What is dopamine in the brain? What happens if you have too much or too little of it?

Neurotransmitter. Dopamine is a neurotransmitter. A chemical that enables a neuron to "communicate" with other neurons in the brain. Dopamine has many varied effects in the brain depending on the location in the brain. It can affect movement, e.g. Parkinson's disease involves too little dopamine. In other locations it can affect attention, pleasure, motivation, the "reward center", mood, and prolactin release.