Developmental neurotoxicity of low dose diazinon exposure of neonatal rats: effects on serotonin systems in adolescence and adulthood

Heart Failure and Gut Microbiota: What Is Cause and Effect?

Emerging evidence highlights the central role of gut microbiota in maintaining physiological homeostasis within the host. Disruptions in gut microbiota can destabilize systemic metabolism and inflammation, driving the onset and progression of cardiometabolic diseases. In heart failure (HF), intestinal dysfunction may induce the release of endotoxins and metabolites, leading to dysbiosis and exacerbating HF through the gut-heart axis. Understanding the relationship between gut microbiota and HF offers critical insights into disease mechanisms and therapeutic opportunities. Current research highlights promising potential to improve patient outcomes by restoring microbiota balance. In this review, we summarize the current studies in understanding the gut microbiota-HF connection and discuss avenues for future investigation.

Assessment of developmental neurotoxicology-associated alterations in neuronal architecture and function using Caenorhabditis elegans

Few of the many chemicals that regulatory agencies are charged with assessing for risk have been carefully tested for developmental neurotoxicity (DNT). To speed up testing efforts, as well as to reduce the use of vertebrate animals, great effort is being devoted to alternate laboratory models for testing DNT. A major mechanism of DNT is altered neuronal architecture resulting from chemical exposure during neurodevelopment. Caenorhabditis elegans is a nematode that has been extensively studied by neurobiologists and developmental biologists, and to a lesser extent by neurotoxicologists. The developmental trajectory of the nervous system in C. elegans is easily visualized, normally entirely invariant, and fully mapped. Therefore, we hypothesized that C. elegans could be a powerful in vivo model to test chemicals for the potential to alter developmental patterning of neuronal architecture. To test whether this might be true, we developed a novel C. elegans DNT testing paradigm that includes exposure throughout development, examines all major neurotransmitter neuronal types for architectural alterations, and tests behaviors specific to dopaminergic, cholinergic, and glutamatergic functions. We used this paradigm to characterize the effects of early-life exposures to the developmental neurotoxicants lead, cadmium, and benzo(a)pyrene (BaP) on dopaminergic, cholinergic, and glutamatergic architecture. We also assessed whether exposures would alter neuronal specification as assessed by expression of reporter genes diagnostic of specific neurotransmitters. We identified no cases in which the apparent neurotransmitter type of the neurons we examined changed, but many in which neuronal morphology was altered. We also found that neuron-specific behaviors were altered during C. elegans mid-adulthood for populations with measured morphological neurodegeneration in earlier stages. The functional changes were consistent with the morphological changes we observed in terms of type of neuron affected. We identified changes consistent with those reported in the mammalian DNT literature, strengthening the case for C. elegans as a DNT model, and made novel observations that should be followed up in future studies.

Environmental toxicants, brown adipose tissue, and potential links to obesity and metabolic disease

Rates of human obesity, type 2 diabetes, and non-alcoholic fatty liver disease (NAFLD) have risen faster than anticipated and cannot solely be explained by excessive caloric intake or physical inactivity. Importantly, this effect is also observed in many other domesticated and non-domesticated mammals, which has led to the hypothesis that synthetic environmental pollutants may be contributing to disease development. While the impact of these chemicals on appetite and adipogenesis has been extensively studied, their potential role in reducing energy expenditure is less studied. An important component of whole-body energy expenditure is adaptive and diet-induced thermogenesis in human brown adipose tissue (BAT). This review summarizes recent evidence that environmental pollutants such as the pesticide chlorpyrifos inhibit BAT function, diet-induced thermogenesis and the potential signaling pathways mediating these effects. Lastly, we discuss the importance of housing experimental mice at thermoneutrality, rather than room temperature, to maximize the translation of findings to humans.

Persistent neurobehavioral and neurochemical anomalies in middle-aged rats after maternal diazinon exposure

Diazinon is an organophosphate pesticide that has a history of wide use. Developmental exposures to organophosphates lead to neurobehavioral changes that emerge early in life and can persist into adulthood. However, preclinical studies have generally evaluated changes through young adulthood, whereas the persistence or progression of deficits into middle age remain poorly understood. The current study evaluated the effects of maternal diazinon exposure on behavior and neurochemistry in middle age, at 1 year postpartum, comparing the results to our previous studies of outcomes at adolescence and in young adulthood (4 months of age) (Hawkey 2020). Female rats received 0, 0.5 or 1.0 mg/kg/day of diazinon via osmotic minipump throughout gestation and into the postpartum period. The offspring were tested on a battery of locomotor, affective, and cognitive tests at young adulthood and during middle age. Some of the neurobehavioral consequences of developmental DZN seen during adolescence and young adulthood faded with continued aging, whereas other neurobehavioral effects emerged with aging. At middle age, the rats showed few locomotor effects, in contrast to the locomotor hyperactivity that had been observed in adolescence. Notably, though, DZN exposure during development impaired reference memory performance in middle-aged males, an effect that had not been seen in the younger animals. Likewise, middle-aged females exposed to DZN showed deficient attentional accuracy, an effect not seen in young adults. Across adulthood, the continued potential for behavioral defects was associated with altered dopaminergic function, characterized by enhanced dopamine utilization that was regionally-selective (striatum but not frontal/parietal cortex). This study shows that the neurobehavioral impairments from maternal low dose exposure to diazinon not only persist, but may continue to evolve as animals enter middle age.

The organophosphate insecticide diazinon and aging: Neurobehavioral and mitochondrial effects in zebrafish exposed as embryos or during aging

Organophosphate (OP) compounds comprise one of the most widely used classes of insecticides worldwide. OPs have been shown to have negative human health impacts, particularly developmental neurotoxicity. However, neurotoxic impacts in later adulthood and during the aging process are relatively uncharacterized. The present study examined diazinon (DZN), an OP, to determine the neurobehavioral consequences, in addition to mitochondrial dysfunction on a macroscale (whole organism basal respiration) and on a microscale (whole organ mitochondrial respiration), using zebrafish (ZF) as a model. One group of 14-month-old adult ZF were exposed acutely as adults (0.4, 1.25, and 4.0 μM) for five days and tested as adults, and another group was exposed developmentally 5-120 h post-fertilization (70, 210, and 700 nM) and tested at larval, adolescent, adult, and aging life stages. ZF exposed acutely as adults did not display many significant neurobehavioral impacts or mitochondrial dysfunction. Conversely, the embryonically exposed ZF showed altered behavioral functions at each stage of life which emerged and attenuated as fish transitioned from each developmental stage to the next. Mitochondrial oxygen consumptions measurement results for developmentally DZN exposed ZF showed significant increases in the low and middle dose groups in organs such as the brain and testes. Overall, there is an indication that early developmental exposure to DZN had continuing adverse neurobehavioral and cellular consequences throughout their lives well into adulthood and aging periods.

Paternal Δ9-Tetrahydrocannabinol Exposure Prior to Mating Elicits Deficits in Cholinergic Synaptic Function in the Offspring

Little attention has been paid to the potential impact of paternal marijuana use on offspring brain development. We administered Δ9-tetrahydrocannabinol (THC, 0, 2, or 4 mg/kg/day) to male rats for 28 days. Two days after the last THC treatment, the males were mated to drug-naïve females. We then assessed the impact on development of acetylcholine (ACh) systems in the offspring, encompassing the period from the onset of adolescence (postnatal day 30) through middle age (postnatal day 150), and including brain regions encompassing the majority of ACh terminals and cell bodies. Δ9-Tetrahydrocannabinol produced a dose-dependent deficit in hemicholinium-3 binding, an index of presynaptic ACh activity, superimposed on regionally selective increases in choline acetyltransferase activity, a biomarker for numbers of ACh terminals. The combined effects produced a persistent decrement in the hemicholinium-3/choline acetyltransferase ratio, an index of impulse activity per nerve terminal. At the low THC dose, the decreased presynaptic activity was partially compensated by upregulation of nicotinic ACh receptors, whereas at the high dose, receptors were subnormal, an effect that would exacerbate the presynaptic defect. Superimposed on these effects, either dose of THC also accelerated the age-related decline in nicotinic ACh receptors. Our studies provide evidence for adverse effects of paternal THC administration on neurodevelopment in the offspring and further demonstrate that adverse impacts of drug exposure on brain development are not limited to effects mediated by the embryonic or fetal chemical environment, but rather that vulnerability is engendered by exposures occurring prior to conception, involving the father as well as the mother.

Reevaluation of Serum Arylesterase Activity in Neurodevelopmental Disorders

Organophosphate compounds (OPs) interfere with neurodevelopment and are neurotoxic for humans and animals. They are first biotransformed to the more toxic oxon form, and then hydrolyzed to specific metabolites by the enzyme paraoxonase/arylesterase, encoded by the gene PON1 located on human chr. 7q21.3. In autism spectrum disorder (ASD) and in attention-deficit/hyperactivity disorder (ADHD), a correlation between OP exposure and disease onset has been reported. In this case-control study, we aimed to replicate our previous work showing reduced levels of serum PON1 arylesterase activity in Italian and Caucasian-American ASD samples, and to extend our analysis to other neurodevelopmental disorders, namely ADHD and developmental language disorder (DLD), also known as specific language impairment (SLI). The arylesterase activity, measured using standard spectrophotometric methods, is significantly reduced in the ADHD, and not in the ASD sample compared with the controls. Our previous results seemingly stem from spuriously high arylesterase levels in the former control sample. Finally, genotyping SNPs rs705379 and rs662 using TDI-FP, a significant effect of rs705379 alleles on the serum arylesterase activity is observed in all of the subgroups tested, regardless of diagnosis, as well as a lack of association between PON1 gene polymorphisms and ASD/ADHD susceptibility in the Italian population. In summary, the serum arylesterase activity is reduced in children and adolescents with ADHD, and this reduction is not due to the functional PON1 gene variants assessed in this study. Based on previous literature, it may more likely reflect enhanced oxidative stress than specific genetic underpinnings.

Gestational exposures to organophosphorus insecticides: From acute poisoning to developmental neurotoxicity

For over three-quarters of a century, organophosphorus (OP) insecticides have been ubiquitously used in agricultural, residential, and commercial settings and in public health programs to mitigate insect-borne diseases. Their broad-spectrum insecticidal effectiveness is accounted for by the irreversible inhibition of acetylcholinesterase (AChE), the enzyme that catalyzes acetylcholine (ACh) hydrolysis, in the nervous system of insects. However, because AChE is evolutionarily conserved, OP insecticides are also toxic to mammals, including humans, and acute OP intoxication remains a major public health concern in countries where OP insecticide usage is poorly regulated. Environmental exposures to OP levels that are generally too low to cause marked inhibition of AChE and to trigger acute signs of intoxication, on the other hand, represent an insidious public health issue worldwide. Gestational exposures to OP insecticides are particularly concerning because of the exquisite sensitivity of the developing brain to these insecticides. The present article overviews and discusses: (i) the health effects and therapeutic management of acute OP poisoning during pregnancy, (ii) epidemiological studies examining associations between environmental OP exposures during gestation and health outcomes of offspring, (iii) preclinical evidence that OP insecticides are developmental neurotoxicants, and (iv) potential mechanisms underlying the developmental neurotoxicity of OP insecticides. Understanding how gestational exposures to different levels of OP insecticides affect pregnancy and childhood development is critical to guiding implementation of preventive measures and direct research aimed at identifying effective therapeutic interventions that can limit the negative impact of these exposures on public health.

Learning and memory retention deficits in prepubertal guinea pigs prenatally exposed to low levels of the organophosphorus insecticide malathion

High doses of malathion, an organophosphorus (OP) insecticide ubiquitously used in agriculture, residential settings, and public health programs worldwide, induce a well-defined toxidrome that results from the inhibition of acetylcholinesterase (AChE). However, prenatal exposures to malathion levels that are below the threshold for AChE inhibition have been associated with increased risks of neurodevelopmental disorders, including autism spectrum disorder with intellectual disability comorbidity. The present study tested the hypothesis that prenatal exposures to a non-AChE-inhibiting dose of malathion are causally related to sex-biased cognitive deficits later in life in a precocial species. To this end, pregnant guinea pigs were injected subcutaneously with malathion (20 mg/kg) or vehicle (peanut oil, 0.5 ml/kg) once daily between approximate gestational days 53 and 63. This malathion dose regimen caused no significant AChE inhibition in the brain or blood of dams and offspring and had no significant effect on the postnatal growth of the offspring. Around postnatal day 30, locomotor activity and habituation, a form of non-associative learning, were comparable between malathion- and peanut oil-exposed offspring. However, in the Morris water maze, malathion-exposed offspring presented significant sex-dependent spatial learning deficits in addition to memory impairments. These results are far-reaching as they indicate that: (i) malathion is a developmental neurotoxicant and (ii) AChE inhibition is not an adequate biomarker to derive safety limits of malathion exposures during gestation. Continued studies are necessary to identify the time and dose dependence of the developmental neurotoxicity of malathion and the mechanisms underlying the detrimental effects of this insecticide in the developing brain.

Gut Microbiome Toxicity: Connecting the Environment and Gut Microbiome-Associated Diseases

The human gut microbiome can be easily disturbed upon exposure to a range of toxic environmental agents. Environmentally induced perturbation in the gut microbiome is strongly associated with human disease risk. Functional gut microbiome alterations that may adversely influence human health is an increasingly appreciated mechanism by which environmental chemicals exert their toxic effects. In this review, we define the functional damage driven by environmental exposure in the gut microbiome as gut microbiome toxicity. The establishment of gut microbiome toxicity links the toxic effects of various environmental agents and microbiota-associated diseases, calling for more comprehensive toxicity evaluation with extended consideration of gut microbiome toxicity.

Gut microbiome: An intermediary to neurotoxicity

There is growing recognition that the gut microbiome is an important regulator for neurological functions. This review provides a summary on the role of gut microbiota in various neurological disorders including neurotoxicity induced by environmental stressors such as drugs, environmental contaminants, and dietary factors. We propose that the gut microbiome remotely senses and regulates CNS signaling through the following mechanisms: 1) intestinal bacteria-mediated biotransformation of neurotoxicants that alters the neuro-reactivity of the parent compounds; 2) altered production of neuro-reactive microbial metabolites following exposure to certain environmental stressors; 3) bi-directional communication within the gut-brain axis to alter the intestinal barrier integrity; and 4) regulation of mucosal immune function. Distinct microbial metabolites may enter systemic circulation and epigenetically reprogram the expression of host genes in the CNS, regulating neuroinflammation, cell survival, or cell death. We will also review the current tools for the study of the gut-brain axis and provide some suggestions to move this field forward in the future.

Gestational and perinatal exposure to diazinon causes long-lasting neurobehavioral consequences in the rat

Diazinon is a widely-used organophosphate pesticide. Pulsatile exposure to diazinon during neonatal development has previously been shown cause long-term neurobehavioral impairments in rats. However, the effects of chronic low concentration exposures during perinatal development remain unclear. This experiment evaluated such effects in Sprague-Dawley rats by implanting osmotic pumps in breeder females prior to conception (N = 13-15 litters per condition) which then delivered chronic, zero order kinetic low-level infusions of 0, 114 or 228 ug/day of diazinon throughout pregnancy. One male and one female from each litter was assessed with a battery of behavioral tests that continued from four weeks of age into adulthood. Litter was used as the unit of variance for the analysis of variance test of significance, with sex as a within litter factor. Diazinon treatment condition was the between subjects factor and time or sessions were repeated measures. Chronic diazinon exposure from pre-mating until the neonatal period caused a significant (p < 0.05) increase in percent of time spent on the open arms of the elevated plus maze, an index of risk-taking behavior. Gestational and lactational diazinon exposure also caused a significant (p < 0.05) degree of hyperactivity in the Figure-8 apparatus during adolescence, specifically affecting the early part of the hour-long test session. This effect had dissipated by the time the rats reached adulthood. Diazinon exposure also caused a significant impairment in novel object recognition, a test of cognitive function. Offspring exposed to 228 ug/day diazinon (p < 0.05) showed significantly less preference for the novel vs. familiar object than controls during the first five minutes of the novel object recognition test.

A combination insecticide at sub-lethal dose debilitated the expression pattern of crucial signalling molecules that facilitate craniofacial patterning in domestic chick Gallus domesticus

Pesticides despite being agents that protect the plants and humans from noxious pests, are infamous for their potential to cause detrimental health issues in nontargeted species. In order to ascertain the latter, a set of experiments were conducted by exposing early chick embryos to a widely used combination insecticide (Ci, 50% chlorpyrifos and 5% cypermethrin). The results revealed a myriad of congenital defects pertaining to craniofacial development such as anophthalmia, microphthalmia, exencephaly as well as deformed beak and cranial structures. These teratological manifestations could be attributed to the Ci induced alteration in the titre of major regulators of neurulation and ossification. Therefore, the mRNA and/or the protein level expression pattern of genes which are reported to be involved in the craniofacial development were studied at selected time points of embryonic development. The analysis of the result showed that there have been significant alternations in the expression patterns of the signalling molecules such as SHH, WNTs, CDH1, CDH2, L1CAM, PAX6, HOX, PCNA, GLI3, BMP7, FGF8, GLIs, SOX9, RUNX2, DLX5, COL10A1, CASPASE3 etc. on embryonic days 2, 4 and/or 10. Concurrently, on day 10, whole-mount skeletal staining and biochemical estimation of hydroxyproline were carried out in the cranial tissues of the embryos. The overall result of the current study indicates that exposure to Ci during early development impede the crucial regulatory signals that orchestrate the morphogenesis of cranial neural crest cells thereby hindering the normal progression of neural tube and endochondral ossification which collectively lead to craniofacial dysmorphism in domestic chicks.

Perinatal diazinon exposure compromises the development of acetylcholine and serotonin systems

Organophosphate pesticides are developmental neurotoxicants. We gave diazinon via osmotic minipumps implanted into dams prior to conception, with exposure continued into the second postnatal week, at doses (0.5 or 1 mg/kg/day) that did not produce detectable brain cholinesterase inhibition. We evaluated the impact on acetylcholine (ACh) and serotonin (5-hydroxytryptamine, 5HT) systems in brain regions from adolescence through full adulthood. Diazinon produced deficits in presynaptic ACh activity with regional and sex selectivity: cerebrocortical regions and the hippocampus were affected to a greater extent than were the striatum, midbrain or brainstem, and females were more sensitive than males. Diazinon also reduced nicotinic ACh receptors and 5HT_1A receptors, with the same regional and sex preferences. These patterns were similar to those of diazinon given in a much more restricted period (postnatal day 1-4) but were of greater magnitude and consistency; this suggests that the brain is vulnerable to diazinon over a wide developmental window. Diazinon's effects differed from those of the related organophosphate, chlorpyrifos, with regard to regional and sex selectivity, and more importantly, to the effects on receptors: chlorpyrifos upregulates nicotinic ACh receptors and 5HT receptors, effects that compensate for the presynaptic ACh deficits. Diazinon can thus be expected to have worse neurodevelopmental outcomes than chlorpyrifos. Further, the disparities between diazinon and chlorpyrifos indicate the problems of predicting the developmental neurotoxicity of organophosphates based on a single compound, and emphasize the inadequacy of cholinesterase inhibition as an index of safety.

Sex-Specific Neurotoxic Effects of Organophosphate Pesticides Across the Life Course

PURPOSE OF REVIEW

This review discusses the sex-specific effects of exposure to various organophosphate (OP) pesticides throughout the life course and potential reasons for the differential vulnerabilities observed across sexes.

RECENT FINDINGS

Sex is a crucial factor in the response to toxicants, yet the sex-specific effects of OP exposure, particularly in juveniles and adults, remain unresolved. This is largely due to study design and inconsistencies in exposure and outcome assessments. Exposure to OPs results in multiple adverse outcomes influenced by many factors including sex. Reported sex-specific effects suggest that males are more susceptible to OPs, which reflects the sex-dependent prevalence of various neurodevelopmental and neurodegenerative disorders such as autism and amyotrophic lateral sclerosis (ALS), in which males are at greater risk. Thus, this review proposes that the biological sex-specific effects elicited by OP exposure may in part underlie the dimorphic susceptibilities observed in neurological disorders. Understanding the immediate and long-term effects of OP exposure across sexes will be critical in advancing our understanding of OP-induced neurotoxicity and disease.

Neurotransmitter changes in rat brain regions following glyphosate exposure

The effects of glyphosate oral exposure (35, 75, 150 and 800mg/kg bw, 6 days) on brain region monoamine levels of male Wistar rats were examined. Glyphosate-treated rats (35, 75, 150 and 800mg/kg bw, 6 days), had no visible injury, i.e., no clinical signs of dysfunction were observed. After last dose of glyphosate, serotonin (5-HT), dopamine (DA) and norepinephrine (NE) and its metabolites levels were determined in the brain regions striatum, hippocampus, prefrontal, cortex, hypothalamus and midbrain, by HPLC. Glyphosate caused statistically significant changes in the 5-HT and its metabolite 5-hydroxy-3-indolacetic acid (5-HIAA), DA and its metabolites 3,4-hydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and NE and its metabolite 3-metoxy-4-hydroxyphenylethyleneglycol (MHPG) levels in a brain regional- and dose-related manner. Moreover, glyphosate, dose-dependent, evoked a statistically significant increase in 5-HT turnover in striatum and hypothalamus and in DA turnover in prefrontal cortex and hippocampus, and a statistically significant decrease in NE turnover in prefrontal cortex and hypothalamus. The present findings indicate that glyphosate significantly altered central nervous system (CNS) monoaminergic neurotransmitters in a brain regional- and dose-related manner, effects that may contribute to the overall spectrum of neurotoxicity caused by this herbicide.

Developmental neurotoxicity of the organophosphorus insecticide chlorpyrifos: from clinical findings to preclinical models and potential mechanisms

Organophosphorus (OP) insecticides are pest-control agents heavily used worldwide. Unfortunately, they are also well known for the toxic effects that they can trigger in humans. Clinical manifestations of an acute exposure of humans to OP insecticides include a well-defined cholinergic crisis that develops as a result of the irreversible inhibition of acetylcholinesterase (AChE), the enzyme that hydrolyzes the neurotransmitter acetylcholine (ACh). Prolonged exposures to levels of OP insecticides that are insufficient to trigger signs of acute intoxication, which are hereafter referred to as subacute exposures, have also been associated with neurological deficits. In particular, epidemiological studies have reported statistically significant correlations between prenatal subacute exposures to OP insecticides, including chlorpyrifos, and neurological deficits that range from cognitive impairments to tremors in childhood. The primary objectives of this article are: (i) to address the short- and long-term neurological issues that have been associated with acute and subacute exposures of humans to OP insecticides, especially early in life (ii) to discuss the translational relevance of animal models of developmental exposure to OP insecticides, and (iii) to review mechanisms that are likely to contribute to the developmental neurotoxicity of OP insecticides. Most of the discussion will be focused on chlorpyrifos, the top-selling OP insecticide in the United States and throughout the world. These points are critical for the identification and development of safe and effective interventions to counter and/or prevent the neurotoxic effects of these chemicals in the developing brain. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Sex-Specific Effects of Organophosphate Diazinon on the Gut Microbiome and Its Metabolic Functions

BACKGROUND

There is growing recognition of the significance of the gut microbiome to human health, and the association between a perturbed gut microbiome with human diseases has been established. Previous studies also show the role of environmental toxicants in perturbing the gut microbiome and its metabolic functions. The wide agricultural use of diazinon, an organophosphate insecticide, has raised serious environmental health concerns since it is a potent neurotoxicant. With studies demonstrating the presence of a microbiome-gut-brain axis, it is possible that gut microbiome perturbation may also contribute to diazinon toxicity.

OBJECTIVES

We investigated the impact of diazinon exposure on the gut microbiome composition and its metabolic functions in C57BL/6 mice.

METHODS

We used a combination of 16S rRNA gene sequencing, metagenomics sequencing, and mass spectrometry-based metabolomics profiling in a mouse model to examine the functional impact of diazinon on the gut microbiome.

RESULTS

16S rRNA gene sequencing revealed that diazinon exposure significantly perturbed the gut microbiome, and metagenomic sequencing found that diazinon exposure altered the functional metagenome. Moreover, metabolomics profiling revealed an altered metabolic profile arising from exposure. Of particular significance, these changes were more pronounced for male mice than for female mice.

CONCLUSIONS

Diazinon exposure perturbed the gut microbiome community structure, functional metagenome, and associated metabolic profiles in a sex-specific manner. These findings may provide novel insights regarding perturbations of the gut microbiome and its functions as a potential new mechanism contributing to diazinon neurotoxicity and, in particular, its sex-selective effects. Citation: Gao B, Bian X, Mahbub R, Lu K. 2017. Sex-specific effects of organophosphate diazinon on the gut microbiome and its metabolic functions. Environ Health Perspect 125:198-206; http://dx.doi.org/10.1289/EHP202.

Ontogeny of SERT Expression and Antidepressant-like Response to Escitalopram in Wild-Type and SERT Mutant Mice

Depression is a disabling affective disorder for which the majority of patients are not effectively treated. This problem is exacerbated in children and adolescents for whom only two antidepressants are approved, both of which are selective serotonin reuptake inhibitor (SSRIs). Unfortunately SSRIs are often less effective in juveniles than in adults; however, the mechanism(s) underlying age-dependent responses to SSRIs is unknown. To this end, we compared the antidepressant-like response to the SSRI escitalopram using the tail suspension test and saturation binding of [(3)H]citalopram to the serotonin transporter (SERT), the primary target of SSRIs, in juvenile [postnatal day (P)21], adolescent (P28), and adult (P90) wild-type (SERT+/+) mice. In addition, to model individuals carrying low-expressing SERT variants, we studied mice with reduced SERT expression (SERT+/-) or lacking SERT (SERT-/-). Maximal antidepressant-like effects were less in P21 mice relative to P90 mice. This was especially apparent in SERT+/- mice. However, the potency for escitalopram to produce antidepressant-like effects in SERT+/+ and SERT+/- mice was greater in P21 and P28 mice than in adults. SERT expression increased with age in terminal regions and decreased with age in cell body regions. Binding affinity values did not change as a function of age or genotype. As expected, in SERT-/- mice escitalopram produced no behavioral effects, and there was no specific [(3)H]citalopram binding. These data reveal age- and genotype-dependent shifts in the dose-response for escitalopram to produce antidepressant-like effects, which vary with SERT expression, and may contribute to the limited therapeutic response to SSRIs in juveniles and adolescents.

The influence of sensitization on mechanisms of organophosphorus pesticide-induced airway hyperreactivity

We previously demonstrated that antigen sensitization increases vulnerability to airway hyperreactivity induced by the organophosphorus pesticide (OP) parathion. Sensitization also changes the mechanism of parathion-induced airway hyperreactivity to one that is dependent on IL-5. To determine whether this effect can be generalized to other OPs, and to other classes of pesticides, we measured airway responsiveness to vagal stimulation or intravenous acetylcholine in nonsensitized and ovalbumin-sensitized guinea pigs 24 hours after a single subcutaneous injection of the OPs diazinon or chlorpyrifos, or the pyrethroid permethrin. Sensitization exacerbated the effects of chlorpyrifos on bronchoconstriction in response to vagal stimulation or intravenous acetylcholine. Pretreatment with function-blocking IL-5 antibody prevented chlorpyrifos-induced airway hyperreactivity in sensitized, but not in nonsensitized, guinea pigs. In sensitized guinea pigs, blocking IL-5 decreased eosinophil activation, as measured by decreased eosinophil major basic protein in the trachea. In contrast, sensitization did not alter diazinon-induced airway hyperreactivity, and permethrin did not cause airway hyperreactivity in either nonsensitized or sensitized guinea pigs. None of the pesticides affected inflammatory cells in the bronchoalveolar lavage fluid or blood. We have previously shown that three different OPs cause airway hyperreactivity via loss of neuronal M2 muscarinic receptor function. Similar to parathion, but unlike diazinon, the mechanism of chlorpyrifos-induced airway hyperreactivity is changed by sensitization. Thus, OP-induced airway hyperreactivity is dependent on sensitization status and on the OP used, which may influence therapeutic approaches.

Developmental exposure to organophosphate flame retardants causes behavioral effects in larval and adult zebrafish

BACKGROUND

Organophosphate flame retardants (OPFRs) have grown in usage since concerns about the health effects of the previously used polybrominated flame retardants led to their being phased out. The potential for OPFRs to cause adverse health effects of their own is still unexamined. Because of their structural similarities to organophosphate pesticides, which have themselves been heavily researched and shown to be neurobehavioral teratogens, we investigated the possibility that developmental exposure to two OPFRs, triphenyl phosphate (TPHP), and tris(1,3-dichloroisopropyl)phosphate (TDCIPP) might lead to behavioral impairment across the lifespan, as has been observed with the organophosphate pesticide chlorpyrifos.

METHODS

Zebrafish were exposed to 0.03 or 0.3 μM of TPHP, TDCIPP, or chlorpyrifos from 0 to 5 days post fertilization. Vehicle control consisted of 0.03% solution of DMSO. At 6 days post fertilization, larvae were tested on a locomotor assay. Separate cohorts of 6 day old larvae that were not tested on the larval assay were allowed to grow to adulthood. At 12 weeks post fertilization, these adult zebrafish were tested on a battery of behavioral assays that included tests of novel environment exploration, startle habituation, social affiliation, and predator escape.

RESULTS

Developmental exposure altered zebrafish behavior across the lifespan. Larval zebrafish exposed to the 0.03 μM doses of chlorpyrifos or TDCIPP exhibited significant (p<0.05) hyperactivity in the locomotor assay. Organophosphate exposure significantly (p<0.05) altered the time course of adult zebrafish behavior in the novel environment, startle habituation, and social affiliation assays. Predator escape behavior was significantly (p<0.05) reduced in fish exposed to the 0.3 μM dose of TDCIPP. Exposure also caused hyperactivity in adult fish, with fish exposed to the 0.3 μM dose of TDCIPP exhibiting significantly (p<0.05) elevated locomotor behavior in the novel environment assay.

DISCUSSION

Early developmental exposure to OPFRs produced behavioral impairment that persisted into adulthood. These findings support broader research investigating the role of organophosphate compounds, including the OPFRs used here, in developmental neurotoxicity.

Gulf War agent exposure causes impairment of long-term memory formation and neuropathological changes in a mouse model of Gulf War Illness

Gulf War Illness (GWI) is a chronic multisymptom illness with a central nervous system component such as memory deficits, neurological, and musculoskeletal problems. There are ample data that demonstrate that exposure to Gulf War (GW) agents, such as pyridostigmine bromide (PB) and pesticides such as permethrin (PER), were key contributors to the etiology of GWI post deployment to the Persian GW. In the current study, we examined the consequences of acute (10 days) exposure to PB and PER in C57BL6 mice. Learning and memory tests were performed at 18 days and at 5 months post-exposure. We investigated the relationship between the cognitive phenotype and neuropathological changes at short and long-term time points post-exposure. No cognitive deficits were observed at the short-term time point, and only minor neuropathological changes were detected. However, cognitive deficits emerged at the later time point and were associated with increased astrogliosis and reduction of synaptophysin staining in the hippocampi and cerebral cortices of exposed mice, 5 months post exposure. In summary, our findings in this mouse model of GW agent exposure are consistent with some GWI symptom manifestations, including delayed onset of symptoms and CNS disturbances observed in GWI veterans.

Assessment of attention and inhibitory control in rodent developmental neurotoxicity studies

In designing screens to assess potential neurotoxicants, the paramount goal is that the selected assessment tools detect dysfunction if it exists. This goal is particularly challenging in the case of cognitive assessments. Cognition is not a unitary phenomenon, and indeed there is growing evidence that different aspects of cognitive functioning are subserved by distinct neural systems. As a result, if a particular neurotoxicant selectively damages certain neural systems but not others, it can impair some cognitive, sensory, or affective functions, but leave many others intact. Accordingly, studies with human subjects use batteries of cognitive tests, cognizant of the fact that no one test is capable of detecting all forms of cognitive dysfunction. In contrast, assessment of cognitive functioning in non-human animal developmental neurotoxicity (DNT) studies typically consists of a single, presumably representative, "learning and memory" task that is expected to detect all potential effects on cognitive functioning. Streamlining the cognitive assessment in these studies saves time and money, but these shortcuts can have serious consequences if the aspect of cognitive functioning that is impaired is not tapped by the single selected task. In particular, executive functioning - a constellation of cognitive functions which enables the organism to focus on multiple streams of information simultaneously, and revise plans as necessary - is poorly assessed in most animal DNT studies. The failure to adequately assess these functions - which include attention, working memory, inhibitory control, and planning - is particularly worrisome in light of evidence that the neural systems that subserve these functions may be uniquely vulnerable to early developmental insults. We illustrate the importance of tapping these areas of functioning in DNT studies by describing the pattern of effects produced by early developmental Pb exposure. Rats exposed to lead (Pb) early in development were tested on a series of automated attention tasks, as well as on a radial arm maze task. The lead-exposed rats were not impaired in this demanding radial arm maze task, despite conditions which tapped the limits of both working and long-term memory. In contrast, the automated tests designed to assess rodent executive functioning revealed selective and functionally important deficits in attention and regulation of emotion or negative affect (produced by committing an error or not receiving an expected reward). This example underscores the importance of including tasks to specifically tap executive functioning in DNT batteries. Such tasks are not only sensitive but can also shed light on the specific nature of the dysfunction, and they can implicate dysfunction of specific neural systems, information which can be used to design therapeutic interventions. Although the use of such tasks increases the time and effort needed to complete the battery, the benefits outweigh the cost, in light of the greater sensitivity of the battery and the more complete characterization of effects.

Monocrotophos induced oxidative stress and alterations in brain dopamine and serotonin receptors in young rats

Human exposure to monocrotophos, an organophosphate pesticide, could occur due to its high use in agriculture to protect crops. Recently, we found that postlactational exposure to monocrotophos impaired cholinergic mechanisms in young rats and such changes persisted even after withdrawal of monocrotophos exposure. In continuation to this, the effect of monocrotophos on noncholinergic targets and role of oxidative stress in its neurotoxicity has been studied. Exposure of rats from postnatal day (PD)22 to PD49 to monocrotophos (0.50 or 1.0 mg kg−1 body weight, perorally) significantly impaired motor activity and motor coordination on PD50 as compared to controls. A significant decrease in the binding of 3H-spiperone to striatal membrane (26%, p < 0.01; 30%, p < 0.05) in rats exposed to monocrotophos at both the doses and increase in the binding of 3H-ketanserin to frontocortical membrane (14%, p > 0.05; 37%, p < 0.05) in those exposed at a higher dose, respectively, was observed on PD50 compared with the controls. Alterations in the binding persisted even after withdrawal of monocrotophos exposure on PD65. Increased oxidative stress in brain regions following exposure of rats to monocrotophos was also observed on PD50 that persisted 15 days after withdrawal of exposure on PD65. The results suggest that monocrotophos exerts its neurobehavioral toxicity by affecting noncholinergic functions involving dopaminergic and serotonergic systems associated with enhanced oxidative stress. The results also exhibit vulnerability of developing brain to monocrotophos as most of the changes persisted even after withdrawal of its exposure.

The dynamics of autism spectrum disorders: how neurotoxic compounds and neurotransmitters interact

In recent years concern has risen about the increasing prevalence of Autism Spectrum Disorders (ASD). Accumulating evidence shows that exposure to neurotoxic compounds is related to ASD. Neurotransmitters might play a key role, as research has indicated a connection between neurotoxic compounds, neurotransmitters and ASD. In the current review a literature overview with respect to neurotoxic exposure and the effects on neurotransmitter systems is presented. The aim was to identify mechanisms and related factors which together might result in ASD. The literature reported in the current review supports the hypothesis that exposure to neurotoxic compounds can lead to alterations in the GABAergic, glutamatergic, serotonergic and dopaminergic system which have been related to ASD in previous work. However, in several studies findings were reported that are not supportive of this hypothesis. Other factors also might be related, possibly altering the mechanisms at work, such as time and length of exposure as well as dose of the compound. Future research should focus on identifying the pathway through which these factors interact with exposure to neurotoxic compounds making use of human studies.

Lower anxiogenic effects of serotonin agonists are associated with lower activation of amygdala and lateral orbital cortex in adolescent male rats

There has been controversy over use of selective serotonin reuptake inhibitors (SSRIs) to treat affective disorders in children and adolescents due to clinical reports of increased risk for suicidal ideation and behavior during treatment, and animal studies showing changes in adult anxiety- and depressive-like behaviors after repeated treatment during adolescence. However, the acute effect of serotonergic drugs on affective behavior during adolescence is poorly understood. We investigated serotonergic modulation of anxiety-like behavior in adolescent (PN28-32) and adult (PN67-73) male rats using the SSRI fluoxetine, the 5-HT(1A) agonist 8-OH DPAT, and the 5-HT₂ agonist mCPP. Acute treatment with fluoxetine (10 mg/kg, i.p.) produced greater anxiogenic effects in adults than adolescents in the light/dark (LD) test for anxiety-like behavior, but fluoxetine (2.5, 5, and 10 mg/kg, i.p.) increased extracellular serotonin in the medial prefrontal cortex similarly in both ages. Adults were also more sensitive to the anxiogenic effects of 8-OH DPAT (0.25 and 0.5 mg/kg, i.p.), but not mCPP (0.5 and 1 mg/kg, i.p.), in the LD test. Fluoxetine (10 mg/kg) stimulated greater increases in c-Fos expression across the extended amygdala in adults than in adolescents, and 8-OH DPAT (0.5 mg/kg) produced greater increases in c-Fos in the lateral orbital cortex and central nucleus of the amygdala in adults. These data show that lower anxiogenic effects of acute SSRIs in adolescents are associated with lesser activation of cortical and amygdala brain regions. This immaturity could contribute to the different profile of behavioral effects observed in adolescents and adults treated with SSRIs.

Methamidophos exposure during the early postnatal period of mice: immediate and late-emergent effects on the cholinergic and serotonergic systems and behavior

Organophosphates (OPs) are among the most used pesticides. Although some OPs have had their use progressively more restricted, other OPs are being used without sufficient investigation of their effects. Here, we investigated the immediate neurochemical and delayed neurochemical and behavioral actions of the OP methamidophos to verify whether there are concerns regarding exposure during early postnatal development. From the third to the nineth postnatal day (PN), Swiss mice were sc injected with methamidophos (1mg/kg). At PN10, we assessed cholinergic and serotonergic biomarkers in the cerebral cortex and brainstem. From PN60 to PN63, mice were submitted to a battery of behavioral tests and subsequently to biochemical analyses. At PN10, the effects were restricted to females and to the cholinergic system: Methamidophos promoted increased choline transporter binding in the brainstem. At PN63, in the brainstem, there was a decrease in choline transporter, a female-only decrease in 5HT1A and a male-only increase in 5HT2 receptor binding. In the cortex, choline acetyltransferase activity was decreased and 5HT2 receptor binding was increased both in males and females. Methamidophos elicited behavioral alterations, suggestive of increased depressive-like behavior and impaired decision making. There were no significant alterations on anxiety-related measures and on memory/learning. Methamidophos elicited cholinergic and serotonergic alterations that depended on brain region, sex, and age of the animals. These outcomes, together with the behavioral effects, indicate that this OP is deleterious to the developing brain and that alterations are indeed identified long after the end of exposure.

Branched-chain amino acids alter neurobehavioral function in rats

Recently, we have described a strong association of branched-chain amino acids (BCAA) and aromatic amino acids (AAA) with obesity and insulin resistance. In the current study, we have investigated the potential impact of BCAA on behavioral functions. We demonstrate that supplementation of either a high-sucrose or a high-fat diet with BCAA induces anxiety-like behavior in rats compared with control groups fed on unsupplemented diets. These behavioral changes are associated with a significant decrease in the concentration of tryptophan (Trp) in brain tissues and a consequent decrease in serotonin but no difference in indices of serotonin synaptic function. The anxiety-like behaviors and decreased levels of Trp in the brain of BCAA-fed rats were reversed by supplementation of Trp in the drinking water but not by administration of fluoxetine, a selective serotonin reuptake inhibitor, suggesting that the behavioral changes are independent of the serotonergic pathway of Trp metabolism. Instead, BCAA supplementation lowers the brain levels of another Trp-derived metabolite, kynurenic acid, and these levels are normalized by Trp supplementation. We conclude that supplementation of high-energy diets with BCAA causes neurobehavioral impairment. Since BCAA are elevated spontaneously in human obesity, our studies suggest a potential mechanism for explaining the strong association of obesity and mood disorders.

Alterations in nitric oxide synthase-expressing neurons in the forebrain regions of rats after developmental exposure to organophosphates

Several mechanisms have been addressed as contributors to the long lasting behavioral deficits after developmental exposure to organophosphate (OP) compounds. Here, the effects of developmental exposure to two common OP insecticides, chlorpyrifos (CPF) and diazinon (DZN), on nitric oxide synthase (NOS)-expressing neurons in the rat forebrain are reported. A daily dose of 1mg/kg of either CPF or DZN was administered to rats during gestational days 15-18 or postnatal days (PND) 1-4. We then assessed NADPH-diaphorase and neuronal NOS (nNOS) immunohistochemistry in forebrain sections on different postnatal days. Prenatal exposure to CPF and DZN induced a transient reduction of NADPH-d(+)/nNOS-immunoreactive (IR) neurons in most cortical regions on PND 4 but exceptionally increased them in the entorhinal/piriform cortex. On PND 15, NADPH-d(+)/nNOS-IR neurons showed morphological abnormalities within entorhinal/piriform cortex of the rats that gestationally exposed to CPF. Postnatal exposure to CPF and DZN did not induce widespread effects on the number of NADPH-d(+)/nNOS-IR neurons on PNDs 7 and 15 but significantly reduced them in most cortical regions and hippocampal subfields on PND 60. The OPs affected NADPH-d(+)/nNOS-IR neurons in a sex independent manner and apparently spared them in the striatum. While the NADPH-d reactivity of microvessels was normally diminished by age, OP treated rats evidently preserved the NADPH-d reactivity of microvessels in the cerebral cortex and hippocampus. The effects of OPs on NADPH-d(+)/nNOS-IR neurons may contribute to the long-lasting behavioral outcomes and expand the neurotransmitter system that need to be considered in OP neurotoxicity evaluations.

Pesticide exposure and neurodevelopmental outcomes: review of the epidemiologic and animal studies

Assessment of whether pesticide exposure is associated with neurodevelopmental outcomes in children can best be addressed with a systematic review of both the human and animal peer-reviewed literature. This review analyzed epidemiologic studies testing the hypothesis that exposure to pesticides during pregnancy and/or early childhood is associated with neurodevelopmental outcomes in children. Studies that directly queried pesticide exposure (e.g., via questionnaire or interview) or measured pesticide or metabolite levels in biological specimens from study participants (e.g., blood, urine, etc.) or their immediate environment (e.g., personal air monitoring, home dust samples, etc.) were eligible for inclusion. Consistency, strength of association, and dose response were key elements of the framework utilized for evaluating epidemiologic studies. As a whole, the epidemiologic studies did not strongly implicate any particular pesticide as being causally related to adverse neurodevelopmental outcomes in infants and children. A few associations were unique for a health outcome and specific pesticide, and alternative hypotheses could not be ruled out. Our survey of the in vivo peer-reviewed published mammalian literature focused on effects of the specific active ingredient of pesticides on functional neurodevelopmental endpoints (i.e., behavior, neuropharmacology and neuropathology). In most cases, effects were noted at dose levels within the same order of magnitude or higher compared to the point of departure used for chronic risk assessments in the United States. Thus, although the published animal studies may have characterized potential neurodevelopmental outcomes using endpoints not required by guideline studies, the effects were generally observed at or above effect levels measured in repeated-dose toxicology studies submitted to the U.S. Environmental Protection Agency (EPA). Suggestions for improved exposure assessment in epidemiology studies and more effective and tiered approaches in animal testing are discussed.

Developmental exposure to chlorpyrifos and diazinon differentially affect passive avoidance performance and nitric oxide synthase-containing neurons in the basolateral complex of the amygdala

Chronic exposure to low doses of organophosphates during brain development can induce persistent neurochemical and behavioral effects. This study sought to determine the long-lasting effects of developmental exposure to chlorpyrifos (CPF) and diazinon (DZN) on passive avoidance (PA) performance and neuronal nitric oxide synthase (nNOS)-containing neurons in the subnuclei within basolateral complex of amygdala (BLC). Developing rats were exposed to daily dose (1mg/kg) of CPF or DZN during gestational days 15-18 and postnatal days (PND) 1-4. PA performance was assessed in young adulthood (PND 60). Brain sections were also processed by NADPH-diaphorase (NADPH-d) and nNOS immunohistochemistry. Gestational exposure to CPF increased NADPH-d(+)/nNOS-immunoreactive (IR) neurons within the basolateral nucleus (BL) and medial paracapsular intercalated cluster, which was along with PA retention impairment in both male and female rats. Prenatal exposure to DZN did not significantly change the number of NADPH-d(+)/nNOS-IR neurons in the BLC while impaired PA retention in females. Postnatal exposure to CPF decreased NADPH-d(+)/NOS-IR neurons in the BL without affecting PA performance. Exposure to DZN during early postnatal period impaired PA retention in both sexes, albeit to a lesser extent in females, and was along with a considerable sex independent reduction of NADPH-d(+)/NOS-IR neurons in all BLC subnuclei. Our data suggest that developmental exposure to apparently subtoxic dose of CPF and DZN elicit long-lasting impairment in PA retention that are associated, but not necessarily correlated with effects on NADPH-d(+)/NOS-IR neurons in BLC of the amygdala.

Effects of monocrotophos pesticide on serotonin metabolism during early development in the sea urchin, Hemicentrotus pulcherrimus

Organophosphate pesticides can interfere with the serotonergic nervous system and potentially lead to malformations and behavioral abnormalities during early development in sea urchin. To investigate the mechanism by which monocrotophos (MCP) pesticide disrupts the serotonergic nervous system, we evaluated its effects on serotonin metabolism. Fertilized embryos of sea urchin were incubated with 40% MCP pesticide at nominal concentrations of 0.01, 0.10 and 1.00mg/L, and the effects on tryptophan hydroxylase of Hemicentrotus pulcherrimus (HpTPH), serotonin reuptake transporter (SERT), monoamine oxidase (MAO), and serotonin levels were investigated. The results indicated that MCP pesticide disturbed the baseline pattern of HpTPH and SERT mRNA expression and MAO activity during early development in H. pulcherrimus. When serotonin should be quickly metabolized at 36-hpf stage, HpTPH and SERT expression was decreased and MAO activity was induced by MCP pesticide, leading to the impairment of serotonergic synaptic activity. But when serotonin should be metabolized at low levels during the other six stages, MCP pesticide induced HpTPH and SERT expression, resulting in the improvement of serotonergic synaptic activity. We concluded that this metabolic disturbance is one of the major mechanisms by which MCP pesticides affect the serotonergic nervous system and potentially contribute to various developmental abnormalities.

Veterinary Pesticides

Veterinary pesticides are used to treat a range of parasitic conditions in companion and farm animals. These products are based on a number of different compounds with different modes of action and different spectra of toxicity. The older agents include the synthetic pyrethroids and organophosphorus compounds, while the newer examples include, for example, representatives of the insect growth promoters, the neonicotinoids, and the oxadiazones. For many of these compounds, toxicity is associated with their pharmacological activity or mode of action. Thus the synthetic pyrethroids and the organophosphorus compounds exert neurotoxic effects. For others, toxicity may be associated with mechanisms that are independent of their mode of action. When used according to the manufacturer's instructions, these products are generally safe and efficacious. However, accidental contamination and misuse can lead to toxicity in operators and treated animals. These compounds are important in the treatment of parasitic disease in animals and their regulation and uses are based on favourable risk-benefit outcomes.

Evaluation of developmental toxicity induced by anticholinesterase insecticide, diazinon in female rats

Developmental toxicities, including birth defects, are significant public health problems. This study was planned to assess the cholinergic and developmental potentials of diazinon that is widely used as an organophosphate insecticide. Pregnant female Sprague-Dawley rats were given diazinon orally at doses of 0, 1.9, 3.8, and 7.6 mg/kg body weight (b.w.)/day on gestation days 6 to 15. Maternal brain acetylcholinesterase activities, measured on gestation day20, were significantly decreased at 3.8 and 7.6 mg/kg b.w./day, but fetal acetylcholinesterase activity was not altered. Maternal toxicities, as evidenced by cholinergic symptoms including diarrhea, tremors, weakness, salivation, and decreased activities, were observed at the 3.8 and 7.6 mg/kg b.w./day dose groups. Net gravid uterine weight was decreased at a dose of 7.6 mg/kg b.w./day. No maternal effects were apparent in the 1.9 mg/kg b.w./day dose group. Maternal toxicity at a dose of 3.8 mg/kg b.w./day did not induce fetotoxicity or teratogeneicity. However, 7.6 mg/kg b.w./day doses significantly resulted in fetal toxicity and malformations in addition to maternal toxicity in animals. In conclusion, teratogenic disorders only outlined by doses that produced marked maternal toxicity. Since the malformations were not morphologically related, they were considered to be secondary to maternal toxicity; hence, the malformations were not related to cholinesterase inhibition.

Repeated exposure to chlorpyrifos alters the performance of adolescent male rats in animal models of depression and anxiety

Chlorpyrifos (CPF) is a broad spectrum, highly effective organophosphorus (OP) pesticide that has been largely used worldwide. Over the past decades, numerous studies have assessed the potential neurotoxic effects of either acute or chronic exposure to CPF on developing brain. Despite being an acetylcholinersterase inhibitor, the effects of CPF are not only confined to cholinergic system, but are involved in a wide variety of neurotransmitter systems, especially the serotonin (5-HT) system, which leads to long-lasting changes in 5-HT-related emotional behaviors. In our present study, 4-week-old adolescent male Sprague-Dawley rats were repeatedly exposed to CPF at daily doses of 10, 20, 40, 80, and 160 mg/kg/day (s.c., 7 days), and then subjected to a battery of emotional behavioral tests that related to serotonergic function in order to determine CPF effects in adolescent rats. Results in behavioral tests demonstrated CPF significantly increased the entries to and time spent in the open arms in the elevated plus-maze test at the dose of 40-160 mg/kg, the number of shocks in the Vogel's conflict test at the dose of 20-160 mg/kg, and significantly decreased the latency to feed in the novelty-suppressed feeding test in both dose range. Interestingly, in the forced swimming test, at the dose of 10mg/kg, CPF significantly increased the immobility time, whereas it significantly decreased the immobility time at the dose of 160 mg/kg. Our data suggest that repeated exposure to CPF elicits alterations of the emotional behaviors related to serotonergic nervous system in adolescent male rats. However, the underlying mechanism needs further investigations.

Diverse neurotoxicants converge on gene expression for neuropeptides and their receptors in an in vitro model of neurodifferentiation: effects of chlorpyrifos, diazinon, dieldrin and divalent nickel in PC12 cells

Unrelated developmental neurotoxicants can produce similar neurobehavioral outcomes. We examined whether disparate agents affect neuromodulators that control numerous neurotransmitters and circuits, employing PC12 cells to explore the targeting of neuroactive peptides by organophosphates (chlorpyrifos, diazinon), an organochlorine (dieldrin) and a metal (Ni(2+)); we utilized microarrays to profile gene expression for the peptides and their receptors. Chlorpyrifos evoked robust upregulation of cholecystokinin, corticotropin releasing hormone, galanin, neuropeptide Y, neurotensin, preproenkephalin and tachykinin 1; this involved a critical period at the commencement of neurodifferentiation, since the effects were much less notable in undifferentiated PC12 cells. Diazinon targeted a similar but smaller repertoire of neuropeptide genes and the magnitude of the effects was also generally less. Surprisingly, dieldrin shared many of the same neuropeptide targets as the organophosphates and concordance analysis showed significant overlap among all three pesticides. However, dieldrin had more notable effects on neuropeptide receptors, and overlap between diazinon and dieldrin for the receptors led to a stronger resemblance of these two agents than of chlorpyrifos and dieldrin. Ni(2+) was unique, evoking upregulation of only one of the peptides affected by the other agents, while causing downregulation of several others. Nevertheless, there was still significant concordance between Ni(2+) and either diazinon or dieldrin, reflecting similarities toward the receptors. Our results show that neuropeptides are likely to be a prominent target for the developmental neurotoxicity of organophosphates and other neurotoxicants, and further, that the convergence of disparate agents on the same genes and pathways may contribute to similar neurobehavioral outcomes.

The neurotoxic effects of monocrotophos on the formation of the serotonergic nervous system and swimming activity in the larvae of the sea urchin Hemicentrotus pulcherrimus

The neurotoxicity of monocrotophos (MCP) in the development of the serotonergic nervous system and swimming activity of larvae of the sea urchin, Hemicentrotus pulcherrimus, was examined. Lethal dose 50% of MCP was 43μg/ml. Overall morphology was not affected in larvae that received up to 30μg/ml of MCP soon after fertilization until the 53h post-fertilization pluteus stage. However, while 70±0.6% of larvae in 5μg/ml MCP swam actively, the proportion decreased to 30±1.7% in 30μg/ml MCP. Accordingly, immunoblotting indicated that MCP decreased the relative intensity of immunoreaction of serotonin receptor protein. Whole-mount immunohistochemistry indicated that MCP inhibited serotonergic axon growth, reduced the number of serotonergic cells at the apical ganglion, and perturbed formation of the serotonin receptor cell network. The present study demonstrated that sea urchin larva is a useful model for evaluating the working mechanism of environmental toxicants in neurogenesis and behavior.

Transcriptional profiles for glutamate transporters reveal differences between organophosphates but similarities with unrelated neurotoxicants

The developmental neurotoxicity of organophosphates involves mechanisms other than their shared property as cholinesterase inhibitors, among which are excitotoxicity and oxidative stress. We used PC12 cells as a neurodevelopmental model to compare the effects of chlorpyrifos and diazinon on the expression of genes encoding glutamate transporters. Chlorpyrifos had a greater effect in cells undergoing nerve growth factor-induced neurodifferentiation as compared to undifferentiated PC12 cells, with peak sensitivity at the initiation of differentiation, reflecting a global upregulation of all the glutamate transporter genes expressed in this cell line. In differentiating cells, chlorpyrifos had a significantly greater effect than did diazinon and concordance analysis indicated no resemblance in their expression patterns. At the same time, the smaller effects of diazinon were highly concordant with those of an organochlorine pesticide (dieldrin) and a metal (divalent nickel). We also performed similar evaluations for the cystine/glutamate exchanger, which provides protection against oxidative stress by moving cystine into the cell; again, chlorpyrifos had the greatest effect, in this case reducing expression in undifferentiated and differentiating cells. Our results point to excitotoxicity and oxidative stress as major contributors to the noncholinesterase mechanisms that distinguish the neurodevelopmental outcomes between different organophosphates while providing a means whereby apparently unrelated neurotoxicants may produce similar outcomes.

Mass spectrometric analyses of organophosphate insecticide oxon protein adducts

OBJECTIVE

Organophosphate (OP) insecticides continue to be used to control insect pests. Acute and chronic exposures to OP insecticides have been documented to cause adverse health effects, but few OP-adducted proteins have been correlated with these illnesses at the molecular level. Our aim was to review the literature covering the current state of the art in mass spectrometry (MS) used to identify OP protein biomarkers.

DATA SOURCES AND EXTRACTION

We identified general and specific research reports related to OP insecticides, OP toxicity, OP structure, and protein MS by searching PubMed and Chemical Abstracts for articles published before December 2008.

DATA SYNTHESIS

A number of OP-based insecticides share common structural elements that result in predictable OP-protein adducts. The resultant OP-protein adducts show an increase in molecular mass that can be identified by MS and correlated with the OP agent. Customized OP-containing probes have also been used to tag and identify protein targets that can be identified by MS.

CONCLUSIONS

MS is a useful and emerging tool for the identification of proteins that are modified by activated organophosphate insecticides. MS can characterize the structure of the OP adduct and also the specific amino acid residue that forms the key bond with the OP. Each protein that is modified in a unique way by an OP represents a unique molecular biomarker that with further research can lead to new correlations with exposure.

Disparate developmental neurotoxicants converge on the cyclic AMP signaling cascade, revealed by transcriptional profiles in vitro and in vivo

Cell-signaling cascades are convergent targets for developmental neurotoxicity of otherwise unrelated agents. We compared organophosphates (chlorpyrifos, diazinon), an organochlorine (dieldrin) and a metal (Ni(2+)) for their effects on neuronotypic PC12 cells, assessing gene transcription involved in the cyclic AMP pathway. Each agent was introduced during neurodifferentiation at a concentration of 30 microM for 24 or 72 h and we assessed 69 genes encoding adenylyl cyclase isoforms and regulators, G-protein alpha-and beta,gamma-subunits, protein kinase A subtypes and the phosphodiesterase family. We found strong concordance among the four agents across all the gene families, with the strongest relationships for the G-proteins, followed by adenylyl cyclase, and lesser concordance for protein kinase A and phosphodiesterase. Superimposed on this pattern, chlorpyrifos and diazinon were surprisingly the least alike, whereas there was strong concordance of dieldrin and Ni(2+) with each other and with each individual organophosphate. Further, the effects of chlorpyrifos differed substantially depending on whether cells were undifferentiated or differentiating. To resolve the disparities between chlorpyrifos and diazinon, we performed analyses in rat brain regions after in vivo neonatal exposures; unlike the in vitro results, there was strong concordance. Our results show that unrelated developmental neurotoxicants can nevertheless produce similar outcomes by targeting cell signaling pathways involved in neurodifferentiation during a critical developmental period of vulnerability. Nevertheless, a full evaluation of concordance between different toxicants requires evaluations of in vitro systems that detect direct effects, as well as in vivo systems that allow for more complex interactions that converge on the same pathway.

Early postnatal parathion exposure in rats causes sex-selective cognitive impairment and neurotransmitter defects which emerge in aging

Developmental exposure of rats to the organophosphate (OP) pesticides leads to altered neurobehavioral function in juvenile and young adult stages. The current study was conducted to determine whether effects of neonatal parathion exposure on cognitive performance persist in older adult and aged rats, and the relationship of behavioral changes to underlying cholinergic and serotonergic mechanisms. We administered parathion to rat pups on postnatal days 1-4, at doses spanning the threshold for the initial signs of systemic toxicity and for barely detectable cholinesterase inhibition (0.1 or 0.2 mg/kg/day). Beginning at 14 months of age and continuing until 19 months, the rats were trained in the 16-arm radial maze. Controls showed the normal sex difference in this spatial learning and memory task, with the males committing significantly fewer working memory errors than females. Neonatal parathion exposure eliminated the sex difference primarily by causing impairment in males. In association with the effects on cognitive performance, neonatal parathion exposure elicited widespread abnormalities in indices of serotonergic (5HT) and cholinergic synaptic function, characterized by upregulation of 5HT(2) receptors and the 5HT transporter, deficits in choline acetyltransferase activity and nicotinic cholinergic receptors, and increases in hemicholinium-3 binding to the presynaptic choline transporter. Within-animal correlations between behavior and neurochemistry indicated a specific correlation between working memory performance and hippocampal hemicholinium-3 binding; parathion exposure eliminated this relationship. Like the behavioral effects, males showed greater effects of parathion on neurochemical parameters. This study demonstrates the sex-selective, long-term behavioral alterations caused by otherwise nontoxic neonatal exposure to parathion, with effects increasingly expressed with aging.

Oxidative stress from diverse developmental neurotoxicants: antioxidants protect against lipid peroxidation without preventing cell loss

Oxidative stress has been hypothesized to provide a mechanism by which apparently unrelated chemicals can nevertheless produce similar developmental neurotoxic outcomes. We used differentiating PC12 cells to compare the effects of agents from four different classes and then to evaluate antioxidant amelioration: fipronil, perfluorooctanesulfonamide (PFOSA), dieldrin and chlorpyrifos. The rank order for lipid peroxidation corresponded to the ability to evoke cell loss: fipronil>PFOSA>dieldrin>chlorpyrifos. The same sequence was found for an index of cell enlargement (protein/DNA ratio) but the effects on neurite outgrowth (membrane/total protein) diverged, with fipronil producing a decrease and PFOSA an increase. Cotreatment with antioxidants reduced (ascorbate) or eliminated (Vitamin E) lipid peroxidation caused by each of the agents but failed to protect against cell loss, with the sole exception of chlorpyrifos, for which we earlier showed partial protection by Vitamin E; addition of higher NGF concentrations protected neither against oxidative stress nor cell loss. Despite the failure to prevent cell loss, ascorbate protected the cells from the effects of PFOSA on neuritic outgrowth; NGF, and to a lesser extent, ascorbate, offset the effects of fipronil on both cell enlargement and neuritogenesis. At the same time, the ameliorant treatments also worsened some of the other toxicant effects. Our results point out the problems in concluding that, just because a neurotoxicant produces oxidative stress, antioxidant therapy will be effective in preventing damage. Instead, additional mechanisms for each agent may provide alternative routes to neurotoxicity, or may be additive or synergistic with oxidative stress.

Neonatal parathion exposure disrupts serotonin and dopamine synaptic function in rat brain regions: modulation by a high-fat diet in adulthood

The consequences of exposure to developmental neurotoxicants are influenced by environmental factors. In the present study, we examined the role of dietary fat intake. We administered parathion to neonatal rats and then evaluated whether a high-fat diet begun in adulthood could modulate the persistent effects on 5HT and DA systems. Neonatal rats received parathion on postnatal days 1-4 at 0.1 or 0.2 mg/kg/day, straddling the cholinesterase inhibition threshold. In adulthood, half the animals in each exposure group were given a high-fat diet for 8 weeks. We assessed 5HT and DA concentrations and turnover in brain regions containing their respective cell bodies and projections. In addition, we monitored 5HT1A and 5HT2 receptor binding and the concentration of 5HT presynaptic transporters. Neonatal parathion exposure evoked widespread increases in neurotransmitter turnover, indicative of presynaptic hyperactivity, further augmented by 5HT receptor upregulation. In control rats, consumption of a high-fat diet recapitulated many of the changes seen with neonatal parathion exposure; the effects represented convergent mechanisms, since the high-fat diet often obtunded further increases caused by parathion. Neonatal parathion exposure causes lasting hyperactivity of 5HT and DA systems accompanied by 5HT receptor upregulation, consistent with "miswiring" of neuronal projections. A high-fat diet obtunds the effect of parathion, in part by eliciting similar changes itself. Thus, dietary factors may produce similar synaptic changes as do developmental neurotoxicants, potentially contributing to the increasing incidence of neurodevelopmental disorders.