Cigarette Use and Striatal Dopamine D2/3 Receptors: Possible Role in the Link between Smoking and Nicotine Dependence

The D1/D2-like receptor antagonist flupentixol and the D2-like receptor antagonist L-741626 decrease operant responding for nicotine and food and locomotor activity in male and female rats

BACKGROUND

The reinforcing properties of nicotine play a critical role in smoking and vaping. There is a need for treatments that decrease the reinforcing properties of nicotine and thereby improve smoking and vaping rates. Dopamine plays a role in the reinforcing properties of nicotine, but little is known about the role of dopamine D2-like receptors in nicotine intake and whether there are sex differences in the effects of dopaminergic drugs on nicotine intake.

AIM

The goal of the present studies was to investigate the effects of the D1/D2-like receptor antagonist flupentixol and the D2-like receptor antagonist L-741626 on nicotine self-administration in male and female rats.

METHODS

The effects of flupentixol and L-741626 on operant responding for nicotine and food and locomotor activity in a small open field were investigated.

RESULTS

There were no sex differences in baseline nicotine intake. The D1/D2-like receptor antagonist flupentixol and the D2-like receptor antagonist L-741626 decreased operant responding for nicotine. Blockade of D1/D2-like receptors and blockade of D2-like receptors also decreased operant responding for food and decreased locomotor activity. Flupentixol induced a greater decrease in operant responding for food in males than females. However, in the other tests, there were no sex differences in the effects of the dopamine receptor antagonists.

CONCLUSIONS

Blockade of D1/D2-like receptors with flupentixol and D2-like receptors with L-741626 decreases nicotine and food intake in rats of both sexes. These compounds also decrease locomotor activity which might be indicative of a sedative effect.

The Effects of Citalopram and Thalamic Dopamine D2/3 Receptor Availability on Decision-Making and Loss Aversion in Alcohol Dependence

Selective serotonin reuptake inhibitors (SSRIs) are commonly prescribed for patients who misuse alcohol, especially in the context of comorbid depressive symptoms. Deficits in impulse control and decision-making are linked to routine alcohol consumption and alcohol dependence. The goal of this study was to determine the effects of a single dose of citalopram on measures of impulsivity, decision-making, and/or brain dopamine receptor availability in alcohol-dependent individuals. A double-blind, placebo-controlled, within-subject, outpatient study was conducted with active alcohol-dependent (DSM-IV-TR criteria) participants (n = 12) and matched healthy controls (n = 13). Serial doses of both citalopram (40 mg) and saline were administered intravenously before laboratory tests of decision-making (Balloon Analogue Risk Task, delay discounting task, and Loss Aversion Gambling Task) and positron emission tomography with [¹⁸F]-fallypride to measure dopamine D_2/3 receptor availability, separated by at least one week. Alcohol-dependent participants demonstrated greater loss aversion than healthy controls, but there were no group differences in risk taking on the Balloon Analogue Risk Task. Citalopram increased delay discounting across groups, with no group difference in the effect. There were no effects of citalopram on risk taking on the Balloon Analogue Risk Task. PET showed a negative correlation between thalamic dopamine D_2/3 receptor availability and loss aversion across groups. The effect of citalopram to decrease the valuation of monetary reward as a function of delay raises the possibility that SSRIs can influence risky decision-making in clinical populations. In addition, these results suggest that altered thalamic dopamine signaling may play an important role in disproportionately valuing losses in patients with Alcohol Use Disorder. This trial is registered under ClinicalTrials.gov registration NCT01657760.

The multitarget FAAH inhibitor/D3 partial agonist ARN15381 decreases nicotine self-administration in male rats

Tobacco use disorder is a worldwide health problem for which available medications show limited efficacy. Nicotine is the psychoactive component of tobacco responsible for its addictive liability. Similar to other addictive drugs, nicotine enhances mesolimbic dopamine transmission. Inhibition of the fatty acid amide hydrolase (FAAH), the enzyme responsible for the degradation of the endocannabinoid anandamide (AEA), palmitoylethanolamide (PEA) and oleoylethanolamide (OEA), reduces nicotine-enhanced dopamine transmission and acquisition of nicotine self-administration in rats. Down-regulation of dopamine transmission by antagonists or partial agonists of the dopamine D3 receptor (DRD3) also reduced nicotine self-administration and conditioned place preference. Based on these premises, we evaluated the effect of ARN15381, a multitarget compound showing FAAH inhibition and DRD3 partial agonist activity in the low nanomolar range, on nicotine self-administration in rats. Pretreatment with ARN15381 dose dependently decreased self-administration of a nicotine dose at the top of the nicotine dose/response (D/R) curve, while it did not affect self-administration of a nicotine dose laying on the descending limb of the D/R curve. Conversely, pretreatment with the selective FAAH inhibitor URB597 and the DRD3 partial agonist CJB090 failed to modify nicotine self-administration independent of the nicotine dose self-administered. Our data indicates that the concomitant FAAH inhibition and DRD3 partial agonism produced by ARN15381 is key to the observed reduction of nicotine self-administration, demonstrating that a multitarget approach may hold clinical importance for the treatment of tobacco use disorder.

Interactions between cigarette smoking and cognitive status on functional connectivity of the cortico-striatal circuits in individuals without dementia: A resting-state functional MRI study

Aims

Cigarette smoking is a modifiable risk factor for Alzheimer's disease (AD), and controlling risk factors may curb the progression of AD. However, the underlying neural mechanisms of the effects of smoking on cognition remain largely unclear. Therefore, we aimed to explore the interaction effects of smoking × cognitive status on cortico‐striatal circuits, which play a crucial role in addiction and cognition, in individuals without dementia.

Methods

We enrolled 304 cognitively normal (CN) non‐smokers, 44 CN smokers, 130 mild cognitive impairment (MCI) non‐smokers, and 33 MCI smokers. The mixed‐effect analysis was performed to explore the interaction effects between smoking and cognitive status (CN vs. MCI) based on functional connectivity (FC) of the striatal subregions (caudate, putamen, and nucleus accumbens [NAc]).

Results

The significant interaction effects of smoking × cognitive status on FC of the striatal subregions were detected in the left inferior parietal lobule (IPL), bilateral cuneus, and bilateral anterior cingulate cortex (ACC). Specifically, increased FC of right caudate to left IPL was found in CN smokers compared with non‐smokers. The MCI smokers showed decreased FC of right caudate to left IPL and of right putamen to bilateral cuneus and increased FC of bilateral NAc to bilateral ACC compared with CN smokers and MCI non‐smokers. Furthermore, a positive correlation between FC of the NAc to ACC with language and memory was detected in MCI smokers.

Conclusions

Cigarette smoking could affect the function of cortico‐striatal circuits in patients with MCI. Our findings suggest that quitting smoking in the prodromal stage of AD may have the potential to prevent disease progression.

Dopaminergic and glutamatergic biomarkers disruption in addiction and regulation by exercise: a mini review

INTRODUCTION

Drug addiction is associated with disruption of a multitude of biomarkers in various brain regions, particularly in the reward center. The most pronounced are dopaminergic and glutamatergic biomarkers, which are affected at various levels. Neuropathological changes in biomarkers alter the homeostasis of the glutamatergic and dopaminergic nervous systems and promote addiction-associated characteristics such as repeated intake, maintenance, withdrawal, reinstatement, and relapse. Exercise has been shown to have a buffering effect on such biomarkers and reverse the effects of addictive substances.

METHODS

A review of the literature searched in PubMed, examining drug addiction and physical exercise in relation to dopaminergic and glutamatergic systems at any of the three biomarker levels (i.e., neurotransmitter, receptor, or transporter).

RESULTS

We review the collective impact of addictive substances on the dopaminergic and glutamatergic systems and the beneficial effect of exercise in terms of reversing the damage to these systems. We propose future directions, including implications of exercise as an add-on therapy, substance use disorder (SUD) prognosis and diagnosis and designing of optimized exercise and pharmaceutical regimens based on the aforementioned biomarkers.

CONCLUSION

Exercise is beneficial for all types of drug addiction at all stages, by reversing molecular damages caused to dopaminergic and glutamatergic systems.

Polypharmacological Approaches for CNS Diseases: Focus on Endocannabinoid Degradation Inhibition

Polypharmacology breaks up the classical paradigm of “one-drug, one target, one disease” electing multitarget compounds as potential therapeutic tools suitable for the treatment of complex diseases, such as metabolic syndrome, psychiatric or degenerative central nervous system (CNS) disorders, and cancer. These diseases often require a combination therapy which may result in positive but also negative synergistic effects. The endocannabinoid system (ECS) is emerging as a particularly attractive therapeutic target in CNS disorders and neurodegenerative diseases including Parkinson’s disease (PD), Alzheimer’s disease (AD), Huntington’s disease (HD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), stroke, traumatic brain injury (TBI), pain, and epilepsy. ECS is an organized neuromodulatory network, composed by endogenous cannabinoids, cannabinoid receptors type 1 and type 2 (CB₁ and CB₂), and the main catabolic enzymes involved in the endocannabinoid inactivation such as fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL). The multiple connections of the ECS with other signaling pathways in the CNS allows the consideration of the ECS as an optimal source of inspiration in the development of innovative polypharmacological compounds. In this review, we focused our attention on the reported polypharmacological examples in which FAAH and MAGL inhibitors are involved.

Cannabis Use and Neuroadaptation: A Call for Δ 9 -Tetrahydrocannabinol Challenge Studies

Currently, the assessment of the neurobehavioral consequences of repeated cannabis use is restricted to studies in which brain function of chronic cannabis users is compared to that of non-cannabis using controls. The assumption of such studies is that changes in brain function of chronic users are caused by repeated and prolonged exposure to acute cannabis intoxication. However, differences in brain function between chronic cannabis users and non-users might also arise from confounding factors such as polydrug use, alcohol use, withdrawal, economic status, or lifestyle conditions. We propose a methodology that highlights the relevance of acute Δ⁹-tetrahydrocannabinol (THC) dosing studies for a direct assessment of neuroadaptations in chronic cannabis users. The approach includes quantification of neurochemical, receptor, and functional brain network changes in response to an acute cannabis challenge, as well as stratification of cannabis using groups ranging from occasional to cannabis-dependent individuals. The methodology allows for an evaluation of THC induced neuroadaptive and neurocognitive changes across cannabis use history, that can inform neurobiological models on reward driven, compulsive cannabis use.

Age Influences Loss Aversion Through Effects on Posterior Cingulate Cortical Thickness

Decision-making strategies shift during normal aging and can profoundly affect wellbeing. Although overweighing losses compared to gains, termed "loss aversion," plays an important role in choice selection, the age trajectory of this effect and how it may be influenced by associated changes in brain structure remain unclear. We therefore investigated the relationship between age and loss aversion, and tested for its mediation by cortical thinning in brain regions that are susceptible to age-related declines and are implicated in loss aversion - the insular, orbitofrontal, and anterior and posterior cingulate cortices. Healthy participants (n = 106, 17-54 years) performed the Loss Aversion Task. A subgroup (n = 78) provided structural magnetic resonance imaging scans. Loss aversion followed a curvilinear trajectory, declining in young adulthood and increasing in middle-age, and thinning of the posterior cingulate cortex mediated this trajectory. The findings suggest that beyond a threshold in middle adulthood, atrophy of the posterior cingulate cortex influences loss aversion.

Smoking Genes: A Case-Control Study of Dopamine Transporter Gene (SLC6A3) and Dopamine Receptor Genes (DRD1, DRD2 and DRD3) Polymorphisms and Smoking Behaviour in a Malay Male Cohort

Dopamine receptor and dopamine transporter genes polymorphisms have been associated with cigarette smoking behaviour in different populations. The aim of this case-control study was to evaluate polymorphisms in the dopamine transporter gene (SLC6A3 (rs27072)) and the dopamine receptor genes (DRD1 (rs686), DRD2 (rs1800497) and DRD3 (rs7653787)) and their contribution to smoking behaviour in a Malay male population. We identified 476 participants over the age of 18 years comprising 238 smokers and 238 non-smokers. Information such as age, height, weight, body mass index, systolic and diastolic blood pressures, marital status, and smoking status of close family members were taken. For the genetic study, we genotyped four genes (SLC6A3 (rs27072), DRD1 (rs686), DRD2 (rs1800497) and DRD3 (rs7653787)) using the polymerase chain reaction-restriction fragment length polymorphism method and further confirmed our findings with sequencing. Dopamine receptor genes (DRD1, DRD2 and DRD3) were found to be associated with smoking behaviour in a Malay male population. The dopamine transporter gene (SLC6A3) did not show this association. Significant differences were observed between smokers' and non-smokers' age, systolic blood pressure, marital status and family members who smoke. Smoking behaviour is significantly influenced by genetic variations of DRD1, DRD2 and DRD3 in a Malay male population.

Effects of Citalopram on Cue-Induced Alcohol Craving and Thalamic D2/3 Dopamine Receptor Availability

BACKGROUND

Selective serotonin reuptake inhibitors are often used in alcohol use disorders. Clinical trials with selective serotonin reuptake inhibitors for alcohol use disorders, however, have yielded mixed results. The goal of this project was to assess whether a single i.v. dose of a selective serotonin reuptake inhibitor reduces craving for alcohol and/or simultaneously increases striatal dopamine concentration in individuals with alcohol dependence.

METHODS

Alcohol-dependent (DSM-IV-TR criteria) volunteers and matched controls (n = 10/group) underwent a double-blind, placebo-controlled, within-subjects study. Participants received i.v. citalopram (40 mg) or saline (counter-balanced) followed by a cue-induced craving assessment and [18F]-fallypride positron emission tomography scanning.

RESULTS

In the alcohol-dependent individuals, the citalopram (compared with saline) resulted in decreased cue-induced craving for alcohol. For the whole study group, cue-induced alcohol craving was inversely correlated with thalamic (but not striatal) dopamine D2/3 receptor availability.

CONCLUSIONS

Acute serotonin reuptake inhibition reduces cue-induced alcohol craving. Furthermore, thalamic dopamine abnormalities and the striatal hyperdopaminergic hypothesis of alcohol use disorder are supported.

Modelling Differential Vulnerability to Substance Use Disorder in Rodents: Neurobiological Mechanisms

Despite the prevalence of drug use within society, only a subset of individuals actively taking addictive drugs lose control over their intake and develop compulsive drug-seeking and intake that typifies substance use disorder (SUD). Although research in this field continues to be an important and dynamic discipline, the specific neuroadaptations that drive compulsive behaviour in humans addicted to drugs and the neurobiological mechanisms that underlie an individual's innate susceptibility to SUD remain surprisingly poorly understood. Nonetheless, it is clear from research within the clinical domain that some behavioural traits are recurrently co-expressed in individuals with SUD, thereby inviting the hypothesis that certain behavioural endophenotypes may be predictive, or at least act in some way, to modify an individual's probability for developing this disorder. The analysis of such endophenotypes and their catalytic relationship to the expression of addiction-related behaviours has been greatly augmented by experimental approaches in rodents that attempt to capture diagnostically relevant aspects of this progressive brain disorder. This work has evolved from an early focus on aberrant drug reinforcement mechanisms to a now much richer account of the putatively impaired cognitive control processes that ultimately determine individual trajectories to compulsive drug-related behaviours. In this chapter we discuss the utility of experimental approaches in rodents designed to elucidate the neurobiological and genetic underpinnings of so-called risk traits and how these innate vulnerabilities collectively contribute to the pathogenesis of SUD.

Addiction and Dopamine: Sex Differences and Insights from Studies of Smoking

Mesolimbic dopaminergic function influences addiction through effects on reinforcement learning, decision-making, and impulsivity. This review covers sex differences in dopaminergic neurochemistry, their hormonal and genetic determinants, and how differences in dopaminergic tone interact with sex and/or ovarian hormone status to affect cognitive functions. Findings from research on cigarette smoking reveal sex differences in striatal and midbrain dopamine D2-type receptor availability and striatal dopamine release that suggest mechanisms of nicotine dependence, and stronger subjective responses to nicotine and efficacy of nicotine replacement therapies in male smokers than in their female counterparts. Opportunities exist to extend such efforts in studies of how sex and hormone status influence other addictions.

The Impact of Combinations of Alcohol, Nicotine, and Cannabis on Dynamic Brain Connectivity

Alcohol, nicotine, and cannabis are among the most commonly used drugs. A prolonged and combined use of these substances can alter normal brain wiring in different ways depending on the consumed cocktail mixture. Brain connectivity alterations and their change with time can be assessed using functional magnetic resonance imaging (fMRI) because of its spatial and temporal content. Here, we estimated dynamic functional network connectivity (dFNC) as derived from fMRI data to investigate the effects of single or combined use of alcohol, nicotine, and cannabis. Data from 534 samples were grouped according to their substance use combination as controls (CTR), smokers (SMK), drinkers (DRN), smoking-and-drinking subjects (SAD), marijuana users (MAR), smoking-and-marijuana users (SAM), marijuana-and-drinking users (MAD), and users of all three substances (ALL). The DRN group tends to exhibit decreased connectivity mainly in areas of sensorial and motor control, a result supported by the dFNC outcome and the alcohol use disorder identification test. This trend dominated the SAD group and in a weaker manner MAD and ALL. Nicotine consumers were characterized by an increment of connectivity between dorsal striatum and sensorimotor areas. Where possible, common and separate effects were identified and characterized by the analysis of dFNC data. Results also suggest that a combination of cannabis and nicotine have more contrasting effects on the brain than a single use of any of these substances. On the other hand, marijuana and alcohol might follow an additive effect trend. We concluded that all of the substances have an impact on brain connectivity, but the effect differs depending on the dFNC state analyzed.

Striatal Dopamine D2/D3 Receptor Availability Varies Across Smoking Status

To assess how tobacco smoking status affects baseline dopamine D2/D3 (D2R) receptor availability and methylphenidate-induced dopamine (DA) release, we retrospectively analyzed D2R availability measures of 8 current smokers, 10 ex-smokers, and 18 nonsmokers who were scanned with positron emission tomography and [11C]raclopride, after administration of an injection of placebo or 0.5 mg/kg i.v. methylphenidate. There was a significant effect of smoking status on baseline striatal D2R availability; with current smokers showing lower striatal D2R availability compared with nonsmokers (caudate, putamen, and ventral striatum) and with ex-smokers (caudate and putamen). Baseline striatal D2R did not differ between nonsmokers and ex-smokers. The effect of smoking status on methylphenidate-induced DA release tended to be lower in smokers but the difference was not significant (p=0.08). For behavioral measures, current smokers showed significantly higher aggression scores compared with both nonsmokers and ex-smokers. These results suggest that with abstinence ex-smokers may recover from low striatal D2R availability and from increased behavioral aggression seen in active smokers. However, longitudinal studies are needed to assess this within abstaining smokers.

Affinity States of Striatal Dopamine D2 Receptors in Antipsychotic-Free Patients with Schizophrenia

BACKGROUND

Dopamine D2 receptors are reported to have high-affinity (D2High) and low-affinity (D2Low) states. Although an increased proportion of D2High has been demonstrated in animal models of schizophrenia, few clinical studies have investigated this alteration of D2High in schizophrenia in vivo.

METHODS

Eleven patients with schizophrenia, including 10 antipsychotic-naive and 1 antipsychotic-free individuals, and 17 healthy controls were investigated. Psychopathology was assessed by Positive and Negative Syndrome Scale, and a 5-factor model was used. Two radioligands, [11C]raclopride and [11C]MNPA, were employed to quantify total dopamine D2 receptor and D2High, respectively, in the striatum by measuring their binding potentials. Binding potential values of [11C]raclopride and [11C]MNPA and the binding potential ratio of [11C]MNPA to [11C]raclopride in the striatal subregions were statistically compared between the 2 diagnostic groups using multivariate analysis of covariance controlling for age, gender, and smoking. Correlations between binding potential and Positive and Negative Syndrome Scale scores were also examined.

RESULTS

Multivariate analysis of covariance demonstrated a significant effect of diagnosis (schizophrenia and control) on the binding potential ratio (P=.018), although the effects of diagnosis on binding potential values obtained with either [11C]raclopride or [11C]MNPA were nonsignificant. Posthoc test showed that the binding potential ratio was significantly higher in the putamen of patients (P=.017). The Positive and Negative Syndrome Scale "depressed" factor in patients was positively correlated with binding potential values of both ligands in the caudate.

CONCLUSIONS

The present study indicates the possibilities of: (1) a higher proportion of D2High in the putamen despite unaltered amounts of total dopamine D2 receptors; and (2) associations between depressive symptoms and amounts of caudate dopamine D2 receptors in patients with schizophrenia.