Glucocorticoids have been recently proposed as input signals of circadian system, although the underlying molecular mechanism remains unclear. This work investigates the role of glucocorticoids as modulators of clock genes expression in the liver of goldfish. In fish maintained under a 12L:12D photoperiod, an intraperitoneal injection at Zeitgeber Time 2 of a glucocorticoid analog, dexamethasone (1 μg/g body weight) induced per1 genes while decreased gbmal1a and gclock1a expression in the liver at 8 h post-injection. A 4-h in vitro exposure of goldfish liver to cortisol (0.1-10 μM) also induced gper1 genes in a concentration-dependent manner. Similarly, the exposure of the goldfish cultured liver to dexamethasone produced a concentration-dependent induction of gper1 genes. Moreover, this glucocorticoid analog led to a decrease in gbmal1a and gclock1a transcripts, while the other clock genes analyzed were unaffected. The induction of gper1a and gper1b by dexamethasone in vitro was observed at short times (2 h), whereas the reductions of gbmal1a and gclock1a transcripts needed longer exposure times (8 h) to the glucocorticoid to be significant. Additionally, a 2-h exposure to dexamethasone in the liver culture was enough to extend the induction of per genes for more than 12 h. Present results indicate that gper1 genes are targets for glucocorticoids in the regulation of goldfish hepatic oscillator, as previously reported in mammals, suggesting a conserved role of glucocorticoids in the functional organization of the peripheral circadian system in vertebrates. The repression of clock1a and bmal1a is not so well established, and suggests that other clock genes could be glucocorticoid targets in the goldfish liver.
Keywords: Circadian system; Clock genes; Cortisol; Dexamethasone; Liver; Teleosts; per1.