Preconditioning of primary rat neuronal cultures against ischemic injury: characterization of the "time window of protection'.

Primary rat neuronal cultures can be preconditioned against ischemic damage by several mechanisms. In the present study we established a new model system in order to characterize the "time window of protection' obtained by preconditioning of neurons with adenosine. Ischemia was simulated by exposure of the cultures to iodoacetate (100 microM) for 150 min, with a post-ischemic reperfusion period of 60 min. Ischemic injury was assessed by the release of lactic dehydrogenase (LDH) to the medium during the ischemic period and ischemia-reperfusion damage by the Trypan blue exclusion test. Exposure of the neuronal cultures to the ischemic or ischemia-reperfusion insult resulted in severe damage to the neurons, manifested for the former insult in a 5.4-fold increase in the release of LDH and for the latter insult in an 8.5-fold increase in the proportion of stained cells by the Trypan blue exclusion test. Preconditioning by short exposure (5 min) of the cultures to iodoacetic acid (simulating sublethal ischemia), or to adenosine (1 mM) and the A1 adenosine receptor agonist N6-(R)-phenylisopropyladenosine (R-PIA; 1 and 100 microM), prior to the insult, partially protected the neurons against the damage. The time-course of the development and waning of the resistance against the two insults following preconditioning exhibited different patterns. The resistance obtained against the ischemic insult developed rapidly, being maximal for all substances at 10 min (the shortest time window studied), and lasted up to 1 h for iodoacetate, 3 h for R-PIA and 24 h for adenosine. In contrast, the protection induced by adenosine and R-PIA against ischemia-reperfusion injury developed relatively slowly, being maximal at 3 h, but lasted longer, up to 48 h. At this time the time-response curve exhibited a second peak of protection. The waning of protection against the two insults was found to continue into a period of increased sensitivity to the insults. This phenomenon was more intense for preconditioning with iodoacetate, and especially against the ischemic injury. The results suggest that in the neurons, different mechanisms may mediate the adenosine-induced preconditioning against the ischemic or ischemia-reperfusion injury. In addition, the results support the possibility that the relatively long "time window of protection', induced by adenosine and R-PIA against ischemia-reperfusion insult, reflects a combination of two different preconditioning mechanisms.