The 3-second auditory conditioned stimulus is a more effective stressor than the 20-second auditory conditioned stimulus in male rats

Why are threatening experiences remembered so well? Insights into memory strengthening from protocols of gradual aversive learning

Aversive experiences often result in strong and persistent memory traces, which can sometimes lead to conditions such as Post-Traumatic Stress Disorder or phobias. Aversive stimulation tests are key tools in psychology and neuroscience for studying learning and memory. These tests typically use electric shocks as the unconditioned stimulus, allowing for precise control over the aversive content of the learning event. This feature has led to extensive research applying these tests with varying shock intensities to examine differences in learning, behavior, and memory formation between low- and high-aversive experiences. This line of research is particularly valuable for understanding the neurobiology underlying memory strengthening, but, to our knowledge, no review has yet compiled and organized the findings from this specific methodology. In this comprehensive review, we focus primarily on animal studies that have employed the same aversive test (i.e. Fear Conditioning, Passive Avoidance, Active Avoidance or Operant boxes) at different intensities. We will first outline and briefly describe the main aversive learning paradigms used in this field. Next, we will examine the relationship between aversiveness and memory strength. Finally, we will explore the neurobiological insights these studies have revealed over the years. Our aim is to gain a better understanding of how the nervous system gradually strengthens memory, while also addressing the remaining gaps and challenges in this area of research.

Approach/Avoidance Behavior to Novel Objects is Correlated with the Serotonergic and Dopaminergic Systems in the Brown Rat (Rattus norvegicus)

The brown rat (Rattus norvegicus) is known to show three types of behavioral responses to novel objects. Whereas some rats are indifferent to novel objects, neophobic and neophilic rats show avoidance and approach behavior, respectively. Here, we compared the dopaminergic, serotonergic, and noradrenergic systems immunohistochemically among these rats. Trapped wild rats and laboratory rats were first individually exposed to the novel objects in their home cage. Wild rats were divided into neophobic and indifferent rats depending on their behavioral responses. Similarly, laboratory rats were divided into neophilic and indifferent rats. Consistent with the behavioral differences, in the paraventricular nucleus of the hypothalamus, Fos expression in corticotropin-releasing hormone-containing neurons was higher in the neophobic rats than in the indifferent rats. In the anterior basal amygdala, the neophobic rats showed higher Fos expression than the indifferent rats. In the posterior basal amygdala, the neophobic and neophilic rats showed lower and higher Fos expressions than the indifferent rats, respectively. When we compared the neuromodulatory systems, in the dorsal raphe, the number of serotonergic neurons and Fos expression in serotonergic neurons increased linearly from neophobic to indifferent to neophilic rats. In the ventral tegmental area, Fos expression in dopaminergic neurons was higher in the neophilic rats than in the indifferent rats. These results demonstrate that approach/avoidance behavior to novel objects is correlated with the serotonergic and dopaminergic systems in the brown rat. We propose that the serotonergic system suppresses avoidance behavior while the dopaminergic system enhances approach behavior to novel objects.

Naloxone increases conditioned fear responses during social buffering in male rats

Social buffering is the phenomenon in which the presence of an affiliative conspecific mitigates stress responses. We previously demonstrated that social buffering completely ameliorates conditioned fear responses in rats. However, the neuromodulators involved in social buffering are poorly understood. Given that opioids, dopamine, oxytocin and vasopressin play an important role in affiliative behaviour, here, we assessed the effects of the most well-known antagonists, naloxone (opioid receptor antagonist), haloperidol (dopamine D2 receptor antagonist), atosiban (oxytocin receptor antagonist) and SR49059 (vasopressin V1a receptor antagonist), on social buffering. In Experiment 1, fear-conditioned male subjects were intraperitoneally administered one of the four antagonists 25 min prior to exposure to a conditioned stimulus with an unfamiliar non-conditioned rat. Naloxone, but not the other three antagonists, increased freezing and decreased walking and investigation as compared with saline administration. In Experiment 2, identical naloxone administration did not affect locomotor activity, anxiety-like behaviour or freezing in an open-field test. In Experiment 3, after confirming that the same naloxone administration again increased conditioned fear responses, as done in Experiment 1, we measured Fos expression in 16 brain regions. Compared with saline, naloxone increased Fos expression in the paraventricular nucleus of the hypothalamus and decreased Fos expression in the nucleus accumbens shell, anterior cingulate cortex and insular cortex and tended to decrease Fos expression in the nucleus accumbens core. Based on these results, we suggest that naloxone blocks social buffering of conditioned fear responses in male rats.

The strain of unfamiliar conspecifics affects stress identification in rats

Humans show distinct social behaviours when we evaluate an individual as being a member of the same group and recognize social similarity to the individual. One example is more accurate identification of emotion in that individual. Our previous studies proposed that rats recognize social similarity to certain strains of unfamiliar rats. It is therefore possible that the strain of unfamiliar conspecifics affects stress identification in rats. Wistar subject rats were allowed to explore a pair of unfamiliar Wistar, Sprague-Dawley (SD), Long-Evans (LE), or Fischer344 (F344) stimulus rats. To induce differences in stress, one of the stimulus rats had received foot shocks immediately before the test. It was found that the subjects showed biased interaction towards the shocked Wistar and SD stimulus rats, but not toward the shocked LE or F344 stimulus rats. Subsequent experiments confirmed that the biased interaction towards the shocked Wistar and SD stimulus rats was driven by stress in these stimulus rats. In addition, the lack of biased interaction towards the shocked LE and F344 stimulus rats did not appear to be due to procedural reasons. The experiment using LE subject rats further confirmed that the shocked LE stimulus rats emitted distress signals. These results suggested that Wistar rats could identify stress in unfamiliar Wistar and SD rats, but not in unfamiliar LE or F344 rats. Therefore, rats appear to recognize social similarity to certain unfamiliar strains of rats.

Mapping of c-Fos expression in the medial amygdala following social buffering in male rats

Social buffering is the phenomenon in which an affiliative conspecific (associate) ameliorates stress responses of a subject. We previously found that social buffering in Wistar subject rats is induced if the strain of the associate is Wistar or a strain derived from Wistar rats. In the present study, we assessed the possible role of medial amygdala (Me) in this strain-dependent induction of social buffering. The subjects were exposed to the conditioned stimulus (CS) that had been paired or unpaired with a foot shock either alone, with an unfamiliar Wistar associate, or with an unfamiliar Fischer 344 (F344) associate. We found that the Wistar associates, but not F344 associates, ameliorated increased freezing and Fos expression in the paraventricular nucleus of the hypothalamus and lateral amygdala caused by the CS. In addition, Fos expression in the posterior complex of the anterior olfactory nucleus and lateral intercalated cell mass of the amygdala was increased simultaneously. These results suggest that Wistar associates, but not F344 associates, induced social buffering. In the Me, we did not find any differences associated with stress responses or amelioration of stress responses. In contrast, a comparison among the unpaired subjects found that the Wistar associates, but not F344 associates, increased exploratory behavior and Fos expression in the posteroventral subdivision of the Me (MePV). Based on these results, we propose that the MePV is involved in the recognition of social similarity with the associates. Taken together, the present study provides information about the possible role of Me in social buffering.

Existence of wild brown rats (Rattus norvegicus) that are indifferent to novel objects

Exposure to novel objects typically evokes avoidance behavior in wild animals, which is called neophobia. We previously found that wild brown rats (Rattus norvegicus) that were trapped in a park in downtown Tokyo, Japan, exhibited neophobia. We also found that this behavior was accompanied by the activation of the basolateral complex of the amygdala (BLA). Previous studies have suggested that genetic factors are the primary determinants of neophobia. Since rats in cities form populations with distinct genetic characteristics, it is reasonable to assume that wild rats caught at different locations in urban centers will exhibit different levels of neophobia. Here we assessed the intensity of neophobia in wild rats trapped at a wholesale market in Tokyo. Although we performed exactly the same experiment in which neophobia was observed in wild rats trapped at the park, the presence of novel objects did not affect the behaviors of wild rats trapped at the market. Conversely, laboratory rats showed approach and exploratory behaviors as seen in the previous study, suggesting that the experiment was performed appropriately. Compared to the laboratory rats, the lack of behavioral changes in the wild rats was accompanied by fewer Fos immunoreactive cells in the BLA. In addition, the numbers of Fos immunoreactive cells in the bed nucleus of the stria terminalis and ventromedial hypothalamus were similar between the two types of rats. The results demonstrated the existence of wild rats that were indifferent to novel objects.

Social buffering enhances extinction of conditioned fear responses by reducing corticosterone levels in male rats

The presence of an affiliative conspecific reduces stress responses to a wide variety of stimuli, which is termed "social buffering." We previously reported that social buffering in male rats ameliorated behavioral responses, as well as hypothalamic-pituitary-adrenal axis activation, elicited by an auditory conditioned stimulus (CS). In addition, subjects that experienced social buffering did not show stress responses when re-exposed to the CS the next day in the absence of an accompanying rat. However, the mechanisms underlying this enhancement of between-session extinction are poorly understood. In Experiment 1, we compared corticosterone levels at 0, 10, and 15 min after extinction training. Subjects that experienced social buffering had lower corticosterone levels than subjects that trained alone at the end of extinction training. However, corticosterone levels at 10 and 15 min after training were not affected by the experience of social buffering. These results suggest that a lower level of corticosterone during extinction training had an important role in the enhancement of extinction. To directly assess this, in Experiment 2, we manipulated the corticosterone level during extinction training. We found that a subcutaneous injection of corticosterone before extinction training blocked the enhancement of extinction by social buffering. These results demonstrate that the enhancement is caused by a low level of corticosterone during the training. Taken together, we suggest that social buffering enhances extinction of conditioned fear responses by reducing corticosterone levels in male rats.

The lateral intercalated cell mass of the amygdala is activated during social buffering of conditioned fear responses in male rats

The presence of an affiliative conspecific reduces stress responses to a wide variety of stimuli. This phenomenon is termed "social buffering". We previously found that the presence of another Wistar rat (associate) suppressed activation of the lateral amygdala (LA) and ameliorated stress responses to an auditory conditioned stimulus (CS) in a fear-conditioned Wistar subject rat. Subsequent analyses suggested that activation of the posterior complex of the anterior olfactory nucleus (AOP) is responsible for the suppression of the LA. However, it remains unclear how the AOP suppresses the LA. To clarify this issue, a fear-conditioned Wistar subject was exposed to the CS either alone or with a Wistar associate. We also prepared a fear-conditioned Wistar subject that was tested with a Fischer344 (F344) associate as an additional control because F344 associates do not induce social buffering. We found that the presence of a Wistar associate induced a reduction of behavioral responses and Fos expression in the paraventricular nucleus of the hypothalamus (PVN) of the subject. Although Fos expression in the AOP was increased, the expression was not biased towards the GABAergic cells. In addition, Fos expression in the lateral intercalated cell mass of the amygdala (lITC) was increased. In contrast, the presence of a F344 associate did not affect Fos expression in subjects' PVN or lITC, whereas behavioral responses were slightly reduced. These results suggest that the lITC was activated during social buffering. Based on these findings, we propose that the AOP indirectly suppresses the LA by activating the lITC.

Bed nucleus of the stria terminalis regulates fear to unpredictable threat signals

The bed nucleus of the stria terminalis (BNST) has been implicated in conditioned fear and anxiety, but the specific factors that engage the BNST in defensive behaviors are unclear. Here we examined whether the BNST mediates freezing to conditioned stimuli (CSs) that poorly predict the onset of aversive unconditioned stimuli (USs) in rats. Reversible inactivation of the BNST selectively reduced freezing to CSs that poorly signaled US onset (e.g., a backward CS that followed the US), but did not eliminate freezing to forward CSs even when they predicted USs of variable intensity. Additionally, backward (but not forward) CSs selectively increased Fos in the ventral BNST and in BNST-projecting neurons in the infralimbic region of the medial prefrontal cortex (mPFC), but not in the hippocampus or amygdala. These data reveal that BNST circuits regulate fear to unpredictable threats, which may be critical to the etiology and expression of anxiety.

A dyad shows mutual changes during social buffering of conditioned fear responses in male rats

The presence of an affiliative conspecific reduces stress responses to a wide variety of stimuli. This phenomenon is termed "social buffering". We previously found that the presence of another naïve rat (associate) reduced conditioned fear responses to an auditory conditioned stimulus in a conditioned subject rat. Although we subsequently conducted a series of studies to examine behavioral, physiological, and neural changes during social buffering in the conditioned subject, the changes in the associate remained unclear. Therefore, in the present study, we investigated the behavioral and neural changes in the associate. Fear-conditioned and non-conditioned rats were re-exposed to the conditioned stimulus with an associate placed in the same enclosure (Experiment 1) or separated by a wire-mesh partition (Experiment 2). In Experiment 1, the associate exhibited increased anogenital contact and allo-grooming, which were accompanied by increased c-Fos expression in the paraventricular nucleus of the hypothalamus and central amygdala. These results suggest that the subject and associate mutually affected each other during social buffering. In contrast, in Experiment 2, we found only a difference in the time course of investigation between associates tested with the conditioned and non-conditioned subjects. These results suggest that the associate was unable to acquire a sufficient amount of signal from the conditioned subject behind the wire-mesh partition necessary to show clear changes in behavior and c-Fos expression. Taken together, the current findings suggest that a dyad shows mutual changes during social buffering of conditioned fear responses in male rats.

Structural differences in the brain between wild and laboratory rats (Rattus norvegicus): Potential contribution to wariness

Wild animals typically exhibit defensive behaviors in response to a wider range and/or a weaker intensity of stimuli compared with domestic animals. However, little is known about the neural mechanisms underlying "wariness" in wild animals. Wild rats are one of the most accessible wild animals for experimental research. Laboratory rats are a domesticated form of wild rat, belonging to the same species, and are therefore considered suitable control animals for wild rats. Based on these factors, we analyzed structural differences in the brain between wild and laboratory rats to elucidate the neural mechanisms underlying wariness. We examined wild rats trapped in Tokyo, and weight-matched laboratory rats. We then prepared brain sections and compared the basolateral complex of the amygdala (BLA), the bed nucleus of the stria terminalis (BNST), the main olfactory bulb and the accessory olfactory bulb. The results revealed that wild rats exhibited larger BLA, BNST and caudal part of the accessory olfactory bulb compared with laboratory rats. These results suggest that the BLA, BNST, and vomeronasal system potentially contribute to wariness in wild rats.

Role of glucocorticoid receptor-mediated mechanisms in cocaine memory enhancement

The basolateral amygdala (BLA) is a critical site for the reconsolidation of labile contextual cocaine memories following retrieval-induced reactivation/destabilization. Here, we examined whether glucocorticoid receptors (GR), which are abundant in the BLA, mediate this phenomenon. Rats were trained to lever press for cocaine reinforcement in a distinct environmental context, followed by extinction training in a different context. Rats were then briefly exposed to the cocaine-paired context (to elicit memory reactivation and reconsolidation) or their home cages (no reactivation control). Exposure to the cocaine-paired context elicited greater serum corticosterone concentrations than home cage stay. Interestingly, the GR antagonist, mifepristone (3-10 ng/hemisphere), administered into the BLA after memory reactivation produced a further, dose-dependent increase in serum corticosterone concentrations during the putative time of cocaine-memory reconsolidation but produced an inverted U-shaped dose-effect curve on subsequent cocaine-seeking behavior 72 h later. This effect was anatomically selective, dependent on memory reactivation (i.e., not observed after home cage exposure), and did not reflect protracted hyperactivity. However, the effect was also observed when mifepristone was administered after novelty stress that mimics drug context-induced hypothalamic-pituitary-adrenal (HPA) axis activation without explicit memory reactivation. Together, these findings suggest that, similar to explicit memory retrieval, a stressful event is sufficient to destabilize cocaine memories and permit their manipulation. Furthermore, BLA GR stimulation exerts inhibitory feedback upon HPA axis activation and thus suppresses cocaine-memory reconsolidation.

Social Odors: Alarm Pheromones and Social Buffering

In this chapter, I describe 2 types of olfactory communication in rats, which appear to arouse anxiety and relief, respectively. In alarm pheromonal communication, rats release 4-methylpentanal and hexanal from their perianal region when they are stressed. These molecules activate the anxiety circuit, including the bed nucleus of the stria terminalis, when 4-methylpentanal and hexanal are simultaneously detected by the vomeronasal system and the main olfactory system, respectively. Consequently, recipient rats show a variety of anxiety responses, depending on the threatening stimuli. In appeasing olfactory communication, non-stressed rats release an appeasing olfactory signal, which is detected by the main olfactory system of other rats. When detected, this olfactory signal suppresses activation of the basolateral complex of the amygdala and, as a result, ameliorates stress responses elicited by an auditory conditioned stimulus during social buffering phenomenon. Because social buffering appears to be based on affinity and attachment to accompanying animals, the appeasing olfactory signal may arouse relief in rats. A definition of social buffering is also proposed as we still have no set definition for the term social buffering yet.

Social buffering ameliorates conditioned fear responses in the presence of an auditory conditioned stimulus

Social buffering is a phenomenon in which stress in an animal is ameliorated when the subject is accompanied by a conspecific animal(s) during exposure to distressing stimuli. Previous studies of social buffering of conditioned fear responses in rats have typically used a 3-s auditory conditioned stimulus (CS) as a stressor, observing stress responses during a specified experimental period. Because a 3-s CS is extremely short compared with a typical experimental period, freezing has thus been observed primarily in the absence of the CS. Therefore, it has been unclear whether social buffering ameliorates conditioned fear responses in the presence of the CS. To clarify this issue, the current study assessed the effects of social buffering on conditioned fear responses in the presence of a 20-s CS. We measured the percentage of time spent freezing during the 20-s period following the onset of the CS. When conditioned subjects were exposed to the 20-s CS alone, they exhibited a high percentage of freezing in the presence of the CS. The presence of another non-conditioned rat completely blocked this response. The same result was observed when freezing was observed primarily in the absence of the 3-s CS. In addition, we confirmed that the presence of an associate ameliorated conditioned fear responses induced by a 20-s CS or 3-s CS when the duration and frequency of fear responses was measured. These findings indicate that social buffering ameliorates conditioned fear responses in the presence of an auditory CS.

Active place avoidance is no more stressful than unreinforced exploration of a familiar environment

Training in the active place avoidance task changes hippocampus synaptic function, the dynamics of hippocampus local field potentials, place cell discharge, and active place avoidance memory is maintained by persistent PKMζ activity. The extent to which these changes reflect memory processes and/or stress responses is unknown. We designed a study to assess stress within the active place avoidance task by measuring serum corticosterone (CORT) at different stages of training. CORT levels did not differ between trained mice that learned to avoid the location of the mild foot shock, and untrained no-shock controls exposed to the same environment for the same amount of time. Yoked mice, that received unavoidable shocks in the same time sequence as the trained mice, had significantly higher CORT levels than mice in the trained and no-shock groups after the first trial. This increase in CORT disappeared by the fourth trial the following day, and levels of CORT for all groups matched that of home cage controls. The data demonstrate that place avoidance training is no more stressful than experiencing a familiar environment. We conclude that changes in neural function as a result of active place avoidance training are likely to reflect learning and memory processes rather than stress. © 2016 Wiley Periodicals, Inc.

Contributions of the Central Extended Amygdala to Fear and Anxiety

It is widely thought that phasic and sustained responses to threat reflect dissociable circuits centered on the central nucleus of the amygdala (Ce) and the bed nucleus of the stria terminalis (BST), the two major subdivisions of the central extended amygdala. Early versions of this hypothesis remain highly influential and have been incorporated into the National Institute of Mental Health Research Research Domain Criteria framework. However, new observations encourage a different perspective. Anatomical studies show that the Ce and BST form a tightly interconnected unit, where different kinds of threat-relevant information can be integrated and used to assemble states of fear and anxiety. Imaging studies in humans and monkeys show that the Ce and BST exhibit similar functional profiles. Both regions are sensitive to a range of aversive challenges, including uncertain or temporally remote threat; both covary with concurrent signs and symptoms of fear and anxiety; both show phasic responses to short-lived threat; and both show heightened activity during sustained exposure to diffusely threatening contexts. Mechanistic studies demonstrate that both regions can control the expression of fear and anxiety during sustained exposure to diffuse threat. These observations compel a reconsideration of the central extended amygdala's contributions to fear and anxiety and its role in neuropsychiatric disease.

The right central amygdala shows greater activation in response to an auditory conditioned stimulus in male rats

Pavlovian fear conditioning is an experimental procedure in which a conditioned stimulus (CS) acquires an ability to elicit fear responses. This type of conditioning depends on the basolateral complex of the amygdala (BLA) and/or central amygdala (CeA). We previously found that rats showed reduced fear responses to an auditory CS when they were subjected to a pre-training chemical lesion of the entire right amygdala as compared with the left amygdala. Based on this finding, we hypothesize that the BLA and/or CeA in the right hemisphere will be more strongly activated by an auditory CS than those in the left hemisphere. To test this hypothesis, we re-exposed fear-conditioned and non-conditioned rats to an auditory CS 1 day after fear conditioning. We assessed Fos expression in the BLA and CeA in each hemisphere. We found that fear-conditioned subjects showed fear responses, such as increased freezing and decreased walking, as well as increased Fos expression in the BLA and CeA. When we compared Fos expression between hemispheres, Fos expression in the CeA, but not the BLA, was greater in the right hemisphere compared with the left hemisphere. These results suggest that the right CeA is more strongly activated by the auditory CS.

Social buffering ameliorates conditioned fear responses in female rats

The stress experienced by an animal is ameliorated when the animal is exposed to distressing stimuli along with a conspecific animal(s). This is known as social buffering. Previously, we found that the presence of an unfamiliar male rat induced social buffering and ameliorated conditioned fear responses of a male rat subjected to an auditory conditioned stimulus (CS). However, because our knowledge of social buffering is highly biased towards findings in male subjects, analyses using female subjects are crucial for comprehensively understanding the social buffering phenomenon. In the present studies, we assessed social buffering of conditioned fear responses in female rats. We found that the estrus cycle did not affect the intensity of the rats' fear responses to the CS or their degree of vigilance due to the presence of a conspecific animal. Based on these findings, we then assessed whether social buffering ameliorated conditioned fear responses in female rats without taking into account their estrus cycles. When fear conditioned female rats were exposed to the CS without the presence of a conspecific, they exhibited behavioral responses, including freezing, and elevated corticosterone levels. By contrast, the presence of an unfamiliar female rat suppressed these responses. Based on these findings, we conclude that social buffering can ameliorate conditioned fear responses in female rats.