The contributions of the orientated photosensitive and other molecules to the absorption of whole retina

Vitamin A activates rhodopsin and sensitizes it to ultraviolet light

The visual pigment, rhodopsin, consists of opsin protein with 11-cis retinal chromophore, covalently bound. Light activates rhodopsin by isomerizing the chromophore to the all-trans conformation. The activated rhodopsin sets in motion a biochemical cascade that evokes an electrical response by the photoreceptor. All-trans retinal is eventually released from the opsin and reduced to vitamin A. Rod and cone photoreceptors contain vast amounts of rhodopsin, so after exposure to bright light, the concentration of vitamin A can reach relatively high levels within their outer segments. Since a retinal analog, β-ionone, is capable of activating some types of visual pigments, we tested whether vitamin A might produce a similar effect. In single-cell recordings from isolated dark-adapted salamander green-sensitive rods, exogenously applied vitamin A decreased circulating current and flash sensitivity and accelerated flash response kinetics. These changes resembled those produced by exposure of rods to steady light. Microspectrophotometric measurements showed that vitamin A accumulated in the outer segments and binding of vitamin A to rhodopsin was confirmed in in vitro assays. In addition, vitamin A improved the sensitivity of photoreceptors to ultraviolet (UV) light. Apparently, the energy of a UV photon absorbed by vitamin A transferred by a radiationless process to the 11-cis retinal chromophore of rhodopsin, which subsequently isomerized. Therefore, our results suggest that vitamin A binds to rhodopsin at an allosteric binding site distinct from the chromophore binding pocket for 11-cis retinal to activate the rhodopsin, and that it serves as a sensitizing chromophore for UV light.

Late stages of visual pigment photolysis in situ: cones vs. rods

Slow photolysis reactions and the regeneration of the dark pigment constitute the mechanisms of dark adaptation whereby photoreceptor cells restore their sensitivity after bright illumination. We present data on the kinetics of the late stages of the photolysis of the visual pigment in intact rods and red- and green-sensitive cones of the goldfish retina. Measurements were made on single photoreceptors by means of a fast-scanning dichroic microspectrophotometer. We show that in cones the hydrolysis of the opsin-all-trans 3-dehydroretinal linkage proceeds with a half-time of approximately 5s at 20 degrees C that is almost two orders of magnitude faster than in rods. 3-Dehydroretinol in cones is produced approximately 3-fold faster than retinol in amphibian rhodopsin rods; the rate of the reaction is limited by the speed of retinal reduction catalyzed by retinoldehydrogenase. The fast hydrolysis of the 3-dehydroretinal/opsin Schiff base and the correspondingly fast appearance of the substrates for dark visual pigment regeneration (free opsin and 3-dehydroretinol) provide essential conditions for faster dark adaptation of cone (diurnal) as compared to rod (nocturnal) vision.

In situ microspectrophotometric studies on the pigments of single retinal rods

Three spectral entities have been observed in single intact frog rod outer segments at 506 mmu, 480 mmu and 380 mmu. It is likely that the peak of 506 mmu was somewhat altered by bleaching reactions and originated at about 510 mmu. This is identified with the 502 mmu frog rhodopsin of digitonin extracts. Spectra in polarized light have the same maximum, identifying the dichroism of rods with rhodopsin. The dichroic ratio is around 6, giving the outer segment an axial density of 0.09/5mu or 0.9 OD total, with a pigment concentration of 2 to 3 mM. The dichroism data are used to compute the angle separating the rhodopsin molecular absorption vectors in rods from perfect restriction to a plane. This angle is 16 degrees or 23 degrees depending on which of two assumptions one chooses for the type of molecular ordering. The spectral peaks at 480 mmu and 380 mmu are thought to correspond respectively to metarhodopsin and retinene. Disappearance of the former is accompanied by accumulation of the latter. This reaction seems to occur more slowly in the intact outer segment than the corresponding reaction in solution. Spread of bleaching spectra from illuminated to dark areas of the same rod did not occur over distances of 2 mu or greater. Spectra were similar from rod to rod and from point to point on the same rod showing that frog rods are spectrally homogeneous both individually and collectively.

The roles of filters in the photophores of oceanic animals and their relation to vision in the oceanic environment

In many of the photophores found in deep-sea fishes and invertebrates, light filters containing pigments lie between the tissues that generate light and the sea. The loss of light within such filters has been measured throughout the visible spectrum for a variety of animals. These filters differ greatly in their spectral absorption characteristics and do not all contain the same pigments. All those from ventral photophores have a transmission band in the blue corresponding to the daylight that penetrates best into oceanic waters. For two fishes it is shown that the light generated inside their photophores is a relatively poor spectral match for the ambient submarine daylight while the light emitted into the sea, after passing through the filters, is a good match. For a third fish a similar improvement in ‘colour match’ is brought about not by passing the light through a filter containing pigments but by reflecting the light into the sea by a blue mirror. All these observations support the hypothesis that the ventral photophores are used for camouflage.Malacosteus nigerAyres 1848 is an oceanic fish which emits red light from a large suborbital photophore. The red light generated inside the photophore is largely absorbed by a coloured filter over its external surface which transmits only a band of light of wavelengths around 700 nm. This is a waveband which is heavily absorbed by oceanic sea water. It is shown, however, that animals that can emit and are sensitive to such far-red light will have very great advantages in being able to see without being seen. The ranges over which such red light can be useful for vision are, however, relatively small. The nature of the pigments found in these various photophores is discussed. It is also calculated that the intensities of penetrating daylight are such that visual acuity could be fairly good down to considerable depths in the mesopelagic zone.

The spectral characteristics of luminous marine organisms

Measurements of the bioluminescent emission spectra of a wide range of marine animals demonstrate considerable differences between taxa in both the position of the peak emission and the half bandwidth. Although most of the measured spectra are unimodal, some species have either two peaks or one main peak with subsidiary shoulders. Such structured emission spectra are present in several systematic groups and in some cases the emission characteristics have been observed to vary with time. The emission maxima of most species fall within the range 450—490 nm, though maxima from 395-545 nm have been recorded. Species found in the pelagic environment are mostly blue-emitting but there is some indication of relative increase in green-emitting species in the benthic environment. Terrestrial organisms are predominantly yellow-green luminescent. The ecological value of the observed spectral differences is discussed. While the characteristics of the emission spectra have considerable adaptive value for certain functions, some minor spectral variations may not be of ecological significance. Selection for increased quantum efficiency of the luminescence may sometimes predominate over spectral considerations.

Adaptations of visual pigments to the photic environment of the deep sea

This is a summary of studies that bear on the problems of the adaptation of visual pigments to the photic environment of the deep sea. The results suggest that the spectral absorption of these retinal pigments is shifted toward the blue in order to match the dim, blue-green downwelling light and/or the bioluminescence of organisms that are critical to the life of the species. Through such a spectral match, greater visual sensitivity is achieved for life in the special photic condition of their habitat. This adaptation has been found for chimaerid fishes, for elasmobranchs, for teleosts, for mammals, and for certain crustaceans and cephalopods. The most convincing evidence for such an adaptive match has been found in teleosts that have red-emitting photophores. In these fishes a photopigment with absorbance shifted toward the red has been found by extraction and microspectrophotometry. A few exceptions to this idea of an adaptive match have appeared in the literature, the cone pigments, especially, being examples of such offset pigments. The malacosteid fishes have been shown to have a red-shifted retinal pigment with 11-cis-3-dehydroretinal as the chromophore and some invertebrates have also adopted this molecule to adjust the spectral absorption to the photic environment or to the bioluminescence. These studies are beginning to reveal that visual biochemistry is basically the same in vertebrates and invertebrates and that the visual pigment protein arose early in phylogeny and has been retained, with appropraite modifications, to the present.

Absorption of light in photoreceptors: transverse incidence

The time variation of the absorption rate (i.e., the number of photons absorbed per see) in a photoreceptor when light is incident perpendicular to its axis has been studied for various species and different conditions. Due to the cylindrical geometry of the photoreceptor the expressions for the absorption rates become very complicated. Hence, simple approximate expressions for the absorption rates in the case of some of the species have been suggested. The present analysis will be useful in analysing the mechanism of the photoreceptor when light is incident perpendicular to the axis.

Linear dichroism of rhodopsin in air-water interface films

Air-water interface films of purified cattle rhodopsin and defined phospholipids are formed by the osmotic lysis of reconstituted membrane vesicles. The interface films thus formed consist of a phospholipid monolayer containing vesicle membrane fragments. Rhodopsin molecules at the interface are restricted within the membrane fragments where they are spectrophotometrically intact and capable of undergoing photoregeneration and chemical regeneration. Multilayers of up to 8 layers can be built from these interface films. The visible absorption band of rhodopsin in these multilayers is linearly dichroic. Quantitative analysis of the linear dichroism reveals that the dipole moment of transition of the retinal chromophore in rhodopsin forms an angle of 15 degrees +/- 4 degrees with the plane of the membrane fragments in the interface film. This orientation of the chromophore relative to the plane of the membrane is essentially the same as that observed in the intact retina. Thus, the orientation of rhodopsin in the interface films is similar to that in the intact disc membranes.

Orientation of acidic polysaccharides and rhodopsin-oligosaccharides in frog retinal rod outer segments

Using topo-optical staining reactions, the presence and molecular order of three structural components of outer segments of frog retina were studied. These components included (1) an acidic polysaccharide texture, (2) free aldehyde groups which arise during formalin fixation and (3) the oligosaccharide chains of rhodopsin. Quantitative measurements of the dye binding and birefringence effects arising from the individual structural components in rod outer segments were made. Results indicated that all three structural components had a rather well-defined orientation within the ROS. The spherulites phagocytized from the apical ends of ROSs by the pigment epithelium also demonstrate preferred orientation of the three structural components investigated.

Light path and photon capture in turtle photoreceptors

1. The directional selectivity of individual cones was examined by intracellular recording in the eye of the turtle. Sensitivites were determined from linear responses to dim flashes of monochromatic light incident on a cell over a range of angles to its long axis. 2. With light near the optimum wave-length, some red- and green-sensitive cones showed a high sensitivity for light entering axially and lower sensitivities for light entering obliquely. In contrast, other cells had lower peak sensitivities and less pronounced directional selectivities. The highest axial sensitivities observed in red receptors were about 320 muV photon(-1) mu2; in these cells, the sensitivity declined to half for rays 6-9 degrees off the axis as measured in the retina. Green receptors had lower axial sensitivities and broader angular profiles. 3. On the assumption that rays at all angles contribute independently to the over-all sensitivity, the sensitivity of a cell to large cones of rays was successfully predicted from the angular selectivity determined with a narrow pencil of rays. The shape of small responses to dim stimuli delivered on and off the axis of the cell was invariant, implying that a cone signals the number of photons absorbed but not their angle of incidence. 4. Short wave-lengths have previously been shown to be filtered out by the oil droplets present in turtle cones. At short wave-lengths, the angular profiles showed a depression in axial sensitivity consistent with this filtering action. 5. Diameters of inner segments, oil droplets, and outer segments were measured in red-, green-, and blue-sensitive cones, since these dimensions are expected to influence the cones' angular acceptances and ability to collect light. The diameters of the structure were in approximately the same proportions for each type of receptor, but the absolute values of the diameters were found to be scaled in relation to the wave-length of maximum sensitivity. 6. Optical determinations of the efficiency with which axial rays are concentrated by red receptors gave a mean value of 55%. 7. Receptors in histological sections of the whole eye were found to be oriented with their long axes directed approximately toward the pupil. 8. The observed directional selectivities and collecting efficiencies agree well with the behaviour of a model retinal cone developed by Winston & Enoch (1971) on a geometrical optical treatment. 9. Effective collecting areas are derived for red-, green- and blue-sensitive cones; these permit conversion of observed flash sensitivities into the mean peak hyperpolarization produced by isomerization of a visual pigment molecule. The figure obtained is about 25 muV for red-sensitive cones and 21muV for green-sensitive cones.

Molecular aspects of photoreceptor function

The description of the molecular processes which underlie visual excitation is the fundamental problem in understanding vision at the level of a single photoreceptor. Thus far only a general outline of photoreceptor function has emerged with little known about actual biochemical and biophysical mechanisms.

Visual pigments of goldfish cones. Spectral properties and dichroism

Freshly isolated retinal photoreceptors of goldfish were studied microspectrophotometrically. Absolute absorptance spectra obtained from dark-adapted cone outer segments reaffirm the existence of three spectrally distinct cone types with absorption maxima at 455 +/- 3,530 +/- 3, and 625 +/- 5 nm. These types were found often recognizable by gross cellular morphology. Side-illuminated cone outer segments were dichroic. The measured dichroic ratio for the main absorption band of each type was 2-3:1. Rapidly bleached cells revealed spectral and dichroic transitions in regions near 400-410, 435-455, and 350-360 nm. These photoproducts decay about fivefold as fast as the intermediates in frog rods. The spectral maxima of photoproducts, combined with other evidence, indicate that retinene(2) is the chromophore of all three cone pigments. The average specific optical density for goldfish cone outer segments was found to be 0.0124 +/- 0.0015/microm. The spectra of the blue-, and green-absorbing cones appeared to match porphyropsin standards with half-band width Deltanu = 4,832 +/- 100 cm(-1). The red-absorbing spectrum was found narrower, having Deltanu = 3,625 +/- 100 cm(-1). The results are consistent with the notion that visual pigment concentration within the outer segments is about the same for frog rods and goldfish cones, but that the blue-, and green-absorbing pigments possess molar extinctions of 30,000 liter/mol cm. The red-absorbing pigment was found to have extinction of 40,000 liter/mol cm, assuming invariance of oscillator strength among the three cone spectra.

Kinetics of slow thermal reactions during the bleaching of rhodopsin in the perfused frog retina

1. Slow thermal reactions occurring in the rhodopsin rods of flash-irradiated frog retinas were investigated spectrophotometrically.2. Five substances were identified as reactants: metarhodopsin II, metarhodopsin III, all-trans-retinal, opsin, and all-trans-retinol.3. Quantitative analysis showed that the transition between these substances are not a series of three consecutive reactions.4. An alternative scheme, compatible with the results, consisted of four reactions and involved two parallel pathways for the decay of metarhodopsin II, viz. conversion into metarhodopsin III, and hydrolysis into retinal and opsin.5. The first-order rate constants for the four reactions were as follows: 1.4 x 10(-2) sec(-1) for the conversion of metarhodopsin II into metarhodopsin III; 7.9 x 10(-3) sec(-1) for the hydrolysis of metarhodopsin II; 1.4 x 10(-3) sec(-1) for the hydrolysis of metarhodopsin III; and 2.6 x 10(-3) sec(-1) for the reduction of retinal into retinol (21 degrees C).6. Two other four-parameter schemes involving an equilibrium between metarhodopsin II and metarhodopsin III were also considered. One was found to be incompatible with the results. The other, though adequate, did not describe the data as well as the model summarized in 4 and 5. It also had the peculiar property of requiring that two apparently independent parameters be equated.

Magnetic anisotropy and the orientation of retinal rods in a homogeneous magnetic field

The reported orientation of retinal rods in a homogeneous magnetic field can be explained by the magnetic anisotropy of oriented molecules in the disc membranes of the rods. The energy of a single rod as a function of orientation in the magnetic field, the time required for alingment of the rod in a viscous medium, and the fluctuations of orientation are calculated. Arguments that rhodopsin is the constituent responsible for the effect are given. The possibility of orientation due to inhomogeneity of the magnetic field is ruled out. The application of magnetic anisotropy as an experimental tool in biology is indicated.

Dichroism of photosensitive pigment in rhabdoms of the crayfish Orconectes

Microspectrophotometric measurements of isolated crayfish rhabdoms illuminated transversely show that their photosensitive absorption exhibits a dichroic ratio of 2 in situ. The major absorption axis matches the axial direction of the closely parallel microvilli comprising the receptor organelle. Since these microvilli are regularly oriented transversely in about 24 layers, with the axes of the microvilli at 90 degrees in alternate layers, transverse illumination of a properly oriented rhabdom displays alternate dichroic and isotropic bands. Because all the microvilli from any one cell share the same orientation, the layers of microvilli constitute two sets of orthogonal polarization analyzers when illuminated along the normal visual axis. Furthermore, since the dichroic ratio is 2 and transverse absorption in isotropic bands is the same as that in the minor absorbing axis of dichroic bands, the simplest explanation of the analyzer action is that the absorbing dipoles of the chromophores, as in rod and cone outer segments, lie parallel to the membrane surface but are otherwise randomly oriented. The rhabdom's functional dichroism thus arises from its specific fine structural geometry.

Visual adaptation of the rhodopsin rods in the frogs retina

1. The threshold of the discharge from single ganglion cells in the excised and opened frog's eye has been measured with on/off stimuli and test parameters that make it possible to activate the rhodopsin rods only. The test stimuli have been restricted to the central part of the receptive field, where no nervous reorganization can be observed with changes in the state of adaptation.2. When such thresholds and the intensities of the background lights are expressed in terms of the number of quanta absorbed per unit time, it is found that three factors can be correlated with the thresholds measured in various states of light- and dark-adaptation: (i) the intensity of a steady background, (ii) the rate of regeneration of rhodopsin, and (iii) the amount of metarhodopsin II present in the rods.3. The threshold is found to be proportional both to the intensity of a background and to the rate of regeneration, whereas there is a linear relationship between the logarithm of the threshold and the amount of metarhodopsin II.4. The presence of metarhodopsin elevates all thresholds, the absolute threshold, increment thresholds and the thresholds elevated by regenerating rhodopsin in the same way.5. The saturation of the rods at high background intensities is found to be correlated with the accumulation of significant amounts of metarhodopsin in the rods, caused by the bleaching effect of the background.6. The effect of metarhodopsin on the threshold is independent of the amount of rhodopsin present in the rods.7. The combined effect of all three factors can be expressed in a general formula, given as eqn. (7) on p. 74.8. A background not only reduces the signals from the rods illuminated, but also those from neighbouring unilluminated rods. This effect is rapidly decreased with increasing distance from rods covered by the background. This kind of lateral spread in the retina probably occurs also when the rate of regeneration affects the threshold. The effect of metarhodopsin, on the other hand, appears restricted to those receptors that contain this substance.

Conductance changes produced by light in rod outer segments

1. Changes in the admittance of rod outer segments produced by illumination with brief flashes were studied by two methods: one, in which maintained changes in real and imaginary parts of admittance were observed in the frequency range 15 c/s-60 kc/s; the other, in which the time course of change in absolute value of admittance (Delta|Y|) was observed at frequencies of 100 kc/s-1.0 Mc/s.2. The response to light absorbed by rhodopsin was resolved into components. One of these components was a transient increase in conductance which arose from a rapid degradation into heat of the light energy. Another component, prominent at high frequencies where the conductivity of the rod interior was accessible to measurement, was produced by the uptake of H(+) by visual pigment in its conversion from metarhodopsin I to metarhodopsin II, causing a change in ionization of buffer.3. Two other components, designated I and II, appeared as maintained changes of admittance involving the organized structure of the rod. Component I appeared as a frequency-independent increase in the real part of admittance (DeltaG), the amplitude of which varied in proportion to the conductivity of the medium, without specificity as to ion species. Component II appeared as a DeltaG which rose linearly with log frequency over the range 1-60 kc/s, while the imaginary part of admittance change (DeltaB) rose to a plateau which was maintained for more than a tenfold frequency range. This component was unaffected by variations in conductivity in the region of low conductivities.4. When rods were suspended in a solution containing 100 mM hydroxylamine, component II no longer appeared as a maintained admittance change while component I was unaffected. Examination of the time course of response showed component II to appear transiently, decaying over the course of 2 sec following a flash.5. Measurements of Delta|Y| for rods in solutions of widely different conductivities showed component II to have a more rapid time course of development than component I and to be only slightly delayed in its early part relative to the buffer component.6. The amplitude of component I varied with temperature to the extent of 4.1%/ degrees C (Q(10) of 1.5) over the range -2-25 degrees C. The amplitude of component II was nearly constant over the range 15-27 degrees C, but fell steeply at temperatures below 10 degrees C, the Q(10) at low temperatures being about 2.4. The effect of temperature on amplitude and time course of component II is consistent with its dependence on the formation and continued presence of metarhodopsin II. The failure of component I to decrease steeply at temperatures below 10 degrees C indicates a dependence on an earlier stage in the thermal conversion of rhodopsin photoproducts.7. With light flashes each bleaching less than 1% of the rhodopsin content of the rod, all components of response were proportional to the amount of rhodopsin bleached (which would be proportional to the light absorbed). For brighter flashes components I and II failed to increase in proportion to the amount of rhodopsin bleached, the deviation from proportionality being greater for component I than for component II. The failure of summation of response extended to successive responses separated by up to 5 min.8. It is suggested that component I arises from a non-selective increase in ionic permeability of the surface membrane of the rod or from a change in rod volume, while component II arises from a change in conduction along the surface membrane.

New evidence supporting the linkage to extracellular space of outer segment saccules of frog cones but not rods

Previous electron microscopic examinations of outer segments of photoreceptors suggest that many flattened saccules of cones are continuous with the cell membrane and that their lumina connect with the extracellular compartment but that most saccules in rods appear to lack these connections. The saccules probably contain photolabile pigment, and certain potentials appear to result from dipole formation during pigment bleaching. The detection of dipoles from rod saccules may require that the lumina of rod saccules connect with extracellular space, and questions have been raised whether the interpretation of micrographs is correct or the isolation of rod saccules is the result of artifact. Accordingly, lanthanum and barium precipitates were produced near fixed and unfixed frog photoreceptors. Lanthanum precipitates appeared to infiltrate the saccules of fixed cones and the few surviving cones exposed prior to fixation, but no rod saccules were infiltrated except occasional, most basal saccules or saccules within narrow zones of probable damage. Barium precipitates did not infiltrate saccules of either variety of unfixed photoreceptor, but they did occasionally infiltrate around the saccules at points of damage in rod outer segments. The results thus support the view of the patency of saccules of frog cones and are consistent with, but do not prove, the isolation of saccules of frog rods.

The renewal of photoreceptor cell outer segments

The utilization of methionine-(3)H by retinal photoreceptor cells has been studied by radioautographic technique in the rat, mouse, and frog. In all three species, the labeled amino acid is concentrated initially in the inner segment of the cell. Within 24 hr, the radioactive material is displaced to the base of the outer segment, where it accumulates as a distinct reaction band. The reaction band then gradually moves along the outer segment and ultimately disappears at the apex of the cell, which is in contact with the retinal pigment epithelium. These findings are interpreted to indicate that the photoreceptor cell outer segment is continually renewed, by the repeated lamellar apposition of material (membranous discs) at the base of the outer segment, in conjunction with a balanced removal of material at its apex. The outer segment renewal rate is accelerated in frogs when ambient temperature is raised, and is elevated in both frogs and rats when the intensity of retinal illumination is increased.