Abstract
Purpose :
M1 intrinsically photosensitive retinal ganglion cells (ipRGCs) are involved in subconscious behaviors while M2-M5 ipRGCs are involved in conscious vision. Despite these differing behavioral influences of M1 vs. M2-M5 ipRGCs, it has been widely assumed that melanopsin phototransduction acts through similar pathways to exert similar physiological influences across ipRGC subtypes. This study examines the phototransduction cascade of M4 ipRGCs and resulting physiological consequences of melanopsin signaling.
Methods :
We utilized electrophysiological recordings of WT or melanopsin null M4 ipRGCs and measured contrast sensitivity or light responses in light intensities ranging from 9-12 log quanta/cm2/s). Deficits in melanopsin null M4 cells were rescued via expression of Gq-DREADDs (AAV2-hSyn-DIO-hM3D(Gq DREADD)-mCherry) in melanopsin null M4 cells exposed to 10 nM clozapine N-oxide (CNO) to activate the Gq pathway in vitro.
Results :
We measured the degree and sensitivity of melanopsin phototransduction's influence on membrane potential and excitability of M4 ipRGCs in background light and found that M4 ipRGCs show a melanopsin-dependent depolarization and increase in intrinsic excitability from scotopic to photopic light intensities. These melanopsin-dependent effects can be mimicked by acute activation of the Gq cascade via Gq-DREADDs in melanopsin null M4 ipRGCs. Surprisingly, melanopsin phototransduction actually decreased the excitability of M1 ipRGCs, suggesting that these two cell types may utilize different phototransduction cascades for melanopsin. In support of this, TRPC3/6/7 null retinas had a complete loss of intrinsic photosensitivity in M1 ipRGC, but no change in the intrinsic light-evoked current in M4 ipRGCs. Moreover, voltage clamp experiments suggest that melanopsin phototransduction in M4 ipRGCs results in a closure of potassium channels. We then examined the physiological consequences for cell signaling in M4 ipRGCs that lack melanopsin phototransduction, and found that melanopsin null M4 cells show decreased contrast sensitivity from scotopic to photopic light intensities.
Conclusions :
This study identifies potassium channels as a novel target of the melanopsin phototransduction cascade in M4 ipRGCs and show that melanopsin is repurposed by different ipRGC subtypes to shape their downstream visual behaviors.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.