Purpose: A brief period of ischemia can induce a remarkably complete state of ischemic tolerance in the retina, a phenomenon known as ischemic preconditioning (IPC). The mechanisms of IPC were studied in the rat retina by examining the role of adenosine as a possible mediator and determining whether IPC protection could be induced more than once in the same rat.
Methods: Retinal ischemia was produced for 60 minutes in ketamine-xylazine-anesthetized Sprague-Dawley rats, and recovery was measured using electroretinography. Twenty-four hours earlier, the IPC stimulus of 5 minutes of ischemia was applied. To test the role of adenosine as a mediator of IPC, the selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.45 mg/kg, 2.25 mg/kg, or 4.5 mg/kg), the A2a antagonist 8-(3-chlorostyryl)caffeine (CSC; 0.1 mg/kg or 1.0 mg/kg), or their cyclodextrin vehicle were administered 15 minutes before IPC. To examine whether exogenous adenosine administration could mimic IPC, animals received intravitreal injections of the adenosine A1 receptor stimulant adenosine amine congener (ADAC) or the A2a stimulant CGS21680, followed by ischemia 24 hours later. To test the hypothesis that IPC could be induced repeatedly without loss of protection, rats were divided to receive IPC or sham IPC, followed 10 days later by IPC or a sham procedure, and 24 hours later by 60 minutes of ischemia.
Results: Adenosine A1 receptor blockade with 4.5 mg/kg DPCPX administered intraperitoneally (IP) before or immediately after 5 minutes of ischemia completely blocked IPC protection, whereas lower doses resulted in partial blockade. CSC at the lowest dose (0.1 mg/kg) had no significant effect on IPC's protective effect, whereas partial blockade was found with 1.0 mg/kg CSC. A1 or A2a receptor stimulation produced partial but significant mimicking of IPC protection, effects that were antagonized by DPCPX or CSC. Ischemic preconditioning applied twice, separated by 10 days, and followed by 60 minutes of ischemia 24 hours after the second IPC stimulus, resulted in nearly identical recovery of function after ischemia compared with IPC performed one time.
Conclusions: Adenosine, acting through the A1 and A2a receptors, is a critical component in the induction of ischemic tolerance after preconditioning in the retina. The neuroprotective effects of IPC in the retina are lost over time but may be reinduced by subsequent application of the IPC stimulus.