(2S,4R)-4-METHYLGLUTAMIC ACID (SYM-2081) - A SELECTIVE, HIGH-AFFINITYLIGAND FOR KAINATE RECEPTORS

Glutamic acid activates ionotropic glutamate receptors that mediate ex citatory transmission in the central nervous system. The introduction of a methyl group at position 4 of glutamic acid imparts selectivity f or kainate receptors, relative to other (N-methyl-D-aspartate and lpha -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) ionotropic glutam ate receptors. Among the stereoisomers of 4-methylglutamic acid, the p otency of the (2S,4R)-isomer (SYM 2081) to inhibit [H-3]kainic acid bi nding to both wild-type (rat forebrain) and recombinant (GluR6) kainat e receptors (IC50 values of similar to 32 and 19 nM, respectively) was comparable to that of kainic acid (IC50 values of similar to 13 and 2 8 nM, respectively), SYM 2081 was similar to 800- and 200-fold less po tent as an inhibitor of radioligand binding to wild-type (rat forebrai n) lpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methy l-D-aspartate receptors, respectively. Preexposure of human embryonic kidney 293 cells stably expressing GluR6 receptors to low concentratio ns of SYM 2081 (30-300 nM) resulted in a reversible blockade of the ra pidly desensitizing currents produced by kainate application. At highe r concentrations, SYM 2081 (EC(50) of similar to 1 mu M) elicited kain ate-like, rapidly desensitizing, inward currents. Pretreatment of reco mbinant GluR6 receptors with concanavalin A both abolished the effect of SYM 2081 to block kainate-induced currents and revealed nondesensit izing currents induced by SYM 2081 alone. The latter observations prov ide strong support for the hypothesis that SYM 2081 blocks kainate-ind uced currents through a process of agonist-induced desensitization. SY M 2081 also activated lpha-amino-3-hydroxy-5-methyl-4-isoxazolepropion ic acid receptor currents in primary cultures of cerebral cortex and, consistent with data obtained by radioligand binding, was similar to 5 -fold less potent than kainate (EC(50) values of 325 and 70 mu M, resp ectively) in this measure. SYM 2081 is a high-affinity, selective, kai nate agonist that may prove useful both as a probe to examine the phys iological functions of kainate receptors and as the prototype of a nov el class of therapeutic agents.