The impact of chronic fluoxetine treatment in adolescence or adulthood on context fear memory and perineuronal nets.

Selective serotonin reuptake inhibitors, such as fluoxetine (Prozac), are commonly prescribed pharmacotherapies for anxiety. Fluoxetine may be a useful adjunct because it can reduce the expression of learned fear in adult rodents. This effect is associated with altered expression of perineuronal nets (PNNs) in the amygdala and hippocampus, two brain regions that regulate fear. However, it is unknown whether fluoxetine has similar effects in adolescents. Here, we investigated the effect of fluoxetine exposure during adolescence or adulthood on context fear memory and PNNs in the basolateral amygdala (BLA), the CA1 subregion of the hippocampus, and the medial prefrontal cortex in rats. Fluoxetine impaired context fear memory in adults but not in adolescents. Further, fluoxetine increased the number of parvalbumin (PV)-expressing neurons surrounded by a PNN in the BLA and CA1, but not in the medial prefrontal cortex, at both ages. Contrary to previous reports, fluoxetine did not shift the percentage of PNNs toward non-PV cells in either the BLA or CA1 in the adults, or adolescents. These findings demonstrate that fluoxetine differentially affects fear memory in adolescent and adult rats but does not appear to have age-specific effects on PNNs.

Effects of social buffering on fear extinction in adolescent rats.

Across social species, the presence of another individual can reduce stress reactions to adverse stimuli, a phenomenon known as social buffering. The present study investigated whether social buffering influences the expression and extinction of learned fear in adolescence, a developmental period of diminished fear inhibition and increased social interaction. Quality of maternal care and degree of social investigation were examined as factors that may influence social buffering. In adolescence, male rats were fear conditioned and then given extinction training either in the presence of a same-age rat or alone. Animals were then tested alone for extinction retention. In two experiments, the presence of a conspecific robustly reduced conditioned fear responses during extinction training. Interestingly, a persistent social buffering effect was observed when the extinction and conditioning contexts had prominent differences in features (Experiment 1), but not when these contexts were relatively similar (Experiment 2). Neither quality of maternal care nor degree of social investigation predicted the effects of social buffering. These findings suggest that social buffering robustly dampens fear responses during adolescence when a peer is present and this suppression can persist, in some instances, even when the peer is absent.

The relationship between salivary Fibroblast Growth Factor-2 and cortisol reactivity and psychological outcomes prior to and during the COVID-19 pandemic.

Background: Fibroblast growth factor-2 (FGF2) is a biomarker that is associated with depression, anxiety and stress in rodents. In humans, we have previously demonstrated that salivary FGF2 increased following stress in a similar pattern to cortisol, and FGF2 (but not cortisol) reactivity predicted repetitive negative thinking, a transdiagnostic risk factor for mental illness. The current study assessed the relationship between FGF2, cortisol, and mental health before and during the COVID-19 pandemic. Methods: We employed a longitudinal correlational design using a convenience sample. We assessed whether FGF2 and cortisol reactivity following the Trier Social Stress Task (TSST) were associated with DASS-21 depression, anxiety and stress, measured at the time of the TSST in 2019-20 (n = 87; time 1), and then again in May 2020 during the first wave of COVID-19 in Sydney (n = 34 of the original sample; time 2). Results: FGF2 reactivity (but not absolute FGF2 levels) at time 1 predicted depression, anxiety, and stress across timepoints. Cortisol reactivity at time 1 was associated with stress over timepoints, and absolute cortisol levels were associated with depression across timepoints. Limitations: The sample was comprised of mostly healthy participants from a student population, and there was high attrition between timepoints. The outcomes need to be replicated in larger, more diverse, samples. Conclusions: FGF2 and cortisol may be uniquely predictive of mental health outcomes in healthy samples, potentially allowing for early identification of at-risk individuals.

Real-World Intake of Dietary Sugars Is Associated with Reduced Cortisol Reactivity Following an Acute Physiological Stressor.

There is increasing academic and clinical interest in understanding the nature of the relation between diet and response to stress exposure as a risk factor for mental illness. Cross-species evidence shows that conditions of chronic and acute stress increase the intake of, and preference for, caloric-dense palatable foods, a phenomenon thought to be explained by the mitigating effects of comfort foods on the activity of the stress-response network. It is largely unknown whether and how real-world dietary intake of saturated fat and sugars impacts stress responsivity in humans. Therefore, here we examined whether real-world dietary intake of saturated fat and sugars predicted salivary cortisol reactivity following an acute physiological stressor. Multilevel modelling of four salivary cortisol measures collected up to 65 min after the stressor on 54 participants (18-49 years old) were analyzed using a quadratic growth curve model. Sugar intake significantly predicted a weaker cortisol response following the Cold Pressor Test (CPT) controlling for BMI and gender, revealing an inhibitory effect of caloric-dense diets on cortisol reactivity to stress. As the consumption of sugar rose individuals had lower post-stressor cortisol levels, a smaller rate of increase in cortisol 20 and 35 min after the CPT, a lower cortisol peak, and an overall weaker quadratic effect. These observations add to a growing body of evidence reporting suppressive effects of high-energy foods on stress-associated glucocorticoids reactivity and are consistent with the comfort food hypothesis, where people are seen as motivated to eat palatable foods to alleviate the detrimental repercussions of stressor exposure.

The Association Between Salivary FGF2 and Physiological and Psychological Components of the Human Stress Response.

Background: Fibroblast Growth Factor 2 (FGF2) is a neurotrophic protein that has been implicated as a biomarker for anxiety and depressive disorders, which comprise a significant component of the global burden of disease. Research using rodents has indicated that FGF2 is part of the stress response, but whether this translates to humans has yet to be investigated. In this study, we aimed to explore the potential role of FGF2 in the human stress response by examining its association with physiological and psychological processes during and following the Trier Social Stress Test (TSST). Methods: Participants in the active stress experiment (N = 87) underwent the TSST, provided saliva samples to obtain levels of cortisol and FGF2, and reported on post-event rumination related to the TSST task over the following week. Participants in the no-stress experiment (N = 25) provided saliva samples for measurement of FGF2 and cortisol across a corresponding time period. Results: Salivary FGF2 levels changed after the TSST and were associated with the pattern of change in salivary cortisol. Cortisol responses in the active stress condition were blunted in females (relative to males), however, sex did not interact with any other effect. FGF2 reactivity (ie, the magnitude of change over time) was not correlated with cortisol reactivity. Lower FGF2 reactivity following the TSST, but not overall FGF2 levels, or cortisol, was associated with higher fear of negative evaluation, repetitive negative thinking and post-event processing, as well as repetitive negative thinking in the week following the TSST. Participants in the no-stress experiment showed a decrease in cortisol, yet no change in their FGF2 levels. Conclusion: These findings suggest that FGF2 is involved in the human stress response and higher levels of FGF2 reactivity may be associated with protective cognitive processes following stress exposure.

Pharmacological Enhancement of Extinction Retention in Non-stressed Adolescent Rats but Not Those Exposed to Chronic Corticosterone.

Individuals exposed to chronic adverse experiences in childhood and adolescence are at increased risk of developing neuropsychiatric illnesses such as mood and anxiety disorders. Symptoms of anxiety disorders can often be reduced through exposure therapy, which is based on the process of extinction. Although chronic stress in adolescence is known to exacerbate the impaired extinction of learned fear during this period of development, it remains unclear whether exposure to stressors in adolescence qualitatively affects the mechanisms underlying fear extinction. Brain-derived neurotrophic factor (BDNF) and its principle receptor, tropomyosin receptor kinase B (TrkB), are involved in neuroplasticity underlying fear extinction. The small-molecule TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) improves fear extinction and reduces fear relapse (reinstatement) in adult mice when administered prior to extinction training but its effects in younger ages are unknown. In this study we tested whether 7,8-DHF enhances extinction retention and leads to less renewal in both stressed and non-stressed adolescent rats. Pre-extinction injection of 7,8-DHF led to lower levels of CS-elicited freezing in both the extinction and conditioning contexts in non-stressed adolescent male rats, but not in those given 7 days of corticosterone. These findings indicate that chronic stress interferes with the effectiveness of pharmacological agonism of TrkB in enhancing fear extinction in adolescence. A greater understanding of the mechanisms underlying extinction in adolescence and the effect of chronic corticosterone exposure on those mechanisms may inform a deeper understanding of the etiology and treatment of pediatric stress-related disorders.

Adults who are more anxious and were anxiously attached as children report later first memories.

Here, we examined retrospective reports of adults' earliest autobiographical memory, the age of this report and whether the reported age was associated with exposure to early life adversity, current anxiety and childhood attachment. Across four studies, we found that reporting a later 'earliest' memory was associated with higher self-reported anxiety in both American (Studies 1, 2 and 4) and Australian (Study 3) samples. Furthermore, in Studies 2-4, we found that reporting a later earliest memory uniquely predicted anxiety when controlling for number of adverse childhood events (a risk factor for the development of anxiety). In Study 4, we established that this relation is partially mediated by childhood anxious attachment. Although we consistently demonstrated that later earliest memories were associated with current anxiety, we found little evidence for a relation between reported age at the time of earliest memory and childhood adversity. We also found no evidence of gender differences in the associations of interest. These results suggest that poorer memory of early childhood is associated with greater childhood anxious attachment and anxiety in adulthood. The implications of this work are discussed in terms of the adaptive nature of autobiographical memory and the development of a coherent life narrative.

Fear extinction learning and retention during adolescence in rats and mice: A systematic review.

Despite exposure-based treatments being recommended for anxiety disorders, these treatments are ineffective for over half of all adolescents who receive them. The limited efficacy of exposure during adolescence may be driven by a deficit in extinction. Although indications of diminished extinction learning during adolescence were first reported over 10 years ago, these findings have yet to be reviewed and compared. This review (k = 34) found a stark inter-species difference in extinction performance: studies of adolescent mice reported deficits in extinction learning and retention of both cued and context fear. In contrast, studies of adolescent rats only reported poor extinction retention specific to cued fear. Adolescent mice and rats appeared to have only one behavioral outcome in common, being poor extinction retention of cued fear. These findings suggest that different behavioral phenotypes are present across rodent species in adolescence and highlight that preclinical work in rats and mice is not interchangeable. Further investigation of these differences offers the opportunity to better understand the etiology, maintenance, and treatment of fear-based disorders.

Maternal care, infant fear memory retention, and the moderating role of variations in separation-induced ultrasonic vocalizations.

Individual differences in parental care predict variations in offspring anxiety across species. Here, we examined whether between- and within-litter variations in maternal licking (a measure of rodent maternal care) predict infant rats' retention of an aversive association (a predictor of later anxiety-like behavior) and whether the relationship between maternal licking and infant fear memory is moderated by variations in infants' solicitation of maternal care. Unique marks were drawn on each pup, coded for fading, and touched up daily across the first week of life. Mark fading was used as an index of maternal licking where greater fading suggested more maternal licking the previous day. Separation-induced ultrasonic vocalizations (USVs) were recorded to measure individual differences in solicitation of maternal care. Infants were fear conditioned at postnatal day (P) 17 and tested for fear of the conditioned stimulus (CS) 1 week later. Across litters, mark fading negatively predicted CS-elicited fear at test for male, but not female, offspring. This relationship was moderated by number of USVs emitted at P1, such that mark fading only predicted CS-elicited fear for males that emitted a low number of USVs. These results suggest that offspring solicitation may moderate the relationship between maternal care and fear/anxiety.

It's all about who you know: Memory retention of a rat's cagemates during infancy negatively predicts adulthood hippocampal FGF2.

Recent research has demonstrated that individual differences in infant fear memory positively predict adulthood anxiety-like behavior and conditioned fear expression. However, the physiological mechanisms underlying this relationship and the effect of environmental (e.g., social) influences on the stability of this relationship have not been explored. In the present study, we examined whether individual differences in infant fear memory predict levels of endogenous fibroblast growth factor-2 (FGF2; a biomarker of fear/anxiety) in adulthood, and whether the mean memory retention of a rat's cagemates predicts conditioned fear expression and FGF2 in adulthood. We conditioned infant rats to associate a white noise with shock, and tested their memory of the association 1 week later. They were then weaned and randomly assigned to cage/cagemates. In adulthood, rats received weak context conditioning (i.e., a single shock) and were tested for fear of the context the following day. Rats were then euthanized and their brains extracted to measure levels of hippocampal FGF2 protein. Across 2 experiments, an individual rat's fear memory during infancy positively predicted their own fear expression in adulthood, but the mean memory retention of their cagemates did not predict fear expression. In contrast, the mean memory retention of a rat's cagemates during infancy negatively predicted hippocampal FGF2 protein in adulthood, but an individual rat's memory retention did not predict their own levels of FGF2. These data support the idea that variations in the fearfulness of a rat's cagemates predict individual differences on physiological measures in adulthood.

Does maternal separation accelerate maturation of perineuronal nets and parvalbumin-containing inhibitory interneurons in male and female rats?

Early life adversity impacts on a range of emotional, cognitive, and psychological processes. A recent theoretical model suggests that at least some of these effects are due to accelerated maturation of specific physiological systems and/or neural circuits. For example, maternal separation (MS), a model of early life adversity in rodents, accelerates maturation of memory systems, and here we examined its impact on maturation of perineuronal nets (PNNs) and parvalbumin (PV)-containing inhibitory interneurons. PNNs are specialized extracellular matrix structures suggested to be involved in stabilizing long-term memories and in the closure of a sensitive period in memory development. PV-containing inhibitory interneurons are the type of cell that PNNs preferentially surround, and are also thought to be involved in memory. In Experiment 1, with male rats, there was an increase in PNNs in both the amygdala and prefrontal cortex with age from infancy to juvenility. Contrary to prediction, MS had no impact on either PNN or PV expression. The same pattern was observed in female rats in Experiment 2. Taken together, these data show that the early maturation of memory in MS infants is not due to an accelerated maturation of PNNs or PV-containing cells in either the amygdala or prefrontal cortex.

Maternal Experience Does Not Predict Fear Extinction and Anxiety-Like Behaviour in Primiparous Rats Post-weaning.

Reproductive experience leads to long-lasting changes in anxiety-like behaviour and fear extinction, the laboratory model of exposure therapy for anxiety disorders. For example, fear extinction is influenced by estrous cycle in nulliparous (no reproductive experience) female rats, but this effect is abolished in primiparous (one reproductive experience) females. It is unclear whether such changes are driven by pregnancy, maternal experience of caring for offspring during the postpartum period, or a combination of both experiences. The present study sought to determine the influence of maternal experience (i.e., exposure to pups and mother-pup interactions) on fear extinction in primiparous rats. In Experiment 1, we tested whether pup exposure is necessary to mitigate estrous effects on fear extinction in primiparous rats. Age-matched nulliparous rats, primiparous rats, and primiparous rats who experienced pregnancy but not pup exposure, underwent fear conditioning on day 1 (2 months post-parturition), extinction training during proestrus (high sex hormones) or metestrus (low sex hormones) on day 2, and extinction recall on day 3. Replicating past research, nulliparous rats showed impaired extinction recall when they were extinguished during metestrus compared to proestrus. In contrast, primiparous rats with and without pup exposure showed comparable extinction recall irrespective of estrous phase. In Experiment 2, we assessed whether naturally-occurring variation in mother-pup interactions predict future fear extinction performance and anxiety-like behaviour. During the first week of lactation, primiparous rats were measured for maternal behaviours toward pups. Primiparous rats were then tested on the light-dark box and elevated plus maze to measure anxiety-like behaviour and underwent a fear extinction protocol 1 month post-weaning. We found no significant correlations between maternal behaviour and fear extinction outcomes or anxiety-like behaviour. Our findings suggest that pregnancy, not maternal experience, mitigates the impact of estrous cycle on fear extinction. In addition, natural variation in maternal experience does not appear to contribute to variability in future fear extinction outcomes or anxiety-like behaviour in primiparous rats.

Is good memory always a good thing? An early offset of infantile amnesia predicts anxiety-like behavior throughout development in rats.

Infants tend to forget experiences much more rapidly than older individuals, a phenomenon referred to as infantile amnesia. This robust, cross-species phenomenon is commonly used to examine memory development. However, in this set of experiments, we examined the novel hypothesis that the expression of infantile amnesia is related to resilience/vulnerability. We conditioned infant rats to associate a white noise with shock. Animals were tested for memory of the association ~1 week later. We found that infants that expressed better memory of the aversive association emitted more vocalizations (indicative of higher levels of distress) when separated from their mother earlier in infancy (Experiment 1). Better expression of memory in infancy also predicted higher levels of conditioned fear (Experiment 2) and anxiety-like behavior (in a light-dark box; Experiment 3) in adulthood. Furthermore, probiotic-treatment administered early in development reduced anxiety-like behavior in animals that exhibited good expression of memory for an aversive association learnt in infancy (Experiment 4). However, the same treatment was ineffective if administered in adulthood. Taken together, these results suggest that individual differences in infants' memory for an aversive association predict anxiety-like behavior throughout development, and that early administration of probiotics can reduce anxiety-like behavior in "at-risk" animals.

Deficits in opioid receptor-mediated prediction error contribute to impaired fear extinction during adolescence.

Adolescent-onset anxiety disorders are more common and costly than those that emerge later in life. Unfortunately, nearly half of adolescents undergoing cognitive behavioural therapies, including exposure therapies, show significant symptom relapse. Such poor treatment outcomes are consistent with preclinical work examining fear extinction, in which adolescents show persistent fear to extinguished cues. Both extinction and exposure are dependent on the generation of prediction error (i.e., the difference between the expected and actual outcome of a cue presentation), a process which involves the opioid system. We investigated the contribution of prediction error signalling to extinction during adolescence using the opioid receptor antagonist naloxone. We demonstrated that unlike in juvenile and adult rats, fear expression during extinction training and test in adolescent rats was unaffected by naloxone, suggesting that adolescent rats are impaired in using prediction error signalling to extinguish fear under typical conditions. However, in two circumstances where adolescents exhibit good extinction retention, opioid receptor blockade impaired extinction retention, suggesting that the recruitment of prediction error signalling mechanisms promotes extinction in this age group, just as it does in adults. Importantly, additional extinction training may be required to enable prediction error mechanisms to be recruited during adolescence.

Developmental differences in the effects of CB1/2R agonist WIN55212-2 on extinction of learned fear.

Adolescence is characterised by substantial changes in emotion regulation and, in particular, impaired extinction consolidation and retention. In this study, we replicated the well-established finding that increasing the activation of cannabinoid receptor 1 (CB1R) via the agonist WIN55212-2 improves fear extinction in adult rodents before examining whether this adjunct would also rescue the extinction retention deficit seen in adolescent rodents. Contrary to the effects in adults, we found that WIN55212-2 impaired within-session acquisition of extinction in adolescent rats with no effect on extinction retention. The same effects of WIN55212-2 were observed for juvenile rats, and did not vary as a function of drug dose. Increased fear expression observed during extinction training was not a result of altered locomotor or anxiety-like behaviour in adolescent rats, as assessed by the open field test. Lastly, we observed a linear decrease in CB1R protein expression across age (i.e., from juveniles, to adolescents, and adults) in both the medial prefrontal cortex and amygdala, two regions implicated in fear expression and extinction, suggesting that there is continued refinement of the endocannabinoid system across development in two regions involved in extinction. Our findings suggest that the expression and extinction of fear in developing rats is differentially affected by CB1R agonism due to an immature endocannabinoid system.

Timing is everything: Developmental differences in the effect of chronic corticosterone exposure on extinction retention.

Adolescence is noted as a time of "storm and stress." In this developmental stage both rodents and humans exhibit an impairment in the extinction of learned fear; however, this impairment can be alleviated, at least in rodents, by increasing the amount of extinction training given or by administering the partial NMDA receptor agonist D-Cycloserine. In the present study we explored whether the benefits of these treatments would be reduced by chronic exogenous corticosterone (a commonly studied stress-related hormone). In 2 experiments, adolescent rats were given pairings of a white noise and shock (acquisition) and then given extinction training (white noise presented alone). In Experiment 1, adolescents exhibited impaired extinction retention even after 2 days of extinction training if they had been exposed to corticosterone in adolescence but not if the exposure occurred when they were juveniles. In Experiment 2, exposure to exogenous corticosterone in adolescence, but not during the juvenile period, reduced the efficacy of the pharmacological adjunct D-Cycloserine at enhancing extinction retention after 1 day of extinction training. Taken together, the results support the idea that adolescence is a time of particular susceptibility to elevated levels of the stress hormone corticosterone. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Early-life stress, microbiota, and brain development: probiotics reverse the effects of maternal separation on neural circuits underpinning fear expression and extinction in infant rats.

Early-life stress has pervasive, typically detrimental, effects on physical and mental health across the lifespan. In rats, maternal-separation stress results in premature expression of an adult-like profile of fear regulation that predisposes stressed rats to persistent fear, one of the hallmarks of clinical anxiety. Probiotic treatment attenuates the effects of maternal separation on fear regulation. However, the neural pathways underlying these behavioral changes are unknown. Here, we examined the neural correlates of stress-induced alterations in fear behavior and their reversal by probiotic treatment. Male Sprague-Dawley rats were exposed to either standard rearing conditions or maternal-separation stress (postnatal days [P] 2-14). Some maternally-separated (MS) animals were also exposed to probiotics (Lactobacillus rhamnosus and L. helveticus) via the maternal drinking water during the period of stress. Using immunohistochemistry, we demonstrated that stressed rat pups prematurely exhibit adult-like engagement of the medial prefrontal cortex during fear regulation, an effect that can be prevented using a probiotic treatment. The present results add to the cross-species evidence that early adversity hastens maturation in emotion-related brain circuits. Importantly, our results also demonstrate that the precocious neural maturation in stressed infants is prevented by a non-invasive probiotic treatment.

Maturational Changes in Prefrontal and Amygdala Circuits in Adolescence: Implications for Understanding Fear Inhibition during a Vulnerable Period of Development.

Anxiety disorders that develop in adolescence represent a significant burden and are particularly challenging to treat, due in no small part to the high occurrence of relapse in this age group following exposure therapy. This pattern of persistent fear is preserved across species; relative to those younger and older, adolescents consistently show poorer extinction, a key process underpinning exposure therapy. This suggests that the neural processes underlying fear extinction are temporarily but profoundly compromised during adolescence. The formation, retrieval, and modification of fear- and extinction-associated memories are regulated by a forebrain network consisting of the prefrontal cortex (PFC), the amygdala, and the hippocampus. These regions undergo robust maturational changes in early life, with unique alterations in structure and function occurring throughout adolescence. In this review, we focus primarily on two of these regions-the PFC and the amygdala-and discuss how changes in plasticity, synaptic transmission, inhibition/excitation, and connectivity (including modulation by hippocampal afferents to the PFC) may contribute to transient deficits in extinction retention. We end with a brief consideration of how exposure to stress during this adolescent window of vulnerability can permanently disrupt neurodevelopment, leading to lasting impairments in pathways of emotional regulation.