| Summary: | Summary: The functional impact of single interneurons on neuronal output in vivo and how interneurons are recruited by physiological activity patterns remain poorly understood. In the cerebellar cortex, molecular layer interneurons and their targets, Purkinje cells, receive excitatory inputs from granule cells and climbing fibers. Using dual patch-clamp recordings from interneurons and Purkinje cells in vivo, we probe the spatiotemporal interactions between these circuit elements. We show that single interneuron spikes can potently inhibit Purkinje cell output, depending on interneuron location. Climbing fiber input activates many interneurons via glutamate spillover but results in inhibition of those interneurons that inhibit the same Purkinje cell receiving the climbing fiber input, forming a disinhibitory motif. These interneuron circuits are engaged during sensory processing, creating diverse pathway-specific response functions. These findings demonstrate how the powerful effect of single interneurons on Purkinje cell output can be sculpted by various interneuron circuit motifs to diversify cerebellar computations. : Using dual two-photon guided patch-clamp recordings from molecular-layer interneurons and Purkinje cells in vivo, Arlt and Häusser show that single-interneuron spikes can potently inhibit Purkinje cell output and uncover the microcircuit logic by which interneurons and their targets are recruited by different excitatory afferents. Keywords: cerebellum, Purkinje cell, interneuron, patch clamp, inhibition, climbing fiber, glutamate spillover, in vivo, two-photon imaging, synaptic integration
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