Defining a subclass of cerebellar molecular layer interneurons by their Purkinje cell feedback

A subset of molecular layer interneurons (MLIs), previously termed Purkinje cell-input MLIs (PC-MLIs), receive inhibitory feedback from Purkinje cells (PCs), the sole output neurons of the cerebellar cortex. Here we have characterized the properties of these cells. Compared to other MLIs and Purkinje layer interneurons, they have distinct axonal and dendritic morphology, intrinsic electrical properties, and expression of marker genes. These interneurons also display distinctive circuit connectivity: they are the only MLI that receives feedback inhibition from PCs and receive minimal inhibition from basket or stellate cells. In turn, they mainly inhibit PCs and basket cells. These interneurons do not form electrical synapses with other neurons but receive the same excitatory inputs-from parallel fibers and climbing fibers -as other MLIs. These properties allow them to shape the output of the cerebellar cortex differently from other interneurons and may account for their proposed role in cerebellar motor learning.