Enrichment of GABAA Receptor α-Subunits on the Axonal Initial Segment Shows Regional Differences.

Yudong Gao,Scott A Heldt

doi:10.3389/fncel.2016.00039

Abstract

Although it is generally recognized that certain α-subunits of γ-aminobutyric acid type A receptors (GABAARs) form enriched clusters on the axonal initial segment (AIS), the degree to which these clusters vary in different brain areas is not well known. In the current study, we quantified the density, size, and enrichment ratio of fluorescently labeled α1-, α2-, or α3-subunits aggregates co-localized with the AIS-marker ankyrin G and compared them to aggregates in non-AIS locations among different brain areas including hippocampal subfields, basal lateral amygdala (BLA), prefrontal cortex (PFC), and sensory cortex (CTX). We found regional differences in the enrichment of GABAAR α-subunits on the AIS. Significant enrichment was identified in the CA3 of hippocampus for α1-subunits, in the CA1, CA3, and BLA for α2-subunits, and in the BLA for α3-subunits. Using α-subunit knock-out (KO) mice, we found that BLA enrichment of α2- and α3-subunits were physiologically independent of each other, as the enrichment of one subunit was unaffected by the genomic deletion of the other. To further investigate the unique pattern of α-subunit enrichment in the BLA, we examined the association of α2- and α3-subunits with the presynaptic vesicular GABA transporter (vGAT) and the anchoring protein gephyrin (Geph). As expected, both α2- and α3-subunits on the AIS within the BLA received prominent GABAergic innervation from vGAT-positive terminals. Further, we found that the association of α2- and α3-subunits with Geph was weaker in AIS versus non-AIS locations, suggesting that Geph might be playing a lesser role in the enrichment of α2- and α3-subunits on the AIS. Overall, these observations suggest that GABAARs on the AIS differ in subunit composition across brain regions. As with somatodendritic GABAARs, the distinctive expression pattern of AIS-located GABAAR α-subunits in the BLA, and other brain areas, likely contribute to unique forms of GABAergic inhibitory transmission and pharmacological profiles seen in different brain areas.

Highlights

Gamma-amino butyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system and acts at synapses through binding to ligand-gated the GABA type A receptors (GABAARs)
Previous studies have shown that clusters of GABAARs containing the α2-subunits are preferentially targeted to the axon initial segment (AIS) compared to those containing the α1-subunits (Nusser et al, 1996)
While it is largely recognized that GABAARs containing α1and α2-subunits form clusters on the AIS, our results confirm that α3-subunits are located on the AIS across many brain area

Summary

Introduction

Gamma-amino butyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system and acts at synapses through binding to ligand-gated the GABA type A receptors (GABAARs). Postsynaptic GABAARs localized on synaptic and extrasynaptic membranes mediate phasic and tonic inhibition, respectively (Farrant and Nusser, 2005). Many of these postsynaptic receptors form symmetric synapses on the shaft. Axonal Initial Segment GABAA Receptors of dendrites or around the somata of a neurons In addition to these somatodendritic locations, GABAARs are located on the axon initial segment (AIS) of principal neurons where they are clustered in rows of synapses and targeted almost exclusively by inputs from chandelier interneurons (Soriano et al, 1990; Freund and Buzsaki, 1996; Woodruff et al, 2009; Inan and Anderson, 2014; Wefelmeyer et al, 2015). A predominantly depolarizing effect upon activation of AIS GABAARs has been shown in various brain regions, including pyramidal neurons in the neocortex (Szabadics et al, 2006), amygdala (Woodruff et al, 2006), and hippocampus (Khirug et al, 2008)

Methods

Results

Discussion

Conclusion