Increased Levels of Extracellular Dopamine in the Nucleus Accumbens and Amygdala of Rats by Ingesting a Low Concentration of a Long-Chain Fatty Acid

An atlas of GPCRs in dopamine neurons: Identification of the free fatty acid receptor 4 as a regulator of food and water intake

Midbrain dopaminergic neurons (DANs) are subject to extensive metabotropic regulation, but the repertoire of G protein-coupled receptors (GPCRs) present in these neurons has not been mapped. Here, we isolate DANs from Dat-eGFP mice to generate a GPCR atlas by unbiased qPCR array expression analysis of 377 GPCRs. Combined with data mining of scRNA-seq databases, we identify multiple receptors in DAN subpopulations with 38 of these receptors representing the majority of transcripts. We identify 41 receptors expressed in midbrain DANs but not in non-DAN midbrain cells, including the free fatty acid receptor 4 (FFAR4). Functional expression of FFAR4 is validated by ex vivo Ca2+ imaging, and in vivo experiments support that FFAR4 negatively regulates food and water intake and bodyweight. In addition to providing a critical framework for understanding metabotropic DAN regulation, our data suggest fatty acid sensing by FFAR4 as a mechanism linking high-energy intake to the dopamine-reward pathway.

Role of the striatal dopamine, GABA and opioid systems in mediating feeding and fat intake

Food intake, which is a highly reinforcing behavior, provides nutrients required for survival in all animals. However, when fat and sugar consumption goes beyond the daily needs, it can favor obesity. The prevalence and severity of this health problem has been increasing with time. Besides covering nutrient and energy needs, food and in particular its highly palatable components, such as fats, also induce feelings of joy and pleasure. Experimental evidence supports a role of the striatal complex and of the mesolimbic dopamine system in both feeding and food-related reward processing, with the nucleus accumbens as a key target for reward or reinforcing-associated signaling during food intake behavior. In this review, we provide insights concerning the impact of feeding, including fat intake, on different types of receptors and neurotransmitters present in the striatal complex. Reciprocally, we also cover the evidence for a modulation of palatable food intake by different neurochemical systems in the striatal complex and in particular the nucleus accumbens, with a focus on dopamine, GABA and the opioid system.

Effect of Beta 2-Adrenergic Receptor Gly16Arg Polymorphism on Taste Preferences in Healthy Young Japanese Adults

The Gly16Arg polymorphism results in a G to C nucleotide mutation in the human beta 2-adrenergic receptor (ADRB2) gene and has a relationship with obesity; however, this substitution’s effects on food preferences are unclear. Therefore, we determined this relationship among healthy young adults (mean age, 23.4; n = 52). To evaluate food preferences, four categories of food (sweet, salty, sour, and bitter) along with high-fat foods were evaluated using a self-reporting questionnaire. Male (n = 26) and female subjects (n = 26) were genotyped for the polymorphism and further divided into three groups (two homozygous groups, GG, CC; and a heterozygous group, GC). Preference for sour foods in the GG group was higher compared with that in the CC group in females (p < 0.05). When sweet foods were classified into low- and high-fat subgroups, preference for high-fat sweet foods in the GG group was higher than that for low-fat sweet foods in all subjects (p < 0.05). The degree of preference for high-fat foods in the GG group was higher than other groups for males (p < 0.05). These results suggest that ADRB2 polymorphism is associated with food preference. Understanding the relationship of ADRB2 substitution to food preference will be valuable for designing individualized anti-obesity strategies.

Dietary lipids as regulators of reward processes: multimodal integration matters

The abundance of energy-dense and palatable diets in the modern food environment tightly contributes to the obesity pandemic. The reward circuit participates to the regulation of body homeostasis by integrating energy-related signals with neural substrates encoding cognitive and motivational components of feeding behaviors. Obesity and lipid-rich diets alter dopamine (DA) transmission leading to reward dysfunctions and food overconsumption. Recent reports indicate that dietary lipids can act, directly and indirectly, as functional modulators of the DA circuit. This raises the possibility that nutritional or genetic conditions affecting 'lipid sensing' mechanisms might lead to maladaptations of the DA system. Here, we discuss the most recent findings connecting dietary lipid sensing with DA signaling and its multimodal influence on circuits regulating food-reward processes.

HIV-Associated Apathy/Depression and Neurocognitive Impairments Reflect Persistent Dopamine Deficits

Individuals living with human immunodeficiency virus type 1 (HIV-1) are often plagued by debilitating neurocognitive impairments and affective alterations;the pathophysiology underlying these deficits likely includes dopaminergic system dysfunction. The present review utilized four interrelated aims to critically examine the evidence for dopaminergic alterations following HIV-1 viral protein exposure. First, basal dopamine (DA) values are dependent upon both brain region andexperimental approach (i.e., high-performance liquid chromatography, microdialysis or fast-scan cyclic voltammetry). Second, neurochemical measurements overwhelmingly support decreased DA concentrations following chronic HIV-1 viral protein exposure. Neurocognitive impairments, including alterations in pre-attentive processes and attention, as well as apathetic behaviors, provide an additional line of evidence for dopaminergic deficits in HIV-1. Third, to date, there is no compelling evidence that combination antiretroviral therapy (cART), the primary treatment regimen for HIV-1 seropositive individuals, has any direct pharmacological action on the dopaminergic system. Fourth, the infection of microglia by HIV-1 viral proteins may mechanistically underlie the dopamine deficit observed following chronic HIV-1 viral protein exposure. An inclusive and critical evaluation of the literature, therefore, supports the fundamental conclusion that long-term HIV-1 viral protein exposure leads to a decreased dopaminergic state, which continues to persist despite the advent of cART. Thus, effective treatment of HIV-1-associated apathy/depression and neurocognitive impairments must focus on strategies for rectifying decreases in dopamine function.

Diet containing stearic acid increases food reward-related behaviors in mice compared with oleic acid

Obesity is currently a worldwide phenomenon. The consumption of calorie-rich foods is responsible for most obesity cases, but not all humans exposed to high-calorie diets develop obesity. According to recent studies, exposure to fat-rich diets may be the actual cause of obesity. Dietary long-chain fatty acids affect brain function and are linked to food intake and motivation-related behaviors. Recently, many studies have shown that different types of fatty acids play different roles in animals. In our study, the effects of stearic acid (a saturated fatty acid) and oleic acid (a monounsaturated fatty acid) in diets on hedonic feeding behaviors were investigated, and changes of feeding-related protein levels in the brain were detected to explore the possible mechanism underlying the effects of these fatty acids. As a result, mice fed a diet containing stearic acid, compared to a diet containing oleic acid, exhibited increased food intake, hedonic eating, and an operant response to sucrose and locomotor activity. Furthermore, stearic acid corresponded to a higher level of leptin in serum than oleic acid. In addition, the stearic acid treated group had lower protein levels of p-JAK2 and p-STAT3 in the VTA and a higher dopamine concentration in the NAc than the oleic acid-treated group. Meanwhile, the protein level of TH in the NAc was higher and the protein level of the DA transporter in the VTA was lower in the stearic acid-fed group than in the oleic acid-fed group. In conclusion, these findings indicated that a diet containing stearic acid can increase hedonic feeding behavior and affect mesolimbic dopamine system signals in mice. Moreover, the lowering of serum leptin and leptin signaling in the VTA may contribute to this effect.

Where Is Dopamine and how do Immune Cells See it?: Dopamine-Mediated Immune Cell Function in Health and Disease

Dopamine is well recognized as a neurotransmitter in the brain, and regulates critical functions in a variety of peripheral systems. Growing research has also shown that dopamine acts as an important regulator of immune function. Many immune cells express dopamine receptors and other dopamine related proteins, enabling them to actively respond to dopamine and suggesting that dopaminergic immunoregulation is an important part of proper immune function. A detailed understanding of the physiological concentrations of dopamine in specific regions of the human body, particularly in peripheral systems, is critical to the development of hypotheses and experiments examining the effects of physiologically relevant dopamine concentrations on immune cells. Unfortunately, the dopamine concentrations to which these immune cells would be exposed in different anatomical regions are not clear. To address this issue, this comprehensive review details the current information regarding concentrations of dopamine found in both the central nervous system and in many regions of the periphery. In addition, we discuss the immune cells present in each region, and how these could interact with dopamine in each compartment described. Finally, the review briefly addresses how changes in these dopamine concentrations could influence immune cell dysfunction in several disease states including Parkinson's disease, multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, as well as the collection of pathologies, cognitive and motor symptoms associated with HIV infection in the central nervous system, known as NeuroHIV. These data will improve our understanding of the interactions between the dopaminergic and immune systems during both homeostatic function and in disease, clarify the effects of existing dopaminergic drugs and promote the creation of new therapeutic strategies based on manipulating immune function through dopaminergic signaling. Graphical Abstract.

Fat Addiction: Psychological and Physiological Trajectory

Obesity has become a major public health concern worldwide due to its high social and economic burden, caused by its related comorbidities, impacting physical and mental health. Dietary fat is an important source of energy along with its rewarding and reinforcing properties. The nutritional recommendations for dietary fat vary from one country to another; however, the dietary reference intake (DRI) recommends not consuming more than 35% of total calories as fat. Food rich in fat is hyperpalatable, and is liable to be consumed in excess amounts. Food addiction as a concept has gained traction in recent years, as some aspects of addiction have been demonstrated for certain varieties of food. Fat addiction can be a diagnosable condition, which has similarities with the construct of addictive disorders, and is distinct from eating disorders or normal eating behaviors. Psychological vulnerabilities like attentional biases have been identified in individuals described to be having such addiction. Animal models have provided an opportunity to explore this concept in an experimental setting. This discussion sheds light on fat addiction, and explores its physiological and psychological implications. The discussion attempts to collate the emerging literature on addiction to fat rich diets as a prominent subset of food addiction. It aims at addressing the clinical relevance at the community level, the psychological correlates of such fat addiction, and the current physiological research directions.

Hypothalamic Fatty Acids and Ketone Bodies Sensing and Role of FAT/CD36 in the Regulation of Food Intake

The obesity and type-2 diabetes epidemic is escalating and represents one of the costliest biomedical challenges confronting modern society. Moreover, the increasing consumption of high fat food is often correlated with an increase in body mass index. In people predisposed to be obese or already obese, the impaired ability of the brain to monitor and respond to alterations in fatty acid (FA) metabolism is increasingly recognized as playing a role in the pathophysiological development of these disorders. The brain senses and regulates metabolism using highly specialized nutrient-sensing neurons located mainly in the hypothalamus. The same neurons are able to detect variation in the extracellular levels of glucose, FA and ketone bodies as a way to monitor nutrient availability and to alter its own activity. In addition, glial cells such as astrocytes create major connections to neurons and form a tight relationship to closely regulate nutrient uptake and metabolism. This review will examine the different pathways by which neurons are able to detect free fatty acids (FFA) to alter its activity and how high fat diet (HFD)-astrocytes induced ketone bodies production interplays with neuronal FA sensing. The role of HFD-induced inflammation and how FA modulate the reward system will also be investigated here.

Oleoylethanolamide: The role of a bioactive lipid amide in modulating eating behaviour

Fatty acid ethanolamides are lipid mediators that regulate a plethora of physiological functions. One such bioactive lipid mediator, oleoylethanolamide (OEA), is a potent agonist of the peroxisome proliferator-activated receptor-alpha (PPAR-α), which modulates increased expression of the fatty acid translocase CD36 that enables the regulation of feeding behaviour. Consumption of dietary fat rich in oleic acid activates taste receptors in the gut activating specific enzymes that lead to the formation of OEA. OEA further combines with PPAR-α to enable fat oxidation in the liver, resulting in enhanced energy production. Evidence suggests that sustained ingestion of a high-fat diet abolishes the anorexic signal of OEA. Additionally, malfunction of the enterocyte that transforms oleic acid produced during fat digestion into OEA might be responsible for reduced satiety and hyperphagia, resulting in overweight and obesity. Thus, OEA anorectic signalling may be an essential element of the physiology and metabolic system regulating dietary fat intake and obesity. The evidence reviewed in this article indicates that intake of oleic acid, and thereby the resulting OEA imparting anorexic properties, is dependent on CD36, PPAR-α, enterocyte fat sensory receptors, histamine, oxytocin and dopamine; leading to increased fat oxidation and enhanced energy expenditure to induce satiety and increase feeding latency; and that a disruption in any of these systems will cease/curb fat-induced satiety.

Behavioral palatability of dietary fatty acids correlates with the intracellular calcium ion levels induced by the fatty acids in GPR120-expressing cells

We recently reported that G-protein-coupled receptor 120 (GPR120) is expressed on taste buds, and that rodents showed preference for long-chain fatty acids (LCFA) at a low concentration. We also showed that the LCFA (1% linoleic acid) increased the extracellular dopamine (DA) level in the nucleus accumbens (NAc), which participates in reward behavior. However, the mechanism underlying the involvement of the GPR120-agonistic activity of LCFA in the palatability of dietary fat remains elusive. Therefore, we examined the association between the GPR120-agonistic activity and palatability of LCFA. First, we measured Ca(2+) signaling in HEK293 cells stably expressing GPR120 under stimulation by various LCFAs. We then assessed the palatability of the various LCFAs by testing the licking behavior in mice and measured the changes in the NAc-DA level by in vivo microdialysis. Consequently, 14- to 22-carbon unsaturated LCFAs showed strong GPR120-agonistic activity. Additionally, mice displayed high licking response to unsaturated 16- and 18-carbon LCFAs, and unsaturated 18-carbon LCFA significantly increased the DA level. The licking rate and the LCFA-dependent increase in DA level also correlated well with the GPR120- agonistic activity. These findings demonstrate that chemoreception of LCFA by GPR120 is involved in the recognition and palatability of dietary fat.

Regulation of the orexigenic neuropeptide, enkephalin, by PPARδ and fatty acids in neurons of the hypothalamus and forebrain

Ingestion of a high-fat diet composed mainly of the saturated fatty acid, palmitic (PA), and the unsaturated fatty acid, oleic (OA), stimulates transcription in the brain of the opioid neuropeptide, enkephalin (ENK), which promotes intake of substances of abuse. To understand possible underlying mechanisms, this study examined the nuclear receptors, peroxisome proliferator-activated receptors (PPARs), and tested in hypothalamic and forebrain neurons from rat embryos whether PPARs regulate endogenous ENK and the fatty acids themselves affect these PPARs and ENK. The first set of experiments demonstrated that knocking down PPARδ, but not PPARα or PPARγ, increased ENK transcription, activation of PPARδ by an agonist decreased ENK levels, and PPARδ neurons coexpressed ENK, suggesting that PPARδ negatively regulates ENK. In the second set of experiments, PA treatment of hypothalamic and forebrain neurons had no effect on PPARδ protein while stimulating ENK mRNA and protein, whereas OA increased both mRNA and protein levels of PPARδ in forebrain neurons while having no effect on ENK mRNA and increasing ENK levels. These findings show that PA has a strong, stimulatory effect on ENK and weak effect on PPARδ protein, whereas OA has a strong stimulatory effect on PPARδ and weak effect on ENK, consistent with the inhibitory effect of PPARδ on ENK. They suggest a function for PPARδ, perhaps protective in nature, in embryonic neurons exposed to fatty acids from a fat-rich diet and provide evidence for a mechanism contributing to differential effects of saturated and monounsaturated fatty acids on neurochemical systems involved in consummatory behavior. Our findings show that PPARδ in forebrain and hypothalamic neurons negatively regulates enkephalin (ENK), a peptide known to promote ingestive behavior. This inverse relationship is consistent with our additional findings, that a saturated (palmitic; PA) compared to a monounsaturated fatty acid (oleic; OA) has a strong stimulatory effect on ENK and weak effect on PPARδ. These results suggest that PPARδ protects against the neuronal effects of fatty acids, which differentially affect neurochemical systems involved in ingestive behavior.

Neural responses to macronutrients: hedonic and homeostatic mechanisms

The brain responds to macronutrients via intricate mechanisms. We review how the brain's neural systems implicated in homeostatic control of feeding and hedonic responses are influenced by the ingestion of specific types of food. We discuss how these neural systems are dysregulated in preclinical models of obesity. Findings from these studies can increase our understanding of overeating and, perhaps in some cases, the development of obesity. In addition, a greater understanding of the neural circuits affected by the consumption of specific macronutrients, and by obesity, might lead to new treatments and strategies for preventing unhealthy weight gain.

Intestinal lipid-derived signals that sense dietary fat

Fat is a vital macronutrient, and its intake is closely monitored by an array of molecular sensors distributed throughout the alimentary canal. In the mouth, dietary fat constituents such as mono- and diunsaturated fatty acids give rise to taste signals that stimulate food intake, in part by enhancing the production of lipid-derived endocannabinoid messengers in the gut. As fat-containing chyme enters the small intestine, it causes the formation of anorexic lipid mediators, such as oleoylethanolamide, which promote satiety. These anatomically and functionally distinct responses may contribute to the homeostatic control and, possibly, the pathological dysregulation of food intake.

A thermal window for yawning in humans: yawning as a brain cooling mechanism

The thermoregulatory theory of yawning posits that yawns function to cool the brain in part due to counter-current heat exchange with the deep inhalation of ambient air. Consequently, yawning should be constrained to an optimal thermal zone or range of temperature, i.e., a thermal window, in which we should expect a lower frequency at extreme temperatures. Previous research shows that yawn frequency diminishes as ambient temperatures rise and approach body temperature, but a lower bound to the thermal window has not been demonstrated. To test this, a total of 120 pedestrians were sampled for susceptibly to self-reported yawn contagion during distinct temperature ranges and seasons (winter: 1.4°C, n=60; summer: 19.4°C, n=60). As predicted, the proportion of pedestrians reporting yawning was significantly lower during winter than in summer (18.3% vs. 41.7%), with temperature being the only significant predictor of these differences across seasons. The underlying mechanism for yawning in humans, both spontaneous and contagious, appears to be involved in brain thermoregulation.

Is fat taste ready for primetime?

Mounting evidence suggests that gustation is important for the orosensory detection of dietary fats, and might contribute to preferences that humans, rodents, and possibly other mammals exhibit for fat-rich foods. In contrast to sweet, sour, salty, bitter, and umami, fat is not widely recognized as a primary taste quality. Recent investigations, however, provide a wealth of information that is helping to elucidate the specific molecular, cellular, and neural mechanisms required for fat detection in mammals. The latest evidence supporting a fat taste will be explored in this review, with a particular focus on recent studies that suggest a surprising role for gut-brain endocannabinoid signaling in controlling intake and preference for fats based on their proposed taste properties.