Feeding-modulatory effects of mu-opioids in the medial prefrontal cortex: a review of recent findings and comparison to opioid actions in the nucleus accumbens

Reward processing in schizophrenia and its relation to Mu opioid receptor availability and negative symptoms: A [11C]-carfentanil PET and fMRI study

BACKGROUND

Reward processing deficits are a core feature of schizophrenia and are thought to underlie negative symptoms. Pre-clinical evidence suggests that opioid neurotransmission is linked to reward processing. However, the contribution of Mu Opioid Receptor (MOR) signalling to the reward processing abnormalities in schizophrenia is unknown. Here, we examined the association between MOR availability and the neural processes underlying reward anticipation in patients with schizophrenia using multimodal neuroimaging.

METHOD

37 subjects (18 with Schizophrenia with moderate severity negative symptoms and 19 age and sex-matched healthy controls) underwent a functional MRI scan while performing the Monetary Incentive Delay (MID) task to measure the neural response to reward anticipation. Participants also had a [11C]-carfentanil PET scan to measure MOR availability.

RESULTS

Reward anticipation was associated with increased neural activation in a widespread network of brain regions including the striatum. Patients with schizophrenia had both significantly lower MOR availability in the striatum as well as striatal hypoactivation during reward anticipation. However, there was no association between MOR availability and striatal neural activity during reward anticipation in either patient or controls (Pearson's Correlation, controls df = 17, r = 0.321, p = 0.18, patients df = 16, r = 0.295, p = 0.24). There was no association between anticipation-related neural activation and negative symptoms (r = -0.120, p = 0.14) or anhedonia severity (social r = -0.365, p = 0.14 physical r = -0.120, p = 0.63).

CONCLUSIONS

Our data suggest reduced MOR availability in schizophrenia might not underlie striatal hypoactivation during reward anticipation in patients with established illness. Therefore, other mechanisms, such as dopamine dysfunction, warrant further investigation as treatment targets for this aspect of the disorder.

Proopiomelanocortin projections to the nucleus accumbens modulate acquisition and maintenance of operant palatable pellet administration in mice

Obesity is a crisis in the United States, producing many co-morbid diseases that can drastically decrease quality of life. While diet is a major focus for therapeutic intervention, the need to understand underlying appetitive neurocircuitry persists. Proopiomelanocortin (POMC) peptides are well-known for their anorexigenic activity, but also mediate reward and learning. The nucleus accumbens (NAcc) is best known for its role in reward-based learning, but the contribution of POMC projections to NAcc on feeding are controversial since the two major POMC-derived peptides (β-endorphin and α-MSH) have opposite effects on food intake. Our objective was to determine the effect of stimulating POMC projections in the NAcc on acquisition and maintenance of operant self-administration of a palatable food. Adult POMCCre mice were microinjected into the NAcc with a Cre-dependent retrograde adeno-associated viral vector expressing Gq Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). Mice were trained to self-administer palatable 20-mg pellets in daily operant sessions. Acquisition of self-administration (fixed ratio 30) and baseline self-administration were measured in daily sessions, with mice receiving injections of either JHU37152 (DREADD agonist) or saline (i.p.) 15 min prior to the sessions. POMC neuron stimulation (JHU injection) before training sessions produced a significant increase in rate of acquisition and accuracy compared to the saline treated group, with no significant effect on rewards earned. Removal of POMC neuron stimulation before sessions initially reduced consumption with a gradual increase in responding for reinforcer over 3 days of saline injections. Reinstatement of POMC neuron stimulation (JHU) before the session resulted in a significant decrease in responding and rewards earned. These results suggest a complex role of POMC peptides within the NAcc that increase reward learning for a novel palatable food while decreasing consumption of the reinforcer following experience with it.

Depressive-like Behavior Is Accompanied by Prefrontal Cortical Innate Immune Fatigue and Dendritic Spine Losses after HIV-1 Tat and Morphine Exposure

Opioid use disorder (OUD) and HIV are comorbid epidemics that can increase depression. HIV and the viral protein Tat can directly induce neuronal injury within reward and emotionality brain circuitry, including the prefrontal cortex (PFC). Such damage involves both excitotoxic mechanisms and more indirect pathways through neuroinflammation, both of which can be worsened by opioid co-exposure. To assess whether excitotoxicity and/or neuroinflammation might drive depressive behaviors in persons infected with HIV (PWH) and those who use opioids, male mice were exposed to HIV-1 Tat for eight weeks, given escalating doses of morphine during the last two weeks, and assessed for depressive-like behavior. Tat expression decreased sucrose consumption and adaptability, whereas morphine administration increased chow consumption and exacerbated Tat-induced decreases in nesting and burrowing—activities associated with well-being. Across all treatment groups, depressive-like behavior correlated with increased proinflammatory cytokines in the PFC. Nevertheless, supporting the theory that innate immune responses adapt to chronic Tat exposure, most proinflammatory cytokines were unaffected by Tat or morphine. Further, Tat increased PFC levels of the anti-inflammatory cytokine IL-10, which were exacerbated by morphine administration. Tat, but not morphine, decreased dendritic spine density on layer V pyramidal neurons in the anterior cingulate. Together, our findings suggest that HIV-1 Tat and morphine differentially induce depressive-like behaviors associated with increased neuroinflammation, synaptic losses, and immune fatigue within the PFC.

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.

Turning the clock forward: New pharmacological and non pharmacological targets for the treatment of obesity

AIMS

Obesity and its main metabolic complication, type 2 diabetes, have attained the status of a global pandemic; there is need for novel strategies aimed at treating obesity and preventing the development of diabetes. A healthy diet and exercise are basic for treatment of obesity but often not enough. Pharmacotherapy can be helpful in maintaining compliance, ameliorating obesity-related health risks, and improving quality of life. In the last two decades, the knowledge of central and peripheral mechanisms underlying homeostatic and hedonic aspects of food intake has significantly increased. Dysregulation of one or more of these components could lead to obesity.

DATA SYNTHESIS

In order to better understand how potential innovative treatment options can affect obesity, homeostatic and reward mechanisms that regulate energy balance has been firstly illustrated. Then, an overview of potential therapeutic targets for obesity, distinguished according to the level of regulation of feeding behavior, has been provided. Moreover, several non-drug therapies have been recently tested in obesity, such as non-invasive neurostimulation: Transcranial Magnetic Stimulation or Transcranial Direct Current Stimulation. All of them are promising for obesity treatment and are almost devoid of side effects, constituting a potential resource for the prevention of metabolic diseases.

CONCLUSIONS

The plethora of current anti-obesity therapies creates the unique challenge for physicians to customize the intervention, according to the specific obesity characteristics and the intervention side effect profiles; moreover, it allows multimodal approaches addressed to treat obesity and metabolic adaptation with complementary mechanisms.

Neural alterations in opioid-exposed infants revealed by edge-centric brain functional networks

Prenatal opioid exposure has been linked to adverse effects spanning multiple neurodevelopmental domains, including cognition, motor development, attention, and vision. However, the neural basis of these abnormalities is largely unknown. A total of 49 infants, including 21 opioid-exposed and 28 controls, were enrolled and underwent MRI (43 ± 6 days old) after birth, including resting state functional MRI. Edge-centric functional networks based on dynamic functional connections were constructed, and machine-learning methods were employed to identify neural features distinguishing opioid-exposed infants from unexposed controls. An accuracy of 73.6% (sensitivity 76.25% and specificity 69.33%) was achieved using 10 times 10-fold cross-validation, which substantially outperformed those obtained using conventional static functional connections (accuracy 56.9%). More importantly, we identified that prenatal opioid exposure preferentially affects inter- rather than intra-network dynamic functional connections, particularly with the visual, subcortical, and default mode networks. Consistent results at the brain regional and connection levels were also observed, where the brain regions and connections associated with visual and higher order cognitive functions played pivotal roles in distinguishing opioid-exposed infants from controls. Our findings support the clinical phenotype of infants exposed to opioids in utero and may potentially explain the higher rates of visual and emotional problems observed in this population. Finally, our findings suggested that edge-centric networks could better capture the neural differences between opioid-exposed infants and controls by abstracting the intrinsic co-fluctuation along edges, which may provide a promising tool for future studies focusing on investigating the effects of prenatal opioid exposure on neurodevelopment.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Developmental Considerations for the Use of Naltrexone in Children and Adolescents

Naltrexone (NTX) is a well-tolerated drug with a wide safety margin and mechanism of action that affords use across a wide variety of indications in adults and children. By antagonizing the opioid reward system, NTX can modulate behaviors that involve compulsivity or impulsivity, such as substance use, obesity, and eating disorders. Evidence regarding the disposition and efficacy of NTX is mainly derived from adult studies of substance use disorders and considerable variability exists. Developmental changes, plausible disease-specific alterations and genetic polymorphisms in NTX disposition, and pharmacodynamic pathways should be taken into consideration when optimizing the use of NTX in the pediatric population. This review highlights the current state of the evidence and gaps in knowledge regarding NTX to facilitate evidence-based pharmacotherapy of mental health conditions, for which few pharmacologic options exist.

Hunger-driven adaptive prioritization of behavior

In order to survive, an animal must adapt its behavioral priorities to accommodate changing internal and external conditions. Hunger, a universally recognized interoceptive signal, promotes food intake though increasingly well-understood neural circuits. Less understood, is how hunger is integrated into the neural computations that guide nonfeeding behaviors. Within the brain, agouti-related peptide neurons in the arcuate nucleus of the hypothalamus have been found to powerfully stimulate feeding in addition to mediating other hunger-driven behavioral phenotypes. In this review, we compile the behavioral plasticity downstream of hunger and present identified or potential molecular and neural circuit mechanisms. We catalogue hunger's ability to increase exploration, decrease anxiety, and alter social behavior, among other phenotypes. Finally, we suggest paths forward for understanding hunger-driven behavioral adaptation and discuss the benefits of understanding state-dependent modulation of neural circuits controlling behavior.

Sex-dependent effects of chronic stress on reinstatement of palatable food seeking and involvement of dopamine D1-like receptors

Recent work in our lab has shown that chronic stress exposure causes sex-dependent changes in subsequent relapse-like behavior in rats with a history of palatable food self-administration. Although these effects are mediated by dopamine D₁-like receptors in male rats, such dopaminergic mechanisms have not been investigated in female animals. Thus, male and female rats were trained to respond for highly palatable food reinforcers in daily sessions. During subsequent extinction training, stress was manipulated (0 or 3 h restraint/day for 7 days). To assess dopaminergic involvement, we administered either SCH-23390 (10.0 μg/kg), a dopamine D₁-like receptor antagonist, or vehicle prior to daily treatments. Rats were then tested for cue- and pellet priming-induced reinstatement. Results showed that a history of chronic stress caused an increase in pellet priming-induced reinstatement in males and a decrease in cue-induced reinstatement in females. SCH-23390 combined with stress prevented those effects in males, but not in females. In females, a history of SCH-23390 administration caused an overall increase in responding that was apparent during cue-, but not pellet priming-, induced reinstatement testing. These results establish that both the effects of chronic stress on reinstatement of food seeking and the involvement of dopamine in those effects are dependent on biological sex. Such findings should inform the development of sex-specific interventions for dietary relapse and other stress-related health problems.

Endogenous opioids facilitate intrinsically-rewarded birdsong

Many songbirds sing in non-reproductive contexts while in flocks. Singing in such gregarious contexts is critical for maintaining and learning songs; however, song is not directed towards other individuals and has no obvious, immediate social consequences. Studies using conditioned place preference (CPP) tests of reward indicate that song production in gregarious contexts correlates positively with a bird’s intrinsic reward state and with opioid markers in the medial preoptic nucleus (mPOA). However, the causal involvement of opioids in gregarious song is unknown. Here we report that the selective mu opioid receptor (MOR) agonist fentanyl dose-dependently facilitates gregarious song and reduces stress/anxiety-related behavior in male and female European starlings. Furthermore, infusion of siRNA targeting MORs specifically in mPOA both suppresses gregarious song and disrupts the positive association between affective state and singing behavior, as revealed using CPP tests of song-associated reward. Results strongly implicate opioids in gregarious song and suggest that endogenous opioids in the mPOA may facilitate song by influencing an individual’s intrinsic reward state.

[Neural pathway between the nucleus accumbens and the rostral ventrolateral medulla in a rat model of anorexia nervosa]

OBJECTIVE

To investigate the potential neural pathway connecting the nucleus accumbens (NAc) and the rostral ventrolateral medulla (RVLM), and whether the pathway participates in the regulation of cardiovascular function in a model rat of anorexia nervosa (AN).

METHODS

Rat models of AN were established by allowing voluntary activity in a running wheel with restricted feeding, with the rats having free access to normal chow without exercise as the control group. FluoroGold (FG) retrograde tracing method and multi-channel simultaneous recording technique were used to explore the possible pathway between the NAc and the RVLM.

RESULTS

The rats in AN group exhibited significantly reduced systolic blood pressure (SBP), mean arterial pressure (MAP) and heart rate (HR) with significantly increased discharge frequency of RVLM neurons in comparison with the control rats. After the injection of FG into the RVLM, retrograde labeled neurons were observed in the NAc of the rats in both the normal control and AN groups. In both groups, SBP and HR were significantly decreased in response to 400 μA electrical stimulation of the NAc accompanied by an obvious increase in the discharge frequency of the RVLM neurons; the diastolic blood pressure (DBP) and MAP were significantly lower in AN model rats than in the normal rats in response to the stimulation.

CONCLUSIONS

We successfully established a rat model of AN via hyperactivity and restricted feeding and confirm the presence of a neural pathway connecting the NAc and the RVLM. This pathway might participate in the regulation of cardiovascular function in AN model rats.

Palatable food access impacts expression of amylin receptor components in the mesocorticolimbic system

NEW FINDINGS

What is the central question of this study? We tested whether intra-nucleus accumbens core amylin receptor (AmyR) activation suppresses feeding and evaluated whether intake of palatable food influences mesocorticolimbic AmyR expression. What is the main finding and its importance? Intra-nucleus accumbens core AmyR activation reduces food intake in some dietary conditions. We showed that all components of the AmyR are expressed in the prefrontal cortex and central nucleus of the amygdala and demonstrated that access to fat impacts AmyR expression in these and other mesocorticolimbic nuclei. These results suggest that the intake of palatable food might alter amylin signalling in the brain and shed further light onto potential sites of action for amylin.

ABSTRACT

Amylin is a pancreas- and brain-derived peptide that acts within the CNS to promote negative energy balance. However, our understanding of the CNS sites of action for amylin remains incomplete. Here, we investigate the effect of amylin receptor (AmyR) activation in the nucleus accumbens core (NAcC) on the intake of bland and palatable foods. Intra-NAcC injection of the AmyR agonist salmon calcitonin or amylin itself in male chow-fed rats had no effect on food intake, meal size or number of meals. However, in chow-fed rats with access to fat solution, although fat intake was not affected by intra-NAcC AmyR activation, subsequent chow intake was suppressed. Given that mesolimbic AmyR activation suppresses energy intake in rats with access to fat solution, we tested whether fat access changes AmyR expression in key mesocorticolimbic nuclei. Fat exposure did not affect NAcC AmyR expression, whereas in the accumbens shell, expression of receptor activity modifying protein (RAMP) 3 was significantly reduced in fat-consuming rats. We show that all components of AmyRs are expressed in the medial prefrontal cortex and central nucleus of the amygdala; fat access significantly reduced expression of calcitonin receptor-A in the central nucleus of the amygdala and RAMP2 in the medial prefrontal cortex. Taken together, these results indicate that intra-NAcC AmyR activation can suppress energy intake and, furthermore, suggest that AmyR signalling in a broader range of mesocorticolimbic sites might have a role in mediating the effects of amylin on food intake and body weight.

The Opioid System and Food Intake: Use of Opiate Antagonists in Treatment of Binge Eating Disorder and Abnormal Eating Behavior

Eating disorders (EDs) and substance use disorders (SUDs) commonly co-occur, especially in conjunction with affective syndromes, yet little is known about opiate abuse and ED symptoms in patients on naltrexone-bupropion therapy. Moreover, evidence suggests that the opioid system can also be regarded as one of the major systems regulating the anticipatory processes preceding binge eating episodes. The lack of evidence in the effectiveness of psychotherapy treatment in addition to psychotropic mediations compounds the difficulties in stabilizing individuals with EDs. This article aims to exhaustively review literature relating to the use of opioid antagonists in the management of binge eating disorder (BED) and other abnormal eating habits and how this can be augmented by the use of psychological approaches to come up with the most effective therapy or combination of therapies to manage these conditions. Although this approach is promising, it has not been evaluated. A review of the literature pertaining to the use of naltrexone in patients with EDs was performed through PubMed, PsycINFO and MEDLINE. We selected 63 relevant articles published between 1981 and 2018 and those written in English. Search terms included “Opioid antagonists”, “naltrexone”, “bupropion” and “Psychotherapy” each combined with “Binge Eating Disorder”, “Bulimia Nervosa”, “Anorexia Nervosa”, “Eating Disorder”, “EDNOS” and “Obesity”. While working with these articles, we also identified several problems related to use of these methods in real clinical practice. Seventy-seven articles were reviewed, and 63 were selected for inclusion. Data obtained from these sources confirmed that the blockade of opioid receptors diminishes food intake. More recent findings also indicate that the combination of bupropion and naltrexone can induce weight loss. Augmentation of this by introducing psychotherapy may lead to better outcomes. Cognitive behavioral therapy (CBT) was the most frequently recommended psychotherapy intervention, showing efficacy for EDs and chemical addictions as documented by most of the studies, but with uncertain efficacy when utilized as augmentation strategy. There are limited data supporting the use of psychotherapy in augmentation of standard therapy in ED; however, there is evidence to support that psychotherapy is safe in this population and has been effective in cases of patients with opiate addiction with and without psychiatric comorbidities as well as BED. More research is needed to establish treatment guidelines. Combining pharmacotherapeutic and psychotherapeutic interventions leads to the achievement of a better outcome in managing patients with EDs. Involving families or the use of support groups increases chances of adherence to the prescribed interventions resulting in higher rates of remission. However, it is clear that all of these interventions must occur in the context of a comprehensive treatment program. We believe that patient-specific psychotherapy may not only facilitate the treatment process, but also cause significant alterations in eating pattern. This approach for BED may lead to more significant treatment outcomes, but this possibility must be tested in larger samples.

Sex-Dependent Gene Expression in Infants with Neonatal Opioid Withdrawal Syndrome

OBJECTIVES

To evaluate salivary biomarkers that elucidate the molecular mechanisms by which in utero opioid exposure exerts sex-specific effects on select hypothalamic and reward genes driving hyperphagia, a hallmark symptom of infants suffering from neonatal opioid withdrawal syndrome (NOWS).

STUDY DESIGN

We prospectively collected saliva from 50 newborns born at ≥34 weeks of gestational age with prenatal opioid exposure and 50 sex- and gestational age-matched infants without exposure. Saliva underwent transcriptomic analysis for 4 select genes involved in homeostatic and hedonic feeding regulation (neuropeptide Y2 receptor [NPY2R], proopiomelanocortin [POMC], leptin receptor [LEPR], dopamine type 2 receptor [DRD2]). Normalized gene expression data were stratified based on sex and correlated with feeding volume on day of life 7 and length of stay in infants with NOWS requiring pharmacotherapy.

RESULTS

Expression of DRD2, a hedonistic/reward regulator, was significantly higher in male newborns compared with female newborns with NOWS (Δ threshold cycle 10.8 ± 3.8 vs 13.9 ± 3.7, P = .01). In NOWS requiring pharmacotherapy expression of leptin receptor, an appetite suppressor, was higher in male subjects than female subjects (Δ threshold cycle 8.4 ± 2.5 vs 12.4 ± 5.1, P = .05), DRD2 expression significantly correlated with intake volume on day of life 7 (r = 0.58, P = .02), and expression of NPY2R, an appetite regulator, negatively correlated with length of stay (r = -0.24, P = .05).

CONCLUSIONS

Prenatal opioid exposure exerts sex-dependent effects on hypothalamic feeding regulatory genes with clinical correlations. Neonatal salivary gene expression analyses may predict hyperphagia, severity of withdrawal state, and length of stay in infants with NOWS.

Medial prefrontal cortex diclofenac-induced antinociception is mediated through GPR55, cannabinoid CB1, and mu-opioid receptors of this area and periaqueductal gray

Supraspinal mechanisms of non-steroidal anti-inflammatory drug (NSAID)-induced antinociception are not well understood. In the present study, the possible antinociceptive mechanisms induced by intra-medial prefrontal cortex (intra-mPFC) microinjection of diclofenac were investigated after blockade of GPR55, cannabinoid CB1, and mu-opioid receptors in this area and ventrolateral periaqueductal gray (vlPAG). For drug delivery, unilateral (left side) of mPFC and bilateral (right and left sides) of vlPAG were surgically cannulated. Formalin test was induced by subcutaneous injection of a diluted formalin solution into the right vibrissa pad. A typical biphasic (neurogenic and inflammatory phases) pain behavior was produced following formalin injection. Microinjection of diclofenac (2.5, 5, and 10 μg/0.25 μL) into the mPFC suppressed both phases of pain. Intra-mPFC microinjection of naloxonazine (a mu-opioid receptor antagonist, 1 μg/0.25 μL) and AM251 (a cannabinoid CB1 receptor antagonist, 1 μg/0.25 μL) increased both phases of pain intensity. In addition, intra-mPFC-microinjected diclofenac-induced antinociception was inhibited by prior intra-mPFC and intra-vlPAG administration of naloxonazine and AM251. On the other hand, intra-mPFC and intra-vlPAG microinjection of AM251 (0.25 μg/0.25 μL) decreased pain severity which was inhibited by prior administration of ML193. The above-mentioned drugs did not alter locomotor activity. In conclusion, diclofenac suppressed both the neurogenic and inflammatory phases of formalin-induced orofacial pain at the level of mPFC. GPR55, cannabinoid CB1, and mu-opioid receptors of the mPFC and vlPAG might be involved in the mPFC analgesic effects of diclofenac.

Voluntary wheel running effects on intra-accumbens opioid driven diet preferences in male and female rats

Palatability driven feeding and voluntary physical activity are mediated by and influence similar neural mechanisms, notably through the actions of opioids within the nucleus accumbens. Recent studies suggest that access to a voluntary running wheel results in sex dependent behavioral and physiological adaptations related to opioid mediated palatability-driven feeding. To explore this relationship, male and female Wistar rats were given either access to a voluntary running wheel (RUN group) or no access (SED group) for one week prior to being stereotaxically implanted with bilateral cannulae targeting the nucleus accumbens. Following 7 days of recovery, with RUN or SED conditions continuing the duration of the experiment, all rats were assessed daily (2 h/day) for feeding behavior of concurrently accessible high-carbohydrate and high-fat diet for one week. Following this week, all rats were administered the μ-opioid receptor agonist D-Ala2, NMe-Phe4, Glyol5-enkephalin (DAMGO) (0.0025  μg, 0.025  μg, or 0.25 μg/0.5 μl/side) or the opioid antagonist naloxone (20 μg/0.5 μl/side) into the nucleus accumbens and given concurrent access (2 h) to both diets. All groups expressed a significant baseline preference for the high-carbohydrate diet. DAMGO administration, compared to saline treatment, led to significant increased consumption of the high-carbohydrate diet in all treatment groups. While high-fat diet consumption also increased following DAMGO administration, the influence of DAMGO was much more robust for the preferred high-carbohydrate diet in all groups. Compared to males, females consumed significantly more of both diets at baseline and following DAMGO treatment. Both male and female rats in the RUN condition consumed more high-carbohydrate diet compared to rats in the SED condition. While males exhibited similar increased consumption of both diets regardless of RUN or SED condition, females in the RUN condition displayed a greater sensitivity to DAMGO-driven consumption of the preferred high-carbohydrate, compared to SED females.

TouchScreen-based phenotyping: altered stimulus/reward association and lower perseveration to gain a reward in mu opioid receptor knockout mice

While the contribution of Mu Opioid Receptors (MORs) to hedonic aspects of reward processing is well-established, the notion that these receptors may also regulate motivation to gain a reward, and possibly other related cognitive dimensions, has been less investigated. The prefrontal cortex (PFC) is a critical site for these processes. Our previous functional magnetic resonance imaging study found alterations of functional connectivity (FC) in reward/aversion networks in MOR knockout mice. Here we pursued voxelwise seed-based FC analyses using the same dataset with a focus on the PFC. We observed significant reduction of PFC FC in mutant mice, predominantly with the nucleus accumbens, supporting the notion of altered reward-driven top-down controls. We tested motivation for palatable food in a classical operant self-administration paradigm, and found delayed performance for mutant mice. We then evaluated motivational and cognitive abilities of MOR knockout mice in TouchScreen-based behavioral tests. Learning was delayed and stimulus/reward association was impaired, suggesting lower hedonic reward value and reduced motivation. Perseverative responses were decreased, while discriminatory behavior and attention were unchanged, indicative of increased inhibitory controls with otherwise intact cognitive performance. Together, our data suggest that MORs contribute to enhance reward-seeking and facilitate perseverative behaviors. The possibility that MOR blockade could reduce maladaptive compulsivity deserves further investigation in addiction and self-control disorder research.

Neuropharmacology of compulsive eating

Compulsive eating behaviour is a transdiagnostic construct observed in certain forms of obesity and eating disorders, as well as in the proposed construct of 'food addiction'. Compulsive eating can be conceptualized as comprising three elements: (i) habitual overeating, (ii) overeating to relieve a negative emotional state, and (iii) overeating despite adverse consequences. Neurobiological processes that include maladaptive habit formation, the emergence of a negative affect, and dysfunctions in inhibitory control are thought to drive the development and persistence of compulsive eating behaviour. These complex psychobehavioural processes are under the control of various neuropharmacological systems. Here, we describe the current evidence implicating these systems in compulsive eating behaviour, and contextualize them within the three elements. A better understanding of the neuropharmacological substrates of compulsive eating behaviour has the potential to significantly advance the pharmacotherapy for feeding-related pathologies.This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

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

When relatively sated, people (and rodents) are still easily tempted to consume calorie-dense foods, particularly those containing fat and sugar. Consumption of such foods while calorically replete likely contributes to obesity. The nucleus accumbens (NAc) opioid system has long been viewed as a critical substrate for this behavior, mainly via contributions to the neural control of consumption and palatability. Here, we test the hypothesis that endogenous NAc opioids also promote appetitive approach to calorie-dense food in states of relatively high satiety. We simultaneously recorded NAc neuronal firing and infused a µ-opioid receptor antagonist into the NAc while rats performed a cued approach task in which appetitive and consummatory phases were well separated. The results reveal elements of a neural mechanism by which NAc opioids promote approach to high-fat food despite the lack of caloric need, demonstrating a potential means by which the brain is biased towards overconsumption of palatable food.

Imagine that you have just finished Thanksgiving dinner. You are completely full, having eaten large portions of turkey, green beans and mashed potatoes. Yet, despite feeling full, you still find yourself tempted by a slice of pie for dessert, maybe even with ice cream on top. Why is it that in such a state of fullness, you desire a slice of pie but not, say, another helping of green beans?

The answer may lie in the way the brain responds to food when we do not need any more calories. At such times, your brain drives you to continue eating only those foods that are tasty and calorie-dense. This preference for fatty and sweet foods may have been helpful in the past when we could not be certain where our next meal would come from. But in modern times, the widespread availability of food makes this preference potentially harmful. For example, the drive to consume fatty and sweet foods even when not hungry may now be contributing to soaring levels of obesity and type 2 diabetes.

What exactly is happening inside the brain to produce this behavior? Previous work has implicated a structure called the nucleus accumbens. When scientists activated proteins called mu opioid receptors within the nucleus accumbens, animals ate more of the foods that they find tasty. However, they were not as interested in eating more of the foods that they are more ambivalent towards.

Caref and Nicola now show that preventing opioid binding makes rats unwilling to respond to a cue to obtain cream, an appetizing, high-fat reward. It also abolishes the brain activity that drives the rats to respond the cue. Crucially, however, this effect only occurs in rats that are not hungry.

It therefore appears that opioid binding in the nucleus accumbens drives animals to approach and eat high-fat foods, but only when they do not need the calories. That is, it increases fat consumption in animals that are not actually hungry. A drug that selectively blocks mu opioid receptors in the nucleus accumbens may reduce this behavior. Such a drug could potentially help to prevent obesity and the health problems associated with it.

Predicting personality from network-based resting-state functional connectivity

Personality is associated with variation in all kinds of mental faculties, including affective, social, executive, and memory functioning. The intrinsic dynamics of neural networks underlying these mental functions are reflected in their functional connectivity at rest (RSFC). We, therefore, aimed to probe whether connectivity in functional networks allows predicting individual scores of the five-factor personality model and potential gender differences thereof. We assessed nine meta-analytically derived functional networks, representing social, affective, executive, and mnemonic systems. RSFC of all networks was computed in a sample of 210 males and 210 well-matched females and in a replication sample of 155 males and 155 females. Personality scores were predicted using relevance vector machine in both samples. Cross-validation prediction accuracy was defined as the correlation between true and predicted scores. RSFC within networks representing social, affective, mnemonic, and executive systems significantly predicted self-reported levels of Extraversion, Neuroticism, Agreeableness, and Openness. RSFC patterns of most networks, however, predicted personality traits only either in males or in females. Personality traits can be predicted by patterns of RSFC in specific functional brain networks, providing new insights into the neurobiology of personality. However, as most associations were gender-specific, RSFC-personality relations should not be considered independently of gender.

Opioid and nondopamine reward circuitry and state-dependent mechanisms

A common notion is that essentially all addictive drugs, including opioids, activate dopaminergic pathways in the brain reward system, and the inappropriate use of such drugs induces drug dependence. However, an opioid reward response is reportedly still observed in several models of dopamine depletion, including in animals that are treated with dopamine blockers, animals that are subjected to dopaminergic neuron lesions, and dopamine-deficient mice. The intracranial self-stimulation response is enhanced by stimulants but reduced by morphine. These findings suggest that dopaminergic neurotransmission may not always be required for opioid reward responses. Previous findings also indicate the possibility that dopamine-independent opioid reward may be observed in opioid-naive states but not in opioid-dependent states. Therefore, a history of opioid use should be considered when evaluating the dopamine dependency of opioid reward.

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.

Neural circuits of eating behaviour: Opportunities for therapeutic development

Understanding of the neural and physiological substrates of hunger and satiety has increased rapidly over the last three decades, and pharmacological targets have already been identified for the treatment of obesity that has moved from pre-clinical screening to therapies approved by regulatory authorities. Initially, this review describes the way in which physiological signals of energy availability interact with hedonic and rewarding properties of food to modulate the neural circuitry that supports eating behaviour. This is followed by a brief account of current and promising targets for drug development and a review of the wide range of preclinical paradigms that model important influences on human eating behaviour, and can be used to guide early stages of the drug development process.

Opioid and orexin hedonic hotspots in rat orbitofrontal cortex and insula

Hedonic hotspots are brain sites where particular neurochemical stimulations causally amplify the hedonic impact of sensory rewards, such as "liking" for sweetness. Here, we report the mapping of two hedonic hotspots in cortex, where mu opioid or orexin stimulations enhance the hedonic impact of sucrose taste. One hedonic hotspot was found in anterior orbitofrontal cortex (OFC), and another was found in posterior insula. A suppressive hedonic coldspot was also found in the form of an intervening strip stretching from the posterior OFC through the anterior and middle insula, bracketed by the two cortical hotspots. Opioid/orexin stimulations in either cortical hotspot activated Fos throughout a distributed "hedonic circuit" involving cortical and subcortical structures. Conversely, cortical coldspot stimulation activated circuitry for "hedonic suppression." Finally, food intake was increased by stimulations at several prefrontal cortical sites, indicating that the anatomical substrates in cortex for enhancing the motivation to eat are discriminable from those for hedonic impact.

Optogenetic activation of amygdala projections to nucleus accumbens can arrest conditioned and unconditioned alcohol consummatory behavior

Following a Pavlovian pairing procedure, alcohol-paired cues come to elicit behavioral responses that lead to alcohol consumption. Here we used an optogenetic approach to activate basolateral amygdala (BLA) axonal terminals targeting the shell of nucleus accumbens (AcbSh) and investigated a possible influence over cue-conditioned alcohol seeking and alcohol drinking, based on the demonstrated roles of these areas in behavioral responding to Pavlovian cues and in feeding behavior. Rats were trained to anticipate alcohol or sucrose following the onset of a discrete conditioned stimulus (CS). Channelrhodopsin-mediated activation of the BLA-to-AcbSh pathway concurrent with each CS disrupted cued alcohol seeking. Activation of the same pathway caused rapid cessation of alcohol drinking from a sipper tube. Neither effect was accompanied by an overall change in locomotion. Finally, the suppressive effect of photoactivation on cued-triggered seeking was also evidenced in animals trained with sucrose. Together these findings suggest that photoactivation of BLA terminals in the AcbSh can override the conditioned motivational properties of reward-predictive cues as well as unconditioned consummatory responses necessary for alcohol drinking. The findings provide evidence for a limbic-striatal influence over motivated behavior for orally consumed rewards, including alcohol.