The neuropharmacology of relapse to food seeking: methodology, main findings, and comparison with relapse to drug seeking

Binge eating, overeating and food addiction: Approaches for examining food overconsumption in laboratory rodents

Overeating ranges in severity from casual overindulgence to an overwhelming drive to consume certain foods. At its most extreme, overeating can manifest as clinical diagnoses such as binge eating disorder or bulimia nervosa, yet subclinical forms of overeating such as emotional eating or uncontrolled eating can still have a profoundly negative impact on health and wellbeing. Although rodent models cannot possibly capture the full spectrum of disordered overeating, studies in laboratory rodents have substantially progressed our understanding of the neurobiology of overconsumption. These experimental approaches range from simple food-exposure protocols that promote binge-like eating and the development of obesity, to more complex operant procedures designed to examine distinct 'addiction-like' endophenotypes for food. This review provides an overview of these experimental approaches, with the view to providing a comprehensive resource for preclinical investigators seeking to utilize behavioural models for studying the neural systems involved in food overconsumption.

Longitudinal study of energy, neurosensory and eating responses durinG pregnancY (ENERGY cohort): A study protocol

BACKGROUND AND AIMS

Physiological changes that occur during pregnancy can have long-term impacts on metabolism and neurosensory responses to food, which can impact nutrition and health outcomes. The ENERGY cohort is a longitudinal study that aims to capitalizes on pregnancy as a natural model of metabolic reprogramming in order to understand the neurosensory mechanisms underpinning links between metabolism and dietary behaviour. The study objectives are to test for multi-sensory shifts during pregnancy, and the effect of sensory changes on dietary choices and bodyweights, and to identify neurosensory mechanisms that determine macronutrient selection before and after pregnancy.

METHODS

A longitudinal cohort study involving 130 pregravid women planning to conceive with the next 12-months and 65 pregravid women with no short-term plans to conceive. Participants will be recruited from Dunedin and Auckland, New Zealand. The study will test for changes in diet, neurosensory outcomes, and metabolism across the reproductive cycle, from pre-pregnancy to 1-year post-pregnancy. Data will be collected at six timepoint throughout the pregnancy which will occur approximately every 3 months. The primary response variables will be changes in supra-threshold sensitivity across modalities, dietary intake, and metabolism between pre-pregnancy and post-pregnancy. Longitudinal data analysis will use linear mixed models to assess changes in the response outcomes over time adjusted for age, ethnicity, and socioeconomic status.

DISCUSSION

Understanding the relationship between metabolism, sensory processing, and macronutrient preferences will provide crucial insights into diet-related health issues, including obesity. This study will lead to the formation of a prospective research cohort that is unique to Aotearoa New Zealand, and will develop multidisciplinary skills that are increasingly necessary to addressing the obesity epidemic.

Incubation of palatable food craving is associated with brain-wide neuronal activation in mice

Studies using rodent models have shown that relapse to drug or food seeking increases progressively during abstinence, a behavioral phenomenon termed "incubation of craving." Mechanistic studies of incubation of craving have focused on specific neurobiological targets within preselected brain areas. Recent methodological advances in whole-brain immunohistochemistry, clearing, and imaging now allow unbiased brain-wide cellular resolution mapping of regions and circuits engaged during learned behaviors. However, these whole-brain imaging approaches were developed for mouse brains, while incubation of drug craving has primarily been studied in rats, and incubation of food craving has not been demonstrated in mice. Here, we established a mouse model of incubation of palatable food craving and examined food reward seeking after 1, 15, and 60 abstinence days. We then used the neuronal activity marker Fos with intact-brain mapping procedures to identify corresponding patterns of brain-wide activation. Relapse to food seeking was significantly higher after 60 abstinence days than after 1 or 15 days. Using unbiased ClearMap analysis, we identified increased activation of multiple brain regions, particularly corticostriatal structures, following 60 but not 1 or 15 abstinence days. We used orthogonal SMART2 analysis to confirm these findings within corticostriatal and thalamocortical subvolumes and applied expert-guided registration to investigate subdivision and layer-specific activation patterns. Overall, we 1) identified brain-wide activity patterns during incubation of food seeking using complementary analytical approaches and 2) provide a single-cell resolution whole-brain atlas that can be used to identify functional networks and global architecture underlying the incubation of food craving.

Opposite effects of stress on effortful motivation in high and low anxiety are mediated by CRHR1 in the VTA

Individuals frequently differ in their behavioral and cognitive responses to stress. However, whether motivation is differently affected by acute stress in different individuals remains to be established. By exploiting natural variation in trait anxiety in outbred Wistar rats, we show that acute stress facilitates effort-related motivation in low anxious animals, while dampening effort in high anxious ones. This model allowed us to address the mechanisms underlying acute stress-induced differences in motivated behavior. We show that CRHR1 expression levels in dopamine neurons of the ventral tegmental area (VTA)-a neuronal type implicated in the regulation of motivation-depend on animals' anxiety, and these differences in CRHR1 expression levels explain the divergent effects of stress on both effortful behavior and the functioning of mesolimbic DA neurons. These findings highlight CRHR1 in VTA DA neurons-whose levels vary with individuals' anxiety-as a switching mechanism determining whether acute stress facilitates or dampens motivation.

Binge eating and psychostimulant addiction

Many of the various factors, characteristics, and variables involved in the addictive process can determine an individual’s vulnerability to develop drug addiction. Hedonic eating, based on pleasure rather than energy needs, modulates the same reward circuits, as do drugs of abuse. According to the last report of the World Health Organization, the worldwide obesity rate has more than doubled since 1980, reaching especially critical levels in children and young people, who are overexposed to high-fat, high-sugar, energy-dense foods. Over the past few decades, there has been an increase in the number of studies focused on how eating disorders can lead to the development of drug addiction and on the comorbidity that exists between the two disorders. Herein, we review the most recent research on the subject, focusing especially on animal models of binge eating disorders and drug addiction. The complex profile of patients with substance use and binge eating disorders requires an integrated response to dually diagnosed patients. Nutritional patterns should be considered an important variable in the treatment of substance use disorders, and future studies need to focus on specific treatments and interventions in individuals who show a special vulnerability to shift from one addiction to the other.

Recovery-from-extinction effects in an anuran amphibian: renewal effect, but no reinstatement

Recovery-from-extinction effects in which a conditioned response returns after extinction have been shown in mammals, birds and fish. Thus, these effects appear to be conserved among vertebrates; however, they have yet to be investigated in amphibians. Using prey catching conditioning in the fire-bellied toad (Bombina orientalis), we tested if renewal and reinstatement occurred after extinction when subjects were respectively re-exposed to the context or reinforcer used during conditioning. For renewal, a different context was used during extinction and thus renewal tests assessed if external contextual cues associated during conditioning stimulated prey catching performance. For reinstatement, the reinforcer withheld during extinction was simply delivered again prior to a test assessing if internal cues associated with recent prey consumption stimulated prey catching performance. Conditioning followed a fixed ratio 5 schedule of reinforcement, where five attempts to capture a cricket stimulus displayed on a computer screen were reinforced by delivery of a single live cricket. Performance was measured as the time to reach five prey catching attempts. A significant improvement in prey catching performance during conditioning followed by deterioration with extinction was seen in the experiments. Upon return to the context used for conditioning after extinction, toads showed a renewal effect whereby they displayed faster performance during testing compared to the end of extinction. Conversely, toads showed no reinstatement effect because pre-feeding of a cricket did not influence performance during the test that followed extinction. Reinstatement could have been lost in amphibian phylogeny due to secondary simplification of the nervous system.

Low-dose polypharmacology targeting dopamine D1 and D3 receptors reduces cue-induced relapse to heroin seeking in rats

Chemical compounds that target dopamine (DA) D1 or D3 receptors have shown promise as potential interventions in animal models of cue-induced relapse. However, undesirable side effects or pharmacodynamic profiles have limited the advancement of new compounds in preclinical studies when administered as independent treatments. In this series of experiments, we explored the effects of coadministration of a D1-receptor partial agonist (SKF 77434) and a D3-receptor antagonist (NGB 2904) in heroin-seeking rats within a "conflict" model of abstinence and cue-induced relapse. Rats were first trained to press a lever to self-administer heroin, and drug delivery was paired contingently with cues (e.g., light and pump noise). Self-initiated abstinence was facilitated by applying electrical current to the flooring in front of the levers. Lastly, a relapse response was provoked by noncontingent presentation of conditioned cues. Prior to provocation, rats received a systemic injection of SKF 77434, NGB 2904, or a combination of both compounds to assess treatment effects on lever pressing. Results indicated that the coadministration of low (i.e., independently ineffective) doses of both compounds was more effective in reducing cue-induced relapse to heroin seeking than either compound alone, with some evidence of drug synergism. Follow-up studies indicated that this reduction was not due to motoric impairment nor enhanced sensitivity to the electrified flooring and that this treatment did not significantly affect motivation for food. Implications for the treatment of opiate use disorder and recommendations for further research are discussed.

Input-specific modulation of murine nucleus accumbens differentially regulates hedonic feeding

Hedonic feeding is driven by the "pleasure" derived from consuming palatable food and occurs in the absence of metabolic need. It plays a critical role in the excessive feeding that underlies obesity. Compared to other pathological motivated behaviors, little is known about the neural circuit mechanisms mediating excessive hedonic feeding. Here, we show that modulation of prefrontal cortex (PFC) and anterior paraventricular thalamus (aPVT) excitatory inputs to the nucleus accumbens (NAc), a key node of reward circuitry, has opposing effects on high fat intake in mice. Prolonged high fat intake leads to input- and cell type-specific changes in synaptic strength. Modifying synaptic strength via plasticity protocols, either in an input-specific optogenetic or non-specific electrical manner, causes sustained changes in high fat intake. These results demonstrate that input-specific NAc circuit adaptations occur with repeated exposure to a potent natural reward and suggest that neuromodulatory interventions may be therapeutically useful for individuals with pathologic hedonic feeding.

Effect of chemogenetic inhibition of lateral habenula neuronal activity on cocaine- and food-seeking behaviors in the rat

A major problem in the treatment of cocaine addiction is high rates of relapse. Relapse is often provoked by acute reexposure to cocaine-associated cues or to cocaine itself. The lateral habenula (LHb), an epithalamic nucleus, regulates midbrain dopaminergic systems that are known to be involved in cocaine taking and seeking behaviors. However, the role of this nucleus in cocaine self-administration and reinstatement of cocaine seeking has not been entirely parsed out. We used an operant self-administration and reinstatement procedure to explore the effect of Designer Receptors Exclusively Activated by Designer Drug (DREADD)-induced transient inhibition of LHb neurons on cocaine taking and seeking. Firstly, rats were injected with adeno-associated viral vectors expressing hM4 Di (a Gi/o -coupled DREADD) into the LHb, trained to self-administer cocaine (0.75 mg/kg/infusion), and the effect of clozapine-N-oxide (an inert ligand that activates DREADDs) was assessed on cocaine self-administration. Secondly, rats were injected with hM4 Di into the LHb, trained to self-administer cocaine; the operant response was extinguished, and cue- and cocaine priming-induced reinstatement was assessed. Thirdly, we tested the generality of the effect of inhibiting LHb neurons by assessing the effect of this manipulation on food-taking and seeking. hM4 Di -induced inhibition of LHb neurons increased cocaine but not food self-administration. In contrast, this manipulation decreased reinstatement of cocaine, but not food-seeking. Taken together, our data suggest that hM4 Di - induced LHb inhibition specifically mediates taking and seeking behaviors reinforced by cocaine but not by natural reinforcers. Further, our data indicate a dissociation in the role of LHb neurons on cocaine self-administration versus reinstatement of cocaine seeking.

The tubular striatum and nucleus accumbens distinctly represent reward-taking and reward-seeking

The ventral striatum regulates motivated behaviors that are essential for survival. The ventral striatum contains both the nucleus accumbens (NAc), which is well established to contribute to motivated behavior, and the adjacent tubular striatum (TuS), which is poorly understood in this context. We reasoned that these ventral striatal subregions may be uniquely specialized in their neural representation of goal-directed behavior. To test this, we simultaneously examined TuS and NAc single-unit activity as male mice engaged in a sucrose self-administration task, which included extinction and cue-induced reinstatement sessions. Although background levels of activity were comparable between regions, more TuS neurons were recruited upon reward-taking, and among recruited neurons, TuS neurons displayed greater changes in their firing during reward-taking and extinction than those in the NAc. Conversely, NAc neurons displayed greater changes in their firing during cue-reinstated reward-seeking. Interestingly, at least in the context of this behavioral paradigm, TuS neural activity predicted reward-seeking, whereas NAc activity did not. Together, by directly comparing their dynamics in several behavioral contexts, this work reveals that the NAc and TuS ventral striatum subregions distinctly represent reward-taking and reward-seeking.NEW & NOTEWORTHY The ventral striatum, considered the reward circuitry "hub," is composed of two regions: the NAc, which is well established for its role in reward processing, and the TuS, which has been largely excluded from such studies. This study provides a first step in directly contextualizing the TuS's activity in relation to that in the NAc and, by doing so, establishes a critical framework for future research seeking to better understand the brain basis for drug addiction.

The neurobiology of abstinence-induced reward-seeking in males and females

Drugs of abuse and highly palatable foods (e.g. high fat or sweet foods) have powerful reinforcing effects, which can lead to compulsive and addictive drives to ingest these substances to the point of psychopathology and self-harm--specifically the development of Substance Use Disorder (SUD) and obesity. Both SUD and binge-like overeating can be defined as disorders in which the salience of the reward (food or drug) becomes exaggerated relative to, and at the expense of, other rewards that promote well-being. A major roadblock in the treatment of these disorders is high rates of relapse after periods of abstinence. It is common, although not universal, for cue-induced craving to increase over time with abstinence, often triggered by cues previously paired with the reinforcing substance. Accumulating evidence suggests that similar neural circuits and cellular mechanisms contribute to abstinence-induced and cue-triggered seeking of drugs and palatable food. Although much research has focused on the important role of corticolimbic circuitry in drug-seeking, our goal is to expand focus to the more recently explored hypothalamic-thalamic-striatal circuitry. Specifically, we review how connections, and neurotransmitters therein, among the lateral hypothalamus, paraventricular nucleus of the thalamus, and the nucleus accumbens contribute to abstinence-induced opioid- and (high fat or sweet) food-seeking. Given that biological sex and gonadal hormones have been implicated in addictive behavior across species, another layer to this review is to compare behaviors and neural circuit-based mechanisms of abstinence-induced opioid- or food-seeking between males and females when such data is available.

Blockade of Orexin Receptors in the Posterior Paraventricular Nucleus of the Thalamus Prevents Stress-Induced Reinstatement of Reward-Seeking Behavior in Rats With a History of Ethanol Dependence

Neural systems involved in processing natural rewards and drugs of abuse overlap and exposure to drugs of abuse induce neuroadaptations that can cause compulsive-like behavior. For example, the recruitment of the orexin (Orx) system by drugs of abuse has been proposed to induce neuroadaptations that in turn alter its function, reflected by maladaptive, compulsive, and addictive behavior. Orexin neurons project to the paraventricular nucleus of the thalamus (PVT)—particularly the posterior part (pPVT), a structure that plays a key role in stress regulation. This study investigated whether Orx transmission in the pPVT plays a role in stress-induced reinstatement of reward-seeking behavior toward ethanol (EtOH) and a highly palatable food reward [sweetened condensed milk (SCM)] in rats and whether this role changes with EtOH dependence. After being trained to orally self-administer EtOH or SCM, the rats were made dependent (EtOH_D and SCM_D) by chronic intermittent EtOH vapor exposure. The control nondependent groups (EtOH_ND and SCM_ND) were exposed to air. Following extinction, the rats were tested for stress-induced reinstatement of EtOH- and SCM-seeking behavior. Stress reinstated EtOH- and SCM-seeking behavior in all groups (EtOH_D/ND and SCM_D/ND). Administration of the dual Orx receptor (OrxR) antagonist TCS1102 (15 μg) in the pPVT prevented stress-induced reinstatement only in dependent rats (EtOH_D and SCM_D). In parallel, the qPCR analysis showed that Orx mRNA expression in the hypothalamus and OrxR1/R2 mRNA expression in the pPVT were increased at the time of testing in the EtOH_D and SCM_D groups. These results are the first to implicate Orx transmission in the pPVT in the stress-induced reinstatement of reward-seeking behavior in EtOH dependent rats and indicate the maladaptive recruitment of Orx transmission in the pPVT by EtOH dependence.

Vulnerability to diet-induced obesity is associated with greater food priming-induced reinstatement of palatable food seeking

We examined whether individual differences in weight gain during exposure to a "junk-food" diet were related to differences in later relapse-like behavior in a rat model. Following free access to a junk-food diet for 7 weeks, rats were trained to press a lever for palatable food pellets. Following extinction training, rats were tested for cue- and pellet priming-induced reinstatement. Results showed that rats prone to obesity while on the junk-food diet displayed greater pellet priming-, but not cue-, induced reinstatement relative to obesity-resistant rats, suggesting that obesity vulnerability is a factor determining one's chances for some types of relapse.

Persistent effects of obesity: a neuroplasticity hypothesis

The obesity epidemic is a leading cause of health problems in the United States, increasing the risk of cardiovascular, endocrine, and psychiatric diseases. Although many people lose weight through changes in diet and lifestyle, keeping the weight off remains a challenge. Here, we discuss a hypothesis that seeks to explain why obesity is so persistent. There is a great degree of overlap in the circuits implicated in substance use disorder and obesity, and neural plasticity of these circuits in response to drugs of abuse is well documented. We hypothesize that obesity is also associated with neural plasticity in these circuits, and this may underlie persistent changes in behavior, energy balance, and body weight. Here, we discuss how obesity-associated reductions in motivation and physical activity may be rooted in neurophysiological alterations in these circuits. Such plasticity may alter how humans and animals use, expend, and store energy, even after weight loss.

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.

Role of impulsivity and reward in the anti-obesity actions of 5-HT2C receptor agonists

The selective 5-HT_2C receptor agonist lorcaserin entered clinical obesity trials with the prevalent view that satiety was a primary mechanism of action. Subsequent Phase II and III trials demonstrated efficacy in terms of weight loss, although the overall effect size (~3% placebo-corrected change) is considered modest. Lorcaserin has been approved by the FDA for the treatment of obesity with lifestyle modification, but since its introduction in 2013 its sales are in decline, probably due to its overall modest effect. However, in some individuals, lorcaserin has a much more clinically significant effect (i.e. >10% placebo-corrected change), although what common features, if any, define these high responders is presently unknown. In the present article we highlight the evidence that alternative mechanisms to satiety may contribute to the anti-obesity effect of lorcaserin, namely effects on constructs of primary and conditioned reward and impulsivity. This may better inform the clinical evaluation of lorcaserin (and any future 5-HT_2C receptor agonists) to subgroups of obese subjects characterized by overeating due to maladaptive impulsivity and reward mechanisms. One such population might be individuals diagnosed with binge eating disorder.

Sugar addiction: is it real? A narrative review

In animal studies, sugar has been found to produce more symptoms than is required to be considered an addictive substance. Animal data has shown significant overlap between the consumption of added sugars and drug-like effects, including bingeing, craving, tolerance, withdrawal, cross-sensitisation, cross-tolerance, cross-dependence, reward and opioid effects. Sugar addiction seems to be dependence to the natural endogenous opioids that get released upon sugar intake. In both animals and humans, the evidence in the literature shows substantial parallels and overlap between drugs of abuse and sugar, from the standpoint of brain neurochemistry as well as behaviour.

Pathological Overeating: Emerging Evidence for a Compulsivity Construct

Compulsive eating behavior is a transdiagnostic construct that is characteristic of medical and psychiatric conditions such as forms of obesity and eating disorders. Although feeding research is moving toward a better understanding of the proposed addictive properties of food, the components and the mechanisms contributing to compulsive eating are not yet clearly defined or understood. Current understanding highlights three elements of compulsive behavior as it applies to pathological overeating: (1) habitual overeating; (2) overeating to relieve a negative emotional state; and (3) overeating despite aversive consequences. These elements emerge through mechanisms involving pathological habit formation through an aberrant learning process, the emergence of a negative emotional state, and dysfunctions in behavioral control. Dysfunctions in systems within neurocircuitries that comprise the basal ganglia, the extended amygdala, and the prefrontal cortex result in compulsive eating behaviors. Here, we present evidence to relate compulsive eating behavior and addiction and to characterize their underlying neurobiological mechanisms. A major need to improve understanding of compulsive eating through the integration of complex motivational, emotional, and cognitive constructs is warranted.

Dissociable effects of systemic and orbitofrontal administration of adrenoceptor antagonists on yohimbine-induced motor impulsivity

The α₂-adrenoceptor antagonist, yohimbine, is commonly used as a pharmacological stressor. Its behavioural effects are typically attributed to elevated noradrenaline release via blockade of central, inhibitory autoreceptors. We have previously reported that yohimbine increases motor impulsivity in rats on the five-choice serial reaction time task (5CSRTT), a cognitive behavioural assessment which measures motor impulsivity and visuospatial attention. Furthermore, this effect depended on cyclic adenomonophosphate (cAMP) signalling via cAMP response element binding (CREB) protein in the orbitofrontal cortex (OFC). However, the role of specific adrenoceptors in this effect is not well-characterised. We therefore investigated whether the pro-impulsive effects of systemic yohimbine could be reproduced by direct administration into the OFC, or attenuated by intra-OFC or systemic administration of prazosin and propranolol-antagonists at the α₁- and β-adrenoceptor, respectively. Male Long-Evans rats were trained on the 5CSRTT and implanted with guide cannulae aimed at the OFC. Systemically administered α₁- or β-adrenoceptor antagonists attenuated yohimbine-induced increases in premature responding. In contrast, local infusion of yohimbine into the OFC reduced such impulsive responding, while blockade of α₁- or β-adrenoceptors within the OFC had no effect on either basal or yohimbine-stimulated motor impulsivity. Direct administration of selective antagonists at the α₁-, α₂- or β-adrenoceptor into the OFC therefore produce clearly dissociable effects from systemic administration. Collectively, these data suggest that the pro-impulsivity effect of yohimbine can be modulated by adrenergic signalling in brain areas outside of the OFC, in addition to non-adrenergic signalling pathways within the OFC.

Cue-induced food seeking after punishment is associated with increased Fos expression in the lateral hypothalamus and basolateral and medial amygdala

In humans, relapse to unhealthy eating habits following dieting is a significant impediment to obesity treatment. Food-associated cues are one of the main triggers of relapse to unhealthy eating during self-imposed abstinence. Here we report a behavioral method examining cue-induced relapse to food seeking following punishment-induced suppression of food taking. We trained male rats to lever press for food pellets that were delivered after a 10-s conditional stimulus (CS) (appetitive). Following training, 25% of reinforced lever presses resulted in the presentation of a compound stimulus consisting of a novel CS (aversive) and the appetitive CS followed by a pellet and footshock. After punishment-imposed abstinence, we tested the rats in an extinction test where lever pressing resulted in the presentation of either the appetitive or aversive CS. We then compared activity of lateral hypothalamus (LH) and associated extrahypothalamic regions following this test. We also assessed Fos expression in LH orexin and GABA neurons. We found that cue-induced relapse of food seeking on test was higher in rats tested with the appetitive CS compared to the aversive CS. Relapse induced by the appetitive CS was associated with increased Fos expression in LH, caudal basolateral amygdala (BLA), and medial amygdala (MeA). This relapse was also associated with increased Fos expression in LH orexin and VGAT-expressing neurons. These data show that relapse to food seeking can be induced by food-associated cues after punishment-imposed abstinence, and this relapse is associated with increased activity in LH, caudal BLA, and MeA. (PsycINFO Database Record

Effect of the CRF1-receptor antagonist pexacerfont on stress-induced eating and food craving

RATIONALE

In rodents, antagonism of receptors for corticotropin-releasing factor (CRF) blocks stress-induced reinstatement of drug or palatable food seeking.

OBJECTIVE

To test anticraving properties of the CRF₁ antagonist pexacerfont in humans.

METHODS

We studied stress-induced eating in people scoring high on dietary restraint (food preoccupation and chronic unsuccessful dieting) with body-mass index (BMI) >22. In a double-blind, between-groups trial, 31 "restrained" eaters were stabilized on either pexacerfont (300 mg/day for 7 days, then 100 mg/day for 21 days) or placebo. On day 15, they underwent a math-test stressor; during three subsequent visits, they heard personalized craving-induction scripts. In each session, stress-induced food consumption and craving were assessed in a bogus taste test and on visual analog scales. We used digital video to monitor daily ingestion of study capsules and nightly rating of food problems/preoccupation on the Yale Food Addiction Scale (YFAS).

RESULTS

The study was stopped early due to an administrative interpretation of US federal law, unrelated to safety or outcome. The bogus taste tests suggested some protective effect of pexacerfont against eating after a laboratory stressor (r _effect = 0.30, 95 % CL = -0.12, 0.63, Bayes factor 11.30). Similarly, nightly YFAS ratings were lower with pexacerfont than placebo (r _effect = 0.39, CI 0.03, 0.66), but this effect should be interpreted with caution because it was present from the first night of pill ingestion, despite pexacerfont's slow pharmacokinetics.

CONCLUSIONS

The findings may support further investigation of the anticraving properties of CRF₁ antagonists, especially for food.

Chronic restraint stress causes a delayed increase in responding for palatable food cues during forced abstinence via a dopamine D1-like receptor-mediated mechanism

Relapse to unhealthy eating habits in dieters is often triggered by stress. Animal models, moreover, have confirmed a causal role for acute stress in relapse. The role of chronic stress in relapse vulnerability, however, has received relatively little attention. Therefore, in the present study, we used an abstinence-based relapse model in rats to test the hypothesis that exposure to chronic stress increases subsequent relapse vulnerability. Rats were trained to press a lever for highly palatable food reinforcers in daily 3-h sessions and then tested for food seeking (i.e., responding for food associated cues) both before and after an acute or chronic restraint stress procedure (3h/day×1day or 10days, respectively) or control procedure (unstressed). The second food seeking test was conducted either 1day or 7days after the last restraint. Because chronic stress causes dopamine D1-like receptor-mediated alterations in prefrontal cortex (a relapse node), we also assessed dopaminergic involvement by administering either SCH-23390 (10.0μg/kg; i.p.), a dopamine D1-like receptor antagonist, or vehicle prior to daily treatments. Results showed that chronically, but not acutely, stressed rats displayed increased food seeking 7days, but not 1day, after the last restraint. Importantly, SCH-23390 combined with chronic stress reversed this effect. These results suggest that drugs targeting D₁-like receptors during chronic stress may help to prevent future relapse in dieters.

Effects of repeated yohimbine administration on reinstatement of palatable food seeking: involvement of dopamine D1 -like receptors and food-associated cues

Acute exposure to the pharmacological stressor yohimbine induces relapse to both food and drug seeking in a rat model. However, no systematic studies on the effects of chronic stress on relapse have been conducted. Because chronic stress causes changes in dopamine D₁ -like receptor-mediated transmission in prefrontal cortex (a relapse node), we tested the hypothesis that chronic exposure to stress increases vulnerability to relapse via dopamine-mediated mechanisms. Additionally, to determine the role of food-conditioned cues in reinstatement of food seeking, we made discrete food-paired cues either available (CS Present) or not available (CS Absent) during extinction and reinstatement testing. Rats responded for palatable food reinforcers in daily 3-hour sessions, and the behavior was extinguished. To model chronic stress, rats were injected daily with yohimbine (0.0, 2.5, or 5.0 mg/kg; i.p.) during the first 7 days of extinction. Injections were combined with SCH-23390 (0.0, 5.0, or 10.0 µg/kg; i.p.), a D₁ -like receptor antagonist. Rats were then tested for reinstatement of food seeking triggered by acute yohimbine (0.0, 1.0, or 2.0 mg/kg; i.p.) and pellet priming. Rats treated previously with chronic yohimbine displayed increased responding following acute yohimbine priming relative to non-chronically stressed rats, but in the CS Absent condition only. Conversely, the lower dose of chronic yohimbine caused an increase in pellet-primed reinstatement, but this effect was more pronounced in the CS Present condition. Importantly, SCH-23390 combined with repeated yohimbine injections attenuated these effects. Thus, chronic stress may increase vulnerability to relapse under specific circumstances via a dopamine D₁ -like receptor-mediated mechanism.

Lorcaserin and CP-809101 reduce motor impulsivity and reinstatement of food seeking behavior in male rats: Implications for understanding the anti-obesity property of 5-HT2C receptor agonists

RATIONALE

The 5-HT2C receptor agonist lorcaserin (Belviq®) has been approved by the FDA for the treatment of obesity. Impulsivity is a contributory feature of some eating disorders.

OBJECTIVE

Experiments investigated the effect of lorcaserin and the highly selective 5-HT2C agonist CP-809101 on measures of impulsivity and on reinstatement of food-seeking behaviour, a model of dietary relapse. The effect of both drugs on 22-h deprivation-induced feeding was also examined, as was the effect of prefeeding in each impulsivity test.

RESULTS

Lorcaserin (0.3-0.6 mg/kg SC) and CP-809101 (0.6-1 mg/kg SC) reduced premature responding in rats trained on the 5-CSRTT and improved accuracy in a Go-NoGo task by reducing false alarms. At equivalent doses, both drugs also reduced reinstatement for food-seeking behaviour. Neither drug altered impulsive choice measured in a delay-discounting task. Lorcaserin (1-3 mg/kg SC) and CP-809101 (3-6 mg/kg SC) reduced deprivation-induced feeding but only at higher doses.

CONCLUSIONS

These results suggest that in addition to previously reported effects on satiety and reward, altered impulse control may represent a contributory factor to the anti-obesity property of 5-HT2C receptor agonists. Lorcaserin may promote weight loss by improving adherence to dietary regimens in individuals otherwise prone to relapse and may be beneficial in cases where obesity is associated with eating disorders tied to impulsive traits, such as binge eating disorder.

Consumption of palatable food primes food approach behavior by rapidly increasing synaptic density in the VTA

In an environment with easy access to highly palatable and energy-dense food, food-related cues drive food-seeking regardless of satiety, an effect that can lead to obesity. The ventral tegmental area (VTA) and its mesolimbic projections are critical structures involved in the learning of environmental cues used to predict motivationally relevant outcomes. Priming effects of food-related advertising and consumption of palatable food can drive food intake. However, the mechanism by which this effect occurs, and whether these priming effects last days after consumption, is unknown. Here, we demonstrate that short-term consumption of palatable food can prime future food approach behaviors and food intake. This effect is mediated by the strengthening of excitatory synaptic transmission onto dopamine neurons that is initially offset by a transient increase in endocannabinoid tone, but lasts days after an initial 24-h exposure to sweetened high-fat food (SHF). This enhanced synaptic strength is mediated by a long-lasting increase in excitatory synaptic density onto VTA dopamine neurons. Administration of insulin into the VTA, which suppresses excitatory synaptic transmission onto dopamine neurons, can abolish food approach behaviors and food intake observed days after 24-h access to SHF. These results suggest that even a short-term exposure to palatable foods can drive future feeding behavior by "rewiring" mesolimbic dopamine neurons.

Ghrelin enhances cue-induced bar pressing for high fat food

Ghrelin is an orexigenic hormone produced by the stomach that acts on growth hormone secretagogue receptors (GHSRs) both peripherally and centrally. The presence of GHSRs in the ventral tegmental area (VTA) suggests that ghrelin signaling at this level may increase the incentive value of palatable foods as well as other natural and artificial rewards. The present investigation sought to determine if ghrelin plays a role in relapse to such foods following a period of abstinence. To achieve this, thirty-six male Long Evans rats were trained to press a lever to obtain a high fat chocolate food reward on a fixed ratio schedule of 1. Following an extinction period during which lever presses were not reinforced, rats were implanted with a cannula connected to a minipump that continuously delivered ghrelin, a GHSR antagonist ([d-Lys-3]-GHRP-6), or saline in the VTA for 14days. One week later, food reward-associated cues, food reward priming, and an overnight fast were used to induce reinstatement of the lever pressing response. Our results indicate that intra-VTA ghrelin enhances cue-induced reinstatement of responses for palatable food pellets. To the extent that the reinstatement paradigm is considered a valid model of relapse in humans, this suggests that ghrelin signaling facilitates relapse to preferred foods in response to food cues through GHSR signaling in the VTA.

Stress-Induced Reinstatement of Drug Seeking: 20 Years of Progress

In human addicts, drug relapse and craving are often provoked by stress. Since 1995, this clinical scenario has been studied using a rat model of stress-induced reinstatement of drug seeking. Here, we first discuss the generality of stress-induced reinstatement to different drugs of abuse, different stressors, and different behavioral procedures. We also discuss neuropharmacological mechanisms, and brain areas and circuits controlling stress-induced reinstatement of drug seeking. We conclude by discussing results from translational human laboratory studies and clinical trials that were inspired by results from rat studies on stress-induced reinstatement. Our main conclusions are (1) The phenomenon of stress-induced reinstatement, first shown with an intermittent footshock stressor in rats trained to self-administer heroin, generalizes to other abused drugs, including cocaine, methamphetamine, nicotine, and alcohol, and is also observed in the conditioned place preference model in rats and mice. This phenomenon, however, is stressor specific and not all stressors induce reinstatement of drug seeking. (2) Neuropharmacological studies indicate the involvement of corticotropin-releasing factor (CRF), noradrenaline, dopamine, glutamate, kappa/dynorphin, and several other peptide and neurotransmitter systems in stress-induced reinstatement. Neuropharmacology and circuitry studies indicate the involvement of CRF and noradrenaline transmission in bed nucleus of stria terminalis and central amygdala, and dopamine, CRF, kappa/dynorphin, and glutamate transmission in other components of the mesocorticolimbic dopamine system (ventral tegmental area, medial prefrontal cortex, orbitofrontal cortex, and nucleus accumbens). (3) Translational human laboratory studies and a recent clinical trial study show the efficacy of alpha-2 adrenoceptor agonists in decreasing stress-induced drug craving and stress-induced initial heroin lapse.

Animal models of drug relapse and craving: From drug priming-induced reinstatement to incubation of craving after voluntary abstinence

High rates of relapse to drug use during abstinence is a defining feature of drug addiction. In abstinent drug users, drug relapse is often precipitated by acute exposure to the self-administered drug, drug-associated cues, stress, as well as by short-term and protracted withdrawal symptoms. In this review, we discuss different animal models that have been used to study behavioral and neuropharmacological mechanisms of these relapse-related phenomena. In the first part, we discuss relapse models in which abstinence is achieved through extinction training, including the established reinstatement model, as well as the reacquisition and resurgence models. In the second part, we discuss recent animal models in which drug relapse is assessed after either forced abstinence (e.g., the incubation of drug craving model) or voluntary (self-imposed) abstinence achieved either by introducing adverse consequences to ongoing drug self-administration (e.g., punishment) or by an alternative nondrug reward using a discrete choice (drug vs. palatable food) procedure. We conclude by briefly discussing the potential implications of the recent developments of animal models of drug relapse after voluntary abstinence to the development of medications for relapse prevention.

Optogenetics in Freely Moving Mammals: Dopamine and Reward

Brain reward systems play a central role in the cognitive and hedonic behaviors of mammals. Multiple neuron types and brain regions are involved in reward processing, posing fascinating scientific questions, and major experimental challenges. Using diverse approaches including genetics, electrophysiology, imaging, and behavioral analysis, a large body of research has focused on both normal functioning of the reward circuitry and on its potential significance in neuropsychiatric diseases. In this introduction, we illustrate a real-world application of optogenetics to mammalian behavior and physiology, delineating procedures and technologies for optogenetic control of individual components of the reward circuitry. We describe the experimental setup and protocol for integrating optogenetic modulation of dopamine neurons with fast-scan cyclic voltammetry, conditioned place preference, and operant conditioning to assess the causal role of well-defined electrical and biochemical signals in reward-related behavior.

Cocaine influences alcohol-seeking behavior and relapse drinking in alcohol-preferring (P) rats

BACKGROUND

The results of several studies suggest that there may be common neurocircuits regulating drug-seeking behaviors. Common biological pathways regulating drug-seeking would explain the phenomenon that seeking for 1 drug can be enhanced by exposure to another drug of abuse. The objective of this study was to assess the time course effects of acute cocaine administration on ethanol (EtOH) seeking and relapse.

METHODS

Alcohol-preferring (P) rats were allowed to self-administer 15% EtOH and water. EtOH-seeking was assessed through the use of the Pavlovian spontaneous recovery (PSR) model, while EtOH-relapse drinking was assessed through the use of the alcohol-deprivation effect.

RESULTS

Cocaine (0, 1, or 10 mg/kg), injected immediately, 30 minutes, or 4 hours prior to the first PSR testing session, dose-dependently increased responding on the EtOH lever compared to extinction responses and responding by saline controls. Under relapse conditions, cocaine given immediately prior to the relapse session had no effect (1 mg/kg) or reduced responding (10 mg/kg). In contrast, cocaine given 4 hours prior to the relapse session markedly enhanced EtOH responding compared to saline.

CONCLUSIONS

The enhanced expression of EtOH-seeking and EtOH-relapse behaviors may be a result of a priming effect of cocaine on neuronal circuits mediating these behaviors. The effect of cocaine on EtOH-relapse drinking is indicative of the complex interactions that can occur between drugs of abuse; production of conflicting behaviors (immediate), and priming of relapse/seeking (4-hour delay).

Effect of yohimbine on reinstatement of operant responding in rats is dependent on cue contingency but not food reward history

Yohimbine is an alpha-2 adrenoceptor antagonist that has been used in numerous studies as a pharmacological stressor in rodents, monkeys and humans. Recently, yohimbine has become the most common stress manipulation in studies on reinstatement of drug and food seeking. However, the wide range of conditions under which yohimbine promotes reward seeking is significantly greater than that of stressors like intermittent footshock. Here, we addressed two fundamental questions regarding yohimbine's effect on reinstatement of reward seeking: (1) whether the drug's effect on operant responding is dependent on previous reward history or cue contingency, and (2) whether yohimbine is aversive or rewarding under conditions typically used in reinstatement studies. We also used in vivo microdialysis to determine yohimbine's effect on dopamine levels in nucleus accumbens (NAc) and medial prefrontal cortex (mPFC). We found that the magnitude of yohimbine-induced (0.5, 1.0, 2.0 mg/kg) operant responding during the reinstatement tests was critically dependent on the contingency between lever pressing and discrete tone-light cue delivery but not the previous history with food reward during training. We also found that yohimbine (2 mg/kg) did not cause conditioned place aversion. Finally, we found that yohimbine modestly increased dopamine levels in mPFC but not NAc. Results suggest that yohimbine's effects on operant responding in reinstatement studies are likely independent of the history of contingent self-administration of food or drug rewards and may not be related to the commonly assumed stress-like effects of yohimbine.

Reinstatement in a cocaine versus food choice situation: reversal of preference between drug and non-drug rewards

Recent studies show that when given a mutually exclusive choice between cocaine and food, rats almost exclusively choose food. The present experiment investigated potential shifts in preference between levers associated with either food or cocaine that might occur during extinction (food and cocaine no longer available) and during footshock-induced, cocaine-primed and food-primed reinstatement. During self-administration sessions where food and cocaine were simultaneously available, rats demonstrated a stable food preference, choosing food over cocaine on 83% of trials. During extinction when neither reinforcer was available, no preference between levers was evident and responding decreased until rats responded on the previously food- and cocaine-associated levers at equally low rates. Footshock resulted in a non-specific reinstatement of responding upon both levers, while cocaine priming resulted in a significant preference for cocaine seeking over food seeking. This suggests that the mechanism underlying footshock-induced reinstatement is distinct from that of cocaine-primed reinstatement. Food priming engendered a mild, non-specific increase in responding on both levers. Although rats generally prefer food over cocaine when presented with a choice between these primary reinforcers, the present results suggest that in certain situations, cocaine-seeking behavior prevails over food-seeking behavior.

Attribution and expression of incentive salience are differentially signaled by ultrasonic vocalizations in rats

During Pavlovian incentive learning, the affective properties of rewards are thought to be transferred to their predicting cues. However, how rewards are represented emotionally in animals is widely unknown. This study sought to determine whether 50-kHz ultrasonic vocalizations (USVs) in rats may signal such a state of incentive motivation to natural, nutritional rewards. To this end, rats learned to anticipate food rewards and, across experiments, the current physiological state (deprived vs. sated), the type of learning mechanism recruited (Pavlovian vs. instrumental), the hedonic properties of UCS (low vs. high palatable food), and the availability of food reward (continued vs. discontinued) were manipulated. Overall, we found that reward-cues elicited 50-kHz calls as they were signaling a putative affective state indicative of incentive motivation in the rat. Attribution and expression of incentive salience, however, seemed not to be an unified process, and could be teased apart in two different ways: 1) under high motivational state (i.e., hunger), the attribution of incentive salience to cues occurred without being expressed at the USVs level, if reward expectations were higher than the outcome; 2) in all experiments when food rewards were devalued by satiation, reward cues were still able to elicit USVs and conditioned anticipatory activity although reward seeking and consumption were drastically weakened. Our results suggest that rats are capable of representing rewards emotionally beyond apparent, immediate physiological demands. These findings may have translational potential in uncovering mechanisms underlying aberrant and persistent motivation as observed in drug addiction, gambling, and eating disorders.

Effect of cafeteria diet history on cue-, pellet-priming-, and stress-induced reinstatement of food seeking in female rats

BACKGROUND

Relapse to unhealthy eating habits is a major problem in human dietary treatment. The individuals most commonly seeking dietary treatment are overweight or obese women, yet the commonly used rat reinstatement model to study relapse to palatable food seeking during dieting primarily uses normal-weight male rats. To increase the clinical relevance of the relapse to palatable food seeking model, here we pre-expose female rats to a calorically-dense cafeteria diet in the home-cage to make them overweight prior to examining the effect of this diet history on cue-, pellet-priming- and footshock-induced reinstatement of food seeking.

METHODS

Post-natal day 32 female Long-Evans rats had seven weeks of home-cage access to either chow only or daily or intermittent cafeteria diet alongside chow. Next, they were trained to self-administer normally preferred 45 mg food pellets accompanied by a tone-light cue. After extinction, all rats were tested for reinstatement induced by discrete cue, pellet-priming, and intermittent footshock under extinction conditions.

RESULTS

Access to daily cafeteria diet and to a lesser degree access to intermittent cafeteria diet decreased food pellet self-administration compared to chow-only. Prior history of these cafeteria diets also reduced extinction responding, cue- and pellet-priming-induced reinstatement. In contrast, modest stress-induced reinstatement was only observed in rats with a history of daily cafeteria diet.

CONCLUSION

A history of cafeteria diet does not increase the propensity for cue- and pellet-priming-induced relapse in the rat reinstatement model but does appear to make rats more susceptible to footshock stress-induced reinstatement.

Inactivation of the nucleus accumbens core or medial shell attenuates reinstatement of sugar-seeking behavior following sugar priming or exposure to food-associated cues

Re-exposure to either palatable food or to conditioned stimuli associated with food is known to reinstate food-seeking after periods of abstinence. The nucleus accumbens core and shell are important for reinstatement in both food- and drug-seeking paradigms, although their potential differential roles have been difficult to delineate due to methodological differences in paradigms across laboratories. The present studies assessed the effects of temporary inactivation of the core or shell on priming- and cue-induced reinstatement of food-seeking in identically-trained rats. Inactivation of either the nucleus accumbens core (Experiment 1A; N = 10) or medial shell (Experiment 1B; N = 12) blocked priming-induced reinstatement in an equivalent manner. Similarly, inactivation of the core or medial shell (Experiments 2A & 2B; N = 11 each) also blocked cue-induced reinstatement, although there was also a significant treatment day X brain region X drug order interaction. Specifically, rats with core inactivation reinstated lever-pressing on the vehicle injection day regardless of whether that was their first or second test, whereas rats that had medial shell inactivation on the first day did not significantly reinstate lever-pressing on the second day of testing (when they received vehicle). Yohimbine, while a reportedly robust pharmacological stressor, was ineffective at inducing reinstatement in the current stress-induced reinstatement procedure. These data suggest that both the nucleus accumbens core and shell serve important roles in reinstatement of food-seeking in response to priming and cues.

Examination of the role of dopamine D₂ and adrenergic α₂ receptors in resurgence of food seeking

Resurgence refers to the reappearance of an extinguished operant behavior when reinforcement for an alternative behavior is also subsequently discontinued. Resurgence has been noted as a source of relapse to problem behavior following interventions involving alternative reinforcement, and has also been recently used as an animal model of relapse to drug seeking induced by reinforcement loss. Existing information about the neuropharmacology of resurgence is scarce, but suggests overlap between relapse observed in the resurgence model and relapse observed in reinstatement and renewal models. In the present experiment rats earned food pellets for pressing a target lever in Phase I. In Phase II lever pressing no longer produced food, but food was delivered for an alterative nose poke response. Finally in Phase III, neither response produced food deliveries. Prior to these Phase III sessions, separate groups of rats were injected with 0, 50, or 100 μg/kg of the dopamine D2 receptor antagonist raclopride or 0, 20, or 40 μg/kg of α 2 agonist clonidine. Both doses of raclopride were effective in blocking resurgence, but there was evidence that the higher dose did so via motor rather than motivational impairment. Only the higher dose of clonidine blocked resurgence, but did so with no evidence of motor impairment. Raclopride significantly impacted extinction of the alternative poke at both doses tested, whereas clonidine had no effect at either dose. The results of the present study provide additional information about the neuropharmacology of resurgence, as well as additional evidence of overlap between resurgence, reinstatement, and renewal.

Effect of recurrent yohimbine on immediate and post-hoc behaviors, stress hormones, and energy homeostatic parameters

Evidence from experimental models has suggested that acute activation of brain stress and anxiety pathways impacts subsequent behaviors that are mediated or modulated by limbic circuitry. There have been limited investigations of prior or chronic activation of these pathways on subsequent limbic-mediated behaviors. In this study, we tested whether recurrent administration of the anxiogenic compound yohimbine (YOH) could have post-injection effects on brain activation, stress hormones, and performance in sucrose self-administration and startle response paradigms. Rats received six injections across two weeks of either 2mg/kg YOH or saline. Behavioral evaluation confirmed the continued efficacy of the YOH regimen, and increased adrenal corticosterone (CORT) was observed. Several days following YOH or SAL administration, cFos, CORT and adrenocorticotropin hormone (ACTH), and behavioral performance were measured. cFos was elevated post-YOH in the hippocampus; ventral tegmental area/zona inserta; and central and medial nuclei of the amygdala. This activation is consistent with a sustained effect of YOH to activate fear and anxiety circuitries in the CNS. CORT but not ACTH was elevated in the YOH-rats following startle testing. Self-administration and startle tests suggested an increase of non-specific activity in the post-YOH rats; there was no increase in sucrose self-administration or startle response per se. Our findings suggest that recurrent YOH administration may prove a useful and reliable model for simulating recurrent stress/anxiety, and that enhancements to the paradigm such as higher or more frequent dosing of YOH could yield stronger or more extensive behavioral effects.

Effects of stressors on the reinforcing efficacy of nicotine in adolescent and adult rats

RATIONALE AND OBJECTIVES

Stress increases drug intake. This depends on the stressor, drug, and aspect of drug seeking assessed. The objectives of these experiments done in adolescent and adult male rats were to (1) examine social defeat effects on acquisition of nicotine self-administration (SA) and the reinforcing efficacy of nicotine and (2) determine the effects of acute exposure to intermittent footshock (FS) or yohimbine on the reinforcing efficacy of nicotine.

METHODS

In experiment 1, rats received four defeat exposures prior to nicotine SA acquisition and progressive ratio (PR) SA sessions (30 μg/kg nicotine/infusion). Exposure to an olfactory cue previously paired with defeat was also tested on responding maintained by nicotine on the PR schedule. In experiments 2 and 3, the effects of FS (5 and 10 min) or yohimbine (0.625 and 1.25 mg/kg, i.p.) on PR responding for nicotine (15, 30, or 60 μg/kg/infusion) were assessed. Adolescents were aged PD34-36 and adults PD81-85 at the beginning of nicotine SA training.

RESULTS

Defeat did not affect nicotine SA acquisition. Prior exposure to defeat or a defeat-paired olfactory cue did not affect PR responding for nicotine. FS modestly decreased PR responding in adolescents at the middle nicotine infusion dose. Yohimbine increased PR responding independent of nicotine infusion dose and age.

CONCLUSIONS

Together with previous work with other drugs, our data indicate that the effects of stress on the reinforcing efficacy of nicotine are stressor- and drug-dependent. This suggests that there is heterogeneity among stressors on how they affect neuronal systems underlying drug intake.

Role of the kappa-opioid receptor system in stress-induced reinstatement of nicotine seeking in rats

RATIONALE

The correlation between stress and smoking is well established. The mechanisms that underlie this relationship are, however, unclear. Recent data suggest that the kappa-opioid system is involved in the mediation of negative affective states associated with stress thereby promoting drug addiction and relapse. Pharmacological treatments targeting the kappa-opioid system and this mechanism may prove to be useful therapeutics for nicotine addiction in the future.

OBJECTIVES

We sought to determine whether there was a stress-specific role of the kappa-opioid system in nicotine seeking behavior.

METHOD

Groups of male Long Evans rats were trained to self-administer nicotine intravenously; their operant responding for nicotine was extinguished prior to tests of reinstatement. Pretreatment with systemic injections of the kappa-opioid receptor (KOR) antagonist nor-binaltorphimine (nor-BNI) was given prior to tests of stress (systemic injections of yohimbine (YOH)) or cue-induced reinstatement of nicotine seeking. Systemic injections of the KOR agonist U50,488 were also given in a test for reinstatement of nicotine seeking.

RESULTS

Nor-BNI pretreatment at 1h and 24h prior to testing was able to block YOH-induced, but not cue-induced reinstatement of nicotine seeking. U50,488 reinstated nicotine seeking behavior in a dose-dependent manner.

CONCLUSIONS

These findings support the hypothesis that the kappa-opioid system is involved in relapse to nicotine seeking induced by stress, but not by conditioned cues. KOR antagonists such as nor-BNI may therefore be useful novel therapeutic agents for decreasing the risk of stress-induced drug relapse.

The use of the reinstatement model to study relapse to palatable food seeking during dieting

Excessive consumption of unhealthy foods is a major public health problem. While many people attempt to control their food intake through dieting, many relapse to unhealthy eating habits within a few months. We have begun to study this clinical condition in rats by adapting the reinstatement model, which has been used extensively to study relapse to drug seeking. In our adaptation of the relapse model, reinstatement of palatable food seeking by exposure to food-pellet priming, food-associated cues, or stress is assessed in food-restricted (to mimic dieting) rats after operant food-pellet self-administration training and subsequent extinction of the food-reinforced responding. In this review, we first outline the clinical problem and discuss a recent study in which we assessed the predictive validity of the reinstatement model for studying relapse to food seeking during dieting by using the anorexigenic drug fenfluramine. Next, we summarize results from our initial studies on the role of several stress- and feeding-related peptides (corticotropin-releasing factor, hypocretin, melanin-concentrating hormone, peptide YY3-36) in reinstatement of palatable food seeking. We then present results from our studies on the role of dopamine and medial prefrontal cortex in stress-induced reinstatement of food seeking. We conclude by discussing potential clinical implications. We offer two main conclusions: (1) the food reinstatement model is a simple, reliable, and valid model to study mechanisms of relapse to palatable food seeking during dieting, and to identify medications to prevent this relapse; (2) mechanisms of relapse to food seeking are often dissociable from mechanisms of ongoing food intake. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Time-dependent decreases in nucleus accumbens AMPA/NMDA ratio and incubation of sucrose craving in adolescent and adult rats

RATIONALE AND OBJECTIVE

There is evidence that cue-induced sucrose seeking progressively increases after cessation of oral sucrose self-administration (incubation of sucrose craving) in both adolescent and adult rats. The synaptic plasticity changes associated with this incubation at different age groups are unknown. We assessed whether incubation of sucrose craving in rats trained to self-administer sucrose as young adolescents, adolescents, or adults is associated with changes in 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA)/N-methyl-D-aspartate (NMDA) ratio (a measure of postsynaptic changes in synaptic strength) in nucleus accumbens.

METHODS

Three age groups initiated oral sucrose self-administration training (10 days) on postnatal day (P) 35 (young adolescents), P42 (adolescents), or P70 (adults). They were then tested for cue-induced sucrose seeking (assessed in an extinction test) on abstinence days 1 and 21. Separate groups of rats were trained to self-administer sucrose or water (a control condition), and assessed for AMPA/NMDA ratio in nucleus accumbens on abstinence days 1-3 and 21.

RESULTS

Adult rats earned more sucrose rewards, but sucrose intake per body weight was higher in young adolescent rats. Time-dependent increases in cue-induced sucrose seeking (incubation of sucrose craving) were more pronounced in adult rats, less pronounced in adolescents, and not detected in young adolescents. On abstinence day 21, but not days 1-3, AMPA/NMDA ratio in nucleus accumbens were decreased in rats that self-administered sucrose as adults and adolescents, but not young adolescents.

CONCLUSIONS

Our data demonstrate age-dependent changes in magnitude of incubation of sucrose craving and nucleus accumbens synaptic plasticity after cessation of sucrose self-administration.

Amygdalostriatal projections in the neurocircuitry for motivation: a neuroanatomical thread through the career of Ann Kelley

In MacLean's triune brain, the amygdala putatively subserves motivated behavior by modulating the "reptilian" basal ganglia. Accordingly, Ann Kelley, with Domesick and Nauta, influentially showed that amygdalostriatal projections are much more extensive than were appreciated. They highlighted that amygdalar projections to the rostral ventromedial striatum converged with projections from the ventral tegmental area and cingulate cortex, forming a "limbic striatum". Caudal of the anterior commissure, the entire striatum receives afferents from deep basal nuclei of the amygdala. Orthologous topographic projections subsequently were observed in fish, amphibians, and reptiles. Subsequent functional studies linked acquired value to action via this neuroanatomical substrate. From Dr. Kelley's work evolved insights into components of the distributed, interconnected network that subserves motivated behavior, including the nucleus accumbens shell and core and the striatal-like extended amygdala macrostructure. These heuristic frameworks provide a neuroanatomical basis for adaptively translating motivation into behavior. The ancient amygdala-to-striatum pathways remain a current functional thread not only for stimulus-response valuation, but also for the psychopathological plasticity that underlies addiction-related memory, craving and relapse.

Individual variation in resisting temptation: implications for addiction

When exposed to the sights, sounds, smells and/or places that have been associated with rewards, such as food or drugs, some individuals have difficulty resisting the temptation to seek out and consume them. Others have less difficulty restraining themselves. Thus, Pavlovian reward cues may motivate maladaptive patterns of behavior to a greater extent in some individuals than in others. We are just beginning to understand the factors underlying individual differences in the extent to which reward cues acquire powerful motivational properties, and therefore, the ability to act as incentive stimuli. Here we review converging evidence from studies in both human and non-human animals suggesting that a subset of individuals are more "cue reactive", in that certain reward cues are more likely to attract these individuals to them and motivate actions to get them. We suggest that those individuals for whom Pavlovian reward cues become especially powerful incentives may be more vulnerable to impulse control disorders, such as binge eating and addiction.