Methylation of the NMDA receptor agonist L-trans-2,3-pyrrolidine-dicarboxylate: Enhanced excitotoxic potency and selectivity

This study investigated the excitotoxic properties of a novel series of NMDA analogues in which a methyl group was introduced to the 5-position of the pyrrolidine ring of L-trans-2,3-PDC, a previously identified NMDA receptor agonist. While all of these compounds induced NMDA-receptor-mediated injury, methylation increased in vivo excitotoxic potency 1000-fold. Injections (1 μl) in rat dorsal hippocampus of cis- and trans-5-methyl-L-trans-2,3-PDC (0.1 nmol) induced 50-70% neuronal damage to areas CA1 and CA4, comparable to that induced by 100 nmol of L-trans-2,3-PDC. Further, cis- and trans-methylated analogues induced distinct patterns of hippocampal pathology consistent with differential excitotoxic vulnerability of neurons expressing NMDA receptors. Neuronal damage produced by the 5-methyl-L-trans-2,3-PDCs could be blocked by coadministration of MK-801 (3 mg/kg ip), but not NBQX (25 nmol). Biochemical and physiological assays confirmed the action of the analogues as NMDA agonists, but did not provide an explanation for differences in excitotoxic potency between the methylated and nonmethylated 2,3-PDCs. For example, the activity of the compounds as inhibitors of ³H-glutamate binding (IC50 values: 0.4, 1.4, and 1.2 μM for cis-5-methyl-, trans-5-methyl-, and L-trans-2,3-PDC, respectively), agonists at NR1A/NR2B receptors (EC₅₀ values: 5, 49, and 16 μM for cis-5-methyl-, trans-5-methyl-, and L-trans-2,3-PDC, respectively), and in vitro excitotoxins in cortical cultures varied only two- to fivefold as a consequence of methylation. Potential roles of NMDA receptor subtypes and transport in these effects are discussed. AS potent and selective NMDA excitotoxins, cis- and trans-5-methyl-L-trans-2,3-PDC will be of value studying excitotoxic mechanisms, NMDA-receptor-mediated pathology, and NMDA receptor heterogeneity.