Figure 2. NMDAR desensitization is weaker with sarcosine than glycine as a co-agonist.

A, currents evoked from one neuron by applying 100 μm NMDA with an EC₅₀ sarcosine (20 μm) or glycine (60 nm) concentration. B, mean percentage desensitization for currents evoked by 100 μm NMDA with an EC₂₀, EC₅₀ or EC₁₀₀ concentration of sarcosine (N= 6–7) or glycine (N= 6–7). Percentage desensitization was calculated by 100 × (peak current amplitude – steady-state current amplitude)/peak current amplitude. *P < 0.0001 by two-tailed paired t test. C, currents evoked from one neuron by applying 100 μm NMDA with an EC₂₀ (6 μm) or an EC₅₀ (20 μm) sarcosine concentration. Lines show a fit to a single exponential with the indicated time constants. D, mean time constant of desensitization for currents evoked by 100 μm NMDA with an EC₂₀, EC₅₀ or EC₁₀₀ concentration of sarcosine (N= 6) or glycine (N= 6). Sarcosine concentrations used in B and D were 6 μm, 20 μm and 300 μm; corresponding glycine concentrations were 30 nm, 60 nm and 3 μm. P < 0.05 for * bars versus all *** bars, and P < 0.05 for ** bar versus* bars, except the EC₂₀ glycine concentration, by one-way ANOVA. E, currents evoked from one neuron by applying 100 μm NMDA with an EC₅₀ sarcosine (20 μm) or glycine (60 nm) concentration. The extracellular solution contained 1.5 mm Ba²⁺ instead of 1.5 mm Ca²⁺. F, mean percentage desensitization for currents evoked by 100 μm NMDA with an EC₅₀ sarcosine (20 μm) or glycine (60 nm) concentration in an extracellular solution containing 1.5 mm Ca²⁺ (N= 6 for both sarcosine and glycine, data from B) or 1.5 mm Ba²⁺ (N= 9 for both sarcosine and glycine). *P < 0.001 by one-way ANOVA.