Expanding treatment of tobacco dependence

Role of orexin peptide system in emotional overeating induced by brain reward stimulation in fed rats

Introduction: The purpose of this work was to prove that the reaction of food self-deprivation in “fed up” rats is a suitable model for studying the emotional overeating in the experiment.

Methods: The self-deprivation reaction, i.e. self-isolation of an animal from food during electrical self-stimulation of the brain, was studied in animals with food deprivation. To reproduce the self-stimulation of the lateral hypothalamus, the male Wistar rats were trained to press a pedal in a Skinner box. After training, the rats received food deprivation, then a feeder was placed in the Skinner box, and a conditioned food reflex was developed in rats within 5 days.

Results and discussion: The food self-deprivation reaction was observed in the ”satiated” rats with a current intensity of 10% and above the threshold for self-stimulation. Hungry animals pressed the pedal for hypothalamic self-stimulation and took no notice of the feeding trough. Sulpiride, a dopamine D2 antagonist (5 and 20 mg/kg i.p.), administered to the “satiated” rats decreased both the eating behavior and self-stimulation in food self-deprivation testing. SB-408124, an orexin A receptor antagonist (0.5 mg/ml, 20 μl intranasally) reduced only the number of pellets eaten, but not the number of pedal presses.

Conclusion: The orexin A receptors are preferably involved in emotional eating compared with orexin B (OX2R TCS-OX2-29) and D2 dopamine receptors. Because emotional eating is significantly related to clinical eating disorders, like bulimia and binge eating disorder, it seems promising to use drugs of the orexin system to treat and prevent the issue.

Functional Upregulation of α4* Nicotinic Acetylcholine Receptors in VTA GABAergic Neurons Increases Sensitivity to Nicotine Reward

Chronic nicotine exposure increases sensitivity to nicotine reward during a withdrawal period, which may facilitate relapse in abstinent smokers, yet the molecular neuroadaptation(s) that contribute to this phenomenon are unknown. Interestingly, chronic nicotine use induces functional upregulation of nicotinic acetylcholine receptors (nAChRs) in the mesocorticolimbic reward pathway potentially linking upregulation to increased drug sensitivity. In the ventral tegmental area (VTA), functional upregulation of nAChRs containing the α4 subunit (α4* nAChRs) is restricted to GABAergic neurons. To test the hypothesis that increased functional expression of α4* nAChRs in these neurons modulates nicotine reward behaviors, we engineered a Cre recombinase-dependent gene expression system to selectively express α4 nAChR subunits harboring a "gain-of-function" mutation [a leucine mutated to a serine residue at the 9' position (Leu9'Ser)] in VTA GABAergic neurons of adult mice. In mice expressing Leu9'Ser α4 nAChR subunits in VTA GABAergic neurons (Gad2(VTA):Leu9'Ser mice), subreward threshold doses of nicotine were sufficient to selectively activate VTA GABAergic neurons and elicit acute hypolocomotion, with subsequent nicotine exposures eliciting tolerance to this effect, compared to control animals. In the conditioned place preference procedure, nicotine was sufficient to condition a significant place preference in Gad2(VTA):Leu9'Ser mice at low nicotine doses that failed to condition control animals. Together, these data indicate that functional upregulation of α4* nAChRs in VTA GABAergic neurons confers increased sensitivity to nicotine reward and points to nAChR subtypes specifically expressed in GABAergic VTA neurons as molecular targets for smoking cessation therapeutics.

Increased CRF signalling in a ventral tegmental area-interpeduncular nucleus-medial habenula circuit induces anxiety during nicotine withdrawal

Increased anxiety is a prominent withdrawal symptom in abstinent smokers, yet the neuroanatomical and molecular bases underlying it are unclear. Here we show that withdrawal-induced anxiety increases activity of neurons in the interpeduncular intermediate (IPI), a subregion of the interpeduncular nucleus (IPN). IPI activation during nicotine withdrawal was mediated by increased corticotropin releasing factor (CRF) receptor-1 expression and signalling, which modulated glutamatergic input from the medial habenula (MHb). Pharmacological blockade of IPN CRF1 receptors or optogenetic silencing of MHb input reduced IPI activation and alleviated withdrawal-induced anxiety; whereas IPN CRF infusion in mice increased anxiety. We identified a mesointerpeduncular circuit, consisting of ventral tegmental area (VTA) dopaminergic neurons projecting to the IPN, as a potential source of CRF. Knockdown of CRF synthesis in the VTA prevented IPI activation and anxiety during nicotine withdrawal. These data indicate that increased CRF receptor signalling within a VTA-IPN-MHb circuit triggers anxiety during nicotine withdrawal.

Attention shift towards smoking cues relates to severity of dependence, smoking behavior and breath carbon monoxide

The aim of this study was to assess the severity of dependence as a factor affecting the attentional bias of smokers towards smoking-related stimuli and to clarify contradictory results of previous studies. A visual dot probe task was administered to 51 smokers and 41 nonsmokers to assess the attentional bias. Smokers were classified into a group of less severely dependent and a group of more severely dependent smokers according to the Fagerström Test for Nicotine Dependence, the number of cigarettes smoked per day or the CO concentration in the expired air. Nicotine craving was assessed as well. The more severely dependent smokers displayed an attentional bias towards smoking-related stimuli, while smokers with less severe nicotine dependence showed a negative attentional bias which was also observed in nonsmokers. A multiple linear regression indicated that CO concentration was the only significant predictor of attentional bias. In the total group of smokers we found a positive association between attentional bias and craving for the rewarding effects of nicotine. Future studies are warranted to further enhance our understanding of factors that affect attentional bias as appetitive responses towards smoking-related stimuli might be an important target for therapeutic interventions.

Cannabinoid-dopamine interaction in the pathophysiology and treatment of CNS disorders

Endocannabinoids and their receptors, mainly the CB(1) receptor type, function as a retrograde signaling system in many synapses within the CNS, particularly in GABAergic and glutamatergic synapses. They also play a modulatory function on dopamine (DA) transmission, although CB(1) receptors do not appear to be located in dopaminergic terminals, at least in the major brain regions receiving dopaminergic innervation, e.g., the caudate-putamen and the nucleus accumbens/prefrontal cortex. Therefore, the effects of cannabinoids on DA transmission and DA-related behaviors are generally indirect and exerted through the modulation of GABA and glutamate inputs received by dopaminergic neurons. Recent evidence suggest, however, that certain eicosanoid-derived cannabinoids may directly activate TRPV(1) receptors, which have been found in some dopaminergic pathways, thus allowing a direct regulation of DA function. Through this direct mechanism or through indirect mechanisms involving GABA or glutamate neurons, cannabinoids may interact with DA transmission in the CNS and this has an important influence in various DA-related neurobiological processes (e.g., control of movement, motivation/reward) and, particularly, on different pathologies affecting these processes like basal ganglia disorders, schizophrenia, and drug addiction. The present review will address the current literature supporting these cannabinoid-DA interactions, with emphasis in aspects dealing with the neurochemical, physiological, and pharmacological/therapeutic bases of these interactions.

The effect of alcohol and nicotine abuse on gene expression in the brain

Alcohol intake at levels posing an acute heath risk is common amongst teenagers. Alcohol abuse is the second most common mental disorder worldwide. The incidence of smoking is decreasing in the Western world but increasing in developing countries and is the leading cause of preventable death worldwide. Considering the longstanding history of alcohol and tobacco consumption in human societies, it might be surprising that the molecular mechanisms underlying alcohol and smoking dependence are still incompletely understood. Effective treatments against the risk of relapse are lacking. Drugs of abuse exert their effect manipulating the dopaminergic mesocorticolimbic system. In this brain region, alcohol has many potential targets including membranes and several ion channels, while other drugs, for example nicotine, act via specific receptors or binding proteins. Repeated consumption of drugs of abuse mediates adaptive changes within this region, resulting in addiction. The high incidence of alcohol and nicotine co-abuse complicates analysis of the molecular basis of the disease. Gene expression profiling is a useful approach to explore novel drug targets in the brain. Several groups have utilised this technology to reveal drug-sensitive pathways in the mesocorticolimbic system of animal models and in human subjects. These studies are the focus of the present review.

Smoking cessation: an economic analysis and review of varenicline

Despite efforts to decrease tobacco use, smoking continues to be a leading cause of preventable morbidity and premature death. The associated economic burden is substantial, both in the form of direct costs (healthcare expenditures) and indirect costs (lost productivity), regardless of whether the burden is assessed from the standpoint of an employer, a health plan, or society as a whole. Cessation programs are considered among the most cost-effective in healthcare, and are often used as a benchmark for other medical interventions. This analysis specifically considers the cost-effectiveness of varenicline, a novel α(4)β(2) partial agonist used for smoking cessation, in comparison to other approved therapies. Clinical trial data have demonstrated that varenicline has the ability to decrease cravings and withdrawal symptoms, and lessen positive reinforcement associated with smoking. Varenicline's novel mechanism has translated into superior efficacy in comparison to other available therapies. For this reason, despite an initial cost that typically exceeds that of other medications, varenicline is a cost-effective option for smoking cessation.

Preliminary evidence for gender-specific effects of topiramate as a potential aid to smoking cessation

AIMS

Study aims were threefold: (i) to determine the feasibility, potential efficacy and safety of topiramate as an aid to smoking cessation; (ii) to examine potential predictors of abstinence including gender; and (iii) to explore topiramate's effects on tobacco withdrawal and post-cessation weight gain.

DESIGN

Randomized, double-blind, placebo-controlled, 11-week clinical trial with a 6-week dosage titration period and 5 weeks of maintenance treatment.

SETTING

Single-site, out-patient, randomized clinical trial.

PARTICIPANTS

Thirty-eight adult male and 49 female chronic smokers who smoked an average of > 10 cigarettes per day and who were motivated to try to quit smoking.

INTERVENTION

Random assignment to receive either topiramate (n = 43) up to 200 mg daily in divided doses or placebo (n = 44) orally combined with brief counseling over an 11-week period.

MEASUREMENTS

Carbon monoxide (CO)-confirmed 4-week prolonged abstinence rate during weeks 8-11. Changes in tobacco withdrawal, body weight and safety parameters were also assessed.

FINDINGS

Overall, no significant increase in the prolonged abstinence rate was detected, but logistic regression analysis indicated significant gender-specific differences. Men treated with topiramate were nearly 16 times more likely to quit smoking than women on topiramate [37.5% versus 3.7%; odds ratio (OR) = 15.6; P = 0.016] and were roughly four times more likely to quit smoking than placebo-treated men (37.5% versus 13.6%; OR = 3.8; P = 0.098). Topiramate-treated men reported significantly lower tobacco withdrawal scores than both women taking topiramate and men on placebo. On average, male cessators on placebo gained 3.30 kg, whereas topiramate led to a 0.72 kg weight loss (P = 0.03). Study discontinuation rates due to adverse events (AEs) were significantly higher in the topiramate group (topiramate 23% versus placebo 2%). The most commonly reported AEs in the topiramate arm were paraesthesia, fatigue, difficulty with concentration/attention and nervousness.

CONCLUSIONS

Topiramate produced gender-specific effects on smoking cessation. Male smokers had markedly greater quit rates than female smokers and men were roughly four times more likely to quit smoking when treated with topiramate as compared to placebo. Topiramate was fairly well tolerated, although higher discontinuation rates were seen. Topiramate's triple effects aiding smoking abstinence, attenuating nicotine withdrawal and preventing post-cessation weight gain might make it a promising agent for treating tobacco addiction, at least in men.

Emerging pharmacotherapies for smoking cessation

PURPOSE

The neurobiology of tobacco dependence and the efficacy and safety of emerging pharmacotherapies for smoking cessation are reviewed.

SUMMARY

Dopamine is pivotal to the major addictive properties of nicotine. The neurotransmitters that influence dopamine activity include gamma-aminobutyric acid, acetylcholine, glutamate, norepinephrine, and serotonin. Opioids and endocannabinoids can also affect dopamine activity. Research on pharmacologic treatments for nicotine dependence has targeted the modulation of these neurotransmitter systems. Current pharmacotherapies for smoking cessation include bupropion and varenicline, both of which target the neurotransmitters involved in nicotine addiction. Several new therapies are emerging as possible treatment options for smoking cessation. Rimonabant, a selective cannabinoid antagonist, blocks dopamine release in the nucleus accumbens, a primary reward center for the brain. Studies have found that rimonabant may not only be effective as a smoking-cessation aid but may also assist in the maintenance of nicotine abstinence. Rimonabant has also demonstrated a weight-loss benefit, which may be attractive to smokers concerned with weight gain associated with smoking cessation. Three nicotine vaccines are currently in development, each acting to sequester nicotine from the bloodstream, thereby preventing its penetration of the central nervous system. Ongoing studies will evaluate their use as established therapies for smoking cessation. New nicotine-replacement formulations are also being developed.

CONCLUSION

There are several promising products in development targeting the mechanisms of tobacco dependence. As failure rates are high and relapse is common, these emerging therapies would offer more therapeutic options for smoking cessation and solutions to the problem of relapse.

Varenicline: the newest agent for smoking cessation

PURPOSE

The pharmacology, pharmacokinetics, clinical efficacy, safety, dosage and administration, and place in therapy of varenicline are reviewed.

SUMMARY

Varenicline is the newest therapy approved by the Food and Drug Administration for smoking cessation and the first in its class targeting the neurobiology of nicotine addiction. Varenicline is selective for the alpha4beta2 acetylcholine-receptor subtype as a partial agonist, thus conferring its effect in limiting the reinforcing aspect of the addictive nicotine molecule. Varenicline is completely absorbed orally and not affected by food. Steady state is reached within four days of administration. Three Phase III clinical trials of varenicline have been published. Two studies compared varenicline with bupropion in patients over age 18 years who smoked more than 10 cigarettes daily. When the data of the two trials were pooled, varenicline use was associated with significant improvements in the four-week carbon-monoxide-confirmed continuous quit rate (44.2% at weeks 9-12 compared with bupropion (29.7%) and placebo (17.7%) (p < 0.0001 for each comparison). The third trial found that continuous quit rates were also significantly higher in patients treated with varenicline versus placebo. Varenicline is generally well tolerated. Varenicline has been administered concurrently with warfarin, digoxin, transdermal nicotine, bupropion, cimetidine, and metformin without any clinically significant drug interactions.

CONCLUSION

Varenicline, a newly approved agent for smoking cessation, offers a new option to patients who cannot tolerate the adverse effects associated with nicotine-replacement therapy and bupropion. It is also an alternative to consider in patients with contraindications to such therapies.