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The regulation of the Glucocorticoid Receptor (GR) transcript was investigated in the gills of Atlantic salmon (Salmo salar) during the parr-smolt transformation. Sampling of parr and smolt fish was performed between December and July and in particular during the smoltification period occurring in spring. Quantification of GR transcripts revealed differences between the two groups in March and at the beginning of April. During these dates, the amounts of GR mRNA in parr gills were respectively three and two fold lower than those measured in smolts. In order to determine which factors are responsible for these differences, we studied the long-term effects of prolactin and Cortisol treatments on GR transcript in the gills of presmolt fish. The plasma levels of these two hormones respectively drop and rise during smoltification. Contrary to Cortisol long-term treatment which did not modify the amount of gill GR transcript, short-term treatment induced a significant decrease within 12 hours. Prolactin long-term treatment caused a significant increase of GR transcript abundance after 13 days of implant treatment. This result is unexpected with regard to those obtained in the smoltification analysis but is in agreement with previous studies performed in mammary gland revealing a positive control of PRL on GR in epithelial cells. Our data suggest that the regulation of the GR transcript during the parr-smolt transformation probably involves several hormonal factors.

The distribution of the glucocorticoid receptor (GR)within the forebrain of rainbow trout has been studied by means of in situ hybridization (ISH) and immunohistochemistry (ICC) using probes and antibodies directed against the AB-domain of the rainbow trout GR (rtGR). Both rtGR mRNA and protein exhibited the same pattern of distribution. A strong labelling was observed in all subdivisions of the preoptic nucleus, in the thalamic region and in the mediobasal hypothalamus (nucleus recessus lateralis, nucleus lateralis tuberis).It was notably found that all magnocellular neurons of the nucleus preopticus pars magnocellularis (NPOmc) were positive with both ISH and ICC. Furthermore, numerous cells of the anterior lobe of the pituitary were positively stained. Double staining studies allowed to identify part of the rtGR expressing cells. At the telencephalic level, it was found that GnRH neurons frequently express rtGR in vitellogenic females. Virtually all tyrosine hydroxylase (TH) cells in the anteroventral part of the preoptic nucleus (NPOav) were rtGR-positive. In this same area, a colocalization between rainbow trout estradiol receptor (rtER) and TH has already been documented, indirectly demonstrating that rtER and rtGR are colocalized. The same holds true for the GTH-II cells of the anterior pituitary which express both types of receptors. These results provide information on the potential sites of cortisol action on the reproductive axis directly at the pituitary level or indirectly on neuroendocrine Systems controllingGTH-II secretion, possibly via GR-ER interactions.