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Restoring the balance between excitatory and inhibitory circuits in the basal ganglia , following the loss of dopaminergic ( DA ) neurons of the substantia nigra pars compacta , represents a major challenge to treat patients affected by Parkinson ' s disease ( PD ) .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
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