Thalamic extrasynaptic GABAA receptors are required for typical absence seizures

Abstract

Aberrant GABAergic inhibition in thalamo-cortical networks has been identified as a potential mechanism for spike-and-wave discharge (SWD) generation. Thalamocortical (TC) neurons in the ventrobasal (VB) thalamus receive both ‘phasic’ and ‘tonic’ GABAA receptor mediated inhibition, generated by synaptic and extrasynaptic delta-subunit containing receptors, respectively [Cope et al., 2005; J. Neurosci. 25: 11553]. We have shown a selective increase of tonic GABAA receptor-mediated inhibition in pharmacological as well as in polygenic and monogenic rat and mouse models of absence epilepsy due to an impairment of GABA transporter-1 (GAT-1) activity [Society for Neuroscience 2007, 142.7, 142.8, 142.9]. Therefore, we suggested that extrasynaptic GABAA receptor gain-of-function in VB TC neurons is a necessary requirement for the appearance of SWDs. To directly test this hypothesis, we have now pharmacologically and genetically targeted extrasynaptic GABAA receptors in VB and monitored EEG and behavioural correlates of absence epilepsy in normal Wistar rats, Genetic Absence Epilepsy Rats from Strasbourg (GAERS) and GABAA receptor delta-subunit knockout mice. All experiments were conducted in accordance with the UK Animal Scientific Procedure Act. Reverse microdialysis of the selective extrasynaptic GABAA agonist THIP (70 and 100 μM, both n=5) and the selective GAT-1 inhibitor NO-711 (200 μM, n=5) into the VB induced SWDs and behavioural correlates of absence seizures in normal Wistar rats. THIP- and NO-711-induced SWDs were suppressed by systemic administration of the anti-absence drug ethosuximide (ETX, 100 mg/kg, n=5). In addition, we “knocked-down” extrasynaptic GABAA receptors in TC cells of GAERS by directly infusing a δ subunit specific antisense oligodeoxynucleotide (ODN, 1 and 2 nMol/μl, n=5 and 6, respectively) into the VB. The antisense ODN produced a marked reduction (~70% at 2 nMol) of the total time spent in seizures and the number of SWDs, whilst infusion of a missense ODN (1-2 nMol/μl, n=5) was ineffective. Lastly, systemic administration of 50 mg/kg of gamma-butyrolactone induced absence seizures in wild type mice, which were significantly reduced in the delta-subunit knockout mice (84% decrease in the total time spent in seizures and 53% reduction in number of SWDs). Our data demonstrate that extrasynaptic GABAA receptor gain-of-function in VB TC neurons is a necessary and sufficient requirement for the appearance of a pure absence epilepsy phenotype, and that GAT-1 critically controls SWD genesis.