Day-night specific binding of 2-[¹²⁵I]Iodomelatonin and melatonin content in gill, small intestine and kidney of three fish species

Some of melatonin's (Mel) well-established physiological effects are mediated via high-affinity cell-membrane receptors belonging to the superfamily of G-protein-coupled receptors. Specific binding of ligand 2-[¹²⁵I]iodomelatonin, using membrane preparations from osmoregulatory tissues of flounder, rainbow trout and sea bream, together with Mel concentrations in the tissues and plasma were studied. The kidney, gill and small intestine samples were collected during the day and at night. The dissociation constants (K d) and maximal binding densities (B max) were calculated for each tissue at 11:00 and 23:00 h. The binding sites with K d values in the tissues in the picomolar range indicated the high affinity. K d and B max values were tissue- and species-dependent. The GTP analogue [Guanosine 5'-O-(3-thiotriphosphate)] treatment significantly reduced the B max value, indicating that the 2-[¹²⁵I]iodomelatonin-binding sites are probably coupled to a G-protein. No daily variations in K d and B max values were observed. These are the first studies of the presence of 2-[¹²⁵I]iodomelatonin-binding sites in the small intestine, kidney tubule and gill of fish. The data strongly suggest new potential targets for Mel action and the influence of Mel on water/ion balance in fish. The intestine seems to be a site of Mel synthesis and/or an active accumulation of the hormone.