Retinomotor Movements Research Articles

Protein kinase A regulation of pigment granule motility in retinal pigment epithelial cells from fish, Lepomis spp.

Retinomotor movements include elongation and contraction of rod and cone photoreceptors, and mass migration of melanin-containing pigment granules (melanosomes) of the retinal pigment epithelium (RPE) within the eyes of fish, frogs, and other lower vertebrates. Eyes of these animals do not contain dilatable pupils; therefore the repositioning of the rods and cones and a moveable curtain of pigment granules serve to modulate light intensity within the eye. RPE from sunfish (Lepomis spp.) can be isolated from the eye and dissociated into single cells, allowing in vitro studies of the cytoskeletal and regulatory mechanisms of organelle movement. Pigment granule aggregation from distal tips of apical projections into the cell body can be triggered by the application of underivatized cAMP, and dispersion is effected by cAMP washout in the presence of dopamine. While the phenomenon of cAMP-dependent pigment granule aggregation in isolated RPE was described many years ago, whether cAMP acts through the canonical cAMP-PKA pathway to stimulate motility has never been demonstrated. Here, we show that pharmacological inhibition of PKA blocks pigment granule aggregation, and microinjection of protein kinase A catalytic subunit triggers pigment granule aggregation. Treatment with a cAMP agonist that activates the Rap GEF, Epac (Effector protein activated by cAMP), had no effect on pigment granule position. Taken together, these results confirm that cAMP activates RPE pigment granule motility by the canonical cAMP-PKA pathway. Isolated RPE cells labeled with antibodies against PKA RIIα and against PKA-phosphorylated serine/threonine amino acids show diffuse, punctate labeling throughout the RPE cell body and apical projections. Immunoblotting of RPE lysates using the anti-PKA substrate antibody demonstrated seven prominent bands; two bands in particular at 27 and 64kD showed increased levels of phosphorylation in the presence of cAMP, indicating their phosphorylation could contribute to the pigment granule aggregation mechanism.

The Role of Tyrosinase in the Time Course of Light‐Induced Cone Contraction in the Goldfish Retina

Dopamine is an important modulator of light‐adaptation in the vertebrate retina. Many teleosts regulate the amount of light incident on the retina through retinomotor movements (RMMs), that include changes in cone and rod length. In the dark, cones elongate and rods contract. In the light, rods elongate and cones contract. It is thought that RMMs are regulated by the release of dopamine in the light from tyrosine hydoxylase‐containing interplexiform cells. However, RMMs are not lost when these dopaminergic retinal neurons are ablated by injection of the neurotoxin 6‐hydroxydopamine (6‐OHDA). A potential alternate source of dopamine in the retina is the retinal pigmented epithelium, due to the activity of tyrosinase.Adult goldfish (C. auratus) were treated with a specific inhibitor of tyrosinase, phenylthiourea (PTU), by placing them in tank water containing 0.2mM PTU for 48 hours. Control and fish treated with PTU were placed in dark for 1hr or light (>100 lux) for 1 hr, or after 1hr darkness then exposed to varying durations of light (1.5 – 30 min). Another set of fish were given intraocular injections of 50μg 6‐OHDA twice on the first 2 days of a 10‐day time course and then subjected to dark/light treatments in the presence or absence of PTU. Fish were sacrificed, eyes removed and fixed in glutaraldehyde. Retinas were isolated and sliced using a razor blade. Cones were labeled by anti‐zpr1 (ZIRC, Eugene, OR) and cone length determined using confocal microscopy. Differences between means (in all case n=10 eyes) were assessed for statistical significance using ANOVA and, when applicable, Tukey's multiple comparison test.In fully light‐ or dark‐adapted conditions, retinas treated with PTU, 6‐OHDA or 6‐OHDA + PTU were indistinguishable from controls, However, both PTU and 6‐OHDA treatment slowed light‐induced cone contraction over the first 15 min of light. This delay was even greater when 6‐OHDA and PTU were combined. In the dark cones were elongated (115 ± 7 μm) and, under control conditions (no PTU, no 6‐OHDA) reached 80% (66 ± 2 μm) of maximum contraction (55 ± 3 μm) after 7.5 min light exposure. In the presence of either PTU or after 6‐OHDA treatment, cone length at 7.5min was significantly (p<0.001) longer (75 ± 3 μm and 79 ± 2 μm, respectively) and was longer still when PTU and 6‐OHDA were combined (85 ± 1 μm).These results suggest that inhibition of tyrosinase (via PTU) or loss of neuronal dopamine (via 6‐OHDA) affects the early phase of light‐induced cone contraction. That neither treatment prevented full contraction may indicate that these treatments did not fully eliminate dopamine signaling in the retina, or that light‐induced cone contraction may also involve other mechanisms independent of dopamine.Support or Funding InformationThis work was supported by NSERC Discovery Grant awarded to WHB.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Cone myoid elongation involves unidirectional microtubule movement mediated by dynein-1.

Teleosts and amphibians exhibit retinomotor movements, morphological changes in photoreceptors regulated by light and circadian rhythms. Cone myoid elongation occurs during dark adaptation, leading to the positioning of the cone outer segment closer to the retinal pigment epithelium. Although it has been shown that microtubules are essential for cone myoid elongation, the underlying mechanism has not been established. In this work, we generated a transgenic line of zebrafish expressing a photoconvertible form of α-tubulin (tdEOS-tubulin) specifically in cone photoreceptors. Using superresolution structured illumination microscopy in conjunction with both pharmacological and genetic manipulation, we show that cytoplasmic dynein-1, which localizes to the junction between the ellipsoid and myoid, functions to shuttle microtubules from the ellipsoid into the myoid during the course of myoid elongation. We propose a novel model by which stationary complexes of cytoplasmic dynein-1 are responsible for the shuttling of microtubules between the ellipsoid and myoid is the underlying force for the morphological change of myoid elongation.

Circadian Rhythms of Retinomotor Movement in a Marine Megapredator, the Atlantic Tarpon, Megalops atlanticus.

Many ecologically and economically important marine fish species worldwide spend portions of their lives in coastal regions that are increasingly inundated by artificial light at night. However, while extensive research illustrates the harmful effects of inappropriate light exposure on biological timing in humans, rodents and birds, comparable studies on marine fish are virtually nonexistent. This study aimed to assess the effects of light on biological clock function in the marine fish retina using the Atlantic tarpon (Megalops atlanticus) as a model. Using anti-opsin immunofluorescence, we observed robust rhythms of photoreceptor outer segment position (retinomotor movement) over the course of the daily light–dark cycle: cone outer segments were contracted toward the inner retina and rods were elongated during the day; the opposite occurred at night. Phase shifting the daily light–dark cycle caused a corresponding shift of retinomotor movement timing, and cone retinomotor movement persisted in constant darkness, indicating control by a circadian clock. Constant light abolished retinomotor movements of both photoreceptor types. Thus, abnormally-timed light exposure may disrupt normal M. atlanticus clock function and harm vision, which in turn may affect prey capture and predator avoidance. These results should help inform efforts to mitigate the effects of coastal light pollution on organisms in marine ecosystems.

Species-specific development of retinal architecture in elopomorph fishes: adaptations for harvesting light in the dark

Elopomorph fishes are distinguished in part by a shared leptocephalus larval form. These glass-clear, ribbon-like larvae are remarkably similar across the elopomorpha, yet they mature to radically different forms as adults. Along the timecourse of development, they transition through a variety of spectrally distinct habitats, occupying very different temporal niches as adults - some diurnal predators in high-light environments, others crepuscular, and others nocturnal. We found that in concert with these changes, the retinas of a variety of elopomorph fish change in similarly dramatic ways over the course of development. While the mature bonefish [Albula vulpes (Linnaeus,1758)] retina is specialized for visual tasks in a high-light environment, the Atlantic tarpon (Megalops atlanticus Valenciennes, 1847) and ladyfish (Elops saurus Linnaeus,1766) exhibit multiple specializations for function at night or otherwise in very dim light conditions. These include stacked rod photoreceptors that are gathered into massive bundles, retinomotor movement of photoreceptor outer segments, and a highly reflective tapetum. In the adult form, the speckled worm eel (Myrophis punctatus Lutken, 1852) maintains a retina with nearly all rods. The dramatic divergence among taxa over the course of development produces species with distinctly specialized visual capabilities. Moreover, the ability to change over the course of development may underlie the capacity for resilience in the face of anthropogenic insults, including light pollution in increasingly developed coastal zones. On the other hand, if light history can drive retinal change, exposure to artificial light at night may be detrimental to the survival of individuals that move between light-polluted and naturally dark locations.

Myosin 7aa−/− mutant zebrafish show mild photoreceptor degeneration and reduced electroretinographic responses

Mutations in myosin VIIa (MYO7A) cause Usher Syndrome 1B (USH1B), a disease characterized by the combination of sensorineural hearing loss and visual impairment termed retinitis pigmentosa (RP). Although the shaker-1 mouse model of USH1B exists, only minor defects in the retina have been observed during its lifespan. Previous studies of the zebrafish mariner mutant, which also carries a mutation in myo7aa, revealed balance and hearing defects in the mutants but the retinal phenotype has not been described. We found elevated cell death in the outer nuclear layer (ONL) of myo7aa−/− mutants. While myo7aa−/− mutants retained visual behaviors in the optokinetic reflex (OKR) assay, electroretinogram (ERG) recordings revealed a significant decrease in both a- and b-wave amplitudes in mutant animals, but not a change in ERG threshold sensitivity. Immunohistochemistry showed mislocalization of rod and blue cone opsins and reduced expression of rod-specific markers in the myo7aa−/− ONL, providing further evidence that the photoreceptor degeneration observed represents the initial stages of the RP. Further, constant light exposure resulted in widespread photoreceptor degeneration and the appearance of large holes in the retinal pigment epithelium (RPE). No differences were observed in the retinomotor movements of the photoreceptors or in melanosome migration within the RPE, suggesting that myo7aa−/− does not function in these processes in teleosts. These results indicate that the zebrafish myo7aa−/− mutant is a useful animal model for the RP seen in humans with USH1B.

Expression and Localization of Multidrug Resistance Protein 4 (MRP4) in the Retinal Tissue of Danio rerio

The retina of Danio rerio (zebrafish) exhibits mechanisms for light and dark adaptation, including morphological changes in photoreceptors and retinal pigment epithelium (RPE). Regulation of dark adaptive retinomotor movements are associated with elevated cAMP levels. The aim of this study is to characterize the spatial and temporal patterns of expression of MRP4, which may transport cAMP, in zebrafish retinal tissue. We hypothesize that MRP4 is localized in the inner segments of the photoreceptors cells, where it could regulate cAMP levels by exporting it into the subretinal space. Once there, cAMP can be taken up by RPE cells to induce and maintain the dark adapted state. In this study, we observed prominent labeling in dark adapted retinal sections using immunohistochemistry. By means of RT‐PCR, we amplified a fragment of the expected size for mRNA encoding MRP4. Furthermore, using qPCR we demonstrated abundant target nucleic acid in samples from various time points of both light and dark adapted retinas. Our observations are consistent with a role for MRP4 in exporting cyclic nucleotides when cytoplasmic levels rise under dark conditions.

A latitudinal cline in the efficacy of endogenous signals: evidence derived from retinal cone contraction in fish

Like many physiological systems synchronised to the light:dark cycle, retinomotor movements in 'lower' vertebrates are controlled by both the ambient illumination and input from endogenous circadian oscillators. In the present study, we examine the relative influence of these two signals in various species of teleost fish with different latitudes of origin. We find equatorial species show very strong endogenous control. The cones of the glowlight tetra, for example, continue to go through undiminished cycles of contraction and relaxation that mirror the previous light:dark cycle for at least two weeks in continual darkness. To quantify the relative effectiveness of the ambient light compared with endogenous signals in causing cone contraction, the degree to which seven teleost species responded to light during the dark phase of their light:dark cycle was examined. In this situation the retina receives conflicting instructions; while the light is acting directly to cause light adaptation, any endogenous signal tends to keep the retinal elements dark adapted. The further from the equator a species originated, the more its cones contracted in response to such illumination, suggesting animals from higher latitudes make little use of endogenous oscillators and rely more on ambient illumination to control behaviours. Equatorial species, however, rely on internal pacemakers to a much greater degree and are relatively insensitive to exogenous light signals. Because these data are consistent with published observations in systems as diverse as melatonin synthesis in Arctic reindeer and the behaviour of regional populations of Drosophila, latitudinal clines in the efficacy of circadian oscillators may be a common feature among animals.

Rhythmic Expression of MicroRNA-26a Regulates the L-type Voltage-gated Calcium Channel α1C Subunit in Chicken Cone Photoreceptors

MicroRNAs (miRNAs) modulate gene expression by degrading or inhibiting translation of messenger RNAs (mRNAs). Here, we demonstrated that chicken microRNA-26a (gga-mir-26a) is a key posttranscriptional regulator of photoreceptor L-type voltage-gated calcium channel alpha1C subunit (L-VGCCalpha1C) expression, and its own expression has a diurnal rhythm, thereby explaining the rhythmic nature of L-VGCCalpha1Cs. Circadian oscillators in retinal photoreceptors provide a mechanism that allows photoreceptors to anticipate daily illumination changes. In photoreceptors, L-VGCC activities are under circadian control, which are higher at night and lower during the day. Interestingly, the mRNA level of VGCCalpha1D oscillates, but those for VGCCalpha1C do not. However, the protein expression of both VGCCalpha1C and alpha1D are higher at night in cone photoreceptors. The underlying mechanism regulating L-VGCCalpha1C protein expression was not clear until now. In vitro targeting reporter assays verified that gga-mir-26a specifically targeted the L-VGCCalpha1C 3'-untranslated region, and gga-mir-26a expression in the retina peaked during the day. After transfection with gga-mir-26a, L-VGCCalpha1C protein expression and L-VGCC current density decreased. Therefore, the rhythmic expression of gga-mir-26a regulated the protein expression of the L-VGCCalpha1C subunit. Additionally, both CLOCK (circadian locomoter output cycles kaput) and CREB (cAMP-response element-binding protein-1) activated gga-mir-26a expression in vitro. This result implies that gga-mir-26a might be a downstream target of circadian oscillators. Our work has uncovered new functional roles for miRNAs in the regulation of circadian rhythms in cone photoreceptors. Circadian regulated miRNAs could serve as the link between the core oscillator and output signaling that further govern biological functions.

CLOCK Is Required for Maintaining the Circadian Rhythms of Opsin mRNA Expression in Photoreceptor Cells

In zebrafish, the expression of long-wavelength cone (LC) opsin mRNA fluctuated rhythmically between the day and night. In a 24-h period, expression was high in the afternoon and low in the early morning. This pattern of fluctuation persisted in zebrafish that were kept in constant darkness, suggesting an involvement of circadian clocks. Functional expression of Clock, a circadian clock gene that contributes to the central circadian pacemaker, was found to play an important role in maintaining the circadian rhythms of LC opsin mRNA expression. In zebrafish embryos, in which the translation of Clock was inhibited by anti-Clock morpholinos, the circadian rhythms of LC opsin mRNA expression diminished. CLOCK may regulate the circadian rhythms of LC opsin mRNA expression via cyclic adenosine monophosphate (cAMP)-dependent signaling pathways. In control retinas, the concentration of cAMP was high in the early morning and low in the remainder of the day and night. Inhibition of Clock translation abolished the fluctuation in the concentration of cAMP, thereby diminishing the circadian rhythms of opsin mRNA expression. Transient increase of cAMP concentrations in the early morning (i.e. by treating the embryos with 8-bromo-cAMP) restored the circadian rhythms of LC opsin mRNA expression in morpholino-treated embryos. Together, the data suggest that Clock plays important roles in regulating the circadian rhythms in photoreceptor cells.

Dim light vision – Morphological and functional adaptations of the eye of the mormyrid fish, Gnathonemus petersii

The African weakly electric fish Gnathonemus petersii is well known for its electrosensory capabilities. These animals can detect and distinguish objects through active electrolocation in complete darkness. Because of their nocturnal lifestyle, a low contribution of vision for orientation and object detection has been expected. However, as we show in this review, the retina of G. petersii is highly specialized with hundreds of rods and tens of cones grouped together in bundles in a complex way, ensheathed by a tapetum lucidum. The structure of the bundles goes beyond what would be expected if only photon catch was supposed to be increased. During daytime, the structure of these “macro-receptors” changes dramatically depending on retinomotor movements. During the day, the rods and cones are located in different compartments of the bundle, separated by a narrow canal in the form of a “bottle neck”. Investigations on cell structure and neurochemistry in the retina indicate a general organization that is simpler in terms of bipolar and ganglion cell diversity than in tetrachromatic species such as goldfish, yet similar in terms of neurochemical differentiation of amacrine cells. In both respects, the inner retina of the elephantnose fish bears the greatest similarity to catfish and some deep-sea fish retinae. Neuronal circuits and bundle structure give hints of possible adaptations for contrast and/or movement detection. Behavioral experiments suggest that, in contrast to the vision specialists Lepomis gibbosus, pattern detection of G. petersii is not affected by higher spatial frequencies. A pattern of low spatial frequencies, however, was equally well detected by G. petersii and L. gibbosus. Optomotor response experiments indicate that motion vision is important for Gnathonemus, narrowing down the search for the functional specialization of the Gnathonemus retina and providing a starting point for work on multisensory integration in these fish.

Expression of Modd muscarinic acetylcholine receptors in the retinas of bluegill and zebra fish

Light adaptation in most vertebrates occurs through adjustments in pupil diameter. Teleosts, however, have a fixed pupil size, so they rely on retinomotor movements to regulate the amount of light incident on the photoreceptors. Retinomotor movements include photoreceptor contraction and elongation as well as the aggregation and dispersion of membrane bound pigment granules located in the retinal pigment epithelium. Previous work has shown that pigment granule dispersion can be induced using the neurotransmitter acetylcholine acting via muscarinic receptors and that dispersion can be inhibited by Modd receptor specific antagonists. In this study we demonstrate Modd receptor expression using immunolabeling with an anti M3 antibody and with in situ hybridization using probes that target the M5 receptor. Labeling with the M3 antibody was seen multiple layers of the bluegill retina and was supported by Western blot analysis using bluegill and zebrafish tissue. In situ hybridization of the M5 sequence indicated a similar distribution of the receptor as that seen with the antibody. These results suggest that multiple muscarinic receptor subtypes are expressed in the fish retina, and lay the foundation for future studies to elucidate the role of muscarinic signaling pathways in the retina, especially as they pertain to light adaptation.

Retinomotor movements in the Antarctic fish Trematomus newnesi Boulenger submitted to different environmental light conditions

The Antarctic fish Trematomus newnesi (Boulenger, 1902) occurs from benthic to pelagic habitats, in seasonally and daily varied photic conditions that induce retinomotor movements. Fish were experimentally kept under constant darkness or light, and 12Light/12Dark for seven days. The retinomotor movement of the pigment epithelium was established through the pigment index, while that of the cones was calculated as the length of the myoid. The retinomotor movement of the pigment epithelium in T.newnesi,revealed that the adaptation to constant light occurred in the one hour of exposure, remaining constant for the next seven days. However, the adaptation to constant darkness, was slower. The difference between the mean values of the pigment indices in the time intervals of sampling was significant in the first hours of the experiment, and only after six hours they were not significant any more. The myoid of cones became elongated in darkness and contracted in light. In the experiments where T.newnesiwas exposed initially to 12 hours light followed by 12 hours darkness 12 was evidenced that the speed and intensity of the retinomotor movements was higher when darkness changed into light, than when light changed into darkness.

Time course and development of light adaptation processes in the outer zebrafish retina

Retinomotor movements are morphological changes in the outer retina in response to changing light conditions. They can be separated into two components: Migration of pigment granules within the microvilli of the retinal pigment epithelium (RPE) and positional changes in photoreceptor cells. These positional changes optimize exposure of the cone and rod photoreceptors to light. The aim of this study was to analyze both the time course of retinomotor movements in the adult zebrafish and the maturation of these processes in the developing fish. We show that retinomotor movements are used as a dark/light adaptation mechanism in zebrafish. In adult zebrafish, melanin granules of the RPE migrate with constant speed and reach the fully light adapted (LA) state approximately after 1 h. In contrast, about two thirds of double cone outer segment movements are finished in 5 min, and are fully completed in 10 to 20 min. During development there are three crucial stages leading to mature retinomotor movements in response to light: at 5 dpf (days post fertilization) the migration of pigment granules begins, at 20 dpf the pigment granules condense in the apical part of the RPE microvilli, and at 28 dpf, concomitant with the functional maturation of rods, the double cones contract as in adult retinas.

Diurnal and circadian retinomotor movements in zebrafish

Key indicators of circadian regulation include the persistence of physiological rhythmicity in the absence of environmental time cues and entrainment of this rhythmicity by the ambient light cycle. In some teleosts, the inner segments of rod and cone photoreceptors contract and elongate according to changes in ambient lighting and the circadian cycle. Pigment granules in the retinal pigment epithelium (RPE) disperse and aggregate in a similar manner. Collectively, these movements are known as retinomotor movements. We report the histological characterization of diurnal and circadian retinomotor movements in zebrafish, Danio rerio. Adult fish subjected to a 14:10 light:dark (LD) cycle, constant darkness (DD), or constant light (LL) were sacrificed at 1-13 h intervals and processed for semithin sectioning of the retina. Using bright-field microscopy, 15 measurements of pigment granule position and the inner segment lengths of 30 rods and 30-45 cones were collected from the central third of the dorso-optic retina per time point. In LD, rods and cones followed a clear diurnal rhythm in their inner segment movements. Short-single, UV-sensitive cones were found to contract significantly 1 h before light onset in LD conditions. In DD conditions, the inner segments movements of short-single and double cones displayed statistically significant rhythms. RPE pigment granule movements are rhythmically regulated in both LD and DD although fluctuations are damped in the absence of photic cues. No significant retinomotor movements were observed in LL. These findings indicate retinomotor movements in zebrafish are differentially regulated by an endogenous oscillator and by light-dependent mechanisms.

The retinal pigment epithelium of the teleost Notopterus notopterus (Pallas): appearance of basal infoldings during prolonged dark-adaptation.

In teleosts, the basal part of the retinal pigment epithelium (RPE) is relatively smooth, i. e., it is free of basal membrane infoldings. In the featherback, Notopterus notopterus, whereas this is the situation in light adaptation, during dark-adaptation, especially when kept for prolonged periods (6-9 hour), numerous infoldings appear at the basal region, as found uniquely by transmission electron microscopy. In this teleost, during retinomotor movements, the rods move vitreally during dark-adaptation, while the cones do not elongate, and remain stationary in both light- and dark-adaptation. The significance of the appearance of basal infoldings in dark-adapted RPE is explained in terms of the pattern of retinomotor responses and the features of RPE and photoreceptors in this species. It is suggested that (1) the thick, impervious tapetal layer present in the RPE, (2) the unusual position of the photoreceptors in the visual cell layer of dark-adapted retina, and (3) the presumably high demand for glucose and O2 of the outer retina during dark-adaptation might contribute to cause this phenomenon in this species. The available evidence tend to associate this phenomenon with the involvement of the RPE in nutrient and O2 delivery to the photoreceptors via the basal infoldings of the RPE in dark-adapted state in this species. This has not been reported for any other teleosts to date.

Nonvisual photoreceptors of the deep brain, pineal organs and retina.

The role of the nonvisual photoreception is to synchronise periodic functions of living organisms to the environmental light periods in order to help survival of various species in different biotopes. In vertebrates, the so-called deep brain (septal and hypothalamic) photoreceptors, the pineal organs (pineal- and parapineal organs, frontal- and parietal eye) and the retina (of the "lateral" eye) are involved in the light-based entrain of endogenous circadian clocks present in various organs. In humans, photoperiodicity was studied in connection with sleep disturbances in shift work, seasonal depression, and in jet-lag of transmeridional travellers. In the present review, experimental and molecular aspects are discussed, focusing on the histological and histochemical basis of the function of nonvisual photoreceptors. We also offer a view about functional changes of these photoreceptors during pre- and postnatal development as well as about its possible evolution. Our scope in some points is different from the generally accepted views on the nonvisual photoreceptive systems. The deep brain photoreceptors are hypothalamic and septal nuclei of the periventricular cerebrospinal fluid (CSF)-contacting neuronal system. Already present in the lancelet and representing the most ancient type of vertebrate nerve cells ("protoneurons"), CSF-contacting neurons are sensory-type cells sitting in the wall of the brain ventricles that send a ciliated dendritic process into the CSF. Various opsins and other members of the phototransduction cascade have been demonstrated in telencephalic and hypothalamic groups of these neurons. In all species examined so far, deep brain photoreceptors play a role in the circadian and circannual regulation of periodic functions. Mainly called pineal "glands" in the last decades, the pineal organs actually represent a differentiated form of encephalic photoreceptors. Supposed to be intra- and extracranially outgrown groups of deep brain photoreceptors, pineal organs also contain neurons and glial elements. Extracranial pineal organs of submammalians are cone-dominated photoreceptors sensitive to different wavelengths of light, while intracranial pineal organs predominantly contain rod-like photoreceptor cells and thus scotopic light receptors. Vitamin B-based light-sensitive cryptochromes localized immunocytochemically in some pineal cells may take part in both the photoreception and the pacemaker function of the pineal organ. In spite of expressing phototransduction cascade molecules and forming outer segment-like cilia in some species, the mammalian pineal is considered by most of the authors as a light-insensitive organ. Expression of phototransduction cascade molecules, predominantly in young animals, is a photoreceptor-like characteristic of pinealocytes in higher vertebrates that may contribute to a light-percepting task in the perinatal entrainment of rhythmic functions. In adult mammals, adrenergic nerves--mediating daily fluctuation of sympathetic activity rather than retinal light information as generally supposed--may sustain circadian periodicity already entrained by light perinatally. Altogether three phases were supposed to exist in pineal entrainment of internal pacemakers: an embryological synchronization by light and in viviparous vertebrates by maternal effects (1); a light-based, postnatal entrainment (2); and in adults, a maintenance of periodicity by daily sympathetic rhythm of the hypothalamus. In addition to its visual function, the lateral eye retina performs a nonvisual task. Nonvisual retinal light perception primarily entrains genetically-determined periodicity, such as rod-cone dominance, EEG rhythms or retinomotor movements. It also influences the suprachiasmatic nucleus, the primary pacemaker of the brain. As neither rods nor cones seem to represent the nonvisual retinal photoreceptors, the presence of additional photoreceptors has been supposed. Cryptochrome 1, a photosensitive molecule identified in retinal nerve cells and in a subpopulation of retinal photoreceptors, is a good candidate for the nonvisual photoreceptor molecule as well as for a member of pacemaker molecules in the retina. When comparing various visual and nonvisual photoreceptors, transitory, "semi visual" (directional) light-perceptive cells can be detected among them, such as those in the parietal eye of reptiles. Measuring diffuse light intensity of the environment, semivisual photoreceptors also possess some directional light perceptive capacity aided by complementary lens-like structures, and screening pigment cells. Semivisual photoreception in aquatic animals may serve for identifying environmental areas of suitable illumination, or in poikilotermic terrestrial species for measuring direct solar irradiation for thermoregulation. As directional photoreceptors were identified among nonvisual light perceptive cells in the lancelet, but eyes are lacking, an early appearance of semivisual function, prior to a visual one (nonvisual --> semivisual --> visual?) in the vertebrate evolution was supposed.

Outer retinal fine structure of the garfish Belone belone (L.) (Belonidae, Teleostei) during light and dark adaptation – photoreceptors, cone patterns and densities

AbstractIn the present EM study, we investigate the retina of Belone belone, a visually‐orientated marine predator living close to the water surface. In the duplex retina, four morphologically different cone types are observed: unequal and equal double cones, long single cones and triple cones. In the light‐adapted state, five different cone patterns occur: row, twisted row, square, pentagonal and hexagonal patterns. High double cone densities are found ventro‐nasally, ventro‐temporally and dorso‐temporally. Throughout the retina the double cone/single cone ratio is 2 : 1, in the ventral part, however, a 1 : 1 ratio occurs. In the vitreous body we found a curtain‐like intraocular septum dividing the retina into two morphologically different regions. In most areas of the dark‐adapted retina the cone patterns are absent at the ellipsoid level, with long single cones standing more vitreally in the light path than double cones. The mosaics are retained, however, in the outer nuclear layer. Typical dark adaptation, i.e. the retinomotor movements of the retinal pigment epithelium and photoreceptors in response to the dark adaptation (light change) is not present in the peripheral ventral and parts of the central ventral area. In both regions we found a twisted row pattern of cones having a vitreal position. The findings are discussed with respect to the photic habitat and feeding habits of this species.

Chapter 34 Light and circadian regulation of retinomotor movement

Chapter 34 Light and circadian regulation of retinomotor movement

Taurine blocks spontaneous cone contraction but not horizontal cell dark suppression in isolated goldfish retina.

The objective of this study was to investigate the effects of taurine on cone retinomotor movements and the responses of cone-driven horizontal cells in dark-adapted teleost retina. In isolated goldfish retina preparations maintained in the dark, cones spontaneously contracted, and the responses of horizontal cells were suppressed. Addition of 5 mM taurine to the physiological solution blocked the spontaneous contraction of cones in the dark but did not block the dark-suppression of horizontal cell responses. These results indicate that the mechanism that leads to horizontal cell dark suppression is not sensitive to taurine. Although both cone retinomotor position and horizontal cell responsiveness are known to be modulated by dopamine, the present results do not support the hypothesis that taurine inhibits dopamine release in the dark because only spontaneous cone contraction was affected by taurine. These results also indicate that spontaneous cone contraction in the dark is not the cause of horizontal cell dark suppression because, in the presence of taurine, cones were elongated yet horizontal cell responses were still suppressed. Consequently, these results make it clear that horizontal cell dark suppression is not an artifact produced by incubating isolated teleost retina preparations in taurine-free physiological solution.