Astrocytes are the major glial cell type in the central nervous system and are vital to support neuronal function and network activity. Astrocytes exhibit complex signalling cascades that are essential for their normal function and to maintain homeostasis of the neural network. Abnormality in astrocyte physiology is closely associated with neurological and psychiatric diseases, suggesting the therapeutic potential of these cells. To further understand astrocyte functions in health and disease, it is important to study astrocyte physiology. Accumulating evidence has shown that visual stimulation elicits Ca2+ responses in astrocytes (Perea et al. 2014; Sonoda et al. 2018). The Ca2+ responses in astrocytes are linked to the activities of interneurons and pyramidal neurons in layers 2/3 of the mouse visual cortex (Perea et al. 2014). Despite the wealth of literature available on how glia support neuronal functions, there are still substantial gaps in our understanding of how stimulated astrocytes regulate information processing of neurons at the cellular level. In a recent article published in The Journal of Physiology, Ryczko et al. (2021) provide evidence that activation of astrocytes regulates the firing of layer 5 pyramidal neurons (L5PNs) by controlling extracellular Ca2+ levels via releasing the Ca2+ binding protein S100β. Previous work has demonstrated that the photostimulation of astrocytes increases excitatory synaptic transmission to layer 2/3 neurons in visual cortex (V1) (Perea et al. 2014). In their study, Ryczko et al. (2021) investigated the firing properties of layer 5 pyramidal neurons (L5PNs) upon photostimulation of proximal astrocytes. L5PNs integrate a plethora of neocortical synaptic inputs along with their dendritic projections, in particular from their distal dendritic tuft that projects to and branches in layer 1; thus L5PNs act as a major source of output from the neocortex to other brain regions. Considering the significance of L5PNs in the cortical network and neuronal output, the authors planned to investigate the role of proximal astrocytes in L5PNs’ distal input processing. To target astrocytes for photostimulation, the authors first generated GFAP-channelrhodopsin-2 (ChR2)-EYFP mice by crossing ChR2-EYFP-lox mice with GFAP-Cre mice. Astrocyte-specific expression of the fusion protein ChR2-EYFP in GFAP-ChR2-EYFP mice was validated by co-staining for EYFP and neuronal markers (MAP2 and NeuN), and 94% of counted neurons at the V1 level did not show any EYFP signal. To further assess the response of ChR2-expressing astrocytes to photostimulation, the authors investigated the astrocyte depolarizations and intracellular Ca2+ transients using GFAP-ChR2-EYFP mouse brain slices. Successful astrocyte depolarization (7.3 ± 4.2 mV) and increased intracellular Ca2+ transients were evoked by a combined stimulus of 440 and 488 nm lasers, where the optogenetic stimulation area was restricted to 2–3 astrocytes. Upon activation of astrocytes using blue light (i.e. a combination of 440 and 488 nm lasers) with 5 s pulses, the L5PNs displayed an evoked rapid firing from 0.4 to 141.9 Hz. Next, the authors studied effects of the activation of proximal astrocytes on L5PNs distal information processing by applying electrical stimulation (1, 5, 10, 20, 30, 40 and 50 Hz) to cortical layer 1 (L1) axons. Under control conditions, L1 axon stimulation elicited both EPSPs and spikes in most of the L5PNs; however, firing was not maintained throughout stimulation at higher stimulation frequencies such as 20, 30, 40 and 50 Hz. Intriguingly, photostimulated astrocytes increased the L5PNs firing response duration and the number of spikes elicited by stimulation of L1 (Fig. 1). In particular, optogenetically stimulated astrocytes overrode the firing adaptation at higher frequencies, thus L5PN displayed continuous spiking during the stimulation train applied to L1. Overall, the data showed that the photostimulated (440–488 nm) astrocytes elicit spiking in L5PNs and improve L5PN distal input processing (Fig. 1). Optogenetic stimulation of astrocytes induced an increase in the intracellular Ca2+ levels, which in turn could activate the vesicular release of gliotransmitters such as glutamate, GABA, ATP and the NMDA co-agonist d-serine (Perea et al. 2014). To investigate whether the observed increase in L5PN spikes could be evoked by gliotransmitters from the astrocytes, the authors applied specific pharmacological blockers against AMPA/kainate (CNQX), Glutamate (AP5), GABA (GABAzine), mGluR (LY-341495) and purinergic (Suramin) receptors. A reduction in the number of L5PN spikes in the range of 50% was observed when the authors co-applied CNQX, AP5 and GABAzine along with the photostimulation of astrocytes. The incomplete block of the effect of astrocyte stimulation by the antagonists above suggests that an additional mechanism might be involved. Previous work has demonstrated that a decrease in extracellular Ca2+ concentration elicits intense and sustained neuronal activity in the brainstem and evidence was obtained that astrocytes regulate the extracellular free Ca2+ levels by releasing S100β (Morquette et al. 2015). The authors thus studied the extracellular Ca2+ levels during spiking episodes elicited by optogenetic stimulation of astrocytes in GFAP-ChR2-EYFP mouse brain slices. Interestingly, upon stimulation of astrocytes, the extracellular Ca2+ levels reduced from 1.24 ± 0.04 to 1.11 ± 0.04 mM compared to the pre-optogenetic stimulation. Remarkably, the time window of the decrease in extracellular Ca2+ coincided with the period during which the L5PNs displayed astrocyte-evoked spikes. Since the extracellular Ca2+ levels modulate persistent sodium current (INaP) dependent neuronal firing (Morquette et al. 2015), the authors next investigated whether astrocyte evoked-spikes of L5PNs rely on INaP. Therefore, the authors incubated brain slices for 7–50 min with 0.1 μM of 4,9-anhydro-tetrodotoxin (4,9-TTX), a Nav1.6 α-subunit blocker. They found that the 4,9-TTX treatment almost removed the effect of photostimulated astrocytes on L5PNs firing properties, thus confirming the involvement of Nav1.6 channels. Next, the authors addressed the role of S100β in L5PN spikes elicited by stimulation of astrocytes by using anti-S100β antibodies in the bath solution. Intriguingly, application of 40 μg/ml of anti-S100β antibodies for 8–15 min drastically decreased L5PN spikes elicited by stimulation of astrocytes, whereas application of anti-myosin antibody, used as a control showed no change. Furthermore, bath application of S100β (129 μM) or the Ca2+ chelator BAPTA (5 mM) reduced extracellular Ca2+ from 1.2 mM to 0.5 and 0.1 mM, respectively, and induced spiking responses in L5PNs that were relatively similar to those evoked by optogenetically stimulated-astrocytes. By plotting the effect of exogenous S100β on the extracellular Ca2+ concentration in the brain slices, the authors estimated that optogenetic stimulation of astrocytes caused a 0.13 mM decrease in extracellular Ca2+ concentration corresponding to the release of 6.7 ± 6.6 pg of S100β. This finding serves as a standard reference for future investigations. Similar to the effects of stimulated-astrocytes on L5PNs, the application of exogenous S100β in L5 overrode the firing adaptation at higher frequencies, resulting in continuous spiking during the whole stimulus train applied to L1 (Fig. 1). Interestingly, S100β on 4,9-TTX (0.1 μM, 6–26 min) pre-treated brain slices evoked considerably fewer spikes in L5PNs, thus prompting an investigation of the interaction between extracellular Ca2+ concentration and Nav1.6 activity. Furthermore, immunostaining signals of S100β were found to be adjacent to the Nav1.6 containing fibres, suggesting that astrocytic S100β release sites are located close enough to locally influence the L5PNs. The data obtained by Ryczko et al. (2021) suggest that release of S100β from depolarized astrocytes induces decreases in extracellular Ca2+ concentration which in turn enhances neuronal responses in L5PNs when processing L1 inputs (Fig. 1). Furthermore, they provided evidence that this increase in firing is evoked via an enhanced activation of the Nav1.6 sodium channels, either via surface charge screening (Frankenhaeuser & Hodgkin, 1957) or direct interactions between Ca2+ and the channels. The remarkable finding of this study is that local activation of astrocytes, as occurs during noradrenaline release during arousal, might locally enhance sensory information processing. The next important steps will be to determine when this mechanism is recruited in vivo, and whether a dysfunction of such mechanism could translate in pathological states. None. All authors contributed equally. All authors approved the final version of the manuscript and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. None The authors would like to thank Prof. Dr. Markus Schwaninger and Dr Irmgard Dietzel-Meyer for valuable inputs while preparing this Journal Club article. Further, we would also like to mention that due to the Journal Club author guidelines we were not able to cite all deserving references.