Cross-adapted sugar responses in the mouse taste cell

Non-nutritive Sweeteners and Their Associations with Obesity and Type 2 Diabetes

Evidence linking the excessive consumption of nutritive sweeteners (NS) to adverse metabolic health outcomes has led to an increase in consumption of non-nutritive sweeteners (NNS), particularly among the obese and individuals with diabetes. NNS are characterized by having zero-to-negligible caloric load, while also having a sweet taste. They are utilized as a replacement for traditional NS to reduce energy intake and to limit carbohydrate-related negative health outcomes. However, recent studies have suggested that NNS may actually contribute to the development or worsening of metabolic diseases, including metabolic syndrome, obesity, type

2 diabetes, and cardiovascular disease. Thus, it is imperative to understand the NNS efficacy and the relationship between NNS and metabolic diseases.

Physiological mechanisms by which non-nutritive sweeteners may impact body weight and metabolism

Evidence linking sugar-sweetened beverage (SSB) consumption to weight gain and other negative health outcomes has prompted many individuals to resort to artificial, non-nutritive sweetener (NNS) substitutes as a means of reducing SSB intake. However, there is a great deal of controversy regarding the biological consequences of NNS use, with accumulating evidence suggesting that NNS consumption may influence feeding and metabolism via a variety of peripheral and central mechanisms. Here we argue that NNSs are not physiologically inert compounds and consider the potential biological mechanisms by which NNS consumption may impact energy balance and metabolic function, including actions on oral and extra-oral sweet taste receptors, and effects on metabolic hormone secretion, cognitive processes (e.g. reward learning, memory, and taste perception), and gut microbiota.

Persistent sucrose stimulation of ovine fetal ingestion: lack of adaptation responses

OBJECTIVE

Sweet taste responsiveness is reduced in adult rats and humans following continued oral sucrose. We have previously demonstrated that sublingual sucrose stimulates near term ovine fetal swallowing, suggesting intact taste responsiveness. We sought to determine if prolonged oral sucrose infusion to the near term ovine fetus will evoke adaptation, as manifested by reduced swallowing stimulation.

METHODS

Time-dated pregnant ewes and fetuses (n = 4) were chronically prepared with fetal vascular and sublingual catheters, and electrocorticogram and esophageal electromyogram electrodes and studied at 129 +/- 1 d gestation. Following an initial 2 h basal period, sucrose (2.5%) was infused sublingually (0.25 ml/min) to the fetus for 8 h. Fetal swallowing activity, blood pressure and heart rate were continuously recorded while maternal and fetal arterial blood samples were taken at timed intervals.

RESULTS

During the basal period, fetal swallowing averaged 0.9 +/- 0.1 swallows/min. Fetal swallowing increased significantly following sublingual 2.5% sucrose infusion and remained significantly elevated at 2, 4, 6 and 8 h after initiation of sucrose infusion (1.3 +/- 0.1, 1.2 +/- 0.1, 1.3 +/- 0.1, 1.3 +/- 0.1 swallows/min; p < 0.001). There were no significant changes in fetal cardiovascular or arterial blood parameters.

CONCLUSIONS

Although oral sucrose significantly stimulates near term ovine fetal ingestive behavior, sweet taste adaptation or habituation does not occur, in contrast to that observed in adult animals and human. The lack of taste adaptation in the fetus/newborn may facilitate increased neonatal food intake and accelerated growth.

Enhanced responses of the chorda tympani nerve to sugars in the ventromedial hypothalamic obese rat

Sweet taste sensitivity in obese rats with lesions of the ventromedial hypothalamus (VMH) was studied by examining chorda tympani nerve responses to various taste stimuli including sugars. In the early progressive phase of obesity (2 wk after creating VMH lesions), there was no significant difference in the nerve responses to any taste stimulus between sham-operated and VMH-lesioned rats. In contrast, in the late phase of obesity (15-18 wk after VMH lesions), the magnitude of responses to sugars (except for fructose) was prominently greater than that in age-matched controls. High-fat diet-induced obese rats and streptozotocin-diabetic rats also showed greater chorda tympani nerve responses to sugars as was observed in VMH-lesioned obese rats, indicating that VMH lesions might not be specifically related to the enhanced gustatory neural responses to sugars. Although it has been demonstrated that the enhanced responses of the chorda tympani nerve to sugars in genetically diabetic db/db mice is largely attributable to the lack of the direct suppressive effect of leptin on the taste receptor cells, plasma leptin levels were not correlated with the changes in chorda tympani responsiveness to sugars in these models of obesity and diabetes. Accordingly, our results suggest that some chronic factors, including high blood glucose, inefficiency of insulin action, or leptin resistance may be related to the enhancement of chorda tympani nerve responses to sugars.

Three sweet receptor genes are clustered in human chromosome 1

A search of the human genome database led us to identify three human candidate taste receptors, hT1R1, hT1R2, and hT1R3, which contain seven transmembrane domains. All three genes map to a small region of Chromosome (Chr) 1. This region is syntenous to the distal end of Chr 4 in mouse, which contains the Sac (saccharin preference) locus that is involved in detecting sweet tastants. A genetic marker, DVL1, which is linked to the Sac locus, is within 1700 bp of human T1R3. Recently, the murine T1Rs and its human ortholog have been independently identified in combination as sweet and umami receptors near the Sac locus. All three hT1Rs genes are expressed selectively in human taste receptor cells in the fungiform papillae, consistent with their role in taste perception.

A candidate taste receptor gene near a sweet taste locus

The mechanisms underlying sweet taste in mammals have been elusive. Although numerous studies have implicated G proteins in sweet taste detection, the expected G protein-coupled receptors have not been found. Here we describe a candidate taste receptor gene, T1r3, that is located at or near the mouse Sac locus, a genetic locus that controls the detection of certain sweet tastants. T1R3 differs in amino acid sequence in mouse strains with different Sac phenotypes ('tasters' versus 'nontasters'). In addition, a perfect correlation exists between two different T1r3 alleles and Sac phenotypes in recombinant inbred mouse strains. The T1r3 gene is expressed in a subset of taste cells in circumvallate, foliate and fungiform taste papillae. In circumvallate and foliate papillae, most T1r3-expressing cells also express a gene encoding a related receptor, T1R2, raising the possibility that these cells recognize more than one ligand, or that the two receptors function as heterodimers.

Effects of acids on neural activity elicited by other taste stimuli in the rat Chorda tympani

The purpose of this study is whether the gustatory neural response of taste cell to a binary mixture with threshold concentration of acid becomes synergistic or antagonistic can be estimated from the whole chorda tympani (CT) nerve in the rat. The present data demonstrate that acids are synergistic enhancer for sugars, and suppressor for NaCl and QHCl, but no effect to glycine and alanine. These results suggest that the acid was modifying the interaction of the other stimulus with its transduction mechanism.

Role of hydrophobic amino acids in gurmarin, a sweetness-suppressing polypeptide

The sweetness-suppressing polypeptide gurmarin isolated from Gymnema sylvestre consists of 35 amino acid residues and contains three intramolecular disulfide bonds. Nuclear magnetic resonance analysis showed that the hydrophobic side chains of Tyr-13, Tyr-14, Trp-28, and Trp-29 in gurmarin are oriented outwardly. Together with the hydrophobic side chains of Leu-9, Ile-11, and Pro-12, they form a hydrophobic cluster, and therefore these hydrophobic groups are assumed to act as the site for interaction with the receptor protein. To examine the roles of these hydrophobic amino acids, they were replaced by Gly. The resulting [Gly13,14,28,29] gurmarin and [Gly9,11,13,14,28,29]-gurmarin did not suppress the responses to sucrose, glucose, fructose, or Gly. This result strongly suggests that these hydrophobic amino acids are involved in the interaction with the receptor protein.

Adaptation of sweeteners in water and in tannic acid solutions

Repeated exposure to a tastant often leads to a decrease in magnitude of the perceived intensity; this phenomenon is termed adaptation. The purpose of this study was to determine the degree of adaptation of the sweet response for a variety of sweeteners in water and in the presence of two levels of tannic acid. Sweetness intensity ratings were given by a trained panel for 14 sweeteners: three sugars (fructose, glucose, sucrose), two polyhydric alcohols (mannitol, sorbitol), two terpenoid glycosides (rebaudioside-A, stevioside), two dipeptide derivatives (alitame, aspartame), one sulfamate (sodium cyclamate), one protein (thaumatin), two N-sulfonyl amides (acesulfame-K, sodium saccharin), and one dihydrochalcone (neohesperidin dihydrochalcone). Panelists were given four isointense concentrations of each sweetener by itself and in the presence of two concentrations of tannic acid. Each sweetener concentration was tasted and rated four consecutive times with a 30 s interval between each taste and a 2 min interval between each concentration. Within a taste session, a series of concentrations of a given sweetener was presented in ascending order of magnitude. Adaptation was calculated as the decrease in intensity from the first to the fourth sample. The greatest adaptation in water solutions was found for acesulfame-K, Na saccharin, rebaudioside-A, and stevioside. This was followed by the dipeptide sweeteners, alitame and aspartame. The least adaptation occurred with the sugars, polyhydric alcohols, and neohesperidin dihydrochalcone. Adaptation was greater in tannic acid solutions than in water for six sweeteners. Adaptation of sweet taste may result from the desensitization of sweetener receptors analogous to the homologous desensitization found in the beta adrenergic system.

Adaptation effect of sucrose on the salt taste response

1. Intracellular recordings from mouse taste receptor cells were made to study cellular adaptation properties. 2. The sugar and salt receptor mechanisms of mammalian taste cells were investigated with cross-adaptation experiments. 3. The responding of taste cells to sucrose as well as to NaCl does not contradict the independency of their binding mechanisms. 4. With a mixture of sucrose and NaCl, different adsorption mechanisms are observed. 5. From these observations, it was concluded that adaptation occurs in the taste receptor cell.

Cross-adapted salt responses in the mouse taste cell

The exact nature of taste adaptation is not known. Intracellular recordings from taste receptor cells are appropriate to clarify cellular adaptation properties. I approached the study of the sugar and salt receptor mechanisms of mammalian taste cells with cross-adaptation experiments. Sucrose pre-adaptation suppresses the cross-adaptation responses to salts. The results show that the taste adaptation is located in the taste receptor cell.

Characteristics of antisweet substances, sweet proteins, and sweetness-inducing proteins

Recent studies on structures and functions of sweetness-inhibiting substances (gymnemic acid, ziziphin, and gurmarin); sweet proteins (monellin, thaumatin and mabinlin); and taste-modifying proteins (miraculin and curculin) were reviewed. Several gymnemic acid homologues and gurmarin were purified from the leaves of Gymnema sylvestre and their structures were determined. Ziziphin was also purified from leaves of Ziziphus jujuba. Gymnemic acid and ziziphin are glycoside of triterpenes that suppress sweetness in human, while gurmarin is a peptide having antisweet activity in rat. Mabinlin is a heat-stable sweet protein. The whole amino acid sequence and the position of disulfide bridges of mabinlin were determined. Miraculin has the unusual property of modifying a sour taste into a sweet taste. Curculin elicits a sweet taste. In addition, water and sour substance elicit a sweet taste after curculin. Their amino acid sequences and subunit structures were determined. These proteins are expected to be used as low-calorie sweeteners.

Mechanisms of chemosensory transduction in taste cells

The application of new techniques to the study of taste cells has revealed much about both the basic physiology of these cells and also about the mechanisms of taste transduction. The taste cells are electrically excitable cells with a variety of voltage-dependent ion currents. These ionic currents have an important role in the transduction of salt taste in mammals and frogs. In mudpuppies different ion channels are involved in the transduction of acidic-sour stimuli. The role of ion currents in the transduction of sweet taste is less clear. Some proposed mechanisms suggest an important role for ion currents and others suggest that the transduction process may be a biochemical event involving cell surface receptors and intracellular second messengers, possibly cAMP. The transduction of bitter taste seems to be a biochemical event involving cell surface receptors and intracellular second messengers in the inositol trisphosphate pathway. Thus, one cannot talk about "the mechanism" of taste transduction. Different taste modalities are transduced by different mechanisms. A corollary to this is that taste cells are not a homogeneous population of cells. In order to provide animals with the ability to discriminate between different taste modalities the taste cells consist of distinct subpopulations of cells based on their primary taste modality. The primary taste modality in a given cell is determined by the receptors and transduction mechanism(s) expressed in that cell. Evidence suggests that modality-specific receptors are expressed in a segregated manner in distinct subpopulations of taste cells. Secondary responses observed in gustatory axons may arise due to a lack of absolute specificity in the transduction processes and nonspecific effects of low pH and high ionic strength and osmolarity on the taste cells. An interesting area for future work will be to elucidate the mechanism(s) by which basal cells become committed to a given taste modality and how the gustatory neurons influence this process of differentiation. The involvement of the gustatory neurons is critical as they must synapse with taste cells of the correct taste modality to preserve the integrity of the information transferred to the CNS. This process of synaptogenesis is presumably mediated by the expression of taste-modality-specific, cell surface antigens on the basolateral domain of a taste cell and receptors on the appropriate neurons, but much work will be necessary to elucidate this process.(ABSTRACT TRUNCATED AT 400 WORDS)