Slices were transferred one at a time to a submersion-recording chamber at 30°C and perfused with the ACSF described above at 2 ml/min. We did not include inhibitors of GABAA receptors in our recording solutions to avoid nonphysiologic network effects resulting from disinhibition (Izumi et al., 2005a, 2007; Tokuda et al., 2011; Izumi and Zorumski, 2020). Extracellular recordings were obtained from the apical dendritic layer (stratum radiatum) of area CA1 to monitor field excitatory postsynaptic potentials (fEPSPs).

fEPSPs in the stratum radiatum of CA1 were evoked once per minute with 0.1-millisecond constant-current pulses administered to the Schaffer collateral pathway via a bipolar stimulating electrode. Stimulus intensity was set to half-maximal based on a baseline input-output (IO) curve. LTP was induced using a 100 Hz × 1 second high-frequency stimulus (HFS) at the same intensity. LTD was induced with low-frequency stimulation (LFS) consisting of 900 single pulses at 1 Hz. IO curves were repeated 60 minutes after HFS or LFS and were the primary outcome measure of synaptic change by comparison with baseline IO curves.

For experiments examining isolated NMDAR-mediated fEPSPs, dendritic responses were recorded in stratum radiatum at low frequency (1/min) in ACSF containing 0.1 mM Mg2+ and 2.5 mM Ca2+ to promote NMDAR activation and 30 μM 6-cyano-7-nitroquinoxaline-2,3-dione to block AMPARs (Izumi et al., 2005b). fEPSPs under these conditions are completely inhibited by the NMDAR antagonist 2-amino-5-phosphonovalerate (Izumi et al., 2005b, 2006). We quantified NMDAR fEPSPs by the rising slope of the field potential (12–24 points) before and at the end of drug treatment, and effects of drugs were based on comparison of the 50% maximal point on IO curves before and after drug exposure.

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