Intact maternal buffering of stress response in infant rats despite altered responsivity towards maternal olfactory cues in the valproic acid model of autism-like behavior.

Disrupted processing of social cues and altered social behaviors are among the core symptoms of autism spectrum disorders (ASDs), and they emerge as early as the first year of life. These differences in sensory abilities may affect the ability of children with ASDs to securely attach to a caregiver and experience caregiver buffering of stress. Prenatal exposure to valproic acid (VPA) has been used to model some aspects of ASDs in rodents. Here, we asked whether prenatal VPA exposure altered infant rats' behavioral responsivity to maternal olfactory cues in an Odor Preference Test (OPT) and affected maternal buffering of infants' stress responsivity to shock. In the odor preference test, 1-week old rats treated with VPA during pregnancy appeared to have impaired social recognition and/or may be less motivated to approach social odors in early infancy. These effects were particularly prominent in female pups. In 2-week old rats, VPA-exposed pups and saline-exposed pups showed similar preferences for home cage bedding. Although VPA-exposed pups may initially have a deficit in this attachment-related behavior they do recover typical responses to home cage bedding in later infancy. Both control and VPA-exposed pups showed robust stress hormone responses to repeated shocks, an effect which was blocked when a calm mother was present during shock exposure. No sex differences in the effect of maternal presence on the stress response to shock and no interactions between sex and prenatal drug exposure were observed. Although VPA-exposed pups may show impaired responsivity to maternal cues in early infancy, maternal presence is still capable of regulating the stress response in VPA-exposed pups. In this study we demonstrate the importance of utilizing multiple batteries of tests in assessing behavior, dissecting the behavior on one test into different components. Our results inform about the underlying behavioral characteristics of some of the ASD phenotypes, including sex differences reported by clinical studies, and could shed light on potential opportunities for intervention.

Acid-sensing ion channel 1a regulates the specificity of reconsolidation of conditioned threat responses.

Recent research on altering threat memory has focused on a reconsolidation window. During reconsolidation, threat memories are retrieved and become labile. Reconsolidation of distinct threat memories is synapse dependent, whereas the underlying regulatory mechanism of the specificity of reconsolidation is poorly understood. We designed a unique behavioral paradigm in which a distinct threat memory can be retrieved through the associated conditioned stimulus. In addition, we proposed a regulatory mechanism by which the activation of acid-sensing ion channels (ASICs) strengthens the distinct memory trace associated with the memory reconsolidation to determine its specificity. The activation of ASICs by CO2 inhalation, when paired with memory retrieval, triggers the reactivation of the distinct memory trace, resulting in greater memory lability. ASICs potentiate the memory trace by altering the amygdala-dependent synaptic transmission and plasticity at selectively targeted synapses. Our results suggest that inhaling CO2 during the retrieval event increases the lability of a threat memory through a synapse-specific reconsolidation process.

Social Fear Learning: from Animal Models to Human Function.

Learning about potential threats is critical for survival. Learned fear responses are acquired either through direct experiences or indirectly through social transmission. Social fear learning (SFL), also known as vicarious fear learning, is a paradigm successfully used for studying the transmission of threat information between individuals. Animal and human studies have begun to elucidate the behavioral, neural and molecular mechanisms of SFL. Recent research suggests that social learning mechanisms underlie a wide range of adaptive and maladaptive phenomena, from supporting flexible avoidance in dynamic environments to intergenerational transmission of trauma and anxiety disorders. This review discusses recent advances in SFL studies and their implications for basic, social and clinical sciences.

The neurobiology of safety and threat learning in infancy.

What an animal needs to learn to survive is altered dramatically as they change from dependence on the parent for protection to independence and reliance on self-defense. This transition occurs in most altricial animals, but our understanding of the behavioral neurobiology has mostly relied on the infant rat. The transformation from dependence to independence occurs over three weeks in pups and is accompanied by complex changes in responses to both natural and learned threats and the supporting neural circuitry. Overall, in early life, the threat system is quiescent and learning is biased towards acquiring attachment related behaviors to support attachment to the caregiver and proximity seeking. Caregiver-associated cues learned in infancy have the ability to provide a sense of safety throughout lifetime. This attachment/safety system is activated by learning involving presumably pleasurable stimuli (food, warmth) but also painful stimuli (tailpinch, moderate shock). At about the midway point to independence, pups begin to have access to the adult-like amygdala-dependent threat system and amygdala-dependent responses to natural dangers such as predator odors. However, pups have the ability to switch between the infant and adult-like system, which is controlled by maternal presence and modification of stress hormones. Specifically, if the pup is alone, it will learn fear but if with the mother it will learn attachment (10-15days of age). As pups begin to approach weaning, pups lose access to the attachment system and rely only on the amygdala-dependent threat system. However, pups learning system is complex and exhibits flexibility that enables the mother to override the control of the attachment circuit, since newborn pups may acquire threat responses from the mother expressing fear in their presence. Together, these data suggest that the development of pups' threat learning system is not only dependent upon maturation of the amygdala, but it is also exquisitely controlled by the environment. Most notably the mother can switch pup learning between attachment to threat learning in a moment's notice. This enables the mother to navigate pup's learning about the world and what is threatening and what is safe.

Neural correlates of the mother-to-infant social transmission of fear.

Although clinical and basic studies show that parental trauma, fear, and anxiety may be transmitted to offspring, the neurobiology of this transmission is still not well understood. We recently demonstrated in an animal model that infant rats acquire threat responses to a distinct cue when a mother expresses fear to this cue in their presence. This ability to acquire maternal fear through social learning is present at birth and, as we previously reported, depends on the pup's amygdala. However, the remaining neural mechanisms underlying social fear learning (SFL) in infancy remain elusive. Here, by using [(14) C]2-deoxyglucose autoradiography, we show that the mother-to-infant transmission of fear in preweaning rats is associated with a significant increase of activity in the subregions of the lateral septum, nucleus accumbens, bed nucleus of stria terminalis, retrosplenial cortex, paraventricular nucleus of the thalamus, mediodorsal and intralaminar thalamic nuclei, medial and the lateral preoptic nuclei of the hypothalamus, and the lateral periaqueductal gray. In contrast to studies of adult SFL demonstrating the role of the anterior cingulate cortex and possibly the insular cortex or research of infant classical fear conditioning showing the role of the posterior piriform cortex, no changes of activation in these areas were observed. Our results indicate that the pup's exposure to maternal fear activates a number of areas involved in processing threat, stress, or pain. This pattern of activation suggests a unique set of neural mechanisms underlying SFL in the developing brain.

Intergenerational transmission of emotional trauma through amygdala-dependent mother-to-infant transfer of specific fear.

Emotional trauma is transmitted across generations. For example, children witnessing their parent expressing fear to specific sounds or images begin to express fear to those cues. Within normal range, this is adaptive, although pathological fear, such as occurs in posttraumatic stress disorder or specific phobias, is also socially transmitted to children and is thus of clinical concern. Here, using a rodent model, we report a mother-to-infant transfer of fear to a novel peppermint odor, which is dependent on the mother expressing fear to that smell in pups' presence. Examination of pups' neural activity using c-Fos early gene expression and (14)C 2-deoxyglucose autoradiography during mother-to-infant fear transmission revealed lateral and basal amygdala nuclei activity, with a causal role highlighted by pharmacological inactivation of pups' amygdala preventing the fear transmission. Maternal presence was not needed for fear transmission, because an elevation of pups' corticosterone induced by the odor of the frightened mother along with a novel peppermint odor was sufficient to produce pups' subsequent aversion to that odor. Disruption of axonal tracts from the Grueneberg ganglion, a structure implicated in alarm chemosignaling, or blockade of pups' alarm odor-induced corticosterone increase prevented transfer of fear. These memories are acquired at younger ages compared with amygdala-dependent odor-shock conditioning and are more enduring following minimal conditioning. Our results provide clues to understanding transmission of specific fears across generations and its dependence upon maternal induction of pups' stress response paired with the cue to induce amygdala-dependent learning plasticity. Results are discussed within the context of caregiver emotional responses and adaptive vs. pathological fears social transmission.

The selectivity of aversive memory reconsolidation and extinction processes depends on the initial encoding of the Pavlovian association.

In reconsolidation studies, memories are typically retrieved by an exposure to a single conditioned stimulus (CS). We have previously demonstrated that reconsolidation processes are CS-selective, suggesting that memories retrieved by the CS exposure are discrete and reconsolidate separately. Here, using a compound stimulus in which two distinct CSs are concomitantly paired with the same aversive unconditioned stimulus (US), we show in rats that reexposure to one of the components of the compound CS triggers extinction or reconsolidation of the other component. This suggests that the original training conditions play a critical role in memory retrieval and reconsolidation.

Sensory-specific associations stored in the lateral amygdala allow for selective alteration of fear memories.

Consolidated long-term fear memories become labile and can be disrupted after being reactivated by the presentation of the unconditioned stimulus (US). Whether this is due to an alteration of the conditioned stimulus (CS) representation in the lateral amygdala (LA) is not known. Here, we show in rats that fear memory reactivation through presentation of the aversive US, like CS presentation, triggers a process which, when disrupted, results in a selective depotentiation of CS-evoked neural responses in the LA in correlation with a selective suppression of CS-elicited fear memory. Thus, an aversive US triggers the reconsolidation of its associated predictor representation in LA. This new finding suggests that sensory-specific associations are stored in the lateral amygdala, allowing for their selective alteration by either element of the association.

Noradrenergic enhancement of reconsolidation in the amygdala impairs extinction of conditioned fear in rats--a possible mechanism for the persistence of traumatic memories in PTSD.

BACKGROUND: Posttraumatic stress disorder (PTSD) is associated with enhanced noradrenergic activity. Animal and human studies demonstrate that noradrenergic stimulation augments consolidation of fear learning. Retrieval of well-established memories by presenting a learned fear cue triggers reconsolidation processes during which memories may be updated, weakened, or strengthened. We previously reported that noradrenergic blockade in the rat amygdala impairs reconsolidation of fear memories. Here we investigated the effects of noradrenergic enhancement on reconsolidation of learned fear. METHODS: Using auditory fear conditioning in rats, we tested the effects of postretrieval intraamygdala infusion of the ß-adrenergic receptor agonist isoproterenol or the antagonist propranolol on conditioned fear in the amygdala. RESULTS: A single intraamygdala infusion of isoproterenol following a retrieval of a well-consolidated memory enhanced fear memory elicited by the learned fear stimulus and impaired extinction of this memory 48 hr later. Intraamygdala infusion of the ß-adrenergic receptor antagonist propranolol following a consecutive retrieval trial blocked the enhancing effects of isoproterenol on fear memory. CONCLUSIONS: Postretrieval ß-adrenergic stimulation in the amygdala enhances reconsolidation of fear memories, making them resistant to extinction. Noradrenergic augmentation during retrieval of fear memories may thus contribute to persistence and severity of traumatic memories. Reconsolidation may be a useful tool in understanding the pathology of PTSD and may thus help in developing new and in modifying existing treatments of traumatic memories.

The amygdala encodes specific sensory features of an aversive reinforcer.

Studies of reconsolidation, in which retrieved memories are altered and restored, offer an approach for exploring the associative structure of fear memory. We found that exposure to the unconditioned stimulus initiates an unconditioned stimulus-specific reconsolidation of learned fear in rats that depended on the amygdala. Thus, specific features of the unconditioned stimulus appear to be encoded in the amygdala as part of fear memories stored there.

Functional emergence of the hippocampus in context fear learning in infant rats.

The hippocampus is a part of the limbic system and is important for the formation of associative memories, such as acquiring information about the context (e.g., the place where an experience occurred) during emotional learning (e.g., fear conditioning). Here, we assess whether the hippocampus is responsible for pups' newly emerging context learning. In all experiments, postnatal day (PN) 21 and PN24 rat pups received 10 pairings of odor-0.5 mA shock or control unpaired odor-shock, odor only, or shock only. Some pups were used for context, cue or odor avoidance tests, while the remaining pups were used for c-Fos immunohistochemistry to assess hippocampal activity during acquisition. Our results show that cue and odor avoidance learning were similar at both ages, while contextual fear learning and learning-associated hippocampal (CA1, CA3, and dentate gyrus) activity (c-Fos) only occurred in PN24 paired pups. To assess a causal relationship between the hippocampus and context conditioning, we infused muscimol into the hippocampus, which blocked acquisition of context fear learning in the PN24 pups. Muscimol or vehicle infusions did not affect cue learning or aversion to the odor at PN21 or PN24. The results suggest that the newly emerging contextual learning exhibited by PN24 pups is supported by the hippocampus.

Phosphorylation of ERK/MAP kinase is required for long-term potentiation in anatomically restricted regions of the lateral amygdala in vivo.

We have previously shown that the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/ MAPK) is transiently activated in anatomically restricted regions of the lateral amygdala (LA) following Pavlovian fear conditioning and that blockade of ERK/MAPK activation in the LA impairs both fear memory consolidation and long-term potentiation (LTP) in the amygdala, in vitro. The present experiments evaluated the role of the ERK/MAPK signaling cascade in LTP at thalamo-LA input synapses, in vivo. We first show that ERK/MAPK is transiently activated/phosphorylated in the LA at 5 min, but not 15 or 60 min, after high-frequency, but not low-frequency, stimulation of the auditory thalamus. ERK activation induced by LTP-inducing stimulation was anatomically restricted to the same regions of the LA previously shown to exhibit ERK regulation following fear conditioning. We next show that intra-LA infusion of U0126, an inhibitor of ERK/MAPK activation, impairs LTP at thalamo-LA input synapses. Collectively, results demonstrate that ERK/MAPK activation is necessary for synaptic plasticity in anatomically defined regions of the LA, in vivo.

From affiliative behaviors to romantic feelings: a role of nanopeptides.

Love is one of the most desired experiences. The quest for understanding human bonds, especially love, was traditionally a domain of the humanities. Recent developments in biological sciences yield new insights into the mechanisms underlying the formation and maintenance of human relationships. Animal models of reproductive behaviors, mother-infant attachment and pair bonding complemented by human studies reveal neuroendocrine foundations of prosocial behaviors and emotions. Amongst various identified neurotransmitters and modulators, which control affiliative behaviors, the particular role of nanopeptides has been indicated. New studies suggest that these chemicals are not only involved in regulating bonding processes in animals but also contribute to generating positive social attitudes and feelings in humans.

Synapse-specific reconsolidation of distinct fear memories in the lateral amygdala.

When reactivated, memories enter a labile, protein synthesis-dependent state, a process referred to as reconsolidation. Here, we show in rats that fear memory retrieval produces a synaptic potentiation in the lateral amygdala that is selective to the reactivated memory, and that disruption of reconsolidation is correlated with a reduction of synaptic potentiation in the lateral amygdala. Thus, both retrieval and reconsolidation alter memories via synaptic plasticity at selectively targeted synapses.

Noradrenergic signaling in the amygdala contributes to the reconsolidation of fear memory: treatment implications for PTSD.

Intrusive memories resulting from an emotional trauma are a defining feature of posttraumatic stress disorder (PTSD). Existing studies demonstrate that an increase of noradrenergic activity during a life-threatening event contributes to strengthening or "overconsolidation" of the memory for trauma. The lateral nucleus of the amygdala (LA) is critical for fear learning. Using classical fear conditioning in rats, we have recently demonstrated that noradrenergic blockade in the LA following reactivation of fear memory by retrieval disrupts memory reconsolidation and lastingly impairs fear memory. This suggests that noradrenergic blockade may be useful in attenuating traumatic memories, even well-consolidated old memories, in PTSD.

Directly reactivated, but not indirectly reactivated, memories undergo reconsolidation in the amygdala.

Memory consolidation refers to a process by which newly learned information is made resistant to disruption. Traditionally, consolidation has been viewed as an event that occurs once in the life of a memory. However, considerable evidence now indicates that consolidated memories, when reactivated through retrieval, become labile (susceptible to disruption) again and undergo reconsolidation. Because memories are often interrelated in complex associative networks rather than stored in isolation, a key question is whether reactivation of one memory makes associated memories labile in a way that requires reconsolidation. We tested this in rats by creating interlinked associative memories using a second-order fear-conditioning task. We found that directly reactivated memories become labile, but indirectly reactivated (i.e., associated) memories do not. This suggests that memory reactivation produces content-limited rather than wholesale changes in a memory and its associations and explains why each time a memory is retrieved and updated, the entire associative structure of the memory is not grossly altered.

Peptides of love and fear: vasopressin and oxytocin modulate the integration of information in the amygdala.

Neuropeptides vasopressin and oxytocin regulate a variety of behaviors ranging from maternal and pair bonding to aggression and fear. Their role in modulating fear responses has been widely recognized, but not yet well understood. Animal and human studies indicate the major role of the amygdala in controlling fear and anxiety. The amygdala is involved in detecting threat stimuli and linking them to defensive behaviors. This is accomplished by projections connecting the central nucleus of the amygdala (CeA) to the brain stem and to hypothalamic structures, which organize fear responses. A recent study by Huber et al demonstrates that vasopressin and oxytocin modulate the excitatory inputs into the CeA in opposite manners. Therefore this finding elucidates the mechanisms through which these neuropeptides may control the expression of fear.

Auditory fear conditioning and long-term potentiation in the lateral amygdala require ERK/MAP kinase signaling in the auditory thalamus: a role for presynaptic plasticity in the fear system.

In the present study, we examined the role of the auditory thalamus [medial division of the medial geniculate nucleus and the adjacent posterior intralaminar nucleus (MGm/PIN)] in auditory pavlovian fear conditioning using pharmacological manipulation of intracellular signaling pathways. In the first experiment, rats were given intrathalamic infusions of the MEK (mitogen-activated protein kinase-kinase) inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto) butadiene (U0126) before fear conditioning. Findings revealed that long-term memory (assessed at 24 h) was impaired, whereas short-term memory (assessed at 1-3 h) of fear conditioning was intact. In the second experiment, rats received immediate posttraining intrathalamic infusion of U0126, the mRNA synthesis inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), or infusion of the protein synthesis inhibitor anisomycin. Posttraining infusion of either U0126 or DRB significantly impaired long-term retention of fear conditioning, whereas infusion of anisomycin had no effect. In the final experiment, rats received intrathalamic infusion of U0126 before long-term potentiation (LTP)-inducing stimulation of thalamic inputs to the lateral nucleus of the amygdala (LA). Findings revealed that thalamic infusion of U0126 impaired LTP in the LA. Together, these results suggest the possibility that MGm/PIN cells that project to the LA contribute to memory formation via ERK (extracellular signal-regulated kinase)-mediated transcription, but that they do so by promoting protein synthesis-dependent plasticity locally in the LA.