Vol. 274, Issue 5, R1324-R1330, May 1998
Enhanced responses of the chorda tympani nerve to nonsugar
sweeteners in the diabetic
db/db
mouse
Yuzo
Ninomiya,
Toshiaki
Imoto,
Akira
Yatabe,
Sanae
Kawamura,
Kiyohito
Nakashima, and
Hideo
Katsukawa
Department of Oral Physiology, Chemistry and Pediatric Dentistry,
Asahi University School of Dentistry, Motosu, Gifu 501-02; and
Department of Physiology, Tottori University School of Medicine,
Tottori 683, Japan
 |
ABSTRACT |
Genetically diabetic
db/db mice show greater neural and
behavioral responses to sugars than lean control mice. The present study examined chorda tympani responses of
db/db mice to nonsugar sweeteners and
their inhibition by a sweet response inhibitor, gurmarin. The results
showed that responses to sucrose, saccharin, glycine,
L-alanine, and
D-tryptophan, but not to
D-phenylalanine, were ~1.5
times greater in db/db mice than in
control mice. Treatment of the tongue with gurmarin suppressed
responses to these sweeteners in db/db
and control mice, but the extent of suppression was considerably smaller in db/db mice. The magnitudes
of gurmarin-sensitive components of the response to sweeteners in
db/db mice were not significantly different from those in control mice, whereas the magnitudes of gurmarin-insensitive components in
db/db mice were about twice as large
as those in control mice. These results suggest that the enhancement of
chorda tympani responses in db/db mice
to sucrose and other nonsugar sweeteners may occur through
gurmarin-insensitive membrane components.
genetically diabetic mice; enhanced sweet taste responses; gurmarin-insensitive taste receptor
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INTRODUCTION |
THE db/db mouse is known as a genetic
model of non-insulin-dependent diabetes, in which a single gene defect
(db gene) leads to the expression of
diabetes with preceding hyperinsulinemia, hyperglycemia, and extreme
obesity (2, 3). Our previous studies (19, 21) demonstrated that the
db/db mice showed greater relative
responses and lower thresholds of the chorda tympani nerve to sugars
than lean control mice. The db/db mice
did not differ from controls in responses to other basic taste
substances (NaCl, HCl, and quinine HCl), suggesting a specific increase
in response to sugars. The greater gustatory neural responses of db/db mice to sugars started to show
at 7-10 days of age, at which time their insulin hypersecretion in
response to glucose stimulation also starts (1). This indicates that
hyperresponsiveness to sugar stimulation simultaneously appears on both
taste and pancreatic B cells at this early stage. However, the
mechanisms underlying genetic induction of the high sugar sensitivities
in these cells have not yet been investigated.
In the present study, to investigate characteristics of the receptor
system responsible for the high sugar sensitivities of the
db/db mouse, we examined whether the
chorda tympani responses of db/db mice
to various nonsugar sweeteners, which would not be recognized by
pancreatic B cells, would also be greater than those of control and
whether the enhanced components of sweeteners responses in
db/db mice would be suppressed by a
new sweet response inhibitor, gurmarin (11, 15). The results suggested
that greater sensitivities in db/db
mice appeared not only to sugars but also to nonsugar sweeteners,
except D-phenylalanine, and the
enhanced sweetener responses probably occurred through
gurmarin-insensitive membrane components.
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MATERIALS AND METHODS |
Subjects
Genetically
diabetic
mice. Diabetic and nondiabetic male
littermates were obtained from mating pairs of the
C57BL/KsJ-db/db mouse strain
originally supplied from the Jackson Laboratories (Bar Harbor, ME). In
this strain, the closely linked mutant fur color gene, misty
(m), has been incorporated into
stocks for maintenance of db in
repulsion
(db+/m+)
to facilitate identification of heterozygotes for breeding. Therefore,
adult diabetic mice with a black fur color
(db+/db+:
8-20 wk of age, 50-62 g body wt) and nondiabetic mice with a
gray (misty) fur color
(m+/m+:
8-20 wk of age, 23-34 g body wt) were selectively obtained
from the stock. Diabetic mice with
db/db genotype are referred to as
db/db mice, whereas nondiabetic
control mice with +/+ genotype are referred to as control mice.
Measurement of blood glucose levels of diabetic and
nondiabetic mice. Before the start of experiments, the
blood glucose level of each mouse was measured by using a blood glucose
autoanalyzer (Reflorax; Manheim-Toho). The blood of the animal was
taken from the tail vein. The obtained blood glucose levels were 11.0 ± 0.98 mmol/l (n = 15) in control
mice and 36.1 ± 6.7 mmol/l (n = 12) in db/db mice.
Electrophysiological Experiment
Recording of the mouse chorda tympani
responses. Mice were anesthetized by an intraperitoneal
injection of pentobarbital sodium (40-50 mg/kg body wt) and
maintained at a surgical level of anesthesia with supplemental
injections. The trachea was cannulated, and the mouse was then fixed in
the supine position with a head holder to allow dissection of the
chorda tympani nerve. The hypoglossal nerve was transected bilaterally
to prevent inadvertent tongue movements. The right chorda tympani nerve
was exposed at its exit from the lingual nerve by removal of the
internal pterygoid muscle. The chorda tympani nerve was then dissected
free from surrounding tissues and cut near its entrance to the bulla.
For whole nerve recording, the entire nerve was placed on a silver wire
electrode. An indifferent electrode was positioned nearby in the wound.
Neural responses resulting from chemical stimulation of the tongue were fed into an amplifier (Iyodenshikogaku K-1) and displayed on an oscilloscope screen (Nihon Kohden VC-10). Whole nerve responses were
integrated and displayed on a recorder (Nihon Kohden WS-641G). The time
constant of the integrator was 1.0 s.
Chemical stimulation. The anterior
one-half of the mouse's tongue was enclosed in a flow chamber made of
silicone rubber (14). Solutions were delivered into the flow chamber by
gravity flow and flowed over the tongue for a controlled period.
Solutions used for chemical stimuli were as follows: 3.0 mM-1.0 M
sucrose, 0.1 mM-0.02 M saccharin sodium, 3.0 mM-1.0 M glycine, 3.0 mM-1.0 M L-alanine, 0.1 mM-0.03 M D-tryptophan, 1.0 mM-0.1 M D-phenylalanine, 0.1 M NH4Cl, 0.1 M NaCl, 0.01 M
HCl, and 0.02 M quinine HCl (all supplied from Nakarai Chemical, Osaka,
Japan). The first six chemicals, which taste sweet to humans, were
behaviorally categorized in the same group ("sweet group") in
this strain of mice by using a conditioned taste-aversion paradigm (Y. Ninomiya and K. Nakashima, unpublished observation). These
chemicals were dissolved in distilled water at 24°C. During
chemical stimulation of the tongue, test solution flowed for ~30 s at
the same flow rate as the distilled water used for rinsing the tongue
(0.5 ml/s). The tongue was rinsed during an interval of ~1 min
between successive stimulations. The stability of each preparation was
monitored by the periodic application of 0.1 M
NH4Cl. A recording was considered
to be stable when the 0.1 M NH4Cl
response magnitudes at the beginning and end of each stimulation series
deviated by no more than 15%. Only responses from stable recordings
were used in the data analysis.
To examine gurmarin inhibition of responses, the tongue was treated
with 1.0-100 µg/ml (~0.24-23.8 µM) gurmarin dissolved in 5 mM phosphate buffer (pH 6.8) for 10 min in the same manner as that
described in our previous reports (15, 16). Gurmarin, a peptide
consisting of 35 amino acids (molecular weight of 4,209), was obtained
from the leaves of the Indian plant Gymnema
sylvestre by using procedures previously reported (11).
Our previous study (15) showed that the time course of recovery from
suppression after gurmarin varies considerably among sweeteners (the
suppressed responses to fructose and saccharin started to recover
within 30 min after gurmarin, whereas those to sucrose and glucose
remained suppressed for >1 h). Therefore, to maintain stable levels
of the gurmarin inhibition, the tongue was repeatedly treated with gurmarin for 1 min with a 5-min intertreatment interval during recording of taste responses. Adaptation of the tongue to the buffer
without gurmarin by itself had no effect on the magnitude of response.
Data analysis. In the analysis of
whole nerve responses, the magnitude of the integrated response at 20 s
after stimulus onset was measured. Relative response magnitude for each
stimulus was calculated when the response magnitude to 0.1 M
NH4Cl was taken as a unity (1.0),
and this was used for statistical analysis.
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RESULTS |
Enhancement of Chorda Tympani Responses of the db/db Mice to
Sweeteners
Figure 1 shows integrated responses of the
chorda tympani nerve to 11 taste stimuli in the
db/db and control mice. In both db/db and control mice, responses to
NH4Cl, NaCl, HCl, and quinine HCl
showed the initial dynamic phase followed by the steady phase. Magnitudes of responses to these stimuli were similar between the two
groups of mice. In contrast, magnitudes of responses to sweeteners,
except D-phenylalanine, were
prominently larger in the db/db mouse
than the control mouse. As shown in Table
1, relative magnitudes of responses to the
five sweeteners in db/db mice were
1.4-1.6 times greater than those in control mice with statistical
significances (t-test,
P < 0.05-0.001), whereas no such difference was observed in responses to NaCl, HCl, quinine HCl,
and D-phenylalanine. The
order of the response magnitude among the six sweeteners in
db/db mice was not different from that
in control mice: 0.02 M saccharin > 0.3 M sucrose > 0.3 M glycine = 0.3 M L-alanine > 0.03 M
D-tryptophan > 0.1 M
D- phenylalanine.
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Fig. 1.
Sample recordings of integrated responses of the chorda tympani nerve
of control (+/+) and diabetic
(db/db) mice to 10 taste stimuli.
Qui, quinine HCl; Suc, sucrose; Sac, saccharin Na; Gly, glycine;
L-Ala,
L-alanine;
D-Try,
D-tryptophan;
D-Phe,
D-phenylalanine.
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Table 1.
Relative response magnitudes of the chorda tympani nerve of db/db and
+/+ mice to 10 taste stimuli when response magnitude to
NH4 Cl was taken as unity
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Concentration-response relationships for six sweeteners in
db/db and control mice are shown in
Fig. 2. At all concentrations tested,
relative magnitudes of responses to sucrose, saccharin, glycine,
L-alanine, and
D-tryptophan were significantly
greater in db/db than in control mice
(t-test,
P < 0.05-0.001). No such difference was observed in responses to
D-phenylalanine [ANOVA, F(1,10) = 0.699, P > 0.05]. Therefore,
D-phenylalanine may be an
exception, a sweetener to which responses are not enhanced in
db/db mice. Table
2 shows the dissociation constant
(Kd value) and
the maximum response
(Vmax value) for
each stimulus in control and db/db mice
measured by using data of the four highest concentrations of each
stimulus. Except for D-phenylalanine,
db/db mice showed lower Kd
values (ranging from 38.5 to 52.9% of that of control mice) and higher
Vmax values than control mice for the other five sweeteners (ranging from 117.4 to 132.2% of that of control mice).
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Fig. 2.
Concentration-response relationships of the chorda tympani nerve of
control ( ) and db/db ( ) mice for
6 sweeteners. Relative responses represented are means ± SD. Data
were obtained from 6 or 7 mice. A:
sucrose; B: saccharin;
C: glycine;
D:
L-alanine;
E:
D-tryptophan;
F:
D-phenylalanine.
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Table 2.
Kd and Vmax obtained from the kinetic analysis
of concentration-response relationships for 6 sweeteners in db/db
and +/+ mice
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Inhibition of Gurmarin on Responses to Sweeteners in db/db and
Control Mice
Figure 3 shows sample
records of the integrated responses to six sweeteners and
NH4Cl before and after the lingual
treatment with 100 µg/ml (~23.8 M) gurmarin for 10 min in
db/db and control mice. In both
db/db and control mice, responses to
the six sweeteners were suppressed by gurmarin, whereas no suppression
was observed in response to NH4Cl.
Although data were not shown, responses to the other basic taste
stimuli, NaCl, HCl, and quinine HCl, were not inhibited by gurmarin,
suggesting the specificity of the inhibitory effect of gurmarin on
sweeteners, as previously reported (11, 15). As shown in Table
3, except for
D-phenylalanine, the normalized
responses (percent responses: control = 100) to the other five
sweeteners after gurmarin were larger in
db/db than in control mice (see also
Fig. 6B). The mean (±SD) of
percent responses to the five sweeteners after 100 µg/ml gurmarin was 66.0 ± 5.1% in db/db mice, which
was significantly larger than that in control mice (52.0 ± 4.5%,
t-test,
P < 0.01).
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Fig. 3.
Sample recordings of integrated responses of the chorda tympani nerve
of control (+/+) and db/db mice to 0.1 M NH4Cl and 6 sweeteners before
and after the lingual treatment with 100 µg/ml gurmarin.
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Table 3.
Normalized control responses of the chorda tympani nerve of db/db and
+/+ mice to 6 sweeteners after the lingual treatment with 100 µg/ml gurmarin
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The concentration dependence of the effect of gurmarin also suggests
that the relative magnitudes of gurmarin suppression of responses to
sucrose are larger in control than in
db/db mice (Fig.
4). Significant suppression of sucrose
responses by gurmarin was observed at 3 µg/ml (~0.7 µM) or more
in control and 30 µg/ml (~7.1 µM) or more in
db/db mice
(t-test,
P < 0.05). In both strains, the
magnitude of gurmarin inhibition of sucrose responses reached a plateau
(percent responses of control: 50.8 ± 7.2% for control mice and
73.4 ± 5.7% for
db/db
mice) at 30 µg/ml (~7.1 µM). Percent responses after each
concentration of 3-100 µg/ml gurmarin in the two strains were
significantly different (t-test,
P < 0.05-0.01).
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Fig. 4.
Normalized response (percent response, control = 100%, before
gurmarin) of the chorda tympani nerve of control () and
db/db () mice to 0.3 M sucrose
after treatment with gurmarin at different concentrations. Effect of
gurmarin at each concentration was tested by using 4-8 mice.
Percent responses represented are means ± SD. Significant
difference from control was tested with data of relative magnitudes of
responses when the magnitude of responses to 0.1 M
NH4Cl was taken as unity (1.0):
t-test,
* P < 0.05;
** P < 0.01;
*** P < 0.001.
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Gurmarin-Sensitive and -Insensitive Components of Responses to
Sweeteners
Figure 5 shows
concentration-response relationships for sucrose of the
gurmarin-sensitive and -insensitive components. The gurmarin-sensitive
component of the response to each stimulus was obtained by subtracting
the residual response after gurmarin (100 µg/ml) from the response
before gurmarin. The gurmarin-insensitive component of each stimulus is
the residual response after gurmarin. It is noted that
gurmarin-insensitive components of sucrose responses in
db/db mice are much larger
(2.0-9.2 times) than those in control mice [ANOVA,
F(1,12) = 118.74, P < 0.001, and
t-test at each concentration, P < 0.001], whereas no
significant difference between the two groups is observed in
gurmarin-sensitive components
[F(1,12) = 1.55, P > 0.05]. The
Kd and
Vmax values of
gurmarin-sensitive components of sucrose responses in
db/db mice were ~61.7 mM and 0.44, respectively, which were not greatly different from those in control
mice (~84.0 mM and 0.53). In contrast, much larger differences
between the two mouse strains were observed in the Kd and
Vmax values of
the gurmarin-insensitive components. The Kd value in
db/db mice was 44.3% (~55.4 mM) of
that of control mice (~125.0 mM), whereas the
Vmax value in
db/db mice (1.22) was about two times
greater than that of control mice (0.60). Furthermore, the
gurmarin-insensitive components of responses to nonsugar sweeteners,
except D-phenylalanine are
~1.6-2.0 times greater in db/db
than control mice (Fig. 6,
t-test,
P < 0.01-0.001), whereas the
gurmarin-sensitive components were not different between the two groups
(t-test,
P > 0.05). These results suggest
that the differences found in chorda tympani responses to sweeteners between db/db and control mice are
derived mainly from the gurmarin-insensitive components of responses.
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Fig. 5.
Concentration-response relationships of gurmarin-sensitive
(A) and -insensitive
(B) components to sucrose in control
() and db/db () mice.
Gurmarin-sensitive component of response to each stimulus was obtained
by subtraction of the residual response after gurmarin (100 µg/ml)
from the response before gurmarin. Gurmarin-insensitive component to
each stimulus is the residual response after gurmarin. Relative
responses represented are means ± SD. Data were obtained from 7 mice.
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Fig. 6.
Gurmarin-sensitive (A) and
-insensitive (B) components of
responses of control (open bars) and
db/db (filled bars) mice to 6 sweeteners. Relative responses represented are means ± SD. Data
were obtained from 6 or 7 mice. Significant difference in response
components for each stimulus between control and
db/db
mice was tested: t-test,
** P < 0.01;
*** P < 0.001.
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DISCUSSION |
Larger Chorda Tympani Responses of db/db Mice to Nonsugar Sweeteners
The present study revealed that db/db
mice had larger responses of the chorda tympani nerve than control mice
not only to sucrose but also to the nonsugar sweeteners, except
D-phenylalanine. Responses to
the other basic taste stimuli, NaCl, HCl, and quinine HCl, were not
different between the two strains, as reported previously (19),
suggesting a specific increase in responses to sweet substances in
db/db mice (the specificity observed
in responses to D-phenylalanine is discussed later). The order of the relative magnitude of responses to six sweeteners tested in db/db mice
was not different from that in control mice. This indicates that
receptor sensitivities to sweet substances may be generally enhanced in
db/db mice. Our previous study (19)
demonstrated that in correspondence to peripheral neural responses
db/db mice showed higher behavioral
preferences to sugars in the two-bottle test than control mice.
Similarly, our recent behavioral study (6) using the two-bottle test
showed that preference scores for saccharin but not for
D-phenylalanine were larger in
db/db than in control mice. Because
the intake for these nonsugar sweeteners must not be on a caloric
basis, differential behavioral preferences to these sweeteners could at
least partially be due to the observed differential gustatory neural
sensitivities to them.
Inhibition of Chorda Tympani Responses to Sweeteners by Gurmarin and
Gurmarin-Sensitive and -Insensitive Response Components
Although chorda tympani responses of
db/db mice to all sweeteners tested
were significantly suppressed by 100 µg/ml gurmarin, the degree of
suppression was smaller and the threshold of gurmarin inhibition was
higher than that in control mice. As shown in Fig. 5, the gurmarin
inhibition on sucrose responses reached the maximum at 30 µg/ml in
both strains. Therefore, differences in the percent suppression cannot
be due to concentration of gurmarin tested (100 µg/ml). Thus the
results may imply relatively low sensitivity of the receptor system of
db/db mice to gurmarin. However,
comparison of magnitudes of the gurmarin-sensitive and -insensitive
components of responses showed that 100 µg/ml gurmarin suppressed
almost equal amounts of responses to various sweeteners in
db/db and control mice (Fig. 6). No
significant difference was observed in the magnitude of
gurmarin-sensitive components to various sweeteners in two strains.
This suggests that gurmarin might fully block the activity of
gurmarin-sensitive sweet receptor system in both strains at
concentrations of 30 µg/ml or more. Because the remaining gurmarin-insensitive responses to sweeteners probably are much larger
in db/db than control mice, this would
result in the observed smaller percent suppression in
db/db mice. The kinetic analysis of
the concentration-response relationship for sucrose further suggests
that differences in the
Kd and
Vmax values for
sucrose responses between control (~125.3 mM and 1.21) and
db/db mice (~48.3 mM and 1.60) may
be solely due to differences in the values for gurmarin-insensitive
components between the two strains [control mice, ~125.0 mM and
0.60 (+0.53 of the gurmarin-sensitive component); db/db mice, ~55.4 mM and 1.22 (+0.44
of the gurmarin-sensitive component)].
Existence of two different receptor systems for sweeteners,
gurmarin-sensitive and -insensitive ones, has been strongly suggested by two major findings in our previous studies in mice (15, 16). One is
the strain difference in gurmarin inhibition. Chorda tympani responses
to sweet substances in C57BL mice were suppressed by gurmarin to
~50% of control, whereas no such inhibition was observed in BALB
mice (15). The other is the nerve-specific inhibition of gurmarin. Even
in C57BL mice, responses of the glossopharyngeal nerve innervating
taste buds in the circumvallate and foliate papillae were not
suppressed by gurmarin (16). These strain and nerve specificities
observed in inhibition of sweet responses by gurmarin were very similar
to those found in inhibition of salt responses by amiloride, a sodium
channel blocker (18, 20). It has been shown that the lingual treatment
with a proteolytic enzyme, pronase, specifically and totally suppressed
chorda tympani responses to various sweeteners ("sweet response
component") in rats (9) and sweet perception in humans (9). In mice,
for most sweeteners tested, their residual responses after gurmarin treatment (~50% of control) were, therefore, further suppressed to
baseline levels by pronase (16, 21). This indicates that the sweet
response component for the sweeteners in mice occurs through both
gurmarin-sensitive and -insensitive receptor components.
The present study showed that chorda tympani responses to
D-phenylalanine, unlike the
other sweeteners, did not differ between db/db and control mice. In our
previous studies in C57BL mice, it was found that responses to
D-phenylalanine were suppressed by gurmarin and pronase to similar extents (~50% of control) (15, 17), and unlike those to the other various sweeteners the residual responses to D-phenylalanine
after gurmarin were not suppressed by further treatment with pronase.
This suggests that the sweet response component to
D-phenylalanine would occur
exclusively through the gurmarin-sensitive sweet receptors. No
significant difference in response to
D-phenylalanine between
db/db and control mice observed in
this study may, therefore, give another indication of no difference in
gurmarin sensitivity in the two strains.
As mentioned above, sweetener responses of the mouse glossopharyngeal
nerve were gurmarin insensitive. Our recent preliminary study showed
that relative magnitudes of responses of the glossopharyngeal nerve to
sucrose and saccharin were also larger in
db/db than in control mice (Y. Ninomiya, M. Inoue, and T. Imoto, unpublished observation). Taken together, the results of this study
suggest that the observed enhancement of sweetener sensitivities in
db/db mice would occur through the
gurmarin-insensitive component of sweet receptor systems.
What Is the Major Determinant for Enhanced Sweet Responses in db/db
Mice?
Previous studies have demonstrated that streptozotocin- and
alloxan-induced diabetic mice and rats showed no detectable difference in chorda tympani responses to sugars compared with the control animals
(8, 15). This suggests that diabetic status itself would not be the
primary influence on the neural responses to sugars. Moreover, it was
found that the db/db mice at 7-9
days of age already have higher chorda tympani responses to sugars, comparable with those in the adult
db/db mice (15). At this age,
db/db mice also started to show
insulin hypersecretion in response to glucose stimulation (1), although
the pathophysiological states of db/db
mice, such as obesity, hyperglycemia, or transient hyperinsulinemia,
become clearly evident at 3-5 wk of age (2, 3). If there is a
common mechanism for the hyperresponsiveness in the two different cell
types, that would not be at the recognition site for stimulants,
because the proposed mechanism of glucose recognition is different
between the two types of cells (13). The taste cell is thought to
recognize glucose at its receptor site, whereas the pancreatic B cell
is believed to metabolize glucose for signal production for insulin
release. Consistently, the present results indicate that greater
responses in db/db mice appeared not
only to sugars but also to nonsugar sweeteners, which would not be
recognized by pancreatic B cells. The factor responsible for
hyperresponsiveness to sugar would be more likely involved in the
intracellular transduction mechanisms.
Recently, it was found that db/db mice
have defects at the receptor for the
ob gene product leptin, which is an
important circulating signal for the regulation of body weight (a
weight-reducing factor) (22). In db/db
mice, the leptin receptor is abnormally spliced and is missing the
cytoplasmic region responsible for signal transduction (12). The leptin
receptor resembles class 1 cytokine receptors, such as the gp130
signal-transducing component of the interleukin-6 receptor, the
granulocyte colony-stimulating factor receptor, and the leukemia
inhibitory factor receptor (5), and is expressed in
various tissues, including adipose tissue, hypothalamus, choroid
plexus, lung, kidney, liver, and muscle (10). More recently, it has
been reported that the full-length leptin receptor, which is believed
to transmit the leptin signal, is expressed in pancreatic islets (4),
and leptin suppressed insulin secretion of the pancreatic B cells by
the activation of ATP-sensitive K+
channels in mice (7). Therefore, it is possible that the leptin receptor is also expressed in taste cells and that the circulating leptin modifies activities of taste cells through the receptor. If the
increasing level of leptin would decrease activities of taste cells in
normal control mice, lack of signal transduction for leptin in
db/db mice may increase their
activities and lead to hyperresponsiveness to sugars. The results
obtained from this study lead to further speculation that the
db gene may express the leptin
receptor specifically in taste cells possessing gurmarin-insensitive sweet receptors. Investigations on possible expression of leptin receptor and modifying effects of leptin in taste cells are now in
progress. Future extensive studies on the action of the
db gene may lead to an answer to these
questions.
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ACKNOWLEDGEMENTS |
We thank Dr. Bruce P. Bryant for valuable comments and suggestions
on the manuscript.
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FOOTNOTES |
This work was supported by Grant-in-Aid 09470407 and 09557147 for
Scientific Research from the Ministry of Education, Science and Culture
of Japan.
Address for reprint requests: Y. Ninomiya, Dept. of Oral Physiology,
Asahi Univ. School of Dentistry, Hozumi, Motosu, Gifu 501-02,
Japan.
Received 1 October 1997; accepted in final form 22 January 1998.
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REFERENCES |
1.
Basabe, J. C.,
L. M. Karabatas,
M. Arata,
O. H. Pivetto,
and
J. C. Cresto.
Secretion and effect of somatostatin in early stages of the diabetic syndrome in C57BL/KsJ-mdb mice.
Diabetologia
29:
485-488,
1986[Medline].
2.
Coleman, D. L.,
and
K. P. Hummel.
Studies with the mutation diabetes in the mouse.
Diabetologia
29:
485-488,
1967.
3.
Coleman, D. L.,
and
K. P. Hummel.
Hyperinsulinemia in the preweaning diabetes (db) mice.
Diabetologia
10:
607-610,
1974.
4.
Emilsson, V.,
Y. Liu,
M. A. Cawthorne,
N. M. Morton,
and
M. Davenport.
Expression of the functional leptin receptor mRNA in pancreatic islets and direct inhibitory action of leptin on insulin secretion.
Diabetes
46:
313-316,
1997[Abstract].
5.
Flier, J. S.
Leptin expression and action: new experimental paradigms.
Proc. Natl. Acad. Sci. USA
94:
4242-4245,
1997[Free Full Text].
6.
Kawamura, S.,
Y. Ninomiya,
A. Yatabe,
S. Yoshida,
and
Y. Fukami.
The effect of age on preference for sweet substances in diabetic db/db mice (Abstract).
Chem. Senses
22:
340,
1997.
7.
Kieffer, T. J.,
R. S. Heller,
C. A. Leech,
G. Hplz,
and
J. F. Habner.
Leptin suppression of insulin secretion by the activation of ATP-sensitive K+ channels in pancreatic cells.
Diabetes
46:
1087-1093,
1997[Abstract].
8.
Hiji, Y.
Gustatory response and preference behavior in alloxan diabetic rats.
Kumamoto Med. J.
22:
109-118,
1969[Medline].
9.
Hiji, Y.
Selective elimination of taste responses to sugars by proteolytic enzymes.
Nature
256:
427-429,
1975[Medline].
10.
Hoggard, N.,
J. G. Mercer,
D. V. Rayner,
and
K. Moar.
Localization of leptin receptor mRNA splice variants in murine peripheral tissues by RT-PCR in situ hybridization.
Biochem. Biophys. Res. Commun.
232:
383-387,
1997[Medline].
11.
Imoto, T.,
A. Miyasaka,
R. Ishima,
and
K. Akasaka.
A novel peptide isolated from the leaves of Gymnema sylvestre. I. Characterization and its suppressive effect on the neural responses to sweet taste stimuli in the rat.
Comp. Biochem. Physiol. A Physiol.
100A:
309-314,
1991.
12.
Lee, G. H.,
R. Proenca,
J. M. Montez,
K. M. Carroll,
J. G. Darvishzadeh,
J. I. Lee,
and
J. M. Friedman.
Abnormal splicing of the leptin receptor in diabetic mice.
Nature
379:
622-625,
1996.
13.
Niki, A.,
H. Niki,
and
T. Hashioka.
Receptors of paraneurons with special reference to glucoreceptors.
Arch. Histol. Cytol.
52:
33-38,
1989.
14.
Ninomiya, Y.,
and
M. Funakoshi.
Responses of rat chorda tympani fibers to electrical stimulation of the tongue.
Jpn. J. Physiol.
31:
559-570,
1981[Medline].
15.
Ninomiya, Y.,
and
T. Imoto.
Gurmarin inhibition of sweet taste responses in mice.
Am. J. Physiol.
268 (Regulatory Integrative Comp. Physiol. 37):
R1019-R1025,
1995[Abstract/Free Full Text].
16.
Ninomiya, Y.,
M. Inoue,
T. Imoto,
and
K. Nakashima.
Lack of gurmarin sensitivity of sweet taste receptors innervated by the glossopharyngeal nerve in C57BL mice.
Am. J. Physiol.
272 (Regulatory Integrative Comp. Physiol. 41):
R1002-R1006,
1997[Abstract/Free Full Text].
17.
Ninomiya, Y.,
T. Nomura,
and
H. Katsukawa.
Genetically variable taste sensitivity to D-amino acids in mice.
Brain Res.
596:
349-352,
1992[Medline].
18.
Ninomiya, Y.,
N. Sako,
and
M. Funakoshi.
Strain differences in amiloride inhibition of NaCl responses in mice.
J. Comp. Physiol. [A]
166:
1-5,
1989[Medline].
19.
Ninomiya, Y.,
N. Sako,
and
Y. Imai.
Enhanced gustatory neural responses to sugars in the diabetic db/db mouse.
Am. J. Physiol.
269 (Regulatory Integrative Comp. Physiol. 38):
R930-R937,
1995[Abstract/Free Full Text].
20.
Ninomiya, Y.,
T. Tanimukai,
S. Yoshida,
and
M. Funakoshi.
Gustatory neural responses in preweanling mice.
Physiol. Behav.
49:
913-918,
1991[Medline].
21.
Sako, N.,
Y. Ninomiya,
and
Y. Fukami.
Analysis of concentration-response relationship for enhanced sugar responses of the chorda tympani nerve in the diabetic db/db mouse.
Chem. Senses
21:
59-63,
1996[Abstract/Free Full Text].
22.
Zhang, Y.,
R. Proenca,
M. Maffei,
M. Barone,
L. Leopold,
and
J. M. Friedman.
Positional cloning of the mouse obese gene and its human homologue.
Nature
372:
425-432,
1994[Medline].
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Abstract
Introduction
