Purkinje cell collaterals preferentially target a subtype of molecular layer interneuron

In addition to providing outputs from the cerebellar cortex, Purkinje cell (PC) axon collaterals target other PCs, molecular layer interneurons (MLIs), and Purkinje layer interneurons (PLIs). It was recently shown that MLIs consist of two subtypes, but the properties of PC synapses onto these subtypes was not known and it was assumed that all PC collateral to MLI synapses would provide positive feedback to PCs. Clarifying the PC connectivity onto MLI subtypes is vital to understating the influence of feedback from PC collaterals because MLI1s primarily inhibit PCs whereas MLI2s mainly inhibit MLI1s and disinhibit PCs. Here we use a combination of serial EM and optogenetic studies to characterize PC synapses onto MLI subtypes in mice. EM reconstructions show that PCs make 53% of their synapses onto other PCs, 32% onto PLIs, 6% onto MLI1s and 7% onto MLI2s. Since there are far more MLI1s than MLI2s, each MLI2 is expected to receive many more synapses than each MLI1. In slice experiments, optogenetic activation of PCs evokes inhibitory currents in most MLI2s, but primarily disinhibits MLI1s. We also find that candelabrum cells, a type of PLI, form many more synapses onto MLI1s than MLI2s. It is therefore expected that both PC-MLI2-MLI1-PC and PC-PLI-MLI1-PC pathways allow increased PC firing to disinhibit MLI1s, which are known to reduce dendritic PC calcium signals and suppress plasticity at granule cell to PC synapses. These pathways provide negative feedback that act in concert with PC-PC synapses to counter elevations in PC firing.