SYNAPTIC ACTIVATION OF CA2-POTENTIALS IN IMMATURE RAT CEREBELLAR GRANULE CELLS IN-SITU( ACTION)

Although numerous Ca2+ channels have been identified in cerebellar gra nule 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 throu ghout the time of development (P4-P21, n = 183), with the aim of ident ifying the role of Ca2+ channels and their activation mechanism. After depolarizing current injection, 46% of granule cells showed Ca2+ acti on potentials, whereas repetitive Na+ spikes were observed in an incre asing proportion of granule cells from P4 to P21. Because Ca2+ action potentials were no longer observed after P21, they characterized an im mature 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 (I-IT S), activating at above -30 mV and resistant to voltage inactivation. Both ITS and I-ITS comprised transient and protracted Ca2+ channel-dep endent depolarizations. The Ca2+ action potentials could be activated synaptically by excitatory postsynaptic potentials, which were signifi cantly slower and had a proportionately greater N-methyl-D-aspartate ( NMDA) receptor-mediated component than those recorded in cells with fa st repetitive Na+ spikes. The NMDA receptor current, by providing a su stained and regenerative current injection, was critical for activatin g the ITS, which was not self-regenerative. Moreover, NMDA receptors d etermined temporal summation of impulses during repetitive messy fiber transmission, raising membrane potential into the range required for generating protracted Ca2+ channel-dependent depolarizations. The natu re of Ca2+ action potentials was considered further using selective io n channel blockers. N-, L-, and P-type Ca2+ channels generated protrac ted depolarizations, whereas the ITS and I-ITS transient phase was gen erated by putative R-type channels (R-ITS and R-HTS, respectively). R- HTS channels had a higher activation threshold and were more resistant to voltage inactivation than R-ITS channels. At a mature stage, most of the Ca2+-dependent effects depended on the N-type current, which pr omoted 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. Synapt ic NMDA receptor-dependent activation of Ca2+ action potentials provid es a sophisticated mechanism for Ca2+ signaling, which might be involv ed in granule cell development and plasticity.