Leptin is famed for controlling our weight and appetite. But the hormone, which is released by fat cells and gives the brain a reading of our fat stores, is also thought to act in brain areas involved in emotion. To explore this link, Xin-Yun Lu and her colleagues at the University of Texas Health Science Center in San Antonio stressed rats by, for example, separating them from other animals. The rats’ leptin levels plunged at the same time that they showed behavioural changes such as losing interest in a sugary drink, the kind of apathy that is often associated with human depression.
The team found that injections of leptin into otherwise healthy animals were as good as at least one known treatment in a test widely used to screen for new antidepressants. Leptin shot to fame in the mid 1990s when scientists discovered that a strain of immensely fat mice that eat voraciously lack a working copy of the gene. They found that leptin injections could help the mice to shed weight, raising the prospect that the hormone might be a miraculous fat fighter for the obese. That hope was dealt a blow when leptin failed to fight flab for most people in clinical trials. Since then, scientists have realized that obese people often have high levels of leptin and seem to have become resistant to its effects.