An orexigenic role for mu-opioid receptors in the lateral parabrachial nucleus

Opioids, sleep, analgesia and respiratory depression: Their convergence on Mu (μ)-opioid receptors in the parabrachial area

Opioids provide analgesia, as well as modulate sleep and respiration, all by possibly acting on the μ-opioid receptors (MOR). MOR's are ubiquitously present throughout the brain, posing a challenge for understanding the precise anatomical substrates that mediate opioid induced respiratory depression (OIRD) that ultimately kills most users. Sleep is a major modulator not only of pain perception, but also for changing the efficacy of opioids as analgesics. Therefore, sleep disturbances are major risk factors for developing opioid overuse, withdrawal, poor treatment response for pain, and addiction relapse. Despite challenges to resolve the neural substrates of respiratory malfunctions during opioid overdose, two main areas, the pre-Bötzinger complex (preBötC) in the medulla and the parabrachial (PB) complex have been implicated in regulating respiratory depression. More recent studies suggest that it is mediation by the PB that causes OIRD. The PB also act as a major node in the upper brain stem that not only receives input from the chemosensory areas in medulla, but also receives nociceptive information from spinal cord. We have previously shown that the PB neurons play an important role in mediating arousal from sleep in response to hypercapnia by its projections to the forebrain arousal centers, and it may also act as a major relay for the pain stimuli. However, due to heterogeneity of cells in the PB, their precise roles in regulating, sleep, analgesia, and respiratory depression, needs addressing. This review sheds light on interactions between sleep and pain, along with dissecting the elements that adversely affects respiration.

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.

Danger and distress: Parabrachial-extended amygdala circuits

Our understanding of the role of the parabrachial nucleus (PBN) has evolved as technology has advanced, in part due to cell-specific studies and complex behavioral assays. This is reflected in the heterogeneous neuronal populations within the PBN to the extended amygdala (EA) circuits which encompass the bed nucleus of the stria terminalis (BNST) and central amygdala (CeA) circuitry, as they differentially modulate aspects of behavior in response to diverse threat-like contexts necessary for survival. Here we review how the PBN→CeA and PBN→BNST pathways differentially modulate fear-like behavior, innate and conditioned, through unique changes in neurotransmission in response to stress-inducing contexts. Furthermore, we hypothesize how in specific instances the PBN→CeA and PBN→BNST circuits are redundant and in part intertwined with their respective reciprocal projections. By deconstructing the interoceptive and exteroceptive components of affect- and stress related behavioral paradigms, evidence suggests that the PBN→CeA circuit modulates innate response to physical stimuli and fear conditioning. Conversely, the PBN→BNST circuit modulates distress-like stress in unpredictable contexts. Thereby, the PBN provides a pathway for alarming interoceptive and exteroceptive stimuli to be processed and relayed to the EA to induce stress-relevant affect. Additionally, we provide a framework for future studies to detail the cell-type specific intricacies of PBN→EA circuits in mediating behavioral responses to threats, and the relevance of the PBN in drug-use as it relates to threat and negative reinforcement. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

Reward signalling in brainstem nuclei under fluctuating blood glucose

Phasic dopamine release from mid-brain dopaminergic neurons is thought to signal errors of reward prediction (RPE). If reward maximisation is to maintain homeostasis, then the value of primary rewards should be coupled to the homeostatic errors they remediate. This leads to the prediction that RPE signals should be configured as a function of homeostatic state and thus diminish with the attenuation of homeostatic error. To test this hypothesis, we collected a large volume of functional MRI data from five human volunteers on four separate days. After fasting for 12 hours, subjects consumed preloads that differed in glucose concentration. Participants then underwent a Pavlovian cue-conditioning paradigm in which the colour of a fixation-cross was stochastically associated with the delivery of water or glucose via a gustometer. This design afforded computation of RPE separately for better- and worse-than expected outcomes during ascending and descending trajectories of serum glucose fluctuations. In the parabrachial nuclei, regional activity coding positive RPEs scaled positively with serum glucose for both ascending and descending glucose levels. The ventral tegmental area and substantia nigra became more sensitive to negative RPEs when glucose levels were ascending. Together, the results suggest that RPE signals in key brainstem structures are modulated by homeostatic trajectories of naturally occurring glycaemic flux, revealing a tight interplay between homeostatic state and the neural encoding of primary reward in the human brain.

Functional and Neurochemical Identification of Ghrelin Receptor (GHSR)-Expressing Cells of the Lateral Parabrachial Nucleus in Mice

The lateral parabrachial nucleus (lPBN), located in the pons, is a well-recognized anorexigenic center harboring, amongst others, the calcitonin gene-related peptide (CGRP)-expressing neurons that play a key role. The receptor for the orexigenic hormone ghrelin (the growth hormone secretagogue receptor, GHSR) is also abundantly expressed in the lPBN and ghrelin delivery to this site has recently been shown to increase food intake and alter food choice. Here we sought to explore whether GHSR-expressing cells in the lPBN (GHSR ^lPBN cells) contribute to feeding control, food choice and body weight gain in mice offered an obesogenic diet, involving studies in which GHSR ^lPBN cells were silenced. We also explored the neurochemical identity of GHSR ^lPBN cells. To silence GHSR ^lPBN cells, Ghsr-IRES-Cre male mice were bilaterally injected intra-lPBN with a Cre-dependent viral vector expressing tetanus toxin-light chain. Unlike control wild-type littermates that significantly increased in body weight on the obesogenic diet (i.e., high-fat high-sugar free choice diet comprising chow, lard and 9% sucrose solution), the heterozygous mice with silenced GHSR ^lPBN cells were resistant to diet-induced weight gain with significantly lower food intake and fat weight. The lean phenotype appeared to result from a decreased food intake compared to controls and caloric efficiency was unaltered. Additionally, silencing the GHSR ^lPBN cells altered food choice, significantly reducing palatable food consumption. RNAscope and immunohistochemical studies of the lPBN revealed considerable co-expression of GHSR with glutamate and pituitary adenylate cyclase-activating peptide (PACAP), and much less with neurotensin, substance P and CGRP. Thus, the GHSR ^lPBN cells are important for diet-induced weight gain and adiposity, as well as in the regulation of food intake and food choice. Most GHSR ^lPBN cells were found to be glutamatergic and the majority (76%) do not belong to the well-characterized anorexigenic CGRP cell population.

Ghrelin Receptor Stimulation of the Lateral Parabrachial Nucleus in Rats Increases Food Intake but not Food Motivation

OBJECTIVE

The lateral parabrachial nucleus (lPBN) in the brainstem has emerged as a key area involved in feeding control that is targeted by several circulating anorexigenic hormones. Here, the objective was to determine whether the lPBN is also a relevant site for the orexigenic hormone ghrelin, inspired by studies in mice and rats showing that there is an abundance of ghrelin receptors in this area.

METHODS

This study first explored whether iPBN cells respond to ghrelin involving Fos mapping and electrophysiological studies in rats. Next, rats were injected acutely with ghrelin, a ghrelin receptor antagonist, or vehicle into the lPBN to investigate feeding-linked behaviors.

RESULTS

Curiously, ghrelin injection (intracerebroventricular or intravenous) increased Fos protein expression in the lPBN yet the predominant electrophysiological response was inhibitory. Intra-lPBN ghrelin injection increased chow or high-fat diet intake, whereas the antagonist decreased chow intake only. In a choice paradigm, intra-lPBN ghrelin increased intake of chow but not lard or sucrose. Intra-lPBN ghrelin did not alter progressive ratio lever pressing for sucrose or conditioned place preference for chocolate.

CONCLUSIONS

The lPBN is a novel locus from which ghrelin can alter consummatory behaviors (food intake and choice) but not appetitive behaviors (food reward and motivation).

The Raphe Dopamine System Controls the Expression of Incentive Memory

The brain dopamine (DA) system participates in forming and expressing memory. Despite a well-established role of DA neurons in the ventral tegmental area in memory formation, the exact DA circuits that control memory expression remain unclear. Here, we show that DA neurons in the dorsal raphe nucleus (DRN) and their medulla input control the expression of incentive memory. DRN DA neurons are activated by both rewarding and aversive stimuli in a learning-dependent manner and exhibit elevated activity during memory recall. Disrupting their physiological activity or DA synthesis blocks the expression of natural appetitive and aversive memories as well as drug memories associated with opioids. Moreover, a glutamatergic pathway from the lateral parabrachial nucleus to the DRN selectively regulates the expression of reward memories associated with opioids or foods. Our study reveals a specialized DA subsystem important for memory expression and suggests new targets for interventions against opioid addiction.

Differential rewarding effects of electrical stimulation of the lateral hypothalamus and parabrachial complex: Functional characterization and the relevance of opioid systems and dopamine

BACKGROUND

Since the discovery of rewarding intracranial self-stimulation by Olds and Milner, extensive data have been published on the biological basis of reward. Although participation of the mesolimbic dopaminergic system is well documented, its precise role has not been fully elucidated, and some authors have proposed the involvement of other neural systems in processing specific aspects of reinforced behaviour.

AIMS AND METHODS

We reviewed published data, including our own findings, on the rewarding effects induced by electrical stimulation of the lateral hypothalamus (LH) and of the external lateral parabrachial area (LPBe) - a brainstem region involved in processing the rewarding properties of natural and artificial substances - and compared its functional characteristics as observed in operant and non-operant behavioural procedures.

RESULTS

Brain circuits involved in the induction of preferences for stimuli associated with electrical stimulation of the LBPe appear to functionally and neurochemically differ from those activated by electrical stimulation of the LH.

INTERPRETATION

We discuss the possible involvement of the LPBe in processing emotional-affective aspects of the brain reward system.

Making Sense of Rodent Models of Anhedonia

A markedly reduced interest or pleasure in activities previously considered pleasurable is a main symptom in mood disorder and psychosis and is often present in other psychiatric disorders and neurodegenerative diseases. This condition can be labeled as "anhedonia," although in its most rigorous connotation the term refers to the lost capacity to feel pleasure that is one aspect of the complex phenomenon of processing and responding to reward. The responses to rewarding stimuli are relatively easy to study in rodents, and the experimental conditions that consistently and persistently impair these responses are used to model anhedonia. To this end, long-term exposure to environmental aversive conditions is primarily used, and the resulting deficits in reward responses are often accompanied by other deficits that are mainly reminiscent of clinical depressive symptoms. The different components of impaired reward responses induced by environmental aversive events can be assessed by different tests or protocols that require different degrees of time allocation, technical resources, and equipment. Rodent models of anhedonia are valuable tools in the study of the neurobiological mechanisms underpinning impaired behavioral responses and in the screening and characterization of drugs that may reverse these behavioral deficits. In particular, the antianhedonic or promotivational effects are relevant features in the spectrum of activities of drugs used in mood disorders or psychosis. Thus, more than the model, it is the choice of tests that is crucial since it influences which facets of anhedonia will be detected and should be tuned to the purpose of the study.

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

RATIONALE

Whereas reward-modulatory opioid actions have been intensively studied in subcortical sites such as the nucleus accumbens (Acb), the role of cortical opioid transmission has received comparatively little attention.

OBJECTIVES

The objective of this study is to describe recent findings on the motivational actions of opioids in the prefrontal cortex (PFC), emphasizing studies of food motivation and ingestion. PFC-based opioid effects will be compared/contrasted to those elicited from the Acb, to glean possible common functional principles. Finally, the motivational effects of opioids will be placed within a network context involving the PFC, Acb, and hypothalamus.

RESULTS

Mu-opioid receptor (μ-OR) stimulation in both the Acb and PFC induces eating and enhances food-seeking instrumental behaviors; μ-OR signaling also enhances taste reactivity within a highly circumscribed zone of medial Acb shell. In both the Acb and PFC, opioid-sensitive zones are aligned topographically with the sectors that project to feeding-modulatory zones of the hypothalamus and intact glutamate transmission in the lateral/perifornical (LH-PeF) hypothalamic areas is required for both Acb- and PFC-driven feeding. Conversely, opioid-mediated feeding responses elicited from the PFC are negatively modulated by AMPA signaling in the Acb shell.

CONCLUSIONS

Opioid signaling in the PFC engages functionally opposed PFC➔hypothalamus and PFC➔Acb circuits, which, respectively, drive and limit non-homeostatic feeding, producing a disorganized and "fragmented" pattern of impulsive food-seeking behaviors and hyperactivity. In addition, opioids act directly in the Acb to facilitate food motivation and taste hedonics. Further study of this cortico-striato-hypothalamic circuit, and incorporation of additional opioid-responsive telencephalic structures, could yield insights with translational relevance for eating disorders and obesity.

Peptide YY signaling in the lateral parabrachial nucleus increases food intake through the Y1 receptor

Although central PYY delivery potently increases food intake, the sites of action and mechanisms mediating these hyperphagic effects are not fully understood. The present studies investigate the contribution of lateral parabrachial nucleus (lPBN) PYY-Y receptor signaling to food intake control, as lPBN neurons express Y receptors and receive PYY fibers and are known to integrate circulating and visceral sensory signals to regulate energy balance. Immunohistochemical results identified a subpopulation of gigantocellular reticular nucleus PYY-producing neurons that project monosynaptically to the lPBN, providing an endogenous source of PYY to the lPBN. lPBN microinjection of PYY-(1-36) or PYY-(3-36) markedly increased food intake by increasing meal size. To determine which receptors mediate these behavioral results, we first performed quantitative real-time PCR to examine the relative levels of Y receptor expression in lPBN tissue. Gene expression analyses revealed that, while Y1, Y2, and Y5 receptors are each expressed in lPBN tissue, Y1 receptor mRNA is expressed at fivefold higher levels than the others. Furthermore, behavioral/pharmacological results demonstrated that the hyperphagic effects of PYY-(3-36) were eliminated by lPBN pretreatment with a selective Y1 receptor antagonist. Together, these results highlight the lPBN as a novel site of action for the intake-stimulatory effects of central PYY-Y1 receptor signaling.

GLP-1 receptor stimulation of the lateral parabrachial nucleus reduces food intake: neuroanatomical, electrophysiological, and behavioral evidence

The parabrachial nucleus (PBN) is a key nucleus for the regulation of feeding behavior. Inhibitory inputs from the hypothalamus to the PBN play a crucial role in the normal maintenance of feeding behavior, because their loss leads to starvation. Viscerosensory stimuli result in neuronal activation of the PBN. However, the origin and neurochemical identity of the excitatory neuronal input to the PBN remain largely unexplored. Here, we hypothesize that hindbrain glucagon-like peptide 1 (GLP-1) neurons provide excitatory inputs to the PBN, activation of which may lead to a reduction in feeding behavior. Our data, obtained from mice expressing the yellow fluorescent protein in GLP-1-producing neurons, revealed that hindbrain GLP-1-producing neurons project to the lateral PBN (lPBN). Stimulation of lPBN GLP-1 receptors (GLP-1Rs) reduced the intake of chow and palatable food and decreased body weight in rats. It also activated lPBN neurons, reflected by an increase in the number of c-Fos-positive cells in this region. Further support for an excitatory role of GLP-1 in the PBN is provided by electrophysiological studies showing a remarkable increase in firing of lPBN neurons after Exendin-4 application. We show that within the PBN, GLP-1R activation increased gene expression of 2 energy balance regulating peptides, calcitonin gene-related peptide (CGRP) and IL-6. Moreover, nearly 70% of the lPBN GLP-1 fibers innervated lPBN CGRP neurons. Direct intra-lPBN CGRP application resulted in anorexia. Collectively, our molecular, anatomical, electrophysiological, pharmacological, and behavioral data provide evidence for a functional role of the GLP-1R for feeding control in the PBN.

Leptin receptor signaling in the lateral parabrachial nucleus contributes to the control of food intake

Pontine parabrachial nucleus (PBN) neurons integrate visceral, oral, and other sensory information, playing an integral role in the neural control of feeding. Current experiments probed whether lateral PBN (lPBN) leptin receptor (LepRb) signaling contributes to this function. Intra-lPBN leptin microinjection significantly reduced cumulative chow intake, average meal size, and body weight in rats, independent of effects on locomotor activity or gastric emptying. In contrast to the effects observed following LepRb activation in other nuclei, lPBN LepRb stimulation did not affect progressive ratio responding for sucrose reward or conditioned place preference for a palatable food. Collectively, results suggest that lPBN LepRb activation reduces food intake by modulating the neural processing of meal size/satiation signaling, and highlight the lPBN as a novel site of action for leptin-mediated food intake control.

Glucagon-like Peptide-1 receptor signaling in the lateral parabrachial nucleus contributes to the control of food intake and motivation to feed

Central glucagon-like peptide-1 receptor (GLP-1R) activation reduces food intake and the motivation to work for food, but the neurons and circuits mediating these effects are not fully understood. Although lateral parabrachial nucleus (lPBN) neurons are implicated in the control of food intake and reward, the specific role of GLP-1R-expressing lPBN neurons is unexplored. Here, neuroanatomical tracing, immunohistochemical, and behavioral/pharmacological techniques are used to test the hypothesis that lPBN neurons contribute to the anorexic effect of central GLP-1R activation. Results indicate that GLP-1-producing neurons in the nucleus tractus solitarius project monosynaptically to the lPBN, providing a potential endogenous mechanism by which lPBN GLP-1R signaling may exert effects on food intake control. Pharmacological activation of GLP-1R in the lPBN reduced food intake, and conversely, antagonism of GLP-1R in the lPBN increased food intake. In addition, lPBN GLP-1R activation reduced the motivation to work for food under a progressive ratio schedule of reinforcement. Taken together, these data establish the lPBN as a novel site of action for GLP-1R-mediated control of food intake and reward.

Advances in the neurobiological bases for food 'liking' versus 'wanting'

The neural basis of food sensory pleasure has become an increasingly studied topic in neuroscience and psychology. Progress has been aided by the discovery of localized brain subregions called hedonic hotspots in the early 2000s, which are able to causally amplify positive affective reactions to palatable tastes ('liking') in response to particular neurochemical or neurobiological stimulations. Those hedonic mechanisms are at least partly distinct from larger mesocorticolimbic circuitry that generates the incentive motivation to eat ('wanting'). In this review, we aim to describe findings on these brain hedonic hotspots, especially in the nucleus accumbens and ventral pallidum, and discuss their role in generating food pleasure and appetite.

Rewarding effects of the electrical stimulation of the parabrachial complex: taste or place preference?

The lateral parabrachial complex has been related to various emotional-affective processes. It has been shown that electrical stimulation of the external Lateral Parabrachial (LPBe) nucleus can induce reinforcing effects in place preference and taste discrimination tasks but does not appear to support self-stimulation. This study examined the relative relevance of place and taste stimuli after electrical stimulation of the LPBe nucleus. A learning discrimination task was conducted that simultaneously included both sensory indexes (taste and place) in order to determine the preference of animals for one or the other. After a taste stimulus reversal task, the rewarding effect of stimulation was found to be preferentially associated with place. These results are discussed in the context of the rewarding action and biological constraints induced by different natural and artificial reinforcing agents.

Lesions of the lateral parabrachial area block the aversive component and induced-flavor preference for the delayed intragastric administration of nutrients in rats: Effects on subsequent food and water intake

The aim of this study was to examine the function of the lateral parabrachial area (LPB) in relation to the intragastric administration of nutrients. The consumption of flavors associated with intragastric nutrient administration and the subsequent food and water intake were measured in rats with lesions in the LPB. The results showed that bilateral LPB lesions prevented development of aversions and induced flavor preference when there was a delay between the presentation of a flavor and the intragastric administration of nutrients. However, these lesions did not disrupt development of the aversive process when there was no delay between the presentations. Likewise, the LPB lesions increased subsequent food intake when there was a delay but not when there was no delay between the presentations. In contrast, the water intake was reduced in both situations. These results are interpreted in terms of a dual visceral system for processing the intragastric effects of foods.

The stereoisomer (+)-naloxone potentiates G-protein coupling and feeding associated with stimulation of mu opioid receptors in the parabrachial nucleus

Classically, opioids produce their effects by activating Gi-proteins that inhibit adenylate cyclase activity. Previous studies proposed that mu-opioid receptors can also stimulate adenylate cyclase due to an initial transient coupling to Gs-proteins. Treatment with ultra-low doses of the nonselective opioid antagonist (-)-naloxone or its inactive enantiomer (+)-naloxone blocks this excitatory effect and enhances Gi-coupling. Previously we reported that infusion of the mu-opioid receptor agonist [D-Ala2, N-Me-Phe4, Glycinol5]-Enkephalin (DAMGO) into the mu-opioid receptor expressing lateral parabrachial nucleus increases feeding. Pretreatment with (-)-naloxone blocks this effect. We used this parabrachial circuit as a model to assess cellular actions of ultra-low doses of (-)-naloxone and (+)-naloxone in modifying the effects of DAMGO. Our results showed that an ultra-low concentration of (-)-naloxone (0.001 nM) and several concentrations of (+)-naloxone (0.01-10 nM) enhanced DAMGO-stimulated guanosine-5'-0-(γ-thio)-triphosphate incorporation in parabrachial sections in vitro. Further, we analyzed the relevance of these effects in vivo. In the present study, we show that (+)-naloxone can potentiate DAMGO-induced feeding at doses at which (-)-naloxone was an antagonist. These results implicated (+)-naloxone as a novel tool for studying mu-opioid receptor functions and suggest that (+)-naloxone may have therapeutic value to enhance clinical actions of opiate drugs.

An examination of the effects of subthalamic nucleus inhibition or μ-opioid receptor stimulation on food-directed motivation in the non-deprived rat

The subthalamic nucleus (STN) serves important functions in regulating movement, cognition, and motivation and is connected with cortical and basal ganglia circuits that process reward and reinforcement. In order to further examine the role of the STN on motivation toward food in non-deprived rats, these experiments studied the effects of pharmacological inhibition or μ-opioid receptor stimulation of the STN on the 2-h intake of a sweetened fat diet, the amount of work exerted to earn sucrose on a progressive ratio 2 (PR-2) schedule of reinforcement, and performance on a differential reinforcement of low-rate responding (DRL) schedule for sucrose reward. Separate behavioral groups (N=6-9) were tested following bilateral inhibition of the STN with the GABA(A) receptor agonist muscimol (at 0-5 ng/0.5 μl/side) or following μ-opioid receptor stimulation with the agonist D-Ala², N-MePhe⁴, Gly-ol-enkephalin (DAMGO; at 0, 0.025 or 0.25 μg/0.5 μl/side). Although STN inhibition increased ambulatory behavior during 2-h feeding sessions, it did not significantly alter intake of the sweetened fat diet. STN inhibition also did not affect the breakpoint for sucrose pellets during a 1-h PR-2 reinforcement schedule or impact the number of reinforcers earned on a 1-h DRL-20s reinforcement schedule in non-deprived rats. In contrast, STN μ-opioid receptor stimulation significantly increased feeding on the palatable diet and reduced the reinforcers earned on a DRL-20 schedule, although DAMGO microinfusions had no effect on PR-2 performance. These data suggest that STN inhibition does not enhance incentive motivation for food in the absence of food restriction and that STN μ-opioid receptors play an important and unique role in motivational processes.

Effects of butorphanol on feeding and neuropeptide Y in the rat

Butorphanol ([BT] an opioid receptor agonist/antagonist) is different from other opioid agonists in that a single dose of BT can elicit up to 12 g of chow intake in a satiated rat whereas most opioid agonists induce a mild feeding response (2-3 g). Here, we first examined whether the effectiveness of BT to elicit feeding was affected by dose, method of infusion and possible tachyphylaxis following administration. Secondly, we examined whether BT administration influenced hypothalamic NPY gene expression and peptide levels. A single dose administration of BT (4 mg/kg) significantly increased food intake at 2, 3 and 6 h after administration. However following repeated injections of BT at 4 mg/kg, the cumulative long-term intake of BT-treated rats did not differ from that of controls, indicating that the animals compensate for the increased feeding following BT injection by decreased feeding at a later time. An ascending dose schedule of repeated BT injections resulted in additional feeding. NPY gene expression in the ARC was influenced by how much food had been consumed, but not by BT. The amount of food consumed and the level of NPY mRNA were inversely correlated. This is consistent with NPY's role in normal feeding. BT treatment did not affect either NPY or leptin RIA levels. We conclude that the feeding produced by BT is sensitive to dose and dosing paradigm. Further, its mechanism of action does not appear to be mediated by NPY or leptin pathways.

μ-Opioid modulation in the rostral solitary nucleus and reticular formation alters taste reactivity: evidence for a suppressive effect on consummatory behavior

The neural control of feeding involves many neuromodulators, including the endogenous opioids that bind μ-opioid receptors (MORs). Injections of the MOR agonist, Damgo, into limbic and hypothalamic forebrain sites increase intake, particularly of palatable foods. Indeed, forebrain Damgo injections increase sucrose-elicited licking but reduce aversive responding (gaping) to quinine, suggesting that MOR activation may enhance taste palatability. A μ-opioid influence on taste reactivity has not been assessed in the brain stem. However, MORs are present in the first-order taste relay, the rostral nucleus of the solitary tract (rNST), and in the immediately subjacent reticular formation (RF), a region known to be essential for consummatory responses. Thus, to evaluate the consequences of rNST/dorsal RF Damgo in this region, we implanted rats with intraoral cannulas, electromyographic electrodes, and brain cannulas aimed at the ventral border of the rNST. Licking and gaping elicited with sucrose, water, and quinine were assessed before and after intramedullary Damgo and saline infusions. Damgo slowed the rate, increased the amplitude, and decreased the size of fluid-induced lick and gape bouts. In addition, the neutral stimulus water, which typically elicits licks, began to evoke gapes. Thus, the current results demonstrate that μ-opioid activation in the rNST/dorsal RF exerts complex effects on oromotor responding that contrast with forebrain effects and are more indicative of a suppressive, rather than a facilitatory effect on ingestion.

Opioidergic consequences of dietary-induced binge eating

Endogenous opioids are involved in the hedonic aspects of eating. Opioid impairments and alterations have been implicated in the pathophysiology of bulimia nervosa and binge eating disorder. Specific contributions by Bartley G. Hoebel have furthered the understanding how cyclical caloric restriction and intermittent optional access to sugar solutions result in opioid-like forebrain neural alterations and dependency in rodents. The present study sought to investigate caudal brainstem and nodose ganglion mu-opioid receptor mRNA alterations in a rodent model of dietary-induced binge eating of sweetened fat (vegetable shortening blended with 10% sucrose). Five groups (n=7 or 8) of adult female Sprague Dawley rats were exposed to various dietary conditions for 6 weeks. As measured by in situ hybridization, there was reduced (approximately 25% from naive) mu-opioid receptor mRNA in the nucleus of the solitary tract (NTS) in the binge access group, which had intermittent calorie restriction and optional limited access to the sweetened fat. A similar reduction in expression was demonstrated in the continuous access group, which has unlimited optional sweetened fat and an obese phenotype. In the nodose ganglion, mu-opioid receptor mRNA was increased (approximately 30% from groups with sweetened fat access) in rats with intermittent caloric restriction alone. Our findings and the body of work from the Hoebel laboratory suggest that dietary-induced binge eating can consequentially alter opioidergic forebrain and hindbrain feeding-related neural pathways. Future work is needed to determine whether similar alterations are involved in the maintenance and progression of binge eating and other related eating pathologies.

Pattern of Fos expression in the brain induced by selective activation of somatostatin receptor 2 in rats

Central activation of somatostatin (sst) receptors by oligosomatostatin analogs inhibits growth hormone and stress-related rise in catecholamine plasma levels while stimulating grooming, feeding behaviors, gastric transit and acid secretion, which can be mimicked by selective sst(2) receptor agonist. To evaluate the pattern of neuronal activation induced by peptide sst receptor agonists, we assessed Fos-expression in rat brain after intracerebroventricular (i.c.v.) injection of a newly developed selective sst(2) agonist compared to the oligosomatostatin ODT8-SST, a pan-sst(1-5) agonist. Ninety min after injection of vehicle (10 microl) or previously established maximal orexigenic dose of peptides (1 microg=1 nmol/rat), brains were assessed for Fos-immunohistochemistry and doublelabeling. Food and water were removed after injection. The sst(2) agonist and ODT8-SST induced a similar Fos distribution pattern except in the arcuate nucleus where only the sst(2) agonist increased Fos. Compared to ODT8-SST, the sst(2) agonist induced higher Fos-expression by 3.7-times in the basolateral amygdaloid nucleus, 1.2-times in the supraoptic nucleus (SON), 1.6-times in the magnocellular paraventricular hypothalamic nucleus (mPVN), 4.1-times in the external lateral parabrachial nucleus, and 2.6-times in both the inferior olivary nucleus and superficial layer of the caudal spinal trigeminal nucleus. Doublelabeling in the hypothalamus showed that ODT8-SST activates 36% of oxytocin, 63% of vasopressin and 79% of sst(2) immunoreactive neurons in the mPVN and 28%, 55% and 25% in the SON, respectively. Selective activation of sst(2) receptor results in a more robust neuronal activation than the pan-sst(1-5) agonist in various brain regions that may have relevance in sst(2) mediated alterations of behavioral, autonomic and endocrine functions.

Hedonic and motivational roles of opioids in food reward: implications for overeating disorders

Food reward can be driven by separable mechanisms of hedonic impact (food 'liking') and incentive motivation (food 'wanting'). Brain mu-opioid systems contribute crucially to both forms of food reward. Yet, opioid signals for food 'liking' and 'wanting' diverge in anatomical substrates, in pathways connecting these sites, and in the firing profiles of single neurons. Divergent neural control of hedonic and motivational processes raises the possibility for joint or separable modulation of food intake in human disorders associated with excessive eating and obesity. Early findings confirm an important role for 'liking' and 'wanting' in human appetitive behaviors, and suggest the intriguing possibility that exaggerated signals for 'wanting,' and perhaps 'liking,' may contribute to forms of overeating.

Chronic suppression of μ-opioid receptor signaling in the nucleus accumbens attenuates development of diet-induced obesity in rats

Objective

To test the hypothesis that mu-opioid receptor signaling in the nucleus accumbens contributes to hedonic (over)eating and obesity. To investigate the effects of chronic mu-opioid antagonism in the nucleus accumbens core or shell on intake of a palatable diet, and the development of diet-induced obesity in rats.

Methods and Design

Chronic blockade of mu-opioid receptor-signaling in the nucleus accumbens core or shell was achieved by means of repeated injections (every 4–5 days) of the irreversible receptor antagonist β-Funaltrexamine (BFNA) over 3–5 weeks. The diet consisted of either a choice of high-fat chow, chocolate-flavored Ensure, and regular chow (each nutritionally complete), or regular chow only. Intake of each food item, body weight, and body fat mass were monitored throughout the study.

Results

BFNA injections aimed at either the core or shell of the nucleus accumbens resulted in significantly attenuated intake of palatable diet, body weight gain, and fat accretion, compared with vehicle control injections. BFNA in the core did not significantly change these parameters in chow-fed control rats. BFNA in the core and shell differentially affected intake of the two palatable food items: in the core BFNA significantly reduced intake of high-fat, but not of Ensure, whereas in the shell, it significantly reduced intake of Ensure, but not of high-fat, compared with vehicle-treatment.

Conclusions

Endogenous mu-opioid receptor-signaling in the nucleus accumbens core and shell is necessary for palatable diet-induced hyperphagia and obesity to fully develop in rats. Sweet and non-sweet fatty foods may be differentially processed in subcomponents of the ventral striatum.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Opioids in the hypothalamic paraventricular nucleus stimulate ethanol intake

BACKGROUND

Specialized hypothalamic systems that increase food intake might also increase ethanol intake. To test this possibility, morphine and receptor-specific opioid agonists were microinjected in the paraventricular nucleus (PVN) of rats that had learned to drink ethanol. To cross-validate the results, naloxone methiodide (m-naloxone), an opioid antagonist, was microinjected with the expectation that it would have the opposite effect of morphine and the specific opioid agonists.

METHODS

Sprague-Dawley rats were trained, without sugar, to drink 4 or 7% ethanol and were then implanted with chronic brain cannulas aimed at the PVN. After recovery, those drinking 7% ethanol, with food and water available, were injected with 2 doses each of morphine or m-naloxone. To test for receptor specificity, 2 doses each of the mu-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-Enkephalin (DAMGO), delta-receptor agonist D-Ala-Gly-Phe-Met-NH2 (DALA), or kappa-receptor agonist U-50,488H were injected. DAMGO was also tested in rats drinking 4% ethanol without food or water available. As an anatomical control for drug reflux, injections were made 2 mm dorsal to the PVN.

RESULTS

A main result was a significant increase in ethanol intake induced by PVN injection of morphine. The opposite effect was produced by m-naloxone. The effects of morphine and m-naloxone were exclusively on intake of ethanol, even though food and water were freely available. In the analysis with specific receptor agonists, PVN injection of the delta-agonist DALA significantly increased 7% ethanol intake without affecting food or water intake. This is in contrast to the kappa-agonist U-50,488H, which decreased ethanol intake, and the mu-agonist DAMGO, which had no effect on ethanol intake in the presence or absence of food and water. In the anatomical control location 2 mm dorsal to the PVN, no drug caused any significant changes in ethanol, food, or water intake, providing evidence that the active site was close to the cannula tip.

CONCLUSIONS

The delta-opioid receptor agonist in the PVN increased ethanol intake in strong preference over food and water, while the kappa-opioid agonist suppressed ethanol intake. Prior studies show that learning to drink ethanol stimulates PVN expression and production of the peptides enkephalin and dynorphin, which are endogenous agonists for the delta- and kappa-receptors, respectively. These results suggest that enkephalin via the delta-opioid system can function locally within a positive feedback circuit to cause ethanol intake to escalate and ultimately contribute to the abuse of ethanol. This is in contrast to dynorphin via the kappa-opioid system, which may act to counter this escalation. Naltrexone therapy for alcoholism may act, in part, by blocking the enkephalin-triggered positive feedback cycle.

Loss of GABAergic signaling by AgRP neurons to the parabrachial nucleus leads to starvation

Neurons in the arcuate nucleus that produce AgRP, NPY, and GABA (AgRP neurons) promote feeding. Ablation of AgRP neurons in adult mice results in Fos activation in postsynaptic neurons and starvation. Loss of GABA is implicated in starvation because chronic subcutaneous delivery of bretazenil (a GABA(A) receptor partial agonist) suppresses Fos activation and maintains feeding during ablation of AgRP neurons. Moreover, under these conditions, direct delivery of bretazenil into the parabrachial nucleus (PBN), a direct target of AgRP neurons that also relays gustatory and visceral sensory information, is sufficient to maintain feeding. Conversely, inactivation of GABA biosynthesis in the ARC or blockade of GABA(A) receptors in the PBN of mice promote anorexia. We suggest that activation of the PBN by AgRP neuron ablation or gastrointestinal malaise inhibits feeding. Chronic delivery of bretazenil during loss of AgRP neurons provides time to establish compensatory mechanisms that eventually allow mice to eat.

Activating mu-opioid receptors in the lateral parabrachial nucleus increases c-Fos expression in forebrain areas associated with caloric regulation, reward and cognition

The pontine parabrachial nucleus (PBN) has been implicated in the modulation of ingestion and contains high levels of mu-opioid receptors (MOPRs). In previous work, stimulating MOPRs by infusing the highly selective MOPR agonist [d-Ala2, N-Me-Phe4, Gly5-ol]enkephalin (DAMGO) into the lateral parabrachial region (LPBN) increased food intake. The highly selective MOPR antagonist d-Phe-Cys-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) prevented the hyperphagic action of DAMGO. The present experiments aimed to analyze both the pattern of neural activation and the underlying cellular processes associated with MOPR activation in the LPBN. Male Sprague-Dawley rats received a unilateral microinfusion of a nearly maximal hyperphagic dose of DAMGO into the LPBN. We then determined the level of c-Fos immunoreactivity in regions throughout the brain. MOPR activation in the LPBN increased c-Fos in the LPBN and in the nucleus accumbens, hypothalamic arcuate nucleus, paraventricular nucleus of the thalamus and hippocampus. Pretreatment with CTAP prevented the increase in c-Fos translation in each of these areas. CTAP also prevented the coupling of MOPRs to their G-proteins which was measured by [(35)S] guanosine 5'-O-[gamma-thio]triphosphate ([(35)S]GTPgammaS) autoradiography. Together, these data strongly suggest that increasing the coupling of MOPRs to their G-proteins in the LPBN disinhibits parabrachial neurons which subsequently leads to excitation of neurons in regions associated with caloric regulation, ingestive reward and cognitive processes in feeding.

Activation of delta-opioid receptors reduces excitatory input to putative gustatory cells within the nucleus of the solitary tract

The rostral nucleus of the solitary tract (NST) is the first central relay in the gustatory pathway and plays a key role in processing and modulation of gustatory information. Here, we investigated the effects of opioid receptor agonists and antagonists on synaptic responses of the gustatory parabrachial nuclei (PbN)-projecting neurons in the rostral NST to electrical stimulation of the solitary tract (ST) using whole cell recordings in the hamster brain stem slices. ST-evoked excitatory postsynaptic currents (EPSCs) were significantly reduced by met-enkephalin (MetE) in a concentration-dependent fashion and this effect was eliminated by naltrexone hydrochloride, a nonselective opioid receptor antagonist. Bath application of naltrindole hydrochloride, a selective delta-opioid receptor antagonist, eliminated MetE-induced reduction of EPSCs, whereas CTOP, a selective mu-opioid receptor antagonist had no effect, indicating that delta-opioid receptors are involved in the reduction of ST-evoked EPSCs induced by MetE. SNC80, a selective delta-opioid receptor agonist, mimicked the effect of MetE. The SNC80-induced reduction of ST-evoked EPSCs was eliminated by 7-benzylidenenaltrexone, a selective delta1-opioid receptor antagonist but not by naltriben mesylate, a selective delta2-opioid receptor antagonist, indicating that delta1-opioid receptors mediate the reduction of ST-evoked EPSCs induced by SNC80. Single-cell reverse transcriptase-polymerase chain reaction analysis revealed the presence of delta1-opioid receptor mRNA in cells that responded to SNC80 with a reduction in ST-evoked EPSCs. Moreover, Western blot analysis demonstrated the presence of 40-kDa delta-opioid receptor proteins in the rostral NST tissue. These results suggest that postsynaptic delta1-opioid receptors are involved in opioid-induced reduction of ST-evoked EPSCs of PbN-projecting rostral NST cells.

Activating parabrachial cannabinoid CB1 receptors selectively stimulates feeding of palatable foods in rats

The endocannabinoid system is emerging as an integral component in central and peripheral regulation of feeding and energy balance. Our investigation analyzed behavioral roles for cannabinoid mechanisms of the pontine parabrachial nucleus (PBN) in modulating intake of presumably palatable foods containing fat and/or sugar. The PBN serves to gate neurotransmission associated with, but not limited to, the gustatory properties of food. Immunofluorescence and in vitro [(35)S]GTPgammaS autoradiography of rat tissue sections containing the PBN revealed the presence of cannabinoid receptors and their functional capability to couple to their G-proteins after incubation with the endocannabinoid 2-arachidonoyl glycerol (2-AG). The selective cannabinoid 1 receptor (CB(1)R) antagonist AM251 [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] prevented the response, demonstrating CB(1)R mediation of 2-AG-induced coupling. Microinfusions of 2-AG into the PBN in behaving rats robustly stimulated feeding of pellets high in content of fat and sucrose (HFS), pure sucrose, and pure fat (Crisco), during the first 30 min after infusion. In contrast, 2-AG failed to increase consumption of standard chow, even when the feeding regimen was manipulated to match baseline intakes of HFS. Orexigenic responses to 2-AG were attenuated by AM251, again indicating CB(1)R mediation of 2-AG actions. Furthermore, responses were regionally specific, because 2-AG failed to alter intake when infused into sites approximately 500 mum caudal to infusions that successfully stimulated feeding. Our data suggest that hedonically positive sensory properties of food enable endocannabinoids at PBN CB(1)Rs to initiate increases in eating, and, more generally, these pathways may serve a larger role in brain functions controlling behavioral responses for natural reward.

A naloxonazine sensitive (mu1 receptor) mechanism in the parabrachial nucleus modulates eating

The parabrachial nucleus (PBN) is an area of the brain stem that controls eating and contains endogenous opioids and their receptors. Previously, we demonstrated that acute activation of mu opioid receptors (MOPR) in the lateral PBN increased food consumption. MOPRs have been divided operationally into mu(1) and mu(2) receptor subtypes on the basis of the ability of naloxonazine (Nlxz) to block the former but not the latter. We used autoradiography to measure whether Nlxz blocks stimulation by the mu(1)/mu(2) agonist DAMGO (D-Ala2, N-Me-Phe4, Gly5-ol-enkephalin) of the incorporation of [(35)S]-guanosine 5'(gamma-thio)triphosphate ([(35)S]-GTPgammaS) into sections of the PBN. In vitro, Nlxz dose dependently inhibited receptor coupling in all areas of the PBN. The 1 muM concentration of Nlxz reduced stimulation by 93.1+/-5% in the lateral inferior PBN (LPBNi) and by 90.5+/-4% in the medial parabrachial subregion (MPBN). Administration of Nlxz directly into the LPBNi decreased both food intake and agonist stimulated coupling, ex vivo, for the 24-h period after infusion. Infusion of Nlxz into the intended area reduced food intake by 42.3% below baseline values. Nlxz infusion prevented DAMGO stimulation of G-protein coupling in LPBNi and markedly reduced this stimulation in the MPBN. The incomplete inhibition of DAMGO-stimulated coupling in the MPBN is most likely due to the limited diffusion of Nlxz from the site of infusion (LPBNi) into this brain region. In conclusion, this study demonstrates that the mu(1) opioid receptor subtype is present in the parabrachial nucleus of the pons and that these receptors serve to modulate feeding in rats.

Central control of thermogenesis in mammals

Thermogenesis, the production of heat energy, is an essential component of the homeostatic repertoire to maintain body temperature in mammals and birds during the challenge of low environmental temperature and plays a key role in elevating body temperature during the febrile response to infection. The primary sources of neurally regulated metabolic heat production are mitochondrial oxidation in brown adipose tissue, increases in heart rate and shivering in skeletal muscle. Thermogenesis is regulated in each of these tissues by parallel networks in the central nervous system, which respond to feedforward afferent signals from cutaneous and core body thermoreceptors and to feedback signals from brain thermosensitive neurons to activate the appropriate sympathetic and somatic efferents. This review summarizes the research leading to a model of the feedforward reflex pathway through which environmental cold stimulates thermogenesis and discusses the influence on this thermoregulatory network of the pyrogenic mediator, prostaglandin E(2), to increase body temperature. The cold thermal afferent circuit from cutaneous thermal receptors ascends via second-order thermosensory neurons in the dorsal horn of the spinal cord to activate neurons in the lateral parabrachial nucleus, which drive GABAergic interneurons in the preoptic area to inhibit warm-sensitive, inhibitory output neurons of the preoptic area. The resulting disinhibition of thermogenesis-promoting neurons in the dorsomedial hypothalamus and possibly of sympathetic and somatic premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, activates excitatory inputs to spinal sympathetic and somatic motor circuits to drive thermogenesis.

Learned preferences induced by electrical stimulation of a food-related area of the parabrachial complex: Effects of naloxone

Electrical stimulation of the External Lateral Parabrachial Subnucleus (LPBe), a food-related area, induced behavioral preferences for associated stimuli in a taste discrimination learning task. Although this stimulation appeared to be ineffective to elicit standard lever press self-stimulation, it induced place preference for one of two training compartments of a rectangular maze in which animals (adult male Wistar rats) received concurrent electrical brain stimulation. In subjects that consistently showed a preference behavior in different trials, administration of the opioid antagonist naloxone (4 mg/ml/kg) blocked concurrent learning when the test was made in a new maze but not in the same maze in which animals had learned the task. These results are discussed in terms of the possible participation of the LPBe subnucleus in different natural and artificial brain reward systems.

Chronic prevention of mu-opioid receptor (MOR) G-protein coupling in the pontine parabrachial nucleus persistently decreases consumption of standard but not palatable food

RATIONALE

Acute pharmacological studies implicate mu-opioid receptors (MORs) in the parabrachial nucleus (PBN) of the brainstem in modulating eating. The long-term effects of preventing the cellular function of parabrachial MORs on food consumption remain to be elucidated.

OBJECTIVES

To determine whether (1) chronic inhibition of MOR-mediated G-protein coupling in the PBN of rats would persistently reduce eating and (2) food properties dictate the effects of MOR blockade.

MATERIALS AND METHODS

We microinfused the irreversible MOR antagonist, beta-funaltrexamine (beta-FNA) into the lateral PBN and measured the intake of standard and calorically dense palatable chow for 1 week. First, rats were given standard chow for 20 h daily and a calorically dense palatable chow for 4 h during the day. We infused the agonist, [D: -Ala(2), N-Me-Phe(4), Glycinol(5)]-Enkephalin (DAMGO), 1 week after beta-FNA to probe the acute effects of exogenous stimulation of MORs on palatable food intake. [(35)S]GTPgammaS autoradiography quantified regional loss of MOR cellular function. Next, we measured the actions of beta-FNA on food intake in rats given only standard or palatable chow for 1 week.

RESULTS

One infusion of beta-FNA persistently decreased consumption of standard but not palatable chow, regardless of feeding regimen. beta-FNA also blocked DAMGO-stimulated palatable chow intake, prevented DAMGO-stimulated G-protein coupling in the central and external lateral subnuclei of the PBN, and decreased coupling in the medial PBN. beta-FNA did not affect kappa-opioid receptors.

CONCLUSIONS

MORs in the lateral PBN serve a physiological role in stimulating consumption of standard food. Properties of the diet, such as high palatability or caloric density, may override the influence of inhibiting MOR function.

Endogenous opioids encode relative taste preference

Endogenous opioid signaling contributes to the neural control of food intake. Opioid signaling is thought to regulate palatability, the reward value of a food item as determined by orosensory cues such as taste and texture. The reward value of a food reflects not only these sensory properties but also the relative value of competing food choices. In the present experiment, we used a consummatory contrast paradigm to manipulate the relative value of a sucrose solution for two groups of rats. Systemic injection of the nonspecific opioid antagonist naltrexone suppressed sucrose intake; for both groups, however, this suppression was selective, occurring only for the relatively more valuable sucrose solution. Our results indicate that endogenous opioid signaling contributes to the encoding of relative reward value.

The animal model in food intake regulation: Examples from the opioid literature

Animal models allow us to investigate the basic mechanisms by which food intake is regulated. There are a host of neuroregulators distributed across a complex central network that control eating behavior. The opioid peptides represent one family of such regulators that have been studied extensively in animals. Using anatomical, biochemical and behavioral methods investigators have found that opioids play an important role in reward-related eating. In this brief review we summarize representative animal studies that utilize a variety of experimental techniques to help explain the role of opioids in ingestive behavior.

Mu-opioid receptor cellular function in the nucleus accumbens is essential for hedonically driven eating

Acute pharmacological studies have implicated mu-opioid receptors (MORs) in the shell of the nucleus accumbens (NAC) in mediating responses for palatable food and other natural and drug-induced rewards. However, the long-term behavioral effects of inactivating signal transduction via accumbal MORs, as quantified by an anatomically defined loss of cellular activity, have never been analysed. We combined microinfusion of the irreversible MOR antagonist, beta-funaltrexamine (beta-FNA; 8.0 nmol/0.8 microL, n=9; controls, n=6) with mapping by [35S]GTPgammaS autoradiography to demonstrate an anatomically specific loss of the coupling of MORs to their G-proteins in the dorsal caudomedial shell of the NAC in rabbits. beta-FNA did not alter the stimulated coupling of kappa-opioid receptors. This selective blockade of the cellular function of MORs persistently decreased consumption of a palatable sucrose solution by 40% during a daily 4-h test conducted 2, 3 and 4 days after infusion. beta-FNA did not alter body weight or 20-h consumption of standard chow or water. In 10 different rabbits, infusion of the selective, competitive MOR antagonist, CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2) into the same locus produced a reversible decrease in sucrose consumption, with normal intakes returning on the next day. Together, these data appear to establish that MORs in this accumbal subregion support responding for orosensory reward. Overall, these results visualize a discrete brain locus where cellular actions of endogenous opioids mediate behaviors involved in self-administration of foods and perhaps other hedonically valued substances, such as ethanol and drugs of abuse.

Medial parabrachial nucleus neurons modulate d-fenfluramine-induced anorexia through 5HT2C receptors

We previously reported that lesions of the medial parabrachial nucleus (PBN) enhanced d-fenfluramine (DFEN)-induced anorexia; a finding that suggests these lesions may potentiate the release of serotonin (5HT) or increase the postsynaptic action of 5HT. In the present study, we used SB 206553 (a 5HT2B/2C receptor antagonist) or m-CPP (a 5HT2C/1B receptor agonist) in a standard behavioral procedure (deprivation-induced feeding) to further explore the role of the medial PBN in drug-induced anorexia. In Experiment 1, DFEN (0 or 1.0 mg/kg) was given alone or in combination with SB 206553 (2.0 or 5.0 mg/kg). In Experiment 2, we investigated the food-suppressive effects of m-CPP (0.5, 1.0 or 2.0 mg/kg). The results of Experiment 1 show that SB 206553, while having no influence on the performance of control subjects, attenuated (2.0 mg/kg) or abolished (5 mg/kg) the potentiating effect of the lesions on DFEN-induced anorexia. In Experiment 2, m-CPP induced a suppression of food intake in nonlesioned animals that was significantly potentiated in rats with medial PBN lesions. These results are consistent with the hypothesis that medial PBN neurons mediate anorexia through 5HT2C receptors.

Palatability-dependent appetite and benzodiazepines: new directions from the pharmacology of GABA(A) receptor subtypes

This paper updates an early review on benzodiazepine-enhanced food intake, published in the first issue of Appetite, and describes the considerable advances since then in the pharmacology of benzodiazepines, their sites and mechanisms of action, and in understanding the psychological processes leading to the increase in food consumption. A great diversity of benzodiazepine receptor ligands have been developed, many of which affect food intake. Agonists can be divided into full agonists (which produce the full spectrum of benzodiazepine effects) and partial agonists (which are more selective in their effects). In addition, inverse agonists have been identified, with high affinity for benzodiazepine receptors but having negative efficacy: these drugs exhibit anorectic properties. Benzodiazepine receptors are part of GABA(A) receptor complexes, and ligands thereby modulate inhibitory neurotransmission in the brain. Molecular approaches have identified a palette of receptor subunits from which GABA(A) receptors are assembled. In all likelihood, benzodiazepine-induced hyperphagia is mediated by the alpha2/alpha3 subtype not the alpha1 subtype. Novel alpha2/alpha3 selective compounds will test this hypothesis. A probable site of action in the caudal brainstem for benzodiazepines is the parabrachial nucleus. Behavioural evidence strongly indicates that a primary action of benzodiazepines is to enhance the positive hedonic evaluation (palatability) of tastes and foodstuffs. This generates the increased food intake and instrumental responding for food rewards. Therapeutic applications may derive from the actions of benzodiazepine agonists and inverse agonists on food procurement and ingestion.

Endocannabinoids and food consumption: comparisons with benzodiazepine and opioid palatability-dependent appetite

The endocannabinoid system consists of several endogenous lipids, including anandamide and 2-arachidonoyl-glycerol (2-AG), and constitute a retrograde signalling system, which modulates neurotransmitter release and synaptic plasticity. Specific brain-type cannabinoid receptors (CB(1)) are widely distributed in the central nervous system, and are localized presynaptically. Mounting evidence, reviewed here, indicates that cannabinoids can act to increase food consumption, and cannabinoid CB(1) receptor antagonists/inverse agonists reduce food intake and suppress operant responding for food rewards. Hence, endocannabinoids provide the first example of a retrograde signalling system, which is strongly implicated in the control of food intake. Benzodiazepine and opioid palatability-dependent appetite are well-established processes supported by several sources of convergent evidence; they provide pharmacological benchmarks against which to evaluate the endocannabinoids. To date, evidence that endocannabinoids specifically modulate palatability as an affective evaluative process is insufficient and not compelling. Endocannabinoids may have important clinical utility in the treatment of human obesity and forms of eating disorders.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Neuropeptide FF exerts pro- and anti-opioid actions in the parabrachial nucleus to modulate food intake

Neurons that synthesize the morphine modulatory peptide neuropeptide FF (NPFF; Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2) densely innervate the parabrachial nucleus (PBN), an area implicated in regulating food intake. We analyzed opioid-related actions of NPFF in feeding in adult male Sprague-Dawley rats. Unilateral infusion of 2 nmol/0.5 microl of the mu-opioid receptor agonist [d-Ala2,NMe-Phe4,glycinol5]enkephalin (DAMGO) into the lateral PBN increased 4-h food intake from 0.7 +/- 0.1 to 3.3 +/- 0.3 g. NPFF (1.25-5.0 nmol) prevented this hyperphagic mu-opioidergic action. In rats fed after 4-h deprivation (baseline = 12.3 +/- 0.3 g/2 h), 5 nmol of NPFF did not alter and larger doses (10 and 20 nmol) actually increased food intake (+36, 54%). Twenty nanomoles also elevated intake of freely feeding rats (from 0.7 +/- 0.1 to 5.1 +/- 1.0 g/4 h). The opioid receptor blocker naloxone (10 nmol) antagonized this increase. These data reveal both pro- and anti-opioid actions of NPFF in the PBN to modulate feeding. The mechanisms for the opposite actions of low and high concentrations of this neuropeptide in parabrachial regulation of food intake remain to be determined.