Much of the versatility of nervous systems resides in the capabilities of neuronal cells to generate and convey signals. However, over the past decade, the neuron-centric viewpoint of information processing in the brain has shifted to a broader perspective that includes other cellular partners, foremost the glial cells. Astrocytes, oligodendrocytes, Schwann cells and microglia, as well as their progenitors, are endowed with ion channels and membrane receptors that allow them to sense and respond to neuronal activity. The most well-studied form of glial signaling involves changes in intracellular Ca 21 levels; the variety of spatial and temporal patterns of these glial Ca 21 transients suggest that, differently from the electrically excitable neurons, glia are ''Ca 21 -excitable'' cells. Although Ca 21 has long been recognized as one of the major second messenger responsible for neuronal chemical synaptic transmission, only recently intra-and intercellular Ca 21 signaling in glial cells have been proposed to play an important role in brain function. These glial Ca 21 transients, that can be evoked or modulated by transmitters released from neurons, can also, per se, trigger ''gliotransmitter'' release (e.g., glutamate, ATP, and D-serine), thus acting on a variety of effectors, including neurons and vasculature. These findings have revolutionized our view of glial cell function and are among the highlights of cellular neuroscience research of the last decade, as also evidenced by the numerous excellent reviews recently published on this topic.
The next step for this field, in our view, is the understanding of how glial cells process information, how their Ca 21 signals are generated and integrated, and subsequently how these cells release signaling molecules. Thus, the collections of articles in these back to back Special Issues were conceptualized to provide the most up-to-date understanding of these related signaling events, from their input components necessary to receive information to their output components involved in the release of gliotransmitters. The way we have chosen to present this information was to invite two or more investigators in a single area to collaborate in writing these articles. In this way the individual articles provide an agreement between competing groups on current knowledge and frontiers. The editors are very grateful that the leaders of these respective fields have agreed to this concept and appreciate the efforts that authors have made to present a consensus viewpoint and to highlight areas of unresolved controversy.
Because of the high diversity of glial cell types in both vertebrate and invertebrate nervous systems (e.g.,astrocytes, oligodendrocytes, microglia, Bergmann glia, radial glia, M⬠uller cells, NG-2 cells, satellite cells, neuropil glia, etc.), the first Special Issue ''Calcium Signaling in Glial Cells'' reviews the current understanding of Ca 21 dynamics in some of these distinct glial cell populations. This collection of articles provides insights about and new directions to our understanding of the input parameters involved in glial Ca 21 signaling. These articles emphasize that the role of cytosolic Ca 21 signaling in the different types of glial cells may vary with functional specialization, e.g. during development, regeneration, synapse formation and pathological processes. Moreover, the comparative aspect is also highlighted in order to raise awareness about the versatile and generalized role of Ca 21 signaling in glial cells.
In Astrocyte Calcium Elevations: Properties, Propagation, and Effects on Brain signaling, Fiacco and McCarthy summarize the various modes of triggering astrocyte Ca 21 transients, the mechanisms involved in the initiation and propagation of Ca 21 waves within and between astrocytes. Different functional aspects of Ca 21 in myelin-forming and progenitor cells and the role of perisynaptic Schwann cells for short-term and long-