Figure 2.
Rod-mediated ganglion cell responses in the Gnat2cpfl3 retina show higher than expected sensitivity to UV light. A1, ON ganglion cell spike responses in the balancing experiment. The green light was turned off for 200 ms as the UV light was turned on to different intensities (dimmest to brightest; trials 1 to 12). When the UV light matched the green light in photoisomeration rate (photoisomerizations per rod per second), there was no response at the transition. When the UV light generated fewer photoisomerizations than the green, there was an “off” response (decreased firing rate), whereas when the UV light generated more photoisomerizations than the green, there was an “on” response (increased firing rate). Responses to UV light onset and offset were measured during times indicated by the magenta and green boxes, respectively. A2, The change in firing rate, from the maintained rate, is plotted versus the estimated ratio of photoisomerizations to the green versus UV lights according to the rhodopsin template (Govardovskii et al., 2000). Responses are from the single set of trials plotted in A1. Responses during the transition (magenta) and afterward (green; boxed responses in A1.) were fit with polynomials. The balance point refers to the intersection of the fitted lines. A3, Histogram of balance points across cells (n = 13). The average balance point was 1.52 ± 0.03 (mean ± SEM), indicating that rhodopsin is ∼52% more sensitive to UV light than predicted by the template. The balance point for each cell was averaged over repeated measurements (typically 2–3 repeats). B1, ON ganglion cell responses to brief (20 ms) green and UV light flashes of variable intensity (dimmest to brightest; trials 1 to 12). Responses were measured during the period indicated by the magenta and green boxes. B2, The firing rate, above the maintained rate, following each flash (see boxed responses in B1) and averaged over two repeats. Responses to UV and green light are each plotted versus the estimated photoisomerizations per rod (Govardovskii et al., 2000) and fit with a Naka–Rushton equation (see Materials and Methods) that differed only in the half-saturation constant (σ). This cell showed higher sensitivity to UV light than to green light. B3, Histogram of relative σ values for green versus UV light. The average ratio was 1.48 ± 0.08, consistent with the balancing experiment. For each cell, responses were typically averaged over two repeats. C, Rhodopsin template (Govardovskii et al., 2000) is plotted on previously published ERG measurements from the mouse eye in vivo (dark-adapted, rod-mediated A wave) (Lyubarsky et al., 1999). The ERG measurements are consistent with a ∼50% elevation in UV light sensitivity relative to the template (green dashed line showing 1.5-fold increase), consistent with the above results. Data from Lyubarsky et al. (1999) are reproduced with permission from the Society for Neuroscience and the senior author of the article (Dr. E. N. Pugh). D, Ganglion cell responses to UV and green light align after correcting for rhodopsin UV sensitivity. Responses were normalized to the maximum firing rate and averaged across cells. Error bars indicate ±1 SD across cells.