Acetyl L-carnitine protects motor neurons and Rohon-Beard sensory neurons against ketamine-induced neurotoxicity in zebrafish embryos

Temperature- and chemical-induced neurotoxicity in zebrafish

Throughout their lives, humans encounter a plethora of substances capable of inducing neurotoxic effects, including drugs, heavy metals and pesticides. Neurotoxicity manifests when exposure to these chemicals disrupts the normal functioning of the nervous system, and some neurotoxic agents have been linked to neurodegenerative pathologies such as Parkinson's and Alzheimer's disease. The growing concern surrounding the neurotoxic impacts of both naturally occurring and man-made toxic substances necessitates the identification of animal models for rapid testing across a wide spectrum of substances and concentrations, and the utilization of tools capable of detecting nervous system alterations spanning from the molecular level up to the behavioural one. Zebrafish (Danio rerio) is gaining prominence in the field of neuroscience due to its versatility. The possibility of analysing all developmental stages (embryo, larva and adult), applying the most common "omics" approaches (transcriptomics, proteomics, lipidomics, etc.) and conducting a wide range of behavioural tests makes zebrafish an excellent model for neurotoxicity studies. This review delves into the main experimental approaches adopted and the main markers analysed in neurotoxicity studies in zebrafish, showing that neurotoxic phenomena can be triggered not only by exposure to chemical substances but also by fluctuations in temperature. The findings presented here serve as a valuable resource for the study of neurotoxicity in zebrafish and define new scenarios in ecotoxicology suggesting that alterations in temperature can synergistically compound the neurotoxic effects of chemical substances, intensifying their detrimental impact on fish populations.

Single-cell analysis of Rohon-Beard neurons implicates Fgf signaling in axon maintenance and cell survival

Peripheral sensory neurons are a critical part of the nervous system that transmit a multitude of sensory stimuli to the central nervous system. During larval and juvenile stages in zebrafish, this function is mediated by Rohon-Beard somatosensory neurons (RBs). RBs are optically accessible and amenable to experimental manipulation, making them a powerful system for mechanistic investigation of sensory neurons. Previous studies provided evidence that RBs fall into multiple subclasses; however, the number and molecular make up of these potential RB subtypes have not been well defined. Using a single-cell RNA sequencing (scRNA-seq) approach, we demonstrate that larval RBs in zebrafish fall into three, largely non-overlapping classes of neurons. We also show that RBs are molecularly distinct from trigeminal neurons in zebrafish. Cross-species transcriptional analysis indicates that one RB subclass is similar to a mammalian group of A-fiber sensory neurons. Another RB subclass is predicted to sense multiple modalities, including mechanical stimulation and chemical irritants. We leveraged our scRNA-seq data to determine that the fibroblast growth factor (Fgf) pathway is active in RBs. Pharmacological and genetic inhibition of this pathway led to defects in axon maintenance and RB cell death. Moreover, this can be phenocopied by treatment with dovitinib, an FDA-approved Fgf inhibitor with a common side effect of peripheral neuropathy. Importantly, dovitinib-mediated axon loss can be suppressed by loss of Sarm1, a positive regulator of neuronal cell death and axonal injury. This offers a molecular target for future clinical intervention to fight neurotoxic effects of this drug.

Chronic exposure to (2 R,6 R)-hydroxynorketamine induces developmental neurotoxicity in hESC-derived cerebral organoids

(R,S)-ketamine (ketamine) has been increasingly used recreationally and medicinally worldwide; however, it cannot be removed by conventional wastewater treatment plants. Both ketamine and its metabolite norketamine have been frequently detected to a significant degree in effluents, aquatic, and even atmospheric environments, which may pose risks to organisms and humans via drinking water and aerosols. Ketamine has been shown to affect the brain development of unborn babies, while it is still elusive whether (2 R,6 R)-hydroxynorketamine (HNK) induces similar neurotoxicity. Here, we investigated the neurotoxic effect of (2 R,6 R)-HNK exposure at the early stages of gestation by applying human cerebral organoids derived from human embryonic stem cells (hESCs). Short-term (2 R,6 R)-HNK exposure did not significantly affect the development of cerebral organoids, but chronic high-concentration (2 R,6 R)-HNK exposure at day 16 inhibited the expansion of organoids by suppressing the proliferation and augmentation of neural precursor cells (NPCs). Notably, the division mode of apical radial glia was unexpectedly switched from vertical to horizontal division planes following chronic (2 R,6 R)-HNK exposure in cerebral organoids. Chronic (2 R,6 R)-HNK exposure at day 44 mainly inhibited the differentiation but not the proliferation of NPCs. Overall, our findings indicate that (2 R,6 R)-HNK administration leads to the abnormal development of cortical organoids, which may be mediated by inhibiting HDAC2. Future clinical studies are needed to explore the neurotoxic effects of (2 R,6 R)-HNK on the early development of the human brain.

Longitudinal effects of ketamine on cell proliferation and death in the CNS of zebrafish

Zebrafish is known for its widespread neurogenesis and regenerative capacity, as well as several biological advantages, which turned it into a relevant animal model in several areas of research, namely in toxicological studies. Ketamine is a well-known anesthetic used both in human as well as veterinary medicine, due to its safety, short duration and unique mode of action. However, ketamine administration is associated with neurotoxic effects and neuronal death, which renders its use on pediatric medicine problematic. Thus, the evaluation of ketamine effects administration at early stages of neurogenesis is of pivotal importance. The 1-4 somites stage of zebrafish embryo development corresponds to the beginning of segmentation and formation of neural tube. In this species, as well as in other vertebrates, longitudinal studies are scarce, and the evaluation of ketamine long-term effects in adults is poorly understood. This study aimed to assess the effects of ketamine administration at the 1-4 somites stage, both in subanesthetic and anesthetic concentrations, in brain cellular proliferation, pluripotency and death mechanisms in place during early and adult neurogenesis. For that purpose, embryos at the 1-4 somites stage (10,5hours post fertilization - hpf) were distributed into study groups and exposed for 20minutes to ketamine concentrations at 0.2/0.8mg/mL. Animals were grown until defined check points, namely 50 hpf, 144 hpf and 7 months adults. The assessment of the expression and distribution patterns of proliferating cell nuclear antigen (PCNA), of sex-determining region Y-box 2 (Sox 2), apoptosis-inducing factor (AIF) and microtubule-associated protein 1 light chain 3 (LC3) was performed by Western-blot and immunohistochemistry. The results evidenced the main alterations in 144 hpf larvae, namely in autophagy and in cellular proliferation at the highest concentration of ketamine (0.8mg/mL). Nonetheless, in adults no significant alterations were seen, pointing to a return to a homeostatic stage. This study allowed clarifying some of the aspects pertaining the longitudinal effects of ketamine administration regarding the CNS capacity to proliferate and activate the appropriate cell death and repair mechanisms leading to homeostasis in zebrafish. Moreover, the results indicate that ketamine administration at 1-4 somites stage in the subanesthetic and anesthetic concentrations despite some transitory detrimental effects at 144 hpf, is long-term safe for CNS, which are newly and promising results in this research field.

Inorganic arsenic alters the development of dopaminergic neurons but not serotonergic neurons and induces motor neuron development via Sonic hedgehog pathway in zebrafish

The mechanism of inorganic arsenic-induced neurotoxicity at the cellular level is not known. In zebrafish, teratological effects of inorganic arsenic have been shown at various concentrations. Here, we used similar concentrations of inorganic arsenic to evaluate the effects on specific neuron types. Exposure of zebrafish embryos at 5 h post fertilization (hpf) to sodium arsenite induced developmental toxicity (reduced body length) in 72 hpf larvae, beginning at a concentration of 300 mg/L concentration. Mortality or overt morphological deformity was detected at 500 mg/L sodium arsenite. While 200 mg/L sodium arsenite induced development of tyrosine hydroxylase-positive (dopaminergic) neurons, there was no significant effect on the development of 5-hydroxytryptamine (serotonergic) neurons. Sodium arsenite reduced acetylcholinesterase activity. In the hb9-GFP transgenic larvae, both 200 and 400 mg/L sodium arsenite produced supernumerary motor neurons in the spinal cord. Inhibition of the Sonic hedgehog (Shh) pathway that is essential for motor neuron development, by Gant61, prevented sodium arsenite-induced supernumerary motor neuron development. Inductively coupled plasma mass spectrometry (ICP-MS) revealed that with 200 mg/L and 400 mg/L sodium arsenite treatment, each larva had an average of 387.8 pg and 847.5 pg arsenic, respectively. The data show for the first time that inorganic arsenic alters the development of dopaminergic and motor neurons in the zebrafish larvae and the latter occurs through the Shh pathway. These results may help understand why arsenic-exposed populations suffer from psychiatric disorders and motor neuron disease and Shh may, potentially, serve as a plasma biomarker of arsenic toxicity.

Zebrafish (Danio rerio): A potential model to assess developmental toxicity of ketamine

Ketamine is a non-competitive antagonist of NMDA glutamate receptor. It is used as an anesthetic, analgesic, sedative, and anti-depressive agent in clinical practice and also an illegal recreational drug. The increasing use has contributed to the measurable levels of ketamine in both wastewaters and hospital effluents, thereby classified as an emergent contaminant. Lately, the potential toxicity of ketamine has raised serious concerns about its iatrogenic or illicit use during pregnancy, neonatal and childhood stages. However, to assess its long-term toxicity potentially by the use of early life stages in human and rodents is limited. In this regard, the zebrafish has been considered as excellent model organism for biosafety assessments of ketamine due to it boasts an in vivo model with the advantages of an in vitro assay. In this review, we summarize the current understanding of the reported toxicity studies with ketamine in early life stage of zebrafish. The adverse effects of ketamine are known to cause overall developmental and multi-organ toxicity, including cardio-, neuro-, and skeletal toxicity. Furthermore, multiple mechanisms are found to be responsible for perpetrating toxicity of ketamine. The current findings confluence to emphasize the zebrafish embryo as an appealing model system for developmental toxicity testing in higher vertebrates.

Ketamine induction of physiological functions alterations in Caenorhabditis elegans by chronic and multigenerational exposure and corresponding aquatic environmental risk assessment

Although ketamine (KET) has been widely detected in aquatic environments, the ecotoxicity data in aquatic invertebrates and associated risk remained unclear. This study aimed to investigate the adverse effects on benthos (Caenorhabditis elegans (C.elegans)) posed by KET from chronic (10 days) and multigenerational (four generations) exposure. Such exposure induced dose-dependent alterations on apoptosis, reactive oxygen species (ROS) induction, locomotion activity, feeding rate, chemotaxis, and brood size of nematodes, showing a cumulative damage through generations. KET posed vulva deformations and worm bags of C. elegans with a dosed-dependent increase. As a consequence, the fecundity and viability of worms would be impaired, which could eventually impact aquatic ecosystem equilibrium. Meanwhile, the bioactivation/detoxification process of xenobiotics and longevity regulating pathway induced by KET might be responsible for the physiological function disorders. Accordingly, the risk quotients (RQ) of KET in surface water in China were calculated using the 90% indicator protection concentration (C_0.1) derived from multiple toxicity indicators cumulative analyses. The results would be more objective considering numerous biomarkers changes of one species in comparison with traditional method using no observed effect concentrations (NOEC) of teratogenesis. The risk in surface water in southern China was up to high level (RQ > 1), suggesting long-term monitoring was imperative.

Effect of ketamine on gene expression in zebrafish embryos

Ketamine is an N-methyl-D-aspartate (NMDA) receptor antagonist. Used as an anesthetic, potential neurotoxic and cardiotoxic effects of ketamine in animal models have been reported. The underlying mechanisms of ketamine-induced toxicity are not clear. The zebrafish is an ideal model for toxicity assays because of its predictive capability in chemical testing, which compares well with that of mammalian models. To gain insight into potential mechanisms of ketamine effects, we performed real-time quantitative polymerase chain reaction-based gene expression array analyses. Gene expression analysis was conducted for multiple genes (a total of 84) related to 10 major signaling pathways including the transforming growth factor β (TGFβ), Wingless and Int-1 (Wnt), nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB), Janus kinase/signal transducers and activators of transcription (JAK/STAT), p53, Notch, Hedgehog, peroxisome proliferator-activated receptor (PPAR), oxidative stress, and hypoxia pathways. Our results show that ketamine altered the expression of specific genes related to hypoxia, p53, Wnt, Notch, TGFβ, PPAR, and oxidative stress pathways. Thus, we can further focus on these specific pathways to elucidate the mechanisms by which ketamine elicits a toxic response.

Neuroprotective Effects of Carnitine and Its Potential Application to Ameliorate Neurotoxicity

Carnitine is an essential metabolite that is absorbed from the diet and synthesized in the kidney, liver, and brain. It ferries fatty acids across the mitochondrial membrane to undergo β-oxidation. Carnitine has been studied as a therapy or protective agent for many neurological diseases and neurotoxicity (e.g., prolonged anesthetic exposure-induced developmental neurotoxicity in preclinical models). Preclinical and clinical data support the notion that carnitine or acetyl carnitine may improve a patient's quality of life through increased mitochondrial respiration, release of neurotransmitters, and global gene expression changes, showing the potential of carnitine beyond its approved use to treat primary and secondary carnitine deficiency. In this review, we summarize the beneficial effects of carnitine or acetyl carnitine on the central nervous system, highlighting protective effects against neurotoxicity-induced damage caused by various chemicals and encouraging a thorough evaluation of carnitine use as a therapy for patients suffering from neurotoxicant exposure.

Antidepressant Actions of Ketamine: Potential Role of L-Type Calcium Channels

Recently, the United States Food and Drug Administration approved esketamine, the S-enantiomer of ketamine, as a fast-acting therapeutic drug for treatment-resistant depression. Although ketamine is known as an N-methyl-d-aspartate (NMDA) receptor antagonist, the underlying mechanisms of how it elicits an antidepressant effect, specifically at subanesthetic doses, are not clear and remain an advancing field of research interest. On the other hand, high-dose (more than the anesthetic dose) ketamine-induced neurotoxicity in animal models has been reported. There has been progress in understanding the potential pathways involved in ketamine-induced antidepressant effects, some of which include NMDA-receptor antagonism, modulation of voltage-gated calcium channels, and brain-derived neurotrophic factor (BDNF) signaling. Often these pathways have been shown to be linked. Voltage-gated L-type calcium channels have been shown to mediate the rapid-acting antidepressant effects of ketamine, especially involving induction of BDNF synthesis downstream, while BDNF deficiency decreases the expression of L-type calcium channels. This review focuses on the reported studies linking ketamine's rapid-acting antidepressant actions to L-type calcium channels with an objective to present a perspective on the importance of the modulation of intracellular calcium in mediating the effects of subanesthetic (antidepressant) versus high-dose ketamine (anesthetic and potential neurotoxicant), the latter having the ability to reduce intracellular calcium by blocking the calcium-permeable NMDA receptors, which is implicated in potential neurotoxicity.

Effects of acetyl L-carnitine on zebrafish embryos: Phenotypic and gene expression studies

Acetyl L-carnitine (ALCAR), a dietary supplement and an antioxidant, plays a vital role in the bioenergetic process that produces ATP. Although there are reports on antioxidant toxicity, there is no information on the potential toxicity of ALCAR. Here, using zebrafish embryos, we explored whether ALCAR modulated ATP synthesis, generation of reactive oxygen species (ROS) and expression of specific genes related to major signaling pathways that control metabolism, growth, differentiation, apoptosis and oxidative stress. First, we show that ALCAR elicits a physiologic response, as ATP levels increased after ALCAR treatment. Simultaneously, an increase in the expression of ROS, a by-product of ATP synthesis, was observed in the ALCAR-treated embryos. Consistent with higher ROS expression, the level of cysteine, a precursor of glutathione, was significantly reduced. ALCAR did not have any drastic effect on overall development and heart rate. Polymerase chain reaction-based gene expression array analyses showed no significant change in the expression of 83 genes related to 10 major signaling pathways including: the transforming growth factor β (TGFβ), Wingless and Int-1 (Wnt), nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB), Janus kinase/signal transducers and activators of transcription (JAK/STAT), p53, Notch, Hedgehog, Peroxisome proliferator-activated receptor (PPAR), oxidative stress, and hypoxia pathways. Our results show that the expression of 83 genes related to these major signaling pathways did not change significantly.

N-acetylcysteine prevents verapamil-induced cardiotoxicity with no effect on the noradrenergic arch-associated neurons in zebrafish

There is a strong association between calcium channel blockers (CCBs) and heart failure. CCB toxicity is very common due to overdose and underlying medical conditions. CCBs also have been shown to affect the nervous system. Recently, we demonstrated that the antioxidant N-acetylcysteine (NAC) prevented ketamine-induced cardiotoxicity, developmental toxicity and neurotoxicity. Functionally, we attributed NAC's beneficial effect to its ability to increase cellular calcium. Here, we hypothesized that if there was an involvement of calcium in NAC's preventative effects on ketamine toxicity, NAC might also ameliorate toxicities induced by verapamil, an L-type CCB used to treat hypertension. Using zebrafish embryos, we show that in the absence of NAC, verapamil (up to 100 μM) dose-dependently reduced heart rate and those effects were prevented by NAC co-treatment. Furthermore, a 2-h treatment with NAC rescued reduction of heart rate induced by pre-treatment of 50 and 100 μM of verapamil for 18 h. Verapamil up to 100 μM and NAC up to 1.5 mM did not have any adverse effects on the expression of tyrosine hydroxylase in the noradrenergic neurons of the arch-associated cluster (AAC) located near the heart. NAC did not change cysteine levels in the embryos suggesting that the beneficial effect of NAC on verapamil toxicity may not involve its antioxidant property. In our search for compounds that can prevent CCB toxicity, this study, for the first time, demonstrates protective effects of NAC against verapamil's adverse effects on the heart.

Impact of ketamine on the behavior and immune system of adult medaka (Oryzias latipes) at environmentally relevant concentrations and eco-risk assessment in surface water

This work for the first time investigated the bioconcentration factor (BCF), toxicity, and eco-risk of KET using adult medaka fish (Oryzias latipes) as model organism after exposure at environmental concentrations (0.05-0.5 μg L^-1) and higher levels (5-100 μg L^-1) for 90 days. The BCF of KET was approximately 1.07- to 10.94- folds. The behavioral functions, including swimming properties, feeding rate, and food preference, were significantly impacted by KET (≥0.05 μg L^-1). After 90-days exposure, KET induced histological abnormalities in liver and kidney tissue at 0.1 and 0.2 μg L^-1, respectively. Additionally, the condition factor, hepatic-somatic index (HSI), and nephric-somatic index (NSI) of medaka were markedly impacted by KET treatment at 0.5, 0.5, and 0.1 μg L^-1, respectively. Morphological inflammation (i.e., haemorrhage and erosion) in the fish body was observed exposed to KET, and the EC₁₀ value was 0.407 μg L^-1. Alterations in the expressions of genes (i.e., cacna1c, oxtr, erk1, and c-fos) and proteins (i.e., OXT and PKA), involved in in calcium ion channels induced by KET, could partly elucidate the underlying mechanism of the toxicity. The inflammatory risk to fish posed by KET in some rivers in southern China was at high level, suggesting the long-term concentration monitoring was required.

Acetyl-l-carnitine does not prevent neurodegeneration in a rodent model of prolonged neonatal anesthesia

Many studies have shown that prolonged or repeated use of general anesthesia early in life can cause an increase in neurodegeneration and lasting changes in behavior. While short periods of general anesthesia appear to be safe, there is a concern about the neurotoxic potential of prolonged or repeated general anesthesia in young children. Unfortunately, the use of general anesthesia in children cannot be avoided. It would be a great benefit to develop a strategy to reduce or reverse anesthesia mitigated neurotoxicity. The mechanisms behind anesthesia related neurotoxicity are unknown, but evidence suggests that mitochondrial dysfunction and abnormal energy utilization are involved. Recent research suggests that a class of compounds known as carnitines may be effective at preventing anesthesia related neurotoxicity by influencing fatty acid metabolism in the mitochondria. However, it is unknown if carnitines can provide protection against changes in behavior associated with early life exposure to anesthesia. Accordingly, we evaluated the neuroprotective potential of acetyl-l-carnitine in 7-day old rats. Rat pups were exposed to 6 h of general anesthesia with sevoflurane or a control condition, with and without acetyl-l-carnitine. The oxygenation level of animals was continuously monitored during sevoflurane exposure, and any animal showing signs of hypoxia was removed from the study. Animals exposed to sevoflurane showed clear signs of neurodegeneration 2 h after sevoflurane exposure. The hippocampus, cortex, thalamus, and caudate putamen all had elevated levels of Fluoro-Jade C staining. Despite the elevated levels of Fluoro-Jade C, few behavioral changes were observed in an independent cohort of animals treated with sevoflurane. Furthermore, acetyl-l-carnitine had little impact on levels of Fluoro-Jade C staining in animals treated with sevoflurane. These data suggest that acetyl-l-carnitine may offer little protection again anesthesia related neurotoxicity in fully oxygenated animals.

Nifedipine toxicity is exacerbated by acetyl l-carnitine but alleviated by low-dose ketamine in zebrafish in vivo

Calcium channel blocker (CCB) poisoning is a common and sometimes life-threatening emergency. Our previous studies have shown that acetyl l-carnitine (ALCAR) prevents cardiotoxicity and developmental toxicity induced by verapamil, a CCB used to treat patients with hypertension. Here, we tested whether toxicities of nifedipine, a dihydropyridine CCB used to treat hypertension, can also be mitigated by co-treatment with ALCAR. In the zebrafish embryos at three different developmental stages, nifedipine induced developmental toxicity with pericardial sac edema in a dose-dependent manner, which were surprisingly exacerbated with ALCAR co-treatment. Even with low-dose nifedipine (5 μm), when the pericardial sac looked normal, ALCAR co-treatment showed pericardial sac edema. We hypothesized that toxicity by nifedipine, a vasodilator, may be prevented by ketamine, a known vasoconstrictor. Nifedipine toxicity in the embryos was effectively prevented by co-treatment with low (subanesthetic) doses (25-100 μm added to the water) of ketamine, although a high dose of ketamine (2 mm added to the water) partially prevented the toxicity.As expected of a CCB, nifedipine either in the presence or absence of ketamine-reduced metabolic reactive oxygen species (ROS), a downstream product of calcium signaling, in the rapidly developing digestive system. However, nifedipine induced ROS in the trunk region that showed significantly stunted growth indicating that the tissues under stress potentially produced pathologic ROS. To the best of our knowledge, these studies for the first time show that nifedipine and the dietary supplement ALCAR together induce adverse effects while providing evidence on the therapeutic efficacy of subanesthetic doses of ketamine against nifedipine toxicity in vivo.

L-carnitine regulated Nrf2/Keap1 activation in vitro and in vivo and protected oxidized fish oil-induced inflammation response by inhibiting the NF-κB signaling pathway in Rhynchocypris lagowski Dybowski

Nrf2/Keap1 pathway is associated with oxidative stress. l-carnitine is currently under preclinical evaluation as a antioxidant, but the use of l-carnitine in aquaculture has been poorly evaluated and so far no mechanism has been demonstrated. Here, we explored the effects of l-carnitine in vitro and in vivo and discussed the possible molecular mechanisms involved. Firstly, Nrf2-siRNA significantly knocked down the mRNA level of Nrf2 in FHM cells. Thus, the activities of antioxidant enzymes (T-SOD, CAT, GSH-PX) and the level of antioxidant substance (GSH) and the level of MDA showed that Nrf2-siRNA pretreatment weakened the protective effect of l-carnitine. Moreover, the mRNA levels of Keap1, Nrf2, Maf and HO-1 indicated that l-carnitine regulated Nrf2/Keap1 activation. Furthermore, oxidized fish oil remarkably suppressed growth in Rhynchocypris lagowski Dybowski, and the lower antioxidant capacity was also observed in liver. According to the results of immune related indexes (the levels of IL-1β, TNF-α, LZM, AKP) in serum and the mRNA levels of immune related genes (NF-κB, IL-1β, TNF-α, IL-8, IL-10 and TGF-β) in liver, oxidized fish oil also induced inflammatory response in fish. Also, l-carnitine supplementation can relieve this bad condition. In conclusion, l-carnitine regulated Nrf2/Keap1 activation in vitro and in vivo and protected oxidized fish oil-induced inflammation response by inhibiting the NF-κB signaling pathway in Rhynchocypris lagowski Dybowski.

Review on the use of zebrafish embryos to study the effects of anesthetics during early development

Over the years, the potential toxicity of anesthetics has raised serious concerns about its safe use during pregnancy. As evidence emerged from research in animal models, showing that some anesthetic drugs are potential teratogenic, the determination of the risk of exposures to anesthetic drugs at early life stages became mandatory. However, due to inaccessibility and ethical constrains related to experimental conditions, the use of early life stages in mammalian models is limited. In this regard, some animal and nonanimal models have been suggested to surpass mammalian use in experimentation. Among them, the zebrafish embryo test has been recognized as a promising alternative in toxicology research, as well as an inexpensive and practical test. Substantial information collected from developmental research following compounds exposure, has contributed to the application of zebrafish assays in research, although only a few studies have focused on the use of early life stages of zebrafish to evaluate the developmental effects of anesthetics. Based on the recent advances of science and technology, there is a clear potential for zebrafish early life stages to provide new insights into anesthetics teratogenicity. This review provides an overview of recent anesthesia research using zebrafish embryos, demonstrating its usefulness to the anesthesia field, discussing the recent findings on various aspects related to the effects of anesthetics during early life development and the strengths and limitations of this model system.

Ketamine-induced attenuation of reactive oxygen species in zebrafish is prevented by acetyl l-carnitine in vivo

Ketamine, an anesthetic, is a non-competitive antagonist of the calcium-permeable N-methyl-d-aspartate (NMDA) receptor. High concentrations of ketamine have been implicated in cardiotoxicity and neurotoxicity. Often, these toxicities are thought to be mediated by reactive oxygen species (ROS). However, findings to the contrary showing ketamine reducing ROS in mammalian cells and neurons in vitro, are emerging. Here, we determined the effects of ketamine on ROS levels in zebrafish larvae in vivo. Based on our earlier studies demonstrating reduction in ATP levels by ketamine, we hypothesized that as a calcium antagonist, ketamine would also prevent ROS generation, which is a by-product of ATP synthesis. To confirm that the detected ROS in a whole organism, such as the zebrafish larva, is specific, we used diphenyleneiodonium (DPI) that blocks ROS production by inhibiting the NADPH Oxidases (NOX). Upon 20 h exposure, DPI (5 and 10 μM) and ketamine at (1 and 2 mM) reduced ROS in the zebrafish larvae in vivo. Using acetyl l-carnitine (ALCAR), a dietary supplement, that induces mitochondrial ATP synthesis, we show elevated ROS generation with increasing ALCAR concentrations. Combined, ketamine and ALCAR counter-balanced ROS generation in the larvae suggesting that ketamine and ALCAR have opposing effects on mitochondrial metabolism, which may be key to maintaining ROS homeostasis in the larvae and affords ALCAR the ability to prevent ketamine toxicity. These results for the first time show ketamine's antioxidative and ALCAR's prooxidative effects in a live vertebrate.

Acetyl-L-carnitine as a putative candidate for the treatment of stress-related psychiatric disorders: Novel evidence from a zebrafish model

Stress-related psychiatric disorders are mental conditions that affect mood, cognition and behavior and arise because of the impact of prolonged stress on the central nervous system (CNS). Acetyl-L-carnitine (ALC) is an acetyl ester of L-carnitine that easily crosses the blood-brain barrier and was recently found to be decreased in patients with major depressive disorder. ALC plays a role in energy metabolism and is widely consumed as a nutritional supplement to improve physical performance. In this study, our objective was to evaluate the effects of ALC treatment (0.1 mg/L, 10 min) for 7 days on behavior and oxidative stress in zebrafish subjected to unpredictable chronic stress (UCS) protocol. Behavioral outcomes were assessed in the novel tank test, and parameters of oxidative status (lipid peroxidation and antioxidant defenses) were evaluated in the brain using colorimetric methods. According to our previous findings, UCS increased anxiety-like behavior and lipid peroxidation, while it decreased non-protein thiol levels and superoxide dismutase activity. However, ALC reversed the anxiety-like behavior and oxidative damage in stressed animals, while it was devoid of effect in control animals. Although our data reinforce the neuroprotective potential of ALC in the treatment of psychiatric disorders related to stress, further investigations are required to clarify its mechanisms of action and confirm its efficacy.

Neuroprotective effects of acetyl-l-carnitine (ALC) in a chronic MPTP-induced Parkinson's disease mouse model: Endothelial and microglial effects

Parkinson's disease (PD) is a progressive motor disease with clinical features emerging due to degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc), which project to the caudate putamen (CPu) where they release dopamine (DA). The current study investigated whether acetyl-l-carnitine (ALC) could ameliorate the pathology seen in an in vivoin vivo chronic 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP)-induced mouse model of PD. Four treatment groups were included: 1) CONTROL receiving probenecid (PROB; 250 mg/kg) only, 2) MPTP (25 mg/kg) + PROB, 3) MPTP + ALC (100 mg/kg), and 4) ALC alone. MPTP-induced losses in tyrosine hydroxylase and DA transporter immunoreactivity in the SNc and CPu were significantly reduced by ALC. HPLC data further suggests that decreases in CPu DA levels produced by MPTP were also attenuated by ALC. Additionally, microglial activation and astrocytic reactivity induced by MPTP were greatly reduced by ALC, indicating protection against neuroinflammation. Glucose transporter-1 and the tight junction proteins occludin and zonula occludins-1 were also protected from MPTP-induced down-regulation by ALC. Together, data suggest that in this model, ALC protects against MPTP-induced damage to endothelial cells and loss of DA neurons in the SNc and CPu, suggesting that ALC therapy may have the potential to slow or ameliorate the progression of PD pathology in a clinical setting.

Metabolomics reveals the role of acetyl-l-carnitine metabolism in γ-Fe2O3 NP-induced embryonic development toxicity via mitochondria damage

Iron oxides nanoparticles (FeO_X NPs), including α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄, are employed in many technological applications. However, very few studies have investigated the embryonic developmental toxicity of FeO_X NPs. In this study, metabolomics analysis were used to uncover the potential mechanisms of FeO_X NPs developmental toxicity on embryo-larval zebrafish and mice. Our results indicated that γ-Fe₂O₃ NP treatment could cause increased mortality, dropped hatching rate, etc., while α-Fe₂O₃ and Fe₃O₄ NPs showed no obvious effect. Through metabolomics analysis, a total of 42 metabolites were found to be significantly changed between the γ-Fe₂O₃ NP-treated group and the control group (p < 0.05). Pathway enrichment analysis indicated the impairment of mitochondria function. γ-Fe₂O₃ NP treatment caused abnormal mitochondrion structure and a decrease in mitochondrial membrane potential in zebrafish embryos. Meanwhile, ATP synthesis was decreased while oxidative stress levels were affected. It is noteworthy that acetyl-l-carnitine (ALCAR) (p = 6.79E - 04) and l-carnitine (p = 1.43E - 03) were identified with minimal p values, the relationship between the two counter-balance was regulated by acetyltransferase (crata). Subsequently, we performed rescue experiments with ALCAR on zebrafish embryos, and found that the mortality rates reduced and hatching rates raised significantly in the γ-Fe₂O₃ NP-treated group. Additionally, γ-Fe₂O₃ exposure could lead to increased absorbed fetus rate, decreased placental weight, lower expression of acetyltransferase (Crat), reduced ATP synthesis as well as increased oxidative stress (p < 0.05). Our findings demonstrated that γ-Fe₂O₃ NP might affect the mitochondrial membrane potential and ATP synthesis by affecting the metabolism of ALCAR, thereby stimulating oxidative stress, cell apoptosis, and causing embryonic development toxicity.

N-acetylcysteine prevents ketamine-induced adverse effects on development, heart rate and monoaminergic neurons in zebrafish

N-acetylcysteine, a precursor molecule of glutathione, is an antioxidant. Ketamine, a pediatric anesthetic, has been implicated in cardiotoxicity and neurotoxicity including modulation of monoaminergic systems in mammals and zebrafish. Here, we show that N-acetylcysteine prevents ketamine's adverse effects on development and monoaminergic neurons in zebrafish embryos. The effects of ketamine and N-acetylcysteine alone or in combination were measured on the heart rate, body length, brain serotonergic neurons and tyrosine hydroxylase-immunoreactive (TH-IR) neurons. In the absence of N-acetylcysteine, a concentration of ketamine that produces an internal embryo exposure level comparable to human anesthetic plasma concentrations significantly reduced heart rate and body length and those effects were prevented by N-acetylcysteine co-treatment. Ketamine also reduced the areas occupied by serotonergic neurons in the brain, whereas N-acetylcysteine co-exposure counteracted this effect. TH-IR neurons in the embryo brain and TH-IR cells in the trunk were significantly reduced with ketamine treatment, but not in the presence of N-acetylcysteine. In our continued search for compounds that can prevent ketamine toxicity, this study using specific endpoints of developmental toxicity, cardiotoxicity and neurotoxicity, demonstrates protective effects of N-acetylcysteine against ketamine's adverse effects. This is the first study that shows the protective effects of N-acetylcysteine on ketamine-induced developmental defects of monoaminergic neurons as observed in a whole organism.

Illicit drug ketamine induces adverse effects from behavioral alterations and oxidative stress to p53-regulated apoptosis in medaka fish under environmentally relevant exposures

With increasing problems of drug abuse worldwide, aquatic ecosystems are contaminated by human pharmaceuticals from the discharge of hospital or municipal effluent. However, ecotoxicity data and related toxic mechanism for neuroactive controlled or illicit drugs are still lacking, so assessing the associated hazardous risk is difficult. This study aims to investigate the behavioral changes, oxidative stress, gene expression and neurotoxic or apoptosis effect(s) in larvae of medaka fish (Oryzias latipes) with environmentally relevant exposures of ketamine (KET) solutions for 1-14 days. KET exposure at an environmentally relevant concentration (0.004 μM) to 40 μM conferred specific patterns in larval swimming behavior during 24 h. At 14 days, such exposure induced dose- and/or time-dependent alteration on reactive oxygen species induction, the activity of antioxidants catalase and superoxide dismutase, glutathione S-transferase and malondialdehyde contents in fish bodies. KET-induced oxidative stress disrupted the expression of acetylcholinesterase and p53-regulated apoptosis pathways and increased caspase expression in medaka larvae. The toxic responses of medaka larvae, in terms of chemical effects, were qualitatively analogous to those of zebrafish and mammals. Our results implicate a toxicological impact of waterborne KET on fish development and human health, for potential ecological risks of directly releasing neuroactive drugs-containing wastewater into the aquatic environment.

Ketamine induction of p53-dependent apoptosis and oxidative stress in zebrafish (Danio rerio) embryos

Ketamine is a widely used pharmaceutical that has been detected in water sources worldwide. Zebrafish embryos were used in this study to investigate the oxidative stress and apoptotic signals following a 24h exposure to different ketamine concentrations (0, 50, 70 and 90 mg L^-1). Early blastula embryos (∼2 h post fertilisation-hpf) were exposed for 24 h and analysed at 8 and 26 hpf. Reactive oxygen species and apoptotic cells were identified in vivo, at 26 hpf. Enzymatic activities (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), lactate dehydrogenase (LDH) and acetylcholinesterase (AChE)), glutathione levels (oxidised (GSSG) and reduced (GSH)), oxidative damage (lipid peroxidation (LPO) and protein carbonyls (CO)) as well as oxidative stress (gclc, gstp1, sod1 and cat), apoptosis (casp3a, casp6, casp8, casp9, aifm1 and tp53) and cell proliferation (pcna) related-genes were evaluated at 8 and 26 hpf. Caspase (3 and 9) activity was also determined at both time-points by colorimetric methods. Superoxide dismutase (SOD), catalase (CAT), glutathione levels (GSSG), caspase-9 and reactive oxygen species (ROS) were shown to be affected by ketamine exposure while in vivo analysis showed no difference in ROS. A significant up-regulation of superoxide dismutase (sod1) and catalase (cat) genes expression was also perceived. Ketamine-induced apoptosis was observed in vivo and confirmed by the apoptotic-related genes up-regulation. The overall results suggest that ketamine induced oxidative stress and apoptosis through the involvement of p53-dependent pathways in zebrafish embryos which could be important for the evaluation of the overall risk of ketamine in aquatic environments.

Ketamine Modulates Zic5 Expression via the Notch Signaling Pathway in Neural Crest Induction

Ketamine is a potent dissociative anesthetic and the most commonly used illicit drug. Many addicts are women at childbearing age. Although ketamine has been extensively studied as a clinical anesthetic, its effects on embryonic development are poorly understood. Here, we applied the Xenopus model to study the effects of ketamine on development. We found that exposure to ketamine from pre-gastrulation (stage 7) to early neural plate (stage 13.5) resulted in disruption of neural crest (NC) derivatives. Ketamine exposure did not affect mesoderm development as indicated by the normal expression of Chordin, Xbra, Wnt8, and Fgf8. However, ketamine treatment significantly inhibited Zic5 and Slug expression at early neural plate stage. Overexpression of Zic5 rescued ketamine-induced Slug inhibition, suggesting the blockage of NC induction was mediated by Zic5. Furthermore, we found Notch signaling was altered by ketamine. Ketamine inhibited the expression of Notch targeted genes including Hes5.2a, Hes5.2b, and ESR1 and ketamine-treated embryos exhibited Notch-deficient somite phenotypes. A 15 bp core binding element upstream of Zic5 was induced by Notch signaling and caused transcriptional activation. These results demonstrated that Zic5 works as a downstream target gene of Notch signaling in Xenopus NC induction. Our study provides a novel teratogenic mechanism whereby ketamine disrupts NC induction via targeting a Notch-Zic5 signaling pathway.

Mechanistic studies on ketamine-induced mitochondrial toxicity in zebrafish embryos

Ketamine, a phencyclidine derivative, is an antagonist of the Ca²⁺-permeable N-methyl-d-aspartate (NMDA)-type glutamate receptors. It is a pediatric anesthetic and has been implicated in developmental neurotoxicity. Ketamine has also been shown to deplete ATP in mammalian cells. Our previous studies showed that acetyl l-carnitine (ALCAR) prevented ketamine-induced cardiotoxicity and neurotoxicity in zebrafish embryos. Based on our finding that ALCAR's protective effect was blunted by oligomycin A, an inhibitor of ATP synthase, we further investigated the effects of ketamine and ALCAR on ATP levels, mitochondria and ATP synthase in zebrafish embryos. The results demonstrated that ketamine reduced ATP levels in the embryos but not in the presence of ALCAR. Ketamine reduced total mitochondrial protein levels and mitochondrial potential, which were prevented with ALCAR co-treatment. To determine the cause of ketamine-induced ATP deficiency, we explored the status of ATP synthase. The results showed that a subunit of ATP synthase, atp5α1, was transcriptionally down-regulated by ketamine, but not in the presence of ALCAR, although ketamine caused a significant upregulation in another ATP synthase subunit, atp5β and total ATP synthase protein levels. Most of the ATP generated by heart mitochondria are utilized for its contraction and relaxation. Ketamine-treated embryos showed abnormal heart structure, which was abolished with ALCAR co-treatment. This study offers evidence for a potential mechanism by which ketamine could cause ATP deficiency mediated by mitochondrial dysfunction.

Cyclosporine exacerbates ketamine toxicity in zebrafish: Mechanistic studies on drug-drug interaction

Cyclosporine A (CsA) is an immunosuppressive drug commonly used in organ transplant patients to prevent allograft rejections. Ketamine is a pediatric anesthetic that noncompetitively inhibits the calcium-permeable N-methyl-d-aspartic acid receptors. Adverse drug-drug interaction effects between ketamine and CsA have been reported in mammals and humans. However, the mechanism of such drug-drug interaction is unclear. We have previously reported adverse effects of combination drugs, such as verapamil/ketamine and shown the mechanism through intervention by other drugs in zebrafish embryos. Here, we show that ketamine and CsA in combination produce developmental toxicity even leading to lethality in zebrafish larvae when exposure began at 24 h post-fertilization (hpf), whereas CsA did not cause any toxicity on its own. We also demonstrate that acetyl l-carnitine (ALCAR) completely reversed the adverse effects. Both ketamine and CsA are CYP3A4 substrates. Although ketamine and CsA independently altered the expression of the hepatic marker CYP3A65, a zebrafish ortholog of human CYP3A4, both drugs together induced further increase in CYP3A65 expression. In the presence of ALCAR, however, CYP3A65 expression was normalized. ALCAR has been shown to prevent ketamine toxicity in mammal and zebrafish. In conclusion, CsA exacerbated ketamine toxicity and ALCAR reversed the effects. These results, providing evidence for the first time on the reversal of the adverse effects of CsA/ketamine interaction by ALCAR, would prove useful in addressing potential occurrences of such toxicities in humans. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.

Acetyl-l-carnitine partially prevents benzene-induced hematotoxicity and oxidative stress in C3H/He mice

Benzene is an environmental pollutant and occupational toxicant which induces hematotoxicity. Our previous metabonomics study suggested that acetyl-l-carnitine (ALCAR) decreased in the mouse plasma and bone marrow (BM) cells due to benzene exposure. In the present study, the topic on whether ALCAR influences hematotoxicity caused by benzene exposure was explored. Thirty-two male C3H/He mice were divided into four groups: control group (C: vehicle, oil), benzene group (150mg/kg body weight (b.w.) benzene), benzene+A1 group (150mg/kg b.w. benzene+100mg/kg b.w. ALCAR), and benzene+A2 group (150mg/kg b.w. benzene+200mg/kg b.w. ALCAR). Benzene was injected subcutaneously, and ALCAR was orally administrated via gavage once daily for 4 weeks consecutively. After the experimental period, the blood routine, BM cell number and frequency of hematopoietic stem/progenitor cell (HS/PC) were assessed. The mitochondrial membrane potential and ATP level were determined to evaluate the mitochondrial function. Reactive oxygen species (ROS), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) levels were also examined, and the comet assay was performed to measure oxidative stress. Results showed that ALCAR intervention can partially reduce the benzene-induced damage on BM and HS/PCs and can simultaneously alleviate the DNA damage by reducing benzene-induced H₂O_2, ROS, and MDA.

Chemical profiling analysis of Maca using UHPLC-ESI-Orbitrap MS coupled with UHPLC-ESI-QqQ MS and the neuroprotective study on its active ingredients

Lepidium meyenii (Maca), originated from Peru, has been cultivated widely in China as a popular health care food. However, the chemical and effective studies of Maca were less in-depth, which restricted its application seriously. To ensure the quality of Maca, a feasible and accurate strategy was established. One hundred and sixty compounds including 30 reference standards were identified in 6 fractions of methanol extract of Maca by UHPLC-ESI-Orbitrap MS. Among them, 15 representative active compounds were simultaneously determined in 17 samples by UHPLC-ESI-QqQ MS. The results suggested that Maca from Yunnan province was the potential substitute for the one from Peru. Meanwhile, the neuroprotective effects of Maca were investigated. Three fractions and two pure compounds showed strong activities in the 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP)-induced zebrafish model. Among them, 80% methanol elution fraction (Fr5) showed significant neuroprotective activity, followed by 100% part (Fr6). The inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) was a possible mechanism of its neuroprotective effect.

Behavioral alterations of zebrafish larvae after early embryonic exposure to ketamine

RATIONALE

Ketamine has been associated with pediatric risks that include neurocognitive impairment and long-term behavioral disorders. However, the neurobehavioral effects of ketamine exposure in early development remain uncertain.

OBJECTIVES

This study aimed to test stage- and dose-dependent effects of ketamine exposure on certain brain functions by evaluating alterations in locomotion, anxiety-like and avoidance behaviors, as well as socialization.

METHODS

Embryos were exposed to different concentrations of ketamine (0, 0.2, 0.4, and 0.8 mg mL^-1) for 20 min during the 256-cell (2.5 h post fertilization-hpf), 50% epiboly (5.5 hpf), and 1-4 somites (10.5 hpf) stages. General exploratory activities, natural escape-like responses, and social interactions were analyzed under continuous light or under a moving light stimulus.

RESULTS

A dose-dependent decrease in the overall mean speed was perceived in the embryos exposed during the 256-cell stage. These results were related to previously observed head and eye malformations, following ketamine exposure at this stage and may indicate possible neurobehavioral disorders when ketamine exposure is performed at this stage. Results also showed that ketamine exposure during the 50% epiboly and 1-4 somites stages induced a significant increment of the anxiety-like behavior and a decrease in avoidance behavior in all exposed groups.

CONCLUSIONS

Overall, the results validate the neurodevelopmental risks of early-life exposure to ketamine.

Acetyl L-carnitine targets adenosine triphosphate synthase in protecting zebrafish embryos from toxicities induced by verapamil and ketamine: An in vivo assessment

Verapamil is a Ca²⁺ channel blocker and is highly prescribed as an anti-anginal, antiarrhythmic and antihypertensive drug. Ketamine, an antagonist of the Ca²⁺ -permeable N-methyl-d-aspartate-type glutamate receptors, is a pediatric anesthetic. Previously we have shown that acetyl l-carnitine (ALCAR) reverses ketamine-induced attenuation of heart rate and neurotoxicity in zebrafish embryos. Here, we used 48 h post-fertilization zebrafish embryos that were exposed to relevant drugs for 2 or 4 h. Heart beat and overall development were monitored in vivo. In 48 h post-fertilization embryos, 2 mm ketamine reduced heart rate in a 2 or 4 h exposure and 0.5 mm ALCAR neutralized this effect. ALCAR could reverse ketamine's effect, possibly through a compensatory mechanism involving extracellular Ca²⁺ entry through L-type Ca²⁺ channels that ALCAR is known to activate. Hence, we used verapamil to block the L-type Ca²⁺ channels. Verapamil was more potent in attenuating heart rate and inducing morphological defects in the embryos compared to ketamine at specific times of exposure. ALCAR reversed cardiotoxicity and developmental toxicity in the embryos exposed to verapamil or verapamil plus ketamine, even in the presence of 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester, an inhibitor of intracellular Ca²⁺ release suggesting that ALCAR acts via effectors downstream of Ca²⁺ . In fact, ALCAR's protective effect was blunted by oligomycin A, an inhibitor of adenosine triphosphate synthase that acts downstream of Ca²⁺ during adenosine triphosphate generation. We have identified, for the first time, using in vivo studies, a downstream effector of ALCAR that is critical in abrogating ketamine- and verapamil-induced developmental toxicities. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.

Exploring Hallucinogen Pharmacology and Psychedelic Medicine with Zebrafish Models

After decades of sociopolitical obstacles, the field of psychiatry is experiencing a revived interest in the use of hallucinogenic agents to treat brain disorders. Along with the use of ketamine for depression, recent pilot studies have highlighted the efficacy of classic serotonergic hallucinogens, such as lysergic acid diethylamide and psilocybin, in treating addiction, post-traumatic stress disorder, and anxiety. However, many basic pharmacological and toxicological questions remain unanswered with regard to these compounds. In this study, we discuss psychedelic medicine as well as the behavioral and toxicological effects of hallucinogenic drugs in zebrafish. We emphasize this aquatic organism as a model ideally suited to assess both the potential toxic and therapeutic effects of major known classes of hallucinogenic compounds. In addition, novel drugs with hallucinogenic properties can be efficiently screened using zebrafish models. Well-designed preclinical studies utilizing zebrafish can contribute to the reemerging treatment paradigm of psychedelic medicine, leading to new avenues of clinical exploration for psychiatric disorders.

Distinct effects of ketamine and acetyl L-carnitine on the dopamine system in zebrafish

Ketamine, a noncompetitive N-methyl-D-aspartic acid (NMDA) receptor antagonist is commonly used as a pediatric anesthetic. We have previously shown that acetyl L-carnitine (ALCAR) prevents ketamine toxicity in zebrafish embryos. In mammals, ketamine is known to modulate the dopaminergic system. NMDA receptor antagonists are considered as promising anti-depressants, but the exact mechanism of their function is unclear. Here, we measured the levels of dopamine (DA) and its metabolites, 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the zebrafish embryos exposed to ketamine in the presence and absence of 0.5 mM ALCAR. Ketamine, at lower doses (0.1-0.3 mM), did not produce significant changes in DA, DOPAC or HVA levels in 52 h post-fertilization embryos treated for 24 h. In these embryos, tyrosine hydroxylase (TH) mRNA expression remained unchanged. However, 2 mM ketamine (internal embryo exposure levels equivalent to human anesthetic plasma concentration) significantly reduced DA level and TH mRNA indicating that DA synthesis was adversely affected. In the presence or absence of 2 mM ketamine, ALCAR showed similar effects on DA level and TH mRNA, but increased DOPAC level compared to control. ALCAR reversed 2 mM ketamine-induced reduction in HVA levels. With ALCAR alone, the expression of genes encoding the DA metabolizing enzymes, MAO (monoamine oxidase) and catechol-O-methyltransferase (COMT), was not affected. However, ketamine altered MAO mRNA expression, except at the 0.1 mM dose. COMT transcripts were reduced in the 2 mM ketamine-treated group. These distinct effects of ketamine and ALCAR on the DA system may shed some light on the mechanism on how ketamine can work as an anti-depressant, especially at sub-anesthetic doses that do not affect DA metabolism and suppress MAO gene expression.

Opposing effects of ketamine and acetyl L-carnitine on the serotonergic system of zebrafish

Ketamine, a pediatric anesthetic, is a noncompetitive N-methyl-D-aspartic acid (NMDA) receptor antagonist. Studies show that ketamine is neurotoxic in developing mammals and zebrafish. In both mammals and zebrafish, acetyl L-carnitine (ALCAR) has been shown to be protective against ketamine toxicity. Ketamine is known to modulate the serotonergic system in mammals. Here, we measured the levels of serotonin (5-HT) and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA) in the embryos exposed to ketamine in the presence and absence of ALCAR. Ketamine, at lower doses, did not produce significant changes in the 5-HT or 5-HIAA levels in 3 dpf (day post-fertilization) embryos. However, 2 mM ketamine (internal embryo exposure levels comparable to human anesthetic plasma concentration) significantly reduced 5-HT level, and 5-HIAA was not detectable indicating that 5-HT metabolism was abolished. In the presence or absence of 2 mM ketamine, ALCAR by itself did not significantly alter 5-HT or 5-HIAA levels compared to the control. Ratios of metabolite/5-HT indicated that 2 mM ketamine inhibited 5-HT metabolism to 5-HIAA whereas lower doses (0.1-0.3 mM) of ketamine did not have any effect. ALCAR reversed the effects of 2 mM ketamine not only by restoring 5-HT and 5-HIAA levels but also 5-HT turnover rate to control levels. Whole mount immunohistochemical studies showed that 2 mM ketamine reduced the serotonergic area in the brain whereas ALCAR expanded it with increased axonal sprouting and branching. These results indicate that ketamine and ALCAR have opposing effects on the zebrafish serotonergic system.

Developmental exposures to waterborne abused drugs alter physiological function and larval locomotion in early life stages of medaka fish

Environmental pollution by neuroactive pharmaceuticals from wastewater discharge is a major threat to aquatic ecosystems. However, the ecotoxicologic effect of waterborne abused drugs remains unclear. Embryos of medaka fish (Oryzias latipes) were exposed to aqueous solutions of 2 hallucinogenic drugs, ketamine (KET) and methamphetamine (MET) (0.004-40μM) to assess developmental toxicity, oxidative stress and behavioral alteration in early life stages. The environmentally relevant concentration (0.004μM) of both KET and MET significantly delayed blood circulation and hatching time in embryos and altered larval swimming behavior (e.g., maximum velocity and relative turn angle). KET and MET induced similar oxidative stress responses in embryos, which were unrecoverable in hatchlings in drug-free solutions. Early life exposure to the 2 drugs conferred distinct patterns in larval locomotion: KET induced hyperactivity and a less tortuous swimming path, but MET-treated larvae showed hypoactivity and a clockwise swimming direction at high doses. The alteration in locomotor responses were generally similar in mammals and zebrafish. We report sensitive biomarkers (e.g., heartbeat, hatching and swimming behavior) by developmental stage of medaka that reflect environmentally relevant exposures of abused drugs. They could be useful for ecological risk assessment of waterborne neuroactive drugs. The toxicity results implicate a potential ecotoxicological impact of controlled or abused drugs on fish development and populations in aquatic environments.

Carbamate nerve agent prophylatics exhibit distinct toxicological effects in the zebrafish embryo model

Pyridostigmine bromide (PB) is an FDA-approved drug for the treatment of myasthenia gravis and a prophylactic pre-treatment for organophosphate nerve agent poisoning. Current methods for evaluating nerve agent treatments include enzymatic studies and mammalian models. Rapid whole animal screening tools for assessing the effects of nerve agent pre-treatment and post-exposure drugs represent an underdeveloped area of research. We used zebrafish as a model for acute and chronic developmental exposure to PB and two related carbamate acetylcholinesterase (AChE) inhibitors, neostigmine bromide (NB) and physostigmine (PS). Lethal doses and gross morphological phenotypes resulting from exposure to sub-lethal doses of these compounds were determined. Quantitative analyses of motility impairment and AChE enzyme inhibition were used to determine optimal dosing conditions for evaluation of the effects of carbamate exposures on neuronal development; ~50% impairment of response to startle stimuli and >50% inhibition of AChE activity were observed at 80 mMPB, 20 mM NB and 0.1 mM PS. PB induced stunted somite length, but no other phenotypic effects were observed. In contrast, NB and PS induced more severe phenotypic morphological defects than PB as well as neurite outgrowth mislocalization. Additionally, NB induced mislocalization of nicotinic acetylcholine receptors, resulting in impaired synapse formation. Taken together, these data suggest that altered patterns of neuronal connectivity contribute to the developmental neurotoxicity of carbamates and demonstrate the utility of the zebrafish model for distinguishing subtle structure-based differential effects of AChE inhibitors, which include nerve agents, pesticides and drugs.

Developmental toxicity assay using high content screening of zebrafish embryos

Typically, time-consuming standard toxicological assays using the zebrafish (Danio rerio) embryo model evaluate mortality and teratogenicity after exposure during the first 2 days post-fertilization. Here we describe an automated image-based high content screening (HCS) assay to identify the teratogenic/embryotoxic potential of compounds in zebrafish embryos in vivo. Automated image acquisition was performed using a high content microscope system. Further automated analysis of embryo length, as a statistically quantifiable endpoint of toxicity, was performed on images post-acquisition. The biological effects of ethanol, nicotine, ketamine, caffeine, dimethyl sulfoxide and temperature on zebrafish embryos were assessed. This automated developmental toxicity assay, based on a growth-retardation endpoint should be suitable for evaluating the effects of potential teratogens and developmental toxicants in a high throughput manner. This approach can significantly expedite the screening of potential teratogens and developmental toxicants, thereby improving the current risk assessment process by decreasing analysis time and required resources.

Ketamine exposure in early development impairs specification of the primary germ cell layers

Preclinical and clinical evidence implicates N-methyl-d-aspartate receptor (NMDAr) signaling in early embryological development. However, the role of NMDAr signaling in early development has not been well studied. Here, we use a mouse embryonic stem cell model to perform a step-wise exploration of the effects of NMDAr signaling on early cell fate specification. We found that antagonism of the NMDAr impaired specification into the neuroectodermal and mesoendodermal cell lineages, with little or no effect on specification of the extraembryonic endoderm cell lineage. Consistent with these findings, exogenous NMDA promoted neuroectodermal differentiation. Finally, NMDAr antagonism modified expression of several key targets of TGF-β superfamily signaling, suggesting a mechanism for these findings. In summary, this study shows that NMDAr antagonism interferes with the normal developmental pathways of embryogenesis, and suggests that interference is most pronounced prior to neuroectodermal and mesoendodermal cell fate specification.