Taste perception of bitter compounds in young and elderly persons: relation to lipophilicity of bitter compounds

Does presbygeusia really exist? An updated narrative review

This review critically assessed the existence of presbygeusia, i.e., the impairment in taste perception occurring in the elderly, as a natural part of the aging process and its potential clinical implications. Several factors might contribute to age-related taste alterations (TAs), including structural changes in taste buds, alterations in saliva composition, central nervous system changes, and oral microbiota dysbiosis. A comprehensive literature review was conducted to disentangle the effects of age from those of the several age-related diseases or conditions promoting TAs. Most of the included studies reported TAs in healthy elderly people, suggesting that presbygeusia is a relatively frequent condition associated with age-related changes in the absence of pathological conditions. However, the impact of TAs on dietary preferences and food choices among the elderly seems to be less relevant when compared to other factors, such as cultural, psychological, and social influences. In conclusion, presbygeusia exists even in the absence of comorbidities or drug side effects, but its impact on dietary choices in the elderly is likely modest.

Histological and immunohistochemical studies of the fungiform and the circumvallate papillae through the life stages from 6- to 72-week-old Sprague-Dawley male rats

Taste sensitivity decreases with age. Therefore, we investigated the histological and immunohistochemical changes in the receptive fields circumvallate papilla (CvP) and fungiform papilla (FfP) to explore the mechanism underlying age-related changes in taste sensitivity in 6- to 72-week-old rats. We analyzed papilla size, the thickness of the keratin layer of the papilla and stratified squamous epithelium, taste bud size, the keratin layer around the taste pores in the CvP and FfP, and the number and distribution of taste buds in the CvP coronal section. We further assessed the expression of marker proteins for Type II and III cells, phospholipase C subtype beta 2 (PLCβ2), and synaptosomal-associated protein 25 (SNAP-25). The cellular activity of these taste cells was examined through co-localization with the senescence cell marker protein-30 (SMP30). There were no differences in the number of taste bud sections in the CvP among the age groups. However, the size of the CvP increased and the density of the taste bud area in the CvP area decreased with increasing age. In contrast, the number of cells with co-expression of SMP30, PLCβ2, and SNAP-25 decreased with age. Furthermore, the morphological structures of the CvP, FfP, and taste buds in these regions changed with age, but not the overall taste bud number in the CvP coronal section. The decrease in cell count with co-expression of SMP30 and PLCβ2, or SNAP-25 may indicate reduced cellular functions of taste cells with aging.

Change in preference for vegetables and their perceptual attributes as a function of age and pickiness

Preference for vegetables is influenced by various factors, including demographic, psychological, socio-environmental, and genetic factors. This study confirmed that age, pickiness, and perceptual attributes were predictors of preference for vegetables and examined how preference for vegetables and their perceptual attributes varies with age and pickiness. Children (8-14 years, n = 420), youth (15-34 years, n = 569), middle-aged adults (35-64 years, n = 726), and older adults (65-85 years, n = 270) were asked which vegetables they liked (or disliked) and which perceptual attributes of each vegetable they liked (or disliked). On the basis of their responses, an overall preference score and a preference sub-score for each perceptual attribute were calculated. Participants in each age group were classified into four statuses (non-, mild, moderate, and severe) according to their pickiness scores. Multiple regression analysis revealed that age and preference sub-scores for eight perceptual attributes (sweetness, sourness, bitterness, umami, pungency, orthonasal aroma, texture, and appearance) were positive predictors of overall preference score and that pickiness score and four perceptual attributes (saltiness, astringency, retronasal aroma, and aftertaste) were negative predictors. In addition, overall preference score and preference sub-scores for perceptual attributes other than saltiness increased with increasing age group and decreasing picker status; however, preference sub-scores for at least one of the six perceptual attributes (bitterness, astringency, pungency, orthonasal aroma, retronasal aroma, and aftertaste) exhibited negative values in children, youth, and pickers (mild, moderate, and severe). The increase in preference for these perceptual attributes might be an indicator of the adultization of food perception and the expansion of food acceptance.

Bitter taste sensitivity in domestic dogs (Canis familiaris) and its relevance to bitter deterrents of ingestion

As the most favoured animal companion of humans, dogs occupy a unique place in society. Understanding the senses of the dog can bring benefits to both the dogs themselves and their owners. In the case of bitter taste, research may provide useful information on sensitivity to, and acceptance of, diets containing bitter tasting materials. It may also help to protect dogs from the accidental ingestion of toxic substances, as in some instances bitter tasting additives are used as deterrents to ingestion. In this study we examined the receptive range of dog bitter taste receptors (Tas2rs). We found that orthologous dog and human receptors do not always share the same receptive ranges using in vitro assays. One bitter chemical often used as a deterrent, denatonium benzoate, is only moderately active against dTas2r4, and is almost completely inactive against other dog Tas2rs, including dTas2r10, a highly sensitive receptor in humans. We substituted amino acids to create chimeric dog-human versions of the Tas2r10 receptor and found the ECL2 region partly determined denatonium sensitivity. We further confirmed the reduced sensitivity of dogs to this compound in vivo. A concentration of 100μM (44.7ppm) denatonium benzoate was effective as a deterrent to dog ingestion in a two-bottle choice test indicating higher concentrations may increase efficacy for dogs. These data can inform the choice and concentration of bitter deterrents added to toxic substances to help reduce the occurrence of accidental dog poisonings.

Association of phenylthiocarbamide perception with anthropometric variables and intake and liking for bitter vegetables

Phenylthiocarbamide (PTC) sensitivity, a sensory trait mediated by the bitter taste receptor 38 (TAS2R38), has been described as a promising biomarker of health status or disease risk. The aim of this cross-sectional study was to evaluate the influence of PTC phenotypes on (1) individual anthropometric and clinical history variables; (2) other basic taste recognition thresholds (RTs), and (3) the hedonic perception and habitual intake of Brassicaceae vegetables in a young adult population (18.9 ± 1.7 years old). The PTC phenotype was determined by the quantitative measure of the PTC recognition threshold (non-tasters, 24.1%; tasters, 52.3%; and super tasters, 23.6%). No significant differences in smoking habits, oral and nasal disorders, family antecedents of diseases related to metabolic syndrome, and Brassicaceae vegetable hedonic perception and consumption were found between the PTC phenotype groups. The average BMI of super-taster females and males was significantly lower compared to non-tasters. In addition, the PTC taster status was a predictor of lower scores for other basic taste RTs. Overall, the defined PTC super-taster cohort could be differentiated from the non-tasters by variables related to weight control such as BMI and sucrose RT.

Reduced Salivary Mucin Binding and Glycosylation in Older Adults Influences Taste in an In Vitro Cell Model

Background: Taste loss is a significant problem in older adults, affecting quality of life and nutrition. Altered salivary rheology and loss of mucin function may contribute to taste loss by reducing mucosal defences in the oral cavity, impairing sensitivity to oral stimulants. This study aimed to investigate the effects of salivary rheology on taste loss in ageing. Salivary mucin glycosylation and binding to the oral epithelium was investigated in older and younger adults. A cell-based model was utilised to consider the role of saliva in taste loss. Methods: Human subjects aged >60 years (n = 25) and 18–30 (n = 30) provided saliva samples which were analysed for viscosity, mucin composition and mucin binding to oral epithelial cells (TR146/MUC1). Oral epithelial cells (TR146/MUC1 and SCC090) provided models for taste receptor activation. Results: Reduced levels and sialylation of MUC7 were evident in saliva of older adults which may lead to reduced viscoelasticity, while viscosity is unaffected. Impaired muco-adhesion of saliva from older adults was also observed. Saliva from older adults facilitated the bitter taste receptor activation less well than saliva from younger adults. The causes of taste dysfunction in older adults are unknown, but this study supports a role of saliva in facilitating the activation of taste receptors.

Sucrose octaacetate

Sucrose octaacetate (SOA) is a United States National Formulary (NF) monograph compendial material (U.S. Pharmacopeia, 2008), and, as shown in Fig. 1, has eight acetate groups attached to a sucrose moiety. It is a natural product that has been extracted from the seeds of Annona cornifolia (Lima et al., 2011). It is nontoxic (Sigma-Aldrich, 2016) and has a number of uses based on its bitter taste. For example, sugar is rendered too bitter is eat at a concentration of 0.06% (w/w) SOA (Mann et al., 1992). SOA can form 255 different possible isomers and degradation products, all of which have a very low molar absorptivity. Its ultraviolet molar absorptivity at 210nm has been reported to be 439 absorption units/cm/M in water and 442 absorption units/cm/M in 30:70 acetonitrile-water.

Detection thresholds for quinine, PTC, and PROP measured using taste strips

PURPOSE

In clinical practice, when ability to perceive bitter taste is studied, quinine is preferred to phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP) as taste stimulus, because many subjects are genetically non-tasters for PTC/PROP. However, it is poorly known how sensitive anterior (front) and posterior (back) parts of the tongue are to different bitter tastants that are detected by different bitter taste receptors (TAS2Rs). In the present study, we aimed to characterize sensitivity to bitter taste at front and back parts of tongue.

METHODS

We measured thresholds for quinine, PTC, and PROP using the "taste strips", employing seven concentrations of each stimulus both at front and back parts of tongue in 203 healthy participants (56% females, mean age 28 years).

RESULTS

Our data confirmed the hypothesis that the inability to perceive quinine was less frequent than the inability to perceive PTC and PROP: People can still perceive the bitter taste of quinine even if they are "non-tasters" for PROP/PTC. As expected, strong correlations between PTC and PROP thresholds were observed. Interestingly, correlations between thresholds for quinine and PTC/PROP also emerged. Overall, the detection thresholds were lower when measured at front part of the tongue.

CONCLUSIONS

Our data suggest that determining taster status for quinine using paper "taste strips", applied to front part of the tongue, represents a suitable method for the screening for ageusia for bitter taste.

Unstimulated saliva: Background noise in taste molecules

Saliva is a highly complex bodily fluid composed of many proteins, peptides, small organic molecules, and ions. Saliva is produced and secreted by the major and minor salivary glands to protect the mouth and to participate in digestion. Generally, a distinction is made between unstimulated saliva that is a result of autonomic stimulation and stimulated saliva that is produced during chewing and taste stimulation. The link between saliva and sensory perception can thus be regarded in two ways: the role of unstimulated saliva as a background taste and the mechanistic role of stimulated saliva during eating. Indeed, unstimulated saliva (and its components) is continuously bathing our oral cavity and as such stimulates our taste receptors, thus playing a role in taste sensitivity. However, the role of unstimulated salivary components in mediating taste has been studied only in very few substances. To explore this question, this review attempts to compare data from the literature on unstimulated salivary composition with those on taste sensitivity. The main conclusion centres around the concept that the gustatory self-adaptation phenomenon may be relevant for only a few salivary compounds. Further studies at the level of the salivary Von Ebner glands and salivary pellicle are necessary before arriving at definitive conclusions on this subject. PRACTICAL APPLICATION: Unstimulated saliva contains taste substances that can influence sensory perception through taste adaptation. However, large inter-individual variability exists in unstimulated salivary composition both qualitatively and quantitatively. These differences may explain the variability in taste perception and thus the food choices and behaviors of an individual. Thus, in the context of providing personalized food and nutrition to the consumer, variability of unstimulated saliva should be considered for specific formulation of food products.

Sucrose Octaacetate Chemical Kinetics and Shelf Lives at Various Formulation pHs

Developing pediatric friendly dosage forms is a high priority worldwide. Sucrose octaacetate (SOA) has been recommended for use as a surrogate for bitter tasting active pharmaceutical ingredients. Even though SOA has found a number of human use applications and has been employed for decades, there are no rigorous chemical kinetic studies reported. A recently reported SOA stability-indicating method was used to perform SOA chemical kinetic and stability studies. As part of the chemical kinetic study, reaction order, activation energies, extrapolated rate constants, pH-rate profiles at 4 and 25°C, and estimated shelf lives at 4 and 25°C at different buffer pHs are provided. The estimated SOA shelf lives at 25°C and pHs 4.00, 5.20, and 6.00 were 25.3, 114, and 27.4 days, respectively. At 4°C, SOA's estimated shelf lives were 0.478, 5.26, and 1.47 years at pHs 4.00, 5.20, and 6.00, respectively. SOA can be formulated at pHs 4 to 6 and stored at 25°C for short-duration (less than 25 days) uses such as a bitter tasting surrogate for fundamental taste mechanism studies or brief taste masking assessment clinical studies. For longer term solution studies, like being used as a bitter tasting control for blinded clinical trials, SOA should be formulated at the optimum pH of 5.40 and refrigerated at 4°C for maximum stability. The reported data can be used as a starting point for developing stable SOA formulations and estimating shelf life.

Statistical Optimization of Evaporative Light Scattering Detection for Molten Sucrose Octaacetate and Comparison With Ultraviolet Diode Array Detection Validation Parameters Using Tandem HPLC Ultraviolet Diode Array Detection/Evaporative Light Scattering Detection-Specific Stability-Indicating Method

A sucrose octaacetate (SOA) gradient HPLC evaporative light scattering detection (ELSD) and low-wavelength UV-diode array detection (UV-DAD)-specific stability-indicating method development and validation comparison is reported. A central composite response surface design and multicriteria optimization was used to maximize molten SOA area-under-the-curve response and signal-to-noise ratio. The ELSD data were also analyzed using multivariate principal component analysis, analysis of variance, and standard least squares effects modeling. The method suitability and validation parameters of both methods were compared. To the authors' knowledge, this is the first report that validates an ELSD method using a molten analyte. SOA exhibited a low molar absorptivity of 439 absorption units/cm/M in water at 210 nm requiring low-wavelength UV-DAD detection. The low-wavelength UV-DAD method provided substantially better intraday and interday precision, intraday and interday goodness-of-fit, detection limit, and quantitation limit than ELSD. ELSD exhibited a 60-fold greater area-under-the-curve response, better resolution, and 58% more theoretical plates. On balance, the UV-DAD method was chosen for SOA chemical kinetic studies. This study illustrates that ELSD may not always be the best alternative to gradient HPLC low-wavelength UV detection.

Sensory Threshold Studies of Picrocrocin, the Major Bitter Compound of Saffron

This study is part of a wider project on the bitter taste of saffron and its preparations. A deeper knowledge on the taste perception of picrocrocin is necessary in order to develop products that satisfy consumer senses and provide them with adequate amounts of saffron major constituents, also appreciated for bioactivity. A systematic approach on the bitterness of picrocrocin, the major responsible compound, was conducted. A panel was trained specifically for the determination of taste detection and recognition thresholds of picrocrocin, which were found to be 5.34 and 7.26 mg/L, respectively, using the Ascending Forced Choice of Limits methodology. The threshold values were examined in water in absence and presence of other saffron constituents and ethanol and were found to decrease when served hot (61 ± 4 °C). Bitterness was enhanced in 40% (v/v) aqueous ethanol. In both aqueous and ethanol extracts, the presence of saffron volatiles improved bitterness perception. The usefulness of the study was tested in the case of commercial saffron based infusions.

Age-related differences in bitter taste and efficacy of bitter blockers

Background

Bitter taste is the primary culprit for rejection of pediatric liquid medications. We probed the underlying biology of bitter sensing and the efficacy of two known bitter blockers in children and adults.

Methods

A racially diverse group of 154 children (3-10 years old) and their mothers (N = 118) evaluated the effectiveness of two bitter blockers, sodium gluconate (NaG) and monosodium glutamate (MSG), for five food-grade bitter compounds (quinine, denatonium benzoate, caffeine, propylthiouracil (PROP), urea) using a forced-choice method of paired comparisons. The trial was registered at clinicaltrials.gov (NCT01407939).

Results

The blockers reduced bitterness in 7 of 10 bitter-blocker combinations for adults but only 3 of 10 for children, suggesting that efficacy depends on age and is also specific to each bitter-blocker combination. Only the bitterness of urea was reduced by both blockers in both age groups, whereas the bitterness of PROP was not reduced by either blocker in either age group regardless of TAS2R38 genotype. Children liked the salty taste of the blocker NaG more than did adults, but both groups liked the savory taste of MSG equally.

Conclusions and Relevance

Bitter blocking was less effective in children, and the efficacy of blocking was both age and compound specific. This knowledge will pave the way for evidence-based strategies to help develop better-tasting medicines and highlights the conclusion that adult panelists and genotyping alone may not always be appropriate in evaluating the taste of a drug geared for children.

Barium versus nonbarium stimuli: differences in taste intensity, chemesthesis, and swallowing behavior in healthy adult women

PURPOSE

The authors examined the impact of barium on the perceived taste intensity of 7 different liquid tastant stimuli and the modulatory effect that these differences in perceived taste intensity have on swallowing behaviors.

METHOD

Participants were 80 healthy women, stratified by age group (<40; >60) and genetic taste status (supertasters; nontasters). Perceived taste intensity and chemesthetic properties (fizziness; burning-stinging) were rated for 7 tastant solutions (each prepared with and without barium) using the general Labeled Magnitude Scale. Tongue-palate pressures and submental surface electromyography (sEMG) were simultaneously measured during swallowing of these same randomized liquids. Path analysis differentiated the effects of stimulus, genetic taste status, age, barium condition, taste intensity, and an effortful saliva swallow strength covariate on swallowing.

RESULTS

Barium stimuli were rated as having reduced taste intensity compared with nonbarium stimuli. Barium also dampened fizziness but did not influence burning-stinging sensation. The amplitudes of tongue-palate pressure or submental sEMG did not differ when swallowing barium versus nonbarium stimuli.

CONCLUSIONS

Despite impacting taste intensity, the addition of barium to liquid stimuli does not appear to alter behavioral parameters of swallowing. Barium solutions can be considered to elicit behaviors that are similar to those used with nonbarium liquids outside the assessment situation.

Effects of age, gender and chronic obstructive pulmonary disease on taste acuity

PRIMARY OBJECTIVE

Multifactoral interactions of age, gender and medical conditions were investigated because of lack of definitive results in previous studies.

METHODS

Absolute detection and recognition were determined thresholds using a three-alternative forced-choice test by younger men (n=50) and women (n=48); and by older men (n=43) and women (n=46) for each of the four basic tastes (sweet, sour, bitter, salty). Data from 46 elderly men with chronic obstructive pulmonary disease (COPD) were included.

RESULTS

Age was consistently associated with elevated recognition thresholds for each of the four tastants for men only. Gender differences were seen for absolute sweet thresholds and for sour, bitter, and sweet recognition thresholds for older participants. Men with COPD had the highest levels for absolute and recognition thresholds for each tastant.

CONCLUSIONS

Gender plays a role in taste perception. COPD diminishes taste.

Ageing and taste

Taste perception has been studied frequently in young and older adult groups. This paper systematically reviews these studies to determine the effect of ageing on taste perception and establish the reported extent of sensory decline. Five databases were searched from 1900 to April 2012. Articles relating to healthy ageing in human subjects were included, reviewed and rated (Downs and Black scoring system). Sixty-nine studies investigated the effect of ageing on taste perception; forty examined detection thresholds of which twenty-three provided sufficient data for meta-analysis, eighteen reported identification thresholds and twenty-five considered supra-threshold intensity perception. Researchers investigating detection thresholds considered between one and thirteen taste compounds per paper. Overall, the consensus was that taste detection thresholds increased with age (Hedges’ g = 0·91, P < 0·001), across all taste modalities. Identification thresholds were reported to be higher for older adults in seventeen out of eighteen studies. Sixteen out of twenty-five studies reported perception of taste intensity at supra-threshold levels to be significantly lower for older adults. However, six out of nine studies concerning sucrose found perceived intensity of sweet taste not to diminish with age. The findings of this systematic review suggest taste perception declines during the healthy ageing process, although the extent of decline varies between studies. Overall, the studies reviewed had low Downs and Black scores (mean 16 (SD 2)) highlighting the need for more robust large scale and longitudinal studies monitoring the impact of ageing on the sensory system, and how this influences the perception of foods and beverages.

Salivary protein profiles and sensitivity to the bitter taste of caffeine

The interindividual variation in the sensitivity to bitterness is attributed in part to genetic polymorphism at the taste receptor level, but other factors, such as saliva composition, might be involved. In order to investigate this, 2 groups of subjects (hyposensitive, hypersensitive) were selected from 29 healthy male volunteers based on their detection thresholds for caffeine, and their salivary proteome composition was compared. Abundance of 26 of the 255 spots detected on saliva electrophoretic patterns was significantly different between hypo- and hypersensitive subjects. Saliva of hypersensitive subjects contained higher levels of amylase fragments, immunoglobulins, and serum albumin and/or serum albumin fragments. It also contained lower levels of cystatin SN, an inhibitor of protease. The results suggest that proteolysis occurring within the oral cavity is an important perireceptor factor associated to the sensitivity to the bitter taste of caffeine.

Age-related functional changes in gustatory and reward processing regions: An fMRI study

Changes in appetite in older adults may result in unhealthy weight change and negatively affect overall nutrition. Research examining gustatory processing in young adults has linked changes in patterns of the hemodynamic response of gustatory and motivation related brain regions to the physiological states of hunger and satiety. Whether the same brain regions are involved in taste processing in older adults is unknown. The current study used functional magnetic resonance imaging (fMRI) to examine age-related changes in gustatory processing during hedonic assessment. Caffeine, citric acid, sucrose, and NaCl were administered orally during two event-related fMRI sessions, one during hunger and one after a pre-load. Participants assessed the pleasantness of the solutions in each session. Increased activity of the insula was seen in both age groups during hunger. Activity of secondary and higher order taste processing and reward regions such as the orbitofrontal cortex, amygdala, hippocampus, thalamus, and caudate nucleus was also observed. Hunger and satiety differentially affected the hemodynamic response, resulting in positive global activation during hunger and negative during satiety in both age groups. While in a state of hunger, the frequency and consistency of positive activation in gustatory and reward processing regions was greater in older adults. Additional regions not commonly associated with taste processing were also activated in older adults. Investigating the neurological response of older adults to taste stimuli under conditions of hunger and satiety may aid in understanding appetite, health, and functional changes in this population.

Heritability and genetic covariation of sensitivity to PROP, SOA, quinine HCl, and caffeine

The perceived bitterness intensity for bitter solutions of propylthiouracil (PROP), sucrose octa-acetate (SOA), quinine HCl and caffeine were examined in a genetically informative sample of 392 females and 313 males (mean age of 17.8 +/- 3.1 years), including 62 monozygotic and 131 dizygotic twin pairs and 237 sib pairs. Broad-sense heritabilities were estimated at 0.72, 0.28, 0.34, and 0.30 for PROP, SOA, quinine, and caffeine, respectively, for perceived intensity measures. Modeling showed 1) a group factor which explained a large amount of the genetic variation in SOA, quinine, and caffeine (22-28% phenotypic variation), 2) a factor responsible for all the genetic variation in PROP (72% phenotypic variation), which only accounted for 1% and 2% of the phenotypic variation in SOA and caffeine, respectively, and 3) a modest specific genetic factor for quinine (12% phenotypic variation). Unique environmental influences for all four compounds were due to a single factor responsible for 7-22% of phenotypic variation. The results suggest that the perception of PROP and the perception of SOA, quinine, and caffeine are influenced by two distinct sets of genes.

The anorexia of aging in humans

Energy intake is reduced in older individuals, with several lines of evidence suggesting that both physiological impairment of food intake regulation and non-physiological mechanisms are important. Non-physiological causes of the anorexia of aging include social (e.g. poverty, isolation), psychological (e.g. depression, dementia), medical (e.g. edentulism, dysphagia), and pharmacological factors. Physiological factors include changes in taste and smell, diminished sensory-specific satiety, delayed gastric emptying, altered digestion-related hormone secretion and hormonal responsiveness, as well as food intake-related regulatory impairments for which specific mechanisms remain largely unknown. Studies in healthy elderly individuals have shown that men who consume diets over several weeks providing either too few or too many calories relative to dietary energy needs subsequently do not compensate for the resulting energy deficit or surplus when provided an ad libitum diet. Healthy elders have also been shown to be less hungry at meal initiation and to become more rapidly satiated during a standard meal compared to younger adults. Studies in animal models are required to investigate potential mechanisms underlying these observations, while human studies should focus on examining the potential consequences of this phenomenon and practical therapeutic strategies for the maintenance of appropriate energy intake with increasing age. In light of this need, we have recently demonstrated that low reported hunger assessed using a simple questionnaire predicts unintentional weight loss in a sample of healthy older women, and thus may provide a clinically useful tool for identifying older individuals at risk for undesirable weight change and therefore at high priority for intervention.

Physiologically based pharmacokinetic (PBPK) modeling of caffeine and theophylline in neonates and adults: implications for assessing children's risks from environmental agents

Children's risks can differ from those in adults for numerous reasons, one being differences in the pharmacokinetic handling of chemicals. Immature metabolism and a variety of other factors in neonates can affect chemical disposition and clearance. These factors can be incorporated into physiologically based pharmacokinetic (PBPK) models that simulate the fate of environmental toxicants in both children and adults. PBPK models are most informative when supported by empirical data, but typically pediatric pharmacokinetic data for toxicants are not available. In contrast, pharmacokinetic data in children are readily available for therapeutic drugs. The current analysis utilizes data for caffeine and theophylline, closely related xanthines that are both cytochrome P-450 (CYP) 1A2 substrates, in developing PBPK models for neonates and adults. Model development involved scale-up of in vitro metabolic parameters to whole liver and adjusting metabolic function for the ontological pattern of CYP1A2 and other CYPs. Model runs were able to simulate the large differences in half-life and clearance between neonates and adults. Further, the models were able to reproduce the faster metabolic clearance of theophylline relative to caffeine in neonates. This differential between xanthines was found to be due primarily to an extra metabolic pathway available to theophylline, back-methylation to caffeine, that is not available to caffeine itself. This pathway is not observed in adults exemplifying the importance of secondary or novel routes of metabolism in the immature liver. Greater CYP2E1 metabolism of theophylline relative to caffeine in neonates also occurs. Neonatal PBPK models developed for these drugs may be adapted to other CYP1A2 substrates (e.g., arylamine toxicants). A stepwise approach for modeling environmental toxicants in children is proposed.

The effects of taste and consistency on swallow physiology in younger and older healthy individuals: a surface electromyographic study

Recent studies done on healthy older individuals have demonstrated elevated taste thresholds for sweet, sour, and salty taste. At suprathreshold levels, such individuals have also shown less ability to discriminate between different intensities of the same tastant. This study was designed to provide information on swallow timing and muscle contraction intensity variables in healthy older people during swallowing of liquid and cottage cheese consistencies in 3 taste conditions (sweet, salty, and sour). Taste and consistency were incorporated in the design of this study to determine not only the effect of taste and consistency independently, but also the effects of taste and consistency in combination. Surface electromyography (EMG) at 3 sites--the orbicularis oris inferior region, submental muscle region, and infrahyoid muscle region--were included in the study. Results revealed that the start of submental muscle activation was significantly later in older participants than in younger participants. The 3 taste conditions had higher EMG levels and shorter activation times than the no-taste condition, although different taste conditions and different muscle sites were affected differently. With thicker consistency, substantially increased EMG amplitude and duration were noted for all three sites. A more pronounced effect of taste was manifested as earlier submental or infrahyoid muscle activation when the three tastants were added to a thicker consistency. Interaction of taste, consistency, and age was also noted for onset time at the submental and infrahyoid sites.

Relationship of papillae number to bitter intensity of quinine and PROP within and between individuals

Subjects were asked to assess the bitterness of one 6-n-propyl-2-thiouracil (PROP) and two quinine HCl (QHCl) concentrations presented via filter papers of varying sizes. The number of taste papillae stimulated by these filter papers was counted in each individual. Whole mouth sensitivity to PROP was determined in a separate session. In support of other demonstrations of spatial summation, these data indicated that perceived bitterness intensity increased as a function of area of stimulation within subjects. Between subjects, there was a significant trend for the perceived bitterness of PROP to increase with the lingual density of fungiform papillae, although this trend was highly variable and was only demonstrable among those who showed at least moderate sensitivity to PROP. On the other hand, the number of stimulated fungiform papillae failed to account for individual differences in perceived bitterness of QHCl.

Covariation in individuals' sensitivities to bitter compounds: evidence supporting multiple receptor/transduction mechanisms

People vary widely in their sensitivities to bitter compounds, but the intercorrelation of these sensitivities is unknown. Our goal was to investigate correlations as a function of individual sensitivities to several bitter compounds representative of different chemical classes and, from these correlations, infer the number and variety of potential bitterness transduction systems for these compounds. Twenty-six subjects rated and ranked quinine HCl, caffeine, (-)-epicatechin, tetralone, L-phenylalanine, L-tryptophan, magnesium sulfate, urea, sucrose octaacetate (SOA), denatonium benzoate, and n-propylthiouracil (PROP) for bitterness. By examining individual differences, ratings and rankings could be grouped into two general clusters--urea/phenylalanine/tryptophan/epicatechin, and quinine/caffeine/SOA/denatonium benzoate/tetralone/magnesium sulfate-none of which contained PROP. When subjects were grouped into the extremes of sensitivity to PROP, a significant difference was found in the bitterness ratings, but not in the rankings. Therefore, there are also subjects who possess diminished absolute sensitivity to bitter stimuli but do not differ from other subjects in their relative sensitivities to these compounds.

The science and complexity of bitter taste

Food choices and eating habits are largely influenced by how foods taste. Without being the dominant taste sensation, bitter taste contributes to the complexity and enjoyment of beverages and foods. Compounds that are perceived as bitter do not share a similar chemical structure. In addition to peptides and salts, bitter compounds in foods may include plant-derived phenols and polyphenols, flavonoids, catechins, and caffeine. Recent studies have shown that humans possess a multitude of bitter taste receptors and that the transduction of bitter taste may differ between one compound and another. Studies of mixture interactions suggest further that bitter compounds suppress or enhance sweet and sour tastes and interact with volatile flavor molecules. Caffeine, a natural ingredient of tea, coffee, and chocolate, has a unique flavor profile. Used as a flavoring agent, it enhances the sensory appeal of beverages. Research developments on the genetics and perception of bitter taste add to our understanding of the role of bitterness in relation to food preference.

The genetics of phenylthiocarbamide perception

The ability to taste the bitter compound phenylthiocarbamide (PTC) and related chemicals is bimodal, and all human populations tested to date contain some people who can and some people who cannot taste PTC. Why this trait has been maintained in the population is uncertain but this polymorphism may influence food selection, nutritional status or thyroid metabolism. The gene product that gives rise to this phenotype is unknown, and its characterization would provide insight into the mechanism of bitter taste perception. Although this trait is often considered a simple Mendelian trait, i.e. one gene two alleles, a recent linkage study found a major locus on chromosome 5p15 and evidence for an additional locus on chromosome 7. The development of methods to identify these genes will provide a good stepping-stone between single-gene disorders and polygenic traits.

Effect of antimicrobial and anti-inflammatory medications on the sense of taste

Elderly individuals and HIV-infected patients have a disproportionate number of taste complaints relative to the general population, and these taste alterations are correlated with the use of medications. Clinical reports of taste disorders have been associated with many drugs, including antimicrobial and anti-inflammatory medications. The purpose of this study was to quantify the taste effects of 6 nonsteroidal anti-inflammatory drugs (NSAIDS) and 13 antimicrobial drugs. The six NSAIDS were: diclofenac sodium salt, fenoprofen calcium salt, ibuprofen, ketoprofen, nabumetone, and sulindac. The 13 antimicrobials were: acyclovir, ampicillin, atovaquone, dapsone, enoxacin, ethambutol, lomefloxacin HCl, ofloxacin, pentamidine isethionate, pyrimethamine, sulfamethoxazole, tetracycline HCl, and trimethoprim. These 19 medications were applied topically to the tongues of unmedicated young and elderly volunteers as well as unmedicated HIV-infected patients to measure the direct effect of the drug on taste receptors. Topical application of drugs to the apical tongue surface was used to mimic the situation in which the drug is secreted into the saliva. The main finding was that the taste qualities of these drugs were perceived as predominantly bitter, metallic, and/or sour, although several did not have a taste. Elderly subjects had higher thresholds than young subjects for one-third of the drugs that were tested. Thresholds for HIV-infected patients were statistically equivalent to young controls; however, HIV-infected patients rated the drugs as more intense at four times above the detection threshold than young subjects. Most of these drugs when applied directly to the tongue also modified the taste intensity of other tastants (e.g., NaCl, citric acid).

Effect of tricyclic antidepressants on taste responses in humans and gerbils

One of the side effects of antidepressant pharmacotherapy reported clinically is impairment of the sense of taste. In this study, the taste effects of four tricyclic antidepressant compounds (clomipramine HCl, desipramine HCl, doxepin HCl, and imipramine HCl) were evaluated experimentally by topical application of the drugs to the tongue. Taste detection threshold concentrations for all four medications ranged from 0.1 mM to 0.2 mM in young persons but were elevated by as much as 7.71 times that in elderly individuals who were taking no concurrent medications. Each compound had a predominantly bitter taste with other qualities including metallic, sour, and sharp-pungent. In addition, each tricyclic antidepressant at concentrations from 1 mM to 5 mM blocked responses to a wide range of taste stimuli in both humans and gerbils. The differential suppression of other tastes by tricyclic antidepressants at the level of the taste receptors may contribute to the clinical reports of dysgeusia and hypogeusia.

Taste effects of lingual application of cardiovascular medications

Medications used to treat cardiovascular diseases such as congestive heart failure, high blood pressure, and arrhythmia, are prescribed extensively in Western countries. However, taste complaints are common side effects of many of these cardiovascular medications. Although clinical observations are helpful in determining potential taste problems from a medication, experimental studies are necessary to obtain quantitative data on taste. In the studies performed here, nine cardiovascular medications (labetalol HCl, captopril, diltiazem HCl, enalapril maleate, hydrochlorothiazide, propranolol HCl, mexiletine HCl, procainamide HCl, and propafenone HCl) were applied to the tongue in human volunteers to measure the direct effect of these drugs on taste receptors. The medications were applied topically to the tongue surface of both young and elderly subjects to mimic the situation in which the drug is secreted into the saliva. Detection thresholds ranged from 0.048 mM (propafenone) to 0.438 mM (procainamide). The detection thresholds of healthy elderly subjects did not significantly differ from young controls. The compounds tested had a predominantly bitter taste with other qualities as well. In addition, topical application of the medications to the tongue affected the taste of one or more taste stimuli, with medications differing in the pattern of taste effects exhibited. The mechanism of taste effects is not fully known, but the results of this study suggest one route may be due to medications' effect on peripheral taste receptors.

Effect of the nucleoside analogs zidovudine, didanosine, stavudine, and lamivudine on the sense of taste

The purpose of this study was to investigate the taste properties of nucleoside analogs, which are among the current medications used to treat human immunodeficiency virus (HIV) infection. Eighteen unmedicated HIV-positive subjects and 41 healthy control subjects participated in threshold and suprathreshold experiments. All of the nucleoside medications tested were perceived as predominantly bitter (along with other qualities such as metallic, medicinal, sour, astringent, and cooling). The nucleoside analog with the lowest detection thresholds was zidovudine; the detection threshold was 1.47 mM for HIV-infected patients and 2.15 mM for control subjects. Detection thresholds for lamivudine were 4.41 mM for HIV-infected patients and 4.36 mM for control subjects. Detection thresholds for stavudine were 6.39 mM for HIV-infected patients and 5.99 mM for control subjects. Detection thresholds for didanosine were 14.29 mM for HIV-infected patients and 24.0 mM for control subjects. The nucleoside analogs also modified the taste perception of KCl and CaCl2. There were no significant differences between HIV-infected subjects and control subjects for detection threshold values for any of the drugs. However, HIV-infected subjects rated lamivudine, zidovudine, and stavudine as significantly more bitter than did the control subjects at concentrations four times higher than their detection thresholds. This result was not due to use of medications by HIV-infected subjects because none of the subjects (neither HIV-infected nor control) were taking medications.

Effect of protease inhibitors on the sense of taste

The purpose of this study was to investigate the taste properties of protease inhibitors which are essential components of drug regimes used to treat human immunodeficiency virus (HIV) infection. In this study, the taste properties of four protease inhibitors (indinavir, ritonavir, saquinavir, and nelfinavir) were investigated in unmedicated HIV-infected patients and healthy controls. Three of the four protease inhibitors (indinavir, ritonavir, and saquinavir) were found to be predominantly bitter (with additional qualities of medicinal, metallic, astringent, sour, and burning). Nelfinavir was found to be relatively tasteless. HIV-infected and uninfected control subjects detected protease inhibitors at similar concentrations, but HIV-infected subjects perceived suprathreshold concentrations as more bitter than controls. Detection thresholds ranged from 0.0061 mM for saquinavir in HIV-infected patients to 0.0702 mM for ritonavir in uninfected control subjects. Suprathreshold studies indicated that protease inhibitors modified the taste perception of a variety of other taste compounds. These results are consistent with clinical findings that protease inhibitors produce taste complaints that can impact patient compliance.

Effect of medications on taste: example of amitriptyline HCl

Use of medications is a major factor that contributes to taste losses in the elderly. Epidemiological studies suggest that community-dwelling elderly over the age of 65 use an average of 2.9 to 3.7 medications, and this number increases significantly for elderly living in retirement and nursing homes. The tricyclic antidepressant amitriptyline HCl is used by at least half a million people aged 65 years or more. In human studies performed here, amitriptyline HCl was found to have a bitter, unpleasant taste of its own. In addition, it blocked responses to other taste stimuli in both humans and gerbils. This blockage in humans was greater when amitriptyline HCl was applied continuously to the tongue than when it was applied intermittently. Continuous application of the drug affected all of the taste qualities to varying degrees, while intermittent application led to taste decrements only for salts. Electrophysiological studies in gerbils also revealed taste decrements after a short adaptation to amitriptyline HCl.

Gustatory neural coding in the cortex of the alert cynomolgus macaque: the quality of bitterness

We sought to define the gustatory neural representation in primates for stimuli that humans describe as predominantly bitter. Thus we analyzed the responses of single neurons from the insular cortex of two alert, male cynomolgus macaques in response to the oral application of four basic taste stimuli (glucose, NaCl, HCl, and quinine HCl) and fruit juice, and to a series of 15 other chemicals to which humans ascribe a bitter component. Gustatory neurons occupied a volume of 109 mm3 across an area of 4.0 mm in the anterposterior plane, 4.4 mm in the mediolateral, and 6.2 mm in the dorsoventral. Taste cells represented 161 (8.6%) of the 1881 neurons tested for chemical sensitivity. Fifty of these could be monitored throughout the delivery of the entire stimulus series, and their responses constitute the data of this study. The mean spontaneous discharge rate of the cortical gustatory cells was 3.2 +/- 3.3 spikes/s (range = 0.2-17.7 spikes/s). The mean breadth-of-tuning coefficient was a moderate 0.77 +/- 0.15 (range = 0.25-0.99). Forty-eight neurons responded to taste stimuli with excitation, and two responded with inhibition. Forty-one of the 50 neurons were able to be classified into one of four functional types based on their responses to the four basic stimuli used here. These were sugar (n = 22), salt (n = 7), acid (n = 7), and quinine (n = 5). A two-dimensional space was generated from correlations among the response profiles elicited by the stimuli array. The 16 bitter chemicals formed a coherent group that was most closely related to HCl, moderately to NaCl, and bore no relationship with glucose. Within the bitter stimuli, six formed a subgroup that was most separated from all nonbitter chemicals: quinine HCl, phenlythiocarbamide, propylthiouracil, caffeine, theophylline, and phenylalanine. Humans describe these stimuli as rather purely bitter. Of the remaining 10 bitter compounds, 4 were on the fringe of the bitter group leading to NaCl: MgCl2, CaCl2, NH4Cl, and arginine. Humans characterize these as bitter-salty. Three were on the fringe leading to HCl: urea, cysteine and vitamin B1. Humans call these bitter-sour. The remaining three (nicotine, histidine, and vitamin B2) occupied the center of the bitter group. Taste quality, inferred from the position of each stimulus in the space, correlated well with human descriptions of the same stimuli, reinforcing the value of the macaque as a neural model for human gustation.

Neural responses to bitter compounds in rats

To determine whether the idiosyncratic distribution of transduction mechanisms for bitter tastants in rat taste receptor cells (TRCs) could be inferred from the neural activity they evoke, single neuron responses to ten bitter-tasting compounds were recorded from rat glossopharyngeal (n = 30) and chorda tympani (n = 22) neurons. Responses to several 'bitter' alkaloids were obtained: 10 mM quinine-HCl, 50 mM caffeine, and 1 mM each nicotine, yohimbine, and strychnine, plus a number of non-alkaloid bitter-tasting compounds: 0.1 M KCl, 0.01 M MgCl2, and 1 mM each phenylthiocarbamide (PTC), L-tyrosine, and denatonium benzoate. To obtain some distinctions with other stimuli NaCl (0.1 M), HCl (pH 2.0), and capsaicin (10 microM) were also tested. It was found that individual neurons in both glossopharyngeal and chorda tympani nerves differed in their relative sensitivities to the various bitter stimuli. To determine relationships among these stimuli, the differences in the evoked responses between each stimulus pair were summarized in a multi-dimensional scaling space. In these analyses neither nerve showed any obvious similarity between the placements of quinine and the other bitter stimuli. Such data suggest that first-order gustatory neurons can discriminate among the above bitter stimuli. For glossopharyngeal neurons, some similarity to quinine was found only for nicotine and denatonium, and for chorda tympani neurons, some similarity to quinine was found only for KCl and MgCl2. Of the bitter compounds tested, quinine evoked the greatest response from glossopharyngeal neurons. We propose this arises because quinine can activate TRCs by more transduction mechanisms than other bitter stimuli. The results from these studies were summarized in a qualitative model for the coding of bitter tastants where the variety of transduction mechanisms for bitters are distributed among various TRCs to account for the heterogeneous responses among the neurons.

Bitterness of sweeteners as a function of concentration

Sixteen trained tasters provided sweetness and bitterness intensity ratings for 19 compounds including: acesulfame-K, alitame, aspartame, fructose, glucose, glycine, lactitol, maltitol, monoammonium glycyrrhizinate, neohesperidin dihydrochalcone, neosugar (fructo-oligosaccharide), palatinit (isomalt), rebaudioside-A, sodium cyclamate, sodium saccharin, stevioside, sucralose, sucrose, and thaumatin. With increasing concentration, high-potency sweeteners including acesulfame-K, neohesperidin dihydrochalcone, sodium saccharin, rebaudioside-A, and stevioside tended to become more bitter. Low-potency sweeteners including fructose, sucrose, and lactitol tended to become less bitter with increasing concentration.

Bitter taste in single chorda tympani taste fibers from chimpanzee

We have found earlier that chimpanzee chorda tympani taste fibers fall into groups that conform with the human taste qualities. This study focuses on bitter taste and its relation to sweet taste. Eight fibers were classified as bitter fibers according to their responses to 31 stimuli. The stimuli included the bitter compounds quinine, denatonium benzoate and caffeine. The results indicate a clear dichotomy between the bitter and sweet fibers. Sweet fibers never responded to the bitter compounds. However, in addition to their responses to the above compounds, some of the bitter fibers were stimulated by other compounds. Most prominent were responses to NaCl-amiloride mixture, KCl and xylitol. In most cases the cause could be assumed to be a bitter taste in the compound. These results suggest that the bitter and sweet tastes are conveyed in specific and separate groups of nerve fibers in the chimpanzee. Because of the closeness between chimpanzee and human, this finding has implications on the question of taste coding in human and the concept of taste qualities.

The effect of sweeteners on bitter taste in young and elderly subjects

The purpose of this study was to quantify the degree of reduction in perceived bitterness by sweeteners at both threshold and suprathreshold concentrations of bitter compounds. Detection and recognition thresholds were determined for six bitter compounds (caffeine, denatonium benzoate, magnesium chloride, quinine hydrochloride, sucrose octaacetate, and urea) in the absence and presence of several suprathreshold concentrations of five sweeteners. The sweeteners were: sucrose, aspartame, sodium saccharin, mannitol, and sorbitol. Polycose was also tested along with the sweeteners. The degree to which bitter thresholds were affected by the addition of sweeteners was dependent on the chemical classification of the sweeteners and their concentrations. In general, the natural sweeteners, sucrose, mannitol, and sorbitol, were more effective than the noncaloric sweeteners, aspartame and sodium saccharin, in elevating the detection and recognition thresholds of the bitter compounds. A sweetness intensity approximating that of 6% sucrose (0.175 M sucrose) or greater was required to elevate thresholds. For elderly subjects, sweeteners did not significantly elevate thresholds for denatonium benzoate and sucrose octaacetate. The degree to which sorbitol and sucrose can decrease the perceived bitterness intensity of suprathreshold concentrations of the six bitter compounds was also determined. The concentrations of sweeteners and bitter compounds were selected to be of moderate to high subjective intensity. The levels of sweeteners used in the mixtures were: sucrose (none, 0.946 M, and 2.13 M) and sorbitol (none, 2.1 M, and 3.68 M). Both sweeteners significantly reduced the bitterness ratings of almost every concentration of the six bitter compounds. The greatest reductions in bitterness were 87.0% for 0.192 microM denatonium benzoate mixed with 2.13 M sucrose and 84.7% for 1.8 M urea mixed with 3.68 M sorbitol.