The study included rats, not people, so it's not yet clear if leptin has an antidepressant effect on humans. The possibility is worth exploring, the researchers write in Proceedings of the National Academy of Sciences.
Leptin is a hormone that's mostly made by the body's fatty tissue; it plays a role in regulating weight and appetite.
The brain is studded with leptin receptors, which could be a clue about the hormones' link to depression, writes Xin-Yun Lu, Ph.D, and colleagues. They work in San Antonio at the University of Texas Health Science Center's pharmacology department.
Depressed Mice Low on Leptin
Obviously, rats can't speak up and describe their depression. But stressed rats act helpless and listless, which researchers use as a model for depression.
Lu's team studied stressed rats. Some of the rats were attacked in their cages by other rats until they gave up and played dead. The rats' physical wounds were limited; the experience was more about psychological stress.
Stressed rats were also less interested in drinking sugar water, their only treat. Loss of pleasure in enjoyable activities is also a hallmark of depression in humans.
The stressed rats had low levels of leptin in their brains, the study shows.
Leptin Injections Tested
Another group of rats was forced to swim, an activity rats don't like. Forced-swim tests are often used to study stress in rats.
The scientists injected the rats' brains with leptin or artificial brain/spinal fluid. The rats that got the leptin shots didn't give up as easily during the swim test.
"Leptin produced antidepressant-like activity in the forced swim test," the researchers write. They add that depressed rats injected with leptin also regained their preference for sugary water over plain water.
Each rat only got one shot of leptin. The injection was given straight into their hippocampus, a brain area involved in memory and emotions.
New Depression Treatments?
Animal models of depression aren't perfect, write Lu and colleagues. But studying the brain's use of leptin could lead to new depression treatments, they note.
Depression is a serious illness that affects up to a fifth of the world's population, the researchers write. Depression is often treatable; the first step is seeking help.
The science behind depression is complex. Low leptin levels may be more important in some depressed people than in others, write Lu and colleagues.
SOURCES: Lu, X. Proceedings of the National Academy of Sciences, early online edition. WebMD Medical Reference provided in collaboration with The Cleveland Clinic: "The Causes of Depression." News release, Proceedings of the National Academy of Sciences.
By Miranda Hitti
Reviewed by Louise Chang, M.D.
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