Abstract
Purpose :
Starburst amacrine cells (SACs) are key neurons for retinal motion detection, communicating object motion information to postsynaptic neurons by releasing both GABA and acetylcholine (ACh). The role of SAC ACh release is not yet fully understood. We previously found that a subset of bipolar cells (BCs) express α7 nicotinic ACh receptors (α7-nAChRs). Because SACs are the only source of ACh in the retina, we examined whether these bipolar cells contribute to direction selectivity in SACs.
Methods :
To identify α7-nAChR expressing ON-BCs, immunohistochemistry was performed using an α7-nAChR marker (α-bungarotoxin) in conjunction with BC type-specific markers including Gus-GFP transgenic mice (Type 7 BCs). Patch clamp recordings were also conducted from BCs while applying PNU282987 puff, an α7-nAChR agonist. We then generated a conditional α7-nAChR knockout from Type 7 BCs using a modified Cre-Lox system, Cre-DOG (dependent-on-GFP). Finally, patch clamp recordings were made from SACs in retinas of wildtype and the α7-knockout mice while giving a moving annulus light stimulus.
Results :
We found that Type 7 ON-BCs, but not other ON-BCs, were positive for α-bungarotoxin staining, indicating that they express α7-nAChRs. Type 7 BCs depolarized to PNU282987 application, confirming that these receptors are active. As Type 7 BCs provide input to ON-SACs, we generated mice of which α7-nAChRs were eliminated from Type 7 BCs to ascertain their role in SAC direction selectivity. In these mice, α-bungarotoxin no longer stained Type 7 BCs. Patch clamp recordings from ON-SACs revealed that preferred direction stimulation excited SACs greater than null direction stimulation in wildtype mice, but this difference was significantly reduced in the Type 7 BC α7-nAChR knockout mice.
Conclusions :
Bipolar cells have thus far been omitted from retinal neural circuits for direction selectivity. We found that α7-nAChRs are expressed in Type 7 BCs, which have been recently shown to provide synaptic inputs to SACs in proximal dendrites. When α7-nAChRs are knocked out of Type 7 BCs, SACs’ ability to distinguish between preferred and null direction object motion is significantly reduced. We have shown for the first time that acetylcholine receptors in a subset of bipolar cells contribute to SACs’ direction selectivity.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.