Takayuki Shoji

Receptor mechanisms of bitter substances.

The receptor mechanism of bitter substances was discussed from the following points of views. (a) Both electrostatic and hydrophobic interactions of bitter substances with taste receptor membranes contribute to reception of bitter substances having a positive charge. (b) In the frog, the responses to bitter substances are easily adapted. The presence of Ca ion in the medium prolongs the responses. (c) Bitter substances elicit electrical responses in nongustatory cells such as neuroblastoma cells and olfactory cells, suggesting that bitter substances induce the response by nonreceptor-mediated mechanism. (d) There is also a possibility that receptors for some bitter substances are G-protein coupled. We cloned G-protein coupled receptors from bovine taste tissues. (e) A specific inhibitor of bitter taste has been desired in pharmaceutical and food sciences, but it has not been available. We found that a lipoprotein made of phosphatidic acid and beta-lactoglobulin selectively inhibits the responses to bitter substances in the frog and humans. Binding of the lipoprotein to the receptor sites for bitter substances leads to suppression of the response.

Identification of c-fos related genes and their induction by neural activation in rainbow trout brain

A number of studies have shown that the induction of c-fos gene is an indicator of the responses of cells and tissues to the environmental stimuli. In the present study, using RT-PCR-based strategy, we isolated, from the brain of the rainbow trout, two partial cDNA clones (RT-fos1 and RT-fos2) that code proteins homologous to c-Fos proteins of higher vertebrates. Sequence analysis of the two clones indicated that the two rainbow trout clones are very similar to each other over the entire cloned region (88% amino acid identity) and showed moderate similarity to c-Fos proteins of higher vertebrates (40% amino acid identity with human c-Fos). Two functionally important domains (i.e. the leucine zipper and zinc finger) are highly conserved among all the vertebrate species analyzed, although the region between the two domains was highly variable between different species. Intraperitoneal administration of kainic acid, a stable agonist of glutamate receptors, transiently induced the mRNAs for both RT-fos1 and RT-fos2 in the rainbow trout brain. These results indicate that the expression pattern of the two clones can be utilized as a suitable anatomical marker for the increased neural activities in salmonid fish brain to investigate the higher order behavior such as the learning and imprinting of odors of the home river.

Olfactory Discriminating Ability of Lacustrine Sockeye and Masu Salmon in Various Freshwaters

Abstract In order to study the olfactory discriminating ability of lacustrine sockeye salmon (Oncorhynchus nerka) and masu salmon (O. masou), the integrated olfactory nerve response to various freshwaters was recorded by electrophysiological techniques. In both species independent of sex and gonadal maturity, each freshwater caused a different olfactory response. Source and effluent waters of the culture pond at Toya Lake Station (the source and culture pond waters) evoked the minimum and maximum response magnitude, respectively. In cross-adaptation experiments, the culture pond water abolished all secondary responses to other freshwaters, and no freshwater abolished the secondary response to the culture pond water. The concentration response study revealed that the minimum concentration (threshold) to induce response to the culture pond water after adaptation to Lake Toya water was between 0.1 and 1.0%. The present study indicates that the olfactory organ of lacustrine salmonids may discriminate different intensities of various freshwater odors.

Turtle olfactory responses are unchanged by perfusing olfactory epithelium with salt free solution

Abstract 1. 1. Turtle olfactory bulbar responses to various odorants were essentially unchanged by perfusing the olfactory epithelium with the salt free solution or solutions containing various concentrations of salts such as NaCl, CaCl2, choline chloride and Tris chloride. 2. 2. It was confirmed that salt concentrations on the surface of olfactory epithelium were sufficiently lowered by perfusing the epithelium with the salt free solution. 3. 3. The results obtained suggested that ion transport across cation channels located at apical membranes of olfactory cells including ciliary membranes does not contribute to generation of olfactory responses in the turtle.

High sensitivity of the turtle olfactory system to nonvolatile substances: comparison of response properties with those in gustatory systems

The olfactory responses of the aquatic turtles, Geocylemys reevesii whose nostrils are closed underwater to nonvolatile substances were measured by recording the olfactory bulbar responses. Various salts, acids and bitter substances elicited large responses, while sugars and amino acids did not elicit the responses. The thresholds for the salts were much lower than those of corresponding salts in the rat gustatory system. The responses to the salts were partially suppressed by amiloride. Various acids induced large responses and the magnitudes greatly depended on the anion species. The thresholds for the bitter substances were much lower than those of corresponding substances in the taste systems. Similar to the responses in taste systems, both electrostatic and hydrophobic interactions contribute to the binding of the substances to the receptor membranes. Similar to the taste systems, the response to quinine hydrochloride showed a sharp temperature dependence having a peak around 25 degrees C, while the responses to odorants did not show such peak. The present results suggest that the olfactory system has similar abilities to respond to salts, acids and bitter substances to those in gustatory systems and that the high sensitivity of the olfactory system to chemical stimuli is not only attributable to the second messenger amplification system, but also to the basic property of the receptor membrane independent of the amplification system.

Transduction Mechanisms in the Olfactory and Vomeronasal Organs of Turtles

It is generally believed that odorants bind to receptor proteins and activate cAMP-dependent channels located in olfactory cilia. In the present paper, we discuss whether or not this mechanism holds for in vivo transduction in the olfactory and vomeronasal organs of a turtle: (1) Elimination of salts from the olfactory epithelium did not affect the responses to odorants; changes in concentration of NaCl or CaCl2 did not affect the olfactory responses. (2) The sensitivity of the vomeronasal system to various general odorants was essentially similar to that of the olfactory system, although the vomeronasal cells have no cilia. Elimination of salts from the surface of the vomeronasal organ, similar to the olfactory system, did not affect responses to odorants. (3) Liposomes having certain lipid compositions responded to odorants with a sensitivity comparable to that in the olfactory system. The liposomes containing phosphatidylserine (PS) exhibited strong responses, especially to fatty acids. (4) Application of PS-containing liposomes to the olfactory epithelium greatly enhanced the responses to fatty acids. The results suggest that the cation channels located at olfactory cilia may not contribute to in vivo olfactory transduction. The results also suggest that lipids in olfactory receptor membranes are important in odor reception.

Molecular recognition and transduction in chemoreceptor systems

Abstract Gustatory and olfactory cells are typical chemical sensors in biological systems, which recognize various species of chemicals in external environments. This paper reviews recent progress in receptor mechanisms of tast and olfaction. It is considered that the initial event of olfactory reception is the binding of odorants to specific receptor proteins in the olfactory cilia. The data presented, however, suggest that lipid layers of the receptor membranes also play an important role in odour reception. For example, membrane fluidity changes induced by a temperature increase of up to 40 °C abolished the ability of olfactory receptors to discriminate odorants having similar structures. Treatment of the olfactory epithelium with phosphatidylserine greatly enhanced the olfactory response to certain species of odorants. The transduction mechanisms of odor reception are also discussed. The paper also deals with characteristics of receptor sites for taste stimuli, the ion dependence of taste responses, a...

Long-term recording of olfactory and vomeronasal stimulant-induced waves from the turtle main olfactory bulb and accessory olfactory bulb.

Recording of stimulant-induced waves (bulbar responses) from the main olfactory bulb is a useful tool for measuring quantitative stable olfactory responses. There is a good relationship between the olfactory bulbar response, olfactory nerve response and electroolfactogram (EOG), suggesting that the bulbar response reflects events in receptor cells. The modern whole-cell recording technique offers direct information on olfactory transduction in single cells, but it requires long experimental periods and many animals. On the other hand, analysis of bulbar responses provides useful information and requires the use of few animals. For example, we found that cAMP-increasing and IP3-increasing odorants were not distinctly received by the turtle olfactory organ by measuring olfactory bulbar responses and analyzed with a multidimensional scaling from about 60 animals. However, to record similar odor responses from isolated turtle olfactory neurons, at least 200 animals would be necessary. Bulbar responses are recorded with electrodes implanted into or located on the main olfactory bulb. When electrodes are located on the olfactory bulb surface, it is possible to record stable responses over a period of 3 days. These methods were applied successfully to the accessory olfactory bulb. In this paper, we describe the protocols used for recording of the stimulant-induced waves from the main and accessory olfactory bulb.

Olfactory Responses of Rainbow Trout Were Unchanged When Fish Was Adapted from Fresh Water to Seawater

Various species of fish migrate between river and sea. Salinity on the olfactory receptor membranes varies greatly in such migration. In order to explore olfactory transduction mechanisms in fish, it is interesting to know how olfactory responses are affected by such large changes in environmental salinity.

Transduction Mechanism in Vertebrate Olfactory Systems

It is generally considered that odorants bind to specific receptor proteins (Buck and Axel, 1991) and activate cAMP-sensitive channels located in olfactory cilia (Pace et al., 1985; Nakamura and Gold, 1987). The present paper discusses whether or not the above mechanism holds in the in vivo olfactory system.