Viral Strategies for In Vivo Optogenetic Stimulation of Purkinje Cell Terminals and Electrophysiological Characterization of Downstream Cerebellar Nuclear Activity in Mice

The cerebellar cortex regulates motor and cognitive function through its sole output neurons, the Purkinje cells (PCs), which inhibit downstream targets, yet precise manipulation of PC activity in vivo remains technically challenging. Here, we describe two viral strategies with differing cell-type specificities for in vivo optogenetic control of PCs and functional assessment of neural activity in downstream target regions. Specifically, we detail strategies for expression of the fast channelrhodopsin ChETA in PCs that enable light-evoked activation of PC axonal terminals in medial deep cerebellar nuclei (mDCN), and demonstrate light-evoked modulation of mDCN neural activity using in vivo extracellular electrophysiology. This protocol provides a reproducible and adaptable framework for cell-type-specific manipulation and functional characterization of cerebellar circuits, with broad applicability to studies of cerebellar physiology, motor and nonmotor behavior, learning and disease.