Hippocampal slice preparation and electrophysiology

Acute 400 μm transverse hippocampal slices were prepared using a Leica VT1200S vibratome as described previously. Slices were maintained at room temperature for at least 2 h in an artificial CSF containing the following (in mm): 118 NaCl, 3.5 KCl, 2.5 CaCl₂, 1.3 MgSO₄,1.25NaH₂PO₄, and 15 glucose, bubbled with 95% O₂/5% CO₂. For electrophysiology, monophasic constant-current stimuli (100 μs) were delivered with a bipolar silver electrode placed in the stratum radiatum of area CA3. Field EPSPs (fEPSPs) were recorded using a glass microelectrode from the stratum radiatum of area CA1. LTP was induced using a high-frequency stimulation (HFS) protocol consisting of two 1 s, 100 Hz trains separated by 60 s. Paired-pulse facilitation (PPF) was conducted using a pair of stimuli with interstimulus intervals (ISIs) of 25, 50, 100, 200, or 300 ms delivered to the CA3, and the respective slopes of the fEPSP were measured. The ratio of the second slope to the first slope, compared with the ISI was plotted. For input/output (I/O) curves, the slopes of fEPSPs and the values of the fiber volley at different stimulation intensities were measured.