Effect of Stimulation of the Laryngopharynx with Water and Salt Solutions on Voluntary Swallowing in Humans: Characteristics of Water Receptors in the Laryngopharyngeal Mucosa
Advance Access publication August
Effect of Stimulation of the Laryngopharynx with Water and Salt Solutions on Voluntary Swallowing in Humans: Characteristics of Water Receptors in the Laryngopharyngeal Mucosa
Yasuyuki Kitada 2 3
Rika Yahagi 1 2
Kazuhisa Okuda-Akabane 0 2
0 Department of Oral Biology, School of Dentistry, Iwate Medical University , 1-3-27 Chuoh-dori, Morioka 020-8505 , Japan
1 Division of Dysphagia Rehabilitation, Department of Oral Biological Science, Niigata University Graduate School of Medical and Dental Sciences , 2-5274 Gakkocho-dori, Chuo-ku, Niigata 951-8514 , Japan
2 a The Author 2010. Published by Oxford University Press. All rights reserved. For permissions , please
3 Morioka Taste and Swallowing Research Institute , 1-15-12 Higashi-Kuroishino, Morioka 020-0108 , Japan
Stimulation of water receptors in the laryngopharynx (LP) with water facilitates voluntary swallowing in humans. Based on measures of swallowing intervals (SIs) in repetitive swallowing, we investigated characteristics of laryngopharyngeal water receptors in humans. Healthy adult volunteers were instructed to perform repetitive swallowing as quickly as possible during infusion of a solution into the LP. Infusion of water shortened SI, suggesting that water excites water receptors. Infusion of 0.3 M NaCl solution prolonged SI, suggesting that the NaCl solution inhibits activity of water receptors. SI increased with increasing concentration of NaCl. Anion or cation substitutions indicated that excitation of water receptors is due to absence or reduced concentration of Cl . With diminution of peripheral inputs, cortical inputs would play a dominant role in voluntary swallowing. With infusion of a nonstimulating solution (0.3 M NaCl at 0.2 mL/min), SI varied greatly from subject to subject, suggesting that the ability of central regulation of swallowing to initiate repetitive voluntary swallowing varies among subjects. Facilitation of swallowing by chemosensory inputs from water receptors appeared strongly in subjects with longer SI with infusion of the nonstimulating solution. It appears that chemosensory activation compensates for the difficulty in initiating swallowing via the central neural mechanism.
human; laryngopharynx; swallowing; taste stimulus; water receptors
Introduction
Swallowing consists of a reflex sequence of muscle
contractions that propels ingested materials and pooled saliva from
the mouth to the stomach. Swallowing simultaneously serves
to protect the respiratory tract from aspiration. Swallowing
can be initiated either voluntarily or reflexively
(Miller 1982)
.
Both inputs from the cerebral cortex and sensory inputs from
mucosal receptors in the oropharynx can regulate the activity
of the swallowing central pattern generator (CPG) located in
the medulla oblongata and the CPG can trigger swallowing
(Jean 2001; Ertekin and Aydogdu 2003)
. Therefore, there
might be interactions between sensory and cortical inputs
for initiation of swallowing. However, the interaction
between the 2 mechanisms has not been fully elucidated.
by NaCl is due to Cl– in rabbits (Shingai 1977) and puppies
(Boggs and Bartlett 1982)
, whereas Cl– is not a crucial factor
for inhibition of water response of water receptors in cats
(Stedman et al. 1980)
and rats
(Shingai 1980)
. Therefore,
excitation of water receptors involves a diversity of
mechanisms. Although taste receptor proteins triggering signal
transduction events were identified by means of
electrophysiology, molecular biology, genetic approaches, and also
screening the mass of genome sequence data
(Chandrashekar
et al. 2006; Sugita 2006)
, molecular mechanism of excitation
of water receptors remains elusive.
Although water applied to the throat has been used to
induce swallowing reflex in humans
(Hughes and Wiles 1996;
Ertekin et al. 2001)
, there are only a few reports on
characteristics of water receptors. In humans, Shingai et al. (1989)
measured the latency of reflex swallowing induced by
application of water or salt solutions to the throat. They showed
that water was the most effective stimulus for elicitation of
swallowing reflex. Stimulation with solutions of various
concentrations of NaCl affected the latency, but their effects
were complicated because salt solutions above 0.05 M
stimulated salt taste receptors in the posterior tongue (PT).
Recently, we were able to stimulate water receptors in the
LP in humans without stimulation of salt taste receptors
in the PT even though a hypertonic saline solution was used
as a stimulant
(Yahagi et al. 2008)
. That is, solutions were
infused into the LP through a fine tube inserted into the
LP orally during voluntary swallowing. Repetitive voluntary
swallowing makes it possible to withdraw the infused
solution from the LP quickly and thereby the solution could
stimulate only a limited area of the LP in which water
receptors are located. Infusion of water or salt solutions into the
LP affected swal (...truncated)