Intra-retinal visual cycle required for rapid and complete cone dark adaptation

Abstract

Daytime vision is mediated by retinal cones, which, unlike rods, remain functional even in bright light and dark-adapt rapidly. These cone properties are enabled by rapid regeneration of their pigment. This in turn requires rapid chromophore recycling that may not be achieved by the canonical retinal pigment epithelium visual cycle. Recent biochemical studies have suggested the presence of a second, cone-specific visual cycle, although its physiological function remains to be established. We found that the Müller cells in the salamander neural retina promote cone-specific pigment regeneration and dark adaptation that are independent of the pigment epithelium. Without this pathway, dark adaptation of cones was slow and incomplete. Notably, the rates of cone pigment regeneration by the retina and pigment epithelium visual cycles were essentially identical, suggesting a possible common rate-limiting step. Finally, we also observed cone dark adaptation in the isolated mouse retina.

Figure 1. Effect of bleach on pigment content in salamander photoreceptors in dissociated and intact retina

Shown are average absorbance spectra of cones from dissociated retina (a, n = 20), cones from intact retina (b, n = 20), and rods from intact retina (c, n = 10) under three different conditions: in dark-adapted state (left), 2 hours after a bleach (middle), and following treatment with exogenous 11-cis retinal (right). In all cases photoreceptors were bleached by identical 40 s white light. Optical density in dark, bleached and 11-cis treated conditions are 0.049 ± 0.004, 0.010 ± 0.002, and 0.042 ± 0.002 respectively for cones of dissociated retina, 0.040 ± 0.002, 0.039 ± 0.002, and 0.040 ± 0.002 for cones of intact retina, and 0.130 ± 0.009, 0.024 ± 0.003, and 0.149 ± 0.006 for rods of intact retina. The fraction of bleached pigment was 82% in rods of intact retina and 79% in cones from dissociated retina. Note the recovery of pigment content after bleach of cones from intact retina (b) but not of cones from dissociated retina (a) or rods from intact retina (c). Error bars give s.e.m.

Figure 2. Effect of bleach on sensitivity in salamander photoreceptors in dissociated and intact retina

Suction recordings of flash intensity-response families from single dissociated cones (a), cones from intact retina (b), and rods from intact retina (c). Cells were stimulated at time 0 with 20-ms flashes of intensity increasing in 0.5 log unit steps. Top panels show test flash responses from cells in dark-adapted state (left), following a 40 s white light bleach (middle), and after treatment with exogenous 11-cis retinal (right). For cones (a and b), red traces represent photoresponses to 6,550 photons µm⁻², 620 nm. For rods (c), red traces represent photoresponses to 119 photons µm⁻², 520 nm. Bottom panels show the corresponding intensity-response relation for each cell, fit with Michaelis-Menten function R/R_max = I/(I+I_O), where R/R_max is the normalized response amplitude, I is the flash intensity, and I_O is the intensity required to produce half-saturating response. Note the recovery of sensitivity of bleached cones from intact retina (b) but not of cones from dissociated retina (a) or rods from intact retina (c).

Figure 3. Effect of the Müller cell inhibitor L-α-AAA on the recovery of cone sensitivity following a bleach

Recordings from cones bleached in isolated intact retina (a) and in eyecup, with retina still attached to the pigment epithelium (b). All retinas were treated for 48 hours with 10 mM L-α-AAA and then transferred to Ringer prior to recordings. Red traces represent photoresponses to 6,550 photons µm⁻², 620 nm. Inhibiting the function of Müller cells blocked the recovery of sensitivity of cones from isolated retina but not of cones from eyecup following exposure to 40 s white bleaching light.

Figure 4. Rod and cone responses from salamander whole-retina ERG recordings

Background adaptation of isolated salamander retina (a), showing distinct rod and cone components. Data is fit with the Weber-Fechner relation S/S_DA = (1 + I_B/I_O)⁻¹, where S is the light-adapted sensitivity, S_DA is the dark-adapted sensitivity, I_B is the intensity of the background, and I_O is the background that reduced sensitivity to 0.5 S_DA. I_O was 0.94 photons µm⁻² s⁻¹ for rods and 4,840 photons µm⁻² s⁻¹ for cones. Insets show a combined rod and cone response in darkness (left), and a cone-only response in background saturating the rods (right). Time course of test flash and background under each trace. Bottom panels show rod (b, d) and cone (c, e) responses from one retina in darkness (top) and following a 40 s white light bleach (bottom). For each trace, a 20-ms flash was delivered at t = 0. Red traces represent photoresponses to 530 photons µm⁻², 520 nm for rods and 2,100 photons µm⁻², 620 nm for cones. Note the significant desensitization of rods and the recovery of sensitivity in cones from the same retina following the bleach. Error bars give s.e.m.

Figure 5. Kinetics of cone dark adaptation from whole-retina ERG recordings

(a) Recovery of salamander cone sensitivity driven by isolated retina in Ringer (black, n = 4) or following 48 hour treatment with L-α-AAA (red, n = 3). Cone sensitivity in Ringer recovered to 64% of its dark-adapted value, corresponding to regeneration of 93% cone pigment. In contrast, bleached cones from retina treated with L-α-AAA recovered only 3.2% of their dark-adapted sensitivity. (b) Recovery of rod sensitivity in eyecup in Ringer (black, n = 4) and following 48 hour treatment with L-α-AAA (red, n = 4). L-α- AAA did not affect the rate or final level of recovery of sensitivity of rods driven by the pigment epithelium. (c) Recovery of cone sensitivity in eyecup in Ringer (black, n = 5), driven by both retina and pigment epithelium visual cycles, or following 48 hour treatment with L-α-AAA (red, n = 5), driven by the pigment epithelium alone. The recovery of cone sensitivity was significantly accelerated and driven to completion by the addition of the retina visual cycle. (d) Recovery of cone pigment content, estimated from (c), using the relation between cone pigment loss and desensitization. The initial rate of pigment regeneration by the retina visual cycle (open circles) was estimated by subtracting the pigment regenerated by the pigment epithelium (red) from the total regenerated pigment (black). Note the comparable rates of pigment regeneration by the two cycles. In all cases identical 40 s white bleaching light was delivered at t = 0. Error bars give s.e.m.

Figure 6. Rod and cone responses from mouse whole-retina ERG recordings

Shown are rod (a, c) and cone (b, d) responses from one retina in darkness (top) and following a 40 s 9.7 ×10⁶ photons µm⁻² s⁻¹ 500 nm bleach (bottom). Test flashes of intensity increasing in 0.5 log unit steps were delivered at t = 0. Red traces represent photoresponses to 1,977 photons µm⁻² for rods and 22,850 photons µm⁻² for cones, both at 500 nm. Note the significant desensitization of rods and the recovery of sensitivity in cones from the same retina following the bleach.