Writeup | NR2D - The inhibitory control subunit
This writeup is about the relevance of NR2D for cognition and inhibitory control.
Introduction to NR2D (and relevance)
The receptors formed by NR2D (aka GluN2D) subunits are known to have higher agonist sensitivity and much slower deactivation kinetics than NR2A-containing receptors, and slower deactivation kinetics than NR2B as well. Furthermore, NR2D is exclusively located on extrasynaptic membranes. [1]
As the density of active synapses increases, the confluence of released glutamate makes EPSC (Excitatory postsynaptic potential) decay much longer by activating more extrasynaptic NR2B- and NR2D-subunit-containing receptors. Long-term potentiation (LTP) induced by successive rounds of burst stimulation is accompanied by a long-term increase in the contribution of extrasynaptic receptors in the afterburst EPSC. [1]
GluN2D-containing NMDARs are present at synapses on hippocampal interneurons and can influence interneuron activity. GluN2D subunits are also selectively expressed in cortical PV+ FSINs (Fast-Spiking Inhibitory Interneurons) [3] with relevance in the dlFPC.
The "superslow" subunit
NR2B subunits are known to be very slow compared to others subunits/receptors (NR2A, AMPA), however NR2D is even slower. Deactivation times in NR2D-expressing receptors can last for potentially hours.
Deactivation times for diheteromeric NMDARs differ in a 50-fold range, following the sequence: NR2A < 2C = 2B << 2D. [12]
Binary complexes of NMDA receptors composed of NR1 and NR2D subunits have been shown to display unique electrophysiological behaviour, and highest affinity for both glutamate and glycine (Ikeda et al. 1992).
Relevance to hedonism and modulation potential
Hedonism can be theorised to be caused by a lack of inhibitory control, and a sub-optimal cognitive heirarchy.
GluN2D positive modulators may have therapeutic potential for disorders involving dysfunctional basal ganglia signaling and inhibitory control. This is because they directly effect inhibitory behavior and also modulate the dorsolateral PFC which is well known to be involved in top-down control.
In one study [2] using PTC-174, a positive allosteric modulator (PAM) of NMDA receptors containing GluN2C/GluN2D subunits, PTC-174 reduced premature and timeout responses in a dose-dependent manner, suggesting improved inhibitory response control.
PTC-174 practically inhibited amphetamine-induced locomotion in the study when tested on rodents (30mg/kg in rats) [2]. PTC-174 does not have the best pharmacological profile with an approximated 300mg dose in humans, however it still can be used to study the effects of NR2D enhancement.
The effects occurred at doses estimated to increase GluN2D activity by 10-50% based on receptor pharmacology, suggesting subtle potentiation is sufficient to alter behavior. This is optimal as too much potentiation could cause excitotoxicity.
The results showed that increasing NR2D function only slightly increased spatial accuracy, however it inhibited premature responses, showing that NR2D is more relevant for inhibitory control than promoting cognition.
Over time, NR2D enhancement may contribute to working memory, as demonstrated in one study, which found that GluN2D is in a positive correspondence with working memory. NR2D mRNA expression was increased 2.17-fold in the high WM group compared to the low WM group (p=0.01). [7]
Another interesting thing about NR2D is that potentiating NR2D function could help counter FSIN (Fast-Spiking Inhibitory Interneuron) hypofunction observed in neurodevelopmental models and postmortem brains from schizophrenia patients. [3]
In acute slices of mouse medial prefrontal cortex (mPFC), the GluN2C/D positive allosteric modulator CIQ(+) increased both the intrinsic excitability of FSINs and the amplitude of NMDA receptor-mediated excitatory postsynaptic currents onto FSINs but not pyramidal neurons. [3] This makes its mechanism unique compared to other NR2 subunits.
Relevance to emotion
NR2D KO has been shown in studies to induce social recognition deficit, social stress, 5-HT2C receptor dysfunction, and anhedonia in mice [4]. This may show a relevance for NR2D in ameriolating anhedonia.
NR2D has also shown to be a key factor in the effects of ketamine, with it potentially being the pathway mediating the psychotomimetic/hallucinagenic activity [5], but potentially seperate from the antidepressant activity.
Targeting NR2D via TTKB1
One method of targeting NR2D is by inhibiting TTBK1. Inhibition of TTKB1 has been shown to increase NR2D and NR2B activity, so the pathway could potentially be used as a selective enhancer of NR2B/NR2D. [6] However, finding a selective inhibitor of TTKB1 is another issue.
One small-molecule inhibitor of TTKB1 called TTBK1-IN-1/2735015-60-6 (aka "compound 31") has an IC50 of 2.7 nM and showed a good selectivity in one study. However, it does not have much more information and it has bad safety predictions.
One compound called picetannol is a potent TTBK1 inhibitor [10], however it has other offtargets (SYK inhibition, cyclin inhibition, etc), which make it less viable as a compound.
Most TTBK1 inhibitors right now have bad pharmacological profiles, so more research should be done as the pathway is definitely very interesting.
Targeting NR2D via Positive Allosteric Modulation
NMDA PAMs are typically more desirable than agonists because they enhance enhance endogenous activation by binding to the allosteric site, which lowers risk for excitotoxicity (as described in my other posts).
Excitotoxicity may not be as big of an issue when potentiating NR2D compared to other NMDA subunits. First, in one study [7] demonstrating differences in NR2D between groups, the high working memory group had twice as much NR2D as the low working memory group. This demonstrates that large differences in NR2D expression already exist between people. Also seeing how NR2D expression only needs to be increased by 10-50% for a cognitive benefit [2], it is not that big of a concern.
Finding selective PAMs for NR2D with good pharmacology is more difficult.
Most selective allosteric modulators for NR2D also effect NR2C due to the similarities of both. This may not be optimal as they seem to have opposing roles in modulating neuronal oscillations [8], however they may also work in tandem. Studies using combined NR2C/NR2D PAMs have shown cognitive enhancement and increased NR2C seems to positively effect dendritic spine density [9].
Here are some novel NR2D PAM compounds:
- PTC-174 - NR2C/NR2D PAM (high effective dose at ~300mg, bad safety predictions)
- CIQ(+) [486427-17-2] - NR2C/NR2D PAM (low half life, high effective dose at ~50-100mg, bad safety predictions)
- R-(+)-EU-1180–453 [2488764-06-1] - NR2C/NR2D PAM (low half life, effective dose of ~5-25mg, meh safety predictions)
R-(+)-EU-1180–453 [11] looks like the most promising novel NR2D PAM at the moment. It is a second-generation advancement using CIQ(+) as a base, however it improves upon the pharmacologic traits of CIQ(+).
R-(+)-EU-1180–453 shows approximately 1 log unit (10x) improved potency compared to CIQ(+), with doubling concentrations in the low micromolar range. It also demonstrates improved aqueous solubility (74 μM vs 8 μM for CIQ(+)), allowing higher concentrations to be reached. This results in an improved solubility:potency ratio (>50:1 vs <1:1).
However, it demonstrated a low half life of about an hour, so it may need to be conjugated with Adamantane to extend the half life to make the compound viable.
R-(+)-EU-1180–453 exhibits reasonable pharmacokinetics, is brain penetrant after i.p. dosing, and shows minimal off-target activity. It looks like the most promising NMDA-NR2D PAM as of now, however it must be remembered that it is still experimental and has not been tested on humans.
Discussion
Increasing NR2D may have some negative effects, including possibly enhancing pain perception [13] (though potentially only when selectively enhanced in the NAc). However, compared to other NMDA subunits, it is more approachable.
Thank you for reading.
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