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 MT 2 and the putative MT 3 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 pD 2 values were between 4.5 6 1.2 and 4.4 6 1.0, respectively. Melatonin hypotensive action was enhanced by the presence of MT 2 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 mM) partially reverses the melatonin action by acting as a selective MT 3 antagonist. However, at 15 nM it acts as an a-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 pD 2 values of 4.6 6 0.2 and 4.9 6 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 2 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.

TRPV4 activation triggers the release of melatonin from human non-pigmented ciliary epithelial cells

“…This melatonin, acting in an autocrine way through melatonin receptors present in the ciliary processes (Alarma-Estrany et al, 2009, 2011, could reduce chloride production from the ciliary epithelium as recently described (Huete-Toral et al, 2015).…”

Rola błonowych receptorów melatoniny w zależnym od melatoniny uwalnianiu oksytocyny z układu podwzgórze-część nerwowa przysadki szczura — badania in vitro oraz in vivo

“…Three concentrations (10 -9 M, 10 -7 M, and 10 -3 M) of the hormone were employed for the study because previous in vitro experiments have PRACE ORYGINALNE (10 -3 M ) concentration melatonin had the opposite effect on the studied process [8][9][10][11][12]. On the other hand, the concentrations of both antagonists (10 -6 M and 10 -5 M) were chosen for our present experiments because they were demonstrated to be the most effective in action [24][25][28][29][30][31]. Several studies have shown that luzindole antagonises the effects of melatonin.…”

Effect of Melatonin and 5-Methoxycarbonylamino-N-Acetyltryptamine on the Intraocular Pressure of Normal and Glaucomatous Mice

“…The first two are negatively coupled to adenylate cyclase, whereas MT 3 seems to be positively linked to the same signal transduction mechanism (Huete-Toral et al, 2015). There is controversy regarding these receptors, since some authors claim that MT 3 receptors are quinone reductase 2 (Nosjean et al, 2000), but this identity has not been demonstrated in ocular tissues.…”

“…Since the mentioned adrenoceptor may contribute to the regulation of IOP as previously described (Pintor, 2009), it is possible that the application of prazosin may reflect an action on a1 adrenergic receptors rather than MT 3 melatonin receptors. To clarify this point, we recently described that when prazosin is topically applied at micromolar concentrations, the intraocular concentration of this antagonist acts mainly on the MT 3 melatonin receptor rather than on the a1 adrenergic receptor, thus discarding the possible interaction with the adrenergic receptor (Huete-Toral et al, 2015).…”