A 60-year-old man has resting tremor, pill-rolling movements, rigidity, and bradykinesia. Which of the following is most likely to be decreased in this man?

The clinical features points toward a diagnosis of Parkinson disease. It is characterized by loss of dopaminergic neurons that project from the substantia nigra to the putamen. Other options: Serotonergic raphe neurons have been implicated in the control of mood and descending pain modulation (analgesia) pathway. GABAergic pathway loss from striatum (caudate nucleus + putamen) results in hyperkinetic disorder, Huntington disease. Loss of nucleus basalis of Meyne in the basal forebrain, a diffuse cholinergic projection to the entire neocoex, the amygdala, and the hippocampus leads to Alzheimer disease.