The frequency of isomerization-like 'dark' events in rhodopsin and porphyropsin rods of the bull-frog retina

Rhodopsin, light-sensor of vision

The light sensor of vertebrate scotopic (low-light) vision, rhodopsin, is a G-protein-coupled receptor comprising a polypeptide chain with bound chromophore, 11-cis-retinal, that exhibits remarkable physicochemical properties. This photopigment is extremely stable in the dark, yet its chromophore isomerises upon photon absorption with 70% efficiency, enabling the activation of its G-protein, transducin, with high efficiency. Rhodopsin's photochemical and biochemical activities occur over very different time-scales: the energy of retinaldehyde's excited state is stored in <1 ps in retinal-protein interactions, but it takes milliseconds for the catalytically active state to form, and many tens of minutes for the resting state to be restored. In this review, we describe the properties of rhodopsin and its role in rod phototransduction. We first introduce rhodopsin's gross structural features, its evolution, and the basic mechanisms of its activation. We then discuss light absorption and spectral sensitivity, photoreceptor electrical responses that result from the activity of individual rhodopsin molecules, and recovery of rhodopsin and the visual system from intense bleaching exposures. We then provide a detailed examination of rhodopsin's molecular structure and function, first in its dark state, and then in the active Meta states that govern its interactions with transducin, rhodopsin kinase and arrestin. While it is clear that rhodopsin's molecular properties are exquisitely honed for phototransduction, from starlight to dawn/dusk intensity levels, our understanding of how its molecular interactions determine the properties of scotopic vision remains incomplete. We describe potential future directions of research, and outline several major problems that remain to be solved.

Phototransduction in Anuran Green Rods: Origins of Extra-Sensitivity

Green rods (GRs) represent a unique type of photoreceptor to be found in the retinas of anuran amphibians. These cells harbor a cone-specific blue-sensitive visual pigment but exhibit morphology of the outer segment typical for classic red rods (RRs), which makes them a perspective model object for studying cone–rod transmutation. In the present study, we performed detailed electrophysiological examination of the light sensitivity, response kinetics and parameters of discrete and continuous dark noise in GRs of the two anuran species: cane toad and marsh frog. Our results confirm that anuran GRs are highly specialized nocturnal vision receptors. Moreover, their rate of phototransduction quenching appeared to be about two-times slower than in RRs, which makes them even more efficient single photon detectors. The operating intensity ranges for two rod types widely overlap supposedly allowing amphibians to discriminate colors in the scotopic region. Unexpectedly for typical cone pigments but in line with some previous reports, the spontaneous isomerization rate of the GR visual pigment was found to be the same as for rhodopsin of RRs. Thus, our results expand the knowledge on anuran GRs and show that these are even more specialized single photon catchers than RRs, which allows us to assign them a status of “super-rods”.

Temporal vision: measures, mechanisms and meaning

Time is largely a hidden variable in vision. It is the condition for seeing interesting things such as spatial forms and patterns, colours and movements in the external world, and yet is not meant to be noticed in itself. Temporal aspects of visual processing have received comparatively little attention in research. Temporal properties have been made explicit mainly in measurements of resolution and integration in simple tasks such as detection of spatially homogeneous flicker or light pulses of varying duration. Only through a mechanistic understanding of their basis in retinal photoreceptors and circuits can such measures guide modelling of natural vision in different species and illuminate functional and evolutionary trade-offs. Temporal vision research would benefit from bridging traditions that speak different languages. Towards that goal, I here review studies from the fields of human psychophysics, retinal physiology and neuroethology, with a focus on fundamental constraints set by early vision.

Summary: Simple measures of temporal vision such as the critical flicker frequency can be useful for modelling natural vision only if their relationship to photoreceptor responses and retinal processing is understood.

Functional modulation of phosphodiesterase-6 by calcium in mouse rod photoreceptors

Phosphodiesterase-6 (PDE6) is a key protein in the G-protein cascade converting photon information to bioelectrical signals in vertebrate photoreceptor cells. Here, we demonstrate that PDE6 is regulated by calcium, contrary to the common view that PDE1 is the unique PDE class whose activity is modulated by intracellular Ca²⁺. To broaden the operating range of photoreceptors, mammalian rod photoresponse recovery is accelerated mainly by two calcium sensor proteins: recoverin, modulating the lifetime of activated rhodopsin, and guanylate cyclase-activating proteins (GCAPs), regulating the cGMP synthesis. We found that decreasing rod intracellular Ca²⁺ concentration accelerates the flash response recovery and increases the basal PDE6 activity (β_dark) maximally by ~ 30% when recording local electroretinography across the rod outer segment layer from GCAPs^-/- recoverin^-/- mice. Our modeling shows that a similar elevation in β_dark can fully explain the observed acceleration of flash response recovery in low Ca²⁺. Additionally, a reduction of the free Ca²⁺ in GCAPs^-/- recoverin^-/- rods shifted the inhibition constants of competitive PDE inhibitor 3-isobutyl-1-methylxanthine (IBMX) against the thermally activated and light-activated forms of PDE6 to opposite directions, indicating a complex interaction between IBMX, PDE6, and calcium. The discovered regulation of PDE6 is a previously unknown mechanism in the Ca²⁺-mediated modulation of rod light sensitivity.

Vitamin A1/A2 chromophore exchange: Its role in spectral tuning and visual plasticity

Vertebrate rod and cone photoreceptors detect light via a specialized organelle called the outer segment. This structure is packed with light-sensitive molecules known as visual pigments that consist of a G-protein-coupled, seven-transmembrane protein known as opsin, and a chromophore prosthetic group, either 11-cis retinal ('A₁') or 11-cis 3,4-didehydroretinal ('A₂'). The enzyme cyp27c1 converts A₁ into A₂ in the retinal pigment epithelium. Replacing A₁ with A₂ in a visual pigment red-shifts its spectral sensitivity and broadens its bandwidth of absorption at the expense of decreased photosensitivity and increased thermal noise. The use of vitamin A₂-based visual pigments is strongly associated with the occupation of aquatic habitats in which the ambient light is red-shifted. By modulating the A₁/A₂ ratio in the retina, an organism can dynamically tune the spectral sensitivity of the visual system to better match the predominant wavelengths of light in its environment. As many as a quarter of all vertebrate species utilize A₂, at least during a part of their life cycle or under certain environmental conditions. A₂ utilization therefore represents an important and widespread mechanism of sensory plasticity. This review provides an up-to-date account of the A₁/A₂ chromophore exchange system.

A frog's eye view: Foundational revelations and future promises

From the mid-19th century until the 1980's, frogs and toads provided important research models for many fundamental questions in visual neuroscience. In the present century, they have been largely neglected. Yet they are animals with highly developed vision, a complex retina built on the basic vertebrate plan, an accessible brain, and an experimentally useful behavioural repertoire. They also offer a rich diversity of species and life histories on a reasonably restricted physiological and evolutionary background. We suggest that important insights may be gained from revisiting classical questions in anurans with state-of-the-art methods. At the input to the system, this especially concerns the molecular evolution of visual pigments and photoreceptors, at the output, the relation between retinal signals, brain processing and behavioural decision-making.

Rejection of the biophoton hypothesis on the origin of photoreceptor dark noise

It has been suggested that retinal “dark light” is caused by photons emitted by the retina itself. The authors show that the “biophoton” radiation from the retina can be detected, but its intensity is ≥100 times lower than necessary to produce the measured physiological noise understood to arise from the spontaneous activation of rhodopsin.

Rod photoreceptors of the vertebrate retina produce, in darkness, spontaneous discrete current waves virtually identical to responses to single photons. The waves comprise an irreducible source of noise (discrete dark noise) that may limit the threshold sensitivity of vision. The waves obviously originate from acts of random activation of single rhodopsin molecules. Until recently, it was generally accepted that the activation occurs due to the rhodopsin thermal motion. Yet, a few years ago it was proposed that rhodopsin molecules are activated not by heat but rather by real photons generated within the retina by chemiluminescence. Using a high-sensitive photomultiplier, we measured intensities of biophoton emission from isolated retinas and eyecups of frogs (Rana ridibunda) and fish (sterlet, Acipenser ruthenus). Retinal samples were placed in a perfusion chamber and emitted photons collected by a high-aperture quartz lens. The collected light was sent to the photomultiplier cathode through a rotating chopper so that a long-lasting synchronous accumulation of the light signal was possible. The absolute intensity of bio-emission was estimated by the response of the measuring system to a calibrated light source. The intensity of the source, in turn, was quantified by measuring rhodopsin bleaching with single-rod microspectrophotometry. We also measured the frequency of discrete dark waves in rods of the two species with suction pipette recordings. Expressed as the rate constant of rhodopsin activation, it was 1.2 × 10⁻¹¹/s in frogs and 7.6 × 10⁻¹¹/s in sterlets. Approximately two thirds of retinal samples of each species produced reliably measurable biophoton emissions. However, its intensity was ≥100 times lower than necessary to produce the discrete dark noise. We argue that this is just a lower estimate of the discrepancy between the hypothesis and experiment. We conclude that the biophoton hypothesis on the origin of discrete dark noise in photoreceptors must be rejected.

Temporal resolution of single-photon responses in primate rod photoreceptors and limits imposed by cellular noise

Sensory receptor noise corrupts sensory signals, contributing to imperfect perception and dictating central processing strategies. For example, noise in rod phototransduction limits our ability to detect light, and minimizing the impact of this noise requires precisely tuned nonlinear processing by the retina. But detection sensitivity is only one aspect of night vision: prompt and accurate behavior also requires that rods reliably encode the timing of photon arrivals. We show here that the temporal resolution of responses of primate rods is much finer than the duration of the light response and identify the key limiting sources of transduction noise. We also find that the thermal activation rate of rhodopsin is lower than previous estimates, implying that other noise sources are more important than previously appreciated. A model of rod single-photon responses reveals that the limiting noise relevant for behavior depends critically on how rod signals are pooled by downstream neurons. NEW & NOTEWORTHY Many studies have focused on the visual system's ability to detect photons, but not on its ability to encode the relative timing of detected photons. Timing is essential for computations such as determining the velocity of moving objects. Here we examine the timing precision of primate rod photoreceptor responses and show that it is more precise than previously appreciated. This motivates an evaluation of whether scotopic vision approaches limits imposed by rod temporal resolution.

Elevated cAMP improves signal-to-noise ratio in amphibian rod photoreceptors

Vertebrate photoreceptors need to distinguish light signals from background noise to convey visual information to downstream bipolar cells. By affecting both signal and noise, Astakhova et al. find that increases in intracellular cAMP can improve the signal-to-noise ratio by twofold.

The absolute sensitivity of vertebrate retinas is set by a background noise, called dark noise, which originates from several different cell types and is generated by different molecular mechanisms. The major share of dark noise is produced by photoreceptors and consists of two components, discrete and continuous. Discrete noise is generated by spontaneous thermal activations of visual pigment. These events are undistinguishable from real single-photon responses (SPRs) and might be considered an equivalent of the signal. Continuous noise is produced by spontaneous fluctuations of the catalytic activity of the cGMP phosphodiesterase. This masks both SPR and spontaneous SPR-like responses. Circadian rhythms affect photoreceptors, among other systems by periodically increasing intracellular cAMP levels ([cAMP]_in), which increases the size and changes the shape of SPRs. Here, we show that forskolin, a tool that increases [cAMP]_in, affects the magnitude and frequency spectrum of the continuous and discrete components of dark noise in photoreceptors. By changing both components of rod signaling, the signal and the noise, cAMP is able to increase the photoreceptor signal-to-noise ratio by twofold. We propose that this results in a substantial improvement of signal detection, without compromising noise rejection, at the rod bipolar cell synapse.

Modulation of thermal noise and spectral sensitivity in Lake Baikal cottoid fish rhodopsins

Lake Baikal is the deepest and one of the most ancient lakes in the world. Its unique ecology has resulted in the colonization of a diversity of depth habitats by a unique fauna that includes a group of teleost fish of the sub-order Cottoidei. This relatively recent radiation of cottoid fishes shows a gradual blue-shift in the wavelength of the absorption maximum of their visual pigments with increasing habitat depth. Here we combine homology modeling and quantum chemical calculations with experimental in vitro measurements of rhodopsins to investigate dim-light adaptation. The calculations, which were able to reproduce the trend of observed absorption maxima in both A1 and A2 rhodopsins, reveal a Barlow-type relationship between the absorption maxima and the thermal isomerization rate suggesting a link between the observed blue-shift and a thermal noise decrease. A Nakanishi point-charge analysis of the electrostatic effects of non-conserved and conserved amino acid residues surrounding the rhodopsin chromophore identified both close and distant sites affecting simultaneously spectral tuning and visual sensitivity. We propose that natural variation at these sites modulate both the thermal noise and spectral shifting in Baikal cottoid visual pigments resulting in adaptations that enable vision in deep water light environments.

How Do Efficient Coding Strategies Depend on Origins of Noise in Neural Circuits?

Neural circuits reliably encode and transmit signals despite the presence of noise at multiple stages of processing. The efficient coding hypothesis, a guiding principle in computational neuroscience, suggests that a neuron or population of neurons allocates its limited range of responses as efficiently as possible to best encode inputs while mitigating the effects of noise. Previous work on this question relies on specific assumptions about where noise enters a circuit, limiting the generality of the resulting conclusions. Here we systematically investigate how noise introduced at different stages of neural processing impacts optimal coding strategies. Using simulations and a flexible analytical approach, we show how these strategies depend on the strength of each noise source, revealing under what conditions the different noise sources have competing or complementary effects. We draw two primary conclusions: (1) differences in encoding strategies between sensory systems-or even adaptational changes in encoding properties within a given system-may be produced by changes in the structure or location of neural noise, and (2) characterization of both circuit nonlinearities as well as noise are necessary to evaluate whether a circuit is performing efficiently.

Biophotons Contribute to Retinal Dark Noise

The discovery of dark noise in retinal photoreceptors resulted in a long-lasting controversy over its origin and the underlying mechanisms. Here, we used a novel ultra-weak biophoton imaging system (UBIS) to detect biophotonic activity (emission) under dark conditions in rat and bullfrog (Rana catesbeiana) retinas in vitro. We found a significant temperature-dependent increase in biophotonic activity that was completely blocked either by removing intracellular and extracellular Ca(2+) together or inhibiting phosphodiesterase 6. These findings suggest that the photon-like component of discrete dark noise may not be caused by a direct contribution of the thermal activation of rhodopsin, but rather by an indirect thermal induction of biophotonic activity, which then activates the retinal chromophore of rhodopsin. Therefore, this study suggests a possible solution regarding the thermal activation energy barrier for discrete dark noise, which has been debated for almost half a century.

Eye spectral sensitivity in fresh- and brackish-water populations of three glacial-relict Mysis species (Crustacea): physiology and genetics of differential tuning

Absorbance spectra of single rhabdoms were studied by microspectrophotometry (MSP) and spectral sensitivities of whole eyes by electroretinography (ERG) in three glacial-relict species of opossum shrimps (Mysis). Among eight populations from Fennoscandian fresh-water lakes (L) and seven populations from the brackish-water Baltic Sea (S), L spectra were systematically red-shifted by 20-30 nm compared with S spectra, save for one L and one S population. The difference holds across species and bears no consistent adaptive relation to the current light environments. In the most extensively studied L-S pair, two populations of M. relicta (L(p) and S(p)) separated for less than 10,000 years, no differences translating into amino acid substitutions have been found in the opsin genes, and the chromophore of the visual pigments as analyzed by HPLC is pure A1. However, MSP experiments with spectrally selective bleaching show the presence of two rhodopsins (λ(max) ≈ 525-530 nm, MWS, and 565-570 nm, LWS) expressed in different proportions. ERG recordings of responses to "red" and "blue" light linearly polarized at orthogonal angles indicate segregation of the pigments into different cells differing in polarization sensitivity. We propose that the pattern of development of LWS and MWS photoreceptors is governed by an ontogenetic switch responsive to some environmental signal(s) other than light that generally differ(s) between lakes and sea, and that this reaction norm is conserved from a common ancestor of all three species.

A Cambrian origin for vertebrate rods

Vertebrates acquired dim-light vision when an ancestral cone evolved into the rod photoreceptor at an unknown stage preceding the last common ancestor of extant jawed vertebrates (∼420 million years ago Ma). The jawless lampreys provide a unique opportunity to constrain the timing of this advance, as their line diverged ∼505 Ma and later displayed high-morphological stability. We recorded with patch electrodes the inner segment photovoltages and with suction electrodes the outer segment photocurrents of Lampetra fluviatilis retinal photoreceptors. Several key functional features of jawed vertebrate rods are present in their phylogenetically homologous photoreceptors in lamprey: crucially, the efficient amplification of the effect of single photons, measured by multiple parameters, and the flow of rod signals into cones. These results make convergent evolution in the jawless and jawed vertebrate lines unlikely and indicate an early origin of rods, implying strong selective pressure toward dim-light vision in Cambrian ecosystems.

DOI:http://dx.doi.org/10.7554/eLife.07166.001

The eyes of humans and many other animals with backbones contain two different types of cells that can detect light, which are known as rod and cone cells. Rod cells are much more sensitive to light than cone cells. The rods allow us to see in dim light by amplifying weak light signals and transmitting information to other cells, including the cones themselves. It is thought that the rod cell evolved from the cone cell in the common ancestors of mammals, fish, and other animals with backbones and jaws at least 420 million years ago.

Lampreys are jawless fish that diverged from the ancestors of jawed animals around 505 million years ago, in the middle of a period of great evolutionary innovation called the Cambrian. They have changed relatively little since that time so they provide a snapshot of what our ancestors' eyes might have been like back then. Like the rod and cone cells of jawed animals, the eyes of adult lampreys also have two types of photoreceptors. However, it was not clear whether the lamprey photoreceptor cells work in a similar way to rod and cone cells. Asteriti et al. collected lampreys in Sweden and France during their breeding season and used patch and suction electrodes to measure the activity of their photoreceptor cells. The experiments show that the short photoreceptor cells are more sensitive to light than the long photoreceptors and are able to amplify weak light signals. Also, the short photoreceptors send signals to the long photoreceptors in a similar way to how rod cells send information to cone cells.

The similarities between lamprey photoreceptor cells and those of jawed animals support the idea that they have a common origin in evolutionary history. Therefore, Asteriti et al. conclude that the ability to see in low light evolved before these groups of animals diverged about 505 million years ago. The picture that emerges is one in which our remote ancestors inhabiting the Cambrian seas already possessed dim-light vision. This would have allowed them to colonize deep waters or to move at twilight, an adaptation suggestive of intense competition or predation from other life forms.

DOI:http://dx.doi.org/10.7554/eLife.07166.002

Origin of the low thermal isomerization rate of rhodopsin chromophore

Low dark noise is a prerequisite for rod cells, which mediate our dim-light vision. The low dark noise is achieved by the extremely stable character of the rod visual pigment, rhodopsin, which evolved from less stable cone visual pigments. We have developed a biochemical method to quickly evaluate the thermal activation rate of visual pigments. Using an isomerization locked chromophore, we confirmed that thermal isomerization of the chromophore is the sole cause of thermal activation. Interestingly, we revealed an unexpected correlation between the thermal stability of the dark state and that of the active intermediate MetaII. Furthermore, we assessed key residues in rhodopsin and cone visual pigments by mutation analysis and identified two critical residues (E122 and I189) in the retinal binding pocket which account for the extremely low thermal activation rate of rhodopsin.

Lake and sea populations of Mysis relicta (Crustacea, Mysida) with different visual-pigment absorbance spectra use the same A1 chromophore

Glacial-relict species of the genus Mysis (opossum shrimps) inhabiting both fresh-water lakes and brackish sea waters in northern Europe show a consistent lake/sea dichotomy in eye spectral sensitivity. The absorbance peak (λmax) recorded by microspectrophotometry in isolated rhabdoms is invariably 20-30 nm red-shifted in "lake" compared with "sea" populations. The dichotomy holds across species, major opsin lineages and light environments. Chromophore exchange from A1 to A2 (retinal → 3,4-didehydroretinal) is a well-known mechanism for red-shifting visual pigments depending on environmental conditions or stages of life history, present not only in fishes and amphibians, but in some crustaceans as well. We tested the hypothesis that the lake/sea dichotomy in Mysis is due to the use of different chromophores, focussing on two populations of M. relicta from, respectively, a Finnish lake and the Baltic Sea. They are genetically very similar, having been separated for less than 10 kyr, and their rhabdoms show a typical lake/sea difference in λmax (554 nm vs. 529 nm). Gene sequencing has revealed no differences translating into amino acid substitutions in the transmembrane parts of their opsins. We determined the chromophore identity (A1 or A2) in the eyes of these two populations by HPLC, using as standards pure chromophores A1 and A2 as well as extracts from bovine (A1) and goldfish (A2) retinas. We found that the visual-pigment chromophore in both populations is A1 exclusively. Thus the spectral difference between these two populations of M. relicta is not due to the use of different chromophores. We argue that this conclusion is likely to hold for all populations of M. relicta as well as its European sibling species.

Spectral tuning by selective chromophore uptake in rods and cones of eight populations of nine-spined stickleback (Pungitius pungitius)

The visual pigments of rods and cones were studied in eight Fennoscandian populations of nine-spined stickleback (Pungitius pungitius). The wavelength of maximum absorbance of the rod pigment (λ(max)) varied between populations from 504 to 530 nm. Gene sequencing showed that the rod opsins of all populations were identical in amino acid composition, implying that the differences were due to varying proportions of chromophores A1 and A2. Four spectral classes of cones were found (two S-cones, M-cones and L-cones), correlating with the four classes of vertebrate cone pigments. For quantitative estimation of chromophore proportions, we considered mainly rods and M-cones. In four populations, spectra of both photoreceptor types indicated A2 dominance (population mean λ(max)=525-530 nm for rods and 535-544 nm for M-cones). In the four remaining populations, however, rod spectra (mean λ(max)=504-511 nm) indicated strong A1 dominance, whereas M-cone spectra (mean λ(max)=519-534 nm) suggested substantial fractions of A2. Quantitative analysis of spectra by three methods confirmed that rods and cones in these populations use significantly different chromophore proportions. The outcome is a shift of M-cone spectra towards longer wavelengths and a better match to the photic environment (light spectra peaking >560 nm in all the habitats) than would result from the chromophore proportions of the rods. Chromophore content was also observed to vary partly independently in M- and L-cones with potential consequences for colour discrimination. This is the first demonstration that selective processing of chromophore in rods and cones, and in different cone types, may be ecologically relevant.

Opsin gene sequence variation across phylogenetic and population histories in Mysis (Crustacea: Mysida) does not match current light environments or visual-pigment absorbance spectra

The hypothesis that selection on the opsin gene is efficient in tuning vision to the ambient light environment of an organism was assessed in 49 populations of 12 Mysis crustacean species, inhabiting arctic marine waters, coastal littoral habitats, freshwater lakes ('glacial relicts') and the deep Caspian Sea. Extensive sequence variation was found within and among taxa, but its patterns did not match expectations based on light environments, spectral sensitivity of the visual pigment measured by microspectrophotometry or the history of species and populations. The main split in the opsin gene tree was between lineages I and II, differing in six amino acids. Lineage I was present in marine and Caspian Sea species and in the North American freshwater Mysis diluviana, whereas lineage II was found in the European and circumarctic fresh- and brackish-water Mysis relicta, Mysis salemaai and Mysis segerstralei. Both lineages were present in some populations of M. salemaai and M. segerstralei. Absorbance spectra of the visual pigment in nine populations of the latter three species showed a dichotomy between lake (λ(max) =554-562 nm) and brackish-water (Baltic Sea) populations (λ(max) = 521-535 nm). Judged by the shape of spectra, this difference was not because of different chromophores (A2 vs. A1), but neither did it coincide with the split in the opsin tree (lineages I/II), species identity or current light environments. In all, adaptive evolution of the opsin gene in Mysis could not be demonstrated, but its sequence variation did not conform to a neutral expectation either, suggesting evolutionary constraints and/or unidentified mechanisms of spectral tuning.

Activation of visual pigments by light and heat

Vision begins with photoisomerization of visual pigments. Thermal energy can complement photon energy to drive photoisomerization, but it also triggers spontaneous pigment activation as noise that interferes with light detection. For half a century, the mechanism underlying this dark noise has remained controversial. We report here a quantitative relation between a pigment's photoactivation energy and its peak-absorption wavelength, λ(max). Using this relation and assuming that pigment activations by light and heat go through the same ground-state isomerization energy barrier, we can predict the relative noise of diverse pigments with multi-vibrational-mode thermal statistics. The agreement between predictions and our measurements strongly suggests that pigment noise arises from canonical isomerization. The predicted high noise for pigments with λ(max) in the infrared presumably explains why they apparently do not exist in nature.

Individual variation in rod absorbance spectra correlated with opsin gene polymorphism in sand goby (Pomatoschistus minutus)

Rod absorbance spectra, characterized by the wavelength of peak absorbance(λmax) were related to the rod opsin sequences of individual sand gobies (Pomatoschistus minutus) from four allopatric populations[Adriatic Sea (A), English Channel (E), Swedish West Coast (S) and Baltic Sea(B)]. Rod λmax differed between populations in a manner correlated with differences in the spectral light transmission of the respective water bodies [λmax: (A)≈503 nm; (E and S)≈505–506 nm; (B)≈508 nm]. A distinguishing feature of B was the wide within-population variation of λmax (505.6–511.3 nm). The rod opsin gene was sequenced in marked individuals whose rod absorbance spectra had been accurately measured. Substitutions were identified using EMBL/GenBank X62405 English sand goby sequence as reference and interpreted using two related rod pigments, the spectrally similar one of the Adriatic P. marmoratus (λmax≈507 nm) and the relatively red-shifted Baltic P. microps(λmax≈515 nm) as outgroups. The opsin sequence of all E individuals was identical to that of the reference, whereas the S and B fish all had the substitution N151N/T or N151T. The B fish showed systematic within-population polymorphism, the sequence of individuals withλ max at 505.6–507.5 nm were identical to S, but those with λmax at 509–511.3 nm additionally had F261F/Y. The substitution F261Y is known to red-shift the rod pigment and was found in all P. microps. We propose that ambiguous selection pressures in the Baltic Sea and/or gene flow from the North Sea preserves polymorphism and is phenotypically evident as a wide variation in λmax.

Temperature dependence of dark-adapted sensitivity and light-adaptation in photoreceptors with A1 visual pigments: a comparison of frog L-cones and rods

Flash responses of L-cones and rods were recorded as ERG mass potentials in the frog retina at different temperatures (2-25 degrees C). The purpose was to elucidate factors that make cones faster and less sensitive than rods, particularly the possible role of thermal activation of L-cone visual pigment in maintaining a "light-adapted" state even in darkness. Up to ca. 15 degrees C, cones and rods were desensitized roughly equally by warming (Q(10) approximately 2.2-2.7), retaining a 5-fold sensitivity difference. In this range, the cone/rod difference must depend on factors other than thermal activation of the visual pigment. Above 15 degrees C, cones showed an additional component of desensitization compared with rods, coupled to accelerated response shut-off. This behavior is consistent with light-adaptation from temperature-dependent intrinsic activity (dark light). The apparent dark light as measured by the minimum background intensities needed to affect sensitivity and/or kinetics increased by ca. 10-fold between 15 and 25 degrees C, whereas reported increases in visual-pigment activation rates over this range are less than 5-fold. We conclude that the dark state of frog L-cones above 15 degrees C may be largely set by thermal activation of the phototransduction machinery, but only part of the experimentally determined dark light can be ascribed to the visual pigment.

Quantal noise from human red cone pigment

The rod pigment, rhodopsin, shows spontaneous isomerization activity. This quantal noise produces a dark light of approximately 0.01 photons s(-1) rod(-1) in human, setting the threshold for rod vision. The spontaneous isomerization activity of human cone pigments has long remained a mystery because the effect of a single isomerized pigment molecule in cones, unlike that in rods, is small and beyond measurement. We have now overcome this problem by expressing human red cone pigment transgenically in mouse rods in order to exploit their large single-photon response, especially after genetic removal of a key negative-feedback regulation. Extrapolating the measured quantal noise of transgenic cone pigment to native human red cones, we obtained a dark rate of approximately 10 false events s(-1) cone(-1), almost 10(3)-fold lower than the overall dark transduction noise previously reported in primate cones. Our measurements provide a rationale for why mammalian red, green and blue cones have comparable sensitivities, unlike their amphibian counterparts.

Visual sensitivity to a conspicuous male cue varies by reproductive state inPhysalaemus pustulosusfemales

The vocal sac is a visually conspicuous attribute of most male frogs, but its role in visual communication has only been demonstrated recently in diurnally displaying frogs. Here we characterized the spectral properties of the inflated vocal sac of male túngara frogs (Physalaemus pustulosus), a nocturnal species, and túngara visual sensitivity to this cue across reproductive state and sex. We measured the spectral and total reflectance of different male body regions, including inflated and non-inflated vocal sacs, along with samples of the visual background against which males are perceived. Inflated vocal sacs were the most reflective of all body parts, being one log unit more reflective than background materials. We utilized an optomotor drum with black stripes and stripes that mimicked the spectral reflectance of the inflated vocal sacs with various nocturnal light intensities to measure the visual sensitivity thresholds of males,non-reproductive females and reproductive females. All three groups exhibited visual sensitivities corresponding to intensities below moonless conditions in open habitats or at the edge of secondary tropical forests. Reproductive females exhibited the greatest visual sensitivity of all groups, and were significantly more sensitive than non-reproductive females. Though the mechanism for this physiological difference between reproductive and non-reproductive females is unknown, it is consistent with previously observed patterns of light-dependent phonotaxic behavior in túngaras. We suggest that the visual ecology of the vocal sac, especially in nocturnal frogs,offers a rich source for investigations of visual ecology and physiological regulation of vision.

Signaling properties of a short-wave cone visual pigment and its role in phototransduction

Although visual pigments play key structural and functional roles in photoreceptors, the relationship between the properties of mammalian cone pigments and those of mammalian cones is not well understood. We generated transgenic mice with rods expressing mouse short-wave cone opsin (S-opsin) to test whether cone pigment can substitute for the structural and functional roles of rhodopsin and to investigate how the biophysical and signaling properties of the short-wave cone pigment (S-pigment) contribute to the specialized function of cones. The transgenic S-opsin was targeted to rod outer segments, and formed a pigment with peak absorption at 360 nm. Expression of S-opsin in rods lacking rhodopsin (rho-/-) promoted outer segment growth and cell survival and restored their ability to respond to light while shifting their action spectrum to 355 nm. Using the spectral separation between S-pigment and rhodopsin, we found that the two pigments produced similar photoresponses. Dark noise did not increase in transgenic rods, indicating that thermal activation of S-pigment might not contribute to the low sensitivity of mouse S-cones. Using rod arrestin knock-out animals (arr1-/-), we found that the physiologically active (meta II) state of S-pigment decays 40 times faster than that of rhodopsin. Interestingly, rod arrestin was efficient in deactivating S-pigment in rods, but its deletion did not have any obvious effect on dim-flash response shutoff in cones. Furthermore, transgenic cone arrestin was not able to rescue the slow shutoff of S-pigment dim-flash response in arr1-/- rods. Thus, the connection between rod/cone arrestins and S-pigment shutoff remains unclear.

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 pigments of Baltic Sea fishes of marine and limnic origin

Absorbance spectra of rods and some cones were measured by microspectrophotometry in 22 fish species from the brackish-water of the Baltic Sea, and when applicable, in the same species from the Atlantic Ocean (3 spp.), the Mediterranean Sea (1 sp.), or Finnish fresh-water lakes (9 spp.). The main purpose was to study whether there were differences suggesting spectral adaptation of rod vision to different photic environments during the short history (<104years) of postglacial isolation of the Baltic Sea and the Finnish lakes. Rod absorbance spectra of the Baltic subspecies/populations of herring (Clupea harengus membras), flounder (Platichthys flesus), and sand goby (Pomatoschistus minutus) were all long-wavelength-shifted (9.8, 1.9, and 5.3 nm, respectively, at the wavelength of maximum absorbance, λmax) compared with their truly marine counterparts, consistent with adaptation for improved quantum catch, and improved signal-to-noise ratio of vision in the Baltic light environment. Judged by the shape of the spectra, the chromophore was pure A1 in all these cases; hence the differences indicate evolutionary tuning of the opsin. In no species of fresh-water origin did we find significant opsin-based spectral shifts specific to the Baltic populations, only spectral differences due to varying A1/A2 chromophore ratio in some. For most species, rod λmaxfell within a wavelength range consistent with high signal-to-noise ratio of vision in the spectral conditions prevailing at depths where light becomes scarce in the respective waters. Exceptions were sandeels in the Baltic Sea, which are active only in bright light, and all species in a “brown” lake, where rod λmaxlay far below the theoretically optimal range.

Physiological properties of rod photoreceptor cells in green-sensitive cone pigment knock-in mice

Rod and cone photoreceptor cells that are responsible for scotopic and photopic vision, respectively, exhibit photoresponses different from each other and contain similar phototransduction proteins with distinctive molecular properties. To investigate the contribution of the different molecular properties of visual pigments to the responses of the photoreceptor cells, we have generated knock-in mice in which rod visual pigment (rhodopsin) was replaced with mouse green-sensitive cone visual pigment (mouse green). The mouse green was successfully transported to the rod outer segments, though the expression of mouse green in homozygous retina was approximately 11% of rhodopsin in wild-type retina. Single-cell recordings of wild-type and homozygous rods suggested that the flash sensitivity and the single-photon responses from mouse green were three to fourfold lower than those from rhodopsin after correction for the differences in cell volume and levels of several signal transduction proteins. Subsequent measurements using heterozygous rods expressing both mouse green and rhodopsin E122Q mutant, where these pigments in the same rod cells can be selectively irradiated due to their distinctive absorption maxima, clearly showed that the photoresponse of mouse green was threefold lower than that of rhodopsin. Noise analysis indicated that the rate of thermal activations of mouse green was 1.7 x 10(-7) s(-1), about 860-fold higher than that of rhodopsin. The increase in thermal activation of mouse green relative to that of rhodopsin results in only 4% reduction of rod photosensitivity for bright lights, but would instead be expected to severely affect the visual threshold under dim-light conditions. Therefore, the abilities of rhodopsin to generate a large single photon response and to retain high thermal stability in darkness are factors that have been necessary for the evolution of scotopic vision.

Crepuscular and nocturnal illumination and its effects on color perception by the nocturnal hawkmoth Deilephila elpenor

Recent studies have shown that certain nocturnal insect and vertebrate species have true color vision under nocturnal illumination. Thus, their vision is potentially affected by changes in the spectral quality of twilight and nocturnal illumination, due to the presence or absence of the moon, artificial light pollution and other factors. We investigated this in the following manner. First we measured the spectral irradiance (from 300 to 700 nm) during the day, sunset, twilight, full moon, new moon, and in the presence of high levels of light pollution. The spectra were then converted to both human-based chromaticities and to relative quantum catches for the nocturnal hawkmoth Deilephila elpenor, which has color vision. The reflectance spectra of various flowers and leaves and the red hindwings of D. elpenor were also converted to chromaticities and relative quantum catches. Finally, the achromatic and chromatic contrasts (with and without von Kries color constancy) of the flowers and hindwings against a leaf background were determined under the various lighting environments. The twilight and nocturnal illuminants were substantially different from each other, resulting in significantly different contrasts. The addition of von Kries color constancy significantly reduced the effect of changing illuminants on chromatic contrast, suggesting that, even in this light-limited environment, the ability of color vision to provide reliable signals under changing illuminants may offset the concurrent threefold decrease in sensitivity and spatial resolution. Given this, color vision may be more common in crepuscular and nocturnal species than previously considered.

Visual pigment absorbance and spectral sensitivity of the Mysis relicta species group (Crustacea, Mysida) in different light environments

Visual-pigment absorbance spectra and eye spectral sensitivities were examined in eight populations of opossum shrimp from different light environments. Four Finnish populations, two from the Baltic Sea and two from freshwater lakes, represent Mysis relicta, sensu stricto. The sibling species M. salemaai and M. diluviana are represented by, respectively, two Baltic Sea populations and two populations from freshwater lakes in Idaho, USA. In M. relicta, the visual pigments of the two lake populations were similar (lambda(max)=554.3+/-0.8 nm and 556.4+/-0.4 nm), but significantly red-shifted compared with the sea populations (at 529 and 535 nm) and with M. salemaai (at 521 and 525 nm). All these pigments had only A2 chromophore and the lake/sea difference indicates adaptive evolution of the opsin. In M. diluviana, lambda(max) varied in the range 505-529 nm and the shapes of spectra suggested varying A1/A2 chromophore proportions, with pure A1 in the 505 nm animals. Eye sensitivity spectra were flatter and peaked at longer wavelengths than the relevant visual-pigment templates, but declined with the same slope beyond ca. 700 nm. The deviations from visual-pigment spectra can be explained by ocular light filters based on three types of identified screening pigments.

The ecology of visual pigment tuning in an Australian marsupial: the honey possum Tarsipes rostratus

While most mammals have no more than two types of cone photoreceptor, four species of Australian marsupial have recently been shown to possess three types, and thus have the potential for trichromatic colour vision. Interestingly, the long-wave cones of the honey possum Tarsipes rostratus are tuned to longer wavelengths than those of the other species measured to date. We tested whether the honey possum's long-wave tuning is adaptive for visual tasks associated with its almost unique diet of nectar and pollen. We modelled three tasks: (1) detecting food-rich 'target' flowers against their natural background of foliage or other vegetation; (2) discriminating target flowers from flowers of non-target species; (3) discerning the maturity of the most important target flowers. Initial comparisons of trichromacy vs dichromacy generally favoured the former, but interestingly dichromacy was no disadvantage in some cases. For tuning, we found that overall the honey possum's long-wave tuning is more adaptive than that of the other marsupial species. Nevertheless, the optimal tuning for tasks 1 and 2 would be at longer wavelengths still, implying that a different pressure or constraint operates against a further long-wave shift of the honey possum's L cone tuning. Our data show that a possible ecological pressure may be provided by the third task--the difficult and potentially critical discrimination of the maturity of the animal's major food supply, the flowers of Banksia attenuata.

The photoactivation energy of the visual pigment in two spectrally different populations of Mysis relicta (Crustacea, Mysida)

We report the first study of the relation between the wavelength of maximum absorbance (lambdamax) and the photoactivation energy (Ea) in invertebrate visual pigments. Two populations of the opossum shrimp Mysis relicta were compared. The two have been separated for 9,000 years and have adapted to different spectral environments ("Sea" and "Lake") with porphyropsins peaking at lambdamax=529 nm and 554 nm, respectively. The estimation of Ea was based on measurement of temperature effects on the spectral sensitivity of the eye. In accordance with theory (Stiles in Transactions of the optical convention of the worshipful company of spectacle makers. Spectacle Makers' Co., London, 1948), relative sensitivity to long wavelengths increased with rising temperature. The estimates calculated from this effect are Ea,529=47.8+/-1.8 kcal/mol and Ea,554=41.5+/-0.7 kcal/mol (different at P<0.01). Thus the red-shift of lambdamax in the "Lake" population, correlating with the long-wavelength dominated light environment, is achieved by changes in the opsin that decrease the energy gap between the ground state and the first excited state of the chromophore. We propose that this will carry a cost in terms of increased thermal noise, and that evolutionary adaptation of the visual pigment to the light environment is directed towards maximizing the signal-to-noise ratio rather than the quantum catch.

Thermal activation and photoactivation of visual pigments

A visual pigment molecule in a retinal photoreceptor cell can be activated not only by absorption of a photon but also "spontaneously" by thermal energy. Current estimates of the activation energies for these two processes in vertebrate rod and cone pigments are on the order of 40-50 kcal/mol for activation by light and 20-25 kcal/mol for activation by heat, which has forced the conclusion that the two follow quite different molecular routes. It is shown here that the latter estimates, derived from the temperature dependence of the rate of pigment-initiated "dark events" in rods, depend on the unrealistic assumption that thermal activation of a complex molecule like rhodopsin (or even its 11-cis retinaldehyde chromophore) happens through a simple process, somewhat like the collision of gas molecules. When the internal energy present in the many vibrational modes of the molecule is taken into account, the thermal energy distribution of the molecules cannot be described by Boltzmann statistics, and conventional Arrhenius analysis gives incorrect estimates for the energy barrier. When the Boltzmann distribution is replaced by one derived by Hinshelwood for complex molecules with many vibrational modes, the same experimental data become consistent with thermal activation energies that are close to or even equal to the photoactivation energies. Thus activation by light and by heat may in fact follow the same molecular route, starting with 11-cis to all-trans isomerization of the chromophore in the native (resting) configuration of the opsin. Most importantly, the same model correctly predicts the empirical correlation between the wavelength of maximum absorbance and the rate of thermal activation in the whole set of visual pigments studied.

On the relation between the photoactivation energy and the absorbance spectrum of visual pigments

We relate the collected experimental data on the minimum energy for photoactivation (E(a)) to the wavelengths of peak absorbance (lambda(max)) of 12 visual pigments. The E(a) values have been determined from the temperature-dependence of spectral sensitivity in the long-wavelength range. As shown previously, the simple physical idea E(a) =const. x (1/lambda(max)) (here termed the Stiles-Lewis-Barlow or SLB relation) does not hold strictly. Yet there is a significant correlation between E(a) and 1/lambda(max) (r(2)=0.73) and the regression slope obtained by an unbiased fit is 84% of the predicted value of the best SLB fit. The correlation can be decomposed into effects of A1 --> A2 chromophore change and effects of opsin differences. For a chromophore change in the same opsin, studied in two A1/A2 pigment pairs, the SLB relation holds nearly perfectly. In seven pigments having different opsins but the same (A2) chromophore, the correlation of E(a) and 1/lambda(max) remained highly significant (r(2)=0.78), but the regression coefficient is only 72% of the best SLB fit. We conclude that (1) when the chromophore is exchanged in the same opsin, the lambda(max) shift directly reflects the difference in photoactivation energies, (2) when the opsin is modified by amino acid substitutions, lambda(max) and E(a) can be tuned partly independently, although there is a dominant tendency for inverse proportionality. In four (A1) rhodopsins with virtually the same lambda(max), E(a) varied over a 4.5 kcal/mol range, which may be taken as a measure of the freedom for independent tuning. Assuming that low E(a) correlates with high thermal noise, we suggest that the leeway in lambda(max) - E(a) coupling is used by natural selection to keep E(a) as high as possible in long-wavelength-sensitive pigments, and that this is why the opsin-dependent E(a) (1/lambda(max))-relation is shallower than predicted.

The thermal contribution to photoactivation in A2 visual pigments studied by temperature effects on spectral properties

Effects of temperature on the spectral properties of visual pigments were measured in the physiological range (5-28 degrees C) in photoreceptor cells of bullfrog (Rana catesbeiana) and crucian carp (Carassius carassius). Absorbance spectra recorded by microspectrophotometry (MSP) in single cells and sensitivity spectra recorded by electroretinography (ERG) across the isolated retina were combined to yield accurate composite spectra from ca. 400 nm to 800 nm. The four photoreceptor types selected for study allowed three comparisons illuminating the properties of pigments using the dehydroretinal (A2) chromophore: (1) the two members of an A1/A2 pigment pair with the same opsin (porphyropsin vs. rhodopsin in bullfrog "red" rods); (2) two A2 pigments with similar spectra (porphyropsin rods of bullfrog and crucian carp); and (3) two A2 pigments with different spectra (rods vs. long-wavelength-sensitive (L-) cones of crucian carp). Qualitatively, the temperature effects on A2 pigments were similar to those described previously for the A1 pigment of toad "red" rods. Warming caused an increase in relative sensitivities at very long wavelengths but additionally a small shift of lambdamax toward shorter wavelengths. The former effect was used for estimating the minimum energy required for photoactivation (Ea) of the pigment. Bullfrog rod opsin with A2 chromophore had Ea = 44.2 +/- 0.9 kcal/mol, significantly lower (one-tailed P < 0.05) than the value Ea = 46.5 +/- 0.8 kcal/mol for the same opsin coupled to A1. The A2 rod pigment of crucian carp had Ea = 42.3 +/- 0.6 kcal/mol, which is significantly higher (one-tailed P < 0.01) than that of the L-cones in the same retina (Ea = 38.3 +/- 0.4 kcal/mol), whereas the difference compared with the bullfrog A2 rod pigment is not statistically significant (two-tailed P = 0.13). No strict connection between lambdamax and Ea appears to exist among A2 pigments any more than among A1 pigments. Still, the A1 --> A2 chromophore substitution in bullfrog opsin causes three changes correlated as originally hypothesized by Barlow (1957): a red-shift of lambdamax, a decrease in Ea, and an increase in thermal noise.

Polymorphism of the rod visual pigment between allopatric populations of the sand goby (Pomatoschistus minutus): a microspectrophotometric study

Absorbance spectra were measured by microspectrophotometry in retinal rods of sand gobies (Pomatoschistus minutus) from four allopatric populations (Baltic Sea, Swedish west coast, English Channel and Adriatic Sea). Mean (+/- S.E.M.) wavelengths of maximum absorbance (lambda(max)) were 508.3+/-0.5 nm, 505.4+/-0.2 nm, 506.2+/-0.3 nm and 503.0+/-0.3 nm, respectively. Pairwise comparison between the populations (post-ANOVA Scheffe's test) shows that each of the lambda(max) differences, except that between the Swedish west coast and the English Channel, is statistically significant (P<0.05). The shapes of the absorbance spectra indicated that the pigments were A1 rhodopsins with no measurable admixture of the A2 chromophore. Thus, the differences indicate polymorphism in the protein part (opsin) of the pigment. Convolution of A1 templates for lambda(max) values 508.3 nm and 503.0 nm with quantum spectra of the downwelling light at two locations at the south-west coast of Finland indicated that a 13-19% improvement in quantum catch would accrue in the Baltic environment from the 5.3 nm red-shift of the rod pigment of Baltic compared with Adriatic sand gobies.

Spectral tuning and evolution of short wave-sensitive cone pigments in cottoid fish from Lake Baikal

The cottoid fishes of Lake Baikal in eastern Siberia provide a unique opportunity to study the evolution of visual pigments in a group of closely related species exposed to different photic environments. Members of this species flock are adapted to different depth habitats down to >1000 m, and both the rod and cone visual pigments display short wave shifts as depth increases. The blue-sensitive cone pigments of the SWS2 class cluster into two species groups with lambda(max) values of 450 and 430 nm, with the pigment in Cottus gobio, a cottoid fish native to Britain, forming a third group with a lambda(max) of 467 nm. The sequences of the SWS2 opsin gene from C. gobio and from two representatives of the 450 and 430 nm Baikal groups are presented. Approximately 6 nm of the spectral difference between C. gobio and the 450 nm Baikal group can be ascribed to the presence of a porphyropsin/rhodopin mixture in C. gobio. Subsequent analysis of amino acid substitutions by site-directed mutagenesis demonstrates that the remainder of the shift from 461 to 450 nm arises from a Thr269Ala substitution and the shift from 450 to 430 nm at least partly from Thr118Ala and Thr118Gly substitutions. The underlying adaptive significance of these substitutions in terms of spectral tuning and signal-to-noise ratio is discussed.

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

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

Measurement of thermal contribution to photoreceptor sensitivity

Activation of a visual pigment molecule to initiate phototransduction requires a minimum energy, Ea, that need not be wholly derived from a photon, but may be supplemented by heat. Theory predicts that absorbance at very long wavelengths declines with the fraction of molecules that have a sufficient complement of thermal energy, and that Ea is inversely related to the wavelength of maximum absorbance (lambda(max)) of the pigment. Consistent with the first of these predictions, warming increases relative visual sensitivity to long wavelengths. Here we measure this effect in amphibian photoreceptors with different pigments to estimate Ea (refs 2, 5-7) and test experimentally the predictions of an inverse relation between Ea and lambda(max). For rods and 'red' cones in the adult frog retina, we find no significant difference in Ea between the two pigments involved, although their lambda(max) values are very different. We also determined Ea for the rhodopsin in toad retinal rods--spectrally similar to frog rhodopsin but differing in amino-acid sequence--and found that it was significantly higher. In addition, we estimated Ea for two pigments whose lambda(max) difference was due only to a chromophore difference (A1 and A2 pigment, in adult and larval frog cones). Here Ea for A2 was lower than for A1. Our results refute the idea of a necessary relation between lambda(max) and Ea, but show that the A1 --> A2 chromophore substitution decreases Ea.

The eyes of deep-sea fish. I: Lens pigmentation, tapeta and visual pigments

Deep-sea fish, defined as those living below 200 m, inhabit a most unusual photic environment, being exposed to two sources of visible radiation; very dim downwelling sunlight and bioluminescence, both of which are, in most cases, maximal at wavelengths around 450-500 nm. This paper summarises the reflective properties of the ocular tapeta often found in these animals, the pigmentation of their lenses and the absorption characteristics of their visual pigments. Deep-sea tapeta usually appear blue to the human observer, reflecting mainly shortwave radiation. However, reflection in other parts of the spectrum is not uncommon and uneven tapetal distribution across the retina is widespread. Perhaps surprisingly, given the fact that they live in a photon limited environment, the lenses of some deep-sea teleosts are bright yellow, absorbing much of the shortwave part of the spectrum. Such lenses contain a variety of biochemically distinct pigments which most likely serve to enhance the visibility of bioluminescent signals. Of the 195 different visual pigments characterised by either detergent extract or microspectrophotometry in the retinae of deep-sea fishes, ca. 87% have peak absorbances within the range 468-494 nm. Modelling shows that this is most likely an adaptation for the detection of bioluminescence. Around 13% of deep-sea fish have retinae containing more than one visual pigment. Of these, we highlight three genera of stomiid dragonfishes, which uniquely produce far red bioluminescence from suborbital photophores. Using a combination of longwave-shifted visual pigments and in one species (Malacosteus niger) a chlorophyll-related photosensitizer, these fish have evolved extreme red sensitivity enabling them to see their own bioluminescence and giving them a private spectral waveband invisible to other inhabitants of the deep-ocean.

Rhodopsins from three frog and toad species: sequences and functional comparisons

The frequency of thermal 'dark events' in the membrane current of rhodopsin rods of the bullfrog, Rana catesbeiana, is considerably lower than observed in rods of two toad species, even though all three rhodopsins have approximately the same absorbance characteristics. In order to map amino acid substitutions possibly associated with thermal stability in the genus Rana, the cDNA's coding for the rhodopsins of Bufo bufo, B. marinus and R. temporaria were sequenced and the conceptually translated protein sequences aligned to the previously sequenced rhodopsins of R. catesbeiana, R. pipiens and Xenopus laevis. Across the six anuran species studied, there are sixteen non-conserved substitutions and six changes that include gain or loss of a hydroxyl group. Serine or threonine at position 220 is unique to the three Rana species, phenylalanine at position 270 is unique to all three Ranas and to X. laevis, and phenylalanine at position 274 is unique to both species of the genus Bufo. This investigation produces a list of substitutions that are candidates for future studies of thermal stability. In addition, a number of amino acids are identified that apparently do not influence absorbance characteristics, at least not cumulatively.

Rod pigment and rod noise in the European toad Bufo bufo

Behavioural experiments and ganglion cell recordings indicate that the visual sensitivity of dark-adapted toads is limited by the occurrence of spontaneous isomerization-like noise events in the rods. The frequency of these "false photons" has previously been studied (with micropipette recording) in the toad species Bufo marinus, while the behavioural thresholds were determined using Bufo bufo toads. Thus, it was necessary to check that the noise event frequency is roughly the same in these two species. Here we show that it is, in both species, close to 0.02 events per second and rod (at 22 degrees C). Using microspectrophotometry we further show that the absorption spectra of these two rhodopsins are very similar, peaking around 503.3 and 501.8 nm for B. marinus and B. bufo, respectively.

Spectral tuning and molecular evolution of rod visual pigments in the species flock of cottoid fish in Lake Baikal

Lake Baikal in Eastern Siberia is the deepest and one of the largest and most ancient lakes in the world. However, even in the deepest regions, oxygenation levels do not fall below 75-80% of the surface levels. This has enabled a remarkable flock of largely endemic teleost fish of the sub-order Cottoidei to colonize all depth habitats. We have previously shown that species that occupy progressively deeper habitats show a blue shift in the peak wavelength of absorbance (lambda max) of both their rod and cone visual pigments; for the rod pigments, a number of stepwise shifts occur from about 516 nm in littoral species to about 484 nm in abyssal species. By sequencing the rod opsin gene from 11 species of Baikal cottoids that include representatives from all depth habitats, we have been able to identify four amino acid substitutions that would account for these shifts. The effect of each substitution on lambda max is approximately additive and each corresponds to a particular lineage of evolution.

Response univariance in bull-frog rods with two visual pigments

Rods in the bull-frog retina contain varying proportions of rhodopsin (lambda max = 502 nm) and porphyropsin (lambda max = 527 nm) in a dorso-ventral gradient from the porphyropsin-rich dorsal rim to the virtually pure rhodopsin fields of the central and ventral retina. We investigated if quantal excitations in the same rod are different depending on whether they are initiated by isomerization of a rhodopsin or a porphyropsin molecule. Current photoresponses were recorded from dark-adapted rods by sucking the outer segment into a recording pipette. The relation between pigment composition and spectral sensitivity was established by comparison with microspectrophotometrically measured absorbance spectra of rods from the same neighbourhood. Rods with suitable porphyropsin: rhodopsin mixtures (ideally between 1:4 and 1:2) were stimulated with flashes of red (608 nm) and blue (465 nm) light, whereby the red light will isomerize porphyropsin much more often than rhodopsin, and the reverse will be true of the blue light. The amplitude and shape of the single-photon response were found to be identical for the "red" and "blue" flash series to within measurement error (ca 10%). This indicates that the quantal responses initiated by the two pigments are identical.

Visual pigments and the photic environment: the cottoid fish of Lake Baikal

The endemic cottoid fish of Lake Baikal in Eastern Siberia offer a singular opportunity for examining within a number of closely related species, the relationships of visual pigments, photoreceptor complements and depth within a deep freshwater environment. The lake, the deepest (1600 m) and one of the largest and most ancient in the world, is unique in that the oxygen levels at the bottom are only reduced to about 80% of the surface levels. We have studied, by light microscopy, microspectrophotometry and visual pigment extraction, the retinas from 17 species of Baikal cottoids that live at different depths within the lake. Generally the retinas contain, in addition to rods, large green-sensitive double cones and small blue-sensitive single cones: surprisingly for freshwater fish, the visual pigments are based on Vitamin A1. The lambda max of both rods and cones are displaced to shorter wavelengths with increasing depth. Surface species have cones with lambda max at about 546, 525 and 450 nm and rods at 523 nm, deeper living species retain cones, but with lambda max shifting towards 500 and 425 nm and with rods at 480 nm, whereas the deepest living fish possess only rods (lambda max 480-500 nm). These data clearly show a correlation between photoreceptor complement, visual pigment lambda max and depth, but question the hypothesis that there is a correlation of pigment lambda max with water colour since, in contrast to oceanic waters, the maximum transmission of Baikal water is between 550 and 600 nm.

Retinal origins of the temperature effect on absolute visual sensitivity in frogs

1. The absolute sensitivity of vision was studied as a function of temperature in two species of frog (Rana temporaria, 9-21 degrees C, and Rana pipiens, 13-28 degrees C). 2. Log behavioural threshold (measured as the lowest light intensity by which frogs trying to escape from a dark box were able to direct their jumping) rose near-linearly with warming with a regression coefficient of 1.26 +/- 0.03 log units per 10 degrees C (Q10 = 18). Threshold retinal illumination corresponded to 0.011 photoisomerizations per rod per second (Rh* s-1) at 16.5 degrees C. 3. The effect of dim backgrounds on jumping thresholds suggested 'dark lights' of 0.011 Rh* s-1 at 16.5 degrees C and 0.080 Rh* s-1 at 23.5 degrees C, corresponding to Q10 = 17. 4. Response thresholds of retinal ganglion cells were extracellularly recorded in the isolated eyecup of R. temporaria. The thresholds of the most sensitive cells when stimulated with large-field steps of light were similar to the behavioural threshold and changed with temperature in a similar manner. 5. The decrease in ganglion cell 'step' sensitivity with warming consisted of a decrease in summation time (by a factor of 2-3 between 10 and 20 degrees C) and an increase in the threshold number of photoisomerizations (a decrease in 'flash' sensitivity, by a factor of 2-5 over the same interval). No effect of temperature changes on spatial summation was found. 6. Frequency-of-response functions of ganglion cells indicated an 11-fold increase in noise-equivalent dark light between 10 and 20 degrees C (mean values in four cells 0.009 vs. 0.10 Rh* s-1). 7. The temperature dependence of ganglion cell flash sensitivity could be strongly decreased with dim background illumination. 8. It is concluded that the desensitization of dark-adapted vision with rising temperature is a retinal effect composed of shortened summation time and lowered flash sensitivity (increased numbers of photons required for a threshold response) in ganglion cells. The desensitization bears no simple relation to the apparent retinal noise increase.

Microspectrophotometric determinations of rod visual pigments in some adult and larval Australian amphibians

Visual pigments from the red rods of adults of eight species of Australian anuran amphibians, from a variety of habitats, were analyzed by microspectrophotometry. The lambda max in all cases fell between 502 nm and 506 nm, and the absorption spectra were well fitted by an A1-based visual pigment template curve. Red rod pigments were also analyzed for a number of tadpoles. In some cases the data were best fitted with an A1-based visual pigment template, in other cases with an A2-based template, and finally some tadpoles appeared to have mixtures of the two pigments.

Noise and the absolute thresholds of cone and rod vision

Literature data on light detection by cone and rod vision at absolute threshold are analysed in order (1) to decide whether the threshold performance of dark-adapted cone vision can, like that of rod vision, be consistently explained as limited by noise from a "dark light"; (2) to obtain comparable estimates of the dark noise and dark light of (foveal) cones and (peripheral) rods. The dark noise was estimated by a maximum-likelihood procedure from frequency-of-seeing data and compared with the dark light derived from increment-threshold functions. In both cone and rod vision, the estimated dark noise coincides with Poisson fluctuations of the estimated dark light if 17% (best estimate) of lambda max-quanta incident at the cornea produce excitations. At that fraction of quanta exciting, dark lights are equivalent to 112 isomerisations per sec in each foveal cone and 0.011 isomerisations per sec in each rod. It is concluded that (1) the threshold performance of dark-adapted cone as well as rod vision can be consistently described as noise-limited, but not by postulating a multi-quantum coincidence requirement for single receptors; (2) the underlying intrinsic activity in both the cone and the rod system is light-like as regards correspondence between noise effect and background adaptation effect. One possibility is that this activity is largely composed of events identical to the single-photon response, originating in the visual pigment, in cones as well as in rods.