4.3. Field Potential Recordings

Extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded from the CA1 stratum radiatum of the hippocampus with glass microelectrodes (0.2–1.0 MΩ). Each slice was stimulated with increasing amplitude currents (25 to 300 μA, 25 μA step), and the amplitude, 20–80% rising phase slope, and fiber volley amplitude (FVs) were measured for each fEPSP. The efficacy of neurotransmission was determined with a sigmoidal Gompertz function as described previously [36]. The stimulation current amplitude for the LTP experiment was 40–50% of the current intensity, inducing population spikes. Stimuli were delivered every 20 s via an A365 stimulus isolator (World Precision Instruments, Sarasota, FL, USA). Responses were amplified by a Model 1800 amplifier (A-M Systems, Carlsborg, WA, USA), then digitized with ADC/DAC NI USB-6211 (National Instruments, Austin, TX, USA) using WinWCP v5.x.x software (University of Strathclyde, Glasgow, UK). The recordings were analyzed using Clampfit 10.2 software (Axon Instruments, San Jose, CA, USA).

Two types of stimulation protocols were used for LTP induction. (1) TBS: five bursts of five pulses with a frequency of 100 Hz, with an interval of 200 ms between bursts. The stimulation was repeated five times every 10 s. (2) HFS: 3 trains consisting of 100 pulses at 100 Hz applied every 20 s.

A 20 min baseline period preceded LTP induction. Potentiated fEPSPs were recorded for 60 min following stimulation. LTP was quantified by calculating the ratio of the average slope of the potentiated fEPSPs (50–60 min after stimulation) and the baseline ones (10 min before stimulation). MK-801 (10 μM), an uncompetitive NMDAR antagonist, ifenprodil (3 μM), a GluN2B subunit-selective NMDAR antagonist, D-serine, a co-agonist of NMDARs were obtained from Sigma (St. Louis, MO, USA). These drugs were diluted in distilled water and bath applied.

The paired-pulse ratio (PPR) was calculated as the second to the first fEPSP amplitude ratio.