Modulation of hierarchical motor sequence learning by alpha-synuclein and the adenosine A_2A receptor

Parkinson's disease (PD) is characterized pathologically by the aggregation of alpha-synuclein (α-Syn) and dopaminergic degeneration and clinically by motor and non-motor deficits, including executive dysfunction and a reduction in the speed of information processing. The neuronal mechanisms underlying executive dysfunction in PD has yet to be elucidated; consequently, there is no effective treatment. In this study, we combined a hierarchical motor sequence learning paradigm, composed of multiple layers of action organization, to simulate the planning, initiation, termination and transition of motor sequence learning, with the focal aggregation of α-Syn in the substantial nigral compacta (SNc). Our goal was to investigate how α-Syn and the adenosine A_2A receptor modulate executive functions. Our analysis revealed that the expression of A53T-α-Syn in SNc impaired executive function, as evidenced by (1) a deficiency in action element learning, (2) a reduction in the intermediate subsequence chunking accuracy of initiation, transition, termination, (3) a decline in high-order sequence execution efficiency, and (4) a reduction in processing speed. Furthermore, pharmacological blockade of the A_2AR improved information processing speed and also reversed α-Syn-induced motor sequence learning deficit. The focal knockdown of A_2ARs in the dorsolateral striatum reversed the α-Syn-induced impairment in motor sequence learning. Collectively, these findings suggest that the aggregation of α-Syn in SNc impaired hierarchical motor sequence learning, thus providing a behavioral model to investigate the executive dysfunction associated with PD. Additionally, the pharmacological blockade of A_2AR reversed hierarchical motor sequence-learning deficit in PD, proposing a promising therapeutic target on PD-associated executive dysfunction.