The basolateral amygdala is critical for the acquisition and extinction of associations between a neutral stimulus and a learned danger signal but not between two neutral stimuli

Lateral hypothalamic GABAergic neurons encode alcohol memories

In alcohol use disorder, the alcohol memories persist during abstinence, and exposure to stimuli associated with alcohol use can lead to relapse. This highlights the importance of investigating the neural substrates underlying not only relapse but also encoding and expression of alcohol memories. GABAergic neurons in the lateral hypothalamus (LH-GABA) have been shown to be critical for food-cue memories and motivation; however, the extent to which this role extends to alcohol-cue memories and motivations remains unexplored. In this study, we aimed to describe how alcohol-related memories are encoded and expressed in LH GABAergic neurons. Our first step was to monitor LH-GABA calcium transients during acquisition, extinction, and reinstatement of an alcohol-cue memory using fiber photometry. We trained the rats on a Pavlovian conditioning task, where one conditioned stimulus (CS+) predicted alcohol (20% EtOH) and another conditioned stimulus (CS-) had no outcome. We then extinguished this association through non-reinforced presentations of the CS+ and CS- and finally, in two different groups, we measured relapse under non-primed and alcohol-primed induced reinstatement. Our results show that initially both cues caused increased LH-GABA activity, and after learning only the alcohol cue increased LH-GABA activity. After extinction, this activity decreases, and we found no differences in LH-GABA activity during reinstatement in either group. Next, we inhibited LH-GABA neurons with optogenetics to show that activity of these neurons is necessary for the formation of an alcohol-cue association. These findings suggest that LH-GABA might be involved in attentional processes modulated by learning.

Lipopolysaccharide (LPS) attenuates the primary conditioning of lithium chloride (LiCl)-induced context aversion but not the secondary conditioning of context aversion or taste avoidance

A first-order association can be formed between toxin-induced nausea and a context, as well as nausea and a taste cue. However, comparatively little is understood about second-order associations. The present study examined if the bacterial endotoxin, LPS, could impair the first- and second-order conditioning of context aversion (anticipatory nausea paradigm) and subsequent conditioned taste avoidance (two-bottle task). Adult male Long Evans rats were treated with LiCl (127 mg/kg, intraperitoneal [i.p.]) or vehicle control (NaCl) and then exposed to a distinct context for 4 first-order conditioning trials. LPS (200 μg/kg, i.p.) or NaCl were administered 24 h after each trial. Seventy-two h after the final first-order conditioning trial, rats underwent 2 second-order conditioning trials where they were treated with 2% saccharin (i.p.) and then exposed to the same context. Twenty-four h after the final second-order conditioning trial, rats were tested in a two-bottle task (2 trials), where they were given a choice between water and a palatable 0.2% saccharin solution. LiCl-treated rats demonstrated a context aversion by the 3rd conditioning trial in the anticipatory nausea paradigm. Rats previously exposed to LiCl also displayed a conditioned taste avoidance of saccharin within the two-bottle task. LPS attenuated first-order context aversion but did not alter either second-order context aversion or conditioned taste avoidance in the two-bottle task. This study demonstrated that a secondary association formed within an aversive context could result in a conditioned taste avoidance. Further, LPS may be able to attenuate primary conditioning, but not secondary conditioning.

Stimulating oxytocin receptors in the basolateral amygdala enhances stimulus processing: Differential and consistent effects for stimuli paired with fear versus sucrose in extinction and reversal learning

Oxytocin (OT) influences a range of social behaviors by enhancing the salience of social cues and regulating the expression of specific social behaviors (e.g., maternal care versus defensive aggression). We previously showed that stimulating OT receptors in the basolateral amygdala of rats also enhanced the salience of fear conditioned stimuli: relative to rats given vehicle infusions, rats infused with [Thr4,Gly7]-oxytocin (TGOT), a selective OT receptor agonist, showed greater discrimination between a cue predictive of danger, and one that signaled safety. In the present series of experiments using male rats, the effects of OT receptor activation in the basolateral amygdala on stimulus processing were examined further using conditioning protocols that consist of changes in stimulus-outcome contingencies (i.e., extinction and reversal), and with stimuli paired with aversive (i.e., foot shock) and appetitive (i.e., sucrose) outcomes. It was revealed that the effects of OTR stimulation diverge for aversive and appetitive learning - enhancing the former but not the latter. However, across both types of learning, OTR stimulation enhanced the detection of conditioned stimuli. Overall, these results are consistent with an emerging view of OT's effects on stimulus salience; facilitating the detection of meaningful stimuli while reducing responding to those that are irrelevant.

What is Learned Determines How Pavlovian Conditioned Fear is Consolidated in the Brain

Activity in the basolateral amygdala complex (BLA) is needed to encode fears acquired through contact with both innate sources of danger (i.e., things that are painful) and learned sources of danger (e.g., being threatened with a gun). However, within the BLA, the molecular processes required to consolidate the two types of fear are not the same: protein synthesis is needed to consolidate the first type of fear (so-called first-order fear) but not the latter (so-called second-order fear). The present study examined why first- and second-order fears differ in this respect. Specifically, it used a range of conditioning protocols in male and female rats, and assessed the effects of a BLA infusion of the protein synthesis inhibitor, cycloheximide, on first- and second-order conditioned fear. The results revealed that the differential protein synthesis requirements for consolidation of first- and second-order fears reflect differences in what is learned in each case. Protein synthesis in the BLA is needed to consolidate fears that result from encoding of relations between stimuli in the environment (stimulus-stimulus associations, typical for first-order fear) but is not needed to consolidate fears that form when environmental stimuli associate directly with fear responses emitted by the animal (stimulus-response associations, typical for second-order fear). Thus, the substrates of Pavlovian fear conditioning in the BLA depend on the way that the environment impinges upon the animal. This is discussed with respect to theories of amygdala function in Pavlovian fear conditioning, and ways in which stimulus-response associations might be consolidated in the brain.

NMDA Receptors in the Basolateral Amygdala Complex Are Engaged for Pavlovian Fear Conditioning When an Animal's Predictions about Danger Are in Error

It is widely accepted that Pavlovian fear conditioning requires activation of NMDA receptors (NMDARs) in the basolateral amygdala complex (BLA). However, it was recently shown that activation of NMDAR in the BLA is only required for fear conditioning when danger occurs unexpectedly; it is not required for fear conditioning when danger occurs as expected. This study tested the hypothesis that NMDARs in the BLA are engaged for Pavlovian fear conditioning when an animal's predictions regarding danger are in error. In each experiment, rats (females in Experiment 1 and males in Experiments 2-5) were conditioned to fear one stimulus, S1, when it was paired with foot-shock (S1→shock), and 48 h later, a second stimulus, S2, when it was presented in sequence with the already-conditioned S1 and foot-shock (S2→S1→shock). Conditioning to S2 occurred under a BLA infusion of the NMDAR antagonist, D-AP5 or vehicle. The subsequent tests of freezing to S2 alone and S1 alone revealed that the antagonist had no effect on conditioning to S2 when the shock occurred exactly as predicted by the S1, but disrupted this conditioning when the shock occurred earlier/later than predicted by S1, or at a stronger/weaker intensity. These results imply that errors in the timing or intensity of a predicted foot-shock engage NMDARs in the BLA for Pavlovian fear conditioning. They are discussed in relation to theories which propose a role for prediction error in determining how experiences are organized in memory and how activation of NMDAR in the BLA might contribute to this organization.SIGNIFICANCE STATEMENT This study is significant in showing that prediction error determines how a new experience is encoded with respect to a past experience and, thereby, whether NMDA receptors (NMDARs) in the basolateral amygdala complex (BLA) encode the new experience. When prediction error is small (e.g., danger occurs as and when expected), the new experience is encoded together with a past experience as part of the same "mental model," and NMDAR activation in the BLA is not needed for this encoding. By contrast, when prediction error is large (e.g., danger occurs at an unexpected intensity or time), the new experience is encoded separately from the past experience as part of a new mental model, and NMDAR activation in the BLA is needed for this encoding.

The Basolateral Amygdala: The Core of a Network for Threat Conditioning, Extinction, and Second-Order Threat Conditioning

Threat conditioning, extinction, and second-order threat conditioning studied in animal models provide insight into the brain-based mechanisms of fear- and anxiety-related disorders and their treatment. Much attention has been paid to the role of the basolateral amygdala (BLA) in such processes, an overview of which is presented in this review. More recent evidence suggests that the BLA serves as the core of a greater network of structures in these forms of learning, including associative and sensory cortices. The BLA is importantly regulated by hippocampal and prefrontal inputs, as well as by the catecholaminergic neuromodulators, norepinephrine and dopamine, that may provide important prediction-error or learning signals for these forms of learning. The sensory cortices may be required for the long-term storage of threat memories. As such, future research may further investigate the potential of the sensory cortices for the long-term storage of extinction and second-order conditioning memories.

Danger Changes the Way the Brain Consolidates Neutral Information; and Does So by Interacting with Processes Involved in the Encoding of That Information

This study examined the effect of danger on consolidation of neutral information in two regions of the rat (male and female) medial temporal lobe: the perirhinal cortex (PRh) and basolateral amygdala complex (BLA). The neutral information was the association that forms between an auditory stimulus and a visual stimulus (labeled S2 and S1) across their pairings in sensory preconditioning. We show that, when the sensory preconditioning session is followed by a shocked context exposure, the danger shifts consolidation of the S2-S1 association from the PRh to the BLA; and does so by interacting with processes involved in encoding of the S2-S1 pairings. Specifically, we show that the initial S2-S1 pairing in sensory preconditioning is encoded in the BLA and not the PRh; whereas the later S2-S1 pairings are encoded in the PRh and not the BLA. When the sensory preconditioning session is followed by a context alone exposure, the BLA-dependent trace of the early S2-S1 pairings decays and the PRh-dependent trace of the later S2-S1 pairings is consolidated in memory. However, when the sensory preconditioning session is followed by a shocked context exposure, the PRh-dependent trace of the later S2-S1 pairings is suppressed and the BLA-dependent trace of the initial S2-S1 pairing is consolidated in memory. These findings are discussed with respect to mutually inhibitory interactions between the PRh and BLA, and the way that these regions support memory in other protocols, including recognition memory in people.SIGNIFICANCE STATEMENT The perirhinal cortex (PRh) and basolateral amygdala complex (BLA) process the pairings of neutral auditory and visual stimuli in sensory preconditioning. The involvement of each region in this processing is determined by the novelty/familiarity of the stimuli as well as events that occur immediately after the preconditioning session. Novel stimuli are represented in the BLA; however, as these stimuli are repeatedly presented without consequence, they come to be represented in the PRh. Whether the BLA- or PRh-dependent representation is consolidated in memory depends on what happens next. When nothing of significance occurs, the PRh-dependent representation is consolidated and the BLA-dependent representation decays; but when danger is encountered, the PRh-dependent representation is inhibited and the BLA-dependent representation is selected for consolidation.

Second-order fear conditioning involves formation of competing stimulus-danger and stimulus-safety associations

How do animals process experiences that provide contradictory information? The present study addressed this question using second-order fear conditioning in rats. In second-order conditioning, rats are conditioned to fear a stimulus, S1, through its pairings with foot-shock (stage 1); and some days later, a second stimulus, S2, through its pairings with the already-conditioned S1 (stage 2). However, as foot-shock is never presented during conditioning to S2, we hypothesized that S2 simultaneously encodes 2 contradictory associations: one that drives fear to S2 (S2-danger) and another that reflects the absence of the expected unconditioned stimulus and partially masks that fear (e.g. S2-safety). We tested this hypothesis by manipulating the substrates of danger and safety learning in the brain (using a chemogenetic approach) and assessing the consequences for second-order fear to S2. Critically, silencing activity in the basolateral amygdala (important for danger learning) reduced fear to S2, whereas silencing activity in the infralimbic cortex (important for safety learning) enhanced fear to S2. These bidirectional changes are consistent with our hypothesis that second-order fear conditioning involves the formation of competing S2-danger and S2-safety associations. More generally, they show that a single set of experiences can produce contradictory associations and that the brain resolves the contradiction by encoding these associations in distinct brain regions.

Higher-order trace conditioning in newborn rabbits

Temporal contingency is a key factor in associative learning but remains weakly investigated early in life. Few data suggest simultaneous presentation is required for young to associate different stimuli, whereas adults can learn them sequentially. Here, we investigated the ability of newborn rabbits to perform sensory preconditioning and second-order conditioning using trace intervals between odor presentations. Strikingly, pups are able to associate odor stimuli with 10- and 30-sec intervals in sensory preconditioning and second-order conditioning, respectively. The effectiveness of higher-order trace conditioning in newborn rabbits reveals that very young animals can display complex learning despite their relative immaturity.

The neural substrates of higher-order conditioning: A review

Sensory preconditioned and second-order conditioned responding are each well-documented. The former occurs in subjects (typically rats) exposed to pairings of two relatively neutral stimuli, S2 and S1, and then to pairings of S1 and a motivationally significant event [an unconditioned stimulus (US)]; the latter occurs when the order of these experiences is reversed with rats being exposed to S1-US pairings and then to S2-S1 pairings. In both cases, rats respond when tested with S2 in a manner appropriate to the affective nature of the US, e.g., approach when the US is appetitive and withdrawal when it is aversive. This paper reviews the neural substrates of sensory preconditioning and second-order conditioning. It identifies commonalities and differences in the substrates of these so-called higher-order conditioning protocols and discusses these commonalities/differences in relation to what is learned. In so doing, the review highlights ways in which these types of conditioning enhance our understanding of how the brain encodes and retrieves different types of information to generate appropriate behavior.

Prediction Error Determines Whether NMDA Receptors in the Basolateral Amygdala Complex Are Involved in Pavlovian Fear Conditioning

It is widely accepted that activation of NMDA receptors (NMDAR) is necessary for the formation of fear memories in the basolateral amygdala complex (BLA). This acceptance is based on findings that blockade of NMDAR in the BLA disrupts Pavlovian fear conditioning in rodents when initially innocuous stimuli are paired with aversive and unexpected events (surprising foot shock). The present study challenges this acceptance by showing that the involvement of NMDAR in Pavlovian fear conditioning is determined by prediction errors in relation to aversive events. In the initial experiments, male rats received a BLA infusion of the NMDAR antagonist, D-AP5 and were then exposed to pairings of a novel target stimulus and foot shock. This infusion disrupted acquisition of fear to the target when the shock was surprising (experiments 1a, 1b, 2a, 2b, 3a, and 3b) but spared fear to the target when the shock was expected based on the context, time and other stimuli that were present (experiments 1a and 1b). Under the latter circumstances, fear to the target required activation of calcium-permeable AMPAR (CP-AMPA; experiments 4a, 4b, and 4c), which, using electrophysiology, were shown to regulate the activity of interneurons in the BLA (experiment 5). Thus, NMDAR activation is not required for fear conditioning when danger occurs as expected given the context, time and stimuli present, but is required for fear conditioning when danger occurs unexpectedly. These findings are related to current theories of NMDAR function and ways that prediction errors might influence the substrates of fear memory formation in the BLA.SIGNIFICANCE STATEMENT It is widely accepted that NMDA receptors (NMDAR) in the basolateral amygdala complex (BLA) are activated by pairings of a conditioned stimulus (CS) and an aversive unconditioned (US) stimulus, leading to the synaptic changes that underlie formation of a CS-US association. The present findings are significant in showing that this theory is incomplete. When the aversive US is unexpected, animals encode all features of the situation (context, time and stimuli present) as a new fear/threat memory, which is regulated by NMDAR in the BLA. However, when the US is expected based on the context, time and stimuli present, the new fear memory is assimilated into networks that represent those features, which occurs independently of NMDAR activation in the BLA.

Understanding Associative Learning Through Higher-Order Conditioning

Associative learning is often considered to require the physical presence of stimuli in the environment in order for them to be linked. This, however, is not a necessary condition for learning. Indeed, associative relationships can form between events that are never directly paired. That is, associative learning can occur by integrating information across different phases of training. Higher-order conditioning provides evidence for such learning through two deceptively similar designs – sensory preconditioning and second-order conditioning. In this review, we detail the procedures and factors that influence learning in these designs, describe the associative relationships that can be acquired, and argue for the importance of this knowledge in studying brain function.

Effects of NMDA receptor antagonists on behavioral economic indices of cocaine self-administration

BACKGROUND

Currently, there are no FDA-approved medications for the treatment of psychostimulant (e.g., cocaine) use disorders. Because the GluN2B subunit of the glutamate N-methyl-D-aspartate (NMDA) receptor is an important mediator of addiction-like behaviors, the goal of the current study was to determine if the GluN2B-selective antagonist Ro 63-1908 is efficacious in attenuating cocaine self-administration.

METHODS

Adult Sprague Dawley rats (24 males and 11 females) were implanted with indwelling catheters and were trained to self-administer cocaine (0.75 mg/kg/inf). Rats were then trained in a threshold procedure, in which the dose of cocaine decreased across six 6-min blocks (0.75, 0.27, 0.08, 0.03, 0.01, 0.003 mg/kg/inf). This procedure allowed for the quantification of behavioral economic indices of drug self-administration. Following training in the threshold procedure, rats were treated with the GluN2B-selective antagonist Ro 63-1908 (0, 0.1, 0.3, 1.0 mg/kg; s.c.). Rats also received treatments of the NMDA receptor channel blocker MK-801 (0, 0.01, 0.03, 0.06 mg/kg; s.c.).

RESULTS

Blocking NMDA receptors decreased initial intake (i.e., consumption during the first block), although Ro 63-1908 and MK-801 increased area under the curve (global measure of demand) and decreased demand elasticity, an effect observed primarily in males. Neither drug affected demand intensity (i.e., consumption of cocaine at a minimally constrained price).

CONCLUSIONS

While blocking the NMDA receptor decreases initial intake of cocaine, NMDA receptor antagonists make cocaine more inelastic with increasing price. These results suggest that NMDA receptor antagonists can exacerbate addiction-like behaviors during self-administration during extinction-like conditions that are observed in later blocks of the threshold procedure.

The Opioid Receptor Antagonist Naloxone Enhances First-Order Fear Conditioning, Second-Order Fear Conditioning and Sensory Preconditioning in Rats

The opioid receptor antagonist naloxone enhances Pavlovian fear conditioning when rats are exposed to pairings of an initially neutral stimulus, such as a tone, and a painful foot shock unconditioned stimulus (US; so-called first-order fear conditioning; Pavlov, 1927). The present series of experiments examined whether naloxone has the same effect when conditioning occurs in the absence of US exposure. In Experiments 1a and 1b, rats were exposed to tone-shock pairings in stage 1 (one trial per day for 4 days) and then to pairings of an initially neutral light with the already conditioned tone in stage 2 (one trial per day for 4 days). Experiment 1a confirmed that this training results in second-order fear of the light; and Experiment 1b showed that naloxone enhances this conditioning: rats injected with naloxone in stage 2 froze more than vehicle-injected controls when tested with the light alone (drug-free). In Experiments 2a and 2b, rats were exposed to light-tone pairings in stage 1 (one trial per day for 4 days) and then to tone-shock pairings in stage 2 (one trial per day for 2 days). Experiment 2a confirmed that this training results in sensory preconditioned fear of the light; and Experiment 2b showed that naloxone enhances sensory preconditioning when injected prior to each of the light-tone pairings: rats injected with naloxone in stage 1 froze more than vehicle-injected controls when tested with the light alone (drug-free). These results were taken to mean that naloxone enhances fear conditioning independently of its effect on US processing; and more generally, that opioids regulate the error-correction mechanisms that underlie associative formation.

Higher-Order Conditioning and Dopamine: Charting a Path Forward

Higher-order conditioning involves learning causal links between multiple events, which then allows one to make novel inferences. For example, observing a correlation between two events (e.g., a neighbor wearing a particular sports jersey), later helps one make new predictions based on this knowledge (e.g., the neighbor's wife's favorite sports team). This type of learning is important because it allows one to benefit maximally from previous experiences and perform adaptively in complex environments where many things are ambiguous or uncertain. Two procedures in the lab are often used to probe this kind of learning, second-order conditioning (SOC) and sensory preconditioning (SPC). In second-order conditioning (SOC), we first teach subjects that there is a relationship between a stimulus and an outcome (e.g., a tone that predicts food). Then, an additional stimulus is taught to precede the predictive stimulus (e.g., a light leads to the food-predictive tone). In sensory preconditioning (SPC), this order of training is reversed. Specifically, the two neutral stimuli (i.e., light and tone) are first paired together and then the tone is paired separately with food. Interestingly, in both SPC and SOC, humans, rodents, and even insects, and other invertebrates will later predict that both the light and tone are likely to lead to food, even though they only experienced the tone directly paired with food. While these processes are procedurally similar, a wealth of research suggests they are associatively and neurobiologically distinct. However, midbrain dopamine, a neurotransmitter long thought to facilitate basic Pavlovian conditioning in a relatively simplistic manner, appears critical for both SOC and SPC. These findings suggest dopamine may contribute to learning in ways that transcend differences in associative and neurological structure. We discuss how research demonstrating that dopamine is critical to both SOC and SPC places it at the center of more complex forms of cognition (e.g., spatial navigation and causal reasoning). Further, we suggest that these more sophisticated learning procedures, coupled with recent advances in recording and manipulating dopamine neurons, represent a new path forward in understanding dopamine's contribution to learning and cognition.

Acquisition and extinction of second-order context conditioned fear: Role of the amygdala

Second-order fear conditioning has been demonstrated in protocols using discrete and simple stimuli, and much is now known about its behavioral and neural characteristics. In contrast, the mechanisms of second-order conditioning to more complex stimuli, such as contexts, are unknown. To address this gap in our knowledge, we conducted a series of experiments to investigate the neural and behavioral characteristics of second-order context fear conditioning in rats. We found that rats acquire fear to a context in which a first-order conditioned stimulus is presented (Experiment 1); neuronal activity in the basolateral amygdala (BLA) is required for the acquisition (Experiment 2) and extinction (Experiment 3) of second-order context fear; second-order context fear can be reduced by extinction of its first-order conditioned stimulus associate (Experiment 4); and that second-order fear reduced in this way is restored when fear of the first-order conditioned stimulus spontaneously recovers or is reconditioned (Experiment 5). Thus, second-order context fear requires neuronal activity in the BLA, and once established, tracks the level of fear to its first-order conditioned stimulus-associate. These results are discussed with respect to the substrates of second-order fear conditioning in other protocols, and the role of the amygdala in different forms of conditioning.

Mechanisms of higher-order learning in the amygdala

Adaptive behaviour is under the potent control of environmental cues. Such cues can acquire value by virtue of their associations with outcomes of motivational significance, be they appetitive or aversive. There are at least two ways through which an environmental cue can acquire value, through first-order and higher-order conditioning. In first-order conditioning, a neutral cue is directly paired with an outcome of motivational significance. In higher-order conditioning, a cue is indirectly associated with motivational events via a directly conditioned first-order stimulus. The present article reviews some of the associations that support learning in first- and higher-order conditioning, as well as the role of the BLA and the molecular mechanisms involved in these two types of learning.

The Conditions under Which Consolidation of Serial-Order Conditioned Fear Requires De Novo Protein Synthesis in the Basolateral Amygdala Complex

Consolidation of conditioned fear to a stimulus (S1) paired with shock requires de novo protein synthesis in the basolateral amygdala complex (BLA), whereas consolidation of conditioned fear to a stimulus (S2) paired with the fear-eliciting S1 requires DNA methylation but not de novo protein synthesis in the BLA. The present experiments merged these protocols by exposing rats to pairings of a serial S2-S1 compound and shock to examine if/when protein synthesis in the BLA is required to consolidate fear to S2. Rats received a BLA infusion of the protein synthesis inhibitor, cycloheximide, immediately after the S2-S1-shock session and were subsequently tested with S2. The infusion disrupted consolidation of fear to S2 when there had been no prior training of S1 (Experiment 1), the prior training had consisted of unpaired presentations of S1 and shock (Experiment 4), or in pairings of S1 and sucrose (Experiment 5). Consolidation of fear to S2 was unaffected by the infusion of cycloheximide but was disrupted by the DNA methyltransferase inhibitor, 5-AZA, when S1 had been previously fear-conditioned (Experiments 2a, 2b, and 3). These findings imply that what has already been learned about S1 determines the BLA processes that consolidate fear to S2. The already-fear-conditioned S1 blocks the S2-shock association that otherwise forms (and whose consolidation requires de novo protein synthesis in the BLA) while simultaneously acting as a learned source of danger for its S2 associate (whose consolidation requires DNA methylation but not de novo protein synthesis in the BLA).SIGNIFICANCE STATEMENT Protein synthesis is widely thought to be crucial for consolidating new learning into stable memories, including the consolidation of conditioned fear memories in the basolateral amygdala complex (BLA). However, our data provide clear evidence that the requirement for protein synthesis to consolidate conditioned fear in the BLA depends on an animal's previous training history, and the type of learning that is consolidated. Further, within the BLA, our data show that DNA methylation, and not protein synthesis, is necessary to consolidate higher-order conditioned fear, indicating that epigenetic mechanisms may provide a more fundamental mnemonic substrate.

Context and topography determine the role of basolateral amygdala metabotropic glutamate receptor 5 in appetitive Pavlovian responding

Preclinical data have shown that the excitatory metabotropic Gα_q-coupled glutamate receptor, mGluR5, has a role in substance abuse and relapse. However, little is known about the contribution of mGluR5 to the expression of conditioned responding elicited by appetitive Pavlovian cues. We investigated this question in rats that were trained to associate a discrete, auditory conditioned stimulus (CS) with a fructose-glucose solution (5.5% fructose/4.5% glucose; "sugar"). In subsequent tests for the expression of conditioned responding without sugar delivery, CS-elicited fluid port entries were elevated in a context associated with sugar, relative to an equally familiar, neutral context. Inhibiting mGluR5 via systemic injections of a negative allosteric modulator (MTEP; 5 mg/kg) reduced CS port entries in both the sugar context and neutral context. Targeting MTEP microinjections (3 µg/side; 0.3 µl/min) to the nucleus accumbens (Acb) core had no effect on CS port entries at test, whereas the same manipulation in the basolateral amygdala (BLA) produced effects that were topographically dependent. Specifically, microinjecting MTEP in the posterior BLA had no effect on behavior, whereas inhibiting mGluR5 in the anterior BLA enhanced the contextual discrimination of CS port entries. These data are the first to show a role of mGluR5 in the context-dependent expression of appetitive Pavlovian conditioned responding, with a topographically defined arrangement of mGluR5 in the BLA being particularly important for context-based responding to a discrete, appetitive cue.

'Online' integration of sensory and fear memories in the rat medial temporal lobe

How does a stimulus never associated with danger become frightening? The present study addressed this question using a sensory preconditioning task with rats. In this task, rats integrate a sound-light memory formed in stage 1 with a light-danger memory formed in stage 2, as they show fear when tested with the sound in stage 3. Here we show that this integration occurs 'online' during stage 2: when activity in the region that consolidated the sound-light memory (perirhinal cortex) was inhibited during formation of the light-danger memory, rats no longer showed fear when tested with the sound but continued to fear the light. Thus, fear that accrues to a stimulus paired with danger simultaneously spreads to its past associates, thereby roping those associates into a fear memory network.

The role of the basolateral amygdala and infralimbic cortex in (re)learning extinction

The basolateral amygdala complex (BLA) and infralimbic region of the prefrontal cortex (IL) play distinct roles in the extinction of Pavlovian conditioned fear in laboratory rodents. In the past decade, research in our laboratory has examined the roles of these brain regions in the re-extinction of conditioned fear: i.e., extinction of fear that is restored through re-conditioning of the conditioned stimulus (CS) or changes in the physical and temporal context of extinction training (i.e., extinction of renewed or spontaneously recovered fear). This paper reviews this research. It has revealed two major findings. First, in contrast to the acquisition of fear extinction, which usually requires neuronal activity in the BLA but not IL, the acquisition of fear re-extinction requires neuronal activity in the IL but can occur independently of neuronal activity in the BLA. Second, the role of the IL in fear extinction is determined by the training history of the CS: i.e., if the CS was novel prior to its fear conditioning (i.e., it had not been trained), the acquisition of fear extinction does not require the IL; if, however, the prior training of the CS included a series of CS-alone exposures (e.g., if the CS had been pre-exposed), the acquisition of fear extinction was facilitated by pharmacological stimulation of the IL. Together, these results were taken to imply that a memory of CS-alone exposures is stored in the IL, survives fear conditioning of the CS, and can be retrieved and strengthened during extinction or re-extinction of that CS (regardless of whether the extinction is first- or second-learned). Hence, under these circumstances, the initial extinction of fear to the CS can be facilitated by pharmacological stimulation of the IL, and re-extinction of fear to the CS can occur in the absence of a functioning BLA.

Early life trauma increases threat response of peri-weaning rats, reduction of axo-somatic synapses formed by parvalbumin cells and perineuronal net in the basolateral nucleus of amygdala

Early life trauma is a risk factor for life-long disorders related to emotional processing, but knowledge underlying its enduring effect is incomplete. This study was motivated by the hypothesis that early life trauma increases amygdala-dependent threat responses via reduction in inhibition by parvalbumin (PV) interneurons and perineuronal nets (PNN) supporting PV cells, thus increasing excitability of the basolateral amygdala (BLA). From postnatal day (PN) 8-12, rat pups of both sexes were reared under normal bedding or under insufficient nest-building materials to induce maternal-to-infant maltreatment trauma (Scarcity-Adversity Model, SAM). At weaning age of PN23, the SAM group exhibited increased threat responses to predator odor. The SAM-induced increase in threat response was recapitulated in normally reared PN22-23 rats that were unilaterally depleted of PNN in the BLA by the enzymes, chondroitinase-ABC plus hyaluronidase at PN19-20. Light and electron microscopic analysis of the BLA revealed that anterior-to-mid levels of SAM group's BLAs exhibited decreased PNN intensity and decreased axo-somatic synapses between PV-to-principal pyramidal-like neurons and PV-to-PV. PV and PNN densities (cells/mm² ) in the BLA of both control (CON) and SAM groups were still low at PN12 and SAM delayed the ontogenetic rise of PV intensity and PNN density. Moreover, PV cell density in the anterior-to-mid BLA correlated negatively with threat response of CON animals, but not for SAM animals. Thus, reduction of PNN-supported, PV-mediated somatic inhibition of pyramidal cells provides a mechanistic support for the enduring effect of early life maltreatment manifested as increasing innate threat response at weaning.

Commonalities and Differences in the Substrates Underlying Consolidation of First- and Second-Order Conditioned Fear

Consolidation of newly formed fear memories requires a series of molecular events within the basolateral complex of the amygdala (BLA). Once consolidated, new information can be assimilated into these established associative networks to form higher-order associations. Much is known about the molecular events involved in consolidating newly acquired fear memories but little is known about the events that consolidate a secondary fear memory. Here, we show that, within the male rat BLA, DNA methylation and gene transcription are crucial for consolidating both the primary and secondary fear memories. We also show that consolidation of the primary, but not the secondary, fear memory requires de novo protein synthesis in the BLA. These findings show that consolidation of a fear memory and its updating to incorporate new information recruit distinct processes in the BLA, and suggest that DNA methylation in the BLA is fundamental to consolidation of both types of conditioned fear.SIGNIFICANCE STATEMENT Our data provide clear evidence that a different set of mechanisms mediate consolidation of learning about cues that signal learned sources of danger (i.e., second-order conditioned fear) compared with those involved in consolidation of learning about cues that signal innate sources of danger (i.e., first-order conditioned fear). These findings carry important implications because second-order learning could underlie aberrant fear-related behaviors (e.g., in anxiety disorders) as a consequence of neutral secondary cues being integrated into associative fear networks established through first-order pairings, and thereby becoming potent conditioned reinforcers and predictors of fear. Therefore, our data suggest that targeting such second-order conditioned triggers of fear may require pharmacological intervention different to that typically used for first-order conditioned cues.

Danger Changes the Way the Mammalian Brain Stores Information About Innocuous Events: A Study of Sensory Preconditioning in Rats

The amygdala is a critical substrate for learning about cues that signal danger. Less is known about its role in processing innocuous or background information. The present study addressed this question using a sensory preconditioning protocol in male rats. In each experiment, rats were exposed to pairings of two innocuous stimuli in stage 1, S2 and S1, and then to pairings of S1 and shock in stage 2. As a consequence of this training, control rats displayed defensive reactions (freezing) when tested with both S2 and S1. The freezing to S2 is a product of two associations formed in training: an S2-S1 association in stage 1 and an S1-shock association in stage 2. We examined the roles of two medial temporal lobe (MTL) structures in consolidation of the S2-S1 association: the perirhinal cortex (PRh) and basolateral complex of the amygdala (BLA). When the S2-S1 association formed in a safe context, its consolidation required neuronal activity in the PRh (but not BLA), including activation of AMPA receptors and MAPK signaling. In contrast, when the S2-S1 association formed in a dangerous context, or when the context was rendered dangerous immediately after the association had formed, its consolidation required neuronal activity in the BLA (but not PRh), including activation of AMPA receptors and MAPK signaling. These roles of the PRh and BLA show that danger changes the way the mammalian brain stores information about innocuous events. They are discussed with respect to danger-induced changes in stimulus processing.

Dexamethasone impairs encoding and expression of aversive conditioning promoted by pentylenetetrazole

Behavioral and neuroendocrine responses following threatening situations promote the release of corticosterone, which is known to modulate trauma-related learning and memory process. However, it remains unknown whether the aversive learning generated by interoceptive fear conditioning is affected by glucocorticoid modulation. Therefore, the present study aimed to investigate the role of dexamethasone suppression in encoding and expression of pentylenetetrazole-induced olfactory fear conditioning (OFC) and in contextual second-order conditioning promoted by the conditioned odor. Adult male Long-Evans rats were treated with dexamethasone 60 min before the encoding or the expression in both OFC and contextual second-order conditioning. Dexamethasone treatment impaired encoding and expression of the OFC, but failed to impair encoding and expression of the contextual second-order conditioning. Altogether, our results show that although OFC and thereafter contextual second-order conditioning may allow the study of traumatic memories, each order of conditioning seems to present specific features related to their pharmacological modulation. These findings highlight the importance of addressing the role of neuromodulatory systems in first-order and second-order conditioning to gain a better understanding of these phenomena and support future therapies related to traumatic memories.

A dangerous context changes the way that rats learn about and discriminate between innocuous events in sensory preconditioning

Four experiments used a sensory preconditioning protocol to examine how a dangerous context influences learning about innocuous events. In Experiments 1, 2, and 3, rats were exposed to presentations of a tone followed immediately or 20-sec later by presentations of a light. These tone-light pairings occurred in a context that was either familiar and safe, or equally familiar but dangerous, that is, it was a context in which rats had been exposed to footshock. Rats were next exposed to parings of the light and shock and then tested with the tone (and light). The experiments showed that a dangerous context permits formation of a tone-light association under circumstances that preclude formation of that same association in a safe context (Experiments 1 and 2), and that this facilitative effect on associative formation depends on the content being currently dangerous rather than having been dangerous in the past (Experiment 3). Experiment 4 examined whether a dangerous context facilitates discrimination between two innocuous events. In a safe or dangerous context, rats were exposed to a tone that signaled the light and then to a white noise presented alone. Subsequent to conditioning of the light, the tests revealed that rats that had been exposed to these tone-light and white noise alone presentations in a dangerous context froze to the tone but not to the noise, whereas those exposed in a safe context froze to both the tone and the white noise. The results were related to previous evidence that the amygdala is critical for processing information about innocuous stimuli in a dangerous but not a safe context. They were attributed to an amygdala-based enhancement of arousal and/or attention in a dangerous context, hence the facilitation of associative formation and enhanced discriminability in this context.

Ifenprodil infusion in agranular insular cortex alters social behavior and vocalizations in rats exposed to moderate levels of ethanol during prenatal development

Moderate exposure to alcohol during development leads to subtle neurobiological and behavioral effects classified under the umbrella term fetal alcohol spectrum disorders (FASDs). Alterations in social behaviors are a frequently observed consequence of maternal drinking, as children with FASDs display inappropriate aggressive behaviors and altered responses to social cues. Rodent models of FASDs mimic the behavioral alterations seen in humans, with rats exposed to ethanol during development displaying increased aggressive behaviors, decreased social investigation, and altered play behavior. Work from our laboratory has observed increased wrestling behavior in adult male rats following prenatal alcohol exposure (PAE), and increased expression of GluN2B-containing NMDA receptors in the agranular insular cortex (AIC). This study was undertaken to determine if ifenprodil, a GluN2B preferring negative allosteric modulator, has a significant effect on social behaviors in PAE rats. Using a voluntary ethanol exposure paradigm, rat dams were allowed to drink a saccharin-sweetened solution of either 0% or 5% ethanol throughout gestation. Offspring at 6-8 months of age were implanted with cannulae into AIC. Animals were isolated for 24h before ifenprodil or vehicle was infused into AIC, and after 15min they were recorded in a social interaction chamber. Ifenprodil treatment altered aspects of wrestling, social investigatory behaviors, and ultrasonic vocalizations in rats exposed to ethanol during development that were not observed in control animals. These data indicate that GluN2B-containing NMDA receptors in AIC play a role in social behaviors and may underlie alterations in behavior and vocalizations observed in PAE animals.

The basolateral amygdala in reward learning and addiction

Sophisticated behavioral paradigms partnered with the emergence of increasingly selective techniques to target the basolateral amygdala (BLA) have resulted in an enhanced understanding of the role of this nucleus in learning and using reward information. Due to the wide variety of behavioral approaches many questions remain on the circumscribed role of BLA in appetitive behavior. In this review, we integrate conclusions of BLA function in reward-related behavior using traditional interference techniques (lesion, pharmacological inactivation) with those using newer methodological approaches in experimental animals that allow in vivo manipulation of cell type-specific populations and neural recordings. Secondly, from a review of appetitive behavioral tasks in rodents and monkeys and recent computational models of reward procurement, we derive evidence for BLA as a neural integrator of reward value, history, and cost parameters. Taken together, BLA codes specific and temporally dynamic outcome representations in a distributed network to orchestrate adaptive responses. We provide evidence that experiences with opiates and psychostimulants alter these outcome representations in BLA, resulting in long-term modified action.

Mapping fear memory consolidation and extinction-specific expression of JunB

Understanding the molecular and cellular process specifically regulated during fear memory consolidation and extinction is a critical step toward development of new strategies in the treatment of human fear disorders. Here we used inhibitory component of AP-1 transcription factor, JunB, in order to map brain regions where JunB-dependent transcription is regulated during consolidation and extinction of contextual fear memory. We found that contextual fear memory consolidation induced JunB expression in the medial nucleus and intercalated cells of the amygdala while extinction training induced JunB in the CA1 and CA3 areas of the dorsal hippocampus. JunB upregulation induced by contextual fear memory extinction was absent in alphaCaMKII autophosphorylation-deficient mice which have impaired contextual fear memory extinction. Thus, our data suggest that JunB expression in the medial nucleus and intercalated cells of the amygdala is involved in fear memory consolidation while alphaCaMKII-autophosphorylation-dependent JunB expression in the areas CA1 and CA3 of the dorsal hippocampus regulates fear memory extinction.

Vagus nerve stimulation enhances extinction of conditioned fear and modulates plasticity in the pathway from the ventromedial prefrontal cortex to the amygdala

Fearful experiences can produce long-lasting and debilitating memories. Extinction of the fear response requires consolidation of new memories that compete with fearful associations. Subjects with posttraumatic stress disorder (PTSD) show impaired extinction of conditioned fear, which is associated with decreased ventromedial prefrontal cortex (vmPFC) control over amygdala activity. Vagus nerve stimulation (VNS) enhances memory consolidation in both rats and humans, and pairing VNS with exposure to conditioned cues enhances the consolidation of extinction learning in rats. Here we investigated whether pairing VNS with extinction learning facilitates plasticity between the infralimbic (IL) medial prefrontal cortex and the basolateral complex of the amygdala (BLA). Rats were trained on an auditory fear conditioning task, which was followed by a retention test and 1 day of extinction training. Vagus nerve stimulation or sham-stimulation was administered concurrently with exposure to the fear-conditioned stimulus and retention of fear conditioning was tested again 24 h later. Vagus nerve stimulation-treated rats demonstrated a significant reduction in freezing after a single extinction training session similar to animals that received 5× the number of extinction pairings. To study plasticity in the IL-BLA pathway, we recorded evoked field potentials (EFPs) in the BLA in anesthetized animals 24 h after retention testing. Brief burst stimulation in the IL produced LTD in the BLA field response in fear-conditioned and sham-treated animals. In contrast, the same stimulation resulted in potentiation of the IL-BLA pathway in the VNS-treated group. The present findings suggest that VNS promotes plasticity in the IL-BLA pathway to facilitate extinction of conditioned fear responses (CFRs).

Mechanisms to medicines: elucidating neural and molecular substrates of fear extinction to identify novel treatments for anxiety disorders

The burden of anxiety disorders is growing, but the efficacy of available anxiolytic treatments remains inadequate. Cognitive behavioural therapy for anxiety disorders focuses on identifying and modifying maladaptive patterns of thinking and behaving, and has a testable analogue in rodents in the form of fear extinction. A large preclinical literature has amassed in recent years describing the neural and molecular basis of fear extinction in rodents. In this review, we discuss how this work is being harnessed to foster translational research on anxiety disorders and facilitate the search for new anxiolytic treatments. We begin by summarizing the anatomical and functional connectivity of a medial prefrontal cortex (mPFC)-amygdala circuit that subserves fear extinction, including new insights from optogenetics. We then cover some of the approaches that have been taken to model impaired fear extinction and associated impairments with mPFC-amygdala dysfunction. The principal goal of the review is to evaluate evidence that various neurotransmitter and neuromodulator systems mediate fear extinction by modulating the mPFC-amygdala circuitry. To that end, we describe studies that have tested how fear extinction is impaired or facilitated by pharmacological manipulations of dopamine, noradrenaline, 5-HT, GABA, glutamate, neuropeptides, endocannabinoids and various other systems, which either directly target the mPFC-amygdala circuit, or produce behavioural effects that are coincident with functional changes in the circuit. We conclude that there are good grounds to be optimistic that the progress in defining the molecular substrates of mPFC-amygdala circuit function can be effectively leveraged to identify plausible candidates for extinction-promoting therapies for anxiety disorders.

Female mice heterozygous for creatine transporter deficiency show moderate cognitive deficits

Creatine transporter (CrT) deficiency (CTD) is an X-linked disorder characterized by intellectual disability and speech delay. There have been reports that show female carriers have clinical symptoms. We have created CrT knockout (CrT(-/y)) mice in which males show severe cognitive deficits as a model of this disorder. The purpose of this study was to examine if the female carrier mice show cognitive deficits. Reductions in Cr levels as well as CrT transcript were observed in the brains of the female CrT(+/-) mice. CrT(+/-) mice show hyperactivity and increased latency to find the cued platform in the Morris water maze (MWM). CrT(+/-) female mice showed deficits in MWM hidden platform acquisition but not during reversal testing. Memory deficits on probe trials were observed during both phases. Novel object recognition memory and contextual fear memory were not affected in female CrT(+/-) mice. Female CrT(+/-) mice show moderate cognitive deficits, which is consistent with some of the human data. Female CrT(+/-) mice could prove to be beneficial in further understanding CTD and testing therapeutic approaches.

Deformations of amygdala morphology in familial pediatric bipolar disorder

OBJECTIVE

Smaller amygdalar volumes have been consistently observed in pediatric bipolar disorder subjects compared to healthy control subjects. Whether smaller amygdalar volume is a consequence or antecedent of the first episode of mania is not known. Additionally, smaller volume has not been localized to specific amygdala subregions.

METHODS

We compared surface contour maps of the amygdala between 22 youths at high risk for bipolar disorder, 26 youths meeting full diagnostic criteria for pediatric familial bipolar disorder, and 24 healthy control subjects matched for age, gender, and intelligence quotient. Amygdalae were manually delineated on three-dimensional spoiled gradient echo images by a blinded rater using established tracing protocols. Statistical surface mesh modeling algorithms supported by permutation statistics were used to identify regional surface differences between the groups.

RESULTS

When compared to high-risk subjects and controls, youth with bipolar disorder showed surface deformations in specific amygdalar subregions, suggesting smaller volume of the basolateral nuclei. The high-risk subjects did not differ from controls in any subregion.

CONCLUSIONS

These findings support previous reports of smaller amygdala volume in pediatric bipolar disorder and map the location of abnormality to specific amygdala subregions. These subregions have been associated with fear conditioning and emotion-enhanced memory. The absence of amygdala size abnormalities in youth at high risk for bipolar disorder suggests that reductions might occur after the onset of mania.

Co-activation of NR2A and NR2B subunits induces resistance to fear extinction

Unpredictable stress is known to profoundly enhance susceptibility to fear and anxiety while reducing the ability to extinguish fear when threat is no longer present. Accordingly, partial aversive reinforcement, via random exposure to footshocks, induces fear that is resistant to extinction. Here we sought to determine the hippocampal mechanisms underlying susceptibility versus resistance to context fear extinction as a result of continuous (CR) and partial (PR) reinforcement, respectively. We focused on N-methyl-D-aspartate receptor (NMDAR) subunits 2A and B (NR2A and NR2B) as well as their downstream signaling effector, extracellular signal-regulated kinase (ERK), based on their critical role in the acquisition and extinction of fear. Pharmacological inactivation of NR2A, but not NR2B, blocked extinction after CR, whereas inactivation of NR2A, NR2B, or both subunits facilitated extinction after PR. The latter finding suggests that co-activation of NR2A and NR2B contributes to persistent fear following PR. In contrast to CR, PR increased membrane levels of ERK and NR2 subunits after the conditioning and extinction sessions, respectively. In parallel, nuclear activation of ERK was significantly reduced after the extinction session. Thus, co-activation and increased surface expression of NR2A and NR2B, possibly mediated by ERK, may cause persistent fear. These findings suggest that patients with post-traumatic stress disorder (PTSD) may benefit from antagonism of specific NR2 subunits.

Sensory preconditioning in newborn rabbits: from common to distinct odor memories

This study evaluated whether olfactory preconditioning is functional in newborn rabbits and based on joined or independent memory of odorants. First, after exposure to odorants A+B, the conditioning of A led to high responsiveness to odorant B. Second, responsiveness to B persisted after amnesia of A. Third, preconditioning was also functional with two overlapping pairs of odorants (A+B and B+C) and amnesia of one odorant did not affect memory of the others. Thus, incidental pairing of odorants allows reinforcement of one odorant to implicitly reinforce the others, the bond then vanishes, and the memory of each element becomes independent.

The basolateral amygdala is critical for learning about neutral stimuli in the presence of danger, and the perirhinal cortex is critical in the absence of danger

The perirhinal cortex (PRh) and basolateral amygdala (BLA) appear to mediate distinct aspects of learning and memory. Here, we used rats to investigate the involvement of the PRh and BLA in acquisition and extinction of associations between two different environmental stimuli (e.g., a tone and a light) in higher-order conditioning. When both stimuli were neutral, infusion of the GABAA, muscimol, or the NMDA receptor (NMDAR) antagonist ifenprodil into the PRh impaired associative formation. However, when one stimulus was neutral and the other was a learned danger signal, acquisition and extinction of the association between them was unaffected by manipulations targeting the PRh. Temporary inactivation of the BLA had the opposite effect: formation and extinction of an association between two stimuli was spared when both stimuli were neutral, but impaired when one stimulus was a learned danger signal. Subsequent experiments showed that the experience of fear per se shifts processing of an association between neutral stimuli from the PRh to the BLA. When training was conducted in a dangerous environment, formation and extinction of an association between neutral stimuli was impaired by BLA inactivation or NMDAR blockade in this region, but was unaffected by PRh inactivation. These double dissociations in the roles of the PRh and BLA in learning under different stimulus and environmental conditions imply that fear-induced activation of the amygdala changes how the brain processes sensory stimuli. Harmless stimuli are treated as potentially harmful, resulting in a shift from cortical to subcortical processing in the BLA.

Temporal factors in the extinction of fear in inbred mouse strains differing in extinction efficacy

BACKGROUND

Various neuropsychiatric conditions, including posttraumatic stress disorder (PTSD), are characterized by deficient fear extinction, but individuals differ greatly in risk for these. While there is growing evidence that fear extinction is influenced by certain procedural variables, it is unclear how these influences might vary across individuals and subpopulations. To model individual differences in fear extinction, prior studies identified a strain of inbred mouse, 129S1/SvImJ (S1), which exhibits a profound deficit in fear extinction, as compared to other inbred strains, such as C57BL/6J (B6).

METHODS

Here, we assessed the effects of procedural variables on the impaired extinction phenotype of the S1 strain and, by comparison, the extinction-intact B6 strain. The variables studied were 1) the interval between conditioning and extinction, 2) the interval between cues during extinction training, 3) single-cue exposure before extinction training, and 4) extinction of a second-order conditioned cue.

RESULTS

Conducting extinction training soon after ('immediately') conditioning attenuated fear retrieval in S1 mice and impaired extinction in B6 mice. Spacing cue presentations with long inter-trial intervals during extinction training augmented fear in S1 and B6 mice. The effect of spacing was lost with one-trial fear conditioning in B6, but not S1 mice. A single exposure to a conditioned cue before extinction training did not alter extinction retrieval, either in B6 or S1 mice. Both the S1 and B6 strains exhibited robust second-order fear conditioning, in which a cue associated with footshock was sufficient to serve as a conditioned exciter to condition a fear association to a second cue. B6 mice extinguished the fear response to the second-order conditioned cue, but S1 mice failed to do so.

CONCLUSIONS

These data provide further evidence that fear extinction is strongly influenced by multiple procedural variables and is so in a highly strain-dependent manner. This suggests that the efficacy of extinction-based behavioral interventions, such as exposure therapy, for trauma-related anxiety disorders will be determined by the procedural parameters employed and the degree to which the patient can extinguish.

The Contribution of the Amygdala to Aversive and Appetitive Pavlovian Processes

Pavlovian cues predict the occurrence of motivationally salient outcomes, thus serving as an important trigger of approach and avoidance behavior. The amygdala is a key substrate of Pavlovian conditioning, and the nature of its contribution varies by the motivational valence of unconditioned stimuli. The literature on aversive Pavlovian learning supports a serial-processing model of amygdalar function, while appetitive studies suggest that Pavlovian associations are processed through parallel circuits in the amygdala. It is proposed that serial and parallel forms of information processing can be attributed to differential recruitment of amygdalar nuclei, with emphasis placed on the lateral amygdala.

Chronic antidepressant treatment impairs the acquisition of fear extinction

BACKGROUND

Like fear conditioning, the acquisition phase of extinction involves new learning that is mediated by the amygdala. During extinction training, the conditioned stimulus is repeatedly presented in the absence of the unconditioned stimulus, and the expression of previously learned fear gradually becomes suppressed. Our previous study revealed that chronic treatment with a selective serotonin reuptake inhibitor (SSRI) impairs the acquisition of auditory fear conditioning. To gain further insight into how SSRIs affect fear learning, we tested the effects of chronic SSRI treatment on the acquisition of extinction.

METHODS

Rats were treated chronically (22 days) or subchronically (9 days) with the SSRI citalopram (10 mg/kg/day) before extinction training. The results were compared with those after chronic and subchronic treatment with tianeptine (10 mg/kg/day), an antidepressant with a different method of action. The expression of the NR2B subunit of the N-methyl-D-aspartate receptor in the amygdala was examined after behavioral testing.

RESULTS

Chronic but not subchronic administration of citalopram impaired the acquisition of extinction and downregulated the NR2B subunit of the N-methyl-D-aspartate receptor in the lateral and basal nuclei of the amygdala. Similar behavioral and molecular changes were found with tianeptine treatment.

CONCLUSIONS

These results provide further evidence that chronic antidepressant treatment can impair amygdala-dependent learning. Our findings are consistent with a role for glutamatergic neurotransmission in the final common pathway of antidepressant treatment.

Individual differences in recovery from traumatic fear

Although exposure to major psychological trauma is unfortunately common, risk for related neuropsychiatric conditions, such as post-traumatic stress disorder (PTSD), varies greatly among individuals. Fear extinction offers a tractable and translatable behavioral readout of individual differences in learned recovery from trauma. Studies in rodent substrains and subpopulations are providing new insights into neural system dysfunctions associated with impaired fear extinction. Rapid progress is also being made in identifying key molecular circuits, epigenetic mechanisms, and gene variants associated with differences in fear extinction. Here, we discuss how this research is informing understanding of the etiology and pathophysiology of individual differences in risk for trauma-related anxiety disorders, and how future work can help identify novel diagnostic biomarkers and pharmacotherapeutics for these disorders.

A model of amygdala-hippocampal-prefrontal interaction in fear conditioning and extinction in animals

Empirical research has shown that the amygdala, hippocampus, and ventromedial prefrontal cortex (vmPFC) are involved in fear conditioning. However, the functional contribution of each brain area and the nature of their interactions are not clearly understood. Here, we extend existing neural network models of the functional roles of the hippocampus in classical conditioning to include interactions with the amygdala and prefrontal cortex. We apply the model to fear conditioning, in which animals learn physiological (e.g. heart rate) and behavioral (e.g. freezing) responses to stimuli that have been paired with a highly aversive event (e.g. electrical shock). The key feature of our model is that learning of these conditioned responses in the central nucleus of the amygdala is modulated by two separate processes, one from basolateral amygdala and signaling a positive prediction error, and one from the vmPFC, via the intercalated cells of the amygdala, and signaling a negative prediction error. In addition, we propose that hippocampal input to both vmPFC and basolateral amygdala is essential for contextual modulation of fear acquisition and extinction. The model is sufficient to account for a body of data from various animal fear conditioning paradigms, including acquisition, extinction, reacquisition, and context specificity effects. Consistent with studies on lesioned animals, our model shows that damage to the vmPFC impairs extinction, while damage to the hippocampus impairs extinction in a different context (e.g., a different conditioning chamber from that used in initial training in animal experiments). We also discuss model limitations and predictions, including the effects of number of training trials on fear conditioning.

Inactivation of basolateral amygdala specifically eliminates palatability-related information in cortical sensory responses

Evidence indirectly implicates the amygdala as the primary processor of emotional information used by cortex to drive appropriate behavioral responses to stimuli. Taste provides an ideal system with which to test this hypothesis directly, as neurons in both basolateral amygdala (BLA) and gustatory cortex (GC)-anatomically interconnected nodes of the gustatory system-code the emotional valence of taste stimuli (i.e., palatability), in firing rate responses that progress similarly through "epochs." The fact that palatability-related firing appears one epoch earlier in BLA than GC is broadly consistent with the hypothesis that such information may propagate from the former to the latter. Here, we provide evidence supporting this hypothesis, assaying taste responses in small GC single-neuron ensembles before, during, and after temporarily inactivating BLA in awake rats. BLA inactivation (BLAx) changed responses in 98% of taste-responsive GC neurons, altering the entirety of every taste response in many neurons. Most changes involved reductions in firing rate, but regardless of the direction of change, the effect of BLAx was epoch-specific: while firing rates were changed, the taste specificity of responses remained stable; information about taste palatability, however, which normally resides in the "Late" epoch, was reduced in magnitude across the entire GC sample and outright eliminated in most neurons. Only in the specific minority of neurons for which BLAx enhanced responses did palatability specificity survive undiminished. Our data therefore provide direct evidence that BLA is a necessary component of GC gustatory processing, and that cortical palatability processing in particular is, in part, a function of BLA activity.

Hypervigilance for fear after basolateral amygdala damage in humans

Recent rodent research has shown that the basolateral amygdala (BLA) inhibits unconditioned, or innate, fear. It is, however, unknown whether the BLA acts in similar ways in humans. In a group of five subjects with a rare genetic syndrome, that is, Urbach-Wiethe disease (UWD), we used a combination of structural and functional neuroimaging, and established focal, bilateral BLA damage, while other amygdala sub-regions are functionally intact. We tested the translational hypothesis that these BLA-damaged UWD-subjects are hypervigilant to facial expressions of fear, which are prototypical innate threat cues in humans. Our data indeed repeatedly confirm fear hypervigilance in these UWD subjects. They show hypervigilant responses to unconsciously presented fearful faces in a modified Stroop task. They attend longer to the eyes of dynamically displayed fearful faces in an eye-tracked emotion recognition task, and in that task recognize facial fear significantly better than control subjects. These findings provide the first direct evidence in humans in support of an inhibitory function of the BLA on the brain's threat vigilance system, which has important implications for the understanding of the amygdala's role in the disorders of fear and anxiety.