Time course of EPSCs in ON‐type starburst amacrine cells is independent of dendritic location

Abstract

Direction selectivity in the retina relies critically on directionally asymmetric GABA release from the dendritic tips of starburst amacrine cells (SBACs). GABA release from each radially directed dendrite is larger for motion outward from the soma toward the dendritic tips than for motion inwards toward the soma. The biophysical mechanisms generating these directional signals remain controversial. A model based on electron-microscopic reconstructions of the mouse retina proposed that an ordered arrangement of kinetically distinct bipolar cell inputs to ON- and OFF-type SBACs could produce directional GABA release. We tested this prediction by measuring the time course of EPSCs in ON-type SBACs in the mouse retina, activated by proximal and distal light stimulation. Contrary to the prediction, the kinetics of the excitatory inputs were independent of dendritic location. Computer simulations based on 3D reconstructions of SBAC dendrites demonstrated that the response kinetics of distal inputs were not significantly altered by dendritic filtering. These direct physiological measurements, do not support the hypothesis that directional signals in SBACs arise from the ordered arrangement of kinetically distinct bipolar cell inputs.

Directional excitatory input to direction‐selective ganglion cells in the rabbit retina

“…We also estimated the electrode series resistance ( R s ) and the electrode capacitance ( C e ). To obtain these estimates, we measured passive charging curves from each cell in response to ±5 mV voltage steps from a holding potential of −70 mV (see Stincic et al, ). We then fitted model charging curves to the real charging curves using a Levenberg‐Marquardt (LM) least‐squares procedure to iteratively adjust the above parameters.…”

“…We then fitted model charging curves to the real charging curves using a Levenberg‐Marquardt (LM) least‐squares procedure to iteratively adjust the above parameters. Convergence on the best fit required 50–300 model runs (Stincic et al, ). The ranges for the best fit parameter values for the five cells were: R i = 76–120 Ω cm, R m = 42,000–83,000 Ω cm 2 , C m = 0.6–0.9 μF/cm 2 , and R s = 9–23 MΩ.…”

Parallel Computations in Insect and Mammalian Visual Motion Processing

“…The direction-selective signals in starburst amacrine cell neurites appear to amplify centrifugal motion (radially outwards) signals cell-autonomously but also show suppression to centripetal motion (radially inwards) signals due to mutually inhibitory GABAergic connections between starburst amacrine cells [102,1108–111]. There may also be a direction-selective contribution to starburst amacrine cell tuning mediated by the temporal order of sustained and transient bipolar cell synapses, at least in the case of OFF starburst amacrine cells [112–114]. The final step in computing direction selectivity occurs in the connections between starburst amacrine cells and direction-selective retinal ganglion cells.…”

Cross-compartmental Modulation of Dendritic Signals for Retinal Direction Selectivity

“…The radial branches of a SAC are considered independent computational units that individually prefer centrifugal motion, from the soma to their distal tips (Euler et al, 2002; Hausselt et al, 2007; Miller and Bloomfield, 1983; Velte and Miller, 1997) (Figure 1B). This centrifugal preference has been attributed to multiple cell intrinsic and synaptic mechanisms, including the distribution of active conductances (Euler et al, 2002; Hausselt et al, 2007; Oesch and Taylor, 2010) and chloride transporters (Gavrikov et al, 2003) along the SAC dendrites, proximal targeting of glutamatergic inputs (Vlasits et al, 2016), segregation of excitatory inputs from distinct bipolar cells (Fransen and Borghuis, 2017; Kim et al, 2014; but see Stincic et al, 2016), and lateral inhibition from neighboring SACs and non-SAC amacrine cells (Chen et al, 2016; Ding et al, 2016; Lee and Zhou, 2006). …”