Active Transition Of Fear Memory Phase From Reconsolidation To Extinction Through ERK-Mediated Prevention Of Reconsolidation
Mar 19, 2022
Contact: Audrey Hu Whatsapp/hp: 0086 13880143964 Email: audrey.hu@wecistanche.com
The retrieval of fear memory induces two opposite memory processes, i.e., reconsolidation and extinction. Brief retrieval induces reconsolidation to maintain or enhance fear memory, while prolonged retrieval extinguishes this memory. Although the mechanisms of reconsolidation and extinction have been investigated, it remains unknown how fear memory phases are switched from reconsolidation to extinction during memory retrieval. Here, we show that an extracellular signal-regulated kinase (ERK)-dependent memory transition process after retrieval regulates the switch of memory phases from reconsolidation to extinction by preventing the induction of reconsolidation in an inhibitory avoidance (IA) task in male mice. First, the transition memory phase, which cancels the induction of reconsolidation, but is insufficient for the acquisition of extinction, was identified after reconsolidation, but before extinction phases. Second, the reconsolidation, transition, and extinction phases after memory retrieval showed distinct molecular and cellular signatures through cAMP-responsive element-binding protein (CREB) and ERK phosphorylation in the amygdala, hippocampus, and medial prefrontal cortex (mPFC). The reconsolidation phase showed increased CREB phosphorylation, while the extinction phase displayed several neural populations with various combinations of CREB and/or ERK phosphorylation, in these brain regions. Interestingly, the three memory phases, including the transition phase, showed transient ERK activation immediately after retrieval. Most importantly, the blockade of ERK in the amygdala, hippocampus, or mPFC at the transition memory phase disinhibited reconsolidation-induced enhancement of IA memory. These observations suggest that the ERK-signaling pathway actively regulates the transition of the memory phase from reconsolidation to extinction and this process functions as a switch that cancels reconsolidation of fear memory.
Keywords: ERK; extinction; fear memory; reconsolidation; transition
1Department of Bioscience, Faculty of Life Sciences, Tokyo University of Agriculture, Tokyo 156-8502, Japan, and
2Graduate School of Agriculture and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan
Significance Statement
Retrieval of fear memory induces two opposite memory processes; reconsolidation and extinction. Reconsolidation maintains/ enhances fear memory, while extinction weakens fear memory. It remains unknown how memory phases are switched from reconsolidation to extinction during retrieval. Here, we identified an active memory transition process functioning as a switch that inhibits reconsolidation. This memory transition phase showed a transient increase of extracellular signal-regulated kinase (ERK) phosphorylation in the amygdala, hippocampus, and medial prefrontal cortex (mPFC). Interestingly, inhibition of ERK in these regions at the transition phase disinhibited the reconsolidation-mediated enhancement of inhibitory avoidance (IA) memory. These findings suggest that the transition memory process actively regulates the switch of fear memory phases of fear memory by preventing the induction of reconsolidation through the activation of the ERK-signaling pathway.
Introduction
Memory retrieval is not a passive process but is rather a dynamic process that allows the maintenance, strengthening, weakening, or altering/updating of an original memory (Misanin et al., 1968; Schneider and Sherman, 1968; Lewis, 1979; Mactutus et al., 1979; Gordon, 1981; Nader et al., 2000; Nader and Hardt, 2009; Dudai, 2012; Fukushima et al., 2014). Importantly, a retrieved conditioned fear memory by brief re-exposure to the conditioned stimulus (CS) becomes labile and requires gene expression-dependent reconsolidation for its maintenance or enhancement (Nader et al., 2000; Dudai, 2002; Kida et al., 2002; Suzuki et al., 2004; Tronel et al., 2005; Fukushima et al., 2014). Conversely, continuous or repeated re-exposure to the CS induces memory extinction, which weakens fear memory (Pavlov, 1927; Rescorla, 2001; Myers and Davis, 2002). Thus, the retrieval of fear memory induces two opposite memory processes, i.e., reconsolidation and extinction, although both processes are induced by re-exposure to an identical CS, but differ according to the duration of re-exposure to the CS.
The common and critical biochemical feature of reconsolidation and extinction is the requirement for cAMP-responsive element-binding protein (CREB)-mediated gene expression (Mamiya et al., 2009). Interestingly, we have shown contrasting molecular, anatomic, and behavioral signatures between the reconsolidation and extinction phases of contextual fear memory (Suzuki et al., 2004; Mamiya et al., 2009). Blocking protein synthesis during the reconsolidation phase disrupts the original fear memory, whereas blocking protein synthesis during the extinction phase fails to do this, although the contextual fear memory was reactivated. The requirement of brain regions displaying the activation of CREB-mediated gene expression differs between reconsolidation and extinction; reconsolidation depends on the amygdala and hippocampus, whereas extinction relies on the amygdala and medial prefrontal cortex (mPFC). However, the time course of amygdaloid CREB activation differs between the reconsolidation and extinction memory phases. These observations suggested that the reconsolidation and extinction phases are not independent, but rather interact with each other. Interestingly, recent studies have identified a time window (transition phase) that shows no extracellular signal-regulated kinase (ERK) activation in the amygdala after reconsolidation, but before the extinction phases following the retrieval of auditory fear memory (Merlo et al., 2018). Taken together, these findings suggest the possible mechanisms by which memory phases are switched from reconsolidation to extinction during the retrieval of fear memory. In other words, it is possible that the memory transition process actively regulates this switch.
In an inhibitory avoidance (IA) task, mice receive an electrical footshock after they enter a dark compartment from a light compartment and form memory to avoid the dark compartment. Previously, by using this task, we showed that the reconsolidation and extinction phases can be discriminated at the time point when a mouse enters a dark compartment from a light compartment during a re-exposure session (Fukushima et al., 2014). Therefore, this task allows us to characterize the perspective molecular signatures of the reconsolidation and extinction phases, in contrast to the classical contextual fear conditioning paradigm in which the reactivation of conditioned fear memory by re-exposure to the CS initiates both reconsolidation and extinction; short (3 min) re-exposure to the conditioned context induces reconsolidation, whereas long (30 min) or repeated re-exposure to this context induces extinction (Eisenberg et al., 2003; Pedreira and Maldonado, 2003; Suzuki et al., 2004; Lee et al., 2008; Mamiya et al., 2009). Furthermore, we found that the retrieved IA memory is enhanced through memory reconsolidation in this task (Fukushima et al., 2014).
To understand the mechanism for the transition from reconsolidation to extinction during the retrieval of fear memory, we aimed to identify and characterize the molecular, cellular, and behavioral signatures of the reconsolidation, transition, and extinction phases of IA memory. We analyzed the activation of CREB and ERK in the amygdala, hippocampus, and mPFC in the reconsolidation, transition, and extinction phases and examined the roles of ERK activation in these memory processes.
Materials and Methods
Mice All experiments were conducted according to the Guide for the Care and Use of Laboratory Animals (Japan Neuroscience Society and Tokyo University of Agriculture). All animal experiments performed in this study were approved by the Animal Care and Use Committee of Tokyo University of Agriculture (authorization #280037). All surgical procedures were performed under Nembutal anesthesia and every effort was made to minimize suffering. Male C57BL/6N mice were obtained from Charles River. The mice were housed in cages of five or six, maintained on a 12/12 h light/dark cycle, and allowed access to food and water ad libitum. The mice were at least eight weeks of age when tested. Testing was performed during the light phase of the cycle. All experiments were conducted blind to the treatment condition of the mice.
IA test The step-through IA apparatus (OHARA Pharmaceutical) consisted of a box with separate light and dark compartments (both 15.5 12.5 11.5 cm). The light compartment was illuminated by a fluorescent light (2500 lux; Fukushima et al., 2008, 2014; Zhang et al., 2011; Ishikawa et al., 2016). Before the commencement of IA training, the mice were handled individually for 2 min each day for one week. During the training sessions, each mouse was allowed to habituate to the light compartment for 30 s, and the guillotine door was raised to allow access to the dark compartment. Latency to enter the dark compartment was considered as a measure of acquisition. As soon as the mouse had entered the dark compartment, the guillotine door was closed. After 5 s, a footshock (0.2 mA) was delivered for a total period of 2 s (training). At 24 h after the training session, the mouse was placed back in the light compartment until it entered the dark compartment (average 459 6 15.49 s). Immediately after the mouse had entered the dark compartment, the guillotine door was closed and the mouse stayed in the dark compartment for a varying length of time (0, 1, or 10 min) without a footshock (reactivation). Memory was assessed 48 h later [postreactivation long-term memory (PR-LTM) test] as the crossover latency for the mouse to enter the dark compartment when replaced in the light compartment, as in reactivation.
For the first experiment, we examined the effect of protein synthesis inhibition after reactivation (re-exposure to the dark compartment for 0, 1, or 10 min; Fig. 1). The protein synthesis inhibitor anisomycin (ANI; Wako) was dissolved in saline (pH adjusted to 7.0–7.4 with NaOH). The mice were trained as described above, and at 24 h later, they received vehicle (VEH) or ANI (150 mg/kg, i.p.) immediately after re-exposure to the dark compartment for 0, 1, or 10 min without a footshock (reactivation). At this dose, ANI inhibits .90% of protein synthesis in the brain during the first 2 h (Flood et al., 1973). At 48 h after the reactivation session, individual mice were once again placed in the light compartment and crossover latency was assessed.
For the second experiment [phosphorylated CREB (pCREB) and phosphorylated ERK (pERK) immunohistochemistry; Figs. 2–5], we examined the brain regions that were activated after re-exposure to the light (until the mice entered the dark compartment, re-exposure to the dark compartment for 0 min) or dark compartment (re-exposure to the dark compartment for 1 or 10 min). The mice were divided into four Fukushima et al. · Transition of Fear Memory Phases after Retrieval J. Neurosci., February 10, 2021, • 41(6):1288–1300 • 1289groups. At 24 h after training, individual mice were re-exposed to the light compartment and then stayed in the dark compartment following their entry from the dark compartment [reactivation: 0 min in the dark compartment, reconsolidation (Recon) group; 1 min, transition (Tran) group; 10 min, extinction (Ext) group]. Another group of mice was not returned to the light/dark compartment [non-reactivated (NR) group]. The mice were then anesthetized with Nembutal (750 mg/kg, i.p.) at 5, 15, or 30 min after reactivation.
For the third experiment (microinfusion of U0126; Figs. 6,7), we examined the effects of ERK inhibition in the amygdala, hippocampus, or mPFC on memory reconsolidation/enhancement, transition, and extinction. The MEK inhibitor U0126 (Sigma-Aldrich) was dissolved in artificial cerebrospinal fluid containing three drops of Tween 80 (Sigma) in 2.5 ml of 7.5% dimethyl sulfoxide (Wako) and adjusted to pH 7.4 with NaOH. The mice were trained as described above, and 24 h later, they were placed back in the light compartment (reactivation). The mice were microinfused with U0126 (1 mg) or VEH into the various brain regions immediately after (Figs. 6A, C, E–H, 7A–C) or at 30 min after (Fig. 6B, D) reactivation. At 48 h after reactivation, individual mice were once again placed in the light compartment and crossover latency was assessed (PRLTM). Microinfusions into the hippocampus and mPFC (0.5 ml) were made at a rate of 0.25ml/min. Microinfusions into the amygdala (0.2 ml) were made at a rate of 0.1ml/min. The injection cannula was left in place for 2 min after microinfusion and the mice were then returned to their home cages. The MEK inhibitor SL327 (Santa Cruz Biotechnology) was dissolved in dimethyl sulfoxide and diluted with saline. The mice were trained as described above, and 24 h later, individual mice were placed back in the light compartment (reactivation). The mice were systemically injected with SL327 (10 or 20 mg/kg) or VEH immediately after reactivation (Fig. 7D–F). At 48 h after reactivation, individual mice were once again placed in the light compartment and crossover latency was assessed (PR-LTM).
Immunohistochemistry was performed as described previously (Mamiya et al., 2009; Suzuki et al., 2011; Zhang et al., 2011; Fukushima et al., 2014; Ishikawa et al., 2016; Hasegawa et al., 2019). After anesthetization, all mice were perfused with 4% paraformaldehyde. The brains were removed, fixed overnight, transferred to 30% sucrose, and stored at 4°C. Coronal sections (30mm) were cut in a cryostat.
For pCREB and pERK staining, free-floating sections were treated with 1%H2O2 and incubated overnight with a rabbit polyclonal anti-phospho-CREB (serine 133; S133) antibody (1:1000; #06-519, Millipore) and/or rabbit monoclonal anti-phospho-ERK1/2 (T202/Y204) antibody (1:300; #4370; Cell Signaling Technology) in blocking solution (phosphate-buffered saline plus 1% goat serum albumin, 1 mg/ml bovine serum albumin, and 0.05% Triton X-100). The sections were washed with phosphate-buffered saline and incubated with horseradish peroxidase-conjugated donkey anti-rabbit IgG (1:500; Jackson ImmunoResearch) for pCREB or horseradish peroxidase-conjugated goat anti-rabbit IgG for pERK for 1 h at room temperature. pCREB signals were amplified by biotin tyramide and visualized using Alexa Fluor-conjugated streptavidin (Invitrogen). pERK signals were amplified with TSA-FCM (Invitrogen). The sections were mounted on slides and coverslipped using a mounting medium (Millipore).
Quantification was performed as described previously (Frankland et al., 2006; Fukushima et al., 2014; Mamiya et al., 2009; Zhang et al., 2011; Suzuki et al., 2008). Structures were defined anatomically according to the atlas of Franklin and Paxinos (1997). All immunoreactive neurons were counted by an experimenter blind to the treatment condition
Results
Characterization of memory phases after retrieval in the IA task
The IA task allows us to discriminate the reconsolidation and extinction phases at the time point when a mouse enters a dark compartment from a light compartment (Fukushima et al., 2014). To understand the mechanism underlying the switch of memory phases from reconsolidation to extinction, we characterized the IA memory phases following memory retrieval by examining the effects of inhibiting the protein synthesis that is required for the reconsolidation and extinction of IA memory (Fukushima et al., 2014). The mice were first placed in the light compartment. At 5 s after they entered the dark compartment, a brief electrical footshock was delivered (training). The mice were re-exposed to the light compartment 24 h after the training (reactivation session; Fig. 1A) and their crossover latency to enter the dark compartment was assessed (Fig. 1B). The mice were returned into their home cages immediately after they entered into the dark compartment from the light compartment (0-min re-exposure to the dark compartment; reconsolidation phase) or stayed in the dark compartment for 1, 3, or 10 min without receiving a footshock (extinction phase; Fig. 1C–E). Immediately after the reactivation session, the mice received a systemic injection of VEH or the protein synthesis inhibitor ANI. At 48 h later, crossover latency was assessed PR-LTM.
Consistent with our previous study (Fukushima et al., 2014), re-exposure to the light compartment (0 min group) induced reconsolidation and enhancement of IA memory. A two-way ANOVA revealed significant effects of time (F(1,24) = 10.433, p = 0.0036), drug (F(1,24) = 23.197, p , 0.0001) and time drug interaction (F(1,24) = 25.022, p , 0.0001; Fig. 1B). Post hoc Bonferroni’s test and paired t-test revealed that the VEH and ANI groups, displayed significantly increased or decreased, respectively, crossover latency at PR-LTM compared with the reactivation session (ps , 0.05; VEH, t(6) = 5.134, p = 0.0021, ANI, t(6) = 4.804, p = 0.003; Fig. 1B). These observations indicate that IA memory retrieval in the light compartment enhanced the memory, while protein synthesis inhibition disrupted the retrieved memory, confirming the previous observation that IA memory retrieval enhances the memory through reconsolidation in a protein synthesis-dependent manner.
In contrast, the re-exposure to the dark compartment induced long-term extinction [two-way ANOVA, time (Fig. 1C, F(1,28) = 9.575, p = 0.004; Fig. 1D, F(1,36) = 11.699, p = 0.0016), drug (Fig. 1C, F(1,28) = 4.674, p = 0.039; Fig. 1D, F(1,36) = 12.285, p = 0.0012), time drug interaction (Fig. 1C, F(1,28) = 7.916, p = 0.009; Fig. 1D, F(1,36) = 7.915, p = 0.0079)], as observed previously (Fukushima et al., 2014). The VEH groups that stayed in the dark compartment for 3 or 10 min showed significantly decreased crossover latency at PR-LTM compared with the reactivation session, whereas the ANI groups displayed comparable crossover latency at PR-LTM compared with the reactivation session and to the VEH groups (post hoc Bonferroni’s test, ps , 0.05; paired t test, Fig. 1C, VEH, t(7) = 4.976, p = 0.0016, ANI, t(7) = 0.796, p . 0.05; Fig. 1D, VEH, t(9) = 10.211, p , 0.0001, ANI, t(9) = 1.02, p . 0.05). These observations indicate that the re-exposure to the dark compartment for 3 or 10 min extinguished IA memory and that inhibition of protein synthesis blocked long-term extinction. Thus, IA memory retrieval in the dark compartment extinguishes IA memory in a gene expression-dependent manner.
Importantly, the VEH group showed comparable crossover latency at PR-LTM compared with the reactivation session and to the ANI group when they stayed in the dark compartment for 1 min [two-way ANOVA, time (F(1,36) = 0.03, p . 0.05), drug (F(1,36) = 0.019, p . 0.05), time drug interaction (F(1,36) = 0.011, p . 0.05); post hoc Bonferroni’s test, ps . 0.05; paired t-test, VEH, t(9) = 0.091, p . 0.05, ANI, t(9) = 0.328, p . 0.05; Fig. 1E]. These observations indicate that the VEH group showed neither enhancement nor extinction of IA memory and that the ANI group showed no disruption of IA memory. Therefore, re-exposure to the dark compartment for 1 min blocked both the enhancement and ANI-induced disruption of the reactivated IA memory, but not extinguished IA memory, suggesting that this 1-min re-exposure cancels the induction of reconsolidation, but is insufficient to extinguish IA memory.
In summary, these results indicated that re-exposure to the light compartment induces the reconsolidation phase, whereas longer re-exposure to the dark compartment (3 or 10 min) induces the extinction phase. More importantly, staying for 1 min in the dark compartment induces the transition phase from reconsolidation to extinction, which, inhibits fear memory reconsolidation without inducing extinction.
Molecular signatures of the reconsolidation, transition, and extinction phases in the amygdala, hippocampus, and mPFC after IA memory retrieval
Reconsolidation and extinction of contextual fear memory show increases in CREB phosphorylation at S133, a marker of gene expression activation required for reconsolidation and long-term extinction, but show distinct dynamics of CREB phosphorylation (Mamiya et al., 2009). Interestingly, recent studies have shown that there is no increase in the phosphorylation of ERK, an upstream regulator of CREB (Impey et al., 1998; Wu et al., 2001), in the basolateral region of the amygdala at the transition from reconsolidation to the extinction of a cued fear memory, although this phosphorylation is increased in the basolateral region when a cued fear memory is reconsolidated and extinguished (Merlo et al., 2014, 2018). Another study indicated that hippocampal ERK is activated only when contextual fear memory is extinguished, but not reconsolidated (Tronson et al., 2009). These findings suggest that reconsolidation, transition, and extinction phases show distinct molecular and cellular signatures. Therefore, we measured and compared the levels of pCREB and pERK in the reconsolidation, transition, and extinction phases using immunohistochemistry. We performed similar experimental schedules as in Figure 1B, D, E using four experimental groups. The mice were re-exposed to the light compartment at 24 h after the training and then stayed in the dark compartment [reactivation: 0 min in the dark compartment, reconsolidation (Recon) group; 1 min, transition (Tran) group; 10 min, extinction (Ext) group]. Another group of mice was not returned to the light/dark compartment (non-reactivated, NR group). We counted pCREB-positive (pCREB1) neurons, pERK-positive (pERK1) neurons, and double-positive (pCREB1/pERK1) neurons in the amygdala, hippocampus, and mPFC at 30 min after the reactivation session.
Amygdala (lateral region) CREB was activated in the extinction and reconsolidation phases, whereas ERK was activated only in the extinction phase (Fig. 2A–C). A one-way ANOVA revealed a significant effect of group (Fig. 2B, F(3,29) = 14.85, p , 0.0001). Similar to previous findings (Mamiya et al., 2009), the post hoc Newman–Keuls test revealed that the Recon and Ext groups showed significantly more pCREB1 neurons than the other groups (p, 0.05). These observations indicated that similar to the observations at the behavioral levels (Fig. 1), exposure to the dark compartment for 1 min (transition phase) cancels the “turning-on” of CREB phosphorylation that would be increased in the reconsolidation phase. In contrast, significantly more pERK1 neurons were observed in the Ext group than in the other groups, although there were much fewer pERK1 neurons than pCREB1 neurons in the Ext group (F(3,29) = 3.793, p = 0.0207; Fig. 2C).
Consistently, significantly more double positive neurons (pCREB1/pERK1) were observed in the Ext group (F(3,29) = 6.698, p = 0.0014; Fig. 2D), whereas significantly more pCREB1/ pERK– (pCREB single positive) neurons were observed in the Recon and Ext groups (F(3,29) = 13.689, p , 0.0001; Fig. 2E). Thus, the reconsolidation phase showed only a single population of pCREB1/pERK– neurons. In contrast, the extinction phase showed two populations of pCREB1/pERK– and pCREB1/ pERK1 neurons, indicating that ERK is activated only in a subset of pCREB1 neurons. Importantly, similar results were observed in the basolateral region of the amygdala (Fig. 2G, F(3,29) = 13.042, p , 0.0001; Fig. 2H, F(3,29) = 3.824, p = 0.0201; Fig. 2I, F(3,29) = 12.633, p , 0.0001; Fig. 2J, F(3,29) = 12.505, p , 0.0001).
Biphasic activation of ERK in the extinction phase ERK is an upstream activator of CREB and thereby, ERK activation is required for the consolidation and reconsolidation of fear memory (Schafe et al., 2000; Duvarci et al., 2005). However, inconsistently, no ERK activation was observed in the amygdala, hippocampus, or mPFC in the reconsolidation phase when pERK was measured at 30 min after the reactivation session (Figs. 2-4). Therefore, we examined the time courses of ERK and CREB phosphorylation. We performed a similar experiment as in Figures 2-4, except that pCREB and pERK levels were measured at 5, 15, and 30 min after the reactivation session (re-exposure to the dark compartment for 0, 1, or 10 min; Fig. 5A). Consistent with the data shown in Figures 2-4, significant increases in pCREB1 neurons were observed at 30 min, but not at 5 min, after the reactivation session in the Recon (amygdala,mPFC and hippocampus) and Ext (amygdala and mPFC) groups, but not the Tran group (Fig. 5E, one-way ANOVA, amygdala, 5 min, F(3,23) = 0.346, p . 0.05, 30 min, F(3,23) = 15.272, p , 0.0001; mPFC, 5 min, F(3,23) = 1.169, p . 0.05, 30 min, F(3,23) = 32.346, p , 0.0001; hippocampus, 5 min, F(3,23) = 0.154, p . 0.05, 30 min, F(3,23) = 16.197, p , 0.0001; unpaired t test, amygdala, reconsolidation, 5 vs 30 min, t(12) = 7.807, p , 0.0001, extinction, 5 vs 30 min, t(12) = 5.405, p = 0.0002; mPFC, reconsolidation, 5 vs 30 min, t(12) = 5.727, p , 0.0001, extinction, 5 vs 30 min, t(12) = 4.188, p = 0.0013; hippocampal CA1 region, reconsolidation, 5 vs 30 min, t(12) = 2.339, p = 0.0374).
Interestingly, significant increases in pERK1 neurons were observed in the amygdala, mPFC and hippocampus of the Recon, Tran, and Ext groups at 5 min after the reactivation session compared with the NR group (Fig. 5F, amygdala, F(3,23) = 10.961, p = 0.0001; mPFC, F(3,23) = 7.525, p = 0.0011; hippocampus, F(3,23) = 6.924, p = 0.0017). These observations indicated that ERK is activated immediately after the reactivation session in all memory phases. However, these increases in the number of pERK1 neurons returned to basal levels (comparable with the NR group) at 15 min after the reactivation session (Fig. 5F, amygdala, F(3,20) = 2.676, p . 0.05; mPFC, F(3,23) = 0.683, p . 0.05; hippocampus, F(3,20) = 0.74, p . 0.05). Furthermore, consistent with the findings shown in Figures 2-4, significantly more pERK1 neurons were observed in the amygdala, mPFC, and hippocampus at 30 min after the reactivation session only in the Ext group (Fig. 5F, amygdala, F(3,23) = 6.616, p = 0.022; mPFC, F(3,23) = 8.012, p = 0.0008; hippocampus, F(3,23) = 6.206, p = 0.003). Thus, the reconsolidation and transition phases show transient activation of ERK only at the early time point (5 min), whereas the extinction phase shows biphasic activation of ERK at the early (5 min) and late (30 min) time points after the reactivation session. These observations indicated that the mechanisms for the regulation of ERK activation differ in the reconsolidation/ transition and extinction phases. Collectively, our observations demonstrated that the reconsolidation, transition, and extinction phases show distinct molecular signatures.
Roles of ERK activation in reconsolidation and extinction phases of IA memory
The reconsolidation/transition and extinction phases showed monophasic and biphasic ERK activation, respectively. We next investigated and compared the roles of early (5 min) and late (30 min) ERK activation in the mPFC in the reconsolidation and extinction phases by examining the effects of inhibiting ERK (Fig. 6).
Discussion
In this study, we investigated the mechanisms for memory transition from the reconsolidation to extinction phases after the retrieval of IA memory. We first characterized the behavioral signatures of IA memory phases after retrieval. Consistent with our previous study (Fukushima et al., 2014), IA memory retrieval-induced reconsolidation and extinction by re-exposure to the light (0 min in the dark compartment) and dark (3 or 10 min) compartments, respectively. Interestingly, IA memory was neither enhanced nor extinguished and showed resistance to the inhibition of protein synthesis when the mice were re-exposed to the dark compartment for only 1 min. Therefore, these observations suggest that a 1-min re-exposure to the dark compartment cancels the induction of reconsolidation, but is insufficient to extinguish IA memory. Furthermore, we found that ERK was activated in the amygdala, hippocampus, and mPFC at an early time point (5 min) after re-exposure to the dark compartment for 0, 1, or 10 min. Consistently, the inhibition of ERK in these brain regions blocked the reconsolidation/enhancement and extinction of IA memory. Most importantly, ERK inhibition in the amygdala, hippocampus, and mPFC following 1-min re-exposure to the dark compartment disinhibited the reconsolidation-mediated enhancement of IA memory, suggesting that ERK activation following brief (1 min) re-exposures to the dark compartment is required for the inhibition of IA memory reconsolidation. Conversely, a 1-min re-exposure to the dark compartment was insufficient to extinguish IA memory, although extended re-exposure to the dark compartment (3 or 10 min) extinguished this memory. Therefore, our results suggest that a 1-min re-exposure to the dark compartment induces a memory transition process that cancels reconsolidation/enhancement but does not initiate extinction learning. Collectively, we suggest that the memory transition process contributes to the switch of memory phases from reconsolidation to extinction through ERK-mediated prevention of reconsolidation.
Similar to our current observations, a recent study using auditory fear conditioning showed that single (1) or prolonged (10) CS presentations induce memory reconsolidation and extinction, respectively, through an increase of pERK levels in the basolateral region of the amygdala. In contrast, intermediate (4–7) CS presentations do not change pERK levels in the basolateral region of the amygdala. Importantly, ERK inhibition at the intermediate CS presentations did not affect fear memory. This study suggested that there is a transition of memory phase from reconsolidation to extinction after fear memory retrieval (Merlo et al., 2018). In the present study, we extended this finding and suggested that the transition phase actively switches memory phases from reconsolidation to extinction through the activation of the ERK signal transduction pathway. In contrast to previous findings (Merlo et al., 2018), we found that the transition phase involves ERK phosphorylation in the amygdala, hippocampus, and mPFC. These discrepancies maybe because of the difference of time points examining ERK phosphorylation; the previous study measured pERK levels at;12 min after CS presentation (Merlo et al., 2018), while our study showed that increased pERK levels returned to the basal level at around this time point (15 min after the re-exposure). Additionally, it is important to note that the IA task enables the observation of the enhancement of IA memory through reconsolidation, thereby leading to our finding that the inhibition of ERK at the transition phase disinhibits the enhancement of IA memory.
Previous studies have shown that ERK phosphorylation is increased in the basolateral region of the amygdala at 20–60 min following extinction learning of cued fear memory (Herry et al., 2006; Merlo et al., 2014, 2018), whereas the hippocampus shows this activation at 1 h following the extinction learning of contextual fear memory (Fischer et al., 2007; Tronson et al., 2009). In the present study, we obtained similar observations that pERK is increased at 30 min after the reactivation session in the extinction phase. These findings suggest that ERK phosphorylation is a common molecular signature of the late extinction phase (20– 60 min).
Furthermore, we observed that ERK activation occurs biphasically at early and late time points (5 and 30 min) after the reactivation session in the extinction phase, whereas this activation occurs mono phasically at the early time point in the reconsolidation phase (Fig. 5). Consistently, inhibiting ERK in brain regions at these time points of the reconsolidation and extinction phases blocked reconsolidation/enhancement and long-term extinction, respectively (Fig. 6A, C–H). These observations suggest that monophasic and biphasic ERK activation is required for the reconsolidation-mediated enhancement and extinction of IA memory, respectively. It is important to note that ERK functions as an upstream regulator of CREB phosphorylation. Therefore, the transient activation of ERK in the early memory phase may, at least in part, contribute to this phosphorylation of CREB, which activates the expression of genes required for reconsolidation and long-term extinction.
Similar to our previous findings using contextual fear conditioning (Mamiya et al., 2009), CREB was activated in the reconsolidation (amygdala/hippocampus/mPFC) and extinction (amygdala/mPFC) memory phases, while ERK was activated only in the extinction phase at 30 min after the reactivation session. Consistently, only a single population of pCREB1/pERK– neurons were observed in the reconsolidation phase, while distinct neuron populations were observed in the extinction phase: pCREB1/pERK– and pCREB1/pERK1 neurons in the amygdala (Fig. 2); pCREB– /pERK1 neurons in the hippocampus (Fig. 3); and pCREB1/pERK–, pCREB– /pERK1, and pCREB1/pERK1 neurons in the mPFC (Fig. 4). These observations, especially the contrasting observation of pCREB–/ pERK1 and pCREB1/pERK– neurons, suggest that the activation of CREB and ERK is regulated differently in each brain area when memory is extinguished and that the late phase activation of ERK plays specific and distinct roles for the extinction of fear memory compared with other memory processes such as consolidation and reconsolidation as discussed below. Interestingly, we observed that pERK1 neurons were more abundant in the mPFC compared with the hippocampus and amygdala, since the mPFC showed a higher ratio of pERK1 neurons (extinction phase) and pCREB1 neurons (reconsolidation phase) compared with the hippocampus and amygdala, suggesting that the activation of ERK in the mPFC plays a more specific role in memory extinction.
It remains unclear whether the same or different populations of neurons are activated in the reconsolidation, transition, and extinction memory phases. A previous study identified the activation of “fear neurons” and “extinction neurons” in the amygdala when cued fear memory is reactivated or extinguished, respectively (Herry et al., 2008). Therefore, it is possible that ERK and CREB are activated in the different populations of neurons with different temporal profiles (i.e., “reconsolidation neurons” and extinction neurons). As discussed above, pCREB1 neurons, including pCREB1/pERK1 neurons, may regulate the reconsolidation and long-term extinction of IA memory through the activation of gene expression as reconsolidation and extinction neurons, respectively. Conversely, ERK activation in pCREB– /pERK1 neurons may contribute to the cancellation of CREB-mediated transcriptional activation that would be required for reconsolidation since this ERK activation is specifically observed in the late extinction phase; ERK has activated in the reconsolidation neurons to cancel the activation of gene expression in the extinction phase. Interestingly, a previous study showed that hippocampal ERK activation blocks the induction of c-fos when contextual fear memory is extinguished (Guedea et al., 2011), raising the possibility that this ERK activation antagonizes the CREB-signaling pathway. It is important to identify the neuronal populations that regulate reconsolidation, transition, and extinction and to investigate the molecular signatures and functional significance of those neurons. Additionally, interactions among neural populations identified in this study remain unknown. It is possible that “extinction (transition) neurons” modulate the function of reconsolidation neurons to cancel prevent reconsolidation through interactions between them (Eisenberg et al., 2003; Merlo et al., 2014). Therefore, it is also important to examine these interactions in and among the amygdala, mPFC, and hippocampus.
Previously, we showed that the hippocampus displays no change of CREB phosphorylation and Arc expression following extinction learning of contextual fear, and consistently, the inhibition of protein synthesis in the hippocampus in the extinction phase fails to block long-term extinction (Mamiya et al., 2009). These observations have raised the possibility that the hippocampus is not required for long-term extinction. However, we showed that ERK is activated in the hippocampus after the extinction learning of IA memory, and consistently, blocking ERK activation in the hippocampus impairs long-term extinction. Therefore, our current observations indicate essential roles for the hippocampus in memory extinction. Taken together with our previous findings, we suggest that the hippocampus is required for memory extinction but not for a consolidation-like process to stabilize an “extinction memory” through the activation of gene expression.
