The role of thyroid hormones in stress response of fish

The role of NMDA receptors in fish stress response: Assessments based on physiology of the caudal neurosecretory system and defensive behavior

Stress strongly influences the physiology and behavior of animals, and leads into a pathological condition and disease. NMDA receptors (NMDARs) play a crucial role in the modulation of neural activity. To understand the role of NMDARs in fish stress response, we used NMDARs agonist aspartate to test the functional role of its input on the Dahlgren cell population in the caudal neurosecretory system (CNSS) of the olive flounder. In addition, the effect of the NMDARs antagonist D-AP5 on the expression of genes of the main secretory products of the CNSS after stress was investigated by using qPCR technology and the effect of the NMDARs antagonist D-AP5 on post-stress behavior was explored by behavioral methods. Ex vivo electrophysiological experiments showed that the NMDARs agonist aspartate enhanced the firing frequency of Dahlgren cells. Additionally, aspartate treatment increased the incidence of cells exhibiting bursting firing pattern, this result is corroborated by the observed upregulation in the expression of ion channels and major hormone genes in the CNSS. Furthermore, the excitatory influence of aspartate was effectively counteracted by NMDARs antagonist D-AP5. Interestingly, NMDARs antagonist D-AP5 treatment also significantly decreased the plasma cortisol levels and the expression of CRH, UI, and UII in CNSS after acute stress. Treatment with D-AP5 effectively attenuated the stress response, as evidenced by alterations in respiratory metabolism, sand-burying behavior, swimming distance, simulated capture, and escape response. In conclusion, modulation of Dahlgren cell excitability in the CNSS by NMDARs contributes to the regulation of the stress response, NMDARs antagonist D-AP5 can effectively suppress stress response in flounder by regulating the stress hormone expression and secretion. CLINICAL TRIAL REGISTRATION: Project code SHOU-DW-2022-032.

Gene expression in the hypothalamic-pituitary-thyroid axis in Seriola rivoliana early larvae development at different temperatures

We analyzed the expression of genes involved in the hypothalamic-pituitary-thyroid axis (HPT-axis) in the longfin yellowtail Seriola rivoliana early larva, including temperature effects (22, 26 and 28 °C) and days of development (day one, day two, and day six after hatching). We aimed to determine if egg and larval incubation at different temperatures could disrupt this critical endocrine axis, which, in an aquaculture context, it could provoke mortality during early metamorphosis. There was a significant interaction between temperature and developmental timing on the relative expression of thyrotropin releasing hormone (trh). Larvae at 22 °C was the longest and increased more trh expression than larvae at higher temperatures. Interestingly, thyrotropin stimulating hormone (tsh) was highly expressed after hatching. Subsequently, it was downregulated at any temperature at least until day four, suggesting a temporal inhibition of the HPT axis. Therefore, we suggest that tsh-binding (tshr) to follicles should have occurred from hatching, creating a further "cascade effect" of upregulation of larval thyroglobulin (tg) from day two in a temperature-dependent manner. Consequently, new thyroid hormones should have been produced after yolk sac absorption. The above may indicate a narrow window of larval survival, where the larval transition from endogenous to exogenous feeding would depend on the correct timing to synthesize tg. Temperature significantly affected the expressions of deiodinase 1 (dio1-downregulated) and deiodinase 2 (dio2-upregulated) after hatching. The expressions of thyroid receptors alpha (trα) and beta (trβ) remained constant after hatching without significant effects of temperature and days of development. Then, the differential expression on day six showed that all HPT-axis transcripts increased their expressions as larvae developed, which suggested a functional HPT. Finally, there was no evidence that any temperature would disrupt the endocrine's larval axis, which indicated that the longfin yellowtail has a wide temperature adaption. Nevertheless, based on tg upregulation, we suggest that larvae should be maintained around 25-26 °C after hatching for a better chance of survival and development.

Effects of L-NAME and air exposure on mitochondrial energetic markers, thyroid hormone receptor/regulator system and stress/ease-responsive receptor expression in the brain/gut axis of zebrafish

As a signal molecule, nitric oxide (NO) has several physiological actions in fish. However, the action of NO on the brain/gut axis, a classic inter-organal axis that bridges the gastrointestinal tract and the CNS, still requires more understanding. The short-term in vivo action of a NO inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME), on mitochondrial energetic markers and the receptor expression of thyroid hormone (TH) and neuroendocrine hormones involved in stress/ease response was tested in the brain/gut axis of zebrafish exposed to either in non-stressed or air-exposed condition. L-NAME treatment decreased the NO content in brain and gut segments in non-stressed fish but rose upon L-NAME treatment in air-exposed fish that corresponded with the activation of inos, nnos, hif1a and hif1an transcript expressions. The brain/gut segments that showed spatial and differential sensitivity to L-NAME, modified the transcript expression patterns of stress (adra2da, adrb1, nr3c2)- and ease-responsive (htr2b, slc6a4a, mtnr1aa) hormone receptors. The expression pattern of the TH receptor/regulator system (thra, thrb, dio1, dio2, dio3) becomes more active in gut segments than brain segments upon L-NAME challenge in stressed zebrafish. The data provide evidence for a novel role of NO as an integrator of brain/gut axis segments in zebrafish, where the endogenously produced NO in mid-brain/posterior-gut axis aligns together upon air-exposure stress, providing a lead role to the posterior gut that activates and directs the neuroendocrine receptor expressions of stress/ease responsive genes. The data further invites studies exploring the therapeutic potential of L-NAME in this biomedical model to control the brain/gut axis segments.

The susceptibility of shi drum juveniles to betanodavirus increases with rearing densities in a process mediated by neuroactive ligand-receptor interaction

Nervous necrosis virus (NNV) is one of the greatest threats to Mediterranean aquaculture, infecting more than 170 fish species and causing mortalities up to 100% in larvae and juveniles of susceptible species. Intensive aquaculture implies stressed conditions that affect the welfare of fish and their ability to fight against infections. In fact, a higher susceptibility to NNV has been related to poor welfare conditions. In order to analyze the physiological link between stressed conditions and increased susceptibility to NNV, as well as its possible role in the pathogenesis of this disease, we reared shi drum (Umbrina cirrosa) juveniles (30.7 ± 3.10 g body weight), which are expected to be asymptomatic upon NNV infection, at three stocking densities (2, 15, and 30 kg/m3) for 27 days and subsequently challenged them with NNV. We firstly characterized the stressed conditions of the specimens before and after infection and recorded the mortalities, demonstrating that stressed specimens reared at 30 kg/m3 suffered mortalities. However, the viral loads in different tissues were similar in all experimental groups, allowing horizontal and vertical transmission of the virus from asymptomatic specimens. All of these data suggest that shi drum tolerates wide ranges of culture densities, although high densities might be a setback for controlling NNV outbreaks in this species. In an attempt to understand the molecular pathways orchestrating this susceptibility change in stressed conditions, we performed a transcriptomic analysis of four tissues under mock- and NNV-infected conditions. In addition to the modification of the exceptive pathways such as cell adhesion, leukocyte migration, cytokine interaction, cell proliferation and survival, and autophagy, we also observed a heavy alteration of the neuroactive ligand-receptor pathway in three of the four tissues analyzed. Our data also point to some of the receptors of this pathway as potential candidates for future pharmacological treatment to avoid the exacerbated immune response that could trigger fish mortalities upon NNV infection.

Supplementation of Saussurea costus root alleviates sodium nitrite-induced hepatorenal toxicity by modulating metabolic profile, inflammation, and apoptosis

Sodium nitrite (NaNO2) is a widely used food ingredient, although excessive concentrations can pose potential health risks. In the present study, we evaluated the deterioration effects of NaNO2 additives on hematology, metabolic profile, liver function, and kidney function of male Wistar rats. We further explored the therapeutic potential of supplementation with S. costus root ethanolic extract (SCREE) to improve NaNO2-induced hepatorenal toxicity. In this regard, 65 adult male rats were divided into eight groups; Group 1: control, Groups 2, 3, and 4 received SCREE in 200, 400, and 600 mg/kg body weight, respectively, Group 5: NaNO2 (6.5 mg/kg body weight), Groups 6, 7 and 8 received NaNO2 (6.5 mg/kg body weight) in combination with SCREE (200, 400, and 600 mg/kg body weight), respectively. Our results revealed that the NaNO2-treated group shows a significant change in deterioration in body and organ weights, hematological parameters, lipid profile, and hepatorenal dysfunction, as well as immunohistochemical and histopathological alterations. Furthermore, the NaNO2-treated group demonstrated a considerable increase in the expression of TNF-α cytokine and tumor suppressor gene P53 in the kidney and liver, while a significant reduction was detected in the anti-inflammatory cytokine IL-4 and the apoptosis suppressor gene BCL-2, compared to the control group. Interestingly, SCREE administration demonstrated the ability to significantly alleviate the toxic effects of NaNO2 and improve liver function in a dose-dependent manner, including hematological parameters, lipid profile, and modulation of histopathological architecture. Additionally, SCREE exhibited the ability to modulate the expression levels of inflammatory cytokines and apoptotic genes in the liver and kidney. The phytochemical analysis revealed a wide set of primary metabolites in SCREE, including phenolics, flavonoids, vitamins, alkaloids, saponins and tannins, while the untargeted UPLC/T-TOF-MS/MS analysis identified 183 metabolites in both positive and negative ionization modes. Together, our findings establish the potential of SCREE in mitigating the toxic effects of NaNO2 by modulating metabolic, inflammatory, and apoptosis. Together, this study underscores the promise of SCREE as a potential natural food detoxifying additive to counteract the harmful impacts of sodium nitrite.

Evaluation of Gamma-Aminobutyric Acid (GABA) as a Functional Feed Ingredient on Growth Performance, Immune Enhancement, and Disease Resistance in Olive Flounder (Paralichthys olivaceus) under High Stocking Density

Gamma-aminobutyric acid (GABA) is a non-protein amino acid that is found in the brain and central nervous system of animals as an inhibitory neurotransmitter. It has been shown to have a variety of physiological functions, including stress reduction and immune enhancement. This study investigated the effects of dietary supplementation with GABA on growth, serum biochemistry, innate immunity, and disease resistance in juvenile olive flounders (Paralichthys olivaceus) challenged with Edwardsiella tarda under high-stocking density. A control diet and three experimental diets were prepared, with 150 mg/kg (GABA150), 200 mg/kg (GABA200), and 250 mg/kg (GABA250) of GABA added to each diet, respectively. Each experimental diet was fed to olive flounders in triplicate with an initial weight of 12.75 g ± 0.3 g in 40 L tanks at two stocking densities: normal density (20 fish/tank) and high density (40 fish/tank). After 8 weeks of the feeding trial, growth, feed utilization, whole-body proximate compositions, blood analyses, and non-specific immune responses were measured, and challenge tests were performed. There were no significant differences in the weight gain (WG) and specific growth rate (SGR) among fish fed the GABA-supplemented diets at the two stocking densities. However, the normal-density groups showed significantly higher WG and SGR than the high-density groups (p < 0.05). There was no significant difference in feed efficiency and protein efficiency ratio among all groups. Moreover, there was no significant difference in the whole-body proximate composition analysis (p > 0.05). There were no significant differences in cortisol levels in fish fed the GABA at both densities, but the high-density group showed a significantly higher cortisol than the low-density group. Blood GABA significantly increased in a dose-dependent manner regardless of the density groups (p < 0.05). Superoxide dismutase activity showed significantly higher levels than the control group, but there was no significant effect of the stocking densities in fish fed the GABA diets (p < 0.05). Myeloperoxidase activities in fish fed the GABA200 and GABA250 diets showed significantly higher levels at both of the stocking densities (p < 0.05). Lysozyme activity was significantly higher in the GABA150 group than in the CON, GABA200, and GABA250 groups (p < 0.05). After 15 days of challenge tests with Edwardsiella tarda, the cumulative survival rates of the GABA150, GABA200, and GABA250 groups were significantly higher than that of the CON group (p < 0.05). The results suggested that the optimal dietary GABA level for juvenile olive flounder culture is 150 mg/kg, regardless of rearing density, to enhance growth, immunity, and disease resistance.

The QseE-QseF two-component system: A key mediator of epinephrine-regulated virulence in the marine pathogen Edwardsiella piscicida

Edwardsiella piscicida is a widespread pathogen that infects various fish species and causes massive hemorrhagic septicemia, resulting in significant property damage to the global aquaculture industry. Type III and VI secretion systems (T3/T6SS), controlled by the master regulator EsrB, are important virulence factors of E. piscicida that enable bacterial colonization and evasion from host immune clearance. In this study, we demonstrate that the QseE-QseF two-component system negatively regulated esrB expression by reanalysis of Tn-seq data. Moreover, the response regulator QseF directly bound to esrB promoter and inhibited the expression of T3/T6SS genes, especially in the presence of epinephrine. Furthermore, in response to the prompt increasing of epinephrine level, the host immune genes were delayed repressed and QseE-QseF timely inhibited the expression of T3/T6SS genes to evade immune clearance. In summary, this study enhances our understanding and knowledge of the conditional pathogenesis mechanism and virulence regulation network of E. piscicida.

Black cardamom (Amomum subulatum) extract improves growth potential, antioxidant status, immune parameters and response to crowding stress in Catla catla

Natural herbs are excellent alternatives to synthetic compounds to enhance the growth performance and health status of fish. This study was conducted to evaluate the efficacy of black cardamom (Amomum subulatum) extract (BCE) as an herbal supplement in improving the growth, antioxidant status, haematology, and serum biochemistry of Catla catla. The acclimatized fish (N = 900; average initial weight = 14.44 ± 0.33 g) were allocated into five groups (60 fish/group in triplicate) in hapas (4 × 2 × 2.5 ft) and provided with feed containing 0 (control), 0.5, 1, 2 and 4 g/kg BCE for 90 days before being subjected to 8 days of crowding stress. After 90 days, significantly higher feed utilization and growth were observed in all BCE-fed treatments compared to the control; however, the maximum values of these parameters were noted in the 2 g/kg BCE-fed treatment. Moreover, the BCE-fed groups exhibited a significant increase in antioxidant indices (glutathione peroxidase, superoxide dismutase and catalase), with a significant reduction in malondialdehyde levels, indicating a higher antioxidant capacity compared to the control. Significant improvements in haematological parameters, such as an increase in haematocrit, haemoglobin and red blood cells and a decrease in white blood cells, were observed in BCE-fed treatments compared to the control. Furthermore, BCE-fed groups showed a significant decrease in serum glucose, cortisol and triglycerides, while total protein, globulin and albumin levels were significantly higher compared with the control. During the feeding trial of 90 days, no mortality was observed; however, the lowest cumulative mortality was noted in the 2 g/kg BCE group after crowding stress of 8 days. In conclusion, the use of BCE in feed can enhance the growth performance, antioxidant status, haematology, and serum biochemistry of C. catla and improve the resistance against crowding stress. The optimum levels of BCE for C. catla were estimated based on weight gain % (1.78 g/kg; R2  = 0.97), feed conversion ratio (1.65 g/kg; R2  = 0.98), MDA content (1.66 g/kg; R2  = 0.93) and serum lysozyme activity (1.72 g/kg; R2  = 1) using broken-line regression analysis.

Neuromolecular responses in disrupted mutualistic cleaning interactions under future environmental conditions

BACKGROUND

Mutualistic interactions, which constitute some of the most advantageous interactions among fish species, are highly vulnerable to environmental changes. A key mutualistic interaction is the cleaning service rendered by the cleaner wrasse, Labroides dimidiatus, which involves intricate processes of social behaviour to remove ectoparasites from client fish and can be altered in near-future environmental conditions. Here, we evaluated the neuromolecular mechanisms behind the behavioural disruption of cleaning interactions in response to future environments. We subjected cleaner wrasses and surgeonfish (Acanthurus leucosternon, serving as clients) to elevated temperature (warming, 32 °C), increased levels of CO2 (high CO2, 1000 ppm), and a combined condition of elevated CO2 and temperature (warming and high CO2, 32 °C, and 1000 ppm) for 28 days.

RESULTS

Each of these conditions resulted in behavioural disruptions concerning the motivation to interact and the quality of interaction (high CO2 - 80.7%, warming - 92.6%, warming and high CO2 - 79.5%, p < 0.001). Using transcriptomics of the fore-, mid-, and hindbrain, we discovered that most transcriptional reprogramming in both species under warming conditions occurred primarily in the hind- and forebrain. The associated functions under warming were linked to stress, heat shock proteins, hypoxia, and behaviour. In contrast, elevated CO2 exposure affected a range of functions associated with GABA, behaviour, visual perception, thyroid hormones and circadian rhythm. Interestingly, in the combined warming and high CO2 condition, we did not observe any expression changes of behaviour. However, we did find signs of endoplasmic reticulum stress and apoptosis, suggesting not only an additive effect of the environmental conditions but also a trade-off between physiological performance and behaviour in the cleaner wrasse.

CONCLUSIONS

We show that impending environmental shifts can affect the behaviour and molecular processes that sustain mutualistic interactions between L. dimidiatus and its clients, which could have a cascading effect on their adaptation potential and possibly cause large-scale impacts on coral reef ecosystems.

Environmentally relevant concentrations of Mn2+ disrupts the endocrine regulation of growth in juvenile Yunlong groupers (Epinephelus moara♀×Epinephelus lanceolatus♂)

Even though manganese is a bioelement essential for metabolism, excessive manganese levels in water can be detrimental to fish development and growth. Therefore, the aim of this study was to evaluate the effects of Mn2+ (0, 0.5,1, 2, and 4 mg·L-1) exposure for 30 d on the growth performance, growth hormone/insulin-like growth factor (GH/IGF) axis, hypothalamic-pituitary-thyroid (HPT) axis, and monoaminergic neurotransmitters of Epinephelus moara♀×Epinephelus lanceolatus♂(Yunlong grouper). Compared with the control and low Mn2+concentration groups of (0.5 and 1 mg·L-1), the high concentration of Mn2+ (4 mg·L-1) significantly reduced body weight (BW), body length (BL), weight gain rate (WGR), and specific growth rate (SGR), increased the feed coefficient rate (FCR) and mortality of Yunlong groupers (P < 0.05). Further, the levels of GH and IGF, along with the expression of ghra and ghrb were significantly reduced after exposure to 2 and 4 mg·L-1 Mn2+for 30 d, whereas the expression of sst5 was significantly up-regulated after exposure to 2 and 4 mg·L-1 Mn2+for 20 and 30 days. Moreover, Mn2+exposure increased thyroid hormone (T3) and thyroid stimulating hormone (TSH) contents, accompanied by increased mRNA levels of dio1 and dio2, however, the T4 level was decreased. Finally, dopamine (DA) and serotonin (5-HT) levels significantly decreased after long-term exposure to higher concentrations of Mn2+, and the levels their metabolites changed as well, suggesting that the synthesis and metabolism of DA and 5-HT were affected. Accordingly, changes in the GH/IGF and HPT axes-related parameters may be the cause of growth inhibition in juvenile groupers under Mn2+ exposure, indicating that the relationship between endocrine disorder and growth inhibition should not be ignored.

Microplastic polyamide toxicity: Neurotoxicity, stress indicators and immune responses in crucian carp, Carassius carassius

This study aimed to determine the toxicity standard and potential risks and effects of polyamide (PA) exposure on neurotoxicity, stress indicators, and immune responses in juvenile crucian carp Carassius carassius. Numerous microplastics (MPs) exists within aquatic environments, leading to diverse detrimental impacts on aquatic organisms. The C. carassius (mean weight, 23.7 ± 1.6 g; mean length, 13.9 ± 1.4 cm) were exposed to PA concentrations of 0, 4, 8, 16, 32 and 64 mg/L for 2 weeks. Among the neurotransmitters, the acetylcholinesterase (AChE) activity in the liver, gill, and intestine of C. carassius was significantly inhibited by PA exposure. Stress indicators such as cortisol and heat shock protein 70 (HSP70) in the liver, gill, and intestine of C. carassius were significantly increased, while immune responses to lysozyme and immunoglobulin M (IgM) were significantly decreased. Our study demonstrates the toxic effects of MP exposure on crucian carp's neurotoxicity, stress indicators, and immune responses.

Reproductive disorders linked to the interaction between sex steroid and thyroid hormonal activities, oxidative stress responses, and the rate of metabolism of tris (1,3-dichloro-2-propyl) phosphate (TDCPP) in zebrafish

The objective of this study was to assess the thyroid hormone disruption and reproductive dysfunction effects of the bioaccumulation and rate of mechanism in zebrafish exposed to tris(1,3-dichloro-2-propyl) phosphate (TDCPP), with stress responsiveness. The fish were exposed to test concentrations of TDCPP (0, 0.06, 0.3, 1.5 µg/mL) for 21 days, in accordance with no observed adverse effect level (i.e., < EC10) for zebrafish embryos. The bioaccumulation of TDCPP was found to be significantly higher in female zebrafish, while the metabolic rate was significantly higher in male zebrafish at all concentrations studied. The thyroid hormone (triiodothyronine [T3] and thyroxine [T4]) levels and sex steroid (i.e., estrogen, androgen, and progesterone) levels were significantly increased only in female zebrafish exposed to TDCPP, and no significant difference was observed in male zebrafish, although their cortisol levels increased. The response to TDCPP can, therefore, be considered sex-specific. The results of this study demonstrate for the first time, that the different response in the bioaccumulation and metabolic rate of TDCPP in males and females. The results also indicate that TDCPP alters thyroid hormone levels, furthermore, as steroidogenesis is related to reproductive function with differing response in males and females. TDCPP can be assumed to exert reproductive toxicity via disruption of thyroid and steroid synthesis through a slow metabolic rate in the whole body after exposure. Consequently, our proposed methodological approach to assess the interactions of thyroid and steroid biosynthesis and metabolic rate of TDCPP with reproductive toxicity will serve a testing strategy to examine the adverse outcomes of emerging environmental chemicals.

Cyhexatin causes developmental toxic effects by disrupting endocrine system and inducing behavioral inhibition, apoptosis and DNA hypomethylation in zebrafish (Danio rerio) larvae

Cyhexatin (CYT), an organotin acaricide, is extensively utilized in developing countries to mitigate plant diseases caused by mites and minimize agricultural crop losses. However, the comprehensive mechanisms underlying the developmental stage of non-target organisms remain largely unexplored. In this study, zebrafish embryos were firstly exposed to CYT (0.06, 0.12, and 0.20 ng/mL, referred to as CYTL, CYTM, and CYTH, respectively) from 2 hpf (hours post fertilization) to 30 dpf (days post fertilization). No developmental toxicity was observed in the CYTL and CYTM groups, except for induced deformed phenotypes in the CYTM group at 120 hpf. However, exposure to CYTH resulted in significant reductions in spontaneous movement (24 hpf), heart rate (48 hpf), hatching rate (48 and 72 hpf), body weight (30 dpf), whole body length (30 dpf), and locomotion (30 dpf). Additionally, CYTH exposure induced morphological malformations, including spinal curvature, pericardial edema, and tail curvature in zebrafish larvae. Moreover, CYTH treatment induced apoptosis, increased reactive oxygen species (ROS) production, and resulted in significant reductions in free T3, cholesterol, estradiol, and testosterone levels in zebrafish larvae, while free T4 levels were increased. RNA-Seq analysis indicated that CYTH exposure led to significant alterations in the genome-wide gene expression profiles of zebrafish, particularly in the thyroid hormone and steroid biosynthesis signaling pathways, indicating endocrine disruption. Furthermore, CYTH exposure induced global DNA hypomethylation, reduced S-adenosylmethionine (SAM) levels and the SAM/S-adenosylhomocysteine (SAH) ratio, elevated SAH levels, and suppressed the mRNA expression of DNA methyltransferases (DNMTs) while also downregulating DNMT1 at both the gene and protein levels in zebrafish larvae. Overall, this study partially elucidated the developmental toxicity and endocrine disruption caused by CYT in zebrafish, providing evidence of the environmental hazards associated with this acaricide.

Thyroid axis participates in high-temperature-induced male sex reversal through its activation by the stress response

Environmental changes alter the sex fate in about 15% of vertebrate orders, mainly in ectotherms such as fish and reptiles. However, the effects of temperature changes on the endocrine and molecular processes controlling gonadal sex determination are not fully understood. Here, we provide evidence that thyroid hormones (THs) act as co-players in heat-induced masculinization through interactions with the stress axis to promote testicular development. We first demonstrated that the thyroid axis (through thyroid-related genes and T3 levels) is highly active in males during the gonadal development in medaka (Oryzias latipes). Similarly, T3 treatments promoted female-to-male sex reversal in XX embryos. Subsequently, embryonic exposure to temperature-induced stress up-regulated the genes related to the thyroid and stress axes with a final increase in T3 levels. In this context, we show that blocking the stress axis response by the loss of function of the corticotropin-releasing hormone receptors suppresses thyroid-stimulating hormone expression, therefore, heat-induced activation of the thyroid axis. Thus, our data showed that early activation of the stress axis and, in consequence, the TH axis, too, leaves us with that both being important endocrine players in inducing female-to-male reversal, which can help predict possible upcoming physiological impacts of global warming on fish populations.

Exposure to environmentally relevant concentrations of TnBP results in tissue-specific bio-accumulation and inhibits growth of silver carp (Hypophthalmichthys molitrix)

Tri-n-butyl phosphate (TnBP) is commonly used as flame retardant and rubber plasticizer, and has been widely detected in aquatic organisms and natural waters. However, the potential toxicity of TnBP in fish remains unclear. In the present study, silver carp (Hypophthalmichthys molitrix) larvae were treated with environmentally relevant concentrations (100 or 1000 ng/L) of TnBP for 60 d and then they were depurated in clean water for 15 d, and the accumulation and depuration of the chemical in six tissues of silver carp were measured. Furthermore, effects on growth were evaluated and potential molecular mechanisms were explored. Results indicated that TnBP could be rapidly accumulated and depurated in silver carp tissues. In addition, the bio-accumulation of TnBP displayed tissue-specificity, where intestine contained the greatest and vertebra had the smallest level of TnBP. Furthermore, exposure to environmentally relevant concentrations of TnBP led to time- and concentration-dependent growth inhibition of silver carp, even though TnBP was completely depurated in tissues. Mechanistic studies suggested that exposure to TnBP up- and down-regulated the expression of ghr and igf1 in liver, respectively, and increased GH contents in plasma of silver carp. TnBP exposure also up-regulated the expression of ugt1ab and dio2 in liver, as well as decreased T4 contents in plasma of silver carp. Our findings provide direct evidence of health hazards of TnBP to fish in natural waters, calling for more attention of environmental risks of TnBP in aquatic environment.

Thyroid endocrine disruption and neurotoxicity of gestodene in adult female mosquitofish (Gambusia affinis)

The frequent detection of progestins in various aquatic environments and their potential endocrine disruptive effects in fish have attracted increasing attention worldwide. However, data on their effects on thyroid function and neurotoxicity in fish are limited, and the underlying mechanisms remain unclear. Here, the effects of gestodene (GES, a common progestin) on the thyroid endocrine and nervous systems of mosquitofish (Gambusia affinis) were studied. Adult female fish were exposed to GES at environmentally relevant concentrations (4.4-378.7 ng/L) for 60 days. The results showed that exposure to 378.7 ng/L GES caused a significant decrease in fish growth compared with the control and a marked reduction in the total distance traveled (50.6%) and swimming velocity (40.1-61.9%). The triiodothyronine (T3) levels were significantly increased by GES in a dose-dependent manner, whereas those of tetraiodothyronine (T4) were significantly decreased only at the G500 concentration. The acetylcholinesterase (AChE) activity was decreased significantly in the 4.42 ng/L GES treatments, but increased significantly at 378.67 ng/L. In the brain, a strong increase in the transcriptional levels of bdnf, trh, and dio2 was observed in fish after the 378.7 ng/L treatment. In addition, chronic exposure to GES caused colloid depletion with a concentration-dependent manner in the thyroid, and angiectasis, congestion, and vacuolar necrosis in the brain. These findings provide a better understanding of the effects of GES and associated underlying mechanisms in G. affinis.

Melatonin integrates multidimensional regulation of Na+/K+-ATPase in ionocytes and promotes stress and ease response in hypoxia-induced air-breathing fish: lessons from integrative approach

As circadian regulator, melatonin is involved in many physiological processes including ionosmotic regulation in fishes. Na⁺/K⁺-ATPase (NKA), an ubiquitous Na⁺/K⁺ transporter in ionocyte epithelia that drives electrochemical Na⁺ gradients and systemic osmotic integration, is a target of stress in fish. However, it is not certain how melatonin regulates NKA functions in ionocyte epithelia and how it modulates the adaptive response such as stress and ease response in fish particularly in hypoxia condition. We, thus, examined the short-term in vivo action of melatonin on the dynamics of NKA regulation in branchial, renal and intestinal ionocytes of hypoxia-induced air-breathing fish (Anabas testudineus Bloch). Interestingly, we found a rise in plasma melatonin in fish when kept for 30 min of forced submergence in water and that indicates a role for melatonin in hypoxia tolerance. A fall in blood [Na⁺ _, K⁺] occurred in these hypoxic fish which later showed a recovery after melatonin treatment. Similarly, melatonin favored the fall in NKA activity in branchial and renal epithelia of hypoxic fish, though it remarkably stimulated its activities in non-stressed fish. Likewise, melatonin that produced differential pattern of mRNA expression in nkaα1-subunit isoforms (nkaα1a, nkaα1b and nkaα1c) and melatonin receptor isoforms (mtnr1a, mtnr1bb, mtnr1bb _x1x2 ) in the tested ionocyte epithelia, showed reversed expression in hypoxic fish. In addition, the rise in NKAα-protein abundance in branchial and renal epithelia of melatonin-treated hypoxic fish indicated a recovery action of melatonin. A higher NKAα-immunoreactivity was found in the immunohistochemical and immunofluorescent images of branchial ionocytes and renal proximal and distal ionocytes of hypoxic fish treated with melatonin. Furthermore, an activation of PKA and PKG-dependent phosphorylation was found in branchial epithelia of hypoxic fish. The generated integrative parabola model showed that melatonin has a maximum targeted action on NKA function in the renal epithelia, suggesting its lead role in the integration of ionosmotic balance during the recovery or ease response. Over all, the data indicate a multidimensional and preferential action of melatonin on NKA regulation in fish ionocytes that integrate the recovery action against hypoxia, thus pointing to a major role for melatonin in stress and ease response in this fish.

Effects of the synthetic progestin levonorgestrel on some aspects of thyroid physiology in common carp (Cyprinus carpio)

The objective of the present study was to assess the effects of levonorgestrel (LNG), a synthetic progestin, on early development and the thyroid system of carp using morphological, histological, immunohistochemical, and gene expression analysis. Fish were exposed to LNG at three levels (3, 31, and 310 ng L^-1) from eggs to the onset of juvenile stage (47 days). LNG had no significant effect on early development in common carp or on the occurrence of morphological anomalies. No pathological alterations of the thyroid follicles were found. Immunohistochemical examination of the thyroid follicles using antibodies against thyroxin did not show any differences in fish exposed to 310 ng L^-1 LNG compared to the controls. mRNA expression of iodothyronine deiodinases (dio1, 2, 3) was differentially affected by LNG treatment during carp development. Most importantly, dio3 was markedly downregulated in fish exposed to all three LNG levels compared to the controls at the conclusion of the experiment (47 days post-fertilization). A decrease in dio1 or dio3 or an increase in dio2 transcription observed at different time points of the study may be a sign of hypothyroidism. mRNA expression of genes npr, esr1, and esr2b in the body and npr and esr2b in the head of fish exposed to 310 ng L^-1 LNG was significantly upregulated compared to the solvent control group at the end of the test. Together, these results show that levonorgestrel caused parallel changes in the hypothalamus-pituitary-thyroid and hypothalamus-pituitary-gonad axes.

Review of ecologically relevant in vitro bioassays to supplement current in vivo tests for whole effluent toxicity testing - Part 2: Non-apical endpoints

Whole effluent toxicity (WET) testing uses whole animal exposures to assess the toxicity of complex mixtures, like wastewater. These assessments typically include four apical endpoints: mortality, growth, development, and reproduction. In the last decade, there has been a shift to alternative methods that align with the 3Rs to replace, reduce, and refine the use of animals in research. In vitro bioassays can provide a cost-effective, high-throughput, ethical alternative to in vivo assays. In addition, they can potentially include additional, more sensitive, environmentally relevant endpoints than traditional toxicity tests. However, the ecological relevance of these endpoints must be established before they are adopted into regulatory frameworks. This is Part 2 of a two-part review that aims to identify in vitro bioassays that are linked to ecologically relevant endpoints that could be included in WET testing. Part 2 of this review focuses on non-apical endpoints that should be incorporated into WET testing. In addition to the four apical endpoints addressed in Part 1, this review identified seven additional toxic outcomes: endocrine disruption, xenobiotic metabolism, carcinogenicity, oxidative stress, inflammation, immunotoxicity and neurotoxicity. For each, the response at the molecular or cellular level measured in vitro was linked to the response at the organism level through a toxicity pathway. Literature from 2015 to 2020 was used to identify suitable bioassays that could be incorporated into WET testing.

Dynamic transcriptome and LC-MS/MS analysis revealed the important roles of taurine and glutamine metabolism in response to environmental salinity changes in gills of rainbow trout (Oncorhynchus mykiss)

Recently, the frequent salinity fluctuation has become a growing threat to fishes. However, the dynamic patterns of gene expression in response to salinity changes remain largely unexplored. In the present study, 18 RNA-Seq datasets were generated from gills of rainbow trout at different salinities, including 0 ‰, 6 ‰, 12 ‰, 18 ‰, 24 ‰ and 30 ‰. Based on the strict thresholds, we have identified 63, 1411, 2096, 1031 and 1041 differentially expressed genes in gills of rainbow trout through pairwise comparisons. Additionally, weighted gene co-expression network analysis was performed to construct 18 independent modules with distinct expression patterns. Of them, green and tan modules were found to be tightly related to salinity changes, several hub genes of which are known as the important regulators in taurine and glutamine metabolism. To further investigate their potential roles in response to salinity changes, taurine, glutamine, and their metabolism-related glutamic acid and α-ketoglutaric acid were accurately quantitated using liquid chromatography-tandem mass spectrometry analysis. Results clearly showed that their concentrations were closely associated with salinity changes. These findings suggested that taurine and glutamine play important roles in response to salinity changes in gills of rainbow trout, providing new insights into the molecular mechanism of fishes in salinity adaptation.

Effects of short-term exposure to the heavy metal, nickel chloride (Nicl2) on gill histology and osmoregulation components of the gray mullet, Mugil cephalus

The gray mullet, Mugil cephalus is an inshore and bottom-feeding fish species of Oman sea. Therefore, the gray mullet may be more exposed to heavy metal contamination, as the toxic impacts of heavy metals mullet has been reported in various studies. This study was conducted to evaluate the toxic effects of the heavy metal, nickel (as NiCl₂) on osmoregulation of the gray mullet by measuring blood biochemicals, hormones, minerals and gill histology. Fish (10 fish/tank) were experimentally exposed to NiCl₂ at three environmentally relevant concentrations of 5, 10 and 15 μg/l for 96 h. Then, fish were challenged with seawater (35 mg/l) for a period of 120 min. The samples (blood and gill tissue) were collected After 96 exposure to NiCl2 and during salinity challenge (30, 60 and 120 min post challenge). The plasma levels of cortisol and glucose significantly increased in NiCl_2-exposed fish. In addition, cortisol increased in all experimental groups 30 min after salinity challenge and then returned gradually to the same levels as the control at 120 min post salinity challenge (PSC). The triiodothyronine (T3) and thyroxine (T4) levels significantly decreased in response to 10 and 15 μg/l NiCl₂. In all groups, the thyroid hormones significantly elevated at 30 min PSC. After 30 min PSC, T3 levels in all NiCl_2-exposed fish and T4 in the treatment, 10 μg/l NiCl₂ remained unchanged throughout the salinity challenge. In the treatment, 5 μg/l NiCl₂, T4 levels were recovered at 120 min PSC and reached the same levels as the control. Exposure of fish to high concentrations of NiCl₂ and salinity stress increased the lactate levels. However, lactate levels in 5 and 10 μg/l NiCl₂ groups were recovered at 120 min PSC and reached the same levels as the control. Furthermore, plasma protein increased in response to 10 and 15 μg/l NiCl₂. At 30 PSC, the protein levels decreased in control and 5 μg/l NiCl₂ group, while it remained unchanged in fish exposed to 10 and 15 μg/l NiCl₂ throughout the salinity challenge. Exposure of fish to NiCl₂ disrupted the electrolyte (Na⁺, Cl^-) balance both before and after salinity challenge, which may be due to gill lesions induced by the heavy metal and following alternations in gill permeability. However, fish in 5 μg/l NiCl₂ re-established the ionic balance in the blood at the end of salinity challenge period. The malondialdehyde (MDA) levels significantly increased in response to 10 and 15 μg/l NiCl₂. The MDA levels returned to the same levels as the control group at 120 min PSC. The results of the present study showed that nickel-induced toxicity (especially at high concentrations) can reduce the osmoregulation capabilities of mullet. However, fish are able to recover from the toxic effects over time, if contamination be eliminated.

IPPD-induced growth inhibition and its mechanism in zebrafish

N-isopropyl-N-phenyl-1,4-phenylenediamine (IPPD) is used as a ubiquitous antioxidant worldwide, it is an additive in tire rubber easily discharged into the surrounding environment. At present, there is no study concerning the subacute toxicity of IPPD on fish. We used zebrafish embryos (2 h post-fertilization) exposed to IPPD for 5 days at concentrations of 0, 0.0012, 0.0120 and 0.1200 mg/L to investigate its toxic effects of embryonic development, disruption of growth hormone/insulin-like growth factor (GH/IGF) and hypothalamic-pituitary-thyroid (HPT) axis. The results showed that IPPD exposure decreased hatchability, weakened movement ability, reduced body length, and caused multiple types of deformities in zebrafish embryos. The expression of genes involved to GH/IGF and HPT axis were altered after exposure to IPPD in zebrafish larvae. Meanwhile, exposure to IPPD significantly decreased thyroxine (T4) and 3,5,3'-triiodothyronine (T3) contents in larvae, which indicated that HPT axis was in a disturbed state. Moreover, treatment of IPPD decreased the enzymatic activities of superoxide dismutase (SOD) and catalase (CAT) as well as levels of glutathione (GSH). While the contents of malondialdehyde (MDA) were elevated after exposure to IPPD. The present study thus demonstrated that IPPD induced oxidative stress, caused developmental toxicity and disrupted the GH/IGF and HPT axis of zebrafish, which could be responsible for developmental impairment and growth inhibition.

Effects of long-term exposure of norfloxacin on the HPG and HPT axes in juvenile common carp

Currently, there is a relatively lack of relevant research on the interference effect of quinolone antibiotics on the endocrine of aquatic animals. In this study, the toxicity of norfloxacin (NOR) on the endocrine system of juvenile common carp (Cyprinus carpio) was evaluated, as well as the hematocyte parameters. Specifically, two important endocrine axes were assessed: the hypothalamus-pituitary-thyroid (HPT) axis and hypothalamus-pituitary-gonadal (HPG) axis. Norfloxacin was used as a representative of quinolone antibiotics. According to the concentration of water pollution areas and considering the bad situation that may be caused by wastewater discharge, a control, 100 ng/L NOR, and 1 mg/L NOR treatment groups were set up. The juvenile carp, as the test animal, was subjected to an exposure experiment for 42 days. Thyroid hormones (T3 and T4) and related genes in HPT axis and sex hormones (11-ketotestosterone [11-KT] and progesterone [PROG]) and related genes in HPG axis and blood count are tested. It was found that the T4 iodine level and conversion process were enhanced after NOR treatment, which in turn led to the increase of T3 content and biological activity in the blood. One hundred nanograms per liter NOR can inhibit the level of sex hormones and inhibit the expression of HPG axis-related genes. In the 1 mg/L NOR treatment group, long-term exposure over a certain concentration range may lead to the development of adaptive mechanisms, making the changes in hormones and related genes insignificant. In conclusion, this study provides reference data for the endocrine interference of quinolone antibiotics on aquatic organisms, and has ecological significance for assessing the health of fish populations of quinolone antibiotics. However, the specific sites and mechanisms of action related to the effects of NOR on the endocrine system remain unclear and require further study.

Inducible Nitric Oxide Synthase/Nitric Oxide System as a Biomarker for Stress and Ease Response in Fish: Implication on Na+ Homeostasis During Hypoxia

The cellular and organismal response to stressor-driven stimuli evokes stress response in vertebrates including fishes. Fishes have evolved varied patterns of stress response, including ionosmotic stress response, due to their sensitivity to both intrinsic and extrinsic stimuli. Fishes that experience hypoxia, a detrimental stressor that imposes systemic and cellular stress response, can evoke disturbed ion homeostasis. In addition, like other vertebrates, fishes have also developed mechanisms to recover from the impact of stress by way of shifting stress response into ease response that could reduce the magnitude of stress response with the aid of certain neuroendocrine signals. Nitric oxide (NO) has been identified as a potent molecule that attenuates the impact of ionosmotic stress response in fish, particularly during hypoxia stress. Limited information is, however, available on this important aspect of ion transport physiology that contributes to the mechanistic understanding of survival during environmental challenges. The present review, thus, discusses the role of NO in Na⁺ homeostasis in fish particularly in stressed conditions. Isoforms of nitric oxide synthase (NOS) are essential for the synthesis and availability of NO at the cellular level. The NOS/NO system, thus, appears as a unique molecular drive that performs both regulatory and integrative mechanisms of control within and across varied fish ionocytes. The activation of the inducible NOS (iNOS)/NO system during hypoxia stress and its action on the dynamics of Na⁺/K⁺-ATPase, an active Na⁺ transporter in fish ionocytes, reveal that the iNOS/NO system controls cellular and systemic Na⁺ transport in stressed fish. In addition, the higher sensitivity of iNOS to varied physical stressors in fishes and the ability of NO to lower the magnitude of ionosmotic stress in hypoxemic fish clearly put forth NO as an ease-promoting signal molecule in fishes. This further points to the signature role of the iNOS/NO system as a biomarker for stress and ease response in the cycle of adaptive response in fish.

The interactive effects of ethinylestradiol and progesterone on transcriptional expression of genes along the hypothalamus-pituitary-thyroid axis in embryonic zebrafish (Danio rerio)

Progestins and estrogens are widespread in various aquatic environments and their potential endocrine disruption effects to aquatic organisms have drawn growing concern. However, their combined effects in aquatic organisms remain elusive. The aim of the present study was to assess the effects of the binary mixtures of gestodene (GES) and 17α-ethinylestradiol (EE2) on the hypothalamic-pituitary-thyroid (HPT) axis of zebrafish (Danio rerio) using the eleuthero-embryos. Embryos were exposed to GES and EE2 alone or in combination at concentrations ranging from 41 to 5329 ng L^-1 (nominal ones from 50 to 5000 ng L^-1) for 48 h, 96 h and 144 h post fertilization (hpf). The results showed that the transcripts of the genes along the HPT axis displayed pronounced alterations. There was no clear pattern in the change of the transcripts of these genes over time and with concentrations. However, in general, the transcripts of the genes were inversely affected by EE2 (increase 0.5 to 4.2-folds) and GES (inhibition 0.4 to 4.9-folds), and their mixtures showed interactive effects in embryonic zebrafish. In addition, physiological data (mortality, malformation, body length and heart rate etc.) denoted higher toxicity of the two chemicals in combination than alone based on the developmental toxicity and neurotoxicity (locomotor behavior). These results indicated that the interactive effects of these two chemicals might be different between at the transcriptional level and at the whole organismal level. In summary, GES and EE2 affect the HPT axis (related genes expression and thyroid hormones (THs) levels) and exhibit developmental toxicity and neurotoxicity.

Di(2-ethylhexyl) phthalate (DEHP) and thyroid: biological mechanisms of interference and possible clinical implications

Di(2-ethylhexyl) phthalate (DEHP) is a ubiquitous environmental endocrine disruptor. DEHP can be absorbed into the human body through the air, food, water, and skin. After entering the human body, DEHP is rapidly converted to mono(2-ethylhexyl) phthalate (MEHP) with greater toxicity than DEHP. An increasing number of studies indicates that DEHP or MEHP can damage the thyroid tissue and disrupt the function, but the mechanisms remain unclear. This article reviews the toxicity of DEHP on thyroid structures and functions and summarizes the potential mechanisms to provide evidence for preventing the thyroid-related diseases.

Thyroid function and immune status in perch (Perca fluviatilis) from lakes contaminated with PFASs or PCBs

The environment contains a multitude of man-made chemicals, some of which can act as endocrine disruptors (EDCs), while others can be immunotoxic. We evaluated thyroid disruption and immunotoxic effects in wild female perch (Perca fluviatilis) collected from two contaminated areas in Sweden; one site contaminated with per- and polyfluoroalkyl substances (PFASs) and two sites contaminated with polychlorinated biphenyls (PCBs), with one reference site included for each area. The hepatic mRNA expression of thyroid receptors α and β, and the thyroid hormone metabolising iodothyronine deiodinases (dio1, dio2 and dio3) were measured using real-time PCR, while the levels of thyroid hormone T3 in plasma was analysed using a radioimmunoassay. In addition, lymphocytes, granulocytes, and thrombocytes were counted microscopically. Our results showed lower levels of T3 as well as lower amounts of lymphocytes and granulocytes in perch collected from the PFAS-contaminated site compared to reference sites. In addition, expressions of mRNA coding for thyroid hormone metabolising enzymes (dio2 and dio3) and thyroid receptor α (thra) were significantly different in these fish compared to their reference site. For perch collected at the two PCB-contaminated sites, there were no significant differences in T3 levels or in expression levels of the thyroid-related genes, compared to the reference fish. Fish from one of the PCB-contaminated sites had higher levels of thrombocytes compared with both the second PCB lake and their reference lake; hence PCBs are unlikely to be the cause of this effect. The current study suggests that lifelong exposure to PFASs could affect both the thyroid hormone status and immune defence of perch in the wild.

Effects of acute low temperature stress on the hormones and gene expression of glucocorticoid receptor of large yellow croaker Larimichthys crocea

The neuroendocrine system of fish responds to low temperature via regulating hormones. To explore the adaptability of Larimichthys crocea to low temperature, the levels of the plasma cortisol, thyroid stimulating hormone (TSH), triiodothyronine (T3), thyroxine (T4), total cholesterol (TC), and glucose were determined after exposure to low temperature and during subsequent rewarming. Furthermore, the mRNA expression of the glucocorticoid receptor (GR) gene was analyzed under the stress. We found that the levels of the plasma cortisol, TSH, T3, glucose, and TC increased under the low temperature stress, suggesting that elevated hormones may be conducive to promoting the mobilization of the glucose and lipid in L. crocea exposed to low temperature. During the rewarming period, the plasma cortisol level decreased, whereas the T3 level was still significantly higher than that in the control group. Notably, the plasma T4 level was unaffected by the temperature changes. Furthermore, the sequence alignment and phylogenetic tree analysis revealed that the GR protein of L. crocea had high homology and a similar protein structure with those from other teleosts. Under the low temperature stress, the GR mRNA expression increased in the brain and head kidney, whereas it basically returned to the control level following rewarming. These findings revealed the changes of the hormones and the potential function of the GR gene in L. crocea following exposure to low temperature, providing some insights into breeding low temperature-resistant varieties of L. crocea.

Hormonal and neuroendocrine control of reproductive function in teleost fish

Reproduction is one of the important physiological events for the maintenance of the species. Hormonal and neuroendocrine regulation of teleost requires multiple and complex interactions along the hypothalamic-pituitary-gonad (HPG) axis. Within this axis, gonadotropin-releasing hormone (GnRH) regulates the synthesis and release of gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Steroidogenesis drives reproduction function in which the development and differentiation of gonads. In recent years, new neuropeptides have become the focus of reproductive physiology research as they are involved in the different regulatory mechanisms of these species' growth, metabolism, and reproduction. However, especially in fish, the role of these neuropeptides in the control of reproductive function is not well studied. The study of hormonal and neuroendocrine events that regulate reproduction is crucial for the development and success of aquaculture.

Modulation of immunity and hepatic antioxidant defense by corticosteroids in pacu (Piaractus mesopotamicus)

We investigated the impact of both the oral administration of hydrocortisone (HC) and an acute stressor on stress, innate immune responses and antioxidant system/oxidative stress responses of juvenile Piaractus mesopotamicus. Fish were either 1) given a commercial feed (C), 2) given a feed supplemented with 400 mg/kg HC, or 3) fed a commercial feed, chased for 2 min and exposed to air for 4 min (S). After initial sampling, fish C and HC were fed and sampled 1, 3, 6, 24 and 72 h post-feeding. Fish S were fed at the same time as the other groups, exposed to a stressor, and sampled 1, 3, 6, 24 and 72 h after. Exposure to the stressor increased circulating glucose and cortisol levels (at 1 and 3 h, respectively), while oral HC increased circulating cortisol at 1 h and glucose at 3 h. The stressor activated respiratory activity of leukocytes (RAL) at 3 h and reduced it at 6 h. HC did not activate RAL, but it did impair it at 6 h. The serum hemolytic activity of the complement system (HAC₅₀) was impaired by the stressor at 1 and 3 h and by HC at 1 h. Regarding the antioxidant system, exposure to the stressor reduced glutathione peroxidase (GPx) and catalase (CAT) activity and decreased concentrations of reduced glutathione (GSH) in the liver up to 6 h. HC only impaired GPx. Additionally, stress induced the accumulation of melano-macrophage (MM) and melano-macrophage centers (MMC), which are biomarkers of oxidative stress, in the spleen. Differences in biomarkers in fish given cortisol and exposed to stress indicate that exogenous hormone was unable to precisely reproduce stress responses.

Effects of Ocean Acidification on Transcriptomes in Asian Seabass Juveniles

Ocean acidification is changing the fate of marine organisms. It is essential to predict the biological responses and evolutionary processes driven by ocean acidification, to maintain the equilibrium of the marine ecosystem and to facilitate aquaculture. However, how marine organisms, particularly the marine fish species, respond to ocean acidification, is still poorly understood. Consequences of ocean acidification on finfish aquaculture are largely not well known. We studied the effects of ocean acidification for 7 days on growth, behaviour and gene expression profiles in the brain, gill and kidney of Asian seabass juveniles. Results showed that growth and behaviour were not affected by short-term ocean acidification. We found tissue-specific differentially expressed genes (DEGs) involving many molecular processes, such as organ development, growth, muscle development, ion homeostasis and neurogenesis and development, as well as behaviours. Most of the DEGs, which were functionally enriched in ion homeostasis, were related to calcium transport, followed by sodium/potassium channels. We found that genes associated with neurogenesis and development were significantly enriched, implying that ocean acidification has also adversely affected the neural regulatory mechanism. Our results indicate that although the short-term ocean acidification does not cause obvious phenotypic and behavioural changes, it causes substantial changes of gene expressions in all three analysed tissues. All these changes of gene expressions may eventually affect physiological fitness. The DEGs identified here should be further investigated to discover DNA markers associated with adaptability to ocean acidification to improve fish's capability to adapt to ocean acidification.

Nitric oxide synthase (NOS) inhibitor L-NAME activates inducible NOS/NO system and drives multidimensional regulation of Na+ /K+ -ATPase in ionocyte epithelia of immersion-stressed air-breathing fish (Anabas testudineus Bloch)

Nitric oxide (NO) has been implicated in Na⁺  homeostatic control in water-breathing fishes. It is, however, uncertain whether air-breathing fish relies on NO to coordinate Na⁺ /K⁺ -ATPase (NKA)-driven Na⁺  transport during acute hypoxemia. We, thus, examined the action of nitric oxide synthase (NOS) inhibitor, L-NAME on NO availability, inducible NOS (iNOS) protein abundance and the regulatory dynamics of NKA in osmoregulatory epithelia of Anabas testudineus kept at induced hypoxemia. As expected in nonstressed fish, in vivo L-NAME (100 ng g^-1 ) challenge for 30 min declined NO production in serum (40%) and osmoregulatory tissues (average 51.6%). Surprisingly, the magnitude of such reduction was less in hypoxemic fish after L-NAME challenge due to the net gain of NO (average 23.7%) in these tissues. Concurrently, higher iNOS protein abundance was found in branchial and intestinal epithelia of these hypoxemic fish. In nonstressed fish, L-NAME treatment inhibited the NKA activity in branchial and intestinal epithelia while stimulating its activity in renal epithelia. Interestingly in hypoxemic fish, L-NAME challenge restored the hypoxemia-inhibited NKA activity in branchial and renal epithelia. Similar recovery response was evident in the NKAα protein abundance in immunoblots and immunofluorescence images of branchial epithelia of these fish. Analysis of Nkaα1 isoform transcript abundance (Nkaα1a, α1b, α1c) also showed spatial and preferential regulation of Nkaα1 isoform switching. Collectively, the data indicate that L-NAME challenge activates iNOS/NO system in the branchial ionocyte epithelia of hypoxemia-stressed Anabas and demands multidimensional regulation of NKA to restore the Na⁺  transport rate probably to defend against acute hypoxemia.

Phenylalanine and Tyrosine as Feed Additives for Reducing Stress and Enhancing Welfare in Gilthead Seabream and Meagre

Simple Summary

Food additives (phenylalanine and tyrosine) were tested in gilthead seabream (Sparus aurata) and meagre (Argyrosomus regius) to improve fish welfare in sea farms. These amino acids improved the stress response in both species, reducing some stress markers, though interspecific differences were detected. The results indicate that these dietary supplements could be provided before submitting fish to typical stress conditions in sea farms (sampling, grading, etc.) for improving animal welfare.

Abstract

Increased aquaculture production is associated with a growing interest in improving fish welfare. For this reason, the search for strategies to mitigate stress has intensified, one of these strategies being food supplementation with amino acids. The objective of this study was to evaluate the effects of dietary phenylalanine (Phe) and Tyrosine (Tyr) on the stress response and metabolism of juvenile gilthead seabreams (Sparus aurata) and meagres (Argyrosomus regius). Fish batches were fed a control diet and two diets supplemented with 5% Phe or Tyr for seven days. At the end of the experiment fish were stressed by air exposure for 3 min and then sacrificed for the extraction of blood and brain. Classical plasma stress markers were analyzed (glucose, lactate, proteins, cortisol), as well as hormones derived from those amino acids (adrenaline, noradrenaline and dopamine). Despite interspecific differences, fish fed the diets supplemented with Phe or Tyr showed a reduction on several stress markers. However, interspecific differences were detected for many indicators. Concretely, hormonal stress markers were significantly attenuated in meagres fed the enriched diets. Moreover, the stress condition favored a mobilization of amino acids towards the brain, especially in supplemented diets, hence this amino acid excess could be used as an energy substrate to cope with stress.

High temperature stress response is not sexually dimorphic at the whole-body level and is dependent on androgens to induce sex reversal

The understanding of the molecular and endocrine mechanisms behind environmentally-induced sex reversal in fish is of great importance in the context of predicting the potential effects of climate change, especially increasing temperature. Here, we demonstrate the global effects of high temperature on genome-wide transcription in medaka (Oryzias latipes) during early development. Interestingly, data analysis did not show sexual dimorphic changes, demonstrating that thermal stress is not dependent on genotypic sex. Additionally, our results revealed significant changes in several pathways under high temperature, such as stress response from brain, steroid biosynthesis, epigenetic mechanisms, and thyroid hormone biosynthesis, among others. These microarray data raised the question of what the exact molecular and hormonal mechanisms of action are for female-to-male sex reversal under high temperatures in fish. Complementary gene expression analysis revealed that androgen-related genes increase in females (XX) experiencing high water temperature. To test the involvement of androgens in thermal-induced sex reversal, an androgen antagonist was used to treat XX medaka under a high-temperature setup. Data clearly demonstrated failure of female-to-male sex reversal when androgen action is inhibited, corroborating the importance of androgens in environmentally-induced sex reversal.

Computational animal welfare: towards cognitive architecture models of animal sentience, emotion and wellbeing

To understand animal wellbeing, we need to consider subjective phenomena and sentience. This is challenging, since these properties are private and cannot be observed directly. Certain motivations, emotions and related internal states can be inferred in animals through experiments that involve choice, learning, generalization and decision-making. Yet, even though there is significant progress in elucidating the neurobiology of human consciousness, animal consciousness is still a mystery. We propose that computational animal welfare science emerges at the intersection of animal behaviour, welfare and computational cognition. By using ideas from cognitive science, we develop a functional and generic definition of subjective phenomena as any process or state of the organism that exists from the first-person perspective and cannot be isolated from the animal subject. We then outline a general cognitive architecture to model simple forms of subjective processes and sentience. This includes evolutionary adaptation which contains top-down attention modulation, predictive processing and subjective simulation by re-entrant (recursive) computations. Thereafter, we show how this approach uses major characteristics of the subjective experience: elementary self-awareness, global workspace and qualia with unity and continuity. This provides a formal framework for process-based modelling of animal needs, subjective states, sentience and wellbeing.

Characterization of Myostain (MSTN) and Myogenic Differentiation Antigen (MyoD) and the Effect of Dexamethasone on Their Expression in Large-Scale Loach Paramisgurnus dabryanus

Myostatin (MSTN) and myogenic differentiation antigen (MyoD) play an essential role in specification and differentiation of skeletal muscle. However, the role of stress in the regulation of MyoD and MSTN has not been fully revealed and more evidence should be provided. Here, we reported the cloning and expressional analyses of MSTN and MyoD in Large-scale Loach Paramisgurnus dabryanus (hereafter PdMSTN and PdMyoD). Injecting individuals with 0, 60, 600, and 1,200 μg/kg dexamethasone (DXM) for five consecutive days resulted in a dose-dependent change of PdMSTN and PdMyoD expression. The expression of PdMSTN was upregulated with increasing DXM concentrations, while PdMyoD expression was downregulated. The changes in the expression of these genes at different time points for 10 consecutive days were studied after individuals were treated with 600 μg/kg DXM. Compared with the control group, PdMSTN expression decreased and PdMyoD expression increased before 12 h, and both PdMSTN and PdMyoD expression levels increased at 24 h, which was significantly higher than those in control group. At a prolonged treatment of 5-10 d, expression levels of PdMSTN and PdMyoD had significantly reduced. The results indicate that both PdMyoD and PdMSTN are involved in DXM-induced stress in Large-scale Loach.

Biochemical responses of a freshwater fish Cirrhinus mrigala exposed to tris(2-chloroethyl) phosphate (TCEP)

Freshwater fish Cirrhinus mrigala were exposed to tris(2-chloroethyl) phosphate (TCEP) with three different concentrations (0.04, 0.2, and 1 mg/L) for a period of 21 days. During the study period, thyroid-stimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4) levels were significantly (p < 0.05) inhibited. The superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST), and lipid peroxidation (LPO) levels were increased significantly (p < 0.05) in gills, liver, and kidney tissues, whereas glutathione (GSH) and glutathione peroxidase (GPx) (except liver tissue) activities were inhibited when compared to the control group. Likewise, exposure to TCEP significantly (p < 0.05) altered the biochemical (glucose and protein) and electrolyte (sodium, potassium, and chloride) levels of fish. Light microscopic studies exhibited series of histopathological anomalies in the gills, liver, and kidney tissues. The present study reveals that TCEP at tested concentrations causes adverse effects on fish and the studied biomarkers could be used for monitoring the ecotoxicity of organophosphate esters (OPEs).

Transcriptomic Analysis of Gill and Kidney from Asian Seabass (Lates calcarifer) Acclimated to Different Salinities Reveals Pathways Involved with Euryhalinity

Asian seabass (or commonly known as barramundi), Lates calcarifer, is a bony euryhaline teleost from the Family Latidae, inhabiting nearshore, estuarine, and marine connected freshwaters throughout the tropical Indo-West Pacific region. The species is catadromous, whereby adults spawn in salinities between 28 and 34 ppt at the mouth of estuaries, with resultant juveniles usually moving into brackish and freshwater systems to mature, before returning to the sea to spawn again as adults. The species lives in both marine and freshwater habitats and can move quickly between the two; thus, the species' ability to tolerate changes in salinity makes it a good candidate for studying the salinity acclimation response in teleosts. In this study, the transcriptome of two major osmoregulatory organs (gills and kidneys) of young juvenile Asian seabass reared in freshwater and seawater were compared. The euryhaline nature of Asian seabass was found to be highly pliable and the moldability of the trait was further confirmed by histological analyses of gills and kidneys. Differences in major expression pathways were observed, with differentially expressed genes including those related to osmoregulation, tissue/organ morphogenesis, and cell volume regulation as central to the osmo-adaptive response. Additionally, genes coding for mucins were upregulated specifically under saline conditions, whereas several genes important for growth and development, as well as circadian entrainment were specifically enriched in fish reared in freshwater. Routing of the circadian rhythm mediated by salinity changes could be the initial step in salinity acclimation and possibly migration in euryhaline fish species such as the Asian seabass.

Dietary GABA enhances hypoxia tolerance of a bottom-dwelling carp, Cirrhinus mrigala by modulating HIF-1α, thyroid hormones and metabolic responses

The Indian major carp, mrigal (Cirrhinus mrigala), is a bottom-dwelling fish that can survive hypoxic episodes in its natural environment. We hypothesise that it can better survive hypoxic conditions by altering metabolic responses through GABA (Gamma-aminobutyric acid) supplementation. In the first experiment, the hypoxia tolerance time of the fishes was evaluated under extreme anoxic conditions after feeding with GABA, which showed that GABA had improved survival time under hypoxia. To study the response of dietary GABA in hypoxia-exposed fish, the branchial HIF-1α expression levels, serum thyroid hormone levels and hepatic metabolic responses were assessed in the subsequent experiment. The treatment groups were fed for 60 days with experimental diets containing 4 levels of GABA (0.00% G, 0.50% G, 0.75% G and 1.0%G) and were subjected to 72-h hypoxia exposure (0.5 ± 0.02 mg L^-1 dissolved oxygen (DO)) whereas a control group was maintained under normoxic conditions (6.0 ± 0.21 mg L^-1 DO). The five treatment groups with three replicates were C0 (0% G + normoxia), H0 (0% G + hypoxia), H0.5 (0.50% G + hypoxia), H0.75 (0.75% G + hypoxia) and H1.0 (1.00% G + hypoxia). The results indicated that GABA supplementation triggered downregulation of HIF 1 alpha expression. When compared with the control group, decreased thyroxine (T4) and triiodothyronine (T3) levels were observed in the GABA-fed hypoxic groups. However, TSH (thyroid stimulating hormone) level remained unchanged in all the treatments. The LDH (lactate dehydrogenase) level in hypoxia-exposed groups was decreased by GABA supplementation. Our study demonstrated that GABA supplementation restores acute hypoxia-induced HIF-1α expression, thyroid hormone levels and LDH activities. On the other hand, it enhanced the citrate synthase (CS) activities at 0.5-1.00%, which showed a sharp decline in hypoxia. Hypoxia caused increase in the serum metabolites such as glucose, lactate, cholesterol and triglycerides. However, GABA supplementation was partially effective in reducing glucose and lactate level while triglycerides and cholesterol values remained unchanged. Overall, our results suggested a potential role of GABA in suppressing metabolism during hypoxia exposure, which can increase the chances of survival of the species Cirrhinus mrigala during hypoxia.

The relationship between ingested thyroid hormones, thyroid homeostasis and iodine metabolism in humans and teleost fish

Oral l-thyroxine (T4) therapy is used to treat human hypothyroidism but T4 fed to teleost fish does not raise plasma thyroid hormone (TH) levels nor induce growth, even though oral 3,5,3'-triiodo-l-thyronine (T3) is effective. This suggests a major difference in TH metabolism between teleosts and humans, often used as a starting thyroid model for lower vertebrates. To gain further insight on the proximate (mechanistic) and ultimate (survival value) factors underlying this difference, the several steps in TH homeostasis from intestinal TH uptake to hypothalamic-hypophyseal regulation were compared between humans and teleosts, and following dietary TH challenges. A major proximate factor limiting trout T4 uptake is a potent constitutive thiol-inhibited intestinal complete T4 deiodination that is ineffective for T3. At the hepatic level, T4 deiodination, conjugation and extensive biliary excretion with negligible T4 enterohepatic recycling can further block teleost T4 uptake to plasma. Such protection of plasma T4 from dietary T4 may be particularly critical for piscivorous fish consuming thyroid tissue, rich in T4 but not T3. It would prevent disruption by unregulated ingested T4 of the characteristic acute and transient changes in teleost plasma T4 due to diel rhythms, food intake and stress-related factors. These marked natural short-term fluctuations in teleost plasma T4 levels are enabled by the relatively small and rapidly-cleared plasma T4 pool, stemming largely from properties of the plasma T4-binding proteins. Humans, however, due mainly to plasma T4-binding globulin, have a relatively massive circulating pool of T4 and an extremely well-buffered free T4 level, consistent with the major TH role in regulating basal metabolic rate. Furthermore, this large well-buffered and slowly-cleared plasma T4 pool, in conjuction with enterohepatic recycling and relaxation of hypothalamic-hypophyseal negative feedback, allows humans to temporarily 'store' ingested T4 in plasma, thereby sparing endogenous TH secretion and conserving thyroidal iodine reserves. Indeed, iodine conservation is likely the key ultimate factor determining the divergent evolution of the human and teleost systems. For humans, ingested iodine in the form of I^-, or TH and their derivatives, is the sole iodine source and may be limiting in many environments. However, most freshwater teleosts, in addition to their ability to assimilate dietary I^-, can derive sufficient I^- from their copious gill irrigation, with no selective advantage in absorbing dietary T4 which would disrupt their natural acute and transient changes in plasma T4. Thus T4 may act also as a vitamin (vitamone) in humans but not in teleosts; in contrast, T3, naturally ingested at much lower levels, may act as a vitamone in both humans and teleosts.

Effects of hyperthyroidism in the development of the appendicular skeleton and muscles of zebrafish, with notes on evolutionary developmental pathology (Evo-Devo-Path)

The hypothalamus-pituitary-thyroid (HPT) axis plays a crucial role in the metabolism, homeostasis, somatic growth and development of teleostean fishes. Thyroid hormones regulate essential biological functions such as growth and development, regulation of stress, energy expenditure, tissue compound, and psychological processes. Teleost thyroid follicles produce the same thyroid hormones as in other vertebrates: thyroxin (T4) and triiodothyronine (T3), making the zebrafish a very useful model to study hypo- and hyperthyroidism in other vertebrate taxa, including humans. Here we investigate morphological changes in T3 hyperthyroid cases in the zebrafish to better understand malformations provoked by alterations of T3 levels. In particular, we describe musculoskeletal abnormalities during the development of the zebrafish appendicular skeleton and muscles, compare our observations with those recently done by us on the normal developmental of the zebrafish, and discuss these comparisons within the context of evolutionary developmental pathology (Evo-Devo-Path), including human pathologies.

Netting the Stress Responses in Fish

In the last decade, the concept of animal stress has been stressed thin to accommodate the effects of short-term changes in cell and tissue physiology, major behavioral syndromes in individuals and ecological disturbances in populations. Seyle's definition of stress as "the nonspecific (common) result of any demand upon the body" now encompasses homeostasis in a broader sense, including all the hierarchical levels in a networked biological system. The heterogeneity of stress responses thus varies within individuals, and stressors become multimodal in terms of typology, source and effects, as well as the responses that each individual elicits to cope with the disturbance. In fish, the time course of changes after stress strongly depends on several factors, including the stressful experiences in early life, the vertical transmission of stressful-prone phenotypes, the degree of individual phenotypic plasticity, the robustness and variety of the epigenetic network related to environmentally induced changes, and the intrinsic behavioral responses (individuality/personality) of each individual. The hierarchical heterogeneity of stress responses demands a code that may decrypt and simplify the analysis of both proximate and evolutionary causes of a particular stress phenotype. We propose an analytical framework, the stressotope, defined as an adaptive scenario dominated by common environmental selective pressures that elicit common multilevel acute stress-induced responses and produce a measurable allostatic load in the organism. The stressotope may constitute a blueprint of embedded interactions between stress-related variations in cell states, molecular mediators and systemic networks, a map of circuits that reflect the inherited and acquired stress responses in an ever-changing, microorganismal-loaded medium. Several features of the proposed model are discussed as a starting point to pin down the maximum common stress responses across immune-neuroendocrine relevant physiological levels and scenarios, including the characterization of behavioral responses, in fish.

Do Metabolic Traits, Vulnerability to Angling, or Capture Method Explain Boldness Variation in Eurasian Perch?

The pace-of-life syndrome (POLS) concept predicts that individuals with high baseline metabolic rates demonstrate high boldness, aggressiveness, and activity, especially in food acquisition, with associated relatively greater energy requirements. In fishes, these behaviors may increase individual vulnerability to angling. To test the predictions of the POLS concept, we quantified individual standard metabolic rate (SMR) and boldness in both wild-caught and hatchery-reared Eurasian perch (Perca fluviatilis). We found both SMR and boldness to be repeatable traits but detected no correlation between them. Individual vulnerability to angling was assessed in the hatchery-reared perch, but we found no difference in boldness or SMR between vulnerable and nonvulnerable perch. Wild-caught perch were ice fished using either natural or artificial bait, and we observed no differences in boldness or SMR with respect to bait type or capture order. Our findings do not support the predictions of the POLS concept and, consistent with earlier studies in perch, suggest that angling may not drive selection against boldness in this species.

Changes in thyroid status of Menidia beryllina exposed to the antifouling booster irgarol: Impacts of temperature and salinity

The triazine-based herbicide irgarol is widely used in antifouling systems as an algicide and has been detected recently in multiple coastal environments. Studies evaluating sub-lethal responses of fish following exposure to irgarol are limited. Moreover, impacts of climate change on fish endocrinology may also contribute to the sublethal toxicity of irgarol. We assessed the effects of irgarol on thyroid endpoints in juveniles of Menidia beryllina under two different treatments of salinity (10 and 20 ‰) and two temperatures (10 and 20°C). Condition factor coefficients (K) of animals were significantly affected by 0.1 to 10 μg/L of irgarol at the higher temperature. Levels of T3 were changed in whole body homogenates from both temperatures at 10‰ following exposure to 1 to 10 μg/L. T4 levels were altered only at 10°C when animals were treated with 1 to 10 μg/L (10 ‰), and in 0.1 and 10 μg/L (20 ‰). Increased transcripts of deiodinase enzymes at 10 °C may be impacted by salinity and alter thyroid hormone homeostasis. Impact on gene expression of thyroid (α and β) and growth hormone receptors were also determined. Our results highlight the relevance of environmental variable that may impact the ecological risk of irgarol in estuarine systems.

Characterization of a β2 adrenergic receptor protein precursor in the European eel (Anguilla anguilla) and its tissue distribution across silvering

This study provides the characterization and tissue distribution of a β2-AR in the female European eel during silvering, aiming to better understand the adrenergic system involvement in this critical maturation event. A putative β2-AR (ADRB2) mRNA was cloned and sequenced. Amino acid residues and motifs important for ligand binding are generally conserved across fish and between fish and mammals, although the occurrence of some sequence variabilities may explain the noted peculiarities of eel AR interaction with pharmacological ligands. The tissue distribution of the ADRB2 gene product was analyzed in five tissues of the eel at different silvering stages and compared with that of the ADRA1 mRNA encoding an α1-AR subtype. On the whole, data suggested that relative ADRA1/ADRB2 tissue expression across silvering is part of the preparatory (molecular) adjustments required to face changes in habitats and migration efforts.

Impacts of Salinity and Temperature on the Thyroidogenic Effects of the Biocide Diuron in Menidia beryllina

Diuron is a herbicide used in agricultural and urban settings and also as an antifouling agent. Recent studies have indicated sublethal responses of diuron in the endocrine system of fish and amphibians. Given the potential of climate change to also alter fish endocrinology, the combination of environmental stressors with diuron may contribute to its sublethal toxicity. In this study, the effects of temperature and salinity on thyroid targets of diuron were assessed in juveniles of the estuarine fish Menidia beryllina under different conditions of salinity (10 and 20‰) and temperature (10 and 20 °C). Environmentally relevant concentrations of diuron affected the growth, and the higher temperature reduced the condition factor of animals. Increased levels of T3 were observed in fish from all treatments, and at 10 °C, T4 levels were augmented at 10‰ but reduced at 20‰. Increased gene expression of deiodinases at 20‰ in both temperatures suggests the influence of salinity on the regulation of hormone imbalance via deiodination pathway activation. Decreased transcripts of thyroid and growth hormone receptors were also observed following diuron treatment. These results indicate that changes in environmental stressors may have significant impacts on the ecological risk of diuron in estuarine fish.

Addressing the impact of mercury estuarine contamination in the European eel (Anguilla anguilla L., 1758) - An early diagnosis in glass eel stage based on erythrocytic nuclear morphology

The decline of the European eel (Anguilla anguilla L., 1758) population throughout Europe has been partially attributed to pollution. As glass eel estuarine migration may represent a considerable threat, the impact of mercury (Hg) contamination at this stage was evaluated through an in situ experiment (7days). Total Hg (tHg) bioaccumulation was evaluated concomitantly with erythrocytic nuclear morphology alterations: erythrocytic nuclear abnormalities assay (ENA), frequency of immature erythrocytes (IE) and the erythrocytic maturity index (EMI). The ENA results suggested a genotoxic pressure at the most contaminated sites, in line with the tHg increase. The EMI data, together with IE frequency, showed that fish exposed to high levels of Hg exhibited alterations of haematological dynamics, translated into an erythropoiesis increment. Despite the presence of these compensatory mechanisms, the present findings suggest a harmful impact of Hg on genome integrity at this early development stage, potentially affecting eels' condition and ultimately the population sustainability.