Specific binding sites for [H Dexamethasone and [H17B-Estradiol [17-Beta (Greek symbol)] in the hypothalamus of juvenile rainbow trout, Oncorhynchus mykiss

In bony fishes such as the rainbow trout, Oncorhynchus mykiss, glucocorticoid (GC) and estrogenic hormones are associated with a variety of physiological functions. Most notable are effects of GCs which allow the animal to respond to stressful events (Donaldson, 1981; Clearwater and Pankhurst, 1997; Stouthart et al., 1998) and the actions of estrogens in the hormonal control of reproduction (Donaldson, 1973; Arcandhoy and Benson, 1998; Aiukwe and Goksoyr, 1998). Circulating levels o f GCs and estrogens are regulated largely by activation of the hypothalamic-pituitary-interrenal (HPI) axis (Donaldson, 1981; Stouthart et ai., 1998), and the hypothalamic-pituitary-gonadal (HPG) axis (Fostier et al., 1983; Blazquez et al., 1998; Kah et al., 1997), respectively In the case o f the HPI axis, the initial stimulus for biosynthesis and release of cortisol, the primary plasnia GC, is corticotropic releasing hormone (CRH). Neurons o f hypothalamic origin project their axons to pituitary target cells, corticotrophs (Peter et al., 1990). CRH released onto corticotrophs stimulates release o f adrenocorticotropic hormone (ACTH) into circulation (Baker et al., 1996; Stouthart et al., 1998). ACTH serves as the final signal in this pathway by stimulating interrenal tissue to synthesize and release cortisol into circulation (Chester-Jones et al., 1969). In turn, the level of plasma GCs may provide a feedback signal to the HPI axis, via activation o f hypothalamic glucorticoid receptors (GRs)