In vivo recordings

Mice were anaesthetised with 2 g/kg urethane (Sigma‐Aldrich, Missouri, USA) diluted 20% w/v in saline. This sedation was supplemented with 0.2 g/kg urethane as needed throughout the experiment (approximately once every 2 h). Mice were left in a dark, quiet, heated environment while the anaesthetic took effect. This was defined as the point at which the animal no longer had any paw pinch or blink reflexes, typically taking up to 1 h 15 min. The following coordinates from bregma were then used to target the CA1 region: anterior/posterior: −1.94 mm, medial/lateral: 1.75 mm, dorsal/ventral: −1.2 mm. A small hole approximately 0.5 mm in diameter was then drilled at the appropriate position using a micro drill (CellPoint Scientific, Maryland, USA). The surface of the exposed brain was covered with saline (0.9% NaCl) for the duration of the experiment.

To record the local field potential (LFP) in vivo, a 2 MΩ tungsten recording electrode (A‐M systems, Hinckley, United Kingdom (UK)) was attached to a 200 μm diameter optical fibre (Thorlabs, New Jersey, USA) with the electrode tip protruding approximately 100 μm past the tip of the optical fibre. This optrode was slowly lowered into the CA1 stratum pyramidale of the hippocampus, and a 473 nm diode‐pumped solid‐state laser (Ciel, Laser Quantum, Cheshire, UK) was used to deliver blue light through the optic fibre, at an intensity of up to 5 mW/mm². A two‐channel amplifier (Microelectrode AC Amplifier Model 1800, A‐M systems) was used to record LFP from the electrode at an acquisition rate of 20 kHz. Data were acquired using an ITC18 acquisition data board (HEKA Instruments, Lambrecht/Pfalz, Germany) and using custom written procedures in Igor Pro (WaveMetrics, Oregon, USA). After a recording session was complete, a 3 mA current was passed through the recording electrode for 2 s, lesioning the recording site. Hippocampal slices were then prepared as described below, and the lesion site was identified under a bright‐field microscope to confirm recording location.