Effect of continuous sweet gustatory stimulation on salivary flow rate over time

Saliva secretion is regarded as an important factor in maintaining a healthy oral environment. Decreased saliva production has been reported to cause dysphagia, masticatory disorders, dysarthria, and dysgeusia (Graf et al., 2021); additionally, increased risks of dental caries and oral candidiasis have been reported. (Ramirez-Amador et al., 1997, Ahmad et al., 2017) In the elderly, decreased saliva production has been reported to lower the retention of denture prostheses (Inamochi et al., 2020), and thus, may lead to various oral dysfunctions and reduce the quality of life of the elderly.

Xerostomia and/or hyposalivation can be a side effect of medical prescriptions or caused by general dehydration or diabetes, even with normally functioning salivary glands (Napeñas et al., 2009). Some conditions and diseases are known to impair salivary gland functions, such as Sjogren’s syndrome and side effects of radiation therapy of the head and neck (Napeñas et al., 2009). Symptomatic treatments are adopted if hyposalivation is not improved by treatment of the causative disease or discontinuation of the drug that causes hyposalivation as a side effect. Such symptomatic treatments include oral application of a moisturizing gel (Nuchit et al., 2020) or use of artificial saliva (Łysik et al., 2019) for alleviating the symptoms of hyposalivation. Although these treatments can relieve the feeling of dry mouth, their effect is reduced by swallowing; thus, frequent application is required to maintain moisture in the oral cavity throughout the day. If the cause is Sjogren’s syndrome or radiation therapy to the head and neck, oral medicines, such as cevimeline and pilocarpine, are therapeutic options. These drugs stimulate parasympathetic nerves and promote salivation; however, they may also induce side effects, such as sweating, headache, nausea, and vomiting (Wiseman and Faulds, 1995, Braga et al., 2009).

Previous studies have investigated treatment methods for alleviating hyposalivation by increasing the amount of saliva secreted by gustatory stimulation (Ishijima et al., 1994, Kuriwada-Satoh and Sasano, 2015). Satoh-Kuriwada et al. (2018) examined the effect of gustatory stimulation on the lip mucosa for 2 min with five basic tastes (umami, sweet, bitter, sour, and salty) and showed increased salivary secretion in the minor salivary glands of the lips. Kusakabe et al. (2020) reported that gustatory stimulation of the tongue with sweeteners, such as sucralose (SC), aspartame (APM), acesulfame potassium (AceK), rebaudioside A, and xylitol, increased salivary production. Mizuhashi et al. (2021) demonstrated increased saliva secretion using a moisturizing gel with sweet and sour tastants. In this case, saliva secretion was maintained for 30 min after application of the moisturizing gel. In contrast, Watanabe (1988) reported that salivary flow rate was highest at 5.5−7.7 s after gustatory stimulation with sucrose and then decreased to 50% of the maximum flow rate after 12 s. They also reported that an adaptation to taste occurred.

Regarding the adaptation, Dawes and Watanabe (1987) reported that saliva flow, which was elicited with a gustatory stimulus, declined exponentially with time. These reports showed the effect of taste on saliva secretion for a relatively short time. However, saliva secretion should be maintained for a longer period to relieve dry mouth sensation in daily life. To use the gustatory-saliva reflex against hyposalivation, we need to elucidate the changes in gustatory sensation and salivation levels by applying gustatory stimulation for a long period in a continuous and quantitative manner.

Therefore, the purpose of this study was to clarify the effect of gustatory stimulation on saliva secretion over time using a continuous and quantitative taste solution feeding system and to evaluate the strength of perceived gustatory stimulation using a visual analog scale (VAS). We hypothesized that there would be no difference in salivary secretion after continuous gustatory stimulation.

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