Dopamine is a neurotransmitter and hormone naturally produced in the nervous system and adrenal glands of the human body. It affects the heart rate, blood pressure, locomotion, pleasure, and motivation.
In 2374, as part of an unethical experiment, Alzen, a Srivani medical researcher, increased Captain Katherine Janeway's dopamine levels to test her "behavioral restraints". (VOY: "Scientific Method")