Pharmacology and characterization of N-methyl-D-aspartate neurotoxicity in developing central nervous system.

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$\sp2$ = 0.99, p $<$ 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$\sp{2+}$ (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.Ph.D.Biological SciencesNeurosciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128827/2/9208613.pd