Abstracts

Rationale: The reduction in GABA-mediated synaptic inhibition that occurs during the prolonged seizures of status epilepticus (SE) is, in part, the result of a decrease in the surface expression and an increase in the intracellular accumulation of GABAA receptors (GABARs). It is not known whether this rapid modification in the GABAR pool results from the selective or non-selective internalization of GABARs. Methods: In this study, biochemical, electrophysiological, and immunohistochemical methods were used to test for subunit-dependent trafficking of GABARs during SE.Results: The surface expression of GABARs in hippocampal slices acutely obtained from male Sprague Dawley rats in SE (SE-treated) and age matched controls was studied by means of a biotinylation pull down assay. The SE-treated slices were obtained at a point when SE is known to be resistant to treatment with benzodiazepines (BDZ). The surface expression of the β2/3 and γ2 GABAR subunits, which are expressed in synaptic GABARs, was diminished in SE-treated slices. In contrast, the surface expression of the δ subunit, which is expressed in extrasynaptic GABARs, was similar in SE-treated and control slices. Complementary electrophysiological findings were observed: GABAergic synaptic currents (mIPSCs) recorded from SE-treated dentate granule cells (DGCs) were smaller in amplitude and less frequent than control mIPSCs. In contrast, extrasynaptic tonic currents in SE-treated DGCs, measured in response to the competitive GABAR antagonists bicuculline and SR 95531 and to the open channel blocker penicillin, was similar to that recorded from control DGCs. Reduced surface expression of GABARs could result from excessive extracellular GABA or increased excitation/neuronal activity. After hippocampal neuronal cultures (> 2 weeks in culture) were incubated in 100 μM GABA for 30 minutes, the surface immunoreactivity of the γ2 subunit remain unchanged. In contrast, increased neuronal activity (induced via a high external concentration of KCl) reduced the surface expression of the γ2 subunit but not that of the δ subunit. Conclusions: These studies demonstrate that there is differential modulation of the surface expression of synaptic and extrasynaptic receptors during SE that occurs in response to the increase in neuronal activity and not ligand binding. As the synaptic receptors are BDZ-sensitive whereas the extrasynaptic receptors are not, subunit-specific, activity-dependent trafficking of GABARs is a potential mechanism to explain the development of BDZ pharmacoresistance during SE.