Astrocytes are the most abundant cell type in the central nervous system. They respond to a variety of stimuli with increases in intracellular calcium concentration ([Ca2+]i) that occur in individual cells or as intercellular waves propagated through many cells. Astrocyte Ca2+ signaling has been implicated in a variety of physiological and pathological processes, including the modulation neuronal synaptic signaling, the modulation of the response of the retina to light, slowly propagated pathological phenomena such as spreading depression, and the multicellular response of the CNS to localized injury. However, the true functions of astrocyte Ca2+ signaling remain poorly understood. Recent studies show that Ca2+ signaling in astrocytes is associated with the release of adenosine 5'-triphosphate (ATP), which acts as a primary intercellular messenger between astrocytes. This proposal focuses on 1) The mechanisms by which astrocytes release ATP in association with Ca2+ signaling, 2) The role of these mechanisms is the sensing and modulation of the extracellular ionic environment, and 3) The role of these mechanism in bidirectional communication between astrocytes and endothelial cells and pericytes that mediates the function of the blood-brain-barrier. Our laboratory has specialized in the techniques of video imaging of [Ca2+]i with simultaneous electrophysiological measurements. We will use theses techniques, in conjunction with other functional assays, to investigate the following hypotheses: 1.) Intercellular Ca2+ signaling in astrocytes is mediated by release of ATP through connexin hemichannels. 2.) ATP release by astrocytes is modulated by diacylglycerol and membrane potential 3.) Astrocytes respond to and modulate extracellular K+ and Ca2+ by intercellular Ca2+ signaling and the release of ATP. 4.) Astrocytes communicate with endothelial cells and pericytes and modulate blood-brain-barrier function via release of ATP. An increased understanding of these novel pathways of astrocyte signaling may provide new insight into cellular mechanisms of a variety of disorders, including epilepsy, migraine and traumatic and ischemic injury, and may identify new therapeutic strategies for these disorders.
