大鼠脑桥臂旁核味觉神经元的生理学特性

采用微电极细胞外记录技术 ,观察和分析了大鼠脑桥臂旁核 (Parabrachialnuclei,PbN)味觉神经元的反应特征。记录到的 5 5个PbN味觉神经元中 ,绝大部分位于脑桥味区并有自发放电。大部分PbN味觉神经元对多种基本味觉刺激起反应 ,其中对氯化钠反应的频率最高。PbN味觉神经元对蔗糖的反应和对其他味觉反应的相关性都较低。根椐最适刺激 ,PbN味觉神经元可分为 :氯化钠优势反应、盐酸优势反应、奎宁优势反应和蔗糖优势反应神经元。结果提示 ,PbN中存在不同类型的神经元 ,它们可能在味觉的传递和编码中发挥着重要作用。     

Regulation of bitter taste responses by tumor necrosis factor

Inflammatory cytokines are important regulators of metabolism and food intake. Over production of inflammatory cytokines during bacterial and viral infections leads to anorexia and reduced food intake. However, it remains unclear whether any inflammatory cytokines are involved in the regulation of taste reception, the sensory mechanism governing food intake. Previously, we showed that tumor necrosis factor (TNF), a potent proinflammatory cytokine, is preferentially expressed in a subset of taste bud cells. The level of TNF in taste cells can be further induced by inflammatory stimuli. To investigate whether TNF plays a role in regulating taste responses, in this study, we performed taste behavioral tests and gustatory nerve recordings in TNF knockout mice. Behavioral tests showed that TNF-deficient mice are significantly less sensitive to the bitter compound quinine than wild-type mice, while their responses to sweet, umami, salty, and sour compounds are comparable to those of wild-type controls. Furthermore, nerve recording experiments showed that the chorda tympani nerve in TNF knockout mice is much less responsive to bitter compounds than that in wild-type mice. Chorda tympani nerve responses to sweet, umami, salty, and sour compounds are similar between TNF knockout and wild-type mice, consistent with the results from behavioral tests. We further showed that taste bud cells express the two known TNF receptors TNFR1 and TNFR2 and, therefore, are potential targets of TNF. Together, our results suggest that TNF signaling preferentially modulates bitter taste responses. This mechanism may contribute to taste dysfunction, particularly taste distortion, associated with infections and some chronic inflammatory diseases.     

Taste coding in the nucleus of the solitary tract of the awake, freely licking rat

It is becoming increasingly clear that the brain processes sensory stimuli differently according to whether they are passively or actively acquired, and these differences can be seen early in the sensory pathway. In the nucleus of the solitary tract (NTS), the first relay in the central gustatory neuraxis, a rich variety of sensory inputs generated by active licking converge. Here, we show that taste responses in the NTS reflect these interactions. Experiments consisted of recordings of taste-related activity in the NTS of awake rats as they freely licked exemplars of the five basic taste qualities (sweet, sour, salty, bitter, umami). Nearly all taste-responsive cells were broadly tuned across taste qualities. A subset responded to taste with long latencies (>1.0 s), suggesting the activation of extraoral chemoreceptors. Analyses of the temporal characteristics of taste responses showed that spike timing conveyed significantly more information than spike count alone in almost one-half of NTS cells, as in anesthetized rats, but with less information per cell. In addition to taste-responsive cells, the NTS contains cells that synchronize with licks. Since the lick pattern per se can convey information, these cells may collaborate with taste-responsive cells to identify taste quality. Other cells become silent during licking. These latter "antilick" cells show a surge in firing rate predicting the beginning and signaling the end of a lick bout. Collectively, the data reveal a complex array of cell types in the NTS, only a portion of which include taste-responsive cells, which work together to acquire sensory information.     

Genetic tracing of the gustatory and trigeminal neural pathways originating from T1R3-expressing taste receptor cells and solitary chemoreceptor cells

We established transgenic mouse lines expressing a transneuronal tracer, wheat germ agglutinin (WGA), under the control of mouse T1R3 gene promoter/enhancer. In the taste buds, WGA transgene was faithfully expressed in T1R3-positive sweet/umami taste receptor cells. WGA protein was transferred not laterally to the synapse-bearing, sour-responsive type III cells in the taste buds but directly to a subset of neurons in the geniculate and nodose/petrosal ganglia, and further conveyed to a rostro-central region of the nucleus of solitary tract. In addition, WGA was expressed in solitary chemoreceptor cells in the nasal epithelium and transferred along the trigeminal sensory pathway to the brainstem neurons. The solitary chemoreceptor cells endogenously expressed T1R3 together with bitter taste receptors T2Rs. This result shows an exceptional signature of receptor expression. Thus, the t1r3-WGA transgenic mice revealed the sweet/umami gustatory pathways from taste receptor cells and the trigeminal neural pathway from solitary chemoreceptor cells.     

Restoration of quinine‐stimulated fos‐immunoreactive neurons in the central nucleus of the amygdala and gustatory cortex following reinnervation or cross‐reinnervation of the lingual taste nerves in rats

Remarkably, when lingual gustatory nerves are surgically rerouted to inappropriate taste fields in the tongue, some taste functions recover. We previously demonstrated that quinine‐stimulated oromotor rejection reflexes and neural activity (assessed by Fos immunoreactivity) in subregions of hindbrain gustatory nuclei were restored if the posterior tongue, which contains receptor cells that respond strongly to bitter compounds, was cross‐reinnervated by the chorda tympani nerve. Such functional recovery was not seen if instead, the anterior tongue, where receptor cells are less responsive to bitter compounds, was cross‐reinnervated by the glossopharyngeal nerve, even though this nerve typically responds robustly to bitter substances. Thus, recovery depended more on the taste field being reinnervated than on the nerve itself. Here, the distribution of quinine‐stimulated Fos‐immunoreactive neurons in two taste‐associated forebrain areas was examined in these same rats. In the central nucleus of the amygdala (CeA), a rostrocaudal gradient characterized the normal quinine‐stimulated Fos response, with the greatest number of labeled cells situated rostrally. Quinine‐stimulated neurons were found throughout the gustatory cortex, but a “hot spot” was observed in its anterior–posterior center in subregions approximating the dysgranular/agranular layers. Fos neurons here and in the rostral CeA were highly correlated with quinine‐elicited gapes. Denervation of the posterior tongue eliminated, and its reinnervation by either nerve restored, numbers of quinine‐stimulated labeled cells in the rostralmost CeA and in the subregion approximating the dysgranular gustatory cortex. These results underscore the remarkable plasticity of the gustatory system and also help clarify the functional anatomy of neural circuits activated by bitter taste stimulation. J. Comp. Neurol. 522:2498–2517, 2014. © 2014 Wiley Periodicals, Inc.     

The Test–Retest Reliability of Fatty Acid Taste Thresholds

Emerging evidence supports the existence of a fat specific oral detection system activated by fatty acids, which conveys the presence of fat in foods. Stability in psychophysical measurement of fatty acids is an essential step in supporting the existence of an oral fat detection system as well as supporting the association between fatty acid taste and development of obesity. This study aimed to determine the test–retest reliability of oral fatty acid thresholds. Seventeen subjects (eight males, age 31?±?2.3 years, BMI 22.9?±?0.6 kg/m², nine females, age 29?±?1.8 years, BMI 23.4?±?0.9 kg/m²) attended 30 laboratory sessions to determine oral detection thresholds for oleic acid (C18:1), linoleic acid (C18:2) and lauric acid (C12:0). Taste thresholds were also performed using sucrose (sweet), citric acid (sour), sodium chloride (salty), caffeine (bitter) and monosodium glutamate (umami). Each stimulus was evaluated on six occasions using ascending forced choice triangle tests, over 2 days. Diet records were also collected prior to each testing session. Fatty acid taste thresholds were determined for all subjects and strong intra-class correlations were found for within day and across day testing sessions for C18:1, C18:2 and C12:0. The strongest correlations were found for across day testing for C18:1 [intra-class correlation (ICC)?=?0.78, confidence interval (CI)?=?0.49–0.91], C18:2 (ICC?=?0.94, CI?=?0.84–0.98) and C12:0 (ICC?=?0.80, CI?=?0.54–0.92). Strong correlations were also found for sweet, sour, salty, bitter and umami tastes (ICC range, 0.7–0.9). This study provides evidence supportive of an oral fatty acid specific detection system.     

Glutamate-induced cobalt uptake reveals non-NMDA receptors in rat taste cells

Taste receptor cells are chemical detectors in the oral cavity. Taste cells form synapses with primary afferent neurons that convey the gustatory information to the central nervous system. Taste cells may also synapse with other taste cells within the taste buds. Furthermore, taste cells may receive efferent connections. However, the neurotransmitters at these synapses have not been identified. Glutamate, a major excitatory neurotransmitter in other sensory organs, might act at synapses in taste buds. We used a cobalt staining technique to detect Ca(2+)-permeable glutamate receptors in taste buds and thus establish whether there might be glutamatergic synapses in gustatory end organs. When 500 microm slices of foliate and vallate papillae were briefly exposed to 1 mM glutamate in the presence of CoCl(2), a subset of spindle-shaped taste cells accumulated Co(2+). Cobalt uptake showed concentration-dependency in the range from 10 microm to 1 mM glutamate. Interestingly, higher glutamate concentrations depressed cobalt uptake. This concentration-response relation for cobalt uptake suggests that synaptic glutamate receptors, not receptors for glutamate taste, were activated. Sensory axons and adjacent non-sensory epithelium were not affected by these procedures. Glutamate-stimulated cobalt uptake in taste cells was antagonized by the non-NMDA receptor antagonist CNQX. Depolarization with 50 mM K(+) and application of NMDA (300 microM) did not increase the number of stained taste cells. This pharmacological characterization of the cobalt uptake suggests that non-NMDA receptors are present in taste cells. These receptors might be autoreceptors at afferent synapses, postsynaptic receptors of a putative efferent system, or postsynaptic receptors at synapses with other taste cells.     

Differential Perception of Caffeine Bitter Taste Depending on Smoking Status

Taste impairment may be associated with tobacco smoking. We assessed taste recognition and intensity among healthy middle-aged current smokers (N?=?94, 21 %), former smokers (N?=?48, 11 %) and non-smokers (N?=?309, 69 %) recruited on a voluntary basis among hospital staff. By means of a whole-mouth gustatory test, participants tasted one concentration of the four basic tastes (NaCl 34 mM, sucrose 58 mM, acetic acid 60 mM, caffeine 1.5 mM for salty, sweet, sour and bitter tastes, respectively) and completed a questionnaire for taste recognition and intensity (ranging from 0 to 10). The recognition of salty, sweet and sour tastes was not influenced by smoking status. Bitter taste recognition was wrong among 13.4, 19.8 and 26.5 % of non-smokers, current smokers and former smokers, respectively (p?=?0.043). The adjusted odds ratio (95 % confidence interval) of correct bitter taste recognition was 0.31 (0.14–0.69) among former and 0.74 (0.35–1.55) among current smokers (p?=?0.016), compared to non-smokers while adjusting for gender, age, year of assessment and bitter taste intensity. The distribution of caffeine’s bitter taste intensity was bimodal regardless of the smoking status. The differential perception of caffeine’s bitter taste by current and former smokers is likely to be caused by a toxic process. As taste impairment persists in former smokers, the bioaccumulation of some tobacco/combustion products might be responsible for the disequilibrium in taste buds regeneration.     

5‐HT3A‐driven green fluorescent protein delineates gustatory fibers innervating sour‐responsive taste cells: A labeled line for sour taste?

Taste buds contain multiple cell types with each type expressing receptors and transduction components for a subset of taste qualities. The sour sensing cells, Type III cells, release serotonin (5‐HT) in response to the presence of sour (acidic) tastants and this released 5‐HT activates 5‐HT₃ receptors on the gustatory nerves. We show here, using 5‐HT_3AGFP mice, that 5‐HT₃‐expressing nerve fibers preferentially contact and receive synaptic contact from Type III taste cells. Further, these 5‐HT₃‐expressing nerve fibers terminate in a restricted central‐lateral portion of the nucleus of the solitary tract (nTS)—the same area that shows increased c‐Fos expression upon presentation of a sour tastant (30 mM citric acid). This acid stimulation also evokes c‐Fos in the laterally adjacent mediodorsal spinal trigeminal nucleus (DMSp5), but this trigeminal activation is not associated with the presence of 5‐HT₃‐expressing nerve fibers as it is in the nTS. Rather, the neuronal activation in the trigeminal complex likely is attributable to direct depolarization of acid‐sensitive trigeminal nerve fibers, for example, polymodal nociceptors, rather than through taste buds. Taken together, these findings suggest that transmission of sour taste information involves communication between Type III taste cells and 5‐HT₃‐expressing afferent nerve fibers that project to a restricted portion of the nTS consistent with a crude mapping of taste quality information in the primary gustatory nucleus.     

Expression of the neural cell adhesion molecule (NCAM) and polysialic acid during taste bud degeneration and regeneration

Taste receptor cells are replaced throughout life, accompanied by continuing synaptogenesis between newly formed taste cells and first-order gustatory fibers. The neural cell adhesion molecule (NCAM) is expressed by a subset of taste cells in adult rodents and appears on gustatory nerve fibers during development prior to differentiation of the taste buds. We employed antibodies against the extracellular domain of the NCAM polypeptide (mAb 3F4) and against polysialic acid (PSA) residues found on embryonic forms of NCAM (mAb 5A5) to investigate the relationship between the expression of these molecules and the innervation of taste buds in adult rats. In unoperated rats, anti-NCAM recognized a subset of cells within the vallate taste buds and also the fibers of the glossopharyngeal (IXth) nerve, including those innervating the gustatory epithelium. Taste bud cells did not express PSA but mAb 5A5 immunoreactivity was observed on some fibers of the IXth nerve, including a few that entered the taste buds. Bilateral crush of the IXth nerve resulted in the loss of NCAM expression from the gustatory epithelium within 8 days. As IXth nerve fibers reinnervated the epithelium, NCAM expression was seen first in the nerve, followed by increased expression in the epithelium as the taste cells differentiated from their precursors. PSA expression by fibers of the IXth nerve did not return to normal until well after the regeneration of the vallate taste buds. The present results demonstrate that taste cell expression of NCAM is dependent upon innervation by the IXth nerve and that NCAM expression appears in the nerve prior to its expression in the differentiating epithelium during regeneration. The occurrence of a similar temporal sequence in the developing taste system suggests that NCAM could play a role in cell-cell interactions that are important for the differentiation of the taste epithelium. Ongoing taste cell turnover and synaptogenesis between IXth nerve fibers and newly differentiating taste cells also requires recognition and adhesion, in which NCAM could play a role. © 1994 Wiley-Liss, Inc.     

Morphologic characterization of rat taste receptor cells that express components of the phospholipase C signaling pathway

Rat taste buds contain three morphologically distinct cell types that are candidates for taste transduction. The physiologic roles of these cells are, however, not clear. Inositol 1,4,5-triphosphate (IP(3)) has been implicated as an important second messenger in bitter, sweet, and umami taste transductions. Previously, we identified the type III IP(3) receptor (IP(3)R3) as the dominant isoform in taste receptor cells. In addition, a recent study showed that phospholipase Cbeta(2) (PLCbeta(2)) is essential for the transduction of bitter, sweet, and umami stimuli. IP(3)R3 and PLCbeta(2) are expressed in the same subset of cells. To identify the taste cell types that express proteins involved in PLC signal transduction, we used 3,3'diaminobenzidine tetrahydrochloride immunoelectron microscopy and fluorescence microscopy to identify cells with IP(3)R3. Confocal microscopy was used to compare IP(3)R3 or PLCbeta(2) immunoreactivity with that of some known cell type markers such as serotonin, protein gene-regulated product 9.5, and neural cell adhesion molecule. Here we show that a large subset of type II cells and a small subset of type III cells display IP(3)R3 immunoreactivity within their cytoplasm. These data suggest that type II cells are the principal transducers of bitter, sweet, and umami taste transduction. However, we did not observe synapses between type II taste cells and nerve fibers. Interestingly, we observed subsurface cisternae of smooth endoplasmic reticulum at the close appositions between the plasma membrane of type II taste cells and nerve processes. We speculate that some type II cells may communicate to the nervous system via subsurface cisternae of smooth endoplasmic reticulum in lieu of conventional synapses.     

Neuron/target plasticity in the peripheral gustatory system

Taste bud volume on the anterior tongue in adult rats is matched by an appropriate number of innervating geniculate ganglion cells. The larger the taste bud, the more geniculate ganglion cells that innervate it. To determine if such a match is perturbed in the regenerated gustatory system under different dietary conditions, taste bud volumes and numbers of innervating neurons were quantified in adult rats after unilateral axotomy of the chorda tympani nerve and/or maintenance on a sodium-restricted diet. The relationship between taste bud size and innervation was eliminated in rats merely fed a sodium-restricted diet; individual taste bud volumes were smaller than predicted by the corresponding number of innervating neurons. Surprisingly, the relationship was disrupted in a similar way on the intact side of the tongue in unilaterally sectioned rats, with no diet-related differences. The mismatch in these groups was due to a decrease in average taste bud volumes and not to a change in numbers of innervating ganglion cells. In contrast, individual taste bud volumes were larger than predicted by the corresponding number of innervating neurons on the regenerated side of the tongue; again, with no diet-related differences. However, the primary variable responsible for disrupting the function on the regenerated side was an approximate 20% decrease in geniculate ganglion cells available to innervate taste buds. Therefore, the neuron/target match in the peripheral gustatory system is susceptible to surgical and/or dietary manipulations that act through multiple mechanisms. This system is ideally suited to model sensory plasticity in adults.     

Transient receptor potential channel M5 and phospholipaseC-beta2 colocalizing in zebrafish taste receptor cells

In mammals, transient receptor potential (TRP) channel M5 (TRPM5) is coexpressed with phospholipaseC-beta2 (PLC-beta2) in the taste receptor cells, and both PLC-beta2 and TRPM5 are essential elements in the signal transduction of sweet, bitter and umami stimuli. In this study, we identified the zebrafish homologue of TRPM5 (zfTRPM5) and examined its expression in the gustatory system by in-situ hybridization. Using a transgenic zebrafish line that expressed green fluorescent protein under the control of the PLC-beta2 promoter, we showed that zfTRPM5 is expressed in green fluorescent protein-labeled cells of the taste buds. These results demonstrate that zfTRPM5 and PLC-beta2 colocalize in zebrafish taste receptor cells, suggesting their crucial roles in taste signaling via the fish taste receptors.     

Intranasal Insulin Boosts Gustatory Sensitivity

Intranasal insulin has been the subject of attention not only with respect to enhancing memory processes, but also for its anorexic effects, as well as its effects on olfactory sensitivity. In the present study, the influence of intranasal insulin on gustatory sensitivity was investigated using intranasal applications of insulin or placebo in a double‐blind manner alongside a control condition without any application. We hypothesised that, because it mediates satiety, intranasal insulin alters gustatory sensitivity, whereas placebo application and the control should not alter gustatory sensitivity. We did not expect the sensitivity to the different taste solutions to differ. Sweet, salty, bitter and sour liquids in four concentrations each were sprayed onto the tongue of healthy male subjects. Additionally, water with no taste was applied to enable calculation of taste sensitivity in terms of parameter d′ of signal detection theory. The task of the subject was to identify the quality of the respective tastant. Gustatory sensitivity and blood parameters were evaluated using repeated‐measures ANOVAs. Gustatory sensitivity (implying all tastants) improved significantly after intranasal insulin application compared to the application of placebo, although it did not reach significance compared to the control condition. Subjects performed best when detecting the sweet taste and worst when detecting the bitter taste. The blood parameters glucose, insulin, homeostatic model assessment and leptin did not differ with respect to insulin or placebo condition, nor did they differ regarding measurements preceding or following intranasal application, in confirmation of preserved peripheral euglycaemia during the experiment. Thus, it can be concluded that the application of intranasal insulin led to an improved gustatory sensitivity compared to placebo.     

Regeneration of taste buds in tongue grafts after reinnervation by neurons in transplanted lumbar sensory ganglia

Taste buds disappear after denervation and reappear after nerve regeneration. Sensory neurons are responsible since reinnervation by motor or autonomic fibers of peripheral nerve fail to induce bud regeneration. However, we do not know whether some neurons in all sensory ganglia can support buds or whether gustatory (i.e., taste bud inducing) neurons are localized to specific cranial ganglia. The present study was therefore pefrormed to determine whether neurons in transplanted spinal ganglia could support taste buds similarly to those in transplanted cranial ganglia. Grafts of lumbar or vagal nodose ganglia were combined with grafts of tongue's vallate papillae in the anterior chamber of rats' eyes and the papillae examined for taste buds 35 days later. Neurons were present in all transplanted ganglia, and all papillae reinnervated by them contained regenerated taste buds. Nerve fibers could be traced from the transplanted ganglia to the epithelium of the tongue grafts which bore the regenerated taste buds. Papillae transplanted without ganglia lacked buds. These findings indicate that some neurons in all sensory ganglia can induce taste bud formation. The present results could occur if gustatory neurons are intrinsically present in all sensory ganglia, but an alternative interpretation is that the tongue grafts transformed some neurons into gustatory neurons and, hence, that neuronal plasticity is involved.