MDMA and MDA cause neurons to release a neurotransmitter called serotonin, which can overactivate serotonin receptors. Serotonin is important to many types of nerve cells, including cells that receive sensory information and cells that control mood, sleep, and memory. Animal studies have taught us that MDMA and MDA can damage fibers from these nerve cells. And even though some of these fibers grow back, they don’t grow back normally. They can wind up in places where they don’t belong.