Dopamine and learned food preferences

Transcutaneous Vagus Nerve Stimulation Effects on Flavor-Evoked Electroencephalogram and Eye-Blink Rate

INTRODUCTION

Chemosensory food signals are carried by the vagus nerve (VN) from the gut to the brain and these signals contribute to communicating fullness and caloric value of the consumed food in regulatory and reward-related contexts. Here, we aimed to explore whether neural responses to flavor can be modulated through noninvasive VN stimulation, which can be done transcutaneously (transcutaneous vagus nerve stimulation [tVNS]) on the outer ear via the auricular branch of VN. The ideal stimulation location on the outer ear for tVNS is not agreed on but two candidate locations are cymba conchae and tragus.

METHODS

In this study, we explore the optimal stimulation location for tVNS (cymba conchae, tragus, and cymba conchae and tragus) and timing of tVNS relative to chocolate milk presentation (during, after) in a within-participants design (15 participants). We examined various measures of efficacy; event-related potential from electroencephalogram, eye-blink rate, perceptual and hedonic aspects of flavor, swallowing behavior, and consumption behavior.

RESULTS

We observed no effect of stimulation location on any of the dependent variables. Unexpectedly, we observed a large effect of food consumption on spontaneous eye-blink rate.

CONCLUSION

In conclusion, overall we did not observe a clear optimal ear location for tVNS-induced modulation of neurophysiological, perceptual, and behavioral variables. Future studies may confirm whether spontaneous eye-blink rate can be a sensitive proxy for food reward-related phasic dopamine shifts.

Deciphering visceral instincts: a scientific quest to unravel food choices from molecules to mind

The study of biological mechanisms, while crucial, cannot fully explain complex phenomena like the instinct to eat. The mind-body connection, as exemplified by the concept of "voodoo death," highlights the profound influence of belief and cultural context on physiology. Indigenous knowledge systems further emphasize the interconnectedness of humans with their environment. Recent discoveries in gut-brain communication reveal the intricate neural circuits that drive our visceral desires, but a holistic approach that integrates both physiological mechanisms and the subjective experience of life, informed by diverse cultural perspectives, will be essential to truly understand what it means to be alive.

AgRP neuron activity promotes associations between sensory and nutritive signals to guide flavor preference

OBJECTIVE

The learned associations between sensory cues (e.g., taste, smell) and nutritive value (e.g., calories, post-ingestive signaling) of foods powerfully influences our eating behavior [1], but the neural circuits that mediate these associations are not well understood. Here, we examined the role of agouti-related protein (AgRP)-expressing neurons - neurons which are critical drivers of feeding behavior [2; 3] - in mediating flavor-nutrient learning (FNL).

METHODS

Because mice prefer flavors associated with AgRP neuron activity suppression [4], we examined how optogenetic stimulation of AgRP neurons during intake influences FNL, and used fiber photometry to determine how endogenous AgRP neuron activity tracks associations between flavors and nutrients.

RESULTS

We unexpectedly found that tonic activity in AgRP neurons during FNL potentiated, rather than prevented, the development of flavor preferences. There were notable sex differences in the mechanisms for this potentiation. Specifically, in male mice, AgRP neuron activity increased flavor consumption during FNL training, thereby strengthening the association between flavors and nutrients. In female mice, AgRP neuron activity enhanced flavor-nutrient preferences independently of consumption during training, suggesting that AgRP neuron activity enhances the reward value of the nutrient-paired flavor. Finally, in vivo neural activity analyses demonstrated that acute AgRP neuron dynamics track the association between flavors and nutrients in both sexes.

CONCLUSIONS

Overall, these data (1) demonstrate that AgRP neuron activity enhances associations between flavors and nutrients in a sex-dependent manner and (2) reveal that AgRP neurons track and rapidly update these associations. Taken together, our findings provide new insight into the role of AgRP neurons in assimilating sensory and nutritive signals for food reinforcement.

AgRP neuron activity promotes associations between sensory and nutritive signals to guide flavor preference

Objective

The learned associations between sensory cues (e.g., taste, smell) and nutritive value (e.g., calories, post-ingestive signaling) of foods powerfully influences our eating behavior [1], but the neural circuits that mediate these associations are not well understood. Here, we examined the role of agouti-related protein (AgRP)-expressing neurons - neurons which are critical drivers of feeding behavior [2; 3] - in mediating flavor-nutrient learning (FNL).

Methods

Because mice prefer flavors associated with AgRP neuron activity suppression [4], we examined how optogenetic stimulation of AgRP neurons during intake influences FNL, and used fiber photometry to determine how endogenous AgRP neuron activity tracks associations between flavors and nutrients.

Results

We unexpectedly found that tonic activity in AgRP neurons during FNL potentiated, rather than prevented, the development of flavor preferences. There were notable sex differences in the mechanisms for this potentiation. Specifically, in male mice, AgRP neuron activity increased flavor consumption during FNL training, thereby strengthening the association between flavors and nutrients. In female mice, AgRP neuron activity enhanced flavor-nutrient preferences independently of consumption during training, suggesting that AgRP neuron activity enhances the reward value of the nutrient-paired flavor. Finally, in vivo neural activity analyses demonstrated that acute AgRP neuron dynamics track the association between flavors and nutrients in both sexes.

Conclusions

Overall, these data (1) demonstrate that AgRP neuron activity enhances associations between flavors and nutrients in a sex-dependent manner and (2) reveal that AgRP neurons track and update these associations on fast timescales. Taken together, our findings provide new insight into the role of AgRP neurons in assimilating sensory and nutritive signals for food reinforcement.

Actions and Consequences of Insulin in the Striatum

Insulin crosses the blood–brain barrier to enter the brain from the periphery. In the brain, insulin has well-established actions in the hypothalamus, as well as at the level of mesolimbic dopamine neurons in the midbrain. Notably, insulin also acts in the striatum, which shows abundant expression of insulin receptors (InsRs) throughout. These receptors are found on interneurons and striatal projections neurons, as well as on glial cells and dopamine axons. A striking functional consequence of insulin elevation in the striatum is promoting an increase in stimulated dopamine release. This boosting of dopamine release involves InsRs on cholinergic interneurons, and requires activation of nicotinic acetylcholine receptors on dopamine axons. Opposing this dopamine-enhancing effect, insulin also increases dopamine uptake through the action of insulin at InsRs on dopamine axons. Insulin acts on other striatal cells as well, including striatal projection neurons and astrocytes that also influence dopaminergic transmission and striatal function. Linking these cellular findings to behavior, striatal insulin signaling is required for the development of flavor–nutrient learning, implicating insulin as a reward signal in the brain. In this review, we discuss these and other actions of insulin in the striatum, including how they are influenced by diet and other physio-logical states.

Lesions of nucleus accumbens shell abolish socially transmitted food preferences

Rats adapt their food choices to conform to their conspecifics' dietary preferences. The nucleus accumbens shell is a relevant brain region to process reward-related and motivated behaviours and social information. Here, we hypothesize that the integrity of the nucleus accumbens shell is necessary to show socially transmitted food preferences. We made excitotoxic and sham lesions of nucleus accumbens shell in male Long-Evans rats who performed a social transmission of food preference task. In this task, observer rats revealed their original preference for one out of two food options. Afterward, they were exposed to a demonstrator rat who was fed with the observer's originally non-preferred food, and the observer's food choices were sampled again. Sham lesioned observer rats changed their food preferences following interaction with the demonstrator, specifically by increasing the intake of their originally non-preferred food type. This interaction-related change in preference was not found after nucleus accumbens shell lesions. The lesion effects on choice were not the consequence of impaired social or non-social motivation, anxiety or sensory or motor function, suggesting that they reflected a genuine deficit in social reward revaluation. These results highlight the role of nucleus accumbens shell in revaluating food rewards to match a conspecific's preferences.

Tasting rewards. Effects of orosensory sweet signals on human error processing

Human research has shown interactions between rewards and cognitive control. In animal models of affective neuroscience, reward administration typically involves administering orosensory sugar signals (OSS) during caloric-deprived states. We adopted this procedure to investigate neurophysiological mechanisms of reward-cognitive control interactions in humans. We predicted that OSS would affect neurophysiological and behavioral indices of error processing oppositely, depending on the relative weight of the OSS-induced 'wanting' and 'liking' components of reward. We, therefore, conducted a double-blind, non-nutritive sweetener-controlled study with a within-subject design. Fasted (16 hr) participants (N = 61) performed a modified Flanker task to assess neurophysiological (error-related negativity [N_e/ERN]) and behavioral (post-error adaptations) measures of error processing. Non-contingent to task performance, we repeatedly administered either a sugar (glucose) or non-nutritive sweetener (aspartame) solution, which had to be expulsed after short oral stimulation to prevent post-oral effects. Consistent with our hypothesis on how 'liking' would affect N_e/ERN amplitude, we found the latter to be decreased for sugar compared to aspartame. Unexpectedly, we found post-error accuracy, instead of post-error slowing, to be reduced by sugar relative to aspartame. Our findings suggest that OSS may interact with error processing through the 'liking' component of rewards. Adopting our reward-induction procedure (i.e. administering OSS in a state of high reward sensitivity [i.e. fasting], non-contingent to task performance) might help future research investigating the neural underpinnings of reward-cognitive control interactions in humans.

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

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

Disruption of lateral hypothalamic calorie detection by a free choice high fat diet

During the last few decades, the consumption of low-calorie sweeteners, as a substitute for caloric sweeteners, has sharply increased. Although research shows that caloric versus low-calorie sweeteners can have differential effects on the brain, it is unknown which neuronal populations are responsible for detecting the difference between the two types of sweeteners. Using in vivo two-photon calcium imaging, we investigated how drinking sucrose or sucralose (a low-calorie sweetener) affects the activity of glutamatergic neurons in the lateral hypothalamus. Furthermore, we explored the consequences of consuming a free-choice high fat diet on the calorie detection abilities of these glutamatergic neurons. We found that glutamatergic neurons indeed can discriminate sucrose from water and sucralose, and that consumption of a free-choice high fat diet shifts the glutamatergic neuronal response from sucrose-specific to sucralose-specific, thereby disrupting calorie detection. These results highlight the disruptive effects of a diet high in saturated fat on calorie detection in the lateral hypothalamus.

Effect of hippocampal 6-OHDA lesions on the contextual modulation of taste recognition memory

Taste recognition memory is evident in rodents because the initial neophobia to novel tastes attenuates across exposures as the taste becomes familiar and safe. This attenuation of taste neophobia (AN) is context-dependent and an auditory background change could induce the recovery of the neophobic response. The AN auditory context-dependency requires the hippocampal integrity but the neurochemical mechanisms underlying the interaction with the taste memory circuit remain unexplored. We have applied pharmacological intervention by 6-hidroxydopamine (6-OHDA) hippocampal lesion for assessing the role of catecholamines in the hippocampal system to Wistar rats that drank a novel 3% vinegar solution for several consecutive days. Additionally, we manipulated the auditory background as a context that could either change or remain constant across all the drinking sessions. We found that a disruption of the context-dependent AN was induced by intracerebral administration of 6-OHDA targeted to the ventral CA1 hippocampus (vCA1). We conclude that the ability of the auditory context to modulate taste recognition memory involves the catecholaminergic activity in the ventral hippocampal circuit for the proper acquisition of safe taste memory.

Reconsolidation of a post-ingestive nutrient memory requires mTOR in the central amygdala

The central control of feeding behavior and metabolic homeostasis has been proposed to involve a form of post-ingestive nutrient learning independent of the gustatory value of food. However, after such learning, it is unknown which brain regions or circuits are activated to retrieve the stored memory and whether this memory undergoes reconsolidation that depends on protein synthesis after its reactivation through retrieval. In the present study, using a conditioned-flavor-preference paradigm by associating flavors with intra-gastric infusion of glucose to minimize the evaluation of the taste of food, we show that retrieval of the post-ingestive nutrient-conditioned flavor memory stimulates multiple brain regions in mice, including the central nucleus of the amygdala (CeA). Moreover, memory retrieval activated the mammalian target of rapamycin complex 1 (mTORC1) in the CeA, while site-specific or systemic inhibition of mTORC1 immediately after retrieval prevented the subsequent expression of the post-ingestive nutrient-associated flavor memory, leading to a long-lasting suppression of reinstatement. Taken together, our findings suggest that the reconsolidation process of a post-ingestive nutrient memory modulates food preferences.

Protein Appetite Drives Macronutrient-Related Differences in Ventral Tegmental Area Neural Activity

Control of protein intake is essential for numerous biological processes as several amino acids cannot be synthesized de novo, however, its neurobiological substrates are still poorly understood. In the present study, we combined in vivo fiber photometry with nutrient-conditioned flavor in a rat model of protein appetite to record neuronal activity in the VTA, a central brain region for the control of food-related processes. In adult male rats, protein restriction increased preference for casein (protein) over maltodextrin (carbohydrate). Moreover, protein consumption was associated with a greater VTA response, relative to carbohydrate. After initial nutrient preference, a switch from a normal balanced diet to protein restriction induced rapid development of protein preference but required extensive exposure to macronutrient solutions to induce elevated VTA responses to casein. Furthermore, prior protein restriction induced long-lasting food preference and VTA responses. This study reveals that VTA circuits are involved in protein appetite in times of need, a crucial process for animals to acquire an adequate amount of protein in their diet.SIGNIFICANCE STATEMENT Acquiring insufficient protein in one's diet has severe consequences for health and ultimately will lead to death. In addition, a low level of dietary protein has been proposed as a driver of obesity as it can leverage up intake of fat and carbohydrate. However, much remains unknown about the role of the brain in ensuring adequate intake of protein. Here, we show that in a state of protein restriction a key node in brain reward circuitry, the VTA, is activated more strongly during consumption of protein than carbohydrate. Moreover, although rats' behavior changed to reflect new protein status, patterns of neural activity were more persistent and only loosely linked to protein status.

Monitoring In Vivo Neural Activity to Understand Gut-Brain Signaling

Appropriate food intake requires exquisite coordination between the gut and the brain. Indeed, it has long been known that gastrointestinal signals communicate with the brain to promote or inhibit feeding behavior. Recent advances in the ability to monitor and manipulate neural activity in awake, behaving rodents has facilitated important discoveries about how gut signaling influences neural activity and feeding behavior. This review emphasizes recent studies that have advanced our knowledge of gut-brain signaling and food intake control, with a focus on how gut signaling influences in vivo neural activity in animal models. Moving forward, dissecting the complex pathways and circuits that transmit nutritive signals from the gut to the brain will reveal fundamental principles of energy balance, ultimately enabling new treatment strategies for diseases rooted in body weight control.

Age-dependent effects of protein restriction on dopamine release

Despite the essential role of protein intake for health and development, very little is known about the impact of protein restriction on neurobiological functions, especially at different stages of the lifespan. The dopamine system is a central actor in the integration of food-related processes and is influenced by physiological state and food-related signals. Moreover, it is highly sensitive to dietary effects during early life periods such as adolescence due to its late maturation. In the present study, we investigated the impact of protein restriction either during adolescence or adulthood on the function of the mesolimbic (nucleus accumbens) and nigrostriatal (dorsal striatum) dopamine pathways using fast-scan cyclic voltammetry in rat brain slices. In the nucleus accumbens, protein restriction in adults increased dopamine release in response to low and high frequency trains of stimulation (1-20 Hz). By contrast, protein restriction during adolescence decreased nucleus accumbens dopamine release. In the dorsal striatum, protein restriction at adulthood has no impact on dopamine release but the same diet during adolescence induced a frequency-dependent increase in stimulated dopamine release. Taken together, our results highlight the sensitivity of the different dopamine pathways to the effect of protein restriction, as well as their vulnerability to deleterious diet effects at different life stages.

tVNS Increases Liking of Orally Sampled Low-Fat Foods: A Pilot Study

Recently a role for the vagus nerve in conditioning food preferences was established in rodents. In a prospective controlled clinical trial in humans, invasive vagus nerve stimulation shifted food choice toward lower fat content. Here we explored whether hedonic aspects of an orally sampled food stimulus can be modulated by non-invasive transcutaneous vagus nerve stimulation (tVNS) in humans. In healthy participants (n = 10, five women, 20-32 years old, no obesity) we tested liking and wanting ratings of food samples with varying fat or sugar content with or without tVNS in a sham-controlled within-participants design. To determine effects of tVNS on food intake, we also measured voluntary consumption of milkshake. Spontaneous eye blink rate was measured as a proxy for dopamine tone. Liking of low-fat, but not high-fat puddings, was higher for tVNS relative to sham stimulation. Other outcomes showed no differences. These findings support a role for the vagus nerve promoting post-ingestive reward signals. Our results suggest that tVNS may be used to increase liking of low-calorie foods, which may support healthier food choices.

Amylin brain circuitry

Amylin is a peptide hormone that is mainly known to be produced by pancreatic β-cells in response to a meal but amylin is also produced by brain cells in discrete brain areas albeit in a lesser amount. Amylin receptor (AMY) is composed of the calcitonin core-receptor (CTR) and one of the 3 receptor activity modifying protein (RAMP), thus forming AMY1-3; RAMP enhances amylin binding properties to the CTR. However, amylin receptor agonist such as salmon calcitonin is able to bind CTR alone. Peripheral amylin's main binding site is located in the area postrema (AP) which then propagate the signal to the nucleus of the solitary tract and lateral parabrachial nucleus (LPBN) and it is then transmitted to the forebrain areas such as central amygdala and bed nucleus of the stria terminalis. Amylin's activation of these different brain areas mediates eating and other metabolic pathways controlling energy expenditure and glucose homeostasis. Peripheral amylin can also bind in the arcuate nucleus of the hypothalamus where it acts independently of the AP to activate POMC and NPY neurons. Amylin activation of NPY neurons has been shown to be transmitted to LPBN neurons to act on eating while amylin POMC signaling affects energy expenditure and locomotor activity. While a large amount of experiments have already been conducted, future studies will have to further investigate how amylin is taken up by forebrain areas and deepen our understanding of amylin action on peripheral metabolism.

Is obesity related to enhanced neural reactivity to visual food cues? A review and meta-analysis

Theoretical work suggests that obesity is related to enhanced incentive salience of food cues. However, evidence from both behavioral and neuroimaging studies on the topic is mixed. In this work we review the literature on cue reactivity in obesity and perform a preregistered meta-analysis of studies investigating effects of obesity on brain responses to passive food pictures viewing. Further, we examine whether age influences brain responses to food cues in obesity. In the meta-analysis we included 13 studies of children and adults that investigated group differences (obese vs. lean) in responses to food vs. non-food pictures viewing. While we found no significant differences in the overall meta-analysis, we show that age significantly influences brain response differences to food cues in the left insula and the left fusiform gyrus. In the left insula, obese vs. lean brain differences in response to food cues decreased with age, while in the left fusiform gyrus the pattern was opposite. Our results suggest that there is little evidence for obesity-related differences in responses to food cues and that such differences might be mediated by additional factors that are often not considered.

FGF21 Signals to Glutamatergic Neurons in the Ventromedial Hypothalamus to Suppress Carbohydrate Intake

Fibroblast growth factor 21 (FGF21) is an endocrine hormone produced by the liver that regulates nutrient and metabolic homeostasis. FGF21 production is increased in response to macronutrient imbalance and signals to the brain to suppress sugar intake and sweet-taste preference. However, the central targets mediating these effects have been unclear. Here, we identify FGF21 target cells in the hypothalamus and reveal that FGF21 signaling to glutamatergic neurons is both necessary and sufficient to mediate FGF21-induced sugar suppression and sweet-taste preference. Moreover, we show that FGF21 acts directly in the ventromedial hypothalamus (VMH) to specifically regulate sucrose intake, but not non-nutritive sweet-taste preference, body weight, or energy expenditure. Finally, our data demonstrate that FGF21 affects neuronal activity by increasing activation and excitability of neurons in the VMH. Thus, FGF21 signaling to glutamatergic neurons in the VMH is an important component of the neurocircuitry that functions to regulate sucrose intake.

Accumbens and amygdala in taste recognition memory: The role of d1 dopamine receptors

The attenuation of taste neophobia (AN) is a good model for studying the structural and neurochemical mechanisms of the emotional component of memory because taste recognition memory exhibits the unique feature of being necessarily linked to hedonic properties. Whilst novel tastes elicit cautious neophobic responses, taste exposures which are not followed by aversive consequences attenuate neophobia as the taste becomes safe and palatable. Given the involvement of the nucleus accumbens in reward and of the amygdala in emotional memories, we applied c-Fos immunohistochemistry as an index of neural activity in Wistar rats that were exposed to a vinegar solution for one, two or six days. An inverse pattern of accumbens nucleus vs amygdala activity was found on the second exposure day on which AN occurred. The number of c-Fos positive cells in the nucleus accumbens shell increased whilst the number of c-Fos positive cells in the basolateral amygdala decreased. Further analyses revealed a positive correlation between AN and the number of c-Fos positive cells in the accumbens shell but a negative correlation in the basolateral amygdala. Furthermore the accumbens-amygdala interplay relevant for AN seems to be mediated by dopamine D1 receptors (D1DR). The injection of SCH23390 (D1DR antagonist) in both the accumbens shell and the basolateral amygdala on the second taste exposure resulted in selectively impaired AN but had opposite long term effects. This finding supports the relevance of a dopaminergic network mediated by D1DRs in the nucleus accumbens shell and basolateral amygdala which is critical for adding the emotional component during the formation of taste memory.

Layers of signals that regulate appetite

All meals come to an end. This is because eating and drinking generate feedback signals that communicate to the brain what and how much has been consumed. Here we review our current understanding of how these feedback signals regulate appetite. We first describe classic studies that surgically manipulated the gastrointestinal tract and measured the effects on behavior. We then highlight recent experiments that have used in vivo neural recordings to directly observe how ingestion modulates circuit dynamics in the brain. A general theme emerging from this work is that eating and drinking generate layers of feedback signals, arising sequentially from different tissues in the body, that converge on individual neurons in the forebrain to regulate hunger and thirst.

Effects of different sweeteners on behavior and neurotransmitters release in mice

Four natural sweeteners (sucrose, stevioside, maltose and xylitol) and six artificial sweeteners (acesulfame, sucralose, aspartame, cyclamate, saccharin and neotame) were used to study the effects of different sweeteners on the behavior and neurotransmitter release of mice with two-bottle preference experiments. The results showed that very significant preference behavior for 8% sucrose solution, 0.3% stevioside solution, 10 mM acesulfame, 10 mM sucralose and 10 mM aspartame solutions (p < 0.01) was observed on mice. Long-term exposure of sucrose solution and acesulfame solution can affect the behavioral indicators such as solution consumption, feed intake, body weight and the release of neurotransmitters in mice. The solution consumption and the release of neurotransmitters were significantly greater (p < 0.05) than that of the control group (water group), but there was no significant difference in feed intake. The acesulfame-A and acesulfame-B groups had no significant difference on the consumption of solution and feed intake, but there was significant difference in the release of neurotransmitters. The result also showed that different sweetener solutions with similar sweetness had the same effect on the neurotransmitters release, and it can be inferred that mice have an addictive behavioral characteristic to sucrose.

Sugar Metabolism Regulates Flavor Preferences and Portal Glucose Sensing

In most species, including humans, food preference is primarily controlled by nutrient value. In particular, glucose-containing sugars exert exquisitely strong effects on food choice via gut-generated signals. However, the identity of the visceral signals underlying glucose’s rewarding effects remains uncertain. In particular, it is unknown whether sugar metabolism mediates the formation of preferences for glucose-containing sugars. Using the mouse as a model organism, we made use of a combination of conditioning schedules, gastrointestinal nutrient administration, and chromatographic/electrochemical methods to assess the behavioral and neural effects of activating the gut with either metabolizable glucose or a non-metabolizable glucose analog. We show that mice display much superior preferences for flavors associated with intra-gastric infusions of glucose compared to flavors associated with intra-gastric infusions of the non-metabolizable glucose analog α-methyl-D-glucopyranoside (“MDG,” an activator of intestinal sodium/glucose co-transporters). These effects were unaffected by surgical bypassing of the duodenum, suggesting glucose-specific post-absorptive sensing mechanisms. Consistently, intra-portal infusions of glucose, but not of MDG, induced significant rises in dopamine (DA) levels within brain reward circuits. Our data reveal that the unmatched rewarding effects of glucose-containing sugars cannot be accounted for by metabolism-independent activation of sodium/glucose cotransporters; rather, they point to glucose metabolism as the physiological mechanism underlying the potent reward value of sugar-sweetened flavored beverages. In particular, no circulating “gut factors” need to be invoked to explain the reward value of ingested glucose. Thus, instead of circulating gut hormones, portal-mesenteric sensing of glucose emerges as the preferential physiological pathway for sugar reward.

The artificial sweetener Splenda intake promotes changes in expression of c-Fos and NeuN in hypothalamus and hippocampus of rats

BACKGROUND

Obesity is the result of the interaction of multiple variables, including the excessive increase of sugar-sweetened beverages consumption. Diets aimed to treat obesity have suggested the use of artificial sweeteners. However, recent evidence has shown several health deficits after intake of artificial sweeteners, including effects in neuronal activity. Therefore, the influence of artificial sweeteners consumption such as Splenda, on the expression of c-Fos and neuronal nuclear protein (NeuN) in hypothalamus and hippocampus remains to be determined.

OBJECTIVES

We investigated the effects on c-Fos or NeuN expression in hypothalamus and hippocampus of Splenda-treated rats.

METHODS

Splenda was diluted in water (25, 75 or 250 mg/100 mL) and orally given to rats during 2 weeks ad libitum. Next, animals were sacrificed by decapitation and brains were collected for analysis of c-Fos or NeuN immunoreactivity.

RESULTS

Consumption of Splenda provoked an inverted U-shaped dose-effect in c-Fos expression in ventromedial hypothalamic nucleus while similar findings were observed in dentate gyrus of hippocampus. In addition, NeuN immunoreactivity was enhanced in ventromedial hypothalamic nucleus at 25 or 75 mg/100 mL of Splenda intake whereas an opposite effect was observed at 250 mg/100 mL of artificial sweetener consumption. Lastly, NeuN positive neurons were increased in CA2/CA3 fields of hippocampus from Splenda-treated rats (25, 75 or 250 mg/100 mL).

CONCLUSION

Consuming Splenda induced effects in neuronal biomarkers expression. To our knowledge, this study is the first description of the impact of intake Splenda on c-Fos and NeuN immunoreactivity in hypothalamus and hippocampus in rats.

Restriction of dietary protein leads to conditioned protein preference and elevated palatability of protein-containing food in rats

The mechanisms by which intake of dietary protein is regulated are poorly understood despite their potential involvement in determining food choice and appetite. In particular, it is unclear whether protein deficiency results in a specific appetite for protein and whether influences on diet are immediate or develop over time. To determine the effects of protein restriction on consumption, preference, and palatability for protein we assessed patterns of intake for casein (protein) and maltodextrin (carbohydrate) solutions in adult rats. To induce a state of protein restriction, rats were maintained on a low protein diet (5% casein) and compared to control rats on non-restricted diet (20% casein). Under these dietary conditions, relative to control rats, protein-restricted rats exhibited hyperphagia without weight gain. After two weeks, on alternate conditioning days, rats were given access to either isocaloric casein or maltodextrin solutions that were saccharin-sweetened and distinctly flavored whilst consumption and licking patterns were recorded. This allowed rats to learn about the post-ingestive nutritional consequences of the two different solutions. Subsequently, during a preference test when rats had access to both solutions, we found that protein-restricted rats exhibited a preference for casein over carbohydrate whereas non-restricted rats did not. Analysis of lick microstructure revealed that this preference was associated with an increase in cluster size and number, reflective of an increase in palatability. In conclusion, protein-restriction induced a conditioned preference for protein, relative to carbohydrate, and this was associated with increased palatability.

The Role of Dorsal Hippocampal Dopamine D1-Type Receptors in Social Learning, Social Interactions, and Food Intake in Male and Female Mice

The neurobiological mechanisms underlying social learning (ie, in which an animal's learning is influenced by another) are slowly being unraveled. Previous work with systemic treatments shows that dopamine (DA) D1-type receptors mediate social learning in the social transmission of food preferences (STFP) in mice. This study examines the involvement of one brain region underlying this effect. The ventral tegmental area has dopaminergic projections to many limbic structures, including the hippocampus-a site important for social learning in the STFP in rodents. In this study, adult male and female CD-1 mice received a dorsal hippocampal microinfusion of the D1-like receptor antagonist SCH23390 at 1, 2, 4, or 6 μg/μl 15 min before a 30 min social interaction with a same-sex conspecific, in which mice had the opportunity to learn a socially transmitted food preference. Results show that social learning was blocked in female mice microinfused with 6 μg/μl, and in males infused with 1, 4, or 6 μg/μl of SCH23390. This social learning impairment could not be explained by changes in total food intake, or olfactory discrimination. A detailed analysis of the social interactions also revealed that although SCH23390 did not affect oronasal investigation for either sex, drug treatments affected other social behaviors in a sex-specific manner; there was primarily a reduction in agonistic-related behaviors among males, and social investigatory-related behaviors among females. Thus, this study shows that dorsal hippocampal D1-type receptors mediate social learning and social behaviors in male and female mice.

Rewarding Effects of Operant Dry-Licking Behavior on Neuronal Firing in the Nucleus Accumbens Core

Certain eating behaviors are characterized by a trend of elevated food consumption. However, neural mechanisms mediating the motivation for food consumption are not fully understood. Food impacts the brain-rewarding-system via both oral-sensory and post-ingestive information. Recent studies have reported an important role of visceral gut information in mediating dopamine (DA) release in the brain rewarding system. This is independent of oral sensation, suggesting a role of the gut-brain-DA-axis in feeding behavior. In this study, we investigated the effects of intra-gastric (IG) self-administration of glucose on neuronal firings in the nucleus accumbens (NA) of water-deprived rats. Rats were trained in an operant-licking paradigm. During training, when the light was on for 2 min (light-period), rats were required to lick a spout to acquire the water oral-intake learning, and either an IG self-infusion of 0.4 M glucose (GLU group) or water (H₂O group). Rats rested in the dark-period (3 min) following the light-period. Four cycles of the operant-licking paradigm consisting of the light–dark periods were performed per day, for 4 consecutive days. In the test session, the same rats licked the same spout to acquire the IG self-administration of the corresponding solutions, without oral water ingestion (dry licking). Behavioral results indicated IG self-administration of glucose elicits more dry-licking behavior than that of water. Neurophysiological results indicated in the dark period, coefficient of variance (CV) measuring the inter-spike interval variability of putative medial spiny neurons (pMSNs) in the NA was reduced in the H₂O group compared to the GLU group, while there was no significant difference in physical behaviors in the dark period between the two groups. Since previous studies reported that DA release increases CV of MSNs, the present results suggest that greater CV of pMSNs in the GLU group reflects greater DA release in the NA and elevated motivation in the GLU group, which might increase lickings in the test session in the GLU group compared to the H₂O group.

Acquisition and expression of fat-conditioned flavor preferences are differentially affected by NMDA receptor antagonism in BALB/c and SWR mice

Conditioned flavor preferences are elicited by fat (Intralipid) in inbred mouse strains with BALB/c and SWR mice displaying among the most robust preferences. Dopamine D₁ and opioid receptor antagonism differentially reduces the acquisition (learning) and expression (maintenance) of fat-conditioned flavor preferences in these two strains. Because noncompetitive NMDA receptor antagonism with MK-801 differentially altered sugar-conditioned flavor preferences in these strains, and because NMDA receptors are involved in fat intake, the present study examined whether MK-801 differentially altered expression and acquisition of fat (Intralipid)-conditioned flavor preferences in BALB/c and SWR mice. In expression studies, food-restricted male mice alternately consumed a flavored (CS+, e.g., cherry, 5 sessions) 5% Intralipid solution and a differently-flavored (CS-, e.g., grape, 5 sessions) 0.5% Intralipid solution. Two-bottle CS choice tests occurred following vehicle or MK-801 (100, 200µg/kg). MK-801 blocked expression of Intralipid-CFP at both doses in BALB/c mice, but only at the 100µg/kg dose in SWR mice. In acquisition studies, groups of BALB/c (0, 100µg/kg) and SWR (0, 100µg/kg) male mice were treated prior to the ten acquisition training sessions followed by six 2-bottle CS choice tests without injections. MK-801 eliminated acquisition of Intralipid-conditioned flavor preferences in BALB/c mice, and actually changed the preference to an avoidance response in SWR mice. Thus, NMDA receptor signaling appears essential especially for the learning of fat-conditioned flavor preferences in both mouse strains.

Gut-brain nutrient sensing in food reward

For the past several decades, vagal and hormonal gut-brain negative feedback signaling mechanisms that promote satiety and subsequent suppression of food intake have been explored. In addition, a separate positive feedback process termed "appetition," involving postoral signaling from the gut to the brain, has been shown to promote food intake and produce flavor-nutrient preference conditioning. Afferent fibers emerging from the vagus nerve form the main pathway by which information is relayed from the abdominal viscera to the hindbrain and eventually other higher brain regions involved in food intake. Using a specialized subdiaphragmatic vagal deafferentation technique, it was observed that gut vagal and splanchnic afferents play a role in the negative feedback control of satiety after nutrient intake; however, these afferents are not required for nutrient reinforcement or flavor-nutrient preference conditioning, thereby highlighting the distinction between the processes of satiation and appetition. By linking these physiological and behavioral processes to a neurochemical mechanism, it was found that striatal dopamine release induced by intragastric glucose infusion is involved in sweet appetite conditioning. The mechanisms underlying appetition are still being investigated but may involve other nondopaminergic neurochemical systems and/or presently undiscovered hormonal mediators. Future work to delineate the biological mechanisms whereby appetition drives increased intake and conditioned food preference in response to ingestion should take a multifaceted approach by integrating hormonal, neurophysiological, and behavioral techniques.

Proceedings of the 2015 ASPEN Research Workshop-Taste Signaling

This article summarizes research findings from 6 experts in the field of taste and feeding that were presented at the 2015 American Society for Parenteral and Enteral Nutrition Research Workshop. The theme was focused on the interaction of taste signals with those of a postingestive origin and how this contributes to regulation of food intake through both physiological and learning processes. Gastric bypass results in exceptional loss of fat mass and increases in circulating levels of key gut peptides, some of which are also expressed along with their cognate receptors in taste buds. Changes in taste preference and food selection in both bariatric surgery patients and rodent models have been reported. Accordingly, the effects of this surgery on taste-related behavior were examined. The conservation of receptor and peptide signaling mechanisms in gustatory and extraoral tissues was discussed in the context of taste responsiveness and the regulation of metabolism. New findings detailing the features of neural circuits between the caudal nucleus of the solitary tract (NST), receiving visceral input from the vagus nerve, and the rostral NST, receiving taste input, were discussed, as was how early life experience with taste stimuli and learned associations between flavor and postoral consequences of nutrients can exert potent and long-lasting effects on feeding.

MCH receptor deletion does not impair glucose-conditioned flavor preferences in mice

The post-oral actions of glucose stimulate intake and condition flavor preferences in rodents. Hypothalamic melanin-concentrating hormone (MCH) neurons are implicated in sugar reward, and this study investigated their involvement in glucose preference conditioning in mice. In Exp. 1 MCH receptor 1 knockout (KO) and C57BL/6 wildtype (WT) mice learned to prefer 8% glucose over an initially more-preferred non-nutritive 0.1% sucralose+saccharin (S+S) solution. In contrast, the KO and WT mice preferred S+S to 8% fructose, which is consistent with this sugar's weak post-oral reinforcing action. In Exp. 2 KO and WT mice were trained to drink a flavored solution (CS+) paired with intragastric (IG) infusion of 16% glucose and a different flavored solution (CS-) paired with IG water. Both groups drank more CS+ than CS- in training and preferred the CS+ to CS- in a 2-bottle test. These results indicate that MCH receptor signaling is not required for flavor preferences conditioned by the post-oral actions of glucose. This contrasts with other findings implicating MCH signaling in other types of sugar reward processing.

Differential effects of naloxone on rewarding electrical stimulation of the central nucleus of the amygdala and parabrachial complex in a place preference study

The central nucleus of the amygdala (CeA) is considered to be involved in different affective, sensory, regulatory, and acquisition processes. This study analyzed whether electrical stimulation of the PB-CeA system induces preferences in a concurrent place preference (cPP) task, as observed after stimulation of the parabrachial-insular cortex (PB-IC) axis. It also examined whether the rewarding effects are naloxone-dependent. The results show that electrical stimulation of the CeA and external lateral parabrachial subnucleus (LPBe) induces consistent preference behaviors in a cPP task. However, subcutaneous administration of an opiate antagonist (naloxone; 4mg/ml/kg) blocked the rewarding effect of the parabrachial stimulation but not that of the amygdala stimulation. These results are interpreted in the context of multiple brain reward systems that appear to differ both anatomically and neurochemically, notably with respect to the opiate system.

Mesolimbic Dopamine Encodes Prediction Errors in a State-Dependent Manner

Mesolimbic dopamine encodes the benefits of a course of action. However, the value of an appetitive reward depends strongly on an animal’s current state. To investigate the relationship between dopamine, value, and physiological state, we monitored sub-second dopamine release in the nucleus accumbens core while rats made choices between food and sucrose solution following selective satiation on one of these reinforcers. Dopamine signals reflected preference for the reinforcers in the new state, decreasing to the devalued reward and, after satiation on food, increasing for the valued sucrose solution. These changes were rapid and selective, with dopamine release returning to pre-satiation patterns when the animals were re-tested in a standard food-restricted state. Such rapid and selective adaptation of dopamine-associated value signals could provide an important signal to promote efficient foraging for a varied diet.

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Dopamine reward prediction errors are shaped by physiological state

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Both choices and dopamine signals rapidly update after selective satiation

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In a new state, dopamine signals mainly only update with experience

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When returning to a familiar state, dopamine immediately signals stored values

Saccharin Taste Conditions Flavor Preference in Weanling Rats

Innate and learned taste/flavor preferences to chemical stimuli in weanling rats are not fully understood. Our previous study showed that weanling rats could establish conditioned flavor preferences when low, but not high, concentrations of sucrose solutions were used as associative rewarding stimuli. Here, we examined whether 3-week-old rats could acquire flavor learning when the rewarding stimulus was saccharin, a non-nutritive artificial sweetener. In the acquisition session, they consumed water with a flavor (cherry or grape) and 0.1% sodium saccharin with another flavor (grape or cherry) for 15 min daily on alternative days over 6 consecutive days. The subsequent test session revealed significant preferences for the flavor previously associated with saccharin. However, they failed to retain the preference when retested in adulthood at the age of 20 weeks. These behavioral results were similar to those previously demonstrated when 2% sucrose was used as an associative sweetener. Although these 2 solutions were equally preferred, the taste quality may not be the same because the weanling rats showed neophobia to 0.1% saccharin and a larger chorda tympani response than 2% sucrose. The present study showed that a conditioned flavor preference was established to saccharin in weanling rats on the basis of flavor-taste association.

Insulin enhances striatal dopamine release by activating cholinergic interneurons and thereby signals reward

Insulin activates insulin receptors (InsRs) in the hypothalamus to signal satiety after a meal. However, the rising incidence of obesity, which results in chronically elevated insulin levels, implies that insulin may also act in brain centres that regulate motivation and reward. We report here that insulin can amplify action potential-dependent dopamine (DA) release in the nucleus accumbens (NAc) and caudate-putamen through an indirect mechanism that involves striatal cholinergic interneurons that express InsRs. Furthermore, two different chronic diet manipulations in rats, food restriction (FR) and an obesogenic (OB) diet, oppositely alter the sensitivity of striatal DA release to insulin, with enhanced responsiveness in FR, but loss of responsiveness in OB. Behavioural studies show that intact insulin levels in the NAc shell are necessary for acquisition of preference for the flavour of a paired glucose solution. Together, these data imply that striatal insulin signalling enhances DA release to influence food choices.

Ghrelin signaling is not essential for sugar or fat conditioned flavor preferences in mice

The oral and post-oral actions of sugar and fat stimulate intake and condition flavor preferences in rodents through a process referred to as appetition. Ghrelin is implicated in food reward processing, and this study investigated its involvement in nutrient conditioning in mice. In Exp. 1 ghrelin receptor-null (GHSR-null) and C57BL/6 wildtype (WT) mice learned to prefer a flavor (CS+) mixed into 8% glucose over another flavor (CS-) mixed into a "sweeter" but non-nutritive 0.1% sucralose+saccharin (S+S) solution. In Exp. 2 treating WT mice with a ghrelin receptor antagonist [(D-Lys3)-GHRP-6] during flavor training did not prevent them from learning to prefer the CS+ glucose over the CS-S+S flavor. GHSR-null and WT mice were trained in Exp. 3 to drink a CS+ paired with intragastric (IG) infusion of 16% glucose and a CS- paired with IG water. Both groups drank more CS+ than CS- in training and preferred the CS+ to CS- in a choice test. The same (Exp. 4) and new (Exp. 5) GHSR-null and WT mice learned to prefer a CS+ flavor paired with IG fat (Intralipid) over a CS- flavor paired with IG water. GHSR-null and WT mice also learned to prefer a CS+ flavor added to 8% fructose over a CS- added to water. Together, these results indicate that ghrelin receptor signaling is not required for flavor preferences conditioned by the oral or post-oral actions of sugar and fat. This contrasts with other findings implicating ghrelin signaling in food reward processing and food-conditioned place preferences.

The role of dopamine in the pursuit of nutritional value

Acquiring enough food to meet energy expenditure is fundamental for all organisms. Thus, mechanisms have evolved to allow foods with high nutritional value to be readily detected, consumed, and remembered. Although taste is often involved in these processes, there is a wealth of evidence supporting the existence of taste-independent nutrient sensing. In particular, post-ingestive mechanisms arising from the arrival of nutrients in the gut are able to drive food intake and behavioural conditioning. The physiological mechanisms underlying these effects are complex but are believed to converge on mesolimbic dopamine signalling to translate post-ingestive sensing of nutrients into reward and reinforcement value. Discerning the role of nutrition is often difficult because food stimulates sensory systems and post-ingestive pathways in concert. In this mini-review, I discuss the various methods that may be used to study post-ingestive processes in isolation including sham-feeding, non-nutritive sweeteners, post-ingestive infusions, and pharmacological and genetic methods. Using this structure, I present the evidence that dopamine is sensitive to nutritional value of certain foods and examine how this affects learning about food, the role of taste, and the implications for human obesity.

Orexin-1 receptor antagonist in central nucleus of the amygdala attenuates the acquisition of flavor-taste preference in rats

Previous studies demonstrated that the intracerebroventricular administration of SB-334867-A, a selective antagonist of orexin OX1R receptors, blocks the acquisition of saccharin-induced conditioned flavor preference (CFP) but not LiCl-induced taste aversion learning (TAL). Orexinergic fibers from the lateral hypothalamus end in the central nucleus of the amygdala (CeA), which expresses orexin OX1R receptors. Taste and sensory inputs also are present in CeA, which may contribute to the development of taste learning. This study analyzed the effect of two doses (1.5 and 6μg/0.5μl) of SB-334867-A administered into the CeA on flavor-taste preference induced by saccharin and on TAL induced by a single administration of LiCl (0.15M, 20ml/kg, i.p.). Outcomes indicate that inactivation of orexinergic receptors in the CeA attenuates flavor-taste preference in a two-bottle test (saccharin vs. water). Intra-amygdalar SB-334867-A does not affect gustatory processing or the preference for the sweet taste of saccharin given that SB-334867-A- and DMSO-treated groups (control animals) increased the intake of the saccharin-associated flavor across training acquisition sessions. Furthermore, SB-334867-A in the CeA does not block TAL acquisition ruling out the possibility that functional inactivation of OX1R receptors interferes with taste processing. Orexin receptors in the CeA appear to intervene in the association of a flavor with orosensory stimuli, e.g., a sweet and pleasant taste, but could be unnecessary when the association is established with visceral stimuli, e.g., lithium chloride. These data suggest that orexinergic projections to the CeA may contribute to the reinforcing signals facilitating the acquisition of taste learning and the change in hedonic evaluation of the taste, which would have important implications for the OX1R-targeted pharmacological treatment of eating disorders.

A relationship between reduced nucleus accumbens shell and enhanced lateral hypothalamic orexin neuronal activation in long-term fructose bingeing behavior

Fructose accounts for 10% of daily calories in the American diet. Fructose, but not glucose, given intracerebroventricularly stimulates homeostatic feeding mechanisms within the hypothalamus; however, little is known about how fructose affects hedonic feeding centers. Repeated ingestion of sucrose, a disaccharide of fructose and glucose, increases neuronal activity in hedonic centers, the nucleus accumbens (NAc) shell and core, but not the hypothalamus. Rats given glucose in the intermittent access model (IAM) display signatures of hedonic feeding including bingeing and altered DA receptor (R) numbers within the NAc. Here we examined whether substituting fructose for glucose in this IAM produces bingeing behavior, alters DA Rs and activates hedonic and homeostatic feeding centers. Following long-term (21-day) exposure to the IAM, rats given 8–12% fructose solutions displayed fructose bingeing but unaltered DA D1R or D2R number. Fructose bingeing rats, as compared to chow bingeing controls, exhibited reduced NAc shell neuron activation, as determined by c-Fos-immunoreactivity (Fos-IR). This activation was negatively correlated with orexin (Orx) neuron activation in the lateral hypothalamus/perifornical area (LH/PeF), a brain region linking homeostatic to hedonic feeding centers. Following short-term (2-day) access to the IAM, rats exhibited bingeing but unchanged Fos-IR, suggesting only long-term fructose bingeing increases Orx release. In long-term fructose bingeing rats, pretreatment with the Ox1R antagonist SB-334867 (30 mg/kg; i.p.) equally reduced fructose bingeing and chow intake, resulting in a 50% reduction in calories. Similarly, in control rats, SB-334867 reduced chow/caloric intake by 60%. Thus, in the IAM, Ox1Rs appear to regulate feeding based on caloric content rather than palatability. Overall, our results, in combination with the literature, suggest individual monosaccharides activate distinct neuronal circuits to promote feeding behavior. Specifically, long-term fructose bingeing activates a hyperphagic circuit composed in part of NAc shell and LH/PeF Orx neurons.

Role of the basolateral amygdala in retrieval of conditioned flavors in the awake rat

Learned association between odor, taste and further post-ingestive consequence is known as flavor nutrient conditioned preference. Amygdala is supposed to be one of the areas involved in these associations. In the present study, one flavor was associated with a 16% glucose (CS(+)) whereas another flavor was paired with less reinforcing 4% glucose (CS(-)). We showed that CS(+) presentation after conditioning increased Fos expression in the basolateral nucleus of amygdala (BLA). Furthermore, we performed electrophysiological recordings in the BLA in free moving rats. After preference acquisition, rats were exposed to either the CS(+) or the CS(-). The proportion of neurons showing a decreased activity during the CS(-) presentation was significantly higher in conditioned rats compared to controls. Among this neuronal population recorded in conditioned rats, we noticed a significant proportion of neurons that also showed a decreased activity during the CS(+) presentation. Our data indicate an involvement of BLA during retrieval of learned flavors. It also suggests that both flavors might have acquired a biological value through conditioning.

Dopamine signaling in the amygdala, increased by food ingestion and GLP-1, regulates feeding behavior

Mesolimbic dopamine plays a critical role in food-related reward processing and learning. The literature focuses primarily on the nucleus accumbens as the key dopaminergic target in which enhanced dopamine signaling is associated with reward. Here, we demonstrate a novel neurobiological mechanism by which dopamine transmission in the amygdala regulates food intake and reward. We show that food intake was associated with increased dopamine turnover in the amygdala. Next, we assess the impact of direct intra-amygdala D1 and D2 receptor activation on food intake and sucrose-driven progressive ratio operant conditioning in rats. Amygdala D2 receptor activation reduced food intake and operant behavior for sucrose, whereas D2 receptor blockade increased food intake but surprisingly reduced operant behavior. In contrast, D1 receptor stimulation or blockade did not alter feeding or operant conditioning for food. The glucagon-like peptide 1 (GLP-1) system, a target for type 2 diabetes treatment, in addition to regulating glucose homeostasis, also reduces food intake. We found that central GLP-1R receptor activation is associated with elevated dopamine turnover in the amygdala, and that part of the anorexic effect of GLP-1 is mediated by D2 receptor signaling in the amygdala. Our findings indicate that amygdala dopamine signaling is activated by both food intake and the anorexic brain-gut peptide GLP-1 and that amygdala D2 receptor activation is necessary and sufficient to change feeding behavior. Collectively these studies indicate a novel mechanism by which the dopamine system affects feeding-oriented behavior at the level of the amygdala.

The reward value of sucrose in leptin-deficient obese mice

Leptin-deficient patients report higher "liking" ratings for food, and leptin replacement therapy normalizes these ratings even before weight loss is achieved. Since animals cannot report their ratings, we studied the relationship between leptin and food reward in leptin-deficient ob/ob mice using a optogenetic assay that quantifies the reward value of sucrose. In this assay, mice chose between one sipper dispensing the artificial sweetener sucralose coupled to optogenetic activation of dopaminergic (DA) neurons, and another sipper dispensing sucrose. We found that the reward value of sucrose was high under a state of leptin deficiency, as well as at a dose of leptin that does not suppress food intake (12.5 ng/h). Treatment with higher doses of leptin decreased the reward value of sucrose before weight loss was achieved (100 ng/h), as seen in leptin-deficient patients. These results phenocopy in mice the behavior of leptin-deficient patients.

Hypothalamic melanin concentrating hormone neurons communicate the nutrient value of sugar

Sugars that contain glucose, such as sucrose, are generally preferred to artificial sweeteners owing to their post-ingestive rewarding effect, which elevates striatal dopamine (DA) release. While the post-ingestive rewarding effect, which artificial sweeteners do not have, signals the nutrient value of sugar and influences food preference, the neural circuitry that mediates the rewarding effect of glucose is unknown. In this study, we show that optogenetic activation of melanin-concentrating hormone (MCH) neurons during intake of the artificial sweetener sucralose increases striatal dopamine levels and inverts the normal preference for sucrose vs sucralose. Conversely, animals with ablation of MCH neurons no longer prefer sucrose to sucralose and show reduced striatal DA release upon sucrose ingestion. We further show that MCH neurons project to reward areas and are required for the post-ingestive rewarding effect of sucrose in sweet-blind Trpm5(-/-) mice. These studies identify an essential component of the neural pathways linking nutrient sensing and food reward. DOI: http://dx.doi.org/10.7554/eLife.01462.001.

Sucrose-conditioned flavor preferences in sweet ageusic T1r3 and Calhm1 knockout mice

The present study compared the ability of sweet ageusic T1r3 knockout (KO) and Calhm1 KO mice to acquire preferences for a sucrose-paired flavor as well as for unflavored sucrose. The KO and wildtype (WT) mice were given 24-h one-bottle access to 8% sucrose containing one flavor CS+, e.g., grape) and to water containing a different flavor (CS-, e.g., cherry) over 4 training days. In subsequent two-bottle tests with the flavors in water only, the T1r3 KO and Calhm1 KO mice, like WT mice, preferred the CS+ to the CS-. After training with flavored solutions, both KO groups also preferred unflavored 8% sucrose to water although Calhm1 KO mice required more sugar experience to match the preference of the T1r3 KO mice. These findings demonstrate that Calhm1 KO mice, like T1r3 KO mice and WT mice, are sensitive to the post-oral preference conditioning actions of sucrose and can discriminate sugar from water. Yet, despite their acquired sucrose preferences, the Calhm1 KO and T1r3 KO mice consumed only half as much sugar per day as did WT mice. Thus, sweet taste signaling elements are not needed in the gut for sugar conditioning, but sweet taste signaling in the mouth is essential for the full expression of sugar appetite.

Effect of dopamine D1 and D2 receptor antagonism in the lateral hypothalamus on the expression and acquisition of fructose-conditioned flavor preference in rats

The attraction to sugar-rich foods is influenced by conditioned flavor preferences (CFP) produced by the sweet taste of sugar (flavor-flavor learning) and the sugar's post-oral actions (flavor-nutrient) learning. Brain dopamine (DA) circuits are involved in both types of flavor learning, but to different degrees. This study investigated the role of DA receptors in the lateral hypothalamus (LH) on the flavor-flavor learning produced the sweet taste of fructose. In an acquisition study, food-restricted rats received bilateral LH injections of a DA D1 receptor antagonist (SCH23390), a D2 antagonist (RAC, raclopride) or vehicle prior to 1-bottle training sessions with a flavored 8% fructose+0.2% saccharin solution (CS+/F) and a less-preferred flavored 0.2% saccharin solution (CS-). Drug-free 2-bottle tests were then conducted with the CS+ and CS- flavors presented in saccharin. The fructose-CFP did not differ among groups given vehicle (76%), 12 nmol SCH (78%), 24 nmol (82%) or 24 nmol RAC (90%) during training. In an expression study with rats trained drug-free, LH injections of 12 or 24 nmol SCH or 12-48 nmol RAC prior to 2-bottle tests did not alter CS+ preferences (77-90%) relative to vehicle injection (86%). Only a 48 nmol SCH dose suppressed the CS+ preference (61%). The minimal effect of LH DA receptor antagonism upon fructose flavor-flavor conditioning differs with the ability of LH SCH injections to block the acquisition of glucose flavor-nutrient learning.

Experience with sugar modifies behavioral but not taste-evoked medullary responses to sweeteners in mice

Dietary exposure to sugars increases the preference for and intake of sugar solutions in mice. We used brief-access lick tests and multiunit electrophysiological recordings from the nucleus of the solitary tract (NST) to investigate the role of taste in diet-induced changes in sucrose responsiveness. We exposed C57BL/6J (B6) and 129X1/SvJ (129) mice to either a sucrose diet (chow, water, and a 500mM sucrose solution) or a control diet (chow and water) for 3 days. In B6 mice, exposure to the sucrose diet decreased the appetitive response (i.e., number of trials initiated) but had no effect on the consummatory response (i.e., rate of licking) to 500mM sucrose and 20mM saccharin. In 129 mice, exposure to the sucrose diet increased the appetitive response but had no effect on the consummatory response to the sweetener solutions. In the NST recordings, the B6 mice exhibited larger multiunit responses to sweeteners than 129 mice, but there was no effect of the sucrose diet in either strain. Our results indicate that sucrose exposure alters the appetitive response of B6 and 129 mice to sweeteners in diametrically opposed ways and that these changes are mediated by structures in the gustatory neuraxis above the NST (e.g., ventral forebrain).

Glucose utilization rates regulate intake levels of artificial sweeteners

It is well established that animals including humans attribute greater reinforcing value to glucose-containing sugars compared to their non-caloric counterparts, generally termed 'artificial sweeteners'. However, much remains to be determined regarding the physiological signals and brain systems mediating the attribution of greater reinforcing value to sweet solutions that contain glucose. Here we show that disruption of glucose utilization in mice produces an enduring inhibitory effect on artificial sweetener intake, an effect that did not depend on sweetness perception or aversion. Indeed, such an effect was not observed in mice presented with a less palatable, yet caloric, glucose solution. Consistently, hungry mice shifted their preferences away from artificial sweeteners and in favour of glucose after experiencing glucose in a hungry state. Glucose intake was found to produce significantly greater levels of dopamine efflux compared to artificial sweetener in dorsal striatum, whereas disrupting glucose oxidation suppressed dorsal striatum dopamine efflux. Conversely, inhibiting striatal dopamine receptor signalling during glucose intake in sweet-naïve animals resulted in reduced, artificial sweetener-like intake of glucose during subsequent gluco-deprivation. Our results demonstrate that glucose oxidation controls intake levels of sweet tastants by modulating extracellular dopamine levels in dorsal striatum, and suggest that glucose utilization is one critical physiological signal involved in the control of goal-directed sweetener intake.