Kinetic and Pharmacological Properties of GABAAReceptors in Single Thalamic Neurons and GABAA Subunit Expression

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Abstract
Synaptic inhibition in the thalamus plays critical roles in sensory processing and thalamocortical rhythm generation. To determine kinetic, pharmacological, and structural properties of thalamic γ-aminobutyric acid type A (GABAA) receptors, we used patch-clamp techniques and single-cell reverse transcriptase polymerase chain reaction (RT-PCR) in neurons from two principal rat thalamic nuclei—the reticular nucleus (nRt) and the ventrobasal (VB) complex. Single-channel recordings identified GABAAchannels with densities threefold higher in VB than nRt neurons, and with mean open time fourfold longer for nRt than VB [14.6 ± 2.5 vs. 3.8 ± 0.7 (SE) ms, respectively]. GABAA receptors in nRt and VB cells were pharmacologically distinct. Zn2+ (100 μM) reduced GABAA channel activity in VB and nRt by 84 and 24%, respectively. Clonazepam (100 nM) increased inhibitory postsynaptic current (IPSC) decay time constants in nRt (from 44.3 to 77.9 ms, P < 0.01) but not in VB. Single-cell RT-PCR revealed subunit heterogeneity between nRt and VB cells. VB neurons expressed α1–α3, α5, β1–3, γ2–3, and δ, while nRt cells expressed α3, α5, γ2–3, and δ. Both cell types expressed more subunits than needed for a single receptor type, suggesting the possibility of GABAA receptor heterogeneity within individual thalamic neurons. β subunits were not detected in nRt cells, which is consistent with very low levels reported in previous in situ hybridization studies but inconsistent with the expected dependence of functional GABAA receptors on β subunits. Different single-channel open times likely underlie distinct IPSC decay time constants in VB and nRt cells. While we can make no conclusion regarding β subunits, our findings do support α subunits, possibly α1 versus α3, as structural determinants of channel deactivation kinetics and clonazepam sensitivity. As the γ2 and δ subunits previously implicated in Zn2+sensitivity are both expressed in each cell type, the observed differential Zn2+ actions at VB versus nRt GABAA receptors may involve other subunit differences.

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