Post-synaptic responses recorded from rat Unipolar Brush Cell: the role of the H-current

The granular layer contains, in addition to granule and Golgi cells, the unipolar brush cells (UBCs). Beside several works revealing the basic synaptic and excitable properties of UBCs (Rossi et al., 1995; Diana et al., 2007), synaptic activation mechanisms remained poorly understood. By using patch-clamp recordings in cerebellar slices of the rat vestibulo- cerebellum, we found that mossy fiber stimulation, in addition to the well known early-onset response (delay ~ 2 ms), in ~70% of cases evoked a late-onset response (tens to hundreds of milliseconds). Both responses culminated with a low-threshold calcium spike (LTS) sustaining a fast action potential burst. While the early-onset response was all-or-none in nature, the delay of the late-onset burst decreased (and the duration increased) by raising mossy fiber stimulation intensity. Moreover, while the early-onset response was sustained by AMPA and NMDA receptor activation, the late-onset response did not required the same synaptic transmission. The late-onset response was primed by a slow depolarizing ramp driven by activation of a ZD7288-sensitive H-current (Ih). These results indicate that mossy fiber activity can regulate Ih gating through a yet unknown neurotransmission mechanism. This novel modality of UBC activation may play an important role for regulating granular layer functions in the vestibulo-cerebellum.