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

Although it is generally recognized that certain alpha-subunits of gamma-aminobutyric acid type A receptors (GABA(A)Rs) 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 alpha 1-, alpha 2-, or alpha 3-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 GABA(A)R alpha-subunits on the AIS. Significant enrichment was identified in the CA3 of hippocampus for alpha 1-subunits, in the CA1, CA3, and BLA for alpha 2-subunits, and in the BLA for alpha 3-subunits. Using alpha-subunit knock-out (KO) mice, we found that BLA enrichment of alpha 2- and alpha 3-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 alpha-subunit enrichment in the BLA, we examined the association of alpha 2- and alpha 3-subunits with the presynaptic vesicular GABA transporter (vGAT) and the anchoring protein gephyrin (Geph). As expected, both alpha 2- and alpha 3-subunits on the AIS within the BLA received prominent GABAergic innervation from vGAT-positive terminals. Further, we found that the association of alpha 2- and alpha 3-subunits with Geph was weaker in AIS versus non-AIS locations, suggesting that Geph might be playing a lesser role in the enrichment of alpha 2- and alpha 3-subunits on the AIS. Overall, these observations suggest that GABA(A)Rs on the AIS differ in subunit composition across brain regions. As with somatodendritic GABA(A)Rs, the distinctive expression pattern of AIS-located GABA(A)R alpha-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.