Animation of the binding of dopamine to a dopamine receptor in a nerve cell membrane, showing the cascade of events it induces. Dopamine is an important neurotransmitter in the brain, involved in pleasure, addiction and reward. The dopamine receptor (left) is a transmembrane protein. Dopamine outside the cell binds to the receptor, inducing a conformational change that allows the part of the receptor inside the cell to bind to a G protein. This complex exchanges bound guanosine diphosphate (GDP) for guanosine triphosphate (GTP), which splits the G-protein trimer into three parts. The active alpha subunit then binds to a membrane-bound adenylyl cyclase enzyme (right), activitating it. This complex converts adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cyclic AMP). The cAMP is a second messenger, which acts inside the cell to effect the response promoted by the dopamine. This receptor system is an important target for many drugs, including those treating Parkinson's d