Melatonin analogue agomelatine reduces rabbit's intraocular pressure in normotensive and hypertensive conditions

Melatonin: Unveiling the functions and implications in ocular health

Melatonin, a versatile hormone produced by the pineal gland, has garnered considerable scientific interest due to its diverse functions. In the eye, melatonin regulates a variety of key processes like inhibiting angiogenesis by reducing vascular endothelial growth factor levels and protecting the blood-retinal barrier (BRB) integrity by enhancing tight junction proteins and pericyte coverage. Melatonin also maintains cell health by modulating autophagy via the Sirt1/mTOR pathways, reduces inflammation, promotes antioxidant enzyme activity, and regulates intraocular pressure fluctuations. Additionally, melatonin protects retinal ganglion cells by modulating aging and inflammatory pathways. Understanding melatonin's multifaceted functions in ocular health could expand the knowledge of ocular pathogenesis, and shed new light on therapeutic approaches in ocular diseases. In this review, we summarize the current evidence of ocular functions and therapeutic potential of melatonin and describe its roles in angiogenesis, BRB integrity maintenance, and modulation of various eye diseases, which leads to a conclusion that melatonin holds promising treatment potential for a wide range of ocular health conditions.

The Therapeutic Trip of Melatonin Eye Drops: From the Ocular Surface to the Retina

Melatonin is a ubiquitous molecule found in living organisms, ranging from bacteria to plants and mammals. It possesses various properties, partly due to its robust antioxidant nature and partly owed to its specific interaction with melatonin receptors present in almost all tissues. Melatonin regulates different physiological functions and contributes to the homeostasis of the entire organism. In the human eye, a small amount of melatonin is also present, produced by cells in the anterior segment and the posterior pole, including the retina. In the eye, melatonin may provide antioxidant protection along with regulating physiological functions of ocular tissues, including intraocular pressure (IOP). Therefore, it is conceivable that the exogenous topical administration of sufficiently high amounts of melatonin to the eye could be beneficial in several instances: for the treatment of eye pathologies like glaucoma, due to the IOP-lowering and neuroprotection effects of melatonin; for the prevention of other dysfunctions, such as dry eye and refractive defects (cataract and myopia) mainly due to its antioxidant properties; for diabetic retinopathy due to its metabolic influence and neuroprotective effects; for macular degeneration due to the antioxidant and neuroprotective properties; and for uveitis, mostly owing to anti-inflammatory and immunomodulatory properties. This paper reviews the scientific evidence supporting the use of melatonin in different ocular districts. Moreover, it provides data suggesting that the topical administration of melatonin as eye drops is a real possibility, utilizing nanotechnological formulations that could improve its solubility and permeation through the eye. This way, its distribution and concentration in different ocular tissues may support its pleiotropic therapeutic effects.

Secondary glaucoma: Toward interventions based on molecular underpinnings

Glaucoma is a heterogeneous group of progressive diseases that leads to irreversible blindness. Secondary glaucoma refers to glaucoma caused by a known underlying condition. Pseudoexfoliation and pigment dispersion syndromes are common causes of secondary glaucoma. Their respective deposits may obstruct the trabecular meshwork, leading to aqueous humor outflow resistance, ocular hypertension, and optic neuropathy. There are no disease-specific interventions available for either. Pseudoexfoliation syndrome is characterized by fibrillar deposits (pseudoexfoliative material) on anterior segment structures. Over a decade of multiomics analyses taken together with the current knowledge on pseudoexfoliative glaucoma warrant a re-think of mechanistic possibilities. We propose that the presence of nucleation centers (e.g., vitamin D binding protein), crosslinking enzymes (e.g., transglutaminase 2), aberrant extracellular matrix, flawed endocytosis, and abnormal aqueous-blood barrier contribute to the formation of proteolytically resistant pseudoexfoliative material. Pigment dispersion syndrome is characterized by abnormal iridolenticular contact that disrupts iris pigment epithelium and liberates melanin granules. Iris melanogenesis is aberrant in this condition. Cytotoxic melanogenesis intermediates leak out of melanosomes and cause iris melanocyte and pigment epithelium cell death. Targeting melanogenesis can likely decrease the risk of pigmentary glaucoma. Skin and melanoma research provides insights into potential therapeutics. We propose that specific prostanoid agonists and fenofibrates may reduce melanogenesis by inhibiting cholesterol internalization and de novo synthesis. Additionally, melatonin is a potent melanogenesis suppressor, antioxidant, and hypotensive agent, rendering it a valuable agent for pigmentary glaucoma. In pseudoexfoliative glaucoma, where environmental insults drive pseudoexfoliative material formation, melatonin's antioxidant and hypotensive properties may offer adjunct therapeutic benefits. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology.

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-Eluting Contact Lenses Effect on Tear Volume: In Vitro and In Vivo Experiments

(1) Background: The purpose of this study was to synthesize melatonin-eluting contact lenses (CLs) and evaluate both the ocular kinetics of the released melatonin and its effect on tear volume and intraocular pressure. (2) Methods: In vitro, melatonin-eluting CLs were synthesized by using non-functionalized (HEMA) and functionalized (HEMA/APMA) monomers. In vivo, a short-term prospective and randomized study was performed on 15 rabbits divided into two groups: 12 rabbits wearing functionalized CLs and 3 rabbits without CLs as a control. The melatonin levels in tears, aqueous humor, vitreous body and retina, tear volume, and intraocular pressure were measured for 8 h. (3) Results: In vitro, both monomers did not show differences in terms of melatonin loading and release (p ≥ 0.05). In vivo, the melatonin concentration was elevated in tears and aqueous humor after 2 and 4 h of wearing CLs, respectively (p < 0.05). Additionally, the CLs increased tear volume for 2 h (p < 0.05). (4) Conclusions: The melatonin-eluting CLs released their content over the ocular surface for at least 2 h, which was associated with a secretagogue effect on tear volume. However, the increased amount of melatonin found in the aqueous humor had no effect on intraocular pressure.

The Roles of Transient Receptor Potential Ion Channels in Pathologies of Glaucoma

Transient receptor ion potential (TRP) channels are a cluster of non-selective cation channels present on cell membranes. They are important mediators of sensory signals to regulate cellular functions and signaling pathways. Alterations and dysfunction of these channels could disrupt physiological processes, thus leading to a broad array of disorders, such as cardiovascular, renal and nervous system diseases. These effects position them as potential targets for drug design and treatment. Because TRP channels can mediate processes such as mechanical conduction, osmotic pressure, and oxidative stress, they have been studied in the context of glaucoma. Glaucoma is an irreversible blinding eye disease caused by an intermittent or sustained increase in intraocular pressure (IOP), which results in the apoptosis of retinal ganglion cells (RGCs), optic nerve atrophy and eventually visual field defects. An increasing number of studies have documented that various TRP subfamilies are abundantly expressed in ocular structures, including the cornea, lens, ciliary body (CB), trabecular meshwork (TM) and retina. In alignment with these findings, there is also mounting evidence supporting the potential role of the TRP family in glaucoma progression. Therefore, it is of great interest and clinical significance to gain an increased understanding of these channels, which in turn could shed more light on the identification of new therapeutic targets for glaucoma. Moreover, this role is not understood completely to date, and whether the activation of TRP channels contributes to glaucoma, or instead aggravates progression, needs to be explored. In this manuscript, we aim to provide a comprehensive overview of recent research on TRP channels in glaucoma and to suggest novel targets for future therapeutic interventions in glaucoma.

The neuroprotective effect of melatonin in glutamate excitotoxicity of R28 cells and mouse retinal ganglion cells

Glaucoma is the leading cause of irreversible blindness. The progressive degeneration of retinal ganglion cells (RGCs) is the major characteristic of glaucoma. Even though the control of intraocular pressure could delay the loss of RGCs, current clinical treatments cannot protect them directly. The overactivation of N-methyl-D-aspartic acid (NMDA) receptors by excess glutamate (Glu) is among the important mechanisms of RGC death in glaucoma progression. Melatonin (MT) is an indole neuroendocrine hormone mainly secreted by the pineal gland. This study aimed to investigate the therapeutic effect of MT on glutamate excitotoxicity of mouse RGCs and R28 cells. The Glu-induced R28 cell excitotoxicity model and NMDA-induced retinal injury model were established. MT was applied to R28 cells and the vitreous cavity of mice by intravitreal injection. Cell counting kit-8 assay and propidium iodide/Hoechst were performed to evaluate cell viability. Reactive oxygen species and glutathione synthesis assays were used to detect the oxidative stress state of R28 cells. Retina immunofluorescence and hematoxylin and eosin staining were applied to assess RGC counts and retinal structure. Flash visual-evoked potential was performed to evaluate visual function in mice. RNA sequencing of the retina was performed to explore the underlying mechanisms of MT protection. Our results found that MT treatment could successfully protect R28 cells from Glu excitotoxicity and decrease reactive oxygen species. Also, MT rescued RGCs from NMDA-induced injury and protected visual function in mice. This study enriches the indications of MT in the treatment of glaucoma, providing practical research ideas for its comprehensive prevention and treatment.

From Oxidative Stress to Inflammation in the Posterior Ocular Diseases: Diagnosis and Treatment

Most irreversible blindness observed with glaucoma and retina-related ocular diseases, including age-related macular degeneration and diabetic retinopathy, have their origin in the posterior segment of the eye, making their physiopathology both complex and interconnected. In addition to the age factor, these diseases share the same mechanism disorder based essentially on oxidative stress. In this context, the imbalance between the production of reactive oxygen species (ROS) mainly by mitochondria and their elimination by protective mechanisms leads to chronic inflammation. Oxidative stress and inflammation share a close pathophysiological process, appearing simultaneously and suggesting a relationship between both mechanisms. The biochemical end point of these two biological alarming systems is the release of different biomarkers that can be used in the diagnosis. Furthermore, oxidative stress, initiating in the vulnerable tissue of the posterior segment, is closely related to mitochondrial dysfunction, apoptosis, autophagy dysfunction, and inflammation, which are involved in each disease progression. In this review, we have analyzed (1) the oxidative stress and inflammatory processes in the back of the eye, (2) the importance of biomarkers, detected in systemic or ocular fluids, for the diagnosis of eye diseases based on recent studies, and (3) the treatment of posterior ocular diseases, based on long-term clinical studies.

Agomelatine prevents angiotensin II-induced endothelial and mononuclear cell adhesion

Agomelatine is a non-selective melatonin receptor agonist and an atypical antidepressant with anti-inflammatory, neuroprotective, and cardioprotective effects. The renin-angiotensin system modulates blood pressure and vascular homeostasis. Angiotensin II (Ang II) and its receptor Ang II type I receptor (AT₁R) are recognized as contributors to the pathogenesis of cardiovascular and cardiometabolic diseases, including diabetes, obesity, and atherosclerosis. The recruitment and attachment of monocytes to the vascular endothelium is a major event in the early stages of atherosclerosis and other cardiovascular diseases. In the present study, we demonstrate that agomelatine reduced Ang II-induced expression of AT₁R while significantly inhibiting the attachment of monocytes to endothelial cells induced by Ang II and mediated by ICAM-1 and VCAM-1. Additionally, Ang II inhibited the expression of the chemokines CXCL1, MCP-1, and CCL5, which are critical in the process of immune cell recruitment and invasion. Agomelatine also suppressed the expression of TNF-α, IL-8, and IL-12, which are proinflammatory cytokines that promote endothelial dysfunction and atherogenesis. Importantly, we demonstrate that the inhibitory effect of agomelatine against the expression of adhesion molecules is mediated through the downregulation of Egr-1 signaling. Together, our findings provide evidence of a novel mechanism of agomelatine that may be practicable in the treatment and prevention of cardiovascular diseases.

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.

A Topical Formulation of Melatoninergic Compounds Exerts Strong Hypotensive and Neuroprotective Effects in a Rat Model of Hypertensive Glaucoma

Melatonin is of great importance for regulating several eye processes, including pressure homeostasis. Melatonin in combination with agomelatine has been recently reported to reduce intraocular pressure (IOP) with higher efficacy than each compound alone. Here, we used the methylcellulose (MCE) rat model of hypertensive glaucoma, an optic neuropathy characterized by the apoptotic death of retinal ganglion cells (RGCs), to evaluate the hypotensive and neuroprotective efficacy of an eye drop nanomicellar formulation containing melatonin/agomelatine. Eye tissue distribution of melatonin/agomelatine in healthy rats was evaluated by HPLC/MS/MS. In the MCE model, we assessed by tonometry the hypotensive efficacy of melatonin/agomelatine. Neuroprotection was revealed by electroretinography; by levels of inflammatory and apoptotic markers; and by RGC density. The effects of melatonin/agomelatine were compared with those of timolol (a beta blocker with prevalent hypotensive activity) or brimonidine (an alpha 2 adrenergic agonist with potential neuroprotective efficacy), two drugs commonly used to treat glaucoma. Both melatonin and agomelatine penetrate the posterior segment of the eye. In the MCE model, IOP elevation was drastically reduced by melatonin/agomelatine with higher efficacy than that of timolol or brimonidine. Concomitantly, gliosis-related inflammation and the Bax-associated apoptosis were partially prevented, thus leading to RGC survival and recovered retinal dysfunction. We suggest that topical melatoninergic compounds might be beneficial for ocular health.

Natural Products: Evidence for Neuroprotection to Be Exploited in Glaucoma

Glaucoma, a leading cause of irreversible blindness worldwide, is an optic neuropathy characterized by the progressive death of retinal ganglion cells (RGCs). Elevated intraocular pressure (IOP) is recognized as the main risk factor. Despite effective IOP-lowering therapies, the disease progresses in a significant number of patients. Therefore, alternative IOP-independent strategies aiming at halting or delaying RGC degeneration is the current therapeutic challenge for glaucoma management. Here, we review the literature on the neuroprotective activities, and the underlying mechanisms, of natural compounds and dietary supplements in experimental and clinical glaucoma.

Contact Lenses as Drug Delivery System for Glaucoma: A Review

Glaucoma is an optical neuropathy associated to a progressive degeneration of retinal ganglion cells with visual field loss and is the main cause of irreversible blindness in the world. The treatment has the aim to reduce intraocular pressure. The first therapy option is to instill drugs on the ocular surface. The main limitation of this is the reduced time of the drug staying on the cornea. This means that high doses are required to ensure its therapeutic effect. A drug-loaded contact lens can diffuse into the post lens tear film in a constant and prolonged flow, resulting in an increased retention of the drug on the surface of the cornea for up to 30 min and thus providing a higher drug bioavailability, increasing the therapeutic efficacy, reducing the amount of administered drug, and thereby provoking fewer adverse events. Several different systems of drug delivery have been studied in recent decades; ranging from more simple methods of impregnating the lenses, such as soaking, to more complex ones, such as molecular imprinting have been proposed. Moreover, different drugs, from those already commercially available to new substances such as melatonin have been studied to improve the glaucoma treatment efficacy. This review describes the role of contact lenses as an innovative drug delivery system to treat glaucoma.

Melatonin attenuates hypertension and oxidative stress in a rat model of L-NAME-induced gestational hypertension

Preeclampsia is a life-threatening multiorgan systemic disease with manifestations including gestational hypertension, oxidative stress, and vascular dysfunction. We aimed to evaluate the therapeutic effects of melatonin on an L-NAME (NLG-nitro-l-arginine methyl ester)-induced rat preeclampsia model. During gestation, L-NAME was added to drinking water at 50 mg/kg/day from gestation day (GD) 8. Rats received the combination of L-NAME with melatonin (10 mg/kg/day), or aspirin (1.5 mg/kg/day), and rats that received only L-NAME or no treatments were used as controls. Aspirin was mixed with rodent chow and melatonin was administered intraperitoneally. Blood pressure and urine protein content were monitored every 3 days. On GD19, blood samples were collected for biochemical analysis. Compared to untreated L-NAME rats, melatonin led to markedly lowered blood pressure and urine protein content, and recovery in the fetus alive ratio, fetal weight, and the fetal weight/placental weight ratio. Compared to untreated L-NAME rats, plasma antioxidant capacity and plasma malondialdehyde were increased and decreased by melatonin, respectively, in L-NAME rats. Melatonin treatment also reduced sFlt-1, increased PlGF, and decreased the sFlt-1/PlGF ratio. In the placenta, melatonin also reduced sFlt-1 levels and increased Nrf2, PlGF, and HO-1 levels. We have demonstrated in a rat model of preeclampsia that melatonin exerts significant protective effects through lowering blood pressure and reducing oxidative stress.

Hypotensive Effect of Nanomicellar Formulation of Melatonin and Agomelatine in a Rat Model: Significance for Glaucoma Therapy

BACKGROUND

Melatoninergic agents are known to reduce intraocular pressure (IOP). The present study was performed to evaluate the effect of nanomicellar formulations of melatoninergic agents on IOP in the rat.

METHODS

Tonometry was used to measure IOP in eyes instilled with melatonin or agomelatine. Ocular hypertension was induced by the injection of methylcellulose in the anterior chamber.

RESULTS

Melatonin formulated in nanomicelles had a longer lasting hypotonizing effect on IOP with respect to melatonin in saline. Nanomicellar formulations of melatonin and agomelatine, either alone or in combination, had lowering effects that did not depend on their concentration or their combination, which, however, resulted in an increased duration of the hypotonizing effect. The duration of the lowering effect was further increased by the addition of lipoic acid.

CONCLUSIONS

We demonstrated the effective hypotonizing activity of melatonin and agomelatine in combination with lipoic acid. Although results in animals cannot be directly translated to humans, the possibility of developing novel therapeutical approaches for patients suffering from hypertensive glaucoma should be considered.

Effect of nutritional supplement based on melatonin on the intraocular pressure in normotensive subjects

PURPOSE

To evaluate the effect of a new nutritional supplement based on melatonin on the intraocular pressure (IOP) in normotensive subjects.

PATIENTS AND METHODS

A short-term prospective study was designed. Sixty-seven normotensive subjects were recruited. Patients were divided into two groups. The daily group (DG) (n = 18) was instructed to take the supplement between 22:00 and 23:00 (before sleeping) for 3 consecutive days. IOP was measured from 10.00 to 11.00 am the day before treatment and during the 3 days of experiment. The acute group (AG) (n = 49) was instructed to take the supplement after the second measure (11.00) of the second day. IOP was measured 1 h and just before the intake of the supplement and 1 and 2 h after. All measurements in this group were taken 1 day before without any supplement (control) and the day of experiment.

RESULTS

The DG group showed a significant decrease in IOP after supplement intake in all days of experiment, from 14.9 ± 3.4 mm Hg to 13.8 ± 2.9 mm Hg after 3 days of experiment (p value < 0.001). For AG, IOP did not change during the control day; however, a reduction of 1 mm Hg was found 2 and 3 h after supplement intake, from 15.7 ± 2.5 mm Hg to 14.7 ± 2.5 mm Hg and 15.1 ± 2.7 mm Hg, respectively, being statistically significant (p value < 0.001).

CONCLUSION

The supplement based on melatonin was able to reduce the IOP in normotensive subjects after 2 h of intake. Moreover, the daily intake showed a reduction in IOP during the 3 days of experiment.

Melatonin and the control of intraocular pressure

Melatonin is not only synthesized by the pineal gland but by several ocular structures. This natural indoleamine is of great importance for regulating several eye processes, among which pressure homeostasis is included. Glaucoma, the most prevalent eye disease, also known as the silent thief of vision, is a multifactorial pathology that is associated to age and, often, to intraocular hypertension (IOP). Indeed IOP is the only modifiable risk factor and as such medications are available to control it; however, novel medications are sought to minimize undesirable side effects. Melatonin and analogues decrease IOP in both normotensive and hypertensive eyes. Melatonin activates its cognate membrane receptors, MT₁ and MT₂, which are present in numerous ocular tissues, including the aqueous-humor-producing ciliary processes. Melatonin receptors belong to the superfamily of G-protein-coupled receptors and their activation would lead to different signalling pathways depending on the tissue. This review describes the molecular mechanisms underlying differential functionalities that are attributed to melatonin receptors. Accordingly, the current work highlights the important role of melatonin and its analogues in the healthy and in the glaucomatous eyes, with special attention to the control of intraocular pressure.

Effect of orally administered melatonin on intraocular pressure of ophthalmologically normal dogs

OBJECTIVE

To determine the effect of orally administered melatonin on the intraocular pressure (IOP) of ophthalmologically normal dogs.

ANIMALS

20 ophthalmologically normal dogs (40 eyes).

PROCEDURES

In a randomized crossover study, each dog received a 7-day regimen of melatonin (0.1 to 0.2 mg/kg, PO, q 12 h) and a placebo (150 mg of lactose powder in a capsule, PO, q 12 h), with a 7-day washout period between treatment regimens. Rebound tonometry was used to measure the IOP in both eyes of each dog 5 times at 2-hour intervals on days 0 (before administration of the first dose), 2, 4, and 7 (after administration of the last dose) of each treatment period. Repeated-measures ANOVA was used to evaluate the effects of treatment, day, and IOP measurement time within day on IOP.

RESULTS

Intraocular pressure was not significantly associated with treatment but was associated with day and the interaction between day and IOP measurement time within day. The mean ± SD IOP was 14.26 ± 2.95 and 14.34 ± 2.69 mm Hg for the melatonin and placebo regimens, respectively. Within each treatment period, the mean IOP tended to decrease from day 0 to 7 as well as within each day, which was attributed to the dogs becoming acclimated to the study protocol and natural diurnal variations in IOP.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that oral administration of melatonin (0.1 to 0.2 mg/kg, q 12 h for 7 d) did not significantly affect the IOP of ophthalmologically normal dogs.

Neuroprotection in Glaucoma: Old and New Promising Treatments

Glaucoma is a major global cause of blindness, but the molecular mechanisms responsible for the neurodegenerative damage are not clear. Undoubtedly, the high intraocular pressure (IOP) and the secondary ischemic and mechanical damage of the optic nerve have a crucial role in retinal ganglion cell (RGC) death. Several studies specifically analyzed the events that lead to nerve fiber layer thinning, showing the importance of both intra- and extracellular factors. In parallel, many neuroprotective substances have been tested for their efficacy and safety in hindering the negative effects that lead to RGC death. New formulations of these compounds, also suitable for chronic oral administration, are likely to be used in clinical practice in the future along with conventional therapies, in order to control the progression of the visual impairment due to primary open-angle glaucoma (POAG). This review illustrates some of these old and new promising agents for the adjuvant treatment of POAG, with particular emphasis on forskolin and melatonin.

The role and therapeutic potential of melatonin in age-related ocular diseases

The eye is continuously exposed to solar UV radiation and pollutants, making it prone to oxidative attacks. In fact, oxidative damage is a major cause of age-related ocular diseases including cataract, glaucoma, age-related macular degeneration, and diabetic retinopathy. As the nature of lens cells, trabecular meshwork cells, retinal ganglion cells, retinal pigment epithelial cells, and photoreceptors is postmitotic, autophagy plays a critical role in their cellular homeostasis. In age-related ocular diseases, this process is impaired, and thus, oxidative damage becomes irreversible. Other conditions such as low-grade chronic inflammation and angiogenesis also contribute to the development of retinal diseases (glaucoma, age-related macular degeneration and diabetic retinopathy). As melatonin is known to have remarkable qualities such as antioxidant/antinitridergic, mitochondrial protector, autophagy modulator, anti-inflammatory, and anti-angiogenic, it can represent a powerful tool to counteract all these diseases. The present review analyzes the role and therapeutic potential of melatonin in age-related ocular diseases, focusing on nitro-oxidative stress, autophagy, inflammation, and angiogenesis mechanisms.

Elevated intraocular pressure increases melatonin levels in the aqueous humour

PURPOSE

To study the levels of melatonin in the aqueous humour of normotensive and hypertensive intraocular pressure (IOP) patients and to compare them to an animal model of glaucoma.

METHODS

A total of 37 eyes of 37 patients who underwent cataract surgery were included in the study and were divided into normotensive patients, with IOP below 21 mmHg (n = 23), and hypertensive patients, with IOP > 21 mmHg (n = 14). Glaucomatous DBA/2J (n = 6) and control C57BL/6J (n = 6) mice presenting 3 and 12 months of age for each strain were also used. Human and mice aqueous humours were aspirated using a 30-gauge Rycroft cannula on a tuberculin syringe and further processed to quantify melatonin by high-performance liquid chromatography analysis.

RESULTS

Melatonin levels in normotensive patients (IOP below 21 mmHg) presented values as medians (first quartile; third quartile) of 14.62 (5.38;37.99) ng/ml (n = 23), while hypertensive patients (IOP above 21 mmHg) showed melatonin concentrations of 46.63 (10.28; 167.28) ng/ml (n = 14; p < 0.039). Glaucoma mice presented melatonin values of 0.37 (0.34; 0.59) ng/ml (at 3 months of age, before the pathology starts), which increased to 1.55 (0.94; 1.88) ng/ml (at 12 months of age, when the pathology is fully developed and IOP is maximum; n = 6, p < 0.001). Control mice did not significantly modified melatonin concentrations between 3 and 12 months of age.

CONCLUSION

Patients with high IOP present increased concentrations of melatonin in their aqueous humour compared to normotensive patients. This has been confirmed in a glaucomatous animal model in which it has been possible to see a correlation between the development of the pathology, with an increase in IOP, and a concomitant elevation of melatonin in the aqueous humour.

TRPV4 Stimulation Induced Melatonin Secretion by Increasing Arylalkymine N-acetyltransferase (AANAT) Protein Level

Melatonin is a molecule which has gained a great deal of interest in many areas of science; its synthesis was classically known to be in the pineal gland. However, many organs synthesize melatonin, such as several ocular structures. Melatonin is known to participate in many functions apart from its main action regulating the circadian rhythm. It is synthesized from serotonin in two steps, with a rate-limiting step carried out by arylalkymine N-acetyltransferase (AANAT). In this report, the role of TRPV4 channel present in human ciliary body epithelial cells in AANAT production was studied. Several experiments were undertaken to verify the adequate time to reach the maximal effect by using the TRPV4 agonist GSK1016790A, together with a dose-response study. An increase of 2.4 folds in AANAT was seen after 18 h of incubation with 10 nM of GSK1016790A (p < 0.001, n = 6). This increment was verified by antagonist assays. In summary, AANAT levels and therefore melatonin synthesis change after TRPV4 channel stimulation. Using this cell model together with human ciliary body tissue it is possible to suggest that AANAT plays an important role in pathologies related to intraocular pressure.

Sustained Delivery of Timolol Maleate for Over 90 Days by Subconjunctival Injection

PURPOSE

Medical treatment of glaucoma relies on intraocular pressure (IOP)-lowering medications, typically administered daily by the patient. While these medications are effective when applied correctly, patient adherence is a major obstacle in glaucoma treatment. We have developed a sustained-release formulation of timolol maleate that can be injected subconjunctivally to avoid patient noncompliance.

METHODS

A biodegradable microsphere formulation for timolol maleate was injected subconjunctivally in normal rabbits. We measured timolol levels in tears, aqueous humor, vitreous humor, and serum of study rabbits. Furthermore, IOP profiles were recorded longitudinally. Tissue compatibility and side effects were evaluated using histochemistry.

RESULTS

The microsphere formulation led to measureable amounts of timolol in the aqueous humor and the tear film for up to 90 days. Timolol was not detectable in the serum at any time. A significant reduction of IOP was observed in treated eyes. Clinically, the subconjunctival administration of the microspheres was well tolerated with no signs of inflammation or infection. The absence of local inflammation was confirmed by histology.

CONCLUSIONS

A single subconjunctival administration of timolol microspheres achieved delivery and IOP reduction in rabbits for up to 90 days without local or systemic inflammation or toxicity. This approach has the potential to improve the management of glaucoma in patient populations, who are challenged to adhere to a regimen of daily eye drops.

24-h monitoring devices and nyctohemeral rhythms of intraocular pressure

Intraocular pressure (IOP) is not a fixed value and varies over both the short term and periods lasting several months or years. In particular, IOP is known to vary throughout the 24-h period of a day, defined as a nyctohemeral rhythm in humans. In clinical practice, it is crucial to evaluate the changes in IOP over 24 h in several situations, including the diagnosis of ocular hypertension and glaucoma (IOP is often higher at night) and to optimize the therapeutic management of glaucoma. Until recently, all evaluations of 24-h IOP rhythm were performed using repeated IOP measurements, requiring individuals to be awakened for nocturnal measurements. This method may be imperfect, because it is not physiologic and disturbs the sleep architecture, and also because it provides a limited number of time point measurements not sufficient to finely asses IOP changes. These limitations may have biased previous descriptions of physiological IOP rhythm. Recently, extraocular and intraocular devices integrating a pressure sensor for continuous IOP monitoring have been developed and are available for use in humans. The objective of this article is to present the contributions of these new 24-h monitoring devices for the study of the nyctohemeral rhythms. In healthy subjects and untreated glaucoma subjects, a nyctohemeral rhythm is consistently found and frequently characterized by a mean diurnal IOP lower than the mean nocturnal IOP, with a diurnal bathyphase - usually in the middle or at the end of the afternoon - and a nocturnal acrophase, usually in the middle or at the end of the night.

Prevention of intraocular pressure elevation with oleuropein rich diet in rabbits, during the general anaesthesia

Background

Oleuropein is known to have anti-oxidant and anti-inflammatory effects. An important aim of anesthetic management in ocular surgery is to keep the intraocular pressure under control. Studies have researched a variety of prophylactic materials used to prevent increases in intraocular pressure. We aimed to research the effects of oleuropein on intraocular pressure (IOP) during general anaesthesia.

Methods

Fourteen New Zealand rabbits were randomly divided into two groups of seven. The rabbits in Group O were given olive leaf extract (OLE) equivalent to a daily dose of 20 mg/kg oleuropein for 15 days. HPLC method used for oleuropein standardization. For anaesthesia induction 1 mg/kg rocuronium was given and after muscle relaxation all animals had a V-gel Rabbit inserted. Anesthetic maintenance was provided by 1 MAC isoflurane. Twenty minutes after rabbits were given 10 mg/kg ketamine, basal IOP values were measured. After the V-gel rabbit was inserted, in the 5th, 10th, 20th, 25th and 30th minutes measurements were repeated.

Results

IOP data variation of OLE group was compared with control group and the measured levels were lower in Group O during the anaesthesia. IOP was 33.8 ± 4 mmHg in Group C and 24.1 ± 8 mmHg in Group O in 25th minute and the difference between the two groups was statistically significant at this time.

Conclusion

We observed that consumption of prophylactic OLE had a reducing effect on IOP in the period before waking in anaesthesia. We believe it is necessary to investigate the effects of OLE on IOP in broad participation patient groups.

Effect of Melatonin and 5-Methoxycarbonylamino-N-Acetyltryptamine on the Intraocular Pressure of Normal and Glaucomatous Mice

Melatonin is a neurohormone that is produced not only by the pineal gland but also by several ocular structures. One of the main physiologic roles of melatonin is the reduction of intraocular pressure (IOP). Using both control C57BL/6J and glaucomatous DBA/2J mice as well as TonoLab tonometry, this study evaluated the effect of melatonin and 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT) when glaucomatous pathology was fully established and compared pharmacological behavior in treated mice versus control mice. In addition, 5-MCA-NAT was tested to determine its effects on ameliorating increased IOP in a glaucoma model. The results demonstrate that melatonin and 5-MCA-NAT can reduce IOP in a concentration-dependent manner. The EC50values for melatonin in control and glaucomatous animals were 34µM and 50µM, respectively. Interestingly, melatonin decreased IOP in 19.4% ± 3.7% and 32.6% ± 6.0% of control and glaucomatous mice, respectively, when the animals were studied at age 12 months. 5-MCA-NAT reduced IOP in the same manner and was able to stop IOP progression in glaucomatous mice. Use of melatonin receptor antagonists showed that hypotensive effects were blocked by the MT2receptor antagonists luzindole and 4-phenyl-2-propionamidotetralin in the case of melatonin and by only 4-phenyl-2-propionamidotetralin in the case of 5-MCA-NAT. In conclusion, melatonin and 5-MCA-NAT can effectively reduce IOP in a glaucoma model, and their hypotensive effects are more profound in the glaucoma model than in control animals.

Evaluation of cationic nanosystems with melatonin using an eye-related bioavailability prediction model

In this study, two types of nanosystems, namely lecithin/chitosan nanoparticles and Pluronic® F127/chitosan micelles, have been prepared and evaluated for their potential for the ocular delivery of melatonin, which is known to exert an ocular hypotensive effect. The melatonin content, particle size, zeta potential and in vitro drug release properties were studied as a function of the presence of chitosan in the nanosystem. Lecithin/chitosan nanoparticles were evaluated in terms of the mucoadhesive properties by a newly established method based on HCE-T cells, also used in in vitro biocompatibility and permeability studies. Lecithin/chitosan nanoparticles were significantly larger than the corresponding F127/chitosan micelles (mean diameter of 241.8 vs. 20.7nm, respectively) and characterised by a higher surface charge (22.7 vs. 4.3mV, respectively). The HCE-T cell viability assay did not show significant toxic effects of nanosystems investigated at the (relevant) chitosan concentration tested. The permeability study results confirmed the permeation enhancing effect of F127, which was hindered in the presence of chitosan. Lecithin/chitosan nanoparticles were characterised by prominent mucoadhesive properties and prolonged melatonin release, which was shown to control melatonin permeation across an in vitro corneal epithelial model. Such properties demonstrate the potential for nanoparticles to provide an extended pre-corneal residence time of melatonin, ensuring higher eye-related bioavailability and extended intraocular pressure reduction compared to melatonin in both aqueous and micelle solutions.

Hybrid formulations of liposomes and bioadhesive polymers improve the hypotensive effect of the melatonin analogue 5-MCA-NAT in rabbit eyes

For the treatment of chronic ocular diseases such as glaucoma, continuous instillations of eye drops are needed. However, frequent administrations of hypotensive topical formulations can produce adverse ocular surface effects due to the active substance or other components of the formulation, such as preservatives or other excipients. Thus the development of unpreserved formulations that are well tolerated after frequent instillations is an important challenge to improve ophthalmic chronic topical therapies. Furthermore, several components can improve the properties of the formulation in terms of efficacy. In order to achieve the mentioned objectives, we have developed formulations of liposomes (150–200 nm) containing components similar to those in the tear film and loaded with the hypotensive melatonin analog 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT, 100 µM). These formulations were combined with mucoadhesive (sodium hyaluronate or carboxymethylcellulose) or amphiphilic block thermosensitive (poloxamer) polymers to prolong the hypotensive efficacy of the drug. In rabbit eyes, the decrease of intraocular pressure with 5-MCA-NAT-loaded liposomes that were dispersed with 0.2% sodium hyaluronate, 39.1±2.2%, was remarkably higher compared to other liposomes formulated without or with other bioadhesive polymers, and the effect lasted more than 8 hours. According to the results obtained in the present work, these technological strategies could provide an improved modality for delivering therapeutic agents in patients with glaucoma.

Melatonin and derivatives as promising tools for glaucoma treatment

Neurohormones melatonin and its analogues are present with an important physiological and pharmacological ability to reduce intraocular pressure (IOP); thus, they are suitable for the treatment of ocular hypertension often associated with glaucoma. It is demonstrated that two of its analogues, 5-MCA-NAT and IIK7, are more effective than melatonin to reduce IOP for a longer period of time. The research for the discovery of better compounds resulted in the development of newer and improved analogues compared to 5-MCA-NAT and IIK7. Furthermore, already commercially available drugs currently used as treatment for other pathologies, presenting a resemblance to the melatonin structure, are being tested as potential glaucoma drugs. In this sense, agomelatine, which is already used as an anti-depressant medicine, is recognized as a worthy candidate since it reduces IOP, even under hypertensive conditions. To sum up, the use of melatonin and its analogues as promising anti-glaucomatous substances is of great importance and should be given serious consideration.

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.