fEPSP Recordings and Induction of LTP

A bipolar stimulation electrode (Fredrick Haer, Bowdowinham, ME, USA) was placed in the Stratum radiatum of the CA1 region of the hippocampus and a glass field recording electrode (impedance: 1–2 MΩ) filled with aCSF was positioned in the CA1 dendritic area.

Test-pulse stimuli (0.025 Hz, 0.2 ms duration) were applied to evoke field excitatory post-synaptic potentials (fEPSPs) that were recorded with a sample rate of 10,000 Hz. For each time point, five responses were averaged. Before recordings were started, a stimulus-response relationship was obtained with a stimulation range of 50–600 μA (in 50 μA steps) to detect the maximal fEPSP. The stimulation strength used for test-pulses was the stimulus intensity that evoked ca. 50% of the maximal fEPSP. After recording basal synaptic transmission for 40 min, LTP was induced by using theta burst stimulation (TBS) with three trains 10 s apart, each consists of 10 bursts of four pulses each at 100 Hz, delivered 200 ms apart (Novkovic et al., 2015).