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.