AGE-RELATED-CHANGES IN 2-[I-125]-IODOMELATONIN BINDING-SITES IN THE BRAIN OF SEA BREAMS (SPARUS-AURATA, L)

The pineal organ of fish, through its 24h rhythmic release of melatoni n, acts as a transducer of the photoperiod, influencing different phys iological functions (e.g., reproduction, growth). The target sites for melatonin are poorly known in fish, especially marine species. A radi oligand study was undertaken using the gilthead sea bream (Sparus aura ta) maintained under natural temperature and photoperiod (at 28 degree s N latitude). This species exhibits the property of changing sex duri ng growth. Brains of one year-old males were collected at 16:00h and b rains of three year-old females at 03:00, 10:00, 16:00 and 23:00h. Mem brane homogenate receptor assays were run using 2-[(125)] iodomelatoni n as a ligand. Binding sites were detected in brains of young and old fish. In the younger, they exhibited a B-max between 3.52 and 4.29 fmo l mg protein(-1) and a K-D between 358-380 pmol l(-1). In the older fi sh, the K-D varied according to a daily pattern: values were three tim es higher at 03:00 and 10:00h (500-600 pmol l(-1)) than at 16:00 and 2 3:00h (150-300 pmol l(-1)). The number of sites also were higher at 03 :00 and 10:00h (180-200 fmol mg protein(-1)) than at 16:00 and 23:00h (95-110 fmol mg protein(-1)). Melatonin and iodomelatonin displaced 2- [I-125]iodomelatonin binding in a dose dependent manner, the second be ing more potent than the first. Binding was also inhibited by GTP. The results provide the first evidence for the presence of membrane melat onin binding sites in the brain of an exclusively marine fish. They su ggest that their number and affinity varies during growth and througho ut a light/dark cycle. Future experiments will aim to precise the anat omical location and role of these binding sites.