Extinction In Rats Research Articles

Vorinostat ameliorates impaired fear extinction possibly via the hippocampal NMDA-CaMKII pathway in an animal model of posttraumatic stress disorder

Given that impairment of fear extinction plays a pivotal role in the pathophysiology of posttraumatic stress disorder (PTSD), drugs that facilitate fear extinction may be useful as novel treatments for PTSD. Histone deacetylase (HDAC) inhibitors have recently been shown to enhance fear extinction in animal studies. Using a single prolonged stress (SPS) paradigm, an animal model of PTSD, we examined whether the HDAC inhibitor vorinostat can facilitate fear extinction in rats, and elucidated the mechanism by which vorinostat enhanced fear extinction, focusing on the N-methyl-D-aspartate (NMDA) receptor signals in the hippocampus. Seven days after SPS, rats received contextual fear conditioning, followed by 2-day extinction training. Vorinostat was intraperitoneally injected immediately after second extinction training session. Contextual fear response was assessed 24h after vorinostat injection. Hippocampal tissues were dissected 2h after vorinostat injection. The levels of mRNA and protein tested were measured by RT-PCR or western blotting, respectively. Systemic administration of vorinostat with extinction training significantly enhanced fear extinction in SPS rats as compared with the controls. Furthermore, vorinostat enhanced the hippocampal levels of NR2B and calcium/calmodulin kinase II (CaMKII) α and β proteins, accompanied by increases in the levels of acetylated histone H3 and H4. These findings suggest that vorinostat ameliorated the impaired fear extinction in SPS rats, and this effect was associated with an increase in histone acetylation and thereby enhancement of NR2B and CaMKII in the hippocampus. Our results may provide new insight into the molecular and therapeutic mechanisms of PTSD.

What Is the Link Between SSRIs and Fear Extinction?

Back to table of contents Previous article Next article Clinical and Research NewsFull AccessWhat Is the Link Between SSRIs and Fear Extinction?Aaron LevinAaron LevinSearch for more papers by this authorPublished Online:15 Mar 2013https://doi.org/10.1176/appi.pn.2013.3b18AbstractA new preclinical study opens doors to better understanding of the role of antidepressants in extinction of learned fear.Antidepressant treatment impairs fear extinction in rats, a finding that may—or may not—point the way to new insights into fear-based psychiatric conditions such as posttraumatic stress disorder (PTSD).“Extinction does not destroy original fear memories but instead involves learning new information about the relationship between the [conditioned stimulus] and the [unconditioned stimulus],” wrote Nesha Burghardt, Ph.D., a postdoctoral researcher at Columbia University College of Physicians and Surgeons, and colleagues online December 20, 2012, in Biological Psychiatry.The best-tested model of extinction in humans is exposure therapy, which is often accompanied by other treatments, typically an SSRI.“Long-term SSRI treatment is known to be effective in the treatment of anxiety disorders,” Burghardt told Psychiatric News. “So I expected chronic citalopram treatment would decrease expression of fear in rats. I was surprised to find that there was no effect on fear expression and instead an impairment in extinction learning.”Nesha Burghardt, Ph.D.,, et al, Biological Psychiatry, December 20, 2012.Burghardt and colleagues treated fear-conditioned rats with a course of the SSRI citalopram, the antidepressant tianeptine, or saline for either 22 or nine consecutive days. Tianeptine is a selective serotonin reuptake enhancer that works through the glutamatergic system. It is approved in Europe but not in the United States.Administering either drug chronically (22 days) impaired the acquisition of fear extinction, but subchronic use (nine days) did not. Apparently, rats, like many humans, take a while to display the effects of antidepressants.“In this way, our rat studies parallel clinical work and provide insight into some of the molecular changes that occur with long-term treatment in people,” Burghardt said.The study may have used rats as subjects, but it raises fundamental questions for the clinical field, said K. Luan Phan, M.D., a professor of psychiatry and director of the Mood and Anxiety Disorders Program at the University of Illinois at Chicago. Phan was not involved in the fear-extinction study.“If the patient takes a medication while undergoing exposure therapy, does it help or hinder in the long run?” Phan asked. “These animal studies will in part help us understand the mechanisms of fear extinction as the results of clinical trials are published.”More clinical trials may be needed to test whether patients with PTSD receiving prolonged exposure therapy should get SSRIs at the same time.Phan, with other researchers, is now in the early stages of testing that question in a clinical trial. The four arms of the trial involve comparisons of an SSRI vs. exposure therapy vs. a combination of the SSRI and exposure vs. exposure plus placebo.Combining SSRIs and cognitive-behavioral therapy is already known to have an additive and possibly synergistic effect in major depressive disorder, he said. For anxiety disorders, adding a drug is sometimes equal, sometimes better, sometimes worse.Results have been mixed in fear extinction, even in rodents. A 2011 study in mice found that antidepressant drugs helped with fear extinction, while the current study in rats finds just the opposite.“We do know that benzodiazepines should not be used if a patient is going on exposure-based therapy,” said Phan.The impairment of extinction learning induced by citalopram was also associated with decreased expression of the NR2B subunit of the N-methyl-D-aspartate [NMDA] receptor in the basal and lateral nuclei of the amygdala, said the researchers.“Our data suggest that the antianxiety and antidepressant effects of SSRIs may depend for their effects on the NR2B subunit of the NMDA receptor,” said Burghardt. This mechanism may also explain the drugs’ potential to impair extinction learning.“Based on our results, it would be ideal to develop a drug that selectively activates the NR2B subunit of the NMDA receptor in the amygdala, to facilitate extinction learning,” she said. D-cycloserine is an NMDA partial agonist that has proved therapeutically effective as an adjunct to exposure-based therapy, suggesting that focusing on the glutamatergic system to facilitate fear extinction is a promising avenue of research.Funding for the study was provided by the National Institutes of Health. ■An abstract of “Chronic Antidepressant Treatment Impairs the Acquisition of Fear Extinction” is posted at http://www.biologicalpsychiatryjournal.com/article/S0006-3223(12)00925-0/abstract. ISSUES NewArchived

Dynamic Regulation of the Synaptic Plasticity in the Prefronto-Amygdala Pathway Following Fear Extinction in Rats

Dynamic Regulation of the Synaptic Plasticity in the Prefronto-Amygdala Pathway Following Fear Extinction in Rats

Differential effects of serotonin-specific and excitotoxic lesions of OFC on conditioned reinforcer devaluation and extinction in rats

The orbitofrontal cortex (OFC) is critical for behavioral adaptation in response to changes in reward value. Here we investigated, in rats, the role of OFC and, specifically, serotonergic neurotransmission within OFC in a reinforcer devaluation task (which measures behavioral flexibility). This task used two visual cues, each predicting one of two foods, with the spatial position (left–right) of the cues above two levers pseudorandomized across trials. An instrumental action (lever press) was required for reinforcer delivery. After training, rats received either excitotoxic OFC lesions made by NMDA (N-methyl-d-aspartic acid), serotonin-specific OFC lesions made by 5,7-DHT (5,7-dihydroxytryptamine), or sham lesions. In sham-lesioned rats, devaluation of one food (by feeding to satiety) significantly decreased responding to the cue associated with that food, when both cues were presented simultaneously during extinction. Both types of OFC lesions disrupted the devaluation effect. In contrast, extinction learning was not affected by serotonin-specific lesions and was only mildly retarded in rats with excitotoxic lesions. Thus, serotonin within OFC is necessary for appropriately adjusting behavior toward cues that predict reward but not for reducing responses in the absence of reward. Our results are the first to demonstrate that serotonin in OFC is necessary for reinforcer devaluation, but not extinction.

Accumbal and pallidal dopamine, glutamate and GABA overflow during cocaine self‐administration and its extinction in rats

We investigated the changes in dopamine (DA), glutamate and γ-aminobutyric acid (GABA) during cocaine self-administration in rats implanted with guide cannulae into the nucleus accumbens and ventral pallidum. After stabilized cocaine self-administration, separate groups of rats underwent extinction (10 days) procedure in which cocaine infusion was replaced by saline injections. With using a 'yoked' procedure, the effects of cocaine or its withdrawal on the level of neurotransmitters were evaluated by dual-probe microdialysis. Repeated cocaine administration reduced basal glutamate levels in the nucleus accumbens and ventral pallidum, whereas it did not affect basal accumbal DA levels. Only rats that self-administered cocaine had increased basal GABA overflow in both examined brain structures. Active or passive cocaine administration elevated extracellular accumbal DA, however, the extent of cocaine-evoked DA level was significantly higher in rats that self-administered cocaine while both groups of animals showed also an attenuation of GABA level in the nucleus accumbens. On day 10 of extinction training, rats previously given cocaine revealed decreases in the basal accumbal concentration of glutamate while the basal GABA levels were significantly enhanced as compared with baseline of saline-yoked controls. Potassium depolarization delayed the reduction of the accumbal and pallidal extracellular glutamate levels in the active and passive cocaine groups. The present data indicate that changes in DA and GABA neurotransmission during maintenance phase mirror the motivational aspects of cocaine intake. Depending on acute (24 hours) or late (10 days) cocaine withdrawal, different neurotransmitter systems (i.e. glutamate or GABA) seem to be involved.

The dopamine beta‐hydroxylase inhibitor nepicastat increases dopamine release and potentiates psychostimulant‐induced dopamine release in the prefrontal cortex

The dopamine-beta-hydroxylase inhibitor nepicastat has been shown to reproduce disulfiram ability to suppress the reinstatement of cocaine seeking after extinction in rats. To clarify its mechanism of action, we examined the effect of nepicastat, given alone or in association with cocaine or amphetamine, on catecholamine release in the medial prefrontal cortex and the nucleus accumbens, two key regions involved in the reinforcing and motivational effects of cocaine and in the reinstatement of cocaine seeking. Nepicastat effect on catecholamines was evaluated by microdialysis in freely moving rats. Nepicastat reduced noradrenaline release both in the medial prefrontal cortex and in the nucleus accumbens, and increased dopamine release in the medial prefrontal cortex but not in the nucleus accumbens. Moreover, nepicastat markedly potentiated cocaine- and amphetamine-induced extracellular dopamine accumulation in the medial prefrontal cortex but not in the nucleus accumbens. Extracellular dopamine accumulation produced by nepicastat alone or by its combination with cocaine or amphetamine was suppressed by the α2 -adrenoceptor agonist clonidine. It is suggested that nepicastat, by suppressing noradrenaline synthesis and release, eliminated the α2 -adrenoceptor mediated inhibitory mechanism that constrains dopamine release and cocaine- and amphetamine-induced dopamine release from noradrenaline or dopamine terminals in the medial prefrontal cortex.

Synaptic Modulation via Basolateral Amygdala on the Rat Hippocampus– Medial Prefrontal Cortex Pathway in Fear Extinction

The present study elucidated the functional role of modulatory effects of basolateral amygdala (BLA) on synaptic transmission in the rat hippocampus–medial prefrontal cortex (mPFC) pathway, compared with the hippocampal dentate gyrus (DG). Exposure to conditioned fear stress (CFS) or prior BLA activation enhanced tetanus-induced long-term potentiation (LTP) in DG. A similar synaptic response was found by low frequency stimulation (LFS) prior to tetanus. In mPFC, they did not affect LTP, but prior BLA activation, as well as pretreatment with the N-methyl-d-aspartate (NMDA)-receptor antagonist MK-801 (0.1 mg/kg, i.p.), suppressed LFS-primed LTP. This BLA-mediated synaptic pattern was mimicked by synaptic changes observed in the fear extinction process; prior BLA activation suppressed the synaptic potentiation responsible for extinction retrieval and attenuated decreases in fear-related freezing behavior. These data suggest that LFS-primed LTP in mPFC is related to the neural basis of extinction. Extinction-related synaptic potentiation did not occur in a juvenile stress model that exhibited extinction deficit. In addition, LFS-primed LTP was suppressed in this model, which was reversed by the NMDA-receptor agonist d-cycloserine (15 mg/kg, i.p.). These findings suggest that modulatory effects of BLA on synaptic function in the hippocampus–mPFC pathway play a significant role in fear extinction in rats.

Prazosin differentially affects extinction of cocaine conditioned place preference on the basis of dose and initial preference

Recent work has shown that α1-adrenergic receptor blockade impairs extinction in fear conditioning paradigms in rodents. However, studies of the role of α1-adrenergic receptors in extinction using other conditioning paradigms, such as those examining the conditioned effects of drug of abuse, have yielded inconsistent results. In this article, we reanalyze and extend previously reported findings of the effect of prazosin, an α1-adrenergic receptor antagonist, on the extinction of a cocaine-induced conditioned place preference in rats, using a median split of performance during the initial test for preference. This new reanalysis, which includes further extinction testing, indicated a paradoxical dose effect. A single post-test administration of a lower dose of prazosin, 0.3 mg/kg intraperitoneally, impaired extinction in rats that showed a below-median preference during initial testing, but had no effect on extinction in rats that showed an above-median preference during initial testing. In contrast, a single post-test administration of a higher dose of prazosin, 1.0 mg/kg intraperitoneally, enhanced extinction in rats that showed an above-median preference during initial testing, but had no effect on extinction in rats that showed a below-median preference during initial testing. Consistent with other studies of fear and drug conditioning, these results suggest the involvement of the α1-adrenergic receptor in the formation of extinction memories, but also indicate a potentially important differential effect on extinction on the basis of the dose of prazosin and the strength of the initial learning.

Combined action of MK-801 and ceftriaxone impairs the acquisition and reinstatement of morphine-induced conditioned place preference, and delays morphine extinction in rats

It is well established that glutamate and its receptors, particularly the N-methyl-D-aspartate receptor (NMDAR), play a significant role in addiction and that the inhibition of glutamatergic hyperfunction reduces addictive behaviors in experimental animals. Specifically, NMDAR antagonists such as MK-801, and an inducer of the expression of glutamate transporter subtype-1 (GLT-1) (ceftriaxone) are known to inhibit addictive behavior. The purpose of this study was to determine whether the combined action of a low dose of MK-801 and a low dose of ceftriaxone provides better inhibition of the acquisition, extinction, and reinstatement of morphine-induced conditioned place preference (CPP) than either compound alone. A morphine-paired CPP experiment was used to study the effects of low doses of MK-801, ceftriaxone and a combination of both on reward-related memory (acquisition, extinction, and reinstatement of morphine preference) in rats. A low dose of neither MK-801 (0.05 mg/kg, i.p.) nor ceftriaxone (25 mg/kg, i.p.) alone effectively impaired CPP behaviors. However, when applied in combination, they reduced the acquisition of morphine-induced CPP and completely prevented morphine reinstatement. Their combination also notably impaired the extinction of morphine-induced CPP. The combined action of a low dose of an NMDAR antagonist (MK-801) and GLT-1 activation by ceftriaxone effectively changed different phases of CPP behavior.

No erasure effect of retrieval–extinction trial on fear memory in the hippocampus-independent and dependent paradigms

Recently, Monfils et al. [9] and Clem and Huganir [3] have shown that an isolated retrieval trial before the extinction sessions (retrieval–extinction) in mice and rats prevented the renewal and spontaneous recovery of the original fear memory by inhibiting reconsolidation in a hippocampus-independent manner. In contrast, Chan et al. [2], using the same paradigm, reported that retrieval extinction in rats augmented the renewal and reinstatement of extinguished fear. However, it remains unclear whether or not retrieval extinction in a hippocampus-independent paradigm erases the original fear memory by inhibiting reconsolidation. We therefore conducted three experiments to investigate whether or not retrieval extinction erases the original fear memory by inhibiting reconsolidation in mice. Our major findings were as follows. (1) Retrieval–extinction in mice did not suppress spontaneous recovery and fear renewal in a hippocampus-independent paradigm. (2) Fear renewal was observed when retrieval–strong extinction in a hippocampus-independent paradigm was performed. (3) Retrieval extinction in a hippocampus-dependent paradigm did not erase the original fear memory. These results suggested that fear extinction after retrieval in mice does not inhibit reconsolidation of previously consolidated fear memory in either a hippocampus-independent or -dependent paradigm.

Stress during puberty boosts metabolic activation associated with fear-extinction learning in hippocampus, basal amygdala and cingulate cortex

Adolescence is characterized by major developmental changes that may render the individual vulnerable to stress and the development of psychopathologies in a sex-specific manner. Earlier we reported lower anxiety-like behavior and higher risk-taking and novelty seeking in rats previously exposed to peri-pubertal stress. Here we studied whether peri-pubertal stress affected the acquisition and extinction of fear memories and/or the associated functional engagement of various brain regions, as assessed with 2-deoxyglucose. We showed that while peri-pubertal stress reduced freezing during the acquisition of fear memories (training) in both sexes, it had a sex-specific effect on extinction of these memories. Moreover hippocampus, basal amygdala and cingulate and motor cortices showed higher metabolic rates during extinction in rats exposed to peri-pubertal stress. Interestingly, activation of the infralimbic cortex was negatively correlated with freezing during extinction only in control males, while only males stressed during puberty showed a significant correlation between behavior during extinction and metabolic activation of hippocampus, amygdala and paraventricular nucleus. No correlations between brain activation and behavior during extinction were observed in females (control or stress). These results indicate that exposure to peri-pubertal stress affects behavior and brain metabolism when the individual is exposed to an additional stressful challenge. Some of these effects are sex-specific.

Central administration of oxytocin receptor ligands affects cued fear extinction in rats and mice in a timepoint-dependent manner

Oxytocin (OXT) has been proposed as a potential therapeutic agent for post-traumatic stress disorder (PTSD). We aimed to verify whether pharmacological manipulation of the brain OXT system affects cued fear conditioning and fear extinction. Male rats and mice were intracerebroventricularly administered synthetic OXT (rats, 0.1 or 1.0 μg/5 μl; mice, 0.1 or 0.5 μg/2 μl) and/or an OXT receptor antagonist (OXTR-A; rats, 0.75 μg/5 μl) either prior to fear conditioning or extinction training. Preconditioning administration of OXT did not affect fear conditioning in rats, but decreased fear expression and facilitated fear extinction. In contrast, preconditioning blockade of OXT neurotransmission by OXTR-A did not affect fear conditioning or fear expression, but impaired fear extinction. When administered before extinction training, OXT impaired fear extinction in both rats and mice, indicating that the effects of OXT on fear extinction are conserved across species. This impairment was OXTR-mediated, as the inhibitory effect of OXT on fear extinction was abolished by prior treatment with OXTR-A. The impaired fear extinction was not a result of reduced locomotion in rats, whereas an apparent decrease in fear expression and facilitation of fear extinction with the higher OXT dose in mice was the result of behavioral hyperactivity. These results suggest that increasing OXT neurotransmission during traumatic events is likely to prevent the formation of aversive memories. In contrast, OXT treatment before fear extinction training, which would be the comparable timepoint for psychotherapy in PTSD patients, rather delays fear extinction and, therefore, caution is needed before recommending OXT for the treatment of PTSD.

The effect of repetitive transcranial magnetic stimulation on fear extinction in rats

Facilitating fear extinction is clinically important to improve the efficacy of current exposure therapies for the treatment of anxiety disorders, such as post-traumatic stress disorder (PTSD). The aim of this study was to determine if repeated transcranial magnetic stimulation (rTMS) facilitates fear extinction in rats, especially when paired with exposure to a conditioned stimulus (CS). Thirty-five rats were conditioned to a tone CS by pairing the tone with an electric foot shock as an aversive unconditioned stimulus (US). We assessed the effects of 10 Hz rTMS before fear extinction (experiment 1) and rTMS paired with CS during extinction (experiment 2) on the following day. Fear responses of the rats were estimated using the level of freezing upon tone stimulus and were compared between the rTMS and corresponding sham groups. The rats treated with rTMS before fear extinction showed no difference in freezing time when compared with the sham group. However, the rats treated with rTMS paired with CS during extinction showed significantly less freezing behavior than the sham group, and this enhancement of fear extinction remained after 24 h without further stimulation. This finding suggests that high-frequency rTMS paired with trauma-reminding stimuli enhances fear extinction and that rTMS in conjunction with exposure therapy is potentially useful for facilitating extinction memory in the treatment of PTSD.

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; 81mg/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 (15mg/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 (15mg/kg, i.p.) or saline control injections were administered for 4days 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 (15mg/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 1h/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.

Chronic treatment with fluoxetine prevents the return of extinguished auditory-cued conditioned fear

We have recently shown that post-extinction exposure of rats to a sub-threshold reminder shock can reactivate extinguished context-related freezing and found that chronic treatment with fluoxetine before fear extinction prevents this phenomenon. In the present study, we examined whether these findings would be confirmed with auditory fear conditioning. Rats were initially submitted to a session of five tone-shock pairings with either a 0.7- or 0.1-mA shock and underwent, 3 days later, a session of 20 tone-alone trials. At the beginning of this latter session, we observed cue-conditioned freezing in rats that received the strong, but not the weak, shock. At the end, both groups (strong and weak shocks) displayed similar low levels of freezing, indicating fear extinction in rats exposed to the strong shock. These rats exhibited again high levels of cue-evoked freezing when exposed to three tone-shock pairings with 0.1-mA shock. This reemergence of cue-conditioned fear was completely abolished by chronic (over a 21-day period) fluoxetine treatment which spared, when administered before the initial fear conditioning, the original tone-shock association. These data extend our previous findings and suggest that chronic fluoxetine treatment favor extinction memory by dampening the reactivation of the original tone-shock association.

Role of beta-adrenergic receptors in the ventromedial prefrontal cortex during contextual fear extinction in rats

It has been reported that stress-related activation of the noradrenergic system strengthens the formation of aversive memories and that beta-adrenergic receptors seem to be involved in this emotional memory processing. In this study, the effects of beta-adrenergic compounds on the extinction of contextual conditioned fear responses were evaluated. Rats were trained with footshock in a conditioning box. In the 3days following the training, the animals were re-exposed to the apparatus and received either a single or repeated intraperitoneal injections of the beta-adrenergic antagonist propranolol, the beta-adrenergic agonist isoproterenol, or saline 30min before (acquisition of extinction) or immediately after (consolidation of extinction) the extinction sessions. A drug-free session was performed on the last day. While repeated isoproterenol treatment facilitated the consolidation of contextual fear extinction, repeated propranolol administration impaired the acquisition and the consolidation of this process. Further, the role of ventromedial prefrontal cortex (vmPFC) in the extinction of contextual conditioned fear was tested with an immunohistochemistry assay. Our results show a reduction in Fos-protein expression between the first and the last extinction session. In a follow-up experiment, intra-vmPFC microinjection of isoproterenol before the first extinction session facilitated the extinction of contextual fear. This facilitation was antagonized by pre-treatment with atenolol, suggesting that this change is mediated by beta-1-adrenergic activity. Our results reinforce the role of the vmPFC in fear extinction mechanisms, suggesting that vmPFC-beta-1-adrenergic receptor activation underlies part of the facilitation of the fear extinction processes.

Strain difference in the effect of infralimbic cortex lesions on fear extinction in rats.

The infralimbic division of the medial prefrontal cortex (IL) has been implicated in the consolidation and retention of extinction memories. However, the effects of IL lesions on the retention of extinction memory are inconsistent. In the present experiments, we examined whether rat strain influences the effects of IL lesions on extinction. In Experiment 1, Sprague-Dawley (SD) or Long-Evans (LE) rats received a standard auditory fear conditioning procedure, which was followed by an extinction session; freezing served as the index of conditional fear. Our results reveal that focal IL lesions impair the retention of extinction in SD, but not LE rats. In addition to the strain difference in sensitivity to IL lesions, LE rats exhibited significantly higher levels of contextual fear before the outset of extinction training than SD rats. In a second experiment we thus examined whether contextual fear influenced the sensitivity of extinction to IL lesions in LE rats. LE rats received the same conditioning as in Experiment 1, and then were either merely exposed to a novel context or administered unsignaled shocks in that context, followed by extinction and test sessions. Our results reveal that LE rats with IL lesions showed normal extinction regardless of the levels of contextual fear manifest before extinction. Thus, we conclude that rat strain is an important variable that influences the role of infralimbic cortex in fear extinction.

COMMENTARY: Breaking down fear memory (Commentary on Meins et al.)

Department of Psychology and Neuroscience Program, University of Michigan, Ann Arbor, MI, USAIt is essential to rapidly learn and unfailingly remember threats in the environment. It is equally important to learn when those threats have passed,as well as the unique contexts in which one is safe from threat. In recent years, considerable progress has been made in understanding the neuralcircuits and molecular mechanisms that underlie the acquisition of fear memory in the mammalian brain (LeDoux, 2000; Maren, 2001). However,much less is known concerning the mechanisms for fear extinction, the learning process that suppresses fear when past threats no longer yieldaversive outcomes.Early work on the neural mechanisms of fear extinction revealed an essential role for N-methyl-d-aspartate (NMDA) receptors in the basolateralamygdala in fear extinction (Falls et al., 1992). More recently, the extinction circuit has expanded to include a network of inhibitory neurons in theamygdala, the intercalated cell masses (ITC), which play an important role in limiting the activity of neurons in the central nucleus (CE) that drivefear responses (Likhtik et al., 2008; Amano et al., 2010). In the current issue, Meins et al. (2010) shed new light on the molecular mechanismswithin these inhibitory amygdala circuits that are involved in the extinction of fear. Using a molecular genetic approach in mice, they first showthat inhibitory interneurons in the CE and ITC express a serine protease inhibitor, protease-nexin 1 (PN-1), which has previously been shown toregulate NMDA receptor function (Kvajo et al., 2004). Much weaker PN-1 expression was found in the basolateral nucleus of the amygdala (BA).Given the localization of PN-1 to inhibitory neurons in ITC and CE, Meins and colleagues next examined fear conditioning and extinction in PN-1knockout mice. Interestingly, PN-1 knockouts exhibited normal fear conditioning, but had marked impairments in the extinction of conditionalfear. Coupled with these behavioral deficits in extinction, PN-1 knockout mice exhibited reduced Fos expression in BA, as well as a reduction inphosphorylated alpha-calcium-calmodulin protein kinase II (aCamKII) in ITC neurons after extinction training. Hence, these data reveal animportant and novel role for PN-1 activity in extinction learning, and reinforce the important role for inhibitory interneurons in the amygdala inthis process.It has been proposed that NMDA receptor-dependent plasticity in the ITC is a mechanism for extinction learning (Amano et al., 2010). Insofaras PN-1 knockout mice exhibit impaired NMDA receptor function, the reduction of ITC c-Fos expression and aCamKII phosphorylation isconsistent with this possibility. Nonetheless, recent data indicate that NMDA receptor antagonism in the CE (and presumably ITC) does not affectthe acquisition of extinction in rats (Zimmerman & Maren, 2010). Further work is clearly required to understand the precise role for amygdaloidNMDA receptors and PN-1 regulation of NMDA receptor function in fear extinction. Nonetheless, the work by Meins and colleagues reveals anew player in the molecular organization of extinction learning within inhibitory interneurons of the amygdala, a finding that yields exciting newavenues for research in this rapidly moving field.

The Role of N-Acetylcysteine in Inhibiting Responding During Extinction in Rats Trained to Self-Administer Cocaine~!2009-12-01~!2010-02-08~!2010-04-09~!

The Role of N-Acetylcysteine in Inhibiting Responding During Extinction in Rats Trained to Self-Administer Cocaine~!2009-12-01~!2010-02-08~!2010-04-09~!

Synaptic correlates of fear extinction in the amygdala

Anxiety disorders such as post-traumatic stress are characterized by an impaired ability to learn that cues previously associated with danger no longer represent a threat. However, the mechanisms underlying fear extinction remain unclear. Here we show in rats that extinction is associated with increased levels of synaptic inhibition in fear output neurons of the central amygdala (CEA). This increased inhibition results from a potentiation of fear input synapses to GABAergic intercalated amygdala neurons that project to CEA. Enhancement of inputs to intercalated cells required prefrontal activity during extinction training and involved a higher transmitter release probability coupled to an altered expression profile of ionotropic glutamate receptors. Overall, our results suggest that intercalated cells constitute a promising target for pharmacological treatments aiming to facilitate the treatment of anxiety disorders.