Plasticity of the nigropallidal pathway in Parkinson's disease

The degeneration of nigrostriatal dopamine neurons in early Parkinson's disease (PD) is compensated in part by increased transmitter turnover in surviving neurons of the pathway. In this F-18-dopa positron emission tomography study, we demonstrate compensatory changes in PD in another midbrain dopamine projection to the basal ganglia, the nigropallidal projection to the internal segment of the globus pallidus (GPi). Increased F-18-dopa uptake in the GPi is seen in early PD which then is lost in advanced PD. Our early PD cases show an absence of significant clinical progression in the face of a continuing loss of nigrostriatal projections. This indicates a compensatory neuronal plasticity that we now show to involve the nigropallidal dopamine pathway to the GPi but not to the external segment of the globus pallidus (GPe). Enhanced function of the dopamine projection to the GPi serves, we propose, to maintain a more normal pattern of pallidal output to ventral thalamus and motor cortex in early PD, whereas loss of this adaptive pathway in advanced disease may be a pivotal step in the progression of the disease.