Distribution and expression of glucocorticoid receptor mRNA in the forebrain of the rainbow trout

The stress - Reproductive axis in fish: The involvement of functional neuroanatomical systems in the brain

The neuroendocrine-stress axis of nonmammalian species is evolutionarily conserved, which makes them useful to serve as important model systems for elucidating the function of the vertebrate stress response. The involvement of hypothalamo-pituitary-adrenal (HPA) axis hormones in regulation of stress and reproduction is well described in different vertebrates. However, the stress response is a complex process, which appears to be controlled by a number of neurochemicals in association with hypothalamo-pituitary-interrenal (HPI) axis or independent of HPI axis in fish. In recent years, the participation of neurohormones other than HPI axis in regulation of stress and reproduction is gaining more attention. This review mainly focuses on the involvement of functional neuroanatomical systems such as the catecholaminergic neurotransmitter dopamine (DA) and opioid peptides in regulation of the stress-reproductive axis in fish. Occurrences of DA and opioid peptides like β-endorphin, enkephalins, dynorphin, and endomorphins have been demonstrated in fish brain, and diverse roles such as pain modulation, social behaviour and reproduction are implicated for these hormones. Neuroanatomical studies using retrograde tracing, immunohistochemical staining and lesion methods have demonstrated that the neurons originating in the preoptic region and the nucleus lateralis tuberis directly innervate the pituitary gland and, therefore, the hypophysiotrophic role of these hormones. In addition, heightened synthetic and secretory activity of the opioidergic and the dopaminergic neurons in hypothalamic areas of the brain during stress exposure suggest potentially intricate relationship with the stress-reproductive axis in fish. Current evidence in early vertebrates like fish provides a novel insight into the underlying neuroendocrine mechanisms as additional pathways along the stress-reproductive axis that seem to be conserved during the course of evolution.

Metabolic control of teleost reproduction by leptin and its complements: Understanding current insights from mammals

Reproduction is expensive. Hence, reproductive physiology is sensitive to an array of endogenous signals that provide information on metabolic and nutritional sufficiency. Although metabolic gating of reproductive function in mammals, as evidenced by studies demonstrating delayed puberty and perturbed fertility, has long been understood to be a function of energy sufficiency, an understanding of the endocrine regulators of this relationship have emerged only within recent decades. Peripheral signals including leptin and cortisol have long been implicated in the physiological integration of metabolism and reproduction. Recent studies have begun to explore possible roles for these two hormones in the regulation of reproduction in teleost fishes, as well as a role for leptin as a catabolic stress hormone. In this review, we briefly explore the reproductive actions of leptin and cortisol in mammals and teleost fishes and possible role of both hormones as putative modulators of the reproductive axis during stress events.

Influence of Stress on Liver Circadian Physiology. A Study in Rainbow Trout, Oncorhynchus mykiss, as Fish Model

In vertebrates stress negatively affects body homeostasis and triggers a battery of metabolic responses, with liver playing a key role. This organ responds with altered metabolism, leading the animal to cope with the stress situation, which involves carbohydrate and lipid mobilization. However, metabolism among other physiological functions is under circadian control within the liver. Then, metabolic homeostasis at system level involves circadian timing systems within tissues and cells, and collaborate with each other. During chronic stress, cortisol maintains the liver metabolic response by modulating carbohydrate- and lipid-related metabolism. Stress also disrupts the circadian oscillator within the liver in mammals, whereas little information is available in other vertebrates, such as fish. To raise the complexity of this process, other candidates may mediate in such effect of stress. In fact, sirtuin1, a link between cellular sensing of energy status and circadian clocks, participates in the response to stress in mammals, but no information is available in fish. Considering the role played by liver in providing energy for the animal to deal with an adverse situation, and the existence of a circadian oscillator within this tissue, jeopardized liver circadian physiology during stress exposure might be expected. Whether the physiological response to stress is a well conserved process through the phylogeny and the mechanisms involved in such response is a question that remains to be elucidated. Then, we provide information at this respect in mammals and show comparable results in rainbow trout as fish animal model. Similar to that in mammals, stress triggers a series of responses in fish that leads the animal to cope with the adverse situation. Stress influences liver physiology in fish, affecting carbohydrate and lipid metabolism-related parameters, and the circadian oscillator as well. In a similar way than that of mammals different mediators participate in the response of liver circadian physiology to stress in fish. Among them, we confirm for the teleost rainbow trout a role of nuclear receptors (rev-erbβ), cortisol, and sirt1. However, further research is needed to evaluate the independent effect of each one, or the existence of any interaction among them.

Seasonal changes in gene expression of corticoid receptors in anadromous and non-anadromous strains of rainbow trout Oncorhynchus mykiss

To clarify the regulation of expression of corticoid receptor (CR) genes during period of parr-smolt transformation of salmonids, seasonal changes in mRNA levels of glucocorticoid receptor (GR)-1, GR-2 and mineralocorticoid receptor (MR) were examined in gill, leucocytes, spleen and brain of anadromous and non-anadromous forms of Oncorhynchus mykiss. Increases in gill Na(+) , K(+) ATPase activity, plasma thyroxine levels and hypo-osmoregulatory ability assessed by 24 h seawater challenge test represented characteristics of smoltification in anadromous O. mykiss from May to June, whereas there was no apparent increase in the values of non-anadromous O. mykiss. Plasma cortisol levels of anadromous O. mykiss were higher than levels of non-anadromous O. mykiss from April to June. In gill of non-anadromous O. mykiss, there were significant increases in mRNA levels of three types of CR in spring. Although there were significant seasonal variations of CR mRNA levels in gill of anadromous O. mykiss, they appear to be less clear than those variations in non-anadromous O. mykiss. In anadromous O. mykiss, significant elevations in mRNA levels of the three types of CR were observed especially in the spleen. In both preoptic area and basal hypothalamus of the brain, there were tendencies to increase in CR mRNA levels from spring to summer in both anadromous and non-anadromous O. mykiss. These results showed difference in regulation of CR gene expression between the two forms of O. mykiss for osmoregulatory, immune and central nervous systems.

Effects of chronic manipulation of adrenocorticotropic hormone levels in Chinook salmon on expression of interrenal steroidogenic acute regulatory protein and steroidogenic enzymes

The effects of chronic exposure to adrenocorticotropic hormone (ACTH) or the synthetic glucocorticoid dexamethasone (DEX) on the expression of genes involved in cortisol synthesis were examined using quantitative RT-PCR and immunohistochemistry. Juvenile Chinook salmon were treated with either ACTH via micro-osmotic pumps or with DEX via a lipid-based sustained release vehicle. Plasma cortisol levels were significantly elevated in ACTH-treated fish after 1 day, with a significant reduction in this effect with increasing treatment duration. ACTH also appeared to cause progressive hyperplasia of interrenal cells. Steroidogenic acute regulatory protein (StAR) and cytochrome P450 side chain cleavage enzyme (P450scc) transcripts but not 3β-hydroxysteroid dehydrogenase-isomerase (3β-HSD) or cytochrome P450 11β-hydroxylase (P45011β) transcripts in head kidneys significantly increased after 5 days of ACTH treatment. Significant linear relationships between plasma cortisol levels and transcript levels were identified at day 1 and day 5 for StAR, and day 5 for P450scc. Increased immunoreactivity for P450scc was observed in interrenal cells of ACTH-treated fish after 5 and 10 days. No effect of ACTH on 3β-HSD immunoreactivity was apparent at any time point. P45011β immunoreactivity was more intense after 5 days treatment with ACTH. DEX significantly reduced resting plasma cortisol levels and induced interrenal cell atrophy. Although no significant effect of treatment with DEX was found for any transcript, immunoreactivity for P450scc and P45011β appeared to be reduced. These results indicate that StAR and P450scc are subject to transcriptional regulation by chronic changes in ACTH levels.

Swimming suppresses hepatic vitellogenesis in European female silver eels as shown by expression of the estrogen receptor 1, vitellogenin1 and vitellogenin2 in the liver

BACKGROUND

When European silver eels (Anguilla anguilla) venture into the Atlantic Ocean for their 6,000 km semelparous spawning run to the Sargasso Sea, they are still in a prepubertal stage. Further sexual development appears to be blocked by dopaminergic inhibition of hypothalamus and pituitary activity. Recently, we found that swimming for several weeks in freshwater stimulated the incorporation of fat droplets in the oocytes. So, it was hypothesized that long term swimming in seawater would release the inhibition further and would also stimulate the production of vitellogenin by the liver.

METHODS

For this study a swim-flume was constructed to allow simulated migration of migratory female silver eels for 3 months (1,420 km) in natural seawater at 20 degrees C. Primers were designed for polymerase chain reactions to measure the mRNA expression of estrogen receptor 1 (esr1), vitellogenin1 (vtg1) and vitellogenin2 (vtg2) genes in the liver of European female silver eels.

RESULTS

In comparison to resting eels, swimming eels showed a diminished expression of esr1, vtg1 and vtg2 in the liver. They also had lower plasma calcium (Ca; indicative of vitellogenin) levels in their blood. This showed that vitellogenesis is more strongly suppressed in swimming than in resting eels. However, when eels were subsequently stimulated by 3 weekly carp pituitary extract injections, the expression of the same genes and plasma levels of Ca strongly increased in both groups to similar levels, thus equalizing the initial differences between resting and swimming.

CONCLUSIONS

It is concluded that vitellogenesis remains suppressed during resting and even more during swimming. The fact that swimming stimulates fat deposition in the oocytes but suppresses vitellogenesis indicates that these events are separated in nature and occur sequentially. Swimming-suppressed vitellogenesis may imply that in nature eels undergo vitellogenesis and final maturation near or at the spawning grounds.

Cloning of partial cDNAs for the chicken glucocorticoid and mineralocorticoid receptors and characterization of mRNA levels in the anterior pituitary gland during chick embryonic development

Virtually nothing is known about glucocorticoid receptor (GR) or mineralocorticoid receptor (MR) gene expression in any avian species. Here we report the cloning of partial cDNAs for chicken GR and MR. These partial cDNAs were used as probes to characterize expression of GR and MR mRNA and to identify the full-length transcripts within the chicken genome. Chicken GR and MR sequences predicted from the genome sequence were compared with those of representatives of other vertebrate classes. GR and MR genes are located on chicken chromosomes 13 and 4, respectively. Northern blotting and reverse transcription-polymerase chain reaction (RT-PCR) results indicate that GR and MR are widely expressed in many tissues. Characterization of mRNA levels in the anterior pituitary gland during chick embryonic development by quantitative real time RT-PCR demonstrates decreased MR and increased GR gene expression between embryonic days 12 and 17. Plasma levels of corticosteroids increased during this same period. This is the first study of GR and MR gene expression in any avian species and the first analysis of changes in pituitary MR gene expression during embryonic development of any species.

Peritoneal cavity phagocytes from the teleost sea bass express a glucocorticoid receptor (cloned and sequenced) involved in genomic modulation of the in vitro chemiluminescence response to zymosan

To gain further insight into the role of cortisol in fish innate immune responses, we cloned and sequenced a 2592bp cDNA from sea bass (Dicentrarchus labrax) peritoneal leukocytes (PCLs) encoding a glucocorticoid receptor (DlGR1). The deduced aminoacid sequence displayed that DlGR1 belong to a multigenic family of steroid hormone receptors, and exhibited high homology (80%) to the Burton's mouth breeder (Haplochromis burtoni) HbGR1. The DlGR1 functional domains presented homologies with those of several vertebrate species. In situ hybridization assay revealed that DlGR1 was expressed in macrophages and neutrophils from the peritoneal cavity. Since in a previous paper, sea bass PCL chemiluminescence response (CL) has been related to increased respiratory burst of phagocytes stimulated with zymosan, PCLs, pre-incubated in vitro with cortisol at various concentrations, were assayed for their CL response. Dose-dependent cortisol inhibitory effects, and significant competitive activity of a low concentration of mifepristone (RU486), a glucocorticoid-receptor blocker, supported that cortisol-GR interaction was involved in modulating CL response via a genomic pathway. Results also indicated that cortisol could be effective through an additional not-genomic way, and showed that high doses of RU486 exerted an inhibitory effect on PCL chemiluminescence activity.

Plasticity of nonapeptidergic neurosecretory cells in fish hypothalamus and neurohypophysis

The structure and function of nonapeptidergic neurosecretory cells (NP-NSC) are considered in terms of comparative morphology. Among NSC of different ergicity for NP-NSC the most characteristic involve massive accumulation and storage of neurohormonal products. Only in NP-NSC are the secretory cycles of functioning clearly expressed. Their highest reactivity is established during experimental and physiological stresses. In contrast, liberinergic, statinergic, and monoaminergic NSC, unlike NP-NSC, are characterized even in the "norm" by a constantly high level of extrusion processes. As signs of maximum NP-NSC plasticity, we consider the largest size of elementary neurosecretory granules, the diversity of secretion forms, and the maximum development of Herring bodies-clear manifestations of secretory cycles of functioning. In particular, phases of massive storage of neurosecretory granules in the extrusion cycle of NP-NSC neurosecretory terminals express accumulation of neurosecretory products. It is concluded that a particularly high degree of plasticity of NP-NSC is provided by their capability for functional reversion. This reversion is manifested first in the form of the restoration of the initial moderate level of functioning and especially in the accumulation of neurosecretory products. The reversion is considered an important mechanism providing a high degree of NSC plasticity. This degree turns out to be sufficient for participation of NP-NSC in the integration of fish reproduction. It is shown that NP-NSC are organized by the principle of a triad of the balanced system. This system consists of two alternative states: accumulation and release of neurosecretory products and the center of control of dynamics of their interrelations, the self-regulating center. In the latter, the key role is probably played by the Golgi complex.

The effects of cortisol administration on social status and brain monoaminergic activity in rainbow troutOncorhynchus mykiss

The hypothesis that circulating cortisol levels influence the outcome of social interactions in rainbow trout was tested. Juvenile rainbow trout Oncorhynchus mykiss were given a single intraperitoneal (i.p.)implant containing either cortisol (110 mg kg–1 fish), or cortisol plus the glucocorticoid receptor antagonist RU486 (mifepristone; 1100 mg kg–1 fish), and sampled after 5 days of social interactions with either a similar sized (<1.5% difference in fork length)or smaller conspecific (>5% difference). Within size-matched pairs of fish,cortisol treatment significantly increased the probability that the treated fish within each pair became subordinate, an effect that was abolished by simultaneous administration of RU486. Cortisol treatment also reduced the usual success of the larger fish within a pair to preferentially become dominant from 86% to 40% of pairs. To investigate one potential mechanism underlying the apparent effect of cortisol in predisposing trout to low social status, fish were treated with cortisol or cortisol+RU486 for 5 days, after which brain monoamines [5-hydroxytryptamine (5-HT); dopamine (DA)] and their major metabolites [5-hydroxyindolacetic acid (5-HIAA);3,4-dihydroxy-phenylacetic acid (DOPAC)] were measured. Significant increases of serotonergic activity ([5-HIAA]/[5-HT] ratio) were detected in the telencephalon with cortisol treatment, an effect that was eliminated by simultaneous administration of RU486. Also, cortisol treatment significantly decreased dopaminergic activity in the telencephalon. Somewhat surprisingly,the effects of cortisol treatment on monoaminergic activity in the hypothalamus were opposite to those in the telencephalon. Moreover, in no case did administration of RU486 abolish these effects. These results suggest that the effects of cortisol on social status in rainbow trout may be mediated via the modulation of central signaling systems and subsequent changes in behaviour and/or competitive ability, although the exact site of action in the brain remains uncertain.

Pathway-specific response to cortisol in the metabolism of catfish

Cortisol produced biochemical pathway-specific effects on metabolic enzymes and other macromolecules in the freshwater catfish, Clarias batrachus. Injection of cortisol increased 1.6-fold activity of citrate synthase (CS) in brain, liver and skeletal muscle of the fish over vehicle-injected control, while administration of metyrapone (a cortisol synthesis inhibitor) reduced CS activity by 52%. Cortisol treatment of metyrapone-treated fish induced CS activity by approximately 2.5-fold, which was blocked after administration of actinomycin D or cycloheximide. This shows de novo synthesis of CS to enhance aerobic capacity of fish. In contrast the activities of glucose-6-phosphate dehydrogenase (G6-PDH) and lactate dehydrogenase (LDH) increased in response to metyrapone and decreased after administration of cortisol in all the three tissues. The cortisol-mediated decrease in G6-PDH and LDH activities reflects reduction in biosynthetic and anaerobic capacity of fish. Administration of metyrapone significantly increased RNA/DNA ratio and protein but cortisol decreased these macromolecular contents in brain, liver and skeletal muscle. It shows cortisol-induced decrease in protein synthesis capacity of fish. The present study suggests that cortisol-induces catabolic and aerobic but inhibits anabolic and anaerobic processes in freshwater catfish. The cortisol-dependent metabolic responses may also be associated with the permissive effect of cortisol on other hormone(s) in fish.

Effects of cortisol on aggression and locomotor activity in rainbow trout

Noninvasive administration of cortisol through the diet resulted in relatively rapid (<1.5 h) and highly reproducible increases in plasma cortisol in rainbow trout, comparable to changes seen in fish subjected to substantial stress. Juvenile rainbow trout were reared in isolation for 1 week, before their daily food ration was replaced by a meal of cortisol-treated food corresponding to 6 mg cortisol kg(-1). All fish were observed for 30 min, beginning at 1 or 48 h following the introduction of cortisol-treated food. Additional cortisol (75% of the original dose on Day 2, and 50% on Day 3) was administered to the long-term cortisol-treated group. The resulting blood plasma concentrations of cortisol were similar in short- and long-term treated fish, and corresponded to those previously seen in stressed rainbow trout. Controls were fed similar food without cortisol. Half of the fish from each treatment group (controls and short- and long-term cortisol) were subjected to an intruder test (a smaller conspecific introduced into the aquarium), while half of the fish were observed in isolation. In fish challenged by a conspecific intruder, short-term cortisol treatment stimulated locomotor activity, while long-term treatment inhibited locomotion. Aggressive behavior was also inhibited by long-term cortisol treatment, but not by short-term exposure to cortisol. Cortisol treatment had no effect on locomotor activity in undisturbed fish, indicating that the behavioral effects of cortisol were mediated through interaction with other signal systems activated during the simulated territorial intrusion test. This study demonstrates for the first time that cortisol has time- and context-dependent effects on behavior in teleost fish.

Differences in behaviour between rainbow trout selected for high- and low-stress responsiveness

Two F1 lines of rainbow trout Oncorhynchus mykiss, divergent for plasma cortisol responsiveness, were generated by individual selection for post-stress cortisol values within the F0 generation. Adult females of the F1 generation were transferred to rearing in social isolation in observation tanks. After 6 days, locomotor activity in high-responding (HR) and low-responding (LR) individuals was quantified as time spent moving during a 20 min observation period. Behavioural observations were repeated the next day with a smaller conspecific intruder present in each observation tank. Differential hypothalamus–pituitary–interrenal axis activity in the two lines was subsequently confirmed by a standardised confinement stress test, which resulted in significantly higher plasma cortisol concentrations in HR than LR fish. HR fish displayed higher levels of locomotor activity than LR fish in the presence of an intruder, but not when in isolation. Aggressive behaviour towards the intruder was not seen, suggesting either a state-dependent lack of territorial aggression, or chronic stress in the experimental fish. A significantly higher incidence of feed intake was seen in LR trout when held in observation tanks (40 % versus 0 % of the fish took food when in isolation), suggesting that these fish acclimated more successfully to the experimental conditions than HR fish did. These results suggest that selection for stress responsiveness in salmonid fish leads to behavioural alterations, which are of potential importance to the performance of these fish in aquaculture rearing operations.

The hypothalamic-pituitary-interrenal axis and the control of food intake in teleost fish

Although environmental, social and physical stressors have been shown to inhibit food intake and feeding behavior in fish, little is known about the mechanisms that mediate the appetite-suppressing effects of stress. Since the hypothalamic-pituitary-interrenal (HPI) axis is activated in response to most forms of stress in fish, components of this axis may be involved in mediating the food intake reductions elicited by stress. Recent investigations into the brain regulation of food intake in fish have identified several signals with orexigenic and anorexigenic properties. Among these appetite-regulating signals are related neuropeptides that can activate the HPI axis, namely corticotropin-releasing factor (CRF) and urotensin I (UI). Central injections of CRF or UI, or treatments that result in an increase in hypothalamic CRF and UI gene expression, can elicit dose-dependent decreases in food intake that can be reversed by pre-treatment with a CRF-receptor antagonist. Evidence also suggests that cortisol, the end product of HPI activation in most fishes (i.e. Osteichthyes), may be involved in the regulation of food intake. Overall, while elements of the HPI axis may mediate some of the appetite-suppressing effects of stress, it is undetermined how either CRF-related peptides, cortisol, or other elements of the stress response interact with the complex circuitry of the hypothalamic feeding center.

Differential expression of corticotropin-releasing factor (CRF) and urotensin I precursor genes, and evidence of CRF gene expression regulated by cortisol in goldfish brain

Corticotropin-releasing factor (CRF) and urotensin I (UI) precursor cDNAs were cloned and sequenced from a goldfish brain cDNA library in order to investigate the distribution of CRF and UI mRNAs in goldfish brain and the regulation of CRF and UI gene expression. The CRF (966-bp) and UI (769-bp) cDNAs encode 163- and 146-amino acid precursors, respectively, and consist of a signal peptide sequence, a cryptic region, and a 41-amino acid mature peptide at the carboxy terminal. The deduced amino acid sequences of the CRF and UI peptides exhibit a sequence identity of 54%. Northern blot analysis revealed a single size of CRF (1.3 kb) and UI (2.0 kb) mRNAs, which are expressed in the telencephalon-preoptic, hypothalamic, optic tectum-thalamus, and posterior brain regions, but not in the pituitary. In addition, while the CRF gene is strongly expressed in the olfactory bulbs, the UI gene is not. In brain regions in which both genes are expressed, the mRNA levels of CRF were three- to sevenfold higher that those of UI. While the low expression levels of the UI gene prevented further analysis of its regulation, the regulation of CRF gene expression by cortisol was examined. In response to intraperitoneal implants of cortisol (300 microg/g BW) the level of CRF mRNA in the telencephalon-preoptic region decreased to 69% of control values at 6 and 24 h posttreatment. In sham-treated fish, in parallel with a transient injection stress-elicited increase in plasma cortisol, CRF mRNA levels declined to 72% of control value at 6 h postinjection and recovered after 24 h. Injection of the glucocorticoid antagonist, RU-486 (100 microg/g BW), prevented the reduction in CRF gene expression associated with the injection stress at 6 h and increased CRF mRNA levels to 145% of control value after 24 h. In contrast, the various implants had no effect on CRF mRNA levels in either the hypothalamus or the optic tectum-thalamus region. These results provide evidence of differential expression of the CRF and UI genes in hypothalamic and extrahypothalamic regions of goldfish brain. Furthermore, they demonstrate that stress levels of plasma cortisol can lead to a decrease in CRF gene expression that is mediated by glucocorticoid receptors in the telencephalon-preoptic region and give an indication of the regional specificity of the regulation of CRF gene expression by cortisol.

Glucocorticoid receptor immunoreactivity in neurons and pituitary cells implicated in reproductive functions in rainbow trout: a double immunohistochemical study

In order to identify the nature of the glucocorticoid receptor (GR)-expressing neurons and pituitary cells that potentially mediate the negative effects of stress on reproductive performance, double immunohistochemical stainings were performed in the brain and pituitary of the rainbow trout (Oncorhynchus mykiss). To avoid possible cross-reactions during the double staining studies, combinations of primary antibodies raised in different species were used, and we report here the generation of an antibody raised in guinea pig against the rainbow trout glucocorticoid receptor (rtGR). The results obtained in vitellogenic females showed that GnRH-positive neurons in the caudal telencephalon/anterior preoptic region consistently exhibited rtGR immunoreactivity. Similarly, in the anterior ventral preoptic region, a group of tyrosine hydroxylase-positive neurons, known for inhibiting gonadotropin (GTH)-2 secretion during vitellogenesis, was consistently shown to strongly express GR. Finally, we show that a large majority of the GTH-1 (FSH-like) and GTH-2 (LH-like) cells of the pituitary exhibit rtGR immunoreactivity. These results indicate that cortisol may affect the neuroendocrine control of the reproductive process of the rainbow trout at multiple sites.

Rainbow trout glucocorticoid receptor overexpression in Escherichia coli: production of antibodies for western blotting and immunohistochemistry

Fragments of cDNA that encode the N-terminal and DNA-binding domains (DBD) of the rainbow trout glucocorticoid receptor (rtGR) were expressed in Escherichia coli as fusion proteins with glutathione-S-transferase (GST). The fusion proteins induced by IPTG could readily be detected as 45- and 40-kDa bands, respectively, in crude extracts, as well as in proteins purified on glutathione-agarose. These purified hybrid proteins were used to immunize rabbits. The antisera produced were tested for specificity by Western blot analysis using extracts from COS-1 cells transfected with an rtGR expression vector and from trout liver cells. The antisera raised against the DBD domain did not detect any bands on Western blots, even at low antiserum dilution. However, the purified DBD fusion protein specifically bound GRE-containing DNA fragments in gel-shift assays, and the retarded complexes were supershifted by these antibodies. The antisera raised against the N-terminal domain consistently detected two protein bands at 104 and 100 kDa in the two cell extracts and allowed specific immunohistochemical staining in fish brain and pituitary. For the first time in fish, these antibodies will allow analysis of GR expression in different cortisol target tissues.