Ca2+ dependence of dark- and light-adapted flash responses in rod photoreceptors

Molecular tuning of calcium dependent processes by neuronal calcium sensor proteins in the retina

Vertebrate photoreceptor cells are exquisite light detectors operating under very dim and bright illumination mediated by phototransduction, which is under control of the two secondary messengers cGMP and Ca²⁺. Feedback mechanisms enable photoreceptor cells to regain their responsiveness after light stimulation and involve neuronal Ca²⁺-sensor proteins, named GCAPs (guanylate cyclase-activating proteins) and recoverins. This review compares the diversity in Ca²⁺-related signaling mediated by GCAP and recoverin variants that exhibit differences in Ca²⁺-sensing, protein conformational changes, myristoyl switch mechanisms, diversity in divalent cation binding and dimer formation. In summary, both subclasses of neuronal Ca²⁺-sensor proteins contribute to a complex signaling network in rod and cone cells, which is perfectly suited to match the requirements for sensitive cell responses and maintaining this responsiveness in the presence of different background light intensities.

A Higher Serum Calcium Level is an Independent Risk Factor for Vision-Threatening Diabetic Retinopathy in Patients with Type 2 Diabetes: Cross-Sectional and Longitudinal Analyses

OBJECTIVE

An elevated serum calcium level is associated with a higher risk of type 2 diabetes (T2D), but its role in microvascular complications remains unclear. This study was conducted to investigate the association between serum calcium levels and vision-threatening diabetic retinopathy (VTDR).

METHODS

This study employed a cross-sectional and longitudinal design. The cross-sectional part included all patients treated for T2D at Shanghai General Hospital between 2007 and 2016, while the longitudinal part involved an overlapping cohort of diabetic patients without VTDR who were followed from their admission until December 2019. Multivariable logistic and Cox proportional hazard regression analyses were performed, respectively. VTDR was defined as severe nonproliferative diabetic retinopathy, proliferative diabetic retinopathy, or clinically significant macular edema.

RESULTS

A total of 3269 patients were included in the cross-sectional analysis, and 649 patients were included in the longitudinal analysis. In the cross-sectional analysis, higher corrected serum calcium (odds ratio: 1.31 per 0.1 mmol/L, 95% confidence interval: 1.16-1.49), younger age, longer diabetes duration, albuminuria, impaired renal function, and lower serum magnesium were independently associated with VTDR. In the longitudinal analysis, 95 subjects developed VTDR during follow-up (9.7 years, interquartile range: 7.4-10.9 years). Higher corrected serum calcium (hazard ratio: 1.38 per 0.1 mmol/L, 95% confidence interval: 1.10-1.72), younger age, longer diabetes duration, sub-VTDR, albuminuria, lower serum magnesium, and higher glycated hemoglobin were identified as independent risk factors for VTDR.

CONCLUSIONS

A higher serum calcium level may be an independent risk factor for VTDR in patients with T2D.

Retinal photoreceptor apoptosis is associated with impaired serum ionized calcium homeostasis in diabetic retinopathy: An in-vivo analysis

PURPOSE

The aim of this work was to study the association of serum ionized calcium with retinal photoreceptor apoptosis on spectral domain optical coherence tomography (SD-OCT) in diabetic retinopathy (DR).

METHODS

Sixty consecutive cases with Type 2 diabetes mellitus were categorized into three groups: no diabetic retinopathy; non-proliferative DR; proliferative DR. The eye with more severe form of the disease was considered. Twenty healthy controls were also included. Best corrected visual acuity (BCVA) was measured on logMAR scale. Retinal photoreceptor apoptosis was defined as disruption of retinal photoreceptor ellipsoid zone (EZ). Ellipsoid zone disruption was assessed using SD-OCT. Serum levels of total and ionized calcium were measured using standard protocol.

RESULTS

EZ disruption was found to be positively associated with serum total calcium and ionized calcium. Also, EZ disruption was found to be positively associated with logMAR BCVA.

CONCLUSION

Increased serum ionized calcium induces retinal photoreceptor apoptosis resulting in increased EZ disruption in DR.

Hyperglycemia potentiates the effect of ionic calcium in photoreceptor ellipsoid zone disruption in diabetic retinopathy

PURPOSE

To study the association of serum ionic calcium and glycated hemoglobin (HbA1c) with retinal photoreceptor ellipsoid zone (EZ) disruption in diabetic retinopathy (DR).

METHODS

This is a tertiary care center-based observational cross-sectional study. Sixty-three consecutive cases, divided into 21 cases each with no diabetic retinopathy, non-proliferative diabetic retinopathy and proliferative diabetic retinopathy were included. Twenty-one healthy controls were also included. Ellipsoid zone disruption was assessed using spectral-domain optical coherence tomography. Serum ionic calcium and HbA1c were measured using standard protocol. Patient data from cases were divided into two groups according to their HbA1c levels: group 1 (HbA1c < 7, n = 26) and group 2 (HbA1c > 7, n = 37). Data were analyzed statistically.

RESULTS

Mean ionic calcium levels in group 1 and group 2 were 1.131 ± 0.073 mmol/dL and 1.170 ± 0.070 mmol/dL, respectively. In group 1, 11 out of 26 had EZ disruption (42.3%). Similarly, in group 2, 29 out of 37 had EZ disruption (78.4%). On logistic regression analysis, as compared to group 1, ellipsoid zone disruption was found to be positively associated with serum ionic calcium (p = 0.01) in group 2 cases.

CONCLUSION

Increased levels of serum ionic calcium are associated with increased EZ disruption in patients with HbA1c > 7 in DR.

Dynamical adaptation in photoreceptors

Adaptation is at the heart of sensation and nowhere is it more salient than in early visual processing. Light adaptation in photoreceptors is doubly dynamical: it depends upon the temporal structure of the input and it affects the temporal structure of the response. We introduce a non-linear dynamical adaptation model of photoreceptors. It is simple enough that it can be solved exactly and simulated with ease; analytical and numerical approaches combined provide both intuition on the behavior of dynamical adaptation and quantitative results to be compared with data. Yet the model is rich enough to capture intricate phenomenology. First, we show that it reproduces the known phenomenology of light response and short-term adaptation. Second, we present new recordings and demonstrate that the model reproduces cone response with great precision. Third, we derive a number of predictions on the response of photoreceptors to sophisticated stimuli such as periodic inputs, various forms of flickering inputs, and natural inputs. In particular, we demonstrate that photoreceptors undergo rapid adaptation of response gain and time scale, over ∼ 300[Formula: see text] ms-i. e., over the time scale of the response itself-and we confirm this prediction with data. For natural inputs, this fast adaptation can modulate the response gain more than tenfold and is hence physiologically relevant.

A calcium-relay mechanism in vertebrate phototransduction

Calcium-signaling in cells requires a fine-tuned system of calcium-transport proteins involving ion channels, exchangers, and ion-pumps but also calcium-sensor proteins and their targets. Thus, control of physiological responses very often depends on incremental changes of the cytoplasmic calcium concentration, which are sensed by calcium-binding proteins and are further transmitted to specific target proteins. This Review will focus on calcium-signaling in vertebrate photoreceptor cells, where recent physiological and biochemical data indicate that a subset of neuronal calcium sensor proteins named guanylate cyclase-activating proteins (GCAPs) operate in a calcium-relay system, namely, to make gradual responses to small changes in calcium. We will further integrate this mechanism in an existing computational model of phototransduction showing that it is consistent and compatible with the dynamics that are characteristic for the precise operation of the phototransduction pathways.

Differential calcium signaling by cone specific guanylate cyclase-activating proteins from the zebrafish retina

Zebrafish express in their retina a higher number of guanylate cyclase-activating proteins (zGCAPs) than mammalians pointing to more complex guanylate cyclase signaling systems. All six zGCAP isoforms show distinct and partial overlapping expression profiles in rods and cones. We determined critical Ca²⁺-dependent parameters of their functional properties using purified zGCAPs after heterologous expression in E.coli. Isoforms 1–4 were strong, 5 and 7 were weak activators of membrane bound guanylate cyclase. They further displayed different Ca²⁺-sensitivities of guanylate cyclase activation, which is half maximal either at a free Ca²⁺ around 30 nM (zGCAP1, 2 and 3) or around 400 nM (zGCAP4, 5 and 7). Zebrafish GCAP isoforms showed also differences in their Ca²⁺/Mg²⁺-dependent conformational changes and in the Ca²⁺-dependent monomer-dimer equilibrium. Direct Ca²⁺-binding revealed that all zGCAPs bound at least three Ca²⁺. The corresponding apparent affinity constants reflect binding of Ca²⁺ with high (≤100 nM), medium (0.1–5 µM) and/or low (≥5 µM) affinity, but were unique for each zGCAP isoform. Our data indicate a Ca²⁺-sensor system in zebrafish rod and cone cells supporting a Ca²⁺-relay model of differential zGCAP operation in these cells.

Dynamics of mouse rod phototransduction and its sensitivity to variation of key parameters

The deep understanding of the biochemical and biophysical basis of visual transduction, makes it ideal for systems-level analysis. A sensitivity analysis is presented for a self-consistent set of parameters involved in mouse phototransduction. The organising framework is a spatio-temporal mathematical model, which includes the geometry of the rod outer segment (ROS), the layered array of the discs, the incisures, the biochemistry of the activation/deactivation cascade and the biophysics of the diffusion of the second messengers in the cytoplasm and the closing of the cyclic guanosine monophosphate (cGMP) gated cationic channels. These modules include essentially all the relevant geometrical, biochemical and biophysical parameters. The parameters are selected from within experimental ranges, to obey basic first principles such as conservation of mass and energy fluxes. By means of the model they are compared to a large set of experimental data, providing a strikingly close match. Following isomerisation of a single rhodopsin R * (single photon response), the sensitivity analysis was carried out on the photo-response, measured both in terms of number of effector molecules produced, and photocurrent suppression, at peak time and the activation and recovery phases of the cascade. The current suppression is found to be very sensitive to variations of the catalytic activities, Hill's coefficients and hydrolysis rates and the geometry of the ROS, including size and shape of the incisures. The activated effector phosphodiesterase (PDE *) is very sensitive to variations of catalytic activity of G-protein activation and the average lifetimes of activated rhodopsin R * and PDE *; however, they are insensitive to geometry and variations of the transduction parameters. Thus the system is separated into two functional modules, activation/deactivation and transduction, each confined in different geometrical domains, communicating through the hydrolysis of cGMP by PDE *, and each sensitive to variations of parameters only in its own module.

Kinetics of turn-offs of frog rod phototransduction cascade

The time course of the light-induced activity of phototrandsuction effector enzyme cGMP-phosphodiesterase (PDE) is shaped by kinetics of rhodopsin and transducin shut-offs. The two processes are among the key factors that set the speed and sensitivity of the photoresponse and whose regulation contributes to light adaptation. The aim of this study was to determine time courses of flash-induced PDE activity in frog rods that were dark adapted or subjected to nonsaturating steady background illumination. PDE activity was computed from the responses recorded from solitary rods with the suction pipette technique in Ca²⁺-clamping solution.

A flash applied in the dark-adapted state elicits a wave of PDE activity whose rising and decaying phases have characteristic times near 0.5 and 2 seconds, respectively. Nonsaturating steady background shortens both phases roughly to the same extent. The acceleration may exceed fivefold at the backgrounds that suppress ≈70% of the dark current.

The time constant of the process that controls the recovery from super-saturating flashes (so-called dominant time constant) is adaptation independent and, hence, cannot be attributed to either of the processes that shape the main part of the PDE wave. We hypothesize that the dominant time constant in frog rods characterizes arrestin binding to rhodopsin partially inactivated by phosphorylation. A mathematical model of the cascade that considers two-stage rhodopsin quenching and transducin inactivation can mimic experimental PDE activity quite well. The effect of light adaptation on the PDE kinetics can be reproduced in the model by concomitant acceleration on both rhodopsin phosphorylation and transducin turn-off, but not by accelerated arrestin binding. This suggests that not only rhodopsin but also transducin shut-off is under adaptation control.

The cone-specific calcium sensor guanylate cyclase activating protein 4 from the zebrafish retina

Guanylate cyclase activating proteins (GCAPs) serve as neuronal Ca(2+)-sensor proteins in vertebrate rod and cone photoreceptor cells. Zebrafish express in their retina a variety of six different GCAPs, of which four are specific for cone cells. One isoform, zGCAP4, is mainly expressed in double cones and long single cones. We cloned the zGCAP4 gene, purified non-myristoylated and myristoylated forms of the protein after heterologous expression in Escherichia coli and studied its properties: zGCAP4 was a strong activator of membrane-bound guanylate cyclases from bovine and zebrafish retina, showing half-maximal activation at 520-570 nM free Ca(2+) concentration. Furthermore, the Ca(2+)-sensitive activation properties of non-myristoylated and myristoylated zGCAP4 were similar, indicating no influence of the myristoyl moiety on Ca(2+)-sensor function. Myristoylated zGCAP4 showed low affinity for membranes and did not exhibit a Ca(2+)-myristoyl switch, a feature typical of some but not all neuronal Ca(2+)-sensor proteins. However, tryptophan fluorescence studies and Ca(2+)-dependent differences in protease accessibility revealed Ca(2+)-induced conformational changes in myristoylated and non-myristoylated zGCAP4, indicating the operation as a Ca(2+) sensor. Thus, expression and biochemical properties of zGCAP4 are in agreement with its function as an efficient Ca(2+)-sensitive regulator of guanylate cyclase activity in cone vision.

Calcium-independent, cGMP-mediated light adaptation in invertebrate ciliary photoreceptors

Calcium is thought to be essential for adaptation of sensory receptor cells. However, the transduction cascade of hyperpolarizing, ciliary photoreceptors of the scallop does not use IP3-mediated Ca release, and the light-sensitive conductance is not measurably permeable to Ca2+. Therefore, two typical mechanisms that couple the light response to [Ca]i changes seem to be lacking in these photoreceptors. Using fluorescent indicators, we determined that, unlike in their microvillar counterparts, photostimulation of ciliary cells under voltage clamp indeed evokes no detectable change in cytosolic Ca. Notwithstanding, these cells exhibit all of the hallmarks of light adaptation, including response range compression, sensitivity shift, and photoresponse acceleration. A possible mediator of Ca-independent sensory adaptation is cGMP, the second messenger that regulates the light-sensitive conductance; cGMP and 8-bromo cGMP not only activate light-dependent K channels but also reduce the amplitude of the light response to an extent greatly in excess of that expected from simple occlusion between light and chemical stimulation. In addition, these substances accelerate the time course of the photocurrent. Tests with pharmacological antagonists suggest that protein kinase G may be a downstream effector that controls, in part, the cGMP-triggered changes in photoresponse properties during light adaptation. However, additional messengers are likely to be implicated, especially in the regulation of response kinetics. These observations suggest a novel feedback inhibition pathway for signaling sensory adaptation.

ATP-regulated module (ARM) of the atrial natriuretic factor receptor guanylate cyclase

ATP is an obligatory agent for the atrial natriuretic factor (ANF) and the type C natriuretic peptide (CNP) signaling of their respective receptor guanylate cyclases, ANF-RGC and CNP-RGC. Through a common mechanism, it binds to a defined ARM domain of the cyclase, activates the cyclase and transduces the signal into generation of the second messenger cyclic GMP. In this presentation, the authors review the ATP-regulated transduction mechanism and refine the previously simulated three-dimensional ARM model (Duda T, Yadav P, Jankowska A, Venkataraman V, Sharma RK. Three dimensional atomic model and experimental validation for the ATP-regulated module (ARM) of the atrial natriuretic factor receptor guanylate cyclase. Mol Cell Biochem 2000;214:7-14; reviewed in: Sharma RK, Yadav P, Duda T. Allosteric regulatory step and configuration of the ATP-binding pocket in atrial natriuretic factor receptor guanylate cyclase transduction mechanism. Can J Physiol Pharmacol 2001;79: 682-91; Sharma RK. Evolution of the membrane guanylate cyclase transduction system. Mol Cell Biochem 2002;230:3-30). The model depicts the ATP-binding dependent configurational changes in the ARM and supports the concept that in the first step, ATP partially activates the cyclase and primes it for the subsequent transduction steps, resulting in full activation of the cyclase.

In intact mammalian photoreceptors, Ca2+-dependent modulation of cGMP-gated ion channels is detectable in cones but not in rods

In the mammalian retina, cone photoreceptors efficiently adapt to changing background light intensity and, therefore, are able to signal small differences in luminance between objects and backgrounds, even when the absolute intensity of the background changes over five to six orders of magnitude. Mammalian rod photoreceptors, in contrast, adapt very little and only at intensities that nearly saturate the amplitude of their photoresponse. In search of a molecular explanation for this observation we assessed Ca²⁺-dependent modulation of ligand sensitivity in cyclic GMP–gated (CNG) ion channels of intact mammalian rods and cones. Solitary photoreceptors were isolated by gentle proteolysis of ground squirrel retina. Rods and cones were distinguished by whether or not their outer segments bind PNA lectin. We measured membrane currents under voltage-clamp in photoreceptors loaded with Diazo-2, a caged Ca²⁺ chelator, and fixed concentrations of 8Br-cGMP. At 600 nM free cytoplasmic Ca²⁺ the midpoint of the cone CNG channels sensitivity to 8BrcGMP, ^8BrcGMPK_1/2, is ∼2.3 μM. The ligand sensitivity is less in rod than in cone channels. Instantly decreasing cytoplasmic Ca²⁺ to <30 nM activates a large inward membrane current in cones, but not in rods. Current activation arises from a Ca²⁺ -dependent modulation of cone CNG channels, presumably because of an increase in their affinity to the cyclic nucleotide. The time course of current activation is temperature dependent; it is well described by a single exponential process of ∼480 ms time constant at 20–21°C and 138 ms at 32°C. The absence of detectable Ca²⁺-dependent CNG current modulation in intact rods, in view of the known channel modulation by calmodulin in-vitro, affirms the modulation in intact rods may only occur at low Ca²⁺ concentrations, those expected at intensities that nearly saturate the rod photoresponse. The correspondence between Ca²⁺ dependence of CNG modulation and the ability to light adapt suggest these events are correlated in photoreceptors.

Selective transmission of single photon responses by saturation at the rod-to-rod bipolar synapse

A threshold-like nonlinearity in signal transfer from mouse rod photoreceptors to rod bipolar cells dramatically improves the absolute sensitivity of the rod signals. The work described here reaches three conclusions about the mechanisms generating this nonlinearity. (1) The nonlinearity is caused primarily by saturation of the feedforward rod-to-rod bipolar synapse and not by feedback from horizontal or amacrine cells. This saturation renders the rod bipolar current insensitive to small changes in transmitter release from the rod. (2) Saturation occurs within the G protein cascade that couples receptors to channels in the rod bipolar dendrites, with little or no contribution from presynaptic mechanisms or saturation of the postsynaptic receptors. (3) Between 0.5 and 2 bipolar transduction channels are open in darkness at each synapse, compared to the approximately 30 channels open at the peak of the single photon response.

Bleaching desensitization: background and current challenges

"Bleaching desensitization" in rod photoreceptors refers to the prolonged depression of phototransduction sensitivity exhibited by rods after their exposure to bright light, i.e., after photolysis (bleaching) of a substantial fraction of rhodopsin in the outer segments. Rod recovery from bleaching desensitization depends critically on operation of the retinoid visual cycle: in particular, on the removal of all-trans retinal bleaching product from opsin and on the delivery of 11-cis retinal to opsin's chromophore binding site. The present paper summarizes representative findings that address the mechanism of bleaching desensitization.

Step response of mouse rod photoreceptors modeled in terms of elemental photic signals

The process of light adaptation in rod photoreceptors enables these sensory cells of the retina to remain responsive to photic stimuli over a broad range of light intensity. Recent studies have employed the technique of paired-flash electroretinography to determine properties of phototransduction, and of light and dark adaptation, in rod photoreceptors in the living eye. Building on these studies, we have developed a theoretical model aimed at explaining the rod electrical response to a step of light based on known physiology. The central feature of the model is its description of the macroscopic (i.e., measured) response in terms of a time-evolving, weighted sum of elemental responses determined under dark-adapted and near fully light-adapted conditions. The model yields a time-dependent function that describes the course of desensitization and putatively represents the cumulative dynamics of underlying biochemical processes involved in light adaptation of the rod.

Tyrosine phosphorylation of rod cyclic nucleotide-gated channels switches off Ca2+/calmodulin inhibition

Cyclic nucleotide-gated (CNG) ion channels are crucial for phototransduction in rod photoreceptors. Light triggers a biochemical cascade that reduces the concentration of cGMP in rods, closing CNG channels, which leads to membrane potential hyperpolarization and a decrease in the concentration of intracellular Ca2+. During light adaptation, the sensitivity of CNG channels to cGMP is decreased by Ca2+, which in conjunction with calmodulin (CaM), binds directly to CNG channels. The cGMP sensitivity of rod CNG channels is also reduced by phosphorylation of specific tyrosine residues in the three CNGA1 subunits and one CNGB1 subunit that comprise the rod channel. Here we show that phosphorylation prevents Ca2+/CaM inhibition. Experiments on native channels in rod outer segments and expressed channels in Xenopus oocytes show that Ca2+/CaM inhibition can be toggled off or on by promoting phosphorylation or dephosphorylation, respectively. Experiments in which the crucial tyrosine phosphorylation sites in CNGA1 and CNGB1 are replaced with phenylalanines show that residue Y498 in CNGA1 is the phosphorylation site responsible for regulating Ca2+/CaM inhibition. Ca2+/CaM inhibits the rod channel by binding to the N terminus of the CNGB1 subunit, causing it to uncouple from the C terminus of CNGA1. We propose that phosphorylation of CNGA1Y498, on the C terminus of CNGA1, triggers an equivalent uncoupling from the C terminus of CNGB1, thereby curtailing Ca2+/CaM inhibition. The control of CaM inhibition by CNG channel phosphorylation may be important for light adaptation and the regulation of phototransduction by IGF-1, a retinal paracrine factor that alters the tyrosine phosphorylation state of rod CNG channels.

Mathematical model of the spatio-temporal dynamics of second messengers in visual transduction

A model describing the role of transversal and longitudinal diffusion of cGMP and Ca(2+) in signaling in the rod outer segment of vertebrates is developed. Utilizing a novel notion of surface-volume reaction and the mathematical theories of homogenization and concentrated capacity, the diffusion of cGMP and Ca(2+) in the inter-disc spaces is shown to be reducible to a one-parameter family of diffusion processes taking place on a single rod cross section; whereas the diffusion in the outer shell is shown to be reducible to a diffusion on a cylindrical surface. Moreover, the exterior flux of the former serves as a source term for the latter, alleviating the assumption of a well-stirred cytosol. A previous model of visual transduction that assumes a well-stirred rod outer segment cytosol (and thus contains no spatial information) can be recovered from this model by imposing a "bulk" assumption. The model shows that upon activation of a single rhodopsin, cGMP changes are local, and exhibit both a longitudinal and a transversal component. Consequently, membrane current is also highly localized. The spatial spread of the single photon response along the longitudinal axis of the outer segment is predicted to be 3-5 microm, consistent with experimental data. This approach represents a tool to analyze point-wise signaling dynamics without requiring averaging over the entire cell by global Michaelis-Menten kinetics.

Calcium and phototransduction

Visual phototransduction, the conversion of incoming light to an electrical signal, takes place in the outer segments of the rod and cone photoreceptor cells. Light reduces the concentration of cGMP, which, in darkness, keeps open cationic channels present in the plasma membrane of the outer segment. Ca2+ plays an important role in phototransduction by modulating the cGMP-gated channels as well as cGMP synthesis and breakdown. Ca2+ is involved in a negative feedback that is essential for photoreceptor adaptation to background illumination. The effects of Ca2+ on the different components of rod phototransduction have been characterized and can quantitatively account for the steady state responses of the rod cell to background illumination. The propagation of the Ca2+ feedback signal from the periphery toward the center of the outer segment depends on the Ca2+ diffusion coefficient, which has a value of 15 +/- 1 microm2 s(-1). This value shows that diffusion of Ca2+ in the radial direction is quite slow providing a significant barrier in the propagation of the feedback signal. Also, because the diffusion coefficient of Ca2+ is much smaller than that of cGMP, the decline of Ca2+ in the longitudinal direction lags behind the propagation of excitation by the decline of cGMP.

Ca2+-dependent control of rhodopsin phosphorylation: recoverin and rhodopsin kinase

Over many years until the middle of the 1980s, the main problem in vision research had been the mechanism of transducing the visual signal from photobleached rhodopsin to the cationic channels in the plasma membrane of a photoreceptor to trigger the electrophysiological response of the cell. After cGMP was proven to be the secondary messenger, the main intriguing question has become the mechanisms of negative feedback in photoreceptors to modulate their response to varying conditions of illumination. Although the mechanisms of light-adaptation are not completely understood, it is obvious that Ca2+ plays a crucial role in these mechanisms and that the effects of Ca2+ can be mediated by several Ca2+-binding proteins. One of them is recoverin. The leading candidate for the role of an intracellular target for recoverin is believed to be rhodopsin kinase, a member of a family of G-protein-coupled receptor kinases. The present review considers recoverin, rhodopsin kinase and their interrelationships in the in vitro as well as in vivo contexts.

Calcium regulation in photoreceptors

In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca2+ is highly compartmentalized. In the outer segment, Ca2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca2+ regulates cells' metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca2+ entry, buffering, sequestration, release from internal stores and Ca2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca2+ homeostasis. We also investigate the various roles played by Ca2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca2+ homeostasis and pathologies leading to retinal dysfunction and blindness.

Measurement of cytoplasmic calcium concentration in the rods of wild-type and transducin knock-out mice

A 10 microm 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 Ca(2+) concentration ([Ca(2+)](i)) and changes in [Ca(2+)](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 alpha-subunit knock-out (Tralpha-/-) 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 (tau~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 [Ca(2+)](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 [Ca(2+)](i). Dissociation constants were measured in vitro for fluo-3, fluo-4 and fluo-5F with and without 1 mM Mg(2+) from 20 to 37 degrees 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 degrees C were used to estimate absolute levels of rod [Ca(2+)](i). All three dyes gave similar values for [Ca(2+)](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 [Ca(2+)](i) between wild-type and Tralpha-/- animals.

Excitation and desensitization of mouse rod photoreceptors in vivo following bright adapting light

Electroretinographic (ERG) methods were used to determine response properties of mouse rod photoreceptors in vivo following adapting illumination that produced a significant extent of rhodopsin bleaching. Bleaching levels prevailing at approximately 10 min and approximately 20 min after the adapting exposure were on average 14 % and 9 %, respectively, based on the analysis of visual cycle retinoids in the eye tissues. Recovery of the rod response to the adapting light was monitored by analysing the ERG a-wave response to a bright probe flash presented at varying times during dark adaptation. A paired-flash procedure, in which the probe flash was presented at defined times after a weak test flash of fixed strength, was used to determine sensitivity of the rod response to the test flash. Recovery of the response to the adapting light was 80 % complete at 13.5 +/- 3.0 min (mean +/- S.D.; n = 7) after adapting light offset. The adapting light caused prolonged desensitization of the weak-flash response derived from paired-flash data. By comparison with results obtained in the absence of the adapting exposure, desensitization determined with a test-probe interval of 80 ms was ~fourfold after 5 min of dark adaptation and approximately twofold after 20 min. The results indicate, for mouse rods in vivo, that the time scale for recovery of weak-flash sensitivity substantially exceeds that for the recovery of circulating current following significant rhodopsin bleaching. The lingering desensitization may reflect a reduced efficiency of signal transmission in the phototransduction cascade distinct from that due to residual excitation.

G proteins and phototransduction

Phototransduction is the process by which a photon of light captured by a molecule of visual pigment generates an electrical response in a photoreceptor cell. Vertebrate rod phototransduction is one of the best-studied G protein signaling pathways. In this pathway the photoreceptor-specific G protein, transducin, mediates between the visual pigment, rhodopsin, and the effector enzyme, cGMP phosphodiesterase. This review focuses on two quantitative features of G protein signaling in phototransduction: signal amplification and response timing. We examine how the interplay between the mechanisms that contribute to amplification and those that govern termination of G protein activity determine the speed and the sensitivity of the cellular response to light.

Massive light-driven translocation of transducin between the two major compartments of rod cells: a novel mechanism of light adaptation

We report a new cellular mechanism of rod photoreceptor adaptation in vivo, which is triggered by daylight levels of illumination. The mechanism involves a massive light-dependent translocation of the photoreceptor-specific G protein, transducin, between the functional compartments of rods. To characterize the mechanism, we developed a novel technique that combines serial tangential cryodissection of the rat retina with Western blot analysis of protein distribution in the sections. Up to 90% of transducin translocates from rod outer segments to other cellular compartments on the time scale of tens of minutes. The reduction in the transducin content of the rod outer segments is accompanied by a corresponding reduction in the amplification of the rod photoresponse, allowing rods to operate in illumination up to 10-fold higher than would otherwise be possible.

Rhodopsin: structural basis of molecular physiology

The crystal structure of rod cell visual pigment rhodopsin was recently solved at 2.8-A resolution. A critical evaluation of a decade of structure-function studies is now possible. It is also possible to begin to explain the structural basis for several unique physiological properties of the vertebrate visual system, including extremely low dark noise levels as well as high gain and color detection. The ligand-binding pocket of rhodopsin is remarkably compact, and several apparent chromophore-protein interactions were not predicted from extensive mutagenesis or spectroscopic studies. The transmembrane helices are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The helix movement model of receptor activation, which might apply to all G protein-coupled receptors (GPCRs) of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor is remarkable for a carboxy-terminal helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. Thus the cytoplasmic surface appears to be approximately the right size to bind to the transducin heterotrimer in a one-to-one complex. Future high-resolution structural studies of rhodopsin and other GPCRs will form a basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

Dynamic and steady-state light adaptation of mouse rod photoreceptors in vivo

1. Electroretinographic (ERG) methods were used to investigate the effects of background illumination on the responses of mouse rod photoreceptors in vivo. A paired-flash procedure, involving the recording and analysis of the ERG a-wave response to a bright probe flash presented after a brief test flash, was used to derive the rod response to the test flash in steady background light. A related, step-plus-probe procedure was used to derive the step response of the rods to backgrounds of defined strength. 2. Steady background light produced a maintained derived response that was graded with background strength. Determinations of the full time course of the derived weak-flash response in steady background light, and of the effect of background strength on the flash response at fixed post-test-flash times, showed that moderate backgrounds reduce the peak amplitude and duration of the flash response. 3. The response to stepped onset of an approximately half-saturating background (1.2 sc cd m(-2)) exhibited a gradual rise over the first 200-300 ms, and an apparent subsequent relaxation to plateau amplitude within 1 s after background onset. Determinations of normalized amplitudes of the derived response to a test flash presented at 50 or 700 ms after background onset indicated substantial development of background-induced shortening of the test flash response within this 1 s period. These findings indicate a time scale of approximately 1 s or less for the near-completion of light adaptation at this background strength. 4. Properties of the derived response to a stepped background and to test flashes presented in steady background light are in general agreement with photocurrent data obtained from mammalian rods in vitro and suggest that the present results describe, to good approximation, the in vivo desensitization of mouse rods by background light.

Phosphorylation of RGS9-1 by an endogenous protein kinase in rod outer segments

Inactivation of the visual G protein transducin, during recovery from photoexcitation, is regulated by RGS9-1, a GTPase-accelerating protein of the ubiquitous RGS protein family. Incubation of dark-adapted bovine rod outer segments with [gamma-(32)P]ATP led to RGS9-1 phosphorylation by an endogenous kinase in rod outer segment membranes, with an average stoichiometry of 0.2-0.45 mol of phosphates/mol of RGS9-1. Mass spectrometry revealed a single major site of phosphorylation, Ser(475). The kinase responsible catalyzed robust phosphorylation of recombinant RGS9-1 and not of an S475A mutant. A synthetic peptide corresponding to the region surrounding Ser(475) was also phosphorylated, and a similar peptide with the S475A substitution inhibited RGS9-1 phosphorylation. The RGS9-1 kinase is a peripheral membrane protein that co-purifies with rhodopsin in sucrose gradients and can be extracted in buffers of high ionic strength. It is not inhibited or activated significantly by a panel of inhibitors or activators of protein kinase A, protein kinase G, rhodopsin kinase, CaM kinase II, casein kinase II, or cyclin-dependent kinase 5, at concentrations 50 or more times higher than their reported IC(50) or K(i) values. It was inhibited by the protein kinase C inhibitor bisindolylmaleimide I and by lowering Ca(2+) to nanomolar levels with EGTA; however, it was not stimulated by the addition of phorbol ester, under conditions that significantly enhanced rhodopsin phosphorylation. A monoclonal antibody specific for the Ser(475)-phosphorylated form of RGS9-1 recognized RGS9-1 in immunoblots of dark-adapted mouse retina. Retinas from light-adapted mice had much lower levels of RGS9-1 phosphorylation. Thus, RGS9-1 is phosphorylated on Ser(475) in vivo, and the phosphorylation level is regulated by light and by [Ca(2+)], suggesting the importance of the modification in light adaptation.

Growth factors regulate phototransduction in retinal rods by modulating cyclic nucleotide-gated channels through dephosphorylation of a specific tyrosine residue

Illumination of vertebrate rod photoreceptors leads to a decrease in the cytoplasmic cGMP concentration and closure of cyclic nucleotide-gated (CNG) channels. Except for Ca(2+), which plays a negative feedback role in adaptation, and 11-cis-retinal, supplied by the retinal pigment epithelium, all of the biochemical machinery of phototransduction is thought to be contained within rod outer segments without involvement of extrinsic regulatory molecules. Here we show that insulin-like growth factor-I (IGF-I), a paracrine factor released from the retinal pigment epithelium, alters phototransduction by rapidly increasing the cGMP sensitivity of CNG channels. The IGF-I-signaling pathway ultimately involves a protein tyrosine phosphatase that catalyzes dephosphorylation of a specific residue in the alpha-subunit of the rod CNG channel protein. IGF-I conjointly accelerates the kinetics and increases the amplitude of the light response, distinct from events that accompany adaptation. These effects of IGF-I could result from the enhancement of the cGMP sensitivity of CNG channels. Hence, in addition to long-term control of development and survival of rods, growth factors regulate phototransduction in the short term by modulating CNG channels.

The delta subunit of type 6 phosphodiesterase reduces light-induced cGMP hydrolysis in rod outer segments

The delta subunit of the rod photoreceptor PDE has previously been shown to copurify with the soluble form of the enzyme and to solubilize the membrane-bound form (). To determine the physiological effect of the delta subunit on the light response of bovine rod outer segments, we measured the real time accumulation of the products of cGMP hydrolysis in a preparation of permeablized rod outer segments. The addition of delta subunit GST fusion protein (delta-GST) to this preparation caused a reduction in the maximal rate of cGMP hydrolysis in response to light. The maximal reduction of the light response was about 80%, and the half-maximal effect occurred at 385 nm delta subunit. Several experiments suggest that this effect was not due to the effects of delta-GST on transducin or rhodopsin kinase. Immunoblots demonstrated that exogenous delta-GST solubilized the majority of the PDE in ROS but did not affect the solubility of transducin. Therefore, changes in the solubility of transducin cannot account for the effects of delta-GST in the pH assay. The reduction in cGMP hydrolysis was independent of ATP, which indicates that it was not due to effects of delta-GST on rhodopsin kinase. In addition to the effect on cGMP hydrolysis, the delta-GST fusion protein slowed the turn-off of the system. This is probably due, at least in part, to an observed reduction in the GTPase rate of transducin in the presence of delta-GST. These results demonstrate that delta-GST can modify the activity of the phototransduction cascade in preparations of broken rod outer segments, probably due to a functional uncoupling of the transducin to PDE step of the signal transduction cascade and suggest that the delta subunit may play a similar role in the intact outer segment.

Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

Timing of Ca(2+) release from intracellular stores and the electrical response of Limulus ventral photoreceptors to dim flashes

Light-induced release of Ca(2+) from stores in Limulus ventral photoreceptors was studied using confocal fluorescence microscopy and the Ca(2+) indicator dyes, Oregon green-5N and fluo-4. Fluorescence was collected from a spot within 4 microm of the microvillar membrane. A dual-flash protocol was used to reconstruct transient elevations of intracellular free calcium ion concentration (Ca(i)) after flashes delivering between 10 and 5 x 10(5) effective photons. Peak Ca(i) increased with flash intensity to 138 +/- 76 microM after flashes delivering approximately 10(4) effective photons, while the latent period of the elevation of Ca(i) fell from approximately 140 to 21 ms. The onset of the light-induced elevation of Ca(i) was always highly correlated with that of the receptor potential. The time for Ca(i) to exceed 2 microM was approximately equal to that for the receptor potential to exceed 8 mV (mean difference; 2.2 +/- 6.4 ms). Ca(i) was also measured during steps of light delivering approximately 10(5) effective photons/s to photoreceptors that had been bleached with hydroxylamine so as to reduce their quantum efficiency. Elevations of Ca(i) were detected at the earliest times of the electrical response to the steps of light, when a significant receptor potential had yet to develop. Successive responses exhibited stochastic variation in their latency of up to 20 ms, but the elevation of Ca(i) and the receptor potential still rose at approximately the same time, indicating a shared process generating the latent period. Light-induced elevations of Ca(i) resulted from Ca(2+) release from intracellular stores, being abolished by cyclopiazonic acid (CPA), an inhibitor of endoplasmic reticulum Ca(2+) pumps, but not by removal of extracellular Ca(2+) ions. CPA also greatly diminished and slowed the receptor potential elicited by dim flashes. The results demonstrate a rapid release of Ca(2+) ions that appears necessary for a highly amplified electrical response to dim flashes.

Longitudinal spread of second messenger signals in isolated rod outer segments of lizards

1. In vertebrate rods activation of the phototransduction cascade by light triggers changes in the concentrations of at least two diffusible intracellular second messengers (cGMP and Ca2+) whose actions depend on how far they spread from their site of production or entry. To address questions about their spatial spread, cell-attached patch current recording and fluorescence imaging of Calcium Green-dextran were used to measure the longitudinal spread of cGMP and Ca2+, respectively, in functionally intact isolated Gecko gecko lizard rod outer segments under whole-cell voltage clamp. 2. The light-evoked changes in cGMP and Ca2+ concentrations decayed with distance from a site of steady focal activation by two-photon absorption of 1064 nm light with similar decay lengths of approximately 3.5 microm. 3. These results can be understood on the basis of a quantitative model of coupled diffusible intracellular messengers, which is likely to have broad relevance for second messenger signalling pathways in general. 4. The decay length for the spread of adaptation from a site of steady local illumination was about 8 microm, i.e. substantially longer than the decay lengths measured for the spread of cGMP and Ca2+. There are a number of factors, however, that could broaden the apparent relationship between functional changes in the light response and the concentration of a diffusible messenger. For these reasons the measured decay length is an upper limit estimate of the spread of adaptation and does not rule out the possibility that Ca2+ and/or cGMP carry the adaptation signal.

Molecular mechanisms of vertebrate photoreceptor light adaptation

An important recent advance in the understanding of vertebrate photoreceptor light adaptation has come from the discovery that as many as eight distinct molecular mechanisms may be involved, and the realization that one of the principal mechanisms is not dependent on calcium. Quantitative analysis of these mechanisms is providing new insights into the nature of rod photoreceptor light adaptation.

Rhodopsin: a prototypical G protein-coupled receptor

A variety of spectroscopic and biochemical studies of recombinant site-directed mutants of rhodopsin and related visual pigments have been reported over the past 9 years. These studies have elucidated key structural elements common to visual pigments. In addition, systematic analysis of the chromophore-binding pocket in rhodopsin and cone pigments has led to an improved understanding of the mechanism of the opsin shift, and of particular molecular determinants underlying color vision in humans. Identification of the conformational changes that occur on rhodopsin photoactivation has been of particular recent concern. Assignments of light-dependent molecular alterations to specific regions of the chromophore have also been attempted by studying native opsins regenerated with synthetic retinal analogs. Site-directed mutagenesis of rhodopsin has also provided useful information about the retinal-binding pocket and the molecular mechanism of rhodopsin photoactivation. Individual molecular groups have been identified to undergo structural alterations or environmental changes during photoactivation. Analysis of particular mutant pigments in which specific groups are locked into their respective "off" or "on" states has provided a framework to identify determinants of the active conformation, as well as the minimal number of intramolecular transitions required to switch between inactive and active conformations. A simple model for the active state of rhodopsin can be compared to structural models of its ground state to localize chromophore-protein interactions that may be important in the photoactivation mechanism. This review focuses on the recent functional characterization of site-directed mutants of bovine rhodopsin and some cone pigments. In addition, an attempt is made to reconcile previous key findings and existing structural models with information gained from the analysis of site-directed mutant pigments.

Kinetics of recovery of the dark-adapted salamander rod photoresponse

The kinetics of the dark-adapted salamander rod photocurrent response to flashes producing from 10 to 10(5) photoisomerizations (Phi) were investigated in normal Ringer's solution, and in a choline solution that clamps calcium near its resting level. For saturating intensities ranging from approximately 10(2) to 10(4) Phi, the recovery phases of the responses in choline were nearly invariant in form. Responses in Ringer's were similarly invariant for saturating intensities from approximately 10(3) to 10(4) Phi. In both solutions, recoveries to flashes in these intensity ranges translated on the time axis a constant amount (tauc) per e-fold increment in flash intensity, and exhibited exponentially decaying "tail phases" with time constant tauc. The difference in recovery half-times for responses in choline and Ringer's to the same saturating flash was 5-7 s. Above approximately 10(4) Phi, recoveries in both solutions were systematically slower, and translation invariance broke down. Theoretical analysis of the translation-invariant responses established that tauc must represent the time constant of inactivation of the disc-associated cascade intermediate (R*, G*, or PDE*) having the longest lifetime, and that the cGMP hydrolysis and cGMP-channel activation reactions are such as to conserve this time constant. Theoretical analysis also demonstrated that the 5-7-s shift in recovery half-times between responses in Ringer's and in choline is largely (4-6 s) accounted for by the calcium-dependent activation of guanylyl cyclase, with the residual (1-2 s) likely caused by an effect of calcium on an intermediate with a nondominant time constant. Analytical expressions for the dim-flash response in calcium clamp and Ringer's are derived, and it is shown that the difference in the responses under the two conditions can be accounted for quantitatively by cyclase activation. Application of these expressions yields an estimate of the calcium buffering capacity of the rod at rest of approximately 20, much lower than previous estimates.

Bleached pigment produces a maintained decrease in outer segment Ca2+ in salamander rods

A spot confocal microscope based on an argon ion laser was used to make measurements of cytoplasmic calcium concentration (Ca2+i) from the outer segment of an isolated rod loaded with the fluorescent calcium indicator fluo-3 during simultaneous suction pipette recording of the photoresponse. The decline in fluo-3 fluorescence from a rod exposed to saturating illumination was best fitted by two exponentials of approximately equal amplitude with time constants of 260 and 2,200 ms. Calibration of fluo-3 fluorescence in situ yielded Ca2+i estimates of 670 +/- 250 nM in a dark-adapted rod and 30 +/- 10 nM during response saturation after exposure to bright light (mean +/- SD). The resting level of Ca2+i was significantly reduced after bleaching by the laser spot, peak fluo-3 fluorescence falling to 56 +/- 5% (SEM, n = 9) of its value in the dark-adapted rod. Regeneration of the photopigment with exogenous 11-cis-retinal restored peak fluo-3 fluorescence to a value not significantly different from that originally measured in darkness, indicating restoration of the dark-adapted level of Ca2+i. These results are consistent with the notion that sustained activation of the transduction cascade by bleached pigment produces a sustained decrease in rod outer segment Ca2+i, which may be responsible for the bleach-induced adaptation of the kinetics and sensitivity of the photoresponse.

G-protein deactivation is rate-limiting for shut-off of the phototransduction cascade

Photoreceptors detect light through a seven-helix receptor (rhodopsin) and heterotrimeric G protein (transducin) coupled to a cyclic GMP phosphodiesterase. Similar pathways are used to amplify responses to hormones, taste and smell. The amplification of phototransduction is reduced by a fall in cytoplasmic Ca2+ , but it is not known how the deactivation of rhodopsin and transducin influence this response and hence the extent and duration of phosphodiesterase activity. Here we investigate this by recording the electrical response to flashes of light in truncated rod photoreceptors. By removing ATP to block the deactivation of rhodopsin by phosphorylation, we show that this reaction limits the amplitude of the response and begins within 3.2 s of a flash in a solution containing 1 microM Ca2+, falling to 0.9 s in a zero-Ca2+ solution. In contrast, the activation and amplitude of the response were unaffected when transducin deactivation by GTP hydrolysis was blocked by replacing GTP with its nonhydrolysable analogue GTP-gammaS, demonstrating that there is little GTP hydrolysis occurring over the period in which photoexcited rhodopsin is quenched. The rapid deactivation of rhodopsin is therefore a Ca2+-sensitive step controlling the amplitude of the light response, whereas transducin deactivation is slower and controls recovery.

Activation and inactivation steps in the visual transduction pathway

Recent genetic, biochemical and electrophysiological evidence has provided insights into the molecular identity of the substance responsible for bleaching desensitization in vision. Studies examining the molecular defects that cause delayed dark adaptation suggest that the desensitizing substance is recognized by rhodopsin kinase and/or arrestin and, therefore, is probably a complex comprising all-trans-retinal and opsin.

Light adaptation in Pecten hyperpolarizing photoreceptors. Insensitivity to calcium manipulations

The ability of scallop hyperpolarizing photoreceptors to respond without attenuation to repetitive flashes, together with their low light sensitivity, lack of resolvable quantum bumps and fast photoresponse kinetics, had prompted the suggestion that these cells may be constitutively in a state akin to light adaptation. We here demonstrate that their photocurrent displays all manifestations of sensory adaptation: (a) The response amplitude to a test flash is decreased in a graded way by background or conditioning lights. This attenuation of the response develops with a time constant of 200-800 ms, inversely related to background intensity. (b) Adapting stimuli shift the stimulus-response curve and reduce the size of the saturating photocurrent. (c) The fall kinetics of the photoresponse are accelerated by light adaptation, and the roll-of of the modulation transfer function is displaced to higher frequencies. This light-induced desensitization exhibits a rapid recovery, on the order of a few seconds. Based on the notion that Ca mediates light adaptation in other cells, we examined the consequences of manipulating this ion. Removal of external Ca reversibly increased the photocurrent amplitude, without affecting light sensitivity, photoresponse kinetics, or susceptibility to background adaptation; the effect, therefore, concerns ion permeation, rather than the regulation of the visual response. Intracellular dialysis with 10 mM BAPTA did not reduce the peak-to-plateau decay of the photocurrent elicited by prolonged light steps, not the background-induced compression of the response amplitude range and the acceleration of its kinetics. Conversely, high levels of buffered free [Ca]i (10 microM) only marginally shifted the sensitivity curve (delta sigma = 0.3 log) and spared all manifestations of light adaptation. These results indicate that hyperpolarizing invertebrate photoreceptors adapt to light, but the underlying mechanisms must utilize pathways that are largely independent of changes in cytosolic Ca. The results are discussed in terms of aspects of commonalty to other ciliary sensory receptor cells.