All of the nervous system relies on neurotransmitters to function. Many drugs and diseases act by changing the levels of neurotransmitters. Look at serotonin and dopamine, explain how each of them typically work in the brain, and find both a disorder and a drug that acts by changing their levels. What do you think happens if someone is born with fewer dopamine receptors than normal? Do you think they would be more or less likely to become addicted to opiates? Why?