The flounder (Platichthys flesus) is capable of full adaptation to sea and freshwater environments. This involves major changes of function in osmoregulatory organs such as gut, gills, bladder and kidney. The neuropeptides urotensin I and II (UI and UII) are major players in this adaptive process (Marshall & Bern, 1981; Loretz et al. 1985). In mammals, UI and UII peptides have been associated with many vital functions such as regulation of glucocorticoid synthesis and cardiovascular homeostasis. In fish, the source of these circulating neurohormones is the caudal neurosecretory system (CNSS), which is located in the terminal eight vertebral segments of the spinal cord. We have cloned the cDNAs encoding UI and UII peptides from flounder by the reverse transcriptase polymerase chain reaction (RT-PCR) and screening of a CNSS cDNA library. Flounder were obtained from Morecambe Bay (Cumbria, UK) and maintained at the University of Manchester in seawater tanks at 10-12 °C under natural photoperiod. Fish were humanely killed according to approved UK Home Office regulations (Schedule 1). RNA was extracted using the guanidine thiocyanate method and utilized for both RT-PCR and library construction. RT-PCR was carried out using degenerate oligonucleotide primers based on the amino acid sequence of flounder UI (Conlon et al. 1990a) and UII (Conlon et al. 1990b). A CNSS cDNA library was constructed into the phage vector l{special}TriplEx2 and screened at high stringency using specific UI and UII probes obtained from degenerate RT-PCR. Results show that the UI precursor consists of 147 amino acid residues and the carboxyl terminus represents the 41 amino acid sequence of the mature peptide, preceded by Lys-Arg and followed by Gly-Lys as putative cleavage sites. The UII precursor consists of 129 amino acid residues and the carboxyl terminus represents the 12 amino acid sequence of the mature peptide, also preceded by Lys-Arg. Using conditions that gave no cross-hybridisation with other genes, Northern blot analysis of a range of tissues confirmed the CNSS as the sole major site of expression of UI and UII genes and also shows the possibility of multiple polyadenylation signals in the 3Ô untranslated region of both UI and UII. RT-PCR using specific primers indicated the presence of UII transcripts in all tissues tested, including brain, spinal cord, intestine and bladder. The primary structure of UI and UII shows a close similarity between fish and humans. The cyclic region of UII, which is responsible for the biological activity of the peptide (McMaster et al. 1992), is fully conserved from fish to humans. From an evolutionary viewpoint these peptides may exert important physiological functions in both fish and humans.

This work was supported by a grant from the BBSRC.