What does it mean to say that someone is addicted to a drug? Why do people become addicted to a drug physically? Mentally? What is a neurotransmitter? What role do neurotransmitters play with regard to brain functions and behavior? What roles do dopamine and serotonin play with regard to drug use? How are dopamine and serotonin levels affected by various drugs? What is meant by the reward pathways or the reward center of the brain? What differences have been found between men and women in terms of effects of psychoactive drugs and addiction?