Astroglial cells respond to trauma and ischemia with reactive gliosis, a reaction characterized by increased astrocytic proliferation and hypertrophy. Although beneficial to a certain extent, excessive gliosis may be detrimental, contributing to neuronal death in neurodegenerative diseases. We have tested the hypothesis that ATP may act as a trigger of reactive gliosis in an in vitro model (rat brain primary astrocytes) where reactive astrogliosis can be quantified as elongation of astrocytic processes. Challenge of cells with the ATP analog alpha,beta methyleneATP (alpha,beta meATP) resulted in concentration dependent elongation of astrocytic processes, an effect that was fully counteracted by the non-selective ATP/P2 receptor antagonists suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS). Signalling studies revealed that alpha,beta meATP-induced gliosis is mediated by a novel G-protein-coupled receptor (a P2Y receptor) coupled to an early release of arachidonic acid. Challenge of cells with alpha,beta meATP also resulted in an increase of inducible cyclooxygenase-2 (COX-2), the activity of which has been reported to be pathologically increased in a variety of neurodegenerative diseases characterized by inflammation and astrocytic activation. Induction of COX-2 by alpha,beta meATP was causally related to reactive astrogliosis, since the selective COX-2 inhibitor NS-398 prevented both the purine-induced elongation of astrocytic processes and the associated COX-2 increase. Preliminary data on the putative receptor-to-nucleus pathways responsible for purine-induced gliosis suggest that induction of the COX-2 gene may occur through the protein kinase C/mitogen activated protein kinase system, and may involve the formation of activated AP-1 transcription complexes. We speculate that antagonists selective at this novel P2Y receptor subtype may represent a novel class of neuroprotective agents able to slow down neurodegeneration by counteracting the inflammatory events contributing to neuronal cell death.