@article {Huete-Toral119, author = {Fernando Huete-Toral and Almudena Crooke and Alejandro Mart{\'\i}nez-{\'A}guila and Jes{\'u}s Pintor}, title = {Melatonin Receptors Trigger cAMP Production and Inhibit Chloride Movements in Nonpigmented Ciliary Epithelial Cells}, volume = {352}, number = {1}, pages = {119--128}, year = {2015}, doi = {10.1124/jpet.114.218263}, publisher = {American Society for Pharmacology and Experimental Therapeutics}, 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 {\textpm} 1.2 and 4.4 {\textpm} 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 {\textmu}M) 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 {\textpm} 0.2 and 4.9 {\textpm} 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.}, issn = {0022-3565}, URL = {https://jpet.aspetjournals.org/content/352/1/119}, eprint = {https://jpet.aspetjournals.org/content/352/1/119.full.pdf}, journal = {Journal of Pharmacology and Experimental Therapeutics} }