Steven G. Gilbert

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

Monkeys were dosed orally with 500 μg/kg/day of lead as lead acetate from Day 1 of life. No overt signs of lead toxicity were observed. At 2–3 years of age, locomotor activity and performance on a multiple fixed interval 8 min-time out schedule of reinforcement were studied. On the fixed interval schedule, the first response after 8 min had elapsed resulted in delivery of fruit juice. Successive intervals were separated by time out periods, during which responses had no scheduled consequences. Treated monkeys behaved differently from controls during initial sessions of the multiple fixed interval-time out schedule, with exposure to lead resulting in increased response rates, decreased median interresponse times, and more responding in the time out period. Subtle differences in the pattern of responding in the fixed interval persisted throughout the course of the study. There were no differences in locomotor activity between control and treated pairs of monkeys. There were no differential effects of d-amphetamine between treated and control monkeys on either locomotor activity or schedule-comtrolled behaviour.

Effects of developmental exposure to methyl mercury on spatial and temporal visual function in monkeys

Detailed characterization of several aspects of visual function was made in two groups of monkeys exposed developmentally to methyl mercury. One group of monkeys (Macaca fascicularis) was dosed from birth onward with 50 micrograms/kg/day of mercury as methyl mercury. Another group was exposed in utero by dosing the mother with 10, 25, or 50 micrograms/kg/day of mercury as methyl mercury, and postnatally until 4.0-4.5 years of age with the same dose the mother had received. Spatial and temporal visual function was tested in both groups. Spatial visual deficits observed in the group dosed beginning postnatally were reported previously (Rice and Gilbert, 1982, Science, 216, 759-761). Monkeys exposed in utero plus postnatally exhibited impaired high- and low-luminance spatial vision. They also exhibited deficits in low-frequency high-luminance temporal vision, while low-luminance temporal vision was superior to that of control monkeys. Monkeys in which exposure began at birth displayed superior low-luminance temporal vision, while high-luminance temporal vision was not impaired. These effects were observed in the absence of constriction of visual fields. These data suggest that the pattern of visual deficits produced by developmental exposure to methyl mercury may be different from that in the adult, and that the developing visual system may be able to remodel as a result of early insult by a neurotoxic agent.

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.

Exposure to methyl mercury from birth to adulthood impairs high-frequency hearing in monkeys

Hearing deficits are a frequent consequence of both developmental and adult methyl mercury exposure in humans. However, a detailed characterization of these deficits has not been performed in either humans or animals. Cynomolgus monkeys (Macaca fascicularis) were dosed from birth to 7 years of age with 50 micrograms/kg/day of mercury as methyl mercuric chloride. Steady-state blood mercury levels during dosing were 0.6-0.9 ppm. When monkeys were 14 years old, pure tone detection thresholds were determined by a psychophysical procedure. Control monkeys exhibited thresholds at frequencies between 125 and 31,500 Hz comparable to previously published values for macaques. One methyl mercury-treated monkey exhibited normal detection thresholds at all frequencies. Three treated monkeys were impaired at the second highest frequency tested (25,000 Hz) and therefore were not tested at 31,500 Hz. The fifth treated monkey displayed severely elevated thresholds at middle frequencies (10,000-12,500 Hz), precluding testing at higher frequencies. These results indicate a selective high-frequency deficit in monkeys exposed to methyl mercury from birth to adulthood and not exposed to methyl mercury in the 7 intervening years before auditory testing. These findings extend previous results in this group of monkeys in which deficits in spatial and temporal visual function were observed.

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.

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.

NOVA SKED II: A behavioral notation language utilizing the Data General Corporation real-time disk operating system

A software system (NOVA SKED) was developed for the experimental control and collection of data from operant behavior experiments that is compatible with the Data General Corporation real-time disk operating system (RDOS). NOVA SKED is based on the SKED state notation language originally implemented on Digital Equipment Corporation PDP-8 series of minicomputers. The system includes a compiler written in FORTRAN, a multitasking run-time system that can be configured to run up to 30 stations, a data back-up system, standard data manipulation programs and subroutines, and user manuals.

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

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

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

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

Acute trimethyltin intoxication in the monkey (Macaca fascicularis)

Adult cynomolgus monkeys were administered trimethyltin (TMT) iv in dosages ranging from 0.75 to 4.0 mg TMT/kg and observed for behavioral changes. Animals were subsequently killed for light and electron microscopic examination. TMT showed a dose-related toxicity, with high dose animals (4.0 and 3.0 mg/kg) dying within 24 hr, and low dose animals (0.75 mg/kg) surviving without morphological effects. Animals given 1.10 mg TMT/kg displayed a reproducible clinical course, characterized by tremor, hyperactivity, and ataxia which progressed to stupor and finally unconsciousness. By light microscopy, neuropathology was most pronounced in the CA-3 and CA-4 regions of Ammons horn. Degenerating pyramidal neurons, micro- and astrogliosis, and neuronophagia were commonly observed. Mild degenerative changes were identified in amygdala, medulla, spinal cord, and Purkinje cells. The fascia dentata remained intact. Ultrastructurally, injured neurons contained accumulations of lysosomes and lysosome-like structures within perikarya and neurites. Demyelination or vascular damage was not observed. Data indicate the monkey to be highly sensitive to TMT, with morphological injury most severe in limbic structures.