Psychophysical and physiological evidence for parallel afferent pathways mediating the sensation of itch

Abstract

The neuronal pathways for itch have been characterized mainly based on responses to histamine. Intracutaneous application of histamine produces intense itch and a large area of axon-reflexive vasodilation ("flare") around the application site. Both phenomena are thought to be mediated through neuronal activity in itch-specific, mechanoinsensitive C-fiber afferents (CMi). However, mechanical and electrical stimuli that do not activate CMi fibers can cause the sensation of itch, and itch may occur without flare, suggesting that other neuronal itch pathways exist. Because cutaneous application of spicules from the plant Mucuna pruriens (cowhage) has been anecdotally reported to produce itch without flare, we performed psychophysical experiments to investigate whether the mechanisms underlying cowhage- and histamine-induced itch differ. Although histamine and cowhage produced itch of similar magnitude, the itch to cowhage was not correlated with the itch to histamine; some subjects had intense itch to cowhage and little itch to histamine and visa versa. Laser Doppler measurements of blood flow revealed that histamine led to a large area of vasodilation, whereas cowhage produced vasodilation restricted to the application site. Pretreatment of the skin with an antihistamine blocked the itch produced by histamine but did not prevent cowhage-induced itch. Desensitization of the skin with topical capsaicin abolished cowhage-induced itch but did not significantly alter histamine-induced itch. These findings indicate that cowhage itch is signaled through a population of capsaicin-sensitive afferent nerve fibers that is distinct from CMi fibers mediating histamine-induced itch. Cowhage may be useful to investigate the neural pathway mediating nonhistaminergic itch.

Figure 1.

Histamine and cowhage produced itch that was greater than saline or inactive cowhage. A, Time course of itch ratings of a typical subject to active and inactive cowhage, histamine, and saline. The itch sensation to active cowhage and histamine peaked during the first few minutes. The itch sensation to cowhage dissipated faster than that to histamine. Inactive cowhage and saline produce only minimal itch. Subjects were asked to assign a number (modulus) to their recollection of the intensity of itch from a mosquito bite. Subjects then rated the intensity of itch sensation relative to this modulus. Ratings were obtained every 10 s and normalized to the mosquito-bite modulus. The mosquito symbol emphasizes mosquito-bite itch at a rating of 1. To combine data across subjects, the average normalized itch ratings over the first 3.5 min (interval 1, indicated by bracket) and the last 3.5 min (interval 2) were computed for each subject. Gray bars below the graph indicate time of laser Doppler scans. B, The averaged itch ratings during interval 1 are plotted (n = 12). The magnitude of itch to histamine and cowhage were comparable. C, The averaged itch ratings during interval 2 are plotted. Cowhage itch during interval 2 was significantly less than during interval 1 (Friedman's repeated-measures ANOVA, p < 0.0001, followed by Dunn's multiple comparison, **p < 0.01, ***p < 0.001; Wilcoxon's matched-pairs signed-ranks test, ^%%p < 0.01, cowhage interval 1 versus cowhage interval 2; median ± 25th and 75th quartiles).

Figure 2.

Histamine but not cowhage produced a large area of flare. The area and the intensity of flare after histamine administration was significantly greater than the area and intensity of flare after active cowhage, inactive cowhage, and saline. A, Analyzed laser Doppler images from each subject (1–12) showing scans obtained 3 min after application of histamine, saline, cowhage, and inactive cowhage. Histamine-induced flare was substantial in each subject. Flares produced by saline, cowhage, and inactive cowhage were minimal. B, The laser Doppler flare area was computed based on the number of “valid pixels” remaining after the threshold procedure was performed. Visual flare was marked on the skin 10 min after the drug applications (Friedman's repeated-measures ANOVA, p < 0.0001, with Dunn's multiple comparisons; median area ± 25th and 75th quartiles). C, Mean flare intensity computed based on the number of pixels above an intensity of 125 PU (repeated-measures ANOVA, p < 0.0001, with Bonferroni's multiple comparisons; **p < 0.01 and ***p < 0.001, histamine vs saline and ^#p < 0.05, ^##p < 0.01, and ^###p < 0.001, histamine vs active cowhage; mean ± SEM).

Figure 3.

Histamine-induced but not cowhage-induced itch was significantly reduced by pretreatment with a topical antihistamine. Subjects (n = 15) rated the intensity of itch sensation from cowhage and histamine at sites pretreated with either a placebo cream or an antihistamine cream. A, Ratings were obtained every 10 s and normalized to the mosquito-bite modulus (median itch ratings ± 25th and 75th quartiles). Gray bars below the graph indicate time of laser Doppler scans. B, Itch ratings were averaged over the 0–3.5 min time period, and the median ± 25th and 75th quartiles are presented (Friedman's repeated-measures ANOVA, p < 0.0001, followed by Dunn's multiple comparison, ***p < 0.001). C, The area of flare at 3 min after histamine injection was significantly less in the area pretreated with the antihistamine (median areas ± 25th and 75th quartiles; Wilcoxon's matched-pairs signed-ranks test, ***p < 0.001).

Figure 4.

Histamine-induced flare is correlated with the magnitude of itch sensation. Cowhage-induced flare is not correlated with itch. The flare area determined from the laser Doppler scan is plotted as function of the averaged itch rating in interval 1 for each subject (n = 14). A, Histamine flare area versus averaged itch rating (0–3.5 min). B, Cowhage flare area versus averages itch rating (0–3.5 min). Note different scale. C, Flare area induced by histamine at the antihistamine-treated site is correlated with the flare area at the placebo-treated site.

Figure 5.

Cowhage-induced itch does not predict histamine-induced itch, suggesting independent mechanisms of action. For each subject, the averaged itch ratings (interval 1) for histamine are plotted against the average itch ratings for cowhage. Data come from study 1 (gray circles, gray dashed line) and the placebo site of study 2 (black squares, black dashed line).

Figure 6.

Capsaicin pretreatment prevented cowhage-induced itch. Subjects (n = 6) rated the intensity of itch sensation from cowhage and histamine at sites pretreated for 3 d with either a placebo cream or 0.1% capsaicin cream. Itch ratings were averaged over the 0–3.5 min time period (median ± 25th and 75th quartiles; *p < 0.05, Wilcoxon's matched-pairs test).