M. Christopher Newland

The clearance of manganese chloride in the primate

Two macaque monkeys inhaled trace amounts of 54MnCl2 for 30 min. Subsequently the chest, head, and fecal radioactivity were monitored for over a year. The chest data curve required a sum of three exponential terms, with half-times ranging from 0.2 to 187 days, to attain a satisfactory fit. Head levels peaked 40 days after acute inhalation exposure and remained elevated for over a year. The excretion of manganese through the feces was best described by a sum of two exponentials. One had a half-time of less than 1 day and the second had a half-time of 50 to 60 days. A third macaque received a 6-week continuous exposure to 54Mn through a subcutaneous osmotic pump. With this route, manganese clearance from the head occurred at a faster rate than after acute inhalation exposure. Fecal elimination following continuous subcutaneous exposure resembled that following acute inhalation. Kinetic analyses suggested that the long half-times of manganese in the head following inhalation reflected both slow disappearance from the head and replenishment from other depots.

Aging unmasks adverse effects of gestational exposure to methylmercury in rats

The consequences of developmental exposure to methylmercury on behavior in aged animals were investigated. Methylmercury exposure was arranged by placing 0, 0.5 or 6.4 ppm Hg in the drinking water of female rats at least 4 weeks before mating and continuing until post-natal (PN) day 16. Brain Hg concentrations in cohorts of low- and high-dose offspring were 0.5 and 9.1 ppm at birth and 0.04 and 0. 52 ppm at weaning (described in another report). Lever pressing of female offspring was maintained under a Multiple Differential Reinforcement of High Rate 9:4 Extinction schedule of food reinforcement (Mult DRH 9:4 EXT). Under the DRH 9:4 schedule, a food reinforcer was delivered when nine responses occurred within 4 s. Under the Extinction schedule, responding had no programmed consequences. No exposure-related differences in reinforcement rate under the DRH schedule or discrimination between the DRH and extinction components were apparent initially. At 950 days of age, the overall response rates of controls had shown a gradual decline over the previous 500 days to about 80% of their beginning levels, but, otherwise, most controls were healthy. A gradual decline in the reinforcement rate began to appear in low- and high-dose rats at about 500 and 800 days of age, respectively. Microanalyses of the nine-response burst maintained by the DRH schedule revealed that the lever-press duration increased, the inter-response time (IRT) was unaffected, and the time between response bursts increased. Overall, the nine-response burst remained intact as a coherent response unit. The increased time between response bursts caused the decline in reinforcement rate. All rats displayed these effects as they aged, but the mercury-exposed rats did so sooner.

Human sensitivity to reinforcement in operant choice: How much do consequences matter?

The results of many human operant conditioning experiments appear to show that humans are less sensitive than nonhumans to operant consequences, suggesting species discontinuities in basic behavioral processes. A reanalysis of 31l data sets from 25 studies employing variable-interval schedules of reinforcement designed to assess sensitivity to reinforcement corroborates the claim that human behavioral allocation among alternatives often deviates from predictions based on rates of experimentally programmed consequences. Close inspection of the studies in question, however, suggests that methodological issues contribute heavily to the differences noted so far between humans and nonhumans and that an explanation based upon species discontinuities is not tenable.

Persistent effects of manganese on effortful responding and their relationship to manganese accumulation in the primate globus pallidus

Manganese produces signs and symptoms that suggest involvement of the basal ganglia, especially the globus pallidus and substantia nigra. Overt neurological signs have been reported in primates exposed to high levels of manganese (over 100 mg/kg) but little is known about the effects of lower doses. To examine these issues, three cebus monkeys were trained to operate a response device with their arms and legs by executing a rowing-like movement against a 3.9- to 4.1-kg spring through an arc length of 10 cm under a multiple fixed-ratio fixed-interval schedule of reinforcement. Over the course of 450 days, these monkeys were administered acute doses of 5 or 10 mg/kg iv of manganese chloride using a multiple baseline experimental design. Doses as low as 5 mg/kg provoked a large increase in the number of incomplete responses. The onset of manganeses effect appeared within days of exposure and developed over the course of several weeks. Its magnitude declined over the course of months, but after a cumulative dose of 10 to 40 mg/kg it did not return to baseline. Action tremor appeared at cumulative doses greater than 40 mg/kg and dystonia was never observed at the cumulative doses examined. Behavioral microanalysis revealed that manganeses effects initially appeared as increased variability of interresponse times and response duration. Later, the response pattern during the fixed ratio component shifted to one of progressively increasing durations through the course of the ratio. Magnetic resonance imaging revealed that the behavioral effects of manganese corresponded to an apparent increase in the manganese content of the globus pallidus and substantia nigra.

Methylmercury and nutrition: Adult effects of fetal exposure in experimental models

Human exposure to the life-span developmental neurotoxicant, methylmercury (MeHg), is primarily via the consumption of fish or marine mammals. Fish are also excellent sources of important nutrients, including selenium and n-3 polyunsaturated fatty acids (PUFAs), such as docosahexaenoic acid (DHA). Laboratory models of developmental MeHg exposure can be employed to assess the roles of nutrients and MeHg and to identify potential mechanisms of action if the appropriate exposure measures are used. When maternal exposure is protracted, relationships between daily intake and brain mercury are consistent and orderly across species, even when large differences in blood:brain ratios exist. It is well established that low-level developmental MeHg produces sensory deficits. Recent studies also show that perseveration in reversal-learning tasks occurs after gestational exposures that produce low micromolar concentrations in the brain. A no-effect level has not been identified for this effect. These exposures do not affect the acquisition or performance of discrimination learning, set shifting (extradimensional shift), or memory. Reversal-learning deficits may be related to enhanced impact of reinforcers as measured using progressive ratio reinforcement schedules, an effect that could result in perseveration. Also reported is enhanced sensitivity to dopamine reuptake inhibitors and diminished sensitivity to pentobarbital, a GABA(A) agonist. Diets rich in PUFAs or selenium do not protect against MeHgs effects on reversal learning but, by themselves, may diminish variability in performance, enhance attention or psychomotor function and may confer some protection against age-related deficits in these areas. It is hypothesized that altered reward processing, dopamine and GABAergic neurotransmitter systems, and cortical regions associated with choice and perseveration are especially sensitive to developmental MeHg at low exposure levels. Human testing for MeHgs neurotoxicity should emphasize these behavioral domains.

Developmental exposure to methylmercury alters behavioral sensitivity to D-amphetamine and pentobarbital in adult rats.

Female rats were exposed to 0, 0.5, or 6.4 ppm methylmercury in their drinking water before mating, and throughout gestation and lactation. When the female offspring were 4-6 months old, they were trained to respond under a multiple differential reinforcement of high rate (DRH) 9:4-- Extinction schedule of reinforcement. No differences among exposure groups were apparent in steady-state behavior. Drug challenges were conducted with multiple doses of D-amphetamine, scopolamine, pentobarbital, haloperidol, and dizocilpine, drugs selected for their different pharmacological effects. The ED(50) values for amphetamines reinforcement rate-reducing effects for the control, 0.5-, and 6.4-ppm groups were 3.1, 1.9, and 0.9 mg amphetamine/kg body weight, respectively, demonstrating an increased sensitivity to D-amphetamine in methylmercury-exposed rats. Rats in the 6.4-ppm group also demonstrated a relative insensitivity to pentobarbital. Further, these exposed rats exhibited an inverted U-shaped dose-effect curve under the pentobarbital dose-effect determination, while controls showed only a declining curve. Exposed rats did not respond differentially to haloperidol, scopolamine, or dizocilpine, suggesting specificity. The present data suggest an involvement of catecholaminergic and GABAergic activity in methylmercurys neurotoxicity.

Dietary selenium protects against selected signs of aging and methylmercury exposure

Acute or short-term exposure to high doses of methylmercury (MeHg) causes a well-characterized syndrome that includes sensory and motor deficits. The environmental threat from MeHg, however, comes from chronic, low-level exposure, the consequences of which are poorly understood. Selenium (Se), an essential nutrient, both increases deposition of mercury (Hg) in neurons and mitigates some of MeHgs neurotoxicity in the short term, but it is unclear whether this deposition produces long-term adverse consequences. To investigate these issues, adult Long-Evans rats were fed a diet containing 0.06 or 0.6 ppm of Se as sodium selenite. After 100 days on these diets, the subjects began consuming 0.0, 0.5, 5.0, or 15 ppm of Hg as methylmercuric chloride in their drinking water for 16 months. Somatosensory sensitivity, grip strength, hindlimb cross (clasping reflex), flexion, and voluntary wheel-running in overnight sessions were among the measures examined. MeHg caused a dose- and time-dependent impairment in all measures. No effects appeared in rats consuming 0 or 0.5 ppm of Hg. Somatosensory function, grip strength, and flexion were among the earliest signs of exposure. Selenium significantly delayed or blunted MeHgs effects. Selenium also increased running in unexposed animals as they aged, a novel finding that may have important clinical implications. Nerve pathology studies revealed axonal atrophy or mild degeneration in peripheral nerve fibers, which is consistent with abnormal sensorimotor function in chronic MeHg neurotoxicity. Lidocaine challenge reproduced the somatosensory deficits but not hindlimb cross or flexion. Together, these results quantify the neurotoxicity of long-term MeHg exposure, support the safety and efficacy of Se in ameliorating MeHgs neurotoxicity, and demonstrate the potential benefits of Se during aging.

High-rate operant behavior in two mouse strains: A response-bout analysis

Operant behavior sometimes occurs in bouts characterized by an initiation rate, within-bout response rate, and bout length. The generality of this structure was tested using high-rate nose-poking in mice. Reinforcement of short interresponse times produced high response rates while a random-interval schedule held reinforcement rates constant. BALB/c mice produced bouts that were more frequent, longer, and contained a higher within-bout rate of responding (nine nose-pokes/s) than did the C57BL/6 mice (five nose-pokes/s). Adding a running wheel decreased total nose-pokes and bout length, and increased bout-initiation rate. Free-feeding reduced nose-poking by decreasing bout-initiation rate. Photoperiod reversal decreased bout-initiation rate but not total nose-poke rate. Despite strain differences in bout structure, both strains responded similarly to the interventions. The three bout measures were correlated with overall rate but not with each other. Log-survival analyses provided independent descriptors of the structure of high-rate responding in these two strains.

Neurobehavioral toxicity of methylmercury and PCBs Effects-profiles and sensitive populations

A large and growing body of literature is available on the neurotoxicity of methylmercury and PCBs as expressed in the behavior of both humans and laboratory animals. Methylmercury and PCBs will be compared with PCBs with attention directed at overlaps and distinctions in their profiles of neurotoxicity. It is possible with methylmercury and, to a lesser extent, with PCBs to characterize the sensory, motor, and cognitive consequences of exposure. Methylmercury is emerging as a life-span developmental neurotoxicant: adverse effects of exposure have been identified in development and during aging in human populations as well as in laboratory animals. Less is known about the PCBs on this count. While the mechanisms of neurotoxicity are not understood for either class of compounds, emerging clues are pointing to the possibility of overlap in some mechanisms of neurotoxicity.

Quantification of motor function in toxicology

Disturbances of movement and other motor functions can result from exposure to toxicants and drugs. Sometimes, as with acute exposure to ethanol or solvents, these effects disappear when exposure ends. Other times, as with manganese, haloperidol, or chronic ethanol, motor disturbances are irreversible and may even lie undetected until after exposure has ended. Motor disturbances can take on many guises, including tremor, difficulty in positioning, fatigue, or rigidity. Techniques for measuring these different endpoints in primates will be addressed. One preparation that enables the simultaneous monitoring of positioning, tremor, and operant behavior in nonhuman primates is described, and tactics for obtaining spectral estimates of tremor from a positioning task are outlined. The spectra obtained from this preparation are reliable and valid: they are stable over a period of a year, they correspond to spectra obtained from accelerometers, and are altered by acute administration of ethanol or oxotremorine. These two drugs had opposite effects on tremor but affected bar positioning in a similar manner.