Four male Long-Evans rats were food restricted and maintained at

a weight of 400–450 g. The behavioral procedure was a modified version of an object-trace-odor paired-associate task in which performance depends on hippocampal (CA1) function (Kesner et al., 2005; for details, see Supplemental Experimental Procedures). The rats were prepared for surgery once they acquired the task and performance was stable (>70% on three consecutive sessions). Following a standard surgical protocol (e.g., Manns et al., 2007 and Komorowski et al., 2009), a 23 tetrode hyperdrive was implanted into the left hemisphere of the rat’s dorsal hippocampus (anterior-posterior [AP] = −3.6 mm; medial-lateral [ML] = 2.8 mm). Each tetrode consisted of four nichrome wires Y-27632 price (12.5 μm diameter; California Fine Wire, Grover Beach, CA, USA) gold plated to lower the impedance to 200 kΩ at 1 kHz. At the end of surgery, each tetrode was lowered ∼850 μm into tissue. After 5–7 days of recovery, the tetrodes were lowered over 7–14 days toward

the CA1 layer, using the progressive increase in θ amplitude, the appearance of sharp-wave events, and finally θ-modulated and complex-cell spiking to localize CA1 (Fox and Ranck, 1981 and Buzsáki et al., 1983). After the experiments, 25 μA of current was passed through each tetrode for Selleck MEK inhibitor 30 s before perfusion and histological confirmation of tetrode placement. Once the tetrodes were placed in their desired location, the rats were tested for 1–2 hr including 72–117 trials for each recording session. The electrical signal recorded from the tips of the tetrodes was referenced to a common skull screw and differentially filtered for single unit activity (154 Hz–8.8 kHz) and LFPs (1.5–400 Hz). The amplified

potentials from each wire were digitized at 40 kHz and monitored with the Multineuron Acquisition Processor (Plexon Inc., Dallas). Action potentials from single neurons were isolated using time-amplitude window discrimination through Offline Sorter (Plexon Inc.). We used conventional methods to identify putative pyramidal neurons and distinguish them from interneurons based on because firing rates and waveforms (Csicsvari et al., 1999; see also Figure S6 for representative waveforms). Individual pyramidal neurons were isolated by visualizing combinations of waveform features (square root of the power, spike-valley, valley, peak, principal components, and time-stamps) extracted from wires making up a single tetrode (i.e., “cluster cutting”). Single-neuron selectivity was verified by the interspike interval histograms that contained no successive spikes within a 2 ms refractory period. Single-neuron stability was verified by comparing clusters across trials. The rat’s behavior was recorded throughout testing with digital video (30 frames/s) using CinePlex Video Tracker (Plexon Inc.).