Downregulation of striatal dopamine D2 receptors in advanced Parkinson's disease contributes to the development of motor fluctuation

The purpose of this study was to evaluate the contribution of the postsynaptic mechanism to the development of motor fluctuation in advanced Parkinson’s disease (PD). We used 123I-iodobenzamide single-photon emission computed tomography to measure the striatal dopamine D2 receptor densities in early levodopa-naïve PD, chronic PD with stable levodopa response, and advanced PD with fluctuating levodopa response. The basal ganglia/frontal cortex ratios at both hemispheres were calculated and averaged. PD patients with fluctuating levodopa response showed a significant decrease in striatal dopamine D2 receptor densities compared to those with early (1.57 ± 0.20 vs. 1.77 ± 0.12, p = 0.009) or chronic stable PD (1.57 ± 0.20 vs. 1.77 ± 0.10, p = 0.024). We conclude that the decreased D2 receptor densities in advanced PD reduced the ‘safety factor’ for synaptic transmission and contributed to the development of motor fluctuation.