Photoregeneration of visual pigments in a moth

Summary The spectral absorbance by the visual pigments in the compound eye of the mothDeilephila elpenor was determined by microphotometry. Two visual pigments and their photoproducts were demonstrated. The photoproducts are thermostable and are reconverted to the visual pigments by light. The concentrations of the visual pigments and the photoproducts at each wavelength are determined by their absorbance coefficients at this wavelength. P 525: The experimental recordings (difference spectra and spectral absorbance changes after exposure to monochromatic lights) were completely reproduced by calculations using nomograms for vertebrate rhodopsin. The identity between experimental recordings and calculations show: One visual pigment absorbs maximally at 525 nm (P 525). The resonance spectrum of the visual pigment is identical to that for a vertebrate rhodopsin (_max at 525 nm). The photoproduct of this pigment absorbs maximally at 480 nm (M 480). It is similar to the acid metarhodopsin in cephalopods. The relative absorbance of P 525 to that of M 480 is 11.75. The quantum efficiency for photoconversion of P 525 to M 480 is nearly equal to that for reconversion of M 480 to P 525. Wavelengths exceeding about 570 nm are absorbed only by P 525, i. e. P 525 is completely converted to M 480. Shorter wavelengths are absorbed both by P 525 and M 480. At these wavelengths a photoequilibrium between the two pigments is formed. Maximal concentration of P 525 is obtained at about 450 nm. P 350: A second visual pigment absorbs maximally at about 350 nm (P 350), and its photoproduct at 450 to 460 nm. In the region of spectral overlap a photoequilibrium between the two pigments is formed.The visual pigment and the photoproduct are similar to those in the neuropteran insectAscalaphus.The work reported in this article was supported by Deutsche Forschungsgemeinschaft, Schwerpunktsprogramm Rezeptorphysiologie Ha 258-10, and SFB 114, by the Swedish Medical Research Council (grant no B 73-04X-104-02B), by Karolinska Institutet, and by a grant (to G. Höglund) from Deutscher Akademischer Austauschdienst.     

Evidence for a two pigment visual system in the fiddler crab, Uca thayeri

Intraocular recordings were made from the eyestalks of dark-adapted fiddler crabs (Uca thayeri) during presentation of monochromatic light flashes of different wavelengths and intensities. Two types of signals were recorded in different experiments: slow potentials (electroretinogram) and fast potentials (spikes). The latter were also recorded in the presence of a continuous green or red adapting light. The resulting visual spectral-sensitivity curves, when fitted to rhodopsin-based visual pigment absorption spectra (from Dartnall nomograms), indicated the presence of two visual pigments, one with an absorption maximum near 430 nm, and the other with a peak absorption between 500 nm and 540 nm. The data also provided evidence for some differential bleaching of the pigments in the presence of a colored adapting light, but most of the adaptation effect was probably due to changes in screening pigment and neural desensitization or inhibition. These two observations suggest that an adequate substrate for color vision may exist in this and other species of fiddler crabs. The electroretinogram and spike-recording methods produced similar visual-sensitivity data, suggesting that latter technique, a much more efficient way of collecting data that is physiologically relevant, may be the method of choice for determining spectral sensitivity in crustaceans.     

Interpreting trans-retinal recordings of spectral sensitivity

A quantitative model is developed to describe spectral sensitivity functions recorded extracellularly from heterogeneous populations of receptors in different states of adaptation. This treatment identifies the most important influences and clarifies several general features of experimental results. The shapes of retinal spectral sensitivity curves in different states of chromatic adaptation depend in predictable fashion on whether the primary effect of the adapting light on individual receptors is to decrease Vmax (response compression) or to increase the quantum demand for half-saturation. Some response compression is necessary in order for one or more receptors to drop out of the response at modest levels of adaptation. The apparent ease of adaptation also depends on the criterion voltage, particularly in the presence of response compression. The technique of selective adaptation of the ERG is capable of revealing the presence of receptors that comprise only a few percent of the total population. The short wavelength absorption of all visual pigments normally makes it impossible to use uv or violet light to adapt selectively those receptors with maximal sensitivity in the uv or violet region of the spectrum while sparing receptors with maximal sensitivity at longer wavelengths. The presence of cone oil droplets absorbing at short wavelengths, however, can effectively screen visual pigments in some of the receptors from uv or violet adapting lights.     

A novel amino acid substitution is responsible for spectral tuning in a rodent violet-sensitive visual pigment

Cone short-wave (SWS1) visual pigments can be divided into two categories that correlate with spectral sensitivity, violet sensitive above 390 nm and ultraviolet sensitive below that wavelength. The evolution and mechanism of spectral tuning of SWS1 opsins are proving more complex than those of other opsin classes. Violet-sensitive pigments probably evolved from an ancestral ultraviolet-sensitive opsin, although in birds ultraviolet sensitivity has re-evolved from violet-sensitive pigments. In certain mammals, a single substitution involving the gain of a polar residue can switch sensitivity from ultraviolet to violet sensitivity, but where such a change is not involved, several substitutions may be required to effect the switch. The guinea pig, Cavia porcellus, is a hystricognathous rodent, a distinct suborder from the Sciurognathi, such as rats and mice. It has been shown by microspectrophotometry to have two cone visual pigments at 530 and 400 nm. We have ascertained the sequence of the short-wave pigment and confirmed its violet sensitivity by expression and reconstitution of the pigment in vitro. Moreover, we have shown by site-directed mutagenesis that a single residue is responsible for wavelength tuning of spectral sensitivity, a Val86Phe causing a 60 nm short-wave shift into the ultraviolet and a Val86Tyr substitution shifting the pigment 8 nm long wave. The convergent evolution of this mammalian VS pigment provides insight into the mechanism of tuning between the violet and UV.     

Single and Multiple Visual Systems in Arthropods

Extraction of two visual pigments from crayfish eyes prompted an electrophysiological examination of the role of visual pigments in the compound eyes of six arthropods. The intact animals were used; in crayfishes isolated eyestalks also. Thresholds were measured in terms of the absolute or relative numbers of photons per flash at various wavelengths needed to evoke a constant amplitude of electroretinogram, usually 50 µv. Two species of crayfish, as well as the green crab, possess blue- and red-sensitive receptors apparently arranged for color discrimination. In the northern crayfish, Orconectes virilis, the spectral sensitivity of the dark-adapted eye is maximal at about 550 mµ, and on adaptation to bright red or blue lights breaks into two functions with λ_max respectively at about 435 and 565 mµ, apparently emanating from different receptors. The swamp crayfish, Procambarus clarkii, displays a maximum sensitivity when dark-adapted at about 570 mµ, that breaks on color adaptation into blue- and red-sensitive functions with λ_max about 450 and 575 mµ, again involving different receptors. Similarly the green crab, Carcinides maenas, presents a dark-adapted sensitivity maximal at about 510 mµ that divides on color adaptation into sensitivity curves maximal near 425 and 565 mµ. Each of these organisms thus possesses an apparatus adequate for at least two-color vision, resembling that of human green-blinds (deuteranopes). The visual pigments of the red-sensitive systems have been extracted from the crayfish eyes. The horse-shoe crab, Limulus, and the lobster each possesses a single visual system, with λ_max respectively at 520 and 525 mµ. Each of these is invariant with color adaptation. In each case the visual pigment had already been identified in extracts. The spider crab, Libinia emarginata, presents another variation. It possesses two visual systems apparently differentiated, not for color discrimination but for use in dim and bright light, like vertebrate rods and cones. The spectral sensitivity of the dark-adapted eye is maximal at about 490 mµ and on light adaptation, whether to blue, red, or white light, is displaced toward shorter wavelengths in what is essentially a reverse Purkinje shift. In all these animals dark adaptation appears to involve two phases: a rapid, hyperbolic fall of log threshold associated probably with visual pigment regeneration, followed by a slow, almost linear fall of log threshold that may be associated with pigment migration.     

Spectral sensitivity of the weakly discharging electric fish Gnathonemus petersi using its electric organ discharges as the response measure

The spectral sensitivity of the weakly electric mormyrid fish Gnathonemus petersi was investigated under dark- and light-adapted conditions using a transient change (startle) in its electric organ discharge (EOD) rate as response measure. The startle was resistant to habituation and graded with light intensity. Under both lighting conditions, the fish responded optimally to a monochromatic light of 525 nm. A porphyropsin pigment (520–540₂) appears to mediate spectral sensitivity over most of the visible spectrum. However, G. petersi responded more strongly to 625- and 675-nm lights (dark- and light-adapted fish) and a 725-nm light (light-adapted fish only) than predicted by the presence of a single rod pigment. These data suggest that at least one additional visual pigment (most likely of cone cells) maximally absorbing long wavelength light (600 nm or longer) is present. The spectral sensitivity data are consistent with the sensitivity hypothesis in that heightened sensitivity to long wavelength light is predicted for fish living in blackwater habitats which are characterized typically by low light levels and transmission of predominantly long wavelengths. Histology of the retina showed photoreceptors grouped into bundles and ensheathed by pigment epithelial cells. Our results demonstrated a functional visual sense in a species of fish much better known and studied for its electrosensory and electromotor abilities.     

Physiological basis of phototaxis to near-infrared light in Nephotettix cincticeps

In a previous study of the phototaxis of green rice leafhoppers, Nephotettix cincticeps (Hemiptera, Cicadellidae), we found positive responses to 735 nm light. Here, we investigated the mechanism underlying this sensitivity to near-infrared light. We first measured the action spectrum using a Y-maze with monochromatic lights from 480 to 740 nm. We thus found that the action spectrum peaks at 520 nm in the tested wavelength range, but that a significant effect is still observed at 740 nm, albeit with a sensitivity 5 log units lower than the peak. Second, we measured the spectral sensitivity of the eye, and found that the sensitivity in the long-wavelength region parallels the behaviorally determined action spectrum. We further identified mRNAs encoding opsins of ultraviolet, blue, and green-absorbing visual pigments, and localized the mRNAs in the ommatidia by in situ hybridization. The electrophysiology, molecular biology and the anatomy of the eye together indicate that the eyes of N. cincticeps do not contain true “red” receptors, but rather that the behavioral response to near-infrared light is mediated by the tail sensitivity of the green receptors in the long-wavelength region of the spectrum.     

The Spectral Sensitivity of Crayfish and Lobster Vision

(1) The spectral sensitivity function for the compound eye of the crayfish has been determined by recording the retinal action potentials elicited by monochromatic stimuli. Its peak lies at approximately 570 mµ. (2) Similar measurements made on lobster eyes yield functions with maxima in the region of 520 to 525 mµ, which agree well with the absorption spectrum of lobster rhodopsin if minor allowances are made for distortion by known screening pigments. (3) The crayfish sensitivity function, since it is unaffected by selective monochromatic light adaptation, must be determined by a single photosensitive pigment. The absorption maximum of this pigment may be inferred with reasonable accuracy from the sensitivity data. (4) The visual pigment of the crayfish thus has its maximum absorption displaced by 50 to 60 mµ towards the red end of the spectrum from that of the lobster and other marine crustacea. This shift parallels that found in both rod and cone pigments between fresh water and marine vertebrates. In the crayfish, however, an altered protein is responsible for the shift and not a new carotenoid chromophore as in the vertebrates. (5) The existence of this situation in a new group of animals (with photoreceptors which have been evolved independently from those of vertebrates) strengthens the view that there may be strong selection for long wavelength visual sensitivity in fresh water.     

Adaptation of a deep-sea cephalopod to the photic environment. Evidence for three visual pigments

Watasenia scintillans, a bioluminescent deep-sea squid, has a specially developed eye with a large open pupil and three visual pigments. Photoreceptor cells (outer segment: 476 micron; inner segment: 99 micron) were long in the small area of the ventral retina receiving downwelling light, whereas they were short (outer segment: 207 micron; inner segment: 44 micron) in the other regions of the retina. The short photoreceptor cells contained the visual pigment with retinal (lambda max approximately 484 nm), probably for the purpose of adapting to their environmental light. The outer segment of the long photoreceptor cells consisted of two strata, a pinkish proximal area and a yellow distal area. The visual pigment with 3-dehydroretinal (lambda max approximately 500 nm) was located in the pinkish proximal area, giving high sensitivity at longer wavelengths. A newly found pigment (lambda max approximately 471 nm) was in the yellow distal area. The small area of the ventral retina containing two visual pigments is thought to have a high and broad spectral sensitivity, which is useful for distinguishing the bioluminescence of squids of the same species in their environmental downwelling light. These findings were obtained by partial bleaching of the extracted pigment from various areas of the retina and by high-performance liquid chromatographic analysis of the chromophore, complemented by microscopic observations.     

Localization of visual pigments within rhabdoms of the compound eye of Spodoptera exempta (Insecta,Noctuidae)

Summary In the noctuid moth Spodoptera exempta, the distribution of visual pigments within the fused rhabdoms of the compound eyes was investigated by electron microscopy. Each ommatidium regularly contains eight receptor cells belonging to three morphological types: one distal, six medial, and one basal cell (Meinecke 1981); four different visual pigments — absorption maxima at approximately 355, 465, 515, and 560 nm — are known to occur within the eye (Langer et al. 1979). The compound eyes were illuminated in situ by use of monochromatic light of different wavelengths. This illumination produced a wide scale of structural changes in the microvilli of the rhabdomeres of individual cells. Preparation of eyes by freeze-substitution revealed the structural changes in the rhabdomeres to be effects of light occurring in vivo.The degree of structural changes may be considerably different in rhabdomeres within the same ommatidium; it was found to depend on the wavelength and the duration of illumination, the intensity received by the ommatidia as well as the spectral sensitivity of the receptor cells. Therefore, it was possible to estimate the spectral sensitivities of the morphological types of receptor cells. Generally, all medial cells are green receptors and all basal cells red receptors; distal cells are blue receptors in about two-thirds of the ommatidia, while in the remaining third of them distal cells are sensitive to ultraviolet light.Supported by Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 114 (Bionach)     

Spectral sensitivity of the green photoreceptor of winged pea aphids

Intracellular recordings are obtained from photoreceptors in the retina of winged (alate) pea aphids Acyrthosiphon pisum (Harris). The responses to monochromatic light, applied in 10‐nm steps over the range 320–650 nm, reveal that all recordings are from green receptors and the spectral sensitivity function of these photoreceptors peaks at 518 nm. A comparison between the spectral sensitivity of the green receptors and extracellular electroretinogram recordings suggests that additional sensitivity to the short‐wavelength light (ultraviolet and/or blue) is also likely to be present in the compound eye of pea aphids. An analysis of the pea aphid genome, comparing its translated nucleotide sequences with the those of the opsin genes of other insect species, supports this electrophysiological finding, although it could not be established whether A. pisum, in addition to the green receptor, has both blue and ultraviolet receptors in the compound eye. The implications of these results for the visual ecology of herbivorous insects are discussed.     

Photopic spectral sensitivity of a teleost fish,the roach (Rutilus rutilus), with special reference to its ultraviolet sensitivity

Summary This study reports photopic spectral sensitivity curves (351–709 nm) for four individual roach,Rutilus rutilus, determined by two choice appetitive training. All four curves show four sensitivity maxima at 361–398 nm, 421–448 nm, 501–544 nm and 634–666 nm which are related to the four known roach photopic visual pigments (Avery et al. 1982). The overall shape of the curves at long wavelengths indicates inhibitory interactions between the red and green cone mechanisms. That the high behavioural sensitivity in the UV is caused by a specific ultraviolet visual pigment and is not due to aberrant stimulation of the other cone types is shown by the redetermination of spectral sensitivity at short wavelengths (351–501 nm) following the selective bleaching of the three longer wavelength visual pigments. This depresses the blue sensitivity to a greater degree than the relatively unaffected UV sensitivity maximum. Spectral transmission data from two corneas and four lenses show that they transmit considerable amounts of light in the near UV.     

Molecular basis of spectral tuning in the red- and green-sensitive (M/LWS) pigments in vertebrates

Vertebrate vision is mediated by five groups of visual pigments, each absorbing a specific wavelength of light between ultraviolet and red. Despite extensive mutagenesis analyses, the mechanisms by which contemporary pigments absorb variable wavelengths of light are poorly understood. We show that the molecular basis of the spectral tuning of contemporary visual pigments can be illuminated only by mutagenesis analyses using ancestral pigments. Following this new principle, we derive the "five-sites" rule that explains the absorption spectra of red and green (M/LWS) pigments that range from 510 to 560 nm. Our findings demonstrate that the evolutionary method should be used in elucidating the mechanisms of spectral tuning of four other pigment groups and, for that matter, functional differentiations of any other proteins.     

Comparative study of spectral and antioxidant properties of pigments from the eyes of two Mysis relicta (Crustacea, Mysidacea) populations, with different light damage resistance

Retinal visual and screening pigments of two populations (one marine and the other freshwater) of the opossum shrimp Mysis relicta Lovén (Crustacea, Mysidacea), which have different ocular tolerance to light, was investigated. Visual pigments were extracted by detergent and their bleaching difference spectra were determined. The difference between the visual pigment absorption maximum of the two populations correlated with their difference in spectral sensitivity. Using buffer or neutral methanol, a yellow pigment was extracted which had absorption maxima at 440 nm and 325 nm and bright blue fluorescence (λ_max 415 nm). A screening pigment (ommochrome) with maximum at 525 nm was extracted by acid methanol, and was probably related to the group of ommines. The eyes of the lake population had 1.8–2.7 times less of this pigment than the eyes of the sea population. The sea population is more resistant to photo-induced accumulation of thiobarbituric acid-reactive substances in eye tissues. This resistance may be due to the higher ommochrome content. Accepted: 8 December 1998     

Genetic basis of spectral tuning in the violet-sensitive visual pigment of African clawed frog, Xenopus laevis

Ultraviolet (UV) and violet vision in vertebrates is mediated by UV and violet visual pigments that absorb light maximally (lambdamax) at approximately 360 and 390-440 nm, respectively. So far, a total of 11 amino acid sites only in transmembrane (TM) helices I-III are known to be involved in the functional differentiation of these short wavelength-sensitive type 1 (SWS1) pigments. Here, we have constructed chimeric pigments between the violet pigment of African clawed frog (Xenopus laevis) and its ancestral UV pigment. The results show that not only are the absorption spectra of these pigments modulated strongly by amino acids in TM I-VII, but also, for unknown reasons, the overall effect of amino acid changes in TM IV-VII on the lambdamax-shift is abolished. The spectral tuning of the contemporary frog pigment is explained by amino acid replacements F86M, V91I, T93P, V109A, E113D, L116V, and S118T, in which V91I and V109A are previously unknown, increasing the total number of critical amino acid sites that are involved in the spectral tuning of SWS1 pigments in vertebrates to 13.     

Visual pigments and spectral sensitivity of the diurnal gecko Gonatodes albogularis

The visual pigments and oil droplets in the retina of the diurnal gecko Gonatodes albogularis were examined microspectrophotometrically, and the spectral sensitivity under various adapting conditions was recorded using electrophysiological responses. Three classes of visual pigments were identified, with _max at about 542, 475, and 362 nm. Spectral sensitivity functions revealed a broad range of sensitivity, with a peak at approximately 530–540 nm. The cornea and oil droplets were found to be transparent across a range from 350–700 nm, but the lens absorbed short wavelength light below 450 nm. Despite the filtering effect of the lens, a secondary peak in spectral sensitivity to ultraviolet wavelengths was found. These results suggest that G. albogularis does possess the visual mechanisms for discrimination of the color pattern of conspecifics based on either hue or brightness. These findings are discussed in terms of the variation in coloration and social behavior of Gonatodes.Abbreviations ERG electroretinogram - MSP microspectrophotometry - UV ultraviolet - _max wavelength of maximum absorbance     

Tuning of photoreceptor spectral sensitivity in fireflies (Coleoptera: Lampyridae)

Sexual communication between male and female fireflies involves the visual detection of species-specific bioluminescent signals. Firefly species vary spectrally in both their emitted light and in the sensitivity of the eye, depending on the time when each is active. Tuning of spectral sensitivity in three firefly species that occupy different photic niches was investigated using light and electron microscopy, microspectrophotometry, and intracellular recording to characterize the location and spectral absorption of the screening pigments that filter incoming light, the visual pigments that receive this filtered light, and the visual spectral sensitivity. Twilight-active species had similar pink screening pigments, but the visual pigment of Photinus pyralis peaked near 545 nm, while that of P. scintillans had a λ_max near 557 nm. The night-active Photuris versicolor had a yellow screening pigment that was uniquely localized, while its visual pigment was similar to that of P. pyralis. These results show that both screening and visual pigments vary among species. Modeling of spectral tuning indicates that the combination of screening and visual pigments found in the retina of each species provides the best possible match of sensitivity to bioluminescent emission. This combination also produced model sensitivity spectra that closely resemble sensitivities measured either with electroretinographic or intracellular techniques. Vision in both species of Photinus appears to be evolutionarily tuned for maximum discrimination of conspecific signals from spectrally broader backgrounds. Ph. versicolor, on the other hand, appears to have a visual system that offers a compromise between maximum sensitivity to, and maximum discrimination of, their signals. Accepted: 29 September 1999     

Colour receptors in the eye of the digger wasp,Sphex cognatus Smith: Evaluation by selective adaptation

Summary Pigment granule migration in pigment cells and retinula cells of the digger wasp Sphex cognatus Smith was analysed morphologically after light adaptation to natural light, dark adaptation and after four selective chromatic adaptations in the range between 358 nm and 580 nm and used as the index of receptor cell sensitivity. The receptor region of each ommatidium consists of nine retinula cells which form a centrally located rhabdom. Two morphologically and physiologically different visual units can be described, defined by the arrangement of the rhabdomeric microvilli, the topographical relationship of the receptor cells with respect to the eye axes and the unique retinula cell screening pigmentation. These two different sets of ommatidia (type A and B) are randomly distributed in a ratio of 13 throughout the eye (Ribi, 1978b). Chromatic adaptation experiments with wavelengths of 358 nm, 443 nm, 523 nm and 580 nm and subsequent histological examination reveal two UV receptors, two blue receptors and four yellow-green receptors in type A ommatidia and two UV receptors and six green to yellow-green receptors in type B ommatidia. The pigments in cells surrounding each ommatidium (two primary pigment cells, 20 secondary pigment cells and four pigmented cone extensions) were not affected significantly by the adaptation experiments.     

Metarhodopsin control by arrestin,light-filtering screening pigments,and visual pigment turnover in invertebrate microvillar photoreceptors

The visual pigments of most invertebrate photoreceptors have two thermostable photo-interconvertible states, the ground state rhodopsin and photo-activated metarhodopsin, which triggers the phototransduction cascade until it binds arrestin. The ratio of the two states in photoequilibrium is determined by their absorbance spectra and the effective spectral distribution of illumination. Calculations indicate that metarhodopsin levels in fly photoreceptors are maintained below ~35% in normal diurnal environments, due to the combination of a blue-green rhodopsin, an orange-absorbing metarhodopsin and red transparent screening pigments. Slow metarhodopsin degradation and rhodopsin regeneration processes further subserve visual pigment maintenance. In most insect eyes, where the majority of photoreceptors have green-absorbing rhodopsins and blue-absorbing metarhodopsins, natural illuminants are predicted to create metarhodopsin levels greater than 60% at high intensities. However, fast metarhodopsin decay and rhodopsin regeneration also play an important role in controlling metarhodopsin in green receptors, resulting in a high rhodopsin content at low light intensities and a reduced overall visual pigment content in bright light. A simple model for the visual pigment–arrestin cycle is used to illustrate the dependence of the visual pigment population states on light intensity, arrestin levels and pigment turnover.     

The Spectral Sensitivities of Single Receptor Cells in the Lateral, Median, and Ventral Eyes of Normal and White-Eyed Limulus

Spectral sensitivity curves can be distorted by screening pigments. We have determined whether this is true for Limulus polyphemus by determining, from receptor potentials recorded using intracellular microelectrodes, spectral sensitivity curves for normal animals and for white-eyed animals (which lack screening pigment). Our results show: (a) In median ocelli, the curve for UV-sensitive receptor cells peaks at 360 nm and does not depend on the presence of screening pigment, (b) The curve for ventral eye photoreceptors is identical to that for retinular cells from the lateral eyes of white-eyed animals and peaks at 520–525 nm. (c) In normal lateral eyes, when the stimulating light passes through screening pigment, the curve indicates relatively more sensitivity in the red region of the spectrum than does the curve for white-eyed animals. Therefore, the screening pigment is probably red-transmitting, (d) In median ocelli, the curve for visible-sensitive cells peaks at 525 nm and is approximately the same whether the ocelli are from normal or white-eyed animals. However, the curve is significantly broader than that for ventral eyes and for lateral eyes from white-eyed animals.