Localization of 2-[125I]lodomelatonin binding sites in the brain of the atlantic salmon, salmo salar L.

The photosensory pineal organ of teleost fish shows a circadian rhythm in melatonin synthesis, and melatonin is known to influence a number of physiological functions. However, the target sites for melatonin are not known. We have investigated the distribution of melatonin binding sites in the brain of the salmon, Salmo salar. Brains were collected for receptor binding assay and autoradiography at each of three time points; just after lights on, just before lights off, and in the dark at midnight (photoperiod light-dark 12: 12, lights on at 08.00 h, lights off at 20.00 h). Specific binding of 2-[125I]iodomelatonin was observed in several brain areas. High densities were associated with (1) the optic tectum, (2) the preoptic area, (3) an area encompassing the magnocellu-lar superficial pretectal nucleus ('nucleus rotundus') and the glomerular complex, (4) the inferior lobes of the hypothalamus, (5) the lateral mesencephalic tegmentum including the torus semicircularis, and (6) the molecular layer of the cerebellum. No binding was observed in the pineal organ or in the pituitary. We observed no differences in labeling between brains collected at different time points, except in the preoptic area where binding was high at 20.00 and 24.00 h, but low at 08.00 h, and in the corpus cerebelli, where labeling in the molecular laver was higher at 24.00 and 08.00 h than at 20.00 h. Saturation experiments with crude brain membranes indicated the presence of a single binding site with no significant differences related to the time of day, with Kd values ranging from 30 to 54 pM, and Bmax values from 7.0 to 10.8 fmol/mg protein. Nonspecific binding, determined with 0.1 μM melatonin, was < 20%. The Kd and Bmax, values are in the same range as those reported for the so-called high-affinity binding site in mammalian neural tissue