Bronwyn M. Graham

The Study of Fear Extinction: Implications for Anxiety Disorders

In this review, the authors propose that the fear extinction model can be used as an experimental tool to cut across symptom dimensions of multiple anxiety disorders to enhance our understanding of the psychopathology of these disorders and potentially facilitate the detection of biomarkers for them. The authors evaluate evidence for this proposition from studies examining the neurocircuitry underlying fear extinction in rodents, healthy humans, and clinical populations. The authors also assess the potential use of the fear extinction model to predict vulnerability for anxiety and treatment response and to improve existing treatments or develop novel ones. Finally, the authors suggest potential directions for future research that will help to further validate extinction as a biomarker for anxiety across diagnostic categories and to bridge the gap between basic neuroscience and clinical practice.

Blockade of Estrogen by Hormonal Contraceptives Impairs Fear Extinction in Female Rats and Women

BACKGROUND Fear extinction is a laboratory model of fear inhibition and is the basis of exposure therapy for anxiety disorders. Emerging evidence from naturally cycling female rodents and women indicates that estrogens are necessary to the consolidation of fear extinction. Hormonal contraceptives (HCs) inhibit estrogen production; yet, their effects on fear extinction are unknown. METHODS We used a cross-species translational approach to investigate the impact of HCs and estradiol supplementation on fear extinction in healthy women (n=76) and female rats (n = 140). RESULTS Women using HCs exhibited significantly poorer extinction recall compared with naturally cycling women. The extinction impairment was also apparent in HC-treated female rats and was associated with reduced serum estradiol levels. The impairment could be rescued in HC-treated rats either by terminating HC treatment after fear learning or by systemic injection of estrogen-receptor agonists before fear extinction, all of which restored serum estradiol levels. Finally, a single administration of estradiol to naturally cycling women significantly enhanced their ability to recall extinction memories. CONCLUSIONS Together, these findings suggest that HCs may impact womens ability to inhibit fear but that this impairment is not permanent and could potentially be alleviated with estrogen treatment.

Pharmacological enhancement of fear reduction: Preclinical models

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

Fibroblast Growth Factor-2 Enhances Extinction and Reduces Renewal of Conditioned Fear

Anxiety disorders are increasingly prevalent in society; hence, there is a need to improve on existing treatments for such disorders. Fibroblast growth factor-2 (FGF2), a mitogen that is involved in brain development and regeneration, has been shown to both facilitate long-term extinction of fear and reduce stress-precipitated relapse in rats. Extinction is the laboratory analog of exposure-based therapies in humans. In this study, we continued to investigate the clinical potential of FGF2 as a pharmacological enhancer of extinction by examining its effect on renewal, a common type of relapse. In all experiments, rats were trained to fear a white noise-conditioned stimulus, and then this learned fear was extinguished the following day. Rats received systemic injections of FGF2 or vehicle immediately after extinction training. At test, on the day after extinction training, levels of freezing elicited by the white noise in either the extinction context or the original training context were measured. FGF2-treated rats showed less renewal of fear when tested in the original training context than did vehicle-treated rats. This pattern occurred even when vehicle rats were given double the amount of extinction training, and when FGF2-treated rats were given equivalent exposure to the extinction context. These results show that FGF2 facilitates long-term extinction and attenuates relapse, and thus highlight its potential as a novel pharmacological adjunct to exposure therapy.

Low estradiol levels: a vulnerability factor for the development of posttraumatic stress disorder.

t a e t m l t r d s c t a P osttraumatic stress disorder (PTSD) affects millions of people who experience trauma in a given year and women are twice as likely to develop PTSD (1). Many preclinical and clinical studies have used fear conditioning and extinction paradigms to investigate the mechanisms underlying PTSD (2). This paradigm is thought to be a good model for PTSD because fear conditioning resembles the occurrence of a traumatic event and fear extinction forms the basis of exposure therapy, which is a commonly used treatment for anxiety. Elucidating the factors that lead to extinction deficits in PTSD may help to develop novel targets for treatment. Over the last decades, much progress has been made in defining the neural and molecular substrates of fear extinction (2). However, most preclinical studies have used male rodents as subjects, and most human studies have collapsed results across sex or not considered potential influences of menstrual cycle. Recent studies indicate that the natural variance of gonadal hormones across the estrous/menstrual cycle regulates fear responses during fear extinction in both female rats and healthy women (3). The aforementioned studies used healthy women to demonstrate the effect of gonadal hormones on fear extinction. In this issue of Biological Psychiatry, Glover et al. (4) took the important next step to ask: do stradiol levels influence fear extinction in women with PTSD? To answer this question, the authors exposed traumatized omen with and without PTSD to a fear conditioning and extincion paradigm. The conditioned stimuli were colored shapes preented on a monitor, the unconditioned stimulus (US) was an air last directed at the larynx, and fear potentiated startle of the eye link muscle contraction after presentation of the startle probe was sed as a measure of conditioned responding. Fear extinction trainng occurred 10 minutes after conditioning during which the conitioned stimuli were presented but no air blast US was delivered. lood samples were collected before startle testing to assess serum evels of estradiol. Women with PTSD were classified into highand ow-estradiol groups based on the median split of estradiol serum evels. The same was also conducted for the trauma-exposed omen without PTSD. Thus, there were a total of four experimental roups. The results showed that all groups acquired fear during onditioning and extinguished fear during extinction training. owever, PTSD women with low estradiol exhibited significantly levated conditioned responses during extinction training relative o all other groups. Moreover, among women with PTSD, those with ow estradiol had greater symptom severity relative to those with igh estradiol. The authors concluded that low levels of estrogen ight be a vulnerability factor for the development of PTSD in omen with trauma histories. This is an important and fundamenal study because it is the first to assess the involvement of estradiol n fear extinction in a clinically anxious population. The study by Glover et al. (4) extends recent findings demontrating a role for estradiol in fear extinction (3). For example, Chang

Acute systemic fibroblast growth factor-2 enhances long-term extinction of fear and reduces reinstatement in rats.

Despite having made substantial advances in the treatment of anxiety disorders over the past few decades it appears that we have now reached a ‘therapeutic impasse’. Further clinical progress requires a greater understanding of the neural mechanisms underlying fear inhibition. In this study, we examined, for the first time, the effects of fibroblast growth factor-2 (FGF2), a mitogen involved in the molecular cascade of memory, on extinction and relapse in rats. In all experiments, rats were first trained to fear a white noise-conditioned stimulus, and then had this learned fear extinguished the following day. Extinction is the process underlying exposure-based therapy in humans. Experiments 1 and 2 demonstrated that FGF2 facilitated the loss of learned fear (ie, extinction) when given either prior to or immediately after extinction but not when given 4 h after extinction. This suggests that FGF2 must be present during the consolidation of the extinction memory to have an effect. Experiment 3 further supported this interpretation by showing that short-term extinction must occur for FGF2 to facilitate long-term extinction, suggesting that FGF2 is facilitating the translation of memory from short-term to long-term storage. In experiment 4 rats given FGF2 immediately after extinction exhibited less shock-induced reinstatement, which is a model preparation of relapse, than did vehicle-treated rats. Together, these experiments demonstrate that FGF2 facilitates extinction and attenuates relapse. Thus, FGF2 may be a novel pharmacological adjunct to exposure therapy.

From Resilience to Vulnerability: Mechanistic Insights into the Effects of Stress on Transitions in Critical Period Plasticity

While early experiences are proposed to be important for the emergence of anxiety and other mental health problems, there is little empirical research examining the impact of such experiences on the development of emotional learning. Of the research that has been performed in this area, however, a complex picture has emerged in which the maturation of emotion circuits is influenced by the early experiences of the animal. For example, under typical laboratory rearing conditions infant rats rapidly forget learned fear associations (infantile amnesia) and express a form of extinction learning which is relapse-resistant (i.e., extinction in infant rats may be due to fear erasure). In contrast, adult rats exhibit very long-lasting memories of past learned fear associations, and express a form of extinction learning that is relapse-prone (i.e., the fear returns in a number of situations). However, when rats are reared under stressful conditions then they exhibit adult-like fear retention and extinction behaviors at an earlier stage of development (i.e., good retention of learned fear and relapse-prone extinction learning). In other words, under typical rearing conditions infant rats appear to be protected from exhibiting anxiety whereas after adverse rearing fear learning appears to make those infants more vulnerable to the later development of anxiety. While the effects of different experiences on infant rats’ fear retention and extinction are becoming better documented, the mechanisms which mediate the early transition seen following stress remain unclear. Here we suggest that rearing stress may lead to an early maturation of the molecular and cellular signals shown to be involved in the closure of critical period plasticity in sensory modalities (e.g., maturation of GABAergic neurons, development of perineuronal nets), and speculate that these signals could be manipulated in adulthood to reopen infant forms of emotional learning (i.e., those that favor resilience).

A window of vulnerability: impaired fear extinction in adolescence.

There have been significant advances made towards understanding the processes mediating extinction of learned fear. However, despite being of clear theoretical and clinical significance, very few studies have examined fear extinction in adolescence, which is often described as a developmental window of vulnerability to psychological disorders. This paper reviews the relatively small body of research examining fear extinction in adolescence. A prominent finding of this work is that adolescents, both humans and rodents, exhibit a marked impairment in extinction relative to both younger (e.g., juvenile) and older (e.g., adult) groups. We then review some potential mechanisms that could produce the striking extinction deficit observed in adolescence. For example, one neurobiological candidate mechanism for impaired extinction in adolescence involves changes in the functional connectivity within the fear extinction circuit, particularly between prefrontal cortical regions and the amygdala. In addition, we review research on emotion regulation and attention processes that suggests that developmental changes in attention bias to threatening cues may be a cognitive mechanism that mediates age-related differences in extinction learning. We also examine how a differential reaction to chronic stress in adolescence impacts upon extinction retention during adolescence as well as in later life. Finally, we consider the findings of several studies illustrating promising approaches that overcome the typically-observed extinction impairments in adolescent rodents and that could be translated to human adolescents.

Inhibition of estradiol synthesis impairs fear extinction in male rats.

Emerging research has demonstrated that the sex hormone estradiol regulates fear extinction in female rodents and women. Estradiol may also regulate fear extinction in males, given its role in synaptic plasticity in both sexes. Here we report that inhibition of estradiol synthesis during extinction training, via the aromatase inhibitor fadrozole, significantly impairs extinction recall in male rats. This deficit in extinction recall is not due to state-dependent memory formation and is completely abolished by coadministration of estradiol. Our data suggest that estradiol may be just as important in the regulation of fear extinction in males as it is in females.

Why are women so vulnerable to anxiety, trauma-related and stress-related disorders? The potential role of sex hormones

Increased prevalence, severity, and burden of anxiety, trauma-related and stress-related disorders in women compared with men has been well documented. Evidence from a variety of fields has emerged suggesting that sex hormones, particularly oestradiol and progesterone, play a significant part in generation of these sex differences. In this Series paper, we aim to integrate the literature reporting on the effects of sex hormones on biological, behavioural, and cognitive pathways, to propose two broad mechanisms by which oestradiol and progesterone influence sex differences in anxiety disorders: augmentation of vulnerability factors associated with anxiety disorder development; and facilitation of the maintenance of anxious symptoms post-development. The implications for future research, along with novel approaches to psychological and pharmacological treatment of anxiety disorders, are also considered.