Increasing CB2 Receptor Activity after Early Life Stress Prevents Depressive Behavior in Female Rats.

Early adversity, the loss of the inhibitory GABAergic interneuron parvalbumin, and elevated neuroinflammation are associated with depression. Individuals with a maltreatment history initiate medicinal cannabis use earlier in life than non-maltreated individuals, suggesting self-medication. Female rats underwent maternal separation (MS) between 2 and 20 days of age to model early adversity or served as colony controls. The prelimbic cortex and behavior were examined to determine whether MS alters the cannabinoid receptor 2 (CB2), which has anti-inflammatory properties. A reduction in the CB2-associated regulatory enzyme MARCH7 leading to increased NLRP3 was observed with Western immunoblots in MS females. Immunohistochemistry with stereology quantified numbers of parvalbumin-immunoreactive cells and CB2 at 25, 40, and 100 days of age, revealing that the CB2 receptor associated with PV neurons initially increases at P25 and subsequently decreases by P40 in MS animals, with no change in controls. Confocal and triple-label microscopy suggest colocalization of these CB2 receptors to microglia wrapped around the parvalbumin neuron. Depressive-like behavior in MS animals was elevated at P40 and reduced with the CB2 agonist HU-308 or a CB2-overexpressing lentivirus microinjected into the prelimbic cortex. These results suggest that increasing CB2 expression by P40 in the prelimbic cortex prevents depressive behavior in MS female rats.

Commentary: Improving the Effectiveness and Utility of the Helping to End Addiction Long-Term (HEAL) Prevention Cooperative: A Full Translational Framework.

The Helping to End Addiction Long-term (HEAL) Prevention Collaborative (HPC) is designed to expedite the development of programs aimed at preventing opioid misuse and opioid use disorder (OUD) in older adolescents and young adults (ages 16-30). Funded by the National Institutes of Health Office of the Director (ODP-NIH), the HPC includes ten outcome studies that focus on distinct interventions to determine their effectiveness and real-world applicability. Also included is a coordinating center at RTI International that supports the individual projects. This commentary highlights the scientific and practical significance of this cooperative and its promise for facilitating the production and implementation of successful interventions. Attributes such as novel program designs, advanced methodologies, addressing unique characteristics of diverse populations, and real-time analysis of data and costs make this cooperative highly innovative. We note, however, that papers in this Supplemental Issue did not specifically address the persistent need to obtain stronger effect sizes than those achieved to date. Existing data captured earlier in development (< 16 years of age) are uncovering interactive neurocognitive and social-contextual mechanisms underlying the phenomena we wish to prevent. HPC projects could be guided by this information to incorporate developmentally appropriate measures of mechanisms shown previously to be influential in targeted outcomes and determine how they are impacted by specific components of their interventions. This mechanistic information can provide a roadmap for constructing interventions that are more precision-based and, thus, more likely to yield greater benefits for a larger number of recipients. Furthermore, an understanding of underlying mechanism(s) promises to shed light on the sources of heterogeneity in outcomes for further intervention refinement. It is quite possible, if not probable, that meaningful measures of underlying processes will reveal subtypes-some with very high effect sizes and others that are much lower-directly enabling program refinements to more directly target mechanisms that portend and explain less favorable outcomes. Described herein is a full-spectrum translational approach which promises to significantly boost effect sizes, a key objective that should be reached prior to scaling.

Neuroinflammation, Early-Life Adversity, and Brain Development.

ABSTRACT: The overarching objective is to review how early exposure to adversity interacts with inflammation to alter brain maturation. Both adversity and inflammation are significant risk factors for psychopathology. Literature relevant to the effects of adversity in children and adolescents on brain development is reviewed. These studies are supported by research in animals exposed to species-relevant stressors during development. While it is known that exposure to adversity at any age increases inflammation, the effects of inflammation are exacerbated at developmental stages when the immature brain is uniquely sensitive to experiences. Microglia play a vital role in this process, as they scavenge cellular debris and prune synapses to optimize performance. In essence, microglia modify the synapse to match environmental demands, which is necessary for someone with a history of adversity. Overall, by piecing together clinical and preclinical research areas, what emerges is a picture of how adversity uniquely sculpts the brain. Microglia interactions with the inhibitory neurotransmitter GABA (specifically, the subtype expressing parvalbumin) are discussed within contexts of development and adversity. A review of inflammation markers in individuals with a history of abuse is combined with preclinical studies to describe their effects on maturation. Inconsistencies within the literature are discussed, with a call for standardizing methodologies relating to the age of assessing adversity effects, measures to quantify stress and inflammation, and more brain-based measures of biochemistry. Preclinical studies pave the way for interventions using anti-inflammation-based agents (COX-2 inhibitors, CB2 agonists, meditation/yoga) by identifying where, when, and how the developmental trajectory goes awry.

Novelty preferences and cocaine-associated cues influence regions associated with the salience network in juvenile female rats.

Preferences for novel environments (novelty-seeking) is a risk factor for addiction, with little known about its underlying circuitry. Exposure to drug cues facilitates addiction maintenance, leading us to hypothesize that exposure to a novel environment activates a shared neural circuitry. Stimulation of the D1 receptor in the prelimbic cortex increases responsivity to drug-associated environments. Here, we use D1 receptor overexpression in the prelimbic cortex to probe brain responses to novelty-preferences (in a free-choice paradigm) and cocaine-associated odors following place conditioning. These same cocaine-conditioned odors were used to study neural circuitry with Blood Oxygen Level Dependent (BOLD) activity. D1 overexpressing females had deactivated BOLD signals related to novelty-preferences within the insula cortex and amygdala and activation in the frontal cortex and dopamine cell bodies. BOLD responses to cocaine cues were also sensitive to D1. Control females demonstrated a place preference for cocaine environments with no significant BOLD response, while D1 overexpressing females demonstrated a place aversion and weak BOLD responses to cocaine-conditioned odor cues within the insula cortex. For comparison, we provide data from an earlier study with juvenile males overexpressing D1 that show a strong preference for cocaine and elevated BOLD responses. The results support the use of a pharmacological manipulation (e.g., D1 overexpression) to probe the neural circuitry downstream from the prelimbic cortex.

This is your teen brain on drugs: In search of biological factors unique to dependence toxicity in adolescence.

Response variability across the lifespan is an important consideration in toxicology and risk assessment, and the toxic effects of drugs and chemicals during adolescence need more research. This paper summarizes a workshop presented in March 2019, at the Society of Toxicology Annual Meeting in Baltimore, Maryland, that brought together experts in research on drug dependence and toxicity related to nicotine, cannabis, cocaine, and other illicit drugs during adolescence. The goal of the workshop was to address the following issues: (1) Do the effects of adolescent exposure differ from the same exposure in adults? (2) Are there unique biological markers of adolescent brain development? If so, what are they and how reliable are they? (3) Since multiple factors influence substance use disorder, can we disentangle risk factors for abuse and/or toxicity? What are the underlying biological susceptibilities that lead to dependence and neurotoxicity? What are the social, psychosocial and environmental factors that contribute to abuse susceptibilities? This paper reviews drug policy and national trends in adolescent substance use; the public health consequences of e-cigarettes; rat models of adolescent-onset nicotine self-administration and persisting effects of gestational nicotine; sex-dependent effects of delta-9-tetrahydrocannabinol on adolescent brain-behavior relationships; and translational approaches for identifying adolescent risk factors for transition to drug dependence. There is strong evidence that drug exposure prior to adulthood has longer lasting effects on behavior and the underlying neural circuitry. These effects, which are sex-dependent and influenced by stress, may be candidates as predictors of adolescent vulnerability. A major challenge to determining if adolescents have a unique susceptibility to dependence is whether and to what extent the human data allow distinction between the increased risk due to biological immaturity, an underlying biological susceptibility to dependence, or psychosocial and environmental factors for substance dependence. Factors important to consider for development of animal models include the timing and pattern of exposure as it relates to adolescence; age of assessment, and direct comparison with similar effects following exposures to adults to demonstrate that these effects are unique to adolescence. Here we provide a roadmap for further research into what makes adolescent brain development unique.

The use of laser capture microdissection to identify specific pathways and mechanisms involved in impulsive choice in rats.

BACKGROUND: Microinjections, lesions, viral-mediated gene transfer, or designer receptors exclusively activated by designer drugs (DREADDs) can identify brain signaling pathways and their pharmacology in research animals. Genetically modified animals are used for more precise assessment of neural circuits. However, only a few of the gene-based pathway modifications are available for use in outbred rat strains. NEW METHOD: Behaviorally characterized Sprague-Dawley rats undergo tract tracing through microinjection of fluorospheres, followed by laser capture microdissection (LCM) and qPCR for detecting mRNA of pathway-associated gene products. Correlations between mRNA expression and behavior identify specific involvement of pharmacologically relevant molecules within cells of interest. Here, we examined this methodology in an impulsive choice paradigm and targeted projections from the orbital and medial prefrontal cortex. RESULTS: In this proof of concept study, we demonstrate relationships between measures of impulsive choice with distinct neurotransmitter receptor expression in cell populations from four different signaling pathways. COMPARISONS WITH EXISTING METHODS: Combining behavior, tract tracing, LCM, and gene expression profiling provides more cellular selectivity than localized lesions and DREADDs, and greater pharmacological specificity than microinjections and viral-mediated gene transfer due to targeting identified neurons. Furthermore, the assessment of inter-individual pathways provides insight into the complex nature of underlying mechanisms involved in typical and atypical behavior. CONCLUSIONS: The novel combination of behavior, tract tracing, LCM, and single gene or potential whole genome transcriptome analysis allows for a more targeted understanding of the interconnection of neural circuitry with behavior, and holds promise to identify more specific drug targets that are relevant to behavioral phenotypes.

Juvenile exposure to methylphenidate and guanfacine in rats: effects on early delay discounting and later cocaine-taking behavior.

RATIONALE: Both methylphenidate (MPH), a catecholamine reuptake blocker, and guanfacine, an alpha2A agonist, are used to treat attention-deficit hyperactivity disorder (ADHD). Childhood impulsivity, including delay discounting, is associated with increased substance use during adolescence. These effects can be mitigated by juvenile exposure to MPH, but less is known about the long-term effects of developmental exposure to guanfacine in males and females. OBJECTIVE: This study aims to determine sex differences and dose-dependent effects of juvenile exposure to MPH or guanfacine on delay-discounting and later cocaine self-administration. METHODS: The dose-dependent effects of vehicle, MPH (0.5, 1, and 2 mg/kg p.o.) or guanfacine (0.003, 0.03, and 0.3 mg/kg, i.p.) on discounting were determined in male and female Sprague-Dawley rats beginning at postnatal day (P)20. At P90, the amount, motivation, and sensitivity to cocaine following early drug exposure were determined with self-administration. RESULTS: Guanfacine, but not MPH, significantly reduced weight by 22.9 ± 4.6% in females. MPH dose dependently decreased delay discounting in both juvenile males and females, while guanfacine was only effective in males. Discounting was associated with cocaine self-administration in vehicle males (R2 = -0.4, P < 0.05) and self-administration was reduced by guanfacine treatment (0.3 mg/kg). Guanfacine significantly decreased cocaine sensitivity in both sexes. CONCLUSIONS: These data suggest that MPH is effective in reducing delay discounting in both sexes. Due to both weight loss and ineffectiveness on discounting in females, guanfacine should be used only in males to reduce delay discounting and later cocaine use.

Stress, sensitive periods, and substance abuse.

Research on the inter-relationship between drug abuse and social stress has primarily focused on the role of stress exposure during adulthood and more recently, adolescence. Adolescence is a time of heightened reward sensitivity, but it is also a time when earlier life experiences are expressed. Exposure to stress early in postnatal life is associated with an accelerated age of onset for drug use. Lifelong addiction is significantly greater if drug use is initiated during early adolescence. Understanding how developmental changes following stress exposure interact with sensitive periods to unfold over the course of maturation is integral to reducing their later impact on substance use. Arousal levels, gender/sex, inflammation, and the timing of stress exposure play a role in the vulnerability of these circuits. The current review focuses on how early postnatal stress impacts brain development during a sensitive period to increase externalizing and internalizing behaviors in adolescence that include social interactions (aggression; sexual activity), working memory impairment, and depression. How stress effects the developmental trajectories of brain circuits that are associated with addiction are discussed for both clinical and preclinical studies.

Anhedonic behavior and γ-amino butyric acid during a sensitive period in female rats exposed to early adversity.

Early life adversity increases depressive behavior that emerges during adolescence. Sensitive periods have been associated with fewer GABAergic interneurons, especially parvalbumin (PV), brain derived growth factor, and its receptor, TrkB. Here, maternal separation (MS) and social isolation (ISO) were used to establish a sensitive period for anhedonic depression using the learned helplessness (LH) paradigm. Female Sprague-Dawley rat pups underwent MS for 4-h/day or received typical care (CON) between postnatal days 2-20; for the ISO condition, separate cohorts were individually housed between days 20-40 or served as controls (CON2). Anhedonia was defined by dichotomizing subjects into two groups based on one standard deviation of the mean number of escapes for the CON group (<14). This approach categorized 22% of CON subjects and 44% of MS subjects as anhedonic (p < 0.05), similar to the prevalence in maltreated human populations. Only 12.5% of ISO rats met criterion versus 28.5% in CON2 rats. Levels of PV and TrkB were reduced in the amygdala and prelimbic prefrontal cortex (PFC) in MS rats with <14 escapes, but elevated in behaviorally resilient MS rats (>13 escapes). The number of escapes in MS subjects significantly correlated with PV and TrkB levels (PFC: r = 0.93 and 0.91 and amygdala: r = 0.63 and 0.81, respectively; n = 9), but not in CON/ISO/CON2 subjects. Calretinin, but not calbindin, was elevated in the amygdala of MS subjects. These data suggest that low levels of PV and TrkB double the risk for anhedonia in females with an MS history compared to normal adolescent females.

Working memory and salivary brain-derived neurotrophic factor as developmental predictors of cocaine seeking in male and female rats.

Poor working memory is linked to future risk-taking behaviors. Lifelong risk of habitual drug use is highest in individuals who initiate use in early adolescence. We sought to determine in rats whether juvenile traits, specifically poor working memory and low salivary brain-derived neurotrophic factor (BDNF), are related to elevated cocaine taking and relapse in adolescence and adulthood. On postnatal day (P) 20, working memory was assessed using the novel object recognition task in male and female rats. Saliva was assayed at P20 for BDNF before cocaine self-administration on P28 [0.75 or 0.25 mg/kg/infusion for 30 days under a fixed-ratio (FR) 1 to FR5 schedule] and on P94 before relapse after 30-day abstinence in adulthood. A separate cohort of P28 male rats was assayed for object discrimination and BDNF in saliva and the medial prefrontal cortex and dorsolateral striatum. Novel object discrimination correlated positively with salivary BDNF on P20 and dorsolateral striatum levels, but negatively with medial prefrontal cortex BDNF in male rats. In female rats, P20 salivary BDNF negatively correlated with object discrimination. Salivary BDNF positively correlated across age in male rats. Male rats earned more cocaine (0.75 mg/kg) at FR5 and responded more at relapse than did female rats. These elevated relapse rates in male rats were significantly associated with P20 object discrimination and salivary BDNF. Relapse after 0.75 and 0.25 mg/kg in female rats correlated only with object discrimination. In conclusion, poor working memory and low salivary BDNF in juvenile male rats may represent biomarkers for later cocaine use. Further research is needed to identify biomarkers for risk in male rats.

Early life stress and later peer distress on depressive behavior in adolescent female rats: Effects of a novel intervention on GABA and D2 receptors.

Early life adversity (ELA) increases the risk of depression during adolescence that may result from a decline in parvalbumin (PVB) secondary to increased neuroinflammation. In this study, we investigated depressive-like behavior following exposure to two different types of stressors that are relevant for their developmental period: 1) chronic ELA (maternal separation; MS) and 2) an acute emotional stressor during adolescence (witnessing their peers receive multiple shocks; WIT), and their interaction. We also determined whether reducing inflammation by cyclooxygenase-2 (COX-2) inhibition would prevent the onset of depressive-like behavior. Female Sprague-Dawley rat pups underwent MS for four-hours/day or received typical care (CON) between postnatal days (P) 2 and P20. A COX-2 inhibitor (COX-2I) or vehicle was administered every other day between P30 and P38. Subjects were tested for learned helplessness to assess depressive-like behavior at P40 (adolescence). MS females demonstrated increased escape latency and decreased PVB in the prefrontal cortex (PFC) and dorsal raphe that were attenuated by COX-2I intervention. Helplessness was also associated with an increase in D2 receptors in the accumbens. In contrast, WIT elevated escape latency in CON, but reduced latency in MS females. Furthermore, COX-2I intervention decreased escape latency in both CON and MS after WIT. WIT reduced PVB levels in the basolateral amygdala and increased PFC levels to CON levels. Our data suggest that decreased PVB in the PFC is important for the expression of depressive-like behavior and suggest that COX-2I intervention may provide a novel prevention for depression.

Sensitive periods of substance abuse: Early risk for the transition to dependence.

Early adolescent substance use dramatically increases the risk of lifelong substance use disorder (SUD). An adolescent sensitive period evolved to allow the development of risk-taking traits that aid in survival; today these may manifest as a vulnerability to drugs of abuse. Early substance use interferes with ongoing neurodevelopment to induce neurobiological changes that further augment SUD risk. Although many individuals use drugs recreationally, only a small percentage transition to SUD. Current theories on the etiology of addiction can lend insights into the risk factors that increase vulnerability from early recreational use to addiction. Building on the work of others, we suggest individual risk for SUD emerges from an immature PFC combined with hyper-reactivity of reward salience, habit, and stress systems. Early identification of risk factors is critical to reducing the occurrence of SUD. We suggest preventative interventions for SUD that can be either tailored to individual risk profiles and/or implemented broadly, prior to the sensitive adolescent period, to maximize resilience to developing substance dependence. Recommendations for future research include a focus on the juvenile and adolescent periods as well as on sex differences to better understand early risk and identify the most efficacious preventions for SUD.

Age-Dependent Effects of Methylphenidate on the Human Dopaminergic System in Young vs Adult Patients With Attention-Deficit/Hyperactivity Disorder: A Randomized Clinical Trial.

IMPORTANCE: Although numerous children receive methylphenidate hydrochloride for the treatment of attention-deficit/hyperactivity disorder (ADHD), little is known about age-dependent and possibly lasting effects of methylphenidate on the human dopaminergic system. OBJECTIVES: To determine whether the effects of methylphenidate on the dopaminergic system are modified by age and to test the hypothesis that methylphenidate treatment of young but not adult patients with ADHD induces lasting effects on the cerebral blood flow response to dopamine challenge, a noninvasive probe for dopamine function. DESIGN, SETTING, AND PARTICIPANTS: A randomized, double-blind, placebo-controlled trial (Effects of Psychotropic Drugs on Developing Brain-Methylphenidate) among ADHD referral centers in the greater Amsterdam area in the Netherlands between June 1, 2011, and June 15, 2015. Additional inclusion criteria were male sex, age 10 to 12 years or 23 to 40 years, and stimulant treatment-naive status. INTERVENTIONS: Treatment with either methylphenidate or a matched placebo for 16 weeks. MAIN OUTCOMES AND MEASURES: Change in the cerebral blood flow response to an acute challenge with methylphenidate, noninvasively assessed using pharmacological magnetic resonance imaging, between baseline and 1 week after treatment. Data were analyzed using intent-to-treat analyses. RESULTS: Among 131 individuals screened for eligibility, 99 patients met DSM-IV criteria for ADHD, and 50 participants were randomized to receive methylphenidate and 49 to placebo. Sixteen weeks of methylphenidate treatment increased the cerebral blood flow response to methylphenidate within the thalamus (mean difference, 6.5; 95% CI, 0.4-12.6; P = .04) of children aged 10 to 12 years old but not in adults or in the placebo group. In the striatum, the methylphenidate condition differed significantly from placebo in children but not in adults (mean difference, 7.7; 95% CI, 0.7-14.8; P = .03). CONCLUSIONS AND RELEVANCE: We confirm preclinical data and demonstrate age-dependent effects of methylphenidate treatment on human extracellular dopamine striatal-thalamic circuitry. Given its societal relevance, these data warrant replication in larger groups with longer follow-up. TRIAL REGISTRATION: identifier: NL34509.000.10 and trialregister.nl identifier: NTR3103.

Commentary on the special issue on the adolescent brain: Adolescence, trajectories, and the importance of prevention.

Adolescence as highlighted in this special issue is a period of tremendous growth, synaptic exuberance, and plasticity, but also a period for the emergence of mental illness and addiction. This commentary aims to stimulate research on prevention science to reduce the impact of early life events that often manifest during adolescence. By promoting a better understanding of what creates a normal and abnormal trajectory, the reviews by van Duijvenvoorde et al., Kilford et al., Lichenstein et al., and Tottenham and Galvan in this special issue comprehensively describe how the adolescent brain develops under typical conditions and how this process can go awry in humans. Preclinical reviews also within this issue describe how adolescents have prolonged extinction periods to maximize learning about their environment (Baker et al.), whereas Schulz and Sisk focus on the importance of puberty and how it interacts with stress (Romeo). Caballero and Tseng then set the stage of describing the neural circuitry that is often central to these changes and psychopathology. Factors that affect the mis-wiring of the brain for illness, including prenatal exposure to anti-mitotic agents (Gomes et al.) and early life stress and inflammation (Schwarz and Brenhouse), are included as examples of how exposure to early adversity manifests. These reviews are synthesized and show how information from the maturational stages that precede or occur during adolescence is likely to hold the key towards optimizing development to produce an adolescent and adult that is resilient and well adapted to their environment.

Preventative treatment in an animal model of ADHD: Behavioral and biochemical effects of methylphenidate and its interactions with ovarian hormones in female rats.

Clinical and preclinical studies on attention deficit hyperactivity disorder (ADHD) show that juvenile males that are exposed to methylphenidate (MPH) show reduced risk for substance use later in life. In contrast, little is known about whether females have the same enduring treatment response to stimulants and how gonadal hormones influence their behavior later in life. Females received either a sham or 6-hydroxydopamine (6-OHDA) microinjection in the prefrontal cortex (PFC) at postnatal day (P)10. Subjects were then treated with Vehicle or MPH (2mg/kg, p.o.) between P20-35 and tested during late adolescence/young adulthood (P60); half of these subjects underwent ovariectomy at P55 to determine hormonal influences. Females with 6-OHDA were depleted of PFC dopamine by 61% and demonstrated increased impulsive choice (delayed discounting) and preferences for cocaine-associated environments relative to control females. Both MPH and ovariectomy reduced impulsive choice and cocaine preferences in 6-OHDA females, but had no enduring effect in Sham females. Ovariectomy itself did not significantly affect impulsivity. Juvenile MPH interacted strongly with 6-OHDA to increase D4, D5, Alpha-1A, Alpha-2A, and 5-HT-1A mRNA receptor expression in the PFC. MPH alone effected D1 mRNA, while 6-OHDA increased BDNF; all markers were decreased by ovariectomy. Together, these data suggest that 6-OHDA changes in dopamine are not only relevant for ADHD-like behaviors, but their long-term modulation by treatment and the influence of cyclical differences in menstrual cycle.

When the party is over: depressive-like states in rats following termination of cortical D1 receptor overexpression.

RATIONALE: Increased activity of prefrontal D1 dopamine receptors (D1R) is involved in reward-related behavior found in bipolar disorder and drug addiction. While the effects of elevated D1R are known, depressive-like behaviors also occur in these disorders after reward-seeking ends. OBJECTIVES: The goal is to characterize how termination of D1R overexpression influences depressive-like behaviors. METHODS: An inducible (Tet.On), lentiviral vector was used to manipulate the expression of the DRD1 gene in glutamate neurons within the prefrontal cortex in male, adult rats. Sexual activity and sucrose preference were studied in both D1R elevated ON and relatively reduced OFF states. Following termination of the D1R ON state, depressive-like behavior was determined in the OFF state. Expression of the transcriptional regulator, cyclic AMP-responsive element-binding protein (CREB), was used as an indication of downstream effects in the nucleus accumbens (NA). RESULTS: ON D1R expression increased sexual activity that returned to baseline in the OFF state. Sucrose preferences increased ~6 % in ON state but fell 11 % below control levels when OFF. Consistent with a depressive-like phenotype, D1R OFF decreased activity by 40 %, impaired the ability to control (43 %) and motivation to escape shock (27 % more impaired) relative to dsRed OFF. CREB increased 29 % in the NA in the D1R OFF state relative to the ON state. CONCLUSIONS: This novel approach demonstrates that elevated D1R expression increased hedonic behavior, whereas the termination of D1R overexpression often resulted in depressive-like behavior. These observations support a role for D1R expression cycling in bipolar-associated behaviors and addiction.

The developmental inter-relationships between activity, novelty preferences, and delay discounting in male and female rats.

Increased locomotion, novelty-seeking, and impulsivity are risk factors associated with substance use. In this study, the inter-relationships between activity, novelty preferences, and delay discounting, a measure of impulsivity, were examined across three stages: juvenile/early adolescence (postnatal Day [P] 15, 19, and 42 for activity, novelty, and impulsivity, respectively), adolescent/late adolescent (P28, 32, 73), and adult (P90, 94, 137) in male and female rats. Our estimates of impulsive choice, where animals were trained to criterion, revealed an age × sex interaction where early adolescent females had the lowest levels of impulsivity. The relationships of activity and novelty to impulsivity significantly changed across age within each sex. Early adolescent males with high activity, but low novelty preferences, were more impulsive; however, low activity and high novelty preferences were related to high impulsivity in adult males. Female activity gradually increased across age, but did not show a strong relationship with impulsivity. Novelty preferences are moderately related to impulsivity into adulthood in females. These data show that males and females have different developmental trajectories for these behaviors. Males show greater sensation-seeking (e.g., activity) and risky behavior (e.g., novelty preferences) earlier in life, whereas these behaviors emerge during adolescence in females.

Sex differences in the ontogeny of CRF receptors during adolescent development in the dorsal raphe nucleus and ventral tegmental area.

Interactions between corticotropin-releasing factor (CRF) and monoaminergic systems originating from the dorsal raphe nucleus (DR) and ventral tegmental area (VTA) have been implicated in the etiology and pathophysiology of several stress-related neuropsychiatric disorders such as depression and substance abuse. Sub-regions within the DR and VTA give rise to specific projections that have unique roles in limbic- and reward-related behaviors. Given that these disorders typically emerge during adolescence, it is surprising that few studies have examined the age-, sex-, and region-dependent expression of CRF receptors throughout multiple stages of adolescence in these stress-relevant circuits. To determine the ontogeny of CRF receptors during adolescent development, three regions of the DR (dorsal, caudal, and ventrolateral parts) and the posterior VTA were microdissected from Sprague-Dawley male and female rats on postnatal day (P) 25, P35, P42, P56, and P90. Tissue was processed and analyzed with qRT-PCR to measure CRF1 and CRF2 receptors. The serotonin and catecholamine enzymes in the DR and VTA, tryptophan hydroxylase 2 (TPH2) and tyrosine hydroxylase, respectively, were also analyzed for maturational differences. This study identified that CRF1 receptors are lower in males than females within the dorsal, ventrolateral region of the DR (DRVL), which is involved in anxiety-, stress-, and panic-related responses. Females had higher CRF2 receptors compared to males in the DRVL only. Levels of TPH2 mRNA in the DRVL were overproduced transiently in females before declining into adulthood. These fundamental studies suggest that sex differences in CRF receptors should be considered when examining stress-related neuropsychiatric disorders and their treatment.