P2X₇Receptors Mediate ATP Release and Amplification of Astrocytic Intercellular Ca²⁺Signaling

TLDR

Astrocytes modulate synaptic transmission and brain microcirculation, and their influence on neuronal activity depends on cytosolic Ca²⁺ fluctuations that trigger gliotransmitter release; however, under pathological conditions other pathways such as connexin hemichannels and P2X7 receptors have been proposed but remain controversial. This study aims to determine whether P2X7 receptors, rather than connexin hemichannels, serve as ATP release sites that amplify intercellular Ca²⁺ signaling in astrocytes. Using genetically modified 1321N1 astrocytoma cells and spinal cord astrocytes from neonatal Cx43‑ and P2X7R‑null mice, the authors demonstrated that P2X7R activation mediates ATP release and Ca²⁺ signal amplification after low divalent cation exposure. The results show that gap junction channel blockers act as P2X7R antagonists, revealing that these compounds cannot be used as evidence for connexin hemichannel involvement in gliotransmitter release.

Abstract

Modulation of synaptic transmission and brain microcirculation are new roles ascribed to astrocytes in CNS function. A mechanism by which astrocytes modify neuronal activity in the healthy brain depends on fluctuations of cytosolic Ca 2+ levels, which regulate the release of “gliotransmitters” via an exocytic pathway. Under pathological conditions, however, the participation of other pathways, including connexin hemichannels and the pore-forming P2X 7 R, have been proposed but remain controversial. Through the use of genetically modified 1321N1 human astrocytoma cells and of spinal cord astrocytes derived from neonatal Cx43- and P2X 7 R-null mice, we provide strong evidence that P2X 7 Rs, but not Cx43 hemichannels, are sites of ATP release that promote the amplification of Ca 2+ signal transmission within the astrocytic network after exposure to low divalent cation solution. Moreover, our results showing that gap junction channel blockers (heptanol, octanol, carbenoxolone, flufenamic acid, and mefloquine) are antagonists of the P2X 7 R indicate the inadequacy of using these compounds as evidence for the participation of connexin hemichannels as sites of gliotransmitter release.