A hypothesis about how early developmental methylmercury exposure disrupts behavior in adulthood

Mercury causes degradation of spatial cognition in a model songbird species

Mercury is a widespread pollutant of increasing global concern that exhibits a broad range of deleterious effects on organisms, including birds. Because the developing brain is well-known to be particularly vulnerable to the neurotoxic insults of mercury, many studies have focused on developmental effects such as on the embryonic brain and resulting behavioral impairment in adults. It is not well understood how the timing of exposure, for example exclusively in ovo versus throughout life, influences the impact of mercury. Using dietary exposure to environmentally relevant methylmercury concentrations, we examined the role that timing and duration of exposure play on spatial learning and memory in a model songbird species, the domesticated zebra finch (Taeniopygia guttata castanotis). We hypothesized that developmental exposure was both necessary and sufficient to disrupt spatial memory in adult finches. We documented profound disruption of memory for locations of hidden food at two spatial scales, cage- and room-sized enclosures, but found that both developmental and ongoing adult exposure were required to exhibit this behavioral impairment. Methylmercury-exposed birds made more mistakes before mastering the spatial task, because they revisited unrewarded locations repeatedly even after discovering the rewarded location. Contrary to our prediction, hippocampal volume was not affected in birds exposed to methylmercury over their lifetimes. The disruption of spatial cognition that we detected is severe and would likely have implications for survival and reproduction in wild birds; however, it appears that individuals that disperse or migrate from a contaminated site might recover later in life if no longer exposed to the toxicant.

Developmental Methylmercury Exposure Induced and Age-Dependent Glutamatergic Neurotoxicity in Caenorhabditis elegans

Developmental methylmercury (MeHg) exposures cause latent neurotoxic effects in adults; however, the mechanisms underlying the latent neurotoxicity are not fully understood. In the current study, we used C. elegans as an animal model to investigate the latent neurotoxic effects of developmental MeHg exposures on glutamatergic neurons. The young larvae stage 1 worms were exposed to MeHg (0.05 ~ 5 µM) for 48 h. The morphological and behavioral endpoints of glutamatergic neurons were compared when worms reached to adult stages including the young adult stage (day 1 adult) and the old adult stage (day 10 adult). Here, we showed that C. elegans glutamatergic neurons were morphologically intact following low or medium MeHg exposures (0.05 ~ 0.5 µM). The morphological damage of glutamatergic neurons appeared to be pronounced in day 10 adults developmentally exposed to 5 µM MeHg. Behavioral assays also showed an age-dependent latent effect of MeHg. In the nose touch response assay, only day 10 adult worms exhibited a functional decline following prior 5 µM MeHg exposure. Moreover, the disruption of NaCl memory appeared only in day 1 adults following MeHg exposures but not in day 10 adults. The expression of C. elegans homologs of mammalian vesicular glutamate transporter (eat-4) was repressed in day 1 adults, while the glutamate receptor homolog (glr-1) was upregulated in day 10 adults with 5 µM MeHg. In the comparison of age-dependent changes in the insulin-like pathway (daf-2/age-1/daf-16) following MeHg exposures, we showed that the daf-2/age-1/daf-16 pathway was mobilized in day 1 adults but repressed in day 10 adults. Collectively, our data supports a conclusion that MeHg-induced glutamatergic neurotoxicity exhibits an age-dependent pattern, possibly related to the prominent changes in age-dependent modulation in the glutamatergic neurotransmission and metabolic pathways.

Past mercury exposure and current symptoms of nervous system dysfunction in adults of a First Nation community (Canada)

BACKGROUND

The watershed in Asubpeeschoseewagong Netum Anishinabek (Grassy Narrows First Nation) territory has been contaminated by mercury (Hg) since 1962, resulting in very high Hg concentrations in fish, central to the community's culture, traditions, economy and diet. Biomarkers of Hg exposure (umbilical cord blood and hair/blood samples), monitored between 1970 and 1997, decreased over time. A recent Grassy Narrows Community Health Assessment (GN-CHA) survey included current symptoms of nervous system dysfunction. The present study aimed to cluster self-reported symptoms and examine their associations with past Hg exposure.

METHODS

The GN-CHA included 391 adults. Symptom clustering used a two-step segmentation approach. Umbilical cord Hg and/or yearly measurements of equivalent hair Hg were available for 242 participants. Structural Equation Models (SEM) displayed the associations between Hg exposure and clusters, with Hg exposure modelled as a latent variable or in separate variables (prenatal, childhood and having had hair Hg ≥ 5 μg/g at least once over the sampling period). Longitudinal Mixed Effects Models (LMEM) served to examine past hair Hg with respect to clusters.

RESULTS

A total of 37 symptoms bonded into 6 clusters, representing Extrapyramidal impairment, Sensory impairment, Cranial nerve disturbances, Gross motor impairment, Neuro-cognitive deficits and Affect/Mood disorders. Median Hg concentrations were 5 μg/L (1-78.5) and 1.1 μg/g (0.2-16) for umbilical cord and childhood hair, respectively. More than one-third (36.6%) had hair Hg ≥ 5 μg/g at least once. In SEM, latent Hg was directly associated with Extrapyramidal and Sensory impairment, Cranial nerve disturbances and Affect/Mood disorders. Direct associations were observed for prenatal exposure with Affect/Mood disorders, for childhood exposure with Extrapyramidal impairment and Cranial nerve disturbances, and for hair Hg ≥ 5 μg/g with Extrapyramidal and Sensory impairment. For all clusters, a further association between past Hg exposure and symptom clusters was mediated by diagnosed nervous system disorders. LMEM showed higher past hair Hg among those with higher scores for all clusters, except Affect/Mood disorders.

CONCLUSION

Our findings provide evidence that in this First Nation community, past Hg exposure from fish consumption was associated with later-life clusters of coexisting symptoms of nervous system dysfunction.

Advances on the Influence of Methylmercury Exposure during Neurodevelopment

Mercury (Hg) is a toxic heavy-metal element, which can be enriched in fauna and flora and transformed into methylmercury (MeHg). MeHg is a widely distributed environmental pollutant that may be harmful to fish-eating populations through enrichment of aquatic food chains. The central nervous system is a primary target of MeHg. Embryos and infants are more sensitive to MeHg, and exposure to MeHg during gestational feeding can significantly impair the homeostasis of offspring, leading to long-term neurodevelopmental defects. At present, MeHg-induced neurodevelopmental toxicity has become a hotspot in the field of neurotoxicology, but its mechanisms are not fully understood. Some evidence point to oxidative damage, excitotoxicity, calcium ion imbalance, mitochondrial dysfunction, epigenetic changes, and other molecular mechanisms that play important roles in MeHg-induced neurodevelopmental toxicity. In this review, advances in the study of neurodevelopmental toxicity of MeHg exposure during pregnancy and the molecular mechanisms of related pathways are summarized, in order to provide more scientific basis for the study of neurodevelopmental toxicity of MeHg.

Methylmercury plus Ethanol Exposure: How Much Does This Combination Affect Emotionality?

Mercury is a heavy metal found in organic and inorganic forms that represents an important toxicant with impact on human health. Mercury can be released in the environment by natural phenoms (i.e., volcanic eruptions), industrial products, waste, or anthropogenic actions (i.e., mining activity). Evidence has pointed to mercury exposure inducing neurological damages related to emotional disturbance, such as anxiety, depression, and insomnia. The mechanisms that underlie these emotional disorders remain poorly understood, although an important role of glutamatergic pathways, alterations in HPA axis, and disturbance in activity of monoamines have been suggested. Ethanol (EtOH) is a psychoactive substance consumed worldwide that induces emotional alterations that have been strongly investigated, and shares common pathophysiological mechanisms with mercury. Concomitant mercury and EtOH intoxication occur in several regions of the world, specially by communities that consume seafood and fish as the principal product of nutrition (i.e., Amazon region). Such affront appears to be more deleterious in critical periods of life, such as the prenatal and adolescence period. Thus, this review aimed to discuss the cellular and behavioral changes displayed by the mercury plus EtOH exposure during adolescence, focused on emotional disorders, to answer the question of whether mercury plus EtOH exposure intensifies depression, anxiety, and insomnia observed by the toxicants in isolation.

Latent effects of early-life methylmercury exposure on motor function in Drosophila

The developmental toxicant, methylmercury (MeHg), can elicit motor deficits that last well into adulthood. Recent studies using Drosophila showed that the developing musculature is sensitive to high doses of MeHg, where a larval feeding paradigm resulted in compromised myotendinous junction (MTJ) formation during development, by a mechanism involving the NG2 homologue, kon-tiki (kon). Low-dose exposures to MeHg that do not produce muscle pathology during development, nevertheless result in impaired flight behavior later in adult life. The present study evaluated the potential for relatively low-dose exposure to produce latent adult muscle pathology and motor impairments, as assayed by climbing and flight, as well as to evaluate molecular mechanisms that may contribute to motor deficits. Wildtype larvae were fed 0, 2, 2.5, or 5 μM MeHg laden food until eclosion. The effect of 5 μM MeHg on MTJ-related gene expression during pupal development was assessed via quantitative RT-qPCR analysis. Upon eclosion, adults were transferred to standard food bottles for 4, 11, or 30 days prior to motor testing. Survivorship (%) was determined from a subset of 200 flies per treatment. Average climbing speed (cm/s) was quantified 4-days post-eclosion (PE). Flight ability was assayed 11- or 30-days PE by measuring landing height (cm) of flies dropped into an adhesive-lined vertical column. In parallel, total body mercury was measured to estimate the influence of residual MeHg at the time of motor testing. Muscle morphology was assessed using immuno-fluorescence microscopy. Exposure to 5uM MeHg significantly reduced climbing speed, and flight ability 4 and 11 - days PE, respectively. While age-related flight deficits were seen in each sex, flight deficits due to MeHg persisted to 30-day PE timepoints exclusively in males. Expression of kon was upregulated across the window of pupal development essential to establishing adult MTJ. However, experimentally restricting the induction of comparable levels of kon to muscle during the same periods did not recapitulate the flight deficits, indicating that muscle-specific induction of kon alone is not sufficient to contribute to latent flight impairments. Adult flight muscle morphology of 11-day PE flies treated with 5 μM MeHg was indistinct from controls, implying muscle structure is not grossly perturbed to impair flight. Collectively, the current data suggest that developmental exposure to 5 μM MeHg reduces flight ability in each sex at 11 day-PE and that latent deficits at 30-day PE are male-specific. It remains to be determined whether the developing MTJ of Drosophila is a sensitive target of MeHg, and whether or not kon acts in conjunction with additional MTJ factors to constitute a MeHg target.

Developmental Exposure to Low Concentrations of Methylmercury Causes Increase in Anxiety-Related Behaviour and Locomotor Impairments in Zebrafish

Methylmercury (MeHg) is a ubiquitous pollutant shown to cause developmental neurotoxicity, even at low levels. However, there is still a large gap in our understanding of the mechanisms linking early-life exposure to life-long behavioural impairments. Our aim was to characterise the short- and long-term effects of developmental exposure to low doses of MeHg on anxiety-related behaviours in zebrafish, and to test the involvement of neurological pathways related to stress-response. Zebrafish embryos were exposed to sub-acute doses of MeHg (0, 5, 10, 15, 30 nM) throughout embryo-development, and tested for anxiety-related behaviours and locomotor activity at larval (light/dark locomotor activity) and adult (novel tank and tap assays) life-stages. Exposure to all doses of MeHg caused increased anxiety-related responses; heightened response to the transition from light to dark in larvae, and a stronger dive response in adults. In addition, impairment in locomotor activity was observed in the higher doses in both larvae and adults. Finally, the expressions of several neural stress-response genes from the HPI-axis and dopaminergic system were found to be disrupted in both life-stages. Our results provide important insights into dose-dependent differences in exposure outcomes, the development of delayed effects over the life-time of exposed individuals and the potential mechanisms underlying these effects.

Selective dopaminergic effects on attention and memory in male mice exposed to Methylmercury during adolescence

Gestational exposure to methylmercury disrupts dopamine-mediated behavior and produces heightened sensitivity to monoamine agonists later in life. This has been reported and replicated following both pre- and post-natal exposure. Impacts of methylmercury when exposure occurs during the sensitive period of adolescence, a key period of dopaminergic development, remain underexplored. There have been variable results thus far in studies investigating links between adolescent exposure to methylmercury and alterations in executive function and altered sensitivity to monoamine agonists. The current study was designed to investigate adolescent exposure by exposing male mice to 0, 0.3, or 3 ppm methylmercury during adolescence and training them in a hybrid task to assess two executive functions, attention and memory, in adulthood. Behavior in these animals was probed with a range of doses of the dopamine agonist, d-amphetamine, and the norepinephrine agonist, desipramine. Attention and memory in these mice were sensitive to disruption by d-amphetamine and interacted with methylmercury exposure. Choice latencies were also longer in the MeHg-exposed mice. Desipramine did not affect behavior in these animals nor did it interact with methylmercury. It is concluded that methylmercury-related inhibition of behavior observed in this study were differentially sensitive to acute disruption in dopamine, but not norepinephrine, neurotransmission.

Adolescent methylmercury exposure: Behavioral mechanisms and effects of sodium butyrate in mice

Methylmercury (MeHg), an environmental neurotoxicant primarily found in fish, produces neurobehavioral impairment when exposure occurs during gestation. Whether other developmental periods, such as adolescence, display enhanced vulnerability to the behavioral effects of MeHg exposure is only beginning to be explored. Further, little is known about the effects of repeated administration of lysine deacetylase inhibitors, such as sodium butyrate (NaB), on operant behavior. In Experiment 1, male C57BL6/n mice were exposed to 0, 0.3, and 3.0 ppm MeHg (n = 12 each) via drinking water from postnatal days 21 to 60 (murine adolescence). As adults, mice were trained to lever press under an ascending series of fixed-ratio schedules of milk reinforcement selected to enable the analysis of three important parameters of operant behavior using the framework provided by Mathematical Principles of Reinforcement. Adolescent MeHg exposure dose-dependently increased saturation rate, a measure of the retroactive reach of a reinforcer, and decreased minimum response time relative to controls. In Experiment 2, the behavioral effects of repeated NaB administration both alone and following adolescent MeHg exposure were examined. Male C57BL6/n mice were given either 0 or 3.0 ppm MeHg during adolescence and, before behavioral testing, two weeks of once daily i.p. injections of saline or 0.6 g/kg NaB (n = 12 in each cell). Adolescent MeHg exposure again increased saturation rate but did not significantly alter minimum response time. NaB also increased saturation rate in both MeHg exposure groups. These data suggest that the behavioral mechanisms of adolescent MeHg exposure and NaB may be related to the impact of reinforcement on prior responses. Specifically, MeHg and NaB concentrated the effects of reinforcers onto the most recent responses.

Cocaine self-administration in male and female rats perinatally exposed to PCBs: Evaluating drug use in an animal model of environmental contaminant exposure

Polychlorinated biphenyls (PCBs) are ubiquitous environmental toxicants known to adversely impact human health. Ortho-substituted PCBs affect the nervous system, including the brain dopaminergic system. The reinforcing effects of psychostimulants are typically modulated via the dopaminergic system, so this study used a preclinical (i.e., rodent) model to evaluate whether developmental contaminant exposure altered intravenous self-administration (IV SA) for the psychostimulant cocaine. Long-Evans rats were perinatally exposed to 6 or 3 mg/kg/day of PCBs throughout gestation and lactation and compared with nonexposed controls. Rats were trained to lever press for a food reinforcer in an operant chamber under a fixed-ratio 5 (FR5) schedule and later underwent jugular catheterization. Food reinforcers were switched for infusions of 250 μg of cocaine, but the response requirement to earn the reinforcer remained. Active lever presses and infusions were higher in males during response acquisition and maintenance. The same sex effect was observed during later sessions which evaluated responding for cocaine doses ranging from 31.25-500 μg. PCB-exposed males (not females) exhibited an increase in cocaine infusions (with a similar trend in active lever presses) during acquisition, but no PCB-related differences were observed during maintenance, examination of the cocaine dose-response relationship, or progressive ratio (PR) sessions. Overall, these results indicated perinatal PCB exposure enhanced early cocaine drug-seeking in this preclinical model of developmental contaminant exposure (particularly the males), but no differences were seen during later cocaine SA sessions. As such, additional questions regarding substance abuse proclivity may be warranted in epidemiological studies evaluating environmental contaminant exposures. (PsycINFO Database Record

Effects of adolescent exposure to methylmercury and d-amphetamine on reversal learning and an extradimensional shift in male mice

Adolescence is associated with the continued maturation of dopamine neurotransmission and is implicated in the etiology of many psychiatric illnesses. Adolescent exposure to neurotoxicants that distort dopamine neurotransmission, such as methylmercury (MeHg), may modify the effects of chronic d-amphetamine (d-AMP) administration on reversal learning and attentional-set shifting. Male C57Bl/6n mice were randomly assigned to two MeHg-exposure groups (0 ppm and 3 ppm) and two d-AMP-exposure groups (saline and 1 mg/kg/day), producing four treatment groups (n = 10-12/group): control, MeHg, d-AMP, and MeHg + d-AMP. MeHg exposure (via drinking water) spanned postnatal days 21-59 (the murine adolescent period), and once daily intraperitoneal injections of d-AMP or saline spanned postnatal days 28-42. As adults, mice were trained on a spatial-discrimination-reversal (SDR) task in which the spatial location of a lever press predicted reinforcement. Following 2 SDRs, a visual-discrimination task (extradimensional shift) was instated in which the presence of a stimulus light above a lever predicted reinforcement. Responding was modeled using a logistic function, which estimated the rate (slope) of a behavioral transition and trials required to complete half a transition (half-max). MeHg, d-AMP, and MeHg + d-AMP exposure increased estimates of half-max on the second reversal. MeHg exposure increased half-max and decreased the slope term following the extradimensional shift, but these effects did not occur following MeHg + d-AMP exposure. MeHg + d-AMP exposure produced more perseverative errors and omissions following a reversal. Adolescent exposure to MeHg can modify the behavioral effects of chronic d-AMP administration. (PsycINFO Database Record

Adolescent methylmercury exposure affects choice and delay discounting in mice

The developing fetus is vulnerable to low-level exposure to methylmercury (MeHg), an environmental neurotoxicant, but the consequences of exposure during the adolescent period remain virtually unknown. The current experiments were designed to assess the effects of low-level MeHg exposure during adolescence on delay discounting, preference for small, immediate reinforcers over large, delayed ones, using a mouse model. Thirty-six male C57BL/6n mice were exposed to 0, 0.3, or 3.0ppm mercury (as MeHg) via drinking water from postnatal day 21 through 59, encompassing the murine adolescent period. As adults, mice lever pressed for a 0.01-cc droplet of milk solution delivered immediately or four 0.01-cc droplets delivered after a delay. Delays ranged from 1.26 to 70.79s, and all were presented within a session. A model based on the Generalized Matching Law indicated that sensitivity to reinforcer magnitude was lower for MeHg-exposed mice relative to controls, indicating that responding in MeHg-exposed mice was relatively indifferent to the larger reinforcer. Sensitivity to reinforcer delay was reduced (delay discounting was decreased) in the 0.3-ppm group, but not in the 3.0-ppm group, compared to controls. Adolescence is a developmental period during which the brain and behavior may be vulnerable to MeHg exposure. As with gestational MeHg exposure, the effects are reflected in the impact of reinforcing stimuli.

A developmental perspective on early-life exposure to neurotoxicants

BACKGROUND

Studies of early-life neurotoxicant exposure have not been designed, analyzed, or interpreted in the context of a fully developmental perspective.

OBJECTIVES

The goal of this paper is to describe the key principles of a developmental perspective and to use examples from the literature to illustrate the relevance of these principles to early-life neurotoxicant exposures.

METHODS

Four principles are discussed: 1) the effects of early-life neurotoxicant exposure depend on a child's developmental context; 2) deficits caused by early-life exposure initiate developmental cascades that can lead to pathologies that differ from those observed initially; 3) early-life neurotoxicant exposure has intra-familial and intergenerational impacts; 4) the impacts of early-life neurotoxicant exposure influence a child's ability to respond to future insults. The first principle is supported by considerable evidence, but the other three have received much less attention.

DISCUSSION

Incorporating a developmental perspective in studies of early-life neurotoxicant exposures requires prospective collection of data on a larger array of covariates than usually considered, using analytical approaches that acknowledge the transactional processes between a child and the environment and the phenomenon of developmental cascades.

CONCLUSION

Consideration of early-life neurotoxicant exposure within a developmental perspective reveals that many issues remain to be explicated if we are to achieve a deep understanding of the societal health burden associated with early-life neurotoxicant exposures.

Revealing Behavioral Learning Deficit Phenotypes Subsequent to In Utero Exposure to Benzo(a)pyrene

To characterize behavioral deficits in pre-adolescent offspring exposed in utero to Benzo(a)pyrene [B(a)P], timed-pregnant Long Evans Hooded rats were treated with B(a)P (150, 300, 600, and 1200 µg/kg BW) or peanut oil (vehicle) on E14, 15, 16, and 17. Following birth, during the pre-weaning period, B(a)P metabolites were examined in plasma and whole brain or cerebral cortex from exposed and control offspring. Tissue concentrations of B(a)P metabolites were (1) dose-dependent and (2) followed a time-dependence for elimination with ∼60% reduction by PND5 in the 1200 µg/kg BW experimental group. Spatial discrimination-reversal learning was utilized to evaluate potential behavioral neurotoxicity in P40-P60 offspring. Late-adolescent offspring exposed in utero to 600 and 1200 µg/kg BW were indistinguishable from their control counterparts for ability to acquire an original discrimination (OD) and reach criterion. However, a dose-dependent effect of in utero B(a)P-exposure was evident upon a discrimination reversal as exposed offspring perseverated on the previously correct response. This newly characterized behavioral deficit phenotype for the first reversal was not apparent in either the (1) OD or (2) subsequent reversal sessions relative to the respective control offspring. Furthermore, the expression of activity related-cytoskeletal-associated protein (Arc), an experience-dependent cortical protein marker known to be up-regulated in response to acquisition of a novel behavior, was greater in B(a)P-exposed offspring included in the spatial discrimination cohort versus home cage controls. Collectively, these findings support the hypothesis that in utero exposure to B(a)P during critical windows of development representing peak periods of neurogenesis results in behavioral deficits in later life.