Hippocampus and amygdala fear memory engrams re-emerge after contextual fear relapse

Abstract

The formation and extinction of fear memories represent two forms of learning that each engage the hippocampus and amygdala. How cell populations in these areas contribute to fear relapse, however, remains unclear. Here, we demonstrate that, in male mice, cells active during fear conditioning in the dentate gyrus of hippocampus exhibit decreased activity during extinction and are re-engaged after contextual fear relapse. In vivo calcium imaging reveals that relapse drives population dynamics in the basolateral amygdala to revert to a network state similar to the state present during fear conditioning. Finally, we find that optogenetic inactivation of neuronal ensembles active during fear conditioning in either the hippocampus or amygdala is sufficient to disrupt fear expression after relapse. These results suggest that fear relapse triggers a partial re-emergence of the original fear memory representation, providing new insight into the neural substrates of fear relapse. IntroductionThe biological capacity to produce adaptive behavioral responses in actively changing environments is critical to an animal's survival. Contextual fear conditioning (CFC) is a form of learning whereby an animal learns to associate a conditioned stimulus (i.e. a context) with an unconditioned aversive stimulus (e.g. foot shocks) to produce a conditioned response to the conditioned stimulus (e.g. freezing). Conditioned responses can be mitigated through extinction learning via repeated exposure to the conditioned context in the absence of the foot shock. However, while extinction learning can be effective at attenuating fear, animals are susceptible to fear relapse under several conditions, including exposure to stressors, the passage of time, and re-exposure to the unconditioned stimulus (

Route-dependent spatial engram tagging in mouse dentate gyrus

“…We found largely no correlation between engram reactivation and behavioral measures (Fig. 5) in agreement with past work showing little relationship between freezing and fear engram ensemble activity (Zaki et al, 2022) or locomotion and Fos levels in hippocampus (VanElzakker et al, 2008). Based on these finding and previous evidence for spatial specificity in DG granule cells, we hypothesize that different routes are encoded by different spatial engram populations.…”

“…We observed a larger Day 1 ensemble relative to the Day 2 ensemble in all groups, consistent with a novelty effect in the Full-T and Half-T groups (Fig. 4A) and with prior work using similar techniques (Zaki et al, 2022). However, we made no formal comparison of this effect due to the difference in cFos detection between IHC and viral tagging methods and because animals ran less distance and at lower speed on the second day (Fig.…”

Hippocampal remapping induced by new behavior is mediated by spatial context

“…Fear conditioning and extinction studies also compare similar outward patterns of behavior before conditioning and after extinction, but it is possible the prior threat association of the environment constitutes a more fundamental change to the memory of that environment than does a different pattern of turn responses; outcomes with different valence relate to different survival instincts, which may engage fundamentally different memory processes (Chen et al, 2020). Even if animals do not display signs of threat expectation after extinction, the condition of the environment is changed in that safety is demonstrably not guaranteed (Zaki et al, 2021). The tasks used in our study may also engage different constellations of neural circuits, as previous work suggests that egocentric, body-turn responses (Turn Right) are supported by striatal circuits while allocentric responses (Go East) are supported by the hippocampus (Chang & Gold, 2003; McDonald & White, 1994; Mcintyre et al, 2003; Packard & McGaugh, 1996), although other studies have proposed that both tasks may be represented as arm-arm location associations in over-trained animals (Futter & Aggleton, 2006).…”

“…All mice were previously used in a fear conditioning and extinction study between four and ten months of age (Zaki et al, 2021; Orlin et al, in preparation). They were allowed to rest undisturbed in their home cage for a minimum of one week, and then handled for an additional week for 10-15min/day before being trained for this study.…”

“…In general, new experiences do not entirely destabilize previously existing hippocampal representations (Cheng & Frank, 2008; Wilson & McNaughton, 1993). Additionally, in familiar environments, a small proportion of place cells can remap their activity to encode new paths (Alvernhe et al, 2011; Lever et al, 2002), reward locations (Boccara et al, 2019; Butler et al, 2019; McKenzie et al, 2013), and fear/threat responses (Moita et al, 2004; Moita et al, 2003; Wang et al, 2012; Zaki et al, 2021). However, in many instances these paradigms do not reinstate a prior behavioral contingency, so in these cases it is challenging to assess whether observed remapping is attributable to representing the new environmental features and behavioral demands, or if it is the result of permanently modifying the previously existing memory representation to reduce interference with new learning.…”