An Extrasynaptic GABA sub(A) 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 sub(A) receptor agonist 4,5,6,7-tetrahydroisothiazolo-[5,4-c]pyridine-3-ol (THIP; 0.1 mu M) and decreased by Zn super(2+) (50 mu M) but was unaffected by zolpidem (0.3 mu M) or midazolam (0.2 mu M). The pharmacological profile of the tonic current is consistent with its generation by activation of GABA sub(A) receptors that do not contain the alpha sub(1) or gamma sub(2) subunits. GABA sub(A) receptors expressed in HEK 293 cells that contained alpha sub(4) beta sub(2) delta subunits showed higher sensitivity to THIP (gaboxadol) and GABA than did receptors made up from alpha sub(1) beta sub(2) delta , alpha sub(4) beta sub(2) gamma sub(2s,) or alpha sub(1) beta sub(2) gamma sub(2s) subunits. Western blot analysis revealed that there is little, if any, alpha sub(3) or alpha sub(5) subunit protein in VB. In addition, co-immunoprecipitation studies showed that antibodies to the delta subunit could precipitate alpha sub(4), but not alpha sub(1) subunit protein. Confocal microscopy of thalamic neurons grown in culture confirmed that alpha sub(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 sub(A) receptors that are highly sensitive to GABA and THIP and that these receptors are most likely made up of alpha sub(4) beta sub(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.