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Deltamethrin (DLM) is a Type II pyrethroid pesticide widely used in agriculture, homes, public spaces, and medicine. Epidemiological studies report that increased pyrethroid exposure during development is associated with neurobehavioral disorders. This raises concern about the safety of these chemicals for children. Few animal studies have explored the long-term effects of developmental exposure to DLM on the brain. Here we review the CNS effects of pyrethroids, with emphasis on DLM. Current data on behavioral and cognitive effects after developmental exposure are emphasized. Although, the acute mechanisms of action of DLM are known, how these translate to long-term effects is only beginning to be understood. But existing data clearly show there are lasting effects on locomotor activity, acoustic startle, learning and memory, apoptosis, and dopamine in mice and rats after early exposure. The most consistent neurochemical findings are reductions in the dopamine transporter and the dopamine D1 receptor. The data show that DLM is developmentally neurotoxic but more research on its mechanisms of long-term effects is needed., (Copyright © 2021 Elsevier Inc. All rights reserved.)

High doses of malathion, an organophosphorus (OP) insecticide ubiquitously used in agriculture, residential settings, and public health programs worldwide, induce a well-defined toxidrome that results from the inhibition of acetylcholinesterase (AChE). However, prenatal exposures to malathion levels that are below the threshold for AChE inhibition have been associated with increased risks of neurodevelopmental disorders, including autism spectrum disorder with intellectual disability comorbidity. The present study tested the hypothesis that prenatal exposures to a non-AChE-inhibiting dose of malathion are causally related to sex-biased cognitive deficits later in life in a precocial species. To this end, pregnant guinea pigs were injected subcutaneously with malathion (20 mg/kg) or vehicle (peanut oil, 0.5 ml/kg) once daily between approximate gestational days 53 and 63. This malathion dose regimen caused no significant AChE inhibition in the brain or blood of dams and offspring and had no significant effect on the postnatal growth of the offspring. Around postnatal day 30, locomotor activity and habituation, a form of non-associative learning, were comparable between malathion- and peanut oil-exposed offspring. However, in the Morris water maze, malathion-exposed offspring presented significant sex-dependent spatial learning deficits in addition to memory impairments. These results are far-reaching as they indicate that: (i) malathion is a developmental neurotoxicant and (ii) AChE inhibition is not an adequate biomarker to derive safety limits of malathion exposures during gestation. Continued studies are necessary to identify the time and dose dependence of the developmental neurotoxicity of malathion and the mechanisms underlying the detrimental effects of this insecticide in the developing brain., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020. Published by Elsevier Inc.)

Recent developments in the field of insecticide exposure have led to a renewed interest in alternative antioxidant therapy. The present study was to investigate the neuroprotective role of syringic acid (SA, 25 mg/kg/day) on the neurotoxicity and oxidative damage induced by deltamethrin (DTM, 1.28 mg/kg/day during two months) in CA1/3 pyramidal neurons. Animals were divided into 4 groups (n = 16/group) (250-270 g) for control, DTM, SA and DTM + SA. DTM and SA were administered by oral gavage daily. Rats that were given sub-chronic DTM had revealed a significant increase in caspase-3 levels, impaired recognition memory, reduced antioxidant activity and enhanced free radicals in the hippocampus. The results showed that SA ameliorated neurobehavioral alterations, reduced reactive oxygen/nitrogen species, pyknosis in the CA1/3 and increased antioxidant enzyme activity. In conclusion, SA (25 mg/kg/day) had potential neuroprotective and therapeutic impacts against sub-chronic DTM exposure via its antioxidant and antiapoptotic efficacy. Therefore, it can be used as a neuroprotective natural plant-derived agent against DTM-induced neurotoxicity., (Copyright © 2019 Elsevier Inc. All rights reserved.)

In early March 1991, shortly after the end of the Gulf War (GW), a munitions dump was destroyed at Khamisiyah, Iraq. Later, in 1996, the dump was found to have contained the organophosphorus (OP) nerve agents sarin and cyclosarin. We previously reported evidence of smaller hippocampal volumes in GW veterans with predicted exposure to the Khamisiyah plume compared to unexposed GW veterans. To investigate whether these macroscopic hippocampal volume changes are accompanied by microstructural alterations in the hippocampus, the current study acquired diffusion-tensor imaging (DTI), T1-, and T2-weighted images from 170 GW veterans (mean age: 53 ± 7 years), 81 of whom had predicted exposure to the Khamisiyah plume according to Department of Defense (DOD) plume modeling. We examined fractional anisotropy (FA), mean diffusivity (MD), and grey matter (GM) density from a hippocampal region of interest (ROI). Results indicate that, even after accounting for total hippocampal GM density (or hippocampal volume), age, sex, apolipoprotein ε4 genotype, and potential confounding OP pesticide exposures, hippocampal MD significantly predicted Khamisiyah exposure status (model p = 0.005, R 2  = 0.215, standardized coefficient β = 0.26, t = 2.85). Hippocampal MD was also inversely correlated with verbal memory learning performance in the entire study sample (p = 0.001). There were no differences in hippocampal FA or GM density; however, veterans with predicted Khamisiyah exposure had smaller hippocampal volumes compared to unexposed veterans. Because MD is sensitive to general microstructural disruptions that lead to increased extracellular spaces due to neuronal death, inflammation and gliosis, and/or to axonal loss or demyelination, these findings suggest that low-level exposure to the Khamisiyah plume has a detrimental, lasting effects on both macro- and micro-structure of the hippocampus., (Published by Elsevier Inc.)

Cognitive tests of learning and memory (L&M) have been required by U.S. Environmental Protection Agency (EPA) developmental neurotoxicity test (DNT) guidelines for more than two decades. To evaluate the utility of these guidelines, the EPA reviewed 69 pesticide DNT studies. This review found that the DNT provided or could provide the point-of-departure for risk assessment by showing the Lowest Observable Adverse Effect Level (LOAEL) in 28 of these studies in relation to other reported end points. Among the behavioral tests, locomotor activity and auditory/acoustic startle provided the most LOAELs, and tests of cognitive function and the Functional Observational Battery (FOB) the fewest. Two issues arose from the review: (1) what is the relative utility of cognitive tests versus tests of unconditioned behavior, and (2) how might cognitive tests be improved? The EPA sponsored a symposium to address this. Bushnell reviewed studies in which both screening (locomotor activity, FOB, reflex ontogeny, etc.) and complex tests (those requiring training) were used within the same study; he found relatively little evidence that complex tests provided a LOAEL lower than screening tests (with exceptions). Levin reviewed reasons for including cognitive tests in regulatory studies and methods and evidence for the radial arm maze and its place in developmental neurotoxicity assessments. Driscoll and Strupp reviewed the value of serial reaction time operant methods for assessing executive function in developmental neurotoxicity studies. Vorhees and Williams reviewed the value of allocentric (spatial) and egocentric cognitive tests and presented methods for using the Morris water maze for spatial and the Cincinnati water maze for egocentric cognitive assessment. They also reviewed the possible use of water radial mazes. The relatively lower impact of cognitive tests in previous DNT studies in the face of the frequency of human complaints of chemical-induced cognitive dysfunction indicates that animal cognitive tests need improvement. The contributors to this symposium suggest that if the guidelines are updated, they be made more specific by recommending preferred tests and providing greater detail on key characteristics of such tests. Additionally, it is recommended that guidance be developed to address important issues with cognitive tests and to provide the information needed to improve the design, conduct, and interpretation of tests of higher function within a regulatory context. These steps will maximize the value of cognitive tests for use in hazard evaluation and risk assessment., (Copyright © 2015 Elsevier Inc. All rights reserved.)

There are a variety of chemicals, including pharmaceuticals, that alter neurobehavior following developmental exposure and guidelines for the conduct of studies to detect such effects by statute in the United States and Europe. Guidelines for Developmental Neurotoxicity Testing (DNT) studies issued by the U.S. Environmental Protection Agency (EPA) under prevailing law and European Organization for Economic Cooperation and Development (OECD) recommendations to member countries provide that such studies include a series of neurobehavioral and neuropathological assessments. Among these are assessment of cognitive function, specifically learning and memory. After reviewing 69 DNT studies submitted to the EPA, tests of learning and memory were noted to have detected the lowest observed adverse effect level (LOAELs) less frequently than behavioral tests of locomotor activity and acoustic/auditory startle, but slightly more than for the developmental Functional Observational Battery (devFOB; which is less extensive than the full FOB), but the reasons for the lower LOAEL detection rate for learning and memory assessment could not be determined. A major concern identified in the review, however, was the adequacy of the methods employed in these studies rather than on the importance of learning and memory to the proper assessment of brain function. Accordingly, a symposium was conducted to consider how the guidelines for tests of learning and memory might be improved. Four laboratories with established histories investigating the effects of chemical exposures during development on learning, memory, and attention, were invited to review the topic and offer recommendations, both theoretical and practical, on approaches to improve the assessment of these vital CNS functions. Reviewers were asked to recommend methods that are grounded in functional importance to CNS integrity, well-validated, reliable, and amenable to the context of regulatory studies as well as to basic research on the underlying processes they measure. This Introduction sets the stage for the reviews by providing the background and regulatory context for improved tests for learning and memory in DNT and other regulatory studies, such as single- or multi-generational studies where similar methods are incorporated., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Background: Developmental exposure to ethanol has long been known to cause persisting neurobehavioral impairment. However, the neural and behavioral mechanisms underlying these deficits and the importance of exposure timing are not well-characterized. Given the importance of timing and sequence in neurodevelopment it would be expected that alcohol intoxication at different developmental periods would result in distinct neurobehavioral consequences., Methods: Zebrafish embryos were exposed to ethanol (0%, 1%, 3%) at either 8-10 or 24-27 h post-fertilization (hpf) then reared to adolescence and evaluated on several behavioral endpoints. Habituation to a repeated environmental stimulus and overall sensorimotor function were assessed using a tap startle test; measurements of anxiety and exploration behavior were made following introduction to a novel tank; and spatial discrimination learning was assessed using aversive control in a three-chambered apparatus. Overt signs of dysmorphogenesis were also scored (i.e. craniofacial malformations, including eye diameter and midbrain-hindbrain boundary morphology)., Results: Ethanol treated fish were more active both at baseline and following a tap stimulus compared to the control fish and were hyperactive when placed in a novel tank. These effects were more prominent following exposure at 24-27 hpf than with the earlier exposure window, for both dose groups. Increases in physical malformation were only present in the 3% ethanol group; all malformed fish were excluded from behavioral testing., Discussion: These results suggest specific domains of behavior are affected following ethanol exposure, with some but not all of the tests revealing significant impairment. The behavioral phenotypes following distinct exposure windows described here can be used to help link cellular and molecular mechanisms of developmental ethanol exposure to functional neurobehavioral effects., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Developmental exposure of rats to the pesticide chlorpyrifos (CPF) causes persistent neurobehavioral impairment. In a parallel series of studies with zebrafish, we have also found persisting behavioral dysfunction after developmental CPF exposure. We have developed a battery of measures of zebrafish behavior, which are reliable and sensitive to toxicant-induced damage. This study determined the critical duration of developmental CPF exposure for causing persisting neurobehavioral effects. Tests of sensorimotor response (tap startle response and habituation), stress response (novel tank diving test) and learning (3-chamber tank spatial discrimination) were conducted with adult zebrafish after early developmental CPF exposure. The CPF exposure level was 100 ng/ml with durations of 0-1, 0-2, 0-3, 0-4 and 0-5 days after fertilization. Developmental CPF exposure had persisting behavioral effects in zebrafish tested as adults. In the tactile startle test, CPF exposed fish showed decreased habituation to startle and a trend toward increased overall startle response. In the novel tank exploration test, exposed fish showed decreased escape diving response and increased swimming activity. In the 3-chamber learning test, the 0-5 day CPF exposure group had a significantly lower learning rate. There was evidence for persisting declines in brain dopamine and norepinepherine levels after developmental CPF exposure. In all of the measures the clearest persistent effects were seen in fish exposed for the full duration of five days after fertilization. In a follow-up experiment there were some indications for persisting behavioral effects after exposure during only the later phase of this developmental window. This study demonstrated the selective long-term neurobehavioral alterations caused by exposure to CPF in zebrafish. The zebrafish model can facilitate the determination of the molecular mechanisms underlying long-term neurobehavioral impairment after developmental toxicant exposure., (Copyright © 2011 Elsevier Inc. All rights reserved.)

As more adults take the stimulant medication methylphenidate to treat attention deficit hyperactivity disorder (ADHD) residual type, the risk arises with regard to exposure during early development if people taking the medication become pregnant. We studied the neurobehavioral effects of methylphenidate in zebrafish. Zebrafish offer cellular reporter systems, continuous visual access and molecular interventions such as morpholinos to help determine critical mechanisms underlying neurobehavioral teratogenicity. Previously, we had seen that persisting neurobehavioral impairment in zebrafish with developmental chlorpyrifos exposure was associated with disturbed dopamine systems. Because methylphenidate is an indirect dopamine agonist, it was thought that it might also cause persistent behavioral impairment after developmental exposure. Zebrafish embryos were exposed to the ADHD stimulant medication methylphenidate 0-5 days post fertilization (12.5-50mg/l). They were tested for long-term behavioral effects as adults. Methylphenidate exposure (50mg/l) caused significant increases in dopamine, norepinepherine and serotonin on day 6 but not day 30 after fertilization. In the novel tank diving test of predatory avoidance developmental methylphenidate (50mg/l) caused a significant reduction in the normal diving response. In the three-chamber spatial learning task early developmental methylphenidate (50mg/l) caused a significant impairment in choice accuracy. These data show that early developmental exposure of zebrafish to methylphenidate causes a long-term impairment in neurobehavioral plasticity. The identification of these functional deficits in zebrafish enables further studies with this model to determine how molecular and cellular mechanisms are disturbed to arrive at this compromised state., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Background: The third trimester in human fetal development represents a critical time of brain maturation referred to as the "brain growth spurt". This period occurs in rats postnatally, and exposure to ethanol during this time can increase the risk of impairments on a variety of cognitive and motor tasks. It has been proposed that one potential mechanism for the teratogenic effects of ethanol is NMDA receptor-mediated excitotoxicity during periods of ethanol withdrawal. In neonatal rats, antagonism of NMDA receptors during ethanol withdrawal, with drugs such as MK-801 and eliprodil, has been shown to mitigate some of the behavioral deficits induced by developmental ethanol exposure. The current study examined whether memantine, an NMDA receptor antagonist and a drug used clinically in Alzheimer's patients, would attenuate impairments associated with binge ethanol exposure in neonatal rats., Methods: On postnatal day 6, rats were exposed to 6 g/kg ethanol via intubation with controls receiving an isocaloric maltose dextrin solution. Twenty-one hours following the ethanol binge, rats received intraperitoneal injections of memantine at 0, 10, 15, or 20 mg/kg. Ethanol's teratogenic effects were assessed using multiple behavioral tasks: open field activity, parallel bars and spatial discrimination reversal learning., Results: Ethanol-treated rats were overactive in the open field and were impaired on both reversal learning and motor performance. Administration of 15 or 20 mg/kg memantine during withdrawal significantly attenuated ethanol's adverse effects on motor coordination, but did not significantly alter activity levels or improve the spatial learning deficits associated with neonatal alcohol exposure., Conclusion: These results indicate that a single memantine administration during ethanol withdrawal can mitigate motor impairments but not spatial learning impairments or overactivity observed following a binge ethanol exposure during development in the rat., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Low weight at birth is a common adverse developmental effect reported in human populations and animal toxicity studies. Epidemiological evidence links low birth weight to a syndrome of metabolic changes that increase later risk for obesity, type 2 diabetes, hypertension, and cardiovascular disease. The present study used a four-treatment cross-over experimental design to evaluate the selective impact of early nutritional deficiency on metabolism and brain function across the lifespan of male Sprague Dawley rats. Undernutrition was induced prenatally by restricting maternal food intake to 50% of ad lib from GD3 to birth. Postnatal undernutrition was induced by fostering pups at birth to naïve dams in large (n=16) litters as opposed to small (n=8) control litters. Body weights were monitored in the early neonatal period, in early adulthood beginning at 5months and through to senescence at 21months of age. In contrast to recent reports, no increase in the prevalence of obesity was seen in animals born to food restricted dams and reared under ad lib feeding conditions. Behavioral tests of locomotion, learning and memory were performed in young, middle-aged, and aged animals. No effects of pre or postnatal nutritional history were detected. Age-dependent reductions in locomotor activity were detected, as well as deficits in spatial learning as measured in the Morris water maze and in context fear conditioning. These findings indicate that moderate fetal undernutrition followed by neonatal adequate nutrition does not appear to lead to obesity or neurological dysfunction in young adulthood or old age.

Methylmercury (MeHg) is a ubiquitous environmental pollutant and has been shown to affect learning in vertebrates following relatively low exposures. Zebrafish were used to model long-term learning deficits after developmental MeHg exposure. Selenomethionine (SeMet) co-exposure was used to evaluate its role in neuroprotection. Embryos were exposed from 2 to 24h post fertilization to (1) MeHg without SeMet, (2) SeMet without MeHg and (3) in combination of MeHg and SeMet. In case (1), the levels of MeHg were 0.00, 0.01, 0.03, 0.06, 0.10, and 0.30 microM. In case (2), the levels of SeMet were 0.00. 0.03, 0.06, 0.10, and 0.30 microM. In case (3), co-exposure levels of (MeHg, SeMet) were (0.03, 0.03), (0.03, 0.06), (0.03, 0.10), (0.03, 0.30), (0.10, 0.03), (0.10, 0.06), (0.10, 0.10), and (0.10, 0.30) microM. Learning functions were tested in individual adults, 4 months after developmental exposure using a spatial alternation paradigm with food delivery on alternating sides of the aquarium. Low levels of MeHg (<0.1 microM) exposure delayed learning in treated fish; fish exposed to higher MeHg levels were unable to learn the task; SeMet co-exposure did not prevent this deficit. These data are consistent with findings in laboratory rodents. The dorsal and lateral telencephalon are the primary brain regions in fish involved in spatial learning and memory. Adult telencephalon cell body density decreased significantly at all MeHg exposures >0.01 microM MeHg. SeMet co-exposure ameliorated but did not prevent changes in telencephalon cell body density. In summary, MeHg affected both learning and brain structure, but SeMet only partially reversed the latter., (Copyright (c) 2009 Elsevier Inc. All rights reserved.)

Acrylamide (ACR) is a relatively potent neurotoxicant. The ingestion of carbohydrate-containing foods cooked at high temperature exposes humans to low levels of ACR virtually daily. At relatively high levels of exposure (i.e., sub-chronic through chronic levels of exposure of >or=20mg/kg body weight/day), ACR has been shown in both rats and humans to produce a variety of effects on the nervous system. The possibility that chronic dietary exposure to ACR might produce brain toxicity which could impede the development of learning skills is a question of current concern. This research evaluated the effects of ACR on learning task performance in Fischer 344 rats exposed daily beginning prenatally and continuing throughout the lifespan. Dams were gavaged with ACR since implantation [gestation day (GD) 6] with 0, 0.1, 0.3, 1.0 or 5.0mg/kg/day through parturition. Gavaging continued at the same doses to pups through weaning on post-natal day (PND) 22 after which dosing continued via their drinking water. One male and one female per litter (8-9 per treatment group) were tested. Using an incremental repeated acquisition (IRA) task to assess learning ability, ACR-exposed Fischer 344 rats exhibited altered performance by 4 months of age. From approximately 1-8 months of age (through approximately PND 240), over 52 testing sessions, a significant treatment effect was found on percent task completed (PTC), with Tukey's post-hoc test (P<0.05) indicating a significantly lower PTC for the 5.0mg/kg/day group compared to controls. While there was no treatment effect on accuracy (P=0.53), a significant decrease in response rate was seen at 5.0mg/kg/day, suggesting that the noted decrease in PTC was due to a decrease in rate of responding, not to an effect on accuracy of task solution. Previous findings in these same animals performing a progressive ratio task for the assessment of motivation, as well as a range of tests of motoric function, suggest that this decreased response rate could be due to subtle motoric effects, or possibly due to decreases in psychomotor speed, but is most likely due to motivational effects., (Copyright (c) 2009 Elsevier Inc. All rights reserved.)

Several studies with animals have shown that even low and medium prenatal and postnatal exposure to methylmercury (MeHg) can result in locomotor, motor coordination and learning deficits. However, some behavioural effects of MeHg remain controversial and the methods to model human MeHg exposure in animal still remain to be optimized. We investigated the neurobehavioral effects of two different patterns of MeHg exposure. MeHg was given mixed in palatable food that mice readily ate. For the first pattern (chronic group), C57BL/6 mice dams were given 1.4 microg/g body weight (BW)/day (n=20) throughout gestation mixed in palatable food. For the second pattern (bolus) dams were given 6.0 microg/g BW/day mixed in palatable food on gestation day 12 and 16 together with a lower chronic dose of 0.85 microg/g BW/day mixed in palatable food on all remaining gestation days (n=20). Day 12 and 16 were chosen because neuron proliferation and the start of migration for many brain regions occur during that period. Behavioural testing on weanling animals started at 8 weeks. Both the chronic and bolus groups showed an impairment of working memory and visual spatial ability in the radial arm maze task. Other tests did not provide clear evidence that methylmercury exposure had significant adverse effects on locomotor activity, motor coordination or emotional reactivity. However, the chronic groups had a tendency for lower performance in most tests including activity in Skinner box and open field trials, as well as a higher number of anxiety-like behaviors. Chronic exposure to lower levels of MeHg combined to acute exposure with high levels of a few days during gestation appears to be less damaging than chronic exposure to slightly higher levels without acute MeHg exposure even though, equal amounts were administered during gestation. Possibly, as indicated by preliminary data, the relatively larger impact of chronic administration of a higher daily dose could be the consequence of a higher brain MeHg burden in pups compared to brain MeHg levels in the pups from dams receiving a smaller daily dose with 2 large doses on gestation day 12 and 16. Alternatively, bolus MeHg could have had a larger impact if administered on different gestation days when some neural development processes are more sensitive to MeHg.

The purpose of the present experiment was to assess the effects of chronic MK-801 (an N-methyl-D-aspartate receptor antagonist) and/or phenytoin (a sodium channel blocker) treatment on behavioral acquisition and performance in rats. Learning, audio/visual discrimination and motivation were modeled using incremental repeated acquisition (IRA), audio/visual discrimination (AVD) and progressive ratio (PR) tasks, respectively. MK-801 and/or phenytoin were administered daily, 7 days/week by orogastric gavage beginning just after weaning on postnatal day (PND) 23 and continuing until PND 306. Monday through Friday behavioral assessments began on PND 27 and continued until PND 430. Throughout treatment, subjects in the high dose MK-801 (1.0 mg/kg/day) and the high dose drug combination (1.0 mg/kg/day MK-801+150 mg/kg/day phenytoin) groups exhibited decreased body weight gains compared to control subjects. For these two affected groups, response rates were also decreased in all tasks. Task acquisition, as evidenced by an increase in response accuracy, was decreased for both these groups in the AVD task, but only for the high dose MK-801 group in the IRA task. The data suggest that chronic MK-801 treatment adversely affects the acquisition of IRA and AVD task performance and that the inclusion of phenytoin in the MK-801 dosing regimen blocks some of the adverse effects of chronic MK-801 treatment on IRA task acquisition. These findings are in marked contrast with those observed in nonhuman primates and suggest important species differences associated with chronic exposure to compounds that block NMDA receptors and/or sodium channels.

At a time when common regulatory pathways are being identified in several different species and genomics is beginning to allow comparisons of genes, how they are arranged on chromosomes and how they are regulated, zebrafish has emerged as a valuable and complementary vertebrate model. Some of the characteristics that prove of value are described and illustrated. Fluorescent transgenic lines of zebrafish embryos are presented for time-line studies with neurotoxicants. While genetic knockout technology has yet to be developed for the model, the anti-sense, morpholino approach allows for knockdown of expression of genes for the 3 day, embryonic period. This can provide for phenocopies of mutant genes for those genes essential to embryonic development or it can provide for a limited inhibition of gene expression that allows subsequent development of the fish. With the zebrafish genomic sequencing effort, microarray technology is now developing for the model system. These resources and technologies allow one to challenge the system with toxicants, and to view the immediate effects of the toxicants with transgenic embryos that fluoresce in part or all of the nervous system. Behavioral and learning protocols have been developed for the organism so that early exposures can be assayed for effects upon adult fish. Microarray technology should allow for one to identify specific genes and pathways affected by a neurotoxicant. In the future, these approaches should provide a working protocol for exploring molecular mechanisms of neurotoxicants. This type of complementary approach should then allow for more efficient examination and testing of mechanisms in mammalian models.

Use of addictive substances by human juveniles and adolescents is common, including use to intoxication and progression from one drug to other drugs. Until quite recently, animal studies have not addressed the periadolescent period as a time of special vulnerability to substance abuse. For ethanol (EtOH), the most studied drug in periadolescent animals, it has become clear that effects of alcohol are similar in some ways, but different in others, compared to the effects seen in adult animals. Sparse data suggest that this conclusion may apply to other drugs as well. Recent work in rats indicates that periadolescent substance use may disrupt normal pubertal development, and may induce stronger effects on systems subserving plasticity and cognition than in adults. Animal data also indicate that some drugs may produce altered subsequent sensitivity to the same or a different drug, changes in adult cognitive capabilities, and even CNS damage. It is now clear that there can be no presumption that all studies of abusable substances carried out in adult animals will generalize readily to periadolescents. Some data suggest that continuing developmental changes in receptor expression may underlie age-related changes in drug effects. However, the biological characteristics of periadolescence which predispose toward consumption to intoxication, and the mechanisms underlying drug progression and persisting drug effects, still remain to be well defined.

This report from a longitudinal study of the effects of prenatal alcohol and marijuana exposure investigates whether these drugs affect neuropsychological development at 10 years of age. Women were recruited from a medical assistance prenatal clinic and interviewed about their substance use at the end of each trimester of pregnancy, at 8 and 18 months, and at 3, 6, 10, 14, and 16 years. Half of the women were African American, and half were Caucasian. The women were generally from lower socioeconomic status families and had obtained high school degrees. At the 10-year follow-up, 593 children completed a neuropsychological battery, which focused on problem solving, learning and memory, mental flexibility, psychomotor speed, attention, and impulsivity. Prenatal alcohol use was found to have a significant negative impact on learning and memory skills, as measured by the WRAML. Prenatal marijuana exposure also had an effect on learning and memory, as well as on impulsivity, as measured by a continuous performance task. The effects of prenatal alcohol and marijuana exposure persisted when other predictors of learning and memory were controlled. We continue to follow these offspring into the adolescent years when further neuropsychological deficits may become evident.

This study investigated the cognitive impact of prenatal exposure to the herbal antidepressant hypericum in CD-1 mice. Hypericum (182 mg/kg/day) or a placebo was consumed in food bars for 2 weeks before mating and throughout gestation. The hypericin content in our hypericum formulation was in the middle range of standardized hypericum products. One offspring per gender from each litter (hypericum 13, placebo 12) was tested on each of the following tasks: juvenile runway with adult memory, adult Morris maze, adult passive avoidance, or adult straight water runway followed by a dry Cincinnati maze. Learning occurred in both genders in all tasks (P<.003) with no significant differences between treatments at the final trial. Female offspring exposed to hypericum, rather than to a placebo, required more time to learn the Morris maze task (P<.05). Postlearning sessions did not show any significant differences. In conclusion, prenatal exposure to a therapeutic dose of hypericum did not have a major impact on certain cognitive tasks in mice offspring.

The present investigation focuses on learning and working memory capabilities of adult male and female Sprague-Dawley rats that were exposed to either cocaine (50 mg/kg/day sc) or distilled water during infancy (postnatal days 11-20). Learning and memory were assessed at 4 months using the eight-arm radial maze. Training was carried out in three phases in order to separate procedural learning from spatial capacity. Once criterion (entering at least seven arms without repeating arms for four out of five trials) was achieved in the first training room (Room 1), testing was moved to a second room (Room 2) with unique visual cues and an identical maze. Upon reaching criterion in Room 2, animals were returned to Room 1 and examined again. Cocaine-pretreated rats were less accurate than vehicle-pretreated rats during the first 10 trials of training. During the first five trials in Room 2 cocaine-pretreated animals made more errors, and made errors earlier within trials, than the vehicle-pretreated animals. Upon return to Room 1, reliable Gender x Pretreatment interactions were found for errors and total arms entered. These data demonstrate that a brief period of postnatal cocaine exposure can impair spatial cognition in adulthood and tentatively suggest that females are more sensitive than males.

Pregnant C57BL/6 mice were orally given daily doses of 4 or 6 mg/kg of methylmercury chloride (MeHg) or vehicle during either gestational days 7-9 (GD7-9) or days 12-14 (GD12-14). Their female offspring were tested between 6 and 16 weeks of age on a variety of behavioral tasks. Motor coordination on the rotarod and visual discrimination learning in the Y maze were not affected by administration of MeHg either at GD7-9 or at GD12-14. In the open field, the total number of square crossings was lower in mice treated with 4 and 6 mg/kg of MeHg at GD12-14 than in control mice whether the environment was new or familiar, but prenatal administration of MeHg at GD7-9 had no effect on this measure. Administration of MeHg either at GD7-9 or at GD12-14 had no effect on the percentage of central square crossings or on the frequency of rearings in the open field. On spatial alternation training in the T maze, both treated groups in Condition GD7-9 and the group treated with 6 mg/kg at GD12-14 required more sessions to reach the learning criterion than their respective vehicle groups. When spatial alternation was tested with delays, treated groups did not differ from their respective control groups. In the radial arm maze, the performance of mice treated at GD7-9 was normal, but reference memory and working memory were impaired by administration of MeHg at GD12-14. In mice treated with 4 mg/kg of MeHg, reference memory was impaired only on the first block of trials, whereas in mice treated with 6 mg/kg, the deficit persisted on all blocks of trials. Overall, these results indicate that prenatal administration of MeHg at GD12-14 had more detrimental effects on behavioral performance than administration at GD7-9. It reduced locomotor activity and impaired reference memory for egocentric and allocentric spatial information as well as working memory for places.

Prenatal exposure to polychlorinated biphenyls (PCBs) was examined by analysis of cord tissue from 435 children from a Faroese birth cohort. Analysis of 50 paired cord blood samples showed excellent correlation with the cord tissue concentration (r=.90). Among 17 neuropsychological outcomes determined at age 7 years, the cord PCB concentration was associated with deficits on the Boston Naming Test (without cues, two-tailed P=.09 not adjusted for mercury; with cues, P=.03), the Continuous Performance Test reaction time (P=.03), and, possibly, on long-term recall on the California Verbal Learning Test (P=.15). The association between cord PCB and cord-blood mercury (r=.42) suggested possible confounding. While no PCB effects were apparent in children with low mercury exposure, PCB-associated deficits within the highest tertile of mercury exposure indicated a possible interaction between the two neurotoxicants. PCB-associated increased thresholds were seen at two of eight frequencies on audiometry, but only on the left side, and no deficits occurred on evoked potentials or contrast sensitivity. The limited PCB-related neurotoxicity in this cohort appears to be affected by concomitant methylmercury exposure.

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.

Long-Evans rats exposed chronically to lead (Pb) acetate (0, 75, or 300 ppm) were tested as adults on an automated, three-choice visual discrimination task as part of a larger study designed to elucidate the cognitive effects of developmental Pb exposure. Median adult BPb levels for the groups were <5, 20, and 36 microgram/dl. The pattern of results suggested a linear effect, with increasing lead dose producing progressively slower learning and an increased incidence of "impaired" individuals. This latter measure proved to be slightly more sensitive than the former, suggesting individual differences in susceptibility to Pb neurotoxicity. Additional analyses revealed that the impairing effect of Pb was seen in both the chance and post-chance learning phases, indicating that the deficit was not limited to (but could include) attentional function. Reaction time on incorrect trials was reduced in the 300-ppm group, whereas no Pb effect was seen for correct trials. The present findings suggest that chronic developmental Pb exposure produces an associative deficit as well as a tendency to respond rapidly, but does not affect information-processing speed.

The National Center for Toxicological Research (NCTR) Operant Test Battery (OTB) has been used extensively in rhesus monkeys to characterize the effects of drugs and toxicants on the performance of tasks designed to model several cognitive functions. Recently, the majority of the OTB tasks have been adapted for use in rats. The current study is the first to examine the effects of a prototypic pharmacological agent previously assessed in monkeys on rat OTB performance. The effects of the dopamine antagonist chlorpromazine (0.56-5.6 mg/kg, i.p.) were assessed in rats performing tasks designed to model auditory-visual-position discrimination, learning, time estimation, and appetitive motivation. All four tasks were equally sensitive to the behavioral effects of chlorpromazine. This pattern of sensitivity was very similar to that obtained when chlorpromazine was tested in monkeys performing the OTB. These data thus suggest that operant tasks designed to model cognitive functions in monkeys can also be used in rats, and that the effects of chlorpromazine on the performance of these tasks may be predictive of results obtained with monkeys. Further characterization of the rat OTB using prototypic pharmacological agents will further determine the extent to which drug effects on rat OTB performance can be generalized to primates.

Evidence from humans suggests that cognitive dysfunction may result from perinatal exposure to polychlorinated biphenyls (PCBs), and the results of some animal research with PCBs have been interpreted in terms of possible impairment of attention. Long-Evans rats were fed 3,3',4,4',5-pentachlorobiphenyl (PCB 126), a coplanar congener, at doses of 0.25 or 1 microgram/kg/day [corrected] throughout gestation and nursing. Male offspring of these rats were trained as adults to perform 2 tests of attention for food reward. First, a cued target-detection task, modeled after Posner's covert orienting method for humans, was used to assess visuospatial attention. In this task, a visual target stimulus was presented in 1 visual hemifield on each trial, preceded either by a valid cue, an invalid cue, or no cue. A valid cue appeared in the same hemifield as the target, and an invalid cue appeared in the opposite hemifield. As expected, valid cues increased accuracy and speed of target detection and invalid cues decreased accuracy and speed; moreover, these effects were systematically related to changes in cue intensity and target duration. However, perinatal exposure to PCB 126 did not affect acquisition or performance of this task. The second task assessed sustained attention by means of a signal detection method in which a brief, spatially-constant but temporally unpredictable, visual signal indicated which of 2 responses would yield food. Varying the intensity of the signal greatly affected the probability of correctly reporting the signal. Perinatal exposure to PCB 126 did not affect acquisition of the response rule or performance of the task. Finally, all rats were challenged with chlordiazepoxide (CDP) at doses of 0, 3, 5, 8, or 12 mg/kg SC, 20 min before testing in the sustained attention task. In control rats, low doses (3, 5, and 8 mg/kg) of CDP reduced accuracy at low signal intensities only, suggesting an increase in visual threshold. The high dose of CDP reduced accuracy at all signal intensities and increased the false-alarm rate as well, suggesting an impairment of attention. The rats exposed perinatally to PCB 126 at 0.25 micrograms/kg [corrected] were unaffected by CDP, and those exposed to PCB 126 at 1 microgram/kg [corrected] showed a smaller decrement in performance after CDP than did the controls. Taken together, these data provide little support for the possibility that perinatal exposure to PCB 126 causes deficits in attention, but suggest that PCB 126 may alter GABA-mediated pathways in the CNS during development.

Phenytoin (PHT) was orally administered in dosages of 50 and 100 mg/kg/day to pregnant rats on days 7-18 of gestation. Offspring were tested on the negative geotaxis test, a figure-eight maze (F8), the Biel water maze (BM), the Morris maze (MM), and the radial maze (RM). In addition, a delayed nonmatching-to-sample (DNMTS) test was employed. The levels of neuropeptides in brain and brain weights were determined. The maturation of negative geotaxis was delayed in both PHT groups. PHT groups showed no differences in F8, BM, and MM. In the RM, the total number of choices was high, whereas the number of correct choices was low. In the DNMTS, PHT groups showed low for correct choices with a long interval. The concentrations of neuropeptides were changed in the mesolimbic cortex, hippocampus, and amygdala. Brain weights were lower at 6 weeks of age in the 100 mg/kg/day PHT group, but were comparable at 16 weeks of age. This study suggests that the RM is a detectable task for the learning/memory impairments induced by PHT. In addition, it is surmised that the learning deficit is due to a working memory impairment arising from abnormal changes in neuropeptides and an injury in the fetal hippocampus.

Chronic exposure to low levels of organophosphate (OP) compounds impairs acetylcholine (ACh) degradation by acetylcholinesterase (AChE) and, in humans, may produce lasting neurotoxicity affecting cognitive function. The present studies examined the ability of such exposure to impair performance of well-learned cognitive tasks in two species, nonhuman primates and rats. During 25 days of exposure to a 0.01 mg/kg dose of the OP diisopropylfluorophosphate (DFP), mature adult monkeys were not impaired in their performance of a well-learned delayed matching-to-sample task (DMTS). However, erythrocyte AChE activity was reduced from predrug levels by 76.26 +/- 3.33% by 14 days after the initiation of DFP administration. Following titration of DFP to a 0.015 mg/kg dose for 15 days, DMTS performance remained at or above baseline levels. DMTS accuracy was moderately, but not significantly, reduced after titration to a dose of 0.02 mg/kg. However, decrements were associated with mild, overt symptoms of OP toxicity and performance returned to baseline levels after withdrawal from OP exposure. In rats, chronic exposure to a low-dose regimen of DFP (0.25 mg/kg/day for 14 days) impaired the ability to initially learn a spatial navigation task, but did not impair performance of previously learned stimulus discrimination and spatial navigation tasks. These data indicate that performance of memory tasks dependent upon reference concepts is not impaired by OP exposure regimens that impair acquisition of novel cognitive tasks prior to the onset of overt toxicity.

A cohort of 1022 consecutive singleton births was generated during 1986-1987 in the Faroe Islands. Increased methylmercury exposure from maternal consumption of pilot whale meat was indicated by mercury concentrations in cord blood and maternal hair. At approximately 7 years of age, 917 of the children underwent detailed neurobehavioral examination. Neuropsychological tests included Finger Tapping; Hand-Eye Coordination; reaction time on a Continuous Performance Test; Wechsler Intelligence Scale for Children-Revised Digit Spans, Similarities, and Block Designs; Bender Visual Motor Gestalt Test; Boston Naming Test; and California Verbal Learning Test (Children). Clinical examination and neurophysiological testing did not reveal any clear-cut mercury-related abnormalities. However, mercury-related neuropsychological dysfunctions were most pronounced in the domains of language, attention, and memory, and to a lesser extent in visuospatial and motor functions. These associations remained after adjustment for covariates and after exclusion of children with maternal hair mercury concentrations above 10 microgram(s) (50 nmol/g). The effects on brain function associated with prenatal methylmercury exposure therefore appear widespread, and early dysfunction is detectable at exposure levels currently considered safe.

Pseudocumene (1,2,4-trimethylbenzene, TMB) is a component of several solvent mixtures. During recent studies on rats we investigated the effect of a 4-week (6 h/day, 5 days/week) inhalation exposure to TMB at concentrations of 0, 25, 100, or 250 ppm on radial maze performance, open field activity, passive avoidance, active two-way avoidance, and shock-induced changes in the pain sensitivity reflecting the magnitude of the shock-induced fear response (hot plate test). The tests were performed between days 14 and 54 after the last exposure. The radial maze performance was not disturbed in any dose group. During testing in the open field grooming was significantly increased in rats exposed to 100 ppm TMB. In rats exposed to 100 and 250 ppm TNB, a foot shock applied after stepping off an elevated platform (a safe area) resulted in a significantly smaller increase in the step-down latency (i.e., passive avoidance, on days 3 and 7 after the foot shock) than in sham-exposed animals. Learning of a two-way active avoidance was slightly retarded in rats exposed to 250 ppm of TMB. Results of the hot plate test revealed no differences between groups in the paw sensitivity to heat (54.5 degrees C) before a 2-min intermittent food shock, but in rats exposed to 100 and 250 ppm of TMB the foot shock-induced fear response persisted apparently longer. These results suggest that inhalation exposure to TMB may lead to long-lasting disturbances in CNS functions.

To explore possible long-term effects of gestational cocaine exposure in a nonhuman primate model, pregnant rhesus monkeys were treated from about 1 month of gestation until term with either 0 (N = 3), 0.3 (N = 3), 1.0 (N = 3), or escalating doses up to 8.5 (N = 3) mg/kg (IM), three times per day, 5 consecutive days per week. Despite these differences in cocaine exposure, the experimental groups did not differ significantly with respect to the postnatal growth of offspring over an 18-month period following birth. Beginning at 6 months of age, the behavior of offspring was monitored using an operant test battery that included five food-reinforced tasks designed to model aspects of learning, color and position discrimination, time estimation, short-term memory and attention, and motivation. Although the acquisition of each operant behavior by offspring progressed significantly during training between 6 and 18 months of age, this acquisition was not differentially affected by gestational cocaine exposure. It was concluded that, in a rhesus monkey model, chronic cocaine exposure during pregnancy had no significant effect on the offsprings' acquisition of operant behaviors.

Although the possible behavioral neurotoxic effects of in utero exposure to cocaine have been the subject of numerous experiments, only a limited number of different types of animal models of cocaine exposure, critical periods, or long-term effects of such exposures have been investigated. In the present experiment, the effects of multiple daily SC exposures to cocaine (20 mg/kg/dose x 5 doses per day) were investigated when administered to gravid Sprague-Dawley CD rats on embryonic days E7-12 or E13-18 compared to weight-matched, vehicle injected, pair-fed controls. Effects of exposure were assessed on general development, olfactory orientation behavior, early locomotion, startle reactivity, spontaneous motor activity, and learning on two different tasks (Morris and Cincinnati water mazes). The multiple cocaine dosing regimen produced maternal peak serum concentrations of cocaine 3 times higher than that of a single dose (approximately 1550 vs. approximately 550 ng/mL). Early-exposed cocaine offspring had lower olfactory orientation scores and reduced postweaning rearing and hole-poke motor activity, whereas late-exposed cocaine offspring had increased postweaning locomotor, rearing, and hole-poke activity. On the Morris hidden platform maze, the cocaine early-exposed females had longer latencies on acquisition than controls. On the Cincinnati multiple-T water maze, the early-exposed cocaine females and the late-exposed cocaine males had increased errors, whereas the early-exposed cocaine males had reduced errors. The effects on measures of learning, when taken together, and in light of their being in the early-exposed group, suggest that embryonic cocaine exposure may have subtle effects on cognition in the offspring as adults. Such effects represent a form of neurotoxicity not previously associated with prenatal cocaine exposure.

Maternal Wistar rats were exposed via inhalation to 0, 50, or 300 ppm styrene for 6 h/day during gestation days 7 to 21, and offspring were subsequently evaluated in several neurobehavioral tests. Preliminary results with a small number of litters revealed significant dose-dependent effects in tests performed prior to weaning (surface righting, pivoting locomotion, and bar holding), as well as in tests performed after weaning (motor coordination, open-field behavior, and motor activity). Exposure to low concentrations of styrene (50 ppm) caused disturbances in motor coordination in addition to delaying some motor and reflex developments. Large doses (300 ppm) led to changes in open-field behavior and increases in spontaneous activity in addition to the delay in neurobehavioral developments. Exposure of dams to styrene did not clearly affect the learning behavior of the offspring. It was also observed that age played a role in the differences in styrene's effects on neurobehavioral function. Only subtle effects were found in both open-field behavior and motor-coordination function when compared with control rats at 120 days of age. These results suggest that the functional neurobehavioral development of progeny of dams exposed to styrene (or other solvents) should be further investigated.

During 1991, the Middle Atlantic Reproduction and Teratology Association (MARTA) conducted a survey of laboratories performing behavioral evaluations as part of GLP developmental toxicity studies. This survey was conducted to determine the extent to which an "industry standard" had evolved for behavioral test batteries. The most commonly used developmental parameters were eye opening, pinna unfolding, and sexual maturation (physical landmarks); surface righting, pupil constriction, and nonautomated acoustic startle (reflexive landmarks). Locomotor activity was used by 80% of the laboratories. The majority (76%) of laboratories conducted at least one learning and/or retention evaluation; nearly one-fourth of the laboratories routinely performed two. The most common learning tests were watermaze (primarily a simple two-choice discrimination task) and passive avoidance. Automated startle paradigms (habituation, prepulse modification, and/or startle elicitation) were evaluated by 28% of the laboratories. This survey showed a remarkable similarity in methodology across laboratories and a progressive increase in the number of GLP studies that included behavioral assessments. The results indicate that behavioral tests have become a common component of developmental toxicity assessments of pharmaceuticals.

The effects of administration by inhalation of metallic mercury vapour (Hg0) to pregnant rats, approximately corresponding to doses of 0.2 mg Hg0/kg/day (high dose) or 0.07 mg Hg0/kg/day (low dose), on the developmental and behavioural repertoire of the offspring were studied. Exposure occurred during days 11-14 plus 17-20 of gestation. The dose levels were selected so as not to induce maternal toxicity. Maturation variables such as surface righting, negative geotaxis, pinna unfolding, and tooth eruption revealed no differences between Hg0-treated offspring and controls. Tests of spontaneous motor activity showed that the Hg0-treated offspring were hypoactive at 3 months of age but hyperactive at 14 months. In spatial learning tasks the prenatally exposed offspring showed retarded acquisition in the radial arm maze but no differences in circular swim maze. A simple test of learning, habituation to a novel environment (activity chambers), indicated a reduced ability to adapt. These data suggest that prenatal exposure to Hg0 vapour results in similar behaviour changes in the offspring as reported for methylmercury.

In humans and animal models there is evidence that prenatal nicotine exposure causes lasting deficits in cognitive performance. The current study examined the cognitive effects of prenatal exposure of rats to 2 mg/kg/day of nicotine. This dose did not cause significant deficits in maternal weight gain, offspring litter size, or pup weight. The control offspring showed the normal ontogeny of spontaneous alternation from near chance (50%) performance to 80%-85% alternation. In contrast, the nicotine-exposed rats had the opposite progression with abnormally high alternation on days 22-30 and abnormally low alternation on days 35-52. Acquisition of choice accuracy performance on the radial-arm maze (RAM) was not altered in a major way by nicotine exposure. Minor nicotine-induced changes in choice accuracy were seen during the initial trials of acquisition. The nicotine exposed female offspring had a significantly longer response duration. Prenatal nicotine exposure did significantly alter the effects of subsequent drug challenges on choice accuracy performance. The nicotine-exposed male offspring were significantly more responsive to the amnestic effects of the nicotinic antagonist mecamylamine. In a subsequent challenge, the effects of the beta-adrenergic antagonist propranolol were examined. A significant dose-related impairment in choice accuracy was seen in the control rats. In contrast, the nicotine-exposed rats did not show any significant response to propranolol. This decreased responsiveness to adrenergic challenge parallels the reduction in adrenergic response to nicotine challenge we previously found in littermates to the rats of the current study. Prenatal nicotine exposure causes subtle alterations in cognitive performance that can be magnified by challenges of nicotinic and adrenergic systems.

Animals exposed prenatally to alcohol (4 g/kg/day) via maternal peroral intubation or control offspring were reared after weaning either alone in standard steel/wire cages or in groups of eight, for 6 weeks. Rats exposed prenatally to alcohol and reared in isolation had a dysmetric stride length indicative of an ataxic gait. However, following postweaning environmental enrichment, prenatal alcohol-exposed rats showed no evidence of ataxia. In addition, the prenatal alcohol-exposed rats showed the same magnitude of improved Morris maze performance after enrichment as did the control offspring. These preliminary results suggest that postnatal environment can influence the expression of alcohol-related birth defects in rats, that rats exposed prenatally to alcohol can benefit from the effects of enriched postweaning environment and that postnatal factors can attenuate some of the deficits due to prenatal alcohol exposure.

Adult exposure to the neurotoxicant 3,3'-iminodipropionitrile (IDPN), induces a hyperkinetic syndrome consisting of spontaneous head movements, abnormal circling, backward locomotion, and sensory disruption. We report here the behavioral effects of developmental exposure to IDPN in the rat. Animals were exposed (IP) to either saline, 75, 150, or 300 mg/kg/day on postnatal days (PND) 5-7. Animals were tested for: Figure-8 maze activity (PND 13-60); olfactory discrimination learning (PND 18 & 24); T-maze alternation and position discrimination learning (PND 25 & 26); acoustic startle response (PND 23, 61, & 62); passive avoidance (PND 70). To better define the dose response, a separate group of animals was exposed to either saline or 225 mg/kg/day (PND 5-7) and tested in the activity, T-maze, and startle paradigms. Animals exposed to 225 mg/kg/day and 300 mg/kg/day had decreased weight gain and lethality was 25% in the latter group. Signs of the IDPN syndrome, evident in the 225 and 300 mg/kg/day groups, persisted throughout the course of the study. IDPN exposed animals (300 mg/kg/day) were hyperactive on PND 17-60, failing to develop habituation in the Figure-eight maze until PND 60. The acoustic startle response was depressed for the 225 and 300 mg/kg/day groups on PND 23 only. Auditory thresholds were elevated for a high-frequency (40 kHz) but not a low-frequency tone (5 kHz) for the 225 and 300 mg/kg/day groups, indicating a hearing loss. IDPN treatment also disrupted performance of olfactory discrimination learning and produced cognitive deficits in T-maze learning in infants (300 mg/kg/day). That cognitive deficits also appeared in adulthood (PND 70) was demonstrated by learning deficits in a passive avoidance task at 150 and 300 mg/kg/day. IDPN (300 mg/kg/day) also caused a decrease in the wet weight of the whole brain (8%) and the cerebellum (12%) but not the hippocampus. These data demonstrate that short-term, neonatal exposure to IDPN in the rat produced persistent alterations in sensory, motor, and cognitive aspects of nervous system function.

Pregnant rats received subcutaneous injections of 15 mg/kg of cocaine twice daily (Cocaine-D), twice daily for two consecutive days at 5-day intervals (Cocaine-I), 0.9% saline (Saline) twice daily, or 1.5 mg/kg amfonelic acid (AFA) daily from gestational days 1-20. Offspring were tested for: rates of spontaneous alteration at postnatal days (PND) 32, 35, 40, and 45; acquisition and retention performance on a water maze task beginning at PND 30 and 60; entrance into and activity in an open-field apparatus at PND 60 and 180. The Cocaine-D offspring were less likely than Control offspring to enter the open field when tested at PND 60. The Cocaine-I offspring were hyperactive in the open-field apparatus when tested at PND 60. The drug treated offspring did not differ from the Saline control animals on all other measures. The failure of the Cocaine-D animals to enter the open field is consistent with neophobic behavior that we have observed before in rats exposed in utero to cocaine.

Young rats were treated by gastric intubation with aluminum lactate (0, 100, and 200 mg Al/kg/day) from postnatal days 5 to 14 to determine the treatment's influence on brain choline acetyltransferase activity and learning abilities. The results indicated that aluminum concentrations in the cerebral areas increased in parallel to plasma aluminum at the dose of 200 mg. In the same case, choline acetyltransferase activity was reduced. At postnatal days 50 and 100, the treated rats did not show alterations in their learning abilities in the 2 tests which are based on different motivations (avoidance of an aversive light or alimentary motivation) and different ways of achievement (pressing on a lever or running in a maze). A low reduction in the general activity, particularly in the radial maze test, was only observed in rats treated with 200 mg Al/kg/day.

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.

Prenatal alcohol exposure is associated with deficits in odor-associative learning in very young rat pups. One alternative explanation for these findings is that rather than a learning deficit per se, alcohol-exposed pups may display a sensory deficit. The present study was designed to examine the effects of prenatal alcohol exposure on behavioral and neuroanatomical components involved in olfaction. The subjects in this study were pups exposed to 35% ethanol-derived calorie (EDC) liquid diet from gestation days (GD) 6-20. Two control groups were included, a 0% EDC pair-fed and an ad lib lab chow group. In Experiment 1, respiratory response to a novel odor was examined in pups tested at either 3, 4, or 10 days of age. The 35% EDC offspring clearly detected the odor. Furthermore, there was an apparent alcohol-related development delay in respiratory rate as shown by a lower baseline respiratory rate at PN 3 relative to controls which was no longer apparent by PN 4. Experiment 2 examined the volume of two neuroanatomical structures involved in olfaction, the main olfactory bulb (MOB) and the vomeronasal organ (VNO) in 3-day-old pups. Prenatal alcohol exposure was associated with a decreased volume of the MOB although the VNO was unaffected.

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.

Maternal phenylketonuria results in a high incidence of children born mentally retarded. We showed that the large neutral amino acids valine, isoleucine, and leucine (VIL) ameliorate the effects of intrauterine hyperphenylalaninemia in rats on a test of complex maze learning. To further test the ameliorative effects of VIL on intrauterine CNS development during hyperphenylalaninemia, gravid rats were administered a phenylalanine/p-chlorophenylalanine (index group) supplemented diet with or without VIL added. Controls were given standard diet with or without VIL. All groups were pair-fed to the index group. As adults, the progeny exposed in utero to hyperphenylalaninemia showed characteristic learning impairments in a complex water (Cincinnati) maze on forced and elective-choice phases of the task and deficits in radial-arm maze and Morris maze acquisition, whereas those exposed to hyperphenylalaninemia combined with VIL showed no deficits in the forced-choice phase of Cincinnati maze learning and no evidence of radial-arm maze deficits. However, the improvement was not complete, with no ameliorative effects obtained on the elective-choice phase of the Cincinnati maze or on the Morris hidden platform test. No deficits were seen on phases containing test trials for memory function (Olton and Morris mazes). The acquisition differences occurred in the absence of any effects of VIL on maternal weight gain during gestation, maternal serum amino acid concentrations of phenylalanine or tyrosine, or effects on offspring growth. VIL alone produced no adverse or enhancing effects on learning or memory. Based on these data it was concluded that the VIL supplement continues to show promise as a potential treatment for intrauterinely acquired mental deficiency associated with maternal phenylketonuria.

Perinatal M.S.G. treatment causes a syndrome characterized by damage to the hypothalamic arcuate nucleus, other circumventricular areas, parts of the visual system and the dentate gyrus of the hippocampus. The resulting hormonal dysfunction may be responsible for developmental anomalies of organ systems, obesity, and alterations in sensory/motor performance. We have shown that some behavioral indicators of M.S.G. toxicity in rats can be masked by rearing them in enriched housing conditions. Here, we evaluated the impact of six housing conditions on M.S.G.-induced alterations of organ systems and behavior. Perinatal M.S.G. treatment reduced adrenal, heart and testes weights, as well as total white blood cell (WBC) counts, and increased tail flick latencies. These measures were unaffected by the housing condition. M.S.G.-induced reductions in body weight, grip strength, water maze and dominance task performance varied as a function of housing. Deficits in water maze performance were most evident following social and isolated single-cage housing. We propose that deficits in water maze performance following perinatal M.S.G. may be attributable to hippocampal damage that can be alleviated by rearing the rats in stimulating environments.

Triethyltin is an organotin compound that is known to produce neurotoxicity in both adult and developing organisms. Although this neurotoxicity has been documented with a variety of behavioral and biological measures, the effects of this compound on learning during early development have been less extensively studied. The present study reports four experiments that examined this question with an odor aversion learning paradigm in which pups received presentations of one odor paired with footshock and an alternate odor without shock. In Experiment 1, Long-Evans rat pups were injected IP on postnatal day 5 (PND 5) with either 0, 3 or 5 mg/kg TET and then tested for olfactory discrimination learning on PND 18. Only the 5-mg/kg dose impaired discrimination learning. In Experiment 2, PND 5 exposure to TET (5 mg/kg) disrupted olfactory learning on PND 18 but not on PND 12, whereas exposure on PND 10 disrupted learning at both ages of testing. In Experiment 3, PND 16 exposure to TET (5 mg/kg) also disrupted acquisition of olfactory learning on PND 18 but had no effect on retention of an olfactory discrimination that was acquired prior to TET exposure (i.e., on PND 14 and PND 15). Unconditioned responses to footshock were also unaffected by TET (Experiment 4). These findings indicate that neonatal exposure to TET impairs associative learning in developing rats and are discussed in relation to other studies of the developmental neurotoxicity of this compound.

Trimethyltin is an organotin compound that produces marked neurotoxicity in both adult and developing animals. The limbic system is a primary CNS target site for this toxicity, and a prominent behavioral effect of TMT is disruption of learning and memory. Impairment of cognitive development has also been suggested by studies showing that rats neonatally exposed to TMT cannot perform spatial working memory tasks during adulthood. However, the question of how early in ontogeny such deficits can be detected has not been addressed. The present study examined this question with a T-maze delayed alternation learning paradigm. Long-Evans rat pups, injected IP on Postnatal Day 10 (PND 10) with 6 mg/kg TMT and tested on PND 18, were unable to learn delayed alternation in the manner shown by vehicle control pups. However, TMT- and vehicle-treated groups were both able to learn a simple position discrimination. These findings indicate a selective impairment of spatial working memory by neonatal TMT exposure and show that this impairment can be demonstrated during the preweanling period in the rat.

A multidisciplinary study to investigate 2,5-hexanedione (2,5-HD)-induced CNS-neurotoxicity and its proposed potentiation by acetone (AC) was conducted. Neurobehavioral testing, estimation of the brain-swelling reaction, neuropathological and morphometric measurements as well as synaptosomal 5-hydroxytryptamine (5-HT) uptake were evaluated. Male Wistar rats were exposed through their drinking water, for seven weeks, either to 0.5% 2,5-HD alone or to 0.5% 2,5-HD in combination with 0.5% AC. This 2,5-HD dose is known to cause neurotoxicity in the peripheral nervous system. Exposed animals were compared to a control group that received tap water. Acquisition but not performance of spatial learning, as measured in the radial 8-arm maze, was significantly inhibited by the coexposure to 2,5-HD and AC. Brain weights of rats exposed to 2,5-HD alone and to 2,5-HD and AC in combination, were slightly, albeit significantly, reduced. Synaptosomal high-affinity 5-HT uptake rate and uptake capacity were significantly reduced by 2,5-HD alone and in combination with AC. The morphometric results are reported in a companion paper. In conclusion, the present findings indicate that 2,5-HD is a CNS-neurotoxicant. The hypothesis about AC potentiation of 2,5-HD CNS-neurotoxicity was supported.

Temporary hypothyroidism was induced in neonatal rats by 0.02% propylthiouracil (PTU) administration to lactating dams during days 0-19 after delivery, and its effects on the behavior and learning of their male offspring were examined. The serum T4 (thyroxine) level was returned to normal around 1 week after the last PTU administration, but the body weight gain was still depressed. The open field and Biel water maze tests at the age of 6 weeks showed an increased number of ambulations and an increase in errors with prolonged swimming time in the PTU rats. The radial arm maze test started at 13 weeks revealed that the PTU animals required more trials until they showed the first well-performed trial. The total number of choices was also larger, with less correct choices, and treatment effects on the response distribution and pattern were significant. Thus, the rats, which had suffered from temporary hypothyroidism in the neonatal period, showed hyperactivity and irreversible impairment in maze learning. These results suggest an involvement of temporary neonatal hypothyroidism in hippocampal dysfunction.