Abstracts

Rationale: Cope and colleagues recently showed that several animal models of absence epilepsy involve increases in GABAergic tonic inhibition (Nat Med 2009; 15:1392). In contrast, a loss of tonic inhibition could also lead to hyperexcitability and seizures. The ?2R43Q mutation of the GABAA receptor causes hereditary Generalized Epilepsy with Febrile Seizures Plus (GEFS ) in humans (Nat Genet 2001; 28:49) and absence-like seizures in knock-in mice (PNAS 2007; 104:17536). In heterologous expression systems, mutant receptors have altered kinetics (Mol Pharmacol 2009;77:35) and expression (J Biol Chem 2004; 279:47034; J Neurosci 2006; 26:2590), differentially affecting phasic and tonic inhibition (J Neurosci 2007; 27:14108). We previously demonstrated this mutation abolishes inhibitory tonic currents in thalamic relay and cortical pyramidal neurons, and decreases currents activated by ?-subunit selective ligands in thalamic relay cells. The loss of this conductance is expected to cause depolarization that could affect the ability of thalamic neurons to fire calcium-dependent bursts. Here we used a multichannel recording technique to examine spontaneous bursting activity throughout the thalamocortical loop in slices from 16-24 day old wild-type (WT) and heterozygous ?2R43Q knock-in mice. Methods: Multichannel recordings were made with two NeuroNexus 16-channel recording arrays placed in thalamus and cortex of 400 m thalamocortical slices. Neurons were isolated using a principal components-based clustering and spike sorting algorithm. Long recordings (> 60 min) consisted mainly of sparse spiking, punctuated by brief periods of high intensity activity, leading to bimodal interspike interval (ISI) distributions with peaks at <10 and >100 ms. We therefore defined a burst as two or more spikes with ISIs of less than 50 ms. For each cell, the tendency to fire in bursts was defined by the burst ratio: (# of bursts events divided by # of lone spikes). Bursting correlational coefficients were computed for all cell combinations for comparison. Results: For WT (n = 5 slices) and ?2R43Q (n = 4 slices) mice, there were no differences in overall firing rates (Hz) for any condition (Kruskal-Wallis test: median, IQR; thal p ? 0.17: WT: 0.05, 0.1: 102 cells; ?2R43Q: 0.03, 0.09: 68 cells; ctx: WT: 0.04, 0.2: 86 cells; ?2R43Q: 0.03, 0.08: 57 cells). Compared to WT, burst ratios were decreased by ~50% for ?2R43Q in both the thalamus and cortex (thal p<0.0001: WT: 0.7, 1.2; ?2R43Q: 0.3, 0.6; ctx p<0.001: WT: 0.2, 0.5; ?2R43Q: 0.1, 0.3). WT mice express strong intra-area and thalamocortical bursting correlations, while RQ mice show decreased correlations in intra-area bursting and thalamocortical burst correlations were completely lost. Conclusions: These results are consistent with the loss of tonic inhibition promoting depolarization of thalamic neurons, causing a switch from burst- to tonic-firing modes, and an apparent breakdown of thalamocortical communication. It remains unclear how such a breakdown might set the stage for hypersynchronous thalamocortical absence seizures.