Microglial cells are the primary immune effector cells in the brain. Extracellular ATP, e.g., released after brain injury, may initiate microglial activation via stimulation of purinergic receptors. In the rat nucleus accumbens (NAc), the involvement of P2X and P2Y receptors in the generation of microglial reaction in vivo was investigated. A stab wound in the NAc increased immunoreactivity (IR) for P2X(1,2,4,7) and P2Y(1,2,4,6,12) receptors on microglial cells when visualized with confocal laser scanning microscopy. A prominent immunolabeling of P2X(7) receptors with antibodies directed against the ecto- or endodomain was found on Griffonia simplicifolia isolectin-B4-positive cells. Additionally, the P2X(7) receptor was colocalized with active caspase 3 but not with the anti-apoptotic marker pAkt. Four days after local application of the agonists alpha,betameATP, ADPbetaS, 2MeSATP, and BzATP, an increase in OX 42- and G. simplicifolia isolectin-IR was observed around the stab wound, quantified both densitometrically and by counting the number of ramified and activated microglial cells, whereas UTPgammaS appeared to be ineffective. The P2 receptor antagonists PPADS and BBG decreased the injury-induced increase of these IRs when given alone and in addition inhibited the agonist effects. Further, the intra-accumbally applied P2X(7) receptor agonist BzATP induced an increase in the number of caspase-3-positive cells. These results indicate that ATP, acting via different P2X and P2Y receptors, is a signaling molecule in microglial cell activation after injury in vivo. The up-regulation of P2X(7)-IR after injury suggests that this receptor is involved in apoptotic rather than proliferative effects.