Channel-mediated astrocytic glutamate release via Bestrophin-1 targets synaptic NMDARs

Kyung-Seok Han,Hyungju Park,Sukwoo Choi,C Justin Lee,Bong-June Yoon,Junsung Woo

doi:10.1186/1756-6606-6-4

Abstract

BackgroundAstrocytes regulate neuronal excitability and synaptic activity by releasing gliotransmitters such as glutamate. Our recent study demonstrated that astrocytes release glutamate upon GPCR activation via Ca2+ activated anion channel, Bestrophin-1 (Best1). The target of Best1-mediated astrocytic glutamate has been shown to be the neuronal NMDA receptors (NMDAR). However, whether it targets synaptically or extra-synaptically localized NMDAR is not known.FindingsWe recorded spontaneous miniature excitatory postsynaptic currents (mEPSCs) from CA1 pyramidal cells to test whether Best1-mediated astrocytic glutamate targets synaptic NMDAR. An agonist of protease activated receptor 1 (PAR1) was used to induce astrocytic Ca2+ increase and glutamate release. Firstly, we found that activation of PAR1 and subsequent release of glutamate from astrocyte does not alone increase the frequency of mEPSCs. Secondly, we found that mEPSC rise time is variable depending on the different electrotonic distances from the somatic recording site to the synaptic region where each mEPSC occurs. Two subgroups of mEPSC from CA1 pyramidal neuron by rise time were selected and analyzed. One group is fast rising mEPSCs with a rise time of 1 ~ 5 ms, representing synaptic activities arising from proximal dendrites. The other group is slowly rising mEPSCs with a rise time of 5 ~ 10 ms, representing synaptic events arising from glutamate release at synapses located in the distal dendrites. We used cell-type specific Best1 gene silencing system by Cre-loxP cleavage to dissociate the effect of neuronal and astrocytic Best1. Astrocytic Best1-mediated glutamate release by PAR1 activation did not affect decay kinetics, frequency, and amplitude of fast rising mEPSC. In contrast, PAR1 activation resulted in an NMDA receptor component to be present on slowly rising mEPSC, but did not alter frequency or amplitude.ConclusionsOur results indicate that astrocytic glutamate via Best1 channel targets and activates synaptic NMDARs.

Highlights

Astrocytes regulate neuronal excitability and synaptic activity by releasing gliotransmitters such as glutamate
Our results indicate that astrocytic glutamate via Best1 channel targets and activates synaptic NMDA receptors (NMDAR)
We demonstrate that Best1mediated astrocytic glutamate by protease activated receptor 1 (PAR1) activation causes an appearance of NMDAR component in slowly rising miniature excitatory postsynaptic currents (mEPSCs), indicating targeting and activation of synaptic NMDARs

Summary

Introduction

Astrocytes regulate neuronal excitability and synaptic activity by releasing gliotransmitters such as glutamate. Several studies have shown that astrocytes can be activated by sensory stimulation [2] or several pathological conditions including brain ischemia or inflammation [1,3,4,5,6]. These stimuli evoke increases in intracellular Ca2+ in astrocytes, which in turn elicit the release of active substances termed gliotransmitters [7,8,9]. We have recently reported that when Gαq-coupled receptors such as PAR1 are activated, astrocytes release glutamate via Ca2+ activated anion channel, Bestrophin (Best1) [12]

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