Figure 2.

Extrasynaptic GABA_ARs and Tonic Inhibition in MGB. A) Confocal image of fluorescent immunolabeling of α₄ (green, top left), δ (red, top right) and colocalized α₄δ (yellow, bottom) GABA_AR subunits in the MGB demonstrate the presence of the subunits necessary for functional tonically active extrasynaptic GABA_AR constructs. Scale bar = 10μm. B) Voltage-clamp recording (CsCl based internal solution, V_hold = −60mV, E_Cl ≈ 0mV) of tonic inhibitory current mediated by extrasynaptic GABA_ARs in the MGB of a control 19 month old FBN rat (red, top) and from an MGB neuron contralateral to the sound exposed ear in a 24 month old FBN rat with behavioral evidence of tinnitus (blue, bottom) in response to 1μM gaboxadol (black bar) and blocked by 50μM gabazine (gray bar). C) Autoradiographs of 250nM [³H]gaboxadol binding in adult FBN rats with and without behavioral evidence of tinnitus (modified from Richardson et al., 2011). Relative optical density spectrum is positioned at right. At these concentrations, [³H]gaboxadol binds preferentially to extrasynaptic GABA_ARs. Binding density is highest in the MGB (outlined with solid line) and lower in auditory cortex (dashed line) and hippocampus (H). D) Binding scatchards and saturation curves (not shown) indicate a significant difference in [³H]gaboxadol binding between the MGB of control and animals with tinnitus and a 20–30% decrease in B_max (x-axis), corresponding to a decrease in extrasynaptic GABA_AR density in the MGB, while K_d (slope) (Kd=231.7±89.6 (control), 175.0±77.9 (contra), 208.3±87,95 (ipsi)) is relatively unaffected (n=5 control, 5 tinnitus).