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

Melatonin and retinal cell damage: molecular and biological functions

The indoleamine hormone, melatonin, is produced in the pineal gland and has an essential role in many physiological functions. The pineal gland is considered to be the most important organ for producing melatonin. Nevertheless, it is important to point out that the eye is also capable of producing melatonin, and has its own circadian rhythm in producing this hormone. Melatonin is mainly produced by a subpopulation of photoreceptors in a diurnal rhythm. Numerous in vitro and in vivo studies have shown the beneficial effects of melatonin in eye-related disorders. These diseases primarily affect retinal cells, highlighting the therapeutic potential of melatonin, especially in the retina. Melatonin's ability to regulate oxidative stress response pathways and modulate the expression of antioxidant genes makes it a promising candidate for mitigating retinal cell damage. Moreover, melatonin can modulate inflammatory pathways such as NF-кB and further reduce retinal damage, as well as affecting programmed cell death such as apoptosis and autophagy in retinal cells. Therefore, the goal of this review is to explore the ways in which melatonin protects retinal cells from damage and ischemia. We discuss the mechanisms involved in order to gain valuable understanding of the possible therapeutic applications of melatonin in protection of retinal cells and treatment of retinal disorders.

Challenging glaucoma with emerging therapies: an overview of advancements against the silent thief of sight

Glaucoma, a leading cause of irreversible blindness, represents a significant challenge in ophthalmology. This review examines recent advancements in glaucoma treatment, focusing on innovative medications and creative strategies. While new agents offer promising methods for lowering intraocular pressure (IOP), they also pose challenges related to efficacy and side effects. Alongside IOP reduction, emerging neuroprotective approaches are being explored to safeguard retinal ganglion cells (RGCs) from glaucoma-induced damage. The review also evaluates the potential of novel drug delivery systems, such as biodegradable implants and nanoparticles, to enhance treatment effectiveness and patient adherence. Additionally, it highlights the role of personalized medicine in identifying new biomarkers and customizing therapies based on individual genetic and environmental factors.

The Effect of Melatonin on Analgesia, Anxiety, and Intraocular Pressure (IOP) in Cataract Surgery Under Topical Anesthesia

PURPOSE

This study investigated the effect of oral melatonin as a premedication before cataract surgery on pain score, anxiety, surgical conditions, and intraocular pressure (IOP) during cataract surgery with phacoemulsification under topical anesthesia.

DESIGN

This randomized, double-blind prospective study was conducted among 120 patients aged 50 to 80.

METHODS

Patients were randomly assigned to receive either sublingual melatonin 3 mg or placebo 60 min before surgery. Verbal anxiety and pain scores, heart rate, systolic and diastolic blood pressure, intraocular pressure, and quality of surgical conditions were recorded. The results were analyzed using SPSS 23 software, and statistical analysis consisted of χ² test and t-test, and a P value less than .05 was considered significant.

FINDINGS

There was a significant difference between the two groups regarding pain after the surgery before being discharged from the recovery room. The mean pain score in the melatonin group was lower than the placebo group in T4 (in the postoperative ward) (P < .05). The anxiety score of patients was lower in the melatonin group in T3 and T4 (during surgery and in the postoperative ward, respectively) (P < .05). IOP was significantly lower in the case group at the end of the surgery after receiving premedication (P = .021).

CONCLUSIONS

This study showed that 3 mg Sublingual melatonin premedication for patients undergoing cataract surgery under topical anesthesia reduced patients' anxiety scores, pain score, and IOP.

Regulation of Aqueous Humor Secretion by Melatonin in Porcine Ciliary Epithelium

Secretion of melatonin, a natural hormone whose receptors are present in the ciliary epithelium, displays diurnal variation in the aqueous humor (AH), potentially contributing to the regulation of intraocular pressure. This study aimed to determine the effects of melatonin on AH secretion in porcine ciliary epithelium. The addition of 100 µM melatonin to both sides of the epithelium significantly increased the short-circuit current (Isc) by ~40%. Stromal administration alone had no effect on the Isc, but aqueous application triggered a 40% increase in Isc, similar to that of bilateral application without additive effect. Pre-treatment with niflumic acid abolished melatonin-induced Isc stimulation. More importantly, melatonin stimulated the fluid secretion across the intact ciliary epithelium by ~80% and elicited a sustained increase (~50-60%) in gap junctional permeability between pigmented ciliary epithelial (PE) cells and non-pigmented ciliary epithelial (NPE) cells. The expression of MT3 receptor was found to be >10-fold higher than that of MT1 and MT2 in porcine ciliary epithelium. Aqueous pre-treatment with MT1/MT2 antagonist luzindole failed to inhibit the melatonin-induced Isc response, while MT3 antagonist prazosin pre-treatment abolished the Isc stimulation. We conclude that melatonin facilitates Cl- and fluid movement from PE to NPE cells, thereby stimulating AH secretion via NPE-cell MT3 receptors.

Melatonin Type 2 Receptor Activation Regulates Blue Light Exposure-Induced Mouse Corneal Epithelial Damage by Modulating Impaired Autophagy and Apoptosis

The MT1/2 receptors, members of the melatonin receptor, belong to G protein-coupled receptors and mainly regulate circadian rhythms and sleep in the brain. Previous studies have shown that in many other cells and tissues, such as HEK293T cells and the retina, MT1/2 receptors can be involved in mitochondrial homeostasis, antioxidant, and anti-inflammatory responses. In our study, we aimed to investigate the effects of blue light (BL) exposure on the expression of melatonin and its receptors in the mouse cornea and to evaluate their functional role in corneal epithelial damage. After exposing 8-week-old C57BL/6 mice to BL at 25 and 100 J/cm² twice a day for 14 days, a significant increase in the expression of 4-HNE and MT2 was observed in the cornea. MT2 antagonist-treated mice exposed to BL showed an increased expression of p62 and decreased expression of BAX and cleaved caspase 3 compared with mice exposed only to BL. In addition, MT2 antagonist-treated mice showed more enhanced MDA and corneal damage. In conclusion, BL exposure can induce MT2 expression in the mouse cornea. MT2 activation can modulate impaired autophagy and apoptosis by increasing the expression of BAX, an apoptosis activator, thereby regulating the progression of corneal epithelial damage induced by BL exposure.

Changes in Ocular Biometric Parameters Over a 24-Hour Period in Ocular Hypertensive Patients

Purpose: To identify 24-h changes in ocular biometric parameters in subjects with ocular hypertension (OHT), and to determine if an intraocular pressure (IOP)-lowering drug alters these parameters. Methods: Thirty volunteers with OHT (58.6 ± 9.2 years of age) were enrolled in this randomized, double-masked, placebo-controlled, crossover study. Participants self-administered 0.2% brimonidine or placebo 3 times daily for 6 weeks. Measurements of seated and supine IOP, central cornea thickness (CCT), anterior chamber depth (ACD), axial length (AXL), and lens thickness were made at 8 am, 3 pm, 8 pm, and 3 am. Statistical tests were Student's 2-tailed paired t-tests or 2-way analysis of variance (ANOVA) followed by one-way ANOVA and post hoc testing. Results: Time of day had a significant effect on IOP, CCT, ACD, and AXL. In placebo-treated eyes, CCT was greater at 3 am than at any other time (P < 0.01), ACD and AXL were greater at 3 am and 8 pm than at 3 pm (P < 0.01). Daytime IOPs were higher than nighttime (seated, P = 0.007; supine, P = 0.018), and supine IOP at night was higher than seated IOP during the day (P < 0.001). Brimonidine did not lower IOP at night nor did it alter the 24-h patterns of CCT, ACD, and AXL. Conclusions: Ocular biometric parameters exhibit characteristic 24-h fluctuations in patients with OHT. At night compared with day, the supine IOP increases, the cornea thickens, the anterior chamber deepens, and the AXL increases. Brimonidine does not alter these parameters at times when it lowers IOP (day) nor when it does not (night). Clinical Trial Registration number: NCT0132419.

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.

Ocular Clocks: Adapting Mechanisms for Eye Functions and Health

Vision is a highly rhythmic function adapted to the extensive changes in light intensity occurring over the 24-hour day. This adaptation relies on rhythms in cellular and molecular processes, which are orchestrated by a network of circadian clocks located within the retina and in the eye, synchronized to the day/night cycle and which, together, fine-tune detection and processing of light information over the 24-hour period and ensure retinal homeostasis. Systematic or high throughput studies revealed a series of genes rhythmically expressed in the retina, pointing at specific functions or pathways under circadian control. Conversely, knockout studies demonstrated that the circadian clock regulates retinal processing of light information. In addition, recent data revealed that it also plays a role in development as well as in aging of the retina. Regarding synchronization by the light/dark cycle, the retina displays the unique property of bringing together light sensitivity, clock machinery, and a wide range of rhythmic outputs. Melatonin and dopamine play a particular role in this system, being both outputs and inputs for clocks. The retinal cellular complexity suggests that mechanisms of regulation by light are diverse and intricate. In the context of the whole eye, the retina looks like a major determinant of phase resetting for other tissues such as the retinal pigmented epithelium or cornea. Understanding the pathways linking the cell-specific molecular machineries to their cognate outputs will be one of the major challenges for the future.

Analysis of Circadian Rhythm Gene Expression With Reference to Diurnal Pattern of Intraocular Pressure in Mice

PURPOSE

To determine the expression of circadian rhythm clock genes in the iris-ciliary body complex of mice and their association with the diurnal pattern of intraocular pressure (IOP).

METHODS

Thirty wild-type C57BL/6 mice were acclimated to a 12-hour light-dark cycle. Intraocular pressure was measured with a rebound tonometer at six time points daily (circadian time [CT] 2, 6, 10, 14, 18, and 22 hours) for five consecutive days. On day 6, mice were euthanized at CT 2, 6, 10, 14, 18, and 22. Eyes were flash-frozen or fixed in 4% phosphate-buffered paraformaldehyde. Total RNA was extracted from the iris-ciliary body complex, and RNA expression of circadian rhythm genes Bmal1, Clock, Cry1, Cry2, Per1, and Per2 was assessed by quantitative real-time PCR. Fixed eyes were paraffin embedded, and immunohistochemistry was performed to localize corresponding proteins (BMAL1, CLOCK, CRY1, CRY2, PER1, and PER2). Linear regression analysis was performed to correlate gene expression with IOP.

RESULTS

Intraocular pressure demonstrated a consistent circadian pattern. The clock genes Bmal1, Clock, Cry1, Cry2, Per1, and Per2 showed a circadian pattern of expression in the iris-ciliary body complex of mice. Bmal1, Clock, Cry1, Per1, and Per2 gene expression demonstrated statistically significant correlations with diurnal variations of IOP. BMAL1, CLOCK, CRY1, CRY2, PER1, and PER2 proteins were found to be expressed locally in the nonpigmented epithelium of the ciliary body.

CONCLUSIONS

Expression patterns of candidate circadian rhythm genes correlates with the diurnal pattern variation of IOP in mouse eyes, indicating a possible mechanism of IOP regulation through these genes.

The effects of melatonin on anxiety and pain scores of patients, intraocular pressure, and operating conditions during cataract surgery under topical anesthesia

Aims:

To evaluate the effects of melatonin premedication on anxiety and pain scores of patients, operating conditions, and intraocular pressure during cataract surgery under topical anesthesia.

Materials and Methods:

Sixty patients were randomly assigned to receive either sublingual melatonin 3 mg or placebo 60 min before surgery. Verbal anxiety scores and verbal pain scores, heart rate, systolic and diastolic blood pressure, intraocular pressure, and quality of operating conditions were recorded.

Results:

Melatonin significantly reduced the anxiety scores (median, interquartile range) from 5 and 5–3 to 3 and 2–4 after premedication and to 3 and 2–3 during surgery and to 0 and 0–1 postoperatively before discharge from the recovery room. There were significant differences between two groups in anxiety scores after premedication (95% CI 3–3.5; P = 0.023), intraoperatively (95% CI 2.5–3.5; P = 0.007), and postoperatively (95% CI 0.5–1; P = 0.007). The surgeon reported better quality of operating conditions in the melatonin group (P = 0.001). No significant difference in intraoperative and postoperative pain scores, intraocular pressure, heart rate, and systolic and diastolic blood pressure between two groups was recorded.

Conclusion:

Sublingual melatonin premedication for patients undergoing cataract surgery under topical anesthesia reduced the anxiety scores in patients and provided excellent operating conditions.

Circadian organization of the mammalian retina: from gene regulation to physiology and diseases

The retinal circadian system represents a unique structure. It contains a complete circadian system and thus the retina represents an ideal model to study fundamental questions of how neural circadian systems are organized and what signaling pathways are used to maintain synchrony of the different structures in the system. In addition, several studies have shown that multiple sites within the retina are capable of generating circadian oscillations. The strength of circadian clock gene expression and the emphasis of rhythmic expression are divergent across vertebrate retinas, with photoreceptors as the primary locus of rhythm generation in amphibians, while in mammals clock activity is most robust in the inner nuclear layer. Melatonin and dopamine serve as signaling molecules to entrain circadian rhythms in the retina and also in other ocular structures. Recent studies have also suggested GABA as an important component of the system that regulates retinal circadian rhythms. These transmitter-driven influences on clock molecules apparently reinforce the autonomous transcription-translation cycling of clock genes. The molecular organization of the retinal clock is similar to what has been reported for the SCN although inter-neural communication among retinal neurons that form the circadian network is apparently weaker than those present in the SCN, and it is more sensitive to genetic disruption than the central brain clock. The melatonin-dopamine system is the signaling pathway that allows the retinal circadian clock to reconfigure retinal circuits to enhance light-adapted cone-mediated visual function during the day and dark-adapted rod-mediated visual signaling at night. Additionally, the retinal circadian clock also controls circadian rhythms in disk shedding and phagocytosis, and possibly intraocular pressure. Emerging experimental data also indicate that circadian clock is also implicated in the pathogenesis of eye disease and compelling experimental data indicate that dysfunction of the retinal circadian system negatively impacts the retina and possibly the cornea and the lens.

Melatonin and its analog 5-methoxycarbonylamino-N-acetyltryptamine potentiate adrenergic receptor-mediated ocular hypotensive effects in rabbits: significance for combination therapy in glaucoma

Melatonin is currently considered a promising drug for glaucoma treatment because of its ocular hypotensive and neuroprotective effects. We have investigated the effect of melatonin and its analog 5-methoxycarbonylamino-N-acetyltryptamine, 5-MCA-NAT, on β₂/α(2A)-adrenergic receptor mRNA as well as protein expression in cultured rabbit nonpigmented ciliary epithelial cells. Quantitative polymerase chain reaction and immunocytochemical assays revealed a significant β₂-adrenergic receptor downregulation as well as α(2A)-adrenergic receptor up-regulation of treated cells (P < 0.001, maximal significant effect). In addition, we have studied the effect of these drugs upon the ocular hypotensive action of a nonselective β-adrenergic receptor (timolol) and a selective α₂-adrenergic receptor agonist (brimonidine) in normotensive rabbits. Intraocular pressure (IOP) experiments showed that the administration of timolol in rabbits pretreated with melatonin or 5-MCA-NAT evoked an additional IOP reduction of 14.02% ± 5.8% or 16.75% ± 5.48% (P < 0.01) in comparison with rabbits treated with timolol alone for 24 hours. Concerning brimonidine hypotensive action, an additional IOP reduction of 29.26% ± 5.21% or 39.07% ± 5.81% (P < 0.001) was observed in rabbits pretreated with melatonin or 5-MCA-NAT when compared with animals treated with brimonidine alone for 24 hours. Additionally, a sustained potentiating effect of a single dose of 5-MCA-NAT was seen in rabbits treated with brimonidine once daily for up 4 days (extra IOP decrease of 15.57% ± 5.15%, P < 0.05, compared with brimonidine alone). These data confirm the indirect action of melatoninergic compounds on adrenergic receptors and their remarkable effect upon the ocular hypotensive action mainly of α₂-adrenergic receptor agonists but also of β-adrenergic antagonists.

Melatonin receptor agonist-induced reduction of SNP-released nitric oxide and cGMP production in isolated human non-pigmented ciliary epithelial cells

The present study was designed to determine the effects of melatonin and its receptor agonists on SNP-released nitric oxide (NO) and cGMP production in aqueous humor producing cells of the ciliary body because these effects may play a role in melatonin receptor-mediated regulation of intraocular pressure (IOP). NO release protocols were carried out using human non-pigmented ciliary epithelial (hNPCE) cells treated in dye free DMEM containing l-arginine (10(-3) M). The cGMP experimental protocols were performed using dye free DMEM containing 3-isobutyl-1-methylxanthine (IBMX, 10(-4) M). The effects of varying concentrations (10(-13), 10(-11), 10(-9), 10(-7), and 10(-5) M) of melatonin, 5-MCA-NAT (putative MT(3) agonist), N-butanoyl-2-(2-methoxy-6H-isoindolo[2, 1-a]indol-11-yl)ethanamine (IIK7; selective MT(2) agonist) or S-27633-1 (selective MT(1) agonist) on sodium nitroprusside (SNP)-released NO or cGMP production were determined in separate experiments. NO and cGMP levels were measured using a colorimetric assay or enzyme immunoassay (EIA), respectively. Melatonin receptor selectivity was evaluated using luzindole (LUZ; nonselective MT(1)/MT(2) antagonist) or 4-phenyl-2-propionamidotetralin (4P-PDOT; selective MT(2) antagonist). Melatonin, 5-MCA-NAT, and IIK7 all caused concentration-dependent reduction of SNP-released NO and cGMP production. The inhibitory actions of melatonin, 5-MCA-NAT and IIK7 were either completely blocked at 10(-13), 10(-11), and 10(-9) M concentrations of the agonists or partially at 10(-7) and 10(-5) M in the presence of luzindole or 4P-PDOT. Results from this study suggest that melatonin and its analogs, 5-MCA-NAT and IIK7 inhibit SNP-released NO and cGMP production via activation of MT(2) receptors in human NPCE cells. These actions may play a role in melatonin agonist-induced regulation of aqueous humor secretion and IOP.

Melatonin: an underappreciated player in retinal physiology and pathophysiology

In the vertebrate retina, melatonin is synthesized by the photoreceptors with high levels of melatonin at night and lower levels during the day. Melatonin exerts its influence by interacting with a family of G-protein-coupled receptors that are negatively coupled with adenylyl cyclase. Melatonin receptors belonging to the subtypes MT(1) and MT(2) have been identified in the mammalian retina. MT(1) and MT(2) receptors are found in all layers of the neural retina and in the retinal pigmented epithelium. Melatonin in the eye is believed to be involved in the modulation of many important retinal functions; it can modulate the electroretinogram (ERG), and administration of exogenous melatonin increases light-induced photoreceptor degeneration. Melatonin may also have protective effects on retinal pigment epithelial cells, photoreceptors and ganglion cells. A series of studies have implicated melatonin in the pathogenesis of age-related macular degeneration, and melatonin administration may represent a useful approach to prevent and treat glaucoma. Melatonin is used by millions of people around the world to retard aging, improve sleep performance, mitigate jet lag symptoms, and treat depression. Administration of exogenous melatonin at night may also be beneficial for ocular health, but additional investigation is needed to establish its potential.

Removal of melatonin receptor type 1 increases intraocular pressure and retinal ganglion cells death in the mouse

Previous studies have demonstrated that melatonin is effective in lowering intraocular pressure and that it may also protect ganglion cells. We have recently reported that, in mice lacking the melatonin receptors type 1, 25-30% ganglion cells die out by 18months of age, suggesting that these receptors might be important for ganglion cells survival. In this study we show that the loss of ganglion cells is specific for melatonin receptors type 1 knock-out since mice lacking the melatonin receptors type 2 did not show any significant change in the number ganglion cells during aging. Furthermore, we report that melatonin receptors type 1 knock-out mice have higher intraocular pressure during the nocturnal hours than control or melatonin receptors type 2 knock-out mice at 3 and 12months of age. Finally, our data indicate that administration of exogenous melatonin in wild-type, but not in melatonin receptors type 1 knock-out, can significantly reduce intraocular pressure. Our studies indicate that the decreased viability of ganglion cells observed in melatonin receptors type 1 knock-out mice may be a consequence of the increases in the nocturnal intraocular pressure thus suggesting that intraocular pressure levels at night and melatonin signaling should be considered as risk factor in the pathogenesis of glaucoma.

Effects of 530 nm green light on refractive status, melatonin, MT1 receptor, and melanopsin in the guinea pig

PURPOSE

To investigate (i) the effect of monochromatic light on inhibiting induction of light-induced melatonin and (ii) the roles of melanopsin and MT1 receptor in light-induced myopia in the guinea pig.

METHODS

Forty-eight guinea pigs were randomly distributed into three treatment groups: white-light (control), green-light (530 nm), and blue-light (480 nm) groups. Levels of pineal gland melatonin were measured twice daily--10:00 a.m. and 10:00 p.m.--10 days after initial light treatment. Thirty additional guinea pigs were also assigned to these groups and treated similarly. For these latter animals, refractive status, ocular length, and vitreous depth were measured before and after light treatment. Eight weeks after light treatment, retinal and sceral levels of melanopsin, melatonin receptor type (MT) 1, and mRNA protein were determined by Western blotting, real-time polymerase chain reaction (RT-PCR), and immunohistochemistry.

RESULTS

The level of pineal gland melatonin in the green-light group was significantly higher than that in the blue-light group. Biometric measurements showed that guinea pigs in the green-light group had a somewhat myopic refractive status. Expressions of retinal melanopsin mRNA and protein were significantly higher in the blue-light group and lower in the green-light group when compared to controls. Conversely, expressions of MT1 receptor mRNA and protein in retina and sclera were significantly higher in the green-light group and lower in the blue-light group when compared to controls.

CONCLUSIONS

Green light appears to suppress induction of melatonin production. In addition, 530 nm of green light is involved in the development of myopia. In the guinea pig, MT1 receptor and melanopsin appear to play roles in the development of myopia induced by 530 nm of light.

New treatments for ocular hypertension

Glaucoma is a neurodegenerative pathology that affects the optic nerve producing blindness. This disease is often a consequence of an abnormal increase of intraocular pressure (IOP) due to a reduction in the ability of the eye to drain a transparent fluid termed aqueous humour. The dynamics of the aqueous humour is highly controlled by the autonomic nervous system, mainly the sympathetic, regulating its production and parasympathetic controlling the evacuation of aqueous humour. This has led pharmaceutical companies to develop chemicals which, by acting via different targets can substantially reduce IOP. Parasympathomimetics, adrenergic antagonists, plus eventually adrenergic agonists, are commonly used for the reduction of IOP and therefore for treatment of glaucoma. New substances linked to the nervous system that innervates the eye are emerging as interesting candidates. Nucleotides, commonly costored with catecholamines or acetylcholine or the indole melatonin, present interesting properties reducing IOP. Moreover new technological ideas such as the use of siRNA (small interference RNA) to silence protein expression demonstrate the relevance of this method to approach ocular hypertension and glaucoma from a different point of view. These three main groups of molecules: nucleotides, melatonins and siRNAs, are reviewed since they appear as firm candidates for the treatment of glaucoma in the near future.

Sympathetic nervous system modulates the ocular hypotensive action of MT2-melatonin receptors in normotensive rabbits

The aim of this study was to investigate the hypotensive effect of the melatonin analogue, N-butanoyl-2-(2-methoxy-6H-isoindolo[2,1-a]indol-11-yl)ethanamine (IIK7), through MT(2)-melatonin receptors and the involvement of the sympathetic nervous system in this action in New Zealand rabbit eyes. The topical application of melatonin or IIK7 produced a reduction in intraocular pressure of 20.2 +/- 5.3% and 38.5 +/- 3.2% respectively. This effect was concentration-dependent; it was blocked by selective MT(2) receptor antagonists and was severely diminished after chemical sympathectomy. Immunohistochemistry and western blot analysis showed the ciliary processes as the site of this action and no co-localization of MT(2)-melatonin receptor with the sympathetic nervous system was observed. The beta-adrenergic agonists, terbutaline and salbutamol, potentiated the hypotensive effect of IIK7 reducing intraocular pressure (IOP) 41.75 +/- 4.26% and 44.7 +/- 5.6% respectively. Also, IIK7 in presence of the nonspecific protein phosphatase inhibitor okadaic acid, lowered IOP 32.2 +/- 4.5% and in presence of forskolin plus 3-isobutyl-1-methylxanthine decreased IOP in 32.2 +/- 5.47%. These data suggest that the melatonin agonist IIK7 reduces intraocular pressure by acting through MT(2)-melatonin receptors presumably decreasing aqueous humour formation. Also, in the presence of beta-adrenoceptor agonists MT(2)-melatonin receptors activity increase their ability to reduce IOP.

Role of CYP1B1 in Glaucoma

Glaucoma is a leading cause of blindness, estimated to affect 60 million people by 2010, and represents a heterogeneous group of neurodegenerative disease. The two major types of glaucoma include primary open-angle glaucoma (POAG) and primary congenital glaucoma (PCG). A genetically heterogeneous group of developmental disorders known as anterior segment dysgenesis (ASD) have been reported to be associated with increased intraocular pressure (IOP) and glaucoma. These include Peters' anomaly, Rieger's anomaly, aniridia, iris hypoplasia, and iridogoniodysgenesis. Genetic linkage analysis and mutation studies have identified CYP1B1 as a causative gene in PCG, as a modifier gene in POAG, and, on rare occasions, as causative gene in POAG as well as in several ASD disorders. CYP1B1-deficient mice exhibit abnormalities in their ocular drainage structure and trabecular meshwork that are similar to those reported in human PCG patients. Accordingly, it is speculated that diminished or absent metabolism of key endogenous CYP1B1 substrates adversely affects the development of the trabecular meshwork. CYP1B1 protein is involved in the metabolism of steroids, retinol and retinal, arachidonate, and melatonin. The conserved expression of CYP1B1 in both murine and human eyes, its higher expression in fetal than adult eyes, and its biochemical properties are consistent with this hypothesis. The exact role of CYP1B1 in the pathogenesis of glaucoma and other ASD disorders remains to be elucidated.

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.

Light, dark, and melatonin: emerging evidence for the importance of melatonin in ocular physiology

Melatonin is a hormone, which is mainly produced by the pineal gland, a vestigial eye. Rather than the rods and cones, it is a newly discovered subgroup of photosensitive retinal ganglion cells, which is responsible for mediating the light-dark cycles, thus regulating melatonin's secretion. One of the correlates of the circadian rhythm of melatonin release is the habitual sleep pattern. Patients with circadian rhythm sleep disorders, including some blind patients with no light-induced suppression of melatonin, benefit from melatonin treatment. Melatonin is synthesized in the retina, lens, ciliary body as well as other parts of the body. In this review, we discuss the physiological roles of melatonin in the eye, as well as the potential therapeutic avenues currently under study.

Melatonin concentrations in aqueous humor of glaucoma patients

PURPOSE

To determine whether glaucoma patients exhibit an abnormal melatonin concentration in aqueous humor.

DESIGN

Case-controlled study, laboratory investigation.

METHODS

Aqueous humor and plasma samples of 28 patients with primary open-angle glaucoma and 31 nonglaucoma control patients were collected during surgery, and additional plasma samples were taken the night preceding surgery. Melatonin concentrations were determined using direct radioimmunoassay.

RESULTS

This study shows detectable concentrations of melatonin in the aqueous humor of healthy humans (45% of subjects) and of glaucoma patients (36% of subjects) sampled in the morning, with similar levels of aqueous humor melatonin concentrations in both groups (6.4 +/- 9.3 standard deviation (SD) pg/ml and 3.6 +/- 1.9 pg/ml, respectively). We find no significant association between the severity of glaucoma and melatonin levels in aqueous humor or in plasma.

CONCLUSIONS

Moderate and severe glaucoma does not appear to be associated with abnormal melatonin concentrations in aqueous humor, at least during the morning sampling period assayed in this study.

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.

Effect of 5-MCA-NAT, a putative melatonin MT3 receptor agonist, on intraocular pressure in glaucomatous monkey eyes

PURPOSE

5-MCA-NAT, a putative melatonin MT3 receptor agonist, reduced intraocular pressure (IOP) in ocular normotensive rabbit eyes. This study evaluates the effect of topical application of 5-MCA-NAT on IOP in monkey eyes with laser-induced unilateral glaucoma.

METHODS

A multiple-dose study was performed in 8 glaucomatous monkey eyes. One 25-microL drop of 5-MCA-NAT (2%) was applied topically to the glaucomatous eye at 9:30 am and 3:30 pm for 5 consecutive days. IOP was measured hourly for 6 hours beginning at 9:30 am for one baseline day, one vehicle-treated day, and treatment days 1, 3, and 5 with 5-MCA-NAT.

RESULTS

Compared with vehicle treatment, twice daily administration of 5-MCA-NAT for 5 days reduced (P < 0.05) IOP from 1 hour to 5 hours after the first dose, and the IOP-lowering effects were shown to last at least 18 hours following administration, based on IOP measurements made after the fourth and eighth doses. The ocular hypotensive effect of 5-MCA-NAT was enhanced with repeated dosing. The maximum reduction (P < 0.001) of IOP occurred at 3 hours after each morning dose, and was 4.0 +/- 0.5 (mean +/- SEM) mm Hg (10%) on day 1, 5.6 +/- 0.8 mm Hg (15%) on day 3, and 7.0 +/- 1.1 mm Hg (19%) on day 5. Adverse ocular or systemic side effects were not observed during the 5 days of treatment.

CONCLUSIONS

5-MCA-NAT, a putative melatonin MT3 receptor agonist, reduces IOP in glaucomatous monkey eyes. Melatonin agonists with activity on the putative MT3 receptor may have clinical potential for treating elevated IOP.

Gene regulation of melatonin and dopamine receptors during eye development

To gain insight into the role of melatonin and dopamine in retinal development, gene expression of two melatonin receptors, MT1 and MT2, as well as five dopamine receptors, D1, D2, D3, D4 and D5, in the rat eye was analyzed by reverse transcription-polymerase chain reaction across various developmental stages. MT1 transcript levels reached maximum levels at embryonic day (E) 16 and then decreased gradually until reaching adult levels by postnatal day (P) 14. MT2 transcript levels similarly peaked at E16, but then decreased dramatically until birth to its lowest levels, which were maintained throughout the postnatal period. Thus, gene expression of both the MT1 and MT2 receptors showed a striking inverse correlation with maturation of the eye. In contrast to melatonin receptors, gene expression of all dopamine receptor subtypes, except for D3, showed only an increase as development proceeds with highest levels in adulthood. The D3 message was not detected throughout the developmental period examined. Gene expression of D1-like receptors, D1 and D5, showed a substantial increase to adult levels during the fetal period at E16 and E20, respectively. Transcript levels of D2-like receptors, D2 and D4, on the other hand, were not detected before birth but increased significantly to adult levels by P7 and P14, respectively. The present findings suggest the presence of unique developmental mechanisms by which transcription of various G protein-coupled receptors are regulated in the eye.

Melatonin receptor expression in the cornea and sclera

The scornea and sclera have been shown to exhibit circadian rhythms in cellular proliferation, wound healing and extracellular matrix synthesis. The distribution of melatonin Mel1a and Mel1c receptors was examined in the cornea and sclera of the Xenopus laevis eye in order to determine whether melatonin may potentially influence the growth and/or development of these ocular tissues. Sections of adult X. laevis eyes were analyzed by immunocytochemistry and confocal microscopy, using antibodies prepared against specific peptide sequences of the Xenopus Mel1a and Mel1c receptor proteins. Antibodies were pre- incubated with their appropriate antigenic peptides to control for non-specific labelling. Analysis of the distribution of Mel1a and Mel1c receptor immunoreactivity in the Xenopus eye revealed that both the Mel1a and Mel1c receptors were located in the outer fibrous layer (OFL) of the sclera, with Mel1c labelling being the most prominent. Similarly, Mel1a and Mel1c (Mel1c mostly) were also located in cells of the inner fibrous layer (IFL) with Mel1c being most abundant. The chondrocytes of the cartilaginous layer also appeared to express Mel1a, Mel1c, or both receptors. Both Mel1a and Mel1c receptor immunoreactivity were observed in the corneal epithelium and endothelium. Whereas the Mel1a antibody labelled the entire corneal epithelial layer, the Mel1c antibody labelled only the most superficial layer of epithelial cells. Cell processes of fibroblasts of the corneal stroma were immunoreactive for either Mel1a or Mel1c receptors. The identification of Mel1a and Mel1c receptors in restricted distributions in the cornea and sclera suggests that melatonin may play a role in the cellular physiology of these ocular tissues.

Ocular hypotensive effects of melatonin receptor agonists in the rabbit: further evidence for an MT3 receptor

(1) Melatonin is involved in the control of intraocular pressure during the night and day photoperiod. We have investigated the receptor that regulates intraocular pressure in New Zealand white rabbits by means of agonists and antagonists of melatonin receptors. (2) Melatonin and its analogues: 2-Phe-melatonin, 6-Cl-melatonin, 2-I-melatonin, 5- methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT) and N-acetyltryptamine all produced a reduction in intraocular pressure. Dose-response analysis for these compounds gave pD(2) values of 9.3+/-0.24 for melatonin; 9.0+/-0.09 for 6-Cl-melatonin; 9.0+/-0.84 for 2-I-melatonin; 8.9+/-0.07 for 5-MCA-NAT; 8.7+/-0.18 for 2-Phe-melatonin and 9.4+/-0.30 for N-acetyltryptamine (all n=8). (3) At a dose of 0.5 nmol (in 10 micro l) melatonin and the selective melatonin MT(3) agonist 5-MCA-NAT, induced greater reductions of intraocular pressure (22.8+/-2.3% and 32.5+/-1.4%, respectively) than the other compounds. (4) The melatonin-receptor antagonists, prazosin, DH-97 and 4-P-PDOT, reversed the effect of 5-MCA-NAT in a dose-dependent manner, with pA(2) values of 13.5+/-0.17 for prazosin, 10.6+/-0.16 for DH-97 and 9.4+/-0.20 for 4-P-PDOT (n=8). (5) Cholinoceptor antagonists (hexamethonium and atropine) and alpha(2)- and beta(2)-adrenoceptor antagonists (yohimbine and ICI 118,551) partially reversed the effects produced by melatonin and 5-MCA-NAT, suggesting the possible involvement of cholinergic and noradrenergic systems in the hypotensive actions mediated by melatonin agonists. The alpha(1)-adrenoceptor antagonist, corynanthine, had no significant effect. (6) The strong hypotensive effect of the MT(3) agonist, 5-MCA-NAT, suggests that this compound may be a useful agent for treating those pathologies where intraocular pressure is abnormally elevated.

Differential distribution of melatonin receptors in the pituitary gland of Xenopus laevis

A major target site for melatonin action is thought to be the pituitary gland. We have detected differential expression and co-localization of the Mel(1a) and Mel(1c) receptors in cells of the Xenopus laevis pituitary gland. Sections of Xenopus pituitary glands were labeled with Mel(1a) and/or Mel(1c) antibodies, in combination with antibodies to arginine vasotocin (AVT), alpha-melanocyte stimulating hormone (alpha-MSH), prolactin (PRL), and luteinizing hormone (LH). Mel(1a) immunoreactivity was localized to cells of the pars intermedia and to elements within the pars nervosa. Mel(1c) immunoreactivity was also localized to the pars nervosa, and significant labeling was also observed in discrete clusters of cells in the pars distalis. Mel(1a) was absent from the pars distalis, while Mel(1c) was absent from the pars intermedia. Mel(1a) and Mel(1c) were co-localized in the pars nervosa. AVT was present in the pars nervosa, and appeared to be localized to the cell clusters of the pars distalis in which the Mel(1c) receptor was localized. alpha-MSH co-localized with the Mel(1a) receptor in the pars intermedia. LH appeared to localize to many of the cells in the pars distalis, with the notable exception of the Mel(1c) receptor-positive clusters of cells. PRL did not appear to co-localize with either melatonin receptor. The pattern of differential expression of the Mel(1a) and Mel(1c) receptors suggests that the receptors specifically mediate the cellular response to melatonin binding in the specific cell populations.

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.