Role of netrin-1 in the organization and function of the mesocorticolimbic dopamine system

Changes in mesocorticolimbic dopamine (DA) neurons and their target cells can be induced throughout life and are important determinants of individual differences in susceptibility to psychopathology. The goal of my research is to gain insight into the nature of the cellularand molecular mechanism underlying the selective plasticity of mesocorticolimbic DA neurons. Here, I review work showing that the guidance cue netrin-1 is implicated in the organization, plasticity and function of mesocorticolimbic DA neurons in rodents. Developmental variations in netrin-1 receptor function result in selective reorganization of medial prefrontal DA circuitry during adolescence and in an adult phenotype protected against schizophrenia-like dopaminergic and behavioural abnormalities. Furthermore, in adulthood, expression of netrin-1 receptors is upregulated by repeated exposure to stimulant drugs of abuse in DA somatodendritic regions and is necessary for drug-induced behavioural plasticity. I propose that risk factors associated with DA-related adult psychiatric disorders alter netrin-1 function.     

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.     

Netrin-1 receptor-deficient mice show enhanced mesocortical dopamine transmission and blunted behavioural responses to amphetamine

The mesocorticolimbic dopamine (DA) system is implicated in neurodevelopmental psychiatric disorders including schizophrenia but it is unknown how disruptions in brain development modify this system and increase predisposition to cognitive and behavioural abnormalities in adulthood. Netrins are guidance cues involved in the proper organization of neuronal connectivity during development. We have hypothesized that variations in the function of DCC (deleted in colorectal cancer), a netrin-1 receptor highly expressed by DA neurones, may result in altered development and organization of mesocorticolimbic DA circuitry, and influence DA function in the adult. To test this hypothesis, we assessed the effects of reduced DCC on several indicators of DA function. Using in-vivo microdialysis, we showed that adult mice that develop with reduced DCC display increased basal DA levels in the medial prefrontal cortex and exaggerated DA release in response to the indirect DA agonist amphetamine. In contrast, these mice exhibit normal levels of DA in the nucleus accumbens but significantly blunted amphetamine-induced DA release. Concomitantly, using conditioned place preference, locomotor activity and prepulse inhibition paradigms, we found that reduced DCC diminishes the rewarding and behavioural-activating effects of amphetamine and protects against amphetamine-induced deficits in sensorimotor gating. Furthermore, we found that adult DCC-deficient mice exhibit altered dendritic spine density in layer V medial prefrontal cortex pyramidal neurones but not in nucleus accumbens medium spiny neurones. These findings demonstrate that reduced DCC during development results in a behavioural phenotype opposite to that observed in developmental models of schizophrenia and identify DCC as a critical factor in the development of DA function.     

Neurobiological consequences of maternal cannabis on human fetal development and its neuropsychiatric outcome

Despite the high prevalence of marijuana use among pregnant women and adolescents, the impact of cannabis on the developing brain is still not well understood. However, growing evidence supports that the endocannabinoid system plays a major role in CNS patterning in structures relevant for mood, cognition, and reward, such as the mesocorticolimbic system. It is thus clear that exposure to cannabis during early ontogeny is not benign and potential compensatory mechanisms that might be expected to occur during neurodevelopment appear insufficient to eliminate vulnerability to neuropsychiatric disorders in certain individuals. Both human longitudinal cohort studies and animal models strongly emphasize the long-term influence of prenatal cannabinoid exposure on behavior and mental health. This review provides an overview of the endocannabinoid system and examines the neurobiological consequences of cannabis exposure in pregnancy and early life by addressing its impact on the development of neurotransmitters systems relevant to neuropsychiatric disorders and its association with these disorders later in life. It posits that studying in utero cannabis exposure in association with genetic mutations of neural systems that have strong relationships to endocannabinoid function, such as the dopamine, opioid, glutamate, and GABA, might help to identify individuals at risk. Such data could add to existing knowledge to guide public health platform in regard to the use of cannabis and its derivatives during pregnancy.     

Personality traits in rats predict vulnerability and resilience to developing stress-induced depression-like behaviors, HPA axis hyper-reactivity and brain changes in pERK1/2 activity

Emerging evidence indicates that certain behavioral traits, such as anxiety, are associated with the development of depression-like behaviors after exposure to chronic stress. However, single traits do not explain the wide variability in vulnerability to stress observed in outbred populations. We hypothesized that a combination of behavioral traits might provide a better characterization of an individual's vulnerability to prolonged stress. Here, we sought to determine whether the characterization of relevant behavioral traits in rats could aid in identifying individuals with different vulnerabilities to developing stress-induced depression-like behavioral alterations. We also investigated whether behavioral traits would be related to the development of alterations in the hypothalamic-pituitary-adrenal axis and in brain activity - as measured through phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2)--in response to an acute stressor following either sub-chronic (2 weeks) or chronic (4 weeks) unpredictable stress (CUS). Sprague-Dawley rats were characterized using a battery of behavioral tasks, and three principal traits were identified: anxiety, exploration and activity. When combined, the first two traits were found to explain the variability in the stress responses. Our findings confirm the increased risk of animals with high anxiety developing certain depression-like behaviors (e.g., increased floating time in the forced swim test) when progressively exposed to stress. In contrast, the behavioral profile based on combined low anxiety and low exploration was resistant to alterations related to social behaviors, while the high anxiety and low exploration profile displayed a particularly vulnerable pattern of physiological and neurobiological responses after sub-chronic stress exposure. Our findings indicate important differences in animals' vulnerability and/or resilience to the effects of repeated stress, particularly during initial or intermediate levels of stress exposure, and they highlight that the behavioral inhibition profile of an animal provides a particular susceptibility to responding in a deleterious manner to stress.     

Dopamine activates HIV in chronically infected T lymphoblasts

Summary. HIV infection is associated with a marked vulnerability of the dopaminergic system. We found recently that dopaminergic substances increase brain pathology in the simian model of HIV infection. In the current study we used the chronically HIV-infected T-lymphoblasts ACH-2 to elucidate the effects of dopamine (DA) on HIV infection. Cells were exposed to various concentrations of DA for 24 hours. Flow cytometry measurements demonstrated that DA induced a concentration-dependent HIV activation. To study the mechanism of action of DA, cells were treated besides DA with glutathione, one of the main components of cellular defense mechanisms against oxidative stress as well as its indirect precursor N-acetylcysteine. Treatment with these antioxidants attenuated DA-induced-HIV activation indicating that changes in cellular redox states might have been the causative factor for the observed effect. Our data suggest that HIV activation is tightly linked to intracellular oxidant/antioxidant levels and that excessive DA exposure may modulate cellular vulnerability to HIV. Received October 26, 2000; accepted November 6, 2000     

Subtle fluctuations in psychotic phenomena as functional states of abnormal dopamine reactivity in individuals at risk.

BACKGROUND: Subjects at increased risk for psychosis experience continuous variation in the intensity of subtle psychotic experiences in response to minor stressors. It was investigated whether this psychotic reactivity in individuals at risk for psychosis is the exophenotypic expression of an underlying endophenotype characterized by a hyperreactive dopamine (DA) system. METHODS: First-degree relatives (n = 47) and control subjects (n = 49) were studied with the Experience Sampling Method (ESM), a structured diary technique assessing current context and psychopathology in daily life, to assess psychotic experiences in response to stress. A metabolic perturbation paradigm (administration of 2-deoxy-D-glucose inducing a mild state of glucoprivation) causing plasma elevation of homovanillic acid (HVA) was used as a proxy of DA reactivity. RESULTS: Multilevel regression analyses revealed that the interaction between HVA reactivity and daily stress in their effect on psychotic experiences differed according to underlying vulnerability. In the first-degree relatives, underlying HVA reactivity modified the psychotic experiences to daily stress, whereas no such effect was found in control subjects. CONCLUSIONS: These results suggest that psychotic experiences in response to minor stresses in the flow of daily life may be functional states of an underlying abnormal DA reactivity in subjects at risk to develop psychosis. The results add credence to the suggestion that abnormal DA reactivity may be part of the substrate that increases risk for psychotic symptoms in individuals at risk.     

Prenatal stress increases HPA axis activity and impairs maternal care in lactating female offspring: implications for postpartum mood disorder

Early life stress is believed to constitute a risk factor for the development of mood disorders later in life. In the present study, we hypothesized that prenatal stress (PS) exerts long-lasting effects in female rat offspring, resulting in impaired adaptations to stress during lactation and, as such, may be a contributory factor to postpartum mood disorders. PS increased anxiety in adult virgin females compared with controls. During lactation, PS dams nursed significantly less and spent less time with pups compared with controls, whereas dams did not differ in pup retrieval or maternal aggression. HPA axis reactivity was elevated in response to a mild stressor in PS dams compared to their controls, but not in virgins, with the delta corticosterone response returning to the higher level seen in virgins. Moreover, corticotropin-releasing hormone (CRH) mRNA expression within the parvocellular region of the paraventricular nucleus (PVN) was increased in both virgins and dams exposed to PS compared with the relative controls, while the attenuation in expression in lactating controls was abolished following PS. In addition, arginine vasopressin (AVP) mRNA was increased in the parvocellular, but not magnocellular part of the PVN, in both PS-exposed virgins and lactating dams compared with their relative controls; although expression was also higher in controls during lactation compared with virgins. Thus, the present study demonstrates that exposure to PS results in long-lasting behavioural and neuroendocrine alterations in the female offspring, which are manifested during the lactation period. Furthermore, it implicates PS as a potential risk factor for the development of postpartum mood disorders, and that alterations in the HPA axis reactivity, at least partially, are involved.     

Desperately driven and no brakes: developmental stress exposure and subsequent risk for substance abuse

Adverse life events are associated with a wide range of psychopathology, including an increased risk for substance abuse. In this review, we focus on the inter-relationship between exposure to adversity and brain development, and relate this to enhanced windows of vulnerability. This review encompasses clinical and preclinical data, drawing evidence from epidemiological studies, morphometric and functional imaging studies, and molecular biology and genetics. The interaction of exposure during a sensitive period and maturational events produces a cascade that leads to the initiation of substance use at younger ages, and increases the likelihood of addiction by adolescence or early adulthood. A stress-incubation/corticolimbic dysfunction model is proposed based on the interplay of stress exposure, development stage, and neuromaturational events that may explain the seeking of specific classes of drugs later in life. Three main factors contribute to this age-based progression of increased drug use: (1) a sensitized stress response system; (2) sensitive periods of vulnerability; and (3) maturational processes during adolescence. Together, these factors may explain why exposure to early adversity increases risk to abuse substances during adolescence.     

Childhood trauma among individuals with co-morbid substance use and post traumatic stress disorder

BACKGROUND: Little is known about the impact of childhood trauma (CT) on the clinical profile of individuals with co-occurring substance use disorder (SUD) and post traumatic stress disorder (PTSD). AIMS: To compare the clinical characteristics of individuals with SUD+PTSD who have a history of CT with SUD+PTSD individuals who have experienced trauma during adulthood only. METHOD: Data were collected on 103 individuals as part of a randomised controlled trial examining the efficacy of an integrated psychosocial treatment for SUD+PTSD. Participants were recruited from substance use treatment services, community referrals and advertising. Data were collected on demographic characteristics, substance use and treatment histories, lifetime trauma exposure, and current physical and mental health functioning. RESULTS: The vast majority (77%) of the sample had experienced at least one trauma before the age of 16, with 55% of those endorsing childhood sexual abuse. As expected individuals with a CT history, as compared to without, evidenced significantly longer duration of PTSD. Those with a CT history also had more extensive lifetime trauma exposure, an earlier age of first intoxication, and reported more severe substance use (e.g., a greater number of drug classes used in their lifetime, higher severity of dependence scores and greater number of drug treatment episodes). CONCLUSION: Individuals with co-morbid SUD+PTSD who have experienced CT present with a more severe and chronic clinical profile in relation to a number of trauma and substance use characteristics, when compared to individuals with adulthood only trauma histories. It is therefore important for SUD+PTSD treatment planning that CT be carefully assessed.     

Amygdalo-nigral circuit mediates stress-induced vulnerability to the parkinsonian toxin MPTP

Aims

The aim was to investigate the effect of mood disorders on parkinsonian toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced motor disability, substantia nigra pars compacta (SNc) dopaminergic (DA) neurons loss. Also, the neural circuit mechanism was elucidated.

Methods

The depression-like (physical stress, PS) and anxiety-like (emotional stress, ES) mouse models were established by the three-chamber social defeat stress (SDS). The features of Parkinson's disease were reproduced by MPTP injection. Viral-based whole-brain mapping was utilized to resolve the stress-induced global changes in direct inputs onto SNc DA neurons. Calcium imaging and chemogenetic techniques were applied to verify the function of the related neural pathway.

Results

We found that PS mice, but not ES mice, showed worse movement performance and more SNc DA neuronal loss than control mice after MPTP administration. The projection from the central amygdala (CeA) to the SNc^DA was significantly increased in PS mice. The activity of SNc-projected CeA neurons was enhanced in PS mice. Activating or inhibiting the CeA-SNc^DA pathway could mimic or block PS-induced vulnerability to MPTP.

Conclusions

These results indicated that projections from CeA to SNc DA neurons contribute to SDS-induced vulnerability to MPTP in mice.     

Gestational Restraint Stress and the Developing Dopaminergic System: An Overview

Prenatal stress exerts a strong impact on fetal brain development in rats impairing adaptation to stressful conditions, subsequent vulnerability to anxiety, altered sexual function, and enhanced propensity to self-administer drugs. Most of these alterations have been attributed to changes in the neurotransmitter dopamine (DA). In humans; dysfunction of dopaminergic system is associated with development of several neurological disorders, such as Parkinson disease, schizophrenia, attention-deficit hyperactivity disorder, and depression. Evidences provided by animal research, as well as retrospective studies in humans, pointed out that exposure to adverse events in early life can alter adult behaviors and neurochemical indicators of midbrain DA activity, suggesting that the development of the DA system is sensitive to disruption by exposure to early stressors. The purpose of this article is to provide a general overview of published studies and our own study related to the effect of prenatal insults on the development of DA metabolism and biology, focusing mainly in articles involving prenatal-restraint stress protocols in rats. We will also attempt to make a correlation between theses alterations and DA-related pathological processes in humans.     

Experimental approach to individual vulnerability to psychostimulant addiction

Vulnerability to the development of drug-intake has been studied by using the acquisition of intravenous amphetamine self-administration in the rat. In a series of neurobiological experiments we provoked imbalances in the functioning of the dopaminergic (DA) network by performing lesions of the DA cell bodies in the ventral tegmental area, DA terminals in the amygdala or median raphe nucleus. These imbalances which resulted in enhanced DA transmission ratio between the nucleus accumbens and the prefrontal cortex led to an increase in the rapidity of self-administration acquisition. With a psychobiological approach, we showed that individual differences in vulnerability to develop self-administration in rats of the same strain were correlated with locomotor responses to stress and to an acute injection of amphetamine. Moreover, activation of DA transmission by repeated amphetamine injections changed animals resistant to drug-intake into vulnerable ones. It is suggested that some inherited or acquired factors, at least in part by affecting the activity of DA network, can predispose individuals to drug abuse.     

Synapses between corticotropin-releasing factor-containing axon terminals and dopaminergic neurons in the ventral tegmental area are predominantly glutamatergic

Interactions between stress and the mesocorticolimbic dopamine (DA) system have been suggested from behavioral and electrophysiological studies. Because corticotropin-releasing factor (CRF) plays a role in stress responses, we investigated possible interactions between neurons containing CRF and those producing DA in the ventral tegmental area (VTA). We first investigated the cellular distribution of CRF in the VTA by immunolabeling VTA sections with anti-CRF antibodies and analyzing these sections by electron microscopy. We found CRF immunoreactivity present mostly in axon terminals establishing either symmetric or asymmetric synapses with VTA dendrites. We established that nearly all CRF asymmetric synapses are glutamatergic, insofar as the CRF-immunolabeled axon terminals in these synapses coexpressed the vesicular glutamate transporter 2, and that the majority of CRF symmetric synapses are GABAergic, insofar as the CRF-immunolabeled axon terminals in these synapses coexpressed glutamic acid decarboxylase, findings that are of functional importance. We then looked for synaptic interactions between CRF- and DA-containing neurons, by using antibodies against CRF and tyrosine hydroxylase (TH; a marker for DA neurons). We found that most synapses between CRF-immunoreactive axon terminals and TH neurons are asymmetric (in the majority likely to be glutamatergic) and suggest that glutamatergic neurons containing CRF may be part of the neuronal circuitry that mediates stress responses involving the mesocorticolimbic DA system. The presence of CRF synapses in the VTA offers a mechanism for interactions between the stress-associated neuropeptide CRF and the mesocorticolimbic DA system.     

Attention-deficit/hyperactivity disorder and the substance use disorders: the nature of the relationship, subtypes at risk, and treatment issues.

There is a strong literature supporting a relationship between ADHD and SUD. Clearly, ADHD adolescents with conduct or bipolar disorder as partof their clinical picture are at the highest risk for SUD. ADHD without comorbidity appears to confer an intermediate risk factor for SUD that appears to manifest in young adults and college students. Both family genetic and self-medication influences may be operational in the development and continuation of SUD in ADHD subjects: however, systematic data are lacking. Patients with ADHD and SUD require multi-modal intervention incorporating addiction and mental health treatment. Pharmacotherapy in individuals with ADHD and SUD needs to take into consideration abuse liability, potential drug interactions, and compliance concerns.Although the existing literature has provided important information on the relationship of ADHD and SUD, it also points to a number of areas in need of further study. The mechanism by which untreated ADHD leads toSUD and the risk reduction of ADHD treatment on later SUD, needs to be understood better. The influence of adequateness of treatment of ADHD on later SUD needs to be delineated. Given the prevalence and major morbidity and impairment caused by SUD and ADHD. prevention and treatment strategies for these patients need to be developed and evaluated further.     

Substance Use Disorders in Schizophrenia��Clinical Implications of Comorbidity

Nearly half of the people suffering from schizophrenia also present with a lifetime history of substance use disorders (SUD), a rate that is much higher than the one seen among unaffected individuals. This phenomenon suggests that the factors influencing SUD risk in schizophrenia may be more numerous and/or complex than those modulating SUD risk in the general population. It is critically important to address this comorbidity because SUD in schizophrenic patients is associated with poorer clinical outcomes and contributes significantly to their morbidity and mortality.     

Stereotaxic localization of the developing nucleus accumbens septi

The nucleus accumbens septi (NAcc) has been implicated as a mediator of a variety of disorders, most notably substance abuse. The development of this system is a critical area for investigation, and has been largely overlooked. Specifically, few studies have focussed on dopamine (DA), its neurochemical pathways and the long-term consequences of manipulating the dopaminergic (DAergic) system in the developing animal. Important insight into the establishment of addiction, its development and time course, may be found by examining the development of the periadolescent DA system, specifically the mesocorticolimbic system. Recent developmental studies demonstrate dramatic changes in DAergic levels, receptor concentrations and transporter levels during periadolescent development. These ontogenetic changes, as well as drug exposure during development, may predispose the adolescent animal to addiction. Given that humans typically experiment with and initiate drug use during the adolescent period it is proposed that developmental alterations in the mesolimbic DA projection areas, specifically the NAcc, are an essential area for investigation in drug addiction. The present paper presents formulas for the weight-based calculation of stereotaxic coordinates for the NAcc in rats across development to facilitate further research in the area.     

l-Theanine Prevents Long-Term Affective and Cognitive Side Effects of Adolescent Δ-9-Tetrahydrocannabinol Exposure and Blocks Associated Molecular and Neuronal Abnormalities in the Mesocorticolimbic Circuitry

Chronic adolescent exposure to Δ-9-tetrahydrocannabinol (THC) is linked to elevated neuropsychiatric risk and induces neuronal, molecular and behavioral abnormalities resembling neuropsychiatric endophenotypes. Previous evidence has revealed that the mesocorticolimbic circuitry, including the prefrontal cortex (PFC) and mesolimbic dopamine (DA) pathway are particularly susceptible to THC-induced pathologic alterations, including dysregulation of DAergic activity states, loss of PFC GABAergic inhibitory control and affective and cognitive abnormalities. There are currently limited pharmacological intervention strategies capable of preventing THC-induced neuropathological adaptations. l-Theanine is an amino acid analog of l-glutamate and l-glutamine derived from various plant sources, including green tea leaves. l-Theanine has previously been shown to modulate levels of GABA, DA, and glutamate in various neural regions and to possess neuroprotective properties. Using a preclinical model of adolescent THC exposure in male rats, we report that l-theanine pretreatment before adolescent THC exposure is capable of preventing long-term, THC-induced dysregulation of both PFC and VTA DAergic activity states, a neuroprotective effect that persists into adulthood. In addition, pretreatment with l-theanine blocked THC-induced downregulation of local GSK-3 (glycogen synthase kinase 3) and Akt signaling pathways directly in the PFC, two biomarkers previously associated with cannabis-related psychiatric risk and subcortical DAergic dysregulation. Finally, l-theanine powerfully blocked the development of both affective and cognitive abnormalities commonly associated with adolescent THC exposure, further demonstrating functional and long-term neuroprotective effects of l-theanine in the mesocorticolimbic system.SIGNIFICANCE STATEMENT With the increasing trend of cannabis legalization and consumption during adolescence, it is essential to expand knowledge on the potential effects of adolescent cannabis exposure on brain development and identify potential pharmacological strategies to minimize Δ-9-tetrahydrocannabinol (THC)-induced neuropathology. Previous evidence demonstrates that adolescent THC exposure induces long-lasting affective and cognitive abnormalities, mesocorticolimbic dysregulation, and schizophrenia-like molecular biomarkers that persist into adulthood. We demonstrate for the first time that l-theanine, an amino acid analog of l-glutamate and l-glutamine, is capable of preventing long-term THC side effects. l-Theanine prevented the development of THC-induced behavioral aberrations, blocked cortical downregulation of local GSK-3 (glycogen synthase kinase 3) and Akt signaling pathways, and normalized dysregulation of both PFC and VTA DAergic activity, demonstrating powerful and functional neuroprotective effects against THC-induced developmental neuropathology.     

Increased phasic activity of VTA dopamine neurons in mice 3 weeks after repeated social defeat

Social defeat is an ethologically relevant stress inducing neuroadaptive changes in the mesocorticolimbic dopaminergic system. Three weeks after 10 days of daily defeat salient behaviors and in vivo dopamine (DA) neuron firing were evaluated in mice. Prior defeat induced social avoidance and hyperphagia and increased ventral tegmental area (VTA) DA neuron bursting activity. These data extend previous studies and suggest that increased phasic DA neuron firing in the VTA could be considered amongst the features defining the lasting imprint of social defeat stress.     

Liability to substance use disorders: 2. A measurement approach

Liabilities to complex disorders, discussed in the accompanying paper, present difficulties in measurement related to the arbitrariness of diagnostic threshold definitions and problems with discrimination between trait values, especially within the 'normal' individuals. The inability to quantitatively estimate the risk for a disorder, such as substance use disorders (SUD), is an obstacle for studying etiological (e.g. genetic) mechanisms and developing efficient prevention and treatment measures. Based on the concept of common liability to SUD, this paper delineates an application of the longitudinal family/high-risk design and item response theory to the development of a continuous index of liability. The method has been tested in both simulation study and empirical data. The approach described affords the opportunity to quantitatively estimate the risk for SUD at an early age and before any drug exposure. This method is also applicable to measuring liabilities to other complex disorders, especially those with relatively late onset.