Chapter 3 Retinomotor movements of photoreceptors and retinal pigment epithelium: Mechanisms and regulation

Photoreceptor calyceal processes accompany the developing outer segment, adopting a stable length despite a dynamic core

Vertebrate photoreceptors detect light through a large cilium-based outer segment, which is filled with photopigment-laden membranous discs. Surrounding the base of the outer segment are microvilli-like calyceal processes (CPs). Although CP disruption has been associated with altered outer segment morphology and photoreceptor degeneration, the role of the CPs remains elusive. Here, we used zebrafish as a model to characterize CPs. We quantified CP parameters and report a strong disparity in outer segment coverage between photoreceptor subtypes. CP length is stable across light and dark conditions, yet heat-shock inducible expression of tagged actin revealed rapid turnover of the CP actin core. Detailed imaging of the embryonic retina uncovered substantial remodeling of the developing photoreceptor apical surface, including a transition from dynamic tangential processes to vertically oriented CPs immediately prior to outer segment formation. Remarkably, we also found a direct connection between apical extensions of the Müller glia and retinal pigment epithelium, arranged as bundles around the ultraviolet sensitive cones. In summary, our data characterize the structure, development and surrounding environment of photoreceptor microvilli in the zebrafish retina.

The expression, localisation and interactome of pigeon CRY2

Cryptochromes (CRY) are highly conserved signalling molecules that regulate circadian rhythms and are candidate radical pair based magnetoreceptors. Birds have at least four cryptochromes (CRY1a, CRY1b, CRY2, and CRY4), but few studies have interrogated their function. Here we investigate the expression, localisation and interactome of clCRY2 in the pigeon retina. We report that clCRY2 has two distinct transcript variants, clCRY2a, and a previously unreported splice isoform, clCRY2b which is larger in size. We show that clCRY2a mRNA is expressed in all retinal layers and clCRY2b is enriched in the inner and outer nuclear layer. To define the localisation and interaction network of clCRY2 we generated and validated a monoclonal antibody that detects both clCRY2 isoforms. Immunohistochemical studies revealed that clCRY2a/b is present in all retinal layers and is enriched in the outer limiting membrane and outer plexiform layer. Proteomic analysis showed clCRY2a/b interacts with typical circadian molecules (PER2, CLOCK, ARTNL), cell junction proteins (CTNNA1, CTNNA2) and components associated with the microtubule motor dynein (DYNC1LI2, DCTN1, DCTN2, DCTN3) within the retina. Collectively these data show that clCRY2 is a component of the avian circadian clock and unexpectedly associates with the microtubule cytoskeleton.

The pupillary light responses of animals; a review of their distribution, dynamics, mechanisms and functions

The timecourse and extent of changes in pupil area in response to light are reviewed in all classes of vertebrate and cephalopods. Although the speed and extent of these responses vary, most species, except the majority of teleost fish, show extensive changes in pupil area related to light exposure. The neuromuscular pathways underlying light-evoked pupil constriction are described and found to be relatively conserved, although the precise autonomic mechanisms differ somewhat between species. In mammals, illumination of only one eye is known to cause constriction in the unilluminated pupil. Such consensual responses occur widely in other animals too, and their function and relation to decussation of the visual pathway is considered. Intrinsic photosensitivity of the iris muscles has long been known in amphibia, but is in fact widespread in other animals. The functions of changes in pupil area are considered. In the majority of species, changes in pupil area serve to balance the conflicting demands of high spatial acuity and increased sensitivity in different light levels. In the few teleosts in which pupil movements occur they do not serve a visual function but play a role in camouflaging the eye of bottom-dwelling species. The occurrence and functions of the light-independent changes in pupil size displayed by many animals are also considered. Finally, the significance of the variations in pupil shape, ranging from circular to various orientations of slits, ovals, and other shapes, is discussed.

Differences in lens optical plasticity in two gadoid fishes meeting in the Arctic

Arctic and boreal/temperate species are likely to be evolutionary adapted to different light regimes. Currently, the boreal/temperate Atlantic cod (Gadus morhua) is coexisting with the native polar cod (Boreogadus saida) in the Arctic waters around Svalbard, Norway. Here, we studied light/dark adaptative optical plasticity of their eye lenses by exposing fish to bright light during the polar night. Schlieren photography, high-definition laser scanning and ray tracing were used to determine the optical properties of excised crystalline lenses. Both species have multifocal lenses, an optical adaptation for improved color vision. In polar cod, the optical properties of the lens were independent of light exposure. In the more southern Atlantic cod, the optical properties of the lens changed within hours upon exposure to light, even after months of darkness. Such fast optical adjustment has previously only been shown in a tropical cichlid. During the polar night the Atlantic cod lens seems to be unregulated and dysfunctional since it had an unsuitable focal length and severe spherical aberration. We present a system, to our knowledge unique, for studying visual plasticity on different timescales in relation to evolutionary history and present the first study on the polar cod visual system.

Ultrastructure of the distal retina of the adult zebrafish, Danio rerio

The organization, morphological characteristics, and synaptic structure of photoreceptors in the adult zebrafish retina were studied using light and electron microscopy. Adult photoreceptors show a typical ordered tier arrangement with rods easily distinguished from cones based on outer segment (OS) morphology. Both rods and cones contain mitochondria within the inner segments (IS), including the large, electron-dense megamitochondria previously described (Kim et al.) Four major ultrastructural differences were observed between zebrafish rods and cones: (1) the membranes of cone lamellar disks showed a wider variety of relationships to the plasma membrane than those of rods, (2) cone pedicles typically had multiple synaptic ribbons, while rod spherules had 1-2 ribbons, (3) synaptic ribbons in rod spherules were ∼2 times longer than ribbons in cone pedicles, and (4) rod spherules had a more electron-dense cytoplasm than cone pedicles. Examination of photoreceptor terminals identified four synaptic relationships at cone pedicles: (1) invaginating contacts postsynaptic to cone ribbons forming dyad, triad, and quadrad synapses, (2) presumed gap junctions connecting adjacent postsynaptic processes invaginating into cone terminals, (3) basal junctions away from synaptic ribbons, and (4) gap junctions between adjacent photoreceptor terminals. More vitread and slightly farther removed from photoreceptor terminals, extracellular microtubule-like structures were identified in association with presumed horizontal cell processes in the OPL. These findings, the first to document the ultrastructure of the distal retina in adult zebrafish, indicate that zebrafish photoreceptors have many characteristics similar to other species, further supporting the use of zebrafish as a model for the vertebrate visual system.

Adaptation in the optical properties of the crystalline lens in the eyes of the Lessepsian migrant Siganus rivulatus

Vision is an important source of information for many animals. The crystalline lens plays a central role in the visual pathway and hence the ecology of fishes. In this study, we tested whether the different light regimes in the Mediterranean and Red Seas have an effect on the optical properties of the lenses in the rivulated rabbitfish, Siganus rivulatus. This species has migrated through the Suez Canal from the Red Sea and established a vital population in the Mediterranean Sea. Longitudinal spherical aberration curves and focal lengths of the fish lenses were measured by laser scans and compared between the two populations. In addition, rivulated rabbitfish from the Mediterranean Sea were exposed to colored light (yellow, green and blue) and unfiltered light for periods of 1 or 13 days to test for short-term adjustments. Lens focal length was significantly longer (3%) in the Rea Sea population. The shorter focal length of the Mediterranean population can be explained as an adaptation to the dimmer light environment, as this difference makes the Mediterranean eyes 5% more sensitive than the eyes of the Red Sea population. The difference may be due to genetic differences or, more likely, adaptive developmental plasticity. Short-term regulatory mechanisms do not seem to be involved.

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.

A complex system of ligaments and a muscle keep the crystalline lens in place in the eyes of bony fishes (teleosts)

The suspension of the crystalline lens in the eye was studied in 11 species of teleost (bony fish) from 10 families and 7 orders by light and electron microscopy. In all species there were 4-5 ligaments in about the equatorial plane of the eye, in which also the tendon of the retractor lentis muscle attaches to the lens. In two cichlid species two additional ligaments were found running from the mid-posterior surface of the lens to the optic nerve head, where they attach to the falciform process. Lens suspension in teleosts is more complex than previously described and well-suited to firmly keep the heavy spherical lens in position for well-focused vision.

Multifocal lenses in coral reef fishes

The optical properties of crystalline lenses were studied in eleven species of coral reef fish from the Red Sea in Eilat, Israel. Three species each of diurnal planktivores, nocturnal planktivores and diurnal herbivores constituted three groups of animals with little within-group variability. In addition we studied two predators, which differed with respect to body size,prey preference, hunting method and diel activity period. All species studied have multifocal lenses. There were statistically significant differences in the optical properties of the lenses between the first three groups and between the predatory species. The properties of the lenses correlate well with known complements of visual pigments and feeding habits. Lenticular zones focusing ultraviolet light were found in two diurnal planktivores. The optical properties of the lens seem to be specifically adapted to the visual needs of each species.

Diurnal changes in exocytosis and the number of synaptic ribbons at active zones of an ON-type bipolar cell terminal

The number and morphology of synaptic ribbons at photoreceptor and bipolar cell terminals has been reported to change on a circadian cycle. Here we sought to determine whether this phenomenon exists at goldfish Mb-type bipolar cell terminals with the aim of exploring the role of ribbons in transmitter release. We examined the physiology and ultrastructure of this terminal around two time points: midday and midnight. Nystatin perforated-patch recordings of membrane capacitance (C(m)) revealed that synaptic vesicle exocytosis evoked by short depolarizations was reduced at night, even though Ca(2+) currents were larger. The efficiency of exocytosis (measured as the DeltaC(m) jump per total Ca(2+) charge influx) was thus significantly lower at night. The paired-pulse ratio remained unchanged, however, suggesting that release probability was not altered. Hence the decreased exocytosis likely reflects a smaller readily releasable vesicle pool at night. Electron microscopy of single sections from intact retinas averaged 65% fewer ribbons at night. Interestingly, the number of active zones did not change from day to night, only the probability of finding a ribbon at an active zone. Additionally, synaptic vesicle halos surrounding the ribbons were more completely filled at night when these on-type bipolar cells are more hyperpolarized. There was no change, however, in the physical dimensions of synaptic ribbons from day to night. These results suggest that the size of the readily releasable vesicle pool and the efficiency of exocytosis are reduced at night when fewer ribbons are present at bipolar cell terminal active zones.

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.

Nitric oxide release is induced by dopamine during illumination of the carp retina: serial neurochemical control of light adaptation

Several lines of indirect evidence have suggested that nitric oxide may play an important role during light adaptation of the vertebrate retina. We aimed to verify directly the effect of light on nitric oxide release in the isolated carp retina and to investigate the relationship between nitric oxide and dopamine, an established neuromodulator of retinal light adaptation. Using a biochemical nitric oxide assay, we found that steady or flicker light stimulation enhanced retinal nitric oxide production from a basal level. The metabotropic glutamate receptor agonist L-amino-4-phosphonobutyric acid, inhibited the light adaptation-induced nitric oxide production suggesting that the underlying cellular pathway involved centre-depolarizing bipolar cell activity. Application of exogenous dopamine to retinas in the dark significantly enhanced the basal production of nitric oxide and importantly, inhibition of endogenous dopaminergic activity completely suppressed the light-evoked nitric oxide release. The effect of dopamine was mediated through the D1 receptor subtype. Imaging of the nitric oxide-sensitive fluorescent indicator 4,5-diaminofluorescein di-acetate in retinal slices revealed that activation of D1 receptors resulted in nitric oxide production from two main spatial sources corresponding to the photoreceptor inner segment region and the inner nuclear layer. The results taken together would suggest that during the progression of retinal light adaptation there is a switch from dopaminergic to nitrergic control, probably to induce further neuromodulatory effects at higher levels of illumination and to enable more efficient spreading of the light adaptive signal.

Adaptive plasticity during the development of colour vision

Colour vision greatly enhances the discriminatory and cognitive capabilities of visual systems and is found in a great majority of vertebrates and many invertebrates. However, colour coding visual systems are confronted with the fact that the external stimuli are ambiguous because they are subject to constant variations of luminance and spectral composition. Furthermore, the transmittance of the ocular media, the spectral sensitivity of visual pigments and the ratio of spectral cone types are also variable. This results in a situation where there is no fixed relationship between a stimulus and a colour percept. Colour constancy has been identified as a powerful mechanism to deal with this set of problems; however, it is active only in a short-term time range. Changes covering longer periods of time require additional tuning mechanisms at the photoreceptor level or at postreceptoral stages of chromatic processing. We have used the trichromatic blue acara (Aequidens pulcher, Cichlidae) as a model system and studied retinal morphology and physiology, and visually evoked behaviour after rearing fish for 1-2 years under various conditions including near monochromatic lights (spectral deprivation) and two intensities of white light (controls). In general, long-term exposure to long wavelengths light had lesser effects than light of middle and short wavelengths. Within the cone photoreceptors, spectral deprivation did not change the absorption characteristics of the visual pigments. By contrast, the outer segment length of middle and long-wave-sensitive cones was markedly increased in the blue rearing group. Furthermore, in the same group, we observed a loss of 65% short-wave-sensitive cones after 2 years. These changes may be interpreted as manifestations of compensatory mechanisms aimed at restoring the balance between the chromatic channels. At the horizontal cell level, the connectivity between short-wave-sensitive cones and the H2 cone horizontal cells, and the spinule dynamics were both affected in the blue light group. This observation rules out the role of spinules as sites of chromatic feedback synapses. The light-evoked responses of H2 horizontal cells were also sensitive to spectral deprivation showing a shift of the neutral point towards short wavelengths in the blue rearing group. Interestingly, we also found an intensity effect because in the group reared in bright white light the neutral point was more towards longer wavelength than in the dim light group. Like the changes in the cones, the reactions of horizontal cells to spectral deprivation in the long wave domain can be characterised as compensatory. We also tested the spectral sensitivity of the various experimental groups of blue acara in visually evoked behaviour using the optomotor response paradigm. In this case, the changes in the relative spectral sensitivity were more complex and could not be explained by a simple extrapolation of the adaptive and compensatory processes in the outer retina. We conclude that the inner retina, and/or the optic tectum are also involved and react to the changes of the spectral environment. In summary, we have shown a considerable developmental plasticity in the colour vision system of the blue acara, where epigenetic adaptive processes at various levels of the visual system respond to the specific spectral composition of the surroundings and provide a powerful mechanism to ensure functional colour vision in different visual environments. We suggest that processes involving an active fine-tuning of the photoreceptors and the postreceptoral processing of chromatic information during ontogenetic development are a general feature of all colour vision systems. Such mechanisms would establish a functional balance between the various chromatic channels. This appears to be an essential condition for the cognitive systems to extract the relevant and stable information from the unstable and changing stimulus situation.

Circadian rhythm of Period1 clock gene expression in NOS amacrine cells of the mouse retina

The vertebrate retina contains self-sustained circadian clocks that broadly influence retinal physiology. In the present study, we have examined the relationship of nitric oxide, GABAergic and glycinergic inner retinal neurons with expression of a reporter for the circadian clock gene Period1 (Per1). Using Per1 : :GFP transgenic mice, we found that 72% of brain nitric oxide synthase (bNOS) expressing amacrine cells (NOS amacrine cells) sampled during the daytime were also immunoreactive for Per1-driven GFP. The number of bright GFP(+) NOS(+) cells was greater at Zeitgeber time (ZT) 10 than at 22, and this pattern persisted in retinas from animals which were placed in constant darkness [Circadian time (CT) 10 vs. 22]. Intensities of GFP-IR for individual NOS amacrine cells were analyzed at ZT4, 10, 16 and 22, with the peak value occurring at ZT10. Similar results were obtained from retinas sampled at CT4, 10, 16 and 22 in constant darkness, indicating that an endogenous circadian clock drives the transcription of the Per1 clock gene within NOS amacrine cells. The predominance of Per1 : :GFP(+) amacrine cells (82%), was immunoreactive to glutamate decarboxylase 65, but no Per1 : :GFP(+) amacrine cells colabeled with a glycine transporter 1 antibody. The results demonstrate circadian rhythms in Per1 promoter activation in nitric oxide (NO) and GABA secreting amacrine cells, and suggest that NO and GABA could be controlled by circadian clock mechanisms in the mammalian retina.

Inhibitory modulation of photoreceptor melatonin synthesis via a nitric oxide-mediated mechanism

Nitric oxide (NO) has been suggested to have many physiological functions in the vertebrate retina, including a role in light-adaptive processes. The aim of this study was to examine the influence of the NO-donor sodium nitroprusside (SNP) on the activity of arylalkylamine-N-acetyltransferase (AA-NAT; EC. 2.3.1.87), the activity of which responds to light and reflects the changes in retinal melatonin synthesis--a key feature of light-adaptive responses in photoreceptors. Incubation of dark-adapted and dark-maintained retinas with SNP lead to the NO-specific suppression of AA-NAT activity, with NO suppressing AA-NAT activity to a level similar to that seen in the presence of dopaminergic agonists or light. Increased levels of cGMP appeared to be causally involved in the suppression of AA-NAT activity by SNP, as non-hydrolysable analogues of cGMP and the cGMP-specific phosphodiesterase (PDE) inhibitor zaprinast also significantly suppressed AA-NAT activity, while an inhibitor of soluble guanylate cyclase blocked the effect of SNP. While this chain of events may not be part of the normal physiology of the retina, it could be important in pathological circumstances that are associated with marked increase in levels of cGMP, as is found to be the case in certain forms photoreceptor degeneration, which are produced by defects in cGMP phosphodiesterase activity.

Rearing in different photic and spectral environments changes the optomotor response to chromatic stimuli in the cichlid fish Aequidens pulcher

Developmental plasticity of spectral processing in vertebrates was investigated in fish by using an innate behavior, the optomotor response. Rearing blue acara (Aequidens pulcher; Cichlidae) under white lights of different intensities as well as deprivation of long wavelengths induced significant changes in the animals' responses to chromatic stimuli. Deprivation of short wavelengths had no effect. With this and previous studies on animals reared under similar conditions, we have demonstrated that developmental plasticity in spectral processing is present at a wide range of neural levels, spanning from photoreceptors to behavior. We hypothesize that earlier studies did not reveal such effects because of the rearing and testing conditions used.

Dopamine mediates circadian clock regulation of rod and cone input to fish retinal horizontal cells

A circadian (24-hour) clock regulates the light responses of fish cone horizontal cells, second order neurones in the retina that receive synaptic contact from cones and not from rods. Due to the action of the clock, cone horizontal cells are driven by cones in the day, but primarily driven by rods at night. We show here that dopamine, a retinal neurotransmitter, acts as a clock signal for the day by increasing cone input and decreasing rod input to cone horizontal cells. The amount of endogenous dopamine released from in vitro retinae was greater during the subjective day than the subjective night. Application of dopamine or quinpirole, a dopamine D(2)-like agonist, during the subjective night increased cone input and eliminated rod input to the cells, a state usually observed during the subjective day. In contrast, application of spiperone, a D(2)-like antagonist, or forskolin, an activator of adenylyl cyclase, during the subjective day reduced cone input and increased rod input. SCH23390, a D(1) antagonist, had no effect. Application of R(p)-cAMPS, an inhibitor of cAMP-dependent protein kinase, or octanol, an alcohol that uncouples gap junctions, during the night increased cone input and decreased rod input. Because D(2)-like receptors are on photoreceptor cells, but not horizontal cells, the results suggest that the clock-induced increase in dopamine release during the day activates D(2)-like receptors on photoreceptor cells. The resultant decrease in intracellular cyclic AMP and protein kinase A activation then mediates the increase in cone input and decrease in rod input.

For whales and seals the ocean is not blue: a visual pigment loss in marine mammals

Most terrestrial mammals have colour vision based on two spectrally different visual pigments located in two types of retinal cone photoreceptors, i.e. they are cone dichromats with long-to-middle-wave-sensitive (commonly green) L-cones and short-wave-sensitive (commonly blue) S-cones. With visual pigment-specific antibodies, we here demonstrate an absence of S-cones in the retinae of all whales and seals studied. The sample includes seven species of toothed whales (Odontoceti) and five species of marine carnivores (eared and earless seals). These marine mammals have only L-cones (cone monochromacy) and hence are essentially colour-blind. For comparison, the study also includes the wolf, ferret and European river otter (Carnivora) as well as the mouflon and pygmy hippopotamus (Artiodactyla), close terrestrial relatives of the seals and whales, respectively. These have a normal complement of S-cones and L-cones. The S-cone loss in marine species from two distant mammalian orders strongly argues for convergent evolution and an adaptive advantage of that trait in the marine visual environment. To us this suggests that the S-cones may have been lost in all whales and seals. However, as the spectral composition of light in clear ocean waters is increasingly blue-shifted with depth, an S-cone loss would seem particularly disadvantageous. We discuss some hypotheses to explain this paradox.

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.

A circadian clock regulates the pH of the fish retina

Although it is generally accepted that the acid/base ratio of tissue, as represented by the pH, is strictly regulated to maintain normal function, recent studies in the nervous system have shown that neuronal activity can result in significant shifts in pH. In the vertebrate retina, many cellular phenomena, including neuronal activity, are regulated by a circadian clock. We thus investigated whether a circadian clock regulates the pH of the retina. pH-sensitive microelectrodes were used to measure the extracellular pH of the in vitro goldfish retina superfused with a bicarbonate-based Ringer solution in the subjective day and night; that is, under conditions of constant darkness. These measurements demonstrated that a circadian clock regulates the pH of the vertebrate retina so that the pH is lower at night compared to the day. This day-night difference in retinal pH was observed at two different values of Ringer solution pH, indicating that the circadian phenomenon is independent of the superfusion conditions. The circadian-induced shift in pH was several times greater than light-induced pH changes and large enough to influence synaptic transmission between retinal neurons. These findings indicate that a circadian clock regulates the pH of the vertebrate retina. Thus, an intrinsic oscillator in neural tissue may modulate metabolic activity and pH as part of normal daily function.

Morphological changes in the retina of Aequidens pulcher (Cichlidae) after rearing in monochromatic light

We investigate the processing of chromatic information in the outer retina of a cichlid fish, Aequidens pulcher. The colour opponent response characteristics of some classes of cone-specific horizontal cells in the fish retina are the result of feedforward-feedback loops with cone photoreceptors. To interfere with the reciprocal transmissions of signals, animals were reared in monochromatic lights which preferentially stimulated the spectrally different cone types. Here we report the effects on the cones. Their absorbance spectra were largely unaffected, indicating no change in photopigment gene expression. Significant changes were observed in the cone outer segment lengths and the frequencies of spectral cone types. Quantum catch efficiency and survival of cones appear to be controlled in a spectrally selective way. Our results suggest that the retina responds to spectral deprivation in a compensatory fashion aimed at balancing the input from the different cone types to second order neurons.

Adenosine stimulates cone photoreceptor myoid elongation via an adenosine A2-like receptor

In several parts of the nervous system, adenosine has been shown to function as an extracellular neuromodulator binding to surface receptors on target cells. This study examines the possible role of adenosine in mediating light and circadian regulation of retinomotor movements in teleost cone photoreceptors. Teleost cones elongate in the dark and contract in the light. In continuous darkness, the cones continue to elongate and contract at subjective dusk and dawn in response to circadian signals. We report here that exogenous adenosine triggers elongation (the dark/night movement) in isolated cone inner segment-cone outer segment preparations (CIS-COS) in vitro. Agonist/antagonist potency profiles indicate that adenosine's effect on cone movement is mediated by an A2-like adenosine receptor, which like other A2 receptors enhances adenylate cyclase activity. Although closest to that expected for A2 receptors, the antagonist potency profile for CIS-COS does not correspond exactly to any known A2 receptor subtype, suggesting that the cone receptor may be a novel A2 subtype. Our findings are consistent with previous reports that retinal adenosine levels are higher in the dark, and further suggest that adenosine could act as a neuromodulatory "dark signal" influencing photoreceptor metabolism and function in the fish retina.

Interocular effect of actin depolymerization on spinule formation in teleost retina

Teleost retinas adapted to light show numerous spinules invaginated in the cone pedicles whereas darkness induces a reduction in the number of spinules. Horizontal cells show nematosomes whose size decreases as the number of spinules increases. We have investigated the involvement of actin filaments in spinule formation, by using cytochalasin D through intraocular injection into an eye. The ultrastructural analysis reveals that cytochalasin D impairs spinule formation and nematosome-size reduction in both treated and contralateral untreated retinas.

Regulation of the phosphorylation state of rhodopsin by dopamine

G protein-coupled receptors (GPCRs) are regulated by kinases and phosphatases that control their phosphorylation state. Here, the possibility that the state of GPCR phosphorylation could be affected by paracrine input was explored. We show that dopamine increased the rate of dephosphorylation of rhodopsin, the light receptor, in intact frog retinas. Further, we found that rod outer segments from dopamine-treated retinas contained increased rhodopsin phosphatase activity, indicating that this effect of dopamine on rhodopsin was mediated by stimulation of rhodopsin phosphatase. Dopamine is a ubiquitous neuromodulator and, in the retina, is released from the inner cell layers. Thus, our results identify a pathway for feedback regulation of rhodopsin from the inner retina and illustrate the involvement of dopamine in paracrine regulation of the sensitivity of a GPCR.

Solar pruning of retinal rods in albino rainbow trout

Morphology of the central retina and scotopic visual sensitivity were compared in juvenile albino and normally pigmented rainbow trout living under natural and reduced daylight. Outdoor albinos avoided exposing their eyes to direct sunlight, whereas normals were indifferent to it. After 4 months outdoors (approximately 10,000 lux in albinos, approximately 100,000 lux in normals), albinos had severely truncated or missing rod outer segments (ROS) and some missing rod ellipsoids, but normal numbers of photoreceptor nuclei and fully intact cones. Albino estimated ROS volume was only 7.1% of normal in July, but increased to 20% by the following February, mainly via an increase in numbers of ROS. However, in albinos moved indoors October 7 and exposed to 10-30 lux ambient daylight, both the number and length of ROS increased significantly, with estimated ROS volume reaching 95% of normal by 34 days. Albinos generally had more phagosomes (approximately 3 x normal) and more macrophages (approximately 2 x normal) in their outer retina. An optomotor reflex was used to define the effect of ROS volume on the ability to respond visually during dark adaptation. In July, albinos and normals from outdoor raceways (3 months) or indoor raceways (35 days) showed equal sensitivity after first being placed in darkness, but after 1 h in darkness, outdoor albinos with 6% of normal ROS volume were 2.0 log units less sensitive than indoor or outdoor normals, whereas indoor albinos with 53% of normal ROS volume were only 0.7 log units less sensitive. This verifies that most rod cell bodies of albino trout can persist without functional ROS in indirect sunlight, and can regrow functional outer segments in dim daylight. This finding is distinct from the extensive retinal light damage observed in albino rats exposed to more moderate cyclic light, in which entire rod cells degenerate early on.

A novel myosin-like protein (myocilin) expressed in the connecting cilium of the photoreceptor: molecular cloning, tissue expression, and chromosomal mapping

We have isolated a human cDNA clone encoding a novel acidic protein of MW 55,000 that we designated "myocilin" since it has homology to myosin and is localized preferentially in the ciliary rootlet and basal body of the connecting cilium of photoreceptor cells. The deduced amino acid sequence of human myocilin showed significant homologies with nonmuscle myosin of Dictyostelium discoideum in the N-terminal region and also with olfactomedin of bullfrog in the C-terminal region. Myocilin contained a leucine zipper-like motif similar to that seen in kinectin and other cytoskeletal proteins. These findings suggest that myocilin is a novel cytoskeletal protein involved in the morphogenesis of ciliated neuroepithelium such as photoreceptor cells. The myocilin gene (MYOC) was mapped to human chromosome 1q23-q24 by fluorescence in situ hybridization.

Light adaptation affects synaptic vesicle density but not the distribution of GABAA receptors in goldfish photoreceptor terminals

GABA is a likely feedback transmitter from H1 horizontal cells to cone photoreceptors in fish retinas. Spinules arise from H1 cell dendrites in light-adapted retinas, are correlated with responses attributed to feedback, and have been proposed to be the GABA release sites. We used mAb 62-3G1, an antibody against the beta 2/beta 3 subunits of the GABAA receptor complex, to visualize GABAA receptor immunoreactivity (GABAr-IR) in photoreceptors as a function of light and dark adaptation at the electron microscopical level. Regardless of adaptation, GABAr-IR was restricted to the synaptic terminals of all cones and most rods; synaptic vesicular membrane and plasma membrane, exhibited GABAr-IR. Contrary to expectations, the density of GABAr-IR was least on the plasma membrane within the invagination, regardless of the presence or absence of spinules. Dense GABAr-IR was observed on the lateral surface of cone pedicles, on cone processes proximal to the invagination, and on presumed telodendria from nearby cones. There was no difference in GABAr-IR of rod plasma membranes within or outside of the invagination or with adaptation. The only novel effect of adaptation was in regards to the density of synaptic vesicles. Cones showed a 29% increase in vesicle density with dark adaptation, whereas rods showed a 17% decrease. We conclude that all goldfish photoreceptors will be GABA-sensitive and that the sensitivity is distributed over the surface of the synaptic terminal rather than localized to within the invagination. The role of spinules in GABA release remains to be determined, but we conclude that spinules are not related to the GABA sensitivity of goldfish photoreceptors.

Effects of different patterns of light adaptation on cellular and synaptic plasticity in teleost retina: comparison of flickering and steady lights

The importance of the pattern of light stimulus in inducing light-adaptive morphological (cellular and sub-cellular) changes in the outer retina of the carp was assessed. Thus, the effects of steady and flickering backgrounds (of the same intensity) on cone photomechanical movements (PMMs) and horizontal cell (HC) spinules were compared by quantitative measurements. For both phenomena, flickering stimuli had a significantly greater effect than steady, although the number of photons delivered by the former was one-half. The results demonstrate clearly that the pattern of light stimulus is indeed an important parameter in determining the extent of adaptation in the outer retina. The effects are discussed in relation to possible neurochemical bases of light-adaptive neural control mechanisms.

Periodic cone cell twists in the walleye, Stizostedion vitreum; a new type of retinomotor activity

A typical retinomotor movements (RM) have been found in a localized area of the retina of the walleye, Stizostedion vitreum. The ellipsoid and outer segment of regions of twin cones in an oblong region of the central retina rotate, twisting about their axes at intervals throughout the 24 hr day. This twist results in a local, cyclic change of the cone mosaic, from the predominant row type to a square type. The cone cell myoids extend slightly, but the cone ellipsoids remain relatively close to the outer limiting membrane (OLM). Throughout the retina, rod cells exhibit ubiquitous extension and contraction of their myoid regions coincident with changes in illumination as is typical of fishes with duplex retinas. These retinomotor movements first appear around 6 weeks of age, and persist throughout growth, becoming more pronounced with maturity. Stizostedion vitreum is the first fish observed with this type of cone RM activity.

Gonadotropin-releasing hormone, a neuropeptide of efferent projections to the teleost retina induces light-adaptive spinule formation on horizontal cell dendrites in dark-adapted preparations kept in vitro

The teleost retina receives efferent projections from neurons of the nucleus olfactoretinalis at the base of the olfactory bulbs. These fibres contain gonadotropin-releasing hormone (GnRH) immunoreactive material and are presynaptic to retinal dopaminergic interplexiform cells. We have incubated isolated dark-adapted retinae and eyecup preparations of roach with salmon-GnRH and found an increase in horizontal cell spinule numbers to 70% light-adaptive levels. This effect was blocked by addition of haloperidol to the incubation medium suggesting that GnRH acts via stimulation of the dopaminergic interplexiform cells. We conclude that GnRH containing efferent fibres are capable of inducing light-adaptive changes in the retina and discuss their implication in the control of endogenous rhythms.

Dark-adaptive cone elongation in the blue acara retina is triggered by green-sensitive cones

In a dichromatic teleost species, we determined the intensity of light of various wavelengths required to prevent cone elongation by exposing fish at the time of their normal "dusk" phase to monochromatic light (479, 623, and 660 nm) at eight to ten different intensities for 75 min. The positions of single and double cones were measured in tangential sections and expressed as cone indices. At all wavelengths, the spectral responses of both cone types were virtually identical. Furthermore, the sensitivity of the blocking effect was highest at shorter wavelengths. When comparing the relative quantal sensitivities of myoid elongation for the two cone types to the spectral sensitivities of the three types of Aequidens pulcher photoreceptor, we found the closest match between the action spectrum and the absorption spectrum of the green-sensitive single cones. This may indicate that this cone type is capable of reacting directly to decreasing levels of illumination. On the other hand, the identical sensitivity of both cone types argues for an indirect control mechanism of dark-adaptive cone elongation, possibly via a neural pathway involving the inner retinal layers, complementary to the neural control of light adaptation. Green-sensitive single cones are well suited to trigger this response, since (1) their sensitivity is inferior to that of double cones; (2) waters inhabited by the blue acara transmit best at long wavelengths; and (3) at dusk, long-wavelength radiation dominates over other parts of the spectrum. Therefore, green-sensitive cone threshold will be reached first at dusk.

Modulation of cone to horizontal cell transmission by calcium and pH in the fish retina

The effects of small changes in the calcium and sodium concentrations and in the pH of superfusion medium on the membrane potential and light-evoked responses of cone horizontal cells in the goldfish retina were examined. Conventional intracellular recording, a bicarbonate-based superfusion medium, and a specially designed superfusion apparatus that reduced pressure wave disturbances were used. An increase in the extracellular calcium concentration, [Ca2+]o, from control levels (0.1 mM) to 1.0 mM hyperpolarized cone horizontal cells and reduced the magnitude of their light responses at all stimulus intensities. Addition of 20 mM NaCl to the 1.0 mM Ca2+ Ringer's solution reversed the hyperpolarizing effect of the 1.0 mM Ca2+ but addition of 20 mM choline, a monovalent cation that does not pass through cyclic GMP-activated channels, did not. Reduction of the superfusate pH from 7.6 to 7.2 by switching from a Ringer's solution gassed with 3% CO2 to one gassed with 10% CO2 hyperpolarized horizontal cells and reduced the magnitude of their light responses at all stimulus intensities for both 0.1 and 1.0 mM Ca2+ Ringer's solutions. An increase in pH to 8.2 by gassing the superfusate with 1% CO2 slightly depolarized the cells in 0.1 mM Ca2+ Ringer's solution but slightly hyperpolarized the cells in the 1.0 mM Ca2+ Ringer's solution. Following pharmacological isolation of the horizontal cells from synaptic input with high doses of glutamate (4-5 mM) and/or Co2+ (4 mM) treatment, no effect on horizontal cell membrane potential due to changes in pHo or [Ca2+]o was observed. These findings are discussed with respect to the cellular mechanisms and sites of action in the outer retina that are affected by changes in pHo and [Ca2+]o.

Protease activities in carp retina

Protease of carp retina were examined by electrophoresis and fluorogenic assays. A 70 kD serine protease with an alkaline pH optimum was detected in gelatin-containing polyacrylamide gels. A similar enzyme was found in carp brain and muscle, but not in lens. Using aminomethylcoumarin (MCA) substrates, activities that hydrolysed Z-Phe-Arg-MCA, Boc-Ala-Gly-Pro-Arg-MCA and various aminoacyl-MCAs were detected. The Z-Phe-Arg-MCA hydrolase was an acidic cysteine protease, whereas the Boc-Ala-Gly-Pro-Arg-MCA hydrolase was an alkaline cysteine protease. All aminoacyl hydrolase activities tested were inhibited by bestatin and o-phenanthroline, but not by inhibitors of serine, cysteine and aspartic proteases, suggesting they are metalloaminopeptidases. Of the substrates tested, Tyr-MCA was the most readily hydrolysed aminoacyl substrate. Preliminary evidence was obtained suggesting that levels of these activities do not differ between light- and dark-adapted retinae. The proteases have a potential involvement in retinal functioning and show similarities to other proteases known to act in the central nervous system. In particular, the Tyr-MCA hydrolase may be related to an enzyme known to remove the N-terminal tyrosine residue from enkephalin.

The circadian component of spinule dynamics in teleost retinal horizontal cells is dependent on the dopaminergic system

During the light phase of a light/dark cycle, dendrites of teleost cone horizontal cells display numerous finger-like projections, called spinules, which are formed at dawn and degraded at dusk, and are thought to be involved in chromatic feedback processes. We have studied the oscillations of these spinules during a normal light/dark cycle and during 48 h of constant darkness in two groups of strongly rhythmic, diurnal fish, Aequidens pulcher. In one group the retinal dopaminergic system had been destroyed by the application of 6-OHDA, while in the other (control) group, the dopaminergic system was intact. In control fish, oscillations of spinule numbers were observed under both normal and constant dark conditions, indicating the presence of a robust circadian rhythm. However, spinule dynamics were severely affected by the absence of retinal dopamine. During the normal light phase, the number of spinules in 6-OHDA injected retinae was strongly reduced, and throughout continual darkness, spinule formation was almost completely suppressed. These results indicate that dopamine is essential for both light-evoked and circadian spinule formation; furthermore, we conclude that there is no circadian oscillator within horizontal cells controlling the formation of spinules.

The effect of dopamine depletion on light-evoked and circadian retinomotor movements in the teleost retina

The retinae of lower vertebrates undergo a number of structural changes during light adaptation, including the photomechanical contraction of cone myoids and the dispersion of melanin granules within the epithelial pigment. Since the application of dopamine to dark-adapted retinae is known to produce morphological changes that are characteristic of light adaptation, dopamine is accepted as a casual mechanism for such retinomotor movements. However, we report here that in the teleost fish, Aequidens pulcher, the intraocular injection of 6-hydroxydopamine (6-OHDA), a substance known to destroy dopaminergic retinal cells, has no effect on the triggering of light-adaptive retinomotor movements of the cones and epithelial pigment and only slightly depresses the final level of light adaptation reached. Furthermore, the retina continues to show circadian retinomotor changes even after 48 h in continual darkness that are similar in both control and 6-OHDA injected fish. Biochemical assay and microscopic examination showed that 6-OHDA had destroyed dopaminergic retinal cells. We conclude, therefore, that although a dopaminergic mechanism is probably involved in the control of light-induced retinomotor movements, it cannot be the only control mechanism, nor can it be the cause of circadian retinomotor migrations. Interestingly, 6-OHDA injected eyes never reached full retinomotor dark adaptation, suggesting that dopamine has a role to play in the retina's response to darkness.

Glutamate antagonists that block hyperpolarizing bipolar cells increase the release of dopamine from turtle retina

Some neurochemical features of the neuronal circuitry regulating dopamine release were examined in the retina of the turtle, Pseudemys scripta elegans. Glutamate antagonists that block hyperpolarizing bipolar cells, such as 2,3 piperidine dicarboxylic acid (PDA), produced dose-dependent dopamine release. In contrast, the glutamate agonist 2-amino-4-phosphonobutyric acid (APB), which blocks depolarizing bipolar cell responses with high specificity, had no effect on the release of dopamine. The gamma-aminobutyric acid (GABA) antagonist, bicuculline, also produced potent dose-dependent release of dopamine. The release of dopamine produced by PDA was blocked by exogenous GABA and muscimol, suggesting that the PDA-mediated release process was polysynaptic and involved a GABAergic synapse interposed between the bipolar and dopaminergic amacrine cells. The only other agents that produced dopamine release were chloride-free media and high extracellular K+; in particular, kainic acid and glutamate itself were ineffective. These results suggest that the primary neuronal chain mediating dopamine release in the turtle retina is: cone----hyperpolarizing bipolar cell----GABAergic amacrine cell----dopaminergic amacrine cell.

Localization and function of dopamine in the adult vertebrate retina

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)

Actin-dependent myoid elongation in teleost rod inner/outer segments occurs in the absence of net actin polymerization

In the retinas of teleost fish, rod photoreceptors elongate in response to light. Light-activated elongation is mediated by the myoid of the rod inner segment and is actin-dependent. Inner segment F-actin filaments form bundles running parallel to the cell's long axis. We examined the mechanism of rod elongation using mechanically-detached rod fragments, consisting of the motile inner segment and sensory outer segment (RIS-ROS). When RIS-ROS are isolated from dark-adapted green sunfish and cultured in the light, they elongate 15 microns at 0.3-0.6 microns/min. Elongation was inhibited 65% by 0.1 microM Cytochalasin D, suggesting a requirement for actin assembly. To determine the extent of assembly during elongation, we used three approaches to measure the F-actin content in RIS-ROS: detection of pelletable actin by SDS-PAGE after detergent-extraction of RIS-ROS; quantification of fluorescein-phalloidin binding by fluorimetry, fluorescence-activated cell sorting and image analysis; estimation of total F-actin filament length by electron microscopy. All three assays indicated that no net assembly of RIS-ROS F-actin accompanied myoid elongation. An increase in F-actin content within the elongated myoid was counterbalanced by a decrease in F-actin content within the 13 microvillus-like calycal processes located at the end of the inner segment opposite to the growing myoid. O'Connor and Burnside (Journal of Cell Biology 89:517-524, 1981) showed that minus-ends of rod F-actin filaments are oriented towards the elongating myoid while plus-ends are oriented towards the shortening calycal processes. Our observations suggest that RIS-ROS elongation entails actin polymerization at the minus-ends of filaments coupled with depolymerization at the filament plus-ends.

Dopamine and plasticity of horizontal cell function in the teleost retina: regulation of a spectral mechanism through D1-receptors

The negative feed-back interaction between horizontal cells (HCs) and cones in the cyprinid fish retina is thought to be mediated by horizontal cell spinules. These are "plastic" structures, largely absent from the dark-adapted retina and formed anew during light adaptation. We have previously shown that horizontal cell feed-back is similarly enhanced by light adaptation. The role of the interplexiform cell transmitter dopamine in both processes has been studied in the roach retina. Application of dopamine to dark-adapted retinae induced spinule formation in a dose-dependent way. The effect of dopamine was mimicked by dibutyryl-cAMP and suppressed selectively by D1 receptor antagonists. The effect of light in inducing spinule formation was lost in retinae depleted of endogenous dopamine. However, application of exogenous dopamine to these retinae triggered normal spinule formation. For all pharmacological treatments used, there was a strong correlation between spinule number and degree of feed-back activity in biphasic horizontal cells. Thus, when the spinule content of the cone pedicles was high, biphasic horizontal cell responses exhibited strong depolarizing components and vice versa. It is concluded that light-evoked formation of spinules in HC dendrites involves the action of dopamine upon D1 receptors. Spinules, in turn, are likely to be presynaptic terminals mediating the dynamic negative feed-back effect of horizontal cells upon cones.

Retinal dopamine D1 and D2 receptors: characterization by binding or pharmacological studies and physiological functions

1. In the retinal inner nuclear layer of the majority of species, a dopaminergic neuronal network has been visualized in either amacrine cells or the so-called interplexiform cells. 2. Binding studies of retinal dopamine receptors have revealed the existence of both D1- as well D2-subtypes. The D1-subtype was characterized by labeled SCH 23390 (Kd ranging from 0.175 to 1.6 nM and Bmax from 16 to 482 fmol/mg protein) and the D2-subtype by labelled spiroperidol (Kd ranging from 0.087 to 1.35 nM and Bmax from 12 to 1500 fmol/mg protein) and more selectively by iodosulpiride (Kd 0.6 nM and Bmax 82 fmol/mg protein) or methylspiperone (Kd 0.14 nM and Bmax 223 fmol/mg protein). 3. Retinal dopamine receptors have been also shown to be positively coupled with adenylate cyclase activity in most species, arguing for the existence of D1-subtype, whereas in some others (lower vertebrates and rats), a negative coupling (D2-subtype) has been also detected in peculiar pharmacological conditions implying various combinations of dopamine or a D2-agonist with a D1-antagonist or a D2-antagonist in the absence or presence of forskolin. 4. A subpopulation of autoreceptors of D2-subtype (probably not coupled to adenylate cyclase) also seems to be involved in the modulation of retinal dopamine synthesis and/or release. 5. Light/darkness conditions can affect the sensitivity of retinal dopamine D1 and/or D2-receptors, as studied in binding or pharmacological experiments (cAMP levels, dopamine synthesis, metabolism and release). 6. Visual function(s) of retinal dopamine receptors were connected with the regulation of electrical activity and communication (through gap junctions) between horizontal cells mediated by D1 and D2 receptor stimulation. Movements of photoreceptor cells and migration of melanin granules in retinal pigment epithelial cells as well as synthesis of melatonin in photoreceptors were on the other hand mediated by the stimulation of D2-receptors. 7. Other physiological functions of dopamine D1-receptors respectively in rabbit and in embryonic avian retina would imply the modulation of acetylcholine release and the inhibition of neuronal growth cones.

Circadian rhythm and light regulate opsin mRNA in rod photoreceptors

Disk membranes in the outer segment of rod photoreceptors are continuously renewed, being assembled at the outer segment base, displaced outward by new disks and eventually shed at the tip. In lower vertebrates, disk assembly occurs with a diurnal rhythm with 2-4% of the outer segment length produced daily. We have discovered that in toad and fish retinas the level of mRNA for opsin, the most abundant protein in rod disks, fluctuates with a daily rhythm and is regulated both by light and by a circadian oscillator. The mRNA level rises before light onset, remains high during the light phase of a diurnal cycle and decreases four to tenfold during the dark phase. In constant darkness, mRNA elevation occurs during subjective daytime. At night, rod opsin mRNA can be elevated by exposure to light.

The interplexiform-horizontal cell system of the fish retina: effects of dopamine, light stimulation and time in the dark

Interplexiform cells contact cone horizontal cells in the fish retina and probably release dopamine at synaptic sites. The effects of dopamine, certain related compounds, and light and dark régimes were tested on the intracellularly recorded activity of horizontal cells in the superfused carp retina to elucidate the functional role of the interplexiform cell. Dopamine application onto retinae kept in the dark for 30-40 min increased the size of the responses of cone horizontal cells to small-spot stimuli but decreased response size to large- and full-field stimuli. Dopamine also altered the response waveform of these cells; the transient at response onset increased in size and the depolarizing afterpotential decreased in size. Haloperidol, a dopamine antagonist, blocked these effects of dopamine application. Forskolin, an adenylate cyclase activator, increased the size of the responses of the cells to small-spot stimuli. Superfusion of vasoactive intestinal peptide did not produce any effects on horizontal cells. The results indicate that dopamine produces multiple physiological effects on cone horizontal cells by activation of an intracellular enzyme system. We propose that some of these effects are probably related to an uncoupling of the gap junctions between horizontal cells, but that other effects are most likely not explained on this basis and reflect additional changes induced in the cells by dopamine. After prolonged periods of darkness (100-110 min), compared with short periods (30-40 min), L-type cone horizontal cells exhibited responses similar to those obtained during dopamine application. Dim flickering or continuous light backgrounds did not mimic the effects of dopamine. Although dopamine application onto retinae after short-term darkness produced dramatic effects on L-type cone horizontal cells, little or no effect was observed when dopamine was applied while the effects of a previous dopamine application were still present or after prolonged darkness. These results suggest that interplexiform cells may release dopamine after prolonged darkness and that interplexiform cells may regulate lateral inhibitory effects mediated by L-type cone horizontal cells as a function of time in the dark.