Search

Arsenic trioxide (As2O3) is a ubiquitous heavy metal in the environment. Exposure to this toxin at low concentrations is unremarkable in developing organisms. Nevertheless, understanding the underlying mechanism of its long-term adverse effects remains a challenge. In this study, embryos were initially exposed to As2O3 from gastrulation to hatching under semi-static conditions. Results showed dose-dependent increased mortality, with exposure to 30–40 µM As2O3 significantly reducing tail-coiling and heart rate at early larval stages. Surviving larvae after 30 µM As2O3 exposure showed deficits in motor behavior without impairment of anxiety-like responses at 6 dpf and a slight impairment in color preference behavior at 11 dpf, which was later evident in adulthood. As2O3 also altered locomotor function, with a loss of directional and color preference in adult zebrafish, which correlated with changes in transcriptional regulation of adsl, shank3a, and tsc1b genes. During these processes, As2O3 mainly induced metabolic changes in lipids, particularly arachidonic acid, docosahexaenoic acid, prostaglandin, and sphinganine-1-phosphate in the post-hatching period of zebrafish. Overall, this study provides new insight into the potential mechanism of arsenic toxicity leading to long-term learning impairment in zebrafish and may benefit future risk assessments of other environmental toxins of concern.

An ideal model organism for neurotoxicology research should meet several characteristics, such as low cost and amenable for high throughput testing. Javanese medaka (JM) has been widely used in the ecotoxicological studies related to the marine and freshwater environment, but rarely utilized for biomedical research. Therefore, in this study, the applicability of using JM in the neurotoxicology research was assessed using biochemical comparison with an established model organism, the zebrafish. Identification of biochemical changes due to the neurotoxic effects of ethanol and endosulfan was assessed using Fourier Transform Infrared (FTIR) analysis. Treatment with ethanol affected the level of lipids, proteins, glycogens and nucleic acids in the brain of JM. Meanwhile, treatment with endosulfan showed alteration in the level of lipids and nucleic acids. For the zebrafish, exposure to ethanol affected the level of protein, fatty acid and amino acid, and exposure to endosulfan induced alteration in the fatty acids, amino acids, nucleic acids and protein in the brain of zebrafish. The sensitive response of the JM toward chemicals exposure proved that it was a valuable model for neurotoxicology research. More studies need to be conducted to further develop JM as an ideal model organism for neurotoxicology research.

Zebrafish is increasingly popular in neurotoxicology research due to their small size, low maintenance and efficient for high throughput testing. However, using nonnative species may raise concern on ecological aspect as accidental introduction of foreign species to our local aquatic environment could cause ecosystem instability. Therefore, the applicability of using Javanese medaka (JM) in the neurotoxicology and developmental neurotoxicology (DNT) research will be assessed by using zebrafish as a reference model. Assessment of the exploratory, anxiety and social behaviour were conducted by using open field test and aquatic light/dark plus maze. Biochemical assessment was conducted using Fourier Transform Infrared (FTIR) analysis. Without exposure to neurotoxicants, JM displayed less exploratory behaviour and showed no difference for anxiety and social behaviour as compared to the zebrafish. JM exhibited light preference while zebrafish showed dark preference. Exposure to 1% ethanol resulted a reduction in exploratory behaviour, induction in anxiety like behaviour and no alteration in the social behaviour of JM. Meanwhile, zebrafish showed a reduction in anxiety like behaviour and no alteration in exploratory and social behaviour. Treatment with 1.6 μg/L endosulfan resulted increment in exploratory behaviour, induction in anxiety like behaviour and decrement in social behaviour for both fishes. Exposure to 100 mg/L caffeine resulted anxiogenic response and decrement in the exploratory behaviour for both fishes. Alteration in social behaviour was only observed in the zebrafish. Both fishes showed alteration in biochemical profile after exposure to ethanol or endosulfan with regard to different macromolecules in the brain tissues. This study also found that, JM is inefficient for DNT research as they are laborious, time-consuming and not amenable for high throughput screening. More studies need to be conducted to further develop JM as an ideal model organism for neur

Exposure to ethanol during critical period of development can cause severe impairments in the central nervous system (CNS). This study was conducted to assess the neurotoxic effects of chronic embryonic exposure to ethanol in the zebrafish, taking into consideration the time dependent effect. Two types of exposure regimen were applied in this study. Withdrawal exposure group received daily exposure starting from gastrulation until hatching, while continuous exposure group received daily exposure from gastrulation until behavioural assessment at 6dpf (days post fertilization). Chronic embryonic exposure to ethanol decreased spontaneous tail coiling at 24hpf (hour post fertilization), heart rate at 48hpf and increased mortality rate at 72hpf. The number of apoptotic cells in the embryos treated with ethanol was significantly increased as compared to the control. We also measured the morphological abnormalities and the most prominent effects can be observed in the treated embryos exposed to 1.50% and 2.00%. The treated embryos showed shorter body length, larger egg yolk, smaller eye diameter and heart edema as compared to the control. Larvae received 0.75% continuous ethanol exposure exhibited decreased swimming activity and increased anxiety related behavior, while withdrawal ethanol exposure showed increased swimming activity and decreased anxiety related behavior as compared to the respective control. Biochemical analysis exhibited that ethanol exposure for both exposure regimens altered proteins, lipids, carbohydrates and nucleic acids of the zebrafish larvae. Our results indicated that time dependent effect of ethanol exposure during development could target the biochemical processes thus leading to induction of apoptosis and neurobehavioral deficits in the zebrafish larvae. Thus it raised our concern about the safe limit of alcohol consumption for pregnant mother especially during critical periods of vulnerability for developing nervous system.

Chronic exposure to mercury (Hg) can lead to cumulative impairments in motor and cognitive functions including alteration in anxiety responses. Although several risk factors have been identified in recent year, little is known about the environmental factors that either due exposure toward low level of inorganic mercury that may led to the developmental disorders. The present study investigated the effects of embryonic exposure of mercury chloride on motor function and anxiety-like behavior. The embryo exposed to 6 different concentrations of HgCl2 (7.5, 15, 30, 100, 125, 250 nM) at 5 hpf until hatching (72 hpf) in a semi-static condition. The mortality rate increased in a dose dependent manner where the chronic embryonic exposure to 100 nM decreased the number of tail coiling, heartbeat, and swimming activity. Aversive stimulus was used to examine the effects of 100 nM interferes with the development of anxiety-related behavior. No elevation in both thigmotaxis and avoidance response of 6 dpf larvae exposed with 100 nM were found. Biochemical analysis showed HgCl2 exposure affects proteins, lipids, carbohydrates and nucleic acids of the zebrafish larvae. These results showed that implication of HgCl2 on locomotor and biochemical defects affects motor performance and anxiety-like responses. Yet, the potential underlying mechanisms these responses need to be further investigated which is crucial to prevent potential hazards on the developing organism due to neurotoxicant exposure.

Carbon nanotubes, which had been discovered in 1991, comprise of two different types of variety; the single walled carbon nanotubes (SWCNTs) and multi walled carbon nanotubes (MWCNTs). Both of these differ in their structures, depending on the number of graphene layer forming the concentric tube. To date, published research related to the carbon nanotubes by using zebrafish as the model organism, were mainly focused on the toxicity effects of carbon nanotubes on the developing embryos. Most of these studies mainly concentrated on the use of SWCNTs as the toxicant. It is likely due to the fact that purified SWCNTs are more toxic than the purified MWCNTs. Nevertheless, it is hard to determine the exact toxicity effects of the carbon nanotubes, since different preparation methods will likely yield different results. In this study, we were investigating the toxicity effects of different concentrations of raw MWCNTs (control, 1ug/L, 10mg/L and 100mg/L) on the anxiety-like behavior of the male adult zebrafish, by using the light-dark plus maze paradigm. Prior to the experiment, the MWCNTs were dispersed in the distilled water for 60 minutes by using the bath sonicator. Then, each of the fish replicates was exposed to different concentrations of MWCNTs about 60 minutes before it was transferred into a black holding cup (placed at the center of the plus maze), for acclimatization period. After 5 minutes, the holding cup was removed and the movement of the fish was recorded by using a camera situated at the top of the maze. The recording was then used for the analysis purpose. The parameters measured include the latency to enter the arms and the mean duration per visit.

Anxiety disorders are prevalent neuropsychiatric conditions that affect human emotion and cognitive functions. Increasing incidence of anxiety in the society caused global economic burden. The causes of anxiety are still unfathomable. Also, anxiety treatment using modern drugs are associated with different types of negative effects. These two situations increased the use of animal models for toxicological and neuropharmacological studies. In this context, a Malaysian native fish species, Javanese medaka (Oryzias javanicus) offer an opportunity to be developed as a model organism for neurotoxicology and neuropharmacology study. In general, anxious zebrafish (Danio rerio) will spend more time in the dark while, bold zebrafish will spend more time in the light. However, currently there is no empirical evidence for this particular condition in Javanese medaka. Since the zebrafish has been established to model anxiety, we will compare anxiety responses in the zebrafish and Javanese medaka to measure the sensitivity of these two species for the assessment of anxiety-related responses. To evaluate anxiety in these two different species, we study the anxiety response of zebrafish and Javanese medaka when introduced to novel environment light/dark plus maze. The aquatic light/dark plus maze is a behavioral testing battery developed based on the tendency of fish to seek dark backgrounds (or avoid light backgrounds) in unfamiliar environments. Fish behavior was observed in the light/dark plus maze for 5 minutes. Based on our preliminary study, we found medaka spend more time in light as compared to the zebrafish.

Arsenic trioxide (As2O3) is a heavy metal that is widely used for therapeutic purpose and is found ubiquitously in the environment. Currently, information about the adverse effects associated with exposure to low levels of inorganic arsenic on the developing organisms is limited. We examined the effects of embryonic exposure of As2O3 on motor functions and anxiety-like behaviors in the developing zebrafish. The embryos were exposed to 4 different concentrations (20, 30, 40, 50 µM) starting from 5 hpf until hatching (72 hpf) in a semi-static condition. The control groups were maintained in the embryo media. The mortality rate increased in a dose dependent manner. We found that chronic embryonic exposure to 30 and 40 µM decreased the number of tail coiling, heartbeat, and swimming activity. Meanwhile, exposure 20 µM did not produce any significant alteration in these parameters. To further understand the effects ofAs2O3 interferences on the development of anxiety-related behavior, we chose 30 µM As2O3 exposed larvae since the 40 µM As2O3 achieved 100% mortality before the behavioral recording was conducted on 6 dpf. However, we found no alterations on thigmotaxis, avoidance response, speed and percentage of resting of 6 dpf larvae. Since we found no alterations in all parameters measured for anxiety related behavior, we evaluated the apoptosis in the eye of the larvae using acridine orange. We found a significant increase in the apoptotic signal particularly in the eye region. These results showed that exposure to microconcentration of As2O3 do not alter the anxiety related responses, yet, inducing apoptosis in the retina and lens, which may contribute to visual deficit. The potential underlying mechanisms and implication of As2O3 on visual acuity, motor performance and anxiety-like responses need further investigation.

Decades of research have pinpointed the pathological consequences of ethanol and we focused to understand the developmental neurotoxicity effects of ethanol on the zebrafish nervous system. The zebrafish embryos were exposed to different concentrations of ethanol (0.25%, 0.5%, 0.75%, 1.5% and 2.00%), in a semi static condition from 5 hours post fertilization (hpf), with daily renewal of the medium until hatching. We have measured the behavior of individual larva; spontaneous locomotor behavior, tail coiling at 24 hpf and locomotor behavior, swimming activity at 6 days post fertilization. We also measured anxiety related behavior in the group of 5 larvae at 6 dpf. Besides that, we also evaluate apoptosis in the zebrafish embryo at 24 hpf. We found that, chronic embryonic exposure to ethanol significantly decreased the number of spontaneous tail coiling as the ethanol concentration increased. The swimming activity in the individual zebrafish zebrafish larva at 6 dpf was significantly increase increased at 0.75%. Analysis of the anxiety related behavior has shown that exposure to ethanol and stimulation with aversive stimulus have significantly decreased in the percentage of outward preference in the larvae. The exposure to ethanol has induced significant decrease in the percentage of edge and down preference of the larvae in the presence of aversive stimulus as compared to control. The exposure to ethanol significantly decreased the percentage of right preference while the swimming speed of the larvae was not affected. The number of apoptotic cells was significantly increased in 1.5% ethanol as compared to the control at 24 hpf. This finding showed that chronic embryonic exposure to ethanol induced behavioral alterations in the zebrafish larvae and also increased apoptosis in the embryo. Further study is needed to understand the mechanism that underlie the behavioral alterations and the induction of apoptosis.