Nicotine increases sucrose self-administration and seeking in rats

Individual variations in motives for nicotine self-administration in male rats: evidence in support for a precision psychopharmacology

The significant heterogeneity in smoking behavior among smokers, coupled with the inconsistent efficacy of approved smoking cessation therapies, supports the presence of individual variations in the mechanisms underlying smoking. This emphasizes the need to shift from standardized to personalized smoking cessation therapies. However, informed precision medicine demands precision fundamental research. Tobacco smoking is influenced and sustained by diverse psychopharmacological interactions between nicotine and environmental stimuli. In the classical experimental rodent model for studying tobacco dependence, namely intravenous self-administration of nicotine, seeking behavior is reinforced by the combined delivery of nicotine and a discrete cue (nicotine+cue). Whether self-administration behavior is driven by the same psychopharmacological mechanisms across individual rats remains unknown and unexplored. To address this, we employed behavioral pharmacology and unbiased cluster analysis to investigate individual differences in the mechanisms supporting classical intravenous nicotine self-administration (0.04 mg/kg/infusion) in male outbred Sprague-Dawley rats. Our analysis identified two clusters: one subset of rats sought nicotine primarily for its reinforcing effects, while the second subset sought nicotine to enhance the reinforcing effects of the discrete cue. Varenicline (1 mg/kg i.p.) reduced seeking behavior in the former group, whereas it tended to increase in the latter group. Crucially, despite this fundamental qualitative difference revealed by behavioral manipulation, the two clusters exhibited quantitatively identical nicotine+cue self-administration behavior. The traditional application of rodent models to study the reinforcing and addictive effects of nicotine may mask individual variability in the underlying motivational mechanisms. Accounting for this variability could significantly enhance the predictive validity of translational research.

Factors modulating the incubation of drug and non-drug craving and their clinical implications

It was suggested in 1986 that cue-induced cocaine craving increases progressively during early abstinence and remains high during extended periods of time. Clinical evidence now supports this hypothesis and that this increase is not specific to cocaine but rather generalize across several drugs of abuse. Investigators have identified an analogous incubation phenomenon in rodents, in which time-dependent increases in cue-induced drug seeking are observed after abstinence from intravenous drug or palatable food self-administration. Incubation of craving is susceptible to variation in magnitude as a function of biological and/or the environmental circumstances surrounding the individual. During the last decade, the neurobiological correlates of the modulatory role of biological (sex, age, genetic factors) and environmental factors (environmental enrichment and physical exercise, sleep architecture, acute and chronic stress, abstinence reinforcement procedures) on incubation of drug craving has been investigated. In this review, we summarized the behavioral procedures adopted, the key underlying neurobiological correlates and clinical implications of these studies.

An opposing role for prelimbic cortical projections to the nucleus accumbens core in incubation of craving for cocaine versus water

BACKGROUND

Both drug and natural reward-seeking have been shown to increase following an extended period of abstinence, a phenomenon termed 'incubation of craving'. Although this phenomenon involves many brain regions, the projections from the prelimbic cortex (PrL) to the nucleus accumbens (NAc) core have been strongly implicated in incubation of cocaine-seeking. However, this circuit has not been investigated in the context of incubation of craving for natural rewards.

METHODS

Male Long Evans rats were trained to self-administer cocaine or water/saline 6 h/d for 14 days and subsequently entered 1 month of experimenter-imposed abstinence. Rats then underwent an optogenetic stimulation protocol used to induce long term depression in the PrL terminals to the NAc core immediately before beginning an extinction test used to assess incubation of craving.

RESULTS

Control cocaine rats showed heightened drug-seeking on day 30 when compared to day 1 of abstinence, demonstrating incubation of craving. Notably, optogenetic stimulation of the PrL to NAc core pathway blocked this behavior in cocaine rats. In contrast, optogenetic stimulation of the PrL to NAc core pathway induced incubation of craving in water/saline rats.

CONCLUSIONS

These findings suggest that neuroadaptations in the PrL to NAc core pathway play opposing roles in the incubation of craving for cocaine versus water.

Flavor additives facilitate oral self-administration of nicotine solution in mice

RATIONALE

Tobacco products are very addictive, partly because they contain nicotine which is reinforcing, but also because they include appealing aromas and tastes. Flavor additives are such sensory stimuli which enhance attractiveness, as well as use and abuse of tobacco and vaping products. Yet, the interaction between these flavor additives and nicotine remains poorly understood.

OBJECTIVES

We want to understand how flavors may reduce nicotine' aversive taste and how it may enhance its voluntary oral self-administration in mice.

METHODS

We first studied the effect of flavor additives on nicotine solution palatability in a free bottle choice paradigm. Second, we investigated the effect of vanilla flavoring on the different stages of nicotine (40 μg/ml) oral self-administration in mice.

RESULTS

We show that adding flavors increase nicotine palatability and facilitate acquisition and maintenance of oral self-administration when compared to nicotine-alone group. Mice adapt their operant behavior depending on changes in nicotine concentration. All mice reinstate nicotine seeking upon presentation of associated cues. Nevertheless, vanilla-flavored nicotine was not more reinforcing than vanilla-flavored water which was reinforcing enough to drive similar operant response rates.

CONCLUSIONS

Flavor additives increase nicotine oral consumption and help maintaining operant behavior in mice. Moreover, flavors can be very attractive and can have high reinforcing value by themselves. Thus, it is crucial that the investigation on how taste signals play an important role in modulating oral nicotine intake in rodent models remains explored.

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.

Incubation of food craving in rats: A review

Incubation of food craving is an abstinence-dependent increase in responding for reward-paired cues. Incubation of craving was first reported for rats responding for cocaine-paired cues, and later generalized to several drugs of abuse and for food. Incubation of drug and food craving has been reported in clinical studies as well. Incubation of food craving by rats has been reported for standard chow as well as for high fat and sucrose reinforcers. Parametric and other evaluations of the incubation of food craving reveal manipulations that reduce incubation, including environmental enrichment and pharmacological manipulation of dopamine, glutamate, and endogenous opiates. Several brain regions are likely involved in the effect, including mesolimbic terminals and the central nucleus of the amygdala. Further study of the incubation of food craving could facilitate development of treatments for cravings that precede relapse characteristic of drug and food addictions.

Not all smokers appear to seek nicotine for the same reasons: implications for preclinical research in nicotine dependence

Tobacco use leads to 6 million deaths every year due to severe long-lasting diseases. The main component of tobacco, nicotine, is recognized as one of the most addictive drugs, making smoking cessation difficult, even when 70 percent of smokers wish to do so. Clinical and preclinical studies have demonstrated consistently that nicotine seeking is a complex behavior involving various psychopharmacological mechanisms. Evidence supports that the population of smokers is heterogeneous, particularly as regards the breadth of motives that determine the urge to smoke. Here, we review converging psychological, genetic and neurobiological data from clinical and preclinical studies supporting that the mechanisms controlling nicotine seeking may vary from individual to individual. It appears timely that basic neuroscience integrates this heterogeneity to refine our understanding of the neurobiology of nicotine seeking, as tremendous progress has been made in modeling the various psychopharmacological mechanisms driving nicotine seeking in rodents. For a better understanding of the mechanisms that drive nicotine seeking, we emphasize the need for individual-based research strategies in which nicotine seeking, and eventually treatment efficacy, are determined while taking into account individual variations in the mechanisms of nicotine seeking.

Peri-adolescent drinking of ethanol and/or nicotine modulates astroglial glutamate transporters and metabotropic glutamate receptor-1 in female alcohol-preferring rats

Impairment in glutamate neurotransmission mediates the development of dependence upon nicotine (NIC) and ethanol (EtOH). Previous work indicates that continuous access to EtOH or phasic exposure to NIC reduces expression of the glutamate transporter-1 (GLT-1) and cystine/glutamate antiporter (xCT) but not the glutamate/aspartate transporter (GLAST). Additionally, metabotropic glutamate receptors (mGluRs) expression was affected following exposure to EtOH or NIC. However, little is known about the effects of EtOH and NIC co-consumption on GLT-1, xCT, GLAST, and mGluR1 expression. In this study, peri-adolescent female alcohol preferring (P) rats were given binge-like access to water, sucrose (SUC), SUC-NIC, EtOH, or EtOH-NIC for four weeks. The present study determined the effects of these reinforcers on GLT-1, xCT, GLAST, and mGluR1 expression in the nucleus accumbens (NAc), hippocampus (HIP) and prefrontal cortex (PFC). GLT-1 and xCT expression were decreased in the NAc following both SUC-NIC and EtOH-NIC. In addition, only xCT expression was downregulated in the HIP in both of these latter groups. Also, glutathione peroxidase (GPx) activity in the HIP was reduced following SUC, SUC-NIC, EtOH, and EtOH-NIC consumption. Similar to previous work, GLAST expression was not altered in any brain region by any of the reinforcers. However, mGluR1 expression was increased in the NAc in the SUC-NIC, EtOH, and EtOH-NIC groups. These results indicate that peri-adolescent binge-like drinking of EtOH or SUC with or without NIC may exert differential effects on astroglial glutamate transporters and receptors. Our data further parallel some of the previous findings observed in adult rats.

Reinforcement enhancement by nicotine in adult rats: behavioral selectivity and relation to mode of delivery and blood nicotine levels

RATIONALE

Reinforcement-enhancing effects of nicotine occur in human subjects and laboratory rats. However, the doses used in animal studies typically exceed smoking-associated levels of exposure, and generalized behavioral activation by nicotine can potentially confound data interpretation.

METHODS

During daily 60-min sessions, male adult rats pressed an "active" lever to illuminate a brief cue light. Pressing on either the active or inactive lever retracted both levers for 60 s. Nicotine (0.025-0.2 mg/kg) was given either by continuous intravenous (IV) infusion, or spaced IV pulses (3-s or 30-s/pulse), or pre-session subcutaneous (SC) injection.

RESULTS

Almost all rats responded preferentially for the cue light for several weeks. After several home-cage nicotine injections, reinforcement enhancement occurred even within the first nicotine test session. Nicotine increased active lever responding without altering inactive lever responding, with effects reliably observed at doses as low as 0.1 mg/kg SC or 0.1 mg/kg/session IV. Within the session, the 0.1 mg/kg dose maximally increased active lever responding by 2-3-fold, coinciding with serum levels of 25 ng/ml. Intravenous nicotine (tested at 0.1 mg/kg/60-min session) was equally effective whether delivered by continuous infusion or in a series of equally spaced 0.003 mg/kg pulses each of 3-s or 30-s duration.

CONCLUSIONS

Low doses of nicotine can potentiate responding for a primary sensory reinforcer without producing a generalized increase in lever pressing. Reinforcer enhancement by nicotine generalized to several modes of drug delivery, appeared to track circulating levels of drug, and occurred even at serum levels within the daytime range of moderate cigarette smokers.

Can We Selectively Reduce Appetite for Energy-Dense Foods? An Overview of Pharmacological Strategies for Modification of Food Preference Behavior

Excessive intake of food, especially palatable and energy-dense carbohydrates and fats, is largely responsible for the growing incidence of obesity worldwide. Although there are a number of candidate antiobesity drugs, only a few of them have been proven able to inhibit appetite for palatable foods without the concurrent reduction in regular food consumption. In this review, we discuss the interrelationships between homeostatic and hedonic food intake control mechanisms in promoting overeating with palatable foods and assess the potential usefulness of systemically administered pharmaceuticals that impinge on the endogenous cannabinoid, opioid, aminergic, cholinergic, and peptidergic systems in the modification of food preference behavior. Also, certain dietary supplements with the potency to reduce specifically palatable food intake are presented. Based on human and animal studies, we indicate the most promising therapies and agents that influence the effectiveness of appetite-modifying drugs. It should be stressed, however, that most of the data included in our review come from preclinical studies; therefore, further investigations aimed at confirming the effectiveness and safety of the aforementioned medications in the treatment of obese humans are necessary.

Recent studies of the effects of sugars on brain systems involved in energy balance and reward: Relevance to low calorie sweeteners

The alarmingly high rates of overweight and obesity pose a serious global health threat. Numerous factors can result in weight gain, one of which is excess consumption of caloric sweeteners. In an effort to aid weight loss efforts, many people have switched from caloric sweeteners to low calorie sweeteners, which provide sweet taste without the accompanying calories. In this review, we present an overview of the animal literature produced in the last 5years highlighting the effects of sugar consumption on neural pathways involved in energy balance regulation and reward processing. We also examine the latest evidence that is beginning to elucidate the effects of low calorie sweeteners on these neural pathways, as well as how homeostatic and hedonic systems interact in response to, or to influence, sugar consumption.

The effects of nicotine self-administration and withdrawal on concurrently available chow and sucrose intake in adult male rats

Carbohydrate intake, preference, and taste thresholds may be altered in current and former cigarette smokers, which may mediate weight gain and risk for obesity in individuals who quit smoking. Attempts to model these effects in rodents have primarily used noncontingent nicotine administration. The purpose of this research was to characterize changes in chow and sucrose intake in rats during a 23-h access model of i.v. nicotine self-administration (NSA), in which rats lever-pressed for chow, sucrose, and nicotine under concurrent fixed-ratio (FR) 1 schedules. Male rats were assigned to one of three groups that differed in food and drug availability. The Nicotine C+S group had concurrent access to nicotine, chow, and sucrose. The Saline C+S group had access to saline, chow, and sucrose. The Nicotine C-Only group had access to nicotine and chow, but not sucrose. Changes in food intake and weight gain were assessed during baseline, NSA, and nicotine withdrawal (i.e., saline extinction). Weight gain was significantly slowed during NSA and increased during withdrawal, but did not differ between the nicotine groups. NSA produced a significant decrease in both chow and sucrose intake. Gradual tolerance to nicotine's effects on sucrose, but not chow intake, occurred. During withdrawal, chow and sucrose intake increased, with a larger percent increase in sucrose intake compared to chow. The proportion of total food intake from sucrose was greater at the end of withdrawal compared to baseline, indicating a history of nicotine intake changed dietary preference. Combined, these results indicate that sucrose intake is more resistant to nicotine's appetite suppressant effects and withdrawal from nicotine produces a greater increase in sweet food intake alongside general increases in chow intake. Changes in overall food intake in current and ex-smokers may lead to increased risk for obesity and other health problems, potentially limiting the benefit of quitting smoking.

Advances in nicotine research in Addiction Biology

The aim of Addiction Biology is to advance our understanding of the action of drugs of abuse and addictive processes via the publication of high-impact clinical and pre-clinical findings resulting from behavioral, molecular, genetic, biochemical, neurobiological and pharmacological research. As of 2013, Addiction Biology is ranked number 1 in the category of Substance Abuse journals (SCI). Occasionally, Addiction Biology likes to highlight via review important findings focused on a particular topic and recently published in the journal. The current review summarizes a number of key publications from Addiction Biology that have contributed to the current knowledge of nicotine research, comprising a wide spectrum of approaches, both clinical and pre-clinical, at the cellular, molecular, systems and behavioral levels. A number of findings from human studies have identified, using imaging techniques, alterations in common brain circuits, as well as morphological and network activity changes, associated with tobacco use. Furthermore, both clinical and pre-clinical studies have characterized a number of mechanistic targets critical to understanding the effects of nicotine and tobacco addiction. Together, these findings will undoubtedly drive future studies examining the dramatic impact of tobacco use and the development of treatments to counter nicotine dependence.

Role of cues and contexts on drug-seeking behaviour

Environmental stimuli are powerful mediators of craving and relapse in substance-abuse disorders. This review examined how animal models have been used to investigate the cognitive mechanisms through which cues are able to affect drug-seeking behaviour. We address how animal models can describe the way drug-associated cues come to facilitate the development and persistence of drug taking, as well as how these cues are critical to the tendency to relapse that characterizes substance-abuse disorders. Drug-associated cues acquire properties of conditioned reinforcement, incentive motivation and discriminative control, which allow them to influence drug-seeking behaviour. Using these models, researchers have been able to investigate the pharmacology subserving the behavioural impact of environmental stimuli, some of which we highlight. Subsequently, we examine whether the impact of drug-associated stimuli can be attenuated via a process of extinction, and how this question is addressed in the laboratory. We discuss how preclinical research has been translated into behavioural therapies targeting substance abuse, as well as highlight potential developments to therapies that might produce more enduring changes in behaviour.

Detrimental effects of acute nicotine on the response-withholding performance of spontaneously hypertensive and Wistar Kyoto rats

RATIONALE

Attention-deficit hyperactivity disorder (ADHD) is associated with a higher prevalence of smoking, which may be related to potential therapeutic effects of nicotine on ADHD symptoms. Whereas nicotine offers robust improvements in sustained attention, the effects of nicotine on impulsivity are unclear.

OBJECTIVES

The present study examined the effects of nicotine on the response inhibition capacity of spontaneously hypertensive rats (SHR), an animal model of ADHD, compared to that of a normotensive control Wistar Kyoto (WKY), using the fixed minimum interval (FMI) schedule of reinforcement.

METHODS

Tests were conducted following acute injections of subcutaneous nicotine (0.1-0.6 mg/kg). On each FMI trial, the first lever press initiated an inter-response time (IRT); a head entry into a food receptacle terminated the IRT. IRTs longer than 6 s were intermittently reinforced with sucrose.

RESULTS

A model that assumes that only a proportion of IRTs are sensitive to the timing contingencies of the FMI provided a close fit to the data, regardless of strain or treatment. No baseline difference in FMI performance was observed between SHR and WKY. Nicotine reduced the duration of timed IRTs and the duration of latencies to the IRT-initiating lever press similarly for both strains. Nicotine dose-dependently increased the proportion of timed IRTs; the dose-response curve was shifted leftwards in SHR relative to WKY.

CONCLUSIONS

These results suggest that nicotine (a) reduces response-inhibition capacity, (b) enhances the reinforcing efficacy of sucrose, and (c) dose-dependently enhances attention-like sensitivity to contingencies of reinforcement, through mechanisms that are yet unknown.

Factors associated with weight changes in successful quitters participating in a smoking cessation program

OBJECTIVE

To identify possible predictors of post-cessation weight gain in smoking abstainers.

PATIENTS AND METHODS

A sample of 607 successful abstainers seen at the Centre for Tobacco-Dependent in Prague, Czech Republic, between 2005 and 2010, was included in this analysis. This sample was followed up for 1year and included 47.9% women (N=291) with the mean age of 48years (18-85).

FINDINGS

Post-cessation weight gain occurred in 88.6% of the 607 abstainers. The mean weight gain after one year post-quit was 5.1kg (95% confidence interval 4.7-5.5kg). Baseline characteristics associated with increased weight gain included a higher baseline smoking rate (p<0.001), more severe cigarette dependence (p=0.003), less physical activity (p=0.008), and a report of increased appetite on the baseline assessment of withdrawal symptoms (p<0.001).

CONCLUSIONS

Smokers who are more dependent and have minimal physical activity are at increased risk for post-cessation weight gain. For these smokers, incorporating interventions targeting the weight issue into tobacco dependence treatment is recommended. Further research should be done to identify reasons for this important quitting complication.

Incubation of saccharin craving and within-session changes in responding for a cue previously associated with saccharin

Time-dependent increases in cue-induced sucrose seeking after forced abstinence have been described in rats with a history of sucrose self-administration, suggesting sucrose craving "incubates". In the present study, we examined whether the incubation of craving generalizes to the artificial sweetener, saccharin. Thirty-one male Long-Evans rats lever pressed for 0.3% saccharin solution 1h/day for 10 days. On either Day 1 or 30 of forced abstinence, rats responded for 1h for presentation of a tone+light cue previously presented with every saccharin delivery during self-administration training. Rats responded more during this cue-reactivity test session following 30 vs. 1 day of forced abstinence ("incubation of craving"). This result is the first demonstration of the "incubation of saccharin craving" and suggests that a post-ingestive caloric consequence of self-administration is not a necessary condition for the development of incubation of sucrose craving. We also examined the time course (within-session decreases) of active-lever responding during the 1-h cue-reactivity test session. Rats in the Day 30 group responded more than rats in the Day 1 group from the beginning of the test session. In addition, within-session decreases in responding were shallower in slope in the Day 30 than the Day 1 group. These results indicate that "incubation of saccharin craving" enhances the persistence of seeking behavior.