Striatal neural ensemble codes for voluntary locomotor and involuntary dyskinetic movements

Classical models of movement control posit that striatal spiny projection neurons of the basal ganglia’s direct and indirect pathways (dSPNs and iSPNs) respectively promote and suppress movement. Supporting this view, physiological recordings have revealed imbalanced dSPN and iSPN activity levels during hypokinetic and hyperkinetic movement conditions. However, in normal brain states, dSPN and iSPN ensembles have approximately equal activation amplitudes and time courses, jointly encoding specific actions. How pathological movement conditions alter such action coding remains poorly understood. Here we imaged the concurrent dynamics of dSPNs and iSPNs in behaving mice across normal, hypokinetic, and hyperkinetic conditions, before and after administration of drug treatments used clinically. Analyses focused on resting periods and neural activity that immediately preceded movement, examining how SPNs encoded upcoming actions. In hypokinetic states, the dSPN population was hypoactive relative to the iSPN population, consistent with prior reports. Moreover, individual dSPNs and iSPNs that encoded upcoming locomotion exhibited a reduced measure of activity compared to the normal state; the extent of this reduction predicted the degree of decline in the occurrence of locomotion. Levodopa (L-DOPA) and amantadine treatments both improved locomotion frequency but acted via distinct mechanisms. L-DOPA rebalanced the activity of the dSPN and iSPN populations, whereas amantadine boosted the activity of individual locomotion-related dSPNs and iSPNs. In hyperkinetic states modeling L-DOPA-induced dyskinesia, dSPN populations were hyperactive relative to iSPN populations. Involuntary dyskinetic movements engaged individual dSPNs and iSPNs distinct from those encoding voluntary locomotion. Amantadine treatment reduced the resting activity of dyskinesia-but not locomotion-related SPNs without improving the overall dSPN and iSPN imbalance. These findings highlight the importance of SPN action coding, not merely the extent of activity balance, for normal and pathological movements. The results delineate two distinct therapeutic mechanisms, one that rebalances the activity of the direct and indirect pathways and another that selectively potentiates or depresses the activity of SPN populations encoding voluntary or involuntary actions. Overall, this study refines the understanding of striatal dysfunction in movement disorders, demonstrates that distinct neural populations underlie normal voluntary locomotion and involuntary dyskinetic movements, and defines two complementary routes for the development of symptomatic treatments.