Taste systems

Studies on the biochemistry of the taste system should take into account results obtained at other levels, such as electrophysiological recordings and, particularly, behavioral responses to taste stimuli. The term sweetness should strictly be used only in studies conducted on humans, because the description of taste modality is a verbal response. It is usually concluded that positive behavioral responses in animals, that is, preferences, or electrophysiological response to a stimulus compound that is known to be sweet to man, are due to the sweet taste. This may not necessarily be true in some cases, because behavioral or electrophysiological response may result from other taste modalities. It is, therefore, critical that comparative aspects be carefully interpreted. 

The function of a sensory system is to select suitable modalities from the multitude presented by the environment, and translate them into corresponding modalities of sensory information that are then projected and processed into the various parts and finally submitted to the central processing-unit, the brain. A working hypothesis of the mechanism by which the taste system senses chemical compounds is that macromolecules that are... 

Devitsina G.V. and Cherova L. (1992). The trigeminal nerve system and its interaction with olfactory and taste system in fishes. In Chemical Signals in Vertebrates 6 (Doty R.L. and Miiller-Schwarze D., eds.). Plenum, New York, pp. 85-88. 

The perception of flavor is a fine balance between the sensory input of both desirable and undesirable flavors. It involves a complex series of biochemical and physiological reactions that occur at the cellular and subcellular level (see Chapters 1-3). Final sensory perception or response to the food is regulated by the action and interaction of flavor compounds and their products on two neur networks, the olfactory and gustatory systems or the smell and taste systems, respectively (Figure 1). The major food flavor components involved in the initiation and transduction of the flavor response are the food s lipids, carbohydrates, and proteins, as well as their reaction products. Since proteins and peptides of meat constitute the major chemical components of muscle foods, they will be the major focus of discussion in this chapter. 

Due to the bias that can occur in the Basic Protocol, results can be as low as 1% of the correct value for the concentration of a volatile in a sample. Most results are <80% of the correct value. This bias is usually caused by differences in the recovery between the internal standard and the analytes. Choosing an internal standard thatis similar to the analyte can reduce this bias but it cannot be completely eliminated unless a separate standard is used for each analyte (see Alternate Protocol). Nevertheless, the Basic Protocol is often sufficiently accurate because the olfactory system, unlike the taste system, is compressive and insensitive to small... 

Electrophysiological studies of the smell and taste systems of fish have likewise demonstrated chemoreceptor cells that are responsive, with varying degrees of specificity, to the amino acids known to elicit feeding behavior.73 75 In addition, a number of fish have receptor cells that respond to bile acids, amphipathic steroid compounds that are used as digestive detergents and that can be released into the environment in substantial quantities. Responses can exhibit both exquisite specificity for the structure of a bile acid, and extreme sensitivity, as best exemplified by the sea lamprey.76 77... 

In this paper the term taste will refer to all the chemical sensory systems of the oral cavity and their sensations. These sensory systems are intimately involved in the selection of food items and in the regulation of food intake. As we shall see, there are a variety of different taste systems attuned to different chemical aspects of food. These taste systems perform an exact and elaborate analysis of the chemical constituents in the food we eat. 

The structure and function of these taste systems will be discussed in the context of a natural nutritional ecosystem, i.e. one in which man is not a disruptive element. Human taste systems are assumed to have developed to function in this natural system and to have changed little as a result of the cultural dietary changes that have < ccurr d in the last 10- 20,000 years. 

Table I Summary of Neurophysiological Investigations on Mammalian Geniculate Ganglion Taste Systems.

Different sensations may arise because of the activation of two distinct neural taste systems (e.g. cat group I and group II), or the differential activation of a single system. Differential activation of the same system could occur when different segments of an ordered population are activated by different chemicals or when one chemical compound excites the neural group and another inhibits. 

Neurophysiology and Stimulus Chemistiy of Mammalian Taste Systems... 

Neurons were divided into 9 groups largely according to stimulus chemistry. A sodium-lithium system was seen in the rat and goat but not the cat and dog. Amino acid responsive neurons were seen in all species except the goat, with major species differences. Amino acid responsive neurons were also, except for the cat, responsive to sugar. A nucleotide system was seen only in the cat. Acid (Br nsted) responsive neurons were seen in all species, but the cat and dog acid taste systems were different from others. A system responsive to furaneol and other compounds present in fruit was seen only in the dog. A system exclusively responsive to alkaloids was found in rat and goat. 

Type of taste systems present can to a certain extent be related to species ecology and dentition. 

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In this report the neurophysiology of mammalian taste systems is reviewed with especial attention to stimulus chemistry. The neurophysiology described is primarily that from our laboratory, since we have been among the few neurophysiologists concerned with stimulus chemistry. The animals that have been investigated in detail are the cat, dog, goat and rat. Work on other animals is included where comparisons are viable. 

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Figure 2. Taste systems of the rat geniculate (GG) and petrosal (PG) ganglia. Location of receptive fields indicated by a dot on tongue for each neuron studied. Examples are shown of elicited spike discharge for neurons from the six different neural groups identified.

The main criteria used to classify the units in Table I were stimulus response measures i.e., the units discharged or were inhibited by different chemical compounds. In addition, other criteria were used to supplement the chemical stimulus response differentiation. Thus, the two main groups in the cat (acid units and amino acid units) can also be differentiated by spontaneous activity measures, latency to electrical stimulation, area of tongue innervated, and differential response to solution temperature (3-5). This comparative work has led to a modular view of peripheral taste systems in which the different neural groups are seen to have distinct receptors responding to distinct types of chemical signals (e.g., Br nsted acids and Br nsted bases), with either excitation or inhibition. The stimulus chemistry of these groups will be briefly described. 

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Acid Responsive Units. All species possessed an acid taste system although this system was not identical from species to species. The system was labeled "acid" because the most stimulating compounds were Br nsted acids and the least stimulating were Br nsted bases. 

BOUDREAU Neuropl /sudogy and StimtdusChemisby of Taste Systems 131... 

The modular taste systems summarized for mammals in Table II are quite similar to the modular taste systems that have been observed for invertebrates, such as lobsters and crayfish (20, 21). The most extensive invertebrate taste research has been performed on caterpillars (22, 23). In 20 different species of caterpillars, 12 different neural groups were distinguished. 

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The cat and the dog, on the other hand, possess taste systems that have little in common with rodents and goats. Not only do they have no sodium system, but their acid and amino acid systems are also markedly distinct. Although the cat and the dog have two systems, the acid and amino acid systems, in common, both also possess a taste system which the other does not the cat a nucleotide system and dog a furaneol system. 

The primates have been inadeguately studied, but those two with adequate single unit data suggest that the organization of primate taste systems is no simple matter. It is not obvious for instance, why the squirrel monkey may have an amino acid system like a carnivore and the macaque one like a rodent. The human taste system further complicates matters since man can best be viewed as a composite, having a sodium system like the rat and goat, carnivore acid and amino acid systems, a furaneol system like the dog and a glutamate system unlike any other mammal studied (14). 

The compounds active on both vertebrate and invertebrate taste systems constitute a select group of low molecular weight compounds. The compounds include organic acids, salts, nucleotides, amino acids and a variety of secondary compounds, notably alkaloids but also others, including here furaneol and ethyl and methyl maltol. Just why certain of these compounds are active on taste systems is often a moot point. The significance of none of the acid systems, for instance, is obvious from an ecological standpoint, nor is it apparent why certain acids are so potent. It is also not clear why the two amino acid systems are so distinct, nor why proline and cysteine should assume such a large role in the carnivore taste system. 

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