Effects of nicotine on body weight, food intake and brown adipose tissue thermogenesis

Opportunities and challenges in the therapeutic activation of human energy expenditure and thermogenesis to manage obesity

The current obesity pandemic results from a physiological imbalance in which energy intake chronically exceeds energy expenditure (EE), and prevention and treatment strategies remain generally ineffective. Approaches designed to increase EE have been informed by decades of experiments in rodent models designed to stimulate adaptive thermogenesis, a long-term increase in metabolism, primarily induced by chronic cold exposure. At the cellular level, thermogenesis is achieved through increased rates of futile cycling, which are observed in several systems, most notably the regulated uncoupling of oxidative phosphorylation from ATP generation by uncoupling protein 1, a tissue-specific protein present in mitochondria of brown adipose tissue (BAT). Physiological activation of BAT and other organ thermogenesis occurs through β-adrenergic receptors (AR), and considerable effort over the past 5 decades has been directed toward developing AR agonists capable of safely achieving a net negative energy balance while avoiding unwanted cardiovascular side effects. Recent discoveries of other BAT futile cycles based on creatine and succinate have provided additional targets. Complicating the current and developing pharmacological-, cold-, and exercise-based methods to increase EE is the emerging evidence for strong physiological drives toward restoring lost weight over the long term. Future studies will need to address technical challenges such as how to accurately measure individual tissue thermogenesis in humans; how to safely activate BAT and other organ thermogenesis; and how to sustain a negative energy balance over many years of treatment.

Effects of nicotine on homeostatic and hedonic components of food intake

Chronic tobacco use leads to nicotine addiction that is characterized by exaggerated urges to use the drug despite the accompanying negative health and socioeconomic burdens. Interestingly, nicotine users are found to be leaner than the general population. Review of the existing literature revealed that nicotine affects energy homeostasis and food consumption via altering the activity of neurons containing orexigenic and anorexigenic peptides in the brain. Hypothalamus is one of the critical brain areas that regulates energy balance via the action of these neuropeptides. The equilibrium between these two groups of peptides can be shifted by nicotine leading to decreased food intake and weight loss. The aim of this article is to review the existing literature on the effect of nicotine on food intake and energy homeostasis and report on the changes that nicotine brings about in the level of these peptides and their receptors that may explain changes in food intake and body weight induced by nicotine. Furthermore, we review the effect of nicotine on the hedonic aspect of food intake. Finally, we discuss the involvement of different subtypes of nicotinic acetylcholine receptors in the regulatory action of nicotine on food intake and energy homeostasis.

Classification of Therapeutic and Experimental Drugs for Brown Adipose Tissue Activation: Potential Treatment Strategies for Diabetes and Obesity

OBJECTIVE

Increasing efforts are being made towards pharmacologic activation of brown adipose tissue (BAT) in animals and humans for potential use in the treatment of obesity and diabetes. We and others have reported a number of animal studies using either experimental or therapeutic drugs. There are now efforts to translate these findings to human studies. The goal of this review is to evaluate the various drugs currently being used that have the potential for BAT activation.

METHODS

Drugs were classified into 4 classes based on their mechanism of action. Class 1 drugs include the use of β3 adrenoceptor agonists for BAT activation. Class 2 drugs include drugs that affect norepinephrine levels and activate BAT with the potential of reducing obesity. Class 3 includes activators of peroxisome proliferator-activated receptor-γ in pursuit of lowering blood sugar, weight loss and diabetes and finally Class 4 includes natural products and other emerging drugs with limited information on BAT activation and their effects on diabetes and weight loss.

RESULTS

Class 1 drugs are high BAT activators followed by Class 2 and 3. Some of these drugs have now been extended to diabetes and obesity animal models and human BAT studies. Drugs in Class 3 are used clinically for Type 2 diabetes, but the extent of BAT involvement is unclear.

CONCLUSION

Further studies on the efficacy of these drugs in diabetes and measuring their effects on BAT activation using noninvasive imaging will help in establishing a clinical role of BAT.

New Insights into the Mechanisms of Action of Cotinine and its Distinctive Effects from Nicotine

Tobacco consumption is far higher among a number of psychiatric and neurological diseases, supporting the notion that some component(s) of tobacco may underlie the oft-reported reduction in associated symptoms during tobacco use. Popular dogma holds that this component is nicotine. However, increasing evidence support theories that cotinine, the main metabolite of nicotine, may underlie at least some of nicotine's actions in the nervous system, apart from its adverse cardiovascular and habit forming effects. Though similarities exist, disparate and even antagonizing actions between cotinine and nicotine have been described both in terms of behavior and physiology, underscoring the need to further characterize this potentially therapeutic compound. Cotinine has been shown to be psychoactive in humans and animals, facilitating memory, cognition, executive function, and emotional responding. Furthermore, recent research shows that cotinine acts as an antidepressant and reduces cognitive-impairment associated with disease and stress-induced dysfunction. Despite these promising findings, continued focus on this potentially safe alternative to tobacco and nicotine use is lacking. Here, we review the effects of cotinine, including comparisons with nicotine, and discuss potential mechanisms of cotinine-specific actions in the central nervous system which are, to date, still being elucidated.

Detection of thermogenesis in rodents in response to anti-obesity drugs and genetic modification

Many compounds and genetic manipulations are claimed to confer resistance to obesity in rodents by raising energy expenditure. Examples taken from recent and older literature, demonstrate that such claims are often based on measurements of energy expenditure after body composition has changed, and depend on comparisons of energy expenditure divided by body weight. This is misleading because white adipose tissue has less influence than lean tissue on energy expenditure. Application of this approach to human data would suggest that human obesity is usually due to a low metabolic rate, which is not an accepted view. Increased energy expenditure per animal is a surer way of demonstrating thermogenesis, but even then it is important to know whether this is due to altered body composition (repartitioning), or increased locomotor activity rather than thermogenesis per se. Regression analysis offers other approaches. The thermogenic response to some compounds has a rapid onset and so cannot be due to altered body composition. These compounds usually mimic or activate the sympathetic nervous system. Thermogenesis occurs in, but may not be confined to, brown adipose tissue. It should not be assumed that weight loss in response to these treatments is due to thermogenesis unless there is a sustained increase in 24-h energy expenditure. Thyroid hormones and fibroblast growth factor 21 also raise energy expenditure before they affect body composition. Some treatments and genetic modifications alter the diurnal rhythm of energy expenditure. It is important to establish whether this is due to altered locomotor activity or efficiency of locomotion. There are no good examples of compounds that do not affect short-term energy expenditure but have a delayed effect. How and under what conditions a genetic modification or compound increases energy expenditure influences the decision on whether to seek drugs for the target or take a candidate drug into clinical studies.

Nicotinic regulation of energy homeostasis

INTRODUCTION

The ability of nicotine, the primary psychoactive substance in tobacco smoke, to regulate appetite and body weight is one of the factors cited by smokers that prevents them from quitting and is the primary reason for smoking initiation in teenage girls. The regulation of feeding and metabolism by nicotine is complex, and recent studies have begun to identify nicotinic acetylcholine receptor (nAChR) subtypes and circuits or cell types involved in this regulation.

DISCUSSION

We will briefly describe the primary anatomical and functional features of the input, output, and central integration structures of the neuroendocrine systems that regulate energy homeostasis. Then, we will describe the nAChR subtypes expressed in these structures in mammals to identify the possible molecular targets for nicotine. Finally, we will review the effects of nicotine and its withdrawal on feeding and energy metabolism and attribute them to potential central and peripheral cellular targets.

The therapeutic potential of nicotine and nicotinic agonists for weight control

Transdermal nicotine patches have been successfully introduced as a safe and powerful aid to smoking cessation; this has contributed to the rising interest in additional therapeutic applications for nicotine and synthetic nicotinic agonists. Nicotine and nicotinic agonists may have a therapeutic potential for a variety of disorders, including Alzheimer's and Parkinson's diseases, depression, attention deficit disorder, Tourette's syndrome and ulcerative colitis. These interests are partially fuelled by the urgent need of the tobacco industry to find new niches for nicotine in a world bound eventually to retire from cigarette smoking. At the same time, there is an increased interest in developing drugs for fighting obesity, a growing affliction of industrialised nations. This review presents data on the potential of nicotine, and in particular synthetic nicotinic agonists, for controlling body weight. Nicotinic agonists may become relatively safe, effective and inexpensive alternatives for several optional drugs currently being developed for treating human obesity, including beta-3-adrenergic agonists, leptin and its agonists, and neuropeptide Y antagonists.

Pharmacological approaches for the treatment of obesity

The high incidence of obesity, its multifactorial nature, the complexity and lack of knowledge of the bodyweight control system, and the scarcity of adequate therapeutics have fuelled anti-obesity drug development during a considerable number of years. Irrespective of the efforts invested by researchers and companies, few products have reached a minimum level of effectiveness, and even fewer are available in medical practice. As a consequence of anti-obesity research, our knowledge of the bodyweight control system increased but, despite this, the pharmacological approaches to the treatment of obesity have not resulted yet in effective drugs. This review provides a panoramic of the multiple different approaches developed to obtain workable drugs. These approaches, however, rely in only four main lines of action: control of energy intake, mainly through modification of appetite;control of energy expenditure, essentially through the increase of thermogenesis;control of the availability of substrates to cells and tissues through hormonal and other metabolic factors controlling the fate of the available energy substrates; andcontrol of fat reserves through modulation of lipogenesis and lipolysis in white adipose tissue. A large proportion of current research is centred on neuropeptidic control of appetite, followed by the development of drugs controlling thermogenic mechanisms and analysis of the factors controlling adipocyte growth and fat storage. The adipocyte is also a fundamental source of metabolic signals, signals that can be intercepted, modulated and used to force the brain to adjust the mass of fat with the physiological means available. The large variety of different approaches used in the search for effective anti-obesity drugs show both the deep involvement of researchers on this field and the large amount of resources devoted to this problem by pharmaceutical companies. Future trends in anti-obesity drug research follow closely the approaches outlined; however, the increasing mass of information on the molecular basis of bodyweight control and obesity will in the end prevail in our search for effective and harmless anti-obesity drugs.

Effects of fluoxetine on weight gain and food intake in smokers who reduce nicotine intake

The effect of fluoxetine hydrochloride, a 5-HT uptake inhibitor (60 mg/day PO), in preventing weight gain associated with nicotine reduction was investigated in participants in a double-blind, placebo-controlled smoking-cessation trial. A lunch of cheese pizza and chocolate bars was offered, and caloric intake was monitored. The analysis focused on subjects (placebo: n = 11; fluoxetine: n = 10) who succeeded in reaching cotinine levels of less than 50% of their starting cotinine levels (signifying a stringent reduction in nicotine intake) and who participated in pre- and post-nicotine reduction lunch sessions 70 days apart. Subjects on placebo gained significantly more weight (mean +/- SEM = +3.3 +/- 0.7 kg) than subjects on fluoxetine (-0.6 +/- 1.2 kg). In fluoxetine-treated subjects, weight gain/loss was strongly correlated with initial body mass index, with higher BMI being associated with greater decreases in weight. A trend towards decreased caloric intake in the fluoxetine group was observed; the change in total calories at lunch was significantly correlated with weight change, an association accounted for principally by change in pizza intake. We conclude that fluoxetine treatment effectively prevents the weight gain that accompanies nicotine reduction and that this phenomenon is mediated, at least in part, by diminished caloric intake.

Acute effects of nicotine on resting metabolic rate in cigarette smokers

The acute effects of nicotine on resting metabolic rate (RMR) were examined to identify a mechanism that may help explain the inverse association between smoking and body weight. Multiple administrations of two nicotine doses (moderate [15 micrograms/kg body wt] and low [7.5 micrograms/kg body wt]) and a placebo (0 micrograms) were presented to 18 male smokers via nasal-spray solution on three separate occasions while RMR was assessed by computerized open-circuit indirect calorimetry. Plasma nicotine levels confirmed the reliability of dosing. RMR increases of 6% above base line after both moderate and low doses were significantly greater than the 3% increase after the placebo. Subsequent examination of the effects of smoking a nonnicotine cigarette suggested that the small placebo effect was due to acute metabolic consequences of inhalation. These results confirm that intake of nicotine, isolated from tobacco smoke, significantly increases RMR in humans. However, the results also indicate that non-pharmacological, behavioral aspects of smoking may also contribute to acutely increasing RMR in smokers.

Chronic nicotine use blocks haloperidol-induced increase in striatal D2-dopamine receptor density

Epidemiologic studies have suggested a positive association in man between nicotine use and the incidence of tardive dyskinesia, a disease characterized by dopaminergic supersensitivity after chronic neuroleptic therapy. In rats, repeated administration of neuroleptics results into dopaminergic supersensitivity and increased density of striatal D2-dopamine receptors. We investigated the effects of 6-week continuous nicotine intake on the neuroleptic (haloperidol)-induced increase in murine striatal D2-dopamine receptor density. Contrary to expectations, our data show that nicotine blocked the increase in D2-dopamine receptor density after neuroleptic administration.

Nicotine increases thermogenesis in brown adipose tissue in rats

This study has tested the hypothesis that nicotine might increase thermogenesis in rats by activating the sympathetic nervous system which supplies brown adipose tissue. Three hours after a single injection of nicotine, both the turnover of norepinephrine and the binding of the purine nucleotide, guanosine 5'-diphosphate (GDP) to mitochondria from brown adipose tissue were significantly increased. After 11 days of treatment with nicotine, the turnover of norepinephrine and the GDP binding to mitochondria from brown adipose tissue both remained elevated but weight gain was not different. These data are consistent with the hypothesis that nicotine may have part of its effect through changes in thermogenesis involving sympathetic nervous activation of peripheral thermogenic tissues such as brown adipose tissue.

Behavioral effects of acute hexamethonium in rats chronically intoxicated with nicotine

To investigate the effects of chronic nicotine administration on feeding behavior, hexamethonium, a nicotinic blocker with mainly peripheral actions, was acutely given to rats during and after chronic nicotine administration. Nicotine decreased both the time spent investigating the food and the amount of food consumed. It also decreased the time spent rearing and grooming and increased the time spent resting. These behaviors returned to control levels after nicotine withdrawal. During nicotine administration, 10 mg/kg of hexamethonium increased the amount of time the nicotine-treated rats spent investigating the food but did not change the amount of food actually eaten. These data show that predominantly peripheral nicotinic blockade can partially alleviate the effects of chronic nicotine administration of feeding behavior, suggesting that at least some of the effects of nicotine on feeding are peripheral. The finding that the investigational and consummatory aspects of feeding behavior can be pharmacologically differentiated implies that some aspects of their neural control may be distinct.