GluN2D-Containing N-methyl-d-Aspartate Receptors Mediate Synaptic Transmission in Hippocampal Interneurons and Regulate Interneuron Activity

Abstract

N-methyl-d-aspartate receptors (NMDARs) are ionotropic glutamatergic receptors that have been implicated in learning, development, and neuropathological conditions. They are typically composed of GluN1 and GluN2A-D subunits. Whereas a great deal is known about the role of GluN2A- and GluN2B-containing NMDARs, much less is known about GluN2D-containing NMDARs. Here we explore the subunit composition of synaptic NMDARs on hippocampal interneurons. GluN2D mRNA was detected by single-cell PCR and in situ hybridization in diverse interneuron subtypes in the CA1 region of the hippocampus. The GluN2D subunit was detectable by immunoblotting and immunohistochemistry in all subfields of the hippocampus in young and adult mice. In whole-cell patch-clamp recordings from acute hippocampal slices, (+)-CIQ, the active enantiomer of the positive allosteric modulator CIQ, significantly enhanced the amplitude of the NMDAR component of miniature excitatory postsynaptic currents (mEPSCs) in CA1 interneurons but not in pyramidal cells. (+)-CIQ had no effect in slices from Grin2d-/- mice, suggesting that GluN2D-containing NMDARs participate in excitatory synaptic transmission onto hippocampal interneurons. The time course of the NMDAR component of the mEPSC was unaffected by (+)-CIQ potentiation and was not accelerated in slices from Grin2d-/- mice compared with wild-type, suggesting that GluN2D does not detectably slow the NMDAR EPSC time course at this age. (+)-CIQ increased the activity of CA1 interneurons as detected by the rate and net charge transfer of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from CA1 pyramidal cells. These data provide evidence that interneurons contain synaptic NMDARs possessing a GluN2D subunit, which can influence interneuron function and signal processing.

In the Telencephalon, GluN2C NMDA Receptor Subunit mRNA is Predominately Expressed in Glial Cells and GluN2D mRNA in Interneurons

“…Given the lack of a layered distribution for GluN2D signal in the cortex and hippocampus, the GluN2D-positive neurons are likely to be interneurons. This result is consistent with demonstrations that GluN2D expression in GABAergic cells in the cerebral cortex and hippocampus [23,26,27]. …”

“…Thirdly, GluN2C-containing NMDARs do not desensitize [4,55], so these receptors would stay responsive to slowly changing or tonic extracellular L-glutamate. Interestingly, GluN2D-containing receptors also display these properties, so they may display a similar role in the detection of low concentrations of extrasynaptic L-glutamate in order to tonically activate inhibitory interneurons [56] in addition to their activation by synaptic activity [26,27]. …”

Mechanisms of ketamine action as an antidepressant

“…This is postulated to be due to the higher frequency of interneuron firing compared with the pyramidal neurons 38 , which allows for increased depolarization-dependent relief of Mg 2+ block, thus permitting ketamine access to bind at the NMDAR channel pore selectively on interneurons 39 . In addition, ketamine is reported to have higher affinity for GluN2D NMDAR subunits 40, 41 , which are highly expressed in forebrain inhibitory interneurons 42, 43 . Inhibition of NMDARs specifically on GABAergic interneurons is predicted to induce a decrease in overall inhibition, leading to pyramidal cell disinhibition and an enhancement of excitatory glutamatergic neurotransmission in the medial prefrontal cortex (mPFC), and potentially other mood-relevant cortico-limbic brain regions 35 (see Figure 1).…”

Kinetic models for activation and modulation of NMDA receptor subtypes

“…Whether splice-isoforms also differ in desensitization is unclear because the model lacks desensitization steps. GluN1/GluN2D receptors participate in synaptic transmission and control cellular excitability in hippocampal inhibitory interneurons [23]. Therefore, it will be important to develop a kinetic model that can describe the entirety of single-channel behaviors for this receptor subtype as well.…”