A 70-year-old man has a history of difficulty walking. On examination, the patient appears stiff and stooped, shows an expressionless face, and speaks in a monotonous voice. A tremor of his fingers is apparent, but ceases when he tries to reach for something. The patient dies 10 years later. At autopsy, the substantia nigra of the patient demonstrates reduced pigmentation. This pathologic finding was associated with reduced secretion of which of the following neurotransmitters?