Neurobiological and psychophysical mechanisms underlying the oral sensation produced by carbonated water

Systematic Review of the Effects of Sparkling Water Consumption on Blood Pressure

PURPOSE OF REVIEW

At present, almost all national health agencies and professional societies recommend reducing dietary sodium to lower blood pressure and prevent cardiovascular disease and stroke. The scientific community has questioned these recommendations multiple times. However, the institutions have clarified that the evidence supporting the reduction of sodium intake remains strong. This review is an examination of the literature on the impact of carbonated water consumption on the blood pressure of hypertensive patients RECENT FINDINGS: Current literature focuses on the act of ingestion and the immediate post-consumption periods, highlighting the significant role of the nervous system on blood pressure increase. This differs from earlier literature, which predominantly focused on hypotheses surrounding the nutritional components responsible for elevated blood pressure Research findings remains uncertainty regarding whether hypertensive patients should avoid the consumption of sparkling water. To contextualize these results, we discuss potential physiological mechanisms, recommendations from relevant organizations, and variations in methodologies and study designs. In conclusion, the question of whether hypertensive patients should steer clear of sparkling water remains unanswered.

Effects of carbonated beverages on sustained swallowing behavior changes in older inpatients

Sensory stimulation, including stimulation with carbonated liquids, has been known to influence swallowing activity. The aim of this study is to determine the effect of sustained changes in the swallowing behavior of hospitalized patients without dysphagia using the cervical auscultatory recordings of swallowing sounds. The study participants were hospitalized older adults without dysphagia. The participants were asked to initially swallow water and then a carbonated beverage. The sustained effects of the carbonated beverage in relation to swallowing movements were evaluated by measuring the swallowing sounds at the following time periods: (1) immediately and (2) at 1 to 9 minutes with intervals of 2 minutes after swallowing the carbonated beverage. The swallowing sounds before and after swallowing the same volume of water were also measured and compared on a different day. Analysis was performed to calculate the time from a sound signal to the beginning of the swallowing sound. Significant shortening of the swallowing sound time of the late pharyngeal phase was observed up to 7 minutes after swallowing the carbonated beverage, except during the 3 minutes after swallowing. Shortening of the whole swallowing sound time was observed only immediately after swallowing the carbonated beverage. Regarding the shortening of the acoustic signal in the late pharyngeal period, the effect of cerebral excitability changes due to carbonic acid stimulation was considered.

Effects of thickened carbonated cola in older patients with dysphagia

Carbonated beverages initiate the swallowing reflex earlier than water and have a shorter pharyngeal transit time. However, the effects of carbonation in thickened beverages of the same flavor on swallowing dynamics have not been reported. Therefore, we investigated the effects of thickened carbonated beverages on swallowing in patients with dysphagia by comparing the swallowing dynamics between thickened carbonated and thickened non-carbonated beverages. We enrolled 38 patients with dysphagia and divided them into two groups. Thickened carbonated and thickened non-carbonated beverages were used. Videoendoscopic swallowing evaluations were performed. Aspiration, penetration, pharyngeal residue, and initiation position of the swallowing reflex were evaluated. The reduction in the amount of residue in both the vallecula (p = 0.007) and pyriform sinus (p = 0.004) was greater after ingestion of thickened carbonated cola than thickened non-carbonated cola. The onset of the swallowing reflex was significantly earlier after ingestion of thickened carbonated cola than thickened non-carbonated cola (p = 0.007). There were no significant differences in the extent of penetration. Thickened carbonated beverages positively affected swallowing compared with thickened non-carbonated beverages. Thus, the use of thickened carbonated beverages may be helpful for patients with dysphagia.

Effects of Carbonated Thickened Drinks on Pharyngeal Swallowing with a Flexible Endoscopic Evaluation of Swallowing in Older Patients with Oropharyngeal Dysphagia

This study aimed to determine the efficacy of carbonated and sweetened drinks added to thickened liquids, which are routinely used for patients with dysphagia to improve dysphagia. Patients swallowed thin liquid (Thin), thickened liquid (Thick), carbonated thin drink (C-Thin), and carbonated thickened drink (C-Thick) in random order. Penetration and/or aspiration were scored using the Penetration−Aspiration Scale (PAS). The residue was scored using the Yale Pharyngeal Residue Severity Rating Scale (YPR-SRS). Swallowing reflex initiation was scored using the Hyodo score. The subjective difficulty of swallowing was scored on a face scale. We analyzed 13 patients with a mean age of 79.6 ± 9.6 years. PAS was significantly lower in the C-Thick group than the Thin group (p < 0.05). Swallowing reflex initiation was significantly different between the Thin and Thick (p < 0.01) groups; moreover, post hoc analysis revealed that it was significantly lower in the C-Thick group than the Thin group (p < 0.01). The subjective difficulty of swallowing in the C-Thick was significantly lower than the Thick group (p < 0.05). C-Thick was easier to swallow than Thick and may improve penetration and/or aspiration in older patients with dysphagia with complex diseases.

Efficacy of sodium bicarbonate ingestion strategies for protecting blinding

Sodium bicarbonate (NaHCO₃) is a widely researched ergogenic aid, but the optimal blinding strategy during randomised placebo-controlled trials is unknown. In this multi-study project, we aimed to determine the most efficacious ingestion strategy for blinding NaHCO₃ research. During study one, 16 physically active adults tasted 0.3 g kg^-1 body mass NaHCO₃ or 0.03 g kg^-1 body mass sodium chloride placebo treatments given in different flavour (orange, blackcurrant) and temperature (chilled, room temperature) solutions. They were required to guess which treatment they had received. During study two, 12 recreational athletes performed time-to-exhaustion (TTE) cycling trials (familiarisation, four experimental). Using a randomised, double-blind design, participants consumed 0.3 g kg^-1 body mass NaHCO₃ or a placebo in 5 mL kg^-1 body mass chilled orange squash/water solutions or capsules and indicated what they believed they had received immediately after consumption, pre-TTE and post-TTE. In study one, NaHCO₃ prepared in chilled orange squash resulted in the most unsure ratings (44%). In study two, giving NaHCO₃ in capsules resulted in more unsure ratings than in solution after consumption (92 vs 33%), pre-TTE (67 vs. 17%) and post-TTE (50 vs. 17%). Administering NaHCO₃ in capsules was the most efficacious blinding strategy which provides important implications for researchers conducting randomised placebo-controlled trials.

The Pressor Response to the Drinking of Cold Water and Cold Carbonated Water in Healthy Younger and Older Adults

Purpose: Water drinking has been proposed for the treatment of orthostatic hypotension because it can increase blood pressure in patients. This study aimed to investigate whether drinking water with a cold or carbonation stimulus would cause a more effective pressor response, and whether it would be greater in older than in younger adults.

Methods: We assessed blood pressure and heart rate from non-invasive arterial pressure (a volume-clamp method) and type II electrocardiography in 13 healthy young adults (6 females, 7 males; mean age, 19.9 ± 1.1 years) and nine healthy older adults (all females; mean age, 71.4 ± 4.2 years) who drank 200 mL of cold, cold carbonated, and room temperature water.

Results: The pressor response to the drinking of cold and cold carbonated water was greater than that to room temperature water in both younger and older participants (p < 0.05; changes in systolic blood pressure of room temperature water, cold water and cold carbonated water in young: 15.31 ± 9.66, 22.56 ± 11.51 and 32.6 ± 17.98 mmHg, respectively; changes in systolic blood pressure of room temperature water, cold water and cold carbonated water in elderly: 21.84 ± 14.31, 41.53 ± 19.82 and 48.16 ± 16.77 mmHg, respectively). In addition, the pressor response to cold and cold carbonated water was persistent during the recovery period by about 5–10 mmHg (p < 0.05). Furthermore, the pressor response during the drinking and recovery periods was greater in the older than in the younger participants (p < 0.05).

Conclusion: Our data suggest that even smaller amounts of water are able to elicit a sustained pressor response, in particular if the water is cold and carbonated. We speculate that the pressor effect may render cold and carbonated water an appropriate first aid method against certain forms of acute hypotension.

Effects of Carbonation on Swallowing: Systematic Review and Meta-Analysis

OBJECTIVES

The effectiveness of the use of carbonation in preventing penetration/aspiration or enhancing swallowing function in adults remains unclear. This systematic review aimed to evaluate the effectiveness of carbonation on improving swallowing function in adult subjects.

METHODS

Literature published before March 2021 was inspected using MEDLINE, CINAHL, Web of Science Core Collection, The Cochrane Library, Cochrane Central Register of Controlled Trials, and Ichushi-web databases. We searched for intervention studies or randomized control trials considering the effects of carbonated liquids on swallowing function. The risk of bias was assessed using the Cochrane tool for assessing the risk of bias for randomized controlled trials and the Risk of Bias Assessment Tool for Nonrandomized Studies.

RESULTS

The systematic review identified 19 studies with a total of 586 participants. The effects of carbonation on swallowing function are diverse. Overall, most studies showed that carbonation promotes swallowing function compared to other liquids. Five studies were included in the quantitative synthesis. Meta-analysis showed that carbonated liquids prevent aspiration (risk difference [RD] -0.27%, 95% confidence interval [CI] -0.44 to -0.10; I²  = 0%; number needed to treat 3.8, 95% CI 2.2 to 15.0; moderate quality of evidence) when compared to noncarbonated thin liquids. Carbonated liquids also increased the duration of swallowing apnea than did noncarbonated liquids (standardized mean difference 0.25 (mean difference 0.36 seconds), 95% CI 0.03 to 0.47; I²  = 0%; low quality of evidence).

CONCLUSIONS

Carbonation had favorable effects on swallowing function. Further in-depth studies are needed to clarify the benefits of carbonation.

LEVEL OF EVIDENCE

NA Laryngoscope, 2022.

Chemogenic Subqualities of Mouthfeel

Mouthfeel refers to the physical or textural sensations in the mouth caused by foods and beverages that are essential to the acceptability of many edible products. The sensory subqualities contributing to mouthfeel are often chemogenic in nature and include heat, burning, cooling, tingling, and numbing. These "chemesthetic" sensations are a result of the chemical activation of receptors that are associated with nerve fibers mediating pain and mechanotransduction. Each of these chemesthetic sensations in the oral cavity are transduced in the nervous system by a combination of different molecular channels/receptors expressed on trigeminal nerve fibers that innervate the mouth and tongue. The molecular profile of these channels and receptors involved in mouthfeel include many transient receptor potential channels, proton-sensitive ion channels, and potassium channels to name a few. During the last several years, studies using molecular and physiological approaches have significantly expanded and enhanced our understanding of the neurobiological basis for these chemesthetic sensations. The purpose of the current review is to integrate older and newer studies to present a comprehensive picture of the channels and receptors involved in mouthfeel. We highlight that there still continue to be important gaps in our overall knowledge on flavor integration and perception involving chemesthetic sensations, and these gaps will continue to drive future research direction and future investigation.

Targeting Chemosensory Ion Channels in Peripheral Swallowing-Related Regions for the Management of Oropharyngeal Dysphagia

Oropharyngeal dysphagia, or difficulty in swallowing, is a major health problem that can lead to serious complications, such as pulmonary aspiration, malnutrition, dehydration, and pneumonia. The current clinical management of oropharyngeal dysphagia mainly focuses on compensatory strategies and swallowing exercises/maneuvers; however, studies have suggested their limited effectiveness for recovering swallowing physiology and for promoting neuroplasticity in swallowing-related neuronal networks. Several new and innovative strategies based on neurostimulation in peripheral and cortical swallowing-related regions have been investigated, and appear promising for the management of oropharyngeal dysphagia. The peripheral chemical neurostimulation strategy is one of the innovative strategies, and targets chemosensory ion channels expressed in peripheral swallowing-related regions. A considerable number of animal and human studies, including randomized clinical trials in patients with oropharyngeal dysphagia, have reported improvements in the efficacy, safety, and physiology of swallowing using this strategy. There is also evidence that neuroplasticity is promoted in swallowing-related neuronal networks with this strategy. The targeting of chemosensory ion channels in peripheral swallowing-related regions may therefore be a promising pharmacological treatment strategy for the management of oropharyngeal dysphagia. In this review, we focus on this strategy, including its possible neurophysiological and molecular mechanisms.

The orotrigeminal system

The trigeminal sensory nerve fiber branches supply afferent information from the skin and mucous membranes of the face and head and the oral cavity regarding information on temperature, touch, and pain. Under normal conditions, the trigeminal nerve serves to provide important information from nerve fibers and tissues using specialized receptors sensitive for irritant and painful stimuli. The current scientific consensus indicates that nerve endings responsible for chemical and thermal sensitivity of the skin and mucous membranes are the same nerves responsible for nociception. This "chemesthetic sense" allows many vertebrates to detect chemical agonists that induce sensations such as touch, burning, stinging, tingling, or changes in temperature. Research has been under way for many years to determine how exposure of the oral and/or nasal cavity to compounds that elicit pungent or irritant sensations can produce these sensations. In addition, these chemicals can alter other sensory information such as taste and smell to affect the flavor of foods and beverages. We now know that these 'chemesthetic molecules' are agonists of molecular receptors, which exist on primary afferent nerve fibers that innervate the orofacial area. However, under pathophysiologic conditions, over- or underexpression or activity of these receptors may lead to painful orotrigeminal syndromes. Some of these individual receptors are discussed in detail, including transient receptor potential channels and acid sensing ion channels, among others.

Guide to Enhancing Swallowing Initiation: Insights from Findings in Healthy Subjects and Dysphagic Patients

PURPOSE OF REVIEW

Difficulty in initiating swallowing is one of the main symptoms of oropharyngeal dysphagia. Therefore, enhancing swallowing initiation is an important approach for the treatment of oropharyngeal dysphagia. This review aims to introduce recent approaches to enhancing swallowing and to discuss their therapeutic potential.

RECENT FINDINGS

Both central interventions such as non-invasive brain stimulation and peripheral interventions such as electrical stimulation to peripheral tissues are conducted to enhance swallowing. Recent studies have paid more attention to generating neuroplasticity to produce long-lasting facilitative effect on swallowing.

SUMMARY

Transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), pharyngeal electrical stimulation (PES), transcutaneous electrical stimulation, and somatic and chemical stimulation were introduced. Considerable evidence supports the therapeutic potential of TMS and PES. Other approaches need further studies to verify their efficacy (e.g., duration of the effect and a limit of effectiveness) and/or possible risk of adverse effects.

Chemical compounds and mechanisms involved in the formation and stabilization of foam in sparkling wines

The visual properties of sparkling wine including foam and bubbles are an indicator of sparkling wine quality. Foam properties, particularly foam height (FH) and foam stability (TS), are significantly influenced by the chemical composition of the wine. This review investigates our current knowledge of specific chemical compounds and, the mechanisms by which they influence the foam properties of sparkling wines. Grape and yeast proteins, amino acids, polysaccharides, phenolic compounds, organic acids, fatty acids, ethanol and sugar are examined with respect to their contribution to foam characteristics in sparkling wines made with the Traditional, Transfer, and Charmat and carbonation methods. Contradictory results have been identified that appear to be due to the analytical methods used to measure and quantify compounds and foam. Biopolymer complexes are discussed and absent knowledge with regards to thaumatin-like proteins (TLPs), polysaccharides, amino acids, oak-derived phenolic compounds and organic acids are identified. Future research is also likely to concentrate on visual analysis of sparkling wines by in-depth imaging analysis and specific sensory analysis techniques.

Exploring the effects of synchronous pharyngeal electrical stimulation with swallowing carbonated water on cortical excitability in the human pharyngeal motor system

BACKGROUND

Previous reports have revealed that excitation of human pharyngeal motor cortex can be induced by pharyngeal electrical stimulation (PES) and swallowing carbonated water (CW). This study investigated whether combining PES with swallowing (of still water, SW or CW) can potentiate this excitation in either cortical and/or brain stem areas assessed with transcranial and transcutaneous magnetic stimulation (TMS).

METHODS

Fourteen healthy volunteers participated and were intubated with an intraluminal catheter to record pharyngeal electromyography and deliver PES. Each participant underwent baseline corticopharyngeal, hand and craniobulbar motor-evoked potential (MEP) measurements. Subjects were then randomized to receive each of four 10-min interventions (PES only, ShamPES+CW, PES+CW, and PES+SW). Corticobulbar, craniobulbar and hand MEPs were then remeasured for up to 60 min and data analyzed using anova and post hoc t-tests.

KEY RESULTS

A two-way rmanova for Interventions × Time-point showed a significant corticopharyngeal interaction (p = 0.010). One-way anova with post hoc t-tests indicated significant cortical changes with PES only at 45 (p = 0.038) and 60 min (p = 0.023) and ShamPES+CW immediately (p = 0.008) but not with PES+CW or PES+SW. By contrast, there were immediate craniobulbar amplitude changes only with PES+CW (p = 0.020) which were not sustained.

CONCLUSIONS & INFERENCES

We conclude that only PES produced long-term changes in corticopharyngeal excitability whereas combination stimuli were less effective. Our data suggest that PES alone rather than in combination, may be better for the patients who have difficulty in performing voluntary swallows.

Brain and behavioral effects of swallowing carbonated water on the human pharyngeal motor system

Chemical stimulation of the swallowing network with carbonation and citric acid has been investigated, showing potential benefits on swallowing of dysphagic patients. Despite this, the underlying mechanisms for these effects are not fully understood. Here we investigated the effects of 5 ml liquid bolus swallows of carbonated, citric acid, and still water on a swallowing reaction-time tasks paradigm in 16 healthy adults (8 male, mean age 33 ± 3.7 yr, protocol 1). We then investigated the net effects of "sensory bolus interventions" (40 repeated swallows every 15 s) of the three different liquid boluses on corticobulbar excitability, as examined with single-pulse transcranial magnetic stimulation (TMS) in 16 participants (8 female, mean age 33 ± 3.7 yr, protocol 2). The findings showed that a larger number of correctly timed swallows (within a predetermined time window) was accomplished mainly with carbonated liquids (z = -2.04, P = 0.04 vs. still water, protocol 1). Both carbonated and citric acid liquid interventions with 40 swallows increased corticobulbar excitability of the stronger pharyngeal projection, suggesting a similar modulatory pathway for the effects on swallowing. However, carbonation showed superiority (P = 0.04, F = 4.75, 2-way ANOVA), with the changes lasting up to 60 min following the intervention. These results hold significance for future further and in-depth physiological investigations of the differences between different stimuli on swallowing neural network.

The influence of bubbles on the perception carbonation bite

Although many people naively assume that the bite of carbonation is due to tactile stimulation of the oral cavity by bubbles, it has become increasingly clear that carbonation bite comes mainly from formation of carbonic acid in the oral mucosa. In Experiment 1, we asked whether bubbles were in fact required to perceive carbonation bite. Subjects rated oral pungency from several concentrations of carbonated water both at normal atmospheric pressure (at which bubbles could form) and at 2.0 atmospheres pressure (at which bubbles did not form). Ratings of carbonation bite under the two pressure conditions were essentially identical, indicating that bubbles are not required for pungency. In Experiment 2, we created controlled streams of air bubbles around the tongue in mildly pungent CO₂ solutions to determine how tactile stimulation from bubbles affects carbonation bite. Since innocuous sensations like light touch and cooling often suppress pain, we predicted that bubbles might reduce rated bite. Contrary to prediction, air bubbles flowing around the tongue significantly enhanced rated bite, without inducing perceived bite in blank (un-carbonated) solutions. Accordingly, though bubbles are clearly not required for carbonation bite, they may well modulate perceived bite. More generally, the results show that innocuous tactile stimulation can enhance chemogenic pain. Possible physiological mechanisms are discussed.

Eugenol and carvacrol induce temporally desensitizing patterns of oral irritation and enhance innocuous warmth and noxious heat sensation on the tongue

Eugenol and carvacrol, from the spices clove and oregano, respectively, are agonists of TRPV3, which is implicated in transduction of warmth and possibly heat pain. We investigated the temporal dynamics of lingual irritation elicited by these agents, and their effects on innocuous warmth and heat pain, using a half-tongue method in human subjects. The irritant sensation elicited by both eugenol and carvacrol decreased across repeated applications at a 1-minute interstimulus interval (self-desensitization) which persisted for at least 10 minutes. Both agents also cross-desensitized capsaicin-evoked irritation. Eugenol and carvacrol significantly increased the magnitude of perceived innocuous warmth (44 °C) for >10 minutes, and briefly (<5 minutes) enhanced heat pain elicited by a 49 °C stimulus. Similar albeit weaker effects were observed when thermal stimuli were applied after the tongue had been desensitized by repeated application of eugenol or carvacrol, indicating that the effect is not due solely to summation of chemoirritant and thermal sensations. Neither chemical affected sensations of innocuous cool or cold pain. A separate group of subjects was asked to subdivide eugenol and carvacrol irritancy into subqualities, the most frequently reported being numbing and warmth, with brief burning, stinging/pricking, and tingle, confirming an earlier study. Eugenol, but not carvacrol, reduced detection of low-threshold mechanical stimuli. Eugenol and carvacrol enhancement of innocuous warmth may involve sensitization of thermal gating of TRPV3 expressed in peripheral warm fibers. The brief heat hyperalgesia following eugenol may involve a TRPV3-mediated enhancement of thermal gating of TRPV1 expressed in lingual polymodal nociceptors.

Examining the role of carbonation and temperature on water swallowing performance: a swallowing reaction-time study

Various therapeutic approaches for dysphagia management are based on modifications of bolus properties to change swallowing biomechanics and increase swallowing safety. Limited evidence exists for the effects of carbonation and bolus temperature on swallowing behavior. Here, we investigated the effects of carbonation and temperature on swallowing behavior using a novel automated and complex swallowing reaction time task via pressure signal recordings in the hypopharynx. Healthy participants (n = 39, 27.7±5 years old) were randomized in two different experiments and asked to perform 10 normal-paced swallows, 10 fast-paced swallows, and 10 challenged swallows within a predetermined time-window of carbonated versus still water (experiment 1) and of cold (4 °C) versus hot (45 °C) versus room temperature (21 °C) water (experiment 2). Quantitative measurements of latencies and percentage of successful challenged swallows were collected and analyzed nonparametrically. An increase in successfully performed challenged swallowing task was observed with carbonated water versus still water (P = 0.021), whereas only cold water shortened the latencies of normally paced swallows compared with room (P = 0.001) and hot (P = 0.004) temperatures. Therefore, it appears that chemothermal stimulation with carbonation and cold are most effective at modulating water swallowing, which in part is likely to be driven by central swallowing afferent activity.

Characterisation of chemosensory trigeminal receptors in the rainbow trout, Oncorhynchus mykiss: responses to chemical irritants and carbon dioxide

Trigeminally innervated, mechanically sensitive chemoreceptors (M) were previously identified in rainbow trout, Oncorhynchus mykiss, but it is not known whether these receptors are responsive only to noxious, chemical irritants or have a general chemosensory function. This study aimed to characterise the stimulus–response properties of these receptors in comparison with polymodal nociceptors (P). Both P and M gave similar response profiles to acetic acid concentrations. The electrophysiological properties were similar between the two different afferent types. To determine whether the receptors have a nociceptive function, a range of chemical stimulants was applied to these receptors, including non-noxious stimuli such as ammonium chloride, bile, sodium bicarbonate and alarm pheromone, and potentially noxious chemical irritants such as acetic acid, carbon dioxide, low pH, citric acid, citric acid phosphate buffer and sodium chloride. Only irritant stimuli evoked a response, confirming their nociceptive function. All receptor afferents tested responded to carbon dioxide (CO2) in the form of mineral water or soda water. The majority responded to 1% acetic acid, 2% citric acid, citric acid phosphate buffer (pH 3) and 5.0 mol l–1 NaCl. CO2 receptors have been characterised in the orobranchial cavity and gill arches in fish; however, this is the first time that external CO2 receptors have been identified on the head of a fish. Because the fish skin is in constant contact with the aqueous environment, contaminants with a low pH or hypercapnia may stimulate the nociceptive system in fish.

Effects of carbonated liquids on oropharyngeal swallowing measures in people with neurogenic dysphagia

Aspiration is common in adults with neurogenic dysphagia and pharyngeal delay. This can lead to dehydration, malnutrition, and aspiration pneumonia. Diet modifications aimed at reducing thin liquid aspiration are partially successful or unpalatable or both. Carbonated liquids show some potential in influencing swallowing behavior. However, there is a paucity of evidence to support this intervention. This study compares the effects of carbonated thin liquids (CTL) with that of noncarbonated thin liquids (NCTL) on oropharyngeal swallowing in adults with neurogenic dysphagia and examines the palatability of the CTL stimulus. Seventeen people with pharyngeal delay attended for videofluoroscopy (VFSS). Outcome measures were oral transit time (OTT), pharyngeal transit time (PTT), stage transition duration (STD), initiation of the pharyngeal swallow (IPS), penetration-aspiration scale (PENASP), and pharyngeal retention (PR). A modification of Quartermaster Hedonic Scale (AQHS) was employed to assess palatability of the CTL. CTL vs. NCTL significantly decreased penetration and aspiration on 5-ml (P = 0.028) and 10-ml (P = 0.037) swallows. CTL had no significant effect on OTT, PTT, IPS, and PR for any volume of bolus. Only one participant disliked the CTL stimulus. These findings support the hypothesis that oropharyngeal swallowing can be modulated in response to sensory stimuli. Implications for research and clinical practice are discussed.

Effects of club soda and ginger brew on linguapalatal pressures in healthy swallowing

Oral chemesthesis is the detection of chemicals that activate temperature and pain receptors in the oral mucosa. Presentation of orally chemesthetic input has been theorized to stimulate a faster, stronger swallow. We measured differences in peak linguapalatal swallowing pressures, pressure durations, and pressure adjustments in response to two volumes of water and carbonation (in Schweppes® Club Soda) and carbonation + gingerol (in Reed's Extra Ginger Brew) in 20 young adult women. There was a main effect of stimulus on linguapalatal swallowing pressure, F(6,74) = 6.247, p = 0.000, hp(2) = 0.536 (Reed's Extra Ginger Brew > Schweppes Club Soda > water). Rising and releasing linguapalatal pressure durations were greater for carbonation + gingerol and carbonation than for water. Our results add to the evidence that orally chemesthetic beverages influence greater neuromotor activity compared to water during the oral stage of swallowing. Our findings also suggest that there may be some benefit to the cumulative addition of chemosensory agents in a beverage. Clinically, this provides a theoretical basis for considering the use of these or chemically similar beverages as facilitating stimuli in patients who aspirate thin liquids.

The cell biology of taste

Taste buds are aggregates of 50–100 polarized neuroepithelial cells that detect nutrients and other compounds. Combined analyses of gene expression and cellular function reveal an elegant cellular organization within the taste bud. This review discusses the functional classes of taste cells, their cell biology, and current thinking on how taste information is transmitted to the brain.

Carbonated beverages and gastrointestinal system: between myth and reality

A wealth of information has appeared on non-scientific publications, some suggesting a positive effect of carbonated beverages on gastrointestinal diseases or health, and others a negative one. The evaluation of the properties of carbonated beverages mainly involves the carbon dioxide with which they are charged. Scientific evidence suggests that the main interactions between carbon dioxide and the gastrointestinal system occur in the oral cavity, the esophagus and the stomach. The impact of carbonation determines modification in terms of the mouthfeel of beverages and has a minor role in tooth erosion. Some surveys showed a weak association between carbonated beverages and gastroesophageal reflux disease; however, the methodology employed was often inadequate and, on the overall, the evidence available on this topic is contradictory. Influence on stomach function appears related to both mechanical and chemical effects. Symptoms related to a gastric mechanical distress appear only when drinking more than 300 ml of a carbonated fluid. In conclusion there is now sufficient scientific evidence to understand the physiological impact of carbonated beverages on the gastrointestinal system, while providing a basis for further investigation on the related pathophysiological aspects. However, more studies are needed, particularly intervention trials, to support any claim on the possible beneficial effects of carbonated beverages on the gastrointestinal system, and clarify how they affect digestion. More epidemiological and mechanistic studies are also needed to evaluate the possible drawbacks of their consumption in terms of risk of tooth erosion and gastric distress.

The taste of carbonation

Carbonated beverages are commonly available and immensely popular, but little is known about the cellular and molecular mechanisms underlying the perception of carbonation in the mouth. In mammals, carbonation elicits both somatosensory and chemosensory responses, including activation of taste neurons. We have identified the cellular and molecular substrates for the taste of carbonation. By targeted genetic ablation and the silencing of synapses in defined populations of taste receptor cells, we demonstrated that the sour-sensing cells act as the taste sensors for carbonation, and showed that carbonic anhydrase 4, a glycosylphosphatidylinositol-anchored enzyme, functions as the principal CO2 taste sensor. Together, these studies reveal the basis of the taste of carbonation as well as the contribution of taste cells in the orosensory response to CO2.

Activation of lumbar spinal wide-dynamic range neurons by a sanshool derivative

The enigmatic sensation of tingle involves the activation of primary sensory neurons by hydroxy-alpha-sanshool, a tingly agent in Szechuan peppers, by inhibiting two-pore potassium channels. Central mechanisms mediating tingle sensation are unknown. We investigated whether a stable derivative of sanshool-isobutylalkenyl amide (IBA)-excites wide-dynamic range (WDR) spinal neurons that participate in transmission of chemesthetic information from the skin. In anesthetized rats, the majority of WDR and low-threshold units responded to intradermal injection of IBA in a dose-related manner over a >5-min time course and exhibited tachyphylaxis at higher concentrations (1 and 10%). Almost all WDR and low-threshold units additionally responded to the pungent agents mustard oil (allyl isothiocyanate) and/or capsaicin, prompting reclassification of the low-threshold cells as WDR. The results are discussed in terms of the functional role of WDR neurons in mediating tingle sensation.

Decreased expression of carbonic anhydrase isozyme II, rather than of isozyme VI, in submandibular glands in long-term zinc-deficient rats

We previously reported that in rats, long-term Zn deficiency significantly reduced taste sensitivity and total carbonic anhydrase (CA) activity in the submandibular gland. We therefore investigated the effects of Zn deficiency on salivary secretion and the expressions of CA isozymes (II and VI) in the rat submandibular gland, since those isozymes are thought to be related to taste sensation and salivary secretion. Male Sprague–Dawley rats, age 4 weeks, were divided into three groups (Zn-def, low-Zn and pair-fed, that were fed a diet containing 2·2, 4·1 or 33·7 mg Zn/kg, respectively, for 42 d). Northern blot analysis indicated that Zn deficiency reduced CA II mRNA expression in the submandibular gland without reducing CA VI mRNA expression. In Western blot analysis, Zn deficiency significantly reduced CA II (erythrocyte CA) protein expression in the submandibular gland without reducing CA VI protein expression. Salivary secretion was lower in the Zn-def group than in the pair-fed group. These results suggest that decreased CA isozyme II expression underlies the decreased CA activity previously reported in the submandibular gland in Zn-def rats, and this may reduce regular salivary secretion.

The neural mechanisms of gustation: a distributed processing code

Whenever food is placed in the mouth, taste receptors are stimulated. Simultaneously, different types of sensory fibre that monitor several food attributes such as texture, temperature and odour are activated. Here, we evaluate taste and oral somatosensory peripheral transduction mechanisms as well as the multi-sensory integrative functions of the central pathways that support the complex sensations that we usually associate with gustation. On the basis of recent experimental data, we argue that these brain circuits make use of distributed ensemble codes that represent the sensory and post-ingestive properties of tastants.

Carbonic anhydrase VI in the mouse nasal gland

Western blotting analysis of mouse nasal tissue using a specific anti-mouse secreted carbonic anhydrase (CA VI) antibody has shown that CA VI is present in this tissue. A single immunoreactive band of 42 kD was observed, as has been found previously for salivary tissues. RT-PCR analysis has shown that nasal mucosa expressed CA VI mRNA. By immunohistochemistry (IHC), CA VI was observed in acinar cells, in duct contents of the anterior gland of the nasal septum, and in the lateral nasal gland. The Bowman's gland, the posterior gland of the nasal septum, and the maxillary sinus gland were negative. Immunoreactivity was also observed in the mucus covering the respiratory and olfactory mucosa and in the lumen of the nasolacrimal duct. In contrast, an anti-rat CA II antibody (that crossreacts with the mouse enzyme) stained only known CA II-positive cells and an occasional olfactory receptor neuron. These results indicate that CA VI is produced by the nasal gland and is secreted over the nasal mucosa. By reversible hydration of CO(2), CA VI is presumed to play a role in mucosal functions such as CO(2) sensation and acid-base balance. It may also play a role in olfactory function as a growth factor in maturation of the olfactory epithelial cells.

Activation of neurons in trigeminal caudalis by noxious oral acidic or salt stimuli is not reduced by amiloride

We investigated the possible role of amiloride-sensitive ion channels of the ENaC/DEGenerin superfamily in the activation of trigeminal nociceptive neurons elicited by noxious chemical stimulation of the oral mucosa using two methodologies, single-unit recording and c-fos immunohistochemistry. In pentobarbital-anesthetized rats, single-unit recordings were made from neurons in superficial laminae of dorsomedial trigeminal subnucleus caudalis (Vc) that responded to noxious thermal and chemical stimuli applied to the dorsal tongue. Successive application of each of three chemicals (250 mM pentanoic acid, n=6 units; 250 mM citric acid, n=8; 5 M NaCl, n=6) evoked responses that were not affected following topical application of amiloride (1 mM). In separate experiments, pentobarbital-anesthetized rats received one of the following stimuli delivered to the dorsal tongue: 250 mM pentanoic acid (n=6); 1 mM amiloride followed by 250 mM pentanoic (N=6); 5 M NaCl (n=5); or 1 mM amiloride followed by 5 M NaCl (n=5). Two hours later they were perfused with 4% paraformaldehyde and the brain stems processed for c-fos immunoreactivity. Both pentanoic acid and 5 M NaCl evoked similar numbers and patterns of fos-like immunoreactivity (FLI) in dorsomedial Vc and other brain stem regions, with no significant difference in counts of FLI in animals pretreated with amiloride. These results suggest that amiloride-sensitive Na(+) channels are not essential in mediating the activation of intraoral trigeminal nociceptors.

Suppression of central taste transmission by oral capsaicin

Because intraoral capsaicin is reported to reduce the perceived intensity of certain taste qualities, we investigated whether it affects the central processing of gustatory information. The responses of gustatory neurons in the nucleus tractus solitarius (NTS) to tastant stimuli were recorded before and after lingual application of capsaicin in anesthetized rats. Thirty-four NTS units were characterized as responding best to sucrose (0.3 m), NaCl (0.1 m), citric acid (0.03 m), monosodium glutamate (0.2 m), or quinine (0.001 m). During lingual application of 330 microm capsaicin for 7 min, the firing rate increased for five units and decreased for four units; the remainder were unaffected. Immediately after capsaicin, responses to each tastant were in nearly all cases depressed (mean, 61.5% of control), followed by recovery in most cases. NTS tastant-evoked unit responses were unaffected by lingual application of vehicle (5% ethanol). Capsaicin elicited an equivalent reduction (to 64.5%) in tastant-evoked responses of nine additional NTS units recorded in rats with bilateral trigeminal ganglionectomy, arguing against a trigeminally mediated central effect. Furthermore, capsaicin elicited a puncate pattern of plasma extravasation in the tongue that matched the distribution of fungiform papillae. These results support a peripheral site of capsaicin suppression of taste possibly via direct or indirect effects on taste transduction or taste receptor cell excitability. The depressant effect of capsaicin on gustatory transmission might underlie its ability to reduce the perceived intensity of some taste qualities.

Sensitization of trigeminal caudalis neuronal responses to intraoral acid and salt stimuli and desensitization by nicotine

In human studies, repeated intraoral application of strong acidic or salt stimuli induces irritation that progressively increases across trials (sensitization), whereas irritation elicited by nicotine progressively decreases (desensitization). We investigated whether nociceptive neurons in trigeminal subnucleus caudalis (Vc) exhibit increasing or decreasing patterns of firing to the intraoral application of these irritants. In rats anesthetized with halothane and thiopental, single-unit recordings were made from nociceptive neurons in superficial layers of dorsomedial Vc that responded to mechanical and noxious thermal and chemical stimulation of the tongue. NaCl (5M), citric acid (300 mM), pentanoic acid (300 mM) or nicotine (600 mM) were separately delivered to the tongue by constant flow (0.32 ml/min) for 15 or 25 min. NaCl, citric acid and pentanoic acid each elicited a progressive, significant increase in Vc neuronal firing over the initial 10 min to a plateau level that was maintained for the stimulus duration. Nicotine induced a significant increase in firing rate of Vc neurons within 6 min, followed by a decline back to the baseline level over the ensuing 10 min. Following a rest period, reapplication of nicotine no longer activated Vc neurons, indicative of self-desensitization. We additionally tested for nicotine cross-desensitization to acid. After recording the responses of Vc neurons to pentanoic acid and noxious heat, nicotine was then applied for 15 min. Post-nicotine responses to pentanoic acid were markedly reduced (to 13% of control), indicative of cross-desensitization; responses to noxious heat were also reduced to a lesser degree (to 71% of control). The progressive increase in Vc neuronal firing elicited by NaCl and acid, and the decline in firing after initial nicotinic excitation, resemble psychophysical patterns of sensitization and desensitization, respectively, and support the involvement of Vc neurons in the signaling of oral irritant sensations.

Acidic stimuli activates two distinct pathways in taste receptor cells from rat fungiform papillae

A sour taste sensation may be produced when acidic stimuli interact with taste receptor cells (TRCs) on the dorsal surface of the tongue. We have searched for pathways in TRCs that may be activated by acidic stimuli using RT-PCR and changes in intracellular calcium (Ca(2+)(I)) induced by acidic stimuli in rat fungiform papillae. RT-PCR revealed the presence of proton-gated subunits ASIC-beta and VR1. Ca(2+) imaging measurements of the TRCs revealed two distinct responses to acidic stimuli: Ca(2+)(i) was increased in 9% (28/308; Type I) and was decreased in 39% (121/308; Type II). Neither of these responses was affected by the removal of extracellular Ca(2+), indicating that the changes arise from the release and sequestration of Ca(2+) from intracellular stores. These responses were also not inhibited by the vanilloid receptor antagonist, capsazepine, suggesting they do not arise from the activation of vanilloid receptors. The Type I, but not the Type II response was inhibited by amiloride. Dose-response measurements for Types I and II responses yielded pH(50%) of 4.8 and 4.9, respectively. Type II responses were inhibited by pertussis toxin, suggesting G-protein involvement. TRCs that exhibit Type II responses could also be activated by quinine (which increased Ca(2+)(I)) thus suggesting a mechanism by which the addition of acid may be suppressive to other chemical stimuli.

Secretion of carbonic anhydrase isoenzyme VI (CA VI) from human and rat lingual serous von Ebner's glands

Salivary carbonic anhydrase VI (CA VI) appears to contribute to taste function by protecting taste receptor cells (TRCs) from apoptosis. The serous von Ebner's glands locating in the posterior tongue deliver their saliva into the bottom of the trenches surrounding the TRC-rich circumvallate and foliate papillae. Because these glands deliver their saliva directly into the immediate vicinity of TRCs, we investigated whether CA VI is secreted by the von Ebner's glands, using immunochemical techniques. The immunohistochemical results showed that CA VI is present in the serous acinar cells, ductal cells, and ductal content of von Ebner's glands and in the demilune and ductal cells plus ductal content of rat lingual mucous glands. More importantly, CA VI was also detected in taste buds and in the taste pores. Western blotting of saliva collected from the orifices of human von Ebner's glands and CAs purified from rat von Ebner's glands confirmed that CA VI is expressed in these glands and secreted to the bottom of the trenches surrounding the circumvallate and foliate papillae. These findings are consistent with the hypothesis that locally secreted CA VI is implicated in the paracrine modulation of taste function and TRC apoptosis. (J Histochem Cytochem 49:657-662, 2001)

Relation between PROP taster status and fat perception, touch, and olfaction

We tested the hypothesis that fat perception (sensitivity to and preferences for fat) may be linked to 6-n-propylthiouracil (PROP) taster status as a result of differences in trigeminal innervation of the oral cavity. In addition, we examined the relationship between taster status and sensitivity to other taste attributes, as well as tactile and olfactory sensitivities. Subjects (40 nontasters, 67 medium tasters, and 40 supertasters of PROP) rated samples (potato chips, chocolate drink, mashed potatoes, and vanilla pudding) varying in fat and flavor concentrations for the intensity of fattiness, saltiness, and sweetness, first without and then with nose clips, and for liking. Tactile sensitivity of the tongue was assessed according to responses to stimulation with Von Frey filaments (2.36, 2.44). Olfactory thresholds were determined for two odors (diacetyl and phenylethyl methyl ethyl carbinol). In general, taster status was not related to the perceptions of fat, saltiness, and sweetness. Subjects were able to accurately assess the fat content of the samples. Increasing the flavor levels in the potato chips and mashed potatoes enhanced the perception of fattiness for these systems. Supertasters were more sensitive to stimulation on the median of the tongue with the no. 2.36 Von Frey filament, and the olfactory thresholds for diacetyl were lower for PROP tasters and supertasters than for nontasters.

Sensitization, desensitization and stimulus-induced recovery of trigeminal neuronal responses to oral capsaicin and nicotine

Repeated application of capsaicin at a 1-min interstimulus interval (ISI) to the tongue induces a progressively increasing irritant sensation (sensitization), followed after a rest period by reduced sensitivity to further capsaicin (desensitization). Sequential reapplication of capsaicin induces irritation that eventually increases to initial levels: stimulus-induced recovery (SIR). In contrast, repeated application of nicotine elicits a declining irritant sensation across trials. To investigate possible neural correlates of these phenomena, we recorded from single units in superficial laminae of the dorsomedial trigeminal subnucleus caudalis (Vc) that responded to noxious thermal (54 degrees C) and chemical (1 M pentanoic acid) stimulation of the tongue of anesthetized rats. We then recorded responses to either capsaicin (330 microM) or nicotine (0.6 M), delivered either once, repeatedly at 1-min ISI, or continually by constant flow. After the initial capsaicin application and a rest period, the capsaicin was reapplied in the identical manner to test for SIR. The mean response of 14 Vc units to sequential application of pentanoic acid did not vary significantly across trials, indicating lack of tachyphylaxis or sensitization. The averaged response of 11 Vc units to repeated capsaicin increased significantly across the first eight trials and then plateaued. Following the rest period, spontaneous firing had returned to the precapsaicin level. With capsaicin reapplication, the averaged response increased again after a significant delay (due to desensitization), but did not reattain the peak firing rate achieved in the initial series (partial SIR). Constant-flow application of capsaicin induced an identical sensitization followed by nearly complete SIR. A single application of capsaicin induced a significant rise in firing in eight other units, but the rate of rise and maximal firing rate were both much lower compared with repetitive or constant-flow capsaicin. When capsaicin was reapplied once after the rest period, there was no change in firing rate indicating absence of SIR. These results indicate that maintenance of the capsaicin concentration induces a progressive increase in neuronal response that parallels sensitization. With recurrent capsaicin application, desensitization can be overcome to result in a delayed recovery of Vc responses similar to SIR. In contrast, the averaged response of 17 Vc units to repeated or constant-flow application of nicotine increased only over the first 3 min, and then decreased to spontaneous levels even as nicotine was still being applied. These results are consistent with the decrease in the perceived irritation elicited by sequential application of nicotine in humans.

Psychophysical and neurobiological evidence that the oral sensation elicited by carbonated water is of chemogenic origin

The sensation produced by carbonated beverages has been attributed to chemical excitation of nociceptors in the oral cavity via the conversion of CO(2) to carbonic acid in a reaction catalyzed by carbonic anhydrase. In separate studies, we tested if the carbonic anyhdrase blocker, acetazolamide, reduced either the intensity of sensation in humans or c-fos expression by trigeminal neurons in rats, evoked by application of carbonated water to the tongue. In the psychophysical experiment, one-half of the dorsal tongue was pretreated with acetazolamide (1 or 2%), after which the tongue was exposed bilaterally to carbonated water. In a two-alternative forced-choice paradigm, subjects chose which side of the tongue yielded a stronger sensation and additionally rated the magnitude of sensation on each side. Pretreatment with acetazolamide reduced the magnitude of sensation elicited by carbonated water in a concentration-dependent manner, since a significant majority of subjects chose the untreated side of the tongue as having a stronger sensation and assigned significantly higher intensity ratings to that side. Acetazolamide did not affect the irritant sensation from citric acid, while capsaicin pretreatment reduced both the sensation elicited by carbonated water and the irritation induced by citric acid application. In a separate experiment using rats, delivery of carbonated water to the tongue significantly increased the number of cells expressing c-fos-like immunoreactivity in the dorsomedial trigeminal nucleus caudalis (versus saline controls); this was significantly reduced by pretreatment with acetazolamide. Our results support the hypothesis that carbonated water activates lingual nociceptors via conversion of CO(2) to carbonic acid; the nociceptors in turn excite trigeminal neurons involved in signaling oral irritation.

Capsaicin, acid and heat-evoked currents in rat trigeminal ganglion neurons: relationship to functional VR1 receptors

Activation of primary trigeminal (TG) neurons by protons, capsaicin, or heat can evoke a variety of sensations, including tingling, stinging, warmth, and burning. Capsaicin and acid are trigeminal stimulants that are important in gustatory physiology. These stimuli can activate H(+)-gated ion channels and heterologously expressed VR1 receptors (vanilloid receptor 1). We have obtained evidence by using electrophysiological and pharmacological measurements on TG neurons that these three stimuli can activate many receptors, and we have determined the extent they behave similarly to VR1 receptors and H(+)-gated channels from the DEGenerin/ENaC superfamily. Whole-cell recordings from rat TG neurons revealed that protons evoked transient (Tp), sustained (Sp), and biphasic (TSp) currents. Tp currents had reversal potentials (Vr) of 24-45 mV, a pH(0.5) range from 5.5 to 6.5, and were inhibited by amiloride, suggesting the presence of functional H(+)-gated channels. Sp currents were inhibited by the VR1 antagonist capsazepine, had Vr's approximately 0 mV, and had pH(0.5) = 6.4. Capsaicin also activated transient (Tc), sustained (Sc), and biphasic (TSc) currents. At pH 5.9, the sensitivity of the Sc currents increased by about a factor of 10, which may partially account for the synergistic responses of acid in foods containing capsaicin. Heating TG neurons evoked a thermally active, capsazepine-inhibitable current with threshold temperature of 43 degrees C and Vr = 5 mV that is also present in neurons activated by and protons (Sp) and capsaicin (Sc). These data suggest that TG neurons have functional receptors that behave similarly to VR1. Activation of such receptors should result in a burning sensation, whereas activation of the transient and biphasic currents should result in other taste descriptors.