of stimulating stainless steel electrodes was implanted in the reticular nucleus pontis oralis (RPO) (8 mm posterior to bregma, 1.5 mm lateral to midline, and 8 mm below the brain surface). Skull screws overlying the nasal bone and cerebellum served as the reference and ground electrode, respectively. Electrode wires and screws were secured with dental cement. Hippocampal theta rhythm was elicited by electrical stimulation of the RPO (0.1 ms square waves at 100 Hz for a duration of 10 s separated by >60 s). Five stimulation intensities were used, evenly distributed between two thresholds empirically identified in each individual rat, at the beginning of the experiment (Li et al. 2007; Ly et al. 2013). The first threshold marked the minimum intensity that produces hippocampal theta rhythm and the second corresponded to the maximal intensity after which theta frequency or amplitude did not change. To identify the exact values of stimulation currents to elicit this standard response, covering the entire range of theta frequencies, at the beginning of experiment a series of various stimulation intensities (range 0.1 mA-1mA) were applied until the thresholds were found. Once the correct range was obtained, five levels of stimulus intensity were applied in random order.
