Researchers from University of California, San Diego School of Medicine have said that chronic depression originates from more ancient mechanisms used by the body to deal with physical injury, such as pain, tissue repair and convalescent behaviour.
Athina Markou professor of psychiatry, and Karen Wager-Smith, a post-doctoral researcher, integrated evidence from diverse clinical, biological and behavioural studies to create a novel theory they hope will lead to a shift in thinking about depression.
"Once we had a theoretical model for the biology of a well-functioning depressive response, it helped make sense of all the myriad differences between depressed and non-depressed subjects that the biomedical approach has painstakingly amassed," said Wager-Smith.
According to the new theory, severe stress and adverse life events, such as losing a job or family member, prompt neurobiological processes that physically alter the brain.
Neurons change shape and connections. Some die, but others sprout as the brain rewires itself.
This neural remodelling employs basic wound-healing mechanisms, which means it can be painful and occasionally incapacitating, even when it's going well.
Real problems occur only "when these restructuring processes go into overdrive, beyond what is necessary and adaptive, and for longer periods of time than needed. Then depression becomes pathological," Markou said.
If psychological and physical pain responses share similar biological mechanisms, then analgesic agents could be useful in treating at least some symptoms of depression.
Similarly, if chronic depression is proven to be a neuroinflammatory condition, then anti-inflammatory treatments should also have some antidepressant effects.
However, Markou cautioned that much more specific research and larger clinical trials are required.
The study is published in the September online edition of Neuroscience and Biobehavioural Review.