Reconstruction of the field excitatory post-synaptic potentials in the dentate gyrus from amperometric biosensor signal

Introduction: Local field potentials (LFP) information can be obtained from amperometric neurochemical recordings. However, conversion from the amperometric high frequency components (HFC) to conventional LFP is a challenging task since the electrode impedance is difficult to determine and the electrical properties of microelectrodes change with the frequency. Objective: To find and test a feasible and reproducible method to reconstruct field excitatory post-synaptic potentials (fEPSPs) in the dentate gyrus of the hippocampal formation from amperometric HFC. Materials and methods: The electrode properties were modelled as an equivalent circuit consisting of 3 resistances and 2 capacitances. A single voltage pulse allowed estimation of the resistances and capacitances values. After determination of the electrode impedance, FFT and Inverse FFT were used to convert the amperometric signal to LFPs. Platinum electrodes and biosensors responses for different voltage pulses at different holding potentials were tested in saline or in PBS. Reconstructions of the evoked potentials elicited in the dentate gyrus of rats (n=3) and mice (n=2) by stimulation of the perforant pathway were compared to electrophysiological recordings obtained subsequently in the same preparations. Long term potentiation (LTP) was induced in rats by high frequency stimulation of the perforant pathway and demonstrated by the reconstructed fEPSPs. Results: The estimated values of the resistances and capacitances of the equivalent circuit for different platinum electrodes and biosensors were found to be quite independent on the amplitude of the voltages steps (0.5-2 mV) and DC values (0-500 mV). Our method allowed perfect overlapping between reconstructed fEPSPs and true voltage recordings. Reconstructed fEPSPs showed typical inversion of the responses on the depth profile. Analysis of the slope of the rising phase of the fEPSPs showed potentiation of the synaptic efficacy in rats after high frequency stimulation. Conclusions: Our results showed that for specific electrophysiological purposes a relatively simple electrode model can work satisfactorily , allowing the reconstruction of the fEPSPs in the dentate gyrus in order to demonstrate LTP during acquisition of neurochemical recordings.