At surgery, animals were anesthetized with isoflurane vapor and an intraperitoneal injection of Equithesin (pentobarbital and chloral hydrate; 1.0 ml/250 g body weight; supplementary doses: 0.15 ml/250 g). Local anesthetic (Xylocaine) was applied to skin before making the incision. For MEC implants, tetrodes were inserted 4.6 mm lateral to midline and ∼0.35 mm anterior to the transverse sinus and tilted ∼9° anteriorly in the sagittal plane. For PPC implants, tetrodes were inserted between −3.9

and −4.2 mm Inhibitor Library cell line posterior to bregma, and 2.3–2.6 mm lateral to midline. All PPC implants were in the right hemisphere, all MEC implants were in the left hemisphere. Bone-tapping stainless steel screws were inserted securely in the skull and dental cement was applied to affix the drives to the skull. One screw served as a ground electrode. All rats were housed individually in Plexiglas cages (45 × 44 × 30 cm) in a humidity and temperature-controlled environment, and kept on a 12 hr light/12 hr dark schedule. Training and testing occurred in the dark phase. Experiments were performed in accordance with the Norwegian Animal Welfare Act and the European Convention for the Protection

of Vertebrate Animals used for Experimental and Other Scientific Purposes. Rats were connected via AC-coupled unity-gain operational amplifiers and counterbalanced cables to an Axona recording system. Tetrodes were lowered in 50 μm steps while the rat rested on a towel in a flower pot on a pedestal. Turning stopped when grid cells RO4929097 price appeared on the MEC

drive (≥1,800 μm) or when well-separated units appeared in PPC (500–1,800 μm). Data collection started when signal amplitudes exceeded approximately five times the noise level (root mean square 20–30 μV) and units were stable for >3 hr. Recordings were performed as rats foraged randomly for crumbs of vanilla cookies on a black mat in a black open-field arena (1.5 × 1.5 × 0.5 Metalloexopeptidase m) surrounded by a black curtain. A white cue-card (95 × 45 cm) hung above the south end of the arena. The animals’ movements were tracked with dual infrared LEDs, spaced 6 cm apart on the head stage (sampling rate of 50 Hz). When the rat regularly covered the entire open field in a 20 min trial (typically after 1–2 weeks), it was trained in a hairpin maze constructed by removing the mat and inserting nine black 135 × 30 × 1 cm Perspex walls in parallel grooves 14 cm apart in the underlying floor (Derdikman et al., 2009). Rats were trained to run from east to west and west to east. Food crumbs were initially administered by the experimenter at the south end of each arm. Once the rats ran regularly (after 2–3 weeks) the food protocol was winnowed to 1 crumb in the final arms.