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, 250nM) at 5hpf until hatching (72hpf) in a semi-static condition. The mortality rate increased in a dose dependent manner where the chronic embryonic exposure to 100nM decreased the number of tail coiling, heartbeat, and swimming activity. Aversive stimulus was used to examine the effects of 100nM interferes with the development of anxiety-related behavior. No elevation in both thigmotaxis and avoidance response of 6dpf larvae exposed with 100nM 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.
Valproic acid (VPA) is a widely prescribed antiepileptic drug with various medicinal efficacies. Accumulated evidence implied that prenatal exposure to VPA is highly associated with autism spectrum disorder (ASD). In this study, the zebrafish were exposed to a set of VPA concentrations (0, 5, 10, 20, 40, 80, 160, 320, 640, 1280, and 2560 μM) at 5 h post fertilization (hpf) to 120 hpf. The adverse effects of VPA were extensively studied through the evaluations on the mortality, heartbeats, spontaneous tail coiling, and hatching rate. Morphological observations were conducted at 120 hpf, following the exposure termination. Basic locomotor responses and anxiety-like behavioral alterations evaluated for behavioral impairments are the hallmark feature of ASD. The exposure to VPA at teratogenic concentrations reduced the aforementioned parameters in a dose-dependent manner (p ≤ .05). At the selected non-teratogenic concentrations of VPA, the treated larvae demonstrated profound alterations of basic locomotor responses. No significant changes of anxiety and thigmotactic behaviors were observed on the VPA-treated fish compared to the control (p ≥ .005). This study depicted that embryonic zebrafish exposure to VPA produced significant toxicity and teratogenicity effects as well as the alterations of basic behavioral responses. Overall, this study provides a fundamental insight of the toxicity effects at morphological and behavioral levels to facilitate the understanding of ASD mechanisms at different molecular levels.