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

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

At the limbic-motor interface: disconnection of basolateral amygdala from nucleus accumbens core and shell reveals dissociable components of incentive motivation

Although it has long been hypothesized that the nucleus accumbens (NAc) acts as an interface between limbic and motor regions, direct evidence for this modulatory role on behavior is lacking. Using a disconnection procedure in rats, we found that basolateral amygdala (BLA) input to the core and medial shell of the NAc separately mediate two distinct incentive processes controlling the performance of goal-directed instrumental actions, respectively: (i) the sensitivity of instrumental responding to changes in the experienced value of the goal or outcome, produced by specific satiety-induced outcome devaluation; and (ii) the effect of reward-related cues on action selection, observed in outcome-specific Pavlovian-instrumental transfer. These results reveal, therefore, that dissociable neural circuits involving BLA inputs to the NAc core and medial shell mediate distinct components of the incentive motivational processes controlling choice and decision-making in instrumental conditioning.

GABA-A and GABA-B receptors mediate feeding elicited by the GABA-B agonist baclofen in the ventral tegmental area and nucleus accumbens shell in rats: reciprocal and regional interactions

Food intake is significantly increased following administration of GABA-B and GABA-A agonists into the nucleus accumbens (NAC) shell and ventral tegmental area (VTA) with receptor-selective antagonist pretreatment capable of blocking these responses within sites. Regional interactions in feeding studies have been evaluated by administering an antagonist in one site of interest prior to administration of the feeding-active agonist in a second site of interest and have identified important relationships, particularly for opioid-opioid interactions. To evaluate whether regional and reciprocal VTA and NAC shell interactions occur for GABA-mediated feeding, the present study examined whether feeding elicited by the GABA-B agonist, baclofen, microinjected into the NAC shell was dose-dependently blocked by pretreatment with either the GABA-B antagonist, saclofen, or the GABA-A antagonist, bicuculline, into the VTA, and then whether VTA baclofen-induced feeding was dose-dependently blocked by NAC shell pretreatment of either saclofen or bicuculline in rats. Rats were stereotaxically implanted with bilateral pairs of cannulae aimed at the VTA and NAC shell and were assessed for food intake following vehicle and baclofen (200 ng) in each site. Baclofen produced similar magnitudes of increased food intake following VTA and NAC shell treatment. Baclofen administration in the VTA and NAC shell was preceded 20 min earlier with administration of bicuculline (0, 7.5, 75, 150, 300 ng) or saclofen (0, 0.5, 1.5, 3, 5 μg) into the other site with intake measured 1, 2 and 4h after agonist treatment. VTA saclofen dose-dependently and significantly blocked feeding elicited by NAC shell baclofen. Correspondingly, NAC shell saclofen dose-dependently and significantly blocked feeding elicited by VTA baclofen, indicating a robust and bidirectional GABA-B/GABA-B receptor interaction between sites. Whereas VTA bicuculline significantly blocked the increased feeding elicited by NAC shell baclofen, NAC shell bicuculline reduced but did not block feeding elicited by VTA baclofen, indicating a unidirectional interaction GABA-B/GABA-A receptor interaction between sites. Unlike within-site receptor specificity governing the ability of GABA agonist mediation of food intake, the present study demonstrates that GABA, like opioids, employs a distributed brain network in mediating its ingestive effects, and that under certain circumstances, uses multiple receptor subtypes to underlie its regional effects.

Evidence for motivational effects elicited by activation of GABA-A or dopamine receptors in the nucleus accumbens shell

Microinjections of the inhibitory GABA-A receptor agonist muscimol into the shell region of the nucleus accumbens (AcbSh) have been reported to induce large increases in food intake, but the effect of these injections on motivational processes is less clear. In the current study, bilateral injections of saline, muscimol (50ng/side) or d-amphetamine (10mug/side) were made into the AcbSh of rats trained to lever press on a progressive ratio schedule for food reward. Injections of both muscimol and amphetamine were found to produce a large increase in the breaking point relative to saline injections. This result suggests that inactivation of the AcbSh does not simply drive ingestive behavior, but also affects motivational processes assessed by the progressive ratio schedule. Breaking points were also increased by injections of amphetamine into the AcbSh.

Desire and dread from the nucleus accumbens: cortical glutamate and subcortical GABA differentially generate motivation and hedonic impact in the rat

BACKGROUND

GABAergic signals to the nucleus accumbens (NAc) shell arise from predominantly subcortical sources whereas glutamatergic signals arise mainly from cortical-related sources. Here we contrasted GABAergic and glutamatergic generation of hedonics versus motivation processes, as a proxy for comparing subcortical and cortical controls of emotion. Local disruptions of either signals in medial shell of NAc generate intense motivated behaviors corresponding to desire and/or dread, along a rostrocaudal gradient. GABA or glutamate disruptions in rostral shell generate appetitive motivation whereas disruptions in caudal shell elicit fearful motivation. However, GABA and glutamate signals in NAc differ in important ways, despite the similarity of their rostrocaudal motivation gradients.

METHODOLOGY/PRINCIPAL FINDINGS

Microinjections of a GABA(A) agonist (muscimol), or of a glutamate AMPA antagonist (DNQX) in medial shell of rats were assessed for generation of hedonic "liking" or "disliking" by measuring orofacial affective reactions to sucrose-quinine taste. Motivation generation was independently assessed measuring effects on eating versus natural defensive behaviors. For GABAergic microinjections, we found that the desire-dread motivation gradient was mirrored by an equivalent hedonic gradient that amplified affective taste "liking" (at rostral sites) versus "disliking" (at caudal sites). However, manipulation of glutamatergic signals completely failed to alter pleasure-displeasure reactions to sensory hedonic impact, despite producing a strong rostrocaudal gradient of motivation.

CONCLUSIONS/SIGNIFICANCE

We conclude that the nucleus accumbens contains two functional affective keyboards for amino-acid signals: a motivation-generating keyboard and a hedonic-generating keyboard. Corticolimbic glutamate signals and subcortical GABA signals equivalently engage the motivation keyboard to generate desire and-or dread. Only subcortical GABA signals additionally engage the hedonic keyboard to amplify affective "liking" and "disliking" reactions. We thus suggest that top-down cortical glutamate signals powerfully regulate motivation components, but are relatively unable to penetrate core hedonic components of emotion. That may carry implications of limits to therapeutic regulation of pathological emotions.

Role of orexin/hypocretin in reward-seeking and addiction: implications for obesity

Orexins (also named hypocretins) are recently discovered neuropeptides made exclusively in the hypothalamus. Recent studies have shown that orexin cells located specifically in lateral hypothalamus (LH) are involved in motivated behavior for drugs of abuse as well as natural rewards. Administration of orexin has been shown to stimulate food consumption, and orexin signaling in VTA has been implicated in intake of high-fat food. In self-administration studies, the orexin 1 receptor antagonist SB-334867 (SB) attenuated operant responding for high-fat pellets, sucrose pellets and ethanol, but not cocaine, demonstrating that signaling at orexin receptors is necessary for reinforcement of specific rewards. The orexin system is also implicated in associations between rewards and relevant stimuli. For example, Fos expression in LH orexin neurons varied in proportion to conditioned place preference (CPP) for food, morphine, or cocaine. This Fos expression was altered accordingly for CPP administered during protracted abstinence from morphine or cocaine, when preference for natural rewards was decreased and drug preference was increased. Additionally, orexin has been shown to be involved in reward-stimulus associations in the self-administration paradigm, where SB attenuated cue-induced reinstatement of extinguished sucrose- or cocaine-seeking. Although the specific circuitry mediating the effects of orexin on food reward remains unknown, VTA seems likely to be a critical target for at least some of these orexin actions. Thus, recent studies have established a role for orexin in reward-based feeding, and further investigation is warranted for determining whether function/dysfunction of the orexin system may contribute to the overeating associated with obesity. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Stimulation of lateral hypothalamic glutamate and acetylcholine efflux by nicotine: implications for mechanisms of nicotine-induced activation of orexin neurons

The hypothalamus is a prominent target of nicotine action. We have previously shown that acute systemic nicotine treatment induces Fos expression in the lateral hypothalamus and perifornical area (LH/PFA), with orexin/hypocretin neurons being particularly responsive. However, the neurochemical correlates of acute nicotine treatment in the LH/PFA have not been described. Anatomical studies have revealed that this area receives afferents from cholinergic, glutamatergic, and GABAergic telencephalic brain regions, suggesting a potential role for these neurotransmitters in mediating the hypothalamic component of nicotine effects on homeostatic phenomena, such as arousal and appetite. Here, we used in vivo microdialysis to determine the effect of acute systemic or local nicotine on glutamate, acetylcholine, and GABA efflux in the LH/PFA of rats. Local administration of nicotine significantly increased acetylcholine and glutamate, but not GABA, in the LH/PFA. Thus, we further tested the role of afferent sources of glutamate and acetylcholine in mediating acute nicotine-induced activation of orexin neurons by unilaterally lesioning the prefrontal cortex or basal forebrain cholinergic regions. Lesioned animals showed reduced Fos-positive orexin neurons following nicotine treatment. These data suggest that both acetylcholine and glutamate may mediate the effects of acute nicotine on the activity of hypothalamic neurons, including orexin/hypocretin cells. Changes in cholinergic or glutamatergic transmission in this region with chronic nicotine may contribute to long-term alterations in functions mediated by LH/PFA neurons, including feeding and arousal.

Homeostatic alterations after IL-1beta microinjection into the nucleus accumbens of the rat

The present study investigates the effects of direct administration of interleukin-1beta (IL-1beta) into the nucleus accumbens (NAcc) on homeostatic regulation. Short- and long-term food intakes (FI), water intakes (WI) and body temperature (BT) were measured before and after bilateral microinjection of IL-1beta (with or without paracetamol pretreatment) into the NAcc of Wistar rats, and the effects were compared with those found in vehicle treated control animals. In addition, blood glucose levels, along with a glucose tolerance test (GTT), and plasma concentrations of metabolic parameters, such as total cholesterol, triglycerides, HDL, LDL and uric acid were determined in cytokine treated and control rats. Short-term FI and WI were suppressed after intraaccumbens application of IL-1beta. A significant increase of BT was also observed after the cytokine microinjection. Pretreatment with paracetamol failed to influence the anorexigenic, adipsogenic, and pyrogenic effects of IL-1beta. A definite glucose intolerance of the cytokine treated animals and their pathologically elevated blood glucose levels became obvious in the acute GTT. Following IL-1beta microinjection, plasma levels of triglycerides, total cholesterol and LDL were found increased. Our present findings show that the NAcc is an important site of action of IL-1beta mediated processes in central homeostatic regulation.

Integrative neurobiology of energy homeostasis-neurocircuits, signals and mediators

Body weight is tightly controlled in a species-specific range from insects to vertebrates and organisms have developed a complex regulatory network in order to avoid either excessive weight gain or chronic weight loss. Energy homeostasis, a term comprising all processes that aim to maintain stability of the metabolic state, requires a constant communication of the different organs involved; i.e. adipose tissue, skeletal muscle, liver, pancreas and the central nervous system (CNS). A tight hormonal network ensures rapid communication to control initiation and cessation of eating, nutrient processing and partitioning of the available energy within different organs and metabolic pathways. Moreover, recent experiments indicate that many of these homeostatic signals modulate the neural circuitry of food reward and motivation. Disturbances in each individual system can affect the maintenance and regulation of the others, making the analysis of energy homeostasis and its dysregulation highly complex. Though this cross-talk has been intensively studied for many years now, we are far from a complete understanding of how energy balance is maintained and multiple key questions remain unanswered. This review summarizes some of the latest developments in the field and focuses on the effects of leptin, insulin, and nutrient-related signals in the central regulation of feeding behavior. The integrated view, how these signals interact and the definition of functional neurocircuits in control of energy homeostasis, will ultimately help to develop new therapeutic interventions within the current obesity epidemic.

Interaction between orexins and the mesolimbic system for overriding satiety

In North American society, it is all too common for the intake of calories to outweigh an individual's energy demands. Such over-consumption where high-energy foods are readily available undoubtedly contributes to the growing problem of obesity. Palatable food stimulates brain circuits similar to those that mediate behavioral responses to drugs of abuse, which may underlie the continuation of food intake long after energy requirements are met. Among the brain areas implicated in reward and food intake, the lateral hypothalamus (LH) has long been recognized as a common region involved in both. It has been suggested that orexin neurons that are expressed exclusively within and adjacent to the LH comprise a major cellular substrate for the functioning of the LH. Here, we review the idea that the orexin neuropeptides play a key role in the rewarding aspects of food intake through interactions with both peripheral and central signals reflecting current energy stores as well as the classic reward pathway--the mesolimbic dopamine system. Furthermore, a possible heterogeneity of orexin neurons is discussed. Uncovering orexin's role in food reinforcement may provide insight into hyperphagia and obesity. In addition, the idea that food intake and substance abuse involve similar brain circuitry suggests potential for a single treatment aiding both obesity and addiction.

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

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

Effects of intracerebroventricular and intra-accumbens melanin-concentrating hormone agonism on food intake and energy expenditure

The brain melanin-concentrating hormone (MCH) system represents an anabolic system involved in energy balance regulation through influences exerted on the homeostatic and nonhomeostatic controls of food intake and energy expenditure. The present study was designed to further delineate the effect of the MCH system on energy balance regulation by assessing the actions of the MCH receptor 1 (MCHR1) agonism on both food intake and energy expenditure after intracerebroventricular (third ventricle) and intra-nucleus-accumbens-shell (intraNAcSH) injections of a MCHR1 agonist. Total energy expenditure and substrate oxidation were assessed following injections in male Wistar rats using indirect calorimetry. Food intake was also measured. Pair-fed groups were added to evaluate changes in thermogenesis that would occur regardless of the meal size and its thermogenic response. Using such experimental conditions, we were able to demonstrate that acute MCH agonism in the brain, besides its orexigenic effect, induced a noticeable change in the utilization of the main metabolic fuels. In pair-fed animals, MCH significantly reduced lipid oxidation when it was injected in the third ventricle. Such an effect was not observed following the injection of MCH in the NAcSH, where MCH nonetheless strongly stimulated appetite. The present results further delineate the influence of MCH on energy expenditure and substrate oxidation while confirming the key role of the NAcSH in the effects of the MCH system on food intake.

How does immune challenge inhibit ingestion of palatable food? Evidence that systemic lipopolysaccharide treatment modulates key nodal points of feeding neurocircuitry

Immune challenge induces behavioral changes including reduced ingestion of palatable food. Multiple pathways likely contribute to this effect, including viscerosensory pathways controlling hypothalamic feeding circuits or by influence on "reward" circuitry previously established to control ingestive behavior. To investigate whether the effects of immune challenge may influence this network, we compared brain activation patterns in animals trained to drink a palatable sweetened milk solution and treated systemically with either the immune stimulant lipopolysaccharide (LPS) or saline. Brain sections were processed for localization of the activation marker c-Fos in neurons of regions implicated in regulation of feeding behavior. Sweetened milk ingestion was associated with increased numbers of c-Fos positive neurons in the caudal core and shell of the nucleus accumbens (NAc), the paraventricular thalamus (PVT), central nucleus of the amygdala (CEA), the basal lateral amygdala (BLA), in orexin-A containing neurons of the lateral hypothalamus (LH), and in cocaine and amphetamine regulated transcript (CART) neurons of the arcuate hypothalamus. In LPS-treated animals sweetened milk consumption was significantly reduced, as was c-Fos induction in the hypothalamic orexin-A and CART neurons, and in the BLA. In addition, induction of c-Fos in the rostral regions of the NAc, the PVT, and CEA was increased following LPS treatment, compared to controls. The findings from this study point to a network of brain regions (LH, PVT, NAc, and BLA) previously implicated in the modulation of feeding behavior, reward, and arousal that may also contribute to neural substrates involved in the reorganization of behavioral priorities that occurs during sickness.

Insulin, leptin, and food reward: update 2008

The hormones insulin and leptin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis at medial hypothalamic sites. In a previous review, we described new research demonstrating that, in addition to these direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and motivation is also a direct and an indirect target for insulin and leptin action. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, i.e., midbrain dopamine and opioidergic pathways. Here we summarize new behavioral, systems, and cellular evidence in support of this hypothesis and in the context of research into the homeostatic roles of both hormones in the CNS. We discuss some current issues in the field that should provide additional insight into this hypothetical model. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Central mechanisms of roles of taste in reward and eating

Taste is unique among sensory systems in its innate association with mechanisms of reward and aversion in addition to its recognition of quality, e.g., sucrose is sweet and preferable, and quinine is bitter and aversive. Taste information is sent to the reward system and feeding center via the prefrontal cortices such as the mediodorsal and ventrolateral prefrontal cortices in rodents and the orbitofrontal cortex in primates. The amygdala, which receives taste inputs, also influences reward and feeding. In terms of neuroactive substances, palatability is closely related to benzodiazepine derivatives and beta-endorphin, both of which facilitate consumption of food and fluid. The reward system contains the ventral tegmental area, nucleus accumbens and ventral pallidum and finally sends information to the lateral hypothalamic area, the feeding center. The dopaminergic system originating from the ventral tegmental area mediates the motivation to consume palatable food. The actual ingestive behavior is promoted by the orexigenic neuropeptides from the hypothalamus. Even palatable food can become aversive and avoided as a consequence of a postingestional unpleasant experience such as malaise. The neural mechanisms of this conditioned taste aversion will also be elucidated.

Functional disturbances in the striatum by region-specific ablation of NMDA receptors

To study the role of NMDA receptors in dopamine signaling of the striatum, the brain area that receives glutamatergic inputs from various cortical areas and most dopaminergic inputs, we generated striatum-specific NMDA receptor-deficient mice. The mutant pups showed reduced food intake and retarded growth starting at the second postnatal week and died on approximately postnatal day 20 (P20). The time course of postnatal lethality is similar to that of compound mutant, double knockout of dopamine D1/D2 receptors, or genetically engineered dopamine-deficient mouse. In vivo electrophysiological recordings in the mutant pups showed that frequencies in the range of gamma oscillation were reduced in the striatal circuits. Moreover, the number of functional dopamine receptors in the striatum as measured by D1- and D2-binding experiments was greatly diminished in the mutants as compared with control animals. A consequence of diminished dopamine binding in the striatum manifested in an increase of locomotor activity. The administration of D1/D2 agonists paradoxically reduced the hyperactivity of the mutant mice as compared with an increase in locomotor activity in control mice. These results demonstrate that the NMDA receptor plays an essential role in the integration of dopamine signaling in the striatum and that is required in behavioral function.

Feeding specific glutamate surge in the rat lateral hypothalamus revealed by low-flow push-pull perfusion

Substantial evidence implicates the lateral hypothalamus (LH) in the control of ingestive behavior and previous studies have found that glutamate release within the LH increases during meals. It is not known, however, whether this effect is selective for feeding, or whether similar changes are also seen during drinking. In this work, we examined this question using low-flow push-pull perfusion which allows sampling from small tissue volumes. Presentation of highly palatable solid or liquid foods to food-deprived rats resulted in an immediate increase in glutamate output of more than 200% over baseline. The response was maximal immediately after food presentation. In contrast, significant changes in glutamate output were not seen when water was presented to water-deprived animals, despite the occurrence of vigorous drinking. These findings confirm reports of feeding related glutamate release in the LH and demonstrate that this effect is specific to feeding, rather than being a general concomitant of all ingestive behaviors. The push-pull technique described here may allow the relevant region of the LH to be identified with greater precision than other methods.

Chronic social stress, hedonism and vulnerability to obesity: lessons from rodents

Obesity is a current health pandemia. Determinants of this pathology are rather complex and include genetic, developmental and environmental factors only partially disclosed. Stress related neuroendocrine dysregulation and overconsumption of high palatable high caloric food and are likely to contribute to this modern health threats. Despite the evidence that psychosocial stress is one of the main sources of stress in humans and may play an important role in the development of the stress disorders, including obesity and metabolic syndrome, animal models focusing on the relationship between chronic stress and energy homeostasis are scattered and most of them encompasses physical rather than psychosocial stress. Aim of the present paper is to review rodent studies on the effect of psychosocial stress throughout life on body weight and food intake regulation. In the second part of the review special focus will be given on the mechanisms linking stress and the reward system.

Role of dopamine D1 and D2 receptors in the nucleus accumbens shell on the acquisition and expression of fructose-conditioned flavor-flavor preferences in rats

Systemic administration of dopamine D1 (SCH23390) and less so D2 (raclopride) receptor antagonists significantly reduce acquisition and expression of fructose-conditioned flavor preferences (CFP). Because dopamine in the nucleus accumbens shell (NAcS) is implicated in food reward, the present study examined whether NAcS D1 or D2 antagonists altered acquisition and/or expression of fructose-CFP. In Experiment 1, food-restricted rats with bilateral NAcS cannulae were trained to drink a fructose (8%)+saccharin (0.2%) solution mixed with one flavor (CS+/Fs) and a less-preferred 0.2% saccharin solution with mixed another flavor (CS-/s). Unlimited two-bottle tests with the two flavors in saccharin (0.2%: CS+/s, CS-/s) occurred 10 min following total bilateral NAcS doses of 0, 12, 24 or 48 nmol of SCH23390 or raclopride. Preference for CS+/s over CS-/s following vehicle treatment (76%) was significantly reduced by SCH23390 (48 nmol, 62%) and raclopride (24 nmol, 63%). In Experiment 2, rats received bilateral NAcS injections (12 nmol) of SCH23390 or raclopride on one-bottle training (16 ml) days. Yoked control rats received vehicle and were limited to the CS intakes of the D1 and D2 groups, whereas untreated controls without injections received their CS ration during training. Subsequent unlimited two-bottle tests revealed initial preferences of CS+/s over CS-/s in all groups that remained stable in untreated and yoked controls, but were lost over the six tests sessions in D1 and D2 groups. These data indicate that NAcS D1 and D2 antagonists significantly attenuated the expression of the fructose-CFP and did not block acquisition, but hastened extinction of fructose-CFP.

A re-examination of septal lesion-induced weight gain in female rats

Previous studies that examined the effects of knife cuts ventral to the septal area, or the pattern of anterograde degeneration after other obesity-inducing lesions or cuts, have implicated the septal area in feeding behavior. However, lesion studies have been inconsistent. In the present study, large electrolytic lesions that destroyed all or nearly all of the septal area resulted in excessive weight gain in female rats (25 g/20 days compared to 1 g/20 days for controls). Lesions confined to the ventral septum produced equal weight gains, but rats with lesions confined to the dorsal septum did not differ from controls. The excessive weight gains were accompanied by modest hyperphagia. The weight gains were not the result of incidental damage to adjacent structures such as the stria terminalis or the shell of the nucleus accumbens. It is concluded that the ventral portion of the septum is part of a limbic-hypothalamic circuitry regulating food intake in female rats.

Anorexia induced by activation of serotonin 5-HT4 receptors is mediated by increases in CART in the nucleus accumbens

Anorexia nervosa is a growing concern in mental health, often inducing death. The potential neuronal deficits that may underlie abnormal inhibitions of food intake, however, remain largely unexplored. We hypothesized that anorexia may involve altered signaling events within the nucleus accumbens (NAc), a brain structure involved in reward. We show here that direct stimulation of serotonin (5-hydroxytryptamine, 5-HT) 4 receptors (5-HT(4)R) in the NAc reduces the physiological drive to eat and increases CART (cocaine- and amphetamine-regulated transcript) mRNA levels in fed and food-deprived mice. It further shows that injecting 5-HT(4)R antagonist or siRNA-mediated 5-HT(4)R knockdown into the NAc induced hyperphagia only in fed mice. This hyperphagia was not associated with changes in CART mRNA expression in the NAc in fed and food-deprived mice. Results include that 5-HT(4)R control CART mRNA expression into the NAc via a cAMP/PKA signaling pathway. Considering that CART may interfere with food- and drug-related rewards, we tested whether the appetite suppressant properties of 3,4-N-methylenedioxymethamphetamine (MDMA, ecstasy) involve the 5-HT(4)R. Using 5-HT(4)R knockout mice, we demonstrate that 5-HT(4)R are required for the anorectic effect of MDMA as well as for the MDMA-induced enhancement of CART mRNA expression in the NAc. Directly injecting CART peptide or CART siRNA into the NAc reduces or increases food consumption, respectively. Finally, stimulating 5-HT(4)R- and MDMA-induced anorexia were both reduced by injecting CART siRNA into the NAc. Collectively, these results demonstrate that 5-HT(4)R-mediated up-regulation of CART in the NAc triggers the appetite-suppressant effects of ecstasy.

Cue-evoked encoding of movement planning and execution in the rat nucleus accumbens

The nucleus accumbens is involved in the modulation of motivated behaviour by reward-associated sensory information. However, little is known about the specific nature of the nucleus accumbens' contribution to generating movement. We investigated motor encoding by nucleus accumbens neurons in rats performing a delayed response task that allowed us to dissociate the effects of sensory and motor events on firing. In a subset of neurons, firing in the delay period preceding movement was highly selective; this selectivity was tightly correlated with the direction of the subsequent movement, but not with the sensory properties of the instructive cue. Direction selectivity in this population of neurons developed over the course of the delay period, with the strongest selectivity apparent just prior to movement onset. Selectivity was also apparent in nucleus accumbens neurons during movement, such that firing showed a tight correlation with movement direction, but not the instructive cue presented nor the spatial destination of the movement. These results are consistent with the hypothesis that a subpopulation of nucleus accumbens neurons contributes to the selection and execution of specific motivated behaviours.

Neural mechanisms underlying obesity and drug addiction

Increasing rates of obesity have alarmed health officials and prompted much public dialogue. While the factors leading to obesity are numerous, an inability to control intake of freely available food is central to the problem. In order to understand this, we need to better define the mechanisms by which the brain regulates food intake, and why it is often difficult to control consumption. From this point of view, it seems valuable to consider the commonalities between food intake and drug abuse. While research in the two fields has historically emphasized different neural substrates, recent data have increased interest in better defining elements that may underlie both drug addiction and obesity. Here we discuss some of these shared elements with an emphasis on emerging areas of research that better define common mechanisms leading to overconsumption.

GABAA and GABAB agonist microinjections into medial accumbens shell increase feeding and induce anxiolysis in an animal model of anxiety

This study investigated the effect of GABAA (muscimol, MUSC) and GABAB (baclofen, BACL) agonist receptors microinjected into medial accumbens shell on feeding and the level of fear in free-feeding rats submitted to the elevated plus-maze (EPM), an animal model of anxiety. Bilateral microinjections of either MUSC (128 pmol/0.2 microl/side) or BACL (128 and 256 pmol/0.2 microl/side) induced an anxiolytic-like effect since they decreased the occurrence of risk assessment, a defensive behaviour positively correlated with the animal anxiety level. Bilateral BACL microinjection (128 pmol), but not MUSC, also increased the head-dipping frequency over the open arms of the EPM, another representative behaviour of anxiety, but negatively correlated with it. In addition to anxiolysis, the present study also showed that the microinjection of MUSC and BACL agonists into rostral sites of the medial Acb shell (AP, +1.2 to +1.6) increased food intake significantly whereas drinking behaviour kept unchanged. Both doses of MUSC and BACL also decreased feeding latency. BACL but not MUSC dose-dependently increased food length. The data indicated a putative role of GABA receptors (especially GABAB) in the medial Acb shell for anxiety modulation in rats.

Pharmacological enhancement of the endocannabinoid system in the nucleus accumbens shell stimulates food intake and increases c-Fos expression in the hypothalamus

BACKGROUND AND PURPOSE

Evidence indicates that the endocannabinoid, 2-arachidonoylglycerol (2-AG), increases food intake when injected into the nucleus accumbens shell (NAcS), thereby potentially activating hypothalamic nuclei involved in food intake regulation. We aimed to evaluate potential orexigenic effects of the endocannabinoid anandamide and of AA5HT, a fatty acid amide hydrolase (FAAH) inhibitor, and OMDM-1, an inhibitor of anandamide uptake, injected in the NAcS, as well as the effect of these treatments on activation of hypothalamic nuclei.

EXPERIMENTAL APPROACH

Drugs were given into the NAcS of rats and food intake quantified during the next 4 h. In other groups, after the same treatments the brains were processed for c-Fos immunohistochemistry with focus on hypothalamic nuclei. Additional groups were used to quantify endocannabinoid levels in the nucleus accumbens and the hypothalamus after AA5HT and OMDM-1 intra-NAcS injections.

KEY RESULTS

Our results indicate that the above treatments stimulate food intake during 4 h post-injection. They also increase c-Fos immunoreactivity in hypothalamic nuclei. The CB(1) antagonist, AM251, blocked these effects. Finally, we found elevated levels of 2-AG, but not anandamide, after intra-NAcS injections of AA5HT.

CONCLUSIONS AND IMPLICATIONS

These data support the involvement of the endocannabinoid system in feeding behavior at the level of the NAcS and hypothalamus. In addition, this is the first experimental demonstration that the pharmacological inhibition of endocannabinoid inactivation in the NAcS stimulates food intake, suggesting that the endocannabinoid degrading proteins can be a target for treating eating disorders.

Chronic fasting-induced changes of neuropeptide Y immunoreactivity in the lateral septum of intact and ovariectomized female rats

The effect of 40% food deprivation for 1 week on the immunohistochemically detectable amount of neuropeptide Y (NPY) was studied in the lateral septum (LS) of intact and ovariectomized (OVX) female rats. Animals were either fed ad libitum or 40% food-deprived. Densitometric analysis of immunostained material showed a significant decrease in NPY-immunoreactivity (NPY-IR) in OVX rats compared to the control group. Food deprivation increased the density of punctate NPY-IR profiles in both intact and OVX animals, however, the density in food-deprived OVX animals was increased compared to baseline but remained reduced compared to intact rats. Our study indicates that the lack of gonadal hormones - most likely estrogen - results in a decrease in the density of NPY-IR axonal fibers within the LS, while food deprivation induced considerable elevation in NPY density. Food restriction-induced changes in the density of NPY-containing neural elements suggest that the LS may play a crucial role in the regulation of food intake and energy balance, in concert with the relevant hypothalamic areas.

Neural network interactions and ingestive behavior control during anorexia

Many models have been proposed over the years to explain how motivated feeding behavior is controlled. One of the most compelling is based on the original concepts of Eliot Stellar whereby sets of interosensory and exterosensory inputs converge on a hypothalamic control network that can either stimulate or inhibit feeding. These inputs arise from information originating in the blood, the viscera, and the telencephalon. In this manner the relative strengths of the hypothalamic stimulatory and inhibitory networks at a particular time dictates how an animal feeds. Anorexia occurs when the balance within the networks consistently favors the restraint of feeding. This article discusses experimental evidence supporting a model whereby the increases in plasma osmolality that result from drinking hypertonic saline activate pathways projecting to neurons in the paraventricular nucleus of the hypothalamus (PVH) and lateral hypothalamic area (LHA). These neurons constitute the hypothalamic controller for ingestive behavior, and receive a set of afferent inputs from regions of the brain that process sensory information that is critical for different aspects of feeding. Important sets of inputs arise in the arcuate nucleus, the hindbrain, and in the telencephalon. Anorexia is generated in dehydrated animals by way of osmosensitive projections to the behavior control neurons in the PVH and LHA, rather than by actions on their afferent inputs.

The nucleus accumbens as part of a basal ganglia action selection circuit

BACKGROUND

The nucleus accumbens is the ventral extent of the striatum, the main input nucleus of the basal ganglia. Recent hypotheses propose that the accumbens and its dopamine projection from the midbrain contribute to appetitive behaviors required to obtain reward. However, the specific nature of this contribution is unclear. In contrast, significant advances have been made in understanding the role of the dorsal striatum in action selection and decision making.

OBJECTIVE

In order to develop a hypothesis of the role of nucleus accumbens dopamine in action selection, the physiology and behavioral pharmacology of the nucleus accumbens are compared to those of the dorsal striatum.

HYPOTHESES

Three hypotheses concerning the role of dopamine in these structures are proposed: (1) that dopamine release in the dorsal striatum serves to facilitate the ability to respond appropriately to temporally predictable stimuli (that is, stimuli that are so predictable that animals engage in anticipatory behavior just prior to the stimulus); (2) that dopamine in the nucleus accumbens facilitates the ability to respond to temporally unpredictable stimuli (which require interruption of ongoing behavior); and (3) that accumbens neurons participate in action selection in response to such stimuli by virtue of their direct (monosynaptic inhibitory) and indirect (polysynaptic excitatory) projections to basal ganglia output nuclei.

Discrete neurochemical coding of distinguishable motivational processes: insights from nucleus accumbens control of feeding

BACKGROUND AND OBJECTIVES

The idea that nucleus accumbens (Acb) dopamine transmission contributes to the neural mediation of reward, at least in a general sense, has achieved wide acceptance. Nevertheless, debate remains over the precise nature of dopamine's role in reward and even over the nature of reward itself. In the present article, evidence is reviewed from studies of food intake, feeding microstructure, instrumental responding for food reinforcement, and dopamine efflux associated with feeding, which suggests that reward processing in the Acb is best understood as an interaction among distinct processes coded by discrete neurotransmitter systems.

RESULTS

In agreement with several theories of Acb dopamine function, it is proposed here that allocation of motor effort in seeking food or food-associated conditioned stimuli can be dissociated from computations relevant to the hedonic evaluation of food during the consummatory act. The former appears to depend upon Acb dopamine transmission and the latter upon striatal opioid peptide release. Moreover, dopamine transmission may play a role in 'stamping in' associations between motor acts and goal attainment and perhaps also neural representations corresponding to rewarding outcomes. Finally, evidence is reviewed that amino acid transmission specifically in the Acb shell acts as a central 'circuit breaker' to flexibly enable or terminate the consummatory act, via descending connections to hypothalamic feeding control systems.

CONCLUSIONS

The heuristic framework outlined above may help explain why dopamine-compromising manipulations that strongly diminish instrumental goal-seeking behaviors leave consummatory activity relatively unaffected.

The neural correlates and role of D1 dopamine receptors in renewal of extinguished alcohol-seeking

We used an ABA renewal design to study the neural correlates, and role of D1 dopamine receptors, in contextual control over extinguished alcohol-seeking. Rats were trained to respond for 4% beer in one context (A), extinguished in a different (B) context, and then tested for responding in the original training context (A) or the extinction context (B). ABA renewal was mediated by D1 dopamine receptors because it was prevented by SCH23390. ABA renewal of alcohol-seeking was associated with selective increases in c-Fos protein induction in basolateral amygdala, ventral accumbens shell, and lateral hypothalamus (renewal-associated Fos). By contrast, being tested was associated with increased c-Fos induction in anterior cingulate, prelimbic and infralimbic cortex, rostral agranular insula, dorsomedial accumbens shell, and accumbens core (test-associated Fos). Renewal-associated Fos in ventral accumbens shell and lateral hypothalamus, but not basolateral amygdala, was D1 dopamine receptor dependent. Double immunofluorescence showed that renewal-associated Fos was expressed in orexin-negative lateral hypothalamic neurons. However, c-Fos induction in either lateral hypothalamic orexin-negative or orexin-positive neurons was positively and significantly correlated with alcohol-seeking. Test-associated c-Fos induction was observed in orexin-positive perifornical neurons. In both regions, c-Fos expression was dependent on D1 dopamine receptors. These results suggest that renewal of extinguished alcohol-seeking depends on a distributed neural circuit involving basolateral amygdala, ventral accumbens shell, and lateral hypothalamus that involves D1 dopamine receptors. Comparison with our previous results [Hamlin AS, Blatchford KE, McNally GP (2006) Renewal of an extinguished instrumental response: Neural correlates and the role of D1 dopamine receptors. Neuroscience 143:25-38] permits identification of similarities and differences in the correlates of renewal of extinguished drug- and natural-reward seeking.

Interactions between the "cognitive" and "metabolic" brain in the control of food intake

If the new environment and modern lifestyle cause obesity in individuals with thrifty genes by increasing energy intake, it is important to know by what mechanisms hyperphagia occurs and why energy balance is not kept in check by the homeostatic regulator. The argument is developed that procuring and ingesting food is an evolutionarily conserved survival mechanism that occupies large parts of the brain's computing capacity including not only the hypothalamus but also a number of cortico-limbic structures. These forebrain systems evolved to engage powerful emotions for guaranteed supply and ingestion of beneficial foods from a sparse and often hostile environment. They are now simply overwhelmed with an abundance of food and food cues that is no longer interrupted by frequent famines. After briefly reviewing structure and functions of the relevant cortico-limbic structures and the better-known hypothalamic homeostatic regulator, the review focuses mainly on interactions between the two systems. Although several cortico-limbic processes are sensitive to metabolic depletion and repletion signals, it appears that they are underlying the same reversible leptin resistance that renders hypothalamic circuits insensible to continuously high leptin levels during periods of feast. It is hypothesized that this naturally occurring leptin resistance allowed temporary neutralization of satiety mechanisms and evolved as a response to survive subsequent periods of famine. With today's continuous and abundant food availability for a segment of the population, the powerful cognitive processes to eat and the resulting overweight can partially escape negative feedback control in prone individuals most strongly expressing such thrifty genes.

Renewal of an extinguished instrumental response: Neural correlates and the role of D1 dopamine receptors

Contexts play an important role in controlling the expression of extinguished behaviors. We used an ABA renewal design to study the neural correlates, and role of D1 dopamine receptors, in contextual control over extinguished instrumental responding. Rats were trained to respond for a sucrose reward in one context (A). Responding was then extinguished in the same (A) or different (B) context. Rats were tested for responding in the original training context (A). Return to the original training context after extinction (group ABA) was associated with a return of responding. Three distinct patterns of Fos induction were detected on test: 1) ABA renewal was associated with selective increases in c-Fos protein induction in basolateral amygdala, ventral accumbens shell, and lateral hypothalamus (but not in orexin- or melanin-concentrating hormone (MCH)-hypothalamic neurons); 2) being placed in the same context as extinction training (AAA or ABB) was associated with a selective decrease in c-Fos induction in rostral agranular insular cortex; 3) being placed in any context on test was associated with the up-regulation of c-Fos induction in anterior cingulate, dorsomedial accumbens shell, accumbens core, lateral septum, and substantia nigra. The return of responding in ABA renewal was prevented by pre-treatment with the D1 dopamine receptor antagonist SCH23390 (10 microg/kg; s.c.). SCH23390 also suppressed basal and renewal-associated c-Fos protein induction throughout accumbens, and, selectively suppressed renewal-associated c-Fos induction in lateral hypothalamus. These results suggest that renewal of extinguished responding for a sucrose reward depends on a distributed neural circuit involving basolateral amygdala, ventral accumbens shell, and lateral hypothalamus. D1 dopamine receptors within this circuit are essential for renewal. The results also suggest that rostral agranular insular cortex may play an important role in suppressing reward-seeking after extinction training.

Expression of prokineticins and their receptors in the adult mouse brain

Prokineticins are a pair of regulatory peptides that have been shown to play important roles in gastrointestinal motility, angiogenesis, circadian rhythms, and, recently, olfactory bulb neurogenesis. Prokineticins exert their functions via activation of two closely related G-protein-coupled receptors. Here we report a comprehensive mRNA distribution for both prokineticins (PK1 and PK2) and their receptors (PKR1 and PKR2) in the adult mouse brain with the use of in situ hybridization. PK2 mRNA is expressed in discrete regions of the brain, including suprachiasmatic nucleus, islands of Calleja and medial preoptic area, olfactory bulb, nucleus accumbens shell, hypothalamic arcuate nucleus, and amygdala. PK1 mRNA is expressed exclusively in the brainstem, with high abundance in the nucleus tractus solitarius. PKR2 mRNA is detected throughout the brain, with prominent expression in olfactory regions, cortex, thalamus and hypothalamus, septum and hippocampus, habenula, amygdala, nucleus tractus solitarius, and circumventricular organs such as subfornical organ, median eminence, and area postrema. PKR2 mRNA is also detected in mammillary nuclei, periaqueductal gray, and dorsal raphe. In contrast, PKR1 mRNA is found in fewer brain regions, with moderate expression in the olfactory regions, dentate gyrus, zona incerta, and dorsal motor vagal nucleus. Both PKR1 and PKR2 are also detected in olfactory ventricle and subventricular zone of the lateral ventricle, both of which are rich sources of neuronal precursors. These extensive expression patterns suggest that prokineticins may have a broad array of functions in the central nervous system, including circadian rhythm, neurogenesis, ingestive behavior, reproduction, and autonomic function.

Effects of amphetamine on serotoninergic and GABAergic expression of developing brain

Roles of age and withdrawal were explored in mechanisms underlying the action of amphetamine (Amph), by monitoring the serotonergic and GABAergic expression in key brain regions of the rat. Postnatal 21 and 60 day-old male rats were intraperitoneally injected with D-Amph, 5 mg/kg, or saline, three times daily for 14 days and then withdrawn from Amph for 0 or 14 days; these animals received single injections on day 15 (W0d) or day 29 (W14d). Following Amph injections, though both age groups exhibited hyperlocomotion, stereotypy and behavioral sensitization, the juvenile showed 100-300% longer latencies to reach and 30%-42% shorter duration of maximal behavioral scores than the adult from day 2-29. Immunocytochemical analysis revealed down-regulation of 42-76% in 5-hydroxytryptamine (HT) immunoreactive processes in motor and somatosensory cortices, and hippocampus of both ages after Amph exposure at W0d. At W14d, the 5-HT resembled saline-control in the Amph-treated juvenile, whereas remained weakened in the adult. By contrast, densities of GAD67 (glutamic acid decarboxylase)-boutons were up-regulated by 35-545% in the neocortical areas, nucleus accumbens, caudate-putamen and hippocampus of all Amph-administered rats. After 14 days withdrawal, the juvenile recovered the decreased 5-HT fibers, but not the increased GABAergic, indicating unique roles of the two systems in response to Amph.

Ingestive effects of NMDA and AMPA-kainate receptor antagonists microinjections into the lateral hypothalamus of the pigeon (Columba livia)

This study examined the ingestive and behavioral effects of NMDA- and AMPA/kainate glutamatergic receptor blockade in the lateral hypothalamic area (LHy) of free-feeding pigeons (Columba livia). Injections of MK-801 (NMDA receptor antagonist; 6 nmol) or CNQX (AMPA/kainate receptor antagonist; 25.8 nmol) into the LHy of free-feeding pigeons induced significant increases in food intake and in feeding duration, as well as reductions in the latency to start feeding. Duration, latency and volume of water intake, as well as duration of sleep-like behavior, alert immobility, locomotion and preening were not changed by these treatments in the LHy. These results indicate that glutamatergic inputs to cells containing NMDA and/or AMPA receptors located in the LHy could modify both the beginning of a feeding bout (or the end of a period of satiety) and its duration (satiation). Our data also suggest that these inhibitory glutamatergic influences on feeding behavior are tonically active in the LHy.

Nucleus accumbens neurons encode Pavlovian approach behaviors: evidence from an autoshaping paradigm

Environmental stimuli predictive of appetitive events can elicit Pavlovian approach responses that enhance an organism's ability to track and secure natural rewards, but may also contribute to the compulsive nature of drug addiction. Here, we examined the activity of individual nucleus accumbens (NAc) neurons during an autoshaping paradigm. One conditioned stimulus (CS+, a retractable lever presented for 10 s) was immediately followed by the delivery of a 45-mg sucrose pellet to a food receptacle, while another stimulus (CS-, a separate retractable lever presented for 10 s) was never followed by sucrose. Approach responses directed at the CS+ and CS- were recorded as lever presses and had no experimental consequence. Rats (n = 9) selectively approached the CS+ on more than 80% of trials and were surgically prepared for electrophysiological recording. Of 76 NAc neurons, 57 cells (75%) exhibited increases and/or decreases in firing rate (i.e. termed 'phasically active') during the CS+ presentation and corresponding approach response. Forty-seven percent of phasically active cells (27 out of 57) were characterized by time-locked but transient increases in cell firing, while 53% (30 out of 57) showed a significant reduction in firing for the duration of the CS+. In contrast, the same excitatory subpopulation exhibited smaller increases in activity relative to CS- onset, while the inhibitory subpopulation showed no change in firing during the CS- period. The magnitude and prevalence of cue-related neural responses reported here indicates that the NAc encodes biologically significant, repetitive approach responses that may model the compulsive nature of drug addiction in humans.

Arousal and reward: a dichotomy in orexin function

The orexins (or hypocretins) are neuropeptide transmitters made exclusively in hypothalamic neurons that have extensive CNS projections. Previous studies reported that this system is most strongly associated with feeding, arousal and the maintenance of waking. We review here recent studies that reveal a novel and important role for the orexin/hypocretin neuronal system in reward processing and addiction. We propose that the current evidence indicates a dichotomy in orexin function, such that orexin neurons in the lateral hypothalamus regulate reward processing for both food and abused drugs, whereas those in the perifornical and dorsomedial hypothalamus regulate arousal and response to stress. Evidence also indicates roles for lateral hypothalamus orexin neurons and ventral tegmental orexin receptors in reward-based learning and memory.

Homeostatic and non-homeostatic pathways involved in the control of food intake and energy balance

A neural network sensitive to leptin and other energy status signals stretching from the hypothalamus to the caudal medulla has been identified as the homeostatic control system for the regulation of food intake and energy balance. While this system is remarkably powerful in defending the lower limits of adiposity, it is weak in curbing appetite in a world of plenty. Another extensive neural system that processes appetitive and rewarding aspects of food intake is mainly interacting with the external world. This non-homeostatic system is constantly attacked by sophisticated signals from the environment, ultimately resulting in increased energy intake in many genetically predisposed individuals. Recent findings suggest a role for accumbens-hypothalamic pathways in the interaction between non-homeostatic and homeostatic factors that control food intake. Identification of the neural pathways that mediate this dominance of cortico-limbic processes over the homeostatic regulatory circuits in the hypothalamus and brainstem will be important for the development of behavioral strategies and pharmacological therapies in the fight against obesity.

Neurochemical modulation of ingestive behavior in the ventral pallidum

The nucleus accumbens and its related circuitry have been shown to play an important role in promoting the intake of hedonically desirable food. A previous report has demonstrated that the blockade of GABAA receptors in the ventral pallidum (VP), a target of GABAergic projection from the nucleus accumbens, greatly increases food, but not water, intake in satiated rats [Stratford et al. (1999)Brain Research, 825, 199-203]. The present study examined which neurotransmission in the VP is specifically involved in the intake of normally preferred taste stimuli. Microinjections of the GABAA antagonist bicuculline selectively increased the intake of saccharin solution but not that of water and quinine solution in water-deprived rats. In contrast, the facilitation of GABAA receptors by microinjections of muscimol in the VP generally suppressed the intake of saccharin, water and quinine. The same injections induced strong aversive taste reactivity responses to oral stimulation with not only quinine but also water and saccharin. The local administration of D-Ala2,N-Me-Phe4,Glyol5-enkephalin, a selective micro-opioid receptor agonist, into the VP had time-dependent effects, decreasing saccharine intake early and increasing intake late. Microinjections of SCH-23390, a dopamine D1 receptor antagonist, in the VP suppressed the intake of saccharin but not water or quinine. Microinjections of sulpiride, the dopamine D2 receptor antagonist, and 6-cyano-7-nitroquinoxaline-2,3-dione, the AMPA/kainate glutamate receptor antagonist, had no effect on fluid intake. These results reveal that GABA, opioid and D1 receptors in the VP are involved in the consumption of hedonically positive taste stimuli.

Projections from bed nuclei of the stria terminalis, dorsomedial nucleus: implications for cerebral hemisphere integration of neuroendocrine, autonomic, and drinking responses

The overall projection pattern of a tiny bed nuclei of the stria terminalis anteromedial group differentiation, the dorsomedial nucleus (BSTdm), was analyzed with the Phaseolus vulgaris-leucoagglutinin anterograde pathway tracing method in rats. Many brain regions receive a relatively moderate to strong input from the BSTdm. They fall into eight general categories: humeral sensory-related (subfornical organ and median preoptic nucleus, involved in initiating drinking behavior and salt appetite), neuroendocrine system (magnocellular: oxytocin, vasopressin; parvicellular: gonadotropin-releasing hormone, somatostatin, thyrotropin-releasing hormone, corticotropin-releasing hormone), central autonomic control network (central amygdalar nucleus, BST anterolateral group, descending paraventricular hypothalamic nucleus, retrochiasmatic area, ventrolateral periaqueductal gray, Barrington's nucleus), hypothalamic visceromotor pattern-generator network (five of six known components), behavior control column (ingestive: descending paraventricular nucleus; reproductive: lateral medial preoptic nucleus; defensive: anterior hypothalamic nucleus; foraging: ventral tegmental area, along with interconnected nucleus accumbens and substantia innominata), orofacial motor control (retrorubral area), thalamocortical feedback loops (paraventricular, central medial, intermediodorsal, and medial mediodorsal nuclei; nucleus reuniens), and behavioral state control (subparaventricular zone, ventrolateral preoptic nucleus, tuberomammillary nucleus, supramammillary nucleus, lateral habenula, and raphé nuclei). This pattern of axonal projections, and what little is known of its inputs suggest that the BSTdm is part of a striatopallidal differentiation involved in coordinating the homeostatic and behavioral responses associated thirst and salt appetite, although clearly it may relate them to other functions as well. The BSTdm generates the densest known inputs directly to the neuroendocrine system from any part of the cerebral hemispheres.

Projections from bed nuclei of the stria terminalis, anteromedial area: cerebral hemisphere integration of neuroendocrine, autonomic, and behavioral aspects of energy balance

The anteromedial area of the bed nuclei of the stria terminalis (BSTam) is the relatively undifferentiated region of the anterior medial (anteromedial) group of the bed nuclei of the stria terminalis (BSTamg), which also includes the more distinct dorsomedial, magnocellular, and ventral nuclei. The overall pattern of axonal projections from the rat BSTam was analyzed with the Phaseolus vulgaris-leucoagglutinin anterograde pathway tracing method. Brain areas receiving relatively moderate to strong inputs from the BSTam fall into five general categories: neuroendocrine system (regions containing pools of magnocellular oxytocin neurons, and parvicellular corticotropin-releasing hormone, thyrotropin-releasing hormone, somatostatin, and dopamine neurons); central autonomic control network (central amygdalar nucleus, descending paraventricular nucleus, and ventrolateral periaqueductal gray); hypothalamic visceromotor pattern generator network (five of six known components); behavior control column (descending paraventricular nucleus and associated arcuate nucleus; ventral tegmental area and associated nucleus accumbens and substantia innominata); and behavioral state control (supramammillary and tuberomammillary nuclei). The BSTam projects lightly to thalamocortical feedback loops (via the medial-midline-intralaminar thalamus). Its pattern of axonal projections, combined with its pattern of neural inputs (the most varied of all BST cell groups), suggests that the BSTam is part of a striatopallidal differentiation involved in coordinating neuroendocrine, autonomic, and behavioral or somatic responses associated with maintaining energy balance homeostasis.

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

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

GABA receptor subtype antagonists in the nucleus accumbens shell and ventral tegmental area differentially alter feeding responses induced by deprivation, glucoprivation and lipoprivation in rats

GABA(A) and GABA(B) receptor agonists stimulate feeding following microinjection into the nucleus accumbens shell and ventral tegmental area, effects blocked selectively and respectively by GABA(A) and GABA(B) receptor antagonists. GABA antagonists also differentially alter opioid-induced feeding responses elicited from these sites. Although GABA agonists and antagonists have been shown to modulate feeding elicited by deprivation or glucoprivation, there has been no systematic examination of feeding elicited by homeostatic challenges following GABA antagonists in these sites. Therefore, the present study examined the dose-dependent ability of GABA(A) (bicuculline, 75-150 ng) and GABA(B) (saclofen, 1.5-3 microg) antagonists administered into the nucleus accumbens shell or ventral tegmental area upon feeding responses elicited by food deprivation (24 h), 2-deoxy-D-glucose-induced glucoprivation (500 mg/kg) or mercaptoacetate-induced lipoprivation (70 mg/kg). A site-specific effect of GABA receptor antagonism was observed for deprivation-induced feeding in that both bicuculline and saclofen administered into the nucleus accumbens shell, but not the ventral tegmental area, produced short-term (1-4 h), but not long-term (24-48 h) effects upon deprivation-induced intake without meaningfully altering body weight recovery. In contrast to the relative inability of GABA receptor antagonism in both sites to alter 2-deoxy-D-glucose-induced intake, mercaptoacetate-induced intake was eliminated by saclofen and significantly reduced by bicuculline in the nucleus accumbens shell and eliminated by both bicuculline and saclofen in the ventral tegmental area. These data reinforce the findings that GABA(A) and GABA(B) receptors in the nucleus accumbens shell and ventral tegmental area are not only important in the modulation of pharmacologically induced feeding responses, but also participate in differentially mediating the short-term feeding response to food deprivation in the nucleus accumbens shell as well strongly modulating lipoprivic, but not glucoprivic feeding responses in both sites.