Anales RANF

Hanan Awad Alkozi @Real Academia Nacional de Farmacia. Spain 52 Figure 2. Scheme showing membrane signalling by melatonin receptors MT 1 and MT 2, both G-protein coupled receptors. On the right is shown the interaction of G i with adenyl cyclase (AC) to decrease cAMP levels and therefore cyclic AMP-dependent protein kinase activity (PKA). On the left is shown the interaction of G q with phospholipase C (PLC) leading to the cleavage of phosphatidyl inositol diphosphate (PIP2) into inositol triphosphate (IP3) and diacylglycerol (DAG). These second messengers stimulate increased intracellular Ca2+ and protein kinase C (PKC), respectively. The downstream effects of these events at the membrane vary with cell type. 2.2. Melatonin and glaucoma One of the most important risk factors for developing glaucoma disease in elevated intraocular pressure. Intraocular pressure consists of an equilibrium between the formation of the aqueous humor in the ciliary process and its drainage by the trabecular meshwork and the to some extent, the uveoscleral pathway (35, 36). The aqueous humor composed mainly of water and electrolytes to complete one of its functions apart from regulating IOP, to provide nutrients to the avascular structures bathe in it (37). Aqueous humor also contains melatonin, which also plays an important role in regulating IOP. Melatonin content in the aqueous humor originates from both the ciliary processes and the crystalline lens. In this sense, both exogenous and endogenous melatonin has been extensively investigated. Before coming into the interesting relationship between melatonin and melanopsin, it is worthy to explain the role melatonin have over IOP when applied or consumed exogenously. Melatonin consumption prior to cataract surgery showed a positive effect on the post-operative outcome and it reduced IOP (38). Moreover, melatonin and its analogue agomelatine showed an ocular hypotensive effect when applied topically to a normotensive and hypertensive model of New Zealand white rabbits (39) as well as in a glaucomatous animal model (40), same effect observed after oral consumption of agomelatine in human glaucoma patients (41). This effect of reducing IOP is mediated by melatonin receptors, since it triggers cAMP production and consequently inhibits chloride efflux in the ciliary processes, hence reduces aqueous humor production (42). Different studies used mice lacking melatonin receptor 1 showed that they suffered from elevated intraocular pressure and retinal ganglion cells death, both signs for developing glaucoma (43). Beside that effect, melatonin is known also as a free radical scavenger, it functions as an oxidative stress and it was shown to have a protective effect against retinal cells death by reducing the damage produced by oxidative stress (44, 45). All of the mentioned studies used melatonin as an external pharmacological agent, however, melatonin content in the aqueous humor without any exogenous addition is not equal among all subjects. Analysis showed that certain conditions could lead to changes in melatonin levels in the aqueous humor. For instance, patients with elevated intraocular pressure have higher levels of melatonin in the aqueous humor. These results are in parallel to the analysis of melatonin levels in a mice model of glaucoma, both before and after developing the disease, and in comparison to the control healthy mice (46). These results were not only observed in the aqueous humor, but also seen in the serum melatonin level, were glaucoma patients had a significant increment of melatonin in blood compared to healthy patients (47). Human donor eyes were examined by immunohystochemistry assay against melatonin synthesizing enzyme AANAT, in both glaucomatous donors and healthy eyes, demonstrated an increased staining of AANAT in the ciliary body of glaucoma donors (48). In order to investigate these results, in vitro studies took place using immortalized human non- pigmented ciliary body epithelial cells. To mimic the elevation of intraocular pressure, a vanilloid channel belonging to to the superfamily of TRPs present in these