Characterization of two channel catfish, Ictalurus punctatus, glucocorticoid receptors and expression following an acute stressor.

Two channel catfish glucocorticoid receptor genes, ipGR1 (NR3C1_1) and ipGR2 (NR3C1_2) were partially characterized. Identification and analysis of the genomic organization of two channel catfish glucocorticoid (GC) receptors (GRs) revealed differences in the lengths of exons 1 and 2 and the addition of an extra 27-bp exon inserted after exon 2 in the GR1 gene, yielding a 9-aa insert in the receptor protein. Sequence of the 9-aa insert in ipGR1 (WRARQNTHG) is unique compared to other teleost fish GRs. Amino acid sequence alignment of the two channel catfish GRs, revealed 55% sequence identity between them, with a high degree of sequence conservation (82%) in the DNA binding and ligand binding domains. Real-time PCR indicated that ipGR1 and ipGR2 were expressed in all tissues evaluated. Channel catfish GR1 was predominantly expressed in the gills, nearly 25-fold higher than in the liver. GR1 expression was higher than GR2 expression in gills, intestine, head kidney and heart (P<0.05). Channel catfish hepatic GR1 mRNA expression was significantly (P<0.05) increased from pre-stress expression 30min following removal of the acute stressor. After 30min of stress and during the 2h recovery period, ipGR1 mRNA expression was higher relative to ipGR2 expression. Hepatic ipGR2 expression was not affected (P>0.05) by the acute stress event. The present study adds to the growing body of information on GR evolution and function and further demonstrates the unique regulation of the GC/GR system in teleost fish.