Layer-5 Pyramidal Cell tLTD Requires Astrocytic Ca2+ and CB1 Receptor Signaling
Timing-dependent long-term depression (tLTD) is a form of spike timing-dependent plasticity (STDP) that has been attributed to presynaptic NMDA and CB1 receptors at synapses between layer 5 (L5) pyramidal cells (PCs) in visual cortex. Here, we asked whether astrocytes, known for gliotransmission and tripartite synapse formation, are also required. Using quadruple whole-cell recordings in acute slices from C57BL/6 mice, we found that L5 PC [->] PC tLTD was abolished by the glial metabolic inhibitor sodium fluoroacetate. Disrupting astrocyte Ca2+ signaling through loss-of-function approaches such as AAV-mediated expression of CalEx and BAPTA loading of astrocyte networks consistently prevented tLTD. Optogenetic activation of astrocyte Gq signaling during tLTD induction abolished tLTD and led to potentiation. Since tLTD relies on endocannabinoid (eCB) signaling and astrocytes express CB1 receptors, we conditionally deleted CB1 receptors from astrocytes and found that this manipulation abolished tLTD. Taken together, our results show that L5 PC [->] PC tLTD requires astrocyte Ca2+ signaling and CB1 receptor activation. These findings suggest that astrocyte-dependent control of STDP may represent a general principle across circuit and synapse types. Significance StatementSpike timing-dependent plasticity (STDP) enables neural circuits to adapt based on the precise timing of activity, with timing-dependent long-term depression (tLTD) promoting competition and stabilization. While tLTD is often considered a two-factor process involving pre- and postsynaptic neurons, we show that astrocytes form a key third element. Using targeted loss-of-function approaches, we demonstrate that tLTD at layer-5 pyramidal cell synapses in visual cortex requires astrocyte Ca2+ signaling and CB1 receptors. Unexpectedly, optogenetic stimulation of astrocyte Gq signaling during induction blocked depression and instead triggered potentiation. These findings call for a revision of existing models of STDP and suggest that astrocyte-dependent plasticity may reflect a general regulatory principle across cortical circuits.