Serotonin is a stimulating neurotransmitter involved in several everyday functions, such as sleep and emotion. In addition, serotonin is important for processing pain stimuli. Serotonin is a neurotransmitter that does not break down when it has been used in the synaptic cleft. Instead, it is re-absorbed into the presynaptic cell. This process is called "re-uptake" of the neurotransmitter.
Serotonin is formed from a substance called tryptophan. This is a substance that can cross the blood-brain barrier by competing with other amino acids. Because of this mechanism, it is possible for food (some of which contains tryptophan) to cause more or less serotonin to be produced in the brain.
Depression is often associated with too low a level of serotonin in the brain. The medications used against depression for this reason are substances that inhibit the reuptake of serotonin. Maybe you have heard of SSRIs (Selective Serotonin Reuptake Inhibitors)? They thus cause serotonin to remain in the synaptic cleft longer, prolonging the action of serotonin.
Another function associated with serotonin is aggression. In fact, when only little serotonin is released by the brain it can be associated with more aggressive behavior. Unfortunately, the correlation found here is not so great that we can predict aggressive behavior based on the amount of serotonin in the brain, or vice versa.
Author: Myrthe Princen (translated by Melanie Smekal)