Parkinson's disease (PD) is a movement disorder caused by the loss of dopaminergic innervation, particularly to the striatum. PD patients often exhibit sensory impairments, yet the underlying network mechanisms are unknown. Here we examined how dopamine (DA) depletion affects sensory processing in the mouse striatum. We used the optopatcher for online identification of direct and indirect pathway projection neurons (MSNs) during in vivo whole-cell recordings. In control mice, MSNs encoded the laterality of sensory inputs with larger and earlier responses to contralateral than ipsilateral whisker deflection. This laterality coding was lost in DA-depleted mice due to adaptive changes in the intrinsic and synaptic properties, mainly, of direct pathway MSNs. L-DOPA treatment restored laterality coding by increasing the separation between ipsilateral and contralateral responses. Our results show that DA depletion impairs bilateral tactile acuity in a pathway-dependent manner, thus providing unexpected insights into the network mechanisms underlying sensory deficits in PD. VIDEO ABSTRACT.

The dorsolateral striatum (DLS) receives excitatory inputs from both sensory and motor cortical regions. In the neocortex, sensory responses are affected by motor activity, however, it is not known whether such sensorimotor interactions occur in the striatum and how they are shaped by dopamine. To determine the impact of motor activity on striatal sensory processing, we performed in vivo whole-cell recordings in the DLS of awake mice during the presentation of tactile stimuli. Striatal medium spiny neurons (MSNs) were activated by both whisker stimulation and spontaneous whisking, however, their responses to whisker deflection during ongoing whisking were attenuated. Dopamine depletion reduced the representation of whisking in direct-pathway MSNs, but not in those of the indirect-pathway. Furthermore, dopamine depletion impaired the discrimination between ipsilateral and contralateral sensory stimulation in both direct and indirect pathway MSNs. Our results show that whisking affects sensory responses in DLS and that striatal representation of both processes is dopamine- and cell type-dependent.

Striatal cholinergic interneurons (CINs) can drive local dopamine release via nicotinic acetylcholine receptors (nAChRs) expressed on dopaminergic axons, but their role in modulating serotonin (5-HT) signaling is poorly understood. Here, we show that synchronous activation of CINs directly triggers local 5-HT release in the dorsal striatum via nAChRs expressed on serotonergic axons. This CIN-5-HT coupling is not detectable in the ventral striatum, despite its substantially denser serotonergic innervation. The nAChR-dependent release not only increases 5-HT levels in the dorsal striatum, but also expands the spatial footprint of serotonergic signaling. In Sapap3 mice, a model of obsessive-compulsive disorder (OCD)-like behavior, this mechanism is exaggerated due to a hypercholinergic state, selectively amplifying the nAChR-dependent component of monoamine release. These findings demonstrate a regionally confined form of acetylcholine-5-HT crosstalk in the striatum and identify CINs as regulators of 5-HT dynamics in both healthy and pathological states.

Midbrain dopamine neurons promote reinforcement learning and movement vigor. An outstanding question is how dopamine-recipient neurons in the striatum parse these heterogeneous signals. Previous work suggests that cholinergic striatal interneurons may gate dopamine-dependent plasticity, but this has not been tested in behaving animals. Here we studied rats performing a decision-making task with reward-related and movement-related events. Optical measurement of dopamine and acetylcholine release in the dorsomedial striatum (DMS) revealed that reward cues evoked cholinergic pauses with different phase relationships relative to dopamine. When dopamine lagged cholinergic dips, dopamine predicted future behavior and DMS firing rates on subsequent trials. In contrast, when dopamine preceded cholinergic dips, there was no observable relationship between dopamine and learning. Finally, when dopamine was coincident with cholinergic bursts, it preceded and predicted the vigor of contralateral orienting movements. Our findings suggest that cholinergic dynamics determine whether dopamine promotes vigor or learning, depending on the instantaneous behavioral context.

Tissue clearing has been widely used for fluorescence imaging of fixed tissues, but its application to live tissues has been limited by toxicity. Here we develop minimally invasive optical clearing media for fluorescence imaging of live mammalian tissues. Light scattering is minimized by adding spherical polymers with low osmolarity to the extracellular medium. A clearing medium containing bovine serum albumin (SeeDB-Live) is compatible with live cells, enabling structural and functional imaging of live tissues, such as spheroids, organoids, acute brain slices and the mouse brains in vivo. SeeDB-Live minimally affects neuronal electrophysiological properties and sensory responses in vivo, and facilitates fluorescence imaging of deep cortical layers in live animals without detectable toxicity to neurons or behavior. We further demonstrate its utility to epifluorescence voltage imaging in acute brain slices and in vivo preparations. Thus, SeeDB-Live expands both the depth and modality range of fluorescence imaging in live mammalian tissues.

Local dendritic computations are thought to critically influence neuronal signaling and plasticity yet remain largely unexplored in vivo due to challenges in stably imaging small structures at ultrafast timescales. We developed a 3D real-time motion correction platform for movement-stabilized, ultrafast two-photon voltage imaging. By co-labeling CA1 pyramidal neurons with voltage and calcium indicators, we simultaneously measured somato-dendritic and electro-calcium coupling at multiple dendritic sites. We characterized isolated dendritic spikes and distance-dependent backpropagation of naturally occurring and photostimulation-evoked bursts and single spikes. We found that bursts backpropagated more reliably than single spikes, validated that somato-dendritic coupling decreases with distance from soma, and showed that electro-calcium coupling decreases with increasing branch order. These findings provide in vivo evidence for distance-dependent invasion of somatic signals into dendrites, highlight the prevalence of isolated dendritic events, and show that dendritic structure isolates voltage from calcium signaling, potentially enabling unique intracellular pathways in distal dendrites.