Regulation of Presynaptic NMDA Responses by External and Intracellular pH Changes at Developing Neuromuscular Synapses

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Abstract
NMDA receptors play important roles in synaptic plasticity and neuronal development. The functions of NMDA receptors are modulated by many endogenous substances, such as external pH (pHe), as well as second messenger systems. In the present study, the nerve–muscle cocultures ofXenopusembryos were used to investigate the effects of both external and intracellular pH (pHi) changes on the functional responses of presynaptic NMDA receptors. Spontaneous synaptic currents (SSCs) were recorded from innervated myocyte using whole-cell recordings. Local perfusion of NMDA at synaptic regions increased the SSC frequency via the activation of presynaptic NMDA receptors. A decrease in pHefrom 7.6 to 6.6 reduced NMDA responses to 23% of the control, and an increase in pHefrom 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 pHechanges: external acidification inhibited and alkalinization potentiated [Ca2+]iincreases induced by NMDA. Intracellular pH changes of single soma were measured by ratio fluorometric method using 2,7-bis (carboxyethyl)-5,6-carboxyfluorescein (BCECF). Cytosolic acidification was used in which NaCl in Ringer’s solution was replaced with weak organic acids. Acetate and propionate but not methylsulfate substitution caused intracellular acidification and potentiated NMDA responses in increasing SSC frequency, intracellular free Ca2+concentration, and NMDA-induced currents. On the other hand, cytosolic alkalinization with NH4Cl did not significantly affect these NMDA responses. These results suggest that the functions of NMDA receptors are modulated by both pHeand pHichanges, which may occur in some physiological or pathological conditions.