Dopamine crystals, differential interference contrast light micrograph. Dopamine is a neurotransmitter and a peripheral hormone. It plays a key role in the control of locomotion, reward-motivated behaviour, learning, working memory, cognition, and emotion. In the brain, altered levels of dopamine activity are linked to Parkinson's disease, schizophrenia, fibromyalgia, and amphetamine and cocaine addiction. It is marketed under various trade names, including Intropin and Revivan. Magnification: x25 when printed at 10 centimetres tall.