An unexpected effect of 5‐MCA‐NAT in chick retinal development

Abstract

Luzindole is an unselective antagonist of the melatonin receptors and melatonin's other binding sites, although some exceptions have been observed in chick retinal neurodevelopment, where this unselective antagonist does not block melatonin's inhibitory effect on the adenylate cyclase enzyme, probably due to the presence of some other melatonin receptor(s) or binding site(s). The present study investigated the modulation of cyclic adenosine 3'-5'-monophosphate (cAMP) levels via MT3 melatonin-binding sites, located within the QR2 (dihydronicotinamide riboside: quinone oxidoreductase 2) enzyme, by observing the response to luzindole. Embryonic and post-hatch retinas, incubated with a selective agonist for the MT3 melatonin-binding site 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT, 10 or 100 nM), had an increase in cAMP accumulation relative to control retinas. Luzindole (5microM) inhibited the 5-MCA-NAT stimulatory effect at all ages tested. The agonist 5-MCA-NAT enhanced the melatonin inhibitory effect on cAMP levels stimulated by forskolin (5microM), but not the stimulatory forskolin effect. The results suggest that MT3 melatonin-binding sites are present in embryonic and post-hatch chick retinas and that luzindole more selectively blocks the 5-MCA-NAT effect on cAMP accumulation than it blocks the melatonin inhibitory effect via G protein-coupled receptors in chick retinal neurodevelopment.

MT3 melatonin binding site, MT1 and MT2 melatonin receptors are present in oocyte, but only MT1 is present in bovine blastocyst produced in vitro

“…Due to the lipophilic nature of melatonin, it is able to function by binding to receptor in the nucleus [5], a cytosolic binding site the calmodulin protein [6] and also by modulating the cytosolic detoxifying enzyme NRH: Quinone Oxireductase 2 (NQO2). It does so through the MT3 melatonin binding site, which was showed to be NQO2 [7]. Furthermore, it has been observed that melatonin modulates the myeloperoxidase enzyme [8].…”

Molecular and cellular pharmacological properties of 5‐methoxycarbonylamino‐N‐acetyltryptamine (MCA‐NAT): a nonspecific MT3 ligand

“…One should then emphasize that this is roughly the affinity of MCA‐NAT for MT 1 and MT 2 . Furthermore, in two of those papers, widely different in their approaches, the authors clearly demonstrated that MCA‐NAT actions in their respective systems were inhibited by a MT receptor antagonist (luzindole [12]) or that MCA‐NAT antagonized the action of melatonin (in chicken retina [26]). One of the major effects of MCA‐NAT is on IOP.…”

Melatonin and the time window for the expression of the α8 nicotinic acetylcholine receptor in the membrane of chick retinal cells in culture

“…Chick retinal cells express three subtypes of melatonin receptors (Mel 1a , Mel 1b and Mel 1c ), activation of which leads to reduction of cAMP levels (Dubocovich et al, 2003; Iuvone et al, 2005; Sampaio, 2008). These receptors are blocked by luzindole (Dubocovich et al, 2003; Sampaio, 2009). In mature chick retina, Mel 1a , Mel 1b and Mel 1c melatonin receptors are located in the cellular and plexiform layers (Natesan and Cassone, 2002).…”

“…During neurodevelopment, melatonin receptor characterized pharmacologically as Mel 1a causes a decrease in cAMP levels observed at embryonic day 8, while active Mel 1b receptor appears only at embryonic day 14, when synaptogenesis arises (Sampaio, 2008). Melatonin also activates a binding site named MT3 located in the enzyme quinone reductase II (Boutin et al, 2008) during all chick retinal differentiation stages and post‐hatched (Sampaio, 2009).…”