An extrasynaptic GABAA receptor mediates tonic inhibition in thalamic VB neurons

Whole cell patch-clamp recordings were obtained from thalamic ventrobasal (VB) and reticular (RTN) neurons in mouse brain slices. A bicuculline-sensitive tonic current was observed in VB, but not in RTN, neurons; this current was increased by the GABA(A) receptor agonist 4,5,6,7-tetrahydroisothiazolo-[5,4-c]pyridine-3-ol (THIP; 0.1 microM) and decreased by Zn(2+) (50 microM) but was unaffected by zolpidem (0.3 microM) or midazolam (0.2 microM). The pharmacological profile of the tonic current is consistent with its generation by activation of GABA(A) receptors that do not contain the alpha(1) or gamma(2) subunits. GABA(A) receptors expressed in HEK 293 cells that contained alpha(4)beta(2)delta subunits showed higher sensitivity to THIP (gaboxadol) and GABA than did receptors made up from alpha(1)beta(2)delta, alpha(4)beta(2)gamma(2s,) or alpha(1)beta(2)gamma(2s) subunits. Western blot analysis revealed that there is little, if any, alpha(3) or alpha(5) subunit protein in VB. In addition, co-immunoprecipitation studies showed that antibodies to the delta subunit could precipitate alpha(4), but not alpha(1) subunit protein. Confocal microscopy of thalamic neurons grown in culture confirmed that alpha(4) and delta subunits are extensively co-localized with one another and are found predominantly, but not exclusively, at extrasynaptic sites. We conclude that thalamic VB neurons express extrasynaptic GABA(A) receptors that are highly sensitive to GABA and THIP and that these receptors are most likely made up of alpha(4)beta(2)delta subunits. In view of the critical role of thalamic neurons in the generation of oscillatory activity associated with sleep, these receptors may represent a principal site of action for the novel hypnotic agent gaboxadol.