| Intrastriatal injection of the glutamate analogue N-methyl-D-aspartate (NMDA, 25 nmol), in postnatal day (PND) 7 rats provided a rapid, sensitive, and reproducible assay in which potential neuroprotective strategies could be examined. Brain injury was quantified 5 days post-injection by comparison of the weights of the injected and contralateral cerebral hemispheres. This index of neuronal injury was linearally related to the amount of NMDA injected (r{dollar}sp2{dollar} = 0.99, p {dollar}<{dollar} 0.001) and was as sensitive and reproducible as 2 widely accepted methods of quantifying brain injury: measurement of (1) tissue choline acetyltransferase activity and (2) regional cross-sectional areas in histopathologic sections.; Peripheral administration of MK-801, a non-competitive NMDA antagonist, reduced NMDA-mediated brain injury by 90% when given 0.5 hours after intrastriatal injection of NMDA. MK-801 was neuroprotective up to 10 hours after injection of NMDA, but the degree of neuroprotection decreased as the post-treatment interval increased. The rank order of neuroprotective potency (relative to MK-801) of three classes of NMDA antagonists when administered 15 min. after NMDA was: MK-801 (1), CGS-19755 (9), CPP (14), PCP (17), DOIPG (27), TCP (38), dextromethrophan (118), ketamine (189), HA-966 (407). Of these compounds only MK-801, TCP and CGS-19755 provided complete neuroprotection. NMDA-mediated brain injury was also reduced by Mg{dollar}sp{lcub}2+{rcub}{dollar} (67% protection) and sigma opioid receptor ligands (+PPP and haloperidol, both 35% protection). In contrast, drugs that reduce presynaptic release of neurotransmitters (adenosine), limit seizure activity (carbamazepine, diazepam, flunarazine, pentobarbital, phenytoin), or enhance neuronal inhibition (baclofen) were not effective against NMDA toxicity.; In contrast to the neuroprotective effects of MK-801 when administered shortly before of after intrastriatal NMDA injections, administration of MK-801 24 hours earlier paradoxically enhanced NMDA-mediated brain injury by 20-30%. After these early treatments, MK-801 upregulated the number of NMDA receptors (30-50% increase). Thus NMDA antagonists may also transiently sensitize the brain to injury as the drug levels decline in the brain. |
