Parkinson's is initially a 'low dopamine problem'.  We know this because it is treated with the dopamine agonist, levodopa, a precursor of dopamine. Also, low dopamine levels are a systemic problem, therefore, low dopamine is experienced in the sensory area of the brain, as well, and downgrades the 'sense of smell' as well as the other four senses. This accounts for the loss of smell in Parkinson's patients. There is a very good different reason, however, why dopamine receptor cells in the Substancia Nigra are dying.