The role of neuronal excitability, allocation to an engram and memory linking in the behavioral generation of a false memory in mice
Graphical abstract
Introduction
Our memories help define who we are. Mnemonic processes allow us to recall the past, function in the present and envision the future. Yet memories are not stored and recalled as exact copies of our experiences. Schacter, 1999, Schacter, in press identified seven “sins” of memory, including the sins of forgetting (i.e., transience, absent-mindedness and blocking), memory persistence (i.e., intrusive, unwanted memories), and memory distortion (i.e., misattribution, suggestibility and bias). Misremembering where we put our keys is an everyday example of a memory distortion that may produce relatively small consequences. However, at the extreme end, are memory distortions that lead to more serious consequences such as when eyewitnesses misidentify innocent individuals (Laney and Loftus, 2013, Wells and Olson, 2003) or when inaccurate memories of childhood sexual abuse are “recovered” (Bremner et al., 2000, McNally and Geraerts, 2009). Different memory distortions may be mediated by different neural mechanisms.
In the lab, human memory may be experimentally distorted via several types of interventions (Brainerd and Reyna, 2005, Gallo, 2006, Roediger and McDermott, 2000, Slotnick and Schacter, 2004). Post-event misinformation, for instance, can be used to contaminate the memory of a previous event (Loftus, 2005). In a classic study, misleading post-event information was often incorporated into subsequent event reports (Loftus, Miller, & Burns, 1978). Moreover, entirely false memories of events that never occurred may also be “implanted” in human participants (Loftus, 2003, Shaw and Porter, 2015). Human neuroimaging studies are beginning to characterize large-scale neural processes associated with memory distortions (Kurkela and Dennis, 2016, Schacter et al., in press). By comparison, the neurobiological mechanisms underlying memory distortions have received far less attention. While optogenetic stimulation of tagged neural circuits has been used to create a false memory in mice (Garner et al., 2012, Ramirez et al., 2013, Vetere et al., 2019, Yokose et al., 2017), we are unaware of studies using purely behavioral manipulations to create a false memory for a discrete cue in rodents.
Schacter, 1999, Schacter et al., 2011) and others (Howe, 2011, Loftus, 2005) suggest that some memory distortions reflect normally advantageous functional memory processes gone awry. To examine this hypothesis, we took advantage of recent findings in mice indicating that two similar events that occur in close temporal proximity can become functionally linked by virtue of neuronal co-allocation to overlapping engrams (Rashid et al., 2016). Here, we investigated whether neuronal co-allocation, a fundamental mechanism mediating the adaptive mnemonic process of memory linking, can be co-opted to create an entirely false cued fear memory in mice using purely behavioral procedures.
The lateral nucleus of the amygdala (LA) is known to be a critical brain region involved in discrete cue fear conditioning (Davis, 1992, Josselyn et al., 2001, LeDoux et al., 1990, Maren and Fanselow, 1996, Maren, 2005). Results from many experiments suggest that within a given brain region, such as the LA, eligible neurons compete for allocation to an engram (or memory trace) supporting a given memory. Moreover, neurons with relatively increased excitability at the time of an event “win” this competition to become “engram neurons” (neurons that are critical components of what is likely a larger engram and are required for subsequent recall of that particular memory) (Cai et al., 2016, Gouty-Colomer et al., 2015, Han et al., 2007, Hsiang et al., 2014, Park et al., 2020, Park et al., 2016, Rashid et al., 2016, Yiu et al., 2014, Zhou et al., 2009). After a training event, allocated engram neurons remain temporarily (~6 h in the LA) more excitable than neighboring neurons (Cai et al., 2016, Pignatelli et al., 2019, Rashid et al., 2016). The enhanced post-training excitability of engram neurons has important implications for neuronal engram allocation to subsequent events. For instance, if a similar event occurs during the time of engram neuron increased excitability (within ~6 h of Event1), these engram neurons (or a subset thereof) supporting Event1 are also allocated to an engram supporting Event2, in a process termed co-allocation. By virtue of co-allocation to overlapping engrams, the memories for these two events become linked (Fig. 1A).
At later time points after an event (>6h-24 h in the LA), homeostatic processes may decrease the excitability of Event1 engram neurons relative to their neighbors, such that these neurons become “refractory” to subsequent allocation. Should Event2 occur in this post-training time window, Event1 engram neurons would be less excitable than their neighbors and a novel population of relatively more excitable neurons would be allocated to an engram supporting Event2. This process is termed dis-allocation. Event1 and Event2 would be not be linked, but remembered separately (Rashid et al., 2016), similar to pattern separation (Fig. 1A).
Importantly, in our previous co-allocation and dis-allocation experiments (Rashid et al., 2016), Event1 and Event2 were cued fear conditioning (in which an initially motivationally neutral conditioned stimulus (CS, typically a tone) was paired with an aversive unconditioned stimulus (US, a footshock)). Here we asked whether a similar neuronal excitability-based co-allocation process could be “hijacked” to generate a false cued fear memory. In the present experiments, we trained mice with cued fear conditioning (Event1) as before, but presented a motivationally-neutral tone (CS2) alone (rather than a tone-footshock Event2) either 3 h or 24 h after auditory fear conditioning (Fig. 1B).
Section snippets
Mice
Adult (>8 weeks of age) male and female F1 hybrid (C57BL/6NTac × 129S6/SvEvTac) wild-type mice were used. All experimental procedures were performed in accordance with policies of the NIH Guidelines on the Care and Use of Laboratory Animals and Canadian Council on Animal Care (CCAC) and approved by the Hospital for Sick Children’s Animal Care and Use Committee.
Gaining access to engram neurons
In “engram tag-and-manipulate” strategies inducible immediate early gene promoters [using minimal Fos, Arc promoters or artificial
Mice show fear responses to a neutral stimulus presented shortly (3 h), but not a longer (24 h) time, after auditory fear conditioning
We first trained all mice on auditory cued fear conditioning during which a tone (CS1) was paired with a footshock. Either 3 h or 24 h later, mice were placed in a new context and a novel, motivationally-neutral tone (CS2) was presented without footshock (Fig. 2A). 24 h later, mice were placed a unique context and CS2 was replayed. During the memory test, mice exposed to CS2 3 h after fear conditioning showed higher CS2 freezing than mice exposed to CS2 24 h after conditioning, even though in
Discussion
Mistaken eyewitness testimony (Smalarz & Wells, 2015), inaccurate or implanted childhood memories (Berkowitz et al., 2008, Hyman et al., 1995, Loftus and Pickrell, 1995), and everyday lapses in our ability to recollect the past reveal the fallibility of memory. However, some memory distortions may reflect (be a “bug” of) normal adaptive memory processes. Here, we investigated whether the adaptive process of memory linking can be hijacked to create an entirely false memory in mice using only
CRediT authorship contribution statement
Jocelyn M.H. Lau: Methodology, Investigation, Visualization, Writing - review & editing. Asim J. Rashid: Resources, Methodology, Investigation, Validation. Alexander D. Jacob: Formal analysis, Investigation. Paul W. Frankland: Supervision, Conceptualization. Daniel L. Schacter: Conceptualization, Writing - review & editing. Sheena A. Josselyn: Writing - original draft, Conceptualization, Supervision.
Acknowledgments
We thank the Josselyn and Frankland labs for helpful discussions on this project.
Disclosures
The authors declare no competing financial interests. This work was supported by grants from the Canadian Institutes of Health Research [CIHR, grant numbers MOP-74650 to SAJ, FDN143227 to PWF], Natural Science and Engineering Council of Canada [NSERCs to SAJ and PWF], CIFAR catalyst award [PWF, SAJ] and NIH (NIMH R01 MH119421) [SAJ]. DLS was supported by NIMH R01 MH060941.
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