One of these systems activates behavior while the other inhibits it. Among other things, the BFS controls the release of a chemical called dopamine. High levels of dopamine tend to cause outgoing, pleasure-seeking behaviors associated with extroverted personality types. Low levels of dopamine have the opposite effect. People with damage to the BFS are less motivated to work for rewards and are also less likely to take stimulant drugs.
Chemicals can also interact with each other.
When MAO levels are high, dopamine levels decrease. The BIS is characterized by inhibitory emotions, such as anxiety or restraint. The chemical that regulates these emotions is serotonin. High levels of serotonin caused by an overactive BIS can cause extreme anxiety. Very low levels of serotonin can cause impulsiveness and oversensitivity, indicated by chronic irritability, extreme aggression, and even sudden violence.