Scientists have shown that the hormone released during periods of long-term stress is directly linked to depression. They already knew that many people with depression had high levels of the stress hormone cortisol, but it remained unclear until now whether the hormone caused the condition or was a consequence of it.

This study, conducted on mice, suggests that a long-term exposure to cortisol may actually contribute to depression. The mice were exposed to acute (24 hours) and chronic (17 to 18 days) doses of the rodent stress hormone corticosterone, putting it into their drinking water.

The mice with chronic exposure, in comparison to those with acute one, took longer to go out of small dark compartments into a brightly lit open field, a common test for anxiety in animals. Those with chronic exposure were more fearful and less willing to explore their new environment, also not showing normal reactions when being startled, another indication that their nervous system was overwhelmed.

Researchers noted that people with Cushing's disease, when too much cortisol is released, suffer depression and anxiety and those receiving corticosteroid therapy for inflammatory conditions also have mood problems.

"If indeed, as our study suggests, long-term exposure to elevated gluco-corticoid levels provokes anxiety-like responses, then this has important implications for depression. Establishing clear links between pathological changes in physiology and subsequent changes in behavior is key to designing novel psychiatric drugs that address the causal factors involved in disease", said the team of researchers at Harvard.