Writeup | The Nootropic Potential of NMDA-NR3A Inhibition
This writeup is about the relevance of NR3A for cognition, schizophrenia, and other mental disorders. Along with other NMDA subsites (e.g NR2), it has a high relevance in the age-based changes that occur to form the adult brain.
Introduction to NR3
NR3A and NR3B are relatively new subunits of the NMDA receptor that form functional receptors when coexpressed with NR1. NR3A was discovered in 1995 alongside NR3B. The NR3 subunit is glycine-binding, which makes it more similar to NR1 than NR2, as NR2 is glutamate-binding.
Fig. 3 - https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP280879
This has important implications because NR3 subunits, like NR2 subunits, are not able to be surface-expressed in the absence of NR1 [Source]. Therefore, a novel type of NMDA receptors composed of NR1 and NR3 subunits would only require glycine and not glutamate for activation.
NR3-containing receptors have much lower permeability to calcium and magnesium than conventional NR1/NR2 NMDA receptors. They are also insensitive to many NMDA receptor antagonists like AP5, memantine and MK-801. NR3B expression is highly enriched in motor neurons, whereas NR3A is more widely distributed in the CNS. [Source]
NR3A protein spatial distribution in the mammalian brain has been reported to parallel that of NR1, NR2A, and NR2B, with some notable exceptions. At an ultrastructural level, NR3A protein is detected specifically localized to postsynaptic membranes in both the cerebral cortex and thalamus, with virtually no labeling in the presynaptic termininal of the cerebral cortex. NR3B protein was found to be expressed in all of the substructures of the hippocampus (CA1, CA3, dentate gyrus), and also layer II of the cerebral cortex. [Source]
NR3A expression is high early in development then decreases, whereas NR3B increases later in development. This developmental regulation may be important for synaptic plasticity. It seems to be particularly relevant in the dorsolateral prefrontal cortex (dlPFC), which is well known for having a very large relevance to high-level cognition.
NR3a in age groups - https://doi.org/10.17615/s4z0-fd80
However, contrary to how it may seem, NR3A is an inhibitory neurotransmitter. Overexpression of NR3A decreases spine density (genetic deletion of NR3A increases spine density) and is common in schizophrenia (NR3A mRNA levels are significantly increased by 32% within subregions of the DLPFC in schizophrenic patients [Mueller and Meador-Woodruff, 2004]).
NR3A levels peak early postnatally in multiple brain regions and decline into adulthood. The normal downregulation of NR3A appears important for maturation of synaptic circuits, as prolonging NR3A expression impairs long-term potentiation and memory consolidation.
In contrast, NR3B levels increase during later postnatal development and remain high in adulthood. The more widespread and persistent expression of NR3B suggests it may fine-tune NMDA receptor signaling in the mature brain. This makes sense as NR3A is prominently expressed during a narrow temporal window of postnatal development that correlates with periods of intense synaptogenesis and pruning and later becomes downregulated, just prior to the onset of critical period plasticity.
The nootropic potential of NR3A modulation
NR3A seems to be more relevant of a role in modulating high-level cognition than its partner NR3B due to distribution and function differences.
NR3A subunits are known to act as dominant negative regulator of the NMDA receptor current and they have been shown to alter the two most prominent properties of the NMDA receptor: calcium permeability and magnesium sensitivity [Source].
Once study generated transgenic mice in which NR3A expression could be prolonged beyond its natural time window in postnatal forebrain neurons. They found that prolonging NR3A expression results in synaptic NMDAR hypofunction, deficits in long-term potentiation (LTP), reduced postsynaptic maturation at Schaffer collateral-CA1 synapses of the hippocampus, and impaired behavioral flexibility and memory consolidation.
Conversely, genetic deletion of endogenous NR3A yielded a premature concentration of NMDARs at postsynaptic sites, enhancing synaptic NMDAR currents and promoting an earlier developmental onset of LTP. Prolonged expression of NR3A subunits decreased the number and size of synaptic contacts, with persistent decreases in PSD length suggesting a dominant role of NR3A in limiting synaptic size.
This brings up the discussion about how significant lowering NR3A expression could be as a nootropic target. Due to NR3A's role in dlPFC function, which is known as a high-level regulator of top-down control, and NR3A's role in reducing synaptic size and reducing LTP, it could definitely be a very desirable receptor to modulate.
To test this hypothesis, scientists knocked out (KO) NR3A receptors in adult mice and measured cognitive and behavioral markers. Receptor knockout is different than receptor inhibition, however the results are still interesting. [Source]
- Adult GluN3A KO mice showed significantly enhanced abilities in learning and memory tasks.
- Hippocampal slices from juvenile and adult GluN3A KO mice showed greater long-term potentiation compared with wild-type slices.
- (GluN3A) knockout (KO) mice showed slow locomotor activity, motor deficits (it was evident that this decreased motor ability was compensated for by prolonged training) and increased pain sensation (not too significant).
Novel object recognition test GluN3A KO - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3630778/pdf/tjp0591-0149.pdf
NR3A knockout significantly enhanced the mice's ability to recognize and have interest in novel objects in the recognition test. The GluN3A KO mice also showed enhanced LTP and CaMKII expression, which is very desirable as synaptic memory requires CaMKII and LTP is one of the major cellular mechanisms that underlies learning and memory.
Behavioral phenotypes of NR3A knockout mice demonstrate increased pre-pulse inhibition, increased object recognition and spatial learning, but lessened odor discrimination, some social deficits and shorter sleep patterns. [Source]
Fig.1 - https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP280879
NR3A also has relevance in addiction and hedonistic behaviour. Alongside NR3A being relevant in dlPFC function which is known as a key brain region regulating self-control and motivation, GRIN3A mRNA levels are increased in the hippocampus and orbitofrontal cortex of individuals with alcoholism (Jin et al. 2014). Risk of nicotine dependence has been linked with a number of rare non-synonymous variants in GRIN3A, as shown in a series of studies in European, African-American and Chinese Han populations. [Source]
In mouse studies, a single cocaine injection drives the insertion of GluN3A-NMDARs at synapses in reward-related regions with subsequent recruitment of calcium-permeable AMPARs, a form of adaptive plasticity involved in relapse (Yuan et al. 2013). Chronic methamphetamine also enhances GluN3A expression, reducing cortical plasticity and impairing motor learning (Huang et al. 2017).
This then brings interest towards NR3A inhibitors / negative allosteric inhibitors as potential nootropic compounds. Allosteric inhibitors would likely be more desirable due to the potential of excitotoxicity of agonists (*).
NR3A Inhibitors
Due to the relatively novel nature of NR3A and the greater focus on NR1/2 in pharmacology, there is less research on NR3a inhibitors than desired. Current NR3A inhibitors are all experimental and most of them have off-targets and have poor selectivity.
Current known negative allosteric modulators of NR3A are:
- EU1180-438 (IC50 = 1.80 ± 0.30 μM)
- WZB117 (IC50 = 1.15 ± 0.34 μM)
- S0859
- WAY200070
All of these compounds are experimental and have offtargets which make them less selective than desirable. Decreasing NR3B is less desirable than decreasing NR3A, which makes things difficult because most NR3A inhibitors are not selective, but are general NR3 inhibitors. Lowering NR3B won't cause severely negative effects as approximately 10% of the normal European-American population lacks NR3B due to homozygous occurrence of a null allele in the gene, however a more NR3A-specific inhibitor is likely a better compound.
WZB117 has been reported to be an irreversable glucose transporter (GluT) inhibitor with some other targets, with an IC50 at A549 and MCF7 cells of 10 μM, which 9x higher than the IC50 of NR3A at 1.15 ± 0.34 μM, meaning it could be a potential compound for NR3A inhibition, however it warrents further investigation.
There is also EU1180-438 which is also quite selective with an IC50 of 1.80 ± 0.30 μM, however it has inhibitory activity at α4β2 and α1β1γδ (less than NR3A/B inhibition, but still significant). It also inhibits NR3B more than NR3A which is not as desirable. To ameriolate these issues, this compound could be combined with a α4β2 PAM/partial agonist, and possible a α1β1γδ positive modulator (if needed), however the offtargets at these receptors is still not optimal.
Also around NR3B inhibitory activity, the (+) or (-) variant of EU1180-438 could potentially have a different selectivity to NR3A/B, however this was not mentioned in the pilot study.
Even with the less-than-optimal compounds for inhibiting NR3A, NR3 is still a very significant and interesting subsite for modulating cognition and neurological disorders. WZB117 and EU1180-438 definitely warrant further investigation as experimental compounds, and this pathway has a very large potential for cognitive enhancement. Also different downstream mechanisms that interact with NR3A should be looked into (e.g PP2A !?).
Thank you for reading. Rather than new writeups being made on NR3A, this page will be continually updated. This writeup was originally posted on reddit, but due to censorship it was removed.
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