Anoxia-induced hippocampal LTP is regeneratively produced by glutamate and nitric oxide from the neuro-glial-endothelial axis.

Transient anoxia causes amnesia and neuronal death. This is attributed to enhanced glutamate release and modeled as anoxia-induced long-term potentiation (aLTP). aLTP is mediated by glutamate receptors and nitric oxide (·NO) and occludes stimulation-induced LTP. We identified a signaling cascade downstream of ·NO leading to glutamate release and a glutamate-·NO loop regeneratively boosting aLTP. aLTP in entothelial ·NO synthase (eNOS)-knockout mice and blocking neuronal NOS (nNOS) activity suggested that both nNOS and eNOS contribute to aLTP. Immunostaining result showed that eNOS is predominantly expressed in vascular endothelia. Transient anoxia induced a long-lasting Ca2+ elevation in astrocytes that mirrored aLTP. Blocking astrocyte metabolism or depletion of the NMDA receptor ligand D-serine abolished eNOS-dependent aLTP, suggesting that astrocytic Ca2+ elevation stimulates D-serine release from endfeet to endothelia, thereby releasing ·NO synthesized by eNOS. Thus, the neuro-glial-endothelial axis is involved in long-term enhancement of glutamate release after transient anoxia.

Ca+ activity maps of astrocytes tagged by axoastrocytic AAV transfer.

Astrocytes exhibit localized Ca2+ microdomain (MD) activity thought to be actively involved in information processing in the brain. However, functional organization of Ca2+ MDs in space and time in relationship to behavior and neuronal activity is poorly understood. Here, we first show that adeno-associated virus (AAV) particles transfer anterogradely from axons to astrocytes. Then, we use this axoastrocytic AAV transfer to express genetically encoded Ca2+ indicators at high-contrast circuit specifically. In combination with two-photon microscopy and unbiased, event-based analysis, we investigated cortical astrocytes embedded in the vibrissal thalamocortical circuit. We found a wide range of Ca2+ MD signals, some of which were ultrafast (≤300 ms). Frequency and size of signals were extensively increased by locomotion but only subtly with sensory stimulation. The overlay of these signals resulted in behavior-dependent maps with characteristic Ca2+ activity hotspots, maybe representing memory engrams. These functional subdomains are stable over days, suggesting subcellular specialization.