Relationships between insulin release and taste

Abstract

Tasting sweet food elicits insulin release prior to increasing plasma glucose levels, known as cephalic phase insulin release (CPIR). The characteristic of CPIR is that plasma insulin secretion occurs before the rise of the plasma glucose level. In this experiment, we examined whether taste stimuli placed on the tongue could induce CPIR. We used female Wistar rats and five basic taste stimuli: sucrose (sweet), sodium chloride (salty), HCl (sour), quinine (bitter) or monosodium glutamate (umami). Rats reliably exhibited CPIR to sucrose. Sodium chloride, HCl, quinine, or monosodium glutamate did not elicit CPIR. The non-nutritive sweetener saccharine elicited CPIR. However, starch, which is nutritive but non-sweet, did not elicit CPIR although rats showed a strong preference for starch which is a source of glucose. In addition, we studied whether CPIR was related to taste receptor cell activity. We carried out the experiment in rats with bilaterally cut chorda tympani nerves, one of the gustatory nerves. After sectioning, CPIR was not observed for sweet stimulation. From these results, we conclude that sweetness information conducted by thistaste nerve provides essential information for eliciting CPIR.

Full text

Biomedical Research 28 (2) 79-83, 2007
Relationships
between
insulin release
and
taste
Kazuyuki
Tonosaki1,
Yasunori
How1,
Yasutake
Shimizu2
and
Keiichi
Tonosaki1
1
Department
of
Oral
Physiology,
School
of
Dentistry,
Meikai
University,
Saitama
350-0283
and
2
Department
of
Veterinary
Physiology,
Faculty of Agriculture, Gifu University, Gifu, Japan
(Received
13
December
2006; and accepted26
January
2007)
ABSTRACT
Tasting sweet food elicits insulin release prior to increasing plasma glucose levels, known as ce
phalic phase insulin release (CPIR). The characteristic
of
CPIR is that plasma insulin secretion oc
curs before the rise of the plasma glucose level. In this experiment, we examined whether taste
stimuli placed on the tongue could induce CPIR. We used female Wistar rats and five basic taste
stimuli: sucrose (sweet), sodium chloride (salty),
HC1
(sour), quinine (bitter) or monosodium glu-
tamate (umami). Rats reliably exhibited CPIR to sucrose. Sodium chloride,
HC1,
quinine, or
monosodium glutamate did not elicit CPIR. The non-nutritive sweetener saccharine elicited CPIR.
However, starch, which is nutritive but non-sweet, did not elicit CPIR although rats showed a
strong preference for starch which is a source
of
glucose. In addition, we studied whether CPIR
was related to taste receptor cell activity. We carried out the experiment in rats with bilaterally cut
chorda tympani nerves, one
of
the gustatory nerves. After sectioning, CPIR was not observed for
sweet stimulation. From these results, we conclude that sweetness information conducted by this
taste nerve provides essential information for eliciting CPIR.
Taste
sensations
have
been
classified
into
five
sub-
modalities: sweet, salty, sour, bitter, and umami,
which typically represent particular categories
of
stimuli. Sweetness is represented by carbohydrates,
sourness by spoilage materials, salt by minerals, bit
terness by toxic substances, and the umami by ami
no acids. On the basis
of
these signals, animals
discriminate
between
nutrient
and
toxic
substances.
The sense
of
taste involves not only responding to
foods and transmitting the chemical information
of
the food to the central nervous system but also set
ting up appropriate caloric intake action and taste
related reflexes (3, 4). For example, strong sourness
increases secretional saliva (4, 6, 9) which helps to
Address correspondence to: Dr. Keiichi Tonosaki
Department
of
Oral Physiology, School
of
Dentistry,
Meikai University, 1-1 Keyakidai, Sakatoshi, Saitama-
ken, Japan 350-0283
Tel (Fax): +81-49-279-2770
E-mail: tonosaki@dent.meikai.ac.jp
prepare for smooth digestion and absorption of food
before
it
reaches
the
stomach.
It is
also
known
that
taste stimuli can produce insulin secretion via the p
cell of the pancreas (1-3, 6). The characteristic of
cephalic phase insulin release (CPIR) is that plasma
insulin secretion occurs before the rise
of
the plas
ma glucose level. The typical characteristic of CPIR
is that plasma insulin is secreted within 2 min after
oral sensory stimulation, peaks at 4 min, and returns
to baseline within
8-10
min after stimulation (4, 6,
14-18). Although many CPIR related experiments
have been conducted using multiple animal species,
including human, the functional role
of
CPIR is not
clearly known (1, 3, 6-10). In CPIR research, food
substances are typically placed in the oral cavity. In
many cases, more attention is paid to "food compo
sition"
than to
"food
taste" (6, 7, 13).
There
is no
report that examines the effect
of
the 5 submodali-
ties
of
taste on CPIR systematically. It is important
to clarify this relationship between qualities
of
tastes
and CPIR. The purpose of this study is to clarify the

80
characteristics of taste specificity of CPIR using the
rat.
MATERIALS
AND
METHODS
Animals.
Male
Wistar
rats
weighting
120-220
g
were housed in plastic cages at
22±1°C
with a
12: 12h
light:
dark cycle(lighton 07 : 00-19 : 00 h).
They were given free access to laboratory chow
(LABO MR Sttock, Nihon-Nosan, Yokohama, Ja
pan) and water.
Taste solutions. Taste stimuli were sucrose (1.0 M,
sweetness), acetic acid (CH3COOH, 0.1 M, sour
ness),
salt
(NaCl, 0.5 M,
salty
taste),
quinine
hydrochloride (QHCl, 0.01 M, bitterness), sodium
glutamate (MSG, 0.2 M, umami), saccharin
(0.01
M),
and starch (5%, Merck KgaA, Germany). Each
chemical
was
dissolved
in
distilled
water.
Neural recordingprocedure. After the rat was suffi
ciently anesthetized with pentobarbital administra
tion (40 mg/kg, i.p.), the trachea was cannulated.
The rat was fixed in a supine position in a head
folder, and the chorda tympani (CT) nerve was ex
posed and cut near its entry into the tympanic bulla.
For
whole nerve recording, the entire nerve was
placed on a bipolar silver wire electrode. The elec
trophysiological recording method as well as the
methods
for
chemical
stimulation
of
the
taste
cells
have been previously described (12). Each chemical
solution was applied to the tongue for 20 s. Solu
tions
were
delivered
at
a
flow
rate
of
0.5
mL/s.
In-
terstimulus intervals were at least 1 min, during
which time the tongue was rinsed with distilled wa
ter.
Behavioral analysis (two bottle preference tests).
Rats were first trained to drink at equal rates from
two bottles
of
distilled water. After training, distilled
water was kept in one bottle while a test solution
was placed in the other. The position
of
the bottle
was switched every 24 h, and the intake volume was
measured every 24 h for 3 days. During the experi
ment, each rat was kept in an individual plastic cage
and
fed
solid
food
ad
lib.
The bilateral sectioning
of
chorda
tympani
nerve
(CT).
Six rats were subjected to surgery for bilateral
sectioning
of
the CT. Rats were anesthetized by i.p.
injection
of
pentobarbital (40 mg/kg). Each animal
was secured with a head holder in a prone position
and an incision was made along the mandible tip.
K.
Tonosaki
et
al.
Then both CT nerves were exposed and bilateral CT
nerves were sectioned. After each operation, the
wound was closed with autoclips and the animal
was returned to its cage for recovery.
Cardiac and oral catheter surgeries. After rats
reached a surgical level
of
anesthesia (pentobarbital,
40 mg/kg, i.p.), cardiac catheterization (0.5 mm in
side diameter, 1.0 mm outer diameter) for blood col
lection was inserted from the right external jugular
vein to the right atrium. The catheter exited and ex
posed about 3.0 cm from the parietal region through
a hypodermic. Simultaneously, the catheter for the
taste stimulation (0.5 mm inside diameter, 1.0 mm
outer diameter) was implanted into the oral cavity
through the right cheek and exited and exposed
about 3.0 cm from the parietal region through a hy
podermic. After each operation, the animal was re
turned to its cage for recovery.
Samples and
measurements.
Taste stimulation and
blood collection were performed under non-anesthe
sia and non-restraint using customary methods. Taste
solutions (1.0 mL) were given for 45 s into the oral
cavity via the oral catheter. After a 12-h fast, blood
samples were obtained from the cardiac catheter at
-5, -1,
1, 3, 5, 7, 9, 11
and
15
min
after taste
stim
ulation. Plasma glucose levels were determined by
the glucose oxidase method (Glucose B-test, Wako
Pure Pharmaceutical, Osaka, Japan). Plasma insulin
concentrations were
determined
by ELISA kits
(Morinaga, Yokohama, Japan).
Statistical analysis. All values are presented as
means ± SE. Statistical significance
was
examined
by an
ANOVA,
with post hoc testing by means of
Duncan's multiple range test. Comparisons between
groups were made by Student's t-test. In all tests,
p < 0.05 was accepted as significant.
RESULTS
Chorda
tympani
(CT)
nerve responses and water in
take
for
five fundamental taste solutions
The tongue was rinsed with distilled water. Gustato
ry CT nerve responses for various taste stimulations
are shown in Fig. 1. 1.0 M sucrose, 0.1 M acetic
acid, 0.5 M NaCl, 0.01 M QHCl and 0.2 M MSG
elicited robust CT responses (12).
In Fig. 2, rats distinguished each taste solution
from distilled water. Rats preferred sucrose solutions
(27.8 ±5.5 mL/day, n = 4) but avoided other solu
tions (acetic acid: 1.1
±0.7
mL/day, n = 4; NaCl:

Insulin
release
and
taste
JX
sucrose
1 M
QHCl
0.01
M
81
Fig. 1 Typical
examples
of integrated
chorda
tympani
nerve
responses
to
sucrose,
acetic acid (CH,COOH). sodium chlo
ride (NaCl), quinine hydrochloride (QHCl)
and
monosodium
glutamate
(MSG).
o
I
i
+>
c
H
a
o
•H
-p
H
O
w
40
30
20
10
I
J
§L
IJ
Distilled
water
fH test solution
ita.
i.
sucrose
CH,COOH
NaCl
QHCl
MSG
1 M
0.1
M
0.5
M
0.01
M
0.2
M
Fig. 2 Oral intake of taste solutions and
distilled
water during 24 h preference tests (n = 4). The open columns are the dis
tilled water intake ratios,
and
the
hatched
columns
are
the
taste
stimulus solution intake ratios
(mean
± SD).
2.2
±0.8
mL/day. n = 4: QHCl: 0.7 ± 0.7 mL/day,
n = 4; MSG: 7.5 ± 2.5 mL/day, n = 4). The water in
take
of
a rat was 25.1 ± 1.7 mL/day.
Fivefundamental taste solutions and
CPIR
In Fig. 3A, 3 min after the sucrose stimulation, there
was a 3 to 4 times increase in plasma insulin con
centration compared to levels prior to stimulation
(before stimulation 3.0
±0.7
ng/mL and after 3 min
12.4
±4.5
ng/mL, n = 5). The rise
of
the plasma in
sulin
concentration
was
transient,
and
declined
with
in 7 min. In Fig. 3B. the change
of
the plasma
glucose level after sucrose stimulation is plotted.
The
transient
increase
in
insulin
secretion
was
ob
served before the rise
of
the glucose level (before
stimulation 88.5 ± 8.5 mg/dL. 3 min after stimula
tion 99.3 ± 8.5 mg/dL, n = 6, 5 min after stimulation
112.2±
6.5
mg/dL.
n = 6.
11
min
after
stimulation
142.8 + 12.5 mg/dL. n = 6). Thus CPIR was induced
by the sucrose stimulation of the tongue. Table 1
presents the results of plasma insulin concentrations
and plasma glucose concentrations for acetic acid.
NaCl, QHCl and MSG. No significant changes were
observed.
Sweetness
and
CPIR
In presenting the 5 fundamental tastes, only sucrose
elicited
CPIR.
However,
sucrose
has
two
character
istics:
sweet
and
nutritive.
Next,
we
tested
whether
'sweet' or 'nutritive' could elicit CPIR. Testing with
the
non-nutritive
sweetener
saccharine
did
elicit
CPIR (Fig. 4A and B). However, the non-sweetener
nutritive starch did not elicit CPIR (Fig. 5A and B).
The
effect
of
bilateral sectioning CT nerve
Finally,
we
studied
whether
CPIR
was
related
to

82
Table
1Plasma insulin and glucose levels in
rats
K.
Tonosaki
et
at.
before
3
minutes
5
minutes
7
minutes
11
minutes
n
Acetic
acid
sour
Insulin (ng/mL)
Glucose (mg/dL)
Insulin
Glucose
2.5
±0.2
98.5
±
6.0
2.4
±
0.5
93.5
±
0.2
2.3
±
0.2
92.5
±
7.5
2.5
±
0.5
92.5
±
7.0
2.5
±
0.5
92.5
±
7.0
6
5
NaCl
salty
3.9
±1.5
111.3
±4.5
3.4
±1.1
110.5
±4.0
3.4
± 1.1
111.3
±4.5
3.4
±1.5
110.5
±4.0
3.4
±1.5
110.5
±4.0
5
5
QHCl
bitter
Insulin
Glucose
3.5
±1.2
98.6
±4.5
2.9
±1.0
98.0
±4.5
3.5
±
1.0
101.0
±2.0
3.5
±1.0
101.0
±2.0
3.5
±1.0
105.0
±3.0
5
5
MSG
umami
Insulin
Glucose
2.5
±0.8
103.5
±4.5
2.5
±
0.8
103.5
±4.5
2.5
±1.0
105.5
±5.0
2.5
±1.0
105.5
±5.0
2.5
±1.5
108.5
±7.0
5
5
The values are indicated at I min before and 3, 5, 7,
11
min after taste stimulation, n: sample numbers. Data are means ± SE.
B
sucrose
Fig. 3 Effect of administration of
sucrose
on
plasma
insu
lin levels (n = 5) (A)
and
plasma
glucose
levels (n = 6) (B)
(mean±SD).
Arrows indicate
the
beginning of
the
taste
stimulation.
Significant
difference
between
conditions:
"P
<
0.01
and
*P
<
0.05.
j/Hfrrrf+M
B
aI
J*
>•
»»
fi
-4
i
o
Jm
ta
5 0 5 10 IS
Time
(min)
starch
-5 0 5 [0 15
Time
(min)
Fig. 5 Effect of administration of starch on plasma insulin
levels (n = 5) (A)
and
plasma
glucose
levels (n = 5) (B)
(mean ± SD). Arrows indicate the beginning of the
taste
stimulation.
Significant
difference
between
conditions:
"P<0.01.
taste receptor cell activity. Experiments were carried
out in rats with bilaterally cut CT nerves, one
of
the
gustatory nerves. After sectioning, CPIR was not
observed
for
sweet
stimulation.
The
results
are
shown in Fig. 6A and B.
DISCUSSION
Five fundamental tastes, sweet, sour, salty, bitter and
umami, were examined in order to clarify the rela
20
.5 10
^fH
-i—*
B
ISO.
8 «o
5
saccharin
-t-*-«-t^«
0 5 10 IS
Time
(min)
Fig. 4 Effect of administration of
saccharine
on
plasma
in
sulin levels (n = 5) (A)
and
plasma
glucose
levels (n = 5) (B)
(mean±SD).
Arrows indicate
the
beginning of
the
taste
stimulation.
Significant
difference
between
conditions:
*P
<
0.05.
B
ISO
J 0 J 10 IS 5 0 5 10 IS
Time
(min)
Time
(min)
sucrose
(CT:CUT)
Fig. 6 Effect of administration of
sucrose
on
plasma
insu
lin levels (n = 6) (A)
and
plasma glucose levels (n = 6) (B)
with bilaterality sectioned the chorda tympani (CT)
nerves
(mean ± SD). An arrow indicates the beginning of the
taste
stimulation.
Significant
difference
between
conditions:
"P
<
0.01
and
*P
<
0.05.
tionship between cephalic phase insulin release
(CPIR) and taste quality since there are no previous
reports examining the relationships between CPIR
and these stimuli. In our experiments, the tongue
was
rinsed
with
distilled
water
and
the
taste
cells
were adapted to distilled water. Sucrose, acetic acid,
NaCl,
QHCl,
MSG
and
saccharin elicited robust CT
responses
(12).
The
characteristic
of
CPIR
is that
plasma insulin secretion (plasma insulin release) oc-

Insulin
release
and
taste
curs within 2 min after oral sensory stimulation, that
is, the transient increase in insulin secretion
was
ob
served before the rise
of
the plasma glucose level.
From
our
results, it is
clear
that
CPIR
was
elicited
only by sucrose stimulation, a sweet stimulus (2, 3,
16). Since sucrose has two characteristics, sweet and
nutritive, it is important to clarify which characteris
tic
is
related
to
CPIR.
We
tested
both
saccharin
and
starch:
the
artificial
sweetener
saccharin
is
sweet
but
not
nutritive
and
starch
is
nutritive
but
not
sweet.
The
non-nutritive
sweetener
saccharine
did
elicit
CPIR
whereas
the
non-sweetener
nutritive
starch
did
not. It has been reported that the rats show a strong
preference for starch which is a source
of
glucose,
but starch did not elicit CT responses (8, 10, 11).
From the results, it became clear that
CPIR
pecu
liarly appeared for sucrose, and it was proven that it
is important that CPIR is elicited by sweet, not by
nutritive
stimuli.
Next
we
studied
whether
CPIR
was related to taste receptor cell activity. We carried
out the experiment in rats with bilaterally cut CT
nerves, one
of
the gustatory nerves. After section
ing, CPIR was not observed for sucrose stimulation.
From
these
results,
we
conclude
that
sweetness
in
formation conducted by this taste nerve provides es
sential information for eliciting CPIR.
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