2.5. Extracellular field recordings

Hippocampal slices were transferred into a recording chamber, under a microscope (Nikon SMZ 745T microscope). This recording chamber was submerged with oxygenated aCSF maintained at 34 °C in a continuous perfusion manner. A bipolar stimulating electrode was placed on CA3 region and a recording glass microelectrode (World Precision Instruments, Sarasota, Florida), filled with aCSF solution was placed on the stratum radiatum of CA1 region of hippocampus to record field excitatory postsynaptic potentials (fEPSP) from the Schaffer collateral pathway. Input-output responses were represented by fEPSP slopes and fiber volley (FV) amplitudes at increasing stimulus intensities. For LTP recording, stimulus intensity was set at 50% of the maximum amplitude at which initial population spike appeared. LTP was induced by Theta Burst Stimulation (TBS) protocol after a stable baseline recording for 10–15 min. The TBS protocol contains 5 sweeps at an interval of 20s, and each sweep contains 10 bursts of stimuli. Each train contains four pulses at 100 Hz, with an inter-burst interval of 200 ms. The recording was continued for 60 min post-TBS (Bloemer et al., 2019; Govindarajulu et al., 2020). LTP was calculated as an average of fEPSP slopes from 50-60 min post induction. Sweep analysis was computed by normalizing the amplitude of the first fEPSP of sweep 2 to sweep 5 with the amplitude of the first fEPSP of the sweep 1. Field potentials were recorded using LTP software with Axoclamp 2B (Axon Instruments, Foster City, CA) and analyzed with Win software (Bristol, UK) (Anderson and Collingridge, 2007).