Factors governing one-trial contextual conditioning

Influence of footshock number and intensity on the behavioral and endocrine response to fear conditioning and cognitive fear generalization in male rats

Foot-shock paradigms have provided valuable insights into the neurobiology of stress and fear conditioning. An extensive body of literature indicates that shock exposure can elicit both conditioned and unconditioned effects, although delineating between the two is a challenging task. This distinction holds crucial implications not only for the theoretical interpretation of fear conditioning, but also for properly evaluating putative preclinical models of post-traumatic stress disorder (PTSD) involving shock exposure. The characteristics of shocks (intensity and number) affect the strength of learning, but how these characteristics interact to influence conditioned and unconditioned consequences of shocks are poorly known. In this study, we aimed to investigate in adult male rats the impact of varying shock number and intensity on the endocrine and behavioral response to contextual fear conditioning and fear generalization to a novel environment markedly distinct from the shock context (i.e., fear generalization). Classical biological markers of stress (i.e., ACTH, corticosterone, and prolactin) were sensitive to manipulations of shock parameters, whereas these parameters had a limited effect on contextual fear conditioning (evaluated by freezing and distance traveled). In contrast, behavior in different novel contexts (fear generalization) was specifically sensitive to shock intensity. Notably, altered behavior in novel contexts markedly improved, but not completely normalized after fear extinction, hypoactivity apparently being the result of both conditioned and unconditioned effects of foot-shock exposure. The present results will contribute to a better understanding of shock exposure as a putative animal model of PTSD.

Long-term d-allose administration ameliorates age-related cognitive impairment and loss of bone strength in male mice

Age-related physical and cognitive decline may be ameliorated by consuming functional foods. d-Allose, reported to have multiple health benefits, may temper aging phenotypes, particularly brain function. We investigated whether d-allose supplementation improves cognitive function. A standard battery of behavioral tests was administered to 18-month-old male mice after consuming diet containing 3 % d-allose for 6 months. Following a wire-hanging test, an open-field test, Morris water maze, fear-conditioning, and an analgesia test were sequentially performed. Bone density and strength were assessed afterwards. Possible mechanism(s) under-lying memory changes in hippocampus were also examined with a DNA microarray. d-Allose failed to influence muscle strength, locomotor activity and anxiety, fear memory, or pain sensitivity. However, d-allose improved hippocampus-dependent spatial learning and memory, and it may contribute to increase bone strength. d-Allose also changed the expression of some genes in hippocampus involved in cognitive functions. Long-term d-allose supplementation appears to modestly change aging phenotypes and improve spatial memory.

Looming stimuli reliably drive innate defensive responses in male rats, but not learned defensive responses

Survival relies on an organism's intrinsic ability to instinctively react to stimuli such as food, water, and threats, ensuring the fundamental ability to feed, drink, and avoid danger even in the absence of prior experience. These natural, unconditioned stimuli can also facilitate associative learning, where pairing them consistently with neutral cues will elicit responses to these cues. Threat conditioning, a well-explored form of associative learning, commonly employs painful electric shocks, mimicking injury, as unconditioned stimuli. It remains elusive whether actual injury or pain is necessary for effective learning, or whether the threat of harm is sufficient. Approaching predators create looming shadows and sounds, triggering strong innate defensive responses like escape and freezing. This study investigates whether visual looming stimuli can induce learned freezing or learned escape responses to a conditioned stimulus in male rats. Surprisingly, pairing a neutral tone with a looming stimulus only weakly evokes learned defensive responses, in contrast to the strong responses observed when the looming stimulus is replaced by a shock. This dissociation sheds light on the boundaries for learned defensive responses thereby impacting our comprehension of learning processes and defensive strategies.

Cue-potentiated feeding in rodents: Implications for weight regulation in obesogenic environments

Cue-potentiated feeding (CPF) describes instances where food intake is increased by exposure to conditioned cues associated with food, often in the absence of hunger. CPF effects have been reported in a range of experimental protocols developed by researchers working across diverse fields spanning behavioural neuroscience, social psychology and ecology. Here we review the evolution of research on cue-potentiated feeding in animal models to identify important behavioural parameters and key neural circuits and pharmacological systems underlying the effect. Overall, evidence indicates that social, discrete and contextual stimuli can be used to elicit CPF effects across multiple species, though effects are often subtle and sensitive to procedural variables. While regular exposure to food cues is thought to be a key risk factor for overeating in so-called 'obesogenic' environments, further work is needed to identify whether CPF promotes positive energy balance and weight gain over the longer term. We suggest several methodological and conceptual areas for inquiry to elucidate the contribution of CPF to the regulation of food choice and energy intake.

Hypersensitivity of the nicotinic acetylcholine receptor subunit (CHRNA2L9'S/L9'S) in female adolescent mice produces deficits in nicotine-induced facilitation of hippocampal-dependent learning and memory

Adolescence is characterized by a critical period of maturation and growth, during which regions of the brain are vulnerable to long-lasting cognitive disturbances. Adolescent exposure to nicotine can lead to deleterious neurological and psychological outcomes. Moreover, the nicotinic acetylcholine receptor (nAChR) has been shown to play a functionally distinct role in the development of the adolescent brain. CHRNA2 encodes for the α2 subunit of nicotinic acetylcholine receptors associated with CA1 oriens lacunosum moleculare GABAergic interneurons and is associated with learning and memory. Previously, we found that adolescent male hypersensitive CHRNA2L9'S/L9' mice had impairments in learning and memory during a pre-exposure-dependent contextual fear conditioning task that could be rescued by low-dose nicotine exposure. In this study, we assessed learning and memory in female adolescent hypersensitive CHRNA2L9'S/L9' mice exposed to saline or a subthreshold dose of nicotine using a hippocampus-dependent task of pre-exposure-dependent contextual fear conditioning. We found that nicotine-treated wild-type female mice had significantly greater improvements in learning and memory than both saline-treated wild-type mice and nicotine-treated CHRNA2L9'S/L9' female mice. Thus, hyperexcitability of CHRNA2 in female adolescent mice ablated the nicotine-mediated potentiation of learning and memory seen in wild-types. Our results indicate that nicotine exposure during adolescence mediates sexually dimorphic patterns of learning and memory, with wild-type female adolescents being more susceptible to the effects of sub-threshold nicotine exposure. To understand the mechanism underlying sexually dimorphic behavior between hyperexcitable CHRNA2 mice, it is critical that further research be conducted.

Sex and the facilitation of cued fear by prior contextual fear conditioning in rats

Previous studies have shown that the formation of new memories can be influenced by prior experience. This includes work using Pavlovian fear conditioning in rodents that has shown that an initial fear conditioning experience can become associated with and facilitate the acquisition of new fear memories, especially when they occur close together in time. However, most of the prior studies used only males as subjects, resulting in questions about the generalizability of the findings from this work. Here we tested whether prior contextual fear conditioning would facilitate later learning of cued fear conditioning in both male and female rats, and if there were differences based on the interval between the two conditioning episodes. Our results showed that levels of cued fear were not influenced by prior contextual fear conditioning or by the interval between training; however, females showed lower levels of cued fear. Freezing behavior in the initial training context differed by sex, with females showing lower levels of contextual fear, and by the type of initial training, with rats given delayed shock showing higher levels of fear than rats given immediate shock during contextual fear conditioning. These results indicate that contextual fear conditioning does not prime subsequent cued fear conditioning and that female rats express lower levels of cued and contextual fear conditioning than males.

17α-Ethinylestradiol exposure disrupts anxiety-like behaviours but not social preference in sea bass larvae (Dicentrarchus labrax)

Endocrine-disrupting chemicals (EDCs) are widespread pollutants known to interfere with hormonal pathways and to disrupt behaviours. Standardised behavioural procedures have been developed in common fish model species to assess the impact of various pollutants on behaviours such as locomotor activity and anxiety-like as well as social behaviours. These procedures need now to be adapted to improve our knowledge on the behavioural effects of EDCs on less studied marine species. In this context, the European sea bass (Dicentrarchus labrax) is emerging as a valuable species representative of the European marine environment. Here, we designed and validated a two-step procedure allowing to sequentially assess anxiety-like behaviours (novel tank test) and social preference (visual social preference test) in sea bass. Thereafter, using this procedure, we evaluated whether social behavioural disruption occurs in 2-month-old larvae after an 8-day exposure to a xenoestrogen, the 17α-ethinylestradiol (EE2 at 0.5 and 50 nM). Our results confirmed previous studies showing that exposure to 50 nM of EE2 induces a significant increase in anxiety-like behaviours in sea bass larvae. On the contrary, social preference seemed unaffected whatever the EE2 concentration, suggesting that social behaviour has more complex mechanical regulations than anxiety.

Defensive behaviors and c-fos expression in the midbrain

Pavlovian fear conditioning serves as a valuable method for investigating species-specific defensive reactions (SSDRs) such as freezing and flight responses. The present study examines the role of white noise under different experimental conditions. Given that white noise has been shown to elicit both conditional (associative) and unconditional (nonassociative) defensive responses, we compared the response to noise following three separate training conditions: shock-only, white noise paired with shock, and context-only. Results showed that baseline freezing level significantly changed across groups: Both the shock-only group and the white noise paired with shock group froze more than the context-only group on the test day. White noise evoked differential freezing between groups on day 2: The shock-only group froze more than the context-only group although both groups were never exposed to white noise during training. Further, an activity burst triggered by white noise was similar for the shock-only and white noise paired with shock groups during testing, although shock-only group was never exposed to white noise stimuli during training. This aligned with c-fos data, indicating similar c-fos activity levels across different periaqueductal gray (PAG) regions for both shock-only and white noise paired with shock groups. However, the driving force behind c-fos activation-whether freezing, activity burst, or a combination of both-remains uncertain, warranting further analysis to explore specific correlations between SSDRs and c-fos activity within the PAG and related brain areas.

Pavlovian or associative sensitization and its biological significance

Pavlovian conditioning is typically distinguished from sensitization but a Pavlovian conditional stimulus (CS) also results in sensitization. A Pavlovian CS can sensitize responding to a probe stimulus that is related to the unconditional stimulus (US) or to the US itself. Pavlovian sensitization has been studied in the defensive, sexual, and feeding systems. In Pavlovian sensitization, the focus is not on a conditional response (CR) directly elicited by the CS but on the response mode that is activated by the CS. Activation of a response mode increases the probability of particular responses and also increases reactivity to various stimuli. Pavlovian sensitization reflects this increased stimulus reactivity. Pavlovian sensitization helps uncover successful learning in situations where a conventional CR does not occur. Pavlovian sensitization also encourages broadening our conceptions of Pavlovian conditioning to include changes in afferent processes. Implications for biological fitness and for basic and translational research are discussed.

Evaluating functions of praise for children diagnosed with autism spectrum disorder

We assessed whether novel praise statements could be used to (a) maintain and increase responses with existing reinforcement histories and (b) teach a previously untaught response among children diagnosed with autism spectrum disorder across two experiments. During response-stimulus pairing, two responses resulted in preferred edibles but only one also produced a praise statement. In the absence of edibles, the response continuing to produce praise tended to persist more. Next, reversing the praise contingency tended to increase the other response. However, in no case did contingent delivery of those same praise statements result in the acquisition of untaught responses. These findings suggest that conditioning praise statements could serve different functions (antecedent or consequence) depending on the reinforcement history for particular responses.

Sex and the facilitation of cued fear by prior contextual fear conditioning in rats

Previous studies have shown that the formation of new memories can be influenced by prior experience. This includes work using pavlovian fear conditioning in rodents that have shown that an initial fear conditioning experience can become associated with and facilitate the acquisition of new fear memories, especially when they occur close together in time. However, most of the prior studies used only males as subjects resulting in questions about the generalizability of the findings from this work. Here we tested whether prior contextual fear conditioning would facilitate later learning of cued fear conditioning in both male and female rats, and if there were differences based on the interval between the two conditioning episodes. Our results showed that levels of cued fear were not influenced by prior contextual fear conditioning or by the interval between training, however, females showed lower levels of cued fear. Freezing behavior in the initial training context differed by sex, with females showing lower levels of contextual fear, and by the type of initial training, with rats given delayed shock showing higher levels of fear than rats given immediate shock during contextual fear conditioning. These results indicate that contextual fear conditioning does not prime subsequent cued fear conditioning and that female rats express lower levels of cued and contextual fear conditioning than males.

Remote memory in a Bayesian model of context fear conditioning (BaconREM)

Here, we propose a model of remote memory (BaconREM), which is an extension of a previously published Bayesian model of context fear learning (BACON) that accounts for many aspects of recently learned context fear. BaconREM simulates most known phenomenology of remote context fear as studied in rodents and makes new predictions. In particular, it predicts the well-known observation that fear that was conditioned to a recently encoded context becomes hippocampus-independent and shows much-enhanced generalization ("hyper-generalization") when systems consolidation occurs (i.e., when memory becomes remote). However, the model also predicts that there should be circumstances under which the generalizability of remote fear may not increase or even decrease. It also predicts the established finding that a "reminder" exposure to a feared context can abolish hyper-generalization while at the same time making remote fear again hippocampus-dependent. This observation has in the past been taken to suggest that reminders facilitate access to detail memory that remains permanently in the hippocampus even after systems consolidation is complete. However, the present model simulates this result even though it totally moves all the contextual memory that it retains to the neo-cortex when context fear becomes remote.

The ventral hippocampus is activated in olfactory but not auditory threat memory

Hippocampal networks required for associative memory formation are involved in cue- and context-dependent threat conditioning. The hippocampus is functionally heterogeneous at its dorsal and ventral poles, and recent investigations have focused on the specific roles required from each sub-region for associative conditioning. Cumulative evidence suggests that contextual and emotional information is processed by the dorsal and ventral hippocampus, respectively. However, it is not well understood how these two divisions engage in threat conditioning with cues of different sensory modalities. Here, we compare the involvement of the dorsal and ventral hippocampus in two types of threat conditioning: olfactory and auditory. Our results suggest that the dorsal hippocampus encodes contextual information and is activated upon recall of an olfactory threat memory only if contextual cues are relevant to the threat. Overnight habituation to the context eliminates dorsal hippocampal activation, implying that this area does not directly support cue-dependent threat conditioning. The ventral hippocampus is activated upon recall of olfactory, but not auditory, threat memory regardless of habituation duration. Concurrent activation of the piriform cortex is consistent with its direct connection with the ventral hippocampus. Together, our study suggests a unique role of the ventral hippocampus in olfactory threat conditioning.

Hippocampal Engrams and Contextual Memory

Memories are not formed in a vacuum and often include rich details about the time and place in which events occur. Contextual stimuli promote the retrieval of events that have previously occurred in the encoding context and limit the retrieval of context-inappropriate information. Contexts that are associated with traumatic or harmful events both directly elicit fear and serve as reminders of aversive events associated with trauma. It has long been appreciated that the hippocampus is involved in contextual learning and memory and is central to contextual fear conditioning. However, little is known about the underlying neuronal mechanisms underlying the encoding and retrieval of contextual fear memories. Recent advancements in neuronal labeling methods, including activity-dependent tagging of cellular ensembles encoding memory ("engrams"), provide unique insight into the neural substrates of memory in the hippocampus. Moreover, these methods allow for the selective manipulation of memory ensembles. Attenuating or erasing fear memories may have considerable therapeutic value for patients with post-traumatic stress disorder or other trauma- or stressor-related conditions. In this chapter, we review the role of the hippocampus in contextual fear conditioning in rodents and explore recent work implicating hippocampal ensembles in the encoding and retrieval of aversive memories.

Synaptic and intrinsic plasticity within overlapping lateral amygdala ensembles following fear conditioning

Introduction

New learning results in modulation of intrinsic plasticity in the underlying brain regions. Such changes in intrinsic plasticity can influence allocation and encoding of future memories such that new memories encoded during the period of enhanced excitability are linked to the original memory. The temporal window during which the two memories interact depends upon the time course of intrinsic plasticity following new learning.

Methods

Using the well-characterized lateral amygdala-dependent auditory fear conditioning as a behavioral paradigm, we investigated the time course of changes in intrinsic excitability within lateral amygdala neurons.

Results

We found transient changes in the intrinsic excitability of amygdala neurons. Neuronal excitability was increased immediately following fear conditioning and persisted for up to 4 days post-learning but was back to naïve levels 10 days following fear conditioning. We also determined the relationship between learning-induced intrinsic and synaptic plasticity. Synaptic plasticity following fear conditioning was evident for up to 24 h but not 4 days later. Importantly, we demonstrated that the enhanced neuronal intrinsic excitability was evident in many of the same neurons that had undergone synaptic plasticity immediately following fear conditioning. Interestingly, such a correlation between synaptic and intrinsic plasticity following fear conditioning was no longer present 24 h post-learning.

Discussion

These data demonstrate that intrinsic and synaptic changes following fear conditioning are transient and co-localized to the same neurons. Since intrinsic plasticity following fear conditioning is an important determinant for the allocation and consolidation of future amygdala-dependent memories, these findings establish a time course during which fear memories may influence each other.

Functional network of contextual and temporal memory has increased amygdala centrality and connectivity with the retrosplenial cortex, thalamus, and hippocampus

In fear conditioning with time intervals between the conditioned (CS) and unconditioned (US) stimuli, a neural representation of the CS must be maintained over time to be associated with the later US. Usually, temporal associations are studied by investigating individual brain regions. It remains unknown, however, the effect of the interval at the network level, uncovering functional connections cooperating for the CS transient memory and its fear association. We investigated the functional network supporting temporal associations using a task in which a 5-s interval separates the contextual CS from the US (CFC-5s). We quantified c-Fos expression in forty-nine brain regions of male rats following the CFC-5s training, used c-Fos correlations to generate functional networks, and analyzed them by graph theory. Control groups were trained in contextual fear conditioning, in which CS and US overlap. The CFC-5s training additionally activated subdivisions of the basolateral, lateral, and medial amygdala; prelimbic, infralimbic, perirhinal, postrhinal, and intermediate entorhinal cortices; ventral CA1 and subiculum. The CFC-5s network had increased amygdala centrality and higher amygdala internal and external connectivity with the retrosplenial cortex, thalamus, and hippocampus. Amygdala and thalamic nuclei were network hubs. Functional connectivity among these brain regions could support CS transient memories and their association.

Estrous cycle state-dependent renewal of appetitive behavior recruits unique patterns of Arc mRNA in female rats

Introduction

Renewal is a behavioral phenomenon wherein extinction learning fails to generalize between different contextual environments, thereby representing a significant challenge to extinction-based rehabilitative therapies. Previously, we have shown that renewal of extinguished appetitive behavior differs across the estrous cycle of the female rat. In this experiment that effect is replicated and extended upon to understand how the estrous cycle may modulate contextual representation at the neuronal population level to drive renewal.

Methods

Estrous cycle stage [i.e., proestrus (P, high hormone) or metestrus/diestrus (M/D, low hormone)] was considered during two important learning and behavioral expression windows: at extinction training and during long-term memory (LTM)/renewal testing. Cellular compartment analysis of temporal activity using fluorescence in situ hybridization (catFISH) for Arc mRNA was conducted after the distinct context-stimulus exposures.

Results

Rats in P during context-dependent extinction training but in a different stage of the estrous cycle during LTM and renewal testing (P-different) were shown to exhibit more renewal of conditioned foodcup (but not conditioned orienting) behavior compared to rats in other estrous cycle groups. Importantly, we discovered this depends on the order of tests. P-different rats showed differential Arc mRNA expression in regions of the prefrontal cortex (PFC), amygdala, and hippocampus (HPC). For each case P-different rats had more co-expression (i.e., expression of both nuclear and cytoplasmic) of Arc mRNA compared to other groups; specific to the dorsal HPC, P-different rats also had a more robust Arc mRNA response to the extinction context exposure.

Conclusion

These data suggest female rats show estrous cycle state-dependent renewal of appetitive behavior, and differences in context and conditioned stimulus representation at the neuronal level may drive this effect.

Social buffering reduces fear expression in Wistar rats when tested in pairs, but not when retested alone

Social buffering is a phenomenon in which the stress response of an individual can be reduced by the presence of another individual. However, little is known about the effect of social buffering on aversive after memory extinction, especially when animals are tested alone afterwards. The aim of this study was to verify the social buffering effect in rats during the extinction session of the contextual fear conditioning model and the fear response when animals are tested alone in the following day. Animals were divided into subjects and associates, with the subjects undergoing the fear conditioning protocol and the associates paired with the subjects during the fear extinction session. Across five different experiments, we tested moderate and high intensity contextual fear conditioning protocols, as well four variations of pairs: (i) two conditioned subjects, (ii) a conditioned subject and a non-conditioned associate, (iii) a conditioned subject and an associate who observed the conditioning of the partner and (iv) two conditioned subjects, with one treated with diazepam. The social buffering effect was found efficient to reduce the fear memory expression during the fear extinction session. In the moderate intensity protocol, the reduction in freezing time occurred only in subjects accompanied by non-conditioned associates and observer associates. In the high intensity protocol, the social buffering effect occurred in subjects accompanied by either conditioned or non-conditioned associates, although the effect was more evident in the presence of non-conditioned subjects. Treatment of the conditioned associates with diazepam did not improve the social buffering effect. Moreover, social buffering effects were not correlated with self-grooming or prosocial behaviors, which indicates that the presence of another animal might decrease freezing by promotion of exploratory activity. Finally, the social buffering effect was not observed in the extinction test, either because the extinction was too effective in the moderate intensity protocol or because the extinction was equally ineffective in the high intensity protocol. Our results suggest that social buffering does not improve fear extinction consolidation.

Contextual fear conditioning with a time interval induces CREB phosphorylation in the dorsal hippocampus and amygdala nuclei that depend on prelimbic cortex activity

In temporal associations, a conditioned stimulus (CS) is separated by a time interval from the unconditioned stimulus (US), which activates the prelimbic cortex (PL) to maintain a CS representation over time. However, it is unknown whether the PL participates, besides the encoding, in the memory consolidation, and thus directly, with activity-dependent changes or indirectly, by modulation of activity-dependent changes in other brain regions. We investigated brain regions supporting the consolidation of associations with intervals and the influence of PL activity in this consolidation process. For this, we observed in Wistar rats the effect of pre-training PL inactivation by muscimol in CREB (cAMP response element-binding protein) phosphorylation, which is essential for memory consolidation, in subdivisions of the medial prefrontal cortex (mPFC), hippocampus, and amygdala 3 h after the training in the contextual fear conditioning (CFC) or CFC with 5-s interval (CFC-5s), fear associations without or with an interval between the CS and US, respectively. Both the CFC-5s and CFC training increased phosphorylation of CREB in the PL and infralimbic cortex (IL); lateral (LA) and basolateral (BLA) amygdala; dorsal CA1 (dCA1); dorsal (dDG), and ventral dentate gyrus, and the CFC-5s training in the central amygdala (CEA). PL activity was necessary for the CREB phosphorylation in the PL, BLA, CEA, dCA1, and dDG only in animals trained in the CFC-5s. The cingulate cortex, ventral CA1, and ventral subiculum did not have learning-induced phosphorylation of CREB. These results suggest that the mPFC, hippocampus, and amygdala support the consolidation of associations with or without intervals and that PL activity influences consolidation in the dorsal hippocampus and amygdala in temporal associations. Thereby, the PL contributes directly and indirectly by modulation to memory consolidation. The time interval engaged the PL early in recent memory consolidation. Results expanded PL's role beyond the time interval and remote memory consolidation.

Genetic and environmental influences on one-trial conditioned context aversion in mice

Anticipatory nausea (AN) is caused by an association between contextual cues and the experience of nausea (the side effects of chemotherapy or radiation treatment) and it develops predominantly in female patients undergoing chemotherapy. Preclinical studies in rodents show that the administration of an illness-inducing agent in the presence of novel contextual cues can cause conditioned context aversion (CCA) and this has been proposed to model AN. The literature also suggests that brief pre-exposure to a novel context prior to shock delivery is critical in the development of contextual fear conditioning in rodents (a phenomenon known as Immediate Shock Deficit), but this has not been assessed in CCA. The aim of present study was to develop a CCA paradigm to assess this in outbred (CD1) and inbred (C57BL/6J) mice and evaluate potential sex differences. The results revealed that a single conditioning trial in which a distinctive context was paired with LiCl-induced illness was sufficient to elicit a conditioned response in both female and male CD1 outbred mice, but not in C57BL/6J inbred mice. In addition, CCA was facilitated when animals had prior experience with the context. Finally, outbred female mice showed longer and more robust retention of CCA than male mice, which parallels clinical findings. The results indicate the importance of using CD1 outbred mice as an animal model of AN as well as examining sex differences in the CCA paradigm. Similar findings in humans encourage the future use of this novel CCA preclinical mouse model.

On the effect of social cue valence in contextual memory persistence

Social cues are valuable sensorial stimuli to the acquisition and retrieval of contextual memories. Here, we asked whether the valence of social cues would impact the formation of contextual memories. Adult male C57/BL6 mice were exposed to either conditioned place preference (CPP) or avoidance (CPA). As positive stimuli we used social interaction with a female (IF), while interaction with a male CD1 mice (IM) was used as negative stimulus. Contextual memory was tested 24 h and 7 days after conditioning. Aggressive behavior of CD1, as well as interaction with the female were quantified along the conditioning sessions. IM, but not IF, was salient enough to induce contextual memory estimated by the difference between the time in the conditioned context during test and habituation. Next, we chose two odors with innate behavioral responses and opposite valence to narrow down the sociability to one of its sensorial sources of information - the olfaction. We used urine from females in proestrus (U) and 2,4,5-trimethyl thiazoline (TMT), a predator odor. TMT decreased and U increased the time in the conditioned context during the test performed 24 h and 7 days after conditioning. Taken together, our results suggest that contextual memories conditioned to social encounters are difficult to stablish in mice, specially the one with positive valence. On the other hand, using odors with ecological relevance is a promising strategy to study long-term contextual memories with opposite valences. Ultimately, the behavioral protocol proposed here offers the advantage of studying contextual memories with opposite valences using unconditioned stimulus from the same sensorial category such as olfaction.

Changes in Memory, Sedation, and Receptor Kinetics Imparted by the β2-N265M and β3-N265M GABAA Receptor Point Mutations

Point mutations in the β2 (N265S) and β3 (N265M) subunits of γ-amino butyric acid type A receptors (GABAARs) that render them insensitive to the general anesthetics etomidate and propofol have been used to link modulation of β2-GABAARs to sedation and β3-GABAARs to surgical immobility. These mutations also alter GABA sensitivity, and mice carrying the β3-N265M mutation have been reported to have impaired baseline memory. Here, we tested the effects of the β2-N265M and β3-N265M mutations on memory, movement, hotplate sensitivity, anxiety, etomidate-induced sedation, and intrinsic kinetics. We found that both β2-N265M and β3-N265M mice exhibited baseline deficits in the Context Preexposure Facilitation Effect learning paradigm. Exploratory activity was slightly greater in β2-N265M mice, but there were no changes in either genotype in anxiety or hotplate sensitivity. β2-N265M mice were highly resistant to etomidate-induced sedation, and heterozygous mice were partially resistant. In rapid solution exchange experiments, both mutations accelerated deactivation two- to three-fold compared to wild type receptors and prevented modulation by etomidate. This degree of change in the receptor deactivation rate is comparable to that produced by an amnestic dose of etomidate but in the opposite direction, indicating that intrinsic characteristics of GABAARs are optimally tuned under baseline conditions to support mnemonic function.

Selective TLR4 Antagonism Prevents and Reverses Morphine-Induced Persistent Postoperative Cognitive Dysfunction, Dysregulation of Synaptic Elements, and Impaired BDNF Signaling in Aged Male Rats

Perioperative neurocognitive disorders (PNDs) are characterized by confusion, difficulty with executive function, and episodic memory impairment in the hours to months following a surgical procedure. Postoperative cognitive dysfunction (POCD) represents such impairments that last beyond 30 d postsurgery and is associated with increased risk of comorbidities, progression to dementia, and higher mortality. While it is clear that neuroinflammation plays a key role in PND development, what factors underlie shorter self-resolving versus persistent PNDs remains unclear. We have previously shown that postoperative morphine treatment extends POCD from 4 d (without morphine) to at least 8 weeks (with morphine) in aged male rats, and that this effect is likely dependent on the proinflammatory capabilities of morphine via activation of toll-like receptor 4 (TLR4). Here, we extend these findings to show that TLR4 blockade, using the selective TLR4 antagonist lipopolysaccharide from the bacterium Rhodobacter sphaeroides (LPS-RS Ultrapure), ameliorates morphine-induced POCD in aged male rats. Using either a single central preoperative treatment or a 1 week postoperative central treatment regimen, we demonstrate that TLR4 antagonism (1) prevents and reverses the long-term memory impairment associated with surgery and morphine treatment, (2) ameliorates morphine-induced dysregulation of the postsynaptic proteins postsynaptic density 95 and synaptopodin, (3) mitigates reductions in mature BDNF, and (4) prevents decreased activation of the BDNF receptor TrkB (tropomyosin-related kinase B), all at 4 weeks postsurgery. We also reveal that LPS-RS Ultrapure likely exerts its beneficial effects by preventing endogenous danger signal HMGB1 (high-mobility group box 1) from activating TLR4, rather than by blocking continuous activation by morphine or its metabolites. These findings suggest TLR4 as a promising therapeutic target to prevent or treat PNDs.SIGNIFICANCE STATEMENT With humans living longer than ever, it is crucial that we identify mechanisms that contribute to aging-related vulnerability to cognitive impairment. Here, we show that the innate immune receptor toll-like receptor 4 (TLR4) is a key mediator of cognitive dysfunction in aged rodents following surgery and postoperative morphine treatment. Inhibition of TLR4 both prevented and reversed surgery plus morphine-associated memory impairment, dysregulation of synaptic elements, and reduced BDNF signaling. Together, these findings implicate TLR4 in the development of postoperative cognitive dysfunction, providing mechanistic insight and novel therapeutic targets for the treatment of cognitive impairments following immune challenges such as surgery in older individuals.

Reactivity to conditioned threat cues is distinct from exploratory drive in the elevated plus maze

Fear and anxiety are adaptive states that allow humans and animals alike to respond appropriately to threatening cues in their environment. Commonly used tasks for studying behaviour akin to fear and anxiety in rodent models are Pavlovian threat conditioning and the elevated plus maze (EPM), respectively. In threat conditioning the rodents learn to associate an aversive event with a specific stimulus or context. The learnt association between the two stimuli (the 'memory') can then be recalled by re-exposing the subject to the conditioned stimulus. The elevated plus maze is argued to measure the agoraphobic avoidance of the brightly lit open maze arms in crepuscular rodents. These two tasks have been used extensively, yet research into whether they interact is scarce. We investigated whether recall of an aversive memory, across contextual, odour or auditory modalities, would potentiate anxiety-like behaviour in the elevated plus maze. The data did not support that memory recall, even over a series of time points, could influence EPM behaviour. Furthermore, there was no correlation between EPM behaviour and conditioned freezing in independent cohorts tested in the EPM before or after auditory threat conditioning. Further analysis found the production of 22 kHz ultrasonic vocalisations revealed the strongest responders to a conditioned threat cue. These results are of particular importance for consideration when using the EPM and threat conditioning to identify individual differences and the possibility to use the tasks in batteries of tests without cross-task interference.

Sex differences in fear responses: Neural circuits

Women have increased vulnerability to PTSD and anxiety disorders compared to men. Understanding the neurobiological underpinnings of these disorders is critical for identifying risk factors and developing appropriate sex-specific interventions. Despite the clear clinical relevance of an examination of sex differences in fear responses, the vast majority of pre-clinical research on fear learning and memory formation has exclusively used male animals. This review highlights sex differences in context and cued fear conditioning, fear extinction and fear generalization with a focus on the neural circuits underlying these behaviors in rodents. There are mixed reports of behavioral sex differences in context and cued fear conditioning paradigms, which can depend upon the behavioral indices of fear. However, there is greater evidence of differential activation of the hippocampus, amygdalar nuclei and the prefrontal cortical regions in male and female rodents during context and cued fear conditioning. The bed nucleus of the stria terminalis (BNST), a sexually dimorphic structure, is of particular interest as it differentially contributes to fear responses in males and females. In addition, while the influence of the estrous cycle on different phases of fear conditioning is delineated, the clearest modulatory effect of estrogen is on fear extinction processes. Examining the variability in neural responses and behavior in both sexes should increase our understanding of how that variability contributes to the neurobiology of affective disorders. This article is part of the Special Issue on 'Fear, anxiety and PTSD'.

Sevoflurane-induced amnesia is associated with inhibition of hippocampal cell ensemble activity after learning

General anesthesia could induce amnesia, however the mechanism remains unclear. We hypothesized that suppression of neuronal ensemble activity in the hippocampus by anesthesia during the post-learning period causes retrograde amnesia. To test this hypothesis, two experiments were conducted with sevoflurane anesthesia (2.5%, 30 min): a hippocampus-dependent memory task, the context pre-exposure facilitation effect (CPFE) procedure to measure memory function and in vivo calcium imaging to observe neural activity in hippocampal CA1 during context exploration and sevoflurane/home cage session. Sevoflurane treatment just after context pre-exposure session impaired the CPFE memory, suggesting sevoflurane induced retrograde amnesia. Calcium imaging showed sevoflurane treatment prevented neuronal activity in CA1. Further analysis of neuronal activity with non-negative matrix factorization, which extracts neural ensemble activity based on synchronous activity, showed that sevoflurane treatment reduced the reactivation of neuronal ensembles between during context exploration just before and one day after sevoflurane inhalation. These results suggest that sevoflurane treatment immediately after learning induces amnesia, resulting from suppression of reactivation of neuronal ensembles.

Reversal of spatial memory impairment by phosphodiesterase 3 inhibitor cilostazol is associated with reduced neuroinflammation and increased cerebral glucose uptake in aged male mice

The nucleotide second messenger 3', 5'-cyclic adenosine monophosphate (cAMP) and 3', 5'-cyclic guanosine monophosphate (cGMP) mediate fundamental functions of the brain, including learning and memory. Phosphodiesterase 3 (PDE3) can hydrolyze both cAMP and cGMP and appears to be involved in the regulation of their contents in cells. We previously demonstrated that long-term administration of cilostazol, a PDE3 inhibitor, maintained good memory performance in aging mice. Here, we report on studies aimed at determining whether cilostazol also reverses already-impaired memory in aged male mice. One month of oral 1.5% cilostazol administration in 22-month-old mice reversed age-related declines in hippocampus-dependent memory tasks, including the object recognition and the Morris water maze. Furthermore, cilostazol reduced neuroinflammation, as evidenced by immunohistochemical staining, and increased glucose uptake in the brain, as evidence by positron emission tomography (PET) with 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG). These results suggest that already-expressed memory impairment in aged male mice that depend on cyclic nucleotide signaling can be reversed by inhibition of PDE3. The reversal of age-related memory impairments may occur in the central nervous system, either through cilostazol-enhanced recall or strengthening of weak memories that otherwise may be resistant to recall.

Hippocampus as a sorter and reverberatory integrator of sensory inputs

The hippocampus must be capable of sorting and integrating multiple sensory inputs separately but simultaneously. However, it remains to be elucidated how the hippocampus executes these processes simultaneously during learning. Here we found that synchrony between conditioned stimulus (CS)-, unconditioned stimulus (US)- and future retrieval-responsible cells occurs in the CA1 during the reverberatory phase that emerges after sensory inputs have ceased, but not during CS and US inputs. Mutant mice lacking N-methyl-D-aspartate receptors (NRs) in CA3 showed a cued-fear memory impairment and a decrease in synchronized reverberatory activities between CS- and US-responsive CA1 cells. Optogenetic CA3 silencing at the reverberatory phase during learning impaired cued-fear memory. Thus, the hippocampus uses reverberatory activity to link CS and US inputs, and avoid crosstalk during sensory inputs.

Behavioural responses of zebrafish with sound stimuli in microfluidics

Neuronal activities of the human brain responsible for cognitive features have been theorized through several animal models that exhibited various complementary spatial learning modes by generating a flexible repertoire of behavioral strategies. However, for such studies associated with a neurodegenerative disease, which can be further manipulated to provide therapeutic strategies, the animal models employed in their developmental stages have been preferred over the adult ones. This pilot work was incepted to underscore the spatial memory capabilities that strengthened the intricate mechanism of memory acquisition potential in one of the low-order evolutionarily conserved species, such as zebrafish larvae. Initially, a reliable and more easily reproducible microfluidic platform integrating simple and intricate paths was designed to learn and test the spatial information in zebrafish larvae of 4-6 d.p.f. under non-invasive acoustic stimuli. Further, to acquire spatial information as the representation of spatial memory formation in zebrafish larvae, the acoustic startle responses were evaluated by quantifying various dynamic behaviors under distinct operating parameters. After significant conditioning sessions, the spatial memory was tested by employing variable 'freezing'. By the end of the 30 min-long test session, 6 d.p.f. larvae were found to exhibit the highest value of freezing of approximately 43% and 20% in the short and long paths, respectively. Even though a substantial rate of memory loss was observed, it can be envisaged to serve several behavioral strategies that process the dynamic cognitive memory among distinct spatiotemporal environments. Further, the proposed behavioral paradigm had the advantage of being more adaptable and reliably replicable by other researchers. As a consequence, different hypotheses can be readily tested to generate more reproducible findings towards distinct neurobehavioral characteristics. Therefore, the proposed paradigm for the consolidation of spatial memory based on the non-invasive spatial avoidance strategies could provide an enduring framework of reference for behavioral studies using zebrafish larvae.

Negative valence systems: sustained threat and the predatory imminence continuum

This review describes the relationship between the National Institute of Mental Health (U.S.A.) Research Domain Criteria (RDoC) Negative Valence System related to responses to threat and the Predatory Imminence Continuum model of antipredator defensive behavior. While the original RDoC constructs of Potential Threat (anxiety) and Acute Threat (fear) fit well with the pre-encounter and post-encounter defense modes of the predatory imminence model, the Sustained Threat construct does not. Early research on the bed nuclei of the stria terminalis (BST) suggested that when fear responding needed to be sustained for a prolonged duration this region was important. However, follow-up studies indicated that the BST becomes critical not because responses needed to be sustained but rather when the stimuli triggering fear were more difficult to learn about, particularly when aversive stimuli were difficult to accurately predict. Instead, it is argued that the BST and the hippocampus act to expand the range of conditions that can trigger post-encounter defense (Acute Threat). It is further suggested that sustained threat refers to situations where the predatory imminence continuum becomes distorted causing defensive behavior to intrude into times when organisms should be engaging in other adaptive behaviors. Stress is seen as something that can cause a long-term disturbance of the continuum and this disturbance is a state of sustained threat.

Biomarkers of Anxiety Acquisition and Generalization in Virtual Reality Experiments

Abstract. Anxiety disorders are characterized by exaggerated responses to a threatening situation and overgeneralization. Context conditioning has been used for the identification of risk factors. This systematic literature search identifies 16 articles published between 1990 and 2021 on differential anxiety conditioning and generalization in humans. Additionally, we provide example data for individuals suffering from panic attacks with and without depressive symptoms. Successful anxiety acquisition (discrimination between anxiety and safety context) was found on the subjective level of anxiety and US-expectancy, on the physiological level of electrodermal activity, and in the defensive behavior of startle response. Anxiety generalization (discrimination between generalization and safety context) was found on the verbal but not on the physiobehavioral level. In sum, we emphasize the impact of virtual reality on anxiety research. Verbal and physiobehavioral responses serve as reliable biomarkers for anxiety. Few studies found ratings to be the best predictor for anxiety generalization. Genetic predisposition or personality traits might foster overgeneralization.

The socially enriched environment test: a new approach to evaluate social behavior in a mouse model of social anxiety disorder

Social anxiety disorder (SAD) is a common anxiety disorder characterized by a marked fear of social situations. Treatments for SAD, including exposure therapy and medication, are not satisfactory for all patients. This has led to the development of several paradigms to study social fear in rodents. However, there are still some social impairments observed in SAD patients that have never been examined in rodent models. Indeed, social situations avoided by SAD patients include not only social interactions but also public performances and being observed by others. Nevertheless, tests used to assess sociability in rodents evaluate mostly social interaction in pairs. Thus, we developed a new test-a socially enriched environment test-that evaluates sociability within a group of three unfamiliar conspecifics in an enriched environment. In this study, we induced a SAD-like behavior (i.e., social fear) in male mice using social fear conditioning (SFC) and then tested social fear using the socially enriched environment test and the three-chamber test. Finally, we tested the effects of fear extinction and acute diazepam treatment in reversing social fear. Results revealed, in conditioned mice, decreased object exploration in proximity to conspecifics, social interaction, and mouse-like object exploration. Extinction training, but not acute diazepam treatment, reversed SFC-induced behavioral changes. These findings demonstrate that the socially enriched environment test provides an appropriate behavioral approach to better understand the etiology of SAD. This test may also have important implications in the exploration of new treatments.

Dose-dependent suppression of hippocampal contextual memory formation, place cells, and spatial engrams by the NMDAR antagonist (R)-CPP

We recently reported that the competitive NMDAR antagonist (R,S)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) does not suppress NMDAR-mediated field EPSPs (fEPSPNMDA) or long-term potentiation (LTP) in vitro at concentrations that block contextual conditioning in vivo. Here we tested one possible explanation for the mismatch - that the hippocampus is relatively resistant to CPP compared to other brain structures engaged in contextual fear conditioning. Using the context pre-exposure facilitation effect (CPFE) paradigm to separate the hippocampal and extra-hippocampal components of contextual learning, we found that the active enantiomer (R)-CPP suppressed the hippocampal component with an IC50 of 3.1 mg/kg, a dose that produces brain concentrations below those required to block fEPSPNMDA or LTP. Moreover, using in-vivo calcium imaging of place cells and spatial engrams to directly assess hippocampal spatial coding, we found that (R)-CPP dose-dependently reduced the development of place cells and interfered with the formation of stable spatial engrams when it was administered prior to exposing mice to a novel context. Both effects occurred at doses that interfered with freezing to context in CPFE experiments. We conclude that (R)-CPP blocks memory formation by interfering with hippocampal function, but that it does so by modulating NMDARs at sites that are not engaged in vitro in the same manner that they are in vivo - perhaps through interneuron circuits that do not contribute to fEPSPs and are not required to elicit LTP using standard induction protocols in vitro, but are essential for successful mnemonic function in vivo.

Prior fear learning enables the rapid assimilation of new fear memories directly into cortical networks

Long-term memory formation involves the reorganization of brain circuits, termed system consolidation. Whether and how a prior fear experience influences system consolidation of new memories is poorly understood. In rats, we found that prior auditory fear learning allows the secondary auditory cortex to immediately encode new auditory memories, with these new memories purely requiring the activation of cellular mechanisms of synaptic consolidation within secondary auditory cortex. Similar results were obtained in the anterior cingulate cortex for contextual fear memories. Moreover, prior learning enabled connections from these cortices to the basolateral amygdala (BLA) to support recent memory retention. We propose that the reorganization of circuits that characterizes system consolidation occurs only in the first instance that an event is learned, subsequently allowing the immediate assimilation of new analogous events in final storage sites.

Reactivating hippocampal-mediated memories during reconsolidation to disrupt fear

Memories are stored in the brain as cellular ensembles activated during learning and reactivated during retrieval. Using the Tet-tag system in mice, we label dorsal dentate gyrus neurons activated by positive, neutral or negative experiences with channelrhodopsin-2. Following fear-conditioning, these cells are artificially reactivated during fear memory recall. Optical stimulation of a competing positive memory is sufficient to update the memory during reconsolidation, thereby reducing conditioned fear acutely and enduringly. Moreover, mice demonstrate operant responding for reactivation of a positive memory, confirming its rewarding properties. These results show that interference from a rewarding experience can counteract negative affective states. While memory-updating, induced by memory reactivation, involves a relatively small set of neurons, we also find that activating a large population of randomly labeled dorsal dentate gyrus neurons is effective in promoting reconsolidation. Importantly, memory-updating is specific to the fear memory. These findings implicate the dorsal dentate gyrus as a potential therapeutic node for modulating memories to suppress fear.

Sex differences in contextual fear learning and generalization: a behavioral and computational analysis of hippocampal functioning

There are sex differences in anxiety disorders with regard to occurrence and severity of episodes such that females tend to experience more frequent and more severe episodes. Contextual fear learning and generalization are especially relevant to anxiety disorders, which are often defined by expressing fear and/or anxiety in safe contexts. In contextual fear conditioning, a representation of the context must first be created, and then that representation must be paired with an aversive consequence. With some variation, the experiments presented here use a 3-d procedure in which day 1 consists of pre-exposure to the to-be-shocked context, day 2 consists of a single context-shock pairing after some placement-to-shock interval (PSI), and day 3 consists of testing in either the same or a novel context. With shorter pre-exposure periods, male rats showed more contextual fear, consistent with previous literature; however, after longer pre-exposure periods, female rats showed greater contextual fear. Additionally, while pre-exposure and PSI are both periods of time prior to the shock, it was found that they were not equivalent to each other. Animals with 120 sec of pre-exposure and a 30-sec PSI show a differential level and time course of fear expression than animals who received no pre-exposure and a 150-sec PSI, and this further depended on sex of the rat. Additionally, an experiment comparing recently versus remotely acquired contextual fear was run. Males were again shown to have greater contextual fear at both time points, and this contextual fear incubated/increased over time in males but not females. To facilitate identification of what processes caused sex differences, we used BaconX, a conceptual and computational model of hippocampal contextual learning. Computational simulations using this model predicted many of our key findings. Furthermore, these simulations suggest potential mechanisms with regard to hippocampal computation; namely, an increased feature sampling rate in males, which may account for the sex differences presented here and in prior literature.

Stress and sex-dependent effects on conditioned inhibition of fear

Anxiety and stress-related disorders are highly prevalent and are characterized by excessive fear to threatening and nonthreatening stimuli. Moreover, there is a large sex bias in vulnerability to anxiety and stress-related disorders-women make up a disproportionately larger number of affected individuals compared with men. Growing evidence suggests that an impaired ability to suppress fear in the presence of safety signals may in part contribute to the development and maintenance of many anxiety and stress-related disorders. However, the sex-dependent impact of stress on conditioned inhibition of fear remains unclear. The present study investigated sex differences in the acquisition and recall of conditioned inhibition in male and female mice with a focus on understanding how stress impacts fear suppression. In these experiments, the training context served as the "fear" cue and an explicit tone served as the "safety" cue. Here, we found a possible sex difference in the training requirements for safety learning, although this effect was not consistent across experiments. Reductions in freezing to the safety cue in female mice were also not due to alternative fear behavior expression such as darting. Next, using footshock as a stressor, we found that males were impaired in conditioned inhibition of freezing when the stress was experienced before, but not after, conditioned inhibition training. Females were unaffected by footshock stress when it was administered at either time. Extended conditioned inhibition training in males eliminated the deficit produced by footshock stress. Finally, exposing male and female mice to swim stress impaired safety learning in male mice only. Thus, we found sex × stress interactions in the learning of conditioned inhibition and sex-dependent effects of stress modality. The present study adds to the growing literature on sex differences in safety learning, which will be critical for developing sex-specific therapies for a variety of fear-related disorders that involve excessive fear and/or impaired fear inhibition.

Darting across space and time: parametric modulators of sex-biased conditioned fear responses

Pavlovian fear conditioning is a widely used behavioral paradigm for studying associative learning in rodents. Despite early recognition that subjects may engage in a variety of both conditioned and unconditioned responses, the last several decades have seen the field narrow its focus to measure freezing as the sole indicator of conditioned fear. We previously reported that female rats were more likely than males to engage in darting, an escape-like conditioned response that is associated with heightened shock reactivity. To determine how experimental parameters contribute to the frequency of darting in both males and females, we manipulated factors such as chamber size, shock intensity, and number of trials. To better capture fear-related behavioral repertoires in our animals, we developed ScaredyRat, an open-source custom Python tool that analyzes Noldus Ethovision-generated raw data files to identify darters and quantify both conditioned and unconditioned responses. We found that, like freezing, conditioned darting occurrences scale with experimental alterations. While most darting occurs in females, we found that with an extended training protocol, darting can emerge in males as well. Collectively, our data suggest that darting reflects a behavioral switch in conditioned responding that is a product of an individual animal's sex, shock reactivity, and experimental parameters, underscoring the need for careful consideration of sex as a biological variable in classic learning paradigms.

Reinforcement Learning Model With Dynamic State Space Tested on Target Search Tasks for Monkeys: Extension to Learning Task Events

Learning is a crucial basis for biological systems to adapt to environments. Environments include various states or episodes, and episode-dependent learning is essential in adaptation to such complex situations. Here, we developed a model for learning a two-target search task used in primate physiological experiments. In the task, the agent is required to gaze one of the four presented light spots. Two neighboring spots are served as the correct target alternately, and the correct target pair is switched after a certain number of consecutive successes. In order for the agent to obtain rewards with a high probability, it is necessary to make decisions based on the actions and results of the previous two trials. Our previous work achieved this by using a dynamic state space. However, to learn a task that includes events such as fixation to the initial central spot, the model framework should be extended. For this purpose, here we propose a "history-in-episode architecture." Specifically, we divide states into episodes and histories, and actions are selected based on the histories within each episode. When we compared the proposed model including the dynamic state space with the conventional SARSA method in the two-target search task, the former performed close to the theoretical optimum, while the latter never achieved target-pair switch because it had to re-learn each correct target each time. The reinforcement learning model including the proposed history-in-episode architecture and dynamic state scape enables episode-dependent learning and provides a basis for highly adaptable learning systems to complex environments.

Neuronal Ensembles Organize Activity to Generate Contextual Memory

Contextual learning is a critical component of episodic memory and important for living in any environment. Context can be described as the attributes of a location that are not the location itself. This includes a variety of non-spatial information that can be derived from sensory systems (sounds, smells, lighting, etc.) and internal state. In this review, we first address the behavioral underpinnings of contextual memory and the development of context memory theory, with a particular focus on the contextual fear conditioning paradigm as a means of assessing contextual learning and the underlying processes contributing to it. We then present the various neural centers that play roles in contextual learning. We continue with a discussion of the current knowledge of the neural circuitry and physiological processes that underlie contextual representations in the Entorhinal cortex-Hippocampal (EC-HPC) circuit, as the most well studied contributor to contextual memory, focusing on the role of ensemble activity as a representation of context with a description of remapping, and pattern separation and completion in the processing of contextual information. We then discuss other critical regions involved in contextual memory formation and retrieval. We finally consider the engram assembly as an indicator of stored contextual memories and discuss its potential contribution to contextual memory.

Gradual decorrelation of CA3 ensembles associated with contextual discrimination learning is impaired by Kv1.2 insufficiency

The associative network of hippocampal CA3 is thought to contribute to rapid formation of contextual memory from one-trial learning, but the network mechanisms underlying decorrelation of neuronal ensembles in CA3 is largely unknown. Kv1.2 expressions in rodent CA3 pyramidal cells (CA3-PCs) are polarized to distal apical dendrites, and its downregulation specifically enhances dendritic responses to perforant pathway (PP) synaptic inputs. We found that haploinsufficiency of Kv1.2 (Kcna2+/-) in CA3-PCs, but not Kv1.1 (Kcna1+/-), lowers the threshold for long-term potentiation (LTP) at PP-CA3 synapses, and that the Kcna2+/- mice are normal in discrimination of distinct contexts but impaired in discrimination of similar but slightly distinct contexts. We further examined the neuronal ensembles in CA3 and dentate gyrus (DG), which represent the two similar contexts using in situ hybridization of immediate early genes, Homer1a and Arc. The size and overlap of CA3 ensembles activated by the first visit to the similar contexts were not different between wild type and Kcna2+/- mice, but these ensemble parameters diverged over training days between genotypes, suggesting that abnormal plastic changes at PP-CA3 synapses of Kcna2+/- mice is responsible for the impaired pattern separation. Unlike CA3, DG ensembles were not different between two genotype mice. The DG ensembles were already separated on the first day, and their overlap did not further evolve. Eventually, the Kcna2+/- mice exhibited larger CA3 ensemble size and overlap upon retrieval of two contexts, compared to wild type or Kcna1+/- mice. These results suggest that sparse LTP at PP-CA3 synapse probably supervised by mossy fiber inputs is essential for gradual decorrelation of CA3 ensembles.

Reinforcement Learning Model With Dynamic State Space Tested on Target Search Tasks for Monkeys: Self-Determination of Previous States Based on Experience Saturation and Decision Uniqueness

The real world is essentially an indefinite environment in which the probability space, i. e., what can happen, cannot be specified in advance. Conventional reinforcement learning models that learn under uncertain conditions are given the state space as prior knowledge. Here, we developed a reinforcement learning model with a dynamic state space and tested it on a two-target search task previously used for monkeys. In the task, two out of four neighboring spots were alternately correct, and the valid pair was switched after consecutive correct trials in the exploitation phase. The agent was required to find a new pair during the exploration phase, but it could not obtain the maximum reward by referring only to the single previous one trial; it needed to select an action based on the two previous trials. To adapt to this task structure without prior knowledge, the model expanded its state space so that it referred to more than one trial as the previous state, based on two explicit criteria for appropriateness of state expansion: experience saturation and decision uniqueness of action selection. The model not only performed comparably to the ideal model given prior knowledge of the task structure, but also performed well on a task that was not envisioned when the models were developed. Moreover, it learned how to search rationally without falling into the exploration–exploitation trade-off. For constructing a learning model that can adapt to an indefinite environment, the method of expanding the state space based on experience saturation and decision uniqueness of action selection used by our model is promising.

A cortical cell ensemble in the posterior parietal cortex controls past experience-dependent memory updating

When processing current sensory inputs, animals refer to related past experiences. Current information is then incorporated into the related neural network to update previously stored memories. However, the neuronal mechanism underlying the impact of memories of prior experiences on current learning is not well understood. Here, we found that a cellular ensemble in the posterior parietal cortex (PPC) that is activated during past experience mediates an interaction between past and current information to update memory through a PPC-anterior cingulate cortex circuit in mice. Moreover, optogenetic silencing of the PPC ensemble immediately after retrieval dissociated the interaction without affecting individual memories stored in the hippocampus and amygdala. Thus, a specific subpopulation of PPC cells represents past information and instructs downstream brain regions to update previous memories.

Animals refer to related past experiences when processing sensory inputs. The authors show that a cellular ensemble in the posterior parietal cortex that is activated during past experience mediates an interaction between past and current information to update memory through a circuit including the anterior cingulate cortex.

The Role of Context Conditioning in the Reinstatement of Responding to an Alcohol-Predictive Conditioned Stimulus

Re-exposure to an unconditioned stimulus (US) can reinstate extinguished conditioned responding elicited by a conditioned stimulus (CS). We tested the hypothesis that the reinstatement of responding to an appetitive CS is driven by an excitatory association formed between the US and the context that the US was ingested in during US re-exposure. Male, Long-Evans rats were acclimated to drinking alcohol (15%, v/v) in the home-cage, then trained to associate an auditory CS with an alcohol-US that was delivered into a fluid port for oral intake. During subsequent extinction sessions, the CS was presented as before, but without alcohol. After extinction, rats were re-exposed to alcohol as in training, but without the CS (alcohol re-exposure). 24h later at test, the CS was presented as in training, but without alcohol. First, we tested the effect of extinguishing the context-alcohol association, formed during alcohol re-exposure, on reinstatement. Conducting four context extinction sessions across four days (spaced extinction) after the alcohol re-exposure session did not impact reinstatement. However, four context extinction sessions conducted across two days (massed extinction) prevented reinstatement. Next, we conducted alcohol re-exposure in a context that either differed from, or was the same as, the test context. One alcohol re-exposure session in a different context did not affect reinstatement, however, three alcohol re-exposure sessions in a different context significantly reduced reinstatement during the first CS trial. These results partially support the view that a context-US association formed during US re-exposure drives the reinstatement of responding to an appetitive, alcohol-predictive CS.

Developmental emergence of persistent memory for contextual and auditory fear in mice

The ability to generate memories that persist throughout a lifetime (that is, memory persistence) emerges in early development across species. Although it has been shown that persistent fear memories emerge between late infancy and adolescence in mice, it is unclear exactly when this transition takes place, and whether two major fear conditioning tasks, contextual and auditory fear, share the same time line of developmental onset. Here, we compared the ontogeny of remote contextual and auditory fear in C57BL/6J mice across early life. Mice at postnatal day (P)15, 21, 25, 28, and 30 underwent either contextual or auditory fear training and were tested for fear retrieval 1 or 30 d later. We found that mice displayed 30-d memory for context- and tone-fear starting at P25. We did not find sex differences in the ontogeny of either type of fear memory. Furthermore, 30-d contextual fear retrieval led to an increase in the number of c-Fos positive cells in the prelimbic region of the prefrontal cortex only at an age in which the contextual fear memory was successfully retrieved. These data delineate a precise time line for the emergence of persistent contextual and auditory fear memories in mice and suggest that the prelimbic cortex is only recruited for remote memory recall upon the onset of memory persistence.

Sexually dimorphic muscarinic acetylcholine receptor modulation of contextual fear learning in the dentate gyrus

Contextual fear conditioning, where the prevailing situational cues become associated with an aversive unconditional stimulus such as electric shock, is sexually dimorphic. Males typically show higher levels of fear than females. There are two components to contextual fear conditioning. First the multiple cues that encompass the context must be integrated into a coherent representation, a process that requires the hippocampus. The second is that representation must be communicated to the basolateral amygdala where it can be associated with shock. If there is inadequate time for forming the representation prior to shock poor conditioning results and this is called the immediate shock deficit. One can isolate the contextual processing component, as well as alleviate the deficit, by providing an opportunity to explore the context without shock prior to the conditioning session. The purpose of the present study was to determine the extent to which cholinergic processes within the dentate gyrus of the hippocampus during contextual processing contribute to the sexual dimorphism. Clozapine-n-oxide (CNO) is a putatively inactive compound that acts only upon synthetic genetically engineered receptors. However, we found that CNO infused into the dentate gyrus prior to exploration eliminated the sexual dimorphism by selectively decreasing freezing in males to the level of females. Biological activity of CNO is usually attributed to metabolism of CNO to clozapine and we found that clozapine, and the muscarinic cholinergic antagonist, scopolamine, produced results similar to CNO, preferentially affecting males. On the other hand, the muscarinic agonist oxotremorine selectively impaired conditioning in females. Overall, the current experiments reveal significant off-target effects of CNO and implicate muscarinic cholinergic receptors in the dentate gyrus as a significant mediator of the sexual dimorphism in contextual fear conditioning.

Functional Aging in Male C57BL/6J Mice Across the Life-Span: A Systematic Behavioral Analysis of Motor, Emotional, and Memory Function to Define an Aging Phenotype

Aging is characterized generally by progressive and overall physiological decline of functions and is observed in all animals. A long line of evidence has established the laboratory mouse as the prime model of human aging. However, relatively little is known about the detailed behavioral and functional changes that occur across their lifespan, and how this maps onto the phenotype of human aging. To better understand age-related changes across the life-span, we characterized functional aging in male C57BL/6J mice of five different ages (3, 6, 12, 18, and 22 months of age) using a multi-domain behavioral test battery. Spatial memory and physical activities, including locomotor activity, gait velocity, and grip strength progressively declined with increasing age, although at different rates; anxiety-like behaviors increased with aging. Estimated age-related patterns showed that these functional alterations across ages are non-linear, and the patterns are unique for each behavioral trait. Physical function progressively declines, starting as early as 6 months of age in mice, while cognitive function begins to decline later, with considerable impairment present at 22 months of age. Importantly, functional aging of male C57BL/6J mouse starts at younger relative ages compared to when it starts in humans. Our study suggests that human-equivalent ages of mouse might be better determined on the basis of its functional capabilities.

The effects of early-life immune activation on microglia-mediated neuronal remodeling and the associated ontogeny of hippocampal-dependent learning in juvenile rats

Many neurodevelopmental disorders and associated learning deficits have been linked to early-life immune activation or ongoing immune dysregulation (Laskaris et al., 2016; O'Connor et al., 2014; Frick et al., 2013). Neuroscientists have begun to understand how the maturation of neural circuits allows for the emergence of cognitive and learning behaviors; yet we know very little about how these developing neural circuits are perturbed by certain events, including risk-factors such as early-life immune activation and immune dysregulation. To answer these questions, we examined the impact of early-life immune activation on the emergence of hippocampal-dependent learning in juvenile male and female rats using a well-characterized hippocampal-dependent learning task and we investigated the corresponding, dynamic multicellular interactions in the hippocampus that may contribute to these learning deficits. We found that even low levels of immune activation can result in hippocampal-depedent learning deficits days later, but only when this activation occurs during a sensitive period of development. The initial immune response and associated cytokine production in the hippocampus resolved within 24 h, several days prior to the observed learning deficit, but notably the initial immune response was followed by altered microglial-neuronal communication and synapse remodeling that changed the structure of hippocampal neurons during this period of juvenile brain development. We conclude that immune activation or dysregulation during a sensitive period of hippocampal development can precipitate the emergence of learning deficits via a multi-cellular process that may be initiated by, but not the direct result of the initial cytokine response. SIGNIFICANCE STATEMENT: Many neurodevelopmental disorders have been linked to early-life immune activation or immune dysregulation; however, very little is known about how dynamic changes in neuroimmune cells mediate the transition from normal brain function to the early stages of cognitive disorders, or how changes in immune signaling are subsequently integrated into developing neuronal networks. The current experiments examined the consequences of immune activation on the cellular and molecular changes that accompany the emergence of learning deficits during a sensitive period of hippocampal development. These findings have the potential to significantly advance our understanding of how early-life immune activation or dysregulation can result in the emergence of cognitive and learning deficits that are the largest source of years lived with disability in humans.

Fear generalization immediately after contextual fear memory consolidation in mice

Fear generalization is a symptom of anxiety-related disorders, including acute stress disorder and post-traumatic stress disorder. Using a contextual fear conditioning paradigm, we found that mice exposed to a similar neutral context but not a different neutral context soon after training showed fear generalization immediately after contextual fear memory consolidation (i.e., 6 h after training). This fear generalization was reflected by a change not only in the total amount but also the pattern of freezing between conditioned and generalized contexts. These results provide insight into the factors that influence fear generalization and can facilitate future studies investigating the underlying pathophysiological mechanisms of anxiety-related disorders.