Multiple opioid receptors mediate feeding elicited by mu and delta opioid receptor subtype agonists in the nucleus accumbens shell in rats

Developmental and adult striatal patterning of nociceptin ligand marks striosomal population with direct dopamine projections

Circuit influences on the midbrain dopamine system are crucial to adaptive behavior and cognition. Recent developments in the study of neuropeptide systems have enabled high-resolution investigations of the intersection of neuromodulatory signals with basal ganglia circuitry, identifying the nociceptin/orphanin FQ (N/OFQ) endogenous opioid peptide system as a prospective regulator of striatal dopamine signaling. Using a prepronociceptin-Cre reporter mouse line, we characterized highly selective striosomal patterning of Pnoc mRNA expression in mouse dorsal striatum, reflecting early developmental expression of Pnoc . In the ventral striatum, Pnoc expression was was clustered across the nucleus accumbens core and medial shell, including in adult striatum. We found that Pnoc tdTomato reporter cells largely comprise a population of dopamine receptor D1 ( Drd1 ) expressing medium spiny projection neurons localized in dorsal striosomes, known to be unique among striatal projections neurons for their direct innervation of midbrain dopamine neurons. These findings provide new understanding of the intersection of the N/OFQ system among basal ganglia circuits with particular implications for developmental regulation or wiring of striatal-nigral circuits.

The role of the nucleus accumbens and ventral pallidum in feeding and obesity

Obesity is a growing global epidemic that stems from the increasing availability of highly-palatable foods and the consequent enhanced calorie consumption. Extensive research has shown that brain regions that are central to reward seeking modulate feeding and evidence linking obesity to pathology in such regions have recently started to accumulate. In this review we focus on the contribution of two major interconnected structures central to reward processing, the nucleus accumbens and the ventral pallidum, to obesity. We first review the known literature linking these structures to feeding behavior, then discuss recent advances connecting pathology in the nucleus accumbens and ventral pallidum to obesity, and finally examine the similarities and differences between drug addiction and obesity in the context of these two structures. The understanding of how pathology in brain regions involved in reward seeking and consumption may drive obesity and how mechanistically similar obesity and addiction are, is only now starting to be revealed. We hope that future research will advance knowledge in the field and open new avenues to studying and treating obesity.

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

A Motivational and Neuropeptidergic Hub: Anatomical and Functional Diversity within the Nucleus Accumbens Shell

The mesocorticolimbic pathway is canonically known as the "reward pathway." Embedded within the center of this circuit is the striatum, a massive and complex network hub that synthesizes motivation, affect, learning, cognition, stress, and sensorimotor information. Although striatal subregions collectively share many anatomical and functional similarities, it has become increasingly clear that it is an extraordinarily heterogeneous region. In particular, the nucleus accumbens (NAc) medial shell has repeatedly demonstrated that the rules dictated by more dorsal aspects of the striatum do not apply or are even reversed in functional logic. These discrepancies are perhaps most easily captured when isolating the functions of various neuromodulatory peptide systems within the striatum. Endogenous peptides are thought to play a critical role in modulating striatal signals to either amplify or dampen evoked behaviors. Here we describe the anatomical-functional backdrop upon which several neuropeptides act within the NAc to modulate behavior, with a specific emphasis on nucleus accumbens medial shell and stress responsivity. Additionally, we propose that, as the field continues to dissect fast neurotransmitter systems within the NAc, we must also provide considerable contextual weight to the roles local peptides play in modulating these circuits to more comprehensively understand how this important subregion gates motivated behaviors.

Early life stress alters opioid receptor mRNA levels within the nucleus accumbens in a sex-dependent manner

Early life stress (ELS) strongly impacts mental health, but little is known about its interaction with biological sex and postnatal development to influence risk and resilience to psychopathologies. A number of psychiatric disorders, such as social anhedonia and drug addiction, involve dysfunctional opioid signaling; moreover, there is evidence for differential central opioid function in males vs. females. The present study examined opioid receptor gene expression in the nucleus accumbens (NAc) and amygdala of male and female rats subjected to a neonatal predator odor exposure (POE) paradigm to model ELS. Brain tissue was collected at two developmental time points: neonatal and juvenile. Results showed that, following the neonatal POE experience, opioid receptor mRNA levels in the NAc were differentially regulated at the neonatal and juvenile time points. POE downregulated neonatal mu- and kappa-opioid receptor mRNA levels in neonatal females, but upregulated mu- and delta-opioid receptor mRNA levels in juvenile females. Intriguingly, POE had no significant effect on NAc opioid receptor mRNA levels in males at either time point, indicating that the impact of POE on opioid system development is sex-dependent. Finally, POE failed to alter amygdalar opioid receptor gene expression in either sex at either time-point. The spatiotemporally- and sex-specific impact of ELS within the developing brain may confer differential risk or resilience for males and females to develop atypical opioid-regulated behaviors associated with conditions such as depression and addiction.

Orexin in Rostral Hotspot of Nucleus Accumbens Enhances Sucrose 'Liking' and Intake but Scopolamine in Caudal Shell Shifts 'Liking' Toward 'Disgust' and 'Fear'

The nucleus accumbens (NAc) contains a hedonic hotspot in the rostral half of medial shell, where opioid agonist microinjections are known to enhance positive hedonic orofacial reactions to the taste of sucrose ('liking' reactions). Within NAc shell, orexin/hypocretin also has been reported to stimulate food intake and is implicated in reward, whereas blockade of muscarinic acetylcholine receptors by scopolamine suppresses intake and may have anti-reward effects. Here, we show that NAc microinjection of orexin-A in medial shell amplifies the hedonic impact of sucrose taste, but only within the same anatomically rostral site, identical to the opioid hotspot. By comparison, at all sites throughout medial shell, orexin microinjections stimulated 'wanting' to eat, as reflected by increases in intake of palatable sweet chocolates. At NAc shell sites outside the hotspot, orexin selectively enhanced 'wanting' to eat without enhancing sweetness 'liking' reactions. In contrast, microinjections of the antagonist scopolamine at all sites in NAc shell suppressed sucrose 'liking' reactions as well as suppressing intake of palatable food. Conversely, scopolamine increased aversive 'disgust' reactions elicited by bitter quinine at all NAc shell sites. Finally, scopolamine microinjections localized to the caudal half of medial shell additionally generated a fear-related anti-predator reaction of defensive treading and burying directed toward the corners of the transparent chamber. Together, these results confirm a rostral hotspot in NAc medial shell as a unique site for orexin induction of hedonic 'liking' enhancement, similar to opioid enhancement. They also reveal distinct roles for orexin and acetylcholine signals in NAc shell for hedonic reactions and motivated behaviors.

Intermittent-access binge consumption of sweet high-fat liquid does not require opioid or dopamine receptors in the nucleus accumbens

Binge eating disorders are characterized by episodes of intense consumption of high-calorie food. In recently developed animal models of binge eating, rats given intermittent access to such food escalate their consumption over time. Consumption of calorie-dense food is associated with neurochemical changes in the nucleus accumbens, including dopamine release and alterations in dopamine and opioid receptor expression. Therefore, we hypothesized that binge-like consumption on intermittent access schedules is dependent on opioid and/or dopamine neurotransmission in the accumbens. To test this hypothesis, we asked whether injection of dopamine and opioid receptor antagonists into the core and shell of the accumbens reduced consumption of a sweet high-fat liquid in rats with and without a history of intermittent binge access to the liquid. Although injection of a μ opioid agonist increased consumption, none of the antagonists (including μ opioid, δ opioid, κ opioid, D1 dopamine and D2 dopamine receptor antagonists, as well as the broad-spectrum opioid receptor antagonist naltrexone) reduced consumption, and this was the case whether or not the animals had a prior history of intermittent access. These results suggest that consumption of sweet, fatty food does not require opioid or dopamine receptor activation in the accumbens even under intermittent access conditions that resemble human binge episodes.

The effects of nucleus accumbens μ-opioid and adenosine 2A receptor stimulation and blockade on instrumental learning

Prior research has shown that glutamate and dopamine receptors in the nucleus accumbens (NAcc) core are critical for the learning of an instrumental response for food reinforcement. It has also been demonstrated that μ-opioid and adenosine A2A receptors within the NAcc impact feeding and motivational processes. In these experiments, we examined the potential roles of NAcc μ-opioid and A2A receptors on instrumental learning and performance. Sprague-Dawley rats were food restricted and trained to lever press following daily intra-accumbens injections of the A2A receptor agonist CGS 21680 (at 0.0, 6.0, or 24.0ng/side), the A2A antagonist pro-drug MSX-3 (at 0.0, 1.0, or 3.0μg/side), the μ-opioid agonist DAMGO (at 0.0, 0.025, or 0.025μg/side), or the opioid receptor antagonist naltrexone (at 0.0, 2.0 or 20.0μg/side). After five days, rats continued training without drug injections until lever pressing rates stabilized, and were then tested with a final drug test to assess potential performance effects. Stimulation, but not inhibition, of NAcc adenosine A2A receptors depressed lever pressing during learning and performance tests, but did not impact lever pressing on non-drug days. Both μ-opioid receptor stimulation and blockade inhibited learning of the lever-press response, though only naltrexone treatment caused impairments in lever-pressing after the task had been learned. The effect of A2A receptor stimulation on learning and performance were consistent with known effects of adenosine on effort-related processes, whereas the pattern of lever presses, magazine approaches, and pellet consumption following opioid receptor manipulations suggested that their effects may have been driven by drug-induced shifts in the incentive value of the sugar reinforcer.

Opioid hedonic hotspot in nucleus accumbens shell: mu, delta, and kappa maps for enhancement of sweetness "liking" and "wanting"

A specialized cubic-millimeter hotspot in the rostrodorsal quadrant of medial shell in nucleus accumbens (NAc) of rats may mediate opioid enhancement of gustatory hedonic impact or "liking". Here, we selectively stimulated the three major subtypes of opioid receptors via agonist microinjections [mu (DAMGO), delta (DPDPE), or kappa (U50488H)] and constructed anatomical maps for functional localizations of consequent changes in hedonic "liking" (assessed by affective orofacial reactions to sucrose taste) versus "wanting" (assessed by changes in food intake). Results indicated that the NAc rostrodorsal quadrant contains a shared opioid hedonic hotspot that similarly mediates enhancements of sucrose "liking" for mu, delta, and kappa stimulations. Within the rostrodorsal hotspot boundaries each type of stimulation generated at least a doubling or higher enhancement of hedonic reactions, with comparable intensities for all three types of opioid stimulation. By contrast, a negative hedonic coldspot was mapped in the caudal half of medial shell, where all three types of opioid stimulation suppressed "liking" reactions to approximately one-half normal levels. Different anatomical patterns were produced for stimulation of food "wanting", reflected in food intake. Altogether, these results indicate that the rostrodorsal hotspot in medial shell is unique for generating opioid-induced hedonic enhancement, and add delta and kappa signals to mu as hedonic generators within the hotspot. Also, the identification of a separable NAc caudal coldspot for hedonic suppression, and separate NAc opioid mechanisms for controlling food "liking" versus "wanting" further highlights NAc anatomical heterogeneity and localizations of function within subregions of medial shell.

Central effects of ethanol interact with endogenous mu-opioid activity to control isolation-induced analgesia in maternally separated infant rats

Endogenous opioid activity plays an important role in ethanol consumption and reinforcement in infant rats. Opioid systems are also involved in mediation and regulation of stress responses. Social isolation is a stressful experience for preweanling rats and changes the effects of ethanol through opioid-dependent mechanisms. The present study assessed effects of intracisternal (i.c.) administration of a selective mu-opioid antagonist (CTOP) and i.p. administration of a nonspecific opioid antagonist (naloxone) on voluntary intake and behavior in socially isolated 12-day-old (P12) pups treated with 0.5 g/kg ethanol. Voluntary intake of 0.1% saccharin or water, locomotion, rearing activity, paw licking and grooming were assessed during short-term isolation from littermates (STSI; 8-min duration). Thermal nociceptive reactivity was measured before and after this intake test, with normalized differences between pre- and post-test latencies of paw withdrawal from a hot plate (49°C) used as an index of isolation-induced analgesia (IIA). Results indicated several effects of social isolation and ethanol mediated through the mu-opioid system. Effects of low dose ethanol (0.5 g/kg) and voluntary consumption of saccharin interacted with endogenous mu-opioid activity associated with STSI. Blockade of mu-opioid receptors on saccharin consumption and paw licking-grooming affected intoxicated animals. Low dose ethanol and ingestion of saccharin blunted effects of CTOP on rearing behavior and nociceptive reactivity. Central injections of CTOP stimulated paw licking and grooming dependent on ethanol dose and type of fluid ingested. Ethanol selectively increased saccharin intake during STSI in females, naloxone and CTOP blocked ethanol-mediated enhancement of saccharin intake. We suggest that enhancement of saccharin intake by ethanol during STSI is the product of synergism between isolation-induced mu-opioid activity that increases the pup's sensitivity to appetitive taste stimulation and the anxiolytic effects of 0.5 g/kg ethanol that decreases behaviors otherwise competing with independent ingestive activity.

Mapping brain circuits of reward and motivation: in the footsteps of Ann Kelley

Ann Kelley was a scientific pioneer in reward neuroscience. Her many notable discoveries included demonstrations of accumbens/striatal circuitry roles in eating behavior and in food reward, explorations of limbic interactions with hypothalamic regulatory circuits, and additional interactions of motivation circuits with learning functions. Ann Kelley's accomplishments inspired other researchers to follow in her footsteps, including our own laboratory group. Here we describe results from several lines of our research that sprang in part from earlier findings by Kelley and colleagues. We describe hedonic hotspots for generating intense pleasure 'liking', separate identities of 'wanting' versus 'liking' systems, a novel role for dorsal neostriatum in generating motivation to eat, a limbic keyboard mechanism in nucleus accumbens for generating intense desire versus intense dread, and dynamic limbic transformations of learned memories into motivation. We describe how origins for each of these themes can be traced to fundamental contributions by Ann Kelley.

Dopamine or opioid stimulation of nucleus accumbens similarly amplify cue-triggered 'wanting' for reward: entire core and medial shell mapped as substrates for PIT enhancement

Pavlovian cues [conditioned stimulus (CS+)] often trigger intense motivation to pursue and consume related reward [unconditioned stimulus (UCS)]. But cues do not always trigger the same intensity of motivation. Encountering a reward cue can be more tempting on some occasions than on others. What makes the same cue trigger more intense motivation to pursue reward on a particular encounter? The answer may be the level of incentive salience ('wanting') that is dynamically generated by mesocorticolimbic brain systems, influenced especially by dopamine and opioid neurotransmission in the nucleus accumbens (NAc) at that moment. We tested the ability of dopamine stimulation (by amphetamine microinjection) vs. mu opioid stimulation [by d-Ala, nMe-Phe, Glyol-enkephalin (DAMGO) microinjection] of either the core or shell of the NAc to amplify cue-triggered levels of motivation to pursue sucrose reward, measured with a Pavlovian-Instrumental Transfer (PIT) procedure, a relatively pure assay of incentive salience. Cue-triggered 'wanting' in PIT was enhanced by amphetamine or DAMGO microinjections equally, and also equally at nearly all sites throughout the entire core and medial shell (except for a small far-rostral strip of shell). NAc dopamine/opioid stimulations specifically enhanced CS+ ability to trigger phasic peaks of 'wanting' to obtain UCS, without altering baseline efforts when CS+ was absent. We conclude that dopamine/opioid stimulation throughout nearly the entire NAc can causally amplify the reactivity of mesocorticolimbic circuits, and so magnify incentive salience or phasic UCS 'wanting' peaks triggered by a CS+. Mesolimbic amplification of incentive salience may explain why a particular cue encounter can become irresistibly tempting, even when previous encounters were successfully resisted before.

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.

Descending projections from the nucleus accumbens shell suppress activity of taste-responsive neurons in the hamster parabrachial nuclei

The parabrachial nuclei (PbN), the second central relay for the gustatory pathway, transfers taste information to various forebrain gustatory nuclei and to the gustatory cortex. The nucleus accumbens is one of the critical neural substrates of the reward system, and the nucleus accumbens shell region (NAcSh) is associated with feeding behavior. Taste-evoked neuronal responses of PbN neurons are modulated by descending projections from the gustatory nuclei in the forebrain. In the present study, we investigated whether taste-responsive neurons in the PbN project to the NAcSh and whether pontine gustatory neurons are subject to modulatory influence from the NAcSh in urethane-anesthetized hamsters. Extracellular single-unit activity was recorded in the PbN, and taste responses were confirmed by the delivery of 32 mM sucrose, NaCl, quinine hydrochloride, and 3.2 mM citric acid to the anterior tongue. The NAcSh was then stimulated (0.5 ms, ≤100 μA) bilaterally using concentric bipolar stimulating electrodes. A total of 98 taste neurons were recorded from the PbN. Eighteen neurons were antidromically invaded from the NAcSh, mostly the ipsilateral NAcSh ( n = 16). Stimulation of the ipsilateral and contralateral NAcSh suppressed the neuronal activity of 88 and 55 neurons, respectively; 52 cells were affected bilaterally. In a subset of pontine neurons tested, electrical stimulation of the NAcSh during taste stimulation also suppressed taste-evoked neuronal firing. These results demonstrated that taste-responsive neurons in the PbN not only project to the NAcSh but also are under substantial descending inhibitory influence from the bilateral NAcSh.

Central kappa opioid receptors modulate salt appetite in rats

The role of the central opioid system in the control of water and salt intake is complex, with both stimulatory and inhibitory effects having been observed. The aim of the present study was to investigate the participation of the central κ-opioid receptors in the control of salt appetite. Male Wistar rats were submitted to two different experimental protocols: sodium deficit produced by the diuretic, furosemide, and brain angiotensinergic stimulation in rats under normal sodium balance. Lateral ventricle (LV) injections of Nor-binaltorphimine (Nor-BNI) at different doses (5, 10 and 20 nmol) inhibited hypertonic saline solution (1.5%) intake in sodium-depleted rats. The salt appetite induced by an LV injection of angiotensin II (Ang II) (10 ng) was also blocked by Nor-BNI injections into the LV, while no significant change was observed in water intake. Furthermore, the decrease in salt intake seems not to have been due to a general inhibition of locomotor activity or to any change in palatability, since central administration of Nor-BNI failed to modify the intake of a 0.1% saccharin solution when the animals were submitted to a "dessert test" or to induce any significant locomotor deficit in the open-field test. Also the central administration of Nor-BNI was unable to modify blood pressure in sodium-depleted animals. The present results suggest that activation of endogenous κ-opioid receptors modulates salt appetite induced by sodium depletion and by central angiotensinergic stimulation in rats.

mu-Opioid receptor stimulation in the nucleus accumbens elevates fatty tastant intake by increasing palatability and suppressing satiety signals

Infusion of a μ-opioid receptor (MOR) agonist into the nucleus accumbens (NAcc) drives voracious food intake, an effect hypothesized to occur through increased tastant palatability. While intake of many palatable foods is elevated by MOR stimulation, this manipulation has a preferential effect on fatty food ingestion. Consumption of high-fat foods is increased by NAcc MOR stimulation even in rats that prefer a carbohydrate-rich alternative under baseline conditions. This suggests that NAcc MOR stimulation may not simply potentiate palatability signals and raises the possibility that mechanisms mediating fat intake may be distinct from those underlying intake of other tastants. The present study was conducted to investigate the physiological mechanisms underlying the effects of NAcc MOR stimulation on fatty food intake. In experiment 1, we analyzed lick microstructure in rats ingesting Intralipid to identify the changes underlying feeding induced by infusion of a MOR-specific agonist into the NAcc. MOR stimulation in the NAcc core, but not shell, increased burst duration and first-minute licks, while simultaneously increasing the rate and duration of Intralipid ingestion. These results suggest that MOR activation in the core increases Intralipid palatability and attenuates inhibitory postingestive feedback. In experiment 2, we measured the effects of MOR stimulation in the NAcc core on consumption of nonnutritive olestra. A MOR-specific agonist dose dependently increased olestra intake, demonstrating that caloric signaling is not required for hyperphagia induced by NAcc MOR stimulation. Feeding induced by drug infusion in both experiments 1 and 2 was blocked by a MOR antagonist. In experiment 3, we determined whether MOR activation in the NAcc core could attenuate satiety-related signaling caused by infusion of the melanocortin agonist MTII into the third ventricle. Suppression of intake caused by MTII was reversed by MOR stimulation. Together, our results suggest that MOR stimulation in the NAcc core elevates fatty food intake through palatability mechanisms dependent on orosensory cues and suppression of satiety signals inhibiting food intake.

Reversible suppression of food reward behavior by chronic mu-opioid receptor antagonism in the nucleus accumbens

Overindulgence in easily available energy-dense palatable foods is thought to be an important factor in the current obesity epidemic but the underlying neural mechanisms are not well understood. Here we demonstrate that mu-opioid receptor signaling in the nucleus accumbens may be important. Protracted suppression of endogenous mu-opioid receptor signaling focused on the nucleus accumbens shell for several days by means of microinjected beta-funaltrexamine (BFNA) diminished both "liking" of sucrose, as indicated by fewer positive hedonic orofacial responses, and the incentive reinforcement value ("wanting") of a food reward, as indicated by lower completion speed and increased time being distracted in the incentive runway. BFNA-treatment also decreased responding to sucrose and corn oil in the brief access lick paradigm, a test measuring a combination of mainly taste-guided "liking" and low-effort "wanting", as well as 4 h intake of sucrose solution. These effects were not due to nonspecific permanent neuronal changes, as they were fully reversible. We conclude that endogenous mu-opioid signaling in the nucleus accumbens is necessary for the full display of palatable food-induced hyperphagia through mechanisms including hedonic, motivational, and reinforcement processes. Development of obesity could be the result of predisposing innate differences in these mechanisms or overstimulation of these mechanisms by external factors.

Intra-accumbens infusion of a muscarinic antagonist reduces food intake without altering the incentive properties of food-associated cues

Previous work has implicated the cholinergic system in modulating feeding behavior; however, its specific function remains unclear. This work aims to characterize potential dissociations between the central cholinergic modulation of the incentive properties of food and food-associated cues, and consummatory behaviors. Three separate experiments demonstrated that intra-accumbens infusion of the muscarinic antagonist scopolamine 3 hr before the testing session significantly decreased food intake. General motor activity in anticipation of food was not diminished. Experiments also showed that scopolamine did not impair operant responding for a food-associated conditioned reinforcer (CR), nor was d-amphetamine potentiation of CR responding altered by scopolamine pretreatment. This study contributes to the growing evidence that goal-seeking behaviors are mediated by a set of neural processes distinct from those governing food reward.

Modulation of feeding and locomotion through mu and delta opioid receptor signaling in the nucleus accumbens

Opioid signaling has been strongly implicated in driving palatable food consumption. The nucleus accumbens (NAcc) is one important site of this effect; hyperphagia elicited by administration of exogenous mu opioid receptor (MOR) ligands in this brain region has been well documented. However, the role that endogenous opioid ligands in the NAcc play in controlling food intake remains poorly understood. Enkephalins, which signal through both the MOR and delta opioid receptor (DOR), are highly expressed within a subset of NAcc neurons, and have been shown to be sensitive to manipulations of diet and motivation. To investigate a potential role for these signaling molecules in regulating palatability-driven consumption, we measured high fat chow intake in rats following a series of pharmacological manipulations of NAcc opioid signaling. NAcc infusion of the MOR agonist [D-Ala2, N-MePHe4, Gly-ol]-enkephalin (DAMGO) robustly increased palatable food intake, as has previously been demonstrated. In contrast, neither infusion of Met-enkephalin, its synthetic analogue [D-Ala2] Met-enkephalin (DALA) nor the DOR-specific ligand [D-Pen2, Pen5]-enkephalin (DPDPE) had significant effects on food intake. However, when administered in combination with DAMGO, DPDPE significantly suppressed the magnitude of DAMGO-evoked feeding. Further analysis of DPDPE effects revealed that the drug strongly increased locomotor activity. Suppressive effects on feeding, then, may have occurred through competition between feeding and locomotion for behavioral expression.

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.

Selective mu- and kappa-opioid receptor antagonists administered into the nucleus accumbens interfere with rapid tolerance to ethanol in rats

BACKGROUND

Previous findings have shown that intra-accumbens injection of naltrexone, a non-selective opioid antagonist, blocks the acquisition of rapid tolerance to ethanol in rats. This study investigates the effects of intra-accumbens injection of the selective mu-, delta-, and kappa-opioid antagonists, respectively, naloxonazine, naltrindole, and nor-binaltorphimine, on rapid tolerance to ethanol.

METHODS

Male Wistar rats with guide cannulae directed to the shell or the core portions of the nucleus accumbens received a microinjection of naloxonazine (2-4 microg), naltrindole (2-4 microg), nor-binaltorphimine (2.5-5 microg), or vehicle. After 5 min, each group was divided in two groups that received ethanol (2.7 g/kg i.p.) or saline. Rats were then tested for motor coordination on the tilting plane apparatus. Twenty four hours later, all rats received a challenge dose of ethanol (2.7 g/kg i.p.) and were tested on the tilt plane again.

RESULTS

Repeated injections of ethanol caused a reduction in motor impairment suggesting the development of tolerance. However, rats injected with 4 microg naloxonazine into either core or shell portions of the nucleus accumbens did not exhibit tolerance when challenged with ethanol on day 2. Rats treated with 5 microg nor-binaltorphimine into accumbens core plus intraperitoneal saline on day 1 showed reduced motor impairment when challenged with ethanol on day 2, suggesting cross-tolerance to ethanol.

CONCLUSIONS

Taken together, our results suggests that mu-opioid receptors in both shell and core portions of the nucleus accumbens, and possibly kappa-opioid in the core, participate in the modulation of rapid tolerance to ethanol.

Opioids in the nucleus accumbens stimulate ethanol intake

The nucleus accumbens (NAc) participates in the control of both motivation and addiction. To test the possibility that opioids in the NAc can cause rats to select ethanol in preference to food, Sprague-Dawley rats with ethanol, food, and water available, were injected with two doses each of morphine, the mu-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-Enkephalin (DAMGO), the delta-receptor agonist D-Ala-Gly-Phe-Met-NH2 (DALA), the k-receptor agonist (+/-)-trans-U-50488 methanesulfonate (U-50,488H), or the opioid antagonist naloxone methiodide (m-naloxone). As an anatomical control for drug reflux, injections were also made 2mm above the NAc. The main result was that morphine in the NAc significantly increased ethanol and food intake, whereas m-naloxone reduced ethanol intake without affecting food or water intake. Of the selective receptor agonists, DALA in the NAc increased ethanol intake in preference to food. This is in contrast to DAMGO, which stimulated food but not ethanol intake, and the k-agonist U-50,488H, which had no effect on intake. When injected in the anatomical control site 2mm dorsal to the NAc, the opioids had no effects on ethanol intake. These results demonstrate that ethanol intake produced by morphine in the NAc is driven in large part by the delta-receptor. In light of other studies showing ethanol intake to increase enkephalin expression in the NAc, the present finding of enkephalin-induced ethanol intake suggests the existence of a positive feedback loop that fosters alcohol abuse. Naltrexone therapy for alcohol abuse may then act, in part, in the NAc by blocking this opioid-triggered cycle of alcohol intake.

Convergent, not serial, striatal and pallidal circuits regulate opioid-induced food intake

Mu opioid receptor (MOR) signaling in the nucleus accumbens (NAcc) elicits marked increases in the consumption of palatable tastants. However, the mechanism and circuitry underlying this effect are not fully understood. Multiple downstream target regions have been implicated in mediating this effect but the role of the ventral pallidum (VP), a primary target of NAcc efferents, has not been well defined. To probe the mechanisms underlying increased consumption, we identified behavioral changes in rats' licking patterns following NAcc MOR stimulation. Because the temporal structure of licking reflects the physiological substrates modulating consumption, these measures provide a useful tool in dissecting the cause of increased consumption following NAcc MOR stimulation. Next, we used a combination of pharmacological inactivation and lesions to define the role of the VP in hyperphagia following infusion of the MOR-specific agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) into the NAcc. In agreement with previous studies, results from lick microstructure analysis suggest that NAcc MOR stimulation augments intake through a palatability-driven mechanism. Our results also demonstrate an important role for the VP in normal feeding behavior: pharmacological inactivation of the VP suppresses baseline and NAcc DAMGO-induced consumption. However, this interaction does not occur through a serial circuit requiring direct projections from the NAcc to the VP. Rather, our results indicate that NAcc and VP circuits converge on a common downstream target that regulates food intake.

The role of amygdalar mu-opioid receptors in anxiety-related responses in two rat models

Amygdala opioids such as enkephalin appear to play some role in the control of anxiety and the anxiolytic effects of benzodiazepines, although the opioid receptor subtypes mediating such effects are unclear. This study compared the influences of mu-opioid receptor (MOR) activation in the central nucleus of the amygdala (CEA) on unconditioned fear or anxiety-like responses in two models, the elevated plus maze, and the defensive burying test. The role of MORs in the anxiolytic actions of the benzodiazepine agonist diazepam was also examined using both models. Either the MOR agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO), or the MOR antagonists Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) or beta-funaltrexamine (FNA) were bilaterally infused into the CEA of rats before testing. The results show that microinjection of DAMGO in the CEA decreased open-arm time in the plus maze, whereas CTAP increased open-arm behaviors. In contrast, DAMGO injections in the CEA reduced burying behaviors and increased rearing following exposure to a predator odor, suggesting a shift in the behavioral response in this context. Amygdala injections of the MOR agonist DAMGO or the MOR antagonist CTAP failed to change the anxiolytic effects of diazepam in either test. Our results demonstrate that MOR activation in the central amygdala exerts distinctive effects in two different models of unconditioned fear or anxiety-like responses, and suggest that opioids may exert context-specific regulation of amygdalar output circuits and behavioral responses during exposure to potential threats (open arms of the maze) vs discrete threats (predator odor).

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.

Muscarinic receptor antagonism causes a functional alteration in nucleus accumbens mu-opiate-mediated feeding behavior

Intra-nucleus accumbens (Acb) infusion of cholinergic muscarinic antagonist, scopolamine (10 microg/0.5 microl), markedly reduced fat intake elicited by intra-Acb treatment of the mu-opioid receptor agonist, DAMGO, with 30 min and 4h pretreatment intervals. Intra-Acb scopolamine infusions also reduced food intake in food-deprived rats, but not water intake in water-deprived rats. Hence, Acb muscarinic manipulations exhibit some specificity for feeding, perhaps via interactions with the striatal opioid system.

The microinjection of AMPA receptor antagonist into the accumbens shell failed to change food intake, but reduced fear-motivated behaviour in free-feeding female rats

This study investigated the effect of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 2.5 and 5.0 nmol/side) microinjected into the core and shell sub-regions of the accumbens (Acb) nucleus, on food intake and the level of anxiety in female rats. Bilateral microinjections of CNQX (5.0 nmol/side) into the Acb shell (AP, +1.08 to +2.04), but not into the Acb core, induced an anxiolytic-like effect in relation to rats microinjected with vehicle, since the animals exhibited low level of SAP in the feeding test. The anxiolytic-like effect induced by 5.0 nmol CNQX microinjection into the Acb shell may not be ascribed to changes in the motor activity of the animals, because the frequency of locomotion, rearing and grooming remained unchanged after the drug microinjection. However, neither Acb shell nor Acb core CNQX microinjections were able to change the animals food intake along 1h feeding behaviour evaluation. Food intake remained unchanged 24h after the drug microinjections either into the Acb shell or into the Acb core. The data suggest that AMPA receptor blockade in the Acb nucleus may differentially change the ingestive and defensive behaviours in female rats.

Nucleus accumbens opioids regulate flavor-based preferences in food consumption

Opioid signaling in the nucleus accumbens (NAcc) regulates feeding behavior, having particularly strong effects on consumption of highly palatable foods. Since macronutrient content may contribute to palatability, it is uncertain whether opioid regulation of food consumption is based primarily on its macronutrient content or its flavor per se. In order to isolate the effect of flavor, we manipulated opioid signaling in the NAcc in rats and quantified consumption of differently flavored but nutritionally identical pellets. When pellets of either flavor were presented alone, microinjection of d-Ala(2),N,Me-Phe(4),Gly-ol(5)-enkephalin (DAMGO (a mu opioid receptor (MOP) agonist)) into the NAcc increased consumption of pellets of both flavors equally. When both flavors of pellets were presented simultaneously, however, DAMGO in the NAcc selectively increased, while naltrexone (a non-selective opioid antagonist) in the NAcc selectively decreased, consumption of the more preferred flavor. Systemic naltrexone injection had no flavor specific effects, decreasing consumption of both flavors equally. Non-selective inactivation of NAcc neurons by local microinjection of muscimol (a GABA(A) agonist) increased consumption of both the more- and less-preferred flavors equally. These results indicate that opioid signaling directly regulates a subset of NAcc neurons that can selectively enhance consumption of preferred palatable foods based exclusively on flavor cues.

Striatal opioid peptide gene expression differentially tracks short-term satiety but does not vary with negative energy balance in a manner opposite to hypothalamic NPY

It has long been known that central opioid systems play an important role in certain aspects of appetite and food intake, particularly with regard to the hedonic or rewarding impact of calorically dense food, such as fat and sugar. Ventral striatal enkephalin may be a key component of this system, as infusions of mu-opiate agonists into this region strongly increase feeding, whereas infusions of opiate antagonists decrease food intake. While pharmacological analysis has consistently supported such a role, direct measurement of enkephalin gene expression in relation to differing food motivational conditions has not been examined. In this study, the effects of a restricted laboratory chow diet (resulting in negative energy balance) as well has recent consumption of chow (short-term satiety) on striatal preproenkephalin (PPE) and prodynorphin (PD) mRNA expression were measured in rats, using both Northern blot analysis and in situ hybridization methods. As a comparison, hypothalamic (arcuate nucleus) neuropeptide Y (NPY) was also measured in these conditions. PPE expression was broadly downregulated throughout the striatum in animals that had recently consumed a meal, whereas it was unaffected by negative energy balance. Expression of an additional striatal peptide gene, PD, did not follow this pattern, although diet restriction caused a decrease in accumbens core dynorphin mRNA. Conversely, as expected, arcuate nucleus NPY mRNA expression was markedly upregulated by negative energy balance, but was unchanged by recent food consumption. This double dissociation between striatal and hypothalamic peptide systems suggests a specific role for striatal PPE in relatively short-term food motivational states, but not in long-term metabolic responses to diet restriction.

Hedonic hot spot in nucleus accumbens shell: where do mu-opioids cause increased hedonic impact of sweetness?

Mu-opioid systems in the medial shell of the nucleus accumbens contribute to hedonic impact ("liking") for sweetness, food, and drug rewards. But does the entire medial shell generate reward hedonic impact? Or is there a specific localized site for opioid enhancement of hedonic "liking" in the medial shell? And how does enhanced taste hedonic impact relate to opioid-stimulated increases in food intake? Here, we used a functional mapping procedure based on microinjection Fos plumes to localize opioid substrates in the medial shell of the nucleus accumbens that cause enhanced "liking" reactions to sweet pleasure and that stimulate food intake. We mapped changes in affective orofacial reactions of "liking"/"disliking" elicited by sucrose or quinine tastes after D-Ala2-N-Me-Phe4-Glycol5-enkephalin (DAMGO) microinjections in rats and compared hedonic increases to food intake stimulated at the same sites. Our maps indicate that opioid-induced increases in sucrose hedonic impact are generated by a localized cubic millimeter site in a rostrodorsal region of the medial shell. In contrast, all regions of the medial shell generated DAMGO-induced robust increases in eating behavior and food intake. Thus, our results identify a locus for opioid amplification of hedonic impact and reveal a distinction between opioid mechanisms of food intake and hedonic impact. Opioid circuits for stimulating food intake are widely distributed, whereas hedonic "liking" circuits are more tightly localized in the rostromedial shell of the nucleus accumbens.

A proposed hypothalamic-thalamic-striatal axis for the integration of energy balance, arousal, and food reward

We elaborate herein a novel theory of basal ganglia function that accounts for why palatable, energy-dense foods retain high incentive value even when immediate physiological energy requirements have been met. Basal ganglia function has been studied from the perspective of topographical segregation of processing within parallel circuits, with primary focus on motor control and cognition. Recent findings suggest, however, that the striatum can act as an integrated unit to modulate motivational state. We describe evidence that the striatal enkephalin system, which regulates the hedonic impact of preferred foods, undergoes coordinated gene expression changes that track current motivational state with regard to food intake. Striatal enkephalin gene expression is also downregulated by an intrastriatal infusion of a cholinergic muscarinic antagonist, a manipulation that greatly suppresses food intake. To account for these findings, we propose that signaling through a hypothalamic-midline thalamic-striatal axis impinges on the cholinergic interneurons of the striatum, which via their large, overlapping axonal fields act as a network to modulate enkephalin-containing striatal output neurons. A key relay in this circuit is the paraventricular thalamic nucleus, which receives convergent input from orexin-coded hypothalamic energy-sensing and behavioral state-regulating neurons, as well as from circadian oscillators, and projects to cholinergic interneurons throughout the striatal complex. We hypothesize that this system evolved to coordinate feeding and arousal, and to prolong the feeding central motivational state beyond the fulfillment of acute energy needs, thereby promoting "overeating" and the consequent development of an energy reserve for potential future food shortages.

Alkylation of opioid receptors by 5′-naltrindole-isothiocyanate injected into the nucleus accumbens of rats: Receptor selectivity and anatomical diffusion

Subtypes of the delta opioid receptor (Oprd1) have been suggested based on pharmacology studies. However, these subtypes have not been confirmed biochemically using either receptor binding assays or molecular cloning. Naltrindole-5'-isothiocyanate (5'-NTII) is an irreversible opioid antagonist that appears to selectively inhibit the actions of a subset of delta opioid agonists in vivo, referred to as putative delta-2 agonists. The biochemical and anatomical selectivity of wash-resistant inhibition of binding of [(3)H]DAMGO (Oprm1), [(3)H]DPDPE (Oprd1, putative subtype 1 agonist), or [(3)H]deltorphin II (Oprd1, putative subytpe 2 agonist) in coronal sections was assessed using quantitative in vitro autoradiography following injection of 5'-NTII into the nucleus accumbens in rats. 5'-NTII decreased [(3)H]deltorphin II to a greater extent than the binding of the other two radioligands following administration of 0.05-2.5 nmol. The effects of 5'-NTII were largely confined to the nucleus accumbens; however, some loss in the ventral caudate was also noted. In contrast, administration of the nonselective opioid receptor alkylating antagonist beta-chlornaltexamine (beta-CNA) over a similar range of doses was found to be nonselective for either delta radioligand, and produced greater inhibition of Oprm1 relative to Oprd1 binding, consistent with the nonselective pharmacological activity of this antagonist. Although 5'-NTII inhibited [(3)H]deltorphin II binding to a greater extent, the binding of the other two radioligands was decreased over a similar range of doses. Absolute conclusions regarding the involvement of delta-2 opioid receptors in pharmacological or physiological effects based on studies with 5'-NTII should therefore be tempered, and for site-directed studies it would be best to employ doses of 0.5 nmol or lower.

Localization of opioid receptor antagonist [3H]-LY255582 binding sites in mouse brain: comparison with the distribution of mu, delta and kappa binding sites

Agonist stimulation of opioid receptors increases feeding in rodents, while opioid antagonists inhibit food intake. The pan-opioid antagonist, LY255582, produces a sustained reduction in food intake and body weight in rodent models of obesity. However, the specific receptor subtype(s) responsible for this activity is unknown. To better characterize the pharmacology of LY255582, we examined the binding of a radiolabeled version of the molecule, [(3)H]-LY255582, in mouse brain using autoradiography. In mouse brain homogenates, the K(d) and B(max) for [(3)H]-LY255582 were 0.156 +/- 0.07 nM and 249 +/- 14 fmol/mg protein, respectively. [(3)H]-LY255582 bound to slide mounted sections of mouse brain with high affinity and low non-specific binding. High levels of binding were seen in areas consistent with the known localization of opioid receptors. These areas included the caudate putamen, nucleus accumbens, claustrum, medial habenula, dorsal endopiriform nucleus, basolateral nucleus of the amygdala, hypothalamus, thalamus and ventral tegmental area. We compared the binding distribution of [(3)H]-LY255582 to the opioid receptor antagonist radioligands [(3)H]-naloxone (mu), [(3)H]-naltrindole (delta) and [(3)H]-norBNI (kappa). The overall distribution of [(3)H]-LY255582 binding sites was similar to that of the other ligands. No specific [(3)H]-LY255582 binding was noted in sections of mu-, delta- and kappa-receptor combinatorial knockout mice. Therefore, it is likely that LY255582 produces its effects on feeding and body weight gain through a combination of mu-, delta- and kappa-receptor activity.

Prolonged morphine treatment selectively increases membrane recruitment of delta-opioid receptors in mouse basal ganglia

In recent years, we demonstrated that prolonged (48-h) treatment of rats or mice with selective m-opioid receptor ((mu)OR) agonists induced a translocation of delta-opioid receptors ((delta)ORs) from intracellular compartments to neuronal plasma membranes in the dorsal horn of the spinal cord. It remained to be determined whether this phenomenon also occurred in the brain. To resolve this issue, we analyzed by immunogold histochemistry the subcellular distribution of (delta)ORs in the nucleus accumbens, dorsal neostriatum, and frontal cortex in mice treated or not with morphine (48 h). We observed that prolonged treatment with morphine induced a translocation of (delta)ORs from intracellular to subplasmalemmal and membrane compartments in dendrites from both the nucleus accumbens and the dorsal neostriatum but not from the frontal cortex. We propose that this (mu)OR-(delta)OR interaction might prolong and modulate the sensitivity of neurons to opiates in specific target regions.

Reciprocal opioid-opioid interactions between the ventral tegmental area and nucleus accumbens regions in mediating mu agonist-induced feeding in rats

Feeding elicited by the mu-selective agonist, [D-Ala2, M-Phe4, Gly-ol5]-encephalin administered into the nucleus accumbens is blocked by accumbal pre-treatment with mu, delta1, delta2 and kappa, but not mu1 opioid antagonists. Correspondingly, mu-agonist-induced feeding elicited from the ventral tegmental area is blocked by ventral tegmental area pre-treatment with mu and kappa, but not delta opioid antagonists. A bi-directional opioid-opioid feeding interaction has been firmly established such that mu-agonist-induced feeding elicited from the ventral tegmental area is blocked by accumbal naltrexone, and that accumbal mu-agonist-induced feeding is blocked by naltrexone pre-treatment in the ventral tegmental area. To determine which opioid receptor subtypes mediate the regional bi-directional opioid-opioid feeding interactions between these two sites, the present study examined the dose-dependent ability of either general (naltrexone), mu (beta-funaltrexamine), kappa (nor-binaltorphamine) or delta (naltrindole) opioid antagonists administered into one site to block mu-agonist-induced feeding elicited from the other site. General, mu and kappa, but not delta opioid receptor antagonist pre-treatment in the ventral tegmental area dose-dependently reduced mu-agonist-induced feeding elicited from the nucleus accumbens. General, mu and delta, and to a lesser degree kappa, opioid receptor antagonist pre-treatment in the nucleus accumbens dose-dependently reduced mu-agonist-induced feeding elicited from the ventral tegmental area. Thus, multiple, but different opioid receptor subtypes are involved in mediating opioid-opioid feeding interactions between the nucleus accumbens and ventral tegmental area regions.

A bi-directional mu-opioid-opioid connection between the nucleus of the accumbens shell and the central nucleus of the amygdala in the rat

The central nucleus of the amygdala (CeA) and the nucleus of the accumbens shell (NAc) have been shown to be involved in opioid-mediated feeding behavior. The present study examined whether mu-opioid signalling between the CeA and NAc affected feeding. Male Sprague-Dawley rats were fitted with one cannula placed in the CeA and two cannulae placed in the NAc, which allowed for coadministration of the mu-opioid receptor agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO) in one site and the opioid antagonist naltrexone (NTX) in the other site. Single injection of DAMGO (2.4 nmol) into the CeA and bilateral injections of DAMGO (2.4 nmol) into the NAc stimulated feeding (P<0.05). The DAMGO-induced increase of food intake following injection into the CeA was decreased by bilateral injection of NTX (13.2 and 26.5 nmol) into the NAc at 2- and 4-h postinjections (P<0.05). In the reverse situation, the DAMGO-induced increase of food intake following injection into the NAc was decreased by injection of NTX (13.2 and 26.5 nmol) into the CeA at 1-, 2-, and 4-h postinjections (P<0.05). These results suggest that a bi-directional mu-opioid-opioid signalling pathway exists between the CeA and the NAc, which influences feeding.

Opioid receptor subtype antagonists differentially alter GABA agonist-induced feeding elicited from either the nucleus accumbens shell or ventral tegmental area regions in rats

Food intake is significantly increased by administration of either GABAA (e.g., muscimol) or GABAB (e.g., baclofen) agonists into either the shell region of the nucleus accumbens (NAC) or the ventral tegmental area (VTA); these responses are selectively blocked by pretreatment with corresponding GABAA and GABAB antagonists. Previous studies found that a single dose (5 microg) of the general opioid antagonist, naltrexone reduced feeding elicited by muscimol, but not baclofen in the NAC shell, and reduced feeding elicited by baclofen, but not muscimol in the VTA. The present study compared feeding responses elicited by either muscimol or baclofen in either the VTA and NAC shell following pretreatment with equimolar doses of selective mu (0.4, 4 microg), delta (0.4, 4 microg), or kappa (0.6, 6 microg) opioid receptor subtype antagonists. Muscimol (25 ng) and baclofen (200 microg) each significantly and equi-effectively increased food intake over 4 h following VTA or NAC shell microinjections. Muscimol-induced feeding elicited from the VTA was significantly enhanced by mu or delta antagonists, and was significantly reduced by kappa antagonists. Baclofen-induced feeding elicited from the VTA was significantly reduced by mu or kappa, but not delta antagonists. Muscimol-induced feeding elicited from the NAC was significantly reduced by either mu, kappa or delta antagonists. Baclofen-induced feeding elicited from the NAC was significantly reduced by kappa or delta, but not mu antagonists. These data indicate differential opioid receptor subtype antagonist-induced mediation of GABA receptor subtype agonist-induced feeding elicited from the VTA and NAC shell. This is consistent with previously demonstrated differential GABA receptor subtype antagonist-induced mediation of opioid-induced feeding elicited from these same sites. Thus, functional relationships exist for the elaborate anatomical and physiological interactions between these two neurochemical systems in the VTA and NAC shell.

Intra-amygdalar injection of DAMGO: effects on c-Fos levels in brain sites associated with feeding behavior

It is well known that the mu opioid agonist, Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO), increases food intake in rats when injected into a variety of brain sites including the central nucleus of the amygdala (CeA). Immunohistochemical studies measuring c-Fos immunoreactivity (IR) suggest that the CeA contributes to opioid-related feeding. In the current study, we injected 2 nmol of DAMGO and measured food intake, c-Fos IR levels in various brain sites involved in feeding behavior, and mu opioid receptor internalization. We also studied the effect of CeA-injected DAMGO on LiCl-induced increases in c-Fos IR in the amygdala. As was expected, intra-CeA injection of DAMGO increased food intake of rats over a 4-h period. DAMGO injection into the CeA also resulted in mu opioid receptor internalization in the CeA, indicating activation of mu opioid receptor expressing neurons in this site. Administration of DAMGO into the CeA increased c-Fos IR levels in the shell of the nucleus accumbens (NAcc), but not in 17 other brain sites that were studied. We also found that intra-CeA injection of DAMGO, prior to LiCl injection, decreased c-Fos IR levels in the CeA compared to vehicle-injected rats. Thus, intra-CeA administration of DAMGO may increase feeding, in part, by activating neurons in the shell of the nucleus accumbens and by inhibiting activity of selected neurons in the CeA.

Alterations in food intake by opioid and dopamine signaling pathways between the ventral tegmental area and the shell of the nucleus accumbens

Reward is an important factor motivating food intake in satiated animals. Two sites involved in the reward response are the ventral tegmental area (VTA) and the nucleus accumbens shell region (sNAcc), between which communication is partially regulated by opioids and dopamine (DA). Previous studies have shown that the mu-opioid agonist Tyr-D-Ala-Gly-MePhe-Gly(ol)-enkephalin (DAMGO) dose-dependently enhances food intake in satiated animals when injected into either the VTA or the sNAcc. The enhanced intake elicited by DAMGO injected into the sNAcc was dose-dependently blocked by injection of naltrexone (NTX) bilaterally into the VTA, indicating an opioid-dependent signaling pathway from the sNAcc to the VTA in mediation of food intake. In the present study, we cannulated animals bilaterally in both the VTA and the sNAcc to further study the nature of opioid- and DA-dependent communication between the sites. Food intake elicited by DAMGO (2 or 5 nmol) injected unilaterally into the VTA was dose-dependently diminished by bilateral injection of NTX (2.5, 5, and 25 g/side) or the D1 antagonist SCH 23390 (3, 1, 0.3, 0.15, 0.05, and 0.015 nmol/side) into the sNAcc. When DAMGO (5 nmol) was injected into the sNAcc, the resulting food intake was decreased by doses of SCH 23390 ranging from 0.05 to 100 nmol/side injected bilaterally into the VTA, but not by equimolar doses of Raclopride, a D2 antagonist. These results, combined with previous findings, suggest a signaling pathway between the VTA and the sNAcc in which opioids and DA facilitate feeding in an interdependent manner.

Endogenous opioids and feeding behavior: a 30-year historical perspective

This invited review, based on the receipt of the Third Gayle A. Olson and Richard D. Olson Prize for the publication of the outstanding behavioral article published in the journal Peptides in 2002, examines the 30-year historical perspective of the role of the endogenous opioid system in feeding behavior. The review focuses on the advances that this field has made over the past 30 years as a result of the timely discoveries that were made concerning this important neuropeptide system, and how these discoveries were quickly applied to the analysis of feeding behavior and attendant homeostatic processes. The discoveries of the opioid receptors and opioid peptides, and the establishment of their relevance to feeding behavior were pivotal in studies performed in the 1970s. The 1980s were characterized by the establishment of opioid receptor subtype agonists and antagonists and their relevance to the modulation of feeding behavior as well as by the use of general opioid antagonists in demonstrating the wide array of ingestive situations and paradigms involving the endogenous opioid system. The more recent work from the 1990s to the present, utilizes the advantages created by the cloning of the opioid receptor genes, the development of knockout and knockdown techniques, the systematic utilization of a systems neuroscience approach, and establishment of the reciprocity of how manipulations of opioid peptides and receptors affect feeding behavior with how feeding states affect levels of opioid peptides and receptors. The role of G-protein effector systems in opioid-mediated feeding responses, which was the subject of the prize-winning article, is then reviewed.

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

The pontine parabrachial nucleus (PBN) has been implicated in regulating ingestion and contains opioids that promote feeding elsewhere in the brain. We tested the actions of the selective mu-opioid receptor (mu-OR) agonist [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) in the PBN on feeding in male rats with free access to food. Infusing DAMGO (0.5-4.0 nmol/0.5 microl) into the lateral parabrachial region (LPBN) increased food intake. The hyperphagic effect was anatomically specific to infusions within the LPBN, dose and time related, and selective for ingestion of chow compared with (nonnutritive) kaolin. The nonselective opioid antagonist naloxone (0.1-10.0 nmol intra-PBN) antagonized DAMGO-induced feeding, with complete blockade by 1.0 nmol and no effect on baseline. The highly selective mu-opioid antagonist d-Phe-Cys-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 1.0 nmol) also prevented this action of DAMGO, but the kappa-antagonist nor-binaltorphimine did not. Naloxone and CTAP (10.0 nmol) decreased intake during scheduled feeding. Thus stimulating mu-ORs in the LPBN increases feeding, whereas antagonizing these sites inhibits feeding. Together, our results implicate mu-ORs in the LPBN in the normal regulation of food intake.

Effects of the opioid antagonist naltrexone on feeding induced by DAMGO in the ventral tegmental area and in the nucleus accumbens shell region in the rat

The nucleus accumbens shell region (sNAcc) and the ventral tegmental area (VTA) are two major nodes in the mesolimbic dopamine pathway, which mediates reward for various survival behaviors, including feeding. Opioids increase and maintain food intake when injected peripherally and centrally. Opioids in the VTA cause increased release of dopamine in the sNAcc, and when injected into either site, cause an increase in food intake. Animals in this study were double cannulated in the VTA and in the sNAcc and injected with various combinations of naltrexone (NTX) (2.5, 5, and 25 microg/side) and Tyr-d-Ala-Gly-(Me)Phe-Gly-ol (DAMGO) (0.1, 0.3, 1, 3, and 5 nmol/side) in both sites. DAMGO was found to dose dependently increase intake to an equal extent when injected into either site. DAMGO-induced increases in food intake when injected into the VTA were blocked to control levels with the highest dose of NTX injected bilaterally into the sNAcc; however, increases in intake when injected into the sNAcc were blocked only partially by the highest dose of NTX injected bilaterally into the VTA. These results indicate opioid-opioid communication between the two sites; however, the communication may be quite indirect, requiring other sites and transmitters to elicit a change in behavior.

Restricted daily consumption of a highly palatable food (chocolate EnsureR) alters striatal enkephalin gene expression

Brain opioid peptide systems are known to play an important role in motivation, emotion, attachment behaviour, the response to stress and pain, and the control of food intake. Opioid peptides within the ventral striatum are thought to play a key role in the latter function, regulating the affective response to highly palatable, energy-dense foods such as those containing fat and sugar. It has been shown previously that stimulation of mu opiate receptors within the ventral striatum increases intake of palatable food. In the present study, we examined enkephalin peptide gene expression within the striatum in rats that had been given restricted daily access to an energy-dense, palatable liquid food, chocolate Ensure(R). Rats maintained on an ad libitum diet of rat chow and water were given 3-h access to Ensure(R) daily for two weeks. One day following the end of this period, preproenkephalin gene expression was measured with quantitative in situ hybridization. Compared with control animals, rats that had been exposed to Ensure(R) had significantly reduced enkephalin gene expression in several striatal regions including the ventral striatum (nucleus accumbens), a finding that was confirmed in a different group with Northern blot analysis. Rats fed this regimen of Ensure(R) did not differ in weight from controls. In contrast to chronic Ensure(R), acute ingestion of Ensure(R) did not appear to affect enkephalin peptide gene expression. These results suggest that repeated consumption of a highly rewarding, energy-dense food induces neuroadaptations in cognitive-motivational circuits.

Lack of intersite GABA receptor subtype antagonist effects upon mu opioid receptor agonist-induced feeding elicited from either the ventral tegmental area or nucleus accumbens shell in rats

Pretreatment with the GABA(A) receptor antagonist, bicuculline or the GABA(B) receptor antagonist, saclofen, into the nucleus accumbens (Nacc) shell, respectively, potentiates and reduces feeding elicited by the mu opioid agonist, [D-Ala(2), Nme(4), Gly-ol(5)]-enkephalin (DAMGO), administered into the same site. DAMGO-induced feeding elicited from the ventral tegmental area (VTA) region is significantly reduced by pretreatment with saclofen into the same site indicating local GABA mediation of opioid-induced feeding in each site. Given the neuroanatomical and functional connections between the two sites, the present study evaluated the dose-dependent actions of bicuculline and saclofen pretreatment in one site upon DAMGO-induced feeding elicited from the second site. Pretreatment of either bicuculline (7.5-75 ng) or saclofen (1.5-10 microg) into the Nacc shell failed to alter the time course or magnitude of DAMGO-induced feeding elicited from the VTA region. DAMGO-induced feeding elicited from the Nacc shell was unaffected by VTA region pretreatment with either bicuculline (7.5-75 ng) or saclofen (1.5-5 microg). A higher (10 microg) saclofen dose prevented significant DAMGO-induced feeding after 1 and 4 h. Thus, although GABA receptor subtype antagonists are capable of differentially modulating DAMGO-induced feeding when both drugs are applied locally in either the VTA region or the Nacc shell, it appears that any effects between the VTA region and the Nacc shell in modulating DAMGO-induced feeding do not depend upon a GABAergic synapse in the other site.

Dietary peptides induce satiety via cholecystokinin-A and peripheral opioid receptors in rats

We hypothesized that the digestion of proteins gives rise to peptides that initiate several satiety signals from the gut, and that the signals arising will be dependent on the protein source. The role of peripheral opioid and cholecystokinin (CCK)-A receptors was investigated. Casein, soy protein, and casein and soy hydrolysates were administered to rats by gavage (0.5 g protein/4 mL water). Food intake was measured over 2 h. The opioid receptor antagonist, naloxone methiodide (1.0 mg/kg) given intraperitoneally (i.p.), increased food intake when given at the same time as the hydrolysate preloads, 25 min after the casein preloads and 55 min after the soy protein preloads. The CCK-A receptor antagonist, devazepide (which reverses protein-induced food intake suppression), when given at 0.25 mg/kg, i.p., 60 min before preloads of each of three soy hydrolysates, also blocked suppression of food intake, but the strength and duration of the interaction depended on the preparation. When the two receptor antagonists were both administered with soy or casein preloads, their effects were additive. We conclude that peptides arising from digestion contribute to satiety by independent activation of both opioid and CCK-A receptors.

Evidence that intermittent, excessive sugar intake causes endogenous opioid dependence

OBJECTIVE

The goal was to determine whether withdrawal from sugar can cause signs of opioid dependence. Because palatable food stimulates neural systems that are implicated in drug addiction, it was hypothesized that intermittent, excessive sugar intake might create dependency, as indicated by withdrawal signs.

RESEARCH METHODS AND PROCEDURES

Male rats were food-deprived for 12 hours daily, including 4 hours in the early dark, and then offered highly palatable 25% glucose in addition to chow for the next 12 hours. Withdrawal was induced by naloxone or food deprivation. Withdrawal signs were measured by observation, ultrasonic recordings, elevated plus maze tests, and in vivo microdialysis.

RESULTS

Naloxone (20 mg/kg intraperitoneally) caused somatic signs, such as teeth chattering, forepaw tremor, and head shakes. Food deprivation for 24 hours caused spontaneous withdrawal signs, such as teeth chattering. Naloxone (3 mg/kg subcutaneously) caused reduced time on the exposed arm of an elevated plus maze, where again significant teeth chattering was recorded. The plus maze anxiety effect was replicated with four control groups for comparison. Accumbens microdialysis revealed that naloxone (10 and 20 mg/kg intraperitoneally) decreased extracellular dopamine (DA), while dose-dependently increasing acetylcholine (ACh). The naloxone-induced DA/ACh imbalance was replicated with 10% sucrose and 3 mg/kg naloxone subcutaneously.

DISCUSSION

Repeated, excessive intake of sugar created a state in which an opioid antagonist caused behavioral and neurochemical signs of opioid withdrawal. The indices of anxiety and DA/ACh imbalance were qualitatively similar to withdrawal from morphine or nicotine, suggesting that the rats had become sugar-dependent.

Alterations in food intake elicited by GABA and opioid agonists and antagonists administered into the ventral tegmental area region of rats

Food intake is significantly increased following administration of mu-selective opioid agonists into the ventral tegmental area (VTA) region acting through multiple local opioid receptor subtypes. Since GABA receptor agonists in the VTA region are capable of eliciting feeding, the present study investigated whether feeding elicited by the mu-selective opioid agonist [D-Ala(2), NMe(4), Gly-ol(5)]-enkephalin (DAMGO) in the VTA region was altered by pretreatment into the same site with equimolar doses of either GABA(A) (bicuculline) or GABA(B) (saclofen) antagonists, and further, whether pretreatment with either general opioid or selective GABA receptor antagonists decreased feeding elicited by GABA(A) (muscimol) or GABA(B) (baclofen) agonists in the VTA region. DAMGO-induced feeding in the VTA region was dose-dependently decreased following pretreatment with either GABA(A) or GABA(B) antagonists in the absence of significant alterations in food intake by the antagonists per se. However, the presence of short-lived seizures following bicuculline in the VTA region suggests that this ingestive effect was caused by nonspecific actions. In contrast, GABA(B) receptors are involved in the full expression of mu-opioid agonist-induced feeding in this region since saclofen failed to elicit either seizure activity or a conditioned taste aversion. Pretreatment with naltrexone in the VTA region reduced intake elicited by baclofen, but not muscimol. Finally, baclofen-induced feeding was significantly reduced by saclofen, but not bicuculline, pretreatment in the VTA region. Therefore, possible coregulation between GABA(B) and opioid receptors in the VTA region, as suggested by immunocytochemical evidence, is supported by these behavioral effects upon ingestion.