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The inability to extinguish a traumatic memory is a key aspect of post-traumatic stress disorder (PTSD). While PTSD affects 10–20% of individuals who experience a trauma, women are particularly susceptible to developing the disorder. Despite this notable female vulnerability, few studies have investigated this particular resistance to fear extinction observed in females. Similar to humans, rodent models of Pavlovian fear learning and extinction show a wide range of individual differences in fear learning and extinction, although female rodents are considerably understudied. Therefore, the present study examined individual differences in fear responses, including freezing behavior and ultrasonic vocalizations (USVs), of female Long–Evans rats during acquisition of fear conditioning and cued fear extinction. Similar to prior studies in males, female rats displayed individual variation in freezing during cued fear extinction and were divided into extinction competent (EC) and extinction resistant (ER) phenotypes. Differences in freezing between ER and EC females were accompanied by shifts in rearing during extinction, but no darting was seen in any trial. Freezing behavior during fear learning did not differ between the EC and ER females. Vocalizations emitted in the 22 and 50 kHz ranges during fear learning and extinction were also examined. Unlike vocalizations seen in previous studies in males, very few 22 kHz distress vocalizations were emitted by female rats during fear acquisition and extinction, with no difference between ER and EC groups. Interestingly, all female rats produced significant levels of 50 kHz USVs, and EC females emitted significantly more 50 kHz USVs than ER rats. This difference in 50 kHz USVs was most apparent during initial exposure to the testing environment. These results suggest that like males, female rodents show individual differences in both freezing and USVs during fear extinction, although females appear to vocalize more in the 50 kHz range, especially during initial periods of exposure to the testing environment, and emit very few of the 22 kHz distress calls that are typically observed in males during fear learning or extinction paradigms. Overall, these findings show that female rodents display fear behavior repertoires divergent from males.

The study of fear conditioning has led to a better understanding of fear and anxiety-based disorders such as post-traumatic stress disorder (PTSD). Despite the fact many of these disorders are more common in women than in men, the vast majority of work investigating fear conditioning in rodents has been conducted in males. The goal of the work presented here was to better understand how biological sex affects contextual fear conditioning and expression. To this end, rats of both sexes were trained to fear a specific context and fear responses were measured upon re-exposure to the conditioning context. In the first experiment, male and female rats were given context fear conditioning and tested the next day during which freezing behavior was measured. In the second experiment, rats were trained and tested in a similar fashion while fear-potentiated startle and defecation were measured. We found that males showed more freezing behavior than females during a fear expression test. The expression of fear-potentiated startle did not differ between sexes, while males exhibited more defecation during a test in a novel context. These data suggest that the expression of defensive behavior differs between sexes and highlight the importance of using multiple measures of fear when comparing between sexes.

The insular cortex (IC), among other brain regions, becomes active when humans experience fear or anxiety. However, few experimental studies in rats have implicated the IC in threat responses. We have recently reported that inactivation of the primary interoceptive cortex (pIC) during pre-training, or the intra-pIC blockade of protein synthesis immediately after training, impaired the consolidation of auditory fear conditioning. The present study was designed to investigate the role of the pIC in innate and learned defensive responses to predator odor. Freezing behavior was elicited by single or repetitive exposures to a collar that had been worn by a domestic cat. Sessions were video-recorded and later scored by video observation. We found that muscimol inactivation of the pIC reduced the expression of freezing reaction in response to a single or repeated exposure to cat odor. We also found that pIC inactivation with muscimol impaired conditioning of fear to the context in which rats were exposed to cat odor. Furthermore, neosaxitoxin inactivation of the pIC resulted in a prolonged and robust reduction in freezing response in subsequent re-exposures to cat odor. In addition, freezing behavior significantly correlated with the neural activity of the IC. The present results suggest that the IC is involved in the expression of both innate and learned fear responses to predator odor.

Early life maltreatment by the caregiver constitutes a major risk factor for the development of later-life psychopathologies, including fear-related pathologies. Here, we used an animal model of early life maltreatment induced by the Scarcity-Adversity Model of low bedding (LB) where the mother is given insufficient bedding for nest building while rat pups were postnatal days (PN) 8–12. To assess effects of maltreatment on the expression of threat-elicited defensive behaviors, animals underwent odor-shock threat conditioning at three developmental stages: late infancy (PN18), adolescence (PN45) or adulthood (>PN75) and tested the next day with odor only presentations (cue test). Results showed that in typically developing rats, the response to threat increases with maturation, although experience with maltreatment in early infancy produced enhanced responding to threat in infancy and adulthood, but a decrease in maltreated adolescents. To better understand the unique features of this decreased threat responding in adolescence, c-Fos expression was assessed within the amygdala and ventromedial prefrontal cortex (vmPFC) associated with the cued expression of threat learning. Fos counts across amygdala subregions were lower in LB rats compared to controls, while enhanced c-Fos expression was observed in the vmPFC prelimbic cortex (PL). Correlational analysis between freezing behavior and Fos revealed freezing levels were correlated with CeA in controls, although more global correlations were detected in LB-reared rats, including the BA, LA, and CeA. Functional connectivity analysis between brain regions showed that LB reared rats exhibited more diffuse interconnectivity across amygdala subnuclei, compared the more heterogeneous patterns observed in controls. In addition, functional connectivity between the IL and LA switched from positive to negative in abused adolescents. Overall, these results suggest that in adolescence, the unique developmental decrease in fear expression following trauma is associated with distinct changes in regional function and long-range connectivity, reminiscent of pathological brain function. These results suggest that early life maltreatment from the caregiver perturbs the developmental trajectory of threat-elicited behavior. Indeed, it is possible that this form of trauma, where the infant’s safety signal or “safe haven” (the caregiver) is actually the source of the threat, produces distinct outcomes across development.

Post-traumatic stress disorder (PTSD) currently has no FDA-approved treatments that reduce symptoms in the majority of patients. The ability to extinguish fear memory associations is impaired in PTSD individuals. As such, the development of extinction-enhancing pharmacological agents to be used in combination with exposure therapies may benefit the treatment of PTSD. Both mGlu5 and CB1 receptors have been implicated in contextual fear extinction. Thus, here we tested the ability of the mGlu5 positive allosteric modulator 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) and cannabidiol (CBD) to reduce both conditioned and unconditioned fear. We used a predator-threat animal model of PTSD which we and others have previously shown to capture the heterogeneity of anxiety responses observed in humans exposed to trauma. Here, 1 week following a 10-min exposure to predator scent stress, rats were classified into stress-Susceptible and stress-Resilient phenotypes using behavioral criteria for elevated plus maze and acoustic startle response performance. Two weeks after classification, rats underwent 3 days of contextual fear extinction and were treated with vehicle, CDPPB or CBD prior to each session. Finally, the light-dark box test was employed to assess phenotypic differences and the effects of CDPPB and CBD on unconditioned anxiety. CDPBB but not CBD, reduced freezing in Susceptible rats relative to vehicle. In the light-dark box test for unconditioned anxiety, CBD, but not CDPPB, reduced anxiety in Susceptible rats. Resilient rats displayed reduced anxiety in the light-dark box relative to Susceptible rats. Taken together, the present data indicate that enhancement of mGlu5 receptor signaling in populations vulnerable to stress may serve to offset a resistance to fear memory extinction without producing anxiogenic effects. Furthermore, in a susceptible population, CBD attenuates unconditioned but not conditioned fear. Taken together, these findings support the use of predator-threat stress exposure in combination with stress-susceptibility phenotype classification as a model for examining the unique drug response profiles and altered neuronal function that emerge as a consequence of the heterogeneity of psychophysiological response to stress.

Throughout life, we learn to associate stimuli with their consequences. But some of the new information that we encounter forces us to abandon what we had previously acquired. This old information is then subject to a new learning process that is called extinction learning. This involves a large number of brain structures (Kattoor et al., 2013; Lissek et al., 2013, Lissek et al.; Merz et al., 2014). Extinction is an unusually complex learning process that can involve both Pavlovian (classical; Pavlov, 1927; Lattal and Lattal, 2012) and operant (instrument) conditioning (Skinner, 1938; Bouton et al., 2012). A further hallmark is its context-dependency (Bouton, 2004) that is likely to rely on a tight interaction between the hippocampus and other brain areas (e.g., Andre et al.; Icenhour et al., 2015). Thus, one of the aims of the present Research Topic was to incorporate studies that analyze the concert of neural structures that enable extinction learning. The old memory trace may be partly, or not at all forgotten during extinction (Ungor and Lachnit, 2006). It tends to re-emerge after a passage of time (spontaneous recovery), when re-exposure to the context of original learning occurs (renewal), or unexpected exposure to the unconditioned stimulus takes place (reinstatement). Such invasive memories are key symptoms of anxiety or pain disorders. They especially occur in individuals with enhanced susceptibility (Mosig et al.; Glombiewski et al., 2015). Although pathological fear in anxiety disorders can be treated through extinction-based approaches, treatment is not always successful in the long-term, underscoring the need to understand the mechanisms underlying impaired extinction. Therefore, the second aim of the Research Topic was to include publications that are situated at the transition between basic and clinical neuroscience. Given the relevance of extinction, it is astonishing how little we know about extinction learning, in terms of its neural fundaments and its development, especially when moving outside the realm of fear extinction in rodents. The third aim of the Research Topic was therefore to include papers on the uncharted territories of extinction learning that involve less-studied entities such as the immune system (Hadamitzky et al., 2016) or hormonal factors (Wolf et al., 2015; Maren and Holmes, 2016), less-studied species (Lengersdorf et al.) or novel paradigms (Wiescholleck et al., 2014). One specific goal of this Research Topic was to offer a basis for trans-species comparisons, as reflected by the spectrum of animals described that range from snails, through mice, rats, and pigeons. Several of the studies also describe extinction learning in humans, including pharmacological approaches. A number of studies (Andre et al.; Lengersdorf et al.; Andre and Manahan-Vaughan; Andrianov et al.; de Oliveira et al.; Lissek et al.) addressed neurotransmitter systems that are known to be involved in other forms of learning (Morris, 2013; Seyedabadi et al., 2014; Bauer, 2015) and in synaptic plasticity that is believed to underlie learning (Harley, 2004; Lesch and Waider, 2012; Park et al., 2013; Hansen and Manahan-Vaughan, 2014; Hagena et al., 2015). Here, for example, antagonism of N-methyl-D-aspartate receptors (NMDAR) prevented appetitive extinction in pigeons (Lengersdorf et al.), and GluN2B-containing NMDAR were found to play a key role in extinction of conditioned suppression of licking in rats (de Oliveira et al.). In an interesting corollary to the latter finding, Shumake and Monfils describe how conditioned suppression of licking is far more sensitive to extinction than freezing behavior, and along with Lee et al. investigated the impact of reactivating the original memory trace on extinction success. Examination of the role of dopamine receptors in appetitive learning in rats (Andre and Manahan-Vaughan) and predictive learning in humans (Lissek et al.), highlight differences that may relate to the species, or the extinction learning paradigm studied. Studies with regard to the neural basis of extinction learning, and its associated brain structures, revealed a specific and experience-dependent role of microcircuitry within the basolateral amygdala (Sangha). In their review article, Giustino and Maren challenge the common assumption that the medial prefrontal cortex (mPFC) mediates the expression, whereas the infralimbic cortex (IL) mediates the suppression of fear responses, whereas Lee at al. offer experimental evidence that extinction learning and retrieval trigger differentiated responses in the mPFC and amygdala. Goodman and Packard differentiated between extinction learning of response and place learning, and provide evidence that the effectivity of the extinction learning strategy depends on the memory system (dorsolateral striatum vs. hippocampus) that encoded the original experience. In line with studies in rats (Gershman et al.), Shiban et al. observed that gradually reducing the frequency of aversive stimuli, in a Pavlovian fear conditioning paradigm in humans, is more effective in averting the return of fear than abrupt stimulus withdrawal, and Zlomuzica et al. demonstrate that improved self-efficacy also improves fear extinction. By contrast, Vervliet and Indekkeu show that low-cost avoidance behavior is resilient to extinction. Earlier studies indicate that extinction learning is reinforced by a context change (Bouton, 2004). Here, Sjouwerman et al. report that the timing of the context change is decisive with regard to the functional outcome with regard to both extinction and renewal. At the structural and/or molecular levels, several studies provided evidence for the direct involvement of the hippocampus in extinction learning (Lissek et al.; de Oliveira et al.; Wille et al.). Whereas, de Oliveira et al. provide evidence of the involvement of the dorsal hippocampus in conditioned suppression, Wille et al. describe how modulation of the expression of chromatin remodeling factors in the ventral hippocampus rescue impaired extinction of conditioned fear. Several studies examined hormonal control of extinction learning in fear, or stress-based, paradigms (Perez-Torres et al.; Hadad-Ophir et al.; Labrenz et al.): aspects that were also addressed in a review article by Stockhorst and Antov and a research perspective by Elsenbruch and Wolf. What becomes apparent from these studies is the emergence of fine-tuning of our understanding as to which neural structures regulate extinction learning, what common denominators (and differences) exist between species, and how the regulation of extinction learning by neurotransmitter systems aligns with current knowledge as to the role of these systems in learning and memory. The papers compiled in this Research Topic offer new and valuable insights into the mechanisms and functional implementation of extinction learning at its different levels of complexity, and form the basis for new concepts and research ideas in this field.

Trauma- and stress-related disorders are among the most common types of mental illness affecting the U.S. population. For many of these disorders, there is a striking sex difference in lifetime prevalence; for instance, women are twice as likely as men to be affected by posttraumatic stress disorder (PTSD). Gonadal steroids and their metabolites have been implicated in sex differences in fear and anxiety. One example, allopregnanolone (ALLO), is a neuroactive metabolite of progesterone that allosterically enhances GABAA receptor activity and has anxiolytic effects. Like other ovarian hormones, it not only occurs at different levels in males and females but also fluctuates over the female reproductive cycle. One brain structure that may be involved in neuroactive steroid regulation of fear and anxiety is the bed nucleus of the stria terminalis (BNST). To explore this question, we examined the consequences of augmenting or reducing ALLO activity in the BNST on the expression of Pavlovian fear conditioning in rats. In Experiment 1, intra-BNST infusions of ALLO in male rats suppressed freezing behavior (a fear response) to the conditioned context, but did not influence freezing to a discrete tone conditioned stimulus (CS). In Experiment 2, intra-BNST infusion of either finasteride, an inhibitor of ALLO synthesis, or 17-phenyl-(3α,5α)-androst-16-en-3-ol, an ALLO antagonist, in female rats enhanced contextual freezing; neither treatment affected freezing to the tone CS. These findings support a role for ALLO in modulating contextual fear via the BNST and suggest that sex differences in fear and anxiety could arise from differential steroid regulation of BNST function. The susceptibility of women to disorders such as PTSD may be linked to cyclic declines in neuroactive steroid activity within fear circuitry.

Maternal immune activation during pregnancy is an environmental risk factor for psychiatric illnesses such as schizophrenia and autism in the offspring. Hence, changes in an array of behaviors, including behavioral flexibility, consistent with altered functioning of cortico-limbic circuits have been reported in rodent models of maternal immune activation. Surprisingly, previous studies have not examined the effect of maternal immune activation on the extinction of fear conditioning which depends on cortico-limbic circuits. Thus, we tested the effects of treating pregnant Long Evans rats with the viral mimetic polyI:C (gestational day 15; 4 mg/kg; i.v.) on fear conditioning and extinction in the male offspring using two different tasks. In the first experiment, we observed no effect of polyI:C treatment on the acquisition or extinction of a classically conditioned fear memory in a non-discriminative auditory cue paradigm. However, polyI:C-treated offspring did increase contextual freezing during the recall of fear extinction in this non-discriminative paradigm. The second experiment utilized a recently developed task to explicitly test the ability of rats to discriminate among cues signifying fear, reward, and safety; a task that requires behavioral flexibility. To our surprise, polyI:C-treated rats acquired the task in a manner similar to saline-treated rats. However, upon subsequent extinction training, they showed significantly faster extinction of the freezing response to the fear cue. In contrast, during the extinction recall test, polyI:C-treated offspring showed enhanced freezing behavior before and after presentation of the fear cue, suggesting an impairment in their ability to regulate fear behavior. These behavioral results are integrated into the literature suggesting impairments in cortico-limbic brain function in the offspring of rats treated with polyI:C during pregnancy.

Pavlovian threat (fear) conditioning (PTC) is an experimental paradigm that couples innate aversive stimuli with neutral cues to elicit learned defensive behavior in response to the neutral cue. PTC is commonly used as a translational model to study neurobiological and behavioral aspects of fear and anxiety disorders, including Posttraumatic Stress Disorder (PTSD). Though PTSD is a complex, multi-faceted construct that cannot be fully captured in animals, PTC is a conceptually valid model for studying the development and maintenance of learned threat responses. Thus it can inform the understanding of PTSD symptomatology. However, there are significant individual differences in posttraumatic stress that are not as of yet accounted for in studies of PTC. Individuals exposed to danger have been shown to follow distinct patterns: some adapt rapidly and completely (resilience), others adapt slowly (recovery), and others failure to adapt (chronic stress response). Identifying similar behavioral outcomes in PTC increases the translatability of this model. In this report, we present a flexible methodology for identifying individual differences in PTC by modeling latent subpopulations, or classes, characterized by defensive behavior during training. We provide evidence from a reanalysis of previously examined PTC learning and extinction data in rats to demonstrate the effectiveness of this methodology in identifying outcomes analogous to those observed in humans exposed to threat. By utilizing Latent Class Growth Analysis to test for heterogeneity in freezing behavior during threat conditioning and extinction learning in adult male outbred rats (n =58), three outcomes were identified: rapid extinction (57.3%), slow extinction (32.3%), and failure to extinguish (10.3%), indicating that heterogeneity analogous to that in naturalistic human studies is present in experimental animal studies, strengthening their translatability in understanding stress responses in humans.

A commentary on A zebrafish model of glucocorticoid resistance shows serotonergic modulation of the stress response by Griffiths, B. B., Schoonheim, P. J., Ziv, L., Voelker, L., Baier, H., and Gahtan, E. (2012). Front. Behav. Neurosci. 6:68. doi: 10.3389/fnbeh.2012.00068 The significance of the glucocorticoid receptor (GR) in stress response regulation and mood disorders has been demonstrated by many studies in animal and human models (Anacker et al., 2011a). The paper by Griffiths' and colleagues adds to this evidence, by investigating GR function and its role in stress response regulation and antidepressant action in an exciting new zebrafish model. The main finding of this study is that mutant zebrafish larvae with impaired GR transactivation, due to a single nucleotide substitution in a region essential for DNA binding (grs357 mutants), exhibit behavioral abnormalities and neuroendocrine dysfunction. Specifically, grs357 mutants show elevated startle responses, a surrogate measure of anxiety-like behavior in fish, as well as increased cortisol levels and impaired HPA axis feedback control, demonstrated by reduced suppression of pituitary pomc upon treatment with the synthetic glucocorticoid, betamethasone. The authors also test whether the selective serotonin reuptake inhibitor (SSRI) antidepressant, fluoxetine, is able to reverse the neuroendocrine and behavioral phenotype of grs357 mutants. Their findings suggest that fluoxetine does not reverse neuroendocrine abnormalities resulting from impaired GR transactivation, while it still effectively counteracts anxiety-like behavior. These findings confirm the crucial role of the GR in neuroendocrine regulation and behavioral stress responses, and are in line with the authors' previous work, in which they have identified stress axis abnormalities and increased freezing behavior upon exposure to a novel environment in the same mutant fish (Ziv et al., 2012). Their new findings thus consolidate the use of the zebrafish model to investigate GR-dependent regulation of the stress response. The data suggest that some of the behavioral effects of fluoxetine on reversing startle response abnormalities may be independent of GR transactivation in fish. However, it is important to note that many GR-mediated effects are indeed independent of direct GR binding to the DNA, particularly the glucocorticoid-mediated regulation of pomc expression (Tuckermann et al., 1999; Bilodeau et al., 2006). It thus remains to be elucidated whether GR protein-protein interactions, such as transrepression of the transcription factors NFkB, AP-1, or cyclic AMP response element binding protein (CREB), may indeed account for some of the SSRI effects on anxiety-like behavior in this study. Moreover, an increasing body of work suggests that a membrane-bound GR may mediate some of the rapid, non-genomic effects of glucocorticoids, particularly glucocorticoid-induced changes in neuronal excitability and memory consolidation (for comprehensive reviews, please see: Haller et al., 2008 and Groeneweg et al., 2012). While previous studies have shown a crucial role for genomic GR effects in antidepressant action (Anacker et al., 2011b), the involvement of the membrane-bound form of the GR in the mechanism of action of antidepressants remains elusive and may possibly account for some of the behavioral changes upon antidepressant treatment in grs357 mutant fish, in which only GR transactivation is abolished. As the authors suggest, the role of serotonergic signaling on the neuroendocrine system, behavior, and the regulation of GR function is likely to be of great importance for the effects of SSRI antidepressants (Laplante et al., 2002). At the same time, potential serotonin-independent molecular mechanisms of antidepressant action, such as liberation of G-proteins from membrane associated lipid rafts, have been suggested (Donati and Rasenick, 2005), and may represent possible alternative pathways involved in stress response regulation and GR activation, which warrant further investigation. Of course, when modeling psychiatric disorders in fish, it needs to be carefully considered that the behavioral repertoire, the neural networks and the psychological responses to stress in teleosts are largely different from respective responses in the mammalian brain. Behavioral studies in higher animals, such as rodents and primates, as well as cellular and molecular studies in human tissues, will therefore indeed remain indispensable for our understanding of stress-induced brain abnormalities, depression pathophysiology, and antidepressant treatment response. To this end, it will be interesting for future investigations to examine if other well established neurobiological correlates of depression, such as alterations in neuronal gene expression or adult neurogenesis, can also be modeled in comparable regions of the fish brain (Kizil et al., 2012). In addition, evolutionary differences in corticosteroid receptor function have been demonstrated and need to be considered when investigating GR function across different phylogenetic taxa (Bury and Sturm, 2007). Nevertheless, the zebrafish is a useful model to investigate principle mechanisms underlying stress response regulation, and it offers a unique possibility for high-throughput drug screening in vivo, facilitating the discovery of new targets that alter neuroendocrine responses and proxy measures of stress-induced behavioral abnormalities. In order to identify new treatment targets and to elucidate the sophisticated heterogeneous mechanisms underlying depression pathogenesis, a cross-species investigation into the several layers of the complex nature of stress-induced biological abnormalities will be essential to our success. The paper by Griffiths' and colleagues, together with previous work on GR function in zebrafish (Ziv et al., 2012), is a great example for such an approach, and offers a novel high-throughput platform in live fish to investigate drug responses and GR function in an in vivo system.

Schiller and Phelps (2011) have provided a thoughtful and comprehensive review in the May issue of Frontiers in Behavioral Neurosciences entitled, “Does reconsolidation occur in humans?” This scholarly paper captures many of the challenges in translating the animal research on reconsolidation to humans. We agree with their main argument that there is little published evidence in humans that meets one important reconsolidation criterion, namely, that the memory-weakening treatment should be administered after the memory reactivation, so as not to influence the preceding memory retrieval process. However, it is unclear whether pre-reactivation propranolol significantly hampers memory retrieval. On the basis of this uncertainty, Schiller and Phelps go on to suggest that the memory impairing effects of pre-reactivation propranolol (e.g., Kindt et al., 2009; Brunet et al., 2011) must be explained by some process other than reconsolidation. Such a conclusion appears illogical, because if propranolol does not impair retrieval sufficiently to preclude reconsolidation, then blockade of memory reconsolidation remains a viable explanation for pre-reactivation propranolol's memory-weakening action. Indeed, the ultimate evidence for successful memory retrieval is measured during reactivation (e.g., freezing behavior in rodent fear conditioning, GSR in human fear conditioning, etc.). Therefore, any impairment of retrieval should be detected. Moreover, as shown recently by Debiec et al. (2011) immediate post-retrieval beta-adrenergic receptor stimulation enhances reconsolidation of fear conditioning in the amygdala. In this scenario, administration of propranolol following memory retrieval (which results in arousal and noradrenergic stimulation) may be too late to be effective. So, a parsimonious explanation is that pre-reactivation propranolol did block reconsolidation to some extent in the above-mentioned studies. Pre-reactivation propranolol studies (and other studies with a similar design) ought to be pursued vigorously because, irrespective of how the memory impairing effect is obtained, they offer the prospect of a novel approach to treating mental disorders that have at their core an emotional, usually traumatic, memory. Although Schiller and Phelps acknowledge some of the challenges of blocking reconsolidation using pharmacological means in humans, it is important to note the reasons for this difficulty. In animals, reconsolidation (i.e., protein synthesis) is believed to begin 3–10 min after memory reactivation (Monfils et al., 2009). Most of it appears to take place within the first 2 h (Przybyslawski et al., 1999) and to be over by the sixth hour (Duvarci and Nader, 2004). Considering that oral propranolol takes about 90 min to reach its peak bioavailability in human blood (Marino, 1987); and considering that only a fraction of this drug will eventually cross the blood–brain barrier to be available to exert the necessary effect, protocols that use post-reactivation propranolol are vulnerable to yielding negative results because not enough protein synthesis will have been blocked by the time the drug reaches its full effect in the human brain. Since conducting our proof-of-concept study in posttraumatic stress disorder (PTSD; Brunet et al., 2008), which used post-reactivation propranolol, we have opted to use pre-reactivation propranolol in an attempt to develop what we hope will be a more potent therapeutic protocol (see Brunet et al., 2011). It is unfortunate that this type of protocol does not meet an important scientific criterion for studies that use it to be labeled as genuine “reconsolidation” studies, but this is the price to pay for conducting sound translational research in patients. Schiller et al. (2010) are to be commended for their efforts to devise a human protocol that circumvents the problems associated with pharmacological blockade of reconsolidation by incorporating new material into a destabilized old memory, rather than simply blocking its reconsolidation. However, they have yet to show that such an approach is sufficiently potent to help patients overcome their psychological symptoms stemming from a traumatic emotional memory.

A neurally oriented conceptual and computational model of fear conditioning manifested by freezing behavior (FRAT), which accounts for many aspects of delay and context conditioning, has been constructed. Conditioning and extinction are the result of neuromodulation-controlled LTP at synapses of thalamic, cortical, and hippocampal afferents on principal cells and inhibitory interneurons of lateral and basal amygdala. The phenomena accounted for by the model (and simulated by the computational version) include conditioning, secondary reinforcement, blocking, the immediate shock deficit, extinction, renewal, and a range of empirically valid effects of pre- and post-training ablation or inactivation of hippocampus or amygdala nuclei.

The Pavlovian conditioned freezing paradigm has become a prominent mouse and rat model of learning and memory, as well as of pathological fear. Due to its efficiency, reproducibility, and well-defined neurobiology, the paradigm has become widely adopted in large-scale genetic and pharmacological screens. However, one major shortcoming of the use of freezing behavior has been that it has required the use of tedious hand scoring, or a variety of proprietary automated methods that are often poorly validated or difficult to obtain and implement. Here we report an extensive validation of the Video Freeze system in mice, a turn-key all-inclusive system for fear conditioning in small animals. Using digital video and near-infrared lighting, the system achieved outstanding performance in scoring both freezing and movement. Given the large-scale adoption of the conditioned freezing paradigm, we encourage similar validation of other automated systems for scoring freezing, or other behaviors.

Gastrointestinal disorders with abdominal pain are associated with central sensitization and psychopathologies that are often exacerbated by stress. Here we investigated the impact of colitis induced by dextran sulfate sodium (DSS) and repeated water avoidance stress (WAS) on spontaneous and nociception-related behavior and molecular signaling in the mouse brain. DSS increased the mechanical pain sensitivity of the abdominal skin while both WAS and DSS enhanced the mechanical and thermal pain sensitivity of the plantar skin. These manifestations of central sensitization were associated with augmented c-Fos expression in spinal cord, thalamus, hypothalamus, amygdala and prefrontal cortex. While WAS stimulated phosphorylation of mitogen-activated protein kinase (MAPK) p42/44, DSS activated another signaling pathway, both of which converged on c-Fos. The DSS- and WAS-induced hyperalgesia in the abdominal and plantar skin and c-Fos expression in the brain disappeared when the mice were subjected to WAS+DSS treatment. Intrarectal allyl isothiocyanate (AITC) evoked aversive behavior (freezing, reduction of locomotion and exploration) in association with p42/44 MAPK and c-Fos activation in spinal cord and brain. These effects were inhibited by morphine, which attests to their relationship with nociception. DSS and WAS exerted opposite effects on AITC-evoked p42/44 MAPK and c-Fos activation, which indicates that these transduction pathways subserve different aspects of visceral pain processing in the brain. In summary, behavioral perturbations caused by colitis and psychological stress are associated with distinct alterations in cerebral signaling. These findings provide novel perspectives on central sensitization and the sensory and emotional processing of visceral pain stimuli in the brain.