Dissecting Rod and Cone Inputs to OFF Transient Alpha Ganglion Cells

Abstract

Purpose : Rod signals can reach retinal ganglion cells (RGCs) through different pathways. It has been proposed that the primary rod bipolar cell [RBC] pathway is the most sensitive and that the secondary (rod-to-cone) pathway operates under intermediate (mesopic) lighting conditions. However, this has never been demonstrated. In addition, the contribution of the tertiary (rod-to-OFF BC) pathway to RGCs remains unknown. Here, we sought to determine the contribution of the different rod and cone inputs to one specific RGC type: the OFF transient alpha RGC (tOFFa RGC), chosen because the primary pathway can be blocked with APB.

Methods : We used mice that lack connexinCx36 (Cx36) specifically in cones or in rods and therefore a secondary rod pathway. Isolated retinas were maintained by perfusion and the light responses of single alpha RGCs were recorded using a loose-patch technique. tOFFa RGCs were identified based on (i) light responses and (ii) morphology and stratification following neurobiotin injection and histochemistry.

Results : In WT littermates, tOFFa RGC threshold was ~ 0.001 R*/rod/s (1 photoisomerization per 1,000 rods/s). Application of APB, an mGluR6 agonist that selectively blocks the primary rod pathway shifted the response-intensity curve by ~ 2 log units to the right, consistent with the primary rod pathway being the most sensitive pathway. In the cone-specific Cx36 knock out, tOFFa RGCs had the same sensitivity, suggesting the primary rod pathway was unchanged. However, APB caused a rightward shift of ~ 3 log units due to the block of the primary pathway by APB combined with the absence of the secondary rod pathway. We do not have a selective tool to block the tertiary rod pathway. However, we can estimate the cone threshold by recording from ON alpha RGCs in the pan-Cx36 knock out mice, which have no primary pathway due to the absence of AII/bipolar gap junctions and no secondary pathway due to the absence of rod/cone gap junctions. ON bipolar cells do not make direct contacts with rods so there is no tertiary ON pathway. This approach yielded the cone threshold at ~ 30 R*/rod/s.

Conclusions : tOFFa RGCs receive convergent inputs from all rod and cone pathways. The inputs have different thresholds with the threshold of the primary rod pathway (~ 0.001 R*/rod/s) < secondary (~ 0.1 R*/rod/s) < tertiary (~ 1 R*/rod/s) < cone pathway (~ 30 R*/rod/s).

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.