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 stress response in fish

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system Nevertheless, substantial questions about the evolution of these mechanisms and control remain

https://people.clas.ufl.edu/devans/files/PRVGill.pdf

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

Cortisol is the principal corticosteriod in teleost fishes and its plasma concentrations rise dramatically during stress. The relationship between this cortisol increase and its metabolic consequences are subject to extensive debate. Much of this debate arises from the different responses of the many species used, the diversity of approaches to manipulate cortisol levels, and the sampling techniques and duration. Given the extreme differences in experimental approach, it is not surprising that inconsistencies exist within the literature. This review attempts to delineate common themes on the physiological and metabolic roles of cortisol in teleost fishes and to suggest new approaches that might overcome some of the inconsistencies on the role of this multifaceted hormone. We detail the dynamics of cortisol, especially the exogenous and endogenous factors modulating production, clearance and tissue availability of the hormone. We focus on the mechanisms of action, the biochemical and physiological impact, and the interaction with other hormones so as to provide a conceptual framework for cortisol under resting and/or stressed states. Interpretation of interactions between cortisol and other glucoregulatory hormones is hampered by the absence of adequate hormone quantification, resulting in correlative rather than causal relationships.

Cortisol in teleosts: dynamics, mechanisms of action, and metabolic regulation

Physical, chemical and perceived stressors can all evoke non-specific responses in fish, which are considered adaptive to enable the fish to cope with the disturbance and maintain its homeostatic state. If the stressor is overly severe or long-lasting to the point that the fish is not capable of regaining homeostasis, then the responses themselves may become maladaptive and threaten the fish's health and well-being. Physiological responses to stress are grouped as primary, which include endocrine changes such as in measurable levels of circulating catecholamines and corticosteroids, and secondary, which include changes in features related to metabolism, hydromineral balance, and cardiovascular, respiratory and immune functions. In some instances, the endocrine responses are directly responsible for these secondary responses resulting in changes in concentration of blood constituents, including metabolites and major ions, and, at the cellular level, the expression of heat-shock or stress proteins. Tertiary or whole-animal changes in performance, such as in growth, disease resistance and behavior, can result from the primary and secondary responses and possibly affect survivorship.Fishes display a wide variation in their physiological responses to stress, which is clearly evident in the plasma corticosteroid changes, chiefly cortisol in actinopterygian fishes, that occur following a stressful event. The characteristic elevation in circulating cortisol during the first hour after an acute disturbance can vary by more than two orders of magnitude among species and genetic history appears to account for much of this interspecific variation. An appreciation of the factors that affect the magnitude, duration and recovery of cortisol and other physiological changes caused by stress in fishes is important for proper interpretation of experimental data and design of effective biological monitoring programs.

/pdf/stress-in-fishes-a-diversity-of-responses-with-particular-q074epxlxf.pdf

Stress in Fishes: A Diversity of Responses with Particular Reference to Changes in Circulating Corticosteroids

Physiological changes in fish from stress in aquaculture with emphasis on the response and effects of corticosteroids

This review focuses on the structure and function of the branchial chloride cell in freshwater fishes. The mitochondria-rich chloride cell is believed to be the principal site of trans-epithelial Ca2+ and Cl- influxes. Though currently debated, there is accruing evidence that the pavement cell is the site of Na+ uptake via channels linked electrically to an apical membrane vacuolar H(+)-ATPase (proton pump). Chloride cells perform an integral role in acid-base regulation. During conditions of alkalosis, the surface area of exposed chloride cells is increased, which serves to enhance base equivalent excretion as the rate of Cl-/HCO3- exchange is increased. Conversely, during acidosis, the chloride cell surface area is diminished by an expansion of the adjacent pavement cells. This response reduces the number of functional Cl-/HCO3- exchangers. Under certain conditions that challenge ion regulation, chloride cells proliferate on the lamellae. This response, while optimizing the Ca2+ and Cl- transport capacity of the gill, causes a thickening of the blood-to-water diffusion barrier and thus impedes respiratory gas transfer.

THE CHLORIDE CELL:Structure and Function in the Gills of Freshwater Fishes

/pdf/the-adrenergic-stress-response-in-fish-control-of-fuw7ou63j0.pdf

The adrenergic stress response in fish: control of catecholamine storage and release

Acid-base balance and CO2 excretion in fish: unanswered questions and emerging models.

Calcium uptake (J Ca in ) in freshwater rainbow trout ( Salmo gairdnen ) under control conditions (external [Ca 2+ ] ≃ 1.8 mequivl −1 , [NaCl] ≃ 0.8 mequiv 1 −1 ) occurred at approximately equal rates (12–15 μequiv kg −1 h −1 ) through the gills and the general body surface in vivo . The gut was not involved. Under the same conditions, in vitro branchial J Ca in in an isolated, saline-perfused head preparation was equal to that in vivo . The cells involved in J in Ca are mainly located on lamellae rather than on filaments since 95 % of J in Ca occurred across the arterio-arterial circulation of the gill. J in Ca , in vitro, displayed Michaelis-Menten kinetics. Acclimation to low external [Ca 2+ ] (50 μequiv 1 −1 ; unchanged [NaCl]) for 1 day caused a five-fold stimulation of J in Ca characterized by decreased K m and increased J max . Longer periods of low [Ca 2+ ] acclimation resulted in changes of J max only. J max gradually returned towards control levels as acclimation time increased, but was still elevated after 30 days. Acclimation to low ambient [Ca 2+ ] caused proliferation and increased exposure of lamellar chloride cells which were correlated with increased J max . Fish exposed to high external [Ca 2+ ] (10 mequivl −1 ; unchanged [NaCl]) displayed reduced J in Ca Similar changes in J in Ca were observed during in vivo experiments. Plasma Ca 2+ concentration remained constant regardless of external [Ca 2+ ], while plasma Na + and Cl − levels were transiently reduced at 1 day low [Ca 2+ ] exposure but had recovered by 7 days. A possible role for cortisol in Ca 2+ regulation is discussed based on observations of cortisol-stimulated lamellar chloride cell proliferation and J in Ca , and elevated plasma [cortisol] in low-[Ca 2+ ] acclimated fish.

Kinetics of Branchial Calcium Uptake in the Rainbow Trout: Effects of Acclimation to Various External Calcium Levels

Daily intramuscular injection of cortisol (4 mg kg−1 body weight) in rainbow trout,Salmo gairdneri, for 10 days caused significant increases in the number and individual apical surface area of gill chloride cells per mm2 of filament epithelium. Concomitantly, whole body influxes of sodium (Na+) and chloride (Cl−) increased. Acute (3 h) intra-arterial infusion of cortisol did not affect whole body Na+ or Cl− influx. A significant correlation was observed between both Na+ and Cl− influxes and the fractional apical surface area of filament chloride cells in control, sham (saline-injected) and experimental (cortisol-injected) fish. The chloride cells displayed similar ultrastructural modifications in trout undergoing cortisol treatment as in trout transferred to ion-deficient water. These findings suggest the existence of structure/function relationships in which branchial chloride cell morphology is an important determinant of Na+ and Cl− transport capacity. We conclude that chronic cortisol treatment enhances whole body Na+ and Cl− influxes by promoting proliferation of branchial chloride cells. The results of correlation analysis indicate that the chloride cell is an important site of NaCl uptake in freshwater rainbow trout.

Steve F. Perry

Papers

THE CHLORIDE CELL:Structure and Function in the Gills of Freshwater Fishes

The adrenergic stress response in fish: control of catecholamine storage and release

Acid-base balance and CO2 excretion in fish: unanswered questions and emerging models.

Kinetics of Branchial Calcium Uptake in the Rainbow Trout: Effects of Acclimation to Various External Calcium Levels

Effects of cortisol on gill chloride cell morphology and ionic uptake in the freshwater trout, Salmo gairdneri