Effect of melatonin on intraocular pressure

Oocyte maturation, blastocyst and embryonic development are mediated and enhanced via hormesis

The present paper provides the first integrative assessment of the capacity of dietary, endogenous and other agents to induce hormetic dose responses in oocytes, their supportive cells such as granulosa cells, blastocyst formation and early stage embryo development with the goal of improving fertility and reproductive success. The analysis showed that numerous agents enhance oocyte maturation and blastocyst/embryonic development in an hormetic fashion. These findings indicate that numerous agents improve oocyte-related biological functioning under normal conditions as well as enhancing its capacity to prevent damage from numerous chemical toxins and related stressor agents, including heat and age-related processes in pre-post conditioning and concurrent exposures. The present assessment suggests that hormetic-based lifestyles and dietary interventions may offer the potential to enhance healthy reproductive performance with applications to animal husbandry and human biology. The present findings also significantly extend the generality of the hormesis dose response concept to multiple fundamental biological processes (i.e., oocyte maturation, fertilization and blastocyst/embryo development).

Mechanosensitive ion channels in glaucoma pathophysiology

Force sensing is a fundamental ability that allows cells and organisms to interact with their physical environment. The eye is constantly subjected to mechanical forces such as blinking and eye movements. Furthermore, elevated intraocular pressure (IOP) can cause mechanical strain at the optic nerve head, resulting in retinal ganglion cell death (RGC) in glaucoma. How mechanical stimuli are sensed and affect cellular physiology in the eye is unclear. Recent studies have shown that mechanosensitive ion channels are expressed in many ocular tissues relevant to glaucoma and may influence IOP regulation and RGC survival. Furthermore, variants in mechanosensitive ion channel genes may be associated with risk for primary open angle glaucoma. These findings suggest that mechanosensitive channels may be important mechanosensors mediating cellular responses to pressure signals in the eye. In this review, we focus on mechanosensitive ion channels from three major channel families-PIEZO, two-pore potassium and transient receptor potential channels. We review the key properties of these channels, their effects on cell function and physiology, and discuss their possible roles in glaucoma pathophysiology.

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.

Validation of a Rapid and Easy-to-Apply Method to Simultaneously Quantify Co-Loaded Dexamethasone and Melatonin PLGA Microspheres by HPLC-UV: Encapsulation Efficiency and In Vitro Release

This paper discusses the development and validation of a rapid method for the reversed phase HPLC-UV quantification of biodegradable poly(D,L-lactic-co-glycolic) acid (PLGA) microspheres co-loaded with two neuroprotective agents (dexamethasone and melatonin) (DX-MEL-MSs) to be intravitreally administered as a promising glaucoma treatment. The study was performed to validate two procedures that quantify the content of the two active substances entrapped into the polymer matrix during an encapsulation efficiency assay and the amount of drugs liberated over time during the in vitro release assay. The reversed-phase method allowed for the simultaneous determination of dexamethasone and melatonin, which were respectively detected at 240.5 and 222.7 nm. Chromatographic separation was performed using an Ascentis^® C18 HPLC Column (25 cm × 4.6 mm, 5 µm) with an isocratic mobile phase composed of methanol-water (70:30, v/v) with 1.0 mL min⁻¹ flow rate. The two procedures were validated analytically in terms of system suitability testing, specificity, linearity, precision, accuracy, sensitivity, and robustness. Both the validated procedures were applied to characterize DX-MEL-MSs and were found appropriate to quantify the drug quantities encapsulated and estimate their release profile over 10 days. The validation study designed in this work can be helpful for planning any other protocols that refer to the quantification of PLGA based drug delivery systems.

Effects of long-term oral administration of melatonin on tear production, intraocular pressure, and tear and serum melatonin concentrations in healthy dogs

OBJECTIVE

To evaluate the effects of long-term (30-day) oral administration of melatonin on tear production, intraocular pressure (IOP), and concentration of melatonin in the tears and serum of healthy dogs.

ANIMALS

20 healthy sexually intact adult male dogs.

PROCEDURES

10 dogs were given melatonin (0.3 mg/kg, PO, q 24 h, administered in food at 9 am), and 10 dogs were given a placebo. Tear and serum melatonin concentrations, IOP, and tear production (determined with a Schirmer tear test) were recorded before (baseline) and 30 minutes, 3 hours, and 5 hours after administration of melatonin or the placebo on day 1 and 30 minutes after administration of melatonin or the placebo on days 8, 15, and 30.

RESULTS

Data collection time had significant effects on tear production, IOP, and tear melatonin concentration but not on serum melatonin concentration. Treatment (melatonin vs placebo) had a significant effect on tear melatonin concentration, but not on tear production, IOP, or serum melatonin concentration; however, tear melatonin concentration was significantly different between groups only 30 minutes after administration on day 1 and not at other times.

CLINICAL RELEVANCE

In healthy dogs, long-term administration of melatonin at a dosage of 0.3 mg/kg, PO, every 24 hours did not have any clinically important effects on tear production, IOP, or serum or tear melatonin concentrations.

Melatonin mitigates disrupted circadian rhythms, lowers intraocular pressure, and improves retinal ganglion cells function in glaucoma

Glaucoma is a progressive optic neuropathy associated with damage to retinal ganglion cells (RGCs) and disrupted circadian rhythms. Melatonin is a promising substance to ameliorate glaucoma-associated compromised circadian rhythms, sleep, mood, and retinal cells function. However, studies estimating melatonin effects in glaucoma are currently lacking. Therefore, In this study, we investigated the effect of long-term (daily at 10:30 pm for 90 days) oral melatonin administration on systemic (Tb) and local to the organ of vision (IOP) circadian rhythms, pattern electroretinogram (PERG), sleep, and mood, depending on glaucoma stage in patients diagnosed with stable or advanced primary open-angle glaucoma. In a laboratory study in 15 of them, 24-hour records of salivary melatonin were obtained and MTNR1B receptor gene polymorphism was assessed. Melatonin increased the stability of the Tb circadian rhythm by improving its phase alignment and alignment with IOP. Melatonin time-dependently decreased IOP and IOP standard deviation (SD). IOP 24-hour mean and IOP SD decreases were more pronounced in individuals with the higher initial 24-hour IOP mean. Melatonin improved RGCs function in advanced glaucoma; N95 amplitude increase correlated positively with RGCs loss. The beneficial effects of melatonin on sleep and mood were greater in advanced glaucoma. Finally, delayed salivary melatonin and Tb phases were observed in MTNR1B G-allele carriers with advanced glaucoma. Combined, these results provide evidence for melatonin efficiency in restoring disrupted circadian rhythms in glaucoma with different effects of melatonin on systemic vs. local circadian rhythms, indicating that a personalized strategy of melatonin administration may further refine its treatment benefits.

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.

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.

Hydroxyindole-O-methyltransferase (HIOMT) activity in the retina of melatonin-proficient mice

Numerous pieces of evidence support the expression by the mammalian retina of Hydroxyindole-O-methyltransferase (HIOMT, EC 2.1.1.4), the enzyme directly responsible for the biosynthesis of the pineal chronobiotic hormone melatonin (MLT). However, conflicting results obtained so far by enzyme-kinetic and immune-detection techniques still make HIOMT presence and relevance in the eye a matter of debate. This work aimed at evaluating unambiguously HIOMT activity in the mouse retina, a valuable model for studying the effects of MLT variations on ocular pathophysiology. Since laboratory mouse strains can bear genetic polymorphisms yielding defective enzymes of MLT biosynthesis, retinas and control pineal glands used in this study were obtained in a MLT-proficient crossing of A/J mice, the A/J/C57BL/10 strain. To improve the radiochemical reference assay, we tested different homogenization procedures coupled with HPLC detection. Concomitantly, we quantified MLT, and its precursor N-acetyl-serotonin (NAS) by HPLC coupled to electrochemical detection in retinas isolated from either light- or dark-adapted mice. Results showed that the standard radio-chemical assay was successful for pineal HIOMT only, whereas specific homogenization buffers and HPLC were required to detect retinal activity, presumably due to interfering methyl-transferases inhibited by NAS. Under present conditions, retinal HIOMT V_max accounted for by ≈ 40 fmol/h/mg protein, 2.6-hundreds-fold lower than the pineal counterpart, displaying equivalent K_Ms (≈10 μM). Moreover, NAS and MLT rapidly decreased in light-exposed isolated retinas, corroborating light-sensitive in-situ MLT formation. Conclusively, we measured mouse retinal HIOMT kinetics under basal conditions, a useful result to elucidate the regulatory patterns, the possible impact on eye health, and therapeutic approaches related to this enzyme.

Localization of melatonin and its receptors (melatonin 1a and 1b receptors) in the mouse inner ear

Background: In the inner ear, evidence has been gathered indicating that melatonin plays important roles in inner ear physiology and pathophysiology. However, no attempt has been made previously to investigate the localization or expression of melatonin and its receptors in the whole inner ear. Aims/objectives: To analyze the presence of melatonin and its receptors in the normal mouse inner ear. Material and methods: C57BL6/J mice were used in this study. The localizations of melatonin, MT1a and MT1b in the inner ear, i.e. cochlea, vestibular end organs, vestibular ganglion and endolymphatic sac (ES), were studied by immunohistochemistry. Results: The organ of Corti, spiral ganglion, vestibular ganglion, vestibular sensory cells, vestibular dark and transitional cells, and ES epithelial cells showed an immunofluorescence reaction to melatonin, MT1a and MT1b. Conclusion and significance: The present findings show that melatonin and its receptors (MT1a and MT1b) are present in the inner ear, thus supporting the hypothesis that melatonin plays a physiological role in the inner ear.

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.

Effect of Melatonin and Its Analogs on Tear Secretion

Melatonin has been shown to enhance tear secretion associated with dinucleotide diadenosine tetraphosphate. This study investigated the isolated action of melatonin and its analogs, agomelatine, N-butanoyl-2-(2-methoxy-6H-isoindolo[2,1-a]indol-11-yl) ethanamine (IIK7), and 5-methoxycarbonylamino-N-cetyltryptamine (5-MCA-NAT) (10 µl at 100 µM), on tear secretion when applied topically in the rabbit cornea and its relationship with the melatonin MT1, MT2, and MT3/quinone reductase QR2 receptors. The results showed a significant increase in tear secretion, with a maximal effect at 60 minutes for the agonists (138.9% ± 6.5%, 128.9% ± 6.4%, and 120.0% ± 5.2%, respectively; P < 0.05; 100% control) but not for melatonin (101.6% ± 7.9%; P > 0.05). Agonist action was tested combined with the antagonists DH97 (MT2 selective), prazosin (MT3/QR2 inhibitor), and luzindole (nonselective MT membrane receptor) (10 µl at 100 µM). DH97 reversed the effect of agomelatine, IIK7, and 5-MCA-NAT up to 30.85% ± 7.6%,108% ± 7.2%, and 87.01% ± 7.6%, respectively (P < 0.05; 100% control). Luzindole antagonized agomelatine and 5-MCA-NAT up to 67.35% ± 7.6% and 92.12% ± 8%, respectively (P < 0.05). Prazosin only reversed 5-MCA-NAT action up to 84.2% ± 7.7% (P < 0.05). These results suggest different pathways for the agonists to act through MT membrane receptors. Therefore, agomelatine, IIK7, and 5-MCA-NAT act through MT membrane receptors as secretagogues of tear secretion, and these analogs could be considered excellent therapeutic candidates for dry eye treatment. SIGNIFICANCE STATEMENT: Currently, dry eye with aqueous deficit is treated by adding artificial tears palliatively. This study shows that topical installation of three melatonin analogs (agomelatine, IIK7, and 5-MCA-NAT), but not melatonin, in therapeutic doses in the rabbit cornea significantly increases tear production, acting through different melatonin membrane receptor subtypes. Therefore, this study suggests that melatoninergic compounds could be considered excellent therapeutic candidates for dry eye treatment and ocular surface diseases occurring with a reduction in tear production.

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.

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.

Melatonin synthesis in the human ciliary body triggered by TRPV4 activation: Involvement of AANAT phosphorylation

Melatonin is a substance synthesized in the pineal gland as well as in other organs. This substance is involved in many ocular functions, giving its synthesis in numerous eye structures. Melatonin is synthesized from serotonin through two enzymes, the first limiting step into the synthesis of melatonin being aralkylamine N-acetyltransferase (AANAT). In this current study, AANAT phosphorylation after the activation of TRPV4 was studied using human non-pigmented epithelial ciliary body cells. Firstly, it was necessary to determine the adequate time and dose of the TRPV4 agonist GSK1016790A to reach the maximal phosphorylation of AANAT. An increase of 72% was observed after 5 min incubation with 10 nM GSK (**p < 0.05, n = 6) with a concomitant rise in N-acetyl serotonin and melatonin synthesis. The involvement of a TRPV4 channel in melatonin synthesis was verified by antagonist and siRNA studies as a previous step to studying intracellular signalling. Studies performed on the second messengers involved in GSK induced AANAT phosphorylation were carried out by inhibiting several pathways. In conclusion, the activation of calmodulin and calmodulin-dependent protein kinase II was confirmed, as shown by the cascade seen in AANAT phosphorylation (***p < 0.001, n = 4). This mechanism was also established by measuring N-acetyl serotonin and melatonin levels. In conclusion, the activation of a TRPV4 present in human ciliary body epithelial cells produced an increase in AANAT phosphorylation and a further melatonin increase by a mechanism in which Ca-calmodulin and the calmodulin-dependent protein kinase II are involved.

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.

Potential role of CYP1B1 in the development and treatment of metabolic diseases

Cytochrome P450 1B1 (CYP1B1), a member of CYP superfamily, is expressed in liver and extrahepatic tissues carries out the metabolism of numerous xenobiotics, including metabolic activation of polycyclic aromatic hydrocarbons. Surprisingly, CYP1B1 was also shown to be important in regulating endogenous metabolic pathways, including the metabolism of steroid hormones, fatty acids, melatonin, and vitamins. CYP1B1 and nuclear receptors including peroxisome proliferator-activated receptors (PPARs), estrogen receptor (ER), and retinoic acid receptors (RAR) contribute to the maintenance of the homeostasis of these endogenous compounds. Many natural flavonoids and synthetic stilbenes show inhibitory activity toward CYP1B1 expression and function, notably isorhamnetin and 2,4,3',5'-tetramethoxystilbene. Accumulating evidence indicates that modulation of CYP1B1 can decrease adipogenesis and tumorigenesis, and prevent obesity, hypertension, atherosclerosis, and cancer. Therefore, it may be feasible to consider CYP1B1 as a therapeutic target for the treatment of metabolic diseases.

Associations of sleep duration with open angle glaucoma in the Korea national health and nutrition examination survey

The aim of this study is to investigate the relationship between sleep duration and glaucoma, stratified by obesity status.This study was conducted using data from the Korean National Health and Nutrition Examination Survey V 2010 to 2012. Open-angle glaucoma was diagnosed according to the International Society of Geographical and Epidemiological Ophthalmology criteria. Subjects were divided into subgroups based on those who were overweight (body mass index ≥25 kg/m or <25 kg/m) or with abdominal obesity (based on waist circumference). Multiple logistic regression analysis was done to estimate the magnitude of the association between sleep duration (<7 h, 7-<9, or ≥9 hours) and prevalence of glaucoma in the total population and in the subgroups.Individuals who slept <5 hours per night had the highest prevalence of glaucoma (5.55 ± 1.09%), followed by those who slept ≥9 hours per night (4.56 ± 0.10%), and then by those who slept 5 to <6 hours per night (4.15 ± 0.68%), which revealed a U-shaped pattern (P for trend = 0.072). Among overweight individuals, subjects who slept <7 hours and those who slept ≥9 hours were significantly more likely to have glaucoma compared with subjects who slept 7 to <9 hours after adjusting for survey year, age, sex, smoking, drinking, exercise, education level, household income, hypertension, intraocular pressure, stress, and depression (odds ratio, 2.41; 95% confidence interval, 1.14-5.03). Unlike for overweight individuals, sleep duration in nonoverweight individuals was not statistically significantly associated with glaucoma.Our results reveal a U-shaped association between sleep duration and the prevalence of glaucoma. An effect of sleep duration on glaucoma was present in the subgroup of overweight patients.

Seasonal changes of 24-hour intraocular pressure rhythm in healthy Shanghai population

The aim of the present study was to investigate and compare the 24-hour intraocular pressure (IOP) rhythms in winter and summer in the healthy population of Shanghai, China.This is a cross-sectional study in which 24-hour IOP measurements were taken for all eligible healthy volunteers in winter and summer, respectively, and the temperature, hours of sunlight (sunlight time), and circulatory parameters, including heart rate, systolic blood pressure, and diastolic blood pressure, were also recorded. The 24-hour IOP curves and IOP parameters (mean, peak, trough, and fluctuation of IOP together with the diurnal-to-nocturnal IOP change) in winter and summer were obtained and compared. The magnitude of IOP changes from summer to winter was also calculated.A total of 29 participants (58 eyes), 14 (48.28%) male and 15 (51.72%) female, aged 43.66 ± 12.20 (19-61) years, were considered eligible for this study. Generally, IOP decreased progressively before noon, increased notably in the nocturnal period, and peaked at 12:00 AM in winter and at 2:00 AM in summer. The pattern of 24-hour IOP in winter and summer was significantly different (P = 0.002). The average IOPs from 4:00 PM to 8:00 AM, except for 6:00 AM, were significantly higher in winter (P < 0.05). However, no significant differences were shown after adjusting for temperature and/or sunlight time. From summer to winter, the extent of IOP increase was mostly around 0 to 3 mm Hg, and the IOPs increased more significantly in the nocturnal period than in the diurnal period (P = 0.05).The 24-hour IOP rhythms were different in winter and summer, with higher IOP level in winter. Temperature and sunlight time, which are independent of heart rate and blood pressure, affected the 24-hour IOP rhythms in healthy people in Shanghai, China. Further investigations are expected for the rhythm of some endogenous substance secretion and the inner mechanism of regulation of IOP.

Diabetes Pathology and Risk of Primary Open-Angle Glaucoma: Evaluating Causal Mechanisms by Using Genetic Information

Although type 2 diabetes (T2D) predicts glaucoma, the potential for unmeasured confounding has hampered causal conclusions. We performed separate sample genetic instrumental variable analyses using the Genetic Epidemiology Research Study on Adult Health and Aging cohort (n = 69,685; 1995-2013) to estimate effects of T2D on primary open-angle glaucoma (POAG; 3,554 cases). Genetic instrumental variables for overall and mechanism-specific (i.e., linked to T2D via influences on adiposity, β-cell function, insulin regulation, or other metabolic processes) T2D risk were constructed by using 39 genetic polymorphisms established to predict T2D in other samples. Instrumental variable estimates indicated that T2D increased POAG risk (odds ratio = 2.53, 95% confidence interval: 1.04, 6.11). The instrumental variable for β-cell dysregulation also significantly predicted POAG (odds ratioβ-cell = 5.26, 95% confidence interval: 1.75, 15.85), even among individuals without diagnosed T2D, suggesting that metabolic dysregulation may increase POAG risk prior to T2D diagnosis. The T2D risk variant in the melatonin receptor 1B gene (MTNR1B) predicted risk of POAG independently of T2D status, indicating possible pleiotropic physiological functions of melatonin, but instrumental variable effect estimates were significant even excluding MTNR1B variants. To our knowledge, this is the first genetic instrumental variable study of T2D and glaucoma, providing a novel approach to evaluating this hypothesized relationship. Our findings substantially bolster observational evidence that T2D increases POAG risk.

Melatonin receptors trigger cAMP production and inhibit chloride movements in nonpigmented ciliary epithelial cells

Melatonin and its analog 5-MCA-NAT (5-methylcarboxyamino-N-acetyl tryptamine) are active compounds reducing intraocular pressure (IOP). This action is mediated through MT2 and the putative MT3 melatonin receptor, producing a transient reduction of IOP that lasts for a few hours and has not yet been characterized. The use of melatonin and its analog are causing a decrease in chloride efflux from rabbit nonpigmented epithelial cells (NPE), possibly explaining the decrease in IOP. Melatonin and 5-MCA-NAT inhibited rabbit NPE chloride release in a concentration-dependent manner, whereas the pD2 values were between 4.5 ± 1.2 and 4.4 ± 1.0, respectively. Melatonin hypotensive action was enhanced by the presence of MT2 antagonists, such as DH97 (N-pentanoyl-2-benzyltryptamine) and 4-P-P-DOT (4-phenyl-2-propionamidotetralin) and by the nonselective melatonin receptor antagonist luzindole. Prazosin (1.5 µM) partially reverses the melatonin action by acting as a selective MT3 antagonist. However, at 15 nM it acts as an α-adrenergic receptor antagonist, enhancing the melatonin effect. Regarding the intracellular pathways triggered by melatonin receptors, neither phospholipase C/protein kinase C pathway nor the canonical reduction of intracellular cAMP was responsible for melatonin or 5-MCA-NAT actions. On the contrary, the application of these substances produced a concentration-dependent increase of cAMP, with pD2 values of 4.6 ± 0.2 and 4.9 ± 0.7 for melatonin and 5-MCA-NAT, respectively. In summary, melatonin reduces the release of chloride concomitantly to cAMP generation. The reduction of Cl(-) secretion accounts for a decrease in the water outflow and therefore a decrease in aqueous humor production. This could be one of the main mechanisms responsible for the reduction of IOP after application of melatonin and 5-MCA-NAT.

Is the melatonin receptor type 1 involved in the pathogenesis of glaucoma?

Melatonin in the mammalian eye is synthesized by the photoreceptors and its levels show a clear daily pattern with high levels at night and lower levels during the day. It is synthesized in the ciliary body and secreted into the aqueous humor with a pattern similar to what has been reported for the retina. It acts by interacting with a family of G-protein coupled receptors that are negatively coupled with adenylate cyclase. Melatonin receptor subtypes MT1 and MT2 have been identified in the retina. Both are found in the inner nuclear layer (horizontal and amacrine cells), in the inner plexiform layer, ganglion cells (RGC) and retinal pigmented epithelium. They are also present in the ciliary body. Several studies implicate melatonin in the rhythmic regulation of intraocular pressure. MT1 and MT2 melatonin receptors are expressed in many parts of the eye. Melatonin receptors are expressed in the iris and ciliary body. Recent studies showed that mice lacking MT1 receptors have elevated intraocular pressure during the night and show a significantly reduced number of RGCs. These new studies suggest that dysfunctional melatonin signaling may be considered a possible risk factor in the pathogenesis of glaucoma and that mice deficient in MT1 receptors may be an animal model of glaucoma.

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 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.

Polymeric nanoparticles augment the ocular hypotensive effect of melatonin in rabbits

Melatonin, a neurohormone secreted by the pineal gland, is able to modulate intraocular pressure (IOP). The aim of this study was to generate nanoparticle (NPs) sustained release formulations that allow to extend the pre-corneal residence time of melatonin, thus prolonging its pharmacological effects. Poly(D,L-lactide-co-glycolide) (PLGA) and PLGA-poly(ethylenglycole) (PEG) nanoparticles (NPs) were used to prepare the new melatonin formulations. Mean particle diameter and zeta potential, determined after freeze-drying in the presence of glucose as a cryoprotectant, ranged between 100 and 400 nm and -32.2/-8.2 mV, respectively for PLGA and PLGA-PEG NPs. Melatonin loading ranged between 44% and 80%. DSC analysis showed a homogeneous molecular dispersion of the drug in the NPs matrix. The hypotensive effect was evaluated by measuring IOP during 24h after instillation in the rabbit eye, in comparison with a melatonin aqueous solution at the same concentration (0.08%, w/v). The tested NPs showed good ocular tolerability in rabbit eye using biomicroscopy. Melatonin-loaded PLGA-PEG NPs were the most effective in reducing IOP up to 8h (maximum IOP reduction: 5 mmHg).

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.

Ocular disorders and the utility of animal models in the discovery of melatoninergic drugs with therapeutic potential

INTRODUCTION

  The pineal indole-derived hormone melatonin is a modulator of circadian and seasonal rhythms with an important role in ocular health and disease. This could be due to specific melatonin receptors that have been identified in structures such as cornea, lens, ciliary body, retina, choroid and sclera. In addition, a local synthesis of melatonin occurs in several of these ocular tissues.

AREAS COVERED

The authors review existing literature on the most common animal models where ocular melatonin actions have been tested. The therapeutic potential of melatonin in diabetic keratopathy and retinopathy, keratitis, cataracts, glaucoma, uveitis, age-related macular degeneration and retinitis pigmentosa is discussed. Furthermore, the authors comment on the usefulness of different animal models for the development of melatoninergic drugs with therapeutic potential.

EXPERT OPINION

The use of animals for the study of ocular diseases and the potentiality of melatonin and its analogs, as future therapeutic drugs, should be performed on the basis of a rationale study. It is important to note that melatonin receptors seem to be widespread all over the eye. This strongly suggests that, in order to modify the physiology and biochemistry of malfunctioning ocular tissue, the melatonin receptors which are present in that tissue must be first identified. Second there is the need to confirm that those receptors targeted perform the desirable responses, and as a third measure, to use selective agonists (or antagonists) instead of melatonin. However, although some animals mimic ocular pathologies relatively well, and these can be used in melatonin studies, there is still a long way to go till some of the results obtained in animal models could be used for human therapy.

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.

The role of melatonin in glaucoma: implications concerning pathophysiological relevance and therapeutic potential

Glaucoma is a frequent ophthalmologic condition leading to chronic progressive optic neuropathy, which can result in visual impairment and blindness. In addition, glaucoma is associated with a dysregulation of circadian rhythms, as well as with a high incidence of sleep disorders, depression, and anxiety. However, because of their high comorbidity in older age, these conditions have not received much scientific attention and are often undertreated. In the current paper, we review the available literature on the role of melatonergic mechanisms in glaucoma, regulation of circadian rhythms, and depression. The literature is presented as a narrative review, providing an overview on the most important and clinically relevant publications. Recently, there has been evidence for a progressive loss of intrinsically photosensitive retinal ganglion cells (ipRGC) because of oxidative stress in glaucoma. As ipRGC are responsible for the photic transduction to the circadian system and subsequent melatonin secretion, and melatonin is involved in the pathophysiology of circadian desynchronization, sleep disorder, and depression, an impairment of photo-dependent melatonergic signaling may be a common pathway connecting glaucoma with these comorbidities. This fact, as well as the proven retinal neuroprotective role of melatonin, suggests that melatonergic drugs provide a potentially promising treatment strategy supplementing the management of intraocular pressure by pharmacological and surgical measures. Additionally, multidisciplinary treatment focusing on depression and normalization of circadian rhythms might be beneficial for glaucoma patients. Furthermore, glaucoma might be a useful model for studying the pathophysiological interactions between the melatonergic, circadian, and mood systems.

Melatonin as a therapeutic tool in ophthalmology: implications for glaucoma and uveitis

Several lines of evidence support the view that increased free radical generation and altered nitric oxide (NO) metabolism play a role in the pathogenesis of highly prevalent ocular diseases, such as glaucoma and uveitis. Data are discussed indicating that melatonin, being an efficient antioxidant that displays antinitridergic properties, has a promising role in the treatment of these ocular dysfunctions. Melatonin synthesis occurs in the eye of most species, and melatonin receptors are localized in different ocular structures. In view of the fact that melatonin lacks significant adverse collateral effects even at high doses, the application of melatonin could potentially protect ocular tissues by effectively scavenging free radicals and excessive amounts of NO generated in the glaucomatous or uveitic eye.

Melatonin: a novel neuroprotectant for the treatment of glaucoma

Glaucoma is a leading cause of blindness. Although ocular hypertension is the most important risk factor, several concomitant factors such as elevation of glutamate and decrease in gamma-aminobutyric acid (GABA) levels, disorganized NO metabolism, and oxidative damage could significantly contribute to the neurodegeneration. The aim of this report was to analyze the effect of melatonin on retinal glutamate clearance, GABA concentrations, NO synthesis, and retinal redox status, as well as on functional and histological alterations provoked by chronic ocular hypertension induced by intracameral injections of hyaluronic acid (HA) in the rat eye. In normal retinas, melatonin increased glutamate uptake, glutamine synthase activity, GABA turnover rate, glutamic acid decarboxylase activity, superoxide dismutase activity, and reduced glutathione (GSH) levels, whereas it decreased NOS activity, L-arginine uptake, and lipid peroxidation. To assess the effect of melatonin on glaucomatous neuropathy, weekly injections of HA were performed in the eye anterior chamber. A pellet of melatonin was implanted subcutaneously 24 hr before the first injection or after six weekly injections of HA. Melatonin, which did not affect intraocular pressure (IOP), prevented and reversed the effect of ocular hypertension on retinal function (assessed by electroretinography) and diminished the vulnerability of retinal ganglion cells to the deleterious effects of ocular hypertension. These results indicate that melatonin could be a promissory resource in the management of glaucoma.

Melatonin provides anxiolysis, enhances analgesia, decreases intraocular pressure, and promotes better operating conditions during cataract surgery under topical anesthesia

BACKGROUND

Melatonin has anxiolytic and potential analgesic effects. In this study, we assessed the effects of melatonin premedication on pain, anxiety, intraocular pressure (IOP), and operative conditions during cataract surgery under topical analgesia.

METHODS

Forty patients undergoing cataract surgery under topical anesthesia were randomly assigned into two groups (20 patients each) to receive either melatonin 10 mg tablet (melatonin group) or placebo tablet (control group) as oral premedication 90 min before surgery. Anxiety scores, verbal pain scores, heart rate, mean arterial blood pressure, and IOP were recorded. In addition, the surgeon was asked to rate operating conditions.

RESULTS

Melatonin significantly reduced the anxiety scores (median, interquartile range) from 5, 3.5-6 to 3, 2-3 after premedication and to 3, 2-3.5 during surgery (P = 0.04 and P = 0.005 compared with the placebo group, respectively). Perioperative verbal pain scores were significantly lower in the melatonin group with less intraoperative fentanyl requirement (median, interquartile range) compared with the control group, 0, 0-32.5 vs 47.5, 30-65 microg, respectively, P = 0.007. Melatonin also decreased IOP (mean +/- sd) significantly from 17.9 +/- 0.9 to 14.2 +/- 1.0 mm Hg after premedication and to 13.8 +/- 1.1 mm Hg during surgery (P < 0.001). It also provided better quality of operative conditions.

CONCLUSION

We concluded that oral melatonin premedication for patients undergoing cataract surgery under topical anesthesia provided anxiolytic effects, enhanced analgesia, and decreased IOP resulting in good operating conditions.

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.

Role of melatonin in the eye and ocular dysfunctions

Melatonin is a ubiquitous molecule and widely distributed in nature, with functional activity occurring in unicellular organisms, plants, fungi, and animals. Several studies have indicated that melatonin synthesis occurs in the retina of most vertebrates, including mammals. The retinal biosynthesis of melatonin and the mechanisms involved in the regulation of this process have been extensively studied. Circadian clocks located in the photoreceptors and retinal neurons regulate melatonin synthesis in the eye. Photoreceptors, dopaminergic amacrine neurons, and horizontal cells of the retina, corneal epithelium, stroma endothelium, and the sclera all have melatonin receptors, indicating a widespread ocular function for melatonin. In addition, melatonin is an effective antioxidant which scavenges free radicals and up-regulates several antioxidant enzymes. It also has a strong antiapoptotic signaling function, an effect that it exerts even during ischemia. Melatonin cytoprotective properties may have practical implications in the treatment of ocular diseases, like glaucoma and age-related macular degeneration.

Melatonin and sleep disorders associated with intellectual disability: a clinical review

BACKGROUND

Melatonin is used to treat sleep disorders in both children and adults with intellectual disability (ID), although it has no product license for such use. The evidence for its efficacy, potential adverse effects and drug interactions are reviewed in the context of prescribing to people with ID.

METHODS

A literature search was performed using multiple electronic databases. More literature was obtained from the reference lists of papers gathered through the searches.

RESULTS

Most of the studies were uncontrolled and the few controlled trials available were of small size. Melatonin appears effective in reducing sleep onset latency and is probably effective in improving total sleep time in children and adolescents with ID. It appears to be ineffective in improving night-time awakenings. Melatonin is relatively safe for short-term use. Its safety for long-term use is not established. Potential drug interactions, possible effects on puberty and concerns regarding the use of melatonin in epilepsy, asthma and depressive disorders are discussed.

CONCLUSIONS

Melatonin appears to be an effective sleep-initiator for children and adolescents with ID and probably has a similar effect for adults. There may be heterogeneity of response depending on the nature of the sleep problem and cause of the ID or associated disabilities. Further studies are necessary before firm conclusions can be drawn and guidelines for the use of melatonin for people with ID formulated.

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

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

Melatonin in the eye: implications for glaucoma

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

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.

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.

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.

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

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