Tonic Inhibition in Principal Cells of the Amygdala: A Central Role for  3 Subunit-Containing GABAA Receptors

Abstract

GABAergic inhibition in the amygdala is essential in regulating fear and anxiety. Although fast "phasic" inhibition arising through the activation of postsynaptic GABA A receptors (GABA A Rs) has been well described in the amygdala, much less is known about extrasynaptic GABA A Rs mediating persistent or tonic inhibition and regulating neuronal excitability. Here, we recorded tonic currents in the basolateral (BLA) nucleus and the lateral (LA) nucleus of the amygdala. While all BLA principal cells expressed a robust GABAergic tonic current, only 70% of LA principal cells showed a tonic current. Immunohistochemical stainings revealed that the ␣3 GABA A R subunit is expressed moderately in the LA and strongly throughout the BLA nucleus, where it is located mostly at extrasynaptic sites. In ␣3 subunit KO mice, tonic currents are significantly reduced in BLA principal cells yet not in LA principal cells. Moreover, the ␣3 GABA A R-selective benzodiazepine site agonist and anxiolytic compound TP003 increases tonic currents and dampens excitability markedly in wild-type BLA principal cells but fails to do so in ␣3KO BLA cells. Interneurons of the LA and BLA nuclei also express a tonic current, but TP003-induced potentiation is seen in only a small fraction of these cells, suggesting that primarily other GABA A R variants underlie tonic inhibition in this cell type. Together, these studies demonstrate that ␣3 GABA A R-mediated tonic inhibition is a central component of the inhibitory force in the amygdala and that tonically activated ␣3 GABA A Rs present an important target for anxiolytic or fear-reducing compounds.

Neurosteroid Agonist at GABAA Receptor Induces Persistent Neuroplasticity in VTA Dopamine Neurons

“…Despite expression of tonic inhibition in both VTA DA and GABA neurons in the presence 500 nM GABA (in the range of physiological extracellular GABA concentration (Murphy and Maidment, 1999)), we believe that tonic inhibition is more effective in VTA GABA interneurons as these neurons have a higher input resistance than DA neurons, allowing the same charge transfer to more effectively change their membrane potential (Margolis et al, 2012). We observed tonic currents with amplitudes between 10 and 30 pA, in line with studies in various brain areas (Marowsky et al, 2012;Stell et al, 2003). Both cell types exhibited GABAergic inhibitory activity that varied in amplitude and event frequency, and some recorded cells expressed no tonic current.…”

“…First, we ascertained whether tonic inhibition is cell-type specific in the VTA, as has been shown to be the case in the hippocampus, amygdala, and thalamic nuclei (Marowsky et al, 2012;Semyanov et al, 2003). Importantly, we used two mouse lines, GAD67-GFP knockin mice and Th-EGFP BAC transgenic mice (Gong et al, 2003;Tamamaki et al, 2003), which helped us to identify the VTA GABA and DA neurons, respectively, independently of their electrophysiological characteristics (Margolis et al, 2012).…”

Neurosteroids increase tonic GABAergic inhibition in the lateral section of the central amygdala in mice

“…In contrast, we did not find any cell type specificity in the CeAl since all the neurons recorded displayed an ongoing and ␦-subunitmediated tonic GABAergic current. GABAergic tonic inhibition has also been shown in LA and BA, where it is mediated by GABA A Rs containing the ␣3-subunit and where the expression of the ␦-subunit is rather low (Marowsky et al 2012). The evidence for ␦-subunit-mediated tonic GABAergic inhibition in CeAl neurons is in agreement with the observed strong effect of the neurosteroids THDOC and allopregnanolone.…”

“…1, F and G). An only minor contribution of the ␣5-subunit is in line with a rather low expression level of this subunit in the CeA (Marowsky et al 2012;Pirker et al 2000;Prut et al 2010; but see Hortnagl et al 2013). A summary of the pharmacological effects is depicted in Fig.…”

“…GABAergic tonic inhibition has been described in many regions of the brain (Farrant and Nusser 2005;Glykys and Mody 2007;Semyanov et al 2004) including the lateral amygdala (LA), the basolateral amygdala (BA; Marowsky et al 2012), and the medial section of the CeA (CeAm; Herman et al 2013). Besides, so far, three GABA A R subunits have been found to be expressed in mouse CeA: ␣1, ␦, and ␣5, the first two of which were functionally related to the generation of tonic inhibition (Herman et al 2013).…”