Long-distance object motion evokes direction selectivity in retinal neurons through nicotinic receptors in bipolar...

In the retina, the direction of a moving object is encoded by starburst amacrine cells (SACs) and ON-OFF direction-selective ganglion cells (ooDSGCs). Multiple neural circuits have accounted for the underlying mechanisms of direction selectivity. We previously found that the cholinergic feedback circuit through α7-nicotinic acetylcholine receptors (α7-nAChRs) in bipolar cells mediates direction selectivity in SACs. We investigated whether distinct visual stimuli differently affected cholinergic feedback through α7-nAChRs. We conducted two-photon Ca²⁺ imaging using Ca²⁺ indicators of Oregon Green BAPTA-1 for SACs and GCaMP6s/f for ooDSGCs. We generated an AAV viral vector inserting the ProD1 promoter and GCaMP6. Ca²⁺ transients were recorded in response to "long" distance and "local" traveling bars. The involvement of nAChRs was assessed using an α7-nAChR antagonist, methyllycaconitine (MLA), and a non-α7-nAChR antagonist, hexamethonium (HEX). The intraocular injection of the ProD1-AAV into C57BL/6J mouse eyes resulted in GCaMP6 expression only by ooDSGCs. The "long" stimulus elicited direction selectivity in ooDSGCs higher than the "local" stimulus did, independent of motion speeds. In SACs, a "long" stimulus also evoked higher direction selectivity than a "local" stimulus. Pharmacological dissection revealed that MLA reduced "long"-evoked direction selectivity, whereas HEX had no effect on SAC direction selectivity. Overall, we found that the long-distance traveling stimulus activates outside of receptive fields for SACs and ooDSGCs to enhance direction selectivity for those cells. In conjunction with our previous results, the α7-nAChRs in bipolar cells likely mediate the signals of a long-distance moving bar to enhance direction selectivity in SACs and ooDSGCs.NEW & NOTEWORTHY Detecting the direction of a moving object is one of the critical functions for retinal neurons. We found that a long-distance traveling moving stimulus evoked an increased direction selectivity in retinal third-order neurons, even though the stimulus activated outside of their receptive fields. Pharmacological examinations revealed that cholinergic feedback from starburst amacrine cells to bipolar cells is the underlying mechanism. Our findings will expand on the classic concept of the receptive field of neurons.