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PubMed1962

Direct action of light in naturally pigmented muscle fibers. I. Action spectrum for contraction in eel iris sphincter.

Contraction due to light in excised eel irises appears to follow a simple first order law. The action spectrum for contraction has a maximum which agrees with the eel rhodopsin absorption maximum. Inasmuch as rhodopsin is the rod pigment-opsin complex and the iris sphincter pupillae evolves from the pigment epithelium of the retina in the region of the iris, the muscle pigment might be the same as the visual pigment. In the human eye the contraction of the iris sphincter is activated only by light incident on the retina and the pupil diameter varies inversely with the square root of the light intensity. The inverse first power relation observed in the present experiments suggests a more primitive origin for the light reaction in eel irises. Relaxation is a much slower process and can be approximated as the sum of two first order processes.

Adaptation, Ocular↗
PubMed1984

Visual pigments and the labile scotopic visual system of fish.

Among mammals, birds, most reptiles and chondrichthians, only rhodopsins are present. Among agnathans, osteichthians, amphibians and certain freshwater turtles there are species having only porphyropsins or only rhodopsins or, more interestingly, both pigments, either sequentially or together. This latter grouping represents the paired-pigment species. Associated with the presence of paired-pigments is the possibility that the proportions of rhodopsin and porphyropsin may change. Depending on the characteristics of each paired-pigment species, naturally occurring changes in visual pigment ratios are related to migrations in anadromous and catadromous teleosts and anadromous cyclostomes and to seasonal variation in several teleosts. In addition, the visual pigment composition of certain species of teleosts has been altered by the specific effects of light, temperature, diet and hormones. Of two possible mechanisms for altering spectral sensitivity, varying the proportion of rhodopsin and porphyropsin is far more common than utilizing a single chromophore and changing the opsin. In addition to the long established evidence that extractable rod pigment ratios may change during the life cycle or in response to specific exogenous factors, there is the more recent recognition from microspectrophotometry that cone pigment ratios may also change in concert. The effect of lighting conditions and temperature on the visual pigment composition of certain paired-pigment species is presented.

Animals↗
PubMed2004

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.

Animals↗
PubMed2004

Ancestral loss of short wave-sensitive cone visual pigment in lorisiform prosimians, contrasting with its strict conservation in other prosimians.

Mammals are basically dichromatic in color vision, possessing middle to long wave-sensitive (M/LWS) and the short wave-sensitive (SWS) cone opsins in the retina, whereas some nocturnal mammals lack functional SWS opsins. Prosimians, primitive primates consisting of three extant groups (Lorisiformes, Lemuriformes, and Tarsiiformes), include many nocturnal species. Among nocturnal prosimians, a species of lorisiforms, the greater galago (Otolemur crassicaudatus), is known to lack a functional SWS opsin gene, while lemuriforms and tarsiiforms appear to retain SWS opsins in the retina. It has not been established, however, whether the loss of SWS opsin is a universal phenomenon among lorisiforms and whether the functional SWS opsin genes of lemuriforms and tarsiiforms are under strict or relaxed selective constraint. To gain better insight into an association between nocturnality and loss of SWS function, we isolated and sequenced the SWS opsin genes from two species of lorisiforms, the slow loris (Nycticebus coucang; nocturnal) and the lesser galago (Galago senegalensis; nocturnal), and one species each of lemuriforms and tarsiiforms, the brown lemur (Eulemur fulvus; cathemeral) and the western tarsier ( Tarsius bancanus; nocturnal), respectively. Our sequence analysis revealed that (1) the SWS opsin gene was disrupted in the common ancestor of galagids and lorisids and (2) the rate of nonsynonymous nucleotide substitution has been kept significantly lower than that of synonymous substitution in tarsier and lemur, demonstrating the presence of strict selective constraint on the SWS opsin genes in tarsiiforms and lemuriforms.

Animals↗
PubMed2005

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.

Animals↗
PubMed1979

Vitamin A deficiency reduces the concentration of visual pigment protein within blowfly photoreceptor membranes.

Visual pigment extracts prepared from rhabdomeric membranes of vitamin A deficient blowflies contain a 5-10 times lower concentration of rhodopsin than extracts from flies which were raised on a vitamin A rich diet. Spectrophotometry showed that digitonin-solubilized rhodopsin from blowfly photoreceptors R1-6 has an absorbance maximum at about 490 nm, but no unusually enhanced beta-band in the ultraviolet. The extracts did not contain detectable concentrations of other visual pigments nor was there any evidence for the presence of photostable vitamin A derivatives. Sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrated that the concentration of opsin in the rhabdomeric membrane is significantly reduced in vitamin A deficient flies compared to normal flies. The results indicate that the synthesis of opsin or its incorporation into the photoreceptor membrane is regulated by the chromophore concentration in the receptor cell. Furthermore, our findings open up the possibility that differences in the spectral absorption and excitability of photoreceptors from normal and vitamin A deficient flies result from the differing opsin content of the rhabdomeres.

Animals↗
PubMed1975

Synthesis of rhodopsin and opsin in vitro.

Isolated bovine retinas have been used to study the synthesis of rhodopsin and the renewal of photoreceptor rod outer segments. Both leucine and glucosamine served as radioactive tracers to follow rhodopsin synthesis. In both cases the rod outer segment preparations contained large amounts of labeled macromolecules chromatographically distinct from rhodopsin, the latter representing only about 10% of the high molecular weight labeled material. However, electrophoresis on polyacrylamide gels with sodium dodecyl sulfate indicated that as much as 60-68% of the radioactivity coincided with opsin, the apoprotein of rhodopsin. Treatment of labeled rod outer segments with 9-cis-retinal caused much of the non-rhodopsin label to be converted to isorhodopsin. After such treatment the fraction of the label in visual pigment rose from about 10 to 51% with leucine as the radioactive tracer and to 78% with glucosamine. Similar treatment of bleached outer segments labeled with leucine gave identical results with complete regeneration of isorhodopsin (lambda max 487 nm) which then accounted for 56% of the labeled macromolecules. No such conversion occurred in controls lacking 9-cis-retinal. Both 9-cis- and 11-cis-retinal were effective but all-trans-retinal was ineffective in producing the conversion. Under in vitro conditions opsin appears to be accumulated in the outer segment prior to the addition of retinal.

Animals↗
PubMed1989

Morphological defects in oraJK84 photoreceptors caused by mutation in R1-6 opsin gene of Drosophila.

The Drosophila mutant, oraJK84, lacks rhabdomeres in the major (R1-6) class of photoreceptors because these rhabdomeres rapidly degenerate in young flies. Genetic analysis reveals that oraJK84 actually contains two mutations (a ninaE and an ort allele) that affect the visual process. The mutation in ort appears to have no effect on photoreceptor structure. The other mutation occurs within the ninaE gene, which encodes the species of rhodopsin found in the R1-6 class of photoreceptors. Our analysis shows that this mutation is responsible for R1-6 rhabdomere degeneration in oraJK84 mutants. We also examined a ninaE mutant, denoted ninaEo117, that produces no ninaE transcript. The morphological phenotype observed in ninaEo117 is similar to that seen in oraJK84 mutants. We conclude that rhodopsin plays a vital role in maintaining photoreceptor structure in Drosophila.

Animals↗
PubMed2002

Adaptive loss of ultraviolet-sensitive/violet-sensitive (UVS/VS) cone opsin in the blind mole rat (Spalax ehrenbergi).

In previous studies, fully functional rod and long-wavelength-sensitive (LWS) cone photopigments have been isolated from the eye of the subterranean blind mole rat (Spalax ehrenbergi superspecies). Spalax possesses subcutaneous atrophied eyes and lacks any ability to respond to visual images. By contrast this animal retains the ability to entrain circadian rhythms of locomotor behaviour to environmental light cues. As this is the only known function of the eye, the rod and LWS photopigments are thought to mediate this response. Most mammals are dichromats possessing, in addition to a single rod photopigment, two classes of cone photopigment, LWS and ultraviolet-sensitive/violet-sensitive (UVS/VS) with differing spectral sensitivities which mediate colour vision. In this paper we explore whether Spalax is a dichromat and has the potential to use colour discrimination for photoentrainment. Using immunocytochemistry and molecular approaches we demonstrate that Spalax is a LWS monochromat. Spalax lacks a functional UVS/VS cone photopigment due to the accumulation of several deleterious mutational changes that have rendered the gene nonfunctional. Using phylogenetic analysis we show that the loss of this class of photoreceptor is likely to have arisen from the visual ecology of this species, and is not an artefact of having an ancestor which lacked a functional UVS/VS cone photopigment. We conclude that colour discrimination is not a prerequisite for photoentrainment in this species.

Adaptation, Biological↗
PubMedDate not supplied

[Spectral sensitivity and visual pigments of the coastal crab Hemigrapsus sanguineus].

It has been shown that the compound eye of the coastal crab has one photosensitive pigment rhodopsin and screening pigments, black and orange one. The orange pigment has lambda max = 480 nm, rhodopsin in digitonin is stable towards hydroxylamin action, has lambda max = 490-495 nm and after bleaching is transformed into free retinene and opsin. The pigments with lambda max = 430 and 475 nm of the receptor part of the eye are also solubilized. These pigments are not photosensitive but they dissociate under the effect of hydroxylamine. The curye of spectral sensitivity of the coastal crab has the basic maximum at approximately 525 nm and the additional one at 450 nm, which seems to be provided by a combination of the visual pigment--rhodopsin (lambda max 500 nm) with a carotinoid filter (lambda max 480-490). Specific features of the visual system of coastal crab are discussed.

Animals↗
PubMed2006

Turning cones off: the role of the 9-methyl group of retinal in red cones.

Our ability to see in bright light depends critically on the rapid rate at which cone photoreceptors detect and adapt to changes in illumination. This is achieved, in part, by their rapid response termination. In this study, we investigate the hypothesis that this rapid termination of the response in red cones is dependent on interactions between the 9-methyl group of retinal and red cone opsin, which are required for timely metarhodopsin (Meta) II decay. We used single-cell electrical recordings of flash responses to assess the kinetics of response termination and to calculate guanylyl cyclase (GC) rates in salamander red cones containing native visual pigment as well as visual pigment regenerated with 11-cis 9-demethyl retinal, an analogue of retinal in which the 9-methyl group is missing. After exposure to bright light that photoactivated more than approximately 0.2% of the pigment, red cones containing the analogue pigment had a slower recovery of both flash response amplitudes and GC rates (up to 10 times slower at high bleaches) than red cones containing 11-cis retinal. This finding is consistent with previously published biochemical data demonstrating that red cone opsin regenerated in vitro with 11-cis 9-demethyl retinal exhibited prolonged activation as a result of slowed Meta II decay. Our results suggest that two different mechanisms regulate the recovery of responsiveness in red cones after exposure to light. We propose a model in which the response recovery in red cones can be regulated (particularly at high light intensities) by the Meta II decay rate if that rate has been inhibited. In red cones, the interaction of the 9-methyl group of retinal with opsin promotes efficient Meta II decay and, thus, the rapid rate of recovery.

1-Methyl-3-isobutylxanthine↗
PubMed1998

Molecular basis of dark adaptation in rod photoreceptors.

Following exposure of the eye to an intense light that 'bleaches' a significant fraction of the rhodopsin, one's visual threshold is initially greatly elevated, and takes tens of minutes to recover to normal. The elevation of visual threshold arises from events occurring within the rod photoreceptors, and the underlying molecular basis of these events and of the rod's recovery is now becoming clearer. Results obtained by exposing isolated toad rods to hydroxylamine solution indicate that, following small bleaches, the primary intermediate causing elevation of visual threshold is metarhodopsin II, in its phosphorylated and arrestin-bound form. This product activates transduction with an efficacy about 100 times greater than that of opsin.

Animals↗
PubMed2000

Functional expression of a locust visual pigment in transgenic Drosophila melanogaster.

The cDNA encoding a visual pigment of the locust Schistocerca gregaria has been inserted into the germline of the ninaE mutant of Drosophila melanogaster by P-element-mediated transformation. Functional expression has been documented by recording light-regulated electroretinograms in transgenic flies. The spectral properties of the expressed visual pigment were determined with detergent-solubilized material, prepared from the eyecups of the transgenic D. melanogaster. The recombinant locust pigment, as well as the genuine pigment of the fruitfly (Rh1) that served as a control for transformation/expression, showed photoreversibility between the pigment and metapigment forms. The absorptions of the difference spectra identify the locust visual pigment as a short wavelength-absorbing, blue-light-sensitive photoreceptor. The absorption maxima are similar to those recorded on living locust animals. These results show that, although locust visual pigments contain 11-cis retinal as chromophore, the expressed protein is able to adopt 3-hydroxyretinal that is provided by the transgenic fruitflies. The electrophysiological recordings reveal that the locust visual pigment is able to induce phototransduction in the fruitfly. The reported results have two important consequences: On the one hand, the binding site of the locust opsin is apparently able to interact with the 3-hydroxyretinal from Drosophila in a way that the biological signal generated by the photoisomerization of the chromophore can be used by the protein to adopt a physiologically active conformation. On the other hand, despite the relatively large phylogenetic distance between both insect species, the extent of conservation between the protein domains thought to be involved in G-protein activation is striking.

Animals↗
PubMed1990

Rhodopsin reconstitution in bleached rod outer segment membranes in the presence of a retinal-binding protein from the honeybee.

The physiological role of a retinal-binding protein from honeybee is investigated. This protein, upon previous loading with all-trans retinal and subsequent irradiation with monochromatic light of wavelength 490 nm, is able to promote the reconstitution of rhodopsin when added to a suspension of opsin membranes from bleached bovine rod outer segments. In this respect this retinal-binding protein could have a role very similar to that postulated for the well-known cephalopod retinochrome, that serves to catalyze the formation in the presence of light of 11-cis retinal in photo-receptor cells and to provide it for the reconstitution of rhodopsin during the visual cycle.

Animals↗
PubMed2006

Divergent selection on opsins drives incipient speciation in Lake Victoria cichlids.

Divergent natural selection acting on ecological traits, which also affect mate choice, is a key element of ecological speciation theory, but has not previously been demonstrated at the molecular gene level to our knowledge. Here we demonstrate parallel evolution in two cichlid genera under strong divergent selection in a gene that affects both. Strong divergent natural selection fixed opsin proteins with different predicted light absorbance properties at opposite ends of an environmental gradient. By expressing them and measuring absorbance, we show that the reciprocal fixation adapts populations to divergent light environments. The divergent evolution of the visual system coincides with divergence in male breeding coloration, consistent with incipient ecological by-product speciation.

Africa, Eastern↗
PubMedDate not supplied

Modulation of the absorption maximum of rhodopsin by amino acids in the C-terminus.

Vision begins when light is absorbed by visual pigments. It is commonly believed that the absorption spectra of visual pigments are modulated by interactions between the retinal and amino acids within or near 4.5 angstroms of the retinal in the transmembrane (TM) segments. However, this dogma has not been rigorously tested. In this study, we show that the retinal-opsin interactions extend well beyond the retinal binding pocket. We found that, although it is positioned outside of TM segments, the C-terminus of the rhodopsin in the rockfish longspine thornyhead (Sebastolobus altivelis) modulates its lambda(max) by interacting mainly with the last TM segment. Our results illustrate how amino acids in the C-terminus are likely to interact with the retinal. We anticipate our analyses to be a starting point for viewing the spectral tuning of visual pigments as interactions between the retinal and key amino acids that are distributed throughout the entire pigment.

Amino Acid Sequence↗
PubMed1988

The fate of immunoreactive opsin following phagocytosis by pigment epithelium in human and monkey retinas.

Polyclonal and monoclonal antibodies to human rhodopsin were used to identify and localize this principal glycoprotein of the photoreceptor outer segment discs on thin sections of human and monkey retinal pigment epithelium (RPE) and on immunoblots of RPE subcellular fractions following gel electrophoresis. Antiopsin was visualized with protein A-gold labeling by electron microscopy or peroxidase-linked second antibody on immunoblots. In immunocytochemical studies using polyclonal antibodies, the rod outer segments (ROS) were heavily labeled whereas cone outer segments labeling was variable and more sparse. Phagosomes and other small bodies in the RPE, interpreted as secondary lysosomes, were labeled. In contrast, lipofuscin granules, osmiophilic residual bodies of the lysosomal system of the RPE, were negative. No reactive sites were found in Bruch's membrane or in drusen. A monoclonal antibody (MAB) specific for the amino terminus of human opsin and another MAB specific for the carboxy terminal region of bovine opsin produced labeling patterns similar to, but about one-half the density obtained with polyclonal antibodies. In the immunoblot analyses, the lipofuscin granule fraction from a sucrose density gradient of human RPE homogenates was positive for rhodopsin only in those specimens that were found, upon ultrastructural examination, to contain recognizable phagosomes. When phagosomes were lacking and therefore did not contaminate the lipofuscin granule fraction, the immunoblots were negative for opsin. Melanolipofuscin granule fractions were uniformly negative for opsin. We conclude that the superficial hydrophilic antigen binding sites on the opsin molecule for which the antibodies are specific have been altered or destroyed by lysosomal enzyme digestion within the phagolysosomal system of the RPE prior to formation of definitive lipofuscin granules. Thus, these antibodies are of limited value in revealing the ultimate fate of the whole rhodopsin molecule, eg, the hydrophobic sequences that are the most likely residues in lipofuscin granules.

Adult↗
PubMed1998

Rhodopsin: a prototypical G protein-coupled receptor.

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

Amino Acid Sequence↗