A hormone released by fat cells that combats diabetes may also act as an anti-depressant, a study has shown.
Mice lacking the protein, adiponectin, were more likely to display symptoms of depression such as social aversion and helplessness.
Injecting the hormone into their brains appeared to have an anti-depressant effect.
Adiponectin is a metabolic hormone secreted by the adipocytes, the fat cells that cause bulging waistlines.
In humans it helps regulate blood sugar and the breakdown of fats. Despite being generated by fat cells, lean people have more of the hormone in their blood than obese individuals.
Low levels of adiponectin are also associated with Type-2 diabetes. Studies have shown a link between diabetes and depression.
The new research, led by Dr Xin-Yun Lu, from the University of Texas at San Antonio, US, is reported in the journal Proceedings of the National Academy of Sciences.
The scientists wrote: "Taken together, these results suggest a critical role of adiponectin in depressive-like behaviours and point to a potential innovative therapeutic approach for depressive disorders."
British expert Dr Clare Stanford, from University College London, said: "This is an interesting paper. It is not at all certain that the behaviours tested in this paper are analogues of depression in humans. Nevertheless, the results do provide convincing evidence to link insulin secretion with the effects of anti-depressants on behaviour.
"A link between insulin secretion and anti-depression was first proposed several years ago but was not widely accepted at that time. This new evidence points the way to a completely new way of treating depression and could help explain why this disorder is often associated with a sudden change in body weight."