| Abstract No.: | B-C2092 |
| Country: | Canada |
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| Title: | PRECONDITIONING ‘BUYS TIME’ BY ALLOWING ADMINISTRATION OF AN
NEUROPROTECTIVE ANTI-EXCITOTOXIC COCKTAIL TO BE DELAYED DURING HARSH OGD
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| Authors/Affiliations: | 1 Joe Tauskela*;
1 National Research Council, Institute for Biological Sciences, Synaptic Pathophysiology Group, Ottawa, ON, Canada
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| Content: | Objectives: Preconditioning prepares the brain to withstand otherwise lethal ischemia in vivo or oxygen-glucose (OGD) in vitro. Preconditioning stimuli include nonlethal exposure to ischemia or chemicals. In considering potential clinical utility of preconditioning, two sets of questions were addressed to understand both mechanism and neuroprotective efficacy. First, identification of a mediator common to diverse preconditioners would assist development of therapeutics based on preconditioning. The search for a common mediator activated by diverse preconditioners has largely failed, but is the response to OGD common to all preconditioners? Second, does neuroprotective efficacy depend on the preconditioning stimulus type? Moreover, will tolerance reverse with harsher OGD and, if so, how does this reversal occur and can an appropriately-timed pharmacological intervention rescue neurons?
Materials and Methods: Cultured rat cortical neurons were preconditioned by exposure to: (i) a K+-channel antagonist, 4-aminopyridine, combined with a GABAA receptor antagonist, bicuculline (4-AP/bic), to increase synaptic activity; (ii) a protein synthesis inhibitor, cycloheximide; (iii) 2-deoxyglucose to induce an ER stress response; (iv) NMDA; (v) desferioxamine to induce HIF-1; (vi) geldanamycin to induce heat-shock proteins; (vii) the polyunsaturated fatty acids, docosahexaenoic acid or linolenic acid; (viii) sulforaphane or tert-butylhydroquinone to induce the antioxidant response element; (ix) a putative mitochondrial KATP channel agonist, diazoxide and; (x) LiCl. Efficacy of preconditioning was tested by exposing cultures to OGD sufficient to kill all neurons (70 min) or longer (90 min). Extracellular buffer was collected at the termination of OGD for glutamate concentration measurement ([glutamate]ex). Where indicated, a receptor antagonist cocktail composed of MK-801, NBQX and nifedipine was added at the 70 min time-point of 90 min OGD. Neurotoxicity was measured 24 h after OGD.
Results: Preconditioning by methods (i)-(vii) provided neuroprotection against 70 min OGD, with methods (i)-(v) providing the most robust neuroprotection, and methods (viii)-(x) provided no neuroprotection. Neuroprotection correlated with the degree to which the [glutamate]ex was suppressed during OGD. In contrast, all preconditioners failed to protect neurons against 90 min OGD, which correlated with increased [glutamate]ex. Adding the anti-excitotoxic receptor antagonist cocktail at the 70 min time-point of 90 min OGD fully restored neuroprotection, but only in those cultures preconditioned by methods (i)-(vii).
Conclusions: A mechanism of action common to diverse preconditioners is identified for the first time. Tolerance to otherwise lethal OGD (70 min) resulted from suppression of a rise in [glutamate]ex, independent of the method of preconditioning. However, preconditioners delayed, but did not prevent, excitotoxic rises in glutamateex levels during more severe OGD (90 min), thereby requiring acute pharmacological intervention. Hence, preconditioning ‘buys time’ by extending the window of opportunity available for acute administration of excito-protective drugs. Clinical translation may be best achieved by focussing on enhancing efficacy (and tolerability), rather than on pursuing novel neuronal signaling activated by preconditioning, since the nature but not the magnitude of the response to OGD seems independent of the preconditioning method.
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