The role of context in the re-extinction of learned fear

d-Cycloserine and estradiol enhance fear extinction in nulliparous but not primiparous female rats

Female reproductive experience has been shown to alter the hormonal, neurobiological and behavioural features of fear extinction, which is the laboratory basis of exposure therapy. This raises uncertainties as to whether pharmacological agents that enhance fear extinction in reproductively inexperienced females are equally effective in reproductively experienced females. The aim of the current study was therefore to compare the effects of two pharmacological enhancers of fear extinction, d-cycloserine (DCS) and estradiol, between nulliparous (virgin) and primiparous (reproductively experienced) female rats. In Experiment 1, nulliparous and primiparous females received systemic administration of either DCS or saline immediately after extinction training, and were tested for extinction recall the following day. DCS enhanced extinction recall in nulliparous females that showed low levels of freezing at the end of extinction training, but not among those that showed high levels of freezing at the end of extinction training. DCS did not enhance fear extinction in primiparous females, regardless of their level of freezing at the end of extinction training. In Experiment 2, nulliparous and primiparous female rats received systemic administration of either estradiol or vehicle prior to extinction training. Estradiol enhanced extinction recall among nulliparous females, but not primiparous females. Increasing the dose of estradiol administered prior to extinction training did not alter the outcomes in primiparous females (Experiment 3). Together, these findings suggest that reproductive status may be an important individual difference factor associated with the response to pharmacological modulators of extinction in rats. The implications of these findings for the pharmacological augmentation of exposure therapy in clinical populations are discussed.

Plasticity of NMDA Receptors at Ventral Hippocampal Synapses in the Infralimbic Cortex Regulates Cued Fear

The medial prefrontal cortex (mPFC) processes contextual information from the hippocampus to generate appropriate fear responses. In rodents, one path for sending contextual information to the mPFC is via the direct projections from the ventral hippocampus (vHC) to the infralimbic cortex (IL). Plasticity in the synaptic communication from the vHC to the IL could contribute to the behavioral changes produced by the acquisition and extinction of conditioned fear. To examine this possibility, we used optogenetic stimulation of vHC synapses in brain slices from trained rats. We found that fear acquisition reduced NMDA receptor (NMDAR) currents at vHC synapses onto IL pyramidal neurons. The depression of NMDAR currents reversed more efficiently after extinction in the conditioning context than extinction in a novel context. Moreover, a cohort of animals that exhibited poor extinction retrieval failed to reverse the plasticity induced by fear conditioning. In addition, ex vivo application of brain-derived neurotrophic factor (BDNF), which is known to simulate extinction in IL, reversed this conditioning-induced plasticity mimicking extinction. Therefore, we have identified a novel mechanism that modulates conditioned fear via changes in NMDAR current at vHC synapses onto IL pyramidal neurons. Disruption of this mechanism could contribute to the abnormal contextual modulation of fear seen in posttraumatic stress disorder (PTSD).

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.

N-Methyl D-aspartate receptor subunit signaling in fear extinction

N-Methyl D-aspartate receptors (NMDAR) are central mediators of glutamate actions underlying learning and memory processes including those required for extinction of fear and fear-related behaviors. Consistent with this view, in animal models, antagonists of NMDAR typically impair fear extinction, whereas partial agonists have facilitating effects. Promoting NMDAR function has thus been recognized as a promising strategy towards reduction of fear symptoms in patients suffering from anxiety disorders and post-traumatic disorder (PTSD). Nevertheless, application of these drugs in clinical trials has proved of limited utility. Here we summarize recent advances in our knowledge of NMDAR pharmacology relevant for fear extinction, focusing on molecular, cellular, and circuit aspects of NMDAR function as they relate to fear extinction at the level of behavior and cognition. We also discuss how these advances from animal models might help to understand and overcome the limitations of existing approaches in human anxiety disorders and how novel, more specific, and personalized approaches might help advance future therapeutic strategies.

Reproductive experience alters the involvement of N-methyl-D-aspartate receptors in fear extinction, but not fear conditioning, in female Sprague Dawley rats

Recently, evidence has emerged showing that the behavioural and hormonal features of fear extinction are altered as a result of reproductive experience in both rats and humans. The current set of experiments sought to determine whether reproductive experience also alters the molecular features of fear extinction. In adult male rats, it has been widely demonstrated that the activation of N-methyl-D-aspartate receptors (NMDAR) is essential for fear extinction. We therefore compared the involvement of NMDAR in fear extinction between nulliparous (virgin) and primiparous (reproductively experienced) female rats. Nulliparous and primiparous females received systemic administrations of either MK-801 (a non-competitive NMDAR antagonist) or saline prior to extinction training. MK-801 was found to impair extinction recall in nulliparous females, but not primiparous females. When the same dose of MK-801 was administered prior to conditioning, both groups of rats showed impaired recall of conditioning the following day. The results of these experiments indicate that the extinction, but not the acquisition of fear, may become NMDAR-independent following reproductive experience.

Estradiol-induced enhancement of fear extinction in female rats: The role of NMDA receptor activation

Converging cross-species evidence indicates that fear extinction (the laboratory basis of exposure therapy for anxiety disorders) in females is modulated by endogenous and exogenous estradiol. The mechanisms underlying estradiol's influences on fear extinction are largely undefined. However, one likely candidate is the NMDA-receptor (NMDAr), activation of which is necessary for estradiol-mediated enhancements in structural and functional neural plasticity, as well as extinction consolidation in males. Here, we demonstrate that systemic co-administration of the non-competitive NMDAr antagonist, MK801, blocked the enhancement of fear extinction by systemic estradiol in ovariectomized rats. In intact rats, MK801 during diestrus (rising estradiol) prevented the enhancement in extinction recall in rats that received extinction training during proestrus (peak estradiol). Systemic administration of the partial NMDAr agonist D-cycloserine (DCS) prior to extinction training facilitated extinction in ovariectomized rats, mimicking the effects of estradiol. In intact rats, DCS administered on the afternoon of proestrus and the morning of estrus (declining estradiol) facilitated extinction in rats that received extinction training during metestrus (low estradiol). Finally, DCS also facilitated extinction in ovariectomized rats when administered immediately after extinction training. Combined, these findings suggest that endogenous and exogenous estradiol enhance fear extinction via NMDAr-dependent mechanisms. Moreover, these findings raise the possibility that fear extinction deficits during periods of low endogenous estradiol levels can be reversed by increasing NMDAr activation via DCS administration, either well prior to, or immediately after, extinction training.

Impaired fear extinction in adolescent rodents: Behavioural and neural analyses

Despite adolescence being a developmental window of vulnerability, up until very recently there were surprisingly few studies on fear extinction during this period. Here we summarise the recent work in this area, focusing on the unique behavioural and neural characteristics of fear extinction in adolescent rodents, and humans where relevant. A prominent hypothesis posits that anxiety disorders peak during late childhood/adolescence due to the non-linear maturation of the fear inhibition neural circuitry. We discuss evidence that impaired extinction retention in adolescence is due to subregions of the medial prefrontal cortex and amygdala mediating fear inhibition being underactive while other subregions that mediate fear expression are overactive. We also review work on various interventions and surprising circumstances which enhance fear extinction in adolescence. This latter work revealed that the neural correlates of extinction in adolescence are different to that in younger and older animals even when extinction retention is not impaired. This growing body of work highlights that adolescence is a unique period of development for fear inhibition.

Interoceptive conditioning with nicotine using extinction and re-extinction to assess stimulus similarity with bupropion

Bupropion is an atypical antidepressant that increases long-term quit rates of tobacco smokers. A better understanding of the relation between nicotine and this first-line medication may provide insight into improving treatment. For all experiments, rats first had nicotine (0.4 mg base/kg) and saline session intermixed; intermittent access to sucrose only occurred on nicotine session. Nicotine in this protocol comes to differentially control "anticipatory" dipper entries. To more closely examine the overlap in the interoceptive stimulus effects of nicotine and bupropion, we assessed whether subsequent prolonged and repeated non-reinforced (extinction) sessions with the bupropion stimulus could weaken responding to nicotine (i.e., transfer of extinction). We also examined whether retraining the discrimination after initial extinction and then conducting extinction again (i.e., re-extinction) with bupropion would affect responding. We found that bupropion (20 and 30 mg/kg) fully substituted for the nicotine stimulus in repeated 20-min extinction sessions. The extent of substitution in extinction did not necessarily predict performance in the transfer test (e.g., nicotine responding unchanged after extinction with 20 mg/kg bupropion). Generalization of extinction back to nicotine was not seen with 20 mg/kg bupropion even after increasing the number of extinction session from 6 to 24. Finally, there was evidence that learning in the initial extinction phase was retained in the re-extinction phase for nicotine and bupropion. These findings indicate that learning involving the nicotine stimuli are complex and that assessment approach for stimulus similarity changes conclusions regarding substitution by bupropion. Further research will be needed to identify whether such differences may be related to different facets of nicotine dependence and/or its treatment.

Pharmacological enhancement of fear reduction: preclinical models

Anxiety disorders have a high prevalence, and despite the substantial advances in the psychological treatment of anxiety, relapse is still a common problem. One approach to improving existing psychological treatments for anxiety has been to develop pharmacological agents that can be used to enhance the processes underlying exposure therapy, which is the most commonly used and empirically validated psychological treatment for anxiety during which individuals are taught to appropriately inhibit fear. Animal models of exposure therapy, particularly fear extinction, have proved to be a very useful way of examining the neural and molecular correlates of fear inhibition, which has in turn led to the identification of numerous drugs that enhance these processes in rats. Several of these drugs have subsequently been tested as novel pharmacological adjuncts to exposure therapy in humans with a range of anxiety disorders. The purpose of this review is to outline the key animal models of exposure therapy and to describe how these have been used to develop potential pharmacological adjuncts for anxiety disorders. Drugs that are currently in clinical use, as well as those currently in the preclinical stages of investigation, are described.

Acute, but not chronic, exposure to d-cycloserine facilitates extinction and modulates spontaneous recovery of a conditioned taste aversion

D-cycloserine, the glutamate N-methyl-D-aspartate receptor partial agonist, has been reported to facilitate the extinction of learned fears acquired in both naturalistic and laboratory settings. The current study extended this literature by evaluating the ability of either chronic or acute administrations of DCS to modulate the extinction and spontaneous recovery of a conditioned taste aversion (CTA). Twenty-three hour fluid-deprived Sprague-Dawley rats acquired a strong CTA following 3 pairings of a conditioned stimulus (CS; 0.3% oral saccharin)+unconditioned stimulus [US; 81 mg/kg (i.p.) lithium chloride (LiCl)]. In separate groups of rats, we then employed 2 different extinction paradigms: (1) CS-only (CSO-EXT) in which saccharin was presented every-other day, or (2) Explicitly Unpaired (EU-EXT) in which both saccharin and LiCl were presented but on alternate days. Previous studies have indicated that the EU-EXT procedure speeds up the extinction process. Further, spontaneous recovery of a CTA emerges following CSO-EXT but the EU-EXT paradigm causes a suppression of spontaneous recovery. DCS (15 mg/kg, i.p.) was administered immediately after daily liquid presentations (saccharin or water, alternate days) during the extinction period. In an acute drug manipulation, DCS (15 mg/kg, i.p.) or saline control injections were administered for 4 days only. This was done during one of 3 different phases of extinction [i.e., static (2-5%), early dynamic (8-16%), or middle dynamic (20-40%) saccharin reacceptance]. Other animals assigned to the chronic DCS condition received daily DCS (15 mg/kg, i.p.) throughout extinction. Changes in saccharin drinking in these animals were compared to the data from rats that received no drug (saline controls). Once rats met our criterion for asymptotic extinction (90% reacceptance of the CS) they entered a 30-day latency period during which they received water for 1 h/day. The day after the completion of the latency period, a final opportunity to drink saccharin was provided (spontaneous recovery test). Saline-treated control rats that went through the EU-EXT procedure achieved asymptotic extinction more quickly than did the CSO-EXT rats and did not exhibit a spontaneous recovery of the CTA. Chronic DCS treatments did not significantly reduce the time to achieve asymptotic CTA extinction in rats exposed to either CSO or EU extinction methods. Further, animals treated with DCS throughout EU-EXT exhibited a spontaneous recovery of the CTA whereas the saline-treated, EU-EXT rats did not. Thus, chronic DCS treatment did not shorten the time to extinguish a CTA and this treatment eliminated the ability of EU-EXT to block spontaneous recovery of the CTA. Acute DCS treatments were more effective in reducing the time required to extinguish a CTA than were chronic drug treatments. Moreover, the timing of these acute DCS treatments affected spontaneous recovery of the CTA depending on the extinction method employed. Acute DCS administrations later in extinction were more effective in reducing spontaneous recovery than were early administrations if the rats went through the CSO-EXT procedure. However, late-in-extinction administrations of DCS facilitated spontaneous recovery of the CTA in rats that experienced the EU-EXT method. These data agree with other findings suggesting that DCS treatments are more effective when administered a limited number of times. Our data extend these findings to the CTA paradigm and further suggest that, depending on the extinction paradigm employed, acute exposure to DCS can speed up CTA extinction and reduce spontaneous recovery of the aversion. The timing of the acute DCS treatment during extinction is generally less important than its duration in predicting the rate of CTA extinction. However, the timing of acute DCS treatments during extinction and the method of extinction employed can interact to affect spontaneous recovery of a CTA.

Comparison of the MK-801-induced appetitive extinction deficit with pressing for reward and associated pERK1/2 staining in prefrontal cortex and nucleus accumbens

Administration of the noncompetitive N-methyl-d-aspartate (NMDA)-receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) has been shown to produce extinction deficits on appetitive operant tasks. The present study sought to further explore this by comparing extinction pressing to pressing for the primary reward and examining associated neural correlates to determine if the MK-801 extinction profile resembled the behavioral and neural profile associated with pressing for primary reward. Immunohistochemical labeling of phosphorylated extracellular signal-regulated kinase-1 and -2(pERK1/2) in the prelimbic (PrL) and infralimbic (IL) cortices and nucleus accumbens shell (AcbSh) and core (AcbC) was examined after rewarded or extinction lever pressing conditions. A dose-response curve revealed a within-day extinction deficit following administration of 0.05 mg/kg MK-801. All doses of MK-801 were associated with reduced IL pERK1/2 staining but only the 0.05 mg/kg dose was associated with elevated AcbSh pERK1/2 labeling. Extinction pressing under the influence of MK-801 was elevated compared to that seen during rewarded pressing-whether on MK-801 or saline. Rewarded pressing following saline or MK-801 was associated with elevated pERK1/2 in the PrL with no similar patterns in the MK-801/extinction group. There was more pERK1/2 labeling in the AcbSh of the MK-801 extinction group than any other condition. These data suggest that the MK-801-induced extinction deficit may be due to the combination of an underactive cortical behavioral inhibition system and an overactive AcbSh reward system.

Fear conditioning, synaptic plasticity and the amygdala: implications for posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is an anxiety disorder that can develop after a traumatic experience such as domestic violence, natural disasters or combat-related trauma. The cost of such disorders on society and the individual can be tremendous. In this article, we review how the neural circuitry implicated in PTSD in humans is related to the neural circuitry of fear. We then discuss how fear conditioning is a suitable model for studying the molecular mechanisms of the fear components that underlie PTSD, and the biology of fear conditioning with a particular focus on the brain-derived neurotrophic factor (BDNF)-tyrosine kinase B (TrkB), GABAergic and glutamatergic ligand-receptor systems. We then summarize how such approaches might help to inform our understanding of PTSD and other stress-related disorders and provide insight to new pharmacological avenues of treatment of PTSD.

New perspectives in glutamate and anxiety

Anxiety and stress-related disorders, namely posttraumatic stress disorder (PTSD), generalized anxiety disorder (GAD), obsessive-compulsive disorder (ODC), social and specific phobias, and panic disorder, are a major public health issue. A growing body of evidence suggests that glutamatergic neurotransmission may be involved in the biological mechanisms underlying stress response and anxiety-related disorders. The glutamatergic system mediates the acquisition and extinction of fear-conditioning. Thus, new drugs targeting glutamatergic neurotransmission may be promising candidates for new pharmacological treatments. In particular, N-methyl-d-aspartate receptors (NMDAR) antagonists (AP5, AP7, CGP37849, CGP39551, LY235959, NPC17742, and MK-801), NMDAR partial agonists (DCS, ACPC), α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) antagonists (topiramate), and several allosteric modulators targeting metabotropic glutamate receptors (mGluRs) mGluR1, mGluR2/3, and mGluR5, have shown anxiolytic-like effects in several animal and human studies. Several studies have suggested that polyamines (agmatine, putrescine, spermidine, and spermine) may be involved in the neurobiological mechanisms underlying stress-response and anxiety-related disorders. This could mainly be attributed to their ability to modulate ionotropic glutamate receptors, especially NR2B subunits. The aim of this review is to establish that glutamate neurotransmission and polyaminergic system play a fundamental role in the onset of anxiety-related disorders. This may open the way for new drugs that may help to treat these conditions.

Differential involvement of protein synthesis and actin rearrangement in the reacquisition of contextual fear conditioning

Extinction learning is associated with a decline of the conditioned fear response (CR). However, re-exposure to the unconditioned stimulus (US, shock) is associated with the return of the fear response. This study aimed to study the role of protein synthesis and actin rearrangement in the CA1 hippocampal subregion and the basolateral amygdala (BLA) in acquisition and reacquisition of contextual fear conditioning. To that end, we trained rats on contextual fear conditioning and extinction, and on the last extinction training session we reconditioned the animals by re-exposure to the US. Immediately after, rats were microinfused with the protein synthesis inhibitor anisomycin or the actin rearrangement inhibitor cytochalasin D into either the BLA or the CA1. The results of this study show differential involvement of anisomycin and cytochalasin D in the acquisition and reacquisition of contextual fear conditioning. Specifically, while the microinfusion of anisomycin into the BLA or the CA1 immediately after reconditioning of fear did not inhibit the return of fear, the microinfusion of cytochalsin D into either the BLA or the CA1 attenuated fear responses. Interestingly, the initial acquisition of contextual fear memory is dependent on intra-BLA and CA1 protein synthesis and cytoskeletal rearrangement, since the microinfusion of these drugs blocked the formation of long-term fear memory. The results suggest that the two processes of acquisition and reacquisition of fear are not identical and they engage different mechanisms.

Fibroblast growth factor-2 alters the nature of extinction

These experiments examined the effects of the NMDA-receptor (NMDAr) antagonist MK801 on reacquisition and re-extinction of a conditioned fear that had been previously extinguished before injection of fibroblast growth factor-2 (FGF2) or vehicle. Recent findings have shown that relearning and re-extinction, unlike initial learning and extinction, do not depend on NMDAr activation. Three experiments demonstrated that FGF2 prevents the switch from NMDAr-dependent to NMDAr-independent reacquisition and re-extinction, suggesting that FGF2 may lead to the partial erasure of the original fear memory. These findings add to a growing body of work suggesting that FGF2 may be a novel pharmacological enhancer of exposure therapy for humans with anxiety disorders.

The effect of D-cycloserine on immediate vs. delayed extinction of learned fear

We compared the effect of D-cycloserine (DCS) on immediate (10 min after conditioning) and delayed (24 h after conditioning) extinction of learned fear in rats. DCS facilitated both immediate and delayed extinction when the drug was administered after extinction training. However, DCS did not facilitate immediate extinction when administered prior to extinction training (i.e., when the interval between drug administration and shock was reduced). In addition, administering five, but not two, shocks prior to extinction training disrupted the facilitating effects of DCS on delayed extinction. These results suggest that aversive experiences prior to DCS administration can prevent it from facilitating extinction.

Extinction in preweanling rats does not involve NMDA receptors

Past research has shown that the N-methyl-d-aspartate receptor (NMDAr) is critically involved in the extinction of learned fear in adult rats with NMDAr antagonists impairing extinction retention and NMDAr agonists enhancing it. In the present study we examined the effects of the NMDAr antagonist MK-801 on extinction in the developing rat. In Experiment 1, rats were given pairings of a white-noise conditioned stimulus (CS) and a shock unconditioned stimulus (US) on postnatal day (P)16. An extinction session, where the CS was presented without the US, occurred on P17 or P24. Prior to extinction rats were injected with MK-801 or saline. All rats were tested for fear of the CS on P25, while drug free. Saline-treated rats exhibited good retention of extinction whether they were extinguished at P17 or P25. Rats treated with MK-801 exhibited impaired extinction retention but only if extinction occurred on P24. These findings show that extinction is NMDAr-dependent at P24 but NMDAr-independent at P17. Experiment 2 further examined the involvement of NMDA receptors in extinction at different stages of development by taking advantage of a finding from several recent studies on re-extinction. These studies have shown, in adult rats, a transition from NMDAr-dependent extinction to NMDAr-independent re-extinction. That is, if rats are trained, extinguished, re-trained to the same CS, and then extinguished again (i.e., re-extinction), then NMDA receptors are not required for extinction the second time. In Experiment 2, rats were trained to fear the CS at P16; this fear was extinguished at either P17 or P24. All rats were then re-trained to fear the CS at P25, re-extinguished at P26, and tested at P27. Prior to re-extinction, rats were injected with MK-801 or saline. Rats initially extinguished at P24, an age where NMDA receptors are involved in extinction, exhibited the transition to an NMDAr-independent re-extinction process. In contrast, rats initially extinguished at P17, an age where NMDA receptors are not involved in extinction, did not (i.e., these rats still exhibited an impairment in extinction retention if given MK-801 prior to re-extinction). Taken together, these findings demonstrate that a qualitatively different system mediates extinction early in life.

The temporal specificity of the switch from NMDAr-dependent extinction to NMDAr-independent re-extinction

Recent findings show that the switch from NMDAr-dependent extinction to NMDAr-independent re-extinction is both context and stimulus specific. In this study we examined whether this switch was temporally specific as well. Re-extinction was found to be NMDAr-independent when it occurred 2 days after initial extinction but NMDAr-dependent when it occurred 21 days following initial extinction, thereby illustrating the importance of time as a type of context that modulates the mechanisms involved in extinction.