Adenosine-mediated inhibition of striatal GABAergic synaptic transmission during in vitro ischaemia

Several reports have shown that energy deprivation, as a result of hypoxia, hypoglycaemia or ischaemia, depresses excitatory synaptic transmission in virtually all brain areas. How this pathological condition affects inhibitory synaptic transmission is still unclear. In the present in vitro study, we coupled whole-cell patch clamp recordings from striatal neurones with focal stimulation of GABAergic nerve terminals in order to characterize the electrophysiological effects of combined oxygen and glucose deprivation (in vitro ischaemia) on inhibitory postsynaptic currents (IPSCs) in this brain area. We found that brief periods (2-5 min) of in vitro ischaemia invariably caused a marked depression of IPSC amplitude. This inhibitory effect was fully reversible on removal of the ischaemic challenge. It was coupled with an increased paired-pulse facilitation, suggesting the involvement of presynaptic mechanisms. Accordingly, the ischaemic inhibition of striatal GABAergic IPSCs was not caused by a shift in the reversal potential of GABA_A-receptor mediated synaptic currents, and was independ- ent of postsynaptic ATP concentrations. Endogenous adenosine, acting on A1 receptors, appeared responsible for this presynaptic action as the ischaemic depression of IPSCs was prevented by CPT [8-(4-chlorophenylthio) adenosine] and DPCPX, two adenosine A1 receptor antagonists, and mimicked by the application of adenosine in the bathing solution. Conversely, ATP-sensitive potassium channels were not involved in the inhibition of IPSCs by ischaemia, as demonstrated by the fact that tolbutamide and glipizide, two blockers of these channels, were ineffective in preventing this electrophysiological effect. The early depression of GABA-mediated synaptic transmission might play a role in the development of irreversible neuronal injury in the course of brain ischaemia.