Novel inhibitory interneurons of the cerebellar cortex

Abstract

The cerebellum is a brain structure important for motor, social and cognitive behaviors. Its main processing unit, the cerebellar cortex, has a simple circuit thought to be composed of few cell types that repeats through the entire cerebellum. However, current circuit models of the cerebellar cortex are incomplete due to a lack of knowledge about inhibitory interneuron types, which play an essential role in cerebellar computations. The largest population of inhibitory interneurons, the molecular layer interneurons (MLIs), provide inhibition to Purkinje cells and other MLIs, and have been traditionally subdivided into two morphological types: basket cells and stellate cells, located in the lower and upper molecular layer, respectively. Using snRNAseq, smFISH and 2-photon microscopy, we found that MLIs are composed of two molecularly distinct populations, MLI1 and MLI2, that surprisingly do not correspond to basket and stellate cells. Instead, both are distributed uniformly across the molecular layer and display a continuum of morphological variation. Additionally, using whole cell recordings we found that MLI1 and MLI2 differ in many of their intrinsic electrical properties, such as spontaneous firing and excitability, as well as network properties like gap junction coupling. We also studied the candelabrum cell (CC), a type of Purkinje layer interneuron (PLI) that has remained obscure since it was discovered in an anatomical study in 1994. We used a combination of electrophysiology, 2-photon microscopy, snRNAseq, smFISH and serial electron microscopy to study the physiological, anatomical and circuit properties of CCs. We found that CCs are the most abundant type of PLI, they are present in all cerebellar lobules, are highly excitable cells and receive input from all stages of cerebellar processing. Finally, we showed CCs primarily target MLIs, and that CC firing controls cerebellar output by disinhibiting Purkinje cells. The properties of these novel interneuron types suggests that they have specialized roles in cerebellar processing. For example, CCs are the only inhibitory interneuron that are directly targeted by both input and output synapses, and therefore could act as a gate for cerebellar output after a mossy fiber input through disinhibition of Purkinje cells. The presence of gap junctions in MLI1s suggest that they can fire synchronously and could be important for timing, whereas MLI2 could be important for providing tonic gain control of the cerebellar circuit.