Is GABA neurotransmission enhanced in auditory thalamus relative to inferior colliculus?

Abstract

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central auditory system. Sensory thalamic structures show high levels of non-desensitizing extrasynaptic GABAA receptors (GABAARs) and a reduction in the redundancy of coded information. The present study compared the inhibitory potency of GABA acting at GABAARs between the inferior colliculus (IC) and the medial geniculate body (MGB) using quantitative in vivo, in vitro, and ex vivo experimental approaches. In vivo single unit studies compared the ability of half maximal inhibitory concentrations of GABA to inhibit sound-evoked temporal responses, and found that GABA was two to three times (P < 0.01) more potent at suppressing MGB single unit responses than IC unit responses. In vitro whole cell patch-clamp slice recordings were used to demonstrate that gaboxadol, a δ-subunit selective GABAAR agonist, was significantly more potent at evoking tonic inhibitory currents from MGB neurons than IC neurons (P < 0.01). These electrophysiological findings were supported by an in vitro receptor binding assay which used the picrotoxin analog [(3)H]TBOB to assess binding in the GABAAR chloride channel. MGB GABAARs had significantly greater total open chloride channel capacity relative to GABAARs in IC (P < 0.05) as shown by increased total [(3)H]TBOB binding. Finally, a comparative ex vivo measurement compared endogenous GABA levels and suggested a trend towards higher GABA concentrations in MGB than in IC. Collectively, these studies suggest that, per unit GABA, high affinity extrasynaptic and synaptic GABAARs confer a significant inhibitory GABAAR advantage to MGB neurons relative to IC neurons. This increased GABA sensitivity likely underpins the vital filtering role of auditory thalamus.

Keywords: GABAA receptor; gamma-aminobutyric acid; inferior colliculus; medial geniculate body.

Fig. 1.

In vivo: effects of the endogenous agonist, gamma-aminobutyric acid (GABA), and the selective agonist, gaboxadol (GBX), application onto medial geniculate body (MGB) units. MGB unit is sensitive to application of the endogenous agonist GABA (A–C, Db, Gb). Ab: 5 nA GABA resulted in a near 50% decrease in spike rate, as shown in dot raster display. B: GABA application showed a selective suppression at or near rate best-modulated frequency (rBMF). C: average total spike change by low-dose GABA (less than 10 nA) for 23 units was 46.24 ± 2.97%. Application of the δ-subunit selective agonist GBX onto MGB units showed varying responses. Average total spikes were 32.59 ± 4.86% under GABA application and 67.15 ± 4.62% under GBX application. F: GABA showed greater reduction of spike rate than GBX for majority units (11/15). The representative unit in D showed profound spike suppression with 0 nA (leaking) GABA and a smaller effect with 20 nA GBX onto the same unit. F: group data indicated that applied GABA was almost twice as potent as GBX (**P < 0.01, independent t-test). There was a small number of units (4/15) that had similar levels of suppression with applied GABA and GBX (G–I). In the representative mixed type unit (G), GABA and GBX show a similar pattern of reduction in discharge rate (15.91% vs. 17.42%) across different modulation frequencies (H). I: group data for this response type indicated the same effects for GABA and GBX (P > 0.5, independent t-test). Data shown as means ± SE. N, number of spikes; n, number of units in all figures.

Fig. 2.

In vivo: ability of half-maximal inhibitory concentrations of GABA to inhibit sound-evoked responses in inferior colliculus (IC) and MGB. Dot raster displays sound-evoked responses inhibited by increasing iontophoretic doses of GABA for representative IC (A) and MGB (C) units. Response rates were plotted against modulated frequency (fm) (B and D) for each unit. E: total spikes for each run were calculated as percent of control. Gray dash-line indicates a 50% reduction from the control condition. Higher dosages of GABA were needed to suppress 50% of the responses for IC units (red line). Group ID₅₀ shown in F, mean values 49.56 ± 6.75 nA for IC and 14.01 ± 3.00 nA for MGB, suggested that MGB units were significantly more sensitive than IC units to GABA application (**P < 0.01, independent t-test). No sensitivity difference was found between dorsal and ventral subdivisions of the MGB (G). Data shown as means ± SE.

Fig. 3.

In vitro: GBX induced tonic inhibition and [³H]t-butylbicycloorthobenzoate ([³H]TBOB) binding in brain slices. Whole cell patch-clamp recordings from 5 IC and 5 MGB neurons from adult slices were used to compare the relative sensitivity of extrasynaptic GABA_ARs in IC and MGB neurons. A: tonic currents were plotted against increasing dosages of GBX. GBX was significantly more potent in activating tonic currents in MGB units than in IC units [EC_50MGB = 2.25 μM; EC_50IC = 4.45 μM; F (1, 54) = 30.17, P < 0.0001; F-test]. B: bath application of increasing doses (0.1–10 μM) of the δ-GABA_AR subunit-selective agonist, GBX, evoked tonic currents in both IC and MGB neurons. Amplitudes of GBX-evoked tonic currents were revealed by addition of 10 μM gabazine (GBZ). C: modulation of [³H]TBOB channel (picrotoxin) binding with increasing concentrations of GABA (0 nM to 5 μM) was performed on IC and MGB slices. 0 nM GABA was set as the control condition and represented the resting/control openings of GABA_AR Cl⁻ channels. At low concentrations, both structures showed increased binding indicative of increased GABA_AR, Cl⁻ channels openings. Peak percent increase in binding occurred at 100 nM for MGB (*P < 0.05, 2-way ANOVA) and 50 nM for IC, with a significantly larger area (shadow) under the MGB curve (black) suggesting a greater MGB neuronal total chloride flux capacity relative to IC neurons. Both MGB and IC showed desensitization (1,000 nM and 5,000 nM) reflecting a greater percentage of closed GABA_AR Cl⁻ channels than in the control condition.

Fig. 4.

Ex vivo: GABA concentrations of IC and MGB. A: absolute GABA levels were obtained from IC and MGB using ¹H-MRS. IC, ventral MGB (vMGB), and dorsal MGB (dMGB) were carefully selected separately in each rat. B: combined values of left and right hemispheres were shown in average group data. Mean GABA concentrations for each structure were IC = 1.50 ± 0.29, dMGB = 3.72 ± 1.13, and vMGB = 3.05 ± 0.86. A nonsignificant but clear trend toward higher GABA levels was found for both vMGB and dMGB compared with GABA levels in IC. An independent t-test showed a P value of 0.05107 (IC vs. dMGB) and 0.0809 (IC vs. vMGB) between IC and MGB. Scale bar = 2 mm.