Stress In Tilapia Research Articles

Proline metabolism is essential for alkaline adaptation of Nile tilapia (Oreochromis niloticus)

BackgroundSaline-alkaline water aquaculture has become a key way to mitigate the reduction of freshwater aquaculture space and meet the increasing global demand for aquatic products. To enhance the comprehensive utilization capability of saline-alkaline water, it is necessary to understand the regulatory mechanisms of aquatic animals coping with saline-alkaline water. In this study, our objective was to elucidate the function of proline metabolism in the alkaline adaptation of Nile tilapia (Oreochromis niloticus).ResultsExpose Nile tilapia to alkaline water of different alkalinity for 2 weeks to observe changes in its growth performance and proline metabolism. Meanwhile, to further clarify the role of proline metabolism, RNA interference experiments were conducted to disrupt the normal operation of proline metabolic axis by knocking down pycr (pyrroline-5-carboxylate reductases), the final rate-limiting enzyme in proline synthesis. The results showed that both the synthesis and degradation of proline were enhanced under carbonate alkalinity stress, and the environmental alkalinity impaired the growth performance of tilapia, and the higher the alkalinity, the greater the impairment. Moreover, environmental alkalinity caused oxidative stress in tilapia, enhanced ion transport, ammonia metabolism, and altered the intensity and form of energy metabolism in tilapia. When the expression level of the pycr gene decreased, the proline metabolism could not operate normally, and the ion transport, antioxidant defense system, and energy metabolism were severely damaged, ultimately leading to liver damage and a decreased survival rate of tilapia under alkalinity stress.ConclusionsThe results indicated that proline metabolism plays an important role in the alkaline adaptation of Nile tilapia and is a key regulatory process in various biochemical and physiological processes.

Transcriptome analysis reveals the molecular mechanisms of heterosis on thermal resistance in hybrid tilapia (Oreochromi niloticus ♀×Oreochromi aureus ♂)

Heterosis is a widely observed biological phenomenon in fishes, in which hybrids exhibit superior traits to their parents. However, the underlying molecular mechanisms for heterosis are not well understood. Tilapia is an economically important aquaculture resource with high commercial value, and the phenomenon of high temperature tolerance in hybrid tilapia (Oreochromis niloticus ♀ × Oreochromis aureus ♂, AN) has been observed in recent studies. Uncovering the molecular mechanisms underlying this phenomenon could provide theoretical support for heterosis in thermal tolerance. To investigate the gene expression patterns and alternative splicing events associated with heat stress in tilapia, RNA Sequencing was conducted at control temperature (28 °C) and heat stress temperatures (36 °C and 39 °C). A total of 280, 260, and 261 genes with significantly divergent alternative splicing events were identified in O.niloticus (NL), O.aureus (AR), and O. niloticus ♀ × O.aureus ♂ (AN), respectively. In the heat stress groups (39 °C groups), 83.59% (23779 out of 28446) of the expressed genes showed non-additive expression patterns, and the highest proportion (35.14%) of these genes exhibited over-dominance expression patterns. KEGG pathway enrichment analysis showed that the overlapping genes among common differentially expressed genes (DEGs) and non-additively expressed genes (NAGs) were significantly enriched in apoptosis, adipocytokine signaling, and insulin signaling pathways. Furthermore, among these overlapping genes, 32 genes had undergone alternative splicing events in AN, which may play an important role in the thermal resistance of hybrid tilapia. These findings shed light on the physiological regulation mechanisms that allow hybrid tilapia to exhibit better thermal resistance than their parents. They provide important insights into the molecular basis of heterosis in tilapia and have potential implications for improving aquaculture practices.

Sub-chronic exposure of Oreochromis niloticus to environmentally relevant concentrations of smaller microplastics: Accumulation and toxico-physiological responses

This study assesses the accumulation and toxic effects of environmentally relevant concentrations (0.01, 0.1 and 1 mg/L) of polystyrene MPs (1 µm) in Oreochromis niloticus (Nile tilapia) for 14 days. The results showed that 1 µm PS-MPs accumulated in the intestine, gills, liver, spleen, muscle, gonad and brain. RBC, Hb and HCT showed a significant decline, while WBC and PLT showed a significant increase after the exposure. Glucose, total protein, A/G ratio, SGOT, SGPT and ALP showed significant increments in 0.1 and 1 mg/L of PS-MPs treated groups. The increase in cortisol level and upregulation of HSP70 gene expression in response to MPs exposure indicate MPs-mediated stress in tilapia. MPs-induced oxidative stress is evident from reduced SOD activity, increased MDA levels and upregulated P53 gene expression. The immune response was enhanced by inducing respiratory burst activity, MPO activity and serum TNF-α and IgM levels. MPs exposure also led to down-regulation of CYP1A gene and decreased AChE activity, GNRH and vitellogenin levels, indicating the toxicity of MPs on the cellular detoxification mechanism, nervous and reproductive systems. The present study highlights the tissue accumulation of PS-MP and its effects on hematological, biochemical, immunological and physiological responses in tilapia with low environmentally relevant concentrations.

Prediction and Experimental Validation of a New Salinity-Responsive Cis-Regulatory Element (CRE) in a Tilapia Cell Line.

Transcriptional regulation is a major mechanism by which organisms integrate gene x environment interactions. It can be achieved by coordinated interplay between cis-regulatory elements (CREs) and transcription factors (TFs). Euryhaline tilapia (Oreochromis mossambicus) tolerate a wide range of salinity and thus are an appropriate model to examine transcriptional regulatory mechanisms during salinity stress in fish. Quantitative proteomics in combination with the transcription inhibitor actinomycin D revealed 19 proteins that are transcriptionally upregulated by hyperosmolality in tilapia brain (OmB) cells. We searched the extended proximal promoter up to intron1 of each corresponding gene for common motifs using motif discovery tools. The top-ranked motif identified (STREME1) represents a binding site for the Forkhead box TF L1 (FoxL1). STREME1 function during hyperosmolality was experimentally validated by choosing two of the 19 genes, chloride intracellular channel 2 (clic2) and uridine phosphorylase 1 (upp1), that are enriched in STREME1 in their extended promoters. Transcriptional induction of these genes during hyperosmolality requires STREME1, as evidenced by motif mutagenesis. We conclude that STREME1 represents a new functional CRE that contributes to gene x environment interactions during salinity stress in tilapia. Moreover, our results indicate that FoxL1 family TFs are contribute to hyperosmotic induction of genes in euryhaline fish.

Genome-wide identification and expression analysis of Bcl-2 gene family under low-temperature stress in tilapia (Oreochromis niloticus)

Low temperature stress can lead to variety of changes, including apoptosis in tilapia (Oreochromis niloticus). The B cell lymphoma-2 (Bcl-2) gene family plays an important role in the process of apoptosis. The present study conducted genome-wide characterization of the Bcl-2 family genes in tilapia and their mRNA expression profiles were analysed in different tissues of tilapia under the low temperature stress (10°C). Twenty-four Bcl-2 family genes were identified, containing 2~8 exons. These genes were classified into two subfamilies (Bcl-2 homologs and BH3-only) based on their conserved domains. Besides, these BCL-2 proteins in tilapia possess at least one of the four conserved BH domains. The phylogenetic analysis showed that the Bcl-2 family genes did not aggregate by species, demonstrating sequence conservation of different types of Bcl-2 family members. Real-time quantitative PCR (RT-qPCR) analysis showed that Bcl-2 family genes were broadly expressed in different tissues of tilapia. When reared at 10 °C, the transcriptional expression levels of most of anti-apoptotic Bcl-2 homologs subgroup members and other BH3-only subgroup members in most tissues of tilapia were higher than those at 30°C. However, most of other Bcl-2 family members revealed a lower expression. The results suggested that hypothermia had significantly induced apoptotic in tilapia.

A mixture of Chinese herbs and a commercial probiotic Bacillus species improves hemato-immunological, stress, and antioxidant parameters, and expression of HSP70 and HIF-1α mRNA to hypoxia, cold, and heat stress in Nile tilapia, Oreochromis niloticus

A 56-day feeding trial was performed to evaluate the effect of herbal-probiotic supplements comprising a mixture of Chinese herbs (CH) (i.e., Astragalus membranaceus, Angelica sinensis, and Crataegus hupehensis) and or commercially prepared probiotic Bacillus species (BS) (i.e., spores of B. subtilis and B.licheniformis) on the physiological response of Nile tilapia. Fish (50 ± 2 g) were fed six diets in duplicates including a control (CT) enriched with BS [at 10 g kg−1], CH [at 10 g kg−1], CHBS1 [CH at 3 g kg−1 and BS at 7 g kg−1], CHBS2 [CH at 5 g kg−1 and BS at 5 g kg−1], and CHBS3 [CH at 7 g kg−1 and BS at 3 g kg−1] for a period of 56 weeks. At the end of the feeding trial, some hemato-immunological parameters and antioxidant indices, as well as stress-related genes in tilapia, were evaluated; and afterwards exposed to hypoxia for 18 h and to cold and heat each for 2 h. The results showed increased hematocrit, red blood cells, haemoglobin, and white blood cells, in fish blood in most of the herbal-probiotic groups compared to the control group (P < 0.05). Lowered cortisol, glucose, as well pyruvate kinase, alanine aminotransferase, aspartate aminotransferase, and lactate dehydrogenase, and increased myeloperoxidase, total antioxidant capacity, superoxide dismutase, and catalase activities were observed in the serum of all herbal-probiotic groups over the control group (P < 0.05). Besides, all herbal-probiotic groups showed increased expression of heat shock protein 70 and hypoxia-inducible factor 1 alpha mRNAs in the livers of fish (P < 0.05). Among the herbal-probiotic groups, tilapia fed with CHBS2 diet showed enhanced values in most of the indices measured under the different stress conditions (P < 0.05). These findings suggest CHBS2 supplement as a prophylactic against the suppressive effects of stress in tilapia.

Genome-Wide Characterization of Alternative Splicing Events and Their Responses to Cold Stress in Tilapia.

Alternative splicing (AS) is an important post-transcriptional regulatory mechanism for cells to generate transcript variability and proteome diversity. No systematic investigation of AS events among different tissues in response to stressors is available for tilapia currently. In this study, AS among different tissues was identified and the cold stress-related AS events were explored in a Nile tilapia (Oreochromis niloticus) line based on 42 RNA-seq datasets using a bioinformatics pipeline. 14,796 (82.76%; SD = 2,840) of the expression genes showed AS events. The two most abundant AS types were alternative transcription start site (TSS) and terminal site (TTS) in tilapia. Testis, brain and kidney possess the most abundant AS gene number, while the blood, muscle and liver possess the least number in each tissue. Furthermore, 208 differentially alternative splicing (DAS) genes in heart and 483 DAS in brain in response to cold stress. The number of AS types for alternative exon end, exon skipping and retention of single intron increased significantly under cold stress. GO enrichment and pathway overrepresentation analysis indicated that many DAS genes, e.g., genes in circadian clock pathway, may influence expression of downstream genes under cold stress. Our study revealed that AS exists extensively in tilapia and plays an important role in cold adaption.

Determination of Effective Dosage of Phyllanthus niruri to Modulate Stress in Tilapia, Oreochromis niloticus

A study was performed to determine the effects of supplementing Phyllanthus niruri, an herbal medicinal plant found worldwide and its optimal dosage to modulate stress in tilapia. Phyllanthus niruri-extract from three different sources (suppliers) was given to study fish through supplementation to commercial fish feed at the rate of 1%, 2%, 5%, and 10% concentration. Administration of 5% Phyllanthus niruri from Source B significantly reduced blood glucose in fish.

The protective role of l-carnitine against cylindrospermopsin-induced oxidative stress in tilapia (Oreochromis niloticus)

Cylindrospermopsin (CYN) is one of the most important cyanotoxins in terms of both human health and environmental quality and is produced by several different species of cyanobacteria, including Aphanizomenon ovalisporum. The principal mechanisms of action of CYN involve inhibition of protein and glutathione synthesis. In addition, CYN-mediated genotoxicity results from DNA fragmentation. The results of both in vivo and in vitro studies suggest that oxidative stress also plays a significant role in CYN pathogenesis in fish. We investigated the protective effects of l-carnitine (LC) pre-treatment on A. ovalisporum-induced oxidative stress in cells containing CYN and deoxy-CYN, or pure standard CYN, in tilapia (Oreochromis niloticus) that had been acutely exposed via oral administration. Various oxidative stress markers, including lipid peroxidation (LPO), protein oxidation, DNA oxidation, and the ratio of reduced glutathione to oxidised glutathione (GSH/GSSG), and the activities of NADPH oxidase, superoxide dismutase (SOD), catalase (CAT), and gamma-glutamyl-cysteine synthetase (γ-GCS), were evaluated in the livers and kidneys of fish in the absence and presence of 400 or 880mgLC/kgfish/day during a 21 day period prior to CYN-intoxication. The results of our study demonstrated for the first time the beneficial antioxidant effects of LC dietary supplementation on oxidative stress status in fish. No pro-oxidant effects were detected at any of the LC doses assayed, suggesting that LC is a chemoprotectant that reduces hepatic and renal oxidative stress and may be effective when used for the prophylaxis and treatment of CYN-related intoxication in fish.

Atividade da catalase e da lactato desidrogenase em tilápias submetidas a estresse de confinamento: efeito da cor do ambiente

Avaliaram-se os efeitos da cor do ambiente sobre o crescimento e a atividade da enzima antioxidante catalase (CAT) e da lactato desidrogenase (LDH) em tilápias do Nilo (n=24; 36,2±3,6g). Oito exemplares foram mortos para determinação da atividade basal das enzimas e os demais permaneceram isolados durante 14 dias sob espectro de luz branca ou azul (n=8 peixes/tratamento). A seguir os peixes foram submetidos a um estresse diário de confinamento de 90 minutos (15° ao 28° dia) e pesados semanalmente para cálculo da taxa de crescimento específico (TCE). A TCE negativa confirmou que o confinamento provocou estresse nos peixes, independentemente da cor do ambiente. O aumento da atividade da LDH no músculo vermelho dos peixes mantidos sob luz branca ou azul indicou mudança do metabolismo aeróbio para anaeróbio. O estresse reduziu a atividade da CAT no músculo branco dos peixes mantidos sob a luz branca ou azul. Na musculatura vermelha, esta redução ocorreu apenas nos animais mantidos sob a luz branca. O confinamento aumenta os processos metabólicos anaeróbios e é adequado para estudos sobre os efeitos do estresse. O espectro de luz azul não evita a redução do crescimento e a demanda energética anaeróbia em situações de estresse, mas preserva a atividade da CAT, contribuindo para o bem-estar da tilápia.

Role of catalase on the hypoxia/reoxygenation stress in the hypoxia-tolerant Nile tilapia

The specific contribution of each antioxidant enzyme to protection against the reoxygenation-associated oxidative stress after periods of hypoxia is not well understood. We assessed the physiological role of catalase during posthypoxic reoxygenation by the combination of two approaches. First, catalase activity of Nile tilapias (Oreochromis niloticus) was 90% suppressed by intraperitoneal injection of 3-amino-1,2,4-triazole (ATZ, 1g/kg). In ATZ-injected fish, liver GSH levels, oxidative stress markers, and activities of other antioxidant enzymes remained unchanged. Second, animals with depleted catalase activity (or those saline-injected) were subjected to a cycle of severe hypoxia (dissolved O(2) = 0.28 mg/l for 3 h) followed by reoxygenation (0.5 to 24 h). Hypoxia did not induce changes in the above-mentioned parameters, either in saline- or in ATZ-injected animals. Reoxygenation increased superoxide dismutase activity in saline-injected fish, whose levels were similar to ATZ-injected animals. The activities of glutathione S-transferase, selenium-dependent glutathione peroxidase, and total-GPX and the levels of GSH-eq, GSSG, and thiobarbituric acid reactive substances remained unchanged during reoxygenation in both saline- and ATZ-injected fish. The GSSG/GSH-eq ratio in ATZ-injected fish increased at 30 min of reoxygenation compared with saline-injected ones. Reoxygenation also increased carbonyl protein levels in saline-injected fish, whose levels were similar to the ATZ-injected group. Our work shows that inhibition of liver tilapia catalase causes a redox imbalance during reoxygenation, which is insufficient to induce further oxidative stress. This indicates the relevance of hepatic catalase for hypoxia/reoxygenation stress in tilapia fish.

Rapid changes in plasma cortisol, osmolality, and respiration in response to salinity stress in tilapia ( Oreochromis mossambicus)

We elucidated a time course for cortisol release in tilapia as it corresponds to changes in plasma osmolytes and respiration. Following exposure of freshwater (FW) tilapia to 25‰ seawater (SW), we measured plasma osmolality, [Na +], [K +], [Cl −], hematocrit, cortisol concentration, oxygen-consumption rate (ṀO2), and ventilation frequency over 5 days and compared them to FW control fish. Cortisol increased rapidly by 3 h and remained elevated for 3 days. Plasma osmolality, [Na +], and [Cl −] were elevated at 6–8 h, peaked 24 h following SW exposure, and then decreased to near-FW levels by 3 days. ṀO2 increased at 24 h post-SW exposure relative to FW, while ventilation frequency increased by 3 h. Overall, we interpret changes in cortisol as resulting from a change in salinity, in contrast to changes in plasma solute concentrations that could be due to adjustments resulting from the fish's cortisol response as it faces osmoregulatory distress. Increases in oxygen-consumption rate at 24 h and ventilation frequency at 3 h are likely as a result of the cellular stress response occurring during salinity stress. No significant changes in blood hematocrit were observed, which suggests that tilapia are capable of rapidly counteracting dehydration during acute hyperosmotic stress.

Time-Course Variations of DNA Damage and Biomarkers of Oxidative Stress in Tilapia (Oreochromis niloticus) Exposed to Effluents From a Swine Industry

DNA damage (Comet assay), lipoperoxidation levels (TBARS), and several biomarkers of oxidative stress such as catalase (CAT), glutathione reductase (GR), glutathione peroxidase (GPx), glutathione S-transferase (GST), and contents of reduced (GSH) and total (TG) glutathione were measured in liver and blood (Comet assay) of tilapia (Oreochromis niloticus) exposed for 7, 15, 30 (subchronic exposure), 60, and 90 days (chronic exposure) to two treatment lagoons of a swine-processing plant, the first an anaerobic lagoon and the second a final treatment lagoon. After the 15th day, TBARS increased in fish exposed to both lagoons, decreased on the 30th day, and on the 90th day remained similar to controls. Fish exposed subchronically and chronically to both effluents showed consistently greater DNA damage. The CAT and GPx activities showed similar profiles and were induced only during the first week and during the first and second months. GST activity was induced throughout the experimental period. On the other hand, GR activities showed inverted profiles, with progressively decreased activities in the liver of fish exposed to the anaerobic lagoon, and progressively increased activities in fish exposed to the final lagoon. GSH showed higher contents in liver after 60 and 90 days of exposure to the final lagoon. GSSG contents were higher in fish exposed to the final lagoon throughout the experimental period. After 15 days, tilapia exposed to both lagoons showed enhanced total glutathione contents. The hepatic antioxidant system and biomarkers of oxidative stress such as DNA fragmentation and TBARS contents of tilapia exposed to both lagoons presented biphasic profiles. These changes in the antioxidant status also indicate that the industrial treatment is not adequate to avoid damaging environmental effects.

Streptococcus agalactiae Vaccination and Infection Stress in Nile Tilapia, Oreochromis niloticus

The stress response following intraperitoneal (IP) injection of a non-adjuvant Streptococcus agalactiae vaccine in cultured warmwater Nile tilapia, Oreochromis niloticus, has not been investigated. Further, little or no information is available on stress following S. agalactiae infection and what effect, if any, vaccination has on susceptibility to infection. The objective of this study was to develop preliminary information on the associations between vaccination, stress, and infection. Blood glucose levels were used to evaluate stress in the fish at different time intervals following vaccination and challenge with S. agalactiae. Blood glucose levels were measured in vaccinates and controls at 0, 2, 6, 24 hours, and 28 days post-immunization (0 hours pre-challenge), and at 2,6, 24,48,72, and 312 hours following challenge with 1.5 × 104 colony forming units (CFU) of S. agalactiae/fish. Significant increases in blood glucose levels were observed only in association with the injection of the vaccine and at 2 hours after injection. After S. agalactiae challenge, both controls and vaccinates had significantly (P < 0.05) higher blood glucose values at 2, 24,48, and 72 hours than at 0 hours. However, blood glucose levels in vaccinates were significantly (P < 0.05) lower than the controls at 24, 48, and 312 hours. Blood glucose levels and mortality of the infected controls were significantly correlated (r2 = 0.9236, P = 0.0134). The cumulative mortality of the vaccinates and controls was 10% and 60% after 13 days post-challenge, respectively. The relative percent survival (RPS) was 83.4. Our results indicate that the vaccine was efficacious against S. agalactiae and induced short-term stress in tilapia. These preliminary results also suggested, for the first time, that vaccination may significantly reduce the infection stress associated with S. agalactiae infection in tilapia.

Ontogeny of corticotropin-releasing factor and of hypothalamic–pituitary–interrenal axis responsiveness to stress in tilapia ( Oreochromis mossambicus; Teleostei)

The ontogeny of the corticotropin-releasing factor (CRF) system and of the ability of the hypothalamic–pituitary–interrenal (HPI) axis to respond to stressors (capture or confinement), or to cortisol treatment was investigated in tilapia ( Oreochromis mossambicus). In 2 days post hatching (dph) larvae, the first developmental stage used for immunohistochemistry, CRF-immunoreactivity (ir) was observed in the nucleus preopticus (npo), and in two hypothalamic nuclei (nlt and nrl). In this stage, CRF- and AVT-ir was found in the neural part of the pituitary, and endocrine cells in the pars distalis and pars intermedia contained POMC-derived peptides. In the ventral telencephalon, CRF-ir cells were first observed 5 dph, whereas projections from these cells into the anterior part of the latero-dorsal telencephalon (Dla) from 7 dph onwards. CRF, ACTH, α-MSH, and cortisol were quantified by radioimmunoassays in homogenates of the anterior-cranial region of the larvae containing brain, pituitary, and headkidneys. CRF contents increased from 43 ± 3 to 1070 ± 70 pg/larvae between 5 and 110 dph. Larvae of age 5, 12, 24, and 42 dph were captured sequentially from a group. All life stages were able to rapidly increase their cortisol content in response to this stressor (ANOVA: P < 0.001). Overall, the developmental stage affected cortisol content (ANOVA: P < 0.001), but developmental stage did not influence the cortisol reaction to stress (ANOVA: P > 0.162). Whole brain CRF content did not change during the 20 min stress period and the relationship between CRF-producing neurons and the initial HPI stress response in early life stages remains to be established. Cortisol feeding of 18 and 29 dph larvae for periods ranging from 2 to 24 days resulted in elevations of the CRF content ( P < 0.003) in comparison to controls. In 18 dph larvae cortisol feeding abolished the cortisol response to capture stress as observed in control fed larvae ( P < 0.008). We propose that cortisol induced upregulation of CRF takes place in the telencephalon and is restricted to a time period during larval development, characterised by the absence of glucocortoid receptor (GR) expression in the telencephalic Dm region in these larvae. Finally, the stress response to 24 h confinement was compared between saltwater adapted and freshwater adapted juveniles (age 77 dph). Confinement stress (24 h) affected cortisol and CRF content (ANOVA: P < 0.001, P < 0.008, respectively), but not ACTH content. Interactions were observed between salinity and confinement regarding cortisol and α-MSH contents (ANOVA: P < 0.02), but not regarding CRF and ACTH contents. The increase in cortisol levels induced by confinement was remarkably high in freshwater adapted larvae (five times higher than in saltwater adapted larvae). Regarding the cortisol response it is concluded that during and after the period of mouth breeding tilapia larvae respond to capture stress in a similar fashion (onset and height) as adults. Previously, we reported that the initial plasma cortisol response to capture stress in adult tilapia occurred independently from changes in plasma ACTH levels. The current finding that also brain CRF contents do not alter during the initial cortisol response in larvae further indicates that the initial cortisol response in this species may be regulated independently from CRF and ACTH.

Metabolic Responses Associated with Confinement Stress in Tilapia: The Role of Cortisol

We examined the effect of 2- or 24-hr confinement stress and cortisol treatment on plasma cortisol, glucose, lactate and free amino acids concentration and hepatic glycogen content and activities of certain enzymes involved in the intermediary metabolism in tilapia ( Oreochromis mossambicus). Confinement of tilapia for 2 or 24 hr resulted in significantly higher plasma cortisol, glucose, lactate (2 hr), total and some of the free amino acids concentration (especially at 24 hr) and hepatic pyruvate kinase (PK), phosphoenolpyruvate carboxykinase (PEPCK; 24 hr) and lactate dehydrogenase activities (24 hr). Hepatic glycogen content was lower at 2 and 24 hr in the confined fish compared with the unstressed fish. Tilapia given cortisol implants (50 mg·kg −1 body wt) had significantly higher plasma cortisol, glucose and some of the free amino acids concentration and hepatic PEPCK and aspartate aminotransferase activities, whereas PK activity ratio was significantly lower compared with the sham group. The results suggest that glucose production 2 hr after confinement may be due to glycogenolysis, whereas the maintenance of higher glucose at 24 hr after confinement is essentially due to gluconeogenesis. Furthermore, the changes in plasma metabolites and hepatic enzyme activities with cortisol implantation suggest that cortisol plays a role in the metabolic adjustment to 24-hr confinement stress in tilapia.

Adrenocorticotropic Hormone in Relation to Interrenal Function during Stress in Tilapia ( Oreochromis mossambicus)

This study examines ACTH-like immunoreactivity in the pituitary pars distalis and pars intermedia of the freshwater teleost Oreochromis mossambicus (tilapia). Two products (tACTH A and tACTH B) were present in both lobes. These two products also accounted for the majority of the ACTH i.r. when in vitro pars distalis incubation medium was analyzed by HPLC. In a homologous bioassay the two tilapia ACTH-like molecules and human ACTH 1-39 possessed similar corticotropic potency. The peptides were quantified using a newly validated radioimmunoassay, which was also used to measure ACTH in plasma of unstressed and stressed fish. Short-term (<12 min) stress rapidly increased plasma cortisol, reaching levels of around 300 ng/ml in 5 min. Surprisingly, this initial elevation was not accompanied by a rise in plasma ACTH levels. A more prolonged (3 hr) confinement in pairs resulted in high plasma cortisol and ACTH levels in one fish of every pair. The second fish had control ACTH levels and only marginally elevated cortisol levels. Therefore, in this species social interactions seem to influence the reaction of the pituitary-interrenal axis to stress. The short-term cortisol response to disturbance could be abolished completely by pretreating fish in vivo with cortisol for 48 hr. This treatment did not alter circulating ACTH levels. It is concluded that tilapia did not rely on circulating ACTH for a rapid elevation of plasma cortisol levels. Both neuronal mechanisms and cortisol feedback may regulate the pituitary-interrenal axis at the level of the interrenal.

Prolactin and acid stress in the teleost Oreochromis (formerly Sarotherodon) mossambicus

Prolactin is probably implicated in the control of physiological adaptation to sublethal acid stress in tilapia. Exposure of fish to water at pH 3 caused death within 72 hr, which was associated with pronounced hemodilution. At pH 4 mortality was low, although a substantial and rapid decrease in plasma osmolality, plasma Na +, and plasma total Ca was observed. These effects were at least partly due to increased permeability of the gills for water and ions. After a few days at pH 4, restoration of plasma osmolality, Na +, and total Ca was noticeable. Control levels were reached after 5 days for Ca, and after 10 days for osmolality, Na +, and branchial osmotic water permeability. Prolactin secretion increased markedly during acid exposure, as was established by morphometrical and biochemical methods. In tilapia, administration of prolactin is known to raise plasma osmolality, Na +, and plasma total Ca. This hormone further has been shown to reduce branchial osmotic water permeability. It is concluded therefore that the restoration of plasma electrolytes and branchial osmotic water permeability during chronic acid stress are causally connected with the observed stimulation of prolactin secretion.