Time-course of the protective effect of selective adenosine A2A receptor agonists and antagonists after brain focal ischemia in the rat

In recent years, evidence indicated that adenosine A2A receptor subtype is of critical importance in stroke. Adenosine A2A receptors located on neuronal and microglial cells of striatum and cortex are overexpressed 24 hours after focal cerebral ischemia. Aims of the work were to investigate the protective effect of the adenosine A2A receptor agonist, CGS21680, and of the adenosine A2A receptor antagonist, SCH58261, chronically administered (0.01 mg/kg, i.p., twice/day for 7 days) after transient (1 hour) focal ischemia induced in the rat by occlusion of the middle cerebral artery (tMCAo). The protective effects of the adenosine A2A receptor agonist,CGS21680 and of the adenosine A2A receptor antagonist, SCH58261 were evaluated by neurological test and by immunohistochemical analysis. CGS21680, administered starting from 4 hours after ischemia, protected from neurological deficit from the first day up to seven days thereafter (score at 7 day: 4.37±0.90, n=4 versus 7.00±0.64, n=9 in vehicle group; p<0.001). Seven days after the ischemic insult, it significantly reduced the volume of the ischemic cortical damage (51.88±10.37 mm3, n=4 versus 75.15±5.13 mm3, n=9 in vehicle group; p<0.02), improved the cytoarchitecture of ischemic cortex and striatum, improved the myelin organization in ischemic striatum, reduced microgliosis and astrogliosis. Two days after tMCAo, a massive infiltration of granulocytes into cerebral ischemic tissue was observed. CGS21680 reduced granulocyte infiltration in the ischemic areas (in cortex: 20.32±2.41 cells/optical field, n=6 versus 37.5±6.8, n=3 in vehicle group; p<0.02). SCH58261, administered starting from 5 min after ischemia significantly protected from the neurological deficit 1 day after tMCAo (p<0.001), but no more after 5 and 7 days. Seven days after tMCAo, SCH58261 has not protected ischemic areas from damage and has not ameliorated myelin organization into the ischemic striatum. Two days after tMCAo, SCH58261 has not reduced blood cell infiltration into ischemic striatal and cortical tissue. Results indicate that CGS21680, chronically administered, protects against ischemic damage by reducing blood cell infiltration and neuroinflammation along the days after ischemia. Moreover, protection by SCH58261 24 hours after ischemia is attributable to reduced excitotoxicity. Seven days after ischemia the early protective effect of the A2A receptor antagonist, likely has been overwhelmed by a secondary damage due to blood cell infiltration and neuroinflammation.