The selective block of adenosine A2B receptors protects synaptic transmission from damage induced by oxygen and glucose deprivation in the CA1 rat hippocampus

During hypoxia or ischemia the extracellular concentrations of adenosine significantly increase reaching micromolar concentrations that activate all adenosine receptor subtypes so far identified: A1, A2A, A2B, andA3 [1]. The A2B receptor is the most enigmatic among all different adenosine receptor subtypes; no data about its involvement in cerebral ischemia are so far available. In this work we characterized the role of adenosine A2B receptors during oxygen and glucose deprivation (OGD) in the CA1 region of rat hippocampus. To this purpose, two selective antagonists of the A2Breceptor subtype, MRS 1754 (N-(4- Cyanophenyl) -2-[4-(2,3,6,7-tetrahydro-2,6-dioxo- 1,3-dipropyl-1H-purin-8-yl) phenoxy] -acetamide) and PSB 603 (8-[4-[4-(4-Chlorophenzyl) piperazide-1- sulfonyl) phenyl]] -1-propylxanthine) were used. Extracellular recordings of CA1 field excitatory post-synaptic potentials (fEPSPs), obtained byelectrical stimulation of Schaffer collaterals, were performed. Application of 7 min OGD induced the appearance of a marked depolarization, known as anoxic depolarization (AD), an unambiguous sign of neuronal damage, in all hippocampal slices recorded (n = 20). Furthermore, after 7 min OGD, fEPSPs did not recover their amplitude after return to normoxic condition. TheA2B antagonistsMRS 1754 or PSB603were applied 15 min before, during and 5 min afterOGD. MRS 1754 (500 nM, n = 9) did not modify synaptic transmission under basal conditions, but completely prevented the appearance of AD in5 out of the 9 slices examined. In the remaining 4 slices, a significant delay in AD development was observed. Furthermore, MRS 1754 allowed a substantial recovery of fEPSPs in all the hippocampal slices underwent to severe OGD, also in the slices in which AD was recorded. Similar results were obtained in the presence of PSB 603 (50 nM, n = 8). The compounddid not affect synaptic transmission before OGD, but significantly delayed the appearance of AD induced by 7 min OGD in 4 out of the 8 slices recorded. In the remaining 4 slices no AD was detected. In any case, in all slices a significant recovery of fEPSPs amplitude was recorded. In order to characterize the role of adenosine A2B receptors on AD development, we prolonged the duration of the OGD from 7 to 30 min; this longer duration is invariably associated with tissue damage [2,3]. We compared the time of the appearance and the magnitude of depolarizing d.c. shift, in the absence or in thepresence of the two A2B adenosine receptor antagonists. Thirty minutes OGD elicited the appearance of AD in all untreated OGD slices, with a mean peak amplitude of -7.3 ± 0.48mV(n = 18) and a mean latency of 6.2 ± 0.16min (n = 18). When OGD was applied in the presenceof 500 nM MRS 1754 (n = 2) or 50 nM PSB 603 (n = 2), the d.c. shifts were always delayed although theAD amplitude was not significantly changed. Data demonstrate for the first time that the selective adenosine A2B receptor antagonism delays the occurrence of AD and improves neuronal survival following severe OGD in the CA1 hippocampus, as demonstrated by the significant recovery of an otherwise disrupted neurotransmission.