Chemical Senses最新文献

Mechanosensory neurons play a crucial role in determining the location of stimuli on the receptor surface, movement, as well as the identification and discrimination of textures. To date, little is known about mechanosensory neuron types that innervate the oral cavity. Here, we recorded from mechanosensitive neurons innervating the oral cavity, to examine their diversity and function mediating touch. We first recorded a rough topographical map to aid in locating mechanosensory neuron types innervating the oral cavity. Electrophysiological mapping indicated that neurons innervating the tongue were located within and between maxillary (V2) and mandibular (V3) receptive fields, resembling a "strip" similar to the anatomical position of the tongue. We found that both rapidly adapting and slowly adapting neurons innervate the anterior tongue and lips. Conduction velocity experiments showed that all the lip-innervating neurons were classified as C-fibers, whereas there was a broader range for the tongue-innervating neurons, ranging from C-fibers to A-fast. Interestingly, we found that rapidly adapting and slowly adapting neurons were present at all 3 conduction velocity classifications. The majority of pressure-sensitive neurons also responded to brush stimulation (91%); however, there was a small subset of neurons that only responded to pressure stimulation (9%). The majority of the A-fibers had receptive fields on the anterior tip of the tongue. Furthermore, we found that when a mechanically sensitive neuron innervated a fungiform papilla, its receptive field did not include the surrounding filiform papillae. Our findings provide important contributions to understanding some of the coding features of mechanosensory neurons that innervate the oral cavity.

Olfactory training (OT), a structured exposure to odors, is commonly used by otorhinolaryngologists to treat olfactory dysfunction. However, OT has been shown to improve cognition of people with cognitive or olfactory impairments and slow the age-related cognitive decline. This study investigated whether OT could enhance cognitive functions in older adults with an intact sense of smell, compared with younger adults. We hypothesized that OT would improve semantic verbal fluency and working memory in the experimental group, with no significant changes in the placebo group (PG). The final sample comprised 184 participants aged 24 to 94 years (Mage = 51.84 ± 23.25 years), including 83 young adults (53% women, Mage = 26.71 ± 2.62 years) and 101 older adults (88% women, Mage = 72.49 ± 5.40 years). For the semantic verbal fluency task, they listed as many items as possible within one of 2 semantic categories: (i) grocery products or (ii) fragrant items, within a 60-s time limit. To measure working memory, participants completed a digit span task where they repeated sequences of digits aloud. In older adults performing OT, it led to a marginal increase in semantic verbal fluency, regardless of semantic category, while no significant changes were observed in the older PG or in young adults. This effect was mainly driven by individuals with lower scores at baseline. Our findings suggest that OT can benefit semantic verbal fluency in the healthy geriatric population with lower baseline scores, but these results urge similar testing in clinical groups with compromised verbal functions.

In cancer chemotherapy, the development of taste disorders is a serious problem because it not only reduces quality of life but also deteriorates patients' nutritional status, sometimes resulting in cessation of chemotherapy. Doxorubicin (DOX), a widely used anticancer anthracycline, reportedly alters patients' taste perception. However, little information exists on the detailed characteristics of DOX-induced taste disorders. In this study, we investigated whether taste responsiveness was altered in DOX-administered mice using a brief-access test and examined the underlying mechanisms. DOX-administered mice exhibited decreased responsiveness to sweet taste, but not to umami, bitter, salty, or sour tastes, on day 7, with recovery by day 14. Although there was no detectable alteration in the number of type II-taste cell marker-immunoreactive cells in the circumvallate papillae (CP), expression of mRNA for sweet taste receptors T1R2, T1R3, and α-gustducin in the CP was greater in DOX-administered mice than in control mice on day 7. Interestingly, in DOX-administered mice on day 7, the lick ratios for a 300-mM sucrose solution showed a significant correlation with the relative expression of T1R3 mRNA in the CP and a trend toward a correlation with food consumption. Together, these findings suggest that the decrease in sweet taste sensitivity in DOX-administered mice might result from dysfunction in signaling downstream of taste perception, with increased sweet taste receptor expression potentially representing a compensatory response to decreased energy intake.

In the oral cavity, oral stereognosis and chemesthesis refer to the abilities to recognize shapes and detect noxious substances, respectively, through various receptors distributed on the tongue. The absence of standardized methods to assess oral somatosensory perception has led to a lack of consensus regarding the relationship between taste and trigeminal sensations. While some studies reported lower trigeminal perception in taste disorders, others did not confirm this correlation. To analyze somatosensation in quantitative and qualitative taste problems, 28 patients (21 females, mean age 59.2 yr) with taste disturbance and 32 participants (20 females, mean age 42.3 yr) with normogeusia identified 3D shapes and letters, sized from 2 to 18 mm, with the anterior part of their tongue, in a threshold test. Additionally, participants rated the intensity of increasing amounts of chili. Gustatory and olfactory functions were also tested. Analyses revealed that patients with taste dysfunction had significantly higher thresholds for lingual tactile sensitivity compared to controls (P < 0.001). Moreover, patients rated the intensity of chili significantly lower than controls (P < 0.001), particularly at higher concentrations. Oral stereognosis threshold scores were negatively correlated with gustatory and olfactory functions, and decreased with increasing age, while intensity for chili correlated with taste scores (P ≤ 0.009). The findings of this study suggest that taste dysfunction is associated with low oral mechanosensory and chemesthetic functions. The testing methods used in this study could be valuable in clinical practice for assessing and monitoring sensory impairments.

In mammals, odors are encoded by a combinatorial code determined by the pattern of responses across hundreds of odorant receptors expressed monogenically and monoallelically in olfactory sensory neurons. The compositions of these receptor response patterns are largely unknown and overlap between them has yet to be explored. Activity-dependent reporter gene expression in freely behaving S100a5-tauGFP mice allowed capture of activated olfactory sensory neurons and identified 168 receptors responsive to moderate concentrations of 1 or more of 12 aliphatic (5 to 8 carbons) ketones, alcohols, and carboxylic acids. These 12 response patterns are remarkably different, with only 19% of the receptors responding to more than 1 of these odorants. This distinctiveness corresponds with the ease of discrimination of these odorants and may help maintain perceptual constancy in the face of response pattern variability, such as across odorant concentrations. This set of 168 receptors is not specific to aliphatic odorants but instead has 16% overlap with the receptors responsive to 7 odors tested previously in vivo, consistent with a receptor repertoire evolved to produce combinatorial codes. Aliphatic odorant response pattern similarity depends more upon odorant functional group than carbon chain length but the impact of chain length increases with the number of carbons. The response patterns to these aliphatic odorants are mostly composed of unrelated receptors, except some patterns contain minor subsets of closely related receptors. These findings argue that the major selective forces driving OR evolution are expansion of the odorant receptor gene family and the production of distinct response patterns.

Umami taste of L-glutamate can be synergistically amplified by the addition of some purine 5'-ribonucleotides, most notably inosine 5'-monophosphate and guanosine 5'-monophosphate (GMP). However, potential synergistic effects of other 5'-ribonucleotides, such as adenosine 5'-monophosphate (AMP), cytidine 5'-monophosphate (CMP), and uridine 5'-monophosphate (UMP), have not been well characterized. Most recently, CMP has been proposed to function as a negative modulator of glutamate taste in some US participants. Here, we examined the effects of mixing these five 5'-ribonucleotides with monopotassium L-glutamate (MPG) on MPG detection threshold and umami intensity using Japanese young female trained participants. Purine 5'-ribonucleotides (IMP, GMP, AMP) significantly decreased MPG detection threshold and enhanced umami taste intensity. UMP showed a weak but significant reduction of MPG detection threshold and a slight but significant enhancement of umami intensity. CMP, however, did not modify MPG detection threshold or umami intensity. The rank order of the effects was GMP ≧IMP > AMP > UMP. Therefore, these results did not support the hypothesis that "CMP functions as a negative modulator of glutamate taste", at least in the Japanese young female trained participants.

With the recent identification of a sour taste receptor Otopetrin1 in mice, interest in the perception of sourness has increased. However, research is limited with regard to the human response to organic acids in combination with organic salts. Additionally, the role of counterions in sourness perception remains underexplored. Here we report on several psychophysical experiments of citric acid and citrate mixtures. A total of 206 prescreened frequent consumers of sour foods and/or beverages (i.e. at least once a month) were recruited for this study. Participants rated attribute intensities on general Labeled Magnitude Scales for aqueous equimolar mixtures of varying citric acid and citrate ratios (monosodium citrate [n = 51 participants], disodium citrate [n = 50], trisodium citrate [n = 55], and tripotassium citrate [n = 50]), each at 3 different concentrations. Results revealed that variations in total concentration, amount of citric acid in solution, and type of citrate affected sourness perception and related attributes. Specifically, the number of counterions (i.e. 1, 2, 3 sodium ions) significantly affected sourness, puckering, and drying. The effect of the counterion type (i.e. sodium and potassium) also affected sourness and saltiness. These differences can be attributed to the chemical structures and the protonation states of the mixtures. Additionally, pH measurements revealed the same sourness levels were achieved at different pH levels. From this finding, solutions can be formulated to provide the same perceived sourness at a higher pH level.

In terrestrial mammals, odorant receptors and associated sensory transduction machinery in olfactory sensory neurons (OSNs) are compartmentalized in the cilia, a critically important organelle for odor detection. The large number and length of olfactory cilia provide an extensive receptive surface for odor detection. The stability of these organelles is critical for olfactory function, as damage to olfactory cilia due to environmental factors, age, or disease impairs odor detection. However, it is unclear if there are innate structural or functional features of olfactory cilia that vary between OSN subtypes and affect the fidelity of the odorant receptive field. Using ciliary-targeted fluorescent probes, we analyzed cilia morphology in live, intact OSNs in situ from mice and rats. This unbiased approach revealed a previously unappreciated constancy of average cilia length and number in OSNs across the olfactory epithelium, measures that were also independent of animal age, sex, genetic background, and even rodent species. However, average OSN cilia length did vary with the cyclic nucleotide they use to transduce olfactory stimuli: OSNs expressing the non-canonical olfactory receptor guanylate cyclase-D, which use cGMP as the second messenger, had dramatically shorter cilia than the canonical odorant receptors M71 or I7 or the trace amine-associated receptor TAAR3, each of which instead employs the second messenger cAMP. These findings suggest that differences in cyclic nucleotide signaling are associated with cilia length in OSNs. Together, the data provide a basis for understanding structure-function relationship between cilia morphology and odorant transduction as a foundation for building a high-fidelity chemosensory organ.

Although animals can reliably locate and recognize odorants embedded in complex environments, the neural circuits for accomplishing these tasks remain incompletely understood. Adaptation is likely to be important as it could allow neurons in a brain area to adjust to the broader sensory environment. Adaptive processes must be flexible enough to allow the brain to make dynamic adjustments, while maintaining sufficient stability so that organisms do not forget important olfactory associations. Processing within the mouse olfactory bulb is likely involved in generating adaptation, although there are conflicting models of how it transforms the glomerular output of the mouse olfactory bulb. Here we performed 2-photon Ca2+ imaging from mitral/tufted glomeruli in awake mice to determine the time course of recovery from adaptation, and whether it acts broadly or selectively across the glomerular population. Individual glomerular responses, as well as the overall population odor representation were similar across imaging sessions. However, odor-concentration pairings presented with interstimulus intervals upwards of 30-s evoked heterogeneous adaptation that was concentration-dependent. We demonstrate that this form of adaptation is unrelated to variations in respiration, and olfactory receptor neuron glomerular measurements indicate that it is unlikely to be inherited from the periphery. Our results indicate that the olfactory bulb output can reliably transmit stable odor representations, but recent odor experiences can selectively shape neural responsiveness for upwards of 30 seconds. We propose that neural circuits that allow for non-uniform adaptation across mitral/tufted glomeruli could be important for making dynamic adjustments in complex odor environments.

Research on odor-color associations provides insights into brain processes that integrate attributes from seemingly independent senses, forming crossmodal correspondences. Since multisensory integration is based, among others, on semantic congruency, we aimed to provide a color profile-encompassing hue, saturation, and lightness-for the standardized MONEX-40 test. This color profile will enable the use of odor-congruent or odor-incongruent colors in olfactory-visual studies. In Study 1, participants completed an online survey, selecting colors corresponding to each descriptor in the smell test. The analysis revealed distinct color patterns, highlighting the importance of learning in descriptor-color associations. Study 2 explored color associations for all 40 odors of the MONEX-40 at different time points. Initially, participants reported color associations based solely on odor perception. Then, they identified the odor from a set of four descriptors and reassigned colors after odor identification. Both qualitative and quantitative methods, including statistical tests and ΔE (CIE 2000) analysis, demonstrated shifts in color associations after odor identification, affecting hue, saturation, and lightness. This research deepens our understanding of how color associations form in response to various stimuli, including descriptors and odors. The findings have scientific relevance, as mentioned above, but also practical applications in product development, particularly in the food and cosmetics industries, where combining specific colors and odors can enhance product appeal.