Serotonin is a naturally occurring chemical found in the cells of the brain, in platelets, and in the intestine. In the central nervous system, it is a key neurotransmitter. In the blood vessels, it is released from platelets when blood vessel walls are damaged. Serotonin is also called 5-hydroxytriptamine.
During a Migraine, levels of serotonin and other neurotransmitters are affected. This is one reason we can have mood issues -- anxiety, depression, panic -- during a Migraine attack.
Additionally, serotonin seems to play a role in Migraine in general that is yet to be understood. It's theorized that the role of serotonin in Migraine is why antidepressants are often good Migraine preventive medications.
When taking some Migraine abortive medications (such as the triptans -- Imitrex, Maxalt, Zomig, etc.), antidepressants, and / or some other medications together, it is possible for a potentially dangerous condition called serotonin syndrome to develop. The "official" warnings about serotonin syndrome do not say that the medications cannot be used together, but do list symptoms that should be reported to our doctors. For more information on serotonin syndrome, see Antidepressants, Triptans, and Serotonin Syndrome.