Behavioral toxicology is the study of behavioral changes induced by exposure to environmental substances. It is a hybrid science, its foundations coming from the behavioral sciences and from toxicology. With an increasing association between the behavioral sciences and neurology, neuro-behavioral toxicology may be an appropriate term to describe the field.
Studies on behavior change in response to drugs, chemicals, or environmental conditions or agents (collectively referred to herein as toxicants). Changes in behavior, as well as changes in sensation, mood, intellectual function and motor coordination, are used by behavioral toxicologists to identify risks associated with exposure to potential toxicants and determine the mechanisms by which toxicants can affect the central nervous system. Information obtained through the behavior of Toxicology can be used by scientists and government agencies to set limits permitted toxicant environmental exposure levels. The techniques used in the behavior of Toxicology include epidemiologic surveys, research-based and laboratory-based experiments using human or non-human subjects.
Studies of the behavioral effects on humans of exposure to toxic substances have lagged behind animal research for at least two reasons.
First, there are ethical limitations on exposing humans to potentially harmful substances. Second, most of the available techniques for measuring the effects of exposure have not proven to be sensitive to low-level exposure. The potential importance of behavioral toxicology lies in the realization that behavioral changes may be the earliest indicator of excessive exposure to a toxic substance.
The earliest symptoms of mercury poisoning in humans include irritability, anxiety, insomnia, hyper-reactivity, shyness, and emotional instability. The gradual mood and character changes go unnoticed by the victim, while producing increasing problems in social relationships, a tendency toward depression, and signs of hypochondria. Psychological testing has shown a positive correlation between length of mercury exposure and neurotic-ism and introversion.
Organic solvents have been investigated extensively to see if they produce early neurobehavioral effects. This research led to the establishment of the new disease organic solvent syndrome (OSS), recognized in Scandinavia as a consequence of exposure to solvents. The symptoms are nearly identical to complaints following heavy-metal exposure and include lack of initiative, inability to concentrate, excessive fatigue, emotional instability, headache, and sexual dysfunction.
One of the most difficult issues confronting behavioral toxicology is the possibility that some individuals may be hypersusceptible to particular substances. There have been claims that certain kinds of food additives and food dyes may cause hyperactivity in children; this is known as the Feingold hypothesis after its discoverer. A small number of hyperactive children do show extreme hypersusceptibility to certain food additives.
Behavioral teratology, a subarea of behavioral toxicology, is defined as the study of the functional effects of toxicant exposure during the development of the nervous system. The exposure may either occur prenatally or postnatally. Some toxicants studied include food additives, therapeutic and drugs of abuse, hormones, alcohol, heavy metals, and pesticides. The early postnatal rat is often used as an animal model, because the development of the rat’s brain in the first week of postnatal life is comparable with the developmental stage of the human brain at the end of the third trimester of pregnancy. Thus, according to Ruppert, “behavioral evaluation in rats following postnatal exposure to potential neurotoxicants can be a useful strategy for assessing the functional consequences of neurotoxic insult during later stages of brain development.”
Drugs ingested prenatally may affect fetal development and produce physical and/or behavioral impairment. For example, the ingestion of alcohol during pregnancy may produce fetal alcohol syndrome (FAS).A number of studies have pointed to the possibility of a neurotoxic effect of aluminum exposure as a potential cause of Alzheimer’s disease (AD). Some evidence supporting such a notion includes the finding of elevated aluminum concentrations in the brains of AD victims, the report that aluminum intoxication may play a role in the phenomenon of dialysis dementia (altered behavior and dementia in kidney disease patients undergoing dialysis), and a variety of animal studies with cats, rabbits, and rats.
As one example of an animal study that found evidence for behavioral changes following aluminum ingestion, Thorne et al. fed adult rats chow containing different amounts of aluminum and found that heightened brain levels of aluminum were correlated with poor performance on a passive-avoidance task and with difficulties on a visual discrimination task with reversal. The highest concentrations of aluminum in the animals were found in the hippo-campus; apparently the hippo campus has an affinity for concentrating metals such as lead and zinc.Because of the enormous quantity of potentially neurotic substances. both synthetic and natural, to which humans are routinely exposed. the field of behavioral toxicology has burgeoned since its inception in 1975.