Failure and rescue of preconditioning-induced neuroprotection in severe stroke-like insults

Preconditioning is a well established neuroprotective modality. However, the mechanism and relative efficacy of neuroprotection between diverse preconditioners is poorly defined. Cultured neurons were preconditioned by 4-aminopyridine and bicuculline (4-AP/bic), rendering neurons tolerant to normally lethal (sufficient to kill most neurons) oxygen-glucose deprivation (OGD) or a chemical OGD-mimic, ouabain/TBOA, by suppression of extracellular glutamate (glutamate\u2091\u2093) elevations. However, subjecting preconditioned neurons to longer-duration supra-lethal insults caused neurotoxic glutamate\u2091\u2093 elevations, thereby identifying a \u2018ceiling\u2019 to neuroprotection. Neuroprotective \u2018rescue\u2019 of neurons could be obtained by administration of an NMDA receptor antagonist, MK-801, just before glutamate\u2091\u2093 rose during these supra-lethal insults. Next, we evaluated if these concepts of glutamate\u2091\u2093 suppression during lethal OGD, and a neuroprotective ceiling requiring MK-801 rescue under supra-lethal OGD, extended to the preconditioning field. In screening a panel of 42 diverse putative preconditioners, neuroprotection against normally lethal OGD was observed in 12 cases, which correlated with glutamate\u2091\u2093 suppression, both of which could be reversed, either by the inclusion of a glutamate uptake inhibitor (TBOA, to increase glutamate\u2091\u2093 levels) during OGD or by exposure to supra-lethal OGD. Administrating MK-801 during the latter stages of supra-lethal OGD again rescued neurons, although to varying degrees dependent on the preconditioning agent. Thus, \u2018stress-testing\u2019 against the harshest ischemic-like insults yet tested identifies the most efficacious preconditioners, which dictates how early MK-801 needs to be administered during the insult in order to maintain neuroprotection. Preconditioning delays a neurotoxic rise in glutamate\u2091\u2093 levels, thereby \u2018buying time\u2019 for acute anti-excitotoxic pharmacologic rescue.Peer reviewed: YesNRC publication: Ye