Karolina P. Skibicka

Ghrelin increases intake of rewarding food in rodents.

We investigated whether ghrelin action at the level of the ventral tegmental area (VTA), a key node in the mesolimbic reward system, is important for the rewarding and motivational aspects of the consumption of rewarding/palatable food. Mice with a disrupted gene encoding the ghrelin receptor (GHS‐R1A) and rats treated peripherally with a GHS‐R1A antagonist both show suppressed intake of rewarding food in a free choice (chow/rewarding food) paradigm. Moreover, accumbal dopamine release induced by rewarding food was absent in GHS‐R1A knockout mice. Acute bilateral intra‐VTA administration of ghrelin increased 1‐hour consumption of rewarding food but not standard chow. In comparison with sham rats, VTA‐lesioned rats had normal intracerebroventricular ghrelin‐induced chow intake, although both intake of and time spent exploring rewarding food was decreased. Finally, the ability of rewarding food to condition a place preference was suppressed by the GHS‐R1A antagonist in rats. Our data support the hypothesis that central ghrelin signaling at the level of the VTA is important for the incentive value of rewarding food.

Ghrelin directly targets the ventral tegmental area to increase food motivation.

Ghrelin, a circulating orexigenic stomach-derived hormone, has recently been implicated in extra-homeostatic feeding, increasing food reward and food-motivated behavior. The precise target site(s) for ghrelins effects on food reward have yet to be elucidated. The neurocircuitry underpinning food-motivated behavior involves, in particular, the dopamine cells of the ventral tegmental area (VTA) that project to the nucleus accumbens (NAcc). Ghrelin stimulation in both of these mesolimbic reward areas increases chow intake. Here we sought to determine if ghrelin acts directly within these mesolimbic reward areas to increase food reward/motivation in studies that combine feeding behavior, pharmacology, and neuroanatomy. We found that motivated behavior for a sucrose reward, assessed in an operant conditioning paradigm in rats, was increased when ghrelin was microinjected directly into the VTA but not into the NAcc. By contrast, ghrelin administration to both areas increased the free feeding of chow. Importantly, in a state of overnight food restriction, where endogenous levels of ghrelin are increased, ghrelin receptor (GHS-R1A) blockade in the VTA was sufficient to decrease the motivation to work for a sugar reward. Blockade of the GHS-R1A in VTA or NAcc was not sufficient to reduce fasting-induced chow hyperphagia. Taken together our data identify the VTA but not the NAcc as a direct, necessary, and sufficient target site for ghrelins action on food motivation.

The Glucagon-Like Peptide 1 (GLP-1) Analogue, Exendin-4, Decreases the Rewarding Value of Food: A New Role for Mesolimbic GLP-1 Receptors

The glucagon-like peptide 1 (GLP-1) system is a recently established target for type 2 diabetes treatment. In addition to regulating glucose homeostasis, GLP-1 also reduces food intake. Previous studies demonstrate that the anorexigenic effects of GLP-1 can be mediated through hypothalamic and brainstem circuits which regulate homeostatic feeding. Here, we demonstrate an entirely novel neurobiological mechanism for GLP-1-induced anorexia in rats, involving direct effects of a GLP-1 agonist, Exendin-4 (EX4) on food reward that are exerted at the level of the mesolimbic reward system. We assessed the impact of peripheral, central, and intramesolimbic EX4 on two models of food reward: conditioned place preference (CPP) and progressive ratio operant-conditioning. Food-reward behavior was reduced in the CPP test by EX4, as rats no longer preferred an environment previously paired to chocolate pellets. EX4 also decreased motivated behavior for sucrose in a progressive ratio operant-conditioning paradigm when administered peripherally. We show that this effect is mediated centrally, via GLP-1 receptors (GLP-1Rs). GLP-1Rs are expressed in several key nodes of the mesolimbic reward system; however, their function remains unexplored. Thus we sought to determine the neurobiological substrates underlying the food-reward effect. We found that the EX4-mediated inhibition of food reward could be driven from two key mesolimbic structures—ventral tegmental area and nucleus accumbens—without inducing concurrent malaise or locomotor impairment. The current findings, that activation of central GLP-1Rs strikingly suppresses food reward/motivation by interacting with the mesolimbic system, indicate an entirely novel mechanism by which the GLP-1R stimulation affects feeding-oriented behavior.

Endogenous leptin signaling in the caudal nucleus tractus solitarius and area postrema is required for energy balance regulation.

Medial nucleus tractus solitarius (mNTS) neurons express leptin receptors (LepRs), and intra-mNTS delivery of leptin reduces food intake and body weight. Here, the contribution of endogenous LepR signaling in mNTS neurons to energy balance control was examined. Knockdown of LepR in mNTS and area postrema (AP) neurons of rats (LepRKD) via adeno-associated virus short hairpin RNA-interference (AAV-shRNAi) resulted in significant hyperphagia for chow, high-fat, and sucrose diets, yielding increased body weight and adiposity. The chronic hyperphagia of mNTS/AP LepRKD rats is likely mediated by a reduction in leptin potentiation of gastrointestinal satiation signaling, as LepRKD rats showed decreased sensitivity to the intake-reducing effects of cholecystokinin. LepRKD rats showed increased basal AMP-kinase activity in mNTS/AP micropunches, and pharmacological data suggest that this increase provides a likely mechanism for their chronic hyperphagia. Overall these findings demonstrate that LepRs in mNTS and AP neurons are required for normal energy balance control.

Role of ghrelin in food reward: impact of ghrelin on sucrose self-administration and mesolimbic dopamine and acetylcholine receptor gene expression

The decision to eat is strongly influenced by non‐homeostatic factors such as food palatability. Indeed, the rewarding and motivational value of food can override homeostatic signals, leading to increased consumption and hence, obesity. Ghrelin, a gut‐derived orexigenic hormone, has a prominent role in homeostatic feeding. Recently, however, it has emerged as a potent modulator of the mesolimbic dopaminergic reward pathway, suggesting a role for ghrelin in food reward. Here, we sought to determine whether ghrelin and its receptors are important for reinforcing motivation for natural sugar reward by examining the role of ghrelin receptor (GHS‐R1A) stimulation and blockade for sucrose progressive ratio operant conditioning, a procedure used to measure motivational drive to obtain a reward. Peripherally and centrally administered ghrelin significantly increased operant responding and therefore, incentive motivation for sucrose. Utilizing the GHS‐R1A antagonist JMV2959, we demonstrated that blockade of GHS‐R1A signaling significantly decreased operant responding for sucrose. We further investigated ghrelins effects on key mesolimbic reward nodes, the ventral tegmental area (VTA) and nucleus accumbens (NAcc), by evaluating the effects of chronic central ghrelin treatment on the expression of genes encoding major reward neurotransmitter receptors, namely dopamine and acetylcholine. Ghrelin treatment was associated with an increased dopamine receptor D5 and acetylcholine receptor nAChRβ2 gene expression in the VTA and decreased expression of D1, D3, D5 and nAChRα3 in the NAcc. Our data indicate that ghrelin plays an important role in motivation and reinforcement for sucrose and impacts on the expression of dopamine and acetylcholine encoding genes in the mesolimbic reward circuitry. These findings suggest that ghrelin antagonists have therapeutic potential for the treatment of obesity and to suppress the overconsumption of sweet food.

Caudal Brainstem Processing Is Sufficient for Behavioral, Sympathetic, and Parasympathetic Responses Driven by Peripheral and Hindbrain Glucagon-Like-Peptide-1 Receptor Stimulation

The effects of peripheral glucagon like peptide-1 receptor (GLP-1R) stimulation on feeding, gastric emptying, and energetic responses involve vagal transmission and central nervous system processing. Despite a lack of studies aimed at determining which central nervous system regions are critical for the GLP-1R response production, hypothalamic/forebrain processing is regarded as essential for these effects. Here the contribution of the caudal brainstem to the control of food intake, core temperature, heart rate, and gastric emptying responses generated by peripheral delivery of the GLP-1R agonist, exendin-4 (Ex-4), was assessed by comparing responses of chronic supracollicular decerebrate (CD) rats to those of pair-fed intact control rats. Responses driven by hindbrain intracerebroventricular (fourth i.c.v) delivery of Ex-4 were also evaluated. Intraperitoneal Ex-4 (1.2 and 3.0 microg/kg) suppressed glucose intake in both CD rats (5.0+/-1.2 and 4.4+/-1.1 ml ingested) and controls (9.4+/-1.5 and 7.7+/-0.8 ml ingested), compared with intakes after vehicle injections (13.1+/-2.5 and 13.2+/-1.7 ml ingested, respectively). Hindbrain ventricular Ex-4 (0.3 microg) also suppressed food intake in CD rats (4.7+/-0.6 ml ingested) and controls (11.0+/-2.9 ml ingested), compared with vehicle intakes (9.3+/-2.1 and 19.3+/-4.3 ml ingested, respectively). Intraperitoneal Ex-4 (0.12, 1.2, 2.4 microg/kg) reduced gastric emptying rates in a dose-related manner similarly for both CD and control rats. Hypothermia followed ip and fourth i.c.v Ex-4 in awake, behaving controls (0.6 and 1.0 C average suppression) and CD rats (1.5 and 2.5 C average suppression). Intraperitoneal Ex-4 triggered tachycardia in both control and CD rats. Results demonstrate that caudal brainstem processing is sufficient for mediating the suppression of intake, core temperature, and gastric emptying rates as well as tachycardia triggered by peripheral GLP-1R activation and also hindbrain-delivered ligand. Contrary to the literature, hypothalamic/forebrain processing and forebrain-caudal brainstem communication is not required for the observed responses.

Hedonic and incentive signals for body weight control

Here we review the emerging neurobiological understanding of the role of the brain’s reward system in the regulation of body weight in health and in disease. Common obesity is characterized by the over-consumption of palatable/rewarding foods, reflecting an imbalance in the relative importance of hedonic versus homeostatic signals. The popular ‘incentive salience theory’ of food reward recognises not only a hedonic/pleasure component (‘liking’) but also an incentive motivation component (‘wanting’ or ‘reward-seeking’). Central to the neurobiology of the reward mechanism is the mesoaccumbal dopamine system that confers incentive motivation not only for natural rewards such as food but also by artificial rewards (eg. addictive drugs). Indeed, this mesoaccumbal dopamine system receives and integrates information about the incentive (rewarding) value of foods with information about metabolic status. Problematic over-eating likely reflects a changing balance in the control exerted by hypothalamic versus reward circuits and/or it could reflect an allostatic shift in the hedonic set point for food reward. Certainly, for obesity to prevail, metabolic satiety signals such as leptin and insulin fail to regain control of appetitive brain networks, including those involved in food reward. On the other hand, metabolic control could reflect increased signalling by the stomach-derived orexigenic hormone, ghrelin. We have shown that ghrelin activates the mesoaccumbal dopamine system and that central ghrelin signalling is required for reward from both chemical drugs (eg alcohol) and also from palatable food. Future therapies for problematic over-eating and obesity may include drugs that interfere with incentive motivation, such as ghrelin antagonists.

Hypothalamic and hindbrain melanocortin receptors contribute to the feeding, thermogenic, and cardiovascular action of melanocortins.

Forebrain ventricular delivery of melanocortin receptor (MC3/4R) agonist increases energy expenditure and decreases food intake (FI). Because forebrain ventricular delivery provides ligand to various anatomically distributed MC3/4R-bearing nuclei, it is unclear which of the receptor subpopulations contributes to the feeding suppression and the sympathetic-thermogenic effects observed. The literature indicates that reexpression of MC4R in the paraventricular nucleus (PVH) affects the feeding but not the energetic phenotype of the MC4R knockout, suggesting that divergent MC4R populations mediate energy expenditure (hindbrain) and FI (hypothalamus) effects of stimulation. Not consistent with this view are data indicating that PVH sympathetic projection neurons express MC4Rs and that feeding effects are induced from hindbrain MC4R sites. Therefore, we hypothesize an opposing perspective: that stimulation of anatomically diverse MC3/4R-bearing nuclei triggers energetic as well as feeding effects. To test this hypothesis, ventricle subthreshold doses of MC3/4R agonist (5 and 10 pmol) were applied in separate experiments to six hindbrain and hypothalamic sites; core temperature (Tc), heart rate (HR), spontaneous activity (SPA), and FI were measured in behaving rats. Nucleus tractus solitarius and PVH stimulation increased Tc, HR, and SPA and decreased FI. Rostral ventrolateral medulla, parabrachial nucleus, and retrochiasmatic area stimulation increased Tc, HR, but not SPA, and decreased FI. The response profile differed to some extent for each nucleus tested, suggesting differential output circuitries for the measured parameters. Data are consistent with the view that energetic and feeding responses are not controlled by regionally divergent MC3/4Rs and can be elicited from multiple, anatomically distributed MC3/4R populations.

Dorsal Hindbrain 5′-Adenosine Monophosphate-Activated Protein Kinase as an Intracellular Mediator of Energy Balance

The fuel-sensing enzyme AMP-activated protein kinase (AMPK) has been implicated in central nervous system control of energy balance. Hypothalamic AMPK activity is increased by food deprivation, and this elevation is inhibited by refeeding or by leptin treatment. The contribution of extrahypothalamic AMPK activity in energy balance control has not been addressed. Here, we investigate the effects of physiological state on the AMPK activity in hindbrain nucleus tractus solitarius (NTS) neurons because treatments that reduce energy availability in these neurons trigger behavioral, endocrine, and autonomic responses to restore energy balance. Food-deprived rats showed significantly increased AMPK activity in both NTS- and hypothalamus-enriched lysates compared with those that were ad libitum fed. Pharmacological inhibition of AMPK activity in medial NTS neurons, by intraparenchymal injection of compound C, suppressed food intake and body weight gain compared with vehicle. Fourth ventricle (4th i.c.v.) compound C delivery increased heart rate and spontaneous activity in free-moving rats. Suppression of AMPK activity has been implicated in leptins anorectic action in the hypothalamus. Given the role of leptin signaling in food intake inhibition within the medial NTS, we also examined whether stimulation of hindbrain AMPK by 4th i.c.v. administration of 5-aminoimidazole-4-carboxamide-riboside (AICAR), an AMP-mimicking promoter of AMPK activity, could attenuate the inhibition of food intake by 4th i.c.v. leptin. The intake-suppressive effects of leptin (at 2 and 4 h) were completely reversed by AICAR. We conclude that 1) hindbrain AMPK activity contributes to energy balance control through regulation of food intake and energy expenditure, 2) leptins intake-reducing effects in the NTS are mediated by AMPK, and 3) central nervous system AMPK controls whole-body homeostasis at anatomically distributed sites across the neuraxis.

Glucagon-like peptide 1 receptor induced suppression of food intake, and body weight is mediated by central IL-1 and IL-6

Significance There is a growing interest in the gut- and hindbrain-produced hormone glucagon-like peptide 1 (GLP-1), and GLP-1–targeting drugs are in clinical trials for treatment of obesity, and already in the clinic for treatment of type 2 diabetes. Therefore, the implications of information arising from our study are clinically relevant and considerable. GLP-1 receptor stimulation decreases feeding and body weight likely via the CNS, effects of unquestioned scientific and clinical importance, considering the alarming rates of obesity. Despite this, there is scarce information about the mediators and mechanisms behind the effects of GLP-1. In this study, we found surprising evidence that two cytokines, interleukin-6 and interleukin-1, mediate antiobesity effects of GLP-1 receptor stimulation at the level of the CNS. Glucagon-like peptide 1 (GLP-1), produced in the intestine and the brain, can stimulate insulin secretion from the pancreas and alleviate type 2 diabetes. The cytokine interleukin-6 (IL-6) may enhance insulin secretion from β-cells by stimulating peripheral GLP-1 production. GLP-1 and its analogs also reduce food intake and body weight, clinically beneficial actions that are likely exerted at the level of the CNS, but otherwise are poorly understood. The cytokines IL-6 and interleukin 1β (IL-1β) may exert an anti-obesity effect in the CNS during health. Here we found that central injection of a clinically used GLP-1 receptor agonist, exendin-4, potently increased the expression of IL-6 in the hypothalamus (11-fold) and the hindbrain (4-fold) and of IL-1β in the hypothalamus, without changing the expression of other inflammation-associated genes. Furthermore, hypothalamic and hindbrain interleukin-associated intracellular signals [phosphorylated signal transducer and activator of transcription-3 (pSTAT3) and suppressor of cytokine signaling-1 (SOCS1)] were also elevated by exendin-4. Pharmacologic disruption of CNS IL-1 receptor or IL-6 biological activity attenuated anorexia and body weight loss induced by central exendin-4 administration in a rat. Simultaneous blockade of IL-1 and IL-6 activity led to a more potent attenuation of exendin-4 effects on food intake. Mice with global IL-1 receptor gene knockout or central IL-6 receptor knockdown showed attenuated decrease in food intake and body weight in response to peripheral exendin-4 treatment. GLP-1 receptor activation in the mouse neuronal Neuro2A cell line also resulted in increased IL-6 expression. These data outline a previously unidentified role of the central IL-1 and IL-6 in mediating the anorexic and body weight loss effects of GLP-1 receptor activation.