Mechanism Involved in Initiation and Propagation of Receptor-Induced Intercellular Calcium Signaling in Cultured Rat Astrocytes

Abstract

The mechanisms involved in the initiation and the propagation of intercellular calcium signaling (calcium waves) were studied in cultured rat astrocytes. The analysis of calcium waves, induced either by mechanical stimulation or by focal application of ionomycin, indicated that initiation was dependent on the presence of external calcium. In addition, pharmacological experiments indicate that intercellular propagation required PLC activation, integrity of IP 3 -sensitive internal calcium stores, and functional gap junctions. An extracellular action of ATP or glutamate and participation of voltage-dependent Ca 2ϩ channels were tested by using enzymatic degradation, receptor antagonists, and channel blockers, respectively. Because neither the speed of propagation nor the extent of the calcium waves was affected by these treatments, these alternate mechanisms were excluded from playing a role in intercellular calcium signaling. Biochemical assays and focal applications of several agonists (methoxamine, carbachol, glutamate) of membrane receptors to neurotransmitters and peptides (endothelin 1) demonstrated that their ability to trigger regenerative calcium waves depended on phospholipase C activity and inositol phosphate production. Thus, in rat astrocytes, initiation and propagation of calcium waves involve a sequence of intra-and intercellular steps in which phospholipase C, inositol trisphosphate, internal calcium stores, and gap junction channels play a critical role. The identification of these different events allows us to determine several targets at which the level of long-range signaling in astrocytes may be controlled.

A Fundamental Role for the Nitric Oxide-G-Kinase Signaling Pathway in Mediating Intercellular Ca2+Waves in Glia

“…7). It has previously been reported that treatment of glial cells with U 73122 alone is sufficient to completely abrogate the stress-induced Ca 2ϩ wave, with only an increase in [Ca 2ϩ ] i being observed in the target cell (Charles et al, 1993;Venance et al, 1997). This is in contrast to the results of our study.…”

“…This result appears to conflict with data from previous studies that demonstrated abrogation of the stress-induced Ca 2ϩ wave by the IP 3 receptor (IP 3 R) antagonist heparin in retinal glial cells (Newman and Zahs, 1997) and also by the PLC inhibitor U 73122 in astrocytes (Charles et al, 1993;Venance et al, 1997), suggesting that the response was mediated principally by IP 3 -induced Ca 2ϩ release from IP 3 R-linked Ca 2ϩ stores. However, it is possible that both IP 3 Rs and RyRs are required in an activated, conducting state to allow the generation and propagation of the Ca 2ϩ wave.…”

Neuronal Activity Regulates Correlated Network Properties of Spontaneous Calcium Transients in AstrocytesIn Situ

“…The present study, which shows that correlation of spontaneous astrocytic Ca 2ϩ activity is independent of gap junctions, together with the fact that evoked Ca 2ϩ waves in acute brain slices are unaffected by gap junction blockers , suggests that different mechanisms correlate Ca 2ϩ activity in cultured and in situ astrocytes. In contrast, inhibition of NMDA and non-NMDA glutamate receptors impairs spontaneous astrocytic activity, which is consistent with other studies that show that activation of glutamate receptors is involved in the propagation of astrocyte Ca 2ϩ signaling (Cornell-Bell et al, 1990;Cornell-Bell and Finkbeiner, 1991;Dani et al, 1992;Finkbeiner, 1992;Venance et al, 1997). Because, in addition to neurons, astrocytes exhibit Ca 2ϩ -dependent glutamate release (Bezzi et al, 1998, Araque et al, 2000Innocenti et al, 2000), astrocytic network activity could be controlled by glutamate derived from both neurons and astrocytes.…”

“…Gap junctions and extracellular messengers, such as glutamate, ATP, and nitric oxide, have been proposed to control the propagation of evoked Ca 2ϩ activation in astrocyte networks (Cornell-Bell et al, 1990;Dani et al, 1992;Finkbeiner, 1992;Venance et al, 1997;Guthrie et al, 1999;Willmott et al, 2000;Newman, 2001;Schipke et al, 2002). To identify the mechanisms that correlate spontaneous activity of astrocytic networks in situ, we used several blocking agents on P5-P7 GFAP/GFP hippocampal slices.…”

Connexin43 is involved in the effect of endothelin‐1 on astrocyte proliferation and glucose uptake

“…The permeability of gap junctions is subject to regulation by several mechanisms (for review, see Giaume and McCarthy, 1996;Rouach et al, 2002). In this sense, the ability of ET-1 to inhibit gap junctional communication between astrocytes both in vitro and in vivo is well documented (Blomstrand and Giaume, 2006;Blomstrand et al, 2004;Giaume et al, 1992;Hasselblatt et al, 2003;Tabernero et al, 1996a;Tabernero et al, 2001;Venance et al, 1997). Indeed, ET-1 downregulates the expression of connexin43 (Cx43) (Tabernero et al, 2006b), the main protein forming gap junction channels in astrocytes (Giaume et al, 1991).…”