Scientists have confirmed that cancers make sufferers depressed, but even before they actually know they have the disease.
Up to 60% of breast cancer patients develop mood disorders including depression and anxiety, but it was never clear whether this was a consequence of knowing the diagnosis and prognosis of the condition, a side-effect of chemotherapy, or possibly a chemical effect of the tumour itself.
Now a paper in the journal PNAS has shed some hormonal light on the problem, by studying rats with breast tumours. University of Chicago researcher Leah Pyter and her colleagues compared groups of rats that has breast tumours with healthy animals of the same age.
In behavioural tests the cancer-affected rats showed many hallmarks of depression and anxiety. They performed less vigorously in a "swim test" indicating that they were more prone to giving up on certain tasks which is suggestive of depression, they found small rewards of sugar solutions less attractive and in a marble-burying test designed to flush out signs of anxiety they also buried more marbles.
To find out what lay at the root of these altered behaviours the team then tested the levels of several hormones in the rats' blood and brains. The animals with cancers had higher levels of inflammatory hormones including IL1-beta, IL6, TNF-alpha and IL10, especially in a region of the brain called the hippocampus. This brain region also had increased levels of receptors for the stress hormone cortisol and, accordingly, the animals produced less blood cortisol when they were exposed to a stressful situation compared with healthy individuals.
This study shows, for the first time, that remote tumours can, in animals at least, directly alter the chemistry and function of the brain, making individuals prone to mood disorders, including anxiety and depression, and a loss of the ability to experience enjoyment.
Whilst cautious about extrapolating their findings to humans, the team point out that since, amongst patients, a diagnosis of depression is linked to reduced cancer survivorship and treatment non-compliance, understanding the mechanisms underlying cancer-associated mood disorders is of profound clinical significance.