|Grau, E. gordon|
Submitted to: Life Sciences
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/6/2005
Publication Date: 4/11/2006
Citation: Takahashi, H., Sakamoto, T., Hyodo, S., Shepherd, B.S., Grau, E. 2006. Expression of glucocorticoid receptor in the intestine of a euryhaline teleost, the mozambique tilapia (Oreochromis mossambicus): effect of seawater exposure and cortisol treatment. Life Sciences 78(20): 2329-2335. Interpretive Summary: In fish, cortisol is an important steroid hormone that controls salt and water balance, metabolism and immune function. For instance, the movement of a fish from fresh water (FW) to seawater (SW) is accompanied by hormonal changes (increases in cortisol and growth hormone levels: GH) that attenuate the physiological stresses (water and ion balance) that result from such movements. Interestingly, in salmonids and tilapias, there is also an increase in growth following the acclimation response to SW. Consequently, the interplay between the stress hormone, cortisol, and GH is of great interest. The intestine is a tissue important to growth (nutrient absorption) and adaptation to the seawater environment; however, the effects of cortisol (and other hormones) on the intestine are not well understood. In this study, we found that blood cortisol levels and levels of its receptor in the intestine were elevated following transfer of tilapia from FW to SW. Treatment with cortisol (via injection) also elevated plasma cortisol and levels of the cortisol receptor in the intestine of FW tilapia, thus demonstrating the direct link between cortisol and its receptor. A better understanding of the effects of cortisol on intestinal function during SW adaptation, and in nutrition and immunity, will increase our knowledge of the interactions of environment with organismal traits and illuminate novel ways to enhance growth (via genetic selection or nutrition) in economically-important fishes.
Technical Abstract: Cortisol plays an important role in controlling intestinal water and ion transport in teleosts possibly through glucocorticoid receptor (GR) and/or mineralocorticoid receptor. To better understand the role of GR in the teleost intestine, in a euryhaline tilapia, Oreochromis mossambicus, we examined (1) the intestinal localizations of GR; (2) the effects of environmental salinity challenge and cortisol treatment on GR mRNA expression. The mRNA abundance of GR in the posterior intestinal region of tilapia was found to be higher than that in the anterior and middle intestine. In the posterior intestine, GR appears to be localized in the mucosal layer. GR mRNA levels in the posterior intestine were elevated after exposure of freshwater fish to seawater for 7 days following an increase in plasma cortisol. Similarly, cortisol implantation in freshwater tilapia for 7 days elevated the intestinal GR mRNA. These results indicate that seawater acclimation is accompanied by upregulation of GR mRNA abundance in intestinal tissue, possibly as a consequence of the elevation of cortisol levels. In contrast, a single intraperitoneal injection of cortisol into freshwater tilapia decreased intestinal GR mRNA. This downregulation of the GR mRNA by cortisol suggests a dual mode of autoregulation of GR expression by cortisol.