ESTROGEN-RECEPTORS IN THE BRAIN-PITUITARY COMPLEX AND THE NEUROENDOCRINE REGULATION OF GONADOTROPIN-RELEASE IN RAINBOW-TROUT

In all vertebrates, steroid feedback is one of the mechanisms by which the brain is constantly kept informed on the physiological status of the periphery. It is, therefore, a very efficient system of synchroniz ation between the different actors of the brain-pituitary-effector axi s, particularly in the context of reproduction. The central effects of steroids are mediated by specific receptors belonging to the superfam ily of intracellular receptors and acting as ligand dependant transcri ption factors to modulate the expression of certain target genes. In o rder to investigate the molecular events underlying steroid actions in the brain/pituitary complex, it is necessary, as a first step, to loc ate precisely steroid expressing cells in the brain/pituitary complex and to identify the phenotype of these cells on the basis of the neuro hormone(s) or hormones they produce. This paper will review recent dat a obtained in our laboratory regarding the distribution and regulation of expression of estrogen receptors in the brain and pituitary gland of the rainbow trout (Oncorhynchus mykiss), using both in situ hybridi zation or immunohistochemistry. In addition, information concerning th e phenotypic identification of cells expressing these receptors is pre sented, especially with respect to the main neuroendocrine systems con trolling reproduction. The functional meaning of these data are discus sed in the context of the reproductive physiology of trout.