Depression: a response to sickness?
However, it’s what is happening in the brain that is so exciting. Inflammation rapidly triggers a change in the responsiveness of a critical mood-regulating network and the way that it connects to other areas of the brain. Remarkably, this is the same network that is impaired in depression. This suggests that depression - at least in some cases - could well be some type of aberrant sickness response.
Although this finding shows that mood changes induced by inflammation use the same brain network as depression, it doesn’t tell us whether inflammation could be a factor in clinical depression, which leads to
more prolonged periods of feeling down.
To study this further, depression researchers teamed up with a surprising group of patients – those with chronic hepatitis-C. The disease doesn’t cause particularly high levels of inflammation but if left untreated it slowly causes liver damage that can lead to cirrhosis and ultimately liver failure. To prevent this, and to potentially cure the hepatitis-C, most people are advised to take a six to 12-month course of antivirals and an immune-system boosting drug called interferon, which helps regulate inflammation.
But interferon injections not only induce sickness symptoms but
can also result in clinical depression. About one in three patients treated with interferon develop depression, typically within six to eight weeks of starting treatment. The depression presents in a similar way to other people suffering with depression, and shows a similar response to anti-depressant medication. This suggests that chronic immune stimulation can cause depression.