TL;DR: Leptin is a hormone that affects satiety and dopamine release. Researchers broke this pathway in mice and they ran a lot more because they didn’t get the dopamine from running.
In an experiment with mice, researchers compared normal mice to those that were genetically engineered to lack a leptin-sensitive protein called STAT3 that relays the leptin signal to release the reward chemical dopamine.
…the genetically engineered mice ran an extraordinary amount, nearly twice as much as the normal mice…
The new study supports previous research in humans that showed that low leptin levels are associated with exercise addiction and fast marathon times.