Developmental neurotoxicity: some old and new issues

Comparability of behavioural assays using zebrafish larvae to assess neurotoxicity

Testing of compounds for neurotoxicity has become increasingly important in recent years. It has been shown that neurological disorders like autism may be related to chemical exposures, which may play a crucial role in the progression of these diseases. Special attention has been be given to the substances causing developmental neurotoxicity as the developing nervous system is more vulnerable to impacts by chemicals than the adult nervous system. The zebrafish (Danio rerio) is a well-established model species in developmental biology and an emerging model in behavioural and neurological studies. Zebrafish larvae display numerous behavioural patterns highly similar to rodents and humans. Their physical characteristics make them well suited for automated high-throughput screening. In the last years, the number of behavioural studies conducted with zebrafish larvae has increased notably. The goal of this review is to provide an overview of behavioural assays commonly used to test substances for developmental neurotoxicity. Literature from 1995 to 2014 was reviewed and focussed on assays performed with zebrafish larvae younger than 7 days post fertilization (dpf). The behavioural tests were scrutinized, and parameters describing the different experimental setups were defined. In the next step, we investigated if differences in the experimental parameters alter the outcome of the test. In order to test the comparability of behavioural assays, we analysed several studies using ethanol, valproate and pentylenetetrazole as model substances. Based on our findings, we provide recommendations which could help improve future behavioural studies performed with zebrafish larvae.

Expanding the test set: Chemicals with potential to disrupt mammalian brain development

High-throughput test methods including molecular, cellular, and alternative species-based assays that examine critical events of normal brain development are being developed for detection of developmental neurotoxicants. As new assays are developed, a "training set" of chemicals is used to evaluate the relevance of individual assays for specific endpoints. Different training sets are necessary for each assay that would comprise a developmental neurotoxicity test battery. In contrast, evaluation of the predictive ability of a comprehensive test battery requires a set of chemicals that have been shown to alter brain development after in vivo exposure ("test set"). Because only a small number of substances have been well documented to alter human neurodevelopment, we have proposed an expanded test set that includes chemicals demonstrated to adversely affect neurodevelopment in animals. To compile a list of potential developmental neurotoxicants, a literature review of compounds that have been examined for effects on the developing nervous system was conducted. The search was limited to mammalian studies published in the peer-reviewed literature and regulatory studies submitted to the U.S. EPA. The definition of developmental neurotoxicity encompassed changes in behavior, brain morphology, and neurochemistry after gestational or lactational exposure. Reports that indicated developmental neurotoxicity was observed only at doses that resulted in significant maternal toxicity or were lethal to the fetus or offspring were not considered. As a basic indication of reproducibility, we only included a chemical if data on its developmental neurotoxicity were available from more than one laboratory (defined as studies originating from laboratories with a different senior investigator). Evidence from human studies was included when available. Approximately 100 developmental neurotoxicity test set chemicals were identified, with 22% having evidence in humans.

Intranasal administration of rotenone in mice attenuated olfactory functions through the lesion of dopaminergic neurons in the olfactory bulb

Many environmental chemicals are thought to affect brain function. It was reported that chemicals in the nasal cavity directly reach the brain through the connection between olfactory neurons and the olfactory bulb (OB). In this 'olfactory transport,' xenobiotics absorbed at the nasal mucosa reach the brain by bypassing some physical barriers and defenses, and thus olfactory transport is suspected to be a vulnerable mechanism of the brain against invasion threats of environmental chemicals. In this study, we focused on the neuronal toxicity of rotenone administered intranasally to mice. The results showed that the mice that were administered rotenone had attenuated olfactory functions. We also found that intranasally administered rotenone induced acute mitochondrial stress at the OB. The repeated administration of rotenone resulted in a decrease in the number of dopaminergic neurons, which are inhibitory interneurons in the OB. Taken together, our findings suggest that the inhalation of environmental toxins induces the neurodegeneration of cranial neurons through olfactory transport, and that olfactory dysfunction may be induced as an earliest symptom of neurodegeneration caused by inhaled neurotoxins.

Mercury exposure in a riverside Amazon population, Brazil: a study of the ototoxicity of methylmercury

Introduction Mercury poisoning causes hearing loss in humans and animals. Acute and long-term exposures produce irreversible peripheral and central auditory system damage, and mercury in its various forms of presentation in the environment is ototoxic.

Objective We investigated the otoacoustic emissions responses in a riverside population exposed to environmental mercury by analyzing the inhibitory effect of the medial olivocochlear system (MOCS) on transient otoacoustic emissions (TEOAE).

Methods The purpose of the research was to evaluate the entire community independently of variables of sex and age. All of the participants were born and lived in a riverside community. After otolaryngologic evaluation, participants were received tympanometry, evaluation of contralateral acoustic reflexes, pure tone audiometry, and recording of TEOAEs with nonlinear click stimulation. Hair samples were collect to measure mercury levels.

Results There was no significant correlation between the inhibitory effect of the MOCS, age, and the level of mercury in the hair.

Conclusions The pathophysiological effects of chronic exposure may be subtle and nonspecific and can have a long period of latency; therefore, it will be important to monitor the effects of mercury exposure in the central auditory system of the Amazon population over time. Longitudinal studies should be performed to determine whether the inhibitory effect of the MOCS on otoacoustic emissions can be an evaluation method and diagnostic tool in populations exposed to mercury.

Assessment of health risks resulting from early-life exposures: Are current chemical toxicity testing protocols and risk assessment methods adequate?

Abstract Over the last couple of decades, the awareness of the potential health impacts associated with early-life exposures has increased. Global regulatory approaches to chemical risk assessment are intended to be protective for the diverse human population including all life stages. However, questions persist as to whether the current testing approaches and risk assessment methodologies are adequately protective for infants and children. Here, we review physiological and developmental differences that may result in differential sensitivity associated with early-life exposures. It is clear that sensitivity to chemical exposures during early-life can be similar, higher, or lower than that of adults, and can change quickly within a short developmental timeframe. Moreover, age-related exposure differences provide an important consideration for overall susceptibility. Differential sensitivity associated with a life stage can reflect the toxicokinetic handling of a xenobiotic exposure, the toxicodynamic response, or both. Each of these is illustrated with chemical-specific examples. The adequacy of current testing protocols, proposed new tools, and risk assessment methods for systemic noncancer endpoints are reviewed in light of the potential for differential risk to infants and young children.

The potential of induced pluripotent stem cells in models of neurological disorders: implications on future therapy

There is an urgent need for new and advanced approaches to modeling the pathological mechanisms of complex human neurological disorders. This is underscored by the decline in pharmaceutical research and development efficiency resulting in a relative decrease in new drug launches in the last several decades. Induced pluripotent stem cells represent a new tool to overcome many of the shortcomings of conventional methods, enabling live human neural cell modeling of complex conditions relating to aberrant neurodevelopment, such as schizophrenia, epilepsy and autism as well as age-associated neurodegeneration. This review considers the current status of induced pluripotent stem cell-based modeling of neurological disorders, canvassing proven and putative advantages, current constraints, and future prospects of next-generation culture systems for biomedical research and translation.

Developmental neurotoxicity - challenges in the 21st century and in vitro opportunities

In recent years neurodevelopmental problems in children have increased at a rate that suggests lifestyle factors and chemical exposures as likely contributors. When environmental chemicals contribute to neurodevelopmental disorders developmental neurotoxicity (DNT) becomes an enormous concern. But how can it be tackled? Current animal test- based guidelines are prohibitively expensive, at $ 1.4 million per substance, while their predictivity for human health effects may be limited, and mechanistic data that would help species extrapolation are not available. A broader screening for substances of concern requires a reliable testing strategy, applicable to larger numbers of substances, and sufficiently predictive to warrant further testing. This review discusses the evidence for possible contributions of environmental chemicals to DNT, limitations of the current test paradigm, emerging concepts and technologies pertinent to in vitro DNT testing and assay evaluation, as well as the prospect of a paradigm shift based on 21st century technologies.

Lead Intoxication Synergies of the Ethanol-Induced Toxic Responses in Neuronal Cells--PC12

Lead (Pb)-induced neurodegeneration and its link with widespread neurobehavioral changes are well documented. Experimental evidences suggest that ethanol could enhance the absorption of metals in the body, and alcohol consumption may increase the susceptibility to metal intoxication in the brain. However, the underlying mechanism of ethanol action in affecting metal toxicity in brain cells is poorly understood. Thus, an attempt was made to investigate the modulatory effect of ethanol on Pb intoxication in PC12 cells, a rat pheochromocytoma. Cells were co-exposed to biological safe doses of Pb (10 μM) and ethanol (200 mM), and data were compared to the response of cells which received independent exposure to these chemicals at similar doses. Ethanol (200 mM) exposure significantly aggravated the Pb-induced alterations in the end points associated with oxidative stress and apoptosis. The finding confirms the involvement of reactive oxygen species (ROS)-mediated oxidative stress, and impairment of mitochondrial membrane potential, which subsequently facilitate the translocation of triggering proteins between cytoplasm and mitochondria. We further confirmed the apoptotic changes due to induction of mitochondria-mediated caspase cascade. These cellular changes were found to recover significantly, if the cells are exposed to N-acetyl cysteine (NAC), a known antioxidant. Our data suggest that ethanol may potentiate Pb-induced cellular damage in brain cells, but such damaging effects could be recovered by inhibition of ROS generation. These results open up further possibilities for the design of new therapeutics based on antioxidants to prevent neurodegeneration and associated health problems.

The effects of prenatal methylmercury exposure on trace element and antioxidant levels in rats following 6-hydroxydopamine-induced neuronal insult

Methylmercury (MeHg) is a metal toxin found commonly in the environment. Studies have shown severe neurotoxic effects of MeHg poisoning especially during pregnancy where it crosses the foetoplacental and the blood brain barrier of the foetus leading to neurodevelopmental deficits in the offspring. These deficits may predispose offspring to neurodegenerative diseases later in life. In this study we investigated the effects of prenatal methylmercury exposure (2.5 mg/L in drinking water from GND 1-GND 21) on the trace element status in the brain of adolescent offspring (PND 28). Total antioxidant capacity (TAC) was measured in their blood plasma. In a separate group of animals that was also exposed prenatally to MeHg, 6-hydroydopamine (6-OHDA) was administered at PND 60 as a model of neuronal insult. Trace element and TAC levels were compared before and after 6-OHDA exposure. Prenatal MeHg treatment alone resulted in significantly higher concentrations of zinc, copper, manganese and selenium in the brain of offspring at PND 28 (p < 0.05), when compared to controls. In contrast, brain iron levels in MeHg-exposed adolescent offspring were significantly lower than their controls (p < 0.05). Following 6-OHDA exposure, the levels of iron, zinc, copper and manganese were increased compared to sham-lesioned offspring (p < 0.05). Prenatal MeHg exposure further increased these trace element levels thereby promoting toxicity (p < 0.05). Total antioxidant capacity was not significantly different in MeHg and control groups prior to lesion. However, following 6-OHDA administration, MeHg-exposed animals had a significantly lower TAC than that of controls (p < 0.05). Brain TAC levels were higher in adult male rats than in female rats during adolescence however male rats that had been exposed to MeHg in utero failed to show this increase at PND 74. Prenatal MeHg exposure results in trace element dyshomeostasis in the brain of offspring and reduces total antioxidant capacity. This may reflect a mechanism by which methylmercury exerts its neurotoxicity and/or predispose offspring to further neurological insults during adulthood.