The reason why cancer patients are more prone to depression has been explained by a new study on rats.
Researchers at the University of Chicago found that tumours produce chemicals, which can produce negative mood swings.
It has long been known that cancer is associated with depression. Experts thought this was likely to be either a result of the trauma of diagnosis, or possibly a side effect of chemotherapy treatment. The new study suggests a third possibility.
"Our research shows that two types of tumour-induced molecules, one secreted by the immune system and another by the stress axis, may be responsible," said Leah Pyter, a postdoctoral fellow and lead author of a paper.
"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," she added.
For the study, the researchers conducted a series of tests on about 100 rats, some of whom had cancer to determine their behavioral responses in tests of emotional state.
They used tests commonly used in testing anti-depressants on rats and found that the rats with tumours became less motivated to escape when submitted to a swimming test, a condition that is similar to depression in humans.
The rats with tumours also were less eager to drink sugar water, a substance that usually attracts the appetites of healthy rats.
The results also showed that the rats with tumours had increased levels of cytokines in their blood and in the hippocampus (the portion of the brain that regulates emotion) when compared with healthy rats.
Cytokines are produced by the immune system, and an increase in cytokines has been linked to depression.
The team also found that stress hormone production also was altered in rats with tumours. The rats with tumours also had dampened production of the stress hormone corticosterone. The hormone helps regulate the impact of cytokines and reducing its production therefore increases the impact of cytokines.
The study is published in the current issue of the Proceedings of the National Academy of Sciences.