Kinetics and components of the flash photocurrent of isolated retinal rods of the larval salamander, Ambystoma tigrinum

Loss of the K+ channel Kv2.1 greatly reduces outward dark current and causes ionic dysregulation and degeneration in rod photoreceptors

Vertebrate retinal photoreceptors signal light by suppressing a circulating “dark current” that maintains their relative depolarization in the dark. This dark current is composed of an inward current through CNG channels and NCKX transporters in the outer segment that is balanced by outward current exiting principally from the inner segment. It has been hypothesized that K_v2.1 channels carry a predominant fraction of the outward current in rods. We examined this hypothesis by comparing whole cell, suction electrode, and electroretinographic recordings from K_v2.1 knockout (K_v2.1^−/−) and wild-type (WT) mouse rods. Single cell recordings revealed flash responses with unusual kinetics, and reduced dark currents that were quantitatively consistent with the measured depolarization of the membrane resting potential in the dark. A two-compartment (outer and inner segment) physiological model based on known ionic mechanisms revealed that the abnormal K_v2.1^−/− rod photoresponses arise principally from the voltage dependencies of the known conductances and the NCKX exchanger, and a highly elevated fraction of inward current carried by Ca²⁺ through CNG channels due to the aberrant depolarization. K_v2.1^−/− rods had shorter outer segments than WT and dysmorphic mitochondria in their inner segments. Optical coherence tomography of knockout animals demonstrated a slow photoreceptor degeneration over a period of 6 mo. Overall, these findings reveal that K_v2.1 channels carry 70–80% of the non-NKX outward dark current of the mouse rod, and that the depolarization caused by the loss of K_v2.1 results in elevated Ca²⁺ influx through CNG channels and elevated free intracellular Ca²⁺, leading to progressive degeneration.

The optoretinogram reveals the primary steps of phototransduction in the living human eye

Photoreceptors initiate vision by converting photons to electrical activity. The onset of the phototransduction cascade is marked by the isomerization of photopigments upon light capture. We revealed that the onset of phototransduction is accompanied by a rapid (<5 ms), nanometer-scale electromechanical deformation in individual human cone photoreceptors. Characterizing this biophysical phenomenon associated with phototransduction in vivo was enabled by high-speed phase-resolved optical coherence tomography in a line-field configuration that allowed sufficient spatiotemporal resolution to visualize the nanometer/millisecond-scale light-induced shape change in photoreceptors. The deformation was explained as the optical manifestation of electrical activity, caused due to rapid charge displacement following isomerization, resulting in changes of electrical potential and surface tension within the photoreceptor disc membranes. These all-optical recordings of light-induced activity in the human retina constitute an optoretinogram and hold remarkable potential to reveal the biophysical correlates of neural activity in health and disease.

Voltage-clamp recordings of light responses from wild-type and mutant mouse cone photoreceptors

We describe the first extensive study of voltage-clamp current responses of cone photoreceptors in unlabeled, dark-adapted mouse retina using only the position and appearance of cone somata as a guide. Identification was confirmed from morphology after dye filling. Photocurrents recorded from wild-type mouse cones were biphasic with a fast cone component and a slower rod component. The rod component could be eliminated with dim background light and was not present in mouse lines lacking the rod transducin-α subunit (Gnat1-/- ) or connexin 36 (Cx36-/- ). Cones from Gnat1-/- or Cx36-/- mice had resting membrane potentials between -45 and -55 mV, peak photocurrents of 20-25 picoamps (pA) at a membrane potential Vm = -50 mV, sensitivities 60-70 times smaller than rods, and a total membrane capacitance two to four times greater than rods. The rate of activation (amplification constant) was largely independent of the brightness of the flash and was 1-2 s-2, less than half that of rods. The role of Ca2+-dependent transduction modulation was investigated by recording from cones in mice lacking rod transducin (Gnat1), recoverin, and/or the guanylyl-cyclase-activating proteins (GCAPs). In confirmation of previous results, responses of Gnat1-/- ;Gcaps-/- cones and triple-mutant Gnat1-/- ;Gcaps-/- ;Rv-/- cones recovered more slowly both to light flashes and steps and were more sensitive than cones expressing the GCAPs. Cones from all four mouse lines showed significant recovery and escaped saturation even in bright background light. This recovery occurred too rapidly to be caused by pigment bleaching or metaII decay and appears to reflect some modulation of response inactivation in addition to those produced by recoverin and the GCAPs. Our experiments now make possible a more detailed understanding of the cellular physiology of mammalian cone photoreceptors and the role of conductances in the inner and outer segment in producing cone light responses.

Implications of dimeric activation of PDE6 for rod phototransduction

We examine the implications of a recent report providing evidence that two transducins must bind to the rod phosphodiesterase to elicit significant hydrolytic activity. To predict the rod photoreceptor's electrical response, we use numerical simulation of the two-dimensional diffusional contact of interacting molecules at the surface of the disc membrane, and then we use the simulated PDE activity as the driving function for the downstream reaction cascade. The results account for a number of aspects of rod phototransduction that have previously been puzzling. For example, they explain the existence of a greater initial delay in rods than in cones. Furthermore, our analysis suggests that the 'continuous' noise recorded in rods in darkness is likely to arise from spontaneous activation of individual molecules of PDE at a rate of a few tens per second per rod, probably as a consequence of spontaneous activation of transducins at a rate of thousands per second per rod. Hence, the dimeric activation of PDE in rods provides immunity against spontaneous transducin activation, thereby reducing the continuous noise. Our analysis also provides a coherent quantitative explanation of the amplification underlying the single photon response. Overall, numerical analysis of the dimeric activation of PDE places rod phototransduction in a new light.

Origins of the phototransduction delay as inferred from stochastic and deterministic simulation of the amplification cascade

PURPOSE

To identify steps of the phototransduction cascade responsible for the delay of the photoresponse.

METHODS

Electrical responses of fish (Carassius) cones and Rana ridibunda frog rods and cones were recorded with a suction pipette technique and as an aspartate-isolated mass receptor potential from isolated perfused retinas. Special attention was paid to sufficiently high temporal resolution (1-ms flash, 700 Hz amplification bandpass). Stochastic simulation of the activation steps from photon absorption to the formation of catalytically active phosphodiesterase (PDE) was performed. In addition, a deterministic mathematical model was fit to the experimental responses. The model included a detailed description of the activation steps of the cascade that enabled identification of the role of individual transduction stages in shaping the initial part of the response.

RESULTS

We found that the apparent delay of the photoresponse gets shorter with increasing stimulus intensity and reaches an asymptotic value of approximately 3 ms in cones and greater than or equal to 10 ms in rods. The result seems paradoxical since it is suggested that the delay occurs in the chain of steps from photon absorption to the formation of active transducin (T*) which in cones is, on average, slower than in rods. Stochastic simulation shows that actually the steps from photon absorption to T* may not contribute perceptibly to the delay. Instead, the delay occurs at the stage that couples the cycle of repetitive activation of T by rhodopsin (R*) with the activation of PDE. These steps include formation of T* (= T _α GTP) out of T _αβγ GTP released from the activation cycle and the subsequent interaction of T* with PDE. This poses a problem. The duration of an average cycle of activation of T in rods is approximately 5 ms and is determined by the frequency of collisions between R* and T in the photoreceptor membrane. The frequency is roughly proportional to the surface packing density of T in the membrane. As the packing density of PDE is approximately 12 times lower than that of T, it could be expected that the rate of the T*-PDE interaction were an order of magnitude slower than that of R* and T. As modeling shows, this is the case in rods. However, the delay in cones is approximately 3 ms which could be achieved only at a T*-PDE interaction time of less than or equal to 5 ms. This means that either the frequency of the collisions of T* and PDE, or the efficiency of collisions, or both in cones are approximately ten times higher than in rods. This may be a challenge to the present model of the molecular organization of the photoreceptor membrane.

CONCLUSIONS

The delay of the photoresponse is mainly set by the rate of interaction of T* with PDE. In cones, the delay is shorter than in rods and, moreover, shorter than the duration of the cycle of repetitive activation of T by R*. This poses a problem for the present model of diffusion interaction of phototransduction proteins in the photoreceptor membrane.

Bright flash response recovery of mammalian rods in vivo is rate limited by RGS9

Peinado Allina et al. measure rod responses in living mice across a wide range of flash strengths and find that responses are much faster in vivo than ex vivo, though the biochemical mechanisms underlying the kinetics appear to be the same in both cases. Although RGS9 overexpression sped recovery from bright flashes, faster rod recovery did not improve the temporal resolution of scotopic vision.

The temporal resolution of scotopic vision is thought to be constrained by the signaling kinetics of retinal rods, which use a highly amplified G-protein cascade to transduce absorbed photons into changes in membrane potential. Much is known about the biochemical mechanisms that determine the kinetics of rod responses ex vivo, but the rate-limiting mechanisms in vivo are unknown. Using paired flash electroretinograms with improved signal-to-noise, we have recorded the amplitude and kinetics of rod responses to a wide range of flash strengths from living mice. Bright rod responses in vivo recovered nearly twice as fast as all previous recordings, although the kinetic consequences of genetic perturbations previously studied ex vivo were qualitatively similar. In vivo, the dominant time constant of recovery from bright flashes was dramatically reduced by overexpression of the RGS9 complex, revealing G-protein deactivation to be rate limiting for recovery. However, unlike previous ex vivo recordings, dim flash responses in vivo were relatively unaffected by RGS9 overexpression, suggesting that other mechanisms, such as calcium feedback dynamics that are strongly regulated by the restricted subretinal microenvironment, act to determine rod dim flash kinetics. To assess the consequences for scotopic vision, we used a nocturnal wheel-running assay to measure the ability of wild-type and RGS9-overexpressing mice to detect dim flickering stimuli and found no improvement when rod recovery was speeded by RGS9 overexpression. These results are important for understanding retinal circuitry, in particular as modeled in the large literature that addresses the relationship between the kinetics and sensitivity of retinal responses and visual perception.

Phototransduction early steps model based on Beer-Lambert optical law

The amount of available rhodopsin on the photoreceptor outer segment and its change over time is not considered in classic models of phototransduction. Thus, those models do not take into account the absorptance variation of the outer segment under different brightness conditions. The relationship between the light absorbed by a medium and its absorptance is well described by the Beer-Lambert law. This newly proposed model implements the absorptance variation phenomenon in a set of equations that admit photons per second as input and results in active rhodopsins per second as output. This study compares the classic model of phototransduction developed by Forti et al. (1989) to this new model by using different light stimuli to measure active rhodopsin and photocurrent. The results show a linear relationship between light stimulus and active rhodopsin in the Forti model and an exponential saturation in the new model. Further, photocurrent values have shown that the new model behaves equivalently to the experimental and theoretical data as published by Forti in dark-adapted rods, but fits significantly better under light-adapted conditions. The new model successfully introduced a physics optical law to the standard model of phototransduction adding a new processing layer that had not been mathematically implemented before. In addition, it describes the physiological concept of saturation and delivers outputs in concordance to input magnitudes.

Information persistence evaluated with low-density dot patterns

After more than a century of study, we do not yet fully understand how shapes and patterns are encoded and identified. Greater progress might result from quantifying stimulus information, thus allowing manipulation of the degree to which a shape or pattern can elicit recognition. The present work used discrete dot patterns that are seen as letters of the alphabet. By adjusting the density of the dots in each pattern, one can determine the probability that it will be recognized. The experiments displayed low-density dot patterns to human respondents, assessing the interval across which non-redundant information provided by two compatible subsets would combine to elicit recognition. This provided a measure of the time required for decay of information persistence. Viewed in the context of prior work, the evidence indicates that the retina mediates initial visibility of the stimulus trace, but the longer-duration persistence required for memory retrieval is mediated by visual cortex.

Hypothalamic gonadotropin-releasing hormone (GnRH) receptor neurons fire in synchrony with the female reproductive cycle

Gonadotropin-releasing hormone (GnRH) controls mammalian reproduction via the hypothalamic-pituitary-gonadal (hpg) axis, acting on gonadotrope cells in the pituitary gland that express the GnRH receptor (GnRHR). Cells expressing the GnRHR have also been identified in the brain. However, the mechanism by which GnRH acts on these potential target cells remains poorly understood due to the difficulty of visualizing and identifying living GnRHR neurons in the central nervous system. We have developed a mouse strain in which GnRHR neurons express a fluorescent marker, enabling the reliable identification of these cells independent of the hormonal status of the animal. In this study, we analyze the GnRHR neurons of the periventricular hypothalamic nucleus in acute brain slices prepared from adult female mice. Strikingly, we find that the action potential firing pattern of these neurons alternates in synchrony with the estrous cycle, with pronounced burst firing during the preovulatory period. We demonstrate that GnRH stimulation is sufficient to trigger the conversion from tonic to burst firing in GnRHR neurons. Furthermore, we show that this switch in the firing pattern is reversed by a potent GnRHR antagonist. These data suggest that endogenous GnRH acts on GnRHR neurons and triggers burst firing in these cells during late proestrus and estrus. Our data have important clinical implications in that they indicate a novel mode of action for GnRHR agonists and antagonists in neurons of the central nervous system that are not part of the classical hpg axis.

Evaluating letter recognition, flicker fusion, and the Talbot-Plateau law using microsecond-duration flashes

Four experiments examined the ability of respondents to identify letters that were displayed on an LED array with flashes lasting little more than a microsecond. The first experiment displayed each letter with a single, simultaneous flash of all the dots forming the letter and established the relation of flash intensity to the probability of letter identification. The second experiment displayed the letters with multiple flashes at different frequencies to determine the probability that the sequence of flashes would be perceived as fused. The third experiment displayed the letters at a frequency that was above the flicker-fusion frequency, varying flash intensity to establish the amount needed to elicit a given probability of letter identification. The fourth experiment displayed each letter twice, once at a frequency where no flicker was perceived and also with steady light emission. The intensity of each flash was fixed and the steady intensity was varied; respondents were asked to judge whether the fused-flicker display and the steady display appeared to be the same brightness. Steady intensity was about double the average flash intensity where the two conditions were perceived as being equal in brightness. This is at odds with Talbot-Plateau law, which predicts that these two values should be equal. The law was formulated relative to a flash lasting half of each period, so it is surprising that it comes this close to being correct where the flash occupies only a millionth of the total period.

Activation and quenching of the phototransduction cascade in retinal cones as inferred from electrophysiology and mathematical modeling

PURPOSE

To experimentally identify and quantify factors responsible for the lower sensitivity of retinal cones compared to rods.

METHODS

Electrical responses of frog rods and fish (Carassius) cones to short flashes of light were recorded using the suction pipette technique. A fast solution changer was used to apply a solution that fixed intracellular Ca2+ concentration at the prestimulus level, thereby disabling Ca2+ feedback, to the outer segment (OS). The results were analyzed with a specially designed mathematical model of phototransduction. The model included all basic processes of activation and quenching of the phototransduction cascade but omitted unnecessary mechanistic details of each step.

RESULTS

Judging from the response versus intensity curves, Carassius cones were two to three orders of magnitude less sensitive than frog rods. There was a large scatter in sensitivity among individual cones, with red-sensitive cones being on average approximately two times less sensitive than green-sensitive ones. The scatter was mostly due to different signal amplification, since the kinetic parameters of the responses among cones were far less variable than sensitivity. We argue that the generally accepted definition of the biochemical amplification in phototransduction cannot be used for comparing amplification in rods and cones, since it depends on an irrelevant factor, that is, the cell's volume. We also show that the routinely used simplified parabolic curve fitting to an initial phase of the response leads to a few-fold underestimate of the amplification. We suggest a new definition of the amplification that only includes molecular parameters of the cascade activation, and show how it can be derived from experimental data. We found that the mathematical model with unrestrained parameters can yield an excellent fit to experimental responses. However, the fits with wildly different sets of parameters can be virtually indistinguishable, and therefore cannot provide meaningful data on underlying mechanisms. Based on results of Ca2+-clamp experiments, we developed an approach to strongly constrain the values of many key parameters that set the time course and sensitivity of the photoresponse (such as the dark turnover rate of cGMP, rates of turnoffs of the photoactivated visual pigment and phosphodiesterase, and kinetics of Ca2+ feedback). We show that applying these constraints to our mathematical model enables accurate determination of the biochemical amplification in phototransduction. It appeared that, contrary to many suggestions, maximum biochemical amplification derived for "best" Carassius cones was as high as in frog rods. On the other hand, all turnoff and recovery reactions in cones proceeded approximately 10 times faster than in rods.

CONCLUSIONS

The main cause of the differing sensitivity of rods and cones is cones' ability to terminate their photoresponse faster.

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.

Discovering electrophysiology in photobiology: A brief overview of several photobiological processes with an emphasis on electrophysiology

The mini-review gives special attention to holistic approach and mechanisms of processes. The physical and chemical frames and background for visual perception and signaling are discussed. Perception of photons by retinal rod cells is described in more detail starting from photon absorption and culminating in ion currents. Dark noise and temperature-dependence of photocurrents in photoreceptor cells are analyzed. Perception of polarized light, its effects and informational importance are discussed based on underlying mechanisms and specialized morphological structures of biological organisms. Role of statistics of photons in photoreception is questioned. The review also pinpoints new and developing directions and raises questions for future research.

Shape recognition elicited by microsecond flashes is not based on photon quantity

It is generally thought that the perceptual impact of a brief flash of light is determined by the quantity of photons the flash delivers. This means that only the total quantity of photons is important below a critical duration of about 30-100 ms. Recent findings have challenged this concept and the present work provides additional evidence that it is not correct. The first experiment reported here delivered a given quantity of photons in under 200 μs, either as a single threshold-intensity flash or as multiple flashes at the same intensity. The single flash was ineffective at eliciting recognition, but multiple flashes became progressively more effective as the number of flashes was increased. A second experiment varied the number of 10 μs flashes. The effectiveness of multiple flashes was far higher than would be expected on the basis of the total quantity of photons being delivered. The results of both experiments suggest that the brief transitions of intensity provided by the flashes are far more important than the quantity of photons. A final experiment examined the combined impact from two threshold-intensity flashes as the interstimulus interval was increased. The pair members were able to combine their influence for at least 100 ms. These results call for more attention to how very brief light flashes generate signals that convey image content.

Speed, sensitivity, and stability of the light response in rod and cone photoreceptors: facts and models

The light responses of rod and cone photoreceptors in the vertebrate retina are quantitatively different, yet extremely stable and reproducible because of the extraordinary regulation of the cascade of enzymatic reactions that link photon absorption and visual pigment excitation to the gating of cGMP-gated ion channels in the outer segment plasma membrane. While the molecular scheme of the phototransduction pathway is essentially the same in rods and cones, the enzymes and protein regulators that constitute the pathway are distinct. These enzymes and regulators can differ in the quantitative features of their functions or in concentration if their functions are similar or both can be true. The molecular identity and distinct function of the molecules of the transduction cascade in rods and cones are summarized. The functional significance of these molecular differences is examined with a mathematical model of the signal-transducing enzymatic cascade. Constrained by available electrophysiological, biochemical and biophysical data, the model simulates photocurrents that match well the electrical photoresponses measured in both rods and cones. Using simulation computed with the mathematical model, the time course of light-dependent changes in enzymatic activities and second messenger concentrations in non-mammalian rods and cones are compared side by side.

Lessons from photoreceptors: turning off g-protein signaling in living cells

Phototransduction in retinal rods is one of the most extensively studied G-protein signaling systems. In recent years, our understanding of the biochemical steps that regulate the deactivation of the rod's response to light has greatly improved. Here, we summarize recent advances and highlight some of the remaining puzzles in this model signaling system.

RGS9 concentration matters in rod phototransduction

The transduction of light by retinal rods and cones is effected by homologous G-protein cascades whose rates of activation and deactivation determine the sensitivity and temporal resolution of photoreceptor signaling. In mouse rods, the rate-limiting step of deactivation is hydrolysis of GTP by the G-protein-effector complex, catalyzed by the RGS9 complex. Here, we incorporate a "Michaelis module" describing the RGS9 reaction into the conventional scheme for phototransduction and show that this augmented scheme can account precisely for the dominant recovery rate of intact rods in which RGS9 expression varies over a 20-fold range. Furthermore, by screening the parameter space of the scheme with maximum-likelihood methodology, we tested specific hypotheses about the rate constant for rhodopsin deactivation, and about the forward, reverse, and catalytic constants for RGS9-mediated G-protein deactivation. These tests reliably exclude lifetimes > approximately 50 ms for rhodopsin, and reveal that the dominant time constant of rod photoresponse recovery is 1/(V(max)/K(m)) for the RGS9 reaction, with k(cat)/K(m) approximately = 0.04 microm(2) s(-1) and k(cat) > 35 s(-1) (or K(m) > 840 microm(-2)). All together, the new kinetic scheme and analysis explain how and why RGS9 concentration matters in rod phototransduction, and they provide a framework for understanding the molecular mechanisms that rate-limit deactivation in other G-protein systems.

Age-related decline in rod phototransduction sensitivity in rhesus monkeys fed an n-3 fatty acid-deficient diet

PURPOSE

Docosahexaenoic acid (DHA), an n-3 fatty acid, is the major polyunsaturate in rod outer segments. The effect of long-term n-3 fatty acid deficiency on rod and cone phototransduction was investigated in the rhesus monkey.

METHODS

From birth to approximately = 9 years rhesus monkeys were fed an n-3-deficient diet (n = 9) known to reduce retinal DHA by 80%. Monkeys in the control group (n = 12) received either 8% alpha-linolenic acid (ALA) or 0.6% DHA, both of which support normal retinal DHA levels. None of the diets contained carotenoids. Photoactivation kinetics were assessed from the rate of increase and a P3 model fit of the ERG a-wave. Maximal cone amplitude and sensitivity were measured from the cone a-wave at 4 ms. The rod photoresponse and rod recovery were derived by using a paired flash

METHOD

RESULTS

Rod sensitivity was reduced by 40% in the n-3-deficient monkeys at 9 but not 4.5 years. The onset of the rising phase of the photoresponse was significantly delayed (P < 0.004) at 9 years. Rod recovery was delayed by 20% in n-3-deficient monkeys at both ages, but only for bright saturating flashes. Cone phototransduction was not altered by n-3 deficiency.

CONCLUSIONS

Long-term dietary n-3 deficiency in the rhesus monkey was associated with two changes in retinal function. First, there was a delay in rod recovery that has remained relatively constant throughout life. Second, there was an age-dependent loss in rod phototransduction sensitivity; the lack of dietary carotenoids may have contributed to this decline.

Ligand binding and activation in a prokaryotic cyclic nucleotide-modulated channel

We designed a technique that directly determines binding of cyclic nucleotides to the prokaryotic cyclic nucleotide modulated ion channel MloK1. The ability to purify large quantities of MloK1 facilitated equilibrium binding assays, which avoided the inherent problem of relatively low affinity binding which hindered the use of eukaryotic channels. We found that MloK1 specifically binds cAMP and cGMP with affinity values in the range of those observed for activity assays for eukaryotic channels. Notably, the concentration of ligand that elicited 50% of maximum response in (86)Rb flux assays (K1/2), also referred to as ligand sensitivity, was smaller than the corresponding value obtained from binding assays (Kd) potentially indicating significant channel activity in partially liganded states. To gain further insight into the mechanism of binding and activation of these channels, we mutated several amino acids in the ligand-binding pocket of MloK1, known from electrophysiological studies of homologous eukaryotic channels to affect ligand selectivity and binding efficacy. The S308V MloK1 mutant (a mutation which decreases cGMP selectivity in eukaryotic channels) decreased both the observed cGMP binding affinity and the sensitivity to cGMP relative to the wild-type (WT) channel, leaving those for cAMP unchanged. Conversely, the A352D MloK1 mutant (a mutation which increases cGMP selectivity in eukaryotic channels) increased both the affinity and the sensitivity for cGMP relative to the WT channel, again leaving those for cAMP unchanged. Mutations at R307 in MloK1, the most conserved residue in the binding pocket of cyclic nucleotide-binding proteins, were not tolerated as these mutants do not form functional channels. Furthermore, for each mutation, changes in binding affinities were mirrored by equivalent changes in ligand sensitivity. These data, together with the evidence that partially liganded channels open significantly, suggested strong coupling between cyclic nucleotide binding and MloK1 channel opening.

Functional characterization and molecular cloning of the K+-dependent Na+/Ca2+ exchanger in intact retinal cone photoreceptors

Light-dependent changes in cytoplasmic free Ca(2+) are much faster in the outer segment of cone than rod photoreceptors in the vertebrate retina. In the limit, this rate is determined by the activity of an electrogenic Na(+)/Ca(2+) exchanger located in the outer segment plasma membrane. We investigate the functional properties of the exchanger activity in intact, single cone photoreceptors isolated from striped bass retina. Exchanger function is characterized through analysis both of the electrogenic exchanger current and cytoplasmic free Ca(2+) measured with optical probes. The exchanger in cones is K(+) dependent and operates both in forward and reverse modes. In the reverse mode, the K(+) dependence of the exchanger is described by binding to a single site with K(1/2) about 3.6 mM. From the retina of the fish we cloned exchanger molecules bassNCKX1 and bassNCKX2. BassNCKX1 is a single class of molecules, homologous to exchangers previously cloned from mammalian rods. BassNCKX2 exists in four splice variants that differ from each other by small sequence differences in the single, large cytoplasmic loop characteristic of these molecules. We used RT-PCR (reverse transcriptase polymerase chain reaction) of individual cells to identify the exchanger molecule specifically expressed in bass single and twin cone photoreceptors. Each and every one of the four bassNCKX2 splice variants is expressed in both single and twin cones indistinguishably. BassNCKX1 is not expressed in cones and, by exclusion, it is likely to be an exchanger expressed in rods.

The pharmacology of cyclic nucleotide-gated channels: emerging from the darkness

Cyclic nucleotide-gated (CNG) ion channels play a central role in vision and olfaction, generating the electrical responses to light in photoreceptors and to odorants in olfactory receptors. These channels have been detected in many other tissues where their functions are largely unclear. The use of gene knockouts and other methods have yielded some information, but there is a pressing need for potent and specific pharmacological agents directed at CNG channels. To date there has been very little systematic effort in this direction - most of what can be termed CNG channel pharmacology arose from testing reagents known to target protein kinases or other ion channels, or by accident when researchers were investigating other intracellular pathways that may regulate the activity of CNG channels. Predictably, these studies have not produced selective agents. However, taking advantage of emerging structural information and the increasing knowledge of the biophysical properties of these channels, some promising compounds and strategies have begun to emerge. In this review we discuss progress on two fronts, cyclic nucleotide analogs as both activators and competitive inhibitors, and inhibitors that target the pore or gating machinery of the channel. We also discuss the potential of these compounds for treating certain forms of retinal degeneration.

Photoreceptors of Nrl -/- mice coexpress functional S- and M-cone opsins having distinct inactivation mechanisms

The retinas of mice null for the neural retina leucine zipper transcription factor (Nrl-/-) contain no rods but are populated instead with photoreceptors that on ultrastructural, histochemical, and molecular criteria appear cone like. To characterize these photoreceptors functionally, responses of single photoreceptors of Nrl-/- mice were recorded with suction pipettes at 35-37 degrees C and compared with the responses of rods of WT mice. Recordings were made either in the conventional manner, with the outer segment (OS) drawn into the pipette ("OS in"), or in a novel configuration with a portion of the inner segment drawn in ("OS out"). Nrl-/- photoreceptor responses recorded in the OS-out configuration were much faster than those of WT rods: for dim-flash responses tpeak = 91 ms vs. 215 ms; for saturating flashes, dominant recovery time constants, tau(D) = 110 ms vs. 240 ms, respectively. Nrl-/- photoreceptors in the OS-in configuration had reduced amplification, sensitivity, and slowed recovery kinetics, but the recording configuration had no effect on rod response properties, suggesting Nrl-/- outer segments to be more susceptible to damage. Functional coexpression of two cone pigments in a single mammalian photoreceptor was established for the first time; the responses of every Nrl-/- cell were driven by both the short-wave (S, lambda(max) approximately 360 nm) and the mid-wave (M, lambda(max) approximately 510 nm) mouse cone pigment; the apparent ratio of coexpressed M-pigment varied from 1:1 to 1:3,000 in a manner reflecting a dorso-ventral retinal position gradient. The role of the G-protein receptor kinase Grk1 in cone pigment inactivation was investigated in recordings from Nrl-/-/Grk1-/- photoreceptors. Dim-flash responses of cells driven by either the S- or the M-cone pigment were slowed 2.8-fold and 7.5-fold, respectively, in the absence of Grk1; the inactivation of the M-pigment response was much more seriously retarded. Thus, Grk1 is essential to normal inactivation of both S- and M-mouse cone opsins, but S-opsin has access to a relatively effective, Grk1-independent inactivation pathway.

Inverted photocurrent responses from amphibian rod photoreceptors: role of membrane voltage in response recovery

We recorded photocurrent responses of retinal rods isolated from cane toads Bufo marinus and clawed frogs Xenopus laevis. With the outer segment drawn part way into the suction pipette, presentation of flashes to the base of the outer segment (outside the pipette) elicited a slow inverted response. Stimulation of the same region, with the outer segment drawn fully in, gave a response of conventional polarity. For moderate to bright flashes a fast transient preceded the slow inverted response. Upon bleaching the tip of the outer segment, the slow inverted response was abolished but the fast initial transient remained, and we attribute this fast component to a capacitive current. Experiments employing simultaneous whole-cell patch-clamp and suction pipette recording revealed that both the fast and slow components of the inverted responses were absent in voltage-clamped cells. In current-clamped cells the slow inverted current response was delayed substantially with respect to the voltage response. We present a computational model for the slow component, in which hyperpolarization leads to increased activity of the Na+ -Ca2+, K+ exchanger, hence lowering the cytoplasmic Ca2+ concentration, activating guanylyl cyclase, raising cyclic GMP concentration, opening cyclic nucleotide-gated channels, and increasing circulating current in the unstimulated region. For the measured voltage response to stimulation of the base, we solve these equations to predict the photocurrent in the tip, and obtain an adequate explanation of the inverted responses. Our work suggests a novel role for membrane voltage in accelerating the inactivation phase of the response to light.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) ion channels were first discovered in rod photoreceptors, where they are responsible for the primary electrical signal of the photoreceptor in response to light. CNG channels are highly specialized membrane proteins that open an ion-permeable pore across the membrane in response to the direct binding of intracellular cyclic nucleotides. CNG channels have been identified in a number of other tissues, including the brain, where their roles are only beginning to be appreciated. Recently, significant progress has been made in understanding the molecular mechanisms underlying their functional specializations. From these studies, a picture is beginning to emerge for how the binding of cyclic nucleotide is transduced into the opening of the pore and how this allosteric transition is modulated by various physiological effectors.

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.

Mouse models to study GCAP functions in intact photoreceptors

In photoreceptor cells cGMP is the second messenger that transduces light into an electrical response. Regulation of cGMP synthesis by Ca2+ is one of the key mechanisms by which Ca2+ exerts negative feedback to the phototransduction cascade in the process of light adaptation. This Ca2+ feedback to retinal guanylyl cyclases (Ret-GCs) is conferred by the guanylate cyclase-activating proteins (GCAPs). Mutations in GCAP1 that disrupt the Ca2+ regulation of Ret-GCs in vitro have been associated with severe human vision disorders. This chapter focuses on recent data obtained from biochemical and electrophysiological studies of GCAP1/GCAP2 knockout mice and other GCAP transgenic mice, addressing: 1. the quantitative aspects of the Ca2+-feedback to Ret-GCs in regulating the light sensitivity and adaptation in intact rods; 2. functional differences between GCAP1 and GCAP2 in intact rod photoreceptors; and 3. whether GCAP mutants with impaired Ca2+ binding lead to retinal disease in vivo by constitutive activation of Ret-GCs and elevation of intracellular cGMP, as predicted from in vitro studies.

Rod and cone contributions to the a-wave of the electroretinogram of the macaque

The electroretinogram (ERG) of anaesthetised dark-adapted macaque monkeys was recorded in response to ganzfeld stimulation and rod- and cone-driven receptoral and postreceptoral components were separated and modelled. The test stimuli were brief (< 4.1 ms) flashes. The cone-driven component was isolated by delivering the stimulus shortly after a rod-saturating background had been extinguished. The rod-driven component was derived by subtracting the cone-driven component from the mixed rod-cone ERG. The initial part of the leading edge of the rod-driven a-wave scaled linearly with stimulus energy when energy was sufficiently low and, for times less than about 12 ms after the stimulus, it was well described by a linear model incorporating a distributed delay and three cascaded low-pass filter elements. Addition of a simple static saturating non-linearity with a characteristic intermediate between a hyperbolic and an exponential function was sufficient to extend application of the model to most of the leading edge of the saturated responses to high energy stimuli. It was not necessary to assume involvement of any other non-linearity or that any significant low-pass filter followed the non-linear stage of the model. A negative inner-retinal component contributed to the later part of the rod-driven a-wave. After suppressing this component by blocking ionotropic glutamate receptors, the entire a-wave up to the time of the first zero-crossing scaled with stimulus energy and was well described by summing the response of the rod model with that of a model describing the leading edge of the rod-bipolar cell response. The negative inner-retinal component essentially cancelled the early part of the rod-bipolar cell component and, for stimuli of moderate energy, made it appear that the photoreceptor current was the only significant component of the leading edge of the a-wave. The leading edge of the cone-driven a-wave included a slow phase that continued up to the peak, and was reduced in amplitude either by a rod-suppressing background or by the glutamate analogue, cis-piperidine-2,3-dicarboxylic acid (PDA). Thus the slow phase represents a postreceptoral component present in addition to a fast component of the a-wave generated by the cones themselves. At high stimulus energies, it appeared less than 5 ms after the stimulus. The leading edge of the cone-driven a-wave was adequately modelled as the sum of the output of a cone photoreceptor model similar to that for rods and a postreceptoral signal obtained by a single integration of the cone output. In addition, the output of the static non-linear stage in the cone model was subject to a low-pass filter with a time constant of no more than 1 ms. In conclusion, postreceptoral components must be taken into account when interpreting the leading edge of the rod- and cone-driven a-waves of the dark-adapted ERG.

Activation, deactivation, and adaptation in vertebrate photoreceptor cells

Visual transduction captures widespread interest because its G-protein signaling motif recurs throughout nature yet is uniquely accessible for study in the photoreceptor cells. The light-activated currents generated at the photoreceptor outer segment provide an easily observed real-time measure of the output of the signaling cascade, and the ease of obtaining pure samples of outer segments in reasonable quantity facilitates biochemical experiments. A quiet revolution in the study of the mechanism has occurred during the past decade with the advent of gene-targeting techniques. These have made it possible to observe how transduction is perturbed by the deletion, overexpression, or mutation of specific components of the transduction apparatus.

Analysis of Ca++-dependent gain changes in PDE activation in vertebrate rod phototransduction

PURPOSE

Recent biochemical and physiological data point to the existence of one or more Ca++-mediated feedback mechanisms modulating gain at stages early in the vertebrate phototransduction cascade, i.e., prior to activation of cGMP-phosphodiesterase (PDE). The present study is a computational analysis that combines quantitative optimization to key data with a qualitative evaluation of each candidate model's ability to capture "signature" features of representative rod responses obtained under a broad range of dark- (DA) and light-adapted (LA) conditions. The primary data motivating the analyses were the two-flash data of Murnick & Lamb. These data exhibited strikingly nonlinear behavior: the period of complete photocurrent saturation (Tsat) in response to a Test flash was reduced substantially when preceded by a less-intense saturating Pre-flash. Depending on the delay between Pre- and Test flashes, the change in Tsat (DTsat) could exceed the magnitude of the delay, and could be reduced by as much as approximately 50%, corresponding to a large reduction in gain by a factor of 10-15. The overall goal of the study was to evaluate what model structure(s) were commensurate with both the Murnick & Lamb data and the salient qualitative features of rod responses obtained under a broad range of DA and LA conditions.

METHODS

Three candidate models were quantitatively optimized to the Murnick & Lamb saturated toad rod flash responses and, simultaneously, to a set of sub-saturated flash responses. Using the parameters from these optimizations, each candidate model was then used to simulate a suite of DA and LA responses.

RESULTS

The analyses showed that: (1) Within the context of a model with Ca++ feedback onto rhodopsin (R*) lifetime (tR), the salient features of the Murnick & Lamb data can only be accounted for if the rate-limiting step is not the Ca++-sensitive step in the early cascade reactions, i.e., if PDE* lifetime, and not tR, is rate-limiting. (2) With tR rate-limiting, the model cannot account for DTsat exceeding the delay. (3) The Ca++-dependent reduction in tR required to effect the large gain is incommensurate with the empirical dynamics of dim-flash responses. (4) Regardless of which reaction is rate-limiting, a model using solely modulation of R* lifetime puts strong constraints on the domain of biochemical parameters commensurate with the large gain changes Murnick & Lamb observed. (5) The analyses show that, in principle, the Murnick & Lamb data can be accounted for when tR is both rate-limiting and Ca++-sensitive if, in addition to the feedback onto tR, there is an earlier, stronger Ca++ feedback that does not affect R* inactivation kinetics (e.g., gain at R* activation or transducin (T*) activation). (6) Ca++-modulation of R* activation or T* activation as the sole early gain mechanism can also account for the Murnick & Lamb data, but fails to predict the data of Matthews, and can thus be rejected along with any model of comparable form.

CONCLUSIONS

The results imply that the Murnick & Lamb data per se are insufficient to rule out rate-limitation by (Ca++-sensitive) R* lifetime; evaluation of a broader set of responses is required. The analyses illustrate the importance of evaluating candidate models in relation to sets of data obtained under the broadest possible range of DA and LA conditions. The analyses are aided by the presence of reproducible signature, qualitative features in the data since these tend to constrain the domain of acceptable model structures and/or parameter sets. Some implications for vertebrate photoreceptor light-adaptation are discussed.

The role of steady phosphodiesterase activity in the kinetics and sensitivity of the light-adapted salamander rod photoresponse

We investigated the kinetics and sensitivity of photocurrent responses of salamander rods, both in darkness and during adaptation to steady backgrounds producing 20-3,000 photoisomerizations per second, using suction pipet recordings. The most intense backgrounds suppressed 80% of the circulating dark current and decreased the flash sensitivity approximately 30-fold. To investigate the underlying transduction mechanism, we expressed the responses as a fraction of the steady level of cGMP-activated current recorded in the background. The fractional responses to flashes of any fixed intensity began rising along a common trajectory, regardless of background intensity. We interpret these invariant initial trajectories to indicate that, at these background intensities, light adaptation does not alter the gain of any of the amplifying steps of phototransduction. For subsaturating flashes of fixed intensity, the fractional responses obtained on backgrounds of different intensity were found to "peel off" from their common initial trajectory in a background-dependent manner: the more intense the background, the earlier the time of peeling off. This behavior is consistent with a background-induced reduction in the effective lifetime of at least one of the three major integrating steps in phototransduction; i.e., an acceleration of one or more of the following: (1) the inactivation of activated rhodopsin (R*); (2) the inactivation of activated phosphodiesterase (E*, representing the complex G(alpha)-PDE of phosphodiesterase with the transducin alpha-subunit); or (3) the hydrolysis of cGMP, with rate constant beta. Our measurements show that, over the range of background intensities we used, beta increased on average to approximately 20 times its dark-adapted value; and our theoretical analysis indicates that this increase in beta is the primary mechanism underlying the measured shortening of time-to-peak of the dim-flash response and the decrease in sensitivity of the fractional response.

Computational analysis of vertebrate phototransduction: combined quantitative and qualitative modeling of dark- and light-adapted responses in amphibian rods

We evaluated the generality of two models of vertebrate phototransduction. The approach was to quantitatively optimize each model to the full waveform of high-quality, dark-adapted (DA), salamander rod flash responses. With the optimal parameters, each model was then used to account for signature, qualitative features of rod responses from three experimental paradigms (stimulus/response, "S/R suite"): (1) step responses; (2) the intensity dependence of the period of photocurrent saturation (Tsat vs. ln(I)); and (3) light-adapted (LA) incremental flash sensitivity as a function of background intensity. The first model was the recent successful model of Nikonov et al. (1998). The second model replaced the instantaneous Ca2+ buffering used in the Nikonov et al. model with a dynamic buffer. The results showed that, in the absence of the dynamic Ca2+ buffer, the Nikonov et al. model does not have sufficient flexibility to provide a good fit to the flash responses, and, using the same parameters, reproduce the salient features of the S/R suite--critical features at step onset and offset are absent; the Tsat function has too shallow a slope; and the model cannot generate the empirically observed I-range of Weber-Fechner LA behavior. Some features could be recovered by changing parameters, but only at the expense of the fit to the reference (Ref) data. When the dynamic buffer is added, the model is able to achieve an acceptable fit to the Ref data while reproducing several features of the S/R suite, including an empirically observed Tsat function, and an extended range of LA flash sensitivity adhering to Weber's law. The overall improved behavior of the model with a dynamic Ca2+ buffer indicates that it is an important mechanism to include in a working model of phototransduction, and that, despite the slow kinetics of amphibian rods, Ca2+ buffering should not be simulated as an instantaneous process. However, neither model was able to capture all the features with the same parameters yielding the optimal fit to the Ref data. In addition, neither model could maintain a good fit to the Ref data when five key biochemical parameters were held at their current known values. Moreover, even after optimization, a number of important parameters remained outside their empirical estimates. We conclude that other mechanisms will need to be added, including additional Ca2+-feedback mechanisms. The present research illustrates the importance of a hybrid qualitative/quantitative approach to model development, and the limitations of modeling restricted sets of data.

UV- and midwave-sensitive cone-driven retinal responses of the mouse: a possible phenotype for coexpression of cone photopigments

Molecular biological, histological and flicker electroretinographic results have established that mice have two cone photopigments, one peaking near 350 nm (UV-cone pigment) and a second near 510 nm [midwave (M)-cone pigment]. The goal of this investigation was to measure the action spectra and absolute sensitivities of the UV-cone- and M-cone-driven b-wave responses of C57BL/6 mice. To achieve this goal, we suppressed rod-driven signals with steady or flashed backgrounds and obtained intensity-response relations for cone-driven b-waves elicited by narrowband flashes between 340 and 600 nm. The derived cone action spectra can be described as retinal1 pigments with peaks at 355 and 508 nm. The UV peak had an absolute sensitivity of approximately 8 nV/(photon microm2) at the cornea, approximately fourfold higher than the M peak. In an attempt to isolate UV-cone-driven responses, it was discovered that an orange conditioning flash (lambda > 530 nm) completely suppressed ERG signals driven by both M pigment- and UV pigment-containing cones. Analysis showed that the orange flash could not have produced a detectable response in the UV-cone pathway were their no linkage between M pigment- and UV pigment-generated signals. Because cones containing predominantly the UV and M pigments have been shown to be located largely in separate parts of the mouse retina (), the most probable linkage is coexpression of M pigment in cones primarily expressing UV pigment. New histological evidence supports this interpretation (). Our data are consistent with an upper bound of approximately 3% coexpression of M pigment in the cones that express mostly the UV pigment.

UV- and midwave-sensitive cone-driven retinal responses of the mouse: a possible phenotype for coexpression of cone photopigments

Molecular biological, histological and flicker electroretinographic results have established that mice have two cone photopigments, one peaking near 350 nm (UV-cone pigment) and a second near 510 nm [midwave (M)-cone pigment]. The goal of this investigation was to measure the action spectra and absolute sensitivities of the UV-cone- and M-cone-driven b-wave responses of C57BL/6 mice. To achieve this goal, we suppressed rod-driven signals with steady or flashed backgrounds and obtained intensity-response relations for cone-driven b-waves elicited by narrowband flashes between 340 and 600 nm. The derived cone action spectra can be described as retinal1 pigments with peaks at 355 and 508 nm. The UV peak had an absolute sensitivity of approximately 8 nV/(photon microm2) at the cornea, approximately fourfold higher than the M peak. In an attempt to isolate UV-cone-driven responses, it was discovered that an orange conditioning flash (lambda > 530 nm) completely suppressed ERG signals driven by both M pigment- and UV pigment-containing cones. Analysis showed that the orange flash could not have produced a detectable response in the UV-cone pathway were their no linkage between M pigment- and UV pigment-generated signals. Because cones containing predominantly the UV and M pigments have been shown to be located largely in separate parts of the mouse retina (), the most probable linkage is coexpression of M pigment in cones primarily expressing UV pigment. New histological evidence supports this interpretation (). Our data are consistent with an upper bound of approximately 3% coexpression of M pigment in the cones that express mostly the UV pigment.

The influence of removing extracellular Ca2+ in the desensitization responses to capsaicin, zingerone and olvanil in rat trigeminal ganglion neurons

Desensitization is a process that describes the diminishing effect of a drug upon repeated applications. In regard to capsaicin, the pungent compound in hot pepper, it is well established that removal of extracellular calcium markedly diminishes desensitization. To explore whether this behavior extends to other analogues of capsaicin, we have determined the effect of removing extracellular calcium with capsaicin analogues, zingerone and olvanil, by whole-cell patch clamping cultured rat trigeminal ganglion neurons. Zingerone, like capsaicin, is pungent but has a shorter acyl chain, whereas olvanil is non-pungent and has a longer acyl chain. The currents evoked by 30-s applications of 30 mM zingerone or 1 microM olvanil repeated every 3 min differ in two important ways from the responses evoked by 1 microM capsaicin under these same conditions. In the presence of extracellular calcium, repeated applications of zingerone and olvanil produce nearly complete desensitization. Also in contrast to capsaicin, removing extracellular calcium for these two agonists does not diminish desensitization. These data analyses suggest the existence of calcium-independent pathways that can result in desensitization, and that pungency is not related to the phenomenon of desensitization.

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.

Actions of Ca2+ on an early stage in phototransduction revealed by the dynamic fall in Ca2+ concentration during the bright flash response

To study the actions of Ca2+ on "early" stages of the transduction cascade, changes in cytoplasmic calcium concentration (Ca2+i) were opposed by manipulating Ca2+ fluxes across the rod outer segment membrane immediately following a bright flash. If the outer segment was exposed to 0 Ca2+/0 Na+ solution for a brief period immediately after the flash, then the period of response saturation was prolonged in comparison with that in Ringer solution. But if the exposure to 0 Ca2+/0Na+ solution instead came before or was delayed until 1 s after the flash then it had little effect. The degree of response prolongation increased with the duration of the exposure to 0 Ca2+/0 Na+ solution, revealing a time constant of 0.49 +/- 0.03 s. By the time the response begins to recover from saturation, Ca2+i seems likely to have fallen to a similar level in each case. Therefore the prolongation of the response when Ca2+i was prevented from changing immediately after the flash seems likely to reflect the abolition of actions of the usual dynamic fall in Ca2+i on an early stage in the transduction cascade at a site which is available for only a brief period after the flash. One possibility is that the observed time constant corresponds to the phosphorylation of photoisomerized rhodopsin.

An alternative phototransduction model for human rod and cone ERG a-waves: normal parameters and variation with age

A quantitative description of the activation reactions in the cGMP phototransduction cascade has been recently developed [Lamb & Pugh (1992). Journal of Physiology, 449, 719-758]. When applied to the human electroretinogram a-wave, the widely used simplified form of this model provides a good description of all waveforms except those elicited with very high energy stimuli. The basis for these misfits at high energies is explored in the current study and an alternative model of phototransduction is derived that retains the quantitative aspects but avoids certain simplifying assumptions previously made. The new model describes well both rod- and cone-isolated a-waves over a large range of stimulus energies extending up to those that cause significant bleaching. To facilitate clinical application of this methodology, a short test protocol is developed and normal data for rod and cone transduction parameters are provided over a wide age range. In the sample of normal subjects studied, maximum amplitude of rod and cone a-waves and sensitivity of the cone a-wave do not change with age. An age-related decline in rod a-wave sensitivity is present and it is greater than that expected from pre-retinal absorption alone.

The kinetics of inactivation of the rod phototransduction cascade with constant Ca2+i

A rich variety of mechanisms govern the inactivation of the rod phototransduction cascade. These include rhodopsin phosphorylation and subsequent binding of arrestin; modulation of rhodopsin kinase by S-modulin (recoverin); regulation of G-protein and phosphodiesterase inactivation by GTPase-activating factors; and modulation of guanylyl cyclase by a high-affinity Ca(2+)-binding protein. The dependence of several of the inactivation mechanisms on Ca2+i makes it difficult to assess the contributions of these mechanisms to the recovery kinetics in situ, where Ca2+i is dynamically modulated during the photoresponse. We recorded the circulating currents of salamander rods, the inner segments of which are held in suction electrodes in Ringer's solution. We characterized the response kinetics to flashes under two conditions: when the outer segments are in Ringer's solution, and when they are in low-Ca2+ choline solutions, which we show clamp Ca2+i very near its resting level. At T = 20-22 degrees C, the recovery phases of responses to saturating flashes producing 10(2.5)-10(4.5) photoisomerizations under both conditions are characterized by a dominant time constant, tau c = 2.4 +/- 0.4 s, the value of which is not dependent on the solution bathing the outer segment and therefore not dependent on Ca2+i. We extended a successful model of activation by incorporating into it a first-order inactivation of R*, and a first-order, simultaneous inactivation of G-protein (G*) and phosphodiesterase (PDE*). We demonstrated that the inactivation kinetics of families of responses obtained with Ca2+i clamped to rest are well characterized by this model, having one of the two inactivation time constants (tau r* or tau PDE*) equal to tau c, and the other time constant equal to 0.4 +/- 0.06 s.

Phototransduction: modeling the primate cone flash response

We have developed a new model of phototransduction that accounts for the dynamics of primate and human cone flash responses in both their linear and saturating range. The model incorporates many of the known elements of the phototransduction cascade in vertebrate photoreceptors. The input stage is a new analytic expression for the activation and inactivation of cGMP-phosphodiesterase (PDE). Although the Lamb and Pugh (1992) model (of a delayed ramp for the rising phase of the PDE response in amphibian rods) provided a good fit for the first 2 log units of stimulus intensity without parameter adjustments, the remaining 4 log units of the data required nonlinear modifications of both delay and gain (slope). We show that this nonlinear behavior is a consequence of the delay approximation and develop a completely linear model to account for the rising phase of amphibian rod photocurrent responses over the full intensity range (approximately 6 log units). We use the same dynamic model to account for primate cone responses by decreasing the time constants of PDE activation and introducing an enhanced inactivation process. This PDE response activates a nonlinear calcium feedback stage that modulates guanylate cyclase synthesis of cyclic GMP. By adjustment of the throughput and feedback parameters, the full model successfully captures most of the features of the primate and human cone flash responses throughout their dynamic range. Our analysis suggests that initial processes in the transduction cascade may be qualitatively different from comparable processes in rods.

Ca2+ flux in retinal rod and cone outer segments: differences in Ca2+ selectivity of the cGMP-gated ion channels and Ca2+ clearance rates

In intact rod and cone photoreceptors of various vertebrate species, depolarization in the dark to > or = +20 mV specifically activates the cGMP-dependent conductance in the outer segment. This activation reflects a voltage-dependent decrease in cytoplasmic Ca2+ and the consequent activation of a Ca(2+)-dependent guanylyl cyclase. The conductance activation in cones is much faster in time course and larger in extent than that in rods. Simulations of the experimental results suggest that these differences arise from differences in Ca2+ homeostasis in the rod and cone outer segments. Direct measurements demonstrate that, indeed, the Ca2+ permeability of the cGMP-gated channels is higher in cones than in rods. Also, as was previously known, the rate of Ca2+ efflux from cone outer segments is higher than that in rods. Therefore, a given light-dependent change in membrane current should cause a much larger and much quicker decrease in Ca2+ concentration in cones than in rods. The activity of every Ca(2+)-dependent biochemical event in the outer segment should, hence, change more rapidly and to a larger extent in cones than in rods. We propose that these kinetic and stoichiometric differences in the function of Ca(2+)-dependent processes is important in explaining the difference in the transduction signal of the two receptor types.

Light adaptation and the rising phase of the flash photocurrent of salamander retinal rods

1. Both theory and analysis of photocurrents in retinal rods show that phosphodiesterase activity after a flash rises initially as a delayed ramp. 2. The effect of light adaptation on the flash-induced rise in phosphodiesterase activity deduced from photocurrent responses was investigated. 3. Background adaptation reduces the deduced rate of rise of phosphodiesterase activity. The effect is most prominent for bright backgrounds and moderate flashes. There is little reduction for bright flashes, even in bright backgrounds. There is no effect for weak backgrounds. 4. Light adaptation after bleaching visual pigment produces a reduction in the deduced rise of phosphodiesterase activity for all flashes. For bright flashes, the reduction is explained by the reduction in quantum catch. For moderate flashes, there is an extra reduction, similar to the reduction produced by the equivalent background. 5. The results provide support for the idea that a reduction in the amplification step of phototransduction functions as part of the mechanism of light adaptation in rods. The dependence on flash intensity of the background-induced reduction in phosphodiesterase activation could imply a feedback mechanism on the activation steps of phototransduction.

Permeability and interaction of Ca2+ with cGMP-gated ion channels differ in retinal rod and cone photoreceptors

We studied the ionic permeability of cGMP-dependent currents in membrane patches detached from the outer segment of retinal cone and rod photoreceptors. Reversal potentials measured in membranes exposed to symmetric Na+ but with varying cytoplasmic Ca2+ concentrations reveal that the permeability ratio, PCa/PNa, is higher in the cGMP-gated channels of cones (7.6 +/- 0.8) than in those of rods (3.1 +/- 1.0). Ca2+ blocks both channels in a voltage-dependent manner. At any Ca2+ concentration, the channel block is maximal near the ionic reversal potential. The maximal block is essentially identical in channels of cones and rods with respect to its extent and voltage and Ca2+ dependence. The Ca2+ block is relieved by voltage, but the features of this relief differ markedly between rods and cones. Whereas the Boltzmann distribution function describes the relief of block by hyperpolarizing voltages, any given voltage is more effective in relieving the Ca2+ block in cones than in rods. Similarly, depolarizing voltages more effectively relieve Ca2+ block in cones than in rods. Our results suggest that channels contain two binding sites for Ca2+, one of which is similar in the two receptor types. The second site either interacts more strongly with Ca2+ than the first one or it is located differently in the membrane, so as to be less sensitive to membrane voltage. The channels in rods and cones differ in the features of this second site. The difference in Ca2+ permeability between the channels is likely to result in light-dependent changes in cytoplasmic Ca2+ concentration that are larger and faster in cones than in rods. The functional differences between channels, therefore, may be critically important in explaining the differences in the phototransduction signal of the two photoreceptor types.

Responses of rod bipolar cells isolated from dogfish retinal slices to concentration-jumps of glutamate

Rod on-bipolar cell light responses are mediated by a class of metabotropic glutamate receptor which is coupled via a G-protein to the control of a cGMP cascade, with cGMP acting to open cation channels, whilst off-bipolar cells possess ionotropic glutamate receptors. Whole-cell voltage-clamp recordings were obtained from on- and off-bipolar cells of dark-adapted dogfish retinal slices, identified by their light responses. Isolated cells were exposed to concentration-jumps of glutamate. At negative voltage-clamp potentials, on-bipolar cells responded to glutamate with outward currents with a mean delay of 10.8 ms, whilst off-bipolar cells responded with inward currents without any delay. Neither cell type showed desensitization to applied steps of glutamate. The dose-response relation for on-bipolar cells showed no gradual saturation, but increased linearly with a sharp cutoff above 200 microM glutamate. This dose-response relation could be fitted with a theoretical expression assuming Michaelis-Menten kinetics for the action of glutamate on receptors and a linear relation between the concentration of receptors bound to glutamate and the fall in cGMP this induces. The dose-response relation of off-bipolar cells showed saturation with a limiting slope of 2 at low glutamate concentrations, suggesting that two molecules of glutamate are required to open each channel by a cooperative mechanism. The glutamate receptor coupled cGMP cascade of rod on-bipolar cells can account for high synaptic voltage gain.

Transduction steps which characterize retinal cone current responses to flash stimuli

There are three prominent characteristics in current responses of retinal cones to flash stimuli when compared with those of rods. First, the time-to-peak of the photocurrent is two to four times faster than for the rod. Second, the photosensitivity of the cone is more than one order of magnitude lower than that of the rod. Third, current responses of cones resemble characteristic under-shoot in the recovery phase. At present, however, it is not known what kinds of mechanisms underlie these characteristics. In the present study, critical steps which characterize cone responses have been investigated by changing rate constants in the model for the signal transduction scheme of cone outer segments under the assumption that the scheme is basically the same as for rods. The computer simulations have shown that rate constants were divided into three categories: those which do not appreciably change photocurrent, those which change only photosensitivity, and those which change both sensitivity and speed. Furthermore, rate constants which characterize undershoot were identified. From these simulations it has been revealed that characteristics of the cone response could be reproduced by changing the rate constants. The critical ones are rate of rhodopsin deactivation, rate of GTP hydrolysis, and deactivation rate of activated phosphodiesterase. These results suggest possibilities that the signal transduction mechanisms in cones are basically the same as those for rods, while rate constants are different.