Neonatal exposure to ultrafine iron but not combined iron and sulfur aerosols recapitulates air pollution-induced impulsivity in mice.

Air pollution (AP) is becoming recognized as a major threat to neurological health across the lifespan with increased risk of both neurodevelopmental and neurodegenerative disorders. AP is a complex mixture of gases and particulate matter, with adsorbed contaminants including metals and trace elements, which may differentially contribute to its neurodevelopmental impacts. Iron (Fe) is one of the most abundant metals found in AP, and Fe concentrations may drive some behavioral deficits observed in children. Furthermore, brains of neonate mice exposed to concentrated ambient ultrafine particulate matter (UFP) show significant brain accumulation of Fe and sulfur (S) supporting the hypothesis that AP exposure may lead to brain metal dyshomeostasis. The current study determined the extent to which behavioral effects of UFP, namely memory deficits and impulsive-like behavior, could be recapitulated with exposure to Fe aerosols with or without concomitant SO2. Male and female neonate mice were either exposed to filtered air or spark discharge-generated ultrafine Fe particles with or without SO2 gas (n = 12/exposure/sex). Inhalation exposures occurred from postnatal day (PND) 4-7 and 10-13 for 4 hr/day, mirroring our previous UFP exposures. Mice were aged to adulthood prior to behavioral testing. While Fe or Fe + SO2 exposure did not affect gross locomotor behavior, Fe + SO2-exposed females displayed consistent thigmotaxis during locomotor testing. Neither exposure affected novel object memory. Fe or Fe + SO2 exposure produced differential outcomes on a fixed-interval reinforcement schedule with males showing higher (Fe-only) or lower (Fe + SO2) response rates and postreinforcement pauses (PRP) and females showing higher (Fe-only) PRP. Lastly, Fe-exposed, but not Fe + SO2-exposed, males showed increased impulsive-like behavior in tasks requiring response inhibition with no such effects in female mice. These findings suggest that: 1) exposure to realistic concentrations of Fe aerosols can recapitulate behavioral effects of UFP exposure, 2) the presence of SO2 can modulate behavioral effects of Fe inhalation, and 3) brain metal dyshomeostasis may be an important factor in AP neurotoxicity.

A behavioral timing intervention upregulates striatal serotonergic markers and reduces impulsive action in adult male mice.

Many studies support the hypothesis that time-based interventions reduce impulsive behavior in rodents. However, few studies have directly assessed 1) how such interventions affect impulsive action rather than impulsive choice, 2) if intervention effects differ by sex, and 3) how time-based interventions affect neurochemistry in regions mediating decision-making and reward. Thus, we assessed how a fixed-interval (FI) intervention initiated during late adolescence and extending into adulthood affected dopaminergic and serotonergic analytes in the frontal cortex and striatum and subsequent impulsive action in adult male and female mice. Beginning on postnatal day (PND) 45, mice were either trained on a progressive series of FI schedules (FI 20, 40, & 60 s) or remained in the home cage. Following the intervention, increases in striatal serotonergic analytes were found in FI-exposed males and females (n = 8/sex/group) with few changes found in the frontal cortex. Impulsive action was assessed in the remaining mice (n = 10/sex/group) using a fixed-ratio waiting-for-reward (FR-wait) task in which completion of an FR-25 component initiated a "free" pellet component in which pellets were delivered at increasing intervals according to a fixed delay increment that varied across sessions. Responses reset the additive delay and initiated a new FR-25 component. FI-exposed males, but not females, showed fewer delay resets and no-wait resets relative to control mice. Importantly, FI-exposure did not affect discrimination reversal performance in either sex. These data suggest that time-based interventions may reduce impulsive action in addition to impulsive choice perhaps with increased male sensitivity. Additionally, time-based interventions appear to operate through striatal serotonergic augmentation.

Developmental exposure to low level ambient ultrafine particle air pollution and cognitive dysfunction.

Developmental exposures to ambient ultrafine particles (UFPs) can produce multiple neuropathological and neurochemical changes that might contribute to persistent alterations in cognitive-type functions. The objective of the current study was to test the hypothesis that developmental UFP exposure produced impairments in learning, memory and impulsive-like behaviors and to determine whether these were selective and thus independent of deficits in other behavioral domains such as motor activity or motivation. Performance on measures of learning (repeated learning), memory (novel object recognition, NOR), impulsive-like behavior (differential reinforcement of low rate (DRL), schedule of reward and delay of reward (DOR)), motor activity (locomotor behavior) and motivation (progressive ratio schedule) were examined in adult mice that had been exposed to concentrated (10-20x) ambient ultrafine particles (CAPS) averaging approximately 45 ug/m3 particle mass concentrations from postnatal day (PND) 4-7 and 10-13 for 4 h/day. Given the number of behavioral tests, animals were tested in different groups. Results showed male-specific alterations in learning and memory functions (repeated learning, NOR and DRL) specifically during transitions in reinforcement contingencies (changes in rules governing behavior) that did not appear to be related to alterations in locomotor function or motivation. Females did not exhibit cognitive-like deficits at these exposure concentrations, but displayed behaviors consistent with altered motivation, including increases in response rates during repeated learning, significantly increased latencies to respond on the delay of reward paradigm, and reductions in the progressive ratio break point. Consistent with our prior findings, male-specific learning and memory-related deficits were seen and occurred even at relatively low level developmental UFP exposures, while females show alterations in motivational behaviors but not final performance. These findings add to the evidence suggesting the need to regulate UFP levels.

Sex- and brain region- specific effects of prenatal stress and lead exposure on permissive and repressive post-translational histone modifications from embryonic development through adulthood.

Developmental exposure to prenatal stress (PS) and lead (Pb) can affect brain development and adversely influence behavior and cognition. Epigenetic-based gene regulation is crucial for normal brain development and mis-regulation, in any form, can result in neurodevelopmental disorders. Post-translational histone modifications (PTHMs) are an integral and dynamic component of the epigenetic machinery involved in gene regulation. Exposures to Pb and/or PS may alter PTHM profiles, promoting lifelong alterations in brain function observed following Pb±PS exposure. Here we examined the effects of Pb±PS on global levels of activating marks H3K9Ac and H3K4Me3 and repressive marks H3K9Me2 and H3K27Me3 at different developmental stages: E18, PND0, PND6 and PND60. Dams were exposed to 0 or 100ppm Pb beginning 2 months prior to breeding followed by no PS (NS) or PS resulting in 4 offspring treatment groups per sex: 0-NS (control), 0-PS, 100-NS and 100-PS. Global levels of PTHMs varied from E18 through adulthood even in control mice, and were influenced by sex and brain-region. The developmental trajectory of these PTHM levels was further modified by Pb±PS in a sex-, brain region- and age-dependent manner. Females showed a preferential response to Pb alone in frontal cortex (FC) and differentially to PS alone and combined Pb+PS in hippocampus (HIPP). In males, PS-induced increases in PTHM levels in FC, whereas PS produced reductions in HIPP. Pb±PS-based changes in PTHM levels continued to be observed in adulthood (PND60), demonstrating the lasting effect of these early life environmental events on these histone marks. These results indicate that epigenetic consequences of Pb±PS and their contribution to mechanisms of toxicity are sex dependent. Additional studies will assist in understanding the functional significance of these changes in PTHM levels on expression of individual genes, functional pathways, and ultimately, their behavioral consequences.

Cognitive Effects of Air Pollution Exposures and Potential Mechanistic Underpinnings.

PURPOSE OF REVIEW: This review sought to address the potential for air pollutants to impair cognition and mechanisms by which that might occur. RECENT FINDINGS: Air pollution has been associated with deficits in cognitive functions across a wide range of epidemiological studies, both with developmental and adult exposures. Studies in animal models are significantly more limited in number, with somewhat inconsistent findings to date for measures of learning, but show more consistent impairments for short-term memory. Potential contributory mechanisms include oxidative stress/inflammation, altered levels of dopamine and/or glutamate, and changes in synaptic plasticity/structure. Epidemiological studies are consistent with adverse effects of air pollutants on cognition, but additional studies and better phenotypic characterization are needed for animal models, including more precise delineation of specific components of cognition that are affected, as well as definitions of critical exposure periods for such effects and the components of air pollution responsible. This would permit development of more circumscribed hypotheses as to potential behavioral and neurobiological mechanisms.

Developmental neurotoxicity of inhaled ambient ultrafine particle air pollution: Parallels with neuropathological and behavioral features of autism and other neurodevelopmental disorders.

Accumulating evidence from both human and animal studies show that brain is a target of air pollution. Multiple epidemiological studies have now linked components of air pollution to diagnosis of autism spectrum disorder (ASD), a linkage with plausibility based on the shared mechanisms of inflammation. Additional plausibility appears to be provided by findings from our studies in mice of exposures from postnatal day (PND) 4-7 and 10-13 (human 3rd trimester equivalent), to concentrated ambient ultrafine (UFP) particles, considered the most reactive component of air pollution, at levels consistent with high traffic areas of major U.S. cities and thus highly relevant to human exposures. These exposures, occurring during a period of marked neuro- and gliogenesis, unexpectedly produced a pattern of developmental neurotoxicity notably similar to multiple hypothesized mechanistic underpinnings of ASD, including its greater impact in males. UFP exposures induced inflammation/microglial activation, reductions in size of the corpus callosum (CC) and associated hypomyelination, aberrant white matter development and/or structural integrity with ventriculomegaly (VM), elevated glutamate and excitatory/inhibitory imbalance, increased amygdala astrocytic activation, and repetitive and impulsive behaviors. Collectively, these findings suggest the human 3rd trimester equivalent as a period of potential vulnerability to neurodevelopmental toxicity to UFP, particularly in males, and point to the possibility that UFP air pollution exposure during periods of rapid neuro- and gliogenesis may be a risk factor not only for ASD, but also for other neurodevelopmental disorders that share features with ASD, such as schizophrenia, attention deficit disorder, and periventricular leukomalacia.

Cerebellar Purkinje cell p75 neurotrophin receptor and autistic behavior.

The p75 neurotrophin receptor (p75NTR) is normally expressed in cerebellar Purkinje cells throughout the lifespan. Children with autism spectrum behavior exhibit apparent cerebellar Purkinje cell loss. Cerebellar transcriptome changes seen in the murine prenatal valproate exposure model of autism include all of the proteins known to constitute the p75NTR interactome. p75NTR is a modulator of cytoplasmic and mitochondrial redox potential, and others have suggested that aberrant response to oxidant stress has a major role in the pathogenesis of autism. We have created Purkinje cell-selective p75NTR knockout mice that are the progeny of hemizygous Cre-Purkinje cell protein 2 C57Bl mice and p75NTR floxed C57Bl mice. These Cre-loxP mice exhibit complete knockout of p75NTR in ~50% of the cerebellar Purkinje cells. Relative to Cre-only mice and wild-type C57Bl mice, this results in a behavioral phenotype characterized by less allogrooming of (P<0.05; one-way analysis of variance) and socialization or fighting with (each P<0.05) other mice; less (1.2-fold) non-ambulatory exploration of their environment than wild-type (P<0.01) or Cre only (P<0.01) mice; and almost twofold more stereotyped jumping behavior than wild-type (P<0.05) or Cre (P<0.02) mice of the same strain. Wild-type mice have more complex dendritic arborization than Cre-loxP mice, with more neurites per unit area (P<0.025, Student's t-test), more perpendicular branches per unit area (P<0.025) and more short branches/long neurite (P<0.0005). Aberrant developmental regulation of expression of p75NTR in cerebellar Purkinje cells may contribute to the pathogenesis of autism.

Enhanced stimulus sequence-dependent repeated learning in male offspring after prenatal stress alone or in conjunction with lead exposure.

Both lead (Pb) exposure and prenatal stress (PS) can produce cognitive deficits, and in a prior study we demonstrated enhanced cognitive deficits in repeated learning of female rats exposed to both of these developmental insults (Cory-Slechta et al., 2010). However, PS can also lead to improved cognitive outcomes that are both gender- and context-dependent. Thus, the current study examined whether Pb ± PS likewise produced repeated learning deficits in males, either after maternal or lifetime Pb exposure. Repeated learning was evaluated using a multiple schedule of repeated learning and performance that required learning 3-response sequences in male offspring that had been subjected to either maternal Pb (0 or 150 ppm) or lifetime Pb exposure (0 or 50 ppm) beginning two months prior to dam breeding, to prenatal immobilization restraint stress (gestational days 16-17), or to both Pb and PS. Blood Pb, corticosterone, hippocampal glucocorticoid receptor density and brain monoamines were also measured. In contrast to outcomes in females, sequence-specific enhancements of repeated learning accuracy were produced by PS, particularly when combined with Pb, results that appeared to be more robust in combination with lifetime than maternal Pb exposure. A common behavioral mechanism of these improvements appears to be an increased reinforcement density associated with increased response rates and shorter session times seen with PS ± Pb that could shorten time to reinforcement. Trends toward lower levels of nucleus accumbens dopamine activity seen after both maternal Pb and lifetime Pb combined with PS suggest a possible role for this region/neurotransmitter in enhanced accuracy, whereas PS ± Pb-induced corticosterone changes did not exhibit an obvious systematic relationship to accuracy enhancements. While PS ± Pb-based increases in accuracy appear to be an improved outcome, the benefits of increased response rate are by no means universal, but highly context-dependent and can lead to adverse behavioral effects in other conditions.

Interactions of lifetime lead exposure and stress: behavioral, neurochemical and HPA axis effects.

Lead (Pb) and stress co-occur as risk factors, share biological substrates and produce common adverse effects. We previously found that prenatal restraint stress (PS) or offspring stress (OS) could enhance maternal Pb-induced behavioral, brain neurotransmitter level and HPA axis changes. The current study examined how lifetime Pb exposure, consistent with human environmental exposure, interacts with stress. Dams were exposed to Pb beginning 2 mos prior to breeding (0, 50 or 150ppm in drinking water), PS on gestational days 16 and 17, or the combination. Offspring continued on the same Pb exposure as the dam. A subset of Pb+PS offspring also received 3 additional stress challenges (OS), yielding 9 exposure groups/gender: 0-NS, 0-PS, 0-OS, 50-NS, 50-PS, 50-OS, 150-NS, 150-PS and 150-OS. As with maternal Pb (Virgolini et al., 2008a), lifetime Pb and stress influenced Fixed Interval (FI) behavior primarily in females. Relative to 0-NS control, reductions in postreinforcement pause (PRP) times were seen only with combined Pb+PS (50-PS, 50-OS, 150-PS). Stress increased FI response rates when Pb alone was without effect (150-PS, 150-OS), but gradually mitigated rate increases produced by Pb alone (50-PS, 50-OS), effects that appear to be due primarily to PS, as they were of comparable magnitude in PS and OS groups. Individual subject data suggest that enhanced Pb and PS effects reflect increasing numbers of subjects shifting to the high end of the normal range of FI performance values, consistent with a dose-response type of Pb+stress additivity. Consistent with reports of cortico-striatal mediation of both interval timing (PRP) and FI rates, principal component analyses suggested potential mediation via altered frontal cortex norepinephrine, reduced nucleus accumbens dopaminergic control and enhanced striatal monoamine control. Altered FI performance, whether occurring through changes in response rate, PRP, or both, represent behavioral inefficiency and potentially sub-optimal or even dysfunctional resource/energy use.

Experimental manipulations blunt time-induced changes in brain monoamine levels and completely reverse stress, but not Pb+/-stress-related modifications to these trajectories.

This study sought to further understand how environmental conditions influence the outcomes of early developmental insults. It compared changes in monoamine levels in frontal cortex, nucleus accumbens and striatum of male and female Long-Evans rat offspring subjected to maternal Pb exposure (0, 50 or 150ppm in drinking water from 2 months pre-breeding until pup weaning)+/-prenatal (PS) (restraint on GD16-17) or PS+offspring stress (OS; three variable stress challenges to young adults) determined at 2 months of age and at 6 months of age in littermates subsequently exposed either to experimental manipulations (EM: daily handling and performance on an operant fixed interval (FI) schedule of food reward), or to no experience (NEM; time alone). Time alone (NEM conditions), even in normal (control) animals, modified the trajectory of neurochemical changes between 2 and 6 months across brain regions and monoamines. EM significantly modified the NEM trajectories, and except NE and striatal DA, which increased, blunted the changes in monoamine levels that occurred over time alone. Pb+/-stress modified the trajectory of monoamine changes in both EM and NEM conditions, but these predominated under NEM conditions. Stress-associated modifications, occurring mainly with NEM OS groups, were fully reversed by EM procedures, while reversals of Pb+/-stress-associated modifications occurred primarily in nucleus accumbens, a region critical to mediation of FI response rates. These results extend the known environmental conditions that modify developmental Pb+/-stress insults, which is critical to ultimately understanding whether early insults lead to adaptive or maladaptive behavior and to devising behavioral therapeutic strategies. That time alone and a set of EM conditions typically used as outcome measures in intervention studies can themselves invoke neurochemical changes, moreover, has significant implications for experimental design of such studies.

Alterations in glucocorticoid negative feedback following maternal Pb, prenatal stress and the combination: a potential biological unifying mechanism for their corresponding disease profiles.

Combined exposures to maternal lead (Pb) and prenatal stress (PS) can act synergistically to enhance behavioral and neurochemical toxicity in offspring. Maternal Pb itself causes permanent dysfunction of the body's major stress system, the hypothalamic pituitary adrenal (HPA) axis. The current study sought to determine the potential involvement of altered negative glucocorticoid feedback as a mechanistic basis of the effects in rats of maternal Pb (0, 50 or 150 ppm in drinking water beginning 2 mo prior to breeding), prenatal stress (PS; restraint on gestational days 16-17) and combined maternal Pb+PS in 8 mo old male and female offspring. Corticosterone changes were measured over 24 h following an i.p. injection stress containing vehicle or 100 or 300 microg/kg (females) or 100 or 150 microg/kg (males) dexamethasone (DEX). Both Pb and PS prolonged the time course of corticosterone reduction following vehicle injection stress. Pb effects were non-monotonic, with a greater impact at 50 vs. 150 ppm, particularly in males, where further enhancement occurred with PS. In accord with these findings, the efficacy of DEX in suppressing corticosterone was reduced by Pb and Pb+PS in both genders, with Pb efficacy enhanced by PS in females, over the first 6 h post-administration. A marked prolongation of DEX effects was found in males. Thus, Pb, PS and Pb+PS, sometimes additively, produced hypercortisolism in both genders, followed by hypocortisolism in males, consistent with HPA axis dysfunction. These findings may provide a plausible unifying biological mechanism for the reported links between Pb exposure and stress-associated diseases and disorders mediated via the HPA axis, including obesity, hypertension, diabetes, anxiety, schizophrenia and depression. They also suggest broadening of Pb screening programs to pregnant women in high stress environments.

Influence of low level maternal Pb exposure and prenatal stress on offspring stress challenge responsivity.

We previously demonstrated potentiated effects of maternal Pb exposure producing blood Pb(PbB) levels averaging 39microg/dl combined with prenatal restraint stress (PS) on stress challenge responsivity of female offspring as adults. The present study sought to determine if: (1) such interactions occurred at lower PbBs, (2) exhibited gender specificity, and (3) corticosterone and neurochemical changes contributed to behavioral outcomes. Rat dams were exposed to 0, 50 or 150ppm Pb acetate drinking water solutions from 2 mos prior to breeding through lactation (pup exposure ended at weaning; mean PbBs of dams at weaning were <1, 11 and 31microg/dl, respectively); a subset in each Pb group underwent prenatal restraint stress (PS) on gestational days 16-17. The effects of variable intermittent stress challenge (restraint, cold, novelty) on Fixed Interval (FI) schedule controlled behavior and corticosterone were examined in offspring when they were adults. Corticosterone changes were also measured in non-behaviorally tested (NFI) littermates. PS alone was associated with FI rate suppression in females and FI rate enhancement in males; Pb exposure blunted these effects in both genders, particularly following restraint stress. PS alone produced modest corticosterone elevation following restraint stress in adult females, but robust enhancements in males following all challenges. Pb exposure blunted these corticosterone changes in females, but further enhanced levels in males. Pb-associated changes showed linear concentration dependence in females, but non-linearity in males, with stronger or selective changes at 50ppm. Statistically, FI performance was associated with corticosterone changes in females, but with frontal cortical dopaminergic and serotonergic changes in males. Corticosterone changes differed markedly in FI vs. NFI groups in both genders, demonstrating a critical role for behavioral history and raising caution about extrapolating biochemical markers across such conditions. These findings demonstrate that maternal Pb interacts with prenatal stress to further modify both behavioral and corticosterone responses to stress challenge, thereby suggesting that studies of Pb in isolation from other disease risk factors will not reveal the extent of its adverse effects. These findings also underscore the critical need to extend screening programs for elevated Pb exposure, now restricted to young children, to pregnant, at risk, women.

CNS effects of developmental Pb exposure are enhanced by combined maternal and offspring stress.

Lead (Pb) exposure and elevated stress are co-occurring risk factors. Both impact brain mesolimbic dopamine/glutamate systems involved in cognitive functions. We previously found that maternal stress can potentiate Pb-related adverse effects in offspring at blood Pb levels averaging approximately 40 microg/dl. The current study of combined Pb exposure and stress sought to extend those results to lower levels of Pb exposure, and to examine relationships among consequences in offspring for fixed interval (FI) schedule-controlled behavior, neurochemistry and corticosterone levels. Dams were exposed to maternal Pb beginning 2 months prior to breeding (0, 50 or 150 ppm in drinking water), maternal restraint stress on gestational days 16 and 17 (MS), or the combination. In addition, a subset of offspring from each resultant treatment group was also exposed intermittently to variable stressors as adults (MS+OS). Marked "Pb-stress"-related increases in response rates on a fixed interval schedule, a behavioral performance with demonstrated sensitivity to Pb, occurred preferentially in female offspring even at mean blood Pb levels of 11 microg/dl when 50 ppm Pb was combined with maternal and offspring stress. Greater sensitivity of females to frontal cortex catecholamine changes may contribute to the elevated FI response rates as mesocorticolimbic systems are critical to the mediation of this behavior. Basal and final corticosterone levels of offspring used to evaluate FI performance differed significantly from those of non-behaviorally tested (NFI) littermates, demonstrating that purported mechanisms of Pb, stress or Pb/stress effects determined in non-behaviorally trained animals cannot necessarily be generalized to animals with behavioral histories. Finally, the persistent and permanent consequences of Pb, stress and Pb+stress in offspring of both genders suggest that Pb screening programs should include pregnant women at risk for elevated Pb exposure, and that stress should be considered as an additional risk factor. Pb+stress effects observed in the absence of either risk factor alone (i.e., potentiated effects) raise questions about the capacity of current hazard identification approaches to adequately identify human health risks posed by neurotoxicants.

New and evolving concepts in the neurotoxicology of lead.

Lead (Pb) is a xenobiotic metal with no known essential function in cellular growth, proliferation, or signaling. Decades of research characterizing the toxicology of Pb have shown it to be a potent neurotoxicant, especially during nervous system development. New concepts in the neurotoxicology of Pb include advances in understanding the mechanisms and cellular specificity of Pb. Experimental studies have shown that stress can significantly alter the effects of Pb, effects that could potentially be mediated through alterations in the interactions of glucocorticoids with the mesocorticolimbic dopamine system of the brain. Elevated stress, with corresponding elevated glucocorticoid levels, has been postulated to contribute to the increased levels of many diseases and dysfunctions in low socioeconomic status populations. Cellular models of learning and memory have been utilized to investigate the potential mechanisms of Pb-induced cognitive deficits. Examination of long-term potentiation in the rodent hippocampus has revealed Pb-induced increases in threshold, decreases in magnitude, and shorter retention times of synaptic plasticity. Structural plasticity in the form of adult neurogenesis in the hippocampus is also impacted by Pb exposure. The action of Pb on glutamate release, NMDA receptor function, or structural plasticity may underlie perturbations in synaptic plasticity and contribute to learning impairments. In addition to providing insight into potential mechanisms of Pb-induced cognitive deficits, cellular models offer an opportunity to investigate direct effects of Pb on isolated biological substrates. A target of interest is the 78-kDa molecular chaperone glucose-regulated protein (GRP78). GRP78 chaperones the secretion of the cytokine interleukin-6 (IL-6) by astrocytes. In vitro evidence shows that Pb strongly binds to GRP78, induces GRP78 aggregation, and blocks IL-6 secretion in astroglial cells. These findings provide evidence for a significant chaperone deficiency in Pb-exposed astrocytes in culture. In the long term, chaperone deficiency could underlie protein conformational diseases such as Alzheimer's Disease (AD). Lead exposure in early life has been implicated in subsequent progression of amyloidogenesis in rodents during old age. This exposure resulted in an increase in proteins associated with AD pathology viz., beta-amyloid precursor protein (beta-APP), and beta-amyloid (Abeta). These four new lines of research comprise compelling evidence that exposures to Pb have adverse effects on the nervous system, that environmental factors increase nervous system susceptibility to Pb, and that exposures in early life may cause neurodegeneration in later life.

Permanent alterations in stress responsivity in female offspring subjected to combined maternal lead exposure and/or stress.

Elevated lead (Pb) exposures preferentially impact low socioeconomic status (SES) populations, the same groups thought to sustain the highest levels of environmental stress. As co-occurring risk factors, therefore, Pb and stress could interact, a possibility further supported by the fact that both act on mesocorticolimbic dopamine systems of the brain. We recently demonstrated in rats that maternal Pb exposure could permanently increase basal corticosterone levels of offspring consistent with altered hypothalamic pituitary adrenal (HPA) axis function. The current study was thus designed to test the hypothesis that stress responsivity of offspring should likewise be altered, with the outcome differing in response to Pb, stress or Pb+stress. The impact of intermittent variable stress challenges (restraint, novelty, cold) on behavior sensitive to Pb exposure (fixed interval (FI) schedule-controlled responding) and on stress-induced corticosterone changes were evaluated in adult female offspring of dams that had been exposed to Pb (150 ppm) in drinking water from 2 months prior to breeding through lactation with or without maternal restraint stress on days 16 and 17 of gestation. This design yielded four treatment groups: (NS/0, no maternal Pb, no maternal stress; S/0, no maternal Pb, maternal stress; NS/150, maternal Pb, no maternal stress; and S/150, maternal Pb exposure and maternal stress). While maternal Pb alone and stress alone each altered components of stress responsivity, the greatest number of effects was seen in response to Pb + stress. This included alterations in FI performance following both restraint and cold stress and in the corticosterone response to cold stress. Collectively, these studies reveal that maternal Pb exposure alone can permanently alter stress responsivity and that the profile of effects produced by maternal Pb differ from those produced by maternal Pb in conjunction with stress, findings which have both mechanistic and risk assessment significance.

Heritability, correlations and in silico mapping of locomotor behavior and neurochemistry in inbred strains of mice.

The midbrain dopamine system mediates normal and pathologic behaviors related to motor activity, attention, motivation/reward and cognition. These are complex, quantitative traits whose variation among individuals is modulated by genetic, epigenetic and environmental factors. Conventional genetic methods have identified several genes important to this system, but the majority of factors contributing to the variation remain unknown. To understand these genetic and environmental factors, we initiated a study measuring 21 behavioral and neurochemical traits in 15 common inbred mouse strains. We report trait data, heritabilities and genetic and non-genetic correlations between pheno-types. In general, the behavioral traits were more heritable than neurochemical traits, and both genetic and non-genetic correlations within these trait sets were high. Surprisingly, there were few significant correlations between the behavioral and the individual neurochemical traits. However, striatal serotonin and one measure of dopamine turnover (DOPAC/DA) were highly correlated with most behavioral measures. The variable accounting for the most variation in behavior was mouse strain and not a specific neurochemical measure, suggesting that additional genetic factors remain to be determined to account for these behavioral differences. We also report the prospective use of the in silico method of quantitative trait loci (QTL) analysis and demonstrate difficulties in the use of this method, which failed to detect significant QTLs for the majority of these traits. These data serve as a framework for further studies of correlations between different midbrain dopamine traits and as a guide for experimental cross designs to identify QTLs and genes that contribute to these traits.

Expression of behavioral sensitization to the cocaine-like fungicide triadimefon is blocked by pretreatment with AMPA, NMDA and DA D1 receptor antagonists.

Triadimefon (TDF) is a triazole fungicide that blocks the reuptake of dopamine (DA), much like cocaine. A recent study in our laboratory found that intermittent injections of TDF led to robust locomotor sensitization in response to challenge TDF after a 2-week withdrawal period. The current study sought to determine whether the expression of TDF behavioral sensitization could be prevented by the DA D1-like receptor antagonist SCH 23390 (SCH), the DA D2-like receptor antagonist remoxipride (Rem), the competitive NMDA antagonist CPP, or the AMPA antagonist NBQX. Adult male C57/BL6 mice were injected with vehicle or 75 mg/kg TDF twice a week for 7 weeks, with locomotor activity measured periodically across the 14 doses. After a 2-week withdrawal period, mice were pretreated with SCH (0.015 mg/kg), Rem (0.3 mg/kg), CPP (2.5 mg/kg) or NBQX (10.0 mg/kg) followed 30 min later by vehicle or 75 mg/kg TDF and tested for the expression of TDF sensitization. Intermittent administration of TDF led to the development and robust expression of behavioral sensitization in terms of vertical activity. Pretreatment with SCH, NBQX and CPP successfully blocked the expression of vertical sensitization to TDF, while Rem pretreatment did not. All four antagonists, however, attenuated the neurochemical changes normally associated with TDF sensitization as measured 8 h after the 2-week TDF challenge. This paper reveals that NMDA, AMPA and DA D1-like receptors are necessary for the behavioral expression of sensitization to the fungicide triadimefon.

Risk factors for dopaminergic neuron loss in human alpha-synuclein transgenic mice.

Genetic background, pesticide exposure, age, gender, diet and lifestyle are implicated risk factors in Parkinson's disease. We demonstrate dopamine neuron loss and other features of Parkinsonism based on the interaction of several of these human risk factors in transgenic mice expressing human alpha-synuclein. Mice expressing different forms of human alpha-synuclein had progressive declines in locomotor activity and abnormal responses to apomorphine that were modified by transgenic status. Stereological counts of tyrosine hydroxylase-positive neurons significantly declined with age only in the transgenic lines, consistent with a constant or decreasing risk, with the line expressing a double-mutant form of human alpha-synuclein more severely affected than the line expressing wild-type human alpha-synuclein. Treatment with Mn2+-ethylenebisdithiocarbamate and paraquat resulted in significantly greater effects in the double-mutant line than the other lines. Inclusions were not identified in the transgenic lines. Overexpression of human alpha-synuclein had adverse effects on substantia nigra pars compacta dopaminergic neurons that were modified by risk factors interacting in humans, including human alpha-synuclein mutations, ageing, and exposure to pesticides.

Development of behavioral sensitization to the cocaine-like fungicide triadimefon is prevented by AMPA, NMDa, DA D1 but not DA D2 receptor antagonists.

Triadimefon (TDF) is a triazole fungicide that blocks the reuptake of dopamine (DA) and leads to increased locomotor activity levels in mice and rats, effects similar to those of indirect DA agonists such as cocaine. We recently found in mice that intermittent TDF administration led to robust locomotor sensitization, a phenomenon reflecting neuronal plasticity, following challenge with the same TDF dose after a 2-week withdrawal period. The current study sought to determine whether antagonists to DA D1-like receptors (SCH 23390; SCH), DA D2-like receptors (remoxipride; Rem), ionotropic glutamate n-methyl-d-aspartate (NMDA) receptors (CPP), or ionotropic glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (NBQX) could prevent the development of TDF behavioral sensitization, therefore indicating their mechanistic involvement in TDF sensitization. Mice were treated with either vehicle, SCH (0.015 mg/kg), remoxipride (Rem, 0.3 mg/kg), CPP (2.5 mg/kg) or NBQX (10.0 mg/kg), followed 30 min later by vehicle or 75 mg/kg TDF (TDF), twice a week for 7 weeks, with locomotor activity measured post-dosing once a week. After a 2-week withdrawal period, mice were challenged with 75 mg/kg TDF or vehicle, to test for the presence of behavioral sensitization. Pretreatment with SCH, CPP, or NBQX, but not Rem, blocked the development of behavioral sensitization to TDF specifically for vertical activity. Antagonists that blocked TDF vertical sensitization also attenuated the increase in extracellular DA turnover (homovanillic acid [HVA]/DA) normally associated with this behavioral response. Therefore, DA D1, NMDA and AMPA receptors appear to be necessary for the development of behavioral sensitization to TDF. As such, TDF may be considered an environmental risk factor for behavioral dysfunctions linked to glutamatergic and dopaminergic systems.