The GABAA receptor is the main inhibitory neurotransmitter receptor in the CNS. Mutations in this receptor are associated with heritable epilepsy. The best studied of these is an arginine-to-glutamine substitution in the y2 subunit (y2R43Q) that confers Childhood Absence Epilepsy in heterozygous human patients. Furthermore, knock-in mice heterozygously expressing y2R43Q (i.e., RQ mice) display absence-like behavioral arrests concurrent with generalized EEG spike-wave discharges, both of which are blocked by anti-absence drugs. In heterologous expression systems, y2R43Q severely interferes with receptor assembly or trafficking, leading to the prediction that synaptic inhibition should be compromised in affected individuals. In sharp contrast to these predictions, however, RQ mice show only subtle changes in Inhibitory Postsynaptic Currents (IPSCs). Interestingly, we have recently discovered that nonsynaptic "tonic" inhibitory currents, activated by endogenous ambient GABA, are completely lost in excitatory cells of the cortex and thalamus of RQ mice, leading us to hypothesize that such loss is responsible for their absence epilepsy phenotype. This prediction, however, is the opposite of the conclusion, by Crunelli and colleagues, that "enhanced tonic inhibition is required for absence seizure generation". Taken together, these seemingly disparate findings lead to the hybrid hypothesis that tonic inhibition must be maintained near an optimal set point to allow correct regulation of thalamocortical function. Our overall hypothesis is therefore that absence seizures arise from dysregulation of tonic inhibition, such that either too much or too little biases the circuit towards absence seizures. The long term goals of this research are a) to understand the precise role of tonic inhibition in regulating thalamocortical function, b) to identify the changes in thalamocortical function that lead to seizures in RQ mice, and c) to identify pharmacological tools that can rescue these changes and restore normal function. We will employ patch clamp and multielectrode recordings in thalamocortical slices, and continuous video/ EEG/EMG monitoring in vivo to determine the specific differences between wild type and mutant cellular, network and behavioral properties. We will then develop pharmacotherapeutic approaches to rescue the mutant properties back to their wild type levels using drugs that specifically target tonic inhibiton in thalamus and cortex.
Our Specific Aims are:
AIM 1 - How does tonic inhibition regulate intrinsic and synaptically driven excitability? AIM 2 - How does tonic inhibition regulate the functional connectivity of the thalamocortical network? AIM 3 - How does tonic inhibition regulate behavioral states, including absence seizures and sleep?
The GABAA receptor regulates excitability in the brain, and mutations in this receptor cause several types of epilepsy. One mutation (y2R43Q) causes childhood absence epilepsy in human patients, and also in a strain of knock-in mice. We will use these mice to understand how the mutation causes changes in cellular and network function that lead to epilepsy, and thereby identify new targets for antiepileptic therapies.