Glutamate exocytosis (glu-exo) from astrocytes strengthens excitatory transmission in the hippocampal dentate gyrus via pre-synaptic NMDA receptors (NMDAR, Jourdain et al., Nat. Neurosci., 2007). The cytokine TNF-a controls glu-exo evoked in cultured astrocytes by purinergic P2Y1 receptor (P2Y1R) stimulation (Domercq et al., J. Biol. Chem., 2006). Here we investigate whether TNF-a has a role in the astrocyte-dependent synaptic potentiation, taking advantage of TNF-deficient mice (Tnf -/- ). In hippocampal slices from wt mice (P23), stimulation of astrocytic P2Y1R (2MeSADP,10 mM), specifically increases mEPSC frequency at dentate granule cell synapses, an effect blocked by: (a) chelating Ca²⁺ selectively in astrocytes with BAPTA; and (b) bath application of ifenprodil, confirming involvement of pre- synaptic NR2B-NMDAR. In slices from Tnf -/- mice, in contrast, P2Y1R stimulation fails to induce neuromodulation. However, the effect is reconstituted by adding pM TNF-a. Parallel monitoring of glu-exo in cultured astrocytes with the specific reporter, VGLUT1pHluorin, showed that P2Y1R-evoked glu-exo proceeds in a synchronous burst in wt astrocytes, but is significantly slower and largely asynchronous in Tnf -/- astrocytes. Again, addition of pM TNF-a reconstitutes the kinetics observed in wt astrocytes, suggesting that TNF-a is required for proper vesicular trafficking/docking. In keeping, in Tnf -/- slices, astrocyte-induced neuromodulation is restored by partial glutamate uptake inhibition (TBOA, 25 ìM), which artificially obviates the lower extracellular glutamate peak produced by desynchronized astrocyte release. Overall, the neuromodulatory action of astrocytes appears to depend on the TNFa-mediated control of the rate of glu-exo. Supported by FNS grant 3100A0- 120398