Kainate-evoked release of adenosine from the hippocampus of the anaesthetised rat: Possible involvement of free radicals

Abstract

Using microdialysis in the hippocampus of anaesthetised rats, the concentration of extracellular adenosine was estimated to be 0.8 μM. Kainic acid (0.1-25 mM) in the perfusate evoked a concentration-dependent release of adenosine with an EC50 of 940 μM. Two 5-min pulses of 1 mM kainic acid in the perfusate increased the dialysate levels with an S2/S1 ratio of 0.52 ± 0.03. Kainate-evoked release of adenosine was reduced significantly by 10 μM tetrodotoxin and by a κ-receptor agonist, U50, 488H (100 μM). The S2/S1 ratio was reduced by 4.5 μM 6-cyano-7-nitroquinoxaline-2,3-dione, a non- NMDA receptor antagonist, but not by the NMDA receptor blockers (+)-MK-801 (dizocilpine; 100 μM) or (±)-2-amino-5-phosphonopentanoic acid (1 mM), indicating a non-NMDA receptor-mediated process. The S2/S1 ratio was also reduced significantly by 10 mM ascorbic acid, 10 mM glutathione (a scavenger of hydroperoxides), and 1 mM oxypurinol (a xanthine oxidase inhibitor), indicating the possible involvement of free radicals. Neither the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (100 μM) nor the A1 adenosine receptor agonist R(-)-N6-(2-phenylisopropyl)adenosine (100 μM) affected release. Adenosine release evoked by kainic acid is therefore mediated by activation of non-NMDA receptors and may involve the propagation of action potentials and the production of free radicals.