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Am J Physiol Regul Integr Comp Physiol 285: R1098-R1106, 2003. First published July 3, 2003; doi:10.1152/ajpregu.00731.2002
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COMPLEX FUNCTION OF THE CENTRAL NERVOUS SYSTEM, SLEEP AND LOCOMOTION

Effects of acetylsalicylic acid treatment on thyroid hormones, prolactins, and the stress response of tilapia (Oreochromis mossambicus)

Rogier D. van Anholt,1 Tom Spanings,1 William Koven,2 and Sjoerd E. Wendelaar Bonga1

1Department of Animal Ecology and Ecophysiology, Faculty of Science, University of Nijmegen, 6525 ED Nijmegen, The Netherlands; and 2Department of Larval Rearing, The National Center for Mariculture, Eilat 88112, Israel

Submitted 2 December 2002 ; accepted in final form 2 July 2003


   ABSTRACT
TOP
 ABSTRACT
MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The cyclooxygenase (COX) pathway converts arachidonic acid (ArA) into prostaglandins (PGs), which interact with the stress response in mammals and possibly in fish as well. Acetylsalicylic acid (ASA) is a COX inhibitor and was used to characterize the effects of PGs on the release of several hormones and the stress response of tilapia (Oreochromis mossambicus). Plasma PGE2 was significantly reduced at 100 mg ASA/kg body wt, and both basal PGE2 and cortisol levels correlated negatively with plasma salicylate. Basal plasma 3,5,3'-triiodothyronine (T3) was reduced by ASA treatment, whereas prolactin (PRL)188 increased at 100 mg ASA/kg body wt. ASA depressed the cortisol response to the mild stress of 5 min of net confinement. As expected, glucose and lactate were elevated in the stressed control fish, but the responses were blunted by ASA treatment. Gill Na+-K+-ATPase activity was not affected by ASA. Plasma osmolarity increased after confinement in all treatments, whereas sodium only increased at the high ASA dose. This is the first time ASA has been administered to fish in vivo, and the altered hormone release and the inhibition of the acute stress response indicated the involvement of PGs in these processes.

arachidonic acid; cortisol; eicosanoids; prostaglandins

PROSTAGLANDINS (PGs) and other eicosanoids control various physiological functions in fish, including respiratory and cardiovascular output (22), ovulation and spawning behavior, oocyte maturation, nervous system function, osmoregulation (23), and immune functions (32). In fish, important sites of PG production are tissues directly involved in fluid and electrolyte regulation, such as the gills, opercular membranes, and kidneys (7, 42). In addition, eicosanoids are known to modulate the response of thyroid tissue to thyroid-stimulating hormone (TSH) and to promote prolactin (PRL) release in mammals (18). PRL is an important osmoregulatory hormone in (freshwater) fish (4). Thyroid hormones have been shown to influence the activity of ion-transporting enzymes, in particular gill Na+-K+-ATPase, in tilapia (4, 28). Furthermore, PGs have been described in human subjects to modulate the sensitivity of the hypothalamus-pituitary-adrenal (HPA) axis, which is responsible for the release of cortisol and corticosterone in the stress response (8, 15, 18, 25). Although direct evidence is still lacking, there are indications that PGs have a similar function in fish by affecting the analogous hypothalamus-pituitary-interrenal (HPI) axis (5).

When the fatty acid arachidonic acid (ArA 20:4n-6) is released from cell membranes by phospholipases, mainly PLA2, it can be metabolized by PGG/H synthase. PGG/H synthase is a membrane-bound protein in the endoplasmatic reticulum of PG-forming cells and exhibits two distinct catalytic activities. The cyclooxygenase (COX) component catalyzes the oxidation of ArA to the intermediate PGG2. The hydroperoxidase component mediates the reduction of the 15-hydroperoxyl group of PGG2 to the highly unstable PG endoperoxide PGH2, which is rapidly converted into PGs and thromboxanes (36). The COX-1 subtype is generally considered to be transcribed constitutively, whereas COX-2 expression is induced by various stimuli and is predominantly involved in pathophysiological processes such as inflammation (18, 40). Recently, it has been demonstrated that in fish, an inducible COX-2 exists as well in addition to a constitutively expressed COX-1 type: both are ~65-84% homologs to the mammalian isozymes (31, 50, 52). Although other fatty acids compete for the same enzymes to produce eicosanoid homologs, enzyme affinity for ArA is higher, resulting in predominantly ArA-derived eicosanoids, mainly PGE2, which are also more biologically active (6, 18).

Acetylsalicylic acid (ASA; aspirin) is a potent and irreversible inhibitor of the COX pathway (36) and provides an excellent tool to determine the functions of PGs in fish. In the present study, we therefore investigated the effects of ASA treatment on the release of several hormones, the osmoregulation, and the response to an acute stressor of a well-studied model species, tilapia (Oreochromis mossambicus; Peters, 1952).


   MATERIAL AND METHODS
TOP
 ABSTRACT
 MATERIAL AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The experiments described in this study were conducted in accordance with the current law on animal experimentation in the Netherlands.

Fish. Male and female freshwater tilapia (O. mossambicus) were obtained from laboratory stock (Univ. of Nijmegen, Nijmegen, The Netherlands). Fish were maintained in aerated, partially recirculated and filtered tap water at 25°C with a 12-h photoperiod. Tilapia were acclimated over a 3- to 4-wk period, during which they were fed standard pellets at 2% of their body weight per day (crude protein 38%, total lipid 10%, ash 10%, moisture 8%; TI-4.5 Tilapia, Trouw Nutrition).

Uptake of ASA. Transparent gelatin capsules (size 4; Lamepro) were used to administer ASA. Capsules were filled with ~100 mg of crushed pellets and the required amount of ASA (Sigma-Aldrich). The filled capsules resembled the standard pellets in size and appearance, and when fed together with pellets they were eaten indiscriminately within 5-10 min, enabling a stress-free administration of ASA. Prior to experimentation, the time course of the uptake of ASA into the bloodstream after oral administration was determined in a separate test. To this end, 16 adult tilapia (174 ± 7 g) from the same cohort were individually housed in 10-liter aquaria, and a 2-day acclimation period was allowed. On the third day, 14 tilapia received a single oral dose of 100 mg ASA/kg body wt together with standard pellets (1% of body wt). As controls, two tilapia were fed gelatin capsules filled with crushed pellets only. Fish were anesthetized in a 1.5% 2-phenoxyethanol solution (Sigma-Aldrich), and, per time point, ~0.2 ml of blood were sampled from the caudal vein with heparinized needles. They were handled as gently as possible, and successive samples were obtained from each fish over a 50-h period. Recovery from the anesthetic always occurred within 5 min after they were returned to their aquaria. No fish died either during the procedure or in the 3 wk that followed the experiment.

ASA and stress challenge. The effect of ASA on the response to net confinement was studied on 120 tilapia (38 ± 1 g) from the same cohort, separated in two consecutive trials. Both trials consisted of 60 tilapia divided over 6 aquaria (80 liters, 10 fish/aquarium), and 2 aquaria were used per treatment. During the acclimation period of 25 days, they were fed 2% of body weight twice a day. Two doses of ASA were used, 10 mg or 100 mg ASA/kg body wt, while the control treatment was fed capsules without ASA. In contrast to the pilot study, the ASA dose was not determined for each individual fish but on the basis of the mean weight of 10 fish per aquarium. ASA was fed twice a day and on the third day, 4 h after their last meal or dose of ASA, several fish were sampled for baseline values. The remaining fish were immediately subjected to 5 min of submerged net confinement, after which they were released back into the aquarium and sampled after either 5 min or 30 min, depending on the parameter. Fish were anesthetized in a 1.5% 2-phenoxyethanol solution, and blood samples were collected using heparinized needles. Plasma samples were separated after centrifugation and stored at -20°C until analysis. Gill arches were dissected and stored frozen in SEI buffer (300 mM sucrose, 10 mM Na2EDTA, 100 mM imidazole, pH 7.4 with HEPES-Tris) until analysis of Na+-K+-ATPase activity.

Salicylate. Levels of plasma salicylate were determined with a commercial kit for human plasma (Sigma-Aldrich). To determine the recovery in tilapia plasma, an additional standard curve was created on the basis of addition of ASA in final concentrations of 0.36-1.81 mmol/l to plasma of a control fish. Up to 0.72 mmol/l salicylate, the regression was linear with a 95% recovery, and all tested samples were within this range. Undiluted samples were measured in triplicates, and assay blanks were used to correct for turbidity. Absorbance was read at 550 nm in a Victor2 Wallac multilabel counter.

PGE2. Plasma levels of PGE2 were assayed with a high-sensitivity chemiluminescence enzyme immunoassay (CLIA 91001; Assay Designs). Plasma samples were diluted five times and assayed according to the manufacturer's protocol. Cross-reactivity of the antibody with PGE1 is 33.2%, PGB1 7.0%, 13,14-dihydro-15-keto-PGF2{alpha} 2.8%, PGE3 <0.01%, and ArA <0.01%.

COX activity. A commercial kit (907-003; Assay Designs) was used to determine whether ASA inhibited the COX activity of tilapia tissue homogenates in vitro. To this end, gill and kidney tissue samples of five control tilapia homogenized in SEI buffer were used. Fifty microliters of homogenate supernatant (~2.5 mg protein/ml) were added to the microplate in duplicates. A concentrated stock solution of 2.8 mM ASA (Sigma-Aldrich) in ethanol was diluted 100x in distilled water, of which 25 µl were added to a second duplicate set of wells to a final concentration of 400 µM. This concentration approximates the average value measured in plasma of the tilapia 4 h after administration of ASA. All samples were incubated for 20 min at room temperature, and chemiluminescence was determined according to protocol using a Victor2 Wallac multilabel counter with an automated dispenser unit. The percentage of inhibition by ASA was calculated as the differences between the COX activity in the control samples and those with ASA, corrected for the nonspecific increase in signal strength caused by ASA alone.

RIAs. Plasma total thyroxine (T4) was determined by RIA using a modification of a procedure developed for human serum and rat plasma (13). Because of the very low levels of T4 in fish plasma, 100-µl samples of plasma (instead of 5 µl) were used for analysis. The standards used for measurement in human serum consisted of 0-360 nmol/l T4 added to 5 µlof T4-free human serum that had been prepared by repeated treatment with charcoal (15 g/100 ml) and centrifugation. For this assay, these standards were supplemented with 100 µl of T4-free fish plasma prepared in the same way. Conversely, 100 µl of unknown fish plasma samples were supplemented with 5 µl of human T4-free serum to provide full matrix compatibility between samples and standards. By using 100-µl samples, the concentration range was 0-18 nmol/l, with a detection range of 0.5 nmol/l. Accuracy was tested by spiking a pooled fish sample with T4. Recoveries were 104 and 97%. At a level of 2.95 nmol/l, the intra-assay variation was 5.6%, and the interassay variation was 10.6%.

Plasma total 3,5,3'-triiodothyronine (T3) was assessed on the Centaur automated immunoassay system (ACS; Bayer) using an acridinium-labeled anti-T3 antibody. Fish plasma was diluted twice, and the interassay variation was <3% at the levels assayed. Plasma levels of the two forms of PRL, PRL188 and PRL177, were measured by homologous RIAs according to previous studies (1, 48).

Plasma cortisol levels were determined with a commercially available antibody, with some minor modifications of the manufacturer's protocol (Campro Scientific). All constituents were in phosphate (P)-EDTA buffer (0.05 M NA2HPO4, 0.01 M Na2EDTA, 0.003 M NaN3). Ten microliters of standards or of 1:5-diluted plasma were incubated for 4 h with 100 µl of cortisol antibody (IgG-F-1, 1:500) in RIA buffer [P-EDTA buffer with 0.1% (wt/vol) 8-anilino-1-naphthalene-sulfonic acid and 0.1% (wt/vol) bovine {gamma}-globulin] and 0.25% normal nonimmune rabbit serum (IgG-NRS). Cross-reactivity of the antibody with 5{alpha}-dihydrocortisol is 27.6%, 5{beta}-dihydrocortisol 9.4%, 11-deoxycortisol 5.9%, corticosterone 1.7%, and cortisone 2.6%. Standards and samples were then incubated overnight with 100 µl of labeled cortisol in RIA buffer [125I-labeled cortisol, 1,700 counts/min per tube; Amersham] and 100 µl of second antibody solution in RIA buffer (IgG-goat anti-rabbit gamma globulin, 1:200). After 1 ml of ice-cold P-EDTA buffer with 2% (wt/vol) BSA and 5% (wt/vol) polyethylene glycol was added, tubes were centrifuged (20 min, 2,000 g, 4°C), supernatants were aspired, and pellets were counted for 3 min (1272 Clinigamma; LKB Wallac).

Glucose and lactate. Plasma glucose levels were determined in duplicates with Sigma's INFINITY glucose reagent (Sigma-Aldrich), and absorbance was read at 340 nm. Plasma concentrations of lactate were assayed in duplicates by a standard colorimetric assay (735-10; Sigma-Aldrich) and read at 550 nm (Victor2 Wallac multilabel counter).

Ions and osmolarity. Plasma concentrations of sodium, potassium, and chloride were determined by flame photometry (Radiometer FLM3). Plasma osmolarity was determined for 50 µl of undiluted plasma samples with an automatic cryoscopic osmometer (Osmomat 030; Gonotec).

Gill Na+-K+-ATPase. Several branchial filaments were homogenized in 250 µl of SEI buffer with aprotinin (5 µl/ml), and, after centrifugation (10 min, 500 g), supernatants were used for analysis. After determination of the protein concentration (BSA standard 1.42 mg/ml; Bio-Rad), saponin was added to each sample (20 µg/mg protein). Na+-K+-ATPase activity was measured on the basis of procedures described previously (11, 39), adapted to 96-well microplates. Five microliters of supernatant (containing 5-9.5 µg of protein, optimal for this assay) were added to each well, with six wells per sample. The reaction was started by adding 100 µl of basic incubation medium (300 mM sucrose, 5 mM MgCl2, 100 mM NaCl, 0.1 mM H2EDTA, 3 mM Na2ATP, 100 mM imidazole, pH 7.4) with either 12.5 mM KCl or 1 mM ouabain (each in triplicates). Plates were incubated at 25°C for 60 min, and the reaction was stopped by adding to all wells 200 µlofa1:1 mixture of ice-cold TCA (17.4%) and color reagent [0.66 M H2SO4 and 9.2 mM (NH4)6Mo7O24 · 4H2O, mixed with 0.33 M FeSO4 · 7H2O]. After 30 min of incubation at 4°C, absorbance was determined at 660 nm (Victor2 Wallac multilabel counter). A standard curve of phosphate (4.84 mM Pi; Sigma-Aldrich) was used as a reference. The difference between the total ATPase activity and the ouabain-insensitive ATPase activity was designated as the ouabain-sensitive potassium ion-dependent Na+-K+-ATPase activity and expressed in micromoles of Pi per hour per milligram protein.

Statistics. Initial analysis revealed no differences between the two trials; all data are expressed as means ± SE as a combination of both tests. Repeated-measures one-way ANOVA was used to test whether levels of salicylate were significantly elevated compared with t = 0.5 h in the ASA uptake test. Differences between treatments for plasma PRL177, PRL188, T3, T4, salicylate, and PGE2 levels were tested with one-way ANOVA. Levene's tests for homogeneity of variances indicated that log transformation was required for cortisol, T3, T4, and PGE2 data. To test whether the responses to net confinement differed between treatments, two-way ANOVA was performed on cortisol, glucose, lactate, plasma ions, and osmolarity data. Post hoc multiple comparison tests (Tukey's honestly significant difference test) were used to determine which treatments or time points differed significantly (SPSS software). Significance was accepted at P <= 0.05, and asterisks were used to indicate significant differences: *P < 0.05, **P < 0.01, and ***P < 0.001.


   RESULTS
TOP
 ABSTRACT
 MATERIAL AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Administration of ASA did not result in mortality during the experimental period or cause changes in motility, behavior, or appetite, and macroscopic analysis of stomachs at the end of the experiments revealed no hemorrhages.

The uptake of ASA in time, reflected by plasma salicylate levels, is shown in Fig. 1. The levels increased 1.5 h after the tilapia had received a single dose of 100 mg ASA/kg body wt, although this was not significant until 3.5 h after administration. Maximum plasma levels were reached after 5.5 h and remained significantly elevated until 15 h later. Repetitive sampling did not alter the salicylate levels of the two control fish, which were 66.4 ± 6.8 µmol/l salicylate for the sampling period.



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  		Fig. 1. Uptake of acetylsalicylic acid (ASA) in time, after a single oral dose of 100 mg ASA/kg body wt at t = 0. Values are means ± SE. Significant differences compared with t = 0.5 h: **P < 0.01 and ***P < 0.001.

 

In the follow-up experiment, two doses of ASA were tested. The administration of 10 mg/kg did not significantly elevate plasma salicylate levels after 4 h compared with the control group (stressed and nonstressed fish combined). However, at 100 mg ASA/kg body wt, plasma salicylate levels were significantly elevated (P = 0.000) compared with the controls (Table 1).


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  		Table 1. Plasma levels of salicylate and PGE2

 

Plasma PGE2 was not significantly affected by the low dose of ASA, and the high ASA dose reduced the average plasma PGE2 by ~34%, which was significantly different from the low dose (P = 0.025; Table 1). Plasma PGE2 levels were significantly negatively correlated (R2 = 0.889, P = 0.001) with the plasma salicylate levels in individual fish (Fig. 2).



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  		Fig. 2. Correlations between plasma salicylate levels and basal levels of cortisol ({bullet}, solid line; R2 = 0.598) and plasma prostaglandin (PG) E2 ({circ}, dashed line; R2 = 0.889) of nonstressed fish measured 4 h after the last dose of 10 or 100 mg ASA/kg body wt.

 

At a concentration of 400 µM in the medium, ASA inhibited the COX activity of kidney homogenates with 43.9 ± 1.9%, but the COX activity in gill homogenates was too low to be detected with this assay.

At 100 mg/kg ASA, the mean plasma T3 level was reduced significantly (P = 0.014) compared with the control, and the low ASA dose had no effect on plasma T3 (P = 0.109; Fig. 3). ASA administration did not significantly affect plasma T4 levels at 10 mg/kg (P = 0.992) or at 100 mg/kg body wt (P = 0.083) compared with the control.



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  		Fig. 3. The effect of ASA (10 or 100 mg/kg body wt) on basal plasma 3,5,3'-triiodothyronine (T3) and thyroxine (T4) levels. Blood samples were collected 4 h after the last dose of ASA (means ± SE). Significant difference from the control group (0 mg ASA/kg body wt): *P < 0.05. Differences between T3 and T4 were not analyzed.

 

Although plasma PRL177 levels increased slightly with increasing ASA dose, this effect was not significant (P = 0.672; Fig. 4). Basal PRL188 levels were significantly enhanced at the high ASA dose compared with the low dose (P = 0.044; Fig. 4).



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  		Fig. 4. The effect of ASA (10 or 100 mg/kg body wt) on basal plasma levels of 2 forms of prolactin, PRL177 and PRL188. Blood samples were collected 4 h after the last dose of ASA (means ± SE). Significant difference from 10 mg ASA/kg body wt group: *P < 0.05. Differences between PRL177 and PRL188 were not analyzed.

 

Administration of ASA did not result in a significant change of mean basal levels of plasma cortisol (Fig. 5). However, when comparing the values of individual fish, there was a significant negative exponential correlation (R2 = 0.598, P = 0.005) between individual plasma salicylate levels and plasma basal cortisol of the nonstressed fish (Fig. 2). Plasma cortisol levels were significantly influenced by both the level of ASA (P = 0.010) and the time of sampling, i.e., confinement (P = 0.000; Fig. 5). At 100 mg/kg ASA, the postconfinement levels were significantly (P = 0.020) lower than the postconfinement levels in the control. Both 5 and 30 min after confinement, cortisol levels were significantly (P = 0.000) increased compared with the unstressed levels in all treatments, but there was no significant difference between 5 and 30 min after confinement (P = 0.051).



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  		Fig. 5. Plasma cortisol responses to 5 min of net confinement. Blood samples were collected 4 h after the last dose of ASA from nonstressed fish (t = 0) and 5 and 30 min after net confinement (means ± SE). Significant difference from the postconfinement levels of the control fish (0 mg ASA/kg body wt): ***P < 0.001.

 

Plasma glucose levels were significantly influenced by both the ASA administration (P = 0.006) and the confinement (P = 0.000). At the low dose of ASA, the response to confinement was significantly (P = 0.007) reduced compared with the controls (Fig. 6). In all treatments, at both 5 and 30 min after confinement, levels were significantly elevated compared with the controls (P = 0.000), and the difference between 5 and 30 min was no longer significant.



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  		Fig. 6. Changes in plasma glucose after ASA administration followed by net confinement. Samples were taken 4 h after the last dose from nonstressed fish (t = 0) and 5 and 30 min after net confinement (means ± SE). Significant difference from the control group (0 mg ASA/kg body wt): **P < 0.01.

 

ASA treatment and time of sampling both significantly (P = 0.000) influenced plasma lactate levels (Fig. 7). There was also a significant interaction effect between time and treatment (P = 0.000). Both the low and high dose of ASA significantly reduced the lactate response compared with the controls (P = 0.010 and 0.024, respectively). Lactate levels were significantly elevated at both 5 and 30 min after confinement compared with the unstressed fish (P = 0.000), and at 30 min the lactate levels were also significantly higher than after 5 min (P = 0.000; Fig. 7).



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  		Fig. 7. Changes in plasma lactate after ASA administration followed by net confinement. Samples were taken 4 h after the last dose from nonstressed fish (t = 0) and 5 and 30 min after net confinement (means ± SE). Significant difference from the control group (0 mg ASA/kg body wt): *P < 0.05.

 

The lowest basal gill Na+-K+-ATPase activity was observed at the high dose of ASA. However, variation was high and the difference with the control group was not significant (Table 2).


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  		Table 2. Total Na+-K+-ATPase activity of gill homogenates, plasma osmolarity, and ion levels before and after net confinement

 

Plasma osmolarity levels exhibited similar transient increases after confinement in all treatments (P = 0.000), but the dose of ASA had no significant influence on this response (Table 2). At 5 and 30 min after confinement, plasma osmolarity was significantly elevated compared with controls (P = 0.000), whereas the increase after 30 min was not significantly different with the levels after 5 min.

Plasma sodium levels were significantly affected by both the dose of ASA as well as the confinement (P = 0.021 and 0.000, respectively; Table 2). Although neither the low nor the high dose of ASA differed significantly from the controls, sodium levels were significantly elevated at 100 mg/kg body wt compared with 10 mg ASA/kg body wt (P = 0.039). Overall, after 5 and 30 min, sodium levels were significantly elevated compared with the unstressed fish (P = 0.010 and 0.000, respectively). Plasma chloride levels were significantly affected by confinement (P = 0.000) but not by the dose of ASA. Within 5 min after confinement, plasma chloride levels were significantly higher compared with unstressed fish (P = 0.000). Thirty minutes after confinement, plasma chloride levels had dropped significantly below the levels at 5 min (P = 0.000) and were no longer significantly elevated compared with the unstressed fish. Plasma potassium levels were not significantly affected by either administration of ASA or confinement.


   DISCUSSION
TOP
 ABSTRACT
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This is the first time, to our knowledge, that ASA was used to study the effects of fatty acid-derived PGs in fish. The administration of 100 mg ASA/kg body wt via gelatin capsules resulted in elevated plasma levels of salicylate within 3.5 h, reaching a maximum within 5.5 h (~650 µmol/l), which was well within the range considered therapeutic for humans (<1,440 µmol/l) and considerably lower than toxic (>2,170 µmol/l) and lethal (>4,340 µmol/l) limits (41). The tilapia did not discriminate between capsules containing ASA and normal pellets, and therefore we consider this method superior to injections or forced feeding, which are always stressful. This was confirmed by the low basal cortisol, glucose, and lactate levels observed both in the controls and after the administration of ASA in our tests. Cortisol is the major glucocorticoid in tilapia and is released from the head kidney after the stress-induced activation of the HPI axis (2, 46). These low basal values also argue against a nonspecific toxic effect of ASA in our experiments, which is supported by the practically identical basal plasma osmolarity and ions levels, the lack of behavioral changes or mortality, and the absence of hemorrhages.

One of three major pathways of ArA metabolism is the COX pathway converting ArA mainly into PGs (PGE2, PGD2, PGF2{alpha}) and thromboxanes (TxA2) (18). ASA has no direct effect on the activity of PLA2 that releases ArA from the membranes (14), but it is a highly specific and irreversible inhibitor of the COX pathway by acetylating the COX enzymes. Hence, new enzyme synthesis is required before new PGs can be produced (36). Four hours after administration of ASA, the levels of circulating PGE2 were significantly lower in the fish that had received the high dose compared with those that received the low dose. The strong negative correlation between individual PGE2 and salicylate levels also suggested a reduction of COX activity in these tilapia, which was supported by the inhibitory effect of ASA on the COX activity of kidney homogenates in vitro.

PGs are known to influence a wide range of physiological processes by enhancing the release of hormones and/or altering the sensitivity of target organs (18, 23). Langer et al. (19) found that ASA (analog) treatment reduced plasma levels of TSH, T3, and T4 in humans, which indicated the involvement of PGs in the release of thyroid hormones. Studies on mammals further showed that PGs enhanced the response of thyroid tissue to TSH, resulting in higher T3 and T4 levels (18). In our study, basal levels of T3 were significantly reduced after ASA treatment, suggesting that PGs have a similar regulatory function in tilapia. Thyroid hormones play an important role in growth and development of fish larvae and particularly flatfish metamorphosis (29, 30). Furthermore, thyroid hormones enhance the osmoregulatory ability, principally in saltwater fish (33) but also in freshwater fish, as they have been reported to promote gill Na+-K+-ATPase activity, an important ion-transporting enzyme, in tilapia (28).

PRL is also a hormone with osmoregulatory functions in fish (1, 16, 34). In mammals eicosanoids are thought to promote PRL release by decreasing the PRL release-inhibiting factor (PIF) and increasing the PRL-releasing factor (PRF) from the hypothalamus, without having a direct effect on the pituitary (26). ASA treatment has also been shown to attenuate the PRL response to exercise in humans, thereby providing indirect evidence for the role of COX metabolites in the control of PRL release (10). Surprisingly, in our experiments PRL levels were not reduced when the COX pathway was blocked, but instead plasma PRL188 levels were elevated after administration of 100 mg ASA/kg body wt. It might be that PRL release is under inhibitory control by PGs in tilapia, or ArA itself has a stimulatory effect on the release of PRL188; several polyunsaturated fatty acids were capable of inducing PRL secretion from rat pituitary cells in vitro, independently of the conversion to PGs (17).

The stress response in fish, similar to that in mammals, is generally characterized by activation of the brain centra that control the release of corticotropin-releasing hormone (CRH) by the hypothalamus, which in turn induces the release of ACTH from the pituitary. ACTH is an important corticortropic hormone inducing the release of cortisol from interrenal cells in fish, although {alpha}-melanocyte-stimulating hormone and {beta}-endorphin appear to be involved as well (3). In our tilapia, 100 mg ASA/kg body wt clearly reduced the cortisol response to net confinement, demonstrating the involvement of PGs in vivo. This supports the observations by Gupta et al. (12), who found that PGs were able to induce the release of cortisol from interrenal tissues of trout in vitro, and Wales (44), who demonstrated that injections of PGE2 stimulated the cortisol release in hagfish in vivo. Several studies on humans have shown that ASA administration resulted in a blunted cortisol response, suggesting that PGs have a stimulatory effect on adrenal steroidogenesis (8). According to these studies, endogenous PGs can generate their effect on the release of cortisol at several levels: PGs can enhance hypothalamic CRH release but also restrain CRH-induced ACTH secretion at the pituitary level as well as enhance the response of the adrenal cortex to ACTH stimulation, resulting in an increase in steroidogenesis (10, 25). In our study, the basal plasma levels of cortisol were slightly reduced by ASA administration, and a strong correlation was found between individual plasma salicylate levels and plasma cortisol levels in the nonstressed fish. This is in contrast to the studies on humans (8), which suggests that basal and induced releases of cortisol are under different controlling mechanisms.

Basal lactate and glucose levels in our tilapia were not affected by the ASA administration, and the high levels of glucose and lactate after confinement in the control group closely resembled those found previously in tilapia in response to stress (43). At a dose of 10 mg/kg body wt, the ASA administration resulted in a blunted glucose response within 5 min. Although cortisol regulates gluconeogenesis in fish, this requires modification of hepatic enzyme activity, which has been shown to take at least 12 h in tilapia. Instead, rapid hyperglycemia is most likely the result of enhanced hepatic glycogenolysis induced by catecholamine release (43). The blunted glucose response by ASA therefore altered either the catecholamine release or, alternatively, the adrenergic response of liver cells, although there is no previous evidence that the latter can occur in fish. Nevertheless, it has been shown in rats that brain PGE2 can mediate the central sympathetic outflow of catecholamines and that other COX products, thromboxanes, are involved in the central adrenomedullary outflow (27, 51).

Although the initial stress-induced rise in plasma lactate remained unaffected, ASA at both doses strongly augmented the lactacidemia observed after 30 min. Lactacidemia points to increased muscle glycolysis or incomplete oxidation as a result of hypoxemia in muscle cells. There is only very limited knowledge on the effects of fatty acids or eicosanoids on lactate metabolism, and no studies have been performed on fish. In humans, ASA treatment had no effect on lactate levels after exercise (10). On the other hand, in young rats, it was shown that feeding n-3 fatty acids attenuated stress-induced lactacidemia, in contrast to n-6 fatty acids that enhanced the lactate response. In that study, increased gluconeogenesis was thought to be responsible for the attenuated lactacidemia, although this was not due to inducible PGs but most likely related to an altered response of liver cells to catecholamines (35).

Several studies have demonstrated both inhibitory or stimulatory effects of PGs and fatty acids on Na+-K+-ATPase (21, 38, 39, 42). Nevertheless, the effects of ASA administration on gill Na+-K+-ATPase, plasma ions, and plasma osmolarity were very limited. While ASA treatment had no effect on plasma chloride and potassium levels, only net confinement in combination with the high dose of ASA resulted in a slight increase in plasma sodium. Generally, plasma ion levels tend to drop in freshwater fish after stress because of an increased permeability of (branchial) membranes. However, during acute stress, plasma water can also move out of the circulation into the tissues, reducing the blood plasma volume (46), explaining the transient increase in plasma ions in our experiments.

Although it appears that the observed effects of ASA administration are the result of reduced PG production, two alternative hypotheses can be proposed. First, ArA itself could be a controlling factor. Once released from cell membranes, free ArA not only functions as a precursor to eicosanoids but also acts directly as a ligand, affecting either negatively or positively the binding of steroid hormones to their specific intracellular receptors. In addition, fatty acids can also coregulate glucocorticoid-dependent gene expression by modulating the activity of protein kinases involved in the phosphorylating transcription factors (37). An intracellular increase in free ArA, as a result of ASA treatment, might therefore be responsible for the augmented cortisol response to the stressor in this study. Second, we should be aware that blocking the COX pathway might have redirected free ArA to other enzymatic pathways, the two main alternative pathways being the oxygenation into leukotrienes and hydroxyeicosatetraenoic acids (HETEs) by lipoxygenase and the conversion into epoxyeicosatraenoic acids (EETs) by epoxygenase (36). These alternative eicosanoids are potential modulators of the HPI axis in fish, as epoxygenase and lipoxygenase metabolites have been shown to stimulate ACTH and {beta}-endorphin secretion from rat pituitary cells (9, 24, 47), and lipoxygenase products stimulated steroidogenesis in bovine adrenal cells in response to ACTH (45, 49). However, in our experiments, these pathways might have been of subdominant importance, as all these studies demonstrated a stimulatory effect of the alternative metabolites on ACTH, {beta}-endorphin, and cortisol secretion, which does not correspond to the attenuated cortisol response after ASA administration in our tilapia. Unfortunately, information on the specific accumulation and conversion of ArA in pituitary and interrenal cells of teleost fish is still lacking.

The results of this study clearly indicated that inhibiting the COX pathway by ASA modified the release of several important hormones and inhibited the response of tilapia to an acute stressor. Administration of ASA in vivo is an excellent tool to investigate the involvement of ArA-derived metabolites in the hormone release and stress response of fish. It enables the differentiation between the PG-controlled and ArA-mediated effects, which will help to further understand the underlying mechanism of dietary control of the stress response and hormone release in fish (20).


   ACKNOWLEDGMENTS
 
We thank Dr. A. Cross (University Hospital St. Radboud, Nijmegen, The Netherlands) for the measurements of thyroid hormones and Prof. T. Hirano (Hawaii Institute of Marine Biology, University of Hawaii) for the PRL determinations.


   FOOTNOTES
 

Address for reprint requests and other correspondence: R. van Anholt, Dept. of Animal Ecology and Ecophysiology, Faculty of Science, Univ. of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands (E-mail: rvanholt{at}sci.kun.nl).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


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