Chronic dietary exposure to arsenic at environmentally relevant concentrations impairs cognitive performance in adult zebrafish (Danio rerio) via oxidative stress and dopaminergic dysfunction

Zebrafish models for studying cognitive enhancers

Cognitive decline is commonly seen both in normal aging and in neurodegenerative and neuropsychiatric diseases. Various experimental animal models represent a valuable tool to study brain cognitive processes and their deficits. Equally important is the search for novel drugs to treat cognitive deficits and improve cognitions. Complementing rodent and clinical findings, studies utilizing zebrafish (Danio rerio) are rapidly gaining popularity in translational cognitive research and neuroactive drug screening. Here, we discuss the value of zebrafish models and assays for screening nootropic (cognitive enhancer) drugs and the discovery of novel nootropics. We also discuss the existing challenges, and outline future directions of research in this field.

Arsenic bioaccumulation and biotransformation in the marine copepod Tigriopus japonicus under chronic dietborne and waterborne exposure

Arsenic (As) can be transferred along the food chain, while little is known about the toxic effects of dietborne As on marine copepods. In this study, we investigated the short-term and long-term effects of waterborne and dietborne As exposure on the bioaccumulation and biotransformation, as well as developmental toxicity of Tigriopus japonicus. Under acute As exposure, As bioaccumulation increased and reached a plateau with increasing exposure concentration. Moreover, As accumulation at dietborne exposure was 4.3 and 5.7 times greater than that at control group for AsIII and AsV, respectively. At chronic As exposure, As accumulation increased continuously with exposure time, with a 2.8-day extension of development time and a 45% reduction in 10-d fecundity under dietborne exposure compared to control, whereas 2.3-day extension of development time and a 20% reduction in 10-d fecundity were observed under waterborne exposure. Among As species, inorganic As had the highest concentrations, but the proportion of inorganic As decreased from 89% to 63% during 4 to 21 d of exposure, suggesting the conversion of inorganic As to organic As. The organic As was dominated by arsenobetaine (AsB, 13-25%), followed by monomethylarsenic (MMA, 8-25%). These results suggest that dietborne exposure has more pronounced toxic effects on T. japonicus, but the toxicity of As could be reduced through biotransformation under chronic exposure. Therefore, the arsenic species should be considered when assessing As toxicity.

Oral arsenite exposure induces inflammation and apoptosis in pulmonary tissue: acute and chronic evaluation in young and adult mice

Inorganic arsenic is a well-known environmental toxicant, and exposure to this metalloid is strongly linked with severe and extensive toxic effects in various organs including the lungs. In the present study, we aimed to investigate the acute and chronic effects of arsenite exposure on pulmonary tissue in young and adult mice. In brief, young and adult female Balb/C mice were exposed to 3 and 30 ppm arsenite daily via drinking water for 30 and 90 days. Subsequently, the animals were sacrificed and various histological and immunohistochemistry (IHC) analyses were performed using lung tissues. Our findings showed arsenite was found to cause dose-dependent pathological changes such as thickening of the alveolar septum, inflammatory cell infiltrations and lung fibrosis in young and adult mice. In addition, arsenite exposure significantly increased the expression of inflammatory markers NF-κB and TNF-α, indicating that arsenite-exposed mice suffered from severe lung inflammation. Moreover, the IHC analysis of fibrotic proteins demonstrated an increased expression of TGF-β1, α-SMA, vimentin and collagen-I in the arsenite-exposed mice compared to the control mice. This was accompanied by apoptosis, which was indicated by the upregulated expression of caspase-3 in arsenite-exposed mice compared to the control. Adult mice were generally found to be more prone to arsenite toxicity during chronic exposure relative to their younger counterparts. Overall, our findings suggest that arsenite in drinking water may induce dose-dependent and age-dependent structural and functional impairment in the lungs through elevating inflammation and fibrotic proteins.

Reproductive and Developmental Effects of Sex-Specific Chronic Exposure to Dietary Arsenic in Zebrafish (Danio rerio)

The present study investigated the reproductive and developmental effects of sex-specific chronic exposure to dietary arsenic in zebrafish. Adult zebrafish (Danio rerio) were exposed to environmentally realistic doses of arsenic via diet [0 (control; no added arsenic), 30 (low), 60 (medium), and 100 (high) μg/g dry weight, as arsenite] for 90 days. Following exposure, arsenic-exposed females from each dietary treatment were mated with control males, and similarly, arsenic-exposed males from each dietary treatment were mated with control females. In females, arsenic exposure resulted in a dose-dependent decrease in reproductive performance (fecundity, fertilization success, and hatching success). Moreover, a dose-dependent increase in developmental toxicity (larval deformities and larval mortality) was observed with maternal exposure to arsenic. In contrast, in males, arsenic exposure also induced similar reproductive and developmental toxicity; however, the adverse effects were mainly evident only in the medium and high dietary arsenic treatment groups. We also examined the sex-specific effects of dietary arsenic exposure on the expression of genes that regulate the hypothalamus-pituitary-gonadal-liver (HPG-L) axis in fish. The gene expression results indicated the downregulation of HPG-L axis genes in females irrespective of the arsenic treatment dose; however, the reduced expression of HPG-L axis genes in males was recorded only in the medium and high arsenic treatment groups. These observations suggest that chronic arsenic exposure in either females or males causes reproductive and developmental toxicity in zebrafish. However, these toxic effects are markedly higher in females than in males. Our results also suggest that arsenic can act as an endocrine disruptor and mediate reproductive and developmental toxicity by disrupting the HPG-L axis in zebrafish.

Effects of sodium arsenite exposure on behavior, ultrastructure and gene expression of brain in adult zebrafish (Danio rerio)

Arsenic, a common metal-like substance, has been demonstrated to pose potential health hazards and induce behavioral changes in humans and rodents. However, the chronic neurotoxic effects of arsenic on aquatic animals are still not fully understood. This study aimed to investigate the effects of arsenic exposure on adult zebrafish by subjecting 3-month-old zebrafish to three different sodium arsenite water concentrations: 0 μg/L (control group), 50 μg/L, and 500 μg/L, over a period of 30 days. To assess the risk associated with arsenic exposure in the aquatic environment, behavior analysis, transmission electron microscopy techniques, and quantitative real-time PCR were employed. The behavior of adult zebrafish was evaluated using six distinct tests: the mirror biting test, shoaling test, novel tank test, social preference test, social recognition test, and T maze. Following the behavioral tests, the brains of zebrafish were dissected and collected for ultrastructural examination and gene expression analysis. The results revealed that sodium arsenite exposure led to a significant reduction in aggression, cohesion, social ability, social cognition ability, learning, and memory capacity of zebrafish. Furthermore, ultrastructure and genes regulating behavior in the zebrafish brain were adversely affected by sodium arsenite exposure.

Zinc attenuates arsenic overdose-induced brain damage via PERK/ATF6 and TLR/MyD88/NF-κB pathways

Exposure to arsenic (As), a widespread non-metallic toxicant in nature, often results in neurotoxicity, although the exact mechanism is unknown. Zinc (Zn) is a powerful nutrient often thought to be beneficial for growth, development and immunity. Whether Zn can rescue brain damage caused by As contamination remains to be demonstrated. Therefore, in this study, a 30-day model of As poisoning (2.83 mg/L) in carp was established and treated with Zn (1 mg/L) to investigate the detoxification mechanism involved. Histological observations showed that As induced the loosening of the molecular layer structure of the cerebellum and the dissolution or even disappearance of nuclei, accompanied by the occurrence of microthrombi in the granular layer, and the addition of Zn attenuated such As-induced damage. Further mechanistic studies indicated that Zn ameliorated As exposure-induced abnormalities in antioxidant capacity (decreased CAT and Cu/Zn-SOD), activation of the Nrf2/keap1 pathway and endoplasmic reticulum stress (ERs), which is a key factor in As-induced brain damage. ERs (high expression of PERK, ATF6, CHOP, eiF2α and GRP78) and inflammation (overexpression of TLR2, TLR4, MyD88, IKK, NF-κB, IL-1β and IL-6 and low expression of IκBα and IL-10). We suggest that Zn can alleviate excessive As-induced brain damage by attenuating As-induced oxidative stress, PERK/ATF6 and TLR/MyD88/NF-κB pathways. The present study fills in the preventive mechanism of As injury in fish and provides the possibility of prevention and control of As pollution-induced brain tissue injury by Zn rescue.