Retrograde Synaptic Signaling Mediated by K+ Efflux through Postsynaptic NMDA Receptors

• Postsynaptic NMDARs are the major source of synaptic K+ efflux
• NMDAR-dependent K+ efflux is enhanced by coincidence of synaptic inputs
• K+ elevation in the synaptic cleft enhances presynaptic Ca2+ transients
• Retrograde K+ signaling is a form of computation in synaptic networks

SummarySynaptic NMDA receptors (NMDARs) carry inward Ca2+ current responsible for postsynaptic signaling and plasticity in dendritic spines. Whether the concurrent K+ efflux through the same receptors into the synaptic cleft has a physiological role is not known. Here, we report that NMDAR-dependent K+ efflux can provide a retrograde signal in the synapse. In hippocampal CA3-CA1 synapses, the bulk of astrocytic K+ current triggered by synaptic activity reflected K+ efflux through local postsynaptic NMDARs. The local extracellular K+ rise produced by activation of postsynaptic NMDARs boosted action potential-evoked presynaptic Ca2+ transients and neurotransmitter release from Schaffer collaterals. Our findings indicate that postsynaptic NMDAR-mediated K+ efflux contributes to use-dependent synaptic facilitation, thus revealing a fundamental form of retrograde synaptic signaling.

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