ON-pathway-dominant glycinergic regulation of cholinergic amacrine cells in the mouse retina

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 Mg(2+) 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.