Perfusion of 5 microM kainate through microdialysis probes induced >2-fold elevation
of extracellular uridine and adenosine concentrations in the hippocampus and in the
thalamus of anaesthetized rats. Administration of uridine via this route produced
an estimated uridine concentration of 50-100 microM around the electrode surface.
This markedly decreased the average firing rate of neurones in the hippocampus, but
not in the thalamus. Activity of separated single hippocampal pyramidal cells was
completely inhibited by uridine. The same amount of adenosine completely blocked neuronal
activity in both hippocampus and thalamus. Uridine administration had no effect on
extracellular adenosine concentration. These findings suggest an important neuromodulatory
role for depolarization-released uridine in the CNS.