Regulation of GH expression in gilthead sea bream pituitary

The GH/IGF-I system is the main endocrine regulator of growth in vertebrates. But, how correlated is the pituitary expression of GH and that of the other components of the system with the animal growth condition? To answer this we have analyzed the changes on pituitary GH and related molecules expression upon different experimental conditions in vivo and in vitro in gilthead sea bream (Sparus aurata). Gilthead sea bream juveniles were treated with recombinant bovine GH (rbGH, Posilac), were fasted during 21 days and refed for 7 days, or were fed a diet containing an amino acid (AA) derivative. Moreover, adult fish pituitaries were treated in vitro with different hormones and AA. Pituitary GH expression in rbGH-treated fish was down-regulated reaching the lowest values after 6 weeks of injection and maintaining low levels during the rest of the experiment, suggesting that sustained high levels of exogenous GH provoked a clear depression on GH synthesis and secretion. On the contrary, 21-days fasting triggered an increase of pituitary GH expression parallel to plasma levels, while GH and IGF-I receptors expression strongly decreased, and refeeding partially recovered the basal levels. The adaptation to food deprivation determined a simultaneous increase of GH gene and protein expression to facilitate its energy mobilization function. Dietary supplementation for 8 weeks with an AA derivative also provoked a tendency to increase GH expression. Finally, hemi-pituitaries treated with Ghrelin or the AA Lysine and Leucine increased, while IGF-I or Somatostatin decreased GH expression. In summary, considering the difficulties to analyze GH circulating levels, these results indicate that pituitary GH expression can be a useful parameter to understand the effects of different physiological conditions on GH/IGF-I system in fish