Current and Calcium Responses to Local Activation of Axonal NMDA Receptors in Developing Cerebellar Molecular Layer Interneurons

Bénédicte Rossi,David Ogden,Yusuf P Tan,Alain Marty,Isabel Llano,Thibault Collin,Fabien Tell

doi:10.1371/journal.pone.0039983

Abstract

In developing cerebellar molecular layer interneurons (MLIs), NMDA increases spontaneous GABA release. This effect had been attributed to either direct activation of presynaptic NMDA receptors (preNMDARs) or an indirect pathway involving activation of somato-dendritic NMDARs followed by passive spread of somatic depolarization along the axon and activation of axonal voltage dependent Ca2+ channels (VDCCs). Using Ca2+ imaging and electrophysiology, we searched for preNMDARs by uncaging NMDAR agonists either broadly throughout the whole field or locally at specific axonal locations. Releasing either NMDA or glutamate in the presence of NBQX using short laser pulses elicited current transients that were highly sensitive to the location of the spot and restricted to a small number of varicosities. The signal was abolished in the presence of high Mg2+ or by the addition of APV. Similar paradigms yielded restricted Ca2+ transients in interneurons loaded with a Ca2+ indicator. We found that the synaptic effects of NMDA were not inhibited by blocking VDCCs but were impaired in the presence of the ryanodine receptor antagonist dantrolene. Furthermore, in voltage clamped cells, bath applied NMDA triggers Ca2+ elevations and induces neurotransmitter release in the axonal compartment. Our results suggest the existence of preNMDARs in developing MLIs and propose their involvement in the NMDA-evoked increase in GABA release by triggering a Ca2+-induced Ca2+ release process mediated by presynaptic Ca2+ stores. Such a mechanism is likely to exert a crucial role in various forms of Ca2+-mediated synaptic plasticity.

Highlights

In the central nervous system, postsynaptic NMDARs are often seen as canonical coincidence detectors in induction of synaptic plasticity
Lonchamp et al [14] have shown that the preNMDARs expressed in parallel fibers (PFs) could mediate an increase in the frequency of miniature IPSCs (mIPSCs) recorded in Purkinje cells
NMDARs has emerged from the observation that bath application of N-methyl D-aspartate (NMDA) strongly increases the frequency of mIPSCs recorded in the two cell types that are innervated by molecular layer interneurons (MLIs) i.e. Purkinje cells and other MLIs [3]

Summary

Introduction

In the central nervous system, postsynaptic NMDARs are often seen as canonical coincidence detectors in induction of synaptic plasticity. Presynaptic NMDARs (preNMDARs) have been anatomically or functionally detected at both GABAergic [3,4,5,6] and glutamatergic [7,8] termini (see [9] for review) Their implication in long-term plasticity has been suggested in various structures including the visual cortex [6], the neocortex [10,11] and the cerebellum [8,12,13]. Application of exogenous NMDA increases the frequency of miniature IPSCs (mIPSCs) [3,12,15] following an elevation of presynaptic Ca2+ [16] This signal is thought to be provided by Ca2+ influx through preNMDARs rather than by activation of voltage-dependent Ca2+ channels (VDCCs) [4,17], and to be amplified through a Ca2+-induced Ca2+ release (CICR) process [4]. Glutamate spillover was proposed to contribute to long term potentiation of GABAergic synapses through preNMDAR stimulation [12] whereas retrograde activation of preNMDARs has been proposed to potentiate GABA release at the MLI-Purkinje cell synapse [4]

Methods

Results

Conclusion