Gao Yudong, Heldt Scott A
Department of Anatomy and Neurobiology, The University of Tennessee Health Science Center, Memphis TN, USA.
Front Cell Neurosci. 2016 Mar 1;10:39. doi: 10.3389/fncel.2016.00039. eCollection 2016.
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.
尽管人们普遍认识到γ-氨基丁酸A型受体（GABAARs）的某些α亚基在轴突起始段（AIS）形成富集簇，但这些簇在不同脑区的变化程度尚不清楚。在当前研究中，我们量化了与AIS标记物锚蛋白G共定位的荧光标记的α1、α2或α3亚基聚集体的密度、大小和富集率，并将它们与包括海马亚区、基底外侧杏仁核（BLA）、前额叶皮层（PFC）和感觉皮层（CTX）在内的不同脑区非AIS位置的聚集体进行比较。我们发现AIS上GABAARα亚基的富集存在区域差异。在海马体的CA3区发现α1亚基有显著富集，在CA1、CA3和BLA区发现α2亚基有显著富集，在BLA区发现α3亚基有显著富集。使用α亚基敲除（KO）小鼠，我们发现BLA区α2和α3亚基的富集在生理上相互独立，因为一个亚基的富集不受另一个亚基基因缺失的影响。为了进一步研究BLA区α亚基富集的独特模式，我们检查了α2和α3亚基与突触前囊泡GABA转运体（vGAT）和锚定蛋白桥连蛋白（Geph）的关联。正如预期的那样，BLA区内AIS上的α2和α3亚基都接受了来自vGAT阳性终末的显著GABA能神经支配。此外，我们发现α2和α3亚基与Geph在AIS位置与非AIS位置的关联较弱，这表明Geph在AIS上α2和α3亚基的富集中可能起较小作用。总体而言，这些观察结果表明AIS上的GABAARs在不同脑区的亚基组成不同。与树突体GABAARs一样，BLA区及其他脑区中AIS定位的GABAARα亚基的独特表达模式可能有助于不同脑区中独特形式的GABA能抑制性传递和药理学特征。
