Prefrontal cortical D1 dopamine receptors modulate subcortical D2 dopamine receptor-mediated stress responsiveness

Depressive-like behavior and impaired synaptic plasticity in the prefrontal cortex as later consequences of prenatal hypoxic-ischemic insult in rats

Perinatal hypoxia-ischemia (HI) is a leading cause of morbidity and mortality among newborns. Infants with HI encephalopathy may experience lasting consequences, such as depression, in adulthood. In this study, we examined depressive-like behavior, neuronal population, and markers of monoaminergic and synaptic plasticity in the prefrontal cortex (PFC) of adolescent rats subjected to a prenatal HI model. Pregnant rats underwent a surgery in which uterine and ovarian blood flow was blocked for 45 min at E18 (HI procedure). Sham-operated subjects were also generated (SH procedure). Behavioral tests were conducted on male and female pups from P41 to P43, and animals were histologically processed or dissected for western blotting at P45. We found that the HI groups consumed less sucrose in the sucrose preference test and remained immobile for longer periods in the forced swim test. Additionally, we observed a significant reduction in neuronal density and PSD95 levels in the HI group, as well as a smaller number of synaptophysin-positive cells. Our results underscore the importance of this model in investigating the effects of HI-induced injuries, as it reproduces an increase in depressive-like behavior and suggests that the HI insult affects circuits involved in mood modulation.

Serotonin disruption at gestation alters expression of genes associated with serotonin synthesis and reuptake at weaning

RATIONALE

Serotonin (5-HT) is a monoamine neuromodulator that plays a key role in the organization of the central nervous system. 5-HT alterations may be associated to the emergence of social deficits and psychiatric disorders, including anxiety, depression, and substance abuse disorders. Notably, disruption of the 5-HT system during sensitive periods of development seems to exert long-term consequences, including altered anxiety responses and problematic use of alcohol.

OBJECTIVE

We analyzed, in mice, the effects of transient 5-HT depletion at gestation (a developmental stage when medial prefrontal cortex (mPFC) 5-HT levels depend exclusively on placental 5-HT availability) on 5-HT central synthesis and reuptake at weaning. We also explored if 5-HT disruption at the embryonic stage influences behavioral outcomes that may serve as a proxy for autistic- or anxiety-like phenotypes.

METHODS

C57/BL6 male and female mice, born from dams treated with a 5-HT synthesis inhibitor (PCPA; 4-Chloro-DL-phenylalanine methyl ester hydrochloride) at gestational days (G)13.5-16.5, were subjected to a behavioral battery that assesses social preference and novelty, compulsive behavior, stereotypies, and ethanol's anti-anxiety effects, at postnatal days (P) 21-28. Afterwards, expression of the genes that encode for 5-HT synthesis (Tph2) and SERT (5-HT transporter) were analyzed in mPFC via real-time RT-PCR. Dopamine 2 receptor (D2R) expression was also analyzed via RT-PCR to further explore possible effects of PCPA on dopaminergic transmission.

RESULTS

Transient 5-HT disruption at G13.5-16.5 reduced Tph2 expression of both male and female mice in mPFC at P23. Notably, female mice also exhibited higher SERT expression and reduced D2R expression in mPFC. Mice derived from 5-HT depleted dams displayed heightened compulsive behavior at P21, when compared to control mice. Alcohol anti-anxiety effects at early adolescence (P28) were exhibited by mice derived from 5-HT depleted dams, but not by control counterparts. No social deficits or stereotyped behaviors were observed.

CONCLUSION

Transient 5-HT inhibition at gestation resulted in altered expression of genes involved in 5-HT synthesis and reuptake in mPFC at weaning, a period in which the 5-HT system is still developing. These alterations may exert lingering effects, which translate to significant compulsivity and heightened sensitivity to the anxiolytic effects of alcohol at early adolescence.

Dopamine Receptors: Is It Possible to Become a Therapeutic Target for Depression?

Dopamine and its receptors are currently recognized targets for the treatment of several neuropsychiatric disorders, including Parkinson’s disease, schizophrenia, some drug use addictions, as well as depression. Dopamine receptors are widely distributed in various regions of the brain, but their role and exact contribution to neuropsychiatric diseases has not yet been thoroughly studied. Based on the types of dopamine receptors and their distribution in different brain regions, this paper reviews the current research status of the molecular, cellular and circuit mechanisms of dopamine and its receptors involved in depression. Multiple lines of investigation of these mechanisms provide a new future direction for understanding the etiology and treatment of depression and potential new targets for antidepressant treatments.

Bold zebrafish (Danio rerio) express higher levels of delta opioid and dopamine D2 receptors in the brain compared to shy fish

Individual variation in coping with environmental challenges is a well-known phenomenon across vertebrates, including teleost fish. Dopamine is the major transmitter in the brain reward networks, and important for motivational processes and stress coping. Functions of the endogenous opioid system are not well studied in teleosts. However, in mammals the activity in the brain reward networks is regulated by the endogenous opioid system. This study aimed at investigating if there was a correlation between risk-taking behavior and the expression of dopamine and opioid receptors in the zebrafish (Danio rerio) brain. Risk-taking behavior was assessed in a novel tank diving test, and the most extreme high risk taking, i.e. bold, and low risk taking, i.e. shy, fish were sampled for qPCR analysis of whole brain gene expression. The expression analysis showed a significantly higher expression of the dopamine D2 receptors (drd2a and drd2b) and the delta opioid receptor (DOR; oprd1b) in bold compared to shy fish. Besides reward and reinforcing properties, DORs are also involved in emotional responses. Dopamine D2 receptors are believed to be important for active stress coping in rodents, and taken together the results of the current study suggest similar functions in zebrafish. However, additional experiments are required to clarify how dopamine and opioid receptor activation affect behavior and stress coping in this species.

COMT Val(158) Met genotype is associated with reward learning: a replication study and meta-analysis

Identifying mechanisms through which individual differences in reward learning emerge offers an opportunity to understand both a fundamental form of adaptive responding as well as etiological pathways through which aberrant reward learning may contribute to maladaptive behaviors and psychopathology. One candidate mechanism through which individual differences in reward learning may emerge is variability in dopaminergic reinforcement signaling. A common functional polymorphism within the catechol-O-methyl transferase gene (COMT; rs4680, Val(158) Met) has been linked to reward learning, where homozygosity for the Met allele (linked to heightened prefrontal dopamine function and decreased dopamine synthesis in the midbrain) has been associated with relatively increased reward learning. Here, we used a probabilistic reward learning task to asses response bias, a behavioral form of reward learning, across three separate samples that were combined for analyses (age: 21.80 ± 3.95; n = 392; 268 female; European-American: n = 208). We replicate prior reports that COMT rs4680 Met allele homozygosity is associated with increased reward learning in European-American participants (β = 0.20, t = 2.75, P < 0.01; ΔR(2) = 0.04). Moreover, a meta-analysis of 4 studies, including the current one, confirmed the association between COMT rs4680 genotype and reward learning (95% CI -0.11 to -0.03; z = 3.2; P < 0.01). These results suggest that variability in dopamine signaling associated with COMT rs4680 influences individual differences in reward which may potentially contribute to psychopathology characterized by reward dysfunction.

Genetic Disruption of Arc/Arg3.1 in Mice Causes Alterations in Dopamine and Neurobehavioral Phenotypes Related to Schizophrenia

Human genetic studies have recently suggested that the postsynaptic activity-regulated cytoskeleton-associated protein (Arc) complex is a convergence signal for several genes implicated in schizophrenia. However, the functional significance of Arc in schizophrenia-related neurobehavioral phenotypes and brain circuits is unclear. Here, we find that, consistent with schizophrenia-related phenotypes, disruption of Arc in mice produces deficits in sensorimotor gating, cognitive functions, social behaviors, and amphetamine-induced psychomotor responses. Furthermore, genetic disruption of Arc leads to concomitant hypoactive mesocortical and hyperactive mesostriatal dopamine pathways. Application of a D1 agonist to the prefrontal cortex or a D2 antagonist in the ventral striatum rescues Arc-dependent cognitive or psychomotor abnormalities, respectively. Our findings demonstrate a role for Arc in the regulation of dopaminergic neurotransmission and related behaviors. The results also provide initial biological support implicating Arc in dopaminergic and behavioral abnormalities related to schizophrenia.

Mesocortical Dopamine Phenotypes in Mice Lacking the Sonic Hedgehog Receptor Cdon

Motivated behaviors and many psychopathologies typically involve changes in dopamine release from the projections of the ventral tegmental area (VTA) and/or the substantia nigra pars compacta (SNc). The morphogen Sonic Hedgehog (Shh) specifies fates of midbrain dopamine neurons, but VTA-specific effects of Shh signaling are also being uncovered. In this study, we assessed the role of the Shh receptor Cdon in the development of VTA and SNc dopamine neurons. We find that Cdon is expressed in the proliferating progenitor zone of the embryonic ventral midbrain and that the number of proliferating cells in this region is increased in mouse Cdon(-/-) embryos. Consistent with a role of Shh in the regulation of neuronal proliferation in this region, we find that the number of tyrosine hydroxylase (TH)-positive neurons is increased in the VTA of Cdon(-/-) mice at birth and that this effect endures into adulthood. In contrast, the number of TH-positive neurons in the SNc is not altered in Cdon(-/-) mice at either age. Moreover, adult Cdon(-/-) mice have a greater number of medial prefrontal cortex (mPFC) dopamine presynaptic sites, and increased baseline concentrations of dopamine and dopamine metabolites selectively in this region. Finally, consistent with increased dopamine function in the mPFC, we find that adult Cdon(-/-) mice fail to exhibit behavioral plasticity upon repeated amphetamine treatment. Based on these data, we suggest that Cdon plays an important role encoding the diversity of dopamine neurons in the midbrain, influencing both the development of the mesocortical dopamine pathway and behavioral outputs that involve this neural circuitry.

D1- and D2-like dopamine receptors within the nucleus accumbens contribute to stress-induced analgesia in formalin-related pain behaviours in rats

BACKGROUND

Stressful experiences can produce analgesia, termed stress-induced analgesia (SIA). Meanwhile, it has been widely established that the mesolimbic dopamine pathway and nucleus accumbens (NAc) have a profound role in pain modulation. In this study, we examined the role of accumbal dopamine receptors in antinociception caused by forced swim stress (FSS) in order to understand more about the function of these receptors within the NAc in FSS-induced analgesia.

METHOD

Stereotaxic surgery was unilaterally performed on adult male Wistar rats weighing 230-250 g (some on the left and some on the right side of the midline). Two supergroups were microinjected into the NAc with a D1-like dopamine receptor antagonist, SCH-23390, at doses of 0.25, 1 and 4 μg/0.5 μl saline per rat or Sulpiride as a D2-like dopamine receptor antagonist at the same doses [0.25, 1 and 4 μg/0.5 μl dimethyl sulfoxide (DMSO) per rat]; while their controls just received intra-accumbal saline or DMSO at 0.5 μl, respectively. The formalin test was performed after rats were subjected to FSS (6 min, 25 ± 1 °C) to assess pain-related behaviours.

RESULTS

The results demonstrated that intra-accumbal infusions of SCH-23390 and Sulpiride dose-dependently reduced FSS-induced antinociception in both phases of the formalin test. However, the percentage decrease in area under the curve (AUC) values calculated for treatment groups compared to formalin-control group was more significant in the late phase than the early phase.

CONCLUSION

Our findings suggest that D1- and D2-like dopamine receptors in the NAc are involved in stress-induced antinociceptive behaviours in the formalin test as an animal model of persistent inflammatory pain.

WHAT DOES THIS STUDY ADD

Forced swim stress (FSS) induces the antinociception in both phases of formalin test. Blockade of accumbal dopamine receptors attenuate the antinociception induced by FSS. Stress-induced analgesia is dose-dependently reduced by dopamine receptor antagonists in both phases, although it is more prominent during the late phase.

Early-Life Stress Affects Stress-Related Prefrontal Dopamine Activity in Healthy Adults, but Not in Individuals with Psychotic Disorder

Early life stress may have a lasting impact on the developmental programming of the dopamine (DA) system implicated in psychosis. Early adversity could promote resilience by calibrating the prefrontal stress-regulatory dopaminergic neurotransmission to improve the individual's fit with the predicted stressful environment. Aberrant reactivity to such match between proximal and distal environments may, however, enhance psychosis disease risk. We explored the combined effects of childhood adversity and adult stress by exposing 12 unmedicated individuals with a diagnosis of non-affective psychotic disorder (NAPD) and 12 healthy controls (HC) to psychosocial stress during an [18F]fallypride positron emission tomography. Childhood trauma divided into early (ages 0-11 years) and late (12-18 years) was assessed retrospectively using a questionnaire. A significant group x childhood trauma interaction on the spatial extent of stress-related [18F]fallypride displacement was observed in the mPFC for early (b = -8.45, t(1,23) = -3.35, p = .004) and late childhood trauma (b = -7.86, t(1,23) = -2.48, p = .023). In healthy individuals, the spatial extent of mPFC DA activity under acute psychosocial stress was positively associated with the severity of early (b = 7.23, t(11) = 3.06, p = .016) as well as late childhood trauma (b = -7.86, t(1,23) = -2.48, p = .023). Additionally, a trend-level main effect of early childhood trauma on subjective stress response emerged within this group (b = -.7, t(11) = -2, p = .07), where higher early trauma correlated with lower subjective stress response to the task. In the NAPD group, childhood trauma was not associated with the spatial extent of the tracer displacement in mPFC (b = -1.22, t(11) = -0.67), nor was there a main effect of trauma on the subjective perception of stress within this group (b = .004, t(11) = .01, p = .99). These findings reveal a potential mechanism of neuroadaptation of prefrontal DA transmission to early life stress and suggest its role in resilience and vulnerability to psychosis.

Resilience to amphetamine in mouse models of netrin-1 haploinsufficiency: role of mesocortical dopamine

RATIONALE

Signaling through the netrin-1 receptor, deleted in colorectal cancer (DCC), in dopamine neurons controls the extent of their innervation to the medial prefrontal cortex (mPFC) during adolescence. In mice, dcc haploinsufficiency results in increased mPFC dopamine innervation and concentrations in adulthood. In turn, dcc haploinsufficiency leads to resilience to the effects of stimulant drugs of abuse on dopamine release in the nucleus accumbens and behavior.

OBJECTIVES

First, we set out to determine whether increased mPFC dopamine innervation causes blunted behavioral responses to amphetamine in adult dcc haploinsufficient mice. Second, we investigated whether unc5c, another netrin-1 receptor expressed by dopamine neurons, is involved in these effects. Third, we assessed whether haploinsufficiency of netrin-1 itself leads to blunted behavioral responding to amphetamine, whether this phenotype emerges before or after adolescence and whether increased mPFC dopamine input is the underlying mechanism.

RESULTS

Adult, but not adolescent, dcc, unc5c and netrin-1 haploinsufficient mice exhibit blunted behavioral responses to amphetamine. Furthermore, adult dcc, unc5c, and netrin-1 haploinsufficient mice have exaggerated mPFC dopamine concentrations in comparison to their wild-type littermates. Importantly, resilience to amphetamine-induced behavioral activation in all the three mouse models is abolished by selective dopamine depletion in the medial prefrontal cortex.

CONCLUSIONS

dcc, unc5c, or netrin-1 haploinsufficiency leads to increased dopamine content in the mPFC and to resilience against amphetamine-induced behavioral activation. Our findings raise the hypothesis that DCC, UNC5C, and netrin-1 act in concert to organize the adolescent development of mesocortical dopamine innervation and, in turn, determine behavioral responses to drugs of abuse.

The dopaminergic response to acute stress in health and psychopathology: A systematic review

Previous work in animals has shown that dopamine (DA) in cortex and striatum plays an essential role in stress processing. For the first time, we systematically reviewed the in vivo evidence for DAergic stress processing in health and psychopathology in humans. All studies included (n studies=25, n observations=324) utilized DA D2/3 positron emission tomography and measured DAergic activity during an acute stress challenge. The evidence in healthy volunteers (HV) suggests that physiological, but not psychological, stress consistently increases striatal DA release. Instead, increased medial prefrontal cortex (mPFC) DAergic activity in HV was observed during psychological stress. Across brain regions, stress-related DAergic activity was correlated with the physiological and psychological intensity of the stressor. The magnitude of stress-induced DA release was dependent on rearing conditions, personality traits and genetic variations in several SNPs. In psychopathology, preliminary evidence was found for stress-related dorsal striatal DAergic hyperactivity in psychosis spectrum and a blunted response in chronic cannabis use and pain-related disorders, but results were inconsistent. Physiological stress-induced DAergic activity in striatum in HV may reflect somatosensory properties of the stressor and readiness for active fight-or-flight behavior. DAergic activity in HV in the ventral striatum and mPFC may be more related to expectations about the stressor and threat evaluation, respectively. Future studies with increased sample size in HV and psychopathology assessing the functional relevance of stress-induced DAergic activity, the association between cortical and subcortical DAergic activity and the direct comparison of different stressors are necessary to conclusively elucidate the role of the DA system in the stress response.

Perceived life stress exposure modulates reward-related medial prefrontal cortex responses to acute stress in depression

INTRODUCTION

Major depressive disorder (MDD) is often precipitated by life stress and growing evidence suggests that stress-induced alterations in reward processing may contribute to such risk. However, no human imaging studies have examined how recent life stress exposure modulates the neural systems that underlie reward processing in depressed and healthy individuals.

METHODS

In this proof-of-concept study, 12 MDD and 10 psychiatrically healthy individuals were interviewed using the Life Events and Difficulties Schedule (LEDS) to assess their perceived levels of recent acute and chronic life stress exposure. Additionally, each participant performed a monetary incentive delay task under baseline (no-stress) and stress (social-evaluative) conditions during functional MRI.

RESULTS

Across groups, medial prefrontal cortex (mPFC) activation to reward feedback was greater during acute stress versus no-stress conditions in individuals with greater perceived stressor severity. Under acute stress, depressed individuals showed a positive correlation between perceived stressor severity levels and reward-related mPFC activation (r=0.79, p=0.004), whereas no effect was found in healthy controls. Moreover, for depressed (but not healthy) individuals, the correlations between the stress (r=0.79) and no-stress (r=-0.48) conditions were significantly different. Finally, relative to controls, depressed participants showed significantly reduced mPFC gray matter, but functional findings remained robust while accounting for structural differences.

LIMITATION

Small sample size, which warrants replication.

CONCLUSION

Depressed individuals experiencing greater recent life stress recruited the mPFC more under stress when processing rewards. Our results represent an initial step toward elucidating mechanisms underlying stress sensitization and recurrence in depression.

No evidence for attenuated stress-induced extrastriatal dopamine signaling in psychotic disorder

Stress is an important risk factor in the etiology of psychotic disorder. Preclinical work has shown that stress primarily increases dopamine (DA) transmission in the frontal cortex. Given that DA-mediated hypofrontality is hypothesized to be a cardinal feature of psychotic disorder, stress-related extrastriatal DA release may be altered in psychotic disorder. Here we quantified for the first time stress-induced extrastriatal DA release and the spatial extent of extrastriatal DA release in individuals with non-affective psychotic disorder (NAPD). Twelve healthy volunteers (HV) and 12 matched drug-free NAPD patients underwent a single infusion [(18)F]fallypride positron emission tomography scan during which they completed the control and stress condition of the Montreal Imaging Stress Task. HV and NAPD did not differ in stress-induced [(18)F]fallypride displacement and the spatial extent of stress-induced [(18)F]fallypride displacement in medial prefrontal cortex (mPFC) and temporal cortex (TC). In the whole sample, the spatial extent of stress-induced radioligand displacement in right ventro-mPFC, but not dorso-mPFC or TC, was positively associated with task-induced subjective stress. Psychotic symptoms during the scan or negative, positive and general subscales of the Positive and Negative Syndrome Scale were not associated with stress-induced [(18)F]fallypride displacement nor the spatial extent of stress-induced [(18)F]fallypride displacement in NAPD. Our results do not offer evidence for altered stress-induced extrastriatal DA signaling in NAPD, nor altered functional relevance. The implications of these findings for the role of the DA system in NAPD and stress processing are discussed.

Motor and behavioral phenotype in conditional mutants with targeted ablation of cortical D1 dopamine receptor-expressing cells

D1-dopamine receptors (Drd1a) are highly expressed in the deep layers of the cerebral cortex and the striatum. A number of human diseases such as Huntington disease and schizophrenia are known to have cortical pathology involving dopamine receptor expressing neurons. To illuminate their functional role, we exploited a Cre/Lox molecular paradigm to generate Emx-1(tox) MUT mice, a transgenic line in which cortical Drd1a-expressing pyramidal neurons were selectively ablated. Emx-1(tox) MUT mice displayed prominent forelimb dystonia, hyperkinesia, ataxia on rotarod testing, heightened anxiety-like behavior, and age-dependent abnormalities in a test of social interaction. The latter occurred in the context of normal working memory on testing in the Y-maze and for novel object recognition. Some motor and behavioral abnormalities in Emx-1(tox) MUT mice overlapped with those in CamKIIα(tox) MUT transgenic mice, a line in which both striatal and cortical Drd1a-expressing cells were ablated. Although Emx-1(tox) MUT mice had normal striatal anatomy, both Emx-1(tox) MUT and CamKIIα(tox) MUT mice displayed selective neuronal loss in cortical layers V and VI. This study shows that loss of cortical Drd1a-expressing cells is sufficient to produce deficits in multiple motor and behavioral domains, independent of striatal mechanisms. Primary cortical changes in the D1 dopamine receptor compartment are therefore likely to model a number of core clinical features in disorders such as Huntington disease and schizophrenia.

The effects of social defeat on behavior and dopaminergic markers in mice

The present study investigated the effects of chronic social defeat stress on several behavioral parameters, and the expression of dopaminergic markers, i.e., dopamine D1 receptors (D1Rs), dopamine D2 receptors (D2Rs), and dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein-32 (DARPP-32), in the prefrontal cortex (PFC), amygdala (AMY), and hippocampus (HIP) of mouse brains. After 10days of social defeat stress, the defeated mice were divided into two groups: one group underwent a series of behavioral tests. The other group was sacrificed on the 11th day and tissue samples were collected for Western blotting. The behavioral tests comprised tests of locomotion, light/dark preference, social interaction, as well as the novel object recognition test (NORT), Morris water maze, and forced swimming test (FST). We measured the expression of D1Rs, D2Rs, total DARPP-32, phospho-Thr34 or Thr75-DARPP-32 using Western blotting. The defeated mice showed increased anxiety- and depression-like behaviors, and impaired cognition. No significant differences in D1Rs and D2Rs expression were shown between defeated and control mice in any area studied. A significantly increased expression in total DARPP-32, and phospho-DARPP-32 was observed in the PFC or AMY of defeated mice. These data suggest that alterations in dopaminergic markers may be involved in anxiety- and depression-like behaviors, and cognitive impairment induced by social defeat stress.

Orbitofrontal dopamine depletion upregulates caudate dopamine and alters behavior via changes in reinforcement sensitivity

Schizophrenia is associated with upregulation of dopamine (DA) release in the caudate nucleus. The caudate has dense connections with the orbitofrontal cortex (OFC) via the frontostriatal loops, and both areas exhibit pathophysiological change in schizophrenia. Despite evidence that abnormalities in dopaminergic neurotransmission and prefrontal cortex function co-occur in schizophrenia, the influence of OFC DA on caudate DA and reinforcement processing is poorly understood. To test the hypothesis that OFC dopaminergic dysfunction disrupts caudate dopamine function, we selectively depleted dopamine from the OFC of marmoset monkeys and measured striatal extracellular dopamine levels (using microdialysis) and dopamine D2/D3 receptor binding (using positron emission tomography), while modeling reinforcement-related behavior in a discrimination learning paradigm. OFC dopamine depletion caused an increase in tonic dopamine levels in the caudate nucleus and a corresponding reduction in D2/D3 receptor binding. Computational modeling of behavior showed that the lesion increased response exploration, reducing the tendency to persist with a recently chosen response side. This effect is akin to increased response switching previously seen in schizophrenia and was correlated with striatal but not OFC D2/D3 receptor binding. These results demonstrate that OFC dopamine depletion is sufficient to induce striatal hyperdopaminergia and changes in reinforcement learning relevant to schizophrenia.

Viral over-expression of D1 dopamine receptors in the prefrontal cortex increase high-risk behaviors in adults: comparison with adolescents

RATIONALE

Adolescents are often described as "lacking brakes" resulting in an increase in several behaviors associated with risk for addiction. Prefrontal cortex dopamine and cortico-limbic interaction play an important role in addiction, and we have previously shown that the dopamine D1 receptor is elevated on prelimbic prefrontal output neurons in adolescent rats. We hypothesized that a constellation of risk-related behaviors is mediated by prefrontal output neuron expression of D1.

OBJECTIVES

We aimed to determine the role of the dopamine D1 receptor in behavioral and neural correlates of risk for addiction that are often observed in adolescents. Therefore, high-risk behaviors as well as subcortical D2 receptor expression were investigated in adult animals with experimentally elevated D1 on prefrontal glutamatergic neurons.

METHODS

A lentiviral vector that selectively expressed the D1 receptor within glutamate neurons was injected in the prelimbic prefrontal cortex of adult male rats. Place conditioning to cocaine, alcohol, and nicotine, as well as delay discounting, novelty preferences, anxiety, cocaine self-administration, and sucrose preferences were assessed.

RESULTS

Virally mediated D1 over-expression in adults leads to stronger drug-cue associations and greater consumption of sweet solutions, elevates bias towards immediate satisfaction rather than delaying gratification, decreases anxiety, and causes rats to work harder for and take more cocaine. Furthermore, elevated cortical D1 reduces D2 receptors in the accumbens (a putative risk marker).

CONCLUSIONS

Together, these data suggest a common mechanism for increased motivational drive to seek and consume substances with hedonic value, consistent with adolescent addictive processes.

Corticolimbic catecholamines in stress: a computational model of the appraisal of controllability

Appraisal of a stressful situation and the possibility to control or avoid it is thought to involve frontal-cortical mechanisms. The precise mechanism underlying this appraisal and its translation into effective stress coping (the regulation of physiological and behavioural responses) are poorly understood. Here, we propose a computational model which involves tuning motivational arousal to the appraised stressing condition. The model provides a causal explanation of the shift from active to passive coping strategies, i.e. from a condition characterised by high motivational arousal, required to deal with a situation appraised as stressful, to a condition characterised by emotional and motivational withdrawal, required when the stressful situation is appraised as uncontrollable/unavoidable. The model is motivated by results acquired via microdialysis recordings in rats and highlights the presence of two competing circuits dominated by different areas of the ventromedial prefrontal cortex: these are shown having opposite effects on several subcortical areas, affecting dopamine outflow in the striatum, and therefore controlling motivation. We start by reviewing published data supporting structure and functioning of the neural model and present the computational model itself with its essential neural mechanisms. Finally, we show the results of a new experiment, involving the condition of repeated inescapable stress, which validate most of the model's predictions.

The effects of olanzapine on genome-wide DNA methylation in the hippocampus and cerebellum

Background

The mechanism of action of olanzapine in treating schizophrenia is not clear. This research reports the effects of a therapeutic equivalent treatment of olanzapine on DNA methylation in a rat model in vivo.

Genome-wide DNA methylation was assessed using a MeDIP-chip analysis. All methylated DNA immunoprecipitation (MeDIP), sample labelling, hybridization and processing were performed by Arraystar Inc (Rockville, MD, USA). The identified gene promoters showing significant alterations to DNA methylation were then subjected to Ingenuity Pathway Analysis (Ingenuity System Inc, CA, USA).

Results

The results show that olanzapine causes an increase in methylation in 1,140, 1,294 and 1,313 genes and a decrease in methylation in 633, 565 and 532 genes in the hippocampus, cerebellum and liver, respectively. Most genes affected are tissue specific. Only 41 affected genes (approximately 3%) showed an increase and no gene showed a decrease in methylation in all three tissues. Further, the two brain regions shared 123 affected genes (approximately 10%). The affected genes are enriched in pathways affecting dopamine signalling, molecular transport, nervous system development and functions in the hippocampus; ephrin receptor signalling and synaptic long-term potentiation in the cerebellum; and tissue morphology, cellular assembly and organization in the liver. Also, the affected genes included those previously implicated in psychosis.

Conclusions

The known functions of affected genes suggest that the observed epigenetic changes may underlie the amelioration of symptoms as well as accounting for certain adverse effects including the metabolic syndrome. The results give insights into the mechanism of action of olanzapine, therapeutic effects and the side effects of antipsychotics.

Role of concomitant inhibition of the norepinephrine transporter for the antipsychotic effect of quetiapine

Quetiapine alleviates both positive and negative symptoms as well as certain cognitive impairments in schizophrenia despite a low D2 receptor occupancy and may also be used as monotherapy in bipolar and major depressive disorder. The mechanisms underlying the broad clinical utility of quetiapine remain to be clarified, but may be related to the potent inhibition of the norepinephrine transporter (NET) by norquetiapine, the major metabolite of quetiapine in humans. Since norquetiapine is not formed in rodents we here investigated in rats whether NET-inhibition may, in principle, contribute to the clinical effectiveness of quetiapine and allow for its low D2 receptor occupancy, by combining quetiapine with the selective NET-inhibitor reboxetine. Antipsychotic-like activity was assessed using the conditioned avoidance response (CAR) test, dopamine output in the medial prefrontal cortex (mPFC) and the nucleus accumbens was measured using in vivo microdialysis, and NMDA receptor-mediated transmission was measured using intracellular electrophysiological recordings in pyramidal cells of the mPFC in vitro. Adjunct reboxetine potentiated the suppression of CAR by quetiapine. Moreover, concomitant administration of quetiapine and reboxetine resulted in a synergistic increase in cortical, but not accumbal, dopamine output. The combination of low, clinically relevant concentrations of quetiapine (60 nM) and reboxetine (20 nM) markedly facilitated cortical NMDA receptor-mediated transmission in contrast to either drug alone, an effect that could be inhibited by the D₁ receptor antagonist SCH23390. We conclude that concomitant NET-inhibition by norquetiapine may contribute to the overall antipsychotic effectiveness of quetiapine in spite of its relatively low level of D₂ occupancy.

COMT Val158Met genotype selectively alters prefrontal [18F]fallypride displacement and subjective feelings of stress in response to a psychosocial stress challenge

Catechol-O-methyltransferase (COMT) plays an essential role in degradation of extracellular dopamine in prefrontal regions of the brain. Although a polymorphism in this gene, COMT Val¹⁵⁸Met, affects human behavior in response to stress little is known about its effect on dopaminergic activity associated with the human stress response, which may be of interest for stress-related psychiatric disorders such as psychosis. We aimed to investigate the effect of variations in COMT genotype on in vivo measures of stress-induced prefrontal cortex (PFC) dopaminergic processing and subjective stress responses. A combined sample of healthy controls and healthy first-degree relatives of psychosis patients (n = 26) were subjected to an [¹⁸F]fallypride Positron Emission Tomography scan. Psychosocial stress during the scan was induced using the Montreal Imaging Stress Task and subjective stress was assessed every 12 minutes. Parametric t-maps, generated using the linear extension of the simplified reference region model, revealed an effect of COMT genotype on the spatial extent of [¹⁸F]fallypride displacement. Detected effects of exposure to psychosocial stress were unilateral and remained restricted to the left superior and right inferior frontal gyrus, with Met-hetero- and homozygotes showing less [¹⁸F]fallypride displacement than Val-homozygotes. Additionally, Met-hetero- and homozygotes experienced larger subjective stress responses than Val-homozygotes. The direction of the effects remained the same when the data was analyzed separately for controls and first-degree relatives. The human stress response may be mediated in part by COMT-dependent dopaminergic PFC activity, providing speculation for the neurobiology underlying COMT-dependent differences in human behaviour following stress. Implications of these results for stress-related psychopathology and models of dopaminergic functioning are discussed.

Schizophrenia as a disorder of too little dopamine: implications for symptoms and treatment

Antipsychotics represent the first effective therapy for schizophrenia, with their benefits linked to dopamine D2 blockade. Schizophrenia was soon identified as a hyperdopaminergic disorder, and antipsychotics proved to be reasonably effective in controlling positive symptoms. However, over the years, schizophrenia has been reconceptualized more broadly, now defined as a heterogeneous disorder with multiple symptom domains. Negative and cognitive features, not particularly responsive to antipsychotic therapy, have taken on increased importance--current thinking suggests that these domains predate the onset of positive symptoms and are more closely tied to functional outcome. That they are better understood in the context of decreased dopamine activity suggests that schizophrenia may fundamentally represent a hypodopaminergic disorder. This shift in thinking has important theoretical implications from the standpoint of etiology and pathophysiology, but also clinically in terms of treatment and drug development.

Psychosocial stress is associated with in vivo dopamine release in human ventromedial prefrontal cortex: a positron emission tomography study using [¹⁸F]fallypride

Rodent studies suggest that prefrontal dopamine neurotransmission plays an important role in the neural processing of psychosocial stress. Human studies investigating stress-induced changes in dopamine levels, however, have focused solely on striatal dopamine transmission. The aim of this study was to investigate in vivo dopamine release in the human prefrontal cortex in response to a psychosocial stress challenge, using the highly selective dopamine D₂/₃ PET radioligand [¹⁸F]fallypride in healthy subjects. Twelve healthy subjects (age (y): 39.8; SD=15.8) underwent a single dynamic Positron Emission Tomography (PET) scanning session after intravenous administration of 185.2 (SD=10.2) MBq [¹⁸F]fallypride. Psychosocial stress was initiated at 100 min postinjection. PET data were analyzed using the linearized simplified reference region model (LSRRM), which accounts for time-dependent changes in [¹⁸F]fallypride displacement. Voxel-based statistical maps, representing specific D₂/₃ binding changes, were computed to localize areas with increased ligand displacement after task initiation, reflecting dopamine release. The psychosocial stress challenge induced detectable amounts of dopamine release throughout the prefrontal cortex, with dopaminergic activity in bilateral ventromedial prefrontal cortex being associated with subjectively rated experiences of psychosocial stress. The novel finding that a mild psychosocial stress in humans induces increased levels of endogenous dopamine in the PFC indicates that the dynamics of the dopamine-related stress response cannot be interpreted by focusing on mesolimbic brain regions alone.

The mesoaccumbens dopamine in coping with stress

Mesoaccumbens dopamine (DA) is involved in the stress response. Although neural mechanisms involved in stress are of paramount importance for both clinical and preclinical research, the results of studies on the stress response by mesoaccumbens DA have received little attention. Therefore, we aimed to review these results and propose a role for mesoaccumbens DA in coping with stress. The data reviewed support the view that fluctuations of tonic levels characterize the mesoaccumbens DA stress response. Stress-induced increase of tonic DA levels in nucleus accumbens (NAc) supports expression of responses aimed at removing and avoiding the stressor through activation of DA D2 receptors, whereas inhibition of DA is associated with cessation of active defensive responses. In novel unescapable/uncontrollable stressful conditions tonic levels of DA in NAc show an initial increase followed by a decrease below pre-stress levels that lasts as long as the stressful situation. This biphasic response fits with the dynamics of the primary and secondary appraisal of a stressor that cannot be removed, escaped or controlled by the organism. In fact, NAc DA fluctuations are controlled by the medial pre-frontal cortex, which is involved in stress appraisal. We propose that enhanced mesoaccumbens DA supports expression of active coping strategies against an event appraised as a stressor and that inhibition of DA is required for passive coping with stressful situations appraised as unescapable/uncontrollable.

Increased phosphorylation of extracellular signal-regulated kinase in the medial prefrontal cortex of the single-prolonged stress rats

The purpose of this study is to investigate the change of phosphorylated p44/42 extracellular signal-regulated kinase (pERK1/2) and c-fos expression induced by single-prolonged stress (SPS) in medial prefrontal cortex (mPFC), and to determine whether extracellular signal-regulated kinase (ERK) pathway plays a role in SPS. Before exposure to SPS, Wistar rats were bilaterally infused with PD98059, inhibitor of ERK, into the mPFC. Animals were then tested in the open field (OF), elevated plus-maze (EPM), and Morris water maze (MWM). Brains were removed for immunohistochemical staining and Western blotting of pERK1/2. And reverse transcription-polymerase chain reaction (RT-PCR) was employed to detect the c-fos mRNA, a member of immediate early genes (IGEs) family. SPS exposure resulted in pronounced anxiety-like behavior compared to control animals. SPS-exposed animals also displayed marked learning and spatial memory impairments. PD98059 significantly ameliorated anxiety-like behavior, learning, and spatial memory impairments. Expression of pERK1/2 in mPFC was significantly increased after rats were exposed to SPS, and the increase induced by SPS was significantly abolished by PD98059. The results of RT-PCR showed that the expression of c-fos mRNA had a significant increase in SPS rats compared with control rats, and the increase was significantly abolished by PD98059. The results suggest that pERK1/2 may be related to signal transduction pathway in single-prolonged stress.