Dopamine and nicotinic receptor binding and the levels of dopamine and homovanillic acid in human brain related to tobacco use

Cholinergic regulation of decision making under risk of punishment

The ability to choose between options that differ in their risks and rewards depends on brain regions within the mesocorticolimbic circuit and regulation of their activity by neurotransmitter systems. Dopamine neurotransmission in particular plays a critical role in modulating such risk-taking behavior; however, the contribution of other major modulatory neurotransmitters, such as acetylcholine, is not as well-defined, especially for decision making in which the risk associated with more rewarding outcomes involves adverse consequences. Consequently, the goal of the current experiments was to examine how cholinergic signaling influences decision making involving risk of explicit punishment. Male and female rats were trained in a decision-making task in which they chose between a small safe food reward and a larger food reward accompanied by a risk of footshock punishment. After training in this task, the effects of nicotinic and muscarinic agonists and antagonists on risk-taking performance were evaluated. Neither nicotine, a nicotinic receptor agonist, nor mecamylamine, a nicotinic receptor antagonist, affected preference for the risky lever, although mecamylamine did alter latencies to press the risky lever and the percentage of omissions. The muscarinic receptor agonist oxotremorine decreased preference for the large, risky lever; similar effects on behavior were observed with the administration of the muscarinic receptor antagonist scopolamine. Control experiments were therefore conducted in which these same muscarinic receptor ligands were administered prior to testing in a reward discrimination task. These experiments revealed that the effects of oxotremorine and scopolamine on risk taking may be due to altered motivational processes rather than to changes in sensitivity to risk of punishment. Importantly, there were no sex differences in the effects of cholinergic manipulations on preference for the large, risky lever. Collectively, these findings suggest that in both males and females, cholinergic signaling via muscarinic receptors is involved in decision making involving risk of explicit punishment, with a specific role in modulating sensitivity to differences in reward magnitude. Future studies will expand upon this work by exploring whether targeting cholinergic receptors has therapeutic potential for psychiatric conditions in which risk taking is pathologically altered.

The Role of Dopamine D3 Receptors in Tobacco Use Disorder: A Synthesis of the Preclinical and Clinical Literature

Tobacco smoking is a significant cause of preventable morbidity and mortality globally. Current pharmacological approaches to treat tobacco use disorder (TUD) are only partly effective and novel approaches are needed. Dopamine has a well-established role in substance use disorders, including TUD, and there has been a long-standing interest in developing agents that target the dopaminergic system to treat substance use disorders. Dopamine has 5 receptor subtypes (DRD1 to DRD5). Given the localization and safety profile of the dopamine receptor D3 (DRD3), it is of therapeutic potential for TUD. In this chapter, the preclinical and clinical literature investigating the role of DRD3 in processes relevant to TUD will be reviewed, including in nicotine reinforcement, drug reinstatement, conditioned stimuli and cue-reactivity, executive function, and withdrawal. Similarities and differences in findings from the animal and human work will be synthesized and findings will be discussed in relation to the therapeutic potential of targeting DRD3 in TUD.

Cigarette smoking in long-term schizophrenia

Purpose

Cigarette smoking is a great health problem and prevalent among subjects with schizophrenia. Our aim was to investigate the prevalence and associations of cigarette smoking in patients with long-term schizophrenia.

Methods

Seven hundred and sixty schizophrenia patients were interviewed and their cigarette smoking was recorded.

Results

Smoking was more prevalent men than in women patients. In logistic regression analysis, male gender, duration of illness (DUI) from 10 to 19 years, being divorced or separated, lower education and high daily doses of neuroleptics (DDN) associated significantly with regular smoking. Heavy smoking associated, in men, with hospital treatment.

Conclusions

In schizophrenia patients, smoking is associated with long DUI, high DDN and institutional care. Interventions for cessation and/or reduction of cigarette smoking should be a part of the treatment for patients with schizophrenia.

More than Smoke and Patches: The Quest for Pharmacotherapies to Treat Tobacco Use Disorder

Tobacco use is a persistent public health issue. It kills up to half its users and is the cause of nearly 90% of all lung cancers. The main psychoactive component of tobacco is nicotine, primarily responsible for its abuse-related effects. Accordingly, most pharmacotherapies for smoking cessation target nicotinic acetylcholine receptors (nAChRs), nicotine's major site of action in the brain. The goal of the current review is twofold: first, to provide a brief overview of the most commonly used behavioral procedures for evaluating smoking cessation pharmacotherapies and an introduction to pharmacokinetic and pharmacodynamic properties of nicotine important for consideration in the development of new pharmacotherapies; and second, to discuss current and potential future pharmacological interventions aimed at decreasing tobacco use. Attention will focus on the potential for allosteric modulators of nAChRs to offer an improvement over currently approved pharmacotherapies. Additionally, given increasing public concern for the potential health consequences of using electronic nicotine delivery systems, which allow users to inhale aerosolized solutions as an alternative to smoking tobacco, an effort will be made throughout this review to address the implications of this relatively new form of nicotine delivery, specifically as it relates to smoking cessation. SIGNIFICANCE STATEMENT: Despite decades of research that have vastly improved our understanding of nicotine and its effects on the body, only a handful of pharmacotherapies have been successfully developed for use in smoking cessation. Thus, investigation of alternative pharmacological strategies for treating tobacco use disorder remains active; allosteric modulators of nicotinic acetylcholine receptors represent one class of compounds currently under development for this purpose.

Characterizing the Structural Pattern of Heavy Smokers Using Multivoxel Pattern Analysis

Background: Smoking addiction is a major public health issue which causes a series of chronic diseases and mortalities worldwide. We aimed to explore the most discriminative gray matter regions between heavy smokers and healthy controls with a data-driven multivoxel pattern analysis technique, and to explore the methodological differences between multivoxel pattern analysis and voxel-based morphometry. Methods: Traditional voxel-based morphometry has continuously contributed to finding smoking addiction-related regions on structural magnetic resonance imaging. However, voxel-based morphometry has its inherent limitations. In this study, a multivoxel pattern analysis using a searchlight algorithm and support vector machine was applied on structural magnetic resonance imaging to identify the spatial pattern of gray matter volume in heavy smokers. Results: Our proposed method yielded a voxel-wise accuracy of at least 81% for classifying heavy smokers from healthy controls. The identified regions were primarily located at the temporal cortex and prefrontal cortex, occipital cortex, thalamus (bilateral), insula (left), anterior and median cingulate gyri, and precuneus (left). Conclusions: Our results suggested that several regions, which were seldomly reported in voxel-based morphometry analysis, might be latently correlated with smoking addiction. Such findings might provide insights for understanding the mechanism of chronic smoking and the creation of effective cessation treatment. Multivoxel pattern analysis can be efficient in locating brain discriminative regions which were neglected by voxel-based morphometry.

Association of Cigarette Smoking with Sleep Disturbance and Neurotransmitters in Cerebrospinal Fluid

BACKGROUND

Cigarette smoking has shown to be associated with sleep disturbance, especially prolonged sleep onset latency (SOL). Cigarette smoking stimulates the release of dopamine (DA) and serotonin (5-HT), which might promote awakening and inhibit rapid eye movement sleep. Dopamine transporter (DAT) and serotonin transporter play a key role in the reuptake of DA and 5-HT from the synaptic cleft into presynaptic neurons. However, the relationship among cigarette smoking, sleep disturbance and neurotransmitters has never been investigated in human cerebrospinal fluid (CSF).

METHODS

A total of 159 Chinese male subjects (81 active smokers and 78 non-smokers) who would undergo lumbar puncture before the surgery of anterior cruciate ligament reconstruction were recruited and 5mL-CSF samples were collected incidentally. CSF levels of DA, DAT, 5-HT, and serotonin transporter were measured using radioimmunoassay and ELISA. Sociodemographic data and the Pittsburgh Sleep Quality Index (PSQI) scale were collected before surgery.

RESULTS

PSQI global scores, SOL, and CSF DA levels were significantly higher in active smokers compared to non-smokers (2.00 [1.00-4.75] scores vs 4.00 [3.00-6.00] scores, p = 0.001; 10.00 [5.00-15.00] minutes vs 15.00 [10.00-30.00] minutes, p = 0.002; 87.20 [82.31-96.06]ng/mL vs 107.45 [92.78-114.38] ng/mL, p < 0.001), while CSF DAT levels were significantly lower in active smokers (0.35 [0.31-0.39] ng/mL vs 0.29 [0.26-0.34] ng/mL, p < 0.001).

CONCLUSION

Cigarette smoking was indeed associated with sleep disturbance, shown by prolonged SOL, higher DA levels and lower DAT levels in CSF of active smokers.

Cerebellar Gray Matter Reductions Associate With Decreased Functional Connectivity in Nicotine-Dependent Individuals

Introduction

Nicotine dependence (ND) is a chronic, relapsing mental disorder characterized by compulsive cigarette seeking and smoking. Although the cerebellum plays an increasingly implicated role in ND, the exact cerebellar alterations in ND remain unclear. Identifying the localization of these cerebellar abnormalities in ND may help to further understand the role of the cerebellum in ND. Thus, we investigated the structural and functional alterations in the cerebellum in a large sample of smokers using the spatially unbiased infratentorial template (SUIT).

Methods

High-resolution structural magnetic resonance imaging (MRI) data were acquired from 85 smokers and 41 nonsmokers. We applied voxel-based morphometry (VBM) and the SUIT cerebellar atlas to compare the cerebellar gray matter (GM) volume between smokers and nonsmokers. Using resting-state functional MRI data, we also performed seed-based functional connectivity (FC) analysis to examine the functional correlates of the GM volume changes.

Results

Both VBM and lobular analyses revealed smaller GM volume in the bilateral Crus I in smokers. The GM volume of the left Crus I was inversely correlated with the severity of nicotine dependence as assessed by Fagerström Test for Nicotine Dependence (r = -.268, p = .013). We also found reduced FC between the bilateral Crus I and brain regions involved in the default mode network and motor system, as well as the frontal and temporal cortex in smokers.

Conclusions

Our results indicate that decreased cerebellar GM volume and corticocerebellar FC are associated with ND, and these may underlie the core ND phenotypes, including automatized smoking behavior, cognitive, and emotional deficits.

Implications

As smoking remains a worldwide public health problem, identifying the related neural alterations may help to understand the pathophysiology of ND. Based on previous findings in the cerebellum, we investigated the localization of the GM differences and related FC changes in ND subjects. Our findings highlight altered corticocerebellar circuits in ND, suggesting an association between the cerebellum and the phenotypes of ND.

Tobacco smoking and dopaminergic function in humans: a meta-analysis of molecular imaging studies

RATIONALE

About 1.1 billion people smoke tobacco globally and tobacco-related health care costs 1.8% of GDP in many countries. The majority of people are unable to quit smoking despite pharmacological intervention, highlighting the need to understand the pathophysiology associated with tobacco smoking to aid the development of new therapeutics. The reinforcing effects of tobacco smoking are thought to be mediated by the dopamine system. However, the nature of dopamine dysfunction seen in smokers is unclear.

OBJECTIVE

To determine the nature and robustness of the evidence for dopaminergic alterations in smokers.

METHODS

The entire MEDLINE, EMBASE, and PsycINFO databases were searched for studies from inception date to November 18, 2018. In vivo human molecular imaging studies of dopamine measures (dopamine synthesis or release capacity, transporter levels, receptor levels) in tobacco smokers were selected. Demographic, clinical, and imaging measures were extracted from each study and meta-analyses, and sensitivity analyses were conducted.

RESULTS

Fourteen studies met inclusion criteria comprising a total sample of 219 tobacco smokers and 297 controls. The meta-analysis showed a significant reduction in dopamine transporter availability in the smokers relative to controls with an effect size of - 0.72 ([95% CI, - 1.38 to - 0.05], p = 0.03). However, there was no difference in D2/3 receptor availability in smokers relative to controls (d = -0.16 ([95% CI, - 0.42 to 0.1], p = 0.23). There were insufficient studies for meta-analysis of other measures. However, findings from the published studies indicated blunted dopamine release and lower D1 receptor availability, while findings for dopamine synthesis capacity were inconsistent.

CONCLUSION

Our data indicate that striatal dopamine transporter availability is lower but D2/3 receptors are unaltered in smokers relative to controls. We discuss the putative mechanisms underlying this and their implications.

CHRNA4 and ANKK1 Polymorphisms Influence Smoking-Induced Nicotinic Acetylcholine Receptor Upregulation

INTRODUCTION

Tobacco smoking leads to increased numbers of β2*-containing nicotinic acetylcholine receptors (β2*-nAChRs) throughout the brain, which return to nonsmoker levels over extended abstinence. The goal of the current study was to determine whether the degree of tobacco smoking-induced changes in β2*-nAChR availability is genetically influenced.

METHODS

In this study, 113 European Americans participated in one or two [(123)I]5-IA-85380 single photon emission computed tomography (SPECT) brain scans. Smokers (n = 58) participated in one scan at 7-9 days of abstinence and those who remained abstinent (n = 27) were imaged again at 6-8 weeks of abstinence. Age- and sex-matched nonsmokers (n = 55) participated in one scan. Blood samples were collected for DNA analysis and genotyped for single nucleotide polymorphisms (SNPs) in the CHRNA4 and ANKK1 gene loci. β2*-nAChR availability was measured in the thalamus, striatum, cortical regions, and cerebellum.

RESULTS

The CHRNA4 SNP rs2236196 and ANKK1 SNP rs4938015 were associated with significantly higher cerebellar and cortical β2*-nAChR availability in smokers versus nonsmokers for specific genotypes. There were no significant differences by carrier status in the change in β2*-nAChR availability in smokers from 7-9 days to 6-8 weeks of abstinence.

CONCLUSION

This study provides evidence for genetic regulation of tobacco smoking-induced changes in β2*-nAChR availability and suggests that β2*-nAChR availability could be an endophenotype mediating influences of CHRNA4 variants on nicotine dependence. These results highlight individual differences in the neurochemistry of nicotine dependence and may suggest the need for individualized programs for smoking cessation.

IMPLICATIONS

This study demonstrates genetic regulation of smoking-induced changes in β2*-nAChRs throughout the brain and highlights the need for personalized programs for smoking cessation.

Sex differences in conditioned nicotine reward are age-specific

Women constitute half of all smokers and many studies suggest that adult males and females differ in factors that maintain tobacco smoking, yet there is limited information about sex differences in nicotine reward during adolescence. Limited studies suggest that adolescent male rats self-administer more nicotine than adults, suggesting that drug administration during adolescence leads to different behavioral effects than during adulthood. In the present study, male rats developed a significant conditioned place preference (CPP) to lower doses of nicotine than females, regardless of age. In addition, adolescents were more sensitive than adults. In female rats, adolescents exhibited a CPP of greater magnitude than adult females. In males, the magnitude of the CPP did not differ as a function of age, but adolescents exhibited CPP to lower doses than adults. There also were differences in nicotinic acetylcholinergic receptor binding in nucleus accumbens and caudate putamen in response to nicotine across age and sex. These findings suggest that it is necessary to consider sex- and age-specific effects of drugs such as nicotine when developing strategies for improving smoking cessation treatments.

The effects of chronic cigarette smoking on gray matter volume: influence of sex

Cigarette smoke contains nicotine and toxic chemicals and may cause significant neurochemical and anatomical brain changes. Voxel-based morphometry studies have examined the effects of smoking on the brain by comparing gray matter volume (GMV) in nicotine dependent individuals (NDs) to nonsmoking individuals with inconsistent results. Although sex differences in neural and behavioral features of nicotine dependence are reported, sex differences in regional GMV remain unknown. The current study examined sex differences in GMV in a large sample of 80 NDs (41 males) and 80 healthy controls (41 males) using voxel-based morphometry. Within NDs, we explored whether GMV was correlated with measures of cigarette use and nicotine dependence. High-resolution T1 structural scans were obtained from all participants. Segmentation and registration were performed in SPM8 using the optimized DARTEL approach. Covariates included age and an estimate of total global GMV. Differences were considered significant at p≤0.001, with a whole brain FWE-corrected cluster probability of p<0.025. Among NDs compared to Controls less GMV was observed in the thalamus and bilateral cerebellum and greater GMV was observed in the bilateral putamen and right parahippocampus. Lower thalamic GMV was observed in both female and male NDs compared to Controls. Female NDs also had lower GMV in the left cerebellum and in the ventral medial and orbitofrontal cortices with no areas of greater GMV. Male NDs had lower GMV in bilateral cerebellum and greater GMV in bilateral parahippocampus and left putamen. Within male NDs, GMV in the left putamen was correlated with number of pack years. This study, conducted in a large cohort, contributes to our knowledge of brain morphology in nicotine addiction and provides additional evidence of sex-specific effects on GMV in NDs. Identifying brain vulnerabilities with respect to sex provides a methodological framework for personalized therapies to improve relapse rates for both sexes.

Effects of smoking on D₂/D₃ striatal receptor availability in alcoholics and social drinkers

Studies have reported lower striatal D₂/D₃ receptor availability in both alcoholics and cigarette smokers relative to healthy controls. These substances are commonly co-abused, yet the relationship between comorbid alcohol/tobacco abuse and striatal D₂/D₃ receptor availability has not been examined. We sought to determine the degree to which dual abuse of alcohol and tobacco is associated with lower D₂/D₃ receptor availability. Eighty-one subjects (34 nontreatment-seeking alcoholic smokers [NTS-S], 21 social-drinking smokers [SD-S], and 26 social-drinking non-smokers [SD-NS]) received baseline [(11)C]raclopride scans. D₂/D₃ binding potential (BPND ≡ Bavail/KD) was estimated for ten anatomically defined striatal regions of interest (ROIs). Significant group effects were detected in bilateral pre-commissural dorsal putamen, bilateral pre-commissural dorsal caudate; and bilateral post-commissural dorsal putamen. Post-hoc testing revealed that, regardless of drinking status, smokers had lower D₂/D₃ receptor availability than non-smoking controls. Chronic tobacco smokers have lower striatal D₂/D₃ receptor availability than non-smokers, independent of alcohol use. Additional studies are needed to identify the mechanisms by which chronic tobacco smoking is associated with striatal dopamine receptor availability.

Dopaminergic contributions to hippocampal pathophysiology in schizophrenia: a computational study

Since the original formulation of the dopamine hypothesis, a number of other cellular-level abnormalities--eg, NMDA receptor hypofunction, GABA system dysfunction, neural connectivity disturbances--have been identified in schizophrenia, but the manner in which these potentially interact with hyperdopaminergia to lead to schizophrenic symptomatology remains uncertain. Previously, we created a neuroanatomically detailed, biophysically realistic computational model of hippocampus in the control (unaffected) and schizophrenic conditions, implemented on a 72-processor supercomputer platform. In the current study, we apply the effects of dopamine (DA), dose-dependently, to both models on the basis of an exhaustive review of the neurophysiologic literature on DA's ion channel and synaptic level effects. To index schizophrenic behavior, we use the specific inability of the model to attune to the 40 Hz (gamma band) frequency, a finding that has been well replicated in the clinical electroencephalography (EEG) and magnetoencephalography literature. In trials using 20 'simulated patients', we find that DA applied to the control model produces modest increases in 40 Hz activity, similar to experimental studies. However, in the schizophrenic model, increasing DA induces a decrement in 40 Hz resonance. This modeling work is significant in that it suggests that DA's effects may vary based on the neural substrate on which it acts, and--via simulated EEG recordings-points to the neurophysiologic mechanisms by which this may occur. We also feel that it makes a methodological contribution, as it exhibits a process by which a large amount of neurobiological data can be integrated to run pharmacologically relevant in silico experiments, using a systems biology approach.

Prenatal nicotine exposure enhances the trigeminocardiac reflex via serotonin receptor facilitation in brainstem pathways

In this study we used a rat model for prenatal nicotine exposure to test whether clinically relevant concentrations of brain nicotine and cotinine are passed from dams exposed to nicotine to her pups, whether this changes the trigeminocardiac reflex (TCR), and whether serotonergic function in the TCR brainstem circuitry is altered. Pregnant Sprague-Dawley dams were exposed to 6 mg·kg(-1)·day(-1) of nicotine via osmotic minipumps for the duration of pregnancy. Following birth dams and pups were killed, blood was collected, and brain nicotine and cotinine levels were measured. A separate group of prenatal nicotine-exposed pups was used for electrophysiological recordings. A horizontal brainstem slice was obtained by carefully preserving the trigeminal nerve with fluorescent identification of cardiac vagal neurons (CVNs) in the nucleus ambiguus. Stimulation of the trigeminal nerve evoked excitatory postsynaptic current in CVNs. Our data demonstrate that prenatal nicotine exposure significantly exaggerates both the TCR-evoked changes in heart rate in conscious unrestrained pups, and the excitatory neurotransmission to CVNs upon trigeminal afferent nerve stimulation within this brainstem reflex circuit. Application of the 5-HT1A receptor antagonist WAY 100635 (100 μM) and 5-HT2A/C receptor antagonist ketanserin (10 μM)significantly decreased neurotransmission, indicating an increased facilitation of 5-HT function in prenatal nicotine-exposed animals. Prenatal nicotine exposure enhances activation of 5-HT receptors and exaggerates the trigeminocardiac reflex.

RIC-3 differentially modulates α4β2 and α7 nicotinic receptor assembly, expression, and nicotine-induced receptor upregulation

Background

Recent work has shown that the chaperone resistant to inhibitors of acetylcholinesterase (RIC-3) is critical for the folding, maturation and functional expression of a variety of neuronal nicotinic acetylcholine receptors. α7 nicotinic receptors can only assemble and functionally express in select lines of cells, provided that RIC-3 is present. In contrast, α4β2 nicotinic receptors can functionally express in many cell lines even without the presence of RIC-3. Depending on the cell line, RIC-3 has differential effects on α4β2 receptor function – enhancement in mammalian cells but inhibition in Xenopus oocytes. Other differences between the two receptor types include nicotine-induced upregulation. When expressed in cell lines, α4β2 receptors readily and robustly upregulate with chronic nicotine exposure. However, α7 nicotinic receptors appear more resistant and require higher concentrations of nicotine to induce upregulation. Could the coexpression of RIC-3 modulate the extent of nicotine-induced upregulation not only for α7 receptors but also α4β2 receptors? We compared and contrasted the effects of RIC-3 on assembly, trafficking, protein expression and nicotine-induced upregulation on both α7 and α4β2 receptors using fluorescent protein tagged nicotinic receptors and Förster resonance energy transfer (FRET) microscopy imaging.

Results

RIC-3 increases assembly and cell surface trafficking of α7 receptors but does not alter α7 protein expression in transfected HEK293T cells. In contrast, RIC-3 does not affect assembly of α4β2 receptors but increases α4 and β2 subunit protein expression. Acute nicotine (30 min exposure) was sufficient to upregulate FRET between α4 and β2 subunits. Surprisingly, when RIC-3 was coexpressed with α4β2 receptors nicotine-induced upregulation was prevented. α7 receptors did not upregulate with acute nicotine in the presence or absence of RIC-3.

Conclusions

These results provide interesting novel data that RIC-3 differentially regulates assembly and expression of different nicotinic receptor subunits. These results also show that nicotine-mediated upregulation of α4β2 receptors can be dynamically regulated by the presence of the chaperone, RIC-3. This could explain a novel mechanism why high affinity α4β2 receptors are upregulated in specific neuronal subtypes in the brain and not others.

Impact of sustained exposure to β-amyloid on calcium homeostasis and neuronal integrity in model nerve cell system expressing α4β2 nicotinic acetylcholine receptors

Although the interaction between β-amyloid (Aβ) and nicotinic acetylcholine receptors has been widely studied, the impact of prolonged exposure to Aβ on nAChR expression and signaling is not known. In this study, we employed a neuronal culture model to better understand the impact of sustained exposure of Aβ on the regulation of cellular and synaptic function. The differentiated rodent neuroblastoma cell line NG108-15 expressing exogenous high-affinity α4β2 nAChRs was exposed to soluble oligomeric Aβ for several days. Ca(2+) responses, expression levels of α4β2 nAChRs, rate of mitochondrial movement, mitochondrial fission, levels of reactive oxygen species, and nuclear integrity were compared between Aβ-treated and untreated cells, transfected or not (mock-transfected) with α4β2 nAChRs. Sustained exposure of Aβ(1-42) to α4β2 nAChR-transfected cells for several days led to increased Ca(2+) responses on subsequent acute stimulation with Aβ(1-42) or nicotine, paralleled by increased expression levels of α4β2 nAChRs, likely the result of enhanced receptor recycling. The rate of mitochondrial movement was sharply reduced, whereas the mitochondrial fission protein pDrp-1 was increased in α4β2 nAChR-transfected cells treated with Aβ(1-42). In addition, the presence of α4β2 nAChRs dramatically enhanced Aβ(1-42)-mediated increases in reactive oxygen species and nuclear fragmentation, eventually leading to apoptosis. Our data thus show disturbed calcium homeostasis coupled with mitochondrial dysfunction and loss of neuronal integrity on prolonged exposure of Aβ in cells transfected with α4β2 nAChRs. Together, the results suggest that the presence of nAChRs sensitizes neurons to the toxic actions of soluble oligomeric Aβ, perhaps contributing to the cholinergic deficit in Alzheimer disease.

Α4β2 and α7 nicotinic acetylcholine receptor binding predicts choice preference in two cost benefit decision-making tasks

Nicotinic receptors have been linked to a wide range of cognitive and behavioral functions, but surprisingly little is known about their involvement in cost benefit decision making. The goal of these experiments was to determine how nicotinic acetylcholine receptor (nAChR) expression is related to two forms of cost benefit decision making. Male Long Evans rats were tested in probability- and delay-discounting tasks, which required discrete trial choices between a small reward and a large reward associated with varying probabilities of omission and varying delays to reward delivery, respectively. Following testing, radioligand binding to α4β2 and α7 nAChR subtypes in brain regions implicated in cost benefit decision making was examined. Significant linear relationships were observed between choice of the large delayed reward in the delay discounting task and α4β2 receptor binding in both the dorsal and ventral hippocampus. Additionally, trends were found suggesting that choice of the large costly reward in both discounting tasks was inversely related to α4β2 receptor binding in the medial prefrontal cortex and nucleus accumbens shell. Similar trends suggested that choice of the large delayed reward in the delay discounting task was inversely related to α4β2 receptor binding in the orbitofrontal cortex, nucleus accumbens core, and basolateral amygdala, as well as to α7 receptor binding in the basolateral amygdala. These data suggest that nAChRs (particularly α4β2) play both unique and common roles in decisions that require consideration of different types of reward costs.

Striatal and extrastriatal dopamine transporter in cannabis and tobacco addiction: a high-resolution PET study

The dopamine (DA) system is known to be involved in the reward and dependence mechanisms of addiction. However, modifications in dopaminergic neurotransmission associated with long-term tobacco and cannabis use have been poorly documented in vivo. In order to assess striatal and extrastriatal dopamine transporter (DAT) availability in tobacco and cannabis addiction, three groups of male age-matched subjects were compared: 11 healthy non-smoker subjects, 14 tobacco-dependent smokers (17.6 ± 5.3 cigarettes/day for 12.1 ± 8.5 years) and 13 cannabis and tobacco smokers (CTS) (4.8 ± 5.3 cannabis joints/day for 8.7 ± 3.9 years). DAT availability was examined in positron emission tomography (HRRT) with a high resolution research tomograph after injection of [11C]PE2I, a selective DAT radioligand. Region of interest and voxel-by-voxel approaches using a simplified reference tissue model were performed for the between-group comparison of DAT availability. Measurements in the dorsal striatum from both analyses were concordant and showed a mean 20% lower DAT availability in drug users compared with controls. Whole-brain analysis also revealed lower DAT availability in the ventral striatum, the midbrain, the middle cingulate and the thalamus (ranging from -15 to -30%). The DAT availability was slightly lower in all regions in CTS than in subjects who smoke tobacco only, but the difference does not reach a significant level. These results support the existence of a decrease in DAT availability associated with tobacco and cannabis addictions involving all dopaminergic brain circuits. These findings are consistent with the idea of a global decrease in cerebral DA activity in dependent subjects.

Placebo-controlled pilot trial of mecamylamine for treatment of autism spectrum disorders

OBJECTIVE

To explore possible benefits of a nicotinic acetylcholine receptor (nAChR) agent for autistic symptoms based on postmortem observation of nAChR abnormalities (deficient α4β2 nAChRs, excess α7 nAChRs) in brains of patients with autism.

METHOD

Mecamylamine, because of its safety record in children with other disorders, was chosen for this first exploration. Twenty children with autism spectrum disorder age 4-12 years were randomly assigned for 14 weeks to placebo (n=8) or mecamylamine (n=12) in ascending fixed doses: 0.5 mg/day for 6 weeks, 2.5 mg for 2 weeks, then 5 mg/day for 6 weeks. Improvement was rated by a blinded independent evaluator. Because of small sample, data analysis was descriptive.

RESULTS

Eighteen participants (10 mecamylamine, 8 placebo) completed the study. All doses were well tolerated; the only side effect of note was constipation (50% compared with 25% of placebo group). Three children had clinically nonsignificant electrocardiographic QT prolongation. Both groups showed modest to moderate improvement, but differences between groups were negligible. On the primary outcome measure, the Ohio Autism Clinical Impressions Scale, 90% of the active treatment group showed improvement at some point (but only 40% sustained it), compared with 62% on placebo. Of the four in active treatment that sustained improvement, three had a maximum dose of 0.13-0.15 mg/kg/day, while those who regressed had doses ≥0.18 mg/kg/day. Graphed means suggested better outcome with lower mg/kg and longer medication duration. Four parents spontaneously reported reduced hyperactivity and irritability and better verbalization and continued mecamylamine at their own expense.

CONCLUSION

Mecamylamine appeared to be safe, but not very effective in autism. The suggestion of better results at lower doses and longer exposure warrants consideration for future trials. The next step would be exploration of a more specific α4β2 nAChR agonist, such as varenicline.

Pharmacogenetics of smoking cessation: role of nicotine target and metabolism genes

Many smokers attempt to quit smoking but few are successful in the long term. The heritability of nicotine addiction and smoking relapse have been documented, and research is focused on identifying specific genetic influences on the ability to quit smoking and response to specific medications. Research in genetically modified cell lines and mice has identified nicotine acetylcholine receptor subtypes that mediate the pharmacological and behavioral effects of nicotine sensitivity and withdrawal. Human genetic association studies have identified single nucleotide polymorphisms (SNPs) in genes encoding nicotine acetylcholine receptor subunits and nicotine metabolizing enzymes that influence smoking cessation phenotypes. There is initial promising evidence for a role in smoking cessation for SNPs in the β2 and α5/α3/β4 nAChR subunit genes; however, effects are small and not consistently replicated. There are reproducible and clinically significant associations of genotypic and phenotypic measures of CYP2A6 enzyme activity and nicotine metabolic rate with smoking cessation as well as response to nicotine replacement therapies and bupropion. Prospective clinical trials to identify associations of genetic variants and gene-gene interactions on smoking cessation are needed to generate the evidence base for both medication development and targeted therapy approaches based on genotype.

Muscarinic and nicotinic acetylcholine receptor agonists and allosteric modulators for the treatment of schizophrenia

Muscarinic and nicotinic acetylcholine (ACh) receptors (mAChRs and nAChRs) are emerging as important targets for the development of novel treatments for the symptoms associated with schizophrenia. Preclinical and early proof-of-concept clinical studies have provided strong evidence that activators of specific mAChR (M(1) and M(4)) and nAChR (α(7) and α(2)β(4)) subtypes are effective in animal models of antipsychotic-like activity and/or cognitive enhancement, and in the treatment of positive and cognitive symptoms in patients with schizophrenia. While early attempts to develop selective mAChR and nAChR agonists provided important preliminary findings, these compounds have ultimately failed in clinical development due to a lack of true subtype selectivity and subsequent dose-limiting adverse effects. In recent years, there have been major advances in the discovery of highly selective activators for the different mAChR and nAChR subtypes with suitable properties for optimization as potential candidates for clinical trials. One novel strategy has been to identify ligands that activate a specific receptor subtype through actions at sites that are distinct from the highly conserved ACh-binding site, termed allosteric sites. These allosteric activators, both allosteric agonists and positive allosteric modulators, of mAChR and nAChR subtypes demonstrate unique mechanisms of action and high selectivity in vivo, and may provide innovative treatment strategies for schizophrenia.

Historical and current perspective on tobacco use and nicotine addiction

Although the addictive influence of tobacco was recognized very early, the modern concepts of nicotine addiction have relied on knowledge of cholinergic neurotransmission and nicotinic acetylcholine receptors (nAChRs). The discovery of the 'receptive substance' by Langley, that would turn out to be nAChRs, and 'Vagusstoff' (acetylcholine) by Loewi, coincided with an exciting time when the concept of chemical synaptic transmission was being formulated. More recently, the application of more powerful techniques and the study of animal models that replicate key features of nicotine dependence have led to important advancements in our understanding of molecular, cellular and systems mechanisms of nicotine addiction. In this review, we present a historical perspective and overview of the research that has led to our present understanding of nicotine addiction.

Nicotine up-regulates alpha4beta2 nicotinic receptors and ER exit sites via stoichiometry-dependent chaperoning

The up-regulation of α4β2* nicotinic acetylcholine receptors (nAChRs) by chronic nicotine is a cell-delimited process and may be necessary and sufficient for the initial events of nicotine dependence. Clinical literature documents an inverse relationship between a person’s history of tobacco use and his or her susceptibility to Parkinson’s disease; this may also result from up-regulation. This study visualizes and quantifies the subcellular mechanisms involved in nicotine-induced nAChR up-regulation by using transfected fluorescent protein (FP)-tagged α4 nAChR subunits and an FP-tagged Sec24D endoplasmic reticulum (ER) exit site marker. Total internal reflection fluorescence microscopy shows that nicotine (0.1 µM for 48 h) up-regulates α4β2 nAChRs at the plasma membrane (PM), despite increasing the fraction of α4β2 nAChRs that remain in near-PM ER. Pixel-resolved normalized Förster resonance energy transfer microscopy between α4-FP subunits shows that nicotine stabilizes the (α4)₂(β2)₃ stoichiometry before the nAChRs reach the trans-Golgi apparatus. Nicotine also induces the formation of additional ER exit sites (ERES). To aid in the mechanistic analysis of these phenomena, we generated a β2_{enhanced-ER-export} mutant subunit that mimics two regions of the β4 subunit sequence: the presence of an ER export motif and the absence of an ER retention/retrieval motif. The α4β2_{enhanced-ER-export} nAChR resembles nicotine-exposed nAChRs with regard to stoichiometry, intracellular mobility, ERES enhancement, and PM localization. Nicotine produces only small additional PM up-regulation of α4β2_{enhanced-ER-export} receptors. The experimental data are simulated with a model incorporating two mechanisms: (1) nicotine acts as a stabilizing pharmacological chaperone for nascent α4β2 nAChRs in the ER, eventually increasing PM receptors despite a bottleneck(s) in ER export; and (2) removal of the bottleneck (e.g., by expression of the β2_{enhanced-ER-export} subunit) is sufficient to increase PM nAChR numbers, even without nicotine. The data also suggest that pharmacological chaperoning of nAChRs by nicotine can alter the physiology of ER processes.

Chronic exposure to nicotine and saquinavir decreases endothelial Notch-4 expression and disrupts blood-brain barrier integrity

Since the advent of HAART, there have been substantial improvements in HIV patient survival; however, the prevalence of HIV associated dementia has increased. Importantly, HIV positive individuals who smoke progress to HIV associated neurological conditions faster than those who do not. Recent in vitro data have shown that pharmacological levels of saquinavir causes endothelial oxidative stress and significantly decreases Notch-4 expression, a primary protein involved in maintaining stability of blood-brain barrier (BBB) endothelium. This is concerning as nicotine can also generate reactive oxygen species in endothelium. It is largely unknown if pharmacological doses of these drugs can cause a similar in vivo down-regulation of Notch-4 and if there is a concurrent destabilization of the integrity of the BBB. The data herein show: (i) nicotine and protease inhibitors cause an additive oxidative stress burden in endothelium; (ii) that the integrity of the BBB is disrupted after concurrent chronic nicotine and protease inhibitor administration; and (iii) that BBB endothelial dysfunction is correlated with a decrease in Notch-4 and ZO-1 expression. Considering the high prevalence of smoking in the HIV infected population (3- to 4-fold higher than in the general population) this data must be followed up to determine if all protease inhibitors cause a similar BBB disruption or if there is a safer alternative. In addition, this data may suggest that the induced BBB disruption may allow foreign molecules to gain access to brain and be a contributing factor to the slow progression of HIV associated dementia.

Why do patients with schizophrenia smoke?

PURPOSE OF REVIEW

Among the mentally ill, smoking prevalence is highest in patients with schizophrenia ( approximately 70-80%). This can impose a significant financial burden on patients, not to speak of increased smoking-related morbidity and mortality. Therefore, it is critical for clinicians to understand why patients with schizophrenia smoke in order to adapt treatment schemes. Understanding the reasons may also help to develop new drugs that target the nicotinic system in the brain as well as smoking cessation programs that are specifically designed for this particular patient population.

RECENT FINDINGS

So far, several reasons have been identified which are believed to explain tobacco consumption in patients with schizophrenia. Originally, it was widely believed that patients with schizophrenia smoke to increase hepatic clearance and to restore the dopamine blockade of certain antipsychotic drugs to diminish their side effects. However, more recently it became obvious that cigarette smoking may also be reinforcing for patients because it improves psychiatric symptoms, most notably negative and cognitive symptoms. The underlying molecular mechanisms of these nicotine effects are currently under intensive investigation.

SUMMARY

Heavy smoking in schizophrenia cannot simply be viewed as a 'bad habit'. Rather, self-medication of clinical symptoms and side effects of antipsychotic drugs appear to play a major role.

Current perspectives on selective dopamine D(3) receptor antagonists as pharmacotherapeutics for addictions and related disorders

Repeated exposure to drugs of abuse produces long-term molecular and neurochemical changes that may explain the core features of addiction, such as the compulsive seeking and taking of the drug, as well as the risk of relapse. A growing number of new molecular and cellular targets of addictive drugs have been identified, and rapid advances are being made in relating those targets to specific behavioral phenotypes in animal models of addiction. In this context, the pattern of expression of the dopamine (DA) D(3) receptor in the rodent and human brain and changes in this pattern in response to drugs of abuse have contributed primarily to direct research efforts toward the development of selective DA D(3) receptor antagonists. Growing preclinical evidence indicates that these compounds may actually regulate the motivation to self-administer drugs and disrupt drug-associated cue-induced craving. This report will be divided into three parts. First, preclinical evidence in support of the efficacy of selective DA D(3) receptor antagonists in animal models of drug addiction will be reviewed. The effects of mixed DA D(2)/D(3) receptor antagonists will not be discussed here because most of these compounds have low selectivity at the D(3) versus D(2) receptor, and their efficacy profile is related primarily to functional antagonism at D(2) receptors and possibly interactions with other neurotransmitter systems. Second, major advances in medicinal chemistry for the identification and optimization of selective DA D(3) receptor antagonists and partial agonists will be analyzed. Third, translational research from preclinical efficacy studies to so-called proof-of-concept studies for drug addiction indications will be discussed.

The history of the cholinergic hypothesis

The cholinergic hypothesis of cognitive impairment and Alzheimer's disease has been for decades a "polar star" for studies on dementia and neurodegenerative diseases. Aim of the present article is to briefly summarize its birth and its evolution throughout years and discoveries. Putting the cholinergic hypothesis in an historical perspective, allows to appreciate the enormous amount of experimental and clinical research that it has stimulated over years and the impressive extent of knowledge generated by this research. While some of the assumptions at the basis of its original formulation are disputable in the light of recent developments, the cholinergic hypothesis has, however, constituted an invaluable stimulus to better understand not only the anatomy and the biochemistry of the cholinergic systems of brain connections but also its developmental biology, its complex relationships with trophic factors, its role in cognitive functions. Thus, rather than being consigned to history, the cholinergic hypothesis will likely contribute to further understanding dementia and neurodegenerative diseases and will hopefully be integrated in novel therapies and treatments.

Association of tobacco and lead exposures with attention-deficit/hyperactivity disorder

OBJECTIVE

The study objective was to determine the independent and joint associations of prenatal tobacco and childhood lead exposures with attention-deficit/hyperactivity disorder (ADHD), as defined by current diagnostic criteria, in a national sample of US children.

METHODS

Data are from the 2001-2004 National Health and Nutrition Examination Survey, a cross-sectional, nationally representative sample of the US population. Participants were 8 to 15 years of age (N = 2588). Prenatal tobacco exposure was measured by report of maternal cigarette use during pregnancy. Lead exposure was assessed by using current blood lead levels. The Diagnostic Interview Schedule for Children was used to ascertain the presence of ADHD in the past year, on the basis of Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, criteria.

RESULTS

A total of 8.7% (95% confidence interval [CI]: 7.3%-10.1%) of children met criteria for ADHD. Prenatal tobacco exposure (adjusted odds ratio [aOR]: 2.4 [95% CI: 1.5-3.7]) and higher current blood lead concentrations (aOR for third versus first tertile: 2.3 [95% CI: 1.5-3.8]) were independently associated with ADHD. Compared with children with neither exposure, children with both exposures (prenatal tobacco exposure and third-tertile lead levels) had an even greater risk of ADHD (aOR: 8.1 [95% CI: 3.5-18.7]) than would be expected if the independent risks were multiplied (tobacco-lead exposure interaction term, P < .001).

CONCLUSIONS

Prenatal tobacco and childhood lead exposures are associated with ADHD in US children, especially among those with both exposures. Reduction of these common toxicant exposures may be an important avenue for ADHD prevention.

Chronic nicotine selectively enhances alpha4beta2* nicotinic acetylcholine receptors in the nigrostriatal dopamine pathway

These electrophysiological experiments, in slices and intact animals, study the effects of in vivo chronic exposure to nicotine on functional alpha4beta2* nAChRs in the nigrostriatal dopaminergic (DA) pathway. Recordings were made in wild-type and alpha4 nicotinic acetylcholine receptor (nAChR) subunit knock-out mice. Chronic nicotine enhanced methyllycaconitine citrate hydrate-resistant, dihydro-beta-erythroidine hydrobromide-sensitive nicotinic currents elicited by 3-1000 mum ACh in GABAergic neurons of the substantia nigra pars reticulata (SNr), but not in DA neurons of the substantia nigra pars compacta (SNc). This enhancement leads to higher firing rates of SNr GABAergic neurons and consequently to increased GABAergic inhibition of the SNc DA neurons. In the dorsal striatum, functional alpha4* nAChRs were not found on the neuronal somata; however, nicotine acts via alpha4beta2* nAChRs in the DA terminals to modulate glutamate release onto the medium spiny neurons. Chronic nicotine also increased the number and/or function of these alpha4beta2* nAChRs. These data suggest that in nigrostriatal DA pathway, chronic nicotine enhancement of alpha4beta2* nAChRs displays selectivity in cell type and in nAChR subtype as well as in cellular compartment. These selective events augment inhibition of SNc DA neurons by SNr GABAergic neurons and also temper the release of glutamate in the dorsal striatum. The effects may reduce the risk of excitotoxicity in SNc DA neurons and may also counteract the increased effectiveness of corticostriatal glutamatergic inputs during degeneration of the DA system. These processes may contribute to the inverse correlation between tobacco use and Parkinson's disease.

The expression of CYP2D22, an ortholog of human CYP2D6, in mouse striatum and its modulation in 1-methyl 4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinson's disease phenotype and nicotine-mediated neuroprotection

The main contributory factors of Parkinson's disease (PD) are aging, genetic factors, and environmental exposure to pesticides and heavy metals. CYP2D22, a mouse ortholog of human CYP2D6, is expected to participate in a chemically induced PD phenotype due to its structural resemblance with CYP2D6. Despite its expected participation in PD, its expression in the nigrostriatal tissues and modulation by the chemicals that induce PD or offer neuroprotection have not yet been investigated. The present study was undertaken to investigate CYP2D22 expression in mouse striatum and to assess its involvement in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD phenotype and nicotine-mediated neuroprotection. The animals were treated intraperitoneally daily with nicotine (1 mg/kg) for 8 weeks, followed by MPTP (20 mg/kg) + nicotine (1 mg/kg) for 2-4 weeks and vice versa, along with respective controls. In some sets of experiments, the animals were treated with ketoconazole (300 mg/kg), a CYP3AH/CYP2D22 inhibitor, along with nicotine and/or MPTP. Tyrosine hydroxylase (TH)-immunoreactivity in the substantia nigra, the expression of nicotinic acetylcholine receptors (nAChRs) alpha6 and alpha4, dopamine content, and 1-methyl-4-phenylpyridinium ion (MPP(+)) level in the striatum were measured to confirm the MPTP-induced PD phenotype and nicotine-mediated neuroprotection. CYP2D22 and nAChRs expressions were measured in the striatum by RT-PCR/western blotting and dopamine level; CYP2D22 catalytic activity and MPP(+) content were determined by high-performance liquid chromatography (HPLC). MPTP increased dopaminergic neuronal degeneration and the striatal MPP(+) level and reduced striatal dopamine content; it attenuated expression/activity of CYP2D22 and nAChRs that were significantly restored in nicotine-treated animals. Ketoconazole reduced the nicotine-mediated increase in CYP2D22 expression and activity, dopamine content, and TH-immunoreactivity. The results indicate the expression of CYP2D22 in mouse striatum and its possible role in the MPTP-induced PD phenotype and nicotine-mediated neuroprotection.

Single photon emission computed tomography experience with (S)-5-[(123)I]iodo-3-(2-azetidinylmethoxy)pyridine in the living human brain of smokers and nonsmokers

(S)-5-[(123)I]iodo-3-(2-azetidinylmethoxy)pyridine (5-[(123)I]IA), a novel potent radioligand for high-affinity alpha4beta2* neuronal nicotinic acetylcholine receptors (nAChRs), provides a means to evaluate the density and the distribution of nAChRs in the living human brain. We sought in healthy adult smokers and nonsmokers to (1) evaluate the safety, tolerability, and efficacy of 5-[(123)I]IA in an open nonblind trial and (2) to estimate the density and the distribution of alpha(4)beta(2)* nAChRs in the brain. Single photon emission computed tomography (SPECT) was performed for 5 h after the i.v. administration of approximately 0.001 microg/kg ( approximately 10 mCi) 5-[(123)I]IA. Blood pressure, heart rate, and neurobehavioral status were monitored before, during, and after the administration of 5-[(123)I]IA to 12 healthy adults (8 men and 4 women) (6 smokers and 6 nonsmokers) ranging in age from 19 to 46 years (mean = 28.25, standard deviation = 8.20). High plasma-nicotine level was significantly associated with low 5-[(123)I]IA binding in: (1) the caudate head, the cerebellum, the cortex, and the putamen, utilizing both the Sign and Mann-Whitney U-tests; (2) the fusiform gyrus, the hippocampus, the parahippocampus, and the pons utilizing the Mann-Whitney U-test; and (3) the thalamus utilizing the Sign test. We conclude that 5-[(123)I]IA is a safe, well-tolerated, and effective pharmacologic agent for human subjects to estimate high-affinity alpha4/beta2 nAChRs in the living human brain.

Greater nicotinic acetylcholine receptor density in smokers than in nonsmokers: a PET study with 2-18F-FA-85380

Assays of human postmortem brain tissue have revealed that smokers have greater densities of high-affinity nicotinic acetylcholine receptors (nAChRs) in several brain regions than do nonsmokers or exsmokers. Quantitative PET imaging of nAChRs in humans has recently been reported using the alpha4beta2* subtype-specific radioligand 2-(18)F-FA-85380 (2FA).

METHODS

We used PET and 2FA to measure total volumes of distribution corrected for the free fraction of 2FA in plasma (V(T)/f(P)) in 10 nonsmokers and 6 heavy smokers (>14 cigarettes/d; abstinent for >36 h). Dynamic PET scans were performed over 8 h, commencing immediately after a bolus injection of 2FA. Anatomic sampling was performed on PET images that were coregistered to MR images acquired from each volunteer. Data were analyzed by Logan plots and by 1- and 2-tissue-compartment models using unbound, unmetabolized arterial 2FA concentration as the input function.

RESULTS

All modeling methods yielded similar results. V(T)/f(P) was significantly higher in smokers than in nonsmokers in all brain regions tested, except the thalamus. We used measures of V(T)/f(P) and estimates of nondisplaceable volume of distribution and found 25%-200% higher values in smokers than in nonsmokers for the volume of distribution for the specific binding compartment in the frontal cortex, midbrain, putamen, pons, cerebellum, and corpus callosum. These findings were consistent with voxel-based analysis using statistical parametric mapping.

CONCLUSION

Our findings suggest that PET with 2FA can be used to study the role of nicotine-induced upregulation of nAChRs in active smokers and during smoking cessation.

SPECT and PET of the dopamine transporter in attention-deficit/hyperactivity disorder

Abnormalities of frontostriatal circuits, which are modulated by dopamine, have been found by brain imaging studies in patients with attention-deficit/hyperactivity disorder (ADHD). With special radiolabeled ligands selective imaging of the dopamine transporter (DAT), which has a key function in dopamine metabolism, can be performed by SPECT and PET. Most of the studies showed a higher DAT availability in untreated patients with ADHD compared with controls. The relationship between DAT availability and a polymorphism of DAT1 gene in patients with ADHD is not clear and the results are controversial. It has been shown that methylphenidate lowers DAT availability very effectively in normal people and in patients with ADHD. First results seem to indicate that nonresponders to methylphenidate among ADHD patients have a low primary DAT availability, whereas patients with a good response to the drug have high DAT. Nicotine seems to lower DAT availability such as stimulant medication; this may explain the high percentage of smokers among patients with ADHD. Zinc is a DAT inhibitor and seems to have a positive therapeutic effect on ADHD symptoms. This article reviews the function and structure of the DAT, the results of DAT imaging with SPECT and PET, the relations between DAT availability and the DAT1 gene polymorphism, the influence of stimulants on DAT and the significance of DAT for therapeutic response, nicotine, zinc and psychotic symptoms in patients with ADHD.

Nicotine effects on mismatch negativity in nonsmoking schizophrenic patients

BACKGROUND

The goal of the present study is to identify the effect of nicotine on auditory automatic processing, as reflected by mismatch negativity (MMN), in nonsmoking schizophrenic patients.

METHODS

Ten nonsmoking schizophrenic patients and 10 healthy volunteers underwent a reference session and 2 test sessions. The test sessions involved administration of a placebo patch and a nicotine skin patch, which were counterbalanced. Nicotine was administered transdermally under controlled dosage.

RESULTS

Nicotine administration shortened the MMN latencies (at Fz on nicotine/placebo: 134.8 +/- 5.7/157.6 +/- 6.4 ms) in healthy volunteers. In contrast, there were no significant differences in MMN latencies in schizophrenic patients (169.6 +/- 5.7/165.0 +/- 6.4 ms).

CONCLUSION

Nicotine activates and accelerates preattentive and automatic processing in healthy controls, whereas there were no such effects observed in nonsmoking patients. The impaired MMN response to nicotine administration in nonsmoking schizophrenic patients may be attributed to low nicotinic receptor function, implicated in dysregulation of the glutamatergic system.

Decreased dopamine transporter availability in male smokers -- a dual isotope SPECT study

INTRODUCTION

Although the mesolimbic dopaminergic system has been shown to play a role in reinforcing tobacco smoking, results of imaging studies that examine the relationship between tobacco smoking and the central dopamine system remain discrepant. To delineate the role of tobacco addiction in central pre- and post-synaptic dopaminergic activities, we analyzed the central D2-family receptors, the dopamine transporters (DAT), and degrees of dependence in male smokers.

METHODS

Eleven male smokers and 11 healthy non-smokers were recruited. The striatal dopamine D2/D3 receptor availability was approximated using SPECT and [123 I] IBZM while the DAT availability was approximated using SPECT and [99m Tc] TRODAT-1. All of the smokers completed the Fagerström Test for Nicotine Dependence (FTND) and other related questionnaires.

RESULTS

A decrease in DAT availability in the striatum of male smokers is noted (p<05). However, the striatal D2/D3 receptor availability in male smokers does not differ from that of non-smokers.

CONCLUSIONS

These findings suggest that cigarette smoking may alter central dopamine functions in males, particularly at the pre-synaptic sites.

Selective loss of dopamine D2 receptors in temporal cortex in dementia with Lewy bodies, association with cognitive decline

Dementia with Lewy bodies (DLB) is a progressive dementia frequently accompanied by psychotic symptoms. Similar symptoms can occur in Alzheimer's disease (AD) to a lesser extent. The use of neuroleptic medication to treat psychosis in both diseases is of modest efficacy and can induce severe adverse reactions in DLB. Dopamine D2 receptors in the cerebral cortex are the putative target for the antipsychotic action of these drugs, but the status of these receptors in DLB is unknown. Autoradiography was used to examine the density D2 receptors in postmortem temporal cortex tissue from prospectively assessed patients with neuropathologically confirmed DLB and AD. D2 receptors were substantially (over 40%) and significantly (P < 0.001) reduced in temporal cortex in DLB, and in DLB with concomitant Alzheimer pathology, but was not significantly changed in AD. This reduction correlated with greater cognitive decline (P < 0.01), but was not significantly related to visual or auditory hallucinations or delusions. D2 receptor density was inversely correlated with cortical Lewy body pathology in the neocortex (P < 0.001). The specific loss of D2 receptors associated with Lewy body pathology, in conjunction with our previous finding of low D2 receptors in striatum in DLB, provides a possible explanation for neuroleptic intolerance. That the reduction of D2 receptors correlated with cognitive decline suggests that neuroleptics, as dopamine D2 receptor antagonists, may have a deleterious effect on cognition in DLB.

Alpha7 nicotinic acetylcholine receptor mRNA expression and binding in postmortem human brain are associated with genetic variation in neuregulin 1

Studies in cell culture and in animals suggest that neuregulin 1 (NRG1), a probable schizophrenia susceptibility gene, regulates the expression of the alpha7 nicotinic acetylcholine receptors (nAChRs). We hypothesized that schizophrenia-associated allelic variations within the NRG1 gene, via their effects on NRG1 isoform expression, would be associated with alterations in nAChR alpha7 receptor levels. We examined the effects of four disease-associated single-nucleotide polymorphisms (SNPs) in the 5' region of the NRG1 gene on nAChR alpha7 mRNA transcript expression in both the dorsolateral prefrontal cortex (DLPFC) and hippocampus of normal controls and patients with schizophrenia using quantitative real-time PCR. NRG1 risk alleles at SNPs SNP8NRG221132 and rs6994992 predicted significantly lower nAChR alpha7 mRNA expression in the DLPFC. Haplotypes containing the risk alleles at the above SNPs were also associated with lower expression of nAChR alpha7 in the DLPFC. The genotype effect for rs6994992 and the haplotype effect were more pronounced within the schizophrenic patient group. To determine whether receptor levels follow that of mRNA expression, we performed receptor binding and autoradiography using [(125)I] alpha-bungarotoxin in the DLPFC. Consistent with the mRNA findings, we found a decrease in binding in risk allele carriers of SNP8NRG221132 as compared with heterozygous individuals. Together, these results suggest that the molecular mechanism of the association between NRG1 risk alleles and schizophrenia may include down-regulation of nAChR alpha7 expression.

Long-term upregulation of protein kinase A and adenylate cyclase levels in human smokers

Repeated injections of cocaine and morphine in laboratory rats cause a variety of molecular neuroadaptations in the cAMP signaling pathway in nucleus accumbens and ventral tegmental area. Here we report similar neuroadaptations in postmortem tissue from the brains of human smokers and former smokers. Activity levels of two major components of cAMP signaling, cAMP-dependent protein kinase A (PKA) and adenylate cyclase, were abnormally elevated in nucleus accumbens of smokers and in ventral midbrain dopaminergic region of both smokers and former smokers. Protein levels of the catalytic subunit of PKA were correspondingly higher in the ventral midbrain dopaminergic region of both smokers and former smokers. Protein levels of other candidate neuroadaptations, including glutamate receptor subunits, tyrosine hydroxylase, and other protein kinases, were within normal range. These findings extend our understanding of addiction-related neuroadaptations of cAMP signaling to tobacco smoking in human subjects and suggest that smoking-induced brain neuroadaptations can persist for significant periods in former smokers.

Basal ganglia cholinergic and dopaminergic function in progressive supranuclear palsy

Progressive Supranuclear Palsy (PSP) is a progressive neurodegenerative disorder. In contrast to Parkinson's disease (PD) and dementia with Lewy bodies (DLB), replacement therapy with dopaminergic and cholinergic agents in PSP has been disappointing. The neurochemical basis for this is unclear. Our objective was to measure dopaminergic and cholinergic receptors in the basal ganglia of PSP and control brains. We measured, autoradiographically, dopaminergic (dopamine transporter, 125I PE2I and dopamine D2 receptors, 125I epidepride) and cholinergic (nicotinic alpha4beta2 receptors, 125I 5IA85380 and muscarinic M1 receptors, 3H pirenzepine) parameters in the striatum and pallidum of pathologically confirmed PSP cases (n=15) and controls (n=32). In PSP, there was a marked loss of dopamine transporter and nicotinic alpha4beta2 binding in the striatum and pallidum, consistent with loss of nigrostriatal neurones. Striatal D2 receptors were increased in the caudate and muscarinic M1 receptors were unchanged compared with controls. These results do not account for the poor response to dopaminergic and cholinergic replacement therapies in PSP, and suggest relative preservation of postsynaptic striatal projection neurones bearing D2/M1 receptors.

Dopamine D3receptor ligands for the treatment of tobacco dependence

This review considers the potential use of the dopamine D(3) receptor (DRD3) as a novel therapeutic target for the treatment of tobacco dependence. Among the 5 dopamine receptors identified, the DRD3 is located in the nucleus accumbens, ventral tegmental area and amygdala: 3 brain structures that are implicated in the motivational control of drug-seeking behaviour and drug-conditioning processes. Although it has been proposed that modulating dopamine transmission would be effective in the treatment of drug dependence, no validation has been provided in humans so far. Several highly selective DRD3 ligands have recently been evaluated in preclinical models of drug dependence. These ligands act as DRD3 antagonists in vivo and are able to decrease the motivation to take various drugs of abuse and reduce the influence of associated drug-conditioned behaviour. Of note is that these effects have been found with nicotine-seeking behaviour and nicotine relapse in rodents, suggesting a potential use of these ligands for the treatment of tobacco smokers. In contrast to nicotine replacement therapy, varenicline and bupropion (which are currently used for the treatment of smokers), DRD3 antagonists do not seem to produce nicotine-like effects in experimental animals and, therefore, may not substitute for nicotine or alleviate nicotine withdrawal symptoms in human smokers. This behavioural profile, which was also reported recently with cannabinoid CB(1) receptor antagonists, may result from effects on specific brain pathways that express DRD3 receptors and are involved in relapse and conditioning processes. These preclinical studies suggest that the clinical evaluation of DRD3 ligands should be performed with clinical trials designed specifically to evaluate the relapse phenomena.

ADHD in adolescence and adulthood, with a special focus on the dopamine transporter and nicotine

The persistence of attention deficit hyperactivity disorder (ADHD) into adolescence and adulthood has now been accepted as a clinical entity. The rate of prevalence among adults is assumed to be from 2% to 4%. With increasing age, a symptom change has to be considered; disturbance of attention becomes more prominent, whereas hyperactivity often diminishes or changes to inactivity. Neuroimaging studies show a high striatal dopamine transporter (DAT) availability in most adults with ADHD; this can be reduced by stimulants. Nicotine seems to have a stimulant-like action on the DAT. In most adults with ADHD, therapy has to be multimodal, combining psychotherapy and medication. Methylphenidate is the first-line drug in adult ADHD; further options are amphetamine and noradrenaline reuptake inhibitors. Nonresponders to methylphenidate seem to have no elevated DAT availability prior to therapy. Combination with other psychiatric disorders occurs frequently in adults with ADHD; in these patients additional pharmacological treatment with special regard to the comorbid disease is recommended.

Cholinergic and glutamatergic drugs in Alzheimer's disease therapy

The pathology and clinical symptoms of Alzheimer's disease are well known and include plaques, tangles, cell loss and dysfunction. The target of current treatments is to improve neuronal dysfunction and produce symptomatic benefits based on a clear understanding of neurotransmitter biochemistry. The purpose of this review is to examine the scientific background to currently available treatments, discuss the clinical experience of employing these drugs in Alzheimer's disease patients and review the socioeconomic influences on their use in the future.

Striatal dopamine D2/D3 receptor availability in male smokers

It has been reported that the activation of the nicotine receptor evokes central endogenous dopamine release. However, whether smoking affects striatal dopamine D(2)/D(3) availability over the long run has not been well established in vivo. Fifteen male smokers and their matching controls were recruited. Striatal dopamine D(2)/D(3) receptor availability was assessed by single photon emission computed tomography with [(123)]IBZM Iodo-benzaimide. There was no significant difference in striatal D(2)/D(3) receptor availability between smokers and controls.

Effect of quinpirole on striatal dopamine release and locomotor activity in nicotine-treated mice

The effect of chronic oral nicotine treatment which in its intermittent delivery resembles human smoking was studied on the sensitivity of dopamine autoreceptors in mice. On the 50th day of nicotine administration in the drinking water or after 23-25 h withdrawal quinpirole (D2/D3 agonist, 0.01-0.1 mg/kg s.c.) was given, and accumbal and dorsal striatal dopamine outflow, locomotor activity and body temperature were measured. Dorsal striatal extracellular dopamine concentration and locomotor activity were found to be elevated during nicotine administration. Chronic nicotine did not alter the effects of small, autoreceptor preferring doses of quinpirole on accumbal or dorsal striatal dopamine, locomotor activity or body temperature. However, quinpirole's locomotor activity reducing effect was slightly diminished in mice treated repeatedly with nicotine (0.4 mg/kg twice daily for 10 days s.c.). Thus, although repeated nicotine treatment for 5-14 days decreases dopamine autoreceptor sensitivity, after long-term oral nicotine treatment such a decrease is not seen. Thus, the changes occurring in the sensitivity of D2-like dopamine receptors probably play a minor role in regulating the dopaminergic transmission during long-term nicotine administration.

Attenuation of Abeta deposition in the entorhinal cortex of normal elderly individuals associated with tobacco smoking

Investigating correlates of tobacco smoking provides the only currently available opportunity of examining effects of long-term exposure of nicotinic receptors on a specific nicotinic agonist in human. Alzheimer-type pathology (Abeta and abnormally phosphorylated tau assessed on the basis of AT8 immunoreactivity) together with vascular markers has been compared in age-matched groups of normal elderly smokers and non-smokers in the entorhinal cortex, an area of noted age-related pathology. The density of total Abeta and diffuse Abeta immunoreactivity, together with formic acid-extractable Abeta42 but not Abeta40, was reduced in smokers (n = 10-18) compared with non-smokers (n = 10-20) (P < 0.05). There was also a reduced percentage of cortical and leptomeningeal vessels with associated Abeta immunoreactivity in smokers (n = 13) compared with non-smokers (n = 14) (P < 0.005 and 0.05, respectively). There was a significant inverse correlation between formic acid-extractable Abeta42 and pack years (n = 34, r = -0.389, P = 0.025), with a similar trend for total Abeta immunoreactivity which did not reach statistical significance (n = 30, r = -0.323, P = 0.082). In contrast, there were no significant group differences for vascular markers (collagen IV, alpha-actin or glucose transporter 1), AT8 immunoreactivity or phosphate-buffered saline-soluble Abeta peptides, and no significant associations with gender for any of the measured parameters. These findings are consistent with previously reported reductions in histologically assessed amyloid plaques in aged human brain associated with tobacco use and dramatic lessening of Abeta deposits in APPsw mice after nicotine treatment. Development of nicotinic drugs to protect against beta-amyloidosis as one of the principal pathological hallmarks of brain ageing and Alzheimer's disease is indicated.

The role of central dopamine D3 receptors in drug addiction: a review of pharmacological evidence

The cDNA for the dopamine D3 receptor was isolated and characterized in 1990. Subsequent studies have indicated that D3 receptors, as well as D3 receptor mRNA, are primarily localized in limbic regions in mammals. This finding led to the postulate that D3 receptors may be involved in drug dependence and addiction. However, this hypothesis has been difficult to test due to the lack of compounds with high selectivity for central D3 receptors. The interpretation of results from studies using mixed D2/D3 agonists and/or antagonists is problematic because these agents have low selectivity for D3 over D2 receptors and it is likely that their actions are primarily related to D2 receptor antagonism and possibly interaction with other neurotransmitter receptors. Currently, with the synthesis and characterization of new highly selective D3 receptor antagonists such as SB-277011-A this difficulty has been surmounted. The purpose of the present article is to review, for the first time, the effects of various putative D3 receptor selective compounds in animal models of drug dependence and addiction. The results obtained with highly selective D3 receptor antagonists such as SB-277011-A, SB-414796, and NGB-2904 indicate that central D3 receptors may play an important role in drug-induced reward, drug-taking, and cue-, drug-, and stress-induced reinstatement of drug-seeking behavior. Provided these results can be extrapolated to human drug addicts, they suggest that selective DA D3 receptor antagonists may prove effective as potential pharmacotherapeutic agents to manage drug dependence and addiction.