The Functional DRD3 Ser9Gly Polymorphism (rs6280) Is Pleiotropic, Affecting Reward as Well as Movement

Abnormalities of motivation and behavior in the context of reward are a fundamental component of addiction and mood disorders. Here we test the effect of a functional missense mutation in the dopamine 3 receptor (DRD3) gene (ser9gly, rs6280) on reward-associated dopamine (DA) release in the striatum. Twenty-six healthy controls (HCs) and 10 unmedicated subjects with major depressive disorder (MDD) completed two positron emission tomography (PET) scans with [11C]raclopride using the bolus plus constant infusion method. On one occasion subjects completed a sensorimotor task (control condition) and on another occasion subjects completed a gambling task (reward condition). A linear regression analysis controlling for age, sex, diagnosis, and self-reported anhedonia indicated that during receipt of unpredictable monetary reward the glycine allele was associated with a greater reduction in D2/3 receptor binding (i.e., increased reward-related DA release) in the middle (anterior) caudate (p<0.01) and the ventral striatum (p<0.05). The possible functional effect of the ser9gly polymorphism on DA release is consistent with previous work demonstrating that the glycine allele yields D3 autoreceptors that have a higher affinity for DA and display more robust intracellular signaling. Preclinical evidence indicates that chronic stress and aversive stimulation induce activation of the DA system, raising the possibility that the glycine allele, by virtue of its facilitatory effect on striatal DA release, increases susceptibility to hyperdopaminergic responses that have previously been associated with stress, addiction, and psychosis

Assessment of reward responsiveness in the response bias probabilistic reward task in rats: implications for cross-species translational research

Mood disorders, such as major depressive disorder, are characterized by abnormal reward responsiveness. The Response Bias Probabilistic Reward Task (hereafter referred to as probabilistic reward task (PRT)) quantifies reward responsiveness in human subjects, and an equivalent animal assessment is needed to facilitate preclinical translational research. Thus, the goals of the present studies were to develop, validate and characterize a rat analog of the PRT. Adult male Wistar and Long–Evans rats were trained in operant testing chambers to discriminate between two tone stimuli that varied in duration (0.5 and 2 s). During a subsequent test session consisting of 100 trials, the two tones were made ambiguous (0.9 and 1.6 s) and correct identification of one tone was reinforced with a food pellet three times more frequently than the other tone. In subsequent experiments, Wistar rats were administered either a low dose of the dopamine D2/D3 receptor agonist pramipexole (0.1 mg kg−1, subcutaneous) or the psychostimulant amphetamine (0.5 mg kg−1, intraperitoneal) before the test session. Similar to human subjects, both rat strains developed a response bias toward the more frequently reinforced stimulus, reflecting robust reward responsiveness. Mirroring prior findings in humans, a low dose of pramipexole blunted response bias. Moreover, in rats, amphetamine potentiated response bias. These results indicate that in rats, reward responsiveness can be quantified and bidirectionally modulated by pharmacological manipulations that alter striatal dopamine transmission. Thus, this new procedure in rats, which is conceptually and procedurally analogous to the one used in humans, provides a reverse translational platform to investigate abnormal reward responsiveness across species

The impact of psychostimulant administration during development on adult brain functions controlling motivation, impulsivity and cognition.

ADHD pharmacotherapy uses methylphenidate (MPH), D-amphetamine (D- amph), two psychostimulants targeting dopamine transporters, or atomoxetine (ATX), specifically targeting norepinephrine transporters. We have assessed the pharmacological mechanisms of these three drugs on the in vitro efflux of neurotransmitters in rat prefrontal cortex (PFC) and striatal slices as well as on the in vivo electrical activities of PFC pyramidal neurons, striatal medium spiny neurons, ventral tegmental area dopamine neurons or dorsal raphe nucleus serotonin neurons, using single cell extracellular electrophysiological recording techniques. We have also tested whether chronic methylphenidate treatment, during either adolescence or adulthood, could have long-lasting consequences on body growth, depression and neuronal functions. Release experiments showed that all ADHD drugs induce dose-dependent dopamine efflux in both the PFC and striatum, with different efficacies, while only D- amph induced cortical norepinephrine efflux. Atomoxetine induced an unexpected massive dopamine outflow in striatal regions, by mechanisms that depend on physiological parameters. Our electrophysiological studies indicate that all three drugs equally stimulate the excitability of PFC pyramidal neurons, in basal and NMDA-evoked conditions, when administered acutely (3 mg/kg). While the electrophysiological effects elicited by psychostimulants may be dependent on D1 receptor activation, those induced by atomoxetine relied on different mechanisms. In the ventral tegmental area (VTA), methylphenidate (2 mg/kg), but not atomoxetine, induced firing and burst activity reductions, through dopamine D2 autoreceptor activation. Reversal of such effects (eticlopride 0.2 mg/kg) revealed an excitatory effect of methylphenidate on midbrain dopamine neurons that appear to be dependent on glutamate pathways and the combination of D1 and alpha-1 receptors. Finally, acute intraperitoneal psychostimulant injections increased vertical locomotor activity as well as NMDA2B protein expression in the striatum. Some animals chronically treated with intraperitoneal administrations (methylphenidate 4 mg/kg/day or saline 1.2 ml/kg/day) showed decreased body weight gain. Voluntary oral methylphenidate intake induces desensitisation to subsequent intravenous methylphenidate challenges, without altering dopamine D2 receptor plasticity. Significant decreases in striatal NMDA2B protein expression were observed in animals chronically treated. After adolescent MPH treatment, midbrain dopaminergic neurons do not display either desensitisation or sensitisation to intravenous methylphenidate re-challenges. However, partial dopamine D2 receptor desensitisation was observed in midbrain dopamine neurons. Using behavioural experiments, cross-sensitisation between adolescent methylphenidate exposure and later-life D-amphetamine challenge was observed. Significant decreases in striatal NMDA2B protein expression were observed in animals chronically treated, while striatal medium spiny neurons showed decreased sensitivities to locally applied NMDA and dopamine. While caffeine is devoid of action on baseline spike generation and burst activity of dopamine neurons, nicotine induces either firing rate enhancement, firing rate reduction, or has no consequences. Adolescent methylphenidate treatment leads to decreased neuronal sensitivities to the combination of nicotine, MPH and eticlopride, compared to controls. Finally, nicotine partially prevented D-amphetamine-induced increase of rearing activities. Our results show that increases in the excitability of PFC neurons in basal conditions and via NMDA receptor activation may be involved in the therapeutic response to ADHD drugs. Long-term consequences were observed after psychostimulant exposure. Such novel findings strengthen the mixed hypothesis in ADHD, whereby both dopamine and glutamate neurotransmissions are dysregulated. Therefore, ADHD therapy may now focus on adequate balancing between glutamate and dopamine

The absence of P2X7 receptors (P2rx7) on non-haematopoietic cells leads to selective alteration in mood-related behaviour with dysregulated gene expression and stress reactivity in mice

The purpose of this study was to explore how genetic deletion and pharmacological antagonism of the P2X7 receptor (P2rx7) alter mood-related behaviour, gene expression and stress reactivity in the brain. The forced swim test (FST), tail suspension test (TST) and amphetamine-induced hyperlocomotion (AH) tests were used in wild-type (P2rx7+/+) and P2rx7-deficient (P2rx7-/-) mice. Biogenic amine levels were analysed in the amygdala and striatum, adrenocorticotropic hormone (ACTH) and corticosterone levels were measured in the plasma and pituitary after restraint stress. Chimeric mice were generated by bone marrow transplantation. A whole genome microarray analysis with real-time polymerase chain reaction validation was performed on the amygdala. In the absence of P2rx7s decreased behavioural despair in the FST, reduced immobility in the TST and attenuated amphetamine-induced hyperactivity were detected. Basal norepinephrine levels were elevated in the amygdala, whereas stress-induced ACTH and corticosterone responses were alleviated in P2rx7-/- mice. Sub-acute treatment with the selective P2rx7 antagonist, Brilliant Blue G, reproduced the effect of genetic deletion in the TST and AH test in P2rx7+/+ but not P2rx7-/- mice. No change in behavioural phenotype was observed in chimeras lacking the P2rx7 in their haematopoietic compartment. Whole genome microarray analysis indicated a widespread up- and down-regulation of genes crucial for synaptic function and neuroplasticity by genetic deletion. Here, we present evidence that the absence of P2rx7s on non-haematopoietic cells leads to a mood-stabilizing phenotype in several behavioural models and suggest a therapeutic potential of P2rx7 antagonists for the treatment of mood disorders

Molecular imaging of depressive disorders

This chapter summarizes findings of a large number of molecular imaging studies in the field of unipolar and bipolar depression (BD). Brain metabolism in depressed unipolar and bipolar patients is generally hypoactive in the middle frontal gyri, the pregenual and posterior anterior cingulate, the superior temporal gyrus, insula, and the cerebellum, while hyperactivity exists in subcortical (caudate nucleus, thalamus), limbic (amygdala, anterior hippocampus), and medial and inferior frontal regions. Interestingly, after depletion of serotonin or noradrenalin/dopamine in vulnerable (recovered) major depressive disorder (MDD) patients, a similar response pattern in metabolism occurs. Findings on the pre- and postsynaptic dopaminergic system show indications that, at least in subgroups of retarded MDD patients, presynaptic dopaminergic markers may be decreased, while postsynaptic markers may be increased. The findings regarding serotonin synthesis, pre- and postsynaptic imaging can be integrated to a presumable loss of serotonin in MDD, while this remains unclear in BD. This reduction of serotonin and dopamine in MDD was recently summarized in a revised version of the monoamine hypothesis, which focuses more on a dysfunction at the level of the MAO enzyme. This should be addressed further in future studies. Furthermore, future longitudinal molecular imaging studies in the same subjects at different clinical mood states are needed to clarify whether the observed changes in transporters and receptors are compensatory reactions or reflect different, potentially causal mechanisms. Several suggestions for future developments are also provided.</p

Regulation of monoaminergic functions by GPCRs with a special emphasis on mental and movement disorders

Dysfunction of the brain’s monoaminergic system has been implicated in many human neurological and psychiatric disorders, such as Parkinson’s disease (PD), major depressive disorder (MDD) and schizophrenia. The monoamines that are most dysregulated in these diseases, are dopamine and serotonin. Monoamines act as signalling molecules through their receptors, which belong predominately to the G-protein coupled receptor (GPCR) superfamily¹. Most of the clinically employed drugs that are used to tackle these diseases, target directly or indirectly the monoaminergic class of GPCRs. This thesis aims to identify the role of four understudied GPCR-signalling related molecules (GPR88, TAAR1, p11 and NURR1) in animal models of PD, MDD and schizophrenia. The main findings relate to the functions of GPR88, TAAR1, p11 and NURR1 in relationship to PD, MDD and schizophrenia. GPR88 has been suggested as crucial suppressor of striatal medium spiny neuron activity. We showed that loss of GPR88 facilitates L- dihydroxyphenylalanine treatment for PD by aiding its therapeutic efficacy without worsening its side effects. TAAR1 has been described as negative regulator of dopamine neurons firing rate. Herein, we report that TAAR1 deletion enhances the response of non-selective monoamine oxidase inhibitors but no other classes of antidepressants. Furthermore, we provide evidence that the antipsychotic action of the pioneering drug, SEP-856, depends in part on TAAR1 agonism. P11 is a small GPCR-adaptor protein that has been linked to MDD and antidepressant treatment response. In the current thesis, we demonstrate that loss of p11 causes an overt response to stress by triggering the activity of hypothalamic-pituitary-adrenal and sympathetic-adrenomedullary axes. Finally, NURR1 is a GPCR regulated transcription factor, which is linked to PD and schizophrenia as a consequence of its fundamental role in coordinating midbrain dopamine neuron development. In the present work, we describe the role of NURR1 in extra-dopaminergic brain structures such as striatum and claustrum. In detail, we show that induced striatal NURR1 is crucial for locomotor sensitization to L-DOPA. Moreover, we revealed that NURR1 is important factor for claustral neuron transcriptional identity without affecting the occurrence of hallucinogen states’ neural correlates. Overall, we explored new avenues in the fields of neurology and psychiatry related molecular neurobiology. These findings may support future drug discovery research on PD, MDD and schizophrenia though the identification of novel pharmaceutical agents to treat these detrimental disorders. Thus, this body of work contributes to the better understanding of both the pharmacology and pathophysiology of mental and movement disorders

Molecular imaging of depressive disorders

This chapter summarizes findings of a large number of molecular imaging studies in the field of unipolar and bipolar depression (BD). Brain metabolism in depressed unipolar and bipolar patients is generally hypoactive in the middle frontal gyri, the pregenual and posterior anterior cingulate, the superior temporal gyrus, the insula, and the cerebellum, while hyperactivity exists in subcortical (caudate nucleus, thalamus), limbic (amygdala, anterior hippocampus), and medial and inferior frontal regions. Interestingly, after depletion of serotonin or noradrenalin/dopamine in vulnerable (recovered) major depressive disorder (MDD) patients, a similar response pattern in metabolism occurs. Findings on the pre-and postsynaptic dopaminergic system show indications that, at least in subgroups of retarded MDD patients, presynaptic dopaminergic markers may be decreased, while postsynaptic markers may be increased. The findings regarding serotonin synthesis, pre-and postsynaptic imaging can be integrated to a presumable loss of serotonin in MDD, while this remains unclear in BD. This reduction of serotonin and dopamine in MDD was recently summarized in a revised version of the monoamine hypothesis, which focuses more on a dysfunction at the level of the MAO enzyme. This should be addressed further in future studies. Nevertheless, it should be acknowledged that the serotonergic and dopaminergic systems appear adaptive; therefore, it remains difficult to distinguish state and trait abnormalities. Therefore, future longitudinal molecular imaging studies in the same subjects at different clinical mood states (preferably with different tracers and imaging modalities) are needed to clarify whether the observed changes in transporters and receptors are compensatory reactions or reflect different, potentially causal mechanisms. Several suggestions for future developments are also provided at the end of this chapter.</p

Bipolar disorders

Bipolar disorder is characterized by (hypo)manic episodes and depressive episodes which alternate with euthymic periods. It causes serious disability with poor outcome, increased suicidality risk, and significant societal costs. This chapter describes the findings of the PET/SPECT research efforts and the current ideas on the pathophysiology of bipolar disorder. First, the cerebral blood flow and cerebral metabolism findings in the prefrontal cortex, limbic system, subcortical structures, and other brain regions are discussed, followed by an overview of the corticolimbic theory of mood disorders that explains these observations. Second, the neurotransmitter studies are discussed. The serotonin transporter alterations are described, and the variation in study results is explained, followed by an overview of the results of the various dopamine receptor and transporter molecules studies, taking into account also the relation to psychosis. Third, a concise overview is given of dominant bipolar disorder pathophysiological models, proposing starting points for future molecular imaging studies. Finally, the most important conclusions are summarized, followed by remarks about the observed molecular imaging study designs specific for bipolar disorder.</p