ABSTRACT
Hormones and neuroendocrine regulatory mechanisms have been highly conserved across vertebrates, making fish good model species for studies on behavioural neuroendocrinology (e.g., Oliveira e t al., 2005b). In teleosts, as in other vertebrates, the neuroendocrine system is organized in a hierarchical fashion with the hypothalamus controlling the activity of the anterior pituitary gland that, in turn, controls the functioning of the numerous peripheral endocrine glands (gonads, anterior kidney, etc.; see Fig. 3.1). As in other vertebrates, the fish pituitary gland consists of two types of tissue, the adenohypophysis and neurohypophysis, and the secretion of the adenohypophysial hormones is under the control of releasing factors produced by hypothalamic neurons (Schreibman, 1986;
Fig. 3 .1) . However, unlike other vertebrates, teleosts lack the hypothalamic-hypophysial portal vascular system, which is used in other vertebrates to pass the releasing factors from the hypothalamus to the pituitary, and the adenohypophysis receives direct innervation from the hypothalamus (Peter, 1990). Therefore, in teleosts, the relevant releasing factors controlling hypophysial function are still produced in neurosecretory hypothalamic neurons that project to the pars distalis of the adenohypophysis, thus making the hypothalamo-hypophysial axis. Within this axis, specific hypothalamic releasing hormones [e.g., gonadotropin-releasing hormone (GnRH), corticotropin-releasing hormone (CRH), growth-hormone-releasing hormone (GHRH)] control the release of specific trophic hormones [e.g., the gonadotropins luteinizing hormone (LH) and follicle stimulating hormone (FSH), adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), growth hormone (GH)] produced by different populations of trophic hormone producing cells in the adenohypophysis (e.g., gonadotropes, corticotropes, thyrotropes, somatotropes). On the other hand, the neurohypophysis receives neural projections from the magnocellular neurons of the preoptic area, which end in a capillary network, where the neurohormones produced by these neurons are released into the bloodstream (Fig. 3.1). All of these vertebrate neurohormones known to date belong to one of two major neurohypophysial hormone families: vasopressin-like peptides and oxytocin-like peptides. In teleosts they are, respectively, arginine-vasotocin and isotocin (Urano et al., 1994).
