The effect of heat conditioning of the primary area before and after induction of hyperalgesia by topical/intradermal capsaicin or by controlled heat injury

The aim of the present study was to test the effect of heat conditioning before and after the induction of hyperalgesia. Three different methods were used for induction of hyperalgesia, topical capsaicin, intradermal capsaicin injection, and a controlled heat injury. The vascular (blood flow and skin temperature) and sensory changes (area of secondary hyperalgesia and ongoing pain) associated with the cutaneous hyperalgesia were compared. Each experiment consisted of two randomized sessions separated by at least 2 days. In one session, pre-conditioning of the skin by heat was performed 30 min before the induction of hyperalgesia using a probe at 45 degrees C for 5 min in the center of the expected primary hyperalgesic area. After the induction of hyperalgesia, heat conditioning was performed twice in the center of the primary hyperalgesic area using a temperature of 2 degrees C above the present individual pain threshold. On the contra-lateral arm, no heat conditioning was applied while hyperalgesia was induced using the same method. This session was evaluated as a control. The preconditioning induced an increased skin temperature in the primary area for both topical capsaicin and the controlled heat injury. Post-conditioning caused increased blood flow in the secondary hyperalgesic area for the topical capsaicin method and increased blood flow in the primary hyperalgesic area for the controlled heat injury method. However, conditioning with heat in an attempt to increase the C-fiber input did not have any effect on the ongoing pain ratings and sensory test results in any of the methods. The results of the present study suggest that there is still a need for a better experimental model with more stable allodynia both between sessions and between subjects while at the same time minimizing discomfort to the volunteer