Rainbow trout (Oncorhynchus mykiss) urotensin-I: structural differences between urotensins-I and urocortins

In bony fishes, both corticotropin-releasing factor (CRF) and urotensin-I play a role in the regulation of interrenal glucocorticoid release. The rainbow trout, Oncorhynchus mykiss, is a useful model for understanding the mechanisms of stress and the hypothalamo-pituitary-interrenal axis because of its phylogenetic position at the base of the euteleostei and its popularity as a food fish. Urotensin-I may act as a glucocorticoid releaser in a mechanism phylogenetically older than that of CRF. The structural and functional relationships of trout urotensin-I have been investigated. The transcript was cloned from a trout brain hypothalamic cDNA library. A single positive clone was isolated and sequenced. It possesses 3218 bases and has the longest 3' untranslated region of all urotensins-I and CRF transcripts found to date. In comparison to the other fish orthologues, it has the closest sequence identity to the mammalian urocortins. The transcript appears to be differentially processed in brain and urophysis as determined by Northern blot analysis and the presence of polyadenylation signals in the 3' untranslated region. Synthetic trout urotensin-I activated both human CRF-R1 and -R2 receptor-transfected CHO cells with a potency similar to that of white sucker (Catostomus commersoni) urotensin-I. Both fish neuropeptides possessed an order of magnitude less potency than human urocortin in CRF-R2 transfected cells.