Changes in agonist concentration dependence that are a function of duration of exposure suggest N-methyl-D-aspartate receptor nonsaturation during synaptic stimulation

Evidence suggests that N-methyl-D-aspartate receptors (NMDARs) have a relat ively high affinity for agonist compared with non-NMDA receptors. Dose-resp onse curves constructed with sustained agonist application suggest that the 50% effective concentration (EC50) for peak glutamate-evoked current at NM DARs is 1 to 10 muM, whereas that of alpha -amino-3-hydroxy-5-methyl-4-isox azole propionate (AMPA) receptors is similar to 500 mM. Given estimates of synaptic cleft glutamate concentration in the millimolar range, it would be expected that NMDARs would be saturated with agonist. However, studies of synaptic NMDAR responses indicate that these receptors may not be saturated during single release events at many synapses. To address this apparent co ntradiction, we have compared the glutamate dose-response curve for the pea k NMDAR current generated by sustained glutamate application with that obta ined during brief synaptic-like pulses of agonist. Our results using both r ecombinant and native NMDARs indicate a marked reduction in glutamate poten cy with reduced agonist application duration (EC50 = 100 to 200 muM with 1 ms application). A kinetic model suggested that the reduction in potency wi th shorter agonist application duration could be attributed to the relative ly slow activation and deactivation rates of the NMDARs. Comparison of room temperature to 37 degreesC indicated that NMDAR activation and deactivatio n were strongly accelerated by increased temperature. However, at 37 degree sC, we still observed a significant increase in potency with longer agonist application duration. We propose that glutamate has a relatively lower pot ency at NMDARs than previously thought from agonist application under equil ibrium conditions. This lower potency would account for data that shows non saturation of NMDARs during synaptic transmission.