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

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 (5 μM) 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 (5 μM), 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.