Chronic nicotine induces a specific appetite for sucrose in rats

Targeting GLP-1 receptors to reduce nicotine use disorder: Preclinical and clinical evidence

Nicotine use disorder (NUD) remains a leading cause of preventable death in the U.S. Unfortunately, current FDA-approved pharmacotherapies for smoking cessation have limited efficacy and are associated with high rates of relapse. One major barrier to long-term smoking abstinence is body weight gain during withdrawal. Nicotine withdrawal-induced body weight gain can also lead to development of chronic disease states like obesity and type II diabetes mellitus. Therefore, it is critical to identify novel pharmacotherapies for NUD that decrease relapse and nicotine withdrawal symptoms including body weight gain. Recent studies demonstrate that glucagon-like peptide-1 receptor (GLP-1R) agonists attenuate voluntary nicotine taking and seeking and prevent withdrawal-induced hyperphagia and body weight gain. Emerging evidence also suggests that GLP-1R agonists improve cognitive deficits, as well as depressive- and anxiety-like behaviors, which contribute to smoking relapse during withdrawal. While further studies are necessary to fully characterize the effects of GLP-1R agonists on NUD and understand the mechanisms by which GLP-1R agonists decrease nicotine withdrawal-mediated behaviors, the current literature supports GLP-1R-based approaches to treating NUD.

Varenicline rescues nicotine-induced decrease in motivation for sucrose reinforcement

Varenicline is one of the top medications used for smoking cessation and is often prescribed before termination of nicotine use. The effect of this combined nicotine and varenicline use on the reward system and motivation for primary reinforcement is underexplored. The goal of this study was to assess the effects of nicotine and varenicline on motivation for a food reinforcer. In Experiment 1, we first assessed the responding for sucrose after pretreatment with nicotine (0, 0.1, or 0.4 mg/kg) and varenicline (0.0, 0.1, 1.0 mg/kg) using a behavioral economics approach. The responding for sucrose was then assessed using a progressive ratio schedule of reinforcement after pretreatment with all possible combinations of nicotine and varenicline doses. In Experiment 2, rats were assessed for the consumption of sucrose in home cages after pretreatment with nicotine and varenicline. We found that (a) nicotine decreased economic demand for sucrose, (b) varenicline rescued nicotine-induced reduction in economic demand for sucrose, and (c) history of varenicline treatment predicted responding for sucrose on a progressive ratio schedule of reinforcement where rats with a history of varenicline treatment responded significantly lower for sucrose across nicotine doses than rats that had not been exposed to varenicline. The results of Experiment 2 largely confirmed that nicotine decreases motivation for sucrose using a passive consumption protocol and that varenicline rescues this effect. Overall, these findings suggest that varenicline interacts with the effects of nicotine by restoring nicotine-induced reduction in motivation for appetitive rewards.

Suppressive and enhancing effects of nicotine on food-seeking behavior

The present study examined how systemic low doses of nicotine affect the microstructure of reinforced food-seeking behavior in rats. Rats were first given an acute saline or nicotine treatment (0.1-0.6mg/kg, with an inter-injection interval of at least 48h), and then a chronic saline or nicotine treatment (0.3mg/kg/day for 10 consecutive days). Immediately after each injection, rats were required to press a lever five times to obtain food that was available at unpredictable times (on average every 80s) with constant probability. Acute nicotine dose-dependently suppressed behavior prior to the delivery of the first reinforcer, but enhanced food-reinforced behavior afterwards. These effects were primarily observed in the time it took rats to initiate food-seeking behavior. Enhancing effects were also observed in the microstructure of food-seeking behavior, with lower nicotine doses (0.1-0.3mg/kg) increasing the rate at which response bouts were initiated, and higher doses (0.3-0.6mg/kg) increasing within-bout response rates. A pre-feeding control suggests that changes in appetite alone cannot explain these effects. Over the course of chronic nicotine exposure, tolerance developed to the suppressive, but not to the enhancing effects of nicotine on food-seeking behavior. These results suggest that (a) lower doses of nicotine enhance the reward value of food and/or food-associated stimuli, (b) higher doses of nicotine enhance motoric activity, and (c) ostensive sensitization effects of nicotine on behavior partially reflect a tolerance to its transient suppressive motoric effects.

Assessing the prevalence of and factors associated with overweight, obesity, and binge eating as a function of ethnicity

OBJECTIVE

This study aimed to examine the prevalence of and factors associated with overweight, obesity, and binge eating as a function of ethnicity among a national sample of college students.

METHODS

Data were drawn from the 2010 Healthy Minds Study sample. Participants were 24,396 college students from 26 colleges and universities. This sample included 62 % females and a broad range of ethnic backgrounds; 30 % were overweight, 10 % were obese, and 7 % met criteria the for binge eating.

RESULTS

While African Americans reported the highest prevalence of overweight (49 %) and obesity (24 %), Arab-Americans reported the highest prevalence of binge eating (10 %). The factors associated with weight problems and binge eating differed by ethnicity.

CONCLUSIONS

Research is warranted to develop more culturally sensitive interventions to target those ethnic minorities at elevated risk. Further study of binge eating among Arab-Americans is particularly important, as the prevalence of this condition was higher among this group over any other.

Effects of economy type and nicotine on the essential value of food in rats

The exponential demand equation proposed by Hursh and Silberberg (2008) provides an estimate of the essential value of a good as a function of price. The model predicts that essential value should remain constant across changes in the magnitude of a reinforcer, but may change as a function of motivational operations. In Experiment 1, rats' demand for food across a sequence of fixed-ratio schedules was assessed during open and closed economy conditions and across one- and two-pellet per reinforcer delivery conditions. The exponential equation was fitted to the relation between fixed-ratio size and the logarithm of the absolute number of reinforcers. Estimates of the rate of change in elasticity of food, the proposed measure of essential value, were compared across conditions. Essential value was equivalent across magnitudes during the closed economy, but showed a slight decrease across magnitudes during the open economy. Experiment 2 explored the behavioral mechanisms of nicotine's effects on consumption with the results from Experiment 1 serving as a within-subject frame of reference. The same subjects were administered nicotine via subcutaneously implanted osmotic minipumps at a dose of 3 mg/kg/day and exposed to both the one- and two-pellet conditions under a closed economy. Although nicotine produced large decreases in demand, essential value was not significantly changed. The data from the present experiments provide further evidence for the adequacy of the exponential demand equation as a tool for quantifying the rate of change in elasticity of a good and for assessing behavioral mechanisms of drug action.

Nicotine increases sucrose self-administration and seeking in rats

Associations between nicotine in cigarettes and food consumption may alter the incentive value of food such that food cue-reactivity is exaggerated during abstinence from smoking. This effect may contribute to the weight gain associated with cessation of smoking. We examined the effects of nicotine (0.4 mg/kg base subcutaneous) paired (NPD) or unpaired (NUP) with 10% sucrose self-administration (SA; 0.2 ml/delivery, 1 h/day for 10 days) on SA response rate and intake as well as sucrose cue-reactivity following either 1 or 30 days of forced abstinence. Rats were administered the training dose of nicotine prior to a second, consecutive cue-reactivity session. NPD rats responded at over three times the rate for sucrose and earned nearly twice the number of sucrose deliveries as NUP rats or saline controls. Sucrose cue-reactivity was greater after 30 days versus 1 day of forced abstinence for all groups. History of nicotine exposure had no effect on sucrose cue-reactivity. However, the subsequent injection of nicotine increased sucrose cue-reactivity only in the NPD groups. There were no abstinent-dependent effects of nicotine challenge on sucrose cue-reactivity. A study conducted in parallel with water as the reinforcer revealed a less dramatic effect of nicotine on intake. There was no history or abstinence-dependent effects of nicotine on water cue-reactivity. Nicotine increases the reinforcing effects of sucrose and sucrose-paired cues when nicotine is present. An implication of these findings is that relapse to nicotine (cigarettes) could substantially elevate food cue-reactivity.

The reinforcement-enhancing effects of nicotine: implications for the relationship between smoking, eating and weight

Concerns about body weight represent an important barrier to public health efforts aimed at reducing smoking. Epidemiological studies have found that current smokers weigh less than non-smokers, smoking cessation results in weight gain, and weight restriction is commonly cited as a reason for smoking. The mechanisms underlying the relationship between smoking and weight are complex and may involve a number of factors including changes in caloric intake, physical activity, metabolic rate, and lipogenesis. Amongst these possible mechanisms, nicotine-induced enhancement of food reinforcement may be particularly important. In this paper, we first review data from our laboratory that highlight two distinct ways in which nicotine impacts reinforced behavior: 1) by acting as a primary reinforcer; and 2) by directly (non-associatively) enhancing the reinforcing effects of other stimuli. We then elaborate on the reinforcement-enhancing effects of nicotine as they pertain to behaviors and stimuli related to food. Data from both laboratory animals and humans support the assertion that nicotine enhances the reinforcing efficacy of food and suggest that the influence of these effects on eating may be most important after nicotine cessation when nicotine's effects on satiety subside. Finally, we discuss the theoretical and clinical implications of this perspective for understanding and addressing the apparent tradeoff between smoking and weight gain. Better understanding of the mechanisms underlying the reinforcement-enhancing effects of nicotine broadly, and the effects on food reinforcement per se, may aid in the development of new treatments with better long term outcomes.

The generality of nicotine as a reinforcer enhancer in rats: effects on responding maintained by primary and conditioned reinforcers and resistance to extinction

RATIONALE

Nicotine may enhance the reinforcing value of other reinforcers. It is unclear whether nicotine enhances responding maintained by all reinforcers or whether there are limits to this role.

OBJECTIVE

The objective of the study is to test the generality of nicotine-induced increases in reinforced responding by using an observing response procedure, which generated measures of responding maintained by food reinforcers, conditioned reinforcers, and responding during extinction. We also examined whether nicotine increased resistance to extinction and whether nicotine's effects could be characterized as rate-dependent.

MATERIALS AND METHODS

Rats received presession subcutaneous injections of Vehicle (n=5), 0.3 (n=6), or 0.56 (n=6) mg/kg nicotine for 70 sessions. Resistance to extinction was also assessed by removing food for five sessions.

RESULTS

Nicotine did not consistently affect food or extinction responding. Both doses of nicotine produced increases in responding maintained by conditioned reinforcers, but did not increase resistance to extinction. Predrug response rates accounted for a small but significant percentage of the variance in the drug effect.

CONCLUSION

Although there was a tendency for nicotine to increase low predrug response rates (i.e., response rates just prior to nicotine administration), 0.3 and 0.56 mg/kg nicotine systematically increased responding maintained by conditioned reinforcers. The results are consistent with a reinforcer-enhancing role of nicotine. However, nicotine did not increase resistance to extinction, nor did it increase food-maintained responses. Nicotine may selectively increase responding maintained by moderately reinforcing stimuli, such as the conditioned reinforcers used in the present study.

18-methoxycoronaridine: a potential new treatment for obesity in rats?

RATIONALE

Excessive eating often leads to obesity. Although a variety of neurotransmitters and brain regions are involved in modulating food intake, a role of accumbal dopamine is thought to be critical for several aspects of this behavior. Since 18-methoxycoronaridine (18-MC), a selective antagonist of alpha3beta4 nicotinic receptors, was previously shown to alter dopamine release in the nucleus accumbens in response to chronic injections of cocaine and morphine, this drug could be a promising therapy for abnormal eating behavior.

OBJECTIVES

Assess the effect of 18-MC on the consumption of sucrose (15%) vs. water in a self-administration paradigm and on the intake of freely available palatable fluids (i.e., 5% sucrose, 0.1% saccharin, and 0.6% saline solutions) as well as on water intake. Determine whether repeated administration of 18-MC (20 mg/kg i.p.) affects weight gain, food intake, and fat deposition in rats drinking 30% sucrose solution.

RESULTS

Acute administration of 18-MC (10-40 mg/kg i.p.) reduced operant responding for sucrose and decreased ad libitum ingestion of sucrose, saccharin, and saline. The highest dose of 18-MC also reduced consumption of water when palatable fluids were not available. In rats having unlimited access to sucrose (30%), chronic treatment with 18-MC (20 mg/kg i.p.) prevented sucrose-induced increases in body weight, decreased fat deposition, and reduced consumption of sucrose while not altering food intake.

CONCLUSIONS

These data suggest that antagonism of alpha3beta4 nicotinic receptors may be involved in the regulation of intake of palatable substances regardless of its caloric value and may participate in maintaining obesity.

Effects of acute and chronic nicotine on responses maintained by primary and conditioned reinforcers in rats

There is growing recognition that nonnicotine factors, such as the sensory stimuli associated with smoking, can play a critical role in the maintenance of cigarette smoking. However, little is known about the effects of nicotine on responding maintained by these stimuli, which are assumed to be conditioned reinforcers. The authors used an animal model to examine the acute and chronic effects of nicotine on responses maintained by food and conditioned reinforcers (i.e., lights) and responses in the absence of programmed consequences (i.e., extinction). During the acute phase, 4 male rats received 5 doses of subcutaneous nicotine. One dose of nicotine was then administered for a minimum of 60 days. Food-maintained and extinction responses did not significantly increase during the acute phase; however, food-maintained responses did increase during the chronic phase. Relative to vehicle, intermediate doses increased responses maintained by conditioned reinforcers during both phases. The results suggest that nicotine enhances responding maintained by conditioned reinforcers and possibly by food.

Effects of acute and chronic nicotine on impulsive choice in rats

Impulsive choice, or preference for small immediate reinforcers over large delayed reinforcers, has been associated with cigarette smoking. The direct effects of nicotine on impulsive choice in laboratory animals are unknown. We examined the effects of acute and chronic nicotine injections, and the termination of injections, on impulsive choice in rats. Five rats made choices between a one- and a three-pellet reinforcer in a discrete trials procedure. The delay to the smaller reinforcer was always 1 s. A computer adjusted the delay to the larger reinforcer until the pattern of choices reflected indifference between the two alternatives. We assessed the effects of acute and chronic nicotine (vehicle, 0.03, 0.1, 0.3 and 1.0 mg/kg nicotine). The latency to make the first response of the session increased under the acute 1.0 mg/kg dose. There were no consistent differences in the effects of acute and chronic nicotine on response latency and lever pressing during the delays between choices. Acute injections of nicotine dose-dependently increased impulsive responding. After chronic injections, impulsive responding was increased equivalently regardless of dose, and it was increased even in the absence of nicotine. After drug injections were terminated, behavior remained impulsive for about 30 drug-free sessions, and then responding gradually returned to baseline levels. The results suggest that increases in impulsive choice were not due to anorectic effects, response biases or changes in conditioned reinforcement. Nicotine may have decreased the value of delayed reinforcers. Chronic nicotine exposure produced long-lasting but reversible increases in impulsive choice.

Nicotine produces selective degeneration in the medial habenula and fasciculus retroflexus

Nicotine's neurotoxic properties in rats were investigated by administering (-)-nicotine tartrate for 5 days either continuously in doses of 5.01, 5.72, 6.44, 7.13, 20.41 and 43.1 mg/kg/day via osmotic minipump or intermittently at 11.32 mg/kg/day via one daily subcutaneous injection. As assessed by silver staining, neurotoxicity was seen almost exclusively in the axons of the medial habenula and its output tract, the fasciculus retroflexus, in all treatment groups except the lowest dose. Within the habenula, the damage was noted in the ventral-medial-most portion of the nucleus which is thought to be dense with the alpha 4 beta 2 and/or alpha 3 beta 4 receptor subtypes. Past research has shown the medial habenula to be highly sensitive to the effects of nicotine, and these findings, in conjunction with related research using dopaminergic stimulants, indicate that the habenula may be a weak link in the neurotoxicity seen following stimulant drugs of abuse.

Selective neurotoxic effects of nicotine on axons in fasciculus retroflexus further support evidence that this a weak link in brain across multiple drugs of abuse

When administered continuously for several days at relatively low plasma levels, a variety of drugs of abuse with strong dopaminergic actions induce degeneration in axons traveling from the lateral habenula through the sheath of fasciculus retroflexus to midbrain monoaminergic nuclei. With some of these drugs, such as cocaine, this is virtually the only degeneration induced in brain. Nicotine given continuously also selectively induces degeneration in fasciculus retroflexus, but in the other half of the tract: the cholinergic axons running from medial habenula in the core of the tract to the interpeduncular nucleus. Fasciculus retroflexus appears to be a weak link in brain for diverse drugs of abuse when administered incessantly for several days. Alterations in this tract would be predicted to be especially important for the genesis of the symptomatology which develops during drug binges, residual effects of such binges, and the processes underlying relapse.

The effects of nicotine and nicotine withdrawal on taste reactivity

The effect of nicotine pretreatment on the palatability of flavored solutions was assessed using the taste reactivity test. In Experiment 1, low doses of nicotine [0.2-0.4 mg/kg, subcutaneously (s.c.)] suppressed the aversive taste properties of quinine and quinine-sucrose mixture and enhanced the hedonic taste properties of sucrose (0.4 mg/kg, s.c.) in rats that were nicotine naive. In Experiment 2, rats were chronically preexposed to nicotine or saline over a period of 21 pretreatment days. Tolerance developed to the ability of nicotine to enhance the palatability of sucrose. Furthermore, rats that were chronically preexposed to nicotine displayed enhanced hedonic evaluation of sucrose 24 h after nicotine was withdrawn. These results confirm human self-reports that withdrawal from nicotine dependency enhances the palatability of sweet-tasting foods.

Effects of tobacco smoking on caloric intake

Reduced body weight due to smoking may be an important factor inhibiting smoking cessation and promoting relapse after cessation in some smokers. It is popularly believed that smoking decreases body weight by suppressing appetite. However, cross-sectional studies show that, despite their lower body weights, smokers do not eat less than non-smokers or ex-smokers and, in fact, tend to eat slightly more. Similarly, laboratory studies show no acute effects of smoking or nicotine intake via other means on caloric intake in smokers, although intake of non-smokers may be reduced after nicotine. In contrast, longitudinal studies show that eating consistently increases in the first weeks after stopping smoking, but may recede to pre-cessation levels with longer-term abstinence, while resumption of smoking after cessation is accompanied by a reduction in eating. A similar pattern of results is seen for self-reported hunger and some, but not all, constituents of diet. Thus, there appear to be no acute or chronic effects of smoking on eating in smokers maintaining regular smoking, but changes in eating are observed concomitant with changes in smoking status (i.e. cessation or relapse). Although tolerance to anorectic effects of nicotine is one potential explanation, these findings may be more parsimoniously explained by viewing changes in eating due to smoking as secondary to an alteration in the set point around which body weight is regulated. According to this notion, cessation is accompanied by increased eating only until a new, higher body weight set point is reached, while relapse (and perhaps initiation of smoking) decreases eating only until a lower set point is reached. Implications of these findings and a set point explanation for them are discussed.

Effects of nicotine on hunger and eating in male and female smokers

We tested whether the inverse relationship between smoking and body weight may be due in part to nicotine's acute effects on reducing hunger and eating. On four mornings, male and female smokers (n = 10 each), abstinent overnight from smoking and food, received one of three nicotine doses (7.5, 15, and 30 micrograms/kg) or placebo (0) via nasal spray every 30 min for 2 h. Self-reported hunger and satiety ("fullness") and craving for cigarettes were obtained after each dose presentation. Subjects subsequently ate ad lib from a large array of food items varying in sweet taste and fat content. For both males and females, nicotine had no effect on self-reported hunger, but cigarette craving was decreased. Rather than being decreased, caloric intake during the meal was unexpectedly increased following nicotine compared with placebo. Cigarette craving increased after the meal, and this increase was unaffected by nicotine dose. There were virtually no differences between males and females in any effects of nicotine. These results indicate that nicotine may not acutely suppress appetite in fasting smokers and suggest that other actions of nicotine or smoking may account for the lower body weights of smokers.

Sweet taste preference in women smokers: comparison with nonsmokers and effects of menstrual phase and nicotine abstinence

Cigarette smokers weigh less than comparably aged nonsmokers, and many gain weight following cessation. Though some evidence suggests that nicotine reduces food intake, with a selective effect on sweet-tasting foods, the issue remains unresolved. In the current study, 64 women (20 smokers, 26 never-smokers, and 18 ex-smokers) were tested for sweet taste preference; 9 of these smokers were studied under conditions of both ad lib smoking and overnight abstinence, in three hormonally verified menstrual phases. 1) Although no overall differences were detected in taste preference among the three groups, significantly more smokers than nonsmokers preferred the higher sucrose concentrations. 2) No significant differences due to menstrual phase were observed. 3) Although preference ratings did not differ significantly between overnight abstinence and ad lib smoking, a subset of smokers who preferred higher sucrose concentrations rated their preference for the solutions significantly higher during the ad lib smoking sessions. Our findings suggest that smoking and nonsmoking women differ with respect to taste preference and that, at least in a subset of female smokers, preference is affected by nicotine abstinence/acute dosing.

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.