Astrocytes are the source of TNF mediating homeostatic synaptic plasticity.

Tumor necrosis factor alpha (TNF) mediates homeostatic synaptic plasticity (HSP) in response to chronic activity blockade, and prior work has established that it is released from glia. Here we demonstrate that astrocytes are the necessary source of TNF during HSP. Hippocampal cultures from rats of both sexes depleted of microglia still will increase TNF levels following activity deprivation, and still express TTX-driven HSP. Slice cultures from mice of either sex with a conditional deletion of TNF from microglia also express HSP, but critically, slice cultures with a conditional deletion of TNF from astrocytes do not. In astrocytes, glutamate signaling is sufficient to reduce NFκB signaling and TNF mRNA levels. Further, chronic TTX treatment increases TNF in an NFκB dependent manner, although NFκB signaling is dispensable for the neuronal response to TTX-driven HSP. Thus, astrocytes can sense neuronal activity through glutamate spillover and increase TNF production when activity falls, to drive HSP through the production of TNF.Significance Statement The inflammatory cytokine tumor necrosis factor alpha (TNF) can mediate homeostatic synaptic plasticity, an adaptive form of circuit level synaptic plasticity, but the source of TNF had not been identified. Here we demonstrate that astrocytes are the critical source of TNF during homeostatic plasticity and define a pathway by which the astrocytes could monitor the local neuronal activity levels. This provides key information on how the astrocyte-neuron circuit functions to maintain neuronal activity within a normal range. This has important mechanistic information about the adaptive changes that occur during development and in response to disruptions of normal neuronal circuit function.