302 Fig. 3 Excitatory effects of endogenous ATP. Bars in the graphs represent the average of fEPSP amplitude in the presence of P2 antagonists: PPADS (30 μM) and MRS 2179 (10 μM). *P<0.05, paired Student’s t-test. (Modified from [15]) Purinergic Signalling (2007) 3:299–310 reduce the damage induced in rats in a model of focal ischemia in vivo. Moreover, RB2 induces the expression of P2X7 receptors on reactive microglia in the remote ipsi and contralateral cingulate and medial frontal cortex and stria- tum. Although a pro-apoptotic role has been attributed to the P2X7 receptor, results suggest that microglial cells express- ing the P2X7 receptor can be implicated in tissue damage as well as in the defence and repairing processes in the remote ipsi and contralateral undamaged areas [36]. In considering the effect of ATP during ischemia, it must be taken into account that ATP itself is involved in control of cerebrovascular regulation [38] and that the metabolite of ATP, ADP, that interacts with P2Y1/P2Y12 receptors, is a potent platelet aggregator. However, clinical studies that checked the antiplatelet therapy of the selective P2Y12 receptor antagonist clopidogrel reported a response variability [39, 40]. Although evidence suggests that ATP exerts a detrimental role under ischemia, there are reports that it may exert a trophic role in terms of cell division and differentiation in both differentiated and undifferentiated cells [41–43], including adult neural stem cells [44]. Interesting data correlate these trophic effects with the occurrence of spontaneous Ca2+ waves that propagate among adjacent cells in a self-renewing manner. Kawano et al. [45] reported that ATP-induced initiation and propagation of intracellular Ca2+ waves in human mesenchymal stem cells (hMSCs) promote activation of transcription factors (e.g. NFAT) that are involved in cell differentiation. In the same study, they demonstrated that ATP-induced Ca2+ waves disappear in the fully differentiated adipogenic phenotype. We recently reported that ATP is spontaneously released from hMSCs during the early stages of culture (P0-P5). Moreover, we reported that ATP de- creases proliferation rate (Fig. 4) and modulates specific ionic current hMSCs [46]. A high extracellular ATP concen- tration at earlier cell culture passages suggests an important role of ATP in regulating cell differentiation. Increased hMSC differentiation may account for an ATP-induced decrease in cell proliferation. Therefore, it can be envisaged that ATP, as an autocrine/ paracrine homeostatic regulator, exerts different effects on Fig. 4a, b Human mesenchymal stem cells in culture spontaneously release ATP that modulates cell proliferation. a Extracellular concen- trations of ATP were measured in the medium containing hMSCs and in control medium not containing cells. Data are expressed as mean ± SE, n=11, unpaired Student’s t-test: *P<0.0001 vs medium alone. b Effect on hMSC proliferation after daily application of ATP (10 μM) and P2 antagonists, PPADS (30 μM) and MRS 2179 (10 μM). Data are expressed as percentage of proliferation. Proliferation of untreated cells was assumed as 100%. The cell number was determined after 5 days of culture by a culture counter. Each column bar represents the mean ± SE of n=4 for each experimental condition. Paired Student’s t-test: *P< 0.05 vs respective control; one-way ANOVA, Newman-Keuls post-test: §P<0.05 vs 10 μM ATP-treated cells. (Modified from [46])