Saliva has many essential functions. As the first digestive fluid in the alimentary canal, saliva is secreted in response to food, assisting intake and initiating the digestion of starch and lipids. During this process, saliva acts as a solvent of taste substances and affects taste sensitivity. Clinically, a more important role is in the maintenance of oral health, including the protection of teeth and mucosa from infections, maintenance of the milieu of taste receptors, and communication ability through speech. Variations in salivary flow can be affected, reversibly or irreversibly, by numerous physiological and pathological factors. Decreased salivary flow results in clinically significant oral discomfort that may manifest as increased caries, susceptibility to oral candidiasis, altered taste sensation or as a host of other problems. Hyposalivation is a condition that is frequently encountered in dental practice. The most common cause is the use of certain systemic medications, which put the elderly at greater risk because they are usually more medicated. Other causes include high doses of radiation and certain diseases such as Sjögren's syndrome. This article reviews the mechanism of salivary secretion, effect of saliva on taste, importance of saliva in oral health, and hyposalivation in relation to ageing, medicine and/or disease and management of hyposalivation.

Saliva is the principal fluid component of the external environment of the taste receptor cells and, as such, could play a role in taste sensitivity. Its main role includes transport of taste substances to and protection of the taste receptor. In the initial process of taste perception, saliva acts as a solvent for taste substances; salivary water dissolves taste substances, and the latter diffuse to the taste receptor sites. During this process, some salivary constituents chemically interact with taste substances. For example, salivary buffers (e.g., bicarbonate ions) decrease the concentration of free hydrogen ions (sour taste), and there are some salivary proteins which may bind with bitter taste substances. Another effect of saliva on taste transduction is that some salivary constituents can continuously stimulate the taste receptor, resulting in an alteration of taste sensitivity. For example, the taste detection threshold for NaCl is slightly above the salivary sodium concentrations with which the taste receptor is continuously stimulated. In contrast, saliva protects the taste receptor from damage brought about by dryness and bacterial infection, and from disuse atrophy via a decrease in transport of taste stimuli to the receptor sites. This is a long-term effect of saliva that may be related to taste disorders. These various effects of saliva on the taste perception differ depending on the anatomical relationship between the taste buds and oral openings of the ducts of the salivary glands. Many taste buds are localized in the trenches of the foliate and circumvallate papillae, where the lingual minor salivary glands (von Ebner's glands) secrete saliva. Taste buds situated at the surface of the anterior part of the tongue and soft palate are bathed with the mixed saliva secreted mainly by the three major salivary glands.

1. Activities of 35 taste-responsive neurons in the cortical gustatory area were recorded with chronically implanted fine wires in freely ingesting Wistar rats. Quantitative analyses were performed on responses to distilled water, food solution, and four taste stimuli: sucrose, NaCl, HCl, and quinine hydrochloride. 2. Taste-responsive neurons were classified into type-1 and type-2 groups according to the response patterns to licking of the six taste stimuli. Type-1 neurons (n = 29) responded in excitatory or inhibitory directions to one or more of the taste stimuli. Type-2 neurons (n = 6) showed responses in different directions depending upon palatability of the liquids to rats: neurons showing excitatory (or inhibitory) responses to palatable stimuli exhibited inhibitory (or excitatory) responses to unpalatable stimuli. 3. Correlation coefficients of responses to pairs of stimuli across neurons suggested that palatable stimuli (water, food solution, sucrose, and NaCl) and unpalatable stimuli (HCl and quinine) elicited reciprocal (excitatory vs. inhibitory) responses in type-2 neurons, whereas type-1 neurons showed positively correlated responses to specific combinations of stimuli such as food solution and NaCl, sucrose and HCl, NaCl and quinine, and HCl and quinine. 4. A tendency toward equalization of effectiveness in eliciting responses among the four basic taste stimuli was detected on the cortex. The ratios of mean evoked responses in 29 type-1 neurons in comparison with spontaneous rate (4.4 spikes/s) were 1.7, 1.9, 1.8, and 1.9 for sucrose, NaCl, HCl, and quinine, respectively. 5. The breadth of responsiveness to the four basic taste stimuli was quantified by means of the entropy measure introduced by Smith and Travers (33). The mean entropy value was 0.540 for 29 type-1 neurons, which was similar to 0.588 previously reported for rat chorda tympani fibers, suggesting that breadth of tuning is not more narrowly tuned in a higher level of the gustatory system in the rat. 6. Convergent inputs of other sensory modalities were detected exclusively in type-1 neurons. Thirteen (45%) of 29 type-1 neurons also responded to cold and/or warm water, but none of 6 type-2 neurons responded to thermal stimuli. Two (7%) of 29 type-1 neurons responded to almond and acetic acid odors, but the 6 type-2 neurons did not. Two (13%) of 16 type-1 neurons responded to interperitoneal injection of LiCl, which is known to induce gastrointestinal disorders, with a latency of approximately 5 min, but 4 type-2 neurons tested were not responsive to this stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

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