Prior observation of fear learning enhances subsequent self-experienced fear learning with an overlapping neuronal ensemble in the dorsal hippocampus

Understanding Others' Distress Through Past Experiences: The Role of Memory Engram Cells in Observational Fear

For individuals to survive and function in society, it is essential that they recognize, interact with, and learn from other conspecifics. Observational fear (OF) is the well-conserved empathic ability of individuals to understand the other's aversive situation. While it is widely known that factors such as prior similar aversive experience and social familiarity with the demonstrator facilitate OF, the neural circuit mechanisms that explicitly regulate experience-dependent OF (Exp OF) were unclear. In this review, we examine the neural circuit mechanisms that regulate OF, with an emphasis on rodent models, and then discuss emerging evidence for the role of fear memory engram cells in the regulation of Exp OF. First, we examine the neural circuit mechanisms that underlie Naive OF, which is when an observer lacks prior experiences relevant to OF. In particular, the anterior cingulate cortex to basolateral amygdala (BLA) neural circuit is essential for Naive OF. Next, we discuss a recent study that developed a behavioral paradigm in mice to examine the neural circuit mechanisms that underlie Exp OF. This study found that fear memory engram cells in the BLA of observers, which form during a prior similar aversive experience with shock, are reactivated by ventral hippocampal neurons in response to shock delivery to the familiar demonstrator to elicit Exp OF. Finally, we discuss the implications of fear memory engram cells in Exp OF and directions of future research that are of both translational and basic interest.

Systems consolidation induces multiple memory engrams for a flexible recall strategy in observational fear memory in male mice

Observers learn to fear the context in which they witnessed a demonstrator's aversive experience, called observational contextual fear conditioning (CFC). The neural mechanisms governing whether recall of the observational CFC memory occurs from the observer's own or from the demonstrator's point of view remain unclear. Here, we show in male mice that recent observational CFC memory is recalled in the observer's context only, but remote memory is recalled in both observer and demonstrator contexts. Recall of recent memory in the observer's context requires dorsal hippocampus activity, while recall of remote memory in both contexts requires the medial prefrontal cortex (mPFC)-basolateral amygdala pathway. Although mPFC neurons activated by observational CFC are involved in remote recall in both contexts, distinct mPFC subpopulations regulate remote recall in each context. Our data provide insights into a flexible recall strategy and the functional reorganization of circuits and memory engram cells underlying observational CFC memory.

Social and asocial learning in zebrafish are encoded by a shared brain network that is differentially modulated by local activation

Group living animals use social and asocial cues to predict the presence of reward or punishment in the environment through associative learning. The degree to which social and asocial learning share the same mechanisms is still a matter of debate. We have used a classical conditioning paradigm in zebrafish, in which a social (fish image) or an asocial (circle image) conditioned stimulus (CS) have been paired with an unconditioned stimulus (US=food), and we have used the expression of the immediate early gene c-fos to map the neural circuits associated with each learning type. Our results show that the learning performance is similar to social and asocial CSs. However, the brain regions activated in each learning type are distinct and a community analysis of brain network data reveals segregated functional submodules, which seem to be associated with different cognitive functions involved in the learning tasks. These results suggest that, despite localized differences in brain activity between social and asocial learning, they share a common learning module and social learning also recruits a specific social stimulus integration module. Therefore, our results support the occurrence of a common general-purpose learning module, that is differentially modulated by localized activation in social and asocial learning.

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.

Socially Transferred Stress Experience Modulates Social Affective Behaviors in Rats

Social communication of affective states between individuals, as well as actual experiences, influences their internal states and behaviors. Although prior stress experiences promote empathy-like behaviors, it remains unclear whether the social transmission of stress events modulates these behaviors. Here, we provide evidence that transferred stress experiences from cage mates modulate socioaffective approach-avoidance behaviors in rats. Male Wistar-Imamichi rats were assigned to one of five experimental groups (Control (n = 15); no shock with shocked cage mates (n = 15); low (0.1 mA, n = 15), middle (0.5 mA, n = 14), and high shock (1.0 mA, n = 14)). Except for the naïve and housed with stressed mate groups, rats received two foot-shocks (5 s for each). The next day, the subjects were allowed to explore two unfamiliar conspecifics; one was a naïve, while the other was a distressed conspecific that received two foot-shocks (1.0 mA, 5 s) immediately before the test. Rats that were housed with stressed mates, as well as those that experienced a higher intensity of foot-shocks, were more likely to approach, while naïve rats avoided, a distressed conspecific. These results suggest that socially transferred stress shifts socioaffective response styles from avoidance to approach toward a stressed conspecific in rats.

Complex Housing, but Not Maternal Deprivation Affects Motivation to Liberate a Trapped Cage-Mate in an Operant Rat Task

Early life environment influences the development of various aspects of social behavior, particularly during sensitive developmental periods. We studied how challenges in the early postnatal period or (early) adolescence affect pro-social behavior. To this end, we designed a lever-operated liberation task, to be able to measure motivation to liberate a trapped conspecific (by progressively increasing required lever pressing for door-opening). Liberation of the trapped rat resulted either in social contact or in liberation into a separate compartment. Additionally, a condition was tested in which both rats could freely move in two separate compartments and lever pressing resulted in social contact. When partners were not trapped, rats were more motivated to press the lever for opening the door than in either of the trapped configurations. Contrary to our expectations, the trapped configuration resulted in a reduced motivation to act. Early postnatal stress (24 h maternal deprivation on postnatal day 3) did not affect behavior in the liberation task. However, rearing rats from early adolescence onwards in complex housing conditions (Marlau cages) reduced the motivation to door opening, both in the trapped and freely moving conditions, while the motivation for a sucrose reward was not affected.

[Histamine signaling restores retrieval of forgotten memories]

Histamine is a biological amine that functions as a neurotransmitter in the brain to regulate arousal, appetite, and cognitive functions. Many pharmacological studies using histamine receptor agonists and antagonists have found that histamine promotes memory consolidation and retrieval. More recently, we have revealed that the activation of the brain histaminergic system by H₃R antagonists/inverse agonists restores retrieval of forgotten long-term memory in mice and humans. The recovery of memory retrieval may involve histamine-induced excitatory effects. Histamine may increase neuronal excitability throughout the neural circuit, including both neurons that are and are not recruited into the memory trace, similar to noise added to the neural circuits for memory retrieval. Stochastic resonance can explain how adding noise to the circuit enhances memory retrieval. Memory is processed not only by consolidation and retrieval, but also by various processes such as maintenance, reconsolidation, extinction, and reinstatement. Further studies that separately analyze the memory processes are needed to elucidate the whole picture of the effects of histamine on learning and memory. Regarding the human histaminergic system, alterations in histamine signaling have been reported in several neuropsychiatric disorders, and these changes have been suggested to be involved in cognitive dysfunction in patients with the neuropsychiatric disorders. Therefore, the drugs that modulate histamine signaling, including H₃R antagonists/inverse agonists, may be effective in the treatment of cognitive dysfunction, including Alzheimer's disease.

Linking Social Cognition to Learning and Memory

Many mammals have evolved to be social creatures. In humans, the ability to learn from others' experiences is essential to survival; and from an early age, individuals are surrounded by a social environment that helps them develop a variety of skills, such as walking, talking, and avoiding danger. Similarly, in rodents, behaviors, such as food preference, exploration of novel contexts, and social approach, can be learned through social interaction. Social encounters facilitate new learning and help modify preexisting memories throughout the lifespan of an organism. Moreover, social encounters can help buffer stress or the effects of negative memories, as well as extinguish maladaptive behaviors. Given the importance of such interactions, there has been increasing work studying social learning and applying its concepts in a wide range of fields, including psychotherapy and medical sociology. The process of social learning, including its neural and behavioral mechanisms, has also been a rapidly growing field of interest in neuroscience. However, the term "social learning" has been loosely applied to a variety of psychological phenomena, often without clear definition or delineations. Therefore, this review gives a definition for specific aspects of social learning, provides an overview of previous work at the circuit, systems, and behavioral levels, and finally, introduces new findings on the social modulation of learning. We contextualize such social processes in the brain both through the role of the hippocampus and its capacity to process "social engrams" as well as through the brainwide realization of social experiences. With the integration of new technologies, such as optogenetics, chemogenetics, and calcium imaging, manipulating social engrams will likely offer a novel therapeutic target to enhance the positive buffering effects of social experiences or to inhibit fear-inducing social stimuli in models of anxiety and post-traumatic stress disorder.

Social Transfer of Fear in Rodents

Social transfer of fear is a potent tool facilitating response to danger in animals forming social groups. With many factors influencing the transfer-such as proximity of the animal receiving information to the donor, familiarity, proximity of danger, and species-specific coping strategies-it allows studies of neuronal correlates of a variety of behavioral responses. Since both the transfer of fear and social modulation of fear responses are impaired in many neuropsychological disorders, the models described in this article could be useful in disentangling the neuronal circuitry involved in the pathogenesis of these disorders. © 2019 by John Wiley & Sons, Inc. Basic Protocol 1: Imminent threat in rats Alternate Protocol 1: Imminent threat in mice Basic Protocol 2: Remote threat in rats Alternate Protocol 2: Remote threat in mice Basic Protocol 3: Social modulation of fear extinction in rats Alternate Protocol 3: Social modulation of fear extinction in mice.

Memory engrams: Recalling the past and imagining the future

In 1904, Richard Semon introduced the term "engram" to describe the neural substrate for storing memories. An experience, Semon proposed, activates a subset of cells that undergo off-line, persistent chemical and/or physical changes to become an engram. Subsequent reactivation of this engram induces memory retrieval. Although Semon's contributions were largely ignored in his lifetime, new technologies that allow researchers to image and manipulate the brain at the level of individual neurons has reinvigorated engram research. We review recent progress in studying engrams, including an evaluation of evidence for the existence of engrams, the importance of intrinsic excitability and synaptic plasticity in engrams, and the lifetime of an engram. Together, these findings are beginning to define an engram as the basic unit of memory.