Central melatonin receptors in the rainbow trout: Comparative distributionof ligand binding and gene expression

To better define the role of melatonin in fish, we have compared in detail the distribution of 2-[I-125]iodomelatonin binding sites with gene expressi on for melatonin receptor subtypes in a widely studied seasonal species, th e rainbow trout. Three distinct partial sequences of the melatonin receptor gene were cloned from trout genomic DNA. Two of the sequences corresponded to the Mel1a receptor subtype, and one corresponded to the Mel1b receptor subtype. Analysis of numerous clones failed to find a sequence equivalent t o the Melic receptor subtype. Comparison of receptor gene expression with 2 -[I-125]iodomelatonin binding distribution indicated dendritic transport of the receptor. Melatonin receptors were associated predominantly with visua lly related areas of the trout brain, such as the thalamic region, the pret ectal area, and the optic tectum. The pituitary was devoid of 2-[I-125] iod omelatonin binding, and melatonin receptor gene expression was not detectab le. It would appear from the results of the present study that melatonin in this species is involved primarily in the processing of visual signals. Ho w melatonin interacts with circannual rhythms of growth and reproduction is unclear, although a direct interaction between melatonin and the hypothala mopituitary axis is not clearly indicated. (C) 1999 Wiley-Liss, Inc.