Astroglial ionotropic NMDA receptor calcium signalling and modulation of 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, 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 reduced Ca2+ responses to whisker stimulation in both astrocytes and neurons. This highlights the importance of NMDAR in astrocyte Ca2+ signalling and astrocyte-neuron communication and suggests that astroglial NMDAR KD could cause deficits in sensory perception. This work contributes to a deeper knowledge of mechanisms underlying astrocyte Ca2+ signalling and provides novel directions to study the role of astrocytes in neuronal circuits.