Levodopa-induced dyskinesia (LID) is a common and disabling complication of dopaminergic therapy for Parkinson disease. Although striatal neuronal ensembles are implicated in the expression of dyskinetic movements, how upstream long-range inputs both recruit and restrain these ensembles has remained unclear. The study used activity-dependent ensemble access to probe afferent control of dyskinesia-linked striatal ensembles and to define synaptic and topographic modifications that accompany chronic levodopa treatment.
The investigators employed a unilateral 6-hydroxydopamine lesion mouse model to emulate parkinsonian dopamine depletion. They used FosTRAP-based ensemble labeling to identify and manipulate striatal neurons active during dyskinesia. This approach enabled projection-wide and pathway-restricted manipulations aimed at testing the causal roles of upstream inputs in promoting or restraining dyskinetic behaviors.
Using the ensemble-access strategy, two major afferent sources to the dyskinesia-associated striatal ensembles were identified: the secondary motor cortex (M2) and the parafascicular thalamus (PF). These inputs were characterized as dominant contributors to the ensemble’s afferent drive but exhibited opposing functional effects on dyskinesia when activated.
Projection-wide activation of M2 inputs enhanced dyskinetic behavior, indicating that cortical recruitment of the striatal ensemble favors LID expression. In contrast, activation of PF projections produced suppression of ongoing dyskinesia and shifted behavior toward non-dyskinetic states, demonstrating that PF exerts a restraining influence on the same ensemble. Thus, M2 and PF form functionally antagonistic long-range control over striatal ensemble activity linked to LID.
Chronic levodopa administration led to a net reduction in overall presynaptic terminal abundance across pathways. Despite this reduction, putative synaptic contacts onto ensemble neurons were preserved, which effectively increased pathway-to-ensemble coupling. In other words, although the total presynaptic footprint diminished, the remaining inputs onto ensemble neurons became more influential, consistent with enhanced functional drive from specific afferents.
The structural and functional remodeling induced by levodopa followed distinct rules depending on the afferent pathway. M2-derived contacts became more spatially dispersed and showed a bias toward NMDAR-mediated excitation, suggesting strengthened glutamatergic recruitment of ensemble neurons by cortical inputs. By contrast, PF inputs were associated with recruitment of stronger polysynaptic inhibition onto the ensemble, consistent with a thalamostriatal brake that limits excessive ensemble activation.
When dyskinesia recurred, ensemble-projecting neurons in M2 were preferentially re-engaged, indicating that the cortical component of the circuit is re-recruited during LID episodes. In PF, however, reactivated neurons were topographically segregated from those PF neurons that directly innervate the ensemble, suggesting that the thalamic suppression of dyskinesia may act via distinct, non-overlapping PF subpopulations or indirect circuits.
Functional tests using selective stimulation revealed that targeted activation of the M2-to-ensemble pathway promoted dyskinesia. In contrast, selective stimulation of the PF-to-ensemble pathway was ineffective at producing the same pro-dyskinetic effect. These results support the view that cortical inputs can directly recruit the dyskinesia-linked ensemble, whereas the thalamic restraint operates through different or indirect mechanisms that are not simply reversed by direct PF-to-ensemble excitation.
To probe the role of NMDAR signaling within the ensemble, the authors performed an ensemble-restricted knockdown of Grin1, an essential NMDA receptor subunit. Grin1 reduction within the ensemble decreased peak dyskinesia and attenuated dyskinesia driven by M2 activation. These findings link ensemble NMDAR-dependent excitation to cortical recruitment mechanisms underlying LID.
Collectively, the data define LID as an imbalance between cortical recruitment of striatal ensembles and a thalamostriatal brake that counteracts that recruitment. The work highlights pathway-specific remodeling after chronic levodopa: enhanced cortical NMDAR-mediated excitation versus thalamically mediated polysynaptic inhibition. Functionally, selective engagement of M2-to-ensemble projections promotes dyskinesia, while PF activation suppresses it, and ensemble NMDAR signaling is necessary for full cortical-driven dyskinesia expression.
These results point to distinct circuit-level targets for potential therapeutic modulation of LID, including pathway-specific interventions that reduce cortical NMDAR-dependent recruitment or augment thalamostriatal restraint. The article is a preprint and has not been peer reviewed; details such as exact experimental parameters, sample sizes, and statistical outcomes are reported in the original preprint but are not reproduced here. Further validation in other models and eventual translation will require peer-reviewed confirmation and additional experiments.