title: Melatonin receptors trigger cAMP production and inhibit chloride movements in nonpigmented ciliary epithelial cells.
abstract: 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.
authors.affiliation: Departamento de Bioquímica y Biología Molecular IV, Facultad de Óptica y Optometría, Universidad Complutense de Madrid, Madrid, Spain.
authors.forename: Fernando
authors.initials: F
authors.lastname: Huete-Toral
authors.affiliation: Departamento de Bioquímica y Biología Molecular IV, Facultad de Óptica y Optometría, Universidad Complutense de Madrid, Madrid, Spain.
authors.forename: Almudena
authors.initials: A
authors.lastname: Crooke
authors.affiliation: Departamento de Bioquímica y Biología Molecular IV, Facultad de Óptica y Optometría, Universidad Complutense de Madrid, Madrid, Spain.
authors.forename: Alejandro
authors.initials: A
authors.lastname: Martínez-Águila
authors.affiliation: Departamento de Bioquímica y Biología Molecular IV, Facultad de Óptica y Optometría, Universidad Complutense de Madrid, Madrid, Spain jpintor@vet.ucm.es.
authors.forename: Jesús
authors.initials: J
authors.lastname: Pintor
mesh_terms: D000818:Animals; D002460:Cell Line; D002712:Chlorides / Q000378:metabolism*; D002924:Ciliary Body / Q000166:cytology; D000242:Cyclic AMP / Q000096:biosynthesis*; D004847:Epithelial Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D008550:Melatonin / Q000494:pharmacology*; D009068:Movement / Q000187:drug effects*; D011817:Rabbits; D018340:Receptors, Adrenergic, alpha-1 / Q000378:metabolism; D044094:Receptors, Melatonin / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015290:Second Messenger Systems / Q000187:drug effects; D014363:Tryptamines / Q000494:pharmacology*
chemical_list: C413586:5-methoxycarbonylamino-N-acetyltryptamine; D002712:Chlorides; D018340:Receptors, Adrenergic, alpha-1; D044094:Receptors, Melatonin; D014363:Tryptamines; D000242:Cyclic AMP; D008550:Melatonin