Chronic stressor exposure impairs extinction of fear in adolescent rats and has associated effects on perineuronal nets and parvalbumin interneurons.

Adolescents, both human and nonhuman, exhibit impairments in the extinction of learned fear, an effect that is exacerbated, at least in rodents, by exposure to chronic stress. However, we have little understanding of the mechanisms underlying this effect. Therefore, here, we examined whether corticosterone exposure, a model of chronic stress, alters the expression of inhibitory neurons expressing parvalbumin (PV) in the basolateral amygdala and prefrontal cortex, two brain regions that have been implicated in fear extinction memories, in adolescent rats. We also examined the expression of perineuronal nets (PNNs), extracellular matrix structures that encompass inhibitory interneurons, in these two regions. These structures might render fear memories resistant to extinction by applying a structural "brake" on the plasticity of fear memories. Corticosterone-exposed adolescent rats exhibited poor extinction retention, as in past work, and were also found to have reduced percentage of PV-positive cells surrounded by PNNs in the basolateral amygdala. PV cells and PNNs were unaffected by corticosterone exposure in the prefrontal cortex. Our results suggest that the altered function of amygdala interneurons may be associated with the impaired extinction performance in stress-exposed adolescent rats. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

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.

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.

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.

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.

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.

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).

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.

A precision medicine approach to pharmacological adjuncts to extinction: a call to broaden research.

There is a pressing need to improve treatments for anxiety. Although exposure-based therapy is currently the gold-standard treatment, many people either do not respond to this therapy or experience a relapse of symptoms after treatment has ceased. In recent years, there have been many novel pharmacological agents identified in preclinical research that have potential as adjuncts for exposure therapy, yet very few of these are regularly integrated into clinical practice. Unfortunately, the robust effects observed in the laboratory animal often do not translate to a clinical population. In this review, we discuss how age, sex, genetics, stress, medications, diet, alcohol, and the microbiome can vary across a clinical population and yet are rarely considered in drug development. While not an exhaustive list, we have focused on these factors because they have been shown to influence an individual's vulnerability to anxiety and alter the neurotransmitter systems often targeted by pharmacological adjuncts to therapy. We argue that for potential adjuncts to be successfully translated from the lab to the clinic empirical research must be broadened to consider how individual difference factors will influence drug efficacy.

d-Cycloserine facilitates fear extinction in adolescent rats and differentially affects medial and lateral prefrontal cortex activation.

Adolescent humans and rodents are impaired in extinguishing learned fear relative to younger and older groups. This impairment could be due to differences in recruitment of medial prefrontal cortex (PFC), orbitofrontal cortex (OFC), or amygdala during extinction. For example, unlike juveniles and adults, adolescent rats do not express extinction-induced increases in phosphorylated mitogen activated protein kinase (pMAPK), a marker of synaptic plasticity, in the medial PFC. The NMDA receptor partial agonist d-cycloserine (DCS) improves extinction retention in adolescent rats. We investigated whether DCS affected recruitment of the PFC and amygdala during extinction by measuring pMAPK-immunoreactive (IR) neurons. Adolescent rats were trained to fear a conditioned stimulus in one context followed by extinction in a second context or equivalent context exposure only (i.e., no extinction). DCS (15 mg/kg, s.c.) or saline was administered systemically immediately after extinction training or context exposure. DCS enhanced extinction learning and this was associated with increased activation of the MAPK signaling pathway in the OFC after extinction training and increased activation in the medial PFC and amygdala at extinction retention. These findings suggest that DCS improves extinction learning in adolescents because it augments OFC contributions to extinction learning, enabling better medial prefrontal contributions to extinction retention.

Differences in the persistence of spatial memory deficits induced by a chronic stressor in adolescents compared to juveniles.

Adolescence is thought of as a stress-sensitive developmental period. While many studies have compared adolescent responses to stress relative to that of adults, a growing body of work has examined stress responses in juveniles. Here we investigated if a chronic stressor has a differential effect on spatial memory in rats depending on whether it occurs during adolescence or the juvenile period. Male rats were exposed to the stress hormone corticosterone (Cort) in their drinking water, a vehicle control (2.5% ethanol), or water, for 7 days before being tested on a novel Object/Place task 6 days or 6 weeks later. Exposure to Cort or ethanol at either age impaired spatial memory at the 6-day test. The ethanol induced impairment was attenuated 6 weeks later. However, rats given Cort during adolescence, but not the juvenile period, were still impaired. Together, these results suggest that adolescence is indeed a stress-sensitive period.

Elucidating the mechanisms of fear extinction in developing animals: a special case of NMDA receptor-independent extinction in adolescent rats.

NMDA receptors (NMDARs) are considered critical for the consolidation of extinction but recent work challenges this assumption. Namely, NMDARs are not required for extinction retention in infant rats as well as when extinction training occurs for a second time (i.e., reextinction) in adult rats. In this study, a possible third instance of NMDAR-independent extinction was tested. Although adolescents typically exhibit impaired extinction retention, rats that are conditioned as juveniles and then given extinction training as adolescents (JuvCond-AdolesExt) have good extinction retention. Unexpectedly, this good extinction retention is not associated with an up-regulation of a synaptic plasticity marker in the medial prefrontal cortex, a region implicated in extinction consolidation. In the current study, rats received either the noncompetitive NMDAR antagonist MK801 (0.1 mg/kg, s.c.) or saline before extinction training. In several experiments, rats conditioned and extinguished as juveniles, adolescents, or adults exhibited impaired extinction retention after MK801 compared to saline, but this effect was not observed in JuvCond-AdolesExt rats. Further experiments ruled out several alternative explanations for why NMDAR antagonism did not affect extinction retention in adolescents extinguishing fear learned as a juvenile. These results illustrate yet another circumstance in which NMDARs are not required for successful extinction retention and highlight the complexity of fear inhibition across development.

The impact of obesity and hypercaloric diet consumption on anxiety and emotional behavior across the lifespan.

Obesity is an increasing problem in young people. Childhood obesity and overweight have increased rapidly on a global scale, and have tripled in the past 30 years, to affect approximately one in five children. Diets high in refined fats and sugar are a major contributor to the development of obesity, and the effects of such obesity-inducing hypercaloric diets on brain function may contribute to the high prevalence of anxiety disorders in people with obesity. Anxiety disorders typically emerge in childhood and adolescence, and symptoms often continue into adulthood. Based on this symptomology, we consider anxiety-related behavioral consequences of hypercaloric diets across development. We review research on the effects of hypercaloric dietary manipulations across the lifespan on emotion regulation and the neurobiological mechanisms that underpin these processes. Cumulatively, the findings reveal that gestation and the juvenile/adolescent developmental periods may be early-life windows of vulnerability for developing anxiety in later life due to the augmented effects of these diets on neuroendocrine stress systems and the maturation of neural circuitry supporting emotion regulation.

The development of perineuronal nets around parvalbumin gabaergic neurons in the medial prefrontal cortex and basolateral amygdala of rats.

Perineuronal nets (PNNs) are extracellular matrix structures that preferentially surround mature GABAergic neurons that express the calcium-binding protein parvalbumin (PV). It has been suggested that aberrant PNN formation in humans may contribute to psychological disorders, many of which emerge during childhood and adolescence. The present experiment investigated the normative developmental trajectory of PNN formation in the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) in juvenile (P24), adolescent (P35-36), and adult (∼P70) rats. Dual-immunofluorescence staining revealed that there was a marked increase in the number of PNNs in both the prelimbic and infralimbic regions of the mPFC across the transition from the juvenile to adolescent period. Although there were no differences in the number of PV neurons across age groups, adolescent and adult rats had more PNNs surrounding PV neurons than juveniles. In contrast to the mPFC, juvenile and adolescent rats had similar total numbers of PNNs in the BLA, and total numbers of PNNs were even higher in adults in this region. In the BLA, adults had more PNNs around non-PV cells whereas the number of PV cells with PNNs did not differ across ages. However, expression patterns differed within subregions of the BLA such that adults had the most PNNs around both PV and non-PV cells in the lateral nucleus, with no age differences observed in the basal nucleus. These findings demonstrate that the juvenile to adolescent developmental period is an important time for the formation of prefrontal PNNs and the maturation of PV inhibitory neurons. (PsycINFO Database Record

Pharmacological evidence that a failure to recruit NMDA receptors contributes to impaired fear extinction retention in adolescent rats.

Adolescents, both humans and rodents, exhibit a marked impairment in extinction of fear relative to younger and older groups which could be caused by a failure to efficiently recruit NMDA receptors (NMDARs) in adolescence. It is well-established that systemic administration of NMDAR antagonists (e.g., MK801) before extinction training impairs the retention of extinction in adult and juvenile rodents, but it is unknown whether this is also the case for adolescents. Therefore, in the present study we investigated the effect of pharmacologically manipulating the NMDAR on extinction retention in adolescent rats. When extinction retention is typically impaired (i.e., after one session of extinction training) adolescent male rats given d-cycloserine (a partial NMDAR agonist) showed enhanced extinction retention relative to saline-treated animals while animals given MK801 (a non-competitive antagonist) did not exhibit any further impairment of extinction retention relative to the controls. In a further two experiments we demonstrated that when two sessions of extinction training separated by either 4 or 24h intervals were given to adolescent rats, saline-treated animals exhibited good extinction retention and the animals given MK801 before the second session exhibited impaired extinction retention. These findings suggest that extinction in adolescence does not initially involve NMDARs and this is a likely mechanism that contributes to the impaired fear inhibition observed at this age. However, NMDARs appear to be recruited with extended extinction training or after administration of a partial agonist, both of which lead to effective extinction retention.

Impaired fear extinction retention and increased anxiety-like behaviours induced by limited daily access to a high-fat/high-sugar diet in male rats: Implications for diet-induced prefrontal cortex dysregulation.

Anxiety disorders and obesity are both common in youth and young adults. Despite increasing evidence that over-consumption of palatable high-fat/high-sugar "junk" foods leads to adverse neurocognitive outcomes, little is known about the effects of palatable diets on emotional memories and fear regulation. In the present experiments we examined the effects of daily 2h consumption of a high-fat/high-sugar (HFHS) food across adolescence on fear inhibition and anxiety-like behaviour in young adult rats. Rats exposed to the HFHS diet exhibited impaired retention of fear extinction and increased anxiety-like behaviour in an emergence test compared to rats fed a standard diet. The HFHS-fed rats displayed diet-induced changes in prefrontal cortex (PFC) function which were detected by altered expression of GABAergic parvalbumin-expressing inhibitory interneurons and the stable transcription factor ΔFosB which accumulates in the PFC in response to chronic stimuli. Immunohistochemical analyses of the medial PFC revealed that animals fed the HFHS diet had fewer parvalbumin-expressing cells and increased levels of FosB/ΔFosB expression in the infralimbic cortex, a region implicated in the consolidation of fear extinction. There was a trend towards increased IBA-1 immunoreactivity, a marker of microglial activation, in the infralimbic cortex after HFHS diet exposure but expression of the extracellular glycoprotein reelin was unaffected. These findings demonstrate that a HFHS diet during adolescence is associated with reductions of prefrontal parvalbumin neurons and impaired fear inhibition in adulthood. Adverse effects of HFHS diets on the mechanisms of fear regulation may precipitate a vulnerability in obese individuals to the development of anxiety disorders.