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

A 40-year analysis of central neuroanatomical and neurochemical circuits mediating homeostatic intake and hedonic intake and preferences in rodents

This perspective review was written in response to the celebration of the 60th anniversary of the journal, Brain Research, and covers the evolving focus of my laboratory's work over 40 years in the neurobiological substrates of ingestive behavior in rodents. Following our initial work examining the effects of systemic and ventricular administration of general and selective opioid receptor agonists and antagonists on food intake under spontaneous, deprivation, glucoprivic and hedonic conditions, my laboratory in close collaboration with Drs. Gavril Pasternak and Ying-Xian Pan utilized an antisense oligodoxynucleotide knock-down technique affecting MOR-1, DOR-1, KOR-1 and ORL-1 genes as well as against G-protein subunits to study receptor mediation of opioid receptor agonist-induced feeding as well as feeding following regulatory challenges. Our laboratory employed intracerebral microinjection techniques to map limbic nucleus accumbens and ventral tegmental area central brain circuits mediating homeostatic and hedonic feeding responses through the use of selective mu, delta1, delta2 and kappa opioid receptor subtype agonists in combination with general and selective opioid, dopamineric, glutamatergic and GABAergic antagonists administered into the same site or the reciprocal site, allowing for the identification of a distributed brain network mediating these ingestive effects. Our laboratory in close collaboration with Dr. Anthony Sclafani then focused on the pharmacological, neuroanatomical and learning mechanisms related to the development of sugar- (sucrose, glucose and fructose) and fat- (corn oil) conditioned flavor preferences (CFP) in rats, and on murine genetic variance in food intake, preferences and the process of appetition.

Dissociable control of μ-opioid-driven hyperphagia vs. food impulsivity across subregions of medial prefrontal, orbitofrontal, and insular cortex

This study explored potentially dissociable functions of mu-opioid receptor (µ-OR) signaling across different cortical territories in the control of anticipatory activity directed toward palatable food, consumption, and impulsive food-seeking behavior in male rats. The µ-OR agonist, DAMGO ([D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin), was infused into infralimbic cortex (ILC), prelimbic cortex (PrL), medial and lateral ventral orbitofrontal cortices (VMO, VLO), and agranular/dysgranular insular (AI/DI) cortex of rats. Intra-ILC DAMGO markedly enhanced contact with a see-through screen behind which sucrose pellets were sequestered; in addition, rats having received intra-ILC and intra-VMO DAMGO exhibited locomotor hyperactivity while the screen was in place. Upon screen removal, intra-ILC and -VMO-treated rats emitted numerous, brief sucrose-intake bouts (yielding increased overall intake) interspersed with significant hyperactivity. In contrast, intra-AI/DI-treated rats consumed large amounts of sucrose in long, uninterrupted bouts with no anticipatory hyperactivity pre-screen removal. Intra-PrL and intra-VLO DAMGO altered neither pre-screen behavior nor sucrose intake. Finally, all rats were tested in a sucrose-reinforced differential reinforcement of low rates (DRL) task, which assesses the ability to advantageously withhold premature responses. DAMGO affected (impaired) DRL performance when infused into ILC only. These site-based dissociations reveal differential control of µ-OR-modulated appetitive/approach vs. consummatory behaviors by ventromedial/orbitofrontal and insular networks, respectively, and suggest a unique role of ILC µ-ORs in modulating inhibitory control.

'Liking' and 'wanting' in eating and food reward: Brain mechanisms and clinical implications

It is becoming clearer how neurobiological mechanisms generate 'liking' and 'wanting' components of food reward. Mesocorticolimbic mechanisms that enhance 'liking' include brain hedonic hotspots, which are specialized subregions that are uniquely able to causally amplify the hedonic impact of palatable tastes. Hedonic hotspots are found in nucleus accumbens medial shell, ventral pallidum, orbitofrontal cortex, insula cortex, and brainstem. In turn, a much larger mesocorticolimbic circuitry generates 'wanting' or incentive motivation to obtain and consume food rewards. Hedonic and motivational circuitry interact together and with hypothalamic homeostatic circuitry, allowing relevant physiological hunger and satiety states to modulate 'liking' and 'wanting' for food rewards. In some conditions such as drug addiction, 'wanting' is known to dramatically detach from 'liking' for the same reward, and this may also occur in over-eating disorders. Via incentive sensitization, 'wanting' selectively becomes higher, especially when triggered by reward cues when encountered in vulnerable states of stress, etc. Emerging evidence suggests that some cases of obesity and binge eating disorders may reflect an incentive-sensitization brain signature of cue hyper-reactivity, causing excessive 'wanting' to eat. Future findings on the neurobiological bases of 'liking' and 'wanting' can continue to improve understanding of both normal food reward and causes of clinical eating disorders.

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.

Endogenous opioids and feeding behavior: A decade of further progress (2004-2014). A Festschrift to Dr. Abba Kastin

Functional elucidation of the endogenous opioid system temporally paralleled the creation and growth of the journal, Peptides, under the leadership of its founding editor, Dr. Abba Kastin. He was prescient in publishing annual and uninterrupted reviews on Endogenous Opiates and Behavior that served as a microcosm for the journal under his stewardship. This author published a 2004 review, "Endogenous opioids and feeding behavior: a thirty-year historical perspective", summarizing research in this field between 1974 and 2003. The present review "closes the circle" by reviewing the last 10 years (2004-2014) of research examining the role of endogenous opioids and feeding behavior. The review summarizes effects upon ingestive behavior following administration of opioid receptor agonists, in opioid receptor knockout animals, following administration of general opioid receptor antagonists, following administration of selective mu, delta, kappa and ORL-1 receptor antagonists, and evaluating opioid peptide and opioid receptor changes in different food intake models.

Effects of secretin on neuronal activity and feeding behavior in central amygdala of rats

Previous studies have shown that secretin and secretin receptors are expressed in central amygdala neurons. By using both in vivo extracellular recording as well as behavioral test, we investigated the direct electrophysiological effects of secretin in the central amygdala and its involvement in feeding behavior. Micro-pressure ejection of secretin increased the spontaneous firing rate by 104.22±26.18% in 13 out of the 27 central amygdala neurons. In other 6 out of the 27 neurons, secretin decreased the firing rate by 68.80±12.10%. Firing patter analysis showed that secretin did not change the firing pattern significantly. Further electrophysiological recordings revealed that secretin decreased the firing rate of glucose-sensitive neurons. In behavioral test, microinjection of secretin into the central amygdala significantly reduced cumulative food intake through cAMP-activated protein kinase activation. Based on the present electrophysiological and behavioral findings, we hypothesized that secretin may suppress food intake by its modulation of spontaneous firing of central amygdala neurons.

Central amygdala opioid transmission is necessary for increased high-fat intake following 24-h food deprivation, but not following intra-accumbens opioid administration

Previous research has demonstrated a dissociation of certain neural mediators that contribute to the increased consumption of a high-fat diet that follows intra-accumbens (Acb) administration of μ-opioid receptor agonists vs. 24-h food deprivation. These two models, both which induce rapid consumption of the diet, have been shown to involve a distributed corticolimbic circuitry, including the amygdala. Specifically, the central amygdala (CeA) has been shown to be involved in high-fat feeding within both opioid and food-deprivation driven models. The present experiments were conducted to examine the more specific role of CeA opioid transmission in mediating high-fat feeding driven by either intra-Acb administration of the μ-opioid agonist d-Ala2-NMe-Phe4-Glyol5-enkephalin (DAMGO) or 24-h home cage food deprivation. Injection of DAMGO into the Acb (0.25 μg/0.5 μl/side) increased consumption of the high-fat diet, but this feeding was unaffected by administration of opioid antagonist, naltrexone (5 μg/0.25 μl/side) administered into the CeA. In contrast, intra-CeA naltrexone administration attenuated high-fat intake driven by 24-h food deprivation, demonstrating a specific role for CeA opioid transmission in high-fat consumption. Intra-CeA naltrexone administration alone had no effect on baseline feeding levels within either feeding model. These findings suggest that CeA opioid transmission mediates consumption of a palatable high-fat diet driven by short-term negative-energy balance (24-h food deprivation), but not intra-Acb opioid receptor activation.

Principles of motivation revealed by the diverse functions of neuropharmacological and neuroanatomical substrates underlying feeding behavior

Circuits that participate in specific subcomponents of feeding (e.g., gustatory perception, peripheral feedback relevant to satiety and energy balance, reward coding, etc.) are found at all levels of the neural axis. Further complexity is conferred by the wide variety of feeding-modulatory neurotransmitters and neuropeptides that act within these circuits. An ongoing challenge has been to refine the understanding of the functional specificity of these neurotransmitters and circuits, and there have been exciting advances in recent years. We focus here on foundational work of Dr. Ann Kelley that identified distinguishable actions of striatal opioid peptide modulation and dopamine transmission in subcomponents of reward processing. We also discuss her work in overlaying these neuropharmacological effects upon anatomical pathways that link the telencephalon (cortex and basal ganglia) with feeding-control circuits in the hypothalamus. Using these seminal contributions as a starting point, we will discuss new findings that expand our understanding of (1) the specific, differentiable motivational processes that are governed by central dopamine and opioid transmission, (2) the manner in which other striatal neuromodulators, specifically acetylcholine, endocannabinoids and adenosine, modulate these motivational processes (including via interactions with opioid systems), and (3) the organization of the cortical-subcortical network that subserves opioid-driven feeding. The findings discussed here strengthen the view that incentive-motivational properties of food are coded by substrates and neural circuits that are distinguishable from those that mediate the acute hedonic experience of food reward. Striatal opioid transmission modulates reward processing by engaging frontotemporal circuits, possibly via a hypothalamic-thalamic axis, that ultimately impinges upon hypothalamic modules dedicated to autonomic function and motor pattern control. We will conclude by discussing implications for understanding disorders of "non-homeostatic" feeding.

What and when to "want"? Amygdala-based focusing of incentive salience upon sugar and sex

RATIONALE

Amygdala-related circuitry helps translate learned Pavlovian associations into appetitive and aversive motivation, especially upon subsequent encounters with cues.

OBJECTIVES

We asked whether μ-opioid stimulation via microinjections of the specific agonist D-Ala(2), N-MePhe(4), Gly-ol)-enkephalin (DAMGO) in central nucleus of amygdala (CeA), or the adjacent basolateral amygdala (BLA) would magnify sucrose or sex "wanting", guided by available cues.

MATERIALS AND METHODS

CeA or BLA DAMGO enhancement of cue-triggered "wanting" was assessed using Pavlovian to instrumental transfer (PIT). Unconditioned food "wanting" was measured via intake, and male sexual "wanting" for an estrous female was measured in a sexual approach test. Sucrose hedonic taste "liking" was measured in a taste reactivity test.

RESULTS

CeA (but not BLA) DAMGO increased the intensity of phasic peaks in instrumental sucrose seeking stimulated by Pavlovian cues over precue levels in PIT, while suppressing seeking at other moments. CeA DAMGO also enhanced food intake, as well as sexual approach and investigation of an estrous female by males. DAMGO "wanting" enhancements were localized to CeA, as indicated by "Fos plume"-based anatomical maps for DAMGO causation of behavioral effects. Despite increasing "wanting", CeA DAMGO decreased the hedonic impact or "liking" for sucrose in a taste reactivity paradigm.

CONCLUSIONS

CeA μ-opioid stimulation specifically enhances incentive salience, which is dynamically guided to food or sex by available cues.

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.

Opioids as facilitators of feeding: can any food be rewarding?

Palatability is one of the most rewarding aspects of consummatory behavior. Opioids, potent facilitators of intake of sweet and fat tastants, are thought to mediate hedonics of feeding. However, the rewarding context of consumption is not limited to palatability, and gratification can be achieved through other means, e.g., eating to satisfy hunger. The current review discusses the role of opioid peptides in food intake regulation by incorporating this expanded concept of feeding reward. We present evidence that, aside from increasing sugar/fat consumption, opioids propel the intake of diets whose gustatory value is low but are nonetheless consumed under circumstances allowing feeding gratification to occur. Opioids enhance reward-driven consumption by acting within the classical reward circuitry and also by signaling reward at sites that regulate other aspects of food intake, such as satiety and aversion. We conclude that, due to the complexity of neural and functional interactions, opioids are capable of enhancing pleasure of eating any food--palatable, bland or even aversive--making any meal into a more rewarding experience, despite possible consequences.

Increased enkephalin in brain of rats prone to overconsuming a fat-rich diet

Recent studies have shown that the opioid enkephalin (ENK), acting in part through the hypothalamic paraventricular nucleus (PVN), can stimulate consumption of a high-fat diet. The objective of the present study was to examine sub-populations of Sprague-Dawley rats naturally prone to overconsuming a high-fat diet and determine whether endogenous ENK, in different brain regions, is altered in these animals and possibly contributes to their behavioral phenotype. An animal model, involving a measure of initial high-fat diet intake during a few days of access that predicts long-term intake, was designed to classify rats at normal weight that are either high-fat consumers (HFC), which ingest 35% more calories of the high-fat than low-fat chow diet, or controls, which consume similar calories of these two diets. Immediately after their initial access to the diet, the HFC compared to control rats exhibited significantly greater expression of ENK mRNA, in the PVN, nucleus accumbens and central nucleus of the amygdala, but not the arcuate nucleus or basolateral amygdala. This site-specific increase in ENK persisted even when the HFC rats were maintained on a chow diet, suggesting that it reflects an inherent characteristic that can be expressed independently of the diet. It was also accompanied by a greater responsiveness of the HFC rats to the stimulatory effect of a PVN-injected, ENK analogue, D-ala2-met-enkephalinamide, compared to saline on consumption of the high-fat diet. Thus, normal-weight rats predicted to overconsume a fat-rich diet exhibit disturbances in endogenous ENK expression and functioning that may contribute to their long-term, behavioral phenotype.

Effect of chronic ethanol on enkephalin in the hypothalamus and extra-hypothalamic areas

BACKGROUND

Ethanol may be consumed for reasons such as reward, anxiety reduction, or caloric content, and the opioid enkephalin (ENK) appears to be involved in many of these functions. Previous studies in Sprague-Dawley rats have demonstrated that ENK in the hypothalamic paraventricular nucleus (PVN) is stimulated by voluntary consumption of ethanol. This suggests that this opioid peptide may be involved in promoting the drinking of ethanol, consistent with our recent findings that PVN injections of ENK analogs stimulate ethanol intake. To broaden our understanding of how this peptide functions throughout the brain to promote ethanol intake, we measured, in rats trained to drink 9% ethanol, the expression of the ENK gene in additional brain areas outside the hypothalamus, namely, the ventral tegmental area (VTA), nucleus accumbens shell (NAcSh) and core (NAcC), medial prefrontal cortex (mPFC), and central nucleus of the amygdala (CeA).

METHODS

In the first experiment, the brains of rats chronically drinking 1 g/kg/d ethanol, 3 g/kg/d ethanol, or water were examined using real-time quantitative polymerase chain reaction (qRT-PCR). In the second experiment, a more detailed, anatomic analysis of changes in gene expression, in rats chronically drinking 3 g/kg/d ethanol compared to water, was performed using radiolabeled in situ hybridization (ISH). The third experiment employed digoxigenin-labeled ISH (DIG) to examine changes in the density of cells expressing ENK and, for comparison, dynorphin (DYN) in rats chronically drinking 3 g/kg/d ethanol versus water.

RESULTS

With qRT-PCR, the rats chronically drinking ethanol plus water compared to water alone showed significantly higher levels of ENK mRNA, not only in the PVN but also in the VTA, NAcSh, NAcC, and mPFC, although not in the CeA. Using radiolabeled ISH, levels of ENK mRNA in rats drinking ethanol were found to be elevated in all areas examined, including the CeA. The experiment using DIG confirmed this effect of ethanol, showing an increase in density of ENK-expressing cells in all areas studied. It additionally revealed a similar change in DYN mRNA in the PVN, mPFC, and CeA, although not in the NAcSh or NAcC.

CONCLUSIONS

While distinguishing the NAc as a site where ENK and DYN respond differentially, these findings lead us to propose that these opioids, in response to voluntary ethanol consumption, are generally elevated in extra-hypothalamic as well as hypothalamic areas, possibly to carry out specific area-related functions that, in turn, drive animals to further consume ethanol. These functions include calorie ingestion in the PVN, reward and motivation in the VTA and NAcSh, response-reinforcement learning in the NAcC, stress reduction in the CeA, and behavioral control in the mPFC.

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.

Hypothalamic injection of non-opioid peptides increases gene expression of the opioid enkephalin in hypothalamic and mesolimbic nuclei: Possible mechanism underlying their behavioral effects

The peptides galanin (GAL) and orexin (OX) share common features with the opioid enkephalin (ENK) in their relationship to ingestive behavior, stimulating consumption of a fat-rich diet and ethanol when injected into the hypothalamus. Since receptors for GAL and OX are dense in areas where ENK-expressing neurons are concentrated, these non-opioid peptides may exert their effects, in part, through the stimulation of endogenous ENK. This study was conducted to determine whether injection of GAL or OX affects the expression of ENK in hypothalamic and mesolimbic nuclei involved in consummatory behavior. Rats were injected with GAL (1 microg), OX-A (1 microg), or saline vehicle just dorsal to the hypothalamic paraventricular nucleus (PVN). They were sacrificed 1h later for analysis of ENK mRNA levels in the PVN, ventral tegmental area (VTA), central nucleus of the amygdala (CeA), and nucleus accumbens (NAc). Both GAL and OX had similar effects, significantly increasing ENK mRNA expression in each of these areas, except for the NAc. This enhanced ENK expression in the PVN, VTA and CeA was demonstrated with real-time quantitative polymerase chain reaction and confirmed in separate groups using radiolabeled and digoxigenin-labeled in situ hybridization. These findings demonstrate that the non-opioid peptides, GAL or OX, which have similar effects on consummatory behavior, are also similar in their effect on endogenous ENK. In light of published findings showing an opioid antagonist to block GAL- and OX-induced feeding, these results provide additional evidence that ENK is involved in mediating the common behavioral effects of these peptides.

FLUOXETINE ALTERS MU OPIOID RECEPTOR EXPRESSION IN OBESE ZUCKER RAT EXTRAHYPOTHALAMIC REGIONS

The aim of this article was to describe the effects of chronic fluoxetine on mu opioid receptor expression in obese Zucker rat extrahypothalamic regions. Male obese Zucker (fa/fa) rats were administered with fluoxetine (10 mg/kg; i.p.) daily for two weeks. Brain regional immunostaining for mu opioid receptor was carried out. An increase in the numbers of neural cells immunostained for mu opioid receptor in caudatus-putamen, dentate gyrus, lateral septum, amygdala, and frontal, parietal, and piriform cortices was observed. Increased mu opioid receptor expression in the central amygdaloid nuclei suggests a decreased opioidergic tone at this level that could be involved in fluoxetine anorectic action.

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.

Amygdalar opioids modulate hypothalamic melanocortin-induced anorexia

We wanted to assess the possibility that opioid activity in the central amygdala (CeA) could modulate the feeding inhibition of melanocortin stimulation of the paraventricular hypothalamus (PVN). The melanocortin system is important in both the acute regulation of satiety and feeding behavior and in the integration of long-term appetite signals. Melanotan II (MTII) is a synthetic MC3R and MC4R agonist which reduces food intake when given intracerebroventricularly (ICV) and into the PVN. Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO), a micro-opioid receptor agonist, increases food intake, while opioid antagonists, like naltrexone (NTX), inhibit food intake after injection into many brain sites involved in appetite regulation, including the CeA. In food-deprived male Sprague-Dawley rats, co-injected intra-PVN MTII partially blocked the orexigenic effect of co-injected intra-CeA DAMGO. Intra-CeA NTX co-injected with intra-PVN MTII reduced food intake significantly more than either alone. NTX administered intra-CeA reduced c-Fos-immunoreactivity (IR) in nucleus accumbens neurons significantly compared to the intra-PVN MTII treated animals, animals co-injected intra-PVN with MTII and intra-CeA with NTX animals, and control animals. Intra-PVN MTII induced c-Fos-IR in significantly more PVN neurons than observed in control animals. Intra-CeA NTX co-injected with intra-PVN MTII induced c-Fos-IR significantly in PVN neurons relative to control and intra-CeA NTX animals. Such data support the significance of opioid action within the CeA as a modulator of the feeding regulation action of melanocortins within the PVN, occurring within the context of a larger appetitive network.

Insulin acts at different CNS sites to decrease acute sucrose intake and sucrose self-administration in rats

Findings from our laboratory and others have demonstrated that the hormone insulin has chronic effects within the CNS to regulate energy homeostasis and to decrease brain reward function. In this study, we compared the acute action of insulin to decrease intake of a palatable food in two different behavioral tasks-progressive ratios sucrose self-administration and micro opioid-stimulated sucrose feeding-when administered into several insulin-receptive sites of the CNS. We tested insulin efficacy within the medial hypothalamic arcuate (ARC) and paraventricular (PVN) nuclei, the nucleus accumbens, and the ventral tegmental area. Administration of insulin at a dose that has no chronic effect on body weight (5 mU) into the ARC significantly suppressed sucrose self-administration (75+/-5% of paired control). However, although the mu opioid DAMGO, [D-Ala2,N-MePhe4,Gly5-ol]-enkephalin acetate salt, stimulated sucrose intake at all four CNS sites, the ventral tegmental area was the only sensitive site for a direct effect of insulin to antagonize acute (60 min) micro opioid-stimulated sucrose feeding: sucrose intake was 53+/-8% of DAMGO-induced feeding, when insulin was coadministered with DAMGO. These findings demonstrate that free feeding of sucrose, and motivated work for sucrose, can be modulated within unique sites of the CNS reward circuitry. Further, they support the interpretation that adiposity signals, such as insulin, can decrease different aspects of ingestion of a palatable food, such as sucrose, in an anatomically specific manner.

Effects of CCK-8 on independent ingestion and central c-Fos-like immunoreactivity in rats on postnatal days 10 and 11

Controls of the independent ingestion of food in the preweanling rat emerge in the second postnatal week. We investigated the effects of CCK-8 (0, 1, 5, or 10 microg/kg IP) on intake and c-Fos-like immunoreactive (CFLI) cells in hindbrain and forebrain on postnatal days 10 and 11. Five micrograms per kilogram decreased intake and increased the number of CFLI cells in four subnuclei of the nucleus tractus solitarius (NTS), in arcuate nucleus (ARC), and in central nucleus of the amygdala (CeA). Ten micrograms per kilogram decreased intake and increased CFLI in three NTS subnuclei as much as 5 microg/kg did, but was more potent than 5 microg/kg in the medial NTS subnucleus. Ten micrograms per kilogram increased CFLI in paraventricular (PVN) and supraoptic (SON) nuclei, but 5 microg/kg did not. Thus, reduction of intake by CCK-8 on days 10 and 11 is associated with increased hindbrain and forebrain CFLI.

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.

Neuropeptide Y administration into the amygdala alters high fat food intake

The orexigenic effects of neuropeptide Y (NPY) are mediated through the hypothalamus, while the anxiolytic effects of NPY appear to be mediated through the amygdala. We hypothesized that intra-amygdalar administration of NPY might alter food preference without changing total food intake. Neuropeptide Y was administered into the central nucleus of the amygdala in both satiated and overnight-fasted rats, and intake and preference for a high fat diet (56%)/low carbohydrate (20%) diet or a low fat (10%)/high carbohydrate (66%) diet were measured. Intra-amygdalar NPY administration in satiated rats did not change total caloric intake, but it did produce a dose-dependent decrease in intake of and preference for high fat diet relative to low fat diet over 24 h. In overnight-fasted rats, intra-amygdalar NPY also decreased the intake and preference for a high fat diet relative to low fat diet over 24 h, without altering total caloric intake. Intra-amygdalar NPY administration did not produce conditioned taste aversions to a novel saccharin solution. These results suggest that amygdalar NPY may have a role in macronutrient selection, without altering total caloric intake.

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.

Cannabinoids, opioids and eating behavior: the molecular face of hedonism?

Obesity represents nowadays one of the most devastating health threats. Published reports even project a decline in life expectancy of US citizens due to the rapidly increasing prevalence of obesity. This alarming increase is intimately linked with recent changes of environment and lifestyle in western countries. In this context, the rewarding or even addictive properties of popular food may represent one of the most serious obstacles to overcome for an effective anti-obesity therapy. Therefore, in addition to molecular networks controlling energy homeostasis, now researchers are starting to define central nervous mechanisms governing hedonic and addictive components of food intake. A recently emerging body of data suggests that the endogenous cannabinoid and opioid systems both represent key circuits responding to the rewarding value of food. This review focuses on the role of these two systems for the homeostatic and hedonic aspects of eating behavior and includes their anatomical and functional interactions. Independent from the degree to which eating can be considered an addiction, cannabinoid and opioid receptor antagonists are promising anti-obesity drugs, since they are targeting both hedonic and homeostatic components of energy balance control.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Acute and chronic administration of clorazepate modifies the cell surface regulation of mu opioid receptors induced by buprenorphine in specific regions of the rat brain

The aim of the present study was to investigate the acute and chronic effects of clorazepate (CRZ) alone or in combination with buprenorphine (BPN) on binding of the selective mu opiate tritiated ligand [3H]-DAMGO in the rat brain. Using 0.1 to 5 nM [3H]-DAMGO concentrations and a beta-imager, Bmax (maximal receptor density) and K(D) (the dissociation constant) were directly determined at different regions of interest (ROI) in the brains of rats treated with BPN and/or CRZ administered either once or over 21 consecutive days. Differences in Bmax and K(D) were related to both treatment and location. Acute BPN induced a down-regulation (62% mean decrease in Bmax observed on the whole brain) of mu opiate receptors. CRZ induced a mean 39% decrease in Bmax associated with substantially decreased affinity, particularly after acute administration (136% mean K(D) increase). Addition of CRZ to BPN [mean Bmax decreases of 34% (acute) and 29% (chronic)] induced significantly less down-regulation than did BPN alone, while altering affinity. These changes were maximal in the amygdaloid nucleus. Significant and persistent decreases in Bmax and affinity were also detected in the hippocampus, hypothalamus and thalamus. In the thalamus, an opposite regulation of Bmax was observed that was maximal with the combination. As the regions where changes were greatest have been specifically implicated in memory and socio-emotional functions and/or vegetative and endocrine adaptations, there is reason to suspect that the addition of CRZ to BPN may have clinical consequences. On the one hand, it may have some impact on drug abuse and misuse behaviors towards treatments including heroin substitute and BZD, and on the other, amplify the BPN effect-particularly hedonic or toxic-mainly after sporadic BPN-BZD abuses. These pharmacodynamic findings may explain, at least in part, the well-established preference of patients for the BPN-benzodiazepine combination and the toxicity with which it is associated.