Novel compounds selectively enhance δ subunit containing GABA_A receptors and increase tonic currents in thalamus

Inhibition in the brain is dominated by the neurotransmitter γ-aminobutyric acid (GABA); operating through GABA_A receptors. This form of neural inhibition was presumed to be mediated by synaptic receptors, however recent evidence has highlighted a previously unappreciated role for extrasynaptic GABA_A receptors in controlling neuronal activity. Synaptic and extrasynaptic GABA_A receptors exhibit distinct pharmacological and biophysical properties that differentially influence brain physiology and behavior. Here we used a fluorescence-based assay and cell lines expressing recombinant GABA_A receptors to identify a novel series of benzamide compounds that selectively enhance, or activate α4β3δ GABA_A receptors (cf. α4β3γ2 and α1β3γ2). Utilising electrophysiological methods, we illustrate that one of these compounds, 4-chloro-N-[6,8-dibromo-2-(2-thienyl)imidazo[1,2-a]pyridine-3-yl benzamide (DS1) potently (low nM) enhances GABA-evoked currents mediated by α4β3δ receptors. At similar concentrations DS1 directly activates this receptor and is the most potent known agonist of α4β3δ receptors. 4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridine-3-yl benzamide (DS2) selectively potentiated GABA responses mediated by α4β3δ receptors, but was not an agonist. Recent studies have revealed a tonic form of inhibition in thalamus mediated by the α4β2δ extrasynaptic GABA_A receptors that may contribute to the regulation of thalamocortical rhythmic activity associated with sleep, wakefulness, vigilance and seizure disorders. In mouse thalamic relay cells DS2 enhanced the tonic current mediated by α4β2δ receptors with no effect on their synaptic GABA_A receptors. Similarly, in mouse cerebellar granule cells DS2 potentiated the tonic current mediated by α6βδ receptors. DS2 is the first selective positive allosteric modulator of δ-GABA_A receptors and such compounds potentially offer novel therapeutic opportunities as analgesics and in the treatment of sleep disorders. Furthermore, these drugs may be valuable in elucidating the physiological and pathophysiological roles played by these extrasynaptic GABA_A receptors.