Estradiol replacement enhances fear memory formation, impairs extinction and reduces COMT expression levels in the hippocampus of ovariectomized female mice

Sex Differences in Contextual Extinction Learning After Single Binge-Like EtOH Exposure in Adolescent C57BL/6J Mice

The relationship between chronic heavy drinking and post-traumatic stress disorder (PTSD) is well-documented; however, the impact of more common drinking patterns, such as a single episode leading to a blood alcohol concentration (BAC) of 0.09 g/dL (moderate intoxication), remains underexplored. Given the frequent co-occurrence of PTSD and alcohol misuse, it is essential to understand the biological and behavioral factors driving this comorbidity. We hypothesize that alcohol's immediate sedative effects are coupled with the development of persistent molecular alcohol tolerance, which may disrupt fear extinction learning. To investigate this, we employed a S ingle E pisode E thanol (SEE) in-vivo exposure to mimic binge-like alcohol consumption over a 6-hour period, following contextual conditioning trials. Extinction trials were conducted 24 hours later to assess the effects on extinction learning. Our findings reveal a significant deficit in fear extinction learning in alcohol-treated adolescent male mice compared to saline-treated controls, with no such effects observed in female adolescent mice. These results suggest that even non-chronic alcohol exposure may contribute to the development of trauma- and stress-related disorders, such as PTSD, in males. Additionally, histological analysis revealed significant alterations in FKBP5, β-catenin, and GSK-3β levels in the hippocampus, striatum, and basolateral amygdala of alcohol-treated mice following extinction. The insights gained from this study could reshape our understanding of the risk factors for PTSD and open new avenues for prevention and treatment, targeting the molecular mechanisms that mediate alcohol tolerance.

Infralimbic YTHDF1 is necessary for the beneficial effects of acute mild exercise on auditory fear extinction retention

Exposure therapy is the most effective approach of behavioral therapy for anxiety and post-traumatic stress disorder (PTSD). But fear is easy to reappear even after successful extinction. So, identifying novel strategies for augmenting exposure therapy is rather important. It was reported that exercise had beneficial effects on cognitive and memory deficits. However, whether exercise could affect fear memory, especially for fear extinction remained elusive. Here, our results showed that exposure to acute mild exercise 1 or 2 h before extinction training can augment recent fear extinction retention and 2 h for the remote fear extinction retention. These beneficial effects could be attributed to increased YTHDF1 expression in medial prefrontal cortex (mPFC). Furthermore, by using an AAV-shRNA-based approach to silence YTHDF1 expression via stereotactic injection in prelimbic cortex (PL) or infralimbic cortex (IL), respectively, we demonstrated that silence YTHDF1 in IL, but not in PL, blunted augmentation of exposure therapy induced by acute mild exercise and accompanied with decreased NR2B and GluR1 expression. Moreover, YTHDF1 modulated dendritic spines remodeling of pyramidal neuron in IL. Collectively, our findings suggested that acute mild exercise acted as an effective strategy in augmenting exposure therapy with possible implications for understanding new treatment underlying PTSD.

The Impacts of Sex Differences and Sex Hormones on Fear Extinction

Fear extinction memories are strongly modulated by sex and hormonal status, but the exact mechanisms are still being discovered. In humans, there are some basal and task-related features in which male and female individuals differ in fear conditioning paradigms. However, analyses considering the effects of sex hormones demonstrate a role for estradiol in fear extinction memory consolidation. Translational studies are taking advantage of the convergent findings between species to understand the brain structures implicated. Nevertheless, the human brain is complex and the transfer of these findings into the clinics remains a challenge. The promising advances in the field together with the standardization of fear extinction methodologies in humans will benefit the design of new personalized therapies.

Sex differences in fear responses: Neural circuits

Women have increased vulnerability to PTSD and anxiety disorders compared to men. Understanding the neurobiological underpinnings of these disorders is critical for identifying risk factors and developing appropriate sex-specific interventions. Despite the clear clinical relevance of an examination of sex differences in fear responses, the vast majority of pre-clinical research on fear learning and memory formation has exclusively used male animals. This review highlights sex differences in context and cued fear conditioning, fear extinction and fear generalization with a focus on the neural circuits underlying these behaviors in rodents. There are mixed reports of behavioral sex differences in context and cued fear conditioning paradigms, which can depend upon the behavioral indices of fear. However, there is greater evidence of differential activation of the hippocampus, amygdalar nuclei and the prefrontal cortical regions in male and female rodents during context and cued fear conditioning. The bed nucleus of the stria terminalis (BNST), a sexually dimorphic structure, is of particular interest as it differentially contributes to fear responses in males and females. In addition, while the influence of the estrous cycle on different phases of fear conditioning is delineated, the clearest modulatory effect of estrogen is on fear extinction processes. Examining the variability in neural responses and behavior in both sexes should increase our understanding of how that variability contributes to the neurobiology of affective disorders. This article is part of the Special Issue on 'Fear, anxiety and PTSD'.

Estradiol and progesterone in female reward-learning, addiction, and therapeutic interventions

Sex steroid hormones like estradiol (E2) and progesterone (P4) guide the sexual organization and activation of the developing brain and control female reproductive behavior throughout the lifecycle; importantly, these hormones modulate functional activity of not just the endocrine system, but most of the nervous system including the brain reward system. The effects of E2 and P4 can be seen in the processing of and memory for rewarding stimuli and in the development of compulsive reward-seeking behaviors like those seen in substance use disorders. Women are at increased risk of developing substance use disorders; however, the origins of this sex difference are not well understood and therapeutic interventions targeting ovarian hormones have produced conflicting results. This article reviews the contribution of the E2 and P4 in females to functional modulation of the brain reward system, their possible roles in origins of addiction vulnerability, and the development and treatment of compulsive reward-seeking behaviors.

Treatment With Nepicastat Decreases Contextual Traumatic Memories Persistence in Post-traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is a common anxiety mental disorder and can be manifested after exposure to a real or perceived life-threatening event. Increased noradrenaline and adrenaline in plasma and urine have been documented in PTSD. Dopamine-β-hydroxylase (DBH) catalyzes the conversion of dopamine to noradrenaline and consequently, DBH inhibition reduces catecholamines. Our aim was to evaluate if nepicastat treatment decreases PTSD signs in an animal model. Wild-type (129x1/SvJ) female mice were submitted to PTSD induction protocol. DBH-inhibitor nepicastat (30 mg/kg) or vehicle (0.2% HPMC) were administered once daily since day 0 until day 7 or 12. The percentage of freezing was calculated on days 0, 1, 2, and 7, and behavioral tests were performed. Quantification of nepicastat in plasma and DBH activity in the adrenal gland was evaluated. Catecholamines were quantified by HPLC with electrochemical detection. mRNA expression of Npas4 and Bdnf in hippocampus was evaluated by qPCR.Mice in the PTSD-group and treated with nepicastat showed a decrease in freezing, and an increase in the time spent and entries in open arms in elevated plus maze test. In mice treated with nepicastat, adrenal gland DBH activity was decreased, and catecholamines were also decreased in plasma and tissues. On day 7, in mice treated with nepicastat, there was an increase of Npas4 and Bdnf mRNA expression in the hippocampus.In conclusion, DBH inhibitor nepicastat has an effect consistent with a decrease in the persistence of traumatic memories and anxiety-like behavior in this PTSD mice model. The disruption of traumatic memories through interference with the formation, consolidation, retrieval, and/or expression processes may be important to decrease PTSD symptoms and signs. The increase in Npas4 and Bdnf mRNA expression in the hippocampus may be important to develop a weaker traumatic contextual memory after nepicastat treatment.

Differential chronic social stress models in male and female mice

Chronic stress creates an allostatic overload that may lead to mood disorders such as anxiety and depression. Modern causes of chronic stress in humans are mostly social in nature, relating to work and relationship stress. Research into neural and molecular mechanisms of vulnerability and resilience following chronic social stress (CSS) is ongoing and uses animal models to discover efficient prevention strategies and treatments. To date, most CSS studies have neglected the female sex and used male-focused aggression-based animal models such as chronic social defeat stress (CSDS). Accumulating evidence on sex differences suggests differences in the stress response, the prevalence of stress-related illness and in response to treatment, indicating that researchers should expand CSS investigation to include female-focused protocols alongside the popular CSDS protocols. Here, we describe a novel female mouse model of CSS and a parallel modified male mouse model of CSDS in C57BL/6 mice. These new models enable the investigation of vulnerability, coping and downstream effectors mediating short-term and long-term consequences of CSS in both sexes. Our data demonstrate differential effects on male and female mice during, soon after, and many weeks after CSS. Female mice are more prone to body weight loss during CSS and hyperactive anxious behaviour following CSS. Both sexes show reduced social interaction, but only stressed male mice show long-term changes in emotional memory and neuroendocrine function. We further discuss future avenues of research using these models to investigate mechanisms pertaining to sensitivity to CSS and treatment response profiles, in a sex-appropriate manner.

Unexpected Role of Physiological Estrogen in Acute Stress-Induced Memory Deficits

Stress may promote emotional and cognitive disturbances, which differ by sex. Adverse outcomes, including memory disturbances, are typically observed following chronic stress, but are now being recognized also after short events, including mass shootings, assault, or natural disasters, events that consist of concurrent multiple acute stresses (MAS). Prior work has established profound and enduring effects of MAS on memory in males. Here we examined the effects of MAS on female mice and probed the role of hormonal fluctuations during the estrous cycle on MAS-induced memory problems and the underlying brain network and cellular mechanisms. Female mice were impacted by MAS in an estrous cycle-dependent manner: MAS impaired hippocampus-dependent spatial memory in early-proestrous mice, characterized by high levels of estradiol, whereas memory of mice stressed during estrus (low estradiol) was spared. As spatial memory requires an intact dorsal hippocampal CA1, we examined synaptic integrity in mice stressed at different cycle phases and found a congruence of dendritic spine density and spatial memory deficits, with reduced spine density only in mice stressed during high estradiol cycle phases. Assessing MAS-induced activation of brain networks interconnected with hippocampus, we identified differential estrous cycle-dependent activation of memory- and stress-related regions, including the amygdala. Network analyses of the cross-correlation of fos expression among these regions uncovered functional connectivity that differentiated impaired mice from those not impaired by MAS. In conclusion, the estrous cycle modulates the impact of MAS on spatial memory, and fluctuating physiological levels of sex hormones may contribute to this effect.SIGNIFICANCE STATEMENT: Effects of stress on brain functions, including memory, are profound and sex-dependent. Acute stressors occurring simultaneously result in spatial memory impairments in males, but effects on females are unknown. Here we identified estrous cycle-dependent effects of such stresses on memory in females. Surprisingly, females with higher physiological estradiol experienced stress-induced memory impairment and a loss of underlying synapses. Memory- and stress-responsive brain regions interconnected with hippocampus were differentially activated across high and low estradiol mice, and predicted memory impairment. Thus, at functional, network, and cellular levels, physiological estradiol influences the effects of stress on memory in females, providing insight into mechanisms of prominent sex differences in stress-related memory disorders, such as post-traumatic stress disorder.

Oestradiol as a neuromodulator of learning and memory

Although hormones such as glucocorticoids have been broadly accepted in recent decades as general neuromodulators of memory processes, sex steroid hormones such as the potent oestrogen 17β-oestradiol have been less well recognized by the scientific community in this capacity. The predominance of females in studies of oestradiol and memory and the general (but erroneous) perception that oestrogens are 'female' hormones have probably prevented oestradiol from being more widely considered as a key memory modulator in both sexes. Indeed, although considerable evidence supports a crucial role for oestradiol in regulating learning and memory in females, a growing body of literature indicates a similar role in males. This Review discusses the mechanisms of oestradiol signalling and provides an overview of the effects of oestradiol on spatial, object recognition, social and fear memories. Although the primary focus is on data collected in females, effects of oestradiol on memory in males will be discussed, as will sex differences in the molecular mechanisms that regulate oestrogenic modulation of memory, which may have important implications for the development of future cognitive therapeutics.

The neurobiological basis of sex differences in learned fear and its inhibition

Learning that certain cues or environments predict threat enhances survival by promoting appropriate fear and the resulting defensive responses. Adapting to changing stimulus contingencies by learning that such cues no longer predict threat, or distinguishing between these threat-related and other innocuous stimuli, also enhances survival by limiting fear responding in an appropriate manner to conserve resources. Importantly, a failure to inhibit fear in response to harmless stimuli is a feature of certain anxiety and trauma-related disorders, which are also associated with dysfunction of the neural circuitry underlying learned fear and its inhibition. Interestingly, these disorders are up to twice as common in women, compared to men. Despite this striking sex difference in disease prevalence, the neurobiological factors involved remain poorly understood. This is due in part to the majority of relevant preclinical studies having neglected to include female subjects alongside males, which has greatly hindered progress in this field. However, more recent studies have begun to redress this imbalance and emerging evidence indicates that there are significant sex differences in the inhibition of learned fear and associated neural circuit function. This paper provides a narrative review on sex differences in learned fear and its inhibition through extinction and discrimination, along with the key gonadal hormone and brain mechanisms involved. Understanding the endocrine and neural basis of sex differences in learned fear inhibition may lead to novel insights on the neurobiological mechanisms underlying the enhanced vulnerability to develop anxiety-related disorders that are observed in women.

Loss of Estrogen Efficacy Against Hippocampus Damage in Long-Term OVX Mice Is Related to the Reduction of Hippocampus Local Estrogen Production and Estrogen Receptor Degradation

Postmenopausal women experience a higher risk for neurodegenerative diseases, including cognitive impairment and ischemic stroke. Many preclinical studies have indicated that estrogen replacement therapy (ERT) may provide protective effects against these neurological diseases. However, the results of Women's Health Initiative (WHI) studies have led to the proposal of "critical period hypothesis," which states that there is a precise window of opportunity for administering beneficial hormone therapy following menopause. However, the underlying molecular mechanisms require further characterization. Here, we explored the effects of ERT on cognition decline and global cerebral ischemia (GCI)-induced hippocampal neuronal damage in mice that had experienced both short-term (ovariectomized (OVX) 1 week) and long-term (OVX 10 weeks) estrogen deprivation. We also further explored the concentration of 17β-estradiol (E2) in the circulation and hippocampus and the expression of aromatase and estrogen receptors (ERα, ERα-Ser118, and ERβ). We found that the neuroprotective effectiveness of ERT against hippocampus damage exhibited in OVX1w mice was totally absent in OVX10w mice. Interestingly, the concentration of hippocampal E2 was irreversibly reduced in OVX10w mice, which was related to the decrease of aromatase expression in the hippocampus. In addition, long-term estrogen deprivation (LTED) led to a decrease in estrogen receptor proteins in the hippocampus. Thus, we concluded that the loss of ERT neuroprotection against hippocampus injury in LTED mice was related to the reduction in hippocampus E2 production and estrogen receptor degradation. These results provide several intervention targets to restore the effectiveness of ERT neuroprotection in elderly post-menopausal women.

Depletion of dietary phytoestrogens reduces hippocampal plasticity and contextual fear memory stability in adult male mouse

Introduction: Phytoestrogens are non-steroidal estrogen analogues and are found primarily in soy products. They have received increasing attention as dietary supplements for estrogen deficiency and as modulators of endogenous estrogen functions, including cognition and emotion. In addition to modifying the levels of circulating sex hormones, phytoestrogens also exert direct effects on estrogen and androgen receptors in the brain and thus effectively modulate the neural circuit functions.Objective: The aim of this study was to investigate the long-term effects of low phytoestrogen intake (∼6 weeks) on the hippocampal plasticity and hippocampus-dependent memory formation in the adult C57BL/6 male mice.Methods and Results: In comparison to mice on a diet with normal phytoestrogen content, mice on low phytoestrogen diet showed a significant reduction in the phosphorylation of NR2B subunit, a molecular correlate of plasticity in the Schaffer collateral-CA1 synapse. We observed a profound decrease in long-term potentiation (LTP) in the ventral hippocampus, whereas no effect on plasticity was evident in its dorsal portion. Furthermore, we demonstrated that acute perfusion of slices with an estrogen analogue equol, an isoflovane metabolized from daidzein produced by the bacterial flora in the gut, was able to rescue the observed LTP deficit. Examining potential behavioral correlates of the plasticity attenuation, we found that mice on phytoestrogen-free diet display decreased contextual fear memory at remote but not at recent time points after training.Conclusions: Our data suggests that nutritional phytoestrogens have profound effects on the plasticity in the ventral hippocampus and ventral hippocampus-dependent memory.

Saikosaponin‑D improves fear memory deficits in ovariectomized rats via the action of estrogen receptor‑α in the hippocampus

Saikosaponin‑D (SSD), which is the main bioactive component in the traditional Chinese medicine Chai Hu (Bupleurum falcatum L), possesses estrogen‑like properties and is widely used in treating estrogen‑related neurological disorders. The current study aimed to investigate the protective effects of SSD on the fear memory deficit in ovariectomized (OVX) rats and the potential underlying mechanism. SSD treatment significantly prolonged freezing time in OVX rats in a manner similar to that of estradiol (E2), whereas this effect was markedly suppressed by co‑administration of ICI182780, a non‑selective estrogen receptor (ER) inhibitor. The expression of ERα in the hippocampus of OVX rats was significantly elevated by SSD; however, Erβ expression and E2 synthesis were not markedly affected by SSD treatment. Collectively, this study demonstrated that SSD‑mediated fear memory improvement in OVX rats may be attributed not to E2 levels or ERβ activity, but to ERα activation in the hippocampus.

Haplotypic and Genotypic Association of Catechol-O-Methyltransferase rs4680 and rs4818 Polymorphisms and Treatment Resistance in Schizophrenia

Treatment-resistant schizophrenia (TRS) continues to be a challenge. It was related to different factors, including alterations in the activity of brain dopaminergic system, which could be influenced by the dopamine-degrading enzyme, catechol-O-methyltransferase (COMT). Variants of the COMT gene have been extensively studied as risk factors for schizophrenia; however, their association with TRS has been poorly investigated. The aim of the present study was to determine the haplotypic and genotypic association of COMT rs4680 and rs4818 polymorphisms with the presence of TRS. Overall, 931 Caucasian patients diagnosed with schizophrenia (386 females and 545 males) were included, while 270 participants met the criteria for TRS. In males, no significant haplotypic and genotypic associations between COMT rs4680 and rs4818 polymorphisms and TRS were detected. However, genotypic analyses demonstrated higher frequency of COMT rs4680 AA genotype carriers compared to G-allele carriers (p = 0.033) and higher frequency of COMT rs4818 CC genotype carriers than G-allele carriers (p = 0.014) in females with TRS. Haplotype analyses confirmed that the presence of the G allele in females was associated with lower risk of TRS. In women with TRS, the high activity G-G/G-G haplotype was rare, while carriers of other haplotypes were overrepresented (p = 0.009). Such associations of COMT rs4680 and rs4818 high-activity (G variants), as well as G-G/G-G haplotype, with the lower risk of TRS in females, but not in males, suggest significant, but sex-specific influence of COMT variants on the development of treatment-resistance in patients with schizophrenia. However, due to relatively low number of females, those findings require replication in a larger sample.

Effects of systemic estradiol on fear extinction in female rats are dependent on interactions between dose, estrous phase, and endogenous estradiol levels

Administering estradiol to females during periods of low endogenous estradiol enhances their ability to extinguish fear, the laboratory basis of exposure therapy for anxiety disorders. It has therefore been proposed that estradiol could be a useful adjunct to enhance exposure therapy outcomes. The present study aimed to clarify the boundary conditions under which estradiol could be used for this purpose, by assessing whether the impact of estradiol, administered systemically prior to extinction training, differs depending on dose and estrous phase in adult female rats. Results demonstrated that in rats extinguished during metestrus (naturally low estradiol), a low dose of estradiol reduced freezing during extinction training and augmented extinction recall the following day, whereas a high dose of estradiol had no effect on either extinction training or recall. In rats extinguished during proestrus (naturally high estradiol), a high dose of estradiol impaired extinction recall, whereas a low dose of estradiol had no effect, or impairing effects, on extinction recall in different experiments. A subsequent analysis revealed that estradiol-treated proestrus rats that exhibited impaired extinction recall had significantly higher pre-treatment serum estradiol levels than those that exhibited good extinction recall. Together, these results indicate that systemically administered estradiol interacts with endogenous estradiol to produce an inverted U shaped dose effect on fear extinction, where low and high estradiol levels lead to poor extinction recall, and moderate estradiol levels lead to good extinction recall. These results highlight potential limitations to the use of estradiol as an adjunct to exposure therapy in clinical settings.

Sex differences in neural responses to subliminal sad and happy faces in healthy individuals: Implications for depression

Twice as many women as men suffer from mood and anxiety disorders, yet the biological underpinnings of this phenomenon have been understudied and remain unclear. We and others have shown that the hemodynamic response to subliminally presented sad or happy faces during functional MRI (fMRI) is a robust biomarker for the attentional bias toward negative information classically observed in major depression. Here we used fMRI to compare the performance of healthy females (n = 28) and healthy males (n = 28) on a backward masking task using a fast event-related design with gradient-recalled, echoplanar imaging with sensitivity encoding. The image data were compared across groups using a region-of-interest analysis with small-volume correction to control for multiple testing (P_corrected  < 0.05, cluster size ≥ 20 voxels). Notably, compared with males, females showed greater BOLD activity in the subgenual anterior cingulate cortex (sgACC) and the right hippocampus when viewing masked sad vs. masked happy faces. Furthermore, females displayed reduced BOLD activity in the right pregenual ACC and left amygdala when viewing masked happy vs. masked neutral faces. Given that we have previously reported similar findings for depressed participants compared with healthy controls (regardless of gender), our results raise the possibility that on average healthy females show subtle emotional processing biases that conceivably reflect a subgroup of women predisposed to depression. Nevertheless, we note that the differences between males and females were small and derived from region-of-interest rather than voxelwise analyses. © 2016 Wiley Periodicals, Inc.

Sex- and Estrus-Dependent Differences in Rat Basolateral Amygdala

Depression and anxiety are diagnosed almost twice as often in women, and the symptomology differs in men and women and is sensitive to sex hormones. The basolateral amygdala (BLA) contributes to emotion-related behaviors that differ between males and females and across the reproductive cycle. This hints at sex- or estrus-dependent features of BLA function, about which very little is known. The purpose of this study was to test whether there are sex differences or estrous cyclicity in rat BLA physiology and to determine their mechanistic correlates. We found substantial sex differences in the activity of neurons in lateral nuclei (LAT) and basal nuclei (BA) of the BLA that were associated with greater excitatory synaptic input in females. We also found strong differences in the activity of LAT and BA neurons across the estrous cycle. These differences were associated with a shift in the inhibition-excitation balance such that LAT had relatively greater inhibition during proestrus which paralleled more rapid cued fear extinction. In contrast, BA had relatively greater inhibition during diestrus that paralleled more rapid contextual fear extinction. These results are the first to demonstrate sex differences in BLA neuronal activity and the impact of estrous cyclicity on these measures. The shift between LAT and BA predominance across the estrous cycle provides a simple construct for understanding the effects of the estrous cycle on BLA-dependent behaviors. These results provide a novel framework to understand the cyclicity of emotional memory and highlight the importance of considering ovarian cycle when studying the BLA of females.SIGNIFICANCE STATEMENT There are differences in emotional responses and many psychiatric symptoms between males and females. This may point to sex differences in limbic brain regions. Here we demonstrate sex differences in neuronal activity in one key limbic region, the basolateral amygdala (BLA), whose activity fluctuates across the estrous cycle due to a shift in the balance of inhibition and excitation across two BLA regions, the lateral and basal nuclei. By uncovering this push-pull shift between lateral and basal nuclei, these results help to explain disparate findings about the effects of biological sex and estrous cyclicity on emotion and provide a framework for understanding fluctuations in emotional memory and psychiatric symptoms.

Astrocyte Activation in Locus Coeruleus Is Involved in Neuropathic Pain Exacerbation Mediated by Maternal Separation and Social Isolation Stress

Our previous studies demonstrated that emotional dysfunction associated with early life stress exacerbated nerve injury-induced mechanical allodynia. Sex differences were observed in several anxiety tests, but not in mechanical allodynia. To elucidate the mechanism underlying these findings, we have now investigated the involvement of astrocytes in emotional dysfunction and enhancement of nerve injury-induced mechanical allodynia in mice subjected to maternal separation combined with social isolation (MSSI) as an early life stress. We measured expression of glial fibrillary acidic protein (GFAP), an astrocyte maker, in each brain area by immunohistochemistry. GFAP expression in the locus coeruleus (LC) of female, but not of male mice, significantly increased after MSSI, corresponding to the behavioral changes at 7 and 12 weeks of age. Lipopolysaccharide (LPS)-treated astrocyte-derived supernatant was administered to local brain regions, including LC. Intra-LC injection of conditioned medium from cultured astrocytes treated with LPS increased GFAP expression, anxiety-like behavior and mechanical allodynia in both male and female mice. Furthermore, increases in anxiety-like behavior correlated with increased mechanical allodynia. These findings demonstrate that emotional dysfunction and enhanced nerve injury-induced mechanical allodynia after exposure to MSSI are mediated, at least in part, by astrocyte activation in the LC. Male but not female mice may show resistance to MSSI stress during growth.

Aromatized testosterone attenuates contextual generalization of fear in male rats

Generalization is a common symptom of many anxiety disorders, and females are 60% more likely to suffer from an anxiety disorder than males. We have previously demonstrated that female rats display significantly accelerated rates of contextual fear generalization compared to male rats; a process driven, in part, by activation of ERβ. The current study was designed to determine the impact of estrogens on contextual fear generalization in male rats. For experiment 1, adult male rats were gonadectomized (GDX) and implanted with a capsule containing testosterone proprionate, estradiol, dihydrotestosterone proprionate (DHT), or an empty capsule. Treatment with testosterone or estradiol maintained memory precision when rats were tested in a different (neutral) context 1day after training. However, male rats treated with DHT or empty capsules displayed significant levels of fear generalization, exhibiting high levels of fear in the neutral context. In Experiment 2, we used acute injections of gonadal hormones at a time known to elicit fear generalization in female rats (e.g. 24h before testing). Injection treatment followed the same pattern of results seen in Experiment 1. Finally, animals given daily injections of the aromatase inhibitor, Fadrozole, displayed significant fear generalization. These data suggest that testosterone attenuates fear generalization likely through the aromatization testosterone into estradiol as animals treated with the non-aromatizable androgen, DHT, or animals treated with Fadrozole, displayed significant generalized fear. Overall, these results demonstrate a sex-dependent effect of estradiol on the generalization of contextual fear.

Perspectives on fear generalization and its implications for emotional disorders

Although generalization to conditioned stimuli is not a new phenomenon, renewed interest in understanding its biological underpinning has stemmed from its association with a number of anxiety disorders. Generalization as it relates to fear processing is a temporally dynamic process in which animals, including humans, display fear in response to similar yet distinct cues or contexts as the time between training and testing increases. This Review surveys the literature on contextual fear generalization and its relation to several views of memory, including systems consolidation, forgetting, and transformation hypothesis, which differentially implicate roles of the hippocampus and neocortex in memory consolidation and retrieval. We discuss recent evidence on the neurobiological mechanisms contributing to the increase in fear generalization over time and how generalized responding may be modulated by acquisition, consolidation, and retrieval mechanisms. Whereas clinical perspectives of generalization emphasize a lack of fear inhibition to CS^- cues or fear toward intermediate CS cues, the time-dependent nature of generalization and its relation to traditional views on memory consolidation and retrieval are often overlooked. Understanding the time-dependent increase in fear generalization has important implications not only for understanding how generalization contributes to anxiety disorders but also for understanding basic long-term memory function. © 2016 Wiley Periodicals, Inc.

Hippocampal cytosolic estrogen receptors regulate fear generalization in females

Generalization of fear responses is a symptom of many anxiety disorders and we have previously demonstrated that female rats generalize fear to a neutral context at a faster rate compared to males. This effect is due in part, to activation of ER and modulation of memory retrieval mechanisms resulting in fear generalization. Given that the effects of estradiol on fear generalization required approximately 24h, our data suggested possible genomic actions on fear generalization. To determine whether these actions were due to cytosolic versus membrane bound receptors, female rats were given infusions of ICI 182,780, a cytosolic estrogen receptor antagonist, into the lateral ventricle or dorsal hippocampus simultaneously with estradiol treatment or with an ER agonist (DPN). Infusions of ICI into the lateral ventricle or the dorsal hippocampus blocked fear generalization induced by peripheral or central treatment with estradiol or DPN, suggesting that estradiol acts through cytosolic ERβ receptors. In further support of these findings, intracerebroventricular or intra-hippocampal infusions of bovine serum conjugated estradiol (E2-BSA), activating membrane-bound estrogen receptors only, did not induce fear generalization. Moreover, rats receiving intra-hippocampal infusions of the ERK/MAPK inhibitor, U0126, continued to display estradiol-induced generalization, again suggesting that membrane-bound estrogen receptors do not contribute to fear generalization. Overall, these data suggest that estradiol-induced enhancements in fear generalization are mediated through activation of cytosolic/nuclear ER within the dorsal hippocampus. This region seems to be an important locus for the effects of estradiol on fear generalization although additional neuroanatomical regions have yet to be identified.

Modulation of Fear Extinction by Stress, Stress Hormones and Estradiol: A Review

Fear acquisition and extinction are valid models for the etiology and treatment of anxiety, trauma- and stressor-related disorders. These disorders are assumed to involve aversive learning under acute and/or chronic stress. Importantly, fear conditioning and stress share common neuronal circuits. The stress response involves multiple changes interacting in a time-dependent manner: (a) the fast first-wave stress response [with central actions of noradrenaline, dopamine, serotonin, corticotropin-releasing hormone (CRH), plus increased sympathetic tone and peripheral catecholamine release] and (b) the second-wave stress response [with peripheral release of glucocorticoids (GCs) after activation of the hypothalamus-pituitary-adrenocortical (HPA) axis]. Control of fear during extinction is also sensitive to these stress-response mediators. In the present review, we will thus examine current animal and human data, addressing the role of stress and single stress-response mediators for successful acquisition, consolidation and recall of fear extinction. We report studies using pharmacological manipulations targeting a number of stress-related neurotransmitters and neuromodulators [monoamines, opioids, endocannabinoids (eCBs), neuropeptide Y, oxytocin, GCs] and behavioral stress induction. As anxiety, trauma- and stressor-related disorders are more common in women, recent research focuses on female sex hormones and identifies a potential role for estradiol in fear extinction. We will thus summarize animal and human data on the role of estradiol and explore possible interactions with stress or stress-response mediators in extinction. This also aims at identifying time-windows of enhanced (or reduced) sensitivity for fear extinction, and thus also for successful exposure therapy.