Rapid recovery from capsaicin desensitization during recurrent stimulation

Inducible desensitization to capsaicin with repeated low-dose exposure in human volunteers

Responses to capsaicin are reduced following repeated exposure, a phenomenon known as capsaicin desensitization. Heavy consumers of chilies consistently report reduced oral burn relative to infrequent consumers, presumably due to chronic desensitization. However, the mechanism(s) underlying capsaicin desensitization remain poorly understood. We hypothesized that reduced response to capsaicin due to repeated oral exposure may result from a change in the expression of the capsaicin receptor (TRPV1) gene. To test this, we conducted two longitudinal desensitization studies in healthy human volunteers. In Study 1, 51 adults completed a 17-day capsaicin desensitization protocol. The study consisted of three in-person visits where they were asked to sample stimuli, including 3, 6, and 9 ppm capsaicin, and rate intensity on a general labeled magnitude scale (gLMS). Between days 3 & 17, participants rinsed at home with 6 ppm capsaicin (n = 31) or a control (n = 20) solution (20 uM sucrose octaccetate; SOA) twice a day. Before and after the oral exposure protocol, a clinician collected fungiform papillae. Participants randomized to the capsaicin rinse showed a statistically significant reduction in oral burn ratings that was not observed in controls, indicating repeated low-dose exposure can systematically induce desensitization. TRPV1 expression was not associated with reported capsaicin burn, and there was no evidence of a decrease in TRPV1 expression following capsaicin exposure. In Study 2, participants (n = 45) rinsed with 6 ppm capsaicin in a similar protocol, rating capsaicin, vanillyl butyl ether (VBE), cinnamaldehyde, ethanol, menthol, and sucrose on days 1, 3, & 17. Burn from capsaicin, VBE, cinnamaldehyde, and ethanol all showed a statistically significant change - capsaicin, VBE and cinnamaldehyde burn all dropped ∼20 %, and a larger reduction was seen for ethanol - while menthol cooling and sucrose sweetness did not change. Collectively, this suggests reductions in oral burn following chronic capsaicin exposure generalizes to other stimuli (i.e., cross desensitization) and this cannot be explained by a change in TRPV1 mRNA expression. More work is needed to elucidate the underlying mechanism for capsaicin desensitization in the oral cavity.

Bitter Taste Disrupts Spatial Discrimination of Piperine-Evoked Burning Sensations: A Pilot Study

Simple Summary

The chemical senses smell, taste, and trigeminal sense enable us to interact with the environment and play an essential role in protecting us from hazardous events. It is theorized that capsaicin and piperine not only elicit burning, but also bitter sensations through bitter taste-responding gustatory receptor cells that possess special channels. Similar psychophysiological responses to capsaicin and piperine suggest that bitter taste might also disrupt the spatial discrimination to piperine-induced burning sensations. Results showed that bitter taste disrupted the spatial discrimination of piperine-evoked burning sensations, providing further evidence for a qualitative similarity between burning and bitter sensations and the usefulness of chemical irritants in spatial discrimination tasks.

Abstract

This study aimed to investigate the perceptual similarity between piperine-induced burning sensations and bitter taste using piperine-impregnated taste strips (PTS). This pilot study included 42 healthy participants. PTS of six ascending concentrations (1 mg, 5 mg, 10 mg, 15 mg, 20 mg, and 25 mg piperine/dL 96% ethanol) were presented at the anterior tongue, and participants rated perceived intensity and duration. Then, participants performed a spatial discrimination task in which they had to report which of the two strips presented to the anterior tongue contained an irritating stimulus when one strip was always a PTS while the other strip was impregnated with either a single taste quality (sweet or bitter) or a blank strip. Repeated measures one-way ANOVA revealed that burning sensations of higher concentrated PTS were perceived more intense and more prolonged compared to lower concentrated PTS. McNemar’s test showed that PTS were identified correctly significantly less often when presented with bitter strips compared to when presented with blank (p = 0.002) or sweet strips (p = 0.017). Our results showed that bitter taste disrupts the spatial discrimination of piperine-evoked burning sensations. PTS might serve as a basis for further studies on disease-specific patterns in chemosensory disorders.

The orotrigeminal system

The trigeminal sensory nerve fiber branches supply afferent information from the skin and mucous membranes of the face and head and the oral cavity regarding information on temperature, touch, and pain. Under normal conditions, the trigeminal nerve serves to provide important information from nerve fibers and tissues using specialized receptors sensitive for irritant and painful stimuli. The current scientific consensus indicates that nerve endings responsible for chemical and thermal sensitivity of the skin and mucous membranes are the same nerves responsible for nociception. This "chemesthetic sense" allows many vertebrates to detect chemical agonists that induce sensations such as touch, burning, stinging, tingling, or changes in temperature. Research has been under way for many years to determine how exposure of the oral and/or nasal cavity to compounds that elicit pungent or irritant sensations can produce these sensations. In addition, these chemicals can alter other sensory information such as taste and smell to affect the flavor of foods and beverages. We now know that these 'chemesthetic molecules' are agonists of molecular receptors, which exist on primary afferent nerve fibers that innervate the orofacial area. However, under pathophysiologic conditions, over- or underexpression or activity of these receptors may lead to painful orotrigeminal syndromes. Some of these individual receptors are discussed in detail, including transient receptor potential channels and acid sensing ion channels, among others.

Behavioral measures of risk tasking, sensation seeking and sensitivity to reward may reflect different motivations for spicy food liking and consumption

Based on work a quarter century ago, it is widely accepted personality traits like sensation seeking are related to the enjoyment and intake of spicy foods; however, data supporting this belief is actually quite limited. Recently, we reported strong to moderate correlations between remembered spicy food liking and two personality traits measured with validated questionnaires. Here, participants consumed capsaicin-containing strawberry jelly to generate acute estimates of spicy food liking. Additionally, we used a laboratory-based behavioral measure of risk taking (the mobile Balloon Analogue Risk Task; mBART) to complement a range of validated self-report measures of risk-related personality traits. Present data confirm Sensation Seeking correlates with overall spicy meal liking and liking of the burn of a spicy meal, and extends prior findings by showing novel correlations with the liking of sampled stimuli. Other personality measures, including Sensitivity to Punishment (SP), Sensitivity to Reward (SR), and the Impulsivity and Risk Taking subscales of the DSM5 Personality Inventory (PID-5) did not show significant relationships with liking of spicy foods, either sampled or remembered. Our behavioral risk taking measure, the mBART, also failed to show a relationship with remembered or sampled liking. However, significant relationships were observed between reported intake of spicy foods and Sensitivity to Reward, and the Risk Taking subscale of the PID-5 (PID5-RT). Based on the observed patterns among various personality measures, and spicy food liking and intake, we propose that personality measures may exert their influence on intake of spicy food via different mechanisms. We also speculate that Sensation Seeking may reflect motivations for consuming spicy foods that are more intrinsic, while the motivations for eating spicy foods measured by SR and PID5-RT may be more extrinsic.

Differential bitterness in capsaicin, piperine, and ethanol associates with polymorphisms in multiple bitter taste receptor genes

To date, the majority of research exploring associations with genetic variability in bitter taste receptors has understandably focused on compounds and foods that are predominantly or solely perceived as bitter. However, other chemosensory stimuli are also known to elicit bitterness as a secondary sensation. Here we investigated whether TAS2R variation explains individual differences in bitterness elicited by chemesthetic stimuli, including capsaicin, piperine and ethanol. We confirmed that capsaicin, piperine and ethanol elicit bitterness in addition to burning/stinging sensations. Variability in perceived bitterness of capsaicin and ethanol were significantly associated with TAS2R38 and TAS2R3/4/5 diplotypes. For TAS2R38, PAV homozygotes perceived greater bitterness from capsaicin and ethanol presented on circumvallate papillae, compared to heterozygotes and AVI homozygotes. For TAS2R3/4/5, CCCAGT homozygotes rated the greatest bitterness, compared to heterozygotes and TTGGAG homozygotes, for both ethanol and capsaicin when presented on circumvallate papillae. Additional work is needed to determine how these and other chemesthetic stimuli differ in bitterness perception across concentrations and presentation methods. Furthermore, it would be beneficial to determine which TAS2R receptors are activated in vitro by chemesthetic compounds.

Integrating TRPV1 Receptor Function with Capsaicin Psychophysics

Capsaicin is a naturally occurring vanilloid that causes a hot, pungent sensation in the human oral cavity. This trigeminal stimulus activates TRPV1 receptors and stimulates an influx of cations into sensory cells. TRPV1 receptors function as homotetramers that also respond to heat, proinflammatory substances, lipoxygenase products, resiniferatoxin, endocannabinoids, protons, and peptide toxins. Kinase-mediated phosphorylation of TRPV1 leads to increased sensitivity to both chemical and thermal stimuli. In contrast, desensitization occurs via a calcium-dependent mechanism that results in receptor dephosphorylation. Human psychophysical studies have shown that capsaicin is detected at nanomole amounts and causes desensitization in the oral cavity. Psychophysical studies further indicate that desensitization can be temporarily reversed in the oral cavity if stimulation with capsaicin is resumed at short interstimulus intervals. Pretreatment of lingual epithelium with capsaicin modulates the perception of several primary taste qualities. Also, sweet taste stimuli may decrease the intensity of capsaicin perception in the oral cavity. In addition, capsaicin perception and hedonic responses may be modified by diet. Psychophysical studies with capsaicin are consistent with recent findings that have identified TRPV1 channel modulation by phosphorylation and interactions with membrane inositol phospholipids. Future studies will further clarify the importance of capsaicin and its receptor in human health and nutrition.

Influence of topical application of capsaicin, menthol and local anesthetics on intraoral somatosensory sensitivity in healthy subjects: temporal and spatial aspects

The aim of this study was to investigate temporal and spatial aspects of somatosensory changes after topical application of capsaicin, menthol and local anesthetics (LA) on the gingiva with the use of intraoral palpometers and thermal devices. Sixteen healthy volunteers (eight male, eight female) participated. Four topical preparations (capsaicin, menthol, LA and Vaseline as a control) were randomly applied to the gingiva around the first premolar in the upper jaw via individual oral templates, which allowed spatial mapping of somatosensory changes at and adjacent to the site of application. The topical drugs were applied for 15 min in a randomized and balanced sequence. The perceived preparation-evoked pain intensity was recorded with the use of 0-10 visual analog scales (VAS). Standardized mechanical and thermal stimuli were applied before, during and up to 30 min after the topical applications, and numerical rating scales (NRS) were used to score the perceived intensity of the stimuli. Peak VAS, area under the curve and mean VAS preparation-evoked pain scores for capsaicin, menthol, LA and control were compared with paired t tests. NRS scores for mechanical and thermal test stimuli were analyzed with four-way repeated measurements analyses of variance. Capsaicin evoked significantly higher VAS pain parameters as well as higher NRS scores to heat stimuli than control (P < 0.029). There were no significant differences in stimulus-evoked NRS scores between the menthol and control conditions (P = 0.518), but LA caused significantly lower stimulus-evoked NRS scores compared with control (P < 0.001). Post hoc tests showed that capsaicin caused sensitization to heat stimuli at and adjacent to the application area. In conclusion, this study for the first time demonstrates the time course of capsaicin-evoked heat hyperalgesia in and outside the site of application at the oral mucosa (primary and secondary hyperalgesia).

The pungent substances piperine, capsaicin, 6-gingerol and polygodial inhibit the human two-pore domain potassium channels TASK-1, TASK-3 and TRESK

For a long time, the focus of trigeminal chemoperception has rested almost exclusively on TRP channels. However, two-pore domain (K_2P) potassium channels have recently been identified as targets for substances associated with typical trigeminal sensations, such as numbing and tingling. In addition, they have been shown to be modulated by several TRP agonists. We investigated whether the pungent substances piperine, capsaicin, 6-gingerol and polygodial have an effect on human K_2P channels. For this purpose, we evaluated the effects of these pungent substances on both wild-type and mutant K_2P channels by means of two-electrode voltage-clamp experiments using Xenopus laevis oocytes. All four pungent substances were found to inhibit the basal activity of TASK-1 (K_2P 3.1), TASK-3 (K_2P 9.1), and TRESK (K_2P 18.1) channels. This inhibitory effect was dose-dependent and, with the exception of polygodial on TASK-1, fully reversible. However, only piperine exhibited an IC₅₀ similar to its reported EC₅₀ on TRP channels. Finally, we observed for TASK-3 that mutating H98 to E markedly decreased the inhibition induced by piperine, capsaicin, and 6-gingerol, but not by polygodial. Our data contribute to the relatively sparse knowledge concerning the pharmacology of K_2P channels and also raise the question of whether K_2P channels could be involved in the pungency perception of piperine.

Somatosensory profiling of intra-oral capsaicin and menthol in healthy subjects

This study was designed to investigate the effect of surrogate orofacial pain models on the quantitative sensory testing (QST) profile in healthy participants. Capsaicin, menthol, or saline (control) were applied topically onto the gingiva of 15 healthy subjects for 15 min. During application, the subjects rated pain intensity on a score of 0-10, on an electronic visual analog scale (VAS). A standardized intra-oral QST protocol was performed before and immediately after application. Data obtained before and after application were compared using rank-sum tests, and QST profiles were made after Z-transformation. Application of capsaicin caused moderate levels of pain (VAS(peak)  = 6.0 ± 0.7), and application of menthol produced mild levels of pain (VAS(peak)  = 1.8 ± 0.6). Capsaicin induced hypersensitivity to warmth, heat pain and cold pain and hyposensitivity to mechanical stimuli. Menthol induced hypersensitivity to cold and warmth. Saline caused hypersensitivity to heat pain and hyposensitivity to mechanical stimuli. However, somatosensory profiles from Z-scores demonstrated sensory gains regarding warmth detection and heat pain only after application of capsaicin. In conclusion, a standardized battery of QST showed somatosensory changes after application of capsaicin, menthol and saline to the gingiva. However, the Z-score-based profiles may only reflect the most prominent somatosensory changes and thus represent a conservative approach for evaluation of data.

Modulation of oral heat and cold pain by irritant chemicals

Common food irritants elicit oral heat or cool sensations via actions at thermosensitive transient receptor potential (TRP) channels. We used a half-tongue, 2-alternative forced-choice procedure coupled with bilateral pain intensity ratings to investigate irritant effects on heat and cold pain. The method was validated in a bilateral thermal difference detection task. Capsaicin, mustard oil, and cinnamaldehyde enhanced lingual heat pain elicited by a 49 degrees C stimulus. Mustard oil and cinnamaldehyde weakly enhanced lingual cold pain (9.5 degrees C), whereas capsaicin had no effect. Menthol significantly enhanced cold pain and weakly reduced heat pain. To address if capsaicin's effect was due to summation of perceptually similar thermal and chemical sensations, one-half of the tongue was desensitized by application of capsaicin. Upon reapplication, capsaicin elicited little or no irritant sensation yet still significantly enhanced heat pain on the capsaicin-treated side, ruling out summation. In a third experiment, capsaicin significantly enhanced pain ratings to graded heat stimuli (47 degrees C to 50 degrees C) resulting in an upward shift of the stimulus-response function. Menthol may induce cold hyperalgesia via enhanced thermal gating of TRPM8 in peripheral fibers. Capsaicin, mustard oil, and cinnamaldehyde may induce heat hyperalgesia via enhanced thermal gating of TRPV1 that is coexpressed with TRPA1 in peripheral nociceptors.

Gastroprotection induced by capsaicin in healthy human subjects

AIM: To evaluate the gastro-protective effect of capsaicin against the ethanol- and indomethacin (IND)-induced gastric mucosal damage in healthy human subjects.

METHODS: The effects of small doses (1-8 μg/mL, 100 mL) of capsaicin on the gastric acid secretion basal acid output (BAO) and its electrolyte concentration, gastric transmucosal potential difference (GTPD), ethanol- (5 mL 300 mL/L i.g.) and IND- (3×25 mg/d) induced gastric mucosal damage were tested in a randomized, prospective study of 84 healthy human subjects. The possible role of desensitization of capsaicin-sensitive afferents was tested by repeated exposures and during a prolonged treatment.

RESULTS: Intragastric application of capsaicin decreased the BAO and enhanced “non-parietal” component (GTPD) in a dose-dependent manner. The decrease of GTPD evoked by ethanol was inhibited by the capsaicin application, which was reproducible. Gastric microbleeding induced by IND was inhibited by co-administration with capsaicin, but was not influenced by two weeks pretreatment with a daily capsaicin dose of 3×400 μg i.g.

CONCLUSION: Capsaicin in low concentration range protects against gastric injuries induced by ethanol or IND, which is attributed to stimulation of the sensory nerve endings.

Effects of piperine, the pungent component of black pepper, at the human vanilloid receptor (TRPV1)

1. We have characterised the effects of piperine, a pungent alkaloid found in black pepper, on the human vanilloid receptor TRPV1 using whole-cell patch-clamp electrophysiology. 2. Piperine produced a clear agonist activity at the human TRPV1 receptor yielding rapidly activating whole-cell currents that were antagonised by the competitive TRPV1 antagonist capsazepine and the non-competitive TRPV1 blocker ruthenium red. 3. The current-voltage relationship of piperine-activated currents showed pronounced outward rectification (25+/-4-fold between -70 and +70 mV) and a reversal potential of 0.0+/-0.4 mV, which was indistinguishable from that of the prototypical TRPV1 agonist capsaicin. 4. Although piperine was a less potent agonist (EC50=37.9+/-1.9 microM) than capsaicin (EC50=0.29+/-0.05 microM), it demonstrated a much greater efficacy (approximately two-fold) at TRPV1. 5. This difference in efficacy did not appear to be related to the proton-mediated regulation of the receptor since a similar degree of potentiation was observed for responses evoked by piperine (230+/-20%, n=11) or capsaicin (284+/-32%, n=8) upon acidification to pH 6.5. 6. The effects of piperine upon receptor desensitisation were also unable to explain this effect since piperine resulted in more pronounced macroscopic desensitisation (t(1/2)=9.9+/-0.7 s) than capsaicin (t(1/2)>20 s) and also caused greater tachyphylaxis in response to repetitive agonist applications. 7. Overall, our data suggest that the effects of piperine at human TRPV1 are similar to those of capsaicin except for its propensity to induce greater receptor desensitisation and, rather remarkably, exhibit a greater efficacy than capsaicin itself. These results may provide insight into the TRPV1-mediated effects of piperine on gastrointestinal function.

Noxious chemical stimulation of rat facial mucosa increases intracranial blood flow through a trigemino-parasympathetic reflex--an experimental model for vascular dysfunctions in cluster headache

Cluster headache is characterized by typical autonomic dysfunctions including facial and intracranial vascular disturbances. Both the trigeminal and the cranial parasympathetic systems may be involved in mediating these dysfunctions. An experimental model was developed in the rat to measure changes in lacrimation and intracranial blood flow following noxious chemical stimulation of facial mucosa. Blood flow was monitored in arteries of the exposed cranial dura mater and the parietal cortex using laser Doppler flowmetry. Capsaicin (0.01-1 mm) applied to oral or nasal mucosa induced increases in dural and cortical blood flow and provoked lacrimation. These responses were blocked by systemic pre-administration of hexamethonium chloride (20 mg/kg). The evoked increases in dural blood flow were also abolished by topical pre-administration of atropine (1 mm) and [Lys1, Pro2,5, Arg3,4, Tyr6]-VIP (0.1 mm), a vasoactive intestinal polypeptide (VIP) antagonist, onto the exposed dura mater. We conclude that noxious stimulation of facial mucosa increases intracranial blood flow and lacrimation via a trigemino-parasympathetic reflex. The blood flow responses seem to be mediated by the release of acetylcholine and VIP within the meninges. Similar mechanisms may be involved in the pathogenesis of cluster headache.

Sensitization of trigeminal caudalis neuronal responses to intraoral acid and salt stimuli and desensitization by nicotine

In human studies, repeated intraoral application of strong acidic or salt stimuli induces irritation that progressively increases across trials (sensitization), whereas irritation elicited by nicotine progressively decreases (desensitization). We investigated whether nociceptive neurons in trigeminal subnucleus caudalis (Vc) exhibit increasing or decreasing patterns of firing to the intraoral application of these irritants. In rats anesthetized with halothane and thiopental, single-unit recordings were made from nociceptive neurons in superficial layers of dorsomedial Vc that responded to mechanical and noxious thermal and chemical stimulation of the tongue. NaCl (5M), citric acid (300 mM), pentanoic acid (300 mM) or nicotine (600 mM) were separately delivered to the tongue by constant flow (0.32 ml/min) for 15 or 25 min. NaCl, citric acid and pentanoic acid each elicited a progressive, significant increase in Vc neuronal firing over the initial 10 min to a plateau level that was maintained for the stimulus duration. Nicotine induced a significant increase in firing rate of Vc neurons within 6 min, followed by a decline back to the baseline level over the ensuing 10 min. Following a rest period, reapplication of nicotine no longer activated Vc neurons, indicative of self-desensitization. We additionally tested for nicotine cross-desensitization to acid. After recording the responses of Vc neurons to pentanoic acid and noxious heat, nicotine was then applied for 15 min. Post-nicotine responses to pentanoic acid were markedly reduced (to 13% of control), indicative of cross-desensitization; responses to noxious heat were also reduced to a lesser degree (to 71% of control). The progressive increase in Vc neuronal firing elicited by NaCl and acid, and the decline in firing after initial nicotinic excitation, resemble psychophysical patterns of sensitization and desensitization, respectively, and support the involvement of Vc neurons in the signaling of oral irritant sensations.

Oral irritation by sodium chloride: sensitization, self-desensitization, and cross-sensitization to capsaicin

Psychophysical methods were used to investigate the irritant sensory properties of concentrated NaCl. The first experiment investigated potential sensitization and desensitization properties. Subjects rated the intensity of the irritation elicited by 10 successive applications of 5 M NaCl on one side of the dorsal surface of the tongue. The mean irritant sensation increased significantly across trials, consistent with sensitization. To test for self- and cross-desensitization effects of unilateral sequential stimulation with NaCl followed by a 10-min rest period, either 5 M NaCl or 10 microM capsaicin was applied bilaterally. In a two-alternative forced-choice (2-AFC) test, subjects indicated which side of the tongue had a stronger irritant sensation. They also rated the intensity of irritation on each side separately. When NaCl was applied bilaterally, the side not previously receiving NaCl was chosen as stronger by a significant majority of subjects and was given significantly higher intensity ratings, consistent with self-desensitization. In contrast, when capsaicin was applied bilaterally, the side that had previously received sequential NaCl was perceived as having a significantly more intense irritation, consistent with cross-sensitization. In a second experiment, the effect of amiloride on NaCl-evoked irritation was studied. One side of the tongue was treated with 1 mM amiloride, after which 5 M NaCl was applied bilaterally and subjects performed the same 2-AFC and rating procedures. Since amiloride significantly reduced the intensity of the irritant sensation, the contribution of amiloride-sensitive ionic currents or the Na+/H+ exchange pump (NHE) are suggested as possible transduction mechanisms in lingual nociceptors mediating NaCl-evoked oral irritation.

Sensitization, desensitization and stimulus-induced recovery of trigeminal neuronal responses to oral capsaicin and nicotine

Repeated application of capsaicin at a 1-min interstimulus interval (ISI) to the tongue induces a progressively increasing irritant sensation (sensitization), followed after a rest period by reduced sensitivity to further capsaicin (desensitization). Sequential reapplication of capsaicin induces irritation that eventually increases to initial levels: stimulus-induced recovery (SIR). In contrast, repeated application of nicotine elicits a declining irritant sensation across trials. To investigate possible neural correlates of these phenomena, we recorded from single units in superficial laminae of the dorsomedial trigeminal subnucleus caudalis (Vc) that responded to noxious thermal (54 degrees C) and chemical (1 M pentanoic acid) stimulation of the tongue of anesthetized rats. We then recorded responses to either capsaicin (330 microM) or nicotine (0.6 M), delivered either once, repeatedly at 1-min ISI, or continually by constant flow. After the initial capsaicin application and a rest period, the capsaicin was reapplied in the identical manner to test for SIR. The mean response of 14 Vc units to sequential application of pentanoic acid did not vary significantly across trials, indicating lack of tachyphylaxis or sensitization. The averaged response of 11 Vc units to repeated capsaicin increased significantly across the first eight trials and then plateaued. Following the rest period, spontaneous firing had returned to the precapsaicin level. With capsaicin reapplication, the averaged response increased again after a significant delay (due to desensitization), but did not reattain the peak firing rate achieved in the initial series (partial SIR). Constant-flow application of capsaicin induced an identical sensitization followed by nearly complete SIR. A single application of capsaicin induced a significant rise in firing in eight other units, but the rate of rise and maximal firing rate were both much lower compared with repetitive or constant-flow capsaicin. When capsaicin was reapplied once after the rest period, there was no change in firing rate indicating absence of SIR. These results indicate that maintenance of the capsaicin concentration induces a progressive increase in neuronal response that parallels sensitization. With recurrent capsaicin application, desensitization can be overcome to result in a delayed recovery of Vc responses similar to SIR. In contrast, the averaged response of 17 Vc units to repeated or constant-flow application of nicotine increased only over the first 3 min, and then decreased to spontaneous levels even as nicotine was still being applied. These results are consistent with the decrease in the perceived irritation elicited by sequential application of nicotine in humans.

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.

Time course of burn to repeated applications of capsaicin

Human subjects rated the burn of 100 ppm capsaicin applied to the tongue via filter paper and replaced every minute. Capsaicin was presented at a constant concentration for 10 min, followed by 10 min of blank stimuli; this sequence was then repeated once. Both sensitization and desensitization were seen. A dynamic model composed of a level detector and change detector predicted the results and also previous data. Thus, the model provides a parsimonious description of responses to constant and intermittent capsaicin stimulation.

Capsaicin desensitization and stimulus-induced recovery on facial compared to lingual skin

It was recently demonstrated that capsaicin desensitization of the tongue can be temporarily reversed during bouts of recurrent or constant stimulation. The present study investigated whether this "stimulus-induced recovery" (SIR) also occurs on skin other than the oral mucosa. Twenty-two subjects received capsaicin treatments on the cheek and on the tongue tip at concentrations (330 and 33 microM) that produced approximately equal sensory irritation on the two sites. Desensitization and SIR occurred on both test sites, although the longer time course of irritation on the face changed the magnitude and form of SIR. There were large individual differences in the extent of desensitization and recovery, and the two phenomena were not correlated across sites, i.e., the capacity for SIR on the tongue was not a good predictor of an individual's capacity for SIR on the face. The results are discussed in terms of possible sources of regional and individual differences, and the implications they may have for the efficacy of topical analgesics that contain capsaicin.

Oral irritant effects of nicotine. Psychophysical evidence for decreased sensation following repeated application of and lack of cross-desensitization to capsaicin

When delivered to the oral mucosa, a variety of naturally occurring chemicals such as capsaicin from red chili peppers, piperine from black pepper, and nicotine from tobacco, cause a diffuse burning sensation often referred to as irritation. The burning sensation evoked by capsaicin increases when delivered repeatedly at 1 min intervals (sensitization), but then decreases markedly following a 10 min rest period (self-desensitization). It is also interesting that following desensitization by capsaicin, irritant sensations evoked by other chemicals are also reduced (cross-desensitization), suggesting that oral irritation from some agents may be mediated by a population of capsaicin-sensitive trigeminal polymodal nociceptors. Although nicotine is a major component in tobacco smoke, little is known about its sensory properties. Accordingly, a study of the oral irritant effects of nicotine as compared with capsaicin was initiated. Whereas capsaicin (0.5 or 3 ppm; repeated at 1 min intervals over 10 min) evoked significantly stronger sensations (sensitization), there was a significant decrement in sensations to repeated application of nicotine (0.1%). After the subjects had received either repeated capsaicin or nicotine on one side of the tongue, a rest period ensued followed by a bilateral application of either capsaicin or nicotine. Subjects were, then, asked to choose which side yielded a stronger sensation (two-alternative forced choice). Following capsaicin pretreatment, all 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 was reported to evoke a significantly weaker sensation on the side of the tongue pretreated with capsaicin (cross-desensitization). In contrast, equal numbers of subjects reported capsaicin to evoke a stronger sensation on either the nicotine-pretreated side or the untreated 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.

Activation of neurons in rat trigeminal subnucleus caudalis by different irritant chemicals applied to oral or ocular mucosa

To investigate the role of trigeminal subnucleus caudalis in neural mechanisms of irritation, we recorded single-unit responses to application of a variety of irritant chemicals to the tongue or ocular mucosa in thiopental-anesthetized rats. Recordings were made from wide dynamic range (WDR) and nociceptive-specific units in superficial layers of the dorsomedial caudalis (0-3 mm caudal to obex) responsive to mechanical stimulation and noxious heating of the ipsilateral tongue ("tongue" units) and from WDR units in ventrolateral caudalis (0-2 caudal to obex) responsive to mechanical and noxious thermal stimulation of cornea-conjunctiva and frequently also surrounding skin ("cornea-conjunctival" units). The following chemicals were delivered topically (0.1 ml) onto the dorsal anterior tongue or instilled into the ipsilateral eye: capsaicin (0.001-1% = 3.3 x 10(-2) to 3.3 x 10(-5) M), ethanol (15-80%), histamine (0.01-10% = 9 x 10(-1) to 9 x 10(-4) M), mustard oil (allyl-isothiocyanate, 4-100% = 4 x 10(-1) to 10 M), NaCl (0.5-5 M), nicotine (0.01-10% = 6 x 10(-1) to 6 x 10(-4) M), acidified phosphate buffer (pH 1-6), piperine (0.01-1% = 3.5 x 10(-2) to 3.5 x 10(-4) M), serotonin (5-HT; 0.3-3% = 1.4 x 10(-1) to 1.4 x 10(-2) M), and carbonated water. The dose-response relationship and possible tachyphylaxis were tested for each chemical. Of 32 tongue units, 31 responded to one or more, and frequently all, chemicals tested. The population responded to 75.3% of the various chemicals tested (</=10 per unit). The incidence of responses was independent of the order of chemicals tested, except for capsaicin, which reduced subsequent responses. Responses to histamine, nicotine, 5-HT, and ethanol had a more rapid onset and shorter duration compared with capsaicin, acid, and mustard oil. Responses to all chemicals increased in a dose-related manner. Successive responses to repeated application decreased significantly for nicotine, 5-HT, capsaicin, and piperine. Spontaneous firing increased significantly 5-10 min after initial application of capsaicin. Of 31 corneal-conjunctival units, 29 responded to one or more chemicals, and the population responded to 65% of all chemicals tested. Responses increased in a dose-related manner for all chemicals, and successive responses decreased significantly for histamine, nicotine, ethanol, acid, and capsaicin. Responses of tongue units to histamine and nicotine were reduced significantly by ceterizine (H1 antagonist) and mecamylamine, respectively. Mecamylamine also significantly reduced responses of corneal-conjunctival units to nicotine. Different classes of irritant chemicals contacting the oral or ocular mucosa can activate individual sensory neurons in caudalis, presumably via independent peripheral transduction mechanisms. Multireceptive units with input from the tongue or cornea-conjunctiva exhibited a similar spectrum of excitability to different irritant chemicals. Such neurons would not be capable of discriminating among different chemically evoked irritant sensations but could contribute to a common chemical sense.

Temporal characteristics of capsaicin desensitization and stimulus-induced recovery in the oral cavity

It was recently discovered that capsaicin desensitization of the tongue can be temporarily reversed during recurrent stimulation ("stimulus-induced recovery," SIR). The effects of concentration and the temporal pattern of stimulation on desensitization and SIR were studied in four experiments. In Experiment 1, three different concentrations of capsaicin (3.3, 33, or 330 microM) were delivered to the tongue in blocks of twelve stimuli at the rate of one per minute (treatment block) followed by a block of 20 exposures to 33 microM capsaicin (test block). The 33- and 330-microM treatments both caused significant desensitization, but by the end of the test block recovery was substantial only for the 33-microM condition. Experiment 2 provided a second test of the effect of concentration while also exploring whether intermittent stimulation was a requirement for SIR, i.e., stimuli were refreshed on the tongue every six minutes rather than every 60 s. SIR occurred but was pronounced only when the concentration of the test stimulus equaled or exceeded that of the treatment stimulus. In the third experiment we delivered capsaicin in a candy formulation to examine SIR under conditions in which stimulation was not interrupted at all. Although desensitization was evident during the first few minutes of succeeding exposures to the candy, peak irritation was unchanged throughout nine exposures over 3 days. In the final experiment, we returned to intermittent stimulation to examine the effect of lengthening ISI on SIR. The results showed that SIR remained virtually complete until ISI was lengthened to 60 s. We conclude that SIR is a robust phenomenon that is maximized by rapid exposure to capsaicin in a concentration at least as high as that of the desensitizing stimulus. The implications of these findings for hypotheses about the mechanism of SIR, its function in endogenous chemonociception, and for clinical use of capsaicin as a topical analgesic are discussed.