GABAA Receptors Containing the α2 Subunit Are Critical for Direction-Selective Inhibition in the Retina

Far from being a simple sensor, the retina actively participates in processing visual signals. One of the best understood aspects of this processing is the detection of motion direction. Direction-selective (DS) retinal circuits include several subtypes of ganglion cells (GCs) and inhibitory interneurons, such as starburst amacrine cells (SACs). Recent studies demonstrated a surprising complexity in the arrangement of synapses in the DS circuit, i.e. between SACs and DS ganglion cells. Thus, to fully understand retinal DS mechanisms, detailed knowledge of all synaptic elements involved, particularly the nature and localization of neurotransmitter receptors, is needed. Since inhibition from SACs onto DSGCs is crucial for generating retinal direction selectivity, we investigate here the nature of the GABA receptors mediating this interaction. We found that in the inner plexiform layer (IPL) of mouse and rabbit retina, GABAA receptor subunit α2 (GABAAR α2) aggregated in synaptic clusters along two bands overlapping the dendritic plexuses of both ON and OFF SACs. On distal dendrites of individually labeled SACs in rabbit, GABAAR α2 was aligned with the majority of varicosities, the cell's output structures, and found postsynaptically on DSGC dendrites, both in the ON and OFF portion of the IPL. In GABAAR α2 knock-out (KO) mice, light responses of retinal GCs recorded with two-photon calcium imaging revealed a significant impairment of DS responses compared to their wild-type littermates. We observed a dramatic drop in the proportion of cells exhibiting DS phenotype in both the ON and ON-OFF populations, which strongly supports our anatomical findings that α2-containing GABAARs are critical for mediating retinal DS inhibition. Our study reveals for the first time, to the best of our knowledge, the precise functional localization of a specific receptor subunit in the retinal DS circuit.

视网膜绝非仅为简单的感光传感器，而是主动参与视觉信号的处理过程。其中，人们对该处理过程中认知最为透彻的环节之一，便是运动方向的检测。方向选择性（direction-selective, DS）视网膜回路包含多种亚型的神经节细胞（ganglion cells, GCs）以及抑制性中间神经元，例如星爆无长突细胞（starburst amacrine cells, SACs）。近期研究表明，方向选择性回路中的突触排布——即星爆无长突细胞与方向选择性神经节细胞之间的突触连接——存在出人意料的复杂性。因此，若要全面解析视网膜方向选择性机制，需掌握所涉所有突触元件的详细信息，尤其是神经递质受体的本质与定位。由于星爆无长突细胞向方向选择性神经节细胞（DS ganglion cells, DSGCs）施加的抑制作用，对于视网膜方向选择性的产生至关重要，本研究聚焦于介导这一相互作用的γ-氨基丁酸A型受体的本质。我们发现，在小鼠与家兔视网膜的内网状层（inner plexiform layer, IPL）中，γ-氨基丁酸A型受体α2亚基（GABAA receptor subunit α2, GABAAR α2）会沿着两条条带聚集为突触簇，该条带与ON型与OFF型星爆无长突细胞的树突丛均存在重叠。在家兔单个标记的星爆无长突细胞的远端树突上，GABAA受体α2亚基与大部分曲张体——该细胞的输出结构——对齐分布，同时在方向选择性神经节细胞的树突上（即内网状层的ON与OFF区域）均检测到该亚基的突触后定位。在GABAA受体α2亚基敲除（knock-out, KO）小鼠中，通过双光子钙成像记录到的视网膜神经节细胞的光响应显示，与同窝野生型小鼠相比，其方向选择性响应出现显著损伤。我们观察到，ON型与ON-OFF型群体中呈现方向选择性表型的细胞比例均大幅下降，这一结果有力佐证了我们的解剖学发现：含有α2亚基的γ-氨基丁酸A型受体，对于介导视网膜方向选择性抑制作用至关重要。据我们所知，本研究首次揭示了特定受体亚基在视网膜方向选择性回路中精准的功能性定位。