Alpha7 nicotinic acetylcholine receptors targeted by cholinergic developmental neurotoxicants: nicotine and chlorpyrifos

Molecular docking analysis of chlorpyrifos at the human α7-nAChR and its potential relationship with neurocytoxicity in SH-SY5Y cells

The organophosphate insecticide chlorpyrifos (CPF), an acetylcholinesterase inhibitor, has raised serious concerns about human safety. Apart from inducing synaptic acetylcholine accumulation, CPF could also act at nicotinic acetylcholine receptors, like the α7-isoform (α7-nAChR), which could potentially be harmful to developing brains. Our aims were to use molecular docking to assess the binding interactions between CPF and α7-nAChR through, to test the neurocytotoxic and oxidative effects of very low concentrations of CPF on SH-SY5Y cells, and to hypothesize about the potential mediation of α7-nAChR. Docking analysis showed a significant binding affinity of CPH for the E fragment of the α7-nAChR (ΔGibbs: -5.63 to -6.85 Kcal/mol). According to the MTT- and Trypan Blue-based viability assays, commercial CPF showed concentration- and time-dependent neurotoxic effects at a concentration range (2.5-20 µM), ten-folds lower than those reported to have crucial effects for sheer CPF. A rise of the production of radical oxygen species (ROS) was seen at even lower concentrations (1-2.5 µM) of CPF after 24h. Notably, our docking analysis supports the antagonistic actions of CPF on α7-nAChR that were recently published. In conclusion, while α7-nAChR is responsible for neuronal survival and neurodevelopmental processes, its activity may also mediate the neurotoxicity of CPF.

Prenatal Exposure to Tobacco and Childhood Cognition and Behavior: Effect Modification by Maternal Folate Intake and Breastfeeding Duration

In this exploratory analysis, we assessed whether nutrition modified the association between prenatal exposure to tobacco and childhood cognition/behavior among 366 Colorado-based mothers and their offspring (born ≥ 37 weeks with birthweights ≥ 2500 g). Interaction by folate ( 5 months, but not for shorter durations. Our findings support the need for smoking cessation campaigns throughout pregnancy and throughout the postpartum period breastfeeding to reduce neurobehavioral risks in the offspring.

Prenatal second-hand smoke exposure and the risk of suspected developmental coordination disorder in preschoolers: A nationwide retrospective cohort study in China

Prenatal exposure to second-hand smoke (SHS) is associated with increased neurodevelopmental problems in children, however, its impact on the risk of developmental coordination disorder (DCD) in preschoolers have not been studied thoroughly. Herein, we probed this association based on a nationwide retrospective cohort study of 149,005 preschoolers in China. We divided the objects into the prenatal SHS-exposed group or the no prenatal smoke exposed group (NS-exposed group). Preschoolers were assessed for motor proficiency by the Chinese version of Little Developmental Coordination Disorder Questionnaire (LDCDQ). Multivariable logistic regression was used to evaluate the associations. The prevalence of prenatal SHS exposure was 23.89%. Generally, the prevalence of suspected DCD was significantly higher in prenatal SHS-exposed group (16.38% VS. 14.19%, P < 0.001). With the increase of age, the mean total scores of LDCDQ of both boys and girls increased gradually; and the prevalence of suspected DCD in girls was higher than that in boys in the same age group. After adjusting for covariates, prenatal SHS exposure had the negative association with the total score of LDCDQ and increased the risk of suspected DCD. Our results suggest a need for interventions designed to reduce maternal SHS exposure during pregnancy, early screen for DCD and increase targeted movement and coordination skill training for vulnerable children.

Nicotinic Receptors in the Brainstem Ascending Arousal System in SIDS With Analysis of Pre-natal Exposures to Maternal Smoking and Alcohol in High-Risk Populations of the Safe Passage Study

Pre-natal exposures to nicotine and alcohol are known risk factors for sudden infant death syndrome (SIDS), the leading cause of post-neonatal infant mortality. Here, we present data on nicotinic receptor binding, as determined by 125I-epibatidine receptor autoradiography, in the brainstems of infants dying of SIDS and of other known causes of death collected from the Safe Passage Study, a prospective, multicenter study with clinical sites in Cape Town, South Africa and 5 United States sites, including 2 American Indian Reservations. We examined 15 pons and medulla regions related to cardiovascular control and arousal in infants dying of SIDS (n = 12) and infants dying from known causes (n = 20, 10 pre-discharge from time of birth, 10 post-discharge). Overall, there was a developmental decrease in 125I-epibatidine binding with increasing postconceptional age in 5 medullary sites [raphe obscurus, gigantocellularis, paragigantocellularis, centralis, and dorsal accessory olive (p = 0.0002-0.03)], three of which are nuclei containing serotonin cells. Comparing SIDS with post-discharge known cause of death (post-KCOD) controls, we found significant decreased binding in SIDS in the nucleus pontis oralis (p = 0.02), a critical component of the cholinergic ascending arousal system of the rostral pons (post-KCOD, 12.1 ± 0.9 fmol/mg and SIDS, 9.1 ± 0.78 fmol/mg). In addition, we found an effect of maternal smoking in SIDS (n = 11) combined with post-KCOD controls (n = 8) on the raphe obscurus (p = 0.01), gigantocellularis (p = 0.02), and the paragigantocellularis (p = 0.002), three medullary sites found in this study to have decreased binding with age and found in previous studies to have abnormal indices of serotonin neurotransmission in SIDS infants. At these sites, 125I-epibatidine binding increased with increasing cigarettes per week. We found no effect of maternal drinking on 125I-epibatidine binding at any site measured. Taken together, these data support changes in nicotinic receptor binding related to development, cause of death, and exposure to maternal cigarette smoking. These data present new evidence in a prospective study supporting the roles of developmental factors, as well as adverse exposure on nicotinic receptors, in serotonergic nuclei of the rostral medulla-a finding that highlights the interwoven and complex relationship between acetylcholine (via nicotinic receptors) and serotonergic neurotransmission in the medulla.

Loss of α7 nicotinic acetylcholine receptors in GABAergic neurons causes sex-dependent decreases in radial glia-like cell quantity and impairments in cognitive and social behavior

The dentate gyrus (DG) is a unique brain structure in that neurons can be generated postnatally and integrated within existing circuitry throughout life. The maturation process of these newly generated neurons (granule cells) is modulated by nicotinic acetylcholine receptors (nAChRs) through a variety of mechanisms such as neural stem pool proliferation, cell survival, signal modulation, and dendritic integration. Disrupted nAChR signaling has been implicated in neuropsychiatric and neurodegenerative disorders, potentially via alterations in DG neurogenesis. GABAergic interneurons are known to express nAChRs, predominantly the α7 subtype, and have been shown to shape development, integration, and circuit reorganization of DG granule cells. Therefore, we examined histological and behavioral effects of knocking out α7 nAChRs in GABAergic neurons. Deletion of α7 nAChRs resulted in a reduction of radial glia-like cells within the subgranular zone of the DG and a concomitant trend towards decreased immature neurons, specifically in male mice, as well as sex-dependent changes in several behaviors, including social recognition and spatial learning. Overall, these findings suggest α7 nAChRs expressed in GABAergic neurons play an important role in regulating the adult neural stem cell pool and behavior in a sex-dependent manner. This provides important insight into the mechanisms by which cholinergic dysfunction contributes to the cognitive and behavioral changes associated with neurodevelopmental and neurodegenerative disorders.

Prenatal Exposure to Tobacco and Offspring Neurocognitive Development in the Healthy Start Study

OBJECTIVE

To explore the associations between prenatal exposure to tobacco and neurocognitive development, in the absence of prematurity or low birth weight.

STUDY DESIGN

We followed mother-child pairs within Healthy Start through 6 years of age. Children were born at ≥37 weeks of gestation with a birth weight of ≥2500 g. Parents completed the Third Edition Ages and Stages Questionnaire (n = 246) and children completed a subset of the National Institutes of Health Toolbox Cognition Battery (n = 200). The Ages and Stages Questionnaire domains were dichotomized as fail/monitor and pass. Maternal urinary cotinine was measured at approximately 27 weeks of gestation. Separate logistic regression models estimated associations between prenatal exposure to tobacco (cotinine below vs above the limit of detection) and the Ages and Stages Questionnaire domains. Separate linear regression models estimated associations between prenatal exposure to tobacco and fully corrected T-scores for inhibitory control, cognitive flexibility, and receptive language, as assessed by the National Institutes of Health Toolbox. A priori covariates included sex, maternal age, maternal education, daily caloric intake during pregnancy, race/ethnicity, household income, maternal psychiatric disorders, and, in secondary models, postnatal exposure to tobacco.

RESULTS

Compared with unexposed offspring, exposed offspring were more likely to receive a fail/monitor score for fine motor skills (OR, 3.9; 95% CI, 1.5-10.3) and decreased inhibitory control (B: -3.0; 95% CI, -6.1 to -0.7). After adjusting for postnatal exposure, only the association with fine motor skills persisted.

CONCLUSIONS

Prenatal and postnatal exposures to tobacco may influence neurocognitive development, in the absence of preterm delivery or low birth weight. Increased developmental screening may be warranted for exposed children.

Impact of maternal prenatal smoking on fetal to infant neurobehavioral development

Despite recent emphasis on the profound importance of the fetal environment in "programming" postnatal development, measurement of offspring development typically begins after birth. Using a novel coding strategy combining direct fetal observation via ultrasound and actocardiography, we investigated the impact of maternal smoking during pregnancy (MSDP) on fetal neurobehavior; we also investigated links between fetal and infant neurobehavior. Participants were 90 pregnant mothers and their infants (52 MSDP-exposed; 51% minorities; ages 18-40). Fetal neurobehavior at baseline and in response to vibro-acoustic stimulus was assessed via ultrasound and actocardiography at M = 35 weeks gestation and coded via the Fetal Neurobehavioral Assessment System (FENS). After delivery, the NICU Network Neurobehavioral Scale was administered up to seven times over the first postnatal month. MSDP was associated with increased fetal activity and fetal limb movements. Fetal activity, complex body movements, and cardiac-somatic coupling were associated with infants' ability to attend to stimuli and to self-regulate over the first postnatal month. Furthermore, differential associations emerged by MSDP group between fetal activity, complex body movements, quality of movement, and coupling, and infant attention and self-regulation. The present study adds to a growing literature establishing the validity of fetal neurobehavioral measures in elucidating fetal programming pathways.

Cigarette smoke exposure effects on the brainstem expression of nicotinic acetylcholine receptors (nAChRs), and on cardiac, respiratory and sleep physiologies

Cigarette smoking during pregnancy is the largest modifiable risk factor for adverse outcomes in the infant. Investigations have focused on the psychoactive component of cigarettes, nicotine. One proposed mechanism leading to adverse effects is the interaction between nicotine and its nicotinic acetylcholine receptors (nAChRs). Much data has been generated over the past three decades on the effects of cigarette smoke exposure (CSE) on the expression of the nAChRs in the brainstem and physiological parameters related to cardiac, respiration and sleep, in the offspring of smoking mothers and animal models of nicotine exposure. This review summarises this data and discusses the main findings, highlighting that findings in animal models closely correlate with those from human studies, and that the major brainstem sites where the expression level for the nAChRs are consistently affected include those that play vital roles in cardiorespiration (hypoglossal nucleus, dorsal motor nucleus of the vagus, nucleus of the solitary tract), chemosensation (nucleus of the solitary tract, arcuate nucleus) and arousal (rostral mesopontine sites such as the locus coeruleus and nucleus pontis oralis). These findings provide evidence for the adverse effects of CSE during and after pregnancy to the infant and the need to continue with the health campaign advising against CSE.

A Drosophila model for developmental nicotine exposure

Despite the known health risks of tobacco smoking, many people including pregnant women continue smoking. The effects of developmental nicotine exposure are known, but the underlying mechanisms are not well understood. Drosophila melanogaster is a model organism that can be used for uncovering genetic and molecular mechanisms for drugs of abuse. Here I show that Drosophila can be a model to elucidate the mechanisms for nicotine’s effects on a developing organism. Drosophila reared on nicotine food display developmental and behavioral effects similar to those in mammals including decreased survival and weight, increased developmental time, and decreased sensitivity to acute nicotine and ethanol. The Drosophila nicotinic acetylcholine receptor subunit alpha 7 (Dα7) mediates some of these effects. A novel role for Dα7 on ethanol sedation in Drosophila is also shown. Future research taking advantage of the genetic and molecular tools for Drosophila will allow additional discovery of the mechanisms behind the effects of nicotine during development.

Chlorpyrifos Toxicity in Mouse Cultured Cerebellar Granule Neurons at Different Stages of Development: Additive Effect on Glutamate-Induced Excitotoxicity

OBJECTIVE

Chlorpyrifos (CPF) is a neurotoxic organophosphorus (OP) insecticide. Its mechanism of action includes oxidative stress, excitotoxicity, and inhibition of the acetylcholinesterase enzyme (AChE). The aim of the present study is to investigate CPF toxicity in mature and immature cerebellar granule neurons (CGNs), as well as its effect on glutamate induced excitotoxicity.

MATERIALS AND METHODS

This study was an in vitro experimental study performed on mice cultured CGNs. Immature and mature neurons were exposed to different concentrations of CPF (1-1000 µM) and glutamate (10-600 µM) for 48 hours after which we used the MTT assay to measure cytotoxicity. Immature neurons had exposure to CPF for 5 days in order to evaluate the cytotoxic effect on developing neurons. Mature neurons received sub-lethal concentrations of CPF (10, 100 µM) combined with different concentrations of glutamate. AChE activity and reactive oxygen species (ROS) generation were assessed after treatments.

RESULTS

Immature CGNs had increased sensitivity to CPF toxicity compared to mature neurons. We observed significantly greater ROS production in immature compared to mature neurons, however AChE activity was more inhibited in mature neurons. Although CPF toxicity was not well correlated with AChE inhibition, it correlated well with ROS production. Glutamate toxicity was potentiated by sub-lethal concentration of CPF, however glutamate induced ROS production was not affected. The results suggested that CPF potentiated glutamate toxicity by mechanisms other than oxidative stress.

CONCLUSION

CPF toxicity differed in mature and immature neurons. Potentiated glutamate toxicity by CPF implied that CPF exposure might be a risk factor for neurodegenerative disease.

Influence of developmental nicotine exposure on the ventilatory and metabolic response to hyperthermia

To determine whether developmental nicotine exposure (DNE) alters the ventilatory and metabolic response to hyperthermia in neonatal rats (postnatal age 2-4 days), pregnant dams were exposed to nicotine (6 mg kg(-1) of nicotine tartrate daily) or saline with an osmotic mini-pump implanted subdermally on day 5 of gestation. Rat pups (a total of 72 controls and 72 DNE pups) were studied under thermoneutral conditions (chamber temperature 33°C) and during moderate thermal stress (37.5°C). In all pups, core temperature was similar to chamber temperature, with no treatment effects. The rates of pulmonary ventilation (V̇(I)), O2 consumption (V̇(O2)) and CO2 production (V̇(CO2)) did not change with hyperthermia in either control or DNE pups. However, V̇(I) was lower in DNE pups at both chamber temperatures, whereas the duration of spontaneous apnoeas was longer in DNE pups than in controls at 33°C. The V̇(I)/V̇(O2) ratio increased at 37.5°C in control pups, although it did not change in DNE pups. To simulate severe thermal stress, additional pups were studied at 33°C and 43°C. V̇(I) increased with heating in control pups but not in DNE pups. As heat stress continued, gasping was evoked in both groups, with no effect of DNE on the gasping pattern. Over a 20 min recovery period at 33°C, V̇(I) returned to baseline in control pups but remained depressed in DNE pups. In addition to altering baseline V̇(I) and apnoea duration, DNE is associated with subtle but significant alterations in the ventilatory response to hyperthermia in neonatal rats.

The long-term impact of early life poverty on orbitofrontal cortex volume in adulthood: results from a prospective study over 25 years

Converging evidence has highlighted the association between poverty and conduct disorder (CD) without specifying neurobiological pathways. Neuroimaging research has emphasized structural and functional alterations in the orbitofrontal cortex (OFC) as one key mechanism underlying this disorder. The present study aimed to clarify the long-term influence of early poverty on OFC volume and its association with CD symptoms in healthy participants of an epidemiological cohort study followed since birth. At age 25 years, voxel-based morphometry was applied to study brain volume differences. Poverty (0=non-exposed (N=134), 1=exposed (N=33)) and smoking during pregnancy were determined using a standardized parent interview, and information on maternal responsiveness was derived from videotaped mother-infant interactions at the age of 3 months. CD symptoms were assessed by diagnostic interview from 8 to 19 years of age. Information on life stress was acquired at each assessment and childhood maltreatment was measured using retrospective self-report at the age of 23 years. Analyses were adjusted for sex, parental psychopathology and delinquency, obstetric adversity, parental education, and current poverty. Individuals exposed to early life poverty exhibited a lower OFC volume. Moreover, we replicated previous findings of increased CD symptoms as a consequence of childhood poverty. This effect proved statistically mediated by OFC volume and exposure to life stress and smoking during pregnancy, but not by childhood maltreatment and maternal responsiveness. These findings underline the importance of studying the impact of early life adversity on brain alterations and highlight the need for programs to decrease income-related disparities.

Pesticide exposure during pregnancy, like nicotine, affects the brainstem α7 nicotinic acetylcholine receptor expression, increasing the risk of sudden unexplained perinatal death

This study indicates the impact of nicotine and pesticides (organochlorine and organophosphate insecticides used in agriculture) on neuronal α7-nicotinic acetylcholine receptor expression in brainstem regions receiving cholinergic projections in human perinatal life. An in-depth anatomopathological examination of the autonomic nervous system and immunohistochemistry to analyze the α7-nicotinic acetylcholine receptor expression in the brainstem from 44 fetuses and newborns were performed. In addition, the presence of selected agricultural pesticides in cerebral cortex samples of the victims was determined by specific analytical procedures. Hypodevelopment of brainstem structures checking the vital functions, frequently associated with α7-nicotinic acetylcholine receptor immunopositivity and smoke absorption in pregnancy, was observed in high percentages of victims of sudden unexpected perinatal death. In nearly 30% of cases however the mothers never smoked, but lived in rural areas. The search for pesticides highlighted in many of these cases traces of both organochlorine and organophosphate pesticides. We detain that exposition to pesticides in pregnancy produces homologous actions to those of nicotine on neuronal α7-nicotinic acetylcholine receptor, allowing to developmental alterations of brainstem vital centers in victims of sudden unexplained death.

Transient suppression of late-stage neuronal progenitor cell differentiation in the hippocampal dentate gyrus of rat offspring after maternal exposure to nicotine

To examine the developmental exposure effect of nicotine (NIC) on hippocampal neurogenesis, pregnant Sprague-Dawley rats were treated with (-)-NIC hydrogen tartrate salt through drinking water at 2, 10 or 50 ppm from gestational day 6 to day 21 after delivery. On postnatal day (PND) 21, immunohistochemically doublecortin (Dcx)(+) cells increased at ≥10 ppm in the dentate subgranular zone (SGZ) as examined in male offspring; however, dihydropyrimidinase-like 3 (TUC4)(+) cells decreased at 2 ppm, and T box brain 2 (Tbr2)(+) cells were unchanged at any dose. Double immunohistochemistry revealed decreases in TUC4(+)/Dcx(+) and TUC4(+)/Dcx(-) cells, an increase in TUC4(-)/Dcx(+) cells at 2 and 10 ppm and an increase in Tbr2(-)/Dcx(+) cells at 50 ppm, suggesting an increase in type-3 progenitor cells at ≥2 ppm and decrease in immature granule cells at 2 and 10 ppm. The number of mature neuron-specific NeuN(-) progenitor cells expressing nicotinic acetylcholine receptor α7 in the SGZ and mRNA levels of Chrna7 and Chrnb2 in the dentate gyrus was unchanged at any dose, suggesting a lack of direct nicotinic stimulation on progenitor cells. In the dentate hilus, glutamic acid decarboxylase 67(+) interneurons increased at ≥10 ppm. All changes disappeared on PND 77. Therefore, maternal exposure to NIC reversibly affects hippocampal neurogenesis targeting late-stage differentiation in rat offspring. An increase in interneurons suggested that their activation affected granule cell differentiation. The lowest observed adverse effect level was at 2 ppm (0.091 mg/kg/day as a free base) by the affection of hippocampal neurogenesis at ≥2 ppm.

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.

Hyper-phosphorylation of GSK-3β: possible roles in chlorpyrifos-induced behavioral alterations in animal model of depression

In recent years, the widespread use of chlorpyrifos (CPF) has aroused concerns regarding its potential neurotoxic effects, especially in developing individuals. One of the major consequences of CPF exposure is mood disorders such as depression. Epidemiological studies have demonstrated susceptibility to depression in populations with a history of CPF exposure. Our previous study indicated that repeated CPF exposure in doses from 10 to 160 mg/kg elicits depression- and anxiety-like alterations. However, whether this alteration is due to persistent inhibition of acetylcholinesterase (AChE) was not determined. In this study, we used lower doses of CPF to avoid evident inhibition of AChE to investigate other potential target systems that contribute to CPF's neurotoxic effect. Four-week-old adolescent male rats were repeatedly exposed to CPF at doses of 2.5, 5, or 10mg/kg (s.c., 10 days) and then were subjected to either neurobehavioral testing or immunoblot analysis. Depression-like behaviors as manifested by increased immobility time was observed in force swimming test, while immunoblot analysis revealed a dramatically increased phosphorylation of glycogen synthase kinase-3β (GSK-3β) in the hippocampus and striatum, with no effect on the levels of Wnt2, GSK-3β, or β-catenin. These results suggest a noncholinergic mechanism, the hyper-phosphorylation of GSK-3β, which may contribute to the cellular neurotoxicity of CPF, thus increasing the susceptibility to mood disorders.

Evaluation of epidemiology and animal data for risk assessment: chlorpyrifos developmental neurobehavioral outcomes

Developmental neurobehavioral outcomes attributed to exposure to chlorpyrifos (CPF) obtained from epidemiologic and animal studies published before June 2010 were reviewed for risk assessment purposes. For epidemiological studies, this review considered (1) overall strength of study design, (2) specificity of CPF exposure biomarkers, (3) potential for bias, and (4) Hill guidelines for causal inference. In the case of animal studies, this review focused on evaluating the consistency of outcomes for developmental neurobehavioral endpoints from in vivo mammalian studies that exposed dams and/or offspring to CPF prior to weaning. Developmental neuropharmacologic and neuropathologic outcomes were also evaluated. Experimental design and methods were examined as part of the weight of evidence. There was insufficient evidence that human developmental exposures to CPF produce adverse neurobehavioral effects in infants and children across different cohort studies that may be relevant to CPF exposure. In animals, few behavioral parameters were affected following gestational exposures to 1 mg/kg-d but were not consistently reported by different laboratories. For postnatal exposures, behavioral effects found in more than one study at 1 mg/kg-d were decreased errors on a radial arm maze in female rats and increased errors in males dosed subcutaneously from postnatal day (PND) 1 to 4. A similar finding was seen in rats exposed orally from PND 1 to 21 with incremental dose levels of 1, 2, and 4 mg/kg-d, but not in rats dosed with constant dose level of 1 mg/kg-d. Neurodevelopmental behavioral, pharmacological, and morphologic effects occurred at doses that produced significant brain or red blood cell acetylcholinesterase inhibition in dams or offspring.

Chlorpyrifos-oxon disrupts zebrafish axonal growth and motor behavior

Axonal morphology is a critical determinant of neuronal connectivity, and perturbation of the rate or extent of axonal growth during development has been linked to neurobehavioral deficits in animal models and humans. We previously demonstrated that the organophosphorus pesticide (OP) chlorpyrifos (CPF) inhibits axonal growth in cultured neurons. In this study, we used a zebrafish model to determine whether CPF, its oxon metabolite (CPFO), or the excreted metabolite trichloro-2-pyridinol (TCPy) alter spatiotemporal patterns of axonal growth in vivo. Static waterborne exposure to CPFO, but not CPF or TCPy, at concentrations ≥ 0.03 μM from 24- to 72-h post fertilization significantly inhibited acetylcholinesterase, and high-performance liquid chromatography detected significantly more TCPy in zebrafish exposed to 0.1 μM CPFO versus 1.0 μM CPF. These data suggest that zebrafish lack the metabolic enzymes to activate CPF during these early developmental stages. Consistent with this, CPFO, but not CPF, significantly inhibited axonal growth of sensory neurons, primary motoneurons, and secondary motoneurons at concentrations ≥ 0.1 μM. Secondary motoneurons were the most sensitive to axonal growth inhibition by CPFO, which was observed at concentrations that did not cause mortality, gross developmental defects, or aberrant somatic muscle differentiation. CPFO effects on axonal growth correlated with adverse effects on touch-induced swimming behavior, suggesting the functional relevance of these structural changes. These data suggest that altered patterns of neuronal connectivity contribute to the developmental neurotoxicity of CPF and demonstrate the relevance of zebrafish as a model for studying OP developmental neurotoxicity.

Alterations in cholinergic sensitivity of respiratory neurons induced by pre-natal nicotine: a mechanism for respiratory dysfunction in neonatal mice

Nicotine may link cigarette smoking during pregnancy with sudden infant death syndrome (SIDS). Pre-natal nicotine leads to diminished ventilatory responses to hypercarbia and reduced central chemoreception in mice at post-natal days 0-3. We studied how pre-natal nicotine exposure changes the cholinergic contribution to central respiratory chemoreception in neonatal isolated brainstem-spinal cord and slice preparations. Osmotic minipumps, implanted subcutaneously into 5-7 days pregnant mice, delivered saline or nicotine ditartrate 60 mg kg(-1) d(-1) for up to 28 days. In control preparations, acidification of the superfusion medium from pH 7.4 to 7.3 increased the frequency and reduced the amplitude of fictive respiration. In nicotine-exposed neonatal mice, the reduction in amplitude induced by acidification was reduced. In control preparations, atropine suppressed respiratory responses to acidification, while hexamethonium did not. By contrast, in nicotine-exposed preparations, hexamethonium blocked chemosensory responses but atropine did not. Our results indicate that pre-natal nicotine exposure switches cholinergic mechanisms of central chemosensory responses from muscarinic receptors to nicotinic receptors. Modification of the cholinergic contribution to central chemoreception may produce respiratory dysfunctions, as suggested by receptor-binding studies in victims of SIDS.

Respiratory dysfunctions induced by prenatal nicotine exposure

1. Maternal tobacco smoking is the principal risk factor associated with sudden infant death syndrome (SIDS), a leading cause of death of infants under 1 year of age. Victims of SIDS show a higher incidence of respiratory control abnormalities, including central apnoeas, delayed arousal responses and diminished ventilatory chemoreflexes. 2. Nicotine is likely the link between maternal tobacco smoking and SIDS. Prenatal nicotine exposure can alter the breathing pattern and can reduce hypoxia- and hypercarbia-induced ventilatory chemoreflexes. In vitro approaches have revealed that prenatal nicotine exposure impairs central chemosensitivity, switching the cholinergic contribution from a muscarinic to a nicotinic receptor-based drive. In addition, serotonergic, noradrenergic, GABAergic, glycinergic and glutamatergic, among others, are affected by prenatal nicotine. 3. Here we propose that prenatal nicotine affects the respiratory network through two main processes: (i) reorganization of neurotransmitter systems; and (ii) remodelling of neural circuits. These changes make breathing more vulnerable to fail in early postnatal life, which could be related to the pathogenesis of SIDS.

Sex differences in nicotine action

Accumulating evidence suggests that the antecedents, consequences, and mechanisms of drug abuse and dependence are not identical in males and females and that gender may be an important variable in treatment and prevention. Although there has been a decline in smoking prevalence in developed countries, females are less successful in quitting. Tobacco use is accepted to be a form of addiction, which manifests sex differences. There is also evidence for sex differences in the central effects of nicotine in laboratory animals. Although social factors impact smoking substantially in humans, findings from nonhuman subjects in controlled experiments provide support that sex differences in nicotine/tobacco addiction have a biological basis. Differences in the pharmacokinetic properties of nicotine or the effect of gonadal hormones may underlie some but not all sex differences observed. Laboratory-based information is very important in developing treatment strategies. Literature findings suggest that including sex as a factor in nicotine/tobacco-related studies will improve our success rates in individually tailored smoking cessation programs.

Prenatal to early postnatal nicotine exposure impairs central chemoreception and modifies breathing pattern in mouse neonates: a probable link to sudden infant death syndrome

Nicotine is a neuroteratogen and is the likely link between maternal cigarette smoking during pregnancy and sudden infant death syndrome (SIDS). Osmotic minipumps were implanted in 5-7 d CF1 pregnant mice to deliver nicotine bitartrate (60 mg Kg(-1) day(-1)) or saline (control) solutions for up to 28 d. Prenatal to early postnatal nicotine exposure did not modify the number of newborns per litter or their postnatal growth; however, nicotine-exposed neonates hypoventilated and had reduced responses to hypercarbia (inhalation of air enriched with 10% CO(2) for 20 min) and hypoxia (inhalation of 100% N(2) for 20 s) at postnatal days 0-3 (P0-P3). In contrast, at postnatal day 8, nicotine-exposed neonates were indistinguishable from controls. Isolated brainstem-spinal cord preparations obtained from P0 to P3 nicotine-exposed neonates showed fictive respiration with respiratory cycles longer and more irregular than those of controls, as indicated by high short- and long-term variability in Poincaré plots. In addition, their responses to acidification were reduced, indicating compromise of central chemoreception. Furthermore, the cholinergic contribution to central chemosensory responses switched from muscarinic receptor to nicotinic receptor-based mechanisms. No significant astrogliosis was detectable in the ventral respiratory group of neurons with glial fibrillary acidic protein immunohistochemistry. These results indicate that nicotine exposure affects the respiratory rhythm pattern generator and causes a decline in central chemoreception during early postnatal life. Consequently, breathing would become highly vulnerable, failing to respond to chemosensory demands. Such impairment could be related to the ventilatory abnormalities observed in SIDS.

The effect of maternal smoking and drinking during pregnancy upon (3)H-nicotine receptor brainstem binding in infants dying of the sudden infant death syndrome: initial observations in a high risk population

The high rate of the sudden infant death syndrome (SIDS) in American Indians in the Northern Plains (3.5/1000) may reflect the high incidence of cigarette smoking and alcohol consumption during pregnancy. Nicotine, a neurotoxic component of cigarettes, and alcohol adversely affect nicotinic receptor binding and subsequent cholinergic development in animals. We measured (3)H-nicotine receptor binding in 16 brainstem nuclei in American Indian SIDS (n = 27) and controls (n = 6). In five nuclei related to cardiorespiratory control, (3)H-nicotinic binding decreased with increasing number of drinks (P < 0.03). There were no differences in binding in SIDS compared with controls, except upon stratification of prenatal exposures. In three mesopontine nuclei critical for arousal there were reductions (P < 0.04) in binding in controls exposed to cigarette smoke compared with controls without exposure; there was no difference between SIDS cases with or without exposure. This study suggests that maternal smoking and alcohol affects (3)H-nicotinic binding in the infant brainstem irrespective of the cause of death. It also suggests that SIDS cases are unable to respond to maternal smoking with the "normal" reduction seen in controls. Future studies are needed to establish the role of adverse prenatal exposures in altered brainstem neurochemistry in SIDS.

Comparative developmental neurotoxicity of organophosphates in vivo: transcriptional responses of pathways for brain cell development, cell signaling, cytotoxicity and neurotransmitter systems

Organophosphates affect mammalian brain development through a variety of mechanisms beyond their shared property of cholinesterase inhibition. We used microarrays to characterize similarities and differences in transcriptional responses to chlorpyrifos and diazinon, assessing defined gene groupings for the pathways known to be associated with the mechanisms and/or outcomes of chlorpyrifos-induced developmental neurotoxicity. We exposed neonatal rats to daily doses of chlorpyrifos (1mg/kg) or diazinon (1 or 2mg/kg) on postnatal days 1-4 and evaluated gene expression profiles in brainstem and forebrain on day 5; these doses produce little or no cholinesterase inhibition. We evaluated pathways for general neural cell development, cell signaling, cytotoxicity and neurotransmitter systems, and identified significant differences for >60% of 252 genes. Chlorpyrifos elicited major transcriptional changes in genes involved in neural cell growth, development of glia and myelin, transcriptional factors involved in neural cell differentiation, cAMP-related cell signaling, apoptosis, oxidative stress, excitotoxicity, and development of neurotransmitter synthesis, storage and receptors for acetylcholine, serotonin, norepinephrine and dopamine. Diazinon had similar effects on many of the same processes but also showed major differences from chlorpyrifos. Our results buttress the idea that different organophosphates target multiple pathways involved in neural cell development but also that they deviate in key aspects that may contribute to disparate neurodevelopmental outcomes. Equally important, these pathways are compromised at exposures that are unrelated to biologically significant cholinesterase inhibition and its associated signs of systemic toxicity. The approach used here demonstrates how planned comparisons with microarrays can be used to screen for developmental neurotoxicity.

Critical periods for the role of oxidative stress in the developmental neurotoxicity of chlorpyrifos and terbutaline, alone or in combination

The developmental neurotoxicity of chlorpyrifos (CPF) involves mechanisms other than inhibition of cholinesterase. In the current study, we examined the ability of CPF to evoke lipid peroxidation in the developing brain of fetal and neonatal rats. CPF given to pregnant rats on gestational days 17-20 or to neonatal rats on postnatal days 1-4, failed to elicit increases in thiobarbituric acid-reactive species (TBARS) in brain regions even when the dose was raised above the threshold for systemic toxicity and hepatic damage. In contrast, CPF administration during the second postnatal week, the peak period of neuronal cell differentiation and synaptogenesis, did evoke significant increases in TBARS even at a dose devoid of systemic toxicity. Terbutaline, which is chemically unrelated to CPF and which stimulates neuronal cell metabolism through direct actions on beta-adrenoceptors, also elicited oxidative damage in the developing brain with greater sensitivity in the second postnatal week. These results indicate that diverse compounds can exert convergent effects on brain development through their shared potential to elicit oxidative stress, and that the net outcome is dependent upon specific developmental stages in which metabolic demand is especially high. Furthermore, given the common use of terbutaline in the therapy of preterm labor, and the nearly ubiquitous exposure of the human population to organophosphorus pesticides, the combined oxidative burden of exposure to both agents may contribute to the worsened neurodevelopmental outcomes noted in animal models of such dual exposures.