Endogenous opioids in the nucleus accumbens promote approach to high-fat food in the absence of caloric need

Endogenous opioid receptors and the feast or famine of maladaptive feeding

Maladaptive feeding comprises unhealthy eating patterns that jeopardize survival, including over- and underconsumption. These behaviors are often coordinated by endogenous opioid receptors (EORs). Here, we explore the involvement of EORs in obesity and anorexia nervosa (AN), two disorders associated with dysregulated feeding behavior and relevant animal models. While seemingly opposing metabo-psychiatric states, our goal is to highlight common circuit and synaptic mechanisms underlying obesity and AN with a focus on EOR functionality. We examine the neural substrates underlying maladaptive feeding and comorbid conditions including pain, suggesting a role for EOR-driven plasticity in the pathogenesis of both obesity and AN.

Sex Differences in the Relationship Between Nucleus Accumbens Volume and Youth Tobacco or Marijuana Use Following Stressful Life Events

Background

Exposure to stressful life events (SLEs) can upset balance and affect the healthy brain development of children and youths. These events may influence substance use by altering brain reward systems, especially the nucleus accumbens (NAc), which plays a key role in motivated behaviors and reward processing. The interaction between sensitization to SLEs, depression, and substance use might vary between male and female youths, potentially due to differences in how each sex responds to SLEs.

Aims

This study aims to examine the effect of sex on the relationship between SLEs, Nucleus Accumbens activity, and substance use in a nationwide sample of young individuals.

Methods

We utilized data from the Adolescent Brain Cognitive Development study (ABCD), a longitudinal study of pre-adolescents aged 9-10 years, comprising 11,795 participants tracked over 36 months. Structured interviews measuring SLEs were conducted using the Kiddie Schedule for Affective Disorders and Schizophrenia (K-SADS). Initial linear regression analyses explored if SLEs could predict volumes of the right and left NAc. Subsequently, Cox regression models were used to investigate how SLEs and NAc volume might predict the initiation of tobacco and marijuana use, with the analysis stratified by sex to address potential sex differences.

Results

Our findings reveal that SLEs significantly predicted marijuana use in males but not in females, and tobacco use was influenced by SLEs in both sexes. A higher number of SLEs was linked with decreased left NAc volume in males, a trend not seen in females. The right NAc volume did not predict substance use in either sex. However, volumes of both the right and left NAc were significant predictors of future tobacco use, with varying relationships across sexes. In females, an inverse relationship was observed between both NAc volumes and the risk of tobacco use. In contrast, a positive correlation existed between the left NAc volume and tobacco and marijuana use in males, with no such relationship for females.

Conclusion

This study underscores that the associations between SLEs, NAc volume, and subsequent substance use are influenced by a nuanced interplay of sex, brain hemisphere, and substance type.

Central effects of opioidergic system on food intake in birds and mammals: a review

Undoubtedly, the food intake process is one of the most necessary physiological functions for the survival of animals and humans. Although; this operation seems simple on the surface, the regulation of the mechanisms involved in it requires the cooperation of many neurotransmitters, peptides, and hormonal factors in the nervous and endocrine systems. Understanding the signals that regulate energy levels and appetite, may open new approaches to therapeutics and drugs used in obesity-related complications. Improving the quality of animal products and health is also possible due to this research. The present review is aimed to sum up the current findings on central effects of opioids on the food consumption of birds and mammals. Based on the reviewed articles, the opioidergic system appears to be one of the key elements in the birds' and mammals' food intake and is closely related to other systems involved in appetite regulation. According to the findings, it seems that the effects of this system on nutritional mechanisms are often applied via kappa- and mu-opioid receptors. Controversial observations have been made regarding opioid receptors, highlighting the need for further studies, especially at the molecular level. The role of opiates in taste or diet craving also showed the efficacy of this system, especially the mu-opioid receptor, on preferences such as diets containing high sugar and fat. Finally, putting the results of this study together with the findings of human experiments and other primates can lead to a correct comprehension of the appetite regulation processes, especially the role of the opioidergic system.

Nucleus Accumbens Shell Neurons Encode the Kinematics of Reward Approach Locomotion

The nucleus accumbens (NAc) is considered an interface between motivation and action, with NAc neurons playing an important role in promoting reward approach. However, the encoding by NAc neurons that contributes to this role remains unknown. We recorded 62 NAc neurons in male Wistar rats (n = 5) running towards rewarded locations in an 8-arm radial maze. Variables related to locomotor approach kinematics were the best predictors of the firing rate for most NAc neurons. Nearly 18% of the recorded neurons were inhibited during the entire approach run (locomotion-off cells), suggesting that reduction in firing of these neurons promotes initiation of locomotor approach. 27% of the neurons presented a peak of activity during acceleration followed by a valley during deceleration (acceleration-on cells). Together, these neurons accounted for most of the speed and acceleration encoding identified in our analysis. In contrast, a further 16% of neurons presented a valley during acceleration followed by a peak just prior to or after reaching reward (deceleration-on cells). These findings suggest that these three classes of NAc neurons influence the time course of speed changes during locomotor approach to reward.

Highly processed foods can be considered addictive substances based on established scientific criteria

BACKGROUND

There is growing evidence that an addictive-eating phenotype may exist. There is significant debate regarding whether highly processed foods (HPFs; foods with refined carbohydrates and/or added fats) are addictive. The lack of scientifically grounded criteria to evaluate the addictive nature of HPFs has hindered the resolution of this debate.

ANALYSIS

The most recent scientific debate regarding a substance's addictive potential centered around tobacco. In 1988, the Surgeon General issued a report identifying tobacco products as addictive based on three primary scientific criteria: their ability to (1) cause highly controlled or compulsive use, (2) cause psychoactive (i.e. mood-altering) effects via their effect on the brain and (3) reinforce behavior. Scientific advances have now identified the ability of tobacco products to (4) trigger strong urges or craving as another important indicator of addictive potential. Here, we propose that these four criteria provide scientifically valid benchmarks that can be used to evaluate the addictiveness of HPFs. Then, we review the evidence regarding whether HPFs meet each criterion. Finally, we consider the implications of labeling HPFs as addictive.

CONCLUSION

Highly processed foods (HPFs) can meet the criteria to be labeled as addictive substances using the standards set for tobacco products. The addictive potential of HPFs may be a key factor contributing to the high public health costs associated with a food environment dominated by cheap, accessible and heavily marketed HPFs.

Appetitive Motivation and Associated Neurobiology Change Differentially across the Life Course of Mouse Offspring Exposed to Peri- and Postnatal High Fat Feeding

Alterations in neural pathways that regulate appetitive motivation may contribute to increased obesity risk in offspring born to mothers fed a high fat (HF) diet. However, current findings on the impact of maternal obesity on motivation in offspring are inconclusive, and there is no information about the long-lasting effects in aged animals. This study examined the longitudinal effect of perinatal and chronic postnatal HF intake on appetitive motivation in young and aged offspring. Female C57Bl/6 were fed either a control (C) or HF diet before mating through to lactation. At weaning, offspring were maintained on the C or HF diet, generating the following four diet groups: C/C, C/HF, HF/C, and HF/HF based on the pre/post weaning diet. At 6 months, motivation was higher in HF/C females, but lower in male and female C/HF and HF/HF mice. By 12 months, this difference was lost, as C-fed animals became less motivated, while motivation increased in HF-fed mice. The mRNA levels of dopamine receptor 1 and 2 increased with age, while cannabinoid receptor 1 and μ-opioid receptor expression remained stable or decreased in mesolimbic and mesocortical dopaminergic pathways. Results from this study suggest that perinatal and chronic postnatal HF feeding produced opposite effects on appetitive motivation in young adult offspring mice, which was also reflected in the shift in motivation over time. These results have significant implications for patterns of hedonic eating across the life course and the relative risk of obesity at different time points.

Eating driven by the gustatory insula: contrasting regulation by infralimbic vs. prelimbic cortices

Subregions within insular cortex and medial prefrontal cortex (mPFC) have been implicated in eating disorders; however, the way these brain regions interact to produce dysfunctional eating is poorly understood. The present study explored how two mPFC subregions, the infralimbic (IL) and prelimbic (PRL) cortices, regulate sucrose hyperphagia elicited specifically by a neurochemical manipulation of the agranular/dysgranular region of gustatory insula (AI/DI). Using intra-AI/DI infusion of the mu-opioid receptor (µ-OR) agonist, DAMGO (1 µg), sucrose hyperphagia was generated in ad-libitum-maintained rats, while in the same rat, either the IL or prelimbic (PRL) subregion of mPFC was inactivated bilaterally with muscimol (30 ng). Intra-IL muscimol markedly potentiated AI/DI DAMGO-induced sucrose hyperphagia by increasing eating bout duration and food consumption per bout. In contrast, PRL attenuated intra-AI/DI DAMGO-driven sucrose intake and feeding duration and eliminated the small DAMGO-induced increase in feeding bout initiation. Intra-IL or -PRL muscimol alone (i.e., without intra-AI/DI DAMGO) did not alter feeding behavior, but slightly reduced exploratory-like rearing in both mPFC subregions. These results reveal anatomical heterogeneity in mPFC regulation of the intense feeding-motivational state engendered by µ-OR signaling in the gustatory insula: IL significantly curtails consummatory activity, while PRL modestly contributes to feeding initiation. Results are discussed with regard to potential circuit-based mechanisms that may underlie the observed results.

Endogenous opiates and behavior: 2019

This paper is the forty-second consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2019 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

The immune-opioid axis in prediabetes: predicting prediabetes with insulin resistance by plasma interleukin-10 and endomorphin-2 to kappa-opioid receptors ratio

BACKGROUND

Prediabetes is characterized by a hemoglobin A1c of 5.7-6.4% and fasting blood glucose of 100-125 mg/dl. A high percentage of prediabetes subjects develop type 2 diabetes mellitus in the next years. The effects of opioid peptides and their receptors, in addition to immunological cytokines, on prediabetes are not well understood. Therefore, molecular, physiological, and clinical studies are required to link the opioid system, immune system, and insulin resistance (IR) in prediabetes. We hypothesize that opioid peptides (endomorphin-2 (EM2), and β-endorphin (βEP)), and their receptors (µ-opioid receptors (MOR) and κ-opioid receptors (KOR)), in addition to the inflammatory cytokines (IL-6) and anti-inflammatory cytokine (IL-10), affect IR parameters in patients with prediabetes.

METHODS

Sixty prediabetes patients with IR (prediabetes+IR) and sixty prediabetes patients without IR (prediabetes-IR), in addition to 58 controls, have participated in the study. IL-6, IL-10, EM2, βEP, MOR, and KOR were measured by the ELISA technique.

RESULTS

In general, most prediabetes subjects have dyslipidemia. The IL-6, IL-10, β-endorphin, MOR, and endomorphin-2 were higher in the prediabetes subgroups than the control group. The immune system was activated in the prediabetes in an IR-dependent manner. Prediabetes+IR can be predicted by the increased levels of IL-10, βEP, and EM2 and by the combination of IL-10 and EM2/KOR with good sensitivity and specificity.

CONCLUSION

Opioid peptides and their receptors were upregulated in patients with prediabetes, depending on the significance of IR and the immune cytokines. The intercorrelation between the immune system, EOS, and insulin in prediabetes was confirmed.

Stimuli predicting high-calorie reward increase dopamine release and drive approach to food in the absence of homeostatic need

Animals and humans are motivated to consume high-fat, high-calorie foods by cues predicting such foods. The neural mechanisms underlying this effect are not well understood.Objective: We tested the hypothesis that cues paired with a food reward, as compared to explicitly unpaired cues, increase rats' food-seeking behavior by potentiating dopamine release in the nucleus accumbens, and that this effect would be less evident under satiety.Methods: We used a simple discriminative stimulus task and electrochemical recordings of dopamine release in freely moving rats.Results: We found that both food-predictive cue and hunger increased conditioned approaches to the receptacle (food-seeking behavior indicated by movement to the food receptacle). In addition, we observed dopamine release when the food-predictive cue (but not the unpaired cue) was presented, independent of hunger or satiety. Finally, we found a positive correlation between dopamine release amplitude and the number of conditioned approaches to the food receptacle in the sated condition, but not in the hungry condition.Discussion: Our results suggest that dopamine could drive seeking behavior for calorie-dense food in absence of homeostatic need, a core aspect of binge eating disorders.

NMDA receptor-dependent plasticity in the nucleus accumbens connects reward-predictive cues to approach responses

Learning associations between environmental cues and rewards is a fundamental adaptive function. Via such learning, reward-predictive cues come to activate approach to locations where reward is available. The nucleus accumbens (NAc) is essential for cued approach behavior in trained subjects, and cue-evoked excitations in NAc neurons are critical for the expression of this behavior. Excitatory synapses within the NAc undergo synaptic plasticity that presumably contributes to cued approach acquisition, but a direct link between synaptic plasticity within the NAc and the development of cue-evoked neural activity during learning has not been established. Here we show that, with repeated cue-reward pairings, cue-evoked excitations in the NAc emerge and grow in the trials prior to the detectable expression of cued approach behavior. We demonstrate that the growth of these signals requires NMDA receptor-dependent plasticity within the NAc, revealing a neural mechanism by which the NAc participates in learning of conditioned reward-seeking behaviors.

Conditioned stimuli elicit phasic changes in nucleus accumbens (NAc) firing that invigorate approach responses to predicted rewards. Here the authors show that NAc neurons acquire cue-evoked responses during learning as a result of excitatory plasticity within the NAc.

Local μ-Opioid Receptor Antagonism Blunts Evoked Phasic Dopamine Release in the Nucleus Accumbens of Rats

μ-opioid receptors (MORs) in the nucleus accumbens (NAc) can regulate reward-related behaviors that are dependent on mesolimbic dopamine, but the precise mechanism of this MOR regulation is unknown. We hypothesized that MORs within the NAc core regulate dopamine release. Specifically, we infused the MOR antagonist CTAP (d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2) into the NAc core while dopamine release was evoked by electrical stimulation of the ventral tegmental area and measured by fast-scan cyclic voltammetry. We report that CTAP dose-dependently inhibited evoked dopamine release, with full blockade achieved with the 8 μg infusion. In contrast, evoked dopamine release increased after nomifensine infusion and was unchanged after vehicle infusion. These findings demonstrate profound local control of dopamine release by MORs within the NAc core, which has implications for regulation of reward processing.

Endocannabinoids in Body Weight Control

Maintenance of body weight is fundamental to maintain one's health and to promote longevity. Nevertheless, it appears that the global obesity epidemic is still constantly increasing. Endocannabinoids (eCBs) are lipid messengers that are involved in overall body weight control by interfering with manifold central and peripheral regulatory circuits that orchestrate energy homeostasis. Initially, blocking of eCB signaling by first generation cannabinoid type 1 receptor (CB1) inverse agonists such as rimonabant revealed body weight-reducing effects in laboratory animals and men. Unfortunately, rimonabant also induced severe psychiatric side effects. At this point, it became clear that future cannabinoid research has to decipher more precisely the underlying central and peripheral mechanisms behind eCB-driven control of feeding behavior and whole body energy metabolism. Here, we will summarize the most recent advances in understanding how central eCBs interfere with circuits in the brain that control food intake and energy expenditure. Next, we will focus on how peripheral eCBs affect food digestion, nutrient transformation and energy expenditure by interfering with signaling cascades in the gastrointestinal tract, liver, pancreas, fat depots and endocrine glands. To finally outline the safe future potential of cannabinoids as medicines, our overall goal is to address the molecular, cellular and pharmacological logic behind central and peripheral eCB-mediated body weight control, and to figure out how these precise mechanistic insights are currently transferred into the development of next generation cannabinoid medicines displaying clearly improved safety profiles, such as significantly reduced side effects.