Furthermore, in combination with learn more the SAC ablation experiments (Amthor et al.,
2002 and Yoshida et al., 2001), it suggested that SACs are a major source for this GABAergic inhibition. While a contribution from other GABAergic amacrine cells cannot be excluded, it seems unlikely that they fit the role of SACs because they typically are not numerous enough to provide the different functional subtypes of DS ganglion cells with adequate input (reviewed in Vaney et al., 2001). This lack of cells may, however, be compensated for by highly localized dendritic processing. To distinguish between different mechanistic hypotheses of direction selectivity, it is important to differentiate between directionally tuned (Figure 5B) and spatially offset inhibition (Figure 5D). While both originate, at least in part, in SACs, they represent different aspects of the DS computation. Spatially offset inhibition as such can be sufficient to render ganglion cell responses DS (Figures 5D and 5E; Borg-Graham and Grzywacz, 1992, Koch et al., 1983 and Barlow and Levick, 1965). Spatially
offset inhibition is a recurrent theme in the retinal DS circuit (Fried et al., 2005). It acts not only at the level of the ganglion cell, but possibly also at the bipolar cell terminals presynaptic to the DS ganglion cells (Figure 5E) and possibly between SACs. The first evidence for DS ganglion Lumacaftor nmr cells receiving spatially offset inhibition from SACs as a consequence of asymmetrical wiring came from paired recordings. These revealed that SACs located on the null side of a ganglion cell provide significantly stronger
inhibition than SACs located on its preferred side (Fried et al., 2002, Lee et al., 2010 and Wei et al., 2011). The anatomical correlate for such asymmetrical wiring has been elusive, since neither the DS cell’s morphology (Amthor et al., 1984) nor the distribution of synaptic inputs (Jeon et al., 2002) allowed researchers to predict its preferred direction. While complex synaptic arrangements between SACs and DS ganglion cells have been described at the ultrastructural level (Dacheux et al., 2003), very recently, a new EM technique for recording tissue volumes of sufficient size and GBA3 resolution (Briggman and Denk, 2006) allowed researchers to statistically analyze the SAC-DS ganglion cell connectivity for the first time (Figure 5D). In their elegant study, Briggman et al. (2011) revealed two important circuit properties: (1) The great majority of SAC synapses a DS ganglion cell receives come from SAC dendrites that roughly point in the ganglion cell’s null direction, meaning that the respective SAC somata are located preferentially on the ganglion cell’s null side. (2) SACs provide input to all DS ganglion cells within their dendritic field but only via dendrites pointing in the null direction of the respective ganglion cell.
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