Adolescent pruning and stabilization of dendritic spines on cortical layer 5 pyramidal neurons do not depend on gonadal hormones

Social experience in adolescence shapes prefrontal cortex structure and function in adulthood

During adolescence, the prefrontal cortex (PFC) undergoes dramatic reorganization. PFC development is profoundly influenced by the social environment, disruptions to which may prime the emergence of psychopathology across the lifespan. We investigated the neurobehavioral consequences of isolation experienced in adolescence in mice, and in particular, the long-term consequences that were detectable even despite normalization of the social milieu. Isolation produced biases toward habit-like behavior at the expense of flexible goal seeking, plus anhedonic-like reward deficits. Behavioral phenomena were accompanied by neuronal dendritic spine over-abundance and hyper-excitability in the ventromedial PFC (vmPFC), which was necessary for the expression of isolation-induced habits and sufficient to trigger behavioral inflexibility in socially reared controls. Isolation activated cytoskeletal regulatory pathways otherwise suppressed during adolescence, such that repression of constituent elements prevented long-term isolation-induced neurosequelae. Altogether, our findings unveil an adolescent critical period and multi-model mechanism by which social experiences facilitate prefrontal cortical maturation.

Sex differences in oxycodone-taking behaviors are linked to disruptions in reward-guided, decision-making functions

Problematic opioid use that emerges in a subset of individuals may be due to pre-existing disruptions in the biobehavioral mechanisms that regulate drug use. The identity of these mechanisms is not known, but emerging evidence suggests that suboptimal decision-making that is observable prior to drug use may contribute to the pathology of addiction and, notably, serve as a powerful phenotype for interrogating biologically based differences in opiate-taking behaviors. The current study investigated the relationship between decision-making phenotypes and opioid-taking behaviors in male and female Long Evans rats. Adaptive decision-making processes were assessed using a probabilistic reversal-learning task and oxycodone- (or vehicle, as a control) taking behaviors assessed for 32 days using a saccharin fading procedure that promoted dynamic intake of oxycodone. Tests of motivation, extinction, and reinstatement were also performed. Computational analyses of decision-making and opioid-taking behaviors revealed that attenuated reward-guided decision-making was associated with greater self-administration of oxycodone and addiction-relevant behaviors. Moreover, pre-existing impairments in reward-guided decision-making observed in female rats was associated with greater oxycodone use and addiction-relevant behaviors when compared to males. These results provide new insights into the biobehavioral mechanisms that regulate opiate-taking behaviors and offer a novel phenotypic approach for interrogating sex differences in addiction susceptibility and opioid use disorders.

The last stage of development: The restructuring and plasticity of the cortex during adolescence especially at puberty

There is considerable evidence of reorganization in the prefrontal cortex during adolescence in humans, as well as in rodents, where the cellular basis can be explored. Studies from my laboratory in the rat medial prefrontal cortex are reviewed here. In general, growth predominates before puberty. Pruning mainly occurs at puberty and after with decreases in the number of synapses, dendrites, and neurons. Perineuronal nets, extracellular structures that control plasticity, are pruned peripubertally only in female rats, which may further open the adolescent prefrontal cortex to environmental influences. This is supported by our recent evidence that exposure to mild stress early, but not late, in adolescence decreases prepulse inhibition. Additionally, exposure to methamphetamine in females early in adolescence increases the number of a major class of inhibitory interneurons, parvalbumin neurons, while the opposite occurs late in adolescence. In females, even estrogen receptor beta mRNA decreases at puberty in the prefrontal cortex. Interestingly, rats of both sexes perform better after puberty on a test of cognitive flexibility in the water maze. Thus, evidence is accruing that adolescence is not a single entity but rather an ongoing set of processes, and environmental effects will differ depending on timing and sex.

Microglia complement signaling promotes neuronal elimination and normal brain functional connectivity

Complement signaling is thought to serve as an opsonization signal to promote the phagocytosis of synapses by microglia. However, while its role in synaptic remodeling has been demonstrated in the retino-thalamic system, it remains unclear whether complement signaling mediates synaptic pruning in the brain more generally. Here we found that mice lacking the Complement receptor 3, the major microglia complement receptor, failed to show a deficit in either synaptic pruning or axon elimination in the developing mouse cortex. Instead, mice lacking Complement receptor 3 exhibited a deficit in the perinatal elimination of neurons in the cortex, a deficit that is associated with increased cortical thickness and enhanced functional connectivity in these regions in adulthood. These data demonstrate a role for complement in promoting neuronal elimination in the developing cortex.

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.

Strength of Excitatory Inputs to Layer 3 Pyramidal Neurons During Synaptic Pruning in the Monkey Prefrontal Cortex: Relevance for the Pathogenesis of Schizophrenia

BACKGROUND

In schizophrenia, layer 3 pyramidal neurons (L3PNs) of the dorsolateral prefrontal cortex exhibit deficits in markers of excitatory synaptic inputs that are thought to disrupt the patterns of neural network activity essential for cognitive function. These deficits are usually interpreted under Irwin Feinberg's hypothesis of altered synaptic pruning, which postulates that normal periadolescent pruning, thought to preferentially eliminate weak/immature synapses, is altered in schizophrenia. However, it remains unknown whether periadolescent pruning on L3PNs in the primate dorsolateral prefrontal cortex selectively eliminates weak excitatory synapses or uniformly eliminates excitatory synapses across the full distribution of synaptic strengths.

METHODS

To distinguish between these alternative models of synaptic pruning, we assessed the densities of dendritic spines, the site of most excitatory inputs to L3PNs, and the distributions of excitatory synaptic strengths in dorsolateral prefrontal cortex L3PNs from male and female monkeys across the periadolescent period of synaptic pruning. We used patch-clamp methods in acute brain slices to record miniature excitatory synaptic currents and intracellular filling with biocytin to quantify dendritic spines.

RESULTS

On L3PNs, dendritic spines exhibited the expected age-related decline in density, but mean synaptic strength and the shape of synaptic strength distributions remained stable with age.

CONCLUSIONS

The absence of age-related differences in mean synaptic strength and synaptic strength distributions supports the model of a uniform pattern of synaptic pruning across the full range of synaptic strengths. The implications of these findings for the pathogenesis and functional consequences of dendritic spine deficits in schizophrenia are discussed.

De-scattering Deep Neural Network Enables Fast Imaging of Spines through Scattering Media by Temporal Focusing Microscopy

Today the gold standard for in vivo imaging through scattering tissue is point-scanning two-photon microscopy (PSTPM). Especially in neuroscience, PSTPM is widely used for deep-tissue imaging in the brain. However, due to sequential scanning, PSTPM is slow. Temporal focusing microscopy (TFM), on the other hand, focuses femtosecond pulsed laser light temporally while keeping wide-field illumination, and is consequently much faster. However, due to the use of a camera detector, TFM suffers from the scattering of emission photons. As a result, TFM produces images of poor quality, obscuring fluorescent signals from small structures such as dendritic spines. In this work, we present a de-scattering deep neural network (DeScatterNet) to improve the quality of TFM images. Using a 3D convolutional neural network (CNN) we build a map from TFM to PSTPM modalities, to enable fast TFM imaging while maintaining high image quality through scattering media. We demonstrate this approach for in vivo imaging of dendritic spines on pyramidal neurons in the mouse visual cortex. We quantitatively show that our trained network rapidly outputs images that recover biologically relevant features previously buried in the scattered fluorescence in the TFM images. In vivo imaging that combines TFM and the proposed neural network is one to two orders of magnitude faster than PSTPM but retains the high quality necessary to analyze small fluorescent structures. The proposed approach could also be beneficial for improving the performance of many speed-demanding deep-tissue imaging applications, such as in vivo voltage imaging.

A biologically informed polygenic score of neuronal plasticity moderates the association between cognitive aptitudes and cortical thickness in adolescents

Although many studies of the adolescent brain identified positive associations between cognitive abilities and cortical thickness, little is known about mechanisms underlying such brain-behavior relationships. With experience-induced plasticity playing an important role in shaping the cerebral cortex throughout life, it is likely that some of the inter-individual variations in cortical thickness could be explained by genetic variations in relevant molecular processes, as indexed by a polygenic score of neuronal plasticity (PGS-NP). Here, we studied associations between PGS-NP, cognitive abilities, and thickness of the cerebral cortex, estimated from magnetic resonance images, in the Saguenay Youth Study (SYS, 533 females, 496 males: age=15.0 ± 1.8 years of age; cross-sectional), and the IMAGEN Study (566 females, 556 males; between 14 and 19 years; longitudinal). Using Gene Ontology, we first identified 199 genes implicated in neuronal plasticity, which mapped to 155,600 single nucleotide polymorphisms (SNPs). Second, we estimated their effect sizes from an educational attainment meta-GWAS to build a PGS-NP. Third, we examined a possible moderating role of PGS-NP in the relationship between performance intelligence quotient (PIQ), and its subtests, and the thickness of 34 cortical regions. In SYS, we observed a significant interaction between PGS-NP and object assembly vis-à-vis thickness in male adolescents (p = 0.026). A median-split analysis showed that, in males with a 'high' PGS-NP, stronger associations between object assembly and thickness were found in regions with larger age-related changes in thickness (r = 0.55, p = 0.00075). Although the interaction between PIQ and PGS-NP was non-significant (p = 0.064), we performed a similar median-split analysis. Again, in the high PGS-NP males, positive associations between PIQ and thickness were observed in regions with larger age-related changes in thickness (r = 0.40, p = 0.018). In the IMAGEN cohort, we did not replicate the first set of results (interaction between PGS-NP and cognitive abilities via-a-vis cortical thickness) while we did observe the same relationship between the brain-behaviour relationship and (longitudinal) changes in cortical thickness (Matrix reasoning: r = 0.63, p = 6.5e-05). No statistically significant results were observed in female adolescents in either cohort. Overall, these cross-sectional and longitudinal results suggest that molecular mechanisms involved in neuronal plasticity may contribute to inter-individual variations of cortical thickness related to cognitive abilities during adolescence in a sex-specific manner.

Dendritic Spines: Synaptogenesis and Synaptic Pruning for the Developmental Organization of Brain Circuits

Synaptic overproduction and elimination is a regular developmental event in the mammalian brain. In the cerebral cortex, synaptic overproduction is almost exclusively correlated with glutamatergic synapses located on dendritic spines. Therefore, analysis of changes in spine density on different parts of the dendritic tree in identified classes of principal neurons could provide insight into developmental reorganization of specific microcircuits.The activity-dependent stabilization and selective elimination of the initially overproduced synapses is a major mechanism for generating diversity of neural connections beyond their genetic determination. The largest number of overproduced synapses was found in the monkey and human cerebral cortex. The highest (exceeding adult values by two- to threefold) and most protracted overproduction (up to third decade of life) was described for associative layer IIIC pyramidal neurons in the human dorsolateral prefrontal cortex.Therefore, the highest proportion and extraordinarily extended phase of synaptic spine overproduction is a hallmark of neural circuitry in human higher-order associative areas. This indicates that microcircuits processing the most complex human cognitive functions have the highest level of developmental plasticity. This finding is the backbone for understanding the effect of environmental impact on the development of the most complex, human-specific cognitive and emotional capacities, and on the late onset of human-specific neuropsychiatric disorders, such as autism and schizophrenia.

Pubertal development underlies optimization of inhibitory control through specialization of ventrolateral prefrontal cortex

Inhibitory control improves into young adulthood after specialization of relevant brain systems during adolescence. However, the biological mechanisms supporting this unique transition are not well understood. Given that adolescence is defined by puberty, we examined relative contributions of chronological age and pubertal maturation to inhibitory control development. 105 8-19-year-olds completed 1-5 longitudinal visits (227 visits total) in which pubertal development was assessed via self-reported Tanner stage and inhibitory control was assessed with an in-scanner antisaccade task. As expected, percentage and latency of correct antisaccade responses improved with age and pubertal stage. When controlling for pubertal stage, chronological age was distinctly associated with correct response rate. In contrast, pubertal stage was uniquely associated with antisaccade latency even when controlling for age. Chronological age was associated with fMRI task activation in several regions including the right dorsolateral prefrontal cortex, while puberty was associated with right ventrolateral prefrontal cortex (VLPFC) activation. Furthermore, task-related connectivity between VLPFC and cingulate was associated with both pubertal stage and response latency. These results suggest that while age-related developmental processes may support maturation of brain systems underlying the ability to inhibit a response, puberty may play a larger role in the effectiveness of generating cognitive control responses.

Areas of Convergence and Divergence in Adolescent Social Isolation and Binge Drinking: A Review

Adolescence is a critical developmental period characterized by enhanced social interactions, ongoing development of the frontal cortex and maturation of synaptic connections throughout the brain. Adolescents spend more time interacting with peers than any other age group and display heightened reward sensitivity, impulsivity and diminished inhibitory self-control, which contribute to increased risky behaviors, including the initiation and progression of alcohol use. Compared to adults, adolescents are less susceptible to the negative effects of ethanol, but are more susceptible to the negative effects of stress, particularly social stress. Juvenile exposure to social isolation or binge ethanol disrupts synaptic connections, dendritic spine morphology, and myelin remodeling in the frontal cortex. These structural effects may underlie the behavioral and cognitive deficits seen later in life, including social and memory deficits, increased anxiety-like behavior and risk for alcohol use disorders (AUD). Although the alcohol and social stress fields are actively investigating the mechanisms through which these effects occur, significant gaps in our understanding exist, particularly in the intersection of the two fields. This review will highlight the areas of convergence and divergence in the fields of adolescent social stress and ethanol exposure. We will focus on how ethanol exposure or social isolation stress can impact the development of the frontal cortex and lead to lasting behavioral changes in adulthood. We call attention to the need for more mechanistic studies and the inclusion of the evaluation of sex differences in these molecular, structural, and behavioral responses.

Computational benefits of structural plasticity, illustrated in songbirds

The plasticity of nervous systems allows animals to quickly adapt to a changing environment. In particular, the structural plasticity of brain networks is often critical to the development of the central nervous system and the acquisition of complex behaviors. As an example, structural plasticity is central to the development of song-related brain circuits and may be critical for song acquisition in juvenile songbirds. Here, we review current evidences for structural plasticity and their significance from a computational point of view. We start by reviewing evidence for structural plasticity across species and categorizing them along the spatial axes as well as the along the time course during development. We introduce the vocal learning circuitry in zebra finches, as a useful example of structural plasticity, and use this specific case to explore the possible contributions of structural plasticity to computational models. Finally, we discuss current modeling studies incorporating structural plasticity and unexplored questions which are raised by such models.

Persistent behavioral and neurobiological consequences of social isolation during adolescence

Meaningful social interactions are a fundamental human need, the lack of which can pose serious risks to an individual's physical and mental health. Across species, peer-oriented social behaviors are dramatically reshaped during adolescence, a developmental period characterized by dynamic changes in brain structure and function as individuals transition into adulthood. Thus, the experience of social isolation during this critical developmental stage may be especially pernicious, as it could permanently derail typical neurobiological processes that are necessary for establishing adaptive adult behaviors. The purpose of this review is to summarize investigations in which rodents were isolated during adolescence, then re-housed in typical social groups prior to testing, thus allowing the investigators to resolve the long-term consequences of social adversity experienced during adolescent sensitive periods, despite subsequent normalization of the social environment. Here, we discuss alterations in social, anxiety-like, cognitive, and decision-making behaviors in previously isolated adult rodents. We then explore corresponding neurobiological findings, focusing on the prefrontal cortex, including changes in synaptic densities and protein levels, white matter and oligodendrocyte function, and neuronal physiology. Made more urgent by the recent wave of social deprivation resulting from the COVID-19 pandemic, especially amongst school-aged adolescents, understanding the mechanisms by which even transient social adversity can negatively impact brain function across the lifespan is of paramount importance.

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.

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.

Cortical reorganization during adolescence: What the rat can tell us about the cellular basis

The human cortex, particularly the prefrontal cortex, decreases in volume during adolescence which indicates considerable pruning. There is consistent evidence from human, monkey and rat tissue that synapses, dendritic spines and dendrites are pruned during this time. However, our work with a rat model of adolescence shows that other cellular components are remodeling at this time as well. Neurons are also pruned and we have found that in female rats, puberty is a key signal for this process. Other critical developmental events occur that are not detectable in gross size changes including the growth of dopaminergic inputs. The changes in the inhibitory GABAergic system, especially the parvalbumin-expressing neuronal subtype, are an essential part of the maturation of the prefrontal cortex. This involves the formation of perineuronal nets around parvalbumin interneurons that allow mature fast spiking. We have found a large increase in perineuronal nets from early adolescence to adulthood in both sexes. We also have seen a temporary pause in this increase at the time of puberty in females. These complicated events cannot be deduced from MRI. The cellular reorganization that is indicated by size changes in the human cortex during adolescence can be informed by work from rodent models.

Sex and Pubertal Status Influence Dendritic Spine Density on Frontal Corticostriatal Projection Neurons in Mice

In humans, nonhuman primates, and rodents, the frontal cortices exhibit grey matter thinning and dendritic spine pruning that extends into adolescence. This maturation is believed to support higher cognition but may also confer psychiatric vulnerability during adolescence. Currently, little is known about how specific cell types in the frontal cortex mature or whether puberty plays a role in the maturation of some cell types but not others. Here, we used mice to characterize the spatial topography and adolescent development of cross-corticostriatal (cSTR) neurons that project through the corpus collosum to the dorsomedial striatum. We found that apical spine density on cSTR neurons in the medial prefrontal cortex decreased significantly between late juvenile (P29) and young adult time points (P60), with females exhibiting higher spine density than males at both ages. Adult males castrated prior to puberty onset had higher spine density compared to sham controls. Adult females ovariectomized before puberty onset showed greater variance in spine density measures on cSTR cells compared to controls, but their mean spine density did not significantly differ from sham controls. Our findings reveal that these cSTR neurons, a subtype of the broader class of intratelencephalic-type neurons, exhibit significant sex differences and suggest that spine pruning on cSTR neurons is regulated by puberty in male mice.

Social Isolation in Adolescence Disrupts Cortical Development and Goal-Dependent Decision-Making in Adulthood, Despite Social Reintegration

The social environment influences neurodevelopment. Investigations using rodents to study this phenomenon commonly isolate subjects, then assess neurobehavioral consequences while animals are still isolated. This approach precludes one from dissociating the effects of on-going versus prior isolation, hindering our complete understanding of the consequences of social experience during particular developmental periods. Here, we socially isolated adolescent mice from postnatal day (P)31 to P60, then re-housed them into social groups. We tested their ability to select actions based on expected outcomes using multiple reinforcer devaluation and instrumental contingency degradation techniques. Social isolation in adolescence (but not adulthood) weakened instrumental response updating, causing mice to defer to habit-like behaviors. Habit biases were associated with glucocorticoid insufficiency in adolescence, oligodendrocyte marker loss throughout cortico-striatal regions, and dendritic spine and synaptic marker excess in the adult orbitofrontal cortex (OFC). Artificial, chemogenetic stimulation of the ventrolateral OFC in typical, healthy mice recapitulated response biases following isolation, causing habit-like behaviors. Meanwhile, correcting dendritic architecture by inhibiting the cytoskeletal regulatory protein ROCK remedied instrumental response updating defects in socially isolated mice. Our findings suggest that adolescence is a critical period during which social experience optimizes one's ability to seek and attain goals later in life. Age-typical dendritic spine elimination appears to be an essential factor, and in its absence, organisms may defer to habit-based behaviors.

α4βδ GABAA Receptors Trigger Synaptic Pruning and Reduce Dendritic Length of Female Mouse CA3 Hippocampal Pyramidal Cells at Puberty

Synaptic pruning during adolescence is critical for optimal cognition. The CA3 hippocampus contains unique spine types and plays a pivotal role in pattern separation and seizure generation, where sex differences exist, but adolescent pruning has only been studied in the male. Thus, for the present study we assessed pruning of specific spine types in the CA3 hippocampus during adolescence and investigated a possible mechanism in the female mouse. To this end, we used Golgi-impregnated brains from pubertal (∼PND 35, assessed by vaginal opening) and post-pubertal (PND 56) mice. Spine density was assessed from z-stack (0.1-μm steps) images taken using a Nikon DS-U3 camera through a Nikon Eclipse Ci-L microscope and analyzed with NIS Elements. Spine density decreased significantly (P < 0.05) during adolescence, with 50-60% decreases in mushroom and stubby spine-types (P < 0.05, ∼PND35 vs. PND56) in non-proestrous mice. This was associated with decreases in kalirin-7, a spine protein which stabilizes the cytoskeleton and is required for spine maintenance. Because our previous findings suggest that pubertal increases in α4βδ GABAA receptors (GABARs) trigger pruning in CA1, we investigated their role in CA3. α4 expression in CA3 hippocampus increased 4-fold at puberty (P < 0.05), assessed by immunostaining and verified electrophysiologically by an increased response to gaboxadol (100 nM), which is selective for α4βδ. Knock-out of α4 prevented the pubertal decrease in kalirin-7 and synaptic pruning and also increased the dendritic length, demonstrating a functional link. These data suggest that pubertal α4βδ GABARs alter dendritic morphology and trigger pruning in female CA3 hippocampus.

Adolescence and "Late Blooming" Synapses of the Prefrontal Cortex

The maturation of the prefrontal cortex (PFC) during adolescence is thought to be important for cognitive and affective development and mental health risk. Whereas many summaries of adolescent development have focused on dendritic spine pruning and gray matter thinning in the PFC during adolescence, we highlight recent rodent data from our laboratory and others to call attention to continued synapse formation and plasticity in the adolescent period in specific cell types and circuits. In particular, we highlight changes in inhibitory neurotransmission onto intratelencephalic (IT-type) projecting cortical neurons and late expansion of connectivity between the amygdala and PFC and the ventral tegmental area and PFC. Continued work on these "late blooming" synapses in specific cell types and circuits, and their interrelationships, will illuminate new opportunities for understanding and shaping the biology of adolescent development. We also address which aspects of adolescent PFC development are dependent on pubertal processes.

Implications of the ABCD study for developmental neuroscience

The Adolescent Brain Cognitive Development Study (ABCD) will capture a breadth of multi-faceted biobehavioral, environmental, familial, and genetic longitudinal developmental open-access data from over 11,000 9–10 year olds throughout the United States of America (USA) for an envisioned ten-year span. This will subsequently represent the largest study ever attempted with this level of brain phenotypic detail. This study holds the opportunity for exciting advances in the understanding of typical adolescent neurodevelopment, discovery of neurodevelopmental underpinnings of mental illness, as well as the neurodevelopmental influences of (and on) social factors, substance use, and critically – their interaction. This project will certainly take unprecedented steps in informing the nature of adolescence and the developing brain. The scale and open-access features of ABCD also necessarily entail areas for consideration to enhance the integrity of the ABCD study, and protect against potential misuse and misinterpretation of ABCD data. Ultimately, with the open-source data, all scientists in the broader community have as much responsibility as the investigators within the Consortium to treat these data with care. It will be fascinating to see what dynamic data these paths generate. ABCD is poised to exemplify how large-scale longitudinal developmental neuroscientific studies can be designed and efficiently conducted.