Regional and Individual Differences in Cutaneous Sensitivity to Chemical Irritants: Capsaicin and Menthol

Assessing regional sensitivity and desensitization to capsaicin among oral cavity mucosae

This study aimed to determine perceptual response differences and characterize desensitization to capsaicin over time across several oral cavity mucosae- the tongue, cheek, hard palate, and lip. For each region, subjects rated the intensity of capsaicin and a vehicle control over a 10-minute period. Following a rest period, capsaicin was reapplied on each pretreated area and subjects indicated which side felt more irritated then rated each side every 30 seconds, over 3.5 minutes. Results from the initial task indicated significantly greater irritation on the tongue than hard palate, hard palate than cheek and lip, but no significant differences between the cheek and lip. Time to max intensity was delayed on the hard palate compared to the tongue, cheek, and lip. Desensitization, as indicated by a significant proportion of subjects choosing the vehicle-pretreated side over capsaicin-pretreated side as having stronger irritation, was exhibited on the tongue and hard palate, but not the cheek and lip. Given these data, a secondary experiment that utilized a higher capsaicin concentration was conducted on the cheek and lip only. Results showed significantly higher overall irritation on the lip than the cheek. Desensitization was exhibited on both areas, although the extent was greater on the lip. Based on differences in sensitivity and the extent of desensitization among these areas, these results indicate oral cavity mucosae respond to, but are impacted differently by, capsaicin exposure.

Differences in the chemesthetic subqualities of capsaicin, ibuprofen, and olive oil

Chemesthetic sensations elicited by ibuprofen, extra-virgin olive oil, and capsaicin were compared to quantify perceptual differences between known agonists of TRPA1 and TRPV1. Extra virgin olive oil contains a phenolic compound, oleocanthal, which is thought to share unique chemesthetic qualities with the nonsteroidal anti-inflammatory drug, ibuprofen. Pilot work suggested participants had difficulty distinguishing between multiple chemesthetic subqualities (e.g., burn, sting, itch, tickle, etc.) in a multiattribute rating task. Here, we assessed overall irritation via direct scaling, and a check all that apply task was used to collect information about chemesthetic subqualities over time. Replicated ratings were collected at discrete intervals using the generalized labeled magnitude scale to generate time-intensity curves; maximum intensity (Imax) and area under the curve were extracted for each participant. Intensity responses varied substantially across participants, and within a participant, the relationship was strongest between ibuprofen and olive oil. However, there were also positive, albeit weaker, correlations between capsaicin and ibuprofen and capsaicin and olive oil. The correlation found between olive oil and capsaicin may suggest the presence of unknown TRPV1 agonists in olive oil. This view was also supported by the qualitative data: capsaicin was described most often as burning and warm/hot, whereas ibuprofen was numbing and tickling. Olive oil shared characteristics with both capsaicin (warm/hot) and ibuprofen (tickle).

Fragrance material review on l-menthol

A toxicologic and dermatologic review of l-menthol when used as a fragrance ingredient is presented.

Evaluation of a quantitative clinical method for assessment of sensory skin irritation

Sensory skin irritation refers to the myriad of symptomatic complaints (e.g., sting and burn) frequently associated with inflammatory skin conditions or skin intolerance to various chemicals or finished products. Sensory irritation is an important factor in consumer acceptance of the products that they buy and use; however, from a safety testing and risk assessment standpoint, it has been difficult to evaluate. Recently, methods have been developed to more quantitatively assess sensory irritation using a semantically-labeled scale of sensation intensity, the labeled magnitude (LM) scale. Using this device, studies were conducted to determine if test subjects' perceptions of recalled or imagined sensory responses (from a series of survey questions) were related to their actual sensory reactivity to chemical challenge. Subjects were presented with 15 skin sensation scenarios of varying intensities and asked to record their self-perceived recalled or imagined responses using the LM scale. Individual and mean responses to each of the 15 survey questions were compared within and across studies. Considerable variation was seen between subjects' responses to the questions, particularly for questions pertaining to stronger stimuli (e.g., scalding water or skin lacerations). There was also little consistency seen in the pattern of individual responses across the questions. However, among 4 different study populations, the group mean scores for each of the 15 survey questions showed a high degree of consistency. Also, in spite of the variability in perceived responses to the recalled/imagined skin sensations, statistically significant dose-response and time-response patterns were observed in chemical (lactic acid and capsaicin) challenge studies. In one capsaicin study, a direct relationship was observed, among 83% of the study subjects, between the mean recall intensity scores and actual responses to subsequent capsaicin challenge. This pattern was not seen in a lactic acid challenge study. However, a similar relationship was seen in this study if only recall stimuli related to sting-type responses were included in the analysis. Hence, use of recall/imagined skin sensation perception data for prediction of actual reactivity to chemical probes may have screening utility depending on the survey questions used. On the whole, the LM scale is of practical use for quantifying subjective sensory irritation responses. Combined with evolving noninvasive instrumental and bioassay procedures for identifying biophysical or inflammatory markers of sensory irritation, better methods are on the horizon for improving our sensory skin irritation testing and risk assessment capabilities.

Intra-individual variations in acute and cumulative skin irritation responses

It is well-known that humans show a wide range of variation in skin reactivity to irritant chemicals. This has been established through population studies, through the examination of inter-subject variability, and (to a limited extent) through studies of skin site variation in response within subjects. However, simple response variability within individual test subjects has not been examined as carefully, and this has implications for our ability to predict irritant reactivity. Some key questions are: (i) how consistently do human beings respond, even within a given study, to different equally irritating chemicals, or to the same chemical when comparing different concentrations or durations of exposure, and, (ii) Do individual test subjects' responses to one chemical (or exposure scenario) correlate with their responses to another? To examine these questions in some detail, we reexamined individual study subjects' responses from earlier published studies involving both acute and cumulative irritation patch test protocols. Acute irritation responses were compared across chemicals with similar irritation profiles. Cumulative irritation responses were compared across different concentrations of the anionic surfactant, sodium dodecyl sulfate (SDS). Acute (high concentration) and cumulative (low concentration) patch test responses to SDS were also compared. The analysis showed that, as might be expected, response correlations were greatest within test types, either when comparing chemicals of similar overall irritancy, or when comparing similar concentrations of a single chemical. However, individually divergent responses were also frequent, reinforcing the conclusion that a given individual's response to one chemical or exposure condition does not always predict their response to another. This has important ramifications for other questions related to population differences in skin reactivity.

Validity and ethics of the human 4-h patch test as an alternative method to assess acute skin irritation potential

For more than 50 years, the Draize rabbit skin irritation test has reigned supreme as the regulatory method of choice for the identification of skin irritant chemicals. To date no in vitro alternative test has been validated as an adequate replacement. However, one potential option, to test the endpoint of concern (skin irritation) in the species of concern (man) has been overlooked. The advent of predictive in vitro tools for the identification of substances corrosive to the skin has opened up the practical possibility of carrying out safe and ethical studies on small panels of humans. The human 4-h patch test has been developed to meet the needs of identifying chemical skin irritation potential, providing data which is inherently superior to that given by a surrogate model, such as the rabbit. This paper reviews in detail the present state of the human 4-h patch test, highlighting its advantages and noting its utility as the 'gold standard' on which to build future in vitro models.

Menthol desensitization of capsaicin irritation. Evidence of a short-term anti-nociceptive effect

Evidence is presented of a short-term antinociceptive effect of menthol that was discovered in the course of investigating menthol's potential to sensitize the mouth to capsaicin. Previous research had shown that treating the tongue with menthol 15 min before exposure to capsaicin could enhance the irritancy of capsaicin, and we wished to learn if this effect would increase as the time between exposure to menthol and capsaicin decreased. We found instead that when capsaicin followed menthol by only 3.5 min, or when it was presented in mixture with menthol for 2-3 min, sensory irritation was reduced rather than enhanced. We examined the duration of this apparent crossdesensitization in a second experiment by varying the delay between exposure to menthol and a block of three consecutive capsaicin stimuli. Cross-desensitization tended to decline as the interstimulus interval (ISI) increased to 5 min, and even when desensitization was maximal, it was significant only for the first of the three capsaicin stimuli. In the final experiment we investigated how menthol self- and cross-desensitization can influence the perception of menthol-capsaicin mixtures. During a series of five, 90-s stimulations, self- and cross-desensitization became evident at the beginning of the second exposure, but the effect on mixture intensity again diminished rapidly as stimulation continued. We infer from these results that method can transiently desensitize capsaicin-sensitive fibers, but that exposure to capsaicin rapidly overrides the effect. The implications these findings have for menthol's potential as a topical analgesic are discussed.

Acetone odor and irritation thresholds obtained from acetone-exposed factory workers and from control (occupationally unexposed) subjects

Sensitivity of olfaction (smell) and chemesthesis (irritation) was evaluated for 2-propanone (acetone) and 1-butanol in acetone-exposed workers (AEW; N = 32) during a workday and unexposed subjects (microES; N = 32). Irritation sensitivity was assessed using a method that relies on the ability of individuals to localize irritants on the body. When a volatile compound is inhaled into one nostril and air into the other, the stimulated side can be determined (lateralized) only after the concentration reaches a level that stimulates the trigeminal nerve (irritation); compounds stimulating olfaction alone cannot be lateralized. Intranasal lateralization thresholds offer an objective measure of sensory irritation elicited by volatile compounds. Test results indicated that neither olfactory nor lateralization thresholds for butanol differed between AEW and microES. Olfactory thresholds to acetone in AEW (855 ppm) were elevated relative to those of microES (41 ppm), as were lateralization thresholds (36,669 ppm and 15,758 ppm, respectively). Within AEW, level of occupational exposure was not correlated with thresholds. Other measures revealed that microES used more irritation descriptors than did AEW on trials where the acetone concentration was below the lateralization threshold. This is noteworthy because microES received lower concentrations of acetone to evaluate than did AEW. These results suggest that exposures to acetone induce changes in acetone sensitivity that are specific to acetone. The acetone concentrations eliciting sensory irritation using the lateralization technique were all well above current occupational exposure standards. The current study indicates that acetone is a weak sensory irritant and that sensory adaptation is an important factor affecting its overall irritancy.