Slice preparation and electrophysiology

For mEPSC recordings: Acute hippocampal slices were prepared from 5–7 month old mice. Mice were decapitated and the brains were removed, and 300 µm sections were cut using a Leica VT1000S vibratome. Sections were cut in an ice cold solution containing (in mM): 206 sucrose, 2.8 KCl, 2 MgSO₄, 1 MgCl₂, 1.25 NaH₂PO₄, 1 CaCl₂, 10 glucose, 26 NaHCO₃, and 0.4 ascorbic acid. Then, sections were incubated in an NMDG-HEPES recovery solution, containing (in mM): 92 NMDG, 92 HCl, 2.5 KCl, 10 MgSO₄, 0.5 CaCl₂, 1.2 NaH₂PO₄, 20 HEPES, 30 NaHCO₃, 25 glucose, 5 sodium ascorbate, 2 thiourea, and 3 sodium pyruvate, for 15 min at 34°C before putting the slices into artificial cerebral spinal fluid (aCSF) for 1 hr at room temperature. aCSF contained (in mM): 124 NaCl, 2.8 KCl, 2 MgSO₄, 1.25 NaH₂PO₄, 2 CaCl₂, 10 glucose, 26 NaHCO₃, and 0.4 ascorbic acid. All solutions were continuously bubbled with 95% O₂/5% CO₂. Neurons were visualized using a customized Scientifica/Olympus microscope. Data were obtained with a Multiclamp 700B amplifier (Axon Instruments, Union City, CA), digitized with Digidata 1440A (Axon Instruments) and collected with Clampex 10.0 (Axon Instruments). Whole-cell patch-clamp recordings were conducted with 4–6 MΩ pipette containing (in mM) 135 K-MeSO₄, 7 NaCl, 10 HEPES, 4 Mg-ATP, 0.3 Li-GTP, and 7 phosphocreatine. Cells were held at -70 mV. aCSF was supplemented during recording with 500 nM tetrodotoxin and 50 µM picrotoxin and warmed to 32°C. mEPSCs were analyzed using Minianalysis (Synaptosoft, Decatur, GA).

For fEPSP recordings: mice (2–3 months old) were decapitated and the hippocampal lobules cut in the same solution as above. Transverse slices (400 µm) of the hippocampus were then cut using a tissue chopper (Stoelting, Kiel, WI). After slicing, sections were incubated in an NMDG-HEPES recovery solution for 15 min at 34°C before putting the slices into aCSF for 1 hr. Slices were then incubated in aCSF at room temperature for at least 1 hr before recording. Then, slices were transferred to a recording chamber, maintained at 32°C and continuously perfused at 1–2 ml/min with oxygenated aCSF. Recording electrodes were pulled from borosilicate capillary glass and filled with 1 M NaCl, 25 mM HEPES (1.5 mm o.d.; Sutter Instruments, Novato, CA). The recording pipette was placed in the in the SL or SR layers of the CA3 region of the hippocampus. Recordings were made with a MultiClamp 700B amplifier, collected using Clampex 10.3, and analyzed using Clampfit 10.3. fEPSPs were evoked using cluster electrodes (FHC) placed in the SL or SR layers of the CA3 region of the hippocampus. Current between 0.1–1 mA for 0.1 s was used to elicit a response. Maximum responses were then used for paired-pulse facilitation experiments. 1 µM DGC-IV was then added to the aCSF and the experiments were repeated after 10 min of perfusion with drug.