Edinburgh Napier University

  Although it is well established that fish possess corticotropin-releasing factor (CRF) and a CRF-like peptide, urotensin I, comparatively little is known about the pharmacology of their cognate receptors. Here we report the isolation and functional expression of two complementary {DNAs} (cDNAs), from the chum salmon Oncorhynchus keta, which encode orthologues of the mammalian and amphibian {CRF} type 1 (CRF1) and type 2 (CRF2) receptors. Radioligand competition binding experiments have revealed that the salmon {CRF1} and {CRF2} receptors bind urotensin I with ∼8-fold higher affinity than rat/human CRF. These two peptides together with two related CRF-like peptides, namely, sauvagine and urocortin, were also tested in cAMP assays; for cells expressing the salmon {CRF1} receptor, {EC50} values for the stimulation of cAMP production were between 4.5±1.8 and 15.3±3.1 nM. For the salmon {CRF2} receptor, the corresponding values were: rat/human CRF, 9.4±0.4 nM; urotensin I, 21.2±2.1 nM; sauvagine, 0.7±0.1 nM; and urocortin, 2.2±0.7 nM. We have also functionally coupled the O. keta {CRF1} receptor, in Xenopus laevis oocytes, to the endogenous Ca2+-activated chloride conductance by co-expression with the G-protein α subunit, Gα16. The {EC50} value for channel activation by rat/human {CRF} (11.2±2.6 nM) agrees well with that obtained in cAMP assays (15.3±3.1 nM). We conclude that although sauvagine is 13- and 30-fold more potent than rat/human {CRF} and urotensin I, respectively, in activating the salmon {CRF2} receptor, neither receptor appears able to discriminate between the native ligands {CRF} and urotensin I.