Rod outer segments elongate in constant light: darkness is required for normal shedding

Regulation of the rhythmic diversity of daily photoreceptor outer segment phagocytosis in vivo

Outer segment phagocytosis (OSP) is a highly-regulated, biological process wherein photoreceptor outer segment (OS) tips are cyclically phagocytosed by the adjacent retinal pigment epithelium (RPE) cells. Often an overlooked retinal process, rhythmic OSP ensures the maintenance of healthy photoreceptors and vision. Daily, the photoreceptors renew OS at their base and the most distal, and likely oldest, OS tips, are phagocytosed by the RPE, preventing the accumulation of photo-oxidative compounds by breaking down phagocytosed OS tips and recycling useful components to the photoreceptors. Light changes often coincide with an escalation of OSP and within hours the phagosomes formed in each RPE cell are resolved. In the last two decades, individual molecular regulators were elucidated. Some of the molecular machinery used by RPE cells for OSP is highly similar to mechanisms used by other phagocytic cells for the clearance of apoptotic cells. Consequently, in the RPE, many molecular regulators of retinal phagocytosis have been elucidated. However, there is still a knowledge gap regarding the key regulators of physiological OSP in vivo between endogenous photoreceptors and the RPE. Understanding the regulation of OSP is of significant clinical interest as age-related macular degeneration (AMD) and inherited retinal diseases (IRD) are linked with altered OSP. Here, we review the in vivo timing of OSP peaks in selected species and focus on the reported in vivo environmental and molecular regulators of OSP.

The cell biology of the retinal pigment epithelium

The retinal pigment epithelium (RPE), a monolayer of post-mitotic polarized epithelial cells, strategically situated between the photoreceptors and the choroid, is the primary caretaker of photoreceptor health and function. Dysfunction of the RPE underlies many inherited and acquired diseases that cause permanent blindness. Decades of research have yielded valuable insight into the cell biology of the RPE. In recent years, new technologies such as live-cell imaging have resulted in major advancement in our understanding of areas such as the daily phagocytosis and clearance of photoreceptor outer segment tips, autophagy, endolysosome function, and the metabolic interplay between the RPE and photoreceptors. In this review, we aim to integrate these studies with an emphasis on appropriate models and techniques to investigate RPE cell biology and metabolism, and discuss how RPE cell biology informs our understanding of retinal disease.

Molecular and Biochemical Aspects of the Retina on Refraction

Mutant mouse models with specific visual pathway defects offer an advantage to comprehensively investigate the role of specific pathways/neurons involved in refractive development. In this review, we will focus on recent studies using mouse models that have provided insight into retinal pathways and neurotransmitters controlling refractive development. Specifically, we will examine the contributions of rod and cone photoreceptors and the ON and OFF retinal pathways to visually driven eye growth with emphasis on dopaminergic mechanisms.

Structure and function of photoreceptor and second-order cell mosaics in the retina of Xenopus

The structure, physiology, synaptology, and neurochemistry of photoreceptors and second-order (horizontal and bipolar) cells of Xenopus laevis retina is reviewed. Rods represent 53% of the photoreceptors; the majority (97%) are green light-sensitive. Cones belong to large long-wavelength-sensitive (86%), large short-wavelength-sensitive (10%), and miniature ultraviolet wavelength-sensitive (4%) groups. Photoreceptors release glutamate tonically in darkness, hyperpolarize upon light stimulation and their transmitter release decreases. Photoreceptors form ribbon synapses with second-order cells where postsynaptic elements are organized into triads. Their overall adaptational status is regulated by ambient light conditions and set by the extracellular dopamine concentration. The activity of photoreceptors is under circadian control and is independent of the central body clock. Bipolar cell density is about 6000 cells/mm2 They receive mixed inputs from rods and cones. Some bipolar cell types violate the rule of ON-OFF segregation, giving off terminal branches in both sublayers of the inner plexiform layer. The majority of them contain glutamate, a small fraction is GABA-positive and accumulates serotonin. Luminosity-type horizontal cells are more frequent (approximately 1,000 cells/mm2) than chromaticity cells (approximately 450 cells/mm2). The dendritic field size of the latter type was threefold bigger than that of the former. Luminosity cells contact all photoreceptor types, whereas chromatic cells receive their inputs from the short-wavelength-sensitive cones and rods. Luminosity cells are involved in generating depolarizing responses in chromatic horizontal cells by red light stimulation which form multiple synapses with blue-light-sensitive cones. Calculations indicate that convergence ratios in Xenopus are similar to those in central retinal regions of mammals, predicting comparable spatial resolution.

Photoreceptor renewal: a role for peripherin/rds

Visual transduction begins with the detection of light within the photoreceptor cell layer of the retina. Within this layer, specialized cells, termed rods and cones, contain the proteins responsible for light capture and its transduction to nerve impulses. The phototransductive proteins reside within an outer segment region that is connected to an inner segment by a thin stalk rich in cytoskeletal elements. A unique property of the outer segments is the presence of an elaborate intracellular membrane system that holds the phototransduction proteins and provides the requisite lipid environment. The maintenance of normal physiological function requires that these postmitotic cells retain the unique structure of the outer segment regions--stacks of membrane saccules in the case of rods and a continuous infolding of membrane in the case of cones. Both photoreceptor rod and cone cells achieve this through a series of coordinated steps. As new membranous material is synthesized, transported, and incorporated into newly forming outer segment membranes, a compensatory shedding of older membranous material occurs, thereby maintaining the segment at a constant length. These processes are collectively referred to as ROS (rod outer segment) or COS (cone outer segment) renewal. We review the cellular and molecular events responsible for these renewal processes and present the recent but compelling evidence, drawn from molecular genetic, biochemical, and biophysical approaches, pointing to an essential role for a unique tetraspanning membrane protein, called peripherin/rds, in the processes of disk morphogenesis.

Symphony of rhythms in the Xenopus laevis retina

The photoreceptor layer in the retina of Xenopus laevis harbors a circadian clock. Many molecular components known to drive the molecular clock in other organisms have been identified in Xenopus, such as XClock, Xper2, and Xcrys, demonstrating phylogenetic conservation. This model system displays a wide array of rhythms, including melatonin release, ERG rhythms, and retinomotor movements, suggesting that the ocular clock is important for proper retinal function. A flow-through culture system allows measurements of retinal rhythms such as melatonin release in vitro over time from a single eyecup. This system is suited for pharmacological perturbations of the clock, and has led to important observations regarding the circadian control of melatonin release, the roles of light and dopamine as entraining agents, and the circadian mechanisms regulating retinomotor movements. The development of a transgenic technique in Xenopus allows precise and reliable molecular perturbations. Since it is possible to follow rhythms in eyecups obtained from adults or tadpoles, the combination of the flow-through culture system and the transgenic technique leads to the fast generation of transgenic tadpoles to monitor the effects of molecular perturbations on the clock.

The role of photoreceptors in the control of refractive state

The current state of research into experimentally induced refractive errors is reviewed. The area is analysed in three components-the transduction of defocus or deprivation, the vector for transmitting the error message from the retina to the outer tunics of the eye, and the identity of the effector for causing growth modulation in the sclera. Anatomical, pharmacological, electrophysiological and optical factors are considered in terms of which elements of the retina are necessary to support a refractive response to deprivation or defocus. Two of the current models are discussed-one emphasizing the role of the choroid in effecting ocular and refractive change, while the second model approaches the problem from the aspect of scleral changes that are associated with growth adaptation without emphasis on the error detection mechanism. A third model is proposed in which the error signal for deprivation or defocus is detected in the outer retina and where error is translated through separate signals for stimulus brightening and darkening into a net signal for fluid flow across and under the active control of the retinal pigment epithelium with the fluid communication between the vitreous chamber and the choroidal lymphatics. The directions of research both fundamental and clinical which are needed to create pharmaceutical or environmental solutions to refractive control are discussed.

A peptide analogue to a fusion domain within photoreceptor peripherin/rds promotes membrane adhesion and depolarization

Photoreceptor peripherin/rds promotes membrane fusion, through a putative fusion domain located within the C-terminus (Boesze-Battaglia et al., Biochemistry 37 (1998) 9477-9487). A peptide analogue to this region, PP-5, competitively inhibits peripherin/rds mediated fusion in a cell free assay system. To characterize how this region is involved in the fusion process we investigated two of the individual steps in membrane fusion, membrane adhesion and membrane destabilization inferred from depolarization studies. Membrane depolarization was measured as the collapse of a valinomycin induced K(+) diffusion potential in model membranes, using a potential sensitive fluorescent probe, diS-C(2)-5. PP-5 induced membrane depolarization in a concentration dependent manner. PP-5 has been shown by Fourier transform infrared spectroscopy to be an amphiphilic alpha-helix. Therefore, the requirement for an amphiphilic alpha-helix to promote depolarization was tested using two mutant peptides designed to disrupt either the amphiphilic nature of PP-5 (PP-5AB) or the alpha-helical structure (PP-5HB). PP-5AB inhibited PP-5 induced depolarization when added in an equimolar ratio to PP-5. Neither mutant peptide alone or in combination with PP-5 had any effect on calcium dependent vesicle aggregation. Using non-denaturing gel electrophoresis and size exclusion chromatography techniques PP-5 was shown to form a tetrameric complex. Equimolar mixtures of PP-5 and PP-5AB formed a heterotetramer which was unable to promote membrane depolarization. The hypothesis that PP-5 tetramers promote membrane depolarization is consistent with the calculated Hill coefficient of 3.725, determined from a Hill analysis of the depolarization data.

Thermal control of rod outer segment length and shedding in a fish, Fundulus zebrinus

The effects of temperature on rod outer segment (ROS) length and membrane shedding were studied in a cyprinodont fish, Fundulus zebrinus. After 30 days in 14L/10D cyclic light and 17 degrees C, ROS length averaged 41.2 microns. Fish were then exposed to 7, 17 or 27 degrees C for 10 and 25 days before being sampled 5 hr before and 1-4 hr after light onset. In 7 degrees C ROS shortened to 83.5% of initial controls within 10 days, then only 4.1% further, to 79.4% by day 25 (34.4, 32.7 microns). ROS length did not change significantly in fish remaining at 17 degrees C (39.7 and 40.7 microns at day 10 and 25) or in fish moved to 27 degrees C (41.7 and 41.6 microns). Phagosomes were most numerous in 7 degrees C and least numerous in 17 degrees C, but varied in overall size among the largest phagosomes being more common after light onset. After light onset at day 25, the estimated volume per phagosome was 1.14, 4.73 and 5.75 microns 3 in 7, 17 and 27 degrees C. Total phagosome volume per 100 microns RPE at 27 degrees C was generally double that at 17 degrees C. Apparently, in F. zebrinus, the number of disks shed from ROS is adjusted during thermal acclimation to stabilize ROS length.

A role for photoreceptor outer segments in the induction of deprivation myopia

An ultrastructural examination of the outer retina and choroid of hatchling chicks reared for periods of 1, 2 or 4 weeks with opaque occluders (MD) covering one eye, was instigated to elucidate the mechanism of deprivation myopia. Refractive myopia (approximately 20 D), retinal and choroidal thinning were induced in all deprived eyes. Electron microscopy showed significant changes in the MD eyes compared to normals. Cone inner segments were markedly thicker and outer segment lamellae more damaged. The rod outer segments were elongated and thicker than normal, such that their distal tips either directly apposed the basal lamina of the retinal pigment epithelium, or indented the cell nuclei. We hypothesize that this "rod-push" mechanism leads to thinning of the choroid in deprived eyes, and may directly contribute to axial myopia.

Photomembrane turnover in frog retina: light intensity and spectral correlates

Light regulates membrane turnover in vertebrate rod photoreceptor cells. Rods shed membrane-filled tips immediately after light onset, with light inhibiting the dark priming phase but initiating the light induction phase. This study examines the intensities and wavelengths of light that control these two shedding requirements, and demonstrates unexpected situations where red or dim lights are simultaneously dark to the dark priming mechanism and light to the light induction process. Since shedding takes place immediately following darkness we asked if dim or red light could substitute for true darkness and dark prime the retinas: our results confirm this. White light, less than 0.7 microE m m-2 sec-1 (0.15 W m2 or 40 lx), allows dark priming, and even 15 microE m-2 sec-1 of red fluorescent light dark primes as effectively as true darkness. Conversely, bright white light and wavelengths from 480 to 560 nm inhibit dark priming, implying that dark priming inhibition is a photopic mechanism transduced by photopigment in the 502-cone. We also asked if dim or red light could induce shedding, substituting for the bright light usually employed: again, the results confirm thus. White light as dim as 0.15 microE m-2 sec-1 induces shedding and red light is an effective light trigger. This light induction is initiated at all wavelengths tested (420-640 nm), with a maximum effect between 540 and 600 nm. Finally, we find that retinas shed continuously in red or dim white light. These lights substitute both for the darkness necessary for dark priming and for the light of light induction, extending shedding from the 20 min dark-light transition period to hours or days. We also find that the dim, red light of natural dawn is as effective a shedding stimulus as the sudden onset of bright laboratory light.

Paracrine control of photomembrane removal

Photomembrane turnover in vertebrate photoreceptors is regulated by light. Rod outer segments (ROS) shed membrane filled tips at light onset, during the coexistence of two light modulated processes: a dark priming factor and a light induction event. Transduction of these two signals is not direct but appears to involve the neural retina and diffusible paracrine molecules. I propose a model wherein three paracrines control this ROS tip shedding. Melatonin, a lipid soluble dark priming molecule, is synthesized in the dark by all photoreceptor cells, diffusing freely and separating the ROS disk membranes. A second paracrine, dopamine is released from the inner retina whenever light is absorbed by the 502 nm-cones, inhibiting melatonin synthesis. Third, a proposed trophic paracrine, "rostrophin", is released in the dark from internal horizontal cells, and stabilizes the photomembrane. Shedding occurs as rostrophin decreases in the presence melatonin; briefly at light onset or continuously in red or dim white light.

Cytoskeletal specializations at the rod photoreceptor distal tip

We have examined microtubules and microtubule-like elements within the toad rod photoreceptor outer segment in order to define regional specializations of the photoreceptor cytoskeleton. "Ciliary" microtubules were localized within the rod outer segment (ROS) by using thin section electron microscopy, immunofluorescence, and rapid-freeze deep-etch microscopy. All three methods showed that ciliary microtubules stop short of the extreme ROS distal tip, although abundant microtubule-like structures distinct from the ciliary microtubules were found within the distal 10-15 microns of the ROS tip. These heretofore undescribed "distal ROS tubules" are clustered at the clefts or incisures of the disk membrane stack and resemble microtubules in overall size and shape, although they are not closely related antigenically to tubulin. The distal ROS tubules are more abundant in green rods than red rods and vary in number during the daily light/dark cycle. Quantitation of these tubules at two time points during the light/dark cycle suggests that there are three- to fourfold more tubules in the ROS tip one hour after light onset than one hour before light onset. Retinas prevented from normal disk membrane shedding by separation of the retina from the adjacent pigment epithelium, failed to develop increased numbers of tubules after light onset. This suggests that the newly described distal ROS tubules may modulate or be modulated by light-induced interactions between the photoreceptors and pigment epithelium, such as those that occur during the disk shedding phase of membrane turnover.

Increased levels of leukotriene C4 in retinal pigment epithelium are correlated with early events in photoreceptor shedding in Xenopus laevis

The levels of 5-lipoxygenase products of arachidonic acid, leukotriene (LT) B4 and LTC4 in retinal pigment epithelia (RPE) from Xenopus laevis were measured by radioimmunoassay (RIA). RPE were isolated during various stages of photoreceptor renewal to determine possible alterations in 5-lipoxygenase activity concurrent with photoreceptor detachment and phagocytosis. Both LTC4 and LTB4 were released to RPE incubation media, although levels of LTB4 in unstimulated RPE were close to the limits of detection by RIA. Incubation of RPE with the calcium ionophore A23187 increased the levels of both LTB4 and LTC4. When animals were maintained on a cycle of 12 hr light/dark, normal photoreceptor shedding, as measured by histological quantitation of the appearance of phagosomes in the RPE, occurred 1 hr after light onset. Levels of LTC4 in RPE were lower 1 hr after light onset, as compared to 1 hr prior to light onset. Due to the low levels of LTB4, no significant differences could be detected. However, when LTB4 levels were elevated with A23187, LTB4 also declined 1 hr after light onset. When animals were maintained in constant light for 5 days, then exposed to 2 hr dark and 2 hr light, a massive shedding response occurred. Levels of LTC4 were stimulated 5 min after light onset (prior to detectable shedding) and declined below dark levels as shedding progressed. These data suggest a correlation between 5-lipoxygenase activity and the events of photoreceptor shedding and phagocytosis.

Addition of the chromophore to rat rhodopsin is an early post-translational event

Rat retinas were labeled either by intravitreal injection of [14C]leucine or by incubation with [3H]-leucine or [35S]-methionine. Subcellular fractions were prepared on linear sucrose gradients and rhodopsin was extracted with detergent and purified by chromatography on ConA-Sepharose. A fraction enriched in rough endoplasmic reticulum (RER) and substantially free of rod outer segments (ROS) was found to contain a light-sensitive protein exhibiting the properties of rhodopsin on ConA-Sepharose or Agarose chromatography and on SDS-polyacrylamide gel electrophoresis, as well as immunologically. Intravitreal injection of [3H]-retinol also labeled the rhodopsin in the RER under conditions in which the rhodopsin in the ROS was not heavily labeled. Thus the chromophore appears to be attached to opsin shortly after the apoprotein is translated in the RER.

Effects of exogenous melatonin on circadian disc shedding in the albino rat retina

The hypothesis that melatonin regulates circadian rod outer segment disc shedding in mammals was tested by determining the effect of exogenous melatonin on histological phagosome counts. Melatonin was administered as a subcutaneous implant or injection to photoentrained albino rats. Retinas of treated rats and paired controls were obtained at various times including the time of expected disc shedding. Phagosome counts on electron micrographs were converted to size-frequency distributions. Injected melatonin did not induce abnormal circadian shedding. However, implanted melatonin increased the frequency of large phagosomes (P less than 0.01). These data further implicate melatonin in the control of disc shedding.

Effects of kynurenate and other excitatory amino acid antagonists as blockers of light- and kainate-induced retinal rod photoreceptor disc shedding

Photoreceptor disc shedding in the retina involves detachment of discs from distal outer segments and phagocytosis of those discs by adjacent pigment epithelial cells. The disc-shedding process balances the continuous renewal of photosensitive membrane. In amphibians, rod disc shedding normally is light-stimulated. However, excitatory amino acids such as kainate stimulate disc shedding independent of a dark-light transition. Excitatory amino acid-induced disc shedding is accompanied by toxic changes within the retina. To evaluate the possible role of an endogenous excitatory amino acid in the regulation of light-evoked disc shedding, we examined the effects of excitatory amino acid antagonists on kainate-induced and light-evoked disc shedding and on retinal toxicity. Using eyecups from Rana pipiens, we found that kynurenate, D-O-phosphoserine, and cis-2,3-piperidine dicarboxylic acid (cis-PDA) all block both the neurotoxic and disc-shedding effects of kainate. Kynurenate and D-O-phosphoserine, but not cis-PDA, also block light-evoked disc shedding. Our analysis suggests that kynurenate blocks the mechanism by which light "triggers" disc shedding rather than directly inhibiting disc detachment and phagocytosis. The observation that cis-PDA antagonizes the effects of kainate, but not light, suggests that the receptor mediating the kainate effect on disc shedding may not be involved in the normal initiation of the response by light. In contrast, our data on kynurenate suggest that it antagonizes an endogenous mechanism involved in the normal control of disc shedding. Thus, analysis of the differences between cis-PDA and kynurenate as antagonists in the retina may yield important insight into the mechanism by which light initiates disc shedding.

Dopamine mediates the light-evoked suppression of serotonin N-acetyltransferase activity in retina

The possible role of dopamine in the light-induced suppression of serotonin N-acetyltransferase (NAT) activity in retinas of the African clawed frog (Xenopus laevis) was investigated using an in vitro eye cup preparation. The nocturnal increase of retinal NAT activity was significantly inhibited by either light exposure or exogenous dopamine. Spiperone, a dopamine receptor blocker, antagonized this inhibitory effect of light on NAT activity, but had no effect in darkness. The effect of spiperone required the presence of cyclic nucleotide phosphodiesterase inhibitors, 3-isobutylmethylxanthine (IBMX), papaverine, or Ro 20-1724. Under the conditions employed in this study, neither spiperone nor the phosphodiesterase inhibitors significantly affected NAT activity when added alone. This observation suggests a synergistic interaction between the dopaminergic antagonists and the phosphodiesterase inhibitors. Other dopamine receptor blockers, including haloperidol, cis-flupenthixol, clozapine and metoclopramide, increased NAT activity of light-exposed retinas incubated in the presence of IBMX. SCH 23390, a D1-selective dopamine receptor antagonist, did not increase NAT activity, nor did the alpha- and beta-adrenergic receptor antagonists tested. The effect of spiperone and IBMX on NAT activity was blocked by apomorphine and by the D2-dopamine receptor agonist LY 171555, but not by the D1-receptor agonist SKF 38393-A. The concentration of 3,4-dihydroxyphenylacetic acid was higher in light-exposed retinas than in dark-adapted retinas, suggesting that light exposure increases dopamine metabolism in Xenopus retina. The results presented in this paper suggest that dopamine, released in response to light exposure and acting on D2-dopamine receptors, is partially responsible for the light-induced suppression of the nocturnal increase in retinal NAT activity.

Hydroxyindole-O-methyltransferase in Y-79 human retinoblastoma cells: effect of cell attachment

Hydroxyindole-O-methyltransferase (HIOMT), the enzyme in the final step of melatonin synthesis, is present in the Y-79 human retinoblastoma cell line. Using electroblot immunolabellings, a single band corresponding to HIOMT was observed. Immunofluorescence, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and quantification of enzyme activity all revealed dramatic increases in HIOMT in cells attached to substrate compared to cells in suspension culture.

Membrane turnover in rod photoreceptors: ensheathment and phagocytosis of outer segment distal tips by pseudopodia of the retinal pigment epithelium

We have documented the ultrastructural changes that occur within the photoreceptor outer segment and the retinal pigment epithelium (RPE) during photosensitive membrane turnover. We employed an in vitro eyecup preparation from Xenopus laevis in which a large shedding event was induced by adding the excitatory amino acid L-aspartate (Greenberger & Besharse 1985; J. comp. Neurol. 239, 361-372). We found that during L-aspartate-induced shedding the RPE cells formed, on their apical domains, previously undescribed processes that were directly involved in disc phagocytosis. These processes are structurally similar to processes formed by macrophages during phagocytosis and are accordingly referred to as pseudopodia. Pseudopodia were distinguishable from the apical villous process normally extended from the RPE in that they were closely applied to the surface of the outer segment, had a cytoplasmic matrix of low electron density that was devoid of most cellular organelles and were enriched in thin (7 nm diameter) filaments. Filament size, specific pseudopodial staining with the actin-specific probe rhodamine phalloidin and inhibition of pseudopod formation by cytochalasin D suggested that the thin filaments were composed of actin. Pseudopodial formation also occurs during a normal light-initiated shedding event. However, the low frequency of shedding, the asynchrony of the individual shedding events and the transient appearance of the pseudopodia prevented a full appreciation of their role during normal disc shedding. Associated with massive shedding and pseudopodial formation, there was an increased adherence between retina and RPE. During L-aspartate treatment, the apical portions of the RPE cells partitioned with the distal outer segment during retinal isolation. This effect was directly related to the development of pseudopodia and may reflect alteration of surface features of the rod outer segment (ROS)-RPE interface related to phagocytosis. Our observations show that transiently forming pseudopodia are the organelles of phagocytosis and that they may play a role in disc detachment as well.

Localization of hydroxyindole-O-methyltransferase-like immunoreactivity in photoreceptors and cone bipolar cells in the human retina: a light and electron microscope study

The localization of the melatonin-synthesizing enzyme hydroxyindole-O-methyltransferase (HIOMT) was examined by light and electron microscopic immunocytochemistry in the human retina. HIOMT-like immunoreactivity was observed in the photoreceptor layers and the inner nuclear layer (INL). The immunoreactive cells in the INL were more numerous in the central retina than in the peripheral retina and sent processes to both the outer plexiform and inner plexiform layers. The HIOMT immunoreactivity in the inner plexiform layer (IPL) appeared as punctate terminals in the proximal and distal one-thirds of that layer. At the ultrastructural level, HIOMT-like immunoreactivity was localized to the cytoplasm of rod and cone photoreceptors and to a population of cone bipolar cells. HIOMT-immunoreactive bipolar cell dendrites were observed to make both invaginating and flat synaptic contacts with cone pedicles. No immunoreactive invaginating contacts in rod spherules were observed. HIOMT immunoreactivity was observed in the bipolar cell cytoplasm in the INL, and in the bipolar synaptic terminals in the IPL. These terminals contained synaptic ribbons, which formed synaptic contacts with unlabeled cells in the IPL. HIOMT radioenzymatic assays confirmed the presence of HIOMT in the human retina. Average HIOMT activity of eight donors was determined to be 15.0 pmol/mg protein/hour +/- 7.2 S.D. The ultrastructural localization of HIOMT observed in this study, combined with reports from other laboratories, suggests that the cytoplasm of the photoreceptors and a population of cone bipolar cells may be the sites of melatonin synthesis in the human retina.

Light-evoked and kainic-acid-induced disc shedding by rod photoreceptors: differential sensitivity to extracellular calcium

In order to study the light and Ca2+ dependence of disc shedding by rod photoreceptors, we have used eyecups prepared from adult Rana pipiens frogs that had been kept in constant light for 4 days. Disc shedding was initiated by a treatment involving 1 hour of darkness followed by exposure to light or by treatment with kainic acid. Maximal L-evoked disc shedding occurred quickly (within 30-60 minutes) after light onset and could be triggered by brief (15 minutes) exposure to light. L-evoked disc shedding was completely blocked by omission of Ca2+ from culture medium or by treatment with 3mM Co2+ or 12 mM Mg2+ in the presence of Ca2+ (2 mM). The response was also blocked by the organic Ca2+ antagonist nifedipine. Experiments designed to distinguish between Ca2+ dependence of the dark- or light-dependent processes necessary for shedding suggest that voltage-sensitive channels mediate a Ca2+-dependent process involved in light-triggering. Kainic acid caused a dose-dependent stimulation of disc shedding under lighting conditions (continuous culture in light or darkness) that did not normally result in a significant response in the absence of the drug. Disc shedding induced by kainic acid was similar in time course and magnitude to that induced by light. However, kainic-acid-induced disc shedding was not inhibited by medium Ca2+ reduction or by the presence of Co2+. The latter observation suggests that kainic acid activates disc shedding directly, by-passing the Ca2+-dependent process involved in the L-evoked response. The Ca2+-dependent process may involve release of an effector of disc shedding that is mimicked by kainic acid.

Melatonin: parallels in pineal gland and retina

It is apparent that several relationships exist between the pineal gland and retina. The similarities in development and morphology have been obvious for many years. A recent resurgence of interest in this field has led to a further understanding of many functional similarities between these two organs. A notable feature of the pineal gland and retina is their common ability to synthesize the indolamine hormone, melatonin. Many investigators suspect that the cyclic rhythm of retinal melatonin synthesis may be related to other cyclic events which normally occur in the retina.

The influence of light on cone disk shedding in the lizard, Sceloporus occidentalis

The lizard, Sceloporus occidentalis has an all-cone retina. In lizards maintained on a 12-h light:12-h dark (12L:12D) cycle, a burst of cone outer segment (COS) shedding occurs 2 h after light offset (1400 h circadian time) (Young, R.W., 1977, J. Ultrastruct. Res. 61:172-72). In this investigation, we studied the effect of different lighting regimes on the pattern of cone disk shedding in this species. When lizards entrained to a 12L:12D cycle are kept in constant darkness (DD), the shedding peak is advanced approximately 2 h and the magnitude of shedding is reduced to 30% of control. COS increased in mean length from 12 micron in controls to 14 micron after one cycle in DD and maintained this length during a second cycle in DD. In constant light (LL), disk shedding was damped to approximately 10% of control values. Shedding synchrony in LL was also perturbed and therefore cyclic shedding bursts could not be distinguished. During LL there was a much larger increase in COS mean length than in DD. After one cycle of LL, COS length was 15 micron and after two cycles COS length exceeded 17 micron. When lizards entrained to 12L:12D are shifted to a 6L:18D regimen, the first shedding cycle is biphasic. The first peak of 5% shedding occurs 2 h after light offset whereas a second larger peak (13%) occurs according to the entrained schedule (1400 h). This manipulation separates out a dark-triggered and circadian shedding component, which is normally superimposed in lizards entrained to a 12L:12D cycle. When entrained lizards are placed in 36 h of LL followed by light offset, the peak shedding response after light offset is double the control response (53% vs. 27%). After 30 h of LL (lights off 90 degrees out of phase), there is a biphasic shedding response similar to the 6L:18D regimen although this time the dark-triggered shedding component is greater in magnitude then the circadian component. COS turnover is estimated by extrapolating from COS mean length increases during LL. From this method we obtained a 2.7-micron increase in COS length during each day in LL. If COS growth is not augmented during LL, this would yield a 4-5-d turnover time for the average 12.5-micron COS.

Some properties of old and new rhodopsin in single Bufo rods

Rod photoreceptors renew the membranous disks of the outer segments (ROS). New disks are assembled at the proximal base and old disks are shed at the distal tip. Rhodopsin, the major protein of the disk, remains with the disk into which it was inserted. Thus, it is true that the oldest rhodopsin is at the tip and the newest at the base. A microspectrophotometer is used to examine the properties of rhodopsin in the two ends of the toad ROS. No differences between the two are found in absorption spectrum, concentration, dichroism, photoconversion rates, or lateral diffusion rates. Regeneration of rhodopsin from the bleached state is also studied but cannot be used to discriminate old from new rhodopsin because the point of entry of regeneration retinoids and/or their concentrations cannot be controlled. However, a new insight into pigment regeneration in the living toad eye is gained: regeneration is faster in the basal disks than in the distal.

Shedding is correlated with disk membrane axial position rather than disk age in Xenopus laevis rod outer segments

Disk membrane synthesis and displacement rates in Xenopus laevis retinal rod outer segments, as measured by light-dark dependent birefringence band periods, are proportional to incubation temperature. Outer segment length, however, is approximately the same for frogs raised at different temperatures. Therefore the age of disks shed from outer segment distal tips is a function of temperature. Unless the rate of disk synthesis coincidentally has the same temperature dependence as putative disk membrane aging processes, this implies that position on the outer segment axis rather than disk age is a sufficient condition for disk membrane shedding in Xenopus laevis maintained under diurnal light-dark conditions.

Increases in volume, fluid content, and lens weight of eyes following systemic administration of melatonin

Melatonin's effects were studied in male golden hamsters (Mesocricetus auratus) distributed among five surgical groups (nonoperated, sham-pinealectomized, sham-pinealectomized plus black plastic shielding of the pineal region, pinealectomized, and pinealectomized plus black plastic shielding of the pineal region) and three injection groups (vehicle only, 25 micrograms melatonin, and 2,500 micrograms melatonin). Injections (s.c.) were daily for 28 d at L11 to L11.75 in a (light:dark) L:D 14:10 artificial photoperiod. Animals (N = 112) were killed and dissected on the day after the last injection (at 55-65 d of age). None of the surgical procedures affected weights of eyes or their parts, nor did they influence the effects of administered melatonin on the eyes. Melatonin caused an increase in absolute and relative eye weight and an increase in fluid content of intraocular space. The magnitudes of these effects were positively related to melatonin dose. These same eyes had a progressively lower weight of nonlenticular tissues with low to high doses of melatonin, probably in relation to greater fluid content, and suspected increase in intraocular pressure. Lens wet and dry weights were significantly greater in animals receiving melatonin, but only at the high dose. These actions of melatonin are likely to be direct and are shown to not require the presence of the pineal. Experiments of other designs are suggested in order to determine whether the effects of the low, near physiological, dose of melatonin represent physiological actions of endogenous melatonin, synthesized and released within the eye. However, effects of large doses of melatonin on the eye are still noteworthy in relation to interpretation of experiments employing such dosages, and of disease states involving changes in intraocular pressure.

Regulation of indoleamine N-acetyltransferase activity in the retina: effects of light and dark, protein synthesis inhibitors and cyclic nucleotide analogs

The regulation of indoleamine N-acetyltransferase (NAT) in the posterior eye was investigated in vivo, and in vitro in cultured eye cups. Surgical separation of neural retina from the retinal pigment epithelium-choroid complex indicated that NAT was localized to neural retina. The activity of retinal NAT fluctuated in vivo in a rhythmic fashion, with peak activity in the dark phase of the light-dark cycle. The rhythm of NAT activity persisted for up to 3 days in constant darkness, with a rhythmic period of approximately 25 h. The rhythm was suppressed by constant light, and could be phase-shifted by exposure to a new light-dark cycle. These observations indicate that retinal NAT activity occurs as a circadian rhythm that is entrained by light and dark. Retinas also responded to light and dark in vitro with changes of NAT activity. A significant increase in retinal NAT activity occurred in eye cups cultured in darkness during the dark phase of the light-dark cycle. This increase was completely suppressed in eye cups cultured at the same time of day in light. The dark-induced increase in NAT was completely blocked by protein synthesis inhibitors, and mimicked in light by cyclic AMP analogs. The similarity of the regulation of NAT activity in retina to that in pineal, and the possible relationship of the retinal NAT rhythm to cyclic metabolism in photoreceptors are discussed.

Methoxyindoles and photoreceptor metabolism: activation of rod shedding

Using an in vitro eye-cup preparation, we have evaluated a potential relationship between methoxyindole metabolism and photoreceptor disk shedding. Melatonin, 6-chloromelatonin, and 5-methoxytryptophol all activate rod disk shedding in culture. The effect is compound specific since serotonin and N-acetylserotonin are without effect, and it is similar to shedding in vivo because it is evoked by light and is quantitatively comparable to a normal intact animal response. The results suggest the involvement of 5-methoxyindoles in the control of rhythmic photoreceptor metabolism.

Effects of cyclic adenosine 3',5'-monophosphate on photoreceptor disc shedding and retinomotor movement. Inhibition of rod shedding and stimulation of cone elongation

As a test of the hypothesis that cyclic nucleotides play a role in the regulation of retinomotor movements and disc shedding in the photoreceptor-pigment epithelial complex, we have used an in vitro eyecup preparation that sustains both disc shedding and cone retinomotor movements, Eyecups were prepared in white light from animals in which both shedding and cone movement had been blocked by 4 d of constant-light treatment. In eyecups incubated for 3 h in light, disc shedding was negligible and cones remained in the light-adapted (contracted) position. In eyecups incubated in darkness, however, a massive shedding response (dominated by rod photoreceptors) was induced, and at the same time cone photoreceptors elongated to their dark-adapted position. In eyecups incubated in light dbcAMP promoted cone elongation and thus mimicked darkness; the dbcAMP effect was potentiated by the phosphodiesterase inhibitors papaverine and 3-isobutylmethylxanthine. In eyecups incubated in darkness, on the other hand, both phosphodiesterase inhibitors and dbcAMP reduced the phagosome content of the pigment epithelium. The effects of dbcAMP on the cone elongation and rod shedding appear to be specific in that dbcGMP, adenosine, and adenosine 5'-monophosphate had no significant effect. Our results suggest that cAMP plays a role in the regulation of both retinomotor movements and disc shedding.

Local stimulation induces shedding throughout the frog retina

Our previous work has demonstrated that rod shedding in the frog retina can be driven by environmental cues such as light onset. Although shedding normally occurs binocularly, we found that shedding could be initiated independently in either eye of the frog by monocular stimulation. Further, rod shedding occurs in vitro in the isolated eyecup under appropriate incubation conditions when provided with a light stimulus following a dark incubation period. Thus, the control mechanism for light induced rod shedding in the frog seems to be located within the eye, and does not seem to be systemically or centrally located. However, the exact link between light onset and shedding of the distal rod tips remains unknown. To elucidate further the control site for initiation of rod shedding, we used a variety of stimulus conditions, including front and rear screens as well as spots and slits projected directly on the retina, to stimulate a small portion of the frog retina with a range of light intensities and stimulus paradigms. In all cases where shedding occurred, it was uniform throughout the retina. Thus, it appears that the light-cued message received by a small population of photoreceptors is sufficient to initiate shedding throughout the retina. These results differ significantly from those found by Easter and Macy for light-induced photomechanical movements, which were found to be locally controlled.

Retinal rhythms in chicks: circadian variation in melantonin and serotonin N-acetyltransferase activity

There is a large-amplitude circadian rhythm of indoleamine metabolism in the retina-pigment epithelium of the chicken. N-Acetyltransferase activity (arylamine acetyltransferase; acetyl-CoA:arylamine N-acetyltransferase, EC 2.3.1.5) and melatonin content are 15-fold higher at night than during the day in a cycle of a 4-fold increase during the subjective night. Light at midnight inactivates N-acetyltransferase and lowers melatonin. N-Acetyltransferase activity is found predominantly in the retina. The circadian rhythm of this enzyme activity persists in pinealectomized chicks. Thus the pineal is not responsible for retinal indoleamine rhythms. Retinal and pineal levels of N-acetyltransferase activity behave similarly under several conditions. In the chicken, the eye is a major site of rhythmic indoleamine metabolic activity.