Summary Cognitive flexibility is an essential ability to adapt to changing environment and circumstances. NMDAR has long been implicated in cognitive flexibility, but the precise molecular and cellular mechanism is not well understood. Here, we report that astrocytes regulate NMDAR tone through Best1-mediated glutamate and D-serine release, which is critical for cognitive flexibility. Co-release of D-serine and glutamate is required for not only homosynaptic LTD but also heterosynaptic LTD, which is induced at unstimulated synapses upon release of norepinephrine and activation of astrocytic α1-AR during homosynaptic LTP. Remarkably, heterosynaptic LTD at unstimulated synapses during memory acquisition is required for later repotentiation LTP during reversal learning, laying a foundation for flexible memory and cognitive flexibility. Our study sheds light on the pivotal role of astrocytes in orchestrating multiple synapses during memory formation and determining the fate of consolidated memory to be retained as a flexible memory. Highlights Astrocytes regulate NMDAR tone via Best1-mediated glutamate and D-serine release Activation of astrocytic α1-AR induces heterosynaptic LTD via NMDAR tone Heterosynaptic LTD is required for repotentiation LTP and spatial reversal learning Astrocytic regulation of NMDAR tone is critical for metaplasticity and flexible memory

Astrocytes render memory flexible