 ± 7.4%, n = 5), compared to slices from the vehicle-treated hypoxic animals (p< 0.001), and were not significantly different from those in slices from the naïve normoxic rats.Conclusions: Our data suggest that hypoxia-induced seizures induce early increases in AMPAR-mediated sEPSCs. Topiramate administration in vivo blocks the development of these changes. These data suggest that ongoing AMPAR activation is required to effect these rapid alterations in function. In a related study (Rakhade et al., AES abstract 2007), NBQX and topiramate block post-translational changes in the GluR1 subunit, providing a potential mechanism for the functional change. AMPAR antagonist treatment following seizures may prevent early epileptogenic changes in neuronal glutamate receptors in the developing brain.