Kinetic and Pharmacological Properties of GABA_AReceptors in Single Thalamic Neurons and GABA_A Subunit Expression

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 gamma-aminobutyric acid type A (GABA(A)) 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 GABA(A) channels 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]. GABA(A) receptors in nRt and VB cells were pharmacologically distinct. Zn(2+) (100 microM) reduced GABA(A) 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 alpha1-alpha3, alpha5, beta1-3, gamma2-3, and delta, while nRt cells expressed alpha3, alpha5, gamma2-3, and delta. Both cell types expressed more subunits than needed for a single receptor type, suggesting the possibility of GABA(A) receptor heterogeneity within individual thalamic neurons. beta 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 GABA(A) receptors on beta 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 beta subunits, our findings do support alpha subunits, possibly alpha1 versus alpha3, as structural determinants of channel deactivation kinetics and clonazepam sensitivity. As the gamma2 and delta subunits previously implicated in Zn(2+) sensitivity are both expressed in each cell type, the observed differential Zn(2+) actions at VB versus nRt GABA(A) receptors may involve other subunit differences.

Differential Postnatal Maturation of GABAA, Glycine Receptor, and Mixed Synaptic Currents in Renshaw Cells and Ventral Spinal Interneurons

“…(Gingrich et al, 1995;Serafini et al, 1998). Decay time constant and total charge transfer in RC GABAergic mPSCs was strikingly similar to those reported in reticular thalamic (Rt) neurons (Browne et al, 2001), another neuron that expresses ␣3/␣5 GABA A receptors. Interestingly, Rt neurons and RCs have common functional features: both provide recurrent negative feedback and respond to excitatory synaptic inputs with high-frequency burst firing.…”

“…These characteristics adjust inhibitory current amplitudes and timing to the integration properties of neurons and synaptic networks (Browne et al, 2001;Pouille and Scanziani, 2001;Semyanov et al, 2004). They also homeostatically regulate the balance between synaptic excitation and inhibition (Turrigiano and Nelson, 2004).…”

“…Thereafter, inhibitory synapses shift their function to synaptic integration, and a bias toward "faster" glycinergic mechanisms seems advantageous for many ventral spinal cord synapses. GABA A receptor recruitment influences GABAergic current amplitudes (Nusser et al, 1997(Nusser et al, , 1998, and their time course is strongly related to subunit composition (Browne et al, 2001;Vicini et al, 2001). Glycinergic currents show an interesting property in that postsynaptic current amplitude is correlated with the amount of postsynaptic gephyrin (Lim et al, 1999;Van Zundert et al, 2004), a glycine receptoranchoring protein Feng et al, 1998;Meier et al, 2000Meier et al, , 2001.…”

Effect of the α subunit subtype on the macroscopic kinetic properties of recombinant GABAA receptors

“…Typically the IPSC decay rate is found to be slower in neurons early in development, changing to more rapid decay with adulthood (Takahashi, 2005). Different decay rates have also been reported for GABAR currents from different kinds of neurons (Xiang et al, 1998;Maric et al, 1999;Browne et al, 2001) and for extrasynaptic vs. synaptic receptors (Banks and Pearce, 2000). Some of these differences might be due to differences in the subunit composition of the receptors.…”

Dynamic GABAAReceptor Subtype-Specific Modulation of the Synchrony and Duration of Thalamic Oscillations

“…The anti-absence drug clonazepam (CZP) suppresses rhythmic activity in thalamic and thalamocortical slices (Huguenard and Prince, 1994;Zhang et al, 1996), but the locus for this suppression is unclear, because CZP modulates IPSCs in TRN and TC neurons (Browne et al, 2001), and effects in thalamocortical slices may reflect cortical actions (Oh et al, 1995). Here we use genetic manipulations to locate the site of action of CZP and elucidate the function of intra-TRN inhibition.…”