ON‐pathway‐dominant glycinergic regulation of cholinergic amacrine cells in the mouse retina. (1st September 2014) - Ishii, Toshiyuki
Abstract Key points Starburst amacrine cells (SACs) in the retina play a key role for the formation of direction selectivity and considered a good model of neural computation. Although SACs distribute in both ON‐ and OFF‐pathway, only the physiological properties of SACs in ON‐pathway have been well characterized. As the physiological properties of SACs in the OFF‐pathway were not well known, in this study, we directly compared the inputs of SACs in ON‐ and OFF‐pathways. Here we report a novel finding that glycinergic inputs onto SACs were ON‐pathway dominant, while glutamatergic inputs were equivalent between ON‐ and OFF‐pathway. Our results suggest that fully defining the physiological properties of SACs in both ON‐ and OFF‐pathway will be critical to understanding and modelling direction selectivity in the retina. Abstract Direction selectivity in the retina has been studied as a model of dendritic computation of neural circuits. Starburst amacrine cells (SACs) have been examined as a model system of dendritic computation as they play a pivotal role in the formation of direction selectivity. Because the difference of anatomical location inside the retina made ON‐SACs an easier target to record, the biophysical properties of ON‐SACs have been used to predict those of OFF‐SACs. In this study, we systematically compared the responses of ON‐ and OFF‐SACs to the two principal neurotransmitters, glycine and glutamate. We found that responses to glycine were significantly larger in ON‐SACs than in OFF‐SACs. In contrast, ON‐ and OFF‐SACs responded similarly to glutamate. The amplitude of glycine responses in ON‐SACs increased after eye opening and the largest amplitude was observed at postnatal day 28. On the other hand, no increase in the amplitude of glycine responses in OFF‐SACs was observed until postnatal day 28. Glycine‐evoked currents were inhibited by the application of strychnine. Glutamate‐evoked currents were mimicked by the application of AMPA or kainite, and responses to N‐methyl‐d‐aspartate were observed in the absence of Mg2+ block. Glutamate‐evoked currents produced an increase in the frequency of GABAergic inhibitory postsynaptic currents. Our results suggest that signal processing in ON‐SACs cannot be directly used to understand the properties of OFF‐SACs. Therefore fully defining the physiological properties of OFF‐SACs will be critical to understanding and modelling direction selectivity in the retina.