Patch-clamp recordings of the light-sensitive dark noise in retinal rods from the lizard and frog

Probing Light-Stimulated Activities in the Retina via Transparent Graphene Electrodes

Graphene has triggered tremendous research due to its superior properties. In particular, the intrinsic high light transmission illustrates the unique advantage in neural biosensing. Here, we combine perforated flexible graphene electrodes with microfluidic platforms to explore real-time extracellular electrical activities of retinal ganglion cells (RGCs). Under light stimulation, the transparent graphene electrodes have demonstrated the capability of recording the electrical activities of stimulated RGCs in direct contact. Different types of RGCs have shown three distinct light induced patterns, ON, OFF, and ON-OFF, which are primarily operated by cone photoreceptors. Moreover, the observed spiking waveforms can be divided into two groups: the biphasic waveform usually occurs at contacts with soma, while the triphasic waveform is likely related to the axon. Under high K+ stimulation, the graphene electrodes exhibit higher electrical sensitivity than gold counterparts with an average 2.5-fold enhancement in spiking amplitude. Furthermore, a strong response has been observed with the firing rate first increasing and then ceasing, which could be due to the potassium-induced neural depolarization. These results show that graphene electrodes can be a promising candidate in the electrophysiology studies of retina and offer a route to engineering future two-dimensional materials-based biosensors.

cGMP in mouse rods: the spatiotemporal dynamics underlying single photon responses

Vertebrate vision begins when retinal photoreceptors transduce photons into electrical signals that are then relayed to other neurons in the eye, and ultimately to the brain. In rod photoreceptors, transduction of single photons is achieved by a well-understood G-protein cascade that modulates cGMP levels, and in turn, cGMP-sensitive inward current. The spatial extent and depth of the decline in cGMP during the single photon response (SPR) have been major issues in phototransduction research since the discovery that single photons elicit substantial and reproducible changes in membrane current. The spatial profile of cGMP decline during the SPR affects signal gain, and thus may contribute to reduction of trial-to-trial fluctuations in the SPR. Here we summarize the general principles of rod phototransduction, emphasizing recent advances in resolving the spatiotemporal dynamics of cGMP during the SPR.

Visual threshold is set by linear and nonlinear mechanisms in the retina that mitigate noise: how neural circuits in the retina improve the signal-to-noise ratio of the single-photon response

In sensory biology, a major outstanding question is how sensory receptor cells minimize noise while maximizing signal to set the detection threshold. This optimization could be problematic because the origin of both the signals and the limiting noise in most sensory systems is believed to lie in stimulus transduction. Signal processing in receptor cells can improve the signal-to-noise ratio. However, neural circuits can further optimize the detection threshold by pooling signals from sensory receptor cells and processing them using a combination of linear and nonlinear filtering mechanisms. In the visual system, noise limiting light detection has been assumed to arise from stimulus transduction in rod photoreceptors. In this context, the evolutionary optimization of the signal-to-noise ratio in the retina has proven critical in allowing visual sensitivity to approach the limits set by the quantal nature of light. Here, we discuss how noise in the mammalian retina is mitigated to allow for highly sensitive night vision.

Chromophore switch from 11-cis-dehydroretinal (A2) to 11-cis-retinal (A1) decreases dark noise in salamander red rods

Dark noise, light-induced noise and responses to brief flashes of light were recorded in the membrane current of isolated rods from larval tiger salamander retina before and after bleaching most of the native visual pigment, which mainly has the 11-cis-3,4-dehydroretinal (A2) chromophore, and regenerating with the 11-cis-retinal (A1) chromophore in the same isolated rods. The purpose was to test the hypothesis that blue-shifting the pigment by switching from A2 to A1 will decrease the rate of spontaneous thermal activations and thus intrinsic light-like noise in the rod. Complete recordings were obtained in five cells (21 degrees C). Based on the wavelength of maximum absorbance, lambda max,A1 = 502 nm and lambda max,A2 = 528 nm, the average A2 : A1 ratio determined from rod spectral sensitivities and absorbances was approximately 0.74 : 0.26 in the native state and approximately 0.09 : 0.91 in the final state. In the native (A2) state, the single-quantum response (SQR) had an amplitude of 0.41 +/- 0.03 pA and an integration time of 3.16 +/- 0.15 s (mean +/- s.e.m.). The low-frequency branch of the dark noise power spectrum was consistent with discrete SQR-like events occurring at a rate of 0.238 +/- 0.026 rod(-1) s(-1). The corresponding values in the final state were 0.57 +/- 0.07 pA (SQR amplitude), 3.47 +/- 0.26 s (SQR integration time), and 0.030 +/- 0.006 rod(-1) s(-1) (rate of dark events). Thus the rate of dark events per rod and the fraction of A2 pigment both changed by ca 8-fold between the native and final states, indicating that the dark events originated mainly in A2 molecules even in the final state. By extrapolating the linear relation between event rates and A2 fraction to 0% A2 (100% A1) and 100% A2 (0% A1), we estimated that the A1 pigment is at least 36 times more stable than the A2 pigment. The noise component attributed to discrete dark events accounted for 73% of the total dark current variance in the native (A2) state and 46% in the final state. The power spectrum of the remaining 'continuous' noise component did not differ between the two states. The smaller and faster SQR in the native (A2) state is consistent with the idea that the rod behaves as if light-adapted by dark events that occur at a rate of nearly one per integration time. Both the decreased level of dark noise and the increased SQR amplitude must significantly improve the reliability of photon detection in dim light in the presence of the A1 chromophore compared to the native (A2) state in salamander rods.

Visual cycle and its metabolic support in gecko photoreceptors

Photoreceptors of nocturnal geckos are transmuted cones that acquired rod morphological and physiological properties but retained cone-type phototransduction proteins. We have used microspectrophotometry and microfluorometry of solitary isolated green-sensitive photoreceptors of Tokay gecko to study the initial stages of the visual cycle within these cells. These stages are the photolysis of the visual pigment, the reduction of all-trans retinal to all-trans retinol, and the clearance of all-trans retinol from the outer segment (OS) into the interphotoreceptor space. We show that the rates of decay of metaproducts (all-trans retinal release) and retinal-to-retinol reduction are intermediate between those of typical rods and cones. Clearance of retinol from the OS proceeds at a rate that is typical of rods and is greatly accelerated by exposure to interphotoreceptor retinoid-binding protein, IRBP. The rate of retinal release from metaproducts is independent of the position within the OS, while its conversion to retinol is strongly spatially non-uniform, being the fastest at the OS base and slowest at the tip. This spatial gradient of retinol production is abolished by dialysis of saponin-permeabilized OSs with exogenous NADPH or substrates for its production by the hexose monophosphate pathway (NADP+glucose-6-phosphate or 6-phosphogluconate, glucose-6-phosphate alone). Following dialysis by these agents, retinol production is accelerated by several-fold compared to the fastest rates observed in intact cells in standard Ringer solution. We propose that the speed of retinol production is set by the availability of NADPH which in turn depends on ATP supply within the outer segment. We also suggest that principal source of this ATP is from mitochondria located within the ellipsoid region of the inner segment.

Tuning outer segment Ca2+ homeostasis to phototransduction in rods and cones

Cone photoreceptors respond to light with less sensitivity, faster kinetics and adapt over a much wider range of intensities than do rods. These differences can be explained, in part, by the quantitative differences in the molecular processes that regulate the cytoplasmic free Ca2+ concentration in the outer segment of both receptor types. Ca2+ concentration is regulated through the kinetic balance between the ions' influx and efflux and the action of intracellular buffers. Influx is passive and mediated by the cyclic-GMP gated ion channels. In cones, Ca2+ ions carry about 35% of the ionic current flowing through the channels in darkness. In rods, in contrast, this fraction is about 20%. We present a kinetic rate model of the ion channels that helps explain the differences in their Ca2+ fractional flux. In cones, but not in rods, the cGMP-sensitivity of the cyclic GMP-gated ion channels changes with Ca2+ at the concentrations expected in dark-adapted photoreceptors. Ca2+ efflux is active and mediated by a Na+ and K+-dependent exchanger. The rate of Ca2+ clearance mediated by the exchanger in cones, regardless of the absolute size of their outer segment is of the order of tens of milliseconds. In rod outer segments, and again independently of their size, Ca2+ clearance rate is of the order of hundreds of milliseconds to seconds. We investigate the functional consequences of these differences in Ca2+ homeostasis using computational models of the phototransduction signal in rods and cones. Consistent with experimental observation, differences in Ca2+ homeostasis can make the cone's flash response faster and less sensitive to light than that of rods. In the simulations, however, changing Ca2+ homeostasis is not sufficient to recreate authentic cone responses. Accelerating the rate of inactivation (but NOT activation) of the enzymes of the transduction cascade, in addition, to changes in Ca2+ homeostasis are needed to explain the differences between rod and cone photosignals. The large gain and precise kinetic control of the electrical photoresponse of rod and cone retinal receptors suggested a long time back that phototransduction is mediated by cytoplasmic second messengers that, in turn, control membrane ionic conductance. (1) The unquestionable identification of cyclic GMP as the phototransduction messenger, however, did not come until the mid 1980's with the discovery that the light-regulated membrane conductance in both rods and cones is gated by this nucleotide (2-4) and is, in fact, an ion channel. (7) The cyclic nucleotide gated (CNG) channels, now we know, are not just the compliant targets of light-dependent change in cytoplasmic cGMP, but actively participate in the regulation transduction through Ca2+ feedback signals. The precise magnitude and time course of the concentration changes of cGMP and Ca2+ in either rods or cones remains controversial. It is clear, however, that whereas cGMP directly controls the opening and closing of the plasma membrane channels, Ca2+ controls the light-sensitivity and kinetics of the transduction signal. (8,9) The modulatory role of Ca2+ is particularly apparent in the process of light adaptation: in light-adapted rods or cones, the transduction signal generated by a given flash is lower in sensitivity and faster in time course than in dark-adapted cells. Light adaptation is compromised if Ca2+ concentration changes are attenuated by cytopiasmic Ca2+ buffers (8,10,11) and does not occur if Ca2+ concentration changes are prevented by manipulation of the solution bathing the cells. (2,4) Several Ca2+-dependent biochemical reactions have been identified in photoreceptors, among them: 1. ATP-dependent deactivation. (15,16) 2 Phodopsin phospshorylation, through the action of recoverin (S-modulin). (17-19) 3. Catalytic activity of guanylyl cyclase, (20-22) through the action of GCAP proteins. (23,24,25) 4. cGMP-sensitivity of the CNG channels. (26-29,30) A challenge in contemporary phototransduction research is to understand the details of these reactions and their role in the control of the phototransduction signal. Transduction signals in cone photoreceptors are faster, lower in light sensitivity, and more robust in their adaptation features than those in rods (for review see refs. 31;32). A detailed molecular explanation for these differences is not at hand. However, biochemical and electrophysiological (33) studies indicate that the elements in the light-activated pathway that hydrolyzes cGMP are quantitatively similar in their function in rods and cones and unlikely to account for the functional differences. Also, within the limited exploration completed todate, the Ca2+-dependence of guanylyl cyclase (34) and visual pigment phosphorylation (19) do not differ in rods and cones. On the other hand, data accumulated over the past few years indicate that cytoplasmic Ca2+ homeostasis, while controlled through essentially identical mechanisms it is quantitatively very different in its features in the two photoreceptor types. Both Ca2+ influx through CNG channels and the rate of Ca2+ clearance from the outer segment differ between the two receptor cells. Also, the Ca2+-dependent modulation of cGMP sensitivity is larger in extent in cones than in rods. Most significantly, the concentration range of this Ca2+ dependence overlaps the physiological range of light-dependent changes in cytoplasmic Ca2+ level in cones, but not in rods. We briefly review some of the evidence that supports these assertions and we then provide a quantitative analysis of the possible significance of these known differences. We conclude that while differences in Ca2+ homeostasis contribute importantly to explaining the differences between the two receptor types, they are alone not sufficient to explain the differences in the photoreceptor's response. It is likely that Ca2+-independent inactivation of the transduction cascade enzymes is more rapid in cones than in rods.

Block of Shaker potassium channels by external calcium ions

We describe an interaction between external Ca(2+) ions and Shaker K channels that is important in the gating of the channels. The interaction was first detected as a partial block of inward K(+) current in elevated Ca(2+), beginning near -40 mV and becoming stronger at more negative voltage. Surprisingly, the time course of the block can be resolved as a rapid decay of inward current magnitude following a repolarizing step. The rapid decay of current is shown to be the result of channel block by using a two-pulse procedure that monitors the time course of gate closing. As a result of block, the decay of the tail current after repolarization is two to three times faster than gate closing. With physiological values for voltage and calcium concentration, block is readily detectable from tail time course, implying that it occurs as a normal concomitant of gate closing in Shaker. The slight voltage dependence of block from -60 to -100 mV suggests that Ca(2+) is bound (with low affinity) near the outer mouth of the channel. Elevated calcium quickens the inward gating current recorded as Shaker channels are closing; this current approximately doubles in amplitude and has a faster time course and quicker rising phase. When combined, the results suggest that calcium accelerates the first step in closing of the channel gate, perhaps by changing the channel's ion-occupancy state.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Engineering aspects of enzymatic signal transduction: photoreceptors in the retina

Identifying the basic module of enzymatic amplification as an irreversible cycle of messenger activation/deactivation by a "push-pull" pair of opposing enzymes, we analyze it in terms of gain, bandwidth, noise, and power consumption. The enzymatic signal transduction cascade is viewed as an information channel, the design of which is governed by the statistical properties of the input and the noise and dynamic range constraints of the output. With the example of vertebrate phototransduction cascade we demonstrate that all of the relevant engineering parameters are controlled by enzyme concentrations and, from functional considerations, derive bounds on the required protein numbers. Conversely, the ability of enzymatic networks to change their response characteristics by varying only the abundance of different enzymes illustrates how functional diversity may be built from nearly conserved molecular components.

Mechanisms of single-photon detection in rod photoreceptors

Rod photoreceptors detect and encode incident photons exceptionally well. They collect sparse photons with high efficiency, maintain a low dark noise, and generate reproducible responses to each absorbed photon. The mechanisms involved in single-photon detection--control of the effective lifetime of a single active receptor molecule, amplification of the activity of this single molecule by a second-messenger cascade, and reliable transmission of small synaptic signals--recur throughout the nervous system. Indeed, several other sensory systems reach or approach limits set by quantization of their input signals. For example, olfactory receptors can detect single odorant molecules. Although our understanding of visual transduction and signal processing has advanced rapidly during the past 10-15 years, fundamental questions still remain: What mechanisms are responsible for the reproducibility of the rod's elementary response? What are the tradeoffs of speed and sensitivity in the transduction cascade? How are the rod single-photon responses reliably transmitted to the rest of the visual system? Future technical innovations, particularly better methods to monitor the activity of intermediate steps in transduction, will play an important role in providing answers.

Mechanism of cGMP-gated channel block by intracellular polyamines

Polyamines block the retinal cyclic nucleotide-gated channel from both the intracellular and extracellular sides. The voltage-dependent mechanism by which intracellular polyamines inhibit the channel current is complex: as membrane voltage is increased in the presence of polyamines, current inhibition is not monotonic, but exhibits a pronounced damped undulation. To understand the blocking mechanism of intracellular polyamines, we systematically studied the endogenous polyamines as well as a series of derivatives. The complex channel-blocking behavior of polyamines can be accounted for by a minimal model whereby a given polyamine species (e.g., spermine) causes multiple blocked channel states. Each blocked state represents a channel occupied by a polyamine molecule with characteristic affinity and probability of traversing the pore, and exhibits a characteristic dependence on membrane voltage and cGMP concentration.

Divalent cation interactions with light-dependent K channels. Kinetics of voltage-dependent block and requirement for an open pore

The light-dependent K conductance of hyperpolarizing Pecten photoreceptors exhibits a pronounced outward rectification that is eliminated by removal of extracellular divalent cations. The voltage-dependent block by Ca(2+) and Mg(2+) that underlies such nonlinearity was investigated. Both divalents reduce the photocurrent amplitude, the potency being significantly higher for Ca(2+) than Mg(2+) (K(1/2) approximately 16 and 61 mM, respectively, at V(m) = -30 mV). Neither cation is measurably permeant. Manipulating the concentration of permeant K ions affects the blockade, suggesting that the mechanism entails occlusion of the permeation pathway. The voltage dependency of Ca(2+) block is consistent with a single binding site located at an electrical distance of delta approximately 0.6 from the outside. Resolution of light-dependent single-channel currents under physiological conditions indicates that blockade must be slow, which prompted the use of perturbation/relaxation methods to analyze its kinetics. Voltage steps during illumination produce a distinct relaxation in the photocurrent (tau = 5-20 ms) that disappears on removal of Ca(2+) and Mg(2+) and thus reflects enhancement or relief of blockade, depending on the polarity of the stimulus. The equilibration kinetics are significantly faster with Ca(2+) than with Mg(2+), suggesting that the process is dominated by the "on" rate, perhaps because of a step requiring dehydration of the blocking ion to access the binding site. Complementary strategies were adopted to investigate the interaction between blockade and channel gating: the photocurrent decay accelerates with hyperpolarization, but the effect requires extracellular divalents. Moreover, conditioning voltage steps terminated immediately before light stimulation failed to affect the photocurrent. These observations suggest that equilibration of block at different voltages requires an open pore. Inducing channels to close during a conditioning hyperpolarization resulted in a slight delay in the rising phase of a subsequent light response; this effect can be interpreted as closure of the channel with a divalent ion trapped inside.

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.

Optimal detection of flash intensity differences using rod photocurrent observations

The rod photocurrent contains two noise components that may limit the detectability of flash intensity increments. The limits imposed by the low- and high-frequency noise components were assessed by computing the performance of an optimal detector of increments in flash intensity. The limits imposed by these noise components depend on the interval of observation of the photocurrent signal. When the entire photocurrent signal, lasting 3 or more seconds, is observed, the low-frequency component of the photocurrent noise (attributed to the quantal noise of the incoming light, as well as random isomerizations of enzymes within the phototransduction cascade) is the most significant limitation on detectability. When only the first 380 ms or less is observed, the high-frequency component of the noise (due to the thermal isomerizations of the cGMP-gated channel) presents a significant limit on the detectability of flashes.

Noise and its effects on photoreceptor temporal contrast sensitivity at low light levels

We studied photoreceptors in the locust (Schistocerca americanus) visual system to determine the extent to which quantal noise and intrinsic neural noise limit temporal sensitivity. Typical computational models of the temporal contrast sensitivity function are deterministic, reflect only filter characteristics, and lack explicit noise sources [J. Opt. Soc. Am. 58, 1133 (1968); Vision Res. 32, 1373 (1992)]. We report here that the temporal contrast sensitivity function, at low light levels, is not simply the reflection of a filter function. Our evidence suggests that, at low backgrounds, noise, in conjunction with temporal filtering, plays a role in shaping the temporal contrast sensitivity function. At a given low adaptation level, quantal noise limits sensitivity at low temporal frequencies, while intrinsic noise limits sensitivity at relatively higher temporal frequencies.

Blockade of a retinal cGMP-gated channel by polyamines

The cyclic nucleotide-gated (CNG) channel in retinal rods converts the light-regulated intracellular cGMP concentration to various levels of membrane potential. Blockade of the channel by cations such as Ca2+ and Mg2+ lowers its effective conductance. Consequently, the membrane potential has very low noise, which enables rods to detect light with extremely high sensitivity. Here, we report that three polyamines (putrescine, spermidine, and spermine), which exist in both the intracellular and extracellular media, also effectively block the CNG channel from both sides of the membrane. Among them, spermine has the greatest potency. Extracellular spermine blocks the channel as a permeant blocker, whereas intracellular spermine appears to block the channel in two conformations-one permeant, and the other non- (or much less) permeant. The membrane potential in rods is typically depolarized to approximately -40 mV in the dark. At this voltage, K1/2 of the CNG channel for extracellular spermine is 3 microM, which is 100-1,000-fold higher affinity than that of the NMDA receptor-channel for extracellular spermine. Blockade of the CNG channel by polyamines may play an important role in suppressing noise in the signal transduction system in rods.

Membrane current noise in dark-adapted and light-adapted isolated retinal rods of the larval tiger salamander

1. Low-frequency light-sensitive membrane current noise in isolated rod photoreceptors of the larval tiger salamander was recorded using suction electrodes, in the dark, and during light adaptation by backgrounds or by bleaching visual pigment. 2. In background light, noise variance increases and then decreases. For rods desensitized to similar levels by bleaching visual pigment, the noise variance either does not change (weak adaptation) or decreases (with stronger adaptation). 3. The power spectral density of the current noise in dark-adapted rods shows a component with half-power cut-off frequency at about 0.1 Hz, attributed to spontaneous single events and continuous noise from dark phosphodiesterase activity. A second component, with half-power cut-off frequency at about 1 Hz, may be due to slow components in the light-sensitive channel gating. 4. The power spectral density of the noise in background light is dominated by noise generated by the background. Background light adapts at least the first component of the noise seen in dark-adapted cells. For cells desensitized by bleaching, light adaptation of both components of the dark-adapted noise is observed. 5. The results confirm that the low-frequency noise in dark-adapted cells arises from the transduction mechanism of the rod, in that both components can be light adapted, and show that, for rods permanently desensitized by bleaching, the desensitization is not due to the presence of active visual pigment molecules similar to those produced by background light.

Linear transduction of natural stimuli by dark-adapted and light-adapted rods of the salamander, Ambystoma tigrinum

1. We examined signal, noise and response properties of salamander rod photoreceptors by measuring: (a) the circulating current of rods which were adapted to darkness and to a wide range of backgrounds; (b) contrasts of natural environments; (c) the effect of adaptation on the linear response range of rods; and (d) the behaviour of rods responding to dynamically modulated stimuli having a range of contrasts found in nature. 2. In the dark, the circulating current contained two noise components analogous to those described in toad. A discrete noise component consisted of events occurring at a rate of 1 event per 32 s (21 degrees C) and had a variance of 0.036 pA2. A continuous noise component contributed 0.022 pA2 to the dark current, roughly equal to the discrete noise variance. 3. Exposure to a wide range of steady backgrounds (suppressing up to 80% of the circulating current), elicited a sustained fluctuating photocurrent having a power spectrum which resembled those of single photon responses and was consistent with the linear summation of single photon events; this indicates that the primary source of noise in the current is caused by the light. 4. Eighty-nine per cent of the contrasts (C) measured in natural environments had magnitude of C < 50%, where C = magnitude of I - Imean/magnitude of Imean. The linear response range elicited by brief flashes expanded with brighter backgrounds, well-encompassing flash contrasts of 100%. 5. Dynamically modulated stimuli and incremental flashes having contrasts similar to those in natural scenes elicited small currents which deviated by a few picoamps about the mean and the transfer functions computed from each type of stimulus-response pair closely corresponded to one another. These results indicate that in natural environments, rods behave as linear small-signal transducers of light.

Block of the cGMP-gated cation channel of catfish rod and cone photoreceptors by organic cations

Tetraalkylammonium compounds and other organic cations were used to probe the structure of the internal and external mouths of the pore of cGMP-gated cation channels from rod and cone photoreceptors. Both rod and cone channels were blocked by tetramethyl- through tetrapentylammonium from the intracellular side in a voltage-dependent fashion at millimolar to micromolar concentrations. The dissociation constant at 0 mV (KD(O)) decreased monotonically with increasing carbon chain length from approximately 80 mM (TMA) to approximately 80 microM (TPeA), where the dissociation constant in rod channels is approximately 50% that of cone channels. N-Methyl-D-glucamine and the buffer Tris also blocked the cone channel in a voltage-dependent fashion at millimolar concentrations, but with lower affinity than similarly sized tetraalkylammonium blockers. Block by tetrahexylammonium (THxA) was voltage-independent, suggesting that the diameter of the intracellular mouth of these channels is less than the size of THxA but larger than TPeA. The location of the binding site for intracellular blockers was approximately 40% across the voltage-drop from the intracellular side. The addition of one carbon to each of the alkyl side chains increased the binding energy by approximately 4 kJ mol-1, consistent with hydrophobic interactions between the blocker and the pore. Cone, but not rod, channels were blocked by millimolar concentrations of extracellular TMA. The location of the extracellular binding site was approximately 13% of the voltage drop from the extracellular side. In cone channels, the two blocker binding sites flank the location of the cation binding site proposed previously.

Molecular origin of continuous dark noise in rod photoreceptors

Noise in the rod photoreceptors limits the ability of the dark-adapted visual system to detect dim lights. We investigated the molecular mechanism of the continuous component of the electrical dark noise in toad rods. Membrane current was recorded from intact, isolated rods or truncated, internally dialyzed rod outer segments. The continuous noise was separated from noise due to thermal activation of rhodopsin and to transitions in the cGMP-activated channels. Selectively disabling different elements of the phototransduction cascade allowed examination of their contributions to the continuous noise. These experiments indicate that the noise is generated by spontaneous activation of cGMP phosphodiesterase (PDE) through a process that does not involve transducin. The addition of recombinant gamma, the inhibitory subunit of PDE, did not suppress the noise, indicating that endogenous gamma does not completely dissociate from the catalytic subunit of PDE during spontaneous activation. Quantitative analysis of the noise provided estimates of the rate constants for spontaneous PDE activation and deactivation and the catalytic activity of a single PDE molecule in situ.

Recoverin, a calcium-binding protein in photoreceptors

Recoverin is a Ca2+-binding protein found primarily in vertebrate photoreceptors. The proposed physiological function of recoverin is based on the finding that recoverin inhibits light-stimulated phosphorylation of rhodopsin. Recoverin interacts with rod outer segment membranes in a Ca2+-dependent manner. This interaction requires N-terminal acylation of recoverin. Four types of fatty acids have been detected on the N-terminus of recoverin, but the functional significance of this heterogeneous acylation is not yet clear.

Future directions for rhodopsin structure and function studies

NMR (nuclear magnetic resonance) may be useful for determining the structure of retinal and its environment in rhodopsin, but not for determining the complete protein structure. Aggregation and low yield of fragments of rhodopsin may make them difficult to study by NMR. A long-term multidisciplinary attack on rhodopsin structure is required.

More answers about cGMP-gated channels pose more questions

Our understanding of the molecular properties and cellular role of cGMP-gated channels in outer segments of vertebrate photo-receptors has come from over a decade of studies which have continuously altered and refined ideas about these channels. Further examination of this current view may lead to future surprises and further refine the understanding of cGMP-gated channels.

Cyclic nucleotides as regulators of light-adaptation in photoreceptors

Cyclic nucleotides can regulate the sensitivity of retinal rods to light through phosducin. The phosphorylation state of phosducin determines the amount of G available for activation by Rho*. Phosducin phosphorylation is regulated by cyclic nucleotides through their activation of cAMP-dependent protein kinase. The regulation of phosphodiesterase activity by the noncatalytic cGMP binding sites as well as Ca2+/calmodulin dependent regulation of cGMP binding to the cation channel are also discussed.

Long term potentiation and CaM-sensitive adenylyl cyclase: Long-term prospects

The type I CaM-sensitive adenylyl cyclase is in a position to integrate signals from multiple inputs, consistent with the requirements for mediating long term potentiation (LTP). Biochemical and genetic evidence supports the idea that this enzyme plays an important role inc LTP. However, more work is needed before we will be certain of the role that CaM-sensitive adenylyl cyclases play in LTP.

Modulation of the cGMP-gated channel by calcium

Calcium acting through calmodulin has been shown to regulate the affinity of cyclic nucleotide-gated channels expressed in cell lines. But is calmodulin the Ca-sensor that normally regulates these channels?

How many light adaptation mechanisms are there?

The generally positive response to our target article indicates that most of the commentators accept our contention that light adaptation consists of multiple and possibly redundant mechanisms. The commentaries fall into three general categories. The first deals with putative mechanisms that we chose not to emphasize. The second is a more extended discussion of the role of calcium in adaptation. Finally, additional aspects of cGMP involvement in adaptation are considered. We discuss each of these points in turn.

Gene therapy, regulatory mechanisms, and protein function in vision

Hereditary retinal degeneration due to mutations in visual genes may be amenable to therapeutic interventions that modulate, either positively or negatively, the amount of protein product. Some of the proteins involved in phototransduction are rapidly moved by a lightdependent mechanism between the inner segment and the outer segment in rod photoreceptor cells, and this phenomenon is important in phototransduction.

A novel protein family of neuronal modulators

A number of proteins homologous to recoverin have been identified in the brains of the several vertebrate species. The brainderived members originally contain four EF-hand domains, but NH2- terminal domain is aberrant. Many of these proteins inhibited light-induced rhodopsin phosphorylation at high [Ca2+], suggesting that the brain-derived members may act as a Ca2+-sensitive modulator of receptor phosphorylation, as recoverin does.

The structure of rhodopsin and mechanisms of visual adaptation

Rapidly advancing studies on rhodopsin have focused on new strategies for crystallization of this integral membrane protein for x-ray analysis and on alternative methods for structural determination from nuclear magnetic resonance data. Functional studies of the interactions between the apoprotein and its chromophore have clarified the role of the chromophore in deactivation of opsin and in photoactivation of the pigment.

Crucial steps in photoreceptor adaptation: Regulation of phosphodiesterase and guanylate cyclase activities and Ca2+-buffering

This commentary discusses the balance of phosphodiesterase and guanylate cyclase activities in vertebrate photoreceptors at moderate light intensities. The rate of cGMP hydrolysis and synthesis seem to equal each other. Ca2+ as regulator of both enzyme activities is also effectively buffered in photoreceptor cells by cytoplasmic buffer components.

The atomic structure of visual rhodopsin: How and when?

Strong arguments are presented by Hargrave suggesting that the crystallization of visual rhodopsin for high resolution analysis by X-ray crystallography or electron microscopy is feasible. However, the effort needed to achieve this goal will most likely exceed the resources of a single laboratory and a concerted approach to the research is necessary.

Molecular insights gained from covalently tethering cGMP to the ligand-binding sites of retinal rod cGMP-gated channels

A photoaffinity analog of cGMP has been used to biochemically identify a new ligand-binding subunit of the retinal rod cGMP-activated ion channel, as well as amino acids in contact with cGMP in the original subunit. Covalent tethering of this probe to channels in excised menbrane patches has revealed a functional heteogeneity in the ligand-binding sites that may arise from the two biochemically identified subunits.

Genetic and functional complexity of inherited retinal degeneration

Recent findings emphasize the complexity, both genetic and functional, of the manifold genes and mutations causing inherited retinal degeneration in humans. Knowledge of the genetic bases of these diseases can contribute to design of rational therapy, as well as elucidating the function of each gene product in normal visual processes.

Channel structure and divalent cation regulation of phototransduction

The identification of additional subunits of the cGMP-gated cation channel suggests exciting questions about their regulatory roles and about structure/functional relationships. How do the different subunits interact? How is the complex assembled into the plasma membrane? Divalent cations have been implicated in the regulation of adaptation. One often overlooked cation is magnesium. Could this ion play a role in phototransduction?

Structure of the cGMP-gated channel

The subunit structure of the cGMP-gated cation channel of rod photoreceptors is rapidly being defined, and in the process the mode of regulation by Ca2+-calmodulin unraveled. Intriguingly, early results suggest that additional subunits of unknown function are associated with the channel and remain to be identified.

Linking genotypes with phenotypes in human retinal degenerations: Implications for future research and treatment

Although undoubtedly it will be incomplete by the time it is published, the target article by Daiger et al. organizes mutations in genes that produce retinal degenerations in humans into categories of clinically relevant phenotypes. Such classifications should help us understand the link between altered photoreceptor cell proteins and subsequent cell death, and they may yield insight into methods for preventing consequent blindness.

Genetic and clinical heterogeneity in tapetal retinal dystrophies

Large scale DNA-mutation screening in patients with hereditary retinal diseases greatly enhances our knowledge about retinal function and diseases. Scientists, clinicians, patients, and families involved with retinal disorders may directly benefit from these developments. However, certain aspects of this expanding knowledge, such as the correlation between genotype and phenotype, may be much more complicated than we expect at present.

The determination of rhodopsin structure may require alternative approaches

The structure of rhodopsin is a subject of intense interest. Solving the structure by traditional methods has proved exceedingly challenging. It may therefore be useful to confront the problem by a combination of alternate techniques. These include FTIR (Fourier transform infrared spectroscopy) and AFM (atomic force microscopy) on the intact protein. Furthermore, additional insights may be gained through structural investigations of discrete rhodopsin domains.