306 possible use of adenosine kinase inhibitors, it should be taken into account that adenosine represents only a small percentage of nucleotide content [134], therefore inhibition of its rephosphorylation to ATP by adenosine kinase inhibitors does not weigh upon the ATP content. The few studies present in the literature concerning the role of A3 receptors in the pathophysiology of cerebral ischemia are rather contradictory. We have demonstrated that selective antagonism of A3 receptors facilitates the recovery of synaptic activity induced by ischemic preconditioning in rat hippocampal slices [81]. A harmful role of A3 receptors during in vitro OGD was confirmed by our observation that blocking the A3 adenosine receptor consistently abolishes or delays the occurrence of anoxic depolarization (AD) and sig- nificantly protects from the irreversible disruption of excit- atory neurotransmission caused by a severe ischemic episode [135]. These results are in agreement with the observation that acute administration of a selective adenosine A3 agonist exacerbates the damage elicited by global ischemia in the gerbil [136]. On the contrary, it was demonstrated that chronic pre-ischemic administration of an A3 agonist protects against ischemic neuronal damage [136]. This effect may be attributed to desensitisation of A3 receptors. In fact, both human and rat A3 receptors are desensitised within a few minutes after agonist exposure [137, 138]. Contrary to the above information, Hentschel and col- leagues [139] demonstrated that under hypoxic conditions, selective activation of A3 adenosine receptors brings about an inhibition of excitatory neurotransmission on cortical neuro- nes, indicating that A3 receptors may sustain the neuro- protective action of adenosine induced by A1 receptors. Consistent with these reports, mice lacking A3 adenosine receptors show increased neurodegeneration in response to repeated episodes of moderate hypoxia [140] or an increase in cerebral infarction after transient ligation of the middle cerebral artery [141]. These opposite results regarding an excitatory or inhibitory role of A3 receptors on synaptic activity under hypoxia/ischemia may be reconciled by our recent data [135], suggesting that in a first phase of ischemia, A3 receptors play a protective synergistic role with A1 receptors. Severe ischemia would transform the A3 receptor- mediated effects from protective to injurious. Taken together, these data suggest that the outcome of A3 receptor stimulation on synaptic transmission during hypoxic/ischemic phenomena depends on the intensity and duration of stimulation. Concluding remarks Purinergic signalling, e.g. adenosine and ATP, by activating specific membrane receptors (P1 and P2 respectively), is strictly correlated in orchestrating brain cell functions either Purinergic Signalling (2007) 3:299–310 under physiological normoxic or ischemic circumstances. However, the respective contribution of each single element is hard to discern from the total outcome, due to the rapid and ubiquitous enzymatic interconversion of these molecules. In the brain, both compounds are likely to play a role in neuronal activity, and the inhibitory action of endogenous adenosine in excitatory neurotransmission (mainly due to A1 receptor activation) is well documented. Conversely our data, in line with previous observations in the literature, suggest that tonic activation of P2 receptors contributes to excitatory transmission in the hippocampus. Such an effect is modest, but it is unmasked by the application of P2 pu- rinergic antagonists that reduce evoked synaptic responses. An inhibitory effect elicited by exogenous ATP applica- tion has been frequently described and is mostly due to enzymatic degradation to adenosine and the subsequent activation of A1 receptors. However, our evidence demon- strates that ATP itself may inhibit hippocampal synaptic transmission since an even more pronounced effect is observed when enzymatic ATP degradation is blocked. Moreover, in our studies in the hippocampus, a potentiation of neurotransmission by the ATP analogue, ATPγS, develops a few minutes after the inhibitory effect has been washed out, and it persists for 45 min to 1 h. Within the brain, ATP is involved in the synaptic plasticity phenom- enon and may be an important mediator of long-lasting ef- fects in synaptic plasticity. Either adenosine or ATP seems to become a particularly important signalling molecule under pathological conditions, such as ischemia, when the extracellular concentration of either compound drastically rises. It is well known that adenosine exerts important neuroprotective effects during brain ischemic insults by activating adenosine A1 receptors, which profoundly inhibit synaptic transmission and in particular the release of glutamate, known to contribute to ischemic damage. Under severe ischemia, a prolonged stim- ulation of both A2A and A3 receptors may be deleterious. The role of endogenously released ATP during cerebral ischemia is mainly deleterious, as found in our work and in the literature, since the application of P2 antagonists always results in a reduction in ischemic damage. The mechanisms underlying these effects are still unknown, but they can be correlated to the tonic excitatory action of endogenous ATP found in many brain regions (for example in the hippo- campus and prefrontal cortex). Purinergic signalling has an ancient phylogenetic origin. Expression of P2 receptors on cell membrane and responses to extracellular ATP are found in primitive prokaryotic species up to evolved animals and plants, suggesting an important and highly conserved role of extracellular purine nucleotides during evolution. Similarly, ontogenetic development in several species seems to involve purinergic signalling, especially during its first stages. Our observation that hMSCs,