The reason that anxiety and depression may be precursors of the appearance of Parkinson’s motor symptoms,it is suggested, may be due to the discovery, a few years ago, that the parkinsonian disease process starts in the brainstem and then advances to the midbrain. The brainstem contains serotonin and adrenergic neurons that could contribute to anxiety and depression, whereas the midbrain contains dopaminergic neurons that contribute to the motor symptoms of Parkinson’s.
The pathological gambling or hypersexuality that some Parkinson’s patients experience appears to be due, however, largely to the dopaminergic medications that patients are prescribed for their motor symptoms—not to the disease process itself..