Astroglial NMDA receptor signaling mediates fast calcium microdomains and modulates cortical neuron activity

Abstract

Astrocytes express different neurotransmitter receptors that serve to integrate these cells into neuronal networks. Many of these receptors, when sensing neuronal activity, induce elevations in intracellular astrocyte Ca2+, which leads to the release of gliotransmitters that modulate nearby neurons. Ionotropic N-methyl-D-aspartate (NMDA) receptors are found on astrocytes and are activated by glutamate and D-serine or glycine, and conduct Ca2+ into astrocytes. In brain slices, astrocyte NMDAR activation causes depolarization and Ca2+ elevations. However, its role in astrocytes Ca2+ transients and feedback modulation to neurons in vivo is not characterized. Therefore, in this study, we used a novel NMDAR knockdown (KD) construct to reduce NMDAR expression specifically in cortical astrocytes. Then, using dual calcium imaging of astrocytes and neurons, each expressing a unique genetically encoded calcium indicator (Lck-GCaMP6f and RCaMP1.07 respectively), we determined the impact of astrocytes NMDARs on astrocytes Ca2+ transients and nearby neuronal activity. Two-photon microscopy of the barrel cortex of awake mice revealed that NMDAR KD reduces fast and delayed Ca2+ microdomains evoked by whisker stimulation in astrocytes. Astrocytic NMDAR KD also reduces overall neuronal response to whisker stimulation which was reflected as animals impaired sensory discrimination ability. This study for the first time, highlights the importance of NMDAR in astrocyte stimulation-evoked Ca2+ microdomains and provides evidence for involvement of astrocytic NMDAR in cortical network activity. This work contributes to a deeper knowledge of mechanisms underlying astrocyte Ca2+ microdomains and their integration in cortical circuits, and provides novel directions to study the role of astrocytes in neuronal networks.