Papillae, bitter taste

Bitter taste is elicited by structurally diverse compounds, including phenols, ions, amino acids and peptides, alkaloids, acylated sugars, glycosides, nitrogenous compounds, and thiocarbamates. Taste receptor cells are primarily associated with papillae on the tongue. The signal transduction mechanisms by which taste perception occurs are well not understood, but are the focus of intensive research as reviewed recently (6). 

Fig. 1 Human bitter taste receptor cells express subsets of TAS2R genes. Dual labelling in situ hybridization of one taste bud in human circumvallate papillae reveals that cells a and b express hTAS2Rl (upper panel). Of the two cells, cell a contains also hTAS2R10 messenger RNA (mRNA), whereas cell b tests negative for hTAS2R10 mRNA...

Dotson CD, Roper SD, Spector AC (2005) PLCbeta2-independent behavioral avoidance of prototypical bitter-tasting ligands. Chem Senses 30 593-600 Farbman AI (1980) Renewal of taste bud cells in rat circumvallate papillae. Cell Tissue Kinet 13 349-357... 

The importance of lipophilicity to bitterness has been well established, both directly and indirectly. The importance of partitioning effects in bitterness perception has been stressed by Rubin and coworkers, and Gardner demonstrated that the threshold concentration of bitter amino acids and peptides correlates very well with molecular connectivity (which is generally regarded as a steric parameter, but is correlated with the octanol-water partition coefficient ). Studies on the surface pressure in monolayers of lipids from bovine, circumvallate papillae also indicated that there is a very good correlation between the concentration of a bitter compound that is necessary in order to give an increase in the surface pressure with the taste threshold in humans. These results and the observations of others suggested that the ability of bitter compounds to penetrate cell membranes is an important factor in bitterness perception. 

PROP status. Individuals classified as tasters of the bitter compound propylthiouracil (PROP) (not found in wine) have been reported to perceive bitterness more intensely and have a higher number of taste pores per taste bud and higher density of fungiform taste papillae on the tongue than non-tasters of PROP (32- 36). Despite this, PROP status has not been demonstrated to affect perception of bitterness or astringency of phenolic compounds in wine (73, 29) or water (75, 77, 79, 30). 

Apart from the five normal basic tastes - sweet, sour, salty, bitter and umami- a great many other actions occur in the mouth, although their importance here is limited. The basic taste sensations are experienced in different areas of the tongue. For example, sweetness is tasted more at the tip of the tongue, sour sensation move on the sides towards the back of the tongue and saltiness on both sides towards the front (Fig. 6.2). Bitter sensations are perceived predominantly towards the back of the tongue, although the precise location varies between individuals, in some cases almost as far back as the throat. This is why it is often difficult to ascertain bitterness. It is best to lick a small sample, so that if possible more papillae are reached. 

The human tongue is studded with small conical bumps, or papillae, which house the taste buds. Each taste bud consists of receptor cells, and extending from each receptor cell are taste hairs with receptor sites for flavor molecules. The receptor cells can distinguish only four general flavors sweet, salty, bitter, and sour. The areas of response to these tastes are located on specific parts of the tongue. 

Over 100 years ago, it was determined that taste cells in the taste buds of different papilla located across the tongue respond to more than one type of stimulus [11]. Although each neuron may respond more strongly to one tastant, it wiU also respond to unlike taste properties. Also, it is thought that no single taste cell contains receptors for both bitter and sweet. Each taste receptor cell is connected through a network of cellular activities to a sensory neuron that travels to the brain. A single sensory neuron may be connected to several taste cells each within different taste buds [7]. 

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