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Department of Behavioral Physiology, Faculty of Human Sciences, Osaka University, Suita, Osaka 565-0871, Japan
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ABSTRACT |
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 Top
 Abstract
 Introduction
Methods
Results
Discussion
References
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Electrophysiological and behavioral studies were performed in rats to analyze the gustatory effects of alcohols, such as methanol, ethanol, ethylene glycol, 1-propanol, 2-propanol, propylene glycol, 1,3-propandiol, and glycerin. When the whole bundle responses to each of the alcohols at 1.0 M were recorded from the chorda tympani (CT) and glossopharyngeal nerve (Gl), the alcohols with two or three hydroxyl groups elicited larger responses than the other alcohols in both nerves. Single-fiber analyses showed that the responses to alcohols were induced dominantly in sucrose-best fibers and were correlated well with sucrose responses in the CT, whereas the responses to alcohols were induced in quinine-best fibers and were correlated well with quinine responses in the Gl. The rats that acquired conditioned taste aversions to alcohols with two or three hydroxyl groups also avoided sucrose and quinine, although the aversion did not generalize to NaCl or HCl. These results suggest that alcohols have a taste similar to the taste of both sucrose and quinine in the rat.
chorda tympani; glossopharyngeal nerve; single-fiber analysis; conditioned taste aversion
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INTRODUCTION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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VARIOUS KINDS OF ALCOHOLIC beverages are consumed in daily life. Although the hedonically preferable feeling produced by alcohol drinking may mainly be derived from direct pharmacological actions of the absorbed alcohol (usually ethanol) to the central nervous system, there is no doubt that alcohol drinkers also enjoy the gustatory effects of alcohol. In contrast to a number of studies focusing on the receptor mechanisms of the conventional five basic taste stimuli, i.e., sucrose (sweet), NaCl (salty), HCl (sour), quinine (bitter), and monosodium glutamate (umami), studies on alcoholic taste are limited and the receptor mechanism subserving taste effectiveness of alcohol is not fully understood.
More than 30 years ago, Hellekant (9, 10) first analyzed in detail the neural responses of the chorda tympani (CT), which innervates taste buds on the anterior part of the tongue in response to lingual application of ethanol in the rat, cat, and dog. The most striking characteristics of the response to ethanol in the CT were the initial depression, the slow onset of discharge, and the strong response to a water rinse after alcohol exposure in the cat and dog. In the rat, the threshold to alcohol was high and the water response after alcohol exposure was not so strong as in the other animals (9). A single-fiber analysis in the cat revealed that ethanol evoked a response in all gustatory fibers tested, but the strong responses at water rinsing after the alcohol were observed only in fibers predominantly responsive to water (10).
After a fairly long silent period, some investigators commenced to study alcoholic taste and analyzed behavioral and electrophysiological responsiveness to ethanol as a tastant. Di Lorenzo et al. (3) recorded single-unit responses of the nucleus of the solitary tract to gustatory stimuli, including ethanol, in the rat and found a weak relationship between responses to ethanol and sucrose through the analysis of the across-unit patterns of response. To test the rat's perception of the gustatory qualities of ethanol, they produced a conditioned taste aversion (CTA) to 6 and 9% ethanol and rats were then tested with all six combinations of four basic taste stimuli. The CTA generalized significantly only to a mixture of sucrose and quinine. This behavioral result was confirmed by Lawrence and Kiefer (19).
More recently, Hellekant and his colleagues recorded single fiber responses to ethanol from the CT (12); the glossopharyngeal nerve (Gl), which innervates taste buds in the posterior part of the tongue (8); and the non-taste-responsive lingual proper nerve (11) in the rhesus monkey. They reported that ethanol stimulated all sweet-best fibers and, at high concentration, some salt-best fibers but never any acid-best and bitter-best fibers. They also found that in mixtures ethanol suppressed the responses to quinine and citric acid in quinine-best fibers and acid-best fibers, respectively (8, 12). They suggested that non-taste-responsive fibers also mediate the sensory effects of ethanol by showing that ~60% of slowly adaptive mechanoreceptors responded to ethanol (11).
No data are available so far concerning the response characteristics of single-taste fibers to alcoholic stimulation of the tongue in the rat. Therefore, the present study was designed to examine and further extend the above findings by recording responses from the whole bundle and single fibers of the CT and Gl to various kinds of alcohols including ethanol. Behavioral experiments using a CTA paradigm were also conducted to elucidate the taste quality of alcohols in this species. Part of the present study has been reported in abstract form (22, 23).
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METHODS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Animals
A total of 68 Wistar male rats (250-300 g) were used: 43 for electrophysiological and 25 for behavioral experiments. They were housed individually in plastic cages and maintained on a 12:12-h light-dark cycle at ~24°C. Food pellets (MF; Oriental Yeast, Osaka, Japan) and tap water were available ad libitum. The macronutrient composition of 100 g MF is (in g) 7.6 water, 24.6 protein, 5.6 fat, 6.3 minerals, 3.1 fiber, and 52.8 starch.Electrophysiological Experiment
Of 43 rats, 23 were used for the whole nerve recording experiment and 20 were used for the single-fiber recording experiment. The animals were deeply anesthetized by an intraperitoneal injection of pentobarbital sodium (60 mg/kg). After surgical level of anesthesia was achieved, each animal was tracheotomized and secured with a head holder. The CT was exposed, freed from surrounding tissues, and cut at the point of its entry to the bulla. The Gl was dissected free and cut near its entry to the posterior lacerated foramen. The whole bundle or single fibers of the nerves were dissected alive with a pair of needles and lifted on a platinum wire recording electrode (0.1 mm diameter). An indifferent electrode was attached to nearby tissues. The whole nerve activities were amplified, displayed on an oscilloscope, and monitored with an audioamplifier. The amplified signal was passed through an integrator with a time constant of 0.3 s and was displayed on a slipchart recorder. Impulse discharges of single fibers were also recorded on digital audio tape for future analysis.The taste solutions were (in M) 0.1 NaCl; 0.5 sucrose; 0.01 HCl; 0.02 quinine hydrochloride (quinine); and 1.0 of eight alcohols, methanol (MtOH), ethanol (EtOH), ethylene glycol (1,2-EtOH), 1-propanol (1-PrOH), 2-propanol (2-PrOH), propylene glycol (1,2-PrOH), 1,3-propandiol (1,3-PrOH), and glycerin, which were made up with distilled water. Each solution and rinsing water flowed for 15 s at a constant flow rate (0.5 ml/s) from a syringe pump at room temperature (25 ± 2°C). To minimize the effects of temperature and tactile transients, rinsing water was switched to taste solution with the continuous flow rate. The tongue was rinsed at least for 45 s between successive stimulations.
Of nine rats, five were used for the pronase experiment and another four rats were used for the gurmarin experiment. The effects of treatment of the tongue with 2% pronase E (35°C, 10 min), a proteolytic enzyme (13), or 50 µM gurmarin, an anti-sweet peptide (14, 21), on CT responses to taste stimuli were examined.
CTA Experiment
The rats were put on a schedule of water deprivation of 20 h/day. On the first training day, each animal was placed in a test box and given free access to distilled water during 1 h from a single drinking tube via a circular window (1 cm diameter). Supplemental water was available for 3 h in the home cage. The spout of polyethylene tubing (4 mm inner diameter) was located 2 mm outside the window. This arrangement prevented contact of the spout with the animal's lip. Licks were detected by a lickometer with a touch sensor and recorded on a slipchart recorder. From the second to fifth days, the training time was reduced from 1 h to 30 min. During this period, the animal was trained to drink distilled water on an interval schedule, consisting of a 20-s presentation of distilled water with 30-s intertrial intervals, resulting in 30-50 trials during the 30-min session. On the sixth day, each animal was given access to one of the four alcohols (1.0 M each), 1,2-EtOH, 1,2-PrOH, 1,3-PrOH, and glycerin, as the conditioned stimulus (CS) and then given an intraperitoneal injection of 0.15 M LiCl (2% of body wt), which induces internal malaise with gastrointestinal distress, as the unconditioned stimulus. Control rats were injected with physiological saline instead of LiCl after ingestion of the CS. The seventh day was a recovery day. On the eighth and ninth days, the number of licks of each test stimulus were counted for 10 s after the first lick to each stimulus. The test stimuli consisted of distilled water, 0.1 M NaCl, 0.5 M sucrose, 1 mM HCl, 0.1 mM quinine, and the following eight alcohols (1.0 M each): MtOH, EtOH, 1,2-EtOH, 1-PrOH, 2-PrOH, 1,2-PrOH, 1,3-PrOH, and glycerin. All the taste solutions were made up with distilled water (25 ± 2°C). Each test solution was presented randomly. The interval between each test solution was 30 s. The mean number of licks was obtained for each of the test stimuli in each rat.Data Analysis
The magnitude of the whole nerve response was measured as the height of integrated response from the baseline at 10 s after onset of stimulation. Responses to taste stimuli were expressed as relative magnitudes of responses when the magnitude of response to 0.1 M NH4Cl was taken as standard. The effects of suppression by pronase E and gurmarin were quantified by expressing them as suppression ratios based on the relative magnitude of responses before and after treatment: suppression ratio = (magnitude of response after treatment)/(magnitude of responses before treatment).The number of impulses occurring before and after each stimulation were counted by WorkBench and Discovery system (BrainWave). We calculated the mean background activity per 5 s during rinsing of the tongue with distilled water. A fiber was considered to be responsive to a stimulus if the nerve impulse rate during the first 5 s of taste stimulation was larger than the mean ± SD of the background rate.
The response magnitude of each fiber to a particular stimulus was the net number of impulses produced for the first 5 s after stimulus onset, which was obtained by subtracting the mean background impulse discharge from the total number of impulses.
The behavioral data were quantified by expressing them as a percent
suppression score based on the ratio of the number of licks to each
taste stimulus shown by an experimental group to that of the control
group, as shown by the following formula: mean licking suppression
ratio (%) = [1 
(licks for 10 s in experimental group)/(licks for 10 s in control group)] × 100.
Suppression effects by pronase E or gurmarin were analyzed by one-way ANOVA and post hoc Student-Newman-Keuls tests with stimulus. Tree clustering method was used for cluster analysis of the responses to single fibers.
In the behavioral experiment, the suppression ratios for each test solution were also analyzed by one-way ANOVA and post hoc Student-Newman-Keuls tests.
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Electrophysiological Experiment
Whole nerve recording. Figure 1 shows sample records of the integrated CT responses to the four basic taste stimuli and eight alcohols. The alcohols elicited tonic responses with small or negligible phasic responses. This characteristic is similar to that of sucrose response but is quite different from that of responses to NaCl, HCl, and quinine, which induced the clear phasic and subsequent tonic responses. Similarly, in the Gl (not shown here), the alcohols elicited tonic responses with small or negligible phasic responses. As for the four basic taste stimuli, only quinine elicited large phasic and tonic responses, whereas other stimuli were less effective in inducing responses in the Gl than in the CT.
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The taste effectiveness of each stimulus is collectively shown in Fig.
2. The mean relative magnitudes of the
integrated responses of the CT to NaCl, sucrose, quinine, and HCl were
1.55, 1.05, 0.67, and 0.64, respectively, when the magnitude of the
response to 0.1 M NH4Cl is taken
as 1.0 (Fig. 2A). As for responses
to alcohols, glycerin (the mean relative magnitude is 1.38) and
1,2-EtOH (1.28) were the most effective, followed by 1,3-PrOH (0.49)
and 1,2-PrOH (0.45). Other alcohols induced very small responses, ranging from 0 to 0.07. The mean relative magnitudes of integrated responses of the Gl to quinine, HCl, sucrose, and NaCl were 3.78, 0.78, 0.58, and 0.31, respectively, when the magnitude of the response to 0.1 M NH4Cl is taken as 1.0 (Fig.
2B). As for responses to alcohols,
1,2-EtOH (1.56) and glycerin (1.42) were the most effective, followed
by 1,2-PrOH (0.81), 1,3-PrOH (0.48), and EtOH (0.35). Responses to the
other alcohols tested were very small, ranging from 0.06 to 0.21.
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Effects of gurmarin, an anti-sweet peptide, and pronase E, a
proteolytic enzyme, on alcoholic responses were examined. Figure 3 shows sample records of the integrated CT
responses to NH4Cl, sucrose,
1,2-EtOH, 1,2-PrOH, 1,3-PrOH, and glycerol before and after treatment
of the tongue with 2% pronase E for 10 min. After this treatment, CT
responses to sucrose, 1,2-EtOH, 1,2-PrOH, and 1,3-PrOH were almost
completely suppressed, although the response to glycerin was suppressed
to about half of the pretreatment level and the response to
NH4Cl was not affected.
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Figure 4A
shows a graphical representation of the effects of 2% pronase E as
expressed by the suppression ratio for each taste stimulus. The
responses to sucrose, 1,2-EtOH, 1,2-PrOH, and 1,3-PrOH were suppressed
to below 20% of the pretreatment level, although this treatment was
less effective in suppressing responses to glycerin, the mean ratio
being 0.71 after pronase E. We analyzed the suppression ratios for each
taste stimulus with one-way ANOVA. The main effect of the group was
significant [F(4,14) = 6.77, P < 0.01], no statistically
significant difference was detected among suppression ratios for
sucrose, 1,2-EtOH, 1,2-PrOH, and 1,3-PrOH, and these suppression ratios
were significantly (P < 0.01, Student-Newman-Keuls tests) smaller than the mean suppression ratio for
glycerin.
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Shown in Fig. 4B is a graphical representation of the effects of 50 µM gurmarin on gustatory responses as expressed by the suppression ratio for each taste stimulus. The responses to sucrose were suppressed to <20% of the pretreatment level, although this treatment was less effective in suppressing responses to the four alcohols. We also analyzed the suppression ratios for each taste stimulus with one-way ANOVA. The main effect of the group was significant [F(4,18) = 4.64, P < 0.01], and statistically significant difference was detected among suppression ratios between sucrose and the four alcohols, 1,2-EtOH, 1,2-PrOH, and 1,3-PrOH, and glycerin (P < 0.01, Student-Newman-Keuls tests).
Single fiber responses. A total of 93 and 31 functionally single fibers were sampled from the CT and Gl,
respectively, and their responses were analyzed. Generally speaking,
fibers responsive to sucrose and quinine also responded to alcohols,
and fibers nonresponsive to these stimuli showed small responses to
alcohols. Sample records obtained in three types of CT and Gl fibers
are shown in Fig. 5. A sucrose-best CT
fiber, which responded most dominantly to sucrose among the four basic
taste stimuli, also responded well to alcohols (Fig.
5A). An NaCl-best CT fiber that responded most dominantly to NaCl elicited essentially no response to
alcohols (Fig. 5B). On the other
hand, a quinine-best Gl fiber responded well to alcohols (Fig.
5C).
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Response profiles of 93 CT fibers are shown in Fig.
6. In this figure, the fibers are
arbitrarily arranged from left to right according to their response
magnitudes to the best responsive stimuli among the four basic tastes.
The magnitudes of responses to the four alcohols were generally smaller
than those to the four basic taste stimuli. The response profiles,
however, suggest that alcohols tend to elicit responses in NaCl-best,
sucrose-best, and quinine-best fibers, but not in HCl-best fibers. The
responsiveness of alcohols in each subset of fibers responding best to
one of the four basic taste stimuli will be described in more detail later.
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Figure 7 shows the response profiles of Gl
fibers. In contrast to CT fibers, which responded dominantly to NaCl,
Gl fibers responded almost exclusively to quinine in four basic tastes. As indicated by the response profiles, the four alcohols elicited larger responses in quinine-best fibers than in other types of fibers.
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Across-fiber pattern. Pearson
product-moment correlations of responses to each of the eight stimuli,
NaCl, sucrose, HCl, quinine, 1,2-EtOH, 1,2-PrOH, 1,3-PrOH, and
glycerol, among all pairs of 93 CT fibers and 31 Gl fibers were
calculated. In CT fibers, as shown in Table
1, the correlation coefficients of
responses between sucrose and each of the four alcohols and among the
four alcohols were very high. In Gl fibers, as shown in Table
2, the correlation coefficients of
responses between quinine and each of the four alcohols and among the
four alcohols were very high, followed by correlations between sucrose
and alcohols.
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The correlation coefficients shown in Tables 1 and 2 were used for a
hierarchical cluster analysis to produce dendrograms. Figure
8A shows a
dendrogram for the CT fibers. When the correlation coefficient of 0.62 (5% significance level) is taken as the critical level, the taste
stimuli used were clustered into the four groups: 1) NaCl,
2) 1,2-EtOH, 1,2-PrOH, 1,3-PrOH,
glycerin, and sucrose, 3) quinine,
and 4) HCl. This result suggests
that the taste information of alcohols is similar to that of sucrose in
the CT fibers. Another dendrogram was constructed for the Gl fibers
(Fig. 8B). When the correlation
coefficient of 0.52 (5% significance level) is taken as the critical
level, the taste stimuli were clustered into the four groups:
1) NaCl,
2) sucrose,
3) 1,2-EtOH, 1,2-PrOH, 1,3-PrOH, glycerin, and quinine, and 4) HCl.
This result suggests that the taste information of alcohols is similar
to that of quinine in the GL fibers.
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Labeled line. The CT and Gl fibers can
be grouped into best-stimulus categories, sucrose best
(n = 23 and 5, respectively), NaCl
best (n = 42 and 3), HCl best
(n = 12 and 6), and quinine best
(n = 16 and 17), depending on which of
the four basic taste stimuli elicited the greatest response. Figure
9 shows mean numbers of impulses/5 s for
eight taste stimuli in each best fibers; the graphs also show relative
magnitudes of responses because the maximum responses in each best
fibers are set unity. These impulses were net impulses that were
subtracted baseline activities. The sucrose-best CT fibers showed good
responses to alcohols. Their percentage responses against the sucrose
response were 42-82%. The quinine-best CT fibers showed smaller
magnitudes (16-49% of the mean quinine response) of responses to
alcohols than the sucrose-best fibers. In the Gl fibers, the
quinine-best fibers showed large responses (42-75% of the quinine
response) to alcohols. The sucrose-best Gl fibers showed smaller
magnitudes (38-60% of the sucrose response) of responses to
alcohols than the quinine-best fibers. NaCl-best and HCl-best fibers in
both nerves elicited small responses to alcohols.
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Behavioral Experiment
When aversive conditioning was established to one of the four alcohols, 1,2-EtOH, 1,2-PrOH, 1.3-PrOH, and glycerin, the self-suppression ratios were 92.4 ± 2.7, 89.2 ± 1.9, 90.9 ± 1.9, and 93.3 ± 1.3 (mean ± SE; n = 5), respectively, indicating that these stimulants are effective CS. As shown in Fig. 10, the aversive conditioning to alcohols generalized to sucrose and quinine with high suppression ratios but not to NaCl and HCl. The mean licking suppression ratios for each test stimuli were analyzed with one-way ANOVA. The main effects of them were significant in all CS groups [F(11,24) = 20.16, P < 0.001 for 1,2-EtOH; F(11,36) = 37.14, P < 0.001 for 1,2-PrOH; F(11,60) = 14.39, P < 0.001 for 1.3-PrOH; and F(11,36) = 9.57, P < 0.001 for glycerin]. The mean licking suppression ratios for sucrose and quinine in each group were significantly higher (P < 0.05, Student-Newman-Keuls tests) than those for NaCl and HCl. When the rats were conditioned to reject each of the four alcohols, they rejected 1,2-PrOH, 1,3-PrOH, and glycerin, but did not reject MtOH and EtOH. The degree of generalization of aversion to 1,2-EtOH, 1-PrOH, and 2-PrOH differs, depending on the kind of alcohols used as the CS.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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The present study through both the taste nerve recording experiment and behavioral conditioned taste aversion experiment has shown that alcohols stimulate taste receptor cells of Wistar rats with different effectiveness depending on the kind of alcohols used.
Whole Nerve Recording
Although it is known that ethanol penetrates the epithelium of the tongue and directly stimulates some mechanoreceptors and thermoreceptors in the monkey (11), the eight alcohols used in the present study at a given concentration (1 M) did stimulate taste receptors. This is based on the present findings that the onset latency for the alcohol responses of the taste nerves was not so sluggish as that reported in the nongustatory fibers (11), and the alcohol responses in the CT were almost completely abolished by treatment of the tongue with a proteolytic enzyme (13), pronase E, at a condition where responses to only sucrose of the four basic taste stimuli were suppressed. The latter finding indicates that the taste receptor for alcohols is composed of a protein that is vulnerable to the action of pronase E. The present study was unable to answer the question whether or not there exists a specific receptor for alcohols on the taste cell membrane, although the sweet-sensitive receptor molecule may partly be involved because alcohol responses were suppressed to ~50% after treatment of the tongue with an anti-sweet peptide suppressant, gurmarin. These findings, together with the present results that alcohol responses in the CT and Gl were very well correlated with responses to sucrose and quinine and taste aversions to alcohols generalized to sucrose and quinine, may indicate that alcohols interact preferably with the sweet-sensitive and bitter-sensitive receptors.The magnitudes of taste nerve responses were different among the eight alcohols used in the present study. The responses to alcohols with two or three hydroxy groups such as 1,2-EtOH, 1,2-PrOH, 1,3-PrOH, and glycerin were larger than those to the other alcohols in both CT and Gl, indicating that these hydroxy groups play a key role in binding to receptor molecules. Methanol and ethanol, which have one hydroxy group with one or two carbons in their structures, were not effective tastants in both CT and Gl. Hellekant (9) also reported that CT responses to ethanol were very small even when strong solutions were used, the maximum response being obtained as high as 6.5 M solution. Such low responsiveness to methanol and ethanol may be one reason why these alcohols were not avoided by rats that had been trained to avoid other alcohols.
It is known that voluntary consumption of ethanol is due to strain differences in rats or mice (1, 2, 16, 17). Some rat lines have been developed based on high and low levels of alcohol consumption. For example, Eriksson (5) discovered alcohol-avoiding and alcohol-preferring lines. Other famous rat lines, alcohol preferring and alcohol nonpreferring, have been used for animal models of alcoholism (18, 20). Sinclair et al. (24) reported that Wistar rats, the same strain used in the present study, were ethanol-preferring rats, preferring concentrations ranging from 0.15 to 20% vol/vol. However, as shown in the present study, our Wistar rats did not show any clear CT and Gl nerve responses to 1.0 M (~10% vol/vol) ethanol. Although there remains a possibility that ethanol induces a large response in the greater superficial petrosal nerve that innervates the taste buds on the soft palate, an alternative explanation for the high preference for ethanol may be attributed to postingestive effects rather than taste effects.
Single-Fiber Analyses
The present single-fiber analyses treated with the across-fiber pattern notion (4) and the labeled-line notion (6, 7) suggest that the responses to alcohols are highly correlated with those to sucrose and quinine in both CT and Gl; more specifically, alcohol responses are correlated well with sucrose responses in the CT and with quinine responses in the Gl. These findings are supported in part by Di Lorenzo et al. (3) who revealed a weak relationship between responses to ethanol and sucrose in the nucleus of the solitary tract of Sprague-Dawley rats. Similarly, in the CT of the rhesus monkey, Hellekant et al. (12) found that ethanol stimulated all sweet-best fibers and, at high concentration, some salt-best fibers but never any other fibers.The present study first demonstrated that alcohols elicited good responses in quinine-best fibers in the Gl. It was difficult to detect how effectively alcohols stimulated quinine-best fibers in the CT because the proportion of these fibers was quite small in the CT. Avoidance of intake of ethanol in a behavioral preference test (19) might be attributed to aversive information transmitted through quinine-best fibers. In another study using a CTA paradigm, Di Lorenzo et al. (3) and Lawrence and Kiefer (19) found that aversion learning to ethanol generalized to a mixture of sucrose and quinine in the rat. Although previous studies in rats (8) or monkeys (8, 11, 12) did not show that quinine-best fibers were responsive to ethanol, interactions between ethanol and quinine responses have been reported, i.e., CT and Gl responses to mixtures of ethanol and quinine were most severely suppressed in quinine-best fibers in the rhesus monkey (8, 12). Taken together, alcohols may interact with bitter-tasting receptors in an excitatory direction (present study) or in a mutually inhibitory direction (8, 12), although the excitatory action of alcohols may vary among species of animals. Interestingly, in mice, Kajiura et al. (15) suggested that ethanol may cause "sweet sensation" in the C57BL/KsJ strain and "bitter sensation" in the BALB/c strain.
Behavioral Experiment
The rats conditioned to reject alcohols, such as 1,2-EtOH, 1,2-PrOH, 1,3-PrOH, and glycerol, also rejected sucrose and quinine solutions. These results are essentially the same as those reported by Di Lorenzo et al. (3), who found in Sprague-Dawley rats that aversive conditioning to ethanol generalized to a mixture of sucrose and quinine. These findings suggest that alcohols do not have a unique taste but have similar tastes to the tastes of both sucrose and quinine. It is noted here that the present behavioral results may have been affected by odors, because most of the alcohols have characteristic odors.From the results of the present study, we may conclude that alcohols exert their gustatory effects by binding to receptor molecules with binding sites for more than two alcoholic hydroxy groups, and that alcohols have a bimodal taste similar to the tastes of sucrose and quinine.
Perspectives
There are many kinds of alcoholic drinks in the world that are consumed with pleasure in daily life, but studies on the alcoholic taste are limited and the receptor mechanism subserving taste effectiveness of alcohol is not fully understood. Taste nerve responses to different structures of alcohols suggest that ethanol elicited small responses, whereas the alcohols with more than two hydroxy groups induced large responses in fibers responding best to sucrose and quinine in the rat. The behavioral experiment using the CTA paradigm also suggests that the tastes of alcohols are similar to the tastes of sucrose and quinine. Future studies should clarify whether alcohols stimulate a specific receptor exclusively sensitive to alcohols or interact with the receptor sites for sweet or bitter tasting substances. Species and strain difference in alcoholic tastes should also be studied in terms of the transduction mechanism occurring in taste receptor cells.| |
ACKNOWLEDGEMENTS |
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This study was supported by grants-in-aid for Science Research (Nos. 09470401 and 09832005) from the Ministry of Education, Science, Sports and Culture of Japan, and by Research for the Future Program (No. JSPS-RFTF97L00906) from the Japan Society for the Promotion of Science.
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: N. Sako, Dept. of Behavioral Physiology, Faculty of Human Sciences, Osaka Univ, 1-2 Yamadaoka, Suita, Osaka 565-0871, Japan.
Received 11 March 1998; accepted in final form 12 October 1998.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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