Scientists already knew that many people with depression had high levels of the stress hormone cortisol. But it was not clear whether cortisol caused the condition or was a consequence of it.
The new study provides strong evidence that long-term exposure to cortisol contributes to the symptoms of depression. The researchers from Harvard Medical School exposed 58 mice to cortisol for both short and long periods of time. The animals were then tested by being placed in a small dark compartment.
Mice given the stress hormone for more than two weeks took significantly longer to emerge from the compartment into a brightly lit open field. They were more fearful and less willing to explore a new environment. Chronic treatment also dulled their reactions to startling stimuli.
The findings, published in the journal Behavioural Neuroscience, fit in with human evidence. People with Cushing's disease, in which too much cortisol is released, commonly suffer depression and anxiety. People receiving corticosteroid therapy for inflammatory conditions and other disorders are also known to have mood problems.