This is what happens to nerve cell transmission when cocaine, shown in red, enters the brain's reward pathway. Cocaine blocks the reuptake pumps which act to remove dopamine from the synapse. More dopamine accumulates in the synapse, resulting in feelings of intense pleasure.
Unfortunately, prolonged cocaine use may cause the brain to adapt, such that it comes to depend on the presence of cocaine to function normally, "downregulating" the amount of dopamine present naturally.
Then, if the person stops using cocaine, there is not enough dopamine in the synapses, and the person experiences the opposite of pleasure--depression, fatigue, and low mood. The immediate, worst symptoms are called withdrawal.
Even long after the person has stopped using cocaine, brain abnormalities can persist, causing feelings of discomfort and craving for more of the drug to relieve these feelings.