Kimberly S. Grant

Comparison of Blood and Brain Mercury Levels in Infant Monkeys Exposed to Methylmercury or Vaccines Containing Thimerosal

Thimerosal is a preservative that has been used in manufacturing vaccines since the 1930s. Reports have indicated that infants can receive ethylmercury (in the form of thimerosal) at or above the U.S. Environmental Protection Agency guidelines for methylmercury exposure, depending on the exact vaccinations, schedule, and size of the infant. In this study we compared the systemic disposition and brain distribution of total and inorganic mercury in infant monkeys after thimerosal exposure with those exposed to MeHg. Monkeys were exposed to MeHg (via oral gavage) or vaccines containing thimerosal (via intramuscular injection) at birth and 1, 2, and 3 weeks of age. Total blood Hg levels were determined 2, 4, and 7 days after each exposure. Total and inorganic brain Hg levels were assessed 2, 4, 7, or 28 days after the last exposure. The initial and terminal half-life of Hg in blood after thimerosal exposure was 2.1 and 8.6 days, respectively, which are significantly shorter than the elimination half-life of Hg after MeHg exposure at 21.5 days. Brain concentrations of total Hg were significantly lower by approximately 3-fold for the thimerosal-exposed monkeys when compared with the MeHg infants, whereas the average brain-to-blood concentration ratio was slightly higher for the thimerosal-exposed monkeys (3.5 ± 0.5 vs. 2.5 ± 0.3). A higher percentage of the total Hg in the brain was in the form of inorganic Hg for the thimerosal-exposed monkeys (34% vs. 7%). The results indicate that MeHg is not a suitable reference for risk assessment from exposure to thimerosal-derived Hg. Knowledge of the toxicokinetics and developmental toxicity of thimerosal is needed to afford a meaningful assessment of the developmental effects of thimerosal-containing vaccines.

Methylmercury exposure and reproductive dysfunction in the nonhuman primate

Macaca fascicularis females were monitored daily through four menstrual cycles then orally administered 0, 50, or 90 micrograms/kg/day methylmercury hydroxide (MeHg). Females were monitored through four additional menstrual cycles and after approximately 124 days of MeHg treatment were time-mated to nontreated males. MeHg treatment did not affect menstrual cycle or menses length. The relationship between dosage of MeHg and reproductive outcome approached but did not reach statistical significance. Reproductive failure (i.e., nonconception, abortion) was, however, related to a significantly higher blood Hg concentration than reproductive success for MeHg-treated females. The offspring of MeHg-treated females tended to be smaller than control infants, but no statistically significant decrease in gestation, birthweight, or crown-rump length was observed. None of the females receiving MeHg exhibited signs of MeHg toxicity during breeding or pregnancy. Daily treatment with 90 micrograms/kg/day MeHg for nearly 1 year, however, produced signs of toxicity in four of seven females. Toxicity was related to increased maternal size, duration of MeHg treatment, and a blood Hg concentration of 2.3 to 2.8 ppm.

Domoic acid: Neurobehavioral consequences of exposure to a prevalent marine biotoxin

Domoic acid (DA), the cause of Amnesic Shellfish Poisoning, is a naturally occurring marine biotoxin that is usually produced by the microscopic algae Pseudo-nitzschia. As is the case for other types of toxic algae, Pseudo-nitzschia outbreaks are becoming more frequent. Acute high-dose symptomology in humans includes vomiting, cramping, coma and death as well as neurological effects such as hallucinations, confusion and memory loss. Experimental studies and medical reports have collectively shown that DA exposure primarily affects the hippocampal regions of the brain and is associated with seizures and the disruption of cognitive processes. The neurobehavioral signature of DA is unique in that it includes transient and permanent changes in memory function that resemble human antegrade amnesia. Experimental studies with adult nonhuman primates have established that DA is a dose-dependent emetic that produces clinical and neuropathological changes consistent with excitotoxicity. Behavioral evaluations of treated rodents have shown that hyperactivity and stereotypical scratching are the first functional markers of toxicity. Mid-dose treatment is associated with memory impairment and behavioral hyperreactivity, suggesting changes in arousal and/or emotionality. At higher doses, DA treatment results in frank neurotoxicity that is characterized by seizures, status epilepticus and death in treated animals. The route of DA exposure is important and influences the severity of effects; intraperitoneal and intravenous treatments produce classic signs of poisoning at significantly lower doses than oral exposure. While developmental studies are few, DA readily crosses the placenta and enters the fetal brain. Domoic acid is not associated with congenital dysmorphia but is linked to persistent changes in motor behavior and cognition in exposed offspring. Comparative research suggests that functional losses associated with DA can be persistent and injuries to the CNS can be progressive. Long-term studies will be necessary to accurately track the expression of DA-related injury, in health and behavior, over the lifespan.

Infant monkeys' visual responses to drawings of normal and distorted faces

Face‐like patterns attract attention from both human and nonhuman primates. The present study explored the facial preferences in infant pigtailed macaques (Macaca nemestrina). Twenty‐five subjects looked at 20 paired drawings of adult conspecific monkey faces, and their looking time was recorded. The facial features in the drawings were arranged in positions ranging from a normal to a scrambled face. The subjects looked at the normal face more than expected by chance (P < .02), suggesting a preference, whereas the distorted faces were observed randomly. The normal face may have been preferred because the eyes were in a normal position within the facial outline. Am. J. Primatol. 44:169–174, 1998.

Methods for studying nonhuman primates in neurobehavioral toxicology and teratology

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

A demonstration of the memory savings effect in infant monkeys

In Experiment I, it was found that a 10-s cumulative study time would elicit a robust novelty response in infant monkeys, whereas a 5-s cumulative study time would not. In Experiment 2, infant monkeys were given a 10-s cumulative familiarization period. Following a 48-hr delay, they then were given 6 visual paired-comparison test trials-3 were preceded by a 5-s cumulative refamiliarization period and 3 were not. The infant monkeys exhibited a novelty response only after refamiliarization. These findings suggest savings (the phenomenon whereby prior exposure to information facilitates the relearning of the material) and parallel E. H. Cornells (1979) work on human infants. Results are discussed in terms of implicit and explicit memory.

Cannabis use during pregnancy: Pharmacokinetics and effects on child development

Abstract The broad‐based legalization of cannabis use has created a strong need to understand its impact on human health and behavior. The risks that may be associated with cannabis use, particularly for sensitive subgroups such as pregnant women, are difficult to define because of a paucity of dose‐response data and the recent increase in cannabis potency. Although there is a large body of evidence detailing the mode of action of &Dgr;9‐tetrahydrocannabinol (THC) in adults, little work has focused on understanding how cannabis use during pregnancy may impact the development of the fetal nervous system and whether additional plant‐derived cannabinoids might participate. This manuscript presents an overview of the historical and contemporary literature focused on the mode of action of THC in the developing brain, comparative pharmacokinetics in both pregnant and nonpregnant model systems and neurodevelopmental outcomes in exposed offspring. Despite growing public health significance, pharmacokinetic studies of THC have focused on nonpregnant adult subjects and there are few published reports on disposition parameters during pregnancy. Data from preclinical species show that THC readily crosses the placenta although fetal exposures appear lower than maternal exposures. The neurodevelopmental data in humans and animals suggest that prenatal exposure to THC may lead to subtle, persistent changes in targeted aspects of higher‐level cognition and psychological well‐being. There is an urgent need for well‐controlled studies in humans and preclinical models on THC as a developmental neurotoxicant. Until more information is available, pregnant women should not assume that using cannabis during pregnancy is safe.

Measuring infant memory: Utility of the visual paired-comparison test paradigm for studies in developmental neurotoxicology

The assessment of brain function and behavior in young infants is central to understanding the effects of chemical exposure on central nervous system development. One approach to infant cognitive assessment, based on the direct observation of infant eye movements, is known as the Visual Paired-Comparison task. The Visual Paired-Comparison test methodology uses selective visual attention as a vehicle to study emerging recognition memory skills. The utility of this procedure to study visual recognition memory has been well established in both human and nonhuman primate infants. The primary outcome measure produced by this assessment technique is known as the Novelty Preference Score, reflecting the amount of time the infant spends actively looking at novel rather than familiar test stimuli. Visual recognition memory testing has demonstrated a strong sensitivity to conditions that may place infants at risk for poor developmental outcome (e.g. preterm birth, Down syndrome) and in humans; performance is significantly related to later measures of I.Q. and language competency. This assessment methodology has been successfully applied to the study of neurobehavioral effects after fetal neurotoxicant exposure. Field and laboratory studies have used tests of visual recognition memory to better understand the effects of compounds such as lead, methylmercury and polychlorinated biphenyls on emergent cognitive processing. The Visual Paired-Comparison paradigm and its capacity to measure recognition memory in preverbal infants provides a valid and theoretically meaningful approach to neurobehavioral assessment for studies in developmental toxicology and teratology.

Four decades of leading‐edge research in the reproductive and developmental sciences: The Infant Primate Research Laboratory at the University of Washington National Primate Research Center

The Infant Primate Research Laboratory (IPRL) was established in 1970 at the University of Washington as a visionary project of Dr. Gene (Jim) P. Sackett. Supported by a collaboration between the Washington National Primate Research Center and the Center on Human Development and Disability, the IPRL operates under the principle that learning more about the causes of abnormal development in macaque monkeys will provide important insights into the origins and treatment of childhood neurodevelopmental disabilities. Over the past 40 years, a broad range of research projects have been conducted at the IPRL. Some have described the expression of normative behaviors in nursery‐reared macaques while others have focused on important biomedical themes in child health and development. This article details the unique scientific history of the IPRL and the contributions produced by research conducted in the laboratory. Past and present investigations have explored the topics of early rearing effects, low‐birth‐weight, prematurity, birth injury, epilepsy, prenatal neurotoxicant exposure, viral infection (pediatric HIV), diarrheal disease, vaccine safety, and assisted reproductive technologies. Data from these studies have helped advance our understanding of both risk and resiliency in primate development. New directions of research at the IPRL include the production of transgenic primate models using our embryonic stem cell‐based technology to better understand and treat heritable forms of human intellectual disabilities such as fragile X. Am. J. Primatol. 75:1063–1083, 2013.

Exposure to Environmental Chemicals and Developmental Risk: Contributions from Studies with Monkeys

Publisher Summary Pregnant women and children are exposed to a vast array of synthetic chemicals in air, water, soil, and food. Effects on exposed children have been documented with a range of environmental pollutants including lead, methylmercury, arsenic, solvents, pesticides, and polychlorinated biphenyls (PCBs). The level of functional disruption associated with exogenous chemical exposure depends on multiple factors that include timing, route of exposure, and dose. In some cases, prenatal exposure can impair function in the offspring while leaving the mother apparently healthy and without evidence of clinical neurotoxicity. Animal models provide a critical link in the developmental consequences of exposure. Selection of the appropriate animal model for studies of environmental chemicals is generally based on factors such as conditions of exposure, public health significance, and economic constraints. Macaque monkeys are Old World primates and share a common evolutionary history with humans, including basic aspects of reproduction and development. Ovarian morphology and physiological control and timing of menses are close to identical in macaques and humans. Thus, the use of the monkey model provides key insights on the impact that chemical agents can have on the growth and development of species closely related to humans.