Visualization of the cortical dopamine transporter in type 1 and 2 alcoholics with human whole hemisphere autoradiography

Lower midbrain dopamine transporter availability in depressed patients: Report from high-resolution PET imaging

BACKGROUND

A reduced presynaptic dopamine neurotransmission has long been implicated in major depressive disorder (MDD). However, molecular imaging studies that assessed the dopamine transporter (DAT) availability have led to inconsistent results, partly due to methodological considerations, and to exclusive focus on the striatum, precluding findings in extra-striatal regions.

METHODS

Herein, we leveraged our database of high-resolution Positron Emission Tomography (PET) images acquired with a highly selective radiotracer, [¹¹C]PE2I, to assess striatal and extra-striatal DAT availability in eight patients treated for depression compared to twenty-four healthy controls.

RESULTS

Statistical parametric mapping and voxel-based analyses of PET images detected a significant lower DAT availability in depressed patients within the superior part of the midbrain (right, p_FWE = 0.002; left, p_FWE = 0.006), a region including the ventral tegmental area and the substantia nigra from where the mesocorticolimbic and nigrostriatal dopamine pathways originate. A similar difference was found in the right dorsal putamen (p_FWE = 0.012).

LIMITATIONS

The statistical power was limited to detect only large effects, due to the size of the patients' sample.

CONCLUSIONS

The findings support the hypothesis that a reduced presynaptic dopamine function plays a role in the pathophysiology of depression, and that extra-striatal dopamine function should be further investigated.

Cross-Species Alterations in Synaptic Dopamine Regulation After Chronic Alcohol Exposure

Alcohol use disorders are a leading public health concern, engendering enormous costs in terms of both economic loss and human suffering. These disorders are characterized by compulsive and excessive alcohol use, as well as negative affect and alcohol craving during abstinence. Extensive research has implicated the dopamine system in both the acute pharmacological effects of alcohol and the symptomology of alcohol use disorders that develop after extended alcohol use. Preclinical research has shed light on many mechanisms by which chronic alcohol exposure dysregulates the dopamine system. However, many of the findings are inconsistent across experimental parameters such as alcohol exposure length, route of administration, and model organism. We propose that the dopaminergic alterations driving the core symptomology of alcohol use disorders are likely to be relatively stable across experimental settings. Recent work has been aimed at using multiple model organisms (mouse, rat, monkey) across various alcohol exposure procedures to search for commonalities. Here, we review recent advances in our understanding of the effects of chronic alcohol use on the dopamine system by highlighting findings that are consistent across experimental setting and species.

Dopamine Transporter Correlates and Occupancy by Modafinil in Cocaine-Dependent Patients: A Controlled Study With High-Resolution PET and [(11)C]-PE2I

Modafinil is a candidate compound for the treatment of cocaine addiction that binds to the dopamine transporter (DAT) in healthy humans, as observed by positron emission tomography (PET). This mechanism, analogous to that of cocaine, might mediate a putative therapeutic effect of modafinil on cocaine dependence, though the binding of modafinil to DAT has never been assessed in cocaine-dependent patients. We aimed at quantifying the DAT availability during a controlled treatment by modafinil, and its clinical and psychometric correlates in cocaine-dependent patients at the onset of abstinence initiation. Twenty-nine cocaine-dependent male patients were enrolled in a 3-month trial for cocaine abstinence. Modafinil was used in a randomized double-blind placebo-controlled design and was administered as follows: 400 mg/day for 26 days, then 300 mg/day for 30 days, and 200 mg/day for 31 days. Participants were examined twice during a 17-day hospitalization for their DAT availability using PET and [(11)C]-PE2I and for assessments of craving, depressive symptoms, working memory, and decision-making. Cocaine abstinence was further assessed during a 10-week outpatient follow-up period. Baseline [(11)C]-PE2I-binding potential covaried with risk taking and craving index in striatal and extrastriatal regions. A 65.6% decrease of binding potential was detected in patients receiving modafinil for 2 weeks, whereas placebo induced no significant change. During hospitalization, an equivalent improvement in clinical outcomes was observed in both treatment groups, and during the outpatient follow-up there were more therapeutic failures in the modafinil-treated group. Therefore, these results do not support the usefulness of modafinil to treat cocaine addiction.

AMPA receptors in post-mortem brains of Cloninger type 1 and 2 alcoholics: a whole-hemisphere autoradiography study

Dysfunction of the brain glutamate system has been associated with alcoholism. Ionotropic glutamatergic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPARs) play an important role in both neurotransmission and post-synaptic plasticity. Alterations in AMPAR densities may also play a role in the neurobiological changes associated with alcoholism. In the present study, [(3)H] AMPA binding density was evaluated in the nucleus accumbens (NAc), frontal cortex, anterior cingulate cortex (ACC), dentate gyrus and hippocampus of Cloninger type 1 (n=9) and 2 (n=8) alcoholics, and compared with non-alcoholic control subjects (n=10) by post-mortem whole-hemisphere autoradiography. The [(3)H] AMPA binding density was significantly higher in the ACC of early onset type 2 alcoholics when compared with controls (p=0.011). There was also a significant negative correlation between [(3)H] AMPA binding and previously published results of dopamine transporter (DAT) density in the ACC in these same brain samples (R=-0.95, p=0.001). Although preliminary, and from a relatively small diagnostic group, the present results help to further explain the pathology of alcohol dependence and impulsive behaviour in type 2 alcoholics.

Psychopathological aspects of dopaminergic gene polymorphisms in adolescence and young adulthood

Dopamine hypotheses of several psychiatric disorders are based upon the clinical benefits of drugs affecting dopamine transporter or receptors, and have prompted intensive candidate gene research within the dopaminergic system during the last two decades. The aim of this review is to survey the most important findings concerning dopaminergic gene polymorphisms in attention deficit hyperactivity disorder (ADHD), Tourette syndrome (TS), obsessive compulsive disorder, and substance abuse. Also, genetic findings of related phenotypes, such as inattention, impulsivity, aggressive behavior, and novelty seeking personality trait are presented, because recent studies have applied quantitative trait measures using questionnaires, symptom scales, or other objective endophenotypes. Unfortunately, genetic variants with minor effects are problematic to detect in these complex inheritance disorders, often leading to contradictory results. The most consistent association findings relate to ADHD and the dopamine transporter and the dopamine D4 receptor genes. Meta-analyses also support the association between substance abuse and the D2 receptor gene. The dopamine catabolizing enzyme genes, such as monoamine oxidase (MAO) A and catechol-O-methyltransferase (COMT) genes, have been linked to aggressive behaviors.

Serotonin-transporter-linked promoter region polymorphism and serotonin transporter binding in drug-naïve patients with major depression

AIMS

Both the serotonin transporter and its genetic regulation by the serotonin-transporter-linked polymorphic region have a role in the pathophysiology of depression. Most of the previous studies have found no influence of serotonin-transporter-linked polymorphic region allelic variation on serotonin transporter binding in healthy controls or patients with major depression. Due to the inconsistency of the previous findings, we compared single photon emission computed tomography imaging with the serotonin-transporter-linked polymorphic region genotype in patients with major depressive disorder.

METHODS

A total of 23 drug-naïve patients with major depressive disorder were genotyped and brain imaged with ([123I])nor-beta-CIT single photon emission computed tomography. The severity of depression was evaluated with the 17-item Hamilton depression rating scale.

RESULTS

Depressed patients homozygous for the short allele had lower ([123I])nor-beta-CIT binding in the medial prefrontal cortex, but not in the midbrain, compared with the other genotypes.

CONCLUSION

The decreased medial prefrontal cortical serotonin transporter binding in the patients homozygous for the short allele may be linked to altered function of the serotonin-transporter-linked polymorphic region gene expressed in these patients, especially in the medial prefrontal cortex.

Dopamine and serotonin transporter availability during acute alcohol withdrawal: effects of comorbid tobacco smoking

Tobacco smoking is highly comorbid with heavy alcohol drinking, yet the interaction of tobacco smoking and alcohol drinking on brain catecholaminergic synaptic markers is unexplored. Here we evaluate the effects of alcohol drinking alone from comorbid alcohol drinking and tobacco smoking on dopamine (DA) and serotonin (5-HT) transporter availability. A total of 14 heavy alcohol drinking smokers (n=6) and nonsmokers (n=8) and 14 age-matched control smokers (n=6) and nonsmokers (n=8) were imaged with [(123)]beta-CIT single photon emission computed tomography. Alcohol drinking smokers and nonsmokers consumed 134.3+/-100.3 and 196.5+/-139.9 drinks, respectively, over the previous month and were imaged during acute withdrawal, eg within 5 days of their last drink. Striatal DA transporter availability was significantly higher (16%, P=0.04) in alcohol drinkers compared to controls. 5-HT transporter availability was also significantly higher in alcohol drinkers vs controls in the brainstem (25%, P=0.001) and the diencephalon (8%, P=0.01). This elevation was restricted to alcohol drinking nonsmokers with higher DA transporter availability in the striatum (26%, P=0.006), and higher 5-HT transporter availability in the diencephalon (26%, P=0.04) and brainstem (42%, P<0.0002). There was a significant positive correlation between days since last drink and 5-HT transporter availability in the diencephalon (r=0.60, P=0.023) and brainstem (r=0.54, P=0.047), in the total group of alcohol drinkers and in the nonsmokers, but not the smokers. During the first week of abstinence, DA and 5-HT transporter availability is higher in alcohol drinking nonsmokers but not in alcohol drinking smokers. Smoking appears to suppress neuroadaptive changes in DA and 5-HT transporters during acute withdrawal from alcohol.

Role of Serotonin and Dopamine System Interactions in the Neurobiology of Impulsive Aggression and its Comorbidity with other Clinical Disorders

Impulsive aggression is characterized by an inability to regulate affect as well as aggressive impulses, and is highly comorbid with other mental disorders including depression, suicidal behavior, and substance abuse. In an effort to elucidate the neurobiological underpinnings of impulsive aggression and to help account for its connections with these other disorders, this paper reviews relevant biochemical, brain imaging, and genetic studies. The review suggests that dysfunctional interactions between serotonin and dopamine systems in the prefrontal cortex may be an important mechanism underlying the link between impulsive aggression and its comorbid disorders. Specifically, serotonin hypofunction may represent a biochemical trait that predisposes individuals to impulsive aggression, with dopamine hyperfunction contributing in an additive fashion to the serotonergic deficit. The current paper proposes a modified diathesis-stress model of impulsive aggression in which the underlying biological diathesis may be deficient serotonergic function in the ventral prefrontal cortex. This underlying disposition can be manifested behaviorally as impulsive aggression towards oneself and others, and as depression under precipitating life stressors. Substance abuse associated with impulsive aggression is understood in the context of dopamine dysregulation resulting from serotonergic deficiency. Also discussed are future research directions in the neurobiology of impulsive aggression and its comorbid disorders.

PE2I: a radiopharmaceutical for in vivo exploration of the dopamine transporter

The membrane dopamine transporter (DAT) has a pivotal role in the regulation of dopamine (DA) neurotransmission involved in a number of physiological functions and brain disorders. Molecular imaging techniques, such as positron emission tomography (PET) and single photon emission computerized tomography (SPECT), are relevant tools to explore the DAT, and we developed the cocaine derivative N-(3-iodopro-2E-enyl)-2beta-carbomethoxy-3beta-(4'-methylphenyl) nortropane (PE2I) that has proved to be a very potent radiopharmaceutical to image the DAT by these techniques. Several methods are available to obtain PE2I labeled with iodine-123 or -125, carbon-11 and tritium. The pharmacological properties of PE2I have demonstrated that it has good affinity for the DAT (4 nM) and is one of the most selective DAT ligands. [(125)I]PE2I characterized postmortem in human brains has revealed very intense and selective binding in the basal ganglia. Ex vivo autoradiography in rats has shown that high level of [(125)I]PE2I accumulates in the striatum and also in the substantia nigra and ventral tegmental area. [(125)I]PE2I accumulation in the rat striatum is rapid, high, and selective, providing a maximum striatum/cerebellum ratio of 10 during the first 30 min post injection. Using SPECT or PET, rapid, high, and selective accumulation of PE2I was found in the caudate nucleus and putamen in monkeys, whereas rapid wash out from the cerebellum was observed. In vivo investigations in healthy humans have demonstrated that PE2I has high striatal uptake, low nonspecific binding, low radiation exposure, and a fairly short scanning time. A number of findings in various animal models of Parkinson's disease in rats and monkeys have demonstrated the high efficacy of PE2I for detection of reduction in the density of DAT, thus showing the potential value of PE2I for early diagnosis and evaluation of treatment of this disease. The excellent properties of PE2I are basis for the development of new DAT tracers for use in future PET explorations using fluor-18.

An orbitofrontostriatopallidal pathway for morality: evidence from postlesion antisocial and obsessive-compulsive disorder

INTRODUCTION

A detailed proposal is made to the effect that nonlesional antisocial personality disorder (APD) is, among other things, a dysfunctional hypomoralism and that nonlesional obsessive-compulsive disorder (OCD) is, among other things, a dysfunctional hypermoralism.

METHOD

To provide an empirical test of this proposal, 25 previously published cases of acquired (post lesion) APD and 39 cases of acquired OCD are reviewed and compared with multivariate inference tests.

RESULTS

The acquired APD patients most often present putamenal or pallidal lesions.

CONCLUSION

The ensemble of neurobiological, endocrine, and behavioural traits in APD and OCD, as well as the distinct lesion sites in the acquired variants, support the notion of an orbitofrontostriatopallidal brain system underlying morality.

Cortical dopamine D(1) receptors in type 1 and type 2 alcoholics measured with human whole hemisphere autoradiography

A large body of evidence indicates the importance of dopamine (DA) activation for ethanol reinforcement, and animal models of alcoholism have implied the involvement of DA D(1) receptors in this context. We studied cortical DA D(1) receptors in nine type 1 alcoholics (late-onset, binge-drinker), eight type 2 alcoholics (early-onset, antisocial) and 10 controls by using [(3)H]SCH23390 as a radioligand in postmortem human whole hemisphere autoradiography. We also evaluated correlations of DA D(1) receptors between the cortical and subcortical areas and between cortical DA transporters and DA D(2) and D(3) receptors by comparing the present results to our earlier studies. On the average, type 2 alcoholics were younger and had more violent causes of death than type 1 alcoholics and controls. There were no statistically significant differences between the groups, suggesting that cortical DA D(1) receptors do not play a major role in alcoholism. However, among type 2 alcoholics, the binding was consistently lower (8.6%-22.3%) than among controls, and the effect sizes showed a large effect in the anterior cingulate (0.90) and frontal (0.87) cortices. Interestingly, among type 2 alcoholics, the correlation of DA D(1) receptors between two ventral midbrain structures (substantia nigra and amygdala) and anterior cingulate cortex was significantly negative, whereas in the type 1 alcoholics and controls, the correlations were significantly positive.

From vice to virtue: insights from sensitization in the nonhuman primate

Repeated, intermittent administration of psychomotor stimulants, or D1 agonists in dopamine-deficient states, induces behavioral sensitization, characterized by an enhanced response to a subsequent acute low dose challenge, which may be manifested in form of altered behavior or cognitive function. Amphetamine sensitization in the nonhuman primate encompasses profound and enduring changes to similar neuronal and neurochemical substrates that occur in rodents. The process of sensitization in the monkey also results in a long-lasting depression in baseline behavioral responding, as well as emergence of hallucinatory-like behaviors reminiscent of human psychosis in response to an acute challenge. Nonhuman primates show a reduction in spine density and dendritic length in prefrontal neurons and a marked reduction in basal dopamine turnover in both prefrontal cortex and striatum. A major hallmark of amphetamine sensitization in both nonhuman primates and rodents is the manifestation of deficits in executive function and working memory which rely upon the integrity of prefrontal cortex and thereby, may yield significant insights into the cognitive dysfunction associated with addiction. Together with evidence from human and rodent studies, it can be concluded that repeated exposure to psychomotor stimulants can lead to a corruption of neuroadaptive systems in the brain by an extraordinary influence on synaptic plasticity, learning, and memory. Actively harnessing this same process by repeated, intermittent D1 agonist administration may be the key to improved working memory and decision making in addiction and other dopamine dysfunctional states, such as schizophrenia.