Reduced local input to fast-spiking interneurons in the somatosensory cortex in the GABA γ2 R43Q mouse model of absence epilepsy.

Objective: Absence seizures in childhood absence epilepsy are initiated in the thalamocortical (TC) system. We investigated if these seizures result from altered development of the TC system before the appearance of seizures in mice containing a point mutation in γ-aminobutyric acid A (GABA ) receptor γ2 subunits linked to childhood absence epilepsy (R43Q). Findings from conditional mutant mice indicate that expression of normal γ2 subunits during preseizure ages protect from later seizures. This indicates that altered development in the presence of the R43Q mutation is a key contributor to the R43Q phenotype. We sought to identify the cellular processes affected by the R43Q mutation during these preseizure ages.

Methods: We examined landmarks of synaptic development at the end of the critical period for somatosensory TC plasticity using electrophysiologic recordings in TC brain slices from wild-type mice and R43Q mice.

Results: We found that the level of TC connectivity to layer 4 (L4) principal cells and the properties of TC synapses were unaltered in R43Q mice. Furthermore, we show that, although TC feedforward inhibition and the total level of GABAergic inhibition were normal, there was a reduction in the local connectivity to cortical interneurons. This reduction leads to altered inhibition during bursts of cortical activity.

Significance: This altered inhibition demonstrates that alterations in cortical circuitry precede the onset of seizures by more than a week.