Selective early-acquired fear memories undergo temporary suppression during adolescence

Termination of convulsion seizures by destabilizing and perturbing seizure memory engrams

Epileptogenesis, arising from alterations in synaptic strength, shares mechanistic and phenotypic parallels with memory formation. However, direct evidence supporting the existence of seizure memory remains scarce. Leveraging a conditioned seizure memory (CSM) paradigm, we found that CSM enabled the environmental cue to trigger seizure repetitively, and activating cue-responding engram cells could generate CSM artificially. Moreover, cue exposure initiated an analogous process of memory reconsolidation driven by mammalian target of rapamycin-brain-derived neurotrophic factor signaling. Pharmacological targeting of the mammalian target of rapamycin pathway within a limited time window reduced seizures in animals and interictal epileptiform discharges in patients with refractory seizures. Our findings reveal a causal link between seizure memory engrams and seizures, which leads us to a deeper understanding of epileptogenesis and points to a promising direction for epilepsy treatment.

Developmentally distinct architectures in top-down circuits

The medial prefrontal cortex (mPFC) plays a key role in learning, mood and decision making, including in how individuals respond to threats 1-6 . mPFC undergoes a uniquely protracted development, with changes in synapse density, cortical thickness, long-range connectivity, and neuronal encoding properties continuing into early adulthood 7-21 . Models suggest that before adulthood, the slow-developing mPFC cannot adequately regulate activity in faster-developing subcortical centers 22,23 . They propose that during development, the enhanced influence of subcortical systems underlies distinctive behavioural strategies of juveniles and adolescents and that increasing mPFC control over subcortical structures eventually allows adult behaviours to emerge. Yet it has remained unclear how a progressive strengthening of top-down control can lead to nonlinear changes in behaviour as individuals mature 24,25 . To address this discrepancy, here we monitored and manipulated activity in the developing brain as animals responded to threats, establishing direct causal links between frontolimbic circuit activity and the behavioural strategies of juvenile, adolescent and adult mice. Rather than a linear strengthening of mPFC synaptic connectivity progressively regulating behaviour, we uncovered multiple developmental switches in the behavioural roles of mPFC circuits targeting the basolateral amygdala (BLA) and nucleus accumbens (NAc). We show these changes are accompanied by axonal pruning coinciding with functional strengthening of synaptic connectivity in the mPFC-BLA and mPFC-NAc pathways, which mature at different rates. Our results reveal how developing mPFC circuits pass through distinct architectures that may make them optimally adapted to the demands of age-specific challenges.

Translating Developmental Neuroscience to Understand Risk for Psychiatric Disorders

The transition from childhood to adulthood represents the developmental time frame in which the majority of psychiatric disorders emerge. Recent efforts to identify risk factors mediating the susceptibility to psychopathology have led to a heightened focus on both typical and atypical trajectories of neural circuit maturation. Mounting evidence has highlighted the immense neural plasticity apparent in the developing brain. Although in many cases adaptive, the capacity for neural circuit alteration also induces a state of vulnerability to environmental perturbations, such that early-life experiences have long-lasting implications for cognitive and emotional functioning in adulthood. The authors outline preclinical and neuroimaging studies of normative human brain circuit development, as well as parallel efforts covered in this issue of the Journal, to identify brain circuit alterations in psychiatric disorders that frequently emerge in developing populations. Continued translational research into the interactive effects of neurobiological development and external factors will be crucial for identifying early-life risk factors that may contribute to the emergence of psychiatric illness and provide the key to optimizing treatments.

Amygdala circuit transitions supporting developmentally-appropriate social behavior

Social behaviors dynamically change throughout the lifespan alongside the maturation of neural circuits. The basolateral region of the amygdala (BLA), in particular, undergoes substantial maturational changes from birth throughout adolescence that are characterized by changes in excitation, inhibition, and dopaminergic modulation. In this review, we detail the trajectory through which BLA circuits mature and are influenced by dopaminergic systems to guide transitions in social behavior in infancy and adolescence using data from rodents. In early life, social behavior is oriented towards approaching the attachment figure, with minimal BLA involvement. Around weaning age, dopaminergic innervation of the BLA introduces avoidance of novel peers into rat pups' behavioral repertoire. In adolescence, social behavior transitions towards peer-peer interactions with a high incidence of social play-related behaviors. This transition coincides with an increasing role of the BLA in the regulation of social behavior. Adolescent BLA maturation can be characterized by an increasing integration and function of local inhibitory GABAergic circuits and their engagement by the medial prefrontal cortex (mPFC). Manipulation of these transitions using viral circuit dissection techniques and early adversity paradigms reveals the sensitivity of this system and its role in producing age-appropriate social behavior.

The lifetime impact of stress on fear regulation and cortical function

A variety of stressful experiences can influence the ability to form and subsequently inhibit fear memory. While nonsocial stress can impact fear learning and memory throughout the lifespan, psychosocial stressors that involve negative social experiences or changes to the social environment have a disproportionately high impact during adolescence. Here, we review converging lines of evidence that suggest that development of prefrontal cortical circuitry necessary for both social experiences and fear learning is altered by stress exposure in a way that impacts both social and fear behaviors throughout the lifespan. Further, we suggest that psychosocial stress, through its impact on the prefrontal cortex, may be especially detrimental during early developmental periods characterized by higher sociability. This article is part of the Special Issue on 'Fear, Anxiety and PTSD'.

Calcium/calmodulin-dependent protein kinase IIα heterozygous knockout mice show electroencephalogram and behavioral changes characteristic of a subpopulation of schizophrenia and intellectual impairment

Cognitive deficit remains an intractable symptom of schizophrenia, accounting for substantial disability. Despite this, little is known about the cause of cognitive dysfunction in schizophrenia. Recent studies suggest that schizophrenia patients show several changes in dentate gyrus structure and functional characteristic of immaturity. The immature dentate gyrus (iDG) has been replicated in several mouse models, most notably the αCaMKII heterozygous mouse (CaMKIIa-hKO). The current study characterizes behavioral phenotypes of CaMKIIa-hKO mice and determines their neurophysiological profile using electroencephalogram (EEG) recording from hippocampus. CaMKIIa-hKO mice were hypoactive in home-cage environment; however, they displayed less anxiety-like phenotype, suggestive of impulsivity-like behavior. In addition, severe cognitive dysfunction was evident in CaMKIIa-hKO mice as examined by novel object recognition and contextual fear conditioning. Several EEG phenomena established in both patients and relevant animal models indicate key pathological changes associated with the disease, include auditory event-related potentials and time-frequency EEG oscillations. CaMKIIa-hKO mice showed altered event-related potentials characterized by an increase in amplitude of the N40 and P80, as well as increased P80 latency. These mice also showed increased power in theta range time-frequency measures. Additionally, CaMKIIa-hKO mice showed spontaneous bursts of spike wave activity, possibly indicating absence seizures. The GABAB agonist baclofen increased, while the GABAB antagonist CGP35348 and the T-Type Ca2⁺ channel blocker Ethosuximide decreased spike wave burst frequency. None of these changes in event-related potentials or EEG oscillations are characteristic of those observed in general population of patients with schizophrenia; yet, CaMKIIa-hKO mice likely model a subpopulation of patients with schizophrenia.

Prelimbic proBDNF Facilitates Retrieval-Dependent Fear Memory Destabilization by Regulation of Synaptic and Neural Functions in Juvenile Rats

Fear regulation changes as a function of the early life is a key developmental period for the continued maturation of fear neural circuitry. The mechanisms of fear retrieval-induced reconsolidation have been investigated but remain poorly understood. The involvement of prelimbic proBDNF in fear memory extinction and its mediated signaling have been reported previously. Specifically, blocking the proBDNF/p75^NTR pathway during the postnatal stage disrupts synaptic development and neuronal activity in adulthood. Given the inherent high expression of proBDNF during the juvenile period, we tested whether the prelimbic proBDNF regulated synaptic and neuronal functions allowing to influencing retrieval-dependent memory processing. By examining the freezing behavior of auditory fear-conditioned rats, we found the high level of the prelimbic proBDNF in juvenile rats enhanced the destabilization of the retrieval-dependent weak but not strong fear memory through activating p75^NTR-GluN2B signaling. This modification of fear memory traces was attributed to the increment in the proportion of thin-type spine and promotion in synaptic function, as evidenced by the facilitation of NMDA-mediated EPSCs and GluN2B-dependent synaptic depression at the prelimbic projection. Furthermore, the strong prelimbic theta- and gamma-oscillation coupling predicted the suppressive effect of juvenile proBDNF on the recall of postretrieval memory. Our results critically emphasize the importance of developmental proBDNF for modification of retrieval-dependent memory and provide a potential critical targeting to inhibit threaten memories associated with neurodevelopment disorders.

Developmental differences in memory reactivation relate to encoding and inference in the human brain

Despite the fact that children can draw on their memories to make novel inferences, it is unknown whether they do so through the same neural mechanisms as adults. We measured memory reinstatement as participants aged 7-30 years learned new, related information. While adults brought memories to mind throughout learning, adolescents did so only transiently, and children not at all. Analysis of trial-wise variability in reactivation showed that discrepant neural mechanisms-and in particular, what we interpret as suppression of interfering memories during learning in early adolescence-are nevertheless beneficial for later inference at each developmental stage. These results suggest that while adults build integrated memories well-suited to informing inference directly, children and adolescents instead must rely on separate memories to be individually referenced at the time of inference decisions.

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.

ERK/MAPK signalling in the developing brain: Perturbations and consequences

The extracellular regulated kinase/microtubule-associated protein kinase (ERK/MAPK) signalling pathway transduces signals that cause an alteration in the ongoing metabolic pathways and modifies gene expression patterns; thus, influencing cellular behaviour. ERK/MAPK signalling is essential for the proper development of the nervous system from neural progenitor cells derived from the embryonic mesoderm. Several signalling molecules that regulate the well-coordinated process of neurodevelopment transduce developmental information through the ERK/MAPK signalling pathway. The ERK/MAPK is a potential novel therapeutic target in several neurodevelopmental disorders, however, despite years of study, there is still significant uncertainty about the exact mechanism by which the ERK/MAPK signalling pathway elicits specific responses in neurodevelopment. Here, we will review the evidence highlighting the role of ERK/MAPK signalling in neurodevelopment. We will also discuss the structural implication and behavioural deficits associated with perturbed ERK/MAPK signalling pathway in cortical development, whilst examining its contribution to the neuropathology of several neurodevelopmental disorders, such as Autism Spectrum Disorder, Schizophrenia, Fragile X, and Attention Deficit Hyperactive Disorder.

Genetic variation in endocannabinoid signaling is associated with differential network-level functional connectivity in youth

The endocannabinoid system is an important regulator of emotional responses such as fear, and a number of studies have implicated endocannabinoid signaling in anxiety. The fatty acid amide hydrolase (FAAH) C385A polymorphism, which is associated with enhanced endocannabinoid signaling in the brain, has been identified across species as a potential protective factor from anxiety. In particular, adults with the variant FAAH 385A allele have greater fronto-amygdala connectivity and lower anxiety symptoms. Whether broader network-level differences in connectivity exist, and when during development this neural phenotype emerges, remains unknown and represents an important next step in understanding how the FAAH C385A polymorphism impacts neurodevelopment and risk for anxiety disorders. Here, we leveraged data from 3,109 participants in the nationwide Adolescent Brain Cognitive Development Study℠ (10.04 ± 0.62 years old; 44.23% female, 55.77% male) and a cross-validated, data-driven approach to examine associations between genetic variation and large-scale resting-state brain networks. Our findings revealed a distributed brain network, comprising functional connections that were both significantly greater (95% CI for p values = [<0.001, <0.001]) and lesser (95% CI for p values = [0.006, <0.001]) in A-allele carriers relative to non-carriers. Furthermore, there was a significant interaction between genotype and the summarized connectivity of functional connections that were greater in A-allele carriers, such that non-carriers with connectivity more similar to A-allele carriers (i.e., greater connectivity) had lower anxiety symptoms (β = -0.041, p = 0.030). These findings provide novel evidence of network-level changes in neural connectivity associated with genetic variation in endocannabinoid signaling and suggest that genotype-associated neural differences may emerge at a younger age than genotype-associated differences in anxiety.

Linking mPFC circuit maturation to the developmental regulation of emotional memory and cognitive flexibility

The medial prefrontal cortex (mPFC) and its abundant connections with other brain regions play key roles in memory, cognition, decision making, social behaviors, and mood. Dysfunction in mPFC is implicated in psychiatric disorders in which these behaviors go awry. The prolonged maturation of mPFC likely enables complex behaviors to emerge, but also increases their vulnerability to disruption. Many foundational studies have characterized either mPFC synaptic or behavioral development without establishing connections between them. Here, we review this rich body of literature, aligning major events in mPFC development with the maturation of complex behaviors. We focus on emotional memory and cognitive flexibility, and highlight new work linking mPFC circuit disruption to alterations of these behaviors in disease models. We advance new hypotheses about the causal connections between mPFC synaptic development and behavioral maturation and propose research strategies to establish an integrated understanding of neural architecture and behavioral repertoires.

Longitudinal network re-organization across learning and development

While it is well understood that the brain experiences changes across short-term experience/learning and long-term development, it is unclear how these two mechanisms interact to produce developmental outcomes. Here we test an interactive model of learning and development where certain learning-related changes are constrained by developmental changes in the brain against an alternative development-as-practice model where outcomes are determined primarily by the accumulation of experience regardless of age. Participants (8-29 years) participated in a three-wave, accelerated longitudinal study during which they completed a feedback learning task during an fMRI scan. Adopting a novel longitudinal modeling approach, we probed the unique and moderated effects of learning, experience, and development simultaneously on behavioral performance and network modularity during the task. We found nonlinear patterns of development for both behavior and brain, and that greater experience supported increased learning and network modularity relative to naïve subjects. We also found changing brain-behavior relationships across adolescent development, where heightened network modularity predicted improved learning, but only following the transition from adolescence to young adulthood. These results present compelling support for an interactive view of experience and development, where changes in the brain impact behavior in context-specific fashion based on developmental goals.

White-matter tract connecting anterior insula to nucleus accumbens predicts greater future motivation in adolescents

The motivation to approach or avoid incentives can change during adolescence. Advances in neuroimaging allow researchers to characterize specific brain circuits that underlie these developmental changes. Whereas activity in the nucleus accumbens (NAcc) can predict approach toward incentive gain, activity in anterior insula (AIns) is associated with avoidance of incentive loss. Recent research characterized the structural white-matter tract connecting the two brain regions, but the tract has neither been characterized in adolescence nor linked to functional activity during incentive anticipation. In this study, we collected diffusion MRI and characterized the tract connecting the AIns to the NAcc for the first time in early adolescents. We then measured NAcc functional activity during a monetary incentive delay task and found that structural coherence of the AIns-NAcc tract is correlated with decreased functional activity at the NAcc terminal of the tract during anticipation of no incentives. In adolescents who completed an assessment 2 years later, we found that AIns-NAcc tract coherence could predict greater future self-reported motivation, and that NAcc functional activity could statistically mediate this association. Together, the findings establish links from brain structure to function to future motivation and provide targets to study the reciprocal development of brain structure and function.

Ontogeny of spontaneous recognition memory in rodents

Spontaneous recognition memory tasks explore thewhat,whereandwhencomponents of recognition memory. These tasks are widely used in rodents to assess cognitive function across the lifespan. While several neurodevelopmental and mental disorders present symptom onset in early life, very little is known about how memories are expressed in early life, and as a consequence how they may be affected in pathological conditions. In this review, we conduct an analysis of the studies examining the expression of spontaneous recognition memory in young rodents. We compiled studies using four different tasks: novel object recognition, object location, temporal order recognition and object place. First, we identify major sources of variability between early life spontaneous recognition studies and classify them for later comparison. Second, we use these classifications to explore the current knowledge on the ontogeny of each of these four spontaneous recognition memory tasks. We conclude by discussing the possible implications of the relative time of onset for each of these tasks and their respective neural correlates. In compiling this research, we hope to advance on establishing a developmental timeline for the emergence of distinct components of recognition memory, while also identifying key areas of focus for future research. Establishing the ontogenetic profile of rodent spontaneous recognition memory tasks will create a necessary blueprint for cognitive assessment in animal models of neurodevelopmental and mental disorders, a first step towards improved and earlier diagnosis as well as novel intervention strategies.

Nested sensitive periods: how plasticity across the microbiota-gut-brain axis interacts to affect the development of learning and memory

There is a growing appreciation for the range of sensitive periods which occur across the brain. These sensitive periods give rise to sensory outcomes, as well as complex higher-order cognitive functions like learning and memory. More recently, an understanding that sensitive periods of development occur outside of the central nervous system (e.g., in the gastrointestinal microbiota) has emerged. Less well understood is how these peripheral sensitive periods may interact with those operating centrally to influence complex behavior. The goal of this paper is to put forward the view that sensitive periods of development occur across the entirety of the microbiota-gut-brain (MGB) axis, and that these nested sensitive periods may interact to influence learning and memory outcomes. Adopting this framework should promote a 'new wave' of thinking in the field which appreciates the complex central and peripheral forces acting on behavior, and uses that understanding to innovate therapies and interventions for disordered learning and memory systems.

Kainate Receptor Auxiliary Subunit NETO2-Related Cued Fear Conditioning Impairments Associate with Defects in Amygdala Development and Excitability

NETO2 is an auxiliary subunit for kainate-type glutamate receptors that mediate normal cued fear expression and extinction. Since the amygdala is critical for these functions, we asked whether Neto2 ^-/- mice have compromised amygdala function. We measured the abundance of molecular markers of neuronal maturation and plasticity, parvalbumin-positive (PV⁺), perineuronal net-positive (PNN⁺), and double positive (PV⁺PNN⁺) cells in the Neto2 ^-/- amygdala. We found that Neto2 ^-/- adult, but not postnatal day (P)23, mice had 7.5% reduction in the fraction of PV⁺PNN⁺ cells within the total PNN⁺ population, and 23.1% reduction in PV staining intensity compared with Neto2 ^+/+ mice, suggesting that PV interneurons in the adult Neto2 ^-/- amygdala remain in an immature state. An immature PV inhibitory network would be predicted to lead to stronger amygdalar excitation. In the amygdala of adult Neto2 ^-/- mice, we identified increased glutamatergic and reduced GABAergic transmission using whole-cell patch-clamp recordings. This was accompanied by increased spine density of thin dendrites in the basal amygdala (BA) compared with Neto2 ^+/+ mice, indicating stronger glutamatergic synapses. Moreover, after fear acquisition Neto2 ^-/- mice had a higher number of c-Fos-positive cells than Neto2 ^+/+ mice in the lateral amygdala (LA), BA, and central amygdala (CE). Altogether, our findings indicate that Neto2 is involved in the maturation of the amygdala PV interneuron network. Our data suggest that this defect, together with other processes influencing amygdala principal neurons, contribute to increased amygdalar excitability, higher fear expression, and delayed extinction in cued fear conditioning, phenotypes that are common in fear-related disorders, including the posttraumatic stress disorder (PTSD).

Early Adversity and the Neotenous Human Brain

Human brain development is optimized to learn from environmental cues. The protracted development of the cortex and its connections with subcortical targets has been argued to permit more opportunity for acquiring complex behaviors. This review uses the example of amygdala-medial prefrontal cortex circuitry development to illustrate a principle of human development-namely, that the extension of the brain's developmental timeline allows for the (species-expected) collaboration between child and parent in co-construction of the human brain. The neurobiology underlying affective learning capitalizes on this protracted timeline to develop a rich affective repertoire in adulthood. Humans are afforded this luxuriously slow development in part by the extended period of caregiving provided by parents, and parents aid in scaffolding the process of maturation during childhood. Just as adequate caregiving is a potent effector of brain development, so is adverse caregiving, which is the largest environmental risk factor for adult mental illness. There are large individual differences in neurobiological outcomes following caregiving adversity, indicating that these pathways are probabilistic, rather than deterministic, and prolonged plasticity in human brain development may also allow for subsequent amelioration by positive experiences. The extant research indicates that the development of mental health cannot be considered without consideration of children in the context of their families.

Regulation of embryonic and adult neurogenesis by Ars2

Neural development is controlled at multiple levels to orchestrate appropriate choices of cell fate and differentiation. Although more attention has been paid to the roles of neural-restricted factors, broadly expressed factors can have compelling impacts on tissue-specific development. Here, we describe in vivo conditional knockout analyses of murine Ars2, which has mostly been studied as a general RNA-processing factor in yeast and cultured cells. Ars2 protein expression is regulated during neural lineage progression, and is required for embryonic neural stem cell (NSC) proliferation. In addition, Ars2 null NSCs can still transition into post-mitotic neurons, but fail to undergo terminal differentiation. Similarly, adult-specific deletion of Ars2 compromises hippocampal neurogenesis and results in specific behavioral defects. To broaden evidence for Ars2 as a chromatin regulator in neural development, we generated Ars2 ChIP-seq data. Notably, Ars2 preferentially occupies DNA enhancers in NSCs, where it colocalizes broadly with NSC regulator SOX2. Ars2 association with chromatin is markedly reduced following NSC differentiation. Altogether, Ars2 is an essential neural regulator that interacts dynamically with DNA and controls neural lineage development.

Sensorimotor developmental factors influencing the performance of laboratory rodents on learning and memory

Behavioral studies in animal models have advanced our knowledge of brain function and the neural mechanisms of human diseases. Commonly used laboratory rodents, such as mice and rats, provide a useful tool for studying the behaviors and mechanisms associated with learning and memory processes which are cooperatively regulated by multiple underlying factors, including sensory and motor performance and emotional/defense innate components. Each of these factors shows unique ontogeny and governs the sustainment of behavioral performance in learning tasks, and thus, understanding the integrative processes of behavioral development are crucial in the accurate interpretation of the functional meaning of learning and memory behaviors expressed in commonly employed behavioral test paradigms. In this review, we will summarize the major findings in the developmental processes of rodent behavior on the basis of the emergence of fundamental components for sustaining learning and memory behaviors. Briefly, most sensory modalities (except for vision) and motor abilities are functional at the juvenile stage, in which several defensive components, including active and passive defensive strategies and risk assessment behavior, emerge. Sex differences are detectable from the juvenile stage through adulthood and are considerable factors that influence behavioral tests. The test paradigms addressed in this review include associative learning (with an emphasis on fear conditioning), spatial learning, and recognition. This basic background information will aid in accurately performing behavioral studies in laboratory rodents and will therefore contribute to reducing inappropriate interpretations of behavioral data and further advance research on learning and memory in rodent models.

Prefrontal somatostatin interneurons encode fear memory

Theories stipulate that memories are encoded within networks of cortical projection neurons. Conversely, GABAergic interneurons are thought to function primarily to inhibit projection neurons and thereby impose network gain control, an important but purely modulatory role. Here we show in male mice that associative fear learning potentiates synaptic transmission and cue-specific activity of medial prefrontal cortex somatostatin (SST) interneurons and that activation of these cells controls both memory encoding and expression. Furthermore, the synaptic organization of SST and parvalbumin interneurons provides a potential circuit basis for SST interneuron-evoked disinhibition of medial prefrontal cortex output neurons and recruitment of remote brain regions associated with defensive behavior. These data suggest that, rather than constrain mnemonic processing, potentiation of SST interneuron activity represents an important causal mechanism for conditioned fear.

A sperm peptide enhances long-term memory in female Drosophila

Can mating influence cognitive functions such as learning and memory in a permanent way? We have addressed this question using a combined behavioral and in vivo imaging approach, finding that aversive long-term memory performance strongly increases in Drosophila females in response to sperm transfer following mating. A peptide in the male sperm, the sex peptide, is known to cause marked changes in female reproductive behavior, as well as other behaviors such as dietary preference. Here, we demonstrate that this sex peptide enhances memory by acting on a single pair of serotonergic brain neurons, in which activation of the sex peptide receptor stimulates the cyclic adenosine monophosphate/protein kinase A pathway. We thus reveal a strong effect of mating on memory via the neuromodulatory action of a sperm peptide on the female brain.

Future considerations for pediatric cancer survivorship: Translational perspectives from developmental neuroscience

Breakthroughs in modern medicine have increased pediatric cancer survival rates throughout the last several decades. Despite enhanced cure rates, a subset of pediatric cancer survivors exhibit life-long psychological side effects. A large body of work has addressed potential mechanisms for secondary symptoms of anxiety, post-traumatic stress, impaired emotion regulation and cognitive deficits in adults. Yet, absent from many studies are the ways in which cancer treatment can impact the developing brain. Additionally, it remains less known whether typical neurobiological changes during adolescence and early adulthood may potentially buffer or exacerbate some of the known negative cancer survivorship outcomes. This review highlights genetic, animal, and human neuroimaging research across development. We focus on the neural circuitry associated with aversive learning, which matures throughout childhood, adolescence and early adulthood. We argue that along with other individual differences, the precise timing of oncological treatment insults on such neural circuitry may expose particular vulnerabilities for pediatric cancer patients. We also explore other moderators of treatment outcomes, including genetic polymorphisms and neural mechanisms underlying memory and cognitive control. We discuss how neural maturation extending into young adulthood may also provide a sensitive period for intervention to improve psychological and cognitive outcomes in pediatric cancer survivors.

From serendipity to clinical relevance: How clinical psychology and neuroscience converged to illuminate psychoneuroendocrinology

Dirk Hellhammer and his colleagues have played a major role in creating the field of psychoneuroendocrinology from their roots in psychology. In this review, using examples from the history of the McEwen laboratory and neuroscience and neuroendocrinology colleagues, I summarize my own perspective as to how the fields of neuroscience and neuroendocrinology have contributed to psychoneuroendocrinology and how they converged with the contributions from Dirk Hellhammer and his colleagues.

Development of Threat Expression Following Infant Maltreatment: Infant and Adult Enhancement but Adolescent Attenuation

Early life maltreatment by the caregiver constitutes a major risk factor for the development of later-life psychopathologies, including fear-related pathologies. Here, we used an animal model of early life maltreatment induced by the Scarcity-Adversity Model of low bedding (LB) where the mother is given insufficient bedding for nest building while rat pups were postnatal days (PN) 8-12. To assess effects of maltreatment on the expression of threat-elicited defensive behaviors, animals underwent odor-shock threat conditioning at three developmental stages: late infancy (PN18), adolescence (PN45) or adulthood (>PN75) and tested the next day with odor only presentations (cue test). Results showed that in typically developing rats, the response to threat increases with maturation, although experience with maltreatment in early infancy produced enhanced responding to threat in infancy and adulthood, but a decrease in maltreated adolescents. To better understand the unique features of this decreased threat responding in adolescence, c-Fos expression was assessed within the amygdala and ventromedial prefrontal cortex (vmPFC) associated with the cued expression of threat learning. Fos counts across amygdala subregions were lower in LB rats compared to controls, while enhanced c-Fos expression was observed in the vmPFC prelimbic cortex (PL). Correlational analysis between freezing behavior and Fos revealed freezing levels were correlated with CeA in controls, although more global correlations were detected in LB-reared rats, including the BA, LA, and CeA. Functional connectivity analysis between brain regions showed that LB reared rats exhibited more diffuse interconnectivity across amygdala subnuclei, compared the more heterogeneous patterns observed in controls. In addition, functional connectivity between the IL and LA switched from positive to negative in abused adolescents. Overall, these results suggest that in adolescence, the unique developmental decrease in fear expression following trauma is associated with distinct changes in regional function and long-range connectivity, reminiscent of pathological brain function. These results suggest that early life maltreatment from the caregiver perturbs the developmental trajectory of threat-elicited behavior. Indeed, it is possible that this form of trauma, where the infant's safety signal or "safe haven" (the caregiver) is actually the source of the threat, produces distinct outcomes across development.

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.

Impaired Fear Extinction Recall in Serotonin Transporter Knockout Rats Is Transiently Alleviated during Adolescence

Adolescence is a developmental phase characterized by emotional turmoil and coincides with the emergence of affective disorders. Inherited serotonin transporter (5-HTT) downregulation in humans increases sensitivity to these disorders. To reveal whether and how 5-HTT gene variance affects fear-driven behavior in adolescence, we tested wildtype and serotonin transporter knockout (5-HTT^-/-) rats of preadolescent, adolescent, and adult age for cued fear extinction and extinction recall. To analyze neural circuit function, we quantified inhibitory synaptic contacts and, through RT-PCR, the expression of c-Fos, brain-derived neurotrophic factor (BDNF), and NDMA receptor subunits, in the medial prefrontal cortex (mPFC) and amygdala. Remarkably, the impaired recall of conditioned fear that characterizes preadolescent and adult 5-HTT^-/- rats was transiently normalized during adolescence. This did not relate to altered inhibitory neurotransmission, since mPFC inhibitory immunoreactivity was reduced in 5-HTT^-/- rats across all ages and unaffected in the amygdala. Rather, since mPFC (but not amygdala) c-Fos expression and NMDA receptor subunit 1 expression were reduced in 5-HTT^-/- rats during adolescence, and since PFC c-Fos correlated negatively with fear extinction recall, the temporary normalization of fear extinction during adolescence could relate to altered plasticity in the developing mPFC.

The ontogeny of memory persistence and specificity

Interest in the ontogeny of memory blossomed in the twentieth century following the initial observations that memories from infancy and early childhood are rapidly forgotten. The intense exploration of infantile amnesia in subsequent years has led to a thorough characterization of its psychological determinants, although the neurobiology of memory persistence has long remained elusive. By contrast, other phenomena in the ontogeny of memory like infantile generalization have received relatively less attention. Despite strong evidence for reduced memory specificity during ontogeny, infantile generalization is poorly understood from psychological and neurobiological perspectives. In this review, we examine the ontogeny of memory persistence and specificity in humans and nonhuman animals at the levels of behavior and the brain. To this end, we first describe the behavioral phenotypes associated with each phenomenon. Looking into the brain, we then discuss neurobiological mechanisms in the hippocampus that contribute to the ontogeny of memory. Hippocampal neurogenesis and critical period mechanisms have recently been discovered to underlie amnesia during early development, and at the same time, we speculate that similar processes may contribute to the early bias towards memory generalization.

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.

Age and sex differences in the innate defensive behaviors of C57BL/6 mice exhibited in a fear conditioning paradigm and upon exposure to a predatory odor

Defensive responses of mice include an array of specific behaviors that involve adaptive components based on the assessment of the threat of confrontation. The freezing response is represented by a motionless moment without any specific posture or behavioral sequence, and it is widely used in the fear conditioning paradigms and other relevant defensive situations. However, freezing measurements include fragmental components of several defensive behaviors that are exhibited during the session, such as behavioral inhibition, crouching, and a moment of risk assessment behavior. From an ethological view, behavioral analyses of C57BL/6 mice of both sexes and three different ages (postnatal days (P) 25, 35, and 65) revealed a rich variety of defensive behaviors during a fear conditioning session and in response to predatory odor exposure as a nonconditioned behavior. P-25 and 35 mice exhibited more behavioral inhibition than P-65 adult mice, and P-65 mice exhibited a crouching posture more often than younger mice. This age difference was more pronounced in males. The stretch-attend posture (SAP) increased with age, except in P-25 males, which exhibited robust SAP in response to a conditioned cue; this response indicates that P-25 males are defensive in a situation-nonmatching manner. Situation-dependent defense strategies were revealed in P-35 and 65 mice: Fear conditioning paradigm induced more robust defensive responses than predatory odor exposure, to which mice primarily exhibited SAP. A sex-based difference was revealed in adult mice. Males tended to show more passive defensive responses, such as crouching and withdrawal, and females exhibited more active responses, such as SAP. These age- and sex-based differences may stem from the ethological demands and illustrate adolescent ontogenetic processes of defense behavior.

Translating Developmental Neuroscience to Understand Risk for Psychiatric Disorders

The transition from childhood to adulthood represents the developmental time frame in which the majority of psychiatric disorders emerge. Recent efforts to identify risk factors mediating the susceptibility to psychopathology have led to a heightened focus on both typical and atypical trajectories of neural circuit maturation. Mounting evidence has highlighted the immense neural plasticity apparent in the developing brain. Although in many cases adaptive, the capacity for neural circuit alteration also induces a state of vulnerability to environmental perturbations, such that early-life experiences have long-lasting implications for cognitive and emotional functioning in adulthood. The authors outline preclinical and neuroimaging studies of normative human brain circuit development, as well as parallel efforts covered in this issue of the Journal, to identify brain circuit alterations in psychiatric disorders that frequently emerge in developing populations. Continued translational research into the interactive effects of neurobiological development and external factors will be crucial for identifying early-life risk factors that may contribute to the emergence of psychiatric illness and provide the key to optimizing treatments.

Using a Developmental Ecology Framework to Align Fear Neurobiology Across Species

Children's development is largely dependent on caregiving; when caregiving is disrupted, children are at increased risk for numerous poor outcomes, in particular psychopathology. Therefore, determining how caregivers regulate children's affective neurobiology is essential for understanding psychopathology etiology and prevention. Much of the research on affective functioning uses fear learning to map maturation trajectories, with both rodent and human studies contributing knowledge. Nonetheless, as no standard framework exists through which to interpret developmental effects across species, research often remains siloed, thus contributing to the current therapeutic impasse. Here, we propose a developmental ecology framework that attempts to understand fear in the ecological context of the child: their relationship with their parent. By referring to developmental goals that are shared across species (to attach to, then, ultimately, separate from the parent), this framework provides a common grounding from which fear systems and their dysfunction can be understood, thus advancing research on psychopathologies and their treatment.

Neural Variability Limits Adolescent Skill Learning

Skill learning is fundamental to the acquisition of many complex behaviors that emerge during development. For example, years of practice give rise to perceptual improvements that contribute to mature speech and language skills. While fully honed learning skills might be thought to offer an advantage during the juvenile period, the ability to learn actually continues to develop through childhood and adolescence, suggesting that the neural mechanisms that support skill learning are slow to mature. To address this issue, we asked whether the rate and magnitude of perceptual learning varies as a function of age as male and female gerbils trained on an auditory task. Adolescents displayed a slower rate of perceptual learning compared with their young and mature counterparts. We recorded auditory cortical neuron activity from a subset of adolescent and adult gerbils as they underwent perceptual training. While training enhanced the sensitivity of most adult units, the sensitivity of many adolescent units remained unchanged, or even declined across training days. Therefore, the average rate of cortical improvement was significantly slower in adolescents compared with adults. Both smaller differences between sound-evoked response magnitudes and greater trial-to-trial response fluctuations contributed to the poorer sensitivity of individual adolescent neurons. Together, these findings suggest that elevated sensory neural variability limits adolescent skill learning.SIGNIFICANCE STATEMENT The ability to learn new skills emerges gradually as children age. This prolonged development, often lasting well into adolescence, suggests that children, teens, and adults may rely on distinct neural strategies to improve their sensory and motor capabilities. Here, we found that practice-based improvement on a sound detection task is slower in adolescent gerbils than in younger or older animals. Neural recordings made during training revealed that practice enhanced the sound sensitivity of adult cortical neurons, but had a weaker effect in adolescents. This latter finding was partially explained by the fact that adolescent neural responses were more variable than in adults. Our results suggest that one mechanistic basis of adult-like skill learning is a reduction in neural response variability.

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.

TLX knockdown in the dorsal dentate gyrus of juvenile rats differentially affects adolescent and adult behaviour

The orphan nuclear receptor TLX is predominantly expressed in the central nervous system and is an important factor regulating the maintenance and self-renewal of neural stem cells from embryonic development through adulthood. In adolescence and adulthood, TLX expression is restricted to the neurogenic niches of the brain: the dentate gyrus of the hippocampus and the subventricular zone. The adolescent period is critical for maturation of the hippocampus with heightened levels of neurogenesis observed in rodents. Therefore, we investigated whether lentiviral silencing of TLX expression (TLX knockdown) in the dorsal dentate gyrus of juvenile rats incurred differential impairments in behaviour during late adolescence and adulthood. Our results showed that knockdown of TLX in the dorsal dentate gyrus led to a decrease in cell proliferation in the dorsal but not ventral dentate gyrus. At a behavioural level we observed differential effects in adolescence and adulthood across a number of parameters. A hyperactive phenotype was present in adolescent but not adult TLX knockdown rats, and an increase in immobility during adolescence and in swimming frequency during adulthood was observed in the forced swim test. There was an increased defecation frequency in the open field during adulthood but not adolescence. There were no changes in cognitive performance on hippocampus-dependent tasks or in anxiety-related behaviours. In conclusion, silencing of TLX in the dorsal dentate gyrus led to impairments in hippocampal-independent behaviours which either did not persist or were reversed during adulthood. The current data highlight the temporal importance and function of the nuclear receptor TLX during development.

Contextual fear memory modulates PSD95 phosphorylation, AMPAr subunits, PKMζ and PI3K differentially between adult and juvenile rats

It is well known that young organisms do not maintain memories as long as adults, but the mechanisms for this ontogenetic difference are undetermined. Previous work has revealed that the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAr) subunits are trafficked into the synaptic membrane following memory retrieval in adults. Additionally, phosphorylated PSD-95-pS295 promotes AMPAr stabilization at the synapse. We investigated these plasticity related proteins as potential mediators in the differential contextual stress memory retrieval capabilities observed between adult and juvenile rats. Rats were assigned to either pedestal stress (1 h) or no stress control (home cage). Each animal was placed alone in an open field for 5 min at the base of a 6 × 6 sq inch pedestal (4ft high). Stress subjects were then placed on this pedestal for 1hr and control subjects were placed in their home cage following initial exploration. Each animal was returned to the open field for 5 min either 1d or 7d following initial exposure. Freezing postures were quantified during the memory retrieval test. The 1d test shows adult (P90) and juvenile (P26) stressed rats increase their freezing time compared to controls. However, the 7d memory retrieval test shows P90 stress rats but not P26 stress rats freeze while in the fear context. Twenty minutes after the memory retrieval test, hippocampi and amygdala were micro-dissected and prepared for western blot analysis. Our results show that 1d fear memory retrieval induced an upregulation of PSD-95 and pS295 in the adult amygdala but not in the juvenile. However, the juvenile animals upregulated PKMζ, PI3K and GluA2/3, GluA1-S845 in the dorsal hippocampus (DH), but the adults did not. Following the 7d memory retrieval test, adults upregulated GluA2 in the amygdala but not the juveniles. In the DH, adults increased PSD-95 and pS295 but not the juveniles. The adults appear to preferentially increase amygdala-driven processing at 1d and increase DH-driven context specific processing at 7d. These data identify molecular processes that may underlie the reduced fear-memory retrieval capability of juveniles. Together these data provide a potential molecular target that could be beneficial in treatment of anxiety disorders and PTSD.

Effects of Trauma in Adulthood and Adolescence on Fear Extinction and Extinction Retention: Advancing Animal Models of Posttraumatic Stress Disorder

Evidence for and against adolescent vulnerability to posttraumatic stress disorder (PTSD) is mounting, but this evidence is largely qualitative, retrospective, or complicated by variation in prior stress exposure and trauma context. Here, we examine the effects of development on trauma vulnerability using adult post-natal (PN) day 61, early adolescent (PN23) and mid adolescence (PN34) rats and two types of trauma: an established animal model of PTSD, single prolonged stress (SPS), and a novel composite model—SPS predation (SPSp) version. We demonstrate that early and mid adolescent rats are capable of fear conditioning and fear extinction, as well as extinction retention. Our results also demonstrate that both types of trauma induced a deficit in the retention of fear extinction in adulthood, a hallmark of PTSD, but not after early or mid adolescence trauma, suggesting that adolescence might convey resilience to SPS and SPSp traumas. Across all three life stages, the effects of SPS exposure and a novel predation trauma model, SPSp, had similar effects on behavior suggesting that trauma type did not affect the likelihood of developing PTSD-like symptoms, and that SPSp is a predation-based trauma model worth exploring.

Neurocognitive Development of Motivated Behavior: Dynamic Changes across Childhood and Adolescence

The ability to anticipate and respond appropriately to the challenges and opportunities present in our environments is critical for adaptive behavior. Recent methodological innovations have led to substantial advances in our understanding of the neurocircuitry supporting such motivated behavior in adulthood. However, the neural circuits and cognitive processes that enable threat- and reward-motivated behavior undergo substantive changes over the course of development, and these changes are less well understood. In this article, we highlight recent research in human and animal models demonstrating how developmental changes in prefrontal-subcortical neural circuits give rise to corresponding changes in the processing of threats and rewards from infancy to adulthood. We discuss how these developmental trajectories are altered by experiential factors, such as early-life stress, and highlight the relevance of this research for understanding the developmental onset and treatment of psychiatric disorders characterized by dysregulation of motivated behavior.

Asymmetric neural tracking of gain and loss magnitude during adolescence

Adolescence has been characterized as a developmental period of heightened reward seeking and attenuated aversive processing. However, it remains unclear how the neural bases of distinct outcome valuation processes shift during this stage of the lifespan. A total of 74 participants ranging in age from 13 to 20 years completed a value-modulated functional magnetic resonance imaging (fMRI) task in which participants earn low and high magnitude monetary outcomes to test whether gain and loss magnitude tracking-the neural representation of relative value in context-change differentially over this age span. Results revealed that gain and loss magnitude tracking follow asymmetric developmental trajectories. Gain magnitude tracking is elevated in the striatum during early adolescence and then decreases with age. By contrast, loss magnitude tracking in the anterior insula follows a quadratic pattern, undergoing a temporary attenuation during mid-late adolescence. A typical comparison of gain vs loss outcomes (collapsing over magnitude effects) showed robust activity across a suite of brain regions sensitive to value based on prior work including the ventral striatum, but they exhibited no changes with age. These findings suggest that value coding subprocesses follow divergent developmental paths across adolescence, which may contribute to normative shifts in adolescent motivated behavior.

Spontaneous recovery of fear differs among early - late adolescent and adult male mice

Adolescence is a vulnerable period for developing anxiety-related mental disorders such as post-traumatic stress disorder (PTSD), which requires a long-term course of therapy when a traumatic event has been experienced during childhood. However, the biological mechanism underlying these age-dependent characteristics remains unclear. In the present study, we used early adolescent, late adolescent and adult (4-, 8-, and 15-week old) male mice to examine age differences in fear memory, fear extinction, and spontaneous recovery of fear. We also measured the activation of extracellular signal-regulated kinase (ERK) 2 in the dorsal hippocampus (dHip) and the basolateral amygdala (BLA) following a spontaneous recovery test. Our major findings were as follows: (1) early adolescent and adult mice did not recover the fear response; only late adolescent mice recovered the fear response. (2) The ERK2 in the dHip was more activated after the spontaneous recovery test in late adolescent mice than in adult mice, and the ERK2 in the BLA was more activated after the spontaneous recovery test in adult mice than in late adolescent mice. These results suggest that there exists a unique period in which spontaneous recovery occurs and that these late adolescent behavioral signatures may be related to alteration in the ERK2 phosphorylation in the dHip and BLA.

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.

Adolescent Isolation Interacts With DISC1 Point Mutation to Impair Adult Social Memory and Synaptic Functions in the Hippocampus

Disrupted-in-schizophrenia 1 (DISC1) is a strong candidate susceptibility gene for a spectrum of neuropsychiatric diseases including schizophrenia, bipolar disorder and major depression, all of which are thought to result from interactions between gene mutations and environmental risk factors such as influenza, trauma and stress. Adolescence is a key period susceptible to stress and stress-related mental illnesses. In a previous study, we found that although DISC1 L100P point mutation mice shows object recognition deficits, their sociability and social memory are relatively normal. Therefore, in this article, we investigated whether the interaction between adolescent stress and DISC1 L100P point mutation affects adult social memory, and we explored the underlying mechanisms. We found that adolescent stress (isolation from 5 weeks to 8 weeks of age) specifically impaired social memory of adult DISC1 L100P mice but not that of WT littermates, which could be rescued by administration of atypical antipsychotic drug clozapine. On the other hand, it did not induce anxiety or depression in adult mice. Adolescent isolation exacerbated adult neurogenesis deficits in the hippocampus of DISC1 L100P mice, while it had no effect on WT mice. In addition, we found that adolescent isolation led to long lasting changes in synaptic transmission and plasticity in the hippocampal circuits, some of which are specific for DISC1 L100P mice. In summary, we identified here the specific interaction between genetic mutation (DISC1 L100P) and adolescence social stress that damages synaptic function and social memory in adult hippocampal circuits.

Highlights–

Adolescent isolation (from 5 weeks to 8 weeks of age) impairs adult social memory when combined with DISC1 L100P point mutation.

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Adolescent isolation exacerbates adult neurogenesis deficit in the hippocampus of L100P mice but has no similar effect on WT mice.

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Adolescent isolation causes long lasting changes in synaptic transmission and plasticity of the hippocampal network in DISC1 L100P mice.

Ablation of the presynaptic organizer Bassoon in excitatory neurons retards dentate gyrus maturation and enhances learning performance

Bassoon is a large scaffolding protein of the presynaptic active zone involved in the development of presynaptic terminals and in the regulation of neurotransmitter release at both excitatory and inhibitory brain synapses. Mice with constitutive ablation of the Bassoon (Bsn) gene display impaired presynaptic function, show sensory deficits and develop severe seizures. To specifically study the role of Bassoon at excitatory forebrain synapses and its relevance for control of behavior, we generated conditional knockout (Bsn cKO) mice by gene ablation through an Emx1 promoter-driven Cre recombinase. In these animals, we confirm selective loss of Bassoon from glutamatergic neurons of the forebrain. Behavioral assessment revealed that, in comparison to wild-type littermates, Bsn cKO mice display selectively enhanced contextual fear memory and increased novelty preference in a spatial discrimination/pattern separation task. These changes are accompanied by an augmentation of baseline synaptic transmission at medial perforant path to dentate gyrus (DG) synapses, as indicated by increased ratios of field excitatory postsynaptic potential slope to fiber volley amplitude. At the structural level, an increased complexity of apical dendrites of DG granule cells can be detected in Bsn cKO mice. In addition, alterations in the expression of cellular maturation markers and a lack of age-dependent decrease in excitability between juvenile and adult Bsn cKO mice are observed. Our data suggest that expression of Bassoon in excitatory forebrain neurons is required for the normal maturation of the DG and important for spatial and contextual memory.

Differential effects of adolescent and adult-initiated voluntary exercise on context and cued fear conditioning

Adolescence is a critical period for postnatal brain maturation and a time during which there is increased susceptibility to developing emotional and cognitive-related disorders. Exercise during adulthood has been shown to increase hippocampal plasticity and enhance cognition. However, the impact of exercise initiated in adolescence, on brain and behaviour in adulthood is not yet fully explored or understood. The aim of this study was to compare the impact of voluntary exercise that was initiated either during adolescence or early adulthood on cognitive performance in hippocampal and amygdala-dependent fear conditioning tasks in adulthood. Adult (eight weeks old) and adolescent (four weeks old) male Sprague Dawley rats had access to a running wheel (exercise) or were left undisturbed (sedentary control) for seven weeks. Adult-initiated exercise enhanced both contextual and cued fear conditioning, while conversely, exercise that began in adolescence did not affect performance in these tasks. These behaviours were accompanied by differential expression of plasticity-related genes in the hippocampus and amygdala in adulthood. Specifically, adolescent-initiated exercise increased the expression of an array of plasticity related genes in the hippocampus including BDNF, synaptophysin, Creb, PSD-95, Arc, TLX and DCX, while adult-initiated exercise did not affect hippocampal plasticity related genes. Together results show that exercise initiated during adolescence has a differential effect on hippocampal and amygdala-dependent behaviour and neuronal plasticity compared to when exercise was initiated in adulthood. These findings reinforce adolescence as a period during which environmental influences have a distinct impact on neuronal plasticity and cognition. This article is part of the Special Issue entitled "Neurobiology of Environmental Enrichment".

Sexual differentiation of contextual fear responses

Development and sex differentiation impart an organizational influence on the neuroanatomy and behavior of mammalian species. Prior studies suggest that brain regions associated with fear motivated defensive behavior undergo a protracted and sex-dependent development. Outside of adult animals, evidence for developmental sex differences in conditioned fear is sparse. Here, we examined in male and female Long-Evans rats how developmental age and sex affect the long-term retention and generalization of Pavlovian fear responses. Experiments 1 and 2 describe under increasing levels of aversive learning (three and five trials) the long-term retrieval of cued and context fear in preadolescent (P24 and P33), periadolescent (P37), and adult (P60 and P90) rats. Experiments 3 and 4 examined contextual processing under minimal aversive learning (1 trial) procedures in infant (P19, P21), preadolescent (P24), and adult (P60) rats. Here, we found that male and female rats display a divergent developmental trajectory in the expression of context-mediated freezing, such that context fear expression in males tends to increase toward adulthood, while females displayed an opposite pattern of decreasing context fear expression toward adulthood. Longer (14 d) retention intervals produced an overall heightened context fear expression relative to shorter (1 d) retention intervals an observation consistent with fear incubation. Male, but not Female rats showed increasing generalization of context fear across development. Collectively, these findings provide an initial demonstration that sexual differentiation of contextual fear conditioning emerges prior to puberty and follows a distinct developmental trajectory toward adulthood that strikingly parallels sex differences in the etiology and epidemiology of anxiety and trauma- and stressor-related disorders.

Redefining neuroendocrinology: Epigenetics of brain-body communication over the life course

The brain is the central organ of stress and adaptation to stress that perceives and determines what is threatening, as well as the behavioral and physiological responses to the stressor, and it does so somewhat differently in males and females. The expression of steroid hormone receptors throughout the brain has broadened the definition of 'neuroendocrinology' to include the reciprocal communication between the entire brain and body via hormonal and neural pathways. Mediated in part via systemic hormonal influences, the adult and developing brain possess remarkable structural and functional plasticity in response to stress, including neuronal replacement, dendritic remodeling, and synapse turnover. This article is both an account of an emerging field elucidating brain-body interactions at multiple levels, from molecules to social organization, as well as a personal account of my laboratory's role and, most importantly, the roles of trainees and colleagues, along with my involvement in interdisciplinary groups working on this topic.

Development of the fear regulation system from early adolescence to young adulthood in female mice

Onset of fear-related disorders such as post-traumatic stress disorder is enhanced from adolescence until adulthood. However, the biological mechanisms underlying this vulnerability remain unclear. Therefore, we investigated contextual fear memory and extinction in 4-, 6-, 8-, 10-, and 15-week-old female mice. We also measured phosphorylation of ERK2 in the medial prefrontal cortex (mPFC) and the dorsal hippocampus following fear conditioning or extinction in 6- and 15-week-old mice. We found that 10- and 15-week-old mice showed stronger fear memory and more resistance to fear extinction than 6-week-old mice. Moreover, 15-week-old mice showed lower ERK2 phosphorylation levels following fear extinction in the mPFC than those 6 weeks old. Our results suggest that female mice acquire strong fear memory and resistance to fear extinction throughout adulthood, which may be related to alteration in ERK2 activation in the mPFC.