Nucleotide-mediated calcium signaling in rat cortical astrocytes: Role of P2X and P2Y receptors

ATP is the dominant messenger for astrocyte-to-astrocyte calcium-mediated communication. Definition of the exact ATP/P2 receptors in astrocytes and of their coupling to intracellular calcium ([Ca2+]i) has important implications for brain physiology and pathology. We show that, with the only exception of the P2X6 receptor, primary rat cortical astrocytes express all cloned ligand-gated P2X (i.e., P2X1-5 and P2X7) and G-protein-coupled P2Y receptors (i.e., P2Y1, P2Y2, P2Y4, P2Y6, and P2Y12). These cells also express the P2Y-like UDP-glucose receptor, which has been recently recognized as the P2Y14 receptor. Single-cell image analysis showed that only some of these receptors are coupled to [Ca2+]i. While ATP induced rapid and transient [Ca2+]i increases (counteracted by the P2 antagonists suramin, pyridoxal-phosphate-6-azophenyl-2′-4′-disulfonic acid and oxidized ATP), the P2X1/P2X3 agonist αβmeATP produced no changes. Conversely, the P2X7 agonist BzATP markedly increased [Ca2+]i; the presence and function of the P2X7 receptor was also confirmed by the formation of the P2X7 pore. ADP and 2meSADP also produced [Ca2+]i increases antagonized by the P2Y1 antagonist MRS2179. Some cells also responded to UTP but not to UDP. Significant responses to sugar-nucleotides were also detected, which represents the first functional response reported for the putative P2Y14 receptor in a native system. Based on agonist preference of known P2 receptors, we conclude that, in rat astrocytes, ATP-induced calcium rises are at least mediated by P2X7 and P2Y1 receptors; additional receptors (i.e., P2X2, P2X4, P2X5, P2Y4, and P2Y14) may also contribute.