A Dynamic Memory Systems Framework for Sex Differences in Fear Memory

Obesogenic diet induces sex-specific alterations of contextual fear memory and associated hippocampal activity in mice

In addition to metabolic and cardiovascular disorders, obesity is associated with cognitive deficits in humans and animal models. We have previously shown that obesogenic high-fat and sugar diet intake during adolescence (adoHFSD) impairs hippocampus (HPC)-dependent memory in rodents. These results were obtained in males only and it remains to evaluate whether adoHFSD has similar effect in females. Therefore, here, we investigated the effects of adoHFSD consumption on HPC-dependent contextual fear memory and associated brain activation in male and female mice. Exposure to adoHFSD increased fat mass accumulation and glucose levels in both males and females but impaired contextual fear memory only in males. Compared with females, contextual fear conditioning induced higher neuronal activation in the dorsal and ventral HPC (CA1 and CA3 subfields) as well as in the medial prefrontal cortex in males. Also, adoHFSD-fed males showed enhanced c-Fos expression in the dorsal HPC, particularly in the dentate gyrus, and in the basolateral amygdala compared with the other groups. Finally, chemogenetic inactivation of the dorsal HPC rescued adoHFSD-induced memory deficits in males. Our results suggest that males are more vulnerable to the effects of adoHFSD on HPC-dependent aversive memory than females, due to overactivation of the dorsal HPC.

Sex difference in the facilitation of fear learning by prior fear conditioning

There is now ample evidence that the strength and underlying mechanisms of memory formation can be drastically altered by prior experience. However, the prior work using rodent models on this topic has used only males as subjects, and as a result, we do know whether or not the effects of prior experience on subsequent learning are similar in both sexes. As a first step towards addressing this shortcoming rats of both sexes were given auditory fear conditioning, or fear conditioning with unsignaled shocks, followed an hour or a day later by a single pairing of light and shock. Fear memory for each experience was assessed by measuring freezing behavior to the auditory cue and fear-potentiated startle to the light. Results showed that males trained with auditory fear conditioning showed facilitated learning to the subsequent visual fear conditioning session when the two training sessions were separated by one hour or one day. Females showed evidence of facilitation in rats given auditory conditioning when they were spaced by an hour, but not when they were spaced by one day. Contextual fear conditioning did not support the facilitation of subsequent learning under any conditions. These results indicate that the mechanism by which prior fear conditioning facilitates subsequent learning differs between sexes, and they set the stage for mechanistic studies to understand the neurobiological basis of this sex difference.

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.

Sexually Dimorphic Alterations in the Transcriptome and Behavior with Loss of Histone Demethylase KDM5C

Chromatin dysregulation has emerged as a major hallmark of neurodevelopmental disorders such as intellectual disability (ID) and autism spectrum disorders (ASD). The prevalence of ID and ASD is higher in males compared to females, with unknown mechanisms. Intellectual developmental disorder, X-linked syndromic, Claes-Jensen type (MRXSCJ), is caused by loss-of-function mutations of lysine demethylase 5C (KDM5C), a histone H3K4 demethylase gene. KDM5C escapes X-inactivation, thereby presenting at a higher level in females. Initially, MRXSCJ was exclusively reported in males, while it is increasingly evident that females with heterozygous KDM5C mutations can show cognitive deficits. The mouse model of MRXSCJ, male Kdm5c-hemizygous knockout animals, recapitulates key features of human male patients. However, the behavioral and molecular traits of Kdm5c-heterozygous female mice remain incompletely characterized. Here, we report that gene expression and behavioral abnormalities are readily detectable in Kdm5c-heterozygous female mice, demonstrating the requirement for a higher KDM5C dose in females. Furthermore, we found both shared and sex-specific consequences of a reduced KDM5C dose in social behavior, gene expression, and genetic interaction with the counteracting enzyme KMT2A. These observations provide an essential insight into the sex-biased manifestation of neurodevelopmental disorders and sex chromosome evolution.

Lactobacillus reuteri normalizes altered fear memory in male Cntnap4 knockout mice

BACKGROUND

Autism spectrum disorder (ASD) is a common neurodevelopmental disease, characterized by deficits in social communication, restricted and repetitive behaviours, and impaired fear memory processing. Severe gastrointestinal dysfunction and altered gut microbiome have been reported in ASD patients and animal models. Contactin associated protein-like 4 (CNTNAP4) has been suggested to be a novel risk gene, though its role in ASD remains unelucidated.

METHODS

Cntnap4^-/- mice were generated to explore its role in ASD-related behavioural abnormalities. Electrophysiological recording was employed to examine GABAergic transmission in the basolateral amygdala (BLA) and prefrontal cortex. RNA-sequencing was performed to assess underlying mechanisms. 16S rDNA analysis was performed to explore changes in faecal microbial composition. Male Cntnap4^-/- mice were fed with Lactobacillus reuteri (L. reuteri) or faecal microbiota to evaluate the effects of microbiota supplementation on the impaired fear conditioning mediated by Cntnap4 deficiency.

FINDINGS

Male Cntnap4^-/- mice manifested deficiency in social behaviours and tone-cue fear conditioning. Notably, reduced GABAergic transmission and GABA receptor expression were found in the BLA but not the prefrontal cortex. In addition, gut Lactobacillus were less abundant in male Cntnap4^-/- mice, and L. reuteri treatment or faecal microbiota transplantation rescued abnormal tone-cued fear memory and improved local GABAergic transmission in the BLA of male Cntnap4^-/- mice.

INTERPRETATION

Cntnap4 shapes GABAergic transmission of amygdala and fear conditioning, and microbial intervention represents a promising therapy in ASD intervention.

FUNDING

National Natural Science Foundation of China, Science and Technology Planning Project of Guangzhou, Guangzhou Medical University, and China Postdoctoral Science Foundation.

Hormonal contraceptives, stress, and the brain: The critical need for animal models

Hormonal contraceptives are among the most important health and economic developments in the 20thCentury, providing unprecedented reproductive control and a range of health benefits including decreased premenstrual symptoms and protections against various cancers. Hormonal contraceptives modulate neural function and stress responsivity. These changes are usually innocuous or even beneficial, including their effects onmood. However, in approximately 4-10% of users, or up to 30 million people at any given time, hormonal contraceptives trigger depression or anxiety symptoms. How hormonal contraceptives contribute to these responses and who is at risk for adverse outcomes remain unknown. In this paper, we discussstudies of hormonal contraceptive use in humans and describe the ways in which laboratory animal models of contraceptive hormone exposure will be an essential tool for expanding findings to understand the precise mechanisms by which hormonal contraceptives influence the brain, stress responses, and depression risk.

Hormonal contraceptive influences on cognition and psychopathology: Past methods, present inferences, and future directions

In the last decade, there has been a remarkable surge in research on the neural and behavioral correlates of hormonal contraceptive use, particularly oral contraceptive use. Questions have evolved swiftly and notably, with studies no longer revealing if hormonal contraceptives matter for the brain and behavior, but rather how, when, and for whom they matter most. Paralleling this shift, the goal of this review is to move beyond an average synthesis of hormonal contraceptive influences on human cognition and psychopathology (and their neural substrates) in order to consider the nature and specificity of effects. Accompanied by an evaluation of study methods and informed by findings from animal models, this consideration uncovers promising areas of research in the next ten years, including potential activational and organizational effects of hormonal contraceptive use, individual differences in effects that matter for the wellbeing of unique individuals, and correlates of intrauterine device use.

The epigenetic role of proteasome subunit RPT6 during memory formation in female rats

Reports of sex differences in the neurobiology of memory formation are becoming more prevalent. Despite this, much remains unknown about the role of sex in this process. We previously reported the first evidence of a novel epigenetic role for proteasome subunit RPT6 during memory formation in the hippocampus of male rodents whereby it associated with monoubiquitinated histone H2B (H2Bubi). Here, we used molecular, biochemical, and behavioral approaches to investigate whether RPT6 has a similar epigenetic role during memory formation in female rats. Following contextual fear conditioning, we found that RPT6 levels and DNA binding at regions coding for c-fos, the previously identified target of RPT6 in males, were unchanged in the hippocampus of females and that loss of RPT6 did not alter learning-induced increases in c-fos However, RPT6 was in complex with H2Bubi in the female hippocampus and this association increased with fear conditioning, suggesting that it could still retain an epigenetic function. Consistent with this, hippocampal siRNA-mediated knockdown of the RPT6-coding gene, Psmc5, impaired memory in females. These results suggest that while RPT6 does associate with epigenetic H2Bubi during memory formation in both males and females, it has sex-specific gene targets during the memory consolidation process.

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.

Sex and age differences in cognitive bias and neural activation in response to cognitive bias testing

Cognitive symptoms of depression, including negative cognitive bias, are more severe in women than in men. Current treatments to reduce negative cognitive bias are not effective and sex differences in the neural activity underlying cognitive bias may play a role. Here we examined sex and age differences in cognitive bias and functional connectivity in a novel paradigm. Male and female rats underwent an 18-day cognitive bias procedure, in which they learned to discriminate between two contexts (shock paired context A, no-shock paired context B), during either adolescence (postnatal day (PD 40)), young adulthood (PD 100), or middle-age (PD 210). Cognitive bias was measured as freezing behaviour in response to an ambiguous context (context C), with freezing levels akin to the shock paired context coded as negative bias. All animals learned to discriminate between the two contexts, regardless of sex or age. However, adults (young adults, middle-aged) displayed a greater negative cognitive bias compared to adolescents, and middle-aged males had a greater negative cognitive bias than middle-aged females. Females had greater neural activation of the nucleus accumbens, amygdala, and hippocampal regions to the ambiguous context compared to males, and young rats (adolescent, young adults) had greater neural activation in these regions compared to middle-aged rats. Functional connectivity between regions involved in cognitive bias differed by age and sex, and only adult males had negative correlations between the frontal regions and hippocampal regions. These findings highlight the importance of examining age and sex when investigating the underpinnings of negative cognitive bias and lay the groundwork for determining what age- and sex-specific regions to target in future cognitive bias studies.

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Middle-aged males had a greater negative cognitive bias than middle-aged females.

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Adult rats displayed a greater negative cognitive bias compared to adolescents.

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Greater neural activity in females than males in limbic and reward regions.

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Greater role of the frontal cortex activation in the cognitive bias of adults.

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Functional connectivity in response to cognitive bias differed by age and sex.

Course and Predictors of Posttraumatic Stress Disorder in the Canadian Armed Forces: A Nationally Representative, 16-Year Follow-up Study: Cours et prédicteurs du trouble de stress post-traumatique dans les Forces armées canadiennes: une étude de suivi de 16 ans nationalement représentative

OBJECTIVE

This study examined baseline risk and protective predictors and interim correlates of the persistence/recurrence, remission, and onset of posttraumatic stress disorder (PTSD) in a 16-year prospective, nationally representative sample of Canadian Forces members and veterans.

METHODS

The 2018 Canadian Armed Forces Members and Veterans Mental Health Follow-up Survey is a prospective study of 2,941 regular force service members and veterans who participated in the 2002 Canadian Community Health Survey on Mental Health and Wellbeing: Canadian Forces Supplement (n = 5,155; ages 15 to 64 years; response rate 68%). PTSD diagnoses in 2002 and 2018 were used to create 4 groups: (1) no lifetime, (2) remitted, (3) new onset, and (4) persistent/recurrent PTSD. Multinomial regressions were conducted to identify predictors of PTSD courses.

RESULTS

Female sex, being a junior noncommissioned member (vs. officer), and land (vs. air) operations in 2002 were associated with all PTSD courses relative to no lifetime PTSD (relative risk ratio [RRR] range: 1.28 to 3.65). After adjusting for sociodemographic variables, baseline predictors of all PTSD courses included lifetime mental disorder, history of mental health care utilization, all trauma type categories (deployment-associated, sexual, "other"), and the number of lifetime traumatic events (RRR range: 1.14 to 8.95). New ("since 2002") traumas, transitioning to veteran status, and alcohol dependence were mostly associated with the new onset and persistent/recurrent PTSD courses (RRR range: 1.79 to 4.31), while mental health care utilization and greater avoidance coping were associated with all PTSD courses (RRR range: 1.10 to 17.87). Protective factors for several PTSD courses at one or both time points included social support, social network size, and problem-focused coping (RRR range: 0.71 to 0.98).

CONCLUSIONS

This is the first population-based survey to examine the longitudinal course of PTSD in Canadian Forces members. Prevention and intervention programs focused on bolstering social support and active coping strategies as possible protective factors/correlates may help mitigate the development and persistence of PTSD.

Three's Company: Neuroimmune activation, sex, and memory at the tripartite synapse

The neuroimmune system is required for normal cognitive functions such as learning and memory in addition to its critical role in detecting and responding to invading pathogens and injury. Understanding the functional convergence of neurons, astrocytes, and microglia at the synapse, particularly in the hippocampus, is key to understanding the nuances of such diverse roles. In the healthy brain, communication between all three cells is important for regulating neuronal activation and synaptic plasticity mechanisms, and during neuroinflammation, the activity and functions of all three cells can produce and be modulated by inflammatory cytokines. An important remaining component to this system is the conclusive evidence of sex differences in hippocampal plasticity mechanisms, hormone modulation of synaptic plasticity, functional properties of hippocampal neurons, and in neuroimmune activation. Sex as a biological variable here is necessary to consider given sex differences in the prevalence of memory-related disorders such as Alzheimer's disease and Post-Traumatic Stress disorder, both of which present with neuroimmune dysregulation. To make meaningful progress towards a deeper understanding of sex biases in memory-related disease prevalence, I propose that the next chapter of psychoneuroimmune research must focus on the signal integration and transduction at the synapse between experience-dependent plasticity mechanisms, neuroimmune activation, and the influence of biological sex.

Age-Related Memory Impairment and Sex-Specific Alterations in Phosphorylation of the Rpt6 Proteasome Subunit and Polyubiquitination in the Basolateral Amygdala and Medial Prefrontal Cortex

Aging is marked by an accumulation of damaged and modified brain proteins, and the ubiquitin-proteasome system (UPS) is important for cellular protein degradation. Recent work has established a critical role for the UPS in memory and synaptic plasticity, but the role of the UPS in age-related cognitive decline remains poorly understood. We trained young, middle-aged, and aged male and female rats using trace fear conditioning (TFC) to investigate the effects of age and sex on memory. We then measured markers of UPS-related protein degradation (phosphorylation of the Rpt6 proteasome regulatory subunit and K48-linked polyubiquitination) using western blots. We found that aged males, but not aged females, showed behavioral deficits at memory retrieval. Aged males also displayed reduced phosphorylation of the Rpt6 proteasome subunit and accumulation of K48 in the basolateral amygdala, while aged females displayed a similar pattern in the medial prefrontal cortex. These findings suggest that markers of UPS function are differentially affected by age and sex in a brain region-dependent manner. Together these results provide an important step toward understanding the UPS and circuit-level differences in aging males and females.

Aplnr knockout mice display sex-specific changes in conditioned fear

The G-protein-coupled receptor APLNR and its ligands apelin and ELABELA/TODDLER/apela comprise the apelinergic system, a signaling pathway that is critical during development and physiological homeostasis. Targeted regulation of the receptor has been proposed to treat several important diseases including heart failure, pulmonary arterial hypertension and metabolic syndrome. The apelinergic system is widely expressed within the central nervous system (CNS). However, the role of this system in the CNS has not been completely elucidated. Utilizing an Aplnr knockout mouse model, we report here results from tests of sensory ability, locomotion, reward preference, social preference, learning and memory, and anxiety. We find that knockout of Aplnr leads to significant effects on acoustic startle response and sex-specific effects on conditioned fear responses without significant changes in baseline anxiety. In particular, male Aplnr knockout mice display enhanced context- and cue-dependent fear responses. Our results complement previous reports that exogenous Apelin administration reduced conditioned fear and freezing responses in rodent models, and future studies will explore the therapeutic benefit of APLNR-targeted drugs in rodent models of PTSD.

On the basis of sex: Differences in safety discrimination vs. conditioned inhibition

Inaccurate discrimination between threat and safety cues is a common symptom of anxiety disorders such as Post-Traumatic Stress Disorder (PTSD). Although females experience higher rates of these disorders than males, the body of literature examining sex differences in safety learning is still growing. Learning to discriminate safety cues from threat cues requires downregulating fear to the safety cue while continuing to express fear to the threat cue. However, successful discrimination between safety and threat cues does not necessarily guarantee that the safety cue can effectively reduce fear to the threat cue when they are presented together. The conditioned inhibitory ability of a safety cue to reduce fear in the presence of both safety and threat is most likely dependent on the ability to discriminate between the two. There are relatively few studies exploring conditioned inhibition as a method of safety learning. Adding to this knowledge gap is the general lack of inclusion of female subjects within these studies. In this review, we provide a qualitative review of our current knowledge of sex differences in safety discrimination versus conditioned inhibition in both humans and rodents. Overall, the literature suggests that while females and males perform similarly in discrimination learning, females show deficits in conditioned inhibition compared to males. Furthermore, while estrogen appears to have a protective effect on safety learning in humans, increased estrogen in female rodents appears to be correlated with impaired safety learning performance.

Cacna1c Hemizygosity Results in Aberrant Fear Conditioning to Neutral Stimuli

CACNA1C, a gene that encodes an alpha-1 subunit of L-type voltage-gated calcium channels, has been strongly associated with psychiatric disorders including schizophrenia and bipolar disorder. An important objective is to understand how variation in this gene can lead to an increased risk of psychopathology. Altered associative learning has also been implicated in the pathology of psychiatric disorders, particularly in the manifestation of psychotic symptoms. In this study, we utilize auditory-cued fear memory paradigms in order to investigate whether associative learning is altered in rats hemizygous for the Cacna1c gene. Cacna1c hemizygous (Cacna1c+/-) rats and their wild-type littermates were exposed to either delay, trace, or unpaired auditory fear conditioning. All rats received a Context Recall (24 h post-conditioning) and a Cue Recall (48 h post-conditioning) to test their fear responses. In the delay condition, which results in strong conditioning to the cue in wild-type animals, Cacna1c+/- rats showed increased fear responses to the context. In the trace condition, which results in strong conditioning to the context in wild-type animals, Cacna1c+/- rats showed increased fear responses to the cue. Finally, in the unpaired condition, Cacna1c+/- rats showed increased fear responses to both context and cue. These results indicate that Cacna1c heterozygous rats show aberrantly enhanced fear responses to inappropriate cues, consistent with key models of psychosis.

Sex-specific effects of the histone variant H2A.Z on fear memory, stress-enhanced fear learning and hypersensitivity to pain

Emerging evidence suggests that histone variants are novel epigenetic regulators of memory, whereby histone H2A.Z suppresses fear memory. However, it is not clear if altered fear memory can also modify risk for PTSD, and whether these effects differ in males and females. Using conditional-inducible H2A.Z knockout (cKO) mice, we showed that H2A.Z binding is higher in females and that H2A.Z cKO enhanced fear memory only in males. However, H2A.Z cKO improved memory on the non-aversive object-in-place task in both sexes, suggesting that H2A.Z suppresses non-stressful memory irrespective of sex. Given that risk for fear-related disorders, such as PTSD, is biased toward females, we examined whether H2A.Z cKO also has sex-specific effects on fear sensitization in the stress-enhanced fear learning (SEFL) model of PTSD, as well as associated changes in pain sensitivity. We found that H2A.Z cKO reduced stress-induced sensitization of fear learning and pain responses preferentially in female mice, indicating that the effects of H2A.Z depend on sex and the type of task, and are influenced by history of stress. These data suggest that H2A.Z may be a sex-specific epigenetic risk factor for PTSD susceptibility, with implications for developing sex-specific therapeutic interventions.

Prenatal exposure to the probiotic Lactococcus lactis decreases anxiety-like behavior and modulates cortical cytoarchitecture in a sex specific manner

Development of the cerebral cortex may be influenced by the composition of the maternal gut microbiota. To test this possibility, we administered probiotic Lactococcus lactis in drinking water to mouse dams from day 10.5 of gestation until pups reached postnatal day 1 (P1). Pups were assessed in a battery of behavioral tests starting at 10 weeks old. We found that females, but not males, exposed to probiotic during prenatal development spent more time in the center of the open field and displayed decreased freezing time in cue associated learning, compared to controls. Furthermore, we found that probiotic exposure changed the density of cortical neurons and increased the density of blood vessels in the cortical plate of P1 pups. Sex-specific differences were observed in the number of mitotic neural progenitor cells, which were increased in probiotic exposed female pups. In addition, we found that probiotic treatment in the latter half of pregnancy significantly increased plasma oxytocin levels in mouse dams, but not in the offspring. These results suggest that exposure of naïve, unstressed dams to probiotic may exert sex-specific long-term effects on cortical development and anxiety related behavior in the offspring.

Sex-Specific Role for Egr3 in Nucleus Accumbens D2-Medium Spiny Neurons Following Long-Term Abstinence From Cocaine Self-administration

BACKGROUND

We previously showed that the transcription factor Egr3 (early growth response 3) is oppositely regulated in nucleus accumbens (NAc) cell subtypes 24 hours following cocaine exposure and bidirectionally mediates cocaine-related behaviors in male rodents. Overexpressing Egr3 in D₂ receptor-containing medium spiny neurons (D2-MSNs) before drug exposure reduces the rewarding and psychomotor sensitization effects of cocaine. However, it is unknown if Egr3 plays a role in long-term neuroadaptations in the NAc and relapse to cocaine seeking.

METHODS

We measured EGR3 protein levels in the NAc following 20 days of forced abstinence from intravenous cocaine self-administration in 10-week-old Sprague Dawley rats and C57BL/6 mice. In 8- to 10-week-old A2A-Cre mice, we used virally mediated Egr3 overexpression in NAc D2-MSNs to test the role of Egr3 on operant responding during seeking, extinction, and drug-induced reinstatement of cocaine self-administration. To evaluate if Egr3 contributed to sex differences to cocaine relapse, we conducted these procedures in both male and female rodents.

RESULTS

We found that EGR3 expression was reduced only in female rodents after 20 days of forced abstinence. Additionally, we showed that our self-administration paradigm in mice recapitulated the sex differences in cocaine intake and relapse demonstrated in humans and rats. Finally, whereas Egr3 overexpression in D2-MSNs during forced abstinence facilitated extinction and blunted drug-induced reinstatement in female mice, it had the opposite effect in male mice.

CONCLUSIONS

We showed that the immediate early gene Egr3 has long-term effects on drug-related behaviors. Our work suggests that changes in Egr3 expression in D2-MSNs contributes to sex differences in cocaine relapse.

The promises and pitfalls of sex difference research

Funding agencies in North America and Europe are recognizing the importance of the integration of sex differences into basic and clinical research. Although these mandates are in place to improve our knowledge of health for both men and women, there have been a number of implementation issues that require vigilance on the part of funders and the research community. Here we discuss issues on simple inclusion of both sexes in studies to specialisation of sex differences with attention paid to statistics and the need for sex-specific treatments. We suggest differing mandates need to be considered regarding simple integration versus the need for studies in the specialisation of sex differences and/or the need for research that recognises the importance of male-specific or female-specific factors that influence subsequent health such as menstruation, menopause or pregnancy.