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D'Angelo, Egidio, Giovanna De Filippi, Paola Rossi, and Vanni Taglietti.Synaptic activation of Ca2+action potentials in immature rat cerebellar granule cells in situ. J. Neurophysiol.78: 1631–1642, 1997. Although numerous Ca2+channels have been identified in cerebellar granule cells, their role in regulating excitability remained unclear. We therefore investigated the excitable response in granule cells using whole cell patch-clamp recordings in acute rat cerebellar slices throughout the time of development (P4–P21, n= 183), with the aim of identifying the role of Ca2+channels and their activation mechanism. After depolarizing current injection, 46% of granule cells showed Ca2+action potentials, whereas repetitive Na+spikes were observed in an increasing proportion of granule cells from P4 to P21. Because Ca2+action potentials were no longer observed after P21, they characterized an immature granule cell functional stage. Ca2+action potentials consisted of an intermediate-threshold spike (ITS) activating at −60/−50 mV and sensitive to voltage inactivation and of a high-threshold spike (HTS), activating at above −30 mV and resistant to voltage inactivation. Both ITS and HTS comprised transient and protracted Ca2+channel-dependent depolarizations. The Ca2+action potentials could be activated synaptically by excitatory postsynaptic potentials, which were significantly slower and had a proportionately greater N-methyl-d-aspartate (NMDA) receptor-mediated component than those recorded in cells with fast repetitive Na+spikes. The NMDA receptor current, by providing a sustained and regenerative current injection, was critical for activating the ITS, which was not self-regenerative. Moreover, NMDA receptors determined temporal summation of impulses during repetitive mossy fiber transmission, raising membrane potential into the range required for generating protracted Ca2+channel-dependent depolarizations. The nature of Ca2+action potentials was considered further using selective ion channel blockers. N-, L-, and P-type Ca2+channels generated protracted depolarizations, whereas the ITS and HTS transient phase was generated by putative R-type channels (RITSand RHTS, respectively). RHTSchannels had a higher activation threshold and were more resistant to voltage inactivation than RITSchannels. At a mature stage, most of the Ca2+-dependent effects depended on the N-type current, which promoted spike repolarization and regulated the Na+-dependent discharge frequency. These observations relate Ca2+channel types with specific neuronal excitable properties and developmental states in situ. Synaptic NMDA receptor-dependent activation of Ca2+action potentials provides a sophisticated mechanism for Ca2+signaling, which might be involved in granule cell development and plasticity.