Maternal deprivation of neonatal rats produces enduring changes in dopamine function

Maternal neglect alters reward-anticipatory behavior, social status stability, and reward circuit activation in adult male rats

Introduction

Adverse early life experiences affect neuronal growth and maturation of reward circuits that modify behavior under reward predicting conditions. Previous studies demonstrate that rats undergoing denial of expected reward in the form of maternal contact (DER-animal model of maternal neglect) during early post-natal life developed anhedonia, aggressive play-fight behaviors and aberrant prefrontal cortex structure and neurochemistry. Although many studies revealed social deficiency following early-life stress most reports focus on individual animal tasks. Thus, attention needs to be given on the social effects during group tasks in animals afflicted by early life adversity.

Methods

To investigate the potential impact of the DER experience on the manifestation of behavioral responses induced by natural rewards, we evaluated: 1) naïve adult male sexual preference and performance, and 2) anticipatory behavior during a group 2-phase food anticipation learning task composed of a context-dependent and a cue-dependent learning period.

Results

DER rats efficiently spent time in the vicinity of and initiated sexual intercourse with receptive females suggesting an intact sexual reward motivation and consummation. Interestingly, during the context-dependent phase of food anticipation training DER rats displayed a modified exploratory activity and lower overall reward-context association. Moreover, during the cue-dependent phase DER rats displayed a mild deficit in context-reward association while increased cue-dependent locomotion. Additionally, DER rats displayed unstable food access priority following food presentation. These abnormal behaviours were accompanied by overactivation of the ventral prefrontal cortex and nucleus accumbens, as assessed by pCREB levels.

Conclusions/discussion

Collectively, these data show that the neonatal DER experience resulted in adulthood in altered activation of the reward circuitry, interfered with the normal formation of context-reward associations, and disrupted normal reward access hierarchy formation. These findings provide additional evidence to the deleterious effects of early life adversity on reward system, social hierarchy formation, and brain function.

The impact of environmental factors during maternal separation on the behaviors of adolescent C57BL/6 mice

Neonatal maternal separation is a widely used method to construct an early-life stress model in rodents. In this method, pups are separated from their mothers for several hours every day during the first 2 weeks of life, which results in adverse early-life events. It is a known fact that maternal separation can exert a significant impact on the behavior and psychological health, such as anxiety and depression, in adolescent offspring. However, environmental conditions during maternal separation can differ such as the presence of other animals or by placing pups in a different dam. To investigate the differential effects of various conditions of maternal separation on the behavior of adolescent mice, we created the following groups: (1) iMS group: pups were moved to an isolated room with no other adult mice in a nearby cage, (2) eDam group: the pups randomly exchanged their dams, (3) OF group: pups were shifted to another cage with the bedding material containing maternal odor (olfactory stimulation), and (4) MS group: pups were shifted to another vivarium. From postnatal day (PND) 2-20 (i.e., 19 consecutive days), pups were separated from the dam daily for 4 h and exposed to various environments (MS, iMS, eDam, and OF) or were left undisturbed [control (CON) group]. A series of behavioral assessments were conducted to evaluate locomotion, anxiety, recognition, learning, and memory in adolescent offspring. The results showed that neonatal maternal separation led to impaired recognition memory, motor coordination, and motor skill learning across all groups. However, the iMS group exhibited anxiety-like behavior in the elevated plus maze test and enhanced the extinction of fear memory in the auditory fear conditioning test. The OF and eDam groups displayed partially recovered short-term working memory in the Y-maze test but exhibited opposite exploratory behaviors. The OF group spent more time in the center, while the eDam group spent less time. These findings demonstrated that exposure to different environmental conditions during maternal separation causes behavioral alterations in adolescent offspring, providing a potential explanation for the variation in behavioral phenotypes observed in the early-life stress models.

Postpartum depression in rats causes poor maternal care and neurochemical alterations on dams and long-lasting impairment in sociability on the offspring

Postpartum depression is a mentally disabling disease with multifactorial etiology that affects women worldwide. It can also influence child development and lead to behavioral and cognitive alterations. Despite the high prevalence, the disease is underdiagnosed and poorly studied. To study the postpartum depression caused by maternal separation model in rats, dams were separated from their litter for 3 h daily starting from lactating day (LD) 2 through LD12. Maternal studies were conducted from LD5 to LD21 and the offspring studies from postnatal day (PND) 2 through PND90. The stress caused by the dam-offspring separation led to poor maternal care and a transient increase in anxiety in the offspring detected during infancy. The female offspring also exhibited a permanent impairment in sociability during adult life. These changes were associated with neurochemical alterations in the prefrontal cortex and hippocampus, and low TSH concentrations in the dams, and in the hypothalamus, hippocampus and striatum of the offspring. These results indicate that the postpartum depression resulted in a depressive phenotype, changes in the brain neurochemistry and in thyroid economy that remained until the end of lactation. Changes observed in the offspring were long-lasting and resemble what is observed in children of depressant mothers.

Neurodevelopmental origins of substance use disorders: Evidence from animal models of early-life adversity and addiction

Addiction is a chronic relapsing disorder with devastating personal, societal, and economic consequences. In humans, early-life adversity (ELA) such as trauma, neglect, and resource scarcity are linked with increased risk of later-life addiction, but the brain mechanisms underlying this link are still poorly understood. Here, we focus on data from rodent models of ELA and addiction, in which causal effects of ELA on later-life responses to drugs and the neurodevelopmental mechanisms by which ELA increases vulnerability to addiction can be determined. We first summarize evidence for a link between ELA and addiction in humans, then describe how ELA is commonly modeled in rodents. Since addiction is a heterogeneous disease with many individually varying behavioral aspects that may be impacted by ELA, we next discuss common rodent assays of addiction-like behaviors. We then summarize the specific addiction-relevant behavioral phenotypes caused by ELA in male and female rodents and discuss some of the underlying changes in brain reward and stress circuits that are likely responsible. By better understanding the behavioral and neural mechanisms by which ELA promotes addiction vulnerability, we hope to facilitate development of new approaches for preventing or treating addiction in those with a history of ELA.

Dopamine Circuit Mechanisms of Addiction-Like Behaviors

Addiction is a complex disease that impacts millions of people around the world. Clinically, addiction is formalized as substance use disorder (SUD), with three primary symptom categories: exaggerated substance use, social or lifestyle impairment, and risky substance use. Considerable efforts have been made to model features of these criteria in non-human animal research subjects, for insight into the underlying neurobiological mechanisms. Here we review evidence from rodent models of SUD-inspired criteria, focusing on the role of the striatal dopamine system. We identify distinct mesostriatal and nigrostriatal dopamine circuit functions in behavioral outcomes that are relevant to addictions and SUDs. This work suggests that striatal dopamine is essential for not only positive symptom features of SUDs, such as elevated intake and craving, but also for impairments in decision making that underlie compulsive behavior, reduced sociality, and risk taking. Understanding the functional heterogeneity of the dopamine system and related networks can offer insight into this complex symptomatology and may lead to more targeted treatments.

Traumatic Injury to the Developing Brain: Emerging Relationship to Early Life Stress

Despite the high incidence of brain injuries in children, we have yet to fully understand the unique vulnerability of a young brain to an injury and key determinants of long-term recovery. Here we consider how early life stress may influence recovery after an early age brain injury. Studies of early life stress alone reveal persistent structural and functional impairments at adulthood. We consider the interacting pathologies imposed by early life stress and subsequent brain injuries during early brain development as well as at adulthood. This review outlines how early life stress primes the immune cells of the brain and periphery to elicit a heightened response to injury. While the focus of this review is on early age traumatic brain injuries, there is also a consideration of preclinical models of neonatal hypoxia and stroke, as each further speaks to the vulnerability of the brain and reinforces those characteristics that are common across each of these injuries. Lastly, we identify a common mechanistic trend; namely, early life stress worsens outcomes independent of its temporal proximity to a brain injury.

Effect of early life social adversity on drug abuse vulnerability: Focus on corticotropin-releasing factor and oxytocin

Early life adversity can set the trajectory for later psychiatric disorders, including substance use disorders. There are a host of neurobiological factors that may play a role in the negative trajectory. The current review examines preclinical evidence suggesting that early life adversity specifically involving social factors (maternal separation, adolescent social isolation and adolescent social defeat) may influence drug abuse vulnerability by strengthening corticotropin-releasing factor (CRF) systems and weakening oxytocin (OT) systems. In adulthood, pharmacological and genetic evidence indicates that both CRF and OT systems are directly involved in drug reward processes. With early life adversity, numerous studies show an increase in drug abuse vulnerability measured in adulthood, along a concomitant strengthening of CRF systems and a weakening of OT systems. Mechanistic studies, while relatively few in number, are generally consistent with the theme that strengthened CRF systems and weakened OT systems mediate, at least in part, the link between early life adversity and drug abuse vulnerability. Establishing a direct role of CRF and OT in mediating the relation between early life social stressors and drug abuse vulnerability will inform clinical researchers and practitioners toward the development of intervention strategies to reduce risk among those suffering from early life adversities. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Impact of preweaning stress on long-term neurobehavioral outcomes in Sprague-Dawley rats: Differential effects of barren cage rearing, pup isolation, and the combination

Two developmental stressors were compared in preweaning rats exposed to either one stressor or both. Stressors were barren cage rearing or maternal separation (pup isolation). 40 gravid Sprague-Dawley CD/IGS rats were randomly assigned to two cage conditions: standard (Std) cage or barren cage (Bar), 20 litters/condition throughout gestation and lactation. After delivery, litters were randomly culled to 4 males and 4 females. The second stressor was maternal separation: Two male/female pairs per litter were isolated from their dam 4 h/day (Iso) and two pairs were not (Norm). Hence, there were 4 conditions: Std-Norm, Std-Iso, Bar-Norm, and Bar-Iso. One pair/litter/stress condition received the following: elevated zero-maze (EZM), open-field, swim channel, Cincinnati water maze, conditioned fear, and open-field with methamphetamine challenge. The second pair/litter/condition received the light-dark test, swim channel, Morris water maze, forced swim, and EZM with diazepam challenge. Barren rearing reduced EZM time-in-open, whereas isolation rearing reduced open-field activity in males and increased it in females. Effects on straight channel swimming were minor. In the Cincinnati water maze test of egocentric learning, isolation rearing increased errors whereas barren cage housing reduced errors in combination with normal rearing. Barren cage with maternal separation (pup isolation) increased Cincinnati water maze escape latency but not errors. Barren cage housing reduced hyperactivity in response to methamphetamine. Isolation rearing increased time in open in the EZM after diazepam challenge. Trends were seen in the Morris water maze. These suggested that barren cage and isolation rearing in combination reduced latency on acquisition on days 1 and 2 in males, whereas females had increased latency on days 2 and 3. Combined exposure to two developmental stressors did not induce additive or synergistic effects, however the data show that these stressors had long-term effects with some evidence that the combination of both caused effects when either stressor alone did not, but synergism was not observed.

Dibutyl phthalate exposure during gestation and lactation in C57BL/6 mice: Maternal behavior and neurodevelopment in pups

OBJECTIVES

Neurotoxic effects of phthalate during pregnancy on immature brain of the offspring or mature brains of the mothers remain unclear. We examined the effect of dibutyl phthalate (DBP) exposure during gestation and lactation on the maternal behavior of mother mice and neurodevelopment in pups.

METHODS

Pregnant mice were treated orally with DBP (0, 50 and 100 mg/kg/day, N = 20 per group) from gestational day 13 to postnatal day (PND) 15. Maternal behavior was measured using pup retrieval and nest shape test at postpartum day 4. For the pups, the neurodevelopment was measured using negative geotaxis, cliff avoidance at PND 7, swimming test and olfactory orientation at PND 14. RNA and protein expressions in the brain cortex of 50 mg/kg/day and control group (0 mg/kg/day) were analyzed using microarray and Western blot analysis. Nissl-stained sections at the coronal level of interaural 2.56 mm, bregma -1,23 mm, were used for counting of dark cortical neurons in mother and pup mice.

RESULTS

DBP treated mother mice (50 and 100 mg/kg/day) showed poor maternal behavior, poor nesting and retrieval behavior compared to the control group (0 mg/kg/day). In brain cortex, DBP-treated mothers showed decrease in protein expression of Nr4a3, Egr1, Arc, BDNF and phosphorylation of AKT and CREB, were also decreased in cortex of DBP-treated mothers. Pups exposed to DBP showed significantly decreased scores in negative geotaxis at PND 7 and swimming scores and olfactory orientation tests at PND 14. The cortex of the DBP exposed pups showed increase in expression of dopamine receptor D2 gene. Nissl staining showed that the dark neurons were increased in cortex of DBP treated mothers and DBP exposed pups. Suggesting that phthalate may delay pup development indirectly through inadequate mothering as well as direct phthalate exposure on the brain.

CONCLUSION

DBP exposure during gestation and lactation cause impairment in maternal behaviors and downregulation of neuronal plasticity and survival signals. Pups of mothers with exposed to DBP, showed delayed neurodevelopment and dark neurons increase in brain cortex, suggesting that phthalate may delay pup development indirectly through inadequate mothering as well as direct phthalate exposure on the brain.

Mechanisms of Shared Vulnerability to Post-traumatic Stress Disorder and Substance Use Disorders

Psychoactive substance use is a nearly universal human behavior, but a significant minority of people who use addictive substances will go on to develop an addictive disorder. Similarly, though ~90% of people experience traumatic events in their lifetime, only ~10% ever develop post-traumatic stress disorder (PTSD). Substance use disorders (SUD) and PTSD are highly comorbid, occurring in the same individual far more often than would be predicted by chance given the respective prevalence of each disorder. Some possible reasons that have been proposed for the relationship between PTSD and SUD are self-medication of anxiety with drugs or alcohol, increased exposure to traumatic events due to activities involved in acquiring illegal substances, or addictive substances altering the brain's stress response systems to make users more vulnerable to PTSD. Yet another possibility is that some people have an intrinsic vulnerability that predisposes them to both PTSD and SUD. In this review, we integrate clinical and animal data to explore these possible etiological links between SUD and PTSD, with an emphasis on interactions between dopaminergic, adrenocorticotropic, GABAergic, and glutamatergic neurobehavioral mechanisms that underlie different emotional learning styles.

Adolescent stress contributes to aberrant dopamine signaling in a heritable rodent model of susceptibility

Evidence suggests that both genetic and environmental factors contribute to the development of schizophrenia. Rodent models of the disorder have been developed that model either genetic or environment factors to recapitulate various aspects of the disease; however, the examination of gene by environment interactions requires a model of susceptibility. We have previously demonstrated that a proportion of the F2 generation of MAM-treated rats display a schizophrenia-like phenotype, defined as an increase in ventral tegmental area (VTA) dopamine neuron population activity. Here we use this model to examine the consequence of adolescent stress (AS), a known risk factor for psychiatric disease, on dopamine neuron activity in the VTA. Specifically, F2 MAM rats were exposed to predator odor, a stressor of high ethological relevance, intermittently over 10 days throughout the adolescent period and VTA dopamine neuron activity was evaluated in adulthood. Both saline and MAM F2 rats exposed to AS displayed significant increases in population activity; however, the proportion of F2 MAM rats exhibiting this increase was significantly greater (approximately 70%) compared to their respective controls. Given that we have previously demonstrated that the augmented dopamine neuron activity in rodent models of psychosis is directly attributable to aberrant activity in the ventral hippocampus (vHipp), we examined whether AS altered activity within the vHipp. Indeed, there was a positive correlation between dopamine neuron activity and hippocampal firing rates, such that those rats that displayed increases in population activity also had increases in the firing rates of vHipp putative pyramidal neurons. Taken together, these data further demonstrate a role for AS as a risk factor for psychosis, particularly in those with a heritable predisposition.

Repeated restraint stress potentiates methylphenidate and modafinil-induced behavioral sensitization in rats

Stress increases the susceptibility of drug abuse and drugs of abuse impair behavioral tolerance. It has been shown that stress exposure enhances the sensitivity to the reinforcing properties of drugs, augments locomotor sensitization effects of drugs of abuse and impairs behavioral tolerance. Previously, it has been shown that long-term administration of psychostimulants (Methylphenidate and Modafinil) induced locomotor sensitization effect that was more pronounced after 13 days of drug administration and was greater at high dose. The present study is designed to investigate the relationship between restraint stress and psychostimulants (Methylphenidate and Modafinil) that induced sensitization. Methylphenidate (10 mg/kg/day twice a day), modafinil (75 mg/kg/day once daily), and saline (0.9% NaCl; 1 ml/kg/day) were administered orally to treated and control animals. Rats were exposed to immobilization stress for 30 days (until locomotor sensitization produced) to monitor any change in drug-induced behavioral sensitization. The motor activity was compared daily by using familiar environment of home cage and weekly by novel environment of open field. The results show that the methylphenidate and modafinil-induced locomotor sensitization is enhanced and impaired behavioral tolerance in repeated restrained rats. It shows that the psychostimulants like methylphenidate and modafinil produce greater locomotor sensitization in stressful environment, suggesting addictive effects of stress and psychostimulants (methylphenidate/modafinil) on dopaminergic neurotransmission. These finding may be helpful to develop potential pharmacotherapies for the patients with co-occurring depression and substance abuse/dependence disorder.

The effects of psychosocial stress on dopaminergic function and the acute stress response

Chronic psychosocial adversity induces vulnerability to mental illnesses. Animal studies demonstrate that this may be mediated by dopaminergic dysfunction. We therefore investigated whether long-term exposure to psychosocial adversity was associated with dopamine dysfunction and its relationship to psychological and physiological responses to acute stress. Using 3,4-dihydroxy-6-[¹⁸F]-fluoro-l-phenylalanine ([¹⁸F]-DOPA) positron emission tomography (PET), we compared dopamine synthesis capacity in n = 17 human participants with high cumulative exposure to psychosocial adversity with n = 17 age- and sex-matched participants with low cumulative exposure. The PET scan took place 2 hr after the induction of acute psychosocial stress using the Montréal Imaging Stress Task to induce acute psychosocial stress. We found that dopamine synthesis correlated with subjective threat and physiological response to acute psychosocial stress in the low exposure group. Long-term exposure to psychosocial adversity was associated with dampened striatal dopaminergic function (p=0.03, d = 0.80) and that psychosocial adversity blunted physiological yet potentiated subjective responses to acute psychosocial stress. Future studies should investigate the roles of these changes in vulnerability to mental illnesses.

The relationship between childhood trauma, dopamine release and dexamphetamine-induced positive psychotic symptoms: a [11C]-(+)-PHNO PET study

Childhood trauma is a risk factor for psychosis. Amphetamine increases synaptic striatal dopamine levels and can induce positive psychotic symptoms in healthy individuals and patients with schizophrenia. Socio-developmental hypotheses of psychosis propose that childhood trauma and other environmental risk factors sensitize the dopamine system to increase the risk of psychotic symptoms, but this remains to be tested in humans. We used [¹¹C]-(+)-PHNO positron emission tomography to measure striatal dopamine-2/3 receptor (D_2/3R) availability and ventral striatal dexamphetamine-induced dopamine release in healthy participants (n = 24). The relationships between dexamphetamine-induced dopamine release, dexamphetamine-induced positive psychotic symptoms using the Positive and Negative Syndrome Scale (PANSS), and childhood trauma using the Childhood Trauma Questionnaire (CTQ) were assessed using linear regression and mediation analyses, with childhood trauma as the independent variable, dexamphetamine-induced dopamine release as the mediator variable, and dexamphetamine-induced symptoms as the dependent variable. There was a significant interaction between childhood trauma and ventral striatal dopamine release in predicting dexamphetamine-induced positive psychotic symptoms (standardized β = 1.83, p = 0.003), but a mediation analysis was not significant (standardized β = -0.18, p = 0.158). There were no significant effects of dopamine release and childhood trauma on change in negative (p = 0.280) or general PANSS symptoms (p = 0.061), and there was no relationship between ventral striatal baseline D_2/3R availability and positive symptoms (p = 0.368). This indicates childhood trauma and dopamine release interact to influence the induction of positive psychotic symptoms. This is not consistent with a simple sensitization hypothesis, but suggests that childhood trauma moderates the cognitive response to dopamine release to make psychotic experiences more likely.

Alterations in adolescent dopaminergic systems as a function of early mother-toddler attachment: A prospective longitudinal examination

Early experiences have the potential for outsized influence on neural development across a wide number of domains. In humans, many of the most important such experiences take place in the context of the mother-child attachment relationship. Work from animal models has highlighted neural changes in dopaminergic systems as a function of early care experiences, but translational research in humans has been limited. Our goal was to fill this gap by examining the longitudinal associations between early attachment experiences (assessed at 2.5 years) and neural responses to risk and rewards during adolescence (assessed at 13 years). Adolescence is a developmental period where sensitivity to rewards has important implications for behavior and long-term outcomes, providing an important window to study potential influences of early attachment experiences on reward processing. In order to address this question, 50 adolescents completed a risk and reward task during an fMRI scan, allowing us to assess differences in neural sensitivity to changes in risk level and reward amount as a function of early attachment experiences. Adolescents with insecure attachment histories showed blunted sensitivity to increasing risk levels in regions of the dorsal striatum, while also showing heightened sensitivity to increasing reward levels in the same region. These results highlight the importance of early attachment experiences for long-term neural development. Specifically, early exposure to more maladaptive relationships with caregivers may confer dual risks prospectively for adolescents, sensitizing them to rewarding outcomes while de-sensitizing them to potential risks associated with those behaviors, perhaps due to stress-related dopaminergic changes early in development.

Early life stress and the propensity to develop addictive behaviors

There is a vast literature on effects of early life manipulations in rodents much of which is aimed at investigating the long-term consequences related to emotion and cognition in adulthood. Less is known about how these manipulations affect responses reflective of alcohol (AUD) and substance (SUD) use disorders. The purpose of this paper is to review the literature of studies that employed early life manipulations and assessed behavioral responses to psychoactive substances, specifically alcohol, opiates, and stimulants, in rodents. While the findings with alcohol are more limited and mixed, studies with opiates and stimulants show strong support for the ability of these manipulations to enhance behavioral responsivity to these substances in line with epidemiological data. Some outcomes show sex differences. The mechanisms that influence these enduring changes may reflect epigenetic alterations. Several studies support a role for altered DNA methylation (and other epigenetic mechanisms) as biological responses to early environmental insults. The chemical changes induced by DNA methylation affect transcriptional activity of DNA and thus can have a long-term impact on the individual's phenotype. Such effects are particularly robust when they occur during sensitive periods of brain development (e.g., first postnatal weeks in rodents). We review this emerging literature as it relates to the known neurobiology of AUDs and SUDs and suggest new avenues of research. Such findings will have implications for the treatment and prevention of AUDs and SUDs and could provide insight into factors that support resiliency.

Exercise attenuates maternal separation-induced mood disorder-like behaviors by enhancing mitochondrial functions and neuroplasticity in the dorsal raphe

Loss of the mother-infant relationship during early childhood affects infant development and is known to increase the infant's vulnerability to neuropsychiatric disorders throughout life. Serotonin deficits and mitochondrial dysfunction in the dorsal raphe may underlie mood disorders such as anxiety and depression. Exercise is known to have a positive effect on brain function. In this study, we investigated the effect of exercise on mitochondrial function, apoptosis, and serotonin levels in the dorsal raphe as well as behavioral changes in cases of maternal separation. Exposure to the stress of maternal separation resulted in mitochondrial dysfunction in the dorsal raphe, including impaired Ca²⁺ homeostasis, an increase in reactive oxygen species such as H₂O₂, and a decrease in the O₂ respiration rate. Exposure to maternal separation stress also decreased tryptophan hydroxylase and 5-hydroxytryptamine positive cells and increased apoptosis, anxiety, and depression. The impairments in mitochondrial function, apoptosis, and serotonin in the dorsal raphe, as well as anxiety and depression, were all improved by exercise. Exercise might alter mitochondrial function, serotonin levels, and the rate of apoptosis in the dorsal raphe. Therefore, exercise might be an important non-pharmacological intervention for the prevention and treatment of the adverse effects of maternal separation.

Resilience priming: Translational models for understanding resiliency and adaptation to early life adversity

Despite the increasing attention to early life adversity and its long-term consequences on health, behavior, and the etiology of neurodevelopmental disorders, our understanding of the adaptations and interventions that promote resiliency and rescue against such insults are underexplored. Specifically, investigations of the perinatal period often focus on negative events/outcomes. In contrast, positive experiences (i.e. enrichment/parental care//healthy nutrition) favorably influence development of the nervous and endocrine systems. Moreover, some stressors result in adaptations and demonstrations of later-life resiliency. This review explores the underlying mechanisms of neuroplasticity that follow some of these early life experiences and translates them into ideas for interventions in pediatric settings. The emerging role of the gut microbiome in mediating stress susceptibility is also discussed. Since many negative outcomes of early experiences are known, it is time to identify mechanisms and mediators that promote resiliency against them. These range from enrichment, quality parental care, dietary interventions and those that target the gut microbiota.

Interaction between childhood adversity and functional polymorphisms in the dopamine pathway on first-episode psychosis

BACKGROUND

There is consistent evidence of a cumulative relationship between childhood adversity and psychosis, with number of adversities experienced increasing the probability of psychosis onset. It is possible that genetic factors moderate the association between childhood adversity and psychosis, potentially by influencing how an individual reacts biologically and/or psychologically following exposure to adversity, in such a way as to set them off on the path to psychosis. However, identifying the specific genetic variants involved and how they interact with childhood adversity remains challenging. We examined whether the association between cumulative exposure to childhood adversity and development of psychotic disorder was moderated by the COMT Val¹⁵⁸Met, AKT1 rs2494732 or DRD2 rs1076560 polymorphisms, known to affect dopamine levels.

METHODS

Participants were 285 first-presentation psychosis cases and 256 geographically-matched controls drawn from the Genetics and Psychosis (GAP) study. Childhood adversity was assessed using the Childhood Experience of Care and Abuse Questionnaire (CECA.Q) and blood- and cheek-derived genotype data were collected.

RESULTS

Our findings revealed no main effect of COMT Val¹⁵⁸Met, AKT1 rs2494732 and DRD2 rs1076560 polymorphisms on psychosis case status or reports of childhood adversity. Individuals reporting a history of multiple adversities were more likely to be psychosis patients than controls, regardless of their genetic risk. There was no evidence of candidate genotype by childhood adversity interactions in relation to psychosis onset.

CONCLUSION

These findings did not provide evidence of a possible role of COMT Val¹⁵⁸Met, AKT1 rs2494732 or DRD2 rs1076560 genotypes in modifying the association between childhood adversity and onset of psychosis.

Pathways Relating the Neurobiology of Attachment to Drug Addiction

Substance use disorders constitute a significant public health problem in North America and worldwide. Specifically, substance addictions in women during pregnancy or in the postpartum period have adverse effects not only on the mother, but also on mother-infant attachment and the child's subsequent development. Additionally, there is growing evidence suggesting that parental addiction may be transmitted intergenerationally, where the child of parents with addiction problems is more likely to experience addiction as an adult. The current review takes a developmental perspective and draws from animal and human studies to examine how compromised early experience, including insecure attachment, early abuse/neglect, and unresolved trauma, may influence the development of neurobiological pathways associated with addictions, ultimately increasing one's susceptibility to addictions later in life. We approach this from three different levels: molecular, neuroendocrine and behavioral; and examine the oxytocin affiliation system, dopamine reward system, and glucocorticoid stress response system in this regard. Increased understanding of these underlying mechanisms may help identify key targets for early prevention efforts and inform needed intervention strategies related to both insecure attachment and addiction.

Early life adversity potentiates expression of addiction-related traits

Many individuals sporadically and circumstantially sample addictive drugs, yet few become addicted. The individual vulnerabilities underlying the development of addiction are not well understood. Correlational findings show that early life adversity is associated with a greater propensity to develop drug addiction. However, the mechanisms by which early life adversity increases addiction vulnerability are unknown. Separate lines of research have found that several traits are associated with addiction. Here, we examined the effects of early life adversity on addiction-related traits in adulthood. We weaned male and female Sprague-Dawley rats (postnatal day - PND21) and randomly assigned them to either a non-adversity group (N-ADV) or an adversity group (ADV). ADV rats experienced adversity from PND 21-35, they were: a) singly housed, b) food restricted for 12h/day, c) subjected to forced-swim sessions, and d) restrained and exposed to predator odour (1h). As adults, rats were tested for impulsivity, anxiety-like behaviour, novelty preference, and attribution of incentive salience to a reward cue. ADV rats showed enhanced novelty preference and attributed greater incentive value to a reward cue. Compared to N-ADV rats, a greater proportion of ADV rats expressed multiple addiction risk traits. Furthermore, fewer ADV rats expressed no addiction risk traits. This effect was most evident in female ADV rats.

Negative consequences of early-life adversity on substance use as mediated by corticotropin-releasing factor modulation of serotonin activity

Early-life adversity is associated with increased risk for substance abuse in later life, with women more likely to report past and current stress as a mediating factor in their substance use and relapse as compared to men. Preclinical models of neonatal and peri-adolescent (early through late adolescence) stress all support a direct relationship between experiences of early-life adversity and adult substance-related behaviors, and provide valuable information regarding the underlying neurobiology. This review will provide an overview of these animal models and how these paradigms alter drug and alcohol consumption and/or seeking in male and female adults. An introduction to the corticotropin-releasing factor (CRF) and serotonin systems, their development and their interactions at the level of the dorsal raphe will be provided, illustrating how this particular stress system is sexually dimorphic, and is well positioned to be affected by stressors early in development and throughout maturation. A model for CRF-serotonin interactions in the dorsal raphe and how these influence dopaminergic activity within the nucleus accumbens and subsequent reward-associated behaviors will be provided, and alterations to the activity of this system following early-life adversity will be identified. Overall, converging findings suggest that early-life adversity has long-term effects on the functioning of the CRF-serotonin system, highlighting a potentially important and targetable mediator linking stress to addiction. Future work should focus on identifying the exact mechanisms that promote long-term changes to the expression and activity of CRF receptors in the dorsal raphe. Moreover, it is important to clarify whether similar neurobiological mechanisms exist for males and females, given the sexual dimorphism both in CRF receptors and serotonin indices in the dorsal raphe and in the behavioral outcomes of early-life adversity.

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Early life stress increases risk for substance abuse in adulthood.

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Stress and drugs increase CRF which alters serotonin release in the brain.

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CRF₂ receptor expression in the dorsal raphe is altered by early life stress.

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Resultant changes to serotonin output facilitates dopamine in the accumbens.

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CRF₂-sertotonin-dopamine interactions may link early life stress with substance abuse.

The effects of early life stress on reward processing

Early life stress (ELS), in the form of childhood maltreatment, abuse, or neglect, increases the risk for psychiatric sequelae later in life. The neurobiology of response to early stress and of reward processing overlap substantially, leading to the prediction that reward processing may be a primary mediator of the effects of early life stress. We describe a growing body of literature investigating the effects of early life stressors on reward processing in animals and humans. Despite variation in the reviewed studies, an emerging pattern of results indicates that ELS results in deficits of ventral striatum-related functions of reward responsiveness and approach motivation, especially when the stressor is experienced in early in development. For stressors experienced later in the juvenile period and adolescence, the animal literature suggests an opposite effect, in which ELS results in increased hedonic drive. Future research in this area will help elucidate the transdiagnostic impact of early life stress, and therefore potentially identify and intervene with at-risk youth, prior to the emergence of clinical psychopathology.

Dopaminergic and behavioural changes in a loss-of-imprinting model of Cdkn1c

The imprinted gene Cdkn1c is expressed exclusively from the maternally inherited allele as a consequences of epigenetic regulation. Cdkn1c exemplifies many of the functional characteristics of imprinted genes, playing a role in foetal growth and placental development. However, Cdkn1c also plays an important role in the brain, being key to the appropriate proliferation and differentiation of midbrain dopaminergic neurons. Using a transgenic model (Cdkn1c^BACx1) with a twofold elevation in Cdkn1c expression that mimics loss‐of‐imprinting, we show that increased expression of Cdkn1c in the brain gives rise to neurobiological and behavioural changes indicative of a functionally altered dopaminergic system. Cdkn1c^BACX1 mice displayed altered expression of dopamine system‐related genes, increased tyrosine hydroxylase (Th) staining and increased tissue content of dopamine in the striatum. In addition, Cdkn1c^BACx1 animals were hypersensitive to amphetamine as showed by c‐fos expression in the nucleus accumbens. Cdkn1c^BACX1 mice had significant changes in behaviours that are dependent on the mesolimbic dopaminergic system. Specifically, increased motivation for palatable food stuffs, as indexed on a progressive ratio task. In addition, Cdkn1c^BACX1 mice displayed enhanced social dominance. These data show, for the first time, the consequence of elevated Cdkn1c expression on dopamine‐related behaviours highlighting the importance of correct dosage of this imprinted gene in the brain. This work has significant relevance for deepening our understanding of the epigenetic factors that can shape neurobiology and behaviour.

Effects of a social stimulus on gene expression in a mouse model of fragile X syndrome

BACKGROUND

People with fragile X syndrome (FXS) often have deficits in social behavior, and a substantial portion meet criteria for autism spectrum disorder. Though the genetic cause of FXS is known to be due to the silencing of FMR1, and the Fmr1 null mouse model representing this lesion has been extensively studied, the contributions of this gene and its protein product, FMRP, to social behavior are not well understood.

METHODS

Fmr1 null mice and wildtype littermates were exposed to a social or non-social stimulus. In one experiment, subjects were assessed for expression of the inducible transcription factor c-Fos in response to the stimulus, to detect brain regions with social-specific activity. In a separate experiment, tissue was taken from those brain regions showing differential activity, and RNA sequencing was performed.

RESULTS

Immunohistochemistry revealed a significantly greater number of c-Fos-positive cells in the lateral amygdala and medial amygdala in the brains of mice exposed to a social stimulus, compared to a non-social stimulus. In the prelimbic cortex, there was no significant effect of social stimulus; although the number of c-Fos-positive cells was lower in the social condition compared to the non-social condition, and negatively correlated with c-Fos in the amygdala. RNA sequencing revealed differentially expressed genes enriched for molecules known to interact with FMRP and also for autism-related genes identified in the Simons Foundation Autism Research Initiative gene database. Ingenuity Pathway Analysis detected enrichment of differentially expressed genes in networks and pathways related to neuronal development, intracellular signaling, and inflammatory response.

CONCLUSIONS

Using the Fmr1 null mouse model of fragile X syndrome, we have identified brain regions, gene networks, and molecular pathways responsive to a social stimulus. These findings, and future experiments following up on the role of specific gene networks, may shed light on the neural mechanisms underlying dysregulated social behaviors in fragile X syndrome and more broadly.

Ventral striatal activity links adversity and reward processing in children

Adversity impacts many aspects of psychological and physical development including reward-based learning and decision-making. Mechanisms relating adversity and reward processing in children, however, remain unclear. Here, we show that adversity is associated with potentiated learning from positive outcomes and impulsive decision-making, but unrelated to learning from negative outcomes. We then show via functional magnetic resonance imaging that the link between adversity and reward processing is partially mediated by differences in ventral striatal response to rewards. The findings suggest that early-life adversity is associated with alterations in the brain's sensitivity to rewards accounting, in part, for the link between adversity and altered reward processing in children.

Annual Research Review: Childhood maltreatment, latent vulnerability and the shift to preventative psychiatry - the contribution of functional brain imaging

BACKGROUND

Childhood maltreatment is a potent predictor of poor mental health across the life span. We argue that there is a need to improve the understanding of the mechanisms that confer psychiatric vulnerability following maltreatment, if we are to progress from simply treating those with a manifest disorder, to developing effective preventative approaches that can help offset the likelihood that such disorders will emerge in the first place.

METHODS

We review extant functional neuroimaging studies of children and adolescents exposed to early neglect and/or maltreatment, including physical, sexual and emotional abuse across four neurocognitive domains: threat processing, reward processing, emotion regulation and executive control. Findings are discussed in the context of 'latent vulnerability', where alterations in neurocognitive function are considered to carry adaptive value in early adverse caregiving environments but confer long-term risk.

RESULTS

Studies on threat processing indicate heightened as well as depressed neural responsiveness in maltreated samples, particularly in the amygdala, thought to reflect threat hypervigilance and avoidance respectively. Studies on reward processing generally report blunted neural response to anticipation and receipt of rewards, particularly in the striatum, patterns associated with depressive symptomatology. Studies on emotion regulation report increased activation of the anterior cingulate cortex (ACC) during active emotion regulation, possibly reflecting greater effortful processing. Finally, studies of executive control report increased dorsal ACC activity during error monitoring and inhibition.

CONCLUSIONS

An emerging body of work indicates that altered neurocognitive functioning following maltreatment: (a) is evident even in the absence of overt psychopathology; (b) is consistent with perturbations seen in individuals presenting with psychiatric disorder; (c) can predict future psychiatric symptomatology. These findings suggest that maltreatment leads to neurocognitive alterations that embed latent vulnerability to psychiatric disorder, establishing a compelling case for identifying those children at most risk and developing mechanistically informed models of preventative intervention. Such interventions should aim to offset the likelihood of any future psychiatric disorder.

Toward understanding the impact of trauma on the early developing human brain

Traumatic experiences early in life predispose animals and humans to later cognitive-behavioral, emotional, and somatic problems. In humans, traumatic experiences are strong predictors of psychiatric illness. A growing body of research has emphasized alterations in neurological structure and function that underscore phenotypic changes following trauma. However, results are mixed and imprecise. We argue that future translation of neurological findings to clinical practice will require: (1) discovery of neurobehavioral associations within a longitudinal context, (2) dissociation of trauma types and of trauma versus chronic stress, and (3) better localization of neural sequelae considerate of the fine resolution of neural circuitry. We provide a brief overview of early brain development and highlight the role of longitudinal research in unearthing brain-behavior relations in youth. We relay an emergent framework in which dissociable trauma types are hypothesized to impact distinct, rationally informed neural systems. In line with this, we discuss the long-standing challenge of separating effects of chronic stress and trauma, as these are often intertwined. We bring to light inconsistencies in localization of neural correlates of trauma, emphasizing results in medial prefrontal regions. We assert that more precise spatial brain localization will help to advance prevailing models of trauma pathways and inform future research.

Effects of maternal separation on nicotine-induced conditioned place preference and subsequent learning and memory in adolescent female rats

Adverse early life experiences can potentially increase risk for drug abuse later in life. However, little research has been conducted studying the effects of maternal separation (MS), an experimental model for early life stress, on the rewarding effects of nicotine. Cognitive function may be affected by MS. So, we also investigated whether nicotine administration affect spatial learning and memory in MS adolescent female rats. Rat pups were subjected to daily MS for 15min (MS15) or 180min (MS180) during the first 2 weeks of life or reared under normal animal facility rearing (AFR) conditions. The place preference test was performed with nicotine (0.6mg/kg,s.c.) or vehicle over a period of 6 conditioning trials during adolescence. Spatial learning and memory performance was evaluated by using Morris water maze (MWM). In our study, adolescent female rats exposed to MS180 shown a significantly greater preference for a nicotine-paired compartment during the testing phase than the MS15 group. Nicotine altered the MS-induced spatial learning defects in the MS180 group. These findings suggest that MS may increase sensitivity to the rewarding effects of nicotine and also it is possible to suggest that nicotine administration may influence learning dysfunction induced by MS in adolescent female rats.

Early adverse experience and substance addiction: dopamine, oxytocin, and glucocorticoid pathways

Substance addiction may follow a chronic, relapsing course and critically undermine the physical and psychological well-being of the affected individual and the social units of which the individual is a member. Despite the public health burden associated with substance addiction, treatment options remain suboptimal, with relapses often seen. The present review synthesizes growing insights from animal and human research to shed light upon developmental and neurobiological pathways that may increase susceptibility to addiction. We examine the dopamine system, the oxytocin system, and the glucocorticoid system, as they are particularly relevant to substance addiction. Our aim is to delineate how early adverse experience may induce long-lasting alterations in each of these systems at molecular, neuroendocrine, and behavioral levels and ultimately lead to heightened vulnerability to substance addiction. We further discuss how substance addiction in adulthood may increase the risk of suboptimal caregiving for the next generation, perpetuating the intergenerational cycle of early adverse experiences and addiction.

Early life stress induces attention-deficit hyperactivity disorder (ADHD)-like behavioral and brain metabolic dysfunctions: functional imaging of methylphenidate treatment in a novel rodent model

In a novel animal model Octodon degus we tested the hypothesis that, in addition to genetic predisposition, early life stress (ELS) contributes to the etiology of attention-deficit hyperactivity disorder-like behavioral symptoms and the associated brain functional deficits. Since previous neurochemical observations revealed that early life stress impairs dopaminergic functions, we predicted that these symptoms can be normalized by treatment with methylphenidate. In line with our hypothesis, the behavioral analysis revealed that repeated ELS induced locomotor hyperactivity and reduced attention towards an emotionally relevant acoustic stimulus. Functional imaging using (¹⁴C)-2-fluoro-deoxyglucose-autoradiography revealed that the behavioral symptoms are paralleled by metabolic hypoactivity of prefrontal, mesolimbic and subcortical brain areas. Finally, the pharmacological intervention provided further evidence that the behavioral and metabolic dysfunctions are due to impaired dopaminergic neurotransmission. Elevating dopamine in ELS animals by methylphenidate normalized locomotor hyperactivity and attention-deficit and ameliorated brain metabolic hypoactivity in a dose-dependent manner.

Brain Reward Pathway Dysfunction in Maternal Depression and Addiction: A Present and Future Transgenerational Risk

Two research areas that could benefit from a greater focus on the role of the reward pathway are maternal depression and maternal addiction. Both depression and addiction in mothers are mediated by deficiencies in the reward pathway and represent substantial risks to the health of offspring and future generations. This targeted review discusses maternal reward deficits in depressed and addicted mothers, neural, genetic, and epigenetic mechanisms, and the transgenerational transmission of these deficits from mother to offspring. Postpartum depression and drug use disorders may entail alterations in the reward pathway, particularly in striatal and prefrontal areas, which may affect maternal attachment to offspring and heighten the risk of transgenerational effects on the oxytocin and dopamine systems. Alterations may involve neural circuitry changes, genetic factors that impact monoaminergic neurotransmission, as well as growth factors such as BDNF and stress-associated signaling in the brain. Improved maternal reward-based preventative measures and treatments may be specifically effective for mothers and their offspring suffering from depression and/or addiction.

Early-life adversity and adolescent depression: mechanisms involving the ventral striatum

Early-life adversity is a well-established risk factor for the development of depression later in life. Here we discuss the relationship between early-life adversity and depression, focusing specifically on effects of early-life caregiver deprivation on alterations in the neural and behavioral substrates of reward-processing. We also examine vulnerability to depression within the context of sensitive periods of neural development and the timing of adverse exposure. We further review the development of the ventral striatum, a limbic structure implicated in reward processing, and its role in depressive outcomes following early-life adversity. Finally, we suggest a potential neurobiological mechanism linking early-life adversity and altered ventral striatal development. Together these findings may help provide further insight into the role of reward circuitry dysfunction in psychopathological outcomes in both clinical and developmental populations.

Perinatal Risks and Childhood Premorbid Indicators of Later Psychosis: Next Steps for Early Psychosocial Interventions

Schizophrenia and affective psychoses are debilitating disorders that together affect 2%-3% of the adult population. Approximately 50%-70% of the offspring of parents with schizophrenia manifest a range of observable difficulties including socioemotional, cognitive, neuromotor, speech-language problems, and psychopathology, and roughly 10% will develop psychosis. Despite the voluminous work on premorbid vulnerabilities to psychosis, especially on schizophrenia, the work on premorbid intervention approaches is scarce. While later interventions during the clinical high-risk (CHR) phase of psychosis, characterized primarily by attenuated positive symptoms, are promising, the CHR period is a relatively late phase of developmental derailment. This article reviews and proposes potential targets for psychosocial interventions during the premorbid period, complementing biological interventions described by others in this Special Theme issue. Beginning with pregnancy, parents with psychoses may benefit from enhanced prenatal care, social support, parenting skills, reduction of symptoms, and programs that are family-centered. For children at risk, we propose preemptive early intervention and cognitive remediation. Empirical research is needed to evaluate these interventions for parents and determine whether interventions for parents and children positively influence the developmental course of the offspring.

Using cross-species comparisons and a neurobiological framework to understand early social deprivation effects on behavioral development

Building upon the transactional model of brain development, we explore the impact of early maternal deprivation on neural development and plasticity in three neural systems: hyperactivity/impulsivity, executive function, and hypothalamic-pituitary-adrenal axis functioning across rodent, nonhuman primate, and human studies. Recognizing the complexity of early maternal-infant interactions, we limit our cross-species comparisons to data from rodent models of artificial rearing, nonhuman primate studies of peer rearing, and the relations between these two experimental approaches and human studies of children exposed to the early severe psychosocial deprivation associated with institutional care. In addition to discussing the strengths and limitations of these paradigms, we present the current state of research on the neurobiological impact of early maternal deprivation and the evidence of sensitive periods, noting methodological challenges. Integrating data across preclinical animal models and human studies, we speculate about the underlying biological mechanisms; the differential impact of deprivation due to temporal factors including onset, offset, and duration of the exposure; and the possibility and consequences of reopening of sensitive periods during adolescence.

Propagation of maternal behavior across generations is associated with changes in non-maternal cognitive and behavioral processes

Over a number of years we have studied the phenomenology of maternal behavior from endocrine, neural, experiential, and ontogenetic perspectives. Here, we focus on the effects of early life experiences with and without the mother on subsequent maternal and non-maternal behaviors of the offspring. We have used an artificial rearing procedure, which entails removing rat pups from their mother and raising them in isolation, while controlling and manipulating several aspects of their upbringing. As adults, mother-reared (MR) and artificially-reared (AR) rats are assessed on their own maternal behavior, as well several other behaviors. While both AR and MR rats nurse and successfully wean their young, the AR rats spend less time licking, grooming, and crouching over their young. Hence, being raised in social isolation does not seem to affect primary maternal motivational dynamics. Instead, isolation rearing produces alterations in the ongoing execution of the behavior and its effective organization. Here, we present evidence that changes in maternal behavior, as a result of social isolation from mother and siblings, are due to changes in top-down (e.g., sustained attention, flexibility) and bottom-up process (e.g., increased stimulus-driven behavior). These changes are likely due to alterations in brain dopamine systems, which are sensitive to early life manipulations and are modulators of bottom-up and top-down processes. Finally, we draw parallels between the rat and human maternal behavior. This article is part of a Special Issue entitled: In Honor of Jerry Hogan.

The different effects of maternal separation on spatial learning and reversal learning in rats

Early postnatal maternal separation (MS) can play an important role in the development of psychopathologies during ontogeny. In the present study, we investigated the effects of repeated MS (4h per day from postnatal day (PND) 1 to 21) on locomotor activity and anxiety behavior in open field, spatial learning and reversal learning in Morris water maze of male and female juvenile (PND 21), adolescent (PND 35) and early adult (PND 56) Wistar rats. The results indicated that MS increased locomotor activity of rats across all ages and reduced anxiety behavior of adolescent rats in open field test. MS also increased swim distance in spatial learning and decreased escape latency in reversal learning in adolescent and early adult rats. Additionally, for socially reared rats, there was increased spontaneous locomotion with age, decreased reversal learning ability with age. The present study provides novel insights into the consequences of MS and demonstrates unique age-dependent changes at the behavioral levels.

Developmental neurobiology of the rat attachment system and its modulation by stress

Stress is a powerful modulator of brain structure and function. While stress is beneficial for survival, inappropriate stress dramatically increases the risk of physical and mental health problems, particularly when experienced during early developmental periods. Here we focus on the neurobiology of the infant rat's odor learning system that enables neonates to learn and approach the maternal odor and describe the unique role of the stress hormone corticosterone in modulating this odor approach learning across development. During the first nine postnatal days, this odor approach learning of infant rats is supported by a wide range of sensory stimuli and ensures attachment to the mother's odor, even when interactions with her are occasionally associated with pain. With maturation and the emergence of a stress- or pain-induced corticosterone response, this odor approach learning terminates and a more adult-like amygdala-dependent fear/avoidance learning emerges. Strikingly, the odor approach and attenuated fear learning of older pups can be re-established by the presence of the mother, due to her ability to suppress her pups' corticosterone release and amygdala activity. This suggests that developmental changes in stress responsiveness and the stimuli that produce a stress response might be critically involved in optimally adapting the pup's attachment system to its respective ecological niche.

Predisposing effects of neonatal visceral pain on abuse-related effects of morphine in adult male Sprague Dawley rats

RATIONALE

Adverse early life experiences are risk factors for drug abuse and addiction. Changes in brain opioid systems have been demonstrated in response to neonatal visceral pain (NVP), but the impact of these changes on abuse-related effects of morphine are unknown. The NVP procedure used models chronic visceral hyperalgesia persisting across development.

OBJECTIVES

Intravenous self-administration, drug discrimination, and locomotor activity were used to compare the abuse-related effects of morphine in NVP and control rats.

METHODS

Rats self-administered 0.3 mg/kg/inj morphine under an FR1 schedule, and dose-effect functions for morphine were then established. Separate rats were trained to discriminate 3.2 mg/kg morphine from saline under an FR20 schedule, and morphine dose-effect functions were then determined in the absence and presence of 0.1 mg/kg naltrexone. A third group of rats was tested with a range of morphine doses in an assay of locomotor activity, then injected daily with 10 mg/kg morphine to assess locomotor sensitization.

RESULTS

NVP rats self-administered more morphine than controls at reinforcing doses. Discriminative stimulus effects of morphine were similar between groups, but in the presence of naltrexone, the ED50 for morphine was more than 12× greater in control rats than in NVP animals. Morphine did not stimulate locomotor activity at any tested dose in NVP rats, although significant effects were observed in controls. Finally, significant locomotor sensitization was observed only in NVP rats.

CONCLUSIONS

NVP-induced changes in brain opioid systems have persistent pharmacological consequences into adulthood and may increase sensitivity to abuse-related effects of opioids across development.

Role of sensory, social, and hormonal signals from the mother on the development of offspring

For mammals, sensory, social, and hormonal experience early in life is essential for the continuity of the infant's development. These experiences come from the mother through maternal care, and have enduring effects on the physiology and behavior of the adult organism. Disturbing the mother-offspring interaction by maternal deprivation (neglect) or exposure to adverse events as chronic stress, maltreatment, or sexual abuse has negative effects on the mental, psychological, physiological, and behavioral health. Indeed, these kinds of negative experiences can be the source of some neuropsychiatric diseases as depression, anxiety, impulsive aggression, and antisocial behavior. The purpose of this chapter is to review the most relevant evidence that supports the participation of cues from the mother and/or littermates during the postnatal preweaning period for the development of nervous system of the offspring. These findings come from the most frequently utilized experimental paradigms used in animal models, such as natural variations in maternal behavior, handling, partial maternal deprivation, and total maternal deprivation and artificial rearing. Through the use of these experimental procedures, it is possible to positively (handling paradigm), or negatively (maternal deprivation paradigms), affect the offspring's development. Finally, this chapter reviews the importance of the hormones that pups ingest through the maternal milk during early lactation on the development of several physiological systems, including the immune, endocrine systems, as well as on the adult behavior of the offspring.

History of childhood adversity is positively associated with ventral striatal dopamine responses to amphetamine

RATIONALE

Childhood exposure to severe or chronic trauma is an important risk factor for the later development of adult mental health problems, such as substance abuse. Even in nonclinical samples of healthy adults, persons with a history of significant childhood adversity seem to experience greater psychological distress than those without this history. Evidence from rodent studies suggests that early life stress may impair dopamine function in ways that increase risks for drug abuse. However, the degree to which these findings translate to other species remains unclear.

OBJECTIVES

This study was conducted to examine associations between childhood adversity and dopamine and subjective responses to amphetamine in humans.

METHODS

Following intake assessment, 28 healthy male and female adults, aged 18-29 years, underwent two consecutive 90-min positron emission tomography studies with high specific activity [(11)C]raclopride. The first scan was preceded by intravenous saline; the second by amphetamine (AMPH 0.3 mg/kg).

RESULTS

Consistent with prior literature, findings showed positive associations between childhood trauma and current levels of perceived stress. Moreover, greater number of traumatic events and higher levels of perceived stress were each associated with higher ventral striatal dopamine responses to AMPH. Findings of mediation analyses further showed that a portion of the relationship between childhood trauma and dopamine release may be mediated by perceived stress.

CONCLUSIONS

Overall, results are consistent with preclinical findings suggesting that early trauma may lead to enhanced sensitivity to psychostimulants and that this mechanism may underlie increased vulnerability for drug abuse.

Neuronal control of breathing: sex and stress hormones

There is a growing public awareness that hormones can have a significant impact on most biological systems, including the control of breathing. This review will focus on the actions of two broad classes of hormones on the neuronal control of breathing: sex hormones and stress hormones. The majority of these hormones are steroids; a striking feature is that both groups are derived from cholesterol. Stress hormones also include many peptides which are produced primarily within the paraventricular nucleus of the hypothalamus (PVN) and secreted into the brain or into the circulatory system. In this article we will first review and discuss the role of sex hormones in respiratory control throughout life, emphasizing how natural fluctuations in hormones are reflected in ventilatory metrics and how disruption of their endogenous cycle can predispose to respiratory disease. These effects may be mediated directly by sex hormone receptors or indirectly by neurotransmitter systems. Next, we will discuss the origins of hypothalamic stress hormones and their relationship with the respiratory control system. This relationship is 2-fold: (i) via direct anatomical connections to brainstem respiratory control centers, and (ii) via steroid hormones released from the adrenal gland in response to signals from the pituitary gland. Finally, the impact of stress on the development of neural circuits involved in breathing is evaluated in animal models, and the consequences of early stress on respiratory health and disease is discussed.