Long-term stabilization of place cell remapping produced by a fearful experience

Abstract

Fear is an emotional response to danger that is highly conserved throughout evolution because it is critical for survival. Accordingly, episodic memory for fearful locations is widely studied using contextual fear conditioning, a hippocampus-dependent task (Kim and Fanselow, 1992; Phillips and LeDoux, 1992). The hippocampus has been implicated in episodic emotional memory and is thought to integrate emotional stimuli within a spatial framework. Physiological evidence supporting the role of the hippocampus in contextual fear indicates that pyramidal cells in this region, which fire in specific locations as an animal moves through an environment, shift their preferred firing locations shortly after the presentation of an aversive stimulus (Moita et al., 2004). However, the long-term physiological mechanisms through which emotional memories are encoded by the hippocampus are unknown. Here we show that during and directly after a fearful experience, new hippocampal representations are established and persist in the long term. We recorded from the same place cells in mouse hippocampal area CA1 over several days during predator odor contextual fear conditioning and found that a subset of cells changed their preferred firing locations in response to the fearful stimulus. Furthermore, the newly formed representations of the fearful context stabilized in the long term. Our results demonstrate that place cells respond to the presence of an aversive stimulus, modify their firing patterns during emotional learning, and stabilize a long-term spatial representation in response to a fearful encounter. The persistent nature of these representations may contribute to the enduring quality of emotional memories.

Figure 1.

a, A fear-conditioning protocol was designed with coyote urine as the US. The neutral context condition was run in a subset of fear-conditioned animals. b–d, Unconditioned responses to coyote odor in the fear-conditioned group comprising speed, avoidance, and elongation, respectively. e, Fear-conditioned animals (n = 25) froze significantly more than control animals (n = 21) exposed to water beginning at 6 h after coyote odor exposure. f, Fear memory acquisition was specific to the training context, as fear-conditioned animals did not freeze in a similar neutral context. bl, Baseline; coy, coyote odor session. Means ± SEM are shown in b–f. *p < 0.05.

Figure 2.

a, Fear conditioning does not affect sampling of the context. Representative examples of trajectories from two fear-conditioned (top panel) and two control (bottom panel) animals recorded in the training context before and after coyote odor exposure. Both animals sample all regions of the environment, which is essential for place cell recordings. The 24 h session is shown because the conditioned freezing response peaks at this time point. b, Example of clusters and waveforms showing long-term recording stability. The two cells shown were recorded for 5 d (120 h) exhibiting minimal or no drift. Features used for cluster cutting included energy (i.e., sum of squared amplitude), peak amplitude, and time.

Figure 3.

a–e, Examples of rate maps generated from cells recorded in fear-conditioned animals. In these maps, yellow indicates areas visited by the animal where the place cell does not fire, whereas increasingly vivid colors indicate higher firing frequencies. Cells exhibited heterogeneous responses during and shortly after fear conditioning: some were stable during predator odor exposure but remapped at 1 h (a), some remapped during coyote odor exposure and again at 1 h (b, c), some remained stable throughout (d), and some remapped in coyote but stabilized the new coyote map at 1 h (e). In all examples, cells became stable in the long term and the map that stabilizes is similar to the one formed directly after coyote odor exposure (1 h session). The blue cluster is the example cell shown in a. f, Example of a rate map generated from a cell recorded in a control animal exposed to water. This place field is stable in the short term (baseline, water, and 1 h sessions) but unstable in the long term (24 h through 120 h). The green cluster is the example cell shown in e. Waveform and cluster constancy indicate stability in the recordings. Peak firing frequency for each session is indicated above each rate map.

Figure 4.

a, Comparison of average correlations between groups during short-term sessions. The fear-conditioned group shows significantly lower short-term correlations due to remapping in a subset of cells. b, Within fear-conditioned animals, short-term remapping induced by fear was more robust than long-term remapping between habituation and baseline. c, d, Between-group comparisons of stable and remapping cells between baseline and coyote odor exposure (c) and baseline and 1 h (d). Unstable cells in the fear-conditioned group remapped significantly more than the few unstable cells in the control group in both sessions. Furthermore, stable cells were also significantly less stable in the fear-conditioned group when comparing the baseline and 1 h sessions (d). e, Top, Pie chart showing percentage of stable (58%) and remapping (42%) cells during coyote odor exposure. Within these two groups, cells are further subdivided into their responses 1 h after conditioning. Bottom, Pie chart showing percentage of stable (90%) and remapping (10%) cells during the conditioning session of the control group. There are no further subdivisions of cells in the control group because no remapping is observed between the conditioning session and the 1 h session. The dotted lines indicate stability threshold (r = 0.21). Means ± SEM are shown. *p < 0.05.

Figure 5.

a, Average place field correlations indicating stability over time. The control group exhibited high short-term stability, while the fear-conditioned group exhibited remapping during both the conditioning and 1 h sessions. In the long term, only cells in the fear-conditioned group displayed increases in stability beginning at 24 h postconditioning. b, Average place field correlations between the 1 h session and each long-term test session. The maps stabilizing in the long term resembled those formed after predator odor exposure, as evidenced by continually high correlations between the 1 h session and each of the long-term sessions. Conversely, control animals exhibited a steady long-term decrease in stability in corresponding sessions. c, All cells tended to form maps in the long term that resembled the 1 h session regardless of whether they showed short-term remapping or stability, and were significantly different from the average correlation between 1 and 120 h of all cells in the control group. Histogram shows average correlations between 1 and 120 h sessions. d, Place field stability in the training context compared with a neutral context in fear-conditioned animals. Between 24 and 72 h, there was an increase in stability that was specific to the training context. hab, Habituation; bl, baseline; coy, coyote. Means ± SEM are shown. *p < 0.05.

Figure 6.

Fear conditioning affects in-field firing rate. a, Histograms showing average in-field firing rate. Fear-conditioned animals displayed significantly higher in-field firing rates during coyote odor, 1 h, and 72 h sessions, with a trend toward significance at other long-term tests (24, 48, 96 h). b, Firing rate changes during the coyote odor and 1 h sessions in the conditioned group. The firing rate change was determined for each cell using the following formula: [session − previous session]/[sum]. The rate change between habituation and baseline sessions was close to zero. During coyote odor exposure, this rate change was significantly higher than the baseline change in firing rate. At 1 h after conditioning, the change in firing rate decreased significantly from the coyote odor session but remained high compared with the baseline session, although this effect did not reach statistical significance (p = 0.058). hab, Habituation; bl, baseline; coy, coyote. Means ± SEM are shown. *p < 0.05.