Abstract
Purpose :
Starburst amacrine cells play a pivotal role in the generation of direction selectivity in the mammalian retina. More than a decade ago starburst dendrites were shown to exhibit direction selective properties, using two photon calcium imaging. However, the mechanisms underlying DS in starburst dendrites remain unclear. Recently, multiple mechanisms, including cell intrinsic and network properties have been hypothesized to mediate dendritic DS in starbursts, but these have received little experimental support. In this study, we aim to develop novel mouse models to test mechanisms underlying starburst DS
Methods :
Three major mechanisms have been proposed to underlie starburst DS. These rely on 1) intrinsic dendritic properties; 2) kinetically distinct bipolar cell inputs and 3) starburst-starburst GABAergic inhibition. To test the role of kinetically distinct bipolar cell inputs, we developed a system in which bipolar inputs are blocked using conventional pharmacology, and starbursts are directly activated using selective expression of ChR2. To test the role of starburst-starburst inhibition, GABAergic inputs were selectively compromised in starbursts using a conditional GABAAα2 receptor KO mouse. To isolate cell intrinsic mechanisms these manipulations were combined. DS in starburst amacrine cell dendrites was assayed by measuring light evoked IPSCs from downstream DSGCs; and 2P calcium imaging of starburst dendrites
Results :
We found that after blocking bipolar cell inputs using conventional pharmacology, stimulation of the starburst network by ChR2 evoked IPSCs in DSGCs that were highly DS. Interestingly, the DS inhibition was roughly similar to that measured in control conditions. Additionally, knocking-out the starburst-starburst GABAergic inputs did not significantly affect DS inhibition. Interestingly, however, when both manipulations were combined to abolish both bipolar inputs as well as starburst-starburst interactions, the resulting IPSCs were almost completely non-DS. These results for the first time demonstrate a method to block DS in starburst dendrites without grossly affecting their output
Conclusions :
Two network mechanisms including those that rely on different bipolar cell inputs and starburst-starburst inhibition appear to work in parallel to generate direction selectivity in starburst dendrites. Under the current experimental conditions, cell intrinsic DS mechanisms were not manifest
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.