The P2X7 receptor as a route for non-exocytotic glutamate release: dependence on the carboxyl tail

P2X7 receptors trigger Ca2+-dependent exocytotic glutamate release, but also function as a route for non-exocytotic glutamate release from neurons or astrocytes. To gain an insight into the mechanisms involving the P2X7 receptor as a direct pathway for glutamate release, we compared the behavior of a full-length rat P2X7 receptor, a truncated rat P2X7 receptor in which the carboxyl tail had been deleted, a rat P2X7 receptor with the 18-amino acid cysteine-rich motif of the carboxyl tail deleted, and a rat P2X2 receptor, all of which are expressed in HEK293 cells. We found that the P2X7 receptor function as a route for glutamate release was antagonized in a non-competitive way by extracellular Mg2+, did not require the recruitment of pore-forming molecules, and was dependent on the carboxyl tail. Indeed, the truncated P2X7 receptor and the P2X7 receptor with the deleted cysteine-rich motif both lost their function as a pathway for glutamate release, while still evoking intracellular Ca2+ elevation. No glutamate efflux was observed through the P2X2 receptor. Notably, HEK293 cells (lacking the machinery for Ca2+-dependent exocytosis), when transfected with P2X7 receptors, appear to be a suitable model for investigating the P2X7 receptor as a route for non-exocytotic glutamate efflux.