REGULATION OF PRESYNAPTIC NMDA RESPONSES BY EXTERNAL AND INTRACELLULAR PH CHANGES AT DEVELOPING NEUROMUSCULAR SYNAPSES

NMDA receptors play important roles in synaptic plasticity and neurona l development. The functions of NMDA receptors are modulated by many e ndogenous substances, such as external pH (pH(e)), as well as second m essenger systems. In the present study, the nerve-muscle cocultures of Xenopus embryos were used to investigate the effects of both external and intracellular pH (pH(i)) changes on the functional responses of p resynaptic NMDA receptors. Spontaneous synaptic currents (SSCs) were r ecorded from innervated myocyte using whole-cell recordings. Local per fusion of NMDA at synaptic regions increased the SSC frequency via the activation of presynaptic NMDA receptors. A decrease in pH(e) from 7. 6 to 6.6 reduced NMDA responses to 23% of the control, and an increase in pH(e) from 7.6 to 8.6 potentiated the NMDA responses in increasing SSC frequency. The effect of NMDA on intracellular Ca2+ concentration ([Ca2+](i)) was also affected by pH(e) changes: external acidificatio n inhibited and alkalinization potentiated [Ca2+](i) increases induced by NMDA. Intracellular pH changes of single soma were measured by rat io fluorometric method using 2,7-bis (carboxyethyl)-5,6-carboxyfluores cein (BCECF). Cytosolic acidification was used in which NaCl in Ringer 's solution was replaced with weak organic acids. Acetate and propiona te but not methylsulfate substitution caused intracellular acidificati on and potentiated NMDA responses in increasing SSC frequency, intrace llular free Ca2+ concentration, and NMDA-induced currents. On the othe r hand, cytosolic alkalinization with NH4Cl did not significantly affe ct these NMDA responses. These results suggest that the functions of N MDA receptors are modulated by both pH(e) and pH(i) changes, which may occur in some physiological or pathological conditions.