Chemosensory processing in the taste - reward pathway

Gustatory-Visual Interaction in Human Brain Cortex: fNIRS Study

BACKGROUND

Many studies, for example, on taste-visual dissonance, have shown that the influence of the visual cortex on taste sensation is enormous. The presented work aims to investigate, using fNIRS, whether a taste stimulus, in this case, the taste of bitter, also causes stimulation of the visual cortex in the fNIRS study.

METHODS

fNIRS was used to examine 51 participants (204 examinations, 9996 records), collecting signals from the left hemisphere. Differences between the maximum and minimum changes in oxyHb concentrations (ΔoxyHb) for the areas of the brain cortex considered responsible for recording visual and gustatory signals were analyzed. Protocols I, II, III, and IV-activation with distillate water, coffee with lower concentration, reference (no stimulation), and coffee with higher concentration, respectively, were used.

RESULTS

We recorded high signals for teste activation on channels covering the gustatory cortex, which confirms the correctness of the choice of research method. As expected, a significant statistical difference was observed between protocols I, II, and IV and reference III (without stimulation). What seems important is the fact that we also received high signals for the channels 45-49, which cover the visual cortex. The statistical analysis shows no differences between protocols I, II, and IV (different taste activation-water, coffee A, and coffee B) for specific channels for analyzing regions of interest. As a result of the analysis of the correlation between the subjective bitterness assessment solutions and the signal ΔoxyHb height, it was observed that a statistically significant correlation, although weak, occurs only for 14 and gustatory channels, only for coffee with a higher concentration. Additionally, the only statistically significant difference between women and men was observed in Protocol I (water), where the ΔoxyHb signal was twice as high in women compared to men.

CONCLUSIONS

In conclusion, we can clearly state that the senses of sight and taste work closely together. Moreover, this cooperation is not one-sided: while visual activation influences taste perception, interestingly, a taste stimulus can also generate a hemodynamic response, activating the visual cortex.

Sensitivity to Sweet and Salty Tastes in Children and Adolescents with Type 1 Diabetes

Taste function impairment is observed in people with type 1 diabetes (T1D). It is most often related to sweet taste. It is associated with such factors as diabetic neuropathy, smoking, age, duration of the disease and a rigorous diet that eliminates easily digestible carbohydrates. The aim of the study was to compare sensitivity to sweet and salty tastes between healthy children and adolescents and children and adolescents with T1D. The study group consisted of children with T1D (n = 35), with at least 5 years of disease history, while the group of healthy children included 46 individuals selected in terms of age, gender and BMI. A study concerning the perception of sweet and salty taste was carried out with the use of the specific gustometry method (examining the recognition and assessment of the intensity of taste sensations, performing a hedonic assessment). Children and adolescents from both groups had trouble recognising tastes. Children and adolescents with T1D were more likely to recognise sweet taste correctly even at its lower concentrations compared to healthy individuals (p = 0.04). Salty taste was significantly more often correctly identified by healthy children compared to T1D patients (p = 0.01). Children and adolescents with T1D reported a stronger intensity of perceived tastes than healthy ones. No significant differences in perceived pleasure were noted at lower sucrose concentrations in any group. The intensity score was higher in individuals with T1D at higher sucrose concentrations. No significant differences occurred in the assessment of salty taste intensity. The hedonic scoring of solutions with higher concentrations of sweet taste was higher in people with T1D than in healthy ones, while salty taste was assessed neutrally. Children and adolescents with T1D were demonstrated to have some taste recognition disorders. Therefore, monitoring taste function in pediatric diabetic clinical practice seems relevant, as it may be associated with important implications for the intake of a particular type of food and for the development of eating habits and preferences.

Sex differences in the taste-evoked functional connectivity network

The central gustatory pathway encompasses multiple subcortical and cortical regions whose neural functional connectivity can be modulated by taste stimulation. While gustatory perception has been previously linked to sex, whether and how the gustatory network differently responds to basic tastes between men and women is unclear. Here, we defined the regions of the central gustatory network by a meta-analysis of 35 fMRI taste activation studies and then analyzed the taste-evoked functional connectivity between these regions in 44 subjects (19 women) in a separate 3 Tesla activation study where sweet and bitter solutions, at five concentrations each, were administered during scanning. From the meta-analysis, a network model was set up, including bilateral anterior, middle and inferior insula, thalamus, precentral gyrus, left amygdala, caudate and dorsolateral prefrontal cortex. Higher functional connectivity than in women was observed in men between the right middle insula and bilateral thalami for bitter taste. Men exhibited higher connectivity than women at low bitter concentrations and middle-high sweet concentrations between bilateral thalamus and insula. A graph-based analysis expressed similar results in terms of nodal characteristics of strength and centrality. Our findings add new insights into the mechanisms of taste processing by highlighting sex differences in the functional connectivity of the gustatory network as modulated by the perception of sweet and bitter tastes. These results shed more light on the neural origin of sex-related differences in gustatory perception and may guide future research on the pathophysiology of taste perception in humans.

An alternative pathway for sweet sensation: possible mechanisms and physiological relevance

Sweet substances are detected by taste-bud cells upon binding to the sweet-taste receptor, a T1R2/T1R3 heterodimeric G protein-coupled receptor. In addition, experiments with mouse models lacking the sweet-taste receptor or its downstream signaling components led to the proposal of a parallel "alternative pathway" that may serve as metabolic sensor and energy regulator. Indeed, these mice showed residual nerve responses and behavioral attraction to sugars and oligosaccharides but not to artificial sweeteners. In analogy to pancreatic β cells, such alternative mechanism, to sense glucose in sweet-sensitive taste cells, might involve glucose transporters and K_ATP channels. Their activation may induce depolarization-dependent Ca²⁺ signals and release of GLP-1, which binds to its receptors on intragemmal nerve fibers. Via unknown neuronal and/or endocrine mechanisms, this pathway may contribute to both, behavioral attraction and/or induction of cephalic-phase insulin release upon oral sweet stimulation. Here, we critically review the evidence for a parallel sweet-sensitive pathway, involved signaling mechanisms, neural processing, interactions with endocrine hormonal mechanisms, and its sensitivity to different stimuli. Finally, we propose its physiological role in detecting the energy content of food and preparing for digestion.

Sensing Senses: Optical Biosensors to Study Gustation

The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca²⁺ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca²⁺, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.

Intensity-related distribution of sweet and bitter taste fMRI responses in the insular cortex

The human gustatory cortex analyzes the chemosensory properties of tastants, particularly the quality, intensity, and affective valence, to determine whether a perceived substance should be ingested or rejected. Among previous studies, the spatial distribution of taste intensity-related activations within the human insula has been scarcely addressed. To spatially characterize a specialized or distributed nature of the cortical responses to taste intensities, a functional magnetic resonance imaging study was performed at 3 T in 44 healthy subjects where sweet and bitter tastants were administered at five increasing concentrations and cortex-based factorial and parametric analyses were performed. Two clusters in the right middle-posterior and left middle insula were found specialized for taste intensity processing, exhibiting a highly nonlinear profile across concentrations. Multiple clusters were found activated by sweet and bitter taste stimuli at most concentrations, in the anterior, middle-posterior, and inferior portion of the bilateral insula. Across these clusters, respectively, for the right and left insula, a superior-to-inferior and an anterior-to-posterior spatial gradient for high-to-low concentrations were observed for the most responsive intensity of both tastes. These findings may gather new insights regarding how the gustatory cortex is spatially organized during the perceptual processing of taste intensity for two basic tastants.

Food Intake and Eating Behavior After Bariatric Surgery

Obesity is an escalating global chronic disease. Bariatric surgery is a very efficacious treatment for obesity and its comorbidities. Alterations to gastrointestinal anatomy during bariatric surgery result in neurological and physiological changes affecting hypothalamic signaling, gut hormones, bile acids, and gut microbiota, which coalesce to exert a profound influence on eating behavior. A thorough understanding of the mechanisms underlying eating behavior is essential in the management of patients after bariatric surgery. Studies investigating candidate mechanisms have expanded dramatically in the last decade. Herein we review the proposed mechanisms governing changes in eating behavior, food intake, and body weight after bariatric surgery. Additive or synergistic effects of both conditioned and unconditioned factors likely account for the complete picture of changes in eating behavior. Considered application of strategies designed to support the underlying principles governing changes in eating behavior holds promise as a means of optimizing responses to surgery and long-term outcomes.

Recent advances in the modification of taste and food preferences following bariatric surgery

There is a large body of evidence indicating that bariatric surgery provides durable weight loss and health benefits to patients who are obese and have comorbidities such as type 2 diabetes (T2D). However, there are still many questions related to mechanisms of metabolic improvement, predictors of success/failure, and long term consequences, which need to be answered. More recently, there has been a particular interest in the modulation of taste and food preferences that occurs after bariatric surgery and how this affects weight loss in different individuals. Animal models as well as human studies have shed some light on the role of taste in changing food preferences and how these changes may affect weight loss after surgery. The goal of this review is to discuss the physiological and behavioral consequences of bariatric surgery as a treatment for obesity and T2D, with particular emphasis on recent studies describing bariatric surgery-induced modifications in taste perception and food preferences.

Sweet taste sensitivity in pre-diabetics, diabetics and normoglycemic controls: a comparative cross sectional study

BACKGROUND

Increasing prevalence of pre-diabetes is an emerging public health problem. Decrease in sweet taste sensitivity which can lead to an increase in sugar intake might be a factor driving them to overt diabetes. The aim of the present study was to assess the sweet taste sensitivity in pre-diabetics in comparison with diabetics and with normoglycemic controls.

METHODS

Forty pre-diabetics, 40 diabetics and 34 normoglycemic controls were studied. The three groups were matched for age, sex and BMI. The division into groups was based on their glycated hemoglobin levels. The detection and recognition thresholds were determined by the multiple forced-choice method using sucrose solutions prepared in ¼ log dilutions. The intensities of perceived sensations for a series of suprathreshold concentrations of sucrose solutions prepared in ½ log dilution were determined by rating on a visual analogue scale. Statistical analyses were performed by SPSS version 21.

RESULTS

The mean (SD) detection thresholds of diabetic, pre-diabetic and normoglycemic groups were 0.025 (0.01), 0.018 (0.01) and 0.015 (0.01) respectively with a significant increase in diabetic group compared to normoglycemic group (p = 0.03). The mean recognition thresholds were not different among the three groups. When the intensity ratings for suprathreshold concentrations of sucrose were compared between the three groups, for all suprathreshold concentrations tested, significant differences were observed across the four concentrations (p < 0.001) and between groups in suprathreshold ratings (p < 0.05). Further analysis showed that the diabetic group had significantly lower suprathreshold ratings than the normoglycemic group (p < 0.001). Although all mean suprathreshold intensity ratings of the pre-diabetic group were between the normoglycemic and diabetic groups, the differences were not significant.

CONCLUSIONS

This is the first study to demonstrate the sweet taste sensitivity in pre-diabetics. The findings of the present study do not support the hypothesis of decreased sweet taste sensitivity of pre-diabetics. However, the results confirm the previous findings of blunted taste response in diabetics. The observation of pre-diabetics having intermediate values for all taste thresholds and suprathreshold ratings warrants a future investigation with a larger pre-diabetic sample recruited with more specific screening criteria to test this hypothesis further.

Speed and accuracy of taste identification and palatability: impact of learning, reward expectancy, and consummatory licking

Despite decades of study, it remains a matter of controversy as to whether in rats taste identification is a rapid process that occurs in about 250-600 ms (one to three licks) or a slow process that evolves over seconds. To address this issue, we trained rats to perform a taste-cued two-response discrimination task (2-RDT). It was found that, after learning, regardless of intensity, the delivery of 10 μl of a tastant (e.g., NaCl or monopotassium glutamate, MPG) was sufficient to identify its taste with maximal accuracy within 400 ms. However, despite overtraining, rats rarely stopped licking in one lick. Thus, a one-drop lick reaction task was developed in which subjects had to rapidly stop licking after release of a stop signal (tastants including water) to obtain rewards. The faster they stopped licking, the greater the reward. Rats did not stop licking after receiving either hedonically positive or negative stop signals, and thus failed to maximize rewards even when reinforced with even larger rewards. In fact, the higher the sucrose concentration given as a stop signal, the greater the number of consummatory licks elicited. However, with a stop signal of 2 mM quinine HCl, they stopped licking in ~370 ms, a time faster than that for sucrose or water, thus showing that in this rapid period, quinine HCl evoked an unpalatable response. Indeed, only when rats licked an empty sipper tube would they usually elicit a single lick to obtain a reward (operant licking). In summary, these data indicate that within 400 ms, taste identification and palatability, must either occur simultaneously or with marked overlap.

Taste preference for amino acids is dependent on internal nutritional state in Drosophila melanogaster

Like mammals, insects need to ingest proteins from foods because they cannot synthesise several amino acids. Amino acids are also essential nutrients for Drosophila melanogaster, especially for female egg production, but how flies detect amino acids and how the feeding response to amino acids is regulated are unknown. In this study, the two-choice preference test, the proboscis extension reflex test and a CAFE assay were performed to explore the ability of D. melanogaster to detect and discriminate amino acids. To determine whether D. melanogaster change their feeding preference to amino acids after being deprived of them, as previously reported in the locust, two groups of flies raised on the usual medium or on glucose medium were compared. Amino-acid-deprived flies demonstrated enhanced preference to an amino acid mixture and to several amino acids. These flies ingested amino acids even when they were replete with glucose. The proboscis extension reflex to particular amino acids was induced only in amino-acid-deprived flies. Our findings indicate that the sensitivity of labellar taste cells to amino acids may change when flies are deficient in amino acid supply, and also reveal that the detection pathways for individual amino acids may differ. We suggest the existence of an amino acid receptor and a monitoring system regulating the feeding responses to amino acids.