Extracellular adenosine concentrations during in vitro ischaemia in rat hippocampal slices

1 The application of an ischaemic insult in hippocampal slices results in t he depression of synaptic transmission, mainly attributed to the activation of A(1) adenosine receptors by adenosine released in the extracellular spa ce. 2 To estimate the concentration of endogenous adenosine acting at the recep tor level during an ischaemic episode, we recorded field e.p.s.ps (fe.p.s.p s) from hippocampal slices, and evaluated the ability of the selective A(1) receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), to revers e the fe.p.s.p. depression induced by in vitro ischaemia. A relationship be tween the IC50 of an antagonist and the endogenous' concentration of a neur otransmitter has been used for pharmacological analysis. 3 The complete and reversible depression of fe.p.s.p, in the CA1 region ind uced by 5 min ischaemia was decreased in the presence of DPCPX (50-500 nM). 8-Phenyltheophylline (10 mu M) abolished the depression of fe.p.s.ps durin g the ischaemic period, while a small (peak effect 12+/-4%) decrease in fe. p.s.ps was observed during the initial phase of reperfusion. 4 In the time-interval of maximal depression of fe.p.s.ps., IC50 and adenos ine concentration changed as function of time with a good degree of correla tion. The maximal value of adenosine concentration was 30 mu M. 5 Our data provide an estimation of the adenosine concentration reached at the receptor level during an ischaemic episode, with a higher time discrimi nation (15 s) than that achieved with any biochemical approach. This estima tion may be useful in order to establish appropriate concentrations of puri nergic compounds to be tested for their pharmacological effects during an i schaemic episode.