Desensitization to Oral Zingerone Irritation: Effects of Stimulus Parameters

Physiology& B V C P 1 0 E U r 0 E 1 I S r $ + PII SO031-9384(96)O0248-X D e t Os o S E Z e P f P S R I ni t sm a S C D &T A R P B S D o z i E s p P humans,repeated oral stimulationwith the isritant capsaicin produces sensitization or desensitization,dependingon the temporalrelationshipand, to a lesser extent, the intensity of the stimuli. We have previously shown that zingerone,an irritant present in ginger, shows only desensitizationacross repeated samples, as well as followinga hiatusin stimulation.Becausethe time-courseof zingeroneirritationdiffersfromthat of capsaicin,it is likelythat optimaltemporal and other stimulationparametersmay also be different. In Experiment1, we examinedthe effects of stimulus intensity (0.5%, 1.0%,2.0%zingerone)and the numberof successivestimuliin a series on psychophysicalresponsesto zingeroneirritationwithin the series and followinga 5-rein hiatus. Experiment2 examinedthe effect of the durationof this hiatus on desensitizationand recovery. Desensitizationwas apparent across the initial series of stimuli in both experiments and, irrespective of zingerone concentration,in Experiment1. Desensitizationalso occurredfollowingthe 5-rein hiatus, evidentprimarilyat the higher concentrations.Precedingtbe hiatuswith 5 or 10stimuliproducedthe greatestposthiatusdesensitization,but a decreasein rated intensity was also evidentfollowinga single stimulus.Experiment2 showedthat the optimalhiatus for demonstratingdesensitizationwas 5 nrin and that, by 15 rein, recoveryhad begun.In both experiments,individualdifferencesin responsewere marked, with some subjectsshowingsensitizationandotherslittle changein responseacrossrepeatedzingeronestimuli.The originof these differences 0 E S I is unclearbut w shownto be relatively stable across multiplesessions. C 6 Zingerone 1 Irritation 1 Desensitization Interstimulusinterval Intensity RESEARCH on the psychophysics of human oral trigeminal chemoreception has demonstrated that multiple, successive samples of capsaicin, a potent oral irritant, produces sensitization, or increases in intensity of the irritant sensations. Conversely, a pause in stimulation following a series of capsaicin solutions, or a sufficiently intense single capsaicin solution, produces a marked decrease in sensation intensity for subsequent capsaicin solutions, an effect known as desensitization (3). Desensitization is thought to occur through the action of capsaicin on sensory receptors, reducing their sensitivity, possibly by depletion of neuropeptides (12). With the limited amount of research in this area, it is unclear whether or not these patterns of response to irritants reflect general properties of the oral trigeminaf system. Investigations of the psychophysical properties of other irntattts have shown that sensitization occurs with repeated NaCl stimuli (7); conversely, the irritation of menthol appears to desensitize over repeated samples (2), and ethanol appears to neither sensitize nor desensitize with repeated samples (4). It has been suggested that zingerone, an irritant present in ginger, neither sensitizes nor desensitizes. For example, capsaicin introduced into the eyes of rats at l-tin intervals initially provoked eye wiping movements, which decreased to zero after several capsaicin doses. Although pretreatment with capsaicin eliminated the Individualdifferences wiping response to zingerone, repeated stimulation with zingerone itself produced only slight reductions in wiping, interpreted as a failure to desensitize ( 18). In human studies, a series of zingerone solutions, and a posthiatus solution 15 min later, on the tongue were found neither to sensitize nor desensitize (6). We have previously demonstrated ( 15) that zingerone produces different patterns of responses to that found with repeated stimulation by capsaicin, despite producing qualitatively simikir sensations. Contrary to previous research with capsaicin (3), desensitization rather than sensitization to zingerone was the group pattern over an initial 10 stimuli, although there was some evidence of “re-sensitization” because the overall pattern across these stimuli was best described by a quadratic function. In addition, desensitization was apparent following a 5-rein hiatus in stimulation. This latter effect was sindar to desensitization observed with capsaicin, although the magnitude of the effect was much smaller. Differences between irritants in the extent to which they show sensitization or desensitization may be, at least partly, a function of the parameters of stimulation. It has been demonstrated, for example, that both capsaicin sensitization and desensitization are dependent on the temporaf relationship of successive capsaicin soiutions. Thus, sensi~ization has been shown to be evident at ‘ Reprint requests sbould be addressed to John Prescott, Sensory Science Research Centre, University of Otago, P.O. Box 56, Dunedin, New Zealand.E-mail:john.prescott@stonebow. otago.ac.nz 1473 f 1474 PRESCOTT AND STEVENSON interstimulus intervals (ISIs) of 0.5 and 1.5 rein, but absent at 1S1sof 3.5 and 5.5 min (5). Green (3), using a series of 3 ppm capsaicin stimuli, explored the effects of altering the hiatus between the series and a final capsaicin solution. He found that desensitization could be demonstrated at intervals of 5 and 10 rein, but not at 2.5 tin, indicating the importance of the duration of this gap. Using a single preexposure to capsaicin, Green (5) noted an optimal hiatus of between 5.5 and 14.5 min to demonstrate desensitization to a subsequent capsaicin sample. Moreover, with a strong 30 ppm capsaicin stimulus, desensitization was less apparent at 14.5 tin than at 9.5 rein, suggesting that some form of recovery had begun at the longer time interval. In our previous studies on zingerone ( 15), we suggested that prior failures to find self-desensitization with zingerone (e.g., 6) may have been the result of the use of a 15-min hiatus, which has been shown to produce capsaicin desensitization (5), but which may be not be optimal for zingerone. The fact that zingerone desensitization was evident with the use of a 5-rein hiatus following a series of zingerone stimuli supports the contention that timing parameters may be important determinants of desensitization. The selection of a 5-rein hiatus was based on the finding that this allowed sufficient time for the irritation of a single zingerone sample to dissipate. However, the role of the such temporal factors could only be inferred from this study, and there has been no investigation of the influence of the duration of hiatus on desensitization to irritants other than capsaicin. One of the aims of the present study was to investigate the temporal parameters for zingerone desensitization and recovery. The number of stimuli in a series may also have an impact on the degree of desensitization observed. In the capsaicin sensitization series shown by Green (3), rated intensity continues to increase with increasing numbers of stimuli (up to 25). Our previous study (15 ) showed desensitization as a function of 10 successive zingerone presentations. We were interested in determining if a pattern of desensitization would be evident over fewer stimuli. In particular, with capsaicin, a single stimulus can lead to desensitization if a hiatus follows (3), and we wished to determine if this effect was rdso characteristic of zingerone irritation. Capsaicin desensitization has been also found to be dependent to some extent on the strength of the stimulus (5,9). Although both sensitization and desensitization have been found to occur at different capsaicin concentrations, the magnitude of the effect varies. Thus, although exposure to a single capsaicin stimulus at 30 ppm was able to produce desensitization to subsequent stimuli, this effect was absent with 3 ppm capsaicin (5). Desensitization of irritation produced by menthol has also been shown to be dependent upon stimulus intensity, with desensitization being present at 0.3%, but not 0.03%, menthol (2). The possibility exists, therefore, that responses to repeated zingerone stimuli are also sensitive to stimulus intensity. If this is the case, then the discrepancy between Green’s (6) failure to find desensitization and our demonstration that it occurs ( 15) may have resulted to some extent from the fact that our use of a whole mouth rinse of 1% zingerone would have produced considerably more irritation than the filter paper presentation method used by Green (6). An additional aim of these studies was to investigate the role of zingerone concentration in producing desensitization. Although there are individual differences in the rates at which sensitization occurs over multiple capsaicin stimuli, an increase in rated intensity is, nevertheless, the predominant pattern (within the optimal 1S1,at least). However, desensitization over an initial series of capsaicin stimuli has been noted for some individurds (6,13). An important characteristic of the data from our previous studies on zingerone (15) was the high inter-individual variability. Over a series of consecutive zingerone sam- ..- ples, some individuals showed marked sensitization and others showed marked desensitization. The reasons for this variability are unclear, but were not explained by differential chili use, sex, or age. Similar variability in response pattern has also been noted with menthol (2). However, in contrast to the effects seen over a series of 10 stimuli in our previous zingerone studies (15), the response to zingerone after the 5-rein hiatus was a consistent desensitization. Again, this is similar to what has been reported with menthol (2). The studies reported here aimed to further characterize individual psychophysical response patterns to zingerone and examine the impact of stimulus parameters on such individual differences. Thus, the present experiments were designed to investigate the role of variations in the parameters of zingerone stimulation on psychophysical responses. Experiment 1 manipulated zingerone concentration and the number of solutions in a series on responses across the series and following a 5-rein hiatus. Experiment 2 manipulated the duration of the hiatus following a zingerone series to examine temporal aspects of desensitization and recovery. EXPERIMENT 1 M S Thirty-three subjects (14 men and 19 women), of whom 10 were CSIRO staff volunteers and 23 paid Macquarie University students, took part. Subjects all completed the Lawless et al. ( 10) questionnaire that assessed their frequency of chili usage at either the start of the experiment or within 3 months of the commencement of the study. Subject details are illustrated in Table 1. M Zingerone (Dragoco) was dissolved in 1.0% PolysorbateTween 80 (Labchem) in distilled water to produce concentrations of 0.5, 1.0, and 2.0910.The zingerone solutions were presented at mouth temperature (37”C) in 10-ml aliquots in 22-ml plastic cups. Because the 2.OVO zingerone tended to come out of solution, it was heated to 37°C in the water bath and then stirred on a precrdibrated stirrer hot-plate during stimulus delivery. P Subjects were assigned, in order of arrival, to 1 of 3 groups, in which they received either ().5~o, 1.OTO,or 2.OVO zingerone in TABLE 1 G C E Z 0.5% C Mean age (SE)* Mean score (SE)T Medianfrequencyof chili use (range)$ 1 C 1.0% 2.0% = = = 27.8 (3.2) 20.2 (2.3) 27.5 (2.9) 16.2(2.1) 25.8 (3.0) 22.3 (2.2) 3 (2-5) 2 (l-5) 3 (l-5) * Not significantlydifferentbetweengroups(ANOVA,by group,F = 0.12); TScoresfrom questionnaireby Lawless et al. (10). These did not differ significantlybetweengroups(ANOVA,by group;F(2,30) = 2.13); $ Frequencyscoresdid not differ significantlybetweengroups(KrnskalWallis test, KW = 2.03). Frequencyvalues: 5 = 3–4 times/week,3 = l–3/montb, 2 = less than I/month, 1 = l/year or less. PARAMETERS OF ZINGERONE DESENSITIZATION + 0.5%Zingerone + 1475 Z O b @ I A o A ● I I o I I I I I I 5 I I I I I I I 012 I I I 01 5 2345 10 (mkts) FIG. 1. Mean intensity ratings for 1, 5, and 10 successivepresentationsof 0.5%, 1.0%, and 2.0% zingerone solutionsfollowed by a subsequent zingeronesolutionafter a 5-rnirrhiatus. The 5% LSD is shown. aII sessions. Each subject then completed 3 sessions, each separated by at least 4 days. Subjects sampled, at their assigned concentration, either 2, 6, or 11 zingerone solutions, with each subject sampling a different number of solutions in each session. The final zingerone solutions in these series were sampled after a 5-rein break. The order of presentation of the 2, 6, or 11 stimuli across sessions was fully counterbalanced. In all sessions, subjects were told they would be receiving a number of zingerone presentations. Detailed written instructions, individually tailored to the number of solutions that the subject was to receive, were then presented. Use of the Labelled Magnitude Scale (LMS) (8) to rate overall intensity was explained and subjects were further instructed not to talk or move their tongues during the zingerone presentation phase. Subjects were then shown a diagram detailing the procedure in total and were then quizzed by the experimenter to check that they completely understood the procedure. To begin the session, the computer sounded a tone as a signal for the subject to pour the first zingerone solution into their mouth. The experimenter then immediately handed them an LMS, which they completed. The subject held the solution still over the tongue untiI 15 s had elapsed, when the computer emitted another tone. The subject then expectorated the solution and immediately completed a second LMS. For subjects receiving just 1 initiaI zingerone solution, a 5-rein break occurred before this procedure was repeated. For subjects receiving 5 or 10 initial zingerone solutions, after a further 15s had elapsed, during which the subjects kept their mouths cIosed and tongues still, the next zingerone stimulus was presented. This cycle was then repeated a further 5 or 10 times, followed by a 5-rnin break, after which the final zingerone stimulus (#6 or#11 ) was presented. RESULTS All ratings made with the LMS were scored between O and 244. Ratings made while the zingerone was in the mouth and after expectoration were highly correlated (median Pearson’s ~ = (),83; rage: 0.66 to 0.92, across all conditions and groups) and were of similar magnitude. Therefore, subjects’ scores were combined for each zingerone presentation and a mean value was used in all reported analyses. The mean intensity ratings for 1, 5, and 10 successive presentations ( 1.0%, and 2 zingerone solutions and a subsequent zingerone solution 5 rnin later are shown in Fig. 1. Using zingerone concentration (0.5%, 1.0%, or 2.0%) as a between-subjects factor, ANOVAs were conducted to examine: 1. Differences between the initial and final zimzerone solutions. with number of stimuli in the initial series ( 1.5. and 10 zingerone solutions, labelled Zl, Z5, or Z1O, respectively) as a within-subjects factor; 2 Effects of successive zingerone solutions in the initial series, separately for Z5 and Z1O; 3 Differences between the last of the initial series (i.e., #5 or #10) and final, posthiatus zingerone solutions, for Z5 and Z1O. All reported statistical effects are significant at least at the 5% level. Post hoc comparisons between means following significant ANOVA effects were conducted using Fisher’s Least Significant Difference (LSD) test. D Between the Initial and Final Zingerone Solutions Comparisons between the ratings for the first and final (posthiatus) zingerone solutions in Zl, Z5, and Z1O showed that (see Fig. 1), for all 3 groups, there was an overall decrease in intensity ratings (main effect of time: F 1,30 = 109.17). There was also a main effect for number of solutions in the series (1, 5, or 10) (F2,60 = 10.48; LSD = 14.19), and a solutions X time interaction (F2,60 = 17.14; LSD = 9.66). As can be seen in Fig. 1, these results reflected a greater decrease from the first to final solutions for Z5 (mean = 51.23) and Z1O (mean = 60.4) than for Z1 (mean = 21.74). There were no significant main effects 1476 PRESCOTT AND STEVENSON or interactions with groups, indicating that these decreases were independent of zingerone concentration. TABLE 2 L C S Differences Between the Pre- and Posthiatus Zingerone Solutions To determine the impact of number of solutions in the initial series on desensitization following the hiatus, comparisons were made between the ratings of final solutions in the initial series in Z5 and Z1O (i.e., #5 or #10) and those of the solutions following the 5-rein hiatus (i.e., #6or#11 ). As can be seen in Fig. 1, rated intensity decreased over the hiatus (main effect of time: F1,30 = 18.94). A main effect for number of solutions (F1,30 = 11.57) reflects the overall lower intensity for Z1O than for Z5, most likely the result of continuing desensitization in Z1O. There was no overall difference between groups. However, a time X group interaction (F2,30 = 4.22) reflected the larger decrease in rated intensity with increasing zingerone concentration. Comparisons between means (LSD = 17.2) revealed that this was due to significant decreases (mean = 25.2) in the l.0~0 zingerone group and an even greater decrease (mean = 48.4) in the 2.0% zingerone group, but no significant decrease in the 0.5~0zingerone group (mean = 4.8). There were no differences between the 3 zingerone concentrations following the 5-rein hiatus, whereas differences were apparent at the final solution in the series with significant differences between 2.09%,and both 1.OYO and 0.570 (see Fig. 1). In other words, group (concentration) differences present in the final solution of the initial series, at the point of maximum desensitization, were not present following the hiatus. Individual Dl~erences Table 2 shows the linear coefficients for each subject’s data for Z5 and Z1O. One sample t-test on these coefficients revealed significant linear slopes for both Z5 (t32 = 2.86) and Z1O (~32 = 3.13). In both cases, there were no significant quadratic trends. As can be seen, in each of the groups, subjects showed both negative and positive linear slopes, indicative of desensitization and sensitization, respectively. ANOVAs on these vrdues revealed no significant differences between groups for either Z5 or Z1O. EXPERIMENT 2 The purpose of this second experiment was to examine the role of the duration of the hiatus following a zingerone series on the degree of desensitization observed over the hiatus. M Subjects Twelve CSIRO staff volunteers, 9 men and 3 women (mean age = 38.1 years), took part. Subjects had all completed the Lawless et al. ( 10) questionnaire within 3 months of the com- E G F R 5 Effects of Successive Zingerone Solutions For treatment Z5, there was a significant decrease in rated intensity over the initial 5 zingerone stimuli (main effect of time: F4,120 = 6.34; 1°:F1,30 = 7.85), but no significant main effects or interactions due to the different zingerone concentrations. For treatment Z1O, there was also a significant decrease over the initial 10 zingerone stimuli (main effect of time: F9,270 = 7.37; 1°: F1,30 = 9.72). Across these stimuli, 2.0% zingerone was rated more intense than 1.0%, which was more intense than 0.5% (main effect of group: F3,30 = 3.36). There was no interaction between time and group, indicating an equivalent decrease in intensity at all zingerone concentrations. C 0 z S O Z 1 S z 2 5 +0.2 –14.1 +3.5 –6.9 –5.9 +1 1.9 –11.8 –1.4 –3.0 –11.5 +2.6 –4.3 +1.8 –1.4 –24.5 +11.4 –17.6 +1.6 – – –5.67 –4.5? Z R I z 5 -1.2 +1.8 + +6.5 +4.4 -0.3 -6.6 –1.7 -2.2 –1.9 –7.2 –12.8 –0.2 +8.0 +5.9 +0.3 +3.3 –9.4 -14.5 –30.4 +7.8 – – – +0.4 –10.8 –8.8 –19.7 –17.6 –8.1 –1.3 –11.7 –1.8 + 1.5 –9.8T –2.87 –2.ot –2.0 –4.1 –0.5 –6.4 –0.3 –5.4 +8.7 +1.1 –9.6 –6.5 +0.1 -2.37 * No. of zingeronestimuli, t Mean. mencement of the study. The mean (SD) score on this questionnaire was 24 (7.1) and the median frequency of chili use was 3.5 (between l-3/month and l/week). Materials Solutions of 1.0% zingerone were made up and presented in the manner described in Experiment 1. Procedure Each subject took part in 4 sessions, each separated by a minimum interval of 4 days. The beginning of each session was identical, in that 5 solutions of 1% zingerone were sampled in the manner described in Experiment 1, that is with 15-s intervals between ingestion and expectoration, at which times ratings of overall intensity were made. However, in this experiment, the time interval between the fifth and sixth solution was systematically varied. Over 4 sessions, all subjects experienced delays of 2.5, 5, 15, and 60 tin before the presentation of the sixth and final zingerone stimulus. The order of the delays was fully counterbalanced across subjects. R Ratings made while the zingerone was in the mouth and after expectoration were highIy correlated (median Pearson’s r = 0.94; range: 0.87 to 0.98, across all conditions) and were of similar magnitude. Therefore, subjects’ scores were combined for each zingerone presentation and a mean value was used in all reported analyses. Effects of Repeated Zingerone Stimuli Figure 2 shows a decrease in rated intensity of approximately 30% over the initial 5 zingerone solutions. Because each subject received 4 series of zingerone solutions, the data from the present experiment allowed an examination of the degree of consistency across repeated sessions of response patterns over the 5 stimuli. ANOVA using repeats and time as factors revealed a significant decrease over the 5 stimuli (main effect of time; F4,44 = 6.35), 1477 PARAMETERS OF ZINGERONE DESENSITIZATION ing to the order in which they undertook the sessions. ANOVA using individual slope values across the initial 5 solutions revealed no evidence that repeated zingerone stimuli produced a consistent effect across sessions. E~ects of ISI z ; > ~ m zw Figure 3 shows an exponential function (chosen to mimic natural decay of the stimulus) fitted to the means of the 4 zingerone series. Also shown are the mean intensities for the zingerone stimuli following the 2.5, 5, 15, and 60 tin 1S1s.As can be seen, the point of maximum desensitization is at 5 tin, while a degree of recovery is evident at 15 min. By 60 rein, rated intensity has returned to 8570 of the rated intensity of the first zingerone stimulus. The effects of the duration of the 1S1on rated zingerone intensity were examined in ANOVAs with time (difference between initial and final, posthiatus zingerone solutions) and 1S1as within-subjects factors revealed a significant main effect of time (Fl,ll = 20.23) and 1S1 (F’3,33 = 4.47), and a time X 1S1 interaction (F3,33 = 3.38). As can be seen in Fig. 3, this reflects the greater desensitization that occurred with shorter 1S1s (2.5 and 5 rein) compared to longer 1S1s(15 and 60 rein). The fact that rated intensity at 2.5- and 5-rein 1S1swas lower than the final zingerone stimulus of the series suggests the possibility that introducing a hiatus between 2 successive zingerone solutions produces an increase in desensitization. This notion was tested by determining, for each subject, the predicted value for 2.5 and 5 min based on individual exponential functions fitted to ~ z 4 u E o 2 1 T ( FIG. 2. Mean intensityratings for the initial series of 5 zingeronesolu- tions in each of the 4 ISI conditionsin Experiment2. but no significant main effect for repeats, or repeat X time interaction, confirming the impression from Fig. 2 that overall group responses were consistent across sessions. To examine the possibility of order effects due to either context ( 17) or desensitization, subjects’ data were grouped accord- o A 1 ! I I I I I I I I I I I I I I I I I I I I I I I I I I 01234567 I I I I I 50 15 10 60 70 TIME (reins) F M i r o 4z s E e f c m n d s been fitted to these values. Also shownare the mean intensitiesfor the zingeronestimuli followingthe 2.5- (0), 5- (A), 15-(A), and60-rnin (0) 1S1s. 1478 PRESCOTT AND STEVENSON their initial 5 intensity ratings. These values were then compared to the actuaf values at 2.5 and 5 min. t-Tests failed to show any difference between the observed and predicted values for either 2.5 tin or 5 min. In other words, there is no evidence that values following either a 2.5- or 5-rnin hiatus were different from predicted ratings if zingerone solutions had continued to be given. Individual Differences For the initial series of 5 stimuli, linear slope values for individual subjects were calculated and are shown in Table 3. As in Experiment 1, despite the overrdl pattern of linear decrease in rated intensity with repeated zingerone stimuli, for individual subjects, patterns of sensitization (positive slope) as well as desensitization (negative slope) were evident. Moreover, in some cases, there were very small positive or negative slopes, suggestive of no real change over successive stimuli. To categorize individuals, SES were calculated for the mean slope values as an estimate of measurement error. Only subjects showing slopes that exceeded this value (positive or negative) were classified as desensitizers or sensitizers, respectively; the remainder were considered to exhibit no change over successive stimuli. According to this criterion (see Table 3), only 6 of the 12 subjects consistently desensitized across all sessions (Ss #1,3,5,7,10,11; see Table 3). One subject (S #2) consistently sensitized, and S #4 showed sensitization over 3 of the 4 sessions, and S #6 desensitized over 3 of the 4 sessions. The remaining 3 subjects showed mixed patterns of sensitization, desensitization, and no change. There was also variability in the patterns of response from the 5th (final zingerone stimulus in the initial series) to the 6th (posthiatus) stimuli. Using a criterion of exceeding the SE of the mean of the differences between these two values, subjects were classified as showing sensitization, desensitization, or no change. There was a tendency for sensitization to be more common at 1S1s of 15 and 60 min than at 1S1s of 2.5 and 5 rein, perhaps reflecting recovery from desensitization. However, even at an 1S1 of 2.5 rein, 2 subjects (#1, 9) showed sensitization from stimuli 5 to 6, despite being clear desensitizers from stimuli 1 to 5. These subjects cannot simply be seen as “early recoverers, ” because both subjects showed desensitization at longer 1S1s. Other subjects showed clear desensitization across the hiatus at all 1S1s, even though one subject (#4) sensitized during the initial series. GENERAL DISCUSSION The present experiments provide data on the psychophysics of zingerone irritation which, following our previous studies (15), further differentiate it from capsaicin irritation. The data from Experiment 1 demonstrate that the predominant response across a series of 5 or 10 successive zingerone stimuli is desensitization, in contrast to the sensitization observed with capsaicin. Compared to our previous findings ( 15), the zingerone desensitization in the present studies showed a more pronounced linear decrease over the series. Given that there are marked individual differences included in the group response, it is possible that this discrepancy is due to the differential contribution of individuals who sensitized over the series. Increasing zingerone concentration had little impact on the degree of desensitization over the initial series, other than the expected increase in rated intensity, most apparent with Z1O. Importantly, because there was no interaction between different ~oncen~ations and time, it confirms that desensitization was equally present for the range of concentrations, representing quite weak to moderately strong levels of irritation. The group data from Experiment 2 were consistent with those from Experiment 1 in that there was art overall decrease in rated intensity over the initiaf series of 5 zingerone stimuli. In addition, the design of the experiment allowed an examination of the consistency of this response within a group. Over time, desensitization was shown to be a consistent group pattern over an initial series of zingerone stimuli. Zingerone desensitization thus appears to be a reliable phenomenon, having been demonstrated in both experiments here, as well as in our previous studies ( 15). Following the 5-rrtin hiatus in stimulation in Experiment 1, desensitization was again the group response. Although this pattern was found even with a single pretreatment with zingerone (Zl ), TABLE 3 LINEARSLOPEC S A D ( I B S S 5 Z P AND POSTHIATIJS (5-6), FOR THE 12 SUBJECTS IN EACH OF THE 4 INTERSTIMULUS INTERVAL (1S1) CONDITIONS (b) 5-6 (A) (a) 1-5 (Slopes) Subject # 1 2 3 4 5 6 7 8 9 10 11 12 2.5 min 5 mill 15 min 60 rnin 2.5 tin 5 min 15 tin 60 min –9.8 –6.7 –18.4 –24.0 S — s s +11.0 –29.9 +6.9 –10.6 –8.4 –10.3 –2.1 –17.0 –20.7 +5.6 –22.5 –0.4 –7.8 +7.6 –17.5 –1.0 –11.3 –19.6 +1 1.3 –17.6 +9.3 –10.6 –11.2 –4.3 +2.0 +1.6 –21.8 +10.6 –9.8 +8.5 –4.9 –10.0 –12.1 –9.5 –8.1 –35.2 — D D D – D D D D s D D D s D S D s D D s s — –23.1 –17.0 –26.0 +2.3 +3.3 +3.5 s — s s D –20.9 D - D D D +3.5 – D — D SESassociatedwiththese slopeanddifferencevalueswere,respectively:3.12,8.06 (2.5rnin 1S1);2.77, 5.88 (5 rnin); 3.62, 11.73(15 rein); 3.83, 11.37(60 rein). Subjectswhose slopeor differencescoredid not exceedthese valueswereconsiderednot to haveexhibited either desensitization(negativeslope/D)or sensitization(positiveslope/S). PARAMETERS OF ZINGERONE DESENSITIZATION 1479 when comparing the pre- and posthiatus solutions, greater decreases were found with larger number of stimuli (Z5 and Z1O). In these latter series, zingerone concentration proved to be a significant factor, with significant reductions in intensity ratings evident over the hiatus, but onfy for the 2 highest concentrations. Thus, in contrast to the pattern of desensitization over the initial series, there was no evidence of desensitization across the hiatus for 0 zingerone. One possibility is that recovery from zingerone desensitization is partly a function of stimulus intensity and that recovery had already commenced at 5 min with the low-intensity zingerone. Further data are needed to confirm this, however. Experiment 2 provided the first data on the time-course of zingerone desensitization and recovery. Maximum desensitization was reached at 5 rnin; by 15 rein, some recovery was evident; by 60 rein, recovery back to the initial rated intensity was substantially complete. By contrast, capsaicin desensitization following a series of 10 3-ppm stimuli is substantial at 10 min and is still evident after 2–3 h (3); greater capsaicin concentrations can produce desensitization that is still evident after severaf days (9). Again, these data are consistent with alf previous data showing that the time-course of zingerone irritation is much briefer than that of capsaicin. What do the current experiments reveal about desensitization as a characteristic of oral trigerninal chemoreception? Capsaicin sensitization and desensitization depend very strongly on the temporal relationship between stimuli. It appears that, if the temporaf conditions are met (e.g., 1S1sof 5–10 rein), capsaicin desensitization always occurs (3,5) and it has been suggested that this is because the trigeminal system has been given sufficient time to recover from prior stimulation (3). In the present studies, desensitization was strongly evident with 1S1s during which considerable irritation would still be present (15). This indicates that, for zingerone, recovery is not a precondition for desensitization to occur during repeated stimulation. Data on the desensitization of menthol also shows that with l-rein 1S1s,desensitization can occur, again suggesting that recovery from prior stimulation is not necesswy (2). Previous research on the parameters of capsaicin desensitization have indicated that prior sensitization is not a prerequisite for it to occur but, rather, that with repeated stimulation using brief 1S1ssensitization acts to delay desensitization. Moreover, following a sufficient hiatus, desensitization is a profound decrease in rated intensity with an impact typically much greater than simply offsetting the impact of sensitization. It has, thus, been thought that the two processes are independent phenomena (5). The question arises of whether or not leaving this hiatus following a zingerone series also produces an impact suggestive of the initiation of an independent process. The present data do not show evidence of independent processes. The zingerone stimuli that were presented at different 1S1s after the initiaf series can be considered as “probe trials” that indicate how the system is functioning overtime. These trials indicated that the initial series had a continuing desensitizing impact on the system up to 5 min following the end of the series. An interpretation that a hiatus in stimulation leads to desensitization greater than would be achieved through continued stimulation was not supported by these data because there was no acceleration of the decrease beyond what would have been expected with continued stimulation. That is, the values at 2.5 and 5 min did not differ from the predicted exponential values. This suggests that the desensitization to the maximum point at 5 min was a continuation of the desensitization observed during the initial series, with the responses at 15 and 60 rnin representing recovery from these effects. However, in considering this issue, the individual differences data also need to be taken into account. A number of Ss who showed desensitization, sensitization, or no change over an initial series all showed desensitization following a hiatus; in other Ss, sensitization occurred following a hiatus, despite desensitization occurring over the initial series. Thus, initial responses over a series of zingerone stimuli were, for many subjects, a poor predictor of responses following a hiatus. These data could result from the operation of distinct physiological processes. It remains uncertain what processes underlie desensitization. Because the desensitization produced by trigeminal stimulants has been shown not to be linked to degree of pungency, it has been suggested that it is not the result of excessive stimulation but, rather, a specific molecular interaction ( 18). If this is the case, different psychophysical responses to different compounds are understandable in terms of different receptor properties. In fact, Green’s (6) demonstration of hypercross-sensitization between capsaicin and zingerone does suggest that capsaicin and zingerone may stimulate different trigeminal receptors or, alternatively, bind differently to the same set of receptors. This latter explanation is perhaps more likely in light of the similarity of the perceptual qualities, but dissimilarity of temporal properties, generated by zingerone to those of capsaicin. Recent studies on the currents activated by capsaicin and zingerone in rat trigeminaf ganglion cells have also suggested that both compounds activate the same receptors and that differences between the compounds relate primarily to the kinetics of activation (1 1). The overall implication of this is that although many compounds may activate trigeminaf nerve fibers, the psychophysical response to repeated stimulation is a combined function of which receptors the particular compound stimulates (or how it stimulates them), together with the temporal properties of stimulation. How the individual differences found here relate to this explanation is unclear. Such differences were not related to stimulation Pammeters. Thus, whether an individual showed a negative or positive slope function across the initial series or an increase or decrease over the hiatus was independent of both stimulus intensity and hiatus duration, suggesting that it is an individual response to irritation. It has also been shown ( 13,15) that degrm of chili use is not a strong predictor of individual response. It is possible that whether sensitization or desensitization is shown may be determined by individual physiological differences, such as PROP taster status ( 1). In addition, individual responses could be influenced to some degree by cognitive or higher perceptual processes. Thus, psychophysical responses to individual capsaicin stimuli have been shown to be related to personality factors ( 16), although such factors have yet to be explored in relation to responses to multiple irritant stimuli. In the pain system generally, individual differences in responses to painful stimuli of increasing intensity have been identified ( 14), and have been suggested to represent variation in individual perceptual styles. Again, the extent to which such findings can shed light on the present data is unclear, but perhaps is a worthwhile avenue for further research. REFERENCES 1. B A 1 2. C D P t P B 5 1 1 m s M e p B G E 5 S i d s 1 1 c p i P G t r G l C p L 1 1 E p K s e 1 b 1 d a t C s m o e c c i G s V I N s M M e PRESCOTT AND STEVENSON 1480 D G Y T c d 1 G s G 1 G u r K E t L g S R r 1 a r 1 o f c c c i p C 1 S S p c c w 1 s r e m N d z p c t C h g P m f S a C 1 S s S g d F p A t P a u I r P C 9 U r i P c P f J I a R o E C p e w 1 c s e Y e 1 S c c p c 1 e D S c 1 i K M c 2 6 M b 4 R p I P r s z B v G V s — g s P o c ( b h1 s B 1 p P f s p P S n S N e T c 1 C S D E i d 1 C i d 4 B S o h 1 o C P – a G o s 1 s B t L S C s 4 B c N 1 S as P r t G h S L a g c t p M s N r 4 P 1 A e