Persistence of impaired reversal learning in young monkeys exposed to low levels of dietary lead.

Lead acetate in milk was fed daily to infant rhesus monkeys at doses averaging 0 (control), 0.287 (low-Pb), or 0.880 (high-Pb) mg/kgd for the first year of life. Pb concentrations in whole blood (PbB) averaged 4.15, 31.71, and 65.17 microgram/dl for the control, low-Pb, and high-Pb groups, respectively, during the year of treatment and declined toward control levels when Pb dosing was stopped. Behavioral observations during the year of treatment had shown that both experimental groups were retarded in their acquisition of object-cue discrimination reversal learning sets. At 4 yr of age, when PbB levels in all animals were normal, the ability of the same monkeys to acquire a series of 3 spatial-cue reversal learning sets was examined; these data form the basis for this report. In the first problem, the high-Pb group was significantly retarded in acquisition of the original discrimination and of most reversals, and the low-Pb group was retarded on reversal 1 only. These deficits declined in severity across the three problems administered, in a manner similar to that seen in the tests given during the first year of life. These data demonstrate that reversal learning retardation, observed early in life, can recur in postadolescent primates with a history of chronic, low-level Pb intoxication during infancy.     

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.     

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 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.     

Prenatal ethanol exposure, generalized learning impairment, and medial prefrontal cortical deficits in rats.

Prenatal ethanol exposure may cause neurological damage and subsequent mental retardation in humans, with learning deficits similar to those following damage to the prefrontal cortex. This study examined cognitive dysfunction and cortical damage after prenatal exposure to ethanol using a chronic administration model. Pregnant Sprague-Dawley rats received one of three diets during gestation: a liquid diet containing 35% ethanol-derived calories (ETOH), an isocaloric liquid diet (ISO), or standard chow (CHOW). Subjects were obtained from ETOH dams with blood alcohol concentrations (BACs) above 90 mg/dl and corresponding ISO and CHOW controls (one male pup/litter; n=6 pups/group). At approximately 90 days of age, subjects began training on a series of unique auditory discrimination problems using a successive go/no-go procedure. A criterion of 85% accuracy determined when a rat continued to the next problem. Subjects completed a varying number of problems within a 30-session limit, after which all rats were tested on a tone/click discrimination and reversal. Subjects were then sacrificed and neuronal number in the medial prefrontal cortex (mPFC) was estimated by the optical fractionator method. Prenatal ethanol exposure induced significant cell loss in the mPFC, which was associated with significantly impaired reversal learning. Poor performance by ETOH subjects on the tone/click reversal indicates a transfer of training deficit that may reflect failures of inhibitory control.     

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.     

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.     

Learning in monkeys exposed perinatally to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

TCDD is an extremely toxic chemical pollutant which bioaccumulates in maternal adipose tissue, and is transferred to the developing organism during gestation and lactation. Long-term cognitive deficits have been reported following perinatal exposure to polychlorinated biphenyls, which are structurally and toxicologically similar to TCDD. In the current study, monkeys exposed to TCDD perinatally were later tested in two cognitive paradigms, discrimination-reversal learning (RL) and delayed spatial alternation (DSA). RL detected effects; whereas DSA, as analyzed, did not. RL consisted of a series of simple spatial reversals, followed by spatial reversals with color and shape as irrelevant cues, then by color reversals and finally by shape reversals. TCDD-exposed monkeys exhibited retarded learning of the shape reversals. The deficit was most pronounced on the first reversal following overtraining. There were no group differences on the spatial or color reversals. However, the number of trials the TCDD-exposed monkeys individually took to learn the spatial reversals was positively correlated with TCDD concentration in body fat. Conversely, the number of trials they took to learn the color reversals was negatively correlated with TCDD in body fat.     

The effects of tetraethyl lead on behavior in the rat

The behavioral effects on rats of various doses of tetraethyl lead, administered intragastrically, were measured on the following: bar press responding for food, two-choice discrimination under negative reinforcement, and emotional responses in the open field. Bar press response rates were drastically curtailed following administration of high doses of lead. Both trials to criterion and mean latency to criterion were detrimentally affected by administration of single doses of lead prior to acquisition of the discrimination. Lead had similar effects on these same measures in reversal of the original discrimination and in retention of the reversal. The performance deficits were not attributable to the tetraethyl radical; control injections of tetraethyl silane were without effect on all behavioral measures in the discrimination task. In addition, the results did not appear to be a function of emotional factors as lead did not influence trials to avoidance criterion or open field behavior. It was concluded that lead given intragastrically can impair learning and memory in the rat.     

Lack of sensitive period for lead-induced behavioral impairment on a spatial delayed alternation task in monkeys

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): vehicle only; dosed with lead from birth onward; dosed with lead from birth to 400 days of age and vehicle thereafter; 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 6-7 years old, they were tested on a spatial delayed alternation task. The task required the monkey to alternate responses between two push buttons. The initial delay was 0.10 sec and was increased in steps to 15 sec by the end of the experiment. All three treated groups were impaired to approximately an equal degree. Deficits were observed in the initial training procedure, and at the longer delay values. These results suggest that there is not an early critical period for lead-induced impairment on this task and that exposure only during infancy results in impairment comparable to ongoing exposure beginning at birth. These results are in contrast to previous findings on a series of nonspatial discrimination reversal tasks, in which the group exposed early in life only was unimpaired, while the group exposed beginning after infancy was less impaired that the group exposed continuously from birth.     

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.     

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.     

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.     

Effects of chronic lead exposure on learning and reaction time in a visual discrimination task

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.     

Dissociable effects of selective 5-HT2A and 5-HT2C receptor antagonists on serial spatial reversal learning in rats.

Serotonin (5-hydroxytryptamine, or 5-HT) is strongly implicated in the ability to shift behavior in response to changing stimulus-reward contingencies. However, there is little information on the contribution of different 5-HT receptors in reversal learning. Thus, we investigated the effects of systemic administration of the 5-HT(2A) antagonist M100907 (0, 0.01, 0.03, and 0.1 mg/kg, i.p.) and the 5-HT(2C) antagonist SB 242084 (0, 0.1, 0.3, and 1.0 mg/kg, i.p.) on the performance of an instrumental two-lever spatial discrimination and serial spatial reversal learning task, where both levers were presented and only one was reinforced. The rat was required to respond on the reinforced lever under a fixed ratio 3 schedule of reinforcement. Following attainment of criterion, a series of within-session reversals was presented. Neither M100907 nor SB 242084 altered performance during spatial discrimination and retention of the previously reinforced contingencies. M100907 significantly impaired reversal learning by increasing both trials to criterion (only at the highest dose) and incorrect responses to criterion in Reversal 1, a pattern of behavior manifested as increased perseverative responding on the previously reinforced lever. In contrast, SB 242084 improved reversal learning by decreasing trials and incorrect responses to criterion in Reversal 1, with significantly fewer perseverative responses. These data support the view that 5-HT(2A) and 5-HT(2C) receptors have distinct roles in cognitive flexibility and response inhibition. The improved performance in reversal learning observed following 5-HT(2C) receptor antagonism suggests these receptors may offer the potential for therapeutic advances in a number of neuropsychiatric disorders where cognitive deficits are a feature, including obsessive-compulsive disorder.     

Exploring a partially enclosed space by lead-exposed female rhesus monkeys

Beginning on Day 8 postpartum, lead acetate was administered to female rhesus monkeys (n=48). Their blood lead levels rose to 35-40 microg/dl (the level maintained for the duration of the study period) by 12 weeks of age. Weekly, these lead-exposed monkeys and their controls (n=23) were placed in a partially enclosed space from the second postnatal week until they escaped three times or were 26 weeks old. The lead-exposed monkeys exhibited more fear, were more likely to be agitated, and climbed more frequently during the first testing session. In subsequent sessions, they more frequently explored the periphery of the test area than the controls. The lead-exposed monkeys also tended to escape sooner although that trend did not consistently reach the.05 level of significance. The increased activity and agitation of the lead-exposed monkeys is suggestive of deficits reported in human children with high blood lead levels.     

Neurobehavioural consequences of prenatal low level exposures to lead

A cohort of 318 children born in three Sydney hospitals between April 1982 and March 1983 were recruited into a five year prospective study designed to investigate the relationship between low level lead exposures and neurobehavioural development. Blood samples were obtained at the time of birth, then at 6 month intervals to 4 years and then at 5 years; neurobehavioural and physical measures were taken at 6 months, 12 months and hence annually to 5 years. This paper presents some of the findings from the first three years of the study and addresses the issue of the relationship between fetal exposures to lead and child development to three years. Maternal and cord blood lead levels were in range 0-29 microgram/dl with the majority less that 15 micrograms/dl. The geometric means were, respectively, 9.1 micrograms/dl and 8.1 micrograms/dl. The analyses presented do not support the hypothesis of a relationship between maternal and cord blood lead levels in this range and developmental deficits in young children to the age of three years.     

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.     

Neurotoxic response of infant monkeys to methylmercury.

Four infant monkeys were dosed orally with 500 microgram Hg/kg body wt./day /as methylmercury (MeHg) chloride dissolved sodium carbonate) beginning at 1 day of age. Neurological and behavioral signs of MeHg toxicity and blood Hg levels were monitored weekly. At first sign of MeHg intoxication, dosing with MeHg was terminated and the infants were monitored to assess reversal of the signs of MeHg toxicity. The first signs of MeHg toxicity, exhibited as a loss in dexterity and locomotor ability, were observed after 28--29 days of treatment; the blood Hg levels were 8.0--9.4 microgram Hg/g blood. Dosing was terminated at 28--29 days of treatment but the signs of MeHg toxicity continued to develop. The infants became ataxic, blind, comatose and were necropsied at 35--43 days after initiating treatment with MgHg. The mercury concentrations in tissues analyzed after necropsy were highest in liver (55.8 +/- 3.2 microgram Hg/g) followed by occipital cortex (35.6 +/- 4.8 microgram Hg/g) renal cortex (32.8 +/- 1.6 microgram Hg/g). The frontal and temporal cortices had 27.0 +/- 3.4 and 29.6 +/- 4.9 microgram Hg/g respectively while the cerebellar Hg concentration averaged 13.0 +/- 1.5 microgram Hg/g. The mean blood/brain ratio was 0.21 +/- 0.4. Histopathologic lesions were marked in the cerebrum with less severe lesions in the cerebellar nuclei. The Purkinje and granular cells of the cerebellar vermis appeared histologically normal. Lesions were not observed in the peripheral nervous system. The signs of MeHg intoxication, the tissue distribution of MeHg and histopathologic lesions observed in the infant monkeys were similar to those reported for adult monkeys.     

Effects of an enkephalin analog on complex learning in the rhesus monkey.

Facilitation of the learning of a discrimination reversal task for a reward of food was found in rhesus monkeys after subcutaneous administration of a potent pentafluorinated enkephalin analog. (D-Ala2)-F5-Phe4-enkephalin-NH2. General activity, short-term memory, startle, and analgesia, however, were not significantly affected. In a within-subject design, each of 6 monkeys (3 males and 3 females) received each of 5 doses of the enkephalin analog (0.1, 1, 10, 100, and 1,000 microgram/kg). One daily injection was made for 7 consecutive days, including pre- and posttests on the first and last days with the diluent control. The enkephalin doses, with the exception of the 0.1 microgram/kg level, produced significantly faster learning than the diluent. Some sex differences were suggested by the data, but these effects are difficult to interpret. The results suggest that relatively small amounts of this analog given systematically can exert a reliable effect on a complex behavior such as reversal learning at doses devoid of opiate effects, due perhaps to enhanced cognitive flexibility rather than improvement in short-term memory or association formation.