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

Abstract

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.

Functional architecture of the retina: Development and disease

“…In particular, asymmetric GABAergic inhibition is generated in the null-direction motion to generate direction-selectivity (Briggman et al, 2011; Fried et al, 2002; Zhou and Lee, 2008). Starburst amacrine cells in the mouse retina contact dendrites of DS retinal ganglion cells at synapses containing GABA A α2 receptors (Auferkorte et al, 2012). Another class of ganglion cells called ‘approach sensitive ganglion cells’ have recently been characterized in mouse retina, and their responses rely on rapid amacrine cell-mediated inhibition that suppresses response to non-approaching objects (Munch et al, 2009).…”

Mapping Synaptic Input Fields of Neurons with Super-Resolution Imaging

“…The substantial prior knowledge of the structure and function of this cell type allows validation of our method, while unresolved structural questions in this system provide an opportunity to test the ability of our approach to extract novel biological information. For example, On-Off DSGCs are known to receive asymmetric inhibitory GABAergic inputs from presynaptic starburst amacrine cells (SACs) during null-direction stimulus movement (Briggman et al, 2011; Fried et al, 2002; Wei et al, 2011), and the α2 subunit of GABA(A) receptor plays an important role in this direction selectivity (Auferkorte et al, 2012). In addition to GABAergic synapses, glycinergic signaling also impacts the response of On-Off DSGCs to the edges of moving stimuli (Caldwell et al, 1978; Jensen, 1999), likely reflecting crossover inhibition between the on and off sublaminae mediated by glycinergic amacrine cells (Kittila and Massey, 1995; Stasheff and Masland, 2002; Werblin, 2010).…”

Identification of Synaptic Patterns of NMDA Receptor Subtypes Upon Direction-Selective Rabbit Retinal Ganglion Cells

“…Compared to typical wild-type mice, mice with a knocked out alpha2 subunit of the GABA A receptor exhibited a remarkable drop in contrast in the light responses of DS RGCs. 46 Apart from the studies of GABAergic input, research on acetylcholine (ACh) release by SACs has also contributed significantly toward our understanding of directional selectivity. A recent study on these topics has suggested that co-released ACh and GABA may be key to implementing directional selectivity, because both ACh and GABA are released by SACs differentially in a Ca 2 +-dependent manner.…”