Clearance of plasma corticosteroids during smoltification of coho salmon, Oncorhynchus kisutch

Neuroendocrine Regulation of Plasma Cortisol Levels During Smoltification and Seawater Acclimation of Atlantic Salmon

Diadromous fishes undergo dramatic changes in osmoregulatory capacity in preparation for migration between freshwater and seawater. One of the primary hormones involved in coordinating these changes is the glucocorticoid hormone, cortisol. In Atlantic salmon (Salmo salar), cortisol levels increase during the spring smoltification period prior to seawater migration; however, the neuroendocrine factors responsible for regulating the hypothalamic-pituitary-interrenal (HPI) axis and plasma cortisol levels during smoltification remain unclear. Therefore, we evaluated seasonal changes in circulating levels of cortisol and its primary secretagogue-adrenocorticotropic hormone (ACTH)-as well as transcript abundance of the major regulators of HPI axis activity in the preoptic area, hypothalamus, and pituitary between migratory smolts and pre-migratory parr. Smolts exhibited higher plasma cortisol levels compared to parr across all timepoints but circulating ACTH levels were only elevated in May. Transcript abundance of preoptic area corticotropin-releasing factor b1 and arginine vasotocin were ~2-fold higher in smolts compared to parr in February through May. Smolts also had ~7-fold greater hypothalamic transcript abundance of urotensin 1 (uts-1a) compared to parr in May through July. When transferred to seawater during peak smolting in May smolts rapidly upregulated hypothalamic uts-1a transcript levels within 24 h, while parr only transiently upregulated uts-1a 96 h post-transfer. In situ hybridization revealed that uts-1a is highly abundant in the lateral tuberal nucleus (NLT) of the hypothalamus, consistent with a role in regulating the HPI axis. Overall, our results highlight the complex, multifactorial regulation of cortisol and provide novel insight into the neuroendocrine mechanisms controlling osmoregulation in teleosts.

Physiological responses of over-wintering common carp (Cyprinus carpio) to disturbance by Eurasian otter (Lutra lutra)

Using a tame animal, the impact of otter (Lutra lutra) disturbance on over-wintering carp (Cyprinus carpio) was monitored in two experiments, 133 and 140 days, respectively, over two consecutive winters (November-April). The level of stress in over-wintering carp exposed to various intensities of disturbance by otters was quantified using biological indicators of stress (cortisol, cortisone, indices of nitrogen, carbohydrate, lipid and mineral metabolism and activity of basic blood plasma enzymes) taken from blood plasma of stocked carp at the end of the winter seasons (when the photoperiod was 12 light:12 dark, respectively, 13L:10D). Moreover, condition (Fulton's coefficient of condition and fat content in muscles) and mortality rate of that carp were measured after over-wintering and also after the subsequent vegetation period. The analysis of blood and tissue samples of experimental fish showed changes in nitrogen, carbohydrate and mineral metabolism as well as levels of hormones and fat reserves. Higher response to stress in metabolism of carp with lower intensity of disturbance by otter suggests that high level of disturbance can lead to metabolic adaptation of carp to stress. The effect of stress on the mortality rate of carp during the over-wintering is not clear. Nevertheless, the negative effect of stress on survival, condition and growth rate of carp in the subsequent vegetation period was not observed.

Rates of cortisol increase and decrease in channel catfish and sunshine bass exposed to an acute confinement stressor

Channel catfish and sunshine bass were exposed to a low-water stress event and allowed to recover in fresh water or a solution of metomidate (dl-1-(1-phenylethyl)-5-(metoxycarbonyl) imidazole hydrochloride), which inhibits the synthesis of cortisol. Change in time of plasma cortisol was used as an index of cortisol secretion and clearance. Plasma cortisol and glucose increased during the exposure to low-water stress in both fish, but the changes of both plasma components were more dramatic in sunshine bass. Exposure to metomidate during recovery resulted in a short-term increase in plasma glucose but differences between controls and metomidate-exposed fish were relatively minor thereafter. Cortisol began to decrease in catfish immediately after the removal of the stress but continued to increase for 15 min in sunshine bass recovering in fresh water and for 5 min in bass recovering in metomidate. Catfish recovering in fresh water had a cortisol elimination rate of -1.28 ng/mL/min compared with -2.45 ng/mL/min for fish recovering in metomidate (P>0.05) while sunshine bass recovering in fresh water had an elimination rate of -6.96 ng/mL/min compared with -4.50 ng/mL/min for fish recovering in metomidate (P>0.05). These data indicate that the rapid decrease of plasma cortisol after removal of the stressor is due to an almost immediate decrease of secretion, tissue uptake and a rapid renal loss due to the absence of a plasma binding protein.

Effects of an advanced temperature cycle on smolt development and endocrinology indicate that temperature is not a zeitgeber for smolting in Atlantic salmon

Atlantic salmon (Salmo salar) juveniles were reared under simulated conditions of normal photoperiod (LDN) or short days (LD 9:15) and ambient temperature (AMB: normal temperature increases in April) or an advanced temperature cycle (ADV: temperature increases in February). Under both photoperiod conditions, the timing of increased and peak levels of gill Na+,K+-ATPase activity were not altered by temperature,although the rate of increase was initially greater under ADV. ADV/LD 9:15 resulted in peak gill Na+,K+-ATPase activity that was half of that seen under normal photoperiod and temperature conditions. Plasma growth hormone (GH) levels increased threefold in late March under ADV/LDN,but not under ADV/LD 9:15, indicating that there is a photoperiod-dependent effect of temperature on levels of this hormone. Plasma insulin-like growth factor I (IGF-I) increased in spring in all groups, with increases occurring significantly earlier in the ADV/LDN group. In each photoperiod condition, the advanced temperature cycle resulted in large decreases in plasma thyroxine(T4) levels in March, which subsequently recovered, whereas plasma 3,5,3′-triiodo-L-thyronine (T3) levels were not substantially affected by either photoperiod or temperature. There was no consistent pattern of change in plasma cortisol levels. The results do not provide support for the role of temperature as a zeitgeber, but do indicate that temperature has a role in the timing of smolting by affecting the rate of development and interacting with the photoperiod.

Characterisation of glucocorticoid receptors in peripheral blood leukocytes of Carp, Cyprinus carpio L

Binding studies with [3H]cortisol revealed the presence of a single class of cortisol-binding sites on carp peripheral blood leukocytes (PBL). These binding sites showed high affinity (Kd of 3.8 nM) and low capacity (490 binding sites per cell), indicative of receptor binding. Affinity for cortisone was 254-fold lower than for cortisol. Affinity for the two synthetic glucocorticoids dexamethasone and triamcinolone acetonide (TA) was 4- and 10-fold higher than for cortisol, respectively. Further evidence for the GR character of the receptor came from results showing that cortisol induced apoptosis, which could be blocked by the glucocorticoid analogue RU486. A single meal of cortisol-containing food elevated plasma cortisol concentrations and decreased GR density in PBL, as measured 3 h later. The percentage of circulating B lymphocytes also decreased. Cortisol-induced redistribution of B lymphocytes from the blood, due to cortisol treatment, may explain the decrease of GR numbers in PBL, although downregulation of available GR remains possible.

The biliary accumulation of corticosteroids in rainbow trout,Oncorhynchus mykiss, during acute and chronic stress

The accumulation of immunoreactive corticosteroids in the bile of rainbow trout during stress was monitored by radioimmunoassay and GUMS. Although plasma cortisol levels were elevated by confinement for 1 hour, biliary levels of free and conjugated steroids in the bile were unaffected. However, after 24 hours confinement, in addition to elevated plasma cortisol levels, free and conjugated steroids in the bile were also significantly higher than in control, unstressed fish. The time-course of change in plasma and biliary corticosteroid levels was determined in rainbow trout subject to 96 hours confinement stress. Free steroid levels in the bile of stressed fish were elevated within 2 hours of the onset of stress, while levels of conjugated steroids were significantly elevated within 4 hours of the onset of confinement. Analysis of bile from stressed fish, by GC/MS, established the major conjugated steroids present to be tetrahydrocortisone (230 μg ml(-1) bile), tetrahydrocortisol (75 μg ml(-1)), cortisone (33.5 μg ml(-1)), cortisol (25 μg ml(-1)) and β-cortolone (5 μg ml(-1)). The data are discussed with reference to the role of cortisone and conjugating enzymes in the clearance of cortisol, and further data are presented to suggest that the analysis of biliary steroid content may provide a suitable means of identifying stressed fish under conditions in which an additional sampling stress is unavoidable.

Cortisol effects on plasma electrolytes and thyroid hormones during smoltification in coho salmon Oncorhynchus kisutch

Plasma concentrations of Na+, K+, triiodothyronine (T3), and thyroxine (T4) and muscle water content were measured in yearling coho salmon. Oncorhynchus kisutch, after injection of cortisol in April, May, and June in fresh water (FW) and during acclimation to seawater (SW). Cortisol (17-21 micrograms/g), injected intraperitoneally in a melted cocoa butter suspension, suppressed the rise of plasma Na+ during SW acclimation in April but not in May or June. Muscle water content increased during SW acclimation in cortisol-treated fish in April and June. These observations suggest a hypoosmoregulatory function for cortisol during SW acclimation. Cortisol treatment also induced elevations of plasma K+ in FW (April only) and SW (April and May only). Cortisol treatment increased plasma T3 during SW acclimation in June and T4 in FW in May. The results suggest that cortisol may modify osmoregulatory processes and thyroid hormone activity during smoltification and acclimation to SW in yearling coho salmon.

Spontaneous and ACTH-induced interrenal steroidogenesis in juvenile coho salmon (Oncorhynchus kisutch): effects of monovalent ions and osmolality in vitro

We determined the in vitro effects of changes in extracellular monovalent ion levels and osmotic pressure on the spontaneous and adrenocorticotropin (ACTH)-stimulated interrenal activity of coho salmon (Oncorhynchus kisutch). We used a perifusion system of incubation and monitored interrenal activity by measuring the effluent cortisol content with a radioimmunoassay. An increase in the medium osmolality with mannitol, from 206 to 290 or 353 mosmol, caused an increase in the spontaneous release of cortisol only slightly (compared with the much greater increase induced by porcine-ACTH). A similar minor increase was observed when NaCl was elevated from 130 to 180 mM. On the other hand, the spontaneous release of cortisol was not affected by increasing the KCl level from 3.2 to 9.6 mM, but was clearly increased when KCl was raised from 3.2 mM to a supraphysiological level of 27.2 mM. Ionic or osmolality changes, within the physiological range observed in coho salmon plasma, did not affect the characteristics of interrenal secretion of cortisol in response to porcine-ACTH. If our results with interrenal cells in vitro are representative of the basic functioning of the cells in vivo, then one would have to conclude that changes in concentrations of plasma monovalent ions or in osmotic pressure may not play a significant physiological role in the regulation of interrenal steroidogenesis or corticosteroid release in coho salmon.

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.

The metabolic clearance rate of estradiol-17 beta in rainbow trout, Salmo gairdneri R., estimated by both single injection and constant infusion methods: increase during oocyte maturation

A dual cannulation of free-swimming rainbow trout is used to estimate the metabolic clearance rate (MCR) of Estradiol-17 beta (E2-17 beta) by both single injection and constant infusion methods. It is shown that E2-17 beta MCR changes significantly during the progress of oogenesis, mainly at the end of the sexual cycle. The same changes in MCR and very similar values are found with both single injection and constant infusion methods: MCR is stable (28.8 ml/hr/kg) from the postovulation period (throughout endogenous vitellogenesis) to early exogenous vitellogenesis. It decreases significantly during advanced exogenous vitellogenesis (18.7 ml/hr/kg) and increases clearly at the onset of oocyte maturation (40.9 ml/hr/kg). A direct relationship between MCR and plasma E2-17 beta occurs: Plasma E2-17 beta levels increase (advanced exogenous vitellogenesis) when MCR decreases. Then estradiol decline takes place at the same time that MCR reaches its highest values (oocyte maturation). An increase in MCR is probably one event required to allow the establishment of an appropriate hormonal environment for oocyte maturation.

A protocol for estimation of cortisol plasma clearance in acid-exposed rainbow trout (Salmo gairdneri)

The cortisol metabolic clearance rate (MCR) and degradation rate (DR) were determined in acid (H2SO4)-stressed and control rainbow trout (260 g) cannulated via the dorsal aorta. Recovery from catheterization, as judged by plasma cortisol, glucose, protein, and packed cell volume (PCV), was complete by 6 days. However, serial blood sampling increased plasma cortisol. Furthermore, although no major or consistent diel change in plasma cortisol occurred in terminally sampled free-swimming control trout, fluctuations were observed in serially bled catheterized trout. These findings preclude cortisol MCR estimation by any serial sampling method. Although plasma cortisol was temporarily elevated by constant infusion (70 microliter hr-1) of saline:ethanol vehicle, a satisfactory protocol was established for determining cortisol MCR by infusion of labeled cortisol to constant plasma specific activity. The MCR for control trout in water, pH 7.7, for 7 days was 30.3 +/- 4.2 ml hr-1 100 g-1, which did not differ from that of trout in water, pH 5.0. However, the cortisol DR was greater at pH 5.0 (2.13 +/- 0.46 micrograms hr-1 100 g-1) than at pH 7.7 (0.56 +/- 0.12) due primarily to increased plasma cortisol at pH 5.0.