Sensory interactions between capsaicin and temperature in the oral cavity

The interaction between thermal and chemical stimulation inthe oral cavity was studied in two experiments by measuringthe perceived intensity of thermal sensations in the presenceof capsaicin, and the perceived intensity of the ‘burning’sensations produced by capsaicin at several solution temperatures.It is demonstrated in the first experiment that capsaicin intensifiessensations of warmth (particularly at moderateto-high temperatures)and slightly but consistently reduces the intensity of perceivedcold. On the other hand, the burning sensation induced by capsaicinis enhanced by warming and inhibited by cooling. The secondexperiment confirmed the existence of a second inhibitory factorin addition to cooling, possibly of tactile origin. Viewed togetherthe results of both experiments indicate that complex sensoryinteractions may take place in the trigeminal system duringsimultaneous chemical, thermal and mechanical stimulation.     

Effects of Prostaglandin E2 and Capsaicin on Behavior and Cerebrospinal Fluid Amino Acid Concentrations of Unanesthetized Rats: A Microdialysis Study

Abstract: Prostaglandin E₂ (PGE₂) delivered to the spinal cord produces an increased sensitivity to noxious (hyperalgesia) and innocuous (allodynia) stimuli. The mechanisms that underlie this effect remain unknown, but a PGE₂-evoked enhancement of spinal neurotransmitter release may be involved. To address this hypothesis, we examined the effect of PGE₂ on CSF concentrations of amino acids and also the modulatory effect of PGE₂ on capsaicin-evoked changes of spinal amino acid concentrations using a microdialysis probe placed in the lumbar subarachnoid space. Amino acids were quantified using HPLC with fluorescence detection. Addition of 1 mM, but not 10 or 100 µM, PGE₂ to the perfusate for a 10-min period (flow rate, 5 µl/min) evoked an immediate increase (80–100%) in glutamate (Glu), aspartate (Asp), taurine (Tau), glycine (Gly), and γ-aminobutyric acid (GABA) concentrations. Similarly, capsaicin infusion (0.1–10 µM) induced a dose-dependent increase in Glu, Asp, Tau, Gly, GABA, and ethanolamine levels. Significant increases in amino acid levels evoked by PGE₂ or capsaicin were associated with a touch-evoked allodynia. The combination of PGE₂ (10 µM) and capsaicin (0.1 or 1.0 µM) at concentrations that individually had no effect together evoked a significant increase (60–100%) in Glu, Asp, Tau, Gly, and GABA concentrations and produced tactile allodynia. These data demonstrate that spinally delivered PGE₂ or capsaicin substantially elevates CSF concentrations of both excitatory and inhibitory amino acids. The capacity of PGE₂ to enhance and prolong capsaicin-evoked amino acid concentrations may be one of the mechanisms by which spinal PGE₂ produces hyperalgesia and allodynia.     

Capsaicin cross-desensitization on the tongue: psychophysical evidence that oral chemical irritation is mediated by more than one sensory pathway

Psychophysical measurements were made of the perceived intensityand quality of sensations of chemical irritation before andafter the tip of the tongue had been desensitized to capsaicin(10 ppm). The results of the first experiment showed that capsaicindesensitization tended to reduce the perceived intensity ofirritation produced by approximately equipotent concentrationsof capsaicin (3 ppm), ethanol (30%), cinnamic aldehyde (2.5%)and NaCl (5M) applied to the tongue on filter paper disks; however,the reduction in irritation was less for the latter three compoundsthan for capsaicin and failed to reach statistical significancefor ethanol. Ratings of sensation quality suggested that thefour irritants produced different quality ‘profiles’,and that ethanol and cinnamic aldehyde were characterized bysensations of numbness as well as by sensations of burning andstinging/pricking. Follow-up experiments in which subjects ratedthe perceived intensity of individual sensation qualities showedthat desensitization dramatically reduced the burning and stinging/prickingcomponents of irritation, but left the sensations of numbnessand chemogenic warmth unchanged. It is concluded that lingualchemesthetic sensations are multidimensional, and mediated byboth capsaicinsensitive and capsaicin-insensitive sensory pathways.     

Capsaicin as a cutaneous stimulus: sensitivity and sensory qualities on hairy skin

An experiment was conducted to investigate the sensitivity ofthe skin to capsaicin. Whereas most previous work on capsaicin'scutaneous (extra-oral) effects have focused on its ability tosensitize or desensitize the skin to subsequent stimulation,the present study measured the absolute sensitivity to, andthe sensations produced by, transient exposures to capsaicin.A wide range of concentrations of capsaicin was presented tothe volar forearm under conditions that prevented significantevaporation for the first 10 min of exposure, and subjects reportedthe sensations they experienced over a 20-min period. The resultsshowed that capsaicin produced a variety of sensations (includingitch, stinging/pricking and burning) that varied in time andfrequency of appearance. Missing from the subjective reportswas a significant thermal component to the sensation; capsaicinapparently failed to stimulate warm fibers either strongly orreliably. Overall, however, the variety of sensations inducedby capsaicin reflects the multi-modal nature of the chemicalsensitivity of the skin.     

Localization of Chemical Stimulation: Capsaicin on Hairy Skin

The ability to localize a chemical stimulus applied to the skin of the forearm was compared to the ability to localize a punctate tactile stimulus. The chemical stimulus was a single, 6-μ1 drop of a 1.0% solution of capsaicin in an ethanol vehicle; the tactile stimulus was a polyester monofilament that exerted 7.5 g of force. Subjects attempted to localize the stimuli at 30-sec intervals for a period of 13.5 min, and rated the perceived intensity and quality of the chemogenic sensations. To avoid generating potentially confounding tactile sensations, localization attempts were made by pointing to the area of sensation with a focused light beam. The results showed that overall, chemical localization was inferior to tactile localization: The absolute error of localization averaged 2.5 cm for capsaicin compared to 1.4 cm for the monofilament. The experiment also revealed that chemical localization (1) varied significantly across arms, (2) exhibited a relatively strong bias toward the elbow, and (3) appeared to be unaffected by the perceived intensity of the sensation. The dominant sensation quality reported was itch. The results are discussed in the context of cutaneous localization in general and localization in the nociceptive system in particular.     

Secondary heat hyperalgesia detected by radiant heat stimuli in humans: Evaluation of stimulus intensity and duration

Diverging observations on secondary hyperalgesia to heat stimuli have been reported in the literature. No studies have investigated the importance of heat stimulus intensity and duration for the assessment of secondary heat hyperalgesia. The present study was designed to investigate systematically (1) if pain sensitivity to radiant heat stimuli (focused Xenon light) is altered in the area of secondary punctuate hyperalgesia induced by intradermal injection of capsaicin and (2) if heat stimulus duration and intensity had an influence on the ability to detect secondary heat hyperalgesia.

Pain ratings to radiant heat stimuli from a focused xenon lamp were assessed within the area of secondary punctuate hyperalgesia in fifteen volunteers before and after intradermal injection of capsaicin. The stimulus conditions were systematically varied between three intensity levels (0.8, 1.0 and 1.2?×?heat pain threshold (PT)) and four duration steps (200, 350, 500 and 750?ms). The present study shows that long duration (350–750?ms) and low intensity (0.8 and 1.0 ×?PT) radiant heat stimuli were adequate to detect secondary heat hyperalgesia.     

Recruitment of occipital cortex during sensory substitution training linked to subjective experience of seeing in people with blindness

Over three months of intensive training with a tactile stimulation device, 18 blind and 10 blindfolded seeing subjects improved in their ability to identify geometric figures by touch. Seven blind subjects spontaneously reported 'visual qualia', the subjective sensation of seeing flashes of light congruent with tactile stimuli. In the latter subjects tactile stimulation evoked activation of occipital cortex on electroencephalography (EEG). None of the blind subjects who failed to experience visual qualia, despite identical tactile stimulation training, showed EEG recruitment of occipital cortex. None of the blindfolded seeing humans reported visual-like sensations during tactile stimulation. These findings support the notion that the conscious experience of seeing is linked to the activation of occipital brain regions in people with blindness. Moreover, the findings indicate that provision of visual information can be achieved through non-visual sensory modalities which may help to minimize the disability of blind individuals, affording them some degree of object recognition and navigation aid.     

Tactile sensitivity,tactile acuity,and affective touch: from childhood to early adolescence

The development of somatosensation and affective touch acquires a central role throughout our lives, for several reasons. In adults, these functions are driven by different, neuroanatomically and functionally segregated fibres. To date, very little is known about the basic features of these fibres in childhood and this lack of knowledge is mirrored in the affective touch domain, where there are no studies on the main physiological features of the tactile processes linked to the stimulation of the hairy skin, namely the preferential site of affective touch. Thus, our study aims to analyze (1) tactile sensitivity and tactile acuity of children’s hairy forearms; (2) a possible dissociation between somatosensation and the affective touch; and (3) the presence/absence of the perception of affective touch already in childhood. To these aims, participants (160 children, aged 6 to 14?years), were administered with the Von Frey (tactile sensitivity) and the 2 Point Discrimination (tactile acuity) tests. Affective touch was measured following the classic protocol and pleasantness ratings were recorded. Our findings showed a correlation between age and somatosensation, suggesting a progressive reduction of sensitivity and acuity as age grows. Further, there was no overlap between affective touch and somatosensation, suggesting a behavioural segregation. Lastly, we found higher pleasantness ratings for Affective versus Neutral stimulations at all ages and an enhanced preference for Affective as age grows. We concluded that both somatosensation and affective touch are already present as two separate components of touch in childhood and change as a function of age.     

Interactions between Chemical and Thermal Cutaneous Stimuli: Inhibition (Counterirritation) and Integration

Methyl salicylate, a commonly used chemical counterirritant, was applied topically to the forearm to determine whether a nonpainful chemical irritation could inhibit the perception of another (weaker) chemical irritation. In the first experiment, sensations of irritation (burning and stinging) produced by a 10% solution of methyl salicylate were significantly attenuated when a 15% solution of the same chemical was applied to the opposite forearm. In the second experiment, neither the perception of warmth nor the heat pain threshold was affected by application of 10% or 15% methyl salicylate to a site 10 cm from the thermal stimulus. Inhibition did, however, occur in the opposite direction: Chemical irritation was reduced after the thermal stimulus reached a painful level. In the third experiment, a 15% solution of methyl salicylate was applied immediately adjacent to the thermal stimulus, with the result that ratings of warmth intensity increased rather than decreased, and perceived irritation was again attenuated following a painful heat stimulus. Overall, the results indicate that (1) chemical counterirritation can occur at nonpainful levels; (2) the resulting inhibition is confined to the nociceptive system; and (3) when the nociceptive and warmth systems are activated together, the tendency is toward integration rather than inhibition.     

Interactions between chemical and thermal cutaneous stimuli: inhibition (counterirritation) and integration.

Methyl salicylate, a commonly used chemical counterirritant, was applied topically to the forearm to determine whether a nonpainful chemical irritation could inhibit the perception of another (weaker) chemical irritation. In the first experiment, sensations of irritation (burning and stinging) produced by a 10% solution of methyl salicylate were significantly attenuated when a 15% solution of the same chemical was applied to the opposite forearm. In the second experiment, neither the perception of warmth nor the heat pain threshold was affected by application of 10% or 15% methyl salicylate to a site 10 cm from the thermal stimulus. Inhibition did, however, occur in the opposite direction: Chemical irritation was reduced after the thermal stimulus reached a painful level. In the third experiment, a 15% solution of methyl salicylate was applied immediately adjacent to the thermal stimulus, with the result that ratings of warmth intensity increased rather than decreased, and perceived irritation was again attenuated following a painful heat stimulus. Overall, the results indicate that (1) chemical counterirritation can occur at nonpainful levels; (2) the resulting inhibition is confined to the nociceptive system; and (3) when the nociceptive and warmth system are activated together, the tendency is toward integration rather than inhibition.     

Implantation of Tumoral XC Cells Induces Chronic, Endothelin-Dependent, Thermal Hyperalgesia in Mice

1. We describe here the alterations in the nociceptive sensitivity of Swiss CD1 mice receiving an intraplantar (i.pl.) administration of XC Rous sarcoma-virus-transformed rat fibroblasts (XC cells). 2. Histological studies reveal that XC cells remain at the injection site 2-3 weeks after implantation, a time at which an inflammatory reaction is also detected. No tumoral growth was found and 5 weeks after inoculation neither XC cells nor inflammatory reaction were observed. 3. Measures to different types of noxious stimuli were performed. At week 1 after XC cell inoculation, hyperalgesia to thermal, but not mechanical, stimuli as well as to capsaicin injection is present in the implanted paw. At week 5 after XC cell implantation, only thermal hyperalgesia is present, and this enhanced reactivity persisted for even 25 weeks after the disappearance of XC tumoral cells. 4. Pharmacological studies on thermal hyperalgesia were conducted at two different stages, week 1 and week 5 after XC cell inoculation. The systemic administration of morphine (1-10 mg/kg i.p. (intraperitoneal); 30 min before testing) prevents this thermal hyperalgesic reaction both at week 1 and week 5. The endothelin type A (ETA) receptor antagonist BQ-123 (10 nmol; i.pl.; 90 min before testing) abolishes both the early (week 1) and the late (week 5) thermal hyperalgesia. In contrast, the selective endothelin type B (ETB) receptor antagonist, BQ-788 (10 nmol; i.pl.; 90 min before) abolishes thermal hyperalgesia only at week 1, but not at week 5 after XC cell inoculation. 5. It might be concluded that endothelins are probably involved in this type of long-term thermal hyperalgesia produced by the transitory presence of the XC tumoral cell line.     

Tactile and thermal detection thresholds of the scalp skin

The tactile and thermal sensitivity of diverse regions of the human body have been documented extensively, with one exception being the scalp. Additionally, sensory changes may accompany the hair loss from the scalp in androgen-related alopecia (ARA), but formal quantitative sensory testing (QST) has not been reported in respect of this. Therefore, light touch detection thresholds were obtained at nine scalp sites and one forehead site, using Semmes–Weinstein filaments (Von Frey hairs), and for warming and cooling from skin baseline temperature, using 28 and 256?mm² thermodes. Affective, thermal, and nociceptive sensations experienced at thermal detection threshold were quantified. Thirty-two male participants were recruited, 10 of whom had normal hair coverage, 12 of whom had shaved scalp but with potentially normal hair coverage, and 10 of whom exhibited ARA to some extent. The scalp was relatively insensitive to tactile and thermal stimulation at all tested sites, especially so along the midline and near the apex of the skull. Threshold level warm stimuli were rated less pleasant, the less sensitive the test site. After correction for age-related changes in sensitivity, bald scalp sites were found more sensitive to cooling than the same sites when shaved, consistent with prior informal reports of increased sensitivity for some scalp sensations in ARA. QST on hair-covered sites was subject to methodological issues that render such testing non-ideal, such as bias in measurement of resting skin temperatures, and the near impossibility of delivering filament stimuli to the scalp skin without disturbing neighboring hairs.     

Focused ultrasound in studies of somatic reception

Focused ultrasound has been used to elicit cutaneous tactile, thermal, specific and nonspecific pain sensations, and also subcutaneous (deep) sensations which included tactile and some pain sensations (muscular and periosteal etc.). It has been found that somatic reception can be attributed to mechanoreception, that the same receptive structures are involved in the sensations of warmth and cold, and that ultrasound has a sensitizing action. Studies have been made of sensation differences from corporal and auricular acupuncture points, and from some chosen skin and subcutaneous points.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, Saint Petersburg. I. P. Pavlov Institute of Physiology, Russian Academy of Sciences, Saint Petersburg. Translated from Neirofiziologiya, Vol. 24, No. 5, pp. 529–534, September–October, 1992.     

Texture perception through direct and indirect touch: an analysis of perceptual space for tactile textures in two modes of exploration

Considerable information about the texture of objects can be perceived remotely through a probe. It is not clear, however, how texture perception with a probe compares with texture perception with the bare finger. Here we investigate the perception of a variety of textured surfaces encountered daily (e.g., corduroy, paper, and rubber) using the two scanning modes - direct touch through the finger and indirect touch through a probe held in the hand - in two tasks. In the first task, subjects rated the overall pair-wise dissimilarity of the textures. In the second task, subjects rated each texture along three continua, namely, perceived roughness, hardness, and stickiness of the surfaces, shown previously as the primary dimensions of texture perception in direct touch. From the dissimilarity judgment experiment, we found that the texture percept is similar though not identical in the two scanning modes. From the adjective rating experiments, we found that while roughness ratings are similar, hardness and stickiness ratings tend to differ between scanning conditions. These differences between the two modes of scanning are apparent in perceptual space for tactile textures based on multidimensional scaling (MDS) analysis. Finally, we demonstrate that three physical quantities, vibratory power, compliance, and friction carry roughness, hardness, and stickiness information, predicting perceived dissimilarity of texture pairs with indirect touch. Given that different types of texture information are processed by separate groups of neurons across direct and indirect touch, we propose that the neural mechanisms underlying texture perception differ between scanning modes.     

The Sensory Effects of l-Menthol on Human Skin

Psychophysical measurements were made of the sensory effects of l-menthol applied topically to the forearm under controlled thermal conditions. In the first experiment, subjects judged the intensity and quality of sensations produced by warming or cooling the skin in the presence of menthol or the vehicle. During cooling, menthol intensified cutaneous sensations and increased reports of burning. During warming, menthol intensified sensations transiently at low temperatures and weakened them lastingly at higher temperatures; the frequency of reports of burning varied with intensity. A second experiment tested the hypothesis that menthol would lower the threshold for warmth and raise the threshold for heat pain. No change in either threshold was observed. The primary sensory effects of l-menthol on hairy skin are therefore to heighten the perception of cooling and to attenuate the perception of moderate warming. In contrast with other common chemical irritants, menthol's pungent qualities appear to be enhanced by cooling and suppressed by warming; this suggests that its sensory irritancy may be attributable to the stimulation of a population of high-threshold cold fibers or cold-sensitive nociceptors.     

Texture perception through direct and indirect touch: An analysis of perceptual space for tactile textures in two modes of exploration

Considerable information about the texture of objects can be perceived remotely through a probe. It is not clear, however, how texture perception with a probe compares with texture perception with the bare finger. Here we investigate the perception of a variety of textured surfaces encountered daily (e.g., corduroy, paper, and rubber) using the two scanning modes—direct touch through the finger and indirect touch through a probe held in the hand—in two tasks. In the first task, subjects rated the overall pair-wise dissimilarity of the textures. In the second task, subjects rated each texture along three continua, namely, perceived roughness, hardness, and stickiness of the surfaces, shown previously as the primary dimensions of texture perception in direct touch. From the dissimilarity judgment experiment, we found that the texture percept is similar though not identical in the two scanning modes. From the adjective rating experiments, we found that while roughness ratings are similar, hardness and stickiness ratings tend to differ between scanning conditions. These differences between the two modes of scanning are apparent in perceptual space for tactile textures based on multidimensional scaling (MDS) analysis. Finally, we demonstrate that three physical quantities, vibratory power, compliance, and friction carry roughness, hardness, and stickiness information, predicting perceived dissimilarity of texture pairs with indirect touch. Given that different types of texture information are processed by separate groups of neurons across direct and indirect touch, we propose that the neural mechanisms underlying texture perception differ between scanning modes.     

Touch inhibits touch: sanshool-induced paradoxical tingling reveals perceptual interaction between somatosensory submodalities

Human perception of touch is mediated by inputs from multiple channels. Classical theories postulate independent contributions of each channel to each tactile feature, with little or no interaction between channels. In contrast to this view, we show that inputs from two sub-modalities of mechanical input channels interact to determine tactile perception. The flutter-range vibration channel was activated anomalously using hydroxy-α-sanshool, a bioactive compound of Szechuan pepper, which chemically induces vibration-like tingling sensations. We tested whether this tingling sensation on the lips was modulated by sustained mechanical pressure. Across four experiments, we show that sustained touch inhibits sanshool tingling sensations in a location-specific, pressure-level and time-dependent manner. Additional experiments ruled out the mediation of this interaction by nociceptive or affective (C-tactile) channels. These results reveal novel inhibitory influence from steady pressure onto flutter-range tactile perceptual channels, consistent with early-stage interactions between mechanoreceptor inputs within the somatosensory pathway.     

Bone hyperalgesia after mechanical impact stimulation: A human experimental pain model

Hyperalgesia in different musculoskeletal structures including bones is a major clinical problem. An experimental bone hyperalgesia model was developed in the present study. Hyperalgesia was induced by three different weights impacted on the shinbone in 16 healthy male and female subjects. The mechanical impact pain threshold (IPT) was measured as the height from which three weights (165, 330, and 660?g) should be dropped to elicit pain at the shinbone. Temporal summation of pain to repeated impact stimuli was assessed. All these stimuli caused bone hyperalgesia. The pressure pain threshold (PPT) was assessed by a computerized pressure algometer using two different probes (1.0 and 0.5?cm²). All parameters were recorded before (0), 24, 72, and 96?h after the initial stimulations. The IPTs were lowest 24?h after hyperalgesia induction for all three weights and the effect lasted up to 72?h (p?2 probe was significantly lower than the PPT obtained with the 0.5?cm² probe, regardless of the time. Females developed more pronounced hyperalgesia reflected in reduced IPTs and PPTs (p?p?相似文献   

Tactile and thermal detection thresholds of the scalp skin

The tactile and thermal sensitivity of diverse regions of the human body have been documented extensively, with one exception being the scalp. Additionally, sensory changes may accompany the hair loss from the scalp in androgen-related alopecia (ARA), but formal quantitative sensory testing (QST) has not been reported in respect of this. Therefore, light touch detection thresholds were obtained at nine scalp sites and one forehead site, using Semmes-Weinstein filaments (Von Frey hairs), and for warming and cooling from skin baseline temperature, using 28 and 256 mm(2) thermodes. Affective, thermal, and nociceptive sensations experienced at thermal detection threshold were quantified. Thirty-two male participants were recruited, 10 of whom had normal hair coverage, 12 of whom had shaved scalp but with potentially normal hair coverage, and 10 of whom exhibited ARA to some extent. The scalp was relatively insensitive to tactile and thermal stimulation at all tested sites, especially so along the midline and near the apex of the skull. Threshold level warm stimuli were rated less pleasant, the less sensitive the test site. After correction for age-related changes in sensitivity, bald scalp sites were found more sensitive to cooling than the same sites when shaved, consistent with prior informal reports of increased sensitivity for some scalp sensations in ARA. QST on hair-covered sites was subject to methodological issues that render such testing non-ideal, such as bias in measurement of resting skin temperatures, and the near impossibility of delivering filament stimuli to the scalp skin without disturbing neighboring hairs.     

Is there modal opponency in the thermal senses?

1. 1.The sensations evoked by pairs of distinct thermal stimuli applied to the back of the hand were studied in 17 volunteer subjects. Four stimulus combinations were used; neutral-cold (NC), neutral-neutral (NN), neutral-warm (NW), and cold-warm (CW).

2. 2.The subjects were first asked to estimate the magnitude of the thermal sensations evoked by the thermal stimuli. On average, the four pairs were reported as increasing magnitude in the following order: NC, CW, NN, and NW, seeming to suggest that the subjects experienced the cold-warm combination as a composite sensation of cold and warmth intermediate between pure cold and pure warmth.

3. 3.When asked only to detect the presence of a cold stimulus, the subjects performed as well for the CW combination as for the CN combination. This second result indicates that the reported composite magnitude of CW does not result from a true opponency of cold and warmth but from a cognitive combination of distinct sensations of cold and warmth.