Chem. Senses 27: 57-65,
2002
© Oxford University Press 2002
Electrophysiological Studies of Salty Taste Modification by Organic Acids in the Labellar Taste Cell of the Blowfly
1 Department of Life Science, Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan 2 Department of Human Environment, Faculty of Human Development, Kobe University, Kobe 657-8501, Japan
Correspondence to be sent to: Naoko Kataoka-Shirasugi, Department of Human Environment, Faculty of Human Development, Kobe University, Kobe 657-8501, Japan. e-mail: naoshika{at}main.h.kobe-u.ac.jp
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Abstract |
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Using the labellar salt receptor cells of the blowfly, Phormia regina, we electrophysiologically showed that the response to NaCl and KCl aqueous solutions was enhanced and depressed by acetic, succinic and citric acids. The organic acid concentrations at which the most enhanced salt response (MESR) was obtained were found to be different: 0.05-1 mM citric acid, 0.5-2 mM succinic acid and 5-50 mM acetic acid. Moreover, the degree of the salt response was not always dependent on the pH values of the stimulating solutions. The salt response was also enhanced by HCl (pH 3.5-3.0) only when the NaCl concentration was greater than the threshold, indicating that the salty taste would be enhanced by the comparatively lower concentrations of hydrogen ions. Another explanation for the enhancement is that the salty taste may also be enhanced by undissociated molecules of the organic acids, because the MESRs were obtained at the pH values lower than the pKa1 or pKa2 values of these organic acids. On the other hand, the salty taste could be depressed by both the lower pH range (pH 2.5-2.0) and the dissociated organic anions from organic acid molecules with at least two carboxyl groups.
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Introduction |
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Salty taste is modified by organic acids in humans. Fabian and Blum showed that the salty taste of 0.1 M NaCl was enhanced by acetic, citric, lactic, malic and tartaric acids at such low concentrations that their taste was not recognized as sour (Fabian and Blum, 1942
). Kamen et al. also showed by sensory evaluations
that, at various concentrations of either NaCl or citric acid, the salty taste
of NaCl was enhanced by citric acid (Kamen
et al., 1961). On the other hand, the salty taste could
be depressed by organic acids because acid seasonings such as vinegar are
experientially applied to weaken the salty taste in food. Though salty taste
modification by organic acids is being understood psychologically or
experientially, it is hardly understood physiologically. Physiological data
concerning salty taste modification by organic acids would be valuable for
evaluating at least two things: (i) whether salty taste is modified by organic
acids peripherally or centrally; and (ii) how salty taste is both enhanced and
depressed by organic acids.
Both salty and sour tastes are directly induced via ion channels on taste
receptor cells. Recently, the ion channel molecule candidates relevant to
salty (Kretz et al.,
1999; Lin et al.,
1999) and sour tastes
(Waldmann et al.,
1997; Chen et al.,
1998; Ugawa et al.,
1998) were identified by immunocytochemical and molecular
biological experiments. Additionally, salty as well as sour tastes are
generally elicited by electrolytes. These common characteristics between salty
and sour tastes prompted us to set up the hypothesis that salty and sour taste
stimuli may interact with each other on the same receptor site of a taste
cell.
The structural simplicity of the taste organs of flies compared to
vertebrates makes them an attractive model system for electrophysiological
studies. Each chemosensillum of a fly possesses four functionally
differentiated taste receptor cells: the salt, sugar, water and fourth taste
receptor cells. Tip (Hodgson et
al., 1955) and sidewall
(Morita, 1959) recording
methods for the labellar chemosensillum of the fly have enabled us to record
the electrophysiological responses of a single taste receptor cell without
cell isolation. The impulses from four taste receptor cells recorded by these
methods are easily distinguished by their amplitude from each other.
Therefore, one can expect that salty taste modification by organic acids in
the fly salt receptor cell would be easily observed by the tip or sidewall
recording methods. Hence, we used the blowfly for elucidating the mechanism
behind the organic-acid-induced change of salty taste.
In the present paper, we electrophysiologically show salty taste modification by organic acids at the taste receptor cell level in the fly and discuss the possible roles of hydrogen ions, organic anions derived from the organic acids and undissociated forms of them on this phenomenon. Three kinds of organic acids, chemically different in the number of carboxyl groups and alkyl chain length, were chosen for this study: acetic acid (CH3COOH), succinic acid (HOOCCH2CH2COOH) and citric acid [HOOCCH2C(OH) (COOH)CH2COOH].
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Materials and methods |
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Fly
Adult blowflies (Phormia regina, 5-12 days old) were used for our experiments. They were reared in the laboratory at 24 ± 1°C. They were fed on chicken liver and yeast bait at the larval stage and 100 mM sucrose and water at the adult stage.
Electrophysiological procedures
Impulses from the taste cells of the blowfly were recorded using the tip
recording method (Hodgson et al.,
1955) and impulses due to organic acid solutions with no salt were
recorded by the sidewall recording method
(Morita, 1959). Two platinum
electrodes were used for the tip recording method: one was an indifferent
electrode, inserted into the isolated head of a fly; the other was a recording
electrode, inserted into a glass capillary containing a stimulating solution.
Impulses, induced by capping the tip of the labellar LL-type chemosensilla
with a glass capillary, were recorded on magnetic tape through a band-path
filter (100-2000 Hz). LL-type are the largest of the labellar chemosensilla
(Wilczek, 1967). The impulses
from the salt, sugar and water cells were distinguished from each other by
their amplitudes. The number of impulses generated during 0.15-0.35 s after
the beginning of stimulation was counted as the magnitude of the response of
the taste cells. In some cases, the magnitude of the salt response was
represented as the relative response normalized to that of 2 M NaCl. A sucrose
solution (50 mM) was used as a sugar stimulus and contained 10 mM NaCl to
maintain electrical conductance. Each stimulus was given at an interval of 3
min or more to avoid adaptation effects. During the recordings, the relative
humidity in the laboratory was >60% in order to maintain the concentration
of the stimulating solution at the tip of the glass capillary. All experiments
were performed at room temperature (22-26°C).
Proboscis extension reflex (PER) test
Flies which had been starved but given water for 36 h were used for the PER
tests in order to avoid the effects of blood sugar levels on PER
(Dethier and Bodenstein, 1958).
The flies were held by the wings with small clothespins. Prior to the test,
the flies had been given sufficient water so that they would not respond to
water. Test solutions were applied to the labellum with a disposal plastic tip
(used for an automatic pipette) having a diameter of 
500 µm. The
number of flies showing the feeding response of full proboscis extension
within 2 s was counted.
Chemicals
All chemicals were of reagent grade and were purchased from Wako Pure Chemicals Industries Ltd (Osaka, Japan).
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Results |
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Identification of impulses
Figure 1 shows a typical record of impulses in response to 200 mM NaCl (the salt response), 50 mM sucrose (the sugar response), 10 mM NaCl (the water response) and 10 mM NaCl plus acetic acid, in the same labellar LL-type sensillum. Impulses of the salt, sugar, water and fourth chemoreceptor cells of a labellar LL-type sensillum are usually identified by comparing their amplitude. Ten millimolar NaCl plus 30 mM acetic acid (pH 3.05) induced two types of impulses differing in their amplitudes: the smaller one and the larger one (Figure 1Ad). The amplitude of the smaller impulse was the same as that of 10 mM NaCl, clearly indicating that the smaller impulse comes from the water receptor cell. On the other hand, the larger impulse was not clearly identified because its amplitude was significantly different (P < 0.001) from 200 mM NaCl, 50 mM sucrose and 10 mM NaCl (Figure 2). The amplitude of the larger impulse was found to be smaller than that of 200 mM NaCl and larger than that of 50 mM sucrose.
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Histological observations of a labellar LL-type chemosensillum
(Dethier, 1976) along with the
generation of four types of injury currents by the chemosensilla tip following
treatment with a strong detergent, soyasaponin I (Amakawa, unpublished data)
confirmed that no other cells are present except the four taste receptor
cells, the so-called sugar, salt, water and fourth chemoreceptor cells in the
labellar chemosensillum of the blowfly. Therefore, the larger impulse induced
by 10 mM NaCl plus 30 mM acetic acid should be formed from the impulses evoked
from any of these four cells. One of the possibilities is that the impulse
generated by any of these four cells may be transformed by some effects that
ultimately led to the generation of an impulse of larger amplitude (see
Discussion).
To identify the origin of the larger impulse induced by 10 mM NaCl plus 30
mM acetic acid, we used another method—the PER test.
Figure 3 shows the results
obtained by the PER test with 50 mM sucrose, 100 mM NaCl and 10 mM NaCl plus
organic acids. Ten millimolar NaCl plus 30 mM acetic acid induced a PER of
only 8% in 50 tested flies, indicating that the fly shows a negative feeding
to 10 mM NaCl plus 30 mM acetic acid, which means that the larger impulse does
not come from the sugar receptor cell. The larger impulse does not come from
the water receptor cell either, because the response to 10 mM NaCl plus 30 mM
acetic acid in Figure 1Ad
consists of the larger impulse and the impulse from the water receptor cell.
Neither does the larger impulse come from the fourth chemoreceptor cell,
because the impulse from the fourth chemoreceptor cell, induced by 250 mM CsCl
(Gillary, 1966b), was also
observed in the response to 10 mM NaCl plus 30 mM acetic acid mixed with 250
mM CsCl different to the larger impulse (data not shown). These results
strongly disagree with the likelihood that the larger impulse induced by 10 mM
NaCl plus 30 mM acetic acid could come from the sugar, water or fourth
chemoreceptor cells; hence, we conclude that the larger impulse is the salt
response. We also conclude, according to the same procedures as in the case of
acetic acid, that the larger impulse induced by the salts plus succinic or
citric acid is the salt response.
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Salt response to NaCl or KCl plus organic acids
Figure 4 shows the change in salt responses to NaCl plus acetic, succinic and citric acids. The salt responses to 10, 50 and 100 mM NaCl were enhanced by acetic, succinic and citric acids, respectively. The organic acid concentrations giving the most enhanced salt response (MESR) were different and found to be in the order citric acid < succinic acid < acetic acid. The MESR to 10 mM NaCl was obtained by 0.5 mM citric acid, 1.5 mM succinic acid and 30 mM acetic acid (Figure 4a); for 50 mM NaCl, the values were 0.5 mM citric acid, 1.0 mM succinic acid and 40 mM acetic acid (Figure 4b); and for 100 mM NaCl, the required concentrations were 0.1 mM citric acid, 0.5 mM succinic acid and 5 mM acetic acid (Figure 4c). At the MESRs, the organic acids enhanced the relative response to NaCl by 0.20-0.30 in 10 and 50 mM NaCl and by 0.10-0.15 in 100 mM NaCl.
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Figure 5 shows the change in the salt responses to KCl plus acetic, succinic and citric acids. The results for KCl were similar to those obtained with NaCl. The salt responses to 10, 50 and 100 mM KCl were enhanced by the three organic acids. Although the order of the organic acid concentrations for the MESR was the same (i.e. citric acid < succinic acid < acetic acids), the organic acid concentrations were different than in case of NaCl. The MESR for 10 mM KCl was obtained by 1 mM citric acid, 2 mM succinic acid and 50 mM acetic acid (Figure 5a), for 50 mM KCl by 1 mM citric acid, 2 mM succinic acid and 10 mM acetic acid (Figure 5b) and for 100 mM KCl by 0.05 mM citric acid, 2 mM succinic acid and 5 mM acetic acid (Figure 5c). At the MESRs, the organic acids enhanced the relative response to KCl by the same degrees as observed in NaCl.
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Each organic acid itself evoked no salt response, a typical record of impulses of which is shown in Figure 1B: the salt response induced by 10 mM NaCl plus 30 mM acetic acid was not observed when 30 mM acetic acid was applied. The salt response to NaCl and KCl was enhanced by the low concentrations of organic acids, while it was depressed by high concentrations.
Figure 1Ae shows a typical record of impulses for the salt response depression: the salt response induced by 10 mM NaCl plus 30 mM acetic acid was hardly observed when 10 mM NaCl plus 500 mM acetic acid (pH 2.42) was applied. The salt response to 50 or 100 mM NaCl and KCl was also greatly depressed by the large amounts of organic acids. Moreover, in the case of 50 mM NaCl and KCl, the salt response was exponentially depressed by each organic acid (Figures 4b and 5b).
pH values at MESR in NaCl
Table 1 shows the pH values of NaCl plus various concentrations of organic acids giving the MESR. The pH values were different among the three kinds of organic acids at all concentrations of NaCl. Especially in the case of 10 and 50 mM NaCl, the pH values with acetic acid were lower than those with succinic and citric acids. The pH values of NaCl plus acetic or succinic acid shown in Table 1 were less than the pKa1 values of each organic acid, indicating that higher proportions of undissociated acetic and succinic acid molecules exist at chemical equilibrium in NaCl solutions than in dissociated ones. For citric acid, the pH values were within the range between pKa1 and pKa2, indicating that the citric acid molecules dissociated at one carboxyl group exist in the highest proportion at chemical equilibrium in the NaCl solutions (see Table 2).
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Salt response to NaCl plus HCl
Figure 6 shows the effects
of pH on the salt response to NaCl aqueous solutions when mixed with HCl. The
salt response to 10 mM NaCl was hardly changed from pH 5.4 through pH 3.0 and
enhanced only at pH <2.5, which was consistent with the results on the
fleshfly (Shiraishi and Morita,
1969). The salt response to 50 mM NaCl was enhanced at pH 3.0. The
enhanced salt response was depressed at pH 2.0, though it recovered to some
extent at pH 1.5. The salt response to 100 mM NaCl was also enhanced at pH
3.0. The enhanced salt response was strongly depressed at pH < 2.0.
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Dose—response curves of NaCl and sodium salts of organic acids
Figure 7 shows the
relationship between sodium salt concentrations and the salt responses to
them. The salt response to NaCl was almost proportional to the log scale of
its concentrations, which followed the results of previous experiments
(Evans and Mellon, 1962;
Gillary, 1966a). Although the
level of saturated salt response to sodium acetate was slightly depressed
compared to NaCl, the dose—response curve of sodium acetate was shifted
to the left of that of NaCl, suggesting that acetate ions raise the
sensitivity of salt receptor cells to sodium ions. On the other hand, the
dose—response curve of sodium succinate or sodium citrate was shifted to
the right of that of NaCl. The level of saturated salt response to sodium
succinate or sodium citrate was strongly depressed compared to NaCl
response.
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Salt response to NaCl plus sodium salts of organic acids
We also investigated whether the sodium salts of organic acids at the concentrations giving the MESR (Figure 4a) could also enhance the salt response to 10 mM NaCl. The results were clearly different between acetate and the others, as shown in Figure 8. Thirty millimolar sodium acetate in 10 mM NaCl evoked almost the same frequency of salt impulses as 100 mM NaCl. On the other hand, 1.5 mM sodium succinate and 0.5 mM sodium citrate in 10 mM NaCl evoked as few salt impulses as 10 mM NaCl. These results are in agreement with the previous idea that acetate ions increase the sensitivity of the salt response to sodium ions.
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Organic acids modify the salty taste at the taste receptor cell
The salty taste modification by organic acids seen in humans was also
observed at the taste cell of the blowfly. Salt response enhancements by the
comparatively lower concentrations of the organic acids in the fly taste
receptor cell were consistent with the results in humans measured by sensory
evaluations (Fabian and Blum,
1942). The salt response depression by the comparatively higher
concentrations of the organic acids also seemed to be consistent with the
phenomenon in humans that the strong salty taste would be depressed by acid
seasonings such as vinegar. Additionally, the salt concentrations affected the
degree of enhancement in the salt response by the organic acids in the fly as
well as in humans. It has been shown
(Kamen et al., 1961)
in the case of citric acid that the degree of enhancement in salty taste
intensity declined in proportion to NaCl concentration. We also found that the
degrees of enhancement in the salt response by organic acids varied with
different salt concentrations (Figures
4 and
5). In both NaCl and KCl, the
response to 10 and 50 mM salt solutions was more enhanced compared to 100 mM
salt solutions. These correspondences suggest that this salty taste
modification by organic acids in humans could be determined peripherally, at
the taste cell, though the concentrations of the organic acids giving the MESR
were different between fly and human. Additionally, both the fly and humans
might possess a common mechanism for salt reception at the taste cell.
Hydrogen ions and undissociated molecules of organic acids are involved in the enhancement of salty taste
Comparatively lower concentrations (3.2 x 10-4-1.0 x 10-3 M) of hydrogen ions are required to enhance the salty taste. As shown in Figure 6, HCl proportionally enhanced salt responses to 50 and 100 mM NaCl from pH 5.4 to pH 3.0. The effect of the same pH range in the enhancement of the salt response by organic acids suggests that hydrogen ions may enhance the salt response to some extent. However, HCl did not enhance the salt response to 10 mM NaCl that elicits little salt response in the blowfly in the same pH range. This result indicates that hydrogen ions have no ability to elicit the salt response by themselves; rather, they act on the enhancement of the salt response only when the salt receptor cells are excited. Additionally, the pH values of NaCl plus the organic acids giving the MESR did not correspond to each other as shown in Table 1, indicating that the enhancement could not be explained by pH dependence. This raises the possibility of the involvement of other molecular species than hydrogen ions in the enhancement of salt responses in the blowfly.
From our experimental results, we predict that undissociated molecules of
the organic acids may be probable candidates.
Table 2 shows the existing
concentrations and percentages of undissociated and dissociated organic acid
molecules in 10 mM NaCl plus the organic acid solutions, calculated from the
pH and pKa values. In the concentration of organic acids
at which the MESR was obtained in 10 mM NaCl, most molecules of each organic
acid existed undissociated: 97.6% of 30 mM acetic acid; 80.0% of 1.5 mM
succinic acid; and 40.6% of 0.5 mM citric acid. It has been reported
(Ogiso et al., 2000)
that undissociated weak acids can be a stimulant for the taste receptor cells,
based on investigations of the chorda tympani nerve response of the rat.
Although their report was related to sour taste reception, the sour taste
reception mechanism is generally proposed to be similar to that of salty taste
and mediated directly via ion channels. Therefore, undissociated organic acids
may be involved in the enhancement of the salt response. However, acetic,
succinic or citric acid did not induce the salt response by itself, which was
recorded by the sidewall recording method
(Figure 1B), showing that the
presence of the salts is necessary for the organic acids to activate the salt
receptor cell.
The concentration differences of acetic, succinic and citric acid giving
the MESR shown in Figures 4 and
5 could depend on the chemical
structure of the organic acids, as their undissociated molecules are involved
in the enhancement of the salt response. It has been suggested
(Hatano et al., 2000)
that the salt response stimulatory capacity of the benzene sulfonic acid
analogs is structurally specific in the taste cells of the fleshfly. The
differences in the chemical structures of the organic acids could be one of
the factors determining the ability of organic acids to activate the salt
receptor cell.
Hydrogen ions and organic anions are involved in the depression of salty taste
Unlike the role of the hydrogen ions in the enhancement of salty taste, comparatively higher concentrations of hydrogen ions (that is to say, the lower pH of the stimulating solution) depresses salty taste. As shown in Figure 6, HCl depressed the enhanced salt response to 50 and 100 mM NaCl from pH 3.0 through pH 2.0. These results suggest that the depression of the salt response by organic acids in the same pH range may be due to the lower pH range. Especially, the pH values of 10 mM NaCl plus acetic acid and 50 mM NaCl plus acetic acid at the MESR (Table 1) almost correspond to pH 3.0, at which the salt response was most enhanced by HCl (Figure 6). This result suggests that hydrogen ions alone might depress the salt response in the case of acetic acid. Hydrogen ions would inactivate the salt receptor cell owing to the denaturation of ion channels in the receptor membrane by the lowering of pH values. However, the pH values of NaCl plus succinic and citric acids at the MESR (Table 1) did not correspond to pH 3.0. In the case of these organic acids, the salt response tended to be depressed over pH 3.0. These results raise the possibility of involvement of other molecular species than hydrogen ions in the depression of salty taste.
Organic anions may also work as effective molecules to depress the salt
response in the case of succinic and citric acids. Sodium succinate and
citrate generally elicited less salt response than NaCl, while sodium acetate
elicited more than NaCl (Figure
7), consistent with the results that the salt response to 10 mM
NaCl was not increased by sodium succinate or citrate, though it was increased
by sodium acetate (Figure 8).
The test solutions used in these experiments covered a wide range of pH
values, i.e. pH 2-8. We can predict that the ionization state of groups on the
membrane surface is affected by the pH of the stimulating solution.
Considering that both succinic and citric acids, beginning to depress the MESR
in NaCl (Figures 4 and
5), were dissociated in the
same order of concentrations—i.e. 0.2-0.5 mM, which values were obtained
by calculation (data not shown)—the depressing effect of the succinate
and citrate ions on the salt response to sodium ions can also be explained,
though the pH values of the stimulating solutions were pH 2-4 in Figures
4 and
5, different from those
containing sodium salts of the organic acids as shown in Figures
7 and
8. The effects of the organic
anions on salty taste at the taste cell have previously been reported
(Ye et al., 1991).
Their report suggested that, in the rat, the reception of salty taste stimuli
involves the paracellular pathway via the tight junctions between the taste
cells, as well as the transcellular pathway via ion channels in the apical
membrane. They also showed that the anions derived from salty taste stimuli
affect the paracellular pathway dependent on their permeability via the tight
junctions. This explanation is not in agreement with our results, since
acetate ions enhance the salt response in the blowfly
(Figure 8). Because of their
larger ionic size, acetate ions are not expected easily to permeate the tight
junction compared to chloride ions. Based on our results, we suggest that
succinate and citrate ions could simply interact with the surface of the
distal membrane related to the transcellular pathway. One can expect that they
may electrically interact with the positively charged groups, such as
—NH3+ groups, on the membrane surface to depress
the salt response.
The response difference between acetate ions and the succinate and citrate ions could depend on the chemical structure of the organic anions, as only succinate and citrate ions are involved in the depression of the salt response. The number of carboxyl groups in the respective organic acid molecules could explain this: acetate, one; succinate, two; citrate, three. Organic anions possessing at least two carboxyl groups would depress the salt response. Acetate ions did not depress the salt response but enhanced it, because acetate ions possess only one carboxyl group.
Inward current may reduce the amplitude of the impulse from the salt receptor cell
The amplitude of the impulses induced by the salts plus organic acids was significantly smaller than that induced by 200 mM NaCl, though these impulses came from the salt receptor cell. To investigate this difference in more detail, the shapes of the impulses were compared with each other by enlarging and superimposing the impulses obtained in Figure 1 (Figure 9). This analysis indicated that the impulse induced by 10 mM NaCl plus 30 mM acetic acid was slightly shorter than that induced by NaCl at the former part of a positive phase (Figure 9d), though the impulses induced by 10 mM NaCl plus 30 mM acetic acid (Figure 9a) and 200 mM NaCl (Figure 9b) were commonly biphasic.
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The biphasic salt impulse of the fly, recorded by the tip-recording method,
is mediated by the summation of the orthodromic and antidromic phases, which
is the backfiring of the orthodromic phase through a dendrite in a labellar
chemosensillum (Morita and Yamashita,
1959). The biphasic salt impulse has been reported to become
smaller because of conduction delaying
(Morita and Yamashita, 1959)
or the decrement (Fujishiro et
al., 1984) of the antidromic phase. However, these two
effects cannot completely explain the result where the negative phase of the
biphasic salt impulse was unchanged. Conduction delaying of the antidromic
phase would reduce the negative phase of the salt impulse as well as the
positive phase. The decrement of the antidromic phase would also reduce the
negative phase of the salt impulse. To reduce only the positive phase of the
biphasic salt impulse, the newly generated negative phase different from the
backfiring of the orthodromic phase is required at the beginning of impulse
formation. Considering that the antidromic phase depends on the excitation of
the membrane at the tip, inward current generated by some ion channels at the
tip could be a candidate for the newly generated negative phase. Further
experiments using the patch-clamp method, as in the fleshfly
(Murakami and Kijima, 2000),
would clarify more elaborately the mechanism of ion channels in the taste
cells of the fly.
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Acknowledgments |
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We are grateful to Dr Arifa Ahamed and Dr Md Abidur Rahman for their helpful advice in the preparation of the present paper, and to Dr Keiitsu Saitoh for advice concerning the calculation of ionic dissociation.
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Accepted September 25, 2001
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