Expression of salmon corticotropin-releasing hormone precursor gene in the preoptic nucleus in stressed rainbow trout

Oreochromis mossambicus (tilapia) corticotropin-releasing hormone: cDNA sequence and bioactivity

Although hypothalamic corticotropin-releasing hormone (CRH) is involved in the stress response in all vertebrate groups, only a limited number of studies on this neuroendocrine peptide deals with non-mammalian neuroendocrine systems. We determined the cDNA sequence of the CRH precursor of the teleost Oreochromis mossambicus (tilapia) and studied the biological potency of the CRH peptide in a homologous teleost bioassay. Polymerase chain reaction (PCR) with degenerate and specific primers yielded fragments of tilapia CRH cDNA. Full-length CRH cDNA (988 nucleotides) was obtained by screening a tilapia hypothalamus cDNA library with the tilapia CRH PCR products. The precursor sequence (167 amino acids) contains a signal peptide, the CRH peptide and a motif conserved among all vertebrate CRH precursors. Tilapia CRH (41 aa) displays between 63% and 80% amino acid sequence identity to CRH from other vertebrates, whereas the degree of identity to members of the urotensin I/urocortin lineage is considerably lower. In a phylogenetic tree, based on alignment of all full CRH peptide precursors presently known, the three teleost CRH precursors (tilapia; sockeye salmon, Oncorhynchus nerka; white sucker, Catostomus commersoni) form a monophyletic group distinct from amphibian and mammalian precursors. Despite the differences between the primary structures of tilapia and rat CRH, maximally effective concentrations of tilapia and rat CRH were equally potent in stimulating adrenocorticotropic hormone (ACTH) and alpha-MSH release by tilapia pituitaries in vitro. The tilapia and salmon CRH sequences show that more variation exists between orthologous vertebrate CRH structures, and teleost CRHs in particular than previously recognized. Whether the structural differences reflect different mechanisms of action of this peptide in the stress response remains to be investigated.

Differential expression of corticotropin-releasing factor (CRF) and urotensin I precursor genes, and evidence of CRF gene expression regulated by cortisol in goldfish brain

Corticotropin-releasing factor (CRF) and urotensin I (UI) precursor cDNAs were cloned and sequenced from a goldfish brain cDNA library in order to investigate the distribution of CRF and UI mRNAs in goldfish brain and the regulation of CRF and UI gene expression. The CRF (966-bp) and UI (769-bp) cDNAs encode 163- and 146-amino acid precursors, respectively, and consist of a signal peptide sequence, a cryptic region, and a 41-amino acid mature peptide at the carboxy terminal. The deduced amino acid sequences of the CRF and UI peptides exhibit a sequence identity of 54%. Northern blot analysis revealed a single size of CRF (1.3 kb) and UI (2.0 kb) mRNAs, which are expressed in the telencephalon-preoptic, hypothalamic, optic tectum-thalamus, and posterior brain regions, but not in the pituitary. In addition, while the CRF gene is strongly expressed in the olfactory bulbs, the UI gene is not. In brain regions in which both genes are expressed, the mRNA levels of CRF were three- to sevenfold higher that those of UI. While the low expression levels of the UI gene prevented further analysis of its regulation, the regulation of CRF gene expression by cortisol was examined. In response to intraperitoneal implants of cortisol (300 microg/g BW) the level of CRF mRNA in the telencephalon-preoptic region decreased to 69% of control values at 6 and 24 h posttreatment. In sham-treated fish, in parallel with a transient injection stress-elicited increase in plasma cortisol, CRF mRNA levels declined to 72% of control value at 6 h postinjection and recovered after 24 h. Injection of the glucocorticoid antagonist, RU-486 (100 microg/g BW), prevented the reduction in CRF gene expression associated with the injection stress at 6 h and increased CRF mRNA levels to 145% of control value after 24 h. In contrast, the various implants had no effect on CRF mRNA levels in either the hypothalamus or the optic tectum-thalamus region. These results provide evidence of differential expression of the CRF and UI genes in hypothalamic and extrahypothalamic regions of goldfish brain. Furthermore, they demonstrate that stress levels of plasma cortisol can lead to a decrease in CRF gene expression that is mediated by glucocorticoid receptors in the telencephalon-preoptic region and give an indication of the regional specificity of the regulation of CRF gene expression by cortisol.

Evolution and physiology of the corticotropin-releasing factor (CRF) family of neuropeptides in vertebrates

Corticotropin-releasing factor (CRF), urotensin-I, urocortin and sauvagine belong to a family of related neuropeptides found throughout chordate taxa and likely stem from an ancestral peptide precursor early in metazoan ancestry. In vertebrates, current evidence suggests that CRF on one hand, and urotensin-I, urocortin and sauvagine, on the other, form paralogous lineages. Urocortin and sauvagine appear to represent tetrapod orthologues of fish urotensin-I. Sauvagine's unique structure may reflect the distinctly derived evolutionary history of the anura and the amphibia in general. The physiological actions of these peptides are mediated by at least two receptor subtypes and a soluble binding protein. Although the earliest functions of these peptides may have been associated with osmoregulation and diuresis, a constellation of physiological effects associated with stress and anxiety, vasoregulation, thermoregulation, growth and metabolism, metamorphosis and reproduction have been identified in various vertebrate species. The elaboration of neural circuitry for each of the two paralogous neuropeptide systems appears to have followed distinct pathways in the actinopterygian and sarcopterygian lineages of vertebrates. A comparision of the functional differences between these two lineages predicts additional functions of these peptides.