Abstracts

Rationale: The GABA receptor (GABAR) 2 subunit mutation Q390X has been associated with the genetic Dravet syndrome, whose patients exhibit generalized and myoclonic seizures with cognitive deficits. In our in vitro studies, mutant 2 (Q390X) subunits were shown to dominant-negatively suppress the biogenesis of functional GABARs and decrease GABA-activated currents. Thalamocortical circuitry has been shown to be involved in generalized seizures, we hypothesized that heterozygous (het) knock-in (KI) of the 2 subunit mutation Q390X would exhibit reduced mIPSCs in the thalamocortical circuitry; However het knock-out (KO) of one 2 subunit would not show any alteration of mIPSCs in the cortex or thalamus. The mIPSC reduction and altered GABAR biogenesis would contribute to homeostatic inhibitory synaptic plasticity and epileptogenesis in human patients.Methods: Transgenic heterozygous (het) knock-in (KI) mouse models (postnatal day 60-120) with the 2 subunit mutation Q390X or het knock-out (KO) mice with one WT 2 subunit were used, together with cortical and thalamic brain slices and whole-cell electrophysiological recordings, to examine inhibitory postsynaptic currents (mIPSCs) in thalamocortical circuitry, homeostatic synaptic plasticity and epileptogenesis mechanism ex vivo. Results: In het KI mice, GABAR-mediated mIPSCs in layer VI pyramidal and thalamic VB neurons were significantly reduced (amplitudes), compared with WT littermates. Meanwhile, mIPSC frequency in neurons from het KI mice was smaller than that in WT neurons in both cortical layer VI and the thalamic VB nucleus. Consistent with these results, GABAR number per synapse (not GABAR single channel conductance) was reduced in het KI mice compared with WT littermates, using peak-scaled nonstationary analysis. In addition, mIPSCs in het KI mice exhibited a longer decay than WT mIPSCs, suggesting that in KI mice GABARs may have a different subunit composition. Moreover, we did not observe paired-pulse depression changes in the cortex, suggesting that presynaptic GABA release mechanism might not be involved. In contrast to KI mice, in het KO mice we did not find any significant differences in mIPSC amplitudes, frequency or decay in cortical layer VI when compared with WT littermate mice, indicating that presence of the mutant 2 subunit plays a role in the reduced GABAR-mIPSCs in het KI mice. In addition, tonic currents were reduced only in the thalamic VB nucleus from het KI mice, not in cortical layer VI neurons. Conclusions: These findings suggest that GABAR subunit assembly and trafficking might be disrupted in het KI mice with a 2 Q390X mutation, which results in a decrease in surface GABARs. These results would lead to homeostatic inhibitory plasticity alteration of GABAergic synapses during different cortical states, linking to epileptogenesis for this particular GABAR subunit mutation in human patients. All these results have started to clarify the thalamo-cortical network mechanisms for seizures and epileptogenesis produced by the GABAR 2 subunit Q390X mutation.