Taste neurons

Taste begins with molecular events at the surface membranes of modified epithelial cells, which share many characteristics with neurons. Taste receptor cells are organized within specialized structures—taste buds— on the tongue and other parts of the oral cavity [for reviews see Kinnamon (1987), Reutter and Witt (1993)]. As described later, taste buds contain cells of different types, which can be distinguished on the basis of their ultrastructure and their molecular phenotypes. 

It is interesting that the stimulus compounds used in the study differ widely in their molecular structures, and yet they all interact with antibodies to thaumatin. It is, therefore, probable that a single receptor-structure responds to all sweet stimuli,there being a variation in the relative effectiveness of sweet stimuli across individual nerve-fibers, and the characteristics of all receptor sites do not appear to be identical. Earlier elec-trophysiological studies of single primary, afferent taste-neurons uniformly agreed that individual fibers very often have multiple sensitivities, and that individual, gustatory receptors are part of the receptive field of more than one afferent fiber. " We have yet to learn how these interact, and the nature of their excitatory, or possible inhibitory, relations, or both. 

The oscillation of membrane current or membrane potential is well-known to occur in biomembranes of neurons and heart cells, and a great number of experimental and theoretical studies on oscillations in biomembranes as well as artificial membranes [1,2] have been carried out from the viewpoint of their biological importance. The oscillation in the membrane system is also related to the sensing and signal transmission of taste and olfaction. Artificial oscillation systems with high sensitivity and selectivity have been pursued in order to develop new sensors [3-8]. 

Taste cells have multiple types of ion channels. TRCs are electrically excitable and capable of generating action-potentials voltage-dependent channels for Na+, Ca2+ and K+, similar to those in neurons, have been detected in vertebrate TRCs. The surface distribution of these channels... 

Researchers have oscillated between emphasizing specificity of neurons ( labeled lines ) and responses to a spectrum of tastants by one cell. More recently, patterns of activation of a number of sensory cells are favored for coding specific taste sensations (Smith and Margolskee, 2001). Neural distinction of different tastes requires simultaneous activation of different cell types. The brain receives a single channel of information, simply bitter for a number of different compounds. 

The taste cells are situated in the lingual epithelium with the apical membrane exposed to the mucosal surface of the oral cavity and the basal surface in contact with the nerve [interstitial fluid] [FIGURE 10]. Within the basolateral surface are the nerves which respond to the chemestiietic stimulants, i.e. direct nerve stimulation. The microvilli at the apical membrane contain receptor proteins which respond to sweeteners, some bitters and possibly coolants. The olfactory cells are bipolar neurons with dendritic ends containing cilia exposed to the surface and axons linked to the brain, where they synapse in the olfactory bulb. The transfer of information from this initial stimulus-receptor interaction to the brain processing centers involves chentical transduction steps in the membrane and within the receptor cells. The potential chemical interactions at the cell membrane and within the cell are schematically outlined in FIGURE 10. 

Bray, S. and Amrein, H. A. (2003). Putative Drosophila pheromone receptor expressed in male-specific taste neurons is required for efficient courtship. Neuron 39 1019-1029. 

The detection of light, smells, and tastes (vision, olfaction, and gustation, respectively) in animals is accomplished by specialized sensory neurons that use signal-transduction mechanisms fundamentally similar to those that detect hormones, neurotransmitters, and growth... 

One method to realize the taste sensor may be the utilization of similar materials to biological systems as the transducer. The biological membrane is composed of proteins and lipids. Proteins are main receptors of taste substances. Especially for sour, salty, or bitter substances, the lipid-membrane part is also suggested to be the receptor site [6]. In biological taste reception, taste stimulus changes the receptor potentials of taste cells, which have various characteristics in reception [7,8]. Then the pattern constructed of receptor potentials is translated into the excitation pattern in taste neurons (across-fiber-pattem theory). 

Derby, C. D., Single unit electrophysiological recordings from crustacean chemoreceptor neurons, in Experimental Biology of Taste and Olfaction Current Techniques and Protocols, Spielman, A. I. and Brand, J. G., Eds., CRC Press, Boca Raton, FL, 1995, 241. 

Heimbeck G., Bugnon V., Gendre N., Haberlin C. and Stocker R. F. (1999) Smell and taste perception in Drosophila melanogaster larva toxin expression studies in chemosensory neurons. J. Neurosci. 19, 6599-6609. 

Insect chemosensory organs have been differentially developed for taste and olfactory sensing. The contact and the distant chemosensory sensilla are responsible for nonvolatile and volatile chemical reception, respectively. The CHCs with long carbon chains are non-volatile, and therefore thought to be received by taste sensilla (Ebbs and Amrein, 2007). However, because of their insolubility in water, it was very difficult to obtain response recordings to them from taste sensilla. Success was recently obtained, however, in Drosophila melanogaster, where a male-specific CHC as a sex-pheromone inhibiting male-male courtship was found to stimulate the bitter taste receptor neuron within the... 

Generally, the taste sensillum contains only small numbers of receptor neurons for fundamental tastes. Hence, the taste sensillum might not be suitable for perception of CHC pheromones that contain many components. For such multi-component pheromone perception, olfactory sensilla with many receptor neurons might be more suitable, even though CHC contact pheromones are non-volatile in most cases. 

In order to make precise kinetic measurements of the relationship between the strength of stimulus and the magnitude of response in each receptor neuron, it is necessary to use adequate stimuli for the targeting receptor neuron. For example, a taste sensillum of flies houses four functionally differentiated chemoreceptor neurons corresponding to insect fundamental tastes sugar, salt, water and bitter taste receptor neurons. These four receptor neurons, when stimulated by adequate stimuli, generate distinguishable impulses by their... 

Thus, using these techniques, it has been suggested that a sex-pheromone CHC of Drosophila stimulates the bitter taste receptor neuron in a CHC concentration-dependent manner. Moreover, the cross adaptation test between the sex-pheromone CHC and an ordinary bitter substance supported the idea that the sex-pheromone CHC tastes bitter (Lacaille el al., 2007). 

Interestingly, bitter molecules (caffeine, quinine, berberine) are detected by the same neurons and when painted on the cuticle of immobilized live males also dose-depend-ently inhibit male-male courtship, suggesting that 7-T tastes bitter to D. melanogaster males (Lacaille el al., 2007). 

For any speciation process based on pheromonal communication to be effective, variation in CHC production should tightly co-evolve with factors regulating CHC processing. This has been observed for the desatl gene in D. melanogaster (Marcillac et al., 2005b). Such coevolution requires that pre-existing sensory structures can detect and respond to new CHC molecules. This basic assumption of the theory of sexual selection is supported by the observation that taste neurons normally used to detect bitter molecules also serve to detect an aversive sex pheromone (Lacaille et al., 2007). It is thus possible that taste neurons that were initially used by the fly to detect noxious food molecules (bitterness is often associated with alkaloids and toxic molecules) have been more recently used to detect inhibitory sex pheromones. 

Lacaille, F Everaerts, C. and Ferveur, J.-F. (2009). Feminization and alteration of Drosophila taste neurons induce reciprocal effects on male avoidance behavior. Behav. Genet., 39, 554-563. 

Figure 6. Spontaneous and evoked spike activity recorded from taste neurons of the geniculate ganglion of the cat. The classification of the three different sensory neurons is indicated by Groups I, II, and III.

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