To fully understand how these pollutants can affect fish at very low levels, it is necessary to outline very briefly how the endocrine system works. It is a control system of the body which responds to internal and external signals to maintain the body in a chemical equilibrium, to regulate sexual development and the seasonal reproductive cycles, and to evoke a stress response to external threats. At its core are the hypothalamus and pituitary, which respond to neural signals from the brain and convert them into hormone messengers which act on the individual glands such as the gonads, the thyroid and the adrenal (Figure 1). The endocrine systems of all vertebrates have essentially the same components, which originated during the early evolution of fish. It is not clear how many of the differences between mammals and fish are due to evolutionary divergence and how much is attributable to the very different habitats in which they have developed. Clearly the regulation of water and mineral balance in the body fluids (osmoregulation) requires very different control in mammals to that of fish inhabiting fresh- or seawater environments, while regulation of reproduction in the predominantly egg-laying (oviparous) fish is very different from that which is required in placental mammals. In many cases it is not the hormones but the uses to which they are put which differs, although some of the steroid hormones in fish do show marked differences to those of mammals.
Once they have elicited the appropriate action in their target tissue, hormones
Figure 1 A schematic diagram of the endocrine system of fish. TRH = thyrotrophin releasing hormone; GnRH = gonadotrophin releasing hormone; CRH = corticotrophin releasing hormone; TSH = thyroid stimulating hormone; GtH = gonadotrophins I and II;
ACTH = adrenocortico-trophic hormone; T4 = thyroxine; T3 = triiodothyronine; E2 = estradiol; T = testosterone; 17,20£P = 17,200-dihydroxy-4-pregnen-3-one; KT = 11-ketotestosterone; VTG = vitellogenin.
are converted by the liver to metabolites which are more easily excreted. There is a negative feedback of the circulating hormones to the pituitary and hypothalamus which maintains the normal hormonal equilibrium.
Fish possess the same essential components of the reproductive endocrine system as mammals in that external cues, such as seasonal changes in temperature or daylength, behaviour patterns of a potential mate, etc., are translated by the brain and hypothalamus into the release of gonadotrophin releasing hormone (GnRH). This in turn causes the pituitary gland, situated at the base of the brain, to release gonadotrophin which stimulates steroid synthesis in the gonads. At least some fish possess two gonadotrophins (GtH-I and GtH-II) analogous to the follicle stimulating and luteinising hormones (FSH and LH) which regulate the female cycle in mammals. GtH-I stimulates the ovary to produce estradiol which induces production of a yolk protein (vitellogenin) by the liver, while GtH-II predominates just before spawning when it stimulates ovarian synthesis of a progestogen (17,20^-dihydroxy-4-pregnen-3-one, usually abbreviated to 17,20^P), which induces maturation of the oocytes prior to ovulation. This progestogen may also play a role in sperm maturation but progesterone, which is an essential hormone in the female mammal, has no known role in fish. Male fish also differ from mammals in that the major product of the testis is 11-ketotestosterone rather than testosterone. In fish, unlike mammals, the gonads of both sexes synthesise testosterone, which may play an important role in feedback to the pituitary. The gonads of fish also have some of the properties associated with the liver in mammals in that they can convert steroid hormones into metabolites. In some species of fish these metabolites may act as sexual signals (pheromones) to members of the opposite sex.
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