SECONDARY HYPERALGESIA AND PERCEPTUAL WIND-UP FOLLOWING INTRADERMAL INJECTION OF CAPSAICIN IN HUMANS

Wind-up and secondary hyperalgesia both are related to central sensiti zation, but whereas the former is explained by homosynaptic facilitati on, the latter is due to heterosynaptic facilitation. To investigate p ossible interactions between both types of facilitation, we tested for alterations of perceptual wind-up in the secondary hyperalgesic skin zone adjacent to a capsaicin injection with light touch (by a cotton w isp) and punctate stimuli (calibrated von Prey hairs and pin pricks). Temporal summation of pain sensation (perceptual wind-up) was only obs erved with a clearly noxious stimulus (pin prick) presented at a repet ition frequency of 0.6 s(-1), but not 0.2 s(-1). Pain ratings to train s of pin pricks reached a plateau after 3-4 repetitions, which was 1.6 5 times the initial rating ('wind-up ratio'). Injection of capsaicin i nduced a tenderness to mechanical stimuli in adjacent uninjured skin ( secondary hyperalgesia), including hyperalgesia to light touch (allody nia) and hyperalgesia to punctate stimuli. Hyperalgesia to punctate st imuli was characterized by a leftward shift of the stimulus response f unction, corresponding to a decrease in pain threshold and an increase of painfulness of suprathreshold stimuli by a factor of 3-4. After ca psaicin, the difference between the ratings of the first and last stim uli of trains of pin pricks was increased, but the ratio was unchanged . This behavior is equivalent to an increase in effective stimulus int ensity, and could be mimicked by increasing the pin prick force from 2 0 mN to 40 and 80 mN in normal skin. Thus, the leftward shift of the s timulus response function fully accounts for all alterations of pain s ensitivity to punctate stimuli in the zone of secondary hyperalgesia. We conclude that when the gain of spinal transmission was changed in s econdary hyperalgesia, the gain of wind-up remained unchanged. These f indings indicate that secondary hyperalgesia (heterotopic facilitation ) and wind-up of pain sensation (homotopic facilitation) are independe nt phenomena. (C) 1998 International Association for the Study of Pain . Published by Elsevier Science B.V.