304 currents by a PKA-dependent mechanism [77]. Finally, facilitation of the onset of epileptiform discharge has been observed in the presence of the selective A3 receptor agonist Cl-IB-MECA [78], and a reduction in such epileptic activity was observed when A3 receptors, activated by endogenously released adenosine during seizures, were blocked by the selective antagonist MRS 1191 [79]. Contrary to previous results, an inhibitory action has been attributed to A3 receptors by Brand and colleagues [80], who demonstrated that, in rat cortical neurones, the selective activation of this adenosine receptor subtype is involved in inhibition of excitatory neurotransmission, suggesting a synergistic action with the inhibitory effect of adenosine brought about by A1 receptor activation. Despite results obtained by A3 receptor stimulation, evidence that a selective block of A3 receptors does not affect neurotransmission in the CA1 region of the hippocampus under normoxic con- ditions indicates that endogenous adenosine at physiological concentrations does not exert tonic activation of A3 receptors [8, 81]. This is in line with evidence that A3 receptor activ- ation requires micromolar levels of extracellular adenosine, which can be reached only during pathological conditions of impairment in energy supply (for example during hypoxia or ischemia [82–85]). Role of adenosine in ischemic conditions Extracellular adenosine concentrations increase dramatical- ly during ischemia [82, 86, 87]. During ischemia, following the imbalance between ATP degradation and resynthesis, the intracellular concentration of adenosine increases. Purinergic Signalling (2007) 3:299–310 Therefore, increased adenosine concentrations in the extra- cellular space during ischemia are likely due to the equil- ibrative transporter that carries adenosine out of cells. Adenosine formation can also take place at the extracellular level, through the hydrolysis of extracellular ATP operated by NTPDases and ecto-5′-nucleotidase (e5′-NTs) (see Fig. 5). Hence, these enzymes have a dual function in modulating purinergic neurotransmission: (1) they rapidly interrupt ATP- mediated signalling by degrading extracellular nucleotides and (2) they give rise to extracellular adenosine, which activates P1 receptors. However, recent evidence in vitro suggests that released ATP does not substantially contribute to the adenosine concentration in the extracellular milieu during ischemia [88]. Adenosine-potentiating agents, which elevate endogenous adenosine levels by either inhibiting its metabolism by adenosine deaminase or kinase [89, 90] or preventing its transport [91, 92], offer protection against ischemic neuronal damage in different in vivo ischemia models. Moreover, adenosine infusion into the ischemic striatum during tran- sient focal ischemia proves to significantly ameliorate the neurological outcome and reduce infarct volume [93]. A temporal correlation exists between adenosine outflow and synaptic potential inhibition in rat hippocampal slices during ischemia-like conditions [84, 88]. Synaptic inhibition during ischemia is greatly dependent on adenosine, which, by stimulating A1 receptors, exerts a protective role by reducing the Ca2+ influx, thus counteracting the presynaptic release of excitatory neurotransmitters [55, 94] and, in particular, glutamate, which exerts an excitotoxic role during ischemia mainly by overstimulation of NMDA receptors [95]. By directly increasing the K+ and Cl− ion conductan- Fig. 5 Schematic drawing of intracellular and extracellular adenosine formation. In the extracellular space, adenosine and ATP act on own purinergic receptor subtypes: P1 and P2 receptors, respectively. ADP adenosine diphosphate, AMP adenosine monophosphate, ATP adenosine triphosphate, e5′-NT ecto-5′-nucleotidase, 5′-NT 5′-nucleotidase, NTDPase ecto-nucleoside triphosphate diphosphohydrolases, P1 adenosine receptor, P2 ATP receptor, SAH S-adenosylhomo- cysteine, T bidirectional nucleoside transporter. (Modified from [142])