Aripiprazole Facilitates Extinction of Conditioned Fear in Adolescent Rats

Minimizing Variability in Developmental Fear Studies in Mice: Toward Improved Replicability in the Field

In rodents, the first weeks of postnatal life feature remarkable changes in fear memory acquisition, retention, extinction, and discrimination. Early development is also marked by profound changes in brain circuits underlying fear memory processing, with heightened sensitivity to environmental influences and stress, providing a powerful model to study the intersection between brain structure, function, and the impacts of stress. Nevertheless, difficulties related to breeding and housing young rodents, preweaning manipulations, and potential increased variability within that population pose considerable challenges to developmental fear research. Here we discuss several factors that may promote variability in studies examining fear conditioning in young rodents and provide recommendations to increase replicability. We focus primarily on experimental conditions, design, and analysis of rodent fear data, with an emphasis on mouse studies. The convergence of anatomical, synaptic, physiological, and behavioral changes during early life may increase variability, but careful practice and transparency in reporting may improve rigor and consensus in the field. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC.

Intermixed safety cues facilitate extinction retention in adult and adolescent mice

Extinction learning is tremendously adaptive as it allows an animal to adjust their behavior in a changing environment. Yet, extinction is not without limitations and fear often reemerges over time (i.e. spontaneous recovery). Relative to adults, adolescent rodents and humans are particularly prone to spontaneous recovery following extinction. In this study, we aimed to address whether combining methods of fear regulation (extinction and conditioned inhibition) can facilitate extinction retention. Early adolescent (29 days old, n = 81) and adult (70 days old, n = 80) mice underwent extinction with or without a safety cue present. Safety cue presentations were systematically varied to overlap with or alternate with fear cue presentations. We found that initial safety learning was faster in adolescent mice. In addition, intermixing safety cues into extinction reduced spontaneous recovery during a test two weeks later. The decrease in spontaneous recovery relative to a standard extinction protocol was greater in adolescents than adults. Together, our findings provide initial evidence that safety learning may be inherently stronger during adolescence. These results inform the parameters by which conditioned safety and extinction learning may be merged to augment the inhibition of fear. While methods to enhance fear regulation are valuable for any age, the potential to do so during adolescence is particularly striking.

Pharmacological Implications of Adjusting Abnormal Fear Memory: Towards the Treatment of Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is a unique clinical mental abnormality presenting a cluster of symptoms in which patients primarily experience flashbacks, nightmares and uncontrollable thoughts about the event that triggered their PTSD. Patients with PTSD may also have comorbid depression and anxiety in an intractable and long-term course, which makes establishing a comprehensive treatment plan difficult and complicated. The present article reviews current pharmacological manipulations for adjusting abnormal fear memory. The roles of the central monoaminergic systems (including serotonin, norepinephrine and dopamine) within the fear circuit areas and the involvement of the hypothalamic-pituitary-adrenal (HPA) axis and glucocorticoid receptor (GR) are explored based on attempts to integrate current clinical and preclinical basic studies. In this review, we explain how these therapeutic paradigms function based on their connections to stages of the abnormal fear memory process from condition to extinction. This may provide useful translational interpretations for clinicians to manage PTSD.

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.

Ameliorating schizophrenia-like symptoms in vasopressin deficient male Brattleboro rat by chronic antipsychotic treatment

Due to its various function vasopressin has been associated with many psychiatric disorders, including schizophrenia. Our previous study confirmed that vasopressin-deficient (di/di) Brattleboro rat can be a good genetic model for schizophrenia. Our present aim was to confirm whether the treatment effects of marketed antipsychotics are similar in di/di rats to those seen in human schizophrenic patients. Chronic subcutaneous administration of aripiprazole (5 mg/kg), clozapine (1 mg/kg), haloperidol (0.1 mg/kg), olanzapine (0.3 mg/kg) or risperidone (0.25 mg/kg) was used for 15 days in control (+/+ Brattleboro) and di/di rats. Social discrimination, social avoidance and prepulse inhibition tests were conducted on day 1, 8 and 15 of the treatment. Vasopressin-deficient rats showed social memory- and sensorimotor gating deficit. All used antipsychotics successfully normalized the reduced prepulse inhibition of di/di animals. However, most were effective only after prolonged treatment. Aripiprazole, clozapine, and olanzapine normalized the social memory deficit, while the effects of haloperidol and risperidone were not unequivocal. All drugs reduced social interest to some extent both in control and in di/di animals, aripiprazole being the less implicated in this regard during the social avoidance test. The restoration of schizophrenia-like behavior by antipsychotic treatment further support the utility of the vasopressin-deficient Brattleboro rat as a good preclinical model. Reduced social interest might be a general side-effect of antipsychotics, and aripiprazole has the most favorable profile in this regard.

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

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

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Periadolescent isolation stress disrupted extinction recall in male adolescents.

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

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Exercise impaired extinction recall in isolated female adolescents.

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Exercise increased hippocampal neurogenesis, except in isolated males.

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Suppression of neurogenesis blocked exercise effects in isolated adolescents.

Aripiprazole and topiramate, alone or in combination, block the expression of ethanol-induced conditioned place preference in mice

BACKGROUND

Ethanol is the most largely consumed drug in the world. Because of its complex mechanisms of action, studies suggest that the combination of drugs with distinct pharmacological effects may be a promising alternative for treating ethanol use disorder. In the present study, we aimed to investigate the effects of topiramate, alone and in combination with aripiprazole, on ethanol-induced conditioned place preference (CPP).

METHODS

Adult male mice were conditioned with ethanol (1.8 g/kg, i.p.) in the conditioned place preference (CPP) apparatus. Animals were then treated with vehicle, topiramate (2.5, 5 or 10 mg/kg, i.p.), aripiprazole (0.025, 0.05, 0.075 or 0.1 mg/kg, i.p.) or a combination of subthreshold doses of topiramate and aripiprazole (5 and 0.075 mg/kg, respectively) in the ethanol-paired compartment for 8 consecutive days. The expression of ethanol-induced CPP was then evaluated during a drug-free test performed 24 h after a re-exposure to ethanol in the ethanol-paired compartment.

RESULTS

Treatment with 10 mg/kg topiramate or 0.1 mg/kg aripiprazole blocked the expression of ethanol-induced CPP. Combined treatment with 5 mg/kg topiramate and 0.075 mg/kg aripiprazole, doses that alone were not effective, also blocked the expression of CPP to ethanol.

CONCLUSIONS

Topiramate and aripiprazole, alone or in combination, blocked the expression of ethanol-induced CPP. By showing that a combination of lower, subthreshold doses or topiramate and aripiprazole was effective in blocking the conditioned reinforcing properties of the ethanol-paired environment in mice, our current findings provide important insights into the therapeutic use of these drugs in ethanol use disorder.

Sex differences in the neurochemistry of frontal cortex: Impact of early life stress

Traumatic events during early life have been linked with later life psychopathology. To understand this risk factor, researchers have studied the effects of prenatal and postnatal early life stress on neurochemical changes. Here we review the rodent literature on sex differences and sex-specific impact of early life stress on frontal cortex neurochemistry. This region is implicated in regulating motivation and emotion, which are often disrupted in psychological disorders. The prefrontal cortex (PFC) in particular is one of the last brain regions to develop, and there are sex differences in the rate of this development. To draw direct comparisons between sexes, our review of the literature was restricted to studies where the effects of prenatal or postnatal stress had been described in male and female littermates. This literature included research describing glutamate, γ-amino butyric acid (GABA), corticosteroids, monoamines, and cannabinoids. We found that sex-dependent effects of stress are mediated by the age at which stress is experienced, age at test, and type of stress endured. More research is required, particularly into the effects of adolescent stress on male and female littermates. We hope that a greater understanding of sex-specific susceptibilities in response to stress across development will help to uncover risk factors for psychological disorders in vulnerable populations.

Assessment of conditioned fear extinction in male and female adolescent rats

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

Clinical Therapeutic Strategy and Neuronal Mechanism Underlying Post-Traumatic Stress Disorder (PTSD)

Post-traumatic stress disorder (PTSD) is characterized by an exaggerated response to contextual memory and impaired fear extinction, with or without mild cognitive impairment, learning deficits, and nightmares. PTSD is often developed by traumatic events, such as war, terrorist attack, natural calamities, etc. Clinical and animal studies suggest that aberrant susceptibility of emotion- and fear-related neurocircuits, including the amygdala, prefrontal cortex (PFC), and hippocampus may contribute to the development and retention of PTSD symptoms. Psychological and pharmacological therapy, such as cognitive behavioral therapy (CBT), and treatment with anti-depressive agents and/or antipsychotics significantly attenuate PTSD symptoms. However, more effective therapeutics are required for improvement of quality of life in PTSD patients. Previous studies have reported that ω3 long-chain polyunsaturated fatty acid (LCPUFA) supplements can suppress the development of PTSD symptoms. Fatty acid binding proteins (FABPs) are essential for LCPUFA intracellular trafficking. In this review, we have introduced Fabp3 null mice as an animal model of PTSD with impaired fear extinction. Moreover, we have addressed the neuronal circuits and novel therapeutic strategies for PTSD symptoms.

Effects of Methamphetamine Exposure on Fear Learning and Memory in Adult and Adolescent Rats

Methamphetamine (meth) use is often comorbid with anxiety disorders, with both conditions predominant during adolescence. Conditioned fear extinction is the most widely used model to study the fear learning and regulation that are relevant for anxiety disorders. The present study investigates how meth binge injections or meth self-administration affect subsequent fear conditioning, extinction and retrieval in adult and adolescent rats. In experiment 1, postnatal day 35 (P35-adolescent) and P70 (adult) rats were intraperitoneally injected with increasing doses of meth across 9 days. At P50 or P85, they underwent fear conditioning followed by extinction and test. In experiments 2a-c, P35 or P70 rats self-administered meth for 11 days then received fear conditioning at P50 or P85, followed by extinction and test. We observed that meth binge exposure caused a significant disruption of extinction retrieval in adult but not adolescent rats. Interestingly, meth self-administration in adolescence or adulthood disrupted acquisition of conditioned freezing in adulthood. Meth self-administration in adolescence did not affect conditioned freezing in adolescence. These results suggest that intraperitoneal injections of high doses of meth and meth self-administration have dissociated effects on fear conditioning and extinction during adulthood, while adolescent fear conditioning and extinction are unaffected.

Multimodal evidence for delayed threat extinction learning in adolescence and young adulthood

Previous research in rodents and humans points to an evolutionarily conserved profile of blunted threat extinction learning during adolescence, underpinned by brain structures such as the amygdala and medial prefrontal cortex (mPFC). In this study, we examine age-related effects on the function and structural connectivity of this system in threat extinction learning in adolescence and young adulthood. Younger age was associated with greater amygdala activity and later engagement of the mPFC to learned threat cues as compared to safety cues. Furthermore, greater structural integrity of the uncinate fasciculus, a white matter tract that connects the amygdala and mPFC, mediated the relationship between age and mPFC engagement during extinction learning. These findings suggest that age-related changes in the structure and function of amygdala-mPFC circuitry may underlie the protracted maturation of threat regulatory processes.

Subchronic administration of aripiprazole improves fear extinction retrieval of Pavlovian conditioning paradigm in rats experiencing psychological trauma

People may suffer from an intruded fear memory when the attributable traumatic events no longer exist. This is of highly clinical relevance to trauma-induced mental disorders, such as posttraumatic stress disorder (PTSD). Mechanism underlying PTSD largely lies in the abnormal process of fear extinction and a functional imbalance within amygdala associated fear circuit areas. Previous evidence suggested central dopamine plays a key role in the regulation of the fear memory process, yet it remains unclear whether the intervention of dopamine modulators would be beneficial for the fear extinction abnormalities. The present study examined the performance of Pavlovian conditioned fear and the changes of dopamine profiles following a subchronic 14-day regimen of aripiprazole (a partial agonist of dopamine D2 receptors to normalize the condition caused by dopamine imbalance) in rats previously experienced a psychologically traumatic procedure of single prolonged stress (SPS). The results demonstrated that aripiprazole at 5.0 mg/kg reversed the SPS-impaired fear memory dysfunction and the SPS-reduced dopamine efflux in the amygdala. The present study suggests a therapeutic potential of subchronic treatment with aripiprazole in managing patients suffered from fear extinction problem.

Poor between-session recall of extinction learning and hippocampal activation and connectivity in children

BACKGROUND

In healthy adults, successful between-session recall of extinction learning depends on the hippocampus and ventromedial prefrontal cortex (vmPFC), especially when tested in the extinction context. Poor extinction recall and dysfunction within hippocampal-vmPFC circuitry are associated with fear-based disorders (e.g., anxiety, posttraumatic stress disorder). Despite the early age of onset of virtually all fear-based disorders and the protracted development of the hippocampus and vmPFC across the first two decades of life, little is known about extinction recall and the underlying neural correlates in children.

METHODS

Here, we tested extinction recall in 43 pre-adolescent children (ages 6-11 yrs) by coupling functional magnetic resonance imaging and virtual reality with a novel interpersonal threat-related two-day (ABBA) fear-extinction paradigm. Conditioned fear responding was assessed at behavioral, subjective, physiological, and neural levels.

RESULTS

Although children demonstrated intact within-session extinction, there was poor between-session recall of extinction learning (retention index: 13.56%), evidenced by elevations in skin conductance, avoidant behavioral responses, and subjective ratings. Elevations in conditioning fear responding were accompanied by activation in the hippocampus and insula, and increased connectivity of the hippocampus with the insula and dorsal anterior cingulate cortex - regions implicated in the return of fear in adult studies. Children who kept more distance from the extinguished cue during extinction subsequently demonstrated heightened hippocampal-cingulate coupling during recall, suggesting that avoidant behavior interferes with extinction retention.

CONCLUSIONS

Poor extinction recall in children may have implications for developmental vulnerability to fear-based disorders, and for the application of therapeutic strategies that rely on principles of extinction (e.g., exposure therapy) to pediatric samples.

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.

Extinction of Conditioned Fear in Adolescents and Adults: A Human fMRI Study

Little is known about the neural correlates of fear learning in adolescents, a population at increased risk for anxiety disorders. Healthy adolescents (mean age 16.26) and adults (mean age 29.85) completed a fear learning paradigm across two stages during functional magnetic resonance imaging (fMRI). Stage 1 involved conditioning and extinction, and stage 2 involved extinction recall, re-conditioning, followed by re-extinction. During extinction recall, we observed a higher skin conductance response to the CS+ relative to CS− in adolescents compared to adults, which was accompanied by a reduction in dorsolateral prefrontal cortex (dlPFC) activity. Relative to adults, adolescents also had significantly reduced activation in the ventromedial PFC, dlPFC, posterior cingulate cortex (PCC), and temporoparietal junction (TPJ) during extinction recall compared to late extinction. Age differences in PCC activation between late extinction and late conditioning were also observed. These results show for the first time that healthy adolescent humans show different behavioral responses, and dampened PFC activity during short-term extinction recall compared to healthy adults. We also identify the PCC and TPJ as novel regions that may be associated with impaired extinction in adolescents. Also, while adults showed significant correlations between differential SCR and BOLD activity in some brain regions during late extinction and recall, adolescents did not show any significant correlations. This study highlights adolescent-specific neural correlates of extinction, which may explain the peak in prevalence of anxiety disorders during adolescence.