Comparing behaviours induced by natural memory retrieval and optogenetic reactivation of an engram ensemble in mice.

Memories are thought to be stored within sparse collections of neurons known as engram ensembles. Neurons active during a training episode are allocated to an engram ensemble ('engram neurons'). Memory retrieval is initiated by external sensory or internal cues present at the time of training reactivating engram neurons. Interestingly, optogenetic reactivation of engram ensemble neurons alone in the absence of external sensory cues is sufficient to induce behaviour consistent with memory retrieval in mice. However, there may exist differences between the behaviours induced by natural retrieval cues or artificial engram reactivation. Here, we compared two defensive behaviours (freezing and the syllable structure of ultrasonic vocalizations, USVs) induced by sensory cues present at training (natural memory retrieval) and optogenetic engram ensemble reactivation (artificial memory retrieval) in a threat conditioning paradigm in the same mice. During natural memory recall, we observed a strong positive correlation between freezing levels and distinct USV syllable features (characterized by an unsupervised algorithm, MUPET (Mouse Ultrasonic Profile ExTraction)). Moreover, we observed strikingly similar behavioural profiles in terms of freezing and USV characteristics between natural memory recall and artificial memory recall in the absence of sensory retrieval cues. Although our analysis focused on two behavioural measures of threat memory (freezing and USV characteristics), these results underscore the similarities between threat memory recall triggered naturally and through optogenetic reactivation of engram ensembles. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Opto-extinction of a threat memory in mice.

Memories of past experiences guide future behaviour. Sparse ensembles of neurons, known as engrams, are thought to store memories in the brain. Neurons involved in a particular engram ("engram neurons") are necessary for subsequent memory expression as memory retrieval is thought to be initiated by an external sensory cue reactivating engram neurons. However, conditions or environments are dynamic, such that future behaviour should be flexible. The role of engrams in mediating flexible behaviour is not understood. Here we examined this question using one type of flexible behaviour, extinction of a threat response. An initially neutral tone is first paired with an aversive footshock such that the tone alone induces defensive freezing. After subsequent repeated tone presentations without the footshock, rodents no longer freeze to the tone. Because the tone cue is thought to reactivate the engram to induce memory retrieval, we examined whether it is possible to induce an extinction-like behavioural effect by optogenetically reactivating the lateral amygdala component of the engram alone (without tone re-exposure). Similar to tone-induced extinction, mice showed decreased freezing to optogenetic stimulation of the lateral amygdala engram in the "extinction training" session. Moreover, "opto-extinguished" mice showed decreased freezing to the tone when subsequently tested for retrieval of the extinction training in the same context, suggesting that the opto-extinction transferred to the actual sensory stimulus. However, unlike tone extinction, in which mice showed renewal of tone-induced freezing when tested in a novel context, opto-extinguished mice continued to show a deficit in tone-induced freezing. Extinction has been characterized as new learning that inhibits the original memory or a phenomenon in which the original memory is "unlearned". Our findings suggest that opto-extinction may silence the original engram to "unlearn" the original memory.

A time-dependent role for the transcription factor CREB in neuronal allocation to an engram underlying a fear memory revealed using a novel in vivo optogenetic tool to modulate CREB function.

The internal representation of an experience is thought to be encoded by long-lasting physical changes to the brain ("engrams") . Previously, we and others showed within the lateral amygdala (LA), a region critical for auditory conditioned fear, eligible neurons compete against one other for allocation to an engram. Neurons with relatively higher function of the transcription factor CREB were more likely to be allocated to the engram. In these studies, though, CREB function was artificially increased for several days before training. Precisely when increased CREB function is important for allocation remains an unanswered question. Here, we took advantage of a novel optogenetic tool (opto-DN-CREB) to gain spatial and temporal control of CREB function in freely behaving mice. We found increasing CREB function in a small, random population of LA principal neurons in the minutes, but not 24 h, before training was sufficient to enhance memory, likely because these neurons were preferentially allocated to the underlying engram. However, similarly increasing CREB activity in a small population of random LA neurons immediately after training disrupted subsequent memory retrieval, likely by disrupting the precise spatial and temporal patterns of offline post-training neuronal activity and/or function required for consolidation. These findings reveal the importance of the timing of CREB activity in regulating allocation and subsequent memory retrieval, and further, highlight the potential of optogenetic approaches to control protein function with temporal specificity in behaving animals.

Sound tuning of amygdala plasticity in auditory fear conditioning.

Various auditory tones have been used as conditioned stimuli (CS) for fear conditioning, but researchers have largely neglected the effect that different types of auditory tones may have on fear memory processing. Here, we report that at lateral amygdala (LA) synapses (a storage site for fear memory), conditioning with different types of auditory CSs (2.8 kHz tone, white noise, FM tone) recruits distinct forms of long-term potentiation (LTP) and inserts calcium permeable AMPA receptor (CP-AMPAR) for variable periods. White noise or FM tone conditioning produced brief insertion (<6 hr after conditioning) of CP-AMPARs, whereas 2.8 kHz tone conditioning induced more persistent insertion (≥6 hr). Consistently, conditioned fear to 2.8 kHz tone but not to white noise or FM tones was erased by reconsolidation-update (which depends on the insertion of CP-AMPARs at LA synapses) when it was performed 6 hr after conditioning. Our data suggest that conditioning with different auditory CSs recruits distinct forms of LA synaptic plasticity, resulting in more malleable fear memory to some tones than to others.

Competition between engrams influences fear memory formation and recall.

Collections of cells called engrams are thought to represent memories. Although there has been progress in identifying and manipulating single engrams, little is known about how multiple engrams interact to influence memory. In lateral amygdala (LA), neurons with increased excitability during training outcompete their neighbors for allocation to an engram. We examined whether competition based on neuronal excitability also governs the interaction between engrams. Mice received two distinct fear conditioning events separated by different intervals. LA neuron excitability was optogenetically manipulated and revealed a transient competitive process that integrates memories for events occurring closely in time (coallocating overlapping populations of neurons to both engrams) and separates memories for events occurring at distal times (disallocating nonoverlapping populations to each engram).

Neuronal Allocation to a Hippocampal Engram.

The dentate gyrus (DG) is important for encoding contextual memories, but little is known about how a population of DG neurons comes to encode and support a particular memory. One possibility is that recruitment into an engram depends on a neuron's excitability. Here, we manipulated excitability by overexpressing CREB in a random population of DG neurons and examined whether this biased their recruitment to an engram supporting a contextual fear memory. To directly assess whether neurons overexpressing CREB at the time of training became critical components of the engram, we examined memory expression while the activity of these neurons was silenced. Chemogenetically (hM4Di, an inhibitory DREADD receptor) or optogenetically (iC++, a light-activated chloride channel) silencing the small number of CREB-overexpressing DG neurons attenuated memory expression, whereas silencing a similar number of random neurons not overexpressing CREB at the time of training did not. As post-encoding reactivation of the activity patterns present during initial experience is thought to be important in memory consolidation, we investigated whether post-training silencing of neurons allocated to an engram disrupted subsequent memory expression. We found that silencing neurons 5 min (but not 24 h) following training disrupted memory expression. Together these results indicate that the rules of neuronal allocation to an engram originally described in the lateral amygdala are followed in different brain regions including DG, and moreover, that disrupting the post-training activity pattern of these neurons prevents memory consolidation.

Structural foundations of optogenetics: Determinants of channelrhodopsin ion selectivity.

The structure-guided design of chloride-conducting channelrhodopsins has illuminated mechanisms underlying ion selectivity of this remarkable family of light-activated ion channels. The first generation of chloride-conducting channelrhodopsins, guided in part by development of a structure-informed electrostatic model for pore selectivity, included both the introduction of amino acids with positively charged side chains into the ion conduction pathway and the removal of residues hypothesized to support negatively charged binding sites for cations. Engineered channels indeed became chloride selective, reversing near -65 mV and enabling a new kind of optogenetic inhibition; however, these first-generation chloride-conducting channels displayed small photocurrents and were not tested for optogenetic inhibition of behavior. Here we report the validation and further development of the channelrhodopsin pore model via crystal structure-guided engineering of next-generation light-activated chloride channels (iC++) and a bistable variant (SwiChR++) with net photocurrents increased more than 15-fold under physiological conditions, reversal potential further decreased by another ∼ 15 mV, inhibition of spiking faithfully tracking chloride gradients and intrinsic cell properties, strong expression in vivo, and the initial microbial opsin channel-inhibitor-based control of freely moving behavior. We further show that inhibition by light-gated chloride channels is mediated mainly by shunting effects, which exert optogenetic control much more efficiently than the hyperpolarization induced by light-activated chloride pumps. The design and functional features of these next-generation chloride-conducting channelrhodopsins provide both chronic and acute timescale tools for reversible optogenetic inhibition, confirm fundamental predictions of the ion selectivity model, and further elucidate electrostatic and steric structure-function relationships of the light-gated pore.

mGluR2/3 in the Lateral Amygdala is Required for Fear Extinction: Cortical Input Synapses onto the Lateral Amygdala as a Target Site of the mGluR2/3 Action.

Various subtypes of metabotropic glutamate receptors (mGluRs) have been implicated in fear extinction, but mGluR2/3 subtype has not been tested. Here, we found that microinjection of an mGluR2/3 antagonist, LY341495, into the lateral amygdala (LA), but not into the adjacent central amygdala (CeA), impaired extinction retention without affecting within-session extinction. In contrast, we failed to detect any significant changes in motility and anxiety during a period when extinction training or retention was performed after LY341495 injection, suggesting that the effect of LY341495 is specific to conditioned responses. Subsequently, on the basis of a previous finding that a long-term potentiation of presynaptic efficacy at cortical input synapses onto the lateral amygdala (C-LA synapses) supports conditioned fear, we tested the hypothesis that activation of mGluR2/3 leads to fear extinction via a long-term weakening of presynaptic functions at C-LA synapses. Fear extinction produced a decrease in C-LA synaptic efficacy, whereas LY341495 infusion into the LA blocked this extinction-induced C-LA efficacy decrease without altering synaptic efficacy at other LA synapses. Furthermore, extinction enhanced paired pulse ratio (PPR) of EPSCs, which inversely correlates with presynaptic release probability, whereas LY341495 infusion into the LA attenuated the extinction-induced increase in PPR, suggesting the presence of mGluR2/3-dependent presynaptic changes after extinction. Consistently, extinction occluded a presynaptic form of depression at C-LA synapses, whereas the LY341495 infusion into the LA rescued this occlusion. Together, our findings suggest that mGluR2/3 is required for extinction retention and that the mGluR2/3 action is mediated by the long-term weakening of release probability at C-LA synapses.

The Effects of Combined Exercise on Health-Related Fitness, Endotoxin, and Immune Function of Postmenopausal Women with Abdominal Obesity.

This study was conducted to examine the effects of combined exercise on health-related fitness, endotoxin concentrations, and immune functions of postmenopausal women with abdominal obesity. 20 voluntary participants were recruited and they were randomly allocated to the combined exercise group (n = 10) or the control group (n = 10). Visceral obesity was defined as a visceral-to-subcutaneous fat ratio ≥ 0.4 based on computed tomography (CT) results. Body composition, exercise stress testing, fitness measurement, CT scan, and blood variables were analyzed to elucidate the effects of combined exercise. The SPSS Statistics 18.0 program was used to calculate means and standard deviations for all variables. Significant differences between the exercise group and control group were determined with 2-way ANOVA and paired t-tests. The exercise group's abdominal obesity was mitigated due to visceral fat reduction; grip strength, push-ups, and oxygen uptake per weight improved; and HDL-C and IgA level also increased, while TNF-α, CD14, and endotoxin levels decreased. Lowered TNF-α after exercise might have an important role in the obesity reduction. Therefore, we can conclude that combined exercise is effective in mitigating abdominal obesity, preventing metabolic diseases, and enhancing immune function.

Group I mGluR-dependent depotentiation in the lateral amygdala does not require the removal of calcium-permeable AMPA receptors.

There is conflicting evidence regarding whether calcium-permeable receptors are removed during group I mGluR-mediated synaptic depression. In support of this hypothesis, AMPAR rectification, a correlative index of the synaptic expression of GluA2-lacking calcium-permeable AMPARs (CP-AMPARs), is known to decrease after the induction of several types of group I mGluR-mediated long-term depression (LTD), suggesting that a significant proportion of synaptic CP-AMPARs is removed during synaptic depression. We have previously demonstrated that fear conditioning-induced synaptic potentiation in the lateral amygdala is reversed by group 1 mGluR-mediated depotentiation. Here, we examined whether CP-AMPARs are removed by mGluR1-mediated depotentiation of fear conditioning-induced synaptic potentiation. The synaptic expression of CP-AMPARs was negligible before, increased significantly 12 h after, and returned to baseline 48 h after fear conditioning, as evidenced by the changes in the sensitivity of lateral amygdala synaptic responses to NASPM. Importantly, the sensitivity to NASPM was not altered after induction of depotentiation. Our findings, together with previous results, suggest that the removal of CP-AMPARs is not required for the depotentiation of fear conditioning-induced synaptic potentiation at lateral amygdala synapses.

ABA renewal involves enhancements in both GluA2-lacking AMPA receptor activity and GluA1 phosphorylation in the lateral amygdala.

Fear renewal, the context-specific relapse of fear following fear extinction, is a leading animal model of post-traumatic stress disorders (PTSD) and fear-related disorders. Although fear extinction can diminish fear responses, this effect is restricted to the context where the extinction is carried out, and the extinguished fear strongly relapses when assessed in the original acquisition context (ABA renewal) or in a context distinct from the conditioning and extinction contexts (ABC renewal). We have previously identified Ser831 phosphorylation of GluA1 subunit in the lateral amygdala (LA) as a key molecular mechanism for ABC renewal. However, molecular mechanisms underlying ABA renewal remain to be elucidated. Here, we found that both the excitatory synaptic efficacy and GluA2-lacking AMPAR activity at thalamic input synapses onto the LA (T-LA synapses) were enhanced upon ABA renewal. GluA2-lacking AMPAR activity was also increased during low-threshold potentiation, a potential cellular substrate of renewal, at T-LA synapses. The microinjection of 1-naphtylacetyl-spermine (NASPM), a selective blocker of GluA2-lacking AMPARs, into the LA attenuated ABA renewal, suggesting a critical role of GluA2-lacking AMPARs in ABA renewal. We also found that Ser831 phosphorylation of GluA1 in the LA was increased upon ABA renewal. We developed a short peptide mimicking the Ser831-containing C-tail region of GluA1, which can be phosphorylated upon renewal (GluA1S); thus, the phosphorylated GluA1S may compete with Ser831-phosphorylated GluA1. This GluA1S peptide blocked the low-threshold potentiation when dialyzed into a recorded neuron. The microinjection of a cell-permeable form of GluA1S peptide into the LA attenuated ABA renewal. In support of the GluA1S experiments, a GluA1D peptide (in which the serine at 831 is replaced with a phosphomimetic amino acid, aspartate) attenuated ABA renewal when microinjected into the LA. These findings suggest that enhancements in both the GluA2-lacking AMPAR activity and GluA1 phosphorylation at Ser831 are required for ABA renewal.

GluA1 phosphorylation at serine 831 in the lateral amygdala is required for fear renewal.

Fear renewal, a widely pursued model of post-traumatic stress disorder and phobias, refers to the context-specific relapse of conditioned fear after extinction. However, its molecular mechanisms are largely unknown. We found that renewal-inducing stimuli, generally believed to be insufficient to induce synaptic plasticity, enhanced excitatory synaptic strength, activity of synaptic GluA2-lacking AMPA receptors and Ser831 phosphorylation of synaptic surface GluA1 in the lateral nucleus of the amygdala (LAn) of fear-extinguished rats. Consistently, the induction threshold for LAn synaptic potentiation was considerably lowered after extinction, and renewal occluded this low-threshold potentiation. The low-threshold potentiation (a potential cellular substrate for renewal), but not long-term potentiation, was attenuated by dialysis into LAn neurons of a GluA1-derived peptide that competes with Ser831-phosphorylated GluA1. Microinjections of the same peptide into the LAn attenuated fear renewal, but not fear learning. Our findings suggest that GluA1 phosphorylation constitutes a promising target for clinical treatment of aberrant fear-related disorders.

Quantitative proteomics of auditory fear conditioning.

Auditory fear conditioning is a well-characterized rodent learning model where a neutral auditory cue is paired with an aversive outcome to induce associative fear memory. The storage of long-term auditory fear memory requires long-term potentiation (LTP) in the lateral amygdala and de novo protein synthesis. Although many studies focused on individual proteins have shown their contribution to LTP and fear conditioning, non-biased genome-wide studies have only recently been possible with microarrays, which nevertheless fall short of measuring changes at the level of proteins. Here we employed quantitative proteomics to examine the expression of hundreds of proteins in the lateral amygdala in response to auditory fear conditioning. We found that various proteins previously implicated in LTP, learning and axon/dendrite growth were regulated by fear conditioning. A substantial number of proteins that were regulated by fear conditioning have not yet been studied specifically in learning or synaptic plasticity.

Ex vivo depotentiation of conditioning-induced potentiation at thalamic input synapses onto the lateral amygdala requires GluN2B-containing NMDA receptors.

We have previously characterized the ex vivo depotentiation (depotentiation(ex vivo)) of conditioning-induced synaptic potentiation at thalamic input synapses onto the lateral amygdala (T-LA synapses) as a potential cellular substrate for fear extinction: both depotentiation(ex vivo) and fear extinction require NMDA receptors, mitogen-activated protein kinases, metabotropic glutamate receptor 1, de novo protein synthesis and AMPA receptor internalization in the amygdala. Surprisingly, as shown in our and other previous studies, ifenprodil, an antagonist of GluN2B-containing NMDA receptors, fails to inhibit depotentiation(ex vivo) at a saturating concentration (10µM), although it has been suggested that GluN2B-containing NMDA receptors are required for fear extinction. Because ifenprodil is also known to act on other molecular targets in addition to GluN2B-containing NMDA receptors, especially at high concentrations (i.e., ≥10µM), the ineffectiveness of 10µM of ifenprodil may be due to its side effects. Therefore, in the present study, we tested Ro25-6981, a more specific antagonist of GluN2B-containing NMDA receptors, and a lower concentration (3µM) of ifenprodil, which may reduce any possible side effects. Ro25-6981 (3µM) blocked both depotentiation(ex vivo) and late-phase long-term potentiation at T-LA synapses. While 10µM ifenprodil failed to inhibit depotentiation(ex vivo), a lower concentration (3µM) of ifenprodil blocked depotentiation(ex vivo). Together, our findings suggest that depotentiation(ex vivo) requires GluN2B-containing NMDA receptors.

Reversible plasticity of fear memory-encoding amygdala synaptic circuits even after fear memory consolidation.

It is generally believed that after memory consolidation, memory-encoding synaptic circuits are persistently modified and become less plastic. This, however, may hinder the remaining capacity of information storage in a given neural circuit. Here we consider the hypothesis that memory-encoding synaptic circuits still retain reversible plasticity even after memory consolidation. To test this, we employed a protocol of auditory fear conditioning which recruited the vast majority of the thalamic input synaptic circuit to the lateral amygdala (T-LA synaptic circuit; a storage site for fear memory) with fear conditioning-induced synaptic plasticity. Subsequently the fear memory-encoding synaptic circuits were challenged with fear extinction and re-conditioning to determine whether these circuits exhibit reversible plasticity. We found that fear memory-encoding T-LA synaptic circuit exhibited dynamic efficacy changes in tight correlation with fear memory strength even after fear memory consolidation. Initial conditioning or re-conditioning brought T-LA synaptic circuit near the ceiling of their modification range (occluding LTP and enhancing depotentiation in brain slices prepared from conditioned or re-conditioned rats), while extinction reversed this change (reinstating LTP and occluding depotentiation in brain slices prepared from extinguished rats). Consistently, fear conditioning-induced synaptic potentiation at T-LA synapses was functionally reversed by extinction and reinstated by subsequent re-conditioning. These results suggest reversible plasticity of fear memory-encoding circuits even after fear memory consolidation. This reversible plasticity of memory-encoding synapses may be involved in updating the contents of original memory even after memory consolidation.

Modulation of fear memory by retrieval and extinction: a clue for memory deconsolidation.

Memories are fragile and easily forgotten at first, but after a consolidation period of hours to weeks, are inscribed in our brains as stable traces, no longer vulnerable to conventional amnesic treatments. Retrieval of a memory renders it labile, akin to the early stages of consolidation. This phenomenon has been explored as memory reactivation, in the sense that the memory is temporarily 'deconsolidated', allowing a short time window for amnesic intervention. This window closes again after reconsolidation, which restores the stability of the memory. In contrast to this 'transient deconsolidation' and the short-spanned amnesic effects of consolidation blockers, some specific treatments can disrupt even consolidated memory, leading to apparent amnesia. We propose the term 'amnesic deconsolidation' to describe such processes that lead to disruption of consolidated memory and/or consolidated memory traces. We review studies of these 'amnesic deconsolidation' treatments that enhance memory extinction, alleviate relapse, and reverse learning-induced plasticity. The transient deconsolidation that memory retrieval induces and the amnesic deconsolidation that these regimes induce both seem to dislodge a component that stabilizes consolidated memory. Characterizing this component, at both molecular and network levels, will provide a key to developing clinical treatments for memory-related disorders and to defining the consolidated memory trace.

Reactivation of fear memory renders consolidated amygdala synapses labile.

It is believed that memory reactivation transiently renders consolidated memory labile and that this labile or deconsolidated memory is reconsolidated in a protein synthesis-dependent manner. The synaptic correlate of memory deconsolidation upon reactivation, however, has not been fully characterized. Here, we show that 3,5-dihydroxyphenylglycine (DHPG), an agonist for group I metabotropic glutamate receptors (mGluRI), induces synaptic depotentiation only at thalamic input synapses onto the lateral amygdala (T-LA synapses) where synaptic potentiation is consolidated, but not at synapses where synaptic potentiation is not consolidated. Using this mGluRI-induced synaptic depotentiation (mGluRI-depotentiation) as a marker of consolidated synapses, we found that mGluRI-depotentiation correlated well with the state of memory deconsolidation and reconsolidation in a predictable manner. DHPG failed to induce mGluRI-depotentiation in slices prepared immediately after reactivation when the reactivated memory was deconsolidated. DHPG induced mGluRI-depotentiation 1 h after reactivation when the reactivated memory was reconsolidated, but it failed to do so when reconsolidation was blocked by a protein synthesis inhibitor. To test the memory-specificity of mGluRI-depotentiation, conditioned fear was acquired twice using two discriminative tones (2.8 and 20 kHz). Under this condition, mGluRI-depotentiation was fully impaired in slices prepared immediately after reactivation with both tones, whereas mGluRI-depotentiation was partially impaired immediately after reactivation with the 20 kHz tone. Consistently, microinjection of DHPG into the LA 1 h after reactivation reduced fear memory retention, whereas DHPG injection immediately after reactivation failed to do so. Our findings suggest that, upon memory reactivation, consolidated T-LA synapses enter a temporary labile state, displaying insensitivity to mGluRI-depotentiation.

Amygdala depotentiation ex vivo requires mitogen-activated protein kinases and protein synthesis.

We have recently characterized a form of ex vivo depotentiation (depotentiationex vivo), which correlates tightly with fear extinction, at thalamic input synapses onto the lateral amygdala. Here, we examined the effects of learning-attenuating drugs, reported to impair fear extinction when microinjected into the basolateral amygdala, on depotentiationex vivo. U0126, a mitogen-activated protein kinase inhibitor, and cycloheximide, a protein synthesis inhibitor, blocked depotentiationex vivo. However, ifenprodil, an NR2B-containing NMDA receptor inhibitor, did not alter depotentiationex vivo, although it blocked amygdala long-term potentiation. These findings indicate that amygdala depotentiation shares some molecular processes with learning and further suggest that different forms of synaptic plasticity in the basolateral amygdala mediate fear extinction.