The sensory-motor responses to environmental acidosis in larval zebrafish: Influences of neurotransmitter and water chemistry

Sublethal effects of acidified water on sensorimotor responses and the transcriptome of zebrafish embryos

Acidification of freshwater due to human activities is a widespread environmental problem. Its effects on the sensorimotor responses of fish, particularly during embryonic stages, may affect population fitness. To address this, zebrafish embryos were exposed to water at pH 7, 5 and 4.5 (adjusted with HCl) for 120 h. Acidic water did not increase mortality or cause obvious morphological abnormalities but reduced the size of the inner ear organs (otic vesicle and otolith) and the eye lens. It also suppressed ion uptake (Na+, Ca2+, K+) and induced embryonic acidosis. Behavioral tests at 4 or 5 days post fertilization revealed significant sensorimotor impairments: reduced touch-evoked escape responses (TEER), decreased acoustic startle responses (ASR) and decreased cadaverine avoidance responses (CAR). There were no effects on speed, acceleration and optomotor responses (OMR). Transcriptomic analyses identified 114 differentially expressed genes (DEGs) associated with ion transport, sensorimotor functions and other physiological processes. Overall, the jeopardizing effect of freshwater acidification threatens survival, highlighting the ecological risks and its potential impacts on fish populations.

Visual toxicity in zebrafish larvae following exposure to 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), triphenyl phosphate (TPhP), and isopropyl phenyl diphenyl phosphate (IPPP)

TPhP and IPPP, alternatives to PBDEs as flame retardants, have been studied for their developmental toxicity, but their visual toxicities are less understood. In this study, zebrafish larvae were exploited to evaluate the potential ocular impairments following exposure to BDE-47, TPhP, and IPPP. The results revealed a range of ocular abnormalities, including malformation, vascular issues within the eyes, and histopathological changes in the retina. Notably, the visually mediated behavioral changes were primarily observed in IPPP and TPhP, indicating that they caused more severe eye malformations and vision impairment than BDE-47. Molecular docking and MD simulations showed stronger binding affinity of TPhP and IPPP to RAR and RBP receptors. Elevated ROS and T3 levels induced by these compounds led to apoptosis in larvae eyes, and increased GABA levels induced by TPhP and IPPP hindered retinal repair. In summary, our results indicate TPhP and IPPP exhibit severer visual toxicity than BDE-47, affecting eye development and visually guided behaviors. The underlying mechanism involves disruptions in RA signaling, retinal neurotransmitters imbalance, thyroid hormones up-regulation, and apoptosis in larvae eyes. This work highlights novel insights into the need for cautious use of these flame retardants due to their potential biological hazards, thereby offering valuable guidance for their safer applications.

Developmental Neurotoxicity of Difenoconazole in Zebrafish Embryos

Difenoconazole is a type of triazole fungicide that is widely used in the treatment of plant diseases. Triazole fungicides have been shown in several studies to impair the development of the nervous system in zebrafish embryos. There is still little known about difenoconazole-induced neurotoxicity in fish. In this study, zebrafish embryos were exposed to 0.25, 0.5, and 1 mg/L of difenoconazole solution until 120 h post-fertilization (hpf). The difenoconazole-exposed groups showed concentration-dependent inhibitory tendencies in heart rate and body length. Malformation rate and spontaneous movement of zebrafish embryos increased, and the locomotor activity decreased in the highest exposure group. The content of dopamine and acetylcholine was reduced significantly in difenoconazole treatment groups. The activity of acetylcholinesterase (AChE) was also increased after treatment with difenoconazole. Furthermore, the expression of genes involved in neurodevelopment was remarkably altered, which corresponded with the alterations of neurotransmitter content and AChE activity. These results indicated that difenoconazole might affect the development of the nervous system through influencing neurotransmitter levels, enzyme activity, and the expression of neural-related genes, ultimately leading to abnormal locomotor activity in the early stages of zebrafish.

Decabromodiphenyl ethane induced hyperactivity in developing zebrafish at environmentally relevant concentrations

Decabromodiphenyl ethane (DBDPE), a widely used novel brominated flame retardant, is gaining concerns due to rapidly increased contents in various environmental and biota samples. In the present study, zebrafish (Danio rerio) embryos were exposed to 2.91, 9.71, 29.14 and 97.12 μg/L of DBDPE until 120 h post-fertilization (hpf) to investigate the potential developmental neurotoxicity and underlying mechanisms. Chemical analysis revealed concentration-dependently increased body burdens of DBDPE in zebrafish larvae, with bioaccumulation factors (BCFs) ranging from 414 to 726. Embryonic exposure to DBDPE caused hyperactivity without affecting the development of secondary motoneuron axons and muscle fibers. However, further results implicated that DBDPE may affect the locomotor regulatory network via different mechanisms at lower and higher concentrations. On the one hand, embryonic exposure to 2.91 μg/L DBDPE transiently promoted spontaneous coiling contractions, but showed no effects on touch-response and swimming activity in zebrafish larvae. The whole-body contents of neurotransmitters were significantly decreased. Significant decreased protein abundances of α1-TUBULIN and SYN2a and molecular docking results pointed out possible interactions of DBDPE with these two proteins. However, these changes may be unconcerned with the transient hyperactivity, and the exact molecular mechanisms need further investigation. On the other hand, 29.14 and 97.12 μg/L DBDPE exposure caused longer-lasting effects in promoting spontaneous coiling contractions, and also touch-response and swimming activity. At the same time, increased ACh contents (without changes of other neurotransmitters) and ChAT activity and inhibited transcription of nAChRs were observed at higher concentrations. Molecular docking indicated direct interaction of DBDPE with ChAT. The results suggested that DBDPE induced hyperactivity at higher concentrations was probably involved with disrupted cholinergic system, with ChAT as a potential target. Given that the body burden of DBDPE in lower concentration group was comparable with those detected in wild fish, the current results may provide useful information for ecological risk assessment.

Toxicological assessment of cadmium-containing quantum dots in developing zebrafish: Physiological performance and neurobehavioral responses

The present research investigated the effects of exposure to sublethal concentrations of cadmium selenide/zinc sulfide (CdSe_core/ZnS_shell)-containing quantum dots (QDs; 0 - 100 µg/L QDs) on the neurophysiological performance of developing zebrafish (Danio rerio). The results suggested that exposure to CdSe QDs for 5 days increased the whole-body content of Cd without affecting the general physiological conditions of larvae. Interestingly, CdSe QD exposure reduced swimming distance but increased swimming velocity of larvae, suggesting that the exposure may lead to burst/episodic swimming. The findings also suggested that CdSe QD exposure reduced the wall-hugging behavior of larvae during a sudden light-to-dark transition test, and that the exposure significantly decreased the locomotor activity of fish during the dark period. On the other hand, control larvae displayed a dark avoidance behavior, whereas CdSe QD-exposed larvae exhibited an increase in the time spent in the dark zone, providing further support that CdSe QDs inhibited anxiety-related responses in larvae. Additional analysis with droplet digital PCR revealed that CdSe QD exposure altered the mRNA levels of genes that are associated with dopamine signaling and oxidative stress response. Collectively, our findings suggested that CdSe QD exposure may induce neurobehavioural toxicity and alters the mRNA abundance of dopamine- and oxidative stress-related genes in developing animals.

Effects of reduced pH on an estuarine penaeid shrimp (Metapenaeus macleayi)

Acid sulfate soils are a major problem in modified coastal floodplains and are thought to have substantial impacts on estuarine species. In New South Wales, Australia, acid sulfate soils occur in every estuary and are thought to impact important fisheries species, such as Eastern School Prawn (Metapenaeus macleayi). These fisheries have experienced declining productivity over the last ten years and increasing occurrence of catchment-derived stressors in estuaries contribute to this problem. We evaluated the effect of pH 4-7.5 on School Prawn survival at two salinities (27 and 14.5), pH 5, 6 and 7.5 on the predation escape response (PER) speed at two salinities (27 and 14.5), and pH 4 and 7.5 on respiration rates. While mortality appeared to be greater in the high salinity treatment, there was no significant relationship between proportional survival and pH for either salinity treatment. Respiration was significantly slower under acidic conditions and the average PER was almost twice as fast at pH 7.5 compared to pH 5 (p < 0.05), indicating prawns may fall prey to predation more easily in acidic conditions. These findings confirm the hypothesised impacts of acidic water on penaeid prawns. Given that the conditions simulated in these experiments reflect those encountered in estuaries, acidic runoff may be contributing to bottlenecks for estuarine species and impacting fisheries productivity.

The neurophysiological effects of iron in early life stages of zebrafish

Trace metal/ion homeostasis, neurophysiological performance, and molecular responses to iron (Fe) exposure were investigated in the model organism zebrafish (Danio rerio). The findings demonstrated that exposure to a sublethal concentration of ferric iron (Fe³⁺) increased Fe contents in both the whole body and head region of developing zebrafish. Among the various trace metals and major ion examined, a dysregulation in manganese, zinc, nickel, and calcium balance was also observed in Fe-exposed larvae. Further biochemical assay and in-vivo imaging revealed that Fe exposure resulted in possible oxidative stress-induced damage, and an increased generation of reactive oxygen species in specific regions of the larvae. Using a droplet digital PCR (ddPCR) technology, it was found that the expression levels of various oxidative stress-responsive genes were temporally modulated by Fe exposure. Additionally, Fe-exposed larvae exhibited an impairment in escape response and a decrease in swimming activity. These larvae also appeared to exhibit a reduced anxiety-like behaviour. Together, our research suggested that larvae experiencing an increased Fe loading exhibited a dysregulation in metal homeostasis and a decrease in neurophysiological performance. These results suggested that neurophysiological assessments are sensitive methods to evaluate Fe toxicity in developing fish.

Respirometry and cutaneous oxygen flux measurements reveal a negligible aerobic cost of ion regulation in larval zebrafish (Danio rerio)

Fishes living in fresh water counter the passive loss of salts by actively absorbing ions through specialized cells termed ionocytes. Ionocytes contain ATP-dependent transporters and are enriched with mitochondria; therefore ionic regulation is an energy-consuming process. The purpose of this study was to assess the aerobic costs of ion transport in larval zebrafish (Danio rerio). We hypothesized that changes in rates of Na+ uptake evoked by acidic or low Na+ rearing conditions would result in corresponding changes in whole-body oxygen consumption (ṀO2 ) and/or cutaneous oxygen flux (JO2 ), measured at the ionocyte-expressing yolk sac epithelium using the scanning micro-optrode technique (SMOT). Larvae at 4 days post-fertilization (dpf) that were reared under low pH (pH 4) conditions exhibited a higher rate of Na+ uptake compared with fish reared under control conditions (pH 7.6), yet they displayed a lower ṀO2  and no difference in cutaneous JO2 Despite a higher Na+ uptake capacity in larvae reared under low Na+ conditions, there were no differences in ṀO2  and JO2  at 4 dpf. Furthermore, although Na+ uptake was nearly abolished in 2 dpf larvae lacking ionocytes after morpholino knockdown of the ionocyte proliferation regulating transcription factor foxi3a, ṀO2 and JO2  were unaffected. Finally, laser ablation of ionocytes did not affect cutaneous JO2 Thus, we conclude that the aerobic costs of ion uptake by ionocytes in larval zebrafish, at least in the case of Na+, are below detection using whole-body respirometry or cutaneous SMOT scans, providing evidence that ion regulation in zebrafish larvae incurs a low aerobic cost.

Effects of state anxiety on gait: a 7.5% carbon dioxide challenge study

We used the 7.5% carbon dioxide (CO₂) model of anxiety induction to investigate the effects of state anxiety on normal gait and gait when navigating an obstacle. Healthy volunteers (n = 22) completed a walking task during inhalations of 7.5% CO₂ and medical air (placebo) in a within-subjects design. The order of inhalation was counterbalanced across participants and the gas was administered double-blind. Over a series of trials, participants walked the length of the laboratory, with each trial requiring participants to navigate through an aperture (width adjusted to participant size), with gait parameters measured via a motion capture system. The main findings were that walking speed was slower, but the adjustment in body orientation was greater, during 7.5% CO₂ inhalation compared to air. These findings indicate changes in locomotor behaviour during heightened state anxiety that may reflect greater caution when moving in an agitated state. Advances in sensing technology offer the opportunity to monitor locomotor behaviour, and these findings suggest that in doing so, we may be able to infer emotional states from movement in naturalistic settings.