P47-S Cold C-fibers sensitization to warm stimulation after ultraviolet B and heat-capsaicin exposure

Abstract

Background Capsaicin, UV-B irradiation and electrical stimulation methods have been used to develop experimental hyperalgesia in Phase-I human studies. These sensitization models contribute to the study of the functional behavior of small fiber C-nociceptors. However, there is no published literature about functional changes of cold C-fibers using these models. Material and methods Skin sensitization was induced at the dorsum of the foot (superficial peroneal nerve) in two healthy volunteers using ultraviolet B radiation in one female and heat/capsaicin in one male. Microneurographic recordings were performed in the intermediate branch of the superficial peroneal nerve. Then trains of cold and warm stimuli, from 32 to 4 and 32 to 48-Celsius degrees respectively, were applied in the skin area of interest before and at least 45 min after induction of the skin sensitization models. Results Preliminary results showed a single cold C-fiber recorded in each subject. During cold stimulation, there were not functional variations in the cold receptors. Warm stimulation induced a reduction of the spontaneous firing rate of the cold fibers during the baseline recording in both models (room temperature at 25 °C). After the UVB and heat-capsaicin induction, the firing rate of the cold C-fibers in response to cold stimuli was increased. All cold C-fibers recorded became also sensitized to warm stimuli. Conclusions The ultraviolet B radiation and the heat-capsaicin sensitization models sensitize cutaneous cold C fibers to warmth. Some cases of warm allodynia in humans could partially be explained by skin sensitization after inflammatory processes (e.g., sunburn or thermal/chemical exposure).