Anesthesia was induced with 4% isoflurane/air mixture followed by 1.3 g/kg of urethane injected i.p. to produce a long-lasting unconscious state with sleep-like brain state alterations [69 (link), 82 (link)]. The depth of anesthesia was controlled by testing the absence of peripheral reflexes of the animal, and a 0.3 g/kg bolus of urethane was given if animal showed signs of elevating level of consciousness, such as muscle movements or vocalizations. Surgery was identical to the chronic implantations except that the opening for the cortical screw was omitted, and the ground and reference electrodes were prepared from TEFLON-insulated 125 µm silver wire. Hippocampal LFP was recorded using a linear 16-channel silicon probe (Neuronexus A1 × 16-5 mm-100–177), which was slowly descended into the dorsal hippocampus to maximal (tip) depth (DV) of 2.1–2.2 mm corresponding to the DG hilus. Correct position was confirmed from the characteristic neurophysiological signals from hippocampal subfields and known anatomical distances: dentate spikes identifying the DG hilus, and SPW-ripples identifying CA1. For some animals, the probe was also dipped into Neurotracer DiI before insertion and the probe position was later histologically confirmed. LFP signals were amplified with Neuralynx HS-18 amplifier and digitized with Neuralynx Digital Lynx SX acquisition system at 30 kHz.
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