Effects of sarcosine and N, N-dimethylglycine on NMDA receptor-mediated excitatory field potentials
© The Author(s). 2017
Received: 29 June 2016
Accepted: 25 December 2016
Published: 28 February 2017
Sarcosine, a glycine transporter type 1 inhibitor and an N-methyl-D-aspartate (NMDA) receptor co-agonist at the glycine binding site, potentiates NMDA receptor function. Structurally similar to sarcosine, N,N-dimethylglycine (DMG) is also N-methyl glycine-derivative amino acid and commonly used as a dietary supplement. The present study compared the effects of sarcosine and DMG on NMDA receptor-mediated excitatory field potentials (EFPs) in mouse medial prefrontal cortex brain slices using a multi-electrode array system.
Glycine, sarcosine and DMG alone did not alter the NMDA receptor-mediated EFPs, but in combination with glutamate, glycine and its N-methyl derivatives significantly increased the frequency and amplitude of EFPs. The enhancing effects of glycine analogs in combination with glutamate on EFPs were remarkably reduced by the glycine binding site antagonist 7-chlorokynurenate (7-CK). However, DMG, but not sarcosine, reduced the frequency and amplitude of EFPs elicited by co-application of glutamate plus glycine. D-cycloserine, a partial agonist at the glycine binding site on NMDA receptors, affected EFPs in a similar manner to DMG. Furthermore, DMG, but not sarcosine, reduced the frequencies and amplitudes of EFPs elicited by glutamate plus D-serine, another endogenous ligand for glycine binding site.
These findings suggest that sarcosine acts as a full agonist, yet DMG is a partial agonist at glycine binding site of NMDA receptors. The molecular docking analysis indicated that the interactions of glycine, sarcosine, and DMG to NMDA receptors are highly similar, supporting that the glycine binding site of NMDA receptors is a critical target site for sarcosine and DMG.
KeywordsN-methylglycine Glycine binding site D-serine D-cycloserine 7-chlorokynurenate
Derivatives of glycine, including N-methylglycine (sarcosine) and N, N-dimethylglycine (DMG) are important intermediates in the metabolism of choline to glycine [30, 40]. Sarcosine and DMG are widely distributed in food and used as dietary supplements. These two structurally similar glycine derivatives were reported to have pharmacological activities in the central nervous system.
Sarcosine is a competitive glycine transporter type-l inhibitor , an N-methyl-D-aspartic acid (NMDA) receptor co-agonist , and a glycine receptor agonist . Based on its enhancing effects on NMDA receptors, sarcosine has been studied for its efficacy in ameliorating negative and cognitive symptoms in patients with schizophrenia, showing promising results [18, 21, 33, 35]. In addition, sarcosine can improve depression-like behaviors in rodent models and in human depression . The toluene-induced behavioral aberrations were also attenuated by sarcosine .
DMG, consisting of one more methyl group than sarcosine, has been reported to decrease oxidative stress , to improve immune responses  and to be promoted as an athletic performance enhancer . DMG also acts on central nervous system since it exhibits anticonvulsant activity in animal models . Recently, it has been reported that DMG has antidepressant-like effect and reduces the ketamine-induced psychotomimetic behaviors in mice . Furthermore, DMG has been tested as a supplement for patients with autism or pervasive developmental disorder [1, 3, 19, 43], although the results from these clinical studies are controversial.
Opening of the NMDA receptor channel complex requires occupation of its glutamate binding site by glutamate and its glycine binding site by glycine or D-serine . Thus, glycine is not only a glycine receptor agonist, but also a co-agonist of NMDA receptors. In addition to inhibiting the glycine transporter 1 and increasing the ambient glycine concentration, sarcosine, simply the glycine molecule with one methyl group added, also directly acts on NMDA receptors . However, it remains unknown whether DMG, with two methyl groups added to glycine, has the capacity to modulate NMDA receptors.
As enhancement of NMDA receptor function (directly or indirectly) has been associated with the therapeutic potential of sarcosine for psychiatric disorders, the present study aimed to compare the effects of srcosine and DMG on NMDA receptor function. A multi-electrode array system was used to measure the NMDA receptor-mediated excitatory field potentials (EFPs) in slice preparations of mouse medial prefrontal cortex because medial prefrontal cortex has been implicated in the processing of a wide range of cognitive and emotional stimuli and is thought to function as a central hub in the brain circuitry mediating symptoms of psychiatric disorders .
In our preparations, with TTX, bicuculline, and in the absence of magnesium ion, the electrophysiological signals are generated by the summed electric current flowing from NMDA receptors in the neurons in the vicinity of the electrode. The frequency of EFPs reflects the open probability of NMDA receptors. The amplitude of field potentials arises from the synchronization of neural activity. In this case the amplitude is representative of the amounts of NMDA receptors are activated simultaneously. When the open probability of the channels in the neurons is dramatically increased, the number of channels opened at the same time will be increased accordingly.
The roles of sarcosine and DMG on NMDA receptor-mediated EFPs were determined by their co-application with glutamate in the absence or presence of endogenous co-agonists glycine. The manifestation of D-cycloserine, a well-known partial agonist at the glycine binding site on NMDA receptors, in the same experimental preparation was illustrated for comparison. In addition, their effects on NMDA receptor-mediated EFPs evoked by glutamate plus D-serine, another endogenous co-agonist for the glycine site of the synaptic NMDA receptors, were assessed. Finally, the molecular docking simulations were used to evaluate the possible binding between NMDA receptor NR1 subunit and these ligands of interests, glycine, sarcosine, and DMG.
Animals and chemicals
Male ICR mice (8–10 weeks) were supplied from BioLASCO (Taiwan) and housed 4 to 5 per cage in a 12 h light/dark cycle (lights on 0700 h) with ad libitum access to water and food during the time the animals were in their home cages. All experiments were performed in accordance with the Republic of China animal protection law (Chapter III: Scientific Application of Animals) and approved by Institutional Animal Care and Use Committee of the National Health Research Institutes (NHRI-IACUC-10430-A).
Glycine and potassium chloride were purchased from J.T. Baker (Mallinckrodt Baker, Inc, Kentucky, USA). Sarcosine, DMG, ketamine, and 7-chlorokynurenic acid (7-CK) and other chemicals were obtained from Sigma (St Louis, MO, USA). The individual reagents were dissolved in an artificial cerebrospinalfluid (ACSF) containing (in mM) NaCl (120), KCl (3.5), CaCl2 (2.5), MgCl2 (1.2), NaHCO3 , NaH2PO4 (1.2), and D-glucose (11.5) at pH 7.4.
Preparation of prefrontal cortex (PFC) slices for electrophysiological recordings
The brains of ICR mice were removed and immersed in an ice-cold ACSF, bubbled with a mixture of 95% O2/5% CO2. Coronal slices (300 μm) were cut from the frontal cortex (3–5.16 mm anterior to bregma) using a vibrating tissue slicer. After recovery for at least 1 h at room temperature, a single slice was transferred to the center area of the coated MED probe (Panasonic, Japan) and positioned to cover the 8 × 8 microelectrode array by a paint brush. The positioned slice was superfused at 2.0 ml/min with ACSF saturated with O2.
For electrophysiological recordings, the MED probe containing the brain slice was placed in a small incubator which was superfused with Mg2+ free-ACSF in 5% CO2/95% O2 at 25 °C and connected to the stimulation/recording component of MED8 multi-electrode array system (Panasonic, Japan). Mg2+-free ACSF was used to minimize Mg2+ block of NMDA receptors. The preparation of the multi-electrode dish has been described previously . Briefly, the MED probe is an array of 64 planar microelectrodes, where each microelectrode has a size of 50 × 50 μm and is arranged in an 8 × 8 pattern. The interpolar distance in this type of probe (MED-P515A) is 150 μm. For sufficient adhesion of the brain slice to the MED probe, the surface of probe was treated with 0.1% polyethylenimine in 25 mM borate buffer for 8 h at room temperature. Then the probe surface was rinsed three times with distilled water for future experiments.
The field potentials at 8 sites in the 64 multi-electrode probe were recorded simultaneously with the multi-channel recording system at a 20 kHz sampling rate. The electrodes in the infralimbic prefrontal cortex were selected as the recording electrodes. In order to prevent the sodium channel-mediated action potential activity and the interference of inhibitory field potentials, all the experiments were performed with tetrodotoxin (TTX) (300 nM) and bicuculine (10 uM). The recording of spontaneous excitatory field potentials (EFPs) in the presence of TTX and bicuculline was first carried out to establish a stable baseline. The changes in the amplitude and frequency of NMDA receptor-mediated EFPs evoked by various ligands, which were continuously applied by adding them into the bath medium, were expressed as percentages of baseline, which were set at 100%.
The frequency and amplitude (>0.2 mV) of field potentials were measured. All data are expressed as mean ± S.E.M. Statistical significance of the difference between groups was determined by one-way or two-way repeated measures ANOVA followed by a Student-Newman-Keuls post-hoc test. p < 0.05 was considered statistically significant.
Molecular docking analysis
Glycine, sarcosine, and DMG were docked with the NMDA receptor NR1 ligand-binding core (PDB ID: 1PB7)  using Glide XP 6.5 [12, 13] from the program in Schrödinger (Schrödinger Suite 2014-4, Schrödinger, LLC, New York, NY, USA, 2014). The missing side chain atoms were built by using Dunbrack rotamer library  in UCSF Chimera (Version 1.6.2. , UC San Francisco, San Francisco, CA, USA, 2012). The water molecules beyond 5 Å of the crystallized glycine were removed. The 3D conformations and protonized states of ligands were generated using LigPrep 3.2 from Schrödinger (Schrödinger Suite 2014-4, Schrödinger, LLC, New York, NY, USA, 2014). The docking box was a cubic box with sides of 17.51 Å long, which centered as the crystalized glycine. To test whether the docking system is feasible for the ligand binding to NMDA receptor, the NMDA-NR1-Glycine complex (PDB ID 1PB7) was initially selected while glycine was redocked back to the complex. The docking structure of glycine was compared to its original crystallographic glycine structure. The root-mean-square distance (RMSD) between these two poses was only 1.49 Å. This result implied that the Glide docking program and docking settings used is suitable for the evaluating the possible binding between NMDA receptor NR1 and ligands of interests, glycine, sarcosine, and DMG.
Characterization of NMDA receptor-mediated excitatory field potentials (EFPs)
Effects of sarcosine and DMG alone or combined with glutamate on excitatory field potentials
Effects of sarcosine and DMG on glutamate/glycine-evoked excitatory field potentials
Effects of D-cycloserine on NMDA receptor-mediated excitatory field potentials
Effects of 7-CK on excitatory field potentials evoked by co-application of glutamate with sarcosine, DMG and D-cycloserine
Effects of sarcosine and DMG on glutamate plus D-serine-evoked excitatory field potentials
Molecular docking of glycine, sarcosine and DMG with NMDA receptors
Comparing these results, the interactions of glycine, sarcosine, and DMG to NMDA receptor were highly similar. However, the number of methyl groups bound to the amine group affected the binding affinity due to the hydrogen bonds and the nitrogen atom at the amine group which matched to our findings through EFPs.
The present study compared the effects of sarcosine and DMG on NMDA receptor–mediated EFPs in the mouse prefrontal cortical slices. Sarcosine and DMG alone, like endogenous co-agonists glycine and D-serine, did not change the EFPs and significantly enhanced the frequency and amplitude of EFPs in the presence of glutamate. The enhancing effects of glycine, sarcosine and DMG were blocked by the competitive NMDA receptor glycine binding site antagonist 7-CK, suggesting these two N-methyl glycine derivatives might act on the glycine binding site. From the results of molecular docking analysis, the binding modes of glycine, sarcosine, and DMG were similar, supporting that the glycine binding site is their target site. However, DMG, but not sarcosine, at higher concentration (100 μM) reduced EFPs in the presence of glutamate plus glycine/D-serine (10 μM) and the inhibitory effect of DMG could be attenuated by elevated glycine concentration (100 μM) (Additional file 1: Figure S1). Furthermore, under the same experimental procedure, the pattern of responses to D-cycloserine, a well known NMDA receptor glycine site partial agonist [22, 39], was similar to DMG. These results reveal that sarcosine is an agonist, whereas DMG acts more like a partial agonist at glycine binding site of the NMDA receptors.
Glutamate, glycine, D-serine, sarcosine, or DMG alone did not change EFPs and co-application of glutamate or NMDA (Additional file 2: Figure S2) with glycine, D-serine, or N-methyl glycine derivatives significantly increased the frequency and amplitude of EFPs. These results support the notion that occupancy of both glutamate and glycine binding sites is necessary for NMDA receptor activation. Moreover, the glutamate and glycine binding sites of NMDA receptors are scarcely occupied in the presence of TTX in acute slice preparations of medial prefrontal cortex and the NMDA receptor-mediated response observed here is independent of glutamate release.
Sarcosine enhanced EFPs with glutamate, but did not affect the glutamate plus glycine/D-serine-evoked EFPs. These results are consistent with previous findings that sarcosine enhances NMDA receptor function by directly acting as a co-agonist  or indirectly inhibiting the glycine transporter to increase glycine availability [17, 25]. It has been reported that sarcosine is less potent than glycine at NMDA receptor-mediated currents using whole-cell voltage clamp recordings from cultured embryonic mouse hippocampal neurons . In the same manner, our data showed that in the presence of glutamate, sarcosine (100 μM) produced EFPs to the same extent as glycine (10 μM) in the prefrontal cortical slices, supporting the lower potency of sarcosine than glycine. The molecular docking analysis demonstrated that the binding modes of glycine, sarcosine and DMG are very similar. Moreover, the number of methyl groups bound to the amine group, which was the main difference between ligands, would affect the role of the nitrogen atom as the hydrogen bond donor and the binding affinity to the glycine binding site. However, the potencies of sarcosine and DMG to co-activate EFPs with glutamate in the brain slice preparations were approximately equal. It is possible that in addition to their direct interaction with NR1 subunit, there are some other factors and certain indirect effects of sarcosine and DMG contributing to their potencies to evoke EFPs with glutamate. To further elucidate the role of DMG in NMDA receptor function, it would be important to measure the whole-cell NMDA receptor-mediated synaptic currents or potential using whole-cell patch-clamp recordings.
NMDA receptors are ligand-gated ion channels assembled from NR1 and NR2 subunits. Although the glycine binding site is located in NR1 subunit, the potencies  and efficacies  of glycine binding site agonists are dependent on the NR2 subunit. In fact, NR2A- and NR2B-containing NMDA receptors are predominantly expressed in the medial prefrontal cortex . Our observations in the infralimbic cortex might only reflect the effects of these N-methyl glycine derivatives on NR2A- together with NR2B-containing NMDA receptors. It remains to be investigated whether these N-methyl glycine derivatives affect NR2C- or NR2D-containing NMDA receptors in the same manner and the NR3-containing NMDA receptors that respond to glycine agonists alone  can be activated by N-methyl glycine derivatives directly. A study for elucidating the differential effects of these glycine analogs on NMDA receptors containing distinct types of NR2 subunits is in progress.
Modulation of NMDA receptor glycine binding site has been proposed as the next wave of drug development for schizophrenia , depression , and autism spectrum disorders . It is generally believed that the augmentation of NMDA receptor transmission can improve the negative and cognitive symptoms in schizophrenia. In fact, sarcosine, when added to an existing regimen of antipsychotic drugs, has shown its efficacy for both chronically stable and acutely ill patients [18, 20, 37]. Therefore, it is postulated that NMDA receptors at key synapses are not saturated with glycine in schizophrenic patients and administration of these glycine derivatives is capable of enhancing NMDA receptor function by increasing agonist occupancy at the glycine binding site. Since DMG can be metabolized to sarcosine, it is of interest to determine if their clinical potency is higher than sarcosine in the treatment of schizophrenia.
Sarcosine can improve depression-like behaviors in rodent models and in human depression . However, it may be not like NMDAR antagonist ketamine with rapid onset and sustained antidepressant effect. In fact, a glycine binding site partial agonist GLYX-13 has been reported to have long-lasting antidepressant-like effects in preclinical study and is currently in clinical trials as an add-on to already approved treatments for treatment-resistant depression patients [2, 26]. Similarly, DMG showed persistent antidepressant-like effects in mice .
DMG has been used as a nutritional supplement for autistic spectrum disorders and there is anecdotal evidence from parents, medical professionals, and caretakers suggesting that DMG does provide benefits for some children with autism The effectiveness of DMG combined with a large dose of vitamin B6 (pyridoxal HCl) and magnesium  has been revealed in young children with autism spectrum disorders although the mechanisms remain unclear. Increasing evidence indicates that dysfunction of NMDA receptors at excitatory synapses is associated with autism spectrum disorders . Treatment with the NMDA receptor glycine site partial agonist D-cycloserine  and GLYX-13 [27, 32] rescued the deficit in the animal models. Our findings demonstrated that DMG acts as a partal agonist at glycine binding site of NMDA receptors, which might explain why it is beneficial to some children with autism.
Our findings revealed that DMG might act as a partial agonist at glycine binding site of NMDA receptors and shed light on the mechanisms that might be responsible for its putative use in autism and extend their potentials in other central nervous system disorders including schizophrenia and depression.
Artificial cerebrospinal fluid
Excitatory field potentials
The authors thank Dr. Hong-Yi Chiu for outstanding technical assistance.
This work was supported by grant from the National Health Research Institutes, Taiwan (NP-103-PP-02) and from Ministry of Science and Technology, Taiwan (MOST-104-2314-B-400 -007 -MY22).
Availability of data and materials
Data available on request.
Conceived and designed the experiments: MHC, HHC. Performed the electrophysiological experiments: MYL, YRL. Performed the molecular docking analysis: YST, YJT. Analyzed and interpreted the data: MYL YJT HHC. Wrote the manuscript: MYL, YJT, MHC, HHC. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Consent for publication
Ethics approval and consent to participate
All experiments were performed in accordance with the Republic of China animal protection law (Chapter III: Scientific Application of Animals) and approved by Institutional Animal Care and Use Committee of the National Health Research Institutes (NHRI-IACUC-10430-A).
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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