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

J Neurophysiol. 1997 Sep;78(3):1631-42. doi: 10.1152/jn.1997.78.3.1631.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Action Potentials / drug effects
  • Action Potentials / physiology
  • Animals
  • Calcium Channel Blockers / pharmacology
  • Calcium Channels / drug effects
  • Calcium Channels / physiology*
  • Cerebellum / cytology
  • Cerebellum / drug effects
  • Cerebellum / physiology*
  • Electrophysiology
  • Excitatory Postsynaptic Potentials / drug effects
  • Excitatory Postsynaptic Potentials / physiology
  • In Vitro Techniques
  • Ion Channel Gating / drug effects
  • Ion Channel Gating / physiology
  • Membrane Potentials / drug effects
  • Membrane Potentials / physiology
  • Nerve Fibers / drug effects
  • Nerve Fibers / physiology*
  • Patch-Clamp Techniques
  • Rats
  • Rats, Wistar
  • Receptors, N-Methyl-D-Aspartate / antagonists & inhibitors
  • Receptors, N-Methyl-D-Aspartate / physiology
  • Synapses / drug effects
  • Synapses / physiology*

Substances

  • Calcium Channel Blockers
  • Calcium Channels
  • Receptors, N-Methyl-D-Aspartate