May 20 (HealthDay News) -- Doctors have long known that people with cancer often suffer from depression.
A new study in rats has found that the cause of the depression may be the properties of the tumor itself, rather than emotional distress over the diagnosis or side effects from chemotherapy.
The study is the first to identify a biological link between tumors and negative mood changes, according to the researchers, who published their study in the May 18 issue of the Proceedings of the National Academy of Sciences.
About 15 percent to 20 percent of patients diagnosed with cancer experience depression, according to the U.S. National Cancer Institute.
Researchers put about 100 rats through a series of exercises commonly used by researchers testing antidepressants in animal models. Some of the rats had cancerous tumors.
The investigators found that the rats with tumors were less motivated to escape when submitted to a swimming test, a condition that is similar to depression in humans. The rats with tumors also were less eager to drink sugar water, a substance that healthy rats usually find irresistible.
"In this case, examining behavioral responses to tumors in non-human animals is particularly useful, because the rats have no awareness of the disease, and thus their behavioral changes were likely the result of purely biological factors," said senior study author Brian Prendergast, associate professor of psychology at the University of Chicago.
The team determined that substances associated with depression are produced in increased quantities by tumors and are transmitted to the brain.
Additionally, pathways that normally moderate the impact of depression-causing substances are disrupted when a tumor develops.
Tumors induce changes in gene expression in the hippocampus, the portion of the brain that regulates emotion.
Rats with tumors had increased levels of cytokines in their blood and in the hippocampus when compared with healthy rats. Previous research has linked high levels of cytokines, which are produced by the immune system, to depression.
The research team also found that stress hormone production was altered in rats with tumors.
The rats with tumors had dampened production of the stress hormone corticosterone, which helps regulate the impact of cytokines. Reducing the production of corticosterone may increase the impact of cytokines.
"Our research shows that two types of tumor-induced molecules, one secreted by the immune system and another by the stress axis, may be responsible," said Leah Pyter, a postdoctoral fellow at the University of Chicago and lead study author. "Both of these substances have been implicated in depression, but neither has been examined over time frames and magnitudes that are characteristic of chronic diseases such as cancer."
SOURCE: University of Chicago, news release, May 18, 2009