Acta Dermato-Venereologica 99-2CompleteContent | Page 16

188 INVESTIGATIVE REPORT Effects of Short-term Temperature Change in the Innocuous Range on Histaminergic and Non-histaminergic Acute Itch Zoe LEWIS 1 , David N. GEORGE 1 , Fiona COWDELL 2 and Henning HOLLE 1 Department of Psychology, University of Hull, Hull, and 2 Birmingham City University, Faculty of Health, Education and Life Sciences, Birmingham, UK 1 While temperatures in the noxious range are well- known to inhibit acute itch, the impact of temperature in the innocuous temperature range is less well under- stood. We investigated the effect of alternating short- term temperature changes in the innocuous range on histamine and cowhage-induced acute itch, taking into account individual differences in baseline skin tempe- rature and sensory thresholds. Results indicate that cooling the skin to the cold threshold causes a tem- porary increase in the intensity of histamine-induced itch, in line with previous findings. Skin warming in- creased cowhage-induced itch intensity. Potential mecha­ nisms of this interaction between thermosen- sation and pruritoception could involve cold-sensitive channels such as TRPM8, TREK-1 or TRPC5 in the case of histamine. The rapid modulation of cowhage indu- ced itch – but not histamine-induced itch – by trans- ient skin warming could be related to the lower tem- perature threshold of pruriceptive polymodal C-fibres (cowhage) as compared to the higher temperature threshold of the mechanoinsensitive C-fibres convey- ing histaminergic itch. Key words: skin temperature; histamine; sensory thresholds; pruritus. Accepted Oct 25, 2018; E-published Oct 25, 2018 Acta Derm Venereol 2019; 99: 188–195. Corr: Dr Henning Holle, Department of Psychology, University of Hull, Cottingham Road, Hull HU6 7RX, U.K. E-mail: [email protected] I t is well documented that thermal counter-stimulation in the noxious range reduces the intensity of acute itch (1–4). However, the effect of brief, less extreme changes of temperature on itch, either by slightly increasing or decreasing the skin temperature from its baseline tem- perature of about 32°C, is less well understood. Deter- mining this relationship is not only of clinical interest, but also relevant for basic research on itch in humans, which often requires an experimental itch model where itch intensity can be quickly modulated (5). Several studies have investigated the effect of in- nocuous warmth on experimentally induced acute itch. Ward et al. (2) induced itch using histamine iontophoresis and found that warming the skin directly adjacent to the itch induction site to 38°C using a thermode did not influence itch intensity. Yosipovitch et al. (4) also used histamine iontophoresis in combination with a thermode applied 3 cm distal to the itch induction site, and again doi: 10.2340/00015555-3077 Acta Derm Venereol 2019; 99: 188–195 SIGNIFICANCE Determining the influence of temperature changes in the non-painful range on itch is not only of clinical interest, but also relevant for basic psychophysical research, which often requires that itch intensity can be reliably modulated within a matter of seconds in an on–off fashion. We cha- racterized the effect of short-term temperature changes on two types of itch sensations; histamine and cowhage-indu- ced itch. Our result shows that both itch pathways can be modulated by short-term changes in temperature, poten- tially enabling statistically powerful neuroimaging studies, to further elucidate the cortical network underpinning the clinically relevant sensation of itch. found that repeatedly warming the skin to 41°C did not modulate itch intensity. Fruhstorfer and colleagues (1) used a slightly different approach in that the thermode was placed directly on the itching skin site. Skin tempe- rature was then slowly increased from baseline at a rate of 0.5°C/s until heat pain was reported. They observed large inter-individual differences in the effect of warmth. The majority of participants (n = 23) reported a decrease in itch intensity as temperature increased. Notably this effect was not restricted to the noxious range, but also evident at sub-noxious temperatures below 40°C. A smal- ler subset of participants (n = 7) reported the opposite pat- tern, with itch increasing as temperature increased. A final subset (n = 4) showed no influence of warmth on itch. In summary, findings about a potential effect of in- nocuous warmth on itch are mixed. The evidence sug- gests that directly warming the affected skin site (1), as opposed to an adjacent skin site (2, 4), may be a more sensitive approach. Another limitation of existing stu- dies is that inter-individual differences in baseline skin temperatures and individual warm thresholds have so far not been taken into account (6). Regarding the effects of cooling on acute itch, findings are, at first glance, contradictory. Yosipovitch et al. (4) observed that repeatedly cooling skin adjacent to the itch site to 15°C (i.e., at the threshold from innocuous to noxious cold) does not influence itch intensity. Fruhstor- fer and colleagues (1) observed that slowly cooling the skin, from baseline at a rate 0.5°C/s until cold pain was perceived, led to a decrease in itch intensity. This effect was consistent across subjects and began to manifest itself already in the innocuous temperature range below This is an open access article under the CC BY-NC license. www.medicaljournals.se/acta Journal Compilation © 2019 Acta Dermato-Venereologica.