Neonatal low-level lead exposure in monkeys: locomotor activity, schedule-controlled behavior, and the effects of amphetamine

A Systematic Review on the Influences of Neurotoxicological Xenobiotic Compounds on Inhibitory Control

Background: Impulsive and compulsive traits represent a variety of maladaptive behaviors defined by the difficulties to stop an improper response and the control of a repeated behavioral pattern without sensitivity to changing contingencies, respectively. Otherwise, human beings are continuously exposed to plenty neurotoxicological agents which have been systematically linked to attentional, learning, and memory dysfunctions, both preclinical and clinical studies. Interestingly, the link between both impulsive and compulsive behaviors and the exposure to the most important xenobiotic compounds have been extensively developed; although the information has been rarely summarized. For this, the present systematic review schedule and analyze in depth the most important works relating different subtypes of the above-mentioned behaviors with 4 of the most important xenobiotic compounds: Lead (Pb), Methylmercury (MeHg), Polychlorinated biphenyls (PCB), and Organophosphates (OP) in both preclinical and clinical models.

Methods: Systematic search strategy on PubMed databases was developed, and the most important information was structured both in text and in separate tables based on rigorous methodological quality assessment.

Results: For Lead, Methylmercury, Polychlorinated biphenyls and organophosphates, a total of 44 (31 preclinical), 34 (21), 38 (23), and 30 (17) studies were accepted for systematic synthesis, respectively. All the compounds showed an important empirical support on their role in the modulation of impulsive and, in lesser degree, compulsive traits, stronger and more solid in animal models with inconclusive results in humans in some cases (i.e., MeHg). However, preclinical and clinical studies have systematically focused on different subtypes of the above-mentioned behaviors, as well as impulsive choice or habit conformations have been rarely studied.

Discussion: The strong empirical support in preclinical studies contrasts with the lack of connection between preclinical and clinical models, as well as the different methodologies used. Further research should be focused on dissipate these differences as well as deeply study impulsive choice, decision making, risk taking, and cognitive flexibility, both in experimental animals and humans.

Developmental stress and lead (Pb): Effects of maternal separation and/or Pb on corticosterone, monoamines, and blood Pb in rats

The level of lead (Pb) exposure in children has decreased dramatically since restrictions on its use were implemented. However, even with restrictions, children are exposed to Pb and still present with cognitive and behavioral deficits. One prominent aspect of the exposome of these children is that many come from low social economic status (SES) conditions, and low SES is associated with stress. In order to compare the combined effects of early stress and Pb, Sprague-Dawley rats were exposed to vehicle or Pb either alone or in combination with maternal separation stress during brain development (i.e., postnatal day (P)4-P11, P19, or P28). Maternally separated/isolated pups had lower body and thymus weights during exposure and had increased levels of blood Pb compared with vehicle controls. Isolation, but not Pb, affected the response to an acute stressor (standing in shallow water) when assessed on P19 and P29, but not earlier on P11. Interactions of Pb and isolation were found on monoamines in the neostriatum, hippocampus, and hypothalamus on turnover but not on levels, and most changes were on dopamine turnover. Isolation had greater short-term effects than Pb. Interactions were dependent on age, sex, and acute stress.

Zebrafish model systems for developmental neurobehavioral toxicology

Zebrafish offer many advantages that complement classic mammalian models for the study of normal development as well as for the teratogenic effects of exposure to hazardous compounds. The clear chorion and embryo of the zebrafish allow for continuous visualization of the anatomical changes associated with development, which, along with short maturation times and the capability of complex behavior, makes this model particularly useful for measuring changes to the developing nervous system. Moreover, the rich array of developmental, behavioral, and molecular benefits offered by the zebrafish have contributed to an increasing demand for the use of zebrafish in behavioral teratology. Essential for this endeavor has been the development of a battery of tests to evaluate a spectrum of behavior in zebrafish. Measures of sensorimotor plasticity, emotional function, cognition and social interaction have been used to characterize the persisting adverse effects of developmental exposure to a variety of chemicals including therapeutic drugs, drugs of abuse and environmental toxicants. In this review, we present and discuss such tests and data from a range of developmental neurobehavioral toxicology studies using zebrafish as a model. Zebrafish provide a key intermediate model between high throughput in vitro screens and the classic mammalian models as they have the accessibility of in vitro models and the complex functional capabilities of mammalian models.

Lead and PCBs as risk factors for attention deficit/hyperactivity disorder

OBJECTIVES

Attention deficit/hyperactivity disorder (ADHD) is the most frequently diagnosed neurobehavioral disorder of childhood, yet its etiology is not well understood. In this review we present evidence that environmental chemicals, particularly polychlorinated biphenyls (PCBs) and lead, are associated with deficits in many neurobehavioral functions that are also impaired in ADHD.

DATA SOURCES

Human and animal studies of developmental PCB or lead exposures that assessed specific functional domains shown to be impaired in ADHD children were identified via searches of PubMed using "lead" or "PCB exposure" in combination with key words, including "attention," "working memory," "response inhibition," "executive function," "cognitive function," "behavior," and "ADHD."

DATA SYNTHESIS

Children and laboratory animals exposed to lead or PCBs show deficits in many aspects of attention and executive function that have been shown to be impaired in children diagnosed with ADHD, including tests of working memory, response inhibition, vigilance, and alertness. Studies conducted to date suggest that lead may reduce both attention and response inhibition, whereas PCBs may impair response inhibition to a greater degree than attention. Low-level lead exposure has been associated with a clinical diagnosis of ADHD in several recent studies. Similar studies of PCBs have not been conducted.

CONCLUSIONS

We speculate that exposures to environmental contaminants, including lead and PCBs, may increase the prevalence of ADHD.

Costs and benefits of enforcing housing policies to prevent childhood lead poisoning

BACKGROUND

Lead poisoning remains an important, yet entirely preventable disease among young children. This article compares the costs and benefits of strict versus limited enforcement of lead poisoning prevention housing policies in preventing additional cases of childhood blood lead elevation.

METHODS

The author conducted decision analysis using population-based data that compared recurrence of childhood lead exposure in 2 urban areas with different enforcement capacity, and cost data from a federal project and from medical and public health literature.

RESULTS

Strict enforcement prevented additional cases, resulting in $45,360 savings from decreased medical and education costs and increased productivity for protected children. The model was robust to changing estimates of followup, housing repairs, relocation, and increases in lead levels over baseline. No cost savings were realized by strict enforcement if the probability of recurrence in limited units was 44 % lower than estimated, or if fewer children were identified in limited versus strict enforcement units. If the discount rate for future productivity losses was > or = 7.5%, strict enforcement did not lower costs.

CONCLUSION

This analysis suggests that strict enforcement of housing policies to prevent childhood blood lead elevation results in decreased societal costs due to the avoidance offuture medical and special education and to increased productivity of resident children.

Lead, attention, and impulsive behavior: changes in a fixed-ratio waiting-for-reward paradigm

Similar to the effects observed in children diagnosed with attention deficit hyperactivity disorder (ADHD), experimental animals exposed to lead (Pb) exhibit behaviors attributed to "impulsivity" and "inability to inhibit inappropriate responding." Such behaviors have led some to suggest that Pb exposure is associated with attention deficit. Based on the hypothesis that attention deficits are related to an ineffectiveness of delayed reinforcement, this study examined the effects of chronic postweaning Pb exposure on an FR waiting-for-reward paradigm. Rats were exposed chronically from weaning to 0, 50, or 150 ppm Pb acetate in water and following 40 days of exposure, trained on a fixed-ratio (FR) wait behavioral baseline. A total of 50 lever press responses (FR 50) produced food delivery. After earning an FR pellet, "free" pellets could be obtained by waiting; emission of another lever press reinitiated the FR requirement. "Free" pellets were delivered at increasing intervals (2 s, 4 s, 6 s, etc.). Pb exposure increased response rates on the FR schedule and decreased the mean longest waiting time, but also resulted in a higher number of responses per reinforcer than exhibited by controls. These Pb-induced differences are consistent with an inability to manage delays of reinforcement.

Low-level lead exposure selectively enhances dopamine overflow in nucleus accumbens: an in vivo electrochemistry time course assessment

Exposures to even very low levels of lead (Pb) alter behavioral and neurochemical functions. The current study was based on the hypothesis that excess synaptic dopamine (DA) availability may contribute to such disturbances and that the mesolimbic DA projection is more sensitive than the nigrostriatal system to Pb-induced DA-based alterations. In vivo electrochemical measurements of potassium chloride-evoked DA overflow and clearance were compared in dorsal striatum (STR) (nigrostriatal system) and nucleus accumbens (NAC)(mesolimbic system) of male rats after 11 weeks or 11 months of postweaning exposure to 0, 50, or 150 ppm Pb acetate drinking solutions. Pb increased evoked DA overflow selectively in NAC, with biphasic effects at 11 weeks, including increases greater than 400% at 50 ppm and concentration-related effects up to 265% of control at 11 months. Considered relative to 11-week control levels, continued exposure tended to attenuate the magnitude of Pb-related increases in DA overflow in NAC. Pb decreased clearance time in both brain regions, with these effects markedly augmented across time. These changes in DA function were observed at blood Pb values of only 15-16 micrograms/dl, underscoring their environmental relevance. The current findings support the hypothesis of excess DA availability as a mechanism of Pb-induced behavioral alterations and of a particular vulnerability of mesolimbic DA systems (NAC) to such effects. They also suggest that different mechanisms underlie Pb-related changes in amplitude and clearance and confirm previous reports of regional differences of DA systems in response to Pb exposure.

Fixed interval/fixed ratio performance in adult monkeys exposed in utero to methylmercury

Previous studies in monkeys and rodents have shown the fixed interval/fixed ratio (FI/FR) schedule to be a sensitive indicator of neurotoxicity. In the present study, monkeys (Macaca fascicularis) were exposed in utero to methylmercury (MeHg). Maternal doses of MeHg of 50, 70, or 90 micrograms/kg b.wt./day resulted in infant blood mercury levels at birth ranging from 1.04 to 2.45 ppm. Monkeys were tested on a multiple FI/FR schedule of reinforcement at 8-10 years of age. Four FI/FR cycles were run per session. Pause time and run rate were calculated for FI and FR components, as well as FI quarter-life and local FI response rates. MeHg treatment and sex effects were investigated by fitting a linear orthogonal polynomial regression to each monkey's profile across sessions and performing two-way ANOVAs on the resulting linear and intercept terms. There were no treatment-related effects on either the FI or FR component for pause time or run rate. Analysis of the quarter-life revealed a significant treatment by sex effect as well as a main effect for sex. Post hoc t-tests revealed a significant difference in quarter-life of treated male and female monkeys and a marginal difference between treated and control males. The FI run rate of the male monkeys was significantly greater than that of the female monkeys whereas the FR run rate of the males was marginally greater. These results indicate that there may be a differential effect of MeHg on male and female monkeys, which could be interpreted as an effect on temporal discrimination. Overall, adult monkeys exposed to in utero MeHg exhibited a very limited sex-related effects on the FI/FR intermittent schedule of reinforcement.

The effects of dopamine agonists on fixed interval schedule-controlled behavior are selectively altered by low-level lead exposure

A previous report of differential effects of catecholaminergic compounds, but not other classes of compounds, on FI (fixed interval) response rates of lead (Pb)-treated pigeons suggests that catecholamine system disturbances might play a role in lead (Pb)-induced changes in FI performance. The current study sought to extend those findings using more selective dopaminergic (DA) D1 and D2-like receptor agonists, Pb-treated rats, and additional classes of compounds. Drug-induced changes in FI performance of rats exposed chronically from weaning to 0, 50, or 150 ppm Pb acetate in drinking water were compared following the administration of drugs known to impact various neurotransmitter systems altered by Pb exposure, including the selective D2-like agonist quinpirole, the D1 agonists SKF38393 and SKF82958, the mu-opioid agonist morphine, the muscarinic cholinergic agonist arecoline, the glutamate agonist NMDA, and the noncompetitive NMDA antagonist MK-801. All drugs except NMDA significantly altered FI performance, but only the effects of DA agonists differed in control and Pb-exposed rats. Pb exposure attenuated the decrements in rates produced by D1 agonists and at 150 ppm modestly altered the rate changes associated with low doses of quinpirole. These data demonstrate functional DA alterations in response to Pb exposure and provide further evidence for the selective involvement of such effects in FI performance.

In utero lead exposure in squirrel monkeys: motor effects seen with schedule-controlled behavior

Timed-pregnant squirrel monkeys were exposed orally to lead during the last 1/2 to 2/3 of gestation such that maternal lead levels ranged from 21 to 70 micrograms/dl in blood. Offspring of these lead-exposed monkeys were compared to gender-matched, untreated controls (blood-lead levels from 4 to 9 micrograms/dl), born at about the same time. When the monkeys were 3 to 7 years old they were trained to pull a T-shaped bar against 1 kg spring through a displacement of 1 cm. This performance was examined during acquisition of different fixed-ratio (1, 5, and 20) and fixed-interval (120", 300", and 600") schedules of reinforcement and during steady state under the fixed-ratio 5 and fixed-interval 600". Monkeys exposed prenatally to lead showed an increased number of responses failing to meet the requirement of pulling against 1 kg spring through a 1 cm displacement when behavior was maintained by a fixed-ratio schedule, which engenders a vigorous, high-rate pattern of responding. This increased number of incomplete responses first appeared in the acquisition of a fixed-ratio 5 and fixed-ratio 20 schedules of reinforcement, remained after the fixed-ratio 5 schedule was allowed to reach steady state, and did not appear under the fixed-interval schedule. Neither body weight not response rate were affected by lead, but it was necessary to control for these variables using multiple regression to isolate lead's effect. The appearance of incomplete responses while the monkeys pulled vigorously against a 1 kg spring suggests that lead exposure during gestation produced subtle motor impairments years after exposure has ended. Deficits in the acquisition of behavior (learning) under Concurrent Random Interval schedules of reinforcement have also been reported with these monkeys. Together, these reports reveal prolonged deficits in learning and motor function resulting from in utero exposure to lead at maternal blood lead levels (21-70 micrograms/dl) that could result from exposure to ambient air in heavily polluted urban environments or in occupational settings meeting current World Health Organization standards.

Neurobehavioral methods used in neurotoxicological research

Exposure to chemicals in the environment and workplace can have adverse effects on the nervous system. Behavioral endpoints are being used with greater frequency in the hazard identification phase of neurotoxicology risk assessment. One reason behavioral procedures are used in animal neurotoxicology studies is that they evaluate neurobiological functions known to be affected in humans exposed to neurotoxic agents, including alterations in sensory, motor, autonomic, and cognitive function. In hazard identification, behavioral tests are used in a tiered-testing context. Tests in the first tier are designed to determine the presence of neurotoxicity. Examples of first-tier behavioral tests include functional observational batteries and motor activity. Second-tier tests are used to characterize neurotoxicant-induced effects on sensory, motor, and cognitive function. Second-tier tests are usually more complex and costly to perform. Reliance on behavioral endpoints in neurotoxicology risk assessment will likely increase in the future.

Behavioral effects of lead in monkeys tested during infancy and adulthood

A total of 12 monkeys (Macaca fascicularis) were dosed orally from birth with 0 or 2000 micrograms/kg/day of lead as lead acetate. Blood lead concentrations of treated monkeys peaked at an average of 115 micrograms/dl by 100 days of age and decreased to a steady state level of 33 micrograms/dl after withdrawal of infant formula at 270 days of age. At 5-6 months of age, they were tested on a nonspatial discrimination reversal paradigm. At 2.5-3.0 years of age, they were tested on a series of nonspatial discrimination reversal problems, including irrelevant cues. As adults, performance was assessed on a differential reinforcement of low rate (DRL) schedule of reinforcement, a spatial delayed alternation task, and during training on a visual discrimination task for a visual psychophysics experiment. There were no or marginal deficits on the discrimination reversal task during infancy. Although lead-treated monkeys were impaired on this task as juveniles, they were less impaired than would have been predicted based on their history of blood lead concentrations. Treated monkeys exhibited decreased interresponse times and a greater ratio of responses per reinforcement on the DRL schedule compared to controls. Four of five treated monkeys were unable to learn the visual discrimination task without a remedial training procedure in which the relevant visual stimuli were arranged to appear as if they were on the response buttons. Treated monkeys were unimpaired on the delayed spatial alternation task. The results are interpreted as suggestive of an interaction between the behavioral history of the monkeys as infants with the results of later behavioral testing.

The three most common occupational exposures reported by pregnant women: an update

Many uncertainties exist in regard to counseling women with occupational exposures during pregnancy. This is due to limited knowledge of the reproductive toxicologic effects of industrial agents, lack of safety standards aimed at protecting the fetus, and limitations in assessing the extent of exposure. The approach to this subject taken by the Motherisk Program and a review of the three most common occupational exposures are presented. Epidemiologic studies and measurements of radiation do not suggest a reproductive hazard for video display terminals. Exposure to organic solvents is hard to quantitate, and information is sparse and sometimes contradictory, and therapeutic decisions are difficult to reach. To date there is no convincing evidence that working with organic solvents within safety regulations would harm a fetus, in contradistinction to detrimental fetal effects of solvent abuse. The reproductive risks of lead are well documented, fetal exposure can be assessed, and effective treatment exists, but its effects on the pregnancy have not been fully established. However, new evidence suggests that maternal levels that are much lower than the accepted adult limits may be damaging to the fetus.

Low level lead exposure increases sensitivity to the stimulus properties of dopamine D1 and D2 agonists

To examine the impact of Pb exposure on dopaminergic (DA) function, weanling rats were chronically exposed to 0, 50 or 250 ppm Pb acetate in drinking water. At 3 months of age, the rats were trained to discriminate the stimulus properties of either the D1 agonist SKF38393 (3.0 mg/kg i.p.; D1/sal) or the D2 agonist quinpirole (0.05 mg/kg i.p., D2/sal) from saline using a standard two-lever operant food-reinforced drug discrimination paradigm. Lead-exposed rats learned the discriminations faster than respective controls. Moreover, they exhibited greater levels of drug lever responding to lower doses of the training drugs (D1/sal and D2/sal), and to selected doses of other direct and indirect DA agonists (D2/sal only), including apomorphine, cocaine and (+)-amphetamine, and less blockade of drug lever responding by haloperidol (D2/sal). Taken together, these findings are consistent with a generalized DA supersensitivity. There were no differential Pb effects when non-DA compounds including morphine, pentobarbital and MK-801 were substituted for the training drugs, indicating the selectivity of the DA effects in the context of these experiments, and the improbability of a non-specific behavioral causation. Pb-exposed rats in the D2/sal group also showed a pronounced enhancement of drug lever responding when NMDA was substituted for quinpirole, suggesting the possibility of a Pb-induced NMDA supersensitivity as well.

Lead-induced behavioral impairment on a spatial discrimination reversal task in monkeys exposed during different periods of development

A total of 52 monkeys (Macaca fascicularis) were dosed orally with vehicle or 1.5 mg/kg/day of lead on one of four dosing regimens (13 monkeys/group): Group 1, vehicle only; Group 2, dosed with lead from birth onward; Group 3, dosed with lead from birth to 400 days of age and vehicle thereafter; Group 4, dosed with vehicle from birth to 300 days of age and lead thereafter. Blood lead concentrations averaged 3-6 micrograms/dl when monkeys were not being dosed with lead, 32-36 micrograms/dl when being dosed with lead and having access to infant formula, and 19-26 micrograms/dl when being dosed with lead after weaning from infant formula. When monkeys were 7-8 years old, they were tested on three spatial discrimination reversal tasks: no irrelevant cues, irrelevant form cues, and irrelevant form and color cues. Fifteen reversals were run for each task. Only Group 2 was impaired in the absence of irrelevant cues, while all three treated groups were impaired in the presence of irrelevant cues. These results are in contrast to results from a series of nonspatial discrimination reversal tasks in these monkeys in which Groups 2 and 4 were impaired and Group 3 was unimpaired. The present results are in agreement with results from another spatial task, delayed alternation, in which all three treated groups were impaired.

Automated behavioral procedures for infant monkeys

A method is described for testing infant monkeys on a variety of operant tasks as soon as they can self-feed, typically within the first week of life. Each infant was housed during the 16-21-hour experimental session in a cage to which operant behavioral equipment was attached. Computer control of the experimental contingencies and data acquisition allowed a relatively large number of monkeys to be tested simultaneously, as well as detailed analysis of response parameters. Infant monkeys are capable of learning a number of tasks that assess learning and memory, including visual discrimination and reversal, simultaneous discrimination, and spatial and nonspatial matching to sample. Infant monkeys also perform like older animals on intermittent schedules of reinforcement. The long experimental sessions allowed determination of feeding pattern over the course of the night. Analyses of these variables have proved sensitive to the effects of developmental exposure to neurotoxicants such as lead and caffeine.

The comparative developmental neurotoxicity of lead in humans and animals

The effects of lead on neurobehavioral development have been extensively investigated in humans as well as animals. This valuable lode of research findings offers a basis for comparing the developmental neurobehavioral toxicity of lead across species and for assessing the validity of animal models of developmental neurotoxicity. Comparisons of human and animal findings suggest that the greatest qualitative similarities involve relatively complex behavioral processes such as cognition and learning. Quantitative comparisons based on dose-response relationships for these endpoints are difficult to make because the relationships are sometimes nonmonotonic (U-shaped) and because blood lead levels may not be directly comparable between species. However, the lowest levels of exposure at which developmental neurobehavioral effects have been observed are similar: 10-15 micrograms/dl in children, less than 15 micrograms/dl in primates, and less than 20 micrograms/dl in rodents. Although the convergence between animal and human findings for other neurobehavioral endpoints is not as striking, sensory-evoked potentials and communicative processes offer two promising areas for continued investigation and cross-species comparison.

Effects of postnatal lead exposure on open field behavior in monkeys

Nursery-reared rhesus monkeys were treated with either no lead or moderate levels of lead from day eight or nine until day 365 after birth. All subjects were at or below 5 micrograms/dl blood lead levels for at least 1.5 years by the time of the present study conducted at 4 years of age. Data were collected in a primate version of the open field test which proved sensitive to several behavioral alterations in the lead-treated monkeys. These significant differences included a failure to habituate as evidenced by number of sectors entered, increased durations of activity and environmental exploration, and a longer latency to enter the open field. Most of these effects were compatible with a hypothesis of hippocampal dysfunction in the lead-treated monkeys and appeared to indicate enduring lead-induced alterations. These results provide further support for a correlation between early lead exposure and hippocampal dysfunction. Additionally, the utility of a primate version of the open field test for behavioral toxicological studies is substantiated.

Sensitive periods for lead-induced behavioral impairment (nonspatial discrimination reversal) in monkeys

A total of 52 nursery-reared monkeys (Macaca fascicularis) were dosed orally with 1.5 mg/kg/day of lead on one of four dosing regimens (13 monkeys/group): Group 1, vehicle only; Group 2, dosed with lead continuously from birth; Group 3, dosed with lead from birth to 400 days of age and vehicle thereafter; and Group 4, dosed with vehicle from birth to 300 days of age and lead thereafter. This dosing regimen allowed evaluation of differential infant vulnerability as well as reversibility of the behavioral toxicity of lead. Blood lead concentrations averaged 3-6 micrograms/dl when monkeys were not being exposed to lead, 32-36 micrograms/dl when being dosed with lead and having access to infant formula, and 19-26 micrograms/dl during lead exposure after weaning from infant formula. When monkeys were 5-6 years old, they were tested on a series of nonspatial discrimination reversal tasks: form, form with irrelevant color cues, color with irrelevant form cues, and alternating form and color. Group 2 exhibited the greatest degree of impairment compared to controls. Group 4 also exhibited impaired performance, although less marked than that of Group 2. Group 3 was not impaired on this series of tasks. These results confirm findings observed in other monkeys exposed continuously to lead and suggest that while exposure beginning after infancy produces impairment, exposure during infancy as well exacerbates the effect.

The persistent threat of lead: medical and sociological issues

Lead exposure is an ancient malady. Its history serves as a useful paradigm through which to understand many other pollutants that our technological society has inserted into the human environment and may guide preventive steps for other agents. Lead poisoning was first recognized in workers exposed to high doses. The discovery of childhood toxicity occurred a century ago in Australia, when children with striking symptoms of paralysis, ophthalmoplegia, or meningitis were found to be highly lead exposed. Encephalopathy generally occurs at blood lead levels of 80 micrograms/dL or more, but unequivocal brain damage has been demonstrated at doses well below this level. At lower doses, the neurocognitive effects of lead are expressed as diminished psychometric intelligence, attention deficits, conduct problems, alterations in the electroencephalogram, school failure, and increased referral rates for special needs. Careful epidemiologic studies, which have controlled for the important confounders, have set the effect level at 10-15 micrograms/dL. Elegant animal studies in which confounding is not an issue have confirmed these findings. Although blood lead levels in the population have dropped over time for a number of reasons, there are some 3-4 million American children with blood lead levels of more than 15 micrograms/dL. Biochemical and functional changes have been demonstrated in the heme biosynthetic pathway and in the renal, cardiovascular, endocrine, immune, and nervous systems. The threshold for effect depends on the sensitivity of the methods used. A no-effect level has not been found. Further, this is not a disease of the poor alone. But the poor are exposed to much more lead than are the more economically favored. Deficiencies in body calcium, zinc, iron, and protein stores are associated with increased uptake. Optimizing nutrition enhances the resistance to lead. All children should be screened for lead at regular intervals, especially those with anemia, growth failure, and developmental or behavioral problems. Treatment protocols are well worked out, but chelation is only part of the therapy. Controlling the environment, strengthening the family's supports, enhancing nutrition, and offering remedial education are essential to a successful therapeutic outcome. Lead control has involved a continuing struggle between vested economic interests and regulatory agencies. In one area, the control of airbone lead, science, and public health have prevailed. In the past decade, the amount of alkyl lead consumed in gasoline additives has been reduced by 99%. Body lead burdens have dropped in close correspondence.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term effects of chronic postnatal lead exposure on delayed spatial alternation in monkeys

Two cohorts of monkeys chronically exposed to lead during the first year after birth and their controls were tested during adulthood for choice accuracy on a learning and memory task, delayed spatial alternation (DSA). Neither cohort showed significant lead-related deficits, as had been seen in a previous experiment with monkeys exposed to similar chronic levels of lead during the first year with an additional high pulse given five-six weeks after birth (18,19). On the contrary, the lead-exposed monkeys in the present experiment actually performed slightly better than controls. In the previous (pulse-chronic) study, the deficit occurred at short intertrial delays, suggesting an attentional rather than mnenomic deficit. A lead-induced decrease in attentiveness could also explain the present results. The lower level lead intoxication may have decreased attentiveness to a lesser degree, so that the monkeys were less susceptible to irrelevant stimuli and performed better.

Quantification of operant behavior

The study of performance on intermittent schedules of reinforcement has proved to be a powerful tool in the fields of experimental psychology and behavioral pharmacology and presently is proving equally valuable in behavioral toxicology. The ability to specify precisely contingencies of reinforcement allows a careful and detailed quantification of performance. Intermittent schedules of reinforcement may be used in behavioral toxicology in a number of ways. A baseline of performance may be established and utilized to monitor acute effects or to track effects of chronic exposure to a toxic agent. Alternatively, schedules of reinforcement may be used in experiments requiring group comparisons, where both terminal performance and acquisition of performance may be of interest. The use of different schedules, generating different rates and patterns of performance, may be compared to elucidate behavioral mechanisms. Use of computers for schedule control and data acquisition allows a detailed analysis of performance, thus increasing the probability of the detection of subtle effects.

Low-level lifetime lead exposure produces behavioral toxicity (spatial discrimination reversal) in adult monkeys

Cynomolgus monkeys (Macaca fascicularis) were dosed from birth with 0, 50, or 100 micrograms/kg/day of lead. This regimen resulted in blood lead concentrations of 3, 15, or 25 micrograms/dl, respectively, before withdrawal of infant formula at 200 days of age. Blood lead concentration declined thereafter over the next 100 to 150 days to steady-state concentrations of 3, 11, or 13 micrograms/dl. At 9 to 10 years of age, these monkeys were tested on a series of spatial discrimination reversal problems. The monkey was required to respond on the right-most of two push buttons in order to receive a fruit-juice reward. When the task was learned, the left-most button became correct for a total of 15 such reversals on each of three tasks. The stimuli for the first task included no irrelevant cues, the second task included irrelevant form cues, and the third task included irrelevant form and color cues. Treated monkeys were impaired relative to controls in the presence but not in the absence of irrelevant cues. Moreover, the lower dose group was impaired only during the first task after the introduction of irrelevant stimuli, but not after irrelevant stimuli were familiar. These findings represent behavioral impairment in adult monkeys as a result of lifetime lead exposure resulting in blood lead concentrations that are typical for humans in industrialized environments.

Pyrethroid effects on schedule-controlled behavior: time and dosage relationships

Pyrethroid insecticides have been divided into Types I and II based on behavioral profiles of toxicity produced by life-threatening dosages. In order to assess potential alterations in acquired (operant) behavior, acute dosage-effect and time-course determinations for permethrin (Type I) and cypermethrin (Type II) were made. Long-Evans rats responded for food according to a multiple schedule consisting of four different variable-interval schedules. Permethrin (100-400 mg/kg) and cypermethrin (7.5-60 mg/kg) were administered PO 1.5 hr pre-session and their effects on response rates and between-component response patterning determined. Permethrin reduced responding in a manner which was independent of the baseline response rate, while the rate reductions following cypermethrin administration showed a dependence on the baseline levels of responding, with low response rates showing differential sensitivity to disruption. When select dosages of each compound were delivered at various pre-session times, onset of and recovery from the rate-decreasing effects were more rapid with cypermethrin, with rates returning to baseline levels by 12 hr post-dosing. Responding was maximally suppressed 24 hr after administration of permethrin and returned to baseline levels 48 hr after administration. The disruption of response patterning following cypermethrin was maximal at 1.5 hr after administration, with complete recovery 12 hr post-dosing. Differential effects on response patterning, in potency, and in the time-course of effects of permethrin and cypermethrin suggest a type-specificity for pyrethroid effects on schedule-controlled behavior at dosages far below those producing lethality in rats.

Longitudinal analyses of prenatal and postnatal lead exposure and early cognitive development

In a prospective cohort study of 249 children from birth to two years of age, we assessed the relation between prenatal and postnatal lead exposure and early cognitive development. On the basis of lead levels in umbilical-cord blood, children were assigned to one of three prenatal-exposure groups: low (less than 3 micrograms per deciliter), medium (6 to 7 micrograms per deciliter), or high (greater than or equal to 10 micrograms per deciliter). Development was assessed semiannually, beginning at the age of six months, with use of the Mental Development Index of the Bayley Scales of Infant Development (mean +/- SD, 100 +/- 16). Capillary-blood samples obtained at the same times provided measures of postnatal lead exposure. Regression methods for longitudinal data were used to evaluate the association between infants' lead levels and their development scores after adjustment for potential confounders. At all ages, infants in the high-prenatal-exposure group scored lower than infants in the other two groups. The estimated difference between the overall performance of the low-exposure and high-exposure groups was 4.8 points (95 percent confidence interval, 2.3 to 7.3). Between the medium- and high-exposure groups, the estimated difference was 3.8 points (95 percent confidence interval, 1.3 to 6.3). Scores were not related to infants' postnatal blood lead levels. It appears that the fetus may be adversely affected at blood lead concentrations well below 25 micrograms per deciliter, the level currently defined by the Centers for Disease Control as the highest acceptable level for young children.

Methodological approaches to primate behavioral toxicological testing

The monkey may often be the best model with which to characterize low-level effects of neurotoxicants, including those produced by developmental exposure. The nervous system of the monkey is very similar to the human and, like humans, monkeys have a long period of gestation, infancy, and sexual immaturity during which the nervous system continues to develop. The sensory systems of monkeys are very similar to humans, and intellectual capabilities may be tested that are not possible using other common laboratory species. The effects of low-level developmental exposure to neurotoxicants have been studied in the cynomolgus monkey using operant conditioning techniques to detect subtle defects. Intermittent schedules, a standard tool in behavioral pharmacology, have proved sensitive indicators of toxicity produced by developmental lead exposure. Such intellectual functions as learning, memory, adaptability, and distractibility have been explored by techniques including discrimination reversal, matching to sample, and delayed alternation. Such techniques have revealed impairment produced by lead similar to that observed in lead-exposed children. Visual deficits produced by methylmercury have been revealed by psychophysical techniques, in the absence of any obvious signs of toxicity.

Psychopharmacological investigations of a lead-induced long-term cognitive deficit in monkeys

This study investigated pharmacological manipulations of the cholinergic (ACh) and dopaminergic (DA) transmitter systems in monkeys with a long-term lead-induced cognitive deficit on delayed spatial alternation (DSA). Both ACh and DA have been found to be affected by developmental lead exposure and to be involved with performance on spatial learning and memory tasks. The lead-induced deficit in performance accuracy on DSA persisted throughout the 2 years of this experiment, which ended more than 8 years after the end of the postnatal lead exposure. Acute administration of agonists and antagonists of the ACh and DA systems did not elicit differential effects from the lead-exposed and control groups in terms of DSA per cent correct performance. The ACh antagonist, scopolamine, caused a dose-related decline in performance in both groups. Significant amelioration of the lead-induced DSA deficit was achieved by chronic treatment with the DA agonist, L-dopa. After withdrawal from L-dopa, the lead-related deficit reappeared. Improvement in performance of the lead-treated group was also seen after chronic amphetamine administration, but this effect was not significant. These data implicate DA mechanisms in the long-lasting cognitive effects of developmental lead exposure. The alleviation of the deficit with chronic administration of a DA precursor points to a possible line of treatment for the cognitive effects of developmental lead exposure.

The behavioral effects of prenatal and early postnatal lead exposure in the primate Macaca fascicularis

Female Macaca fascicularis monkeys were given daily oral doses of 3.0 and 4.25 mg/kg lead, as lead nitrate, prior to breeding, and exposure was maintained throughout gestation and terminated at parturition. No overt signs of lead toxicity were observed in the maternal monkeys. The offspring were tested at 6 to 18 months of age on a three-choice non-spatial form discrimination paradigm, and at 19 to 26 months of age on a response inhibition task. When compared to controls, the lead-exposed infants showed a deficit in form discrimination performance which continued during the entire period of testing. A deficit in response inhibition performance was also observed in exposed infants when compared to controls. However, the deficit was temporary, as the performance of all groups was similar at the termination of testing. The observation of a persistent deficit in form discrimination up to 18 months following the termination of exposure suggests that lead-induced behavioral changes may be permanent. These findings are cause for concern since behavioral deficits in the offspring were observed at maternal gestational whole blood lead levels within a concentration range (30 to 70 micrograms/dl) associated with occupational exposure.

Discriminative stimulus properties of amphetamine and other stimulants in lead-exposed and normal rats

The present study examined the discriminative stimulus properties of amphetamine (AMP) at progressively lower doses in lead-exposed and normal rats. In addition, generalization gradients of AMP, apomorphine, methylphenidate, and caffeine to both high and low training doses of AMP were determined in these rats. Under the high AMP training dose condition (1.0 mg/kg, IP) generalization gradients of AMP were similar for lead-exposed and control rats. When the training doses were progressively lowered, the lead-exposed rats tended to require a higher range of AMP doses (0.24-0.49 mg/kg) than did control rats (0.18-0.32 mg/kg) to maintain discriminative control. In parallel with this, the minimal discriminable doses tended to be higher for lead-exposed rats than for control rats. Methylphenidate generalization gradients were different for lead-exposed and control rats under the high AMP training condition but became similar under the low AMP training condition. No differences attributable to training dose or lead exposure were evident for apomorphine or caffeine.

Chronic low-lead exposure from birth produces deficits in discrimination reversal in monkeys

Cynomolgus monkeys (Macaca fascicularis) were dosed from birth with 100, 50, or 0 micrograms/kg/day of lead. This protocol resulted in blood lead concentrations of 25, 15, or 3 micrograms/dl, respectively, before withdrawal of infant formula at 200 days of age. Blood lead concentration declined thereafter over the next 100 to 150 days to steady-state levels of 13, 11, or 3 micrograms/dl. At approximately 3 years of age, monkeys were tested on a series of three discrimination reversal tasks: nonspatial form discrimination, nonspatial color discrimination with irrelevant form cues, and nonspatial form discrimination with irrelevant color cues. The higher dose group was impaired relative to controls over the entire experiment (all three tasks combined), the two form discrimination tasks combined, and the form discrimination with no irrelevant cues. Deficits were most marked over the first several reversals. The lower dose group was impaired on the color discrimination task and on the last several reversals of all tasks combined. In addition, the higher dose group was impaired relative to the lower dose group over the entire experiment.

Early postnatal lead exposure: behavioral effects in common tern chicks (Sterna hirundo)

Exposure to lead early in life is known to affect behavioral and intellectual development. To develop an animal model we chose the common tern, Sterna hirundo, a species whose early developmental landmarks are well known. One potential for avian models lies in the reliance of birds on visual and acoustic rather than olfactory (and ultrasonic) modes of communication. One randomly chosen member of each of 8 pairs of young common tern chicks was injected with lead nitrate solution at a concentration of 0.2 mg/g. The pairs were not siblings but were matched for age (+/- 1 d) and weight (+/- 3 g). The second member of each pair was injected with an equal volume of sterile saline. Behavioral tests performed examined locomotion, balance and righting response, feeding tasks and begging, depth perception and response on a visual cliff, and behavioral thermoregulation. In each pair the control chick was heavier at 4 wk of age. For most behavioral measures, except begging and movement on a stationary incline, the lead-injected chicks performed less well than the control chicks. When presented with a novel feeding situation (reversal of fish position), the lead-injected chicks required significantly more time to eat the same number of fish. The single injection of lead thus affected a variety of behavioral patterns, with effects apparent within 5 d after injection.

Behavioral deficit (delayed matching to sample) in monkeys exposed from birth to low levels of lead

Monkeys (Macaca fascicularis) were dosed po from birth with 500 micrograms/kg/day of lead as lead acetate. Blood levels peaked at an average of 55 micrograms/dl by 100 days of age, and dropped after 200 days of age to a steady-state level averaging 33 micrograms/dl. No overt signs of lead toxicity were observed. Between 3 and 3.5 years of age, monkeys were tested on both a nonspatial (color) and a spatial delayed matching to sample paradigm. For the nonspatial paradigm, the monkey was required to press a button that was lit with one of three colors a specified number of times, which turned the light off. After a specified delay period (varying from 0 seconds to several minutes), three test buttons were lit, one with each of the three colors. The monkey pressed the button corresponding to the color that had appeared on the sample button in order to receive a fruit juice reward. For the spatial matching to sample, one of the three test buttons was lit. The monkey responded on this button a specified number of times, which turned the light off. After a predetermined delay of variable duration, all three test buttons were lit and the monkey responded on the previously lit one in order to be reinforced. The treated monkeys were impaired relative to controls on both the spatial and nonspatial matching tasks, even though they learned the matching tasks as readily as controls and performed as well as 0 sec delay.

Delayed behavioral toxicity of lead with increasing exposure concentration

Twenty-one-day-old male Long-Evans hooded rats were exposed chronically to drinking solutions containing 500 ppm sodium acetate (controls) or 50, 100, or 500 ppm lead acetate. Performance on a fixed interval 1-min schedule of food reinforcement was assessed over 155 sessions. Blood lead values were monitored serially and brain lead determinations made at the end of testing (335 days of lead exposure). The lowest concentration of lead in the drinking water was associated with increases in the rate of fixed-interval responding over the first 30 sessions. At the two higher concentrations, response rate values similar to controls over the first 40 sessions were followed by rate increases. The latency to maximum rate depended on concentration; the highest concentration was associated with the longest latency. Response rates of rats exposed to 50 ppm lead gradually returned to control levels after 120 sessions, while increased rates were sustained in rats at 100 and 500 ppm lead, even 100 days after lead exposure was terminated. Marked individual differences in susceptibility to lead-induced rate increases were observed in all treatment lead values also reflected exposure concentrations. Blood-brain ratios averaged 0.743 to 0.913, which agree with other data for the rat and human. These results confirm the vulnerability to lead of rats beyond the neonatal period, and extend the range of conditions under which such effects occur.

Reversal learning deficits in young monkeys exposed to lead

The reversal learning capacity of young rhesus monkeys in visual discrimination tasks was examined during daily exposure to dietary lead acetate throughout the first year of life. While not affected in physical development, all lead-treated monkeys showed performance deficits on reversal learning tasks. These deficits were independent of lead-induced changes in motivation. Over a series of problems, the overall learning rate of monkeys with blood lead concentrations in the range of 70-90 microgram/dl was retarded, which resulted partly from a pronounced difficulty in attaining criterion on the first of a series of reversals within a given problem. This latter deficit resulted from an increase in errors, balks, and total trials to criterion on the first reversal. Monkeys exposed to blood lead concentrations of 40-60 microgram/dl required significantly more trials to finish all problems, but did not show the first-reversal deficit. Theoretical implications of these data were discussed.