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 librar ies. Northern blot analysis showed that the corresponding messengers a re detected in the brain of trout with a size 7.5 kb similar to the si ze 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 protei ns were detected with a strong overlapping mainly in the: (a) preoptic region; (b) mediobasal hypothalamus; and (c) anterior pituitary, conf irming their implication in the neuroendocrine control of pituitary fu nctions. 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 s ame 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 glucocorti coid-responsive elements (GRE) and that cortisol inhibits the oestradi ol-stimulated ER expression in the liver, the possibility exists for m odulation of ER gene expression by GR in the hypothalamo-pituitary com plex. This could explain some of the well documented effects of stress on the reproductive performance in salmonids. (C) 1998 Elsevier Scien ce Ireland Ltd. All rights reserved.