Long-term potentiation

Field long-term potentiation (LTP) was recorded using a planar multi-electrode recording setup (MED64 system; Alpha Med Sciences, Tokyo, Japan). Animals were decapitated and brains were immediately placed in ice-cold slicing buffer (124 mM NaCl, 3.3 mM KCl, 1.2 mM KH₂PO₄, 7 mM MgSO₄, 0.5 mM CaCl₂, 20 mM NaHCO₃ and 10 mM glucose; constantly gassed with 95% O₂/5% CO₂). Coronal hippocampal slices were cut using a vibrating microtome at 400 µM and then placed in a chamber containing artificial cerebrospinal fluid (aCSF: 124 mM NaCl, 3.3 mM KCl, 1.2 mM KHPO₄, 1.3 mM MgSO₄, 2.4 mM CaCl₂, 20 mM NaHCO₃ and 10 mM glucose; constantly gassed with 95% O₂/5% CO₂). Slices were left in the buffer for at least 30 min before recording. The slices were placed on an 8 × 8 multi-electrode array containing P5155 probes (Alpha Med Sciences; inter-electrode distance 150 µM) and 500 µL aCSF was added to the moist chamber which was constantly gassed with 95%O₂/5% CO₂. Correct placement of the array over the CA1 area was done using a microscope (SZ61, Olympus, Japan) and an image of the placement was acquired for all the recorded slices. Slices were held in place using a platinum harp. During recording, the chamber with the slice was constantly perfused with oxygenated aCSF at flow rate 2 mL/min at RT. From the 64 electrodes, one electrode on the afferent side of the CA1 area was chosen for stimulation. All other electrodes recorded, but only the electrodes that were in the direct lane of stimulation were used for analysis (5–8 electrodes per recording). Slices with LTP measurements below baseline level of 100% were excluded and numbers did not differ between groups (data not shown). After maintaining a stable baseline recording for at least 10 min, LTP was evoked by 3 × 100 Hz stimulation (tetanus) of 1 s separated by 20 s and the slope and amplitude were measured for 60 min. LTP was expressed as percentage of baseline. All data collection and processing were performed blinded.