Thus, by using short focal 2MeSADP applications, we succeeded in inducing local [Ca2+]i rises in astrocytic processes with spatial-temporal characteristics reproducing the P2Y1R-dependent Ca2+ signals evoked by endogenous synaptic activity and involved in its modulation (Chuquet

et al., 2010). These 2MeSADP-induced local Ca2+ signals were in all similar in WT and in Tnf−/− slices, although in the latter local or even bath application of the P2Y1R agonist did not produce any synaptic modulation. In keeping, fast submembrane [Ca2+]i elevations evoked by 2MeSADP in cultured astrocytes, which correlate in space and time to exocytic fusions of glutamatergic vesicles ( Marchaland et al., 2008), were identical in WT and Tnf−/− astrocytes, although in the latter cells, P2Y1R-evoked vesicle fusions and glutamate Cabozantinib ic50 release were dramatically altered. Therefore, our data demonstrate Selleckchem FK228 that the induction of [Ca2+]i elevation in astrocytes, even when produced by stimulation of the appropriate

GPCR, is not “necessary and sufficient” for functional gliotransmission to occur ( Araque et al., 1998) if a downstream control mechanism is altered. Thus, we identify the existence of permissive/homeostatic factors like TNFα that control stimulus-secretion coupling in astrocytes and its synaptic effects independently of, and in addition to, [Ca2+]i elevations. We believe that this finding represents a relevant contribution to the understanding of the process of gliotransmission, particularly in view of recent conflicting results. Indeed, parallel investigations

using different experimental paradigms succeeded or failed in detecting an astrocytic control on synaptic transmission and Levetiracetam plasticity ( Agulhon et al., 2010, Fellin et al., 2004, Fiacco et al., 2007, Henneberger et al., 2010 and Perea and Araque, 2007). While the present debate focuses on the required characteristics of astrocytic [Ca2+]i elevations for the control to occur ( Hamilton and Attwell, 2010 and Kirchhoff, 2010), our findings call for attention also to the role of additional factors. Our study reveals a complexity and dose dependency of the TNFα effects on astrocyte glutamate release and on mEPSC activity in general. The effects on P2Y1R- or CXCR4-evoked glutamate release observed in the present study, which affect presynaptic excitatory function, depend on the presence of constitutive TNFα and are “reconstituted” in Tnf−/− astrocytes by adding low picomolar concentrations of the cytokine. However, in line with our previous observations ( Bezzi et al., 1998 and Bezzi et al., 2001), at higher (nanomolar) concentrations, the cytokine induces exocytosis of glutamatergic vesicles directly, suggesting that its impact on excitatory transmission may change at these concentrations (see below).