Phospholipase Cgamma in distinct regions of the ventral tegmental area differentially modulates mood-related behaviors

Characterization of the brain virome in human immunodeficiency virus infection and substance use disorder

Viruses can infect the brain in individuals with and without HIV-infection: however, the brain virome is poorly characterized. Metabolic alterations have been identified which predispose people to substance use disorder (SUD), but whether these could be triggered by viral infection of the brain is unknown. We used a target-enrichment, deep sequencing platform and bioinformatic pipeline named "ViroFind", for the unbiased characterization of DNA and RNA viruses in brain samples obtained from the National Neuro-AIDS Tissue Consortium. We analyzed fresh frozen post-mortem prefrontal cortex from 72 individuals without known viral infection of the brain, including 16 HIV+/SUD+, 20 HIV+/SUD-, 16 HIV-/SUD+, and 20 HIV-/SUD-. The average age was 52.3 y and 62.5% were males. We identified sequences from 26 viruses belonging to 11 viral taxa. These included viruses with and without known pathogenic potential or tropism to the nervous system, with sequence coverage ranging from 0.03 to 99.73% of the viral genomes. In SUD+ people, HIV-infection was associated with a higher total number of viruses, and HIV+/SUD+ compared to HIV-/SUD+ individuals had an increased frequency of Adenovirus (68.8 vs 0%; p<0.001) and Epstein-Barr virus (EBV) (43.8 vs 6.3%; p=0.037) as well as an increase in Torque Teno virus (TTV) burden. Conversely, in HIV+ people, SUD was associated with an increase in frequency of Hepatitis C virus, (25 in HIV+/SUD+ vs 0% in HIV+/SUD-; p=0.031). Finally, HIV+/SUD- compared to HIV-/SUD- individuals had an increased frequency of EBV (50 vs 0%; p<0.001) and an increase in TTV viral burden, but a decreased Adenovirus viral burden. These data demonstrate an unexpectedly high variety in the human brain virome, identifying targets for future research into the impact of these taxa on the central nervous system. ViroFind could become a valuable tool for monitoring viral dynamics in various compartments, monitoring outbreaks, and informing vaccine development.

PLCγ1 in dopamine neurons critically regulates striatal dopamine release via VMAT2 and synapsin III

Dopamine neurons are essential for voluntary movement, reward learning, and motivation, and their dysfunction is closely linked to various psychological and neurodegenerative diseases. Hence, understanding the detailed signaling mechanisms that functionally modulate dopamine neurons is crucial for the development of better therapeutic strategies against dopamine-related disorders. Phospholipase Cγ1 (PLCγ1) is a key enzyme in intracellular signaling that regulates diverse neuronal functions in the brain. It was proposed that PLCγ1 is implicated in the development of dopaminergic neurons, while the physiological function of PLCγ1 remains to be determined. In this study, we investigated the physiological role of PLCγ1, one of the key effector enzymes in intracellular signaling, in regulating dopaminergic function in vivo. We found that cell type-specific deletion of PLCγ1 does not adversely affect the development and cellular morphology of midbrain dopamine neurons but does facilitate dopamine release from dopaminergic axon terminals in the striatum. The enhancement of dopamine release was accompanied by increased colocalization of vesicular monoamine transporter 2 (VMAT2) at dopaminergic axon terminals. Notably, dopamine neuron-specific knockout of PLCγ1 also led to heightened expression and colocalization of synapsin III, which controls the trafficking of synaptic vesicles. Furthermore, the knockdown of VMAT2 and synapsin III in dopamine neurons resulted in a significant attenuation of dopamine release, while this attenuation was less severe in PLCγ1 cKO mice. Our findings suggest that PLCγ1 in dopamine neurons could critically modulate dopamine release at axon terminals by directly or indirectly interacting with synaptic machinery, including VMAT2 and synapsin III.

Viral tools for neuroscience

Recombinant viruses are the workhorse of modern neuroscience. Whether one would like to understand a neuron's morphology, natural activity patterns, molecular composition, connectivity or behavioural and physiologic function, most studies begin with the injection of an engineered virus, often an adeno-associated virus or herpes simplex virus, among many other types. Recombinant viruses currently enable some combination of cell type-specific, circuit-selective, activity-dependent and spatiotemporally resolved transgene expression. Viruses are now used routinely to study the molecular and cellular functions of a gene within an identified cell type in the brain, and enable the application of optogenetics, chemogenetics, calcium imaging and related approaches. These advantageous properties of engineered viruses thus enable characterization of neuronal function at unprecedented resolution. However, each virus has specific advantages and disadvantages, which makes viral tool selection paramount for properly designing and executing experiments within the central nervous system. In the current Review, we discuss the key principles and uses of engineered viruses and highlight innovations that are needed moving forward.

Motor skill learning and reward consumption differentially affect VTA activation

Dopamine release from the ventral tegmental area (VTA) terminals in the primary motor cortex (M1) enables motor skill acquisition. Here, we test the hypothesis that dopaminergic VTA neurons projecting to M1 are activated when rewards are obtained during motor skill acquisition, but not during task execution at plateau performance, or by rewards obtained without performing skilled movements. Rats were trained to perform a skilled reaching task for 3 days (acquisition) or 7 days (plateau). In combination with retrograde labelling of VTA-to-M1 projection neurons, double immunofluorescence for c-fos and tyrosine hydroxylase (TH) was used to assess activation of dopaminergic and non-dopaminergic VTA neurons. Dopaminergic VTA-to-M1 projection neurons were indeed activated during successful motor skill acquisition, but not when rats failed to learn or had reached plateau performance, nor by food rewards alone. By contrast, dopaminergic VTA neurons that did not project to M1 were activated by both skilled reaching and food rewards. Non-dopaminergic neurons were found to be activated by motor task performance at plateau, but not during skill acquisition. These results indicate that distinct populations of VTA neurons are activated by motor skill acquisition and task performance. Moreover, this activation is not merely related to consumption of food rewards.

BDNF-TrkB controls cocaine-induced dendritic spines in rodent nucleus accumbens dissociated from increases in addictive behaviors

Chronic cocaine use is associated with prominent morphological changes in nucleus accumbens shell (NACsh) neurons, including increases in dendritic spine density along with enhanced motivation for cocaine, but a functional relationship between these morphological and behavioral phenomena has not been shown. Here we show that brain-derived neurotrophic factor (BDNF) signaling through tyrosine kinase B (TrkB) receptors in NACsh neurons is necessary for cocaine-induced dendritic spine formation by using either localized TrkB knockout or viral-mediated expression of a dominant negative, kinase-dead TrkB mutant. Interestingly, augmenting wild-type TrkB expression after chronic cocaine self-administration reverses the sustained increase in dendritic spine density, an effect mediated by TrkB signaling pathways that converge on extracellular regulated kinase. Loss of TrkB function after cocaine self-administration, however, leaves spine density intact but markedly enhances the motivation for cocaine, an effect mediated by specific loss of TrkB signaling through phospholipase Cgamma1 (PLCγ1). Conversely, overexpression of PLCγ1 both reduces the motivation for cocaine and reverses dendritic spine density, suggesting a potential target for the treatment of addiction in chronic users. Together, these findings indicate that BDNF-TrkB signaling both mediates and reverses cocaine-induced increases in dendritic spine density in NACsh neurons, and these morphological changes are entirely dissociable from changes in addictive behavior.

Inhibition of TrkB at the nucleus accumbens, using ANA-12, regulates basal and stress-induced orexin A expression within the mesolimbic system and affects anxiety, sociability and motivation

Repeated stress exposure can lead to the development of anxiety and mood disorders. An emerging biological substrate of depression and associated pathology is the nucleus accumbens (NAc), which through interactions with limbic, cognitive and motor circuits can regulate a variety of stress responses. Within these circuits, orexin neurons are involved in arousal and stress adaptability, effects proposed mediated via brain-derived neurotrophic factor signaling. This study tested the hypotheses that 1) repeated exposure to heterotypic stress alters social ability and preference and passive avoidant behaviors, 2) TrkB receptors at the NAc shell regulates stress-induced behavioral responses and orexin expression within the mesocorticolimbic system. Our findings indicate that ANA-12 (0.25 μg/0.5 μl) enhanced sociability during the social interaction test, although treatment had no effect on social preference. The development of conditioned place preference, and fear retention in the passive avoidance test were also facilitated by ANA-12. Biochemical assessments on brain tissues collected within 2 h of a forced swim exposure revealed that ANA-12 increased orexin A immunoreactivity (ir) in the hypothalamic perifornical area, while expression was reduced in the ventral portion of the hippocampal CA1 layer, irrespective of the stress condition. This contrasts changes at the VTA characterized by elevated versus reduced orexin A-ir in ANA-12-treated stress and non-stress rats, respectively. Colocalized orexin A- and tyrosine hydroxylase (TH)-ir at the VTA supports a different temporal expression post stress, TH-ir being unaffected 9 days post stress. These findings support a role for TrkB receptors in regulating basal and stress-induced social, cognitive and motivational behavior, and modulatory actions of BDNF, via TrkB signaling, on orexin A signaling upon stress exposure.

Blockade of TrkB receptors in the nucleus accumbens prior to heterotypic stress alters corticotropin-releasing hormone (CRH), vesicular glutamate transporter 2 (vGluT2) and glucocorticoid receptor (GR) within the mesolimbic pathway

Inhibition of stress-induced elevations in brain-derived neurotrophic factor (BDNF) or its primary receptor tyrosine-related kinase B (TrkB) within the reward pathway may modulate vulnerability to anxiety and mood disorders. The current study examined the role of BDNF/TrkB signaling on biochemistry and behavior under basal conditions and following exposure to a 10-day heterotypic stress paradigm in male rats. Effects of intra-accumbal administration of TrkB antagonist ANA-12 (0.25μg/0.5μl/min) on anxiety, and expression of Trk-B, corticotropin-releasing hormone (CRH), vesicular glutamate transporter 2 (vGluT2) and glucocorticoid receptor (GR) within the mesolimbic pathway were determined. Notably, ANA-12 attenuated anxiety-like behavior in stress rats while increasing anxiety in the non-stress group in the elevated plus maze (EPM). At the neurochemical level, ANA-12 blocked the increased vGluT2 and CRH expressions in the hypothalamic PVN and basolateral amygdala in stress rats, while it enhanced vGluT2 and CRH expressions in non-stress rats. ANA-12 also showed state-dependent effects at the NAc core, attenuating TrkB-ir in non-stress rats while reversing reduced expression in stressed rats. At the cingulate cortex, ANA-12 normalized stress-induced increase in TrkB expression. Notably, ANA-12 showed region-specific effects on GR-ir at the NAc core and shell, with increased GR-ir in non-stress rats, although the drug attenuated stress-induced GR-ir expression only in the core portion of the NAc, while having no impact at the cingulate cortex. Elevated blood CORT levels post-stress was not influenced by ANA-12 treatment. Together, these findings suggest that BDNF-mediated TrkB activation exerts differential impact in regulating emotional response under basal and stress conditions.

The antero-posterior heterogeneity of the ventral tegmental area

The ventral tegmental area (VTA) is a brain region processing salient sensory and emotional information, controlling motivated behaviors, natural or drug-related reward, reward-related learning, mood, and participating in their associated psychopathologies. Mostly studied for its dopamine neurons, the VTA also includes functionally important GABA and glutamate cell populations. Behavioral evidence supports the presence of functional differences between the anterior VTA (aVTA) and the posterior VTA (pVTA), which is the topic of this review. This antero-posterior heterogeneity concerns locomotor activity, conditioned place preference and intracranial self-administration, and can be seen in response to ethanol, acetaldehyde, salsolinol, opioids including morphine, cholinergic agonists including nicotine, cocaine, cannabinoids and after local manipulation of GABA and serotonin receptors. It has also been observed after viral-mediated manipulation of GluR1, phospholipase Cγ (PLCγ) and cAMP response element binding protein (CREB) expression, with impact on reward and aversion-related responses, on anxiety and depression-related behaviors and on pain sensitivity. In this review, the substrates potentially underlying these aVTA/pVTA differences are discussed, including the VTA sub-nuclei and the heterogeneity in connectivity, cell types and molecular characteristics. We also review the role of the tail of the VTA (tVTA), or rostromedial tegmental nucleus (RMTg), which may also participate to the observed antero-posterior heterogeneity of the VTA. This region, partly located within the pVTA, is an inhibitory control center for dopamine activity. It controls VTA and substantia nigra dopamine cells, thus exerting a major influence on basal ganglia functions. This review highlights the need for a more comprehensive analysis of VTA heterogeneity.

Neurotrophins in the ventral tegmental area: Role in social stress, mood disorders and drug abuse

This review discusses the impact of neurotrophins and other trophic factors, including fibroblast growth factor and glial cell line-derived neurotrophic factor, on mood disorders, weight regulation and drug abuse, with an emphasis on stress- and drug-induced changes in the ventral tegmental area (VTA). Neurotrophins, comprising nerve growth factor, brain-derived neurotrophic factor (BDNF), and neurotrophins 3 and 4/5 play important roles in neuronal plasticity and the development of different psychopathologies. In the VTA, most research has focused on the role of BDNF, because other neurotrophins are not found there in significant quantities. BDNF originating in the VTA provides trophic support to dopamine neurons. The diverse intracellular signaling pathways activated by BDNF may underlie precise physiological functions specific to the VTA. In general, VTA BDNF expression increases after psychostimulant exposures, and enhanced BDNF level in the VTA facilitates psychostimulant effects. The impact of VTA BDNF on the behavioral effects of psychostimulants relies primarily on its action within the mesocorticolimbic circuit. In the case of opiates, VTA BDNF expression and effects seem to be dependent on whether an animal is drug-naïve or has a history of drug use, only the latter of which is related to dopamine mechanisms. Social defeat stress that is continuous in mice or intermittent in rats increases VTA BDNF expression, and is associated with depressive and social avoidance behaviors. Intermittent social defeat stress induces persistent VTA BDNF expression that triggers psychostimulant cross-sensitization. Understanding the cellular and molecular substrates of neurotrophin effects may lead to novel therapeutic approaches for the prevention and treatment of substance use and mood disorders.

Effects of psychotropic drugs on second messenger signaling and preference for nicotine in juvenile male mice

RATIONALE

A common treatment strategy for pediatric attention deficit/hyperactivity disorder (ADHD) and major depressive disorder (MDD) is combined methylphenidate (MPH) and fluoxetine (FLX). This has raised concerns because MPH + FLX treatment may have pharmacodynamic properties similar to cocaine, potentially increasing drug abuse liability.

OBJECTIVES

To examine the short- and long-term consequences of repeated vehicle, MPH, FLX, MPH + FLX, and cocaine treatment on gene expression in juvenile (postnatal days [PD] 20-34) and adult (PD 70-84) male mice. We further assessed whether juvenile drug treatment influenced subsequent sensitivity for nicotine in adulthood.

METHODS

Juvenile and adult C57BL/6J mice received vehicle, MPH, FLX, MPH + FLX, or cocaine twice-daily for 15 consecutive days. Mice were sacrificed 24 h or 2 months after the last drug injection to assess drug-induced effects on the extracellular signal-regulated protein kinase-1/2 (ERK) pathway within the ventral tegmental area. Subsequent sensitivity for nicotine (0.05, 0.07, and 0.09 mg/kg) was measured using the place-conditioning paradigm (CPP) 24 h and 2 months after juvenile drug exposure.

RESULTS

MPH + FLX, or cocaine exposure in juvenile mice increased mRNA expression of ERK2 and its downstream targets (CREB, cFos, and Zif268), and increased protein phosphorylation of ERK2 and CREB 2 months after drug exposure. Similar mRNA findings were observed in the adult-treated mice. Findings on gene expression 24 h following drug treatment were variable. Juvenile drug exposure increased preference for nicotine when tested in adulthood.

CONCLUSIONS

Early-life MPH + FLX, or cocaine exposure similarly disrupts the ERK pathway, a signaling cascade implicated in motivation and mood regulation, and increases sensitivity for nicotine in adulthood.

Life-long consequences of juvenile exposure to psychotropic drugs on brain and behavior

Psychostimulants such as methylphenidate (MPH) and antidepressants such as fluoxetine (FLX) are widely used in the treatment of various mental disorders or as cognitive enhancers. These medications are often combined, for example, to treat comorbid disorders. There is a considerable body of evidence from animal models indicating that individually these psychotropic medications can have detrimental effects on the brain and behavior, especially when given during sensitive periods of brain development. However, almost no studies investigate possible interactions between these drugs. This is surprising given that their combined neurochemical effects (enhanced dopamine and serotonin neurotransmission) mimic some effects of illicit drugs such as cocaine and amphetamine. Here, we summarize recent studies in juvenile rats on the molecular effects in the mid- and forebrain and associated behavioral changes, after such combination treatments. Our findings indicate that these combined MPH+FLX treatments can produce similar molecular changes as seen after cocaine exposure while inducing behavioral changes indicative of dysregulated mood and motivation, effects that often endure into adulthood.

The brain reward circuitry in mood disorders

Mood disorders are common and debilitating conditions characterized in part by profound deficits in reward-related behavioural domains. A recent literature has identified important structural and functional alterations within the brain's reward circuitry--particularly in the ventral tegmental area-nucleus accumbens pathway--that are associated with symptoms such as anhedonia and aberrant reward-associated perception and memory. This Review synthesizes recent data from human and rodent studies from which emerges a circuit-level framework for understanding reward deficits in depression. We also discuss some of the molecular and cellular underpinnings of this framework, ranging from adaptations in glutamatergic synapses and neurotrophic factors to transcriptional and epigenetic mechanisms.

Pharmacological recruitment of the GABAergic tail of the ventral tegmental area by acute drug exposure

BACKGROUND AND PURPOSE

The tail of the ventral tegmental area (tVTA), also called the rostromedial tegmental nucleus, is a newly defined brain structure and a potential control centre for dopaminergic activity. It was identified by the induction of DeltaFosB following chronic cocaine exposure. In this work, we screened 20 drugs for their ability to induce FosB/DeltaFosB in the tVTA.

EXPERIMENTAL APPROACH

Immunohistochemistry following systemic drug administration was used to study FosB/DeltaFosB induction in the tVTA of adult rats. Double-staining was used to determine whether dopamine or GABA neurones are involved in this induction.

KEY RESULTS

The acute injection of the psychostimulant drugs cocaine, D-amphetamine, (+/-)-3,4-methylenedioxymethamphetamine (MDMA), methylphenidate or caffeine, induced the expression of FosB/DeltaFosB in the tVTA GABAergic cells. No induction was observed following exposure to ethanol, diazepam, γ-hydroxybutyric acid (GHB), morphine, ketamine, phencyclidine (PCP), Δ(9)-tetrahydrocannabinol (THC), sodium valproic acid or gabapentin. To evaluate the role of monoamine transporters in the psychostimulant-induced expression of FosB/DeltaFosB, we tested the antidepressant drugs reboxetine, nortriptyline, fluoxetine and venlafaxine (which target the noradrenaline and/or the 5-hydroxytryptamine transporters), the 5-hydroxytryptamine releasing agent dexfenfluramine, and the dopamine transporter inhibitor GBR12909. Only GBR12909 was able to induce FosB/DeltaFosB expression in the tVTA, showing that this induction is mediated by dopamine.

CONCLUSIONS AND IMPLICATIONS

Newly described brain structures may help to increase our knowledge of brain function, pathology and targets for treatments. FosB/DeltaFosB induction in the tVTA is a common feature of drugs sharing psychostimulant properties but not of drugs sharing risk of abuse.

Individual Differences in Nucleus Accumbens Dopamine Receptors Predict Development of Addiction-Like Behavior: A Computational Approach

Clinical and experimental observations show individual differences in the development of addiction. Increasing evidence supports the hypothesis that dopamine receptor availability in the nucleus accumbens (NAc) predisposes drug reinforcement. Here, modeling striatal-midbrain dopaminergic circuit, we propose a reinforcement learning model for addiction based on the actor-critic model of striatum. Modeling dopamine receptors in the NAc as modulators of learning rate for appetitive—but not aversive—stimuli in the critic—but not the actor—we define vulnerability to addiction as a relatively lower learning rate for the appetitive stimuli, compared to aversive stimuli, in the critic. We hypothesize that an imbalance in this learning parameter used by appetitive and aversive learning systems can result in addiction. We elucidate that the interaction between the degree of individual vulnerability and the duration of exposure to drug has two progressive consequences: deterioration of the imbalance and establishment of an abnormal habitual response in the actor. Using computational language, the proposed model describes how development of compulsive behavior can be a function of both degree of drug exposure and individual vulnerability. Moreover, the model describes how involvement of the dorsal striatum in addiction can be augmented progressively. The model also interprets other forms of addiction, such as obesity and pathological gambling, in a common mechanism with drug addiction. Finally, the model provides an answer for the question of why behavioral addictions are triggered in Parkinson's disease patients by D2 dopamine agonist treatments.

Viral-mediated expression of extracellular signal-regulated kinase-2 in the ventral tegmental area modulates behavioral responses to cocaine

Chronic exposure to cocaine increases the activity of extracellular signal-regulated kinase (ERK1/2) in the ventral tegmental area (VTA), a neural substrate for drugs of abuse. However, the functional significance of changes in ERK1/2 activity in this brain region is unknown. Using herpes simplex virus-mediated gene transfer to regulate ERK2 activity within the VTA in male rats, we show that overexpressing ERK2 increases preference for environments previously paired with low doses of cocaine and enhances cocaine-induced locomotion, whereas blocking ERK2 activity blocks cocaine-induced place conditioning and locomotor activity. These results demonstrate that ERK2-signaling within the VTA is a key modulator of functional responses to cocaine.

From synapse to nucleus: novel targets for treating depression

The need for newer compounds to treat depression is an ever-growing concern due to the enormous societal and financial ramifications of this disorder. Here, we review some of the candidate systems that could potentially be involved in depression, or an inherent resistance to depression termed resilience, and the numerous protein targets for these systems. A substantial body of literature provides strong evidence that neurotrophic factors, glutamate receptors, hypothalamic feeding peptides, nuclear hormone receptors, and epigenetic mechanisms, among others, will make for interesting targets when examining depressive behavior or resilience in preclinical models, and eventually clinical trials. Although some of these targets for depression already appear promising, new waves of more selective compounds for any molecular system should promote a better understanding of this complex disease and perhaps improved treatments.

Differential effects of dopamine D2 and GABA(A) receptor antagonists on dopamine neurons between the anterior and posterior ventral tegmental area of female Wistar rats

Previous findings indicated differences in neuronal circuitries mediating drug reinforcement between the anterior and posterior ventral tegmental area (VTA). The objective of the present study was to examine the effects of the dopamine D2 antagonist sulpiride and the GABA(A) antagonist picrotoxin administered in the anterior and posterior VTA on the activity of mesoaccumbal dopamine neurons in female Wistar rats. Sulpiride and picrotoxin were administered in the anterior and posterior VTA. Extracellular dopamine levels were measured in sub-regions of the VTA and nucleus accumbens (ACB). Reverse-microdialysis of sulpiride (100 microM) into the posterior VTA increased extracellular dopamine levels locally (80% above baseline) and in the ACB shell and core (70% above baseline), whereas reverse-microdialysis into the anterior VTA produced a much smaller effect locally (30% above baseline) and in the ACB shell and core. In contrast, microinjection of picrotoxin (80 and 160 microM) into the anterior, but not posterior VTA, increased dopamine release in the ACB shell. The results suggest that dopamine neurons in the posterior VTA, compared to the anterior VTA, may be under greater D2 receptor-mediated tonic inhibition, whereas dopamine neurons in the anterior VTA, compared to the posterior VTA, may be under greater GABA(A) receptor-mediated tonic inhibition.

Afferents to the GABAergic tail of the ventral tegmental area in the rat

We previously showed that chronic psychostimulant exposure induces the transcription factor DeltaFosB in gamma-aminobutyric acid (GABA)ergic neurons of the caudal tier of the ventral tegmental area (VTA). This subregion was defined as the tail of the VTA (tVTA). In the present study, we showed that tVTA can also be visualized by analyzing FosB/DeltaFosB response following acute cocaine injection. This induction occurs in GABAergic neurons, as identified by glutamic acid decarboxylase (GAD) expression. To characterize tVTA further, we mapped its inputs by using the retrograde tracers Fluoro-Gold or cholera toxin B subunit. Retrogradely labeled neurons were observed in the medial prefrontal cortex, the lateral septum, the ventral pallidum, the bed nucleus of the stria terminalis, the substantia innominata, the medial and lateral preoptic areas, the lateral and dorsal hypothalamic areas, the lateral habenula, the intermediate layers of the superior colliculus, the dorsal raphe, the periaqueductal gray, and the mesencephalic and pontine reticular formation. Projections from the prefrontal cortex, the hypothalamus, and the lateral habenula to the tVTA were also shown by using the anterograde tracer biotinylated dextran amine (BDA). We showed that the central nucleus of the amygdala innervates the anterior extent of the VTA but not the tVTA. Moreover, the tVTA mainly receives non-aminergic inputs from the dorsal raphe and the locus coeruleus. Although the tVTA has a low density of dopaminergic neurons, its afferents are mostly similar to those targeting the rest of the VTA. This suggests that the tVTA can be considered as a VTA subregion despite its caudal location.

Phasic excitation of dopamine neurons in ventral VTA by noxious stimuli

Midbrain dopamine neurons play central roles in reward processing. It is widely assumed that all dopamine neurons encode the same information. Some evidence, however, suggests functional differences between subgroups of dopamine neurons, particularly with respect to processing nonrewarding, aversive stimuli. To directly test this possibility, we recorded from and juxtacellularly labeled individual ventral tegmental area (VTA) dopamine neurons in anesthetized rats so that we could link precise anatomical position and neurochemical identity with coding for noxious stimuli. Here, we show that dopamine neurons in the dorsal VTA are inhibited by noxious footshocks, consistent with their role in reward processing. In contrast, we find that dopamine neurons in the ventral VTA are phasically excited by footshocks. This observation can explain a number of previously confusing findings that suggested a role for dopamine in processing both rewarding and aversive events. Taken together, our results indicate that there are 2 functionally and anatomically distinct VTA dopamine systems.

Phosphorylation of DeltaFosB mediates its stability in vivo

The transcription factor, DeltaFosB, accumulates in a region-specific manner in brain in response to many types of chronic stimulation due to the unusual stability of the protein. The phosphorylation of Ser27 in DeltaFosB has been shown to promote this stability in vitro. We show here that this phosphorylation reaction is also important for DeltaFosB's stability in the brain in vivo and for the unique behavioral plasticity mediated by this transcription factor.

AKT signaling within the ventral tegmental area regulates cellular and behavioral responses to stressful stimuli

BACKGROUND

The neurobiological mechanisms by which only a minority of stress-exposed individuals develop psychiatric diseases remain largely unknown. Recent evidence suggests that dopaminergic neurons of the ventral tegmental area (VTA) play a key role in the manifestation of stress vulnerability.

METHODS

Using a social defeat paradigm, we segregated susceptible mice (socially avoidant) from unsusceptible mice (socially interactive) and examined VTA punches for changes in neurotrophic signaling. Employing a series of viral vectors, we sought to causally implicate these neurotrophic changes in the development of avoidance behavior.

RESULTS

Susceptibility to social defeat was associated with a significant reduction in levels of active/phosphorylated AKT (thymoma viral proto-oncogene) within the VTA, whereas chronic antidepressant treatment (in mice and humans) increased active AKT levels. This defeat-induced reduction in AKT activation in susceptible mice was both necessary and sufficient to recapitulate depressive behaviors associated with susceptibility. Pharmacologic reductions in AKT activity also significantly raised the firing frequency of VTA dopamine neurons, an important electrophysiologic hallmark of the susceptible phenotype.

CONCLUSIONS

These studies highlight a crucial role for decreases in VTA AKT signaling as a key mediator of the maladaptive cellular and behavioral response to chronic stress.

Neurotrophic factors and structural plasticity in addiction

Drugs of abuse produce widespread effects on the structure and function of neurons throughout the brain's reward circuitry, and these changes are believed to underlie the long-lasting behavioral phenotypes that characterize addiction. Although the intracellular mechanisms regulating the structural plasticity of neurons are not fully understood, accumulating evidence suggests an essential role for neurotrophic factor signaling in the neuronal remodeling which occurs after chronic drug administration. Brain-derived neurotrophic factor (BDNF), a growth factor enriched in brain and highly regulated by several drugs of abuse, regulates the phosphatidylinositol 3'-kinase (PI3K), mitogen-activated protein kinase (MAPK), phospholipase Cgamma (PLCgamma), and nuclear factor kappa B (NFkappaB) signaling pathways, which influence a range of cellular functions including neuronal survival, growth, differentiation, and structure. This review discusses recent advances in our understanding of how BDNF and its signaling pathways regulate structural and behavioral plasticity in the context of drug addiction.

Dopamine reward circuitry: two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex

Anatomical and functional refinements of the meso-limbic dopamine system of the rat are discussed. Present experiments suggest that dopaminergic neurons localized in the posteromedial ventral tegmental area (VTA) and central linear nucleus raphe selectively project to the ventromedial striatum (medial olfactory tubercle and medial nucleus accumbens shell), whereas the anteromedial VTA has few if any projections to the ventral striatum, and the lateral VTA largely projects to the ventrolateral striatum (accumbens core, lateral shell and lateral tubercle). These findings complement the recent behavioral findings that cocaine and amphetamine are more rewarding when administered into the ventromedial striatum than into the ventrolateral striatum. Drugs such as nicotine and opiates are more rewarding when administered into the posterior VTA or the central linear nucleus than into the anterior VTA. A review of the literature suggests that (1) the midbrain has corresponding zones for the accumbens core and medial shell; (2) the striatal portion of the olfactory tubercle is a ventral extension of the nucleus accumbens shell; and (3) a model of two dopamine projection systems from the ventral midbrain to the ventral striatum is useful for understanding reward function. The medial projection system is important in the regulation of arousal characterized by affect and drive and plays a different role in goal-directed learning than the lateral projection system, as described in the variation-selection hypothesis of striatal functional organization.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Regulation of neuronal PLCgamma by chronic morphine

Alterations in neurotrophic signaling pathways may contribute to the changes in the mesolimbic dopamine system induced by chronic morphine exposure. In a rat model of morphine dependence, we previously identified increased levels of phospholipase C gamma-1 (PLCgamma1) immunoreactivity specifically within the ventral tegmental area (VTA) following chronic morphine treatment. Using an antibody specific for the tyrosine-phosphorylated, activated form of PLCgamma1, we now show that chronic morphine also significantly upregulates PLCgamma1 activity in the VTA, as well as in the nucleus accumbens and hippocampus, regions which are also implicated in the reinforcing properties of morphine. In contrast, no increase in PLCgamma1 activity was found in the substantia nigra or dorsal striatum. HSV-mediated overexpression of PLCgamma1 in PC12 cells induced ERK activation via a mechanism dependent, in part, on both MAP-ERK kinase (MEK) and protein kinase C. PLCgamma1 overexpression in the VTA similarly induced ERK activation in the VTA in vivo. As chronic morphine treatment has been shown to increase ERK activity within the VTA, the current results suggest that increased PLCgamma1 activity may be an upstream mediator of this effect.

Electrical stimulation of the ventral tegmental area exerts opposite effects on male rat sexual behaviour expression depending on the stimulated sub region

The effects of electrical stimulation of the ventral tegmental area (VTA) on male rat sexual behaviour were studied. Stimulation of dorsal VTA facilitated copulation of sexually experienced animals; while ventral VTA stimulation inhibited sexual behaviour. In sexually exhausted rats, stimulation at either region lacked of an effect. It is concluded that different loci within the VTA exert opposite influences on male sexual behaviour expression and that copulation to satiation modifies this VTA control.

Topographical organization of GABAergic neurons within the ventral tegmental area of the rat

The ventral tegmental area (VTA), the origin of dopaminergic cell bodies that comprise the mesocorticolimibic DA system, is widely implicated in drug and natural reward, cognition, and several psychiatric disorders. In addition to dopaminergic neurons, this region is populated by GABAergic neurons, which both regulate the firing of their dopaminergic counterparts and send projections throughout the brain. Although the dopaminergic neurons of the VTA have been extensively characterized neuroanatomically, much less is known about the GABAergic neurons in this region. Recent data suggest that the rostro-caudal topographic organization of these GABAergic neurons may correspond to their ability to regulate drug reward. In the present study, we used immunohistochemical techniques to examine the frequency and topography of GABAergic neurons throughout the rostro-caudal axis of the VTA and the extent to which they coexpress other proteins, including tyrosine hydroxylase (a marker of DA neurons), cholecystokinin, parvalbumin, calretinin, and calbindin d 28k.

IRS2-Akt pathway in midbrain dopamine neurons regulates behavioral and cellular responses to opiates

Chronic morphine administration (via subcutaneous pellet) decreases the size of dopamine neurons in the ventral tegmental area (VTA), a key reward region in the brain, yet the molecular basis and functional consequences of this effect are unknown. In this study, we used viral-mediated gene transfer in rat to show that chronic morphine-induced downregulation of the insulin receptor substrate 2 (IRS2)-thymoma viral proto-oncogene (Akt) signaling pathway in the VTA mediates the decrease in dopamine cell size seen after morphine exposure and that this downregulation diminishes morphine reward, as measured by conditioned place preference. We further show that the reduction in size of VTA dopamine neurons persists up to 2 weeks after morphine withdrawal, which parallels the tolerance to morphine's rewarding effects caused by previous chronic morphine exposure. These findings directly implicate the IRS2-Akt signaling pathway as a critical regulator of dopamine cell morphology and opiate reward.

The mesolimbic dopamine reward circuit in depression

The neural circuitry that mediates mood under normal and abnormal conditions remains incompletely understood. Most attention in the field has focused on hippocampal and frontal cortical regions for their role in depression and antidepressant action. While these regions no doubt play important roles in these phenomena, there is compelling evidence that other brain regions are also involved. Here we focus on the potential role of the nucleus accumbens (NAc; ventral striatum) and its dopaminergic input from the ventral tegmental area (VTA), which form the mesolimbic dopamine system, in depression. The mesolimbic dopamine system is most often associated with the rewarding effects of food, sex, and drugs of abuse. Given the prominence of anhedonia, reduced motivation, and decreased energy level in most individuals with depression, we propose that the NAc and VTA contribute importantly to the pathophysiology and symptomatology of depression and may even be involved in its etiology. We review recent studies showing that manipulations of key proteins (e.g. CREB, dynorphin, BDNF, MCH, or Clock) within the VTA-NAc circuit of rodents produce unique behavioral phenotypes, some of which are directly relevant to depression. Studies of these and other proteins in the mesolimbic dopamine system have established novel approaches to modeling key symptoms of depression in animals, and could enable the development of antidepressant medications with fundamentally new mechanisms of action.

Afferents of the ventral tegmental area in the rat-anatomical substratum for integrative functions

The ventral tegmental area (VTA) is critically important to an organism's capacity to detect rewards and novelty and to enlist appropriate behavioral responses. Although there has been substantial progress concerning information processing at the single cell and molecular levels in the VTA, our knowledge of its overall afferent connections is based principally on the benchmark description by Phillipson ([1979] J. Comp. Neurol. 187:117-144). Given that, since then, the sensitivity of tracing methods and knowledge about the organization of brain structures have increased considerably, we undertook to reevaluate the VTA afferents of the rat. The retrograde tracer Fluoro-Gold was injected into different parts of the VTA, and labeled neurons were visualized by immunocytochemistry. Retrogradely labeled neurons were not confined to nuclei but rather constituted an elongated formation stretching from the prefrontal cortex rostrally to the medulla oblongata caudally. In the case of descending afferents, this formation was centered on the medial forebrain bundle and the fasciculus retroflexus. The input to the VTA in general was bilateral, with a smaller descending and comparable ascending projection from the contralateral side. Injections of the anterograde tracers Phaseolus vulgaris-leucoagglutinin or biotinylated dextran amine into selected forebrain structures revealed a surprisingly sparse terminal arborization in the VTA. Furthermore, structures projecting to the VTA innervate other brain areas with similar or greater robustness, which in turn also provide a strong input to the VTA, indicating an anatomical network. Given the importance of the VTA in basic behaviors, this organization might provide a basis for an extraordinary level of afferent integration.

Phospholipase C gamma in distinct regions of the ventral tegmental area differentially regulates morphine-induced locomotor activity

Neurotrophic factors and the signaling pathways they activate play a role in mediating long-term molecular, cellular, and behavioral adaptations associated with drug addiction. Here we mimicked the biological response of phospholipase C-gamma (PLC gamma) induction in the ventral tegmental area (VTA) observed after chronic morphine using viral-mediated gene transfer. Using a behavioral sensitization paradigm, we demonstrate that microinjections of PLC gamma 1 into distinct (rostral vs. caudal) regions of the VTA result in differential locomotor responses to morphine.

In vivo up-regulation of brain-derived neurotrophic factor in specific brain areas by chronic exposure to Delta9-tetrahydrocannabinol

Cannabinoids are widely abused drugs. Here we show that chronic administration of Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the active psychotropic agent in marijuana and hashish, at 1.5 mg per kg per day intraperitoneally for 7 days, increases the expression, at both mRNA and protein levels, of brain-derived neurotrophic factor (BDNF), in specific rat brain areas, notably in those involved in reward and addiction. Real-time PCR revealed a 10-fold up-regulation of BDNF mRNA in the nucleus accumbens (NAc) upon chronic Delta(9)-THC treatment, but there was no change at 3 or 24 h after a single injection. Smaller increases in mRNA levels were found in the ventral tegmental area (VTA), medial prefrontal cortex and paraventricular nucleus (PVN). Immunohistochemistry showed large increases in BDNF-stained cells in the NAc (5.5-fold), posterior VTA (4-fold) and PVN (1.7-fold), but no change was observed in the anterior VTA, hippocampus or dorsal striatum. Altogether, our study indicates that chronic exposure to Delta(9)-THC up-regulates BDNF in specific brain areas involved with reward, and provides evidence for different BDNF expression in the anterior and posterior VTA. Moreover, BDNF is known to modulate synaptic plasticity and adaptive processes underlying learning and memory, leading to long-term functional and structural modification of synaptic connections. We suggest that Delta(9)-THC up-regulation of BDNF expression has an important role in inducing the neuroadaptive processes taking place upon exposure to cannabinoids.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Mu opioid receptor: a gateway to drug addiction

Mu opioid receptors mediate positive reinforcement following direct (morphine) or indirect (alcohol, cannabinoids, nicotine) activation, and our understanding of mu receptor function is central to the development of addiction therapies. Recent data obtained in native neurons confirm that mu receptor signaling and regulation are strongly agonist-dependent. Current functional mapping reveals morphine-activated neurons in the extended amygdala and early genomic approaches have identified novel mu receptor-associated proteins. A classification of about 30 genes either promoting or counteracting the addictive properties of morphine is proposed from the analysis of knockout mice data. The targeting of effectors or regulatory proteins, beyond the mu receptor itself, might provide valuable strategies to treat addictive disorders.

Molecular basis for catecholaminergic neuron diversity

Catecholaminergic neurons control diverse cognitive, motor, and endocrine functions and are associated with multiple psychiatric and neurodegenerative disorders. We present global gene-expression profiles that define the four major classes of dopaminergic (DA) and noradrenergic neurons in the brain. Hypothalamic DA neurons and noradrenergic neurons in the locus coeruleus display distinct group-specific signatures of transporters, channels, transcription, plasticity, axon-guidance, and survival factors. In contrast, the transcriptomes of midbrain DA neurons of the substantia nigra and the ventral tegmental area are closely related with <1% of differentially expressed genes. Transcripts implicated in neural plasticity and survival are enriched in ventral tegmental area neurons, consistent with their role in schizophrenia and addiction and their decreased vulnerability in Parkinson's disease. The molecular profiles presented provide a basis for understanding the common and population-specific properties of catecholaminergic neurons and will facilitate the development of selective drugs.

Catabolic action of insulin in rat arcuate nucleus is not enhanced by exogenous "tub" expression

The central nervous system (CNS) protein "tub" has been identified from the genetically obese "tubby" mouse. Although the native function of tub in situ is not understood, cell-based studies suggest that one of its roles may be as an intracellular signaling target for insulin. In normal animals, insulin acts at the hypothalamic arcuate nucleus (ARC) to regulate energy balance. Here we used a Herpes Simplex viral expression system to evaluate whether tub overexpression in the ARC of normal rats enhances this action of insulin. In chow-fed rats, tub overexpression had no effect on insulin action. In rats fed a high-fat diet snack in addition to chow, simulating the diet of Westernized societies, the body weight regulatory action of insulin was impaired, and tub overexpression further impaired insulin action. Thus an excess of tub at the ARC does not enhance the in vivo effectiveness of insulin and is not able to compensate for the "downstream" consequences of a high-fat diet to impair CNS body weight regulatory mechanisms.

Molecular neurobiology of drug addiction

Addiction can be viewed as a form of drug-induced neural plasticity. One of the best-established molecular mechanisms of addiction is upregulation of the cAMP second messenger pathway, which occurs in many neuronal cell types in response to chronic administration of opiates or other drugs of abuse. This upregulation and the resulting activation of the transcription factor CREB appear to mediate aspects of tolerance and dependence. In contrast, induction of another transcription factor, termed DeltaFosB, exerts the opposite effect and may contribute to sensitized responses to drug exposure. Knowledge of these mechanisms could lead to more effective treatments for addictive disorders.

Sexual behavior and sex-associated environmental cues activate the mesolimbic system in male rats

The mesolimbic system plays an important role in the regulation of both pathological behaviors such as drug addiction and normal motivated behaviors such as sexual behavior. The present study investigated the mechanism by which this system is endogenously activated during sexual behavior. Specifically, the effects of sexual experience and sex-related environmental cues on the activation of several components of the mesolimbic system were studied. The mesolimbic system consists of a dopaminergic projection from the ventral tegmental area (VTA) to the nucleus accumbens (NAc). Previous studies suggest that these neurons are under tonic inhibition by local GABA interneurons, which are in turn modulated by mu opioid receptor (MOR) ligands. To test the hypothesis that opioids are acting in the VTA during sexual behavior, visualization of MOR internalization in VTA was used as a marker for ligand-induced activation of the receptor. Significant increases in MOR internalization were observed following copulation or exposure to sex-related environmental cues. The next goal was to determine if sexual behavior activates dopamine neurons in the VTA, using tyrosine hydroxylase as a marker for dopaminergic neurons and Fos-immunoreactivity as a marker for neuronal activation. Significant increases in the percentage of activated dopaminergic neurons were observed following copulation or exposure to sex-related environmental cues. In addition, mating and sex-related cues activated a large population of nondopaminergic neurons in VTA as well as neurons in both the NAc Core and Shell. Taken together, our results provide functional neuroanatomical evidence that the mesolimbic system is activated by both sexual behavior and exposure to sex-related environmental cues.