A comparative study of picomolar affinity 2-[125I]iodomelatonin binding sites in the hearts of three salmonid species

The hearts of three cultured salmonid species, collected at either mid-light or mid-dark were studied for their binding to 2-[125I]iodomelatonin, a specific melatonin agonist. The binding was saturable, reversible, and highly specific. The equilibrium dissociation constant (Kd) ranged from 30.1 ± 3.0 pmole 1−1 in Arctic charr (Salvelinus alpinus) to 40.5 ± 2.3 pmole 1−1 in rainbow trout (Oncorhynchus mykiss) indicating a high binding affinity. The maximum density of binding (Bmax) was at the low femtomolar level of 0.57 to 0.87 fmole mg−1 protein. Higher Bmax appeared to be demonstrated in the mid-light samples when compared to the mid-dark samples but the difference was not significant (p > 0.05). Competition study with various indoles showed the following order of potency: 2-iodomelatonin > melatonin > 6-chloromelatonin ≫ N-acetylserotonin ⋙ serotonin. Guanosine 5′-O-(3-thiotriphosphate) (GTPγS) strongly inhibited the binding (IC50 = 0.66 μmole 1−1) in the rainbow trout heart, suggesting that these binding sites belong to the superfamily of G-protein linked receptors. Our results suggest the presence of melatonin receptors in the fish heart. In addition, there was no marked intraspecies differences in Kd, Bmax and specificity that could be correlated with the phylogeny or life history of the salmonid species.