Serotonin-driven long-range inhibitory connections in the cerebellar cortex

Disturbances of the serotoninergic neuromodulation in the cerebellar cortex have been involved in several types of ataxia, but the physiological actio n of serotonin in this structure remains poorly understood. We report that in slices of the rat cerebellar vermis, serotonin triggers the firing of an inhibitory interneuron presynaptic to Golgi cells. The Lugaro cell, a negl ected interneuronal type, satisfies the expected criteria for this input, w hereas basket cells, stellate cells, or Golgi cells do not. Lugaro cells ar e selectively excited by serotonin, and their firing behavior (sustained st eady frequency in the 5-15 Hz range) resembles the pattern of occurrence of serotonin-evoked IPSCs in Golgi cells. Immunohistochemical stainings and s ingle cell reconstructions show that Lugaro cell axons form a parasagittal plexus but also extend long transverse branches that run parallel to the pa rallel fibers and are partly myelinated. Electrophysiological data suggest that these transverse axons participate in synaptic contacts of the Lugaro cells with Golgi cells, and we calculated that in the intact cerebellum a g iven Lugaro cell contacts >100 Golgi cells. Serotonin modulation of Lugaro cells may constitute an intracortical switch involved in information patter ning at the level of Golgi cells and granule cells populations, and particu larly in synchronizations recorded along the transverse axis in vivo.