Activation of Glutamatergic Fibers in the Anterior NAc Shell Modulates Reward Activity in the aNAcSh, the Lateral Hypothalamus, and Medial Prefrontal Cortex and Transiently Stops Feeding

Stimulation of Dopamine D4 Receptors in the Nucleus Accumbens Shell Increases Palatable Food Intake in Satiated Male Rats: Modulation by NMDA and AMPA Receptors

BACKGROUND/OBJECTIVES

Palatability significantly influences food consumption, often leading to overeating and obesity by activating the brain's reward systems. The nucleus accumbens (NAc) plays a central role in this process, modulating reward mechanisms primarily via dopamine through D2-like receptors (D2R, D3R, D4R). While the involvement of D2 receptors in feeding is well-documented, the role of D4 receptors (D4Rs) is less clear.

METHODS

Male Wistar rats received intra-NAc shell microinjections of the D4R agonist PD-168077 and the antagonist L-745870. This study also examined the modulation between D4R and glutamatergic transmission by administration of NMDA, NMDA receptor antagonist AP-5, AMPA, and AMPA receptor antagonist CNQX.

RESULTS

PD-168077 increased sweet solution intake by 46%, an effect that was reversed by L-745870. Pre-treatment with NMDA prevented the stimulatory effect of PD-168077, whereas the NMDA receptor antagonist AP-5 had no such effect. Additionally, AMPA administration reduced sweet solution intake by 63%, counteracting the effect of PD-168077, while the AMPA receptor antagonist CNQX, on its own, increased intake by 40%.

CONCLUSIONS

These findings suggest that D4Rs promote hedonic feeding by modulating glutamatergic transmission in the NAc shell, highlighting the complexity of D4R involvement in food intake regulation. This study underscores the potential of targeting D4Rs for therapeutic interventions in eating disorders and obesity, though further research is essential to clarify the precise mechanisms through which D4R modulates AMPA and NMDA receptor activity in feeding behavior.

The flow of reward information through neuronal ensembles in the accumbens

The nucleus accumbens shell (NAcSh) integrates reward information through diverse and specialized neuronal ensembles, influencing decision-making. By training rats in a probabilistic choice task and recording NAcSh neuronal activity, we found that rats adapt their choices based solely on the presence or absence of a sucrose reward, suggesting they build an internal representation of reward likelihood. We further demonstrate that NAcSh ensembles dynamically process different aspects of reward-guided behavior, with changes in composition and functional connections observed throughout the reinforcement learning process. The NAcSh forms a highly connected network characterized by a heavy-tailed distribution and the presence of neuronal hubs, facilitating efficient information flow. Reward delivery enhances mutual information, indicating increased communication between ensembles and network synchronization, whereas reward omission decreases it. Our findings reveal how reward information flows through dynamic NAcSh ensembles, whose flexible membership adapts as the rat learns to obtain rewards (energy) in an ever-changing environment.

A hippocampus-accumbens code guides goal-directed appetitive behavior

The dorsal hippocampus (dHPC) is a key brain region for the expression of spatial memories, such as navigating towards a learned reward location. The nucleus accumbens (NAc) is a prominent projection target of dHPC and implicated in value-based action selection. Yet, the contents of the dHPC→NAc information stream and their acute role in behavior remain largely unknown. Here, we found that optogenetic stimulation of the dHPC→NAc pathway while mice navigated towards a learned reward location was both necessary and sufficient for spatial memory-related appetitive behaviors. To understand the task-relevant coding properties of individual NAc-projecting hippocampal neurons (dHPC→NAc), we used in vivo dual-color two-photon imaging. In contrast to other dHPC neurons, the dHPC→NAc subpopulation contained more place cells, with enriched spatial tuning properties. This subpopulation also showed enhanced coding of non-spatial task-relevant behaviors such as deceleration and appetitive licking. A generalized linear model revealed enhanced conjunctive coding in dHPC→NAc neurons which improved the identification of the reward zone. We propose that dHPC routes specific reward-related spatial and behavioral state information to guide NAc action selection.

Optical Intracranial Self-Stimulation (oICSS): A New Behavioral Model for Studying Drug Reward and Aversion in Rodents

Brain-stimulation reward, also known as intracranial self-stimulation (ICSS), is a commonly used procedure for studying brain reward function and drug reward. In electrical ICSS (eICSS), an electrode is surgically implanted into the medial forebrain bundle (MFB) in the lateral hypothalamus or the ventral tegmental area (VTA) in the midbrain. Operant lever responding leads to the delivery of electrical pulse stimulation. The alteration in the stimulation frequency-lever response curve is used to evaluate the impact of pharmacological agents on brain reward function. If a test drug induces a leftward or upward shift in the eICSS response curve, it implies a reward-enhancing or abuse-like effect. Conversely, if a drug causes a rightward or downward shift in the functional response curve, it suggests a reward-attenuating or aversive effect. A significant drawback of eICSS is the lack of cellular selectivity in understanding the neural substrates underlying this behavior. Excitingly, recent advancements in optical ICSS (oICSS) have facilitated the development of at least three cell type-specific oICSS models-dopamine-, glutamate-, and GABA-dependent oICSS. In these new models, a comparable stimulation frequency-lever response curve has been established and employed to study the substrate-specific mechanisms underlying brain reward function and a drug's rewarding versus aversive effects. In this review article, we summarize recent progress in this exciting research area. The findings in oICSS have not only increased our understanding of the neural mechanisms underlying drug reward and addiction but have also introduced a novel behavioral model in preclinical medication development for treating substance use disorders.

Synergistic photoactivation of VTA-catecholaminergic and BLA-glutamatergic projections induces long-term potentiation in the insular cortex

The presentation of novel stimuli induces a reliable dopamine release in the insular cortex (IC) from the ventral tegmental area (VTA). The novel stimuli could be associated with motivational and emotional signals induced by cortical glutamate release from the basolateral amygdala (BLA). Dopamine and glutamate are essential for acquiring and maintaining behavioral tasks, including visual and taste recognition memories. In this study, we hypothesize that the simultaneous activation of dopaminergic and glutamatergic projections to the neocortex can underlie synaptic plasticity. High-frequency stimulation of the BLA-IC circuit has demonstrated a reliable long-term potentiation (LTP), a widely acknowledged synaptic plasticity that underlies memory consolidation. Therefore, the concurrent optogenetic stimulation of the insula's glutamatergic and dopaminergic terminal fibers would induce reliable LTP. Our results confirmed that combined photostimulation of the VTA and BLA projections to the IC induces a slow-onset LTP. We also found that optogenetically-induced LTP in the IC relies on both glutamatergic NMDA receptors and dopaminergic D1/D5 receptors, suggesting that the combined effects of these neurotransmitters can trigger synaptic plasticity in the neocortex. Overall, our findings provide compelling evidence supporting the essential role of both dopaminergic and glutamatergic projections in modulating synaptic plasticity within the IC. Furthermore, our results suggest that the synergistic actions of these projections have a pivotal influence on the formation of motivational memories.

Early-life influences of low-calorie sweetener consumption on sugar taste

Children and adolescents are the highest consumers of added sugars, particularly from sugar-sweetened beverages (SSB). Regular consumption of SSB early in life induces a variety of negative consequences on health that can last into adulthood. Low-calorie sweeteners (LCS) are increasingly used as an alternative to added sugars because they provide a sweet sensation without adding calories to the diet. However, the long-term effects of early-life consumption of LCS are not well understood. Considering LCS engage at least one of the same taste receptors as sugars and potentially modulate cellular mechanisms of glucose transport and metabolism, it is especially important to understand how early-life LCS consumption impacts intake of and regulatory responses to caloric sugars. In our recent study, we found that habitual intake of LCS during the juvenile-adolescence period significantly changed how rats responded to sugar later in life. Here, we review evidence that LCS and sugars are sensed via common and distinct gustatory pathways, and then discuss the implications this has for shaping sugar-associated appetitive, consummatory, and physiological responses. Ultimately, the review highlights the diverse gaps in knowledge that will be necessary to fill to understand the consequences of regular LCS consumption during important phases of development.

Glial cells in anorexia

Anorexia is a loss of appetite or an inability to eat and is often associated with eating disorders. However, animal anorexia is physiologically regulated as a part of the life cycle; for instance, during hibernation, migration or incubation. Anorexia nervosa (AN), on the other hand, is a common eating disorder among adolescent females that experience an intense fear of gaining weight due to body image distortion that results in voluntary avoidance of food intake and, thus, severe weight loss. It has been shown that the neurobiology of feeding extends beyond the hypothalamus. The prefrontal cortex (PFC) is involved in food choice and body image perception, both relevant in AN. However, little is known about the neurobiology of AN, and the lack of effective treatments justifies the use of animal models. Glial cells, the dominant population of nerve cells in the central nervous system, are key in maintaining brain homeostasis. Accordingly, recent studies suggest that glial function may be compromised by anorexia. In this review, we summarize recent findings about anorexia and glial cells.

The PACAP Paradox: Dynamic and Surprisingly Pleiotropic Actions in the Central Regulation of Energy Homeostasis

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP), a pleiotropic neuropeptide, is widely distributed throughout the body. The abundance of PACAP expression in the central and peripheral nervous systems, and years of accompanying experimental evidence, indicates that PACAP plays crucial roles in diverse biological processes ranging from autonomic regulation to neuroprotection. In addition, PACAP is also abundantly expressed in the hypothalamic areas like the ventromedial and arcuate nuclei (VMN and ARC, respectively), as well as other brain regions such as the nucleus accumbens (NAc), bed nucleus of stria terminalis (BNST), and ventral tegmental area (VTA) - suggesting that PACAP is capable of regulating energy homeostasis via both the homeostatic and hedonic energy balance circuitries. The evidence gathered over the years has increased our appreciation for its function in controlling energy balance. Therefore, this review aims to further probe how the pleiotropic actions of PACAP in regulating energy homeostasis is influenced by sex and dynamic changes in energy status. We start with a general overview of energy homeostasis, and then introduce the integral components of the homeostatic and hedonic energy balance circuitries. Next, we discuss sex differences inherent to the regulation of energy homeostasis via these two circuitries, as well as the activational effects of sex steroid hormones that bring about these intrinsic disparities between males and females. Finally, we explore the multifaceted role of PACAP in regulating homeostatic and hedonic feeding through its actions in regions like the NAc, BNST, and in particular the ARC, VMN and VTA that occur in sex- and energy status-dependent ways.

The role of the nucleus accumbens and ventral pallidum in feeding and obesity

Obesity is a growing global epidemic that stems from the increasing availability of highly-palatable foods and the consequent enhanced calorie consumption. Extensive research has shown that brain regions that are central to reward seeking modulate feeding and evidence linking obesity to pathology in such regions have recently started to accumulate. In this review we focus on the contribution of two major interconnected structures central to reward processing, the nucleus accumbens and the ventral pallidum, to obesity. We first review the known literature linking these structures to feeding behavior, then discuss recent advances connecting pathology in the nucleus accumbens and ventral pallidum to obesity, and finally examine the similarities and differences between drug addiction and obesity in the context of these two structures. The understanding of how pathology in brain regions involved in reward seeking and consumption may drive obesity and how mechanistically similar obesity and addiction are, is only now starting to be revealed. We hope that future research will advance knowledge in the field and open new avenues to studying and treating obesity.

Input-specific modulation of murine nucleus accumbens differentially regulates hedonic feeding

Hedonic feeding is driven by the "pleasure" derived from consuming palatable food and occurs in the absence of metabolic need. It plays a critical role in the excessive feeding that underlies obesity. Compared to other pathological motivated behaviors, little is known about the neural circuit mechanisms mediating excessive hedonic feeding. Here, we show that modulation of prefrontal cortex (PFC) and anterior paraventricular thalamus (aPVT) excitatory inputs to the nucleus accumbens (NAc), a key node of reward circuitry, has opposing effects on high fat intake in mice. Prolonged high fat intake leads to input- and cell type-specific changes in synaptic strength. Modifying synaptic strength via plasticity protocols, either in an input-specific optogenetic or non-specific electrical manner, causes sustained changes in high fat intake. These results demonstrate that input-specific NAc circuit adaptations occur with repeated exposure to a potent natural reward and suggest that neuromodulatory interventions may be therapeutically useful for individuals with pathologic hedonic feeding.

The convergence of aversion and reward signals in individual neurons of the mice lateral habenula

The lateral habenula (LHb) and ventral tegmental area (VTA) are two structures closely connected, and they serve as aversion and reward junction of the brain, respectively. This study investigated whether single neurons in the LHb/VTA respond to both aversion and reward stimuli and how these neurons regulate aversion and reward processing. Using optogenetic combined with multi-channel recording of LHb / VTA neuronal discharge, we found that most single neurons in the LHb/ VTA respond to both aversion and reward stimuli. Interestingly, majority of neurons in LHb were aversion-activated and reward-inhibited neurons, consisting mainly of glutamatergic neurons, while most neurons in VTA were reward-activated and aversion-inhibited neurons, which inhibited by glutamatergic neurons in the LHb. Furthermore, optogenetic activation or inhibition of glutamatergic neurons in LHb and their terminals in VTA could induce aversive or reward behaviors. These results indicate that identical neurons in the LHb and VTA have different responses to reward and aversion stimuli. The aversion behaviors induced by activating LHb glutamatergic neurons may be due to its inhibition on reward-activated neurons in VTA. This study suggests that interplay between the LHb and VTA neurons may play a key role in regulating reward and aversion behaviors.

Projections from the nucleus accumbens shell to the ventral pallidum are involved in the control of sucrose intake in adult female rats

In rodents, stimulation of the nucleus accumbens shell (AcbSh) directly or via its projection to the lateral hypothalamus (LH) attenuates food intake. The ventral pallidum (VP) receives dense projections from the AcbSh and is sensitive to the hedonic aspect of food and motivation for reward. However, the role of accumbal projections to the VP in the regulation of food intake was not well investigated. In the present study conducted on female rats, we examined the effects of stimulation of the AcbSh using optogenetics, or pharmacological inhibition of the rostral VP, or stimulation of projections from the AcbSh to the rostral VP using optogenetics on the consumption of 10% sucrose, lick microstructure and the expression of c-fos mRNA. Stimulation of the AcbSh, inhibition of the rostral VP with muscimol, or stimulation of axonal terminals from the AcbSh to the rostral VP resulted in a decrease in sucrose intake, meal duration, and total number of licks. The licking microstructure analysis showed that optogenetic stimulation of AcbSh or axonal terminals from the AcbSh to the rostral VP decreased the hedonic value of the sucrose. However, inhibition of the rostral VP decreased the motivation, whereas stimulation of the accumbal projections in the rostral VP increased the motivation to drink. This difference could be due to differential involvement of GABAergic and glutamatergic VP neurons. Stimulation of the AcbSh resulted in a decrease of c-fos mRNA expression in the LH and rostral VP, and stimulation of axonal terminals from the AcbSh to the rostral VP decreased c-fos mRNA expression only in the rostral VP. This study demonstrates that in adult female rats, in addition to the already known role of the AcbSh projections to the LH, AcbSh projections to the VP play a major role in the regulation of sucrose intake.

The neuroscience of sugars in taste, gut-reward, feeding circuits, and obesity

Throughout the animal kingdom sucrose is one of the most palatable and preferred tastants. From an evolutionary perspective, this is not surprising as it is a primary source of energy. However, its overconsumption can result in obesity and an associated cornucopia of maladies, including type 2 diabetes and cardiovascular disease. Here we describe three physiological levels of processing sucrose that are involved in the decision to ingest it: the tongue, gut, and brain. The first section describes the peripheral cellular and molecular mechanisms of sweet taste identification that project to higher brain centers. We argue that stimulation of the tongue with sucrose triggers the formation of three distinct pathways that convey sensory attributes about its quality, palatability, and intensity that results in a perception of sweet taste. We also discuss the coding of sucrose throughout the gustatory pathway. The second section reviews how sucrose, and other palatable foods, interact with the gut-brain axis either through the hepatoportal system and/or vagal pathways in a manner that encodes both the rewarding and of nutritional value of foods. The third section reviews the homeostatic, hedonic, and aversive brain circuits involved in the control of food intake. Finally, we discuss evidence that overconsumption of sugars (or high fat diets) blunts taste perception, the post-ingestive nutritional reward value, and the circuits that control feeding in a manner that can lead to the development of obesity.

Medial Nucleus Accumbens Projections to the Ventral Tegmental Area Control Food Consumption

Decades of research have shown that the NAc is a critical region influencing addiction, mood, and food consumption through its effects on reinforcement learning, motivation, and hedonic experience. Pharmacological studies have demonstrated that inhibition of the NAc shell induces voracious feeding, leading to the hypothesis that the inhibitory projections that emerge from the NAc normally act to restrict feeding. While much of this work has focused on projections to the lateral hypothalamus, the role of NAc projections to the VTA in the control food intake has been largely unexplored. Using a retrograde viral labeling technique and real-time monitoring of neural activity with fiber photometry, we find that medial NAc shell projections to the VTA (mNAc→VTA) are inhibited during food-seeking and food consumption in male mice. We also demonstrate that this circuit bidirectionally controls feeding: optogenetic activation of NAc projections to the VTA inhibits food-seeking and food intake (in both sexes), while optogenetic inhibition of this circuit potentiates food-seeking behavior. Additionally, we show that activity of the NAc to VTA pathway is necessary for adaptive inhibition of food intake in response to external cues. These data provide new insight into NAc control over feeding in mice, and contribute to an emerging literature elucidating the role of inhibitory midbrain feedback within the mesolimbic circuit.SIGNIFICANCE STATEMENT The medial NAc has long been known to control consummatory behavior, with particular focus on accumbens projections to the lateral hypothalamus. Conversely, NAc projections to the VTA have mainly been studied in the context of drug reward. We show that NAc projections to the VTA bidirectionally control food intake, consistent with a permissive role in feeding. Additionally, we show that this circuit is normally inactivated during consumption and food-seeking. Together, these findings elucidate how mesolimbic circuits control food consumption.

Hippocampal Input to the Nucleus Accumbens Shell Enhances Food Palatability

BACKGROUND

Insight into the neural basis of hedonic processing has come from studies of food palatability in rodents. Pharmacological manipulations of the nucleus accumbens shell (NAcSh) have repeatedly been demonstrated to increase hedonic taste reactivity, yet the contribution of specific NAcSh circuit components is unknown.

METHODS

Bidirectional optogenetic manipulations were targeted to the principal NAcSh projection neurons and afferent pathways in mice during free feeding assays. Number of licks per bout of consumption was used as a measure of food palatability as it was confirmed to track sucrose concentration and subjective flavor preferences.

RESULTS

Photoinhibition of NAcSh neurons, whether general or cell-type specific, was found to alter consumption without affecting its hedonic impact. Among the principal excitatory afferent pathways, we showed that ventral hippocampal (vHipp) input alone enhances palatability upon low-frequency photostimulation time-locked to consumption. This enhancement in palatability was independent of opioid signaling and not recapitulated by NAcSh or dopamine neuron photostimulation. We further demonstrated that vHipp input photostimulation is sufficient to condition a flavor preference, while its inhibition impedes sucrose-driven flavor preference conditioning.

CONCLUSIONS

These results demonstrate a novel contribution of vHipp-NAcSh pathway activity to palatability that may relate to its innervation of a particular region or neuronal ensemble in the NAcSh. These findings are consistent with the evidence that vHipp-NAcSh activity is relevant to the pathophysiology of anhedonia and depression as well as the increasing appreciation of hippocampal involvement in people's food pleasantness ratings, hunger, and weight.

Glutamatergic basolateral amygdala to anterior insular cortex circuitry maintains rewarding contextual memory

Findings have shown that anterior insular cortex (aIC) lesions disrupt the maintenance of drug addiction, while imaging studies suggest that connections between amygdala and aIC participate in drug-seeking. However, the role of the BLA → aIC pathway in rewarding contextual memory has not been assessed. Using a cre-recombinase under the tyrosine hydroxylase (TH+) promoter mouse model to induce a real-time conditioned place preference (rtCPP), we show that photoactivation of TH+ neurons induced electrophysiological responses in VTA neurons, dopamine release and neuronal modulation in the aIC. Conversely, memory retrieval induced a strong release of glutamate, dopamine, and norepinephrine in the aIC. Only intra-aIC blockade of the glutamatergic N-methyl-D-aspartate receptor accelerated rtCPP extinction. Finally, photoinhibition of glutamatergic BLA → aIC pathway produced disinhibition of local circuits in the aIC, accelerating rtCPP extinction and impairing reinstatement. Thus, activity of the glutamatergic projection from the BLA to the aIC is critical for maintenance of rewarding contextual memory.

The Triple Combination Phentermine Plus 5-HTP/Carbidopa Leads to Greater Weight Loss, With Fewer Psychomotor Side Effects Than Each Drug Alone

Obesity has become a serious public health problem. Although diet, surgery, and exercise are the primary treatments for obesity, these activities are often supplemented using appetite suppressants. A previous study reported that obesity specialists frequently prescribed a new drug combination for its treatment that includes phentermine (Phen; dopaminergic appetite suppressant), a serotonin (5-HT) precursor 5-hydroxytryptophan (5-HTP; an appetite suppressant that increases the 5-HT concentration), and carbidopa (CB; peripheral blocker of conversion of 5-HTP to 5-HT). Despite its widespread use, there is neither a preclinical study confirming the drug efficacy nor studies of its effects on the brain. To fill this gap, in rats for seven consecutive days, we administered Phen intraperitoneally at different doses either alone or in combination with a fixed dose of 5-HTP/CB. In a different group, we infused drugs via an intraperitoneal catheter while extracellular-recordings were performed in the nucleus accumbens shell (NAcSh), a brain region with dopamine-releasing effects that is involved in the action of appetite suppressants. We found that the triple-drug combination leads to greater weight-loss than each drug alone. Moreover, and as the treatment progresses, the triple drug combination partially reversed psychomotor side-effects induced by Phen. Electrophysiological results revealed that Phen alone evoked a net inhibitory imbalance in NAcSh population activity that correlated with the onset of psychomotor effects. In addition, and unlike the greater weight loss, the addition of 5-HTP/CB did not alter the Phen-evoked inhibitory imbalance in NAcSh responses. Subsequent experiments shed light on the underlying mechanism. That is the majority of NAcSh neurons modulated by 5-HTP/CB were suppressed by Phen. Notably, and despite acting via a different mechanism of action (DA for Phen vs. 5-HT for 5-HTP/CB), both drugs recruited largely overlapping NAcSh neuronal ensembles. These data suggest that the neural correlates of the greater weight loss could be located outside the NAcSh, in other brain circuits. Furthermore, we conclude that Phen + 5-HTP/CB is a potential treatment for overweight and obesity.

Oral supplementation with ginseng polysaccharide promotes food intake in mice

INTRODUCTION

Ginseng polysaccharide (GPS, same as Panax polysaccharide) is a kind of polysaccharide extracted from ginseng. It has been reported that GPS has the ability to activate innate immunity, regulates blood sugar balance, and improves antioxidant capacity, but the effect on feeding behavior and its mechanism remains unclear.

METHOD

To investigate the possible effect of GPS on feeding behavior of animals, mice were supplied with GPS in water, and food intake, hedonic feeding behavior, anxiety-like behavior, expression of appetite-regulation peptides in the central nervous system and glucose-related hormone levels in the serum of mice were measured.

RESULTS

Ginseng polysaccharide significantly increased the average daily food intake in mice and promoted hedonic eating behavior. Meanwhile, the levels of serum glucose and glucagon were significantly reduced by GPS, and GPS promoted hypothalamic neuropeptide Y expression, inhibited proopiomelanocortin (POMC) expression, and reduced dopamine D1 receptor (DRD1) levels in the midbrain. We also found that the anxiety level of mice was significantly lower after GPS intake. In conclusion, oral supplementation with GPS promoted food intake in mice, most likely through the regulation of circulating glucose levels.

Convergent neural connectivity in motor impulsivity and high-fat food binge-like eating in male Sprague-Dawley rats

Food intake is essential for survival, but maladaptive patterns of intake, possibly encoded by a preexisting vulnerability coupled with the influence of environmental variables, can modify the reward value of food. Impulsivity, a predisposition toward rapid unplanned reactions to stimuli, is one of the multifaceted determinants underlying the etiology of dysregulated eating and its evolving pathogenesis. The medial prefrontal cortex (mPFC) is a major neural director of reward-driven behavior and impulsivity. Compromised signaling between the mPFC and nucleus accumbens shell (NAcSh) is thought to underlie the cognitive inability to withhold prepotent responses (motor impulsivity) and binge intake of high-fat food (HFF) seen in binge eating disorder. To explore the relationship between motor impulsivity and binge-like eating in rodents, we identified high (HI) and low impulsive (LI) rats in the 1-choice serial reaction time task and employed a rat model of binge-like eating behavior. HFF binge rats consumed significantly greater calories relative to control rats maintained on continual access to standard food or HFF. HI rats repeatedly exhibited significantly higher bingeing on HFF vs. LI rats. Next, we employed dual viral vector chemogenetic technology which allows for the targeted and isolated modulation of ventral mPFC (vmPFC) neurons that project to the NAcSh. Chemogenetic activation of the vmPFC to NAcSh pathway significantly suppressed motor impulsivity and binge-like intake for high-fat food. Thus, inherent motor impulsivity and binge-like eating are linked and the vmPFC to NAcSh pathway serves as a 'brake' over both behaviors.

Reward-representing D1-type neurons in the medial shell of the accumbens nucleus regulate palatable food intake

BACKGROUND/OBJECTIVES

Dysfunction in reward-related aspects of feeding, and consequent overeating in humans, is a major contributor to obesity. Intrauterine undernutrition and overnutrition are among the predisposing factors, but the exact mechanism of how overeating develops is still unclear. Consummatory behavior is regulated by the medial shell (mSh) of the accumbens nucleus (Nac) through direct connections with the rostral part of the lateral hypothalamic area (LHA). Our aim was to investigate whether an altered Nac-LHA circuit may underlie hyperphagic behavior.

SUBJECTS/METHODS

Intrauterine protein-restricted (PR) male Wistar rats were used as models for hyperphagia. The experiments were performed using young adult control (normally nourished) and PR animals. Sweet condensed milk (SCM) served as a reward to test consumption and subsequent activation (Fos+) of Nac and LHA neurons. Expression levels of type 1 and 2 dopamine receptors (D1R, D2R) in the Nac, as well as tyrosine hydroxylase (TH) levels in the ventral tegmental area, were determined. The D1R agonist SKF82958 was injected into the mSh-Nac of control rats to test the effect of D1R signaling on SCM intake and neuronal cell activation in the LHA.

RESULTS

A group of food reward-representing D1R+ neurons was identified in the mSh-Nac. Activation (Fos+) of these neurons was highly proportional to the consumed palatable food. D1R agonist treatment attenuated SCM intake and diminished the number of SCM-activated cells in the LHA. Hyperphagic PR rats showed increased intake of SCM, reduced D1R expression, and an impaired response to SCM-evoked neuronal activation in the mSh-Nac, accompanied by an elevated number of Fos+ neurons in the LHA compared to controls.

CONCLUSIONS

Sensitivity of food reward-representing neurons in the mSh-Nac determines the level of satisfaction that governs cessation of consumption, probably through connections with the LHA. D1R signaling is a key element in this function, and is impaired in obesity-prone rats.

Nucleus Accumbens Modulation in Reward and Aversion

The nucleus accumbens (NAc) is a key node of the brain’s circuitry that is responsible for translating motivation into action. It has been implicated in playing critical roles in virtually all forms of adaptive and pathological motivated behaviors. It is subject to modulation by a broad array of inputs that influence NAc activity in complex ways that are still poorly understood. Here, we briefly review current knowledge about the behavioral consequences of NAc modulation, focusing on recent studies that use novel techniques developed and implemented over the last decade.

Encoding of Sucrose's Palatability in the Nucleus Accumbens Shell and Its Modulation by Exteroceptive Auditory Cues

Although the palatability of sucrose is the primary reason for why it is over consumed, it is not well understood how it is encoded in the nucleus accumbens shell (NAcSh), a brain region involved in reward, feeding, and sensory/motor transformations. Similarly, untouched are issues regarding how an external auditory stimulus affects sucrose palatability and, in the NAcSh, the neuronal correlates of this behavior. To address these questions in behaving rats, we investigated how food-related auditory cues modulate sucrose's palatability. The goals are to determine whether NAcSh neuronal responses would track sucrose's palatability (as measured by the increase in hedonically positive oromotor responses lick rate), sucrose concentration, and how it processes auditory information. Using brief-access tests, we found that sucrose's palatability was enhanced by exteroceptive auditory cues that signal the start and the end of a reward epoch. With only the start cue the rejection of water was accelerated, and the sucrose/water ratio was enhanced, indicating greater palatability. However, the start cue also fragmented licking patterns and decreased caloric intake. In the presence of both start and stop cues, the animals fed continuously and increased their caloric intake. Analysis of the licking microstructure confirmed that auditory cues (either signaling the start alone or start/stop) enhanced sucrose's oromotor-palatability responses. Recordings of extracellular single-unit activity identified several distinct populations of NAcSh responses that tracked either the sucrose palatability responses or the sucrose concentrations by increasing or decreasing their activity. Another neural population fired synchronously with licking and exhibited an enhancement in their coherence with increasing sucrose concentrations. The population of NAcSh's Palatability-related and Lick-Inactive neurons were the most important for decoding sucrose's palatability. Only the Lick-Inactive neurons were phasically activated by both auditory cues and may play a sentinel role monitoring relevant auditory cues to increase caloric intake and sucrose's palatability. In summary, we found that auditory cues that signal the availability of sucrose modulate its palatability and caloric intake in a task dependent-manner and had neural correlates in the NAcSh. These findings show that exteroceptive cues associated with feeding may enhance positive hedonic oromotor-responses elicited by sucrose's palatability.

Optogenetic activation of amygdala projections to nucleus accumbens can arrest conditioned and unconditioned alcohol consummatory behavior

Following a Pavlovian pairing procedure, alcohol-paired cues come to elicit behavioral responses that lead to alcohol consumption. Here we used an optogenetic approach to activate basolateral amygdala (BLA) axonal terminals targeting the shell of nucleus accumbens (AcbSh) and investigated a possible influence over cue-conditioned alcohol seeking and alcohol drinking, based on the demonstrated roles of these areas in behavioral responding to Pavlovian cues and in feeding behavior. Rats were trained to anticipate alcohol or sucrose following the onset of a discrete conditioned stimulus (CS). Channelrhodopsin-mediated activation of the BLA-to-AcbSh pathway concurrent with each CS disrupted cued alcohol seeking. Activation of the same pathway caused rapid cessation of alcohol drinking from a sipper tube. Neither effect was accompanied by an overall change in locomotion. Finally, the suppressive effect of photoactivation on cued-triggered seeking was also evidenced in animals trained with sucrose. Together these findings suggest that photoactivation of BLA terminals in the AcbSh can override the conditioned motivational properties of reward-predictive cues as well as unconditioned consummatory responses necessary for alcohol drinking. The findings provide evidence for a limbic-striatal influence over motivated behavior for orally consumed rewards, including alcohol.

Brownian Optogenetic-Noise-Photostimulation on the Brain Amplifies Somatosensory-Evoked Field Potentials

Stochastic resonance (SR) is an inherent and counter-intuitive mechanism of signal-to-noise ratio (SNR) facilitation in biological systems associated with the application of an intermediate level of noise. As a first step to investigate in detail this phenomenon in the somatosensory system, here we examined whether the direct application of noisy light on pyramidal neurons from the mouse-barrel cortex expressing a light-gated channel channelrhodopsin-2 (ChR2) can produce facilitation in somatosensory evoked field potentials. Using anesthetized Thy1-ChR2-YFP transgenic mice, and a new neural technology, that we called Brownian optogenetic-noise-photostimulation (BONP), we provide evidence for how BONP directly applied on the barrel cortex modulates the SNR in the amplitude of whisker-evoked field potentials (whisker-EFP). In all transgenic mice, we found that the SNR in the amplitude of whisker-EFP (at 30% of the maximal whisker-EFP) exhibited an inverted U-like shape as a function of the BONP level. As a control, we also applied the same experimental paradigm, but in wild-type mice, as expected, we did not find any facilitation effects. Our results show that the application of an intermediate intensity of BONP on the barrel cortex of ChR2 transgenic mice amplifies the SNR of somatosensory whisker-EFPs. This result may be relevant to explain the improvements found in sensory detection in humans produced by the application of transcranial-random-noise-stimulation (tRNS) on the scalp.