Climbing fibers selectively recruit disinhibitory interneurons to enhance dendritic calcium signals in cerebellar Purkinje cells

Abstract

It is well established that the cerebellar cortex is the locus for several types of associative learning. Inputs from the two main afferent pathways, climbing fibers (CF) and mossy fibers (MF), get integrated in individual Purkinje cells (PCs). CFs convey instructive signals for learning and plasticity by elevating dendritic calcium, while MFs relay sensorimotor information. Paradoxically, CFs also activate molecular layer interneurons type 1 (MLI1s) that produce strong inhibition of PCs that could prevent plasticity. On the other hand, molecular layer interneurons type 2 (MLI2s) inhibit MLI1s and in consequence disinhibits PCs. The extent and how CFs drive each MLI subtype is unknown.To address how can CFs drive MLIs without disrupting calcium influx necessary for PCs plasticity, I investigated the functional interactions between CFs and specific MLI subtypes in vivo. First, large scale electrophysiological recordings of the mouse cerebellar cortex allowed me to characterize the spontaneous interactions of MLI1s, MLI2s, PCs and PC complex spikes (PCcs), a widely used and established proxy for CF firing. I found that MLI2s were strongly activated by spontaneous PCcs, MLI1s were strongly inhibited following PCcs onset and PCs were disinhibited. Next, I explored the effect of evoked PCcs on the activity of MLIs. PCcs were evoked by corneal airpuffs, a learning relevant stimulus widely used in eyeblink conditioning. This time, the responses of MLI1s were diverse. Some MLI1s were excited and some suppressed, however; MLI2s were only excited. This apparent contradictory effect was explained by the airpuff evoked activity of the MF sensory pathway that provide direct excitation to MLIs. Despite this, MLI1s that received synchronous PCcs inputs were strongly inhibited. Finally, two-photon imaging of PCs dendrites activated synchronously in response to an airpuff revealed higher calcium signals necessary for plasticity. Thus, my experiments revealed a novel disinhibitory circuitry in the cerebellar cortex that facilitates calcium signaling required for associative learning.