The dopamine and serotonin pathways are two brain systems affected by drugs of abuse. They are illustrated here. By altering activity in these pathways, abused substances can influence their function. Dopamine neurons (shown in yellow) influence pleasure, motivation, motor function and saliency of stimuli or events. Serotonin (shown in red) plays a role in learning, memory, sleep and mood.
As a result of scientific research, we know that addiction is a disease that affects both brain and behavior.