Sex differences in learning processes of classical and operant conditioning

Associative learning via eyeblink conditioning differs by age from infancy to adulthood

Associative learning is a key feature of adaptive behaviour and mental health, enabling individuals to adjust their actions in anticipation of future events. Comprehensive documentation of this essential component of human cognitive development throughout different developmental periods is needed. Here, we investigated age-related changes in associative learning in key developmental stages, including infancy, childhood, adolescence, and adulthood. We employed a classical delay eyeblink conditioning paradigm that consisted of two sessions with a total of 48 paired trials. Our initial hypothesis was that performance in associative learning would increase linearly with age. However, our findings suggest that performance peaks during the primary school years: Children in this age-group exhibited superior performance compared to all other age-groups and displayed the most consistent and least variable learning. Adults and adolescents exhibited faster association learning than infants. An additional learning session supported learning in infants and adolescents indicating that during these developmental stages, consolidation processes are vital for learning. A comprehensive account of the development of associative learning may inform theories on aetiology and treatment options in clinical psychology and neurosciences.

Childhood adversity is associated with reduced threat-safety discrimination and increased fear generalization in 12- to 16-year-olds

BACKGROUND

Childhood adversity poses a major transdiagnostic risk for a host of psychiatric disorders. Altered threat-related information processing has been put forward as a potential process underlying the association between childhood adversity and psychiatric disorders, with previous research providing support for decreased discrimination between threat and safety cues, in both children and adults exposed to adversity. This altered threat-safety discrimination has been hypothesized to stem from increased generalization of fear, yet to date, this hypothesis has not been tested in youth.

METHODS

Here, we investigate whether childhood adversity is associated with fear generalization during adolescence. 119 adolescents between 12 and 16 years of age (mean = 13.95), of whom 63 exposed to childhood adversity, completed a fear generalization paradigm. Fear conditioning was assessed through trial-by-trial US expectancy ratings and post-experimental ratings of fear, valence and arousal. Additionally, we administered a perceptual discrimination task to assess the potential impact of perceptual discrimination abilities upon fear generalization.

RESULTS

In line with our hypotheses, results showed that childhood adversity is associated with (1) reduced threat-safety differentiation during fear acquisition and (2) increased fear generalization in both boys and girls, albeit to a different extent, as boys showed more generalization towards safety cues while girls showed more generalization towards dangerous cues. Moreover, this overgeneralization of fear could not be attributed to group differences in perceptual discrimination.

CONCLUSIONS

Altered fear learning may be an important process through which adversity increases risk for the development of psychopathology. Longitudinal research is essential to elucidate risk and resilience patterns following childhood adversity.

Leveraging research into sex differences and steroid hormones to improve brain health

Sex differences, driven in part by steroid hormones, shape the structure and function of the brain throughout the lifespan and manifest across brain health and disease. The influence of steroid hormones on neuroplasticity, particularly in the adult hippocampus, differs between the sexes, which has important implications for disorders and diseases that compromise hippocampus integrity, such as depression and Alzheimer disease. This Review outlines the intricate relationship between steroid hormones and hippocampal neuroplasticity across the adult lifespan and explores how the unique physiology of male and female individuals can affect health and disease. Despite calls to include sex and gender in research, only 5% of neuroscience studies published in 2019 directly investigated the influence of sex. Drawing on insights from depression, Alzheimer disease and relevant hippocampal plasticity, this Review underscores the importance of considering sex and steroid hormones to achieve a comprehensive understanding of disease susceptibility and mechanisms. Such consideration will enable the discovery of personalized treatments, ultimately leading to improved health outcomes for all.

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.

In search of prosociality in rodents: A scoping review

Studying prosociality in rodents can provide insight into brain mechanisms potentially related to neurodevelopmental disorders known to impact social behaviors (e.g., autism spectrum disorder). While many studies have been published suggesting promising models, current knowledge remains scattered, including potential factors mediating prosocial behaviors in rodents. Prosocial behavior is characterized by an action done to benefit another or promote their well-being. The goal of this scoping review is to characterize current findings regarding prosocial paradigms in rodents, highlight current gaps in reporting, and identify factors shown to be important in mediating prosocial responses in rodents. Five databases were consulted in search of relevant studies published between 2000 and 2020 (APA PsycInfo, Embase, MEDLINE, Scopus, Web of Science). An update using a semi-supervised machine learning approach (ASReview) was then conducted to collect studies from 2021-2023. In total, 80 articles were included. Findings were the following: (1) Three categories of prosocial paradigm were extracted: cooperation, helping, and sharing tasks, (2) Rodents showed the ability to perform prosocial actions in all three categories, (3) Significant gaps in reported methodologies (e.g., failure to report animals' characteristics, housing conditions, and/or experimental protocol) and mediating factors (e.g., sex, strain, housing, food restriction) were found, and (4) Behaviors are determinant when investigating prosociality in rodents, however many studies omitted to include such analyses. Together these results inform future studies on the impact of mediating factors and the importance of behavioral analyses on the expression of prosocial behaviors in rodents.

Zoographics in the Journal of the Experimental Analysis of Behavior: Increasing inclusion of female animals

We examined the zoographics, or the characteristics of nonhuman animal subjects, reported for the entirety of the Journal of the Experimental Analysis of Behavior (JEAB) through the most recent complete year (1958-2023). Animal sex in particular was evaluated to determine whether the lack of inclusion of female subjects in other disciplines extends to JEAB. Through systematic coding of all nonhuman empirical articles, we found consistent underreporting of most zoographics and a disproportionate use of male subjects relative to female subjects. Additionally, animal sex was commonly unreported and the inclusion of both male and female subjects was sparse. Recent years show some improvement, but greater inclusion is required. Lack of female subjects in research as well as underreporting of zoographics can generate unrepresentative results and hamper replication, generalization, and translation. We provide resources to guide future research and reporting suggestions such as equal inclusion and disaggregation of data by sex. We also clarify misunderstandings about the use of both sexes in research such as beliefs that it necessarily increases the cost of research.

Place conditioning as evaluation of affective valence in piglets

The emotional valence of animals is challenging to assess, despite being a key component of welfare. In this study, we attempted to assess emotional valence through memory in 1- and 3-week-old piglets. It was hypothesized that piglets would spend less time in a pen where they experienced a negative event (castration) and more time in a pen where they experienced a positive event (enrichment). A testing apparatus was designed with three equally sized pens: two outer sections serving as treatment pens containing unique visual and tactile cues and a center section remaining neutral. Piglets received either negative or positive condition in one outer treatment pen and a sham treatment in the opposite. Various methods were tested (age of piglets, number and length of conditioning sessions, passive vs. active conditioning). Contrary to expectations, piglets did not decrease their time in the pen associated with the negative condition or increase their time in the pen associated with the positive condition. However, when exposed to the positive condition, results indicate older piglets developed an aversion towards the sham treatment. This study provides methodological groundwork for the application of place conditioning in piglets and highlights the nuances important for the use of cognitive tests to assess animal welfare.

Understanding sex differences and the translational value of models of persistent substance use despite negative consequences

Persistent substance use despite negative consequences is a key facet of substance use disorder. The last decade has seen the preclinical field adopt the use of punishment to model adverse consequences associated with substance use. This has largely involved the pairing of drug use with either electric foot shock or quinine, a bitter tastant. Whilst at face value, these punishers may model aspects of the physical and psychological consequences of substance use, such models are yet to assist the development of approved medications for treatment. This review discusses progress made with animal models of punishment to understand the behavioral consequences of persistent substance use despite negative consequences. We highlight the importance of examining sex differences, especially when the behavioral response to punishment changes following drug exposure. Finally, we critique the translational value these models provide for the substance use disorder field.

Behavioral outputs and overlapping circuits between conditional fear and active avoidance

Aversive learning can produce a wide variety of defensive behavioral responses depending on the circumstances, ranging from reactive responses like freezing to proactive avoidance responses. While most of this initial learning is behaviorally supported by an expectancy of an aversive outcome and neurally supported by activity within the basolateral amygdala, activity in other brain regions become necessary for the execution of defensive strategies that emerge in other aversive learning paradigms such as active avoidance. Here, we review the neural circuits that support both reactive and proactive defensive behaviors that are motivated by aversive learning, and identify commonalities between the neural substrates of these distinct (and often exclusive) behavioral strategies.

Targeting fear memories: Examining pharmacological disruption in a generalized fear framework

Labilization-reconsolidation, which relies on retrieval, has been considered an opportunity to attenuate the negative aspects of traumatic memories. A therapeutic strategy based on reconsolidation blockade is deemed more effective than current therapies relying on memory extinction. Nevertheless, extremely stressful memories frequently prove resistant to this process. Here, after inducing robust fear memory in mice through strong fear conditioning, we examined the possibility of rendering it susceptible to pharmacological modulation based on the degree of generalized fear (GF). To achieve this, we established an ordered gradient of GF, determined by the perceptual similarity between the associated context (CA) and non-associated contexts (CB, CC, CD, and CE) to the aversive event. We observed that as the exposure context became less similar to CA, the defensive pattern shifted from passive to active behaviors in both male and female mice. Subsequently, in conditioned animals, we administered propranolol after exposure to the different contexts (CA, CB, CC, CD or CE). In males, propranolol treatment resulted in reduced freezing time and enhanced risk assessment behaviors when administered following exposure to CA or CB, but not after CC, CD, or CE, compared to the control group. In females, a similar change in behavioral pattern was observed with propranolol administered after exposure to CC, but not after the other contexts. These results highlight the possibility of indirectly manipulating a robust contextual fear memory by controlling the level of generalization during recall. Additionally, it was demonstrated that the effect of propranolol on reconsolidation would not lead to a reduction in fear memory per se, but rather to its reorganization resulting in greater behavioral flexibility (from passive to active behaviors). Finally, from a clinical viewpoint, this would be of considerable relevance since following this strategy could make the treatment of psychiatric disorders associated with traumatic memory formation more effective and less stressful.

Sex differences in discrimination behavior and orbitofrontal engagement during context-gated reward prediction

Animals, including humans, rely on contextual information to interpret ambiguous stimuli. Impaired context processing is a hallmark of several neuropsychiatric disorders, including schizophrenia, autism spectrum disorders, post-traumatic stress disorder, and addiction. While sex differences in the prevalence and manifestations of these disorders are well established, potential sex differences in context processing remain uncertain. Here, we examined sex differences in the contextual control over cue-evoked reward seeking and its neural correlates, in rats. Male and female rats were trained in a bidirectional occasion-setting preparation in which the validity of two auditory reward-predictive cues was informed by the presence, or absence, of a visual contextual feature (LIGHT: X+/DARK: X-/LIGHT: Y-/DARK: Y+). Females were significantly slower to acquire contextual control over cue-evoked reward seeking. However, once established, the contextual control over behavior was more robust in female rats; it showed less within-session variability (less influence of prior reward) and greater resistance to acute stress. This superior contextual control achieved by females was accompanied by an increased activation of the orbitofrontal cortex (OFC) compared to males. Critically, these behavioral and neural sex differences were specific to the contextual modulation process and not observed in simple, context-independent, reward prediction tasks. These results indicate a sex-biased trade-off between the speed of acquisition and the robustness of performance in the contextual modulation of cued reward seeking. The different distribution of sexes along the fast learning ↔ steady performance continuum might reflect different levels of engagement of the OFC, and might have implications for our understanding of sex differences in psychiatric disorders.

Impact of hindlimb length variation on jumping dynamics in the Longshanks mouse

Distantly related mammals (e.g. jerboa, tarsiers, kangaroos) have convergently evolved elongated hindlimbs relative to body size. Limb elongation is hypothesized to make these species more effective jumpers by increasing their kinetic energy output (through greater forces or acceleration distances), thereby increasing take-off velocity and jump distance. This hypothesis, however, has rarely been tested at the population level, where natural selection operates. We examined the relationship between limb length, muscular traits and dynamics using Longshanks mice, which were selectively bred over 22 generations for longer tibiae. Longshanks mice have approximately 15% longer tibiae and 10% longer femora compared with random-bred Control mice from the same genetic background. We collected in vivo measures of locomotor kinematics and force production, in combination with behavioral data and muscle morphology, to examine how changes in bone and muscle structure observed in Longshanks mice affect their hindlimb dynamics during jumping and clambering. Longshanks mice achieved higher mean and maximum lunge-jump heights than Control mice. When jumping to a standardized height (14 cm), Longshanks mice had lower maximum ground reaction forces, prolonged contact times and greater impulses, without significant differences in average force, power or whole-body velocity. While Longshanks mice have longer plantarflexor muscle bodies and tendons than Control mice, there were no consistent differences in muscular cross-sectional area or overall muscle volume; improved lunge-jumping performance in Longshanks mice is not accomplished by simply possessing larger muscles. Independent of other morphological or behavioral changes, our results point to the benefit of longer hindlimbs for performing dynamic locomotion.

Rodent tests of depression and anxiety: Construct validity and translational relevance

Behavioral testing constitutes the primary method to measure the emotional states of nonhuman animals in preclinical research. Emerging as the characteristic tool of the behaviorist school of psychology, behavioral testing of animals, particularly rodents, is employed to understand the complex cognitive and affective symptoms of neuropsychiatric disorders. Following the symptom-based diagnosis model of the DSM, rodent models and tests of depression and anxiety focus on behavioral patterns that resemble the superficial symptoms of these disorders. While these practices provided researchers with a platform to screen novel antidepressant and anxiolytic drug candidates, their construct validity-involving relevant underlying mechanisms-has been questioned. In this review, we present the laboratory procedures used to assess depressive- and anxiety-like behaviors in rats and mice. These include constructs that rely on stress-triggered responses, such as behavioral despair, and those that emerge with nonaversive training, such as cognitive bias. We describe the specific behavioral tests that are used to assess these constructs and discuss the criticisms on their theoretical background. We review specific concerns about the construct validity and translational relevance of individual behavioral tests, outline the limitations of the traditional, symptom-based interpretation, and introduce novel, ethologically relevant frameworks that emphasize simple behavioral patterns. Finally, we explore behavioral monitoring and morphological analysis methods that can be integrated into behavioral testing and discuss how they can enhance the construct validity of these tests.

A developmental framework for understanding the influence of sex and gender on health: Pediatric pain as an exemplar

Sex differences are a robust finding in many areas of adult health, including cardiovascular disease, psychiatric disorders, and chronic pain. However, many sex differences are not consistently observed until after the onset of puberty. This has led to the hypothesis that hormones are primary contributors to sex differences in health outcomes, largely ignoring the relative contributions of early developmental influences, emerging psychosocial factors, gender, and the interaction between these variables. In this paper, we argue that a comprehensive understanding of sex and gender contributions to health outcomes should start as early as conception and take an iterative biopsychosocial-developmental perspective that considers intersecting social positions. We present a conceptual framework, informed by a review of the literature in basic, clinical, and social science that captures how critical developmental stages for both sex and gender can affect children's health and longer-term outcomes. The literature on pediatric chronic pain is used as a worked example of how the framework can be applied to understanding different chronic conditions.

Beyond the binary: Characterizing the relationships between sex and neuropeptide receptor binding density measures in the rat brain

Sex differences exist in numerous parameters of the brain. Yet, sex-related factors are part of a large set of variables that interact to affect many aspects of brain structure and function. This raises questions regarding how to interpret findings of sex differences at the level of single brain measures and the brain as a whole. In the present study, we reanalyzed two datasets consisting of measures of oxytocin, vasopressin V1a, and mu opioid receptor binding densities in multiple brain regions in rats. At the level of single brain measures, we found that sex differences were rarely dimorphic and were largely persistent across estrous stage and parental status but not across age or context. At the level of aggregates of brain measures showing sex differences, we tested whether individual brains are 'mosaics' of female-typical and male-typical measures or are internally consistent, having either only female-typical or only male-typical measures. We found mosaicism for measures showing overlap between females and males. Mosaicism was higher a) with a larger number of measures, b) with smaller effect sizes of the sex difference in these measures, and c) in rats with more diverse life experiences. Together, these results highlight the limitations of the binary framework for interpreting sex effects on the brain and suggest two complementary pathways to studying the contribution of sex to brain function: (1) focusing on measures showing dimorphic and persistent sex differences and (2) exploring the relations between specific brain mosaics and specific endpoints.

Chemogenetic activation of CRF neurons as a model of chronic stress produces sex-specific physiological and behavioral effects

Trauma and chronic stress exposure are the strongest predictors of lifetime neuropsychiatric disease presentation. These disorders often have significant sex biases, with females having higher incidences of affective disorders such as major depression, anxiety, and PTSD. Understanding the mechanisms by which stress exposure heightens disease vulnerability is essential for developing novel interventions. Current rodent stress models consist of a battery of sensory, homeostatic, and psychological stressors that are ultimately integrated by corticotropin-releasing factor (CRF) neurons to trigger corticosteroid release. These stress paradigms, however, often differ between research groups in the type, timing, and duration of stressors utilized. These inconsistencies, along with the variability of individual animals' perception and response to each stressor, present challenges for reproducibility and translational relevance. Here, we hypothesized that a more direct approach using chemogenetic activation of CRF neurons would recapitulate the effects of traditional stress paradigms and provide a high-throughput method for examining stress-relevant phenotypes. Using a transgenic approach to express the Gq-coupled Designer Receptor Exclusively Activated by Designer Drugs (DREADD) receptor hM3Dq in CRF-neurons, we found that the DREADD ligand clozapine-N-oxide (CNO) produced an acute and robust activation of the hypothalamic-pituitary-adrenal (HPA) axis, as predicted. Interestingly, chronic treatment with this method of direct CRF activation uncovered a novel sex-specific dissociation of glucocorticoid levels with stress-related outcomes. Despite hM3Dq-expressing females producing greater corticosterone levels in response to CNO than males, hM3Dq-expressing males showed significant typical physiological stress sensitivity with reductions in body and thymus weights. hM3Dq-expressing females while resistant to the physiological effects of chronic CRF activation, showed significant increases in baseline and fear-conditioned freezing behaviors. These data establish a novel mouse model for interrogating stress-relevant phenotypes and highlight sex-specific stress circuitry distinct for physiological and limbic control that may underlie disease risk.

Sex Differences in Stress Response: Classical Mechanisms and Beyond

Neuropsychiatric disorders, which are associated with stress hormone dysregulation, occur at different rates in men and women. Moreover, nowadays, preclinical and clinical evidence demonstrates that sex and gender can lead to differences in stress responses that predispose males and females to different expressions of similar pathologies. In this curated review, we focus on what is known about sex differences in classic mechanisms of stress response, such as glucocorticoid hormones and corticotrophin-releasing factor (CRF), which are components of the hypothalamicpituitary- adrenal (HPA) axis. Then, we present sex differences in neurotransmitter levels, such as serotonin, dopamine, glutamate and GABA, as well as indices of neurodegeneration, such as amyloid β and Tau. Gonadal hormone effects, such as estrogens and testosterone, are also discussed throughout the review. We also review in detail preclinical data investigating sex differences caused by recentlyrecognized regulators of stress and disease, such as the immune system, genetic and epigenetic mechanisms, as well neurosteroids. Finally, we discuss how understanding sex differences in stress responses, as well as in pharmacology, can be leveraged into novel, more efficacious therapeutics for all. Based on the supporting evidence, it is obvious that incorporating sex as a biological variable into preclinical research is imperative for the understanding and treatment of stress-related neuropsychiatric disorders, such as depression, anxiety and Alzheimer's disease.

Capsaicin alleviates neuronal apoptosis and schizophrenia-like behavioral abnormalities induced by early life stress

Early life stress (ELS) is associated with the later development of schizophrenia. In the rodent model, the maternal separation (MS) stress may induce neuronal apoptosis and schizophrenia-like behavior. Although the TRPV1 agonist capsaicin (CAP) has been reported to reduce apoptosis in the central nervous system, its effect in MS models is unclear. Twenty-four hours of MS of Wistar rat pups on postnatal day (PND9) was used as an ELS. Male rats in the adult stage were the subjects of the study. CAP (1 mg/kg/day) intraperitoneal injection pretreatment was undertaken before behavioral tests for 1 week and continued during the tests. Behavioral tests included open field, novel object recognition, Barnes maze test, and pre-pulse inhibition (PPI) test. MS rats showed behavioral deficits and cognitive impairments mimicking symptoms of schizophrenia compared with controls. MS decreased the expression of TRPV1 in the frontal association cortex (FrA) and in the hippocampal CA1, CA3, and dentate gyrus (DG) regions compared with the control group resulting in the increase of pro-apoptotic proteins (BAX, Caspase3, Cleaved-Caspase3) and the decrease of anti-apoptotic proteins (Bcl-2). The number of NeuN++TUNEL+ cells increased in the MS group in the FrA, CA1, CA3, and DG compared with the control group. Neuronal and behavioral impairments of MS were reversed by treatment with CAP. Exposure to ELS may lead to increased neuronal apoptosis and impaired cognitive function with decreased TRPV1 expression in the prefrontal cortex and hippocampus in adulthood. Sustained low-dose administration of CAP improved neuronal apoptosis and cognitive function. Our results provide evidence for future clinical trials of chili peppers or CAP as dietary supplements for the reversal treatment of schizophrenia.

Lack of sex and estrous stage effects on compulsive behavior assessed using a schedule-induced polydipsia procedure in rats

Preclinical behavior models used for screening pharmacological treatments for mental disorders have generally used only male research subjects, and for studies that have included female subjects, few have utilized sex as a study variable. In fact, many mental disorders vary by prevalence and symptomatology between sexes, creating a need to evaluate established subject models for sex differences. Compulsive behavior is a feature shared across many mental disorders and effective treatments have been examined pre-clinically using the schedule-induced polydipsia procedure in rats. Drugs effective for reducing polydipsia include psychostimulants, such as d -amphetamine. Virtually no studies have examined sex differences using this procedure. For the present study, male and female rats were examined in the schedule-induced polydipsia paradigm. Rats were food-restricted and trained on a fixed-interval food reinforcement schedule and given free access to water during experimental sessions. Estrous stages were assessed during training and test sessions. The psychostimulant d -amphetamine was also tested once stable water consumption occurred. Excessive water intake developed over the course of training. Females required significantly more sessions to reach a stable level of drinking. Treatment with d -amphetamine (1.0 mg/kg, but not 0.25 or 0.5 mg/kg) significantly reduced drinking in both male and female rats. No sex differences were observed across other study variables including comparisons between diestrus and proestrus stages. Overall, these findings suggest that schedule-induced polydipsia procedures that employ similar methods can produce results generalizable across male and female subjects.

Sex differences in anxiety and threat avoidance in GAD65 knock-out mice

Anxiety disorders have been linked to a disbalance of excitation and inhibition in a network of brain structures comprising frontal cortical regions, the amygdala and the hippocampus, among others. Recent imaging studies suggest sex differences in the activation of this anxiety network during the processing of emotional information. Rodent models with genetically altered ϒ-amino butyric acid (GABA) neurotransmission allow studying the neuronal basis of such activation shifts and their relation to anxiety endophenotypes, but to date sex effects have rarely been addressed. Using mice with a null mutation of the GABA synthetizing enzyme glutamate decarboxylase 65 (GAD65-/-), we started to compare anxiety-like behavior and avoidance in male vs. female GAD65-/- mice and their wildtype littermates. In an open field, female GAD65-/- mice displayed increased activity, while male GAD65-/- mice showed an increased adaptation of anxiety-like behavior over time. GAD65-/- mice of both sexes had a higher preference for social interaction partners, which was further heightened in male mice. In male mice higher escape responses were observed during an active avoidance task. Together, female mice showed more stable emotional responses despite GAD65 deficiency. To gain insights into interneuron function in network structures controlling anxiety and threat perception, fast oscillations (10-45 Hz) were measured in ex vivo slice preparations of the anterior cingulate cortex (ACC). GAD65-/- mice of both sexes displayed increased gamma power in the ACC and a higher density of PV-positive interneurons, which are crucial for generating such rhythmic activity. In addition, GAD65-/- mice had lower numbers of somatostatin-positive interneurons in the basolateral amygdala and in the dorsal dentate gyrus especially in male mice, two key regions important for anxiety and active avoidance responses. Our data suggest sex differences in the configuration of GABAergic interneurons in a cortico-amygdala-hippocampal network controlling network activity patterns, anxiety and threat avoidance behavior.

Sex differences in learning and performing the Go/NoGo tasks

BACKGROUND

The quality of learning and post-learning performances is critical for daily life. The behavioral flexibility is equally important for adapting the changing circumstances. The learning process requires repeated practices, which enhances prompt and proper behavioral responses, in turn, which promotes habits formation as well. Despite the well-documented sex differences in learning and performances, contradictory results were reported. A possible cause might be a systematic analysis due to specific research interests, regardless of the continuity of natural acquisition process. Here, we investigate the potential sex differences in learning, performances and adjustments of habited behaviors with regular and reversal Go/NoGo tasks.

METHODS

Both male and female Sprague-Dawley rats were used in this study. All rats were trained for a regular rodent Go/NoGo task and a subset of rats were trained for a reversal rodent Go/NoGo task, both with strict elimination criteria. The behavioral performance data were stored in PC for off-line analysis. Multiple behavioral indices were analyzed for both passed and retired rats.

RESULTS

The ability of learning the regular the reversal Go/NoGo tasks was similar for both male and female rats, however, the female rats took longer time to master the task principles in later stages for both tasks. In the regular Go/NoGo task, the female rats spent more time on completing the trial in performance optimization phases, which implied female rats were more cautious than male rats. Along with the progression of training, both male and female rats developed Go-preference strategies to perform the regular Go/NoGo task, which induced failure to meet the setting success criteria. The retired male rats exhibited shorter RTs and MTs than the retired female rats after developing Go-preference. Moreover, the time needed to complete the Go trials was significantly prolonged for male rats in the reversal Go/NoGo task.

CONCLUSIONS

Overall, we conclude that distinctive strategies were employed in performing Go/NoGo tasks for both male and female rats. Male rats required less time to stabilize the performance in behavioral optimization phase. In addition, male rats were more accurate in estimating time elapsing. In contrast, female rats took more cautious considerations in performing the task, through which minimal influences were manifested in the reversal version of task.

Delay eyeblink conditioning performance and brain-wide c-Fos expression in male and female mice

Delay eyeblink conditioning has been extensively used to study associative learning and the cerebellar circuits underlying this task have been largely identified. However, there is a little knowledge on how factors such as strain, sex and innate behaviour influence performance during this type of learning. In this study, we used male and female mice of C57BL/6J (B6) and B6CBAF1 strains to investigate the effect of sex, strain and locomotion in delay eyeblink conditioning. We performed a short and a long delay eyeblink conditioning paradigm and used a c-Fos immunostaining approach to explore the involvement of different brain areas in this task. We found that both B6 and B6CBAF1 females reach higher learning scores compared to males in the initial stages of learning. This sex-dependent difference was no longer present as the learning progressed. Moreover, we found a strong positive correlation between learning scores and voluntary locomotion irrespective of the training duration. c-Fos immunostainings after the short paradigm showed positive correlations between c-Fos expression and learning scores in the cerebellar cortex and brainstem, as well as previously unreported areas. By contrast, after the long paradigm, c-Fos expression was only significantly elevated in the brainstem. Taken together, we show that differences in voluntary locomotion and activity across brain areas correlate with performance in delay eyeblink conditioning across strains and sexes.

Importance of home cage condition for contextual fear memory, fear extinction and spontaneous recovery: Cage size and bedding material

Home cage condition influences the central nervous system of experimental animals. However, little is known about the effect of home cage size and bedding material on fear-related behaviors. Thus, in this study, the effects of home cage size (large or small) and/or bedding material (paper or wood) on acquisition, retrieval, extinction, and spontaneous recovery of contextual fear memory were investigated in both male and female mice. The present study demonstrated that males housed in small cages with wood bedding showed a low fear response during fear extinction when compared to males housed in small or large cages with paper bedding. In females, mice housed in small cages with wood bedding showed low fear response during fear conditioning and extinction when compared to mice housed in large cages with paper bedding. Moreover, small cages with wood bedding, but not small or large cages with paper bedding, prevented the spontaneous recovery of fear memory in females. Thus, home cage conditions, and particularly bedding material, influence contextual fear extinction and spontaneous recovery. This finding may help to obtain reproducibility of results by researchers and explain discrepancies of results among research groups.

Environmental conditions of recognition memory testing induce neurovascular changes in the hippocampus in a sex-specific manner in mice

Experiences are linked to emotions impacting memory consolidation and associated brain neuronal circuits. Posttraumatic stress disorder is an example of strong negative emotions affecting memory processes by flashbacks of past traumas. Stress-related memory deficits are also observed in major depressive disorder (MDD). We recently highlighted that sex-specific blood-brain barrier (BBB) alterations underlie stress responses in mice and human depression. However, little is known about the relationship between emotional valence, memory encoding and BBB gene expression. Here, we investigated the effects of novel object recognition (NOR) test, an experience considered of neutral emotional valence, on BBB properties in dorsal vs ventral hippocampus (HIPP) in the context of various environmental conditions (arena size, handling, age). The HIPP is a brain area central for learning and memory processes with the dorsal and ventral subregions being associated with working memory vs reference memory retrieval, respectively. Expression of genes related to BBB integrity are altered in line with learning and memory processes in a region- and sex-specific manner. We observed correlations between poor learning, anxiety, stress-induced corticosterone release and changes in BBB-associated gene expression. Comparison of BBB transcriptomes between sexes also revealed profound differences at baseline in both ventral and dorsal HIPP. Finally, we identified circulating vascular biomarkers, such as sE-selectin and matrix metallopeptidase 9 (MMP-9), altered following NOR exposure supporting that recognition memory formation has an impact on the neurovasculature. Although deemed as a neutral valence test, NOR experimental conditions can shift it toward a negative valence, impacting performance and highlighting the need to minimize anxiety when performing this commonly used test in mice.

Learning processes in relapse to alcohol use: lessons from animal models

RATIONALE

Alcohol use is reliably preceded by discrete and contextual stimuli which, through diverse learning processes, acquire the capacity to promote alcohol use and relapse to alcohol use.

OBJECTIVE

We review contemporary extinction, renewal, reinstatement, occasion setting, and sex differences research within a conditioning framework of relapse to alcohol use to inform the development of behavioural and pharmacological therapies.

KEY FINDINGS

Diverse learning processes and corresponding neurobiological substrates contribute to relapse to alcohol use. Results from animal models indicate that cortical, thalamic, accumbal, hypothalamic, mesolimbic, glutamatergic, opioidergic, and dopaminergic circuitries contribute to alcohol relapse through separable learning processes. Behavioural therapies could be improved by increasing the endurance and generalizability of extinction learning and should incorporate whether discrete cues and contexts influence behaviour through direct excitatory conditioning or occasion setting mechanisms. The types of learning processes that most effectively influence responding for alcohol differ in female and male rats.

CONCLUSION

Sophisticated conditioning experiments suggest that diverse learning processes are mediated by distinct neural circuits and contribute to relapse to alcohol use. These experiments also suggest that gender-specific behavioural and pharmacological interventions are a way towards efficacious therapies to prevent relapse to alcohol use.

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.

Human threat circuits: Threats of pain, aggressive conspecific, and predator elicit distinct BOLD activations in the amygdala and hypothalamus

Introduction

Threat processing, enabled by threat circuits, is supported by a remarkably conserved neural architecture across mammals. Threatening stimuli relevant for most species include the threat of being attacked by a predator or an aggressive conspecific and the threat of pain. Extensive studies in rodents have associated the threats of pain, predator attack and aggressive conspecific attack with distinct neural circuits in subregions of the amygdala, the hypothalamus and the periaqueductal gray. Bearing in mind the considerable conservation of both the anatomy of these regions and defensive behaviors across mammalian species, we hypothesized that distinct brain activity corresponding to the threats of pain, predator attack and aggressive conspecific attack would also exist in human subcortical brain regions.

Methods

Forty healthy female subjects underwent fMRI scanning during aversive classical conditioning. In close analogy to rodent studies, threat stimuli consisted of painful electric shocks, a short video clip of an attacking bear and a short video clip of an attacking man. Threat processing was conceptualized as the expectation of the aversive stimulus during the presentation of the conditioned stimulus.

Results

Our results demonstrate differential brain activations in the left and right amygdala as well as in the left hypothalamus for the threats of pain, predator attack and aggressive conspecific attack, for the first time showing distinct threat-related brain activity within the human subcortical brain. Specifically, the threat of pain showed an increase of activity in the left and right amygdala and the left hypothalamus compared to the threat of conspecific attack (pain > conspecific), and increased activity in the left amygdala compared to the threat of predator attack (pain > predator). Threat of conspecific attack revealed heightened activity in the right amygdala, both in comparison to threat of pain (conspecific > pain) and threat of predator attack (conspecific > predator). Finally, for the condition threat of predator attack we found increased activity in the bilateral amygdala and the hypothalamus when compared to threat of conspecific attack (predator > conspecific). No significant clusters were found for the contrast predator attack > pain.

Conclusion

Results suggest that threat type-specific circuits identified in rodents might be conserved in the human brain.

A sex difference in mouse dopaminergic projections from the midbrain to basolateral amygdala

BACKGROUND

Dopaminergic circuits play important roles in the motivational control of behavior and dysfunction in dopaminergic circuits have been implicated in several psychiatric disorders, such as schizophrenia and depression. While these disorders exhibit different incidence rates in men and women, the potential sex differences in the underlying neural circuits are not well-understood. Previous anatomical tracing studies in mammalian species have revealed a prominent circuit projection connecting the dopaminergic midbrain ventral tegmental area (VTA) to the basolateral amygdala (BLA), which is involved in emotional processing and associative learning. However, whether there is any sex difference in this anatomical circuit remains unknown.

METHODS

To study the potential sex differences in the VTA-to-BLA dopaminergic circuit, we injected two viral vectors encoding fluorescent reporters of axons and synaptic boutons (AAV-FLEX-tdTomato and AAV-FLEX-SynaptophysinGFP, respectively) into the VTA of a mouse transgenic driver line (tyrosine hydroxylase promoter-driven Cre, or TH-Cre), which restricts the reporter expression to dopaminergic neurons. We then used confocal fluorescent microscopy to image the distribution and density of dopaminergic axons and synaptic boutons in serial sections of both male and female mouse brain.

RESULTS

We found that the overall labeling intensity of VTA-to-BLA dopaminergic projections is intermediate among forebrain dopaminergic pathways, significantly higher than the projections to the prefrontal cortex, but lower than the projections to the nucleus accumbens. Within the amygdala areas, dopaminergic axons are concentrated in BLA. Although the size of BLA and the density of dopaminergic axons within BLA are similar between male and female mice, the density of dopaminergic synaptic boutons in BLA is significantly higher in male brain than female brain.

CONCLUSIONS

Our results demonstrate an anatomical sex difference in mouse dopaminergic innervations from the VTA to BLA. This finding may provide a structural foundation to study neural circuit mechanisms underlying sex differences in motivational and emotional behaviors and related psychiatric dysfunctions.

Nanoselenium improved learning, memory, and brain-derived neurotrophic factor and attenuated nitric oxide, and oxidative stress in the brain of juvenile hypothyroid rats

BACKGROUND

Nanoselenium (Nan S) is a form of selenium element that acts with high absorption and low toxicity. However, few studies have examined the effects of Nan S on cognitive impairment. On the other hand, hypothyroidism is a common disease that causes cognitive disorders. Therefore, this study aimed to investigate the effect of Nan S on memory impairment in rats due to propylthiouracil (PTU) - induced hypothyroidism. The roles of brain-derived neurotrophic factor (BDNF), nitric oxide (NO), and oxidative stress were also challenged.

MATERIALS AND METHODS

The animals were randomly divided into 4 groups: (1) Control group (normal saline), (2) hypothyroid (Hypo) group: where 0.05% PTU was added to drinking water, (3) and (4) Hypo-Nan S 50, Hypo-Nan S 100 in which 50 or 100 µg/ kg of Nan S were injected respectively. After 6 weeks, spatial and avoidance memory was measured by Morris water maze (MWM) and passive avoidance (PA) tests. The animals then underwent deep anesthesia and the serum samples and the hippocampus and cortex were collected to be used for thyroxin and biochemical measurements including malondialdehyde (MDA), NO, thiol, superoxide dismutase (SOD), catalase (CAT), and BDNF.

RESULTS

The rats showed an increase in the escape latency and traveled path in MWM in the Hypo group compare with the Control group and these parameters were decreased in both Hypo-Nan S 50 and Hypo-Nan S 100 groups compared to the Hypo group. The rats of both Hypo-Nan S 50 and Hypo-Nan S 100 groups spent longer time and traveled longer distances in the target area during the probe trial of MWM than the Hypo group. In addition, the latency to enter the dark box in the PA test was lower in the Hypo group than in the Control group, which was significantly improved after Nan S treatment. Furthermore, the hippocampal and cortical lipid peroxide marker (MDA) levels and NO metabolites of the Hypo group were significantly increased and the antioxidant markers (total thiol, SOD, and CAT) were significantly inhibited compared to the Control group. Compared with the Hypo group, Nan S administration could significantly decrease the oxidant factors and increase the activities antioxidant system and concentration of BDNF.

CONCLUSION

It is concluded that Nan S might be able to enhance endogenous antioxidant proteins due to its antioxidant activity, thereby improving BDNF and spatial and avoidance memory in the hypothyroidism-induced memory impairment model however, more studies are still necessary to elucidate the exact mechanism(s).

Deficits in Cerebellum-Dependent Learning and Cerebellar Morphology in Male and Female BTBR Autism Model Mice

Recently, there has been increased interest in the role of the cerebellum in autism spectrum disorder (ASD). To better understand the pathophysiological role of the cerebellum in ASD, it is necessary to have a variety of mouse models that have face validity for cerebellar disruption in humans. Here, we add to the literature on the cerebellum in mouse models of autism with the characterization of the cerebellum in the idiopathic BTBR T + Itpr3tf/J (BTBR) inbred mouse strain, which has behavioral phenotypes that are reminiscent of ASD in patients. When we examined both male and female BTBR mice in comparison to C57BL/6J (C57) controls, we noted that both sexes of BTBR mice showed motor coordination deficits characteristic of cerebellar dysfunction, but only the male mice showed differences in delay eyeblink conditioning, a cerebellum-dependent learning task that is known to be disrupted in ASD patients. Both male and female BTBR mice showed considerable expansion of, and abnormal foliation in, the cerebellum vermis—including a significant expansion of specific lobules in the anterior cerebellum. In addition, we found a slight but significant decrease in Purkinje cell density in both male and female BTBR mice, irrespective of the lobule. Finally, there was a marked reduction of Purkinje cell dendritic spine density in both male and female BTBR mice. These findings suggest that, for the most part, the BTBR mouse model phenocopies many of the characteristics of the subpopulation of ASD patients that have a hypertrophic cerebellum. We discuss the significance of strain differences in the cerebellum as well as the importance of this first effort to identify both similarities and differences between male and female BTBR mice with regard to the cerebellum.

A Learning Experience? Enjoyment at Sexual Debut and the Gender Gap in Sexual Desire among Emerging Adults

Gender differences in experience of first intercourse are among the largest in sexuality research, with women recalling less pleasure and satisfaction than men. This "enjoyment gap" has not been considered in explanations of gender differences in sexual desire. Yet, reinforcement and incentive learning feature prominently in recent models of women's sexual desire, and nonhuman animal models demonstrate their impact at sexual debut. We examined whether women's lower sexual desire is explained by their gender or by gendered experience of enjoyment at sexual debut. Emerging adults (N = 838) provided retrospective accounts of physical (orgasm) and affective (satisfaction) enjoyment at (hetero)sexual debut. We replicated gender differences across behavioral, general, and multidimensional measures of trait sexual desire; however, they were contingent on experience and measurement method. When its cognitive multidimensional properties were appreciated, women's sexual desire varied with experience of orgasm at sexual debut and diverged from men's only when orgasm did not occur. Such effects were not observed for satisfaction, nor for men. Nor did effects of a control event - masturbatory debut - extend beyond solitary sexual desire. Findings underscore the importance of orgasm equality, and suggest its absence at sexual debut may play an unacknowledged role in differentiating sexual desire.

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.

The effects of curcumin in learning and memory impairment associated with hypothyroidism in juvenile rats: the role of nitric oxide, oxidative stress, and brain-derived neurotrophic factor

The effect of curcumin (Cur) on cognitive impairment and the possible role of brain tissue oxidative stress, nitric oxide (NO) levels, and brain-derived neurotrophic factor (BDNF) were investigated in juvenile hypothyroid rats. The juvenile rats (21 days old) were allocated into the following groups: (1) control; (2) hypothyroid (0.05% propylthiouracil (PTU) in drinking water); (3-5) hypothyroid-Cur 50, 100, and 150, which in these groups 50, 100, or 150 mg/kg, Cur was orally administered by gavage during 6 weeks. In the hypothyroid rats, the time elapsed and the traveled distance to locate the hidden platform in the learning trials of Morris water maze (MWM) increased, and on the probe day, the amount of time spent in the target quadrant and the distance traveled in there was decreased. Hypothyroidism also decreased the latency and increased the time spent in the darkroom of the passive avoidance (PA) test. Compared with the hypothyroid group, Cur enhanced the performance of the rats in both MWM and PA tests. In addition, Cur reduced malondialdehyde concentration and NO metabolites; however, it increased thiol content as well as the activity of catalase (CAT) and superoxide dismutase enzymes in both the cortex and hippocampus. Cur also increased hippocampal synthesis of BDNF in hypothyroid rats. The beneficial effects of Cur cognitive function in juvenile hypothyroid rats might be attributed to its protective effect against oxidative stress and potentiation of BDNF production.

Hormonal contraceptive usage influences stress hormone effects on cognition and emotion

Men and women partially differ in how they respond to stress and how stress in return affects their cognition and emotion. The influence of hormonal contraceptives (HCs) on this interaction has received little attention, which is surprising given the prevalence of HC usage. This selective review illustrates how HC usage modulates the effects of stress hormones on cognition and emotion. As three examples, we discuss stress hormone effects on episodic memory, fear conditioning and cognitive emotion regulation. The identified studies revealed that stress effects on cognitive-emotional processes in women using HCs were at times reduced or even absent when compared to men or naturally cycling women. Especially striking were the few examples of reversed effects in HC women. As underlying neuroendocrine mechanisms, we discuss influences of HCs on the neuroendocrine stress response and effects of HCs on central glucocorticoid sensitivity. The summarized findings emphasize the need for additional translational research.

Sex-specific Effects of Endocrine-disrupting Chemicals on Brain Monoamines and Cognitive Behavior

The period of brain sexual differentiation is characterized by the development of hormone-sensitive neural circuits that govern the subsequent presentation of sexually dimorphic behavior in adulthood. Perturbations of hormones by endocrine-disrupting chemicals (EDCs) during this developmental period interfere with an organism's endocrine function and can disrupt the normative organization of male- or female-typical neural circuitry. This is well characterized for reproductive and social behaviors and their underlying circuitry in the hypothalamus and other limbic regions of the brain; however, cognitive behaviors are also sexually dimorphic, with their underlying neural circuitry potentially vulnerable to EDC exposure during critical periods of brain development. This review provides recent evidence for sex-specific changes to the brain's monoaminergic systems (dopamine, serotonin, norepinephrine) after developmental EDC exposure and relates these outcomes to sex differences in cognition such as affective, attentional, and learning/memory behaviors.

Sex as a Biological Variable in Nutrition Research: From Human Studies to Animal Models

Biological sex is a fundamental source of phenotypic variability across species. Males and females have different nutritional needs and exhibit differences in nutrient digestion and utilization, leading to different health outcomes throughout life. With personalized nutrition gaining popularity in scientific research and clinical practice, it is important to understand the fundamentals of sex differences in nutrition research. Here, we review key studies that investigate sex dimorphism in nutrition research: sex differences in nutrient intake and metabolism, sex-dimorphic response in nutrient-restricted conditions, and sex differences in diet and gut microbiome interactions. Within each area above, factors from sex chromosomes, sex hormones, and sex-specific loci are highlighted.

Sex differences in long-term fear and anxiety-like responses in a preclinical model of PTSD

With a high prevalence of posttraumatic stress disorder (PTSD) in females, studying sex differences in preclinical models is of substantial importance. We have previously employed behavioural criteria to identify and characterize a subpopulation of rats that presented impaired fear extinction and long-term fear and anxiety responses following fear conditioning. We now exposed male and female rats to fear conditioning and extinction and segregated the animals into weak- (WE) and strong-extinction (SE) groups based on behavioural scores during extinction. Animals were subsequently tested for tone and context recall, as well as anxiety-like responses in the marble burying and novelty suppression of feeding (NSF) tests. Vaginal lavages were collected to characterize the phase of the estrous cycle during fear extinction. We found that females had reduced freezing during tone recall and a lower latency to feed in the NSF test. No differences were found in females undergoing extinction during high and low estrogen phases of the cycle in any of the performed tests. Overall, the percentage of animals that presented WE and SE phenotypes was similar in males and females. Both, WE males and females had increased freezing during tone and context recall. Along with our previous reports, WE males presented anxiety-like responses, particularly in the NSF compared to SE animals. In contrast, WE females buried less marbles than their SE mates. Future investigation including a larger number of behavioural tests are certainly required to corroborate our findings and ascertain potential mechanisms to explain the differences observed in our study.

Sex differences in the immediate extinction deficit and renewal of extinguished fear in rats

Extinction learning is central to exposure-based behavioral therapies for reducing fear and anxiety in humans. However, patients with fear and anxiety disorders are often resistant to extinction. Moreover, trauma and stress-related disorders are highly prone to relapse and are twice as likely to occur in females compared to males, suggesting that females may be more susceptible to extinction deficits and fear relapse phenomena. In this report, we tested this hypothesis by examining sex differences in a stress-induced extinction learning impairment, the immediate extinction deficit (IED), and renewal, a common form of fear relapse. In contrast to our hypothesis, there were no sex differences in the magnitude of the immediate extinction deficit in two different rat strains (Long-Evans and Wistar). However, we did observe a sex difference in the renewal of fear when the extinguished conditioned stimulus was presented outside the extinction context. Male Wistar rats exhibited significantly greater renewal than female rats, a sex difference that has previously been reported after appetitive extinction. Collectively, these data reveal that stress-induced extinction impairments are similar in male and female rats, though the context-dependence of extinction is more pronounced in males.

Sex Differences in the Activity of Basolateral Amygdalar Neurons That Project to the Bed Nucleus of the Stria Terminalis and Their Role in Anticipatory Anxiety

Abnormal fear and anxiety can manifest as psychiatric disorders. The bed nucleus of the stria terminalis (BNST) is implicated in sustained responding to, or anticipation of, an aversive event which can be expressed as anticipatory anxiety. The BLA is also active during anticipatory anxiety and sends projections to the BNST. However, little is known about the role for BLA neurons that project to BNST (BLA-BNST) in anticipatory anxiety in rodents. To address this, we tested whether chemogenetic inactivation of the BLA-BNST pathway attenuates sustained conditioned responses produced by anticipation of an aversive stimulus. For comparison, we also assessed BLA-BNST inactivation during social interaction, which is sensitive to unlearned anxiety. We found that BLA-BNST inactivation reduced conditioned sustained freezing and increased social behaviors, but surprisingly, only in males. To determine whether sex differences in BLA-BNST neuronal activity contribute to the differences in behavior, we used in vivo and ex vivo electrophysiological approaches. In males, BLA-BNST projection neurons were more active and excitable, which coincided with a smaller after-hyperpolarization current (I AHP) compared with other BLA neurons; whereas in females, BLA-BNST neurons were less excitable and had larger I AHP compared with other BLA neurons. These findings demonstrate that activity of BLA-BNST neurons mediates conditioned anticipatory anxiety-like behavior in males. The lack of a role of BLA-BNST in females in this behavior, possibly because of low excitability of these neurons, also highlights the need for caution when generalizing the role of specific neurocircuits in fear and anxiety.SIGNIFICANCE STATEMENT Anxiety disorders disproportionately affect women. This hints toward sex differences within anxiety neurocircuitry, yet most of our understanding is derived from male rodents. Furthermore, debilitating anticipation of adverse events is among the most severe anxiety symptoms, but little is known about anticipatory anxiety neurocircuitry. Here we demonstrated that BLA-BNST activity is required for anticipatory anxiety to a prolonged aversive cue, but only in males. Moreover, BLA-BNST neurons are hypoactive and less excitable in females. These results uncover BLA-BNST as a key component of anticipatory anxiety circuitry, and cellular differences may explain the sex-dependent role of this circuit. Uncovering this disparity provides evidence that the assumed basic circuitry of an anxiety behavior might not readily transpose from males to females.

Cyclic estrogen and progesterone during instrumental acquisition contributes to habit formation in female rats

Habit formation is thought to involve two parallel processes that are mediated by distinct neural substates: one that suppresses goal-directed behavior, and one that facilitates stimulus-response (S-R) learning, which underscores habitual behavior. In previous studies we showed that habitual responding emerges early during instrumental training in gonadally-intact female, compared to male, rats. The present study aimed to determine the role of ovarian hormones during instrumental acquisition in the transition from goal-directed to habitual behavior in female rats. Ovariectomized (OVX) female rats were given subcutaneous silastic capsules that released low levels of 17-β estradiol (E2) to maintain estrogen receptor availability. Rats were assigned to one of three hormone treatment conditions: no additional hormone replacement (Control group), replacement with high E2 (High E2 group), or replacement with high E2 followed by progesterone (High E2 + P4 group). Hormone replacement occurred twice during acquisition to mimic natural hormone fluctuations. At test, the Control and High E2 groups demonstrated responding that was sensitive to devaluation by lithium chloride-induced illness, indicating goal-directed behavior. In contrast, the High E2 + P4 group exhibited a pattern of devaluation-insensitive, habitual responding, that suggested the suppression of goal-directed processes. In a follow-up experiment, similar procedures were conducted, however during acquisition, OVX rats were given cyclic high E2 plus medroxy-progesterone (MPA), a form of progesterone that does not metabolize to neuroactive metabolites. In this group, goal-directed behavior was observed. These data indicate that habit formation is not facilitated in low estrogen states, nor in the presence of cyclic high E2. However, cyclic high E2, together with progesterone during acquisition, appears to facilitate the early emergence of habitual responding. Furthermore, these data suggest that a neuroactive progesterone metabolite, like allopregnanolone, in combination with high cyclic E2, supports this phenomenon.

Neural, physiological, and psychological markers of appetitive conditioning in anorexia nervosa: a study protocol

BACKGROUND

Anorexia nervosa (AN) is a chronic and disabling psychiatric condition characterized by low hedonic drive towards food, and is thought to be inclusive of altered dimensions of reward processing. Whether there exists a fundamental aberrancy in the capacity to acquire and maintain de novo hedonic associations-a critical component of hedonic responding-has never been studied in AN.

METHODS

This multi-modal study will employ a 2-day Pavlovian appetitive conditioning paradigm to interrogate the (1) acquisition, (2) extinction, (3) spontaneous recovery and (4) reinstatement of appetitive learning in adolescents and young adults with AN. Participants will be 30 currently ill, underweight individuals with AN; 30 weight-restored individuals with AN; and 30 age-matched healthy controls, all aged 12-22 years. All subjects will undergo clinical assessment, followed by the 2-day appetitive conditioning task during which fMRI, pupillometry, heart rate deceleration, and subjective ratings will be acquired.

DISCUSSION

This study will be the first to interrogate appetitive conditioning in AN-a disorder characterized by altered hedonic responding to food. Results will help establish objective biomarkers of appetitive conditioning in AN and lay the groundwork for developing novel lines of treatment for AN and other psychiatric disorders involving diminished ability to experience pleasure and reward.

TRIAL REGISTRATION

Pending.

INTENDED REGISTRY

Clinicaltrials.gov.

Stress-related impairment of fear memory acquisition and disruption of risk assessment behavior in female but not in male mice

Stress encompasses reactions to stimuli that promote negative and positive effects on cognitive functions, such as learning and memory processes. Herein, we investigate the effect of restraint stress on learning, memory, anxiety levels and locomotor activity of male and female mice. We used the plus-maze discriminative avoidance task (PMDAT), a behavioral task based on the innate exploratory response of rodents to new environments. Moreover, this task is used to simultaneously evaluate learning, memory, anxiety-like behavior and locomotor activity. Male and female mice were tested after repeated daily restraint stress (4h/day for 3 days). The results showed stress-induced deficits on aversive memory retrieval only in female mice, suggesting a sexual dimorphism on memory acquisition. Furthermore, stressed females exhibited increased anxiety-like behavior and decreased exploratory behavior. Plasma corticosterone levels were similarly increased by restraint stress in both sexes, suggesting that the behavioral outcome was not related to hormonal secretion. Our findings corroborate previous studies, showing a sexually dimorphic effect of restraint stress on cognition. In addition, our study suggests that stress-related acquisition deficit may be the consequence of elevated emotional response in females.

The role of context on responding to an alcohol-predictive cue in female and male rats

In male rats, physical contexts that are associated with alcohol can amplify the response to a discrete, alcohol-predictive conditioned stimulus (CS), and amplify prime-induced reinstatement. Here, we examined these effects as a function of biological sex. Male and female Long-Evans rats were acclimated to drinking ethanol (15% v/v) in their home cages. Next, they were trained to associate an auditory conditioned stimulus (CS) (10 s; white noise or clicker; 15 trials per session) with ethanol delivery (0.2 mL per CS; 3.0 mL per session) into a fluid port for oral intake. Training occurred in a distinctive context containing specific visual, olfactory, and tactile stimuli. During alternating sessions, rats were exposed to a second context wherein they did not receive ethanol. At test, CS trials occurred in both contexts without ethanol delivery. Rats then underwent extinction using repeated unreinforced presentations of the CS in both contexts. An alcohol-primed reinstatement test was then conducted, in which 0.2 mL of ethanol was presented at the start of the session and during the first CS trial, after which no ethanol was delivered for the remainder of the session. At both test and reinstatement, male rats made significantly more CS port-entries in the context associated with alcohol delivery than in the context in which alcohol was never experienced. Unlike males, female rats made a similar number of CS port-entries at the test in both the alcohol context and the neutral context. The reinstatement observed in female rats was also not affected by context. These findings suggest that the capacity of an alcohol-associated context to modulate responding to a discrete, alcohol-predictive cue is less pronounced in female than male rats.

The effects of vitamin D on learning and memory of hypothyroid juvenile rats and brain tissue acetylcholinesterase activity and oxidative stress indicators

Apart from a role as a key regulator of calcium/phosphate homeostasis, vitamin D (Vit D) is suggested to be a potential player in nervous system growth and function. This study aimed to assess the impacts of Vit D administration on memory impairment, oxidative damage, and acetylcholinesterase (AchE) overactivity in hypothyroid juvenile rats. The animals were randomly grouped as (1) Control; (2) Hypothyroid; (3) Hypothyroid-Vit D100, and (4) Hypothyroid-Vit D 500. Propylthiouracil (PTU) was added to their drinking water (0.05%) for 6 weeks, and Vit D (100 or 500 IU/kg) treatment was performed daily by gavage. Morris water maze (MWM) and passive avoidance (PA) tests were performed. The brains were removed under deep anesthesia, then the hippocampal and cortical tissues were separated to assess biochemical parameters. Hypothyroidism was significantly associated with learning and memory impairment in MWM and PA tests. Hypothyroidism was also accompanied by an elevation in AChE activity and malondialdehyde (MDA) content and a reduced level of thiol content and superoxide dismutase (SOD) activity in the brain. Treatment with Vit D recovered hypothyroidism-induced cognitive impairment and improved memory performance in MWM and PA tasks. On the other hand, Vit D alleviated AChE activity and MDA level, whereas increased SOD activity and thiol content in the hippocampal and cortical tissues. In conclusion, these outcomes suggest an association between the oral administrations of Vit D and learning and memory improvement of hypothyroid rats, which was accompanied by decreasing AChE activity and brain tissue oxidative damage.

Sex differences in behavior, response to LPS, and glucose homeostasis in middle-aged mice

Sex and age have distinct influences and roles in behavior and immune reactivity; yet, most studies use adult male rodents with little attention to middle age, a time associated with key physiological transitions in both sexes. Thus, this study investigated sex differences during middle age in behavior, immune response to lipopolysaccharide (LPS), and glucose regulation in C57BL/6 mice with GFP-tagged monocytes/microglia. Behaviorally, males performed better in tests of motor function (Open Field [OF], Grip Strength, Sticker Removal, Gait, and Pole tests) and displayed less depressive- and anxiety-like behaviors across multiple mood tests (OF, Elevated Zero Maze, Sucrose Preference, and Swim test). However, females performed better in tests of cognition (Barnes Maze and Novel Object Recognition). Following behavioral assessment, mice were given LPS to characterize sex-dependent inflammagen responses. Females displayed greater sickness behavior in the OF, higher levels of peripheral cytokines, and subtle neuroinflammation in the cortex, striatum, and hippocampus. A separate middle-aged cohort was used for glucose tolerance and insulin sensitivity testing. Both sexes had excessive blood glucose rebound after insulin challenge, but displayed differences following glucose administration, where males had higher baseline glucose and females remained hyperglycemic. This study suggests that during middle-age male mice have better emotional regulation and motor function, but not cognitive ability than females. Further, males are less sensitive than females to the acute effects of LPS peripherally and centrally, but both sexes showed sex-specific impairments in blood glucose regulation. Overall, it appears that middle age is an important transition point with multiple sex differences, some of which are unique to this stage of life.

Adolescent Alcohol Exposure Results in Sex-specific Alterations in Conditioned Fear Learning and Memory in Adulthood

The present study used auditory fear conditioning to assess the impact of repeated binge-like episodes of alcohol exposure during adolescence on conditioned fear in adulthood. Male and female Long-Evans rats were subjected to adolescent intermittent ethanol (AIE) exposure by vapor inhalation between post-natal day 28 and 44. After aging into adulthood, rats then underwent fear conditioning by exposure to a series of tone-shock pairings. This was followed by cued-tone extinction training, and then testing of fear recovery. In male rats, AIE exposure enhanced conditioned freezing but did not alter the time-course of extinction of cued-tone freezing. During subsequent assessment of fear recovery, AIE exposed rats exhibited less freezing during contextual fear renewal, but greater freezing during extinction recall and spontaneous recovery. Compared to males, female rats exhibited significantly lower levels of freezing during fear conditioning, more rapid extinction of freezing behavior, and significantly lower levels of freezing during the tests of fear recovery. Unlike males that were all classified as high conditioners; female rats could be parsed into either a high or low conditioning group. However, irrespective of their level of conditioned freezing, both the high and low conditioning groups of female rats exhibited rapid extinction of conditioned freezing behavior and comparatively low levels of freezing in tests of fear recovery. Regardless of group classification, AIE had no effect on freezing behavior in female rats during acquisition, extinction, or fear recovery. Lastly, exposure of male rats to the mGlu5 positive allosteric modulator CDPPB prevented AIE-induced alterations in freezing. Taken together, these observations demonstrate sex-specific changes in conditioned fear behaviors that are reversible by pharmacological interventions that target mGlu5 receptor activation.

[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.

Behavior and Fos activation reveal that male and female rats differentially assess affective valence during CTA learning and expression

Females are more affected by psychiatric illnesses including eating disorders, depression, and post-traumatic stress disorder than males. However, the neural mechanisms mediating these sex differences are poorly understood. Animal models can be useful in exploring such neural mechanisms. Conditioned taste aversion (CTA) is a behavioral task that assesses how animals process the competition between associated reinforcing and aversive stimuli in subsequent task performance, a process critical to healthy behavior in many domains. The purpose of the present study was to identify sex differences in this behavior and associated neural responses. We hypothesized that females would value the rewarding stimulus (Boost®) relative to the aversive stimulus (LiCl) more than males in performing CTA. We evaluated behavior (Boost® intake, LiCl-induced behaviors, ultrasonic vocalizations (USVs), CTA performance) and Fos activation in relevant brain regions after the acute stimuli [acute Boost® (AB), acute LiCl (AL)] and the context-only task control (COT), Boost® only task (BOT) and Boost®-LiCl task (BLT). Acutely, females drank more Boost® than males but showed similar aversive behaviors after LiCl. Females and males performed CTA similarly. Both sexes produced 55 kHz USVs anticipating BOT and inhibited these calls in the BLT. However, more females emitted both 22 kHz and 55 kHz USVs in the BLT than males: the latter correlated with less CTA. Estrous cycle stage also influenced 55 kHz USVs. Fos responses were similar in males and females after AB or AL. Females engaged the gustatory cortex and ventral tegmental area (VTA) more than males during the BOT and males engaged the amygdala more than females in both the BOT and BLT. Network analysis of correlated Fos responses across brain regions identified two unique networks characterizing the BOT and BLT, in both of which the VTA played a central role. In situ hybridization with RNAscope identified a population of D1-receptor expressing cells in the CeA that responded to Boost® and D2 receptor-expressing cells that responded to LiCl. The present study suggests that males and females differentially process the affective valence of a stimulus to produce the same goal-directed behavior.