There is no question about the fact that astrocytes and other glial cells release neurotransmitters that activate receptors on neurons, glia and vascular cells, and that calcium is an important second messenger regulating the release. This occurs in cell culture, tissue slice and in vivo. Negative results from informative experiments designed to test the mechanism of calcium-dependent neurotransmitter release from astrocytes and the ensuing effects on synaptic transmission, have been cited as evidence calling into question whether astrocytes release neurotransmitters under normal circumstances with effects on synaptic transmission. The special feature section in this issue of Neuron Glia Biology addresses these issues and other aspects of neurotransmitter release from astrocytes in communicating with neurons and glial cells. Together these studies suggest that application of vocabulary and concepts developed for synaptic communication between neurons can lead to confusion and apparent paradoxes with respect to communication by extracellular signaling molecules released from glia in response to functional activity.

There is a growing body of evidence suggesting a functional relationship between Ca2+ signals generated in astroglia and the functioning of nearby excitatory synapses. Interference with endogenous Ca2+ homeostasis inside individual astrocytes has been shown to affect synaptic transmission and its use-dependent changes. However, establishing the causal link between source-specific, physiologically relevant intracellular Ca2+ signals, the astrocytic release machinery and the consequent effects on synaptic transmission has proved difficult. Improved methods of Ca2+ monitoring in situ will be essential for resolving the ambiguity in understanding the underlying Ca2+ signalling cascades.

Astrocytes constitute a major group of glial cells which were long regarded as passive elements, fulfilling nutritive and structural functions for neurons. Calcium rise in astrocytes propagating to neurons was the first demonstration of direct interaction between the two cell types. Since then, calcium has been widely used, not only as an indicator of astrocytic activity but also as a stimulator switch to control astrocyte physiology. As a result, astrocytes have been elevated from auxiliaries to neurons, to cells involved in processing synaptic information. Curiously, while there is evidence that astrocytes play an important role in synaptic plasticity, the data relating to calcium's pivotal role are inconsistent. In this review, we will detail the various mechanisms of calcium flux in astrocytes, then briefly present the calcium-dependent mechanisms of gliotransmitter release. Finally, we will discuss the role of calcium in plasticity and present alternative explanations that could reconcile the conflicting results published recently.

Extensive literature documented that astrocytes release neurotransmitters, cytokines and other signaling molecules to modulate migration, maturation and myelin synthesis of oligodendrocytes through mechanisms primarily converging on cytosolic [Ca2+] transients. Considering the long-term effects, it is expected that astrocyte-conditioned medium is a major regulator of gene expression in oligodendrocytes even in the absence of cytosol-to-cytosol communication via astrocyte–oligodendrocyte gap junction channels. Indeed, by comparing the transcriptomes of immortalized precursor oligodendrocyte (Oli-neu) cells when cultured alone and co-cultured with non-touching astrocytes we found profound changes in the gene expression level, control and networking. Remarkably, the astrocyte proximity was more effective in remodeling the myelination (MYE) gene fabric and its control by cytokine receptor (CYR)-modulated intercellular Ca2+-signaling (ICS) transcriptomic network than the dibutyryl-cAMP (db-cAMP) treatment-induced transformation into myelin-associated glycoprotein-positive oligodendrocyte-like cells. Moreover, astrocyte proximity up-regulated 37 MYE genes and switched on another 14 MYE, 23 ICS and 4 CYR genes, enhancing the roles of the leukemia inhibitory factor receptor and connexins Cx29 and Cx47. The novel prominent gene analysis identified the enhancer of zeste homolog 2 as the most relevant MYE gene in the astrocyte proximity, notch gene homolog 1 in control and B-cell leukemia/lymphoma 2 in differentiated Oli-neu cells.

Cell bodies of trigeminal nerves, which are located in the trigeminal ganglion, are completely surrounded by satellite glial cells and together form a functional unit that regulates neuronal excitability. The goals of this study were to investigate the cellular organization of the rat trigeminal ganglia during postnatal development and correlate those findings with expression of proteins implicated in neuron–glia interactions. During postnatal development there was an increase in the volume of the neuronal cell body, which correlated with a steady increase in the number of glial cells associated with an individual neuron from an average of 2.16 at birth to 7.35 on day 56 in young adults. Interestingly, while the levels of the inwardly rectifying K+ channel Kir4.1 were barely detectable during the first week, its expression in satellite glial cells increased by day 9 and correlated with initial formation of functional units. Similarly, expression of the vesicle docking protein SNAP-25 and neuropeptide calcitonin gene-related peptide was readily detected beginning on day 9 and remained elevated throughout postnatal development. Based on our findings, we propose that the expression of proteins involved in facilitating neuron–glia interactions temporally correlates with the formation of mature functional units during postnatal development of trigeminal ganglion.

Astrocytes are involved in synaptic and cerebrovascular regulation in the brain. These functions are regulated by intracellular calcium signalling that is thought to reflect a form of astrocyte excitability. In a recent study, we reported modification of the genetically encoded calcium indicator (GECI) GCaMP2 with a membrane-tethering domain, Lck, to generate Lck-GCaMP2. This GECI allowed us to detect novel microdomain calcium signals. The microdomains were random and ‘spotty’ in nature. In order to detect such signals more reliably, in the present study we further modified Lck-GCaMP2 to carry three mutations in the GCaMP2 moiety (M153K, T203V within EGFP and N60D in the CaM domain) to generate Lck-GCaMP3. We directly compared Lck-GCaMP2 and Lck-GCaMP3 by assessing their ability to monitor several types of astrocyte calcium signals with a focus on spotty microdomains. Our data show that Lck-GCaMP3 is between two- and four-times better than Lck-GCaMP2 in terms of its basal fluorescence intensity, signal-to-noise and its ability to detect microdomains. The use of Lck-GCaMP3 thus represents a significantly improved way to monitor astrocyte calcium signals, including microdomains, and will facilitate detailed exploration of their molecular mechanisms and physiological roles.

In the cerebellum, lamellar Bergmann glial (BG) appendages wrap tightly around almost every Purkinje cell dendritic spine. The function of this glial ensheathment of spines is not entirely understood. The development of ensheathment begins near the onset of synaptogenesis, when motility of both BG processes and dendritic spines are high. By the end of the synaptogenic period, ensheathment is complete and motility of the BG processes decreases, correlating with the decreased motility of dendritic spines. We therefore have hypothesized that ensheathment is intimately involved in capping synaptogenesis, possibly by stabilizing synapses. To test this hypothesis, we misexpressed GluR2 in an adenoviral vector in BG towards the end of the synaptogenic period, rendering the BG α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) Ca2+-impermeable and causing glial sheath retraction. We then measured the resulting spine motility, spine density and synapse number. Although we found that decreasing ensheathment at this time does not alter spine motility, we did find a significant increase in both synaptic pucta and dendritic spine density. These results indicate that consistent spine coverage by BG in the cerebellum is not necessary for stabilization of spine dynamics, but is very important in the regulation of synapse number.

It was recently reported that in one of the adult neurogenetic zones, the subventricular zone (SVZ), astrocyte-like cells release glutamate upon intracellular Ca2+ increases. However, the signals that control Ca2+ activity and glutamate release from SVZ astrocytes are not known. Here, we examined whether prostaglandin E2 (PGE2), which induces glutamate release from mature astrocytes, is such a signal. Using the gramicidin-perforated patch-clamp technique, we show that the activity of N-Methyl-D-Aspartate receptor (NMDAR) channel in neuroblasts is a high fidelity sensor of ambient glutamate levels. Using such sensors, we found that application of PGE2 led to increased ambient glutamate levels in the SVZ. In parallel experiments, PGE2 induced an increase in intracellular Ca2+ levels in SVZ cells, in particular astrocyte-like cells, as shown using Ca2+ imaging. Finally, a PGE2 enzyme immunoassay showed that the choroid plexus of the lateral ventricle and to a lesser extent the SVZ (ten-fold less) released PGE2. These findings suggest that PGE2 is a physiological signal for inducing glutamate release from SVZ astrocytes that is important for controlling neuroblast survival and proliferation. This signal may be accentuated following ischemia or injury-induced PGE2 release and may contribute to the injury-associated increased neurogenesis.