The anxiogenic drug yohimbine reinstates palatable food seeking in a rat relapse model: a role of CRF1 receptors

The anxiogenic drug yohimbine is a reinforcer in male and female rats

The indole alkaloid yohimbine is an anxiogenic drug that activates stress-responsive systems in the brain. However, because yohimbine also elicits approach behaviors, this study employed male and female Sprague-Dawley rats to explore its potential reinforcing effects. Thus, it was first determined if intravenous (IV) infusions of yohimbine (0.25 mg/kg/infusion) could maintain lever pressing, whether intake could be modulated by dose/infusion, and if lever pressing would persist in the absence of yohimbine or yohimbine-paired cues. Next, to assess yohimbine's effect on memory consolidation, 0.3, 1.25 or 3 mg/kg yohimbine was administered post-training using an object recognition memory task. Finally, place conditioning assessed whether doses of yohimbine that elevate blood serum corticosterone levels (1.25 or 3 mg/kg) could elicit a conditioned place preference. It was found that both sexes acquired yohimbine IV self-administration, that intake was modulated by dose/infusion, and that lever pressing persisted during extinction and in the absence of the yohimbine-paired cue. As well, post-training injections of 1.25 mg/kg yohimbine enhanced consolidation of object memory, and 1.25 and 3 mg/kg elevated corticosterone levels and elicited a place preference in both sexes. Finally, in behavioral tests of psychomotor functions, acute yohimbine increased lever pressing for a visual cue and elevated locomotor activity. These findings reveal a profile of yohimbine's behavioral effects that is consistent with that of psychostimulant reinforcing drugs.

After a period of forced abstinence, rats treated with the norepinephrine neurotoxin DSP-4 still exhibit preserved food-seeking behavior and prefrontal cortex fos-expressing neurons

Aims

Relapse is a common characteristic of compulsive behaviors like addiction, where individuals tend to return to drug use or overeating after a period of abstinence. PFC (prefrontal cortex) neuronal ensembles are required for drug and food-seeking behaviors and are partially regulated by Norepinephrine (NE). However, the contributions of neuromodulators, such as the adrenergic system, in food-seeking behavior are not fully understood.

Main methods

To investigate this, we trained male and female rats to press a lever in an operant chamber to obtain banana-flavored food pellets for ten days. We then administered DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride), a neurotoxin that diminishes norepinephrine levels in the brain. The rats were kept in their home cages for ten more days before being returned to the operant chambers to measure food-seeking behavior.

Key findings

Despite receiving DSP-4, the PFC neuronal ensembles measured by Fos and food-seeking behavior did not differ between groups, but rather sex.

Significance

Although both NE and Fos expressing neurons are implicated in food-seeking, they do not seem to be involved in a cue-contextual induced re-exposure response.

Pharmacological activation of the amygdala, but not single prolonged footshock-induced acute stress, interferes with cue-induced motivation toward food rewards in rats

In the face of threats, animals adapt their behaviors to cope with the situation. Under such circumstances, irrelevant behaviors are usually suppressed. In this study, we examined whether food-seeking motivation would decrease under activation of the amygdala, an important nucleus in the regulation of stress response in the central nervous system, or after a physical acute stress session. In Experiment 1, we pharmacologically activated the basolateral nucleus (BLA) or the central nucleus of the amygdala (CeA) before a cue-induced reinstatement test in rats. Our results showed that activation of the BLA or the CeA abolished cue-induced motivation toward food rewards, while locomotor activity and free food intake were not affected. In Experiments 2 and 3, we further assessed anxiety and despair levels, as well as cue-induced reinstatement, after a single prolonged footshock-induced acute stress in rats. Behaviorally, acute stress did not affect anxiety level, despair level, or cue-induced motivation toward food rewards. Physiologically, there was no difference in cellular activities of the amygdala immediately after acute stress. To conclude, our results suggested that pharmacological activation of the amygdala decreased cue-induced motivation toward food reward. However, physiological acute stress did not immediately interfere with the negative emotions, motivation, or amygdala activities of the animals.

In silico evaluation of the role of lisdexamfetamine on attention-deficit/hyperactivity disorder common psychiatric comorbidities: mechanistic insights on binge eating disorder and depression

Attention-deficit/hyperactivity disorder (ADHD) is a psychiatric condition well recognized in the pediatric population that can persist into adulthood. The vast majority of patients with ADHD present psychiatric comorbidities that have been suggested to share, to some extent, the pathophysiological mechanism of ADHD. Lisdexamfetamine (LDX) is a stimulant prodrug approved for treating ADHD and, in the US, also for binge eating disorder (BED). Herein, we evaluated, through a systems biology-based in silico method, the efficacy of a virtual model of LDX (vLDX) as ADHD treatment to improve five common ADHD psychiatric comorbidities in adults and children, and we explored the molecular mechanisms behind LDX's predicted efficacy. After the molecular characterization of vLDX and the comorbidities (anxiety, BED, bipolar disorder, depression, and tics disorder), we created a protein-protein interaction human network to which we applied artificial neural networks (ANN) algorithms. We also generated virtual populations of adults and children-adolescents totaling 2,600 individuals and obtained the predicted protein activity from Therapeutic Performance Mapping System models. The latter showed that ADHD molecular description shared 53% of its protein effectors with at least one studied psychiatric comorbidity. According to the ANN analysis, proteins targeted by vLDX are predicted to have a high probability of being related to BED and depression. In BED, vLDX was modeled to act upon neurotransmission and neuroplasticity regulators, and, in depression, vLDX regulated the hypothalamic-pituitary-adrenal axis, neuroinflammation, oxidative stress, and glutamatergic excitotoxicity. In conclusion, our modeling results, despite their limitations and although requiring in vitro or in vivo validation, could supplement the design of preclinical and potentially clinical studies that investigate treatment for patients with ADHD with psychiatric comorbidities, especially from a molecular point of view.

G, H.L. H, D. P, T. S, S.L. D, K. B, W.P. K, C.W. H, T.L. K. Disentangling the effects of Corticotrophin Releasing Factor and GABA release from the ventral bed nucleus of the stria terminalis on ethanol self-administration in mice

Excessive alcohol use causes a great deal of harm and negative health outcomes. Corticotrophin releasing factor (CRF), a stress-related neuropeptide, has been implicated in binge ethanol intake and ethanol dependence. CRF containing neurons in the bed nucleus of the stria terminalis (BNSTCRF) can control ethanol consumption. These BNSTCRF neurons also release GABA, raising the question, is it CRF or GABA release or both that is controlling alcohol consumption. Here, we used viral vectors to separate the effects of CRF and GABA release from BNSTCRF neurons on the escalation of ethanol intake in an operant self-administration paradigm in male and female mice. We found that CRF deletion in BNST neurons reduces ethanol intake in both sexes, with a stronger effect in males. For sucrose self-administration there was no effect of CRF deletion. Suppression of GABA release, via knockdown of vGAT, from BNSTCRF produced a transient increase in ethanol operant self-administration following in male mice, and reduced in motivation to work for sucrose on a progressive ratio schedule of reinforcement in a sex-dependent manner. Together, these results highlight how different signaling molecules from the same populations of neurons can bidirectionally control behavior. Moreover, they suggest that BNST CRF release is important for high intensity ethanol drinking that precedes dependence, whereas GABA release from these neurons may play a role in regulating motivation.

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.

Brain responses to food viewing in women during pregnancy and post partum and their relationship with metabolic health: study protocol for the FOODY Brain Study, a prospective observational study

INTRODUCTION

Food cravings are common in pregnancy and along with emotional eating and eating in the absence of hunger, they are associated with excessive weight gain and adverse effects on metabolic health including gestational diabetes mellitus (GDM). Women with GDM also show poorer mental health, which further can contribute to dysregulated eating behaviour. Food cravings can lead to greater activity in brain centres known to be involved in food 'wanting' and reward valuation as well as emotional eating. They are also related to gestational weight gain. Thus, there is a great need to link implicit brain responses to food with explicit measures of food intake behaviour, especially in the perinatal period. The aim of this study is to investigate the spatiotemporal brain dynamics to visual presentations of food in women during pregnancy and in the post partum, and link these brain responses to the eating behaviour and metabolic health outcomes in women with and without GDM.

METHODS AND ANALYSIS

This prospective observational study will include 20 women with and 20 without GDM, that have valid data for the primary outcomes. Data will be assessed at 24-36 weeks gestational age and at 6 months post partum. The primary outcomes are brain responses to food pictures of varying carbohydrate and fat content during pregnancy and in the post partum using electroencephalography. Secondary outcomes including depressive symptoms, current mood and eating behaviours will be assessed with questionnaires, objective eating behaviours will be measured using Auracle and stress will be measured with heart rate and heart rate variability (Actiheart). Other secondary outcome measures include body composition and glycaemic control parameters.

ETHICS AND DISSEMINATION

The Human Research Ethics Committee of the Canton de Vaud approved the study protocol (2021-01976). Study results will be presented at public and scientific conferences and in peer-reviewed journals.

Food addiction, hormones and blood biomarkers in humans: A systematic literature review

BACKGROUND

Food addiction may play a role in rising obesity rates in connection with obesogenic environments and processed food availability, however the concept of food addiction remains controversial. While animal studies show evidence for addictive processes in relation to processed foods, most human studies are psychologically focussed and there is a need to better understand evidence for biological mechanisms of food addiction in humans. Several key hormones are implicated in models of food addiction, due to their key roles in feeding, energy metabolism, stress and addictive behaviours. This systematic literature review examines evidence for relationships between food addiction, hormones and other blood biomarkers.

METHODS

A series of literature searches was performed in Scopus, PsychInfo, MedLine, ProQuest, CINAHL and Web of Science. A total of 3111 articles were found, of which 1045 were duplicates. Articles were included if they contained a psychometric measurement of food addiction, such as the Yale Food Addiction Scale, as well as addressed the association between FA and hormones or blood biomarkers in humans. Articles were assessed for eligibility by two independent reviewers.

RESULTS

Sixteen studies were identified that examined relationships between food addiction and blood biomarkers, published between 2015 and 2021. Significant findings were reported for leptin, ghrelin, cortisol, insulin and glucose, oxytocin, cholesterol, plasma dopamine, thyroid stimulating hormone (TSH), haemoglobin A1c (HbA1c), triglyceride (TG), amylin, tumour necrosis factor alpha (TNF- α) and cholecystokinin (CCK). Methodological issues included small sample sizes and variation in obesity status, sex and mental health-related comorbidities. Due to methodological limitations, definite connections between FA, hormones and other blood biomarkers cannot yet be determined.

CONCLUSION

This systematic review identified preliminary evidence linking FA symptoms to hormones and other blood biomarkers related to feeding, addiction, and stress. However, due to the small number of studies and methodological limitations, further research is needed to evaluate biopsychosocial models of FA and to resolve controversies.

Reinstatement of Pavlovian responses to alcohol cues by stress

RATIONALE

Stress may contribute to relapse to alcohol use in part by enhancing reactivity to cues previously paired with alcohol. Yet, standard models of stress-induced reinstatement generally use contingent presentations of alcohol-paired cues to reinforce instrumental behaviors, making it difficult to isolate the ability of cues to invigorate alcohol-seeking.

OBJECTIVE

Here we sought to test the impact of stress on behavioral responses to alcohol-paired cues, using a model of stress-induced reinstatement of Pavlovian conditioned approach, inspired by Nadia Chaudhri's work on context-induced reinstatement.

METHODS

Long Evans rats were trained to associate one auditory cue with delivery of alcohol or sucrose and an alternative auditory cue with no reward. Following extinction training, rats were exposed to a stressor prior to being re-exposed to the cues under extinction conditions. We assessed the effects of yohimbine, intermittent footshock and olfactory cues paired with social defeat on responses to alcohol-paired cues and the effects of yohimbine on responses to sucrose-paired cues.

RESULTS

The pharmacological stressor, yohimbine, enhanced alcohol seeking in a Pavlovian setting, but not in a cue-selective manner. Intermittent footshock and social defeat cues did not enhance alcohol seeking in this paradigm.

CONCLUSIONS

While yohimbine elicited reinstatement of reward-seeking in a Pavlovian setting, these effects may be unrelated to activation of stress systems or to interactions with specific cues.

The Antagonism of Corticotropin-Releasing Factor Receptor-1 in Brain Suppress Stress-Induced Propofol Self-Administration in Rats

Propofol addiction has been detected in humans and rats, which may be facilitated by stress. Corticotropin-releasing factor acts through the corticotropin-releasing factor (CRF) receptor-1 (CRF1R) and CRF2 receptor-2 (CRF2R) and is a crucial candidate target for the interaction between stress and drug abuse, but its role on propofol addiction remains unknown. Tail clip stressful stimulation was performed in rats to test the stress on the establishment of the propofol self-administration behavioral model. Thereafter, the rats were pretreated before the testing session at the bilateral lateral ventricle with one of the doses of antalarmin (CRF1R antagonist, 100–500 ng/site), antisauvagine 30 (CRF2R antagonist, 100–500 ng/site), and RU486 (glucocorticoid receptor antagonist, 100–500 ng/site) or vehicle. The dopamine D1 receptor (D1R) in the nucleus accumbens (NAc) was detected to explore the underlying molecular mechanism. The sucrose self-administration establishment and maintenance, and locomotor activities were also examined to determine the specificity. We found that the establishment of propofol self-administration was promoted in the tail clip treated group (the stress group), which was inhibited by antalarmin at the dose of 100–500 ng/site but was not by antisauvagine 30 or RU486. Accordingly, the expression of D1R in the NAc was attenuated by antalarmin, dose-dependently. Moreover, pretreatments fail to change sucrose self-administration behavior or locomotor activities. This study supports the role of CRF1R in the brain in mediating the central reward processing through D1R in the NAc and provided a possibility that CRF1R antagonist may be a new therapeutic approach for the treatment of propofol addiction.

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.

Neural substrates of appetitive and aversive prediction error

Prediction error, defined by the discrepancy between real and expected outcomes, lies at the core of associative learning. Behavioural investigations have provided evidence that prediction error up- and down-regulates associative relationships, and allocates attention to stimuli to enable learning. These behavioural advances have recently been followed by investigations into the neurobiological substrates of prediction error. In the present paper, we review neuroscience data obtained using causal and recording neural methods from a variety of key behavioural designs. We explore the neurobiology of both appetitive (reward) and aversive (fear) prediction error with a focus on the mesolimbic dopamine system, the amygdala, ventrolateral periaqueductal gray, hippocampus, cortex and locus coeruleus noradrenaline. New questions and avenues for research are considered.

The Neural Network of Neuropeptide S (NPS): Implications in Food Intake and Gastrointestinal Functions

The Neuropeptide S (NPS), a 20 amino acids peptide, is recognized as the endogenous ligand of a previously orphan G protein-coupled receptor, now termed NPS receptor (NPSR). The limited distribution of the NPS-expressing neurons in few regions of the brainstem is in contrast with the extensive expression of NPSR in the rodent central nervous system, suggesting the involvement of this receptor in several brain functions. In particular, NPS promotes locomotor activity, behavioral arousal, wakefulness, and unexpectedly, at the same time, it exerts anxiolytic-like properties. Intriguingly, the NPS system is implicated in the rewarding properties of drugs of abuse and in the regulation of food intake. Here, we focus on the anorexigenic effect of NPS, centrally injected in different brain areas, in both sated and fasted animals, fed with standard or palatable food, and, in addition, on its influence in the gastrointestinal tract. Further investigations, regarding the role of the NPS/NPSR system and its potential interaction with other neurotransmitters could be useful to understand the mechanisms underlying its action and to develop novel pharmacological tools for the treatment of aberrant feeding patterns and obesity.

The pharmacological stressor yohimbine, but not U50,488, increases responding for conditioned reinforcers paired with ethanol or sucrose

RATIONALE

Environmental stimuli paired with alcohol can function as conditioned reinforcers (CRfs) and trigger relapse to alcohol-seeking. In animal models, pharmacological stressors can enhance alcohol consumption and reinstate alcohol-seeking, but it is unknown whether stress can potentiate the conditioned reinforcing properties of alcohol-paired stimuli.

OBJECTIVES

We examined whether the pharmacological stressors, the α-2 adrenoreceptor antagonist yohimbine (vehicle, 1.25, 2.5 mg/kg; IP) and the K-opioid receptor agonist U50,488 (vehicle, 1.25, 2.5 mg/kg; SC), increase responding for conditioned reinforcement, and if their effects interact with the nature of the reward (alcohol vs. sucrose). We subsequently examined the effects of yohimbine (vehicle, 1.25, 2.5 mg/kg; IP) on responding for sensory reinforcement.

METHODS

Male Long-Evans underwent Pavlovian conditioning, wherein a tone-light conditioned stimulus (CS) was repeatedly paired with 12% EtOH or 21.7% sucrose. Next, tests of responding for a CRf were conducted. Responding on the CRf lever delivered the CS, whereas responding on the other lever had no consequences. In a separate cohort of rats, the effects of yohimbine on responding just for the sensory reinforcer were examined.

RESULTS

Both doses of yohimbine, but not U50,488, increased responding for conditioned reinforcement. This enhancement occurred independently of the nature of the reward used during Pavlovian conditioning. Yohimbine-enhanced responding for a CRf was reproducible and stable over five tests; it even persisted when the CS was omitted. Both doses of yohimbine also increased responding for sensory reinforcement.

CONCLUSIONS

Yohimbine increases operant responding for a variety of sensory and conditioned reinforcers. This enhancement may be independent of its stress-like effects.

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.

Protective effects of morphine in a rat model of post-traumatic stress disorder: Role of hypothalamic-pituitary-adrenal axis and beta- adrenergic system

Post-traumatic stress disorder (PTSD) arises after tremendous traumatic experiences. Recently, we have reported that morphine has time-dependent protective effects against behavioral and morphological deficits in the single prolonged stress (SPS) as an experimental model of PTSD in adult male rats. To find the mechanisms underlying the protective effects of morphine against SPS-induced PTSD-like symptoms, the present study investigated the interaction between morphine and hypothalamic-pituitary-adrenal (HPA) axis and beta - adrenergic system, which crucially involved in the stress response, on PTSD-like symptoms in male rats. The animals were exposed to the SPS procedure (restraint for 2 h, forced swimming for 20 min, and ether anesthesia) and morphine (10 mg/kg) or saline was injected 24 h following the SPS. The glucocorticoid receptor antagonist RU486 (20 mg/kg), the mineralocorticoid receptor antagonist spironolactone (50 mg/kg), and the corticosterone synthesis inhibitor metyrapone (50 mg/kg) were injected 90 min before morphine administration to block the HPA axis activity. The beta - adrenergic receptor blocker propranolol (10 mg/kg) and the peripheral beta-adrenergic receptor blocker nadolol (5 mg/kg) were administered 30 min before morphine injection to block the beta - adrenergic system. Anxiety-like behaviors were evaluated using the elevated plus maze (EPM) 11 days after the SPS. After that, animals were conditioned in a fear-conditioning task and extinction training was performed on days 1, 2, 3, 4 and 11 after fear conditioning. SPS increased anxiety-like behaviors and impaired fear extinction. Morphine injection 24 h after SPS significantly improved anxiety-like behaviors and enhanced fear extinction. The RU486, spironolactone and metyrapone prevented the protective effects of morphine on both SPS-induced anxiety-like behaviors and impaired fear extinction. The propranolol, and nadolol did not prevent the effect of morphine on anxiety-like behaviors, but the propranolol prevented morphine effects on fear extinction in SPS animals. These findings together suggest that the protective effects of morphine on PTSD-like symptoms in rats require a certain level of the HPA axis and central beta - adrenergic activity and any alteration in the function of these systems can impede the protective effects of morphine.

Sweet taste does not modulate pain perception in adult humans

Background: Sugar is routinely used to comfort neonates undergoing painful procedures, and animal studies have shown that sucrose increases the time to withdrawal from painful stimuli. However, there are no published studies examining the effects of sweet substances on heat pain thresholds and percept in adult humans. Methods: Healthy adult volunteers (n=27, aged 18-48 years) were recruited to a controlled, double-blind, randomised, cross-over study to characterise the effect of tasting solutions of equivalent sweetness (10% sucrose and 0.016% sucralose) on warm detection and heat pain thresholds and the percept ratings of painfully hot stimuli. The effect of anticipation of a sweet taste on heat pain threshold was also assessed. Results: Tasting either sucrose or sucralose had no significant effect on the percept of an individually titrated hot stimulus (54.5±4.2 and 54.9±3.2 vs 53.2±3.5 for water, 0-100 visual analogue scale), on the warm detection or heat pain threshold (43.3±0.8, 43.2±0.8 vs 43.0±0.8°C). Anticipation of a sweet substance similarly did not affect heat pain thresholds. Conclusions: Sucrose and sucralose solutions had no analgesic effect when assessed using heat detection thresholds and percept ratings of painfully hot stimuli despite being perceived as sweeter and more pleasant than water. These findings are in contrast to results reported from previous animal studies in which thermal analgesia from sweet solutions is robust. Given the ubiquitous availability of sugar rich drinks in the modern environment, the lack of observable effect may be due to an insufficient hedonic value of the test solutions when compared to the experience of a laboratory rodent. Alternatively, sweet tastes may have a specific effect on pain tolerance rather than the threshold and acute percept measures assayed in this study.

Sweet taste does not modulate pain perception in adult humans

Background: It is commonly observed that humans who are in pain or discomfort seek solace in the form of sweet foods and drinks. Sugar is routinely used to comfort neonates undergoing painful procedures, and animal studies have shown that sucrose increases the time to withdrawal from painful stimuli. However, there are no published studies examining the effects of sweet substances on heat pain thresholds and percept in adult humans. Methods: Healthy adult volunteers (n=27, aged 18-48 years) were recruited to a controlled, double-blind, randomised, cross-over study to characterise the effect of tasting solutions of equivalent sweetness (10% sucrose and 0.016% sucralose) on warm detection and heat pain thresholds and the percept ratings of painfully hot stimuli. The effect of anticipation of a sweet taste on heat pain threshold was also assessed. Results: Tasting either sucrose or sucralose had no significant effect on the percept of an individually titrated hot stimulus (54.5±4.2 and 54.9±3.2 vs 53.2±3.5 for water, 0-100 visual analogue scale), on the warm detection or heat pain threshold (43.3±0.8, 43.2±0.8 vs 43.0±0.8°C). Anticipation of a sweet substance similarly did not affect heat pain thresholds. Conclusions: Sucrose and sucralose solutions had no analgesic effect when assessed using heat detection thresholds and percept ratings of painfully hot stimuli despite being perceived as sweeter and more pleasant than water. These findings are in contrast to results reported from previous animal studies in which thermal analgesia from sweet solutions is robust. Given the ubiquitous availability of sugar rich drinks in the modern environment, the lack of observable effect may be due to an insufficient hedonic value of the test solutions when compared to the experience of a laboratory rodent. Alternatively, sweet tastes may have a specific effect on pain tolerance rather than the threshold and acute percept measures assayed in this study.

Effects of chronic stress on reinstatement of palatable food seeking: Sex differences and relationship to trait anxiety

Previous research in our lab has established a causal role for chronic stress exposure in subsequent increases in relapse-like behaviors in male rats with a history of palatable food self-administration. Given that many of the neurobehavioral consequences of stress are sex dependent, we aimed to determine whether sex differences exist with regard to the effects of chronic stress on relapse. Additionally, because high trait anxiety confers vulnerability to stress-related disorders, we examined whether individual differences in trait anxiety were related to differences in relapse-like behavior after chronic stress exposure. Following elevated plus maze testing for classification into high- or low-anxiety phenotypes, male and female rats responded for highly palatable food pellets. During subsequent extinction training, stress was manipulated (0 or 90 min restraint/day for 7 days). Rats were then tested for cue- and pellet priming-induced reinstatement of palatable food seeking. Results showed that female rats displayed higher levels of responding during cue-induced reinstatement tests compared to males, and that a history of chronic stress caused an attenuation of cue-induced reinstatement in female, but not male, rats. Regarding pellet priming-induced reinstatement, there was a three-way interaction such that neither stress history nor anxiety phenotype was related to reinstatement in females, but a history of stress in males caused increased and decreased responding in low- and high-anxiety rats, respectively. These results suggest that biological sex and trait anxiety level may help to explain differences in vulnerability to relapse among individuals exposed to chronic stress. Such information may be useful in designing more personalized and effective treatments for obesity and eating disorders.

Effects of environmental enrichment on reinstatement of methamphetamine-induced conditioned place preference

OBJECTIVE

The influence of environmental enrichment (EE) on reinstatement to methamphetamine (METH) seeking in rat model was investigated.

METHODS

Wistar rats were divided to receive saline (1 ml/kg) or METH (1 mg/kg, i.p.) for 8 days during the conditioning training in the conditioned place preference (CPP) paradigm, which is one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. Rats were then kept in either isolated (IE) or enriched environment (EE) for 30 days during the extinction training. Animals were finally examined for reinstatement provoked by i.p. injections of METH.

RESULTS

Saline injections during the conditioning phase did not change CPP during reinstatement in animals of IE or EE control groups. METH injections reinstated place preference in the IE animal group. Interestingly, EE significantly blocked this reinstatement effects of METH.

CONCLUSION

These results show the important role of social interactions and positive environment conditions in preventing reinstatement to drug use.

Glutamate receptor interacting protein acts within the prefrontal cortex to blunt cocaine seeking

Glutamate receptor interacting protein (GRIP) is a neuronal scaffolding protein that anchors GluA2-containing AMPA receptors to the cell membrane. GRIP plays a critical role in activity-dependent synaptic plasticity, including that which occurs after drug exposure. Given that cocaine administration alters glutamate receptor trafficking within the prefrontal cortex (PFC), a better understanding of the role of receptor trafficking proteins could lead to a more complete understanding of addictive phenotypes. AMPA receptor trafficking in general, and GRIP specifically, is known to play a role in cocaine seeking and conditioned reward in the nucleus accumbens, but its role in the PFC has not been characterized. The current study demonstrates that conditional deletion of GRIP1 in the medial prefrontal cortex increases the motivation for cocaine and potentiates cue-induced reinstatement of cocaine seeking in male and female mice. As no effects of PFC GRIP1 deletion were seen in reinstatement of food seeking, strategy set-shifting, or reversal learning the effects on cocaine seeking are not related to generalized alterations in cognitive function. While disrupting GRIP1 might be expected to lead to decreased AMPA transmission, our electrophysiological data indicate an increase in sEPSC amplitude in the prefrontal cortex and a corresponding decrease in paired pulse facilitation in the nucleus accumbens. Taken together this suggests a strengthening of the PFC to NAc input following prefrontal GRIP1 deletion that may mediate the enhanced drug seeking behavior.

A medium throughput rodent model of relapse from addiction with behavioral and pharmacological specificity

One of most formidable problems in the treatment of addiction is the high rate of relapse. The discovery of medicines to help mitigate relapse are aided by animal models that currently involve weeks of training and require surgical preparations and drug delivery devices. The present set of experiments was initiated to investigate a rapid 8-day screening method that utilizes food instead of intravenous drug administration. Male Sprague-Dawley rats were trained in a reinstatement paradigm in which every lever press produced a 45 mg food pellet concurrently paired with a light and tone. Behavior was subsequently extinguished with lever responses producing neither food nor food-associated stimuli. Reinstatement of responding was evaluated under conditions in which the first three responses of every 5 min time bin produced a food pellet along with food-associated stimuli. The mGlu₅ receptor antagonists MPEP and MTEP produced a significant reduction in reinstatement while failing to alter responding where every response produced food. The cannabinoid CB₁ receptor antagonist rimonabant and the mGlu_2/3 receptor agonist LY379268 also selectively reduced reinstatement. Other compounds including clozapine, d-amphetamine, chlordiazepoxide, ABT-431, naltrexone and citalopram were without effect. The results suggest that relapse-like behavioral effects can be extended to non-pharmacological reinforcers. Drug effects demonstrated both behavioral and pharmacological specificity. The present experimental design thus allows for efficient and rapid assessment of the effects of drugs that might be useful in the treatment of addiction-associated relapse.

The intersection of stress and reward: BNST modulation of aversive and appetitive states

The bed nucleus of the stria terminalis (BNST) is widely acknowledged as a brain structure that regulates stress and anxiety states, as well as aversive and appetitive behaviours. The diverse roles of the BNST are afforded by its highly modular organisation, neurochemical heterogeneity, and complex intrinsic and extrinsic circuitry. There has been growing interest in the BNST in relation to psychopathologies such as anxiety and addiction. Although research on the human BNST is still in its infancy, there have been extensive preclinical studies examining the molecular signature and hodology of the BNST and their involvement in stress and reward seeking behaviour. This review examines the neurochemical phenotype and connectivity of the BNST, as well as electrophysiological correlates of plasticity in the BNST mediated by stress and/or drugs of abuse.

Intermittent, extended access to preferred food leads to escalated food reinforcement and cyclic whole-body metabolism in rats: Sex differences and individual vulnerability

Compulsive binge eating is a hallmark of binge eating disorder and bulimia nervosa and is implicated in some obesity cases. Eating disorders are sexually dimorphic, with females more often affected than males. Animal models of binge-like eating based on intermittent access to palatable food exist; but, little is known regarding sex differences or individual vulnerability in these models with respect to the reinforcing efficacy of food, the development of compulsive- and binge-like eating, or associated changes in whole-body metabolism or body composition. Adolescent male (n = 24) and female (n = 32) Wistar rats were maintained on chow or a preferred, high-sucrose, chocolate-flavored diet in continuous or intermittent, extended access conditions. Body weight and composition, intake, fixed- and progressive-ratio operant self-administration, and whole body energy expenditure and respiratory exchange ratios were measured across an 11-week study period. Subgroup analyses were conducted to differentiate compulsive-like "high responder" intermittent access rats that escalated to extreme progressive-ratio self-administration performance vs. more resistant "low responders." Female rats had greater reinforcing efficacy of food than males in all diet conditions and were more often classified as "high responders". In both sexes, rats with intermittent access showed cycling of fuel substrate utilization and whole-body energy expenditure. Further, "high-responding" intermittent access female rats had especially elevated respiratory exchange ratios, indicating a fat-sparing phenotype. Future studies are needed to better understand the molecular and neurobiological basis of the sex and individual differences we have observed in rats and their translational impact for humans with compulsive, binge eating disorders.

Food Addiction and Binge Eating: Lessons Learned from Animal Models

The feeding process is required for basic life, influenced by environment cues and tightly regulated according to demands of the internal milieu by regulatory brain circuits. Although eating behaviour cannot be considered "addictive" under normal circumstances, people can become "addicted" to this behaviour, similarly to how some people are addicted to drugs. The symptoms, cravings and causes of "eating addiction" are remarkably similar to those experienced by drug addicts, and both drug-seeking behaviour as eating addiction share the same neural pathways. However, while the drug addiction process has been highly characterised, eating addiction is a nascent field. In fact, there is still a great controversy over the concept of "food addiction". This review aims to summarize the most relevant animal models of "eating addictive behaviour", emphasising binge eating disorder, that could help us to understand the neurobiological mechanisms hidden under this behaviour, and to improve the psychotherapy and pharmacological treatment in patients suffering from these pathologies.

Hypothalamic CRF1 receptor mechanisms are not sufficient to account for binge-like palatable food consumption in female rats

OBJECTIVE

The present study evaluated the effect of systemic injection of the CRF1 receptor antagonist R121919, the corticosterone synthesis inhibitor metyrapone and central amygdala (CeA) injections of the nonselective CRF antagonist D-Phe-CRF_(12-41) in rats in which binge eating was evoked by stress and cycles of food restriction.

METHOD

Female rats were subjected or not to repeated cycles of regular chow food restriction/ad libitum feeding during which they were also given limited access (2 h) to palatable food. On the test day, rats were either exposed or not to the sight of the palatable food for 15 min without allowing access, before assessing food consumption.

RESULTS

Systemic injections of R121919, but not of the metyrapone, blocked binge-like eating behavior. Restricted and stressed rats showed up-regulation of crh1 receptor mRNA signal in the bed nucleus of the stria terminalis and CeA but not in basolateral amygdala (BLA) or in the paraventricular nucleus. Injection D-Phe-CRF_(12-41) in CeA but not in the BLA-blocked binge-like eating behavior.

DISCUSSION

These findings demonstrate that extra-hypothalamic CRF1 receptors, rather than those involved in endocrine functions, are involved in binge eating and the crucial role of CRF receptors in CeA. CRF1 receptor antagonism may represent a novel pharmacological treatment for binge-related eating disorders.

Corticotropin-Releasing Factor (CRF) and Addictive Behaviors

Drug addiction is a complex disorder that is characterized by compulsivity to seek and take the drug, loss of control in limiting intake of the drug, and emergence of a withdrawal syndrome in the absence of the drug. The transition from casual drug use to dependence is mediated by changes in reward and brain stress functions and has been linked to a shift from positive reinforcement to negative reinforcement. The recruitment of brain stress systems mediates the negative emotional state produced by dependence that drives drug seeking through negative reinforcement mechanisms, defined as the "dark side" of addiction. In this chapter we focus on behavioral and cellular neuropharmacological studies that have implicated brain stress systems (i.e., corticotropin-releasing factor [CRF]) in the transition to addiction and the predominant brain regions involved. We also discuss the implication of CRF recruitment in compulsive eating disorders.

Role of addiction and stress neurobiology on food intake and obesity

The US remains at the forefront of a global obesity epidemic with a significant negative impact on public health. While it is well known that a balance between energy intake and expenditure is homeostatically regulated to control weight, growing evidence points to multifactorial social, neurobehavioral and metabolic determinants of food intake that influence obesity risk. This review presents factors such as the ubiquitous presence of rewarding foods in the environment and increased salience of such foods that stimulate brain reward motivation and stress circuits to influence eating behaviors. These rewarding foods via conditioned and reinforcing effects stimulate not only metabolic, but also stress hormones, that, in turn, hijack the brain emotional (limbic) and motivational (striatal) pathways, to promote food craving and excessive food intake. Furthermore, the impact of high levels of stress and trauma and altered metabolic environment (e.g. higher weight, altered insulin sensitivity) on prefrontal cortical self-control processes that regulate emotional, motivational and visceral homeostatic mechanisms of food intake and obesity risk are also discussed. A heuristic framework is presented in which the interactive dynamic effects of neurobehavioral adaptations in metabolic, motivation and stress neurobiology may further support food craving, excessive food intake and weight gain in a complex feed-forward manner. Implications of such adaptations in brain addictive-motivational and stress pathways and their effects on excessive food intake and weight gain are discussed to highlight key questions that requires future research attention in order to better understand and address the growing obesity epidemic.

Don't stress about CRF: assessing the translational failures of CRF1antagonists

BACKGROUND

Dr. Athina Markou sought treatments for a common neural substrate shared by depression and drug dependence. Antagonists of corticotropin-releasing factor (CRF) receptors, a target of interest to her, have not reached the clinic despite strong preclinical rationale and sustained translational efforts.

METHODS

We explore potential causes for the failure of CRF1 antagonists and review recent findings concerning CRF-CRF1 systems in psychopathology.

RESULTS

Potential causes for negative outcomes include (1) poor safety and efficacy of initial drug candidates due to bad pharmacokinetic and physicochemical properties, (2) specificity problems with preclinical screens, (3) the acute nature of screens vs. late-presenting patients, (4) positive preclinical results limited to certain models and conditions with dynamic CRF-CRF1 activation not homologous to tested patients, (5) repeated CRF1 activation-induced plasticity that reduces the importance of ongoing CRF1 agonist stimulation, and (6) therapeutic silencing which may need to address CRF2 receptor or CRF-binding protein molecules, constitutive CRF1 activity, or molecules that influence agonist-independent activity or to target structural regions other than the allosteric site bound by all drug candidates. We describe potential markers of activation towards individualized treatment, human genetic, and functional data that still implicate CRF1 systems in emotional disturbance, sex differences, and suggestive clinical findings for CRF1 antagonists in food craving and CRF-driven HPA-axis overactivation.

CONCLUSION

The therapeutic scope of selective CRF1 antagonists now appears narrower than had been hoped. Yet, much remains to be learned about CRF's role in the neurobiology of dysphoria and addiction and the potential for novel anti-CRF therapies therein.

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

Obesity: Current and potential pharmacotherapeutics and targets

Obesity is a global epidemic that contributes to a number of health complications including cardiovascular disease, type 2 diabetes, cancer and neuropsychiatric disorders. Pharmacotherapeutic strategies to treat obesity are urgently needed. Research over the past two decades has increased substantially our knowledge of central and peripheral mechanisms underlying homeostatic energy balance. Homeostatic mechanisms involve multiple components including neuronal circuits, some originating in hypothalamus and brain stem, as well as peripherally-derived satiety, hunger and adiposity signals that modulate neural activity and regulate eating behavior. Dysregulation of one or more of these homeostatic components results in obesity. Coincident with obesity, reward mechanisms that regulate hedonic aspects of food intake override the homeostatic regulation of eating. In addition to functional interactions between homeostatic and reward systems in the regulation of food intake, homeostatic signals have the ability to alter vulnerability to drug abuse. Regarding the treatment of obesity, pharmacological monotherapies primarily focus on a single protein target. FDA-approved monotherapy options include phentermine (Adipex-P®), orlistat (Xenical®), lorcaserin (Belviq®) and liraglutide (Saxenda®). However, monotherapies have limited efficacy, in part due to the recruitment of alternate and counter-regulatory pathways. Consequently, a multi-target approach may provide greater benefit. Recently, two combination products have been approved by the FDA to treat obesity, including phentermine/topiramate (Qsymia®) and naltrexone/bupropion (Contrave®). The current review provides an overview of homeostatic and reward mechanisms that regulate energy balance, potential therapeutic targets for obesity and current treatment options, including some candidate therapeutics in clinical development. Finally, challenges in anti-obesity drug development are discussed.

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.

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.

Baclofen blocks yohimbine-induced increases in ethanol-reinforced responding in rats

Chronic or repeated stress increases alcohol consumption. The GABA-B agonist baclofen decreases alcohol consumption and may be most effective for individuals with comorbid anxiety/stress disorders. The present study sought to determine if baclofen blocks stress-induced increases in ethanol self-administration as modeled by repeated yohimbine injections in rats. Rats were trained to respond for 15% w/v ethanol in operant chambers using a method that applies neither water deprivation nor saccharin/sucrose fading. Following training, the rats received 6 injections of 1.25mg/kg yohimbine were given immediately prior to the operant sessions during a 2-week time period. Subsequently, some rats were pair-matched to receive either 1.25mg/kg yohimbine or saline in the presence of 0.3, 1, and 3mg/kg baclofen prior to sessions. Acquisition of ethanol self-administration was poor. Pretreatment with yohimbine consistently increased responding across repeated injections. Yohimbine's effect on ethanol intake unexpectedly diverged from the effect on responding as the rats failed to consume all reinforcers earned. Smaller doses of baclofen paired with saline injections had no effect on ethanol responding; only 3mg/kg baclofen reduced ethanol self-administration. The smallest baclofen dose of 0.3mg/kg failed to block the yohimbine-induced increase in self-administration. The large baclofen dose of 3mg/kg continued to suppress ethanol self-administration when given with yohimbine. Baclofen 1mg/kg blocked the effect of yohimbine even though it had no effect when given in the absence of yohimbine. Exposure to high ethanol concentrations may induce self-administration only in certain conditions. The dissociation between responding and intake suggests that repeated yohimbine injections may initiate other behavioral or physiological mechanisms that confound its effects as a pharmacological stressor. Furthermore, an optimal baclofen dose range may specifically protect against stress-induced alcohol self-administration, highlighting a specific contribution of GABA-B receptors and a potential therapeutic efficacy of GABA-B agonists at a non-sedating dose.

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.

Intensity of anxiety is modified via complex integrative stress circuitries

Escalation of anxious behavior while environmentally and socially relevant contextual events amplify the intensity of emotional response produces a testable gradient of anxiety shaped by integrative circuitries. Apprehension of the Stress-Alternatives Model apparatus (SAM) oval open field (OF) is measured by the active latency to escape, and is delayed by unfamiliarity with the passageway. Familiar OF escape is the least anxious behavior along the continuum, which can be reduced by anxiolytics such as icv neuropeptide S (NPS). Social aggression increases anxiousness in the SAM, reducing the number of mice willing to escape by 50%. The apprehension accompanying escape during social aggression is diminished by anxiolytics, such as exercise and corticotropin releasing-factor receptor 1 (CRF1) antagonism, but exacerbated by anxiogenic treatment, like antagonism of α2-adrenoreceptors. What is more, the anxiolytic CRF1 and anxiogenic α2-adrenoreceptor antagonists also modify behavioral phenotypes, with CRF1 antagonism allowing escape by previously submissive animals, and α2-adrenoreceptor antagonism hindering escape in mice that previously engaged in it. Gene expression of NPS and brain-derived neurotrophic factor (BDNF) in the central amygdala (CeA), as well as corticosterone secretion, increased concomitantly with the escalating anxious content of the mouse-specific anxiety continuum. The general trend of CeA NPS and BDNF expression suggested that NPS production was promoted by increasing anxiousness, and that BDNF synthesis was associated with learning about ever-more anxious conditions. The intensity gradient for anxious behavior resulting from varying contextual conditions may yield an improved conceptualization of the complexity of mechanisms producing the natural continuum of human anxious conditions, and potential therapies that arise therefrom.

Differential effects of the pharmacological stressor yohimbine on impulsive decision making and response inhibition

RATIONALE

High levels of impulsivity have been associated with psychiatric disorders such as attention-deficit/hyperactivity disorder (ADHD) and substance abuse. In addition, acute stress is known to exacerbate many psychiatric symptoms in impulse control disorders.

OBJECTIVES

The purpose of the current study was to investigate the acute effects of the pharmacological stressor yohimbine on response inhibition and impulsive choice.

METHODS

A group of male rats (n = 12) was trained in the delayed reward task (DRT) to assess impulsive choice. A separate group (n = 10) was trained in the stop-signal task (SST) to measure response inhibition. Upon stable responding, the effects of yohimbine (0, 1.25, 2.5, and 5 mg/kg i.p.) were tested in a Latin square design.

RESULTS

Acute yohimbine significantly increased the preference for the large and delayed reinforcer in the DRT, indicating a decrease in impulsive choice. On the contrary, the effect size of 1.25 mg/kg yohimbine on stop-signal reaction times correlated negatively with baseline performance, suggesting a baseline-dependent effect on response inhibition as measured in the SST.

CONCLUSIONS

The current data suggest that the effects of the pharmacological stressor yohimbine on impulse control strongly depend on the type of impulsive behavior. Pharmacological stress decreased impulsive decision making, an observation that is in line with previously published rodent studies. By contrast, the lowest dose of yohimbine revealed a baseline-dependent effect on response inhibition. As such, the effects of yohimbine are largely comparable to the effects of psychostimulants on impulsivity and may support the notion of cross sensitization of stress and psychostimulants.

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.

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.

Modulation of Food Reward by Endocrine and Environmental Factors: Update and Perspective

OBJECTIVE

Palatable foods are frequently high in energy density. Chronic consumption of high-energy density foods can contribute to the development of cardiometabolic pathology including obesity, diabetes, and cardiovascular disease. This article reviews the contributions of extrinsic and intrinsic factors that influence the reward components of food intake.

METHODS

A narrative review was conducted to determine the behavioral and central nervous system (CNS) related processes involved in the reward components of high-energy density food intake.

RESULTS

The rewarding aspects of food, particularly palatable and preferred foods, are regulated by CNS circuitry. Overlaying this regulation is modulation by intrinsic endocrine systems and metabolic hormones relating to energy homeostasis, developmental stage, or gender. It is now recognized that extrinsic or environmental factors, including ambient diet composition and the provocation of stress or anxiety, also contribute substantially to the expression of food reward behaviors such as motivation for, and seeking of, preferred foods.

CONCLUSIONS

High-energy density food intake is influenced by both physiological and pathophysiological processes. Contextual, behavioral, and psychological factors and CNS-related processes represent potential targets for multiple types of therapeutic intervention.

Enhanced motivation for food reward induced by stress and attenuation by corticotrophin-releasing factor receptor antagonism in rats: implications for overeating and obesity

RATIONALE

Overeating beyond individuals' homeostatic needs critically contributes to obesity. The neurobehavioral mechanisms underlying the motivation to consume excessive foods with high calories are not fully understood.

OBJECTIVE

The present study examined whether a pharmacological stressor, yohimbine, enhances the motivation to procure food reward with an emphasis on comparisons between standard lab chow and high-fat foods. The effects of corticotropin-releasing factor (CRF) receptor blockade by a CRF1-selective antagonist NBI on the stress-enhanced motivation for food reward were also assessed.

METHODS

Male Sprague-Dawley rats with chow available ad libitum in their home cages were trained to press a lever under a progressive ratio schedule for deliveries of either standard or high-fat food pellets. For testing yohimbine stress effects, rats received an intraperitoneal administration of yohimbine 10 min before start of the test sessions. For testing effects of CRF1 receptor blockade on stress responses, NBI was administered 20 min prior to yohimbine challenge.

RESULTS

The rats emitted higher levels of lever responses to procure the high-fat food pellets compared with their counterparts on standard food pellets. Yohimbine challenge facilitated lever responses for the reward in all of the rats, whereas the effect was more robust in the rats on high-fat food pellets compared with their counterparts on standard food pellets. An inhibitory effect of pretreatment with NBI was observed on the enhancing effect of yohimbine challenge but not on the responses under baseline condition without yohimbine administration.

CONCLUSIONS

Stress challenge significantly enhanced the motivation of satiated rats to procure extra food reward, especially the high-fat food pellets. Activation of CRF1 receptors is required for the stress-enhanced motivation for food reward. These results may have implications for our better understanding of the biobehavioral mechanisms of overeating and obesity.

A test of stress, cues, and re-exposure to large wins as potential reinstaters of suboptimal decision making in rats

The present experiment investigated potential reinstaters of suboptimal economic decision making in rats. Rats were first trained on a version of the rat Gambling Task under conditions designed to promote choice of a suboptimal option that occasionally resulted in large “wins” (four sucrose pellets). In a second phase, preference for this economically suboptimal option was reduced by substantially increasing the probability of punishment when this option was chosen. Then, three events were tested for their ability to reinstate choice of the suboptimal option. A brief period of re-exposure to a high frequency of large wins significantly increased choice of the suboptimal option. The pharmacological stressor yohimbine did not reinstate suboptimal choice, but did increase impulsive action as indexed by premature responding. Presentation of cues previously associated with large wins did not alter behavior. Results suggest reinstaters of suboptimal choice may differ from reinstaters of extinguished drug- and food-seeking behavior.

Antagonism of corticotrophin-releasing factor type 1 receptors attenuates caloric intake of free feeding subordinate female rhesus monkeys in a rich dietary environment

Social subordination in macaque females is a known chronic stressor and previous studies have shown that socially subordinate female rhesus monkeys consume fewer kilocalories than dominant animals when a typical laboratory chow diet is available. However, in a rich dietary environment that provides access to chow in combination with a more palatable diet (i.e. high in fat and refined sugar), subordinate animals consume significantly more daily kilocalories than dominant conspecifics. Substantial literature is available supporting the role of stress hormone signals in shaping dietary preferences and promoting the consumption of palatable, energy-dense foods. The present study was conducted using stable groups of adult female rhesus monkeys to test the hypothesis that pharmacological treatment with a brain penetrable corticotrophin-releasing factor type 1 receptor (CRF1) antagonist would attenuate the stress-induced consumption of a palatable diet among subordinate animals in a rich dietary environment but would be without effect in dominant females. The results show that administration of the CRF1 receptor antagonist significantly reduced daily caloric intake of both available diets among subordinate females compared to dominant females. Importantly, multiple regression analyses showed that the attenuation in caloric intake in response to Antalarmin (Sigma-Aldrich, St Louis, MO, USA) was significantly predicted by the frequency of submissive and aggressive behaviour emitted by females, independent of social status. Taken together, the findings support the involvement of activation of CRF1 receptors in the stress-induced consumption of excess calories in a rich dietary environment and also support the growing literature concerning the importance of CRF for sustaining emotional feeding.

Role of bed nucleus of the stria terminalis corticotrophin-releasing factor receptors in frustration stress-induced binge-like palatable food consumption in female rats with a history of food restriction

We developed recently a binge-eating model in which female rats with a history of intermittent food restriction show binge-like palatable food consumption after 15 min exposure to the sight of the palatable food. This "frustration stress" manipulation also activates the hypothalamic-pituitary-adrenal stress axis. Here, we determined the role of the stress neurohormone corticotropin-releasing factor (CRF) in stress-induced binge eating in our model. We also assessed the role of CRF receptors in the bed nucleus of the stria terminalis (BNST), a brain region implicated in stress responses and stress-induced drug seeking, in stress-induced binge eating. We used four groups that were first exposed or not exposed to repeated intermittent cycles of regular chow food restriction during which they were also given intermittent access to high-caloric palatable food. On the test day, we either exposed or did not expose the rats to the sight of the palatable food for 15 min (frustration stress) before assessing food consumption for 2 h. We found that systemic injections of the CRF1 receptor antagonist R121919 (2,5-dimethyl-3-(6-dimethyl-4-methylpyridin-3-yl)-7 dipropylamino pyrazolo[1,5-a]pyrimidine) (10-20 mg/kg) and BNST (25-50 ng/side) or ventricular (1000 ng) injections of the nonselective CRF receptor antagonist D-Phe-CRF(12-41) decreased frustration stress-induced binge eating in rats with a history of food restriction. Frustration stress also increased Fos (a neuronal activity marker) expression in ventral and dorsal BNST. Results demonstrate a critical role of CRF receptors in BNST in stress-induced binge eating in our rat model. CRF1 receptor antagonists may represent a novel pharmacological treatment for bingeing-related eating disorders.

Animal models of compulsive eating behavior

Eating disorders are multifactorial conditions that can involve a combination of genetic, metabolic, environmental, and behavioral factors. Studies in humans and laboratory animals show that eating can also be regulated by factors unrelated to metabolic control. Several studies suggest a link between stress, access to highly palatable food, and eating disorders. Eating "comfort foods" in response to a negative emotional state, for example, suggests that some individuals overeat to self-medicate. Clinical data suggest that some individuals may develop addiction-like behaviors from consuming palatable foods. Based on this observation, "food addiction" has emerged as an area of intense scientific research. A growing body of evidence suggests that some aspects of food addiction, such as compulsive eating behavior, can be modeled in animals. Moreover, several areas of the brain, including various neurotransmitter systems, are involved in the reinforcement effects of both food and drugs, suggesting that natural and pharmacological stimuli activate similar neural systems. In addition, several recent studies have identified a putative connection between neural circuits activated in the seeking and intake of both palatable food and drugs. The development of well-characterized animal models will increase our understanding of the etiological factors of food addiction and will help identify the neural substrates involved in eating disorders such as compulsive overeating. Such models will facilitate the development and validation of targeted pharmacological therapies.

Knockdown of CRF1 receptors in the ventral tegmental area attenuates cue- and acute food deprivation stress-induced cocaine seeking in mice

Corticotrophin-releasing factor (CRF) modulates the influence of stress on cocaine reward and reward seeking acting at multiple sites, including the ventral tegmental area (VTA). There is controversy, however, concerning the contribution of CRF receptor type 1 (CRFR1) to this effect and whether CRF within the VTA is involved in other aspects of reward seeking independent of acute stress. Here we examine the role of CRFR1 within the VTA in relation to cocaine and natural reward using viral delivery of short hairpin RNAs (lenti-shCRFR1) and investigate the effect on operant self-administration and motivation to self-administer, as well as stress- and cue-induced reward seeking in mice. While knockdown of CRFR1 in the VTA had no effect on self-administration behavior for either cocaine or sucrose, it effectively blocked acute food deprivation stress-induced reinstatement of cocaine seeking. We also observed reduced cue-induced cocaine seeking assessed in a single extinction session after extended abstinence, but cue-induced sucrose seeking was unaffected, suggesting dissociation between the contribution of CRFR1 in the VTA in cocaine reward and sucrose and cocaine seeking. Further, our data indicate a role for VTA CRFR1 signaling in cocaine seeking associated with, and independent of, stress potentially involving conditioning and/or salience attribution of cocaine reward-related cues. CRFR1 signaling in the VTA therefore presents a target for convergent effects of both cue- and stress-induced cocaine-seeking pathways.

Impaired flexibility in decision making in rats after administration of the pharmacological stressor yohimbine

RATIONALE

Stress-induced disruption of decision making has been hypothesized to contribute to drug-seeking behaviors and addiction. Noradrenergic signaling plays a central role in mediating stress responses. However, the effects of acute stress on decision making, and the role of noradrenergic signaling in regulating these effects, have not been well characterized.

OBJECTIVE

To characterize changes in decision making caused by acute pharmacological stress, the effects of yohimbine (an α2-adrenergic antagonist) were examined in a delay discounting task. Noradrenergic contributions to decision making were further characterized by examining the effects of propranolol (a β antagonist), prazosin (an α1 antagonist), and guanfacine (an α2 agonist).

METHODS

Sprague-Dawley rats were administered drugs prior to performance on a delay discounting task, in which the delay preceding the large reward increased within each session (ascending delays). To dissociate drug-induced changes in delay sensitivity from behavioral inflexibility, drug effects were subsequently tested in a modified version of the discounting task, in which the delay preceding the large reward decreased within each session (descending delays).

RESULTS

Yohimbine increased choice of the large reward when tested with ascending delays but decreased choice of the same large reward when tested with descending delays, suggesting that drug effects could be attributed to perseverative choice of the lever preferred at the beginning of the session. Propranolol increased choice of the large reward when tested with ascending delays. Prazosin and guanfacine had no effect on reward choice.

CONCLUSIONS

The stress-like effects of yohimbine administration may impair decision making by causing inflexible, perseverative behavior.

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.

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.