The Journal of Neurophysiology Vol. 78 No. 3 September 1997, pp. 1631-1642
Copyright ©1997 The American Physiological Society

Synaptic Activation of Ca²⁺ Action Potentials in Immature Rat Cerebellar Granule Cells In Situ

Egidio D'Angelo, Giovanna De Filippi, Paola Rossi, and Vanni Taglietti

Istituto di Fisiologia Generale, I-27100 Pavia, Italy

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 (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 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.

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