The stress response in teleost fish shows many similarities to that of the terrestrial vertebrates. These concern the principal messengers of the brain-sympathetic-chromaffin cell axis (equivalent of the brain-sympathetic-adrenal medulla axis) and the brain-pituitary-interrenal axis (equivalent of the brain-pituitary-adrenal axis), as well as their functions, involving stimulation of oxygen uptake and transfer, mobilization of energy substrates, reallocation of energy away from growth and reproduction, and mainly suppressive effects on immune functions. There is also growing evidence for intensive interaction between the neuroendocrine system and the immune system in fish. Conspicuous differences, however, are present, and these are primarily related to the aquatic environment of fishes. For example, stressors increase the permeability of the surface epithelia, including the gills, to water and ions, and thus induce systemic hydromineral disturbances. High circulating catecholamine levels as well as structural damage to the gills and perhaps the skin are prime causal factors. This is associated with increased cellular turnover in these organs. In fish, cortisol combines glucocorticoid and mineralocorticoid actions, with the latter being essential for the restoration of hydromineral homeostasis, in concert with hormones such as prolactin (in freshwater) and growth hormone (in seawater). Toxic stressors are part of the stress literature in fish more so than in mammals. This is mainly related to the fact that fish are exposed to aquatic pollutants via the extensive and delicate respiratory surface of the gills and, in seawater, also via drinking. The high bioavailability of many chemicals in water is an additional factor. Together with the variety of highly sensitive perceptive mechanisms in the integument, this may explain why so many pollutants evoke an integrated stress response in fish in addition to their toxic effects at the cell and tissue levels. Exposure to chemicals may also directly compromise the stress response by interfering with specific neuroendocrine control mechanisms. Because hydromineral disturbance is inherent to stress in fish, external factors such as water pH, mineral composition, and ionic calcium levels have a significant impact on stressor intensity. Although the species studied comprise a small and nonrepresentative sample of the almost 20,000 known teleost species, there are many indications that the stress response is variable and flexible in fish, in line with the great diversity of adaptations that enable these animals to live in a large variety of aquatic habitats.

The recent characterization of the corticotropin-releasing hormone (CRH) prehormone of the fish tilapia (Oreochromis mossambicus) showed that more variation exists between vertebrate CRH amino acid sequences than recognized before. The present study investigates whether the deviating composition of tilapia CRH coincides with an atypical distribution of CRH in the brain. For this purpose we applied immunohistochemistry, as well as radioimmunoassay (RIA) quantification in brain slices. The results are plotted in a new atlas and reconstruction of the tilapia brain. The largest population of CRH-immunoreactive (ir) neurons is present in the lateral part of the ventral telencephalon (Vl). Approximately tenfold less CRH-ir neurons are observed in the preoptic and tuberal region. The CRH-ir neurons observed in the preoptic region are parvocellular and do not, or hardly, display arginine-vasotocin (AVT) immunoreactivity. CRH-ir neurons are also present in the glomerular layer of the olfactory bulb, in the periventricular layer of the optic tectum, and caudal to the glomerular nucleus. A very dense plexus of CRH-ir terminals is located in the most rostral part of the dorsal telencephalon. This region has not been described in other teleosts and is in the present study subdivided into the anterior part of the dorsal telencephalon (Da) and the anterior part of the laterodorsal telencephalon (Dla). High densities of CRH-ir terminals were observed in and around Vl, in the tuberal region, around the rostral part of the lateral recess, and in the caudal part of the vagal lobe. In the pituitary, CRH-ir terminals are concentrated in the neuro-intermediate lobe. Overall, the immunohistochemical and quantitative data correlated well, as the RIA CRH profile in serial 160-microm slices revealed four peaks, which corresponded with major ir-cell groups and terminal fields. Our results strongly suggest that the CRH-ir cells of Vl project to the rostro-dorsal telencephalon. Consequently, they may not be primarily involved in regulation of pituitary cell types but may subserve other functions. The presence of a CRH-containing Vl-Da/Dla projection seems to be restricted to the most modern group of teleosts, i.e., the Acanthopterygians. Further anatomic indications for non-pituitary-related functions of CRH are found in the vagal lobe and the optic tectum of tilapia. Although the low CRH content of the preoptic region reported here for tilapia may be typical for unstressed fish, the fact remains that remarkably few CRH-ir neurons are involved in regulating the pituitary. Overall, the CRH distribution in the brain of tilapia is more widespread than previously reported for other teleosts.

Fish store large quantities of vitamin D in their liver and fat tissues, including the fat associated with muscle, and this makes fish an important dietary source of vitamin D. Fish do not synthesize vitamin D and are fully dependent on dietary sources to meet their requirement. Under natural circumstances planktonic vitamin D accumulates in the aquatic food chain. In aquaculture, formulated diets are used and vitamin D intake can be manipulated. The minimum dietary requirement for vitamin D has been established in several fish species. The role of vitamin D in fish physiology is still enigmatic. Till the 1970s there was consensus that fish accumulate but not metabolize vitamin D. There is substantial evidence now, four decades later, that fish have a vitamin D endocrine system with similar functions as in mammals. We here summarize the major characteristics of the fish vitamin D endocrine system. The hydroxylation of vitamin D to more polar metabolites, their plasma levels and protein-bound transport in blood plasma will be discussed. The vitamin D receptor profile, receptor distribution and responses to vitamin D are reviewed for the key target tissues (gill, intestine, kidney and bone). We will address the classical slower actions through transcription factors as well as the faster G-protein coupled (membrane) receptor mediated effects. We then review the dietary requirement of vitamin D in aquaculture and address some lesser known functions of the vitamin D endocrine system. KEY WORDS

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A d r e n o c o r tic o tr o p ic H o r m o n e in R e la tio n to In te rre n a l F u n c tio n d u r in g S t r e s s in T ila p ia ( O r e o c h r o m is m o s s a m b i c u s )