Changes in plasma corticosteroids during smoltification of coho salmon, Oncorhynchus kisutch

Endocrine control of gill ionocyte function in euryhaline fishes

The endocrine system is an essential regulator of the osmoregulatory organs that enable euryhaline fishes to maintain hydromineral balance in a broad range of environmental salinities. Because branchial ionocytes are the primary site for the active exchange of Na+, Cl-, and Ca2+ with the external environment, their functional regulation is inextricably linked with adaptive responses to changes in salinity. Here, we review the molecular-level processes that connect osmoregulatory hormones with branchial ion transport. We focus on how factors such as prolactin, growth hormone, cortisol, and insulin-like growth-factors operate through their cognate receptors to direct the expression of specific ion transporters/channels, Na+/K+-ATPases, tight-junction proteins, and aquaporins in ion-absorptive (freshwater-type) and ion-secretory (seawater-type) ionocytes. While these connections have historically been deduced in teleost models, more recently, increased attention has been given to understanding the nature of these connections in basal lineages. We conclude our review by proposing areas for future investigation that aim to fill gaps in the collective understanding of how hormonal signaling underlies ionocyte-based processes.

Thyroid and Corticosteroid Signaling in Amphibian Metamorphosis

In multicellular organisms, development is based in part on the integration of communication systems. Two neuroendocrine axes, the hypothalamic–pituitary–thyroid and the hypothalamic–pituitary–adrenal/interrenal axes, are central players in orchestrating body morphogenesis. In all vertebrates, the hypothalamic–pituitary–thyroid axis controls thyroid hormone production and release, whereas the hypothalamic–pituitary–adrenal/interrenal axis regulates the production and release of corticosteroids. One of the most salient effects of thyroid hormones and corticosteroids in post-embryonic developmental processes is their critical role in metamorphosis in anuran amphibians. Metamorphosis involves modifications to the morphological and biochemical characteristics of all larval tissues to enable the transition from one life stage to the next life stage that coincides with an ecological niche switch. This transition in amphibians is an example of a widespread phenomenon among vertebrates, where thyroid hormones and corticosteroids coordinate a post-embryonic developmental transition. The review addresses the functions and interactions of thyroid hormone and corticosteroid signaling in amphibian development (metamorphosis) as well as the developmental roles of these two pathways in vertebrate evolution.

Interdependence of Thyroid and Corticosteroid Signaling in Vertebrate Developmental Transitions

Post-embryonic acute developmental processes mainly allow the transition from one life stage in a specific ecological niche to the next life stage in a different ecological niche. Metamorphosis, an emblematic type of these post-embryonic developmental processes, has occurred repeatedly and independently in various phylogenetic groups throughout metazoan evolution, such as in cnidarian, insects, molluscs, tunicates, or vertebrates. This review will focus on metamorphoses and developmental transitions in vertebrates, including typical larval metamorphosis in anuran amphibians, larval and secondary metamorphoses in teleost fishes, egg hatching in sauropsids and birth in mammals. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in the regulation of these life transitions. The review will address the molecular and functional evolution of these axes and their interactions. Mechanisms of integration of internal and environmental cues, and activation of these neuroendocrine axes represent key questions in an “eco-evo-devo” perspective of metamorphosis. The roles played by developmental transitions in the innovation, adaptation, and plasticity of life cycles throughout vertebrates will be discussed. In the current context of global climate change and habitat destruction, the review will also address the impact of environmental factors, such as global warming and endocrine disruptors on hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal/interrenal axes, and regulation of developmental transitions.

Exposure of Persian sturgeon (Acipenser persicus) to cadmium results in biochemical, histological and transcriptional alterations

Sturgeon is one of the endangered families of fish in the Caspian Sea region, where there is up to 80% of their global caching. Unfortunately, in recent years, increase of pollutants has been resulted in their total population reduction. Due to their benthic nature, sturgeons are at great risk of exposing to contaminants such as cadmium. Despite their endangered status in the Caspian Sea, there are only a few studies on characterizing the relative sensitivity of sturgeons to cadmium. Adverse effects associated with pollution on angiogenesis are mediated by hypoxia inducing factor-1 (HIF-1) and vascular endothelial growth factor (VEGF). In this investigation, gene expression of two distinct HIFs-1, HIF-1α and HIF-2α, and VEGF was investigated at the mRNA transcript levels after exposure of Persian sturgeon (Acipenser persicus) to cadmium. VEGF, HIF-1α and HIF-2α expressions in treated Persian sturgeon were greater than controls. Significant increases (P<0.05) were also observed in cortisol and glucose levels compared to the control group especially in the fish exposed to higher cadmium concentration (800 μg/L). Plasma aspartate aminotransferase (AST), alanine aminotransferase (ALT) and lactic acid dehydrogenase (LDH) levels were increased in the cadmium-exposed fish, although the observed increases were not significant between the control and 200 μg/L cadmium treatment at some sampling time points. Gill tissues also showed histopathological changes in the cadmium treatments. Overall, results indicated that cadmium resulted in some alterations in biochemical parameters, mRNA transcript level expression of two important angiogenesis related genes as well as histological alterations in Persian sturgeon.

Impact of a short-term diazinon exposure on the osmoregulation potentiality of Caspian roach (Rutilus rutilus) fingerlings

The stocks of Caspian roach (Rutilus rutilus), an economically important species in the Caspian Sea, are depleting. Each year millions of artificially produced fingerlings of this species are restocked in the mouth of rivers of the Southern Caspian Sea (e.g. Qare Soo River), where they are exposed to pesticides originating from regional rice and orchard fields. This early exposure to pesticides could affect the hypo-osmoregulatory ability of juvenile fish. Thus, in this study, Caspian roach fingerlings were exposed to environmentally-relevant concentrations of the organophosphate insecticide diazinon for 96 h in fresh water and then transferred to diazinon-free brackish water (BW) for another 96 h. We report that cortisol and glucose levels were significantly increased in all diazinon treatments at all sampling time points in comparison to the control group. Moreover, the thyroid hormone levels of TSH, T4, and T3 significantly decreased in diazinon-exposed fish even after the transfer to BW. The electrolytes were differentially affected during the exposure to diazinon and after the transfer to BW. The number of chloride cells in the gill tissue was significantly increased during diazinon exposure at the higher concentrations and decreased to control levels after transfer to BW. Finally, gill and kidney tissues showed many histopathological changes in diazinon-exposed fish even after 240 h in BW. These results suggest that the release of Caspian roach fingerlings into the diazinon-contaminated Caspian Sea regions may alter their physiology and jeopardize their survival, which could lead to a failure in rebuilding the Caspian roach stocks in the Caspian Sea.

Effect of parental mate choice and semi-natural early rearing environment on the growth performance and seawater tolerance of Chinook salmon Oncorhynchus tshawytscha

To assess whether parental mate choice and early rearing in a semi-natural spawning channel may benefit the culture of Chinook salmon Oncorhynchus tshawytscha, 90 day growth trials were conducted using hatchery O. tshawytscha (hatchery), mate choice O. tshawytscha (i.e. the offspring of parents allowed to choose their own mate) that spent 6 months in a spawning channel prior to hatchery rearing (channel) and mate choice O. tshawytscha transferred to the hatchery as fertilized eggs (transfer). During the growth trials, all O. tshawytscha stocks were reared separately or in either mixed channel and hatchery or transfer and hatchery groups for comparison of performance to traditional practices. After 60 days in fresh water, all O. tshawytscha were transferred to seawater for an additional 30 days. Reared separately, all stocks grew c. 4.5 fold over 90 days but specific growth rate (G) and food conversion efficiency were higher in fresh water than after seawater transfer on day 60. In contrast, hatchery O. tshawytscha from mixed hatchery and channel and hatchery and transfer growth trials had a larger mass and length gain than their counterparts on day 60, but reduced G in seawater. In general, plasma levels of growth hormone, insulin-like growth factor I and cortisol did not differ among any O. tshawytscha groups in either the separate or mixed growth trials. Despite some differences in gill Na(+),K(+)-ATPase activity, all O. tshawytscha had a high degree of seawater tolerance and experienced virtually no perturbation in plasma chloride following seawater transfer. Overall, all O. tshawytscha exhibited similar growth and seawater performance under traditional hatchery conditions and any benefit derived from either parental mate choice or semi-natural early rearing environment was only observed in the presence of mutual competition with hatchery O. tshawytscha.

Metamorphosis in teleosts

Teleosts are the largest and most diverse group of vertebrates, and many species undergo morphological, physiological, and behavioral transitions, "metamorphoses," as they progress between morphologically divergent life stages. The larval metamorphosis that generally occurs as teleosts mature from larva to juvenile involves the loss of embryo-specific features, the development of new adult features, major remodeling of different organ systems, and changes in physical proportions and overall phenotype. Yet, in contrast to anuran amphibians, for example, teleost metamorphosis can entail morphological change that is either sudden and profound, or relatively gradual and subtle. Here, we review the definition of metamorphosis in teleosts, the diversity of teleost metamorphic strategies and the transitions they involve, and what is known of their underlying endocrine and genetic bases. We suggest that teleost metamorphosis offers an outstanding opportunity for integrating our understanding of endocrine mechanisms, cellular processes of morphogenesis and differentiation, and the evolution of diverse morphologies and life histories.

Intestinal fluid absorption in anadromous salmonids: importance of tight junctions and aquaporins

The anadromous salmonid life cycle includes both fresh water (FW) and seawater (SW) stages. The parr-smolt transformation (smoltification) pre-adapt the fish to SW while still in FW. The osmoregulatory organs change their mode of action from a role of preventing water inflow in FW, to absorb ions to replace water lost by osmosis in SW. During smoltification, the drinking rate increases, in the intestine the ion and fluid transport increases and is further elevated after SW entry. In SW, the intestine absorbs ions to create an inwardly directed water flow which is accomplished by increased Na⁺, K⁺-ATPase (NKA) activity in the basolateral membrane, driving ion absorption via ion channels and/or co-transporters. This review will aim at discussing the expression patterns of the ion transporting proteins involved in intestinal fluid absorption in the FW stage, during smoltification and after SW entry. Of equal importance for intestinal fluid absorption as the active absorption of ions is the permeability of the epithelium to ions and water. During the smoltification the increase in NKA activity and water uptake in SW is accompanied by decreased paracellular permeability suggesting a redirection of the fluid movement from a paracellular route in FW, to a transcellular route in SW. Increased transcellular fluid absorption could be achieved by incorporation of aquaporins (AQPs) into the enterocyte membranes and/or by a change in fatty acid profile of the enterocyte lipid bilayer. An increased incorporation of unsaturated fatty acids into the membrane phospholipids will increase water permeability by enhancing the fluidity of the membrane. A second aim of the present review is therefore to discuss the presence and regulation of expression of AQPs in the enterocyte membrane as well as to discuss the profile of fatty acids present in the membrane phospholipids during different stages of the salmonid lifecycle.

The hypothalamic-pituitary-thyroid (HPT) axis in fish and its role in fish development and reproduction

Bony fishes represent the largest vertebrate class and are a very diverse animal group. This chapter provides a thorough review of the available scientific literature on the thyroid system in these important vertebrate animals. The molecular components of the hypothalamic-pituitary-thyroid (HPT) axis in this group correspond closely to those of mammals. The thyroid tissue in the fishes is organized as diffuse follicles, with a few exceptions, rather than as an encapsulated gland as is found in most other vertebrate species. The features of this diffuse tissue in fishes are reviewed with an emphasis on feedback relationships within the HPT axis, the molecular biology of the thyroid system in fishes, and comparisons versus the thyroid systems of other vertebrate taxa. A review of the role of thyroid hormone in fish development and reproduction is included. Available information about the HPT axis in fishes is quite detailed for some species and rather limited or absent in others. This review focuses on species that have been intensively studied for their value as laboratory models in assays to investigate disruption in normal function of the thyroid system. In addition, in vitro and in vivo assay methods for screening chemicals for their potential to interfere with the thyroid system are reviewed. It is concluded that there are currently no in vitro or in vivo assays in fish species that are sufficiently developed to warrant recommendation for use to efficiently screen chemicals for thyroid disruption. Methods are available that can be used to measure thyroid hormones, although our ability to interpret the causes and implications of potential alterations in T4 or T3 levels in fishes is nonetheless limited without further research.

Larval exposure to 4-nonylphenol and 17beta-estradiol affects physiological and behavioral development of seawater adaptation in Atlantic salmon smolts

Population declines of anadromous salmonids are attributed to anthropogenic disturbances including dams, commercial and recreational fisheries, and pollutants, such as estrogenic compounds. Nonylphenol (NP), a xenoestrogen, is widespread in the aquatic environment due to its use in agricultural, industrial, and household products. We exposed Atlantic salmon yolk-sac larvae to waterborne 10 or 100 microg L(-1) NP (NP-L or NP-H, respectively), 2 microg L(-1) 17beta-estradiol (E2), or vehicle, for 21 days to investigate their effects on smolt physiology and behavior 1 year later. NP-H caused approximately 50% mortality during exposure, 30 days after exposure, and 60 days after exposure. Mortality rates of NP-L and E2 fish were not affected until 60 days after treatment, when they were 4-fold greater than those of controls. Treatment with NP-L or E2 as yolk-sac larvae decreased gill sodium-potassium-activated adenosine triphosphatase (Na+,K(+)-ATPase) activity and seawater (SW) tolerance during smolt development, 1 year after exposure. Exposure to NP-L and E2 resulted in a latency to enter SW and reduced preference for SW approximately 2- and 5-fold, respectively. NP-L-exposed fish had 20% lower plasma insulin-like growth factor I (IGF-I) levels and 35% lower plasma triiodothyronine (T3). Plasma growth hormone and thyroxine (T4) were unaffected. Exposure to E2 did not affect plasma levels of IGF-I, GH, T3, or T4. Both treatment groups exhibited increased plasma cortisol and decreased osmoregulatory capacity in response to a handling stressor. These results suggest that early exposure to environmentally relevant concentrations of NP, and other estrogenic compounds, can cause direct and delayed mortalities and that this exposure can have long-term, "organizational" effects on life-history events in salmonids.

Tissue culture of sockeye salmon intestine: functional response of Na+-K+-ATPase to cortisol

A method to culture tissue explants of the intestine from freshwater-adapted sockeye salmon ( Oncorhynchus nerka) was developed to assess possible direct effects of cortisol on Na+-K+-ATPase activity. As judged by several criteria, explants from pyloric ceca and the posterior region of the intestine remained viable during short-term (6-day) culture, although Na+-K+-ATPase activity declined and basolateral components of the enterocytes were observed to be partially degraded. Addition of cortisol to the culture medium maintained Na+-K+-ATPase activity (over 2–12 days) above that of control explants and, in some cases, was similar to levels before culture. The response to cortisol was dose dependent (0.001–10 μg/ml). Within the physiological range, the response was specific for cortisol and showed the following hierarchy: dexamethasone ≥ cortisol > 11-deoxycortisol > cortisone. Insulin maintained Na+-K+-ATPase activity over controls in explants of ceca but not posterior intestine. To compare in vivo and in vitro responses, slow-release implants of cortisol (50 μg/g) were administered to salmon for 7 days. This treatment elevated plasma cortisol levels and stimulated Na+-K+-ATPase activity in both intestinal regions. The results demonstrate that the teleost intestine is a direct target of cortisol, this corticosteroid protects in vitro functionality of Na+-K+-ATPase, and explants retain cortisol responsiveness during short-term culture.

Central administration of corticotropin-releasing hormone alters downstream movement in an artificial stream in juvenile chinook salmon (Oncorhynchus tshawytscha)

We evaluated the effect of corticotropin-releasing hormone (CRH) on spatial distribution and downstream movement in an artificial stream in juvenile Chinook salmon (Oncorhynchus tshawytscha) during the period when the fish were able to tolerate seawater. An intracerebroventricular (ICV) injection of CRH (500 ng) to hatchery fish significantly increased the proportion of fish that were distributed downstream of a mid-stream release site. A second group of hatchery fish were given ICV injections of saline (control) or CRH (500 ng) and released near the top of the stream. The time taken to enter a trap at the lower end of the stream was recorded. In all cases the groups given CRH had a higher proportion of fish that did not enter the trap within 60 min of release. However, in those fish that did enter the trap, treatment with CRH increased the speed of downstream movement to this point relative to control fish. Wild sub-yearling Chinook salmon were captured during their downstream migration to the estuary and given ICV injections of saline or CRH (500 ng) either 2, 3, or 7 days after transport from the river. As with hatchery fish, a significantly higher proportion of wild fish that were administered CRH did not enter the trap at the lower end of the stream. The mean time of passage for control fish decreased on each successive day (day 2 > day 3 > day 7). In contrast, the mean passage time of the wild fish that were given CRH was relatively constant through time, and was only significantly faster than control fish on day 2. The current study provides evidence that CRH alters the downstream movement of juvenile Chinook in a simulated stream environment, and produces behavioral effects similar to those of juvenile salmonids that are stressed during their downstream migration.

Survey of the Adrenal Homolog in Teleosts

The adrenal homolog of teleosts is not a compact organ as the adrenal glands of most vertebrates but is composed by aminergic chromaffin and interrenal steroidogenic cells located mostly inside the head kidney that, in this taxon, generally has a hematopoietic function. The two tissues can be mixed, adjacent, or completely separated and line the endothelium of the venous vessels or are located in close proximity. The chromaffin cells in some species are also present in the posterior kidney. Histological and ultrastructural work revealed cytological peculiarities of both types of cells as compared to those of other vertebrate species. In particular, the interrenal ones can show some variations in ultrastructure depending on sex, time of the year, and relation to stress events. A periodic renewal of the whole gland tissue is also sustained by some studies. Research regarding development is scanty as compared to mammals and most studies go back to the early years of the past century. The adrenal homolog of teleosts is under hormonal and neuronal control. Moreover, local paracrine interactions may play an important role in modulating a system involved in stress response and osmoregulation. Most previous studies involved a few species with the object of intensive rearing for commercial purposes; in fact cortisol, the main hormone secreted by the interrenal cells, can also influence reproduction and growth. This review summarizes data from morphocytological work and refers to other excellent reviews regarding physiology. Some of the results are compared to data available from other fishes and vertebrate classes with the aim of including them in an evolutionary and environmental framework.

Glucocorticoids, thyroid hormones, and iodothyronine deiodinases in embryonic saltwater crocodiles

We investigated the relationship between glucocorticoids, thyroid hormones, and outer ring and inner ring deiodinases (ORD and IRD) during embryonic development in the saltwater crocodile (Crocodylus porosus). We treated the embryos with the synthetic glucocorticoid dexamethasone (Dex), 3,3',5-triiodothyronine (T(3)), and a combination of these two hormones (Dex + T(3)). The effects of these treatments were specific in different tissues and at different stages of development and also brought about changes in plasma concentrations of free thyroid hormones and corticosterone. Administration of Dex to crocodile eggs resulted in a decrease in 3,3',5,5'-tetraiodothyronine (T(4)) ORD activities in liver and kidney microsomes, and a decrease in the high-K(m) rT(3) ORD activity in kidney microsomes, on day 60 of incubation. Dex treatment increased the T(4) ORD activity in liver microsomes, but not kidney microsomes, on day 75 of incubation. Dex administration decreased T(3) IRD activity in liver microsomes. However, this decrease did not change plasma-free T(3) concentrations, which suggests that free thyroid hormone levels are likely to be tightly regulated during development.

Parr-smolt transformation in Atlantic salmon: thyroid hormone deiodination in liver and brain and endocrine correlates of change in rheotactic behavior

We tested the hypothesis that metabolism of thyroid hormones by the brain of Atlantic salmon (Salmo salar) changes when rheotactic behavior reverses during parr-smolt transformation (PST). We measured brain and liver thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3) outer-ring deiodination (ORD) and inner-ring deiodination (IRD) activities and plasma T4 and T3 levels in Atlantic salmon held under natural photoperiod in fresh water at 10°C in the spring of 1993 and 1994. We also measured changes in T4, T3, and cortisol levels during the change in rheotactic behavior. Condition factor decreased while salinity tolerance improved from mid-March to late April. The turbidity-induced transition from upstream to downstream swimming occurred in mid to late April. The main changes in brain deiodination were reduced T3IRD (1993 study) and elevated T4ORD (1994 study). In both years, a high ratio of T4ORD/T3IRD activities in the brain indicated an increased potential for T3 production in the brain during advanced PST. Liver deiodination profiles differed between years, but during advanced PST the low T4ORD activity and low T4ORD/T3IRD activity ratio suggested a low potential for hepatic, and hence systemic, T3 production. However, plasma T4 was increased in downstream swimmers at 1 d (1993) and 4 h (1994) after the turbidity increase. Since at this time brain deiodination pathways were poised towards T3 production, the surge in plasma T4 would likely increase local T3 formation in brain. We conclude that during PST there is no major change in hepatic deiodination and hence probably no major change in systemic T3 availability. But deiodination properties in brain during late PST indicate the potential for local T3 formation. This may be significant when plasma T4 increases at the time of downstream migration.

Regulation of gill cytosolic corticosteroid receptors in juvenile Atlantic salmon: interaction effects of growth hormone with prolactin and triiodothyronine

The potential effects of growth hormone (GH), prolactin (Prl), and triiodothyronine (T3) on gill Na+,K+-ATPase activity and corticosteroid receptor (CR) concentration (Bmax) and dissociation constant (Kd) were examined in juvenile Atlantic salmon (Salmo salar). Compared to controls, fish injected with GH (ovine, 5.0 microgram g-1) had significantly greater gill Na+,K+-ATPase activity after 7 and 14 days. Gill CR Bmax and Kd were significantly elevated on day 7, but not day 14. T3 also significantly increased CR Bmax. The effect of GH on CR Bmax was also additive with T3 (5.0 microgram g-1) treatment. There was a synergistic effect on CR Bmax when purified coho salmon GH (csGH, 0.1 microgram g-1) was injected in combination with T3 (1.6 microgram g-1). Prl (ovine, 5.0 microgram g-1; purified coho salmon, 0.1 microgram g-1) did not significantly alter gill CR Bmax. Although Prl limited the increase in CR Bmax by GH, the effect was not signicant. T3 and Prl did not have an effect on Kd. GH significantly increased gill Na+,K+-ATPase activity, T3 administration did not have a significant effect, and Prl-treated fish had significantly lower gill Na+,K+-ATPase activity. The results indicate that T3 acts additively with GH, while Prl has no effect in regulating CR Bmax. An increase in cytosolic CR by GH and T3, but not Prl, may regulate gill responsiveness to cortisol and be an important mechanism in the endocrine control of physiological changes during the parr-smolt transformation.

Localization of cortisol receptor in branchial chloride cells in chum salmon fry

To clarify the involvement of cortisol in functional differentiation of branchial chloride cells, cellular gene expression and localization of cortisol receptor were examined in chum salmon (Oncorhynchus keta) fry in freshwater (FW) and those adapted to seawater (SW) by in situ hybridization and immunocytochemical staining. Sodium-potassium adenosinetriphosphatase (Na+,K(+)-ATPase) activity in the whole gill homogenate was significantly higher in SW fish than in FW fish. There were no significant differences in plasma cortisol levels nor in the expression of cortisol receptor mRNA, examined by Northern blot analysis, between SW and FW fish. The receptor gene expression, examined by in situ hybridization with biotin-labeled synthetic oligonucleotide probe, and specific immunostaining with anticortisol receptor serum were found in two types of chloride cells distributed in the gill filaments and lamellae, which were also labeled with anti-Na+,K(+)-ATPase serum, indicating that cortisol may be one of the important factors regulating chloride cell functions. Gene expression of cortisol receptor in filament chloride cells, which were highly activated in SW-adapted fry, was significantly greater in the fry adapted to SW than in FW-adapted fry, reflecting their specific role in salt secretion in SW. Cortisol receptors were also present in undifferentiated cells in the interlamellar regions adjacent to the central venous sinus, also suggesting the involvement of cortisol in the functional differentiation of chloride cells.

Cortisol inhibits glycosaminoglycan synthesis in cultured rainbow trout cartilage

In vitro actions of corticosteroids (cortisol, 11-deoxycortisol, cortisone, and corticosterone) as well as interaction between cortisol and triiodothyronine (T3) or recombinant human insulin-like growth factor-I (rhIGF-I) on cartilage glycosaminoglycan (GAG) synthesis in rainbow trout (Oncorhynchus mykiss) were examined. Uptake of [35S]sulfate by isolated branchial cartilage was measured as a marker for GAG synthesis. In vitro exposure of cartilage to cortisol at concentrations of 10, 100, and 1000 nM for 6 days dose-dependently inhibited sulfate uptake, while exposure to 0.1 and 1 nM cortisol had no effect. Corticosterone and 11-deoxycortisol at concentrations of 10 and 100 nM inhibited sulfate uptake slightly but not dose-dependently. Cortisone (1, 10, and 100 nM) had no effect. When cortisol (1, 10, and 100 nM) and T3 (0.075 and 0.75 nM) were simultaneously added to the culture, the T3-induced sulfate uptake was dose-dependently reduced by the presence of 10 and 100 nM cortisol. When cortisol (1, 10, and 100 nM) and rhIGF-I (0.1 and 1 nM) were added together, the sulfate uptake induced by 0.1 nM rhIGF-I was only slightly inhibited by 100 nM cortisol, but 1 nM rhIGF-I completely masked the inhibitory effect of cortisol. These data suggest that GAG synthesis in the rainbow trout cartilage is controlled by multiple interactions among stimulative hormones, such as T3 and IGF-I, and inhibitory hormones, such as cortisol.

Cortisol-induced changes in oxygen consumption and ionic regulation in coastal cutthroat trout (Oncorhynchus clarki clarki) parr

The influence of cortisol on oxygen consumption and osmoregulatory variables was examined in coastal cutthroat trout (Oncorhynchus clarki clarki) parr kept in fresh water (FW) and transferred to seawater (SW). Intraperitoneal implants containing cortisol (50 μg g(-1)) in vegetable oil resulted in elevated plasma cortisol titres similar to those observed in fish following a 24h SW exposure. Cortisol treatment significantly increased the oxygen consumption and plasma glucose levels of trout in FW, consistent with the glucocorticoid role of cortisol. Cortisol treatment did not cause any changes in plasma ion concentrations or gill Na(+),K(+)-ATPase activity in FW after 10 days. Cortisol-implanted fish exposed to SW for 24h showed slightly improved ion regulatory ability compare to non-implanted controls. The results of this study suggest that during SW transfer in juvenile salmonids, increases in cortisol may act as both a mineralocorticoid and a glucocorticoid, depending on the developmental state of the fish (e.g., smolt versus parr). Furthermore, the relative energetic costs of osmoregulation and that of the stress associated SW transfer cannot be discerned using whole-animal oxygen consumption rates.

Distribution and changes in mu- and kappa-opiate receptors during the midlife neurodevelopmental period of Coho salmon, Oncorhynchus kisutch

Parr-smolt transformation (PST) in coho salmon is associated with a plasma thyroid hormone (PT4) surge and a critical period of neural development that includes axonal sprouting, neurogenesis, and surges of select neurotransmitters. Here we provide a description of the selectivity, distribution, and the changes in the density of mu- and kappa-opiate receptors during PST, as revealed by quantitative in vitro autoradiography of [3H]Tyr-D-Ala-Gly-NMe-Phe-Gly-ol ([3H]DAMGO) and [3H]ethylketocyclazocine ([3H]EKC), respectively. The concentration of mu-receptors increased significantly in select cell groups in the early stages of parr-smolt transformation, until a peak was reached at the time coinciding with the peak of the PT4 surge. In other cell groups, the peaks occurred 1 or 2 weeks later. With one exception, this increase was followed by a decrease in concentration. The brain areas showing the highest concentrations are the dorsal nucleus of the ventral telencephalic area, the glomerular region, the granular layer of the valvula cerebelli, the nucleus diffuses of the inferior lobe, and the nucleus diffuses of the torus lateralis. Other regions with distinctly elevated mu-receptor concentrations are the stratum griseum centrale of the optic tectum and the preoptic area. The distribution of kappa-receptors is more diffuse, and the densities are considerably lower. The overlap in distribution of mu- and kappa-receptors is considerable, but significant exceptions are noted. For example, the dorsomedial nucleus of the dorsal telencephalic area, the habenular nucleus, and the dorsomedial nucleus of the thalamus exhibit a surge in density of kappa-receptors at the time of the PT4 surge, while the density of mu-receptors in these nuclei remain very low throughout parr-smolt transformation. The kappa-receptor containing cell groups are not identifiable until 3 weeks before the PT4 surge because of low densities. The most prominently labeled kappa-receptor regions are the ventral and dorsal nuclei of the ventral telencephalic area, the medial and dorsal zones of the dorsal telencephalic area, the optic tectum (all layers), the dorsomedial nucleus of the thalamus, the torus lateralis of the ventral hypothalamus, and the preoptic area. An increase of mu- and kappa-opiate receptor densities in specific brain regions may reflect roles in the alteration of brain organization, olfactory imprinting, neuroendocrine activity or other physiological activities. The overall distribution of these receptors are relatively more extensive in salmon than in other vertebrates so far studied.

Cortisol stimulates intestinal fluid uptake in Atlantic salmon (Salmo salar) in the post-smolt stage

The fluid uptake rate of the posterior intestine of salmonids increases during the parr-smolt transformation. Intestinal fluid uptake in post-smolt Atlantic salmon was investigated after treatment with cortisol and growth hormone (GH), alone or together. Two replicate experiments were conducted in August 1991 and August 1992. Cortisol was emulsified in vegetable shortening and vegetable oil (1:1) and implanted into the peritoneal cavity. GH was administered as intraperitoneal injections in a saline vehicle on days 0 and 2. On days 5 and 6, plasma cortisol levels, gill Na(+),K(+)-ATPase activity, andin vitro measurements of fluid transport rate (Jv) across the posterior intestine were measured. Implants of cortisol elevated the plasma cortisol levels within a physiological range, and resulted in elevated gill Na(+),K(+)-ATPase activity, as expected. The fluid uptake rate across the posterior intestine was roughly doubled by cortisol treatment. GH treatment did not affect intestinal fluid transport, gill Na(+),K(+)-ATPase activity, or plasma cortisol concentrations. The seawater-adapting increase in the rate of fluid uptake by the posterior intestine of smolting salmon is probably stimulated by elevated plasma cortisol concentrations.

Changes in intestinal fluid transport in Atlantic salmon (Salmo salar L) during parr-smolt transformation

We examined changes in fluid transport by the intestine of Atlantic salmon (Salmo salar L) undergoing parrsmolt transformation during springtime. In vitro measurements of fluid transport rate (Jv) across non-everted middle and posterior intestinal sac preparations were made in late April and early June 1990 and from February through June 1991 for juvenile smolting fish. Intestinal Jv was also compared between parr- and smolt-stage salmon in both years. To evaluate the osmoregulatory role of the intestine, Jv was measured for smolts adapted to seawater and their cohorts remaining in fresh water. The middle intestine of smolting fish underwent a significant decrease in fluid transport during the springtime, while posterior intestinal Jv significantly increased. Parr-stage fish decreased Jv in the middle intestine during springtime similar to smolts. However, the posterior intestinal Jv of smolts showed a significant increase over the parr around the peak smolt period in both years. Seawater-adapted smolts generally exhibited posterior intestinal Jv approximately double that of freshwater cohorts. A decrease over time shown for the middle intestine, together with the increased Jv in the posterior intestine preceding and after seawater entry, suggests the development of a functional regionalization during parr-smolt transformation, with the posterior intestine taking on increased importance in osmoregulation in seawater.

Plasma somatolactin levels in coho salmon (Oncorhynchus kisutch) during smoltification and sexual maturation

Somatolactin (SL) is a novel pituitary hormone recently characterized in several fish species. Structural analyses have shown that SL belongs to the growth hormone/prolactin family, and that it is a highly conserved protein. SL is synthesized by the periodic acid/Schiff-positive cells in the pars intermedia, but has an as yet unidentified function(s). We have recently developed a homologous radioimmunoassay for coho salmon SL and measured plasma levels of SL during two stages of the coho salmon life cycle, smoltification and sexual maturation. During smoltification, plasma levels of SL changed almost in parallel with plasma levels of thyroxine; levels increased as morphological indices of smoltification appeared and decreased as smoltification was completed. Following this period, SL levels remained low until the spring prior to spawning. In a separate study, plasma levels of SL were measured in sexually maturing coho salmon that remained in fresh water throughout their life cycle. During the year of sexual maturation, plasma levels of SL gradually increased from the spring onward, reaching peak levels at the time of spawning in November and December. These data are similar to those previously reported for sexually maturing coho salmon that were maintained in seawater prior to spawning (Rand-Weaver et al. 1992). Therefore, increases in plasma SL levels occurred in sexually maturing fish irrespective of whether they were maintained in fresh water or seawater. Peak levels at spawning were higher than those observed during smoltification. Possible roles for SL in metabolism and reproduction are discussed.

Serum thyroxine, estradiol-17β, and testosterone profiles during the parr-smolt transformation of masu salmon, Oncorhynchus masou

Changes in serum thyroxine (T4), estradiol-17β (E2) and testosterone (T) levels during the parr-smolt transformation (smoltification) were investigated in the masu salmon (Oncorhynchus masou) in 1985 and 1987. T4 showed a peak in levels at the early stage of smoltification and E2 and T levels peaked almost at the same time. There were no significant differences between the concentrations of serum hormones in female and males. During smoltification, germ cells in the peri-nucleolus and spermatogonia stage were present in the ovary and testis, respectively. These results suggest that E2 and T may be involved in smoltification in the masu salmon.

Seasonal changes in cortisol sensitivity and glucocorticoid receptor affinity and number in leukocytes of coho salmon

To determine if there were organ-specific changes in immune responses or immune-endocrine interaction, we monitored in vitro immune response, cortisol sensitivity and number and affinity of glucocorticoid receptors (GR) in leukocytes from freshwater-adapted juvenile coho salmon (Oncorhynchus kisutch) during the physiological changes that prepare them to enter the marine environment. During this period, absolute immune response declined, but splenic leukocytes generated more antibody-producing cells than did cells from anterior kidney. Splenic leukocytes were initially more sensitive to the suppressive effects of cortisol and had fewer GR than leukocytes from the anterior kidney. Leukocytes from the anterior kidney were initially insensitive to cortisol but developed sensitivity at about the same time as the dissociation constant and number of GR increased. In vitro incubation of anterior kidney leukocytes in cortisol altered GR variables when experiments were conducted during March through September but not during November through February. In some years, changes in GR or immune responses were correlated with plasma cortisol titers, but in other years there was no correlation. Thus, the exact relation between cortisol, GR and immune response in anadromous salmonids is unclear and other factors are involved.

Developmental differences in the responsiveness of gill Na+,K(+)-ATPase to cortisol in salmonids

The ability of cortisol to increase gill Na+,K(+)-ATPase activity was examined in several salmonid species during development. Coho salmon (Oncorhynchus kisutch) parr were unresponsive to cortisol in vitro (10 micrograms/ml for 2 days) in November. Responsiveness was significant from January to March, peaking in January just prior to seasonal increases in gill Na+,K(+)-ATPase activity. Gill tissue became unresponsive to in vitro cortisol in April when in vivo gill Na+,K(+)-ATPase activity peaked. The ability of cortisol to stimulate gill, Na+,K(+)-ATPase activity in postemergent fry (2-3 months after hatching) was examined in chum (O. keta), chinook (O. tschawytscha), coho, and Atlantic salmon (Salmo salar). Initial levels of gill Na+,K(+)-ATPase activity were elevated in chum salmon, which normally migrate as fry. Cortisol (10 micrograms/ml for 4 days in vitro) increased gill Na+,K(+)-ATPase activity in chum salmon fry (48% above initial levels), had a limited but significant effect in chinook salmon fry, and had no effect in coho and Atlantic salmon fry. In an in vivo experiment, Atlantic salmon previously exposed to simulated natural photoperiod (SNP) and continuous light (L24) received four cortisol injections of 2 micrograms.g-1 every third day. SNP fish responded with increased gill Na+,K(+)-ATPase activity (+66%), whereas L24 fish were not affected. Atlantic salmon presmolts with initially low levels of gill Na+,K(+)-ATPase activity responded to cortisol in vitro, whereas smolts with initially high levels of gill Na+,K(+)-ATPase activity were unresponsive.(ABSTRACT TRUNCATED AT 250 WORDS)

Intestinal water transport in juvenile Atlantic salmon (Salmo salar L.) during smolting and following transfer to seawater

1. The rate of mucosal to serosal water movement was measured in vitro in non-everted midgut segments in Atlantic salmon (Salmo salar L.) during parr-smolt transformation (February-July) and following transfer of smolts to seawater in May. 2. The rate increased significantly during smolting from 5.61 microliters/cm2/hr in the parr (February) to 11.03 microliters/cm2/hr in smolts in May. 3. Measured at intervals over a period of 20 days in seawater, the rate of water transport was not significantly different from that found in the freshwater-adapted smolts (11.20 microliters/cm2/hr). 4. Intestinal water transport is sodium-linked and inhibited by ouabain but is not stimulated by cortisol.

Smoltification and seawater adaptation in coho salmon (Oncorhynchus kisutch): plasma prolactin, growth hormone, thyroid hormones, and cortisol

The status of circulating growth hormone and prolactin during the parr-smolt transformation and during seawater adaptation of coho salmon (Oncorhynchus kisutch) was investigated in relation to changes in plasma levels of thyroxine, triiodothyronine, and cortisol, and in hypoosmoregulatory ability. Sampling (biweekly or monthly) occurred between early February and October. When peak hypoosmoregulatory ability was achieved (mid-April), one group of fish was acclimated to seawater over a period of 18 hr and was sampled 1, 3, and 7 days after the introduction of fish to seawater and biweekly thereafter. Plasma prolactin levels rose steadily from the first sampling date to a peak of 15 ng/ml in early April, declined rapidly, and remained low until June when a second increase occurred. Prolactin declined to 2 ng/ml within 1 day of the beginning of seawater adaptation. Growth hormone increased twofold from February to late March, and achieved plateau levels of 20 ng/ml in the period from mid-April to July and then gradually declined to 10 ng/ml in September and October. Plasma levels of growth hormone in seawater-acclimated fish were similar to those of freshwater coho, but with larger fluctuations; no increase was apparent during the first week of seawater acclimation. Plasma cortisol and plasma triiodothyronine increased at the same time as plasma growth hormone; increases in plasma thyroxine occurred later. In general, both growth hormone and cortisol levels were elevated when hypoosmoregulatory ability was high. Conversely, prolactin levels generally showed a negative relationship with hypoosmoregulatory ability.

Smoltification and seawater adaptation in Atlantic salmon (Salmo salar): plasma prolactin, growth hormone, and thyroid hormones

To obtain more information on the role of prolactin and growth hormone during the parr-smolt transformation of Atlantic salmon, a population of fish in fresh water was sampled from January to June during two consecutive years. Gill Na+,K+-ATPase activity increased steadily during smoltification and a plasma thyroxine peak was observed 2-3 weeks before the gill Na+,K+-ATPase peak. On the basis of these two parameters, smoltification was considered complete in our populations in April 1985 and May 1986. Two peaks in plasma growth hormone levels occurred in 1986, one in mid-April and the second in mid-May. In both cases, these peaks coincided with a peak in plasma triiodothyronine and preceded the thyroxine peak by 1-2 weeks. Moreover, the second peak which lasted for 1 month coincided with maximal gill Na+,K+-ATPase activity. A decrease in plasma prolactin levels was observed during smoltification of Atlantic salmon in 2 consecutive years. During this period of decreasing and low plasma prolactin levels, gill Na+,K+-ATPase activity increased to its highest values. Atlantic salmon smolts were also directly transferred into seawater. After 2 days or more in seawater, plasma prolactin levels were not significantly different from those on Day 0, whereas in fresh water they showed large fluctuations. All these data indicate that growth hormone may play an important role in the development of hypoosmoregulatory activity. Increased hypoosmoregulatory ability also appears to be associated with low prolactin levels.

Regulation of the interrenal of fishes: non-classical control mechanisms

The regulation of the interrenal of teleostean fishes is reviewed from the perspective of non-classical control mechanisms and new evidence is presented suggesting gonadotropic control of the interrenal. Cortisol secretion by the interrenal, in addition to regulation by ACTH, appears to be mediated by other hormones. Physiologically relevant, direct control of interrenal function by hydromineral factors is unclear.In vitro experiments with interrenals of coho salmon (Oncorhynchus kisutch) indicate that salmon gonadotropin is extremely corticotropic and both ACTH and gonadotropin stimulate the secretion of large quantities of androstenedione from the interrenal.

Enhanced response of the interrenal of coho salmon (Oncorhynchus kisutch) to ACTH after growth hormone treatment in vivo and in vitro

The effects of ovine growth hormone (GH) on the response of the interrenal of yearling coho salmon (Oncorhynchus kisutch) to ACTH were examined in two series of experiments. In the first series, GH was administered intraperitoneally (2 micrograms/g body wt, three daily injections) to coho salmon which had not undergone the parr-smolt transformation. Animals were sacrificed 30 hr after the final injection and head kidney tissue was prepared for in vitro incubation. GH had no effect on plasma cortisol levels but tissue taken from treated fish had an enhanced sensitivity to ACTH in vitro and its steroidogenic capacity was increased. In the second series of experiments, interrenal tissue was preincubated with GH in the dose range of 10-1000 ng/ml and was then challenged with ACTH or was incubated with 1-10 microM pregnenolone. GH-pretreated tissue displayed an enhanced response to ACTH. Cortisol production by tissue pretreated with 500 ng/ml GH was significantly enhanced after incubation with pregnenolone. No significant effect of GH was found after simultaneous exposure of interrenal tissue to combinations of ACTH and GH for 3 hr. Pretreatment of interrenal tissue with salmon prolactin had no significant effect on the subsequent response of tissue to ACTH. These results indicate that GH regulates interrenal activity in coho salmon and that one probable site of action is on the biosynthetic pathway between pregnenolone and cortisol.

Response of the interrenal to adrenocorticotropic hormone after short-term thyroxine treatment of coho salmon (Oncorhynchus kisutch)

The effects of experimentally elevated plasma thyroxine levels on the subsequent response of interrenals of coho salmon (Oncorhynchus kisutch) to adrenocorticotropic hormone (ACTH) in vitro were examined. Animals were treated with thyroxine by immersion (200 micrograms/L) for 3 days, which resulted in physiological elevations in circulating thyroxine. In animals treated before the parr-smolt transformation was completed (early smolts), thyroxine had no effect on plasma cortisol levels but significantly enhanced the sensitivity of the interrenal to ACTH in vitro. In animals treated after the period of smoltification (postsmolts), plasma cortisol levels were significantly higher than those of controls; both experimental and control animals had plasma cortisol levels higher than normally observed at this stage of development. The response of the interrenals of thyroxine-treated postsmolts to ACTH in vitro was significantly lower than that of controls. Results from the experiments using early smolts are in agreement with studies in other vertebrates showing that thyroid hormones are involved in maintaining the normal functioning of corticosteroidogenic tissue and suggest that thyroid hormones may be involved in the activation of the interrenal that occurs during smoltification. The results obtained using postsmolts are more difficult to interpret because of the possibility that these animals were physiologically stressed by the treatment. Increased ACTH release in vivo resulting from stress may have led to a depression of interrenal sensitivity to ACTH in vitro and may have masked a refractoriness of the pituitary-interrenal axis to thyroxine.

Interrenal secretion of corticosteroids and plasma cortisol and cortisone concentrations after acute stress and during seawater acclimation in juvenile coho salmon (Oncorhynchus kisutch)

We determined the major corticosteroids secreted by interrenal tissue and those present in plasma of juvenile coho salmon (Oncorhynchus kisutch). Incubation medium of interrenal tissue, unstimulated or stimulated with exogenous ACTH in vitro, and plasma of resting or acutely stressed salmon were extracted and qualitatively analyzed for steroid composition using high-performance liquid chromatography. Concentrations of plasma cortisone and cortisol following an acute handling stress or exposure to seawater were quantitatively measured by radioimmunoassay (RIA). Cortisol was the only corticosteroid detected by HPLC in media after incubation of interrenal tissue in the absence or presence of ACTH in vitro. However, both cortisone and cortisol were detected by HPLC in plasma sampled 1 hr after fish were acutely stressed by handling. Stress and seawater acclimation produced marked elevations in plasma levels of both steroids as determined by RIA and also resulted in long-lasting changes in the plasma cortisone:cortisol ratios. In resting fish, cortisone concentrations were similar or higher than cortisol levels. We concluded that cortisol is the primary steroid secreted by the interrenal tissue of coho salmon, and that plasma cortisone arises primarily from the peripheral conversion of cortisol to cortisone. The relatively high levels of cortisone in resting fish and its increase following stress and seawater acclimation suggest the possibility of a biologically significant role for this hormone.

Physiological responses to continuous swimming in wild salmon (Salmo salar L.) parr and smolt

Wild salmon (Salmo salar) parr and smolt were forced to swim against constant flow (50 cm.s(-1)) for 8 hours. Physiological properties describing the hormonal status, the energy metabolism and the ionic and osmotic balance of fish were measured from the fish prior to and at the end of the swimming test.Plasma cortisol levels were elevated in response to enforced swimming; the response of the smolt was clearly greater than that of the parr. Plasma thyroxine concentration increased in the parr but stayed at the initial level in the smolts. The parr consumed much of their coelomic fat, but the glycogen stores stayed nearly constant. The smolts had very low fat stores, and the glycogen stores were depleted in the test. The ionic and osmotic balance of the parr was stable in the test, but in smolts, the plasma Cl(-1) and osmotic concentrations decreased and muscle moisture increased.The results indicate that downstream migration smolts have markedly lower physiological capacity for continuous swimming than parr.

Effects of cortisol and growth hormone replacement on osmoregulation in hypophysectomized coho salmon (Oncorhynchus kisutch)

Both cortisol and GH were able to reverse partially the effects of hypophysectomy on coho salmon (Oncorhynchus kisutch) as judged by improved seawater (SW) tolerance after long-term treatment; however, neither hormone significantly restored gill Na+, Na+,K+-ATPase activity. In the first experiment, gill Na+,K+-ATPase activity remained low in hypophysectomized (Hx) coho treated with cortisol (15 micrograms/g body wt, suspended in cocoa butter) for 96 hr (48 hr in fresh water followed by 48 hr in seawater). In addition, plasma sodium levels were higher in the cortisol-treated Hx animals compared with those in untreated controls. In the second experiment, treatment with cortisol-filled Silastic capsules and implants of bovine GH (mixed with cholesterol in a ratio calculated to yield a dose of 1.5 microgram/g body wt/week) for 12 days also failed to alter gill Na+,K+-ATPase activity, but did reduce plasma sodium levels in animals transferred to SW for 48 hr. These results suggest that these hormones may be involved in the development of hypoosmoregulatory ability in coho salmon. However, the failure of either hormone to restore gill Na+,K+-ATPase activity suggests that a combination of these hormones and/or an additional hormone(s) acting in a synchronized fashion may be required for full effectiveness.

Effects of cortisol and growth hormone on osmoregulation in pre- and desmoltified coho salmon (Oncorhynchus kisutch)

Salmonid species which undergo smoltification show a concurrent enhancement in saltwater (SW) osmoregulatory ability. This developmental change is marked by an increase in SW tolerance and gill Na+,K+-ATPase activity which appears to result, in part, from an increase in gill chloride cell density. Previous studies have suggested that cortisol and growth hormone (GH) may stimulate SW osmoregulatory mechanisms in salmonids. In this study, these hormones were examined for their ability to induce smoltification-associated osmoregulatory changes in pre- and desmoltified coho salmon (Oncorhynchus kisutch). Cortisol treatment for 12 days increased gill Na+,K+-ATPase activity in presmolts and gill residual (Na+,K+-independent) ATPase activity in both groups. Chloride cell density in presmolt primary and secondary lamellae and in desmolt secondary lamellae was increased as well. The rise in plasma sodium levels in fish transferred to SW was reduced only in desmolts. Treatment with bovine GH for 12-13 days increased gill Na+,K+-ATPase activity in presmolts and in desmolts. However, GH treatment in either group did not increase gill residual ATPase activity or alter plasma sodium levels in SW-transferred animals. Gill chloride cell density in presmolts also was unaffected (desmolts were not examined). Thus, both cortisol and GH are partially able to produce changes similar to those observed during smoltification. The contrasting effects of these hormones on gill chloride cell density and gill residual ATPase activity suggest that cortisol may stimulate chloride cell proliferation and/or differentiation, whereas GH may act specifically to increase gill Na+,K+-ATPase activity.

Cortisol secretion in vitro by the interrenal of coho salmon (Oncorhynchus kisutch) during smoltification: relationship with plasma thyroxine and plasma cortisol

An in vitro system for the incubation of interrenal tissue (head kidney fragments) from coho salmon, Oncorhynchus kisutch, was developed in order to examine changes in interrenal sensitivity to ACTH1-24 during smoltification, using cortisol secretion as the endpoint. Time-course studies indicated that maximal cortisol accumulation in incubation media was achieved after 3 hr exposure to ACTH. There was no correlation between head kidney weight, body weight, or sex and the response of the interrenal to ACTH1-24 in vitro. Approximately monthly or bi-weekly experiments were performed during the smoltification period (February-July): tissue was preincubated in hormone-free media for 3 hr, washed twice, and then challenged with 5 X 10(-10)-5 X 10(-7) M (1.5-1500 ng/ml) ACTH1-24 for 3 hr. The pattern of cortisol secretion was similar in February, early March, and late March in the dose range of 5 X 10(-10)-5 X 10(-8) M ACTH1-24. A marked, significant increase in sensitivity to ACTH and in the steroidogenic capacity of the tissue occurred in April, but by May the response was similar to that in the pre-April period. Enhanced sensitivity and steroidogenic capacity were found in interrenal tissue taken from coho salmon in June and July. Maximal in vitro responsiveness of interrenal tissue to ACTH in April was correlated with peak plasma thyroxine titers and enhanced hypoosmoregulatory ability, but not with peak plasma cortisol titers, which occurred in May.