Changes in the light-sensitive current of salamander rods upon manipulation of putative pH-regulating mechanisms in the inner and outer segment

Bicarbonate boosts flash response amplitude to augment absolute sensitivity and extend dynamic range in murine retinal rods

Rod photoreceptors in the retina adjust their responsiveness and sensitivity so that they can continue to provide meaningful information over a wide range of light intensities. By stimulating membrane guanylate cyclases in the outer segment to synthesize cGMP at a faster rate in a Ca²⁺-dependent fashion, bicarbonate increases the circulating "dark" current and accelerates flash response kinetics in amphibian rods. Compared to amphibian rods, mammalian rods are smaller in size, operate at a higher temperature, and express visual cascade proteins with somewhat different biochemical properties. Here, we evaluated the role of bicarbonate in rods of cpfl3 mice. These mice are deficient in their expression of functional cone transducin, Gnat2, making cones very insensitive to light, so the rod response to light could be observed in isolation in electroretinogram recordings. Bicarbonate increased the dark current and absolute sensitivity and quickened flash response recovery in mouse rods to a greater extent than in amphibian rods. In addition, bicarbonate enabled mouse rods to respond over a range that extended to dimmer flashes. Larger flash responses may have resulted in part from a bicarbonate-induced elevation in intracellular pH. However, high pH alone had little effect on flash response recovery kinetics and even suppressed the accelerating effect of bicarbonate, consistent with a direct, modulatory action of bicarbonate on Ca²⁺- dependent, membrane guanylate cyclase activity.

Effects of cell size and bicarbonate on single photon response variability in retinal rods

Accurate photon counting requires that rods generate highly amplified, reproducible single photon responses (SPRs). The SPR is generated within the rod outer segment (ROS), a multilayered structure built from membranous disks that house rhodopsin. Photoisomerization of rhodopsin at the disk rim causes a local depletion of cGMP that closes ion channels in the plasmalemma located nearby with relative rapidity. In contrast, a photoisomerization at the disk center, distant from the plasmalemma, has a delayed impact on the ion channels due to the time required for cGMP redistribution. Radial differences should be greatest in large diameter rods. By affecting membrane guanylate cyclase activity, bicarbonate could impact spatial inhomogeneity in cGMP content. It was previously known that in the absence of bicarbonate, SPRs are larger and faster at the base of a toad ROS (where the ROS attaches to the rest of the cell) than at the distal tip. Given that bicarbonate enters the ROS at the base and diffuses to the tip and that it expedites flash response recovery, there should be an axial concentration gradient for bicarbonate that would accentuate the base-to-tip SPR differences. Seeking to understand how ROS geometry and bicarbonate affect SPR variability, we used mathematical modeling and made electrophysiological recordings of single rods. Modeling predicted and our experiments confirmed minor radial SPR variability in large diameter, salamander rods that was essentially unchanged by bicarbonate. SPRs elicited at the base and tip of salamander rods were similar in the absence of bicarbonate, but when treated with 30 mM bicarbonate, SPRs at the base became slightly faster than those at the tip, verifying the existence of an axial gradient for bicarbonate. The differences were small and unlikely to undermine visual signaling. However, in toad rods with longer ROSs, bicarbonate somehow suppressed the substantial, axial SPR variability that is naturally present in the absence of bicarbonate. Modeling suggested that the axial gradient of bicarbonate might dampen the primary phototransduction cascade at the base of the ROS. This novel effect of bicarbonate solves a mystery as to how toad vision is able to function effectively in extremely dim light.

Modes of Accessing Bicarbonate for the Regulation of Membrane Guanylate Cyclase (ROS-GC) in Retinal Rods and Cones

The membrane guanylate cyclase, ROS-GC, that synthesizes cyclic GMP for use as a second messenger for visual transduction in retinal rods and cones, is stimulated by bicarbonate. Bicarbonate acts directly on ROS-GC1, because it enhanced the enzymatic activity of a purified, recombinant fragment of bovine ROS-GC1 consisting solely of the core catalytic domain. Moreover, recombinant ROS-GC1 proved to be a true sensor of bicarbonate, rather than a sensor for CO₂. Access to bicarbonate differed in rods and cones of larval salamander, Ambystoma tigrinum, of unknown sex. In rods, bicarbonate entered at the synapse and diffused to the outer segment, where it was removed by Cl^--dependent exchange. In contrast, cones generated bicarbonate internally from endogenous CO₂ or from exogenous CO₂ that was present in extracellular solutions of bicarbonate. Bicarbonate production from both sources of CO₂ was blocked by the carbonic anhydrase inhibitor, acetazolamide. Carbonic anhydrase II expression was verified immunohistochemically in cones but not in rods. In addition, cones acquired bicarbonate at their outer segments as well as at their inner segments. The multiple pathways for access in cones may support greater uptake of bicarbonate than in rods and buffer changes in its intracellular concentration.

Bicarbonate Modulates Photoreceptor Guanylate Cyclase (ROS-GC) Catalytic Activity

By generating the second messenger cGMP in retinal rods and cones, ROS-GC plays a central role in visual transduction. Guanylate cyclase-activating proteins (GCAPs) link cGMP synthesis to the light-induced fall in [Ca(2+)]i to help set absolute sensitivity and assure prompt recovery of the response to light. The present report discloses a surprising feature of this system: ROS-GC is a sensor of bicarbonate. Recombinant ROS-GCs synthesized cGMP from GTP at faster rates in the presence of bicarbonate with an ED50 of 27 mM for ROS-GC1 and 39 mM for ROS-GC2. The effect required neither Ca(2+) nor use of the GCAPs domains; however, stimulation of ROS-GC1 was more powerful in the presence of GCAP1 or GCAP2 at low [Ca(2+)]. When applied to retinal photoreceptors, bicarbonate enhanced the circulating current, decreased sensitivity to flashes, and accelerated flash response kinetics. Bicarbonate was effective when applied either to the outer or inner segment of red-sensitive cones. In contrast, bicarbonate exerted an effect when applied to the inner segment of rods but had little efficacy when applied to the outer segment. The findings define a new regulatory mechanism of the ROS-GC system that affects visual transduction and is likely to affect the course of retinal diseases caused by cGMP toxicity.

Calcium stores in vertebrate photoreceptors

This review lays out the emerging evidence for the fundamental role of Ca(2+) stores and store-operated channels in the Ca(2+) homeostasis of rods and cones. Calcium-induced calcium release (CICR) is a major contributor to steady-state and light-evoked photoreceptor Ca(2+) homeostasis in the darkness whereas store-operated Ca(2+) channels play a more significant role under sustained illumination conditions. The homeostatic response includes dynamic interactions between the plasma membrane, endoplasmic reticulum (ER), mitochondria and/or outer segment disk organelles which dynamically sequester, accumulate and release Ca(2+). Coordinated activation of SERCA transporters, ryanodine receptors (RyR), inositol triphosphate receptors (IP3Rs) and TRPC channels amplifies cytosolic voltage-operated signals but also provides a memory trace of previous exposures to light. Store-operated channels, activated by the STIM1 sensor, prevent pathological decrease in [Ca(2+)]i mediated by excessive activation of PMCA transporters in saturating light. CICR and SOCE may also modulate the transmission of afferent and efferent signals in the outer retina. Thus, Ca(2+) stores provide additional complexity, adaptability, tuneability and speed to photoreceptor signaling.

Method of targeted delivery of laser beam to isolated retinal rods by fiber optics

A method of controllable light delivery to retinal rod cells using an optical fiber is described. Photo-induced current of the living rod cells was measured with the suction electrode technique. The approach was tested with measurements relating the spatial distribution of the light intensity to photo-induced current. In addition, the ion current responses of rod cells to polarized light at two different orientation geometries of the cells were studied.

Ectopic expression of cone-specific G-protein-coupled receptor kinase GRK7 in zebrafish rods leads to lower photosensitivity and altered responses

To investigate the roles of G-protein receptor kinases (GRKs) in the light responses of vertebrate photoreceptors, we generated transgenic zebrafish lines, the rods of which express either cone GRK (GRK7) or rod GRK (GRK1) in addition to the endogenous GRK1, and we then measured the electrophysiological characteristics of single-cell responses and the behavioural responses of intact animals. Our study establishes the zebrafish expression system as a convenient platform for the investigation of specific components of the phototransduction cascade. The addition of GRK1 led to minor changes in rod responses. However, exogenous GRK7 in GRK7-tg animals led to lowered rod sensitivity, as occurs in cones, but surprisingly to slower response kinetics. Examination of responses to long series of very dim flashes suggested the possibility that the GRK7-tg rods generated two classes of single-photon response, perhaps corresponding to the interaction of activated rhodopsin with GRK1 (giving a standard response) or with GRK7(giving a very small response). Behavioural measurement of optokinetic responses (OKR) in intact GRK7-tg zebrafish larvae showed that the overall rod visual pathway was less sensitive, in accord with the lowered sensitivity of the rods. These results help provide an understanding for the molecular basis of the electrophysiological differences between cones and rods.

Intracellular pH modulates inner segment calcium homeostasis in vertebrate photoreceptors

Neuronal metabolic and electrical activity is associated with shifts in intracellular pH (pH(i)) proton activity and state-dependent changes in activation of signaling pathways in the plasma membrane, cytosol, and intracellular compartments. We investigated interactions between two intracellular messenger ions, protons and calcium (Ca²(+)), in salamander photoreceptor inner segments loaded with Ca²(+) and pH indicator dyes. Resting cytosolic pH in rods and cones in HEPES-based saline was acidified by ∼0.4 pH units with respect to pH of the superfusing saline (pH = 7.6), indicating that dissociated inner segments experience continuous acid loading. Cytosolic alkalinization with ammonium chloride (NH₄Cl) depolarized photoreceptors and stimulated Ca²(+) release from internal stores, yet paradoxically also evoked dose-dependent, reversible decreases in [Ca²(+)](i). Alkalinization-evoked [Ca²(+)](i) decreases were independent of voltage-operated and store-operated Ca²(+) entry, plasma membrane Ca²(+) extrusion, and Ca²(+) sequestration into internal stores. The [Ca²(+)](i)-suppressive effects of alkalinization were antagonized by the fast Ca²(+) buffer BAPTA, suggesting that pH(i) directly regulates Ca²(+) binding to internal anionic sites. In summary, this data suggest that endogenously produced protons continually modulate the membrane potential, release from Ca²(+) stores, and intracellular Ca²(+) buffering in rod and cone inner segments.

Light responses and light adaptation in rat retinal rods at different temperatures

Rod responses to brief pulses of light were recorded as electroretinogram (ERG) mass potentials across isolated, aspartate-superfused rat retinas at different temperatures and intensities of steady background light. The objective was to clarify to what extent differences in sensitivity, response kinetics and light adaptation between mammalian and amphibian rods can be explained by temperature and outer-segment size without assuming functional differences in the phototransduction molecules. Corresponding information for amphibian rods from the literature was supplemented by new recordings from toad retina. All light intensities were expressed as photoisomerizations per rod (Rh*). In the rat retina, an estimated 34% of incident photons at the wavelength of peak sensitivity caused isomerizations in rods, as the (hexagonally packed) outer segments measured 1.7 microm x 22 microm and had specific absorbance of 0.016 microm(-1) on average. Fractional sensitivity (S) in darkness increased with cooling in a similar manner in rat and toad rods, but the rat function as a whole was displaced to a ca 0.7 log unit higher sensitivity level. This difference can be fully explained by the smaller dimensions of rat rod outer segments, since the same rate of phosphodiesterase (PDE) activation by activated rhodopsin will produce a faster drop in cGMP concentration, hence a larger response in rat than in toad. In the range 15-25 degrees C, the waveform and absolute time scale of dark-adapted dim-flash photoresponses at any given temperature were similar in rat and toad, although the overall temperature dependence of the time to peak (t(p)) was somewhat steeper in rat (Q(10) approximately 4 versus 2-3). Light adaptation was similar in rat and amphibian rods when measured at the same temperature. The mean background intensity that depressed S by 1 log unit at 12 degrees C was in the range 20-50 Rh* s(-1) in both, compared with ca 4500 Rh* s(-1) in rat rods at 36 degrees C. We conclude that it is not necessary to assume major differences in the functional properties of the phototransduction molecules to account for the differences in response properties of mammalian and amphibian rods.

Acid-sensing ion channel 2 is important for retinal function and protects against light-induced retinal degeneration

pH variations in the retina are thought to be involved in the fine-tuning of visual perception. We show that both photoreceptors and neurons of the mouse retina express the H+-gated cation channel subunits acid-sensing ion channel 2a (ASIC2a) and ASIC2b. Inactivation of the ASIC2 gene in mice leads to an increase in the rod electroretinogram a- and b-waves and thus to an enhanced gain of visual transduction. ASIC2 knock-out mice are also more sensitive to light-induced retinal degeneration. We suggest that ASIC2 is a negative modulator of rod phototransduction, and that functional ASIC2 channels are beneficial for the maintenance of retinal integrity.

Therapeutic photobiomodulation for methanol-induced retinal toxicity

Methanol intoxication produces toxic injury to the retina and optic nerve, resulting in blindness. The toxic metabolite in methanol intoxication is formic acid, a mitochondrial toxin known to inhibit the essential mitochondrial enzyme, cytochrome oxidase. Photobiomodulation by red to near-IR radiation has been demonstrated to enhance mitochondrial activity and promote cell survival in vitro by stimulation of cytochrome oxidase activity. The present studies were undertaken to test the hypothesis that exposure to monochromatic red radiation from light-emitting diode (LED) arrays would protect the retina against the toxic actions of methanol-derived formic acid in a rodent model of methanol toxicity. Using the electroretinogram as a sensitive indicator of retinal function, we demonstrated that three brief (2 min, 24 s) 670-nm LED treatments (4 J/cm(2)), delivered at 5, 25, and 50 h of methanol intoxication, attenuated the retinotoxic effects of methanol-derived formate. Our studies document a significant recovery of rod- and cone-mediated function in LED-treated, methanol-intoxicated rats. We further show that LED treatment protected the retina from the histopathologic changes induced by methanol-derived formate. These findings provide a link between the actions of monochromatic red to near-IR light on mitochondrial oxidative metabolism in vitro and retinoprotection in vivo. They also suggest that photobiomodulation may enhance recovery from retinal injury and other ocular diseases in which mitochondrial dysfunction is postulated to play a role.

pH and rate of "dark" events in toad retinal rods: test of a hypothesis on the molecular origin of photoreceptor noise

Thermal activation of the visual pigment constitutes a fundamental constraint on visual sensitivity. Its electrical correlate in the membrane current of dark-adapted rods are randomly occurring discrete "dark events" indistinguishable from responses to single photons. It has been proposed that thermal activation occurs in a small subpopulation of rhodopsin molecules where the Schiff base linking the chromophore to the protein part is unprotonated. On this hypothesis, rates of thermal activation should increase strongly with rising pH. The hypothesis has been tested by measuring the effect of pH changes on the frequency of discrete dark events in red rods of the common toad Bufo bufo. Dark noise was recorded from isolated rods using the suction pipette technique. Changes in cytoplasmic pH upon manipulations of extracellular pH were quantified by measuring, using fast single-cell microspectrophotometry, the pH-dependent metarhodopsin I-metarhodopsin II equilibrium and subsequent metarhodopsin III formation. These measurements show that, in the conditions of the electrophysiological experiments, changing perfusion pH from 6.5 to 9.3 resulted in a cytoplasmic pH shift from 7.6 to 8.5 that was readily sensed by the rhodopsin. This shift, which implies an 8-fold decrease in cytoplasmic [H(+)], did not increase the rate of dark events. The results contradict the hypothesis that thermal pigment activation depends on prior deprotonation of the Schiff base.

Reducing extracellular Cl- suppresses dihydropyridine-sensitive Ca2+ currents and synaptic transmission in amphibian photoreceptors

A reduction in extracellular chloride suppresses light-evoked currents of second-order retinal neurons (bipolar and horizontal cells) by reducing release of glutamate from photoreceptors. The underlying mechanisms responsible for this action of reduced extracellular Cl- were studied with a combination of electrophysiological recordings from single neurons in a retinal slice preparation and image analyses of intracellular Ca2+ (Fura-2) and pH [2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, acetoxymethyl ester] in dissociated photoreceptors. The results show that reducing extracellular Cl- suppresses a dihydropyridine (DHP)-sensitive Ca2+ current (I(Ca)) in photoreceptors. It is proposed that suppression of I(Ca) results in suppression of photoreceptor neurotransmission. The suppressive effect of low Cl- on I(Ca) is not due to antagonism by the substituting anion nor is it mediated by changes in extracellular or intracellular pH. We conclude that normal extracellular levels of Cl- are important for maintenance of the voltage-gated Ca2+ channels that support neurotransmission from photoreceptors. Several ideas are presented about the mechanisms by which Cl- supports photoreceptor neurotransmission and the possibility that modulations of Cl- might play a physiological role in the regulation of Ca2+ channels in photoreceptors and, hence, photoreceptor function.

Regulation of intracellular pH in salamander retinal rods

1. We measured intracellular pH (pHi) in rods isolated from the retina of the axolotl salamander, Ambystoma mexicanum, using the fluorescent indicator 2',7'-bis(carboxyethyl)-5(and -6)-carboxyfluorescein (BCECF). 2. The light exposures associated with data acquisition had no marked effect on pHi. There was no sharp change between the value obtained from the first exposure of dark-adapted rods and subsequent readings. Increasing the acquisition frequency from 1 to 10 min-1 either had no effect, or brought about a slow acidification, which was stopped or reversed when the low frequency was restored. 3. In nominally HCO3(-)-free solution at pH 7.5, the rods had a steady-state pHi of 7.09 +/- 0.02 (n = 46) and a buffering power (beta i) of 24 +/- 1 mM (pH unit)-1 (n = 48). The buffering power was virtually constant in the pH range 6.6-8.0. In the same range, pHi dependent linearly on perfusion pH (pHo) with regression coefficients of 0.4-0.5. 4. There were no significant differences between the inner and outer segment of intact rods as regards steady-state pHi or responses to experimental treatments. 5. Recovery from an intracellular acid load imposed by sodium propionate or an NH4Cl prepulse in nominally bicarbonate-free perfusate was completely blocked by decreasing the extracellular Na+ concentration to 7 mM, and slowed by 86% by applying 1 mM amiloride. 6. Introduction of 2% CO2-13 mM HCO3- caused an alkalinization that was often preceded by a transient acidification. Steady-state pHi was on average 0.1 pH units higher than in nominally bicarbonate-free solution. The mean acid extrusion rate, calculated on the assumption that CO2-HCO3- behaves as an open system, was 19% higher (31 +/- 2 mM h-1) than in a solution buffered only by Hepes (26 +/- 2 mM h-1). 7. In the presence of CO2-HCO3-, 100 microM 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) decreased the acid extrusion rate by 20% on average. Lowering the extracellular Cl-concentration to 7 mM raised pHi, but did not significantly affect the acid extrusion rate. 8. We conclude that retinal rods regulate pHi by both Na(+)-H+ exchange and mechanism(s) involving HCO3(-)-Cl- exchange. In the present conditions, the Na(+)-H+ exchanger appears as the dominant mechanism for acid extrusion.

pH changes in frog rods upon manipulation of putative pH-regulating transport mechanisms

Rod intracellular pH (pHi) in the intact frog retina was measured fluorometrically with the dye 2',7'-bis(2-carboxyethyl)-5(and-6)-carboxyfluorescein under treatments chosen to affect putative pH-regulating transport mechanisms in the plasma membrane. The purpose was to relate possible pHi changes to previously reported effects on photoresponses. In nominally bicarbonate-free Ringer, application of amiloride (1 mM) or substitution of 95 mM external Na+ by K+ or choline triggered monotonic but reversible acidifications, consistent with inhibition of Na+/H+ exchange. Bicarbonate-dependent mechanisms were characterized as follows: (1) Replacing half of a 12 mM phosphate buffer by bicarbonate caused a sustained rise of pHi. (2) Subsequent application of the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2',2'-disulphonic acid (DIDS, 0.2 mM) set off a slow acidification. (3) Substitution of external Cl- by gluconate (95 mM) caused a rapid pHi rise both in normal Na+ and low-Na+ perfusion. (4) This effect was inhibited by DIDS. The results support a consistent explanation of parallel electrophysiological experiments on the assumption that intracellular acidifications reduce and alkalinizations (in a certain range) augment photoresponses. It is concluded that both Na+/H+ exchange and bicarbonate transport control rod pHi, modulating the light-sensitive current. Part of the bicarbonate transport is by Na(+)-independent HCO3-/Cl- exchange, but a further Na(+)-coupled bicarbonate import mechanism is implicated.

Removal of extracellular chloride suppresses transmitter release from photoreceptor terminals in the mudpuppy retina

Removal of extracellular Cl- has been shown to suppress light-evoked voltage responses of ON bipolar and horizontal cells, but not photoreceptors or OFF bipolar cells, in the amphibian retina. A substantial amount of experimental evidence has demonstrated that the photoreceptor transmitter, L-glutamate, activates cation, not Cl-, channels in these cells. The mechanism for Cl-free effects was therefore reexamined in a superfused retinal slice preparation from the mudpuppy (Necturus maculosus) using whole-cell voltage and current clamp techniques. In a Cl-free medium, light-evoked currents were maintained in rod and cone photoreceptors but suppressed in horizontal, ON bipolar, and OFF bipolar cells. Changes in input resistance and dark current in bipolar and horizontal cells were consistent with the hypothesis that removal of Cl- suppresses tonic glutamate release from photoreceptors. The persistence of light-evoked voltage responses in OFF bipolar cells, despite the suppression of light-evoked currents, is due to a compensatory increase in input resistance. Focal application of hyperosmotic sucrose to photoreceptor terminals produced currents in bipolar and horizontal cells arising from two sources: (a) evoked glutamate release and (b) direct actions of the hyperosmotic solution on postsynaptic neurons. The inward currents resulting from osmotically evoked release of glutamate in OFF bipolar and horizontal cells were suppressed in a Cl-free medium. For ON bipolar cells, both the direct and evoked components of the hyperosmotic response resulted in outward currents and were thus difficult to separate. However, in some cells, removal of extracellular Cl- suppressed the outward current consistent with a suppression of presynaptic glutamate release. The results of this study suggest that removal of extracellular Cl- suppresses glutamate release from photoreceptor terminals. Thus, it is possible that control of [Cl-] in and around photoreceptors may regulate glutamate release from these cells.