Genomic structure of the proopiomelanocortin gene and expression during acute low-water stress in channel catfish.

Proopiomelanocortin (POMC) is an important gene involved in the stress response of the hypothalamic-pituitary-adrenal axis. It is a precursor of several peptide hormones including adrenocorticotropic hormone, melanocyte stimulating hormones, and beta-endorphin. Our study aims to determine genomic structure and expression of POMC gene during temporal stress in channel catfish (Ictalurus punctatus). The catfish POMC gene consisting of three exons and two introns has a similar structural organization to that of other species. The catfish and mammalian POMC promoters do not exhibit regions of conservation except that of one TATA box. Genomic Southern blot analysis indicated POMC is present as a single copy gene in the catfish genome. Real-time PCR allowed us to monitor temporal expression of the POMC mRNA in catfish pituitary during low-water stress. Plasma cortisol concentrations were also measured as an indicator of stress. Within 15 min after the onset of low-water stress, POMC mRNA expression was elevated 1.87-fold above the control value. The POMC mRNA level had declined after 30 min (1.29-fold) and 1h (1.1-fold) at which time stress was removed. After 1h recovery, a significant increase in the POMC mRNA expression was detected (2.44-fold, P<0.05) followed by a decline 2h later (1.52-fold) when the experiment was terminated. Plasma cortisol levels in stressed fish were significantly above the cortisol levels in control fish during stress application (t=15 min, t=30 min, and t=1h, P<0.05), which then returned to normal during recovery. We conclude that POMC and cortisol are both involved in the low-water stress response during which cortisol may serve as a negative regulator of POMC expression in catfish.