GLUTAMATE-INDUCED CALCIUM SIGNALING IN ASTROCYTES

Astrocytes respond to the excitatory neurotransmitter glutamate with d ynamic spatio-temporal changes in intracellular calcium [Ca2+](i). Alt hough they share a common wave-like appearance, the different [Ca2+](i ) changes-an initial spike, sustained elevation, oscillatory intracell ular waves, and regenerative intercellular waves-are actually separate and distinct phenomena. These separate components of the astrocytic C a2+ response appear to be generated by two different signal transducti on pathways. The metabotropic response evokes an initial spatial Ca2spike that can propagate rapidly from cell to cell and appears to invo lve IP3. The metabotropic response can also produce oscillatory intrac ellular waves of various amplitudes and frequencies that propagate wit hin cells and are sustained only in the presence of external Ca2+. The ionotropic response, however, evokes a sustained elevation in [Ca2+]( i) associated with receptor-mediated Na+ and Ca2+ influx, depolarizati on, and voltage-dependent Ca2+ influx. In addition, the ionotropic res ponse can lead to regenerative intercellular waves that propagate smoo thly and nondecrementally from cell to cell, possibly involving Na+/Ca 2+ exchange. All these astrocytic [Ca2+](i) changes tend to appear wav e-like, traveling from region to region as a transient rise in [Ca2+]( i). Nevertheless, as our understanding of the cellular events that und erlie these [Ca2+](i) changes grows, it becomes increasingly dear that glutamate-induced Ca2+ signaling is a composite of separate and disti nct phenomena, which may be distinguished not based on appearance alon e, but rather on their underlying mechanisms. (C) 1994 Wiley-Liss, Inc .