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Answer 1
  1. Dopamine. The pathologic hallmark of Parkinson’s disease is degeneration of pigmented dopaminergic neurons in the pars compacta substantiae nigrae.1 Considerable degeneration of nigral neurons precedes the onset of symptoms, with a greater than 60% depletion of the neurons being the generally accepted figure. Reduction in striatal dopamine content by more than 80% is considered the threshold that must be surpassed prior to the development of clinical symptoms of Parkinson’s disease.2

    1. Hughes AJ. Clinicopathological aspects of Parkinson’s disease. Eur Neurol 1997;38 Suppl 2:13-20.

    2. Koller WC, Montgomery EB. Issues in the early diagnosis of Parkinson’s disease. Neurology 1997;49(1 Supp 1):S10-25.

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