Reversal learning deficits in young monkeys exposed to lead

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.

Feasibility of repeated testing for learning ability in juvenile primates for pediatric safety assessment

Assessment of learning ability in nonhuman primate (NHP) models is sometimes requested by regulatory authorities. The double choice object discrimination task using a Wisconsin General Testing Apparatus (WGTA) approach is typically being applied. In this study, the WGTA approach was performed on 66 juvenile cynomolgus monkeys aged 8-9 months in the predose phase of juvenile toxicity assessment. In addition, reversal learning data of seven control animals/gender were obtained for the weeks 25 and 52 of dosing. Gender differences in the number of days required to pass the habituation, learning or reversal learning phases were statistically comparable, males and females may be combined for statistical analysis. At first instance, the habituation phase was passed on average after 6.4 days, and the learning test on average after 8.6 days with improvement to 2.0-2.6 days for habituation and 6.4-6.7 days for learning in weeks 52. Power analysis (α = 0.05, one-sided t-test) revealed a sample size of 8 and 41 to predict a 50% and 20% difference, respectively. In conclusion, examination for learning ability, but not for memory ability (during repeated testing) is feasible in juvenile NHPs using the WGTA approach.

Object discrimination reversal as a method to assess cognitive impairment in nonhuman primate enhanced pre- and postnatal developmental (ePPND) studies: statistical power analysis

An important aspect of the enhanced pre- and postnatal developmental (ePPND) toxicity study in nonhuman primates (NHP) is that it combines in utero and postnatal assessments in a single study. However, it is unclear if NHP ePPND studies are suitable to perform all of the evaluations incorporated into rodent PPND studies. To understand the value of including cognitive assessment in a NHP ePPND toxicity study, we performed a power analysis of object discrimination reversal task data using a modified Wisconsin General Testing Apparatus (ODR-WGTA) from two NHP ePPND studies. ODR-WGTA endpoints evaluated were days to learning and to first reversal, and number of reversals. With α = 0.05 and a one-sided t-test, a sample of seven provided 80% power to predict a 100% increase in all three of the ODR-WGTA endpoints; a sample of 25 provided 80% power to predict a 50% increase. Similar power analyses were performed with data from the Cincinnati Water Maze (CWM) and passive avoidance tests from three rat PPND toxicity studies. Groups of 5 and 15 in the CWM and passive avoidance test, respectively, provided 80% power to detect a 100% change. While the power of the CWM is not far superior to the NHP ODR-WGTA, a clear advantage is the routine use of larger sample size, with a group of 20 rats the CWM provides ~90% power to detect a 50% change. Due to the limitations on the number of animals, the ODR-WGTA may not be suitable for assessing cognitive impairment in NHP ePPND studies.

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.

Sex, status, and criminality: a theoretical nexus

This article offers a theoretical explanation for relationships between social status and involvement in serious and persistent criminal behavior from an evolutionary perspective. The theory's central premise is that natural selection has produced females who bias their mating choices toward males who strive for status. This bias has resulted in males devoting greater time and energy to status striving (relative to females). To account for why nearly all "victimizing" forms of criminality are more common among males than among females, the theory asserts that status striving exists along a continuum of competitive/victimizing behavior. One end of this continuum is epitomized by crude (criminal) forms of the behavior that societies generally discourage and even punish. The other end consists of sophisticated (commercial) forms that societies tolerate and even encourage. According to the theory, most males begin to exhibit non-playful forms of competitive/victimizing behavior around the onset of puberty as they start their reproductive careers. Adolescent males with the greatest abilities to learn will transition quickly from crude forms of competitive/victimizing behavior to more sophisticated forms, while males who have the greatest difficulties learning will transition more slowly. A major deduction from the theory is that genes on the Y-chromosome must be affecting the brain in ways that promote status-striving behavior. This deduction needs empirical scrutiny, although it is consistent with evidence (a) that the Y-chromosome transforms would-be ovaries into testes, the latter being specialized organs for the production of testosterone, and (b) that testosterone alters brain functioning in ways that contribute to both status striving and criminality.

Methods for studying nonhuman primates in neurobehavioral toxicology and teratology

The behavioral repertoire of nonhuman primates is highly evolved and includes advanced problem-solving capabilities, complex social relationships, and sensory acuity equal or superior to humans. These factors make nonhuman primates valuable animal models for studies of the functional effects of neurotoxicants. This review provides descriptions of tests designed to study learning, memory, schedule-controlled behavior, information processing, social behavior, sensory functioning, and visual-motor coordination and/or visuospatial orientation in macaque monkeys. Whenever possible, the results of studies in primate behavioral toxicology are provided for individual test measures. The primate model is especially useful for studies of developmental exposures because monkeys, like humans, have relatively prolonged periods of gestation, infancy, and adolescence. In recognition of this, a special section is provided for tasks that are specifically designed to study behavioral processes in infant monkeys.

Enduring effects of early lead exposure: evidence for a specific deficit in associative ability

Long-Evans dams were exposed to Pb acetate in the drinking water during both gestation and lactation, or lactation only. This report presents the results of an automated, olfactory, serial reversal task administered to the adult offspring. Although overall learning rate was not significantly affected by Pb exposure, analyses of specific phases of the learning process revealed that all three exposed groups required significantly more trials than controls to reach criterion from the point at which perseverative responding to the previously correct cue ended. These in-depth analyses revealed that the reversal learning impairment of the Pb-exposed animals was not due to a deficit in inhibiting responses to the previously correct cue, the mechanism commonly assumed to underlie impaired reversal learning. Instead, the analyses revealed that two other independent Pb effects were responsible for the prolonged postperseverative learning period: a response bias and an impaired ability to associate cues and/or actions with affective consequences. The contribution of these two factors varied as a function of the timing and intensity of the Pb exposure. It is hypothesized that the Pb-induced associative deficit may reflect lasting damage to the amygdala and/or nucleus accumbens, which comprise a system thought to modulate the process by which environmental cues acquire affective significance.

Lead effects on neurobehavioral development in the neonatal rhesus monkey (Macaca mulatta)

Effects of lead exposure on behavioral development during the first month of postnatal life were examined in rhesus monkeys using a multi-item assessment scale developed for the evaluation of neonatal rhesus monkeys. Lead was administered daily beginning at day 8 postpartum at levels that produced blood lead levels of about 20 microg/dl by week 4 (n = 48); controls were treated identically but given vehicle only (n = 24). All monkeys were tested once a week for the first 4 weeks postpartum. The first principal component explained a substantial portion of the variance and was relatively consistent across ages for both groups. Analyses of the individual items and of both conceptually derived and empirically defined summary scores yielded no significant effects of lead. Furthermore, there were no systematic relationships between blood lead level and performance on the test. Correlation coefficients indicated more similarity across age for control monkeys than for lead-exposed monkeys suggesting that continuity of development, as measured by this test, was disrupted by lead. The relationship between outcome on these early assessments and later behavior will be explored in subsequent studies of these monkeys.

Operant test battery performance in children: correlation with IQ

The relationship between intelligence and money-(nickel-)reinforced operant behaviors were compared in 115 six year old children. The Operant Test Battery (OTB) consists of tasks thought to engender responses dependent upon specific brain functions that include motivation, color and position discrimination, learning, short-term memory, and time estimation. OTB endpoints were compared with Full Scale, Verbal and Performance IQ scores. Highly significant correlations were noted between several OTB measures (e.g., color and position discrimination accuracy) and IQ scores, but not in others (e.g., motivation task response rate). The results demonstrate the relevance of these measures as metrics of important brain functions. Additionally, since laboratory animals can readily perform these same tasks, these kinds of behaviors in laboratory animals should be useful in studying the effects of neuroactive/neurotoxic compounds on aspects of cognitive function in animals and in predicting adverse effects of such agents on related brain functions in humans.

Lead-induced changes in NMDA receptor complex binding: correlations with learning accuracy and with sensitivity to learning impairments caused by MK-801 and NMDA administration

This study sought to further evaluate potential mechanistic relationships between Pb-induced alterations in glutamate neurotransmission and behavioral toxicity. It examined correlations between Pb-induced changes in [3H]MK-801 and [3H]CGP-39653 binding sites in 4 different brain regions (frontal cortex, dentate gyrus, CA1 and striatum) and (1) changes in learning accuracy on a multiple repeated acquisition and performance schedule, and (2) sensitivity to the accuracy-impairing effects of MK-801 and NMDA on this learning baseline. All data were obtained from a single population of rats that had been chronically exposed from weaning to 0, 50 or 250 ppm Pb acetate in drinking water and demonstrated selective learning impairments and altered sensitivity to the effects of MK-801 and NMDA on learning accuracy. Pb exposure decreased MK-801 binding and possibly increased CGP-39653 binding, effects statistically significant in some brain regions, but generally exhibiting similar trends across regions. At 0 ppm, higher levels, particularly of MK-801 binding, were associated with higher accuracy levels in the learning paradigm and with greater decrements in learning accuracy following MK-801 or NMDA administration. These linear correlations were negated and in some cases even reversed by 50 and 250 ppm Pb, an effect that might be attributable to an alteration of NMDA receptor complex subunit composition and thus, ligand binding. Of the 4 brain regions examined, striatal MK-801 binding proved to be the best predictor of learning accuracy levels. These data provide additional support for an involvement of the NMDA receptor complex in Pb-induced learning impairments. The fact that these effects were noted most frequently in striatum also raises the possibility that dopamine-glutamatergic interactions contribute to Pb's effects.

Subsensitivity of lead-exposed rats to the accuracy-impairing and rate-altering effects of MK-801 on a multiple schedule of repeated learning and performance

Exposure to lead (Pb) has been reported to inhibit MK-801 binding and to alter other NMDA receptor complex-associated functions. These reported changes are provocative since both NMDA receptor antagonism and Pb exposure are know to impair learning processes. Whether the Pb-induced changes in NMDA function relate to the learning impairments associated with Pb exposure, however, has not been explored. The contention of this study was that if changes in NMDA function produced by Pb serve as the basis of Pb-associated learning impairments, then such changes should be of sufficient biological magnitude and clinical relevance to induce alterations in sensitivity at the level of the whole animal, i.e., changes in behavioral sensitivity to glutamatergic compounds. Thus, in this study, dose-effect curves of control and Pb-treated rats working on a multiple schedule of repeated learning (repeated acquisition, RA) and performance (P) were compared following acute administration of MK-801, the non-competitive NMDA antagonist. Based on the nature of the reported effects of Pb on NMDA systems, it was expected that the curves of Pb-exposed rats would be right-shifted relative to controls, if differential behavioral sensitivity was evident. Rats were chronically exposed to 0, 50 or 250 ppm Pb acetate in drinking water from weaning and trained on the multiple RA and P schedule beginning at 55 days old. The RA component required the rat to learn a new 3-member sequence of responses during each experimental session (center right left, RLC, CLR, RCL, or LRC), while the correct sequence of responses for the P component was constant across sessions (LCR), requiring performance of an already learned response. Acute administration of MK-801 (0.05-0.3 mg/k, i.p.) resulted in decrements in accuracy in both the RA and P components of the schedule, indicative of non-specific effects on behavior rather than selective effects on learning. The declines in accuracy during the RA component of the schedule were primarily the result of increased perseverative responding, i.e., repetitive responding on a single lever. Both the decline in RA accuracy and the increases in perseverative responding produced by MK-801 were attenuated by Pb exposure. Moreover, dose-effect curves relating MK-801 dose to changes in rates of responding were significantly shifted to the right in Pb-exposed rats relative to controls. Taken together, these data demonstrate a subsensitivity of Pb-exposed rats to both the accuracy-impairing and response rate-altering properties of MK-801.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Use of the lesion model for examining toxicant effects on cognitive behavior

It is often beneficial to use a model to help understand unknown effects and relate those effects to an existing body of knowledge. In much of the early development of behavioral toxicology, the pharmacological model has served as a valuable theoretical guide, especially with regard to dosing and kinetic parameters. However, as with any model, it has certain limitations. The lesion model has complementary features which provide valuable insights into the behavioral effects of toxicants. This is particularly true for effects which persist long after the end of toxicant exposure. There is much literature describing effects of brain lesions on behavior. By comparing results from toxicology studies to those of lesion studies, one can take advantage of this trove of information to gain a better insight into the possible loci of toxic effects, and to identify tests which would be useful in further describing the nature of the toxic effects. In this article, we examine the theoretical and practical utility of the lesion model. Examples are given showing how it has proven useful in interpreting the cognitive effects of exposure of monkeys to lead and polychlorinated biphenyls (PCBs). These exposures produced syndromes that closely resemble the effects of lesions in the frontal cortex or limbic system.

Serial spatial reversal learning in rats: comparison of instrumental and automaintenance procedures

Serial reversals of a spatial discrimination were trained in rats under automaintenance conditions, in which food reward occurred regardless of responding. This automaintained reversal learning was compared to instrumental reversal learning in other rats trained under a similar procedure which required responding for reward. In the automaintenance (AU) procedure, rats received food after every retraction of a "positive" response lever (S+); retraction of a second, "neutral" lever (So) was not paired with food delivery. Responses to the S+ were elicited at fairly constant rates during daily 100-trial conditioning sessions. Responses to the So occurred early in each session but rapidly diminished across trials. When the valences of the levers were reversed, responding shifted to the new S+ and diminished on the new So. Criterion for reversal was defined as a discrimination ratio (DR) of at least 90% responding to the S+ in two consecutive 10-trial blocks. With repeated reversals, acquisition of criterion performance occurred with increasing rapidity, reaching an asymptote below that required for the original discrimination. A second group of rats was trained on a similar instrumental schedule, in which at least one response to the S+ was required for food delivery. Response rates in this instrumental (IN) group were approximately double those of the AU group. However, ratios of S+ to So response rates were similar to those of the AU group, and the serial reversal curves generated were qualitatively similar. Thus rats can show improvement across serial reversals of a spatial discrimination based entirely on pairings of stimulus events (automaintenance), in a manner similar to that observed in instrumental procedures, in which reward is contingent upon correct responding.

Toxic effects of lead on neuronal development and function

The effects of lead on the development of the nervous system are of immediate concern to human health. While it is clear that lead can affect neuronal development at levels of exposure within the range found in the environment, the particular mechanism of the disruption is not readily ascertained. Lack of knowledge of the mechanisms of lead-induced damaged hampers its treatment and prevention. The goal of our research is to develop a model system in which the effects of lead on central nervous system development can be demonstrated. The complexity of the brain hampers such investigations because often it is not clear if apparent toxic effects represents changes secondary to somatic changes, such as endocrine or hematological defects, that could alter brain development, or even transneuronal effects caused by toxicity at a distal site that deprives a brain area of a synaptic input needed for its proper development. A related problem is the redundancy of compensatory systems in the brain. Such system may disguise the severity of the initial toxic insult and themselves can cause functional disturbances. To study neuronal development in a system that minimizes such difficulties, we have grafted discrete brain regions derived from rat fetuses into the anterior chamber of the eye of adult hosts. The brain pieces continue organotypic development of the eye, but are isolated from possible secondary changes due to alterations in the development of the endocrine and other somatic systems because the adult host has these systems already fully developed. Similarly, effects mediated by connecting brain areas are minimized since the transplant is isolated in the anterior chamber of the eye.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

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.

Selective blockade of NMDA-activated channel currents may be implicated in learning deficits caused by lead

The effect of Pb2+ on glutamate receptor activity in rat hippocampal neurons was investigated with a view of explaining the cognitive and learning deficits produced by this heavy metal. Pb2+ (2.5-50 microM) selectively inhibited N-methyl-D-aspartate (NMDA)-induced whole-cell and single-channel currents in a concentration-dependent but voltage-independent manner, without significantly altering currents induced by either quisqualate or kainate. The frequency of NMDA-induced channel activation was decreased by Pb2+. Neither glycine (10-100 microM), nor Ca2+ (10 mM) reversed the effect of Pb2+. Pb2+ also inhibited the [3H]MK-801 binding to rat hippocampal membranes in vitro. The elucidation of the actions of Pb2+ on the NMDA receptor ion channel complex provides important insights into the clinical and toxic effects of this cation.

Glutamate in the mammalian CNS

The excitatory amino acid glutamate plays an important role in the mammalian CNS. Studies conducted from 1940 to 1950 suggested that oral administration of glutamate could have a beneficial effect on normal and retardate intelligence. The neurotoxic nature of glutamate resulting in excitotoxic lesions (neuronal death) is thought possibly to underlie several neurological diseases including Huntington's disease, status epilepticus. Alzheimer's dementia and olivopontocerebellar atrophy. This neurodegenerative effect of glutamate also appears to regulate the formation, modulation and degeneration of brain cytoarchitecture during normal development and adult plasticity, by altering neuronal outgrowth and synaptogenesis. In addition to its function as a neurotransmitter in several regions of the CNS, glutamate seems to be specifically implicated in the memory process. Long-term potentiation (LTP) and long-term depression (LTD), two forms of synaptic plasticity associated with learning and memory, both involve glutamate receptors. Studies with antagonists of glutamate receptors reveal a highly selective dependency of LTP and LTD on the N-methyl-D-aspartate and quisqualate receptors respectively. The therapeutic value of glutamate receptor antagonists is being actively investigated. The most promising results have been obtained in epilepsy and to some extent in ischaemia and stroke. The major drawback remains the inability of antagonists to permeate the blood-brain barrier when administered systemically. Efforts should be directed towards finding antagonists that are lipid soluble and able to cross the blood-brain barrier and to find precursors that would yield the antagonist intracerebrally.

Effects of chronic developmental lead exposure on monkey neuroanatomy: visual system

The effects of lead on specific areas of the visual system were examined in two groups of monkeys (Macaca fascicularis). The first group (N = 3) received 2000 micrograms Pb/kg/day from infancy onward, while the second group (N = 4) received 25 micrograms Pb/kg/day from birth onward. Monkeys were killed at approximately 6 years of age. Areas of the visual system, including optic nerve, lateral geniculate nucleus, and primary area V1 and one visual projection area V2, were examined by a combination of light and electron microscopy and Golgi impregnation. No effect of lead on optic nerve was identified, nor were there consistent differences in the lateral geniculate nucleus. Within areas V1 and V2, the neuronal volume density was significantly reduced in the high dose compared to the low dose group. Moreover, analysis of the dendritic arborization by Golgi analysis revealed a relative decrease in the number of arborizations among pyramidal neurons in both areas V1 and V2. These data suggest that lead exposure beginning during the early postnatal period may result in changes in cytoarchitecture in visual areas V1 and V2.

Chronic neuroleptic effects on spatial reversal learning in monkeys

Cebus apella monkeys were chronically administered the antipsychotic drug fluphenazine decanoate for periods ranging from 3.5 to 5.5 years. In the present study, four of these monkeys and two controls were tested for cognitive abilities on a spatial learning task, which consisted of an original discrimination and four reversals of that discrimination. No effect of fluphenazine administration was seen in the rate of learning the original discrimination, but the carryover of learning across discrimination reversals was significantly reduced by fluphenazine. After overtraining on the original discrimination, the controls showed the normal difficulty in learning the first reversal. The fluphenazine-treated monkeys showed no such disruption. On subsequent reversals, the controls showed continually improving performance, so that on the third and fourth reversals they had near-perfect scores. On the other hand, the fluphenazine-treated monkeys showed no change over the four reversals. Unlike normal monkeys, their learning did not improve with practice. Although simple forms of learning seem to be relatively unaffected by chronic fluphenazine administration, more complex learning is disrupted.

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.

Behavioral effects of developmental lead exposure in rhesus monkeys

Postnatal lead exposure has been found to cause long-term learning and memory deficits in monkeys. Pulse-chronic exposure, consisting of acute high-level exposure followed by chronic lower-level exposure, has been particularly effective in causing these impairments. We investigated possible antecedents of lead-induced cognitive dysfunction by evaluating the behavioral effects of pulse-chronic lead exposure in rhesus monkeys during the first 6 months of postnatal life. Blood lead concentrations in the monkeys reached a peak of 55.8 +/- 7.8 ug/dl during week 5 after birth and then averaged between 33.1 and 42.9 ug/dl during the rest of the first 6 months after birth. Zinc protoporphyrin levels were increased by lead exposure, but hematocrits were unaffected. Significant lead-related effects were detected on a visual exploration test and a neonatal behavioral assessment battery. Lead-treated monkeys exhibited decreased looking behavior on the visual exploration test and decreased muscle tonus and increased arousal or agitation on the behavioral assessment battery. No effects were seen on a Piagetian object permanence task and no toxic effects on health or growth were detected. In addition to providing indices of behavioral dysfunction during postnatal lead exposure, performance on these early behavioral tests may predict later lead-induced cognitive dysfunction.

Lifetime low-level lead exposure produces deficits in delayed alternation in adult monkeys

Cynomolgus monkeys (Macaca fascicularis) were dosed continuously from birth onward with 100, 50, or 0 micrograms/kg/day of lead. This 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 concentrations declined thereafter over the next 100-150 days to steady-state concentrations of 13, 11, or 3 micrograms/dl. At seven to eight years of age, monkeys were tested on a delayed alternation task. The task required the monkey to alternate responses between two pushbuttons; each alternation was rewarded with a small amount of apple juice. After each monkey learned the task, a delay was instituted between trials. The initial delay was 100 msec, and was increased in steps to 15 sec by the end of the experiment. Treated monkeys were impaired in their ability to learn the alternation task, but were not different from controls at short delay values (1 and 3 sec). At longer delay values (5 and 15 sec), treated monkeys again exhibited impairment. At the 15 sec delay value, some individuals in both treated groups exhibited marked perseveration, responding on the same button in some instances for hours at a time. Treated monkeys were also more variable in their performance across sessions than were controls. The data are interpreted as indicative of spatial learning and short-term memory deficits in the lead-exposed monkeys.

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.

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.

Effect of lead on Na+,K+ATPase activity in the developing brain of intra-uterine growth-retarded rats

Lead (Pb) intoxication in developing mammals, including humans, produces serious brain damage. In addition, it is known that nutritional status influences the susceptibility to Pb toxicity. We developed an in utero undernutrition model based on restriction of blood supply to fetuses on d 17 of pregnancy (IUGR rats). The aim of this study was to investigate in vitro the possible effect of Pb on Na+, K+ATPase activity in the brain of developing IUGR and control rats from 6 to 60 d after birth. In addition, we measured the stimulation of Na+, K+ATPase by the monoamines noradrenaline and serotonin. Our results show that: The neurotoxic effect of Pb is an age-related phenomenon. Both IUGR and control rats were more sensitive to Pb in the first week of life. In adults, Pb had a weak inhibitory potency; the delayed matured brain in IUGR animals seemed less sensitive to Pb when compared to age-paired control rats; in the IUGR group, at 15 and 22 d, low doses of Pb had a stimulatory effect on Na+, K+ATPase instead of an inhibitory effect; noradrenaline and serotonin stimulated Na+, K+ATPase activity to an equivalent extent, but this was greater in IUGR than control rats; and at low Pb concentrations, the studied monoamines reversed Pb-induced inhibition.

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.

Low lead exposure from birth produces behavioral toxicity (DRL) in monkeys

Cynomolgus monkeys (Macaca fascicularis) were dosed from birth with 100, 50, or 0 micrograms/kg/day of lead. This treatment resulted in blood lead concentrations of 25, 15, or 3 micrograms/dl, respectively, before withdrawal of infant formula at 200 days of age, and steady-state concentrations of 13, 11, or 3 micrograms/dl. At approximately 3 years of age, monkeys were tested on an intermittent schedule, differential reinforcement of low rate (DRL). This schedule required the monkey to withhold responding for a specific time in order to be reinforced. The performance of treated monkeys did not improve as rapidly as controls as measured by increase in reinforced responses and decrease in nonreinforced responses during initial sessions. In addition, treated monkeys exhibited greater between session variability during terminal sessions. These effects were dose related. The results of the present experiment in conjunction with those of previous experiments with this same group of monkeys suggest that blood lead concentrations presently found routinely in the human population may produce neurotoxicity.

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.

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.

Relationship between hematopoietic parameters and behavioral measures in lead-exposed rats

The effects of low level lead (Pb) exposure on learning tasks in developing rats were investigated and the results correlated with individual hematopoietic indices. Pups received exposure via the dams milk; dams were exposed to either 0-, 545-, or 1090-ppm Pb during the lactation period. At Day 30 of age, half of the high Pb group was placed on distilled water; the remaining groups continued on the same exposure regimens as their dams. On Days 20, 30, and 90, blood samples for all rats were obtained via cardiac puncture. Each sample was analyzed for Pb concentration, free erythrocyte protoporphyrin (FEPs), hematocrit, and hemoglobin. Beginning at Day 90, all rats were tested on a battery of tasks designed to investigate the following questions: (1) to what degree lead exposure interferes with reversal learning; (2) whether changing of task requirements adversely affects acquisition of a new task; (3) to what extent task difficulty contributes to lead-induced deficits; and (4) whether lead exposure affects the capacity to retain information over short or long periods of time. The actual testing paradigms included spatial discrimination with reversal, visual discrimination with reversal, and visual discrimination task with delay. No significant differences were observed among any of the groups on any of the tasks. Correlation of individual learning scores with individual measures of hematopoietic function also failed to reach significance. These findings indicate that at low exposure levels, lead has little appreciable effect on learning and memory function as measured by these tasks.

Postnatal lead exposure alters spontaneous cerebellar Purkinje neuron discharge

The effects of early postnatal lead administration on the spontaneous activity of cerebellar Purkinje neurons were studied electrophysiologically in adult male Sprague-Dawley rats. Newborn rat litters were divided into three groups and injected daily from Day 1 to Day 20 after birth with 8 mg NaAc/kg, 1 mg PbAc/kg, or 8 mg PbAc/kg intraperitoneally. Purkinje neurons in all three groups showed regular and sustained discharge. However, the mean spontaneous firing rate was significantly lower in the 8-mg PbAc/kg group (26.13 +/- 1.28 Hz) compared to the NaAc controls (32.39 +/- 1.93, P greater than 0.01). Furthermore, the distribution of the firing rates of the Purkinje cells were different in the two groups, with an obvious loss of faster firing cells in the 8-mg PbAc/kg group. No differences were seen between the 1-mg PbAc/kg group and the NaAc controls. As the 8-mg PbAc/kg group weighed significantly less than the NaAc group, malnourished animals were produced by using oversized litters, to control for any changes caused by the lower body weight. However, when compared to concomitantly raised controls, no changes could be seen in Purkinje cell discharge in the malnourished animals. It is concluded that neonatal exposure to lead can cause permanent changes in spontaneous Purkinje cell discharge.

Lead at low dose and the behavior of children

Over 3,000 children attending first and second grade in public school were classified as to past lead exposure according to the concentrations of lead in their teeth. Those in the highest and lowest deciles for lead were evaluated by a broad panel of neuropsychologic outcome measures. Controlled either by matching or analysis of covariance were 39 other variables which could effect outcome. High lead children were significantly impaired on IQ auditory processing and reaction time under varying intervals of delay. Teacher's rating scales showed a dose-related increase in non-adaptive classroom behavior with no evidence of a threshold. EEG scores and observations of children in class demonstrated differences in high and low lead subjects.

Neuropsychological studies in children with elevated tooth-lead concentrations. I. Pilot study

In contrast to blood-lead (PbB), tooth-lead concentrations (PbT) provide retrospective information about longterm, cumulative childhood lead-intake. From a basic sample of 458 school age children from the city of Duisburg (FRG), whose lead-concentrations in shed incisor teeth had been measured (means = 4.6 ppm; range: 1.4-12.7 ppm), two extreme-groups of 26 children each (mean age: 8.5 years) with low (means = 2.4 ppm) and elevated (means = 9.2 ppm) PbT were selected. After pair-matching both groups for age, sex, and father's occupational status, these children were tested under double-blind precautions for intellectual performance (German WISC), for perceptual-motor integration (Göttinger Formreproduktionstest = GFT, Diagnostikum für Cerebralschädigung = DCS, Benton-Test), and for gross motor-coordination (Körper-Koordinationstest für Kinder = KTK). Significant (P less than 0.05) inferiority of the lead-children was found in two tests of perceptual-motor integration (increased GFT-errors; lower success rate for DCS). In addition a near significant (P less than 0.1) reduction of 5-7 IQ-points was determined in these children. Although this pilot study has provided some evidence for an association between childhood lead-exposure and neuropsychological impairment, this association cannot yet be considered proven, because the observed effects were discrete and statistically confirmed only in part, and because there was a slight prevalence of perinatal risk factors in the lead group. Further research to clarify the issue is necessary.