Levodopa Disrupts Activity Patterns and Encoding of Movement in Striatal Cholinergic Interneurons of Behaving Mice

Objectives: Levodopa treatment, the gold standard therapy for Parkinson's disease, can lead to levodopa-induced dyskinesias (LIDs). Recent studies in acute brain slices have implicated the neural dynamics of striatal cholinergic interneurons (CINs) in the pathophysiology of LID, reporting that their discharge becomes more bursty in dyskinetic mice. However, little is known about how parkinsonism and levodopa affect CINs in vivo. Methods: We used microendoscopic imaging in the dorsal striatum of the genetically encoded Ca²⁺ indicator, GCaMP6f, expressed in cholinergic neurons. We imaged the collective, simultaneous activity of several molecularly identified CINs in the dorsal striatum, in control mice, as well as in mice rendered parkinsonian with 6-hydroxydopamine (6-OHDA). Results: We found that 6-OHDA drove a levodopa-insensitive increase in the baseline rate of activity of CINs and a reduction in their responsiveness to self-initiated movement. Furthermore, levodopa treatment (even before the appearance of LIDs) drove an increase in the amplitude of the individual Ca²⁺ events (perhaps indicative of more burst-like discharge), which correlated with a further reduction in the number of CINs responsive to movement. Conclusions: 6-OHDA treatment disrupts the robust encoding of movement by CINs. Levodopa disrupts the activity of CINs, possibly making them more bursty in vivo, even before the manifestation of LIDs. Thus, molecularly-identified CINs are implicated in the pathophysiology of parkinsonism. Not only does levodopa fail to restore their normal dynamics, but it actually exacerbates their abnormal burstiness.