Abstracts

Rationale: Status epilepticus (SE) is a neurological emergency characterized by a prolonged, self-sustaining seizure that can result in death or long-term neurological dysfunction. SE commonly occurs at ages at which the brain is still developing; however, laboratory studies investigating the pathogenesis of SE have been performed predominantly in older animals after the development of the GABAergic synaptic system is complete. In addition, although the recent focus has been on changes in the trafficking of GABA_A receptors during SE, there are likely multiple levels of dysfunction that affect both the ability of the presynaptic inhibitory interneuron to release GABA as well as the response of the postsynaptic principal neuron to the GABA that is released. In this study, we have used electrophysiological and biochemical techniques to begin to understand the cellular mechanisms that contribute to GABA-mediated dysfunction during SE in younger animals. Methods: SE was induced by lithium followed by pilocarpine in the postnatal day 15-17 Sprague Dawley rat (SE-treated). This age was chosen as prior studies demonstrated that the GABAergic synapse in these animals undergoes a protracted development that is not complete until the end of the 3rd postnatal week. Spontaneous (sIPSCs) and miniature (mIPSCs) GABAergic synaptic currents were recorded using standard whole cell patch clamp techniques from CA1 pyramidal neurons in hippocampal slices acutely obtained from SE-treated animals and from naïve controls. A biotinylation pull down assay was used to measure the surface expression of the γ2 subunit in SE-treated and control slices. Results: Compared to controls (n=7), the frequency (1.1 ± 0.4 Hz vs. 6.5 ± 0.8 Hz) and amplitude (52 ± 6.8 pA vs. 75 ± 6.4 pA) of sIPSCs recorded from SE-treated pyramidal neurons (n=7) were reduced. Rise time and weighted decay were similar in both groups. The amplitude distribution histogram for the pooled control sIPSCs was best fit to a sum of five Gaussians (peaks at 37, 53, 76, 118, and 160 pA) with each Gaussian contributing 15 to 29% of the overall population with the single quantal event accounting for 18%. The amplitude of pooled SE-treated sIPSCs was best fit to three Gaussians (peaks 35, 60, and 84 pA) with single quantal events accounting for 71% of the total events. In contrast to the changes observed in sIPSCs, the mIPSC frequency (0.96 ± 0.38 Hz) and amplitude (45.8 ± 6.0 pA) recorded for SE-treated pyramidal neurons (n=6) were similar to the mIPSC frequency (0.94 ± 0.39 Hz) and amplitude (44.7 ± 6.9 pA) recorded from controls (n=5). Furthermore, the surface expression of the γ2 subunit in the SE-treated slices was similar to controls. Conclusions: As evidenced by the alteration in sIPSC characteristics, this study demonstrates that a rapid alteration in GABA-mediated synaptic inhibition occurred during SE in young animals. However, in contrast to older animals, these findings may be best explained by a modification in the release of GABA and not a rapid modification in the surface expression of benzodiazepine-sensitive GABA_A receptor population.