GABA-mediated spatial and temporal asymmetries that contribute to the directionally selective light responses of starburst amacrine cells in retina.

Starburst amacrine cells (SACs) are an essential component of the mechanism that generates direction selectivity in the retina. SACs exhibit opposite polarity, directionally selective (DS) light responses, depolarizing to stimuli that move centrifugally away from the cell through the receptive field surround, but hyperpolarizing to stimuli that move centripetally towards the cell through the surround.Recent findings suggest that (1) the intracellular chloride concentration (Cl(−)) is high in SAC proximal, but low in SAC distal dendritic compartments, so that GABA depolarizes and hyperpolarizes the proximal and distal compartments, respectively, and (2) this Cl(−) gradient plays an essential role in generating SAC DS light responses. Employing a biophysically realistic, computational model of SACs, which incorporated experimental measurements of SAC electrical properties and GABA and glutamate responses, we further investigated whether and how a Cl(−) gradient along SAC dendrites produces their DS responses. Our computational analysis suggests that robust DS light responses would be generated in both the SAC soma and distal dendrites if (1) the Cl(−) equilibrium potential is more positive in the proximal dendrite and more negative in the distal dendrite than the resting membrane potential, so that GABA depolarizes and hyperpolarizes the proximal and distal compartments, respectively, and (2) the GABA-evoked increase in the Cl(−) conductance lasts longer than the glutamate-evoked increase in cation conductance. The combination of these two specific GABA-associated spatial and temporal asymmetries, in conjunction with symmetric glutamate excitation, may underlie the opposite polarity, DS light responses of SACs.