Dopamine
Dopamine is a neurotransmitter that plays many important roles in the human brain and body. If you have ADHD, scientists think that you got it from one of your parents' genes. Typically, a number of genes are involved. Many affect the dopamine neurotransmission. It is a chemical which is triggered by the feeling of being in love, or feeling affection. In most cases, it is also activated by nicotine or cocaine. It can also be activated by marijuana.