Evidence of a functional relationship between the nucleus accumbens shell and lateral hypothalamus subserving the control of feeding behavior

Hyperphagia induced by GABAA receptor-mediated inhibition of the nucleus accumbens shell: dependence on intact neural output from the central amygdaloid region

To investigate the role of corticolimbic input in modulating feeding-related nucleus accumbens (Acb) circuitry, researchers temporarily deactivated sites within the basolateral amygdaloid complex (BLA) or central amygdaloid region (CeA) via GABA(A) agonist (muscimol) infusions and measured feeding responses following muscimol infusions into the Acb shell. Hyperphagia elicited by intra-Acb shell muscimol was not altered by coinfusions of intra-BLA muscimol. In contrast, muscimol infusions into the CeA dose-dependently reduced feeding elicited either by intra-Acb shell GABA(A) receptor stimulation or by food deprivation and produced a syndrome of forepaw treading. Intra-CeA tetrodotoxin infusions also blocked intra-Acb shell muscimol-induced hyperphagia. Hence, feeding elicited by intra-Acb shell GABA(A) receptor stimulation requires intact neural output from the CeA but not the BLA.

Endocannabinoids in the regulation of appetite and body weight

The discovery of cannabinoid receptors, together with the development of selective cannabinoid receptor antagonists, has encouraged a resurgence of cannabinoid pharmacology. With the identification of endogenous agonists, such as anandamide, scientists have sought to uncover the biological role of endocannabinoid systems; initially guided by the long-established actions of cannabis and exogenous cannabinoids such as delta9-tetrahydrocannabinol (THC). In particular, considerable research has examined endocannabinoid involvement in appetite, eating behaviour and body weight regulation. It is now confirmed that endocannabinoids, acting at brain CB1 cannabinoid receptors, stimulate appetite and ingestive behaviours, partly through interactions with more established orexigenic and anorexigenic signals. Key structures such as the nucleus accumbens and hypothalamic nuclei are sensitive sites for the hyperphagic actions of these substances, and endocannabinoid activity in these regions varies in relation to nutritional status and feeding expression. Behavioural studies indicate that endocannabinoids increase eating motivation by enhancing the incentive salience and hedonic evaluation of ingesta. Moreover, there is strong evidence of an endocannabinoid role in energy metabolism and fuel storage. Recent developments point to potential clinical benefits of cannabinoid receptor antagonists in the management of obesity, and of agonists in the treatment of other disorders of eating and body weight regulation.

NMDA receptors mediate feeding elicited by neuropeptide Y in the lateral and perifornical hypothalamus

Neuropeptide Y (NPY) and N-methyl-d-aspartate (NMDA) receptors in the lateral (LH) and perifornical hypothalamus (PFH) are believed to be involved in the stimulation of feeding behavior. To investigate the possibility that neurons with these receptors interact to stimulate eating, the NMDA receptor antagonists d-(-)-2-amino-5-phosphonopentanoic acid (D-AP5) or 7-chlorokynurenic acid (7-CK) were injected into the LH or PFH of satiated rats 5 min prior to NPY in the same site and subsequent food intake was measured 1, 2, and 4 h postinjection. The injection of NPY (78 pmol/0.3 microl aCSF) in the PFH produced an average food intake of 9.7 g in 4 h, compared to the intake of 1.3 g after the artificial cerebrospinal fluid (aCSF) vehicle. D-AP5 (1, 10, or 20 nmol/0.3 microl aCSF) pretreatment suppressed NPY-induced eating, with the 20 nmol dose of D-AP5 producing up to an 80% suppression of elicited food intake down to 1.9 g in 4 h. Similar effects were produced with the LH as the injection site. Illustrating the specificity of the NMDA receptor antagonist's suppression of NPY-elicited feeding, D-AP5 suppressed NMDA-elicited feeding but did not affect the eating response induced by kainic acid. Consistent with the effects of D-AP5, the NMDA receptor antagonist 7-CK (40 nmol/0.3 microl dimethyl sulfoxide, DMSO) suppressed feeding elicited by NPY in the LH by 78%. Collectively, the findings suggest that the feeding elicited by NPY is dependent upon the activation of the NMDA receptors in the LH and PFH.

Hormonal regulation of food intake

Our knowledge of the physiological systems controlling energy homeostasis has increased dramatically over the last decade. The roles of peripheral signals from adipose tissue, pancreas, and the gastrointestinal tract reflecting short- and long-term nutritional status are now being described. Such signals influence central circuits in the hypothalamus, brain stem, and limbic system to modulate neuropeptide release and hence food intake and energy expenditure. This review discusses the peripheral hormones and central neuronal pathways that contribute to control of appetite.

Orexin A in the nucleus accumbens stimulates feeding and locomotor activity

Due to the nature of processing within the accumbens shell (AccSh) and the presence of orexin receptors and varicosities within the AccSh, we hypothesized that orexin A may partly regulate feeding behavior and locomotor activity via signaling in this site. To test this hypothesis, male Sprague-Dawley rats were implanted with guide cannulae directed to the medial portion of the AccSh. Orexin A (0, 100, 500, and 1000 pmol, in 0.5 microl artificial cerebrospinal fluid) was infused into the AccSh and feeding behavior and locomotor activity were monitored. The effect of pretreatment with an orexin 1 receptor antagonist (SB334867A) on orexin A-induced feeding and locomotor activity was assessed. Orexin A augmented feeding in the 0-1 h and 1-2 h post-infusion interval (P = 0.0058 and P = 0.025, respectively) and stimulated locomotor activity in the 30-60 min, 60-90 min, and 90-120 min post-infusion intervals (P <or= 0.0001, P = 0.0056 and P = 0.046, respectively). Orexin A-induced feeding was significantly attenuated by preadministration of SB334867A in the 0-1 h post-infusion time interval (P = 0.03). Orexin A-induced locomotor activity was not affected by SB334867A. These data support the hypothesis that the AccSh is a site of orexin A modulation of feeding behavior and locomotor activity.

Activation of feeding-related neural circuitry after unilateral injections of muscimol into the nucleus accumbens shell

Chemical inhibition of neurons in the nucleus accumbens shell (AcbSh) elicits intense, behaviorally specific, feeding in satiated rats. We have demonstrated previously that this treatment activates a number of brain regions, most significantly the lateral hypothalamus (LH). This activation could be elicited through a direct neural connection with the AcbSh or secondarily through changes in autonomic activity, stress, or circulating levels of orexigenic or satiety factors. In the present study, we used the immunohistochemical localization of Fos protein to map neuronal activation after unilateral muscimol injections into the AcbSh to determine whether AcbSh-mediated Fos expression remains lateralized in the circuit and whether secondary systemic changes in the rat can be excluded as primary factors in the activation of downstream component nuclei. Rats receiving only saline injections exhibited very little Fos immunoreactivity. In contrast, unilateral injections of muscimol into the AcbSh consistently increased Fos expression in several brain regions. Three distinct patterns of expression were observed. Fos synthesis in the LH was increased only on the side of the brain ipsilateral to the muscimol injection. Fos expression remained primarily ipsilateral to the injection site in the septohypothalamic, paraventricular hypothalamic (PVN), paratenial thalamic, and lateral habenular nuclei, and medial substantia nigra, but was increased bilaterally in the piriform cortex, supraoptic nucleus, central nucleus of the amygdala, and nucleus of the solitary tract. Smaller numbers of Fos-immunoreactive cells were seen unilaterally in the bed nucleus of the stria terminalis, medial ventral pallidum, arcuate nucleus, and ventral tegmental area and bilaterally in the supraoptic and tuberomammillary nuclei. The labeling in the LH, PVN, and other unilaterally labeled structures provides evidence that these brain regions are components of an AcbSh-mediated neural circuit and suggests that they may be involved in the expression of AcbSh-mediated feeding behavior.

A cocaine-and-amphetamine-regulated-transcript peptide projection from the lateral hypothalamus to the ventral tegmental area

Cocaine-and-amphetamine-regulated-transcript peptides play a role in the modulation of feeding and psychomotor stimulant-like behaviors. The ventral tegmental area and the lateral hypothalamus are likely structures where cocaine-and-amphetamine-regulated-transcript peptides mediate both of these functions. Although lateral hypothalamus inputs to the ventral tegmental area have long been known, the chemical nature of this pathway remains poorly understood. To address this issue, we tested the possibility that cocaine-and-amphetamine-regulated-transcript peptide-containing neurons in the lateral hypothalamus project to the ventral tegmental area using the retrograde transport of cholera toxin subunit B combined with cocaine-and-amphetamine-regulated-transcript peptide immunostaining. The largest density of retrogradely-labeled neurons in the hypothalamus after cholera toxin subunit B injection in the ventral tegmental area was found, ipsi- and contralaterally, in the lateral hypothalamus/perifornical area, although substantial numbers of retrogradely-labeled cells were also found in the medial preoptic area, lateral preoptic area, paraventricular nucleus, dorsomedial hypothalamus and ventromedial hypothalamus. More than 80% of the retrogradely-labeled cocaine-and-amphetamine-regulated-transcript peptide-immunoreactive neurons in the hypothalamus were found in the lateral hypothalamus/perifornical area both ipsilateral and contralateral to the injection sites. Although retrogradely-labeled neurons were seen in the amygdala, locus coeruleus, and raphe nucleus, none of them displayed cocaine-and-amphetamine-regulated-transcript peptide immunoreactivity. Therefore, the hypothalamic projection to the ventral tegmental area provides a substrate whereby cocaine-and-amphetamine-regulated-transcript peptides could mediate the rewarding aspects of feeding and psychomotor stimulant-like behaviors. These findings, combined with the fact that the lateral hypothalamus receives strong inputs from the shell of the nucleus accumbens and ventral pallidum, suggest that these structures are part of integrative functional loops that control reward and appetitive behaviors.

Dopaminergic regulation of orexin neurons

Orexin/hypocretin neurons in the lateral hypothalamus and adjacent perifornical area (LH/PFA) innervate midbrain dopamine (DA) neurons that project to corticolimbic sites and subserve psychostimulant-induced locomotor activity. However, it is not known whether dopamine neurons in turn regulate the activity of orexin cells. We examined the ability of dopamine agonists to activate orexin neurons in the rat, as reflected by induction of Fos. The mixed dopamine agonist apomorphine increased Fos expression in orexin cells, with a greater effect on orexin neurons located medial to the fornix. Both the selective D1-like agonist, A-77636, and the D2-like agonist, quinpirole, also induced Fos in orexin cells, suggesting that stimulation of either receptor subtype is sufficient to activate orexin neurons. Consistent with this finding, combined SCH 23390 (D1 antagonist)-haloperidol (D2 antagonist) pretreatment blocked apomorphine-induced activation of medial as well as lateral orexin neurons; in contrast, pretreatment with either the D1-like or D2-like antagonists alone did not attenuate apomorphine-induced activation of medial orexin cells. In situ hybridization histochemistry revealed that LH/PFA cells rarely express mRNAs encoding dopamine receptors, suggesting that orexin cells are transsynaptically activated by apomorphine. We therefore lesioned the nucleus accumbens, a site known to regulate orexin cells, but this treatment did not alter apomorphine-elicited activation of medial or lateral orexin neurons. Interestingly, apomorphine failed to activate orexin cells in isoflurane-anaesthetized animals. These data suggest that apomorphine-induced arousal but not accumbens-mediated hyperactivity is required for dopamine to transsynaptically activate orexin neurons.

Entrainment by a palatable meal induces food-anticipatory activity and c-Fos expression in reward-related areas of the brain

Rats maintained under restricted feeding schedules (RFS) develop food-anticipatory activity and entrainment of physiological parameters. Food entrainment is independent of the suprachiasmatic nucleus and depends on food-entrainable oscillators (FEO). Restricted feeding schedules lead animals toward a catabolic state and to increase their food driven motivation, suggesting that in this process metabolic- and reward-related mechanisms are implicated. This study explored if motivation driven by a palatable meal is sufficient to produce food-entrainment. To address this question, we evaluated whether daily fixed access to a highly palatable meal entrained (PME) locomotor activity, serum glucose and free fatty acids concentrations in rats maintained without food deprivation. The entrained response of PME rats was compared with rats entrained to RFS. In a second experiment, we used c-Fos-IR to identify structures in the central nervous system involved with PME. Rats showed anticipatory activity to a daily palatable meal, with a lower intensity than rats entrained to RFS. Anticipatory activity persisted at least for four cycles after interrupting palatable meal, suggesting that this persistence depends on an endogenous oscillator. Glucose and free fatty acids were not entrained in PME rats. c-Fos expression in limbic system nuclei was in phase with PME time, but not in the hypothalamus. Results suggest 1) that food deprivation, i.e. a catabolic state is not necessary for the expression of anticipatory activity; 2) that an increase in the motivational state due to taste and/or nutritional contents of palatable meal is sufficient to entrain behavior; and 3) that structures in the limbic system are involved in this entrainment process. The present study indicates that metabolic and motivational mechanisms are involved in food entrainment, and suggests that the FEO may be a multi-oscillatory system distributed over different regulatory systems in the brain.

Differential expression of dopamine D2 and D4 receptor and tyrosine hydroxylase mRNA in mice prone, or resistant, to chronic high-fat diet-induced obesity

The present study examined brain dopamine D2 and D4 receptor and tyrosine hydroxylase (TH) mRNA expression in chronic high-fat diet-induced obese (cDIO) and obese-resistant (cDR) mice. Twenty-eight mice were fed a high-fat diet (HF: 40% of calories from fat) for 6 weeks and then classified as cDIO (n = 8) or cDR (n = 8) mice according to the highest and lowest body weight gainers, respectively. Seven mice were fed a low-fat diet (LF: 10% of calories from fat) and used as controls. After 20 weeks of feeding, visceral fat per gram of initial body weight was significantly higher in the cDIO group (ratio: 0.25, 0.09, and 0.04; P < 0.01 cDIO vs. cDR and LF, respectively). Using quantitative in situ hybridization techniques, the levels of D2 and D4 receptor and tyrosine hydroxylase (TH) mRNAs were measured in multiple brain sections. The cDIO mice had a significantly higher level of D2 receptor mRNA expression in the core of the nucleus accumbens (AcbC, +16%) and ventral parts of caudate putamen (CPu, 21% and 24%) compared to the cDR and LF mice. The levels of D2 receptor mRNA expression in the AcbC and ventromedial part of the CPu were positively related to the final body weight. This study is the first to systematically examine the D4 mRNA expression in the mouse brain using in situ hybridization method. D4 receptor mRNA expression in the ventromedial hypothalamic nucleus (VMH) and the ventral part of the lateral septal nucleus were also significantly higher in the cDIO mice compared to the cDR and LF mice (+31% and +60%; P < 0.05). TH mRNA expression was significantly higher in the ventral tegmental area (+17%, P </= 0.05) and locus caeruleus (+15%, P </= 0.05) of the cDIO mice compared to cDR mice. In conclusion, this study has demonstrated differentially regulated levels of D2 and D4 receptor and TH mRNA expression in specific brain regions of cDIO and cDR mice. It provides evidence that D4 receptors may play an important role influencing satiety via the mesohypothalamic pathway while the D2 receptor may regulate reward and motor centers via mesolimbic and nigrostriatal pathways. These findings contribute to the understanding of the role of these receptors in susceptibility, or resistance, to diet-induced obesity.

Orexin A-induced feeding is augmented by caloric challenge

Orexin neurons are stimulated by conditions that are glucoprivic, suggesting that orexin signaling may be increased during nutritional duress. We have previously shown that injection of orexin A (OxA) into the rostral lateral hypothalamic area (rLHa) robustly and dose-dependently increases feeding behavior. Thus we hypothesized that exogenous administration of orexin A would induce a greater feeding response after acute food deprivation or perceived caloric duress achieved through 2-deoxyglucose (2DG) administration. To test our hypothesis, male Sprague-Dawley rats implanted with internal guide cannulas directed to the rLHa were exposed to varying degrees of food deprivation (0, 3, 12, 24 h) and 2DG (200 mg/kg) before intra-rLHa OxA (500 pmol) infusion. We also performed a dose-response study using graded doses of OxA (0, 31.25, 125, and 500 pmol) in fed and 24-h fasted rats. OxA administration in conjunction with the highest level of prior food deprivation (24 h) resulted in the greatest feeding response (above baseline means; 0 h deprivation: 1.9 +/- 0.6; 24 h deprivation: 4.4 +/- 0.8; P = 0.0034) and showed a dose-dependent enhancement of feeding. Additionally, 2DG administration before OxA administration resulted in a significantly higher feeding response (above baseline means: 2DG = 1.8 +/- 0.5; OxA = 1.8 +/- 0.4; 2DG + OxA = 5.1 +/- 0.6; P < 0.0001). These data support the hypothesis that orexin signaling may be important in modulating the feeding network under times of nutritional duress.

NPY mediates the feeding elicited by muscimol injections into the nucleus accumbens shell

Injections of muscimol into the nucleus accumbens shell (AcbSh) induce large increases in food intake in satiated rats and also activate neurons in a number of feeding-related brain regions, including NPY-containing neurons in the arcuate hypothalamic nucleus and cells in the paraventricular hypothalamic nucleus. This suggests that the NPY system may participate in the expression of AcbSh-mediated feeding behavior. Therefore, we examined the effects of intraventricular administration of the Y1 receptor antagonist 1229U91 or the Y5 receptor antagonist L-152,804 on AcbSh-mediated food intake. Intra-AcbSh muscimol elicited a large increase in food intake which was potently suppressed by blocking either central Y1 or Y5 receptors. Our results suggest that the AcbSh influences food intake, in part, through the release of NPY.

Origins of lateral hypothalamic afferents associated with N-methyl-d-aspartic acid-elicited eating studied using reverse microdialysis of NMDA and Fluorogold

Afferent projections to the tuberal lateral hypothalamus (tLH), where excitatory amino acid application is most effective in eliciting feeding, and to the anterior, posterior and medial regions of the hypothalamus were studied using reverse microdialysis of N-methyl-D-aspartic acid (NMDA) and Fluorogold (FG). NMDA at 660 microM delivered for 10 min was effective in stimulating food intake only when administered into the tLH, causing a mean intake of 9.3 g compared to less than 1 g in any other site. Subsequent administration of FG through the dialysis probe retrogradely in labeled neurons in brain structures associated with the feeding response including the frontal cortex, amygdala, nucleus accumbens (NA), preoptic areas, substantia nigra, ventral tegmental area (VTA), parabrachial nucleus, and the nucleus of the solitary tract (NST). Labeling after anterior and posterior LH infusion of FG was similar to that seen after tLH delivery with some apparent differences, whereas FG administration into the medial hypothalamus produced a distinctly different pattern of labeling compared to the other groups. Some of the observed labeling appeared to be almost exclusively associated with the tLH where NMDA elicits feeding. In particular, amygdala, preoptic area and shell of the accumbens labeling was noticeably denser in tLH eaters than in all other groups. These findings are consistent with the role of LH glutamate and NMDA receptors in the regulation of food intake and identify afferents to the region which possibly mediate endogenous LH glutamate's effects on feeding.

Differential role of the accumbens Shell and Core subterritories in food-entrained rhythms of rats

Restricted feeding schedules (RFS) entrain behavioral and physiological rhythms even in suprachiasmatic nucleus ablated animals, suggesting the existence of a food-entrained oscillator. The nucleus accumbens is an important structure for the expression of motivational behaviors and because its anatomical subterritories, Shell (Acc-Sh) and Core (Acc-Co) establish connections with different functional systems, they may participate in a differential way in food-entrainment. A first experiment, explored the role of Acc-Sh and Acc-Co in food-entrainment using the immunohistochemical detection of the protein c-Fos as a transcriptional activation marker. Experiment 2 tested the differential effect of Acc-Sh and Acc-Co, NMDA excitotoxic lesions. Lesioned rats were entrained to RFS and locomotor activity and free fatty acids (FFA) concentrations were evaluated. Results data show that in the Acc-Sh there is an increase of c-Fos immunoreactivity in food-entrained rats principally during feeding, whereas c-Fos expression in the Acc-Co region was increased during feeding and also anticipating mealtime. FFA were entrained in both lesioned groups, but the basal level was lower in Core-lesion rats. All rats exhibited food anticipatory activity (FAA). However, FAA was increased in Shell-lesioned animals and was almost abolished in the Core-lesion rats. These data indicate that the accumbens nucleus is involved with behavioral and metabolic food-entrainment, and that there is a differential role between both subregions.

Cocaine- and amphetamine-regulated transcript in the nucleus accumbens participates in the regulation of feeding behavior in rats

The present studies aimed to determine whether cocaine- and amphetamine-regulated transcript (CART) peptide in the nucleus of accumbens shell (AcbSh) is implicated in the regulation of food intake. Bilateral intranuclear injections of CART peptide (55-102, 1 microg/microl/side) into the AcbSh decreased food intake with no change in locomotion activity and attenuated the orexigenic effect of the GABA(A) agonist muscimol (100 ng/microl/side) in male Sprague-Dawley rats. Decreased food intake after bilateral intranuclear injections of CART was more sustained in freely fed rats than in food-deprived rats, suggesting fuel availability is an important factor in modulating the function of CART in the regulation of feeding. Our anatomical findings indicate that in addition to the perifornical region and the arcuate nucleus, some neurons within the AcbSh also project within the AcbSh. Moreover, many of these efferent cells contain CART immunoreactivity, including those which reside within the AcbSh, suggesting that accumbal CART circuitry is involved in the central function of the nucleus accumbens. Furthermore, fasting suppressed CART mRNA levels in the AcbSh, paraventricular nucleus of the hypothalamus, arcuate nucleus, and the perifornical region, indicating that the Acb is sensitive to fuel availability to an extent similar to those regions in the hypothalamus. Our findings are the first to demonstrate that CART mRNA in the AcbSh is sensitive to metabolic challenges and that injection of CART peptide into the AcbSh has an inhibitory effect on food intake.

NMDA receptor function within the anterior piriform cortex and lateral hypothalamus in rats on the control of intake of amino acid-deficient diets

Animals decrease intake of an indispensable amino acid (AA)-deficient or devoid diet, due in part to decreased dietary limiting AA (DLAA) concentrations within the anterior piriform cortex (APC), and to a recognition process that occurs as early as 20 min following exposure to AA deficiencies. Glutamate levels within the APC change in response to AA deficiencies. The APC projects to the lateral hypothalamus (LH), where glutamate acts to stimulate food intake. We hypothesize that the APC, through glutamatergic projections to the LH, inhibits the LH, which signals to reject the AA-deficient or devoid diet, and trigger aversions to the AA-deficient or devoid diet via an ascending pathway to the APC. We examined the effects of (1) bilateral APC and LH blockade of glutamate's NMDA receptors with the antagonist, D-AP5, (2) APC blockade of AMPA receptors with the antagonist, NBQX, to block glutamate transmission from the APC, and (3) direct injection of the agonist, NMDA, into the LH on intake of the AA-deficient, devoid, or corrected diet. Administration of D-AP5 into the APC increased intake of AA-deficient diet by 6 h, but D-AP5 in the LH decreased AA-devoid diet preferentially over AA corrected intake sooner. NBQX in the APC increased AA-deficient diet intake, also at 6 h. NMDA injection into the LH-stimulated intake of the AA corrected diet by 3 h, but did not affect AA-devoid diet intake. Thus, the glutamate receptors in the APC and LH are involved in the feeding responses to AA-deficient diet, albeit with regional differences. We suggest that glutamate mediates the anorectic responses to AA-deficient diets through recognition of AA-devoid diet with the glutamatergic output cells of the APC sending glutamate-based signals for changes in food intake within the LH and through learned avoidance of AA-deficient diet within the APC, as indicated through the more immediate and prolonged periods of activation within the LH and APC, respectively.

High-fat diet induced adiposity in mice with targeted disruption of the dopamine-3 receptor gene

Dopamine (DA) signaling has been implicated in the control of energy balance and ingestive behavior. In the present study, we sought to characterize body weight, body fat and food intake regulation in a mouse with a targeted disruption of the dopamine-3 receptor gene (Drd3). In the first set of experiments male and female wild-type and mutant (Drd3-/-) mice were given access to two different diets varying in fat content. Body weight, food intake, carcass analysis and plasma levels of leptin and insulin were measured. Male Drd3-/- mice have increased body weight and body fat when given access to high fat (HF) diet but not standard rodent chow. The female Drd3-/- mice did not demonstrate increased body weight when given access to either diet, but did have increased body fat on both diets. Plasma leptin and insulin levels reflected the increased adiposity demonstrated in each genotype and gender. These findings suggest the D3-R signaling is involved in the regulation of body weight and body fat when mice are given access to diets differing in palatability and fat content.

Ventral striatal control of appetitive motivation: role in ingestive behavior and reward-related learning

The nucleus accumbens is a brain region that participates in the control of behaviors related to natural reinforcers, such as ingestion, sexual behavior, incentive and instrumental learning, and that also plays a role in addictive processes. This paper comprises a review of work from our laboratory that focuses on two main research areas: (i). the role of the nucleus accumbens in food motivation, and (ii). its putative functions in cellular plasticity underlying appetitive learning. First, work within a number of different behavioral paradigms has shown that accumbens neurochemical systems play specific and dissociable roles in different aspects of food seeking and food intake, and part of this function depends on integration with the lateral hypothalamus and amygdala. We propose that the nucleus accumbens integrates information related to cognitive, sensory, and emotional processing with hypothalamic mechanisms mediating energy balance. This system as a whole enables complex hierarchical control of adaptive ingestive behavior. Regarding the second research area, our studies examining acquisition of lever-pressing for food in rats have shown that activation of glutamate N-methyl-d-aspartate (NMDA) receptors, within broadly distributed but interconnected regions (nucleus accumbens core, posterior striatum, prefrontal cortex, basolateral and central amygdala), is critical for such learning to occur. This receptor stimulation triggers intracellular cascades that involve protein phosphorylation and new protein synthesis. It is hypothesized that activity in this distributed network (including D1 receptor activity) computes coincident events and thus enhances the probability that temporally related actions and events (e.g. lever pressing and delivery of reward) become associated. Such basic mechanisms of plasticity within this reinforcement learning network also appear to be profoundly affected in addiction.

Activation of a subpopulation of orexin/hypocretin-containing hypothalamic neurons by GABAA receptor-mediated inhibition of the nucleus accumbens shell, but not by exposure to a novel environment

Gamma-amino butyric acid (GABA)A receptor stimulation in the nucleus accumbens shell produces intense hyperphagia in rats and increases Fos expression in the lateral hypothalamus. To explore the involvement of hypothalamic orexin/hypocretin- or melanin concentrating hormone-immunoreactive neurons in this effect, the GABAA agonist, muscimol (0, 50 ng), was infused directly into the nucleus accumbens shell of rats; 90 min later, their brains were collected and subsequently processed for immunohistochemistry. A group exposed to a novel environment was included to evaluate the specificity of Fos expression changes with regard to general arousal. Alternating sections through the hypothalamus were double-stained for orexin/hypocretin-Fos or melanin concentrating hormone-Fos combinations. Intra-accumbens shell muscimol treatment significantly increased the percentage of orexin/hypocretin-containing neurons expressing Fos in the lateral, but not medial, portion of the perifornical/lateral hypothalamic area. Regardless of treatment condition, greater percentages of orexin/hypocretin-containing neurons in the medial portion of the hypothalamus expressed Fos relative to cells located more laterally. None of the manipulations increased Fos expression in melanin concentrating hormone-immunoreactive neurons. Muscimol treatment also markedly increased Fos expression in the arcuate nucleus, which connects reciprocally to the lateral/perifornical hypothalamic area. Thus, orexin/hypocretin-containing neurons in lateral sectors of the hypothalamus, along with cells in the arcuate nucleus, display phasic increases in Fos expression after an orexigenic pharmacological manipulation of the nucleus accumbens shell, but to a lesser degree after the heightened arousal associated with exposure to a novel environment.

Low dietary protein is associated with an increase in food intake and a decrease in the in vitro release of radiolabeled glutamate and GABA from the lateral hypothalamus

Moderately low-protein diets lead to a rapid increase in food intake and body fat. The increase in feeding is associated with a decrease in the concentration of serum urea nitrogen, suggesting that the low-protein-induced increase in food intake may be related to the decreased metabolism of nitrogen from amino acids. We hypothesized that low dietary protein would be associated with a decrease in the synaptic release of two nitrogen-containing neurotransmitters, GABA and glutamate, whose nitrogen can be derived from amino acids. In this study, we examined the effects of a low-protein diet (10% casein) in Sprague-Dawley rats on the in vitro release of 3H-GABA and 14C-glutamate from the lateral and medial hypothalamus. The low-protein diet increased food intake by about 25% after one day. After four days, the in vitro release of radiolabeled GABA and glutamate was assessed. The calcium-dependent, potassium-stimulated release of radiolabeled GABA and glutamate from the lateral hypothalamus was decreased in rats fed the low-protein diet. The magnitude of neurotransmitter release from the lateral hypothalamus inversely correlated with food intake. No dietary differences in the release of neurotransmitters from the medial hypothalamus were observed. These results support the contention that alterations in nitrogen metabolism are associated with low-protein-induced feeding.

Increases in food intake or food-seeking behavior induced by GABAergic, opioid, or dopaminergic stimulation of the nucleus accumbens: is it hunger?

RATIONALE

Previous work has shown that stimulation of GABAergic, opioid, or dopaminergic systems within the nucleus accumbens modulates food intake and food-seeking behavior. However, it is not known whether such stimulation mimics a motivational state of food deprivation that commonly enables animals to learn a new operant response to obtain food.

OBJECTIVES

In order to address this question, acquisition of lever pressing for food in hungry animals was compared with acquisition in non-food-deprived rats subjected to various nucleus accumbens drug treatments.

METHODS

All animals were given the opportunity to learn an instrumental response (a lever press) to obtain a food pellet. Prior to training, ad lib-fed rats were infused with the gamma-aminobutyric acid (GABA)A agonist muscimol (100 ng/0.5 microl per side) or the mu-opioid receptor agonist D-Ala2, N-me-Phe4, Gly-ol5-enkephalin (DAMGO, 0.25 microg/0.5 microl per side), or saline into the nucleus accumbens shell (AcbSh). The indirect dopamine agonist amphetamine (10 microg/0.5 microl per side) was infused into the AcbSh or nucleus accumbens core (AcbC) of ad lib-fed rats. An additional group was food deprived and infused with saline in the AcbSh. Chow and sugar pellet intake responses after drug treatments were also evaluated in free-feeding tests.

RESULTS

Muscimol, DAMGO, or amphetamine did not facilitate acquisition of lever pressing for food, despite clearly increasing food intake in free-feeding tests. In contrast, food-deprived animals rapidly learned the task.

CONCLUSIONS

These findings suggest that pharmacological stimulation of any of these neurochemical systems in isolation is insufficient to enable acquisition of a food-reinforced operant task. Thus, these selective processes, while likely involved in control of food intake and food-seeking behavior, appear unable to recapitulate the conditions necessary to mimic the state of negative energy balance.

Modulation of feeding behaviour by blocking purinergic receptors in the rat nucleus accumbens: a combined microdialysis, electroencephalographic and behavioural study

The nonspecific P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), the nonspecific P1 receptor antagonist 8-(p-sulphophenyl)-theophylline (8-SPT) and the combination of both were applied by retrograde microdialysis into the nucleus accumbens (NAc) before and during feeding of 18-h food-deprived rats. In addition to the registration of behavioural parameters, such as the amount and duration of food intake, the feeding-induced changes in dopamine (DA) concentration and the concomitant changes of neuronal activity in the NAc and the ventral tegmental area (VTA) were simultaneously determined. The perfusion with PPADS (20 microm) diminished the amount of food intake and the duration of feeding. Furthermore, the P2 receptor antagonist blocked the feeding-induced DA release and prevented the feeding-elicited changes of the electroencephalography (EEG) power distribution which was characterised by an increase in the power of the 8.0-13.0-Hz frequency band in the NAc and the VTA. The effects of PPADS could be completely prevented by the concomitantly perfused adenosine receptor antagonist 8-SPT (100 microm). When given alone, 8-SPT increased the amount of food ingested, the duration of feeding and the EEG power of the higher frequency range, particularly between 19.0 and 30.0 Hz, in both the NAc and the VTA. The feeding-elicited DA release was supplemented to the enhanced DA level caused by the perfusion with 8-SPT in an additive manner. The P2 and P1 receptor antagonists interact antagonistically in the modulation of feeding behaviour and the feeding-induced changes of EEG activity suggesting that both endogenous extracellular ATP and adenosine are involved in the regulation of the feeding-associated mesolimbic neuronal activity in a functionally antagonistic manner.

Endocannabinoid Receptor Antagonists

Obesity has been described as a global epidemic. Its increasing prevalence is matched by growing costs, not only to the health of the individual, but also to the medical services required to treat a range of obesity-related diseases. In most instances, obesity is a product of progressively less energetic lifestyles and the over-consumption of readily available, palatable, and highly caloric foods. Past decades have seen massive investment in the search for effective anti-obesity therapies, so far with limited success. An important part of the process of developing new pharmacologic treatments for obesity lies in improving our understanding of the psychologic and physiologic processes that govern appetite and bodyweight regulation. Recent discoveries concerning the endogenous cannabinoids are beginning to give greater insight into these processes. Current research indicates that endocannabinoids may be key to the appetitive and consummatory aspects of eating motivation, possibly mediating the craving for and enjoyment of the most desired, most fattening foods. Additionally, endocannabinoids appear to modulate central and peripheral processes associated with fat and glucose metabolism. Selective cannabinoid receptor antagonists have been shown to suppress the motivation to eat, and preferentially reduce the consumption of palatable, energy-dense foods. Additionally, these agents act to reduce adiposity through metabolic mechanisms that are independent of changes in food intake. Given the current state of evidence, we conclude that the endocannabinoids represent an exciting target for new anti-obesity therapies.

Oral sucrose stimulation increases accumbens dopamine in the rat

Although taste can influence meal size and body weight, the neural substrate for these effects remains obscure. Dopamine, particularly in the nucleus accumbens, has been implicated in both natural and nonnatural rewards. To isolate the orosensory effects of taste from possible postingestive consequences, we investigated the quantitative relationship between sham feeding of sucrose and extracellular dopamine in the nucleus accumbens with microdialysis in rats. Sucrose intake linearly increased as a function of concentration (0.03 M, 18.07 +/- 2.41 ml; 0.1 M, 30.92 +/- 2.60 ml; 0.3 M, 43.28 +/- 2.88 ml). Sham feeding also stimulated accumbens dopamine overflow as a function of sucrose solution concentration (0.03 M, 120.76 +/- 2.6%; 0.1 M, 140.28 +/- 7.8%; 0.3 M, 146.27 +/- 5.05%). A second experiment used the same protocol but clamped the amount of sucrose ingested and revealed a similar, concentration-dependent dopamine activation in the nucleus accumbens. This is the first demonstration of a quantitative relationship between the concentration-dependent rewarding effect of orosensory stimulation by sucrose during eating and the overflow of dopamine in the nucleus accumbens. This finding provides new and strong support for accumbens dopamine in the rewarding effect of sucrose.

Dual roles in feeding for AMPA/kainate receptors: receptor activation or inactivation within distinct hypothalamic regions elicits feeding behavior

We have previously shown that hypothalamic injections of glutamate, or agonists of its ionotropic receptors (iGluRs), elicit intense feeding responses in satiated rats [Brain Res. 613 (1993) 88, Brain Res. 630 (1993) 41]. While attempting to clarify the role of the AMPA and kainate (KA) receptor subtypes in glutamatergic feeding systems, we discovered that lateral hypothalamic (LH) injection of high doses of the competitive AMPA/KA receptor antagonist, NBQX (10 and 30 nmol), elicited a pronounced feeding response. We questioned whether this effect was due to inactivation of AMPA or possibly KA receptors. To determine whether other AMPA/KA antagonists can also elicit feeding, we tested whether injection of CNQX, another AMPA/KA receptor antagonist, also stimulates eating and whether these feeding stimulatory effects were due to antagonists' actions in the LH or in other hypothalamic sites. Here we report that NBQX and CNQX elicit feeding in a dose dependent manner and are most effective when injected into the perifornical hypothalamus (PFH), or into the paraventricular nucleus (PVN) and, to a lesser extent, into the LH of satiated rats. In contrast, AMPA was most effective in stimulating feeding when injected into the LH, confirming previous reports. These data suggest that either activation or inactivation of AMPA/KA receptors in distinct but overlapping hypothalamic sites may be sufficient to induce feeding behavior, indicating a broadened role for glutamate in hypothalamic feeding mechanisms.

Effects of an adenosine A2A receptor blockade in the nucleus accumbens on locomotion, feeding, and prepulse inhibition in rats

The nucleus accumbens (NAc) subserves behaviors governed by natural rewards, i.e., feeding or exploration, and has been implicated in control of prepulse inhibition (PPI), a measure of sensorimotor gating. The present study sought to determine whether a tonic stimulation of adenosine A(2A) receptors in the rat NAc is involved in control of spontaneous locomotor activity, feeding behavior, and PPI. To this end, bilateral microinfusions of a prodrug (MSX-3) (3 microg and 5 microg in 1 microl per side) of the selective A(2A) receptor antagonist MSX-2 or vehicle (1 microl per side) were administered into the NAc. Results show that blockade of intra-NAc adenosine A(2A) receptors by a high (5 microg), but not by a low (3 microg), dose of MSX-3 increased locomotor activity in an open field, reduced food intake, and delayed intake onset in food-deprived rats examined in a test cage with standard laboratory chow. Furthermore, PPI was significantly disrupted after intra-NAc infusion of 5 microg, but not 3 microg, MSX-3. These findings suggest that locomotor activity as well as intact PPI and feeding behavior rely on tonic activation of intra-NAc A(2A) receptors. The data add further support to the view that adenosine is a tonically active modulator of striatal function through actions on A(2A) receptors.

Projections from the rhomboid nucleus of the bed nuclei of the stria terminalis: implications for cerebral hemisphere regulation of ingestive behaviors

The basic organization of an exceptionally complex pattern of axonal projections from one distinct cell group of the bed nuclei of the stria terminalis, the rhomboid nucleus (BSTrh), was analyzed with the PHAL anterograde tract-tracing method in rats. Brain areas that receive a strong to moderate input from the BSTrh fall into nine general categories: central autonomic control network (central amygdalar nucleus, descending hypothalamic paraventricular nucleus, parasubthalamic nucleus and dorsal lateral hypothalamic area, ventrolateral periaqueductal gray, lateral parabrachial nucleus and caudal nucleus of the solitary tract, dorsal motor nucleus of the vagus nerve, and salivatory nuclei), gustatory system (rostral nucleus of the solitary tract and medial parabrachial nucleus), neuroendocrine system (periventricular and paraventricular hypothalamic nuclei, hypothalamic visceromotor pattern generator network), orofaciopharyngeal motor control (rostral tip of the dorsal nucleus ambiguus, parvicellular reticular nucleus, retrorubral area, and lateral mesencephalic reticular nucleus), respiratory control (lateral nucleus of the solitary tract), locomotor or exploratory behavior control and reward prediction (nucleus accumbens, substantia innominata, and ventral tegmental area), ingestive behavior control (descending paraventricular nucleus and dorsal lateral hypothalamic area), thalamocortical feedback loops (medial-midline-intralaminar thalamus), and behavioral state control (dorsal raphé and locus coeruleus). Its pattern of axonal projections and its position in the basal telencephalon suggest that the BSTrh is part of a striatopallidal differentiation involved in modulating the expression of ingestive behaviors, although it may have other functions as well.

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.

Lateral hypothalamic neuropeptides in reward and drug addiction

The hypothalamus has been long considered important in feeding and other motivated behaviors. The identification of neuropeptides expressed in the hypothalamus has initiated efforts to better elucidate the underlying molecular mechanisms involved. The neuropeptides orexin and melanin-concentrating hormone (MCH) are expressed in the lateral hypothalamus (LH) and have been implicated in regulation of feeding behavior. Neurons expressing these neuropeptides have extensive projections to regions of the brain important for behavioral responses to drugs of abuse, raising the possibility that the pathways may also be important in addiction. Regulation of LH intracellular signaling pathways in response to drugs of abuse supports a role for the LH neuropeptides in addiction.

Effects of intracerebroventricular ethanol on ingestive behavior and induction of c-Fos immunoreactivity in selected brain regions

The early changes in the central nervous system (CNS) following drinking of ethanol (ETOH) are poorly understood. It is known that chronic intracerebroventricular (ICV) administration of ethanol to rats induces preference for imbibed alcohol solutions. These results suggest that ICV ethanol could alter taste preference. In the present study, we tested whether ETOH[ICV] could induce a conditioned taste preference (CTP) or aversion (CTA) and alter c-Fos immunoreactivity (c-Fos-IR) in brain regions associated with feeding, aversion, and/or reward. Acute ETOH[ICV], as tested in the ETOH-naïve rat, did not induce CTA nor affect the amount of water imbibed by treated rats. The effects of ETOH[ICV] on intake and preference were determined using a novel palatable (i.e. sweet) noncaloric 0.1% saccharin solution. A single dose of ETOH[ICV] in the ETOH-nai;ve animal induced a CTP for saccharin. ETOH[ICV] significantly increased c-Fos-IR in a number of brain sites associated with feeding and reward including the bed nucleus of the stria terminalis, lateral dorsal area (BSTLD); nucleus accumbens, shell area (AcbSh); hypothalamic paraventricular nucleus (PVN); and lateral septum, ventral area (LSV). Thus, ETOH induced a CTP, not CTA, via central mechanisms; it increased c-Fos-IR in specific sites associated with feeding and reward.

Peptides that regulate food intake: appetite-inducing accumbens manipulation activates hypothalamic orexin neurons and inhibits POMC neurons

Corticolimbic circuits involving the prefrontal cortex, amygdala, and ventral striatum determine the reward value of food and might play a role in environmentally induced obesity. Chemical manipulation of the nucleus accumbens shell (AcbSh) has been shown to elicit robust feeding and Fos expression in the hypothalamus and other brain areas of satiated rats. To determine the neurochemical phenotype of hypothalamic neurons receiving input from the AcbSh, we carried out c-Fos/peptide double-labeling immunohistochemistry in various hypothalamic areas known to contain feeding peptides, from rats that exhibited a significant feeding response after AcbSh microinjection of the GABA(A) agonist muscimol. In the perifornical area, a significantly higher percentage of orexin neurons expressed Fos after muscimol compared with saline injection. In contrast, Fos expression was not induced in melanin-concentrating hormone and cocaine-amphetamine-related transcript (CART) neurons. In the arcuate nucleus, Fos activation was significantly lower in neurons coexpressing CART and proopiomelanocortin, and there was a tendency for higher Fos expression in neuropeptide Y neurons. In the paraventricular nucleus, no significant activation of oxytocin and CART neurons was found. Thus AcbSh manipulation may elicit food intake through coordinated stimulation of hypothalamic neurons expressing orexigenic peptides and suppression of neurons expressing anorexigenic peptides. However, activation of many neurons not expressing these peptides suggests that additional peptides/transmitters in the lateral hypothalamus and accumbens projections to other brain areas might also be involved.

Nucleus accumbens opioid, GABaergic, and dopaminergic modulation of palatable food motivation: contrasting effects revealed by a progressive ratio study in the rat

The current studies were designed to evaluate whether incentive motivation for palatable food is altered after manipulations of opioid, GABAergic, and dopaminergic transmission within the nucleus accumbens. A progressive ratio schedule was used to measure lever-pressing for sugar pellets after microinfusion of drugs into the nucleus accumbens in non-food-deprived rats. The mu opioid agonist D-Ala2, NMe-Phe4, Glyo15-enkephalin and the indirect dopamine agonist amphetamine induced a marked increase in break point and correct lever-presses; the GABA(A) agonist muscimol did not affect breakpoint or lever-presses. The data suggest that opioid, dopaminergic, and GABAergic systems within the accumbens differentially modulate food-seeking behavior through mechanisms related to hedonic evaluation of food, incentive salience, and control of motor feeding circuits, respectively.

Extracellular glutamate increases in the lateral hypothalamus during meal initiation, and GABA peaks during satiation: microdialysis measurements every 30 s

Glutamate injected into the lateral hypothalamus can initiate eating, and gamma-aminobutyric acid (GABA) can stop it. This leads to the hypothesis that glutamate inputs are active at the beginning of a meal, and GABA is released at the end. To test this theory, the authors used microdialysis to sample glutamate and GABA simultaneously before, during, and after a meal. Food-deprived rats ate a meal of chow. Glutamate increased during the first third of the meal, then decreased to below baseline while the rats were still eating. GABA also increased at the start of the meal but continued rising and peaked during the last third of the meal. Glutamate may drive a hypothalamic system for eating, and GABA may oppose it.

Inhibition of rostral basal forebrain neurons promotes wakefulness and induces FOS in orexin neurons

The present study examined whether the activities of the rostral basal forebrain neurons alter the activities of the orexin (also known as hypocretin) neurons in the tuberal part of the hypothalamus in rats. We performed microdialysis perfusion of the ventromedial portion of the rostral basal forebrain with the GABAA receptor agonist muscimol to inhibit focally the neuronal activities in the rostral basal forebrain. Then, we monitored sleep/wake behaviour and investigated the pattern of activities of orexin neurons by examining the expression of FOS as an indicator of cellular activation. Bilateral perfusion with muscimol (5, 15, and 50 micro m) produced a dose-dependent decrease in the amount of sleep. This perfusion with muscimol at 50 micro m produced FOS-like immunoreactivity in 37% of the orexin neurons located in the tuberal part of the hypothalamus, whereas the FOS-like immunoreactivity was sparse in orexin neurons of the sleeping control rats (P = 0.001 by Mann-Whitney U-test). Unilateral perfusion with muscimol (50 micro m) also suppressed sleep. In this case, FOS-like immunoreactivity was seen in 40% of the orexin neurons on the side ipsilateral to the perfusion site but only in 10% of orexin neurons on the contralateral side (P = 0.018 by Wilcoxon signed rank test). These functional data suggested that a sleep-generating element in the ventromedial part of the rostral basal forebrain provides an inhibitory influence on the activities of the orexin neurons in the tuberal part of the hypothalamus.

Adiposity signals and food reward: expanding the CNS roles of insulin and leptin

The hormones insulin and leptin have been proposed to act in the central nervous system (CNS) as adiposity signals as part of a theoretical negative feedback loop that senses the caloric stores of an animal and orchestrates adjustments in energy balance and food intake. Much research has provided support for both the existence of such a feedback loop and the specific roles that insulin and leptin may play. Most studies have focused on hypothalamic sites, which historically are implicated in the regulation of energy balance, and on the brain stem, which is a target for neural and humoral signals relating to ingestive acts. More recent lines of research, including studies from our lab, suggest that in addition to these CNS sites, brain reward circuitry may be a target for insulin and leptin action. These studies are reviewed together here with the goals of providing a historical overview of the findings that have substantiated the originally hypothesized negative feedback model and of opening up new lines of investigation that will build on these findings and allow further refinement of the model of adiposity signal/CNS feedback loop. The understanding of how motivational circuitry and its endocrine or neuroendocrine modulation contributes to normal energy balance regulation should expand possibilities for future therapeutic approaches to obesity and may lead to important insights into mental illnesses such as substance abuse or eating disorders.

Distinct pattern of c-fos mRNA expression after systemic and intra-accumbens amphetamine and MK-801

Pharmacological manipulation of both dopamine and glutamate systems affects motor responses in laboratory animals. The two systems, however, seem to act in opposite ways, since direct or indirect activation of dopamine receptors induces similar stimulatory effects to those seen following blockade of N-methyl-D-aspartate receptors. In the present study we compared the pattern of c-fos activation induced by systemic and intra-accumbens administration of the non-competitive N-methyl-D-aspartate antagonist MK-801 and the indirect dopamine agonist amphetamine. Systemic MK-801 induced c-fos mRNA expression in the motor cortex and preferentially in the motor thalamus, i.e. ventrolateral nucleus. Systemic amphetamine, on the other hand, enhanced c-fos mRNA expression in the shell of the accumbens and in limbic thalamic nuclei such as the anteroventral and anterodorsal nuclei. The main effect observed after intra-accumbens administrations of either drug was enhanced c-fos expression in the thalamus, somewhat similar to what seen following systemic administration. In fact also in this case there was a preferential activation of the limbic thalamus by amphetamine and the motor thalamus by MK-801. The present results confirm that different neural substrates underlie behavioral effects induced by systemic administrations of N-methyl-D-aspartate receptor antagonists and dopamine agonists. Further they suggest that intra-accumbens manipulation of the two neural systems could affect different efferent pathways from this structure activating different thalamic targets.

Endocannabinoid signaling via cannabinoid receptor 1 is involved in ethanol preference and its age-dependent decline in mice

Cannabinoids and ethanol can activate the same reward pathways, which could suggest endocannabinoid involvement in the rewarding effects of ethanol. The high ethanol preference of young (6-10 weeks) C57BL6J mice is reduced by the cannabinoid receptor 1 (CB1) antagonist SR141716A to levels observed in their CB1 knockout littermates or in old (26-48 weeks) wild-type mice, in both of which ethanol preference is unaffected by SR141716A. Similarly, SR141716A inhibits food intake in food-restricted young, but not old, wild-type mice. There are no age-dependent differences in the tissue levels of the endocannabinoids anandamide and 2-arachidonoylglycerol or the density of CB1 in the hypothalamus, limbic forebrain, amygdala, and cerebellum. CB1-stimulated guanosine 5'-[gamma-thio]triphosphate (GTP[gammaS]) binding is selectively reduced in the limbic forebrain of old compared with young wild-type mice. There is no age-dependent difference in G(i) or G(o) subunit protein expression in the limbic forebrain, and the selective reduction in GTP[gammaS] labeling in tissue from old mice is maintained in a receptorG protein reconstitution assay by using functional bovine brain G protein. These findings suggest that endocannabinoids acting at CB1 contribute to ethanol preference, and decreased coupling of CB1 to G proteins in the limbic forebrain by mechanisms other than altered receptor or G protein levels may be involved in the age-dependent decline in the appetite for both ethanol and food.

Effects of selective dopamine D1 or D2 receptor blockade within nucleus accumbens subregions on ingestive behavior and associated motor activity

Two anatomically and neurochemically distinguishable regions of the nucleus accumbens (Acb), the core and the shell, have been shown to differentially regulate feeding behavior. Nevertheless, despite the well-known role of Acb dopamine in the modulation of motivated behaviors, there have been no studies directly comparing the effects of acute dopamine receptor blockade in the Acb core versus the Acb shell on feeding. In this study, D1- or D2-selective dopamine receptor antagonists were infused bilaterally into the Acb core or shell of hungry rats, whereupon feeding, drinking, and spontaneous motor activity were monitored. Both the D1 antagonist SCH 23390 (0, 1, and 2 microg/0.5 microl) and the D2 antagonist raclopride (0, 1, and 2 microg/0.5 microl) markedly suppressed ambulation and rearing when infused into either the Acb core or shell. Total food intake and latency to begin feeding were unaffected by either drug in either site. SCH 23390 in the Acb shell, and raclopride in the Acb core or shell, significantly decreased the total number of feeding bouts. In the Acb core, raclopride produced a small but statistically significant increase in overall feeding duration. Dopamine receptor blockade in either site tended to increase mean feeding bout duration. Measures of drinking behavior were generally unaffected. Hence, dopamine receptor blockade in either the Acb core or shell of hungry rats suppressed spontaneous motor activity and shifted the structure of feeding towards longer bout durations, but did not alter the total amount of food consumed. In the Acb shell, the effects of D1 receptor blockade tended to be of greater magnitude than the effects of D2 receptor blockade, although major differences between core and shell effects were not observed. These results are discussed with regard to current theories of dopaminergic control of feeding behavior, and with reference to the functional heterogeneity of Acb subregions.

Nicotinic receptor-mediated effects on appetite and food intake

It is well known, although not well understood, that smoking and eating just do not go together. Smoking is associated with decreased food intake and lower body weight. Nicotine, administered either by smoking or by smokeless routes, is considered the major appetite-suppressing component of tobacco. Perhaps the most renowned example of nicotine's influence on appetite and feeding behavior is the significant weight gain associated with smoking cessation. This article presents an overview of the literature at, or near, the interface of nicotinic receptors and appetite regulation. We first consider some of the possible sites of nicotine's action along the complex network of neural and non-neural regulators of feeding. We then present the hypothesis that the lateral hypothalamus is a particularly important locus of the anorectic effects of nicotine. Finally, we discuss the potential role of endogenous cholinergic systems in motivational feeding, focusing on cholinergic pathways in the lateral hypothalamus.

The need to feed: homeostatic and hedonic control of eating

Feeding provides substrate for energy metabolism, which is vital to the survival of every living animal and therefore is subject to intense regulation by brain homeostatic and hedonic systems. Over the last decade, our understanding of the circuits and molecules involved in this process has changed dramatically, in large part due to the availability of animal models with genetic lesions. In this review, we examine the role played in homeostatic regulation of feeding by systemic mediators such as leptin and ghrelin, which act on brain systems utilizing neuropeptide Y, agouti-related peptide, melanocortins, orexins, and melanin concentrating hormone, among other mediators. We also examine the mechanisms for taste and reward systems that provide food with its intrinsically reinforcing properties and explore the links between the homeostatic and hedonic systems that ensure intake of adequate nutrition.

Anatomical substrates of orexin-dopamine interactions: lateral hypothalamic projections to the ventral tegmental area

Dopaminergic projections to the forebrain arising from the mesencephalic ventral tegmentum modulate information processing in cortical and limbic sites. The lateral hypothalamus is crucial for the coordination of behavioral responses to interoceptive cues. The presence of a hypothalamic input to the ventral tegmental area has been known for some time, but the organization of this pathway has received little attention. Among the neuropeptides found in the hypothalamus are the orexins, which are selectively expressed in the lateral hypothalamus and adjacent perifornical area and are critically involved in homeostatic regulatory processes, including arousal and feeding. We examined the anatomical relationships between orexin and dopamine neurons in rats, with particular attention to characterizing the lateral hypothalamic projection to midbrain dopamine neurons. Iontophoretic deposits of the retrograde tracer FluoroGold into the ventral tegmental area revealed a large number of retrogradely-labeled cells that formed a band extending from the medial perifornical area arching dorsally over the fornix and then ventrolaterally into the lateral hypothalamus; approximately 20% of these cells expressed orexin A-like immunoreactivity. Moreover, axons that were anterogradely labeled from the lateral hypothalamus were seen throughout the ventral tegmental area, and were often in close proximity to the dendrites and somata of dopamine neurons. Dopamine and orexin fibers were found to codistribute in the medial prefrontal cortex; orexin fibers were present in lower density in the medial shell of the nucleus accumbens, and the central and posterior basolateral nuclei of the amygdala. We conclude that the lateral hypothalamic/perifornical projection represents an anatomical substrate by which interoceptive-related signals may influence forebrain dopamine function.

Opioid modulation of taste hedonics within the ventral striatum

There is a long-standing interest in the role of endogenous opioid peptides in feeding behavior and, in particular, in the modulation of food reward and palatability. Since drugs such as heroin, morphine, alcohol, and cannabinoids, interact with this system, there may be important common neural substrates between food and drug reward with regard to the brain's opioid systems. In this paper, we review the proposed functional role of opioid neurotransmission and mu opiate receptors within the nucleus accumbens and surrounding ventral striatum. Opioid compounds, particularly those selective for the mu receptor, induce a potent increase in food intake, sucrose, salt, saccharin, and ethanol intake. We have explored this phenomenon with regard to macronutrient selection, regional specificity, role of output structures, Fos mapping, analysis of motivational state, and enkephalin gene expression. We hypothesize that opioid-mediated mechanisms within ventral striatal medium spiny neurons mediate the affective or hedonic response to food ('liking' or food 'pleasure'). A further refinement of this hypothesis is that activation of ventral striatal opioids specifically encodes positive affect induced by tasty and/or calorically dense foods (such as sugar and fat), and promotes behaviors associated with this enhanced palatability. It is proposed that this brain mechanism was beneficial in evolutionary development for ensuring the consumption of relatively scarce, high-energy food sources. However, in modern times, with unlimited supplies of high-calorie food, it has contributed to the present epidemic of obesity.

Multiple neural systems controlling food intake and body weight

Discovery of the leptin receptor and its downstream peptidergic pathways has reconfirmed the crucial role of the hypothalamus in the regulation of food intake and energy balance. Strategically located in the midst of the mammalian neuraxis, the hypothalamus receives at least three distinct types of relevant information via direct or indirect neural connections as well as hormone receptors and substrate sensors bestowed on hypothalamic neurons. First, the medial and to a lesser extent the lateral hypothalamus receive a rich mix of information pertaining to the internal state of relative energy repletion/depletion. Second, specific hypothalamic nuclei receive information about the behavioral state, such as diurnal clock, physical activity-level, reproductive cycle, developmental stage, as well as imminent (e.g. fight and flight) and chronic (e.g. infection) stressors, that can potentially impact on short-term availability of fuels and long-term energy balance. Third, the hypothalamus, particularly its lateral aspects, receives information from areas in the forebrain involved in the acquisition, storage, and retrieval of sensory representations of the external food space and internal food experience, as well as from the executive forebrain involved in behavior selection and initiation. In addition, rich intrahypothalamic connections facilitate further distribution of incoming information to various hypothalamic nuclei. On the other hand, the hypothalamus has widespread neural projections to the same cortical areas it receives inputs, and many hypothalamic neurons are one synapse away from most endocrine systems and from both sympathetic and parasympathetic effector organs involved in the flux, storage, mobilization, and utilization of fuels. It is argued that processing within cortico-limbic areas and communication with hypothalamic areas are particularly important in human food intake control that is more and more guided by cognitive rather than metabolic aspects in the obesigenic environment of affluent societies. A distributed neural network for the control of food intake and energy balance consisting of a central processor and several parallel processing loops is hypothesized. Detailed neurochemical, anatomical, and functional analysis of reciprocal connections of the numerous peptidergic neuron populations in the hypothalamus with extrahypothalamic brain areas will be necessary to better understand what hypothalamus, forebrain, and brainstem tell each other and who is in charge under specific conditions of internal and external nutrient availability.

Endocannabinoid levels in rat limbic forebrain and hypothalamus in relation to fasting, feeding and satiation: stimulation of eating by 2-arachidonoyl glycerol

Endocannabinoids are implicated in appetite and body weight regulation. In rodents, anandamide stimulates eating by actions at central CB1 receptors, and hypothalamic endocannabinoids may be under the negative control of leptin. However, changes to brain endocannabinoid levels in direct relation to feeding or changing nutritional status have not been investigated. We measured anandamide and 2-arachidonoyl glycerol (2-AG) levels in feeding-associated brain regions of rats, during fasting, feeding of a palatable food, or after satiation. Endocannabinoid levels were compared to those in rats fed ad libitum, at a point in their daily cycle when motivation to eat was absent. Fasting increased levels of anandamide and 2-AG in the limbic forebrain and, to a lesser extent, of 2-AG in the hypothalamus. By contrast, hypothalamic 2-AG declined as animals ate. No changes were detected in satiated rats. Endocannabinoid levels in the cerebellum, a control region not directly involved in the control of food intake, were unaffected by any manipulation. As 2-AG was most sensitive to variation during feeding, and to leptin regulation in a previous study, we examined the behavioural effects of 2-AG when injected into the nucleus accumbens shell, a limbic forebrain area strongly linked to eating motivation. 2-AG potently, and dose-dependently, stimulated feeding. This effect was attenuated by the CB1 receptor antagonist SR141716. These findings provide the first direct evidence of altered brain levels of endocannabinoids, and of 2-AG in particular, during fasting and feeding. The nature of these effects supports a role for endocannabinoids in the control of appetitive motivation.

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.

Regional differences in feeding and other behaviors elicited by N-methyl-D-aspartic acid in the rodent hypothalamus: a reverse microdialysis mapping study

Regional differences in the feeding stimulatory actions of hypothalamically delivered N-methyl-D-aspartate (NMDA) were investigated. NMDA (660 microM intraprobe) delivered by reverse microdialysis into the tuberal lateral hypothalamus (tLH) reliably elicited feeding in satiated rats. The average food intake was 8.6 g in 50 min, and during the infusion rats spent 26% of the time eating, compared to less than 1% before NMDA treatment. In contrast, NMDA did not affect feeding when reverse dialyzed into the anterior LH (aLH), posterior LH (pLH) or the medial hypothalamus (MH). NMDA had no apparent behavioral effect in the aLH; in contrast, it significantly decreased the time spent resting/sleeping when infused into each of the other three areas tested. Additionally, in the medial hypothalamus, NMDA infusions increased time spent grooming; while in the pLH only alertness was significantly increased. These data underscore the functional and anatomical heterogeneity of the hypothalamus, and implicate glutamate and NMDA receptors in different portions of the hypothalamus in the control of eating, grooming and arousal.

Reverse microdialysis of N-methyl-D-aspartic acid into the lateral hypothalamus of rats: effects on feeding and other behaviors

The effects of reverse microdialysis of N-methyl-D-aspartic acid (NMDA) into the lateral hypothalamus (LH) on feeding and other behaviors were examined. Consistent with similar studies utilizing central microinjections, NMDA reverse microdialysed into the rat LH rapidly elicited a strong concentration-dependent stimulation of feeding. The minimum perfusate concentration of NMDA needed to elicit feeding with reverse microdialysis was 660 microM, a concentration 1/50 of that needed with pressure injections. Further, eating responses could be consistently elicited in sequential tests separated by 2-4 h in the same subject, and the magnitude of the eating in the first and second tests was highly correlated (r=0.87). Behavioral analysis revealed that the main response to NMDA consisted of eating without the concomitant hyperactivity produced by central microinjections of this agonist. The other behaviors exhibited during NMDA administration were those that normally occur during spontaneous feeding in rats. Also, rats precisely compensated for the increased food intake elicited by NMDA by reducing spontaneous feeding during the subsequent nocturnal phase, so as to maintain normal daily intakes. In contrast, N-methyl-L-aspartate (NMLA) reverse microdialysed in to the LH (660 microM) did not elicit feeding nor affect any other behaviors we examined. These data support a role for LH glutamate and NMDA receptors in the control of feeding.

Excitatory amino acid receptor subtype agonists induce feeding in the nucleus accumbens shell in rats: opioid antagonist actions and interactions with mu-opioid agonists

Administration of mu-opioid receptor subtype agonists into the nucleus accumbens shell elicits feeding which is dependent upon the normal function of mu-, delta- and kappa-opioid receptors, D(1) dopamine receptors and GABA(B) receptors in the nucleus accumbens shell for its full expression. Whereas the AMPA antagonist, DNQX administered into the nucleus accumbens shell elicits a transient, though intense feeding response, feeding is elicited by excitatory amino acid agonists administered into the lateral hypothalamus. The present study examined whether excitatory amino acid agonists elicited feeding following administration into the nucleus accumbens shell of rats, whether such feeding responses were altered by opioid antagonist pretreatment, and whether such feeding responses interacted with feeding elicited by mu-opioid agonists. Both AMPA (0.25-0.5 microg) and NMDA (1 microg) in the nucleus accumbens shell significantly and dose-dependently increased food intake over 4 h. Both feeding responses were blocked by naltrexone pretreatment in the nucleus accumbens shell. The mu-opioid agonist, [D-Ala(2),NMe-Phe(4),Gly-ol(5)]-enkephalin in the nucleus accumbens shell significantly increased food intake which was significantly enhanced by AMPA cotreatment. This enhanced feeding response was in turn blocked by pretreatment with either general or mu-selective opioid antagonists. In contrast, cotreatment of NMDA and the mu-opioid agonist in the nucleus accumbens shell elicited feeding which was significantly less than that elicited by either treatment alone. These data indicate the presence of important interactions between excitatory amino acid receptors and mu-opioid receptors in the nucleus accumbens shell in mediating feeding responses in nondeprived, ad libitum-fed rats.