Adrenergic transmission facilitates extinction of conditional fear in mice

The use of cognitive enhancers in animal models of fear extinction

In anxiety disorders, such as posttraumatic stress disorders and phobias, classical conditioning pairs natural (unconditioned) fear-eliciting stimuli with contextual or discrete cues resulting in enduring fear responses to multiple stimuli. Extinction is an active learning process that results in a reduction of conditioned fear responses after conditioned stimuli are no longer paired with unconditioned stimuli. Fear extinction often produces incomplete effects and this highlights the relative permanence of bonds between conditioned stimuli and conditioned fear responses. The animal research literature is rich in its demonstration of cognitive enhancing agents that alter fear extinction. This review specifically examines the fear extinguishing effects of cognitive enhancers that act on gamma-aminobutyric acid (GABA), glutamatergic, cholinergic, adrenergic, dopaminergic, and cannabinoid signaling pathways. It also examines the effects of compounds that alter epigenetic and neurotrophic mechanisms in fear extinction. Of these cognitive enhancers, glutamatergic N-methyl d-aspartate (NMDA) receptor agonists, such as D-cycloserine, have enhanced fear extinction in a context-, dose- and time-dependent manner. Agents that function as glutamatergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor agonists, alpha2-adrenergic receptor antagonists (such as yohimbine), neurotrophic factors (brain derived neurotrophic factor or BDNF) and histone deacetylase inhibitors (valproate and sodium butyrate) also improve fear extinction in animals. However, some have anxiogenic effects and their contextual and temporal effects need to be more reliably demonstrated. Various cognitive enhancers produce changes in cortico-amygdala synaptic plasticity through multiple mechanisms and these neural changes enhance fear extinction. We need to better define the changes in neural plasticity produced by these agents in order to develop more effective compounds. In the clinical setting, such use of effective cognitive enhancers with cue exposure therapy, using compounds derived from animal model studies, provides great hope for the future treatment of anxiety disorders.

Pharmacological enhancement of memory and executive functioning in laboratory animals

Investigating how different pharmacological compounds may enhance learning, memory, and higher-order cognitive functions in laboratory animals is the first critical step toward the development of cognitive enhancers that may be used to ameliorate impairments in these functions in patients suffering from neuropsychiatric disorders. Rather than focus on one aspect of cognition, or class of drug, in this review we provide a broad overview of how distinct classes of pharmacological compounds may enhance different types of memory and executive functioning, particularly those mediated by the prefrontal cortex. These include recognition memory, attention, working memory, and different components of behavioral flexibility. A key emphasis is placed on comparing and contrasting the effects of certain drugs on different cognitive and mnemonic functions, highlighting methodological issues associated with this type of research, tasks used to investigate these functions, and avenues for future research. Viewed collectively, studies of the neuropharmacological basis of cognition in rodents and non-human primates have identified targets that will hopefully open new avenues for the treatment of cognitive disabilities in persons affected by mental disorders.

Deepened extinction following compound stimulus presentation: noradrenergic modulation

Behavioral extinction is an active form of new learning involving the prediction of nonreward where reward has previously been present. The expression of extinction learning can be disrupted by the presentation of reward itself or reward-predictive stimuli (reinstatement) as well as the passage of time (spontaneous recovery) or contextual changes (renewal). The following experiments replicated the demonstration that presenting multiple previously rewarded stimuli in compound during extinction enhances extinction learning. To explore the pharmacological basis for this we next examined the effects of pharmacological treatments that either facilitated or blocked noradrenergic activity to test the hypothesis that increased noradrenergic activity at the time of extinction training would improve, whereas blockade of noradrenergic activity would impair the extinction of appetitive stimulus-reward memories. Different groups of rats were trained in a discriminative stimulus paradigm to lever-press for food reward. Once stable responding was achieved, responding was extinguished for 2 d. Prior to a third extinction session, rats received systemic administration of either saline, yohimbine (α2 antagonist), atomoxetine (norepinephrine reuptake inhibitor), or propranolol (β-receptor antagonist). Spontaneous recovery of responding to the stimuli was tested 4 wk later. Our results indicate that increasing noradrenergic activity during extinction augments extinction learning resulting in less recovery of responding at test. These results have important implications for models of relapse to drug seeking and the development of extinction-based therapies.

Cognitive enhancers for anxiety disorders

Cognitive-behavioral therapy is an effective intervention for anxiety disorders. However, a significant number of people do not respond or only show partial response even after an adequate course of the treatment. Recent research has shown that the efficacy of the intervention can be improved by the use of cognitive enhancers that augment the core learning processes of cognitive-behavior therapy. This manuscript provides a review of the current state of cognitive enhancers for the treatment of anxiety disorders.

Rescue of impaired fear extinction and normalization of cortico-amygdala circuit dysfunction in a genetic mouse model by dietary zinc restriction

Fear extinction is impaired in neuropsychiatric disorders, including posttraumatic stress disorder. Identifying drugs that facilitate fear extinction in animal models provides leads for novel pharmacological treatments for these disorders. Zinc (Zn) is expressed in neurons in a cortico-amygdala circuit mediating fear extinction, and modulates neurotransmitter systems regulating extinction. We previously found that the 129S1/SvImJ mouse strain (S1) exhibited a profound impairment in fear extinction, coupled with abnormalities in the activation of the extinction circuit. Here, we tested the role of Zn in fear extinction in S1 and C57BL/6N reference strain (B6) by feeding the mice a Zn-restricted diet (ZnR) and testing for fear extinction, as well as neuronal activation of the extinction circuit via quantification of the immediate-early genes c-Fos and Zif268. Results showed that (preconditioning or postconditioning) ZnR completely rescued deficient extinction learning and long-term extinction retrieval in S1 and expedited extinction learning in B6, without affecting fear acquisition or fear expression. The extinction-facilitating effects of ZnR were associated with the normalization of Zif268 and/or c-Fos expression in cortico-amygdala regions of S1. Specifically, ZnR increased activity in infralimbic cortex, lateral and basolateral amygdala nuclei, and lateral central amygdala nucleus, and decreased activity in prelimbic and insular cortices and medial central amygdala nucleus. ZnR also increased activation in the main intercalated nucleus and decreased activation of the medial paracapsular intercalated mass in S1. Our findings reveal a novel role for Zn in fear extinction and further support the utility of the S1 model for identifying extinction facilitating drugs.

Virtual reality exposure therapy in anxiety disorders: a systematic review of process-and-outcome studies

In recent years, virtual reality exposure therapy (VRET) has become an interesting alternative for the treatment of anxiety disorders. Research has focused on the efficacy of VRET in treating anxiety disorders: phobias, panic disorder, and posttraumatic stress disorder. In this systematic review, strict methodological criteria are used to give an overview of the controlled trials regarding the efficacy of VRET in patients with anxiety disorders. Furthermore, research into process variables such as the therapeutic alliance and cognitions and enhancement of therapy effects through cognitive enhancers is discussed. The implications for implementation into clinical practice are considered.

A role for alpha-adrenergic receptors in extinction of conditioned fear and cocaine conditioned place preference

Previous work has demonstrated an important role for adrenergic receptors in memory processes in fear and drug conditioning paradigms. Recent studies have also demonstrated alterations in extinction in these paradigms using drug treatments targeting beta- and alpha2-adrenergic receptors, but little is known about the role of alpha-adrenergic receptors in extinction. The current study examined whether antagonism of alpha-adrenergic receptors would impair the consolidation of extinction in fear and cocaine conditioned place preference paradigms. After contextual fear conditioning, injections of the alpha-adrenergic receptor antagonist prazosin (1.0 or 3.0 mg/kg) following nonreinforced context exposures slowed the loss of conditioned freezing over the course of 5 extinction sessions (Experiment 1). After cocaine place conditioning, prazosin had no effect on the rate of extinction over 8 nonreinforced test sessions. Following postextinction reconditioning, however, prazosin-treated mice showed a robust place preference, but vehicle-treated mice did not, suggesting that prazosin reduced the persistent effects of extinction (Experiment 2). These results confirm the involvement of the alpha-adrenergic receptor in extinction processes in both appetitive and aversive preparations.

Combined Pharmacotherapy and Cognitive-Behavioral Therapy for Anxiety Disorders: Medication Effects, Glucocorticoids, and Attenuated Treatment Outcomes

Despite the success of both pharmacologic and cognitive-behavioral interventions for the treatment of anxiety disorders, the combination of these modalities in adults has not resulted in substantial improvements in outcome relative to either strategy alone, raising questions about whether there are interfering effects that attenuate the magnitude of combination treatment benefits. In this article, we introduce an accounting of potential interference effects that expands upon arguments asserting the necessity of arousal for successful fear exposure. Specifically, recent advances in the study of the effects of cortisol on memory--suggesting that glucocorticoids are crucial to the learning of emotional material--have led us to posit that the attenuation of glucocorticoid activity by anxiolytic medications may interfere with extinction learning in exposure-based therapies. Implications for the efficacy of combination treatments for the anxiety disorders are discussed.

Pharmacological facilitation of fear extinction and the search for adjunct treatments for anxiety disorders--the case of yohimbine

There is current interest in identifying drugs that facilitate fear extinction, as this form of learning is the basis of certain cognitive therapies for anxiety disorders. Following an initial report several years ago that the alpha2-adrenoreceptor antagonist yohimbine facilitated extinction in mice, more recent studies have shown mixed effects or even impairment. It has become clear that the effect of yohimbine on extinction depends on a number of factors, including genetic background, contextual variables and the presence of competing behaviors. To what extent theses effects of yohimbine are mediated through the alpha2-adrenoreceptor, as opposed to other sites of action, is also uncertain. More work is needed before this drug can be approved as a pharmacological adjunct for extinction-based therapies. More generally, the case of yohimbine may serve as a model for the development of other extinction facilitators.

Noradrenergic modulation of extinction learning and exposure therapy

Memory consolidation is enhanced by emotional arousal, an effect mediated by noradrenergic beta-receptor signaling. Norepinephrine strengthens consolidation of both appetitive and aversive learning, and is implicated in extinction of conditioned responses. In this review, we summarize work on the noradrenergic mechanisms of extinction learning and implications for extinction-based exposure therapy. The evidence suggests that norepinephrine release evoked by conditioned stimuli during extinction strengthens extinction memory via beta-receptor signaling. The modulatory effect of norepinephrine during extinction depends on predictable presentation of conditioned stimuli and optimal levels of norepinephrine release. Mechanistically, norepinephrine acts to increase cellular excitability and enhance synaptic plasticity within extinction-related neural circuitry. Currently, drugs that modulate norepinephrine are being used to treat symptoms of anxiety disorders, and are now being tested as pharmacotherapeutic prophalactics in the prevention of chronic posttraumatic stress reactions and as adjuncts to extinction-based exposure therapy. Studies of these new applications of noradrenergic drugs show a converging pattern of results with basic science suggesting ways in which basic laboratory findings can be translated into procedures to enhance clinical outcomes.

Post-retrieval disruption of a cocaine conditioned place preference by systemic and intrabasolateral amygdala beta2- and alpha1-adrenergic antagonists

Previous work has demonstrated post-retrieval impairment in associative learning paradigms, including those mediated by drugs of abuse, using nonspecific beta-adrenergic receptor (beta-AR) antagonists. Remarkably little is known about the role of the specific beta-AR subtypes, or other adrenergic receptors, in these effects. The current study examined the effects of beta(1) and beta(2), as well as alpha(1)-adrenergic receptor antagonism following retrieval of a cocaine conditioned place preference (CPP). We found that rats administered the beta(2) antagonist ICI 118,551 (8 mg/kg intraperitoneal [IP]) or the alpha(1) antagonist prazosin (1 mg/kg IP) following a drug-free test for CPP showed attenuated preference during a subsequent test, while the beta(1) antagonist betaxolol (5 or 10 mg/kg IP) and a lower dose of prazosin (0.3 mg/kg IP) had no effect. Furthermore, post-test microinfusion of ICI 118,551 (6 nmol/side) or prazosin (0.5 nmol/side) into the basolateral amygdala (BLA) also impaired a subsequent preference. Systemic or intra-BLA ICI 118,551 or prazosin administered to rats in their home cages, in the absence of a preference test, had no effect on CPP 24 h later. ICI 118,551 also attenuated the FOS response in the BLA induced by the CPP test. These results are the first to demonstrate a role for alpha(1)- and beta(2)-specific adrenergic mechanisms in post-retrieval memory processes. These systemic and site-specific injections, as well as the FOS immunohistochemical analyses, implicate the importance of specific noradrenergic signaling mechanisms within the BLA in post-retrieval plasticity.

Modulation of glutamatergic synaptic transmission in the bed nucleus of the stria terminalis

Glutamate, catecholamine and neuropeptide signaling within the bed nucleus of the stria terminalis (BNST) have all been identified as key participants in anxiety-like behaviors and behaviors related to withdrawal from exposure to substances of abuse. The BNST is thought to serve as a key relay between limbic cognitive centers and reward, stress and anxiety nuclei. Human studies and animal models have demonstrated that stressors and drugs of abuse can result in long term behavioral modifications that can culminate in psychological diseases such as addiction and post-traumatic stress disorder. The ability of catecholamines and neuropeptides to influence synaptic glutamatergic transmission (stemming from cognitive centers) within the BNST may have profound consequences over these behaviors. In this review we highlight studies examining synaptic plasticity and modulation of excitatory transmission within the BNST, emphasizing how such modulation may result in alterations in anxiety and reward related behavior.

The effects of yohimbine and amphetamine on fear expression and extinction in rats

RATIONALE

Psychostimulants, such as yohimbine and amphetamine, can enhance learning and memory. Extinction of conditioned fear involves new learning, so we asked whether psychostimulants could enhance this learning. Previous work suggests that yohimbine facilitates extinction, using freezing as a fear measure. However, psychostimulant-induced alterations in locomotion can confound freezing measurements. Furthermore, the effects of amphetamine on fear extinction have never been examined.

OBJECTIVE

We evaluated the effectiveness of yohimbine and amphetamine in enhancing fear extinction. In addition to freezing, we measured bar-press suppression, which is less sensitive to changes in locomotion. We asked: Do psychostimulants reduce fear during extinction training when drug is present? Does learning extinction with psychostimulants result in better extinction retention?

MATERIALS AND METHODS

Rats received fear conditioning on day 1 followed by partial extinction training on days 2 and 3. Yohimbine (1.0, 2.0, or 5.0 mg/kg, i.p.), amphetamine (1.0 mg/kg, i.p.), or vehicle were injected prior to extinction on day 2.

RESULTS

Yohimbine dose-dependently reduced freezing during extinction training on day 2, whereas bar-press suppression was reduced at the highest dose only. When tested drug-free, yohimbine-treated rats showed equivalent levels of freezing and suppression to controls. Amphetamine also decreased freezing during extinction, but did not decrease suppression. During the drug-free test, there was no difference between amphetamine-treated rats and controls in either measure.

CONCLUSIONS

Although yohimbine and amphetamine are capable of decreasing freezing, neither drug strengthened retention of fear extinction. Based on these rodent findings, psychostimulants may not be suitable adjuncts to extinction-based therapies for the treatment of anxiety disorders.

Systemic propranolol acts centrally to reduce conditioned fear in rats without impairing extinction

BACKGROUND

Previous work has implicated noradrenergic beta-receptors in the consolidation and reconsolidation of conditioned fear. Less is known, however, about their role in fear expression and extinction. The beta-receptor blocker propranolol has been used clinically to reduce anxiety. With an auditory fear conditioning task in rats, we assessed the effects of systemic propranolol on the expression and extinction of two measures of conditioned fear: freezing and suppression of bar-pressing.

METHODS

One day after receiving auditory fear conditioning, rats were injected with saline, propranolol, or peripheral beta-receptor blocker sotalol (both 10 mg/kg, IP). Twenty minutes after injection, rats were given either 6 or 12 extinction trials and were tested for extinction retention the following day. The effect of propranolol on the firing rate of neurons in prelimbic (PL) prefrontal cortex was also assessed.

RESULTS

Propranolol reduced freezing by more than 50%, an effect that was evident from the first extinction trial. Suppression was also significantly reduced. Despite this, propranolol had no effect on the acquisition or retention of extinction. Unlike propranolol, sotalol did not affect fear expression, although both drugs significantly reduced heart rate. This suggests that propranolol acts centrally to reduce fear. Consistent with this, propranolol reduced the firing rate of PL neurons.

CONCLUSION

Propranolol reduced the expression of conditioned fear, without interfering with extinction learning. Reduced fear with intact extinction suggests a possible use for propranolol in reducing anxiety during extinction-based exposure therapies, without interfering with long-term clinical response.

Explicit disassociation of a conditioned stimulus and unconditioned stimulus during extinction training reduces both time to asymptotic extinction and spontaneous recovery of a conditioned taste aversion

Conditioned taste aversions (CTAs) may be acquired when an animal consumes a novel taste (CS) and then experiences the symptoms of poisoning (US). This aversion may be extinguished by repeated exposure to the CS alone. However, following a latency period in which the CS is not presented, the CTA will spontaneously recover (SR). In the current study we employed an explicitly unpaired extinction procedure (EU-EXT) to determine if it could thwart SR of a CTA. Sprague-Dawley rats acquired a strong CTA after 3 pairings of saccharin (SAC the CS) and Lithium Chloride (LiCl the US). CTA acquisition was followed by extinction (EXT) training consisting of either (a) CS-only exposure (CSO) or, (b) exposure to saccharin and Lithium Chloride on alternate days (i.e., explicitly unpaired: EU). Both extinction procedures resulted in >/= 90% reacceptance of SAC, although the EU extinction procedure (EU-EXT) significantly decreased the time necessary for rats to reach this criterion (compared to CSO controls). Rats were subsequently tested for SR of the CTA upon re-exposure to SAC following a 30-day latency period of water drinking. Rats that acquired a CTA and then underwent the CSO extinction procedure exhibited a significant suppression of SAC drinking during the SR test (as compared to their SAC drinking at the end of extinction). However, animals in the EU-EXT group did not show such suppression in drinking compared to CSO controls. These data suggest that the EU-EXT procedure may be useful in reducing both time to extinction and the spontaneous recovery of fears.

mGluR5 has a critical role in inhibitory learning

The mechanisms that contribute to the extinction of previously acquired memories are not well understood. These processes, often referred to as inhibitory learning, are thought to be parallel learning mechanisms that require a reacquisition of new information and suppression of previously acquired experiences in order to adapt to novel situations. Using newly generated metabotropic glutamate receptor 5 (mGluR5) knock-out mice, we investigated the role of mGluR5 in the acquisition and reversal of an associative conditioned task and a spatial reference task. We found that acquisition of fear conditioning is partially impaired in mice lacking mGluR5. More markedly, we found that extinction of both contextual and auditory fear was completely abolished in mGluR5 knock-out mice. In the Morris Water Maze test (MWM), mGluR5 knock-out mice exhibited mild deficits in the rate of acquisition of the regular water maze task, but again had significant deficits in the reversal task, despite overall spatial memory being intact. Together, these results demonstrate that mGluR5 is critical to the function of neural circuits that are required for inhibitory learning mechanisms, and suggest that targeting metabotropic receptors may be useful in treating psychiatric disorders in which aversive memories are inappropriately retained.

Facilitation of fear extinction in phobic participants with a novel cognitive enhancer: a randomized placebo controlled trial of yohimbine augmentation

Preliminary animal research suggests that yohimbine hydrochloride, a selective competitive alpha2-adrenergic receptor antagonist, accelerates fear extinction and converts ineffective extinction regimens (long intertrial intervals) to effective ones. This randomized placebo controlled study examined the potential exposure enhancing effect of yohimbine hydrochloride in claustrophobic humans. Participants (71% undergraduate students and 29% community volunteers) displaying marked claustrophobic fear (n=24) were treated with 2 1-h in vivo exposure sessions. Participants were randomly allocated to take 10.8mg yohimbine hydrochloride (n=12) or placebo (n=12) prior to each exposure session. Outcome measures included peak fear during a behavioral avoidance task, the Claustrophobia Questionnaire, and the Claustrophobic Concerns Questionnaire. Results showed that both conditions improved significantly at post-treatment with no significant difference between groups. Consistent with prediction the group that took yohimbine hydrochloride prior to exposure sessions showed significantly greater improvement in peak fear at the one-week follow-up behavioral assessment (d=1.68). This was also true across other outcome measures with large to very large effect sizes. These data provide initial support for exposure enhancing effect of single-dose yohimbine hydrochloride in a clinical application.

Repetitive TMS combined with exposure therapy for PTSD: a preliminary study

Treatment for anxiety and post-traumatic stress disorder (PTSD) includes exposure therapy and medications, but some patients are refractory. Few studies of repetitive transcranial magnetic stimulation (rTMS) for anxiety or PTSD exist. In this preliminary report, rTMS was combined with exposure therapy for PTSD. Nine subjects with chronic, treatment-refractory PTSD were studied in a placebo-controlled, crossover design of imaginal exposure therapy with rTMS (1Hz) versus sham. PTSD symptoms, serum and 24h urine were obtained and analyzed. Effect sizes for PTSD symptoms were determined using Cohen's d. Active rTMS showed a larger effect size of improvement for hyperarousal symptoms compared to sham; 24-h urinary norepinephrine and serum T4 increased; serum prolactin decreased. Active rTMS with exposure may have symptomatic and physiological effects. Larger studies are needed to confirm these preliminary findings and verify whether rTMS plus exposure therapy has a role in the treatment of PTSD.

Social modulation of associative fear learning by pheromone communication

Mice communicate through visual, vocal, and olfactory cues that influence innate, nonassociative behavior. We here report that exposure to a recently fear-conditioned familiar mouse impairs acquisition of conditioned fear and facilitates fear extinction, effects mimicked by both an olfactory chemosignal emitted by a recently fear-conditioned familiar mouse and by the putative stress-related anxiogenic pheromone beta-phenylethylamine (beta-PEA). Together, these findings suggest social modulation of higher-order cognitive processing through pheromone communication and support the concurrent excitor hypothesis of extinction learning.

The lasting effect of words on feelings: words may facilitate exposure effects to threatening images

Previous studies have shown that mere words, particularly affective words, can dampen emotional responses. However, the effect of affective labels on emotional responding in the long term is unknown. The authors examined whether repeated exposure to aversive images would lead to more reduction in autonomic reactivity a week later if the images were exposed with single-word labels than without labels. In Experiment 1, healthy individuals were exposed to pictures of disturbing scenes with or without labels on Day 1. On Day 8, the same pictures from the previous week were exposed, this time without labels. In Experiment 2, participants were spider fearful and were exposed to pictures of spiders. In both experiments, although repeated exposure to aversive images (without labels) led to long-term attenuation of autonomic reactivity, exposure plus affective labels, but not nonaffective labels, led to more attenuation than exposure alone. Thus, affective labels may help dampen emotional reactivity in both the short and long terms. Implications for exposure therapy and translational studies are discussed.

Impaired fear extinction learning and cortico-amygdala circuit abnormalities in a common genetic mouse strain

Fear extinction is a form of new learning that results in the inhibition of conditioned fear. Trait deficits in fear extinction are a risk factor for anxiety disorders. There are few examples of naturally occurring animal models of impaired extinction. The present study compared fear extinction in a panel of inbred mouse strains. This strain survey revealed an impairment in fear extinction in 129/SvImJ (129S1). The phenotypic specificity of this deficit was evaluated by comparing 129S1 and C57BL/6J for one-trial and multitrial fear conditioning, nociception, and extinction of conditioned taste aversion and an appetitive instrumental response. 129S1 were tested for sensitivity to the extinction-facilitating effects of extended training, as well as d-cycloserine and yohimbine treatment. To elucidate the neural basis of impaired 129S1 fear extinction, c-Fos and Zif268 expression was mapped after extinction recall. Results showed that impaired fear extinction in 129S1 was unrelated to altered fear conditioning or nociception, and was dissociable from intact appetitive extinction. Yohimbine treatment facilitated extinction in 129S1, but neither extended extinction training nor d-cycloserine treatment improved 129S1 extinction. After extinction recall, 129S1 showed reduced c-Fos and Zif268 expression in the infralimbic cortex and basolateral amygdala, and elevated c-Fos or Zif268 expression in central nucleus of the amygdala and medial paracapsular intercalated cell mass, relative to C57BL/6J. Collectively, these data demonstrate a deficit in fear extinction in 129S1 associated with a failure to properly engage corticolimbic extinction circuitry. This common inbred strain provides a novel model for studying impaired fear extinction in anxiety disorders.

Yohimbine impairs extinction of cocaine-conditioned place preference in an alpha2-adrenergic receptor independent process

Extinction, a form of learning that has the ability to reshape learned behavior based on new experiences, has been heavily studied utilizing fear learning paradigms. Mechanisms underlying extinction of positive-valence associations, such as drug self-administration and place preference, are poorly understood yet may have important relevance to addiction treatment. Data suggest a major role for the noradrenergic system in extinction of fear-based learning. Employing both pharmacological and genetic approaches, we investigated the role of the alpha(2)-adrenergic receptor (alpha(2)-AR) in extinction of cocaine-conditioned place preference (CPP) and glutamatergic transmission in the bed nucleus of the stria terminalis (BNST). We found that pre-extinction systemic treatment with the alpha(2)-AR antagonist yohimbine impaired cocaine CPP extinction in C57BL/6J mice, an effect that was not mimicked by the more selective alpha(2)-AR antagonist, atipamezole. Moreover, alpha(2A)-AR knockout mice exhibited similar cocaine CPP extinction and exacerbated extinction impairing effects of yohimbine. Using acute brain slices and electrophysiological approaches, we found that yohimbine produces a slowly evolving depression of glutamatergic transmission in the BNST that was not mimicked by atipamezole. Further, this action was extant in slices from alpha(2A)-AR knockout mice. Our data strongly suggest that extinction-modifying effects of yohimbine are unlikely to be due to actions at alpha(2A)-ARs.

Recent advances in the neurobiology of anxiety disorders: implications for novel therapeutics

Anxiety disorders are a highly prevalent and disabling class of psychiatric disorders. This review focuses on new directions in neurobiological research and implications for the development of novel psychopharmacological treatments. Neuroanatomical and neuroimaging research in anxiety disorders has centered on the role of the amygdala, reciprocal connections between the amygdala and the prefrontal cortex, and, most recently, alterations in interoceptive processing by the anterior insula. Anxiety disorders are characterized by alterations in a diverse range of neurochemical systems, suggesting ample novel targets for drug therapies. Corticotropin-releasing factor (CRF) concentrations are elevated in a subset of anxiety disorders, which suggests the potential utility of CRF receptor antagonists. Pharmacological blockade of the memory-enhancing effects of stress hormones such as glucocorticoids and noradrenaline holds promise as a preventative approach for trauma-related anxiety. The glutamatergic system has been largely overlooked as a potential pharmacological target, although convergent preclinical, neuroimaging, and early clinical findings suggest that glutamate receptor antagonists may have potent anxiolytic effects. Glutamatergic receptor agonists (e.g., D-cycloserine) also have an emerging role in the treatment of anxiety as facilitators of fear extinction during concurrent behavioral interventions. The neuropeptides substance P, neuropeptide Y, oxytocin, orexin, and galanin are each implicated in anxiety pathways, and neuropeptide analogs or antagonists show early promise as anxiolytics in preclinical and/or clinical research. Each of these active areas of research holds promise for expanding and improving evidence-based treatment options for individuals suffering with clinical anxiety.

Effect of post-retrieval propranolol on psychophysiologic responding during subsequent script-driven traumatic imagery in post-traumatic stress disorder

The beta-adrenergic blocker propranolol given within hours of a psychologically traumatic event reduces physiologic responses during subsequent mental imagery of the event. Here we tested the effect of propranolol given after the retrieval of memories of past traumatic events. Subjects with chronic post-traumatic stress disorder described their traumatic event during a script preparation session and then received a one-day dose of propranolol (n=9) or placebo (n=10), randomized and double-blind. A week later, they engaged in script-driven mental imagery of their traumatic event while heart rate, skin conductance, and left corrugator electromyogram were measured. Physiologic responses were significantly smaller in the subjects who had received post-reactivation propranolol a week earlier. Propranolol given after reactivation of the memory of a past traumatic event reduces physiologic responding during subsequent mental imagery of the event in a similar manner to propranolol given shortly after the occurrence of a traumatic event.

Alteration of dopaminergic markers in gastrointestinal tract of different rodent models of Parkinson's disease

Parkinson's disease (PD) is a progressive neurological disorder that is often associated with various gastrointestinal (GI) symptoms. The link between the alteration of dopaminergic system and the symptoms of the GI tract in PD is complicated. To determine the changes in the dopaminergic system in the GI tract in PD, two kinds of rodent PD models were used in the present study. One was 6-hydroxydopamine (6-OHDA) -treated rats in which 6-OHDA was microinjected in the bilateral substantia nigra (SN). The other was 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) -treated mice in which MPTP was injected intraperitoneally. Immunofluorescence, reverse transcription (RT)-real time polymerase chain reaction (PCR) and Western blot were used to evaluate and compare the levels of mRNA and protein expression of tyrosine hydroxylase (TH) and dopamine transporter (DAT) in the GI tract between normal and rodent PD models, as well as between 6-OHDA-treated rats and MPTP-treated mice. The results indicated that TH- and DAT-positive cells were widely distributed in the GI tract. There were significant differences in TH and DAT expression in the GI tract between normal and PD models, as well as between 6-OHDA-treated rats and MPTP-treated mice. The protein levels of TH and DAT in the GI tract were significantly increased in 6-OHDA-treated rats, but the protein level of TH was significantly decreased in MPTP-treated mice. In addition, there was visible atrophy of gastric epithelial parietal cells in MPTP-treated mice, although the protein level of DAT was not significantly changed. The different alterations of dopaminergic system in the GI tract of the two kinds of PD models might underline the differences in GI symptoms in PD patients and might be correlated with the disease severity and disease process affecting the GI tract.

Noradrenergic signaling in infralimbic cortex increases cell excitability and strengthens memory for fear extinction

Emotional arousal strengthens memory. This is most apparent in aversive conditioning, in which the stress-related neurotransmitter norepinephrine (NE) enhances associations between sensory stimuli and fear-inducing events. In contrast to conditioning, extinction decreases fear responses, and is thought to form a new memory. It is not known, however, whether NE is necessary for extinction learning. Previous work has shown that the infralimbic prefrontal cortex (IL) is a site of extinction consolidation. Here, we show that blocking noradrenergic beta-receptors in IL before extinction training impaired retrieval of extinction the following day, consistent with a weakened extinction memory. We further found that the sequelae of beta-receptor activation, including protein kinase A (PKA), gene transcription and translation in IL, are necessary for extinction. To determine whether activation of this cascade modulates IL excitability, we measured the response of IL pyramidal neurons to injected current. NE increased the excitability of IL neurons in a beta-receptor- and PKA-dependent manner. We suggest that NE released in IL during fear extinction activates a PKA-mediated molecular cascade that strengthens extinction memory. Thus, emotional arousal evoked by conditioned fear paradoxically promotes the subsequent extinction of that fear, thereby ensuring behavioral flexibility.

The histone deacetylase inhibitor valproic acid enhances acquisition, extinction, and reconsolidation of conditioned fear

Histone modifications contribute to the epigenetic regulation of gene expression, a process now recognized to be important for the consolidation of long-term memory. Valproic acid (VPA), used for many years as an anticonvulsant and a mood stabilizer, has effects on learning and memory and enhances the extinction of conditioned fear through its function as a histone deacetylase inhibitor (HDAC). Here we report that VPA enhances long-term memory for both acquisition and extinction of cued-fear. Interestingly, VPA enhances extinction, but also enhances renewal of the original conditioned fear when tested in a within-subjects design. This effect appears to be related to a reconsolidation-like process since a single CS reminder in the presence of VPA can enhance long-term memory for the original fear in the context in which fear conditioning takes place. We also show that by modifying the intertrial interval during extinction training, VPA can strengthen reconsolidation of the original fear memory or enhance long-term memory for extinction such that it becomes independent of context. These findings have important implications for the use of HDAC inhibitors as adjuncts to behavior therapy in the treatment of phobia and related anxiety disorders.

Neural mechanisms of extinction learning and retrieval

Emotional learning is necessary for individuals to survive and prosper. Once acquired, however, emotional associations are not always expressed. Indeed, the regulation of emotional expression under varying environmental conditions is essential for mental health. The simplest form of emotional regulation is extinction, in which conditioned responding to a stimulus decreases when the reinforcer is omitted. Two decades of research on the neural mechanisms of fear conditioning have laid the groundwork for understanding extinction. In this review, we summarize recent work on the neural mechanisms of extinction learning. Like other forms of learning, extinction occurs in three phases: acquisition, consolidation, and retrieval, each of which depends on specific structures (amygdala, prefrontal cortex, hippocampus) and molecular mechanisms (receptors and signaling pathways). Pharmacological methods to facilitate consolidation and retrieval of extinction, for both aversive and appetitive conditioning, are setting the stage for novel treatments for anxiety disorders and addictions.

Effects of ethanol on encoding, consolidation, and expression of extinction following contextual fear conditioning

Studies of contextual fear conditioning have found that ethanol administered prior to a conditioning session impairs the conditioned freezing response during a test session the next day. The present experiments examined the effects of ethanol on extinction, the loss of conditioned responding that occurs as the animal learns that a previously conditioned context no longer signals shock. Ethanol (1.5 g/kg) administered prior to single (Experiment 1) or multiple (Experiment 2) extinction sessions impaired extinction. Ethanol administered prior to a test session disrupted the expression of freezing after extinction (Experiments 3-5). There was some evidence that ethanol served as an internal stimulus signaling the operation of conditioning or extinction contingencies (Experiments 4-5). In Experiment 6, postsession injections of 1.5 g/kg ethanol had no effect on extinction with brief (3 min) or long (24 min) exposures to the context, but injections of 3 g/kg after long exposures impaired extinction. Together, these results indicate that ethanol affects extinction by acting on multiple learning and performance processes, including attention, memory encoding, and memory expression.

Translating findings from basic fear research to clinical psychiatry in Puerto Rico

Recent advances in the neuroscience of classical fear conditioning from both rodent and human studies are beginning to be translated to the psychiatry clinic. In particular, our understanding of fear extinction as a form of "safety learning" holds promise for the treatment of anxiety disorders in which extinction learning is thought to be compromised. The Department of Psychiatry at the UPR, School of Medicine promotes the development of innovative strategies for treating mental health problems. Given the burden resulting from anxiety disorders in Puerto Rico, and the lack of evidence-based treatment practices, there is a pressing need for a future center specializing in the treatment of anxiety related disorders. This center would also serve research and training functions, with the ultimate goal of translating extinction research into clinical practice. This review presents the current developments in extinction research and its relationship to anxiety disorders and treatment. We also analyze the available literature on the epidemiology of anxiety disorders and the existing evidence-based treatments for these conditions.

Modulation of fear and anxiety by the endogenous cannabinoid system

The last decade has witnessed remarkable progress in the understanding of the mammalian cannabinoid system, from the cloning of the endogenous cannabinoid receptor to the discovery of new pharmacologic compounds acting on this receptor. Current and planned studies in humans include compounds with effects ranging from direct antagonists to inhibitors of reuptake and breakdown. This progress has been accompanied by a much greater understanding of the role of the cannabinoid system in modulating the neural circuitry that mediates anxiety and fear responses. This review focuses on the neural circuitry and pharmacology of the cannabinoid system as it relates to the acquisition, expression, and extinction of conditioned fear as a model of human anxiety. Preclinical studies suggest that these may provide important emerging targets for new treatments of anxiety disorders.

Mechanisms of fear extinction

Excessive fear and anxiety are hallmarks of a variety of disabling anxiety disorders that affect millions of people throughout the world. Hence, a greater understanding of the brain mechanisms involved in the inhibition of fear and anxiety is attracting increasing interest in the research community. In the laboratory, fear inhibition most often is studied through a procedure in which a previously fear conditioned organism is exposed to a fear-eliciting cue in the absence of any aversive event. This procedure results in a decline in conditioned fear responses that is attributed to a process called fear extinction. Extensive empirical work by behavioral psychologists has revealed basic behavioral characteristics of extinction, and theoretical accounts have emphasized extinction as a form of inhibitory learning as opposed to an erasure of acquired fear. Guided by this work, neuroscientists have begun to dissect the neural mechanisms involved, including the regions in which extinction-related plasticity occurs and the cellular and molecular processes that are engaged. The present paper will cover behavioral, theoretical and neurobiological work, and will conclude with a discussion of clinical implications.

Facilitation of extinction learning for contextual fear memory by PEPA: a potentiator of AMPA receptors

Contextual fear memory is attenuated by the re-exposure of mice to the context without aversive stimulus. This phenomenon is called extinction. Here, we report that a potentiator of AMPA receptors, 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluorophenoxyacetamide (PEPA), potently facilitates extinction learning in mice. C57BL/6J mice were exposed to novel context and stimulated by electrical footshock. After 24 h (extinction training) and 72 h (extinction test), the mice were repeatedly exposed to the context without footshock and the duration of their freezing response was measured. The duration of freezing response in the extinction test was consistently shorter than the value in extinction training. Intraperitoneal injection of PEPA 15 min before extinction training remarkably reduced the duration of freezing responses during the extinction training and test, compared with the vehicle-injected control mice. This action of PEPA on extinction was dose-dependent and inhibited by NBQX (1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide), an AMPA receptor antagonist. PEPA had no effect on acquisition and consolidation of fear memory itself. Electrophysiological studies suggested that PEPA activates the neural network much more potently in the medial prefrontal cortex (mPFC) than in the basolateral amygdala and hippocampal CA1 field. Quantitative PCR studies suggested the pronounced expression of PEPA-preferring AMPA receptor subunits (GluR3 and GluR4) and a splice variant (flop) in the mPFC. An intra-mPFC injection of PEPA facilitated the extinction much more potently than an intra-amygdala injection of PEPA did. These results suggest that PEPA facilitates extinction learning through AMPA receptor activation mainly in the mPFC.

Memory is not extinguished along with CS presentation but within a few seconds after CS-offset

Prior work with the crab's contextual memory model showed that CS-US conditioned animals undergoing an unreinforced CS presentation would either reconsolidate or extinguish the CS-US memory, depending on the length of the reexposure to the CS. Either memory process is only triggered once the CS is terminated. Based on these results, the following questions are raised. First, when is extinction memory acquired, if not along extinction training, and how long does it take? Second, can acquisition and consolidation of extinction memory be pharmacologically dissected? Here we address these questions performing three series of experiments: a first one aimed to study systematically the relationship between extinction and increasing periods of unreinforced CS presentations, a second one to determine the time boundaries of the extinction memory acquisition, and the third one to assay the requirement for protein synthesis and NMDA-like receptors of acquisition and consolidation of extinction memory. Our results confirm that it is CS-offset and not the mere retrieval (CS-onset) that triggers acquisition of extinction memory and that it is completed in less than 45 sec after CS-offset. In addition, protein synthesis is required for consolidation but not for acquisition of this memory and, conversely, NMDA-like receptor activity is required for its acquisition but not for its consolidation. Finally, we offer an interpretative scheme of our results and we discuss to what extent it could apply to multitrial extinction.

Combining pharmacotherapy with cognitive behavioral therapy: traditional and new approaches

Given the ever-increasing sources of trauma both nationally and globally, it is imperative to develop new and better treatments for anxiety disorders such as posttraumatic stress disorder (PTSD). This review is a collection of presentations that seek to do just that, either by using pharmacotherapy to try to prevent or erase the formation of traumatic fear memories, or to enhance exposure-based cognitive-behavioral therapy using pharmacological agents that have been effective in enhanced extinction of fear in rodents.

Brain mechanisms of fear extinction: historical perspectives on the contribution of prefrontal cortex

What brain regions are involved in regulating behavior when the emotional consequence of a stimulus changes from harmful to harmless? One way to address this question is to study the neural mechanisms underlying extinction of Pavlovian fear conditioning, an important form of emotional regulation that has direct relevance to the treatment of human fear and anxiety disorders. In fear extinction, the capacity of a conditioned stimulus to elicit fear is gradually reduced by repeatedly presenting it in the absence of any aversive consequence. In recent years there has been a dramatic increase in research on the brain mechanisms of fear extinction. One region that has received considerable attention as a component of the brain's extinction circuitry is the medial prefrontal cortex (mPFC). In the present article, we review the historical foundations of the modern notion that the mPFC plays a critical role in emotional regulation, a literature that was largely responsible for studies that explored the role of the mPFC in fear extinction. We also consider the role of the mPFC in a broader neural circuit for extinction that includes the amygdala and hippocampus.

Prefrontal mechanisms in extinction of conditioned fear

Interest in the medial prefrontal cortex (mPFC) as a source of behavioral inhibition has increased with the mounting evidence for a functional role of the mPFC in extinction of conditioned fear. In fear extinction, a tone-conditioned stimulus (CS) previously paired with a footshock is presented repeatedly in the absence of footshock, causing fear responses to diminish. Here, we review converging evidence from different laboratories implicating the mPFC in memory circuits for fear extinction: (1) lesions of mPFC impair recall of extinction under various conditions, (2) extinction potentiates mPFC physiological responses to the CS, (3) mPFC potentiation is correlated with extinction behavior, and (4) stimulation of mPFC strengthens extinction memory. These findings support Pavlov's original notion that extinction is new learning, rather than erasure of conditioning. In people suffering from posttraumatic stress disorder (PTSD), homologous areas of ventral mPFC show morphological and functional abnormalities, suggesting that extinction circuits are compromised in PTSD. Strategies for augmenting prefrontal function for clinical benefit are discussed.

Pharmacological treatments that facilitate extinction of fear: relevance to psychotherapy

SUMMARY

A great deal is now known about the mechanisms of conditioned fear acquisition and expression. More recently, the mechanisms of inhibition of conditioned fear have become the subject of intensive study. The major model system for the study of fear inhibition in the laboratory is extinction, in which a previously fear conditioned organism is exposed repeatedly to the fear-eliciting cue in the absence of any aversive event and the fear conditioned response declines. It is well established that extinction is a form of new learning as opposed to forgetting or "unlearning" of conditioned fear, and it is hypothesized that extinction develops when sensory pathways conveying sensory information to the amygdala come to engage GABAergic interneurons through forms of experience-dependent plasticity such as long-term potentiation. Several laboratories currently are investigating methods of facilitating fear extinction in animals with the hope that such treatments might ultimately prove to be useful in facilitating exposure-based therapy for anxiety disorders in clinical populations. This review discusses the advances that have been made in this field and presents the findings of the first major clinical study to examine the therapeutic utility of a drug that facilitates extinction in animals. It is concluded that extinction is an excellent model system for the study of fear inhibition and an indispensable tool for the screening of putative pharmacotherapies for clinical use.

Beta-adrenoceptor mediated inhibition of long-term reward-related memory reconsolidation

Well-consolidated fear-related memories, once retrieved, are susceptible to disruption and require reconsolidation in order to be maintained. We examined whether reactivated reward-related memories are also susceptible to interference by evaluating the effect of propranolol (PROP), a beta-adrenergic antagonist known to impair reconsolidation of fear-related memories, on context-induced sucrose seeking. PROP administration upon reactivation reduced sucrose seeking behaviour 3 weeks post-training, indicating that reconsolidation of reward-related memories can be disrupted after a long post-training interval.

The dual origins of affect in nightmares: The roles of physiological homeostasis and memory

Strong negative affect is a key and distressing ingredient of nightmares. Affect in nightmares arises either from the new generation of affective states due to physiological imbalances that occur during sleep or from the reactivation of affect-laden memories. The disruption of physiological balance produces a negative hedonic state, restoration of this balance produces a positive hedonic state, and when balance is attained, a neutral hedonic state results. As a result, hedonic states provoke behaviors in defense of homeostasis, then guide and terminate them. When, due to inadvertent behavior, a pronounced disruption of homeostasis occurs after sleep onset, the resultant strong negative hedonic state is likely to precipitate a nightmare and may lead to awakening. During normal wakefulness, associations of the interplay between stimuli and behaviors that disrupt homeostasis, those that restore homeostasis, and the affective states generated in the process, are committed to memory as affecto-cognitive ensembles. Sleep serves to build or rebuild neural architecture to effect development or to compensate for use- or disease-related wear (e.g. repair oxidative damage). Dreaming serves to synchronize or resynchronize such modified neural circuits with each other and those not modified. Hence, during dreaming, affecto-cognitive ensembles may get reactivated as part of the synchronization process. Where such an ensemble contains strong negative affect (i.e., due to strong affect generated during the original experience), a nightmare may be precipitated. Although both can occur throughout life, the latter type of nightmare is more likely in adults and the former in young children. For the latter memory-based behavioral therapy and for the former education and care are expected to be useful. For both types of nightmare, because strong negative affect is deemed dependent on noradrenergic outflow from the locus coeruleus, the administration of alpha-adrenergic antagonists will provide relief subject to certain caveats.

Fear extinction in rodents: basic insight to clinical promise

Fear extinction, the reduction of fear by repeated exposure to the object of fear, is a crucial paradigm of inhibitory learning and the acknowledged preclinical model for behavior therapy of human anxiety. Recent insights have clarified roles for infralimbic prefrontal cortex, hippocampus and periaqueductal gray in extinction learning, while maintaining a central role for the basolateral amygdaloid nucleus in the acquisition and storage of this learning. Simultaneously, molecular insights have implicated several neurotransmitter and second messenger systems in extinction learning, and revealed that extinction is surprisingly easy to improve, yielding the promise of a novel approach to improved psychiatric treatments for a variety of human anxiety disorders.

The L-type calcium channel blocker nifedipine impairs extinction, but not reduced contingency effects, in mice

We recently reported that fear extinction, a form of inhibitory learning, is selectively blocked by systemic administration of L-type voltage-gated calcium channel (LVGCC) antagonists, including nifedipine, in mice. We here replicate this finding and examine three reduced contingency effects after vehicle or nifedipine (40 mg/kg) administration. In the first experiment, contingency reduction was achieved by adding USs to the training protocol (degraded contingency), a phenomenon thought to be independent of behavioral inhibition. In the second experiment, contingency reduction was achieved by varying the percentage of CS-US pairing, a phenomenon thought to be weakly dependent on behavioral inhibition. In the third and fourth experiments, contingency reduction was achieved by adding CSs to the training protocol (partial reinforcement), a phenomenon thought to be completely dependent on behavioral inhibition. We found that none of these reduced contingency effects was impaired by nifedipine. In a final experiment, we found that extinction conducted 1 or 3 h post-acquisition, but not immediately, was LVGCC-dependent. Taken together, the results suggest that reduced contingency effects and extinction depend on different molecular mechanisms and that LVGCC dependence of behavioral inhibition develops with time after associative CS-US learning.

Systemic blockade of D2-like dopamine receptors facilitates extinction of conditioned fear in mice

Extinction of conditioned fear in animals is the explicit model of behavior therapy for human anxiety disorders, including panic disorder, obsessive-compulsive disorder, and post-traumatic stress disorder. Based on previous data indicating that fear extinction in rats is blocked by quinpirole, an agonist of dopamine D2 receptors, we hypothesized that blockade of D2 receptors might facilitate extinction in mice, while agonists should block extinction, as they do in rats. One day after fear conditioning mice with three pairings of a white noise conditional stimulus (CS) with moderate footshock, we injected the D2 antagonist, sulpiride, the D2 agonist, quinpirole, or vehicle, just before repeated CS presentations to generate extinction. We assayed fear by measuring behavioral freezing during extinction presentations and then drug-free during CS presentations 1 d later. We found that sulpiride injections before extinction training facilitated extinction memory 24 h later, while quinpirole partially blocked extinction memory compared with vehicle-injected controls. Notably, sulpiride treatment yielded significant extinction after spaced CS presentations, which yield no extinction at all in vehicle-treated mice. These findings suggest that dopamine D2-mediated signaling contributes physiological inhibition of extinction, and that D2 antagonists may be useful adjuncts to behavior therapy of human anxiety disorders.

A requirement for memory retrieval during and after long-term extinction learning

Current learning theories are based on the idea that learning is driven by the difference between expectations and experience (the delta rule). In extinction, one learns that certain expectations no longer apply. Here, we test the potential validity of the delta rule by manipulating memory retrieval (and thus expectations) during extinction learning. Adrenergic signaling is critical for the time-limited retrieval (but not acquisition or consolidation) of contextual fear. Using genetic and pharmacologic approaches to manipulate adrenergic signaling, we find that long-term extinction requires memory retrieval but not conditioned responding. Identical manipulations of the adrenergic system that do not affect memory retrieval do not alter extinction. The results provide substantial support for the delta rule of learning theory. In addition, the timing over which extinction is sensitive to adrenergic manipulation suggests a model whereby memory retrieval occurs during, and several hours after, extinction learning to consolidate long-term extinction memory.