Effects of Hg sublethal exposure in the brain of peacock blennies Salaria pavo: Molecular, physiological and histopathological analysis

Toxicity Evaluation, Oxidative, and Immune Responses of Mercury on Nile Tilapia: Modulatory Role of Dietary Nannochloropsis oculata

The current study evaluated the potential ameliorative effect of a dietary immune modulator, Nannochloropsis oculata microalga, on the mercuric chloride (HgCl2)-induced toxicity of Nile tilapia. Nile tilapia (45-50 g) were fed a control diet or exposed to ¼ LC50 of HgCl2 (0.3 mg/L) and fed on a medicated feed supplemented with N. oculata (5% and 10% (50 or 100 g/kg dry feed)) for 21 days. Growth and somatic indices, Hg2+ bioaccumulation in muscles, and serum acetylcholinesterase (AChE) activity were investigated. Antioxidant and stress-related gene expression analyses were carried out in gills and intestines. Histopathological examinations of gills and intestines were performed to monitor the traits associated with Hg2+ toxicity or refer to detoxification. Hg2+ toxicity led to significant musculature bioaccumulation, inhibited AChE activity, downregulated genes related to antioxidants and stress, and elicited histopathological changes in the gills and intestine. Supplementation with N. oculata at 10% was able to upregulate the anti-oxidative-related genes while downregulated the stress apoptotic genes in gills and intestines compared to the unexposed group. In addition, minor to no histopathological traits were detected in the gills and intestines of the N. oculata-supplemented diets. Our data showed the benefit of dietary N. oculata in suppressing Hg2+ toxicity, which might support its efficacy as therapeutic/preventive agent to overcome environmental heavy metal pollution in aquatic habitats.

Comparative neurotoxicity of dietary methylmercury and waterborne inorganic mercury in fish: Evidence of optic tectum vulnerability through morphometric and histopathological assessments

This work investigated the effects of inorganic mercury (iHg) and methylmercury (MeHg) on the fish optic tectum morphology, viz. in relation to: (i) vulnerability of specific optic tectum layers; (ii) preferential targeting of Hg forms to neurons or glial cells; (iii) comparative toxicity of iHg and MeHg in this brain area that is in the maintenance of several fish behaviors. Two experiments exposing juvenile white seabream (Diplodus sargus) to waterborne iHg [HgCl₂ (2 μg L^-1)] and dietary MeHg (8.7 μg g^-1) were performed, comprising both exposure (7 and 14 days; E7 and E14, respectively) and post-exposure (28 days; PE28) periods. Morphometric assessments were performed using stereological methods where the layers of the optic tectum were outlined, while its area and the number of neurons and glial cells were estimated. A histopathological assessment was also performed per section and per layer of optic tectum. iHg exposure did not trigger the loss of neurons during the exposure periods, while a decrease of glial cells was detected in a single layer of the optic tectum at E14. Differently, upon MeHg exposure, a decrease on the number of neurons and glial cells was found in several layers of optic tectum. In the post-exposure, both Hg forms triggered the loss of neurons, while only MeHg exposure led to a decrease on the number of glia cells. The histopathological assessment pointed out a higher toxicity of MeHg in the optic tectum layers, particularly in the post-exposure period, while no significant alterations were found in fish exposed to iHg. Hg forms targeted preferentially neurons. iHg and MeHg are relevant neurotoxicants to fish, with MeHg exposure leading to a higher toxicity than iHg in the optic tectum. After 28 days of post-exposure, iHg and MeHg neurotoxicity remained prominent, suggesting long-term effects of these toxicants.

Evaluation of cadmium and mercury on cardiovascular and neurological systems: Effects on humans and fish

Chemicals are at the top of public health concerns and metals have received much attention in terms of toxicological studies. Cadmium (Cd) and mercury (Hg) are among the most toxic heavy metals and are widely distributed in the environment. They are considered important factors involved in several organ disturbances. Heart and brain tissues are not among the first exposure sites to Cd and Hg but they are directly affected and may manifest intoxication reactions leading to death. Many cases of human intoxication with Cd and Hg showed that these metals have potential cardiotoxic and neurotoxic effects. Human exposure to heavy metals is through fish consumption which is considered as an excellent source of human nutrients. In the current review, we will summarize the most known cases of human intoxication with Cd and Hg, highlight their toxic effects on fish, and investigate the common signal pathways of both Cd and Hg to affect heart and brain tissues. Also, we will present the most common biomarkers used in the assessment of cardiotoxicity and neurotoxicity using Zebrafish model.

Blood Flow Disturbance and Morphological Alterations Following the Right Atrial Ligation in the Chick Embryo

Collectively known as congenital heart defects (CHDs), cardiac abnormalities at birth are the most common forms of neonatal defects. Being principally responsible for the heart‘s pumping power, ventricles are particularly affected by developmental abnormalities, such as flow disturbances or genomic defects. Hypoplastic Right Heart Syndrome (HRHS) is a rare disease where the right ventricle is underdeveloped. In this study, we introduce a surgical procedure performed on chick embryo, termed right atrial ligation (RAL) for disturbing hemodynamics within the right heart aiming in order to generate an animal model of HRHS. RAL is a new surgical manipulation, similar to the well-studied left atrial ligation (LAL) surgery but it induces the hemodynamic change into the right side of the heart. After inducing RAL, We utilized techniques such as Doppler ultrasound, x-ray micro-CT, histology, and computational fluid dynamics (CFD) analysis, for a comprehensive functional and structural analysis of a developing heart. Our results displayed that RAL does not induce severe flow disturbance and ventricular abnormalities consistent with clinical findings. This study allows us to better understand the hemodynamics-driven CHD development and sensitivities of ventricles under disturbed flows.

Microplastics alone or co-exposed with copper induce neurotoxicity and behavioral alterations on zebrafish larvae after a subchronic exposure

Microplastics (MPs, <5 mm) have been frequently detected in aquatic ecosystems, representing both health and ecological concerns. However data about the combined effects of MPs and other contaminants is still limited. This study aimed to evaluate the impact of MPs and the heavy metal copper (Cu) on zebrafish (Danio rerio) larvae development and behavior. Zebrafish embryos were subchronically exposed to MPs (2 mg/L), two sub-lethal concentrations of Cu (60 and 125 µg/L) and binary mixtures of MPs and Cu using the same concentrations, from 2-h post fertilization until 14 days post fertilization. Lethal and sub-lethal responses (mortality, hatching, body length) were evaluated during the embryogenesis period, and locomotor, avoidance, anxiety and shoaling behaviors, and acetylcholinesterase (AChE) activity were measured at 14 dpf. The results showed that survival of larvae was reduced in groups exposed to MPs, Cu and Cu+MPs. Regarding the behavioral patterns, the higher Cu concentration and mixtures decreased significantly the mean speed, the total distance traveled and the absolute turn angle, demonstrating an adverse effect on swimming competence of zebrafish larvae. Exposure to MPs and Cu, alone or combined, also affected avoidance behavior of zebrafish, with larvae not reacting to the aversive stimulus. There was a significant inhibition of AChE activity in larvae exposed to all experimental groups, compared to the control group. Moreover, a higher inhibition of AChE was noticed in larvae exposed to MPs and both Cu+MPs groups, comparatively to the Cu alone groups. Our findings demonstrate the adverse effects of MPs, alone or co-exposed with Cu, on fish early life stages behavior. This study highlights that MPs and heavy metals may have significant impacts on fish population fitness by disrupting locomotor and avoidance behaviors.

Neurobehavioral and immune-toxic impairments induced by organic methyl mercury dietary exposure in Nile tilapia Oreochromis niloticus

Although substantial knowledge of mercury toxicity in fish has been assembled; until now, studies investigating the toxic impacts in Nile tilapia (Oreochromis niloticus) following dietary exposure to organic methyl mercury (MeHg) are less prolific. Accordingly, the current study aimed to evaluate the impacts of MeHg on neurobehavioral and immune integrity in Nile tilapia after dietary exposure. Two hundred and twenty-five juvenile Nile tilapia (19.99 ± 0.33 g) were allocated into five groups in triplicates (15 fish/replicate). G1, G2, G3, G4, and G5. O. niloticus were fed corresponding basal diets containing 0, 0.5, 1, 1.5, and 2 mg/kg diet MeHg chloride (MeHgCl) daily for 30 days, zero value represented the control G1 group. The results showed that MeHg induced significant alterations in O. niloticus behavior, the swimming behavior was significantly decreased, while scratching, biting, and fin tugging behaviors were significantly augmented. Moreover; chasing, mouth pushing, and butting behaviors were significantly increased in all the exposed groups. MeHg significantly decreased brain acetylcholine esterase (AChE) and serum immunoglobulin M (IgM) levels in all the exposed groups. Meanwhile, serum levels of lysozyme (LYZ), nitric oxide (NO), superoxide dismutase (SOD) malondialdehyde (MDA), protein carbonyl (PCO), and 8 hydroxy 2 deoxyguanosine (8OH2dG) were significantly elevated in all the exposed groups except for serum reduced glutathione (GSH) content was significantly decreased implying oxidative stress (OS), lipid peroxidation (LPO), protein, DNA damage and impaired immune response of the exposed tilapia. MeHg significantly altered transcriptional expression of immune-related genes including (TNF-α, IL-1β, and IL-8, and IL-10) in all the exposed groups. From the obtained outcomes, the present research is the premier to investigate that dietary MeHg exposure in O. niloticus significantly induced neurobehavioral and immune defense impairments in a dose-related manner. This study exhibits that dietary MeHg may pose a potential threat to the O. niloticus populations.

Visual system: An understudied target of aquatic toxicology

Visual system is increasingly recognized as a sensitive target of xenobiotics in aquatic ecosystems. Various environmental pollutants of distinct physicochemical properties are able to impair the retinal development and function of teleost fishes, including dioxin-like pollutants, flame retardants, pesticides, perfluoroalkyl acids, retinoic acids and metals. Considering the availability of developmental and functional database, zebrafish has been the most frequently used as the teleost model to study aquatic visual toxicology. A diversity of visual deficits has been displayed for fishes across multiple levels of biological organizations (e.g., molecule, cell, histology, physiology and behavior). Covering sensitive developmental windows of eyes during early embryogenesis, acute or chronic exposure to xenobiotics can disturb the expressions of visual gene and protein markers, which affect the retinal neurogenesis and induce degeneration of neurons. Morphological structures and physiological responses of retina and optic tectum are then disorganized, eventually compromising the performance of visually-mediated behaviors and recruitment of individuals. Environmental pollutants can cross the blood-retina barrier and accumulate in eyes, which might impact visual system directly. In addition, pollutants are very likely to interrupt retinal development and function indirectly by disturbing the signaling of retinoids and thyroid. However, exact mechanisms of visual toxicity are largely unknown currently. In this review, the development and structure of retina and available tools for studying visual science are described briefly. Advances in visual toxicology are summarized in detail and outlooks for future visual toxicity studies are discussed.

Optimization of a Digestion Method to Determine Total Mercury in Fish Tissue by Cold Vapor Atomic Fluorescence Spectrophotometry

Several microwave-assisted digestion methods were tested at the Centro de Estudios Aplicados en Química laboratory in Quito, Ecuador, to determine the accuracy and performance efficiency of the mineralization process for the determination of total mercury in fish tissue by cold vapor atomic fluorescence spectrophotometry. The use of MARSEasyPrep high-pressure vessels, low amounts of reagents (1 cm³ HNO₃, 1 cm³ H₂O₂, and 1 cm³ HClO₄), an irradiation temperature of 210 °C, and 35 min of mineralization time resulted in accurate performance, with recoveries of certified reference material DORM-4 between 90.1% and 105.8%. This is better than the Association of Official Analytical Chemists 2015.01 method, which has a reported accuracy of 81%. The repeatability precision and intermediate precision were established at three concentration levels (0.167, 0.500, and 0.833 mg·kg⁻¹) and expressed as the percentage of the relative standard deviation ranging from 1.5% to 3.0% and 1.7% to 4.2%, respectively. Further, the method was satisfactorily applied to analyze fortified samples of tilapia (Oreochromis niloticus), with recoveries ranging from 98.3% to 104.3%. The instrumental limits of detection and quantification were 0.118 µg·dm⁻³ and 0.394 µg·dm⁻³, respectively.

Diphenyl diselenide dietary supplementation alleviates behavior impairment and brain damage in grass carp (Ctenopharyngodon idella) exposed to methylmercury chloride

Mercury (Hg) induces neurobehavioral disorders through reactive oxygen species (ROS) elevation and impairment of brain key enzyme activities. Nevertheless, the therapeutic and toxic selenium concentrations for fish are very close; diphenyl diselenide (Ph₂Se₂), an organoselenium compound with neuroprotective effects, may be an alternative to elemental Se. Therefore, the aim of this study was to determine whether dietary supplementation with Ph₂Se₂ prevented or reduced the neurobehavioral alterations and oxidative damage elicited by CH₃HgCl in grass carp Ctenopharyngodon idella. Fish exposed to CH₃HgCl exhibited significantly reduced distance travelled and swimming speed compared to the control group, as well as augmented cortisol and ROS levels and xanthine oxidase (XO) activities. CH₃HgCl exposure significantly increased lipid peroxidation (LOOH) and protein carbonylation (PC) levels compared to those of the control group, while acetylcholinesterase (AChE) and sodium-potassium pump (Na⁺, K⁺-ATPase) activities were inhibited. Dietary supplementation with 3 mg/kg Ph₂Se₂ ameliorated locomotor activity impairment and prevented the augmented brain cortisol and ROS levels as well as XO activity. The supplement reduced lipid and protein damage elicited by CH₃HgCl and exerted protective effects on brain AChE and Na⁺, K⁺-ATPase activities. Exposure to an environmental concentration of CH₃HgCl elicited neurobehavioral alterations linked to reduced locomotor activity, a finding that can be explained by oxidative damage and reduced activity of AChE and Na⁺, K⁺-ATPase in telencephalon and mesencephalon structures. Dietary supplementation with Ph₂Se₂ prevented CH₃HgCl-induced locomotor impairment. This effect appeared to be mediated by antioxidant action. Ph₂Se₂ may be a viable approach to prevention or reduction CH₃HgCl-mediated neurotoxic effects.

A multidimensional concept for mercury neuronal and sensory toxicity in fish - From toxicokinetics and biochemistry to morphometry and behavior

BACKGROUND

Neuronal and sensory toxicity of mercury (Hg) compounds has been largely investigated in humans/mammals with a focus on public health, while research in fish is less prolific and dispersed by different species. Well-established premises for mammals have been governing fish research, but some contradictory findings suggest that knowledge translation between these animal groups needs prudence [e.g. the relative higher neurotoxicity of methylmercury (MeHg) vs. inorganic Hg (iHg)]. Biochemical/physiological differences between the groups (e.g. higher brain regeneration in fish) may determine distinct patterns. This review undertakes the challenge of identifying sensitive cellular targets, Hg-driven biochemical/physiological vulnerabilities in fish, while discriminating specificities for Hg forms.

SCOPE OF REVIEW

A functional neuroanatomical perspective was conceived, comprising: (i) Hg occurrence in the aquatic environment; (ii) toxicokinetics on central nervous system (CNS)/sensory organs; (iii) effects on neurotransmission; (iv) biochemical/physiological effects on CNS/sensory organs; (v) morpho-structural changes on CNS/sensory organs; (vi) behavioral effects. The literature was also analyzed to generate a multidimensional conceptualization translated into a Rubik's Cube where key factors/processes were proposed.

MAJOR CONCLUSIONS

Hg neurosensory toxicity was unequivocally demonstrated. Some correspondence with toxicity mechanisms described for mammals (mainly at biochemical level) was identified. Although the research has been dispersed by numerous fish species, 29 key factors/processes were pinpointed.

GENERAL SIGNIFICANCE

Future trends were identified and translated into 25 factors/processes to be addressed. Unveiling the neurosensory toxicity of Hg in fish has a major motivation of protecting ichtyopopulations and ecosystems, but can also provide fundamental knowledge to the field of human neurodevelopment.