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C. V. Starr Laboratory for Molecular Neuropharmacology, Department of Anesthesiology, Weill Medical College, Cornell University, New York, New York
Submitted 25 April 2005; accepted in final form 4 September 2005
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 GABAA receptor agonist 4,5,6,7-tetrahydroisothiazolo-[5,4-c]pyridine-3-ol (THIP; 0.1 µM) and decreased by Zn2+ (50 µM) but was unaffected by zolpidem (0.3 µM) or midazolam (0.2 µM). The pharmacological profile of the tonic current is consistent with its generation by activation of GABAA receptors that do not contain the
1 or
2 subunits. GABAA receptors expressed in HEK 293 cells that contained
4
2
subunits showed higher sensitivity to THIP (gaboxadol) and GABA than did receptors made up from
1
2
,
4
2
2s, or
1
2
2s subunits. Western blot analysis revealed that there is little, if any,
3 or
5 subunit protein in VB. In addition, co-immunoprecipitation studies showed that antibodies to the
subunit could precipitate
4, but not
1 subunit protein. Confocal microscopy of thalamic neurons grown in culture confirmed that
4 and
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 GABAA receptors that are highly sensitive to GABA and THIP and that these receptors are most likely made up of
4
2
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.
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