Astrocytes Inhibit Nitric Oxide-Dependent Ca2+ Dynamics in Activated Microglia: Involvement of ATP Released Via Pannexin 1 Channels

Under inflammatory conditions, microglia exhibit increased levels of free intracellular Ca2+ and produce high amounts of nitric oxide (NO). However, whether NO, Ca2+ dynamics, and gliotransmitter release are reciprocally modulated is not fully understood. More importantly, the effect of astrocytes in the potentiation or suppression of such signaling is unknown. Our aim was to address if astrocytes could regulate NO-dependent Ca2+ dynamics and ATP release in LPS-stimulated microglia. Griess assays and Fura-2AM time-lapse fluorescence images of microglia revealed that LPS produced an increased basal [Ca2+](i) that depended on the sequential activation of iNOS, COXs, and EP1 receptor. TGF1 released by astrocytes inhibited the abovementioned responses and also abolished LPS-induced ATP release by microglia. Luciferin/luciferase assays and dye uptake experiments showed that release of ATP from LPS-stimulated microglia occurred via pannexin 1 (Panx1) channels, but not connexin 43 hemichannels. Moreover, in LPS-stimulated microglia, exogenous ATP triggered activation of purinergic P2Y(1) receptors resulting in Ca2+ release from intracellular stores. Interestingly, TGF1 released by astrocytes inhibited ATP-induced Ca2+ response in LPS-stimulated microglia to that observed in control microglia. Finally, COX/EP1 receptor signaling and activation of P2 receptors via ATP released through Panx1 channels were critical for the increased NO production in LPS-stimulated microglia. Thus, Ca2+ dynamics depended on the inflammatory profile of microglia and could be modulated by astrocytes. The understanding of mechanisms underlying glial cell regulatory crosstalk could contribute to the development of new treatments to reduce inflammatory cytotoxicity in several brain pathologies.