Fig. 3.

Increasing bathing GLY concentrations ([GLY]) occlude HCY's desensitization effects. A: neurons were stimulated with 100 μM NMDA with or without HCY (1 mM = 500 μM l-HCY isomer). As extracellular GLY was raised, desensitization in the black control traces was reduced. HCY caused a modest enhancement of charge transfer at moderate (1 μM) GLY levels and a nonsignificant reduction of peak amplitude. HCY no longer affected the amplitude of NMDAR responses (0% reduction) and did not provide additional enhancement of charge transfer when GLY was at a saturating 10 μM (N = 5 for 1 μM GLY, N = 4 for 10 μM GLY). B and C: charge transfer ratios and peak amplitude ratios, respectively, plotted against extracellular [GLY] show that HCY lost its ability to modulate NMDA currents as GLY was raised (i.e., peak amplitude ratios and charge transfer ratios approach 1 at high [GLY]). These data suggest a common mode of action for GLY and HCY (for [GLY] = 200 nM, 1 μM, 10 μM, N = 10, 5, 4).