Abstract
Purpose :
Motion detection is performed by a unique retinal neural network comprised principally of starburst amacrine cells (SACs) and direction selective ganglion cells (DSGCs). Underlying direction selective mechanisms, however, remain unclear. We recently found that some bipolar cells (BCs), including type 7 BCs, express a7 nicotinic acetylcholine receptors (a7 nAChRs). As SACs are the sole source of acetylcholine release, BCs expressing a7 nAChRs likely play a role in motion detection. We tested this hypothesis by patch clamp recording from ON-OFF DSGCs (ooDSGCs), and by assessing the optomotor response (OMR).
Methods :
Patch clamp recordings were conducted from BCs in transgenic mice where channelrhodopsin-2 is expressed by SACs (Ai32-ChAT-cre line), and from ooDSGCs in C57BL/6J and mutant mice of which a7 nAChRs were eliminated from type-7 BCs using cre-loxP and cre-DOG (Cre-dependent-on-GFP) technologies. An antagonist of a7 nAChRs, MLA, was bath applied. Direction selectivity was assessed by calculating the direction selectivity index (DSI) and vector sum. OMRs were measured, allowing for assessment of contrast sensitivity and the acuity.
Results :
Using wholemount retinal preparation from Ai32-ChAT-cre mice, a bright light stimulus in the presence of glutamate receptor antagonists evoked inward currents in type 7 BCs. The currents were eliminated with MLA, confirming that these BCs receive acetylcholine-mediated synaptic inputs from SACs. Then, we recorded moving light-evoked spiking and inhibitory currents (IPSCs) from ooDSGCs. In the mutant mice, immunohistochemistry confirmed that a great majority of a7 nAChRs were eliminated from type 7 BCs. Direction selectivity of ON spiking in mutant ooDSGCs was significantly reduced compared to those in C57 mice. In addition, direction selectivity measured by IPSCs in C57 mice was reduced by MLA application. These results indicate that a7 nAChRs in type 7 BCs contribute to direction selectivity in ooDSGCs. OMR revealed that the acuity was decreased in the mutant mice compared to the one in control mice.
Conclusions :
With respect to the motion detection circuit, bipolar cell contributions have not been understood. We demonstrated for the first time that acetylcholine receptors in a subset of bipolar cells are critical for direction selectivity in retinal ganglion cells, as evidenced via cellular recordings and in vivo behavioral studies.
This is a 2020 ARVO Annual Meeting abstract.