Summary Why do vertebrates use rods and cones that hyperpolarize, when in insect eyes a single depolarizing photoreceptor can function at all light levels [1, 2]? We answer this question at least in part with a comprehensive assessment of ATP consumption for mammalian rods from voltages and currents and recently published physiological and biochemical data. In darkness, rods consume 108 ATP s−1, about the same as Drosophila photoreceptors [3]. Ion fluxes associated with phototransduction and synaptic transmission dominate; as in CNS [4], the contribution of enzymes of the second-messenger cascade is surprisingly small. Suppression of rod responses in daylight closes light-gated channels and reduces total energy consumption by >75%, but in Drosophila light opens channels and increases consumption five-fold [5]. Rods therefore provide an energy-efficient mechanism for high-sensitivity vision in dim light not present in rhabdomeric photoreceptors. Rods are metabolically less “costly” than cones, since cones do not saturate in bright light [6, 7] and use more ATP s−1 for transducin activation [8] and rhodopsin phosphorylation [9]. This helps to explain why the vertebrate retina is duplex, and why some diurnal animals like primates have a small number of cones, concentrated in a region of high acuity.

Previous work established retinal expression of channelrhodopsin-2 (ChR2), an algal cation channel gated by light, restored physiological and behavioral visual responses in otherwise blind rd1 mice. However, a viable ChR2-based human therapy must meet several key criteria: (i) ChR2 expression must be targeted, robust, and long-term, (ii) ChR2 must provide long-term and continuous therapeutic efficacy, and (iii) both viral vector delivery and ChR2 expression must be safe. Here, we demonstrate the development of a clinically relevant therapy for late stage retinal degeneration using ChR2. We achieved specific and stable expression of ChR2 in ON bipolar cells using a recombinant adeno-associated viral vector (rAAV) packaged in a tyrosine-mutated capsid. Targeted expression led to ChR2-driven electrophysiological ON responses in postsynaptic retinal ganglion cells and significant improvement in visually guided behavior for multiple models of blindness up to 10 months postinjection. Light levels to elicit visually guided behavioral responses were within the physiological range of cone photoreceptors. Finally, chronic ChR2 expression was nontoxic, with transgene biodistribution limited to the eye. No measurable immune or inflammatory response was observed following intraocular vector administration. Together, these data indicate that virally delivered ChR2 can provide a viable and efficacious clinical therapy for photoreceptor disease-related blindness.

A 10 μm spot of argon laser light was focused onto the outer segments of intact mouse rods loaded with fluo‐3, fluo‐4 or fluo‐5F, to estimate dark, resting free Ca2+ concentration ([Ca2+]i) and changes in [Ca2+]i upon illumination. Dye concentration was adjusted to preserve the normal physiology of the rod, and the laser intensity was selected to minimise bleaching of the fluorescent dye. Wild‐type mouse rods illuminated continuously with laser light showed a progressive decrease in fluorescence well fitted by two exponentials with mean time constants of 154 and 540 ms. Rods from transducin α‐subunit knock‐out (Trα–/–) animals showed no light‐dependent decline in fluorescence but exhibited an initial rapid component of fluorescence increase which could be fitted with a single exponential (τ∼1–4 ms). This fluorescence increase was triggered by rhodopsin bleaching, since its amplitude was reduced by pre‐exposure to bright bleaching light and its time constant decreased with increasing laser intensity. The rapid component was however unaffected by incorporation of the calcium chelator BAPTA and seemed therefore not to reflect an actual increase in [Ca2+]i. A similar rapid increase in fluorescence was also seen in the rods of wild‐type mice just preceding the fall in fluorescence produced by the light‐dependent decrease in [Ca2+]i. Dissociation constants were measured in vitro for fluo‐3, fluo‐4 and fluo‐5F with and without 1 mm Mg2+ from 20 to 37 °C. All three dyes showed a strong temperature dependence, with the dissociation constant changing by a factor of 3–4 over this range. Values at 37 °C were used to estimate absolute levels of rod [Ca2+]i. All three dyes gave similar values for [Ca2+]i in wild‐type rods of 250 ± 20 nm in darkness and 23 ± 2 nm after exposure to saturating light. There was no significant difference in dark [Ca2+]i between wild‐type and Trα–/– animals.

Effective sensory processing requires matching the gain of neural responses to the range of signals encountered. For rod vision, gain controls operate at light levels at which photons arrive rarely at individual rods, light levels too low to cause adaptation in rod phototransduction. Under these conditions, adaptation within a conserved pathway in mammalian retina maintains sensitivity as light levels change. To relate retinal signals to behavioral work on detection at low light levels, we measured how background light affects the gain and noise of primate ganglion cells. To determine where and how gain is controlled, we tracked rod-mediated signals across the mouse retina. These experiments led to three main conclusions: (1) the primary site of adaptation at low light levels is the synapse between rod bipolar and AII amacrine cells; (2) cellular noise after the gain control is nearly independent of background intensity; and (3) at low backgrounds, noise in the circuitry, rather than rod noise or fluctuations in arriving photons, limits ganglion cell sensitivity. This work provides physiological insights into the rich history of experiments characterizing how rod vision avoids saturation as light levels increase.

Vision at absolute threshold is based on signals produced in a tiny fraction of the rod photoreceptors. This requires the rods to signal the absorption of single photons and the resulting signals to be transmitted across the retina and encoded in the activity sent from the retina to the brain. Behavioral and ganglion cell sensitivity has often been interpreted to indicate that these biophysical events occur effectively noiselessly -i.e. that vision reaches limits to sensitivity imposed by the division of light into discrete photons and occasional photon-like noise events generated in the rod photoreceptors. We will argue that this interpretation is not unique and provide a more conservative view of the constraints behavior and ganglion cell experiments impose on phototransduction and retinal processing. We will then summarize what is known about how these constraints are met and identify some of the outstanding open issues.