Real-time monitoring of extracellular l-glutamate levels released by high-frequency stimulation at region CA1 of hippocampal slices with a glass capillary-based l-glutamate sensor

Real-time monitoring of l-glutamate released by high-frequency stimulation in region CA1 of mouse hippocampal slices was performed with a glass capillary-based sensor, in combination with the recoding of excitatory postsynaptic potentials (fEPSPs). A method for extracting l-glutamate currents from the recorded ones was described and applied for determining the level of extracellular l-glutamate released by 100 Hz stimulation. Recording of an l-glutamate current with a current sampling interval of 1 Hz was found to be useful for acquiring a Faradaic current that reflects l-glutamate level released by the high-frequency stimulation of 7 trains, each 20 stimuli at 100 Hz and inter-train interval of 3 s. The l-glutamate level was obtained as 15 ± 6 μM (n = 8) for the persistent enhancement of fEPSPs, i.e., the induction of long-term potentiation (LTP), and 3 ± 1 μM (n = 5) for the case of no LTP induction. Based on these observations, the level of the extracellular l-glutamate was shown to play a crucial role in the induction of LTP.