Mark Gudesblatt, MD, responds:
Serotonin is a neurotransmitter found mainly in the brain, bowels, and blood platelets that carries signals along and between nerves. The chemical is linked to several functions, including cognition, mood, sexual behavior, pain, and the sleep-wake cycle. It is best known for helping to regulate mood; too little serotonin may cause depression. Drugs called selective serotonin reuptake inhibitors (SSRIs) are often prescribed to treat depression.
Too much serotonin can interfere with brain function, sometimes causing serotonin syndrome, an unpredictable, serious, and fatal complication that can make a person appear drugged, drunk, or delirious. It's not clear what causes the syndrome; the causes may be different for different people, including too high a dose, an allergic reaction, or difficulty metabolizing serotonin.
Beyond that, we're just beginning to understand serotonin's role in various neurologic conditions, including autism, migraine, Parkinson's disease, seizures, spinal cord damage and recovery as well as processing of pain signals in the peripheral nerves and spinal cord. For example, low levels of serotonin have been linked to migraines, a discovery that led to the development of a class of medications called triptans, which interact with serotonin pathways. Serotonin receptors are in various parts of the body as well as the brain and blood vessels within the brain. Triptans may work by narrowing blood vessels in the brain that become dilated during a migraine.
SSRIs also might be useful for treating Parkinson's-associated tremors by reducing anxiety and stress, which can exacerbate tremors. But it's difficult to know which receptor to target and which medication to use because of our limited understanding of all of the pathways and neurotransmitters involved in these complex processes.
Improved understanding of the role of serotonin in these disorders might lead to identifying additional pathways for treatment. For example, multiple medications are available to treat Parkinson's disease and other conditions that cause tremor but not for other movement disorders. Most of the current treatments for Parkinson's disease increase the level of dopamine, another brain chemical. Targeting serotonin pathways might provide additional effective therapies for managing tremors, freezing, or dyskinesias (involuntary movement) as well as mood problems that impact quality of life.
In a study published in the January 4, 2017, issue of the Journal of Neuroscience, researchers developed an atlas of serotonin receptors and transporters in the brain, based on MRI and positron emission tomography (PET) scans from more than 200 participants. The atlas may be a valuable tool for future studies investigating the brain's serotonin system.
Dr. Gudesblatt is medical director of the Comprehensive Multiple Sclerosis Care Center at South Shore Neurologic Associates in Islip, NY. He is also a member of the Practice Committee of the American Academy of Neurology and the Huntington Study Group and serves on the Science Advisory Board for the Spastic Paraplegia Foundation.
