Replay, the sequential reactivation within a neuronal ensemble, is a central hippocampal mechanism postulated to drive memory processing. While both rate and place representations are used by hippocampal place cells to encode behavioral episodes, replay has been largely defined by only the latter – based on the fidelity of sequential activity across neighboring place fields. Here, we show that dorsal CA1 place cells in rats can modulate their firing rate between replay events of two different contexts. This experience-dependent phenomenon mirrors the same pattern of rate modulation observed during behavior and can be used independently from place information within replay sequences to discriminate between contexts. Our results reveal the existence of two complementary neural representations available for memory processes.

Animals

Five male Lister-Hooded rats (350-450g) were implanted with a microdrive with 24 independently moveable tetrodes. Prior to surgery, rats were kept at 90% of their free-feeding weight and housed in pairs on a 12-hour light/dark cycle, with 1 hour of simulated dusk/dawn. All experimental procedures and postoperative care were approved and carried out in accordance with the UK Home Office, subject to the restrictions and provisions contained within the Animal (Scientific Procedures) Act of 1986

Surgery

Animals were deeply anaesthetized under isoflurane anesthesia (1.5-3% at 2L/min) and implanted with a custom-made microdrive array carrying 24 independently moveable tetrodes (modified from microdrive first published by (Davidson et al., 2009)). Each tetrode consisted of a twisted bundle of four tungsten microwires (12µm diameter, Tungsten 99.95% CS, California Fine Wire), gold-plated to reduce impedance to < 200kΩ. Three rats were implanted with a dual-hippocampal microdrive targeting both dorsal hippocampal CA1 areas (AP: -3.48mm, ML: +/-2.4mm from Bregma), each output carrying 12 tetrodes. The two remaining rats were implanted with a microdrive targeting the right dorsal hippocampal CA1 area (AP: 3.72mm, ML: 2.5mm from Bregma) and the left primary visual cortex (AP: -5.76mm, ML: -3.8mm from Bregma), using 16 and 8 tetrodes respectively. After surgery, animals were housed individually and allowed to recover with food and water ad libitum for a week before returning to being kept at 90% of their free-feeding weight.

Experimental design

A given recording session started with a 1-hour rest period in which the rats were placed in a quiet, remote location (rest pot), to which they had been previously habituated. The rest pot consisted of a black circular enclosure of 20cm of diameter, surrounded by a 50cm tall black plastic sheet that isolated them from the surroundings. The animals went through one of the two following protocols:

1. Following the rest period, the rats were exposed to two novel 2m linear tracks in which they were allowed to run back and forth for 15 min, except for one session in which the animal ran for 30 min in the second track. (Rat1 session1; Rat2 and Rat3 all sessions)
2. Following the rest period, the rats were exposed to three novel 2m linear tracks in which they were allowed to run back and forth for 15 min. Data from the first track has been removed for this study to ensure consistency between protocols, such as temporal proximity to final rest session, and for all analyses. (Rat1 session2; Rat4 and Rat5 all sessions)

Liquid reward was dispensed at each end of the track (0.1mL chocolate flavored soy milk) to encourage the animals to traverse the entirety of the track. In all except one session (Rat2 session2), the exposures to the two tracks were separated by a 10min rest period in the rest pot. The recording session finished with a final 2-hours rest period inside the rest pot.

To simulate novel environments, the shape of the tracks was changed between recording sessions and their surfaces covered with different textured fabrics. In each session, the room was surrounded by black curtains with different high contrast visual cues. The tracks were separated using view-obstructing dividers.

Spike detection and unit isolation

Spiking data was sorted using the semi-automatic clustering software KlustaKwik 2.0 (K.Harris, http://klustakwik.sourceforge.net/) and then manually curated with either Phy-GUI (https://github.com/kwikteam/phy) or Klustaviewa (https://github.com/klusta-team/klustaviewa). Putative single units were discriminated based on the spike waveform, a clean inter-spike interval, and their stability across the recording session. The rest of the clustered activity was classified in either multi-unit activity or noise.

local field potentials

The power spectral density (PSD) of the LFP was calculated using Welch’s method (pwelch, MATLAB) to identify the channels with higher power for theta (4–12 Hz) and ripple (125–300 Hz) oscillations, as well as the channel with the largest difference in normalized theta to ripple power. The LFP of selected channels was next downsampled from 30 kHz to 1 kHz and band-passed filtered (MATLAB command filtfilt). The instantaneous phases were estimated using Hilbert transform.

Files can be viewed in Matlab. Details available in companion README.txt