Diminished Fear Extinction in Adolescents Is Associated With an Altered Somatostatin Interneuron-Mediated Inhibition in the Infralimbic Cortex

Age and sex independently influence safety learning in mice

Difficulty discriminating between threat and safety cues is a hallmark symptom of many fear-related disorders including anxiety, which are highly prevalent during adolescence. Moreover, females are disproportionately affected by fear and anxiety disorders. Correspondingly, growing evidence suggests that sex differences in fear responding and regulation are readily apparent in adults. Yet, it remains unclear when these differences emerge throughout development and what behavioral factors may drive them. Using adult (postnatal day/PND 69-128) and adolescent (PND 29-34) mice, we set out to study age and sex differences in learning about cues explicitly indicating safety (i.e., safety learning). Our results revealed that female mice of both ages froze more and discriminated less than males of both ages during initial discriminative conditioning. All mice showed evidence of conditioned inhibition during a summation test, though the magnitude of suppression was smaller in females. During an expanded summation test that included novel cues and different combinations of stimulus parameters, females of both ages exhibited higher fear generalization to novel cues. In addition, adolescents of both sexes failed to inhibit fear using a safety cue, suggesting that adolescent learning may be disrupted by complex experimental design. Finally, neural activity (cFos expression) was greater in the prefrontal cortex and ventral hippocampus of adolescents relative to adults, and the retrosplenial cortex and ventral hippocampus of males relative to females. Together, these findings illustrate the potential to take age and sex into account in the identification and treatment of fear and anxiety disorders.

Adolescent alcohol exposure promotes mechanical allodynia and alters synaptic function at inputs from the basolateral amygdala to the prelimbic cortex

Binge drinking is common among adolescents despite mounting evidence linking it to various adverse health outcomes that include heightened pain perception. The prelimbic (PrL) cortex is vulnerable to insult from adolescent alcohol exposure and receives input from the basolateral amygdala (BLA) while sending projections to the ventrolateral periaqueductal gray (vlPAG) - two brain regions implicated in nociception. In this study, adolescent intermittent ethanol (AIE) exposure was carried out in male and female rats using a vapor inhalation procedure. Assessments of mechanical and thermal sensitivity revealed that AIE exposure-induced protracted mechanical allodynia. To investigate synaptic function at BLA inputs onto defined populations of PrL neurons, retrobeads and viral labeling were combined with optogenetics and slice electrophysiology. Recordings from retrobead labeled cells in the PrL revealed AIE reduced BLA-driven feedforward inhibition of neurons projecting from the PrL to the vlPAG, resulting in augmented excitation/inhibition (E/I) balance and increased intrinsic excitability. Consistent with this finding, recordings from virally tagged PrL parvalbumin interneurons (PVINs) demonstrated that AIE exposure reduced both E/I balance at BLA inputs onto PVINs and PVIN intrinsic excitability. These findings provide compelling evidence that AIE alters synaptic function and intrinsic excitability within a prefrontal nociceptive circuit.

Preclinical behavioral and pharmacological treatments for enhancing fear extinction in adolescence

Adolescence is a window of vulnerability for the development of anxiety disorders but also a window of opportunity for treatments to minimize the long-term impact of such disorders. Current first-line treatments, primarily exposure-based cognitive-behavioral therapy (CBT), have limited long-term efficacy in adolescents. The urgent need for more effective interventions is underscored by the frequent reports of extinction impairments in adolescents as well as the rising anxiety rates in youth, particularly post-COVID-19. Preclinical research on the extinction of learned fear in adolescents may contribute to developing better treatment approaches to anxiety in this age group. Unfortunately, this is still a largely under-explored area. However, both pharmacological and behavioral augmentation strategies can be used to enhance extinction learning and consolidation. Here we describe work exploring such adjuncts, focusing on pre-clinical work with rodents. Much of the research to date shows striking developmental differences in response to various pharmacological treatments, with only a few shown to be effective in adolescents. Further, recent experience of stress reduces the efficacy of these treatments in adolescence. This review highlights the necessity for tailored strategies, especially when it comes to pharmacological adjuncts, that address developmental differences in drug responses as well as the impact of stressful experiences on treatment efficacy.

The beautiful adolescent brain: An evolutionary developmental perspective

The adolescent brain has been characterized as a defective car, with no brakes or steering wheel-only an accelerator. This characterization has been used to explain the impulsive and risky behavior of this transient developmental period. But why do adolescents respond to the world the way they do? In this article, we consider adolescent-specific changes in the brain and behavior from a developmental evolutionary viewpoint in how they might be adaptive. We suggest ways in which the adolescent brain has evolved to explore and learn from new and changing environments as the adolescent gains independence from the caregiver and transitions into an adult. We highlight adolescent-specific changes in the brain and behavior in response to emotional and social cues that may facilitate learning to independently secure resources (e.g., food, water, and shelter) and to establish new social bonds beyond the family or pack for their own survival. Specifically, we focus on how rewards, social cues, and threats in the environment influence behavior and may serve an adaptive role for the adolescent.

Environmental Enrichment Has Age- and Sex-Specific Effects on Fear Regulation in Mice

Previous research has sought to understand the mechanisms by which early life adversity disrupts later behavioral function. Yet less has been done to investigate the effects of positive experiences such as environmental enrichment. Here, we examined the effects of brief (2 weeks) or extended (8 weeks) environmental enrichment on discrimination, fear inhibition, and fear extinction during adolescence or adulthood. Two conditions of enrichment were used: a "Hut" group received a polycarbonate Hut in the home cage throughout the duration of the experiment, while a "Variable" group received a polycarbonate Hut, as well as a series of rotating enrichment features. Our data show that in females, brief Variable enrichment increased the rate of learning about cues that explicitly indicate safety during adolescence while disrupting this type of learning in adults. In males, enrichment did not influence learning about safety cues. Conversely, in males, brief Variable enrichment initially improved extinction, but extended enrichment (Variable or Hut) was necessary to see sustained improvements across extinction. This pattern was apparent for both adolescents and adults. In females, Variable enrichment did not affect extinction, while Hut enrichment increased freezing for all mice initially and adolescents as extinction continued. This work has the potential to inform treatments and interventions for fear-related disorders, such as anxiety and post-traumatic stress disorder, tailored for both specific developmental ages and sex.

Adolescent alcohol exposure promotes mechanical allodynia and alters synaptic function at inputs from the basolateral amygdala to the prelimbic cortex

Binge drinking is common among adolescents despite mounting evidence linking it to various adverse health outcomes that include heightened pain perception. The prelimbic (PrL) cortex is vulnerable to insult from adolescent alcohol exposure and receives input from the basolateral amygdala (BLA) while sending projections to the ventrolateral periaqueductal gray (vlPAG) - two brain regions implicated in nociception. In this study, adolescent intermittent ethanol (AIE) exposure was carried out in male and female rats using a vapor inhalation procedure. Assessments of mechanical and thermal sensitivity revealed that AIE exposure induced protracted mechanical allodynia. To investigate synaptic function at BLA inputs onto defined populations of PrL neurons, retrobeads and viral labelling were combined with optogenetics and slice electrophysiology. Recordings from retrobead labelled cells in the PrL revealed AIE reduced BLA driven feedforward inhibition of neurons projecting from the PrL to the vlPAG, resulting in augmented excitation/inhibition (E/I) balance and increased intrinsic excitability. Consistent with this finding, recordings from virally tagged PrL parvalbumin interneurons (PVINs) demonstrated that AIE exposure reduced both E/I balance at BLA inputs onto PVINs and PVIN intrinsic excitability. These findings provide compelling evidence that AIE alters synaptic function and intrinsic excitability within a prefrontal nociceptive circuit.

Examining threat responses through a developmental lens

Adolescence has been characterized by risk taking and fearlessness. Yet, the emergence of anxiety disorders that are associated with fear peaks during this developmental period. Moreover, adolescents show heightened sensitivity to stress relative to children and adults. To address inconsistencies between the common characterization of adolescents as fearless and the evidence of heightened anxiety and stress during this time, we build upon foundational discoveries of threat-related circuitry and behavior in adult rodents by Joseph LeDoux and colleagues. Specifically, the conservation of this circuitry across species has provided opportunities for identifying mechanisms underlying threat responses that we have extended to developing humans and rodents. We elucidate situations in which adolescents show heightened threat responses and others where they appear fearless and link them to developmental changes of threat circuitry during this period. We discuss the potential adaptiveness of these threat responses for survival of the individual and species but also the potential risks for anxiety and stress. We end by offering potential new ways in which behavioral treatments for youth with anxiety and stress-related disorders may be optimized to target the developing vs developed brain.

Adolescent Alcohol Exposure Dysregulates Developing Cortical GABA Circuits

Adolescence is a critical developmental period during which physical, behavioral, and neurobiological maturation occurs. Within the brain, the prefrontal cortex is one of the last brain regions to undergo remodeling, often into adulthood. These relatively late developmental changes leave the prefrontal cortex uniquely vulnerable to insults beginning in adolescence-including alcohol exposure. Adolescents initiate alcohol consumption at a high rate, increasing the risk of lasting consequences through impairing the typical development of the prefrontal cortex. In this chapter, we discuss the development of prefrontal circuitry and the current literature investigating how alcohol influences prefrontal development. We primarily focus on preclinical studies in rodent models, which allow for the study of specific populations of neurons in the prefrontal cortex. We identify several future directions for adolescent alcohol research, including greater focus on neuropeptides and stronger understanding of sex differences in brain maturation and alcohol consumption.

Sex-Dependent Effects of Angiotensin Type 2 Receptor-Expressing Medial Prefrontal Cortex Interneurons in Fear Extinction Learning

Background

The renin-angiotensin system has been identified as a potential therapeutic target for posttraumatic stress disorder, although its mechanisms are not well understood. Brain angiotensin type 2 receptors (AT2Rs) are a subtype of angiotensin II receptors located in stress and anxiety-related regions, including the medial prefrontal cortex (mPFC), but their function and mechanism in the mPFC remain unexplored. Therefore, we used a combination of imaging, cre/lox, and behavioral methods to investigate mPFC-AT2R-expressing neurons in fear and stess related behavior.

Methods

To characterize mPFC-AT2R-expressing neurons in the mPFC, AT2R-Cre/tdTomato male and female mice were used for immunohistochemistry. mPFC brain sections were stained with glutamatergic or interneuron markers, and density of AT2R+ cells and colocalization with each marker were quantified. To assess fear-related behaviors in AT2R-flox mice, we selectively deleted AT2R from mPFC neurons using a Cre-expressing adeno-associated virus. Mice then underwent Pavlovian auditory fear conditioning, elevated plus maze, and open field testing.

Results

Immunohistochemistry results revealed that AT2R was densely expressed throughout the mPFC and primarily expressed in somatostatin interneurons in a sex-dependent manner. Following fear conditioning, mPFC-AT2R Cre-lox deletion impaired extinction and increased exploratory behavior in female but not male mice, while locomotion was unaltered by mPFC-AT2R deletion in both sexes.

Conclusions

These results identify mPFC-AT2R+ neurons as a novel subgroup of somatostatin interneurons and reveal their role in regulating fear learning in a sex-dependent manner, potentially offering insights into novel therapeutic targets for posttraumatic stress disorder.

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

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

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.

Sex-dependent effects of angiotensin type 2 receptor expressing medial prefrontal cortex (mPFC) interneurons in fear extinction learning

Background

The renin-angiotensin system (RAS) has been identified as a potential therapeutic target for PTSD, though its mechanisms are not well understood. Brain angiotensin type 2 receptors (AT2Rs) are a subtype of angiotensin II receptors located in stress and anxiety-related regions, including the medial prefrontal cortex (mPFC), but their function and mechanism in the mPFC remain unexplored. We therefore used a combination of imaging, cre/lox, and behavioral methods to investigate mPFC-AT2R-expressing neuron involvement in fear learning.

Methods

To characterize mPFC-AT2R-expressing neurons in the mPFC, AT2R-Cre/td-Tomato male and female mice were used for immunohistochemistry (IHC). mPFC brain sections were stained with glutamatergic or interneuron markers, and density of AT2R+ cells and colocalization with each marker was quantified. To assess fear-related behaviors in AT2R-flox mice, we selectively deleted AT2R from mPFC neurons using an AAV-Cre virus. Mice then underwent Pavlovian auditory fear conditioning, approach/avoidance, and locomotion testing.

Results

IHC results revealed that AT2R is densely expressed in the mPFC. Furthermore, AT2R is primarily expressed in somatostatin interneurons in females but not males. Following fear conditioning, mPFC-AT2R deletion impaired extinction in female but not male mice. Locomotion was unaltered by mPFC-AT2R deletion in males or females, while AT2R-deleted females had increased exploratory behavior.

Conclusion

These results lend support for mPFC-AT2R+ neurons as a novel subgroup of somatostatin interneurons that influence fear extinction in a sex-dependent manner. This furthers underscores the role of mPFC in top-down regulation and a unique role for peptidergic (ie., angiotensin) mPFC regulation of fear and sex differences.

Sex-specific modulation of the medial prefrontal cortex by glutamatergic median raphe neurons

A substantial proportion of raphe neurons are glutamatergic. However, little is known about how these glutamatergic neurons modulate the forebrain. We investigated how glutamatergic median raphe nucleus (MRN) input modulates the medial prefrontal cortex (mPFC), a critical component of fear circuitry. We show that vesicular glutamate transporter 3 (VGLUT3)-expressing MRN neurons activate VGLUT3- and somatostatin-expressing neurons in the mPFC. Consistent with this modulation of mPFC GABAergic neurons, activation of MRN (VGLUT3) neurons enhances GABAergic transmission in mPFC pyramidal neurons and attenuates fear memory in female but not male mice. Serotonin plays a key role in MRN (VGLUT3) neuron-mediated GABAergic plasticity in the mPFC. In agreement with these female-specific effects, we observed sex differences in glutamatergic transmission onto MRN (VGLUT3) neurons and in mPFC (VGLUT3) neuron-mediated dual release of glutamate and GABA. Our results demonstrate a cell type-specific modulation of the mPFC by MRN (VGLUT3) neurons and reveal a sex-specific role of this neuromodulation in mPFC synaptic plasticity.

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.

Sex-specific modulation of the medial prefrontal cortex by glutamatergic median raphe neurons

The current understanding of the neuromodulatory role of the median raphe nucleus (MRN) is primarily based on its putative serotonergic output. However, a significant proportion of raphe neurons are glutamatergic. The present study investigated how glutamatergic MRN input modulates the medial prefrontal cortex (mPFC), a critical component of the fear circuitry. Our studies show that VGLUT3-expressing MRN neurons modulate VGLUT3- and somatostatin-expressing neurons in the mPFC. Consistent with this modulation of mPFC GABAergic neurons, activation of MRN (VGLUT3) neurons suppresses mPFC pyramidal neuron activity and attenuates fear memory in female but not male mice. In agreement with these female-specific effects, we observed sex differences in glutamatergic transmission onto MRN (VGLUT3) neurons and mPFC (VGLUT3) neuron-mediated dual release of glutamate and GABA. Thus, our results demonstrate a cell type-specific modulation of the mPFC by MRN (VGLUT3) neurons and reveal a sex-specific role of this neuromodulation in mPFC synaptic plasticity and fear memory.

Adolescent binge drinking leads to long-lasting changes in cortical microcircuits in mice

Adolescent drug consumption has increased risks to the individual compared to consumption in adulthood, due to the likelihood of long-term and permanent behavioral and neurological adaptations. However, little is known about how adolescent alcohol consumption influences the maturation and trajectory of cortical circuit development. Here, we explore the consequences of adolescent binge drinking on somatostatin (SST) neuronal function in superficial layers of the prelimbic (PL) cortex in male and female SST-Ai9 mice. We find that adolescent drinking-in-the-dark (DID) produces sex-dependent increases in intrinsic excitability of SST neurons, with no change in overall SST cell number, persisting well into adulthood. While we did not find evidence of altered GABA release from SST neurons onto other neurons within the circuit, we found a complementary reduction in layer II/III pyramidal neuron excitability immediately after binge drinking; however, this hypoexcitability rebounded towards increased pyramidal neuron activity in adulthood in females, suggesting long-term homeostatic adaptations in this circuit. Together, this suggests that binge drinking during key developmental timepoints leads to permanent changes in PL microcircuitry function, which may have broad behavioral implications.

Function and development of interneurons involved in brain tissue oxygen regulation

The regulation of oxygen in brain tissue is one of the most important fundamental questions in neuroscience and medicine. The brain is a metabolically demanding organ, and its health directly depends on maintaining oxygen concentrations within a relatively narrow range that is both sufficiently high to prevent hypoxia, and low enough to restrict the overproduction of oxygen species. Neurovascular interactions, which are responsible for oxygen delivery, consist of neuronal and glial components. GABAergic interneurons play a particularly important role in neurovascular interactions. The involvement of interneurons extends beyond the perspective of inhibition, which prevents excessive neuronal activity and oxygen consumption, and includes direct modulation of the microvasculature depending upon their sub-type. Namely, nitric oxide synthase-expressing (NOS), vasoactive intestinal peptide-expressing (VIP), and somatostatin-expressing (SST) interneurons have shown modulatory effects on microvessels. VIP interneurons are known to elicit vasodilation, SST interneurons typically cause vasoconstriction, and NOS interneurons have to propensity to induce both effects. Given the importance and heterogeneity of interneurons in regulating local brain tissue oxygen concentrations, we review their differing functions and developmental trajectories. Importantly, VIP and SST interneurons display key developmental milestones in adolescence, while NOS interneurons mature much earlier. The implications of these findings point to different periods of critical development of the interneuron-mediated oxygen regulatory systems. Such that interference with normal maturation processes early in development may effect NOS interneuron neurovascular interactions to a greater degree, while insults later in development may be more targeted toward VIP- and SST-mediated mechanisms of oxygen regulation.

Infralimbic medial prefrontal cortex signalling to calbindin 1 positive neurons in posterior basolateral amygdala suppresses anxiety- and depression-like behaviours

Generalization is a fundamental cognitive ability of organisms to deal with the uncertainty in real-world situations. Excessive fear generalization and impaired reward generalization are closely related to many psychiatric disorders. However, the neural circuit mechanism for reward generalization and its role in anxiety-like behaviours remain elusive. Here, we found a robust activation of calbindin 1-neurons (Calb 1) in the posterior basolateral amygdala (pBLA), simultaneous with reward generalization to an ambiguous cue after reward conditioning in mice. We identify the infralimbic medial prefrontal cortex (IL) to the pBLA^Calb1 (Calb 1 neurons in the pBLA) pathway as being involved in reward generalization for the ambiguity. Activating IL–pBLA inputs strengthens reward generalization and reduces chronic unpredictable mild stress-induced anxiety- and depression-like behaviours in a manner dependent on pBLA^Calb1 neuron activation. These findings suggest that the IL–pBLA^Calb1 circuit could be a target to promote stress resilience via reward generalization and consequently ameliorate anxiety- and depression-like behaviours.

The neural mechanisms for reward generalization are not fully understood. Here the authors investigate the role of posterior basolateral amygdala calbindin-expressing cells in modulating behavioural responses related to reward and aversion.

Decoding Shared Versus Divergent Transcriptomic Signatures Across Cortico-Amygdala Circuitry in PTSD and Depressive Disorders

OBJECTIVE

Posttraumatic stress disorder (PTSD) is a debilitating neuropsychiatric disease that is highly comorbid with major depressive disorder (MDD) and bipolar disorder. The overlap in symptoms is hypothesized to stem from partially shared genetics and underlying neurobiological mechanisms. To delineate conservation between transcriptional patterns across PTSD and MDD, the authors examined gene expression in the human cortex and amygdala in these disorders.

METHODS

RNA sequencing was performed in the postmortem brain of two prefrontal cortex regions and two amygdala regions from donors diagnosed with PTSD (N=107) or MDD (N=109) as well as from neurotypical donors (N=109).

RESULTS

The authors identified a limited number of differentially expressed genes (DEGs) specific to PTSD, with nearly all mapping to cortical versus amygdala regions. PTSD-specific DEGs were enriched in gene sets associated with downregulated immune-related pathways and microglia as well as with subpopulations of GABAergic inhibitory neurons. While a greater number of DEGs associated with MDD were identified, most overlapped with PTSD, and only a few were MDD specific. The authors used weighted gene coexpression network analysis as an orthogonal approach to confirm the observed cellular and molecular associations.

CONCLUSIONS

These findings provide supporting evidence for involvement of decreased immune signaling and neuroinflammation in MDD and PTSD pathophysiology, and extend evidence that GABAergic neurons have functional significance in PTSD.

Adolescence alcohol exposure impairs fear extinction and alters medial prefrontal cortex plasticity

After experiencing a traumatic event people often turn to alcohol to cope with symptoms. In those with post-traumatic stress disorder (PTSD) and a co-occurring alcohol use disorder (AUD), PTSD symptoms can worsen, suggesting that alcohol changes how traumatic memory is expressed. The objective of this series of experiments is to identify how alcohol drinking (EtOH), following cued fear conditioning and extinction, impacts fear expression in mice. Molecular (activity-regulated cytoskeleton-associated protein, Arc/arg3.1) and structural (dendrite and spine morphometry) markers of neuronal plasticity were measured following remote extinction retrieval. Mouse age (adolescent and adult) and sex were included as interacting variables in a full factorial design. Females drank more EtOH than males and adolescents drank more EtOH than adults. Adolescent females escalated EtOH intake across drinking days. Adolescent drinkers exhibited more conditioned freezing during extinction retrieval, an effect that persisted for at least 20 days. Heightened cued freezing in the adolescent group was associated with greater Arc/arg3.1 expression in layer (L) 2/3 prelimbic (PL) cortex, greater spine density, and reduced basal dendrite complexity. In adults, drinking was associated with reduced L2/3 infralimbic (IL) Arc expression but no behavioral differences. Few sex interactions were uncovered throughout. Overall, these data identify prolonged age-related differences in alcohol-induced fear extinction impairment and medial prefrontal cortex neuroadaptations.

Adolescent Alcohol and Stress Exposure Rewires Key Cortical Neurocircuitry

Human adolescence is a period of development characterized by wide ranging emotions and behavioral risk taking, including binge drinking (Konrad et al., 2013). These behavioral manifestations of adolescence are complemented by growth in the neuroarchitecture of the brain, including synaptic pruning (Spear, 2013) and increases in overall white matter volume (Perrin et al., 2008). During this period of profound physiological maturation, the adolescent brain has a unique vulnerability to negative perturbations. Alcohol consumption and stress exposure, both of which are heightened during adolescence, can individually and synergistically alter these neurodevelopmental trajectories in positive and negative ways (conferring both resiliency and susceptibility) and influence already changing neurotransmitter systems and circuits. Importantly, the literature is rapidly changing and evolving in our understanding of basal sex differences in the brain, as well as the interaction between biological sex and life experiences. The animal literature provides the distinctive opportunity to explore sex-specific stress- and alcohol- induced changes in neurocircuits on a relatively rapid time scale. In addition, animal models allow for the investigation of individual neurons and signaling molecules otherwise inaccessible in the human brain. Here, we review the human and rodent literature with a focus on cortical development, neurotransmitters, peptides, and steroids, to characterize the field's current understanding of the interaction between adolescence, biological sex, and exposure to stress and alcohol.

Aberrant maturation and connectivity of prefrontal cortex in schizophrenia-contribution of NMDA receptor development and hypofunction

The neurobiology of schizophrenia involves multiple facets of pathophysiology, ranging from its genetic basis over changes in neurochemistry and neurophysiology, to the systemic level of neural circuits. Although the precise mechanisms associated with the neuropathophysiology remain elusive, one essential aspect is the aberrant maturation and connectivity of the prefrontal cortex that leads to complex symptoms in various stages of the disease. Here, we focus on how early developmental dysfunction, especially N-methyl-D-aspartate receptor (NMDAR) development and hypofunction, may lead to the dysfunction of both local circuitry within the prefrontal cortex and its long-range connectivity. More specifically, we will focus on an "all roads lead to Rome" hypothesis, i.e., how NMDAR hypofunction during development acts as a convergence point and leads to local gamma-aminobutyric acid (GABA) deficits and input-output dysconnectivity in the prefrontal cortex, which eventually induce cognitive and social deficits. Many outstanding questions and hypothetical mechanisms are listed for future investigations of this intriguing hypothesis that may lead to a better understanding of the aberrant maturation and connectivity associated with the prefrontal cortex.

Coming of age in the frontal cortex: The role of puberty in cortical maturation

Across species, adolescence is a period of growing independence that is associated with the maturation of cognitive, social, and affective processing. Reorganization of neural circuits within the frontal cortex is believed to contribute to the emergence of adolescent changes in cognition and behavior. While puberty coincides with adolescence, relatively little is known about which aspects of frontal cortex maturation are driven by pubertal development and gonadal hormones. In this review, we highlight existing work that suggests puberty plays a role in the maturation of specific cell types in the medial prefrontal cortex (mPFC) of rodents, and highlight possible routes by which gonadal hormones influence frontal cortical circuit development.

Development, Diversity, and Death of MGE-Derived Cortical Interneurons

In the mammalian brain, cortical interneurons (INs) are a highly diverse group of cells. A key neurophysiological question concerns how each class of INs contributes to cortical circuit function and whether specific roles can be attributed to a selective cell type. To address this question, researchers are integrating knowledge derived from transcriptomic, histological, electrophysiological, developmental, and functional experiments to extensively characterise the different classes of INs. Our hope is that such knowledge permits the selective targeting of cell types for therapeutic endeavours. This review will focus on two of the main types of INs, namely the parvalbumin (PV⁺) or somatostatin (SOM⁺)-containing cells, and summarise the research to date on these classes.

Memories are not written in stone: Re-writing fear memories by means of non-invasive brain stimulation and optogenetic manipulations

The acquisition of fear associative memory requires brain processes of coordinated neural activity within the amygdala, prefrontal cortex (PFC), hippocampus, thalamus and brainstem. After fear consolidation, a suppression of fear memory in the absence of danger is crucial to permit adaptive coping behavior. Acquisition and maintenance of fear extinction critically depend on amygdala-PFC projections. The robust correspondence between the brain networks encompassed cortical and subcortical hubs involved into fear processing in humans and in other species underscores the potential utility of comparing the modulation of brain circuitry in humans and animals, as a crucial step to inform the comprehension of fear mechanisms and the development of treatments for fear-related disorders. The present review is aimed at providing a comprehensive description of the literature on recent clinical and experimental researches regarding the noninvasive brain stimulation and optogenetics. These innovative manipulations applied over specific hubs of fear matrix during fear acquisition, consolidation, reconsolidation and extinction allow an accurate characterization of specific brain circuits and their peculiar interaction within the specific fear processing.

An Adolescent Sensitive Period for Threat Responding: Impacts of Stress and Sex

Anxiety and fear-related disorders peak in prevalence during adolescence, a window of rapid behavioral development and neural remodeling. However, understanding of the development of threat responding and the underlying neural circuits remains limited. Preclinical models of threat conditioning and extinction have provided an unparalleled glimpse into the developing brain. In this review we discuss mouse and rat studies on the development of threat response regulation, with a focus on the adolescent period. Evidence of nonlinear patterns of threat responding during adolescence and the continued development of the underlying circuitry is highly indicative of an adolescent sensitive period for threat response regulation. While we highlight literature in support of this unique developmental window, we also emphasize the need for causal studies to clarify the parameters defining such a sensitive period. In doing so, we explore how stress and biological sex affect the development and expression of threat response regulation during adolescence and beyond. Ultimately, a deeper understanding of how these factors interact with and affect developmental trajectories of learning and memory will inform treatment and prevention strategies for pediatric anxiety disorders.

Development-Dependent Plasticity in Vasoactive Intestinal Polypeptide Neurons in the Infralimbic Cortex

The onset of several neuropsychiatric disorders including anxiety disorders coincides with adolescence. Consistently, threat extinction, which plays a key role in the regulation of anxiety-related behaviors, is diminished during adolescence. Furthermore, this attenuated threat extinction during adolescence is associated with an altered synaptic plasticity in the infralimbic medial prefrontal cortex (IL-mPFC), a brain region critical for threat extinction. However, the mechanism underlying the altered plasticity in the IL-mPFC during adolescence is unclear. Given the purported role of vasoactive intestinal polypeptide expressing interneurons (VIPINs) in disinhibition and hence their potential to affect cortical plasticity, we examined whether VIPINs exhibit an adolescence-specific plasticity in the IL-mPFC. We observed an increase in GABAergic transmission and a decrease in excitability in VIPINs during adolescence. Male mice show a significantly higher VIPIN-pyramidal neuron GABAergic transmission compared with female mice. The observed increase in GABAergic transmission and a decrease in membrane excitability in VIPINs during adolescence could play a role in the altered plasticity in the adolescent IL-mPFC. Furthermore, the suppression of VIPIN-mediated GABAergic transmission in females might be relevant to sex differences in anxiety disorders.

Stress impacts corticoamygdalar connectivity in an age-dependent manner

Stress is a socio-environmental risk factor for the development of psychiatric disorders, with the age of exposure potentially determining the outcome. Several brain regions mediate stress responsivity, with a prominent role of the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) and their reciprocal inhibitory connectivity. Here we investigated the impact of stress exposure during adolescence and adulthood on the activity of putative pyramidal neurons in the BLA and corticoamygdalar plasticity using in vivo electrophysiology. 155 male Sprague-Dawley rats were subjected to a combination of footshock/restraint stress in either adolescence (postnatal day 31-40) or adulthood (postnatal day 65-74). Both adolescent and adult stress increased the number of spontaneously active putative BLA pyramidal neurons 1-2 weeks, but not 5-6 weeks post stress. High-frequency stimulation (HFS) of BLA and mPFC depressed evoked spike probability in the mPFC and BLA, respectively, in adult but not adolescent rats. In contrast, an adult-like BLA HFS-induced decrease in spike probability of mPFC neurons was found 1-2 weeks post-adolescent stress. Changes in mPFC and BLA neuron discharge were found 1-2 weeks post-adult stress after BLA and mPFC HFS, respectively. All these changes were transient since they were not found 5-6 weeks post adolescent or adult stress. Our findings indicate that stress during adolescence may accelerate the development of BLA-PFC plasticity, probably due to BLA hyperactivity, which can also disrupt the reciprocal communication of BLA-mPFC after adult stress. Therefore, precocious BLA-mPFC connectivity alterations may represent an early adaptive stress response that ultimately may contribute to vulnerability to adult psychiatric disorders.

Increased thin-spine density in frontal cortex pyramidal neurons in a genetic rat model of schizophrenia-relevant features

The cellular mechanisms altered during brain wiring leading to cognitive disturbances in neurodevelopmental disorders remain unknown. We have previously reported altered cortical expression of neurodevelopmentally regulated synaptic markers in a genetic animal model of schizophrenia-relevant behavioral features, the Roman-High Avoidance rat strain (RHA-I). To further explore this phenotype, we looked at dendritic spines in cortical pyramidal neurons, as changes in spine density and morphology are one of the main processes taking place during adolescence. An HSV-viral vector carrying green fluorescent protein (GFP) was injected into the frontal cortex (FC) of a group of 11 RHA-I and 12 Roman-Low Avoidance (RLA-I) male rats. GFP labeled dendrites from pyramidal cells were 3D reconstructed and number and types of spines quantified. We observed an increased spine density in the RHA-I, corresponding to a larger fraction of immature thin spines, with no differences in stubby and mushroom spines. Glia cells, parvalbumin (PV) and somatostatin (SST) interneurons and surrounding perineuronal net (PNN) density are known to participate in FC and pyramidal neuron dendritic spine maturation. We determined by stereological-based quantification a significantly higher number of GFAP-positive astrocytes in the FC of the RHA-I strain, with no difference in microglia (Iba1-positive cells). The number of inhibitory PV, SST interneurons or PNN density, on the contrary, was unchanged. Results support our belief that the RHA-I strain presents a more immature FC, with some structural features like those observed during adolescence, adding construct validity to this strain as a genetic behavioral model of neurodevelopmental disorders.

Maturation of amygdala inputs regulate shifts in social and fear behaviors: A substrate for developmental effects of stress

Stress can negatively impact brain function and behaviors across the lifespan. However, stressors during adolescence have particularly harmful effects on brain maturation, and on fear and social behaviors that extend beyond adolescence. Throughout development, social behaviors are refined and the ability to suppress fear increases, both of which are dependent on amygdala activity. We review rodent literature focusing on developmental changes in social and fear behaviors, cortico-amygdala circuits underlying these changes, and how this circuitry is altered by stress. We first describe changes in fear and social behaviors from adolescence to adulthood and parallel developmental changes in cortico-amygdala circuitry. We propose a framework in which maturation of cortical inputs to the amygdala promote changes in social drive and fear regulation, and the particularly damaging effects of stress during adolescence may occur through lasting changes in this circuit. This framework may explain why anxiety and social pathologies commonly co-occur, adolescents are especially vulnerable to stressors impacting social and fear behaviors, and predisposed towards psychiatric disorders related to abnormal cortico-amygdala circuits.

Distinct subtypes of inhibitory interneurons differentially promote the propagation of rate and temporal codes in the feedforward neural network

Sensory information is believed to be encoded in neuronal spikes using two different neural codes, the rate code (spike firing rate) and the temporal code (precisely-timed spikes). Since the sensory cortex has a highly hierarchical feedforward structure, sensory information-carrying neural codes should reliably propagate across the feedforward network (FFN) of the cortex. Experimental evidence suggests that inhibitory interneurons, such as the parvalbumin-positive (PV) and somatostatin-positive (SST) interneurons, that have distinctively different electrophysiological and synaptic properties, modulate the neural codes during sensory information processing in the cortex. However, how PV and SST interneurons impact on the neural code propagation in the cortical FFN is unknown. We address this question by building a five-layer FFN model consisting of a physiologically realistic Hodgkin-Huxley-type models of excitatory neurons and PV/SST interneurons at different ratios. In response to different firing rate inputs (20-80 Hz), a higher ratio of PV over SST interneurons promoted a reliable propagation of all ranges of firing rate inputs. In contrast, in response to a range of precisely-timed spikes in the form of pulse-packets [with a different number of spikes (α, 40-400 spikes) and degree of dispersion (σ, 0-20 ms)], a higher ratio of SST over PV interneurons promoted a reliable propagation of pulse-packets. Our simulation results show that PV and SST interneurons differentially promote a reliable propagation of the rate and temporal codes, respectively, indicating that the dynamic recruitment of PV and SST interneurons may play critical roles in a reliable propagation of sensory information-carrying neural codes in the cortical FFN.