Melatonin receptor RNA is expressed in photoreceptors and displays a diurnal rhythm in Xenopus retina

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

Hibernation with Rhythmicity in the Retina, Brain, and Plasma but Not in the Liver of Hibernating Giant Spiny Frogs (Quasipaa spinosa)

Simple Summary

Aquatic ectotherms experience hypoxia under water during hibernation, which enables them to move denoting some level of consciousness, unlike terrestrial hibernators. However, how aquatic ectotherms modulate their clocks and clock-controlled genes in different tissues and plasma melatonin and corticosterone in light-dark cycles under natural environments before and during hibernation, remains to be largely unexplored. To achieve these, in this study, we investigated circadian clock genes, circadian clock-controlled genes, antioxidant enzyme genes, and related hormones in giant spiny frog (Quasipaa spinosa). Our results demonstrated that, despite the hypometabolic state of hibernation, the retina and the brain displayed some circadian rhythms of clock and antioxidant genes, as well as melatonin, while the liver was inactive. These novel findings may contribute to an understanding of how aquatic ectotherms use their circadian system differentially to modulate their physiology in escaping hypoxia during hibernation and preparing for arousal.

Abstract

Hibernation in ectotherms is well known, however, it is unclear how the circadian clock regulates endocrine and antioxidative defense systems of aquatic hibernators. Using the giant spiny frog (Quasipaa spinosa), we studied mRNA expression levels of (1) circadian core clock (Bmal1, Clock, Cry1 and Per2), clock-controlled (Ror-α, Mel-1c and AANAT), and antioxidant enzyme (AOE) (SOD1, SOD2, CAT and GPx) genes in retina, brain, and liver; and (2) plasma melatonin (MT) and corticosterone (CORT) levels, over a 24-hour period at six intervals pre-hibernation and during hibernation. Our results showed that brain Bmal1, Cry1, Per2 and Mel-1c were rhythmic pre-hibernation and Clock and Ror-α during hibernation. However, the retina Bmal1, Clock and Mel-1c, and plasma MT became rhythmic during hibernation. All brain AOEs (SOD1, SOD2, CAT and GPx) were rhythmic pre-hibernation and became non-rhythmic but upregulated, except SOD1, during hibernation. However, plasma CORT and liver clocks and AOEs were non-rhythmic in both periods. The mRNA expression levels of AOEs closely resembled those of Ror-α but not plasma MT oscillations. In the hibernating aquatic frogs, these modulations of melatonin, as well as clock and clock-controlled genes and AOEs might be fundamental for them to remain relatively inactive, increase tolerance, and escape hypoxia, and to prepare for arousal.

Pre-mRNA Processing Factors and Retinitis Pigmentosa: RNA Splicing and Beyond

Retinitis pigmentosa (RP) is the most common inherited retinal disease characterized by progressive degeneration of photoreceptors and/or retinal pigment epithelium that eventually results in blindness. Mutations in pre-mRNA processing factors (PRPF3, 4, 6, 8, 31, SNRNP200, and RP9) have been linked to 15–20% of autosomal dominant RP (adRP) cases. Current evidence indicates that PRPF mutations cause retinal specific global spliceosome dysregulation, leading to mis-splicing of numerous genes that are involved in a variety of retina-specific functions and/or general biological processes, including phototransduction, retinol metabolism, photoreceptor disk morphogenesis, retinal cell polarity, ciliogenesis, cytoskeleton and tight junction organization, waste disposal, inflammation, and apoptosis. Importantly, additional PRPF functions beyond RNA splicing have been documented recently, suggesting a more complex mechanism underlying PRPF-RPs driven disease pathogenesis. The current review focuses on the key RP-PRPF genes, depicting the current understanding of their roles in RNA splicing, impact of their mutations on retinal cell’s transcriptome and phenome, discussed in the context of model species including yeast, zebrafish, and mice. Importantly, information on PRPF functions beyond RNA splicing are discussed, aiming at a holistic investigation of PRPF-RP pathogenesis. Finally, work performed in human patient-specific lab models and developing gene and cell-based replacement therapies for the treatment of PRPF-RPs are thoroughly discussed to allow the reader to get a deeper understanding of the disease mechanisms, which we believe will facilitate the establishment of novel and better therapeutic strategies for PRPF-RP patients.

Influence of Circadian Rhythm in the Eye: Significance of Melatonin in Glaucoma

Circadian rhythm and the molecules involved in it, such as melanopsin and melatonin, play an important role in the eye to regulate the homeostasis and even to treat some ocular conditions. As a result, many ocular pathologies like dry eye, corneal wound healing, cataracts, myopia, retinal diseases, and glaucoma are affected by this cycle. This review will summarize the current scientific literature about the influence of circadian patterns on the eye, focusing on its relationship with increased intraocular pressure (IOP) fluctuations and glaucoma. Regarding treatments, two ways should be studied: the first one, to analyze if some treatments could improve their effect on the ocular disease when their posology is established in function of circadian patterns, and the second one, to evaluate new drugs to treat eye pathologies related to the circadian rhythm, as it has been stated with melatonin or its analogs, that not only could be used as the main treatment but as coadjutant, improving the circadian pattern or its antioxidant and antiangiogenic properties.

Cellular localization of melatonin receptor Mel1b in pigeon retina

Melatonin, an important neuromodulator involved in circadian rhythms, modulates a series of physiological processes via activating its specific receptors, namely Mel1a (MT₁), Mel1b (MT₂) and Mel1c receptors. In this work, the localization of Mel1b receptor was studied in pigeon retina using double immunohistochemistry staining and confocal scanning microscopy. Our results showed that Mel1b receptor widely existed in the outer segment of photoreceptors and in the somata of dopaminergic amacrine cells, cholinergic amacrine cells, glycinergic AII amacrine cells, conventional ganglion cells and intrinsically photosensitive retinal ganglion cells, while horizontal cells, bipolar cells and Müller glial cells seemed to lack immunoreactivity of Mel1b receptor. That multiple types of retinal cells expressing Mel1b receptor suggests melatonin may directly modulate the activities of retina via activating Mel1b receptor.

Role of melatonin and its receptors in the vertebrate retina

Melatonin is a chemical signal of darkness that is produced by retinal photoreceptors and pinealocytes. In the retina, melatonin diffuses from the photoreceptors to bind to specific receptors on a variety of inner retinal neurons to modify their activity. Potential target cells for melatonin in the inner retina are amacrine cells, bipolar cells, horizontal cells, and ganglion cells. Melatonin inhibits the release of dopamine from amacrine cells and increases the light sensitivity of horizontal cells. Melatonin receptor subtypes show differential, cell-specific patterns of expression that are likely to underlie differential functional modulation of specific retinal pathways. Melatonin potentiates rod signals to ON-type bipolar cells, via activation of the melatonin MT2 (Mel1b) receptor, suggesting that melatonin modulates the function of specific retinal circuits based on the differential distribution of its receptors. The selective and differential expression of melatonin receptor subtypes in cone circuits suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons and thus promote dark adaptation.

Melatonin receptors are anatomically organized to modulate transmission specifically to cone pathways in the retina of Xenopus laevis

Melatonin receptors have been identified in several retinal cell types, including photoreceptors, horizontal cells, amacrine cells, and ganglion cells. Recent reports suggest that melatonin potentiates signaling from rods to inner retinal neurons. However, the organization of the melatonin receptors mediating this action in the outer plexiform layer (OPL) is not clear. To assess melatonin receptor localization in the OPL, double-label confocal immunohistochemistry for Mel1a or Mel1b melatonin receptors was performed in combination with markers for cone photoreceptors (calbindin, XAP-1) and ON bipolar cells (guanine nucleotide binding protein alpha, Goα) on the retina of Xenopus laevis. Both Mel1a and Mel1b receptors were specifically associated with processes contacting the pedicles of cones, but localized to processes from different sets of second-order neurons. Mel1a receptors localized to the large axonal processes of horizontal cells, while Mel1b receptors localized to the dendrites of OFF bipolar cells. Both receptors also localized to third-order amacrine and ganglion cells and their processes in the inner plexiform layer. This study indicates that Mel1a and Mel1b melatonin receptors are expressed specifically in the Xenopus OPL to modulate transmission from cones to horizontal cells and OFF bipolar cells, respectively; they are second-order neurons that predominantly contact ribbon synapses and display OFF responses to light. When combined with results from recent physiological studies, the current results suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons across species, although the precise mechanisms by which melatonin enhances this transmission are likely to vary in a species-dependent manner.

Melatonin receptors in a pleuronectiform species, Solea senegalensis: Cloning, tissue expression, day-night and seasonal variations

Melatonin receptors are expressed in neural and peripheral tissues and mediate melatonin actions on the synchronization of circadian and circannual rhythms. In this study we have cloned three melatonin receptor subtypes (MT1, MT2 and Mel1c) in the Senegalese sole and analyzed their central and peripheral tissue distribution. The full-length MT1 (1452 nt), MT2 (1728 nt) and Mel1c (1980 nt) cDNAs encode different proteins of 345, 373, 355 amino acids, respectively. They were mainly expressed in retina, brain and pituitary, but MT1 was also expressed in gill, liver, intestine, kidney, spleen, heart and skin. At peripheral level, MT2 expression was only evident in gill, kidney and skin whereas Mel1c expression was restricted to the muscle and skin. This pattern of expression was not markedly different between sexes or among the times of day analyzed. The real-time quantitative PCR analyses showed that MT1 displayed higher expression at night than during the day in the retina and optic tectum. Seasonal MT1 expression was characterized by higher mRNA levels in spring and autumn equinoxes for the retina, and in winter and summer solstices for the optic tectum. An almost similar expression profile was found for MT2, but differences were less conspicuous. No day-night differences in MT1 and MT2 expression were observed in the pituitary but a seasonal variation was detected, being mRNA levels higher in summer for both receptors. Mel1c expression did not exhibit significant day-night variation in retina and optic tectum but showed seasonal variations, with higher transcript levels in summer (optic tectum) and autumn (retina). Our results suggest that day-night and seasonal variations in melatonin receptor expression could also be mediating circadian and circannual rhythms in sole.

Expression of the melatonin receptor Mel1c in neural tissues of the reef fish Siganus guttatus

The golden rabbitfish, Siganus guttatus, is a reef fish exhibiting a restricted lunar-related rhythm in behavior and reproduction. Here, to understand the circadian rhythm of this lunar-synchronized spawner, a melatonin receptor subtype-Mel(1c)-was cloned. The full-length Mel(1c) melatonin receptor cDNA comprised 1747 bp with a single open reading frame (1062 bp) that encodes a 353-amino acid protein, which included 7 presumed transmembrane domains. Real-time PCR revealed high Mel(1c) mRNA expression in the retina and brain but not in the peripheral tissues. When the fish were reared under light/dark (LD 12:12) conditions, Mel(1c) mRNA in the retina and brain was expressed with daily variations and increased during nighttime. Similar variations were noted under constant conditions, suggesting that Mel(1c) mRNA expression is regulated by the circadian clock system. Daily variations of Mel(1c) mRNA expression with a peak at zeitgeber time (ZT) 12 were observed in the cultured pineal gland under LD 12:12. Exposure of the cultured pineal gland to light at ZT17 resulted in a decrease in Mel(1c) mRNA expression. When light was obstructed at ZT5, the opposite effect was obtained. These results suggest that light exerts certain effects on Mel(1c) mRNA expression directly or indirectly through melatonin actions.

Melatonin receptors in the eye: location, second messengers and role in ocular physiology

The pineal hormone melatonin, an important regulator of circadian and seasonal rhythms, has a role in ocular pathophysiology. In addition to the pineal gland, melatonin synthesis is carried out in several ocular structures. Moreover, specific melatonin receptors have been located in the retina, cornea, ciliary body, lens, choroid and sclera, which suggests that cells in these tissues may be targets for melatonin action. This review summarizes the current knowledge about melatonin receptor subtypes with the emphasis on those melatonin receptors, which have been identified in ocular tissues and their possible roles in biochemical and physiological processes in the eye.

Melatonin in the eye: implications for glaucoma

Melatonin synthesis occurs in the retina of most animals as well as in humans. Circadian oscillators that control retinal melatonin synthesis have been identified in the eyes of different animal species. The presence of melatonin receptors is demonstrable by immunocytochemical studies of ocular tissues. These receptors may have different functional roles in different parts of the eye. In view that melatonin is a potent antioxidant molecule, it can be effective in scavenging free radicals that are generated in ocular tissues. By this mechanism melatonin could protect the ocular tissues against disorders like glaucoma, age-related macular degeneration, retinopathy of prematurity, photo-keratitis and cataracts. Although an increased intraocular pressure is an important risk factor in glaucoma, other concomitant phenomena like increased glutamate levels, altered nitric oxide metabolism and increased free radical generation seem to play a significant role in its pathogenesis. Data are discussed indicating that melatonin, being an efficient antioxidant with antinitridergic properties, has a promising role in the treatment and management of glaucoma.

Effects of bilateral and unilateral ophthalmectomy on plasma melatonin in Rana tadpoles and froglets under various experimental conditions

The effect of ophthalmectomy (enucleation) on plasma melatonin in Rana tadpoles and froglets was studied under various experimental conditions to determine if ocular melatonin is released into the circulation from the eyes and to study the factors which might affect this process. Where operations occurred in early or mid-photophase on a 12 light:12 dark (12L:12D) cycle (light onset at 08:00 h), sampling in mid-light and mid-dark revealed that scotophase plasma melatonin was reduced at all developmental stages, with the more significant effects occurring before metamorphic climax. Experiments sampling prometamorphic tadpoles six times in a 24h period on 18L:6D, 12L:12D, or 6L:18D five days after enucleation also showed a significant lowering of plasma melatonin in the dark, so that the scotophase peak was virtually eliminated on all the LD cycles. These findings indicated that the reduction in plasma melatonin after bilateral eye removal was independent of the LD cycle and the metamorphic stage, and that it abolished the diel melatonin rhythm at the expense of the scotophase peak. Experiments carried out for 5 weeks suggested that compensatory secretion of melatonin by other organs after eye removal might partially restore the plasma melatonin level over time. Unilateral ophthalmectomy tended to reduce, but not eliminate, the night peak of plasma melatonin, and did not result in a compensatory increase in ocular melatonin in the remaining eye. Ophthalmectomized tadpoles exhibited darkening of the skin after the operation, which was not associated with a significant change in pituitary alpha-melanotropin. The findings overall indicate that the eyes in Rana tadpoles and froglets contribute up to somewhat over one-half of the circulating melatonin, particularly during the scotophase, and provide experimental evidence for ocular secretion into the blood for the first time in the Amphibia.

Development and optimization of a Q-RT PCR method to quantify CYP19 mRNA expression in testis of male adult Xenopus laevis: Comparisons with aromatase enzyme activity

Due to limitations of the currently used enzymatic assays, it is difficult to determine aromatase activity in testicular tissue of amphibians. Quantitative reverse transcription polymerase chain reaction (Q-RT PCR) is a sensitive and reliable technique to detect low amounts of mRNA for specific genes. This study was designed to develop and optimize a SYBR Green I-based Q-RT PCR method to quantify CYP19 mRNA in testicular tissue from male Xenopus laevis. Four quantification methods for measuring CYP19 mRNA expression were compared. The established test system proved to be highly sensitive (detectable mRNA copies < 10), reproducible (interassay CV < 5.4%, intraassay CV < 0.9%), precise and specific for the CYP19 gene. To confirm the validity of the applied test system, an ex vivo testicular and ovarian explant study with a known inducer of aromatase, forskolin, was conducted. Forskolin induced CYP19 gene expression in both ovarian (3.7-fold) and testicular (2.6-fold) explants. Of the four quantification methods, the absolute standard curve and the comparative CT method appear to be optimal as indicated by their highly significant correlation (r2 = 0.998, p < 0.001). In conclusion, we recommend the comparative CT method over the standard curve method because it is more economical in terms of both cost and labor. Although both aromatase activity and CYP19 mRNA were clearly detectable in testes of X. laevis, both aromatase enzyme activity and CYP19 gene expression were very low. Also, no significant relationships were found between aromatase enzyme activity and gene expression. This is likely due the fact that the aromatase enzyme may have been dormant at the developmental stage the frogs were in during the experiment.

Stimulation of Melatonin Receptors Decreases Calcium Levels in Xenopus Tectal Cells by Activating GABAC Receptors

To investigate the physiological effects of melatonin receptors in the Xenopus tectum, we have used the fluorescent indicator Fluo-4 AM to monitor calcium dynamics of cells in tectal slices. Bath application of KCl elicited fluorescence increases that were reduced by melatonin. This effect was stronger at the end of the light period than at the end of the dark period. Melatonin increased γ-aminobutyric acid-C (GABAC)–receptor activity, as demonstrated by the ability of the GABAC-receptor antagonists, picrotoxin and TPMPA, to abolish the effects of melatonin. In contrast, neither the GABAA-receptor antagonist bicuculline nor the GABAB-receptor antagonist CGP 35348 diminished the effects of melatonin. RT-PCR analyses revealed expression of the 3 known melatonin receptors, MT1 (Mel1a), MT2 (Mel1b), and Mel1c. Because the effect of melatonin on tectal calcium increases was antagonized by an MT2-selective antagonist, 4-P-PDOT, we performed Western blot analyses with an antibody to the MT2 receptor; the data indicate that the MT2 receptor is expressed primarily as a dimeric complex and is glycosylated. The receptor is present in higher amounts at the end of the light period than at the end of the dark period, in a pattern complementary to the changes in melatonin levels, which are higher during the night than during the day. These results imply that melatonin, acting by MT2 receptors, modulates GABAC receptor activity in the optic tectum and that this effect is influenced by the light–dark cycle.

Melatonin decreases calcium levels in retinotectal axons of Xenopus laevis by indirect activation of group III metabotropic glutamate receptors

Melatonin is a neuromodulator that binds to receptors in the retinotectal laminae of the amphibian optic tectum. The effect of melatonin on calcium dynamics in Xenopus retinotectal axons was investigated by imaging retinotectal axons labeled with the fluorescent indicator Fluo-4. Melatonin exerted an inhibitory influence on depolarization-evoked calcium increases, and the melatonin receptor antagonist 4-P-PDOT blocked this effect. Blockade of group III metabotropic receptors (mGluRs) counteracted the effect of melatonin on retinotectal axons. Application of the group II/group III mGluR antagonist MSPG or the group III-selective antagonist MSOP abolished the effect of melatonin. Conversely, this effect was not significantly affected by the group I mGluR antagonist LY367385 nor by EGLU or LY341495 at concentrations that specifically inhibit group II mGluRs. Furthermore, a higher concentration of LY341495 that affects group III mGluRs inhibited the effect of melatonin. The data therefore support the hypothesis that, in retinotectal axons, melatonin reduces cAMP levels, thereby relieving PKA-induced inhibition of group III mGluRs; the newly activated mGluRs in turn inhibit voltage-sensitive calcium channels, leading to a decrease in Ca2+ concentrations. The role of GABA(C) receptors in retinotectal responses was also evaluated. GABA(C) receptor antagonists did not block the effects of melatonin but instead were additive. Moreover, while other studies have shown that in Xenopus tectal cells, GABA(C) receptors mediate inhibition, in retinotectal axons, the opposite appears to occur since depolarization-evoked calcium rises in retinotectal axons were inhibited by GABA(C) receptor blockade. This result suggests that activation of GABA(C) receptors produces an increase in the synaptic excitability of retinotectal axon terminals.

Binding characteristics and daily rhythms of melatonin receptors are distinct in the retina and the brain areas of the European sea bass retina (Dicentrarchus labrax)

Melatonin is synthesized, with a circadian rhythm, in the pineal organ of vertebrates, high levels being produced during the scotophase and low levels during the photophase. The retina also produces melatonin, although in the case of the European sea bass, its secretion pattern appears to be inverted. In the study described here, radioreceptor assay techniques were used to characterize the melatonin binding sites, their regional distribution and their daily variations. Brain and retina membrane preparations were used in all the binding assays and 2-[125I]iodomelatonin ([125I]Mel) as radioligand at 25 degrees C. The specific binding of [125I]Mel was seen to be saturable, reversible, specific and of high affinity. In all the tissues assayed, the power of the ligands to inhibit [125I]Mel binding decreased in the following order: melatonin>>4-P-PDOT>luzindole> or =N-acetylserotonin, which points to the presence of Mel1-like receptors. The inhibition curves of 4-P-PDOT suggested the presence of two different binding sites in the brain areas, but only one type of site of low affinity in the neural retina. No daily variations in [125I]Mel binding capacity (Bmax) or affinity (Kd) were detected in the brain areas, while a clear rhythm in Kd melatonin receptor affinity and Bmax binding capacity was observed in the retina. Kd and Bmax retinal rhythms were out of phase with the lowest Kd and the highest Bmax occurring at scotophase. This result suggests that retinal melatonin is a paracrine factor able to control receptor desensitization during photophase when ocular melatonin is higher in this species.

2-[125I]-melatonin binding sites in the central nervous system and neural retina of the frog Rana perezi: regulation by light and temperature

The objective of the present study is to test daily and seasonal changes in 2-[125I]-Melatonin ([125I]-Mel) binding in different brain areas and the retina of the frog Rana perezi as well as the possible effect of light and temperature on melatonin receptors. During the day-night cycle, binding of [125I]-Mel showed a clear rhythm in the optic tectum, diencephalon, telencephalon, and neural retina, the binding being higher in the light phase than in the dark phase. By contrast, melatonin receptors did not show any significant summer-winter differences in any of the four tissues studied. In the neural retina, but not in the brain, exposure of frogs to 24 h darkness for one week leads to significantly less [125I]-Mel binding than 24 h light exposure. This darkness-induced reduction of [125I]-Mel binding is not due to a desensitisation of binding sites by high melatonin levels. Thermal acclimation to either 5 or 22 degrees C for one month did not change the affinity (Kd) and density (Bmax) of [125I]-Mel binding sites either in the brain or the retina. All these results indicate that there is a daily rhythm in melatonin receptors in the frog brain and retina, and that the light/dark cycle can drive this rhythm in [125I]-Mel binding in the retina. Temperature apparently did not modify [125I]-Mel binding in frogs.

Localization of Mel1b melatonin receptor-like immunoreactivity in ocular tissues of Xenopus laevis

The circadian signaling molecule, melatonin, is produced by pinealocytes and retinal photoreceptors. In the retina, melatonin is thought to diffuse into the inner retina to act as a paracrine signal of darkness by binding to specific receptors in retinal neurons. The retinal cell locations of the Mel1a and Mel1c melatonin receptor types have been reported, but the localization of the Mel1b receptor, which is the most highly expressed melatonin receptor type in the retina, is unknown. To determine the cellular distribution of Mel1b melatonin receptor protein in the Xenopus laevis retina and other ocular tissues, polyclonal antibodies were raised against a peptide fragment of the X. laevis Mel1b receptor. Western blot analysis of several ocular tissues revealed the presence of one or more immunoreactive bands in the sclera, cornea, lens, retinal pigment epithelium (RPE)/choroid, and neural retina. In the neural retina, the major immunoreactive bands displayed electrophoretic mobilities corresponding to approximately 35, 42, 45, and 80 Kd. Sections of X. laevis eyes were analyzed by immunocytochemistry and confocal microscopy, in combination with antibodies against the Mel1a melatonin receptor, a rod photoreceptor-specific protein, opsin, and two amacrine cell-specific markers, tyrosine hydroxylase (TOH; dopaminergic cells) and glutamic acid decarboxylase (GAD; GABA-ergic cells). Mel1b immunoreactivity was localized to the apical membranes of RPE cells, and punctate Mel1b immunoreactivity was observed in both rod and cone photoreceptor inner segments. Presumptive horizontal cells that ramify in the outer plexiform layer (OPL) were immunoreactive for Mel1b, and were exclusive of the Mel1a immunoreactivity present in the OPL. Neither TOH nor GAD co-localized with the Mel1b immunoreactivity that was present in the inner plexiform layer (IPL), suggesting that Mel1b is not expressed in dopaminergic or GABA-ergic amacrine cells. Mel1b immunoreactivity was observed in ganglion cells of the retina, a population of cells covering the outer surface of the outer fibrous layer of the sclera, and in lens fibers located in the outer regions of the lens. These results suggest that melatonin may influence retinal function by binding to receptors on RPE and photoreceptor cells, and by acting on neurons of the inner retina that do not use dopamine or GABA as a neurotransmitter. Furthermore, melatonin may bind to receptors on cells located in the sclera and lens, perhaps to modify the growth or function of these ocular tissues.

Embryonic and post-embryonic expression of arylalkylamine N-acetyltransferase and melatonin receptor genes in the eye and brain of chum salmon (Oncorhynchus keta)

Melatonin and arylalkylamine N-acetyltransferase (AANAT), the rate-limiting enzyme in melatonin synthesis, have taken on special importance in vertebrate circadian biology. Recent identification of genes encoding two AANAT (AANAT(1) and AANAT(2)) and two subtypes of melatonin receptor (Mel-R; Mel(1a) and Mel(1b)) in several fish species has led to rapid advances in characterizing the physiological roles of melatonin. In the present study, partial cDNAs encoding these four genes were cloned from the eye and brain of chum salmon (Oncorhynchus keta). Based on the nucleotide sequences, we developed highly sensitive real-time PCR systems for these four mRNAs. The development of daily rhythmicity in AANAT(1), AANAT(2), Mel(1a), and Mel(1b) transcript levels was examined in the eye and brain of chum salmon during embryonic and post-embryonic stages (from day -9 to day +180). In a parallel experiment, ocular and brain melatonin levels were measured by radioimmunoassay. Parallelism in developmental changes and in circadian rhythms of AANAT mRNAs and melatonin levels in the eye and the brain supports a hypothesis that the developmental increases of nocturnal melatonin levels results partly from the elevated transcription of AANAT genes. Moreover, abundant expression of AANAT and Mel-R mRNAs in the optic tectum, thalamus, hypothalamus, cerebellum, and eye indicates possible roles of melatonin in visual processing and neuroendocrine regulation, through which melatonin might be involved in migratory behavior of chum salmon.

A circadian clock in the fish retina regulates dopamine release via activation of melatonin receptors

Although many biochemical, morphological and physiological processes in the vertebrate retina are controlled by a circadian (24 h) clock, the location of the clock and how the clock alters retinal function are unclear. For instance, several observations have suggested that dopamine, a retinal neuromodulator, may play an important role in retinal rhythmicity but the link between dopamine and a clock located within or outside the retina remains to be established. We found that endogenous dopamine release from isolated goldfish retinae cultured in continuous darkness for 56 h clearly exhibited a circadian rhythm with high values during the subjective day. The continuous presence of melatonin (1 nM) in the culture medium abolished the circadian rhythm of dopamine release and kept values constantly low and equal to the night-time values. The selective melatonin antagonist luzindole (1 microM) also abolished the dopamine rhythm but the values were high and equal to the daytime values. Melatonin application during the late subjective day introduced rod input and reduced cone input to fish cone horizontal cells, a state usually observed during the subjective night. In contrast, luzindole application during the subjective night decreased rod input and increased cone input. Prior application of dopamine or spiperone, a selective dopamine D(2)-like antagonist, blocked the above effects of melatonin and luzindole, respectively. These findings indicate that a circadian clock in the vertebrate retina regulates dopamine release by the activation of melatonin receptors and that endogenous melatonin modulates rod and cone pathways through dopamine-mediated D(2)-like receptor activation.

Differential distribution of Mel(1a) and Mel(1c) melatonin receptors in Xenopus laevis retina

The hormone melatonin is an output signal of an endogenous circadian clock in retinal photoreceptors. Melatonin may act as a paracrine and/or intracrine neurohormone by binding to specific receptors in the eye. The distribution of Mel(1a) and Mel(1c) melatonin receptors in the Xenopus laevis retina was examined by immunocytochemistry, using antibodies prepared against specific sequences of the Xenopus receptor proteins. Antibodies that label dopaminergic and GABA-ergic amacrine cells were used in double-label experiments with the melatonin receptor antibodies. The distribution of Mel(1a) and Mel(1c) receptor immunoreactivity was similar insofar as the two receptors were localized in the inner plexiform layer. However, the Mel(1c) receptor displayed some immunoreactivity in the photoreceptor cells, whereas the Mel(1a) receptor displayed little if any photoreceptor labelling. The Mel(1c) antibody, but not the Mel(1a), labelled a population of ganglion cells. While both receptors were localized to the outer plexiform layer, they did not appear to localize to the identical cell types. These results demonstrate that the Mel(1a) and Mel(1c) receptor proteins are present in cells of the X. laevis retina, and their distribution in the photoreceptors and inner retina is very similar to that reported in the human retina. The differential pattern of expression of the melatonin receptors suggests that melatonin may convey differential effects on various target cells in the retina.

Regulation of gene expression by melatonin: a microarray survey of the rat retina

The pineal secretory product melatonin is synthesized by pinealocytes and retinal photoreceptors on a cyclic rhythm, with highest levels occurring at night. Our previous work has demonstrated that melatonin treatment increases the sensitivity of the rat retina to light-induced photoreceptor cell death. This raises the possibility that inappropriate exposure of photoreceptors to melatonin may result in visual impairment, caused by a loss of retinal photoreceptors. We hypothesize that retinal genes whose expression levels are altered in response to melatonin may be involved in processes that contribute to light-induced photoreceptor cell death. To identify retinal genes that are up- or down-regulated in response to melatonin receptor binding, rats were treated with or without melatonin, and the RNA from the neural retinas and retinal pigment epithelium (RPE) were analyzed for differential gene expression by hybridization of labeled cRNA probes to an Affymetrix rat genome microarray set. GeneChip algorithms were applied to measured hybridization intensities of compared samples and showed that in the neural retina, six genes were up-regulated, and eight were down-regulated. In the RPE, 15 genes were up-regulated, and two genes were down-regulated. The protein products of these specific genes are potentially involved in the molecular mechanism of melatonin action in the retina, and may play a role in the effect of melatonin on light-induced photoreceptor cell death. Identification of these candidate genes and their response to melatonin administration may provide a foundation for further studies on gene regulation by melatonin, the function of melatonin in the retina, and the role of circadian signaling in inherited and environmentally induced photoreceptor degenerations.

Melatonin MT-1-receptor immunoreactivity in the human eye

AIM

To examine the distribution of melatonin 1a (MT1) receptors in the human eye.

METHODS

Seven normal human eyes were examined by immunohistochemical staining of paraffin sections, using an anti-MT1 primary antibody and an ABC detection system.

RESULTS

MT1 receptor immunoreactivity (MT1-IR) was detected primarily in the inner segments of rods and cones and in retinal ganglion cells. In addition, MT1-IR was present in the adventitia of retinal arteries and veins, including the papillary region, but absent in ciliary and choroidal vessels. Mild staining of corneal endothelial cells and keratocytes was observed in all but two eyes.

CONCLUSION

MT1-IR is present in various ocular tissues with the highest density in photoreceptor cells and ganglion cells. The physiological function of these receptors deserves further investigation.

Melatonin induces alterations in protein expression in the Xenopus laevis retina

The hormone melatonin is synthesized by pinealocytes and retinal photoreceptors with a diurnal rhythm. Melatonin produced in the retina at night is thought to exert local modulatory effects by binding to specific receptors in several different retinal cell types. The mechanisms by which melatonin influences circadian activity in retinal cells is poorly understood. Suppression of cyclic AMP synthesis appears to be a major signaling pathway in response to melatonin receptor binding in many tissues. A potential downstream consequence of melatonin-induced changes in cyclic AMP concentrations and protein phosphorylation is the up- or down-regulation of expression of specific genes. In this report, we examined the changes in expression levels of specific proteins in the neural retina and retinal pigment epithelium (RPE) in response to melatonin treatment, because both of these tissues express melatonin receptors. Neural retina and RPE isolated from the eyes of Xenopus laevis were treated with or without 1 microM melatonin for 6 hr, then the rapidly synthesized tissue proteins were radiolabeled by a 15 min incubation with 35S-methionine, and the proteins were subsequently analyzed by two-dimensional gel electrophoresis and autoradiography. In both the neural retina and RPE, the densities of some specific proteins were altered in response to melatonin treatment, and the few protein spots that were altered were distinct between the two tissues. These results support the concept that one function of melatonin may be to regulate the expression of specific genes and the consequent protein levels, and that the target genes may differ according to the cell or tissue type.

Melatonin receptor mRNA and protein expression in Xenopus laevis nonpigmented ciliary epithelial cells

Melatonin is an output signal of the circadian clock, and may regulate diurnal rhythms in ocular tissues. A role for melatonin has been suggested in the circadian changes in intraocular pressure (IOP). Changes in IOP may be due partially to changes in the rate of aqueous humor secretion, which is produced by the nonpigmented epithelium of the ciliary body. To examine the mechanism by which melatonin may influence ciliary epithelium function and perhaps the IOP diurnal rhythm, immunocytochemistry with an antibody directed against the Mel(1c) melatonin receptor subtype was performed on sections of Xenopus eyes. Melatonin receptor immunoreactivity was observed in the basolateral regions of the nonpigmented epithelial cells of the ciliary body. Receptor immunoreactivity was also observed in cells of the retina, as has been previously reported. Specific immunoreactivity was not observed in the epithelium of the iris or pigmented ciliary epithelium. In situ hybridization of the Xenopus eye revealed expression of Mel(1c) but not Mel(1b) receptor mRNA in the nonpigmented ciliary epithelium. These results provide evidence that the nonpigmented epithelia of the ciliary body are direct targets for melatonin, and supports previous work that melatonin may influence the rate of aqueous humor secretion by ciliary epithelium, and perhaps the circadian rhythm of IOP.