Identification of potential sites of cortisol actions on the reproductive axis in rainbow trout

The full length cDNA encoding a rainbow trout glucocorticoid receptor (rtGR) has been obtained from rainbow trout liver and intestine libraries. Northern blot analysis showed that the corresponding messengers are detected in the brain of trout with a size 7.5 kb similar to the size of rtGR mRNA in other target tissues. The distribution of the rtGR mRNA and protein was studied in the forebrain of the trout by means of both in situ hybridization and immunohistochemistry and compared with that of the oestrogen receptor (rtER). The GR and ER mRNAs and proteins were detected with a strong overlapping mainly in the: (a) preoptic region; (b) mediobasal hypothalamus; and (c) anterior pituitary, confirming their implication in the neuroendocrine control of pituitary functions. In both diencephalon and pituitary, the peptidergic phenotype of some neuron or cell categories expressing either type of receptors could be determined by double staining. Furthermore, double staining studies have demonstrated colocalization of the two receptors in the same neurons or pituitary cells. The rtER and rtGR were found to be co-expressed in the dopaminergic neurons inhibiting GTH2 secretion and in pituitary cells of the anterior lobe--notably the gonadotrophs. Given that the promoter of the ER gene contains several potential glucocorticoid-responsive elements (GRE) and that cortisol inhibits the oestradiol-stimulated ER expression in the liver, the possibility exists for modulation of ER gene expression by GR in the hypothalamo-pituitary complex. This could explain some of the well documented effects of stress on the reproductive performance in salmonids.