Figure 1.
Receptor Binding Assay Indicating High Levels of δ-containing GABAARs on MGB Neurons:
Representative autoradiographs of [3H] gaboxadol binding in young adult rats. Warm colors (red) indicate higher levels of binding while cooler colors (blue) represent lower levels (referenced to Relative Optical Density spectrum at left). At all three concentrations shown here (75 nM, 125 nM and 250 nM), [3H]gaboxadol binds selectively to GABAARs in MGB with little binding in brain regions shown in this coronal section, except for hippocampus and upper layers of neocortex. The MGB and hippocampus are indicated by arrows labeled “MGB” and “HP”, respectively with primary auditory cortex labeled as “A1”.
Figure 2.
GABAAR Mediated Tonic Inhibition in MGB Neurons:
A) Representative traces of gaboxadol-induced tonic Cl− currents (outward) revealed by gabazine block, resulting in an inward shift in baseline current for MGB neurons held at −10 mV. The solid black line above the first trace represents the continuous focal application of (50 µM) gabazine for all traces. B) Bar graph of tonic current amplitude changes revealed by focal application of gabazine in the presence of increasing concentrations of GABAAR agonist, gaboxadol (GBX), applied to the ACSF. Current amplitudes are represented on the y-axis with the concentration of gaboxadol on the x-axis. (*p<0.005 when compared to Control using Dunnett's post-hoc analysis, data underwent first-order Winsorization; n = control: 8; 0.1 µM: 4; 0.3 µM: 6; 1 µM: 6; 2 µM: 6; 5 µM: 6).
Figure 3.
Summary Illustration of GABAARs on MGB Neurons:
Note receptor location relative to the presynaptic GABAergic terminal (IC or TRN) with classic α1γ-subunit containing GABAARs located within the synapse and α4δ-subunit containing GABAARs outside of the synapse. The concentration of GABA to which each receptor type is typically exposed and the nature of the current mediated by each subtype of receptor is also depicted. For sample traces, the internal solution used here is CsCl-based (140 mM) and the membrane potential is clamped at −60 mV. As a result, GABAAR currents are inward and blocked by gabazine. The phasic response (left trace) is expanded from within the trace of the tonic response (right trace). The inward shift in baseline current is induced by upregulating extracellular GABA through inhibition of GABA uptake via the application of neuronal and glial GABA transporters with NNC-711 and SNAP 5114, respectively (solid line, right trace). These sample recordings were obtained from an MGB neuron of a 7-month-old FBN rat.