Final Report Summary - MULTIMOSSY (Multimodal mossy fiber input and its role in information processing in the cerebellar granule cell layer) The cerebellum is thought to influence motor systems by assessing disparities between intention and action. This is achieved by adjusting the operation of motor centers in the cortex and brainstem while a movement is in progress as well as during repetitions of the same movement. In order to process this information, the input layer of the cerebellar cortex receives sensory inputs from almost every sensory system, together with information from different cortical areas about motor commands and sensory feedback. The recoding of diverse sensory and cortical signals (efferent copy) by the granule cell layer is critical for the function of cerebellar circuits, yet the nature of these transformations and their significance for cerebellar information processing remain poorly understood. Sensory and motor command signals are conveyed by mossy fibers, which arise from multiple brainstem nuclei and make glutamatergic synapses onto Golgi cells and granule cells in the granule cell layer. Golgi cells are inhibitory interneurons and provide the sole neuronal source of inhibition for granule cells. Although the gross structure of the cerebellum is well characterized, the fine scale connectivity between mossy fibers arising from the different precerebellar nuclei and Golgi cells and granule cells, which determine how sensory and motor signals are combined and transformed, have only partially been studied. The aim of the present project was to characterize the anatomical connectivity and functional properties of mossy fiber inputs conveying different modalities in the granule cell layer.For this purpose, we took advantage of optogenetics to perform a functional mapping and selectively activate mossy fibers arising from specific precerebellar nuclei in the spinal trigeminal nuclei (Sp5; which conveys sensory information about the face area) and the pontine nucleus (PGN; which carries motor and sensory signals from the cortex). To identify the morphological and functional connectivity rules of the mossy fiber connectivity to Golgi cells and granule cells, we recorded light-evoked synaptic responses in voltage-clamped Golgi cells and granule cells from Crus I and II regions. We discovered differences in the amplitude and short term dynamics of mossy fiber synaptic inputs arising from PGN and Sp5 onto granule cells. Golgi cells are the sole source of inhibition onto granule cells and have been shown to be activated by mossy fiber inputs. We asked whether Golgi cells receive multimodal inputs and showed that Sp5 and PGN inputs innervated Golgi cells. Granule cells innervated by Sp5 or PGN mossy fiber inputs were also rapidly inhibited by Golgi cells activated by the same mossy fiber inputs indicating the presence of input-specific feedforward inhibition. Our experiments also revealed that stimulation of individual Sp5 or PGN mossy fiber inputs was not able to trigger spike activity in granule cells at the different frequencies tested. These experiments suggest that a combination of mossy fiber inputs carrying sensory information and copies of the motor command from the cortex needs to be integrated by granule cell in order to trigger a spike output. Knowledge of whether granule cells are unimodal or multimodal integrators and whether Golgi cells integrate similar or different types of inputs is essential for understanding the transformations performed by the cerebellar input layer. Our experiments have revealed some of the fine scale connectivity rules and functional properties of identified mossy fiber inputs, thereby providing a better understanding of how incoming information is combined and transformed in the cerebellum.
