Sex-dependent impacts of low-level lead exposure and prenatal stress on impulsive choice behavior and associated biochemical and neurochemical manifestations.

A prior study demonstrated increased overall response rates on a fixed interval (FI) schedule of reward in female offspring that had been subjected to maternal lead (Pb) exposure, prenatal stress (PS) and offspring stress challenge relative to control, prenatal stress alone, lead alone and lead+prenatal stress alone (Virgolini et al., 2008). Response rates on FI schedules have been shown to directly relate to measures of self-control (impulsivity) in children and in infants (Darcheville et al., 1992, 1993). The current study sought to determine whether enhanced effects of Pb±PS would therefore be seen in a more direct measure of impulsive choice behavior, i.e., a delay discounting paradigm. Offspring of dams exposed to 0 or 50ppm Pb acetate from 2 to 3 months prior to breeding through lactation, with or without immobilization restraint stress (PS) on gestational days 16 and 17, were trained on a delay discounting paradigm that offered a choice between a large reward (three 45mg food pellets) after a long delay or a small reward (one 45mg food pellet) after a short delay, with the long delay value increased from 0s to 30s across sessions. Alterations in extinction of this performance, and its subsequent re-acquisition after reinforcement delivery was reinstated were also examined. Brains of littermates of behaviorally-trained offspring were utilized to examine corresponding changes in monoamines and in levels of brain derived neurotrophic factor (BDNF), the serotonin transporter (SERT) and the N-methyl-d-aspartate receptor (NMDAR) 2A in brain regions associated with impulsive choice behavior. Results showed that Pb±PS-induced changes in delay discounting occurred almost exclusively in males. In addition to increasing percent long delay responding at the indifference point (i.e., reduced impulsive choice behavior), Pb±PS slowed acquisition of delayed discounting performance, and increased numbers of both failures to and latencies to initiate trials. Overall, the profile of these alterations were more consistent with impaired learning/behavioral flexibility and/or with enhanced sensitivity to the downshift in reward opportunities imposed by the transition from delay discounting training conditions to delay discounting choice response contingencies. Consistent with these behavioral changes, Pb±PS treated males also showed reductions in brain serotonin function in all mesocorticolimbic regions, broad monoamine changes in nucleus accumbens, and reductions in both BDNF and NMDAR 2A levels and increases in SERT in frontal cortex, i.e., in regions and neurotransmitter systems known to mediate learning/behavioral flexibility, and which were of greater impact in males. The current findings do not fully support a generality of the enhancement of Pb effects by PS, as previously seen with FI performance in females (Virgolini et al., 2008), and suggest a dissociation of the behaviors controlled by FI and delay discounting paradigms, at least in response to Pb±PS in rats. Collectively, however, the findings remain consistent with sex-dependent differences in the impacts of both Pb and PS and with the need to understand both the role of contingencies of reinforcement and underlying neurobiological effects in these sex differences.

Early postnatal exposure to ultrafine particulate matter air pollution: persistent ventriculomegaly, neurochemical disruption, and glial activation preferentially in male mice.

BACKGROUND: Air pollution has been associated with adverse neurological and behavioral health effects in children and adults. Recent studies link air pollutant exposure to adverse neurodevelopmental outcomes, including increased risk for autism, cognitive decline, ischemic stroke, schizophrenia, and depression. OBJECTIVES: We sought to investigate the mechanism(s) by which exposure to ultrafine concentrated ambient particles (CAPs) adversely influences central nervous system (CNS) development. METHODS: We exposed C57BL6/J mice to ultrafine (< 100 nm) CAPs using the Harvard University Concentrated Ambient Particle System or to filtered air on postnatal days (PNDs) 4-7 and 10-13, and the animals were euthanized either 24 hr or 40 days after cessation of exposure. Another group of males was exposed at PND270, and lateral ventricle area, glial activation, CNS cytokines, and monoamine and amino acid neurotransmitters were quantified. RESULTS: We observed ventriculomegaly (i.e., lateral ventricle dilation) preferentially in male mice exposed to CAPs, and it persisted through young adulthood. In addition, CAPs-exposed males generally showed decreases in developmentally important CNS cytokines, whereas in CAPs-exposed females, we observed a neuroinflammatory response as indicated by increases in CNS cytokines. We also saw changes in CNS neurotransmitters and glial activation across multiple brain regions in a sex-dependent manner and increased hippocampal glutamate in CAPs-exposed males. CONCLUSIONS: We observed brain region- and sex-dependent alterations in cytokines and neurotransmitters in both male and female CAPs-exposed mice. Lateral ventricle dilation (i.e., ventriculomegaly) was observed only in CAPs-exposed male mice. Ventriculomegaly is a neuropathology that has been associated with poor neurodevelopmental outcome, autism, and schizophrenia. Our findings suggest alteration of developmentally important neurochemicals and lateral ventricle dilation may be mechanistically related to observations linking ambient air pollutant exposure and adverse neurological/neurodevelopmental outcomes in humans.

Developmental exposure to concentrated ambient ultrafine particulate matter air pollution in mice results in persistent and sex-dependent behavioral neurotoxicity and glial activation.

The brain appears to be a target of air pollution. This study aimed to further ascertain behavioral and neurobiological mechanisms of our previously observed preference for immediate reward (Allen, J. L., Conrad, K., Oberdorster, G., Johnston, C. J., Sleezer, B., and Cory-Slechta, D. A. (2013). Developmental exposure to concentrated ambient particles and preference for immediate reward in mice. Environ. Health Perspect. 121, 32-38), a phenotype consistent with impulsivity, in mice developmentally exposed to inhaled ultrafine particles. It examined the impact of postnatal and/or adult concentrated ambient ultrafine particles (CAPS) or filtered air on another behavior thought to reflect impulsivity, Fixed interval (FI) schedule-controlled performance, and extended the assessment to learning/memory (novel object recognition (NOR)), and locomotor activity to assist in understanding behavioral mechanisms of action. In addition, levels of brain monoamines and amino acids, and markers of glial presence and activation (GFAP, IBA-1) were assessed in mesocorticolimbic brain regions mediating these cognitive functions. This design produced four treatment groups/sex of postnatal/adult exposure: Air/Air, Air/CAPS, CAPS/Air, and CAPS/CAPS. FI performance was adversely influenced by CAPS/Air in males, but by Air/CAPS in females, effects that appeared to reflect corresponding changes in brain mesocorticolimbic dopamine/glutamate systems that mediate FI performance. Both sexes showed impaired short-term memory on the NOR. Mechanistically, cortical and hippocampal changes in amino acids raised the potential for excitotoxicity, and persistent glial activation was seen in frontal cortex and corpus callosum of both sexes. Collectively, neurodevelopment and/or adulthood CAPS can produce enduring and sex-dependent neurotoxicity. Although mechanisms of these effects remain to be fully elucidated, findings suggest that neurodevelopment and/or adulthood air pollution exposure may represent a significant underexplored risk factor for central nervous system diseases/disorders and thus a significant public health threat even beyond current appreciation.

Consequences of developmental exposure to concentrated ambient ultrafine particle air pollution combined with the adult paraquat and maneb model of the Parkinson's disease phenotype in male mice.

Current evidence suggests suceptibility of both the substantia nigra and striatum to exposure to components of air pollution. Further, air pollution has been associated with increased risk of PD diagnsosis in humans or PD-like pathology in animals. This study examined whether exposure of mice to concentrated ambient ultrafine particles (CAPS; <100nm diameter) during the first two weeks of life would alter susceptibility to induction of the Parkinson's disease phenyotype (PDP) in a pesticide-based paraquat and maneb (PQ+MB) model during adulthood utilizing i.p. injections of 10mg/kg PQ and 30mg/kg MB 2× per week for 6 weeks. Evidence of CAPS-induced enhancement of the PQ+MB PDP was limited primarily to delayed recovery of locomotor activity 24 post-injection of PQ+MB that could be related to alterations in striatal GABA inhibitory function. Absence of more extensive interactions might also reflect the finding that CAPS and PQ+MB appeared to differentially target the nigrostriatal dopamine and amino acid systems, with CAPS impacting striatum and PQ+MB impacting dopamine-glutamate function in midbrain; both CAPS and PQ+MB elevated glutamate levels in these specific regions, consistent with potential excitotoxicity. These findings demonstrate the ability of postnatal CAPS to produce locomotor dysfunction and dopaminergic and glutamateric changes, independent of PQ+MB, in brain regions involved in the PDP.

Brain hemispheric differences in the neurochemical effects of lead, prenatal stress, and the combination and their amelioration by behavioral experience.

Brain lateralization, critical to mediation of cognitive functions and to "multitasking," is disrupted in conditions such as attention deficit disorder and schizophrenia. Both low-level lead (Pb) exposure and prenatal stress (PS) have been associated with mesocorticolimbic system-mediated executive-function cognitive and attention deficits. Mesocorticolimbic systems demonstrate significant laterality. Thus, altered brain lateralization could play a role in this behavioral toxicity. This study examined laterality of mesocorticolimbic monoamines (frontal cortex, nucleus accumbens, striatum, midbrain) and amino acids (frontal cortex) in male and female rats subjected to lifetime Pb exposure (0 or 50 ppm in drinking water), PS (restraint stress on gestational days 16-17), or the combination with and without repeated learning behavioral experience. Control males exhibited prominent laterality, particularly in midbrain and also in frontal cortex and striatum; females exhibited less laterality, and this was primarily striatal. Lateralized Pb ± PS induced neurotransmitter changes were assessed only in males because of limited sample sizes of Pb + PS females. In males, Pb ± PS changes occurred in left hemisphere of frontal cortex and right hemisphere of midbrain. Behavioral experience modified the laterality of Pb ± PS-induced neurotransmitter changes in a region-dependent manner. Notably, behavioral experience eliminated Pb ± PS neurotransmitter changes in males. These findings underscore the critical need to evaluate both sexes and brain hemispheres for the mechanistic understanding of sex-dependent differences in neuro- and behavioral toxicity. Furthermore, assessment of central nervous system mechanisms in the absence of behavioral experience, shown here for males, may constitute less relevant models of human health effects.

Variations in the nature of behavioral experience can differentially alter the consequences of developmental exposures to lead, prenatal stress, and the combination.

Behavioral experience (BE) can critically influence later behavior and brain function, but the central nervous system (CNS) consequences of most developmental neurotoxicants are examined in the absence of any such context. We previously demonstrated marked differences in neurotransmitter changes produced by developmental lead (Pb) exposure ± prenatal stress (PS) depending upon whether or not rats had been given BE (Cory-Slechta, D. A., Virgolini, M. B., Rossi-George, A., Weston, D., and Thiruchelvam, M. (2009). The current study examined the hypothesis that the nature of the BE itself would be a critical determinant of outcome in mice that had been continually exposed to 0 or 100 ppm Pb acetate in drinking water alone or in combination with prenatal restraint stress. Half of the offspring in each of the four resulting groups/gender were exposed to positively reinforced (food-rewarded Fixed Interval schedule-controlled behavior) or negatively reinforced (inescapable forced swim) BE. Brain monoamines and amino acids differed significantly in relation to BE, even in control animals, as did the trajectory of effects of Pb ± PS, particularly in frontal cortex, hippocampus (both genders), and midbrain (males). In males, Pb ± PS-related changes in neurotransmitters correlated with behavioral performance. These findings suggest that CNS consequences of developmental toxicants studied in the absence of a broader spectrum of BEs may not necessarily be predictive of human outcomes. Evaluating the role of specific BEs as a modulator of neurodevelopmental insults offers the opportunity to determine what specific BEs may ameliorate the associated impacts and can assist in establishing underlying neurobiological mechanisms.

Lifetime consequences of combined maternal lead and stress.

Elevated lead (Pb) exposure and high stress both target low socio-economic status populations. Both also act on the hypothalamic-pituitary-adrenal (HPA) axis. Pb disrupts cognition through effects on the mesocorticolimbic dopamine pathway. Stress hormones act on this same pathway via the HPA axis. The fact that Pb and stress are likely interactive risk factors served as the rationale for a series of studies in our laboratory. These demonstrate that stress can modify Pb effects, that Pb can modify stress responsivity, and, notably, that Pb + stress effects can occur in the absence of an effect of either alone in rats. Furthermore, maternal only Pb exposure can permanently alter basal corticosterone levels, stress responsivity (i.e. permanent modification of HPA axis function) and brain catecholamines in offspring of both genders. Interactive effects of Pb + stress are not limited to early development: even Pb exposures initiated post-weaning alter basal corticosterone and stress responsivity. Outcomes differ in relation to gender, brain region, stressor and time of measurement, making Pb + stress interactions complex. Altered HPA axis function may serve as a mechanism for the behavioural and catecholaminergic neurotoxicity associated with Pb, as well as for the increased incidence of disease and dysfunctions associated with low socio-economic status. The permanent consequences of maternal only Pb exposure suggest that Pb screening programmes 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 raise questions about the capacity of current hazard identification approaches to adequately identify human health risks posed by neurotoxicants.