The functional organization of excitation and inhibition in the dendritic arbors of retinal direction-selective ganglion cells

Recent studies indicate that the precise timing and location of excitation
and inhibition (E/I) within active dendritic trees can significantly
impact neuronal function. How excitatory and inhibitory inputs are
functionally organized at the subcellular level in intact circuits remains
unclear. To address this issue, we took advantage of the retinal
direction-selective ganglion cell circuit, in which directionally tuned
inhibitory GABAergic input arising from starburst amacrine cells shape
direction-selective dendritic responses. We combined two-photon calcium
imaging with genetic, pharmacological, and single-cell ablation methods to
examine local E/I. We demonstrate that when active dendritic conductances
are blocked, direction selectivity emerges semi-independently within
unusually small dendritic segments (<10 µm). Impressively, the
direction encoded by each segment is relatively homogenous throughout the
ganglion cell’s dendritic tree. Together the results demonstrate a precise
subcellular functional organization of excitatory and inhibitory input,
which suggests that the parallel processing scheme proposed for direction
encoding could be more fine-grained than previously envisioned.