Functional neuroimaging of umami taste: what makes umami pleasant?

Enhancing saltiness perception by chemosensory interaction: an fMRI study

Neuroimaging studies that focus on taste, odor, and their interactions can specify their capability to elicit brain regions responsible for flavor perception and reward. Such information would be useful for formulating healthy food products, such as low salt food. In this study, a sensory experiment was conducted to investigate the capability of cheddar cheese odor, monosodium glutamate (MSG), and their interactions to enhance saltiness perception and preference of NaCl solutions. The activated brain areas in response to odor-taste-taste interactions were then investigated using an fMRI study. The results of the sensory tests showed that saltiness and preference of NaCl solutions were enhanced in the presence of MSG + cheddar cheese odor. According to the fMRI study, the stimulus with a higher salty rate activated the rolandic operculum, and the stimulus with a higher preference activated the rectus, medial orbitofrontal cortex, and substantia nigra. Moreover, the activation of multiple regions, such as the orbitofrontal cortex (OFC), anterior cingulate cortex (ACC), temporal pole, and amygdala was observed in response to (cheddar cheese odor + MSG + NaCl) minus (odorless air + NaCl).

The orbitofrontal cortex, food reward, body weight and obesity

In primates including humans, the orbitofrontal cortex is the key brain region representing the reward value and subjective pleasantness of the sight, smell, taste and texture of food. At stages of processing before this, in the insular taste cortex and inferior temporal visual cortex, the identity of the food is represented, but not its affective value. In rodents, the whole organisation of reward systems appears to be different, with reward value reflected earlier in processing systems. In primates and humans, the amygdala is overshadowed by the great development of the orbitofrontal cortex. Social and cognitive factors exert a top-down influence on the orbitofrontal cortex, to modulate the reward value of food that is represented in the orbitofrontal cortex. Recent evidence shows that even in the resting state, with no food present as a stimulus, the liking for food, and probably as a consequence of that body mass index, is correlated with the functional connectivity of the orbitofrontal cortex and ventromedial prefrontal cortex. This suggests that individual differences in these orbitofrontal cortex reward systems contribute to individual differences in food pleasantness and obesity. Implications of how these reward systems in the brain operate for understanding, preventing and treating obesity are described.

Healthy Eating in Population Models of Nutrition: Asian Diet Style Summary

The population of Japan and Okinawa is known for the longest life expectancy, which many researchers rightly associate with the nature of nutrition existing in these territories. The Japanese diet and Okinawan diet, along with other traditional diets, are real examples of historically established sustainable patterns of healthy eating. Asian eating styles have marked differences from European eating patterns, not only in differences in food sources, but also in eating habits. The article presents the historical, climatic and cultural features of these diets; the issues of food composition, energy and nutritional value of these models of nutrition are considered in detail with an analysis of the differences existing between them; highlights the benefits of products grown mainly in Japan, which are ration-forming for the population of this country; as well as the results of scientific studies on the protective effect of the Japanese and Okinawan diets on human health and disease prevention.

Interconnected sub-networks of the macaque monkey gustatory connectome

Macroscopic taste processing connectivity was investigated using functional magnetic resonance imaging during the presentation of sour, salty, and sweet tastants in anesthetized macaque monkeys. This examination of taste processing affords the opportunity to study the interactions between sensory regions, central integrators, and effector areas. Here, 58 brain regions associated with gustatory processing in primates were aggregated, collectively forming the gustatory connectome. Regional regression coefficients (or β-series) obtained during taste stimulation were correlated to infer functional connectivity. This connectivity was then evaluated by assessing its laterality, modularity and centrality. Our results indicate significant correlations between same region pairs across hemispheres in a bilaterally interconnected scheme for taste processing throughout the gustatory connectome. Using unbiased community detection, three bilateral sub-networks were detected within the graph of the connectome. This analysis revealed clustering of 16 medial cortical structures, 24 lateral structures, and 18 subcortical structures. Across the three sub-networks, a similar pattern was observed in the differential processing of taste qualities. In all cases, the amplitude of the response was greatest for sweet, but the network connectivity was strongest for sour and salty tastants. The importance of each region in taste processing was computed using node centrality measures within the connectome graph, showing centrality to be correlated across hemispheres and, to a smaller extent, region volume. Connectome hubs exhibited varying degrees of centrality with a prominent leftward increase in insular cortex centrality. Taken together, these criteria illustrate quantifiable characteristics of the macaque monkey gustatory connectome and its organization as a tri-modular network, which may reflect the general medial-lateral-subcortical organization of salience and interoception processing networks.

Sensory Stimulation Via the Visual, Auditory, Olfactory and Gustatory Systems Can Modulate Mood and Depression

Depression is one of the most common mental disorders, predicted to be the leading cause of disease burden by the next decade. There is great deal of emphasis on the central origin and potential therapeutics of depression whereby the symptomatology of depression has been interpreted and treated as brain generated dysfunctions filtering down to the periphery. This top-down approach has found strong support from clinical work and basic neuroscientific research. Nevertheless, despite great advances in our knowledge of the etiology and therapeutics of depression, success in treatment is still by no means assured.. As a consequence, a wide net has been cast by both clinicians and researchers in search of more efficient therapies for mood disorders. As a complementary view, the present integrative review advocates approaching mood and depression from the opposite perspective: a bottom-up view that starts from the periphery. Specifically, evidence is provided to show that sensory stimulation via the visual, auditory, olfactory and gustatory systems can modulate depression. The review shows how -depending on several parameters- unisensory stimulation via these modalities can ameliorate or aggravate depressive symptoms. Moreover, the review emphasizes the bidirectional relationship between sensory stimulation and depression. Just as peripheral stimulation can modulate depression, depression in turn affects-and in most cases impairs-sensory reception. Furthermore, the review suggests that combined use of multisensory stimulation may have synergistic ameliorative effects on depressive symptoms over and above what has so far been documented for unisensory stimulation.

A systematic review and meta-analyses of food preference modifications after bariatric surgery

This systematic review and meta-analyses aimed to synthesize evidence of the link between bariatric surgery and changes in food preferences, considering the method of assessment. MEDLINE, Cochrane Library, Web of Science, Cinahl, PsychINFO, ProQuest, and Open grey were searched incorporating two blocks of terms ("Intervention" and "Food Preferences"). Interventional or observational studies involving patients (BMI ≥ 35 kg m^-2 ) with sleeve gastrectomy (SG) or Roux-en-Y Gastric Bypass (RYGB) and a control group were included. Meta-analyses were performed comparing the standardized daily mean percentage energy from proteins, carbohydrates, and lipids between preoperative and postoperative patients. Fifty-seven studies concerning 2,271 patients with RYGB and 903 patients with SG met the inclusion criteria, of which 24 were eligible for meta-analysis. Despite a total reduction in macronutrient intakes, the meta-analyses revealed a postoperative increase in percentage energy from proteins at 12 months (0.24, 95% CI: 0.03, 0.46, {I² } = 73%) and a decrease in percentage energy from fat at 1 month (-0.47, 95% CI: 0.86, 0.09, {I² } = 72%), up to 24 months (-0.20, 95% CI: -0.31, 0.08, {I² } = 0%). In conclusion, the present systematic review and meta-analyses showed changes of food preferences in terms of macronutrient, food selection and, overall food appreciation up to 5 years following bariatric surgery.

Omega-3 Rich Oils Attenuate ADHD-Like Behaviour Induced by Dietary Monosodium Glutamate in Rats

<b>Background and Objective:</b> Attention Deficit Hyperactivity Disorder (ADHD) is a neurodevelopmental disorder characterized by inattention, hyperactivity and cognitive dysfunction. The present study was designed to examine the possible modulatory effect of Fish, Walnuts or Fenugreek Oils against Attention Deficit Hyperactivity Disorder (ADHD)-like Behavior induced by Monosodium Glutamate (MSG) in Rats. <b>Materials and Methods:</b> Fifty weaning rats were divided into five groups, (each group contain 10 rats) as follows: Group 1: Normal control rats were fed on a balanced diet. Groups from 2-5 rats were fed on a balanced diet+MSG (0.4 g kg<sup></sup><sup>1</sup> diet), Group 2 served as a positive control group whereas group 3, 4 and 5 treated with Fish, Walnuts and Fenugreek oil, respectively, (200 mg kg<sup></sup><sup>1</sup> b.wt.) by intra-gastric tube. Biochemical and behavioural parameters were tested as well as microscopic examination of brain tissue was done. <b>Results:</b> MSG ingestion caused marked disruption in locomotors activity, memory function and brain tissue structure along with significant abnormalities in some bio-markers and reduction in the gene expression level of Bcl-2 in brain tissue. However, treatment with the tested oils showed remarkable effect by reversing the condition. <b>Conclusion:</b> Dietary supplementation with walnut; fenugreek or fish oils at the tested dose could modulate the condition of ADHD in rats.

Mechanisms of umami taste perception: From molecular level to brain imaging

Due to unique characteristics, umami substances have gained much attention in the food industry during the past decade as potential replacers to sodium or fat to increase food palatability. Umami is not only known to increase appetite, but also to increase satiety, and hence could be used to control food intake. Therefore, it is important to understand the mechanism(s) involved in umami taste perception. This review discusses current knowledge of the mechanism(s) of umami perception from receptor level to human brain imaging. New findings regarding the molecular mechanisms for detecting umami tastes and their pathway(s), and the peripheral and central coding to umami taste are reviewed. The representation of umami in the human brain and the individual variation in detecting umami taste and associations with genotype are discussed. The presence of umami taste receptors in the gastrointestinal tract, and the interactions between the brain and gut are highlighted. The review concludes that more research is required into umami taste perception to include not only oral umami taste perception, but also the wider "whole body" signaling mechanisms, to explore the interaction between the brain and gut in response to umami perception and ingestion.

The use of 'artificial saliva' as a neutral control condition in gustatory research: Artificial saliva is not a neutral gustatory stimulus

Distilled water with NaHCO₃ and KCl is a solution often referred to as 'artificial saliva' because its chemical composition mimics human saliva. It is often used as a control stimulus in gustatory research, especially in neuroimaging, owing to the claim that it does not produce a response in primary gustatory cortex Yet evidence that human research volunteers perceive this liquid as affectively neutral is lacking. Unpublished data from our lab suggested that this solution might be perceived as aversive. This study set out to systematically test the parameters influencing taste neutrality. We used two different concentrations of distilled water with NaHCO₃ and KCl, as well as bottled water as a control stimulus. Healthy adults rated all stimuli on two separate scales to rule out an interpretation based on the specifics of a single scale. Our participants rated artificial saliva as aversive on both scales. The bottled water was rated as neutral in valence on both scales, and as significantly less intense in sensation than both concentrations of the artificial saliva. This is the first study to have directly tested the subjective feelings that accompany the ingestion of these oft-used solutions on a trial-by-trial basis. We found that these stimuli, which were previously assumed to be neutral, may not be perceived as such by research participants. Therefore, future gustatory studies should take care when using this solution as a neutral baseline. It is advised that trial-by-trial ratings are collected. Also, depending on the nature of future studies, bottled water may be considered as a preferable neutral baseline.

Food Folio by Columbia Center for Eating Disorders: A Freely Available Food Image Database

Food images are useful stimuli for the study of cognitive processes as well as eating behavior. To enhance rigor and reproducibility in task-based research, it is advantageous to have stimulus sets that are publicly available and well characterized. Food Folio by Columbia Center for Eating Disorders is a publicly available set of 138 images of Western food items. The set was developed for the study of eating disorders, particularly for use in tasks that capture eating behavior characteristic of these illnesses. It contains foods that are typically eaten, as well as those typically avoided, by individuals with eating disorders. Each image has now been rated across 17 different attributes by a large general United States population sample via Amazon's Mechanical Turk (n = 1054). Ratings included subjective attributes (e.g., tastiness, healthiness, and favorable texture) as well as estimates of nutrient content (e.g., fat and carbohydrate). Each participant rated a subset of stimulus set food items (46 foods) on all 17 dimensions. Additional description of the image set is provided in terms of physical image information and accurate nutritional information. Correlations between subjective ratings were calculated and an exploratory factor analysis and exploratory cluster analysis completed. Outcomes of the factor analysis suggested foods may be described along three latent factors of healthiness, tastiness, and umami taste; the cluster analysis highlighted five distinct clusters of foods varying on these same dimensions. Descriptive outcomes indicated that the stimulus set includes a range of foods that vary along multiple dimensions and thus is likely to be useful in addressing various research questions surrounding eating behavior and cognition in healthy populations, as well as in those with eating disorders. The provision of comprehensive descriptive information allows for stimulus selection that is optimized for a given research question and promotes strong inference.

Exploring Anhedonia in Kennelled Dogs: Could Coping Styles Affect Hedonic Preferences for Sweet and Umami Flavours?

Simple Summary

Kennelled dogs are susceptible to suffer chronic stress when social interactions with conspecifics and spatial needs are long-term restricted. Chronic stress may affect pleasure perception of food and solutions in dogs as observed in several animals, a phenomenon known as anhedonia. However, little information exists on how different coping styles could prevent the onset of anhedonia. Fourteen kennelled Beagle dogs were used to study the acceptability and preference for different dilute palatable sucrose and monosodium glutamate (MSG) solutions. Coping style of animals was previously evaluated through a modified human-approach test. This test consisted in assessing whether or not dogs approached an unfamiliar human when a feeding opportunity was presented, classifying them as close dogs (CD; proactive) or distant dogs (DD; reactive) respectively. It was observed that DD presented a lower intake of both sucrose and MSG dilute solutions compared with CD. Moreover, DD exhibited a higher consumption of MSG than CD at the highest concentrations, supporting that their intake depends on solution palatability. Finally, DD did not prefer sucrose or MSG solutions over water at any dilute solution offered. Together, these results suggest that dogs that are categorized as reactive animals could diminish their ability to perceive dilute palatable solutions reflecting depressive-like behaviours such as anhedonia.

Abstract

Kennelled dogs are at risk of suffering chronic stress due to long-term spatial, social and feeding restrictions. Chronic stress may decrease the dogs’ capacity to feel pleasure when facing hedonic experiences, modifying their perception for palatable ingredients. However, different abilities to cope with environmental stressors could prevent the onset of anhedonia. Fourteen kennelled Beagle dogs were used to study the acceptability and preference for different dilute sucrose and monosodium glutamate (MSG) solutions. Coping style of animals was previously evaluated through a human approach test (HAT) and classified as close dogs (CD; proactive) or distant dogs (DD; reactive) according to whether or not they approached an unfamiliar human when a feeding opportunity was presented. Consumption results were analysed taking into account the sucrose/MSG concentrations, HAT (CD or DD), age, and weight of the animals. DD presented a lower intake of sucrose (p = 0.041) and MSG (p = 0.069) solutions compared with CD. However, DD exhibited a higher consumption of MSG than CD at its highest concentrations, supporting that their intake depends on solution palatability. Finally, DD did not prefer sucrose or MSG solutions over water at any dilute solution offered. Together, these results suggest that dogs that are categorized as reactive animals could diminish their ability to perceive dilute palatable solutions, reflecting depressive-like behaviours as anhedonia.

A Systematic Review and Activation Likelihood Estimation Meta-Analysis of fMRI Studies on Sweet Taste in Humans

BACKGROUND

The reward value of palatable foods is often cited as an important influence on eating behaviors, including intake of sugars. However, human neuroimaging studies have generated conflicting evidence on the basic neural representation of taste and reward responses to caloric sweeteners (sucrose and glucose), and most relevant studies have used small subject numbers.

OBJECTIVE

We conducted a systematic review and a coordinate-based meta-analysis of studies reporting brain responses to oral sugar solutions.

METHODS

A systematic search of MEDLINE, Scopus, and PsycINFO through October 2019 identified fMRI studies (in healthy human adults, including those with overweight or obesity) assessing differences in responses to purified sweet and nonsweet taste stimuli. Data were extracted with the primary objective of quantifying evidence for the activation of brain regions associated with caloric sweet taste sensation. We used activation likelihood estimation meta-analysis methods. We also performed multiple sensitivity analyses to assess the generality of effects.

RESULTS

Of 455 unique articles, 15 met the criteria for inclusion. These contributed to 2 primary meta-analyses: 1) sucrose (13 experiments, 179 coordinates, n = 241) and 2) sucrose + glucose (16 experiments, 209 coordinates, n = 262). Consistent activation was apparent in primary taste areas: insula (69.2% of studies) and opercular cortex (76.9% of studies), precentral gyri (53.9% of studies), and globus pallidus and postcentral gyrus (30.8% of studies for each). Evidence of reward activity (caudate) was seen in the primary analyses (30.8% of studies) but not in sensitivity analysis.

CONCLUSIONS

We confirm the importance of primary taste areas for gustatory processing in human adults. We also provide tentative evidence for reward-related caudate activity in relation to the sweet taste of caloric sugars. A number of factors affect the observation and interpretation of brain responses, including reward-related activity. Firm conclusions require confirmation with large data set studies.

The texture and taste of food in the brain

Oral texture is represented in the brain areas that represent taste, including the primary taste cortex, the orbitofrontal cortex, and the amygdala. Some neurons represent viscosity, and their responses correlate with the subjective thickness of a food. Other neurons represent fat in the mouth, and represent it by its texture not by its chemical composition, in that they also respond to paraffin oil and silicone in the mouth. The discovery has been made that these fat-responsive neurons encode the coefficient of sliding friction and not viscosity, and this opens the way for the development of new foods with the pleasant mouth feel of fat and with health-promoting designed nutritional properties. A few other neurons respond to free fatty acids (such as linoleic acid), do not respond to fat in the mouth, and may contribute to some "off" tastes in the mouth. Some other neurons code for astringency. Others neurons respond to other aspects of texture such as the crisp fresh texture of a slice of apple versus the same apple after blending. Different neurons respond to different combinations of these texture properties, oral temperature, taste, and in the orbitofrontal cortex to olfactory and visual properties of food. In the orbitofrontal cortex, the pleasantness and reward value of the food is represented, but the primary taste cortex represents taste and texture independently of value. These discoveries were made in macaques that have similar cortical brain areas for taste and texture processing as humans, and complementary human functional neuroimaging studies are described.

Taste and smell processing in the brain

Taste pathways in humans and other primates project from the nucleus of the solitary tract directly to the taste thalamus, and then to the taste insula. The taste cortex in the anterior insula provides separate and combined representations of the taste, temperature, and texture of food in the mouth independently of hunger and thus of reward value and pleasantness. One synapse on, in the orbitofrontal cortex, these sensory inputs are for some neurons combined by associative learning with olfactory inputs received from the pyriform cortex, and visual inputs from the temporal lobe, and these neurons encode food reward value in that they only respond to food when hungry, and in that activations correlate linearly with subjective pleasantness. Cognitive factors, including word-level descriptions, and selective attention to affective value, modulate the representation of the reward value of taste, olfactory and flavor stimuli in the orbitofrontal cortex and a region to which it projects, the anterior cingulate cortex. These food reward representations are important in the control of appetite, and the liking of food. Individual differences in these reward representations may contribute to obesity, and there are age-related differences in these reward representations.

Neuropsychopharmacological aesthetics: A theoretical consideration of pharmacological approaches to causative brain study in aesthetics and art

Recent developments in neuroaesthetics have heightened the need for causative approaches to more deeply understand the mechanism underlying perception, emotion, and aesthetic experiences. This has recently been the topic for empirical work, employing several causative methods for changing brain activity, as well as comparative assessments of individuals with brain damage or disease. However, one area of study with high potential, and indeed a long history of often nonscientific use in the area of aesthetics and art, employing psychopharmacological chemicals as means of changing brain function, has not been systematically utilized. This chapter reviews the literature on this topic, analyzing neuroendocrinological (neurochemical) approaches and mechanisms that might be used to causatively study the aesthetic brain. We focus on four relevant neuromodulatory systems potentially related to aesthetic experience: the dopaminergic, serotonergic, cannabinoid, and the opioidergic system. We build a bridge to psychopharmacological methods and review drug-induced behavioral and neurobiological consequences. We conclude with a discussion of hypotheses and suggestions for future research.

Functions of the anterior insula in taste, autonomic, and related functions

The anterior insula contains the primary taste cortex, in which neurons in primates respond to different combinations providing a distributed representation of different prototypical tastes, oral texture including fat texture, and oral temperature. These taste neurons do not represent food reward value, in that feeding to satiety does not reduce their responses to zero, in contrast to the next stage of processing, the orbitofrontal cortex, where food reward value is represented. Corresponding results are found with fMRI in humans. A more ventral part of the anterior insula is implicated using fMRI in autonomic-visceral functions. 'Salient' stimuli, including rewarding, punishing, non-rewarding, and novel stimuli may activate this viscero-autonomic system, via inputs received from regions that represent these stimuli such as the orbitofrontal and anterior cingulate cortex. More posteriorly in the insula, there is an oral somatosensory region, and posterior to this somatosensory regions that respond to touch to the body. These taste and somatosensory representations in the insula provide representations that are about the external world (touch), are intermediate (oral taste and texture), and are about internal signals related to visceral and autonomic function. This functionality needs to be taken into account when considering activations of the insula found in cognitive tasks.

Umami the Fifth Basic Taste: History of Studies on Receptor Mechanisms and Role as a Food Flavor

Three umami substances (glutamate, 5'-inosinate, and 5'-guanylate) were found by Japanese scientists, but umami has not been recognized in Europe and America for a long time. In the late 1900s, umami was internationally recognized as the fifth basic taste based on psychophysical, electrophysiological, and biochemical studies. Three umami receptors (T1R1 + T1R3, mGluR4, and mGluR1) were identified. There is a synergism between glutamate and the 5'-nucleotides. Among the above receptors, only T1R1 + T1R3 receptor exhibits the synergism. In rats, the response to a mixture of glutamate and 5'-inosinate is about 1.7 times larger than that to glutamate alone. In human, the response to the mixture is about 8 times larger than that to glutamate alone. Since glutamate and 5'-inosinate are contained in various foods, we taste umami induced by the synergism in daily eating. Hence umami taste induced by the synergism is a main umami taste in human.

Functional organization of human subgenual cortical areas: Relationship between architectonical segregation and connectional heterogeneity

Human subgenual anterior cingulate cortex (sACC) is involved in affective experiences and fear processing. Functional neuroimaging studies view it as a homogeneous cortical entity. However, sACC comprises several distinct cyto- and receptorarchitectonical areas: 25, s24, s32, and the ventral portion of area 33. Thus, we hypothesized that the areas may also be connectionally and functionally distinct. We performed structural post mortem and functional in vivo analyses. We computed probabilistic maps of each area based on cytoarchitectonical analysis of ten post mortem brains. Maps, publicly available via the JuBrain atlas and the Anatomy Toolbox, were used to define seed regions of task-dependent functional connectivity profiles and quantitative functional decoding. sACC areas presented distinct co-activation patterns within widespread networks encompassing cortical and subcortical regions. They shared common functional domains related to emotion, perception and cognition. A more specific analysis of these domains revealed an association of s24 with sadness, and of s32 with fear processing. Both areas were activated during taste evaluation, and co-activated with the amygdala, a key node of the affective network. s32 co-activated with areas of the executive control network, and was associated with tasks probing cognition in which stimuli did not have an emotional component. Area 33 was activated by painful stimuli, and co-activated with areas of the sensorimotor network. These results support the concept of a connectional and functional specificity of the cyto- and receptorarchitectonically defined areas within the sACC, which can no longer be seen as a structurally and functionally homogeneous brain region.

Limbic systems for emotion and for memory, but no single limbic system

The concept of a (single) limbic system is shown to be outmoded. Instead, anatomical, neurophysiological, functional neuroimaging, and neuropsychological evidence is described that anterior limbic and related structures including the orbitofrontal cortex and amygdala are involved in emotion, reward valuation, and reward-related decision-making (but not memory), with the value representations transmitted to the anterior cingulate cortex for action-outcome learning. In this 'emotion limbic system' a computational principle is that feedforward pattern association networks learn associations from visual, olfactory and auditory stimuli, to primary reinforcers such as taste, touch, and pain. In primates including humans this learning can be very rapid and rule-based, with the orbitofrontal cortex overshadowing the amygdala in this learning important for social and emotional behaviour. Complementary evidence is described showing that the hippocampus and limbic structures to which it is connected including the posterior cingulate cortex and the fornix-mammillary body-anterior thalamus-posterior cingulate circuit are involved in episodic or event memory, but not emotion. This 'hippocampal system' receives information from neocortical areas about spatial location, and objects, and can rapidly associate this information together by the different computational principle of autoassociation in the CA3 region of the hippocampus involving feedback. The system can later recall the whole of this information in the CA3 region from any component, a feedback process, and can recall the information back to neocortical areas, again a feedback (to neocortex) recall process. Emotion can enter this memory system from the orbitofrontal cortex etc., and be recalled back to the orbitofrontal cortex etc. during memory recall, but the emotional and hippocampal networks or 'limbic systems' operate by different computational principles, and operate independently of each other except insofar as an emotional state or reward value attribute may be part of an episodic memory.

Lateral hypothalamus contains two types of palatability-related taste responses with distinct dynamics

The taste of foods, in particular the palatability of these tastes, exerts a powerful influence on our feeding choices. Although the lateral hypothalamus (LH) has long been known to regulate feeding behavior, taste processing in LH remains relatively understudied. Here, we examined single-unit LH responses in rats subjected to a battery of taste stimuli that differed in both chemical composition and palatability. Like neurons in cortex and amygdala, LH neurons produced a brief epoch of nonspecific responses followed by a protracted period of taste-specific firing. Unlike in cortex, however, where palatability-related information only appears 500 ms after the onset of taste-specific firing, taste specificity in LH was dominated by palatability-related firing, consistent with LH's role as a feeding center. Upon closer inspection, taste-specific LH neurons fell reliably into one of two subtypes: the first type showed a reliable affinity for palatable tastes, low spontaneous firing rates, phasic responses, and relatively narrow tuning; the second type showed strongest modulation to aversive tastes, high spontaneous firing rates, protracted responses, and broader tuning. Although neurons producing both types of responses were found within the same regions of LH, cross-correlation analyses suggest that they may participate in distinct functional networks. Our data shed light on the implementation of palatability processing both within LH and throughout the taste circuit, and may ultimately have implications for LH's role in the formation and maintenance of taste preferences and aversions.

Lateral hypothalamus contains two types of palatability-related taste responses with distinct dynamics

The taste of foods, in particular the palatability of these tastes, exerts a powerful influence on our feeding choices. Although the lateral hypothalamus (LH) has long been known to regulate feeding behavior, taste processing in LH remains relatively understudied. Here, we examined single-unit LH responses in rats subjected to a battery of taste stimuli that differed in both chemical composition and palatability. Like neurons in cortex and amygdala, LH neurons produced a brief epoch of nonspecific responses followed by a protracted period of taste-specific firing. Unlike in cortex, however, where palatability-related information only appears 500 ms after the onset of taste-specific firing, taste specificity in LH was dominated by palatability-related firing, consistent with LH's role as a feeding center. Upon closer inspection, taste-specific LH neurons fell reliably into one of two subtypes: the first type showed a reliable affinity for palatable tastes, low spontaneous firing rates, phasic responses, and relatively narrow tuning; the second type showed strongest modulation to aversive tastes, high spontaneous firing rates, protracted responses, and broader tuning. Although neurons producing both types of responses were found within the same regions of LH, cross-correlation analyses suggest that they may participate in distinct functional networks. Our data shed light on the implementation of palatability processing both within LH and throughout the taste circuit, and may ultimately have implications for LH's role in the formation and maintenance of taste preferences and aversions.

Excitotoxicity: bridge to various triggers in neurodegenerative disorders

Glutamate is one of the most prominent neurotransmitter in the body, present in over 50% of nervous tissue and plays an important role in neuronal excitation. This neuronal excitation is short-lived and is followed by depression. Multiple abnormal triggers such as energy deficiency, oxidative stress, mitochondrial dysfunction, calcium overload, etc can lead to aberration in neuronal excitation process. Such an aberration, serves as a common pool or bridge between abnormal triggers and deleterious signaling processes with which central neurons cannot cope up, leading to death. Excitotoxicity is the pathological process by which nerve cells are damaged and killed by excessive stimulation by neurotransmitters such as glutamate and similar substances. Such excitotoxic neuronal death has been implicated in spinal cord injury, stroke, traumatic brain injury, hearing loss and in neurodegenerative diseases of the central nervous system such as stroke, epilepsy, multiple sclerosis, Alzheimer disease, Amyltropic lateral sclerosis, Parkinson's disease, Huntington disease and alcohol withdrawal. This review mainly emphasizes the triggering events which sustain neuronal excitation, role of calcium, mitochondrial dysfunction, ROS, NO, chloride homeostasis and eicosanoids pathways. Further, a brief introduction about the recent research occurring in the treatment of various neurodegenerative diseases, including a summary of the presumed physiologic mechanisms behind the pharmacology of these disorders.

Taste, olfactory and food texture reward processing in the brain and the control of appetite

Complementary neuronal recordings and functional neuroimaging in human subjects show that the primary taste cortex in the anterior insula provides separate and combined representations of the taste, temperature and texture (including fat texture) of food in the mouth independently of hunger and thus of reward value and pleasantness. One synapse on, in the orbitofrontal cortex (OFC), these sensory inputs are for some neurons combined by learning with olfactory and visual inputs, and these neurons encode food reward in that they only respond to food when hungry, and in that activations correlate with subjective pleasantness. Cognitive factors, including word-level descriptions, and attention modulate the representation of the reward value of food in the OFC and a region to which it projects, the anterior cingulate cortex. Further, there are individual differences in the representation of the reward value of food in the OFC. It is argued that over-eating and obesity are related in many cases to an increased reward value of the sensory inputs produced by foods, and their modulation by cognition and attention that over-ride existing satiety signals. It is proposed that control of all rather than one or several of these factors that influence food reward and eating may be important in the prevention and treatment of overeating and obesity.

Disrupted action perception in autism: behavioral evidence, neuroendophenotypes, and diagnostic utility

Disruptions in the visual perception of biological motion are emerging as a hallmark of autism spectrum disorder (ASD), consistent with the pathognomonic social deficits of this neurodevelopmental disorder. Accumulating evidence suggests an early and marked divergence in ASD from the typical developmental tuning of brain regions to process social information. In this review, we discuss a relatively recent yet substantial literature of behavioral and neuroimaging studies that consistently indicates impairments in biological motion perception in ASD. We then illustrate the fundamental disruption in this form of social perception in autism, drawing connections between a genetic liability to develop autism and disrupted associated brain mechanisms, as we describe neuroendophenotypes of autism derived from an fMRI study of biological motion perception in children with autism and their unaffected siblings. Finally, we demonstrate the diagnostic utility of brain responses to biological motion. With the ability to measure brain function in the first year of life comes the potential to chart the development of disrupted biological motion processing in ASD and to specify the gene-brain-behavior interactions shaping this atypical trajectory. We propose that a comprehensive understanding of the development of impaired responses to biological motion in ASD can inform future diagnosis and treatment approaches.

Basic taste stimuli elicit unique responses in facial skin blood flow

Facial expression changes characteristically with the emotions induced by basic tastes in humans. We tested the hypothesis that the five basic tastes also elicit unique responses in facial skin blood flow. Facial skin blood flow was measured using laser speckle flowgraphy in 16 healthy subjects before and during the application of basic taste stimuli in the oral cavity for 20 s. The skin blood flow in the eyelid increased in response to sweet and umami taste stimuli, while that in the nose decreased in response to a bitter stimulus. There was a significant correlation between the subjective hedonic scores accompanying these taste stimuli and the above changes in skin blood flow. These results demonstrate that sweet, umami, and bitter tastes induce unique changes in facial skin blood flow that reflect subjective hedonic scores.

Pleasant human touch is represented in pregenual anterior cingulate cortex

Touch massage (TM) is a form of pleasant touch stimulation used as treatment in clinical settings and found to improve well-being and decrease anxiety, stress, and pain. Emotional responses reported during and after TM have been studied, but the underlying mechanisms are still largely unexplored. In this study, we used functional magnetic resonance (fMRI) to test the hypothesis that the combination of human touch (i.e. skin-to-skin contact) with movement is eliciting a specific response in brain areas coding for pleasant sensations. The design included four different touch conditions; human touch with or without movement and rubber glove with or without movement. Force (2.5 N) and velocity (1.5 cm/s) were held constant across conditions. The pleasantness of the four different touch stimulations was rated on a visual analog scale (VAS-scale) and human touch was rated as most pleasant, particularly in combination with movement. The fMRI results revealed that TM stimulation most strongly activated the pregenual anterior cingulate cortex (pgACC). These results are consistent with findings showing pgACC activation during various rewarding pleasant stimulations. This area is also known to be activated by both opioid analgesia and placebo. Together with these prior results, our finding furthers the understanding of the basis for positive TM treatment effects.

Common cellular and molecular mechanisms in obesity and drug addiction

The hedonic properties of food can stimulate feeding behaviour even when energy requirements have been met, contributing to weight gain and obesity. Similarly, the hedonic effects of drugs of abuse can motivate their excessive intake, culminating in addiction. Common brain substrates regulate the hedonic properties of palatable food and addictive drugs, and recent reports suggest that excessive consumption of food or drugs of abuse induces similar neuroadaptive responses in brain reward circuitries. Here, we review evidence suggesting that obesity and drug addiction may share common molecular, cellular and systems-level mechanisms.

Chemosensory learning in the cortex

Taste is a primary reinforcer. Olfactory–taste and visual–taste association learning takes place in the primate including human orbitofrontal cortex to build representations of flavor. Rapid reversal of this learning can occur using a rule-based learning system that can be reset when an expected taste or flavor reward is not obtained, that is by negative reward prediction error, to which a population of neurons in the orbitofrontal cortex responds. The representation in the orbitofrontal cortex but not the primary taste or olfactory cortex is of the reward value of the visual/olfactory/taste input as shown by devaluation experiments in which food is fed to satiety, and by correlations of the activations with subjective pleasantness ratings in humans. Sensory-specific satiety for taste, olfactory, visual, and oral somatosensory inputs produced by feeding a particular food to satiety is implemented it is proposed by medium-term synaptic adaptation in the orbitofrontal cortex. Cognitive factors, including word-level descriptions, modulate the representation of the reward value of food in the orbitofrontal cortex, and this effect is learned it is proposed by associative modification of top-down synapses onto neurons activated by bottom-up taste and olfactory inputs when both are active in the orbitofrontal cortex. A similar associative synaptic learning process is proposed to be part of the mechanism for the top-down attentional control to the reward value vs. the sensory properties such as intensity of taste and olfactory inputs in the orbitofrontal cortex, as part of a biased activation theory of selective attention.

Localization of brain activation by umami taste in humans

There are no credible data to support the notion that individual taste qualities have dedicated pathways leading from the tongue to the end of the pathway in the brain. Moreover, the insular cortex is activated not only by taste but also by non-taste information from oral stimuli. These responses are invariably excitatory, and it is difficult to determine whether they are sensory, motor, or proprioceptive in origin. Furthermore, umami is a more unfamiliar and complex taste than other basic tastes. Considering these issues, it may be effective to minimize somatosensory stimuli, oral movement, and psychological effects in a neuroimaging study to elicit cerebral activity by pure umami on the human tongue. For this purpose, we developed an original taste delivery system for functional magnetic resonance imaging (fMRI) studies for umami. Then, we compared the results produced by two authorized models, namely, the block design model and event-related design model, to decide the appropriate model for detecting activation by umami. Activation by the umami taste was well localized in the insular cortex using our new system and block design model analysis. The peaks of the activated areas in the middle insular cortex by umami were very close to another prototypical taste quality (salty). Although we have to carefully interpret the perceiving intensities and brain activations by taste from different sessions, this study design might be effective for detecting the accession area in the cortex of pure umami taste on the tongue.

Collecting, comparing, and computing sequences: the making of Margaret O. Dayhoff's Atlas of Protein Sequence and Structure, 1954-1965

Collecting, comparing, and computing molecular sequences are among the most prevalent practices in contemporary biological research. They represent a specific way of producing knowledge. This paper explores the historical development of these practices, focusing on the work of Margaret O. Dayhoff, Richard V. Eck, and Robert S. Ledley, who produced the first computer-based collection of protein sequences, published in book format in 1965 as the Atlas of Protein Sequence and Structure. While these practices are generally associated with the rise of molecular evolution in the 1960s, this paper shows that they grew out of research agendas from the previous decade, including the biochemical investigation of the relations between the structures and function of proteins and the theoretical attempt to decipher the genetic code. It also shows how computers became essential for the handling and analysis of sequence data. Finally, this paper reflects on the relationships between experimenting and collecting as two distinct "ways of knowing" that were essential for the transformation of the life sciences in the twentieth century.

History of glutamate production

In 1907 Kikunae Ikeda, a professor at the Tokyo Imperial University, began his research to identify the umami component in kelp. Within a year, he had succeeded in isolating, purifying, and identifying the principal component of umami and quickly obtained a production patent. In 1909 Saburosuke Suzuki, an entrepreneur, and Ikeda began the industrial production of monosodium l-glutamate (MSG). The first industrial production process was an extraction method in which vegetable proteins were treated with hydrochloric acid to disrupt peptide bonds. l-Glutamic acid hydrochloride was then isolated from this material and purified as MSG. Initial production of MSG was limited because of the technical drawbacks of this method. Better methods did not emerge until the 1950s. One of these was direct chemical synthesis, which was used from 1962 to 1973. In this procedure, acrylonitrile was the starting material, and optical resolution of dl-glutamic acid was achieved by preferential crystallization. In 1956 a direct fermentation method to produce glutamate was introduced. The advantages of the fermentation method (eg, reduction of production costs and environmental load) were large enough to cause all glutamate manufacturers to shift to fermentation. Today, total world production of MSG by fermentation is estimated to be 2 million tons/y (2 billion kg/y). However, future production growth will likely require further innovation.

Umami taste transduction mechanisms

l-Glutamate elicits the umami taste sensation, now recognized as a fifth distinct taste quality. A characteristic feature of umami taste is its potentiation by 5'-ribonucleotides such as guanosine-5'-monophosphate and inosine 5'-monophosphate, which also elicit the umami taste on their own. Recent data suggest that multiple G protein-coupled receptors contribute to umami taste. This review will focus on events downstream of the umami taste receptors. Ligand binding leads to Gbetagamma activation of phospholipase C beta2, which produces the second messengers inositol trisphosphate and diacylglycerol. Inositol trisphosphate binds to the type III inositol trisphosphate receptor, which causes the release of Ca(2+) from intracellular stores and Ca(2+)-dependent activation of a monovalent-selective cation channel, TRPM5. TRPM5 is believed to depolarize taste cells, which leads to the release of ATP, which activates ionotropic purinergic receptors on gustatory afferent nerve fibers. This model is supported by knockout of the relevant signaling effectors as well as physiologic studies of isolated taste cells. Concomitant with the molecular studies, physiologic studies show that l-glutamate elicits increases in intracellular Ca(2+) in isolated taste cells and that the source of the Ca(2+) is release from intracellular stores. Both Galpha gustducin and Galpha transducin are involved in umami signaling, because the knockout of either subunit compromises responses to umami stimuli. Both alpha-gustducin and alpha-transducin activate phosphodiesterases to decrease intracellular cAMP. The target of cAMP in umami transduction is not known, but membrane-permeant analogs of cAMP antagonize electrophysiologic responses to umami stimuli in isolated taste cells, which suggests that cAMP may have a modulatory role in umami signaling.

Can dietary supplementation of monosodium glutamate improve the health of the elderly?

Dietary free l-glutamate has been known for a century to improve taste and palatability. Recent evidence suggests that this effect is mediated through specific l-glutamate receptors located on the taste buds. However, l-glutamate receptors are also present elsewhere in the gastrointestinal tract, such as the stomach. Here, l-glutamate exerts physiologic actions beneficial to gut function by stimulating l-glutamate receptors linked to the gastric vagus nerve. In addition, dietary l-glutamate also appears to be an important energy substrate for gut tissue. Can such l-glutamate effects on taste and gut function be clinically useful? Elderly people often develop health problems related to their nutritional status that can be linked to insufficient energy and nutrient intake. A number of studies have examined the potential usefulness of l-glutamate, added to food in the form of monosodium glutamate (MSG), in promoting better nutrition in the elderly and in patients with poor nutrition. Some positive effects have been observed. This article reviews the physiologic roles of dietary l-glutamate in relation to alimentation and examines the evidence linking the utility of MSG supplementation to the improvement of nutrition in elderly and hospitalized patients.

Taste signaling elements expressed in gut enteroendocrine cells regulate nutrient-responsive secretion of gut hormones

Many of the receptors and downstream signaling elements involved in taste detection and transduction are also expressed in enteroendocrine cells where they underlie the chemosensory functions of the gut. In one well-known example of gastrointestinal chemosensation (the "incretin effect"), it is known that glucose that is given orally, but not systemically, induces secretion of glucagon-like peptide 1 and glucose-dependent insulinotropic peptide (the incretin hormones), which in turn regulate appetite, insulin secretion, and gut motility. Duodenal L cells express sweet taste receptors, the taste G protein gustducin, and several other taste transduction elements. Knockout mice that lack gustducin or the sweet taste receptor subunit T1r3 have deficiencies in secretion of glucagon-like peptide 1 and glucose-dependent insulinotropic peptide and in the regulation of plasma concentrations of insulin and glucose in response to orally ingested carbohydrate-ie, their incretin effect is dysfunctional. Isolated small intestine and intestinal villi from gustducin null mice displayed markedly defective glucagon-like peptide 1 secretion in response to glucose, indicating that this is a local circuit of sugar detection by intestinal cells followed by hormone secretion from these same cells. Modulating hormone secretion from gut "taste cells" may provide novel treatments for obesity, diabetes, and malabsorption syndromes.

Perceptual variation in umami taste and polymorphisms in TAS1R taste receptor genes

BACKGROUND

The TAS1R1 and TAS1R3 G protein-coupled receptors are believed to function in combination as a heteromeric glutamate taste receptor in humans.

OBJECTIVE

We hypothesized that variations in the umami perception of glutamate would correlate with variations in the sequence of these 2 genes, if they contribute directly to umami taste.

DESIGN

In this study, we first characterized the general sensitivity to glutamate in a sample population of 242 subjects. We performed these experiments by sequencing the coding regions of the genomic TAS1R1 and TAS1R3 genes in a separate set of 87 individuals who were tested repeatedly with monopotassium glutamate (MPG) solutions. Last, we tested the role of the candidate umami taste receptor hTAS1R1-hTAS1R3 in a functional expression assay.

RESULTS

A subset of subjects displays extremes of sensitivity, and a battery of different psychophysical tests validated this observation. Statistical analysis showed that the rare T allele of single nucleotide polymorphism (SNP) R757C in TAS1R3 led to a doubling of umami ratings of 25 mmol MPG/L. Other suggestive SNPs of TAS1R3 include the A allele of A5T and the A allele of R247H, which both resulted in an approximate doubling of umami ratings of 200 mmol MPG/L. We confirmed the potential role of the human TAS1R1-TAS1R3 heteromer receptor in umami taste by recording responses, specifically to l-glutamate and inosine 5'-monophosphate (IMP) mixtures in a heterologous expression assay in HEK (human embryonic kidney) T cells.

CONCLUSIONS

There is a reliable and valid variation in human umami taste of l-glutamate. Variations in perception of umami taste correlated with variations in the human TAS1R3 gene. The putative human taste receptor TAS1R1-TAS1R3 responds specifically to l-glutamate mixed with the ribonucleotide IMP. Thus, this receptor likely contributes to human umami taste perception.

The blood-brain barrier and glutamate

Glutamate concentrations in plasma are 50-100 micromol/L; in whole brain, they are 10,000-12,000 micromol/L but only 0.5-2 micromol/L in extracellular fluids (ECFs). The low ECF concentrations, which are essential for optimal brain function, are maintained by neurons, astrocytes, and the blood-brain barrier (BBB). Cerebral capillary endothelial cells form the BBB that surrounds the entire central nervous system. Tight junctions connect endothelial cells and separate the BBB into luminal and abluminal domains. Molecules entering or leaving the brain thus must pass 2 membranes, and each membrane has distinct properties. Facilitative carriers exist only in luminal membranes, and Na(+)-dependent glutamate cotransporters (excitatory amino acid transporters; EAATs) exist exclusively in abluminal membranes. The EAATs are secondary transporters that couple the Na(+) gradient between the ECF and the endothelial cell to move glutamate against the existing electrochemical gradient. Thus, the EAATs in the abluminal membrane shift glutamate from the ECF to the endothelial cell where glutamate is free to diffuse into blood on facilitative carriers. This organization does not allow net glutamate entry to the brain; rather, it promotes the removal of glutamate and the maintenance of low glutamate concentrations in the ECF. This explains studies that show that the BBB is impermeable to glutamate, even at high concentrations, except in a few small areas that have fenestrated capillaries (circumventricular organs). Recently, the question of whether the BBB becomes permeable in diabetes has arisen. This issue was tested in rats with diet-induced obesity and insulin resistance or with streptozotocin-induced diabetes. Neither condition produced any detectable effect on BBB glutamate transport.

Nonsynonymous single nucleotide polymorphisms in human tas1r1, tas1r3, and mGluR1 and individual taste sensitivity to glutamate

Several studies indicate an essential role of the heterodimer Tas1R1-Tas1R3 for monosodium l-glutamate (MSG) detection, although others suggest alternative receptors. Human subjects show different taste sensitivities to MSG, and some are unable to detect the presence of glutamate. Our objective was to study possible relations between phenotype (sensitivity to glutamate) and genotype (polymorphisms in candidate glutamate taste receptors tas1r1, tas1r3, mGluR4, and mGluR1) at the individual level. The sensitivity was measured with a battery of tests to distinguish the effect of sodium ions from the effect of glutamate ions in MSG. A total of 142 genetically unrelated white French subjects were categorized into 27 nontasters (specific ageusia), 21 hypotasters, and 94 tasters. Reverse transcriptase polymerase chain reaction and immunohistochemistry showed expression of tas1r1, tas1r3, and alpha-gustducin in fungiform papillae in all 12 subjects tested, including subjects who presented specific ageusia for glutamate. Amplification and sequencing of cDNA and genomic DNA allowed the identification of 10 nonsynonymous single nucleotide polymorphisms (nsSNPs) in tas1r1 (n = 3), tas1r3 (n = 3), and mGluR1 (n = 4). In our sample of subjects, the frequencies of 2 nsSNPs, C329T in tas1r1 and C2269T in tas1r3, were significantly higher in nontasters than expected, whereas G1114A in tas1r1 was more frequent in tasters. These nsSNPs along with minor variants and other nsSNPs in mGluR1, including T2977C, account for only part of the interindividual variance, which indicates that other factors, possibly including additional receptors, contribute to glutamate sensitivity.