Functional and pharmacological properties of triheteromeric GluN1/2B/2D NMDA receptors

Abstract

Key points Triheteromeric NMDA receptors contain two GluN1 and two distinct GluN2 subunits and mediate excitatory neurotransmission in the CNS. Triheteromeric GluN1/2B/2D receptors have functional properties intermediate to those of diheteromeric GluN1/2B and GluN1/2D receptors. GluN1/2B/2D receptors are more sensitive to channel blockade by ketamine and memantine compared to GluN1/2B receptors in the presence of physiological Mg2+. GluN2B‐selective antagonists produce robust inhibition of GluN1/2B/2D receptors, and the GluN2B‐selective positive allosteric modulator spermine enhances responses from GluN1/2B/2D but not GluN1/2A/2B receptors. These insights into the properties of triheteromeric GluN1/2B/2D receptors are necessary to appreciate their physiological roles in neural circuit function and the actions of therapeutic agents targeting NMDA receptors. Abstract Triheteromeric NMDA‐type glutamate receptors that contain two GluN1 and two different GluN2 subunits contribute to excitatory neurotransmission in the adult CNS. In the present study, we report properties of the triheteromeric GluN1/2B/2D NMDA receptor subtype that is expressed in distinct neuronal populations throughout the CNS. We show that neither GluN2B, nor GluN2D dominate the functional properties of GluN1/2B/2D receptors because agonist potencies, open probability and the glutamate deactivation time course of GluN1/2B/2D receptors are intermediate to those of diheteromeric GluN1/2B and GluN1/2D receptors. Furthermore, channel blockade of GluN1/2B/2D by extracellular Mg2+ is intermediate compared to GluN1/2B and GluN1/2D, although GluN1/2B/2D is more sensitive to blockade by ketamine and memantine compared to GluN1/2B in the presence of physiological Mg2+. Subunit‐selective allosteric modulators have distinct activity at GluN1/2B/2D receptors, including GluN2B‐selective antagonists, ifenprodil, EVT‐101 and CP‐101‐606, which inhibit with similar potencies but with different efficacies at GluN1/2B/2D (∼65% inhibition) compared to GluN1/2B (∼95% inhibition). Furthermore, the GluN2B‐selective positive allosteric modulator spermine enhances responses from GluN1/2B/2D but not GluN1/2A/2B receptors. We show that these key features of allosteric modulation of recombinant GluN1/2B/2D receptors are also observed for NMDA receptors in hippocampal interneurons but not CA1 pyramidal cells, which is consistent with the expression of GluN1/2B/2D receptors in interneurons and GluN1/2A/2B receptors in pyramidal cells. Altogether, we uncover previously unknown functional and pharmacological properties of triheteromeric GluN1/2B/2D receptors that can facilitate advances in our understanding of their physiological roles in neural circuit function and therapeutic drug actions.

Contribution of NMDA Receptors to Synaptic Function in Rat Hippocampal Interneurons

“…In CA1 PyrCs, following a 10 min wash-in, ifenprodil produced a strong 55 ± 21% block of native NMDAR-EPSCs, reducing the average amplitude from 136 ± 78 to 54 ± 30 pA ( N = 13 rats; r = 0.60, −82.4; CI, −137.4, −27.4; p = 0.009, paired Student’s t test; Fig. 5 C ), consistent with the presence of GluN2B receptor subunits ( Gray et al, 2011 ; Yi et al, 2019 ), but a greater block than for triheteromeric receptors alone ( Hansen et al, 2014 ). This ifenprodil block in CA1 PyrCs was stronger in GluN2A-null rats at 83 ± 13% ( N = 8 rats; r = 0.48, −25.2; CI, −48.7, −1.7; p = 0.039, paired Student’s t test), which is consistent with a loss of GluN2A subunit containing triheteromeric receptors ( Hansen et al, 2014 ).…”

“…Pharmacology for the specific NMDAR subunits was as follows: GluN2B-ifenprodil tartrate (10 μ m ) or NAB-14 (10 μ m ), and both were bath applied. As NAB-14 is highly lipophilic ( Yi et al, 2019 ), all tubing was rinsed with 100% ethanol and liberal amounts of distilled water between recordings to ensure full removal of the drug from the surfaces of perfusion tubes. All EPSC amplitudes were recorded at the peak of the EPSC, as measured over a 2 ms peak average.…”

GluN2B but Not GluN2A for Basal Dendritic Growth of Cortical Pyramidal Neurons

“…This is in contrast to our findings that neither the block of GluN2B nor of GluN2A could alter interneuronal dendritic complexity. Opposed to GluN1/2A/2Bcontaining pyramidal cells, interneurons are more enriched for GluN1/2B/2D-containing receptors, and the efficacy of ifenprodil at GluN1/2B/2D receptors is much less than at GluN1/2B receptors (Yi et al, 2019). A recent study in mouse cortex has revealed that parvalbuminergic interneurons mature via tonically active GluN2C/2D-containing receptors which transform the depolarizing signaling of ambient glutamate into an increase of dendritic complexity.…”

DMV extrasynaptic NMDA receptors regulate caloric intake in rats

“…It should also be noted, however, that, similar to their synaptic counterparts, NMDARex may also display a voltage-dependent Mg 2+ block. Although the extent of this block may be dependent upon the receptor subunit composition (27)(28)(29), it is also possible that some additional mechanism by which DMV neuronal excitability is modulated may also be involved. Given the observed plasticity occurs in glutamatergic synapses, the most metabolically economical means for removal of the Mg 2+ -dependent block of NMDARex and NMDARs may involve the activation of postsynaptic metabotropic glutamate receptors (56,57) which are distributed throughout the vagal brainstem (58)(59)(60)(61)(62)(63)(64).…”

“…Since antagonists of NMDARex have been shown previously to have variable selectivity for NMDARs (27)(28)(29), and memantine in particular may antagonize α7 containing nicotinic receptors (30) as well as 5-HT3 receptors ( 31) which may modulate central vagal neurocircuits (32)(33)(34)(35)(36)(37)(38)(39), a series of experiments were conducted in which the effects of the NMDARex antagonist, memantine (30µM), as well as the NR2B subunit-containing NMDA receptor (primarily extrasynaptic) antagonist, ifenprodil (3M) (40) were assessed on evoked (synaptic) NMDAR-mediated currents.…”