Nicotine suppression of gustatory responses of neurons in the nucleus of the solitary tract

Differential Effect of TRPV1 Modulators on Neural and Behavioral Responses to Taste Stimuli

In our diet, we ingest a variety of compounds that are TRPV1 modulators. It is important to understand if these compounds alter neural and behavioral responses to taste stimuli representing all taste qualities. Here, we will summarize the effects of capsaicin, resiniferatoxin, cetylpyridinium chloride, ethanol, nicotine, N-geranyl cyclopropylcarboxamide, Kokumi taste peptides, pH, and temperature on neural and behavioral responses to taste stimuli in rodent models and on human taste perception. The above TRPV1 agonists produced characteristic biphasic effects on chorda tympani taste nerve responses to NaCl in the presence of amiloride, an epithelial Na+ channel blocker, at low concentrations enhancing and at high concentrations inhibiting the response. Biphasic responses were also observed with KCl, NH4Cl, and CaCl2. In the presence of multiple stimuli, the effect is additive. These responses are blocked by TRPV1 antagonists and are not observed in TRPV1 knockout mice. Some TRPV1 modulators also increase neural responses to glutamate but at concentrations much above the concentrations that enhance salt responses. These modulators also alter human salt and glutamate taste perceptions at different concentration ranges. Glutamate responses are TRPV1-independent. Sweet and bitter responses are TRPV1-independent but the off-taste of sweeteners is TRPV1-dependent. Aversive responses to acids and ethanol are absent in animals in which both the taste system and the TRPV1-trigeminal system are eliminated. Thus, TRPV1 modulators differentially alter responses to taste stimuli.

Comparison of smoking conventional cigarettes and using heated tobacco products on the olfactory and gustatory function in healthy young adults: A cross-sectional study

INTRODUCTION

Smoking has a negative impact on the chemosensory function. The aim of the study was to evaluate the effects of smoking experience and type of tobacco products on gustatory and olfactory function.

METHODS

This study included 30 conventional cigarette smokers, 30 heated tobacco products (HTPs) users, and 30 non-smokers. Olfactory function was assessed with a 'Sniffin' Sticks Screening 12 Test' while the taste function was assessed with 'taste strips' for the basic tastes of sweet, sour, salty, and bitter. The lifetime exposure to smoking was calculated using the Brinkman index (BI).

RESULTS

Conventional cigarette smokers demonstrated decreased olfactory function in comparison to non-smokers (median: 10 vs 11; p=0.001) but HTPs users did not differ between those two groups. Overall gustatory function was lower in conventional cigarette smokers (median: 9.5) and HTPs users (median: 10) than in non-smokers (median: 14; p<0.001). A difference was detected in the sour, salty and bitter taste but without significant difference between HTPs users and conventional cigarette smokers. Negative linear correlations were found between the BI and olfactory function, overall gustatory function, sour, salty, and bitter taste (r= -0.317 - -0.585; p≤0.002). In multiple linear regression, BI was the only predictor of olfactory dysfunction when controlling for the effect of tobacco products, age, and gender accounting for 11% of variance (p=0.024, R2=0.123). For gustatory dysfunction, BI was the strongest predictor followed by gender and tobacco products accounting for 11%, 5%, and 4% (p<0.001, R2=0.259).

CONCLUSIONS

Olfactory and gustatory function are adversely associated with smoking, more depending on BI than tobacco product.

Examining the Association between Cigarette Smoking Quantity and Subjective Salt Taste Preference and Salt-Related Eating Behavior

BACKGROUND

Smoking can reduce taste sensitivity, the ability to sense various tastes, and diet quality and can increase the incidence of diseases such as hypertension. This study aimed to analyze the association between the smoking amount, subjective salt preference, and salt-related eating behaviors.

METHODS

Data of more than 16 million individuals from the Korean Community Health Survey were used. Forest plots were drawn to compare the cumulative odds ratios of salt taste preference and salt-related eating behaviors, adjusted for sex, age, body mass index, education level, household income, marital status, and drinking status at various smoking levels.

RESULTS

Subjective salt preference and salt-related eating behaviors increased with smoking amount; the adjusted odds ratios (AORs) for smoking >20 cigarettes were higher than those for smoking <20 cigarettes. For daily smokers, the AOR was 1.27 (95% confidence interval [CI], 1.22-1.31) for 1-5 cigarettes per day and 1.68 (95% CI, 1.65-1.71) for 16-20 cigarettes per day (P<0.001). Smokers were more likely to have more frequent salt-related eating behaviors than nonsmokers.

CONCLUSION

The subjective salt preference of smokers was higher than that of nonsmokers. Additionally, smokers used salt or soy sauce and dipped fried food in soy sauce more frequently than nonsmokers, which was also related to smoking amount.

The effects of cigarette smoking and alcohol drinking on salty taste preferences based on Korean Community Health Survey data

BACKGROUND/OBJECTIVES

Excessive sodium intake, cigarette smoking, and alcohol consumption are risk factors for a wide range of diseases. This study aimed to determine whether smokers and drinkers are more likely to enjoy their food with more salt, and whether the combination of smoking and drinking is associated with salty taste preferences.

SUBJECTS/METHODS

This study analyzed the data of over 16 million Koreans from two four-year Korean Community Health Survey cycles (i.e., 2010 to 2013 and 2014 to 2017). The respondents' preferences for salty foods (i.e., their salt intake levels, whether they added salt or soy sauce to foods served on the table, and whether they dipped fried foods in salt or soy sauce), and the odds ratio (OR) of their preference were examined among smokers and drinkers when adjusted for sex, age, body mass index, educational level, household income, marital status, and cigarette smoking or alcohol consumption status.

RESULTS

Cigarette smoking and alcohol consumption were correlated with the consumption of salty food. Based on the adjusted model, cigarette smokers and alcohol drinkers preferred adding salt or soy sauce or dipping fried foods in soybean more than non-smokers and non-drinkers. In addition, people who smoked and consumed alcohol reported a more significant stacking effect regarding the salty taste preference.

CONCLUSION

This large population-based study found that both cigarette smoking and alcohol consumption were correlated with salty taste preferences, which may cause excessive sodium intake.

Differences in Oral Lesions Associated with Tobacco Smoking, E-Cigarette Use and COVID-19 Infection among Adolescents and Young People in Nigeria

COVID-19 infection is associated with oral lesions which may be exacerbated by tobacco smoking or e-cigarette use. This study assessed the oral lesions associated with the use of e-cigarettes, tobacco smoking, and COVID-19 among adolescents and young people in Nigeria. A national survey recruited 11-23-year-old participants from the 36 States of Nigeria and the Federal Capital Territory, Abuja. Data were collected using Survey Monkey®. Binary logistic regression analysis was conducted. Statistical significance was set at p-value less than 0.05. There were 2870 participants, of which 386 (13.4%) were tobacco smokers, 167 (5.8%) e-cigarette users, and 401 (14.0%) were both e-cigarette and tobacco users; and 344 (12.0%) had ever tested positive to COVID-19. Adolescents and young people who smoked tobacco had more than twice the odds of reporting gingival inflammation, oral ulcers, dry mouth, and changes in taste than those who did not smoke. Those who used e-cigarettes had 1.5 times higher odds of reporting oral lesions. Respondents who had COVID-19 infection had higher odds of reporting gingival inflammation and lower odds of reporting dry mouth than those who did not have COVID-19 infection. These findings were significant, and may help clinicians to screen for tobacco use and COVID-19 among adolescents and young people in Nigeria.

Chemosensory Contributions of E-Cigarette Additives on Nicotine Use

While rates of smoking combustible cigarettes in the United States have trended down in recent years, use of electronic cigarettes (e-cigarettes) has dramatically increased, especially among adolescents. The vast majority of e-cigarette users consume "flavored" products that contain a variety of chemosensory-rich additives, and recent literature suggests that these additives have led to the current "teen vaping epidemic." This review, covering research from both human and rodent models, provides a comprehensive overview of the sensory implications of e-cigarette additives and what is currently known about their impact on nicotine use. In doing so, we specifically address the oronasal sensory contributions of e-cigarette additives. Finally, we summarize the existing gaps in the field and highlight future directions needed to better understand the powerful influence of these additives on nicotine use.

Sensory Effects of Nicotine and Tobacco

INTRODUCTION

Ingestion of nicotine by smoking, vaping, or other means elicits various effects including reward, antinociception, and aversion due to irritation, bitter taste, and unpleasant side effects such as nausea and dizziness.

AIMS AND METHODS

Here we review the sensory effects of nicotine and the underlying neurobiological processes.

RESULTS AND CONCLUSIONS

Nicotine elicits oral irritation and pain via the activation of neuronal nicotinic acetylcholine receptors (nAChRs) expressed by trigeminal nociceptors. These nociceptors excite neurons in the trigeminal subnucleus caudalis (Vc) and other brainstem regions in a manner that is significantly reduced by the nAChR antagonist mecamylamine. Vc neurons are excited by lingual application of nicotine and exhibit a progressive decline in firing to subsequent applications, consistent with desensitization of peripheral sensory neurons and progressively declining ratings of oral irritation in human psychophysical experiments. Nicotine also elicits a nAChR-mediated bitter taste via excitation of gustatory afferents. Nicotine solutions are avoided even when sweeteners are added. Studies employing oral self-administration have yielded mixed results: Some studies show avoidance of nicotine while others report increased nicotine intake over time, particularly in adolescents and females. Nicotine is consistently reported to increase human pain threshold and tolerance levels. In animal studies, nicotine is antinociceptive when delivered by inhalation of tobacco smoke or systemic infusion, intrathecally, and by intracranial microinjection in the pedunculopontine tegmentum, ventrolateral periaqueductal gray, and rostral ventromedial medulla. The antinociception is thought to be mediated by descending inhibition of spinal nociceptive transmission. Menthol cross-desensitizes nicotine-evoked oral irritation, reducing harshness that may account for its popularity as a flavor additive to tobacco products.

IMPLICATIONS

Nicotine activates brain systems underlying reward and antinociception, but at the same time elicits aversive sensory effects including oral irritation and pain, bitter taste, and other unpleasant side effects mediated largely by nicotinic acetylcholine receptors (nAChRs). This review discusses the competing aversive and antinociceptive effects of nicotine and exposure to tobacco smoke, and the underlying neurobiology. An improved understanding of the interacting effects of nicotine will hopefully inform novel approaches to mitigate nicotine and tobacco use.

Neurobiological Mechanisms of Nicotine Reward and Aversion

Neuronal nicotinic acetylcholine receptors (nAChRs) regulate the rewarding actions of nicotine contained in tobacco that establish and maintain the smoking habit. nAChRs also regulate the aversive properties of nicotine, sensitivity to which decreases tobacco use and protects against tobacco use disorder. These opposing behavioral actions of nicotine reflect nAChR expression in brain reward and aversion circuits. nAChRs containing α4 and β2 subunits are responsible for the high-affinity nicotine binding sites in the brain and are densely expressed by reward-relevant neurons, most notably dopaminergic, GABAergic, and glutamatergic neurons in the ventral tegmental area. High-affinity nAChRs can incorporate additional subunits, including β3, α6, or α5 subunits, with the resulting nAChR subtypes playing discrete and dissociable roles in the stimulatory actions of nicotine on brain dopamine transmission. nAChRs in brain dopamine circuits also participate in aversive reactions to nicotine and the negative affective state experienced during nicotine withdrawal. nAChRs containing α3 and β4 subunits are responsible for the low-affinity nicotine binding sites in the brain and are enriched in brain sites involved in aversion, including the medial habenula, interpeduncular nucleus, and nucleus of the solitary tract, brain sites in which α5 nAChR subunits are also expressed. These aversion-related brain sites regulate nicotine avoidance behaviors, and genetic variation that modifies the function of nAChRs in these sites increases vulnerability to tobacco dependence and smoking-related diseases. Here, we review the molecular, cellular, and circuit-level mechanisms through which nicotine elicits reward and aversion and the adaptations in these processes that drive the development of nicotine dependence. SIGNIFICANCE STATEMENT: Tobacco use disorder in the form of habitual cigarette smoking or regular use of other tobacco-related products is a major cause of death and disease worldwide. This article reviews the actions of nicotine in the brain that contribute to tobacco use disorder.

Impact of bitter tastant sub-qualities on retronasal coffee aroma perception

The impact of different bitter taste compounds on the retronasal perception of coffee aroma was investigated. A sorted napping experiment was carried out on thirteen compounds at iso-intense bitter concentrations. Differences in perceptual bitter sub-qualities among the compounds were reported by Multidimensional Scaling (MDS) and Multiple Factor Analysis (MFA) analyses. Seven exemplar compounds were further selected to investigate the impact of taste sub-qualities on cross-modal flavor interactions. In general, the different bitter compounds, when paired with a coffee aroma isolate, significantly modified the perception of the retronasal coffee aroma profile. Interestingly, the three bitter compounds endogenous to coffee had the most similar impact on the coffee aroma profile. Further sensory analysis of these sample sets indicated no significant effect of the bitter compounds on the orthonasal perception. Gas Chromatography/Mass Spectrometry (GC/MS) analysis of the volatile composition of the samples headspace also indicated negligible impact of the bitter compounds on aroma release. Altogether evidence of cross-modal interactions occurring at a higher cognitive level were demonstrated in a complex food sample, supporting the importance of multi-modal sensory integration on flavor perception.

Effect of using tobacco on taste perception

Background:

Impaired taste perception has impact on quality of life. Tobacco is a perilous factor that contributes to an impaired taste.

Objective:

To evaluate and compare taste perception among tobacco chewers and nonchewers.

Materials and Methods:

Sixty subjects (30 tobacco chewers + 30 nonchewers as controls) were enrolled in the study for evaluating taste perception. Taste identification time using four aqueous solutions of basic tastes – sweet, salty, sour, and bitter – was recorded (in seconds) and compared between tobacco chewers and controls. The data were analyzed using Student's t-test and analysis of variance using SPSS 20 version software.

Results:

A statistically significant increase in taste identification time for salty taste in tobacco chewers (12.32 s) was noted compared with nonchewers (10.21 s) (P = 0.03). The average taste identification time was higher for tobacco chewers than nonchewers for sweet and salty taste. However, the average taste identification time was lower for tobacco chewers than nonchewers for sour and bitter taste. In nonchewers, the average taste identification time was 13.01, 10.21, 8.43, and 7.56 s for sweet, salty, bitter, and sour taste, respectively. In tobacco chewers, the average taste identification time was 15.16, 12.32, 7.75, and 7.04 s for sweet, salty, bitter, and sour taste, respectively.

Conclusion:

The results of the study demonstrated noticeable decrease in taste perception to salty taste among tobacco chewers when compared with tobacco nonchewers. There is a remarkable difference in taste perception to basic tastes among tobacco chewers and controls.

To flourish or perish: evolutionary TRiPs into the sensory biology of plant-herbivore interactions

The interactions between plants and their herbivores are highly complex systems generating on one side an extraordinary diversity of plant protection mechanisms and on the other side sophisticated consumer feeding strategies. Herbivores have evolved complex, integrative sensory systems that allow them to distinguish between food sources having mere bad flavors from the actually toxic ones. These systems are based on the senses of taste, olfaction and somatosensation in the oral and nasal cavities, and on post-ingestive chemosensory mechanisms. The potential ability of plant defensive chemical traits to induce tissue damage in foragers is mainly encoded in the latter through chemesthetic sensations such as burning, pain, itch, irritation, tingling, and numbness, all of which induce innate aversive behavioral responses. Here, we discuss the involvement of transient receptor potential (TRP) channels in the chemosensory mechanisms that are at the core of complex and fascinating plant-herbivore ecological networks. We review how "sensory" TRPs are activated by a myriad of plant-derived compounds, leading to cation influx, membrane depolarization, and excitation of sensory nerve fibers of the oronasal cavities in mammals and bitter-sensing cells in insects. We also illustrate how TRP channel expression patterns and functionalities vary between species, leading to intriguing evolutionary adaptations to the specific habitats and life cycles of individual organisms.

Brief Exposures to the Taste of Ethanol (EtOH) and Quinine Promote Subsequent Acceptance of EtOH in a Paradigm that Minimizes Postingestive Consequences

BACKGROUND

Aversion to the orosensory properties of concentrated ethanol (EtOH) solutions is often cited as a primary barrier to initiation of drinking and may contribute to abstention. These aversive properties include gustatory processes which encompass both bitter-like taste qualities and trigeminal-mediated irritation. Chronic intermittent EtOH access (CIA) results in substantial and persistent increases in EtOH consumption, but the degree to which this facilitation involves sensory responding to EtOH and other bitter stimuli is currently undetermined.

METHODS

Long-Evans rats were given brief-access licking tests designed to examine the immediate, taste-guided assessment of the palatability of EtOH and quinine solutions. Rats were assessed once in a naïve state and again following previous brief-access exposure, or following 4 weeks of CIA. The relationship between the sensitivity to the aversive orosensory properties of EtOH and quinine following EtOH access and the impact of antecedent quinine exposure on the acceptance of EtOH were determined in 2 parallel studies.

RESULTS

Both brief access to EtOH and 4-week CIA resulted in substantial rightward shifts in the concentration-response function of brief-access EtOH licking, indicating that EtOH exposure increased acceptance of the taste of EtOH. The initial sensitivity to the aversive orosensory properties of EtOH and quinine was positively correlated in naïve rats, such that rats that were initially more accepting of quinine were also more accepting of EtOH. Rats that sampled quinine immediately prior to tasting EtOH exhibited successive positive contrast in that they were more accepting of highly concentrated EtOH, relative to a water-control group.

CONCLUSIONS

Increased EtOH acceptance following exposure is, at least in part, facilitated by a decrease in its aversive sensory properties. Both long- and short-term access increase the palatability of the taste of EtOH in brief-access licking tests. Moreover, the sensitivity to the bitterness of quinine was predictive of acceptance of EtOH indicating some commonality in the sensory mechanisms that mediate the initial acceptance of the 2 stimuli. Accordingly, immediate prior exposure to quinine results in increased acceptance of EtOH, suggesting that successive positive contrast between oral stimuli may contribute to increased alcohol consumption.

Nicotinic acetylcholine receptors (nAChRs) are expressed in Trpm5 positive taste receptor cells (TRCs)

Nicotine evokes chorda tympani (CT) taste nerve responses and an aversive behavior in Trpm5 knockout (KO) mice. The agonists and antagonists of nicotinic acetylcholine receptors (nAChRs) modulate neural and behavioral responses to nicotine in wildtype (WT) mice, Trpm5 KO mice and rats. This indicates that nicotine evokes bitter taste by activating a Trpm5-dependent pathway and a Trpm5-independent but nAChR-dependent pathway. Rat CT responses to ethanol are also partially inhibited by nAChR blockers, mecamylamine and dihydro-β-erythroidine. This indicates that a component of the bitter taste of ethanol is also nAChR-dependent. However, at present the expression and localization of nAChR subunits has not been investigated in detail in taste receptor cells (TRCs). To this end, in situ hybridization, immunohistochemistry and q-RT-PCR techniques were utilized to localize nAChR subunits in fungiform and circumvallate TRCs in WT mice, Trpm5-GFP transgenic mice, nAChR KO mice, and rats. The expression of mRNAs for α7, β2 and β4 nAChR subunits was observed in a subset of rat and WT mouse circumvallate and fungiform TRCs. Specific α3, α4, α7, β2, and β4 antibodies localized to a subset of WT mouse circumvallate and fungiform TRCs. In Trpm5-GFP mice α3, α4, α7, and β4 antibody binding was observed in a subset of Trpm5-positive circumvallate TRCs. Giving nicotine (100 μg/ml) in drinking water to WT mice for 3 weeks differentially increased the expression of α3, α4, α5, α6, α7, β2 and β4 mRNAs in circumvallate TRCs to varying degrees. Giving ethanol (5%) in drinking water to WT mice induced an increase in the expression of α5 and β4 mRNAs in circumvallate TRCs with a significant decrease in the expression of α3, α6 and β2 mRNAs. We conclude that nAChR subunits are expressed in Trpm5-positive TRCs and their expression levels are differentially altered by chronic oral exposure to nicotine and ethanol.

Electrogustometry Thresholds, Tongue Tip Vascularization, Density, and Form of the Fungiform Papillae Following Smoking Cessation

The objective of this study was to evaluate differences in gustatory function and in shape, density, and vascularization of the fungiform papillae (fPap) of smokers' tongue before and after smoking cessation. In 24 smokers (19 males, 5 females; median age: 54.6 ± 2.9 years) electrogustometry (EGM) thresholds at the chorda tympani area, at the soft palate area and at the area of the vallate papillae were recorded bilaterally. Morphology and density of the fungiform papillae (fPap) and blood vessels' density and morphology at the tip of the tongue were examined using contact endoscopy (CE). Follow-up exams (EGM and CE) were performed on average 3.2 months after smoking cessation. Findings were compared between the 2 conditions as well as to those of a group of 24 non-smokers (median age: 55.2 ± 3.4 years; 19 males, 5 females). After smoking cessation, EGM thresholds decreased significantly (P = 0.02 or P = 0.03 depending on the tested area) but nonetheless still were quite different from those of non-smokers (P = 0.05 or 0.04 depending on the site of EGM measurement). Under CE the fPap density was higher after quitting smoking (P = 0.05) and the shape and vascularization of fPap also exhibited a trend to improvement (P = 0.05) after smoking cessation. Chronic exposure to cigarette smoke infers long lasting, although to a large extent reversible, alterations in morphology of taste buds in fungiform papillae, but rather irreversible EGM-related functional gustatory compromise, suggesting a profound physiologic effect on human peripheral taste organs.

Effect of cigarette smoke on gustatory sensitivity, evaluation of the deficit and of the recovery time-course after smoking cessation

BACKGROUND

Study results have shown that chronic exposure to cigarette smoke affects the taste function in humans. However, neither the quantitative impact on taste sensitivity nor the time-course of taste recovery on stopping smoking have been precisely examined.

METHODS

The experimental design included 2 phases, (i) a case-control phase comparing the taste sensitivity level measured by Electrogustometric (EGM) thresholds from various parts of the tongue (locus) between smokers (n = 83) and non-smokers (n = 48), (ii) a follow-up study looking at the taste sensitivity recovery in smokers after smoking cessation (n = 24) and compared with non-smokers.

RESULTS

Smokers exhibited significantly lower taste sensitivity than non-smokers - the higher the nicotine dependence (Fagerström scores), the lower the taste sensitivity. After smoking cessation, EGM thresholds decreased progressively, and reached the taste sensitivity range of non-smokers depending on locus and time. After 2 weeks a recovery could be observed on the 3 Tip and the 2 edge loci; the recovery in the posterior loci was complete after 9 weeks, and in the dorsal loci recovery was observed only after 2 months or more.

CONCLUSIONS

Smoking cessation does lead to a rapid recovery of taste sensitivity among smokers, with recovery time found to differ based on the sensitivity of loci of the tongue. The use of EGM could potentially be explored as a motivational tool for smoking cessation.

Psychophysical and Vasomotor Responses of the Oral Tissues: A Nicotine Dose-Response and Menthol Interaction Study

INTRODUCTION

This study implemented an intra-oral test-platform to assess the sensory, psychophysical, and vasomotor responses to nicotine and menthol, alone or in combination.

METHODS

Two double-blinded, placebo-controlled, randomized, cross-over studies, including healthy nonsmoking participants were performed. Study I: A dose-response relationship (N = 20) between 0, 2, and 4 mg nicotine gum. Study II: An interaction response (N = 22) to 30 mg menthol and 4 mg nicotine alone or in combination. Heart rate, blood pressure, tactile and thermosensory thresholds, intra-oral blood flow and temperature, pain/irritation intensities/locations, McGill Pain Questionnaire, and taste experience were assessed before, during or after the completion of a standardized chewing regime.

RESULTS

A dose-response elevation in heart rate was attenuated when nicotine was combined with menthol. Blood flow, temperature, and warm-detection thresholds, as assessed on the tongue, similarly increased for all gums. Pain intensity and taste experiences were similar between nicotine doses. Nicotine attenuated the sweet, cooling, and freshening sensation of menthol. Within the first 4 minutes, menthol reduced the intensity but not the area of nicotine-induced pain and irritation. The 4-mg nicotine dose led to a continued increase in the intensity and area of irritation in the throat post-chewing. Moreover, one-half of participants responded to menthol as an irritant, and these individuals demonstrated larger areas of nicotine-induced irritation in the throat post-chewing.

CONCLUSIONS

The intra-oral test platform provides a basis to optimize the assessment of nicotine-related taste and sensory experiences and can be used in future studies for profiling nicotine gum.

Nicotinic Acetylcholine Receptor (nAChR) Dependent Chorda Tympani Taste Nerve Responses to Nicotine, Ethanol and Acetylcholine

Nicotine elicits bitter taste by activating TRPM5-dependent and TRPM5-independent but neuronal nAChR-dependent pathways. The nAChRs represent common targets at which acetylcholine, nicotine and ethanol functionally interact in the central nervous system. Here, we investigated if the nAChRs also represent a common pathway through which the bitter taste of nicotine, ethanol and acetylcholine is transduced. To this end, chorda tympani (CT) taste nerve responses were monitored in rats, wild-type mice and TRPM5 knockout (KO) mice following lingual stimulation with nicotine free base, ethanol, and acetylcholine, in the absence and presence of nAChR agonists and antagonists. The nAChR modulators: mecamylamine, dihydro-β-erythroidine, and CP-601932 (a partial agonist of the α3β4* nAChR), inhibited CT responses to nicotine, ethanol, and acetylcholine. CT responses to nicotine and ethanol were also inhibited by topical lingual application of 8-chlorophenylthio (CPT)-cAMP and loading taste cells with [Ca²⁺]_i by topical lingual application of ionomycin + CaCl₂. In contrast, CT responses to nicotine were enhanced when TRC [Ca²⁺]_i was reduced by topical lingual application of BAPTA-AM. In patch-clamp experiments, only a subset of isolated rat fungiform taste cells exposed to nicotine responded with an increase in mecamylamine-sensitive inward currents. We conclude that nAChRs expressed in a subset of taste cells serve as common receptors for the detection of the TRPM5-independent bitter taste of nicotine, acetylcholine and ethanol.

Sociodemographic, psychological, and lifestyle characteristics are associated with a liking for salty and sweet tastes in French adults

BACKGROUND

Various studies have shown that sensory liking influences dietary behavior and that individual characteristics are related to food intake and weight status, but little is known about individual profiles associated with salt and sweet liking.

OBJECTIVE

The aim of the present study was to investigate the association between a liking for salty and sweet tastes (i.e., a liking for foods rich in salt or sugar and preferred amounts of salt or sugar seasoning in foods) and sociodemographic, psychological, and lifestyle characteristics in a large sample.

METHODS

Individual factors and liking scores were collected by validated questionnaires from 37,181 French adults participating in the NutriNet-Santé study, a large web-based observational cohort launched in 2009 that studies relations between nutrition and health. The associations were assessed by multivariable multinomial logistic regression models adjusted for socioeconomic, anthropometric, and health variables.

RESULTS

In both genders, with increasing age, individuals were more likely to have a high salt liking (men, OR: 1.24; 95% CI: 1.18, 1.30; women, OR: 1.14, 95% CI: 1.09, 1.19), whereas they were less likely to have a strong sweet liking (men, OR: 0.87; 95% CI: 0.83, 0.91; women, OR: 0.66; 95% CI: 0.64, 0.68). Current smokers (men, OR: 2.30; 95% CI: 1.90, 2.78; women, OR: 1.50; 95% CI: 1.36, 1.66) and heavy drinkers (men, OR: 2.92; 95% CI: 2.37, 3.58; women, OR: 2.57, 95% CI: 2.22, 2.98) were more likely to like salt than nonsmokers and alcohol abstainers. Regarding the sweet taste, women smokers were less likely to like sweets (OR: 0.80; 95% CI: 0.72, 0.89). Highly uncontrolled eaters [men, OR: 2.39; 95% CI: 2.04, 2.80; women, OR: 2.22; 95% CI: 1.99, 2.47) and highly emotional women (OR: 1.35; 95% CI: 1.18, 1.55) were more likely to have a high liking for sweets than slightly uncontrolled eaters and nonemotional eaters, whereas those with high cognitive restraint (men, OR: 0.39; 95% CI: 0.33, 0.46; women, OR: 0.55; 95% CI: 0.50, 0.60) and former weight-loss dieters (men, OR: 0.60; 95% CI: 0.52, 0.70; women, OR: 0.68; 95% CI: 0.62, 0.73) were less likely to have a strong sweet liking compared with those with low cognitive restraint and never-dieters.

CONCLUSION

An unhealthy lifestyle that includes smoking and alcohol consumption may influence salt liking, and eating behavior may have an impact on sweet liking. Further research is needed to study the influence of individual factors and sensory liking on dietary intake and weight status. This study was registered at the European Clinical Trials Database as 2013-000929-31.

Relationships of PROP Taste Phenotype, Taste Receptor Genotype, and Oral Nicotine Replacement Use

INTRODUCTION

Recommended dosage of oral nicotine replacement therapy (NRT) product is often not achieved in smoking cessation attempts. n-6-propylthiouracil (PROP) bitter taste phenotype may be a potential risk factor for non-adherence to oral NRT products due to their bitter taste. There is limited literature on this phenotype in the context of smoking and none in relation to oral NRT pharmacotherapy.

METHODS

The association of PROP taste phenotype with NRT usage and sensory response to products was examined. In a cross-over experimental design, 120 participants received a 1 week supply of nicotine inhalers and 1 week of nicotine lozenges with random assignment to order. Mixed effects linear model analyses were conducted.

RESULTS

PROP taste phenotype and taste receptor genotype were not associated with NRT usage or sensory response to NRT, after adjusting for other factors. However, PROP non-tasters used a higher number of lozenges per day (continuous exposure) than nicotine cartridges (intermittent exposure). Unexpectedly, half of baseline PROP non-tasters shifted to taster phenotype 2 weeks after smoking cessation or reduction. Menthol cigarette smokers identified higher NRT strength of sensation scores than nonmenthol smokers. Taste receptor genotype was related to PROP taste phenotype (Kendall τ = .591, p = .0001).

CONCLUSIONS

A nonsignificant relationship of PROP phenotype and NRT usage may be associated with NRT under-dosing and limited variance in the outcome variable. PROP non-tasters' greater use of lozenges is consistent with nicotine exposure being less aversive to non-tasters. Further research of this and other factors impacting NRT usage are warranted to effectively inform smoking cessation pharmacotherapy.

Amiloride-sensitive sodium currents in fungiform taste cells of rats chronically exposed to nicotine

Many studies have demonstrated that chronic exposure to nicotine, one of the main components of tobacco smoke, has profound effects on the functionality of the mammalian taste system. However, the mechanisms underlying nicotine action are poorly understood. In particular no information is available on the chronic effect of nicotine on the functioning of taste cells, the peripheral detectors which transduce food chemicals into electrical signals to the brain. To address this issue, I studied the membrane properties of rat fungiform taste cells and evaluated the effect of long-term exposure to nicotine on the amiloride-sensitive sodium currents (ASSCs). These currents are mediated by the epithelial sodium channels (ENaC) thought to be important, at least in part, in the transduction of salty stimuli. Patch-clamp recording data indicated that ASSCs in taste cells from rats chronically treated with nicotine had a reduced amplitude compared to controls. The pharmacological and biophysical analysis of ASSCs revealed that amplitude reduction was not dependent on changes in amiloride sensitivity or channel ionic permeability, but likely derived from a decrease in the activity of ENaCs. Since these channels are considered to be sodium receptors in taste cells, my results suggest that chronic exposure to nicotine hampers the capability of these cells to respond to sodium ions. This might represent a possible cellular mechanism underlying the reduced taste sensitivity to salt typically found in smokers.

Enhanced corticobulbar excitability in chronic smokers during visual exposure to cigarette smoking cues

BACKGROUND

Neuroimaging studies of chronic smokers report altered activity of several neural regions involved in the processing of rewarding outcomes. Neuroanatomical evidence suggests that these regions are directly connected to the tongue muscle through the corticobulbar pathways. Accordingly, we examined whether corticobulbar excitability might be considered a somatic marker for nicotine craving.

METHODS

We compared motor-evoked potential (MEP) amplitudes recorded from the tongue and the extensor carpi radialis (control muscle) of chronic smokers under drug withdrawal and intake conditions as well as a nonsmoker group. All participants were tested during passive exposure to pictures showing a smoking cue or a meaningless stimulus. In the intake condition, chronic smokers were asked to smoke a real cigarette (CSn: group 1) or a placebo (CSp: group 2).

RESULTS

Results show that MEP amplitudes recorded from the tongues of participants in the CSn and CSp groups under the withdrawal condition were selectively enhanced during exposure to a smoking cue. However, this effect on tongue MEP amplitudes disappeared in the intake condition for both the CSn and CSp groups.

LIMITATIONS

Limitations include the fact that the study was conducted in 2 different laboratories, the small sample size, the absence of data on chronic smoker craving strength and the different tastes of the real and placebo cigarettes.

CONCLUSION

These results suggest that, in chronic smokers, tongue muscle MEP amplitudes are sensitive to neural processes active under the physiological status of nicotine craving. This finding implicates a possible functional link between neural excitability of the corticobulbar pathway and the reward system in chronic smokers.

Nicotinic regulation of energy homeostasis

INTRODUCTION

The ability of nicotine, the primary psychoactive substance in tobacco smoke, to regulate appetite and body weight is one of the factors cited by smokers that prevents them from quitting and is the primary reason for smoking initiation in teenage girls. The regulation of feeding and metabolism by nicotine is complex, and recent studies have begun to identify nicotinic acetylcholine receptor (nAChR) subtypes and circuits or cell types involved in this regulation.

DISCUSSION

We will briefly describe the primary anatomical and functional features of the input, output, and central integration structures of the neuroendocrine systems that regulate energy homeostasis. Then, we will describe the nAChR subtypes expressed in these structures in mammals to identify the possible molecular targets for nicotine. Finally, we will review the effects of nicotine and its withdrawal on feeding and energy metabolism and attribute them to potential central and peripheral cellular targets.

Gustatory, trigeminal, and olfactory aspects of nicotine intake in three mouse strains

Studies of nicotine consumption in rodents often intend to investigate nicotine's post-absorptive effects, yet little is known about the pre-absorptive sensory experience of nicotine drinking, including gustatory, trigeminal, and olfactory influences. We conditioned taste aversion (CTA) to nicotine in males of 3 inbred mouse strains: C57BL/6J, DBA/2J, and 129X1/SvJ by repeatedly pairing 150 μg/ml nicotine drinking with lithium chloride injections. Generalization to a variety of bitter, sour, sweet, salty, and irritant solutions and to nicotine odor was then examined. Nicotine CTA generalized to the bitter stimulus quinine hydrochloride and the chemosensory irritant spilanthol in all strains. It also showed strain specificity, generalizing to hydrogen peroxide (an activator of TRPA1) in C57BL/6J mice and to the olfactory cue of nicotine in DBA/2J mice. These behavioral assays demonstrate that the sensory properties of nicotine are complex and include multiple gustatory, irritant, and olfactory components. How these qualities combine at the level of perception remains to be assessed, but sensory factors clearly exert an important influence on nicotine ingestion and their contribution to net intake of nicotine should not be neglected in animal or human studies.

Taste function after stapes surgery

OBJECTIVE

A manipulation of chorda tympani nerve (CTN) is frequently necessary during the surgical therapy of stapedial ankylosis. The aim of this study was to re-assess the taste function before and after stapes surgery in patients with unilateral stapes ankylosis.

METHODS

Eighteen patients (14 female and 4 male) with unilateral stapedial ankylosis were included. Taste and olfactory function were measured preoperatively, 3 days and 3 months after surgery by questionnaire, chemical taste test, electrogustometry, and Sniffin'Sticks. The patients who reported deterioration in taste and revealed pathological taste test results were re-investigated 8-12 months after surgery. Postoperatively 11 patients were treated intravenously for 3 days for inner ear protection with 1g of cortisone. The gustatory and olfactory results were compared with age and sex specific normative data.

RESULTS

A significant transient decrease of gustatory measurement values was found on the ipsilateral two thirds of the anterior tongue. 12 out of 17 patients whose CTN was slightly manipulated during stapes surgery reported tongue sensations, such as numbness or gustatory blindness of the ipsilateral tongue side. The measures of chemical taste test significantly decrease in lateralized taste test on the ipsilateral side and in whole mouth testing. There was no significant increase of the EGM measures at the ipslateral tongue. In 11 of 12 patients with symptoms, complaints recovered within one year after surgery completely. The factor "cortisone" did not have a significant effect on the taste test results after surgery.

CONCLUSION

In conclusion, even after minor CTN manipulation the rate of postoperative taste disorders or tongue symptoms after stapes surgery is high. The symptoms appear to be transient. Therefore CTN should be preserved in stapes surgery after mild trauma of the CTN. Transient taste alteration should be mentioned prior to stapes surgery.

Dental Afferents Project onto Gustatory Neurons in the Nucleus of the Solitary Tract

The aim of this study was to investigate the inferior alveolar nerve (IAN) and chorda tympani (CT) projections onto gustatory neurons of the nucleus of the solitary tract (NST) in the rat by immunochemical and electrophysiological techniques. IAN afferents were retrogradely labeled. NST neurons were labeled either by retrograde tracer injection into the parabrachial nucleus (PBN) or by c-Fos mapping after CT activation. NST neurons responding to tastant stimulation were recorded in vivo before and after electrical stimulation of the IAN. Results from the immunolabeling approach showed IAN boutons “en passant” apposed to retrogradely labeled neurons from PBN and to CT-activated neurons in the NST. Recordings of single NST neurons showed that the electrical stimulation of the IAN significantly decreased CT gustatory responses. Analysis of these data provides an anatomical and physiological basis to support trigeminal dental and gustatory interactions within the brainstem.

Common cellular and molecular mechanisms in obesity and drug addiction

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

Effects of acute abstinence and nicotine administration on taste perception in cigarette smokers

We investigated the effects of short-term abstinence from smoking and acute nicotine administration on taste perception in smokers. We assessed sensitivity for salt and sucrose solutions and the self-reported intensity and pleasantness of these tastes, using a previously validated model of taste perception. This was in order to investigate mechanisms by which cigarette smoking and smoking cessation may modulate dietary behaviour. Male and female daily smokers attended a single testing session. Participants were randomised to either abstain for smoking for 12 h or smoke as usual on the morning of testing. At the testing session, participants completed subjective ratings of mood and ratings of intensity and pleasantness of salt and sucrose solutions, followed by measurement of the threshold at which these solutions could be detected on the tongue. Participants were then randomised to smoking either a nicotine-containing or denicotinised cigarette, after which they completed the same measures as previously. Our data suggest that following cigarette smoking, lower taste thresholds are obtained after smoking a denicotinised cigarette compared with a nicotinised cigarette, but among females only. This effect was not observed among males and did not differ as a function of abstinence condition. In addition, among non-abstinent smokers, females demonstrated higher taste thresholds (i.e. reduced sensitivity) for salt than males, but this sex difference was not observed among abstinent smokers.

Gestational exposure to nicotine in drinking water: teratogenic effects and methodological issues

Individual differences in nicotine effects lead to questions about appropriate experimental procedures for prenatal nicotine exposure in rodent models. The objective of this study was to develop a method for gestational studies in rats based on oral nicotine exposure, and to evaluate the neurodevelopmental effects. Female Lister hooded rats were exposed to nicotine solutions both before and during pregnancy. These female rats were divided into groups consuming solutions of different concentrations such that animals that initially consumed the solutions most readily were exposed to progressively higher concentrations. Offspring of these female rats were evaluated in a test battery measuring maturational and developmental milestones. Female rats ingested nicotine solutions at levels that provided blood nicotine concentrations of 10-60 ng/ml, at daily dose levels of 2.9-6.2 mg/kg. Solutions with concentrations below 0.06 mg/ml were well tolerated with some moderate adverse effects at the highest dose. Concentrations above 0.08 mg/ml led to a large drop in fluid consumption and in body weight. Strong teratogenic effects of prenatal nicotine exposure were observed at concentrations above 0.04 mg/ml, including developmental and maturational delays shown by measures of pinnae detachment, fur appearance, incisor eruption, eye opening and righting reflex. Negative geotaxis, grip strength and weight gain were impaired and postnatal mortality was increased. This study design provides a model for the impact of prenatal exposure to nicotine at blood levels comparable with those in medium and heavy smokers. There were marked developmental and behavioural deficits induced in the offspring of nicotine-exposed female rats.

Nicotine activates the chemosensory cation channel TRPA1

Topical application of nicotine, as used in nicotine replacement therapies, causes irritation of the mucosa and skin. This reaction has been attributed to activation of nicotinic acetylcholine receptors (nAChRs) in chemosensory neurons. In contrast with this view, we found that the chemosensory cation channel transient receptor potential A1 (TRPA1) is crucially involved in nicotine-induced irritation. We found that micromolar concentrations of nicotine activated heterologously expressed mouse and human TRPA1. Nicotine acted in a membrane-delimited manner, stabilizing the open state(s) and destabilizing the closed state(s) of the channel. In the presence of the general nAChR blocker hexamethonium, nociceptive neurons showed nicotine-induced responses that were strongly reduced in TRPA1-deficient mice. Finally, TRPA1 mediated the mouse airway constriction reflex to nasal instillation of nicotine. The identification of TRPA1 as a nicotine target suggests that existing models of nicotine-induced irritation should be revised and may facilitate the development of smoking cessation therapies with less adverse effects.

Nicotine activates TRPM5-dependent and independent taste pathways

The orosensory responses elicited by nicotine are relevant for the development and maintenance of addiction to tobacco products. However, although nicotine is described as bitter tasting, the molecular and neural substrates encoding the taste of nicotine are unclear. Here, rats and mice were used to determine whether nicotine activates peripheral and central taste pathways via TRPM5-dependent mechanisms, which are essential for responses to other bitter tastants such as quinine, and/or via nicotinic acetylcholine receptors (nAChRs). When compared with wild-type mice, Trpm5(-/-) mice had reduced, but not abolished, chorda tympani (CT) responses to nicotine. In both genotypes, lingual application of mecamylamine, a nAChR-antagonist, inhibited CT nerve responses to nicotine and reduced behavioral responses of aversion to this stimulus. In accordance with these findings, rats were shown to discriminate between nicotine and quinine presented at intensity-paired concentrations. Moreover, rat gustatory cortex (GC) neural ensemble activity could also discriminate between these two bitter tastants. Mecamylamine reduced both behavioral and GC neural discrimination between nicotine and quinine. In summary, nicotine elicits taste responses through peripheral TRPM5-dependent pathways, common to other bitter tastants, and nAChR-dependent and TRPM5-independent pathways, thus creating a unique sensory representation that contributes to the sensory experience of tobacco products.

Distribution of cholinergic cells in guinea pig brainstem

We used an antibody to choline acetyltransferase (ChAT) to label cholinergic cells in guinea pig brainstem. ChAT-immunoreactive (IR) cells comprise several prominent groups, including the pedunculopontine tegmental nucleus, laterodorsal tegmental nucleus, and parabigeminal nucleus, as well as the cranial nerve somatic motor and parasympathetic nuclei. Additional concentrations are present in the parabrachial nuclei and superior colliculus. Among auditory nuclei, the majority of ChAT-IR cells are in the superior olive, particularly in and around the lateral superior olive, the ventral nucleus of the trapezoid body and the superior paraolivary nucleus. A discrete group of ChAT-IR cells is located in the sagulum, and additional cells are scattered in the nucleus of the brachium of the inferior colliculus. A group of ChAT-IR cells lies dorsal to the dorsal nucleus of the lateral lemniscus. A few ChAT-IR cells are found in the cochlear nucleus and the ventral nucleus of the lateral lemniscus. The distribution of cholinergic cells in guinea pigs is largely similar to that of other species; differences occur mainly in cell groups that have few ChAT-IR cells. The results provide a basis for further studies to characterize the connections of these cholinergic groups.

Multisensory Processing of Gustatory Stimuli

Gustatory perception is inherently multimodal, since approximately the same time that intra-oral stimuli activate taste receptors, somatosensory information is concurrently sent to the CNS. We review evidence that gustatory perception is intrinsically linked to concurrent somatosensory processing. We will show that processing of multisensory information can occur at the level of the taste cells through to the gustatory cortex. We will also focus on the fact that the same chemical and physical stimuli that activate the taste system also activate the somatosensory system (SS), but they may provide different types of information to guide behavior.

Effects of cigarette smoking and family history of alcoholism on sweet taste perception and food cravings in women

BACKGROUND

Despite popular beliefs that smoking affects the sensitivity and liking of sweet-tasting foods and beverages, few psychophysical studies have examined this phenomenon and none have taken into account the individual's family history of alcoholism (FH+), a predictor of heightened sweet preferences.

METHODS

A within- and between-subjects study was conducted to determine the effect of both cigarette smoking and an acute exposure to nicotine on sweet taste sensitivity and preferences in women. Two groups were studied on 2 days separated by 1 week: women who were current smokers (n = 27, 18 were FH+) and those who never smoked in their lifetime (n = 22, 9 were FH+). Current smokers smoked nicotine-containing cigarettes during 1 test session and nicotine-free cigarettes during the other. The procedures were identical during both test sessions for the group of never smokers, with the exception that they did not smoke. Two-alternative staircase methods and forced-choice tracking procedures were used to assess sucrose thresholds and preferences, respectively, during both test session. Standardized questionnaires were administered to assess food cravings as well as smoking and alcohol usage and dependence. The Family Interview for Genetic Studies was used to detect alcoholism according to the DSM III criteria for family members up to second-degree relatives.

RESULTS

Acute exposure to nicotine did not affect sucrose detection thresholds or preferences, but smokers had significantly higher sucrose detection thresholds than never smokers. The greater the smoking dose in pack-years, the lower the sucrose sensitivity. Regardless of smoking status, women who were FH+ preferred significantly higher sucrose concentrations and craved sweets more often than women who were not.

CONCLUSIONS

Both smoking and having a family history of alcoholism had differential effects on sweet taste. Smoking was associated with decreased sweet taste sensitivity whereas having a family history of alcoholism was associated with heightened sweet preferences. These findings suggests that future research on the effects of smoking on food habits and cravings should take into account family history of alcoholism given its association with sweet liking and the increased likelihood to develop a tobacco disorder.

Effect of nicotine on chorda tympani responses to salty and sour stimuli

The effect of nicotine on the benzamil (Bz)-insensitive (transient receptor potential vanilloid-1 variant cation channel, TRPV1t) and the Bz-sensitive (epithelial Na(+) channel, ENaC) salt taste receptors and sour taste was investigated by monitoring intracellular Na(+) and H(+) activity (pH(i)) in polarized fungiform taste receptor cells (TRCs) and the chorda tympani (CT) nerve responses to NaCl, KCl, and HCl. CT responses in Sprague-Dawley rats and both wildtype and TRPV1 knockout (KO) mice were recorded in the presence and absence of agonists [resiniferatoxin (RTX) and elevated temperature] and an antagonist (SB-366791) of TRPV1t, the ENaC blocker (Bz), and varying apical pH (pH(o)). At concentrations <0.015 M, nicotine enhanced and at >0.015 M, it inhibited CT responses to KCl and NaCl. Nicotine produced maximum enhancement in the Bz-insensitive NaCl CT response at pH(o) between 6 and 7. RTX and elevated temperature increased the sensitivity of the CT response to nicotine in salt-containing media, and SB-366791 inhibited these effects. TRPV1 KO mice demonstrated no Bz-insensitive CT response to NaCl and no sensitivity to nicotine, RTX, and elevated temperature. We conclude that nicotine modulates salt responses by direct interaction with TRPV1t. At pH(o) >8, the apical membrane permeability of nicotine was increased significantly, resulting in increase in TRC pH(i) and volume, activation of ENaC, and enhancement of the Bz-sensitive NaCl CT response. At pH(o) >8, nicotine also inhibited the phasic component of the HCl CT response. We conclude that the effects of nicotine on ENaC and the phasic HCl CT response arise from increases in TRC pH(i) and volume.

Long-term effects of nicotine on rat fungiform taste buds

Nicotine, an alkaloid found in tobacco smoke, has been recognized as capable of inducing changes in taste functionality in conditions of chronic exposure. The mechanisms underlying these sensory alterations, however, are currently unknown. We addressed this issue by studying the long-term effects of nicotine on the anatomical features of taste buds, the peripheral end-organs of taste, in rat fungiform papillae. Nicotine was administered to rats via drinking water over a period of 3 weeks, which represents a standard method to achieve chronic drug exposure in laboratory animals. We found that prolonged administration of nicotine induced a significant reduction in the size of fungiform taste buds, without affecting their total number on the rat tongue. Morphometric measurements as well as evaluations of taste cell membrane capacitance suggested that the reduced size of taste organs was determined by a decrease in the number of cells per taste bud. In addition, chronic treatment with nicotine caused an increase in the relative density of cells expressing gustducin, a specific G protein alpha-subunit found in some taste cells and involved in bitter/sweet transduction. Interestingly, changes in the expression pattern of gustducin turned out to be more pronounced in periadolescent/adolescent than in adult rats. As a whole, our data indicate that long-term nicotine administration induces significant changes in the anatomical properties of taste buds in rat fungiform papillae. These changes could have a profound impact on the sensory information relayed to the brain; therefore, they may be responsible, at least in part, for the alterations in taste functionality observed during chronic nicotine exposure, a condition found in regular smokers.

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.