Effect of sympathetic activity on capsaicin-evoked pain, hyperalgesia, andvasodilatation

Background: Painful nerve and tissue injuries can be exacerbated by activit y in sympathetic neurons. The mechanisms of sympathetically maintained pain (SMP) are unclear. Objective: To determine the effect of cutaneous sympath etic activity on pain induced by primary afferent C-nociceptor sensitizatio n with capsaicin in humans. Methods: In healthy volunteers capsaicin was ap plied topically (n = 12) or injected into the forearm skin (n = 10) to indu ce spontaneous pain, dynamic and punctate mechanical hyperalgesia, and anti dromic (axon reflex) vasodilatation (flare). Intensity of pain and hyperalg esia, axon reflex vasodilatation (laser Doppler), and flare size and area o f hyperalgesia (planimetry) were assessed. The local skin temperature at th e application and measurement sites was kept constant at 35 degrees C. In e ach individual the analyses were performed during the presence of high and low sympathetic skin activity induced by whole-body cooling and warming wit h a thermal suit. By this method sympathetic vasoconstrictor activity is mo dulated in the widest range that can be achieved physiologically. The degre e of vasoconstrictor discharge was monitored by measuring skin blood flow ( laser Doppler) and temperature (infrared thermometry) at the index finger. Results: The intensity and spatial distribution of capsaicin-evoked spontan eous pain and dynamic and punctate mechanical hyperalgesia were identical d uring the presence of high and low sympathetic discharge. Antidromic vasodi latation and flare size were significantly diminished when sympathetic vaso constrictor neurons were excited. Conclusions: Cutaneous sympathetic vasoco nstrictor activity does not influence spontaneous pain and mechanical hyper algesia after capsaicin-induced C-nociceptor sensitization. When using phys iologic stimulation of sympathetic activity, the capsaicin model is not use ful for elucidating mechanisms of SMP. In neuropathic pain states with SMP, different mechanisms may be present.