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Effects and Mechanisms of Tastants on the Gustatory-Salivary Reflex in Human Minor Salivary Glands

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The effects and mechanisms of tastes on labial minor salivary gland (LMSG) secretion were investigated in 59 healthy individuals. Stimulation with each of the five basic tastes (i.e., sweet, salty, sour, bitter, and umami) onto the tongue induced LMSG secretion in a dose-dependent manner. Umami and sour tastes evoked greater secretion than did the other tastes. A synergistic effect of umami on LMSG secretion was recognized: a much greater increase in secretion was observed by a mixed solution of monosodium glutamate and inosine 5′-monophosphate than by each separate stimulation. Blood flow (BF) in the nearby labial mucosa also increased following stimulation by each taste except bitter. The BF change and LMSG secretion in each participant showed a significant positive correlation with all tastes, including bitter. Administration of cevimeline hydrochloride hydrate to the labial mucosa evoked a significant increase in both LMSG secretion and BF, while adrenaline, atropine, and pirenzepine decreased LMSG secretion and BF. The change in LMSG secretion and BF induced by each autonomic agent was significantly correlated in each participant. These results indicate that basic tastes can induce the gustatory-salivary reflex in human LMSGs and that parasympathetic regulation is involved in this mechanism.
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Research Article
Effects and Mechanisms of Tastants on the Gustatory-Salivary
Reflex in Human Minor Salivary Glands
Shizuko Satoh-Kuriwada , Noriaki Shoji, Hiroyuki Miyake,
Chiyo Watanabe, and Takashi Sasano
Division of Oral Diagnosis, Department of Oral Medicine and Surgery, Tohoku University Graduate School of Dentistry,
4-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan
Correspondence should be addressed to Shizuko Satoh-Kuriwada; kuri-shu@dent.tohoku.ac.jp
Received 20 October 2017; Accepted 26 December 2017; Published 31 January 2018
Academic Editor: Elsa Lamy
Copyright ©  Shizuko Satoh-Kuriwada et al. is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium,provided the original work is properly cited.
e eects and mechanisms of tastes on labial minor salivary gland (LMSG) secretion were investigated in  healthy individuals.
Stimulation with each of the ve basic tastes (i.e., sweet, salty, sour, bitter, and umami) onto the tongue induced LMSG secretion in a
dose-dependent manner. Umami and sour tastes evoked greater secretion than did the other tastes. A synergistic eect of umami on
LMSG secretion was recognized: a much greater increase in secretion was observed by a mixed solution of monosodium glutamate
and inosine 󸀠-monophosphate than by each separate stimulation. Blood ow (BF) in the nearby labial mucosa also increased
following stimulation by each taste except bitter. e BF change and LMSG secretion in each participant showed a signicant
positive correlation with all tastes, including bitter. Administration of cevimeline hydrochloride hydrate to the labial mucosa evoked
a signicant increase in both LMSG secretion and BF, while adrenaline, atropine, and pirenzepine decreased LMSG secretion and
BF. e change in LMSG secretion and BF induced by each autonomic agent was signicantly correlated in each participant. ese
results indicate that basic tastes can induce the gustatory-salivary reex in human LMSGs and that parasympathetic regulation is
involved in this mechanism.
1. Introduction
e minor salivary glands are vital for the maintenance of oral
health because they secrete abundant mucin, which acts as
a lubricant [], and are involved in immunoactivity through
secretion of immunoglobulin A []. Although the minor
salivary glands contain less volume than the major salivary
glands [], they are widely distributed throughout the oral
mucosa [].
Eating is a strong stimulus for the secretion of saliva by
themajorsalivaryglands[].Largevolumesofsalivaare
secreted before, during, and aer eating via the gustatory-
salivary reex, masticatory-salivary reex, olfactory-salivary
reex, and esophageal-salivary reex. Parasympathetic eer-
ent activities induced by taste stimuli have been shown to
involve salivation and vasodilation in the major salivary
glands []. However, the details of secretion mechanisms
in the minor salivary glands remain unclear because of
diculties in collecting and quantifying the minute secretion
volume from the minor salivary glands. We previously devel-
oped a new technique for measuring the minor salivary gland
ow using a simple iodine-starch lter paper method [] and
demonstrated that the subjective feeling of dry mouth was
more strongly related to a reduction in minor salivary gland
ow than in whole salivary ow []. is nding suggests an
important role of the minor salivary glands in xerostomia.
In the present study, we examined the eects of ve
basic taste stimuli (sweet, salty, sour, bitter, and umami) on
reex salivation in the human labial minor salivary glands
(LMSGs). Specically, we studied the synergistic eect of the
umami taste on reexive LMSG secretion because the com-
bined umami taste of monosodium glutamate (MSG) and
inosine 󸀠-monophosphate (IMP) is widely known to have
a strong eect on taste perception as a characteristic feature
of the umami taste []. Additionally, we investigated the
nervous control of LMSG secretion using autonomic agents
while monitoring the nearby blood ow (BF) in the labial
mucosawhereLMSGsecretionwasobserved.
Hindawi
BioMed Research International
Volume 2018, Article ID 3847075, 12 pages
https://doi.org/10.1155/2018/3847075
BioMed Research International
2. Materials and Methods
2.1. Participants and Exclusion Criteria. In total,  healthy
participants were initially recruited from the students at
Tohoku University and the residents and sta members
at Tohoku University Hospital. Individuals with systemic
disease (e.g., endocrine, infectious, or immunological disease
or a history of chemotherapy and/or radiation therapy for
headandneckcancer),whohadbeenprescribedmedi-
cations that could directly aect dry mouth, or who had
a feeling of oral dryness were excluded. Individuals with
hyposalivation identied by measuring the LMSG ow and
those with a history of psychological problems were also
excluded aer careful interviews and psychological testing
(Self-Rating Depression Scale) to avoid psychogenic oral
dryness. Consequently,  individuals (average age, 31.2±8.3
years; age range, – years;  men,  women) were nally
included. ese participants were divided into four groups to
evaluate the LMSG responses to taste (𝑛=21), the synergistic
eect of umami (𝑛=10), the relationship between LMSG
secretionandtheBFchangewheretheLMSGarelocated
(𝑛=14), and the involvement of the autonomic nervous
system in LMSG secretion and the BF change (𝑛=11). is
study was designed and conducted in complete accordance
with the World Medical Association Declaration of Helsinki
(http://www.wma.net) and was approved by the Ethics Com-
mittee of the Tohoku University Graduate School of Den-
tistry. Written consent was obtained from each participant
aer they had received an explanation of the purpose of the
study.
2.2. Quantication of LMSG Secretion. e LMSG secretion
responses to distilled water (DW), tastants, or autonomic
agents were quantied using the iodine-starch lter paper
method as previously described []. Briey, a strip of test
paper ( × cm, Filter paper ; Toyo Roshi Kaisha, Ltd.,
Tokyo, Japan) painted with a solution of iodine in absolute
alcohol and a ne starch powder mixed with castor oil was
appliedontothelowerlipformin[].eblackenedareasof
each test paper, imprinted by the iodine-starch reaction, were
scanned and digitized at  bits using a GT- ART image
scanner (Seiko Epson Corp., Nagoya, Japan) with the scan-
ning resolution set at  dpi. e total area was measured
using image analysis soware (Scion Image Beta .; Scion
Corporation, Frederick, MD, USA). Each total area value was
converted to a ow rate (𝜇L/cm2/min) using the calibration
line 𝑌 = −0.084 +24.992𝑋 (𝑌,areainmm
2;𝑋,volumein𝜇L)
previously described [].
2.3. BF Measurements in Labial Mucosa. BF changes in the
lower labial mucosa, where quantication of the LMSG
secretion was undertaken, were continuously monitored
using reection-mode laser Doppler owmetry (SNF;
Cyber Firm Med, Inc., Tokyo, Japan) before and aer the
administration of DW, tastants, or autonomic agents. During
measurement of the BF, the participants were asked to keep
their mouths open, and the lower lip was everted using an
angle widener. e test areas were isolated with rolled gauze
andthendriedwithacottongauzepadimmediatelybefore
the recording was performed. A sensor probe was rmly
anchored to the angle widener with surgical tape, and the
tip of the probe was kept at a distance of .mm from the
lip surface. All recordings were electrically calibrated to zero
BF. Laser Doppler signals from the lower labial mucosa were
continuously monitored, together with the systemic blood
pressure (BP) (Finometer; Finapres Medical Systems, Ams-
terdam, Netherlands). e outputs from the owmeter and
BP monitor were recorded on a multichannel chart recorder
(Recti-Horiz-K; NEC San-ei, Tokyo, Japan).
2.4. Taste Stimulation
2.4.1. Five Basic Tastes. Five well-established taste substances
were used. For the four basic tastes (sweet, salty, sour, and
bitter), ready-made test solutions of Taste Disc(Sanwa
ChemicalCo.,Ltd.,Nagoya,Japan)wereused.Eachconcen-
tration of the four basic tastes was as follows:
(i)Sweet(sucrose):.mM(No.:S),.mM(No.:
S), . mM (No. : S), . mM (No. : S), and
. mM (No. : S)
(ii) Salty (NaCl): . mM (No. : N), . mM (No. :
N),.mM(No.:N),.mM(No.:N),and
. mM (No. : N)
(iii) Sour (tartaric acid): . mM (No. : T), . mM (No.
: T), . mM (No. : T), . mM (No. : T),
and . mM (No. : T)
(iv) Bitter (quinine): . mM (No. : Q), . mM (No. :
Q),.mM(No.:Q),.mM(No.:Q),and
. mM (No. : Q)
For umami taste, MSG aqueous solution previously devel-
opedforanumamitastesensitivitytest[]wasused.e
concentrations of the umami taste were  mM (No. : G),
mM(No.:G),mM(No.:G),mM(No.:G),and
mM(No.:G).Eachtastenumber(Nos.)wassetup
so that the intensity of the participant’s perception of the taste
was equivalent in spite of the dierent taste qualities based
on previously reported data of the Taste Disc [] and our
previously described ndings regarding umami []. us,
the intensity of perception of the same taste number (Nos.
–) was equal among the ve tastes.
A  mm diameter cotton ball containing  𝜇L of each
taste solution or DW was applied onto the posterior tongue
for  min, and the LMSG secretion was then measured. e
participants were asked to rinse their mouth with water for
at least  min between each taste stimulation. e next taste
stimulation was applied aer the measurement value had
returned to the baseline level. All participants (𝑛=21)
were asked to refrain from eating or drinking (except water),
smoking, and brushing their teeth for at least  h before
testing.
2.4.2. Combined Umami Tastes of MSG and IMP. To inv e s-
tigate the well-known synergistic eect of combined umami
tastes on LMSG secretion,  mM of MSG aqueous solution,
 mM of IMP aqueous solution, and a solution containing
a mixture of the two ( mM MSG and  mM IMP) were
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prepared. Combined umami tastes that have been shown to
evoke a synergistic eect [] were made using an aqueous
solution containing  mM MSG and  mM IMP. Changes
in LMSG secretion were quantied following each admin-
istration of MSG, IMP, or MSG + IMP solution onto the
posterior tongue of  participants. e procedure was similar
to the above-described experiment involving the ve basic
tastes.
2.5. Relationship between LMSG Secretion and Nearby BF
Change following Taste Stimulation. e LMSG secretion and
nearby BF changes in the labial mucosa following application
of the highest concentration (No. ) of each of the ve basic
taste solutions onto the posterior tongue were observed for
 participants. e LMSG secretion was rst measured, and
then the BF change to the tastant was measured until the BF
had recovered to the prestimulus value.
2.6. Use of Autonomic Agents. e following four autonomic
agents were prepared:
(i) .% adrenaline: a sympathomimetic agent (Adren-
aline Injection .%; Terumo Corporation, Tokyo,
Japan)
(ii) % cevimeline hydrochloride aqueous solution: a
muscarinic (M) receptor agonist (Saligrencapsule
 mg; Nippon Kayaku, Tokyo, Japan)
(iii) % atropine sulfate hydrate: a cholinergic blocking
agent (atropine ophthalmic solution %; Nitten Phar-
maceutical Co., Ltd., Nagoya, Japan)
(iv) .% pirenzepine hydrochloride aqueous solution:
a muscarinic (M) receptor antagonist (pirenzepine
hydrochloride tablets  mg; Nichi-Iko Pharmaceuti-
cal Co., Ltd., Toyoma, Japan)
e concentration of each autonomic agent was based
on the manufacturer’s medical package insert for clinical
use. e LMSG ow rate and BF were measured following
application of a  ×cm lter paper soaked in 𝜇Lof
eachagentorDWonthelabialmucosaforminin
participants. Stimulation with the next agent was applied
aer the measurement values had returned to the baseline
level. e participants were asked to rinse their mouth with
water,andanintervalofatleastminwassetbetweeneach
stimulation.
2.7. Data Analysis. e LMSG secretion aer the administra-
tion of DW, tastants, or autonomic agents is presented as a
percentage of the resting value (mean ±standard deviation).
To compare each mean to the control (DW) mean, the data
were analyzed by one-way analysis of variance followed by
Dunnetts multiple-comparison test. Tukey’s honestly signif-
icant dierence test was used to analyze the dierences in
LMSG secretion and BF changes following stimulation with
various tastants.
e BF changes in the labial mucosa aer the administra-
tion of DW, tastants, or autonomic agents are presented as a
percentage of the baseline value recorded with no adminis-
tration (mean ±standard deviation). To compare each mean
tothecontrol(DW)mean,thedatawereanalyzedbyone-
way analysis of variance followed by Dunnett’s multiple-
comparison test.
e normality of the data was assessed using the Shapiro-
Wilk test, and the correlation between the changes in
LMSG secretion and the nearby BF was then statistically
analyzed using Spearman’s rank correlation. All statistical
analyses were performed using SPSS . (SPSS Inc., Chicago,
IL, USA). e criterion for signicance was dened as
𝑝 < 0.05.
3. Results
3.1. Changes in LMSG Secretion following Stimulation with
Five Basic Tastes. Low concentrations of the ve basic tastes
(sweet, salty, sour, bitter, and umami) caused no signi-
cantchangesinLMSGsecretion;however,highconcentra-
tions (Nos. –) of all tastes evoked signicant increases
in LMSG secretion (Figure ). Table  shows the detailed
results.
As shown in Table , sour and umami tastes evoked
signicantly larger increases in LMSG secretion than did the
other tastes (sweet, salty, and bitter) at high concentrations
(Nos.  and ), although low concentrations (Nos. –) they
did not.
3.2. Changes in LMSG Secretion following Stimulation with
Mixed Umami Substances. Mixed umami substances of
mM MSG and  mM IMP caused a signicant increase in
LMSG secretion (𝑝 < 0.0001), while each solution alone
elicited no signicant change (MSG G: 104.1 ± 6.2,𝑝=
0.985;IMP:106.6 ± 6.8,𝑝 = 0.886)ascomparedwithDW
stimulation (101.8 ± 4.5) (Figure ).
3.3. Relationship between LMSG and Nearby Lip BF following
Taste Stimulation. e highest concentration (No. ) of each
of the ve basic tastes evoked a signicant increase in LMSG
secretion (sweet S: 131.3±19.3,𝑝 = 0.045;saltyN:130.8±
11.1,𝑝 = 0.049;sourT:264.8 ± 76.4,𝑝 < 0.0001;bitterQ:
131.0 ± 36.3,𝑝 = 0.048;umami:266.8 ± 47.6,𝑝 < 0.0001)
compared with DW stimulation (96.7 ± 0.8) (Figure (a)).
All tastes except bitter evoked a signicant increase in lip BF
(sweet S: 134.5 ± 9.5,𝑝 = 0.013;saltyN:128.9 ± 8.6,𝑝=
0.047;sourT:238.5 ± 43.8,𝑝 < 0.0001;umamiG:224.4 ±
56.4,𝑝 < 0.0001) compared with DW stimulation (99.0±2.5),
while bitter did not elicit a signicant change in BF (118.9 ±
29.1,𝑝 = 0.295) (Figure (b)). As shown in Tables  and ,
sour and umami tastes evoked signicantly larger increases
in both LMSG secretion and BF changes than did the other
tastes (sweet, salty, and bitter). Some participants showed
increasesinbothLMSGsecretionandBFchangeinresponse
to bitter taste, but others showed decreases in both LMSG
secretion and BF change. Comparison of the changes in the
same participants revealed a signicant correlation between
theamountofchangesinsalivationandBFinresponseto
each taste stimulus (sweet: 𝑟 = 0.802;salty:𝑟 = 0.751;
sour: 𝑟 = 0.806;bitter:𝑟 = 0.805;umamitaste:𝑟 = 0.853)
(Figure ).
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S2 S3 S4 S5DW S1
0
50
100
150
200
250
300
Saliva secreted from LMSGs (%)
∗∗
∗∗ ∗∗
(a) Sweet stimulation
N2
0
50
100
150
200
250
300
Saliva secreted from LMSGs (%)
N3 N4 N5DW N1
∗∗
∗∗
∗∗
(b) Salty stimulation
T2 T3 T4 T5DW T1
0
50
100
150
200
250
300
Saliva secreted from LMSGs (%)
∗∗
∗∗
(c) Sour stimulation
Q2 Q3 Q4 Q5DW Q1
0
50
100
150
200
250
300
Saliva secreted from LMSGs (%)
∗∗
∗∗
(d) Bitter stimulation
G2 G3 G4 G5DW G1
0
50
100
150
200
250
300
Saliva secreted from LMSG (%)
∗∗
∗∗
∗∗
(e) Umami stimulation
F : Changes in LMSG secretion following stimulation with ve basic tastes. High concentrations (Nos. –) of each of the ve basic
tastes (S, N, T, Q, and G) elicited a signicant increase in LMSG secretion in human participants (𝑛=21), although lower concentrations
(Nos.  and ) of each solution caused no signicant change. Ordinate: a percentage (%) of the resting saliva. 𝑝< 0.05,∗∗𝑝< 0.0001.
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T : Changes in LMSG secretion induced by DW or tastants.
DW 
S(sweet)
%change 99.5 ± 2.3 99.1 ± 2.6 100.4 ± 4.1 112.8 ± 8.7 125.5 ± 16.6 126.4 ± 25.5
𝑝values - . . <.1∗∗ <.1∗∗ <.1∗∗
N(salty)
%change 100.6 ± 3.0 98.5 ± 2.2 108.9 ± 8.7 114.1 ± 12.5 118.7 ± 14.1 119.8 ± 16.6
𝑝values - . . <.1∗∗ <.1∗∗ <.1∗∗
T (sour)
%change 98.7 ± 2.8 99.3 ± 3.2 111.1 ± 7.2 134.2 ± 15.8 174.2 ± 35.7 272.4 ± 42.5
𝑝values - . . 0.007<.1∗∗ <.1∗∗
Q(bitter)
%change 104.0 ± 1.9 97.7 ± 1.9 105.1 ± 9.7 115.1 ± 18.9 119.0 ± 26.8 121.1 ± 34.3
𝑝values - . . 0.001<.1∗∗ <.1∗∗
G (ummai)
%change 98.5 ± 2.9 99.6 ± 3.1 109.4 ± 5.4 122.5 ± 18.6 163.7 ± 32.5 268.6 ± 42.1
𝑝values - . . <.1∗∗ <.1∗∗ <.1∗∗
Statistical dierences were analysed by one-way ANOVA and Dunnett’s multiple-comparison test (𝑝<0.05,∗∗𝑝< 0.001); %change indicates a percentage
oftherestingvalue(mean±standard deviation); 𝑛=21.
T : Increases in LMSG secretion induced by high concentration of tastants (No.  and ).
S (sweet) N (salty) T (sour) Q (bitter) G (umami)
No. 
S (sweet) - . <.1. <.1
N(salty) - <.1. <.1
T(sour) - <.1.
Q(bitter) -<.1
G (umami) -
No. 
S (sweet) - . <.1. <.1
N(salty) - <.1. <.1
T(sour) - <.1.
Q(bitter) -<.1
G (umami) -
Statistical dierences were analysed by one-way ANOVA and Tukey’s honestly signicant dierence test; numerical value means 𝑝value between one tastant
and another: (asterisk) means signicant; 𝑛=21.
3.4. Changes in LMSG Secretion and Nearby Lip BF Change fol-
lowing Stimulation with Autonomic Agents. Administration
of cevimeline chloride (parasympathetic agonist) caused a
signicant increase in LMSG secretion (170.3 ± 22.1,𝑝<
0.0001), while adrenaline (sympathetic agonist) (33.2 ± 3.8,
𝑝 < 0.0001), atropine (parasympathetic inhibitor) (64.0±6.1,
𝑝 < 0.0001), and pirenzepine (parasympathetic antagonist)
(42.3 ± 8.4,𝑝 < 0.0001) evoked a signicant decrease in
LMSG secretion compared with DW stimulation (103.4±3.4)
(Figure (a)). ese changes induced by the dierent agents
were consistent with those of nearby lip BF changes; that is,
cevimeline chloride caused a signicant increase in the BF
(198.9 ± 37.1,𝑝 < 0.0001), while adrenaline (37.7 ± 9.2,𝑝<
0.0001), atropine (61.9 ± 12.2,𝑝 < 0.0001), and pirenzepine
(49.6 ± 18.8,𝑝 < 0.0001) elicited a signicant decrease
in the BF compared with DW stimulation (106.8 ± 7.2)
(Figure (b)). Signicant correlations were found between
theamountofchangeinLMSGsecretionandtheBFfor
each autonomic agent in the same participant (adrenaline:
𝑟 = 0.893; cevimeline: 𝑟 = 0.882;atropine:𝑟 = 0.797;
pirenzepine: 𝑟 = 0.788) (Figure ).
4. Discussion
4.1. Responses of Minor Salivary Gland Secretion to Stimulation
with Five Dierent Tastes. e gustatory-salivary reex (i.e.,
taste-initiated secretion of saliva) is important for tasting,
masticating, and swallowing food. is vital reex has been
mainly studied in the saliva secreted from the major salivary
glandsormixedsalivasecretedfromthemajorandminor
salivary glands. Kerr [] showed that the human major
salivary ow response to citric acid, salt, and sucrose was
, , and  times higher than the resting saliva response,
respectively. Hodson and Linden [] also demonstrated that
the ve basic taste qualities (sweet, salty, sour, bitter, and
umami) induced the gustatory-salivary reex in the parotid
gland and that parotid salivary ow increased in a dose-
dependent manner in response to umami taste (MSG).
BioMed Research International
T : Increases in LMSG secretion induced by tastant of No. .
S (sweet) N (salty) T (sour) Q (bitter) G (umami)
No. 
S (sweet) - . <.1. <.1
N(salty) - <.1. <.1
T(sour) - <.1.
Q(bitter) -<.1
G (ummai) -
Statistical dierences were analysed by one-way ANOVA and Tukey’s honestly signicant dierence test; numerical value means 𝑝value between one tastant
and another: (asterisk) means signicant; 𝑛=14.
T : Increases in BF change induced by tastant of No. .
S (sweet) N (salty) T (sour) Q (bitter) G (umami)
No. 
S (sweet) - . <.1. <.1
N(salty) - <.1. <.1
T(sour) - <.1.
Q(bitter) -<.1
G (ummai) -
Statistical dierences were analysed by one-way ANOVA and Tukey’s honestly signicant dierence test; numerical value means 𝑝value between one tastant
and another: (asterisk)means signicant; 𝑛=14.
Saliva secreted from LMSGs
DW MSG (G2) IMP MSG + IMP
0
50
100
150
200
250
(%)
F : Changes in LMSG secretion following stimulation with
umami substances. Neither MSG (G) nor IMP elicited a signicant
change in LMSG secretion, while mixed umami substance (mM
MSG +  mM IMP) caused a signicant increase in LMSG secretion
(𝑝 < 0.0001)(𝑛=10). 𝑝< 0.0001. Ordinate: a percentage (%) of
the resting saliva.
Few reports have provided a detailed comparison of
gustatory-salivary reex salivation in response to the dif-
ferent taste stimuli in the minor salivary glands, except
our preliminary report [], because of the diculty in
measurement of the minute secretion volume from the
minor salivary glands. In the present study, we used a newly
developed method for measuring the LMSG ow rate []
and demonstrated that () each of the ve basic taste stimuli
elicited a signicant increase in saliva secreted from the
LMSG in a dose-dependent manner, and () sour and umami
tastes elicited signicantly larger increases in LMSG secretion
than did sweet, salty, or bitter. ese results are consistent
with previous reports demonstrating the major salivary ow
response [, , ].
Allen [] reported a correlation between gustatory-
salivary reex salivation in the parotid gland and the intensity
ofthetastestimulus.erefore,thetasteintensityofeach
dierent taste quality must be equivalent to compare the
dierences in the amount of saliva produced by the gustatory-
salivary reex. In the present study, each dierent taste
quality solution, including umami, was administered at ve
dierent intensities (Nos. –) based on a previous study
that established the cumulative distribution of each tastant
[, ]. For example, the specic taste quality of the No. 
concentration of each taste solution can be recognized by
% of participants. us, using the same number of taste test
solutions, it becomes possible to supply an equal intensity of
taste perception in spite of the dierences in taste quality.
4.2. Responses of LMSG Secretion to Stimulation with Mixed
Umami Substance. Mixed umami solution containing MSG
and IMP caused a signicant increase in LMSG secretion,
whereas stimulation with MSG or IMP alone did not increase
LMSG secretion at these concentrations. e synergism of
umami tastes between MSG and guanylate was rst reported
by Kuninaka [, ], and Yamaguchi and Ninomiya [] indi-
cated that the detection threshold of umami taste perception
ofMSGwasmarkedlylowerinthepresenceofIMP.Arecent
electrophysiological study involving mice demonstrated the
occurrence of marked enhancement of the glossopharyngeal
nerve responses to MSG by the addition of guanylate [, ].
isisinlinewithourresultonthesynergismofumami
tastes when the posterior tongue is stimulated by a mixture
of MSG and another umami substance (e.g., IMP). It has
been suggested that the human taste receptor, a TR + TR
heterodimer, induces potentiation of the synergism between
MSG and IMP. A recent study suggested the existence of
separate binding sites for MSG and IMP within the same TR
Venus ytrap domain, which is important for umami taste
synergism [, ]. In TR-knockout mice, the synergism
BioMed Research International
Saliva secreted from LMSGs
DW S5 N5 T5 Q5 G5
0
50
100
150
200
250
300
350
(%)
∗∗ ∗∗
(a)
0
50
100
150
200
250
300
(%)
DW S5 N5 T5 Q5 G5
∗∗ ∗∗
Labial BF change
(b)
F : Changes in (a) LMSG secretion and (b) nearby BF change following stimulation with ve basic tastes. (a) e highest concentration
(No. ) of each of the ve basic tastes (S, N, T, Q, and G) evoked signicant increases in MSG secretion (𝑛=14). (b) e same concentration
of each of the basic tastes except bitter elicited a signicant increase, but not a signicant change, in labial mucosal BF (𝑛=14). Ordinate: a
percentage (%) of (a) the resting saliva and (b) the baseline BF value. 𝑝< 0.05,∗∗𝑝< 0.0001.
betweenMSGandIMPisconsiderablyreducedinthe
anterior tongue []. us, the umami taste has a quite
distinguished synergistic eect exhibited by no other taste
quality. We demonstrated that the synergistic eect of umami
not only showed sensory perception but also evoked the
gustatory-salivary reex in the LMSGs. is synergistic eect
has also been shown to be elicited not only between MSG
and IMP but also between MSG and other nucleotides of
guanylate [, ]. erefore, further studies of the eect of
dierent mixtures of MSG and other nucleotides on reex
secretion in the LMSGs are needed.
Umami has another specic characteristic, that is, its
residual aertaste, which diers from other taste qualities
[]. In a preliminary study, we examined the time course
of the salivary ow of LMSG secretion in response to the
ve basic tastes and found that the umami taste evoked a
long-lasting increase in LMSG, whereas sour taste evoked
a prominent increase in the LMSG ow that immediately
diminished []. It seems likely that these long-lasting eects
on LMSG secretion incidental to the umami taste are due to
the residual aertaste. e synergism and residual aertaste
of the umami taste may be benecial for patients with dry
mouth based on our previous nding that xerostomia is more
strongly related to the LMSG ow than the major salivary
gland ow [].
4.3. Relationship between LMSG Secretion and Nearby Lip
BF Change following Taste Stimulation. e salivary glands
are supplied by a dense capillary network equivalent to
that of the heart; thus, vasodilatation of these capillaries
surrounding the salivary glands might be necessary to ensure
that large volumes of saliva are produced by the secretory
cells [, ]. We considered that the circulation surrounding
theLMSGsiscloselyrelatedtotheLMSGsecretorysystem.
Consequently, we examined the nearby lip BF where the
LMSG secretion measurement was performed using laser
Dopplerowmetry.OurmeasurementoftheBFincluded
the labial glandular BF because laser Doppler owmetry
can measure the erythrocyte ux through an approximately
mm
3volume of the capillary bed without touching the
tissues[].iscanbeaccomplishedbecausetheLMSGs
densely exit via the supercial oral mucosa, which is very
thin. We demonstrated that stimulation with all tastes except
bitter caused an increase in the nearby lip BF consistent
with the increase in the LMSG secretion. In addition, sour
and umami tastes induced prominent increases in the BF in
thesamemannerastheLMSGsecretion.Tastestimulation
evoked no BP changes, indicating that vasodilation in the
stimulated area was induced.
Interestingly, we observed a correlation between the rate
of changes in the BF and the LMSG response to each
taste stimulus in dierent participants (Figure ). As shown
in Figure , sweet, salty, sour, and umami tastes evoked
correlatedincreasesintheBFandLMSGsecretion(>%
in the gure) in all participants; however, bitter caused a
correlated decrease in the BF and LMSG secretion (<%
in the gure) in some participants. us, bitter only evoked
aBFdecreaseinsomeparticipants.Apreviousstudyshowed
that the BF in the orofacial area is uniquely controlled by a
double autonomic system; that is, vasoconstriction mediated
via sympathetic nerve bers and vasodilation mediated via
parasympathetic eerent nerve bers []. Bitter taste can
evoke both sympathetically induced reexive vasoconstric-
tion and parasympathetically mediated vasodilation, while
the other tastes prominently induce reex vasodilation. In
this respect, the hedonic dimension to the taste report-
edly plays various roles in the many taste-mediated whole-
body responses. Interestingly, an unpleasant bitter taste can
BioMed Research International
80 120 16040
LMSG (%)
40
80
120
160
BF (%)
r = 0.802
(a) Sweet stimulation
40
80
120
160
BF (%)
80 120 16040
LMSG (%)
r = 0.751
(b) Salty stimulation
100 200 300 4000
LMSG (%)
0
100
200
300
BF (%)
r = 0.806
(c) Sour stimulation
40 80 120 1600
LMSG (%)
0
40
80
120
160
BF (%)
r = 0.805
(d) Bitter stimulation
0
100
200
300
BF (%)
100 200 300 4000
LMSG (%)
r = 0.853
(e) Umami stimulation
F : Relationship between LMSG secretion and BF response in lip following taste stimulation. Signicant correlations were present
between the amount of change in LMSG secretion and BF evoked by the highest concentration (No. ) of each taste stimulus (𝑛=14).
Ordinate: a percentage (%) of the baseline BF value; Abscissa: a percentage (%) of the resting LMSG saliva.
reportedly induce sympathetically mediated physiological
changesinskinBFandskintemperature,instantaneous
heart rate, and skin potential and skin resistance much more
strongly than other taste qualities (sweet, salty, and sour) [].
Additionally, pleasant stimuli were found to elicit approach
and acceptance, whereas unpleasant stimuli induced avoid-
ance and rejection, thus determining taste preferences and
aversions []. Although we did not record the participants
liking of each tastant in this experiment, some participants
indeed hated the bitter taste. Such individuals may show
strongerdecreasesinLMSGsecretionandtheBFasa
sympathetic eect. Further studies are needed to clarify the
role of unpleasant taste sensations in the control of taste-
mediated responses related to food rejection.
BioMed Research International
Saliva secreted from LMSGs
0
50
100
150
200
(%)
Pirenzepine
Atropine
Cevimeline
Adrenaline
DW
(a)
0
50
100
150
200
250
(%)
Pirenzepine
Atropine
Cevimeline
Adrenaline
DW
Labial BF change
(b)
F : Changes in (a) LMSG secretion and (b) nearby BF change following stimulation with autonomic agents onto the lip. (a)
Administration of cevimeline chloride caused a signicant increase in LMSG secretion (𝑝 < 0.001), while adrenaline, atropine, and
pirenzepine evoked signicant decreases in LMSG secretion (𝑛=11). (b) Cevimeline chloride caused a signicant increase in labial mucosal
BF, while adrenaline, atropine, and pirenzepine elicited signicant decreases (𝑛=11). Ordinate: a percentage (%) of the (a) resting saliva and
(b) baseline BF. 𝑝< 0.0001.
Overall, our results indicate that the BF change surround-
ing the LMSGs is an important factor in the salivary secretory
system in the LMSGs.
4.4. LMSG Secretion and Nearby Lip BF Changes Mediated
by the Autonomic Nervous System. Salivary secretion is
controlled by the parasympathetic and sympathetic auto-
nomic nervous systems []. In the human parotid gland,
the gustatory-salivary reex involves the activity of both
types of autonomic nerves, while the masticatory-salivary
reex preferentially activates the parasympathetic nerves
[]. Mobilization of the intracellular messenger calcium by
stimulation of muscarinic receptors (M, M) is associated
with uid secretion, particularly large volumes in response
to muscarinic agonists, via exocytosis in the rat parotid gland
[]. However, the neural regulation of the gustatory-salivary
reexinhumanLMSGsremainsunknown.Inthepresent
study, application of cevimeline hydrochloride hydrate (an
agonist of the muscarinic M receptor) onto the lip elicited
an increase in LMSG secretion. Furthermore, pirenzepine
(an antagonist of the muscarinic M receptor) and atropine
(a competitive inhibitor of the muscarinic acetylcholine
receptor) elicited a decrease in LMSG secretion. us, we
conclude that muscarinic receptors (M, M) are engaged
in human LMSG secretion. However, the application of
adrenaline (an agonist of 𝛼and 𝛽adrenergic receptors)
certainly decreased LMSG secretion. is phenomenon dif-
fers from that described in a report on sympathetic nerve-
induced secretion by the parotid gland, suggesting that the
human LMSGs may lack sympathetic secretion. is idea
is supported by a histochemical study indicating that few
adrenergic nerves have been identied in the human LMSGs
[].
Nervous control of the orofacial BF is regulated by both
the parasympathetic and sympathetic autonomic nervous
systems []. In the cat, labial BF is controlled by two
groups of parasympathetic bers (the facial and glossopha-
ryngeal nerves) for vasodilatation [] and by sympathetic 𝛼-
adrenergic bers for vasoconstriction []. We also examined
the neural regulation of the BF in the human labial mucosa
because salivary secretion appears to be related to the nearby
BF, as mentioned above. In our pharmacological analysis,
application of cevimeline hydrochloride hydrate (an agonist
of the muscarinic M receptor) elicited a prominent increase
in the BF without a change in the BP, and pirenzepine (an
antagonist of the muscarinic M receptor) and atropine (a
competitive inhibitor of the muscarinic acetylcholine recep-
tor) elicited a signicant decrease in the BF without a change
in the BP, indicating that muscarinic receptors (M, M)
are engaged in vasodilatation in the human labial mucosal
tissues surrounding the LMSGs. Furthermore, adrenaline
(an agonist of 𝛼and 𝛽adrenergic receptors) elicited a
signicant decrease in the nearby BF without a change in
the BP, indicating that 𝛼-adrenergic receptors are involved
in vasoconstriction in this region. Interestingly, correlations
were found between the dynamics of the saliva secreted
fromtheLMSGsandthenearbylipBFchangesinresponse
to each chemical agent in the same participants (Figure ),
although vascular responses monitored by laser Doppler
owmetry should include not only the labial glandular BF
 BioMed Research International
0
20
40
60
BF (%)
10 20 30 400
LMSG (%)
r = 0.893
(a) Adrenaline
0
100
200
300
BF (%)
50 100 150 200 2500
LMSG (%)
r = 0.882
(b) Cevimeline
20 40 60 800
LMSG (%)
0
20
40
60
80
100
BF (%)
r = 0.797
(c) Atropine
0
20
40
60
80
BF (%)
20 40 600
LMSG (%)
r = 0.788
(d) Pirenzepine
F : Relationship between LMSG secretion and BF response in lip following stimulation with autonomic agent. Signicant correlations
were present between the amounts of change in LMSG secretion and BF evoked by each autonomic agent in the same participant (𝑛=11).
Ordinate: a percentage (%) of the baseline BF value; abscissa: a percentage (%) of the resting LMSG saliva.
but also the mucosal capillary BF. ese results show that
parasympathetic activation can simultaneously increase the
salivary secretion from the LMSGs and induce vasodilatation
in the mucosal tissues surrounding the LMSGs. Conversely,
decreases in the saliva secreted by the LMSGs may be caused
byadecreaseinBFincidentaltothevasoconstrictionbecause
the human LMSGs possibly lack sympathetic secretion, as
discussed above []. us, we consider that LMSG secretion
is strongly inuenced by the nearby BF. Further detailed
studies are necessary to clarify the eects of the relationship
between LMSG secretion and nearby BF changes on the
autonomic nervous system.
e present study has shown that each of the ve basic
tastesensationscaninducehumanLMSGsecretion.is
LMSG secretion is an autonomic nervous system-induced
reex that spontaneously arises at meals and may be bene-
cial to various functions of eating, such as smooth chewing
and formation of a food bolus. Moreover, LMSG secretion
provides lubrication and protection of the oral mucosa
because the LMSG secretions contain high concentrations of
protective substances such as mucin and immunoglobulin A.
isLMSGsecretioninducedbytastesubstancescontained
in food at meals would thus be benecial for maintaining oral
health.
5. Conclusions
Taste stimulation can cause a gustatory-reex secretion in
the human LMSGs. In particular, sour and umami tastes
cause larger increases in LMSG secretion than do other tastes.
Umami has a synergistic eect on the LMSG secretion reex.
Parasympathetic regulation is involved in the gustatory-
salivaryreexintheLMSGsinassociationwiththechanges
in BF near the LMSGs.
BioMed Research International 
Conflicts of Interest
All authors declare no conicts of interest regarding the
publication of this paper.
Acknowledgments
e authors greatly appreciate the participation of all indi-
viduals in this study. is study was supported in part by
Grants-in-Aid for Developmental Scientic Research from
the Ministry of Education, Science and Culture of Japan
(nos.  and  to S. Satoh-Kuriwada) and
was funded in part by the Ajinomoto Company (Kanagawa,
Japan). Finally, the authors thank Angela Morben, DVM,
ELS, from Edanz Group for editing a dra of this manuscript.
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This series of experiments was performed in order to evaluate the physiological characteristics and patterns of after-taste of various taste substances. The durations of after-taste following monosodium glutamate (MSG), inosine-5′-monophosphate (IMP) and guanosine-5′-monophosphate (GMP) (umami substances) were longer than those for sucrose, NaCl, tartaric acid and quinine-HCI at concentrations corresponding to the recognition threshold. The periods of after-taste of solutions of MSG and IMP, and MSG and GMP, were longer than those for the single component solutions. Most subjects recognized sucrose as sweet, NaCl as salty, tartaric acid as sour and quinine-HCI as bitter, both in terms of immediate taste and after-taste. According to the patterns of after-taste for umami substances, the subjects were divided into three groups. In group A, umami (appealing, savory taste in Japanese cuisine) was the main quality of the after-taste sensation; in group B, an indefinite, equivocal taste was the characteristic quality of the aftertaste; and no difference was reported in group C between the immediate taste and after-taste. These results suggest that the characteristics of after-taste for MSG, IMP and GMP are different from those of the four basic tastes.