Gustatory Pathways Research Articles

The Contribution of the Corpus Callosum to the Symmetrical Representation of Taste in the Human Brain: An fMRI Study of Callosotomized Patients

The present study was designed to establish the contribution of the corpus callosum (CC) to the cortical representation of unilateral taste stimuli in the human primary gustatory area (GI). Unilateral taste stimulation of the tongue was applied to eight patients with partial or total callosal resection by placing a small cotton pad soaked in a salty solution on either side of the tongue. Functional images were acquired with a 1.5 Tesla machine. Diffusion tensor imaging and tractography were also performed. Unilateral taste stimuli evoked bilateral activation of the GI area in all patients, including those with total resection of the CC, with a prevalence in the ipsilateral hemisphere to the stimulated tongue side. Bilateral activation was also observed in the primary somatic sensory cortex (SI) in most patients, which was more intense in the contralateral SI. This report confirms previous functional studies carried out in control subjects and neuropsychological findings in callosotomized patients, showing that gustatory pathways from tongue to cortex are bilaterally distributed, with an ipsilateral predominance. It has been shown that the CC does play a role, although not an exclusive one, in the bilateral symmetrical representation of gustatory sensitivity in the GI area, at least for afferents from one side of the tongue.

Early-life influences of low-calorie sweetener consumption on sugar taste

Children and adolescents are the highest consumers of added sugars, particularly from sugar-sweetened beverages (SSB). Regular consumption of SSB early in life induces a variety of negative consequences on health that can last into adulthood. Low-calorie sweeteners (LCS) are increasingly used as an alternative to added sugars because they provide a sweet sensation without adding calories to the diet. However, the long-term effects of early-life consumption of LCS are not well understood. Considering LCS engage at least one of the same taste receptors as sugars and potentially modulate cellular mechanisms of glucose transport and metabolism, it is especially important to understand how early-life LCS consumption impacts intake of and regulatory responses to caloric sugars. In our recent study, we found that habitual intake of LCS during the juvenile-adolescence period significantly changed how rats responded to sugar later in life. Here, we review evidence that LCS and sugars are sensed via common and distinct gustatory pathways, and then discuss the implications this has for shaping sugar-associated appetitive, consummatory, and physiological responses. Ultimately, the review highlights the diverse gaps in knowledge that will be necessary to fill to understand the consequences of regular LCS consumption during important phases of development.

Habituation of Sugar-Induced Proboscis Extension Reflex and Yeast-Induced Habituation Override in Drosophila melanogaster.

Habituation, the process by which animals learn to ignore insignificant stimuli, facilitates engagement with salient features of the environment. However, neural mechanisms underlying habituation also allow responses to familiar stimuli to be reinstated when such stimuli become potentially significant. Thus, the habituated state must allow a mechanism for habituation override. The remarkably precise knowledge of cell identity, connectivity, and information coding in Drosophila sensory circuits, as well as the availability of tools to genetically target these cells, makes Drosophila a valuable and important organism for analysis of habituation and habituation-override mechanisms. Studies of olfactory and gustatory habituation in Drosophila suggest that potentiation of GABAergic neurons underlies certain timescales of habituation and have specified some elements of a gustatory habituation-override pathway. More detailed understanding of gustatory habituation and habituation-override mechanisms will benefit from access to robust behavioral assays for (a) the proboscis extension reflex (PER) elicited by a sweet stimulus, (b) exposure paradigms that result in PER habituation, and, most critically, (c) manipulations that result in PER-habituation override. Here, we describe simple protocols for persistent sucrose exposure of tarsal hairs that lead to habituation of proboscis extension and for presentation of a novel appetitive stimuli that reinstate robust PER to habituated flies. This detailed protocol of gustatory habituation provides (a) a simple method to induce habituation by continuous exposure of the flies to sucrose for 10 min without leading to ingestion and (b) a novel method to override habituation by presenting yeast to the proboscis. Key features • A protocol for stimulation of Drosophila's taste (sugar) sensory neurons that induces gustatory habituation without satiation due to ingestion. • A chemical (yeast) stimulation protocol that rapidly induces habituation override/dishabituation in sugar-habituated Drosophila.

Discrete roles of Ir76b ionotropic coreceptor impact olfaction, blood feeding, and mating in the malaria vector mosquito Anopheles coluzzii

Anopheline mosquitoes rely on their highly sensitive chemosensory apparatus to detect diverse chemical stimuli that drive the host-seeking and blood-feeding behaviors required to vector pathogens for malaria and other diseases. This process incorporates a variety of chemosensory receptors and transduction pathways. We used advanced in vivo gene-editing and -labeling approaches to localize and functionally characterize the ionotropic coreceptor AcIr76b in the malaria mosquito Anopheles coluzzii, where it impacts both olfactory and gustatory systems. AcIr76b has a broad expression pattern in female adult antennal grooved pegs, coeloconic sensilla, and T1 and T2 sensilla on the labellum, stylets, and tarsi, as well as the larval sensory peg. AcIr76b is colocalized with the Orco odorant receptor (OR) coreceptor in a subset of cells across the female antennae and labella. In contrast to Orco and Ir8a, chemosensory coreceptors that appear essential for the activity of their respective sets of chemosensory neurons in mosquitoes, AcIr76b−/− mutants maintain wild-type peripheral responses to volatile amines on the adult palps, labellum, and larval sensory cone. Interestingly, AcIr76b−/− mutants display significantly increased responses to amines in antennal grooved peg sensilla, while coeloconic sensilla reveal significant deficits in responses to several acids and amines. Behaviorally, AcIr76b mutants manifest significantly female-specific insemination deficits, and although AcIr76b−/− mutant females can locate, alight on, and probe artificial blood hosts, they are incapable of blood feeding successfully. Taken together, our findings reveal a multidimensional functionality of Ir76b in anopheline olfactory and gustatory pathways that directly impacts the vectorial capacity of these mosquitoes.

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.

The taste of the pandemic-contemporary review on the current state of research on gustation in coronavirus disease 2019 (COVID-19).

Subjectively perceived impairment of taste is a common and distinct symptom of coronavirus disease 2019 (COVID‐19). Large meta‐analyses identified this symptom in approximately 50% of cases. However, this high prevalence is not supported by blinded and validated psychophysical gustatory testing, which showed a much lower prevalence in up to 26% of patients. This discrepancy may be due to misinterpretation of impaired retronasal olfaction as gustatory dysfunction. In addition, we hypothesized that COVID‐19–associated hyposmia is involved in the decrease of gustatory function, as found for hyposmia of different origin. This indirect mechanism would be based on the central‐nervous mutual amplification between the chemical senses, which fails in COVID‐19–associated olfactory loss. However, further research is necessary on how severe acute respiratory syndrome‐coronavirus‐2 (SARS‐CoV‐2) may directly impair the gustatory pathway as well as its subjective perception.

Neuronal architecture and functional mapping of the taste center of larval Helicoverpa armigera (Lepidoptera: Noctuidae).

The sense of taste plays a crucial role in herbivorous insects by discriminating nutrients from complex plant metabolic compounds. The peripheral coding of taste has been thoroughly studied in many insect species, but the central gustatory pathways are poorly described. In the present study, we characterized single neurons in the gnathal ganglion of Helicoverpa armigera larvae using the intracellular recording/staining technique. We identified different types of neurons, including sensory neurons, interneurons, and motor neurons. The morphologies of these neurons were largely diverse and their arborizations seemingly covered the whole gnathal ganglion. The representation of the single neurons responding to the relevant stimuli of sweet and bitter cues showed no distinct patterns in the gnathal ganglion. We postulate that taste signals may be processed in a manner consistent with the principle of population coding in the gnathal ganglion of H. armigera larvae.

Smell and Taste Disturbance in COVID-19 Patients: A Prospective Multicenteric Review.

To study the incidence of the smell and taste disturbance in the COVID-19 patients and a follow up at 4 months to observe for the duration of resolution of these symptoms. This is a multicentric prospective study carried out in 3 different countries, from April, 2020 to January, 2021. The COVID-19 positive patients, aged between 15 and 60 years, were inquired about the presence of any smell or taste related symptoms. The same patients were followed up with the telephonic interview after 2 months and then after 4 months, respectively. The duration of resolution of the smell and taste disturbance symptoms was noted. Total of 188 COVID-19 positive patients, average age 33.1 ± 1.7 years, 54.2% males and 45.8% female were included in the study. The smell disturbance was present in 60.6% (hyposmia 36.1%, anosmia 20.2%, and parosmia 4.2%) and taste disturbance in 28.7% of patients (hypogeusia 20.2%, ageusia 6.9%, and parageusia 1.6%). There was improvement of anosmia by 97.4, hyposmia by 95.6%, parosmia by 100%, ageusia by 100%, hypogeusia 94.8%, and parageusia by 66.7%, at 4 months follow up. The present study concludes that the smell and taste disturbances are one of the main early presenting features of the COVID-19 infection. The temporary effect of the COVID-19 infection on the olfactory and gustatory pathway was also highlighted with more than 95% patients improving at 4 months of follow up.

Evidence of central nervous system infection and neuroinvasive routes, as well as neurological involvement, in the lethality of SARS-CoV-2 infection.

The outbreak of coronavirus disease 2019 (COVID‐19), caused by severe acute respiratory syndrome coronavirus‐2 (SARS‐CoV‐2), has become a significant and urgent threat to global health. This review provided strong support for central nervous system (CNS) infection with SARS‐CoV‐2 and shed light on the neurological mechanism underlying the lethality of SARS‐CoV‐2 infection. Among the published data, only 1.28% COVID‐19 patients who underwent cerebrospinal fluid (CSF) tests were positive for SARS‐CoV‐2 in CSF. However, this does not mean the absence of CNS infection in most COVID‐19 patients because postmortem studies revealed that some patients with CNS infection showed negative results in CSF tests for SARS‐CoV‐2. Among 20 neuropathological studies reported so far, SARS‐CoV‐2 was detected in the brain of 58 cases in nine studies, and three studies have provided sufficient details on the CNS infection in COVID‐19 patients. Almost all in vitro and in vivo experiments support the neuroinvasive potential of SARS‐CoV‐2. In infected animals, SARS‐CoV‐2 was found within neurons in different brain areas with a wide spectrum of neuropathology, consistent with the reported clinical symptoms in COVID‐19 patients. Several lines of evidence indicate that SARS‐CoV‐2 used the hematopoietic route to enter the CNS. But more evidence supports the trans‐neuronal hypothesis. SARS‐CoV‐2 has been found to invade the brain via the olfactory, gustatory, and trigeminal pathways, especially at the early stage of infection. Severe COVID‐19 patients with neurological deficits are at a higher risk of mortality, and only the infected animals showing neurological symptoms became dead, suggesting that neurological involvement may be one cause of death.

Ocean acidification impedes gustation-mediated feeding behavior by disrupting gustatory signal transduction in the black sea bream, Acanthopagrus schlegelii

Growing evidence suggests that ocean acidification (OA) may affect animal behaviors such as feeding. Although gustation plays a crucial role in evaluating the quality and palatability of food and ultimately influences whether or not teleosts consume the food, the potential impact of OA on gustation-mediated feeding behavior remains unknown. In this study, gustation mediated-feeding behavior, as indicated by the consumption rate (CR) and swallowing rate (SR) of agar pellets with or without feed upon OA exposure was investigated in black sea bream (Acanthopagrus schlegelii). Results showed that the exposure to acidified seawater led to significant reductions in the CR and SR of feed-containing agar pellets. In addition, the in vivo contents of three neurotransmitters and expression of genes from the gustatory signal transduction pathway were all significantly suppressed by the OA treatment. In general, the data obtained indicated that OA may hinder the gustation-mediated feeding behavior of A. schlegelii by disrupting gustatory signal transduction, which may aggravate the issue of food shortage for wild populations of black sea bream.

The Gustatory Sensory G-Protein GNAT3 Suppresses Pancreatic Cancer Progression in Mice.

Background & AimsPancreatic ductal adenocarcinoma (PDA) initiation and progression are accompanied by an immunosuppressive inflammatory response. Here, we evaluated the immunomodulatory role of chemosensory signaling in metaplastic tuft cells (MTCs) by analyzing the role of GNAT3, a gustatory pathway G-protein expressed by MTCs, during PDA progression.MethodsGnat3-null (Gnat3-/-) mice were crossbred with animals harboring a Cre-inducible KrasLSL-G12D/+ allele with either Ptf1aCre/+ (KC) or tamoxifen-inducible Ptf1aCreERT/+ (KCERT) mice to drive oncogenic KRAS expression in the pancreas. Ex vivo organoid conditioned medium generated from KC and Gnat3-/-;KC acinar cells was analyzed for cytokine secretion. Experimental pancreatitis was induced in KCERT and Gnat3-/-;KCERT mice to accelerate tumorigenesis, followed by analysis using mass cytometry and single-cell RNA sequencing. To study PDA progression, KC and Gnat3-/-;KC mice were aged to morbidity or 52 weeks.ResultsAblation of Gnat3 in KC organoids increased release of tumor-promoting cytokines in conditioned media, including CXCL1 and CXCL2. Analysis of Gnat3-/-;KCERT pancreata found altered expression of immunomodulatory genes in Cxcr2 expressing myeloid-derived suppressor cells (MDSCs) and an increased number of granulocytic MDSCs, a subset of tumor promoting MDSCs. Importantly, expression levels of CXCL1 and CXCL2, known ligands for CXCR2, were also elevated in Gnat3-/-;KCERT pancreata. Consistent with the tumor-promoting role of MDSCs, aged Gnat3-/-;KC mice progressed more rapidly to metastatic carcinoma compared with KC controls.ConclusionsCompromised gustatory sensing, achieved by Gnat3 ablation, enhanced the CXCL1/2–CXCR2 axis to alter the MDSC population and promoted the progression of metastatic PDA.

Salt Appetite and its Effects on Cardiovascular Risk in Primary Aldosteronism.

First described in 1955 by Jerome W. Conn, primary aldosteronism (PA) today is well established as a relevant cause of secondary hypertension and accounts for about 5-10 % of hypertensives. The importance of considering PA is based on its deleterious target organ damage far beyond the effect of elevated blood pressure and on PA being a potentially curable form of hypertension. Aside the established contributory role of high dietary salt intake to arterial hypertension and cardiovascular disease, high salt intake is mandatory for aldosterone-mediated deleterious effects on target-organ damage in patients with primary aldosteronism. Consequently, counselling patients on the need to reduce salt intake represents a major component in the treatment of PA to minimize cardiovascular damage. Unfortunately, in PA patients salt intake is high and far beyond the target values of 5 g per day, recommended by the World Health Organization. Insufficient patient motivation for lifestyle interventions can be further complicated by enhancing effects of aldosterone on salt appetite, via central and gustatory pathways. In this context, treatment for PA by adrenalectomy results in a spontaneous decrease in dietary salt intake and might therefore provide further reduction of cardiovascular risk in PA than specific medical treatment alone. Furthermore, there is evidence from clinical studies that even after sufficient treatment of PA dietary salt intake remains a relevant prognostic factor for cardiovascular risk. This review will focus on the synergistic benefits derived from both blockade of aldosterone-mediated effects and reduction in dietary salt intake on cardiovascular risk.

The neuroscience of sugars in taste, gut-reward, feeding circuits, and obesity.

Throughout the animal kingdom sucrose is one of the most palatable and preferred tastants. From an evolutionary perspective, this is not surprising as it is a primary source of energy. However, its overconsumption can result in obesity and an associated cornucopia of maladies, including type 2 diabetes and cardiovascular disease. Here we describe three physiological levels of processing sucrose that are involved in the decision to ingest it: the tongue, gut, and brain. The first section describes the peripheral cellular and molecular mechanisms of sweet taste identification that project to higher brain centers. We argue that stimulation of the tongue with sucrose triggers the formation of three distinct pathways that convey sensory attributes about its quality, palatability, and intensity that results in a perception of sweet taste. We also discuss the coding of sucrose throughout the gustatory pathway. The second section reviews how sucrose, and other palatable foods, interact with the gut-brain axis either through the hepatoportal system and/or vagal pathways in a manner that encodes both the rewarding and of nutritional value of foods. The third section reviews the homeostatic, hedonic, and aversive brain circuits involved in the control of food intake. Finally, we discuss evidence that overconsumption of sugars (or high fat diets) blunts taste perception, the post-ingestive nutritional reward value, and the circuits that control feeding in a manner that can lead to the development of obesity.

Recognizing Taste: Coding Patterns Along the Neural Axis in Mammals.

The gustatory system encodes information about chemical identity, nutritional value, and concentration of sensory stimuli before transmitting the signal from taste buds to central neurons that process and transform the signal. Deciphering the coding logic for taste quality requires examining responses at each level along the neural axis-from peripheral sensory organs to gustatory cortex. From the earliest single-fiber recordings, it was clear that some afferent neurons respond uniquely and others to stimuli of multiple qualities. There is frequently a "best stimulus" for a given neuron, leading to the suggestion that taste exhibits "labeled line coding." In the extreme, a strict "labeled line" requires neurons and pathways dedicated to single qualities (e.g., sweet, bitter, etc.). At the other end of the spectrum, "across-fiber," "combinatorial," or "ensemble" coding requires minimal specific information to be imparted by a single neuron. Instead, taste quality information is encoded by simultaneous activity in ensembles of afferent fibers. Further, "temporal coding" models have proposed that certain features of taste quality may be embedded in the cadence of impulse activity. Taste receptor proteins are often expressed in nonoverlapping sets of cells in taste buds apparently supporting "labeled lines." Yet, taste buds include both narrowly and broadly tuned cells. As gustatory signals proceed to the hindbrain and on to higher centers, coding becomes more distributed and temporal patterns of activity become important. Here, we present the conundrum of taste coding in the light of current electrophysiological and imaging techniques at several levels of the gustatory processing pathway.

Mouse Parabrachial Neurons Signal a Relationship between Bitter Taste and Nociceptive Stimuli.

Taste and somatosensation both mediate protective behaviors. Bitter taste guides avoidance of ingestion of toxins while pain sensations, such as noxious heat, signal adverse conditions to ward off harm. Although brain pathways for taste and somatosensation are typically studied independently, prior data suggest that they intersect, potentially reflecting their common protective role. To investigate this, we applied electrophysiologic and optogenetic techniques in anesthetized mice of both sexes to evaluate relationships between oral somatosensory and taste activity in the parabrachial nucleus (PbN), implicated for roles in gustation and pain. Spikes were recorded from taste-active PbN neurons tested with oral delivery of thermal and chemesthetic stimuli, including agonists of nocisensitive transient receptor potential (TRP) ion channels on somatosensory fibers. Gustatory neurons were also tested to follow electrical pulse stimulation of an oral somatosensory region of the spinal trigeminal subnucleus caudalis (Vc), which projects to the PbN. Neurons composed classic taste groups, including sodium, electrolyte, appetitive, or bitter cells. Across groups, most neurons spiked to Vc pulse stimulation, implying that trigeminal projections reach PbN gustatory neurons. Among such cells, a subpopulation responsive to the bitter taste stimuli quinine and cycloheximide, and aversive concentrations of sodium, cofired to agonists of nocisensitive TRP channels, including capsaicin, mustard oil, and noxious heat. Such neurons populated the lateral PbN. Further, nociceptive activity in PbN bitter taste neurons was suppressed during optogenetic-assisted inhibition of the Vc, implying convergent trigeminal input contributed to such activity. Our results reveal a novel role for PbN gustatory cells in cross-system signaling related to protection.SIGNIFICANCE STATEMENT Prior data suggest that gustatory and trigeminal neural pathways intersect and overlap in the parabrachial area. However, no study has directly examined such overlap and why it may exist. Here we found that parabrachial gustatory neurons can receive afferent projections from trigeminal nuclei and fire to oral nociceptive stimuli that excite somatosensory receptors and fibers. Activation to aversive nociceptive stimuli in gustatory cells was associated with responding to behaviorally avoided bitter tastants. We were further able to show that silencing trigeminal projections inhibited nociceptive activity in parabrachial bitter taste neurons. Our results imply that in the parabrachial area, there is predictable overlap between taste and somatosensory processing related to protective coding and that classically defined taste neurons contribute to this process.

A low-cost open-architecture taste delivery system for gustatory fMRI and BCI experiments

BackgroundTasting is a complex process involving chemosensory perception and cognitive evaluation. Different experimental designs and solution delivery approaches may in part explain the variability reported in literature. These technical aspects certainly limit the development of taste-related brain computer interface devices. New MethodWe propose a novel modular, scalable and low-cost device for rapid injection of small volumes of taste solutions during fMRI experiments that gathers the possibility to flexibly increase the number of channels, allowing complex multi-dimensional taste experiments. We provide the full description of the hardware and software architecture and illustrate the application of the working prototype in single-subject event-related fMRI experiments by showing the BOLD responses to basic taste qualities and to five intensities of tastes during the course of perception. ResultsThe device is shown to be effective in activating multiple clusters within the gustatory pathway and a precise time-resolved event-related analysis is shown to be possible by the impulsive nature of the induced perception. Comparison with Existing Method(s)This gustometer represents the first implementation of a low-cost, easily replicable and portable device that is suitable for all kinds of fMRI taste experiments. Its scalability will boost the experimental design of more complex multi-dimensional fMRI studies of the human taste pathway. ConclusionsThe gustometer represents a valid open-architecture alternative to other available devices and its spread and development may contribute to an increased standardization of experimental designs in human fMRI studies of taste perception and pave the way to the development of novel taste-related BCIs.

Fatty Acid Lingual Application Activates Gustatory and Reward Brain Circuits in the Mouse.

The origin of spontaneous preference for dietary lipids in humans and rodents is debated, though recent compelling evidence has shown the existence of fat taste that might be considered a sixth taste quality. We investigated the implication of gustatory and reward brain circuits, triggered by linoleic acid (LA), a long-chain fatty acid. The LA was applied onto the circumvallate papillae for 30 min in conscious C57BL/6J mice, and neuronal activation was assessed using c-Fos immunohistochemistry. By using real-time reverse transcription polymerase chain reaction (RT-qPCR), we also studied the expression of mRNA encoding brain-derived neurotrophic factor (BDNF), Zif-268, and Glut-1 in some brain areas of these animals. LA induced a significant increase in c-Fos expression in the nucleus of solitary tract (NST), parabrachial nucleus (PBN), and ventroposterior medialis parvocellularis (VPMPC) of the thalamus, which are the regions known to be activated by gustatory signals. LA also triggered c-Fos expression in the central amygdala and ventral tegmental area (VTA), involved in food reward, in conjunction with emotional traits. Interestingly, we noticed a high expression of BDNF, Zif-268, and Glut-1 mRNA in the arcuate nucleus (Arc) and hippocampus (Hipp), where neuronal activation leads to memory formation. Our study demonstrates that oral lipid taste perception might trigger the activation of canonical gustatory and reward pathways.

Gustatory pathway in humans: A review of models of taste perception and their potential lateralization.

The focus of this review is to interpret recent advances in human gustatory pathways with respect to the laterality of gustatory responses. Psychophysical, neuroimaging, and clinical anatomical studies published in peer reviewed scientific journals were examined. From the anatomical and neuroimaging studies a total of six models are outlined and discussed in the light of some recent psychophysical and clinical results. In the specific of the salt condition and right preferred hand the outcomes have revealed a predominant left ipsilateral pathway with evidences of ipsilateral projection from the left primary gustatory cortex (PGC) to the orbitofrontal cortex, while a bilateral projection from the right oral cavity to the left and right insula seems to be more consistent. Also, the right side predominance of the chemosensory perception is objected. Additionally, the gustatory response appears to be dependent on the taste quality, supporting the idea of a chemotopical organization of the PGC as well as the Labelled-Line Model theory of peripheral taste quality encoding. However, where the fibers branch along the ascending pathway is not unequivocally established. Interestingly, factors like handedness appear to be remarkable when studying the lateralization of brain functions. Finally we suggest that further studies must include handedness and taste quality as distinctive factors that can help to interpret the results in a unique way.

A chronic LPS-induced low-grade inflammation fails to reproduce in lean mice the impairment of preference for oily solution found in diet-induced obese mice

Diet-induced obesity (DIO) is associated with a decreased oral fat detection in rodents. This alteration has been explained by an impairment of the lipid-mediated signaling in taste bud cells (TBC). However, factors responsible for this defect remain elusive. Diet rich in saturated fatty acids is known to elicit a metabolic inflammation by promoting intestinal permeation to lipopolysaccharides (LPS), Gram-negative bacteria-derived endotoxins. To determine whether a local inflammation of the gustatory tissue might explain the obese-induced impairment of the oro-sensory detection of lipids, mice were subjected to a DIO protocol. Using a combination of behavioral tests, transcriptomic analyses of gustatory papillae and biochemical assays, we have found that i) DIO elicits a pro-inflammatory genic profile in the circumvallate papillae (CVP), known to house the highest density of lingual taste buds, ii) NFkB, a key player of inflammatory process, might play a role in this transcriptomic pattern, iii) plasma LPS levels are negatively correlated with the preference for oily solution, and iv) a chronic infusion of LPS at a level similar to that found in DIO mice is not sufficient to alter the spontaneous preference for fat in lean mice. Taken together these data bring the demonstration that a saturated high fat diet elicits an inflammatory response at the level of peripheral gustatory pathway and a LPS-induced low-grade endotoxemia alone does not explain the change in the preference for dietary lipids observed in DIO mice.

Contributory role of sex differences in the variations of gustatory function.

Remarkable variability between males and females occurs for an array of taste-guided behaviors in both rodents and humans. Sex differences have been noted for taste preference, detection thresholds, and reactivity to taste stimuli. Manipulating sex hormones during early postnatal development or altering the amount of circulating estrogen in adulthood can dramatically alter the pattern of these behaviors. Receptors for sex hormones appear to be prominent in several nuclei associated with central gustatory pathways, indicating that steroid hormones may modulate central taste processing. Electrophysiological recordings from the rat brainstem suggest that taste-elicited activity to sweet stimuli is organized by hormones during early development, and activity during bitter stimulation is altered by circulating ovarian hormones. Sex differences in gustatory function appear to emerge at the level of the taste bud. Among ovariectomized rats, estradiol treatment decreases activity in the chorda tympani nerve during NaCl stimulation. Although there is no evidence that chorda tympani responses to NaCl differ between intact male and female rats, glossopharyngeal nerve responses are lower in intact females for both NaCl and sodium acetate. Responses in the glossopharyngeal nerve to citric acid stimulation are also higher in female rats relative to males. These findings suggest that, in addition to differential central modulation of taste input based on sex, taste information from the periphery varies between males and females. Although the extent of sex differences in taste processing and the underlying causal mechanisms require further clarification, it is clear that studying one sex alone provides an incomplete picture of gustatory function. © 2016 Wiley Periodicals, Inc.