Differences between Oral Dryness and Thirst

Abstract

Saliva is secreted from the salivary glands. A decrease in saliva secretion produces a decrease in oral wetness and elicits the feeling of oral dryness. On the other hand, although the oral ca— vity may be wet enough, people often experience thirst. If the feeling of oral dryness with hyposalivation and/ or thirst persists, the condition is called xerostomia (or dry mouth). Salivary secretion is controlled by the autonomic nervous system. Acetylcholine released by the parasympathetic nervous system stimulates muscarinic receptors and triggers the secretion of serous saliva. We found that intraperitoneally injected pilocarpine, which is known as a cholinergic muscarinic receptor agonist and a sialogue, increases not only saliva secretion but also drinking behavior in rats. This indicates that while pilocarpine increases wetness in the oral cavity, it also evokes the sensation of thirst and elicits water intake. Therefore, the thirst sensation can be evoked independently of oral wetness. Further, we found that nicotine, which is a stimulant for cholinergic nicotinic receptors, evokes the thirst sensation. We demonstrated that thirst was induced both by nicotine acting on nicotinic receptors in the brain and by pilocarpine acting on muscarinic receptors int he brainb yu singb ehavi o.lra electrophysiological and molecular biological experimenst. In this review, we discuss the relationship between oral dryness and thirst and the different mecha— nisms underlying these two phenomena. We propose that while the symptom of xerostomia ineludes botho ral dryness and thi .rtsit isi mportantt ou nderstand thato ral dryness and the sensation of thirst are intrinsically different from each other.