N-methyl-D-aspartate receptor (NMDAR) serves for both high cortical function and fundamental CNS mechanisms. NMDAR-mediated neurotransmission
is the molecular engine for CNS development and plasticity; it is also the molecular underpin of learning, memory and cognition.
Due to its critical role in CNS function, either over- or under-activation of NMDAR-mediated neurotransmission contributes significantly
to the development of CNS disorders. In this issue of Current Pharmaceutical Design, the authors discuss the involvement of NMDARmediated
neurotransmission in a variety of CNS disorders including schizophrenia , cognitive deficits in schizophrenia [2, 3], depression
, aging , mild cognitive impairment and Alzheimer’s dementia , attention deficit hyperactivity disorder , frontal lobe synaptic plasticity
 as well as autism spectrum disorder .
NMDAR distinguish itself in two ways: first, it is both ligand-gated and voltage-dependent; second, it requires co-activation by two ligands:
glutamate or aspartate and either D-serine or glycine. The experience-dependent learning originates from these two critical coincidental
mechanisms. First, the activation of non-NMDA glutamate receptor will relieve the magnesium blockade and allow the opening of NMDAR
channel ionophore and calcium influx. This “coincidence” mechanism between NMDA and non-NMDA receptors provides a transduction
and transformation of excitatory input from multiple modalities of stimulation through non-NMDARs into the molecular machinery of
NMDAR that mediates complex CNS behaviors. In this regard, NMDAR serves as a high order integrator to summate the signals from the
EPSP carried by the non-NMDARs. Second, D-serine and glycine, as obligatory co-agonists, provide another dimension of dynamics in the
neocortex. D-serine and the racemic enzyme converting L-serine to D-serine are enriched in the corticolimbic regions . D-serine plays a
key role in NMDAR activation for high order cognitive functions, while glycine’s localization is much less specific, which is also enriched in
the brain stem and spinal cord other than the forebrain.
There are a variety of approaches to regulate NMDAR-mediated neurotransmission; not only by the electrophysiological and molecular coincidental
mechanisms mentioned above, NMDAR-mediated neurotransmission also has multiple regulatory mechanisms. As in the aminergic
or GABAergic system, the regulation can happen at the level of precursor and neurotransmitter synthesis and release, or termination of action
by uptake or catabolism. Parallel to the aminergic or GABAergic systems (Table 1), administration of D-serine or glycine, can activation
NMDAR to improve cognitive and psychotic symptoms as tryptophan loading can facilitate the synthesis of serotonin and improves the depressive
symptoms. Blocking the high efficient uptake site of glycine transporter-1 (GlyT-1) in the forebrain  potentiates NMDA function
similar to selective serotonin uptake inhibitor’s (SSRI’s) action to raise serotonin tone. However, D-serine appears not to have a high efficient
uptake site; instead, there are low affinity exchangers alanine serine cysteine transporter-1 (ASC-1) and -2 which physiological role is unclear
. Therefore, D-serine regulation may provide a tone that slower in time scale than the high efficient mechanism like GlyT-1. The tonicclonic
coordination between D-serine and glycine can provide another dimension of complexity, and thus possibilities of regulation.The occupation of the co-agonist site is obligatory for the activation of NMDAR. However, the presence of both glycine and D-serine is not
essential since both co-agonists are full agonists with strong potency. These two co-agonists not only provide redundancy for the activation of
NMDAR, it also provides an unique opportunities to regulate NMDAR . Although glycine and D-serine have similar potency, the anatomic
specificity favors D-serine. Other than their different anatomical localization at the gross regional level, their microanatomical and
physiological functions are also different; synaptic NMDAR 2A subunit-containing and extrasynaptic 2B-containing NMDARs have different
co-agonists: D-serine for synaptic NMDARs and glycine for extrasynaptic NMDARs . In addition, it has been found that synaptic and
extrasynaptic NMDARs have opposing effects in determining the fate of neurons; the mechanisms of cell destruction or cell survival in response
to the activation of NMDAR depend in part on calcium and its route of entry, and more significantly on the subunit composition and
localization of the NMDARs. Overall, the synaptic NMDAR activation is involved in neuroprotection, the stimulation of extrasynaptic
NMDARs, triggers cell destruction pathways and may play a key role in the neurodegeneration associated with excitotoxicity.
The multi-dimensional complexity of the physiology and pathology of NMDAR can be best exampled by these two co-agonists. Although the
microscopic availability of the co-agonists matches the preferential affinity of synaptic NMDARs for D-serine and extrasynaptic NMDARs
for glycine, this dichotomy is not universal. For example, long-term potentiation rely on synaptic NMDARs, but both glycine and D-serine
can be involved . Conversely, long-term depression requires both synaptic and extrasynaptic receptors. While the initial thought that Dserine
originates from astrocytes, recent evidences indicate D-serine is also neuronal in origin . Neuronal D-serine is required for NMDAR-dependent, long-term potentiation at the hippocampal CA1-CA3 synapses and proper synapse formation in the cerebral cortex.
However, glycine is present on both forebrain and hindbrain, for both inhibitory and excitatory neurotransmission.
Based upon the prediction that enhancement of NMDA function will improve the pathological state induced by NMDAR antagonists like
phencyclidine and ketamine, glycine, a full agonist, was the first to be tested in schizophrenia . However, glycine has poor efficacy and
requires large amount of administration (>= 60 grams/day) to have a modest effect . We first proposed D-cycloserine, a partial agonist,
would be a better NMDA agent than glycine due to its CNS bioavailability. We found D-cycloserine offered an inverted-U dose-response
curve consistent with its partial agonist activity in a dose-finding trial of schizophrenia . To facilitate higher NMDA activation, we further
conducted trials with full agonists, D-serine  and D-alanine . As we predicted, the full agonists were able to elicit a higher level of
NMDA activation and clinical improvement than the partial agonist. The dose-response of NMDA activation by the treatment of full agonist
was later supported by an open-label trial, indicating 60 or 120 mg/kg D-serine has better efficacy than 30 mg/kg in symptom reduction and
cognitive improvement .
We further hypothesized that the facilitation of NMDAR activation can be achieved by blocking the reuptake of the agonist, either glycine or
D-serine. However, no high affinity uptake site for D-serine has been identified, therefore we focused on the GlyT-1 which is enriched in
corticolimbic region, unlike GlyT-2 which is not present in the corticolimbic region. We also predicted, the competitive antagonist will elicit
a safer pharmacological profile than the noncompetitive antagonist, for the concern that the high affinity blockade of the GlyT-1 may overactivate
NMDAR, particularly when extrasynaptic NMDAR that mediates toxicity is involved. In addition, strong inhibition by noncompetitive
antagonism can induce endocytosis . The prototype GlyT-1 inhibitor we applied, sarcosine, is a naturally occurring amino acid, discovered
at high concentration in tissues including CNS. Sarcocine’s efficacy had been proved in several small scale double blind, placebo controlled
studies [21-25]. Supporting the advantage of competitive vs. noncompetitive GlyT-1 antagonist, all noncompetitive antagonists had
failed the development so far, including bitopertin, which gave a weak signal at the Phase II study and did not meet its endpoints of the improvement
of negative symptoms in two Phase III trials . Infact, bitopertin provides a inverted-U dose-response, which also discourage
the therapeutic approach of noncompetitive antagonism .
In addition to its efficacy in the main symptom domains of schizophrenia like positive, negative and cognitive symptoms, the depressive
symptoms are also improved by NMDA enhancement treatments . To determine whether the antidepressant effect is primary or secondary
to the improvement of other symptom domains, we has conducted both rodent behavior studies and a trial of sarcosine treatment in major
depression. In which, sarcosine treatment not only elicits an antidepressant-like behavior profile in both acute and chronic stress model of
depression, but also reach a much higher remission rate than a standard SSRI treatment in major depression .
It had been well known that magnesium infusion can quickly relieve migraine and eclampsia, likely due to its blockade of NMDAR. Given
the recent findings that NMDAR antagonists can improve the symptoms of depression, the mechanistic question was raised why both NMDA
enhancement and blockade can improve the symptoms of depression . It is possible that both treatments share a final common target, like
m-TOR or BDNF, through some unidentified intermediate mechanism. However, NMDAR agonist and antagonist have different time scales
in improving the depressive symptoms; NMDAR antagonist elicits an almost immediate effect, which is much faster than agonist treatment.
At the same time, the underline molecular mechanism of NMDAR antagonists is unclear given that the proposed BDNF activation and synaptogenesis
will take days to weeks to develop, while ketamine’s effect is immediate.
The efficacy of NMDA treatment is not limited to schizophrenia and depression. The efficacy had been shown in improving the symptoms of
dementia and obsessive compulsive disorder (OCD) by sarcosine treatment , which is consistent with the involvement of glutamatergic
neurotransmission in dementia and memory and the circuitry of OCD. NMDA neurotransmission is ubiquitous and involved in many
fundamental function of CNS including psychosis, cognition, rewarding, motor, etc. Its modulation can certainly offer beneficial outcome in
the symptoms involving these circuitries. In the hindsight, though NMDA-enhancement treatment is particularly relevant to schizophrenia
given that the NMDAR antagonists generate “schizophrenia-like” symptoms, it is not surprising that the treatment is also beneficial for a
variety of CNS disorders.
While looking back the development of aminergic and GABAergic treatments, I saw the history of glutamatergic treatments, developed in the
past two to three decades, could follow a similar path. All three lines of treatments can involve neurotransmitter and its precursor (chloroziapoxide,
tryptophan), agonist/antagonist (L-dopa, chlopromazine), uptake blocker (imipramine, fluoxetine, bupropion), catabolism inhibitor
(iproniazid, selegiline) (Table 1). In analogy to the aminergic and GABAergic treatments, I saw the missing NMDA treatment options of:
first, the neurotransmitter uptake inhibition by GlyT-1 inhibitor; second, NMDAR antagonists; third, the inhibition of the D-amino acid oxidase
(DAAO), which metabolize D-serine . The treatment of DAAO inhibition is analogous to monoamin oxidase inhibitors (MAOI),
which upregulates monoamine for CNS disorders like depression and Parkinson disease. In another word, DAAO inhibitors are similar to
MAOI in raising the tone of neurotransmitter of interest, by inhibiting the catabolism enzyme.
The legendary biochemist, Sir Hans Adolf Krebs discovered D-amino acid deaminase and considered the enzyme “in search of function”
. Almost eighty years later, the enzyme, now known as DAAO, has gained attention as a critical regulator of CNS neurotransmission.
Its main substrate, D-serine is a critically important co-agonist of the NMDAR. We recently demonstrated that sodium benzoate, as a
DAAO inhibitor, can substantially improve the symptoms and neurocognition of schizophrenia, presumably enhance NMDA function by
raising D-serine level . In mild cognitive impairment (MCI), sodium benzoate also improves the cognition and function . In late 19
century, the planet Neptune was mathematically predicted before it was directly observed; working from Le Verrier's calculations, telescopic
observations of Neptune was confirming afterwards. In developing the NMDA treatment for CNS disorder, we predicted DAAO inhibition
could be the last missing approach in regulating the grand regulator of CNS, NMDAR-mediated neurotransmission. To search for novel
DAAO inhibitors, one of the articles in this issue discuss an innovative informatics method to determine the potential activity of DAAO inhibitors
in this new frontier of CNS drug development .