However, there is always concern about how much one can interpret cell culture data in an in situ context. Thus, moving forward, it will be important to perform an extremely technically challenging study and provide conclusive evidence supporting exocytosis in situ and evaluate whether burst mode release can be imaged within the confines

of a brain slice. Additionally, one would like to image extracellular glutamate using recently developed FRET sensors of extracellular glutamate (Dulla et al., 2008) to test whether TNFα-dependent modulation of glutamate accumulations following an astrocytic Ca2+ signal can be observed. This study significantly advances the field AZD6244 by providing new insights into the complexity of the control of astrocyte-neuron interactions. Additionally, it points to the importance of carefully controlling the timing of experiments given that TNFα is regulated in a diurnal manner. Since TNFα rises during wakefulness and falls during sleep it is not difficult to envision that there will be time of day differences in glutamate-mediated gliotransmission that is gated by this cytokine. Thus, it will be critical to record and state the time of day that experiments

were performed, as well as the timing of the light/dark cycle that the experimental animals were housed. An area of considerable concern about glutamate mediated gliotransmission has always been how this transmitter could escape the avid transporters that are responsible for its uptake. While this study points Bcl-2 inhibitor to one possibility, another is the potential for gliotransmission to regulate the availability of transporters for synaptically released glutamate. For example, by being scavenged by glutamate transporters, asynchronous astrocytic glutamate release could influence their availability for subsequent glutamate arising from synaptic transmission and thereby influence spillover of synaptic transmitter. The

Cell press linkage between TNFα and gliotransmission adds intriguing pieces to the developing puzzle of how astrocytes contribute to neuronal function and ultimately behavior. First, it sheds new light on recent controversy about the presence of Ca2+-dependent glutamate release from astrocytes. This study clearly demonstrates that the presence of a cytokine can gate whether Ca2+ dependent gliotransmission is able to act on neurons. Second, the importance of TNFα in modulating gliotransmission points to the involvement of astrocytic signals in sleep related processes. Astrocytes have been previously demonstrated to contribute to sleep homeostasis through the activation of neuronal A1 adenosine receptors (Halassa et al., 2009). Sleep homeostasis, a process by which the duration of wakefulness provides a feedback drive to sleep, is also regulated by TNFα. Indeed, TNFα exhibits a diurnal rhythm, and TNFα infusion can promote sleep (Krueger, 2008). Furthermore, glial-derived TNFα regulates synaptic scaling (Kaneko et al.