Striatal Dopamine Actions and Movement: Inferences from Parkinson Disease

The nature of motor deficits in Parkinson disease (PD) and aspects of their improvements with ʟ-DOPA replacement therapy (LDRT) offer potential insights into striatal dopamine actions. The defining and most LDRT responsive feature of PD, bradykinesia, is a complex phenomenon exhibiting impairments of both simple and complex limb movements. LDRT significantly remediates the former but not the latter. LDRT pharmacodynamics has two major components, the Short Duration Response (SDR), with a time course of seconds to minutes, and the Long Duration Response (LDR), with a time course of days to weeks. LDRT pharmacodynamics suggests different striatal dopamine actions on different time scales. While many studies used PD subjects to investigate striatal dopamine actions, few take LDRT pharmacodynamics into account. Correlating bradykinesia features and LDRT pharmacodynamics with our present understanding of striatal dopamine actions suggests that LDRT failure to improve complex movement performance reflects loss of phasic dopaminergic signaling. The SDR is likely a result of partially restored tonic/volume striatal dopamine neurotransmission. There is no existing explanation for the LDR. Recent experiments isolating the LDR cast doubt on prior accounts of how LDRT remediates bradykinesia. Interactions between the SDR and LDR may give rise to novel properties. Longer duration effects of striatal dopaminergic signaling need to be incorporated into studies of striatal dopamine actions in both preclinical and clinical experiments. Careful studies of PD bradykinesia and LDRT pharmacodynamics offer potential avenues for future explorations of striatal dopamine actions. Systematic preclinical experiments are needed to optimize design of clinical experiments.