Effects of sarcosine and N, N-dimethylglycine on NMDA receptor-mediated excitatory field potentials

Abstract

Background: 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.

Results: 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.

Conclusions: 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.

Keywords: 7-chlorokynurenate; D-cycloserine; D-serine; Glycine binding site; N-methylglycine.

Fig. 1

Characterization of NMDA receptor-mediated excitatory field potentials. The experimental protocol (a) and the representative field potential recordings (b) of excitatory filed potentials in an individual prefrontal cortical slice in the presence of TTX (0.3 μM) and bicuculline (10 μM) were shown after the application of glutamate (100 μM) and glycine (10 μM) alone, and then glutamate plus glycine, followed by co-application of glutamate (100 μM) plus glycine (10 μM) with ketamine (10 μM), 7-CK (1 μM), CNQX (10 μM) or D-AP5 (100 μM)

Fig. 2

Effects of sarcosine and DMG alone or combined with glutamate on excitatory field potentials. The effects of glutamate (100 μM), glycine (10 μM), and glutamate plus glycine were examined prior to the glycine methyl derivatives to verify the slice activity. The experimental protocol (a), the representative field potential recordings (b), and the frequency and amplitude of NMDA receptor-mediated excitatory field potentials after perfusion with sarcosine or DMG (100 μM) alone and various concentrations of sarcosine (c) or DMG (d) (10, 30, and 100 μM) plus glutamate (100 μM). (Frequency: sarcosine F_7,21 = 29.43, p < 0.001; DMG F_7,21 = 48.94, p < 0.001; Amplitude: sarcosine F_7,21 = 28.17, p < 0.001; DMG F_7,21 = 5.28, p < 0.001). Data are expressed as percentage of baseline, mean ± SEM (n = 4). ***p < 0.001 compared with baseline

Fig. 3

Effects of sarcosine or DMG on glutamate/glycine-evoked excitatory field potentials. The experimental protocol (a), the representative field potential recordings (b), and the frequency and amplitude of NMDA receptor-mediated excitatory field potentials (c) after application of glutamate, glycine, glutamate (100 μM) plus glycine (10 μM) and various concentrations (10, 30 and 100 μM) of sarcosine and DMG sequentially with glutamate plus glycine. The data were analyzed by one-way repeated measures ANOVA (Frequency: F_9,27 = 50.58, p < 0.001; Amplitude: F_9,27 = 54.64, p < 0.001). Data are expressed as percentage of baseline, mean ± SEM (n = 4). ***p < 0.001 compared with baseline. ^### p < 0.001 compared with glutamate plus glycine

Fig. 4

Effects of D-cycloserine on NMDA receptor-mediated excitatory field potentials. The experimental protocol (a), the representative field potential recordings (b), and the frequency and amplitude of NMDA receptor-mediated excitatory field potentials (c) after the application D-cycloserine alone, co-application of D-cycloserine (10 and 30 μM) with glutamate or glutamate plus glycine. The data were analyzed by one-way repeated measures ANOVA (Frequency: F_8,24 = 70.25, p < 0.001; Amplitude: F_8,24 = 100.8, p < 0.001). Data are expressed as percentage of baseline, mean ± SEM (n = 4). ***p < 0.001 compared with baseline. ^### p < 0.001 compared with glutamate plus glycine. DCS:D-cycloserine

Fig. 5

Effects of 7-CK on excitatory field potentials evoked by co-application of glutamate with sarcosine, DMG and D-cycloserine. The experimental protocol (a) and the frequency and amplitude of NMDA receptor-mediated excitatory field potentials (b) after the application of sarcosine (100 μM), DMG (100 μM), and D-cycloserine (30 μM) plus glutamate with or without 7-CK (1 μM). Two-way repeated measures ANOVA demonstrated significant effects of 7-CK (Frequency: F_1,9 = 645.38, p < 0.001; Amplitude: F_1,9 = 206.5, p < 0.001). Data are expressed as percentage of baseline, mean ± SEM (n = 4).***p < 0.001 compared with respective controls

Fig. 6

Effects of sarcosine or DMG on glutamate plus D-serine-evoked excitatory field potentials. The experimental protocol (a) and the frequency and amplitude of NMDA receptor-mediated excitatory field potentials (b) after the application of D-serine alone, D-serine plus glutamate, and co-application of sarcosine or DMG with glutamate plus D-serine were presented. The data were analyzed by one-way repeated measures ANOVA (Frequency: F_5,15 = 39.19.43, p < 0.001; Amplitude: F_5,15 = 36.79, p < 0.001). Data are expressed as percentage of baseline, mean ± SEM (n = 4). **p < 0.01, ***p < 0.01 compared with baseline. ^# p < 0.05, ^### p < 0.001 compared with glutamate plus D-serine

Fig. 7

The docked poses of glycine (a), sarcosine (b), and DMG (c) with the NDMA receptor NR1 ligand-binding core (PDB ID: 1 PB7). Ligands were presented as cyan sticks. Potential interacting residues were presented as green lines and labeled with their residue codes and indices. Yellow dashed lines were presented for indicating potential hydrogen bonds (distance <3.5 Å)