A University of Chicago team has found tumours produce chemicals which can lead to negative mood swings.
That perhaps explains cancer patients are especially vulnerable to depression, apart from apprehensions of impending death. Side-effects of chemotherapy could also be a contributory factor.
Now researchers say 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.
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."
The study features in Proceedings of the National Academy of Sciences.
It will be a while before it is established that what is true in the case of rats could also hold good in the case of humans, but the findings could lead to better cancer care.