Dopamine drug crystals. Polarized light micrograph of crystals of the drug dopamine hydrochloride. 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. Field of view size: 297 x 445 micrometres.