SUMMARY
The cerebellum is characterized by stereotyped cytoarchitecture across all of its lobules. However, recent evidence
has revealed new complexity in these circuits, including variation in cells types, connectivity patterns, and receptor
expression. Within the vestibulocerebellum, there is an excitatory glutamatergic interneuron cell type known as the
unipolar brush cell (UBC). UBCs are heavily enriched within regions of the cerebellum controlling eye movements and
vestibular processing. UBCs receive input from mossy fibers entering the cerebellum and form a local recurrent feed-
forward excitatory network. This connectivity suggests that UBCs may perform large scale transformations on inputs to
the cerebellum. Several distinct functions for these cells within the cerebellum have been proposed, but the resulting
hypotheses have been difficult to test without selective genetic tools for targeting UBCs. We have found a transgenic
mouse line that selectively labels the most common subtype of UBCs. In addition, our preliminary data show that
vestibulocerebellar Purkinje cells make functional inhibitory synapses onto UBCs. The goals of this project are to
examine how UBCs modulate information flow to the cerebellum, using a combination of optogenetic,
electrophysiology, and imaging techniques and to characterize a novel feedback circuit from Purkinje cells to UBCs.
Aim 1 is to characterize the responses of Purkinje cells to optogenetic activation of UBCs. This will be accomplished
by using light-gated opsin ChR2 to activate UBCs directly and measuring either the firing responses or synaptic
currents in Purkinje cells. We will also use inhibitory opsins to transiently inactivate UBCs and examine how Purkinje
cells respond to mossy fiber input in the presence or absence of UBC activity. Aim 2 will examine new synaptic
connections between Purkinje cells and UBCs, using promoter-restricted specific optogenetic activation of Purkinje
cells with recordings from genetically identified UBCs. Using stimulation of mossy fiber inputs, we will record the
spatiotemporal dynamics of UBC activity. This will be done with both cell-attached recordings and rapid 3D light-sheet
imaging of calcium dynamics to image activity in dozens of UBCs simultaneously. To measure how circuit level
Purkinje cell inhibition of the granular layer neurons alters UBC activity, in interleaved trials, we will combine mossy
fiber activation with optogenetically evoked Purkinje cell firing. Carrying out these experiments will build a conceptual
framework to explain how UBCs operate within feedback circuits to regulate cerebellar output, relevant to motor
control in physiological and pathological conditions.