Inappropriate recollections and responses in stressful conditions are hallmarks of post-traumatic stress disorder and other anxiety and mood disorders, but how stress contributes to the disorders is unclear. Here we show that stress itself reactivates memories even if the memory is unrelated to the stressful experience. Forced-swim stress one day after learning enhanced memory recall. One-day post-learning amnestic treatments were ineffective unless administered soon after the swim, indicating that a stressful experience itself can reactivate unrelated consolidated memories. The swim also triggered inter-hemispheric transfer of a lateralized memory, confirming stress reactivates stable memories. These novel effects of stress on memory required the hippocampus although the memories themselves did not, indicating hippocampus-dependent modulation of extrahippocampal memories. These findings that a stressful experience itself can activate memory suggest the novel hypothesis that traumatic stress reactivates pre-trauma memories, linking them to memory for the trauma and pathological facilitation of post-traumatic recall.
Inappropriate negative responses in emotionally neutral circumstances and the development of negative associations to harmless stimuli are core, debilitating features of post-traumatic stress disorder (PTSD), depression, and a host of anxiety and mood disorders. The possibility that stress itself might promote inappropriate associations between unrelated memories and events has not been explored, although a central role for stress and memory in these disorders is established [1]–[3]. Here we demonstrate that a single stressful experience can activate already consolidated memories outside of their appropriate context. These findings provide the basis for a novel hypothesis: by triggering out-of-context activation of memories stressful events themselves create opportunities for inappropriate associations to form, thereby promoting and perpetuating anxiety and mood disorders.
Stress modulates memories that are undergoing cellular consolidation [4] and may play a role in the swim-induced enhancement of memory. Experiments 4 and 5 were designed to test if the day-old memory is undergoing cellular consolidation at the time of swim. If the memories were consolidating, then amnesic treatments by electro-convulsive shock (ECS) [5] or the beta-adrenergic antagonist Propranolol [6] 24 h after conditioning should impair retention. Temporary inactivation of hippocampus with long-acting (6–10 h) tetrodotoxin (TTX) [24] or short-acting (∼30 min) lidocaine [25] was used to test if hippocampus is important for OCAM. The TTX injection was determined to block neural activity in both the dorsal and the ventral hippocampi [24].
The results of this set of experiments suggest that stress can activate memory, even if the memory is unrelated to the stressful experience. We use the term “memory activation” in the established sense that the term is used in the consolidation and reconsolidation literatures, to mean that memory is in a labile (“active”) state rather than an inert (“inactive”) state [9]. We provided multiple lines of evidence that a stressful swim returned a consolidated memory to a labile state. As a result, expression of the memory was strengthened, and if the memory was lateralized, the swim triggered its interhemispheric transfer. The activation by forced-swim stress was independent of the conditioned and external contextual stimuli that were present during learning, leading us to call the phenomenon OCAM (alternative interpretations are considered and rejected in the section that follows). OCAM seems to be a general phenomenon that does not depend on whether the conditioned response is rapidly extinguished (Experiment 1) or persistent (Experiments 2–10) or whether the activated memory is acquired during single (Experiment 5) or multiple (all other experiments) appetitively (Experiment 1) or aversively (all other experiments) conditioned trials that reinforce an inhibitory (Experiment 5) or an active (all other experiments) conditioned response. OCAM also seems to occur whether or not memory expression depends on the hippocampus (inhibitory avoidance) or the neocortex (left/right discrimination). Both beta-adrenergic activation (Figure 3A) and Dexamethasone-suppressible HPA activity (Figure 3B) were required for the OCAM effect, indicating a central role for stress and two key stress mediators. This is consistent with the idea that stress and arousal act together to modulate memory mechanisms [26]–[28]. The forced swim modified a consolidated memory and no anterograde learning effects were detected (Experiment 3b). The results of the first three experiments that assayed enhanced memory expression did not distinguish between whether the forced swim had its effect on memory storage or the process of memory retrieval, including for example by inducing perseveration during the reversal tests for memory strength. However, the results of Experiments 4 and 5 with amnestic agents indicate the swim-induced modifications only occurred soon after the swim but not after a 5-h delay. This strongly suggests the effect of swim was not on retrieval itself, which occurred a day later. The results of Experiment 2b are also consistent with an effect on storage rather than retrieval, because 6 d after the swim, we also observed the enhanced expression of intensively conditioned left/right discrimination memory (Figure 1D). Swim-induced increases in circulating hormones are unlikely to persist for 6 d (corticosterone returns to baseline levels within a day of the forced swim; Figure 1B). This is additional evidence that the effect of the swim was not on the retrieval process itself. Because IHT is conventionally interpreted as indicating memory formation in a “naïve” brain site, perhaps the strongest evidence that the swim altered memory storage and not retrieval is that IHT was induced during the swim (Experiments 7 and 10). An effect on storage rather than retrieval would be consistent with the effects of consolidation, reconsolidation, and protein synthesis inhibition [31], which are all also believed to affect memory storage. The forced swim activated consolidated memories that were 24 h old, to the best of our knowledge mimicking the basic phenomenon of reconsolidation, possibly with an important distinction. Reconsolidation is said to occur when a consolidated memory is retrieved and the activation converts the memory from a biochemically stable state to a labile state [10], [32] that is characterized by additional memory formation [33] in which the original memory can be modified, strengthened, or changed (see [34] for review). Curiously, we did not observe any memory disruption due to the forced swim stress, which seemed to activate, strengthen, or expand the localization of established memories. Further work is necessary to determine whether the stress-induced activation of memory we observed is biochemically identical to consolidation or reconsolidation, a pair of related but biochemically distinguishable phenomena [35]–[38]. It is important in the present context to point out that both consolidation and reconsolidation are specific to the memory that was directly activated by learning or retrieval [39], whereas we observed that the stressful forced swim activated several different memories, none of which were related to the stressful experience. This distinguishes the stress-induced activation of memory phenomenon we describe from conscious, recall-triggered activation of memory. For example, after CS2 → CS1 → US second-order-conditioning, recall elicited by CS2 causes the directly activated CS2 → CS1 association to become labile without altering the indirectly activated CS1 → US association [39]. We investigated the OCAM effect in several memories, but we only assayed each memory in isolation, so whether stress activates all or a subset of the rat' s memories remains an open question. We suspect that the answer will be complex because whether and how a memory is modified after retrieval depends on the strength and age of the memory [40], the brain regions involved in information storage [41], as well as the duration of the reactivation and whether extinction occurred [42]. A model of memory that attempts to synthesize the consolidation and reconsolidation literature [35] states that learning creates a memory trace, and both learning and reactivation evokes memory modulation events. The stabilization of memory is a graded function of the amount of modulation for each memory. This view predicts that forced swim will be more likely to activate recent memories than remote ones. Providing evidence for this hypothesis will require extensive experiments that manipulate both the strength of the memory and the interval between learning and forced swim. However, regardless of whether or not there is a restricted time window during which the stressful swim can cause the modification of a once consolidated memory, the present data demonstrate that OCAM occurs at the very least for consolidated memories that are 24 h old. Blocking hippocampal activity during the swim prevented both the swim-induced memory enhancement and the swim-induced IHT of lateralized memory for left/right discrimination, the learning or expression of which is insensitive to hippocampal inactivation. This suggests that hippocampal activity during the swim was necessary for the out-of-context activation of an extra-hippocampal memory. The results do not indicate whether the role of hippocampus was only to mediate the response to stress or whether hippocampal memories were specifically activated. The data demonstrate the hippocampus plays a role in memory beyond its role in associative memory storage [43]–[45], adding to the evidence that hippocampus modifies recent memories that are stored elsewhere in the brain [8], [46] and is a site along with amygdala for the combined roles of stress and arousal in mediating memory modulation [26], [28]. The results of Experiments 8,9, and 10 suggest that OCAM is a hippocampus-dependent process that appears to alter memory in extrahippocampal sites. OCAM is a common feature of human conscious recollection, but despite a recent suggestion that hippocampus is important for recollection in rats [47], there are alternative interpretations of those data and whether rats recollect remains controversial [48]. To our knowledge OCAM has never been described in non-humans. Future studies will determine whether swim-induced OCAM in rats is related to human out-of-context recollection in part by investigating whether the same hippocampal-neocortical networks are engaged. The electrophysiological re-expression of recently expressed hippocampal and neocortical activity patterns has been recorded from monkey [49] and rat during sleep [50], [51] and during conscious human recall [52]. It is of substantial interest whether such electrophysiological reactivation is the expression of memory and whether it will occur during swim-induced OCAM. Indeed, our extensive use of the intensive aversive L/R discrimination protocol was motivated in part by the fact that it generates stereotyped behavior that is amenable to searching during the stress for replay of the place cell ensemble activity sequences that are expressed during memory formation, as the rat runs up the start arm to the choice point. The findings presented here indicate that under acute stress, the hippocampus is involved in activating a set of arbitrary memories that can be stored at both hippocampal [41] and extrahippocampal sites. Although we evaluated the effect of stress on single memories, one at a time, we assume that stress can concurrently activate many memories for two reasons. First, the forced swim had little in common with the learning and retrieval experiences we investigated, suggesting stress affects memory in general rather than just memories of specific, stress-related experiences. Second, we observed that the stressful swim enhanced a variety of associations that included a weak appetitively L/R discrimination (Experiment 1A), as well as more persistent aversively conditioned L/R discrimination and inhibitory avoidance responses. These findings extend our understanding of the consequences of memory consolidation and reconsolidation, which our data demonstrate can be modified by stress. While further investigations of the effects of stress on multiple, concurrent memories are warranted, our observations indicate that stressful experience alters diverse associative memories. We only found evidence of memory enhancement, for both weak and strong associations; it however remains possible that other forms of memory that we did not test were weakened by the stress. Nonetheless, at this point, our observations suggest that in stress-induced OCAM, stress acts to generally strengthen memory rather than acting to strengthen some and weaken others. If confirmed, this may help understand the memory dysfunction in PTSD and other stress-related mood disorders. We hypothesize that stress-triggered memory activation creates a condition where multiple memories coactivate, and through mechanisms of synaptic plasticity [53] that include both long-term potentiation and depression [54]–[57], consolidation and reconsolidation, their subsequent expression is enhanced. We point out that there is evidence that recall which activates a consolidated memory can cause additional information to become incorporated into that memory via the molecular events associated with consolidation [58] but not reconsolidation [59]. According to our hypothesis, already strong traumatic memories or the stress itself can become inappropriately associated with other memories of everyday experience, making the subsequent experience and recall of everyday events more likely to trigger unwanted recall of the traumatic memory.
The experiments were conducted in accordance with Institutional (SUNY, IACUC 07-197-05) and NIH guidelines, and the directive of the European Communities Council (6/609/EEC). Male rats of the Long-Evans strain weighing 350–450 g were used. The experiments were performed during the light period (07: 00 to 19: 00) of a 12 h: 12 h cycle. Rats were habituated to handling by the experimenter for 3–5 d prior to behavioral testing. Trunk blood was collected under Halothane anesthesia. After overnight storage, the blood was centrifuged at 4,000 rpm for 10 min; the supernatant was withdrawn and then stored frozen until assayed by radioimmunoassay. More than 10 experiments were performed requiring the use of a large number of behavioral and experimental manipulations. Here in the Methods we describe the procedures themselves, and to optimize the clarity of the report, we describe each experiment' s protocol in an introduction to the individual experiment in the Results. Rats were placed in a plastic holding cage next to the forced swim bucket. A pair of electrodes was clipped to the ears and an ECS (50 mA, 50 Hz, 1 s) was delivered. After the treatment, the rats were returned to their home cage to recover. Average measures ± SEM are reported. Significant differences confirmed by ANOVA were followed by Newman-Keuls post hoc tests. The results of these pair-wise comparisons are reported in the main text, and the statistical details are given in the corresponding figure legends. Chi-square and t tests were also used as indicated in the text.
This work identifies a powerful effect of stressful experiences on memories. We report that a single intensely stressful experience can activate memories in a situation that has essentially no physical or motivational relationship to the stressful experience. Using a forced-swim test as a stressor in rats, we find that this treatment was able to activate unrelated memories formed 24 hours earlier. We also find that the hippocampus of the brain is required for this effect of stress but that recall of the memories themselves does not. The ability of stress to activate memories that are unrelated to the stressful event may help to explain how memories can sometimes become pathological and uncontrollable following traumatic events, as in post-traumatic stress disorder. Our findings suggest the novel hypothesis that the stress of the traumatic event activates neutral, unrelated memories, which then become associated with the traumatic event. Subsequent normal recall of the neutral memories can more easily trigger inappropriate recall of the traumatic event, initiating another bout of stress and inappropriate associations of neutral and traumatic memories.
neuroscience/behavioral neuroscience
neuroscience/neurobiology of disease and regeneration
neuroscience/animal cognition
neuroscience/experimental psychology
Stress-Induced Out-of-Context Activation of Memory