1 The application of an ischaemic insult in hippocampal slices results in the depression of synaptic transmission, mainly attributed to the activation of A 1 adenosine receptors by adenosine released in the extracellular space. 2 To estimate the concentration of endogenous adenosine acting at the receptor level during an ischaemic episode, we recorded ®eld e.p.s.ps (fe.p.s.ps) from hippocampal slices, and evaluated the ability of the selective A 1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), to reverse the fe.p.s.p. depression induced by in vitro ischaemia. A relationship between the IC 50 of an antagonist and the endogenous concentration of a neurotransmitter has been used for pharmacological analysis. 3 The complete and reversible depression of fe.p.s.p. in the CA1 region induced by 5 min ischaemia was decreased in the presence of DPCPX (50 ± 500 nM). 8-Phenyltheophylline (10 mM) abolished the depression of fe.p.s.ps during the ischaemic period, while a small (peak eect 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., IC 50 and adenosine concentration changed as function of time with a good degree of correlation. The maximal value of adenosine concentration was 30 mM. 5 Our data provide an estimation of the adenosine concentration reached at the receptor level during an ischaemic episode, with a higher time discrimination (15 s) than that achieved with any biochemical approach. This estimation may be useful in order to establish appropriate concentrations of purinergic compounds to be tested for their pharmacological eects during an ischaemic episode.

No abstract

Various experimental approaches have been used to determine the concentration of adenosine in extracellular brain fluid. The cortical cup technique or the microdialysis technique, when adenosine concentrations are evaluated 24 hours after implantation of the microdialysis probe, are able to measure adenosine in the nM range under normoxic conditions and in the μM range under ischemia. In vitro estimation of adenosine show that it can reach 30 μM at the receptor level during ischemia, a concentration able to stimulate all adenosine receptor subtypes so far identified. Although the protective role of A1 receptors in ischemia seems consistent, the protective role of A2A receptors appears to be controversial. Both A2A agonists and antagonists have been shown to be neuroprotective in various in vivo ischemia models. Although A2A agonists may be protective, mainly through peripherally mediated effects, A2A antagonists may be protective through local brain mediated effects. It is possible that A2A receptors are tonically activated following a prolonged increase of adenosine concentration, such as occurs during ischemia. A2A receptor activation desensitizes A1 receptors and reduces A1 mediated effects. Under these conditions A2A receptor antagonists may be protective by potentiating all the neuroprotective A1 mediated effects, including decreased neurotoxicity due to reduced ischemia induced glutamate outflow.

1 In the present study we investigated the role of A 2A adenosine receptors in hippocampal synaptic transmission under in vitro ischaemia-like conditions. 2 The e ects of adenosine, of the selective A 2A receptor agonist, CGS 21680 (2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoadenosine), and of selective A 2A receptor antagonists, ZM 241385 (4-(2-[7-amino-2-(2-furyl)-{1,2,4}-triazolo{2,3-a}{1,3,5}triazin-5-ylamino]ethyl)phenol) and SCH 58261 (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine), have been evaluated on the depression of ®eld e.p.s.ps induced by an in vitro ischaemic episode. 3 The application of 2 min of in vitro ischaemia brought about a rapid and reversible depression of ®eld e.p.s.ps, which was completely prevented in the presence of the A 1 receptor antagonist DPCPX (1,3-dipropyl-8-cyclopentylxanthine) (100 nM). On the other hand both A 2A receptor antagonists, ZM 241385 and SCH 58261, by themselves did not modify the ®eld e.p.s.ps depression induced by in vitro ischaemia. 4 A prolonged application of either adenosine (100 mM) or CGS 21680 (30, 100 nM) before the in vitro ischaemic episode, signi®cantly reduced the synaptic depression. These e ects were antagonized in the presence of ZM 241385 (100 nM). 5 SCH 58261 (1 and 50 nM) did not antagonize the e ect of 30 nM CGS 21680 on the ischaemiainduced depression. 6 These results indicate that in the CA1 area of the hippocampus the stimulation of A 2A adenosine receptors attenuates the A 1 -mediated depression of synaptic transmission induced by in vitro ischaemia.

No abstract