Chronic stress and sex differences on the recall of fear conditioning and extinction

Stress-induced changes in the molecular processes underlying fear memories: implications for PTSD and relevant animal models

Most of the fear literature on humans and animals tests healthy individuals. However, fear memories can differ between healthy individuals and those previously exposed to traumatic stress, such as a car accident, sexual abuse, military combat and personal assault. Traumatic stress can lead to post-traumatic stress disorder (PTSD) which presents alterations in fear memories, such as an impairment of fear extinction and extinction recall. PTSD-like animal models are exposed to a single highly stressful experience in the laboratory, such as stress immobilization or single-prolonged stress. Some days later, animals exposed to a PTSD-like model can be tested in fear procedures that help uncover molecular mechanisms of fear memories. In this review, there are discussed the molecular mechanisms in stress-induced fear memories of patients with PTSD and PTSD-like animal models. The focus is on the effects of estradiol and cortisol/corticosterone hormones and of different genes, such as FKBP prolyl isomerase 5 gene (FKBP5) - FK506 binding protein 51 (FKBP51), pituitary adenylate cyclase-activating peptide (PACAP) - pituitary adenylate cyclase-activating polypeptide type I receptor (PAC1R), endocannabinoid (eCB) system and the tropomyosin receptor kinase B (TrkB) - brain-derived neurotrophic factor (BDNF). The conclusion is that greater emphasis should be placed on investigating the molecular mechanisms of fear memories in PTSD, through direct testing of patients with PTSD or the use of relevant PTSD-like models.

Preclinical behavioral and pharmacological treatments for enhancing fear extinction in adolescence

Adolescence is a window of vulnerability for the development of anxiety disorders but also a window of opportunity for treatments to minimize the long-term impact of such disorders. Current first-line treatments, primarily exposure-based cognitive-behavioral therapy (CBT), have limited long-term efficacy in adolescents. The urgent need for more effective interventions is underscored by the frequent reports of extinction impairments in adolescents as well as the rising anxiety rates in youth, particularly post-COVID-19. Preclinical research on the extinction of learned fear in adolescents may contribute to developing better treatment approaches to anxiety in this age group. Unfortunately, this is still a largely under-explored area. However, both pharmacological and behavioral augmentation strategies can be used to enhance extinction learning and consolidation. Here we describe work exploring such adjuncts, focusing on pre-clinical work with rodents. Much of the research to date shows striking developmental differences in response to various pharmacological treatments, with only a few shown to be effective in adolescents. Further, recent experience of stress reduces the efficacy of these treatments in adolescence. This review highlights the necessity for tailored strategies, especially when it comes to pharmacological adjuncts, that address developmental differences in drug responses as well as the impact of stressful experiences on treatment efficacy.

Environmental Enrichment Has Age- and Sex-Specific Effects on Fear Regulation in Mice

Previous research has sought to understand the mechanisms by which early life adversity disrupts later behavioral function. Yet less has been done to investigate the effects of positive experiences such as environmental enrichment. Here, we examined the effects of brief (2 weeks) or extended (8 weeks) environmental enrichment on discrimination, fear inhibition, and fear extinction during adolescence or adulthood. Two conditions of enrichment were used: a "Hut" group received a polycarbonate Hut in the home cage throughout the duration of the experiment, while a "Variable" group received a polycarbonate Hut, as well as a series of rotating enrichment features. Our data show that in females, brief Variable enrichment increased the rate of learning about cues that explicitly indicate safety during adolescence while disrupting this type of learning in adults. In males, enrichment did not influence learning about safety cues. Conversely, in males, brief Variable enrichment initially improved extinction, but extended enrichment (Variable or Hut) was necessary to see sustained improvements across extinction. This pattern was apparent for both adolescents and adults. In females, Variable enrichment did not affect extinction, while Hut enrichment increased freezing for all mice initially and adolescents as extinction continued. This work has the potential to inform treatments and interventions for fear-related disorders, such as anxiety and post-traumatic stress disorder, tailored for both specific developmental ages and sex.

Inhibition of prefrontal glutamatergic neuron activity during the recovery period following chronic stress disrupts fear memory in male rats: potential role of the infralimbic cortex

Chronic stress typically leads to deficits in fear extinction. However, when a delay occurs from the end of chronic stress and the start of fear conditioning (a "recovery"), rats show improved context-cue discrimination, compared to recently stressed rats or nonstressed rats. The infralimbic cortex (IL) is important for fear extinction and undergoes neuronal remodeling after chronic stress ends, which could drive improved context-cue discrimination. Here, glutamatergic IL neurons of Sprague-Dawley male rats were targeted for inhibition using inhibitory designer receptors exclusively activated by designer drugs (DREADDs) and daily injections of clozapine N-oxide (CNO) during a 21-day recovery period from chronic stress. Histological verification confirmed DREADDs in the IL with some spread to nearby medial prefrontal cortex (PFC) regions. CNO administration was then discontinued before fear conditioning started and behavioral testing thereafter so that behavioral assessments occurred without neuronal inhibition. Fear conditioning involved presenting male rats with three tone-foot shock pairings on 1 day, which was followed by 2 days of 15 tone-alone extinction sessions. Daily and repeated inhibition of mainly IL neurons during the 21-day recovery period did not disrupt fear learning or fear extinction in all groups (controls, stressed rats without a recovery, and stressed rats with a recovery). However, chronically stressed rats given a recovery and with DREADD activation showed impaired spontaneous recovery, indicating a failure to form a tone-foot shock association. The findings show that daily inhibition of mainly IL neurons prior to fear conditioning and extinction depends upon the changes that occur during the recovery period following the end of chronic stress.

Post-conditioning sleep deprivation facilitates delay and trace fear memory extinction

Trace and delay auditory fear conditioning involve different memory association strategies based on working memory involvement; however, their differences in long-term processing through sleep and extinction training remain unclear. While females often exhibit more persistent fear, complicating psychiatric treatment, most studies have primarily focused on how sleep affects initial recall in male mice. We investigated the three-way interaction between tests (trace vs. delay), sleep states, and sex during initial recall, extinction, and post-extinction remote recall. A six-hour post-conditioning sleep deprivation (SD) did not affect freezing behavior during the following day's extinction training of delay fear memory. However, during post-extinction remote recall of delay fear memory, SD prevented spontaneous recovery in males and reduced persistent freezing in females. In contrast, SD rapidly facilitated extinction of trace fear memory. In summary, SD enhances extinction both in the short-term and long-term, depending on the conditioning protocol. These findings highlight the importance of long-term assessments to explore interactions among emotional memory, sleep, and sex differences, with implications for individualized mechanisms underlying post-traumatic stress disorder (PTSD) and its treatments.

Sex differences in fear expression and persistence in an animal model of Post-Traumatic Stress Disorder

Post-Traumatic Stress Disorder (PTSD) is a complex psychiatric condition arising from traumatic experiences, marked by abnormal fear memories. Despite women are twice as likely as men to develop PTSD, the biological mechanisms underlying this disparity remain inadequately explored, particularly in preclinical studies involving female subjects. Previous research shows that female rats exhibit active fear responses, while males display passive behaviors. Additionally, sex differences in ultrasonic vocalizations (USVs) during fear conditioning have been observed, indicating varying emotional responses. Here, we validated a traumatic stress model consisting of footshock exposure paired with social isolation - originally developed in male rats - on females for the first time, focusing on sex differences in fear memory expression, retention and extinction. Our findings reveal that only during trauma exposure, males predominantly exhibited passive responses, whereas females demonstrated more active responses, despite both sexes emitting similar numbers of alarm USVs. Females also showed lower levels of freezing and USV emissions throughout extinction sessions and displayed a higher extinction rate compared to males. Notably, only males displayed a conditioned fear response when triggered by a single mild stressor. These findings highlight sex differences in trauma responses and fear memory processes. The study emphasizes the importance of incorporating 22-kHz USV evaluations along with other behavioral metrics for a comprehensive understanding of fear memory. This research contributes to the existing literature on traumatic stress models as well as underscores the necessity of including female subjects in preclinical studies to better inform treatment and prevention strategies tailored to both sexes.

Oxytocin receptor control social information about fear expression of others in mice

The social functions of oxytocin are diverse, and the specific aspects of information processing involved in emotional contagion remain unclear. We compared some fear-related behaviors among oxytocin receptor knockout mice and oxytocin-receptor-reduced mice with that of wild-type mice. In the observational fear assay, which reflects fear emotional contagion, mice that observed other individuals receiving electric shocks exhibited vicarious freezing. Mice with reduced or knockout oxytocin receptor expression showed reduced vicarious freezing. In the emotional discrimination assay, which reflects the ability to perceive others' emotional cues, we compared approach and scent-sniffing behaviors toward fear and emotionally neutral individuals. While wild-type mice were able to detect the fear emotion of others, mice with reduced or knocked-out oxytocin receptors showed reduced discrimination ability. In the fear behavior assays, which do not present social cues, we did not find these differences in oxytocin receptor expression in the brain. These findings indicate that oxytocin plays a role in emotional contagion by perceiving the emotions of others.

Effects of early social isolation and adolescent single prolonged stress on anxiety-like behaviors and voluntary ethanol consumption in female Long Evans rats

BACKGROUND

Exposure to stress during childhood and adolescence is a risk factor for alcohol use disorder (AUD) and comorbid conditions, including posttraumatic stress disorder (PTSD). We previously established an adolescent social isolation (SI) model that leads to the emergence of a wide range of behavioral risk factors for AUD, including increased anxiety-like behavior, locomotor activity, and ethanol consumption in male and female rats. Here, we sought to test the hypothesis that SI may increase vulnerability to single prolonged stress (SPS), a rodent model of PTSD.

METHODS

Female Long Evans rats (n = 8/group) were either single-housed or group-housed (GH) (4/cage) on postnatal day 21. One week later, rats underwent testing in the open field test (OFT), elevated plus-maze (EPM), and successive alleys test (SAT). Following initial behavioral testing, a subset of SI/GH rats were exposed to SPS. All rats were then tested on the novelty-suppressed feeding test (NSFT) followed by fear conditioning and home cage two-bottle choice to assess ethanol consumption.

RESULTS

SI significantly increased activity in the OFT and anxiety-like behavior on the SAT, but not the EPM. While SI and SPS alone had no effect on the NSFT, exposure to both stressors significantly increased approach latency. Complex effects of stress history were observed across a 3-day fear conditioning paradigm and no group differences were observed with home cage ethanol consumption, regardless of prior ethanol exposure.

CONCLUSIONS

The results from this study provide novel evidence that SI interacts with SPS in female rats to influence behavior in assays of unconditioned anxiety-like behavior (NSFT) and conditioned fear. Surprisingly, stress exposure had no effect on home cage ethanol consumption. Ultimately, these models provide useful avenues to examine the interaction between stressful experiences, alcohol exposure, biological sex, and the neurobiological adaptations underlying potential risk factors for psychiatric conditions.

Sex divergent behavioral responses in platform-mediated avoidance and glucocorticoid receptor blockade

Women are more likely than men to develop anxiety or stress-related disorders. A core behavioral symptom of all anxiety disorders is avoidance of fear or anxiety eliciting cues. Recent rodent models of avoidance show reliable reproduction of this behavioral phenomenon in response to learned aversive associations. Here, a modified version of platform-mediated avoidance that lacked an appetitive task was utilized to investigate the learning and extinction of avoidance in male and female C57BL6/J mice. Here, we found a robust sex difference in the acquisition and extinction of platform-mediated avoidance. Across three experiments, 63.7% of female mice acquired avoidance according to our criterion, whereas 83.8% of males acquired it successfully. Of those females that acquired avoidance, they displayed persistent avoidance after extinction compared to males. Given their role in regulating stress responses and habitual behaviors, we investigated if glucocorticoid receptors (GR) mediated avoidance learning in males and females. We found that a subcutaneous injection (25 mg/kg) of the GR antagonist, RU486 (Mifepristone), significantly reduced persistent avoidance in females but did not further reduce avoidance in males after extinction. These data suggest that GR activation during avoidance learning may contribute to persistent avoidance in females that is resistant to extinction.

Fear-Conditioning to Unpredictable Threats Reveals Sex and Strain Differences in Rat Fear-Potentiated Startle (FPS)

Fear-potentiated startle (FPS) has been widely used to study fear processing in humans and rodents. Human studies showed higher startle amplitudes and exaggerated fear reactivity to unpredictable vs. predictable threats in individuals suffering from post-traumatic stress disorder (PTSD). Although human FPS studies use both sexes, a surprisingly limited number of rodent FPS studies use females. Here we investigate the effects of signal-threat contingency, signal-threat order and threat predictability on FPS in both sexes. We use a classic fear-conditioning protocol (100% contingency of cue and shock pairings, with forward conditioning such that the cue co-terminates with the shock) and compare it to modified fear-conditioning protocols (70% contingency; backward conditioning; or cue and shock un-paired). Although there are no sex differences in the startle amplitudes when corrected for body weight, females consistently demonstrate higher shock reactivity during fear-conditioning. Both sexes and strains demonstrate comparable levels of cued, non-cued, and contextual fear in the classic FPS and FPS following fear-conditioning with 70% contingency or backward order (cue co-starts with shock). However, in the classic FPS, Sprague-Dawley females show reduced proportion between cued fear and cue-elicited vigilant state than males. Lastly, a prominent sex difference is uncovered following unpredictable fear-conditioning (cue and shock un-paired), with Wistar, but not Sprague-Dawley, females showing significantly higher startle overall during the FPS recall, regardless of trial type, and higher contextual fear than males. This striking sex difference in processing unpredictable threats in rodent FPS might help to understand the mechanisms underlying higher incidence of PTSD in women.

Sex divergent behavioral responses in platform-mediated avoidance and glucocorticoid receptor blockade

Women are more likely than men to develop anxiety or stress-related disorders. A core behavioral symptom of all anxiety disorders is avoidance of fear or anxiety eliciting cues. Recent rodent models of avoidance show reliable reproduction of this behavioral phenomenon in response to learned aversive associations. Here, a modified version of platform-mediated avoidance that lacked an appetitive task was utilized to investigate the learning and extinction of avoidance in male and female C57BL6/J mice. Here, we found a robust sex difference in the acquisition and extinction of platform-mediated avoidance. Across three experiments, 63.7% of female mice acquired avoidance according to our criterion, whereas 83.8% of males acquired it successfully. Of those females that acquired avoidance, they displayed persistent avoidance after extinction compared to males. Given their role in regulating stress responses and habitual behaviors, we investigated if glucocorticoid receptors (GR) mediated avoidance learning in males and females. Here we found that a subcutaneous injection (25mg/kg) of the GR antagonist, RU486 (mifepristone), significantly reduced persistent avoidance in females but did not further reduce avoidance in males after extinction. These data suggest that GR activation during avoidance learning may contribute to persistent avoidance in females that is resistant to extinction.

On making (and turning adaptive to) maladaptive aversive memories in laboratory rodents

Fear conditioning and avoidance tasks usually elicit adaptive aversive memories. Traumatic memories are more intense, generalized, inflexible, and resistant to attenuation via extinction- and reconsolidation-based strategies. Inducing and assessing these dysfunctional, maladaptive features in the laboratory are crucial to interrogating posttraumatic stress disorder's neurobiology and exploring innovative treatments. Here we analyze over 350 studies addressing this question in adult rats and mice. There is a growing interest in modeling several qualitative and quantitative memory changes by exposing already stressed animals to freezing- and avoidance-related tests or using a relatively high aversive training magnitude. Other options combine aversive/fearful tasks with post-acquisition or post-retrieval administration of one or more drugs provoking neurochemical or epigenetic alterations reported in the trauma aftermath. It is potentially instructive to integrate these procedures and incorporate the measurement of autonomic and endocrine parameters. Factors to consider when defining the organismic and procedural variables, partially neglected aspects (sex-dependent differences and recent vs. remote data comparison) and suggestions for future research (identifying reliable individual risk and treatment-response predictors) are discussed.

Fear-conditioning to unpredictable threats reveals sex differences in rat fear-potentiated startle (FPS)

Fear-potentiated startle (FPS) has been widely used to study fear processing in humans and rodents. Human studies have shown higher startle amplitudes and exaggerated fear reactivity to unpredictable vs. predictable threats in individuals suffering from post-traumatic stress disorder (PTSD). Although human FPS studies often use both sexes, a surprisingly limited number of rodent FPS studies use females. Here we investigate the effects of signal-threat contingency, signal-threat order and threat predictability on FPS in both sexes. We use a classic fear-conditioning protocol (100% contingency of cue and shock pairings, with forward conditioning such that the cue co-terminates with the shock) and compare it to modified fear-conditioning protocols (70% contingency; backward conditioning; or cue and shock unpaired). Although there are no sex differences in the startle amplitudes when corrected for body weight, females demonstrate higher shock reactivity during fear-conditioning. Both sexes demonstrate comparable levels of cued, non-cued, and contextual fear in the classic FPS but females show reduced fear discrimination vs. males. Fear-conditioning with 70% contingency or backward order (cue co-starts with shock) induces similar levels of cued, non-cued, and contextual fear in both sexes but they differ in contextual fear extinction. Lastly, a prominent sex difference is uncovered following unpredictable fear-conditioning protocol (cue and shock un-paired), with females showing significantly higher startle overall during the FPS recall, regardless of trial type, and higher contextual fear than males. This striking sex difference in processing unpredictable threats in rodent FPS might help to understand the mechanisms underlying higher incidence of PTSD in women.

Highlights

Male and female rats have comparable startle amplitudes when corrected for body weightFemale rats show higher foot-shock reactivity than males during fear-conditioningFemale rats show reduced fear discrimination vs. males in the classic FPSReversed signal-threat order increases contextual fear in both sexesExposure to unpredictable threats increases startle in general and contextual fear only in females.

Persistent disruption of overexpectation learning after inactivation of the lateral orbitofrontal cortex in male rats

RATIONALE AND OBJECTIVE

Learning to inhibit acquired fear responses is fundamental to adaptive behavior. Two procedures that support such learning are extinction and overexpectation. In extinction, an expected outcome is omitted, whereas in overexpectation two individually trained cues are presented in compound to induce an expectation of a greater outcome than that delivered. Previously, we showed that inactivation of the lateral orbitofrontal cortex (lOFC) in experimentally naïve male rats causes a mild impairment in extinction learning but a profound one in overexpectation. The mild extinction impairment was also transient; that is, it was absent in a cohort of rats that had prior history of inhibitory training (overexpectation, extinction) and their associated controls. This raised the question whether lOFC involvement in overexpectation could likewise be attenuated by prior experience.

METHODS

Using a muscimol/baclofen cocktail, we inactivated the lOFC during overexpectation training in rats with prior associative learning history (extinction, overexpectation, control) and examined its contribution to reducing learned fear.

RESULTS

Inactivating the lOFC during compound training in overexpectation persistently disrupted fear reduction on test in naïve rats and regardless of prior experience. Additionally, we confirm that silencing the lOFC only resulted in a mild impairment in extinction learning in naïve rats.

CONCLUSION

We show that prior associative learning experience did not mitigate the deficit in overexpectation caused by lOFC inactivation. Our findings emphasize the importance of this region for this particular form of fear reduction and broaden our understanding of the conditions in which the lOFC modulates behavioral inhibition.

Patterns of Arc mRNA expression in the rat brain following dual recall of fear- and reward-based socially acquired information

Learning can occur via direct experience or through observation of another individual (i.e., social learning). While research focused on understanding the neural mechanisms of direct learning is prevalent, less work has examined the brain circuitry mediating the acquisition and recall of socially acquired information. Here, we aimed to further elucidate the mechanisms underlying recall of socially acquired information by having male and female rats sequentially recall a socially transmitted food preference (STFP) and a fear association via fear conditioning by-proxy (FCbP). Brain tissue was processed for mRNA expression of the immediate early gene (IEG) Arc, which expresses in the nucleus following transcription before migrating to the cytoplasm over the next 25 min. Given this timeframe, we could identify whether Arc transcription was triggered by STFP recall, FCbP recall, or both. Contrary to past research, we found no differences in any Arc expression measures across a number of prefrontal regions and the ventral CA3 of the hippocampus between controls, demonstrators, and observers. We theorize that these results may indicate that relatively little Arc-dependent neural restructuring is taking place in the prefrontal cortices and ventral CA3 following recall of recently socially acquired information or directly acquired fear associations in these areas.

ChAT::Cre transgenic rats show sex-dependent altered fear behaviors, ultrasonic vocalizations and cholinergic marker expression

The cholinergic system is a critical regulator of Pavlovian fear learning and extinction. As such, we have begun investigating the cholinergic system's involvement in individual differences in cued fear extinction using a transgenic ChAT::Cre rat model. The current study extends behavioral phenotyping of a transgenic ChAT::Cre rat line by examining both freezing behavior and ultrasonic vocalizations (USVs) during a Pavlovian cued fear learning and extinction paradigm. Freezing, 22 kHz USVs, and 50 kHz USVs were compared between male and female transgenic ChAT::Cre+ rats and their wildtype (Cre-) littermates during fear learning, contextual and cue-conditioned fear recall, cued fear extinction, and generalization to a novel tone. During contextual and cued fear recall ChAT::Cre+ rats froze slightly more than their Cre- littermates, and displayed significant sex differences in contextual and cue-conditioned freezing, 22 kHz USVs, and 50 kHz USVs. Females showed more freezing than males in fear recall trials, but fewer 22 kHz distress calls during fear learning and recall. Females also produced more 50 kHz USVs during exposure to the testing chambers prior to tone (or shock) presentation compared with males, but this effect was blunted in ChAT::Cre+ females. Corroborating previous studies, ChAT::Cre+ transgenic rats overexpressed vesicular acetylcholine transporter immunolabeling in basal forebrain, striatum, basolateral amygdala, and hippocampus, but had similar levels of acetylcholinesterase and numbers of ChAT+ neurons as Cre- rats. This study suggests that variance in behavior between ChAT::Cre+ and wildtype rats is sex dependent and advances theories that distinct neural circuits and processes regulate sexually divergent fear responses.

Pimavanserin reverses multiple measures of anxiety in a rodent model of post-traumatic stress disorder

Pimavanserin is a highly selective 5-HT_2A inverse agonist in current medical use. Prior studies suggest that 5-HT_2A serotonin receptors may play a role in anxiety and emotional memory. Therefore, pimavanserin was tested in a rat model of PTSD to determine whether it might ameliorate PTSD-like symptoms. The lifetime prevalence of PTSD is estimated to be 125% higher in women than men. Consequently, in an effort to create a robust model of PTSD that was more representative of human PTSD prevalence, 20-week old female rats of the emotionally hyperreactive Lewis strain were used for these studies. The rats were single-housed and exposed twice to restraint stress coupled with predator odor or to a sham-stressed condition. Twenty days after the second stress or sham-stress exposure, rats were injected with saline alone or with 0.3 or 1.0 mg/kg pimavanserin, doses that were confirmed to substantially block 5-HT_2A receptor activity in this study without causing any non-specific behavioral or adverse effects. One hour later, rats were tested for anxiety through acoustic startle response, the elevated plus-maze and three parameters of open field behavior. Five days later, blood was sampled for plasma corticosterone. The stressed/saline-injected rats had higher anxiety scores and corticosterone levels than sham-stressed/saline-injected rats. Pimavanserin significantly and generally dose-dependently reversed these persistent stress effects, but had no significant effect on the behavioral measures in normal, non-stressed rats. These results, consistent with a role for the 5-HT_2A receptor, suggest that pimavanserin might have potential to reduce some consequences of traumatic stress.

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'.

The sex-dependent response to psychosocial stress and ischaemic heart disease

Stress is an important risk factor for modern chronic diseases, with distinct influences in males and females. The sex specificity of the mammalian stress response contributes to the sex-dependent development and impacts of coronary artery disease (CAD). Compared to men, women appear to have greater susceptibility to chronic forms of psychosocial stress, extending beyond an increased incidence of mood disorders to include a 2- to 4-fold higher risk of stress-dependent myocardial infarction in women, and up to 10-fold higher risk of Takotsubo syndrome-a stress-dependent coronary-myocardial disorder most prevalent in post-menopausal women. Sex differences arise at all levels of the stress response: from initial perception of stress to behavioural, cognitive, and affective responses and longer-term disease outcomes. These fundamental differences involve interactions between chromosomal and gonadal determinants, (mal)adaptive epigenetic modulation across the lifespan (particularly in early life), and the extrinsic influences of socio-cultural, economic, and environmental factors. Pre-clinical investigations of biological mechanisms support distinct early life programming and a heightened corticolimbic-noradrenaline-neuroinflammatory reactivity in females vs. males, among implicated determinants of the chronic stress response. Unravelling the intrinsic molecular, cellular and systems biological basis of these differences, and their interactions with external lifestyle/socio-cultural determinants, can guide preventative and therapeutic strategies to better target coronary heart disease in a tailored sex-specific manner.

Involvement of D2-like dopaminergic receptors in contextual fear conditioning in female rats: influence of estrous cycle

Introduction: Dopamine has been increasingly recognized as a key neurotransmitter regulating fear/anxiety states. Nevertheless, the influence of sex and estrous cycle differences on the role of dopamine in fear responses needs further investigation. We aimed to evaluate the effects of sulpiride (a dopaminergic D2-like receptor antagonist) on contextual fear conditioning in females while exploring the influence of the estrous cycle. Methods: First, using a contextual fear conditioning paradigm, we assessed potential differences in acquisition, expression, and extinction of the conditioned freezing response in male and female (split in proestrus/estrus and metestrus/diestrus) Wistar rats. In a second cohort, we evaluated the effects of sulpiride (20 and 40 mg/kg) on contextual conditioned fear in females during proestrus/estrus and metestrus/diestrus. Potential nonspecific effects were assessed in motor activity assays (catalepsy and open-field tests). Results: No sex differences nor estrous cycle effects on freezing behavior were observed during the fear conditioning phases. Sulpiride reduced freezing expression in female rats. Moreover, females during the proestrus/estrus phases of the estrous cycle were more sensitive to the effects of sulpiride than females in metestrus/diestrus. Sulpiride did not cause motor impairments. Discussion: Although no sex or estrous cycle differences were observed in basal conditioned fear expression and extinction, the estrous cycle seems to influence the effects of D2-like antagonists on contextual fear conditioning.

Childhood Maltreatment in Females Is Associated with Enhanced Fear Acquisition and an Overgeneralization of Fear

Childhood maltreatment may alter fear neurocircuitry, which results in pathological anxiety and depression. One alteration of fear-related behaviors that has been observed in several psychiatric populations is an overgeneralization of fear. Thus, we examined the association between childhood maltreatment and fear generalization in a non-clinical sample of young adults. Two hundred and ninety-one participants underwent differential fear conditioning in a fear-potentiated startle paradigm. One visual stimulus (CS+), but not another (CS-), was associated with an aversive airblast to the throat (US) during acquisition. The next day, participants were tested for their fear responses to the CS+, CS-, and several generalization stimuli (GS) without the presence of the US. Participants also completed questionnaires that assessed symptoms of childhood maltreatment, anxiety, depression, and post-traumatic stress disorder (PTSD). Participants reporting high childhood maltreatment (n = 71; 23 males, 48 females) exhibited significantly greater anxiety, depression, and symptoms of PTSD than participants reporting low childhood maltreatment (n = 220; 133 males, 87 females). Females reporting high childhood maltreatment demonstrated significantly enhanced fear learning and greater fear generalization, based on their fear-potentiated startle responses. Our findings suggest that childhood maltreatment may sex-dependently influence the development of fear neurocircuitry and result in greater fear generalization in maltreated females.

Effects of sex and retention interval on the retrieval and extinction of auditory fear conditioning

Fear memory retrieval is relevant to psychiatric disorders such as post-traumatic stress disorder (PTSD). One of the hallmark symptoms of PTSD is the repeated retrieval and re-experiencing of the initial fear memory even long after the traumatic event has occurred. Women are nearly twice as likely to develop PTSD following a trauma than men, thus sex differences in the retrieval of fear memories is highly relevant for understanding the development and maintenance of PTSD. In the current study, we aimed to examine sex differences in the retrieval and extinction of either recent or remote fear memories. To do so, we conditioned male and female rats either 1 day (recent) or 28 days (remote) prior to testing retrieval and extinction. While there was no effect of sex or retention interval on initial retrieval, we found that remotely conditioned females exhibited higher rates of freezing than remotely conditioned males in later retrieval/extinction sessions, suggesting a sex difference in the retrieval and/or extinction of remote, but not recent, fear memories. Overall, these results are the first to demonstrate a sex difference in the extinction of remote fear memory, and this may contribute to the differential expression of fear-related disorders like PTSD in men and women.

Sex-specific endocrine regulation of seasonal aggression in Siberian hamsters

Coordinating physiological and behavioural processes across the annual cycle is essential in enabling individuals to maximize fitness. While the mechanisms underlying seasonal reproduction and its associated behaviours are well characterized, fewer studies have examined the hormonal basis of non-reproductive social behaviours (e.g. aggression) on a seasonal time scale. Our previous work suggests that the pineal hormone melatonin facilitates a 'seasonal switch' in neuroendocrine regulation of aggression in male and female Siberian hamsters (Phodopus sungorus), specifically by acting on the adrenal glands to increase the production of the androgen dehydroepiandrosterone (DHEA) during the short-day (SD) photoperiods of the non-breeding season. Here, we provide evidence that the activity of 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase (3β-HSD), a key enzyme within the steroidogenic pathway that mediates DHEA synthesis and metabolism, varies in a sex-specific and melatonin-dependent manner. Although both male and female hamsters displayed increased aggression in response to SDs and SD-like melatonin, only males showed an increase in adrenal 3β-HSD activity. Conversely, SD and melatonin-treated females exhibited reductions in both adrenal and neural 3β-HSD activity. Collectively, these results suggest a potential role for 3β-HSD in modulating non-breeding aggression and, more broadly, demonstrate how distinct neuroendocrine mechanisms may underlie the same behavioural phenotype in males and females.

Morning blue light treatment improves sleep complaints, symptom severity, and retention of fear extinction memory in post-traumatic stress disorder

Disrupted sleep is a major feature in numerous clinical disorders and is related to decrements in affective memory processing. The prevalence of sleep disruption in post-traumatic stress disorder (PTSD) is suggested to be a key feature that exacerbates the impaired ability to recall extinction memories during experimental fear conditioning. We hypothesized that an intervention employing blue-wavelength light therapy (BLT) to regulate sleep and stabilize circadian rhythms in patients with PTSD (i.e., via regulated morning exposure) would be associated with PTSD symptom improvement, decreased sleep-related complaints, as well as improved consolidation and retention of extinction memories relative to a fear conditioning/extinction paradigm. Eighty-two individuals with PTSD underwent a well-validated fear conditioning/extinction protocol with subsequent assignment to receive morning BLUE (BLT) or placebo AMBER (ALT) light therapy daily for 30-min over 6-weeks. Participants returned after the intervention for post-treatment extinction recall, comprised of exposure to the previously conditioned stimuli, with the difference in skin conductance response between the "extinguished" and the "never-extinguished" stimuli at follow-up. Participants also viewed previously conditioned stimuli in a novel context during a functional magnetic resonance imaging (fMRI) scan. BLUE light therapy was associated with improvements relative to correlated decreases between PTSD symptoms and sleep-related complaints. Participants receiving BLT also sustained retention of the extinction memory, while those in the placebo amber light treatment group showed impairment, characterized by the restoration of the extinguished fear response after 6-weeks. Participants in the ALT also demonstrated greater reactivity in the left insula when viewing the previously extinguished fear-conditioned stimuli in a novel context. Daily BLUE-wavelength morning light exposure was associated with greater retention of extinction learning in patients with PTSD when compared to ALT, as supported by both autonomic and neurobiological reactivity. We speculate that improved sleep facilitated by a stabilized circadian rhythm, after fear-learning, led to greater consolidation of the fear extinction memory, decreased PTSD symptom presentation, and associated decreases in sleep-related complaints. Prominent exposure treatments for PTSD incorporate principles of fear extinction, and our findings suggest that blue light treatment may facilitate treatment gains by promoting the consolidation of extinction memories via improved sleep.

Structural, functional, and behavioral significance of sex and gonadal hormones in the basolateral amygdala: A review of preclinical literature

The basolateral amygdala (BLA) is intimately involved in the development of neuropsychiatric disorders such as anxiety and alcohol use disorder (AUD). These disorders have clear sex biases, with women more likely to develop an anxiety disorder and men more likely to develop AUD. Preclinical models have largely confirmed these sex-specific vulnerabilities and emphasize the effects of sex hormones on behaviors influenced by the BLA. This review will discuss sex differences in BLA-related behaviors and highlight potential mechanisms mediated by altered BLA structure and function, including the composition of GABAergic interneuron subpopulations, glutamatergic pyramidal neuron morphology, glutamate/GABA neurotransmission, and neuromodulators. Further, sex hormones differentially organize dimorphic circuits during sensitive developmental periods (organizational effects) and initiate more transient effects throughout adulthood (activational effects). Current literature indicates that estradiol and allopregnanolone, a neuroactive progestogen, generally reduce BLA-related behaviors through a variety of mechanisms, including activation of estrogen receptors or facilitation of GABA_A-mediated inhibition, respectively. This enhanced GABAergic inhibition may protect BLA pyramidal neurons from the excitability associated with anxiety and alcohol withdrawal. Understanding sex differences and the effects of sex hormones on BLA structure and function may help explain sex-specific vulnerabilities in BLA-related behaviors and ultimately improve treatments for anxiety and AUD.

[Research for sex differences in fear extinction]

Fear memory has an important role in survival, but excess fear memory could be cause to fear-related disorders such as post traumatic disorder (PTSD). People who undergo a fearful event do not always develop PTSD, suggesting that there are vulnerable factors to develop the disorder. It is believed that sex, in particular female, is one of the factors as it is known that the prevalence rate of PTSD is higher in women than in men. However, the biological mechanisms underlying the sex differences in their prevalence rate remain unclear. Because previous studies reported that patients with fear-related disorders show an impairment of fear extinction, investigation of sex differences in fear extinction may be useful for understanding the greater vulnerability of women to fear-related disorders. Although number of papers investigating sex differences in fear extinction, these reports have increased recently. This review introduces fear extinction, sex differences in fear extinction is low, and the molecular mechanisms of sex differences in fear extinction. In the molecular mechanisms, we focused brain-derived neurotrophic factor (BDNF)-tropomyosin-related kinase B (TrkB) receptor signal which has been investigated by independent research groups.

Environmental certainty influences the neural systems regulating responses to threat and stress

Flexible calibration of threat responding in accordance with the environment is an adaptive process that allows an animal to avoid harm while also maintaining engagement of other goal-directed actions. This calibration process, referred to as threat response regulation, requires an animal to calculate the probability that a given encounter will result in a threat so they can respond accordingly. Here we review the neural correlates of two highly studied forms of threat response suppression: extinction and safety conditioning. We focus on how relative levels of certainty or uncertainty in the surrounding environment alter the acquisition and application of these processes. We also discuss evidence indicating altered threat response regulation following stress exposure, including enhanced fear conditioning, and disrupted extinction and safety conditioning. To conclude, we discuss research using an animal model of coping that examines the impact of stressor controllability on threat responding, highlighting the potential for previous experiences with control, or other forms of coping, to protect against the effects of future adversity.

Sexually Dimorphic Expression of Fear-conditioned Analgesia in Rats and Associated Alterations in the Endocannabinoid System in the Periaqueductal Grey

The endocannabinoid system within the periaqueductal grey (PAG) has been implicated in fear-conditioned analgesia (FCA), the profound suppression of pain upon re-exposure to a context previously paired with an aversive stimulus. Since the endocannabinoid and nociceptive systems exhibit sexual dimorphism, the aim of the present study was to assess possible sex differences in the expression of FCA, fear in the presence of nociceptive tone, and associated sex-dependent alterations in the endocannabinoid system within the PAG. Male and female Sprague-Dawley rats received footshock (10 × 1s; 0.4 mA; every 60 s) or no-footshock in a conditioning arena and 23.5 h later received intraplantar injection of formalin (2.5%) under brief isoflourane anaesthetic into the right hind paw. Nociceptive and fear-related behaviours were assessed 30 min later. Levels of endocannabinoids, N-acylethanolamines and neurotransmitters in the PAG were assessed by LC-MS/MS and expression of endocannabinoid system-related proteins by Western immunoblotting. Male, but not female, rats exhibited robust FCA and greater expression of fear-related behaviours than females. Fear-conditioned formalin-treated males, but not females, had higher levels of N-oleoylethanolamine (OEA) and γ-aminobutyric acid (GABA) in the PAG, compared with non-fear-conditioned controls. There was no effect of fear conditioning on the levels of FAAH or CB₁ receptor expression (CB1R) in the PAG of male or female formalin-treated rats. Non-fear-conditioned females had higher levels of CB1R and PPARγ expression than non-fear-conditioned male counterparts. In summary, our results provide evidence of sexual dimorphism in the expression of FCA and fear-related behaviours, and associated alterations in components of the endocannabinoid system and GABA within the PAG.

Sex differences in anxiety and depression: circuits and mechanisms

Epidemiological sex differences in anxiety disorders and major depression are well characterized. Yet the circuits and mechanisms that contribute to these differences are understudied, because preclinical studies have historically excluded female rodents. This oversight is beginning to be addressed, and recent studies that include male and female rodents are identifying sex differences in neurobiological processes that underlie features of these disorders, including conflict anxiety, fear processing, arousal, social avoidance, learned helplessness and anhedonia. These findings allow us to conceptualize various types of sex differences in the brain, which in turn have broader implications for considering sex as a biological variable. Importantly, comparing the sexes could aid in the discovery of novel therapeutics.

Fear Extinction-Based Inter-Individual and Sex Differences in Pain-Related Vocalizations and Anxiety-like Behaviors but Not Nocifensive Reflexes

Inter-individual and sex differences in pain responses are recognized but their mechanisms are not well understood. This study was intended to provide the behavioral framework for analyses of pain mechanisms using fear extinction learning as a predictor of phenotypic and sex differences in sensory (mechanical withdrawal thresholds) and emotional-affective aspects (open field tests for anxiety-like behaviors and audible and ultrasonic components of vocalizations) of acute and chronic pain. In acute arthritis and chronic neuropathic pain models, greater increases in vocalizations were found in females than males and in females with poor fear extinction abilities than females with strong fear extinction, particularly in the neuropathic pain model. Female rats showed higher anxiety-like behavior than males under baseline conditions but no inter-individual or sex differences were seen in the pain models. No inter-individual and sex differences in mechanosensitivity were observed. The data suggest that vocalizations are uniquely suited to detect inter-individual and sex differences in pain models, particularly in chronic neuropathic pain, whereas no such differences were found for mechanosensitivity, and baseline differences in anxiety-like behaviors disappeared in the pain models.

Gonadal steroid hormones and emotional memory consolidation: A systematic review and meta-analysis

Anxiety and stress-related disorders are more prevalent in women and associated with negative emotional memory consolidation as well as impaired fear extinction recall. Recent research has identified a role of gonadal steroid hormones in influencing emotional memories and fear extinction, however most individual studies have small samples and employed various protocols. A systematic review and meta-analysis were conducted on studies that examined sex hormones (estrogen, progesterone, testosterone, allopregnanolone, dehydroepiandrosterone) on four aspects of memory, namely, intentional recall (k = 13), recognition memory (k = 7), intrusive memories (k = 9), and extinction recall (k = 11). The meta-analysis on natural cycling women revealed that progesterone level was positively associated with negative recall and negative intrusive memories, and this effect on intentional recall was enhanced under stress induction. Estradiol level was positively associated with extinction recall. This study reveals an important role of progesterone and estradiol in influencing emotional memory consolidation. It highlights the need to control for these hormonal effects and examine progesterone and estradiol concurrently across all menstrual phases in future emotional memory paradigms.

Individual Differences in Conditioned Fear and Extinction in Female Rats

The inability to extinguish a traumatic memory is a key aspect of post-traumatic stress disorder (PTSD). While PTSD affects 10–20% of individuals who experience a trauma, women are particularly susceptible to developing the disorder. Despite this notable female vulnerability, few studies have investigated this particular resistance to fear extinction observed in females. Similar to humans, rodent models of Pavlovian fear learning and extinction show a wide range of individual differences in fear learning and extinction, although female rodents are considerably understudied. Therefore, the present study examined individual differences in fear responses, including freezing behavior and ultrasonic vocalizations (USVs), of female Long–Evans rats during acquisition of fear conditioning and cued fear extinction. Similar to prior studies in males, female rats displayed individual variation in freezing during cued fear extinction and were divided into extinction competent (EC) and extinction resistant (ER) phenotypes. Differences in freezing between ER and EC females were accompanied by shifts in rearing during extinction, but no darting was seen in any trial. Freezing behavior during fear learning did not differ between the EC and ER females. Vocalizations emitted in the 22 and 50 kHz ranges during fear learning and extinction were also examined. Unlike vocalizations seen in previous studies in males, very few 22 kHz distress vocalizations were emitted by female rats during fear acquisition and extinction, with no difference between ER and EC groups. Interestingly, all female rats produced significant levels of 50 kHz USVs, and EC females emitted significantly more 50 kHz USVs than ER rats. This difference in 50 kHz USVs was most apparent during initial exposure to the testing environment. These results suggest that like males, female rodents show individual differences in both freezing and USVs during fear extinction, although females appear to vocalize more in the 50 kHz range, especially during initial periods of exposure to the testing environment, and emit very few of the 22 kHz distress calls that are typically observed in males during fear learning or extinction paradigms. Overall, these findings show that female rodents display fear behavior repertoires divergent from males.

Effect of Estrous Cycle on Behavior of Females in Rodent Tests of Anxiety

Anxiety disorders are more prevalent in women than in men. In women the menstrual cycle introduces another variable; indeed, some conditions e.g., premenstrual syndrome, are menstrual cycle specific. Animal models of fear and anxiety, which form the basis for research into drug treatments, have been developed almost exclusively, using males. There remains a paucity of work using females and the available literature presents a confusing picture. One confound is the estrous cycle in females, which some authors consider, but many do not. Importantly, there are no accepted standardized criteria for defining cycle phase, which is important given the rapidly changing hormonal profile during the 4-day cycle of rodents. Moreover, since many behavioral tests that involve a learning component or that consider extinction of a previously acquired association require several days to complete; the outcome may depend on the phase of the cycle on the days of training as well as on test days. In this article we consider responsiveness of females compared to males in a number of commonly used behavioral tests of anxiety and fear that were developed in male rodents. We conclude that females perform in a qualitatively similar manner to males in most tests although there may be sex and strain differences in sensitivity. Tests based on unconditioned threatening stimuli are significantly influenced by estrous cycle phase with animals displaying increased responsiveness in the late diestrus phase of the cycle (similar to the premenstrual phase in women). Tests that utilize conditioned fear paradigms, which involve a learning component appear to be less impacted by the estrous cycle although sex and cycle-related differences in responding can still be detected. Ethologically-relevant tests appear to have more translational value in females. However, even when sex differences in behavior are not detected, the same outward behavioral response may be mediated by different brain mechanisms. In order to progress basic research in the field of female psychiatry and psychopharmacology, there is a pressing need to validate and standardize experimental protocols for using female animal models of anxiety-related states.

Hippocampal neurogenesis mediates sex-specific effects of social isolation and exercise on fear extinction in adolescence

Impaired extinction of conditioned fear is associated with anxiety disorders. Common lifestyle factors, like isolation stress and exercise, may alter the ability to extinguish fear. However, the effect of and interplay between these factors on adolescent fear extinction, and the relevant underlying neural mechanisms are unknown. Here we examined the effects of periadolescent social isolation and physical activity on adolescent fear extinction in rats and explored neurogenesis as a potential mechanism. Isolation stress impaired extinction recall in male adolescents, an effect prevented by exercise. Extinction recall in female adolescents was unaffected by isolation stress. However, exercise disrupted extinction recall in isolated females. Extinction recall in isolated females was positively correlated to the number of immature neurons in the ventral hippocampus, suggesting that exercise affected extinction recall via neurogenesis in females. Pharmacologically suppressing cellular proliferation in isolated adolescents using temozolomide blocked the effect of exercise on extinction recall in both sexes. Together, these findings highlight sex-specific outcomes of isolation stress and exercise on adolescent brain and behavior, and highlights neurogenesis as a potential mechanism underlying lifestyle effects on adolescent fear extinction.

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Periadolescent isolation stress disrupted extinction recall in male adolescents.

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Running prevented isolation-induced extinction recall deficit in male adolescents.

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Exercise impaired extinction recall in isolated female adolescents.

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Exercise increased hippocampal neurogenesis, except in isolated males.

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Suppression of neurogenesis blocked exercise effects in isolated adolescents.

Sex Differences in Behavioral Sensitivities After Traumatic Brain Injury

Traumatic brain injury (TBI) is associated with high rates of post-injury psychiatric and neurological comorbidities. TBI is more common in males than females despite females reporting more symptoms and longer recovery following TBI and concussion. Both pain and mental health conditions like anxiety and post-traumatic stress disorder (PTSD) are more common in women in the general population, however the dimorphic comorbidity in the TBI population is not well-understood. TBI may predispose the development of maladaptive anxiety or PTSD following a traumatic stressor, and the impact of sex on this interaction has not been investigated. We have shown that white noise is noxious to male rats following fluid percussion injury (FPI) and increases fear learning when used in auditory fear conditioning, but it is unclear whether females exhibit a similar phenotype. Adult female and male rats received either lateral FPI or sham surgery and 48 h later received behavioral training. We first investigated sex differences in response to 75 dB white noise followed by white noise-signaled fear conditioning. FPI groups exhibited defensive behavior to the white noise, which was significantly more robust in females, suggesting FPI increased auditory sensitivity. In another experiment, we asked how FPI affects contextual fear learning in females and males following unsignaled footshocks of either strong (0.9 mA) or weaker (0.5 mA) intensity. We saw that FPI led to rapid acquisition of contextual fear compared to sham. A consistent pattern of increased contextual fear after TBI was apparent in both sexes across experiments under differing conditioning protocols. Using a light gradient open field task we found that FPI females showed a defensive photophobia response to light, a novel finding supporting TBI enhanced sensory sensitivity across modalities in females. General behavioral differences among our measures were observed between sexes and discussed with respect to interpretations of TBI effects for each sex. Together our data support enhanced fear following a traumatic stressor after TBI in both sexes, where females show greater sensitivity to sensory stimuli across multiple modalities. These data demonstrate sex differences in emergent defensive phenotypes following TBI that may contribute to comorbid PTSD, anxiety, and other neurological comorbidities.

Sex differences in neuro(auto)immunity and chronic sciatic nerve pain

Chronic pain occurs with greater frequency in women, with a parallel sexually dimorphic trend reported in sufferers of many autoimmune diseases. There is a need to continue examining neuro-immune-endocrine crosstalk in the context of sexual dimorphisms in chronic pain. Several phenomena in particular need to be further explored. In patients, autoantibodies to neural antigens have been associated with sensory pathway hyper-excitability, and the role of self-antigens released by damaged nerves remains to be defined. In addition, specific immune cells release pro-nociceptive cytokines that directly influence neural firing, while T lymphocytes activated by specific antigens secrete factors that either support nerve repair or exacerbate the damage. Modulating specific immune cell populations could therefore be a means to promote nerve recovery, with sex-specific outcomes. Understanding biological sex differences that maintain, or fail to maintain, neuroimmune homeostasis may inform the selection of sex-specific treatment regimens, improving chronic pain management by rebalancing neuroimmune feedback. Given the significance of interactions between nerves and immune cells in the generation and maintenance of neuropathic pain, this review focuses on sex differences and possible links with persistent autoimmune activity using sciatica as an example.

The infralimbic cortex and mGlu5 mediate the effects of chronic intermittent ethanol exposure on fear learning and memory

RATIONALE AND OBJECTIVES

Alcohol use disorder (AUD) and post-traumatic stress disorder (PTSD) often occur comorbidly. While the incidence of these disorders is increasing, there is little investigation into the interacting neural mechanisms between these disorders. These studies aim to identify cognitive deficits that occur as a consequence of fear and ethanol exposure, implement a novel pharmaceutical intervention, and determine relevant underlying neurocircuitry. Additionally, due to clinical sex differences in PTSD prevalence and alcohol abuse, these studies examine the nature of this relationship in rodent models.

METHODS

Animals were exposed to a model of PTSD+AUD using auditory fear conditioning followed by chronic intermittent ethanol exposure (CIE). Then, rats received extinction training consisting of multiple conditioned stimulus presentations in absence of the shock. Extinction recall and context-induced freezing were measured in subsequent tests. CDPPB, a metabotropic glutamate receptor 5 (mGlu5) positive allosteric modulator, was used to treat these deficits, and region-specific effects were determined using microinjections.

RESULTS

These studies determined that CIE exposure led to deficits in fear extinction learning and heightened context-induced freezing while sex differences emerged in fear conditioning and extinction cue recall tests. Furthermore, using CDPPB, these studies found that enhancement of infralimbic (IfL) mGlu5 activity was able to recover CIE-induced deficits in both males and females.

CONCLUSIONS

These studies show that CIE induces deficits in fear-related behaviors and that enhancement of IfL glutamatergic activity can facilitate learning during extinction. Additionally, we identify novel pharmacological targets for the treatment of individuals who suffer from PTSD and AUD.

Sex Differences in the Development of the Rodent Corticolimbic System

In recent years, a growing body of research has shown sex differences in the prevalence and symptomatology of psychopathologies, such as depression, anxiety, and fear-related disorders, all of which show high incidence rates in early life. This has highlighted the importance of including female subjects in animal studies, as well as delineating sex differences in neural processing across development. Of particular interest is the corticolimbic system, comprising the hippocampus, amygdala, and medial prefrontal cortex. In rodents, these corticolimbic regions undergo dynamic changes in early life, and disruption to their normative development is believed to underlie the age and sex-dependent effects of stress on affective processing. In this review, we consolidate research on sex differences in the hippocampus, amygdala, and medial prefrontal cortex across early development. First, we briefly introduce current principles on sexual differentiation of the rodent brain. We then showcase corticolimbic regional sex differences in volume, morphology, synaptic organization, cell proliferation, microglia, and GABAergic signaling, and explain how these differences are influenced by perinatal and pubertal gonadal hormones. In compiling this research, we outline evidence of what and when sex differences emerge in the developing corticolimbic system, and illustrate how temporal dynamics of its maturational trajectory may differ in male and female rodents. This will help provide insight into potential neural mechanisms underlying sex-specific critical windows for stress susceptibility and behavioral emergence.

Chronic stress has lasting effects on improved cued discrimination early in extinction

Chronic stress typically leads to deficits in fear extinction when tested soon after chronic stress ends. Given the importance of extinction in updating fear memories, the current study examined whether fear extinction was impaired in rats that were chronically stressed and then given a break from the end of chronic stress to the start of fear conditioning and extinction. Male rats were chronically stressed by restraint (6 h/d/21 d) and tested soon (termed immediate, STR-IMM), or 3 or 6 wk after a rest period from restraint (termed rest or "R," STR-R3, STR-R6). In Experiment 1, STR-R3 and STR-R6 discriminated between the cue and nonshock context better than STR-IMM or control. Interestingly, STR-IMM showed high freezing to the nonshock context. Consequently, Experiment 2 investigated whether STR-IMM generalized across contexts, which was not supported. Experiment 3 determined whether STR-IMM were susceptible to second-order conditioning to a novel context, but showed that the level of second-order conditioning was similar for all groups. These findings reveal that rats exposed to chronic stress and then given a rest period of 3 or 6 wk, express unique fear extinction profiles compared to control and STR-IMM. Specifically, STR-R demonstrated excellent cue and context discrimination during extinction, and perhaps showed a stress inoculation effect. For STR-IMM, the heightened freezing under these extensive acclimation parameters was not attributed to generalization nor to second-order fear conditioning to "safe" contexts and, instead, may reflect hypervigilance.

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.

A novel social fear conditioning procedure alters social behavior and mTOR signaling in differentially housed adolescent rats

Vulnerabilities to fear-related disorders can be enhanced following early life adversity. This study sought to determine whether post-weaning social isolation (PSI), an animal model of early life adversity, alters the development of social fear in an innovative model of conditioned social fear. Male and female Sprague-Dawley rats underwent either social rearing (SR) or PSI for 4 weeks following weaning. Rats were then assigned to groups consisting of either Footshock only, Social conditioned stimulus (CS) only, or Paired footshock with a social CS. Social behavior was assessed the next day. We observed a novel behavioral response in PSI rats, running in circles, that was rarely observed in SR rats; moreover, this behavior was augmented after Paired treatment in PSI rats. Other social behaviors were altered by both PSI and Paired footshock and social CS. The mammalian target of rapamycin (mTOR) pathway was assessed using immunohistochemistry for phosphorylated ribosomal protein S6 (pS6) in subregions of the prefrontal cortex (PFC) and amygdala. Paired treatment produced opposite effects in the PFC and amygdala in males, but no differences were observed in females. Conditioned social fear produced alterations in social behavior and the mTOR pathway that are dependent upon rearing condition and sex.

Assessment of conditioned fear extinction in male and female adolescent rats

Pavlovian fear conditioning and extinction have been widely studied across many species to understand emotional learning and memory. Importantly, it is becoming clear that these processes are affected by sex and age. In adult rodents and humans, sex differences are evident in extinction, with estradiol playing a significant role. In adolescence, an extinction deficit has been reported in rodents and humans. However, the influence of sex on extinction during adolescence is unknown. This is surprising, since adolescence coincides with the onset of hormone cycling, and therefore it might be expected that hormones fluctuations exert a more profound effect at this time. Therefore, we examined Pavlovian fear conditioning and extinction in adolescent male and female rats. In experiment 1, 35-day-old male and female rats were exposed to 6 pairings of a conditioned stimulus (CS, a tone) with an aversive unconditioned stimulus (US, a footshock). The next day they were extinguished in a contextually distinct chamber, via 60 presentations of the CS without the US. Extinction recall was tested 24 hours later in the extinction context. Estrous phase was monitored by cytology on vaginal smears taken 1 hour after each behavioral session. In experiment 2, male and female rats were given sham surgery or gonadectomy at 21 days of age. They were then trained and tested as for experiment 1. We observed that females in proestrus or met/diestrus during extinction showed delayed extinction and impaired extinction recall the next day compared to males. Ovariectomy enhanced extinction for female rats, but orchidectomy delayed extinction for males. Plasma analyses showed that met/di/proestrus phases were associated with high estradiol levels. These findings suggest that high plasma estradiol levels impair extinction for adolescent females. These results contradict what is reported in adult animals, suggesting that hormonal influences on extinction are dependent on age. Given that impaired extinction is widely used as a model to understand resistance to exposure-based therapies, our findings have important implications for understanding mental health treatments in adolescents.

Sex differences in behavioral responses during a conditioned flight paradigm

Females exhibit greater susceptibility to trauma- and stress-related disorders compared to males; therefore, it is imperative to study sex differences in the mode and magnitude of defensive responses in the face of threat. To test for sex differences in defensive behavior, we used a modified Pavlovian fear conditioning paradigm that elicits clear transitions between freezing and flight behaviors within individual subjects. Female mice subjected to this paradigm exhibited more freezing behavior compared to males, especially during the intertrial interval period. Female mice also exhibited more freezing in response to conditioned auditory stimuli in the last block of extinction training. Furthermore, there were sex differences in the expression of other adaptive behaviors during fear conditioning. Assaying rearing, grooming, and tail rattling behaviors during the conditioned flight paradigm yielded measurable differences across sessions and between males and females. Overall, these results provide insight into sex-dependent alterations in mouse behavior induced by fear conditioning.

Retrieval-Extinction and Relapse Prevention: Rewriting Maladaptive Drug Memories?

Addicted individuals are highly susceptible to relapse when exposed to drug-associated conditioned stimuli (CSs; "drug cues") even after extensive periods of abstinence. Until recently, these maladaptive emotional drug memories were believed to be permanent and resistant to change. The rediscovery of the phenomenon of memory reconsolidation-by which retrieval of the memory can, under certain conditions, destabilize the previously stable memory before it restabilizes in its new, updated form-has led to the hypothesis that it may be possible to disrupt the strong maladaptive drug-memories that trigger a relapse. Furthermore, recent work has suggested that extinction training "within the reconsolidation window" may lead to a long-term reduction in relapse without the requirement for pharmacological amnestic agents. However, this so-called "retrieval-extinction" effect has been inconsistently observed in the literature, leading some to speculate that rather than reflecting memory updating, it may be the product of facilitation of extinction. In this mini review article, we will focus on factors that might be responsible for the retrieval-extinction effects on preventing drug-seeking relapse and how inter-individual differences may influence this therapeutically promising effect. A better understanding of the psychological and neurobiological mechanisms underpinning the "retrieval-extinction" paradigm, and individual differences in boundary conditions, should provide insights with the potential to optimize the translation of "retrieval-extinction" to clinical populations.

Reverse translation of major depressive disorder symptoms: A framework for the behavioural phenotyping of putative biomarkers

BACKGROUND

Reverse translating putative biomarkers of depression from patients to animals is complex because Major Depressive Disorder (MDD) is a highly heterogenous condition. This review proposes an approach to reverse translation based on relating relevant bio-behavioural functions in laboratory rodents to MDD symptoms.

METHODS

This systematic review outlines symptom clusters assessed by psychometric tests of MDD and antidepressant treatment response including the Montgomery-Åsberg Depression Rating Scale, the Hamilton Depression Rating Scale, and the Beck Depression Inventory. Symptoms were related to relevant behavioural assays in laboratory rodents.

RESULTS

The resulting battery of tests includes passive coping, anxiety-like behaviours, sleep, caloric intake, cognition, psychomotor functions, hedonic reactivity and aversive learning. These assays are discussed alongside relevant clinical symptoms of MDD, providing a framework through which reverse translation of a biomarker can be interpreted.

LIMITATIONS

Certain aspects of MDD may not be quantified by tests in laboratory rodents, and their biological significance may not always be of clinical relevance.

CONCLUSIONS

Using this reverse translation approach, it is possible to clarify the functional significance of a putative biomarker in rodents and hence translate its contribution to specific clinical symptoms, or clusters of symptoms.

Prior stress followed by a novel stress challenge results in sex-specific deficits in behavioral flexibility and changes in gene expression in rat medial prefrontal cortex

Chronic stress leads to sex-specific changes in the structure and function of rat medial prefrontal cortex (mPFC). Little is known about whether these effects persist following the cessation of chronic stress, or how these initial effects may impact responses to future stressors. Here we examined attentional set-shifting in male and female rats following chronic restraint stress, a post-chronic stress rest period, and an acute novel stress challenge. Chronic stress resulted in a reversible impairment in extradimensional set-shifting in males, but had no effect on attentional set-shifting in females. Surprisingly, chronically stressed female, but not male, rats had impaired extradimensional set-shifting following a novel stress challenge. Alterations in the balance of excitation and inhibition of mPFC have been implicated in behavioral deficits following chronic stress. Thus, in a separate group of rats, we examined changes in the expression of genes related to glutamatergic (NR1, NR2A, NR2B, GluR1) and GABAergic (Gad67, parvalbumin, somatostatin) neurotransmission in mPFC after acute and chronic stress, rest, and their combination. Stress significantly altered the expression of NR1, GluR1, Gad67, and parvalbumin. Notably, the pattern of stress effects on NR1, Gad67, and parvalbumin expression differed between males and females. In males, these genes were upregulated following the post-chronic stress rest period, while minimal changes were found in females. In contrast, both males and females had greater GluR1 expression following a rest period. These findings suggest that chronic stress leads to sex-specific stress adaptation mechanisms that may contribute to sex differences in response to subsequent stress exposure.

Food restriction in adolescence increases emotional disorder-like behaviors in adult rats

This study was designed to investigate the neuronal mechanism underlying the influence of early-life food restriction on the central nervous system and subsequent behaviors in adult rats. Several behavioral paradigms were tested in rats, including sucrose negative contrast test, forced swimming test (FST) and elevated plus maze test (EPM). in vivo intracellular electrophysiological recordings were conducted in the lateral nucleus of the amygdala (LAT). Finally, the levels of neuropeptide Y (NPY) were examined using immunohistochemistry. Food restriction during adolescence reduced sucrose preference in adult rats. Adolescent food restriction increased total immobile time in the FST and reduced the latency in rats to the first bout of immobility. In the EPM test, rats that experienced food restriction in adolescence and tested four weeks later spent less time than unrestricted controls in the open arm. In addition, chronic food restriction in adolescence increased in vivo LAT neuronal excitability in adulthood. Finally, NPY immunoreactivity in the LAT was reduced in rats that experienced chronic food restriction in adolescence compared to controls. Our results suggest that food restriction in adolescence increases emotional disorder-like behaviors in adult life, in which NPY production regulates the LAT-dependent behaviors and may underly the vulnerability to emotional disorders.