Abstract
Purpose :
To dissect how the different rod pathways contribute to the light-adapted flicker ERG in mouse retina.
Methods :
Full-field flicker ERGs were elicited with sine-wave stimuli of constant contrast (100%) and flicker frequencies of 8 and 16 Hz. Mean luminance was the independent variable (range 10-4 to 103 cd/m2). We compared the magnitude of the fundamental response of wild-type C57BL/6J mice to those with no functional cones (Gnat2 KO, Ronning et al., 2018) or no functional rods (Gnat1 KO, Calvert et al., 2000 phototransduction), without Cx36 gap junctions in their inner and outer retinas (Cx36 KO, Deans et al., 2002), without Cx36 between rods and cones (conditional Cone-Cx36 XO mice, Jin et al., 2020), and with deficient rod to rod bipolar synapses (ELFN1 KO, Cao et al., 2015). We applied unpaired t-tests to compare magnitudes at peak luminance (n = 4 mice per genetic line).
Results :
In WT mice, 8 Hz flicker elicited response magnitudes with two distinct peaks: at low (0.1 cd/m2) and high (100 cd/m2) luminance values (Lei, 2012, Nusinowitz et al., 2007). The responses of Gnat2 KO mice exhibited a single peak at 0.1 cd/m2 while the responses of Gnat1 KO mice exhibited a single peak at 100 cd/m2. These results are consistent with the notion that the flicker responses at the low and high luminance peaks are driven largely by rods and cones, respectively. For Cx36-mediated signals, the low luminance peak decreased from 47 ± 8 μV in WT to 14 ± 3 μV in pan Cx36 KO mice (p=0.029), but was unchanged in Cone-Cx36 XO mice (p>0.05). These results suggest that Cx36 gap junctions in the inner but not in the outer retina shape the flicker responses. In ELFN1 KO mice, the magnitude of the low luminance peak was attenuated to 8 ± 3μV (p=0.009), indicating that the flicker responses at the peak are mediated largely, but not exclusively, by the rod to rod bipolar synapses of the primary rod pathway.
Conclusions :
We show that the primary rod pathway determines the low luminance response, mediated by rod-to-rod bipolar synapses and inner retina Cx36 gap junctions. The contributions of the secondary rod pathway appear minimal, since the response is independent of rod-cone Cx36 gap junctions. This may reflect closure of the gap junctions at the mean luminance levels of the flicker. The involvement of the tertiary rod pathway is suggested by the attenuated responses observed in ELFN1 KO, reflecting rod to cone OFF bipolars signaling as proposed by Bo Lei (2012).
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.