Most drugs of abuse--nicotine, cocaine, marijuana, and others--affect the brain's "" circuit, which is part of the limbic system.

Most drugs of abuse--nicotine, cocaine, marijuana, and others--affect the brain's "ventral tegmental area” circuit. This ventral tegmental area is located in the midbrain.  

Explanation:

Nicotine is a naturally occurring substance which effect the Working of brain. It triggers the ventral part of the brain which contains dopamine. This substance when released give the feeling of pleasure and joy. The effect of nicotine can be effective till 6 to 7 hours.

Marijuana is also known as Cannabis. Marijuana is a psychoactive drug which can also be used in medicine. But sometimes people also use it for their pleasure.

which of what? i am confused

this is true my friend.