Chronic postweaning lead exposure and response duration performance

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.

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.

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.

Behavioral toxicology and risk assessment

In order to establish safe exposure levels to toxic chemicals, risk assessment guidelines have been developed. These guidelines evaluate epidemiologic and animal research data on a particular chemical, as well as dose-response relationships, animal to human extrapolation and assessment of exposure levels of populations. Using the guidelines, risk characterization is established in order to determine a strategy for reducing undesirable risk to human populations. Using both human neonatal lead exposure data and results from rodent and primate studies, this review examines the possibility that behavioral measurements are sufficient to provide adequate risk assessment guidelines for lead intoxication of the developing organism. The overall trend in these data during the past 10 years has been to show that exposures to inorganic lead at levels previously considered safe have long-lasting significant alterations in behavioral measures, suggesting that central nervous system function has been altered irreversibly. The conclusion is drawn that behavioral toxicology can provide sensitive, quantitative and reliable data for risk assessment and that in the future these methodologies could be used to set exposure guidelines for other neurotoxic chemicals.

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.

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)

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.

Dietary administration of nickel: effects on behavior and metallothionein levels

Adult male rats were fed either 0, 10, or 20 mg Ni/kg body weight (as NiCl2) via a 10 g daily food ration. Following 14 days of exposure, animals were trained over a period of 61 days to lever press for food on a VI-2 operant training schedule while continuing to experience the indicated daily doses. Those rats treated with 20 mg/kg Ni lever pressed at a significantly lower rate than controls. Group 10 mg/kg subjects were not significantly different on this behavioral measure than control subjects. Atomic absorption spectrophotometric analysis revealed a dose/response accumulation of Ni in the kidney, but analyses of blood, bone, brain, hair, small intestine, liver, and testes did not show differential agent accumulations. Neither hepatic nor renal metallothionein levels were increased above control levels. These findings were discussed within the framework of other recent behavioral and biochemical studies of heavy metal toxicity.

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.

The effects of oral cadmium exposure on passive avoidance performance in the adult rat

Step-down passive avoidance acquisition and 24 h retention performance were examined in adult rats given daily doses of either 0, 1, or 5 mg/kg cadmium (Cd) (as CdCl2) via the diet. Results indicated that subjects exposed to the 5 mg/kg Cd diet were less likely to step off a safe platform onto an electrified grid floor than controls. The 1 mg/kg subjects did not differ from controls on this acquisition task. None of the groups showed differential performances on a retention test administered 24 h after acquisition training. These results are discussed in terms of recent claims that Cd exposure enhances emotional reactivity in animals.

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.