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

ABSTRACT

The behavior of genes encoding the corticotropin-releasing hormone (CRH) precursor in response to stress has not been extensively studied in teleosts. To clarify this problem, CRH cDNAs were isolated from a hypothalamic cDNA library of sockeye salmon, Oncorhynchus nerka, by screening with PCR products amplified from the hypothalamic mRNA with primers deduced from the sequence of the sucker CRH precursor. Two types of PCR products with a high degree of sequence homology were identified (CRH-I and CRH-II). A cDNA encompassing the entire coding sequence of the salmon CRH-I precursor was isolated. The salmon CRH-I cDNA encodes a 167-amino-acid precursor, which consists of a signal sequence, a cryptic peptide, and the carboxyl terminal 41-amino-acid sequence of CRH. The deduced amino acid sequence of salmon CRH peptide exhibits 66 to 80% homology with mammalian, Xenopus, and sucker CRHs, whereas it shows about 50% homology with sucker, carp, or sole urotensin I, a CRH-related neuropeptide in teleost fish. In situ hybridization histochemistry demonstrated CRH mRNA-positive perikarya in the preoptic nucleus in rainbow trout, Oncorhynchus mykiss, when the fish were stressed by confinement. Adjacent sections hybridized with probes for salmon vasotocin (VT) precursor showed many VT mRNA-positive neurons also in the preoptic nucleus, suggesting a colocalization of CRH and VT mRNAs in the same magnocellular neurons in the rainbow trout brain. The present results suggest that CRH may have important roles in the control of stress responses in salmonid fish.