Laboratory Rat - Use in Research

Use in Research

Over the years, rats have been used in many experimental studies, which have added to our understanding of genetics, diseases, the effects of drugs, and other topics in health and medicine. Laboratory rats have also proved valuable in psychological studies of learning and other mental processes. The historical importance of this species to scientific research is reflected by the amount of literature on it, roughly 50% more than that on mice.

Domestic rats differ from wild rats in many ways: They are calmer and less likely to bite, they can tolerate greater crowding, they breed earlier and produce more offspring, and their brains, livers, kidneys, adrenal glands, and hearts are smaller.

Scientists have bred many strains or "lines" of rats specifically for experimentation. Most are derived from the albino Wistar rat, which is still widely used. Other common strains are the Sprague Dawley, Fischer 344, Holtzman albino strains, the Long-Evans, and Lister black hooded rats. Inbred strains are also available but are not as commonly used as inbred mice.

Rat strains are generally not transgenic, or genetically modified, because the gene knockout and embryonic stem cell techniques that work in mice are relatively difficult in rats. This has disadvantaged many investigators, who regard many aspects of behavior and physiology in rats as more relevant to humans and easier to observe than in mice and who wish to trace their observations to underlying genes. As a result, many have been forced to study questions in mice that might be better pursued in rats. In October 2003, however, researchers succeeded in cloning two laboratory rats by nuclear transfer. So rats may begin to see more use as genetic research subjects. Much of the genome of Rattus norvegicus has been sequenced.

A 1972 study compared neoplasms in “Sprague-Dawley” rats from 6 different commercial suppliers and found highly significant differences in the incidences of endocrine and mammary tumors. There were even significant variations in the incidences of adrenal medulla tumors among rats from the same source raised in different laboratories. All but 1 of the testicular tumors occurred in the rats from a single supplier. The researchers found that the incidence of tumors in Sprague-Dawley rats from different commercial sources varied as much from each other as from the other strains of rats. The authors of the study "stressed the need for extreme caution in evaluation of carcinogenicity studies conducted at different laboratories and/or on rats from different sources."