Receptor and transduction processes for umami taste

Identification of compounds contributing to umami taste of pea protein isolate

The umami taste of pea protein ingredients can be desirable or undesirable based on the food application. The compounds contributing to the umami perception of pea protein isolate (PPI) were investigated. Sensory-guided prep-liquid chromatography fractionation of a 10% aqueous PPI solution revealed one well-known compound, monosodium glutamate (MSG), however, it was reported at a subthreshold concentration. Two umami enhancing compounds 5'-adenosine monophosphate (AMP) and 5'-uridine monophosphate (UMP) were subsequently identified after the LC fractions were re-evaluated with MSG. Sensory recombination studies, utilizing the aqueous PPI solution as the base, confirmed AMP and UMP were umami enhancers of MSG and contributed approximately 81% of the perceived umami intensity. However UMP was only reported to enhance umami perception in combination with AMP (not individually) indicating synergistic interactions were observed between the two enhancer compounds. Therefore the presence of all three compounds are important for umami perception and provide an improved basis to tailor the flavor profile in PPI products.

Pharmacology of the Umami Taste Receptor

Umami, the fifth taste, has been recognized as a legitimate taste modality only recently relative to the other tastes. Dozens of compounds from vastly different chemical classes elicit a savory (also called umami) taste. The prototypical umami substance glutamic acid or its salt monosodium glutamate (MSG) is present in numerous savory food sources or ingredients such as kombu (edible kelp), beans, soy sauce, tomatoes, cheeses, mushrooms, and certain meats and fish. Derivatives of glutamate (Glu), other amino acids, nucleotides, and small peptides can also elicit or modulate umami taste. In addition, many potent umami tasting compounds structurally unrelated to amino acids, nucleotides, and MSG have been either synthesized or discovered as naturally occurring in plants and other substances. Over the last 20 years several receptors have been suggested to mediate umami taste, including members of the metabotropic and ionotropic Glu receptor families, and more recently, the heterodimeric G protein-coupled receptor, T1R1/T1R3. Careful assessment of representative umami tasting molecules from several different chemical classes shows activation of T1R1/T1R3 with the expected rank order of potency in cell-based assays. Moreover, 5'-ribonucleotides, molecules known to enhance the savory note of Glu, considerably enhance the effect of MSG on T1R1/T1R3 in vitro. Binding sites are found on at least 4 distinct locations on T1R1/T1R3, explaining the propensity of the receptor to being activated or modulated by many structurally distinct compounds and these binding sites allosterically interact to modulate receptor activity. Activation of T1R1/T1R3 by all known umami substances evaluated and the receptor's pharmacological properties are sufficient to explain the basic human sensory experience of savory taste and it is therefore unlikely that other receptors are involved.

Temporal profile of flavor enhancers MAG, MSG, GMP, and IMP, and their ability to enhance salty taste, in different reductions of sodium chloride

We evaluated the temporal profile of the flavor enhancers monosodium glutamate (MSG), disodium inosinate (IMP), disodium guanylate (GMP), and monoammonium glutamate (MAG). We also evaluated the ability of these flavor enhancers to enhance salty taste in solutions containing different reductions of sodium chloride. Four experiments were conducted using Central Composite Rotational Design (CCRD) with focus on two objectives: concentration of flavor enhancers (0% to 1%) and reduction of sodium chloride content (0% to 100%). A 0.75% saline solution of NaCl was used as a control. In each experiment, the treatments were evaluated by the intensity of salty and umami tastes using an intensity scale. Treatments, selected according to the results of CCRD, were analyzed using time-intensity (TI) and temporal dominance of sensations (TDS) analyses. Glutamates (MSG/MAG) showed greater capacity to enhance salty taste than treatments containing nucleotides (IMP/GMP). The intensity of umami taste, using all the examined flavor enhancers, showed a similar sensory profile. Temporal perception curves (TI and TDS) of salty and umami tastes also showed a similar temporal profile. The glutamic acid amino acids were better able to improve salty taste than nucleotides in any range of sodium chloride reduction. Flavor enhancers showed greater ability to increase salty taste in smaller reductions in sodium chloride content. PRACTICAL APPLICATION: This research expand the knowledge about the ability to enhance the salty taste of flavor enhancers in different reductions in sodium content, Beside that, will provide information about the time profile of flavor enhancers. This study provides scientific technical information on the ability to intensify the salty taste of flavor enhancers and can assist the industry to develop new low sodium products and encourage the scientific community to conduct future research on this subject.

Clofibrate inhibits the umami-savory taste of glutamate

In humans, umami taste can increase the palatability of foods rich in the amino acids glutamate and aspartate and the 5'-ribonucleotides IMP and GMP. Umami taste is transduced, in part, by T1R1-T1R3, a heteromeric G-protein coupled receptor. Umami perception is inhibited by sodium lactisole, which binds to the T1R3 subunit in vitro. Lactisole is structurally similar to the fibrate drugs. Clofibric acid, a lipid lowering drug, also binds the T1R3 subunit in vitro. The purpose of this study was to determine whether clofibric acid inhibits the umami taste of glutamate in human subjects. Ten participants rated the umami taste intensity elicited by 20 mM monosodium glutamate (MSG) mixed with varying concentrations of clofibric acid (0 to 16 mM). In addition, fourteen participants rated the effect of 1.4 mM clofibric acid on umami enhancement by 5' ribonucleotides. Participants were instructed to rate perceived intensity using a general Labeled Magnitude Scale (gLMS). Each participant was tested in triplicate. Clofibric acid inhibited umami taste intensity from 20 mM MSG in a dose dependent manner. Whereas MSG neat elicited "moderate" umami taste intensity, the addition of 16 mM clofibric acid elicited only "weak" umami intensity on average, and in some subjects no umami taste was elicited. We further show that 1.4 mM clofibric acid suppressed umami enhancement from GMP, but not from IMP. This study provides in vivo evidence that clofibric acid inhibits glutamate taste perception, presumably via T1R1-T1R3 inhibition, and lends further evidence that the T1R1-T1R3 receptor is the principal umami receptor in humans. T1R receptors are expressed extra-orally throughout the alimentary tract and in regulatory organs and are known to influence glucose and lipid metabolism. Whether clofibric acid as a lipid-lowering drug affects human metabolism, in part, through T1R inhibition warrants further examination.

Genetics of Amino Acid Taste and Appetite

The consumption of amino acids by animals is controlled by both oral and postoral mechanisms. We used a genetic approach to investigate these mechanisms. Our studies have shown that inbred mouse strains differ in voluntary amino acid consumption, and these differences depend on sensory and nutritive properties of amino acids. Like humans, mice perceive some amino acids as having a sweet (sucrose-like) taste and others as having an umami (glutamate-like) taste. Mouse strain differences in the consumption of some sweet-tasting amino acids (d-phenylalanine, d-tryptophan, and l-proline) are associated with polymorphisms of a taste receptor, type 1, member 3 gene (Tas1r3), and involve differential peripheral taste responsiveness. Strain differences in the consumption of some other sweet-tasting amino acids (glycine, l-alanine, l-glutamine, and l-threonine) do not depend on Tas1r3 polymorphisms and so must be due to allelic variation in other, as yet unknown, genes involved in sweet taste. Strain differences in the consumption of l-glutamate may depend on postingestive rather than taste mechanisms. Thus, genes and physiologic mechanisms responsible for strain differences in the consumption of each amino acid depend on the nature of its taste and postingestive properties. Overall, mouse strain differences in amino acid taste and appetite have a complex genetic architecture. In addition to the Tas1r3 gene, these differences depend on other genes likely involved in determining the taste and postingestive effects of amino acids. The identification of these genes may lead to the discovery of novel mechanisms that regulate amino acid taste and appetite.

Metabotropic glutamate receptors are involved in the detection of IMP and L-amino acids by mouse taste sensory cells

G-protein-coupled receptors are thought to be involved in the detection of umami and L-amino acid taste. These include the heterodimer taste receptor type 1 member 1 (T1r1)+taste receptor type 1 member 3 (T1r3), taste and brain variants of mGluR4 and mGluR1, and calcium sensors. While several studies suggest T1r1+T1r3 is a broadly tuned lLamino acid receptor, little is known about the function of metabotropic glutamate receptors (mGluRs) in L-amino acid taste transduction. Calcium imaging of isolated taste sensory cells (TSCs) of T1r3-GFP and T1r3 knock-out (T1r3 KO) mice was performed using the ratiometric dye Fura 2 AM to investigate the role of different mGluRs in detecting various L-amino acids and inosine 5' monophosphate (IMP). Using agonists selective for various mGluRs such as (RS)-3,5-dihydroxyphenylglycine (DHPG) (an mGluR1 agonist) and L-(+)-2-amino-4-phosphonobutyric acid (l-AP4) (an mGluR4 agonist), we evaluated TSCs to determine if they might respond to these agonists, IMP, and three L-amino acids (monopotassium L-glutamate, L-serine and L-arginine). Additionally, we used selective antagonists against different mGluRs such as (RS)-L-aminoindan-1,5-dicarboxylic acid (AIDA) (an mGluR1 antagonist), and (RS)-α-methylserine-O-phosphate (MSOP) (an mGluR4 antagonist) to determine if they can block responses elicited by these L-amino acids and IMP. We found that L-amino acid- and IMP-responsive cells also responded to each agonist. Antagonists for mGluR4 and mGluR1 significantly blocked the responses elicited by IMP and each of the L-amino acids. Collectively, these data provide evidence for the involvement of taste and brain variants of mGluR1 and mGluR4 in L-amino acid and IMP taste responses in mice, and support the concept that multiple receptors contribute to IMP and L-amino acid taste.

Genetics of taste receptors

Taste receptors function as one of the interfaces between internal and external milieus. Taste receptors for sweet and umami (T1R [taste receptor, type 1]), bitter (T2R [taste receptor, type 2]), and salty (ENaC [epithelial sodium channel]) have been discovered in the recent years, but transduction mechanisms of sour taste and ENaC-independent salt taste are still poorly understood. In addition to these five main taste qualities, the taste system detects such noncanonical "tastes" as water, fat, and complex carbohydrates, but their reception mechanisms require further research. Variations in taste receptor genes between and within vertebrate species contribute to individual and species differences in taste-related behaviors. These variations are shaped by evolutionary forces and reflect species adaptations to their chemical environments and feeding ecology. Principles of drug discovery can be applied to taste receptors as targets in order to develop novel taste compounds to satisfy demand in better artificial sweeteners, enhancers of sugar and sodium taste, and blockers of bitterness of food ingredients and oral medications.

Effects of oral monosodium glutamate in mouse models of asthma

The available evidence from numerous clinical studies has failed to demonstrate a clear and consistent relationship between monosodium glutamate (MSG) and asthma. The objective of this study was to investigate the effects of MSG on bronchial inflammation by measuring cytological, histological and functional changes in an ovalbumin-induced asthma mouse model. BALB/c mice with experimentally induced asthma were fed a diet containing 0.5% or 5% MSG the week before the first ovalbumin injection and for the subsequent 3-week period. MSG feeding did not affect pulmonary eosinophil infiltration, production of Th2 cytokines, circulating IgE concentrations or airway hyperresponsiveness (induced by methacholine). Histological observations did not reveal pulmonary inflammation, including secondary changes, in the asthmatic mice. An oral gavage challenge with an MSG solution (0.5% or 5%, w/w) did not exert any acute effects on lung inflammation or airway hyperresponsiveness in the asthmatic mice. The results of this study suggest that MSG is not involved in the development of asthma or in acute asthmatic responses, and they support previous observations from well-designed clinical studies.

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.

Taste perception of monosodium glutamate and inosine monophosphate by 129P3/J and C57BL/6ByJ mice

Our previous studies have shown that in long-term two-bottle preference tests, mice from the C57BL/6ByJ (B6) inbred strain drink more monosodium glutamate (MSG) and inosine monophosphate (IMP) than mice from the 129P3/J (129) inbred strain. The goal of this study was to examine whether this variation in consumption could be attributed to strain differences in perception of the taste quality of MSG and IMP. We developed a conditioned taste aversion (CTA) in B6 and 129 mice to 100 mM MSG or 10 mM IMP and used a brief-access taste assay to examine CTA generalization. B6 and 129 mice did not differ in the generalization patterns following CTA to MSG: mice from both strains generalized CTA from MSG to NaCl. In contrast, strain differences in the generalization patterns were evident following the CTA to IMP: while mice from both strains generalized CTA from IMP to MSG, 129 mice tended to have stronger CTA generalization to saccharin and d-tryptophan, both of which are perceived as sweet by humans. These data suggest that the strain differences in MSG consumption are not due to variation in perception of the taste quality of MSG. Instead, the differential intake of IMP likely reflects strain differences in the way the taste quality of IMP is perceived. Our data suggest that mice perceive MSG and IMP as complex taste stimuli: some taste components are shared between these two substances, but their relative intensity seems to be different for MSG and IMP. The amiloride-sensitive salt taste component is more prevalent in MSG than in IMP taste, and in B6 compared with 129 mice.

Taste receptor genes

In the past several years, tremendous progress has been achieved with the discovery and characterization of vertebrate taste receptors from the T1R and T2R families, which are involved in recognition of bitter, sweet, and umami taste stimuli. Individual differences in taste, at least in some cases, can be attributed to allelic variants of the T1R and T2R genes. Progress with understanding how T1R and T2R receptors interact with taste stimuli and with identifying their patterns of expression in taste cells sheds light on coding of taste information by the nervous system. Candidate mechanisms for detection of salts, acids, fat, complex carbohydrates, and water have also been proposed, but further studies are needed to prove their identity.

Glutamate concentration in whole saliva and taste responses to monosodium glutamate in humans

It is universally accepted that saliva plays an important role in taste sensations. However, interactions between constituents of whole saliva and the five basic taste modalities are still poorly understood. The aim of the present study was to evaluate possible relationship between endogenous glutamate (Glu) levels in whole saliva and taste responses to a prototypic umami substance, monosodium glutamate (MSG; 0.03-10.0%). Rated intensity and pleasantness of MSG taste was studied in healthy volunteers divided into a high glutamate (HG) in saliva (HG; n = 19) and low glutamate in saliva (LG; n = 18) group based on the median split level of salivary Glu. The HG and LG group did not differ in terms of electrogustometric thresholds, rated intensity of the MSG samples and pleasantness of distilled water and the lower MSG concentrations (0.03-1.0%). Perceived intensity of water taste was significantly (P < 0.05) higher in the LG subjects. The LG group rated the higher MSG concentrations (3.0-10.0%) as more unpleasant (P < 0.01). The difference remained significant after controlling for a between-group difference in age. The present results suggest that individual differences in salivary Glu levels may alter hedonic responses to suprathreshold MSG concentrations.

Polycose taste pre-exposure fails to influence behavioral and neural indices of taste novelty

Taste novelty can strongly modulate the speed and efficacy of taste aversion learning. Novel sweet tastes enhance c-Fos-like immunoreactivity (FLI) in the central amygdala and insular cortex. The present studies examined whether this neural correlate of novelty extends to different taste types by measuring FLI signals after exposure to novel and familiar polysaccharide (Polycose) and salt (NaCl) tastes. Novel Polycose not only failed to elevate FLI expression in central amygdala and insular cortex, but also failed to induce stronger taste aversion learning than familiar Polycose. Novel NaCl, on the other hand, showed patterns of FLI activation and aversion learning similar to that of novel sweet tastes. Possible reasons for the resistance of Polycose to typical pre-exposure effects are discussed.

The sixth taste?

Five taste qualities are recognized in humans: sweet, bitter, sour, salty, and umami. Rats and some other species may also have a sixth taste. Behavioral and electrophysiological data suggest that rats can taste polysaccharides derived from starch. Furthermore, the tastes of sugars and polysaccharides appear to differ in quality. Rats also discriminate different types of polysaccharide and starch molecules. Recent studies indicate that sweet taste is mediated by a T1R2 and T1R3 receptor complex but the identity of the hypothesized polysaccharide taste receptor remains to be established.

TASTE RESPONSES TO MONOSODIUM GLUTAMATE AFTER ALCOHOL EXPOSURE

AIMS

The aim of the present study was to evaluate the effects of acute and chronic exposure to alcohol on taste responses to a prototypic umami substance, monosodium glutamate (MSG).

METHODS

The rated intensity and pleasantness of MSG taste (0.03-10.0%) was compared in chronic male alcoholics (n = 35) and control subjects (n = 25). In a separate experiment, the effects of acute exposure of the oral mucosa to ethanol rinse (0.5-4.0%) on MSG taste (0.3-3.0%) were studied in 10 social drinkers.

RESULTS

The alcoholic and control group did not differ in terms of the rated intensity and pleasantness of MSG taste. Electrogustometric thresholds were significantly (P < 0.01) higher, i.e. worse, in the alcohol-dependent subjects. The difference remained significant after controlling for between-group differences in cigarette smoking and coffee drinking. Rinsing with ethanol did not alter either intensity or pleasantness of MSG taste in social drinkers.

CONCLUSIONS

The present results suggest that: (i) neither acute nor chronic alcohol exposure modifies taste responses to MSG; (ii) alcohol dependence may be associated with deficit in threshold taste reactivity, as assessed by electrogustometry.

Oral delivery of medications to companion animals: palatability considerations

There is an increased need for highly palatable solid oral dosage forms for companion animals, which are voluntarily accepted by the dog or cat, either from a feeding bowl or from the outstretched hand of the pet owner. Such dosage forms represent an emerging trend in companion animal formulations with major impact on medical needs such as convenience and compliance, particularly for chronically administered medications, and on marketing needs such as product differentiation. This review focuses on the science of taste, food and flavor preferences of dogs and cats, and palatability testing, in the context of applying these principles to the development of an oral palatable tablet for companion animals.

Gustatory responsiveness to monosodium glutamate and sodium chloride in four species of nonhuman primates

The taste responsiveness of six squirrel monkeys, five pigtail macaques, four olive baboons and four spider monkeys to monsodium glutamate (MSG) and to sodium chloride was assessed in two-bottle preference tests of brief duration (2 min). When given the choice between tap water and defined concentrations of the two tastants dissolved in tap water, the animals were found to significantly discriminate concentrations of MSG as low as 2 mM (spider monkeys and olive baboons), 50 mM (pigtail macaques) and 300 mM (squirrel monkeys) from the solvent. With sodium chloride, taste preference thresholds were found to be 1 mM (spider monkeys), 20 mM (pigtail macaques), 50 mM (olive baboons), and 200 mM (squirrel monkeys), respectively. Across-species comparisons of the degree of preference for MSG and sodium chloride displayed by the four primate species showed the same order of spider monkeys>olive baboons>pigtail macaques>squirrel monkeys. When presented with equimolar concentrations of different tastants, all four species preferred sucrose as well as a mixture of sucrose and sodium chloride over MSG, and--at least at one concentration--they preferred MSG over sodium chloride. The results support the assertion that the taste responsiveness of the four primate species to MSG and sodium chloride might reflect an evolutionary adaptation to their respective dietary habits.

Design of virtual libraries of umami-tasting molecules

The introduction of molecular tools in food research offers the possibility to the food industry to benefit from the experience gained in the field by pharmaceutical companies. In this work we are showing how in silico virtual screening techniques based on molecular similarity were applied for identifying novel umami-tasting compounds. The results obtained suggest that 5'-ribonucleotides and monosodium glutamate might elicit the fifth basic taste via the same molecular mechanism. New algorithms were developed and used in this work, such as the dimension reduction of data sets by singular value decomposition and the introduction of the correlation dimension as a natural dimension of a chemical space. It is shown that the representations of molecular data sets in chemical spaces possess self-similar properties, characteristic of fractal objects.

II. Glutamine and glutamate

Glutamine and glutamate with proline, histidine, arginine and ornithine, comprise 25% of the dietary amino acid intake and constitute the "glutamate family" of amino acids, which are disposed of through conversion to glutamate. Although glutamine has been classified as a nonessential amino acid, in major trauma, major surgery, sepsis, bone marrow transplantation, intense chemotherapy and radiotherapy, when its consumption exceeds its synthesis, it becomes a conditionally essential amino acid. In mammals the physiological levels of glutamine is 650 micromol/l and it is one of the most important substrate for ammoniagenesis in the gut and in the kidney due to its important role in the regulation of acid-base homeostasis. In cells, glutamine is a key link between carbon metabolism of carbohydrates and proteins and plays an important role in the growth of fibroblasts, lymphocytes and enterocytes. It improves nitrogen balance and preserves the concentration of glutamine in skeletal muscle. Deamidation of glutamine via glutaminase produces glutamate a precursor of gamma-amino butyric acid, a neurotransmission inhibitor. L-Glutamic acid is a ubiquitous amino acid present in many foods either in free form or in peptides and proteins. Animal protein may contain from 11 to 22% and plants protein as much as 40% glutamate by weight. The sodium salt of glutamic acid is added to several foods to enhance flavor. L-Glutamate is the most abundant free amino acid in brain and it is the major excitatory neurotransmitter of the vertebrate central nervous system. Most free L-glutamic acid in brain is derived from local synthesis from L-glutamine and Kreb's cycle intermediates. It clearly plays an important role in neuronal differentiation, migration and survival in the developing brain via facilitated Ca++ transport. Glutamate also plays a critical role in synaptic maintenance and plasticity. It contributes to learning and memory through use-dependent changes in synaptic efficacy and plays a role in the formation and function of the cytoskeleton. Glutamine via glutamate is converted to alpha-ketoglutarate, an integral component of the citric acid cycle. It is a component of the antioxidant glutathione and of the polyglutamated folic acid. The cyclization of glutamate produces proline, an amino acid important for synthesis of collagen and connective tissue. Our aim here is to review on some amino acids with high functional priority such as glutamine and to define their effective activity in human health and pathologies.

[Taste disorders]

PURPOSE

This review focuses on the present state of the question about taste disorders with reference to their associated factors, diagnostic methods, and potential effects.

CURRENT KNOWLEDGE AND KEY POINTS

Taste disorders may be induced by many drugs and are associated to a number of acute or chronic diseases. Patients may be asked about their taste complaints, and taste thresholds may be determined by electrogustometry or chemical gustometry. Taste impairment may provide a good indicator to the course of some diseases such as diabetes mellitus in which hypogeusia predicts occurrence of degenerative complications. Dysgeusia may induce nutritional disorders and contribute to wasting in chronic liver disease, cancer, or human immunodeficiency virus infected patients. Mechanisms involved in dysgeusia are more than one in a patient. Taste disturbance may be secondary to a variety of causes that include zinc deficiency, lesions of the lingual epithelium, neurological impairment, and a pharmacological effect.

FUTURE PROSPECTS AND PROJECTS

A better understanding of the transduction mechanisms of the gustatory signal and the main pathogenic factors involved in dysgeusia may possibly improve the follow-up of the concerned patients notably in terms of nutritional status.

Receptors and transduction in taste

Taste is the sensory system devoted primarily to a quality check of food to be ingested. Although aided by smell and visual inspection, the final recognition and selection relies on chemoreceptive events in the mouth. Emotional states of acute pleasure or displeasure guide the selection and contribute much to our quality of life. Membrane proteins that serve as receptors for the transduction of taste have for a long time remained elusive. But screening the mass of genome sequence data that have recently become available has provided a new means to identify key receptors for bitter and sweet taste. Molecular biology has also identified receptors for salty, sour and umami taste.

Taste as a factor in the management of nutrition

The sense of taste lies at the interface between the external and internal milieux, at the point at which the animal must decide which chemicals from the environment to incorporate into itself. Accordingly, taste is organized along a neural dimension of nutrients versus toxins, which corresponds to a behavioral dimension of acceptance versus rejection, and to a hedonic dimension of appetitive versus aversive qualities. Reflexive responses, cognitive analyses, and hedonic reactions appear to be managed at different levels of the nervous system. At the first central relay, the nucleus of the solitary tract, somatic reflexes for acceptance or rejection, and autonomic reflexes anticipating digestion are orchestrated. At the second, the parabrachial nucleus of the rodent, associative mechanisms important to the development of conditioned aversions and sodium appetite are manifested. In the thalamic taste relay, gustatory memories associated with non-visceral events may be formed. Primary taste cortex appears to be the site for a cognitive evaluation of gustatory quality and intensity. Finally, a hedonic assessment of the chemical may be made in secondary taste cortex and in the ventral forebrain sites to which it projects. With this assessment comes integration of the gustatory signal with those from other senses, perhaps to create a perception of flavor. Therefore, a sequence that begins with an analysis of the molecular structure of a chemical in the mouth serves to incorporate that gustatory component into an appreciation of flavor, and to participate in the control of motivational processes that guide dietary selection.

Glutamic acid, twenty years later

This review examines progress in understanding the physiologic functions of glutamic acid in the body since the first symposium on glutamic acid physiology and biochemistry was held at the Mario Negri Institute in Milan in 1978. The topics reviewed, although not exhaustive, include the metabolism of glutamic acid, umami taste, the role of glutamic acid as a neurotransmitter, glutamate safety and the development of new drugs resulting from the knowledge of the neurodegeneration induced by high doses of glutamic acid.