Acute and longer term effects of meso-2,3 dimercaptosuccinic acid (DMSA) on the behavior of lead-exposed and control mice

Animal Models of Childhood Exposure to Lead or Manganese: Evidence for Impaired Attention, Impulse Control, and Affect Regulation and Assessment of Potential Therapies

Behavioral disorders involving attention and impulse control dysfunction, such as ADHD, are among the most prevalent disorders in children and adolescents, with significant impact on their lives. The etiology of these disorders is not well understood, but is recognized to be multifactorial, with studies reporting associations with polygenic and environmental risk factors, including toxicant exposure. Environmental epidemiological studies, while good at establishing associations with a variety of environmental and genetic risk factors, cannot establish causality. Animal models of behavioral disorders, when properly designed, can play an essential role in establishing causal relationships between environmental risk factors and a disorder, as well as provide model systems for elucidating underlying neural mechanisms and testing therapies. Here, we review how animal model studies of developmental lead or manganese exposure have been pivotal in (1) establishing a causal relationship between developmental exposure and lasting dysfunction in the domains of attention, impulse control, and affect regulation, and (2) testing the efficacy of specific therapeutic approaches for alleviating the lasting deficits. The lead and manganese case studies illustrate how animal models can advance knowledge in ways that are not possible in human studies. For example, in contrast to the Treatment of Lead Poisoned Children (TLC) human clinical trial evaluating succimer chelation efficacy to improve cognitive functioning in lead-exposed children, our developmental lead exposure animal model showed that succimer chelation can produce lasting cognitive benefits if chelation sufficiently reduces brain lead levels. In addition, this study revealed that succimer treatment in the absence of lead exposure produces lasting cognitive dysfunction, highlighting potential risks of chelation in off-label uses, such as the treatment of autistic children without a history of lead exposure. Our animal model of developmental manganese exposure has demonstrated that manganese can cause lasting attentional and sensorimotor deficits, akin to an ADHD-inattentive behavioral phenotype, thereby providing insights into the role of environmental exposures as contributors to ADHD. These studies have also shown that oral methylphenidate (Ritalin) can fully alleviate the deficits produced by early developmental Mn exposure. Future work should continue to focus on the development and use of animal models that appropriately recapitulate the complex behavioral phenotypes of behavioral disorders, in order to determine the mechanistic basis for the behavioral deficits caused by developmental exposure to environmental toxicants, and the efficacy of existing and emerging therapies.

A role for amyloid precursor protein translation to restore iron homeostasis and ameliorate lead (Pb) neurotoxicity

Iron supplementation ameliorates the neurotoxicity of the environmental contaminant lead (Pb); however, the mechanism remains undefined. Iron is an essential nutrient but high levels are toxic due to the catalytic generation of destructive hydroxyl radicals. Using human neuroblastoma SH-SY5Y cells to model human neurons, we investigated the effect of Pb on proteins of iron homeostasis: the Alzheimer's amyloid precursor protein (APP), which stabilizes the iron exporter ferroportin 1; and, the heavy subunit of the iron-storage protein, ferritin (FTH). Lead (Pb(II) and Pb(IV) inhibited APP translation and raised cytosolic iron(II). Lead also increased iron regulatory protein-1 binding to the cognate 5'untranslated region-specific iron-responsive element (IRE) of APP and FTH mRNAs. Concurrent iron treatment rescued cells from Pb toxicity by specifically restoring APP synthesis, i.e. levels of the APP-related protein, APLP-2, were unchanged. Significantly, iron/IRE-independent over-expression of APP695  protected SH-SY5Y cells from Pb toxicity, demonstrating that APP plays a key role in maintaining safe levels of intracellular iron. Overall, our data support a model of neurotoxicity where Pb enhances iron regulatory protein/IRE-mediated repression of APP and FTH translation. We propose novel treatment options for Pb poisoning to include chelators and the use of small molecules to maintain APP and FTH translation. We propose the following cascade for Lead (Pb) toxicity to neurons; by targeting the interaction between Iron regulatory protein-1 and Iron-responsive elements, Pb caused translational repression of proteins that control intracellular iron homeostasis, including the Alzheimer's amyloid precursor protein (APP) that stabilizes the iron exporter ferroportin, and the ferroxidase heavy subunit of the iron-storage protein, ferritin. When unregulated, IRE-independent over-expression of APP695 protected SH-SY5Y neurons from Pb toxicity. There is a novel and key role for APP in maintaining safe levels of intracellular iron pertinent to lead toxicity.

Cross-species coherence in effects and modes of action in support of causality determinations in the U.S. Environmental Protection Agency's Integrated Science Assessment for Lead

The peer-reviewed literature on the health and ecological effects of lead (Pb) indicates common effects and underlying modes of action across multiple organisms for several endpoints. Based on such observations, the United States (U.S.) Environmental Protection Agency (EPA) applied a cross-species approach in the 2013 Integrated Science Assessment (ISA) for Lead for evaluating the causality of relationships between Pb exposure and specific endpoints that are shared by humans, laboratory animals, and ecological receptors (i.e., hematological effects, reproductive and developmental effects, and nervous system effects). Other effects of Pb (i.e., cardiovascular, renal, and inflammatory responses) are less commonly assessed in aquatic and terrestrial wildlife limiting the application of cross-species comparisons. Determinations of causality in ISAs are guided by a framework for classifying the weight of evidence across scientific disciplines and across related effects by considering aspects such as biological plausibility and coherence. As illustrated for effects of Pb where evidence across species exists, the integration of coherent effects and common underlying modes of action can serve as a means to substantiate conclusions regarding the causal nature of the health and ecological effects of environmental toxicants.

Learning on Jupiter, learning on the Moon: the dark side of the G-force. Effects of gravity changes on neurovascular unit and modulation of learning and memory

On earth, gravity vector conditions the development of all living beings by physically imposing an axis along which to build their organism. Thus, during their whole life, they have to fight against this force not only to maintain their architectural organization but also to coordinate the communication between organs and keep their physiology in a balanced steady-state. In space, astronauts show physiological, psychological, and cognitive deregulations, ranging from bone decalcification or decrease of musculature, to depressive-like disorders, and spatial disorientation. Nonetheless, they are confronted to a great amount of physical changes in their environment such as solar radiations, loss of light-dark cycle, lack of spatial landmarks, confinement, and obviously a dramatic decrease of gravity force. It is thus very hard to selectively discriminate the strict role of gravity level alterations on physiological, and particularly cerebral, dysfunction. To this purpose, it is important to design autonomous models and apparatuses for behavioral phenotyping utilizable under modified gravity environments. Our team actually aims at working on this area of research.

Succimer chelation normalizes reactivity to reward omission and errors in lead-exposed rats

This study evaluated the efficacy of a 3-week course of succimer treatment to alleviate behavioral deficits in rats exposed to lead (Pb) for the first 4 weeks of life. A 3 x 2 factorial design was used: three levels of lead exposure (No Pb, Moderate, and High Pb) and two levels of chelation (succimer or vehicle). Behavioral testing was conducted following chelation therapy, from 2 to 9 months of age; this report presents the results of two of the administered tasks: (1) a conditional olfactory discrimination task (baseline task), and (2) a conditional olfactory discrimination task with periodic reward omission on some correct trials (RO task). In the RO task, the performance disruption produced by committing an error on the previous trial was significantly greater for both unchelated lead-exposed groups than for controls. The High Pb rats were also more sensitive to reward omission than controls, providing converging evidence for impaired regulation of arousal or emotion. Importantly, succimer treatment was effective in normalizing the heightened reactivity of the lead-exposed animals to both errors and reward omission. In addition, non-lead-exposed rats that were treated with succimer tended to be more affected by a prior error than controls in their latency to respond on post-error trials. In sum, these findings provide new evidence that succimer chelation can significantly lessen the lasting neurobehavioral dysfunction produced by early lead exposure, but also suggest that there may be risks of administering the drug to individuals without elevated blood lead levels.

Succimer chelation improves learning, attention, and arousal regulation in lead-exposed rats but produces lasting cognitive impairment in the absence of lead exposure

BACKGROUND

There is growing pressure for clinicians to prescribe chelation therapy at only slightly elevated blood lead levels. However, very few studies have evaluated whether chelation improves cognitive outcomes in Pb-exposed children, or whether these agents have adverse effects that may affect brain development in the absence of Pb exposure.

OBJECTIVES

The present study was designed to answer these questions, using a rodent model of early childhood Pb exposure and treatment with succimer, a widely used chelating agent for the treatment of Pb poisoning.

RESULTS

Pb exposure produced lasting impairments in learning, attention, inhibitory control, and arousal regulation, paralleling the areas of dysfunction seen in Pb-exposed children. Succimer treatment of the Pb-exposed rats significantly improved learning, attention, and arousal regulation, although the efficacy of the treatment varied as a function of the Pb exposure level and the specific functional deficit. In contrast, succimer treatment of rats not previously exposed to Pb produced lasting and pervasive cognitive and affective dysfunction comparable in magnitude to that produced by the higher Pb exposure regimen.

CONCLUSIONS

These are the first data, to our knowledge, to show that treatment with any chelating agent can alleviate cognitive deficits due to Pb exposure. These findings suggest that it may be possible to identify a succimer treatment protocol that improves cognitive outcomes in Pb-exposed children. However, they also suggest that succimer treatment should be strongly discouraged for children who do not have elevated tissue levels of Pb or other heavy metals.

Changes in brain biogenic amines and haem biosynthesis and their response to combined administration of succimers and Centella asiatica in lead poisoned rats

This study was designed to investigate the therapeutic potential of meso 2,3-dimercaptosuccinic acid (DMSA) and one of its monoesters, monoisoamyl DMSA (MiADMSA), individually or when administered in combination with an extract of Centella asiatica against experimental lead intoxication in rats. Biochemical variables indicative of alterations in the central nervous system and haem biosynthesis were investigated to determine the toxicity in male Wistar rats. Thirty five rats were exposed to 0.2% lead acetate for 10 weeks, followed by 10 days of treatment with DMSA and MiADMSA (50 mg kg(-1), i.p., once daily) alone and in combination with C. asiatica (200 mg kg(-1), p.o., once daily). Biochemical variables indicative of oxidative stress and brain biogenic amines, along with lead concentration in blood and brain, were measured. Lead exposure caused a significant depletion of blood and brain delta-aminolevulinic acid dehydratase (ALAD) activity, an important enzyme of the haem biosynthesis pathway, and glutathione (GSH) level. These changes were accompanied by a marked increase in reactive oxygen species (ROS) level, thiobarbituric acid reactive substances (TBARS), delta-aminolevulinic acid synthase (ALAS) and oxidized glutathione (GSSG) activity in blood and brain. Significant depletion of brain noradrenaline (norepinephrine, NE), 5-hydroxytryptamine (5-HT), dopamine (DA) and acetylcholinesterase (AChE) also were observed following lead exposure. Also seen was a significant depletion in brain glutathione peroxidase (GPx), glutathione S-transferase (GST) and monoamine oxidase activity, as well as blood and brain superoxide dismutase (SOD) activity. These biochemical changes were correlated with an increased uptake of lead in blood and brain. Combined administration of MiADMSA and C. asiatica was most effective in reducing these alterations, including biogenic amines, besides reducing body lead burden, compared with individual treatment with MiADMSA. Certain other biochemical variables responded favourably to combination therapy and monotherapy with MiADMSA. Thus, supplementation of C. asiatica during chelation could be recommended for achieving optimum effects of chelation therapy.

Waterborne Lead Exposure Affects Brain Endocannabinoid Content in Male but not Female Fathead Minnows (Pimephales promelas)

There are several similarities between the behavioral and neurochemical effects of lead (Pb2+) and the cannabinoids. Both Pb2+ exposure and cannabinoid treatment decrease exploratory behavior. Pb2+-induced hyperactivity has been observed in rats and fish. By comparison, cannabinoids increase locomotor activity at higher doses in rats. Moreover, Pb2+ exposure produces learning and memory impairments as do the cannabinoids. Many of the behavioral effects of Pb2+ are thought to be due, in part, to the ability of Pb2+ to either inhibit or mimic the actions of calcium (Ca2+). At low concentrations, Pb2+ enhances basal release of neurotransmitter from presynaptic terminals by increasing intracellular free Ca2+ concentrations. Pb2+ also decreases evoked neurotransmitter release due to blockade of voltage-gated Ca2+ channels. Interestingly, the endocannabinoids (eCBs) including N-arachidonylethanolamine (AEA) and 2-arachidonylglycerol (2-AG) are synthesized in response to increases in intracellular Ca2+ and activate the CB1 receptor that inhibits voltage-gated Ca2+ channels. We tested the hypothesis that waterborne Pb2+ exposure significantly affects whole-brain eCB content in adult male and female fathead minnows (Pimephales promelas). Waterborne Pb2+ exposure (1.0 ppm) resulted in a time-dependent accumulation of Pb2+ in bone in both males and females. Brain AEA and 2-AG content were significantly greater in females compared to males. Pb2+ exposure increased brain AEA content in males at 7 and 14 days of exposure and increased brain 2-AG content at 14 days. Pb2+ exposure had no effect on either brain AEA or 2-AG content in females at any of the time points examined. As eCBs serve as activity-dependent retrograde inhibitors of neurotransmitter release, the increase in brain eCB content would accentuate Pb2+-induced decreases in evoked neurotransmitter release in male but not female fathead minnows.

Effects of dietary lead and/or dimercaptosuccinic acid exposure on regional serotonin and serotonin metabolite content in rainbow trout (Oncorhynchus mykiss)

The lead (Pb) chelator, meso-2,3-dimercaptosuccinic acid (DMSA) may be effective in reversing some of the adverse effects of Pb exposure. Pb-induced behavioral deficits observed in fish are due to disruptions in the integrative functioning of the medulla, cerebellum, and optic tectum. Pb exposure increased serotonin (5-HT) content in all three brain regions without an effect on 5-hydroxy-3-indoleacetic acid (5-HIAA). Pb exposure followed by no Pb in the diet increased 5-HT and 5-HIAA content in all three brain regions. The replacement of dietary Pb with DMSA had no effect on 5-HT and increased 5-HIAA content. DMSA increased 5-HIAA content in all three brain regions and increased 5-HT content only the optic tectum. Treatment with DMSA may be more effective than removal of Pb from the diet in reversing Pb-induced alterations in 5-HT.

Short-term exposure to dietary Pb and/or DMSA affects dopamine and dopamine metabolite levels in the medulla, optic tectum, and cerebellum of rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss) were randomly assigned to one of the following dietary exposure conditions: lead (Pb) solvent (2% nitric acid), meso-2,3-dimercaptosuccinic acid (DMSA) solvent (0.1 N NaOH), Pb, DMSA, Pb followed by Pb solvent, or Pb followed by DMSA. Medulla, cerebellum, and optic tectum homogenates were analyzed for dopamine (DA), homovanillic acid (HVA), and 3,4-dihydroxyphenylacetic acid (DOPAC). DA levels in all brain regions tended to be highest for trout exposed to dietary Pb followed by dietary DMSA. DA levels were elevated for trout exposed to dietary DMSA and Pb followed by Pb solvent. DA levels were below control levels for trout exposed to Pb only. HVA levels varied across brain regions. However, HVA levels in all brain regions tended to be elevated for trout exposed to dietary DMSA and Pb followed by Pb solvent. DOPAC levels across all brain regions were below control levels for trout dietary exposed to DMSA, Pb only, Pb followed by Pb solvent, and Pb followed by DMSA. These data indicate that Pb and/or DMSA have the potential of altering DA, HVA, and DOPAC levels in the medulla, cerebellum, and optic tectum. The animal model of short-term dietary exposure to Pb and DMSA, both alone and sequentially, to mimic dietary exposure to Pb and the oral delivery of DMSA, that our laboratory has developed, may be useful in future studies aimed at characterizing the neurobiological mechanisms by which Pb and/or DMSA alter neurotransmitter levels and behavior.

Antidepressant-like effect of lead in adult mice

It has been reported that lead can cause behavioral impairment by inhibiting the N-methyl-D-aspartate (NMDA) receptor complex. MK-801, a noncompetitive NMDA receptor antagonist, exhibits an antidepressant-like action in the forced swimming test. The purpose of the present study was to determine whether subacute lead exposure in adult male Swiss mice weighing 30-35 g causes an antidepressant-like action in a forced swimming test. Mice were injected intraperitoneally (ip) with 10 mg/kg lead acetate or saline daily for 7 consecutive days. Twenty-four hours after the last treatment, the saline and lead-treated mice received an injection of MK-801 (0.01 mg/kg, ip) or saline and were tested in forced swimming and in open-field tests. Immobility time was similarly reduced in the saline-MK-801, Pb-saline and Pb-MK-801 groups compared to the saline-saline group (mean +/- SEM; 197.3 +/- 18.5, 193.5 +/- 15.8, 191.3 +/- 12.3 and 264.0 +/- 14.4 s, respectively; N = 9). These data indicate that lead may exert its effect on the forced swimming test by directly or indirectly inhibiting the NMDA receptor complex. Lead treatment caused no deficit in memory of habituation and did not affect locomotor activity in an open-field (N = 14). However, mice that received MK-801 after lead exhibited a deficit in habituation (22% reduction in rearing responses between session 3 and 1; N = 14) as compared to control (41% reduction in rearing responses; N = 15), further suggesting that lead may have affected the NMDA receptor activity. Forced-swim immobility in a basin in two daily consecutive sessions was also significantly decreased by lead exposure (mean +/- SEM; day 1 = 10.6 +/- 3.2, day 2 = 19.6 +/- 3.6; N = 16) as compared to control (day 1 = 18.4 +/- 3.8, day 2 = 34.0 +/- 3.7; N = 17), whereas the number of crossings was not affected by lead treatment, further indicating a specific antidepressant-like action of lead.