Extracellular pH impacts on neuronal activity, which is in turn an important determinant of extracellular H+ concentration. The aim of this study is to describe the spatio-temporal dynamics of extracellular pH at synaptic sites during neuronal hyperexcitability. To address this issue we created ex.E2GFP, a membrane-targeted extracellular ratiometric pH indicator exquisitely sensitive to acidic shifts. By monitoring ex.E2GFP fluorescence in real time in primary cortical neurons we were able to quantify pH fluctuations during network hyperexcitability induced by convulsant drugs or high frequency electrical stimulation. Sustained hyperactivity caused a pH decrease that was reversible upon silencing of neuronal activity and localized to active synapses. This acidic shift was not attributable to the outflow of synaptic vesicle protons into the cleft nor to the activity of membrane-exposed H+-vATPase, but rather to the activity of the Na+/H+-exchanger. Our data demonstrate that extracellular synaptic pH shifts take place during epileptic-like activity of neural cultures, underlying the strict links existing between synaptic activity and synaptic pH. This evidence may contribute to the understanding of the physio-pathological mechanisms associated with hyperexcitability in the epileptic brain.
