INVESTIGATIONS INTO THE ADENOSINE OUTFLOW FROM HIPPOCAMPAL SLICES EVOKED BY ISCHEMIA-LIKE CONDITIONS

The characteristics of adenosine and inosine outflow evoked by 5 min o f ischemia-like conditions in vitro (superfusion with glucose-free Kre bs solution gassed with 95% N2/5% CO2) were investigated on rat hippoc ampal slices. The viability of the slices after ''ischemia'' was evalu ated by extracellular recording of the evoked synaptic responses in th e CA1 region. The evoked dendritic field potentials were abolished aft er 5 min of superfusion under ''ischemia'' but a complete recovery occ urred after 5 min of reperfusion with normal oxygenated Krebs solution . No recovery took place after 1 0 min of ''ischemia.'' The addition o f the adenosine A, receptor antagonist 8-phenyltheophylline to the sup erfusate antagonized the depression of the evoked field potentials cau sed by 5 min of ''ischemia.'' Five minutes of ''ischemia'' brought abo ut a six- and fivefold increase in adenosine and inosine outflow, resp ectively, within 1 0 min. Tetrodotoxin reduced the outflow of adenosin e and inosine by 42 and 33%, respectively, whereas the removal of Ca2 caused a further increase. The NMDA receptor antagonist D(-)-2-amino- 7-phoshonoheptanoic acid and the non-NMDA antagonist 6,7-dinitroquinox aline-2,3-dione brought about small, not statistically significant dec reases of adenosine and inosine outflow. The glutamate uptake inhibito r dihydrokainate did not affect the outflow of adenosine and inosine. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP did not affect basal adenosine outflow but potentiated ''ischemia''-e voked adenosine outflow. It is concluded that ischemia-like conditions in vitro evoke a Ca2+-independent adenosine and inosine outflow, thro ugh a mechanism that partly depends on propagated nervous activity but does not involve excitatory amino acids. The efflux of adenosine is p robably responsible for the depression of the evoked synaptic electric al activity during ''ischemia'' in the hippocampal slices.