Chemical Senses - current issue

Regional Specificity of Chlorhexidine Effects on Taste Perception

Grover, R., Frank, M. E. — 2008-05-09

Chlorhexidine (CHX) gluconate, a bitter bis-biguanide antiseptic, reduces the intensity of the salty taste of NaCl and bitter taste of quinine in humans. This study addresses regional specificity of CHX's effects on taste. Perceptual intensity and quality were measured for separate taste bud containing oral loci innervated either by afferent fibers of cranial nerve (CN) VII or CN IX. Measurements were obtained following three 1-min oral rinses with either 1.34 mM CHX or water, the control rinse. CHX rinse reduced the intensity of NaCl more at the tongue tip and palate than at posterior oral sites. Thus, fungiform and palatal salt-taste receptors may differ from salt-taste receptors of the foliate and circumvallate taste papillae. The intensity of quinine·HCl was reduced equally by CHX at all sites tested but was frequently tasteless on the less sensitive anterior sites, suggesting quinine receptor diversity. In rodents, a portion of NaCl-taste receptors in the receptive field of CN VII is sensitive to the epithelial Na⁺ channel blocker amiloride and a portion is amiloride insensitive; all CN IX receptors are amiloride insensitive. The current results are the first to suggest that there may also be distinct, regionally specific populations of NaCl-taste receptors in humans.

A Genome-Wide Screen for Hyposmia Susceptibility Loci

Pinto, J. M., Thanaviratananich, S., Hayes, M. G., Naclerio, R. M., Ober, C. — 2008-05-09

Olfactory dysfunction is an important public health problem in the United States, with approximately 14 million elderly Americans having chronic olfactory impairment. We performed a genome-wide linkage scan for loci influencing susceptibility to hyposmia in the Hutterites, a founder population of European ancestry. Using interviews regarding the olfactory medical history and psychophysical smell testing, we identified 25 individuals with severe hyposmia. Elimination of subjects with confounding conditions yielded 7 hyposmics for analysis. A 52-member pedigree including all affected individuals was constructed from the larger, >1623-member pedigree, and a genome-wide screen for loci influencing the trait of hyposmia using 1123 markers was performed. The most significant evidence for linkage with hyposmia extended over a 45 cM region on chromosome 4q (P = 0.0013). Although this signal meets the criteria for suggestive linkage only and will require replication, these results offer the strongest data to date on the effects of genetic variation on olfactory dysfunction.

Cholinergic Modulation of Dopaminergic Neurons in the Mouse Olfactory Bulb

Pignatelli, A., Belluzzi, O. — 2008-05-09

Considerable evidence exists for an extrinsic cholinergic influence in the maturation and function of the main olfactory bulb. In this study, we addressed the muscarinic modulation of dopaminergic neurons in this structure. We used different patch-clamp techniques to characterize the diverse roles of muscarinic agonists on identified dopaminergic neurons in a transgenic animal model expressing a reporter protein (green fluorescent protein) under the tyrosine hydroxylase promoter. Bath application of acetylcholine (1 mM) in slices and in enzymatically dissociated cells reduced the spontaneous firing of dopaminergic neurons recorded in cell-attached mode. In whole-cell configuration no effect of the agonist was observed, unless using the perforated patch technique, thus suggesting the involvement of a diffusible second messenger. The effect was mediated by metabotropic receptors as it was blocked by atropine and mimicked by the m2 agonist oxotremorine (10 µM). The reduction of periglomerular cell firing by muscarinic activation results from a membrane-potential hyperpolarization caused by activation of a potassium conductance. This modulation of dopaminergic interneurons may be important in the processing of sensory information and may be relevant to understand the mechanisms underlying the olfactory dysfunctions occurring in neurodegenerative diseases affecting the dopaminergic and/or cholinergic systems.

Xenopus V1R Vomeronasal Receptor Family Is Expressed in the Main Olfactory System

Date-Ito, A., Ohara, H., Ichikawa, M., Mori, Y., Hagino-Yamagishi, K. — 2008-05-09

To date, over 100 vomeronasal receptor type 1 (V1R) genes have been identified in rodents. V1R is specifically expressed in the rodent vomeronasal organ (VNO) and is thought to be responsible for pheromone reception. Recently, 21 putatively functional V1R genes were identified in the genome database of the amphibian Xenopus tropicalis. Amphibians are the first vertebrates to possess a VNO. In order to determine at which point during evolution the vertebrate V1R genes began to function in the vomeronasal system, we analyzed the expression of all putatively functional V1R genes in Xenopus olfactory organs. We found that V1R expression was not detected in the VNO but was specifically detected in the main olfactory epithelium (MOE). We also observed that V1R-expressing cells in the MOE coexpressed Gi2, thus suggesting that the V1R-Gi2–mediated signal transduction pathway, which is considered to play an important role in pheromone reception in the rodent VNO, exists in the amphibian MOE. These results suggest that V1R-mediated signal transduction pathway functions in Xenopus main olfactory system.

Nitric Oxide Modulates Salt and Sugar Responses via Different Signaling Pathways

Newland, P. L., Yates, P. — 2008-05-09

Locusts lay their eggs by digging into a substrate using rhythmic opening and closing movements of ovipositor valves at the end of the abdomen. The digging rhythm is inhibited by chemosensory stimulation of chemoreceptors on the valves. Nitric oxide (NO) modulated the effects of chemosensory stimulation on the rhythm. Stimulation with either sucrose or sodium chloride (NaCl) stopped the digging rhythm, whereas simultaneous bath application of the NO inhibitor, N-nitro-L-arginine methyl ester (L-NAME), increased the duration for which the digging rhythm stopped. Increasing NO levels caused a significant reduction in the cessation of the rhythm in response to the same 2 chemicals. Bath applying cyclic guanosine monophosphate (cGMP), the soluble guanylate inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), and the generic protein kinase inhibitor H-7 had no effect on the duration for which the rhythm stopped in response to NaCl stimulation. Conversely, bath application of cGMP and ODQ resulted in a significant decrease and increase, respectively, in the duration for which the digging rhythm stopped when stimulated with sucrose. Moreover, bath application of the selective protein kinase G (PKG) inhibitor KT-5823 also resulted in a significant increase in the duration of cessation of the rhythm when stimulated with sucrose. Results suggest that NO modulates the behavioral responses to NaCl via a cGMP/PKG-independent pathway while modulating the responses to sucrose via a NO-cGMP/PKG-dependent pathway.

Regional Differences in Taste Bud Distribution and {alpha}-Gustducin Expression Patterns in the Mouse Fungiform Papilla

Zhang, G.-H., Zhang, H.-Y., Deng, S.-P., Qin, Y.-M. — 2008-05-09

The regional differences between distribution patterns and -gustducin expression patterns of the fungiform (FF) taste buds were investigated in the adult mouse, using hematoxylin-eosin staining and immunofluorescence histochemistry on the most anterior region of the tongue (the first millimeter) through the intermediate region of the tongue (the last 1–4 mm). Paraffin sections were prepared from the tip to posterior regions (anterior and intermediate region containing the FF taste buds) of the adult mouse tongue. Results indicate that there were significant regional differences in size and density of taste buds, the cell counts of the single taste bud, and the -gustducin–immunoreactive taste buds between the 2 regions. The taste bud had a characteristic onion-like appearance, and the -gustducin–immunoreactive cell was spindle shaped with elongated processes extending from the base to the pore of the taste buds. These results provide a detailed insight to better understand regional descriptions of mouse taste bud density and size and -gustducin expression with the mouse model.

Synergistic Mixture Interactions in Detection of Perithreshold Odors by Humans

Miyazawa, T., Gallagher, M., Preti, G., Wise, P. M. — 2008-05-09

Laboratory demonstrations of synergistic mixture interactions in human odor perception have been rare. The current study examined perithreshold mixture interactions between maple lactone (ML) and selected carboxylic acids. An air-dilution olfactometer allowed precise stimulus control. Experimenters measured stimulus concentrations in vapor phase using a combination of solid-phase microextraction and gas chromatography/mass spectrometry. A probability of detection versus concentration, or a psychometric, functions was measured for pure ML. Psychometric functions were also measured for ML with the addition of fixed, subthreshold concentrations of carboxylic acids. Relative to statistical independence in detection, clear synergy occurred over a range of ML concentrations. To the best of our knowledge, the current results constitute the first clear demonstration of synergy in odor detection by humans from an experiment that combined precise stimulus control, vapor-phase calibration of stimuli, and a clear statistical definition of synergy.

Electrical Properties and Gustatory Responses of Various Taste Disk Cells of Frog Fungiform Papillae

Sato, T., Nishishita, K., Okada, Y., Toda, K. — 2008-05-09

We compared the electrical properties and gustatory response profiles of types Ia cell (mucus cell), Ib cell (wing cell), and II/III cell (receptor cell) in the taste disks of the frog fungiform papillae. The large depolarizing responses of all types of cell induced by 1 M NaCl were accompanied by a large decrease in the membrane resistance and had the same reversal potential of approximately +5 mV. The large depolarizing responses of all cell types for 1 mM acetic acid were accompanied by a small decrease in the membrane resistance. The small depolarizing responses of all cell types for 10 mM quinine–HCl (Q-HCl) were accompanied by an increase in the membrane resistance, but those for 1 M sucrose were accompanied by a decrease in the membrane resistance. The reversal potential of sucrose responses in all cell types were approximately +12 mV. Taken together, depolarizing responses of Ia, Ib, and II/III cells for each taste stimulus are likely to be generated by the same mechanisms. Gustatory depolarizing response profiles indicated that 1) each of Ia, Ib, and II/III cells responded 100% to 1 M NaCl and 1 mM acetic acid with depolarizing responses, 2) approximately 50% of each cell type responded to 10 mM Q-HCl with depolarizations, and 3) each approximately 40% of Ia and Ib cells and approximately 90% of II/III cells responded to 1 M sucrose with depolarizations. These results suggest that the receptor molecules for NaCl, acid, and Q-HCl stimuli are equivalently distributed on all cell types, but the receptor molecules for sugar stimuli are richer on II/III cells than on Ia and Ib cells. Type III cells having afferent synapses may play a main role in gustatory transduction and transmission.

Antennal and Behavioral Responses of Cis boleti to Fungal Odor of Trametes gibbosa

Thakeow, P., Angeli, S., Weissbecker, B., Schutz, S. — 2008-05-09

Cis boleti (Coleoptera: Ciidae) preferentially colonizes fungi from the genus Trametes that are known as important wood decomposers. The aim of our research was to investigate if C. boleti uses the chemical volatile composition of its fungal host, Trametes gibbosa, as a key attraction factor. Therefore, the T. gibbosa fruiting body volatiles were analysed by using gas chromatography–mass spectrometry, with parallel electroantennographic detection (GC-MS/EAD) using adults of C. boleti. Furthermore, we examined the behavioral responses of C. boleti to the T. gibbosa volatile compounds. The dominant component of the T. gibbosa fruiting body bouquet was 1-octen-3-ol. Other volatiles, like the aldehydes hexanal, nonanal, and (E,E)-2,4-decadienal and the terpene -bisabolol, were present in minor quantities. 1-Octen-3-ol was released with a ratio of the (R)- and (S)-enantiomers of 93:7, respectively. Electroantennography (EAG) employing C. boleti antennae yielded consistently dominant responses to 1-octen-3-ol. GC–EAD and EAG responses to pure standard compounds showed that C. boleti also perceived other host fungal volatiles. A highly significant attraction to 1-octen-3-ol was observed in behavioral tests. Female beetles were significantly attracted to the (S)-(+)- enantiomer at 10 times lower doses than male beetles. Our finding is the first direct proof that ciid beetles use 1-octen-3-ol as a key cue for host finding.

Just Noticeable Differences in Component Concentrations Modify the Odor Quality of a Blending Mixture

2008-05-09

The odors we perceive are mainly the result of mixtures of odorants that, however, are commonly perceived as single undivided entities; nevertheless, the processes involved remain poorly explored. It has been recently reported that perceptual blending based on configural olfactory processing can cause odorant mixtures to give rise to an emergent odor not present in the components. The present study examined whether specific component proportions are required to elicit an emergent odor. Starting from the composition of a ternary target mixture in which an emergent pineapple odor was perceived, 4 concentration levels of each component were chosen to elicit just noticeable differences (JNDs). Each combination of levels was used to design sample mixtures. Fifteen subjects evaluated the intensity, typicality, and pleasantness of each sample mixture against the target mixture in a paired-comparison protocol. Statistical modeling showed that a variation of less than 1 JND in one of the components was sufficient to induce a significant decrease in pineapple odor typicality in the ternary mixture. This finding confirms previous findings on perceptual blending in simple odorant mixtures and underscores the human ability to discriminate between odor percepts induced by mixtures including very similar odorant proportions.

Electroolfactogram Responses from Organotypic Cultures of the Olfactory Epithelium from Postnatal Mice

Pinato, G., Rievaj, J., Pifferi, S., Dibattista, M., Masten, L., Menini, A. — 2008-05-09

Organotypic cultures of the mouse olfactory epithelium connected to the olfactory bulb were obtained with the roller tube technique from postnatal mice aged between 13 and 66 days. To test the functionality of the cultures, we measured electroolfactograms (EOGs) at different days in vitro (DIV), up to 7 DIV, and we compared them with EOGs from identical acute preparations (0 DIV). Average amplitudes of EOG responses to 2 mixtures of various odorants at concentrations of 1 mM or 100 µM decreased in cultures between 2 and 5 DIV compared with 0 DIV. The percentage of responsive cultures was 57%. We also used the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) to trigger the olfactory transduction cascade bypassing odorant receptor activation. Average amplitudes of EOG responses to 500 µM IBMX were not significantly different in cultures up to 6 DIV or 0 DIV, and the average percentage of responsive cultures between 2 and 5 DIV was 72%. The dose–response curve to IBMX measured in cultures up to 7 DIV was similar to that at 0 DIV. Moreover, the percentage of EOG response to IBMX blocked by niflumic acid, a blocker of Ca-activated Cl channels, was not significantly different in cultured or acute preparations.