Purkinje Cell Collaterals Enable Output Signals from the Cerebellar Cortex to Feed Back to Purkinje Cells and Interneurons

Purkinje cells (PCs) provide the sole output from the cerebellar cortex. Although PCs are well characterized on many levels, surprisingly little is known about their axon collaterals and their target neurons within the cerebellar cortex. It has been proposed that PC collaterals transiently control circuit assembly in early development, but it is thought that PC-to-PC connections are subsequently pruned. Here, we find that all PCs have collaterals in young, juvenile, and adult mice. Collaterals are restricted to the parasagittal plane, and most synapses are located in close proximity to PCs. Using optogenetics and electrophysiology, we find that in juveniles and adults, PCs make synapses onto other PCs, molecular layer interneurons, and Lugaro cells, but not onto Golgi cells. These findings establish that PC output can feed back and regulate numerous circuit elements within the cerebellar cortex and is well suited to contribute to processing in parasagittal zones. •In adults, all Purkinje cells have axon collaterals confined to a parasagittal plane•Collaterals contact Purkinje cells, Lugaro cells, and molecular layer interneurons•Collaterals allow cerebellar output to regulate processing in parasagittal zones•Collaterals could regulate firing rate and synchrony in the cerebellum Witter et al. show that in the juvenile and adult cerebellum, Purkinje cells have collaterals that provide inhibitory feedback to neighboring Purkinje cells and interneurons. These collaterals could allow cerebellar output to feed back and regulate firing in parasagittal zones.