Mechanisms of chemosensory transduction in taste cells

Classification of genes and putative biomarker identification using distribution metrics on expression profiles

Background

Identification of genes with switch-like properties will facilitate discovery of regulatory mechanisms that underlie these properties, and will provide knowledge for the appropriate application of Boolean networks in gene regulatory models. As switch-like behavior is likely associated with tissue-specific expression, these gene products are expected to be plausible candidates as tissue-specific biomarkers.

Methodology/Principal Findings

In a systematic classification of genes and search for biomarkers, gene expression profiles (GEPs) of more than 16,000 genes from 2,145 mouse array samples were analyzed. Four distribution metrics (mean, standard deviation, kurtosis and skewness) were used to classify GEPs into four categories: predominantly-off, predominantly-on, graded (rheostatic), and switch-like genes. The arrays under study were also grouped and examined by tissue type. For example, arrays were categorized as ‘brain group’ and ‘non-brain group’; the Kolmogorov-Smirnov distance and Pearson correlation coefficient were then used to compare GEPs between brain and non-brain for each gene. We were thus able to identify tissue-specific biomarker candidate genes.

Conclusions/Significance

The methodology employed here may be used to facilitate disease-specific biomarker discovery.

Astringency: mechanisms and perception

Astringency plays an important role in the sensory experience of many foods and beverages, ranging from wine to nuts. Given the recent trend toward fortifying consumables with astringent compounds and the evidence regarding the health benefits of some astringents, the mechanisms and perceptual characteristics of astringency warrant further discussion and investigation. This paper reviews the current state of the literature, including consideration of new methods for describing and measuring astringency, and provides an overview of research concerned with elucidating the physical, physiological, and psychological factors that underlie and mediate perception of this sensation.

Neurobiology of the gustatory systems of Drosophila and some terrestrial insects

Insects have been favorites for the study of taste perception in the last few decades. They have been used for anatomical, behavioral, developmental, genetic, and physiological studies related to gustation and feeding response. Several genes known to affect the formation of gustatory sensilla or alter the feeding behavior of insects such as Drosophila are known. Studies related to signal transduction, coding of gustatory information, and the nature and constitution of genes involved in taste perception have also been taken up with insects in recent years. The understanding of basic mechanisms of taste perception in insects is likely to lead to better management of useful as well as harmful insects.

Effect of lipid-derived second messengers on electrophysiological taste responses in the gerbil

Integrated chorda tympani (CT) recordings were made to salty, sour, sweet, bitter, and glutamate tastants before and after a 4-min application of modulators of lipid-derived second messenger systems. The modulators included two membrane-permeable analogues of DAG, 1-oleoyl-2-acetyl glycerol (OAG) and dioctanoyl glycerol (DiC8); thapsigargin, which releases Ca++ from intracellular stores; ionomycin, a calcium ionophore; lanthanum chloride, an inorganic calcium channel blocker; nifedipine, a dihydropyridine calcium channel blocker; quinacrine diHCl, a phospholipase A2 antagonist; melittin, a phospholipase A2 agonist; and indomethacin, which decreases the release of prostaglandins by inhibiting the enzyme cyclo-oxygenase. The main findings were: OAG (125 microM) and DiC8 (100 microM) blocked the responses of several bitter compounds while enhancing the taste response to several sweeteners. Lanthanum chloride blocked all responses, which may be due to the fact that it blocks tight junctions. Quinacrine (1 mM) suppressed several bitter responses while enhancing the response to several sweeteners. The enhancement of sweet taste responses by DAG analogues suggests that there is cross-talk between the adenylate cyclase system and one (or more) pathways involving lipid-derived second messengers in taste cells.

Detection thresholds of potassium salts are related to the molar conductivity of the anion

In a previous study, we found that human taste detection thresholds for Na+ salts were linearly correlated with molar conductivity values at infinite dilution of their anions. In the present study, detection threshold concentrations for potassium salts were also found to be linearly correlated (r = -0.92) with the molar conductivity of the anion of the salt. Detection thresholds were determined here for nine potassium salts with the same anions as the sodium salts previously tested. The mean detection thresholds for these potassium salts were found to be: K Acetate (0.00311 M), K Carbonate (0.00286 M), K Chloride (0.00242 M), K Citrate (0.000300 M), K Phosphate (0.00196 M), K Sulfate (0.00090 M), K Tartrate (0.00164 M), K Glutamate (0.00153 M), and K Ascorbate (0.00375 M). The rank order correlation between the detection threshold values for sodium and potassium salts was 0.88. This finding suggests that detection thresholds for both Na+ and K+ salts are determined by the charge mobility of the anion.

Application of serial sectioning and three-dimensional reconstruction to the study of taste bud ultrastructure and organization

The lingual taste buds of mammals are complex organs containing dozens of cells of varying morphology and numerous nerve fibers that are intermingled among the cellular processes. Some of the taste bud cells form synaptic contacts with these nerve fibers. Important questions remain to be answered regarding the structure and function of the cells of various types within taste buds and the means by which responses to gustatory stimuli are transmitted to the nerve fibers that communicate with the brain. Using both conventional and high voltage electron microscopy, we have examined serially sectioned taste buds from the tongues of mice and rabbits in order to address these issues and to obtain more complete information than that available from sampling of sections. The technique of computer-assisted 3-D reconstruction was used to generate models of whole taste buds and individual cellular and neural elements within taste buds from the serial sections. Analysis of serially sectioned taste buds from mice and rabbits has revealed that in both of these species relatively few (30% or less) of the cells within the taste buds form synaptic contacts with nerve fibers. In the foliate taste buds of rabbits, all of the cells that are presynaptic to nerve fibers are of a single morphological type (type III). The cells that are presynaptic to nerve fibers within the taste buds of mice are morphologically diverse. A pattern of synaptic connectivity exists within murine taste buds such that a given nerve fiber receives synaptic input only from taste cells that are ultrastructurally similar. In the taste buds of both mice and rabbits, we have observed both divergence and convergence of synaptic input from the putative taste receptor cells onto nerve fibers, suggesting that at the level of the taste bud there is some integration of the information generated by individual receptor cells. In addition to typical chemical synapses, other cytoplasmic specializations (such as subsurface cisternae and atypical mitochondria) may be involved in interactions between taste bud cells and nerve fibers.

Information processing in mammalian gustatory systems

The taste system has multiple functions that are carried by three cranial nerves. It is now apparent that these functions cannot be accommodated by a single coding mechanism for taste quality. A current view emphasizes the likely existence of coding channels activated by specific sets of receptors.

Effects of inorganic constituents of saliva on taste responses of the rat chorda tympani nerve

The effects of saliva on the taste responses of the chorda tympani nerve to the 4 standard chemical stimuli (sucrose, NaCl, HCl, and quinine hydrochloride) and water were investigated in anesthetized rats. When the tongue was adapted to pilocarpine-stimulated whole saliva (pH 8.7), the magnitude of neural response to sucrose was about 2 times that obtained when the tongue was adapted to distilled water. Under saliva-adapted conditions, the magnitude of responses to other taste stimuli was reduced by 10-30%, and the water response appeared. These changes were dependent on the concentration of electrolytes (Na+, K+, Cl-, and HCO3-) and on the pH of the saliva. When the tongue was adapted to 10-30 mM NaHCO3 (pH 8.4-8.6), taste and water responses were similar to those under saliva-adapted conditions. Single fiber analyses revealed that the enhancement of the sucrose response after adaptation to NaHCO3 was produced by an increased overall activity of sucrose-responsive fibers. The correlation coefficients of the magnitude of the taste responses between the 4 taste stimuli remained unchanged, but the water response showed a high correlation to HCl and quinine hydrochloride responses after adaptation. Possible mechanisms for the effects of saliva on taste and water responses were discussed.