Absorption Characteristics of Vertebrate Non-Visual Opsin, Opn3

Most animals possess multiple opsins which sense light for visual and non-visual functions. Here, we show spectral characteristics of non-visual opsins, vertebrate Opn3s, which are widely distributed among vertebrates. We successfully expressed zebrafish Opn3 in mammalian cultured cells and measured its absorption spectrum spectroscopically. When incubated with 11-cis retinal, zebrafish Opn3 formed a blue-sensitive photopigment with an absorption maximum around 465 nm. The Opn3 converts to an all-trans retinal-bearing photoproduct with an absorption spectrum similar to the dark state following brief blue-light irradiation. The photoproduct experienced a remarkable blue-shift, with changes in position of the isosbestic point, during further irradiation. We then used a cAMP-dependent luciferase reporter assay to investigate light-dependent cAMP responses in cultured cells expressing zebrafish, pufferfish, anole and chicken Opn3. The wild type opsins did not produce responses, but cells expressing chimera mutants (WT Opn3s in which the third intracellular loops were replaced with the third intracellular loop of a Gs-coupled jellyfish opsin) displayed light-dependent changes in cAMP. The results suggest that Opn3 is capable of activating G protein(s) in a light-dependent manner. Finally, we used this assay to measure the relative wavelength-dependent response of cells expressing Opn3 chimeras to multiple quantally-matched stimuli. The inferred spectral sensitivity curve of zebrafish Opn3 accurately matched the measured absorption spectrum. We were unable to estimate the spectral sensitivity curve of mouse or anole Opn3, but, like zebrafish Opn3, the chicken and pufferfish Opn3-JiL3 chimeras also formed blue-sensitive pigments. These findings suggest that vertebrate Opn3s may form blue-sensitive G protein-coupled pigments. Further, we suggest that the method described here, combining a cAMP-dependent luciferase reporter assay with chimeric opsins possessing the third intracellular loop of jellyfish opsin, is a versatile approach for estimating absorption spectra of opsins with unknown signaling cascades or for which absorption spectra are difficult to obtain.

Classical Photoreceptors Are Primarily Responsible for the Pupillary Light Reflex in Mouse

Pupillary light reflex (PLR) is an important clinical tool to assess the integrity of visual pathways. The available evidence suggests that melanopsin-expressing retinal ganglion cells (mRGCs) mediate PLR—driven by the classical photoreceptors (rods and cones) at low irradiances and by melanopsin activation at high irradiances. However, genetic or pharmacological elimination of melanopsin does not completely abolish PLR at high irradiances, raising the possibility that classical photoreceptors may have a role even at high irradiances. Using an inducible mouse model of photoreceptor degeneration, we asked whether classical photoreceptors are responsible for PLR at all irradiances, and found that the PLR was severely attenuated at all irradiances. Using multiple approaches, we show that the residual PLR at high irradiances in this mouse was primarily from the remnant rods and cones, with a minor contribution from melanopsin activation. In contrast, in rd1 mouse where classical photoreceptor degeneration occurs during development, the PLR was absent at low irradiances but intact at high irradiances, as reported previously. Since mRGCs receive inputs from classical photoreceptors, we also asked whether developmental loss of classical photoreceptors as in rd1 mouse leads to compensatory takeover of the high-irradiance PLR by mRGCs. Specifically, we looked at a distinct subpopulation of mRGCs that express Brn3b transcription factor, which has been shown to mediate PLR. We found that rd1 mouse had a significantly higher proportion of Brn3b-expressing M1 type of mRGCs than in the inducible model. Interestingly, inducing classical photoreceptor degeneration during development also resulted in a higher proportion of Brn3b-expressing M1 cells and partially rescued PLR at high irradiances. These results suggest that classical photoreceptors are primarily responsible for PLR at all irradiances, while melanopsin activation makes a minor contribution at very high irradiances.

Impaired circadian photosensitivity in mice lacking glutamate transmission from retinal melanopsin cells

Intrinsically photoreceptive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin and convey retinal light inputs to the circadian system via the retinohypothalamic tract (RHT) projection to the suprachiasmatic nucleus (SCN). The principal neurotransmitter of this projection is glutamate, and ipRGCs use the vesicular glutamate transporter 2 (VGLUT2) to package glutamate into synaptic vesicles. However, these neurons contain other potential neurotransmitters, such as pituitary adenylate cyclase activating polypeptide (PACAP). To test the role of glutamate in mediating ipRGC light inputs into the SCN, we crossed mice in which Cre-recombinase expression is driven by the melanopsin promotor (Opn4(Cre/+)) with mice in which the second exon of VGLUT2 is flanked by loxP sites (VGLUT2(fl/fl)), producing ipRGCs that are unable to package glutamate into synaptic vesicles. Such mice had free-running circadian rhythms that did not entrain to a 12:12 light-dark (12:12 LD) cycle, nor did they show a phase delay after a 45-min light pulse administered at circadian time (CT) 14. A small subset of the mice did appear to entrain to the 12:12 LD cycle with a positive phase angle to lights-off; a similar entrainment pattern could be achieved in free-running mice if they were exposed to a 12:12 LD cycle with light of a greater intensity. Glutamate transmission from the ipRGCs is necessary for normal light entrainment of the SCN at moderate (0.35 W/m(2)) light levels, but residual transmission (possibly by PACAP in ipRGCs or by other RGCs) can weakly entrain animals, particularly at very high (6.53 W/m(2)) light levels, although it may be less effective at suppressing locomotor activity (light masking).

Distribution of mammalian-like melanopsin in cyclostome retinas exhibiting a different extent of visual functions

Mammals contain 1 melanopsin (Opn4) gene that is expressed in a subset of retinal ganglion cells to serve as a photopigment involved in non-image-forming vision such as photoentrainment of circadian rhythms. In contrast, most nonmammalian vertebrates possess multiple melanopsins that are distributed in various types of retinal cells; however, their functions remain unclear. We previously found that the lamprey has only 1 type of mammalian-like melanopsin gene, which is similar to that observed in mammals. Here we investigated the molecular properties and localization of melanopsin in the lamprey and other cyclostome hagfish retinas, which contribute to visual functions including image-forming vision and mainly to non-image-forming vision, respectively. We isolated 1 type of mammalian-like melanopsin cDNA from the eyes of each species. We showed that the recombinant lamprey melanopsin was a blue light-sensitive pigment and that both the lamprey and hagfish melanopsins caused light-dependent increases in calcium ion concentration in cultured cells in a manner that was similar to that observed for mammalian melanopsins. We observed that melanopsin was distributed in several types of retinal cells, including horizontal cells and ganglion cells, in the lamprey retina, despite the existence of only 1 melanopsin gene in the lamprey. In contrast, melanopsin was almost specifically distributed to retinal ganglion cells in the hagfish retina. Furthermore, we found that the melanopsin-expressing horizontal cells connected to the rhodopsin-containing short photoreceptor cells in the lamprey. Taken together, our findings suggest that in cyclostomes, the global distribution of melanopsin in retinal cells might not be related to the melanopsin gene number but to the extent of retinal contribution to visual function.

Dendritic morphology of caudal periaqueductal gray projecting retinal ganglion cells in Mongolian gerbil (Meriones unguiculatus)

In this study we investigated the morphological features of the caudal periaqueductal gray (cPAG)-projecting retinal ganglion cells (RGCs) in Mongolian gerbils using retrograde labeling, in vitro intracellular injection, confocal microscopy and three-dimensional reconstruction approaches. cPAG-projecting RGCs exhibit small somata (10–17 µm) and irregular dendritic fields (201–298 µm). Sizes of somata and dendritic fields do not show obvious variation at different distance from the optic disk (eccentricity). Dendrites are moderately branched. Morphological analysis (n = 23) reveals that cPAG-projecting RGCs ramified in sublamina a and b in the inner plexiform layer. These cells exhibit different stratification patterns based on the thickness of dendritic bands in sublaminas a and b: majority of analyzed cells (16 out of 23) have two bands of arborizations share similar thickness. The rest of analyzed cells (7 out of 23) exhibit thinner band in sublamina a than in sublamina b. Together, the present study suggests that cPAG of Mongolian gerbil could receive direct retinal inputs from two types of bistratified RGCs. Furthermore, a small subset of melanopsin-expressing RGCs (total 41 in 6 animals) is shown to innervate the rostral PAG (rPAG). Functional characteristics of these non-visual center projecting RGCs remain to be determined.

Loss of melanopsin-containing retinal ganglion cells in a rat glaucoma model

BACKGROUND

Glaucoma can cause progressive damage to retinal ganglion cells. These cells can be classified as cells projecting to the superior colliculus and melanopsin-containing retinal ganglion cells, which project to the suprachiasmatic nucleus. This study was to investigate the effects of chronic intraocular pressure elevation on melanopsin-containing retinal ganglion cells in rats.

METHODS

Chronic intraocular pressure elevation was induced in one eye of adult Wistar rats by cauterization of three episcleral veins. Intraocular pressure was measured at different intervals with a rebound tonometer. Superior collicular retinal ganglion cells were retrogradely labeled from the superior colliculus with Fluorogold. Melanopsin-containing retinal ganglion cells were visualized by free-floating immunohistochemistry on whole-mount retinas. The number of labeled superior collicular and melanopsin-containing retinal ganglion cells were counted in the sample areas on flat-mounted retinas.

RESULTS

Compared with contralateral control eyes, the numbers of both superior collicular and melanopsin-containing retinal ganglion cells were significantly reduced after 12 weeks of experimental intraocular pressure elevation ((2317.41 +/- 29.96)/mm(2) vs (1815.82 +/- 24.25)/mm(2); (26.20 +/- 2.10)/mm(2) vs (20.62 +/- 1.52)/mm(2), respectively). The extent of cell loss of the two types of retinal ganglion cells was similar. However, no morphologic changes were found in melanopsin-containing retinal ganglion cells.

CONCLUSION

Both melanopsin-containing and superior collicular retinal ganglion cells were damaged by chronic ocular hypertension, indicating that glaucomatous neural degeneration involves the non-image-forming visual pathway.

Gene duplication is an evolutionary mechanism for expanding spectral diversity in the long-wavelength photopigments of butterflies

Butterfly long-wavelength (L) photopigments are interesting for comparative studies of adaptive evolution because of the tremendous phenotypic variation that exists in their wavelength of peak absorbance (lambda(max) value). Here we present a comprehensive survey of L photopigment variation by measuring lambda(max) in 12 nymphalid and 1 riodinid species using epi-microspectrophotometry. Together with previous data, we find that L photopigment lambda(max) varies from 510-565 nm in 22 nymphalids, with an even broader 505- to 600-nm range in riodinids. We then surveyed the L opsin genes for which lambda(max) values are available as well as from related taxa and found 2 instances of L opsin gene duplication within nymphalids, in Hermeuptychia hermes and Amathusia phidippus, and 1 instance within riodinids, in the metalmark butterfly Apodemia mormo. Using maximum parsimony and maximum likelihood ancestral state reconstructions to map the evolution of spectral shifts within the L photopigments of nymphalids, we estimate the ancestral pigment had a lambda(max) = 540 nm +/- 10 nm standard error and that blueshifts in wavelength have occurred at least 4 times within the family. We used ancestral state reconstructions to investigate the importance of several amino acid substitutions (Ile17Met, Ala64Ser, Asn70Ser, and Ser137Ala) previously shown to have evolved under positive selection that are correlated with blue spectral shifts. These reconstructions suggest that the Ala64Ser substitution has indeed occurred along the newly identified blueshifted L photopigment lineages. Substitutions at the other 3 sites may also be involved in the functional diversification of L photopigments. Our data strongly suggest that there are limits to the evolution of L photopigment spectral shifts among species with only one L opsin gene and that opsin gene duplication broadens the potential range of lambda(max) values.

Presence of glutamate, glycine, and gamma-aminobutyric acid in the retina of the larval sea lamprey: comparative immunohistochemical study of classical neurotransmitters in larval and postmetamorphic retinas

The neurochemistry of the retina of the larval and postmetamorphic sea lamprey was studied via immunocytochemistry using antibodies directed against the major candidate neurotransmitters [glutamate, glycine, gamma-aminobutyric acid (GABA), aspartate, dopamine, serotonin] and the neurotransmitter-synthesizing enzyme tyrosine hydroxylase. Immunoreactivity to rod opsin and calretinin was also used to distinguish some retinal cells. Two retinal regions are present in larvae: the central retina, with opsin-immunoreactive photoreceptors, and the lateral retina, which lacks photoreceptors and is mainly neuroblastic. We observed calretinin-immunostained ganglion cells in both retinal regions; immunolabeled bipolar cells were detected in the central retina only. Glutamate immunoreactivity was present in photoreceptors, ganglion cells, and bipolar cells. Faint to moderate glycine immunostaining was observed in photoreceptors and some cells of the ganglion cell/inner plexiform layer. No GABA-immunolabeled perikarya were observed. GABA-immunoreactive centrifugal fibers were present in the central and lateral retina. These centrifugal fibers contacted glutamate-immunostained ganglion cells. No aspartate, serotonin, dopamine, or TH immunoreactivity was observed in larvae, whereas these molecules, as well as GABA, glycine, and glutamate, were detected in neurons of the retina of recently transformed lamprey. Immunoreactivity to GABA was observed in outer horizontal cells, some bipolar cells, and numerous amacrine cells, whereas immunoreactivity to glycine was found in amacrine cells and interplexiform cells. Dopamine and serotonin immunoreactivity was found in scattered amacrine cells. Amacrine and horizontal cells did not express classical neurotransmitters (with the possible exception of glycine) during larval life, so transmitter-expressing cells of the larval retina appear to participate only in the vertical processing pathway.

Parietal-eye phototransduction components and their potential evolutionary implications

The parietal-eye photoreceptor is unique because it has two antagonistic light signaling pathways in the same cell-a hyperpolarizing pathway maximally sensitive to blue light and a depolarizing pathway maximally sensitive to green light. Here, we report the molecular components of these two pathways. We found two opsins in the same cell: the blue-sensitive pinopsin and a previously unidentified green-sensitive opsin, which we name parietopsin. Signaling components included gustducin-alpha and Galphao, but not rod or cone transducin-alpha. Single-cell recordings demonstrated that Go mediates the depolarizing response. Gustducin-alpha resembles transducin-alpha functionally and likely mediates the hyperpolarizing response. The parietopsin-Go signaling pair provides clues about how rod and cone phototransduction might have evolved.

Exon-skipping variant of RGR opsin in human retina and pigment epithelium

An extraneous exon-skipping mRNA encodes an altered form of a light-absorbing opsin in human retina and pigment epithelium (RPE). The predicted protein variant differs from full-length RPE-retinal G protein-coupled receptor (RGR) by having an in-frame deletion of exon 6, which contains the entire sixth transmembrane domain. To verify that the exon 6-deleted RGR protein (RGR-d) exists in human retinas, we have produced RGR-d antibody probes. In Western blot assays, the RGR-d protein was detected in retinas of a large proportion ( approximately 53%) of individual donors, including patients with age-related macular degeneration (AMD). The relative abundance of RGR-d varied significantly between individuals. The altered protein is expressed in RPE cells and has a more basal subcellular localization that is remarkably different from that of normal RGR opsin. The presence of this exon-skipping variant of RGR in humans may contribute to the progressive derangement of the RPE.

Physiological characterization of the compound eye in monarch butterflies with focus on the dorsal rim area

The spectral, angular and polarization sensitivities of photoreceptors in the compound eye of the monarch butterfly (Danaus plexippus) are examined using electrophysiological methods. Intracellular recordings reveal a spectrally homogenous population of UV receptors with optical axes directed upwards and >or=10 degrees to the contralateral side. Based on optical considerations and on the opsin expression pattern (Sauman et al. 2005), we conclude that these UV receptors belong to the anatomically specialized dorsal rim area (DRA) of the eye. Photoreceptors in the main retina with optical axes <10 degrees contralateral or ipsilateral have maximal sensitivities in the UV (lambda(max)<or=340 nm), the blue (lambda(max)=435 nm) or in the long-wave range (green, lambda(max)=540 nm). The polarization sensitivity (PS) of the UV receptors in the DRA is much higher (PS=9.4) than in the UV cells (PS=2.9) or green cells (PS=2.8) of the main retina. The physiological properties of the photoreceptors in the DRA and in the main retina fit closely with the anatomy and the opsin expression patterns described in these eye regions. The data are discussed in the light of present knowledge about polarized skylight navigation in Lepidopterans.

Expression of UV-, blue-, long-wavelength-sensitive opsins and melatonin in extraretinal photoreceptors of the optic lobes of hawk moths

Lepidopterans display biological rhythms associated with egg laying, eclosion and flight activity but the photoreceptors that mediate these behavioural patterns are largely unknown. To further our progress in identifying candidate light-input channels for the lepidopteran circadian system, we have developed polyclonal antibodies against ultraviolet (UV)-, blue- and extraretinal long-wavelength (LW)-sensitive opsins and examined opsin immunoreactivity in the adult optic lobes of four hawk moths, Manduca sexta, Acherontia atropos, Agrius convolvuli and Hippotion celerio. Outside the retina, UV and blue opsin protein expression is restricted to the adult stemmata, with no apparent expression elsewhere in the brain. Melatonin, which is known to have a seasonal influence on reproduction and behaviour, is expressed with opsins in adult stemmata together with visual arrestin and chaoptin. By contrast, the LW opsin protein is not expressed in the retina or stemmata but rather exhibits a distinct and widespread distribution in dorsal and ventral neurons of the optic lobes. The lamina, medulla, lobula and lobula plate, accessory medulla and adjacent neurons innervating this structure also exhibit strong LW opsin immunoreactivity. Together with the adult stemmata, these neurons appear to be functional photoreceptors, as visual arrestin, chaoptin and melatonin are also co-expressed with LW opsin. These findings are the first to suggest a role for three spectrally distinct classes of opsin in the extraretinal detection of changes in ambient light and to show melatonin-mediated neuroendocrine output in the entrainment of sphingid moth circadian and/or photoperiodic rhythms.

Ciliary photoreceptors with a vertebrate-type opsin in an invertebrate brain

For vision, insect and vertebrate eyes use rhabdomeric and ciliary photoreceptor cells, respectively. These cells show distinct architecture and transduce the light signal by different phototransductory cascades. In the marine rag-worm Platynereis, we find both cell types: rhabdomeric photoreceptor cells in the eyes and ciliary photoreceptor cells in the brain. The latter use a photopigment closely related to vertebrate rod and cone opsins. Comparative analysis indicates that both types of photoreceptors, with distinct opsins, coexisted in Urbilateria, the last common ancestor of insects and vertebrates, and sheds new light on vertebrate eye evolution.

Axotomized mouse retinal ganglion cells containing melanopsin show enhanced survival, but not enhanced axon regrowth into a peripheral nerve graft

Melanopsin is found in only approximately 2% of mouse retinal ganglion cells (RGCs), making these RGCs uniquely and directly photosensitive. Given that the majority of RGCs die after axotomy and that grafting of a peripheral nerve to the eye provides a permissive environment for axon regrowth, the present study examined the survival and axonal regrowth of melanopsin-containing RGCs in mice. One month after optic nerve transection and grafting, RGCs with regrown axons were labeled from the grafts and retinae were processed to visualize melanopsin and TUJ1. Melanopsin-positive and negative RGCs were counted and compared to axotomized RGCs from ungrafted eyes and uninjured RGCs. Melanopsin-positive RGCs showed a 3-fold increase in survival rate compared to non-melanopsin RGCs. Despite this enhanced survival, melanopsin-containing RGCs did not show increased axon regrowth into nerve grafts.

The rod opsin pigments from two marsupial species, the South American bare-tailed woolly opossum and the Australian fat-tailed dunnart

Rod visual pigment genes have been studied in a wide range of vertebrates including a number of mammalian species. However, no marsupials have yet been examined. To correct this omission, we have studied the rod pigments in two marsupial species, the nocturnal and frugivorous bare-tailed woolly opossum, Caluromys philander, from Central and South America, and the arhythmic and insectivorous fat-tailed dunnart, Sminthopsis crassicaudata, from Australia. Phylogenetic analysis establishes that the cloned opsin sequences are orthologues of rod opsin genes from other vertebrate species. The deduced amino acid sequences show that both possess glutamate at residue 122, a feature of rod opsins, and the corresponding gene follows the typical vertebrate rod opsin pattern of five exons separated by four introns. Compared to other vertebrates, a stretch of five residues near the C-terminus is deleted in the rod opsin of both marsupials and all eutherian mammals. From microspectrophotometric measurements, the pigments in the two species show an 8 nm difference in peak absorbance; the molecular basis for this spectral shift is discussed and two candidate substitutions are identified.

Origin of the vertebrate visual cycle: II. Visual cycle proteins are localized in whole brain including photoreceptor cells of a primitive chordate

The visual cycle system in a primitive chordate, ascidian Ciona intestinalis, was studied by whole-mount in situ hybridization and by whole-mount immunohistochemistry. Three visual cycle proteins, Ciona homologue of RGR (Ci-opsin3), CRALBP (Ci-CRALBP), and BCO/RPE65 (Ci-BCO/RPE65) were widely distributed in the brain vesicle and visceral ganglion. To identify the visual cycle system in a primitive chordate, we compared the localization of photoreceptor-specific proteins (visual pigment and arrestin) and visual cycle proteins (Ci-opsin3 and Ci-CRALBP). The ascidian visual cycle is composed of two cellular compartments, the photoreceptors and the brain vesicle, but some photoreceptor cells also contain visual cycle proteins.

Absorption spectra of reconstituted visual pigments of a nocturnal prosimian, Otolemur crassicaudatus

Absorption spectra of visual pigments characterize animal vision. The association between absorption spectra and amino acid (aa) sequences of pigments has been well established for the middle- to-long-wave-sensitive (M/LWS) class of cone visual pigments of vertebrates, known as the "five-sites" rule where amino acid residues at the 180th, 197th, 277th, 285th, and 308th sites mostly determine the spectra. For primate M/LWS pigments, however, applicability of the rule is not clear because of the scarcity of absorbance data collected directly from purified pigments. In particular, no prosimian pigment has been examined in vitro. In this study, we reconstituted visual pigments of a nocturnal prosimian, the greater galago (Otolemur crassicaudatus), which has the M/LWS cone and the rod visual pigments in its retina. The five residues of the galago M/LWS pigment are Ala, His, Tyr, Ala, and Ala, respectively, and its peak absorption spectra (lambda(max)) was measured to be 539 nm, which is virtually identical to the expected value from the rule (538 nm), showing that the five-sites rule holds for this prosimian. The lambda(max) of the rod visual pigment was measured as 502 nm. Accurate estimate of lambda(max) values is essential in establishing the molecular basis of visual pigment evolution.

Retinal ganglion cell projections to the hamster suprachiasmatic nucleus, intergeniculate leaflet, and visual midbrain: bifurcation and melanopsin immunoreactivity

The circadian clock in the suprachiasmatic nucleus (SCN) receives direct retinal input via the retinohypothalamic tract (RHT), and the retinal ganglion cells contributing to this projection may be specialized with respect to direct regulation of the circadian clock. However, some ganglion cells forming the RHT bifurcate, sending axon collaterals to the intergeniculate leaflet (IGL) through which light has secondary access to the circadian clock. The present studies provide a more extensive examination of ganglion cell bifurcation and evaluate whether ganglion cells projecting to several subcortical visual nuclei contain melanopsin, a putative ganglion cell photopigment. The results showed that retinal ganglion cells projecting to the SCN send collaterals to the IGL, olivary pretectal nucleus, and superior colliculus, among other places. Melanopsin-immunoreactive (IR) ganglion cells are present in the hamster retina, and some of these cells project to the SCN, IGL, olivary pretectal nucleus, or superior colliculus. Triple-label analysis showed that melanopsin-IR cells bifurcate and project bilaterally to each SCN, but not to the other visual nuclei evaluated. The melanopsin-IR cells have photoreceptive characteristics optimal for circadian rhythm regulation. However, the presence of moderately widespread bifurcation among ganglion cells projecting to the SCN, and projection by melanopsin-IR cells to locations distinct from the SCN and without known rhythm function, suggest that this ganglion cell type is generalized, rather than specialized, with respect to the conveyance of photic information to the brain.

Seeing More Clearly: Recent Advances in Understanding Retinal Circuitry

Among 10 breakthroughs that Science announced at the end of 2002 was the discovery of a photosensing (melanopsin-containing) retinal ganglion cell (RGC) and its role in entraining the circadian clock. This breakthrough exemplifies the ultimate goal of neuroscience: to understand the nervous system from molecules to behavior. Light-sensing RGCs constitute one of a dozen discrete RGC populations coding various aspects of visual scenes by virtue of their unique morphology, physiology, and coverage of the retina. Interestingly, the function of the melanopsin-containing RGCs in entraining the circadian clock need not involve much retinal processing, making it the simplest form of processing in the retina. This review focuses on recent advances in our understanding of retinal circuitry, visual processing, and retinal development demonstrated by innovative experimental techniques. It also discusses the advantages of using the retina as a model system to address some of the key questions in neuroscience.

Melanopsin retinal ganglion cells receive bipolar and amacrine cell synapses

Melanopsin is a novel opsin synthesized in a small subset of retinal ganglion cells. Ganglion cells expressing melanopsin are capable of depolarizing in response to light in the absence of rod or cone input and are thus intrinsically light sensitive. Melanopsin ganglion cells convey information regarding general levels of environmental illumination to the suprachiasmatic nucleus, the intergeniculate leaflet, and the pretectum. Typically, retinal ganglion cells communicate information to central visual structures by receiving input from retinal photoreceptors via bipolar and amacrine cells. Because melanopsin ganglion cells do not require synaptic input to generate light-induced signals, these cells need not receive synapses from other neurons in the retina. In this study, we examined the ultrastructure of melanopsin ganglion cells in the mouse retina to determine the type (if any) of synaptic input these cells receive. Melanopsin immunoreaction product was associated primarily with the plasma membrane of (1) perikarya in the ganglion cell layer, (2) dendritic processes in the inner plexiform layer (IPL), and (3) axons in the optic fiber layer. Melanopsin-immunoreactive dendrites in the inner (ON) region of the IPL were postsynaptic to bipolar and amacrine terminals, whereas melanopsin dendrites stratifying in the outer (OFF) region of the IPL received only amacrine terminals. These observations suggested that rod and/or cone signals may be capable of modifying the intrinsic light response in melanopsin-expressing retinal ganglion cells.

The photoreceptive capacity of the developing pineal gland and eye of the golden hamster (Mesocricetus auratus)

Anatomical and physiological studies have suggested that the pineal gland of neonatal mammals has a photoreceptive capacity. Using the golden hamster (Mesocricetus auratus) as our model, we applied biochemical approaches to look for a functional photopigment within the pineal during early development. Immunocytochemistry and enzyme-linked immunosorbent assay (ELISA) were used to localize and quantify opsin, and high-performance liquid chromatography (HPLC) to identify photopigment chromophore (11-cis and all-trans retinaldehyde) in the developing eye and pineal. For HPLC analysis, retinaldehydes were converted to their corresponding retinoid oximes. Eluted retinoids were identified by comparison with standard vitamin A1 retinoid oxime isomers on the basis of relative elution sequence and characteristic absorbance spectra. Both immunocytochemistry and ELISA suggested an increase in the opsin content of the pineal during the first week of life. In the eye, 11-cis retinaldehyde was first detected between days 3 and 5 after birth. In three separate extractions, and using a considerable excess of pineal tissue, we failed to identify chromophore within the pineal during the first week of postnatal development. The appearance of 11-cis retinaldehyde within the eye between postnatal days 3-5 is consistent with the hypothesis that retinol isomerase activity is coordinated with outer segment development. The failure to identify chromophore within the neonatal pineal suggests that this gland lacks a functional opsin-based photopigment. These data contradict physiological evidence suggesting that the neonatal pineal of mammals contains photoreceptors.

Isolation and characterization of two teleost melanopsin genes and their differential expression within the inner retina and brain

Melanopsin is a newly discovered photopigment that is believed to be involved in the regulation of circadian rhythms in tetrapods. Here we describe the characterization of the first two teleost melanopsins (opn4a and opn4b) isolated from Atlantic cod (Gadus morhua). These two teleost genes belong to a subgroup of melanopsins that also include members from Xenopus, chicken, and Takifugu. In situ hybridization revealed that opn4a and opn4b are differentially expressed within the retina and brain. In the larval and adult retina, both melanopsins are expressed in a subset of cells in the inner retina, resembling amacrine and ganglion cells. In addition, opn4a is expressed in the horizontal cells, indicating a separate task for this gene. In the brain, the two melanopsins are separately expressed in two major retinal and extraretinal photosensitive integration centers, namely, the suprachiasmatic nucleus (opn4a) and the habenula (opn4b). The expression of opn4a in the suprachiasmatic nucleus in cod is similar to the melanopsin expression found in Xenopus. This suggests a conserved role for this opsin and an involvement in mediation of nonvisual photoreceptive tasks, such as entraining circadian rhythms and/or hypophysiotrophic systems. The differential expression of opn4b in the habenula suggests that this gene plays a role similar to that of opn4a, in that it is also situated in an area that integrates photic inputs from the pineal as well as other brain regions. Thus, the habenula may be an additional region that mediates photic cues in teleosts.

Differentiation of human neural stem cells into retinal cells

We have previously reported that transplanted human neural stem cells (HNSCs) display extensive migration and positional incorporation into the aged rat brain, which is associated with an improvement in cognitive function. In the current study, to investigate whether HNSCs are capable of differentiating into retinal cells, we treated HNSCs with human transforming growth factor-beta3 (TGF-beta3) under a serum-free differentiation condition. After 5 days of differentiation in vitro we detected opsin-immunopositive cells in the culture treated with TGF-beta3. We also transplanted TGF-beta3-treated HNSCs into the rat vitreous cavity. The donor cells migrated and differentiated into opsin-positive cells in the host retinal cell layer. Here we show for the first time that TGF-beta3-treated HNSCs differentiate into retinal cells.

Multiple photopigments from the Mexican blind cavefish, Astyanax fasciatus: a microspectrophotometric study

The cave-dwelling (hypogean) form of the teleost Astyanax fasciatus is blind, having only subdermal eye rudiments, but nevertheless maintains intact opsin genes. Second generation offspring of a cross between these and the normally sighted surface (epigean) form inherit opsin genes from both ancestries. A study of the expressed hypogean opsins of the hybrids, in comparison to the epigean forms, was undertaken by microspectrophotometry. The hybrid population showed considerable variation in the visual pigments of double cones, with evidence for two groups of cells with lambda(max) intermediate to those of the epigean pigments. Possible explanations for these intermediate pigments are discussed, including the hypothesis that they may represent hybrid genes similar to the genes for anomalous cone pigments in humans. Evidence was also found for ultraviolet-sensitive single cones and for an additional MWS pigment.

Topography of Photoreceptors and Retinal Ganglion Cells in the Spotted Hyena (Crocuta crocuta)

The spatial distributions of photoreceptors and retinal ganglion cells were examined in the spotted hyena (Crocuta crocuta). Two populations of cones were identified by immunocytochemical labeling. The hyena retina contains approximately 2.3 million middle- to long-wavelength sensitive (M/L) cones that reach peak densities of about 7,500/mm(2) in the vicinity of the optic nerve head. A sparser population of short-wavelength sensitive (S) cones, totaling about 0.3 million, was also detected. There is a striking disparity in the spatial distributions of the two cone types with S cones achieving peak density in a region located well below the optic nerve head. The differences in the spatial distributions of the two cone types have implications both for visual sensitivity and for color vision. Hyena rods outnumber cones by about 100:1 with rod density falling off modestly along a central-peripheral gradient. Ganglion cells were identified in retinal wholemounts by Nissl staining patterns. Their distribution defines a prominent visual streak with highest spatial packing (approx. 4,200/mm(2)) in an area centralis that is located in the temporal retina. The total number of ganglion cells is estimated at about 260,000. Using standard assumptions the maximum spatial resolution of the spotted hyena is calculated to be about 8.4 cycles/degree, a value similar to estimates obtained for other terrestrial carnivores.

Developmental changes in the cone visual pigments of black bream Acanthopagrus butcheri

The spectral absorption characteristics of the visual pigments in the photoreceptors of the black bream Acanthopagrus butcheri Munro(Sparidae, Teleostei), were measured using microspectrophotometry. A single cohort of fish aged 5-172 days post-hatch (dph), aquarium-reared adults and wild-caught juveniles were investigated. During the larval stage and in juveniles younger than 100 dph, two classes of visual pigment were found, with wavelengths of maximum absorbance (λmax) at approximately 425 nm and 535 nm. Following double cone formation, from 40 dph onwards, the short wavelength-sensitive pigment was recorded in single cones and the longer wavelength-sensitive pigment in double cones. From 100 dph, a gradual shift in the λmax towards longer wavelengths was observed in both cone types. By 160 dph, and in adults, all single cones had aλ max at approximately 475 nm while theλ max in double cones ranged from 545 to 575 nm. The relationships between the λmax and the ratio of bandwidth:λmax, for changes in either chromophore or opsin,were modelled mathematically for the long-wavelength-sensitive visual pigments. Comparing our data with the models indicated that changes inλ max were not mediated by a switch from an A1 to A2 chromophore, rather a change in opsin expression was most likely. The shifts in the λmax of the visual pigments occur at a stage when the juvenile fish begin feeding in deeper, tanninstained estuarine waters, which transmit predominantly longer wavelengths, so the spectral sensitivity changes may represent an adaptation by the fish to the changing light environment.

The circadian photopigment melanopsin is expressed in the blind subterranean mole rat, Spalax

Despite severe degeneration of its eyes, the blind subterranean mole rat, Spalax, is able to adjust circadian rhythms to the environmental light/dark cycle due to a conserved retinohypothalamic tract (RHT). The photopigment mediating the circadian photoreception and it cellular localisation is unknown in the Spalax retina. Here we show, using in situ hybridization and immunohistochemistry, that melanopsin, a recently identified opsin, is expressed in retinal ganglion cells which also co-store PACAP, a neurotransmitter of the RHT. The melanopsin-component of retinal ganglion cells in the Spalax retina is well conserved resulting in a relatively higher density of melanopsin positive cells per area compared to the rat. The results show that the Spalax, as sighted animals expresses melanopsin in ganglion cells projecting to the circadian clock supporting a role of melanopsin as a circadian photopigment.

Multiple mechanisms of rhabdom shedding in the lateral eye of Limulus polyphemus

Rhabdom shedding in horseshoe crab lateral eye photoreceptors was studied with anti-opsin and anti-arrestin immunocytochemistry. Two, possibly three, distinct shedding mechanisms were revealed in animals maintained in natural lighting. Transient rhabdom shedding, triggered by dawn, is a brief, synchronous event that removes up to 10% of the rhabdom membrane. Whorls of rhabdomeral membrane break into vesicles and form compact multivesicular bodies. These debris particles are immunoreactive for opsin and are of a relatively uniform size, averaging approximately 2 microm(2) in area. Transient shedding requires that input from circadian efferent fibers to the retina precedes the light trigger, and cutting the optic nerve blocks efferent input and transient shedding. Light-driven rhabdom shedding is a progressive process. Rhabdomeral membrane is removed by coated vesicles that accumulate into loosely packed multivesicular bodies. These debris particles label with antibodies directed against opsin, arrestin, and adaptin, and they have a large distribution of sizes, averaging almost 6 microm(2) in area and ranging up to 25 microm(2) or more. The amount of rhabdomeral membrane removed by light-driven shedding has seasonal variation and depends on latitude. Light-driven shedding does not require circadian efferent input. A possible third shedding mechanism, light-independent shedding, is observed when transient shedding is blocked either by 48 hours of darkness or by cutting the optic nerve. Small particles, averaging 1.8 microm(2) in area, exhibiting opsin but not arrestin immunoreactivity can then be found in the cytoplasm surrounding the rhabdom. The nature of light-independent shedding is not yet clear.

Opsin expression in adult, developing, and regenerating newt retinas

Japanese common newts (Cynops pyrrhogaster) have an ability to regenerate their neural retina even as adults. Although extensive research has been carried out attempting to understand this retinal regeneration, the molecules characterized in newt retina are limited. We isolated cDNAs encoding three putative opsins (Cp-Rh, -LWS and -SWS1), in addition to Cp-SWS2 [Takahashi et al., FEBS Lett. 501 (2001) 151-155] from a cDNA library of adult newt retina. Our immunohistochemical and in situ hybridization studies demonstrated that Cp-Rh is selectively expressed in rods, whereas the other opsins are expressed in cones. The distribution of opsin mRNAs in normal and regenerated retinas is very similar. In both developing and regenerating retinas, Cp-Rh and its mRNA first appeared in immature rods at the beginning or just after the formation of plexiform layers. Cp-Rh was initially found isotropically in the plasma membrane, and then translocalized to the apical region along with the maturation of regenerating rods. This suggests that reorganization of the intracellular structure takes place during maturation of the regenerating newt photoreceptors.

Spatial control of rhabdom shedding in the lateral eye of the American horseshoe crab, Limulus polyphemus

Membrane leaves the rhabdom of Limulusphotoreceptors either by transient shedding at dawn or throughout the day by light-driven shedding. We examined whether the light trigger for transient shedding and the light drive for light-driven shedding are localized properties of the illuminated photoreceptors or whether they are an array property of the retina. Four experiments were conducted during which the lateral eye was exposed to one of a variety of different illumination patterns for a day, fixed, dissected and cut into serial frozen sections. Immunocytochemistry with different antibodies to Limulus opsin and arrestin revealed the results of the two processes in a distinguishable way. Eyes stimulated with whole-eye illumination had both types of shedding or just light-driven shedding when transient shedding was blocked by cutting the optic nerve. Eyes exposed to whole-eye darkness had neither type of shedding. However, when only half of an eye was exposed to light, the dark half had the same kinds of shedding as the lighted half. We conclude that the signals to trigger or drive shedding must be communicated laterally from illuminated ommatidia to unilluminated ommatidia. Rhabdom shedding is an array property.

Visual pigment coexpression in Guinea pig cones: a microspectrophotometric study

PURPOSE

To determine the visual pigment content of the rods and cones of the guinea pig (Cavia porcellus) and to quantify the level of coexpression of pigments within individual cones.

METHODS

Microspectrophotometry was used to measure the absorbance spectrum of visual pigments in individual rods and cones from three retinal regions: dorsal, ventral, and a subequatorial transition zone. Partial bleaching was used to establish whether two spectrally distinct visual pigments were present within a single cone.

RESULTS

Rods possessed a pigment with a wavelength of maximum absorbance (lambda(max)) close to 500 nm. A population of middle-wave-sensitive cones (M cones) contained a pigment with lambda(max) at approximately 530 nm, and a short-wave-sensitive cone population (S cones) contained a pigment with lambda(max) close to 400 nm. The majority of cones in all regions were M cones. Approximately 10% of cones in the transition region were found to coexpress the M and S cone pigments in a ratio of approximately 4:1. Coexpression was not detected in S cones.

CONCLUSIONS

In C. porcellus, coexpression of cone pigments occurs in a small number of cells but is biased in favor of the M pigment. Given the relatively low level of coexpression, detectable in only approximately 10% of the cones in the transition region, it is unlikely to cause any significant detriment to dichromatic color vision.

Distribution and density of medium- and short-wavelength selective cones in the domestic pig retina

The topography of medium (M)- and short (S)-wavelength sensitive cone photoreceptors was studied in the domestic pig retina. Antisera specific for M or S opsin as well as cone photoreceptor proteins arrestin and alpha-transducin were used to label cone types. Retinal wholemounts and their blood vessel patterns were drawn and specific regions removed. The wholemounts were immunocytochemically labelled to detect both M and S cones, and the specific regions labelled to detect S cones. Cones were counted in a 1 mm grid pattern, using the drawings as a guide. Pig retina has a high cone density retinal streak extending across the retina covering the optic disc (OD) and horizontal meridian. Densities in the streak are 20,000-35,000 mm(-2). Two higher peaks occur in the streak, one in temporal retina near the OD (39,000 mm(-2)) and the other in nasal retina 5-7 mm from the OD (40,500 mm(-2)). The lowest cone density is in far peripheral inferior retina (7000 mm(-2)). The total number of cones in pig retina is 17-20 million. Both types of cones are found throughout the retina, with S cone percentages ranging from 7.4 to 17.5% in no consistent topographical pattern. S cones have an irregular local distribution which can vary from a regular hexagonal pattern to small clusters of adjacent S cones to small areas lacking S cones. Double-label immunocytochemistry found that virtually all S cone outer segments (OS) contain some M opsin. M cone OS do not label at detectible levels for S opsin. Domestic pig retina is widely available, large, has a high cone density and has two types of cones. This tissue should be an excellent source for biochemical analysis of cone proteins, and for in vitro approaches to understanding cone survival factors.

Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity

The primary circadian pacemaker, in the suprachiasmatic nucleus (SCN) of the mammalian brain, is photoentrained by light signals from the eyes through the retinohypothalamic tract. Retinal rod and cone cells are not required for photoentrainment. Recent evidence suggests that the entraining photoreceptors are retinal ganglion cells (RGCs) that project to the SCN. The visual pigment for this photoreceptor may be melanopsin, an opsin-like protein whose coding messenger RNA is found in a subset of mammalian RGCs. By cloning rat melanopsin and generating specific antibodies, we show that melanopsin is present in cell bodies, dendrites, and proximal axonal segments of a subset of rat RGCs. In mice heterozygous for tau-lacZ targeted to the melanopsin gene locus, beta-galactosidase-positive RGC axons projected to the SCN and other brain nuclei involved in circadian photoentrainment or the pupillary light reflex. Rat RGCs that exhibited intrinsic photosensitivity invariably expressed melanopsin. Hence, melanopsin is most likely the visual pigment of phototransducing RGCs that set the circadian clock and initiate other non-image-forming visual functions.

Phototransduction by retinal ganglion cells that set the circadian clock

Light synchronizes mammalian circadian rhythms with environmental time by modulating retinal input to the circadian pacemaker-the suprachiasmatic nucleus (SCN) of the hypothalamus. Such photic entrainment requires neither rods nor cones, the only known retinal photoreceptors. Here, we show that retinal ganglion cells innervating the SCN are intrinsically photosensitive. Unlike other ganglion cells, they depolarized in response to light even when all synaptic input from rods and cones was blocked. The sensitivity, spectral tuning, and slow kinetics of this light response matched those of the photic entrainment mechanism, suggesting that these ganglion cells may be the primary photoreceptors for this system.

[Experimental studies of effects of retinoic acid on the proliferation of retinal cells]

OBJECTIVE

To determine whether the application of exogenous retinoic acid (RA) may induce the proliferation of retinal cells in adult rat.

METHODS

Thirty-two healthy adult Sprague-Dawley (SD) rats were randomly divided into 4 groups. In Group 1 and Group 2, all-trans RA (5 microliter, 0.001 mol/L) was injected into the subretinal space. In Group 3 and Group 4, all-trans RA (10 microliter, 0.001 mol/L) was injected into epiretinal vitreous space. In the Group 1 and Group 3, transient ischemic-reperfusion injuries of the experimental eyes were induced by ligating ophthalmic artery prior to RA treatment. In the control group, 5 SD rats were treated by ischemia-reperfusion injuries but no exogenous RA application. The treated eyes were enucleated for light microscopic analysis and immunohistochemical assays after 2 - 4 weeks of RA application.

RESULTS

In group 1, the number of the retinal cells expressing rod-specific opsin marker in the subretinal space was significantly increased and the thickness of inner nuclear layer was also increased after the RA treatment for 16 days. However, no cell proliferation was detected in group 2. There were also no changes within population of retinal cells in Group 3 and Group 4 in which RA was injected into epiretinal vitreous space no matter under the ischemia-reperfusion or non-ischemia-reperfusion. In the control group, there were no significant morphological changes within the neural retinal layers as well as photoreceptor proliferation.

CONCLUSIONS

The application of RA in the subretinal space can induce photoreceptor proliferation in adult rat under an ischemic-reperfusion injury condition. It will provide a new idea for the regeneration of neural retinal cells.

Microenvironmental regulation of visual pigment expression in the chick retina

Visual pigment (VP) expression in the chick embryo retina was investigated in ovo, in dissociated and explant cultures, and in cDNAs from individual cells. While VP mRNA is not detectable by in situ hybridization until embryonic day (ED) 14-16 in ovo, analysis of VP expression by RT-PCR showed that VP messages are present in the retina as many as 7-10 days before they become detectable by in situ hybridization, and are also detected in other regions of the embryonic CNS. On the other hand, red opsin expression is markedly accelerated when cells are isolated from their intraocular microenvironment at ED 6, and placed in pigment epithelium-free dissociated or explant cultures. This acceleration occurs regardless of cell density, birth date, or serum presence in the medium, suggesting that many photoreceptors are already programmed to express red opsin on or before ED 6, and that microenvironmental inhibitory factors prevent implementation of this program until ED 14 in ovo. The selectivity of this phenomenon is suggested by the finding that other VPs are not observed by in situ hybridization in ED 6 cultures, although they are detectable in cultures of older retinas. Taken together, these findings suggest that red opsin expression may be constitutive for many developing photoreceptor cells in the chick.

Evidence for two distinct mechanisms of neurogenesis and cellular pattern formation in regenerated goldfish retinas

After its destruction by intraocular injection of ouabain, the goldfish retina regenerates, but little is known about the histogenesis of the new tissue, including the structure and formation of regenerated cell mosaic patterns. In an effort to determine how retinal cells are generated and spatially organized within retina regenerated after ouabain injection, in situ hybridization and immunocytochemical techniques were combined with computational analyses of two-dimensional spatial patterns of identified neurons. Labeling with specific opsin riboprobes revealed two distinct cone patterns in the ouabain-injected eyes, each of which was different from the relatively orderly cone patterns of native retina. Central, regenerated regions had sparse aggregates of cones, and a relatively lower density of each cone type. Peripheral regions of experimental retina, likely derived from the circumferential germinal zone, had high densities of all cone types, each of which tended to be distributed randomly. The spatial patterns of inner retinal neurons in experimental eyes were also disorganized with respect to native retina. These results indicate that although some aspects of retinal regeneration resemble normal retinal development and growth, ouabain-induced regeneration does not produce well-organized mosaics of neurons, indicating a failure of the developmental interactions needed for proper pattern formation, which in turn could compromise visual recovery. Furthermore, the distinct cone patterns in different regions of experimental retina support the hypothesis that new goldfish retina arises via two spatially and cellularly distinct mechanisms after exposure to ouabain.

Direct innervation of GnRH neurons by encephalic photoreceptors in birds

In nonmammalian vertebrates, photic cues that regulate the timing of seasonal reproductive cyclicity are detected by nonretinal, nonpineal deep brain photoreceptors. It has long been assumed that the underlying mechanism involves the transmission of photic information from the photoreceptor to a circadian system, and thence to the reproductive axis. An alternative hypothesis is that there is direct communication between the brain photoreceptor and the reproductive axis. In the present study, light and confocal microscopy reveal that gonadotropin releasing hormone (GnRH) neurons and processes are scattered among photoreceptor cells (identified by their opsin-immunoreactivity) in the lateral septum (SL). In the median eminence (ME), opsin and GnRH immunoreactive fibers overlap extensively. Single and double label ultrastructural immunocytochemistry indicate that in the SL and preoptic area (POA), opsin positive terminals form axo-dendritic synapses onto GnRH dendrites. In the ME, opsin and GnRH terminals lie adjacent to each other, make contact with tanycytes, or terminate on the hypophyseal portal capillaries. These results reveal thatbrain photoreceptors communicate directly with GnRH-neurons; this represents a means by which photoperiodic information reaches the reproductive axis.

Rod-type transducin alpha-subunit mediates a phototransduction pathway in the chicken pineal gland

The chicken pineal gland is a photosensitive neuroendocrine organ producing melatonin in circadian clock-regulated and light-sensitive manners. To understand the relationship between the photoreceptive molecule pinopsin and the light-dependent melatonin suppression that is sensitive to pertussis toxin treatment, we have searched for pertussis toxin-sensitive G protein alpha-subunits expressed in the chicken pineal gland. Here we report the cDNA cloning of the pineal transducin alpha-subunit (Gtalpha), which is highly homologous to human retinal rod cell-specific Gt(1)alpha. Concurrent cDNA cloning of chicken retinal Gt(1)alpha and Gt(2)alpha (rod and cone cell-specific alpha-subunits of transducin, respectively) revealed that the chicken pineal Gtalpha is identical to the retinal Gt(1)alpha. Double-immunostaining analysis of the chicken pineal sections localized Gt(1)alpha-immunoreactivity in the rudimentary outer segments of both follicular and parafollicular pinealocytes that were immunopositive to anti-pinopsin antibody. To examine whether pineal Gt(1)alpha is involved in the pineal phototransduction pathway, trypsin protection assay was applied for detecting the conversion of GDP-bound Gt(1)alpha into the guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS)-bound form in the pineal membrane homogenate. It was clearly demonstrated that the pineal Gt(1)alpha is activated in a light-dependent manner in the presence of GTPgammaS. These data together suggest strongly that pineal Gt(1)alpha mediates the phototransduction pathway triggered by pinopsin in the chicken pinealocytes.

Vertebrate ancient (VA) opsin and extraretinal photoreception in the Atlantic salmon (Salmo salar)

A member of a new photopigment family first isolated from teleost fish, vertebrate ancient (VA) opsin, has recently been shown to form a functional photopigment and to be expressed within a subset of horizontal and amacrine cells of the inner retina. These sites of expression (and structural features) of VA opsin suggest that this photopigment might mediate non-image-forming light-detection tasks. We attempted to gain support for this hypothesis by examining the expression of VA opsin within the central nervous system (CNS) (pineal and deep brain) of the Atlantic salmon Salmo salar. In addition, we examined the sites of rod-opsin, cone-opsin and α -transducin expression within the salmon CNS to provide a more complete description of the extraretinal photoreceptors of a teleost vertebrate. We show that multiple populations of cells within the salmon CNS appear to contain photoreceptors: VA opsin was strongly expressed in the pineal organ and in bilateral columns of subependymal cells in the epithalamus; anti-cone-opsin antibodies labelled cells within the pineal and numerous cells in the anterior hypothalamus (suprachiasmatic nucleus, nucleus preopticus magnocellularis, nucleus preopticus parvocellularis); anti-rod-opsin antibodies labelled cells within the pineal but no other areas within the central brain; and anti- α -transducin antibodies labelled cells within the pineal and the ventral telencephalon. Collectively, our results suggest that VA opsin is a photopigment specialised for irradiance detection tasks within the eye, pineal and central brain, and that the salmon has multiple and varied populations of photoreceptors within the CNS. We review the significance of these findings within the broad context of vertebrate extraretinal photoreception.

Rhodopsin transport in the membrane of the connecting cilium of mammalian photoreceptor cells

The transport of the photopigment rhodopsin from the inner segment to the photosensitive outer segment of vertebrate photoreceptor cells has been one of the main remaining mysteries in photoreceptor cell biology. Because of the lack of any direct evidence for the pathway through the photoreceptor cilium, alternative extracellular pathways have been proposed. Our primary aim in the present study was to resolve rhodopsin trafficking from the inner to the outer segment. We demonstrate, predominantly by high-sensitive immunoelectron microscopy, that rhodopsin is also densely packed in the membrane of the photoreceptor connecting cilium. Present prominent labeling of rhodopsin in the ciliary membrane provides the first striking evidence that rhodopsin is translocated from the inner segment to the outer segment of wild type photoreceptors via the ciliary membrane. At the ciliary membrane rhodopsin co-localizes with the unconventional myosin VIIa, the product of human Usher syndrome 1B gene. Furthermore, axonemal actin was identified in the photoreceptor cilium, which is spatially co-localized with myosin VIIa and opsin. This actin cytoskeleton of the cilium may provide the structural bases for myosin VIIa-linked ciliary trafficking of membrane components, including rhodopsin.

Lutein and zeaxanthin concentrations in rod outer segment membranes from perifoveal and peripheral human retina

PURPOSE

In addition to acting as an optical filter, macular (carotenoid) pigment has been hypothesized to function as an antioxidant in the human retina by inhibiting the peroxidation of long-chain polyunsaturated fatty acids. However, at its location of highest density in the inner (prereceptoral) layers of the foveal retina, a specific requirement for antioxidant protection would not be predicted. The purpose of this study was to determine whether lutein and zeaxanthin, the major carotenoids comprising the macular pigment, are present in rod outer segment (ROS) membranes where the concentration of long-chain polyunsaturated fatty acids, and susceptibility to oxidation, is highest.

METHODS

Retinas from human donor eyes were dissected to obtain two regions: an annular ring of 1.5- to 4-mm eccentricity representing the area centralis excluding the fovea (perifoveal retina) and the remaining retina outside this region (peripheral retina). ROS and residual (ROS-depleted) retinal membranes were isolated from these regions by differential centrifugation and their purity checked by polyacrylamide gel electrophoresis and fatty acid analysis. Lutein and zeaxanthin were analyzed by high-performance liquid chromatography and their concentrations expressed relative to membrane protein. Preparation of membranes and analysis of carotenoids were performed in parallel on bovine retinas for comparison to a nonprimate species. Carotenoid concentrations were also determined for retinal pigment epithelium harvested from human eyes.

RESULTS

ROS membranes prepared from perifoveal and peripheral regions of human retina were found to be of high purity as indicated by the presence of a dense opsin band on protein gels. Fatty acid analysis of human ROS membranes showed a characteristic enrichment of docosahexaenoic acid relative to residual membranes. Membranes prepared from bovine retinas had protein profiles and fatty acid composition similar to those from human retinas. Carotenoid analysis showed that lutein and zeaxanthin were present in ROS and residual human retinal membranes. The combined concentration of lutein plus zeaxanthin was 70% higher in human ROS than in residual membranes. Lutein plus zeaxanthin in human ROS membranes was 2.7 times more concentrated in the perifoveal than the peripheral retinal region. Lutein and zeaxanthin were consistently detected in human retinal pigment epithelium at relatively low concentrations.

CONCLUSIONS

The presence of lutein and zeaxanthin in human ROS membranes raises the possibility that they function as antioxidants in this cell compartment. The finding of a higher concentration of these carotenoids in ROS of the perifoveal retina lends support to their proposed protective role in age-related macular degeneration.

Colocalization of pinopsin with two types of G-protein alpha-subunits in the chicken pineal gland

Pinopsin is a photoreceptive molecule present in the outer segments of chicken pinealocytes. In this paper, the localization of alpha-subunits of G-proteins, rod transducin (Gt1) and Gq/11, was examined by immunoelectron microscopy to investigate whether these G-proteins colocalize with pinopsin in the outer segments. Ultrathin sections of the chicken pineal gland were double-immunolabeled with antibodies to pinopsin and either Gt1alpha or Gq/11alpha. As shown previously, the outer segments around the follicular lumen exhibited divergent morphology with ciliary, bulbous, or lamellate shapes, and most of them displayed pinopsin immunoreactivity. The majority (>90%) of pinopsin-immunopositive outer segments were labeled by anti-Gt1alpha and/or anti-Gq/11alpha antibodies. Application of double-immunolabeling to serial sections demonstrated that a large number of the pinopsin-immunopositive outer segments contained both Gt1alpha and Gq/11alpha immunoreactivities. These results suggest that Gt1alpha and Gq/11alpha are functionally coupled with light-activated pinopsin within a single outer segment.

Diversity of opsin immunoreactivities in the extraretinal tissues of four anuran amphibians

The pineal complex, deep brain, and skin have been known to function as extraretinal photoreceptors in non-mammalian vertebrates. To see the diversity of localization of extraretinal photoreceptors in lower vertebrates having different habitats, we analyzed the opsin-like immunoreactivities in anuran amphibians, Xenopus laevis, Rana catesbeiana, Rana nigromaculata, and Bufo japonicus. An antiserum (toad Rh-AS) was raised against rhodopsin purified from the retinas of Japanese toad, B. japonicus. In the retina of all the anurans examined, the outer segments of rods were immunopositive to toad Rh-AS. The outer segments of most pinealocytes were immunopositive in R. catesbeiana, R. nigromaculata, and B. japonicus. The outer segments of photoreceptor-like cells within the frontal organ of R. nigromaculata were immunostained. Interestingly, toad Rh-AS immunostained many secretory cells of mucous glands in the head skin of B. japonicus, implying the presence of a novel photoreceptive molecule. Within the hypothalamus, toad Rh-AS immunostained many cells in the magnocellular preoptic nucleus of R. catesbeiana and B. japonicus. Toad Rh-AS also labeled cerebrospinal fluid (CSF)-contacting cells in the anterior preoptic nucleus of R. nigromaculata and those adjacent to the lateral ventricle within the septum of R. catesbeiana. Thus the distribution patterns of the rhodopsin-like immunoreactivities among the anurans were highly diverged, and there was no relationship between the distribution patterns and their habitats. J. Exp. Zool. 286:136-142, 2000.

Extraretinal photoreceptors at the compound eye's posterior margin in Drosophila melanogaster

Many invertebrates have supplementary extraocular photoreceptors that often are implicated in circadian rhythms. An extraretinal group of candidate photoreceptors in the fruit fly, Drosophila melanogaster, has been revealed previously at the posterior margin of the compound eye by using a photoreceptor-specific monoclonal antibody (Hofbauer and Buchner [1989] Naturwissen 76:335-336), but it never has been characterized. Here, we report the fine structure of this cell cluster reported by Hofbauer and Buchner, which is called "eyelet," as well as the further candidacy of their visual pigment and neurotransmitter. Eyelet forms a specialized, pigmented organ with cells that have numerous microvilli arranged into coherent rhabdomeres. The presence of rhabdomeric microvilli is a defining feature of a photoreceptor, reported here for the first time in eyelet. The rhabdomeres exhibit Rh6 opsin-like immunoreactivity, which provides evidence that the photoreceptors are functional: they fail to immunostain with antibodies against NINAE (Rh1), Rh4, or Rh5. The photoreceptors have been shown previously to exhibit histamine-like immunoreactivity, but they also stain with a monoclonal antiserum raised against Drosophila choline acetyltransferase (ChAT), suggesting that the photoreceptors not only may contain histamine but also can synthesize acetylcholine. A ChAT-immunoreactive axon bundle originating from eyelet terminates in the cortex of the anterior medulla. This bundle also is seen with reduced silver stains. Electron microscopic examination revealed four axon profiles of similar size in this bundle, indicating that eyelet contains at least four photoreceptors. The pathway of eyelet's axon bundle coincides with the precocious pathway of Bolwig's nerve that arises from the larval organ of sight. The origin and possible function of eyelet are discussed.

Visual pigments and oil droplets in the retina of a passerine bird, the canary Serinus canaria: microspectrophotometry and opsin sequences

The visual receptors of the passeriform bird Serinus canaria, the canary, have been examined microspectrophotometrically and the sequences of the opsins determined. Rods have a maximum absorbance (lambda max) at 506 nm. Four spectral classes of single cone are present: long-wave-sensitive (LWS) containing a photopigment with lambda max at 569 nm, middle-wave-sensitive (MWS) with lambda max at 505 nm, short-wave-sensitive (SWS) with lambda max at 442 nm, and ultraviolet-sensitive (UVS) with lambda max at about 366 nm. Double cones possess the 569-nm pigment in both members. Typical combinations of photopigment and oil droplet occur in most cone classes. An ambiguity exists in the oil droplet of the single LWS cones. In some birds, LWS cones are paired with an R-type droplet, whereas in the majority of canaries the LWS pigment is paired with a droplet similar to the P-type of double cones. Mechanisms of spectral tuning within each opsin class are discussed.

Encephalopsin: a novel mammalian extraretinal opsin discretely localized in the brain

We have identified a mammalian opsin, encephalopsin, that shows strong and specific expression in the brain. Encephalopsin defines a new family of opsins and shows highest homology to vertebrate retinal and pineal opsins. Encephalopsin is highly expressed in the preoptic area and paraventricular nucleus of the hypothalamus, both regions implicated in encephalic photoreception in nonmammalian vertebrates. In addition, encephalopsin shows highly patterned expression in other regions of the brain, being enriched in selected regions of the cerebral cortex, cerebellar Purkinje cells, a subset of striatal neurons, selected thalamic nuclei, and a subset of interneurons in the ventral horn of the spinal cord. Rostrocaudal gradients of encephalopsin expression are present in the cortex, cerebellum, and striatum. Radial stripes of encephalopsin expression are seen in the cerebellum. In the cortex and cerebellum, encephalopsin expression is considerably higher and more highly patterned in the adult than in the neonate. Encephalopsin is the first putative extraocular opsin identified in mammals and may play a role in encephalic photoreception.