1. A treatment for Parkinson's Disease comprises: diagnosing a subject with Parkinson's disease; providing at least one stimulus electrode implanted in a subthalamic nucleus of the subject; providing at least one chemosensor implanted in a globus pallidus pars interna (GPi) of the subject; sensing a level of a neurochemical with the chemosensor; comparing the level of the neurochemical to a desired level of the neurochemical; and when the level of the neurochemical is less than the desired level, providing an electrical stimulation to the stimulus electrode.