Cancer patients often struggle with depression
Scientists have shown there may be biochemical reasons - quite apart from the mental trauma of diagnosis - why cancer patients can become depressed.
A University of Chicago team found tumours produce chemicals which can produce negative mood swings.
They say the findings shed new light on why depression is such a risk for many cancer patients.
The study, carried out on rats, features in Proceedings of the National Academy of Sciences.
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 Chicago study suggests a third possibility.
The researchers found that substances associated with depression are produced in increased quantities by tumours, and then are transmitted to the brain where they impact on the hippocampus - the area which regulates emotion.
In addition, chemical pathways which normally put a brake on the impact of these depression-causing substances appear to be disrupted when a tumour develops.
The researchers carried out tests on about 100 rats, some of which had cancer, to determine their emotional state.
Lack of motivation
They found animals with tumours were less motivated to try to escape when submitted to a swimming test - a condition similar to depression in humans.
Rats with tumours were also less eager to drink sugar water, a substance that usually attracts the appetites of healthy rats.
Further tests revealed that the rats with tumours had increased levels of cytokines in their blood and in the hippocampus when compared with healthy rats.
Cytokines are produced by the immune system, and an increase in cytokines has been linked to depression.
They also produced lower levels of the stress hormone corticosterone, which helps to regulate the impact of cytokines.
Lead researcher Dr Brian Prendergast said: "Rats are commonly used to test drugs that are being studied for potential human benefits, such as treating depression.
"In this case, examining behavioural responses to tumours in non-human animals is particularly useful because the rats have no awareness of the disease, and thus their behavioural changes were likely the result of purely biological factors."
Cancer Research UK senior science information officer Dr Alison Ross said: "As this study looking at cancer and depression was carried out in rats, we don't know whether the results will hold true in cancer patients.
"About one in 10 people with cancer gets clinical depression and the root causes are likely to be complex, but this study provides an intriguing suggestion that the cancer itself may have a part to play."