Researcher's naturalistic study depresses mouse-sized rodents

Arizona Summer Wildcat Research Instructor William Lesley Castro is doing post-doctoral work on Siberian dwarf hamsters to determine if mate separation causes unusual behavior and physiology distress. The bond between male and female hamsters is strong, like that of humans

In his laboratory at the Arizona Health Sciences Center, research instructor William Lesley Castro is separating mated pairs of Siberian dwarf hamsters to study the effect this stressful event has on the rodents' behavior and physiology.

Castro, who holds a doctorate in zoology from Arizona State University, is at the UA on a two-year fellowship doing post-doctoral research.

His principal interest is neurological and endocrine system functions and how stress affects those physiological processes.

The Siberian dwarf hamster is a tail-less, mouse-sized rodent native to southwest Siberia, Mongolia, and northern China. It is a nocturnal burrowing creature that usually forms monogamous male-female bonds.

The Siberian dwarf is unique among hamsters in this respect, Castro said. They select mates and remain paired for most of their 1-to-2-year lives, with the males participating in nesting and the care of pups.

Because the pair bond appears very strong, Castro believes separating a hamster couple is a significant, stressful event for the rodents.

In an experiment involving 40 animals, which includes a control group of hamster couples that stay together, Castro has observed that male hamsters, when separated from their mate, show a dramatic shift in behavior.

They begin to overeat, become lethargic and sleep more, and they gain weight.

"They become couch potatoes," Castro said. "There is a decrease in overall activity and we see a 10 percent to 25 percent increase in body weight within the first week after separation."

The male hamsters that remain paired with females do not experience any of these changes, and, curiously, neither do the female hamsters that are separated from their male mates, he said.

Castro also monitors hormonal changes, as well as changes in brain and endocrine system activity.

He has found significant increases in activity of the hypothalamic, pituitary, and adrenal glands (known to researchers as the HPA-axis), as well as a sharp increase in the level of the stress-hormone Cortosol in separated male hamsters.

Cortosol has been well-established in human and animal research to be linked with high levels of stress, and stress is also associated with increased HPA activity, Castro said.

Although many experiments study stress in animals, they often deal with laboratory stresses, such as electric shock, Castro said.

He finds the hamster model interesting because it is naturalistic. The break of a mated pair does occur in the wild, when one member is killed by a predator, for example.

The separation of a hamster couple is a social event that induces a whole set of physiological and behavioral changes that resemble some forms of depression in humans.

Depressive behavior that includes increased appetite and weight gain, decreased activity, and an increase in sleep, is known to psychologists as atypical depression.

This depressed population of hamsters, therefore, "may be a very good model for testing the efficacy of antidepressant drugs, and it may prove useful in developing preventative treatments for depression or obesity," Castro said.

Currently, however, what Castro learns about stress is being used to understand how stress may affect arthritis.

According to Dr. David E. Yocum, director of the Arizona Arthritis Center, people with rheumatoid arthritis report dramatic changes in the activity of their disease during periods of high stress.

Physicians and scientists have few clues about why arthritis strikes, what factors cause it, or how to treat it effectively, but it appears that stress plays a role.

Also, it seems that many forms of the disease may be caused by the body's own immune system attacking the joints.

Yocum says a study of stress in hamsters can help researchers better understand the immune system and hormonal changes associated with stress.

"Many stress hormones are small, short-lived proteins that last seconds to minutes in the body," said Yocum. "The immune system monitors these hormones. It recognizes their existence and responds to them. This study (of hamsters) may give us an idea about where to look in humans for the immune receptors and help us understand and identify some of these small proteins," Yocum said.

Castro will give an informal lecture "The Siberian Dwarf Hamster Blues: the Life and Times of an Animal Model of Depression" on Aug. 14 at noon. It will be hosted by the Arizona chapter of the American Association for Laboratory Animal Science and held in Arizona Health Sciences Center Room 3505.