Oral Irritant Effects of Nicotine: Psychophysical
Evidence for Decreased Sensation Following
Repeated Application and Lack of
Cross-desensitization to Capsaicin
J.-M. Dessirier, M. O'Mahony and E. Carstens1
Department of Food Science and Section of Neurobiology, Physiology and Behavior, University of California,
Davis, Davis, CA 95616, USA
.
Abstract
Psychophysical methods were used to assess changes in the intensity of irritant sensations elicited by repeated
application of capsaicin and nicotine delivered unilaterally to the tongue of human subjects. Whereas capsaicin (0.5
or 3 p.p.m.; repeated at 1 min intervals over 10 min) evoked progressively stronger ratings of irritation (sensitization),
there was a significant decrement in irritation ratings (desensitization) to repeated application of nicotine (0.1%). A
two-alternative forced-choice (2-AFC) procedure was additionally used to test for self- and cross-desensitization. After
the subjects had received either repeated capsaicin or nicotine, a rest period ensued followed by the 2-AFC
procedure. Either capsaicin or nicotine was delivered bilaterally to the tongue and subjects were asked to choose
which side yielded a stronger sensation. Following capsaicin pretreatment, subjects reported that capsaicin evoked a
stronger sensation on the previously untreated side (capsaicin self-desensitization). Similar self-desensitization was
observed with nicotine. Furthermore, nicotine evoked a significantly weaker sensation on the side of the tongue
pretreated with capsaicin (cross-desensitization). In contrast, capsaicin did not consistently evoke a weaker sensation
on the nicotine-pretreated side, indicating an absence of cross-desensitization. These results are discussed in terms of
physiological mechanisms that might underlie the contrasting sensory effects of nicotine versus capsaicin. Chem.
Senses 22: 483-492,1997.
Introduction
A variety of naturally occurring chemicals, such as capsaicin from red chili peppers, piperine from black pepper,
and nicotine from tobacco, cause irritation when delivered
to the oral mucosa (for recent review, see Green and
Lawless, 1991). Capsaicin in low concentration (3 p.p.m.)
© Oxford University Press
evokes a burning sensation which increases when delivered
repeatedly at 1 min intervals (sensitization), but then
decreases markedly following a 10-15 min rest period
(self-desensitization) (Green, 1989, 1991b, 1996; Karrer and
Bartoshuk, 1991; reviewed in Holzer, 1991). It is interesting
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Correspondence to be sent to: E. Carstens, Section of Neurobiology, Physiology and Behavior, University of
California, Davis, Davis, CA 95616, USA
484
I J.-M. Dessirier, M. O'Mahony and E. Carstens
Materials and methods
Subjects
Subjects were drawn from a pool of 59 (22 males, 37
females, age 18-42 years) students and staff at the
University of California at Davis who voluntarily
participated in the study. All subjects signed an informed
consent form approved by the UC Davis Human Subjects
Review committee which explained the general nature of the
study, but subjects were naive as to its specific goals. All
were non-smokers and were asked not to eat spicy food for 3
days prior to testing. Each subject participated in only one
individual session of data collection per day, which typically
lasted <30 min. Some subjects returned to participate in
subsequent sessions; individual sessions were conducted a
minimum of 3 days apart.
Stimulus application
From a stock solution of 1% capsaicin (vanillyl nonamide;
98-100%, Sigma Chemical Co., St Louis, MO) in 80%
ethanol (from 100% punctilious; Quantum, Los Angeles,
CA), 0.5-10 p.p.m. capsaicin solutions were made by
diluting with distilled water. A 15 ul aliquot of the diluted
capsaicin was pipetted onto small (78.5 mm2) and 35 ul onto
large (176.7 mm2) circular filter papers (Whatmann,
Maidstone, UK, product # 1001-0105 and 1001-0155).
To avoid any effect of ethanol, the filter papers were airdried before application. Nicotine (free base, 98-100%,
Sigma) was diluted to 0.1 or 0.12% in distilled water, and
was pipetted in the same volumes onto the small or large
filter papers just prior to application. Subjects reported
the nicotine and capsaicin to be tasteless at these
concentrations.
Sequential stimulation procedure
In all subjects, the larger size filter paper containing either
capsaicin (experiments 1, 3) or nicotine (experiments 2, 3)
was applied with forceps onto one side of the dorsal surface
of the tongue. After application the mouth was closed. The
side of stimulation was counterbalanced across subjects. At
various intervals following application of the filter paper
(see Table 1), the subject was asked to rate the intensity of
the irritant sensation using a VAS (Price et al., 1983) which
was modified to have the descriptors 'no sensation' at the
lower end and 'most intense irritation imaginable' at the
upper end. The rating was quantified in terms of
centimeters read from a 0-15 cm ruler on the back of the
VAS. We have prior experience using the VAS (Douglass et
al., 1992). Thirty seconds after stimulus onset, the filter
paper was removed. Thirty seconds later, a fresh filter paper
was applied to the same area of the tongue. Thus, a new
filter paper was applied at successive 1 min intervals for a
total of 10 repetitions.
Because capsaicin stimulation increases salivary flow
(Duner-Engstrom et al, 1986), a suction device (Saliva
Ejector, 6" clear; Sullivan Dental Products Inc., Sacramento,
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that following desensitization of the oral (Green, 1991a;
Gilmore and Green, 1993) or nasal (Geppetti et al, 1993)
mucosa by capsaicin, irritant sensations evoked by other
chemicals such as cinnamic aldehyde, NaCl at concentrations above 1 M, or citric acid were also reduced
(cross-desensitization). These results suggest that oral
irritation from some agents may be mediated by a
population of capsaicin-sensitive trigeminal polymodal
nociceptors innervating the oral mucosa.
Nicotine is also well known to evoke a burning pain
sensation when applied to skin (Keele and Armstrong, 1964)
or to the oral (Jarvik and Assil, 1988), nasal (Hummel et al.,
1992; Greiff et al., 1993) or ocular (Maclver and Tanelian,
1993) mucosa. The oral irritation from nicotine appears to
act via nicotinic cholinergic receptors (Jansco et al, 1961;
Jarvik and Assil, 1988) presumably residing on nociceptor
terminals (Steen and Reeh, 1993). In the present study, we
wished to investigate further the irritant effect of nicotine
delivered sequentially to the oral cavity, and its possible
interactions with capsaicin. We have modified a paradigm
used by Green (1989) in which subjects gave magnitude
ratings of irritation elicited by capsaicin delivered
sequentially to the surface of the tongue bilaterally. In the
present study, nicotine or capsaicin was delivered to only
one side of the tongue and subjects rated the magnitude of
irritation using a visual analog scale (VAS). The unilateral
application allowed us to employ a two-alternative forced
choice (2-AFC) to test for desensitization. That is, following
sequential unilateral application of one of the chemicals and
an ensuing rest period, the same chemical was then applied
to both sides of the tongue at once and subjects had to
choose which side yielded the stronger sensation. We have
used this method to verify the sensitizing and selfdesensitizing effects of capsaicin, to determine if nicotine
has similar effects, and to test if these chemicals exhibit
cross-desensitization with one another.
Oral Irritant Effects of Nicotine I
Table 1
485
Experimental details; specific details are provided for each experiment (see text for explanation)
Experiment
No. of subjects
1a: Cap self-desens.
20
1 b: Cap-nic cross desens. 20
2a: nic self-desens.
14
2b: nic-cap cross-desens. 20
3a: cap-nic cross-desens. 10
3b: nic-cap cross-desens. 20
Sequential stim.
(10x)
Time of rating(s)
Rest (min)
Bilateral stim. (1 x)
2-AFC (min)
cap 3 p.p.m.
cap 3 p.p.m.
nic 0.1%
nic 0.1%
cap 0.5 p.p.m.
nic 0.12%
25
25, 40,55
40, 55
40, 55
25
25
10
cap 3 p.p.m.
nic 0.1%
nic 0.1%
cap 10 p.p.m.
nic 0.12%
cap 0.5 p.p.m.
5
5
5
5
3
3
CA) was placed in the mouth to remove saliva. This freed
the subject from having to swallow or spit, and avoided
spreading the chemical solution across the tongue. Subjects
were instructed to use the suction device at any time, except
for the 10 s period prior to giving VAS or 2-AFC responses.
This avoided a possible influence of cooling caused by
opening the mouth.
period of 2-AFC testing. The various procedural changes
for each experiment are listed in Table 1.
Experiment 1: capsaicin
1a. Sensitization and self-desensitization
In these experiments we sought to reproduce the finding
(Green, 1989) that sequential application of capsaicin leads
to successively increasing intensity ratings (sensitization),
and to decreased ratings following a rest period (selfdesensitization).
Capsaicin (3 p.p.m.) was successively applied in 10
subjects, and ratings were taken at 25 s following stimulus
onset. After a 10 min rest period, self-desensitization was
tested using the 2-AFC procedure (Table 1).
1b. Sensitization and nicotine cross-desensitization
Capsaicin (3 p.p.m.) was again applied successively (see
Table 1). After a 10 min rest period, nicotine (0.1%) was
applied onto both sides of the tongue and the 2-AFC
procedure was performed. Of the 20 subjects tested, three
had performed in experiment la.
Experiment 2: nicotine
In these experiments, we tested if sequential application of
nicotine also gives rise to increasing ratings of irritation like
capsaicin, and if nicotine exhibits self-desensitization and
cross-desensitization to capsaicin.
2a. Sequential stimulation and self-desensitization
Nicotine (0.1%) was applied sequentially as above, and after
a 5 min resting period self-desensitization was tested using
the 2-AFC procedure as before (Table 1). Of the 14 subjects
tested, nine had participated in experiment la.
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Two-alternative forced choice (2-AFC)
procedure
After the 10 sequential stimulus applications, the subjects
rested for 10 min following capsaicin (Green, 1989), or for 5
min following nicotine, since pilot experiments indicated
that nicotine had a short-lasting desensitizing effect. During
the waiting period the subjects sat quietly without speaking.
Then two small size filter papers either containing the same
chemical as applied previously (self-desensitization) or a
different chemical (cross-desensitization) were applied
simultaneously onto both sides of the tongue at a location
corresponding to the one where the larger filter papers were
applied earlier (see Figure 2, inset above). After application,
the mouth was closed. Subjects were then asked to choose
which side of the tongue gave rise to a stronger sensation
(2-AFC; Green and Swets, 1966). This initial 2-AFC rating
provided the data shown in Figure 2. The 2-AFC testing
procedure was continued for 5 min in experiments 1 and 2
and was shortened to 3 min in experiment 3. During this
period, subjects were instructed to indicate any time that the
side of stronger sensation changed, and the time of such
occurrence was noted. From this, the total time that each
side of the tongue yielded a stronger sensation was
determined for the entire period. In experiment 1, capsaicin
filter papers remained on the tongue for 5 min. In
experiment 2 with nicotine, the filter papers were removed
after 30 s. In experiment 3, both the capsaicin and nicotine
filter papers remained on the tongue for the entire 3 min
10
5
5
10
5
486
I J.-M. Dessirier, M. O'Mahony and E. Carstens
2b. Sequential stimulation and cross-desensitiza- nicotine in an identical subject population. An additional 10
naive subjects were added to increase the number of
tion with capsaicin
Nicotine (0.1%) was applied sequentially as before, and after
a 5 min rest period capsaicin (10 p.p.m.) was applied and the
2-AFC procedure was conducted as before (Table 1).
Thirteen of the 14 subjects tested in experiment 2a
(including nine who had participated in experiment la) were
tested, and an additional seven subjects were added (total:
20 subjects) to increase the number of observations for the
2-AFC test.
Experiment 3: nicotine and capsaicin
cross-desensitization
3a. Sequential stimulation with capsaicin and
cross-desensitization with nicotine
With 10 naive subjects capsaicin (0.5 p.p.m.) was applied
sequentially as described above, and after a 10 min rest
period nicotine (0.12%) was applied and the 2-AFC
procedure carried out with the following changes. The
period of 2-AFC testing was shortened to 3 min since the
earlier experiments indicated that the cross-desensitization
effect wore off by this time. Also, for the initial 2-AFC test,
subjects were asked if they really detected a difference or if
they chose randomly. In addition, they were asked to report
when any spontaneous laterality change in sensory intensity
occurred, or when they could no longer detect any laterality
difference. When the latter occurred, subjects were nonetheless asked to choose which side yielded a stronger
sensation as required by 2-AFC.
3b. Sequential stimulation with nicotine and
cross-desensitization with capsaicin
This experiment used the same 10 subjects from experiment
3a to allow comparison of results with capsaicin and
Statistical analyses
For sequential stimulation, a two-factor (application trial,
subjects) or three-factor (application trial, time of rating,
and subject) analysis of variance (ANOVA) was performed.
For the 2-AFC data, a binomial test was performed. The
mean total time that each side of the tongue yielded a
stronger sensation across the period of 2-AFC testing was
compared using a Student's Mest. For all tests the
significance level was taken as P < 0.05.
Results
Experiment 1: capsaicin
1a. Capsaicin sensitization and self-desensitization
Our results confirm a sensitization by capsaicin as reported
earlier (Green, 1989) (Figure 1A). Mean ratings of
perceived intensity of irritation increased across trials of
capsaicin application. This is illustrated in Figure 1A. There
was a significant main effect of application trial (P < 0.001
for both experiment la, n - 10 subjects, and lb, n - 20
subjects; ANOVA), indicating a significant increase in
ratings from the first to 10th trial. This was confirmed by
the observation that ratings increased across trials in a
significant majority (18/20) of subjects {P < 0.001,
binomial). There was also a significant main effect of time
of rating (P < 0.001, ANOVA); a Fisher's LSD test revealed
significant differences among all three times of rating
(Figure 1A).
Results with 2-AFC testing confirm self-desensitization
by capsaicin (Green, 1989). A significant majority of
subjects (9/10; P = 0.02, binomial) initially reported a
stronger irritant sensation on the previously unstimulated
side of the tongue following bilateral testing with capsaicin
(Figure 2, first hatched bar). During the 5 min period of
2-AFC testing, the total time that the irritation was
perceived to be stronger on the previously unstimulated
side was significantly greater compared with the other
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This experiment was performed to replicate our findings of
experiments 1 and 2 using concentrations of capsaicin and
nicotine that were matched more closely in terms of the
intensity of the irritant sensation. In experiments 1 and 2,
the capsaicin concentrations were chosen with a view to
confirming earlier studies (e.g. Green, 1989; Karrer and
Bartoshuk, 1991), and yielded higher ratings compared with
0.1% nicotine. In experiment 3 we used the highest nicotine
concentration allowed by FDA regulations (0.12%) and a
lower capsaicin concentration (0.5 p.p.m.) to achieve
approximately matched sensory ratings.
observations in the 2-AFC procedure. Nicotine (0.12%) was
applied sequentially as before, and after a 5 min rest period
capsaicin (0.5 p.p.m.) was applied and the 2-AFC procedure
carried out as above (Table 1).
Oral Irritant Effects of Nicotine I
487
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10
10
IL
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8
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8
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40 sec
55 sec
is
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Si
0
1
2
3
4
5
6
7
8
9
10
TRIAL NUMBER (CAP.)
2
1
2
3
4
5
6
7
8
9
10
TRIAL NUMBER (NIC.)
Figure 1 Different effects of sequential stimulation with capsaicin or nicotine. (A) Sensitization in sequential ratings of irritation intensity by capsaicin.
Graph plots mean ratings of perceived irritation (n = 20 subjects) versus the trial of capsaicin application (1 min intervals). Three ratings were given at 25, 40
and 55 s after each application. (B) Decrement in sequential ratings of irritation intensity by nicotine. Graph as in (A) for ratings (n = 20 subjects) across trials.
Ratings were given at 40 and 55 s after each application.
1b. Cross-desensitization with nicotine
Sequential application of capsaicin also led to a crossdesensitization of the irritant effect of nicotine. Following
bilateral testing with nicotine, a significant majority of
subjects (18/20, P < 0.001, binomial) reported the side not
previously stimulated with capsaicin to yield a stronger
irritant sensation (Figure 2, second hatched bar). After
nicotine application, the total time that the previously
unstimulated side was reported to yield a stronger sensation
was significantly larger compared with the capsaicindesensitized side (means: 4.5 versus 0.5 min; P < 0.001,
Mest).
time of rating was significant (P < 0.001, ANOVA). For the
13 subjects participating in experiments 2a and 2b, the
results were reproducible. This was verified in a four-factor
ANOVA (subject, application trial, time of rating, and
session as factors) for which the effect of session was not
significant.
Sequential application of nicotine also resulted in an
apparent self-desensitization. All (14/14) subjects reported a
stronger irritant sensation on the previously unstimulated
side of the tongue when nicotine was tested bilaterally after
the rest period (Figure 2,firstsolid bar); this was statistically
significant (P < 0.001, binomial). Furthermore, the total
time that subjects reported a stronger sensation on the
previously unstimulated side was significantly greater
compared with the other side (mean 4.8 versus 0.2 min; P <
0.001, f-test).
Experiment 2: nicotine
2a. Sequential stimulation and self-desensitization 2b. Lack of cross-desensitization with capsaicin
In contrast to the sensitizing effect of capsaicin, sequential
application of nicotine led to a progressive reduction
(desensitization) in the intensity of irritation (Figure IB).
This was observed in experiment 2a with 14 subjects and
again in experiment 2b with 20 subjects (13 of whom had
participated in experiment 2a). The main effect of
application trial was significant (P < 0.001, ANOVA),
indicating that mean ratings decreased significantly from the
first to 10th trial. This was the case in a significant majority
(19/20) of subjects (P < 0.001, binomial). The main effect of
In contrast to the cross-desensitization of nicotine by
capsaicin, the results do not support cross-desensitization
of capsaicin by nicotine. Following bilateral testing with
capsaicin, only six of 13 subjects used in experiment 2a
(46%) indicated a stronger sensation on the side that did not
receive prior nicotine. To confirm this, seven additional
subjects were tested and 11 of 20 of the total subject sample
initially reported the previously unstimulated side to yield a
stronger irritant sensation (Figure 2, second solid bar),
while nine reported the opposite (P > 0.8, binomial). The
Downloaded from chemse.oxfordjournals.org by guest on July 14, 2011
(capsaicin-desensitized) side (means: 4.6 versus 0.4 min; P <
0.001, Mest).
I
J.-M. Dessirier, M. O'Mahony and E. Carstens
1st Stimulus
10 times
unilaterally
2nd Stimulus
bilaterally after
rest period
I/lean 1Magnitude of Oral Irritatior
488
Non-Treated
Side
*
100n
(N=10)
10
9
/
8
7
\
6
Capsaicin 0.5 ppm
J
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5
Nicotine 0.12%
4
\
^a
<J
3
0
1
2
3
4
5
6
7
8
9
10
Trial Number
Figure 3 Differential responses to repeated application of nicotine and
capsaicin at concentrations matched for intensity. Graph as in Figure 1
plotting mean ratings for each of 10 sequential applications of 0.12%
nicotine (O) and 0.5 p.p.m. capsaicin (•
) .
50
Cap (0
Nic •
Nic©
1a
1b
2a
Nic*
1
Nic (D> Cap (It
Cap*
Nic •
2b
3a
Nic (I
Cap*
3b
Figure 2 Cross-desen'sitization studies using 2-AFC Bar graph plots the
probability that subjects reported the non-pretreated side of the tongue to
yield a stronger sensation of irritation, under six different experimental
conditions. Experiment 1 (hatched bars): capsaicin followed by capsaicin
(la) or nicotine (1 b). Experiment 2 (filled bars): nicotine followed by nicotine
(2a) or capsaicin (2b). Experiment 3 (open bars): capsaicin followed by
nicotine (3a) and nicotine followed by capsaicin (3b). Upper diagrams of the
tongue illustrate the 2-AFC procedure. First, either capsaicin or nicotine was
delivered sequentially to one side of the tongue (hatched circle in upper left
diagram) 10 times at 1 min intervals. After a rest period, the second test
stimulus (capsaicin or nicotine) was applied bilaterally at sites corresponding
to the previously stimulated area (filled circles in upper right diagram).
Subjects were asked to report which side of the tongue gave rise to a more
intense sensation. Therefore, in the graph 100% means that all subjects
reported the non-pretreated side to yield a stronger sensation, while 50%
indicates that subjects chose the untreated and treated sides with equal
probability.
average total time that either side of the tongue was judged
to yield a stronger irritant sensation was virtually identical.
Experiment 3: nicotine and capsaicin
cross-desensitization
This last experiment was carried out for four reasons: (i) to
retest capsaicin and nicotine at concentrations that were
better matched for sensory intensity; (ii) to check that a
lower (0.5 p.p.m.) capsaicin concentration cross-desensitized
irritation produced by nicotine; (iii) to confirm that nicotine
does not cross-desensitize irritation evoked by capsaicin at
this low concentration; and (iv) to compare effects of
capsaicin and nicotine in an identical subject sample.
3a. Sequential stimulation with capsaicin and
cross-desensitization with nicotine
Sensitization was again demonstrated with 0.5 p.p.m.
capsaicin (Figure 3). The main effect of application trial was
significant (P < 0.001, ANOVA), and increasing ratings
across trials were obtained in a significant majority (19/20)
of subjects (P < 0.001, binomial).
The 2-AFC procedure revealed that 0.5 p.p.m. capsaicin
led to cross-desensitization of the irritant effect of 0.12%
nicotine. A significant majority of subjects (9/10, P = 0.02,
binomial) initially reported a stronger sensation on the
previously unstimulated side following bilateral testing with
nicotine (Figure 2, first open bar). In addition, the total time
that subjects reported a stronger sensation on the previously
unstimulated side was significantly greater (mean 2.3 versus
0.6 min; P = 0.01, /-test).
To examine the time course of cross-desensitization, the 3
min 2-AFC test period was broken into three 1 min
segments. During the initial segment, nine of 10 subjects
reported a stronger sensation on the previously unstimulated side for a significantly greater total amount of time
(mean 0.9 versus 0.1 min, P - 0.003, /-test). A similar result
was obtained during the second 1 min segment (mean 0.78
versus 0.22 min, P = 0.03, /-test). However, during the last 1
min segment the difference was no longer significant (7/10
subjects chose the unstimulated side; P = 0.17, binomial;
mean time 0.73 versus 0.27 min, P = 0.14, /-test). This
indicates that the cross-desensitization began to wear off
toward the end of the 3 min 2-AFC test period. This is supported by the mean time of occurrence of a 'no difference'
Downloaded from chemse.oxfordjournals.org by guest on July 14, 2011
Experiment:
Cap (Hi
Cap*
Oral Irritant Effects of Nicotine I
judgement, which was 1.51 min (although two subjects
continued to rate one side as consistently more intense for
the entire 3 min period).
Discussion
The present results confirm that sequential intra-oral
stimulation with capsaicin elicits progressively stronger
irritant sensations (sensitization). In contrast, repeated
application of nicotine evokes progressively weaker irritant
sensations which we will refer to as desensitization. In
addition, an asymmetric cross-desensitization effect was
observed whereby capsaicin reduced the perceived intensity
of irritation evoked by nicotine, but not vice versa. These
findings are discussed in terms of methodology and possible
physiological mechanisms.
Sequential sensitization and
self-desensitization by capsaicin
The present data confirm the sensitizing and selfdesensitizing effects of capsaicin reported by Green (1989).
We presume that capsaicin and nicotine, which were judged
to be tasteless at the concentrations used (see also Lawless
and Stevens, 1990), activated trigeminal nociceptors in the
oral cavity, although activation of gustatory receptors
cannot be ruled out. Capsaicin binds specific molecular
capsaicin ('vanilloid') receptors in the nociceptor terminal
(Szallasi et al., 1994; reviewed in Szallasi, 1994) to evoke a
depolarization (Wood et al., 1988; Cholewinski et al., 1993;
Liu and Simon, 1994,1996b; Chard et al., 1995). The means
by which capsaicin can sensitize yet also desensitize
nociceptors is not known; desensitization appears to require
Ca2+ influx (Cholewinski et al. ,1993; Chard et al., 1995; Liu
and Simon, 1996b). Recent studies indicate that the
relationship between capsaicin sensitization and desensitization is complex, since desensitization (i) is masked or
delayed in the presence of capsaicin; (ii) only manifests itself
after capsaicin is removed (Green, 1989, 1991b) even if a
non-self-desensitizing irritant such as zingerone is present
(Green, 1993); and (iii) can be overcome by recurrent
capsaicin application (Green, 1996).
Sequential desensitization and
self-desensitization by nicotine
Using the same testing procedure, we found that ratings of
nicotine irritation decreased, rather than increased, across
trials (desensitization) and that this effect persisted for at
least 10 min (self-desensitization). Cliff and Green (1994,
1996) similarly reported decreasing ratings of irritation
evoked by intra-oral menthol across trials (at 1 or 5 min
interstimulus intervals), consistent with desensitization.
Sub-populations of cutaneous (Brown and Gray, 1948;
Douglas and Gray, 1953; Douglas and Ritchie, 1960;
Fjallbrant and Iggo, 1961; Steen and Reeh, 1993) and
lingual receptors (Wang et al., 1993), as well as trigeminal
ganglion neurons (Sucher et al, 1990; Liu et al., 1993), are
sensitive to nicotine and other cholinergic agents via a
'neuronal' nicotinic receptor (Sucher et al., 1990; Liu et al.,
1993; Wang et al., 1993; for recent reviews, see Ochoa and
McNamee, 1990; Deneris et al., 1991; Sargent, 1993). The
role of neuronal nicotinic receptors is supported by
previous (Jarvik and Assil, 1988) work, as well as current
work in our laboratory, showing that nicotine irritation is
reduced by the ganglionic blocker, mecamylamine.
Downloaded from chemse.oxfordjournals.org by guest on July 14, 2011
3b. Sequential stimulation with nicotine and lack
of cross-desensitization to capsaicin
In confirmation of experiment 2a, sequential application of
nicotine led to a decrease in the perceived intensity of
irritation (Figure 3). The main effect of application trial was
significant (P < 0.001 ANOVA), and decreasing ratings
across trials were obtained in a significant majority (17/20)
of subjects (P - 0.003, binomial).
Data from the 2-AFC procedure were again consistent
with an absence of cross-desensitization of nicotine to
capsaicin. For the 10 subjects participating in experiment 3a,
five reported a stronger sensation on the nicotine-pretreated
side, and five on the untreated side, immediately after
capsaicin was delivered bilaterally. This was verified in a
larger population with 10 additional naive subjects. Eleven
of the 20 subjects (55%) reported that the side of the tongue
not previously exposed to nicotine yielded a stronger
sensation immediately after capsaicin was delivered
bilaterally (Figure 2, second open bar); this was not
significant (P = 0.50, binomial). This ratio remained
unchanged 45 s later, arguing against the possibility that the
sensation required time to develop. The total time that
subjects reported a stronger sensation on the previously
unstimulated side was not significantly greater during the
entire 3 min 2-AFC test period (mean 1.86 versus 1.14 min,
P > 0.8, r-test), or during any of the 1 min segments. Finally,
the mean time to a 'no difference' judgement was 0.9 min,
and only one subject reported a consistent difference over
the entire 3 min test period.
489
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I J.-M. Dessirier, M. O'Mahony and E. Carstens
Cross-desensitization
2-AFC and desensitization/cross-desensitization
Our use of 2-AFC with half-tongue stimuli to investigate
oral irritation is novel, although a 2-AFC method has been
employed recently to assess sensations evoked by chemical
irritants versus solvent when both were applied simultaneously to the face (Green and Bluth, 1995). From a
theoretical viewpoint, 2-AFC has the advantage of greater
sensitivity to small differences in sensation compared with
scaling methods, and constitutes one of the most powerful
discrimination methods available (O'Mahony, 1992).
A potential confound associated with bilateral stimulus
application is that the larger of two simultaneously
presented noxious stimuli reduces irritation from the weaker
stimulus (Green, 1991c). However, such a 'counterirritation'
effect would enhance any differences in perceived intensity
of simultaneously presented irritants, and this difference
should arguably be detected more readily using the present
2-AFC method. A drawback of the 2-AFC procedure as
presently used is that it assesses if there is a bilateral
difference in the intensity of sensation, but not the
magnitude of the difference. The relative magnitude of
self-desensitization effects by capsaicin and nicotine
therefore cannot be compared. The feasibility of combining
a magnitude rating procedure with 2-AFC to evaluate
differences in the magnitude of sensations on the two sides
of the tongue is currently being examined.
ACKNOWLEDGEMENTS
This work was supported by a grant from the California Tobacco-Related Disease Research Program # 4IT-0087.
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