Potencies and unblocking kinetic properties of antagonists at recombinant human NMDA receptors in a Xenopus oocytes model

Abstract

N-methyl-D-aspartate (NMDA) receptor channels are implicated in a wide range of physiological and pathophysiological processes, and a large number of pharmacological agents have been introduced that target the receptor via diverse mechanisms of action. Amongst others, subunit selectivity (in particular for the NR2B receptor subunit) and rapid unblocking kinetics have been put forward as favourable pharmacological properties of NMDA receptor-targeting drugs. Here, we describe a pharmacological characterization of human recombinant NMDA receptors expressed in Xenopus oocytes in an electrophysiological set-up. Using this approach, we compare inhibitor potencies of several known NMDA receptor ligands as well as unblocking kinetic properties of selected compounds. All compounds tested had similar potencies at receptors containing NR2A or NR2B receptors with the exception of traxoprodil, which was selective for NR2B. The rank order of potency was (+)MK-801 > phencyclidine (PCP) ≈ traxoprodil > memantine ≈ ketamine > duloxetine. In line with its proposed rapid dissociation properties, the relatively well-tolerated drug memantine exhibits markedly faster unblocking than ketamine and PCP, similar to the low-affinity compound, duloxetine. Electrophysiological recording in Xenopus oocytes thus allows a relatively convenient comparison of key pharmacological parameters at recombinant human NMDA receptors.

Is Memantine Effective as an NMDA Receptor Antagonist in Adjunctive Therapy for Schizophrenia?

“…Parsons et al reported that memantine, amantadine, and dextromethorphan, which have low affinities for NMDA receptors, exhibit obviously faster open-channel blocking/unblocking kinetics than NMDA receptor antagonists associated with severe psychotropic effects, such as PCP or (+)MK-801, which have high affinities for NMDA receptors, and unblocking kinetics are directly related to affinity, namely, lower affinity NMDA receptor antagonists have faster unblocking kinetics [ 50 , 51 ]. Heusler et al also reported that memantine has faster unblocking kinetics than PCP and ketamine, similar to duloxetine, which is also a low-affinity antagonist of NMDA receptors, on recombinant human NMDA receptors [ 52 ] (it is described in more detail in the later Section 4.2 Mode of Fast Off-Rate and Section 4.3 Voltage Dependency).…”

“…Low affinity/fast off-rate [45,50,51,53] Voltage dependency/fast unblocking kinetics [50][51][52][53][54][55] Partial trapping [54][55][56] Uncompetitive antagonism/ agonist concentration dependency [53,[57][58][59][60] Preferential inhibition of extrasynaptic receptors [65][66][67][68][69][70]…”

GM-1020: a novel, orally bioavailable NMDA receptor antagonist with rapid and robust antidepressant-like effects at well-tolerated doses in rodents

“…The affinity for NMDAR was determined using [ 3 H]MK-801 radioligand binding in rat cerebrum homogenate as previously described [ 15 , 16 ]. A comparison of the inhibition of glutamate/glycine currents in oocytes expressing human NR1/NR2A or NR1/NR2B NMDAR was made using patch clamp as previously described [ 17 ]. Patch clamp electrophysiology studies to investigate concentration and voltage dependence of NMDAR currents were conducted in HEK293 NMDA NR1/2A TET-inducible cells as previously described [ 18 , 19 ].…”