Purinergic Signalling (2007) 3:299–310 301 Fig. 1a, b The inhibitory effect induced by ATP on fEPSP amplitude is potentiated in the presence of different NTPDase inhibitors. a Time- course of fEPSP amplitude before, during and after the application of ATP in the absence or in the presence of the NTPDase1,2,3 inhibitor PV4. Each point in the graph represents the mean ± SE of fEPSP value measured as percent of baseline, pre-drug level. b Columns in the graph summarize the average amplitude (mean ± SE) of evoked fEPSP recorded from CA1 hippocampal region in control conditions, 5 min after superfusion of ATP alone and 5 min after ATP superfusion in the presence of different ecto-ATPases inhibitors. Note that the inhibitory effect of ATP on fEPSP amplitude is potentiated by BGO 136, PV4 and ARL 67156. *P<0.05 one-way ANOVA, Newman-Keuls multiple comparison post-hoc test versus pre-drug value. §P<0.05, one- way ANOVA, Newman-Keuls multiple comparison post-hoc test versus 10 μM ATP treated slices. (Modified from [15]) Slices superfused with P2 antagonists show a small but significant reduction in synaptic transmission (Fig. 3). These data demonstrate that tonic activation of P2 receptors contributes to glutamatergic excitatory neurotransmission in the hippocampus, an observation that is in line with previous work [27]. Roles of ATP in ischemic conditions The role of ATP may become critical during pathological conditions such as ischemia, when extracellular ATP concen- trations increase. An enhanced outflow of radioactive ATP from hippocampal slices during in vitro ischemic-like insults was first reported by Juranyi and co-workers [28], and the first demonstration that ATP outflow increases in vivo during the induction of focal ischemia in the rat was reported by Melani and colleagues [29]. Thus, at the ischemic site, levels of extracellular nucleotides may remain elevated for long periods of time after injury. Evidence supports the idea that, under such pathological conditions, released ATP may exert an excitotoxic role by acting on its receptors, thus enhancing Ca2+ inward currents and altering synaptic activity and cellular plasticity. Non-selective antagonists of P2 receptors, suramin and PPADS, and the selective antagonists, BBG and MRS 2179 of P2X7 and P2Y1 receptors respectively, prevent the irreversible failure of neurotransmission induced by a prolonged period of OGD in hippocampal slices [15]. Moreover, they protect from development of anoxic depo- larization (AD), which is a rapid and regenerative wave of depolarization that propagates in tissue and represents an unequivocal sign of sufferance [15]. Antagonists of P2 receptors have been proved protective against cell death induced by either ATP itself, hypoglycemia or glutamate exposure in primary cultures of brain-derived neurones [30– 33]. Intrastriatal ATP injection in rats induced, 24 h later, a clearly lesioned area [34]. The ATP-induced damage was concentration-dependent, mimicked by ATPγS and α,β meATP (but not by ADP or adenosine) and blocked by RB2, a non-specific P2 antagonist [34]. Suramin, another non-specific P2 receptor antagonist, administered 30 min before occlusion of the middle cerebral artery, resulted in a significant decrease in infarct and oedema volume 6 h after brain injury [35]. In agreement, it was demonstrated that RB2 [36] and PPADS [37] improve neurological deficit and Fig. 2 Inhibitory and excitatory effects of the stable ATP analogue ATPγS. Averaged time-course (n =4) of PS amplitude before, during and after the application of different concentrations of ATPγS. PS amplitude (mean ± SE) is measured as percent of baseline level. Upper panels represent single traces recorded in a typical experiment before, during and after ATPγS application at different concentrations