Inner, not outer, retinal Cx36 governs absolute psychophysical threshold

Abstract

Purpose : Connexin36 (Cx36) is a major constituent of interneuronal gap junctions. In the inner retina, Cx36 couples AII amacrine cells to one another and to ON cone bipolar cells, while Cx36 couples rod and cone photoreceptors in the outer retina. Thus, Cx36 gap junctions have the capacity to govern the absolute psychophysical visual threshold. We directly assessed the impact of both inner and outer retinal Cx36 on the absolute threshold using two transgenic mice: a global Cx36 knockout mouse (KO; Deans et al. 2002) and a photoreceptor-specific Cx36 knockout mouse (cKO; Jin et al. 2022).

Methods : We compared thresholds of knock-out mice to their wild-type (WT) littermates. We measured absolute visual thresholds using an operant 1-alternative forced choice task (LaMagna et al. 2021). Stimuli were flashes of durations (t_flash) ranging from 0.06 s to 2 s. Statistical comparisons of thresholds between genotypes were performed using 2-way ANOVA (α = 0.05). Nonlinear regression was used to fit threshold-vs-t_flash models. AIC were scores were calculated to compare model fits.

Results : At 0.06 s, the average threshold of all WT mice (n = 10) was -1.4 ± 0.3 log(R*/rod/s) decreasing to -3.4 ± 0.1 log(R*/rod/s) at 0.5 s. From 0.5 s to 2 s, thresholds remained constant. Strikingly, we found no significant difference (p = 0.16) between thresholds of cKO mice (n = 7) and their WT littermates (n = 4). In contrast, KO (n = 8) thresholds were significantly elevated (p = 0.005) compared to their WT littermates (n = 6) by 0.96±0.1 log(R*/rod/s). For mechanistic insight concerning this elevation, we modelled threshold-vs-t_flash as a chain of exponential decays, characterized by three parameters: the number of stages in the chain (N), a scaling parameter (I₀), and a time constant (t_c). In our best fit model, I₀ is the only parameter dependent on genotype: WT I₀ = -3.7 [-3.9, -3.4] log(R*/rod/s), and KO I₀ = -2.7 [-3.0, -2.4] log(R*/rod/s), with N = 3.6 [1.9, 5.4] and t_c = 0.22 [0.05, 0.38] s.

Conclusions : We conclude that inner, not outer, retinal Cx36 governs absolute psychophysical threshold. Our model explains how the loss of sensitivity in the KO ganglion cell (Deans et al. 2002) influences the visual threshold. Specifically, loss of inner-retinal Cx36 results in a change in scaling factor that manifests as a rise in the absolute threshold without affecting temporal processing.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.

 

Average log-threshold intensities (±SE) and model fits.