The rhodopsin-porphyropsin system in freshwater fishes—1. Effects of age and photic environment

The freshwater fish Scardinius erythrophthalmus L. has a mixture of rhodopsin (λ_max 507 nm) and porphyropsin (λ_max, 535 nm) in its retina. Fish kept in continuous light for several weeks convert nearly all their porphyropsin into rhodopsin. In darkness this rhodopsin is reconverted to porphyropsin. If, in the light, each fish is fitted with an opaque cap over one eye, then this eye increases its proportion of porphyropsin while there is no change in the exposed contralateral eye. “Dark fish”, with porphyropsin-dominated retinas, converted this pigment into rhodopsin in the light. Conversion was markedly retarded in capped eyes but not in contralateral uncapped ones. Thus light converted prophyropsin into rhodopsin by acting locally within the ocular tissues and not by centrally-controlled endocrine or neuro-endocrine mechanisms. This does not rule out the possibility that endocrine factors also determine visual pigment composition under some circumstances. Thus there is a sharp increase in the proportion of porphyropsin in fish between 5 and 8 yr, although this might reflect a change in the pattern of ocular dehydrogenases converting retinol into 3-dehydroretinol. In any event, it appears that the seasonal visual pigment variation, manifested by a rhodopsin increase in the summer, is absent or much reduced in older fish. The effects of monocular thyroxine injections and exposure to intermittent photic conditions are also investigated and discussed.     

Effects of light and darkness on the visual pigments of amphibian tadpoles

The effects of light and darkness on the visual pigment composition in the eyes of tadpoles ofRana clamitans, Rana catesbeiana, Ambystoma maculatum andXenopus laevis have been investigated. Ranid tadpoles have mainly porphyropsin in the light, but in darkness the proportion of rhodopsin gradually increases over a period of several weeks. The process is reversible over several light-dark cycles. The light effect is extremely rapid: exposure to illumination of 85–150 ft-c. can return the system to nearly pure porphyropsin after 24–48 hr. The reaction appears to be the same at all developmental stages examined, i.e. from Taylor-Kollros stage IV–XV (early prometamorphosis). Illumination retards but does not prevent the switch to rhodopsin at natural or thyroxine-induced metamorphic climax (i.e. forelimb emergence). The dark increase of rhodopsin is not prevented by 0.04% thiourea. If tadpoles which have been permitted to increase their rhodopsin in the dark are illuminated after one eye in each specimen has been covered with black vaseline, the exposed eyes synthesize more porphyropsin. The difference is not observed if normal, clear vaseline is used instead, and it is therefore concluded that the light-stimulated formation of porphyropsin is a local response of the ocular tissues and is not mediated by hormones (such as thyroxine) or extraocular receptors (such as the stirnorgan). The quantities of visual pigment molecules appear to be the same in tadpoles kept in light and darkness, irrespective of the composition of the mixture. Like their adult forms, bullfrog tadpoles at or approaching metamorphic climax always had more porphyropsin in the dorsal retina. The premetamorphic tadpoles kept in the light did not show any significant differences between dorsal and ventral retinal areas, but those kept in darkness sometimes did. No porphyropsin was detectable in any part of the retinas of adultR. clamitans, R. pipiens, R. palustris, R. sylvatica, Hyla crucifer andH. versicolor. No dorsoventral gradient of visual pigment composition was found inR. clamitans tadpoles orXenopus laevis adults. UnlikeRana, Ambystoma tadpoles have pure rhodopsin like the adults.Xenopus tadpoles (stages 58–59) have virtually pure porphyropsin, like the adults. Neither species changes its visual pigments when kept for several weeks in light or darkness. It is suggested that the increase of rhodopsin in the dark occurs because rhodopsin is the major pigment that is incorporated into the outer segment during the renewal process. On the other hand, the rise of porphyropsin in the light is partly a consequence of the interchange of retinol and 3-dehydroretinol between retinal visual pigment and stores in the pigment epithelium, which are composed predominantly of 3-dehydroretinol even in tadpoles kept in the dark. However, ultimately there is total disappearance of retinol and rhodopsin from the eyes of tadpoles kept in continuous light, so it is believed that a further light-driven reaction must be implicated.     

Scotopic visual pigment composition in the retinas and vitamins A in the pigment epithelium of the goldfish

Recently, the retinas of goldfish from Grassyfork Fisheries (Martinsville, Indiana 46151, U.S.A.) were reported to possess a rhodopsin with an absorbance maximum at 499 nm, in addition to the known porphyropsin. Rhodopsin and porphyropsin are visual pigments respectively derived from the conjugation of vitamin A₁ aldehyde (retinal) and vitamin A₂ aldehyde (3-dehydroretinal) to a species-specific opsin. In this study, goldfish from a different supplier (Ozark Fisheries, Stoutland, Missouri 65567, U.S.A.) were also confirmed to possess a rhodopsin with an absorbance maximum at 499 nm. In addition, this paper describes how external factors (photoperiod, light intensity, temperature and exogenous thyroxine) affected the composition of these visual pigments in the goldfish retinas and the vitamins A (retinol/3-dehydroretinol) in their pigment epithelium.After 50 days of acclimation, constant illumination (at 30°C with a 7·5 W incandescent light bulb) and constant darkness (at 30°C) both favored high proportions of porphyropsin (more than 85%) whereas fish held under a 16L/8D cycle (at 30°C with a 7·5 W light bulb) had predominately rhodopsin (less than 10% porphyropsin). Testing at 16L/8D and 30°C, light intensities higher and lower than the equivalent of a 7·5 W bulb (i.e. 6×10¹³ photons/cm²-sec, 400–750 nm, measured at water surface) both favored significantly higher proportions of porphyropsin. Lower water temperature (10°C and 20°C) or the introduction of thyroxine to the tank water (at a concentration of 100 μg l-thyroxine per liter tank water) also resulted in significantly higher proportions of porphyropsin.Despite the influence of these external factors on the visual pigment composition in the goldfish retinas, the vitamins A (retinol and 3-dehydroretinol) in their pigment epithelium remained predominately 3-dehydroretinol. Although the rhodopsin dominated retinas were always associated with pigment epithelium of slightly lowered 3-dehydroretinol proportions, these results do not agree with previous reports on other species where the proportions of retinol and 3-dehydroretinol mirrored the ratios of rhodopsin to porphyropsin.     

Visual pigments and vitamins A in the adult bullfrog

Adult bullfrogs, Rana catesbeiana, have a pair of scotopic visual pigments: a rhodopsin and a porphyropsin. Partial bleaching experiments performed on rhodopsin and porphyropsin rich retinal extracts from the bullfrog revealed that the absorbance maxima (λ_max) are 499 and 522 nm respectively. The rhodopsin absorbance maximum of 499 nm is slightly lower than the values (i.e. 500–503 nm) previously reported.After subjecting groups of bullfrogs to different light-temperature environments for 40 days, some frogs, when considering the retina as a whole, had predominately rhodopsin, whereas others possessed up to 50% porphyropsin because of the presence of porphyropsin in the dorsal part of the retina. The visual pigment composition in the dorsal (superior one third) part of the retina changed from predominately porphyropsin to predominately rhodopsin in response to specific light and temperature regimes. The ventral part of the retina of these frogs always remained rhodopsin rich.The vitamin A composition (i.e. the relative proportions of vitamin A₂ to vitamin A₁) of the associating pigment epithelium was, in most cases, similar to the visual pigment composition (i.e. the relative proportions of porphyropsin to rhodopsin) in both the dorsal and ventral parts of the retina. However, exceptions were found in dorsal retinas of frogs held in constant darkness (for 40 days) and in dorsal retinas which become rhodopsin rich in response to specific light and temperature treatments. In both instances, the vitamin A₂ proportions in the pigment epithelium were higher than the porphyropsin proportions in the retinas. The liver of all frogs had predominately vitamin A₁.This article provides the first report on a successful attempt to induce changes in the visual pigment and vitamin A compositions in the eyes of an adult amphibian using specific combinations of light and temperatures.     

Storage, distribution and utilization of vitamins A in the eyes of adult amphibians and their tadpoles

This study has investigated the quantities, isomeric configuration and ocular distribution of the two retinols (vitamins A₁ and A₂) in the eyes of goldfish. Xenopus. frogs and tadpoles, with particular emphasis on the shifting relationships of rhodopsin, porphyropsin. retinol and 3-dehydroretinol in tadpoles kept in light and darkness. The amounts of retinols (mainly esterified) stored in the dark-adapted eyes range from 41 moles per mole of visual pigment in the goldfish down to 0.4 mole in the Xenopus. The cleanly isolated frog retina has 0.07 mole of 11-cis and all-trans vitamin A (50 per cent esterified) per mole of ROS rhodopsin, the remainder (1 per cent esterified) being found in the pigment epithelium, 80 per cent in the oil-droplets separated by centrifugal flotation. A maximum of 3 per cent of the rhodopsin in the ROS is apparently in the pigment epithelium. During light-adaptation, retinol formed by rhodopsin bleaching in the ROS leaves the retina, enters the pigment epithelium where most of it passes into the oil-droplets. The dark-adapted stores always contain 11-cis isomer, which is present in least amount in eyes that have undergone one cycle of light- and dark-adaptation. During prolonged periods in the dark, there was an increase of 11-cis at the expense of all-trans retinyl ester. In illuminated tadpoles retinol and 3-dehydroretinol are utilized for visual pigment regeneration in the proportions present in the pigment epithelium. This is not observed in the dark, where the situation is dominated by the visual pigment renewal process. The prosthetic group used for visual pigment biosynthesis in the tadpole retina is mainly retinol, even though there is an overwhelming preponderance of 3-dehydroretinol in the pigment epithelium.     

Competition between retinal and 3-dehydroretinal for opsin in the regeneration of visual pigment

Rhodopsin regenerated faster than porphyropsin in all preparations of bullfrog opsin, bullfrog rod outer segment membrane and cattle opsin. When opsin was incubated with excess amount of an equimolar mixture of 11-cis-retinal and 11-cis-3-dehydroretinal, the composition of the regenerated pigment was simply dependent on the ratio of regeneration rates of rhodopsin and porphyropsin. This result can provide a mechanism to account for the discrepancy in vitamin A1/A2 composition between the retina and the pigment epithelium. The property of opsin preferring retinal to 3-dehydroretinal may be one of the basic factors affecting vitamin A1/A2 visual pigment systems.     

The presence of a porphyropsin-based visual pigment in the juvenile lemon shark (Negaprion brevirostris)

The visual pigment from the juvenile lemon shark has been extracted and is a homogeneous vitamin A₂-based porphyropsin with maximum absorption at 522 nm. This is the first report of a porphyropsin visual pigment extracted from the retina of an elasmobranch. In contrast, the visual pigment from the adult lemon shark yields a homogeneous vitamin A₁-based rhodopsin with maximum absorption at 501 nm. We conclude that the porphyropsin of the juvenile lemon shark changes over to a rhodopsin as the animal matures.     

The visual pigment composition of rainbow trout.

The relative amounts of rhodopsin and porphyropsin (visual pigment composition) in rainbow trout are known to be affected by light and temperature. By transferring fish to new photic and thermal environments and sampling at intervals, the dynamics of the changing visual pigment composition were studied. Within 30–50 days of acclimation, visual pigment compositions in different groups subjected to the same treatment converged and stabilized. Groups of fish held under ten different light and temperature regimes for 60 days were then analysed for visual pigment proportions. These “steady-state visual pigment compositions” reflect accurately the effects of light and temperature on the visual pigment composition in rainbow trout.     

Spectral sensitivity of the system controlling visual pigment composition in tadpole eyes

Rana clamitans tadpoles were kept in incubators in complete darkness or illuminated with different intensities of light of dominant wavelengths 482, 564 and 690 nm. At various times batches of tadpoles were taken from each incubator, dark-adapted and their visual pigments analyzed. While in the dark there was a steady drift of porphyropsin proportion down to about 30 per cent; in the light the visual pigment composition stabilized at various porphyropsin percentages above this value, increasing with increasing intensity of illumination of a given color. Hence a series of curves relating porphyropsin percentage to intensity could be plotted, and selection of several “criterion levels” (between 65 and 77 per cent) permitted direct comparison of the sensitivity of the system to each of the three wavelengths. Blue light was most effective in maintaining high levels of porphyropsin, the sensitivity lying about 3.4 log units above the red. The yellow-green was about 2.7 log units above the red. Tadpoles were transferred from two intensities of red light to two intensities of blue light approx 3 log units lower. No change of visual pigment composition occurred over a 6-day period, confirming the “matching values” deduced from the intensity-response curves. Since the red sensitivity is approx 2.5 log units below the published photopic luminosity curve for tadpoles containing 70 % porphyropsin, the participation of cones is unlikely. There is tolerable agreement with the scotopic sensitivity curve, which is fitted within±0.2 log units, and it is concluded that the maintenance of porphyropsin in the tadpole eye is most probably mediated by light absorbed in the rods.     

The presence of a porphyropsin-based visual pigment in the juvenile lemon shark (negaprion brevirostris)

The visual pigment from the juvenile lemon shark has been extracted and is a homogeneous vitamin A₂-based porphyropsin with maximum absorption at 522 nm. This is the first report of a porphyropsin visual pigment extracted from the retina of an elasmobranch. In contrast, the visual pigment from the adult lemon shark yields a homogeneous vitamin A₁-based rhodopsin with maximum absorption at 501 nm. We conclude that the porphyropsin of the juvenile lemon shark changes over to a rhodopsin as the animal matures.     

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.     

Rod and cone visual pigments in the goldfish

This microspectrophotometric (MSP) study has confirmed that goldfish, acclimated under a certain light and temperature regimen, possess mixtures of porphyropsin and rhodopsin in their rod outer segments. Moreover, the MSP spectra from the red, green and blue cones of these acclimated goldfish also shifted to shorter wavelengths from spectra of pure 3-dehydroretinal based visual pigments. Thus, goldfish retinas may possess a total of eight visual pigments (two rod pigments and six cone pigments) instead of four (one rod pigment and three cone pigments). Possibly, a common control mechanism, governed by external conditions, regulates the utilization of retinal and 3-dehydroretinal for the biosynthesis and/or regeneration of visual pigments in both rods and cones.     

The distribution and proportions of vitamin A compounds during the visual cycle in the rat

Radioisotopes and t.l.c. were used to indirectly determine the distribution and proportions of Vitamin A compounds in the rat eye. In the dark adapted eye there are Vitamin A compounds in the pigment epithelium; these compounds are likely within the rod outer segment discs being digested there. During light adaptation. 11-cis retinal decreases, alltrans retinal increases and is reduced to retinol, and retinol is esterified to fatty acids in the retina and in the pigment epithelium. Essentially the reverse occurs during dark adaptation. It is suggested that the metabolism of Vitamin A compounds in the eye functions to return chromophore to the newly synthesized visual pigment at the base of the rod outer segment.     

Hereditary retinal dystrophy in the rat: rhodopsin, retinol, vitamin A deficiency.

Measurements were performed on young hooded (B) and albino (Ba) rats with inherited visual cell degeneration of the Bourne-Campbell-Tansley type, in comparison to heterozygotes. The animals were reared and maintained in a dark environment. Rhodopsin of the dark-maintained B and Ba increased between 16 and 35 days to levels twice those of controls. These high levels were maintained up to the age of 100 days. Estimates indicate that no more than 50% of the rhodopsin is contained in surviving visual cells at the age of 25 days, only 25% at 35 days and virtually none at 80 days. The transfer of retinol from bleached rhodopsin to the pigment epithelium (Pe) is increasingly slowed from 17 days on as a function of the accumulation of debris. The conversion of retinal from bleached rhodopsin to retinol also occurs at a lower rate; about 45% of the retinal is converted to retinol at the end of a 10-min period of light exposure in 37-day-old Ba compared to 80% in the controls. The rate of rhodopsin regeneration after 1 hr exposure to strongly bleaching light is decreased after the age of 17 days; it is only about 30% normal for 6 hr in darkness at 37 days. Maintenance on a vitamin A deficient diet accelerates the rate of visual cell death and ERG decline as early as 12 days after withholding vitamin A from weaned B rats.     

Proceedings: Evolution of visual pigments

Rod visual pigments exhibit a variety of absorption maxima, determined partly by the nature of the prosthetic group, retinal or 3-dehydroretinal, and partly by the apoprotein opsin. Some animal taxa exhibit little diversity. On the other hand, selective pressures arising from the different light habitats of aquatic environments are believed to have produced the correspondingly wide range of λ_max in fish visual pigments, which often extends over 80 nm. Opsins appear to have similar molecular weights. Latest estimates of molecular weight suggest that they have approximately 315 amino acid residues. Some species, notably members of the salmonidae, have visual pigments with different λ_max yet have diverged only during the past several million years. This would indicate a very high rate of protein evolution in circumstances where we should expect that amino acid substitution would be severely restricted. Certain fish species (e.g. the deepwater sculpin, Myoxocephalus) have mixtures of visual pigments with λ_max separated by only 6 nm. The physiological usefulness of such a pair is dubious, but it may indicate polymorphism at the gene locus coding for opsin.The use of retinol and 3-dehydroretinol, the basis of rhodopsin and porphyropsin, may vary between quite closely related species. Extraneous factors such as light and hormones are known to be important in amphibian larvae and fishes, but there is also clear evidence for genetic influences, as revealed by the divergent evolution of the system in various populations of originally anadromous smelt after only 10 000 years of isolation.     

Retinal light damage in rats exposed to intermittent light. Comparison with continuous light exposure

Visible light-induced photoreceptor cell damage resulting from exposure to multiple intermittent light-dark periods was compared with damage resulting from continuous light in albino rats maintained in a weak cyclic-light environment or in darkness before light treatment. The time course of retinal damage was determined by correlative measurements of rhodopsin and visual cell DNA at various times after light exposure, and by histopathological evaluation. The effect of intense light exposures on rhodopsin regeneration and on the level of rod outer segment docosahexaenoic acid was also determined. For rats previously maintained in weak cyclic light, 50% visual cell loss was measured 2 weeks after 12 1 hr light/2 hr dark periods, or following 24 hr of continuous light. A comparable 50% loss of visual cells was found in dark-reared rats after only 5 hr of continuous illumination or 2-3 hr of intermittent light. As judged by histology, cyclic-light-reared rats incurred less retinal pigment epithelial cell damage than dark-reared animals. In both experimental rat models intermittent light exposure resulted in greater visual cell damage than continuous exposure. Visual cell damage from intermittent light was found to depend on the duration of light exposure and on the number of light doses administered. Measurements of rhodopsin and DNA 2 hr and 2 weeks after light exposure of up to 8 hr duration revealed that visual cell loss occurs largely during the 2 week dark period following light treatment. The loss of docosahexaenoic acid from rod outer segments was also greater in rats exposed to intermittent light than in animals treated with continuous light. It is concluded that intermittent light exposure exacerbates Type I light damage in rats (involving the retina and retinal pigment epithelium) and the schedule of intense light exposure is a determinant of visual cell death.     

Visual cycle in the mammalian eye. Retinoid-binding proteins and the distribution of 11-cis retinoids

This work was designed to provide an insight into the mammalian visual cycle by investigating the possible function of retinoid-binding proteins in this system, and the distribution and type of 11-cis retinoids present in the interphotoreceptor matrix and the cytosols of the retinal pigment epithelium and retina. The total retinol and retinal in the soluble fractions from these three compartments was 8% (3.31 nmol/eye) of the retinyl palmitate and stearate stored in the pigment epithelium membrane fractions (39 nmol/eye). Only small amounts of retinoids were detected in the rod outer segment cytosol. The insoluble fractions also contained retinol, nearly all of which was found in the retina. The retinoids in the soluble fractions appeared to be bound to cellular retinol-binding protein (CRBP), cellular retinal-binding protein (CRA1BP) and interstitial retinol-binding protein (IRBP, a high-Mr glycoprotein). Using immunospecific precipitation, immunoblot and immunocytochemical techniques it was demonstrated that IRBP was localized in the interphotoreceptor matrix and was synthesized and secreted by the retina, a process that did not require the protein to be glycosylated. The amount of retinol bound to IRBP increased if the eyes were exposed to light, when it was estimated that the protein carried up to 30% of its full capacity for all-trans retinol. In addition to all-trans retinol, IRBP carried smaller amounts of 11-cis retinol. The proportion of 11-cis retinol was frequently higher in eyes that had been protected from illumination, suggesting that IRBP plays a role in rhodopsin regeneration during dark-adaptation. Additionally, endogenous 11-cis retinoids in the retina and RPE cytosols were bound to an Mr 33,000 protein tentatively identified as CRA1BP. The 11-cis retinoid in the retina cytosol was mainly in the form of retinol, while in the RPE cytosol it was mainly in the form of retinal. Substantial amounts of 11-cis retinol were also found in the insoluble (membrane) fraction from the retina. It is suggested that in the mammalian retina 11-cis retinol is generated from all-trans retinol (possibly in the Muller cells). Lack of an 11-cis retinol oxidoreductase in the retina prevents it from being utilized for rhodopsin regeneration until it has been transported to the pigment epithelium, where it is converted to 11-cis retinal and returned to the rod outer segments. It is also suggested that IRBP may be implicated in the transport of retinoids between the rod outer segments, the Muller cells and the pigment epithelium.(ABSTRACT TRUNCATED AT 400 WORDS)     

Rhodopsin regeneration in the normal and in the detached/replaced retina of the skate.

The bleaching and regeneration of rhodopsin in the skate retina was studied by means of fundus reflectometry, both in the normal eyecup preparation and after the retina had been detached and then replaced on the surface of the pigment epithelium (RPE). After bleaching virtually all the rhodopsin in the retinal test area of the normal eyecup, more than 90% of the photopigment was reformed after about 2 hr in darkness; over most of this time course, rhodopsin density rose linearly at a rate of 0.875% min-1 with a half-time of 55 min. Detaching the retina from its pigment epithelium resulted in a number of abnormalities, both structural and functional. Histological examination of the detached/replaced (D/R) retina showed striking alterations in the structural integrity of the RPE cells at their interface with the neural retina. The cells appeared vacuolated and misshapen, and the apical processes of the RPE, which normally ensheath the receptor outer segments, were shredded and free of their association with the visual cells. These morphological changes, as well as dilution of the IRBP content of the subretinal space caused by separation of the tissues, appear to be the main factors contributing to the functional abnormalities in rhodopsin kinetics. But despite these abnormalities and the persistent detachment, the rate of regeneration and the amount of rhodopsin reformed after bleaching were reduced by less than 50% of their normal values. The fact that a significant fraction of the bleached rhodopsin was regenerated under these conditions indicates that 11-cis retinal formed in the RPE was able to traverse a much greater than normal subretinal space to reach the opsin-bearing photoreceptor membranes.     

Spatial distribution of visual pigment and dopamine in the bullfrog retina

Visual pigments and a neurotransmitter, dopamine, were quantitatively investigated in the retina of adult bullfrog, Rana catesbeiana. The adult bullfrog (body length 15-16 cm, body weight 375 +/- 52 g, n = 10) had 21.4 +/- 4.2 nmol visual pigment and 209 +/- 28 pmol dopamine in retinal areas of 266 +/- 27 mm2. Greater pigment densities were recorded in a semicircular band around the optic disc, extending to the nasal and temporal peripheries of the ventral retina. The area with the highest concentration of visual pigment was found in the middle of the dorsal retina, 3-4 mm dorsal to the optic disc. A high concentration of vitamin A2-based pigment was found in the dorsal quarter of the retina (porphyropsin zone); the zone extended up to the most ventral part along peripheral regions of the retina. There was also a band with higher dopamine concentrations although it was not so prominent as that of the visual pigment; the highest concentration of dopamine was found in the area immediately dorsotemporal to the optic disc. Fluorescence micrography indicated that the distribution pattern of catecholamine-containing amacrine cells paralleled that of the dopamine content. The topographic map of dopamine was slightly different from that of visual pigment in the bullfrog retina.     

Visual pigment changes in salmonid fishes in response to exogenous L-thyroxine, bovine TSH and 3-dehydroretinol

Kokanee salmon, a “paired-pigment” species having mostly rhodopsin, receiving L-thryoxine do not immediately have an increase in the percentage of porphyropsin but do within 36–48 hr post-injection if held in a light-dark cycle. Thyroxine treated fish held in continual darkness do not have a substantial increase in porphyropsin until exposed to a light period. Bovine thyrotropic hormone is also effective in promoting the rhodopsin to porphyropsin conversion presumably by stimulating the release of thyroid hormone. Sexually mature kokanee are as responsive as juveniles to both hormones. 3-Dehydroretinol from walleye liver when administered in sequential 300–600 μg doses promotes a significant increase in porphyropsin and becomes the dominant vitamin A in the liver and pylorci caeca. Coho salmon panalso respond toL-thyroxine.